Transcriptional Coactivator p300 Stimulates Cell Type-specific Gene Expression in Cardiac Myocytes*

Terminal differentiation is characterized by cell cycle arrest and the expression of cell type-specific genes. Previous work has suggested that the p300 family of transcriptional coactivators plays an important role in preventing the re-initiation of DNA synthesis in terminally differentiated cardiac myocytes. In this study, we investigated whether p300 proteins are also involved in the transcriptional activation of cell type-specific genes in these cells. Since p300 function can be abrogated through direct binding by the adenovirus E1A protein, we overexpressed E1A in cardiac myocytes using recombinant adenoviral vectors. The expression of transfected reporter genes driven by α- or β-myosin heavy chain promoters was markedly diminished by expression of the 12 S E1A protein. In contrast, the activity of a promoter derived from the ubiquitously expressed β-actin gene was affected only modestly. While an E1A mutant unable to bind members of the retinoblastoma family of pocket proteins decreased the activity of α- and β-myosin heavy chain promoters to nearly the same extent as wild type 12 S E1A, transcriptional repression by a mutant defective for p300 binding was severely impaired. Furthermore, overexpression of p300 and, to an even greater extent, p300del33, a mutant lacking residues required for binding by E1A, relieved E1A’s repression of β-myosin heavy chain promoter activity while having no effect on the activity of the β-actin promoter. Thus, E1A’s transcriptional repression of cell type-specific genes in cardiac myocytes is mediated through its binding of p300 proteins, and these proteins appear to be involved in maintaining both cell type-specific gene expression and cell cycle arrest in cardiac myocytes.