Altered Nuclear Receptor Corepressor Expression Attenuates Vitamin D Receptor Signaling in Breast Cancer Cells

Purpose: We hypothesized that deregulated corepressor actions, with associated histone deacetylation activity, epigenetically suppressed vitamin D receptor (VDR) responsiveness and drives resistance towards 1α,25-dihydroxyvitamin D 3 . Experimental Design: Profiling, transcriptional, and proliferation assays were undertaken in 1α,25(OH) 2 D 3 -sensitive MCF-12A nonmalignant breast epithelial cells, a panel of breast cancer cell lines, and a cohort of primary breast cancer tumors ( n = 21). Results: Elevated NCoR1 mRNA levels correlated with suppressed regulation of VDR target genes and the ability of cells to undergo arrest in G 1 of the cell cycle. A similar increased ratio of corepressor mRNA to VDR occurred in matched primary tumor and normal cells, noticeably in estrogen receptor α–negative ( n = 7) tumors. 1α,25(OH) 2 D 3 resistance in cancer cell lines was targeted by cotreatments with either 1α,25(OH) 2 D 3 or a metabolically stable analogue (RO-26-2198) in combination with either trichostatin A (TSA; histone deacetylation inhibitor) or 5-aza-2′-deoxycytidine (DNA methyltransferase inhibitor). Combinations of vitamin D 3 compounds with TSA restored VDR antiproliferative signaling (target gene regulation, cell cycle arrest, and antiproliferative effects in liquid culture) to levels which were indistinguishable from MCF-12A cells. Conclusions: Increased NCoR1 mRNA is a novel molecular lesion in breast cancer cells, which acts to suppress responsiveness of VDR target genes, resulting in 1α,25(OH) 2 D 3 resistance and seems to be particularly associated with estrogen receptor negativity. This lesion provides a novel molecular diagnostic and can be targeted by combinations of vitamin D 3 compounds and low doses of TSA.