INDUCTION OF CYCLIN-E AND CYCLIN-A IN RESPONSE TO MITOGEN REMOVAL - ABASIC ALTERATION ASSOCIATED WITH THE ARREST OF DIFFERENTIATION OF C2 MYOBLASTS TRANSFORMED BY SIMIAN-VIRUS-40 LARGE T-ANTIGEN

We previously showed that C2 myoblasts transformed by simian virus 40 large T antigen (SVLT) stop the myogenic process after the induction o f myogenin and of high Rb levels; the induced Rb, however, becomes not ably phosphorylated. We have analyzed the protein levels and activitie s of cyclin-dependent kinases (cdks) in untransformed C2 cells and in transformants of either SVLT or the cytoplasmic mutant NKT1 (which per mits differentiation) upon a shift from growth medium (GM) to mitogen- poor differentiation medium (DM). After the shift, cdk4 levels remaine d constant and cdk6 levels decreased in all cell types; cdk2 minimally increased only in SVLT cells. Cyclin D1 was downregulated in DM in al l cell types, and cyclin D3 was upregulated (albeit less strongly in S VLT cells than in the others). In contrast, a dramatic difference betw een SVLT cells and the other cells was observed for cyclins E and A, w hich essentially disappeared (as protein and RNA) in normal C2 and NKT 1 cells upon the shift from GM to DM, whereas they increased in SVLT c ells. Concurrently, cdk2 activity ceased in C2 and NKT1 cells in DM, w hereas it persisted at 20% of the GM level in SVLT cells. cdk4 activit y was detectable in all cells only in GM. Cyclin E and A induction thu s appeared to sustain enough Rb phosphorylation to interfere with tiss ue-specific expression, with cdk activity not high enough to activate cyclin self-regulation. In DM, cdk2 complexed to D3 was underphosphory lated in all cells, and SVLT allowed strong inductions of p21 and p27 without affecting their complexes with cdks.