The dual carbonyl reductase activities of 17β-HSD12a during ovarian development in Japanese eel (Anguilla japonica).

17β-hydroxysteroid dehydrogenase subtype 12 (17β-HSD12) is responsible for the reactions between carbonyl and hydroxyl groups at the C17 position of sex steroids. Our previous research revealed that hsd17b12a was expressed at a high level during follicular maturation in Anguilla japonica, peaking at the migrating nucleus stage. This indicated the potential of 17β-HSD12 may convert 17α-hydroxyprogesterone (17OHP) into 17α,20β-dihydroxy-4-pregnen-3-one (DHP), a maturation-inducing steroid. To elucidate the functions of hsd17b12a in A. japonica, the spatiotemporal expression patterns of hsd17b12a were investigated, as well as its role in sex steroid hormone synthesis, and its regulation by upstream gonadotropins. The results showed that hsd17b12a expression was the highest in the ovaries compared to other tissues. Within the ovary, hsd17b12a expression peaked at the migrating nucleus stage, followed by the mid-vitellogenic stage, both significantly higher than other stages. HEK293T cells transfected with a plasmid expressing A. japonica hsd17b12a converted estrone, androstenedione, and 17OHP into 17β-estradiol, testosterone, and DHP, with the conversion rates of 23.70 ± 10.65, 14.95 ± 4.42, and 22.35 ± 0.53 %, respectively. In vitro experiment showed that stimulation with 10 ng/mL 17OHP and 100 µg/mL carp pituitary extract greatly significantly increased hsd17b12a expression of follicles with migrating nucleus stage, although DHP synthesis did not change significantly. The present study demonstrates that the enzyme encoded by eel hsd17b12a possesses both C17 and C20 carbonyl reductase activities. These findings provide new insights into the reproductive endocrine regulatory mechanisms and offer theoretical support for optimizing artificial maturation and spawning in eels.