TRANSFORMING GROWTH FACTOR-BETA-1 (TGF-BETA-1) REDUCES CELLULAR-LEVELS OF P34CDC2, AND THIS EFFECT IS ABROGATED BY ADENOVIRUS INDEPENDENTLY OF THE E1A-ASSOCIATED PRB BINDING-ACTIVITY

We have used E1A probes to study the roles of the p34cdc2 kinase and the retinoblastoma tumor susceptibility gene product (pRB) in transforming growth factor-beta-1(TGF-beta-1)-mediated growth suppression in mink lung epithelial (Mv1Lu) cells. In agreement with previous reports, we see a decline in p34cdc2 kinase activity and a loss of pRB phosphorylation after TGF-beta-1 treatment. We report here that TGF-beta-1 induces not only a change in p34cdc2 kinase activity but a strong repression of p34cdc2 synthesis. Loss of p34cdc2 kinase activity is not seen until the steady-state level of p34cdc2 declines, suggesting that the intra-cellular signals induced by TGF-beta-1 affect p34cdc2 at the level of expression, rather than by altering the posttranslational modifications of p34cdc2 that regulate its kinase activity. Infection with adenovirus expressing either wild-type EIA or a mutant E1A (pm928) defective for pRB binding alleviated TGF-beta-1-mediated suppression of DNA synthesis, indicating that E1A does not need to bind pRB physically to keep cell growth-suppressing functions from being activated by TGF-beta-1. The E1A.928 mutant virus is able to maintain p34cdc2 expression and kinase activity, as well as pRB phosphorylation in the presence of TGF-beta-1, which may account for its ability to maintain cell cycle activity without directly sequestering pRB. Overall our results suggest that TGF-beta-1 acts by signaling changes at the level of control of G1 gene expression, not at the level of posttranslational modification of p34cdc2 or its substrates.