Interleukin-6 signaling via four transcription factors binding palindromic enhancers of different genes

Interleukin-6 (IL-6) released by macrophages induces IL-6 secretion in the human colon cancer HT-29 cell line

The level of interleukin-6 (IL-6) is correlated with prognosis and liver metastasis in colon cancer. However, the relationship between macrophage-derived and tumor-derived IL-6 in colon cancer remains unclear. We harvested the macrophage supernatant and studied the IL-6-inducing ability of the macrophage supernatant on the colon cancer cell line HT-29. The macrophage supernatant effectively induced IL-6 secretion of colon cancer cells in vitro. The macrophage supernatant and recombinant IL-6 neutralized with anti-IL-6 or ant-gp130 antibodies dramatically decreased the IL-6-induction ability of cancer cells. IL-6-induction occurred through phosphorylation of STAT3. We analyzed the surgical specimens of 126 patients with colon cancer using an immunohistochemical staining method and demonstrated the colocalization of macrophages and the expression of IL-6 in colon cancer patients. These results indicate that macrophages in tumor infiltrates could release IL-6, which in turn conditions colon cancer cells, causing them to secrete IL-6 themselves via phosphorylation of STAT3.

Macrophage-derived interleukin-6 up-regulates MUC1, but down-regulates MUC2 expression in the human colon cancer HT-29 cell line

Little is known regarding the effects of IL-6 released by tumor-infiltrating macrophages on the mucin expression of colon cancer cells. We isolated macrophages from healthy donors and harvested the supernatant after 48-h cultures. Using flow cytometry and intracellular staining methods, we found that macrophage supernatant effectively induced MUC1 up-regulation and MUC2 down-regulation of colon cancer cells in vitro. Western blotting analysis using monoclonal antibody against IL-6 or gp 130 verified that this IL-6-driven activity was through the activation of tyrosine phosphorylation (Tyr(705)) of STAT3 in cancer cells. We analyzed the surgical specimens of 29 patients with colon cancer by an immunohistochemical staining method and demonstrated the co-localization of macrophages, and the expression of IL-6, CD68, and MUC1 in colon cancer patients. Therefore, macrophage-derived IL-6 modulates the mucin expression of colorectal cancer cells that might in turn produce a permissive milieu favorable to cancer spread.

The comparative roles of suppressor of cytokine signaling-1 and -3 in the inhibition and desensitization of cytokine signaling

Negative feedback is a mechanism commonly employed in biological processes as a means of maintaining homeostasis. We have investigated the roles of suppressor of cytokine signaling (SOCS) proteins in regulating the kinetics of negative feedback in response to cytokine signaling. In mouse livers and bone marrow-derived macrophages, both interferon-gamma (IFNgamma) and interleukin-6 (IL-6) rapidly induced the tyrosine phosphorylation of signal transducer and activator of transcription-1 (STAT1) and STAT3. STAT3 tyrosine phosphorylation was bi-phasic in response to continuous IL-6 signaling. In macrophages lacking Socs3, however, continuous IL-6 signaling induced uniformly high levels of STAT3 tyrosine phosphorylation, and early IL-6-inducible genes were inappropriately expressed at intermediate time points. SOCS3 therefore imposes bi-phasic kinetics upon IL-6 signaling. Compared with Socs3 mRNA, Socs1 mRNA was induced relatively slowly, and SOCS1 simply attenuated the duration of IFNgamma signaling. Surprisingly, heightened Socs1 mRNA expression but minimal STAT1 tyrosine phosphorylation was observed after prolonged stimulation with IFNgamma, indicating that STAT1 may not play a large role in inducing Socs1 mRNA during steady-state IFNgamma signaling. We also demonstrate that both SOCS1 and SOCS3 can desensitize primary bone marrow-derived macrophages to IFNgamma and IL-6 signaling, respectively. Consistent with the kinetics with which Socs1 and Socs3 mRNAs were induced, SOCS3 desensitized cells to IL-6 rapidly, whereas SOCS1-mediated desensitization to IFNgamma occurred at later time points. The kinetics with which SOCS proteins are induced by cytokine may therefore be a parameter that is "hard-wired" into specific cytokine signaling pathways as a means of tailoring the kinetics with which cells become desensitized.

