Geranylgeranylacetone induces antiviral gene expression in human hepatoma cells

Geranylgeranylacetone (GGA), an isoprenoid compound, is used clinically as an anti-ulcer drug. Since some isoprenoids including retinoids have anti-tumor and anti-viral activities in a variety of cell types, we investigated whether GGA could induce anti-viral proteins in human hepatoma cells. The HuH-7 and HepG2 cells were treated with GGA, and expression of anti-viral proteins such as 2'5'-oligoadenylate synthetase (2'5'-OAS) and double-stranded RNA-dependent protein kinase (PKR) in these cells was analyzed. GGA stimulated 2'5'-OAS and PKR gene expression at the transcriptional level through the formation of interferon-stimulated gene factor 3 (ISGF3), which regulates both gene transcription. By Western blotting, GGA induced expression of signal transducers and activators of transcription 1, 2 (STAT1, STAT2) and p48 proteins, components of ISGF3, together with the phosphorylation of STAT1. These results suggest that GGA acts as a potent inducer of anti-viral gene expression by stimulating the ISGF3 formation in human hepatoma cells.

Expression of the transcription factor STAT-1 alpha in insulinoma cells protects against cytotoxic effects of multiple cytokines

Destruction of pancreatic islet beta-cells in type 1 diabetes appears to result from direct contact with infiltrating T-cells and macrophages and exposure to inflammatory cytokines such as interferon (IFN)-gamma, interleukin (IL)-1 beta, and tumor necrosis factor TNF-alpha that such cells produce. We recently reported on a method for selection of insulinoma cells that are resistant to the cytotoxic effects of inflammatory cytokines (INS-1(res)), involving their growth in progressively increasing concentrations of IL-1 beta plus IFN-gamma, and selection of surviving cells. In the current study, we have investigated the molecular mechanism of cytokine resistance in INS-1(res) cells. By focusing on the known components of the IFN-gamma receptor signaling pathway, we have discovered that expression levels of signal transducer and activator of transcription (STAT)-1 alpha are closely correlated with the cytokine-resistant and -sensitive phenotypes. That STAT-1 alpha is directly involved in development of cytokine resistance is demonstrated by an increase of viability from 10 +/- 2% in control cells to 50 +/- 6% in cells with adenovirus-mediated overexpression of STAT-1 alpha (p < 0.001) after culture of both cell groups in the presence of 100 units/ml IFN-gamma plus 10 ng/ml IL-1 beta for 48 h. The resistance to IL-1 beta plus IFN-gamma in STAT-1 alpha-expressing cells is due in part to interference with IL-1 beta-mediated stimulation of inducible nitric-oxide synthase expression and nitric oxide production. Furthermore, overexpression of STAT-1 alpha does not impair robust glucose-stimulated insulin secretion in the INS-1-derived cell line 832/13. We conclude that expression of STAT-1 alpha may be a means of protecting insulin-producing cell lines from cytokine damage, which, in conjunction with appropriate cell-impermeant macroencapsulation devices, may allow such cells to be used for insulin replacement in type 1 diabetes.

Interferon-tau activates multiple signal transducer and activator of transcription proteins and has complex effects on interferon-responsive gene transcription in ovine endometrial epithelial cells

Interferon-tau (IFNtau), a type I IFN produced by sheep conceptus trophectoderm, is the signal for maternal recognition of pregnancy. Although it is clear that IFNtau suppresses transcription of the estrogen receptor alpha and oxytocin receptor genes and induces expression of various IFN-stimulated genes within the endometrial epithelium, little is known of the signal transduction pathway activated by the hormone. This study determined the effects of IFNtau on signal transducer and activator of transcription (STAT) activation, expression, DNA binding, and transcriptional activation using an ovine endometrial epithelial cell line. IFNtau induced persistent tyrosine phosphorylation and nuclear translocation of STAT1 and -2 (10 min to 48 h), but transient phosphorylation and nuclear translocation of STAT3, -5a/b, and -6 (10 to <60 min). IFNtau increased expression of STAT1 and -2, but not STAT3, -5a/b, and -6. IFN-stimulated gene factor-3 and STAT1 homodimers formed and bound an IFN-stimulated response element (ISRE) and gamma-activated sequence (GAS) element, respectively. IFNtau increased transcription of GAS-driven promoters at 3 h, but suppressed their activity at 24 h. In contrast, the activity of an ISRE-driven promoter was increased at 3 and 24 h. These results indicate that IFNtau activates multiple STATs and has differential effects on ISRE- and GAS-driven gene transcription.

