Differential Expression of OATP1B3 Mediates Unconjugated Testosterone Influx

Abstract

Castration-resistant prostate cancer (CRPC) has greater intratumoral testosterone concentrations than similar tumors from eugonadal men; simple diffusion does not account for this observation. This study was undertaken to ascertain the androgen uptake kinetics, functional, and clinical relevance of <i>de novo</i> expression of the steroid hormone transporter OATP1B3 (<i>SLCO1B3</i>). Experiments testing the cellular uptake of androgens suggest that testosterone is an excellent substrate of OATP1B3 (<i>K</i>_m = 23.2 μmol/L; <i>V</i>_max = 321.6 pmol/mg/minute), and cells expressing a doxycycline-inducible <i>SLCO1B3</i> construct had greater uptake of a clinically relevant concentration of 3H-testosterone (50 nmol/L; 1.6-fold, <i>P</i> = 0.0027). When compared with <i>Slco1b2</i> (-/-) mice, <i>Slco1b2</i> (-/-)/<i>hSLCO1B3</i> knockins had greater hepatic uptake (15% greater AUC, <i>P</i> = 0.0040) and lower plasma exposure to 3H-testosterone (17% lower AUC, <i>P</i> = 0.0030). Of 82 transporters genes, <i>SLCO1B3</i> is the second-most differentially expressed transporter in CRPC cell lines (116-fold vs. androgen-sensitive cells), with a differentially spliced cancer-type ct-<i>SLCO1B3</i> making up the majority of <i>SLCO1B3</i> expression. Overexpression of <i>SLCO1B3</i> in androgen-responsive cells results in 1.5- to 2-fold greater testosterone uptake, whereas siRNA knockdown of <i>SLCO1B3</i> in CRPC cells did not change intracellular testosterone concentration. Primary human prostate tumors express <i>SLCO1B3</i> to a greater extent than <i>ct-SLCO1B3</i> (26% of total <i>SLCO1B3</i> expression vs. 0.08%), suggesting that androgen uptake in these tumor cells also is greater. Non-liver tumors do not differentially express SLCO1B3.<b>Implications:</b> This study suggests that <i>de novo</i> OATP1B3 expression in prostate cancer drives greater androgen uptake and is consistent with previous observations that greater OATP1B3 activity results in the development of androgen deprivation therapy resistance and shorter overall survival. <i>Mol Cancer Res; 15(8); 1096-105. ©2017 AACR</i>.