The importance of
estrogens in bone metabolism is illustrated by the accelerated bone loss and increase in
osteoporotic fractures associated with postmenopausal
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 reactions using
mRNA from an in vitro differentiating human cell line (SV-HFO) were performed to assess
mRNA expression of the
enzymes aromatase, different subtypes of
17beta-hydroxysteroid dehydrogenase (17beta-HSD), and
steroid sulfatase.
Aromatase,
sulfatase, and 17beta-HSD type 2 and 4 were found to be expressed throughout differentiation. Expression of 17beta-HSD type 3, however, was relatively weak, except for early time points in differentiation. Type 1 17beta-HSD expression was not detected.
Aromatase activity decreased during differentiation, as was demonstrated by the conversion of
androstenedione (A) and
testosterone (T) into
estrone (E(1)) and
estradiol (E(2)), respectively. The 17beta-HSD
isozymes catalysing a reductive reaction convert
androstenedione and
estrone into
testosterone and
estradiol, respectively. Their activity declined with differentiation. Analysis of 17beta-HSD 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. Both activities declined as cells moved toward a differentiating mineralizing phenotype. However, the oxidative reaction was increasingly in favor of the reductive reaction at all times during differentiation.
Sulfatase activity, as demonstrated by the conversion of
estrone-sulfate into
estrone, was constant during differentiation. In conclusion, we have demonstrated that all
enzymes necessary for
estrogen metabolism are expressed and biologically active in differentiating human osteoblasts. The activity of
aromatase and 17beta-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 ultimate change in rate of bone turnover after menopause, as well as the development of
osteoporosis. Moreover, the
enzymes involved in the metabolism of
estradiol may form a target for intervention.