[Effect of estradiol on 5 alpha-reductase activity in osteoblast-like cell (UMR106-01)]

Bone is known to be a target organ of not only estrogens, but also androgens. The mechanism by which these steroids exert their action on bone cells is still poorly understood. In the present study, the effect of 17 beta-estradiol (E2) on 5 alpha-reductase activity, converting testosterone (T) to a more potent biological androgen, dihydrotestosterone (DHT), was assessed in an osteoblast-like cell line of rat origin (UMR106-01). Cells were incubated under standardized conditions with varying concentrations of E2 (10(-12)-10(-6) M) for 48 hours. Incubation medium was replaced when the cells were preconfluent and thereafter at 24 hour intervals. Then the cells were harvested. Each cell homogenate was incubated with [4-14C]-T. DHT was detected as a single metabolite on silicagel thin layer chromatography. 5 alpha-reductase activity was determined by measuring the amount of labeled DHT from T. The radiochemical purity of DHT recovered after incubation was confirmed by recrystallization to constant specific activity. Under the conditions used, no estrogen was detected. Production of insulin-like growth factor-I and alkaline phosphatase in UMR106-01 was increased when E2 was added into the culture medium, however, 5 alpha-reductase activity was significantly decreased by the addition of 10(-12) M to 10(-6) M of E2. Maximum inhibition was noticed at 10(-10) M. Our results demonstrate that UMR106-01 cells have a capacity to transform T into the biologically more potent androgen, DHT. The result, that the enzyme activity was influenced by E2, suggests the regulatory mechanism of both sex steroids on the steroid metabolism in osteoblasts.