Rapamycin inhibits cdk4 activation, p 21WAF1/CIP1 expression and G1‐phase progression in transformed mouse fibroblasts

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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 p21WAF1/CIP1 as well as that of p21WAF1/CIP1 associated with the cyclin D1/cdk4 complexes. Besides its inhibitory activity toward cdk, p21WAF1/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 p21WAF1/CIP1 is a mechanism by which rapamycin inhibits the triggering of the cdk cascade in the BP‐A31 cells.