Rationale: We previously identified conserved G/C Repressor elements in the promoters of most smooth muscle cell (SMC) marker genes and demonstrated that mutation of this element within the SM22α promoter nearly abrogated repression of this transgene after vascular wire injury or within lesions of ApoE-/- mice. However, the mechanisms regulating the activity of the G/C Repressor are unknown, although we have previously shown that phenotypic switching of cultured SMC is dependent on Krupple-like factor (KLF)4.
Objective: The goals of the present studies were to ascertain if (1) injury-induced repression of SM22α gene after vascular injury is mediated through KLF4 binding to the G/C Repressor element and (2) the transcriptional repressor activity of KLF4 on SMC marker genes is dependent on cooperative binding with pELK-1 (downstream activator of the mitogen-activated protein kinase pathway) and subsequent recruitment of histone de-acetylase 2 (HDAC2), which mediates epigenetic gene silencing.
Methods and results: Chromatin immunoprecipitation (ChIP) assays were performed on chromatin derived from carotid arteries of mice having either a wild-type or G/C Repressor mutant SM22α promoter-LacZ transgene. KLF4 and pELK-1 binding to the SM22α promoter was markedly increased after vascular injury and was G/C Repressor dependent. Sequential ChIP assays and proximity ligation analyses in cultured SMC treated with platelet-derived growth factor BB or oxidized phospholipids showed formation of a KLF4, pELK-1, and HDAC2 multiprotein complex dependent on the SM22α G/C Repressor element.
Conclusions: Silencing of SMC marker genes during phenotypic switching is partially mediated by sequential binding of pELK-1 and KLF4 to G/C Repressor elements. The pELK-1-KLF4 complex in turn recruits HDAC2, leading to reduced histone acetylation and epigenetic silencing.