Use of Resistant or Hypersensitive Variant Clones of Friend Cells in Analysis of Mode of Action of Inducers1

Twenty-one variant clones of Friend erythroleukemia cell were isolated and characterized. Seventeen clones were resistant to induction of hemoglobin synthesis by one of eight drugs that have been reported to induce erythroid differentiation in clone 745. Four clones re sponded hypersensitively to dimethyl sulfoxide or hypoxanthine. The variant clones were treated with inducing drugs to determine whether patterns of cross-resistance or cross-sensitivity could be identified. On the basis of the results obtained, we divided the drugs into three classes. The first, arbitrarily named Class A, was the most common and included dimethyl sulfoxide, hypoxanthine, hexamethylene bisacetamide, 1-methyl-2-pyrrolidinone, tetramethylurea, pyridine N-oxide, N-methylacetamide, and 6thioguanine. Every clone resistant to induction of hemo globin synthesis by one of these drugs was found to be cross-resistant to induction by all other inducers of this class. Clones hypersensitive to dimethyl sulfoxide were also hypersensitive to hypoxanthine, and hypoxanthinehypersensitive clones were hypersensitive to dimethyl sulfoxide. Clones that could not be induced to synthesize hemoglobin by butyric acid, a Class B inducer, could be induced to synthesize hemoglobin both by drugs of Class A and by hemin. Clones hypersensitive to dimethyl sulf oxide and hypoxanthine were not hypersensitive to induc tion by butyric acid. Butyric acid could induce synthesis of hemoglobin in clones resistant to induction by Class A inducers and in clones noninducible by hemin. Hemin, a Class C inducer, characteristically induced the synthesis of hemoglobin containing a and ßminor globin chains, but not ßmajor, both in wild-type cells and in some clones resistant to induction of hemoglobin synthesis by Class A and B inducers. Clones selected for survival in the pres ence of hemin were inducible by Class B inducers, but response to Class A inducers varied, and the composition of the globin chains induced differed from that of the wild-