Type 2 Diabetes - Cell Biology

Introduction to Type 2 Diabetes

Type 2 Diabetes Mellitus is a chronic and systemic metabolic disorder distinguished by high blood glucose (hyperglycemia), insulin resistance, and insulin deficiency. [1][2]Previously, it was referred to as non-insulin-dependent diabetes mellitus or adult-onset diabetes mellitus. [1][2]

Type 2 diabetes affects 25.8 million people in the U.S., or 8.3% of the total U.S. population. [3] Type 2 diabetes accounts for approximately 90-95% (23.2 million-24.5 million people) of the total number of diabetes cases. [3] It is estimated that of the 25.8 million U.S. residents that have Type 2 diabetes, 7 million of them are undiagnosed. (Statistics as of 2010) [3]

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Figure 1. Global Prevalence of Type 2 Diabetes in 2000 and estimated prevalence in 2030.
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Causes of Diabetes/ Risk Factors

Type 2 diabetes develops when the body becomes insulin resistant or when the pancreas stops producing enough insulin.[12] The exact cause is unknown, however excess body weight and inactivity are contributing factors.[12] Refer to the following table for major risk factors associated with Type 2 diabetes.

Table 1. Risk Factors for the Development of Type 2 Diabetes[2][12]


Risk Factors for Type 2 Diabetes
Positive family history
Ethnic origin: African American, Native American, Hispanic, Asian American, Pacific Islander
Obesity
Increasing age (>45 years)
Habitural physical activity; sedentary lifestyle
Previous history of gestational diabetes or deliver of babies weighing >9lbs
Presence of other clinical conditions associated with insulin resistance (e.g., polycystic ovary syndrome)
History of vascular disease
Previously identified impaired fasting glucose (IFG) or impaired glucose tolerance (IGT)
Hypertension (> 140/90 mm Hg in adults)
HDL cholesterol level <35 mg/dl and/or triglyceride level >250 mg/dl
Cigarette smoking
Pre-diabetes

Clinical Manifestation

In order to be diagnosed with Type 2 Diabetes, an individual must present with all of the following: 1) Classic symptoms of diabetes mellitus, including polyuria, polydipsia, polyphagia, weight loss, and random plasma glucose ≥ 200 mg/dL, 2) Fasting plasma glucose ≥126 mg/dL, and 3) Two hour post glucose load (75g) plasma glucose ≥200 mg/dL, and confirmed by repeat test.[14]

Table 2. Common Features of Type 2 Diabetes


Features Presentation in Type 2 Diabetes
Age at Onset Usually > 40 years old
Type of Onset Gradual
Etiologic Factors Obesity; associated insulin resistance
HLA Association No
Insulin Antibodies No
Bodyweight at Onset Majority are obese (80%)
Ketoacidosis Rare
Treatment Diet, oral hypoglycemic agents, exercise, insulin, weight control

*HLA (human leukocyte antigen, associated with auto-immune disorders) [2]

The individual with Type 2 Diabetes typically goes undiagnosed for years because the onset is gradual and signs of hyperglycemia is not noticed. Individuals commonly experience visual blurring, neuropathic complications, infections, fatigue and significant blood lipid abnormalities.[2][12] Type 2 Diabetes is typically diagnosed when the patient is receiving medical care for another problem. The patient often presents with a long-term complication of type 2 diabetes, such as cardiovascular disease (CVD), neuropathy, retinopathy, or nephropathy.[2]
Cardinal signs and symptoms of type 2 diabetes include: 1) atherosclerosis, 2) CVD, 3) retinopathy, 4) nephropathy and end-stage renal disease (ESRD), 5) increased risk of infection, 6) musculoskeletal complications of the hands, shoulders, spine, and feet, and 7) sensory, motor, and autonomic neuropathy.[2]

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Figure 2. Characteristic symptoms of type 2 diabetes.
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The long-term presence of type 2 diabetes impacts the large and small blood vessels and nerves throughout the body. Chronic hyperglycemia can lead to macrovascular disease, which affects the arteries supplying the heart, brain, and lower extremities.[2] Type 2 diabetes is also associated with the development of microvascular pathologies in the retina, renal glomerulus, and peripheral nerves. Hyperglycemia associated with diabetes can cause abnormalities in blood flow and increased vascular permeability.[2]

Normal Cellular Function

A. Energy Consumption

Carbohydrates are a main form of energy consumed and utilized by the body.[4] They enter the body in the form of food and are broken down into the molecule glucose by the digestive system.[4] For more on the digestive system refer to here. Glucose enters the bloodstream after digestion and may be used immediately by various tissues or stored for later use as glycogen (See Carbohydrate Storage: Glycogen), depending on metabolic demands.[4] When glucose levels rise in the bloodstream, the pancreas secretes insulin into the blood.[4] In normal healthy tissues, insulin then enables glucose to enter cells through the insulin signaling pathway (see next section for more information).[5] Once inside the cell, glucose is converted into energy (ATP) by three processes: Glycolysis, the Citric Acid Cycle, and the Electron Transport Chain, resulting in a total of 36 ATP units.[63] For more information on Carbohydrate Metabolism and ATP, click here.[63]

B. Insulin Signaling Pathway

The insulin signaling pathway refers to the complex biological process of insulin reacting with target cells such as muscle, fat, or liver cells and the resulting intracellular effects that result, leading to various functional effects observed at the multicellular level.
Insulin is a hormone that is well-known for regulating glucose within the bloodstream.[5][7][8] It also has been shown to mediate lipid and protein metabolism, transcription of specific genes, cell growth and differentiation.[8] The hormone is produced endogenously by Beta Islet cells in the pancreas and is typically released into the bloodstream after consuming carbohydrates (sugar).[6]

Overview of insulin effects on carbohydrate, lipid, and protein metabolism within the body:

Insulin effects on carbohydrate metabolism:

  • increases the rate of glucose transport across the cell membrane in muscle and adipose tissue by activating Glut4 (See Glut4 in Table of Contents)
  • stimulates glycogen synthesis in muscle, fat and liver tissue
  • increases the rate of glycolysis in muscle and adipose tissue
  • inhibits glycogenolysis and gluconeogenesis in the liver[9]

***For information on glycogen synthesis (glycogenesis), glycogenolysis, and glyconeogenesis, see Carbohydrate Storage: Glycogen.