The role of STAT1 in activation of IL-3- and IL-5-induced eosinophils by interferon gamma

Tyrosine phosphorylation of STAT1alpha in eosinophils after IFN-gamma stimulation has been shown, but the biological significance of eosinophil STAT1alpha activation in transmitting the signals through the IFN-gamma receptor remains unknown. The purpose of this study is to determine whether STAT1 is involved in the regulation of eosinophils by IFN-gamma-IFN-gamma receptor interaction. rhIL-3- and rhIL-5-induced eosinophils from CD34+ cells of cord blood on day 28 of culture were used. The cells were washed and further incubated in IL-3- and IL-5-free medium for 48 h. The induced eosinophils constitutively expressed CD69 and lost this expression after a further 48-hour incubation without the cytokines. IFN-gamma significantly upregulated CD69 expression on the 48-hour incubated cells. In inhibitory experiments on STAT1, a phosphorothioate oligo antisense DNA against STAT1alpha was added to IL-3- and IL-5-containing medium from day 15 to day 28 of culture. The oligo DNAs altered neither the expressions of myeloid cell marker CD9 and 13 nor the expression of IFN-gamma receptor on the cells. The added STAT1alpha antisense, but not sense, DNA significantly reduced STAT1alpha mRNA expression in the cells. The STAT1 antisense also significantly inhibited IFN-gamma-induced CD69 expression on the 48-hour incubated eosinophils. In conclusion, these results indicate that IFN-gamma induces CD69 expression in the induced eosinophils through STAT1alpha, suggesting that STAT1alpha may play a significant role in eosinophil regulation by IFN-gamma.

Immune response in Stat2 knockout mice

Type I IFNs induce gene expression through Stat1 and Stat2, which can in turn associate either to form Stat1 homodimers or the transcription factor ISGF-3. Stat1 homodimers also transduce signals for IFN-gamma. To explore the unique properties of Stat2 and ISGF-3 in type I IFN signaling, its gene was targeted for deletion. Stat2 null mice exhibit a number of defects in immune response. This includes an increased susceptibility to viral infection and the loss of a type I IFN autocrine/ paracrine loop, which in turn regulates several aspects of immune response. Intriguingly, Stat2-deficient fibroblasts exhibit a more significant defect in their response to type I IFNs than macrophages, highlighting tissue-specific differences in the response to this family of ligands.

The neutralization of type I IFN biologic actions by anti-IFNAR-2 monoclonal antibodies is not entirely due to inhibition of Jak-Stat tyrosine phosphorylation

A panel of monoclonal antibodies (mAb) derived against human interferon-alpha/beta receptor-2 (IFNAR-2) was evaluated for their ability to antagonize the biologic effects of type 1 interferons (IFN-alpha1, IFN-alpha2a, and IFN-beta). Anti-IFNAR-2 mAb 117.7, 35.9, 53.2, and 51.44 neutralized type I IFN-mediated antiviral, antiproliferative, and major histocompatibility complex (MHC) class I upregulation functions. However, only mAb 51.44 neutralized IFN-alpha2a and IFN-beta-mediated natural killer (NK) cell cytotoxicity. In BIAcore and cell binding studies, only mAb 51.44 and 234.28 inhibited IFN-alpha2a and IFN-beta binding to its receptor. The receptor blockade by mAb 51.44 and 234.28 resulted in the inhibition of IFN-alpha2a and IFN-beta-induced tyrosine phosphorylation of Jak1, Tyk2, Stat1/2/3, and IFNAR-1/2 and inhibition of IFN-stimulated gene factor 3 (ISGF3) formation. mAb 117.7, 35.9, and 53.2, although antagonists of IFN's biologic activities, did not block the binding of IFN-alpha/beta to its receptor. The 117.7 mAb, representative of this class of receptor nonblocking mAb, induced hyper-tyrosine phosphorylation of IFNAR-2 in the presence of IFN-alpha/beta but did not inhibit IFN-alpha/beta-induced Jak-Stat tyrosine phosphorylation and ISGF3 complex formation. These results show that the neutralization of type I IFN biologic actions by anti-IFNAR-2 mAb cannot be entirely explained by inhibition of Jak-Stat tyrosine phosphorylation.

Regulation of the promoter activity of interferon regulatory factor-7 gene. Activation by interferon snd silencing by hypermethylation

The molecular mechanism by which virus induces expression of the early inflammatory genes has not yet been completely elucidated. Previous studies indicated that the virus-mediated transcription of type I interferon (IFN) genes required activation of two members of IFN regulatory factor (IRF) family, IRF-3 and IRF-7, where the expression of IRF-7 was found to be indispensable for the induction of IFNA genes. To determine the factors that regulate expression of IRF-7 gene, as well as its inducibility by type I IFNs, we have isolated and characterized the promoter and first intron of the human IRF-7 gene. This region shows a presence of two potential interferon-sensitive response elements (ISRE/IRF-E). However, only the ISRE present in the first intron was functional and conferred interferon inducibility in a transient transfection assay. Using a pull-down assay with an oligodeoxynucleotide corresponding to this ISRE immobilized to magnetic beads, we have demonstrated that this ISRE binds ISGF3 complex and IRF-1 from the extract of IFN-treated cells but not from the untreated cells. We have further shown that the previously observed lack of expression of IRF-7 in 2fTGH fibrosarcoma cell line, correlated with hypermethylation of the CpG island in the human IRF-7 promoter. The repression of the promoter activity was relieved by treatment with DNA methyltransferase inhibitor 5-aza-deoxycytidine. In vitro methylation of IRF-7 promoter silenced IRF-7 directed expression of luciferase gene in HeLa cells that express endogenous IRF-7 gene. Whether silencing of IRF-7 by methylation is instrumental for the process of tumorigenesis remains to be determined.

