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Vol. 11, Issue 6, 2117-2130, June 2000




and
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Molecular Oncology Program and *Clinical Investigation Program, H. Lee Moffitt Cancer Center, and Departments of The cyclin/cyclin-dependent kinase (cdk) inhibitor
p27kip1 is thought to be responsible for the onset and
maintenance of the quiescent state. It is possible, however, that cells
respond differently to p27kip1 in different conditions, and
using a BALB/c-3T3 cell line (termed p27-47) that inducibly expresses
high levels of this protein, we show that the effect of
p27kip1 on cell cycle traverse is determined by cell
density. We found that ectopic expression of p27kip1
blocked the proliferation of p27-47 cells at high density but had
little effect on the growth of cells at low density whether exponentially cycling or stimulated from quiescence. Regardless of cell
density, the activities of cdk4 and cdk2 were markedly repressed by
p27kip1 expression, as was the cdk4-dependent dissociation
of E2F4/p130 complexes. Infection of cells with SV40, a DNA tumor virus
known to abrogate formation of p130- and Rb-containing complexes,
allowed dense cultures to proliferate in the presence of
supraphysiological amounts of p27kip1 but did not stimulate
cell cycle traverse when cultures were cotreated with the potent cdk2
inhibitor roscovitine. Our data suggest that residual levels of
cyclin/cdk activity persist in p27kip1-expressing p27-47
cells and are sufficient for the growth of low-density cells and of
high-density cells infected with SV40, and that effective disruption of
p130 and/or Rb complexes is obligatory for the proliferation of
high-density cultures.
Pathology
and
Biochemistry and Molecular Biology, University of
South Florida College of Medicine, Tampa, Florida 33612
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