Abstract A069: Mechanisms governing the transcriptional regulation of the liver-specific transporter OATP1B3 in prostate cancer

Abstract

Abstract Background: The testosterone uptake transporter OATP1B3 is expressed de novo in prostate tumors and SLCO1B3 gene (encoding OATP1B3) variants are associated with clinical outcomes in patients with prostate cancer receiving androgen deprivation therapy (ADT) and in castration-resistant disease. These findings support the role of OATP1B3 as a major physiologic contributor to androgen distribution and a contributor to resistance to ADT. We have previously shown CBP/p300-mediated SLCO1B3 expression; however, specific transcriptional regulation of de novo expression remains to be elucidated. We characterized the SLCO1B3 promoter in order to better understand the regulatory mechanisms that govern its expression in prostate cancer. Methods: Functional analysis of the SLCO1B3 promoter was conducted using 5’ deletion mutagenesis. Transcriptional activity of the SLCO1B3 gene was measured using the SLCO1B3 promoter-luciferase reporter plasmids, transient transfections and luciferase reporter assays. Promoter activity was modulated by co-transfection with the p300 expression plasmid or treatment with various p300 inhibitor compounds (chetomin, HATi II, or C646). Results: We detected different transcriptional profiles across multiple prostate cancer cells (22Rv1, LNCaP, PC3) and identified a conserved domain responsible for SLCO1B3 transcriptional activity. Variable transcriptional changes in response to p300 inhibitor treatments were observed in both the conserved domain plasmid and the full-length reporter with the HATi II compound showing a consistent 2 to 3-fold increase in luciferase activity. Putative transcription factor binding sites are being evaluated by site-directed mutagenesis to determine potential regulatory elements involved in transcriptional activity of the gene. Conclusions: Our data suggest that several multi-protein transcription factor complexes assemble at distinct regulatory elements in the SLCO1B3 promoter, driving tissue-specific expression of OATP1B3 in prostate cancer. Understanding the underlying regulatory mechanisms of OATP1B3 expression and the transporter’s role in prostate cancer progression will aid in its development as a potential therapeutic target. Citation Format: Roberto H Barbier, Edel M McCrea, Jonathan D Strope, Phoebe A Huang, Tristan M Sissung, Douglas K Price, Cindy H Chau, William D Figg. Mechanisms governing the transcriptional regulation of the liver-specific transporter OATP1B3 in prostate cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A069. doi:10.1158/1535-7163.TARG-19-A069