Characteristics of the dextran-coated charcoal assay for estradiol receptor in breast cancer preparations.

The measurement of E2 receptor (E2R) in human breast cancer cytosol is significantly influenced by conditions usually employed in the dextran-coated charcoal assay. The incubation time and temperature have an influence on the rate of binding and stability of the receptor. Since lower temperatures preserve the integrity of the receptor, a 2 hr incubation at 4 degrees was selected as the standard incubation procedure. These conditions allow for the detection of at least 80% of the E2R. With supernatants from high-speed centrifugation of HBT biopsies or the human breast cancer cell line MCF-7, reducing agents increased the apparent E2R binding in the order: DTT greater than G-SH greater than MTG. The maximum enhancement of specific E2R binding by a given thiol agent was dependent on its concentration in the incubation medium. The optimum DTT level (7.5 mM) for MCF-7 cell homogenization and cytosol equilibration with tritiated E2 increased E2R to two times control (no DTT). For the HBT 150,000 g supernatant, 1 mM DTT was required to optimize the E2R quantitation. The duration of the dextran-coated charcoal extraction of the cytosol-[3H]E2 incubation had no effect on the level of E2R up to 21 hr. Minimum levels of nonspecific binding of [3H]E2 could be obtained after 4 hr extraction. Maximum depletion of specific [3H]E2 binding could be obtained by adding between 200- and 1000-fold molar excess of unlabeled E2. Greater amounts of unlabeled steroid displaced the radioactive E2 from the dextran-coated charcoal, thereby artifactually increasing the apparent nonspecific binding. This phenomenon may be overcome by utilizing more dextran-coated charcoal in the extraction. However, there was a 9% loss of specifically bound [3H]E2 per milligram of dextran-coated charcoal (1:10 dextran to charcoal by weight) when the cytosol protein was below 90 microgram per incubation. Supplementation with 200 microgram or more albumin per incubation prevented this loss. The dextran:charcoal ratio also prevented E2R loss in the order: 1:1 greater than 1:10 greater than 1:100. One milligram of dextran-coated charcoal (1:10) has the capacity to adsorb 0.3 to 0.4 microgram of free E2. Other unlabeled competitors are capable of displacing [3H]E2 on the receptor. Although DES was as effective as E2, U11,100A and estrone were inefficient competitors. It appeared that the levels of these two estrogen analogues required to maximally displace [3H]E2 on receptor also eluted labeled E2 from the dextran-coated charcoal. DES, however, was unable to displace significant quantities of the [3H] E2 from dextran-coated charcoal even at a molar excess of 50,000:1.