Colon lamina propria dendritic cells induce a proinflammatory cytokine response in lamina propria T cells in the SCID mouse model of colitis

Intestinal immune responses are normally regulated to maintain a state of immune balance. Dendritic cells (DC) are antigen-presenting cells, which induce immune responses against microbes and other stimuli and are key players in the regulation of tolerance in the gut. These cells influence the differentiation of cytokine responses in T cells, and in the gut, in particular, such interactions may be critical to the course of inflammatory bowel disease (IBD). Using the CD45RBhi CD4+ T cell-reconstituted severe combined immunodeficient mouse model of colitis, we investigated the ability of isolated colon DC to stimulate immune responses in syngeneic and allogeneic spleen CD4+ T cells, as well as in colon T cells isolated from the same tissue as DC in IBD mice. We found that the frequency of DC in IBD mice colons and spleens was elevated in comparison with control mice, but colon and spleen DC exhibited different phenotypic and functional properties. Colon DC stimulated significantly higher levels of interferon-gamma and interleukin-6 when cocultured with autologous colon T cells than in cocultures with syngeneic or allogeneic spleen T cells. These data suggest that in the IBD colon, DC-T cell interactions may create conditions with an abundance of proinflammatory cytokines, which favor the inflammatory state.

Recruitment of STAT3 for production of IL-10 by colon carcinoma cells induced by macrophage-derived IL-6

The immunosuppressive cytokine IL-10 is associated with poor prognosis in colon cancer. Although macrophages are involved in antitumor defenses, production of IL-10 by tumor cells may permit malignant cells escape to cell-mediated immune defenses. To investigate interactions between macrophages and tumor cells in humans, we cultured macrophages isolated from patients and tested the effect of these macrophages on the production of IL-10 by several tumor cell lines. Macrophages were isolated from pleural effusions of patients with malignancy and from noncancer control patients. We demonstrated that culture supernatants of macrophages from both sources strongly stimulated IL-10 production by the three different human colon adenocarcinoma cell lines, Colo 205, Colo 320, and HT29. Recombinant IL-6, but not IL-10, TNF-alpha, and IFN-alpha, stimulated the secretion of IL-10 by colon tumor cells. mAbs against IL-6 and IL-6R prevented the effect of macrophage culture supernatants and of rIL-6, respectively, on the production of IL-10 by the three cell lines. Cocultures of macrophages and colon cancer cells showed that these tumor cells first stimulated macrophages to produce IL-6, which was then followed by IL-6-induced IL-10 production by colon cancer cells. Finally, we showed that IL-10 gene regulation was mediated by STAT3, which was phosphorylated after the binding of IL-6 to IL-6R. This is the first demonstration that IL-6, secreted by macrophages, can induce a STAT3-mediated IL-10 production by colon tumor cells.

STAT3 exerts two-way regulation in the biological effects of IL-6 in M1 leukemia cells

The signal transducer and activator of transcription (STAT) proteins have been implicated in cytokine-regulated proliferation, differentiation and cell survival. Interleukin-6 (IL-6), a pleiotropic cytokine, induces a robust and sustained activation of STAT3 in M1 acute myeloid leukemia cells, which in turn undergo growth arrest, terminal differentiation and apoptosis in response to IL-6. The roles of STAT3 activation in IL-6-mediated responses in M1 cells are not fully understood. We introduced STAT3 antisense cDNA into M1 cells. STAT3 antisense cDNA blocked the expression and IL-6-induced tyrosine phosphorylation and DNA binding of STAT3, and resulted in reduction of both IL-6-induced growth arrest at G(0)/G(1) phase and macrophage differentiation in the M1 transformants. This observation is in accordance with previous reports and confirms that STAT3 plays an essential role in IL-6-induced growth arrest and terminal differentiation in M1 leukemia cells. On the other hand, STAT3 antisense cDNA augmented IL-6-induced apoptosis of M1 cells, which was supported by the cell cycle assay, DNA fragmentation assay and detection of the p17 active fragment of Caspase 3. As proliferation inhibition and differentiation induction stands for a negative signal, while survival maintenance stands for a positive signal, we conclude that STAT3 exerts two-way regulation on the biological effects of IL-6 in M1 leukemia cells.

