Chen Jiyuan, Kitchen Chad M, Streb Jeffrey W, Miano Joseph M
Center for Cardiovascular Research, University of Rochester Medical Center, 601 Elmwood Avenue, Rochester, NY 14642, USA.
J Mol Cell Cardiol. 2002 Oct;34(10):1345-56. doi: 10.1006/jmcc.2002.2086.
A hallmark of smooth muscle cells (SMCs) in culture and the injured vessel wall is their phenotypic modulation from a differentiated state to one of heightened growth, migration, and matrix synthesis. The transcriptional mechanisms underlying this altered genetic program have yet to be elucidated. Serum response factor (SRF) has emerged as a critical regulator of SMC-restricted gene expression via its interaction with proximal CArG elements; however, levels of SRF protein do not change during SMC phenotypic modulation, suggesting a role for other factors or events in this process. One such factor could be myocardin, a novel SRF coactivator recently cloned from cardiac tissue. Levels of myocardin are abundantly expressed in rat aortic media along with key SMC-restricted genes. In several SMC lines, myocardin mRNA levels decrease in parallel with the loss or attenuation of SMC marker expression. Transient transfection experiments with CMV-driven myocardin in both SMC and non-SMC reveal CArG-dependent transactivation of the SM-Calp promoter-enhancer. Several additional CArG-dependent SMC promoters show variable activation in a cell-and promoter-context dependent manner. To determine whether myocardin could activate an endogenous program of SMC differentiation, we stably transfected L6 myoblasts and assessed SMC marker expression and growth. Results reveal the expression of several SMC markers concomitant with a lower growth potential. Collectively, these studies suggest that myocardin is an important component of a molecular switch for the SMC differentiation program.
培养的平滑肌细胞（SMC）和受损血管壁的一个标志是其表型从分化状态转变为生长、迁移和基质合成增强的状态。这种改变的基因程序背后的转录机制尚未阐明。血清反应因子（SRF）通过与近端CArG元件相互作用，已成为SMC限制性基因表达的关键调节因子；然而，在SMC表型调节过程中，SRF蛋白水平并未改变，这表明在此过程中其他因素或事件发挥了作用。其中一个这样的因素可能是心肌素，一种最近从心脏组织中克隆的新型SRF共激活因子。心肌素水平与关键的SMC限制性基因一起在大鼠主动脉中膜大量表达。在几个SMC系中，心肌素mRNA水平随着SMC标志物表达的丧失或减弱而平行下降。在SMC和非SMC中用CMV驱动的心肌素进行瞬时转染实验，揭示了SM-Calp启动子-增强子的CArG依赖性反式激活。其他几个CArG依赖性SMC启动子在细胞和启动子背景依赖性方式下表现出可变激活。为了确定心肌素是否能激活内源性SMC分化程序，我们稳定转染了L6成肌细胞并评估了SMC标志物表达和生长情况。结果显示几种SMC标志物的表达伴随着较低的生长潜能。总的来说，这些研究表明心肌素是SMC分化程序分子开关的重要组成部分。
