Purpose:We hypothesized that deregulated corepressor actions, with associated histone deacetylation activity, epigenetically suppressed vitamin D receptor (VDR) responsiveness and drives resistance towards 1a,25-dihydroxyvitamin D 3 . Experimental Design: Profiling, transcriptional, and proliferation assays were undertaken in 1a,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 a^negative (n = 7) tumors. 1a,25(OH) 2 D 3 resistance in cancer cell lines was targeted by cotreatments with either 1a,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-2V-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 1a,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.The nuclear receptor superfamily regulates diverse signals that are central to the formation and homeostasis of the mammary gland. The postgenomic description of the superfamily conjoined with profiling approaches (1) reveals that breast myoepithelial and epithelial cells express a rich cohort of nuclear receptors, many of which display overt nutrient-sensing capacity for micronutrients and macronutrients alongside the estrogen receptors (ERa and ERh; refs. 2 -6).Several of the diet-sensing nuclear receptors, such as the VDR, preferentially form heterodimers with retinoid X receptors whereas the ERs preferentially heterodimerize with one another. For the nuclear receptors to regulate transcriptional programs, these dimers must be contained as subunits in either large gene coactivator or corepressor complexes. In the absence of ligand, receptors exist in an apo state as part of large complexes (f2.0 MDa; ref. 7), associated with corepressors (e.g., NCoR1, NCoR2/SMRT, and TRIP15/Alien), and bound to response element sequences. These complexes actively recruit a range of enzymes that posttranslationally modify histone tails (e.g., histone deacetylases and methyltransferases) and thereby maintain a locally closed chromatin structure around response element sequences (8). Ligand binding induces a...