Breast Cancer Cell Bio

1. Overview

Breast cancer is the most common cancer among American women, excluding non-melanoma skin cancers.1 2 In a woman’s lifetime, she has approximately a 1 in 8 (12%) chance of developing invasive breast cancer.2 In 2010, The American Cancer Society released estimates that approximately 207,090 new cases of invasive breast cancer and 54,010 new cases of carcinoma in situ (CIS) would be diagnosed that year.2 The cases of male breast cancer are rare, making up just under 1% of all newly diagnosed breast cancer (0.94%).3 Breast cancer was estimated to account for 15% of all cancer deaths in women in 2010, this was second only to lung cancer (26%).3 The deaths related to breast cancer have been steadily declining since 1990. Even larger decreases are noted in women over 50 years old, which is thought to be related to earlier detection, increased awareness, and improved treatment.2 It may also be because of the reduction in use of hormone therapy after menopause.2 The reduction in the use of hormone replacement therapy with menopause is a result of the Women's Health Initiative, which was published in 2002.2 They linked the use of hormone replacement therapy to an increased risk of breast cancer and heart diseases.2 This is uplifting information as approximately 70% of those diagnosed with breast cancer are over 50 years old.4 Currently there are over 2.5 million breast cancer survivors in the United States alone.2

80% of breast cancers originate in the ducts that bring the milk to the nipple, 8% stem from the lobules, which are the milk-producing glands of the breast, and 12% originate in combination.5 Breast carcinomas are most commonly adenocarcinomas that originate in the single layer of epithelial cells that line the ductal and lobular systems of all milk ducts.4 It is believed that germline mutations of the BRCA1, BRCA2, and other genes account for 5-10% of breast cancer cases reported and may be inherited.4 6 There are eight different types of breast cancer: ductal carcinoma in situ (DCIS), invasive (infiltrating) ductal carcinoma (IDC), invasive (infiltrating) lobular carcinoma (ILC), inflammatory breast disease, Paget’s disease of the breast, male breast cancer, and Phyllodes tumor of the breast.7 Breast cancer presents similar biologically in both men and women, although typically it is diagnosed in the later stages with older men.4 While both genders develop the same types of breast cancer, lobular carcinoma is rare in men due to the absence of lobules in their breast.4

The most common presenting symptom in breast cancer is a palpable lump or nodule and approximately 90% of the masses are found by the woman herself.4 The typical location for the cancerous mass to develop is either centrally behind the areola or in the outer upper quadrant for women, and in the center behind the areola for men (see figure 1).4 The tumor typically feels firm and irregular if it is a carcinoma or smooth and rubbery if it is benign.4 Other clinical manifestations include:

  • A change in breast contour or texture
  • Nipple discharge and retraction or inversion
  • Local skin dimpling
  • Erythema
  • A local rash or ulceration
  • Lymphadenopathy may also be possible

Figure 1 -Frequency of Breast Cancer by quadrants: The maximum amount of cases originate in the Upper Outer Quadrant (UOQ) and in the areolar area. (

Table of Contents

2. Normal Breast Anatomy and Function

a. Anatomy

Breast tissue is an elemental component of both male and female human anatomy, although function of the breast as a whole is considerably different. Male breast tissue, including the mammary glands, is essentially functionless and consists of only a few small ducts.8 Additionally, the fat tissue in the male breast is consistent with the rest of the body’s subcutaneous fat tissue and glandular development is not considered normal.8 Both male and female breast anatomy is characterized by the nipple at its center surrounded by a circular area of increased pigmentation known as the areola.8

Female breasts are more prominent relative to males and consist of a much more intricate layout of fat and glandular tissue due to increased functional relevance.8 The female breast occupies the space on the chest from the lateral border of the sternum to the midaxillary line and vertically from the second through 6th ribs.8 Thick fascia covering the pectoralis major and serratus anterior make up the bed of the breast.8 Between this thick fascia and the actual breast tissue itself is the retromammary space, which allows a small degree of movement of the breast on the pectoral fascia.8 Female mammary glands are suspended within the breast tissue by the suspensory ligaments of Cooper.8 Each lobule of the mammary gland is drained by a lactiferous duct, which opens on the nipple.8 Just deep to the areola is the lactiferous sinus, a holding center for milk from lactating mothers.8 Blood to the breast tissue is supplied by the medial mammary branches and anterior intercostal branches of the internal thoracic artery as well as the lateral thoracic and thoracoacromial branches of the axillary artery.8 Additional blood is supplied from the posterior intercostal branches of the thoracic aorta.8 Most venous drainage occurs via the axillary vein.8 Lymphatic drainage passes through the subareolar lymphatic plexus to the axillary lymph nodes and on into lymphatic circulation.8 Remaining lymph may flow to the opposite breast.8 Breast innervation occurs via the 4th through 6th intercostal nerves.8

Figure 2 - Basic Breast Anatomy (


b. Primary Function

The primary function of the breast, namely the mammary glands, is lactation.9 The breasts contain many series of alveoli-duct complexes surrounded by contractile myoepithelial cellls.9 The alveoli are the site of milk secretion.9 At the onset of menarche, duct development and branching occurs in response to increased estrogen levels.9 During pregnancy, increased levels of estrogen, progesterone, prolactin, and human placental lactogen contribute to alveoli enlargement.9 Prolactin specifically, released from the anterior pituitary following prolactin-releasing hormone stimulation, is the major stimulator for milk production in the alveoli.9 Estrogen and progesterone, released in large quantities from the placenta, inhibit the action of prolactin on the breast during pregnancy.9 Delivery, therefore, removes this inhibition and milk production occurs.9 The myoepithelial cells, under the control of the hormone oxytocin, contract around the alveoli in the breasts to move the milk into the ducts where it is available for a nursing infant.9 Interestingly, suckling provides a natural form of birth control via its effect on the hypothalamo-putuitary-ovarian axis and subsequent blocking of ovulation.9

3. Risk Factors

Risk factors for developing breast cancer are believed to be multi-factorial. Below is a chart that outlines the risk factors. Information for the chart was provided by Goodman CG et al.4 located on page 764.

Risk Factors for Breast Cancer
Key Risk Factors
Age > 60
First menstruation < 12 equals a great risk
Age when giving birth the first time (> 35 increases risk)
Number of first-degree relatives with breast cancer
Number of previous breast biopsies (does not matter if they were positive or negative)
Having at least one biopsy with atypical (ductal or lobular) hyperplasia or radial scar
Potential Additional Risk Factors
History of fibrocystic breast disease
Ethnicity (whites have greater incidence; blacks have more deaths)
Late menopause (>50 years old)
Nulliparity, infertility
DES(diethylstilbestrol) exposure
Alcohol (>2 drinks a day of beer, wine, or hard liquor)
Postmenopausal weight gain (more than 45 lbs since age 18); obesity
High does of chest radiation before age 30
Environmental exposures
High-fat diets
Long-term use of oral contraceptives or combined hormone replacement therapy
High bone density (postmenopausal women); circulating estrogen promotes bone formation

4. Genetic Mutations


5-10% of all breast cancers can be traced to a hereditary link, meaning the cancer syndromes were caused by inherited cancer-susceptibility gene mutations.10 Of these, 80% of cases are caused by BRCA gene mutations.10

i. BRCA1

BRCA1 is a large tumor suppressor gene located on chromosome 17q12-21.10 BRCA1 gene expression has been found in testis, thymus, breast, and ovarian tissues, specifically in differentiating epithelial cells. The proteins encoded by BRCA1, including acid nuclear phosphoprotein 220kDa, are involved in DNA repair by interacting with the RAD51 protein.10 The RAD51 protein is active in DNA recombination and double-stranded DNA-break repairs. BRCA1 can mutate in multiple ways, but truncating mutation types are associated with the greatest incidence of breast cancer. With loss of the DNA repair functions of BRCA1, other mutations of these fast-replicating breast cells go unchecked, allowing for aggressive tumor growth.10 In fact, researchers have found that BRCA1 tumors are more aggressive than other forms of breast cancer and are likely to be at least grade III by the time they are diagnosed. They are more likely to be estrogen-receptor negative, have a high frequency of TP53 mutations, and infrequently demonstrate HER2/neu or cyclin D1 gene amplification.10

BRCA1 risk: 60-65% of BRCA gene mutations are BRCA1-related; therefore, 48-52% of all hereditary breast cancers are caused by BRCA1 mutations.10 For women in families with BRCA1 hereditary risk, there is an 85% risk of acquiring breast cancer by age 70.10 In men with familial history of BRCA1 mutations, the risk of breast cancer is 58 times that of men in non-BRCA1 risk groups.10

BRCA1 animal model: Mice embryos with homozygous deletion of BRCA1 died after 7.5 days of gestation due to decreased cell proliferation and inhibited cell growth.10

ii. BRCA2

BRCA2 is another tumor suppressor gene, located on chromosome 12q12.10 BRCA2 is involved in the homologous recombination pathway of DNA repair; like BRCA1, BRCA2 interacts with RAD51. Recently, Badie and colleagues demonstrated BRCA2 interacts with telomeres during S and G2 phases of the cell cycle.11 Inactivation of BRCA2 caused shortened telomeres and formation of fragmented telomeric signals, which has been associated with defective DNA replication. Human breast tumors with mutated BRCA2 demonstrated shorter telomeres than tumors with BRCA1 mutations, indicating that telomere dysfunction may play a role in the genomic instability of BRCA2-deficient tumors.11 Most BRCA2 mutations result from truncated proteins.10

BRCA2 risk: BRCA2 mutations account for 35-40% of all BRCA gene mutations, or 28-32% of all hereditary breast cancers.10

BRCA2 animal model: If germ-line mutations are made in mouse embryos, the mice do not survive past 7.5 days gestation.10 Mice with heterozygous BRCA2 mutations do not have increased susceptibility to breast cancer, whereas individuals with homozygous BRCA2 mutations are frequently affected.10 Mouse breast tumors that lacked the BRCA2 protein accumulated telomere dysfunction-induced foci.11

b. TP53

TP53 is a tumor suppressor gene located on chromosome 17p12 that is responsible for the coding of protein p53.10 TP53 stops cells from replicating damaged DNA through the G1/S cell cycle checkpoint; therefore, TP53 mutations may lead to unregulated cell proliferation. In addition, protein p53 has tetramers that bind to DNA and can activate the transcription of reporter genes.10 When TP53 mutates and is no longer to encode protein p53, cells may not properly undergo apoptosis.

