MYOD INDUCES RETINOBLASTOMA GENE-EXPRESSION DURING MYOGENIC DIFFERENTIATION

In skeletal muscle cells permanent withdrawal from the cell cycle is a prerequisite for terminal differentiation. The muscle-specific transc ription factor MyoD can activate downstream muscle structural genes an d myogenic conversion in many different cell types. It has been demons trated that the product of the retinoblastoma susceptibility gene, wit h its growth-suppressive activity, is involved in the myogenic functio n of MyoD (Caruso et al., 1993; Gu et al., 1993). The present study ch aracterises the modulation of retinoblastoma (Rb1) mRNA levels during myogenic differentiation of the murine C2 cell line and provides evide nce that the muscle-specific regulatory factor MyoD enhances Rb1 gene transcription. We demonstrate that MyoD mediates the transactivation o f a CAT construct whose expression is driven by the human Rbl gene pro moter, and that this is not a consequence of direct binding of MyoD to an E-box DNA sequence motif present in the Rb1 promoter sequences. In addition we have tested the capability of several MyoD mutant protein s of inducing the Rb1 promoter CAT construct. Our results indicate tha t the MyoD function required for induction of Rb1 promoter activity is distinct from its myogenic function.