17β-hydroxysteroid dehydrogenases in human bone cells

Interconversion of estrogens by osteoblasts may play a role in regulating bone mass. As a first step toward exploring this possibility, we investigated the expression and activity of 17β-hydroxysteroid dehydrogenases (17β-HSDs) in cultured human osteoblasts (HOB) and osteoblast-like osteosarcoma cells (MG63, TE85, and SaOS-2). Significant 17β-HSD activity was detected in cell-free extracts of all bone cells with oxidation of estradiol to estrone predominating over reduction. Reverse transcription-polymerase chain reaction (RT-PCR) experiments showed that the mRNA for 17β-HSD I was detectable only in MG63 cells, albeit at low levels, while 17β-HSD II was present in MG63, TE85, and HOB, but not SaOS-2, and 17β-HSD III was absent from each bone cell type. 17β-HSD IV was the only isoform present in all bone cells analyzed. Further analysis of the expression of 17β-HSD IV in these bone cells by immunoblotting revealed both the full-length 83 kDa protein and the proteolytic 38 kDa form. The kinetic parameters for estradiol oxidation by purified recombinant 17β-HSD IV (Km = 49.7 μM, Vmax = 79.4 nmol/minute/mg of protein) and its HSD-domain (Km = 79.4 μM, Vmax = 476 nmol/minute/mg of protein) were significantly higher than previously reported, but consistent with the values obtained with crude cell-free extracts of SaOS-2 cells (Km = 98.8 μM, Vmax = 0.07 nmol/minute/mg of protein) which contain only 17β-HSD IV based on RT-PCR. These studies show that bone cells have the capacity to interconvert circulating estrogens and suggest that bone cell 17β-HSDs serve primarily to attenuate the continuing actions of estradiol through conversion to its less potent form, estrone, under certain conditions.