TP53 risk: TP53 mutations are the most common known gene mutation in all human cancers; 20-40% of individuals with breast cancer demonstrate TP53 mutations. However, only a fraction of these gene mutations are inherited.10

TP53 animal model: Mice engineered to have mutations in one or both TP53 alleles have significantly increased tumor risk, demonstrating that both heterozygous and homozygous TP53 mutations increase risk of breast cancer.10


PTEN is a tumor suppressor gene at chromosomal site 10q22-33.10 Loss of heterozygosity of PTEN results in hamartomas (benign focal malformations that resemble neoplasms) and benign tumors in many tissues, including the breast. The hereditary form of PTEN mutation is known as Cowden Disease, an autosomal dominant genetic condition. Mutations in PTEN cause upregulation in the phosphatidylinositorl 3’-kinase/Akt signaling pathway, which has a significant role in oncogenesis.10

PTEN risk: In individuals with the hereditary mutation of PTEN, lifetime risk of breast cancer is between 30 and 50%.10 The frequency of PTEN mutation in all breast cancer patients, whether or not the mutation was inherited, is estimated at 9-39%.10

d. ATM

ATM is a gene located on chromosome 11q22-33 that forms the ATM protein kinase.10 The Mre11-Rad50-Nbs1 (MRN) DNA repair complex activates ATM protein kinase in response to DNA double-strand breaks.10 The MRN complex begins signaling cascades to initiate DNA damage response. Cells lacking ATM due to mutations have increased susceptibility to damage caused by oxidative stress because ATM is a sensor of reactive oxygen species within human cells.12 Without this system of reactive oxygen species recognition, oxidative stress damage may go unchecked.12

ATM risk: ATM mutations are present in 1 in 40,000 to 100,000 live births.10 Individuals with ATM homozygous mutations have 100 times the risk of developing cancer when compared with the general population.10 Loss of heterozygosity of ATM has been identified in 30-40% of spontaneous breast cancers.10

5. Epigenetics

Epigenetics are a specialized area of molecular biology that studies inherited changes in phenotype, mitotically stable states, and gene activity that does not involve actual changes to the underlying DNA sequence.13 14 Important mechanisms in essential biological processes include X-chromosome inactivation, genomic imprinting, position effect variegation, reprogramming of genomes, and post-transcription gene silencing by RNA interference.13 Recent literature has indicated two epigenetic processes that are primarily responsible for the initiation and progression of cancer, and in this case, breast cancer. These two processes are DNA methylation and histone tail modification and are of particular interest because they are thought to be reversible.13 14 15

a. DNA methylation

DNA methylation is a process in which specific enzymes, DNA methyltransferases, cause a methyl group (-CH3) to attach onto the 5-carbon ring of cytosines of a CpG dinucleotide.13 16 Individual DNA methyltransferases will be discussed later. In general, genes that are transcriptionally active, such as housekeeping genes, are unmethylated but developmental and tissue-specific genes are methylated.13 Therefore, the CpG nucleotides in humans are typically methylated.16 In these terms, genes can either become hypo- or hypermethylated. Hypomethylated genes have been associated with gene reactivation and chromosomal instability, while hypermethylated genes have been associated with gene repression and genomic instability.14 In cancer, both hypo- and hypermethylated genes have been connected to specific pathological functions. Hypomethylation of the CpG segments can lead to upregulation and overexpression of cancer causing genes.14 Hypermethylation of certain CpG segments can lead to inhibition of tumor-suppressing genes, including Rb1, GSTP1, and CDH1.14 16

b. Selective advantage and selective targeting

The process by which CpG segments (islands) are targeted for abnormal (un-)methylation remains unclear. In one study, 78 genes out of 400 studied were found to be altered from their normal status.17 Of these 78 genes, 35 were found to be associated with only 1 tumor type.17 However, 7 hypermethylated genes were found to be present in multiple tumor types: TSPYL, PAX8, LEP, PHOX2B, TMPRSS2, MYOD1, and PAX5.17 These findings would indicate both a level of specialization between tumor types and the possibility of a universal mechanism standard for most cancer types.

Two general theories have been proposed to explain the central mechanism responsible for CpG island methylation: selective advantage and selective targeting. The selective advantage theory operates on the premise that initial methylation of CpG islands is a random process as a result of deregulation of the “methylation machinery”.14 15 This deregulation results in repression in the promoter region (where most CpGs in humans are found) of genes whose function is to limit the survival or proliferation of cells (i.e. tumor-suppressing genes). These events lead to highly expressive phenotypes giving cancerous cells the “selective advantage” in the sense of allowing unrestricted growth and proliferation of the aberrant phenotypes or metastasis to secondary sites in progressive stages.14 15 18

In selective targeting, it is proposed that certain regions of the genome contain specific intrinsic factors that make them susceptible to abnormal methylation. Under this theory, the process of de novo methylation is a non-random process and is carried out by specific enzymes called DNA methyltransferases (DNMTs).13 14 15 De novo methylation simply refers to specific DNMTs adding a methyl group to a cytosine along the DNA strand and is primarily involved in embryonic development. The other type of methylation is categorized as maintenance and is necessary to preserve the methylation of daughter cells in DNA replication. Specific DNMTs, their functions, and roles in cancer are described below.

c. DNA methyltransferases

DNMT1: DNMT1 is considered a maintenance methyltransferase. Its role in normal function is to maintain CpG methylation after DNA replication by methylation of the daughter cell DNA strand.15 Inactivation of this enzyme can result in global demethylation. Abnormal regulation of DNMT1 by cell-cycle regulator proteins, p21 and Rb, can result in irregular methylation patterns during the DNA replication phase of cell division. Excessive regulation by p21 can block DNMT1 binding, while loss of Rb could lead to abnormal de novo methylation.15

DNMT3a and DNMT3b: Considered de novo methyltransferases, these enzymes catalyze the methylation of unmethylated CpG sequences. They are primarily active during embryonic development, decreasing in concentration after cell differentiation, but have also been found in high concentrations in tumor cells.15

DNMTs in Cancer: Excessive amounts of DNMT1 can result in abnormal de novo methylation of CpG segments that should be unmethylated under normal conditions. The increased expression of this DNMT is theorized to inhibit or downregulate the genetic sequences of tumor-suppressing genes. DNMT3b has been suggested in causing de novo hypermethylation of the CpG islands on tumor-suppressing genes, rendering them inactive. Literature has indicated only DNMT3b of the DNA methyltransferases in the development and progression of breast cancer specifically.15

d. Histone modification

Histones are proteins that are found within the nucleus that are responsible for structuring DNA into sequenced nucleosomes. Histone modification and DNA methylation are both interrelated functions, but whereas DNA methylation deals with the C-terminal, histone modification deals with the N-terminal tails. These tails provide locations for several types of protein modifications including: acetylation, methylation, phosphorylation, sumoylation, ubiquination, and ADP ribosylation.13. These modifications are essential in regulating open (euchromatin) and closed (heterochromatin) chromatin states, which determine the accessibility of transcription mechanisms, with heterochromatin being harder to access. Typically, methylation of the CpG islands happens concurrently with histone modification, indicating a role in gene expression.19

Parrella indicates that the most common histone modification is acetylation of lysine, which creates a more open chromatin state.19 This (de-) acetylation process is performed by two proteins, with histone acetyltransferase (HAT) performing acetylation and causing an open chromatin state, and histone deacetylase (HDAC) removing acetyl groups causing the chromatin to enter a closed state.19 Other common modifications include methylation of histone 3 (H3) at lysine 4 resulting in open chromatin states, methylation of H3 at lysine 9, lysine 27, and H4 result in transcriptional repression.19 Synthesizing this information, a general conclusion can be made that hyperacetylation results in increased transcription, and that hypoacetylation causes inhibition of gene expression. It appears that HDAC activity directly or indirectly functions with DNMTs to function in the regulation of silencing tumor-suppressing genes, or in expressing of methylated proto-oncogenes.14 Literature suggests that DNA methylation precedes and initiates the histone acetylation, marking specific genome sites for deacetylation (i.e. repress transcription activity).13

e. micro RNA

Micro RNA (miRNA) are a noncoding regulatory RNA up to 40 nucleotides in length, typically 18-25. Primary roles include cellular proliferation, differentiation, and apoptosis.20 They can induce “gene silencing” by pairing with target mRNA, this is also known as “translational repression.”20 The exact mechanism of how this happens is unknown at this time. The study of miRNA in relation to breast cancer is a recent phenomenon and they are thought to have multiple potential roles in risk stratification, detection, and treatment of breast cancer.20

Some miRNAs act as tumor suppressors (i.e. miR-10b,miR 145) and therefore downregulate in the presence of cancer, others are upregulated (i.e. miR-21, miR-155) and act as oncogenes working against the tumor supressor genes.20

Current studies are focused on miR-21 in particular as it is thought to serve as an early biomarker as well as an indication of how effective certain chemotherapy agents would be on a particular cancer type.20 Proposed genetic therapies designed to suppress miR-21 are thought to have the potential to make chemotherapy treatments more effective.20

6. Pathogenesis of Breast Cancer

a. Immune System/Inflammatory Response

i. Cytokines

Cytokines mediate intracellular communication locally to cause an integrated response to stimuli. They are rapidly synthesized and can be secreted by healthy and diseased cells alike. Their job is to regulate the survival, growth, differentiation, and effector functions of the cell, thus playing an important role in the immune response.21 Due to the actions of cytokines involving mediating the effector response from both innate and adaptive cellular immunities, it is likely that they are involved in the mechanism of tumor cell evasion of the immunosurveillance system.21 In breast cancer carcinogenesis, many cytokines have a role in expression of cancer cells or are produced in the microenviroment of the primary or metastatic tumor.22 Typically, the local production of cytokines within the tumor microenvironment is crucial in mounting an immune response against tumor cells, however certain suppressive cytokines may inhibit this response.23 The inability to produce particular cytokines may inhibit the functions of lymphocytes that are associated with tumor suppression and may, in turn, promote tumor cell growth.23

Figure 3 - Cytokines and their role in tumor progression (


1. Interleukins

Interleukins are a type of cytokine that has been studied with breast cancer patients. The interleukins IL-1, IL-6, IL-8, IL-11, and IL-13 favor tumor growth, while IL-2, IL-10, IL-12 mainly interfere with the cell-mediated immunity response.22 Below in the chart are some interlukins that have been found to play a role in breast cancer.