On lipid metabolism:

  • inhibits lipolysis and decreased plasma fatty acids
  • stimulates fatty acid and triacylglycerol synthesis
  • increases the rate of Low-density lipid (LDL) formation in the liver
  • increases the rate of triglyceride uptake from the bloodstream to muscle and adipose tissue
  • decreases the rate of fatty acid oxidation inmuscle and liver tissue
  • increases the rate of cholesterol synthesis in the liver[9]

On protein metabolism:

  • increases the rate of amino acid transport into tissues from the bloodstream
  • increases the rate of protein synthesis in muscle, adipose and liver tissue
  • decreases the rate of protein degradation in muscle and other tissues
  • decreases urea formation[9]

Intracellular substrates of the insulin signaling pathway:

Insulin works by binding its specific receptor on cell surfaces throughout the body, such as on liver, muscle or adipose cells.[5] The insulin receptor is a tyrosine kinase protein that undergoes autophosphorylation of its tyrosine residues that located on its cytoplasmic face once activated by insulin. This in turn phosphorylates a panel of substrates such as insulin receptor substrates (IRS) 1 and 2 and Shc that play an important role in the subsequent cascade of intracellular events described below.[7] There are two main subpathways in insulin signaling pathway: PI3k and MAPK.[8]

PI3K Subpathway:

The phosphorylated IRS proteins activate PI 3-kinase (pI3k) and its downstream targets such as PKB, mTOR, p70 S6 kinase and atypical PKCs.[8]

One of the earliest steps in the insulin signaling pathway is PI 3-kinase (PI3k) activation.[8] It plays a major role in many insulin regulated responses including stimulation of glucose uptake via Glut4 translocation, glycogen synthesis by inhibiting CSK-3 phosphorylation, synthesis of growth-specific proteins, cell growth and proliferation.[8]

Protein kinase B (PKB) is a cellular homologue to the transforming oncogene v-Akt.[8] The activation of PI3k is necessary for the activation of PKB.[8] PKB activation involves an interaction with PI(3,4,5)P3 and/or PI(3,4)P2 through the PH domain. Through PKB’s isoforms α, β, and γ, it plays role in mediating glycogen synthase kinase-3, metabolic actions of insulin, and Glut4 translocation.[8][66] It is debated whether PKB plays a significant role in insulin resistance with diabetes. Some studies have observed alterations in phosphorylation or enzymatic activity of PKB when comparing type 2 diabetic patients to controls while others have not.[66]

The p70 ribosomal S6 kinase (p70 S6k) is a serine/threonine kinase. Mounting evidence has shown that PI3k and PKB activation participate in the stimulation of p70 S6k. Other substrates that promote p70 S6k are PDK1 and mTOR.[8]

PI3k also contributes to the activation of the atypical PKC enzymes PKCζ and PKCλ. These activations are independent of the PI3k’s cascade branch involving PKB, PDK1, mTOR, and p70 S6k (see figure below).[8]

In total, the activation of the PI3K subpathway mediates several insulin-induced responses including GLUT4 activation, glycogen synthesis by inhibiting CSK-3 phosphorylation, and lipogenesis by up-regulation of fatty-acid synthase gene expression.

MAPK Subpathway:

MAPK is other main subpathway that is activated after IRS-1 and 2 phosphorylation that begins with small adaptor proteins Grb2 and SHP2 that lead to further substrate activation downstream. These proteins activate Ras, which recruits serine/threonine kinase Raf to the plasma membrane by direct interaction with its effector region. Raf phosphorylates MEK, a dual-specificity kinase of tyrosine and threonine that activates mitogen-activated protein kinase (MAPK). The MAPK pathway is well known within the insulin signaling cascade, but is not very sensitive to insulin or involved in most of the hormone’s important metabolic responses.[8] The MAPK subpathway has some evidence showing it functions to exert feedback regulation on the PI3k subpathway and is involved in the process of insulin resistance.

PI3K and MAPK Subpathways:
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Figure 4. The PI3K subpathway substrate sequence: IRS proteins (IRS-1,2 and Shc) -> PI3K ->PKB -> p70 S6K. This subpathway also has a second branch: PI3K -> PKCζ and PKCλ. The PI3k subpathway functions to mediate glut4 activation, glycogen synthesis, and lipogenesis. The MAPK subpathway substrate sequence: IRS-1,2 -> Grb2 and SHP2 -> Ras -> Raf ->MEK -> MAPK -> p90rsk. The MAPK subpathway may serve to regulate the PI3k subpathway and may be involved in insulin resistance, but more research is needed to prove this.

Substrates leading to glucose transport:

Insulin-mediated Glucose transport is primarily accounted for through the translocation of glucose transporters to the plasma membrane, most of which is GLUT4 within muscle and adipose cells. Insulin increases the transporters’ cycle to and from the cell surface by promoting exocytosis and inhibiting endocytosis. It has been shown that tyrosine kinase activity and IRS-1-protein phosphorylation are two essential processes in normal glucose transport. Within these pathways, PI3k, PKB, and the atypical PKCs play an particularly key roles in the process of glucose uptake into cells. In contrast, the downstream constituents of PKB such as p70 S6k have been shown to have no immediate effects on glucose uptake. mTOR/p70 S6k may however be implicated in some longterm effects of insulin such as synthesis of Glut1 in specific cells.[8] The MAPK pathway is not likely to be involved in most cases of rapid glucose transport based on the work of Haussdorff et al and Fingar et al.[8]

For more on Glut4, see the GLUT4 section below.

Glucose Transporter Promotion in the Insulin Signaling Pathway
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Figure 5. The PI3K pathway (key substrates- PI3k, PKB and atypical PKCs) mediates Glut4 activation.
The mTOR pathway mediates Glut1 activation.
The MAPK pathway mediates Glut3 activation.

