The role of intracellular equilibria and the effect of antiestrogens on estrogen-receptor dissociation kinetics from perfused cultures of human breast cancer cells.

Abstract

Dissociation of [3H]estradiol from perfused monolayer cultures of estrogen receptor-positive MCF-7 human breast cancer cells is described. A kinetic model is proposed in which loss of estradiol from the cells occurs from an intracellular compartment of free estradiol that is in dynamic equilibrium with estradiol bound to both specific (receptor) and nonspecific cellular binding sites. We found that the estrogen receptor in MCF-7 cells has little effect on the dissociation rate constant of estradiol from MCF-7 cells by comparing the dissociation of estradiol from MCF-7 cells with estrogen receptor-negative MDA-MB-231 cells. The major determinant of the rate constant of estradiol dissociation from both MCF-7 and MDA-MB-231 cells is the equilibrium between free intracellular estradiol and estradiol bound to slowly dissociating nonspecific binding sites. This result indicates that slow dissociation of estradiol from nonspecific cellular binding sites may prolong the occupancy of specific receptor si tes. Thus...