Cell type-specific expression of 17β-hydroxysteroid dehydrogenase type 2 in human placenta and fetal liver

The enzymatic actions of the 17β-hydroxysteroid dehydrogenase (17βHSD) isozymes are crucial in steroid hormone metabolism/physiology. The type 1 isozyme catalyzes the conversion of the biologically inactive C₁₈ steroid, estrone, to the active estrogen, 17β-estradiol, and the enzyme is predominantly expressed in the syncytiotrophoblast of the placenta and the granulosa cells of the ovary. 17βHSD type 2 is highly expressed in placenta, liver, and secretory endometrium and catalyzes the conversion of bioactive estrogens and androgens to biologically inactive 17-ketosteroid counterparts. The expression pattern of 17βHSD type 2 protein was determined in human term placenta and fetal liver by immunohistochemical analysis using monoclonal antibodies directed against distinct epitopes of the 17βHSD type 2 protein. In placenta, the protein was detected in the endothelial cells of fetal capillaries, but not in cytotrophoblasts or syncytiotrophoblast. There was dichotomous immunostaining seen among pairs of cotyledonary vessels and chorionic vessels. In the liver, on the other hand, staining was detected in the hepatocytes, but not in the cells lining blood vessels. We conclude that the cell type-specific localization of 17βHSD type 2 is in accord with the proposed physiological role of the enzyme, namely to protect tissues, in this case the fetus, from bioactive estrogen and androgen.