Human chorionic gonadotropin-dependent regulation of 17β-hydroxysteroid dehydrogenase type 4 in preovulatory follicles and its potential role in follicular luteinization

17beta-Hydroxysteroid dehydrogenase type 4 (17betaHSD4) has a unique multidomain structure, with one domain involved in 17beta-estradiol inactivation. The objective of the study was to investigate the regulation of 17betaHSD4 during human chorionic gonadotropin (hCG)-induced ovulation/luteinization. The equine 17betaHSD4 cDNA was cloned and was shown to encode a 735-amino acid protein that is highly conserved ( 81-87% identity) compared with other mammalian orthologs. RT-PCR/ Southern blot analyses were performed to study the regulation of 17betaHSD4 transcripts in equine preovulatory follicles isolated between 0 - 39 h after hCG treatment. Results showed the presence of basal 17betaHSD4 mRNA expression before hCG treatment, but an increase was observed in follicles obtained 24 h after hCG ( P < 0.05). Analyses of isolated preparations of granulosa and theca interna cells identified basal mRNA expression in both layers, but granulosa cells appeared as the predominant site of follicular 17 beta HSD4 mRNA induction. A specific polyclonal antibody was raised against a fragment of the equine protein and used to study regulation of the 17 beta HSD4 protein. Immunoblots showed an increase in full-length 17 beta HSD4 protein in follicles 24 h after hCG ( P < 0.05), in keeping with mRNA results. Immunohistochemical data confirmed the induction of the enzyme in follicular cells after hCG treatment. Collectively, these results demonstrate that the gonadotropin-dependent induction of follicular luteinization is accompanied by an increase in 17betaHSD4 expression. Considering the estrogen-inactivating function of 17betaHSD4, its regulated expression in luteinizing preovulatory follicles appears as a potential complementary mechanism to reduce circulating levels of 17beta-estradiol after the LH surge.