Functional Cross-modulation between SOCS Proteins Can Stimulate Cytokine Signaling

Regulation of CD4+ T-cell polarization by suppressor of cytokine signalling proteins

Suppressors of cytokine signalling (SOCS) proteins are induced in responses to many stimuli and by binding to cytokine receptors and associated janus kinase (JAK) proteins, directly regulate the activation of the signal transducers and activators of transcription (STATs). STAT proteins regulate the expression of many genes required for the differentiation of various CD4(+) T helper cell lineages, and there is now accumulating evidence that SOCS also play essential roles in the regulation and maintenance of CD4(+) T-cell polarization. As it is now clear that CD4(+) T cells are more plastic than initially thought, it is of particular importance to understand the molecular mechanisms regulating CD4(+) T-cell differentiation. Here we review the current understanding of how STATs and SOCS act in concert to influence the polarization of CD4(+) T cells and highlight the relevance of this in disease.

Fish Suppressors of Cytokine Signaling (SOCS): Gene Discovery, Modulation of Expression and Function

The intracellular suppressors of cytokine signaling (SOCS) family members, including CISH and SOCS1 to 7 in mammals, are important regulators of cytokine signaling pathways. So far, the orthologues of all the eight mammalian SOCS members have been identified in fish, with several of them having multiple copies. Whilst fish CISH, SOCS3, and SOCS5 paralogues are possibly the result of the fish-specific whole genome duplication event, gene duplication or lineage-specific genome duplication may also contribute to some paralogues, as with the three trout SOCS2s and three zebrafish SOCS5s. Fish SOCS genes are broadly expressed and also show species-specific expression patterns. They can be upregulated by cytokines, such as IFN-γ, TNF-α, IL-1β, IL-6, and IL-21, by immune stimulants such as LPS, poly I:C, and PMA, as well as by viral, bacterial, and parasitic infections in member- and species-dependent manners. Initial functional studies demonstrate conserved mechanisms of fish SOCS action via JAK/STAT pathways.

Identification of interaction sites for dimerization and adapter recruitment in Toll/interleukin-1 receptor (TIR) domain of Toll-like receptor 4

Toll-like receptor signaling requires interactions of the Toll/IL-1 receptor (TIR) domains of the receptor and adapter proteins. Using the mammalian protein-protein interaction trap strategy, homology modeling, and site-directed mutagenesis, we identify the interaction surfaces in the TLR4 TIR domain for the TLR4-TLR4, TLR4-MyD88 adapter-like (MAL), and TLR4-TRIF-related adapter molecule (TRAM) interaction. Two binding sites are equally important for TLR4 dimerization and adapter recruitment. In a model based on the crystal structure of the dimeric TLR10 TIR domain, the first binding site mediates TLR4-TLR4 TIR-TIR interaction. Upon dimerization, two identical second binding sites of the TLR4 TIR domain are juxtaposed and form an extended binding platform for both MAL and TRAM. In our mammalian protein-protein interaction trap assay, MAL and TRAM compete for binding to this platform. Our data suggest that adapter binding can stabilize the TLR4 TIR dimerization.

MAPPIT: a protein interaction toolbox built on insights in cytokine receptor signaling

MAPPIT (mammalian protein-protein interaction trap) is a two-hybrid interaction mapping technique based on functional complementation of a type I cytokine receptor signaling pathway. Over the last decade, the technology has been extended into a platform of complementary assays for the detection of interactions among proteins and between chemical compounds and proteins, and for the identification of small molecules that interfere with protein-protein interactions. Additionally, several screening approaches have been developed to broaden the utility of the platform. In this review we provide an overview of the different components of the MAPPIT toolbox and highlight a number of applications in interactomics, drug screening and compound target profiling.

Suppressor of cytokine signalling-6 promotes neurite outgrowth via JAK2/STAT5-mediated signalling pathway, involving negative feedback inhibition

Background

Suppressors of cytokine signalling (SOCS) protein family are key regulators of cellular responses to cytokines and play an important role in the nervous system. The SOCS6 protein, a less extensively studied SOCS family member, has been shown to induce insulin resistance in the retina and promote survival of the retinal neurons. But no reports are available about the role of SOCS6 in neuritogenesis. In this study, we examined the role of SOCS6 in neurite outgrowth and neuronal cell signalling.

