The 17beta-hydroxysteroid dehydrogenases (17beta-HSDs) play a key role in the synthesis of sex steroids. The hallmark of this family of enzymes is the interconversion, through their oxydoreductive reactivity at position C17, of 17-keto- and 17beta-hydroxy-steroids. Because this reaction essentially transforms steroids having low binding activity for the steroid receptor to their more potent 17beta-hydroxysteroids isoforms, it is crucial to the control of the physiological activities of both estrogens and androgens. The human placenta produces large amounts of progesterone and estrogens throughout pregnancy. The placental type 1 17beta-HSD enzyme (E17beta-HSD) catalyzes the reduction of the low activity estrogen, estrone, into the potent estrogen, estradiol. We studied the cell-specific expression of type 1 17beta-HSD in human term placental villous tissue by combining in situ hybridization to localize type 1 17beta-HSD mRNA with immunohistochemistry using an antibody against human placental lactogen, a trophoblast marker. Immunolocalization of E17beta-HSD was also performed. To ascertain whether other steroidogenic enzymes are present in the same cell type, cytochrome P450 cholesterol side-chain cleavage (P450scc), P450 aromatase, and type 1 3beta-hydroxysteroid dehydrogenase (3beta-HSD) were also localized by immunostaining. Our results showed that the syncytium is the major steroidogenic unit of the fetal term villi. In fact, type 1 17beta-HSD mRNA and protein, as well as P450scc, P450 aromatase, and 3beta-HSD immunoreactivities were found in these cells. In addition, our results revealed undoubtedly that extravillous cytotrophoblasts (CTBs), e.g. those from which cell columns of anchoring villous originate, also express the type 1 17beta-HSD gene. However, CTBs lying beneath the syncytial layer, e.g. those from which syncytiotrophoblasts develop, contained barely detectable amounts of type 1 17beta-HSD mRNA as determined by in situ hybridization. These findings, along with those from other laboratories confirm the primordial role of the syncytium in the synthesis of steroids during pregnancy. In addition, our results indicate for the first time that CTBs differentiating along the invasive pathway contain type 1 17beta-HSD mRNA.

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