Down‐regulation of GSTA1 by IL‐1 beta is mediated by overexpression of the transdominant repressor vHNF‐1C

Abstract

Interleukin-1 (IL-1) beta markedly suppresses glutathione S-transferase (GST) A1 expression, which has profound implications on protection against cytotoxic byproducts of inflammation. We investigated the role of hepatic nuclear factor 1 (HNF-1) alpha in the regulation of human GSTA1 by IL-1 beta in Caco-2 cells. Overexpression of HNF-1 alpha increased GSTA1 promoter activity in luciferase reporter assays through a HNF-1 response element (HRE) in the proximal promoter. IL-1 beta repressed GSTA1 transcriptional activity in reporter assays but not when the HRE was mutated in luciferase contsructs. Both GST and HNF-1 alpha mRNA and protein levels progressively increased as cells reached 7 days post-confluency, however, only GSTA1 mRNA levels were reduced by IL-1 beta. Overexpression of HNF-1 alpha failed to counteract IL-1 beta-mediated repression of GSTA1 transcription both in reporter assays and at the mRNA level. Moreover, IL-1 beta had no effect on HNF-1 alpha binding to the HRE in gel shift assays. Messenger RNA levels of the C isoform of variant HNF-1 (vHNF-1C), the transdominant repressor variant form, were 60 fold higher relative to the A and B forms (transcriptional activator forms) in pre-confluent cells but only 20 fold higher in confluent cells. IL-1 beta significantly up-regulated vHNF-1C expression in pre-confluent and confluent cells to 80 fold and 30 fold respectively. These findings indicate that IL-1 beta represses GSTA1 transcription via a mechanism involving overexpression of vHNF-1C. Supported by the Canadian Institutes for Health Research