L-type Voltage-Gated Ca ²⁺ Channels Modulate Expression of Smooth Muscle Differentiation Marker Genes via a Rho Kinase/Myocardin/SRF–Dependent Mechanism

Abstract

Vascular smooth muscle cell (SMC) contraction is mediated in part by calcium influx through L-type voltage-gated Ca2+ channels (VGCC) and activation of the RhoA/Rho kinase (ROK) signaling cascade. We tested the hypothesis that Ca2+ influx through VGCCs regulates SMC differentiation marker expression and that these effects are dependent on RhoA/ROK signaling. Depolarization-induced activation of VGCCs resulted in a nifedipine-sensitive increase in endogenous smooth muscle myosin heavy chain (SMMHC) and SM alpha-actin expression and CArG-dependent promoter activity, as well as c-fos promoter activity. The ROK inhibitor, Y-27632, prevented depolarization-induced increase in SMMHC/SM alpha-actin but had no effect on c-fos expression. Conversely, the Ca2+/calmodulin-dependent kinase inhibitor, KN93, prevented depolarization-induced increases in c-fos expression with no effect on SMMHC/SM alpha-actin. Depolarization increased expression of myocardin, a coactivator of SRF that mediates CArG-dependent transcription of SMC marker gene promoters containing paired CArG cis regulatory elements (SMMHC/SM alpha-actin). Both nifedipine and Y-27632 prevented the depolarization-induced increase in myocardin expression. Moreover, short interfering RNA (siRNA) specific for myocardin attenuated depolarization-induced SMMHC/SM alpha-actin transcription. Chromatin immunoprecipitation (ChIP) assays revealed that depolarization increased SRF enrichment of the CArG regions in the SMMHC, SM alpha-actin, and c-fos promoters in intact chromatin. Whereas Y-27632 decreased basal and depolarization-induced SRF enrichment in the SMMHC/SM alpha-actin promoter regions, it had no effect of SRF enrichment of c-fos. Taken together, these results provide evidence for a novel mechanism whereby Ca2+ influx via VGCCs stimulates expression of SMC differentiation marker genes through mechanisms that are dependent on ROK, myocardin, and increased binding of SRF to CArG cis regulatory elements.