Abstract 2686: Is ARV7 a regulator of prostate cancer stemness and plasticity

Abstract Therapy options for advanced and castration-resistant prostate cancer (CRPC) include the use of drugs targeting directly/indirectly the ligand binding domain (LBD) of androgen receptor (AR) such as enzalutamide (Enza) and abiraterone acetate. However, primary or acquired treatment failures are frequently observed and have been associated with the expression of truncated AR variants devoid of the LBD. In particular, the AR variant AR3, also known as ARV7, is detected in over 80% of CRPC specimen and is increasingly expressed in metastatic lesions. In PC models ARV7 expression causes treatment resistance to Enza. Moreover, ARV7 expression leads to increased expression of mesenchymal markers and stem cell signature genes. CDH2 (N-Cadherin) expression is increased by ARV7 binding to regulatory elements in intron 1 of CDH2, which is antagonized by full length (FL) AR. Based on these observations we hypothesize that ARV7 is a marker and regulator of PC stemness and/or plasticity. ARFL or ARV7 mRNA and protein was detected in cell lines and tissue samples by standard procedures including qRT-PCR, RNA in situ hybridization (RISH), immunofluorescence (IF) and western blotting using the monoclonal ARV7 antibody RM7. ARFL or ARV7 stably expressing LNCaP were obtained by lentiviral transduction. LNCaP cells resistant to Enza (LNCaP-Enza) were generated by exposure to increasing concentrations (up to 5 µM) of the drug. Transcriptome analysis in LNCaP-Enza vs. LNCaP was done by RNAseq and Gene Set Enrichment Analysis. ARV7 mRNA was detectable in LNCaP, albeit gene expression was 2.3% - 2.6% of the known ARV7 positive cell lines 22Rv1 and VCaP, respectively. RISH and IF demonstrated that only a fraction (~10%) of LNCaP cells expressed ARV7 mRNA and nuclear localized ARV7 protein. ARV7 mRNA was also detected in isolated epithelial and stromal areas in treatment-naïve prostate cancer specimen by RISH. In contrast, all LNCaP-Enza cells were positive for nuclear ARV7 protein. Transcriptome analysis of LNCaP-Enza cells showed enrichments for published ARV7 gene expression signatures. Insular spots of ARV7 mRNA were detected in treatment-naïve tissue and in isolated cells of the castration-sensitive cell line LNCaP suggesting that these cells could be intrinsically resistant against LBD-targeting drugs and responsible for the failure of various endocrine therapies. This project will further analyze whether this subpopulation possesses stem cell (-like)/plasticity characteristics and whether these are regulated by ARV7. Citation Format: Tobias Furlan, Florian Handle, Marcus V. Cronauer, Frédéric R. Santer. Is ARV7 a regulator of prostate cancer stemness and plasticity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2686.