論文

Phosphorylation of the α subunit of eukaryotic translation initiation factor 2 (eIF2α) at serine 51 inhibits protein synthesis in cells subjected to various forms of stress including virus infection. The human papillomavirus (HPV) E6 oncoprotein contributes to virus-induced pathogenicity through multiple mechanisms including the inhibition of apoptosis and the blockade of interferon (IFN) action. We have investigated a possible functional relationship between the E6 oncoprotein and eIF2α phosphorylation by an inducible-dimerization form of the IFN-inducible protein kinase PKR. Herein, we demonstrate that HPV type 18 E6 protein synthesis is rapidly repressed upon eIF2α phosphorylation caused by the conditional activation of the kinase. The remainder of E6, however, can rescue cells from PKR-mediated inhibition of protein synthesis and induction of apoptosis. E6 physically associates with GADD34/PP1 holophosphatase complex, which mediates translational recovery, and facilitates eIF2α dephosphorylation. Inhibition of eIF2α phosphorylation by E6 mitigates eIF2α-dependent responses to transcription and translation of proapoptotic genes. These findings demonstrate, for the first time, a role of the oncogenic E6 in apoptotic signaling induced by PKR and eIF2α phosphorylation. The functional interaction between E6 and the eIF2α phosphorylation pathway may have important implications for HPV infection and associated pathogenesis.

Human papilloma viruses (HPVs) are small double-stranded DNA viruses that infect mucosal and cutaneous epithelium and induce cervical cancer. It has been shown that interferon (IFN)γ suppresses proliferation of HPV-infected cells by suppressing expression of HPV E7. Here, we found that IFNγ induces not only suppression of E7 transcription but also proteasome-dependent degradation. Suppressor of cytokine signaling-1 (SOCS1)/JAB, a suppressor of cytokine signaling, is known to be induced by IFNγ, and functions as an antioncogene against various hematopoietic oncogenic proteins. SOCS1 contains the SOCS-box, which is shown to recruit ubiquitin transferase to the molecules that interact with SOCS1. We found that SOCS1 interacted with HPV E7 protein and induced ubiquitination and degradation of E7 in a SOCS-box-dependent manner. SOCS1 overexpression also increased Rb protein levels and suppressed proliferation of cervical cancer cell lines infected with HPV. Moreover, E7 protein levels were higher and Rb protein levels were lower in SOCS1-deficient fibroblasts infected with retrovirus vector carrying E7 gene than in wild-type fibroblasts. E7 induced anchorage-independent growth in SOCS1-deficient fibroblasts, but not in wild-type cells. These data suggested that SOCS1 plays an important role in regulating the levels of E7 protein and their transforming potential, and could be a new therapeutic tool for HPV-mediated tumors.

SOCS-3 (suppressor of cytokine signaling 3) is an intracellular protein that is selectively and rapidly induced by appropriate agonists and that modulates responses of immune cells to cytokines by interfering with the Janus kinase/signal transducer and activator of transcription (Jak/STAT) pathway. On the basis of the observations that interferon γ (IFNγ) up-regulates SOCS-3 gene and protein expression in primary mouse macrophages, J774 macrophage cell line and embryonal fibroblasts, we investigated which sequences of the 5′ SOCS-3 gene are responsive to IFNγ. By promoter deletion analysis we identified a functional IFNγ-responsive element, located at nucleotides -72/-64 upstream from the transcription initiation, whose presence and integrity is necessary to ensure responsiveness to IFNγ. This element contains a STAT consensus binding sequence (SOCS-3/STAT-binding element (SBE)) whose specific mutation totally abolished the responsiveness to IFNγ. In contrast, discrete deletion of other 5' regions of the SOCS-3 promoter did not substantially modify the inducibility by IFNγ. Electromobility shift assay analyses revealed that IFNγ promotes specific DNA binding activities to an oligonucleotide probe containing the SOCS-3/SBE sequence. Even though EFNγ triggered tyrosine phosphorylation of both STAT1 and STAT3 in macrophages and J774 cells, only STAT1 was appropriately activated and thus found to specifically bind to the SOCS-3/SBE oligonucleotide probe. Accordingly, IFNγ-induced SOCS-3 protein expression was not impaired in STAT3-deficient embryonal fibroblasts. Taken together, these results demonstrate that the induction of SOCS-3 by IFNγ depends upon the presence of a STAT-binding element in the SOCS-3 promoter that is specifically activated by STAT1.

Immune and inflammatory systems are controlled by multiple cytokines, including interleukins and interferons. These cytokines exert their biological functions through Janus tyrosine kinases and signal transducer and activator of transcription factors. The cytokine-inducible Src homology 2 protein (CIS) and suppressors of cytokine signaling (SOCS) are a family of intracellular proteins, several of which have emerged as key physiological regulators of cytokine responses, including those that regulate the inflammatory systems. In this short review, we focused on the molecular mechanism of the action of CIS/SOCS family proteins and their roles in Toll-like receptor signal regulation and inflammatory diseases.

