Endocrine Abstracts

Hydroxysteroid (17-β) dehydrogenase enzymes (HSD17Bs) have an important role in sex steroid hormone metabolism. They are known to catalyze reactions between highly active 17β-hydroxy steroids and less active 17-keto steroids. In addition to sex steroid metabolism, it is becoming evident that HSD17Bs have functions in other pathways as well, e.g. lipid and bile acid metabolism. HSD17B type 7 enzyme is known to catalyze in vitro the activation of estrone (E1) to estradiol (E2). However, it has also been shown that HSD17B7 catalyzes in vitro the conversion of zymosterone to zymosterol, which is an essential step in cholesterol biosynthesis. The hydroxysteroid (17-β) dehydrogenase 7 deficient mice (HSD17B7KO) are known to be embryonic lethal and they die at embryonic day 10.5 (E10.5). In the present study, we analyzed HSD17B7KO mouse embryos in order to find out the physiological function of the enzyme in vivo. We could show that the function of HSD17B7 enzyme in cholesterol biosynthesis takes also place in vivo. Accordingly, the amount of lanosterol and squalene were accumulated, and concentrations of the cholesterol biosynthesis late intermediates in the HSD17B7KO embryos were markedly reduced. The total cholesterol concentration in the HSD17B7KO embryos was unaltered at the time of death, indicating that the normal maternal cholesterol supply was not able to rescue the embryonic survival in the absence of de novo cholesterol synthesis. In line with the normal cholesterol concentration in the embryo, sonic hedgehog and two of its down stream targets, Ptch1 and Smo, showed normal expression pattern at E8.5. The vascularization in the HSD17B7 deficient yolk sacs was disrupted, and by E10.5, the embryos had pericardial effusion, reduced number of cardiomyocytes, and the complexity of the vasculature was reduced. Increasing apoptosis was observed in the neural structures of the HSD17B7KO mice at E9.5, and by the age of E11.5, the HSD17BKO embryos were resorbed. The phenotype observed in the HSD17B7KO embryos indicates the importance of HSD17B7 enzyme in sterol metabolism rather than in steroid metabolism. This study, together with previously published mouse models with disrupted cholesterol biosynthesis, shows that HSD17B7 is the last essential enzyme in cholesterol biosynthesis pathway needed for embryonic survival, and more specifically, for the proper differentiation of brain and cardiovascular structures.