Smooth Muscle α-Actin CArG Elements Coordinate Formation of a Smooth Muscle Cell–Selective, Serum Response Factor–Containing Activation Complex

Abstract

Abstract —Previous studies have shown that multiple serum response factor (SRF)-binding CArG elements were required for smooth muscle cell (SMC)-specific regulation of smooth muscle (SM) α-actin expression. However, a critical question remains as to the mechanisms whereby a ubiquitously expressed transcription factor such as SRF might contribute to SMC-specific expression. The goal of the present study was to investigate the hypothesis that SMC-selective expression of SM α-actin is due at least in part to (1) unique CArG flanking sequences that distinguish the SM α-actin CArGs from other ubiquitously expressed CArG-dependent genes such as c- fos , (2) cooperative interactions between CArG elements, and (3) SRF-dependent binding of SMC-selective proteins to the CArG-containing regions of the promoter. Results demonstrated that specific sequences flanking CArG B were important for promoter activity in SMCs but not in bovine aortic endothelial cells. We also provided evidence indicating that the structural orientation between CArGs A and B was an important determinant of promoter function. Electrophoretic mobility shift assays and methylation interference footprinting demonstrated that a unique SRF-containing complex formed that was selective for SMCs and, furthermore, that this complex was probably stabilized by protein-protein interactions and not by specific interactions with CArG flanking sequences. Taken together, the results of these studies provide evidence that SM α-actin expression in SMCs is complex and may involve the formation of a unique multiprotein initiation complex that is coordinated by SRF complexes bound to multiple CArG elements.