Interleukins Normal response Response in Breast Cancer
IL-2 They are produced by activated T-cells and cause proliferation of the T-cells as well as differentiation of cytostatic T-lymphocytes.22 They also act on NK cells, B cells, monocyte/macrophages and neutrophils.22 IL-2 mainly interferes with cell mediated immunity response.22
IL-6 Important cells thought to be the source of IL-6 are fibroblasts, macrophages, and lympohcytes.21 There have been high concentrations found in breast tumor samples and human breast cancer cell lines.21 IL-6 promotes tumor growth through upregulating antiapoptotic and angiogenic proteins in the tumor cells.22
IL-8 IL-8 is a key factor in the inflammatory response and is produced by macrophages as well as other cell types like epithelial cells. A member of angiogenesis CXC chemokine family that is a potent chemoattractant which can cause breast cancer progression as a mitogenic, angiogentic, and motogenic factor.22 IL-8 contributes to protumigenic activities like angiogenesis, tumor proliferation, and local tumor invasion.22 It stimulates osteoclastogenesis and resorption of bone. Elevated IL-8 levels can predict early metastatic spread of breast cancer.22
IL-10 Produced primarily by activated macrophages and can inhibit them by negative feedback.22 IL-10 has inhibitory effects on T-cell proliferation and function.23 IL-10 mainly interferes with the cell-mediated immunity response and its existence in tumors has been correlated with increased tumorigenicity and immune suppression.22 24 This IL is secreted by many cancer cells and at an even higher rate by metastatic cancer cells.22 In a paper by Llanes-Fernandez et al.25, a direct association was noted between IL-10 and an increase in CD3-polypeptides.25 In addition, IL-10 mRNA was found in more than 50% of tumors contrary to the prevalence of type 1 cytokines in regional nodes (40%).25 A downregulation on the function of T-cells in tumors is suggested due to the lack of lyphocyte activation proteins expressed and increase in IL-10.25 The authors conclude that the results above are useful in understanding the local immune response that is key in controlling progression of tumors.25
IL-11 In 1990, IL-11 was initially described as a bone marrow stroma-derived hemopoietic cytokine.26 A variety of tissues express IL-11 including: the gut, brain, spinal cord neurons, and testes.26 Because of this, IL-11 may have a physiological role in these organs.26 Significantly higher levels of IL-11 in primary breast cancer are associated with poor prognostic index, high historical grade and poor survival.22 26 IL-11 may play a significant role in bone metastasis as breast cancer cells are known to secrete it, and IL-11 has been shown to stimulate osteoclasts.26 A study by Hanavadi et al26 found significantly higher transcript levels of IL-11 in node-positive tumor samples when compared with node-negative samples.26
IL-12 The main sources of IL-12 are macrophages, monocytes, dendritic cells, neutrophils, and to a lesser extent, B cells.22 There are reports that breast cancer causes a local immune response that is likely directed towards tumor-associated antigens.22 Mononuclear cells have a defective IL-12 production capability.25 This is the main cytokine that controls differentiation of CD4+ T-cells to the Th1 phenotype that, in turn, produces IFN-γ.22 A study by Vitolo et al.27 reported that IL-12 and IFN-γ were demonstrated by reverse transcriptase-polymerase chain reaction in each of the 10 cases of infiltrating ductal carcinoma.27
IL-18 IL-18 is an important regulator of innate and acquired immune responses. It has been found to be expressed at sites of chronic inflammation, in a variety of cancers, in autoimmune diseases, and in the context of numerous infectious diseases.28 Nicolini et al.21 reported that elevated serum levels of IL-18 were found in breast cancer patients over controls and the values were higher in those who were in the advanced stages rather than those in the early stages of the disease.21 Higher serum levels were also noted in metastatic verses non-metastatic patients.21

2. Interferons

Interferons (IFNs) are cytokines that belong to the large class of glycoprotiens. They are proteins that are made and released by lymphocytes during a response to the presence of a pathogen, this includes tumor cells. INFs are named for their ability to “interfere” with viral replication.29 IFNs act as the communication link for infected and nearby cells to trigger the immunce system response so the pathogens or tumors can be eradicated. Other functions for which IFNs are responsible include: activation of immune cells (i.e. natural killer cells and macrophages), up-regulation of antigen presentation to T-lymphocytes to increase recognition of infection or tumor cells, inhibition of B-lymphocyte activation, and enabling the uninfected host cells to resist infection.29 There are three forms of IFNs named IFN-alpha (α), IFN-beta (β), and IFN-gamma (γ). These three have been classified into two seprate types. Type I includes the alpha and beta forms, while type II contains the gamma form. Type I is able to be produced by most cells once stimulated by a virus, with their primary function being to make cells that are resistant to that virus.29 The type II IFN is only secreted by natural killer cells and T-lymphocytes, and its main job is to signal the immune system to activate because of infectious agents or cancer growth.29

In breast cancer cells, IFN-α has been found to be involved in anti-proliferate and anti-adhesive operations.22 These anti-proliferative effects are synergistic with anti-estrogens.22 In addition, IFN-β also has an important role in the anit-proliferative effect on those with breast cancer no matter if there are estrogen receptors.22 The IFN-1, α and β, proteins have anti-tumor activity, inducing tumor suppression genes and down-regulation of oncogene expression.21 22 In addition, they cause an increase in the expression of major histocompatibility complex (MHC) class-1 molecules, which in tumor cells can enhance immune recognition.22 INF-1 proteins help promote Th1 type of immunoresponse which include cytotoxic lymphocytes and dendritic cells.22 The IFN regulatory factor 1 (IRF-1) is a transcription factor in the IFN-γ signal transduction pathway. IFN-γ also can affect cellular apoptosis by inducing the expression of FasR mRNA and FsaR protein, which both promote apoptosis.22 This particular regulatory factor causes apoptosis by acting as a tumor suppressor gene.22 The loss of the ability to regulate IFR-1 appears to directly relate to anti estrogen resistance.22 In vitro, patients who have advanced breast cancer and were treated with low doses of IFN-β show an increase in estrogen and progesterone.22

3. TNF

Tumor necrosis factor (TNF) is a cytokine that plays a vital role in the angiogenesis of breast cancer. It is a product of lymphocytes and macrophages that cause lysis of cancer cells.30 TNF is involved with systemic inflammation and simulates the acute phase reaction. Its primary role is to regulate immune cells. TNF has the ability to cause apoptosis and inflammation, as well as inhibit tumorigenesis and replication of viruses.

It has been discovered that high doses of TNF cause tumor destruction, while low and chronic production of this cytokine sustains tumor growth.31 In fact, TNF-alpha, which is primarily produced by mononuclear phagocytes, has been linked to the intratumoral regulation of angiogenesis.22 32 In high doses it inhibits angiogenesis, however in low doses it seems to promote it.32 Angiogenesis provides new vascularization to the tumor site, aiding in its growth and eventual metastasis through the blood stream. In cancer patients, higher levels of TNF are linked to poor prognosis.31

While small amounts of TNF could be produced by the tumor cells directly within the microenviroment of the tumor, the most fitting source of TNF is from infiltrating leukocytes.31 A study conducted by Sangaletti et al.31 looked at TNF in spontaneous mammary cancer in the HER2/neuT transgenic mouse model.31 Their results indicated that when treated with anti-TNF blocking antibody, tumor growth halted for at least one month after treatment was discontinued, then resumed.31 In addition, they concluded that in the absence of TNF, leukocyte infiltration and cytokine production are altered at the tumor site.31 This animal model insinuates that this treatment could be beneficial in humans, and TNF has a definite role in tumor growth.

ii. Macrophages

Macrophages are specialized cells that adopt tissue-specific functions that are important for normal tissue homeostasis and response to physiological challenges. They play a role in normal development of certain organs, including the breast. However, they are also found in the stroma of breast tumors, where they promote tumor growth and metastasis. High macrophage density indicates a poor prognosis in breast cancer patients.33

There are two classifications of macrophages based on their task within the body. M1 macrophages are “classically activated” by interferons and TNF, carrying out phagocytosis and pro-inflammatory cytokine production. M2 macrophages are “alternatively activated” by IL-4, IL-13, glucocorticoids, and tranforming growth factor. These play a role in tissue modeling and repair, as well as immunoregulation and angiogenesis, which all may be involved in tumor growth and metastasis.33

