Rapamycin inhibition of interleukin-2-dependent p33^cdk2 and p34^cdc2 kinase activation in T lymphocytes

The immunosuppressant rapamycin (RAP) is a potent inhibitor of the entry of interleukin (IL)-2-stimulated T cells into S-phase. Earlier results indicated that RAP treatment arrested the growth of the murine IL-2-dependent T cell line CTLL-2 in late G₁-phase. To explore further the interactions of RAP with the cell cycle control machinery in T cells, we examined the effects of RAP treatment on the activation of the cyclin-dependent kinases p34^cdc2 and p33^cdk2 in G₁-phase CTLL-2 cells. Stimulation of factor-deprived cells with IL-2 led to the assembly of high molecular weight complexes containing active p34^cdc2 and p33^cdk2. The appearance of these complexes was explained, at least in part, by the association of both cyclin-dependent kinases with IL-2-induced cyclin A. RAP treatment profoundly inhibited both cyclin A expression and the appearance of active cyclin A-cyclin-dependent kinase complexes in IL-2-stimulated, late G₁-phase CTLL-2 cells. Although p34^cdc2 activation was largely dependent on association with cyclin A, a significant proportion of the active p33^cdk2 pool was complexed with cyclin E. In contrast to cyclin A, the IL-2-induced accumulation of cyclin E in G₁-phase cells was only partially suppressed by RAP, and cyclin E-p33^cdk2 complexes were readily detected in drug-treated cells. These cyclin E-cyclin-dependent kinase complexes were nonetheless devoid of histone H1 kinase activity. The inhibitory effects of RAP on the activation of cyclin E- and cyclin A-associated cyclin-dependent kinases suggest that one or both events participate in the regulation of T cell entry into S-phase.