Cyclin D1 Overexpression Induces Progestin Resistance in T-47D Breast Cancer Cells Despite p27^Kip1 Association with Cyclin E-Cdk2

Abstract

Long-term growth inhibition, arrest in G1 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 p18INK4c and increased association of the CDK inhibitors p21WAF1/Cip1 and p27Kip1 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 ∼60%. This was accompanied by p27Kip1 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 p27Kip1 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 p27Kip1. 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.