Due to these varied activations, macrophages can play both an antitumor and protumor role within the body. When carrying out antitumor activity, macrophages secrete immunity cytokines, including interferons, interleukins, and TNF alpha. They also induce antigen presentation to the T-cells by macrophage phagocytosis, where the foreign antigen is expressed on the macrophage surface, allowing them to recognize and kill the the tumor cells. They may also play a primary role in cytotoxicity by performing macrophage-mediated tumor cytotoxicity (MTC) and antibody-dependent cellular toxicity (ADCC). In MTC, the macrophage secretes TNF-alpha and serine proteases directly onto the tumor cells resulting in the cells' lysis. ADCC uses a similar process, but rather than direct secretion, the tumor cell and macrophage both bind to an antibody. This provides cross-linking for deposit of factors to the tumor cell, again resulting in cell lysis. Protumor activity of macrophages involves secretion of mitogenic cytokines, resulting in tumor growth and metastasis, by suppression of the immune response on the tumor cells. Angiogensis is increased in tumor cells by macrophage activity including release of angiogenic cytokines and proteolytic enzymes which degrade the extracellular matrix, resulting in increased systemic spread of tumor cells.34

iii. TLR

Toll-like receptors (TLR) are proteins expressed on the cell membrane that function to recognize specific pathogen molecular patterns in order to signal an intricate inflammatory response. This response includes the secretion of various cytokines and chemokines in order to protect infected cells and prevent their spread.35 36 Increased expression of TLRs (namely TLR-3, TLR-4, and TLR-9) has been correlated with increased breast tumor metastasis.36 This has been suggested to be associated with a cyclic inflammatory cascade induced by TLRs in which cytokines and chemokines stimulate the release of further inflammatory factors.36 Further, it has been suggested that TLRs contribute to the release of TNF-∝, interleukins, NF-kappaβ, and malleoproteases leading to tumor proliferation.36

b. Mitochondrial Dysfunction

i. Free Radicals

Reactive oxygen species (ROS) or free radicals are highly reactive molecules containing oxygen and unpaired electrons in their outer shell. Free radicals are products of normal cell metabolism and may also result from ionizing environmental factors. It has been suggested that free radicals interact with cellular components, including DNA, causing genetic alterations.40 41 42 These alterations lead to loss of regulation of a number of cellular mechanisms and development of cancer. 40 41 42 Therefore, loss of balance between free radicals and antioxidant protection places an individual at risk for cancer.40 41 42 Sinha and colleagues conducted a study to investigate the balance of free radical activity and antioxidant concentration in human breast cancer.40 Antioxidants are molecules capable of neutralizing the harmful effects from oxidative reactions.42 The group found significantly increased levels of lipid peroxide free radicals and decreased concentration of antioxidants in individuals with malignant breast cancer relative to controls with benign breast disease.40 Additionally, in the malignant group, subjects with more progressive disease, as indicated by TNM staging, had significantly increased levels of free radicals with associated decrease in antioxidants.40 This research suggests a correlation between free radical concentration and progressive disease in addition to reductions in antioxidant protection mechanisms.40

The exact mechanism with which free radical oxidative stress induces DNA damage is not well understood. Two rather complex mechanisms have been described in a review by Valko et al.41 and relate to modulation of gene expression, direct genetic mutation, and chromosomal rearrangement 41 Additionally, oxidation of cell membrane fatty acids has been shown to yield reactive products that cause harmful interactions with other cellular components.41 Free radical-induced alteration of genetic material is also thought to be a prime player in carcinogenesis when target cells include essential oncogenes and tumor suppressor genes.41 43 44 BRCA1, one of the most potent breast cancer susceptibility genes, encodes a tumor suppressor protein with responsibilities vital to the elimination of ROS intracellularly.45 Studies have shown that intracellular ROS levels are reduced in breast carcinoma tissue and non-tumor breast tissue when BRCA1 remains viable. 45 Additionally, BRCA1 seems to reduce levels of protein nitration and other DNA lesions indicative of oxidative damage.45 This evidence suggests the gene’s key role in improving antioxidant gene expression and protection against oxidative stress. It also suggests the increased vulnerability of individuals with a mutant form of BRCA1 to ROS stress and potential carcinogenesis.44 45 46 The ATM gene is another possible genetic link to free radical damage susceptibility.12 Research has shown that ATM is a sensor of ROS within human cells. Therefore, cells with ATM mutations demonstrate increased vulnerability to damage caused by oxidative stress because the free radical damage may go undetected without this system of ROS recognition.12

ii. Warburg Effect

The Warburg Effect describes the process of energy production observed in replicating cancer cells. Cancer cells generally rely on aerobic glycolysis for ATP production rather than oxidative phosphorylation, which is the most efficient pathway of ATP production in healthy cells. Although aerobic glycolysis produces significantly less ATP than oxidative phosphorylation, it allows cancer cells to thrive in the hypoxic tumor microenvironment.47 Additionally, aerobic glycolysis provides sufficient energy to maintain cell proliferation. Glycolysis provides the carbon skeletons needed for biosynthesis. Cell building blocks including nucleic acids, lipids, and proteins required for proliferation are produced in this pathway.48 In a glucose rich environment, production of biomass may be more important to the replicating cancer cells than ATP production. Understanding this effect has led to additional options in cancer treatment, including the use of glycolytic inhibitors, in addition to traditional cancer treatments. Researchers have also identified Tumor M2-PK, a pyruvate kinase enzyme that, when inhibited, slows proliferation of cancer cells.49

c. Endocrine System/Hormones

i. Estrogen

Estrogen and its increased prevalence in breast tissue and surrounding adipose tissue is believed to play an important role in breast cancer, specifically hormone-dependent tumors.50 51 Estrogen is an important mediator of breast cancer development.52 Estrogen binds to estrogen receptors (ER) which, once bound, regulates the transcription of genes that promote tumor formation and progress.52 One main type of estrogen, 17beta-estradiol or E2, is more biologically active than other types of estrogen.52

Estrogen Synthesis

Estrogen synthesis is proven to be increased in obese and older individuals which explains the increased risk for these populations.50 Estrogen synthesis requires enzymes found not only in normal and cancerous breast tissue, but also in surrounding adipose tissue.50 Cytokines are mediators of this process.50 In the process of peripheral estrogen synthesis, estrone is converted from adrenal androstenedione via the aromatase enzyme complex.50 40-50% of breast tumors show a marked increase in aromatase enzyme activity indicating active synthesizing of estrogen.50 Estrone is either converted to estradiol through the estradiol-17beta-hydoxysteriod dehydrogenase (E2DH-type1) or is stored as estrone sulfate via estrone sulfatransferase as a estrone reserve.50 Estrone sulfate is found in most breast tumors and a higher prevalence of estrone found in breast tumors is produced via the sulfatase pathway.50 The increased risk for obese individuals is from an increased conversion of androstenedione to estrone in the periphery.50 1% of androstenedione is synthesized into estrone in normal weight subjects whereas 10% is converted in obese patients.50 Furthermore, there has been evidence to show an increase in peripheral synthesis of estrogen that occurs with aging.50 Breast cancer tumors contain a high level of estradiol compared to a low level present in plasma post-meopause.50 Evidence has shown that tumors were not able to readily metabolize estradiol but could easily convert estrone to estradiol by E2DH.50 E2DH is actively present in breast tumors and a positive correlation has been found between E2DH activity and extent of obesity.50 There is evidence that breast tumors induce estrogen metabolism and activate enzymes important for estrogen synthesis.50 Another significant correlation between E2DH activity in surrounding adipose tissue and the size of the malignancy.50 Overall, all activity involved in the synthesis of estrogen (aromatase, E2DH, and estrone sulfatase) are higher in malignant breast tissue.50

Cytokine Regulation

Estrogen synthesis is regulated by specific cytokines, IL-6 and TNF-alpha, which appear to work in a synergistic pattern.50 Insulin-like growth factors I and II and IL-1 also have a role in activating estrogen synthesis via human serum albumin.50 Human serum albumin increases the activity of E2DH and aromatase enzymes complex.50 Il-6 needs its soluble receptor (IL-6sR) in order to increase aromatase activity in stromal cells.50 Aromatase activity in breast tumors correlates with increased activity of DNA polymerage alpha activity which increases cell proliferation.50 Studies have shown both cytokines activate aromatase, E2DH and estrone sulftase activity in breast cancer cells.50 Breast tumor cytosol has been show to have a preferential activation of E2DH not seen in normal breast cells.50