Expression of protein mediators of type I interferon signaling in human squamous cell carcinoma of the skin

IFN-based therapy has been shown to be active in the treatment of squamous cell carcinoma (SCC) of the skin and has promise for chemoprevention and treatment of several other cancers. In an effort to better understand the molecular mechanism of this activity, we have determined the expression pattern of several of the protein mediators of type I IFN signaling by immunohistochemistry in cutaneous SCC, SCC metastases, and adjacent nonmalignant epithelium from patient biopsies. All of the proteins, signal transducer and activator of transcription (STAT) 1alpha/beta, STAT2, p48, STAT3a, and STAT3beta, are expressed at varying levels in the adjacent epidermis, as well as in other epidermal and dermal cell types. For the majority of samples tested, the expression of one or more of these proteins was reduced in SCC primary tumors compared with the adjacent nonmalignant epithelial cells, as determined by manual scoring. Quantitative densitometry of several samples revealed differences that are statistically significant. Our study provides the first direct evidence for the expression of the IFN-stimulated gene factor 3 (STAT1alpha/beta, STAT2, and p48) and STAT3alpha and STAT3beta mediators of IFN-alpha/beta signaling in human skin and skin-derived SCCs. These data have led to the hypothesis that the loss of IFN sensitivity may contribute to the development and progression of skin SCC.

Differentiation of monocytes to macrophages switches the Mycobacterium tuberculosis effect on HIV-1 replication from stimulation to inhibition: modulation of interferon response and CCAAT/enhancer binding protein beta expression

HIV-1 replication is inhibited in uninflamed lung macrophages and is stimulated during tuberculosis. Attempts to recapitulate activation of HIV-1 replication in primary monocytes and macrophages ex vivo and in the untreated and PMA-treated THP-1 cell line model in vitro have produced opposite results depending on the state of differentiation of the cells. After infection with Mycobacterium tuberculosis, monocytes enhanced HIV-1 replication and produced a stimulatory 37-kDa CCAAT/enhancer binding protein beta (C/EBPbeta) transcription factor, whereas macrophages suppressed HIV-1 replication and produced an inhibitory 16-kDa C/EBPbeta transcription factor. IFN-beta induced inhibitory 16-kDa C/EBPbeta in macrophages, but had no effect on C/EBPbeta expression in monocytes. Macrophages, but not monocytes, were able to activate IFN-stimulated gene factor-3 (ISGF-3), a transcription factor composed of STAT-1, STAT-2, and IFN regulatory factor (IRF)-9, after infection with M. tuberculosis or stimulation with type I IFN. Macrophages expressed IRF-9 DNA-binding activity, but monocytes did not, and addition of the IRF-9 component reconstituted ISGF-3 in extracts of IFN-treated monocytes. Modulation of IFN responsiveness upon differentiation occurred at least in part through a post-transcriptionally regulated increase in IRF-9 expression. Both monocytes and macrophages maintained IFN responsiveness, activating STAT-1 homodimer formation and transcription of the STAT-1 gene after IFN stimulation. In addition, both monocytes and macrophages were able to activate NF-kappaB upon infection with M. tuberculosis. These results show that induction of ISGF-3, expression of the inhibitory 16-kDa C/EBPbeta, and suppression of HIV-1 replication via a transcriptional mechanism are macrophage-specific responses to infection with M. tuberculosis.

Role of the intracellular domain of the human type I interferon receptor 2 chain (IFNAR2c) in interferon signaling. Expression of IFNAR2c truncation mutants in U5A cells

A human cell line (U5A) lacking the type I interferon (IFN) receptor chain 2 (IFNAR2c) was used to determine the role of the IFNAR2c cytoplasmic domain in regulating IFN-dependent STAT activation, interferon-stimulated gene factor 3 (ISGF3) and c-sis-inducible factor (SIF) complex formation, gene expression, and antiproliferative effects. A panel of U5A cells expressing truncation mutants of IFNAR2c on their cell surface were generated for study. Janus kinase (JAK) activation was detected in all mutant cell lines; however, STAT1 and STAT2 activation was observed only in U5A cells expressing full-length IFNAR2c and IFNAR2c truncated at residue 462 (R2.462). IFNAR2c mutants truncated at residues 417 (R2. 417) and 346 (R2.346) or IFNAR2c mutant lacking tyrosine residues in its cytoplasmic domain (R2.Y-F) render the receptor inactive. A similar pattern was observed for IFN-inducible STAT activation, STAT complex formation, and STAT-DNA binding. Consistent with these data, IFN-inducible gene expression was ablated in U5A, R2.Y-F, R2.417, and R2.346 cell lines. The implications are that tyrosine phosphorylation and the 462-417 region of IFNAR2c are independently obligatory for receptor activation. In addition, the distal 53 amino acids of the intracellular domain of IFNAR2c are not required for IFN-receptor mediated STAT activation, ISFG3 or SIF complex formation, induction of gene expression, and inhibition of thymidine incorporation. These data demonstrate for the first time that both tyrosine phosphorylation and a specific domain of IFNAR2c are required in human cells for IFN-dependent coupling of JAK activation to STAT phosphorylation, gene induction, and antiproliferative effects. In addition, human and murine cells appear to require different regions of the cytoplasmic domain of IFNAR2c for regulation of IFN responses.

