17β-Hydroxysteroid dehydrogenase type 12 is responsible for maturation-inducing steroid synthesis during oocyte maturation in Nile tilapia

17 alpha, 20 beta-Dihydroxy-4-pregnen-3-one (DHP) is a maturation-inducing steroid in many teleost fish. Carbonyl reductase-like 20 beta-hydroxysteroid dehydrogenase (CR/20 beta-HSD) is a candidate enzyme responsible for DHP production during oocyte maturation in various fish, including Nile tilapia. However, a novel type of 17 beta-hydroxysteroid dehydrogenase, type 12-like (17 beta-HSD12L), is responsible for DHP production during oocyte maturation in masu salmon. 17 beta-HSD12 (presumably orthologous to salmon 17 beta-HSD12L) has been detected in Nile tilapia; however, its enzymatic activity and specific ability to convert the DHP substrate 17 alpha-hydroxyprogesterone (17OHP) have not been examined. This study aimed to determine whether CR/20 beta-HSD or 17 beta-HSD12 is responsible for DHP production during oocyte maturation in the Nile tilapia. Mammalian expression vectors containing tilapia hsd17b12 or CR/20bhsd were transfected into HEK293T cells, followed by incubation with 170HP. HEK293T cells transfected with hsd17b12 exhibited a strong ability to convert exogenous 17OHP to DHP (73.8% yield). Cells transfected with CR/20bhsd or the control vector converted only 7.4% and 7.5% of 17OHP to DHP, respectively. In addition, based on LC-MS/MS analyses, 17 beta-HSD12 did not convert any substrates other than 17OHP, including DHP, adrenosterone, androstenedione, estrone, testosterone, 11-ketotestosterone, and estradiol-17 beta. CR/20 beta-HSD showed strong 17 beta-HSD oxidoreductase activity especially with adrenosterone and androstenedione. Tissue-specific hsd17b12 expression analyzed by RT-PCR showed that hsd17b12 mRNA was strongest amplification in full-grown follicles. Finally, full-grown ovarian follicles were incubated with salmon pituitary extract (SPE, 100 mu g/mL) or human chorionic gonadotropin (HCG, 100 IU/mL) to induce 2013-HSD activity in vitro, and enzyme activity was assessed by co-incubation with 100 ng/mL 17OHP for 2, 4, 8, and 16 h. Conversion of 17OHP to DHP by ovarian follicles incubated with SPE and HCG peaked at 16 h, subsequent with increased follicular hsd17b12 mRNA levels, which were significantly higher than those in control incubations. However, the levels of CR/20bhsd mRNA remained low and did not differ among time points. The present study strongly suggests that 17 beta-HSD12, and not CR/20 beta-HSD, is the 20 beta-FISD responsible for DHP production by ovarian follicles during oocyte maturation in Nile tilapia.