17β-hydroxysteroid dehydrogenase type 12-like is associated with maturation-inducing steroid synthesis during induced oocyte maturation and ovulation in sturgeons

17α, 20β-dihydroxy-4-pregnen-3-one (DHP) has been identified as a maturation-inducing steroid (MIS) in several teleosts. DHP is converted from 17α-hydroxyprogesterone (17OHP) by 20β-hydroxysteroid dehydrogenase (20β-HSD). A recent study reported that 17β-hydroxysteroid dehydrogenase type 12-like (hsd17b12L) is responsible for DHP production, and its expression is upregulated by gonadotropin during oocyte maturation in masu salmon and Nile tilapia. However, whether the hsd17b12L gene encodes 20β-HSD in Actinopterygii remains unknown. Sturgeons belong to ancient fish and are suggested to have diverged 200 million years ago. Currently, the MIS of sturgeons and its associated mechanism remain unknown. This study aimed to understand the evolution of Actinopterygii hsd17b12L and determine its function during sturgeon oocyte maturation. In the Amur sturgeon and sterlet, two hsd17b12L types were repeatedly isolated. Mammalian expression vectors containing Amur sturgeon hsd17b12L type I and sterlet hsd17b12L type I were transfected into HEK293T cells, which then exhibited strong 20β-oxidoreductase activity, converting 17OHP to DHP. LC/MS analyses indicated that Amur sturgeon hsd17b12L did not convert DHP, androstenedione, testosterone, estrone, estradiol-17β, adrenosterone, and 11-ketotestosterone, confirming that Amur sturgeon hsd17b12L has 20β-oxidoreductase activity, but not 20β-dehydrogenase activity toward DHP neither17β-HSD activity. Masu salmon hsd17b12L and Nile tilapia hsd17b12 expression were limited to the follicles and testes at the maturation stage, while Amur sturgeon hsd17b12L was upregulated in all tissues examined. In individuals that reached ovulation, serum DHP concentrations during oocyte maturation increased, and the Amur sturgeon hsd17b12L mRNA levels of follicles during oocyte maturation were higher than prior to maturation. Finally, sterlet and Amur sturgeon ovarian follicles were incubated with chum salmon pituitary extract (SPE, 500 μg/mL) for different times. The DHP concentration in the medium was increased by SPE. However, hsd17b12L mRNA levels were not significantly different following SPE stimulation. The present study demonstrated that sturgeon hsd17b12L encodes 20β-HSD, suggesting that the hsd17b12L gene encodes 20β-HSD in related fish species that evolved after the sturgeon. An in vivo experiment revealed that sturgeon hsd17b12L was associated with oocyte maturation and DHP production, but its mRNA expression in vitro did not exhibit a rapid increase during oocyte maturation as previously observed in masu salmon, suggesting that hsd17b12L does not solely regulate sturgeon DHP production.