Abstract 1000: Platelet-Derived Growth Factor-BB Reduces Smooth Muscle Marker Gene Expression through the Dissociation of MKL1 from the Smooth Muscle Marker Gene Promoters
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 factor, MKL1 in SMC phenotypic switching remains unknown. The aims of the present studies were to determine if:
MKL1 contributes to the expression of SMC marker gene expression in cultured SMCs; and
repression of SMC marker genes by platelet-derived growth factor-BB (PDGF-BB), a potent inducer of SMC phenotypic switching, is mediated by the suppression of MKL1 function.
Results showed that expression of MKL1 was unaltered during the induction of SMC differentiation in multiple cultured cell systems including a novel embryonic stem cell-derived embryoid body model. Results of gain- and loss-of-function experiments showed that MKL1 regulated the expression of single and multiple CArG-containing SMC marker genes such as SM α-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 in cultured SMCs, while it had no effects on expression and intracellular localization of MKL1. However, of interest, chromatin immunoprecipitation assays revealed that PDGF-BB induced the dissociation of MKL1 from the CArG-containing promoter at the SMC marker gene loci, and this dissociation was caused by the competition between MKL1 and Elk-1 at the early phase, and subsequently by the histone deacetylase 4/5-induced reduction of acetylated histone H4 at the SMC marker gene loci. Results provide novel evidence that MKL1 contributes to the expression of SMC marker genes in SMCs and SMC phenotypic switching is occurred in part by suppression of MKL1 function.