Abstract 1590: High intranuclear mobility of AR-v7 reveals distinct mode of transcriptional activity in prostate cancer with important therapeutic implications
Authors: Kim Seaho , Jamalruddin Mohd Azrin , Giannakakou Paraskevi 
Published: July 2017

Abstract
It is well established that androgen receptor (AR) signaling, is a key driver of prostate cancer (PC) growth and metastatic progression. Therefore, androgen deprivation therapy (ADT) is the first line of treatment for PC. However, most patients develop castration resistant prostate cancer (CRPC). Interestingly, AR signaling remains active in CRPC, due to the expression of transcriptionally active AR splice variants (AR-Vs), which lack the ligand binding domain (LBD) and constitutively translocate to the nucleus even in castrate conditions. AR-v7 is the most prevalent AR-V expressed in about 60% of CRPC tumors. AR-v7 expression was clinically correlated with poor prognosis of CRPC patients and with resistance to next-generation AR signaling inhibitors, which are part of standard clinical care. Therefore, inhibition of AR-v7 function is urgently needed for the treatment of CRPC. Currently, there is no therapeutic modality that can inhibit AR-v7 expression or activity. Mechanistically, AR-v7 transcriptional targets largely overlap with those of AR-fl, with the exception of a few AR-v7 unique targets. However, the exact mechanism by which transcription is activated by AR-v7 is not known. In this study we sought to investigate the mechanisms underlying the transcriptional activity of the ligand-independent AR-v7 in comparison to liganded AR-fl. We used live cell imaging to monitor the dynamics and intranuclear mobility of fluorescently-tagged AR-v7 or AR-fl. Fluorescent recovery after photobleaching (FRAP) revealed that AR-v7 intranuclear mobility was significantly faster than that of liganded AR-fl, with t1/2 3s versus several minutes, respectively. To precisely map the spatial distribution and chromatin-binding kinetics of AR-fl and AR-v7, we generated expression plasmids with AR tagged with green-to-red mEos4 photo-convertible proteins. Photoconversion of a discrete, subnuclear pool of AR-fl in the presence of its ligand (R1881) followed by 1 hr of time-lapse imaging in 5 min intervals, revealed absence of mobility indicating tight chromatin binding and active transcription. In contrast, similar photoconversion experiment for AR-v7, revealed immediate redistribution throughout the nucleus in less than 9 s, suggesting a “hit-and-run” mode of interaction with DNA with uncertain transcriptional output. QRT-PCR of endogenous target genes and ARE-mcherry reporter assays showed similar transcriptional activity of the two proteins. We are currently investigating the relationship between rates of intranuclear mobility and transcriptional activity and the mechanisms underlying the distinct mobility patterns. These data suggest that AR-v7 has a distinct mode of interaction with DNA and gene promoters, which may identify novel targetable pathways for its inhibition in CRPC.
Citation Format: Seaho Kim, Mohd Azrin Jamalruddin, Paraskevi Giannakakou. High intranuclear mobility of AR-v7 reveals distinct mode of transcriptional activity in prostate cancer with important therapeutic implications [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 1590. doi:10.1158/1538-7445.AM2017-1590