Substrates leading to Glycogen Synthase

Most of glucose that enters human muscle in response to insulin is desposited as Glycogen (see Carbohydrate Storage: Glycogen for more information). Insulin causes stable Glycogen Synthase (GS) activation by causing dephosphorylation at multiple sites within the enzyme. There is still much debate on the exact pathways for GS activation.

The PI3k pathway has been implicated as two modes of GS promotion. PKB activates the mTOR/p70 S6k cascade, which is thought to directly promote GS in specific cells. PKB has also been shown to directly inhibit GSK-3, a well-known inhibitor of GS, thereby promoting GS. MAPK has been implicated in activating GS through phosphorylation of p90 Ribosomal S6 kinase 2 (p90 rsk2) and glycogen bound protein phosphatase-1 (PP1G) downstream. PP1G has many phosphorylation sites that insulin has been shown to augment, but its exact role in GS promotion is not fully understood.

Glycogen Synthase production in the Insulin Signaling Pathway
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Figure 6. PI3k subpathway has two main branches that promote GS production: PI3K -> PKB -> mTOR/p70 S6k (?)-> GS and PI3K -> PKB -| GSK-3 -| GS. The former branch is implicated in direct promotion of GS although the specific relationship between mTOR/p70 S6K and GS are not clear. The later branch is implicated GS promotion by inhibition of the well-established inhibitor of GS, GSK-3.
The MAPK pathway is also implicated in GS production: Ras -> Raf -> MEK -> MAPK -> p90 rsk2 -> PP1G (?)-> GS although the relationship of the endstream substrates p90 rsk2/PP1G and GS is not clear, like mTOR/p70 S6K. Through these three subpathways, the insulin signaling pathway promotes GS and glycogen synthesis.

C. Carbohydrate Storage: Glycogen

Carbohydrates are stored as glycogen in humans through a process called glycogenesis.[10] Glycogen is a polymer of glucose, containing up to 120,000 glucose units.[10] Glycogen is primarily stored in the liver and muscles.[10]

Once glucose enters liver or muscle cells, it is converted to glucose-6-phosphate (G6P) by local hexokinase or glucokinase enzymes.[11] G6P is converted into glucose-1-phosphate (G1P) by enzyme phosphoglutomutase.[11] G1P and uridine-5'-triphosphate (UTP) is synthesized into to UDP-glucose by enzyme G1P-uridyltransferase.[11] The gycogen synthase enzyme completes glycogenesis by removing glucose from UDP-glucose and depositing it onto existing glycogen stores.[11]

Glycogenesis:
Glycogenesis.gif
The conversion process from glucose to glycogen: Glucose (1)-> G6P (2)-> G1P (3)-> UDP-glucose (4)-> added to existing glycogen stores. Enzymes responsible for glycogenesis 1 through 4 respectively: Hexokinase/Glucokinase, Phosphoglutomutase, G1P-Uridyltransferase, and Glycogen Synthase.

Glycogenolysis is the process of breaking down glycogen for energy use when blood glucose levels are low and carbohydrates are not available by consumption.[64] Glycogenolysis is stimulated by Glucagon, a hormone created within the Alpha Islet Cells of the pancreas.[6] Whereas Insulin facilitating uptake to lower its blood glucose, Glucagon stimulates the breakdown of carbohydrate stores to increase blood glucose.[6] See figure below for diagram of pancreas secretion of Glucagon and Insulin working together to regulate blood glucose.

Regulating Blood Glucose Levels with Insulin and Glucagon
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Figure 7. The Pancreas excretes both Insulin and Glucagon to regulate blood glucose. Insulin lowers blood glucose by facilitating absorption into cells and Glucagon raises blood glucose by facilitating the breakdown of Glycogen, carbohydrate stores in muscle and liver cells.*[6]

The biochemical process of glycolysis reverses many of the steps of Glycogenesis with different enzymes[64]. Glycogen links are converted to G1P by Glycogen phosphorylase[64]. G1P is converted to G6P by Phosphoglutomutase. G6P is then converted to glucose by Glucose-6 Phosphotase (in the liver)[64]. Conversion from glucose is not always a necessary step as many tissues can utilize locally generated G6P for energy processes.[64]

Glycogenolysis
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Figure 8. The conversion process from glycogen to glucose (in the liver): Glycogen (1)-> G1P (2)-> G6P (3)-> Glucose. Ezymes responsible for Glycogenolysis 1 through 3 respectively: Glycogen phosphorylase, Phosphoglutomutase, Phosphoglutomutase, and Glucose-6 Phosphotase.

Gluconeogenesis is an alternative process of obtaining glucose through the biochemical conversion of amino acids and trans-fatty acids from protein and fat stores.[64] It mainly occurs in the liver and is necessary when there is not enough glucose available in the blood for tissues that require glucose exclusively as an energy source, such as the brain.[64] For more on Glycolysis, gluconeogenesis, and glycogenolysis, click here.[64]

Normal Immune System Function

The Immune System of the human body is comprised of two different systems, the aquired immune system and innate immune system. The aquired immune system is your immunity your body build up from being exposed to foreign invaders, and the innate immune system is the body's natural unspecific defense against new foreign invaders that the body has not built up immunity against. [2]

A. Innate Immunity

The innate immune system is the body’s first-line of defense against invaders including infections and physical or chemical injury. [68][69][2] When an invasion takes place, a set of reactions happens (that results in inflammation) to prevent tissue damage, kill the invaders, and initiate the repair processes. [68][69][2] With the inflammation there is a systemic reaction referred to as the acute-phase response that results in changes in the amounts of circulating proteins and other substances (acute-phase reactants). [68][69] The acute-phse proteins, C-reactive protein (CRP), serum amyloid A, α1-acid glycoprotein, haptoglobin, and fibrinogen typically increase when there is an invader or injury. [68][69] The amount of albumin (an acute-phase reactant) decreases following an acute-phase response. [68][69] Production of the acute-phase proteins takes place in the liver and is initiated by cytokines including interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α). [68][69] Simply put, the acute-phase proteins limit injury and promote healing. [68][69]

