Aromatase activity of membrane fractions of human adipose tissue stromal cells and adipocytes

Adipose tissue is the principal site of extraglandular estrogen formation in nonpregnant women. The importance of adipose tissue as a site of estrogen formation is emphasized by the finding that increased body weight is associated with an increased incidence of endometrial carcinoma. In the present study, the kinetics of estrogen formation from androstenedione by adipocytes and by stromal cells isolated from human adipose tissue as well as by membrane fractions prepared from these cells were investigated. Subcutaneous adipose tissue samples obtained from women were dispersed by collagenase treatment, and aromatase activity was assayed by the incorporation of tritium from [1-³H]androstenedione into [³H]water. As previously reported, aromatase activity was found in intact stromal cells of adipose tissue, whereas little aromatase activity was detected in intact adipocytes. When crude membrane fractions (100, 000 × g pellet) of stromal cells and adipocytes were incubated in the presence of [1-³H]androstenedione and an NADPH-generating system, however, aromatase activity was found in membrane fractions of both stromal cells and adipocytes, and estrogen formation increased in a linear manner as a function of time and membrane protein concentration. The apparent Michaelis constant (K_m) of aromatase for androstenedione of intact stromal cells was 0.03 μm. whereas intact adipocytes did not convert androstenedione to estrone at substrate concentrations up to 3.0 μm. In membrane fractions of stromal cells, the rate of aromatization as a function of androstenedione concentration did not follow simple Michaelis-Menten kinetics, and both low affinity (K_m = 1.03 μm) and high affinity (K_m = 0.10 μm) components were observed. The affinity of androstenedione for aromatase of adipocyte membrane fractions was low; the rate of aromatization was not saturable at concentrations of androstenedione up to 3.0 μm. When intact adipocytes were incubated with [1-³H]androstenedione, then homogenized, and the homogenate was treated by differential centrifugation, the radioactivity that was added to the medium was found almost entirely in the lipid fraction of the cells. This finding is indicative that the low aromatase activity in intact adipocytes is the result of sequestration of steroid in lipid droplets in the cells. We suggest that the stromal cells of adipose tissue are a major source of the increased estrogen production in obese persons; a role for adipocytes in the regulation of adipose tissue estrogen formation, however, cannot be excluded at this time.