Abstract 925: Insulin Inhibits β-adrenergic Receptor Resensitization Through PI3KγInsulin Inhibits β-adrenergic Receptor Resensitization Through PI3Kγ

Abstract

Insulin regulates cardiac metabolism, myocyte survival, and cardiac growth. In addition, insulin is also known to modulate beta adrenergic receptors (βARs) function in the heart regulating cardiac function. βAR function in response to its agonist epinephrine/norepinephrine is regulated by G-protein coupled receptor kinase (GRKs) driven desensitization and protein phosphatase 2A (PP2A) mediated resensitization. Although insulin is known to modulate βAR function through GRKs, less is known about role in insulin in regulating resensitization mechanisms. Phosphoinositide 3-kinase γ (PI3Kγ) is a key regulator of resensitization at it inhibits PP2A activity and therefore, we tested whether insulin could mediate βAR dysfunction through inhibition of resensitization. Insulin stimulation of HEK 293 cells stably expressing β 2 ARs resulted in significant β 2 AR phosphorylation. This β 2 AR phosphorylation was completely abolished with use of pharmacologic inhibitors of PI3K wortmannin or LY 294002 and knock-down of PI3Kγ showing an essential role for PI3Kγ in regulation of βAR function by insulin. Activation of insulin receptor induces downstream signaling via recruitment of the adaptor insulin receptor substrates (IRS). Interestingly, our data suggests that cross talk between the IRs and B 2 ARs are dependent on interaction between PI3Kγ, GRK2 and IRS as identified by co-immunoprecipitation studies and use of surface plasmon resonance using purified proteins. Since cardiomyocytes and fibroblasts represent major cell-types in the heart, primary adult cardiomyocytes and cardiac fibroblasts (CFs) were isolated from WT and PI3Kγ KO hearts for insulin stimulation. While β 2 AR phosphorylation was completely abolished in primary CFs from PI3Kγ KO compared to WT CFs, cardiomyocytes did not show any differences in β 2 AR phosphorylation in response to insulin. These data suggest that PI3Kγ may differentially regulate pathways in cardiomyocytes and CFs in the heart indicating the existence of cell-specific signaling mechanisms that may underlie response to insulin and thereby the cardio-metabolic process.