Androgen receptor pathway inhibition through targeting PARP-2 in castration-resistant prostate cancer

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ABSTRACT Androgen receptor (AR) is ligand-activated transcription factor and a driver of prostate cancer (PCa). Understanding the molecular mechanisms of AR-mediated transcription is a key for the development of novel therapeutic strategies for both castration-sensitive prostate cancer (CSPC) and castration-resistant prostate cancer (CRPC). It is well-known that AR transcriptional activity is prominently dictated by the transcription activator FOXA1, which acts as a ‘pioneer’ factor opening the condensed chromatin and facilitating the recruitment of AR. Genome sequencing studies have revealed that FOXA1 is one of the most frequently mutated genes in primary PCa and even more common in metastatic CRPC. Aberrant FOXA1 function is implicated in PCa development and progression likely through its impact on AR signaling. Therefore, inhibition of AR through targeting FOXA1 is a promising therapeutic approach for CRPC. However, to date FOXA1 has been deemed undruggable. We recently reported that critical to the function of FOXA1 is its modulation by poly-(ADP-ribose) polymerase 2 (PARP-2), conventionally known as a DNA repair protein. Our studies have demonstrated that PARP-2 is a critical component in AR signaling through interacting with FOXA1 and facilitating AR recruitment to prostate-specific enhancers. Expression of PARP-2 is significantly elevated in primary PCa tumors compared to benign prostate tissues, and even higher in CRPC tumors. Selective targeting of PARP-2 by genetic or pharmacological means blocks PARP-2/FOXA1 interaction, which in turn attenuates AR-mediated gene expression and PCa growth. These results lead us to the hypothesis that PARP- 2 plays a central role in AR-mediated transcription through interacting with FOXA1. Therefore, PARP-2 Inhibition attenuates AR signaling through disrupting FOXA1 function, which provides an alternative therapeutic strategy for AR inhibition without involving AR ligand binding. The overall objective of this project is to determine the molecular mechanisms by which selective targeting of PARP-2 inhibits CRPC growth through disruption of FOXA1 function and define PARP-2 as an alternative therapeutic target for CRPC. To attain the overall objective, we propose two specific aims: Aim 1: Determine the molecular mechanisms by which targeting the PARP-2/FOXA1 interaction inhibits AR signaling. Aim 2: Determine to what extent selective targeting of PARP-2 inhibits CRPC tumor growth in preclinical models. The successful implementation of this project will greatly advance our understanding of multifaceted biology of PARP proteins and their evolving impact on cancer therapeutics. More specifically, the results from the proposed research are expected to provide a strong basis for future development and clinical application of selective PARP-2 inhibitors benefiting patients with incurable metastatic CRPC.