Medroxyprogesterone Acetate Induces Cell Proliferation through Up-Regulation of Cyclin D1 Expression via Phosphatidylinositol 3-Kinase/Akt/Nuclear Factor-κB Cascade in Human Breast Cancer Cells

The mechanism of medroxyprogesterone acetate (MPA)-induced cell proliferation in human breast cancer cells remains elusive. We examined the mechanism by which MPA affects the cyclin D1 expression in progesterone receptor (PR)-positive T47D human breast cancer cells. MPA (10 nm) treatment for 48 h induced proliferation of the cells (1.6-fold induction). MPA induced cyclin D1 expression (3.3-fold induction), and RU486, a selective PR antagonist, blocked the MPA-induced cell proliferation and cyclin D1 expression (23% inhibition). MPA increased both the protein level (2.2-fold induction) and promoter activity (2.7-fold induction) of cyclin D1 in MCF-7 cells transfected with PRB but not with PRA. Although MPA transcriptionally activated cyclin D1 expression, cyclin D1 promoter does not have progesterone-responsive element-related sequence. We further examined the mechanism for the regulation of the cyclin D1 expression. Because the cyclin D1 promoter contains three putative nuclear factor-κB (NFκB)-binding motifs and NFκB is a substrate of Akt, we investigated the effect of the phosphatidylinositol 3-kinase (PI3K)/Akt/NFκB cascade on the responses of cyclin D1 to MPA. MPA induced the transient phosphorylation of Akt (2.7-fold induction at 5 min), and treatment with PI3K inhibitor (wortmannin) attenuated the MPA-induced up-regulation of cyclin D1 expression (40% inhibition) and cell proliferation (40% inhibition). MPA also induced phosphorylation of inhibitor of NFκBα (IκBα) (2.3-fold induction), and treatment with wortmannin attenuated the MPA-induced IκBα phosphorylation (60% inhibition). Treatment with an IκBα phosphorylation inhibitor (BAY 11-7085) or a specific NFκB nuclear translocation inhibitor (SN-50) attenuated the MPA-induced up-regulation of both cyclin D1 expression (80 and 50% inhibition, respectively) and cell proliferation (55 and 34% inhibition, respectively). Because MPA induced a transient phosphorylation of Akt and the cyclin D1 promoter contains no progesterone-responsive element-related sequence, the MPA-induced cell proliferation through PRB by up-regulation of cyclin D1 expression via the PI3K/Akt/NFκB cascade may be a nongenomic mechanism.