Density-dependent growth inhibition of fibroblasts ectopically expressing p27(kip1)

The cyclin/cyclin-dependent kinase (cdk) inhibitor p27(kip1) is thought to be responsible for the onset and maintenance of the quiescent state. It is possible, however, that cells respond differently to p27(kip1) in different conditions, and using a BALB/c-3T3 cell line (termed p27-47) that inducibl y expresses high levels of this protein, we show that the effect of p27(kip 1) On cell cycle traverse is determined by cell density. We found that ecto pic expression of p27(kip1) blocked the proliferation of p27-47 cells at hi gh density but had little effect on the growth of cells at low density whet her exponentially cycling or stimulated from quiescence. Regardless of cell density, the activities of cdk4 and cdk2 were markedly repressed by p27(ki p1) expression, as was the cdk4-dependent dissociation of E2F4/p130 complex es. Infection of cells with SV40, a DNA tumor virus known to abrogate forma tion of p130- and Rb-containing complexes, allowed dense cultures to prolif erate in the presence of supraphysiological amounts of p27(kip1) but did no t stimulate cell cycle traverse when cultures were cotreated with the poten t cdk2 inhibitor roscovitine. Our data suggest that residual levels of cycl in/cdk activity persist in p27(kip1)-expressing p27-47 cells and are suffic ient for the growth of low-density cells and of high-density cells infected with SV40, and that effective disruption of p130 and/or Iib complexes is o bligatory for the proliferation of high-density cultures.