Administration of Con A induces liver injury that is considered to be an experimental model for human autoimmune or viral hepatitis, where immunopathology plays roles mediated by activated lymphocytes, especially NK1.1+ CD3+ NKT cells, and inflammatory cytokines, including IFN-γ and IL-4. In the present study we investigated the role of WSX-1, a component of IL-27R, in Con A-induced hepatitis by taking advantage of WSX-1 knockout mice. WSX-1-deficient mice were more susceptible to Con A treatment than wild-type mice, showing serum alanine aminotransferase elevation and massive necrosis in the liver. Although the development of NKT cells appeared normal in WSX-1 knockout mice, purified NKT cells from the knockout mice produced more IFN-γ and IL-4 than those from wild-type mice in response to stimulation with Con A both in vitro and in vivo. In addition, hyperproduction of proinflammatory cytokines, including IL-1, IL-6, and TNF-α, was observed in the knockout mice after Con A administration. These data revealed a novel role for WSX-1 as an inhibitory regulator of cytokine production and inflammation in Con A-induced hepatitis.

In midgestation mouse embryos, the aorta-gonad-mesonephros (AGM) region generates hematopoietic stem cells and definitive hematopoiesis is regulated by cell-cell interaction and signaling molecules. We showed that a Ras/mitogen-activated protein (MAP) kinase signaling-specific inhibitor and a dominant negative mutant Ras blocked the production of CD45+ hematopoietic cells in embryonic day 11.5 AGM culture, indicating an essential role for the MAP kinase pathway in AGM hematopoiesis. Overexpression of the Ras/MAP kinase pathway regulator, Spred-2, in the AGM culture significantly reduced the number of CD45+ cells. In contrast, production of CD45+ cells from the AGM region of Spred-2-null mice was up-regulated as compared with wild-type littermates. Furthermore, Spred-2-deficient mice exhibited elevated hematopoietic colony formation from vascular endothelial-cadherin+ cells. These data indicate that Spred-2 functions as a negative regulator of AGM hematopoiesis by inhibiting hematopoietic cytokine signaling.

The immediate early response of cells treated with IL-6 (interleukin-6) is the activation of the signal transducer and activator of transcription (STAT)3. The Src homology domain 2 (SH2)-containing protein tyrosine phosphatase SHP2 and the feedback inhibitor SOCS3 (suppressor of cytokine signalling) are potent inhibitors of IL-6 signal transduction. Impaired function of SOCS3 or SHP2 leads to enhanced and prolonged IL-6 signalling. The inhibitory function of both proteins depends on their recruitment to the tyrosine motif 759 within glycoprotein gp130. In contrast to inactivation, desensitization of signal transduction is regarded as impaired responsiveness due to prestimulation. Usually, after activation the sensing receptor becomes inactivated by modifications such as phosphorylation, internalization or degradation. We designed an experimental approach which allows discrimination between desensitization and inactivation of IL-6 signal transduction. We observed that pre-stimulation with IL-6 renders cells less sensitive to further stimulation with IL-6. After several hours, the cells become sensitive again. We show that not only signal transduction through previously activated receptors is affected by desensitization but signalling through receptors which were not targeted by the first stimulation was also attenuated (transdesensitization). Interestingly, in contrast to inhibition, desensitization does not depend on the presence of functional SHP2. Furthermore, cells lacking SOCS3 show constitutive STAT3 activation which is not affected by pre-stimulation with IL-6. All these observations suggest that desensitization and inhibition of signalling are mechanistically distinct.

The suppressor of cytokine signaling-3 (SOCS3/CIS3) has been shown to be an important negative regulator of cytokines, especially cytokines that activate STAT3. To examine the role of SOCS3 in neutrophils and the granulocyte colony-stimulating factor (G-CSF) signaling in vivo, we compared neutrophils from two types of conditional knockout mice, LysM-Cre:SOCS3fl/fl mice and Tie2-Cre:SOCS3fl/fl mice, in which the Socs3 gene had been deleted in mature neutrophils and hematopoietic stem cells, respectively. The size of the G-CSF-dependent colonies from Tie2-Cre:SOCS3fl/fl mouse bone marrow was much larger than that of colonies from control wild-type mice, while the size of interleukin-3-dependent colonies was similar. Moreover, LysM-Cre:SOCS3fl/fl mice had more neutrophils than SOCS3 fl/fl mice, suggesting that SOCS3 is a negative regulator of G-CSF signaling in neutrophils. Consistent with this notion, G-CSF-induced STAT3 as well as mitogen-activated protein kinase activation was much stronger and prolonged in SOCS3-deficient mature neutrophils than in wildtype neutrophils. The preventive effect of G-CSF on apoptosis was more prominent in SOCS3-deficient mature neutrophils than in control neutrophils. These data indicate that SOCS3 negatively regulates granulopoiesis and G-CSF signaling in neutrophils and may contribute to neutrophilia or neutropenia.

