Abstract
Platelet-derived growth factor BB (PDGF-BB) has been shown to be an extremely potent negative regulator of smooth muscle cell (SMC) differentiation. Moreover, previous studies have demonstrated that the Kruppel-like transcription factor (KLF) 4 potently represses the expression of multiple SMC genes. However, the mechanisms whereby KLF4 suppresses SMC gene expression are not known, nor is it clear whether KLF4 contributes to PDGF-BB-induced down-regulation of SMC genes. The goals of the present studies were to determine the molecular mechanisms by which KLF4 represses expression of SMC genes and whether it contributes to PDGF-BB-induced suppression of these genes. Results demonstrated that KLF4 markedly repressed both myocardin-induced activation of SMC genes and expression of myocardin. KLF4 was rapidly up-regulated in PDGF-BB-treated, cultured SMC, and a small interfering RNA to KLF4 partially blocked PDGF-BB-induced SMC gene repression. Both PDGF-BB and KLF4 markedly reduced serum response factor binding to CArG containing regions within intact chromatin. Finally, KLF4, which is normally not expressed in differentiated SMC in vivo, was rapidly up-regulated in vivo in response to vascular injury. Taken together, results indicate that KLF4 represses SMC genes by both down-regulating myocardin expression and preventing serum response factor/myocardin from associating with SMC gene promoters, and suggest that KLF4 may be a key effector of PDGF-BB and injury-induced phenotypic switching of SMC.
Highlights
Alterations in the differentiated state of the smooth muscle cell (SMC),1 or phenotypic switching, has been shown to play a key role in the repair of tissue damage and in the development of a variety of major human diseases, including atherosclerosis, restenosis, and asthma [1, 2]
The goals of the present studies were: 1) to determine molecular mechanisms whereby KLF4 inhibits expression of SMC marker genes, including testing the hypothesis that it interferes with CArG/SRF/myocardin-dependent gene expression and recruitment of SRF to CArG containing regions of SMC genes within intact chromatin and 2) to determine whether KLF4 expression is induced by treatment of cultured SMC with Platelet-derived growth factor BB (PDGF-BB) and/or within phenotypically modulated SMC in vivo following vessel injury
KLF4 Dramatically Repressed SRF/Myocardin-induced Activation of SM ␣-Actin Promoter, and the Repression Could Be Rescued by Overexpression of SRF—Given that KLF4 was originally identified based on its binding to TGF control element (TCE), which are found adjacent to CArG elements within SMC promoters [19], we hypothesized that KLF4 may act, in part, through direct inhibition of SRF/CArG-dependent transcription
Summary
Alterations in the differentiated state of the smooth muscle cell (SMC), or phenotypic switching, has been shown to play a key role in the repair of tissue damage and in the development of a variety of major human diseases, including atherosclerosis, restenosis, and asthma [1, 2]. An especially exciting recent discovery was the demonstration (by our laboratory and others (4 – 8)) that the SMC/cardiac myocyte-restricted SRF co-activator myocardin is required for the expression of multiple SMC marker genes by inducing CArG/SRF-dependent transcription of SMC genes in SMC and many embryonic stem cells/fibroblasts and, most importantly, that it is necessary for SMC differentiation in vivo. It is not clear whether down-regulation of SMC marker genes associated with phenotypic switching is due to the decreased expression of a transcriptional activator, such as myocardin, and/or stimulation of the expression of cell-selective gene repressors. The goals of the present studies were: 1) to determine molecular mechanisms whereby KLF4 inhibits expression of SMC marker genes, including testing the hypothesis that it interferes with CArG/SRF/myocardin-dependent gene expression and recruitment of SRF to CArG containing regions of SMC genes within intact chromatin and 2) to determine whether KLF4 expression is induced by treatment of cultured SMC with PDGF-BB and/or within phenotypically modulated SMC in vivo following vessel injury
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
