ANTIPROGESTIN INHIBITION OF CELL-CYCLE PROGRESSION IN T-47D BREAST-CANCER CELLS IS ACCOMPANIED BY INDUCTION OF THE CYCLIN-DEPENDENT KINASE INHIBITOR P21

Progestin antagonists inhibit the proliferation of progesterone recept or-positive cells, including breast cancer cells, by G(1) phase-specif ic actions, but the molecular targets involved are not defined. Reduce d phosphorylation of pRB, a substrate for G(1) cyclin-dependent kinase s (CDKs) in vivo, was apparent after 9 h treatment of T-47D breast can cer cells with the antiprogestins RU 486 or ORG 31710, accompanying ch anges in S phase fraction. Although the abundance of cyclin D1, Cdk4, and Cdk6 did not decrease, cyclin D1-associated kinase activity was re duced by similar to 50% at 9-18 h. Similarly, cyclin E-associated kina se activity decreased by similar to 60% at 12-24 h in the absence of s ignificant changes in the abundance of cyclin E and Cdk2. The CDK inhi bitor p21 increased in mRNA and protein abundance and was present at i ncreased levels in cyclin D1 and cyclin E complexes at times when thei r kinase activity was decreased. Increased p21 protein abundance was o bserved in another antiprogestin-sensitive cell line, BT 474, but not in two breast cancer cell lines insensitive to antiprogestins. These d ata suggest increased p21 abundance and concurrent inhibition of CDK a ctivity as a mechanism for antiprogestin induction of growth arrest. A ntiprogestin effects on proliferation were markedly reduced after ecto pic expression of cyclin D1, indicating that inhibition of cyclin D1 f unction is a critical element in antiprogestin inhibition of prolifera tion. However, these data also implicate regulation of cyclin E functi on in antiprogestin regulation of cell cycle progression.