A new gene expression system in mammalian cells was developed by using the glucocorticoid receptor (GR) as an inducible positive feedback factor. Mouse Ltk- cells were transfected with plasmids carrying the GR-encoding gene and the lacZ reporter gene, both of which were fused with the glucocorticoid-inducible enhancer/promotor of the mouse mammary tumor virus (MTV). The GR gene was first induced to supply the receptor protein, which further induced the expression of both GR and reporter genes. Stable transformants induced with dexamethasone, a synthetic glucocorticoid hormone, demonstrated β-galactosidase activity 60-140-fold higher than uninduced controls. Similarly, the human α-interferon-encoding gene fused with the MTV enhancer/promoter was induced more than 12 000-fold. This system allowed us to increase the expression of the reporter or target genes without augmenting basal levels of expression significantly, and may be useful to investigate the unknown function of a cloned gene, particularly when the gene product of interest is cytotoxic or growth-inhibiting.
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