Tetracycline Repressor, tetR, rather than the tetR–Mammalian Cell Transcription Factor Fusion Derivatives, Regulates Inducible Gene Expression in Mammalian Cells

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Abstract
This article describes the first (to our knowledge) tetracycline-inducible regulatory system that demonstrates that the tetracycline repressor (tetR) alone, rather than tetR-mammalian cell transcription factor fusion derivatives, can function as a potent trans-modulator to regulate gene expression in mammalian cells. With proper positioning of tetracycline operators downstream of the TATA element and of human epidermal growth factor (hEGF) as a reporter, we show that gene expression from the tetracycline operator-bearing hCMV major immediate-early enhancer-promoter (pcmvtetO) can be regulated by tetR over three orders of magnitude in response to tetracycline when (1) the reporter was cotransfected with tetR-expressing plasmid in transient expression assays, and (2) the reporter unit was stably integrated into the chromosome of a tetR-expressing cell line. This level of tetR-mediated inducible gene regulation is significantly higher than that of other repression-based mammalian cell transcription switch systems. In an in vivo porcine wound model, close to 60-fold tetR-mediated regulatory effects were detected and it was reversed when tetracycline was administered. Collectively, this study provides a direct implementation of this tetracycline-inducible regulatory switch for controlling gene expression in vitro, in vivo, and in gene therapy. Regulation of transgene expression in target cells represents a critical and challenging aspect of gene therapy. Using the hCMV major immediate-early promoter as a prototype mammalian cell promoter, this study demonstrates that tetR alone, rather than the previously used tetR–mammalian cell transcription factor fusion derivatives, can function as a potent repressor of expression of genes under the tet operator-containing hCMV major immediate-early promoter, while its natural promoter activity is preserved. Specifically, with hEGF as a secretable reporter, more than 1000-fold tetracycline-reversible regulation can be detected in transient transfection assays, and in tetR-expressing stable cell lines with a chromosomally integrated hEGF reporter unit. These observations suggest a direct implementation of this biological switch in regulating the expression of transgenes in cell biology, molecular virology, and gene therapy.