Department of Medicine, Masonic Cancer Center, University of Minnesota, Minneapolis, Minnesota, USA.
Progesterone action contributes to the signaling of many growth factor pathways relevant to breast cancer tumor biology, including the insulin-like growth factor (IGF) system. Previous work has shown that insulin receptor substrate-2 (IRS-2) but not IRS-1 levels were regulated by progestin in progesterone receptor-B (PR-B) isoform expressing MCF-7 cells (C4-12 PR-B). Furthermore, type 1 IGF receptor (IGF1R) signaling via IRS-2 correlated with the increased cell migration observed in a number of breast cancer cell lines. Consequently, in this study, we examined whether the elevation of IRS-2 protein induced by progestin was sufficient to promote IGF-I-stimulated cell motility. Treatment of C4-12 PR-B cells with progestin shifted the balance of phosphorylation from IRS-1 to IRS-2 in response to IGF-I. This shift in IRS-2 activation was associated with enhanced migration in C4-12 PR-B cells pretreated with progestin, but had no effect on cell proliferation or survival. Treatment of C4-12 PR-B cells with RU486, an antiprogestin, inhibited IGF-induced cell migration. Attenuation of IRS-2 expression using small interfering RNA resulted in decreased IGF-stimulated motility. In addition, IRS-2 knockdown resulted in an abrogation of PKB/Akt phosphorylation but not mitogen-activated protein kinase. Consequently, LY294002, a phosphoinositide-3-kinase inhibitor, abolished IGF-induced cell motility in progestin-treated C4-12 PR-B cells. These data show a role for the PR in functionally promoting growth factor signaling, showing that levels of IRS proteins can determine IGF-mediated biology, PR-B signaling regulates IRS-2 expression, and that IRS-2 can mediate IGF-induced cell migration via phosphoinositide-3-kinase in breast cancer cells.