Differential regulation of p34cdc2 and p33cdk2 by transforming growth factor-beta 1 in murine mammary epithelial cells.

Cyclin-dependent kinases (cdks) are a family of proteins whose function plays a critical role in cell cycle traverse. Transforming growth factor-beta 1 (TGF-beta 1) is a potent growth inhibitor of epithelial cells. Since cdks have been suggested as possible biochemical markers for TGF-beta growth inhibition, we investigated the effect of TGF-beta 1 on cdc2 and cdk2 in a normal mouse mammary epithelial cell line (MME) and a TGF-beta-resistant MME cell line (BG18.2). TGF-beta 1 decreases newly synthesized cdc2 protein levels within 6 h after addition. Coincident with this decrease in newly synthesized cdc2 protein was a marked reduction in its ability to phosphorylate histone H1. This decrease in kinase activity is not due to a change in steady-state levels of cdc2 protein, since mRNA and total protein levels of cdc2 are not reduced until 12 h after TGF-beta 1 addition. This suggests that the kinase activity of cdc2 is dependent on newly synthesized cdc2 protein. Moreover, the protein synthesis of another cyclin-dependent kinase, cdk2, is not effected by TGF-beta 1 addition, but its kinase activity is substantially reduced. Thus, it appears that TGF-beta decreases the kinase activity of both cdc2 and cdk2 by distinct mechanisms.