The G-protein-coupled estrogen receptor, a gene co-expressed with ERα in breast tumors, is regulated by estrogen-ERα signalling in ERα positive breast cancer cells

Abstract

Purpose: The purpose of this study was to assess the relationship between GPER, a seven transmembrane G-protein coupled estrogen receptor, and ERα in breast tumors, and to make inroads into the mechanistic basis and clinical significance. Methods: TCGA-BRCA data was mined to examine the relationship between GPER and ERα expression. GPER mRNA, and protein expression were analyzed in ERα-positive or -negative breast tumors from two cohorts using immunohistochemistry, western blotting, or RT-qPCR. The Kaplan-Meier Plotter was employed for survival analysis. The influence of estrogen in vivo was studied by examining GPER expression levels in estrus or diestrus mouse mammary tissues, and the impact of 17β-estradiol (E2) administration in juvenile or adult mice. The effect of E2, or propylpyrazoletriol (PPT, an ERα agonist) stimulation on GPER expression was studied in MCF-7 and T47D cells, with or without tamoxifen or ERα knockdown. ERα-binding to the GPER locus was explored by analysing ChIP-seq data (ERP000380), in silico prediction of estrogen response elements, and chromatin immunoprecipitation assay. Results: Clinical data revealed significant positive association between GPER and ERα expression in breast tumors. The median GPER expression in ERα-positive tumors was significantly higher than ERα-negative tumors. High GPER expression was significantly associated with longer overall survival of patients with ERα-positive tumors. In vivo experiments showed a positive effect of E2 on GPER expression. E2 induced GPER expression in MCF-7 and T47D cells; an effect mimicked by PPT. Tamoxifen or ERα-knockdown blocked the induction of GPER. Estrogen-mediated induction was associated with increased ERα occupancy in the upstream region of GPER. Conclusion: GPER expression is positively associated with ERα in breast tumors, and a transcriptional target of the estrogen-ERα signalling axis. More in-depth studies are required to establish the significance of GPER-ERα co-expression, and their interplay in breast tumor development, progression, and treatment.