Progesterone markedly inhibits the functions of human macrophages and T lymphocytes, and acts as an immunosuppressant during pregnancy. It is important to examine progesterone metabolites to understand the over-all bioactive properties of this sex steroid. However, progesterone metabolism has not been examined in human immune cells. The human leukemic monoblast U937 cell line exhibits monocytic lineage and provides a valuable model to analyze monocyte-macrophage differentiation. Therefore, in this study, we analyzed progesterone metabolism in U937 cells by thin-layer chromatography. Progesterone was metabolized to 5α-pregnan-3β,6α-diol-20-one via 5α-dihydroprogesterone and 5α-pregnan-3β-ol-20-one, and 5α-pregnan-3β,20α-diol was also detected as a final metabolic product via 20α-dihydroprogesterone and 5α-pregnan-20α-ol-3 -one. 5α-reduction (5α-reductase type 1) and 20α-reduction were involved in the first step of metabolism. To identify the enzyme responsible for the 20α-reduction, we screened an U937 cDNA library, and obtained a clone (1.2 kb), which was identical to the human hepatic bile acid-binding protein or 20α-hydroxysteroid dehydrogenase (20α-HSD). 293 cells transfected with this cDNA demonstrated marked 20α-reduction of progesterone to 20αDHP, but 20α-oxidative, 3α-HSD or 17β-HSD activity was found to be negligible. In experimental animals, the importance of 20α-HSD has been reported to be involved in the protection of immune cells from the toxic effects of progesterone. Therefore, our present data suggest that 20α-HSD plays an important role in the regulation of progesterone actions in human immune cells.
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