Ca2 + signal-induced cardiomyocyte hypertrophy through activation of myocardin

Abstract

Hypertrophic growth of cardiomyocytes in response to pressure overload is an important stage during the development of many cardiac diseases. Ca2+ overload as well as subsequent activation of Ca2+ signaling pathways has been reported to induce cardiac hypertrophy. Myocardin, a transcription cofactor of serum response factor (SRF), is a key transducer of hypertrophic signals. However, the direct role of myocardin in Ca2+ signal-induced cardiomyocyte hypertrophy has not been explained clearly. In the present study, we discovered that embryonic rat heart-derived H9c2 cells responded to the stimulation of calcium ionophore A23187 with a cell surface area enlargement and an increased expression of cardiac hypertrophy marker genes. Increased Ca2+ also induces an organization of sarcomeres in neonatal rat cardiomyocytes, as revealed by α-actinin staining. Increased Ca2+ could upregulate the expression of myocardin. Knockdown of myocardin by shRNA attenuates hypertrophic responses triggered by increased intracellular Ca2+, suggesting that Ca2+ signals induce cardiomyocyte hypertrophy partly through activation of myocardin. Furthermore, A23187 treatment directly activates myocardin promoter, chelation of Ca2+ by EGTA inhibits this activation and knockdown of myocardin expression using shRNA also abrogates A23187-induced ANF and SK-α-actin promoter activity. CSA (calcineurin inhibitor) and KN93 (CaMKII inhibitor) inhibit A23187-induced the increase in myocardin expression. These results suggest that myocardin plays a critical role in Ca2+ signal-induced cardiomyocyte hypertrophy, which may serve as a novel mechanism that is important for cardiac hypertrophy.