Sympathetic control of cardiac myosin heavy chain gene expression

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Abstract
Several neuroendocrine factors have been shown to influence the muscle phenotype. Various physiological reports have suggested the role of adrenergic nervous system for cardiac myosin heavy chain (MHC) expression. We have used cultured fetal rat heart myocytes to investigate the role of cAMP on the α- and (β-MHC gene expression. In low density cultures, addition of 1 mM 8 Br CAMP resulted in up regulation of α-MHC and down regulation of β-MHC mRNA. This antithetic effect of cAMP depends on the basal expression of both MHC transcripts. In transient transfection analysis employing a series of α-MHC gene promoter/reporter constructs, we identified a 13 by E-box M-CAT hybrid motif (EM element) which conferred a basal muscle specific and cAMP- inducible expression of the α-MHC gene. Data obtained from the mobility gel-shift analysis indicated that one of the factor(s) binding to the EM element is related to troponin T M-CAT binding factor (TEF-1). To test whether the protein binding to this sequence could be a substrate for cAMP-dependent phosphorylation, the cardiac nuclear proteins were preincubated in a kinase reaction buffer either with a catalytic subunit of PKA (CatPKA) or with cAMP, and binding activity of proteins to the EM element was evaluated by mobility gel shift assay. In a concentration dependent manner, a twofold increase in the intensity of the retarded band was observed. Furthermore, at 100 units of CatPKA, an additional band of faster mobility was observed which was not present either when phosphorylated nuclear extract was incubated with alkaline phosphatase or when ATP was absent in kinase reaction buffer. These results strongly suggest that factor(s) binding to the EM element is a substrate for cAMP dependent phosphorylation.

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