Co-Activation of Nuclear Factor-kappaB and Myocardin/Serum Response Factor Conveys the Hypertrophy Signal of High Insulin Levels in Cardiac Myoblasts

Abstract

Background and Objective: Hyperinsulinemia contributes to cardiac hypertrophy and heart failure in patients with the metabolic syndrome and type 2 diabetes. Here high circulating levels of tumor necrosis factor (TNF)-alpha may synergize with insulin in signaling inflammation and cardiac hypertrophy. We tested whether high insulin affects activation of TNF-alpha-induced NF-kappaB and myocardin/serum response factor (SRF) to convey hypertrophy signaling in cardiac myoblasts. Methods and Results: In canine cardiac myoblasts, treatment with high insulin (10-8-10-7 M) for 0-24 h increased insulin receptor substrate (IRS)-1 phosphorylation at Ser307, decreased protein levels of chaperone-associated ubiquitin (Ub) E3 ligase C terminus of heat shock protein 70-interacting protein (CHIP), increased SRF activity, as well as beta-myosin heavy chain (MHC) and myocardin expressions. Here siRNAs to myocardin or NF-kappaB, as well as CHIP overexpression prevented - while siRNA-mediated CHIP disruption potentiated - high insulin-induced SR element (SRE) activation and beta-MHC expression. Insulin markedly potentiated TNF-alpha-induced NF-kappaB activation Compared with insulin alone, insulin+TNF-alpha increased SRF/SRE binding and beta-MHC expression, which was reversed by the NF-kappaB inhibitor pyrrolidine dithiocarbamate (PDTC) and by NF-kappaB silencing. In the hearts of db/db diabetic mice, in which Akt phosphorylation was decreased, p38MAPK, Akt1 and IRS-1 phosphorylation at Ser307 were increased, together with myocardin expression as well as SRE and NF-kappaB activities. Conclusion: In response to high insulin, cardiac myoblasts increase the expression or the promyogenic transcription factors myocardin/SRF in a CHIP-dependent manner. Insulin potentiates TNF-alpha in inducing NF-kappaB and SRF/SRE activities. In hyperinsulinemic states, myocardin may act as a nuclear effector of insulin, promoting cardiac hypertrophy.