Metabolic effects of vanadyl sulfate in humans with non-insulin-dependent diabetes mellitus: in vivo and in vitro studies.

To investigate the efficacy and mechanism of action of vanadium salts as oral hypoglycemic agents, 16 type 2 diabetic patients were studied before and after 6 weeks of vanadyl sulfate (VOSO4) treatment at three doses. Glucose metabolism during a euglycemic insulin clamp did not increase at 75 mg/d, but improved in 3 of 5 subjects receiving 150 mg VOSO4 and 4 of 8 subjects receiving 300 mg VOSO4. Basal hepatic glucose production (HGP) and suppression of HGP by insulin were unchanged at all doses. Fasting glucose and hemoglobin A1c (HbA1c) decreased significantly in the 150- and 300-mg VOSO4 groups. At the highest dose, total cholesterol decreased, associated with a decrease in high-density lipoprotein (HDL). There was no change in systolic, diastolic, or mean arterial blood pressure on 24-hour ambulatory monitors at any dose. There was no apparent correlation between the clinical response and peak serum level of vanadium. The 150- and 300-mg vanadyl doses caused some gastrointestinal intolerance but did not increase tissue oxidative stress as assessed by thiobarbituric acid-reactive substances (TBARS). In muscle obtained during clamp studies prior to vanadium therapy, insulin stimulated the tyrosine phosphorylation of the insulin receptor, insulin receptor substrate-1 (IRS-1), and Shc proteins by 2- to 3-fold, while phosphatidylinositol 3-kinase (PI 3-kinase) activity associated with IRS-1 increased 4.7-fold during insulin stimulation (P = .02). Following vanadium, there was a consistent trend for increased basal levels of insulin receptor, Shc, and IRS-1 protein tyrosine phosphorylation and IRS-1-associated PI 3-kinase, but no further increase with insulin. There was no discernible correlation between tyrosine phosphorylation patterns and glucose disposal responses to vanadyl. While glycogen synthase fractional activity increased 1.5-fold following insulin infusion, there was no change in basal or insulin-stimulated activity after vanadyl. There was no increase in the protein phosphatase activity of muscle homogenates to exogenous substrate after vanadyl. Vanadyl sulfate appears safe at these doses for 6 weeks, but at the tolerated doses, it does not dramatically improve insulin sensitivity or glycemic control. Vanadyl modifies proteins in human skeletal muscle involved in early insulin signaling, including basal insulin receptor and substrate tyrosine phosphorylation and activation of PI 3-kinase, and is not additive or synergistic with insulin at these steps. Vanadyl sulfate does not modify the action of insulin to stimulate glycogen synthesis. Since glucose utilization is improved in some patients, vanadyl must also act at other steps of insulin action.

Activation of the hexosamine pathway by glucosamine in vivo induces insulin resistance of early postreceptor insulin signaling events in skeletal muscle.

To explore potential cellular mechanisms by which activation of the hexosamine pathway induces insulin resistance, we have evaluated insulin signaling in conscious fasted rats infused for 2-6 h with saline, insulin (18 mU x kg(-1) x min(-1)), or insulin and glucosamine (30 micromol x kg(-1) x min(-1)) under euglycemic conditions. Glucosamine infusion increased muscle UDP-N-acetylglucosamine concentrations 3.9- and 4.3-fold over saline- or insulin-infused animals, respectively (P < 0.001). Glucosamine induced significant insulin resistance to glucose uptake both at the level of the whole body and in rectus abdominis muscle, and it blunted the insulin-induced increase in muscle glycogen content. At a cellular level, these metabolic effects were paralleled by inhibition of postreceptor insulin signaling critical for glucose transport and glycogen storage, including a 45% reduction in insulin-stimulated insulin receptor substrate (IRS)-1 tyrosine phosphorylation (P = 0.02), a 44% decrease in IRS-1 association with the p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase (P = 0.03), a 34% reduction in IRS-1-associated PI 3-kinase activity (P = 0.03), and a 51% reduction in insulin-stimulated glycogen synthase activity (P = 0.03). These alterations in postreceptor insulin signaling were time-dependent and paralleled closely the progressive inhibition of systemic glucose disposal from 2 to 6 h of glucosamine infusion. We also demonstrated that glucosamine infusion results in O-linked N-acetylglucosamine modification of IRS-1 and IRS-2. These data indicate that activation of the hexosamine pathway may directly modulate early postreceptor insulin signal transduction, perhaps via posttranslation modification of IRS proteins, and thus contribute to the insulin resistance induced by chronic hyperglycemia.

Bidirectional modulation of insulin action by amino acids.

Amino acids have been shown to stimulate protein synthesis, inhibit proteolysis, and decrease whole-body and forearm glucose disposal. Using cultured hepatoma and myotube cells, we demonstrate that amino acids act as novel signaling elements in insulin target tissues. Exposure of cells to high physiologic concentrations of amino acids activates intermediates important in the initiation of protein synthesis, including p70 S6 kinase and PHAS-I, in synergy with insulin. This stimulatory effect is largely due to branched chain amino acids, particularly leucine, and can be reproduced by its transamination product, ketoisocaproic acid. Concurrently, amino acids inhibit early steps in insulin action critical for glucose transport and inhibition of gluconeogenesis, including decreased insulin-stimulated tyrosine phosphorylation of IRS-1 and IRS-2, decreased binding of grb 2 and the p85 subunit of phosphatidylinositol 3-kinase to IRS-1 and IRS-2, and a marked inhibition of insulin-stimulated phosphatidylinositol 3-kinase. Taken together, these data support the hypothesis that amino acids act as specific positive signals for maintenance of protein stores, while inhibiting other actions of insulin at multiple levels. This bidirectional modulation of insulin action indicates crosstalk between hormonal and nutritional signals and demonstrates a novel mechanism by which nutritional factors contribute to insulin resistance.