Methodology/Principal Findings

The effect of SOCS6 in neural stem cells differentiation was studied in neural stem cells and PC12 cell line. Highly elevated levels of SOCS6 were found upon neural cell differentiation both at the mRNA and protein level. Furthermore, SOCS6 over-expression lead to increase in neurite outgrowth and degree of branching, whereas SOCS6 knockdown with specific siRNAs, lead to a significant decrease in neurite initiation and extension. Insulin-like growth factor-1 (IGF-1) stimulation which enhanced neurite outgrowth of neural cells resulted in further enhancement of SOCS6 expression. Jak/Stat (Janus Kinase/Signal Transducer And Activator Of Transcription) pathway was found to be involved in the SOCS6 mediated neurite outgrowth. Bioinformatics study revealed presence of putative Stat binding sites in the SOCS6 promoter region. Transcription factors Stat5a and Stat5b were involved in SOCS6 gene upregulation leading to neuronal differentiation. Following differentiation, SOCS6 was found to form a ternary complex with IGFR (Insulin Like Growth Factor-1 Receptor) and JAK2 which acted in a negative feedback loop to inhibit pStat5 activation.

Conclusion/Significance

The current paradigm for the first time states that SOCS6, a SOCS family member, plays an important role in the process of neuronal differentiation. These findings define a novel molecular mechanism for Jak2/Stat5 mediated SOCS6 signalling.

RNF41 (Nrdp1) controls type 1 cytokine receptor degradation and ectodomain shedding

Cytokines, such as interferons, erythropoietin, leptin and most interleukins, signal through type 1 cytokine receptors and activate the canonical JAK–STAT pathway. Aberrant cytokine signalling underlies numerous pathologies and adequate, temporary receptor activation is therefore under tight control. Negative-feedback mechanisms are very well studied, but cellular sensitivity also depends on the number of receptors exposed at the cell surface. This is determined by the equilibrium between receptor synthesis and transport to the plasma membrane, internalisation and recycling, degradation and ectodomain shedding, but the molecular basis of how cells establish steady state receptor levels is poorly understood. Here, we report that ring finger protein 41 (RNF41, also known as E3 ubiquitin-protein ligase Nrdp1) interacts with JAK2-associated cytokine receptor complexes and modulates their cell surface exposure and signalling. Moreover, ectopic expression of RNF41 affected turnover of leptin, leukaemia inhibitory factor and interleukin-6 receptor in a dual way: it blocked intracellular cathepsin-L-dependent receptor cleavage and concomitantly enhanced receptor shedding by metalloproteases of the ADAM family. Receptor degradation and shedding are thus interconnected phenomena with a single protein, RNF41, determining the balance.

SOCS2 regulates T helper type 2 differentiation and the generation of type 2 allergic responses

The incidence of allergy and asthma in developed countries is on the increase and this trend looks likely to continue. CD4(+) T helper 2 (Th2) cells are major drivers of these diseases and their commitment is controlled by cytokines such as interleukin 4, which are in turn regulated by the suppressor of cytokine signaling (SOCS) proteins. We report that SOCS2(-/-) CD4(+) T cells show markedly enhanced Th2 differentiation. SOCS2(-/-) mice, as well as RAG-1(-/-) mice transferred with SOCS2(-/-) CD4(+) T cells, exhibit elevated type 2 responses after helminth antigen challenge. Moreover, in in vivo models of atopic dermatitis and allergen-induced airway inflammation, SOCS2(-/-) mice show significantly elevated IgE, eosinophilia, type 2 responses, and inflammatory pathology relative to wild-type mice. Finally, after T cell activation, markedly enhanced STAT6 and STAT5 phosphorylation is observed in SOCS2(-/-) T cells, whereas STAT3 phosphorylation is blunted. Thus, we provide the first evidence that SOCS2 plays an important role in regulating Th2 cell expansion and development of the type 2 allergic responses.

