Du Kevin L, Chen Mary, Li Jian, Lepore John J, Mericko Patricia, Parmacek Michael S
Division of Cardiovascular Medicine, Department of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
J Biol Chem. 2004 Apr 23;279(17):17578-86. doi: 10.1074/jbc.M400961200. Epub 2004 Feb 17.
The SAP domain transcription factor myocardin plays a critical role in the transcriptional program regulating smooth muscle cell differentiation. In this report, we describe the capacity of myocardin to physically associate with megakaryoblastic leukemia factor-1 (MKL1) and characterize the function of MKL1 in smooth muscle cells (SMCs). The MKL1 gene is expressed in most human tissues and myocardin and MKL are co-expressed in SMCs. MKL1 and myocardin physically associate via conserved leucine zipper domains. Overexpression of MKL1 transactivates serum response factor (SRF)-dependent SMC-restricted transcriptional regulatory elements including the SM22alpha promoter, smooth muscle myosin heavy chain promoter/enhancer, and SM-alpha-actin promoter/enhancer in non-SMCs. Moreover, forced expression of MKL1 and SRF in undifferentiated SRF(-/-) embryonic stem cells activates multiple endogenous SMC-restricted genes at levels equivalent to, or exceeding, myocardin. Forced expression of a dominant-negative MKL1 mutant reduces myocardin-induced activation of the SMC-specific SM22alpha promoter. In NIH3T3 fibroblasts MKL1 localizes to the cytoplasm and translocates to the nucleus in response to serum stimulation, actin treadmilling, and RhoA signaling. In contrast, in SMCs MKL1 is observed exclusively in the nucleus regardless of serum conditions or RhoA signaling. However, when actin polymerization is disrupted MKL1 translocates from the nucleus to the cytoplasm in SMCs. Together, these data were consistent with a model wherein MKL1 transduces signals from the cytoskeleton to the nucleus in SMCs and regulates SRF-dependent SMC differentiation autonomously or in concert with myocardin.
SAP结构域转录因子心肌素在调节平滑肌细胞分化的转录程序中起关键作用。在本报告中，我们描述了心肌素与巨核细胞白血病因子-1（MKL1）物理结合的能力，并对MKL1在平滑肌细胞（SMC）中的功能进行了表征。MKL1基因在大多数人类组织中表达，心肌素和MKL在SMC中共表达。MKL1和心肌素通过保守的亮氨酸拉链结构域进行物理结合。在非SMC中，MKL1的过表达可反式激活血清反应因子（SRF）依赖性的SMC限制性转录调控元件，包括SM22α启动子、平滑肌肌球蛋白重链启动子/增强子和SM-α-肌动蛋白启动子/增强子。此外，在未分化的SRF（-/-）胚胎干细胞中强制表达MKL1和SRF可激活多个内源性SMC限制性基因，其水平与心肌素相当或超过心肌素。强制表达显性负性MKL1突变体可降低心肌素诱导的SMC特异性SM22α启动子的激活。在NIH3T3成纤维细胞中，MKL1定位于细胞质，并在血清刺激、肌动蛋白踏车运动和RhoA信号传导的作用下转位至细胞核。相反，在SMC中，无论血清条件或RhoA信号如何，MKL1仅在细胞核中观察到。然而，当肌动蛋白聚合被破坏时，MKL1在SMC中从细胞核转位至细胞质。总之，这些数据与一个模型一致，即MKL1在SMC中从细胞骨架向细胞核转导信号，并自主或与心肌素协同调节SRF依赖性的SMC分化。
