A novel inhibitor of cyclin-Cdk activity detected in transforming growth factor β-arrested epithelial cells

Transforming growth factor β (TGF-β) is a potent inhibitor of epithelial cell growth. Cyclins E and A in association with Cdk2 have been shown to play a role in the G₁-to-S phase transition in mammalian cells. We have studied the effects of TGF-β-mediated growth arrest on G₁/S cyclins E and A. Inhibition of cyclin A-associated kinase by TGF-β is primarily due to a decrease in cyclin A mRNA and protein. By contrast, while TGF-β inhibits accumulation of cyclin E mRNA, the reduction in cyclin E protein is minimal. Instead, we find that the activation of cyclin E-associated kinase that normally accompanies the G₁-to-S phase transition is inhibited. A novel inhibitor of cyclin-Cdk complexes was detected in TGF-β-treated cell lysates. Inhibition is mediated by a heat-stable protein that targets both Cdk2 and Cdc2 kinases. In G₀-arrested cells, a similar inhibitor of Cdk2 kinase was detected. These data suggest the existence of an inhibitor of cyclin-dependent kinases induced under different conditions to mediate antiproliferative responses.