Mol. Cell. Biol., 07 1995, 3463-3469, Vol 15, No. 7
D Resnitzky and SI Reed
Ectopic expression of cyclins D1 and E was previously shown to accelerate
the G1/S-phase transition, indicating that both classes of G1 cyclin
control an event(s) that is rate limiting for entry into S phase. In order
to determine whether cyclins D1 and E control the same or two different
rate-limiting events, we have created cell lines that express both cyclins
in an inducible manner. We show here that ectopic expression of both
cyclins E and D1 in the same cell has an additive effect on shortening of
the G1 interval relative to expression of any single cyclin. In order to
further explore the molecular basis for G1 cyclin action, we used cell
lines capable of expressing cyclin D1, E, or both prematurely and measured
the effect of cyclin expression in early G1 on phosphorylation of the
retinoblastoma susceptibility gene product (pRb). We show here that while
premature expression of either cyclin alone advances the G1/S-phase
transition to the same extent, premature expression of cyclin D1 leads to
immediate appearance of hyperphosphorylated pRb, while premature expression
of cyclin E does not. Ectopic expression of both cyclins E and D1 in the
same cell has an additive effect on shortening of the G1 interval, while
the effect on pRb phosphorylation is similar to the effect of cyclin D1
alone. These results suggest that cyclins E and D1 control two different
events, both rate limiting for the G1/S-phase transition, and that pRb
phosphorylation might be the rate-limiting event controlled by cyclin D1.
Copyright © 1995, American Society for Microbiology
Different roles for cyclins D1 and E in regulation of the G1-to-S transition
Scripps Research Institute, La Jolla, California 92037, USA.
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