Putting out the fire: coordinated suppression of the innate and adaptive immune systems by SOCS1 and SOCS3 proteins

Summary: The mounting of an effective immune response requires the coordinated function of both the innate and the adaptive arm of the immune system. Cells from both types of immunity respond to antigenic stimuli through a variety of surface and intracellular receptors and produce cytokines that tightly orchestrate the inflammatory response. The operation of feedback control mechanisms that regulate the duration and the amplitude of antigenic and cytokine receptor signaling is therefore required to prevent hyper-activation of the immune system that could lead to tissue destruction or autoimmunity. Suppressor of cytokine signaling (SOCS) proteins have been identified as a negative feedback loop to cytokine signaling. Recently, the generation of genetically engineered mouse models permitted the evaluation of their function in different processes of the immune responses. In this article, we review new insights into the modular structure of SOCS proteins and the function of SOCS1 and SOCS3 to negatively regulate activation and/or differentiation pathways in macrophages, dendritic cells, and T lymphocytes. Thus, SOCS family proteins are components of an emerging immunoregulatory mechanism that maintains the coordinated balance of both innate and adaptive immune responses.     

Kinase inhibition,competitive binding and proteasomal degradation: resolving the molecular function of the suppressor of cytokine signaling (SOCS) proteins

The suppressor of cytokine signaling (SOCS) family of proteins are key negative regulators of cytokine and growth factor signaling. They act at the receptor complex to modulate the intracellular signaling cascade, preventing excessive signaling and restoring homeostasis. This regulation is critical to the normal cessation of signaling, highlighted by the complex inflammatory phenotypes exhibited by mice deficient in SOCS1 or SOCS3. These two SOCS proteins remain the best characterized of the eight family members (CIS, SOCS1-7), and in particular, we now possess a sound understanding of the mechanism of action for SOCS3. Here, we review the mechanistic role of the SOCS proteins and identify examples where clear, definitive data have been generated and discuss areas where the information is less clear. From this functional viewpoint, we discuss how the SOCS proteins achieve exquisite and specific regulation of cytokine signaling and highlight outstanding questions regarding the function of the less well-studied SOCS family members.     

Phosphorylation of a novel SOCS-box regulates assembly of the HIV-1 Vif-Cul5 complex that promotes APOBEC3G degradation

HIV-1 Vif (viral infectivity factor) protein overcomes the antiviral activity of the DNA deaminase APOBEC3G by targeting it for proteasomal degradation. We report here that Vif targets APOBEC3G for degradation by forming an SCF-like E3 ubiquitin ligase containing Cullin 5 and Elongins B and C (Cul5-EloB-EloC) through a novel SOCS (suppressor of cytokine signaling)-box that binds EloC. Vif binding to EloC is negatively regulated by serine phosphorylation in the BC-box motif of the SOCS-box. Vif ubiquitination is promoted by Cul5 in vitro and in vivo, and requires an intact SOCS-box. Thus, autoubiquitination of Vif occurs within the assembled Vif-Cul5 complex, analogous to F-box proteins that are autoubiquitinated within their SCF (Skp1-Cullin-F-box) complex. These findings suggest mechanisms that regulate the assembly and activity of Cul5 E3 complexes through phosphorylation or autoubiquitination of the SOCS-box protein, and identify interactions between Vif and host cell proteins that may be therapeutic targets.     

Regulation of multiple cytokine signalling pathways by SOCS3 is independent of SOCS2

Suppressor of cytokine signalling (SOCS) 3 is an essential regulator of cytokine signalling, and in turn its expression is tightly regulated. Data from overexpression studies in cell lines suggest that SOCS2 regulates SOCS3 protein degradation, by forming a molecular bridge to an E3 ubiquitin-ligase complex. Whether this regulation is relevant in primary cells is unknown. In this study, we utilized Socs2 ^{? / ?} mice to examine the role of SOCS2 in modulating SOCS3 expression and degradation, and its impact on interleukin-2 (IL-2) and IL-6 signalling in primary haemopoietic cells. Both biochemical and biological analyses demonstrated unperturbed SOCS3 expression and cytokine signalling in the absence of SOCS2. Our results suggest that SOCS2 is not a physiological regulator of SOCS3 expression and action in primary haemopoietic cells.     

Molecular and physiological roles of Pellino E3 ubiquitin ligases in immunity

The sensing of foreign agents by the innate and adaptive immune system triggers complex signal transduction cascades that culminate in expression of gene patterns that facilitate host protection from the invading agent. Post-translational modification of intracellular signaling proteins in these pathways is a key regulatory mechanism with ubiquitination being one of the important processes that controls levels and activities of signaling molecules. E3 ubiquitin ligases are the determining enzymes in dictating the ubiquitination status of individual proteins. Among these hundred E3 ubiquitin ligases are a family of Pellino proteins that are emerging to be important players in immunity and beyond. Herein, we review the roles of the Pellino E3 ubiquitin ligases in innate and adaptive immunity. We discuss their early discovery and characterization and how this has been aided by the highly conserved nature of innate immune signaling across evolution. We describe the molecular roles of Pellino proteins in immune signaling with particular emphasis on their involvement in pathogen recognition receptor (PRR) signaling. The growing appreciation of the importance of Pellino proteins in a wide range of immune-mediated diseases are also evaluated.     

