Elevated NCOR1 disrupts PPAR{alpha}/{gamma} signaling in prostate cancer and forms a targetable epigenetic lesion.

The loss of anti-proliferative responsiveness in prostate cancer cell lines towards ligands for VDR, RARs/RXRs, and PPARalpha/gamma may entail underlying epigenetic events, as ligand insensitivity reflects significantly altered mRNA expression of co-repressors and histone modifying enzymes. Expression patterns were dependent on phases of the cell cycle and associated with repressed basal gene expression of VDR and PPARalpha/gamma target genes, for example CDKN1A (encodes p21((waf1/cip1))). Elevated NCOR1 and NCOR2/SMRT protein levels were detected in prostate cancer cell lines compared to non-malignant counterparts. Knockdown of the co-repressor NCOR1 significantly elevated basal expression of a cohort of target genes, including CDKN1A. Both chemical (HDACi) and NCOR1 knockdown targeting enhanced anti-proliferative sensitivity towards PPARalpha/gamma ligands in prostate cancer cell lines. Pursuing PPARalpha/gamma signaling, microarray approaches were undertaken to identify pathways and genes regulated uniquely by a combination of PPARalpha/gamma activation and HDAC inhibition. Again, HDACi and knockdown approaches demonstrated that elevated NCOR1 expression and activity distorted PPARalpha/gamma gene targets centered on, for example, cell cycle control, including CDKN1A and TGFBRAP1. Q-RT-PCR validation and ChIP assays both confirmed that elevated NCOR1 disrupted the ability of PPARalpha/gamma to regulate key target genes (CDKN1A and TGFBRAP1). Interrogation of these relationships in prostate cancer samples using principal component and partial correlation analyses established significant inter-dependent relationships between NCOR1-PPARalpha/gamma and representative target genes, independently of AR expression. Therefore we conclude that elevated NCOR1 distorts the actions of PPARalpha/gamma selectively and generates a potential epigenetic lesion with diagnostic and prognostic significance.