Estrogen and antiestrogen interaction with estrogen receptor of MCF-7 cells--relationship between processing and estrogenicity.

Overnight preincubation of MCF-7 cells with 2 x 10(-10) M estradiol (E2) produces a dramatic reduction of their specific [3H]E2 binding capacity. Scatchard plot analysis revealed that this loss of estrogen receptor (ER) concentration, usually termed "processing", occurs without any significant modification of binding properties of the unprocessed receptors. Direct measurement of ER (ER-EIA from Abbott) gave residual receptor concentrations close to those established by binding assay indicating that processing involves the loss of at least one epitope other than the steroid binding site. Incubation with increasing amounts of E2 (0.1 to 5 x 10(-10) M) resulted in an increasing reduction of binding capacity indicating that the extent of processing is associated with the hormone concentration. Steroidal estrogens other than E2 as well as antiestrogens of the triphenylethylene category behaved similarly in this regard although the latter compounds usually acted only when at higher concentrations. The processing capacity of a large series of ligands was compared with the corresponding binding affinity for ER as assessed by classical competitive inhibition of [3H]E2 binding in both cytosol and whole cells. For steroidal estrogens, a large spectrum of concordant values was found which correlated with the known uterotrophic activity of the compounds. On the contrary, weak estrogen and antiestrogens of the triphenylethylene category displayed low processing capacities which were in the order of magnitude of the binding affinities established in whole cells; these values were considerably lower than the corresponding values measured in the cytosol. These observations are consistent with the concept that the capacity of a ligand to process ER is related to its agonistic activity. They also support our hypothesis (J. steroid Biochem. 25 (1986) 677-682) that assessment of the ability of a ligand to inhibit the binding of [3H]E2 in whole cells provides an estimate of its agonistic activity, an estimate which can not be established in the corresponding cytosol assay.