Linear ubiquitination in immunity

Linear ubiquitination is a post-translational protein modification recently discovered to be crucial for innate and adaptive immune signaling. The function of linear ubiquitin chains is regulated at multiple levels: generation, recognition, and removal. These chains are generated by the linear ubiquitin chain assembly complex (LUBAC), the only known ubiquitin E3 capable of forming the linear ubiquitin linkage de novo. LUBAC is not only relevant for activation of nuclear factor-κB (NF-κB) and mitogen-activated protein kinases (MAPKs) in various signaling pathways, but importantly, it also regulates cell death downstream of immune receptors capable of inducing this response. Recognition of the linear ubiquitin linkage is specifically mediated by certain ubiquitin receptors, which is crucial for translation into the intended signaling outputs. LUBAC deficiency results in attenuated gene activation and increased cell death, causing pathologic conditions in both, mice, and humans. Removal of ubiquitin chains is mediated by deubiquitinases (DUBs). Two of them, OTULIN and CYLD, are constitutively associated with LUBAC. Here, we review the current knowledge on linear ubiquitination in immune signaling pathways and the biochemical mechanisms as to how linear polyubiquitin exerts its functions distinctly from those of other ubiquitin linkage types.     

UPS-E3泛素连接酶ITCH与胰岛素抵抗的关系

糖尿病已成为严重的世界性公共卫生问题,预计至2045年全球糖尿病患者将超过6亿人[1]。胰岛素抵抗是2型糖尿病的重要表型,是指各种原因导致胰岛素促进葡萄糖摄取和利用率下降,机体代偿性地分泌过多胰岛素,产生高胰岛素血症。胰岛素是一种蛋白类激素,促进糖原、脂质和蛋白质合成等多种生物代谢过程,其作用的发挥依赖于胰岛素信号正常转导;胰岛素重要靶器官如肝脏、脂肪、肌肉、胰腺和脑中胰岛素信号通路阻断将导致胰岛素抵抗。     

Therapeutic hope for psoriasis offered by SOCS (suppressor of cytokine signaling) mimetic peptide

The cytokine network has a pivotal role in maintaining skin homeostasis, and the disturbance of this network can lead to skin pathogenesis. Evidence published in this issue of the European Journal of Immunology by Madonna et al. [Eur. J. Immunol. 2013. 43: 1883–1895], together with other recent data, focuses attention on a negative cytokine regulator, namely SOCS 1 (suppressor of cytokine signaling 1) in immune‐mediated skin disease. In addition, Madonna et al. bring new perspectives regarding the therapeutic control of IFN‐γ‐mediated skin pathogenesis by use of a SOCS 1 mimetic peptide.     

The E3 ubiquitin ligase Itch in T cell activation, differentiation, and tolerance

Tagging a small molecule ubiquitin to a protein substrate, or protein ubiquitination, plays an important role in the immune responses. This process is catalyzed by a cascade of enzymatic reactions, with the E3 ubiquitin ligases being the critical enzymes that determine the specificity of substrate recognition. The E3 ligase Itch was identified from a mutant mouse which displays skin scratching and abnormal immune disorders. In the past few years, much progress has been made in our understanding of Itch-promoted protein ubiquitination, modulation of its ligase activity by upstream kinases, and the kinase-ligase interaction in T cell differentiation and tolerance induction.     

亚低温对大鼠缺血性脑损伤后SOCS3表达的影响

目的观察大鼠局灶性脑缺血再灌注后不同时间点细胞因子信号转导抑制因子-3(supressor of cytokine signaling 3,SOCS3)的表达情况及实施亚低温后的变化,进一步探讨亚低温的脑保护作用。方法线栓法制作大鼠大脑中动脉栓塞局灶性脑缺血再灌注模型,同时给予亚低温治疗。HE染色观察病理形态改变,免疫组化法检测SOCS3的表达,TUNEL法检测凋亡细胞。结果与假手术组相比,常温缺血组于再灌注3 h后SOCS3的表达开始增强,至24 h达高峰,7天时仍有表达;亚低温缺血组各时间点表达均明显高于常温缺血组(P<0.05);常温缺血组凋亡阳性细胞数随再灌注时间的延长而逐渐增多,至72h达高峰;亚低温缺血组各时间点的表达均明显少于常温缺血组(P<0.05)。结论脑缺血再灌注损伤后SOCS3的表达增强,亚低温可能通过促进SOCS3的表达发挥缺血后抗神经元凋亡的作用。     

SOCS signaling in autoimmune diseases: Molecular mechanisms and therapeutic implications

Suppressor of cytokine signaling (SOCS) proteins are mainly induced by various cytokines and have been described as classical inhibitors of cytokine signaling. SOCS signaling is involved in the regulation of immune cells, and recent findings suggest that SOCS proteins, especially SOCS1 and SOCS3, are often dysregulated in a wide variety of autoimmune diseases, including systemic lupus erythematosus, rheumatoid arthritis, type 1 diabetes, psoriasis, and multiple sclerosis. Recent studies suggest that SOCS signaling could be therapeutically targeted in various autoimmune diseases. In this review, we discuss recent studies on the role of SOCS proteins in the development and pathogenesis of autoimmune diseases, as well as their clinical implications.     

