Aim: Estrogen receptor-α (ER-α) activation drives the progression of luminal breast cancers. Signaling by transforming growth factor-β (TGF-β) typically opposes the actions of ER-α; it also induces epithelial-mesenchymal transition (EMT) programs that promote breast cancer dissemination, stemness, and chemoresistance. The impact of EMT programs on nongenomic ER-α signaling remains unknown and was studied herein.
Methods: MCF-7 and BT474 cells were stimulated with TGF-β to induce EMT programs, at which point ER-α expression, localization, and nongenomic interactions with receptor tyrosine kinases and MAP kinases (MAPKs) were determined. Cell sensitivity to anti-estrogens both before and after traversing the EMT program was also investigated.
Results: TGF-β stimulated MCF-7 and BT474 cells to acquire EMT phenotypes, which enhanced cytoplasmic accumulation of ER-α without altering its expression. Post-EMT cells exhibited (i) elevated expression of EGFR and IGF1R, which together with Src formed cytoplasmic complexes with ER-α; (ii) enhanced coupling of EGF, IGF-1 and estrogen to the activation of MAPKs; and (iii) reduced sensitivity to tamoxifen, an event reversed by administration of small molecule inhibitors against the receptors for TGF-β, EGF, and IGF-1, as well as those against MAPKs.
Conclusion: EMT stimulated by TGF-β promotes anti-estrogen resistance by activating EGFR-, IGF1R-, and MAPK-dependent nongenomic ER-α signaling.
Keywords: Breast cancer; EMT; Estrogen receptor-α; Growth factor; Signal transduction; TGF-β; tamoxifen resistance.