Two classes of mutant mammary tumor virus-infected HTC cell with defects in glucocorticoid-regulated gene expression.

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Abstract
We have isolated mutant derivatives of M1.54 (a mammary tumor virus [MTV]-infected rat hepatoma [HTC] cell line containing multiple integrated proviruses) that fail to express hormone-inducible cell surface viral glycoproteins. In wild-type M1.54, the synthetic glucocorticoid dexamethasone selectively stimulates the rate of synthesis of MTV RNA. In addition, dexamethasone is essential for posttranslational maturation of three of the four cell surface viral glycoproteins processed from the MTV glycosylated precursor polyprotein; the fourth mature species is produced constitutively. Two mutant phenotypes are described; each contains glucocorticoid receptors that are indistinguishable from the wild-type receptor with respect to hormone affinity, intracellular concentration, nuclear translocation efficiency, DNA-cellulose chromatography, and sedimentation rate. In one class, represented by the mutant line CR1, dexamethasone fails to stimulate the low basal rate of MTV gene transcription; surprisingly, hormonal regulation of tyrosine aminotransferase activity is also defective in CR1, whereas several other cellular responses to dexamethasone are normal. In the second class of mutants, represented by CR4, dexamethasone stimulates synthesis of MTV transcripts indistinguishable from those produced in M1.54, but only the constitutive cell surface viral glycoprotein is expressed. Thus, these mutants define two distinct and novel aspects of glucocorticoid regulated gene expression in HTC cells: CR4 contains a defect in a hormone inducible protein maturation pathway that acts on specific viral (and presumably cellular) precursor polypeptides, whereas the lesion in CR1 appears to affect the expression of a subset of the gene products normally under glucocorticoid control in M1.54.