IDENTIFICATION OF THE ACTIVATION-LABILE GENE - A SINGLE-POINT MUTATION IN THE HUMAN GLUCOCORTICOID RECEPTOR PRESENTS AS 2 DISTINCT RECEPTORPHENOTYPES

CCRF-CEM-C7 is a well characterized human leukemic clonal cell line wh ich is lysed by dexamethasone (dex). Originating from the wild-type CE M-C7 cells are two dex-resistant clones which are not lysed by 1 muM d ex and have functionally defective glucocorticoid receptors (GR). They are receptorless ICR27TK.3 and activation-labile 4R4 cells. ICR27TK.3 and 4R4 cells have distinct cellular phenotypes, as indicated by diss imilar numbers of dex-binding sites despite similar levels of GR mRNA and immunochemically detectable GR. We have now investigated the molec ular defects in the GR of ICR27TK.3 and 4R4 cells by determining the n ucleotide sequence of their GR. Our results support the biochemical ev idence previously reported by others f or the presence of both a norma l (GR+) and a mutant (GR) allele in CEM-C7 cells. We clearly show tha t the wild-type CEM-C7 cells express two alleles of GR, the normal GR and the abnormal GR, which has a Leu753-->Phe753 mutation. We demons trate that both ICR27TK.3 and 4R4 cells contain only the abnormal GR and that the normal GR+ gene is deleted in both of these GR defective clones. Our results further show that the GR is basically an activati on-labile receptor and has diminished functional capability in a trans fection assay measuring GR-driven transcription. Thus, these two pheno typically different cell lines express similar amounts of an identical GR containing a single point mutation at amino acid 753. A single po int mutation in the steroid-binding domain of the GR, therefore, may b ehave differently, depending on the cellular milieu in which it is exp ressed.