Abstract A6: Androgen receptor-mediated transcription is reprogrammed after hormone depletion
Authors: Decker Keith F. , Zheng Dali , Edwards John R. , Jia Li 
Published: May 2013

Androgen receptor (AR) is a ligand-induced transcription factor, which binds to thousands of genomic loci and activates a cell-type specific gene expression program. Androgen deprivation therapy has been the cornerstone of treatment for advanced prostate cancer for 70 years. Despite initial response, an incurable castration-resistant prostate cancer (CRPC) inevitably develops as a result of restored AR activity. Given the importance of AR signaling in CRPC, there has been a dedicated interest in dissecting the mechanisms of AR function after androgen deprivation.

In CRPC, AR activity remains critical for tumor growth despite androgen deprivation. However, the gene expression program characteristic of AR signaling in androgen-dependent prostate cancer (ADPC) is attenuated, indicating AR signaling may be altered after androgen deprivation. Recent studies indicate that AR targets may be altered through direct reprogramming of ligand-induced AR binding (Wang et al, Cell 2009, 138: 245) or through reprogramming of FoxA1 (Wang et al, Nature 2011, 474:390) in the presence of androgen. In this study, we show that in the absence of ligand AR binds a distinct set of genomic loci that drive a gene expression program necessary for CRPC growth and may be an important alternative therapeutic target when androgen-deprivation therapies fail.

We characterized AR action in the presence or absence of androgen (DHT) in LNCaP and C4-2B cells. Using chromatin immunoprecipitation sequencing (ChIP-seq) we identified thousands of distinct AR binding events in the absence of DHT in CRPC C4-2B cells, which direct a unique gene expression program in CRPC cells. Whole transcriptome sequencing (RNA-seq) revealed ∼ 450 genes were basally upregulated in the absence of DHT in C4-2B compared to the parental androgen-dependent LNCaP cell line. These genes are strongly correlated with androgen-independent AR binding sites. Quantitative chromosome conformation capture (3C) assays demonstrated a strong physical interaction between androgen-independent AR binding sites and nearby basally upregulated genes. Interestingly, in androgen-depleted conditions, the AR occupies genomic loci with constitutively open chromatin structures that lack the canonical androgen response element (ARE) and are not directed by FoxA1, a transcription factor involved in ligand-dependent AR targeting. After DHT treatment, androgen-independent AR binding was diminished as AR binding switched to canonical androgen-dependent AR binding sites.

The novel androgen-independent AR target genes, which showed significant enrichment for cell-cycle pathways, are required for the survival and proliferation of CRPC cells after androgen withdrawal. RNA interference experiments showed significant inhibitory effects on proliferation, and a corresponding increase in apoptosis in the absence of or at low concentrations of androgen. Finally, we found that androgen-independent AR genes were to be significantly over-represented in castration-resistant metastatic patient samples exhibiting moderate AR expression.

Current treatments for CRPC have focused on limiting androgen synthesis and blocking AR ligand binding based on the notion that persistent ligand-dependent AR signaling drives tumor growth. Our results show that androgen depletion results in a dramatic androgen-independent alteration of genome-wide AR occupancies and reprogramming of AR-mediated gene expression, which may be a critical alternative mechanism for CRPC growth.

Citation Format: Keith F. Decker, Dali Zheng, John R. Edwards, Li Jia. Androgen receptor-mediated transcription is reprogrammed after hormone depletion [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr A6.