B. Innate Immunity and Type 2 Diabetes

Research has shown that circulating concentrations of acute-phase reactants is increased in type 2 diabetic patients when compared to nondiabetic subjects. [68][69] The acute-phase reactants that were increased in the type 2 diabetic subjects included CRP, serum amyloid A, cortisol, α1-acid glycoprotein, and sialic acid. [68][69] It was also shown that the greater number of symptoms of type 2 diabetes (obesity, coronary heart disease, hypertension, hypertriglyceridemia, and low levels of HDL cholesterol) that a subject experienced, regardless of if they had diabetes or not, the higher the levels of acute-phase reactants and IL-6. [68][69] Many studies have shown a link between elevated levels of acute-phase reactants and type 2 diabetes. [68][69] However, the exact effect of inflammatory cytokines on glucose metabolism and insulin resistance in humans is still unclear. [68][69] It is known that subcutaneous and intrabdominal adipose tissue is a major source of cytokine (TNF-α and IL-6) production, which could lead to the increased inflammation seen in type 2 diabetes. [68][69] Some research has shown that insulin acts as an inhibitor of acute-phase protein synthesis, which means that non-diabetic patients with normal insulin production and utilization inhibit the inflammatory response. [68][69] Diabetic animal models have shown that the acute-phase inflammation response is increased by insulin deficiency, therefore patients with diabetes have increased inflammation due to their decreased insulin production and/or utilization. [68][69] Research suggests that there is a positive feedback found in patients with type 2 diabetes, such that cytokine-induced insulin resistance further promotes the acute-phase response. [68][69] Due to conflicting evidence, further research is warranted to figure out if the acute-phase response of increased inflammation actually causes type 2 diabetes by causing insulin resistance or if the inflammation is a secondary response to type 2 diabetes. [68][69]

C. TNF-α

TNF-α is a proinflammatory cytokine that is induced in response to injury and infection.[37][38] It plays a role in the mediation of the inflammatory processes in a variety of inflammatory disorders.[37][38] TNF-α has been found to have an effect on insulin signaling, lipid metabolism, and adipocyte function and play a central role in various components of metabolic syndrome, including obesity-induced insulin resistance.[37][39][40] Research indicates that TNF-α levels in adipose tissue of human subjects are positively correlated with body mass index (BMI) and hyperinsulinemia, and extended administration of TNF-α may result in hyperinsulinemia.[37][41][42] TNF-α contributes to insulin resistance by a variety of mechanisms, including inhibition of insulin receptor signaling, inhibition of glucose transport, and regulating lipid metabolism.[37][39][40]

TNF-α offered the first piece of molecular evidence that linked obesity, inflammation, and insulin resistance.[37][43] TNF-α mRNA levels have been found to be elevated in adipose tissue of obese human subjects, and may play a role in obesity-mediated insulin resistance.[37][40][41][44]

The Insulin Resistance Atherosclerosis Study (IRAS)[37] investigated the relationships insulin resistance, cardiovascular risk factors, and cardiovascular disease in a multiethnic population across varying statuses of glucose tolerance. Participants demonstrated normal glucose tolerance (NGT), impaired glucose tolerance (IGT), or type 2 diabetes mellitus.[37] Measures of insulin sensitivity and insulin secretion were obtained from all participants during two 4-hour visits, occurring approximately one week apart. Results indicated that circulating levels of TNF-α were higher in participants with type 2 diabetes and IGT compared with those with NGT, regardless of ethnicity.[37] Correlations between TNF-α and waist circumference, fasting insulin, HDL, plasminogen activator inhibitor-1 (PAI-1, the principal inhibitor of tPA), and triglycerides were found across all participants, indicating that TNF-α is more closely associated with increased insulin resistance than with decreased insulin secretion and defects in β-cell function.[37]

Cellular Dysfunction / Dysregulation Associated with Type 2 Diabetes

A. Mitochondrial Dysfunction

1. Reactive Oxygen Species

Mitochondria are the major source of reactive oxygen species (ROS) production in the cell. Increased levels of ROS are a likely cause in a variety of pathophysiological conditions, including type 2 diabetes.[16] Oxidative stress to the mitochondria can come from many sources. ROS are produced in larger amounts by islet cells from patients with type 2 diabetes than by those from non-diabetic patients.[17] Although some ROS are produced in the peroxisomes, the major source of ROS production in cells is the mitochondria. Evidence suggests that obesity and hyperglycemia are associated with increased production of ROS.[17][30] Mitochondrial production of ROS may also be involved in determining skeletal muscle insulin sensitivity.[31] Research on cultured cells suggests that ROS may have an inhibitory effect on insulin signaling.[31][32]

If ROS are not immediately eliminated from the mitochondria, they can cause damage to the mitochondria by promoting DNA fragmentation, protein cross-linking, peroxidation (oxidative degradation of lipids) of membrane phospholipids, and activation of a series of stress pathways.[17] Therefore, in islets in patients with type 2 diabetes, β-cell mitochondria have been found to exhibit morphological abnormalities such as hypertrophy, a rounded shape, and higher density compared to β-cell mitochondria in control subjects.[17]

Research indicates that increased ROS levels are associated with altered mitochondrial morphology in both myotubes cultured in high glucose conditions and in diet-induced diabetic mice.[16] In addition, increased oxidative stress in mitochondria may contribute to increased lipid peroxidation and damage to cell membranes and DNA. This, in turn, activates a cascade of events that further aggravates the severity of the type 2 diabetes.[16]

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Figure 9. Mitochondrial ROS leads to a vicious cycle of damage.
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2. Apoptosis

Apoptosis is a genetically directed process of cell self-destruction marked by the fragmentation of nuclear DNA.[45] It is a form of cell death during which a programmed sequence of events leads to the elimination of cells without releasing harmful substances into the surrounding area. Apoptosis plays a role in developing and maintaining health by eliminating old, unnecessary, and damaged cells.[46]

