ERRα/VDR Axis Promotes Calcitriol Degradation and Estrogen Signaling Activation and Correlates with Poor Prognosis in Basal-like Breast Cancer Patients.

Abstract Background Vitamin D is used to reduce cancer risk and improve the outcome of cancer patients, but the VDR pathway needs to be functionally intact to ensure the biological effects of circulating calcitriol. Besides ERα, another nuclear receptor, ERRα, has recently been shown to interfere with the VDR pathway, but its role in the cytotoxicity and transactivation activity of calcitriol is completely unknown in breast cancer (BC). Methods We investigated the function of ERRα on BC cell proliferation and calcitriol cytotoxicity and transactivation activity by silencing ERRα expression. We then performed a colony formation assay and cell cycle analysis to assess cell proliferation, and western blot and RT-PCR to investigate underlying mechanisms of cytotoxicity and VDR genomic action. Immunofluorescence was used to investigate VDR and ERRα cellular distribution. Bioinformatics analyses were performed to uncover the interaction network of VDR and ERRα. The translational significance of both bioinformatics and in vitro results were studied in the TCGA-BRCA (BReast CAncer) cohort. Results ERRα functionally supported the proliferation of BC cell lines and acted as a calcitriol-induced co-activator of the VDR complex. As such, ERRα deregulated the calcitriol/VDR genomic action by enhancing CYP24A1 and both ESR1 and aromatase (CYP19A1) expression in calcitriol-treated cells. In contrast, ERRα functionally supported calcitriol cytotoxicity by enhancing calcitriol- induced G0/G1 phase cell cycle arrest and by affecting the expression of cyclin D1 and p21/Waf 1 . The interactome analysis suggested PPARGC1A and PELP-1 were key players in genomic actions of the calcitriol/VDR/ERRα axis. Evaluation of patients’ outcome in the TCGA dataset definitely showed the translational significance of VDR/ERRα biological effects, highlighting that VDR-CYP24A1-ESRRA overexpression correlates with poor prognosis in basal-like breast cancer setting. Conclusions Collectively, our findings identified a novel VDR/ERRα axis in breast cancer through which ERRα promotes the corruption of VDR genomic action and drives worsening of prognosis in BC patients.