Activation-induced inhibition of interleukin 6-mediated T cell survival and signal transducer and activator of transcription 1 signaling

The cytokines interleukin (IL)-2, IL-4, IL-6, IL-7, and IL-15 have all previously been shown to inhibit resting T cell death in vitro. We have found a difference in the response of T cells to IL-6, depending on the activation status of the cells. IL-6 inhibited the death of naive T cells, but had no effect on the death of either superantigen-activated T cells, or T cells bearing memory markers. This was true even when the resting and activated T cells were isolated from the same animal; thus, the determining factor for IL-6 insensitivity was the activation status or activation history of the cell, and not the milieu in the animal from which the cells were isolated. Activated T cells expressed lower levels of IL-6 receptors on their surfaces, yet there were sufficient levels of receptors for signaling, as we observed similar levels of signal transducer and activator of transcription (Stat)3 phosphorylation in resting and activated T cells treated with IL-6. However, there was profound inhibition of IL-6-induced Stat1 phosphorylation in activated T cells compared with resting T cells. These data suggest that there is activation-induced inhibition of IL-6 receptor signaling in T cells. This inhibition appears to be specific for some but not all of the IL-6-mediated signaling cascades in these cells.

Interferon- Activates Multiple STAT Proteins and Upregulates Proliferation-Associated IL-2R, c-myc, and pim-1 Genes in Human T Cells

Interferon- (IFN-) is a pleiotropic cytokine that has antiviral, antiproliferative, and immunoregulatory functions. There is increasing evidence that IFN- has an important role in T-cell biology. We have analyzed the expression ofIL-2R, c-myc, and pim-1 genes in anti-CD3–activated human T lymphocytes. The induction of these genes is associated with interleukin-2 (IL-2)–induced T-cell proliferation. Treatment of T lymphocytes with IFN-, IL-2, IL-12, and IL-15 upregulated IL-2R, c-myc, andpim-1 gene expression. IFN- also sensitized T cells to IL-2–induced proliferation, further suggesting that IFN- may be involved in the regulation of T-cell mitogenesis. When we analyzed the nature of STAT proteins capable of binding to IL-2R,pim-1, and IRF-1 GAS elements after cytokine stimulation, we observed IFN-–induced binding of STAT1, STAT3, and STAT4, but not STAT5 to all of these elements. Yet, IFN- was able to activate binding of STAT5 to the high-affinity IFP53 GAS site. IFN- enhanced tyrosine phosphorylation of STAT1, STAT3, STAT4, STAT5a, and STAT5b. IL-12 induced STAT4 and IL-2 and IL-15 induced STAT5 binding to the GAS elements. Taken together, our results suggest that IFN-, IL-2, IL-12, and IL-15 have overlapping activities on human T cells. These findings thus emphasize the importance of IFN- as a T-cell regulatory cytokine.

Highly conserved amino-acid sequence between murine STAT3 and a revised human STAT3 sequence

Signal transducer and activator of transcription 3 (STAT3) is an important mediator of cytokine signaling, whose cDNA and protein sequences have been fully characterized. We sequenced the whole human STAT3 cDNA isolated from HepG2 cells. The new sequence determined contains 43 nucleotide changes overall, corresponding to six modifications at the amino-acid level. The revised amino-acid sequence of human STAT3 is now completely identical to the mouse sequence, except for a single amino-acid change at position 760. Thus STAT3 now results as one of the most evolutionarily conserved among known proteins. By using specific RT-PCR we could discriminate between the original sequence and the new variant. Amplification of regions within the src-homology domain 2 (SH2) of STAT3, from the RNAs of 11 different tissues or cells, revealed only the expression of the new SH2 variant. Besides, only this SH2 variant was amplified from human genomic DNA. We conclude that the new sequence we have determined in this study represents a revised sequence of hSTAT3 or, less likely, a new predominant allele.

