Molecular cloning and characterization of a nuclear androgen receptor activated by 11-ketotestosterone

Androgens have critical physiological roles in male sexual differentiation and in the development of male secondary sex characteristics. Androgens primarily mediate their actions through interactions with receptors belonging to the steroid hormone receptor super-family. Androgen receptors (AR) have been identified in many vertebrates and demonstrate similar binding characteristics in the various species investigated. Most vertebrates have one AR with high specificity for 5α-dihydrotestosterone (DHT). However, some teleosts have two AR with high binding affinities for either testosterone (T) or DHT [1,2]. 11-ketotestosterone (KT) is a major androgen in many teleosts and it often occurs at higher levels in the circulation and has a higher potency in inducing male reproductive functions than other androgens [3]. Although several AR have been isolated and characterized from teleost species, none of these have characteristics expected in a specific KT receptor. Consequently, an explanation for the high androgenic potency of KT in teleosts is currently not available.AR have been cloned and characterized from several teleosts, including Japanese eel (Anguilla japonica), rainbow trout (Oncorhynchus mykiss), tilapia (Oreochromis niloticus) and red seabream (Pagrus major) [4-7]. Rainbow trout, tilapia and Japanese eel have two AR isoforms and while only one isoform (ARα) is a functional AR in rainbow trout, both Japanese eel AR are functional receptors [6,7]. Following transfection of human embryonic kidney 293 cells with either of the Japanese eel AR cDNAs it was observed that KT and DHT were equally potent activators of both AR isoforms, while T was significantly less potent in activating an MMTV-LTR driven luciferase reporter vector [4,6]. In contrast, the red sea bream AR was equally activated by T and KT via an MMTV promoter system in transfected COS-7 cells [5]. Similarly, a rainbow trout ARα reporter system in a carp EPC cell line was equally activated by DHT, KT and T