Metamorphosis and the regenerative capacity of spinal cord axons in Xenopus laevis

Throughout the vertebrate subphylum, the regenerative potential of central nervous system axons is greatest in embryonic stages and declines as development progresses. For example, Xenopus laevis can functionally recover from complete transection of the spinal cord as a tadpole but is unable to do so after metamorphosing into a frog. Neurons of the reticular formation and raphe nucleus are among those that regenerate axons most reliably in tadpole and that lose this ability after metamorphosis. To identify molecular factors associated with the success and failure of spinal cord axon regeneration, we pharmacologically manipulated thyroid hormone (TH) levels using methimazole or triiodothyronine, to either keep tadpoles in a permanently larval state or induce precocious metamorphosis, respectively. Following complete spinal cord transection, serotonergic axons crossed the lesion site and tadpole swimming ability was restored when metamorphosis was inhibited, but these events failed to occur when metamorphosis was prematurely induced. Thus, the metamorphic events controlled by TH led directly to the loss of regenerative potential. Microarray analysis identified changes in hindbrain gene expression that accompanied regeneration-permissive and -inhibitory conditions, including many genes in the permissive condition that have been previously associated with axon outgrowth and neuroprotection. These data demonstrate that changes in gene expression occur within regenerating neurons in response to axotomy under regeneration-permissive conditions in which normal development has been suspended, and they identify candidate genes for future studies of how central nervous system axons can successfully regenerate in some vertebrates.

Nuclear beta-catenin and CD44 upregulation characterize invasive cell populations in non-aggressive MCF-7 breast cancer cells

Background

In breast cancer cells, the metastatic cell state is strongly correlated to epithelial-to-mesenchymal transition (EMT) and the CD44⁺/CD24^- stem cell phenotype. However, the MCF-7 cell line, which has a luminal epithelial-like phenotype and lacks a CD44⁺/CD24^- subpopulation, has rare cell populations with higher Matrigel invasive ability. Thus, what are the potentially important differences between invasive and non-invasive breast cancer cells, and are the differences related to EMT or CD44/CD24 expression?

Methods

Throughout the sequential selection process using Matrigel, we obtained MCF-7-14 cells of opposite migratory and invasive capabilities from MCF-7 cells. Comparative analysis of epithelial and mesenchymal marker expression was performed between parental MCF-7, selected MCF-7-14, and aggressive mesenchymal MDA-MB-231 cells. Furthermore, using microarray expression profiles of these cells, we selected differentially expressed genes for their invasive potential, and performed pathway and network analysis to identify a set of interesting genes, which were evaluated by RT-PCR, flow cytometry or function-blocking antibody treatment.

Results

MCF-7-14 cells had enhanced migratory and invasive ability compared with MCF-7 cells. Although MCF-7-14 cells, similar to MCF-7 cells, expressed E-cadherin but neither vimentin nor fibronectin, β-catenin was expressed not only on the cell membrane but also in the nucleus. Furthermore, using gene expression profiles of MCF-7, MCF-7-14 and MDA-MB-231 cells, we demonstrated that MCF-7-14 cells have alterations in signaling pathways regulating cell migration and identified a set of genes (PIK3R1, SOCS2, BMP7, CD44 and CD24). Interestingly, MCF-7-14 and its invasive clone CL6 cells displayed increased CD44 expression and downregulated CD24 expression compared with MCF-7 cells. Anti-CD44 antibody treatment significantly decreased cell migration and invasion in both MCF-7-14 and MCF-7-14 CL6 cells as well as MDA-MB-231 cells.

Conclusions

MCF-7-14 cells are a novel model for breast cancer metastasis without requiring constitutive EMT and are categorized as a "metastable phenotype", which can be distinguished from both epithelial and mesenchymal cells. The alterations and characteristics of MCF-7-14 cells, especially nuclear β-catenin and CD44 upregulation, may characterize invasive cell populations in breast cancer.

