Progestin antagonists inhibit the proliferation of
progesterone receptor-positive cells, including
breast cancer cells, by G1 phase-specific actions, but the molecular targets involved are not defined. Reduced phosphorylation of pRB, a substrate for G1
cyclin-dependent kinases (CDKs) in vivo, was apparent after 9 h treatment of T-47D
breast cancer cells with the antiprogestins
RU 486 or
ORG 31710, accompanying changes in S phase fraction. Although the abundance of
cyclin D1, Cdk4, and Cdk6 did not decrease
cyclin D1-associated
kinase activity was reduced by approximately 50% at 9-18 h. Similarly,
cyclin E-associated
kinase activity decreased by approximately 60% at 12-24 h in the absence of significant changes in the abundance of
cyclin E and Cdk2. The CDK inhibitor p21 increased in
mRNA and
protein abundance and was present at increased levels in
cyclin D1 and
cyclin E complexes at times when their
kinase activity was decreased. Increased p21
protein abundance was observed in another antiprogestin-sensitive cell line, BT 474, but not in two
breast cancer cell lines insensitive to antiprogestins. These data suggest increased p21 abundance and concurrent inhibition of CDK activity as a mechanism for antiprogestin induction of growth arrest. Antiprogestin effects on proliferation were markedly reduced after ectopic expression of
cyclin D1, indicating that inhibition of
cyclin D1 function is a critical
element in antiprogestin inhibition of proliferation. However, these data also implicate regulation of
cyclin E function in antiprogestin regulation of cell cycle progression.