Cytokine and interferon research in Israel

From its inception, the field of interferons and cytokines has occupied an important position in Israeli biological science. With the Second Joint Meeting of the International Society for Interferon and Cytokine Research and the International Cytokine Society taking place in Jerusalem in 1998, it is timely to review here current Israeli research on the biology, gene regulation, receptors, signal transduction, mode of action and clinical aspects of interferons and cytokines.

Interleukin-6-specific activation of the C/EBPdelta gene in hepatocytes is mediated by Stat3 and Sp1

C/EBPdelta (CCAAT/enhancer binding protein delta) has been implicated as a regulator of acute-phase response (APR) genes in hepatocytes. Its expression increases dramatically in liver during the APR and can be induced in hepatic cell lines by interleukin-6 (IL-6), an acute-phase mediator that activates transcription of many APR genes. Here we have investigated the mechanism by which C/EBPdelta expression is regulated by IL-6 in hepatoma cells. C/EBPdelta promoter sequences to -125 bp are sufficient for IL-6 inducibility of a reporter gene and include an APR element (APRE) that is essential for IL-6 responsiveness. DNA binding experiments and transactivation assays demonstrate that Stat3, but not Stat1, interacts with this APRE. Two Sp1 sites, one of which is adjacent to the APRE, are required for IL-6 induction and transactivation by Stat3. Thus, Stat3 and Sp1 function cooperatively to activate the C/EBPdelta promoter. Replacement of the APRE with Stat binding elements (SBEs) from the ICAM-1 or C/EBPbeta promoter, both of which recognize both Stat1 and Stat3, confers responsiveness to gamma interferon, a cytokine that selectively activates Stat1. Sequence comparisons suggest that the distinct Stat binding specificities of the C/EBPdelta and C/EBPbeta SBEs are determined primarily by a single base pair difference. Our findings indicate that the cytokine specificity of C/EBPdelta gene expression is governed by the APRE sequence.

The competitive binding of STAT3 and NF-kappaB on an overlapping DNA binding site

Interleukin-1 (IL-1) and interleukin-6 (IL-6), two early-response cytokines expressed during an acute inflammatory reaction, regulate the expression of several acute phase proteins (APP) in the liver. IL-1 relays its signal to specific genes via NF-kappaB, whereas IL-6 sends its signal to the nucleus via STAT1alpha and STAT3. Interestingly, overlapping binding sites for STAT3 and NF-kappaB can be found on promoters of several APP genes. We show here that both STAT3 and NF-kappaB are active during inflammation and are capable of binding to a STAT3/NF-kappaB overlapping DNA motif derived from the alpha2-macroglobulin gene promoter. In vitro binding assays demonstrated that NF-kappaB competes with STAT3 binding on this probe. Our results suggest that these transcription factors regulate each others' function through competition for overlapping DNA binding sites.

Multiple prolactin (PRL) receptor cytoplasmic residues and Stat1 mediate PRL signaling to the interferon regulatory factor-1 promoter

The Nb2 PRL receptor (PRL-R) is known to mediate PRL signaling to the interferon (IFN) regulatory factor-1 (IRF-1) gene via the family of signal transducers and activators of transcription or Stats. To analyze the components of the PRL-R/Stat/IRF-1 signaling pathway, various PRL-R, Stat, and IRF-1-CAT reporter constructs were transiently cotransfected into COS cells. First, mutations in the IFNgamma-activated sequence (GAS), either multimerized or in the context of the 1.7-kb IRF-1 promoter, failed to mediate a PRL response, showing that the IRF-1 GAS is a target of PRL signaling. Next, pairwise alanine substitutions into conserved residues in the proline-rich motif or Box 1 region and two tyrosine mutations, Y308F and Y382F, in the PRL-R intracellular domain all impaired PRL signaling to multimerized GAS or to the 1.7-kb IRF-1 promoter. Furthermore, these PRL-R mutants mediated reduced Stat1 binding to the IRF-1 GAS. Transfection of Stat1 further enhanced PRL signaling to the IRF-1 promoter, suggesting that Stat1 is a positive mediator of PRL action. These studies show that both membrane proximal and distal residues of the PRL-R are involved in signaling to the IRF-1 gene. Further, Stat1 and the GAS element are important for PRL activation of the IRF-1 gene.

