Cytoplasmic 17β-[3H]Estradiol Binding in Rat Adipose Tissues*

Abstract

Ovarian hormones, particularly 17.beta.-estradiol, have important effects on body fat levels in rats, but it is not known whether 17.beta.-estradiol can act directly on various fat deposits to affect adiposity or whether these effects are entirely indirect (e.g., via food intake, exercise, or various metabolic actions). High affinity, estrogen-specific macromolecular binding of 17.beta.-[3H]estradiol was found in the cytoplasmic fraction of adipose tissues from ovariectomized rats. Saturation analysis indicated a Kd of 7.4 .times. 10-10 M, and binding was inhibited by unlabeled 17.beta.estradiol or 11.beta.-methoxy-17-ethynyl-1,3,5(10)-triene-3,17.beta.-diol (R2858) but not by progesterone, 5.alpha.-dihydrotestosterone, or cortisol. 17.beta.-[H]Estradiol binding was virtually abolished by incubation with pronase but not with DNase or RNase, indicating that the binding macromolecule was probably a protein. Binding was seen in all adipose tissues studied, including abdominal, s.c. and brown fat. Binding site concentration was highest in parametrial fat pads, followed by retroperitoneal, brown, omental and inguinal deposits. Binding was also seen in the cytoplasmic fraction of isolated parametrial adipocytes. The various adipose tissues might be estrogen target tissues in rats. It is possible that estrogenic effects on body weight and composition could be mediated in part by direct estrogen action on adipose tissues.