Estrogen-Receptor Signaling

Once estrogen is synthesized into biologically active estrogen, estradoil, it acts on nuclear DNA to alter gene expression.51 Estrogen binds to nuclear estrogen receptors, which then bind as dimers to estrogen response elements of estrogen-responsive genes.51 The response elements then interact with basal transcription factors, coactivators, and corepressors to act on gene expression.51 Estrogen receptors act with coactivator proteins to initiate other transcription factors.51 Estrogen also increases the mitochondrial DNA (mtDNA) transcription levels.51 However, the mechanism used by estrogen-receptors is not known.51 Estrogen facilitates the cross-talk between estrogen-receptor signaling and other signal-transduction pathways by activating protein kinases and increasing the level of second messengers.51 This impacts the control estrogen has over cell growth and death.51

ii. Prolactin

Prolactin increases the survival of breast cancer cells due to its role in tumor cell proliferation.39 Additionally, prolactin inhibits apoptosis through activation of Akt, a serine/threonine protein kinase.37

iii. HPA Axis


Recent evidence points to stress as a risk factor for the growth and advancement of cancer.37 In animal models, experimentally imposed stress plays a role in cancer progression.37 Stress causes a release of catecholamines and corticosteroids, such as glucocorticoids, originating from the hypothalamic-pituitary-adrenal axis (HPA axis).37 Chronic alterations in these neuroendocrine dynamics from stress have an influence on different physiological processes that are critical in tumor pathogenesis.37 As a result of stimulation of cancer cells from catecholamines, there is an increased production of vascular endothelial growth factor (VEGF).38 This growth factor is a mediator of blood vessel growth in tumors.38 Tumors are unable to grow beyond 1 mm without an increase in blood supply.38 Stress has also been shown to increase catecholamine release which increases the blood supply of tumors.38 Certain neurotransmitters such as substance P, dopamine, and norepinephrine increase the migratory effects of breast cancer cells.39 However, norepinephrine is the only neurotransmitter that exhibits a chemotactic response on breast cancer cells.39 Epinephrine decreases the sensitivity of breast cancer cells to apoptosis via the interaction with beta-adrenergic-2 receptors (ADRB2) and the protein kinase A-dependent BAD phosphorylation.39 Chronic stress reduces dopamine release in patients and dopamine inhibits cancer cell growth.39 Dopamine’s role in the inhibition of breast tumor growth can be attributed to various mechanisms, including the activation of dopamine receptors or dopamine auto-oxidation.37 Additionally, in vivo models have shown that tumor proliferation can also be inhibited by the obstruction of the vascular permealizing and angiogenic mechanisms of VEGF.37. VEGF receptor 2 is important in increasing angiogenesis, but is endocytosed via dopmine acting on D2 dopamine receptors.37 Tumor size and vessel density in in vivo models are decreased in rats with hyperactive dopaminergic systems indicating dopamine’s role in tumor growth inhibition.37

d. Protein Kinases

Protein kinases are enzymes that modify proteins through a process called phosphorylation. During phosphorylation, a phosphate group is added to the polypeptide chain through the breakdown of ATP into ADP, altering that protein’s structure and function. There are multiple groups of protein kinases, classified depending on the type of molecule contained in its structure. The two classifications most relevant to the carcinogenesis processes discussion are the regulation of tyrosine-specific and serine/threonine-specific kinases.53 54 Alterations in the regulation of protein kinases have been linked to the initiation and proliferation multiple cancer types.54 These alterations typically involve being stuck in “on” or “off” cycles.

i. Serine/Threonine Kinase

Protein Kinase C: Of the serine/threonine class, protein kinase C (PKC) has been indicated as a primary effector in the signaling pathway for cancer cell proliferation.53 Normal regulatory functions of PKC include cellular proliferation, differentiation, apoptosis, and angiogenesis.53 The three classifications of PKCs are novel, classic, and atypical. Each classification contains several isozymes, but knowing and understanding each individual isozyme is not relevant to this discussion. The general process by which PKC is involved in signaling pathways is as follows: Tyrosine-kinase and g-protein membrane receptors are activated by extracellular ligands. Stimulation of these receptors results in activation of phospholipase C (PLC), which causes an increase in diaclyglycerol (DAG), a lipid secondary messenger. Increased membrane DAG activates more PKC, which ultimately leads to activation of MEK-ERK (MAPK) and P13K-Akt pathways.53 The MAPK (see below) and P13K-Akt pathways provide a means of communicating growth factor signals from the cell membrane to the nucleus. Activation of these pathways can result in uncontrolled growth or apoptosis. In breast cancer, PKC- α and PKC-β have been found to have primary roles in tumor proliferation. The mechanisms of PKC-α in breast cancer are still unknown, but PKC- β is primarily responsible for vascular endothelial cell proliferation.53

Mitogen-Activated Protein Kinase (MAPK) Pathway: MAPKs are the constituents that drive the MAPK pathway (once called MEK-ERK pathway) involved in tumor cell proliferation.55 MAPKs are regulated by mitogen-activated protein kinase phosphatases (MKP) and have been found to be overexpressed in cancer cells.55 The MAPK pathway is broken down into three branches that activate three separate kinases: extracellular regulated kinases (ERK), c-Jun N-terminal kinases (JNK), and p38.55 The ERK branch is activated by growth factors, while JNK and p38 are activated by growth factors, cytokines, and cellular stress. The ERK branch signals the binding of transcription factors to DNA to stimulate apoptosis or changes to cell motility and angiogenesis, depending on the target genes. JNK can promote cell proliferation through microtubule assembly or can promote cell death by inhibiting mitotic spindle stabilization or by activating ubiquination factors.55 The downstream activity of p38 activation is relatively unclear. Haagenson et al.55 state that it has a negative effect on RNA binding effectors, therefore increasing their mRNA targets' stability.55

ii. Tyrosine Kinase

Protein tyrosine kinases are involved in cellular processes such as proliferation, differentiation, death, motility, adhesion, and inter-cellular communication.54 The two types of tyrosine kinases are receptor tyrosine kinase (RTK) and nonreceptor tyrosine kinase (NRTK). Poor regulation of these proteins can lead to many forms of cancer, with as many as 70% of onco- and proto-oncogenes positive for tyrosine kinase activation.54

Receptor Tyrosine Kinases: RTKs are often associated with gene mutation by chromosome translocation or through overexpression, leading to initiation of oncogenesis.54 There are three cellular components associated with RTKs: an extracellular ligand binding domain, transmembrane domain, and intracellular catalytic domain.54 The extracellular domain is located on the membrane and can bind insulin or various types of growth factors. The growth factors include epidermal growth factor, fibroblast growth factor, platelet-derived growth factor, and vascular endothelial growth factor. In order to activate RTKs, extracellular growth factor ligands must bind to the membrane domains. Phosphorylation occurs across the membrane, which recruits a component of an intracellular signaling pathway to the intracellular catalytic domain, allowing communication into the nucleus.54

Nonreceptor Tyrosine Kinases: NRTKs are unlike RTKs in that they are purely intracellular, being located in the cytoplasm, nucleus, or attached to the inner plasma membrane. Because of their location, NRTKs contain only the intracellular catalytic domain component.54 There are two largely important kinases within the NRTK family that are prominent in breast cancer cells: Focal adhesion kinase (FAK) and Rak.

FAK is an intracellular kinase associated with and attracted to focal membrane adhesions, and is therefore important in signal transduction through membrane receptors for integrins (necessary for adhesions with surrounding structures), growth factors, and cytokines.56 Cellular functions that are relevant to FAK include regulation of cell proliferation, adhesion, migration, invasion, survival, differentiation, and angiogenesis. FAK is mainly activated by phosphorylation by clusters of integrin at the membrane, but can also be activated in the presence of epidermal growth factor, platelet-derived growth factor, hepatocyte growth factor, cytokines, G-protein coupled receptors, phospholipid and lipid mediators.56 Downstream, activation of FAK ultimately leads to activation of enzymes that begin the kinase cascade of the MAPK pathway. Additional downstream activity includes interaction with multiple signaling molecules and associated pathways to regulate the different cellular functions described previously.56 It is then suggested that dysregulation of this kinase is an early step in the initiation, progression, and metastasis of breast cancer cells regardless of type, with some exception.56

Rak is member of the Src NRTK family and is found in many epithelial-derived germ cell lines.54 This kinase is mostly found within the nucleus but may be found in the cellular periphery in certain cases. Following phosphorylation, Rak is a prominent component of intracellular kinase autoregulation and intercellular signaling pathways. Unlike other members of the Src family, Rak has two unique characteristics that provide it with a different function when phosphorylated. Following phosphorylation at the specific sites, Rak can inhibit kinase activity or it can signal kinases to localized to the nuclear membrane (versus the inner cellular membrane), ultimately allowing it tumor-suppressor-like qualities via limitation of intercellular signaling.54 Therefore, overexpression of Rak is associated with decreased cell proliferation, cell transformation, invasiveness, and tumor formation, whereas deletion or downregulation of Rak is associated with increases in these functions.54

7. Clinical Presentation

a. Overview

As mentioned previously, the most common presenting symptom in breast cancer is a palpable lump or nodule found by the woman herself.4 Other clinical manifestations may include: a change in breast contour or texture, nipple discharge and retraction or inversion, local skin dimpling, erythema, a local rash or ulceration, and lymphadenopathy may also be possible.4 While these are the initial presenting symptoms, there are also some clinical signs that suggest metastases. This are normally not present at the initial complaint, but include the following:

  • Upper extremity edema
  • Bone pain
  • Jaundice
  • Weight loss

Medical professionals use clinical signs and symptoms as well as a battery of tests to determine if the patient has breast cancer. One of the primary ways they do this is by looking for diagnostic markers. Diagnostic markers play a key role in breast cancer care as they can be used to detect, diagnose, stage, and help determine appropriate treatment of the disease.57

