p21^Waf1 inhibits the activity of cyclin dependent Kinase 2 by preventing its activating phosphorylation

Abstract

Prostaglandin A2 (PGA2), a potent inhibitor of the growth of many cell types, inhibits G1 phase cyclin dependent kinases (cdk). Although PGA2 suppresses cyclin D1 and elevates p21^Waf1 levels, it was the failure of cdk2 to become activated by phosphorylation which correlated best with growth inhibition. In kinetic studies, cdk2 activation was inhibited efficiently only if p21^Waf1 levels increased prior to the activating phosphorylation; suggesting that p21^Waf1 had blocked this phosphorylation. This model was confirmed in cells from p21^Waf1 knockout mice where PGA2 was completely unable to block the activating phosphorylation of cdk2, or inhibit cdk2 activity. As expected, growth inhibition of p21^Waf1−/− cells was not observed at PGA2 concentrations which inhibited cdk2 activity and growth of p21^Waf1+/+ cells.