Acquired tamoxifen (TAM) resistance limits the therapeutic benefit of TAM in patients with hormone-dependent breast cancer. The switch from estrogen-dependent to growth factor-dependent growth is a critical step in this process. However, the molecular mechanisms underlying this switch remain poorly understood. In this study, we established a TAM resistant cell sub line (MCF-7/TAM) from estrogen receptor-α (ER-α66) positive breast cancer MCF-7 cells by culturing ER-α66-positive MCF-7 cells in medium plus 1 μM TAM over 6 months. MCF-7/TAM cells were then found to exhibit accelerated proliferation rate together with enhanced in vitro migratory and invasive ability. And the estrogen receptor-α36 (ER-α36), a novel 36-kDa variant of ER-α66, was dramatically overexpressed in this in vitro model, compared to the parental MCF-7 cells. Meanwhile, the expression of epidermal growth factor receptor (EGFR) in MCF-7/TAM cells was significantly up-regulated both in mRNA level and protein level, and the expression of ER-α66 was greatly down-regulated oppositely. In the subsequent studies, we overexpressed ER-α36 in MCF-7 cells by stable transfection and found that ER-α36 transfected MCF-7 cells (MCF-7/ER-α36) similarly exhibited decreased sensitivity to TAM, accelerated proliferative rate and enhanced in vitro migratory and invasive ability, compared to empty vector transfected MCF-7 cells (MCF-7/V). Real-time qPCR and Western blotting analysis revealed that MCF-7/ER-α36 cells possessed increased EGFR expression but decreased ER-α66 expression both in mRNA level and protein level, compared to MCF-7/V cells. This change in MCF-7/ER-α36 cells could be reversed by neutralizing anti-ER-α36 antibody treatment. Furthermore, knock-down of ER-α36 expression in MCF-7/TAM cells resulted in reduced proliferation rate together with decreased in vitro migratory and invasive ability. Decreased EGFR mRNA and protein expression as well as increased ER-α66 mRNA expression were also observed in MCF-7/TAM cells with down-regulated ER-α36 expression. In addition, blocking EGFR/ERK signaling in MCF-7/ER-α36 cells could restore the expression of ER-α66 partly, suggesting a regulatory function of EGFR/ERK signaling in down-regulation of ER-α66 expression. In conclusion, our results indicated for the first time a regulatory role of ER-α36 in up-regulation of EGFR expression and down-regulation of ER-α66 expression, which could be an underlying mechanism for the growth status switch in breast tumors that contribute to the generation of acquired TAM resistance. And ER-α36 could be considered a potential new therapeutic target in breast tumors which have acquired resistance to TAM.
Copyright © 2013 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.