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| | | | | | Diabetes is a chronic metabolic disorder that affects about 5% of the population in industrialized nations and accounts for over $100 billion in medical costs. Type l diabetes often manifests in childhood and may result from autoimmune destruction of b-cells. Type II diabetes, a more widespread metabolic disorder, generally manifests after the age of 40 and involves progressive development of insulin resistance leading to overt hyperglycemia. Insulin is the major hormone that counters the concerted action of a number of hyperglycemia-generating hormones. It enhances glucose uptake in muscle and adipose tissue, and reduces gluconeogenesis and lipolysis. Insulin resistance, caused by obesity, can result in elevated fasting and postprandial glucose levels and predispose individuals to the risk of type II diabetes.
Action of insulin on target cells is mediated via its interaction with insulin receptor (IR), a heterotetrameric glycoprotein consisting of two extracellular a-subunits (135 kDa) and two transmembrane b-subunits (95 kDa). IR functions as an allosteric enzyme in which the a-subunit inhibits the tyrosine kinase activity of the b-subunit. Insulin binding to the a-subunits results in the stimulation of the tyrosine kinase activity of the b-subunits. The kinase domains of the b-subunits are juxtaposed to the a-subunits, which permit autophosphorylation of Tyr1158, Tyr1162, and Tyr1163, the first step in receptor activation. IR transphosphorylates tyrosine residues on several immediate substrates including insulin receptor substrate (IRS) proteins 1-4, Shc, Grb-2 associated binder-1 (Gab1), and APS adapter protein, all of which provide specific docking sites for other signaling proteins containing SH2 domains. These events lead to the activation of downstream signaling molecules, including PI 3-kinase (PI 3-K). The four IRS proteins exhibit a high degree of homology. IRS-1-knockout mice exhibit growth retardation and impaired glucose tolerance due to resistance to insulin and insulin-like growth factor-1 (IGF-1). IRS-2-knockout mice show severe insulin resistance in the liver and peripheral tissues and develop overt type II diabetes. In addition to tyrosine phosphorylation, both IR and IRS proteins undergo serine phosphorylation by PKC, GSK-3, Akt, and mTOR, which attenuate insulin signaling by blocking insulin-stimulated tyrosine phosphorylation. This serves as a negative feedback loop for insulin signal transduction and allows crosstalk with other pathways that may mediate insulin resistance. Recently, inhibitors of PI 3-K were shown to block the degradation of IRS-1 and the insulin-stimulated increase in Ser312 phosphorylation.
PI 3-K plays a critical role in the metabolic actions of insulin. Inhibitors of class Ia PI 3-K, such as LY 294002, or transfections with dominant negative constructs of the enzyme, block most metabolic actions of insulin, including stimulation of glucose transport, and glycogen and lipid synthesis. Activated PI 3-K specifically phosphorylates PI substrates generating PIP2 and PIP3, which then enlist PI 3-K-dependent kinase (PDK1) and Akt from the cytoplasm to the plasma membrane. This leads to conformational changes in Akt, allowing it to be phosphorylated on Thr308 and Ser473 by PDK1 and PDK2 (mTOR), respectively, to achieve full activation. Akt phosphorylates GSK-3 and inactivates it, which then allows the activation of glycogen synthase to proceed. Activation of Akt also results in the translocation of GLUT4 vesicles to the cell membrane where they participate in the transport of glucose.
Insulin resistance observed in obesity and type II diabetes is characterized by defects at many levels, including decreases in the number of insulin receptors and their tyrosine kinase activity, the concentration and phosphorylation of IRS-1 and -2, PI 3-K activity, and an impairment in insulin-stimulated recruitment of GLUT4 transporter from its intracellular storage compartment to the cell surface. These abnormalities result in a variety of metabolic defects, including hyperglycemia, hyperlipidemia, and hyperinsulinemia. Adipose tissue plays a vital role in the development of insulin resistance and associated abnormalities. A higher circulating level of free fatty acids (FFA), as observed in obesity and type II diabetes, is considered to be an important contributor to insulin resistance. Elevated FFA levels cause a reduction in insulin-stimulated IRS-1 phosphorylation, IRS-1-associated PI 3-K activity, and increased hepatic glucose production via gluconeogenesis. Higher levels of FFA shift substrate preference from glucose to FFA in the muscle tissue oxidation, further contributing to hyperglycemia. Long-term exposure of pancreatic b-cells to FFAs diminishes their insulin secretory response to glucose. Adipose tissue also secretes a variety of hormones (adipokines) that regulate various cellular processes, including energy expenditure. A higher expression of TNF-a in adipose tissue of obese subjects has been linked to insulin resistance. TNF-a is known to impair insulin signaling through IRS-1 serine phosphorylation and through reduced expression of IRS-1 and GLUT4. Deficiency of leptin, another hormone of adipose origin, is also linked with insulin resistance in db/db and ob/ob mice. Leptin replacement improves glycemic control and reduces circulating lipid levels. Resistin, another hormone of adipose origin, is found at much higher levels in animal models of diabetes and obesity, and treatments with insulin sensitizing agents, such as thiazolidinediones (TZD) reduces circulating levels of resistin. TZDs also reduce the expression of adiponectin, an insulin-sensitizing factor in adipose tissue, which reduces serum FFAs by promoting their flux into adipose tissue.
TZDs belong to a new class of insulin sensitizers that are used for the treatment of type II diabetes. They act as direct, high-affinity ligands of peroxisome proliferator-activated receptor g (PPARg) - an adipocyte-specific nuclear hormone receptor. Although PPARg is expressed in most organs, the level of PPARg mRNA is about 50-fold higher in adipose tissue. When compared to some natural ligands, such as 15-deoxy-D 12, 14-prostaglandin J2, TZDs exhibit much higher affinity for PPARg (EC50 = 20-400 nM). In the cell, PPARg forms a heterodimer with the retinoid X receptor (RXR). Without TZD binding the heterodimer is associated with a co-repressor complex that includes a histone deacetylase, which keeps DNA in a transcriptionally repressed state. Upon TZD binding to PPARg, the co-repressor complex dissociates and a co-activator complex containing histone acetylase associates. This promotes binding of the PPARg-RXR complex to PPAR response elements (PPRE) in target genes resulting in modification of the transcription of these genes. PPREs are commonly found in genes involved in lipid metabolism and energy balance, including those encoding lipoprotein lipase, adipocyte fatty acid binding protein, fatty acyl-CoA synthase, glucokinase, and the glucose transporter GLUT4.
TZDs may also have cardiovascular benefits in type II diabetic subjects who exhibit metabolic syndrome X, which is characterized by clustering of atherosclerotic cardiovascular disease risk factors, including insulin resistance, obesity, hypertension, and hyperlipidemia. The characteristic features of metabolic syndrome X emerge from interactions between molecular pathways of glucose and lipid metabolism and blood pressure control. Insulin is known to promote the activity of lipoprotein lipase, which participates in converting VLDL into LDL. A few clinical studies have shown that TZDs raise HDL levels, reduce triglyceride levels, and improve endothelium-mediated vasodilatation. TZD-induced activation of PPARg triggers signaling from adipocytes to skeletal muscle, which ameliorates insulin resistance. This may be linked to a significant reduction of FFA levels by TZDs. It is widely recognized that cardiovascular complications observed in type II diabetes develop through inflammatory and procoagulant pathways with increased oxidative stress. In addition to their insulin-sensitizing effects, TZDs also exhibit antioxidant, anti-inflammatory, and anti-procoagulant properties. These important links have increased our understanding of the relationship between hyperglycemia, insulin resistance, obesity, and the onset of cardiovascular disease. | | | | | |
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