Estradiol formation by human osteoblasts via multiple pathways: Relation with osteoblast function

The importance of estrogens in bone metabolism is illustrated by the accele rated bone loss and increase in osteoporotic fractures associated with post menopausal estrogen deficiency. In this study, the expression and activity of the enzymes involved in estrogen metabolism in human osteoblastic cells were investigated in relation to differentiation of these cells. PCR reacti ons using mRNA from an in vitro differentiating human cell line (SV-HFO) we re performed to assess mRNA expression of the enzymes aromatase, different subtypes of 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD), and steroid sulfatase. Aromatase, sulfatase, and 17 beta-HSD type 2 and 4 were found t o be expressed throughout differentiation. Expression of 17 beta-HSD type 3 , however, was relatively weak, except for early time points in differentia tion. Type 1 17 beta-HSD expression was not detected. Aromatase activity de creased during differentiation, as was demonstrated by the conversion of an drostenedione (A) and testosterone (T) into estrone (E-1) and estradiol (E- 2), respectively. The 17 beta-HSD isozymes catalysing a reductive reaction convert androstenedione and estrone into testosterone and estradiol, respec tively. Their activity declined with differentiation. Analysis of 17 beta-H SD activity indicated both oxidative (E-2 to E-1; T to A) and reductive (E- 1 to E-2; A to T) metabolism at all stages of osteoblast differentiation. B oth activities declined as cells moved toward a differentiating mineralizin g phenotype. However, the oxidative reaction was increasingly in favor of t he reductive reaction at all times during differentiation. Sulfatase activi ty, as demonstrated by the conversion of estrone-sulfate into estrone, was constant during differentiation. In conclusion, we have demonstrated that a ll enzymes necessary for estrogen metabolism are expressed and biologically active in differentiating human osteoblasts. The activity of aromatase and 17 beta-HSD was found to be dependent on the stage of cell differentiation . In addition, human osteoblasts effectively convert estradiol into estrone . The efficacy of osteoblasts to synthesize estradiol may determine the ult imate change in rate of bone turnover after menopause, as well as the devel opment of osteoporosis. Moreover, the enzymes involved in the metabolism of estradiol may form a target for intervention. J. Cell. Biochem. 75:528-537 , 1999. (C) 1999 Wiley-Liss, Inc.