Sanjay K. Pandey

Potential mechanism (S) involved in the regulation of glycogen synthesis by insulin

Stimulation of glycogen synthesis is one of the major physiological responses modulated by insulin. Although, details of the precise mechanism by which insulin action on glycogen synthesis is mediated remains uncertain, significant advances have been made to understand several steps in this process. Most importantly, recent studies have focussed on the possible role of glycogen synthase kinase-3 (GSK-3) and glycogen bound protein phosphatase-1 (PP-1G) in the activation of glycogen synthase (GS) - a key enzyme of glycogen metabolism. Evidence is also accumulating to establish a link between insulin receptor induced signaling pathway(s) and glycogen synthesis. This article summarizes the potential contribution of various elements of insulin signaling pathway such as mitogen activated protein kinase (MAPK), protein kinase B (PKB), and phosphatidyl inositol 3-kinase (PI3-K) in the activation of GS and glycogen synthesis.

Insulin signal mimicry as a mechanism for the insulin-like effects of vanadium

Among several metals, vanadium has emerged as an extremely potent agent with insulin-like properties. These insulin-like properties have been demonstrated in isolated cells, tissues different animal models of type I and type II diabetes as well as a limited number of human subjects. Vanadium treatment has been found to improve abnormalities of carbohydrate and lipid metabolism and of gene expression in rodent models of diabetes. In isolated cells, it enhances glucose transport, glycogen and lipid synthesis, and inhibits gluconeogenesis and lipolysis. The molecular mechanism responsible for the insulin-like effects of vanadium compounds have been shown to involve the activation of several key components of insulin-signaling pathways that include the mitogen-activated-protein kinases (MAPKs) extracellular signal-regulated kinase 1/2 (ERK1/2) and p38MAPK, and phosphatidylinositol 3-kinase (PI3-K)/protein kinase B (PKB). It is interesting that the vanadium effect on these signaling systems is independent of insulin receptor protein tyrosine kinase activity, but it is associated with enhanced tyrosine phosphorylation of insulin receptor substrate-1. These actions seem to be secondary to vanadium-induced inhibition of protein tyrosine phosphatases. Because MAPK and PI3-K/PKB pathways are implicated in mediating the mitogenic and metabolic effects of insulin, respectively, it is plausible that mimicry of these pathways by vanadium serves as a mechanism for its insulin-like responses.

Vanadium salts stimulate mitogen-activated protein (MAP) kinases and ribosomal S6 kinases

Effect of several vanadium salts, sodium orthovanadate, vanadyl sulfate and sodium metavanadate on protein tyrosine phosphorylation and serine/threonine kinases in chinese hamster ovary (CHO) cells overexpressing a normal human insulin receptor was examined. All the compounds stimulated protein tyrosine phosphorylation of two major proteins with molecular masses of 42 kDa (p42) and 44 kDa (p44). The phosphorylation of p42 and p44 was associated with an activation of mitogen activated protein (MAP) kinase as well as increased protein tyrosine phosphorylation of p42mapk and p44mapk. Vanadinm salts also activated the 90 kDa ribosomal s6 kinase (p90rsk) and 70 kDa ribosomal s6 kinase (p70s6k). Among the three vanadium salts tested, vanadyl sulfate appeared to be slightly more potent than others in stimulating MAP kinases and p70s6k activity. It is suggested that vanadium-induced activation of MAP kinases and ribosomal s6 kinases may be one of the mechanisms by which insulin like effects of this trace element are mediated.

