Estrogen-induced proliferation in estrogen receptor (ER)-positive breast cancer cells is primarily mediated through two distinct intracellular receptors, ER? and ER?. Although tumor necrosis factor alpha (TNFalpha) and E2/ERalpha are known to exert opposing effects on cell proliferation in MCF-7 cells, the mechanism by which TNFalpha antagonizes E2/ERalpha-mediated cell proliferation is not well understood. The present study suggests that reduced cell survival in response to TNFalpha treatment in MCF-7 cells may be associated with the down-regulation of ERalpha protein. The decrease in ERalpha protein level was accompanied by an inhibition of ERalpha gene transcription. Cell viability was decreased synergistically by the combined treatment with ERalpha-siRNA and TNFalpha. Furthermore, pretreatment of cells with the PI3-kinase (PI3K)/ Akt inhibitor, LY294002, markedly enhanced TNF?-induced down-regulation of the ERalpha protein, suggesting that the PI3K/Akt pathway might be involved in control of the ERalpha level. Moreover, down-regulation of ERalpha by TNF? was not inhibited in cells that were pretreated with the proteasome inhibitors, MG132 and MG152, which suggests that proteasome-dependent proteolysis does not significantly influence TNFalpha-induced down-regulation of ERalpha protein. In contrast, the effect of the PI3K/Akt inhibitor on ERalpha was blocked in cells that were treated with LY294002 in the presence of the proteasome inhibitors. Collectively, our findings show that the TNFalpha may partly regulate the growth of MCF-7 breast cancer cells through the down-regulation of ERalpha expression, which is primarily mediated by a PI3K/Akt signaling.

Keywords: Apoptosis, Cell Viability, EV alpha, Gene Silencing, Phosphatidylinositol 3-kinase, Proteasome, TNF alpha, Transcription