Interleukin-6-induced STAT3 and AP-1 amplify hepatocyte nuclear factor-1-mediated transactivation of hepatic genes, an adaptive response to liver injury

The insulin-like growth factor binding protein-1 (IGFBP-1) gene is highly expressed in fetal and regenerating liver. Upregulation is transcriptionally mediated in regenerating liver and occurs in the first few minutes to hours after partial hepatectomy. In transgenic mice a 970 bp region from −776 to +151 of the IGFBP-1 promoter was sufficient for tissue specific and induced expression of the gene in fetal and hepatectomized livers. However weak and/or poorly regulated expression in some transgenic lines suggested the existence of other regulatory regions. Novel tissue-specific sites that interacted with C/EBP and HNF3 transcription factors were identified in the −3100 region. A hepatectomy induced DNA binding complex containing the transcription factor USF1 was identified within the −100 to −300 region of the promoter. However, cytokine stimulation of hepatic IGFBP-1 production, in particular the stimulatory effect of interleukin-6 (IL-6), is a proposed mechanism to account for the disruption of the acute inverse relationship between insulin and IGFBP-1 levels reported in a few clinical conditions. Evidence for a biologic role of IL-6 in IGFBP-1 upregulation was demonstrated by increased expression of hepatic IGFBP-1 in IL-6 transgenic and following injection of IL-6 into non-fasting animals, and reduced expression in IL-6−/− livers posthepatectomy. In both hepatic and nonhepatic cells, IL-6 mediated IGFBP-1 promoter activation was via an intact hepatocyte nuclear factor (HNF)-1 site and was dependent on the presence of HNF-1 and induced factors STAT3 and AP-1 (c-Fos/c-Jun). HNF-1/c-Fos and HNF-1/STAT3 protein complexes were detected in mouse livers and in hepatic and nonhepatic cell lines overexpressing STAT3/c-Fos/HNF-1, and further confirmed in vitro, with recombinant proteins, and in vivo, during transient transfection. Direct physical interactions between HNF-1α/STAT3, HNF-1α/c-Fos, and STAT3/c-Fos were verified using bacterially expressed STAT3, c-Fos, and GST-HNF-1α. HNF-1/IL-6/STAT3/AP-1 mediated transactivation of hepatic gene expression is a general phenomenon after liver injury as verified using glucose-6 phosphatase and α-fibrinogen promoters. These results demonstrate that the two classes of transcription factors, growth induced (STAT3 and AP-1) and tissue specific (HNF-1), can interact as an adaptive response to liver injury to amplify expression of hepatic genes important for the homeostatic response during organ repair.