b. Diagnostic Markers

i. Established Markers

Cancer Specific Autoantibodies (CSA) CSAs are most useful for risk stratification of breast cancer. The presence of CSAs indicate the body has successfully eliminated cancerous cells. An increased value can indicate an anti-tumor response, which can indicate an increased risk of cancer development, or paradoxically, a decreased risk due to high levels of increased elimination of cancerous cells.57
Carcinoembryonic Antigen (CEA) First identified in 1965, CEA is a glycoprotein from the immunoglobulin family of genes involved in cell adhesion. It is not a good marker for early detection and has a high false positive rate in healthy individuals. It is not specific to breast cancer but is more commonly seen in ductal cancers versus lobular tumors. In present day medicine CEA best serves as a test for metastasis rather than initial diagnosis.57
Estrogen Receptor (ER) ER is considered a vital biomarker in breast cancer as it can indicate the sensitivity to endocrine treatment and guide the choice of treatment. 80% of breast cancers are ER+ and therefore estradiol serves as their main growth stimulus. Treatment with the drug Tamoxifen increases the 5 year survival rate by 31% in ER+ patients, while no increase in survival is seen in ER- patients using this approach.58
HER2/neu protein (HER2) HER2/neu is a protein whose antibody, when detected in the blood, is associated with an aggressive breast cancer phenotype with poorer outcomes and greater resistance to treatments. 15% of primary breast tumors are HER2+.58 HER2+ cancer patients are more sensitive to treatment with trastuzumab with studies showing increased survival rates and decreased recurrence rates.59
Mucin 1 Glycoprotein (MUC1) MUC1 serves in a protective capacity and binds to pathogens. An elevated level has been observed in certain carcinomas. The most common tests for breast cancer include the carcinoma antigen (CA) 15.3 and CA 27.29 both of which are derived from MUC1.57 Like CEA it suffers from low sensitivity and specificity and serves primarily as a measure of disease in a metastatic setting.60
Progesterone Receptor (PgR) PgR is an important biomarker that can, like ER, indicate the most appropriate course of treatment. It typically exists in conjunction with ER+, <1% of all breast cancers are ER- and PgR+. Studies indicate asti-estrogen treatment is more effective when both ER+ and PgR+ cells are present in the tumor than ER+ only.58
Tumor Protein 53 (TP53) TP53 normally functions as a tumor suppressor. Increased levels of serum autoantibodies for TP53 have been detected in patients with carcinoma in situ of the breast, with the elevated values occurring early in the course of disease making TP53 a tool for early detection.60

ii. Emerging Markers

Cyclin D1 Cyclin D1 is overexpressed in 50% of breast cancer tumors. Its primary function is the regulation of estrogen receptive cell proliferation. In cases of “gene amplification”, which occurs in 15% of tumors, poorer responses to anti-estogen treatments have been seen. This is a new biomarker meriting further study according to the literature.58
Cyclin E Cyclin E is an emerging biomarker that is believed to play a role in tumorigenesis. Increased values are thought to positively correlate with increasing stage and tumor grades. Initial studies show a higher Cyclin E value indicates chemotherputic agents such as cisplatin and paclitaxel to be the most effective treatment choices. Conversely, the effectiveness of anti-estrogen treatment is reduced as Cyclin E values increase.58
Ki67 Ki67 is a nuclear nonhistone protein related to cell proliferation and is involved with RNA transcription. Ki67 assay has been combined with the established biomakers ER, PgR, and HER2 to differentiate between Luminal A and Luminal B tumor subtypes.58 Ki67 is measured as the % of a cancer cell nuclei sample that is positively stained. A value above 13.25% indicates the Luminal B subtype which carries a worse prognosis. This test is not widely utilized and is still a developing method.58

c. Tumor Classes and Cancer Staging

i. Primary Tumors

1. Ductal Carcinoma in situ (DCIS)

Ductal carcinoma in situ is the most common type of non-invasive breast cancer.61 Ductal refers to the cancerous cells residing within the milk ducts of the breast. In situ means that the cancer is localized to the tissue of origin and has not spread to adjacent tissues to form a tumor. DCIS accounts for approximately 17% of all breast cancers diagnosed annually in the United States.5 The chances of recurrence and subsequent invasive cancer are <30%.61 Detection of DCIS most often occurs via mammogram with normal treatment being a lumpectomy of the cancerous cells. Mastectomy is an option in more complicated cases or when other risk factors are present. Hormonal interventions are often utilized if the cancer cells are ER+ and/or PgR+.62

2. Lobular Carcinoma in situ (LCIS)

Lobular carcinoma in situ is a non-invasive cancer that resides in the milk producing glands of the breast. LCIS is most often diagnosed prior to menopause, between the ages of 40 and 50.63 It is far less common than DCIS, is not typically detectable on a mammogram, and is usually found during a breast biopsy done for other reasons.63 LCIS makes up 2% of all breast cancers diagnosed annually in the United States.5 LCIS is not always considered a true cancer but a cellular abnormality that increases the risk of developing breast cancer, however its statistics are included with other forms of breast cancer.63 Women with LCIS have a 3x higher risk of developing invasive breast cancer at some point in their lives as compared to women without LCIS, however this usually does not occur until 10-20 years after the LCIS is present.64 Treatment for LCIS normally includes careful observation by an oncologist including bi-annual clinical breast exams, annual mammograms, and potentially MRI if additional breast cancer risk factors exist.65 Medications such as tamoxifen may be prescribed to further reduce the chances of invasive breast cancer developing.65 Prophylactic mastectomy is an option if LCIS exists concurrently with BRCA1 or BRCA2 gene mutations or a strong familial history of breast cancer.65

3. Invasive Ductal Carcinoma (IDC)

a. Typical IDC

Invasive ductal carcinoma is the most common type of invasive breast cancer and accounts for 67% (including subtypes) of all breast cancers diagnosed annually in the United States.5 IDC begins as Ductal carcinoma in situ and breaks through the wall of the milk duct into the surrounding tissues. While women of any age can be affected, the majority of cases are seen in individuals aged 55 or older with a slightly younger average onset than ILC.66 Detection typically occurs via physical exam with a lump-like presentation or via mammogram.67 Much like ILC, treatment options include surgical management, radiation, pharmacological intervention or combinations thereof.68 This is dependent upon cancer type, hormone sensitivity, and spread of the cancer. Follow up is similar to ILC with consultation every 4-6 months for the first 5 years post-treatment and annually thereafter.69

b. Rare IDC Subtypes

There are several less common types of IDC which differ in appearance and cellular behavior versus typical IDC cancer cells.

Variation vs. typical IDC
Cribriform Carcinoma of the Breast Occurs in the connective tissue of the breast, known as stroma, between the ducts and lobules. Due to this formation a swiss cheese-like appearance results. This is considered a low grade and less aggressive form of IDC.70
Medullary Carcinoma of the Breast Medullary carcinoma earns its name due to its bulbous appearance that resembles the brain’s medulla. Occurrence is most common between the ages of 40 and 60 in women who possess the BRCA1 mutation. While the tumors have the physical appearance of an aggressive strain, they are low grade in their behavior and easier to treat than the majority of invasive breast cancers.71
Mucinous Carcinoma of the Breast Mucinous carcinomas involve cancerous cells that float in the mucin, a component of mucus. Onset typically occurs post menopause in women >60 years of age. Mucinous carcinoma is less likely to spread to the lymph nodes than most breast cancers and is therefore considered more treatable.72
Papillary Carcinoma of the Breast Papillary carcinoma is a tumor with well defined finger-like projections into the surrounding tissue. It is typically seen in post-menopausal patients.73
Tubular Carcinoma of the Breast Tubular carcinomas are small low-grade tumors with tube shaped cells. It is considered a highly treatable form due to it being less likely to spread beyond the breast than typical IDC. Average age of diagnosis ranges from 45-70 years.74

4. Invasive Lobular Carcinoma (ILC)

a. Typical ILC

Invasive lobular carcinoma (including subtypes) is the second most common form of invasive breast cancer diagnosed in the United States accounting for roughly 6% of all new breast cancer diagnoses.5 ILC begins as a LCIS that then breaks through the wall of the lobule and spreads to surrounding tissues. ILC is more common in women 55 years of age or older and tends to occur later in life compared to Invasive Ductal Carcinoma.75 Detection frequently occurs on physical exam, where a hardening of the breast tissue rather than a lump is felt. Mammography or ultrasound is also useful in the detection of ILC.76 Ultrasound is more sensitive than mammography due to the cancerous cells normally growing in a single-file formation and therefore being less visible on a mammogram.76 Treatment options include surgical management, radiation, pharmacological intervention or combinations of multiple approaches depending upon the specific type, hormone sensitivity, and presence of metastases.77 Medical follow up continues every 4-6 months following cessation of treatment and then annually after 5 years.78

b. Rare ILC subtypes

ILC includes several rare subtypes that differ by either a non single-file cellular growth pattern or have unique cellular characteristics.79

Variation vs. typical ILC
Solid Cells grow in large sheets with few stroma in between.
Alveolar Cancer cells are grouped by 20 or more.
Tubulolobular Some cancer cell growth occurs in the typical single-file formation while others occur in small tube-like formations.
Pleomorphic Cells are larger than in normal ILC and the nuclei differ in appearance.
Signet ring cell Some tumor cells contain mucus that displaces the cell's nucleus to one side.

5. Inflammatory Breast Cancer

Inflammatory breast cancer is a rare and aggressive form that is typically detected later in the disease process, occurs in younger women, and has less favorable outcomes as compared to other breast cancers.80 Inflammatory breast cancer accounts for 1% of all cases of invasive breast cancer diagnosed between 2003-2007.5 Detection often occurs after specific symptoms including one or more of the following: swelling, redness, itching, orange peel-like skin texture, nipple retraction, warmth to the touch, and breast pain.80 Inflammatory breast cancer is often initially diagnosed as mastitis (breast infection/inflammation) which can delay treatment.80 Imaging such as mammogram, MRI, and PET scan are used to support diagnosis with biopsy utilized to confirm diagnosis. Treatment includes chemotherapy in conjunction with surgery, radiation, or hormonal intervention depending upon the amount of spread and ER and PgR status.80

6. Male Breast Cancer

Male breast cancer accounts for slightly less than 1% of all breast cancers diagnosed annually and is most often of the invasive ductal variety.3 5 Outlook, while once considered worse for men vs. women with breast cancer, is now understood to be on par with that for women.81 Risk factors beyond those present in women include Klinefelter syndrome (>1 X-chromosome in men), history of estrogen treatment for prostate cancer, undescended testicle(s), and history of orchiectomy (testicular resection).81 Diagnostic procedures, imaging techniques, and treatment options are generally the same as those utilized with female breast cancer patients. One notable exception is the practice of orchiectomy in males whose breast cancer is androgen sensitive, though this practice has slowed in recent years due to advances in Leutinizing hormone-releasing hormone (LHRH) analogs. LHRH analogs act as anti-androgens, thereby achieving a similar effect to orchiectomy without invasive surgery.81