Reductions in β-cell mass and function both contribute to the pathogenesis of β-cell failure in type 2 diabetes.[17] The islets of individuals with type 2 diabetes have significantly reduced β-cell volume.[17][47][48][49] In addition, the decreased β-cell volume found in subjects with fasting hyperglycemia is associated with increased β-cell death by apoptosis.[17][50]

The onset of type 2 diabetes is accompanied by a progressive decline in β-cell mass due to a significant increase in β-cell apoptosis.[17][50] Mitochondria play a fundamental role in regulating apoptotic cell death.[17] During apoptosis, pro-apoptotic stimuli induce the release of cytochrome c from the mitochondria into the cytoplasm.[17][51] Cytochrome c then participates in the formation of apoptosomes (proteins formed during apoptosis), which activate caspase-9.[17][51] Caspase-9, the initiator caspase, then activates the executioner caspaces 3,6, and 7, which dismantle the cell during apoptosis.[17][51]

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Figure 10. The process of apoptosis and its relation to the mitochondria.
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Evidence suggests that the release of cytochrome c from the mitochondria results from direct action of ROS on cardiolipin, a mitochondrial phospholipid which is located in the inner mitochondrial membrane.[17][52][53] During the early phase of apoptosis, mitochondrial ROS production is stimulated and cardiolipin is oxidized (loses electrons). This causes cytochrome c to detach from the mitochondrial membrane and be released into the cytoplasm of the cell.[17][52] Cardiolipin is a target of the proapoptotic protein tBid, which is a member of the Bcl-2 family produced from Bid by the activation of caspase-8.[17][54] This then results in the activation of the mitochondrial death pathway once apoptosis is induced via engagement of death receptors.[17][54] Cardiolipin is the mitochondrial target of tBid, which then promotes the formation of pores in the outer mitochondrial membrane by Bax or Bak. The process of pore formation is inhibited by Bcl-2 or Bcl-XL.[17][55]

Cardiolipin is a central participant in regulating apoptosis and alterations of mitochondrial cardiolipin have been recognized as being involved in the development of type 2 diabetes and other pathological conditions.[17][56][57]

Specifically within renal cells, increased apoptosis due to hyperglycemia is shown to be associated with end stage renal failure in type 2 diabetics.[70] Hyperglycemia has been shown to downregulate Akt substrate activation within the PI3K subpathway and corresponding increase in p38 MAPK activity of the insulin signaling pathway.[70] Akt is a serine/threonine kinase that regulates a number of cellular functions such as glucose metabolism, glycogen synthesis, protein synthesis, cell proliferation, cell hypertrophy, and cell death.[70] p38 is a protein substrate of the MAPK subpathway which has implications in insulin resistance and influences of PI3K subpathway when significantly altered.[8] The changes in Akt activation coincides with a significant increase in ROS generation within the mitochondria, leading to increase apoptosis of renal cells of the kidney.[70] For more on the PI3k and MAPK subpathways, see the Insulin Signaling Pathway above.

B. Insulin Resistance

1. HbA1c

Hemoglobin is an iron-containing protein in red blood cells (erythrocytes) that carries oxygen from the lungs to other tissues in the body.[2],[22] When hemoglobin molecules combine with glucose in the body, they form the chemical called HbA1c in a process known as glycation.[21],[22] This occurs intra- and extracellularly via a non-enzymatic pathway when hemoglobin is exposed to high levels of blood plasma glucose.[22],[23] In related literature, HbA1c is also commonly referred to as glycohemoglobin, glycated hemoglobin, or glycosylated hemoglobin. The amount of hemoglobin that forms HbA1c depends on the amount of glucose that hemoglobin is exposed to over time.[22],[23] For example, hemoglobin exposed to high levels of glucose for long periods of time results in greater amounts of glycation. This is directly related to continuous breakdown and replacement of erythrocytes in the body. There is a mixture of older and younger erythrocytes in the bloodstream at any point in time.[22] The average lifespan of a red blood cell is about 120 days.[22],[24] Younger erythrocytes have been exposed to recent blood glucose levels (past 30 days); whereas older erythrocytes have been exposed to recent blood glucose levels as well as those from 3-4 months earlier. Blood glucose levels from the past 30 days contribute about 50% to an individual’s HbA1c level, while the remaining 50% are attributable to blood glucose levels up to 120 days ago.[2],[22],[23] Therefore, HbA1c testing reflects an average level; over time (6-12 weeks) of how many red blood cells have glucose attached to them.[2],[21],[22],[23],[24]

Although HbA1c is directly related to blood glucose levels, it is important to realize that blood glucose and HbA1c are not the same. HbA1c levels reflect long-term glucose control. Blood glucose testing reflects measurements of acute changes in blood glucose, or what an individual’s blood glucose level is at the exact time of testing.[22],[25] In regards to assessing blood glucose control in patients with diabetes, previous literature has suggested that HbA1c testing may be a more accurate and stable method than fasting blood glucose tests.[26] The key strength of HbA1c testing is its ability to assess chronic glucose control without results being affected by specific day-to-day variables (ie: time of day for testing, pre/post meal factors, effects of an acute exercise bout).[21],[22],[27] Due to this, HbA1c become a common biological marker of interest for assessing glucose control in patients with diabetes. Additionally, two recently published systematic reviews by the World Health Organization (WHO) moderately support the use of HbA1c testing as a diagnostic tool for diabetes, as well as a predictor of diabetes-related symptoms and/or conditions.[27],[28]

Healthy, non-diabetic individuals with good glucose control have HbA1c levels ranging from 2.5% to 5.9% (meaning <2.5%-5.9% of hemoglobin has glucose bound to it).[2] Due to insulin resistance and/or deficiency, patients with type 2 diabetes are more susceptible to having high levels of circulating blood glucose; and thus, higher HbA1c levels. The WHO currently recommends an HbA1c level of 6.5% as a cut point for diagnosing diabetes.[25],[27] Additionally, the American Diabetes Association (ADA) states that HbA1c levels ranging from 5.7% to 6.4% place individuals at high risk for diabetes.[25],[27] However, controversy remains in relation to interpretation of HbA1c levels near the WHO’s proposed cut point and their clinical significance.[27],[28]

