Endocrine Abstracts

The retinoic acid receptor gamma (RARg, encoded by NR1B3/RARG) is significantly downregulated but not mutated in the MSKCC and PRAD prostate cancer (PCa) cohorts in the Cancer Genome Atlas (TCGA) data. We investigated the consequences of modifying RARg expression.

Independent of ligand, stable RARg knockdown stimulated RWPE-1 and LNCaP cells proliferation, changed the cell cycle profile and significantly altered global gene expression patterns that were enriched for AR and NF-kB signaling. RARg ChIP-Seq in RWPE-1 cells identified ~1300 significant binding sites, in the absence of ligand. ~800 of these sites significantly overlapped with active enhancer histone modifications (H3K27ac, H3K4me1), AR and p65 (unit of NF-kB) binding and were coincident with the ONECUT2 motif. Combining RARg cistrome and transcriptome data and applying bootstrapping demonstrated RARg sustained target gene expression. Reducing RARg levels in HPr-1AR cells disrupted AR-dependent gene regulation; ~1700 genes including MYC targets. RARg directly bound MYC, suggesting RARg directly regulates MYC, and co-regulates the AR capacity to repress MYC networks. Reduced RARg target gene expression in the TCGA-PRAD cohort associated with higher Gleason grade PCa.

RARg reduction was driven by elevated miR-96 in PCa cells, mouse models, and TCGAPRAD, which associated with worse disease free survival. MiR-96 mimics stimulated cell cycle progression and biotin-miR-96 pulldown identified 360 miR-96 targets. The most altered miR-96 target genes in TCGA-PRAD consisted of a RARg-centric network. Finally, tumors with low RARg-network and high miR-96 expression displayed repression of RARg target genes (e.g. SOX15) and significantly worse disease free survival.

Together, these findings support the concept that, ligand independent RARg binds at gene enhancer regions to govern AR signaling, including MYC network repression, and control proliferation. Elevated miR-96 represses the RARg-centered network and drives aggressive PCa. Conceptually, identifying miRNA that govern NR genomic functions, independently of ligand, has the capacity to change how these key transcription factors are studied and therapeutically exploited.