The COOH-terminal nuclear localization sequence of interferon gamma regulates STAT1 alpha nuclear translocation at an intracellular site

We have recently shown that the nuclear localization of IFN gamma is mediated by a polybasic nuclear localization sequence (NLS) in its C terminus. This NLS is required for the full expression of biological activity of IFN gamma, both extracellularly and intracellularly. We now show that this NLS plays an integral intracellular role in the nuclear translocation of the transcription factor STAT1 alpha activated by IFN gamma. Treatment of IFN gamma with antibodies to the C-terminal region (95–133) containing the NLS blocked the induction of STAT1 alpha nuclear translocation. The antibodies had no effect on nuclear translocation of STAT1 alpha in IFN gamma treated cells. A deletion mutant of human IFN gamma, IFN gamma (1–123), which is devoid of the C-terminal NLS region was found to be biologically inactive, but was still able to bind to the IFN gamma receptor complex on cells with a K(d) similar to that of the wild-type protein. Deletion of the NLS specifically abolished the ability of IFN gamma(1–123) to initiate the nuclear translocation of STAT1 alpha, which is required for the biological activities of IFN gamma following binding to the IFN gamma receptor complex. Thus, the NLS region appears to contribute minimally to extracellular high-affinity receptor-ligand binding, yet exerts a strong functional role in STAT1 alpha nuclear localization. A high-affinity site for the interaction of the C-terminal NLS domain of IFN gamma with a K(d) approx. 3 × 10(−8) M(−1) has been described by previous studies on the intracellular cytoplasmic domain of the IFN gamma receptor alpha-chain. To examine the role of the NLS at the intracellular level, we microinjected neutralizing antibodies raised against the C-terminal NLS domain of IFN gamma into the cytoplasm of cells before treatment of cells with IFN gamma. These intracellular antibodies specifically blocked the nuclear translocation of STAT1 alpha following the subsequent treatment of these cells extracellularly with IFN gamma. These data show that the NLS domain of IFN gamma interacts at an intracellular site to regulate STAT1 alpha nuclear import. A C-terminal peptide of murine IFN gamma, IFN gamma(95–133), that contains the NLS motif, induced nuclear translocation of STAT1 alpha when taken up intracellularly by a murine macrophage cell line. Deletion of the NLS motif specifically abrogated the ability of this intracellular peptide to cause STAT1 alpha nuclear translocation. In cells activated with IFN gamma, IFN gamma was found to as part of a complex that contained STAT1 alpha and the importin-alpha analog Npi-1, which mediates STAT1 alpha nuclear import. The tyrosine phosphorylation of STAT1 alpha, the formation of the complex IFN gamma/Npi-1/STAT1 alpha complex and the subsequent nuclear translocation of STAT1 alpha were all found to be dependent on the presence of the IFN gamma NLS. Thus, the NLS of IFN gamma functions intracellularly to directly regulate the activation and ultimate nuclear translocation STAT1 alpha.

Increased effect of interferon gamma on PDGF-induced c-fos gene transcription in glomerular mesangial cells: differential effect of the transcriptional coactivator CBP on STAT1alpha activation

We have previously shown that interferon gamma (IFNgamma) synergistically increases PDGF-induced DNA synthesis in mesangial cells. To examine the mechanism, we studied its effect on PDGF-induced c-fos gene transcription using a reporter mesangial cell in which firefly luciferase gene is driven by c-fos promoter. IFNgamma significantly enhanced PDGF-induced c-fos transcription. We have shown previously that PDGF-induced c-fos transcription in mesangial cells is mediated by the ternary complex factor Elk-1. Using a GAL-4 DNA binding-domain-Elk-1 transactivation domain fusion protein-based reporter assay we showed that the increased effect of IFNgamma was not mediated by Elk-1 transactivation. Gel mobility shift assay of lysates of mesangial cells treated with a combination of IFNgamma and PDGF using sis-inducible DNA element (SIE) showed increased STAT1alpha-SIE complex formation as compared to the PDGF alone. To investigate the transcriptional consequences of this observation, stable reporter mesangial cells in which luciferase gene is driven by four copies of SIE was used. IFNgamma and PDGF in combination significantly increased SIE-dependent transcription as compared to PDGF or IFNgamma alone. Using an antibody in the gel mobility shift assay we showed that the PDGF-induced SIE-STAT1alpha complex recruited the transcriptional coactivator CBP. However, the STAT1alpha-SIE complex formed in the presence of IFNgamma and PDGF did not contain CBP. Taken together, our data provide the first evidence that the synergistic effect of IFNgamma on PDGF-induced DNA synthesis may be the result of increased c-fos gene transcription via SIE. This effect occurs in the presence of increased activation of STAT1alpha without recruitment of the transcriptional coactivator CBP.

