Cellular mechanisms of insulin resistance
The cellular events via which insulin initiates its stimulatory effect on glucose metabolism start with binding of the hormone to specific receptors that are present on the cell surface of all insulin target tissues [1–3,211–214]. After insulin has bound to and activated its receptor, “second messengers” are generated and these second messengers activate a cascade of phosphorylation-dephosphorylation reactions that result in stimulation of intracellular glucose metabolism. The first step in glucose utilization involves activation of the glucose transport system, leading to glucose influx into insulin target tissues, primarily muscle. The free glucose, which has entered the cell, subsequently is metabolized by a series of enzymatic steps that are under the control of insulin. Of these, the most important are glucose phosphorylation (catalyzed by hexokinase), glycogen synthase (which controls glycogen synthesis), and phosphofructokinase (PFK) and PDH (which regulate glycolysis and glucose oxidation, respectively).
Insulin receptor/insulin receptor tyrosine kinase
The insulin receptor is a glycoprotein which consists of two α-subunits and two β-subunits linked by disulfide bonds [1–3, 211–214] (Figure 25.10). The two α-subunits of the insulin receptor are entirely extracellular and contain the insulin binding domain. The β-subunits have an extracellular domain, a transmembrane domain, and an intracellular domain that expresses insulin-stimulated kinase activity directed towards its own tyrosine residues. Phosphorylation of the β-subunit, with subsequent activation of insulin receptor tyrosine kinase, represents the first step in the action of insulin on glucose metabolism. Mutagenesis of any of the three major phosphorylation sites (at residues 1158, 1163, and 1162) impairs insulin receptor kinase activity, leading a decrease in the metabolic and growth promoting effects of insulin .
Insulin receptor signal transduction
Following its activation, insulin receptor tyrosine kinase phosphorylates specific intracellular proteins, of which at least nine have been identified [212,213,235]. In muscle insulin-receptor substrate-1 (IRS-1) serves as the major docking protein that interacts with the insulin receptor tyrosine kinase and undergoes tyrosine phosphorylation in regions containing specific amino acid sequence motifs that, when phosphorylated, serve as recognition sites for proteins containing src-homology 2 (SH2) domains. Mutation of these specific tyrosines severely impairs the ability of insulin to stimulate muscle glycogen synthesis, glucose oxidation, and other acute metabolic and growth promoting effects of insulin . In liver, IRS-2 serves as the primary docking protein that undergoes tyrosine phosphorylation and mediates the effect of insulin on hepatic glucose production, gluconeogenesis, and glycogen formation .
In muscle, the phosphorylated tyrosine residues of IRS-1 mediate an association with the 85-kDa regulatory subunit of phosphatidylinositol-3 kinase (PI-3 kinase), leading to activation of the enzyme [1–3,211–214,220,237] (Figure 25.10). PI-3 kinase is comprised of an 85-kDa regulatory subunit and a 110-kDa catalytic subunit. The latter catalyzes the 3-prime phosphorylation of phosphatidylinositol (PI), PI-4-phosphate, and PI-4,5-diphosphate, resulting in the stimulation of glucose transport. Activation of PI-3 kinase by phosphorylated IRS-1 also leads to activation of glycogen synthase, via a process that involves activation of PKB/Akt and subsequent inhibition of kinases, such as glycogen synthase kinase (GSK)-3, and activation of protein phosphatase 1 (PP1). Inhibitors of PI-3 kinase impair glucose transport and block the activation of glycogen synthase and hexokinase (HK)-II expression [211–214,220,237–239]. The action of insulin to increase protein synthesis and inhibit protein degradation also is mediated by PI-3 kinase.
Other proteins with SH2 domains, including the adapter protein Grb2 and Shc, also interact with IRS-1 and become phosphorylated following exposure to insulin [211–214,220]. Grb2 and Shc link IRS-1/IRS-2 to the mitogen-activated protein (MAP) signaling pathway (Figure 25.11), which plays an important role in the generation of transcription factors and promotes cell growth, proliferation, and differentiation [212,220]. Inhibition of the MAP kinase pathway prevents the stimulation of cell growth by insulin but has no effect on the metabolic actions of the hormone .
Under anabolic conditions insulin augments glycogen synthesis by simultaneously activating glycogen synthase and inhibiting glycogen phosphorylase [241,242]. The effect of insulin is mediated via the PI-3 kinase pathway which inactivates kinases, such as glycogen synthase kinase-3, and activates phosphatases, particularly protein phosphatase 1 (PP1). PP1 is believed to be the primary regulator of glycogen metabolism. In skeletal muscle, PP1 associates with a specific glycogen-binding regulatory subunit, causing dephosphorylation (activation) of glycogen synthase. PP1 also phosphorylates (inactivates) glycogen phosphorylase. Multiple studies have demonstrated convincingly that inhibitors of PI-3 kinase inhibit glycogen synthase activity and abolish glycogen synthesis [213,243].
Insulin receptor signal transduction defects in type 2 diabetes
Insulin receptor number and affinity
Both receptor and postreceptor defects contribute to insulin resistance in individuals with T2DM. Some studies have demonstrated a modest 20–30% reduction in insulin binding to monocytes and adipocytes from T2DM patients, but this has not been a consistent finding [1–3,244–247]. The decrease in insulin binding is due to a reduction in the number of insulin receptors without change in insulin receptor affinity.
