Abstract 13412: Differential Insulin-dependent Signaling Pathways Modulated by Hydrogen Peroxide in Cardiac Myocytes

Abstract

Hydrogen peroxide (H 2 O 2 ) has long been implicated in insulin signal transduction, yet the mechanisms whereby insulin regulates intracellular H 2 O 2 to modulate physiological responses remain obscure. Here we explore the roles of H 2 O 2 in regulation of insulin pathways in cardiac myocytes. We found that insulin inhibits cardiac myocyte beta adrenergic physiological and biochemical responses. We found that catalase (which degrades H 2 O 2 ) completely blocks the ability of insulin to activate the insulin-responsive kinases Akt/mTOR and also to attenuate beta adrenergic receptor-mediated increases in myocyte contractility. These insulin responses are abrogated in a mouse model of type 2 diabetes, establishing a key role for H 2 O 2 -dependent pathways in the diabetic heart. To directly probe intracellular H 2 O 2 responses to insulin in cardiac myocytes, we constructed a novel recombinant AAV9 vector expressing the fluorescent H 2 O 2 biosensor HyPer, which we used to infect mice in vivo . We isolated cardiac myocytes from mice infected with this biosensor and performed quantitative imaging studies. This approach revealed robust insulin-dependent increases in cardiac myocyte H 2 O 2 in response to insulin in wild-type mice. Attenuated H 2 O 2 responses were observed in myocytes isolated from mouse lines deficient in the NADPH oxidase isoforms Nox2 or Nox4 that had been infected with HyPer-AAV9. In Nox2 knockout myocytes, we found that insulin-promoted phosphorylation of kinases Akt and mTor is markedly attenuated, while insulin-dependent inhibition of beta adrenergic contractility is unaffected. In contrast, in cardiac myocytes isolated from the Nox4 knockout mouse, insulin-dependent Akt/mTor phosphorylation responses are unaffected, while the ability of insulin to attenuate the beta adrenergic increase in myocyte contractility was lost. These studies represent an important advance in our understanding of insulin action in the heart by identifying a proximal point of bifurcation in insulin signaling via the differential activation of two NADPH oxidase isoforms. The work also provides insight into mechanisms of diabetic cardiomyopathy, and suggest that insulin resistance in the diabetic heart may unmask potentially deleterious beta adrenergic responses.