Immunobiochemical assay for determination of nuclear steroid receptors

Knowledge of receptor status is important for therapeutic strategies in hormone-dependent tumors. Therefore, methods specifically predicting biological response to endocrine therapy are essential. We investigated the receptor modulation of two breast tumors and one endometrial tumor in the nude mice model after injection of 20 micrograms 17 beta-estradiol. To differentiate the unoccupied and the occupied receptor sites, we used the enzyme immunoassay (EIA) for estrogen receptors (ER) under low- and high-salt conditions. With low-salt extraction we found a sharp decrease of the ER-EIA values within the first hour after estradiol treatment. This decrease continued for 24 hr until recovery to pretreatment levels occurred. In contrast, the ligand-receptor complexes tightly bound to acceptor sites on the DNA increased more than three times within 1 hr. These high levels could be measured for almost 12 hr, and then pretreatment levels were reestablished. The PgR-EIA values under low-salt conditions increased 10-fold within 12 hr, indicating an intact receptor mechanism. We conclude that this immunobiochemical method is a useful tool in determining receptor sites: those both unbound and those tightly bound to nuclear acceptors.