Effect of serum and mechanical stretch on skeletal alpha-actin gene regulation in cultured primary muscle cells

The purpose of this study was to determine whether mechanical stretch or se rum availability alters pretranslational regulation of skeletal alpha-actin (SkA) in cultured striated muscle cells. Chicken primary skeletal myoblast s and cardiac myocytes were plated on collagenized Silastic membranes adher ent to nylon supports and stretched 8-20% of initial length 96 h postplatin g. Serum dependence of SkA. gene regulation was determined by maintaining d ifferentiated muscle cells in growth/differentiation (G/D; skeletal myotube s, 10% horse serum-2% chick embryo extract; cardiac myocytes, 10% horse ser um) or growth-limiting (G-L; 0.5% horse serum) medium. Skeletal myotubes ha d higher SkA mRNA and SkA promoter activity in G/D than in G-L medium. Card iac myocyte SkA mRNA was higher in G-L than in G/D medium. Serum response f actor (SRF) protein binding to serum response element 1 (SRE1) of SkA. prom oter increased in skeletal cultures in G/D compared with G-L medium. Wester n blot analysis demonstrated that increased SRF-SRE1 binding was due, in pa rt, to increased SRF protein. Stretching skeletal myotubes in G-L medium re duced SkA mRNA and repressed SkA promoter activity. The first 100 bp of SkA promoter were sufficient for stretch-induced repression of SkA promoter ac tivity, and an intact transcriptional enhancer factor 1 (TEF-1) binding sit e was necessary for this response. Serum and stretch appear to repress SkA promoter activity in skeletal myotubes through different DNA binding elemen ts, the SRE1 and TEF-1 sites, respectively. Stretching increased SkA mRNA i n cardiac myocytes in G-L medium but did not alter SkA mRNA level in cardia c cells in G/D medium. These results demonstrate that stretch and serum int eract differently to alter SkA expression in cultured cardiac and skeletal muscle cells.