Highlights d ARfl and ARv7 genomic binding is interdependent and colocalized d ARv7, unlike ARfl, preferentially represses transcription d Expression of ARv7-repressed genes negatively correlates with recurrence d Re-expression of ARv7-repressed genes may serve as a biomarker of ARv7 inhibition

Androgen receptor (AR) splice variants are described as one of the potential drivers of lethal castration-resistant prostate cancer. Androgen receptor splice variant 7 (ARv7) is the most commonly observed isoform and strongly correlates with resistance to second-generation antiandrogens. Despite this clinical evidence, the interplay between ARv7 and the highly expressed full-length AR (ARfl) remains unclear. In this work we show that ARfl/ARv7 heterodimers readily form in the nucleus via an intermolecular N/C interaction that brings the four termini of the proteins in close proximity. Combining FRET and FRAP we demonstrate that these heterodimers undergo conformational changes following DNA binding indicating dynamic nuclear receptor interaction. Although transcriptionally active, ARv7 can only form short-term interactions with DNA at highly accessible, high-occupancy ARfl binding sites. Dimerization with ARfl does not affect ARv7 binding dynamics suggesting that DNA binding occupancy is determined by the individual protein monomers and not the homo- or heterodimer complex. Overall, these biophysical studies reveal detailed properties of ARv7 dynamics as both a homodimer or heterodimer with ARfl.

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Prostate cancer is an extremely common disease that affects one in every seven men in their lifetime. The standard care for late-stage cancer is designed to inhibit the activation of androgen receptor (AR). While this therapeutic approach is initially efficient, the cancer almost always develops resistance and gives rise to castration-resistant prostate cancer (CRPC). When therapy fails, the median survival of patients who suffer from CRPC is 12-16 months. It is critical to understand the mechanism of CRPC better to ameliorate the outcome of late-stage prostate cancer patients. AR is a transcription factor that consists of an N-terminal domain (NTD), a DNA binding domain (DBD) and a ligand binding domain (LBD). Upon activation by androgen, an intramolecular interaction between NTD and LBD occurs. The AR translocates into the nucleus where it forms a homodimer. It binds to the promoter and enhancer elements of the AR target genes and triggers gene transcription. It has been demonstrated in many clinical studies that AR is important for the progression of prostate cancer including CRPC. In CRPC, although androgen production and binding is inhibited, how the AR can still be activated is a fundamental question to be answered. One of the several different mechanisms proposed is the alternative splicing of the AR generating truncated AR variants that lack LBD. These variants do not require androgens to be activated and they are intrinsically resistant to clinically approved therapeutics. Despite the fact that the AR variants have been shown to be critical in late-stage prostate cancer, the mechanism of gene transcription initiation by the AR variants is not well-known. We propose to characterize the mechanism of the AR variant activation. Since ARV7 is the most commonly observed variant, we utilize ARV7. We develop a FRET based methodology to identify how ARV7 dimers form and to test if full-length AR is necessary for transcriptional activation mediated by ARV7. We are able to demonstrate possible intermolecular interaction between the full length AR and ARV7 measuring the FRET intensity between the fluorophores attached to the N and C terminus of two proteins. FRET measurements are analyzed by tracking single cells and recording the FRET intensities during the process. Our preliminary data suggest that the full-length AR and ARV7 may interact in the presence of androgen. ARV7 is always localized in the nucleus whereas the AR requires androgen to be activated and translocates into the nucleus. Being in close proximity, putative interaction between the AR and ARV7 occurs in the nucleus. Additionally, we use NTD and DBD point mutants to determine critical positions of intermolecular interaction between them. Supporting the significance of ARV7 in prostate cancer, our results clarify the dimerization process. It gives an opportunity to test clinical drugs currently in development to effectively inhibit ARV7 activation. Citation Format: Fatma Özgün, Zeynep Kaya, Halil Bayraktar, Selen Manioğlu, Doğancan Özturan, Nathan Lack. Structural characterization of androgen receptor variant 7 in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3522. doi:10.1158/1538-7445.AM2017-3522