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1 Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, Virginia, United States
2 Molecular physiology and Biological Physics, University of Virginia, Charlottesville, Virginia, United States
* To whom correspondence should be addressed. E-mail: gko{at}virginia.edu.
A hallmark of smooth muscle cell (SMC) phenotypic switching is suppression of SMC maker gene expression. Although myocardin has been shown to be a key regulator of this process, the role of its related factors, MKL1 and MKL2, in SMC phenotypic switching remains unknown. The present studies were aimed at determining if: (1) MKL factors contribute to the expression of SMC marker genes in cultured SMCs; and (2) platelet-derived growth factor-BB (PDGF-BB)-induced repression of SMC marker genes is mediated by suppression of MKL factors. Results of gain- and loss-of-function experiments showed that MKL factors regulated the expression of single and multiple CArG-containing SMC marker genes such as SM 
-actin and telokin, but not CArG-independent SMC marker genes such as smoothelin-B. Treatment with PDGF-BB reduced the expression of CArG-containing SMC marker genes as well as myocardin expression in cultured SMCs, while it had no effect on expression of MKL1 and MKL2. However, of interest, PDGF-BB induced the dissociation of MKL factors from the CArG-containing region of SMC marker genes as determined by chromatin immunoprecipitation assays. This dissociation was caused by the competition between MKL factors and phosphorylated Elk-1 at early time points, but subsequently by the reduction in acetylated histone H4 levels at these promoter regions mediated by histone deacetylases, HDAC2, HDAC4, and HDAC5. Results provide novel evidence that PDGF-BB-induced repression of SMC marker genes is mediated through combinatorial mechanisms including down-regulation of myocardin expression and inhibition of the association of myocardin/MKL factors to CArG-containing SMC marker gene promoters.
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