In summary, postbinding defects in insulin action primarily are responsible for the insulin resistance in type 2 diabetes. Diminished insulin binding, when present, is modest and secondary to down-regulation of the insulin receptor by chronic hyperinsulinemia. In type 2 diabetic patients with overt fasting hyperglycemia, a number of postbinding defects have been demonstrated, including reduced insulin receptor tyrosine kinase activity, insulin signal transduction abnormalities, decreased glucose transport, diminished glucose phosphorylation, and impaired glycogen synthase activity. The glycolytic/glucose oxidative pathway is largely intact and, when defects are observed, they appear to be acquired secondary to enhanced FFA/lipid oxidation. From the quantitative standpoint, impaired glycogen synthesis represents the major pathway responsible for the insulin resistance in type 2 diabetes and is present long before the onset of overt diabetes, that is, in normal glucose-tolerant, insulin-resistant prediabetic subjects and in individuals with IGT. Recent studies link the impairment in glycogen synthase activation to a defect in the ability of insulin to phosphorylate IRS-1, causing a reduced association of the p85 subunit of PI 3-kinase with IRS-1 and decreased activation of the enzyme PI3K.
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