Factors that modify insulin action

摘要

Abstract ¿Glucose toxicity¿ accounts for insulin resistance in patients with uncontrolled type 1 diabetes and contributes to it in type 2 diabetes. It contributes to the vascular complications, the major causes of morbidity and mortality in diabetic patients. Sustained exposure of cells to high glucose increases flux via the hexosamine synthesis pathway, enhancing the production of UDP-N-acetyl glucosamine (UDP-GlcNAc), the substrate of O-GlcNAc transferase (OGT). OGT catalyzes the reversible, single addition of O-GlcNAc to specific Ser/Thr residues. OGlcNAcylation and O-phosphorylation are often reciprocal. We have recently identified, for the first time, four sites of O-GlcNAcylation on IRS-1 (as well as 11 novel Ser/Thr phosphorylation sites) by mass spectrometry. Preliminary data suggest that O-GlcNAc may affect IRS-1 signal transduction. Our major objective in Spec. Aim 1 is to firmly establish whether the O-GlcNAc modification alters IRS-1 signaling. In in vitro studies Ser will be mutated to Ala at the four O-GlcNAc sites, singly and in combination, wild type and mutated IRS-1 will be expressed in Hek-293 cells and their interactions with IRS-1 binding partners in response to insulin or to IGF-1 studied. In in vivo studies endogenous mouse IRS-1 will be knocked out with shRNA adenovirus and substituted with wild type or mutant IRS-1-expressing adenovirus. The effect of these manipulations on glucose and insulin tolerance tests and the expression of hepatic gluconeogenic enzymes will be studied. In Spec Aim 2 studies will be continued in a model of high glucose/ low dose insulin-induced insulin resistance of glucose transport and Akt activation in 3T3-L1 adipocytes. Insulin signaling to PI(3)K is largely maintained, but Akt activation is markedly impaired. Preliminary data suggest that PTEN protein expression is increased and insulin stimulated PtdIns(3,4,5)P3 is diminished. mTORC-1 activation and cPKC play a role, but JNK does not. Several mechanisms which may synergize with mTOR will be addressed, including dysregulation of actin dynamics and possible activation of a phosphoprotein phosphatase. The analysis of the modus operandi in this model will be contrasted with the insulin resistance of glucose transport elicited by exposing the cells to FFA. Insights gained from this model will then be applied to L-6 myotubes and to intact rats. Understanding how different excess nutrients modify insulin¿s signalling may lead to the rational development of novel therapeutic targets.