ISGF3 gamma p48, a specificity switch for interferon activated transcription factors

Type I interferons are essential mediators of apoptotic death in virally infected cells

BACKGROUND

The interferons (IFNs) have been extensively studied in the context of host defence against viral infection. In the established model of IFN action, virally infected cells secrete type I IFNs (IFN-alpha/beta) which induce an antiviral state in uninfected cells. However, it is not clear how IFNs function on the infected cells. It has been reported that cells infected by some viruses die by apoptosis.

RESULTS

In the present study, we found that three types of viruses commonly induce apoptosis in primary cell cultures. Importantly, we observed that virus-induced apoptosis was inhibited by anti-IFN-alpha/beta antibodies, and in cells lacking either the type I IFN receptor 1 (IFNAR1) or its downstream mediator, Stat1 (Signal transducer and activator of transcription 1). IFN-alpha treatment by itself did not induce apoptosis unless it was combined with transfection by double-stranded RNA (dsRNA), which is normally generated during the course of viral infection.

CONCLUSION

These results indicate a novel antiviral function of the type I IFNs, i.e. the selective induction of apoptosis in virally infected cells. In effect, these IFNs have a bifunctional role in limiting the spread of virus; eliciting an antiviral state in uninfected cells while promoting apoptosis in infected cells. Our results may help explain why IFNs are sometimes useful in the treatment of viral diseases and will provide further insight into the mechanisms of virus-induced pathogenesis.

Induction of hepatocyte growth factor/scatter factor by interferon-gamma in human leukemia cells

Induction of hepatocyte growth factor/scatter factor (HGF/SF) may be one of the critical steps in organ regeneration, wound healing, and embryogenesis. We previously reported the production of HGF/SF from various human leukemia cell lines and a high level of the growth factor in blood and bone marrow plasma from patients with various types of leukemia. We determined here the effects of hematopoietic cytokines on HGF/SF production in human leukemia cell lines, KG-1, a myeloid cell line, and RPMI-8226, a B cell line. Interferon (IFN)-gamma remarkably stimulated HGF/SF production in both cell lines at concentrations of more than 0.1 or 1 IU/ml. IFN-alpha and IFN-beta were as effective as IFN-gamma in RPMI-8226 cells, but less than IFN-gamma in KG-1 cells. HGF/SF gene expression in KG-1 cells was also up-regulated by IFN-gamma. Granulocyte colony-stimulating factor (G-CSF), granulocyte/macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-5 and IL-6 had no effect on HGF/SF production in the 2 leukemia cell lines. We also determined the effects of HGF/SF inducers known for human fibroblasts on the growth factor production in leukemia cells. Out of phorbol 12-myristate 13-acetate (PMA), cholera toxin, IL-1 beta, and tumor necrosis factor (TNF)-alpha, the former three were as effective as IFN-gamma in KG-1 cells, but only TNF-alpha stimulated HGF/SF production in RPMI-8226 cells, whose effect was less than those of IFN-alpha, IFN-beta, and IFN-gamma. The effect of IFN-gamma in KG-1 cells was synergistic with that of PMA. In contrast with the effect in leukemia cells, HGF/SF induction by IFN-gamma in human skin fibroblasts was much less than that by PMA or cholera toxin. These results indicated that IFN-gamma is a potent inducer of HGF/SF in human leukemia cells. This finding suggests the presence of a homeostatic control mechanism in liver regeneration and repair: hepatic injury, DNA synthesis inhibition, or apoptosis caused by IFN-gamma is subsequently overcome by cytokine-induced HGF/SF, a potent promoter of liver DNA synthesis.

The growing family of interferon regulatory factors

Interferons (IFN) exert their multiple biological effects through the induction of expression of over 30 genes encoding proteins with antiviral, antiproliferative and immunomodulatory functions. Among the many IFN-inducible proteins are the Interferon Regulatory Factors (IRFs), a family of transcription regulators, originally consisting of the well-characterized IRF-1 and IRF-2 proteins; the family has now expanded to over 10 members and is still growing. The present review provides a detailed description of recently characterized IRF family members. Studies analyzing IRF-expressing cell lines and IRF knockout mice reveal that each member of the IRF family exerts distinct roles in biological processes such as pathogen response, cytokine signalling, cell growth regulation and hematopoietic development. Understanding the molecular mechanisms by which the IRFs affect these important cellular events and IFN expression will contribute to a greater understanding of events leading to various viral, immune and malignant disease states and will suggest novel strategies for antiviral and immune modulatory therapy.