IFN-beta induces serine phosphorylation of Stat-1 in Ewing's sarcoma cells and mediates apoptosis via induction of IRF-1 and activation of caspase-7

Four human cell lines derived from Ewing's sarcoma, EW-7, EW-1, COH and ORS, were investigated to establish the effects of human recombinant interferon-alpha2a and human recombinant interferon-beta on cell proliferation and apoptosis. All four cell lines were much more sensitive to the antiproliferative effects of IFN-beta than of IFN-alpha. Analysis of the early signals triggered by IFN-alpha and IFN-beta demonstrated that the two IFNs were similarly effective in inducing tyrosine phosphorylation of the Jak-1 and Tyk-2 kinases and the transcription factors Stat-1 and Stat-2. Interestingly, an additional rapid phosphorylation of Stat-1 on serine was observed after IFN-beta treatment, with concomitant activation of p38 mitogen-activated protein kinase. In these cells, Stat-1 Ser727 phosphorylation in response to IFN-beta was found to be impaired by p38 MAPkinase inhibitor (SB203580). IFN-beta induced the formation of the Interferon Stimulated Gene Factor 3 complex more efficiently than IFN-alpha, as well as sustained induction of IRF-1, which may account for its greater induction of 2'5'oligo(A)synthetase and greater inhibition of cell proliferation. IFN-beta, but not IFN-alpha, induced apoptosis in wild-type p53 EW-7 and COH cell lines, but not in the mutated p53 EW-1 or ORS cell lines. The apoptosis induced by IFN-beta in EW-7 and COH cell lines appeared to be mediated by IRF-1 and involved the activation of caspase-7. Ectopic expression of IRF-1 induced apoptosis in all four cell lines which correlated with the activation of caspase-7 and with the downregulation of the Bcl-2 oncoprotein, as observed for IFN-beta-induced apoptosis in parental EW-7 and COH cell lines.

Interleukin-6 modulates interferon-regulated gene expression by inducing the ISGF3 gamma gene using CCAAT/enhancer binding protein-beta(C/EBP-beta)

Although interleukin-6 (IL-6) alone does not induce the expression of IFN stimulated genes (ISG), a low dose priming of cells with IL-6 strongly enhances the cellular responses to interferon-alpha (IFN-alpha). This effect of IL-6 is not due to superstimulation of the JAK-STAT pathway. Rather, IL-6 induces expression of ISGF3 gamma (p48), a subunit of the multimeric transcription factor ISGF3. As a result IFN-alpha robustly activates gene transcription in IL-6 primed cells. We have shown earlier that the transcription of ISGF3 gamma gene is regulated through a novel element GATE (gamma-IFN activated transcriptional element). We show here IL-6 induces the ISGF3 gamma gene through GATE. Transcription factor C/EBP-beta is required for inducing ISGF3 gamma gene expression through GATE. A mutant C/EBP-beta inhibits the IL-6 inducible ISGF3 gamma gene expression through GATE. Together, these results establish a molecular basis for the synergy between IFNs and IL-6.

Differential nuclear localization of the IFNGR-1 and IFNGR-2 subunits of the IFN-gamma receptor complex following activation by IFN-gamma

We have recently identified a nuclear localization sequence (NLS) in the C-terminus of murine type II interferon (IFN), IFN-gamma, that is responsible for the internalization and nuclear translocation of extracellularly added IFN-gamma. Because the uptake of IFN-gamma is a receptor-mediated endocytotic process, we examined in this study the fate of both the receptor subunits (IFNGR-1 and IFNGR-2) of the heterodimeric IFN-gamma receptor complex. Human IFN-gamma (HuIFN-gamma) was also found to contain a polybasic NLS in a conserved C-terminal region capable of directing its nuclear translocation. Like the ligand, the IFNGR-1 subunit of the receptor complex on WISH cells was found to be translocated to the nucleus on treatment with HuIFN-gamma. Using a combination of immunoprecipitation and immunofluorescence techniques, we found the nuclear accumulation of IFNGR-1 to be ligand dependent, and it was evident within 10-20 min after ligand stimulation. IFNGR-1 was found to colocalize, in a time-dependent and dose-dependent fashion, with the nuclear translocation of the transcription factor Stat1alpha, which is activated by this ligand-receptor system. In addition, Stat1alpha was found to be complexed with IFNGR-1 over the time period of its nuclear translocation. In marked contrast, IFNGR-2 was not transported to the nucleus. The surface immunofluorescence pattern of IFNGR-2 suggested that, following ligand stimulation, the majority of IFNGR-2 remains at the cell surface, whereas IFNGR-1 is endocytosed and targeted to the cell nucleus. These findings suggest that IFNGR-1 plays an active intracellular role in signal transduction events subsequent to the binding of ligand to the dimeric receptor complex. Furthermore, these studies provide the first example of the selective endocytosis and nuclear translocation of a subunit of a multimeric receptor complex.

