Insulin resistance is the major metabolic abnormality associated with burn injury. All insulin-mediated effects, including glucose uptake in tissues, protein synthesis, inhibition of gluconeogenesis and anti-inflammatory functions, are markedly attenuated. Supraphysiologic doses of exogenous insulin to counter burn-induced insulin resistance produce deleterious effects, including increased CO2 production and hepatic steatosis. Burn injury alters the insulin signaling pathway at multiple points, via the insulin receptor, insulin receptor substrate-1 (IRS-1), phosphatidylinositol 3-kinase (PI3-K), Akt/PKB (protein kinase B), and glycogen synthase kinase-32 (GSK-32). Although inducible nitric oxide synthase (iNOS) is thought to play an important role in the deranged insulin-signaling, the molecular mechanisms by which iNOS mediates these changes are unknown.
Specific Aim 1 will test the hypothesis that the de-nitrosylation reaction, regulated by S-nitrosoglutathione reductase (GSNOR), plays an important protective role in burn-induced insulin resistance in mice. It is hypothesized that nitrosative stress by iNOS leads to increased protein S-nitrosylation (post-translational modification) of the insulin-signaling proteins resulting in depressed insulin signaling. To test this hypothesis, insulin sensitivity, glucose uptake in muscle, insulin signaling, and S-nitrosylation (by proteomics) of the insulin-signaling proteins will be evaluated in muscle from sham-burned or burned wild-type, GSNOR knockout (-/-) and GSNOR/ iNOS- /- double knockout mice. The protective role of GSK-32 inhibitors in obesity-induced insulin resistance is established, but the molecular mechanism of GSK-32 activation and the salutary effects of GSK-32 inhibitors remain to be investigated particularly in skeletal muscle.
Specific Aim 2 will test the hypothesis that (1) iNOS- mediated increased activity of GSK-32 plays an important role in burn-induced insulin resistance;(2) S- nitrosylation is involved in iNOS-mediated GSK-32 activation after burn injury;and (3) GSK-32 activation reduces IRS-1 expression as a downstream effector of iNOS in skeletal muscle.
Specific Aim 3 will test the hypothesis that endoplasmic reticulum (ER) stress response plays an important role in muscle insulin resistance of burns, and that iNOS functions both as a downstream effector and an upstream enhancer of ER stress in skeletal muscle. The planned studies will use: XBP-1 , skeletal muscle-specific ORP150 over- expressing transgenic, GSNOR-/-, iNOS-/- and GSNOR/iNOS-/- double knockout mice to test these hypotheses. (XBP-1 is a transcription factor regulating ER chaperones and ORP150 protects cells from ER stress.) Thus, these studies will apply an integrated molecular, pharmacologic, and proteomic approach to elucidate the molecular mechanism by which iNOS, GSNOR, GSK-32, and ER stress interrelate to produce insulin resistance, and will provide a rationale and preclinical data for novel therapeutic interventions to treat insulin resistance in muscle after burns.
The proposed studies will apply integrated molecular, pharmacologic, and proteomic approaches to elucidate the molecular mechanism by which inducible nitric oxide and endoplasmic stress reticulum interact with each other and insulin signaling proteins to cause insulin resistance in burns
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