Epithelial induction of porcine suppressor of cytokine signaling 2 (SOCS2) gene expression in response to Entamoeba histolytica

Suppressor of cytokine signaling (SOCS) proteins are key physiological regulators of both innate and adaptive immunity. These proteins belong to the three major classes of modulators of cytokines signaling. In the following article, we used porcine polarized intestinal cells to study early response to the protozoan, Entamoeba histolytica, and we identified by rapid amplification of cDNA ends (RACE) PCR porcine SOCS1, SOCS4, SOCS5 and SOCS6 encoding sequences. With more than 92% identity predicted porcine SOCS proteins are very similar to their human counterparts. Among SOCS transcripts, only SOCS2 mRNA was significantly induced in epithelial intestinal cells in response to the cytolytic activity of the parasite. The transcriptomic profile obtained after 3h of co-culture of polarized intestinal cells with E. histolytica was clearly oriented toward inflammation and the recruitment of neutrophils. These transcriptomic data have been normalized with accuracy by the utilisation of multiple validated reference genes. The analysis offers a first set of reference genes useful for future studies in porcine intestinal cells. Our data shed light on the understanding of the early response of polarized intestinal cells to E. histolytica and identified a potential involvement of SOCS2 in the parasite regulation of the host response.

Palmitate attenuates insulin signaling and induces endoplasmic reticulum stress and apoptosis in hypothalamic neurons: rescue of resistance and apoptosis through adenosine 5' monophosphate-activated protein kinase activation

Hypothalamic insulin signaling is essential to the maintenance of glucose and energy homeostasis. During pathological states, such as obesity and type 2 diabetes mellitus, insulin signaling is impaired. One key mechanism involved in the development of insulin resistance is lipotoxicity, through increased circulating saturated fatty acids. Although many studies have begun to determine the underlying mechanisms of lipotoxicity in peripheral tissues, little is known about the effects of excess lipids in the brain. We used a hypothalamic, neuronal cell model, mHypoE-44, to understand how the highly prevalent nonesterified fatty acid, palmitate, affects neuronal insulin signaling. Through Western blot analysis, we discerned that prolonged exposure to palmitate impairs insulin activation, as assessed by phosphorylation of Akt. We investigated the role of endoplasmic reticulum (ER) stress, which is known to promote cellular insulin resistance and apoptosis in peripheral tissues. Palmitate treatment induced ER stress through a c-Jun N-terminal kinase (JNK)-dependent pathway because a selective JNK inhibitor blocked palmitate activation of the ER stress pathways eIF2 alpha and X-box binding protein-1. Interestingly, JNK inhibition did not prevent the palmitate-mediated cleaved caspase-3 increase, an apoptotic marker, or insulin signaling attenuation. However, pretreatment with the AMP kinase activator, aminoimidazole carboxamide ribonucleotide, blocked JNK phosphorylation and importantly prevented caspase-3 cleavage and restored insulin signaling during short-term exposure to palmitate. Thus, activation of AMP kinase prevents the deleterious effects of palmitate on hypothalamic neurons by inhibiting the onset of insulin resistance and apoptosis.

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.

Suppressors of cytokine signaling (SOCS) in T cell differentiation, maturation, and function

Cytokines are key modulators of T cell biology, but their influence can be attenuated by suppressors of cytokine signaling (SOCS), a family of proteins consisting of eight members, SOCS1-7 and CIS. SOCS proteins regulate cytokine signals that control the polarization of CD4(+) T cells into Th1, Th2, Th17, and T regulatory cell lineages, the maturation of CD8(+) T cells from naïve to "stem-cell memory" (Tscm), central memory (Tcm), and effector memory (Tem) states, and the activation of these lymphocytes. Understanding how SOCS family members regulate T cell maturation, differentiation, and function might prove critical in improving adoptive immunotherapy for cancer and therapies aimed at treating autoimmune and infectious diseases.

