cyclin-d1 and 16-alpha-ethyl-21-hydroxy-19-nor-4-pregnene-3-20-dione

Long-term growth inhibition, arrest in G(1) phase and reduced activity of both cyclin D1-Cdk4 and cyclin E-Cdk2 are elicited by progestin treatment of breast cancer cells in culture. Decreased cyclin expression, induction of p18(INK4c) and increased association of the CDK inhibitors p21(WAF1/Cip1) and p27(Kip1) with cyclin E-Cdk2 have been implicated in these responses. To determine the role of decreased cyclin expression, T-47D human breast cancer cells constitutively expressing cyclin D1 or cyclin E were treated with the progestin ORG 2058. Overexpression of cyclin E had only a modest effect on growth inhibition. Although cyclin E expression was maintained during progestin treatment, cyclin E-Cdk2 activity decreased by approximately 60%. This was accompanied by p27(Kip1) association with cyclin E-Cdk2, indicating that both cyclin E down-regulation and p27(Kip1) recruitment contribute to the decrease in activity. In contrast, overexpression of cyclin D1 induced progestin resistance and cell proliferation continued despite decreased cyclin E-Cdk2 activity. Progestin treatment of cyclin D1-overexpressing cells was associated with increased p27(Kip1) association with cyclin E-Cdk2. Thus the ability of cyclin D1 to confer progestin resistance does not depend on sequestration of p27(Kip1) away from cyclin E-Cdk2, providing evidence for a critical function of cyclin D1 other than as a high-capacity "sink" for p27(Kip1). These data indicate that regulation of cyclin D1 is a critical element of progestin inhibition in breast cancer cells and suggest that breast cancers overexpressing cyclin D1 may respond poorly to progestin therapy.

Topics: Animals; Blotting, Western; Breast Neoplasms; CDC2-CDC28 Kinases; Cell Cycle Proteins; Cell Division; Cyclin D1; Cyclin E; Cyclin-Dependent Kinase 2; Cyclin-Dependent Kinase Inhibitor p27; Cyclin-Dependent Kinases; Dose-Response Relationship, Drug; Down-Regulation; Drug Resistance; Drug Resistance, Neoplasm; Flow Cytometry; G1 Phase; Phosphorylation; Precipitin Tests; Pregnenediones; Progesterone Congeners; Progestins; Prognosis; Protein Binding; Protein Serine-Threonine Kinases; Retroviridae; S Phase; Time Factors; Tumor Suppressor Proteins

Possible mechanisms by which the progestin antagonist RU 486 inhibits cell growth were investigated by comparing the effects of the antiprogestin with those of progestin and antiestrogen. Exposure of T-47D breast cancer cells to RU 486 caused a decline in the proportion of cells in S phase, indicative of a block to cell cycle progression in G1 phase. This was accompanied by a marked decrease in c-myc expression but no change in cyclin D1 expression. The cell kinetic data suggest that progestin antagonist inhibition of proliferation and progestin stimulation of proliferation are mediated by opposing effects on the same mechanism. Both estrogen antagonists and progestin antagonists appear to act at a similar part of G1 phase but there are clear differences in their effects on cyclin D1 expression, suggesting that the mechanisms by which these compounds inhibit proliferation are distinct.

Topics: Breast Neoplasms; Cell Cycle; Cyclin D1; Cyclins; Dose-Response Relationship, Drug; Estradiol; Humans; Mifepristone; Oncogene Proteins; Polyunsaturated Alkamides; Pregnenediones; Progesterone Congeners; Progestins; Proto-Oncogene Proteins c-myc; RNA, Messenger; S Phase; Tumor Cells, Cultured