Pervanadate mimics IFNgamma-mediated induction of ICAM-1 expression via activation of STAT proteins

Differential expression of intercellular adhesion molecule-1 (ICAM-1) in the epidermis plays a critical role in the regulation of cutaneous inflammation, immunologic reactions, and tissue repair. Transcriptional upregulation of ICAM-1 in response to interferon-gamma (IFNgamma) occurs through a palindromic response element pIgammaRE. pIgammaRE is homologous to IFNgamma-activated sequences, which bind to tyrosine phosphorylated members of the transcription factor family known as signal transducers and activators of transcription (STAT). The importance of tyrosine phosphorylation events in the STAT pathway led us to investigate the effect of the protein tyrosine phosphatase inhibitor, pervanadate, on ICAM-1 expression. We show that treatment of A431 cells and human keratinocytes with pervanadate stimulates protein complex formation on pIgammaRE in a time- and concentration-dependent manner. As demonstrated by mobility supershift assays, the pervanadate-stimulated complex is similar to the IFNgamma-stimulated complex and contains Stat1. Pervanadate treatment also led to an increase in overall protein tyrosine phosphorylation and phosphorylation of Stat1, as well as the subsequent increase in ICAM-1 mRNA and cell surface protein levels. These data show that pervanadate can mimic each step in the IFNgamma-mediated pathway leading to ICAM-1 expression, demonstrate the ability of a pharmacologic agent to bypass the standard cytokine-receptor interaction required for increased ICAM-1 expression, and emphasize the importance of protein tyrosine phosphatases and protein tyrosine kinases in mediating inflammatory responses in the skin.

GAS elements: a few nucleotides with a major impact on cytokine-induced gene expression

Gamma interferon activation site (GAS) elements are short stretches of DNA, originally defined as a requirement for the rapid transcriptional induction of genes in response to interferon-gamma (IFN-gamma). The protein complex binding to GAS sequences in IFN-gamma-treated cells, the gamma interferon activation factor (GAF), is a dimer of Stat1, the prototype of a family of cytokine-responsive transcription factors, the signal transducers and activators of transcription. To date, seven different Stats are known (excluding alternatively spliced or processed forms), six of which recognize the same small palindromic consensus sequence TTCN2-4 GAA that defines a GAS element. Because one or several Stats take part in nuclear signaling in response to most cytokines or growth factors, the GAS sequence has changed from being viewed as a specific site for IFN-activated GAF to becoming the general nuclear end of the Jak-Stat signaling pathways. This review focuses on the identification and definition of GAS elements, their interaction with Stat transcription factors, and their contribution to the specificity of cytokine-induced gene expression.

The interferon (IFN)-stimulated gene Sp100 promoter contains an IFN-gamma activation site and an imperfect IFN-stimulated response element which mediate type I IFN inducibility

Expression of the nuclear domain-associated proteins Sp100, PML, and NDP52, is enhanced by interferons (IFNs) on the mRNA and protein level. Increase both of Sp100 and PML mRNA is due to enhanced transcription of the corresponding genes which occurs independently of cellular protein synthesis immediately upon IFN-beta addition. Here, we describe the molecular cloning and functional analysis of the Sp100 promoter. DNA sequence analysis revealed potential binding sites for several constitutive and IFN-inducible transcription factors. Consistent with the absence of a TATA box and an initiator element, several transcription initiation sites were found. Transient expression studies identified an imperfect IFN-stimulated response element within the first 100 nucleotides upstream of the major transcription start site. This element rendered a heterologous promoter IFN-beta-inducible and bound IFN-stimulated gene factor 2 strongly but IFN-stimulated gene factor 3 only weakly. An IFN-gamma activation site approximately 500 base pairs upstream of the IFN-stimulated response element was found to bind three IFN-alpha/beta activation factors upon IFN-beta induction and conferred both type I and type II IFN inducibility upon a heterologous promoter. These data demonstrate a novel arrangement of a nonoverlapping IFN-gamma activation site and an IFN-stimulated response element mediating type I IFN inducibility, previously not reported for other IFN-stimulable promoters.