Constitutive expression of suppressor of cytokine signalling-3 in skeletal muscle leads to reduced mobility and overweight in mice

AIMS/HYPOTHESIS

Due to their ability to regulate various signalling pathways (cytokines, hormones, growth factors), the suppressor of cytokine signalling (SOCS) proteins are thought to be promising therapeutic targets for metabolic and inflammatory disorders. Hence, their role in vivo has to be precisely determined.

METHODS

We generated transgenic mice constitutively producing SOCS-3 in skeletal muscle to define whether the sole abundance of SOCS-3 is sufficient to induce metabolic disorders and whether SOCS-3 is implicated in physiological roles distinct from metabolism.

RESULTS

We demonstrate here that chronic expression of SOCS-3 in skeletal muscle leads to overweight in mice and worsening of high-fat diet-induced systemic insulin resistance. Counter-intuitively, insulin sensitivity in muscle of transgenic mice appears to be unaltered. However, following constitutive SOCS-3 production, several genes had deregulated expression, among them other members of the SOCS family. This could maintain the insulin signal into skeletal muscle. Interestingly, we found that SOCS-3 interacts with calcineurin, which has been implicated in muscle contractility. In Socs-3 transgenic muscle, this leads to delocalisation of calcineurin to the fibre periphery. Relevant to this finding, Socs-3 transgenic animals had dilatation of the sarcoplasmic reticulum associated with swollen mitochondria and decreased voluntary activity.

CONCLUSIONS/INTERPRETATION

Our results show that constitutive SOCS-3 production in skeletal muscle is not in itself sufficient to induce the establishment of metabolic disorders such as diabetes. In contrast, we reveal a novel role of SOCS-3, which appears to be important for muscle integrity and locomotor activity.

Ciliary neurotrophic factor recruitment of glucagon-like peptide-1 mediates neurogenesis, allowing immortalization of adult murine hypothalamic neurons

The distinct lack of cell lines derived from the adult brain is evident. Ciliary neurotrophic factor (CNTF) triggers neurogenesis in primary culture from adult mouse hypothalamus, as detected by bromodeoxyuridine and Ki67 immunostaining. Using SV-40 T-antigen, we immortalized dividing neurons and generated clonal cell lines expressing neuropeptides and receptors involved in neuroendocrine function. We hypothesized that proglucagon-derived peptides may be the mechanistic downstream effectors of CNTF due to documented neuroprotective and proliferative effects. Indeed, proglucagon gene expression was induced by CNTF, and exposure of primary cells to glucagon-like peptide-1 receptor (GLP-1) agonist, exendin-4, induced cell proliferation. Intracerebroventricular injection of CNTF into adult mice caused increased expression of proglucagon peptide in the hypothalamus. Using a specific GLP-1-receptor antagonist, we found that neurogenesis was significantly attenuated and primary culture from GLP-1-receptor-knockout mice lacked CNTF-mediated neuronal proliferation, thus linking the induction of neurogenesis in the hypothalamus to GLP-1-receptor signaling.

Heterogeneous expression of suppressor of cytokine signalling 2 (SOCS-2) in liver tissue

Suppressor of cytokine signalling 2 (SOCS-2), a dual effector of growth hormone signalling, was found to be heterogeneously expressed in murine liver parenchyma. Data from Affymetrix gene arrays, confirmed by quantitative RT-PCR using preparations of periportal and pericentral hepatocyte subpopulations as well as immunohistochemical detection, showed a preferential expression of SOCS-2 in pericentral hepatocytes. Stimulation of cultured periportal and pericentral hepatocyte subpopulations by different concentrations of growth hormone for 1 h resulted at 100 ng mL(-1) in a 1.6-fold and 4.3-fold increase of SOCS-2 mRNA, respectively. Likewise, insulin-like growth factor-1, another physiological target of growth hormone, was stimulated preferentially in pericentral hepatocytes. As growth hormone receptor was found to be homogeneously expressed in mouse liver parenchyma, our data indicate that growth hormone signalling downstream of growth hormone receptor is more sensitive and/or effective in pericentral than in periportal hepatocytes. Presumably, the heterogeneous distribution of SOCS-2 may contribute to the pericentral preference of growth hormone action via differential feedback.