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Figure 3. The Diabetes Control Card is a quick reference for patients diagnosed with diabetes to assess glucose control. Patients with type 2 diabetes are commonly recommended to maintain HbA1c levels <8.0% to prevent and/or reduce risk of complications related to chronic hyperglycemia.[21] Healthy, non-diabetic patients are recommended to maintain HbA1c levels <5.7% to reduce risk of developing type 2 diabetes.[25],[27]

Evidence suggests that elevated HbA1c levels are associated with multiple diabetic complications, such as retinopathy, neuropathy, and nephropathy.[23],[26],[27],[28] Previous literature also suggests elevated HbA1c levels are associated with an increased risk of cardiovascular disease in diabetics.[23],[25],[26] Meta-analysis of related prospective cohort studies found that for each 1% increase in HbA1c, relative risk for a cardiovascular event is 1.18 for patients with type 2 diabetes.[26] Furthermore, excessive levels of HbA1c may increase low-density lipoprotein (LDL) cholesterol, subsequently resulting in development of atherosclerosis and an increase in free radical activity.[23] Evidence supporting the association between elevated HbA1c levels and related pathologies remains unclear due to lack of strong study designs. However, related literature collectively agrees that risk of diabetic complications increases with further increases in HbA1c.[25],[26],[27],[28]

HbA1c testing provides clinicians and patients with type 2 diabetes an objective measurement average glycemic control over time.[23] The ADA recommends that HbA1c testing be performed twice a year in diabetic patients with stable glucose control, and once every three months in diabetic patients who are failing to meet their treatment goals.[23] This information can be used to set patient goals to reduce the risks associated with chronic hyperglycemia. It can also provide information about the efficacy of interventions in patients managing type 2 diabetes.[23] A recently published review of related studies clearly shows that patients with type 2 diabetes can reduce HbA1c levels and the associated risk of cardiovascular disease via numerous therapy options.[29] Therapy options found to reduce HbA1c levels include lifestyle modification (diet and exercise) and numerous antidiabetic drugs.[29]

2. IMCL

A portion of the metabolic stress seen in Type 2 Diabetes may originate from myocellular fat storage. In muscle tissue, lipids are stored as either extramyocellular lipids (EMCL) or intramyocellular lipids (IMCL). EMCL is metabolically static, but IMCL stores are built up, mobilized, and used within hours. Research has found a strong negative correlation between accumulation of IMCL and insulin sensitivity (IS) in humans, both those with and without diabetes, glucose-tolerant and –intolerant, and in individuals with or without obesity.[15]

The combination of increased IMCL and low oxidative capacity are key features in the development of muscular insulin resistance, which is one of the earliest signs of Type 2 Diabetes.[13] High IMCL levels combined with compromised mitochondrial function may play a role in reduced insulin sensitivity.[13] Research has found that IMCL content correlates negatively with insulin sensitivity; therefore, IMCL is a marker for insulin resistance.[13][15]
A four month study investigating the relationship between insulin sensitivity (IS) and IMCL content in Zucker diabetic fatty rats (ZDF) confirmed the relationship between IS and IMCL content seen in humans. The study monitored the IMCL levels in lean and obese ZDF male rats (at 6, 8, 10, 14, 18, and 22 weeks), and detected IMCL concentrations were significantly increased in the obese ZDF rats compared to their lean counterparts.[15]

fatrats.jpg
Figure 11. An obese Zucker diabetic fatty rat has significantly higher IMCL concentrations than its lean counterpart.
http://www.iub.edu/~k536/images/etiology/fatrats.jpg

C. Pathway Dysregulation

1. AMP (adenosine monophosphate)-Activated Protein Kinase (AMPK)

AMPK is a protein kinase, that combines signals to monitor and balance both systemic and cellular energy. It functions to balance energy demand with nutrient supply. [33][67]

a. Structure of AMPK

Activators and inhibitors of AMPK
*AMPK is activated resulting in increased ATP production or inhibited resulting in ATP dephosphorylation to ADP in order to balance nutrient supply with energy demand.[67]

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Figure 12. Upstream kinases (LKB1), transforming growth factor-β activated kinase 1 (Tak1) and Ca2+/calmoudulin activated kinase (CAMKK) activate AMPK through phosphorylation (adding a phosphate) of the α subunit. Protein phosphatases 2A/C (PP2A/C) inhibit AMPK through dephosphorylation of α subunit. Glycogen can bind to the β subunit to inhibit AMPK. Small molecular activators AICAR/SMP and AMP directly activate the γ subunit. Indirect activators (metformin, dinitrophenol (DNP), and rotenone) work by increasing AMP:ATP ratio, compound C works by inhibiting activation of AICAR. [67]

It is composed of 3 different subunits α, β, and γ appearing in several isoforms (different forms of the same protein) with similar functions.[33][67]

  • The α-subunit contains the catalyst site that helps to speed up the activation of AMPK through binding with protein kinases (seen in figure 12).[33][67]
  • The regulatory β- and γ-subunits are important to maintain the stability of the complex.[33][67]
  • The β-subunit contains the region that allows AMPK to bind to glycogen.[33][67]
  • The γ-subunit binds two molecules of AMP or ATP. This binding of AMP activates AMPK through direct activation and phosphorylation of the α-subunit.[33][67]
  • At times of high energy demand the γ subunit rapidly responds to changes in the AMP to ATP ratio to maintain energy balance. AMPK promotes ATP producing catabolic pathways and also inhibits ATP consuming anabolic pathways (seen in figure 12).[33] [67]

*This entire process works to regulate energy homeostasis by balancing nutrient supply with energy demand. In cases of hyperglycemia and sedentary lifestyles, that are common in patients with type 2 diabetes, the AMPK pathway is not activated as effectively as in patients with normal blood glucose levels. [33] [67]

b. The AMPK pathway

Target of AMPK activation
*Activation of AMPK causes increased GLUT4 activation resulting in increased glucose uptake from the blood into the cell.[67]

