RB FUNCTIONS TO INHIBIT APOPTOSIS DURING MYOCYTE DIFFERENTIATION

During in vitro myogenesis, a portion of myoblasts undergo apoptosis, whereas others continue with their differentiation program and form my otubes that are resistant to cell death, Previous work has shown that the expression of the Cdk inhibitor p21 correlates with enhanced resis tance to apoptosis and that forced expression of p21 will confer this phenotype on differentiating myocytes, Here we examine the role of the retinoblastoma gene (Rb) in myocyte survival. Compared with wild-type myocytes, CC42 (Rb-/-) myocytes undergo higher frequencies of apoptos is during mitogen deprivation-induced myogenesis. Despite these featur es, Rb-/- myocytes display normal up-regulation of p21 and downregulat ion of Cdk activities upon differentiation. Adenoviral constructs expr essing the Cdk inhibitors p21 or p16 inhibit apoptosis in wild-type bu t not Rb -/- myocyte cultures. On the other hand, a Ph-expressing aden oviral construct inhibited apoptosis in both cell types, These data de monstrate that Rb functions downstream from the Cdk inhibitors to coor dinate cell cycle withdrawal with programmed cell death during myocyte differentiation.