Department of Cell Biology Vanderbilt University , Nashville , Tennessee , USA .
J Biol Chem 271 : 11253-60 ( 1996)
Abstract
Stimulation of quiescent Balb/c 3T3 fibroblasts into S phase requires the synergistic action of platelet-derived growth factor ( PDGF ) and progression factors found in platelet-poor plasma ( PPP ) .
Traverse of the G1/S phase boundary and the initiation of DNA replication require functional cyclin E-cyclin-dependent kinase ( Cdk ) 2 and cyclin A-Cdk2 complexes ; however , the mechanisms by which PDGF and PPP regulate Cdk2 activation are not known .
Density-arrested fibroblasts contain low levels of cyclins E and A , and high levels of the Cdk inhibitor p27kip1 .
Exposure of PDGF , which stimulates cell cycle entry but not progression through G1 , induces the formation of cyclin D1-Cdk4 complexes that bind p27kip1 and titrate the pool of Kip1 available to inhibit Cdk2 .
In addition , PDGF stimulates a moderate transient reduction in the abundance of p27kip1 protein .
However , limited expression of cyclin E and cyclin A is observed after PDGF treatment , and in the absence of PPP , p27 levels are sufficient to bind and inactivate existing cyclin-Cdk complexes .
Although plasma does not significantly increase the proportion of Kip1 bound to cyclin D1-Cdk4 , stimulation of PDGF-treated cells with plasma does overcome the threshold inhibition of p27kip1 by further increasing the expression of cyclins E and A and decreasing the amount of Kip1 over a prolonged time period .
Our results indicate that the distinct mitogenic activities of PDGF and PPP differentially influence the activation of cyclin E- and cyclin A-associated kinases that ultimately regulate entry into S phase .