Two subtypes of androgen and progestogen receptors in fish testes

Two subtypes (alpha and beta) of androgen (AR) and progestogen receptors (PR) are present in the testis of Japanese eel (Anguilla japonica). Amino acid homology of the open reading frames between alpha and beta in AR or PR is approximately 40%, but the DNA- and ligand-binding domains show high homology between subtypes. Judging from these structures, alpha and beta are not isoforms derived from translational initiation at two in-phase ATG codons, alternative splicing, or tetraploidy. In transient transfection assays using a reporter construct containing a steroid-responsive promoter, each subtype showed its corresponding hormone-dependent transactivation. The ligand affinity for transactivation between AR and PR subtypes was similar for physiological ligands. Tissue distribution of both subtype mRNAs was different. Protein interaction between subtypes was demonstrated in vitro by GST pull-down assays. These results clearly indicate that two functional subtypes of AR and PR exist in eel. These findings will advance our understanding of the mechanisms underlying sex steroid signaling.