Activation of the MAPK pathway enhances sensitivity of MCF-7 breast cancer cells to the mitogenic effect of estradiol

Abstract

Long-term estrogen deprivation causes human breast cancer cells to develop hypersensitivity to the mitogenic effect of estradiol (E2). Our prior studies demonstrated an association between enhanced MAPK activation and hypersensitivity in long-term estrogen-deprived (LTED) MCF-7 cells. Herein, we report that MAPK is constitutively activated in LTED cells and not dependent on serum factors. Additionally, activated MAPK levels fall upon reversion of the hypersensitivity. Importantly, we now provide direct evidence that enhanced MAPK causes hypersensitivity to E2. We activated MAPK in wild-type MCF-7 cells using TGFα, and demonstrated a 2-3 log enhancement of sensitivity to E2. PD98059 abrogated the TGFα-induced effect, indicating that MAPK activation is responsible for E2 hypersensitivity. To determine the level at which MAPK activation enhanced E2 sensitivity, we examined the dose-response effects of E2 on several transcriptional readouts, including ERE-reporter activity and the levels of progesterone receptor and pS2. Wild-type and LTED cells exhibited nearly identical responses to E2, suggesting that mechanisms downstream of estrogen receptor-mediated transcription are involved in inducing hypersensitivity. In support of this possibility, LTED and TGFα-treated wild-type cells were hypersensitive to the effects of E2 on the key cell cycle regulator, E2F1.