Role of angiotensin II in activation of the JAK/STAT pathway induced by acute pressure overload in the rat heart

This study was designed to determine whether the JAK/STAT (indicating just another kinase/signal transducer and activator of transcription) pathway is activated in cardiac hypertrophy induced in vivo by pressure overload in rats and to demonstrate whether angiotensin II is involved in the activation of the JAK/STAT pathway. Acute pressure overload was produced by constricting the abdominal aorta of Wistar rats. Immunoprecipitation-Western blot analysis revealed that pressure overload activated JAK1, JAK2, and Tyk2 as early as 5 minutes and that STAT1, STAT2, and STAT3 were tyrosine-phosphorylated rapidly after exposure to the pressure overload. Phosphorylation of STAT1 and STAT2 peaked in the early stage at 5 to 15 minutes, whereas that of STAT3 peaked in the late stage at 60 minutes. Gel mobility shift of the interferon gamma activation site/interferon alpha-stimulating response element was observed immediately after the aortic banding, whereas the band of sis-inducing element was shifted in the late stage at 60 minutes. Both cilazapril (angiotensin II-converting enzyme inhibitor) and E4177 (angiotensin II type 1 [AT1] receptor antagonist) significantly suppressed the phosphorylation of Tyk2 and partially inhibited the phosphorylation of JAK2, but neither affected JAK1. Coimmunoprecipitation of the AT1 receptor with JAK2 or Tyk2 was clearly observed at 5 minutes and peaked at 15 minutes (20-fold the control value). These results indicate that the JAK/STAT pathway is activated by acute pressure overload in rats and that angiotensin II is involved in activating Tyk2, and partially activating JAK2, via the AT1 receptor. Both angiotensin II-dependent and -independent pathways take part in activating the JAK/STAT pathway in the pressure-overloaded rat heart.

Regulation of interferon-alpha responsiveness by the duration of Janus kinase activity

Daudi B lymphoblastoid cells are highly sensitive to the anti-growth and anti-viral effects of interferon (IFN). Unlike many cell lines, these cells show prolonged transcription of IFN-stimulated genes following treatment with IFN-alpha. This prolonged response correlated with the continued presence of the activated transcription factor, IFN-stimulated gene factor 3 (ISGF3). Pulse-chase labeling experiments indicated that the half-life of the phosphorylation of signal transducers and activators of transcription (Stat)1 and Stat2 was short (<2 h) although the turnover of the proteins themselves was slow (>24 h), indicative of a constitutive phosphatase activity. The administration of protein-tyrosine kinase inhibitors at any time point during IFN stimulation led to rapid inhibition of the response, indicating that tyrosine kinase activity was continuously required. Catalytic activity of Jak1 and Tyk2 kinases remained elevated for prolonged periods following stimulation. Continuous presence of IFN-alpha was necessary for maintaining prolonged activation of ISGF3 and of Janus kinases, an activity that was blocked by antibodies to IFN-alpha or by cycloheximide. Conditioned medium of IFN-alpha-stimulated cells was capable of stimulating STAT activation in naive cells. Taken together, these results suggest that the response to IFN-alpha is controlled by the duration of stimulated Janus kinase activity over the background of constitutive dephosphorylation and that this response can be sustained by autocrine secretion of IFN-alpha.

Physical association between STAT1 and the interferon-inducible protein kinase PKR and implications for interferon and double-stranded RNA signaling pathways

The interferon-inducible double-stranded RNA protein kinase PKR controls protein synthesis through the phosphorylation of eukaryotic translation initiation factor (eIF)-2. In addition to its demonstrated role in translational control, several reports have suggested a transcriptional role for PKR. Here we report that PKR is involved in IFN- and dsRNA-signaling pathways by modulating the function of the signal transducer and activator of transcription STAT1. We also show that PKR associates with STAT1 in mouse and human cells. The association is not a kinase-substrate interaction since STAT1 phosphorylation is not modified by PKR in vitro or in vivo. In addition, the formation of the PKR-STAT1 complex is not dependent upon the enzymatic activity of PKR but does require the dsRNA-binding domain of PKR. Moreover, there is a concomitant decrease in PKR-STAT1 interaction and increase in STAT1 DNA binding in response to IFNs or dsRNA. These findings suggest that PKR plays an important role in IFN and dsRNA-signaling pathways by modulating the transcriptional function of STAT1.

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.

Stat2 is a transcriptional activator that requires sequence-specific contacts provided by stat1 and p48 for stable interaction with DNA

Transcriptional responses to interferon (IFN) are mediated by tyrosine phosphorylation and nuclear translocation of transcription factors of the signal transducer and activator of transcription (Stat) family. The Stat1 protein is required for all transcriptional responses to IFN (both type I and type II). Responses to type I IFN (alpha and beta) also require Stat2 and the IFN regulatory factor family protein p48, which form a heterotrimeric transcription complex with Stat1 termed ISGF3. Stat1 homodimers formed in response to IFN-gamma treatment can also interact with p48 and function as transcriptional activators. We now show that Stat2 is capable of forming a stable homodimer that interacts with p48, can be recruited to DNA, and can activate transcription, raising a question of why Stat1 is required. Analysis of the transcriptional competence, affinity, and specificity of Stat2-p48 complexes compared with other Stat protein-containing transcription factor complexes suggests distinct roles for each component. Although Stat2 is a potent transactivator, it does not interact stably with DNA in complex with p48 alone. Adding Stat1 increases the affinity and alters the sequence selectivity of p48-DNA interactions by contacting a half-site of its palindromic recognition motif adjacent to a p48 interaction sequence. Thus, ISGF3 assembly involves p48 functioning as an adaptor protein to recruit Stat1 and Stat2 to an IFN-alpha-stimulated response element, Stat2 contributes a potent transactivation domain but is unable to directly contact DNA, while Stat1 stabilizes the heteromeric complex by contacting DNA directly.