Transcriptional regulation of the MHC II gene DRA in untransformed human thyrocytes

MHC class II molecules are heterodimeric, polymorphic transmembrane glycoproteins physiologically expressed on cells of the immune system and pathologically expressed on the affected target cells of autoimmunity. Their function is to present processed peptides to antigen-specific CD4(+) T cells. To understand the molecular mechanism of the regulation of class II genes in autoimmune target cell thyrocytes, we investigated the transcriptional regulation of DRA on untransformed, differentiated human thyroid cells following IFN-gamma stimulation, which is potentially relevant to the inappropriate class II expression found in Graves' disease. Data from this study show that IFN-gamma enhances a promoter Y box binding protein and induces an X box binding protein in untransformed thyrocytes, but not in SV-40-transfected thyrocytes. Initial characterization of the proteins has indicated that the Y box binding protein is approximately 132 kDa in size while the X box binding protein binds to the X2 region and is approximately 116 kDa. The X box binding protein may correspond to poly(ADP-ribose) polymerase, a recently described component of the X2 box binding protein, X2BP. In addition, the signal transducer and activator of transcription 1alpha protein (STAT1alpha) is also induced by IFN-gamma in these cells. These results further suggest that there are differences in class II gene regulation between differentiated cells and transformed cell lines.

Lactobacilli and Streptococci activate NF-kappa B and STAT signaling pathways in human macrophages

Gram-positive bacteria induce the production of several cytokines in human leukocytes. The molecular mechanisms involved in Gram-positive bacteria-induced cytokine production have been poorly characterized. In this work we demonstrate that both nonpathogenic Lactobacillus rhamnosus GG and pathogenic Streptococcus pyogenes (group A streptococci) induce NF-kappa B and STAT DNA-binding activity in human primary macrophages as analyzed by EMSA. NF-kappa B activation was rapid and was not inhibited by a protein synthesis inhibitor cycloheximide, suggesting that these bacteria could directly activate NF-kappa B. STAT1, STAT3, and IFN regulatory factor-1 DNA binding was induced by both bacteria with delayed kinetics compared with NF-kappa B. In addition, streptococci induced the formation of IFN-alpha-specific transcription factor complex and IFN-stimulated gene factor-3 (ISGF3). STAT1 and STAT3 activation and ISGF3 complex formation were inhibited by cycloheximide or by neutralization with IFN-alpha/beta-specific Abs. Streptococci were more potent than lactobacilli in inducing STAT1, ISGF3, and IFN regulatory factor-1 DNA binding. Accordingly, only streptococci induced IFN-alpha production. The activation of the IFN-alpha signaling pathway by streptococci could play a role in the pathogenesis of these bacteria. These results indicate that extracellular Gram-positive bacteria activate transcription factors involved in cytokine signaling by two mechanisms: directly, leading to NF-kappa B activation, and indirectly via cytokines, leading to STAT activation.

The tetracycline-responsive promoter contains functional interferon-inducible response elements

Tetracycline (tet)-responsive expression vectors allow controlled inducible expression of proteins in mammalian cells. This system is widely used for experimental research both in vivo and in vitro. In our attempts to use this system to study the antiviral effect of IFNalpha on hepatitis B virus, we discovered an unexpected feature of the tet-responsive promoter (tet promoter) of the currently available expression vectors. IFNalphawas found to stimulate tet promoter activity after transient transfection in a dose- and cell type-dependent manner. By sequence inspection, an IFNalpha-stimulated response element (ISRE)-like sequence was identified in the linker regions located between the heptameric tet operator sequences. Gel shift assays revealed binding of IFN-stimulated gene factors to these sequences, indicating that they mediate the IFNalpha-mediated promoter stimulation. These data demonstrate an unexpected feature of the tet-responsive expression system which needs to be taken into account when using this system for analysis of cytokine functions in vitro and in vivo. The data also imply that the tet promoter-based expression system can be rendered non-responsive to IFNalpha by mutagenesis of the ISREs and this may be essential when considering gene therapy in vivo.

