Study On The Effect Of Estrogen Receptor-α36during Development Of Acquired Tamoxifen Resistance In Breast Cancer

Purpose:The purpose of this study is to uncover the effect of ER-a36during development of acquired tamoxifen resistance in ER-a66positive breast cancer.Methods&Results:In this study, we established a TAM resistant cell sub line (MCF-7/TAM) from estrogen receptor-a (ER-a66) positive breast cancer MCF-7cells by culturing ER-a66-positive MCF-7cells in medium plus1μM TAM over6months. 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-a36(ER-α36), a novel36-kDa variant of ER-a66, was dramatically overexpressed in this in vitro model, compared to the parental MCF-7cells. 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-a66was greatly down-regulated oppositely. In the subsequent studies, we overexpressed ER-a36in MCF-7cells by stable transfection and found that ER-a36transfected MCF-7cells (MCF-7/ER-α436) similarly exhibited decreased sensitivity to TAM, accelerated proliferative rate and enhanced in vitro migratory and invasive ability, compared to empty vector transfected MCF-7cells (MCF-7/V). Real-time qPCR and Western blotting analysis revealed that MCF-7/ER-a36cells possessed increased EGFR expression but decreased ER-a66expression both in mRNA level and protein level, compared to MCF-7/V cells. This change in MCF-7/ER-a36cells could be reversed by neutralizing anti-ER-a36antibody treatment. Furthermore, knock-down of ER-a36expression 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-a66mRNA expression were also observed in MCF-7/TAM cells with down-regulated ER-a36expression. In addition, blocking EGFR/ERK signaling in MCF-7/ER-a36cells could restore the expression of ER-a66partly, suggesting a regulatory function of EGFR/ERK signaling in down-regulation of ER-a66expression.Conclusion:This sduty indicated for the first time a regulatory role of ER-a36in up-regulation of EGFR expression and down-regulation of ER-a66expression, 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-a36could be considered a potential new therapeutic target in breast tumors which have acquired resistance to TAM.