Abstract
Fifty-four independent dexamethasone-resistant clones were isolated from the clonal, glucocorticoid-sensitive human leukemic T-cell line CEM-C7. Resistance to 1 microM dexamethasone was acquired spontaneously at a rate of 2.6 X 10(-5) per cell per generation as determined by fluctuation analysis. After mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), the phenotypic expression time for dexamethasone resistance was determined to be 3 days. Spontaneous acquisition of resistance to 0.1 mM 6-thioguanine appeared to occur at a much slower rate, 1.6 X 10(-6) per cell per generation. However, the expression time after MNNG mutagenesis for this resistant phenotype was greater than 11 days, suggesting that the different rates of acquisition for the two phenotypes measured by fluctuation analysis were the results of the disparate expression times. The mutagens ICR 191 and MNNG were effective in increasing the dexamethasone-resistant fraction of cells in mutagenized cultures; ICR 191 produced a 35.6-fold increase, and MNNG produced an 8.5-fold increase. All the spontaneous dexamethasone-resistant clones contained glucocorticoid receptors, usually less than half of the amount found in the parental clone. They are therefore strikingly different from dexamethasone-resistant clones derived from the mouse cell lines S49 and W7. Dexamethasone-resistant clones isolated after mutagenesis of CEM-C7 contained, on the average, lower concentrations of receptor than did those isolated spontaneously, and one clone contained no detectable receptor. These results are consistent with a mutational origin for dexamethasone resistance in these human cells at a haploid or functionally hemizygous locus. They also suggest that this is a useful system for mutation assay.