RAPAMYCIN INHIBITION OF THE G(1) TO S-TRANSITION IS MEDIATED BY EFFECTS ON CYCLIN D1 MESSENGER-RNA AND PROTEIN STABILITY

The immunosuppressant rapamycin has been shown previously to inhibit t he G(1)/S transition in several cell types by prolonging the G, phase of the cell cycle. This process appears to be controlled in part, by t he rapamycin-sensitive FK506-binding protein-rapamycin-associated prot ein-p70 S6 kinase (p70(S6k)) pathway and the cyclin-dependent kinases (Cdk). We now show that in serum-stimulated NIH 3T3 cells, rapamycin t reatment delays the accumulation of cyclin D1 mRNA during progression through G,. Rapamycin also appears to affect stability of the transcri pt. The combined transcriptional and post-transcriptional effects of t he drug ultimately result in decreased levels of cyclin D1 protein. Mo reover, degradation of newly synthesized cyclin D1 protein is accelera ted by rapamycin, a process prevented by inclusion of the proteasome i nhibitor, N-acetyl-Leu-Leu-norleucinal. The overall effect of rapamyci n on cyclin D1 leads, in turn, to impaired formation of active complex es with Cdk4, a process which triggers retargeting of the p27(Kip1) in hibitor to cyclin E/Cdk2. In view of this novel experimental evidence, we discuss a possible mechanism for the rapamycin-induced cell cycle arrest at the G(1)/S transition.