ACCELERATION OF THE G(1) S PHASE-TRANSITION BY EXPRESSION OF CYCLIN-D1 AND CYCLIN-E WITH AN INDUCIBLE SYSTEM/

Conditional overexpression of human cyclins B1, D1, and E was accompli shed by using a synthetic cDNA expression system based on the Escheric hia coli tetracycline repressor. After induction of these cyclins in a synchronous Rat-1 fibroblasts, a decrease in the length of the G(1) in terval was observed for cyclins D1 and E, consistent with an accelerat ion of the G(1)/S phase transition. We observed, in addition, a compen satory lengthening of S phase and G(2) so that the mean cell cycle len gth in populations constitutively expressing these cyclins was unchang ed relative to those of their uninduced counterparts. We found that ex pression of cyclin B1 had no effect on cell cycle dynamics, despite el evated levels of cyclin B-associated histone H1 kinase activity. Induc tion of cyclins D1 and E also accelerated entry into S phase for synch ronized cultures emerging from quiescence. However, whereas cyclin E e xerted a greater effect than cyclin D1 in asynchronous cycling cells, cyclin D1 conferred a greater effect upon stimulation from quiescence, suggesting a specific role for cyclin D1 in the G(0)-to-G(1) transiti on. Overexpression of cyclins did not prevent cells from entering into quiescence upon serum starvation, although a slight delay in attainme nt of quiescence was observed for cells expressing either cyclin D1 or cyclin E. These results suggest that cyclins D1 and E are rate-limiti ng activators of the G(1)-to-S phase transition and that cyclin D1 mig ht play a specialized role in facilitating emergence from quiescence.