SCO-ping out the mechanisms underlying the etiology of hydrocephalus

The heterogeneous nature of congenital hydrocephalus has hampered our understanding of the molecular basis of this common clinical problem. However, disease gene identification and characterization of multiple transgenic mouse models has highlighted the importance of the subcommissural organ (SCO) and the ventricular ependymal (vel) cells. Here, we review how altered development and function of the SCO and vel cells contributes to hydrocephalus.

Changes in the expression of suppressor of cytokine signalling (SOCS) 2 in the colonic mucosa of acromegalic patients are associated with hyperplastic polyps

BACKGROUND

Acromegalic patients have increased prevalence of colonic polyps. Development of hyperplastic polyps was related to suppressor of cytokine signalling (SOCS) 2 haploinsufficiency in animal models of acromegaly.

OBJECTIVE AND PATIENTS

To evaluate whether variations in SOCS2 expression in the colonic mucosa of acromegalic patients might be associated to hyperplastic polyps, patients with active acromegaly or disease in remission with or without hyperplastic polyps were studied; controls were non-acromegalic subjects age- and sex- matched with or without polyps.

MEASUREMENTS

Expression of SOCS1-3 was evaluated by RT-PCR, immunofluorescence and Western blot in the colonic mucosa. Coimmunoprecipatiton was used to evaluate multimeric protein complexes.

RESULTS

Acromegalic patients with active disease and hyperplastic polyps had higher levels of SOCS2 transcripts; on the contrary, SOCS1 and SOCS3 transcripts did not differ among the study groups. While the expression of SOCS2 and SOCS3 protein was indistinguishable with that of the corresponding transcripts, SOCS1 protein expression was reduced in active acromegalic patients with polyps. SOCS1 protein was reduced owing to its increased proteasome degradation mediated by SOCS2. The increased SOCS2 and reduced SOCS1 led to increased STAT5b expression, suggesting a higher GH signalling transduction.

CONCLUSIONS

Acromegalic patients with active disease and hyperplastic polyps have high levels of SOCS2 and increased SOCS1 degradation, leading to reduced negative feedback on GH signalling, likely favouring a hyperplastic polyps phenotype.

Cytokine-induced SOCS expression is inhibited by cAMP analogue: impact on regeneration in injured retina

Injured adult retinal ganglion cells (RGCs) regrow axons into peripheral nerve (PN) grafted onto cut optic nerve. Survival and regeneration of RGCs is increased by intraocular injections of ciliary neurotrophic factor (CNTF) and axonal regeneration is further enhanced by co-injection of a cyclic AMP analogue (CPT-cAMP). Based on these data, and because cytokine signaling is negatively regulated by suppressor of cytokine signaling (SOCS) proteins, we set out to determine whether CNTF injections increase retinal SOCS expression and whether any changes are attenuated by co-injection with CPT-cAMP. Using quantitative PCR we found increased SOCS1, SOCS2 and SOCS3 mRNA levels at various times after a single CNTF injection. Expression remained high for many days. SOCS protein levels were also increased. In situ hybridization revealed that RGCs express SOCS3 mRNA, and SOCS expression in cultured RGCs was increased by CNTF. Co-injection of CPT-cAMP reduced CNTF induced expression of SOCS1 and SOCS3 mRNA and decreased SOCS3 protein expression. CNTF injection also transiently increased retinal leukemia inhibitory factor (LIF) expression, an effect that was also moderated by CPT-cAMP. We propose that, along with known reparative effects of elevated cAMP on neurons, reducing SOCS upregulation may be an additional way in which cyclic nucleotides augment cytokine-induced regenerative responses in the injured CNS.

Leptin signaling: A key pathway in immune responses

Leptin is a hormone whose central role is to regulate endocrine functions and to control energy expenditure. After the discovery that leptin can also have pro-inflammatory effects, several studies have tried to address - at the molecular level - the pathways involved in leptin-induced modulation of the immune functions in normal and pathologic conditions. The signaling events influenced by leptin after its binding to the leptin receptor have been under scrutiny in the past few years, and considerable experimental work has elucidated the consequences of leptin effects on immune cells. This review examines the biochemistry, function and regulation of leptin signaling in view of possible intervention on this molecule for a better management and therapy of immune-mediated diseases.