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Figure 13. AMPK phosphorylates TBC1D1 which increases activity of GLUT4, resulting in increased glucose uptake. AMPK also inhibits glycogen synthase (GS) in order to decrease glycogen synthesis. During times of high glucose uptake, increased amounts of glucose-6-phosphate (G6P) leads to an increase in glycogen synthesis. [67]

AMPK is activated by physical activity in such a way that increased intensity results in increased activation. [33][67] It is known that increased muscle contraction results in increased energy/ATP demand, which results in increased activation of AMPK. [33][67] As physical activity intensity increases, ATP demand increases as well, causing increased activation of the AMPK pathway.[33][67]

It has been shown that AMPK directly phosphorylates and activates PGC-1 α. [33] We know that PGC-1 α activity is decreased in type 2 diabetes, and therefore activation of AMPK could be used to increase PGC-1 α activity in patients with type 2 diabetes. [33] Increased activation of PGC-1 α leads to an increase in mitochondrial biogenesis, which improves metabolism of carbohydrates, fats and proteins.[33][67]

Research shows that AMPK activation may enhance the effects of exercise on insulin sensitivity and glucose transports in skeletal muscle as well as additional metabolic benefits in the liver.[33] Disruption of AMPK activation has been shown to be a key factor in metabolic disorders, including type 2 diabetes.[33]

Reduction of AMPK activity promotes the development of insulin resistance and glucose intolerance, disturbs muscle energy balance during exercise, and decreases mitochondrial biogenesis (mitochondria’s ability to make ATP).[33] In insulin-resistant rodents, increased AMPK activity has been linked with improved blood glucose homeostasis, lipid profile and blood pressure. [33

2. PGC-1α

PGC-1α (Peroxisome Proliferator-Activated Receptor (PPAR)- gamma coactivator (PGC)-1 alpha) is a protein transcription coactivator that assists by coactivating many transcription factor involved in many aspects of cellular functions.[18][19][20] Of the many cellular functions PGC-1α helps with is metabolism of multiple energy substrates. In mice with over expressed PGC-1α there was an enhanced mRNA content of fat oxidation enzymes like carnitine almitoyltransferase I and medium-chain acyl-coenzyme A dehydrogenase, whereas PGC-1α knockout mice has a decrease in these fat oxidation enzymes. PGC-1α assists with glucose metabolism, by increasing the amount of Glut4 which allows glucose transport in to the cell. Gluconeogenisis is assisted by PGC-1α which has been shown that mice with over expressed PGC-1α had improve glycogen utilization during endurance exercise compared to knockout mice.[18][19]

Multiple changes in skeletal muscle have been associated with PGC-1α such as conversion of type IIb (glycolytic) muscle fibers to type IIa and type I (oxidative) muslce fibers.[18][20] The fiber type change in the skeletal muscle changes the substrate utilization to an increase in oxidative fatty acid metabolism. Angiogenesis has also been found in mice with over expressed PGC-1α, these mice have an increase in vascular endothelial growth factor (VEGF) and an increase capillary density compared to knockout mice.[18][19] The fiber type conversion and angiogenisis is functionally supported in over expressed PGC-1α mice who have increased running endurance and VO2 max compared to knockout mice.[18][19]

PGC-1α promotes mitochondrial biogenesis and has been linked with increased mitochondrial transcription factor (tFAM) and nuclear respiratory factor (NRF1, NRF 2).[20] Skeletal muscle of over expressed mice showed a change in color and increase mRNA in lab testing compared to knockout mice.[18][19] Studies have shown PGC-1α to have a decrease in mRNA and reduced protein content of mitochondrial respiratory chain proteins and ATP synthase.[18][20] This change in mitochondrial respiratory chain proteins and ATP synthase also assists in the defense against anti-oxidants.[18] With increased oxidative capacity of mitochondrial there is better control of reactive oxygen species (ROS).[18]

In human studies with people with type 2 diabeties have a decreased PGC-1α expression compared to healthy subjects.[20] The skeletal muscle of people with type 2 diabetes and their asymptomatic relatives have a decrese in mRNA expression for PGC-1α.[19]

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Figure 14: Cellular functions involving PGC-1α

3. Glucose Transport 4 (GLUT4)

GLUT4 is a glucose transporter protein found mostly within muscle and fat cells.[34] Glucose transporters facilitate delivery of glucose from the bloodstream to various bodily tissues and thus are important in regulating blood glucose homeostasis.[34] Within skeletal muscle and adipose tissue, GLUT4 is the predominant glucose transporter.[35] For muscle, both insulin and muscle contraction can activate GLUT4 by translocating it from intracellular sites such to the plasma membrane where it can actively deliver glucose into the cell.[36] There is evidence suggesting that exercise and insulin stimulate different stores of GLUT4 within muscle cells and thus have additive effects when working together to translocate the transporter.[36]

Defects in the cycling of GLUT4 from intracellular cites to the plasma membrane and back is associated with insulin resistance in type 2 diabetes. [36] These defects may arise from alterations in the translocation, docking, or fusion of the glucose transporters at the plasma membrane or T tubules, or potentially from changes in the specific activity of the transporters. [36] PI3K and one of its downstream substrates p85 are implicated in glucose transporter interruption as well. [36] Despite the changes implicated in GLUT4 cycling and insulin signaling pathway substrates, studies have shown that exercise-induced activation of Glut4 remains unimpaired, allowing glucose to enter muscle cells at normal or even higher levels compared to diabetic controls. [36]

Glut4 translocation from intracellular sites to the plasma membrane in skeletal muscle via Insulin and Exercise induced stimulus
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Figure 15. Insulin-induced Glut4 translocation: occurs via the PI3K subpathway.
Exercise-induced Glut4 translocation: occurs via mechanical contraction and calcium presence from the sarcoplasmic reticulum, although these pathways are less understood than insulin-induced translocation.

