ECTOPIC EXPRESSION OF CYCLIN D1 PREVENTS ACTIVATION OF GENE-TRANSCRIPTION BY MYOGENIC BASIC HELIX-LOOP-HELIX REGULATORS

Activation of muscle gene transcription in differentiating skeletal my oblasts requires their withdrawal from the cell cycle. The effects of ectopic cyclin expression on activation of muscle gene transcription b y myogenic basic helix-loop-helix (bHLH) regulators were investigated. Ectopic expression of cyclin D1, but not cyclins A, B1, B2, C, D3, an d E, inhibited transcriptional activation of muscle gene reporter cons tructs by myogenic bHLH regulators in a dose-dependent manner. Ectopic expression of cyclin D1 inhibited the activity of a myogenic bHLH reg ulator mutant lacking the basic region protein kinase C site, indicati ng that phosphorylation of this site is not relevant to the mechanism of inhibition. Analysis of cyclin D1 mutants revealed that the C-termi nal acidic region was required for inhibition of myogenic bHLH regulat or activity, whereas an intact N-terminal pRb binding motif was not es sential. Together, these results implicate expression of cyclin D1 as a central determinant of a putatively novel mechanism that links posit ive control of cell cycle progression to negative regulation of genes expressed in differentiated myocytes.