GABP, HCF-1 and YY1 are involved in Rb gene expression during myogenesis

Muscle cell differentiation, or myogenesis, is a well-characterized process and involves the expression of specific sets of genes in an orderly manner. A prerequisite for myogenesis is the exit from the cell cycle, which is associated with the up-regulation of the tumor suppressor Rb. In this study, we set to investigate the regulatory mechanism of the Rb promoter that allows adequate up-regulation in differentiating myoblasts. We report that Rb expression is regulated by the transcription factors GABP, HCF-1 and YY1. Before induction of differentiation, Rb is expressed at a low level and GABP and YY1 are both present on the promoter. YY1, which exerts an inhibitory effect on Rb expression, is removed from the promoter as cells advance through myogenesis and translocates from the nucleus to the cytoplasm. On the other hand, upon induction of differentiation, the GABP cofactor HCF-1 is recruited to and coactivates the promoter with GABP. RNAi-mediated knock-down of HCF-1 results in inhibition of Rb up-regulation as well as myotube formation. These results indicate that the Rb promoter is subject to regulation by positive and negative factors and that this intricate activation mechanism is critical to allow the accurate Rb gene up-regulation observed during myogenesis.