DIFFERENT ROLES FOR CYCLIN-D1 AND CYCLIN-E IN REGULATION OF THE G(1)-TO-S TRANSITION

Ectopic expression of cyclins D1 and E was previously shown to acceler ate the G(1)/S-phase transition, indicating that both classes of G(1) cyclin control an event(s) that is rate limiting for entry into S phas e. In order to determine whether cyclins D1 and E control the same or two different rate-limiting events, we have created cell lines that ex press both cyclins in an inducible manner. We show here that ectopic e xpression of both cyclins E and D1 in the same cell has an additive ef fect on shortening of the G(1) interval relative to expression of any single cyclin, In order to further explore the molecular basis for G(1 ) cyclin action, we used cell lines capable of expressing cyclin D1, E , or both prematurely and measured the effect of cyclin expression in early G(1) on phosphorylation of the retinoblastoma susceptibility gen e product (pRb). We show here that while premature expression of eithe r cyclin alone advances the G(1)/S-phase transition to the same extent , premature expression of cyclin D1 leads to immediate appearance of h yperphosphorylated pRb, while premature expression of cyclin E does no t. Ectopic expression of both cyclins E and D1 in the same cell has an additive effect on shortening of the G(1) interval, while the effect on pRb phosphorylation is similar to the effect of cyclin D1 alone. Th ese results suggest that cyclins E and D1 control two different events , both rate limiting for the G(1)/S-phase transition, and that pRb pho sphorylation might be the rate-limiting event controlled by cyclin D1.