However, caution should be employed in interpreting these studies, since muscle and liver, not adipocytes, are the major tissues responsible for the regulation of glucose homeostasis in vivo and insulin binding to solubilized receptors obtained from skeletal muscle and liver has been shown to be normal in obese and lean diabetic individuals [245,246,248]. Moreover, a decrease in insulin receptor number cannot be demonstrated in over half of type 2 diabetic subjects, and it has been difficult to demonstrate a correlation between reduced insulin binding and the severity of insulin resistance [249–251]. A variety of defects in insulin receptor internalization and processing have been described in syndromes of severe insulin resistance and diabetes. However, the insulin receptor gene has been sequenced in T2DM patients from diverse ethnic populations and, with very rare exceptions, physiologically significant mutations in the insulin receptor gene have not been observed [252,253]. This excludes a structural gene abnormality in the insulin receptor as a cause of common T2DM.
Insulin receptor tyrosine kinase activity
Insulin receptor tyrosine kinase activity has been examined in skeletal muscle, adipocytes, and hepatocytes from normal-weight and obese diabetic subjects. Most [1–3,210, 246,249,254], but not all , investigators have found a reduction in tyrosine kinase activity (Figure 25.11) that cannot be explained by alterations in insulin receptor number or insulin receptor binding affinity. However, restoration of normoglycemia by weight loss has been shown to correct the defect in insulin receptor tyrosine kinase activity , suggesting that the defect in tyrosine kinase is acquired secondary to some combination of hyperglycemia, distributed intracellular glucose metabolism, hyperinsulinemia, and insulin resistance—all of which improved after weight loss. Exposure of cultured fibroblasts to high glucose concentration also inhibits insulin receptor tyrosine kinase activity . Since insulin receptor tyrosine kinase activity assays are performed in vitro, the results of these assays could provide misleading information with regard to insulin receptor function in vivo. To circumvent this problem, investigators have employed the euglycemic hyperinsulinemic clamp with muscle biopsies and antiphosphotyrosine immunoblot analysis to provide a “snapshot” of the insulin-stimulated tyrosine phosphorylation state of the receptor in vivo . In insulin-resistant obese nondiabetic and type 2 diabetic subjects a substantial decrease in insulin receptor tyrosine phosphorylation has been demonstrated. However, when insulin-stimulated insulin receptor tyrosine phosphorylation was examined in normal glucose-tolerant, insulin-resistant individuals (offspring of two diabetic parents) at high risk of developing T2DM, a normal increase in tyrosine phosphorylation of the insulin receptor was observed . These findings are consistent with the concept that impaired insulin receptor tyrosine kinase activity in type 2 diabetic patients is acquired secondary to hyperglycemia or some other metabolic disturbance.
Insulin signaling (IRS-1 and PI-3 kinase) defects
In insulin-resistant obese nondiabetic subjects, the ability of insulin to activate insulin receptor and IRS-1 tyrosine phosphorylation in muscle is modestly reduced, while in T2DM individuals insulin-stimulated insulin receptor and IRS-1 tyrosine phosphorylation are severely impaired  (Figure 25.11). Association of the p85 subunit of PI-3 kinase with IRS-1 and activation of PI-3 kinase also are greatly attenuated in obese nondiabetic and type 2 diabetic subjects compared to lean healthy controls [210,220,221] (Figure 25.11). The decrease in insulin-stimulated association of the p85 regulatory subunit of PI-3 kinase with IRS-1 is closely correlated with the reduction in insulin-stimulated muscle glycogen synthase activity and in vivo insulin-stimulated glucose disposal . Impaired regulation of PI-3 kinase gene expression by insulin also has been demonstrated in skeletal muscle and adipose tissue of type 2 diabetic subjects . In animal models of diabetes, an 80–90% decrease in insulin-stimulated IRS-1 phosphorylation and PI-3 kinase activity has been reported .
In the insulin-resistant, normal glucose-tolerant offspring of two type 2 diabetic parents, IRS-1 tyrosine phosphorylation and the association of p85 protein/PI-3 kinase activity with IRS-1 are markedly decreased despite normal tyrosine phosphorylation of the insulin receptor; these insulin signaling defects are correlated closely with the severity of insulin resistance, measured with the euglycemic insulin clamp technique . In summary, impaired association of PI-3 kinase with IRS-1 and its subsequent activation are characteristic abnormalities in type 2 diabetics, and these defects are correlated closely with in vivo muscle insulin resistance. A common mutation in the IRS-1 gene (Gly 972 Arg) has been associated with T2DM, insulin resistance, and obesity, but the physiologic significance of this mutation remains to be established .
Insulin resistance of the PI-3 kinase signaling pathway contrasts with an intact stimulation of the MAP kinase pathway by insulin in insulin-resistant type 2 diabetic and obese nondiabetic individuals [1–3,209–211,220]. Physiologic hyperinsulinemia increases MEK1 activity and ERK1/2 phosphorylation and activity similarly in lean healthy subjects and in insulin-resistant obese nondiabetic and type 2 diabetic patients. Intact stimulation of the MAP kinase pathway by insulin in the presence of insulin resistance in the PI-3 kinase pathway may play an important role in the development of atherosclerosis . If the metabolic (PI-3 kinase) pathway is impaired, plasma glucose levels rise, resulting in increased insulin secretion and hyperinsulinemia. Because insulin receptor function is normal or only modestly impaired, especially early in the natural history of T2DM, this leads to excessive stimulation of the MAP kinase (mitogenic) pathway in vascular tissues, with resultant proliferation of vascular smooth muscle cells, increased collagen formation, and increased production of growth factors and inflammatory cytokines [211,226,260].