Estrogen receptor-beta sensitizes breast cancer cells to the anti-estrogenic actions of endoxifen.

ABSTRACT: We have previously demonstrated that endoxifen is the most important tamoxifen metabolite responsible for eliciting the anti-estrogenic effects of this drug in breast cancer cells expressing estrogen receptor-alpha (ER?). However, the relevance of ER? in mediating endoxifen action has yet to be explored. Here, we characterize the molecular actions of endoxifen in breast cancer cells expressing ER? and examine its effectiveness as an anti-estrogenic agent in these cell lines.MCF7, Hs578T and U2OS cells were stably transfected with full-length ER?. ER? protein stability, dimer formation with ER? and expression of known ER target genes were characterized following endoxifen exposure. The ability of various endoxifen concentrations to block estrogen-induced proliferation of MCF7 parental and ER?-expressing cells was determined. The global gene expression profiles of these two cell lines was monitored following estrogen and endoxifen exposure and biological pathway analysis of these data sets was conducted to identify altered cellular processes.Our data demonstrate that endoxifen stabilizes ER? protein, unlike its targeted degradation of ER?, and induces ER?/ER? heterodimerization in a concentration dependent manner. Endoxifen is also shown to be a more potent inhibitor of estrogen target genes when ER? is expressed. Additionally, low concentrations of endoxifen observed in tamoxifen treated patients with deficient CYP2D6 activity (20 to 40 nM) markedly inhibit estrogen-induced cell proliferation rates in the presence of ER?, whereas much higher endoxifen concentrations are needed when ER? is absent. Microarray analyses reveal substantial differences in the global gene expression profiles induced by endoxifen at low concentrations (40 nM) when comparing MCF7 cells which express ER? to those that do not. These profiles implicate pathways related to cell proliferation and apoptosis in mediating endoxifen effectiveness at these lower concentrations.Taken together, these data demonstrate that the presence of ER? enhances the sensitivity of breast cancer cells to the anti-estrogenic effects of endoxifen likely through the molecular actions of ER?/? heterodimers. These findings underscore the need to further elucidate the role of ER? in the biology and treatment of breast cancer and suggest that the importance of pharmacologic variation in endoxifen concentrations may differ according to ER? expression.

SUBMITTER: Wu X 

PROVIDER: S-EPMC3219188 | biostudies-literature | 2011 Mar

REPOSITORIES: biostudies-literature