No abstract

Interleukin-6 (IL-6), leukemia inhibitory factor, oncostatin M, interleukin-11, and ciliary neurotrophic factor bind to receptor complexes that share the signal transducer gp130. Upon binding, the ligands rapidly activate DNA binding of acute-phase response factor (APRF), a protein antigenically related to the p91 subunit of the interferon-stimulated gene factor-3 alpha (ISGF-3 alpha). These cytokines caused tyrosine phosphorylation of APRF and ISGF-3 alpha p91. Protein kinases of the Jak family were also rapidly tyrosine phosphorylated, and both APRF and Jak1 associated with gp130. These data indicate that Jak family protein kinases may participate in IL-6 signaling and that APRF may be activated in a complex with gp130.

Interleukin-6 (IL-6) is known to be a major mediator of the acute-phase response in liver. We show here that IL-6 triggers the rapid activation of a nuclear factor, termed acute-phase response factor (APRF), both in rat liver in vivo and in human hepatoma (HepG2) cells in vitro. APRF bound to IL-6 response elements in the 5'-flanking regions of various acute-phase protein genes (e.g., the %-macroglobulin, fibrinogen, and al-acid glycoprotein genes). These elements contain a characteristic hexanucleotide motif, CTGGGA, known to be required for the IL-6 responsiveness of these genes. Analysis of the binding specificity of APRF revealed that it is different from NF-IL6 and NF-KB, transcription factors known to be regulated by cytokines and involved in the transcriptional regulation of acute-phase protein genes. In HepG2 cells, activation of APRF was observed within minutes after stimulation with IL-6 or leukemia-inhibitory factor and did not require ongoing protein synthesis. Therefore, a preexisting inactive form of APRF is activated by a posttranslational mechanism. We present evidence that this activation occurs in the cytoplasm and that a phosphorylation is involved. These results lead to the conclusions that APRF is an immediate target of the IL-6 signalling cascade and is likely to play a central role in the transcriptional regulation of many IL-6-induced genes.During an acute inflammation, cytokines released by different cell types, including monocytes, fibroblasts, and endothelial cells, stimulate the synthesis and secretion of a set of plasma proteins, the so-called acute-phase proteins, by the liver. These proteins play a protective role during the acute-phase reaction, e.g., by inactivating proteases, supporting the wound-healing process, or scavenging free oxygen radicals (for a review, see reference 26). According to their regulation by different cytokines, acute-phase proteins have been divided into two subclasses (8). The synthesis of class 1 acute-phase proteins (e.g., a1-acid glycoprotein, C-reactive protein, haptoglobin, and serum amyloid A) is induced by interleukin-1 (IL-1) or combinations of IL-1 and IL-6, whereas the genes for class 2 acute-phase proteins (e.g., a2-macroglobulin, cxl-antichymotrypsin, and fibrinogen) are regulated mainly by IL-6 and glucocorticoids. The 5'-flanking regions of many acute-phase protein genes have been studied in detail, with the goal of identifying regulatory elements required for the cytokine induction of these genes. One type of cytokine response elements found in the promoters of several class 1 acute-phase protein genes represents binding sites for members of the C/EBP family of transcription factors (15,21,49,51,62 (36). In addition, NF-iB or NF-KB-like factors were found to be involved in the regulation of the angiotensinogen, serum amyloid A, and complement factor B genes by 48,54).Less is known about transcription factors regulating the promoters of class 2 acute-phase protein genes. On the basis of a sequence comparison of the 5'-flanking regions of the...

The protein tyrosine kinases JAK1, JAK2 and Tyk2 and STATs (signal transducers and activators of transcription) 1 and 3 are activated in response to interleukin‐6 (IL‐6) in human fibrosarcoma cells. In mutant cells lacking JAK1, JAK2 or Tyk2, the absence of one kinase does not prevent activation of the others; activation does not, therefore, involve a sequential three‐kinase cascade. In the absence of JAK1, the phosphorylation of the gp130 subunit of the IL‐6 receptor and the activation of STATs 1 and 3 are greatly reduced. JAK1 is also necessary for the induction of IRF1 mRNA, thus establishing a requirement for the JAK/STAT pathway in the IL‐6 response. JAK2 and Tyk2 although activated cannot, in the absence of JAK1, efficiently mediate activation of STATs 1 and 3. A kinase‐negative mutant of JAK2 can, however, inhibit such activation, and ancillary roles for JAK2 and Tyk2 are not excluded. A major role for JAK1 and the nonequivalence of JAK1 and JAK2 in the IL‐6 response pathway are, nevertheless, clearly established for these cells.