7. Paget's Disease of the Nipple

Paget’s disease of the nipple is a rare form of breast cancer accounting for less than 1% of all breast cancers5 and occurs concurrently with an underlying form of breast cancer in 95% of those it inhabits.82 Most cases arise in individuals over the age of 50.82 Detection typically occurs when the patient experiences red, flaky, and scaly skin on the nipple. The symptoms spontaneously resolve and recur in the early stages which allow them to be confused with common skin irritation.82 In approximately 50% of cases a lump can be palpated in the breast in addition to the nipple-related symptoms.82 When Paget’s disease of the nipple is suspected a biopsy is taken, if confirmed additional tests (i.e. mammogram) are conducted to detect any additional pathology in the underlying breast tissue.82 Treatment is normally surgical in nature, sometimes in conjunction with radiation therapy, and varies depending upon the spread of the cancer and any underlying tumors.82

8. Phyllodes Tumor of the Breast

Phyllodes tumors of the breast are rare growths that are benign in nature the majority of the time and accounted for only 564 cases (0.2%) of breast malignancies documented between 2003 and 2007 in the United States.5 Physical presentation often occurs as a painless lump felt in the breast.83 Growth occurs quickly and can stretch and even cause an ulceration of the skin.84 Treatment is normally lumpectomy with wide margins (1-2cm) or mastectomy. Response to non-surgical means such as radiation and chemotherapy (poor responses) or hormonal therapy (no response) is limited and is not typically utilized.83

ii. Secondary Tumors

Secondary breast cancers, also known as metastases, occur when cancer cells spread from the original tumor to distant sites via the lymph and blood.85 Cancer cells normally retain the cellular characteristics of the primary tumor in the remote sites which make them recognizable as secondary tumors of a remote location as opposed to primary tumors of the site.85 Once metastases are present the cancer has reached stage IV and the patient’s prognosis is much poorer than in other stages. The most common sites for breast cancer metastases in general are the liver, lungs, bones, and the brain.85 Treatment is normally palliative rather than curative at this stage, however some hormonal and chemotherapy interventions have resulted in long term survival in select studies.86 Radiation therapy is not usually indicated in metastaic disease unless the metastases are limited and symptomatic in nature.86 Stage IV patients often have the option to participate in clinical trials of developing and experimental interventions designed to address late stage metastatic disease.86

iii. Tumor Staging

1. Overview of the Tumor, Nodes, Metastases (TNM) System

Breast Cancer is staged using the Tumor, Nodes, Metastases (TNM) system as defined by the American Joint Committee on Cancer (AJCC) and was most recently updated in 2010.87 88 It is the most widely utilized staging method used in clinical practice and during communication with patients.87 The system specifically cites the size and type of tumor, extent of nodal involvement (if any), and the existence (or lack of) metastases. Based on this classification the cancer can be staged as non-invasive Stage 0 ( in situ ), invasive non-metastatic Stages I-III, or invasive metastatic Stage IV. These stages, as well as cancer type and location, assists in selection of intervention as well as determination of prognosis.87 TNM staging is done prior to any clinical intervention. The below tables detail how TNM information is translated into a breast cancer stage and survival rate by stage at initiation of treatment.87 88 Specifics on determining the T, N, and M classifcations are conatined in the subsequent sections.

Stage T Classification N Classification M classification
0 Tis N0 M0
IA T1 N0 M0
IB T0 N1mi M0
T1 N1mi M0
IIA T0 N1 M0
T1 N1 M0
T2 N0 M0
IIB T2 N1 M0
T3 N0 M0
T1 N2 M0
T2 N2 M0
T3 N1 M0
T3 N2 M0
T4 N1 M0
T4 N2 M0
IIIC Any T N3 M0
IV Any T Any N M1

An empty space indicates same stage as the above box(es).

Stage 5-year survival rate (approximate)87
Stage 0 95%
Stage I 90%
Stage II 78%
Stage III 65%
Stage IV 20%

2. Primary Tumor Classifications87 88

T Category Definition
TX Primary tumor cannot be assessed
T0 No evidence of primary tumor
Tis Carcinoma in situ
Tis (DCIS) Ductal Carcinoma in situ
Tis (LCIS) Lobular Carcinoma in situ
Tis (Paget) Paget disease of the nipple not associated with invasive carcinoma, DCIS or LCIS
T1 Tumor ≤20 mm in greatest dimension
T1 mi Tumor ≤1 mm in greatest dimension.
T1a Tumor >1 mm but ≤5 mm in greatest dimension
T1b Tumor >5 mm but ≤10 mm in greatest dimension
T1c Tumor >10 mm but ≤20 mm in greatest dimension
T2 Tumor >20 mm but ≤50 mm in greatest dimension
T3 Tumor >50 mm in greatest dimension
T4 Tumor of any size with direct extension to the chest wall and/or to the skin (ulceration or skin nodules).
T4a Extension to the chest wall, not including only pectoralis muscle adherence/invasion
T4b Ulceration and/or ipsilateral satellite nodules and/or edema (including peau d’orange) of the skin, which do not meet the criteria for inflammatory carcinoma
T4c Both T4a and T4b
T4d Inflammatory Carcinoma

3. Nodal Classificaitons87 88

N Category Definition
NX Regional lymph nodes cannot be assessed.
N0 No evidence of primary tumor.
N1 Metastases to moveable ipsilateral level I, II axillary node(s)
N2 Metastases in ipsilateral level I, II axillary nodes that are fixed or matted
Metastases in clinically detected ipsilateral internal mammary nodes in the absence of clinically evident axially node metastases
N2a Metastases in ipsilateral level I, II axillary nodes matted to one another or other structures
N2b Metastases only in clinically detected ipsilateral internal mammary nodes and in the absence of clinically evident level I, II axillary metastases
N3 Metastases in ipsilateral infraclavicular (level III axillary) or node(s) with or without level I, II axillary node involvement
Metastases in clinically detected ipsilateral mammary node(s) with clinically evident level I, II axillary node metastases
Metastases in ipsilateral supraclavicular node(s) with or without axillary or internal mammary node involvement
N3a Metastases in ipsilateral infraclavicular node(s)
N3b Metastases in ipsilateral internal mammary nodes(s) and axillary node(s)
N3c Metastases in ipsilateral supraclavicular node(s)

An empty space indicates same category as the above box(es).

4. Metastases Classifications87 88

M Category Definition
M0 No clinical or radiographic evidence of distant metastases
cM0(i+) No clinical or radiographic evidence of distant metastases, but deposits of molecularly or microscopically detected tumor cells in circulating blood, bone marrow, or other non-regional nodal tissue that are ≤0.2mm in a patient without symptoms or signs of metastases.
M1 Distant detectable metastases as determined by classic clinical and radiographic means and/or histologically proven >0.2mm.

8. Medical Management

a. Surgical

Surgery is the most common breast cancer treatment.89 Surgery aims to remove the cancerous tissue from the patient, thus ridding the body of the faulty cellular functioning and subsequent detrimental effects of the disease.89 Surgical types include:

i. Breast-Sparing Surgery

These operations remove the tumor while leaving the breast as intact as possible. Different options include a lumpectomy (just the tumor is removed) or a segmental/partial mastectomy (the section of the breast with the tumor is removed).90 With all surgeries, patients may additionally receive radiation therapy or chemotherapy to destroy any cancerous cells left in the body.89 Long term study follow up has shown no differences in survival or cancer recurrence in patients who received breast-sparing surgery versus radical mastectomies.89

ii. Mastectomy

As much of the breast tissue as possible is removed in attempts to ensure no cancerous cells are left behind. Lymph nodes from under the arm are often also excised with mastectomies.89 In a total (simple) mastectomy, the entire breast is removed and some lymph nodes from the underarm may also be included.90 With a modified radical mastectomy, the surgery involves the entire breast, most or all underarm lymph nodes, and possibly the chest muscle lining.90 The last option is a full radical mastectomy, where the entire breast, every underarm lymph node, and the underlying chest musculature are all removed.90

iii. Hormonal Surgery

If the patient has not gone through menopause, she may have a bilateral oophorectomy, or the surgical removal of her ovaries.89 Because the ovaries are the body’s primary producer of estrogen, removing the ovaries cuts the hormone off at the source.89 However, early menopause caused by this surgery may lead to increased incidence of severe osteoporosis and cardiovascular artherosclerosis.89

b. Radiation

Radiation therapy is an adjunct treatment for cancer that utilizes ionizing radiation (IR) energy to damage DNA and destroy cancerous cells.90 When used in conjunction with tumor removal surgery, internal or external radiation can decrease the patient’s risk of cancer recurrence by 50-66%.91

Radiation energy can be provided by either a photon or a charged particle and the damage done to the DNA may either be direct or indirect ionization of atoms in the DNA sequence.92 With indirect ionization, water molecules are ionized to form hydroxyl free radicals that damage the DNA through single-stranded breaks or double-stranded breaks (DSB). If the IR induces a DSB on an essential gene, it can result in cell death through severe damage to the genome or by initiating apoptosis.93 IR also causes the release of cytokines and growth factors, resulting in altered signal transduction, which can spread the IR-induced effects to neighboring cells.94 The effects of DSBs result in a signal cascade consisting of sensors, transducers, and effectors, with the downstream result of cellular arrest, DNA repair, or apoptosis.93 Each of the sensor, transducer, and effector components of the cascade represents a mechanism called a checkpoint, with each checkpoint being mediated by proteins. Simplified, the cascade consists of the following steps: sensors recognize and bind the damage from the DSBs and subsequent chromatin changes, which activates transducer kinases such as ATM and ATR, and ultimately phosphorylates effector kinases.93 The transducer, ATM, and effector kinases phosphorylate P53 and MDM2, preventing the binding of the two proteins and increasing the stability of P53 (typically, MDM2 binds to and degrades, or ubiquinates, P53).93

