Diaphanous 1 and 2 regulate smooth muscle cell differentiation by activating the myocardin-related transcription factors.

We have previously shown that smooth muscle cell (SMC) differentiation marker gene expression is regulated by the small GTPase, RhoA. The objective of the present study was to determine the contributions of the RhoA effectors, diaphanous 1 and 2 (mDia1 and mDia2), to this regulatory mechanism. mDia1 and mDia2 are expressed highly in aortic SMCs and in a number of SMC-containing organs including bladder, lung, and esophagus. Activation of mDia1/2 signaling by RhoA strongly stimulated SMC-specific promoter activity in multiple cell-types including primary aortic SMCs, and stimulated endogenous SM alpha-actin expression in 10T1/2 cells. Expression of a dominant negative Dia1 variant that inhibits both mDia1 and mDia2 significantly decreased SMC-specific transcription in SMCs. The effects of mDia1 and mDia2 required the presence of SRF and the activity of the myocardin transcription factors and were dependent on changes in actin polymerization. Importantly, stimulation of mDia1/2 signaling synergistically enhanced the activities of the myocardin-related transcription factors, MRTF-A and MRTF-B, and this effect was attributable to increased nuclear localization of these factors. These results indicate that RhoA-dependent signaling through mDia1/2 and the MRTFs is important for SMC-specific gene expression in SMCs.