To characterize the immunoreactive glucocorticoid receptor (GR) protein present in “receptorless” (r-) mutants isolated from the glucocorticoid-sensitive (dexs) human leukemic cell line CEM-C7, binding of [3H] dexamethasone was determined in extracts prepared from the sensitive cell line 6TG1.1 and the r- mutant ICR27TK.3 after gentle freeze-thaw lysis and low-speed centrifugation. Under these conditions there was significant high-affinity binding activity in r- extracts assayed at 4°C but not at 23°C. Loss of binding at 23°C was not a function of GR proteolysis or denaturation of the steroid-binding site and could be prevented by the addition of sodium molybdate. Dissociation of ligand from either activated or unactivated receptors in r- extracts was significantly more rapid than from receptors in extracts prepared from normal cells, suggesting that the defect in receptors in r- cells is the result of mutation in the ligand-binding site. While the rate of dissociation from unactivated receptors in r- extracts was linear, dissociation from receptors in extracts of 6TG1.1 cells was biphasic. Analysis of these dissociation curves, as well as dissociation from receptors in the B-cell line IM-9, indicated that the mutant gene present in r- cells is also present in the dexs parental cell line. This conclusion is consistent with our previous hypothesis (J. M. Harmon et al., Mol. Endocrinol., 3: 734–743, 1989) that glucocorticoid-sensitive CCRF-CEM cells express both a normal (GR+) and a mutant (GR*) allele.
This investigation was supported by USPHS Grant CA32226 awarded by the National Cancer Institute.