Medical Management

A. Drug-Free Management of Type 2 Diabetes

Diet - The use of a balanced diet consisting of primarily non-starch whole grain carbohydrates high in fiber and low in saturated fats can help control slightly elevated blood glucose.[58]
An increase in overall fat intake and excess adipose tissue decreases insulin binding, glucose transportation, and reduced glycogen synthesis. Saturated fat increase has been linked to higher fasting insulin levels and decreased insulin sensitivity, where as non-saturated vegetable fats have had an opposite effect. A positive correlation has been seen with an increase in BMI (greater than 18.5-24.9 kg/m2) and waist to hip ratio and Type 2 Diabetes in various epidemiological studies.[58]
An increase in carbohydrates with high glycemic index scores and low in dietary fiber have an increase risk for diabetes and decrease insulin sensitivity. A variety of studies have shown improve glycemic control and decreased LDL with diets high in carbohydrates and low in fat compared to diets high in fat and low in carbohydrates.[58]

B. Exercise

The American College of Sports Medicine (ACSM) recommends 150 minutes of exercise per week to help manage blood glucose levels.[59] For more information, visit the Type 2 Diabetes Exercise page.

C. Management of Type 2 Diabetes via Medicine

Oral medications
Metformin- A medication which lowers blood glucose levels by suppressing hepatic gluconeogenesis, increases glucose utilization in the periphery, and increases insulin sensitivity. Metformin is contraindicated in patients with renal impairments or vascular disease.[60]
Thiazolidinediones – A medication which increases receptor molecules, specifically peroxisome proliferator-activated receptors. These molecules help the transcription of genes for fat and glucose metabolism. Adverse effects include increased edema and congestive heart failure.[61]
Dipeptidyl peptidase-4 inhibitors - A medication which reduces blood glucose by blocking DPP-4, which reduces glucagon production and increases insulin secretion. Side effects include slowed gastric emptying.[60]
α-glucosidase inhibitors – A medication which reduces postprandial blood glucose by reducing the amount of glucose digested through competitive inhibition of enzymes for carbohydrate digestion in the intestines. This is contraindicated in gastrointestinal disease or severe renal and hepatic disease.[60]
Sulfonylureas – A drug class that stimulates insulin secretion in a glucose-dependent manner. Side effects include hypoglycemia; therefore, the patient must be monitored.[60]
Repaglinide – A drug class administered prior to eating that stimulates insulin secretion with a shorter half life compared to sulfonylureases.[60]

A systematic review of oral medication found that most guidelines for oral medications support metformin as a top tier hypogylcemic drug with minimal adverse conditions compared to other oral medications.[60]

D. Insulin therapy

When the pancreas can no longer maintain optimal blood glucose levels with using the various oral medications, many doctors will add insulin injections to assist in controlling glucose levels. Normally insulin is injected in to adipose tissue of the abdomen or pumped in to the abdomen tissue through a pump. A variety of types of insulin are used based on the onset time for insulin to reach the blood stream, peak time for the insulin to reach maximum strength, and the duration of time required for insulin to lower blood glucose.[62]
Insulin therapy normally begins with one injection per day plus the use of oral medication. When blood glucose is no longer controlled with one injection, multiple injections may be added.[62]

E. Combination of therapies

Metformin and insulin in combination had better control over glucose levels than insulin alone.[62]
Sulphonylurea and insulin had similar or better control in combination compared to insulin alone.[62]

Conclusion

Type 2 diabetes is a chronic, systemic, metabolic disorder distinguished by hyperglycemia, insulin resistance, and insulin deficiency.[2] Type 2 diabetes develops when the pancreas does not produce enough insulin (insulin deficiency) or cells in the body fail to respond to insulin (insulin resistance).[1] When insulin fails to enable glucose transport into cells, glucose may excessively accumulate in the blood (hyperglycemia).[1] Collectively, this decreases the body’s ability to control glucose levels and utilize glucose to adequately fuel cells in the body.[1] Uncontrolled chronic hyperglycemia can lead to diabetic complications such as cardiovascular disease, retinopathy, neuropathy, nephropathy, recurring infections, and musculoskeletal disorders of the spine, shoulders, and distal extremities.[12] However, onset is gradual and clinical signs are often unnoticed in early development.[2][12]

Type 2 diabetes is the most common form of diabetes, currently affecting more than 25 million people in the U.S. It is considered a global epidemic by the WHO, and it is estimated that the worldwide prevalence will increase substantially within the next 20 years.[1],[3] Currently, there is no cure for type 2 diabetes.[12] Therefore, prevention and disease management are critical for patients who are diagnosed or at risk of developing type 2 diabetes. Major components of treatment include lifestyle modifications, with an emphasis on eating a balanced diet and performing regular exercise.[29],[58],[59] Additional medical interventions include multiple pharmacotherapy options, insulin replacement therapy, or a combination of both.[29],[60],[61],[62]

The exact cause of type 2 diabetes remains unknown. However, obesity and sedentary lifestyle are highly associated with development of the disease.[12] On the cellular level, evidence is mounting relating to the insulin signaling pathway, its associated substrates, and their roles in insulin resistance with diabetes. Activation of the PI3k and MAPK subpathways promote glucose transport and production of glucose synthase.[8] Alterations in these signaling pathways may account for decreased insulin function with type 2 diabetes. Mitochondrial dysfunction also appears to be an important factor related to interruption of the insulin signaling pathway and alterations in cellular apoptosis. Both human and animal studies have demonstrated an association between compromised mitochondrial health and type 2 diabetes.[16],[17],[50],[70] Research has also shown additional cellular structures to have associations with the disease; including HbA1c, TNF-α, IMCL, AMPK, PCC-1α, and Glut4. Each of these has been used as a biological marker of interest in previous literature. For example, treatment aimed at increasing expression PCC-1α may be of particular importance with type 2 diabetes due to its ability to promote glucose metabolism and mitochondrial biogenesis.[18][19],[34] However, the exact cellular mechanisms attributable to type 2 diabetes remain unclear. Further research is warranted to better understand the underlying cellular processes of the disease and determine what interventions are best suited to manage it.

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