The difference in time course of Ras and mitogen activated protein kinase (MAPK) cascade by different growth factors is considered to be the cause of different cellular responses. We have developed the computer simulation of Ras-MAPK signal transduction pathway containing newly identified negative feedback system, Sprouty, and adaptor molecules. Unexpectedly, negative feedback system did not profoundly affect time course of MAPK activation. We propose the key role of fibroblast growth factor receptor substrate 2 (FRS2) in NGF/FGF pathway for sustained MAPK activation. More Grb2-SOS complexes were recruited to the plasma membrane by binding to membrane-bound FRS2 in FGF pathway than in EGF pathway and caused sustained activation of ERK. The EGF pathway with high concentration of EGF receptor also induced sustained MAPK activation, which is consistent with the results in the PC12 cell overexpressing the EGF receptors. The simulated time courses of FRS2 knock-out cells were consistent with those of the reported experimental results. © 2004 Elsevier Inc. All rights reserved.

The tumor suppressor p53, a sensor of multiple forms of cellular stress, is regulated by post-translational mechanisms to induce cell-cycle arrest, senescence, or apoptosis. We demonstrate that endoplasmic reticulum (ER) stress inhibits p53-mediated apoptosis. The mechanism of inhibition involves the increased cytoplasmic localization of p53 due to phosphorylation at serine 315 and serine 376, which is mediated by glycogen synthase kinase-3 β (GSK-3β). ER stress induces GSK-3β binding to p53 in the nucleus and enhances the cytoplasmic localization of the tumor suppressor. Inhibition of apoptosis caused by ER stress requires GSK-3β and does not occur in cells expressing p53 with mutation(s) of serine 315 and/or serine 376 to alanine(s). As a result of the increased cytoplasmic localization, ER stress prevents p53 stabilization and p53-mediated apoptosis upon DNA damage. It is concluded that inactivation of p53 is a protective mechanism utilized by cells to adapt to ER stress.

The signal transducer and activator of transcription (STAT) family proteins are transcription factors critical in mediating cytokine signaling. Among them, STAT3 is frequently activated in a number of human cancers and transformed cell lines and is implicated in tumorigenesis. However, although constitutively activated STAT3 mutant (STAT3C) leads to cellular transformation, its transformation potential such as colony-forming activity in soft-agar is much weaker than that of v-src. To identify tumorigenic factors that cooperatively induce cellular transformation with STAT3C, we screened the retroviral cDNA library. We found that the microphthalmia-associated transcription factor (MITF), an essential transcription factor for melanocyte development and pigmentation, induces anchorage-independent growth of NIH-3T3 cells in cooperation with STAT3C. Microarray analysis revealed that c-fos is highly expressed in transformants expressing STAT3C and MITF. Promoter analysis and chromatin immunoprecipitation assay suggested that both STAT3 and MITF can cooperatively upregulate the c-fos gene. In addition, the transformation of NIH-3T3 cells by both MITF and STAT3C was significantly suppressed by a dominant-negative AP-1 retrovirus. These data indicate that MITF and STAT3 cooperatively induce c-fos, resulting in cellular transformation.

Immune and inflammatory systems are controlled by multiple cytokines, including interleukins (ILs) and interferons. These cytokines exert their biological functions through Janus tyrosine kinases (JAKs) and STAT transcription factors. The CIS (cytokine-inducible SH2 protein) and SOCS (suppressors of cytokine signaling) are a family of intracellular proteins, several of which have emerged as key physiological regulators of cytokine responses, including those that regulate the inflammatory systems. In this review, we focused on the molecular mechanism of the action of CIS/SOCS family proteins and their roles in inflammatory diseases. Furthermore, we illustrate several approaches for treating inflammatory diseases by modulating extracellular and intracellular signaling pathways.

The suppressors of cytokine signaling (SOCS) and cytokine-inducible SH2 protein are key physiological regulators of the immune system. Principally, SOCS1 and SOCS3 regulate T cells as well as antigen-presenting cells, including macrophages and dendritic cells. Here we review the function of SOCS1 and SOCS3 in innate and adaptive immunity, with particular emphasis on the relationship between immune regulation and SOCS.