The von Hippel-Lindau tumor suppressor protein is a component of an E3 ubiquitin-protein ligase activity.

pVHL, the product of the VHL tumor suppressor gene, plays an important role in the regulation of cell growth and differentiation of human kidney cells, and inactivation of the VHL gene is the most frequent genetic event in human kidney cancer. The biochemical function of pVHL is unknown. Here we report that pVHL exists in vivo in a complex that displays ubiquitination-promoting activity in conjunction with the universally required components E1, E2, and ubiquitin. pVHL-associated ubiquitination activity requires, at a minimum, pVHL to bind elongin C and Cul-2, relatives of core components of SCF (Skp1-Cdc53/Cul-1-F-box protein) E3 ligase complexes. Notably, certain tumor-derived mutants of pVHL demonstrate loss of associated ubiquitination promoting activity. These results identify pVHL as a component of a potential SCF-like E3 ubiquitin-protein ligase complex and suggest a direct link between pVHL tumor suppressor and the process of ubiquitination.     

Suppressor of cytokine signaling-3 and intimal hyperplasia in porcine coronary arteries following coronary intervention

Aims

The growth and differentiation of cells is regulated by cytokines by binding to cell-surface receptors and activating intracellular signal transduction cascade. Suppressor of cytokine signaling (SOCS)-3 is a negative regulator of cytokines. In this study we examined the expression of SOCS-3 in porcine coronary artery smooth muscle cells (PCASMCs) in vitro and in proliferating smooth muscle cells of neointimal lesions after coronary artery intervention in a swine model.

Methods and results

PCASMCs were cultured and stimulated with TNF-α and/or IGF-1 individually or in combination. Protein expression of SOCS-3 was examined using Western blot. For in vivo studies, six female Yucatan miniswine were fed with special high cholesterol diet for 8 months. At 4 months of high cholesterol diet, animals underwent coronary balloon angioplasty. At the end of 8 months animals were euthanized, coronary arteries were isolated and morphological and histological studies were performed. Western blot data revealed significantly high SOCS-3 expression in PCASMCs in the presence of either TNF-α or IGF-1 (5–6 fold) alone. However, in the presence of both TNF-α and IGF-1 the SOCS-3 expression was significantly decreased (4–5 fold). Results from morphological studies including, H&E and Masson's trichrome stain showed typical lesions with significant neointimal proliferation. Histological evaluation showed expression of smooth muscle α-actin and significantly increased proliferating cell nuclear antigen (PCNA) in neointimal lesion. Interestingly, there was significantly decreased expression of SOCS-3 in smooth muscle cells of neointima as compared to control.

Conclusions

These data suggest that SOCS-3 expression is decreased in proliferating smooth muscle cells of neointimal lesions. This leads to uncontrolled growth of vascular smooth muscle cells in injured arteries leading to restenosis. Therefore, local delivery of SOCS-3 gene at the site of injury after coronary artery intervention could regulate the proliferation of vascular smooth muscle cells and help in preventing the neointimal hyperplasia and restenosis.     

细胞因子信号转导抑制因子3在脓毒症小鼠肝脏和脾脏中的表达

目的 探讨细胞因子信号转导抑制因子3(SOCS3)在脓毒症小鼠肝脏和脾脏中的表达情况及其可能的作用机制.方法 采用盲肠结扎并穿刺术(CLP)制作小鼠脓毒症模型.检测肝脏和脾脏组织的SOCS3 mRNA及蛋门表达,采用RT-PCR检测组织中SOCS3 mRNA的相对含量,用免疫印迹方法测定组织中SOCS3相对蛋白含量.用SPSS统计软件对上述指标间的变化关系进行分析.结果 脓毒症手术后SOCS3在肝脏内基因和蛋白表达量有升高趋势,但与对照组比较筹异无统计学意义(P>0.05).SOCS3在脾脏内的基因和蛋白表达在术后2 h迅速升高.至12 h达峰值.在肝脏和脾脏中SOCS3的基因表达和蛋白表达均正相关(r=0.353、0.731,P均<0.05).结论 CLP导致的脓毒症可以诱导SOCS3在小鼠肝脏和脾脏中表达增多,提示SOCS3在脓毒症后的免疫变化中有重要作用.