Physiological control of smooth muscle-specific gene expression through regulated nuclear translocation of serum response factor

Prolonged serum deprivation induces a structurally and functionally contrac tile phenotype in about 1/6 of cultured airway myocytes, which exhibit morp hological elongation and accumulate abundant contractile apparatus-associat ed proteins. We tested the hypothesis that transcriptional activation of ge nes encoding these proteins accounts for their accumulation during this phe notypic transition by measuring the transcriptional activities of the murin e SM22 and human smooth muscle myosin heavy chain promoters during transien t transfection in subconfluent, serum fed or 7 day serum-deprived cultured canine tracheal smooth muscle cells. Contrary to our expectation, SM22 and smooth muscle myosin heavy chain promoter activities (but not viral murine sarcoma virus-long terminal repeat promoter activity) were decreased in lon g term serum-deprived myocytes by at least 8-fold. Because serum response f actor (SRF) is a required transcriptional activator of these and other smoo th muscle-specific promoters, we evaluated the expression and function of S RF in subconfluent and long term serum-deprived cells. Whole cell SRP mRNA and protein were maintained at high levels in serum-deprived myocytes, but SRF transcription-promoting activity, nuclear SRF binding to consensus CArG sequences, and nuclear SRF protein were reduced. Furthermore, immunocytoch emistry revealed extranuclear redistribution of SRF in serum-deprived myocy tes; nuclear localization of SRF was restored after serum refeeding. These results uncover a novel mechanism for physiological control of smooth muscl e-specific gene expression through extranuclear redistribution of SRF and c onsequent down-regulation of its transcription-promoting activity.