Contractile activity of the cardiac myocyte is required for maintaining cell mass and phenotype, including expression of the cardiac-specific ␣-myosin heavy chain (␣-MHC) gene. An E-box hemodynamic response element (HME) located at position Ϫ47 within the ␣-MHC promoter is both necessary and sufficient to confer contractile responsiveness to the gene and has been shown to bind upstream stimulatory factor-1 (USF1). When studied in spontaneously contracting cardiac myocytes, there is enhanced

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... ing of USF1 to the HME compared with quiescent cells, which correlates with a threefold increase in ␣-MHC promoter activity. A molecular mechanism by which contractile function modulates ␣-MHC transcriptional activity may involve signaling via phosphorylation of USF1. The present studies showed that purified rat USF1 was phosphorylated in vitro by protein kinase C (PKC) and cAMPdependent protein kinase (PKA) but not casein kinase II. Phosphorylated USF1 by either PKC or PKA had increased DNA binding activity to the HME. PKC-mediated phosphorylation also leads to the formation of USF1 multimers as assessed by gel shift assay. Analysis of in vivo phosphorylated nuclear proteins from cultured ventricular myocytes showed that USF1 was phosphorylated, and resolution by two-dimensional gel electrophoresis identified at least two distinct phosphorylated USF1 molecules. These results suggest that endogenous kinases can covalently modify USF1 and provide a potential molecular mechanism by which the contractile stimulus mediates changes in myocyte gene transcription. ventricular myocytes; contractile activity; protein kinase C; cAMP-dependent protein kinase; E-box