Abstract
Down-regulation of glutathione transferase A1 (GSTA1) expression has profound implications in cytoprotection against toxic by-products of lipid peroxidation produced during inflammation. We investigated the role of hepatic nuclear factor 1 (HNF-1) in repression of human GSTA1 expression by interleukin (IL)-1β in Caco-2 cells. In luciferase reporter assays, overexpression of HNF-1α increased GSTA1 transcriptional activity via an HNF-1 response element (HRE) in the proximal promoter. In addition, constitutive mRNA levels of GSTA1 and HNF-1α rose concurrently in Caco-2 cells with increasing stage of confluence. IL-1β reduced GSTA1 mRNA levels at all stages of confluence; however, HNF-1α mRNA levels were not altered. IL-1β repressed GSTA1 transcriptional activity, an effect that was abolished by mutating the HRE. Similar results were observed in HT-29 and HepG2 cells. Overexpression of HNF-1α did not counteract IL-1β-mediated repression of GSTA1 transcription either in reporter assays or at the mRNA level. Involvement of the transdominant repressor C isoform of variant HNF-1 (vHNF-1C) in GSTA1 repression was demonstrated, because vHNF-1C overexpression significantly reduced GSTA1 transcriptional activity. Finally, IL-1β caused concentration-related up-regulation of vHNF-1C mRNA levels and increased binding of vHNF-1C protein to the HRE, whereas HNF-1α-HRE complex formation was reduced. These findings indicate that IL-1β represses GSTA1 transcription via a mechanism involving overexpression of vHNF-1C.
Footnotes
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This research was funded by the Canadian Institutes for Health Research
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ABBREVIATIONS: TNF, tumor necrosis factor; GST, glutathione transferase; IL, interleukin; HNF-1, hepatic nuclear factor 1; vHNF, variant hepatic nuclear factor; HRE, hepatic nuclear factor response element; RT-PCR, reverse transcription-polymerase chain reaction; PCR, polymerase chain reaction; DTT, dithiothreitol; PMSF, phenylmethylsulfonyl fluoride; EMSA, electrophoretic mobility shift assay.
- Received July 10, 2006.
- Accepted October 4, 2006.
- The American Society for Pharmacology and Experimental Therapeutics
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