Modulation of G-protein expression by the angiotensin converting enzyme inhibitor captopril in hearts from spontaneously hypertensive rats Relationship with adenylyl cyclase

We have recently shown an enhanced expression of Gi alpha-2 and Gi alpha-3 at protein and mRNA levels and their relationship with adenylyl cyclase regulation in hearts and aorta from spontaneously hypertensive rats (SHR). The present studies were undertaken to examine if the antihypertensive action of captopril, an angiotensin I converting enzyme (ACE) inhibitor, is associated with the interaction and modulation of G-proteins and adenylyl cyclase activity. SHR and age-matched WKY were divided into two groups. One group of rats received captopril (10 mg/kg body weight) intravenously, whereas the other group received only vehicle (0.9% saline). The levels of Gi alpha-2 and Gi alpha-3 proteins were determined by immunoblotting technique using specific antisera against these proteins. The levels of Gi alpha-2 and Gi alpha-3 proteins were significantly enhanced in hearts from SHR as compared to WKY and captopril treatment restored the enhanced levels of Gi alpha-2 and Gi alpha-3 observed in SHR by about 70% to 80% towards WKY control rats. However, captopril slightly decreased the levels of Gi alpha 2 and Gi alpha 3 protein in normotensive WKY. In addition, the diminished stimulation of adenylyl cyclase by isoproterenol, glucagon and N-ethyl carboxamide adenosine and enhanced inhibition by inhibitory hormones such as C-type natriuretic peptide and angiotensin II observed in SHR was restored significantly by captopril treatment. These results suggest that one of the mechanisms by which captopril lowers the blood pressure may be due to its ability to modulate the levels of G-proteins and adenylyl cyclase activity.

Insulin-induced Ca2+ entry in hepatocytes is important for PI 3-kinase activation, but not for insulin receptor and IRS-1 tyrosine phosphorylation

Insulin produces an influx of Ca(2+) into isolated rat hepatocyte couplets that is important to couple its tyrosine kinase receptor to MAPK activity (Benzeroual et al., Am. J. Physiol. 272, (1997) G1425-G1432. In the present study, we have examined the implication of Ca(2+) in the phosphorylation state of the insulin receptor (IR) beta-subunit and of insulin receptor substrate-1 (IRS-1), as well as in the stimulation of PI 3-kinase activity in cultured hepatocytes. External Ca(2+) chelation (EGTA 4 mM) or administration of Ca(2+) channel inhibitors gadolinium 50 microM or nickel 500 microM inhibited insulin-induced PI 3-kinase activation by 85, 50 and 50%, respectively, whereas 200 microM verapamil was without effect. In contrast, the insulin-induced tyrosine phosphorylation of IR beta-subunit and of IRS-1 was not affected by any of the experimental conditions. Our data demonstrate that the stimulation of PI 3-kinase activity by the activated insulin receptor, but not the phosphorylation of IR beta-subunit and IRS-1, requires an influx of Ca(2+). Ca(2+) thus appears to play an important role as a second messenger in insulin signaling in liver cells.

Stimulation of Mitogen-Activated Protein Kinases ERK-1 and ERK-2 by H2O2 in Vascular Smooth Muscle Cells

The possible roles of the mitogen-activated protein kinases (MAPKs) ERK-1 and ERK-2 in mediating growth-promoting and hypertrophic responses were investigated by examining the effect of H2O2 on ERK-1 and ERK-2 phosphorylation and activation in an established vascular smooth muscle cell (VSMC) line (A10). H2O2 treatment of VSMCs stimulated the phosphorylation of both ERK-1 and ERK-2 in a concentration-dependent manner with maximal impact at the dose level of 2mM H2O2. Treatment of cells with aminotriazole, an inhibitor of catalase, potentiated the effect of H2O2 and reduced the H2O2 concentration required to elicit the maximum response. In addition, PD98059, an inhibitor of MAPK kinase (MEK), and wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3K), attenuated the H2O2-induced activation of ERK-1 and ERK-2. H2O2 was also found to stimulate PI3K activity, which was inhibited by wortmannin. Taken together, these data demonstrate that oxidative stress-induced ERK activation is mediated, at least in part, through a PI3K-dependent pathway and suggest a role of this pathway in eliciting growth-promoting and hypertrophic responses to H2O2 in VSMCs.