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 1 alpha,25-dihydroxyvitamin D-3. Experimental Design: Profiling, transcriptional, and proliferation assays were undertaken in 1 alpha,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, of the cell cycle. A similar increased ratio of corepressor m RNA to VDR occurred in matched primary tumor and normal cells, noticeably in estrogen receptor alpha- negative (n = 7) tumors. 1 alpha,25(OH)(2)D-3 resistance in cancer cell lines was targeted by cotreatments with either 1 alpha,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 1 alpha,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.