TRANSFORMING GROWTH-FACTOR BETA-INDUCED ACTIVATION OF CYCLIN E-CDK2 KINASE AND DOWN-REGULATION OF P27(KIP1) IN C3H 10T1 2 MOUSE FIBROBLASTS/

Transforming growth factor (TGF-beta)-stimulated induction of DNA synt hesis is preceded by the activation of cyclin E/cyclin-dependent kinas e (cdk)2 kinase in late G(1) in C3H 10T1/2 mouse fibroblasts. TGF-beta has no effect on the steady-state level of cdk4, while having only a modest inductive effect on cyclin D1 expression, TGF-beta stimulation does, however, lead to the striking down-regulation of p27(Kip1) expre ssion during G(1) in a manner consistent with the timing of cyclin E-e dk2 activation. Co-immunoprecipitation analysis reveals that the amoun t of p27(Kip1) in complexes with the cdk2 catalytic subunit is drastic ally reduced at the time in late G(1) when cyclin E-cdk2 activity is m aximal. These data indicate that cyclin E-cdk2 is inhibited by p27(Kip 1) in the growth-arrested state and that TGF-beta relieves this inhibi tion by down-regulating the steady-state level of the p27(Kip1) inhibi tor protein, thus reducing the level of inhibitor present in complexes with cdk2.