Abstract
Rapamycin, a bacterial macrolide antibiotic, is a potent immunosuppressant agent that blocks cell proliferation by inhibiting the G1/S transition in several cell types. In sensitive cells, rapamycin inhibits the phosphorylation of p70 S6K and of Rb; however, the precise mechanisms involved have not been elucidated. In the mouse BP‐A31 fibroblasts, synchronised in G0/G1 phase by serum starvation and induced to reinitiate the G1‐phase progression, rapamycin inhibited the entry into S phase. The effect of rapamycin was situated in early G1 phase. The assembly of the cyclin D1/cdk4 complexes that phosphorylate Rb early in the G1 phase was not modified by the drug. Nevertheless, an inhibition of the activation of cyclin D1/cdk4 and cyclin E/cdk2 as well as of Rb phosphorylation accompanied the cell cycle arrest. Remarkably, rapamycin reduced the level of total p21^WAF1/CIP1^ as well as that of p21^WAF1/CIP1^ associated with the cyclin D1/cdk4 complexes. Besides its inhibitory activity toward cdk, p21^WAF1/CIP1^ has been recently found to participate in the formation/stabilisation/nuclear translocation of cyclin D1/cdk4 complexes. We propose that the inhibition of the expression of p21^WAF1/CIP1^ is a mechanism by which rapamycin inhibits the triggering of the cdk cascade in the BP‐A31 cells. © 2003 Wiley‐Liss, Inc.