Upregulation of CYP1B1 by hypoxia is mediated by ERα activation in breast cancer cells

Abstract Background: Endocrine therapy for breast cancer often leads to drug resistance and tumor recurrence; tumor hypoxia is also associated with mortality and tumor relapse. Cytochrome P450 1B1 (CYP1B1) regulates estrogen metabolism in breast cells and is known to be overexpressed in breast cancer tissue. Although the individual association of hypoxia-induced hypoxia-inducible factor-1α (HIF-1α) and CYP1B1 with tumorigenesis is well known, the association between HIF-1α and CYP1B1 leading to tumorigenesis has not been investigated. Here, we investigated the correlation between hypoxia and CYP1B1 expression in breast cancer cells for tumorigenesis-related mechanisms. Methods: Hypoxia was induced in the human breast cancer cell lines MCF-7 (Er-positive) and MDA-MB-231 (triple-negative) and the normal breast epithelial cell line MCF10A, and then subjected to immunoblotting, transient transfection and luciferase assays, gene silencing using small interfering RNA, PCR analysis, and chromatin immunoprecipitation, co-immunoprecipitation, and mammalian two-hybrid assays. Furthermore, immunofluorescence analysis of tumor microarrays was performed and the pub2015 and The cancer genome atlas patient datasets were analyzed.Results: HIF-1α expression in response to hypoxia occurred in both normal and breast cancer cells, whereas CYP1B1 was induced only in estrogen receptor α (ERα)-positive breast cancer cells under hypoxia. HIF-1α activated ERα by direct binding as well as in a ligand-independent manner to promote CYP1B1 expression. Conclusions: Therefore, we established the mechanism by which hypoxia and ER-positivity orchestrate breast cancer relapse.