Immunoregulatory effects of interferon-beta and interacting cytokines on human vascular endothelial cells. Implications for multiple sclerosis autoimmune diseases

The mechanism(s) of action responsible for the anti-inflammatory effects mediated by interferon (IFN)-beta are still elusive although suggestions include anti-viral effects, the enhancement of natural killer (NK) or suppressor T cell activity and opposition to the effects of inflammatory cytokines. As vascular endothelial cells are active participants in inflammatory and demyelinating processes, we decided to examine the effects of IFN-beta on the expression of major histocompatibility complex (MHC) gene products and intercellular adhesion molecule (ICAM)-1 on human vascular endothelial cells (ECs). Human umbilical ECs demonstrated constitutive expression of ICAM-1 and MHC class I molecules but did not express MHC class II molecules. Basal expression of ICAM-1 molecules was enhanced by TNF alpha and to a lesser extent by IFN-beta, but was not affected by IFN-gamma. MHC class I expression on ECs was enhanced by IFN-beta, IFN-gamma, and tumor necrosis factor (TNF)-alpha. Furthermore, a synergistic effect was observed to combinations of these interacting cytokines. Incubation of ECs with IFN-gamma, but not IFN-beta, induced class II expression in a dose dependent manner. Moreover, co-incubation of ECs with IFN-beta and IFN-gamma resulted in significant down-regulation of class II molecules expression which was directly dependent on IFN-beta concentration. Northern blot analysis of DR alpha and Beta 2-microglobulin mRNA expression suggested that cytokine-mediated regulation of MHC molecules is at the transcriptional level, while modulation of ICAM-1 expression appears to be at the transcriptional as well as post-transcriptional level. Thus, our study demonstrated that IFN-beta and interacting cytokines exert complex immunoregulatory effects on endothelial cells with differential modulatory effects on various cell surface markers. Understanding the biological significance of these immunomodulatory effects mediated by IFN-beta may have important implications for cytokine-based strategies in the treatment of inflammatory and autoimmune diseases.

Activation of the STAT3/acute phase response factor transcription factor by interleukin-5

The receptor for interleukin-5 (IL-5R) is composed of a unique alpha chain (IL-5R alpha) expressed on eosinophils and basophils, associated with a beta c subunit, which is shared by the receptors for IL-3 and granulocyte macrophage-colony stimulating factor. One of the molecular events activated via the IL-5R is the JAK/STAT signaling pathway. Recent reports have shown that IL-5 induces tyrosine phosphorylation of JAK2 followed by the subsequent cell type-specific activation of either STAT1 alpha or STAT5. To identify additional STAT proteins activated by IL-5, we co-transfected the IL-5R with STAT cDNAs in COS cells. We found that IL-5 induces binding of STAT3 to the intercellular adhesion molecule-1 pIRE, and activates STAT3-dependent transcription. Moreover, endogenous STAT3 was tyrosine phosphorylated and activated in human IL-5-stimulated BaF3 cells ectopically expressing the human IL-5R (BaF3/IL5R). These data imply that multiple STAT proteins are involved in gene regulation by IL-5 in a cell type-specific manner. We further demonstrate using C-terminal truncations of the alpha and beta c subunits of the IL-5R that the membrane-proximal STAT activation. Interestingly, a beta c receptor mutant lacking intracellular tyrosine residues is able to mediate STAT3 activation, suggesting that tyrosine phosphorylation of the beta c receptor is not essential for STAT3 activation.

