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Siu, Chu-Sin
Enhancer elements and transcription factors mediating the suppressive effect of IL-1 on CYP1A1 transcriptional activation.

Order No. 9726318

Total P450 content and related activities are known to decrease in cultured rat hepatocytes in response to IL-1 treatment. Previous studies reported that IL-1 suppressed the induction of CYP1A1 and CYP1A2 through a transcriptional mechanism. In order to identify the cis-acting element which mediates IL-1 effect, we analyzed the promoter activity of the 5'-flanking region of CYP1A1 gene. Two elements were identified: xenobiotic responsive element (XRE) and HNF-4 binding site. Transient transfection experiment using primary hepatocytes transfected with CAT reporter genes carrying either XRE1 or XRE2 gave direct evidence that XREs can mediate IL-1 action, although the level of AH receptor binding was not affected. By deletion analysis of the 3.1 kb regulatory region of the CYP1A1 gene, a 36 bp IL-1 responsive region was identified. The region is a composite of three distinct sites: XRE, Sp1-like, and IL-1 responsive element (ILRE). Gel mobility shift assays demonstrated that the ILRE binds constitutively a liver-enriched protein designated as IL-1 responsive protein (ILRP), whose binding activity is reduced by IL-1. Antiserum to the rat HNF-4 transcription factor supershifted the DNA-protein complex formed between ILRE and ILRP. Cotransfection with an HNF-4 expression plasmid increased transcriptional activity of the CYP1A1 minimal promoter carrying one copy of ILRE (about 1.7-fold), or three copies of ILRE (about 2.7-fold) in HepG2 cells. These data suggested that ILRP is in fact HNF-4. Although the transactivation potential of HNF-4 is weak in the context of CYP1A1 promoter, its reduced binding activity upon IL-1 treatment suggests that it may mediate IL-1 action in down- regulating CYP1A1 induction. This is the first report that showed the binding activity of HNF-4 can be down-regulated by IL-1. In summary, IL-1 down-regulation of CYP1A1 transcriptional activation is mediated by XREs, however the mechanism by which this occurs is not known. The HNF-4 binding site may also mediate IL-1 action but more direct evidence is needed. Source: DAI, 58, no. 03B, (1997): 1125

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