Analysis of transcription factors regulating induction of indoleamine 2,3-dioxygenase by IFN-gamma

IFN-gamma treatment of the human carcinoma cell line ME180 causes cell death due to induction of indoleamine 2,3-dioxygenase (IDO) and resulting starvation for tryptophan. A mutant cell line 3B6A derived from ME180 was resistant to IFN-gamma because of loss of IDO activity. Cotransfecting an IDO promoter-chloramphenicol acetyl transferase (CAT) construct with IFN regulatory factor-1 (IRF-1) resulted in induction of CAT activity in both ME180 and 3B6A cells even in the absence of IFN-gamma. This induction was reduced by cotransfection with IRF-2. However, IRF-1 was not able to restore IDO activity, suggesting a possible repressor site outside the IDO promoter region. Stat1alpha (p91) restored both CAT and IDO activities in 3B6A cells following IFN-gamma treatment. 3B6A cells doubly treated with IFN-gamma and IFN-alpha or IFN-beta restored IDO activity, although neither cytokine on its own could induce IDO. Western blot analysis showed that both constitutive expression and induction of Stat1alpha by IFN-gamma were reduced in 3B6A cells, and double treatment of IFN-gamma with IFN-alpha or IFN-beta restored the expression level of Statla. Electrophoretic mobility shift assays indicated that Stat1 binds to the IFN-gamma-activated sequence (GAS) region in the IDO promoter in ME180 cells following IFN-gamma treatment. Our results indicated that the defect in 3B6A cells was reduced expression of Stat1alpha and that IRF-1, NF-kappaB, and PKR were all involved to some extent in the induction of IDO following IFN-gamma treatment.

Regulation of CD26/DPPIV gene expression by interferons and retinoic acid in tumor B cells

Interferons (IFNs alpha, beta and gamma) and all trans retinoic acid (RA) have the ability to activate genes with GAS sites. We have found that the promoter of CD26/dipeptidylpeptidase IV (DPPIV) contains a consensus GAS site TTCnnnGAA located at bp-35 to -27, and computer analysis confirmed this sequence to be a putative Stat binding site. Consistent with this finding, we show that IFNs and RA rapidly enhanced CD26 gene and protein expression in chronic B lymphocytic leukemia (B-CLL) cells. Immunoblot analyses revealed that unstimulated B-CLL cells expressed detectable levels of serine/tyrosine-phosphorylated Stat1alpha, and RA and IFN-gamma treatment led to increased levels of tyrosine phosphorylation of Stat1alpha and its nuclear accumulation. As shown by electrophoretic mobility shift assay, RA and IFN-gamma increased the binding of a nuclear protein to the GAS-CD26 element. Shift-Western blotting identified Stat1alpha as the GAS-CD26 binding factor. Augmented levels of CD26 protein in malignant B cells cultured with IFNs or RA coincided with the enhancement of DPPIV activity. Taken together, our results are in favor of the IFN-/RA-mediated upregulation of CD26/DPPIV in B-CLL through the signaling pathway involving Stat1alpha and the GAS response element of CD26 promoter.

Functional significance of globotriaosyl ceramide in interferon-alpha(2)/type 1 interferon receptor-mediated antiviral activity

The N-terminus of the type 1 interferon receptor subunit, IFNAR1, has high amino acid sequence similarity to the receptor binding B subunit of the Escherichia coli-derived verotoxin 1, VT1. The glycolipid, globotriaosyl ceramide (Gb(3): Gal alpha(1) --> 4 Gal beta 1 --> 4 Glu beta 1 --> 1 Cer) is the specific cell receptor for VT1. Gb(3)-deficient variant cells selected for VT resistance are cross-resistant to interferon-alpha (IFN-alpha)-mediated antiproliferative activity. The association of eIFNAR1 with Gal alpha 1 --> 4 Gal containing glycolipids has been previously shown to be important for the receptor-mediated IFN-alpha signal transduction for growth inhibition. The crucial role of Gb(3) for the signal transduction of IFN-alpha-mediated antiviral activity is now reported. IFN-alpha-mediated antiviral activity, nuclear translocation of activated Stat1, and increased expression of PKR were defective in Gb(3)-deficient vero mutant cells, although the surface expression of IFNAR1 was unaltered. The VT1B subunit was found to inhibit IFN-alpha-mediated antiviral activity, Stat1 nuclear translocation and PKR upregulation. Unlike VT1 cytotoxicity, IFN-alpha-induced Stat1 nuclear translocation was not inhibited when RME was prevented, suggesting that the accessory function of Gb(3) occurs at the plasma membrane. IFN-alpha antiviral activity was also studied in Gb(3)-positive MRC-5 cells, which are resistant to IFN-alpha growth inhibition, partially resistant to VT1 but still remain fully sensitive to IFN-alpha antiviral activity, and two astrocytoma cell lines expressing different Gb(3) fatty acid isoforms. In both systems, long chain fatty acid-containing Gb(3) isoforms, which are less effective to mediate VT1 cytotoxicity, were found to correlate with higher IFN-alpha-mediated antiviral activity. Inhibition of Gb(3) synthesis in toto prevented IFN-alpha antiviral activity in all cells. We propose that the long chain Gb(3) fatty isoforms preferentially remain in the plasma membrane, and by associating with IFNAR1, mediate IFN-alpha antiviral signaling, whereas short chain Gb(3) fatty acid isoforms are preferentially internalized to mediate VT1 cytotoxicity and IFNAR1-dependent IFN-alpha growth inhibition.