Characterization of the IL-6 responsive elements in the gamma fibrinogen gene promoter

Fibrinogen, a hepatically derived class II acute phase protein, is the product of three separate genes, (A alpha, B beta, and gamma). The fibrinogen genes are expressed constitutively; however, their transcription can be significantly up-regulated by interleukin-6 (IL-6) and glucocorticoid. Inspection of the promoter region of the fibrinogen gamma gene revealed three hexanucleotide clusters of CTGGGA that are recognized as class II IL-6 responsive elements. Functional analyses of these regions (designated here as site I, site II, and site III according to their position in the promoter) were performed using luciferase reporter constructs and show a hierarchy of IL-6 response in which site II was the preferred functional site, site I was the next important site, and site III was the site least responsive to IL-6. Gel mobility shift assays using 25-base pair oligonucleotide probes derived from these three regions with the CTGGGA positioned in the middle and nuclear extracts from IL-6-treated primary hepatocytes reveal the presence of IL-6-induced high molecular weight complexes appearing 5 min after cytokine treatment. Supershift assays using anti-Stat3 antibody indicate that Stat3 is part of the IL-6-induced complex formed on the three gamma chain probes. The binding of Stat3 to the IL-6 responsive elements of the gamma probes is significantly weaker than to an alpha 2-macroglobulin probe. These findings show for the first time that Stat3 is involved in associating with the IL-6 responsive elements of fibrinogen gamma chain, a class II acute phase gene other than alpha 2-macroglobulin.

Separate signaling mechanisms are involved in the control of STAT protein activation and gene regulation via the interleukin 6 response element by the box 3 motif of gp130

The cytoplasmic receptor sequences required for the transcriptional control via the IL-6 response element (IL-6RE) and the hematopoietin receptor response element (HRRE) in hepatoma cells were defined by transient expression of wild-type and mutant granulocyte-colony stimulating factor receptor-gp130 chimeric receptors. gp130 generated two separate transcriptional signals, one of which was directed to IL-6RE and required an intact box 3 motif, and another, which was directed to HRRE and was box 3-independent. The activation of DNA-binding of STAT3 required the same gp130 domains as the IL-6RE response. A box 3-independent activation of STAT proteins was achieved by overexpression of the kinases JAK2 or TYK2. The increase in the DNA-binding activity of STAT proteins, however, did not result in a corresponding increase in transcription via either IL-6RE or HRRE. The data indicate that activation of the DNA-binding potential of STAT proteins via gp130 is not sufficient to achieve transcriptional up-regulation of specific target genes.

Detection of a novel transcription factor for the A alpha fibrinogen gene in response to interleukin-6

The three fibrinogen genes belong to the class II hepatic acute phase proteins that are regulated in part by members of the interleukin-6 (IL-6) family of cytokines and glucocorticoids. The common DNA sequence that characterizes this group of proteins is a hexanucleotide CTGGGA residing in the promoter regions of these genes. Investigations of IL-6 control of the A alpha fibrinogen gene by electrophoretic mobility shift assays using a 30-base pair DNA probe containing the CTGGGA element revealed that a novel protein is associated with this site during non-IL-6-stimulated conditions. Sensitive time-course studies of IL-6 stimulation using primary hepatocyte cultures, high resolution polyacrylamide gel electrophoresis, and site-directed mutagenesis show that upon IL-6 stimulation of hepatocytes, this DNA binding protein transiently leaves the CTGGGA site and binds 12 base pairs down-stream but then begins to re-associate with the original DNA site at 1 h and is completed by 2 h. A recently characterized and cloned IL-6-activated transcription factor, Stat-3, which has been reported to bind a CT-GGGAA site in the alpha-2 macroglobulin gene, another member of the class II acute phase proteins, does not bind to the CTGGGA sequence in the A alpha fibrinogen gene. These findings reveal the presence of a previously undefined IL-6-regulated event, which involves a new DNA binding protein and demonstrates for the first time additional details of the kinetics of IL-6 control of fibrinogen gene expression.