Inflammation is progressed by the action of pro-inflammatory cytokines, including IL-1, TNF, IFN-γ and IL-6, and is resolved by anti-inflammatory cytokines, such as IL-4, IL-10, IL-13, IFN-α and transforming growth factor-β (TGF-β). Intracellular signal transduction pathways initiated by these cytokines have been studied extensively; these pathways ultimately activate transcription factors, such as NF-κB, Smad and STATs. Recently, negative feedback regulation of these pathways, mediated by molecules such as A-20, Smad7 and CIS/SOCS family proteins, has been identified. Negative regulation of cytokine signaling is quite important for inflammation.

WSX-1 is a class I cytokine receptor with homology to the IL-12 receptors and is essential for resistance to Leishmania major infection. In the present study, we demonstrated that WSX-1 was also required for resistance to Trypanosoma cruzi. WSX-1-/- mice exhibited prolonged parasitemia, severe liver injury, and increased mortality over wild-type mice. WSX-1 -/- splenocytes produced enhanced levels of Th2 cytokines, which were responsible for the prolonged parasitemia. Massive necroinflammatory lesions were observed in the liver of infected WSX-1-/- mice, and IFN-γ that was overproduced in WSX-1-/- mice compared with wild-type mice was responsible for the lesions. In addition, vast amounts of various proinflammatory cytokines, including IL-6 and TNF-α, were produced by liver mononuclear cells in WSX-1-/- mice. Thus, during T. cruzi infection, WSX-1 suppresses liver injury by regulating production of proinflammatory cytokines, while controlling parasitemia by suppression of Th2 responses, demonstrating its novel role as an inhibitory regulator of cytokine production.

Suppressor of cytokine signaling-1 (SOCS1/JAB) negatively regulates not only the cytokine-signaling pathway but also lipopolysaccharide (LPS)-induced macrophage activation. We found that SOCS1-deficient dendritic cells (DCs) were also hyperresponsive to interferon-γ and interleukin-4. To define the role of SOCS1-deficient DCs in vivo, we generated mice in which the SOCS1 expression was restored in T and B cells on a SOCS1-/- background. In these mice, DCs were accumulated in the thymus and spleen and produced high levels of BAFF/BLyS and APRIL, resulting in the aberrant expansion of B cells and autoreactive antibody production. SOCS1-deficient DCs efficiently stimulated B cell proliferation in vitro and autoantibody production in vivo. These results indicate that SOCS1 plays an essential role in the normal DC functions and suppression of systemic autoimmunity.

Immune and inflammatory systems are controlled by multiple cytokines, including interleukins and interferons. These cytokines exert their biological functions through Janus tyrosine kinases and signal transducer and activators of transcription (STATs). The cytokine-inducible SH2 proteins (CIS) and suppressors of cytokine signaling (SOCS) are a family of intracellular proteins, several of which have emerged as key physiological regulators of cytokine responses, including those that regulate the inflammatory system. Several recent advances have been made in this field, including treatment of inflammatory diseases by modulating extracellular and intracellular signaling pathways.

To unravel the mechanism of interleukin-13 (IL-13)-specific functions, we sought to identify IL-13 receptor (IL-13R) binding molecules. A novel human IL-13Rα1 binding protein (IL13RBP1) has been identified using yeast tri-hybrid system, which was found to encode the same protein as MIP-T3 (microtubule interacting protein that associates with tumor necrosis factor (TNF) receptor associating factor-3 (TRAF3)). It constitutively associates with IL-13Rα1 and suppresses IL-4/13-induced signal transducer and activator of transcription-6 (STAT6) phosphorylation. IL-13-induced STAT6 activation was also inhibited as determined by dual luciferase assay and electrophoretic mobility shift assay (EMSA). These results suggest that MIP-T3 is a novel inhibitor of IL-13 signaling and may be a useful molecule in ameliorating various conditions in which IL-13 plays a central role. © 2003 Published by Elsevier B.V. on behalf of the Federation of European Biochemical Societies.

Members of the suppressor of cytokine signaling (SOCS) family are involved in the pathogenesis of many inflammatory diseases. SOCS-3 is predominantly expressed in T-helper type 2 (TH2) cells, but its role in T H2-related allergic diseases remains to be investigated. In this study we provide a strong correlation between SOCS-3 expression and the pathology of asthma and atopic dermatitis, as well as serum IgE levels in allergic human patients. SOCS-3 transgenic mice showed increased TH2 responses and multiple pathological features characteristic of asthma in an airway hypersensitivity model system. In contrast, dominant-negative mutant SOCS-3 transgenic mice, as well as mice with a heterozygous deletion of Socs3, had decreased TH2 development. These data indicate that SOCS-3 has an important role in regulating the onset and maintenance of T H2-mediated allergic immune disease, and suggest that SOCS-3 may be a new therapeutic target for the development of antiallergic drugs.