A Stat1alpha factor regulates the expression of the human vimentin gene by IFN-gamma

Vimentin is an intermediate filament protein normally expressed in cells of mesenchymal origin. Here, we report an increase in vimentin gene transcription induced by the cytokine interferon-y (IFN-gamma). Northern blot analysis and reporter gene assays reveal that IFN-gamma induces vimentin gene transcription in HeLa cells. However, no increase in vimentin mRNA synthesis was observed de novo in MCF-7 cells, which do not already express vimentin. Band shift analysis shows that the Stat1alpha protein mediates vimentin induction by IFN-gamma. A human mutant fibroblast cell line (U3A), which lacks Stat1alpha but expresses vimentin mRNA, yields no increase in vimentin mRNA levels on the addition of IFN-gamma. These results suggest that the induction of vimentin gene expression might be an important part of a complex cellular response to IFN-gamma.

Suppression of interferon response gene expression in cells persistently infected with mumps virus, and restoration from its suppression by treatment with ribavirin

Persistent infections with mumps virus were established in human B-lymphoid cell line Akata and in the human chronic myelogenous leukaemia cell line K562. Even after IFN treatment a drastic decrease in STAT-1alpha (signal transducers and activators of transcription-1alpha), STAT-2 and p48 (ISGF-3gamma: IFN-stimulated gene factor-3gamma), which are closely correlated with the IFN-signaling pathway, was found in these persistently infected cells (Akata-MP1 and K-MTP). Therefore, the IFN-signaling pathway is thought to be defective in these persistently infected cells. In other words, most of the IFN-inducible genes in these cells persistently infected with mumps virus may not be able to respond to IFN treatment. Indeed, poor induction of 2',5'-oligoadenylate synthetase (2-5AS), dsRNA activated protein kinase (PKR), and MxA protein mRNAs were demonstrated in these cell lines after IFN treatment. Expression of MHC class-I antigen was also significantly reduced in the persistently infected cell lines as compared with that of uninfected control cells. HLA antigen was augmented by IFN-alpha in Akata and K562 cells, but not in persistently infected cells. Furthermore, suppression of IFN-induced 2-5AS induction and MHC class-I expression was restored by treatment of persistently infected cells with ribavirin through inhibition of virus replication. The result of restoration was also confirmed by IFN-induced STAT-1 induction in persistently infected cells treated with ribavirin.

The V protein of simian virus 5 inhibits interferon signalling by targeting STAT1 for proteasome-mediated degradation

To replicate in vivo, viruses must circumvent cellular antiviral defense mechanisms, including those induced by the interferons (IFNs). Here we demonstrate that simian virus 5 (SV5) blocks IFN signalling in human cells by inhibiting the formation of the IFN-stimulated gene factor 3 and gamma-activated factor transcription complexes that are involved in activating IFN-alpha/beta- and IFN-gamma-responsive genes, respectively. SV5 inhibits the formation of these complexes by specifically targeting STAT1, a component common to both transcription complexes, for proteasome-mediated degradation. Expression of the SV5 structural protein V, in the absence of other virus proteins, also inhibited IFN signalling and induced the degradation of STAT1. Following infection with SV5, STAT1 was degraded in the absence of virus protein synthesis and remained undetectable for up to 4 days postinfection. Furthermore, STAT1 was also degraded in IFN-pretreated cells, even though the cells were in an antiviral state. Since pretreatment of cells with IFN delayed but did not prevent virus replication and protein synthesis, these observations suggest that following infection of IFN-pretreated cells, SV5 remains viable within the cells until they eventually go out of the antiviral state.

The human papilloma virus (HPV)-18 E6 oncoprotein physically associates with Tyk2 and impairs Jak-STAT activation by interferon-alpha

We have examined the effects of human papilloma virus (HPV) E6 proteins on interferon (IFN) signaling. Here we show that expression of the 'malignant' HPV-18 E6 in human HT1080 cells results in inhibition of Jak-STAT activation in response to IFN-alpha but not IFN-gamma. This inhibitory effect is not shared by the 'benign' HPV-11 E6. The DNA-binding and transactivation capacities of the transcription factor ISGF3 are diminished in cells expressing HPV-18 E6 after IFN-alpha treatment as a result of decreased tyrosine phosphorylation of Tyk2, STAT2 and STAT1. However, HPV-18 E6 does not affect the induction of tyrosine phosphorylation and DNA-binding of STAT1 by IFN-gamma. In addition, HPV E6 proteins physically interact with Tyk2. This interaction takes place preferably with HPV-18 E6 and to a lesser extent with HPV-11 E6. The E6/Tyk2 interaction requires the JH6-JH7 domains of Tyk2, which are important for Tyk2 binding to the cytoplasmic portion of IFN-alpha receptor 1 (IFNAR1). These findings demonstrate an inhibitory role of HPV-18 E6 in the IFN-alpha-induced Jak-STAT pathway, which may be explained, at least in part, by the ability of E6 to interact with and impair Tyk2 activation.