Differential ligand selectivity of androgen receptors α and β from Murray-Darling rainbowfish (Melanotaenia fluviatilis)

Androgen receptors (ARs) mediate the physiological effects of androgens in vertebrates. In fishes, AR-mediated pathways can be modulated by aquatic contaminants, resulting in the masculinisation of female fish or diminished secondary sex characteristics in males. The Murray-Darling rainbowfish (Melanotaenia fluviatilis) is a small-bodied freshwater teleost used in Australia as a test species for environmental toxicology research. We determined concentration-response profiles for selected agonists and antagonists of rainbowfish AR alpha and AR beta using transient transactivation assays. For both AR alpha and AR beta, the order of potency of natural agonists was 11-ketotestosterone (11-KT) > 5 alpha-dihydrotestosterone > testosterone > androstenedione. Methyltestosterone was a highly potent agonist of both receptors relative to 11-KT. The relative potency of the veterinary growth-promoting androgen, 17 beta-trenbolone, varied by more than a factor of 5 between AR alpha and AR beta. The non-steroidal anti-androgen bicalutamide exhibited high inhibitory potency relative to the structurally related model anti-androgen, flutamide. The inhibitory potency of the agricultural fungicide, vinclozolin, was approximately 1.7-fold relative to flutamide for AR alpha, but over 20-fold in the case of AR beta. Fluorescent protein tagging of ARs showed that the rainbowfish AR alpha subtype is constitutively localised to the nucleus, while AR beta is cytoplasmic in the absence of ligand, an observation which agrees with the reported subcellular localisation of AR subtypes from other teleost species. Collectively, these data suggest that M. fluviatilis AR alpha and AR beta respond differently to environmental AR modulators and that in vivo sensitivity to contaminants may depend on the tissue distribution of the AR subtypes at the time of exposure. (C) 2015 Elsevier Inc. All rights reserved.