Evolutionarily conserved promoter region containing CArG*-like elements is crucial for smooth muscle myosin heavy chain gene expression.

Abstract

In recent years, significant progress has been made toward understanding skeletal muscle development. However, the mechanisms that regulate smooth muscle development and differentiation are presently unknown. To better understand smooth muscle-specific gene expression, we have focused our studies on the smooth muscle myosin heavy chain (SMHC) gene, a highly specific marker of differentiated smooth muscle cells. The goal of the present study was to isolate and characterize the mouse SMHC gene promoter, since the mouse promoter would be particularly suited for in vivo promoter analyses in transgenic mice and would serve as a tool for targeting genes of interest into smooth muscle cells. We report here the isolation and characterization of the mouse SMHC promoter and its 5' flanking region. DNA sequence analysis of a 2.6-kb portion of the promoter identified several potential binding sites for known transcription factors. Transient transfection analysis of promoter deletion constructs in primary cultures of smooth muscle cells showed that the region between -1208 and -1050 bp is critical for maximal SMHC promoter activity. A comparison of SMHC promoter sequences from mouse, rat, and rabbit revealed the presence of a highly conserved region located between -967 and -1208 bp. This region includes three CArG/CArG*-like elements, two SP-1 binding sites, a NF-1-like element, an Nkx2-5 binding site, and an Elk-1 binding site. Gel mobility shift assay and DNase I footprinting analyses show that all three CArG/CArG*-like elements can form DNA-protein complexes with nuclear extract from vascular smooth muscle cells. Protein binding to the CArG* elements can be competed out by either serum response element or by an authentic CArG element from the cardiac alpha-actin gene. Using a serum response factor (SRF) antibody, we demonstrate that SRF is part of the protein complex. In addition, we show that cotransfection with the SRF dominant-negative mutant expression vector abolishes SMHC promoter activity, suggesting that SRF protein plays a critical role in SMHC gene regulation.