Regulation of cdk2 activity in endothelial cells that are inhibited from growth by cell contact

Endothelial cells (ECs) are quiescent in normal blood vessels but undergo r apid bursts of proliferation after vascular injury and during angiogenesis. Here we show that the activity of cyclin-dependent kinase-2 (cdk2), a key regulator of the G1 and S phases of the cell cycle, is expressed at high le vels in proliferating ECs but at low levels in ECs that are contact-inhibit ed for growth. Despite these differences in kinase activity, the protein le vels of cdk2 and 1 of its activating subunits, cyclin E, are not modulated by these different growth conditions. The cdk inhibitor p27 is highly expre ssed in contact-inhibited but not proliferating ECs, whereas the level of c yclin A protein is preferentially expressed in proliferating ECs. p27 prote in was detected in immunoprecipitable complexes with cdk2 or cyclin E in cu ltures that were contact-inhibited for growth. The functional significance of the p27 induction was indicated by the detection of a heat-stable cdk2 i nhibitory activity that was induced by endothelial cell-cell contact and co uld be immunodepleted with anti-p27 antibodies. In a confluent EC monolayer , cdk2 kinase activity was activated by a scraping injury that led to cell migration and proliferation. The injury-induced activation of cdk2 coincide d with the downregulation of p27 and the induction of cyclin A. These data demonstrate that p27 is induced in confluent cultures of ECs. They also ind icate that both p27 induction and cyclin A downregulation contribute to the inhibition of cdk2 and cell proliferation by cell-cell contact in ECs.