Estrogen-Binding Parameters of Cytoplasmic and Nuclear Receptors in an Established Rat Endometrial Cell Line and Tumor

Abstract

We have consistently found receptors for estradiol in both the cytosol and nuclear extracts of a rat endometrial cell line and in transplantable tumors derived from this cell line. The equilibrium dissociation constants (Kd) and the rate constants for the receptor-estradiol interaction in these cells and tumors did not differ significantly from those of the cytosol receptor in the rat uterus. A mean Kd of 3 × 10⁻¹⁰ M with a rate of association (ka) of 3 × 10⁵ M⁻¹ sec-1 and a rate of dissociation (kd) of 1.5 × 10⁻⁵ sec⁻¹ were obtained for nuclear and cytosol receptors for both tumors and cells. For uterine cytosol, a Kd of 8 × 10⁻¹⁰ M, ka = 2.8 × 10⁵ M⁻¹sec⁻¹ and kd = 1 × 10⁻⁵ sec⁻¹ were obtained. Although no differences were seen in equilibrium and kinetic parameters for estradiol-17β binding between the nuclear and cytosol receptors of tumors and cells, an apparent difference in the relative affinities of nuclear and cytosol receptors for estrone was detected. This suggests that the binding site in nuclear receptors may have been modified. Implications of this observation with regard to receptor translocation and the mechanism of action of sex hormones are being considered.