Inducible eukaryotic promoters, particularly those responsive to glucocorticoids or heavy metals, have been extensively used to study the consequences of induction of a target gene in mammalian cells. An alternative approach, intended to improve the selectivity of gene induction and to minimize perturbation of chromatin structure, is to utilize elements from prokaryotic regulatory systems that are unlikely to be shared by mammalian cells. We and others previously have shown that the lac repressor can function in mammalian cells and repress expression of a reporter gene controlled by a eukaryotic promoter containing a lac operator sequence. The reporter gene can be specifically activated by administration of the lactose analogue isopropyl β-d-thiogalactoside. The target genes tested so far encode the biochemical and histochemical markers, chloramphenicol acetyltransferase and β-galactosidase. As a model system to establish whether or not the lactose regulatory system can also be used to effectively modulate a cellular phenotype, NIH 3T3 cells were made transgenic for a constitutively expressed lacI gene, encoding lac repressor, and an activated human Ha-ras gene directed by a simian virus 40 promoter within which a lac operator sequence had been embedded. In the absence of inducer, cells were phenotypically untransformed. Consequent to isopropyl β-d-thiogalactoside administration, four biological end points characteristic of a transformed phenotype were observed. Consistent with transformation, the cells assumed an altered morphology; they displayed a reduced density inhibition of growth; they acquired the capacity to grow in soft agar; and they were released from a G0 block following serum deprivation. The data demonstrate that regulation of gene expression in mammalian cells by the lactose regulatory system affords a sensitive means for modulating cellular phenotype.

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This work was aided by a grant from the Jane Coffin Childs Memorial Fund, National Institute of Environmental Health Services training grant ES07250, NIH grants ES05204 and CA36897, and American Cancer Society CH470. H. S. is a fellow of the Jane Coffin Childs Memorial Fund for Medical Research.

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