Activation and stabilization of P53 can result in cell arrest, repair, or apoptosis as previously mentioned. In cellular arrest, specific cell cycle phases are delayed, resulting in the halt of DNA replication. Through indirect and direct means, P53 influences the timing of the cyclin-dependent kinases (CDK), interfering with the cellular transition mechanisms.93 By altering the CDK cycles, P53 can delay the G1, S, and G2/M transition cell phases.93 Further discussion of the effect on the cell cycle phases requires further research or is beyond the scope of this document. P53 activation can also trigger DNA repair through processes named homologous recombination and non-homologous end-joining.93 In the event of irreparable or improper repair of the DSBs, apoptosis is mediated by the P53 by way of activation of proapoptotic genes. These genes activate proteins that include CD95, PERP, KILLER/DR5 at the cell membrane, cytoplasmic genes encode the proteins PIDD AND PIG, and lastly, a group of genes encode proteins localized at the mitochondria including BAX, BID, PUMA, NOXA, and P53Aip1.93 It is, however, unclear how P53 interacts with these genes and proteins to mediate apoptosis, and whether they are the primary effectors. It has been suggested that apoptosis is mediated through activation of BCL-2 and BCL-XL to induce mitochondrial apoptosis.93

Although the mode of action of external radiation and internal radiation are the same, the delivery mechanisms differ:

i. External Radiation

External radiation is provided from an outside machine directed at the tissue. Patients must travel to a hospital or clinic to receive treatment, typically 5 days per week for several weeks.89

ii. Internal Radiation or Brachytherapy

With internal radiation, the patient’s breast is implanted with a thin plastic tube, needle, seed, wire, or catheter filled with radioactive material, so the radiation therapy comes from within the patient.89 90 The patient will remain in the hospital for several days until the implant is removed.

c. Pharmacological

i. Chemotherapy

Chemotherapy is the use of chemotoxic drugs to kill cancer cells, given as a pill or by intravenous injection.89 The drugs enter the bloodstream and therefore are not targeted specifically at the cancer cells, leading to death of other cells in the body.89 While this systemic effect causes undesirable side effects of the drugs, including hair loss, it additionally leads to destruction of micrometastases, or cancer cells removed from the site of the initial tumor which may have gone undetected.

Cyclophosphamide, methotrexate, 5-fluorouracil, anthracyclines doxorubicin and epirubicin, and taxanes docetaxel and paclitaxel are the most frequently utilized chemotherapeutic drugs with breast cancer treatment.

Cyclophosphamide, or cytophosphane, is a chemotherapeutic prodrug from the oxazophorines group.95 As a prodrug, cyclophosphamide is converted in the liver to its active form by mixed-function oxidase enzymes.96 It is a nitrogen mustard alkylating agent which alters DNA by adding an alkyl group to the guanine base at the N-7 nitrogen atom of the imidazole ring.95 Only cells with low levels of ALDH will form the metabolite phosphoramide mustard from the drug, which causes irreversible cell death through its interaction with the N-7 guanine.

Methotrexate is an antimetabolite and antifolate drug that inhibits the metabolism of folic acid.97 Methotrexate competitively binds with the dihydrofolate reductase (DHFR) enzyme with a thousand times the binding affinity of folate, preventing the conversion of dihydrofolate to active tetrahydrofolate.97 The lack of tetrahydrofolate interrupts DNA synthesis because tetrahydrofolate is required for thymidine production.97 Methotrexate primarily works during the S-phase of the cell cycle.98

5-fluorouracil is a pyrimidine analogue of the antimetabolite drug family, which noncompetitively inhibits thymidylate synthase. Similar to methotrexate, 5- fluorouracil is S-phase specific. It is formed in the cell into cytotoxic metabolites that interfere with DNA and RNA synthesis, causing cell cycle arrest and eventual apoptosis.99 5-fluorouracil additionally inhibits the exosome complex, which is essential for cell survival.99

Anthracyclines, including drugs doxorubicin and epirubicin, are a class of chemotherapeutic drugs that create a chemotoxic effect by intercalating DNA. Doxorubicin inhibits macromolecular biosynthesis and subsequently interferes with the actions of topoisomerase II.100 Topoisomerase II is responsible for uncoiling DNA in preparation for transcription and recoiling the DNA following transcription. Doxorubicin stops topoisomerase II from resealing the DNA double helix.100 While epirubicin has a similar mechanism of action as doxorubicin, its structure includes an altered orientation of the hydroxyl group at the 4’ carbon of the sugar which allows it to be eliminated from the body more quickly and therefore epirubicin has fewer side effects than doxorubicin.101

Taxanes, a drug class including paclitaxel and docetaxel, work by stabilizing microtubules, preventing their breakdown during cell division and subsequently halting mitosis.102 Paclitaxel stabilizes GDP-bound tubulin in microtubules, causing the cell to either get stuck in the G1-phase without dividing or to trigger apoptosis and destroy the cell entirely.102 Docetaxel works by preventing microtuble depolymerization without GTP, leading to accumulation of microtubules within the cell and causing apoptosis.103 Docetaxel additionally inhibits the Bcl-2 oncoprotein.105

ii. Hormonal Therapy

Hormonal therapy prevents the breast cancer cells from absorbing the hormones they require for growth.89 Certain drugs block the woman’s natural hormone through a variety of mechanisms, including tamoxifen, aromatase inhibitors, and luteinizing hormone-releasing hormone (LHRH) agonists.

Tamoxifen is a non-steroidal estrogen receptor modulator that is particularly effective on tumors with prominent hormone receptor expression.89 Tamoxifen selectively binds to estrogen receptors, preventing the hormone from binding to cancerous cells and therefore stunting tumor growth.89 However, as tamoxifen has a systemic effect, estrogen binding is additionally prevented in the uterus, cardiovascular system, cerebrovascular system, and osseous tissues, leading to the drug’s negative side effects.89

Aromatase inhibitors (AI) are another form of endocrine therapy which prevent the patient from producing estradiol, a nonovarian type of estrogen, through peripheral conversion of adrenal component.89 Specifically, the AIs stop the enzyme aromatase from converting androgen into estradiol.90 Nonovarian hormone production primarily occurs in postmenopausal women, limiting the scope of this treatment option. Regardless, research indicates AIs are a beneficial adjunct to tamoxifen to prevent the recurrence of breast cancer in appropriate patients.89

LHRH agonists disrupt the HPG (hypothalamic-pituitary-gonodal) axis and thus indirectly suppresses the ovarian production of estrogen.89 In a normally functioning woman, the hypothalamus manufactures LHRH, which stimulates gonadotropin-releasing hormone (GnRH) from the pituitary and, in turn, affects estrogen production of the ovaries.89 LHRH agonists bind to GnRH receptors, leading to an initial increase in estrogen production, followed by a block of natural LHRH action. The benefit of this therapy is that this suppression of the ovaries and subsequent early menopause is entirely reversible, minimizing the detrimental effects of early menopause caused by permanent solutions like oophorectomy.89

iii. Biological Therapy

Biological therapy, or targeted therapy, takes advantage of the body’s own immune system to fight tumors. These drugs specifically identify and interact with cancer cells, decreasing their proliferation and/or killing the cells outright.90 By reducing the growth and spread of the tumor, the immune system is better able to keep up with the cancer cell proliferation; without intervention, cancer cells typically spread too rapidly for the immune system’s disposal mechanisms to be effective.90

One form of biological therapy for breast cancer is trastuzumab. Trastuzumab is a monoclonal antibody that will bind to HER2 proteins, which are a type of receptor tyrosine kinase.90 105 This drug will only be used in patients who test positive for large amounts of HER2, approximately 25% of all breast cancer patients.90 Trastuzumab binds to HER2, blocking the protein and slowing or stopping the rapid proliferation and growth of cancer cells by interfering with signal transduction to the cell.90 After binding, trastuzumab will increase p27, a protein that interferes with cell proliferation.105 If these tumors continue to progress, lapatinib may be tried. Lapatinib is a tyrosine kinase inhibitor which blocks growth factors from binding to receptor tyrosine kinase and subsequently slows the growth of the tumor.90

9. Conclusion

Breast cancer is a fairly prominent disease in women, affecting approximately 1 in 8 females and accounting for 12% of cancer deaths in women.2 As discussed above, there are a variety of cellular mechanisms which have been identified in the development, progression, detection, and treatment of breast cancer. Cytokines, which typically play a role in the immune response, can end up working in ways that promote tumor growth and metastasis. For instance, TNF-α, which is primarily produced by mononuclear phygocytes, has been linked to the intratumoral regulation of angiogenesis.22 32 Varying serum levels of particular cytokines (i.e. interferons and interleukins) can assist in the prognosis of breast cancer and its propensity to metastasize.21 22 25 26 There has also been research in animal models to suggest that treatment with anti-TNF blocking antibody could halt the grown of cancer cells, however more research is needed in this area.31

Research has shown that 5-10% of breast cancers stem from inherited genetic mutations, while the rest of cases are spontaneous due to other faulty cellular mechanisms.4 Biomarkers are a critical component of breast cancer detection and treatment—they serve to detect, diagnose, stage, and guide treatment of the disease. With the inverse relationship between elevating cancer stages and survival rates, early detection is the best means by which to combat this common form of cancer in women. Current studies involving emerging biomarkers and microRNA, among others, show promise in the ongoing efforts to better detect breast cancer. As research on breast cancer continues to progress, understanding additional cellular mechanisms involved in the progression of the disease will be key to improve upon the present treatments for breast cancer, thus optimizing results and potentially leading to a cure for this widespread disease.

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