MAPPIT: a versatile tool to study cytokine receptor signalling

MAPPIT (mammalian protein–protein interaction trap) is a cytokine receptor-based two-hybrid method that operates in intact mammalian cells. A bait is fused C-terminally to a STAT (signal transducer and activator of transcription) recruitment-deficient receptor, whereas the prey is linked to functional STAT-binding sites. When bait and prey interact a ligand-dependent complementation of the STAT recruitment deficiency occurs, leading to activation of a STAT-responsive reporter. MAPPIT is very well suited to study protein interactions involving activated cytokine receptors as the technique allows modification of the bait protein in a physiologically optimal environment.

The many faces of the SOCS box

The suppressors of cytokine signalling (SOCS) box is a structural domain found at the C-terminus of over 70 human proteins. It is usually coupled to a protein interaction module such as an SH2 domain in case of SOCS proteins, a family of modulators of cytokine signaling. The SOCS box participates in the formation of E3 ligase complexes, marking activated cytokine receptor complexes for proteasomal degradation. A similar mechanism was recently uncovered for controlling SOCS activity itself, since SOCS2 was found to enhance the turnover of other SOCS proteins. The SOCS box can also add unique features to individual SOCS proteins: it can function as an adaptor domain as was demonstrated for SOCS3, or as a modulator of substrate binding in case of CIS. In this review we discuss these multiple roles of the SOCS box, which emerges as a versatile module controlling cytokine signaling via multiple mechanisms.

SOCS regulation of the JAK/STAT signalling pathway

The suppressor of cytokine signalling (SOCS) proteins were, as their name suggests, first described as inhibitors of cytokine signalling. While their actions clearly now extend to other intracellular pathways, they remain key negative regulators of cytokine and growth factor signalling. In this review we focus on the mechanics of SOCS action and the complexities of the mouse models that have underpinned our current understanding of SOCS biology.

Elongin B/C recruitment regulates substrate binding by CIS

SOCS proteins play a major role in the regulation of cytokine signaling. They are recruited to activated receptors and can suppress signaling by different mechanisms including targeting of the receptor complex for proteasomal degradation. The activity of SOCS proteins is regulated at different levels including transcriptional control and posttranslational modification. We describe here a novel regulatory mechanism for CIS, one of the members of this protein family. A CIS mutant deficient in recruitment of the Elongin B/C complex completely failed to suppress STAT5 activation. This deficiency was not caused by altered turnover of CIS but by loss of cytokine receptor interaction. Intriguingly, no such effect was seen for binding to MyD88. The interaction between CIS and the Elongin B/C complex, which depends on the levels of uncomplexed Elongin B/C, was easily disrupted. This regulatory mechanism may be unique for CIS, as similar mutations in SOCS1, -2, -3, -6, and -7 had no functional impact. Our findings indicate that the SOCS box not only plays a role in the formation of E3 ligase complexes but, at least for CIS, can also regulate the binding modus of SOCS box-containing proteins.

Erythropoietin promotes axonal growth in a model of neuronal polarization

Erythropoietin (EPO) enhances neurogenesis, neuroprotection and regeneration. Here, we examined the effects of EPO on axonal and dendritic growth in a model of neuronal polarization. EPO did not effect survival or the polarized morphology of hippocampal neurons but its effect on neurite outgrowth depended upon the stage of polarization. When added to neurons in the process of establishing polarity (0-2 days in vitro (DIV)), it enhanced axonal and dendritic growth, while EPO added to early polarized cultures at 3-4 DIV promoted the growth of axons but not dendrites. EPO stimulated the phosphorylation of Akt at serine-473 and co-incubation of the Akt/PI-3 kinase pathway inhibitor LY294002 with EPO abolished its effects on Akt phosphorylation and axonal growth. However, while Leukemia Inhibitory Factor (LIF) similarly stimulated phosphorylation of Akt, it had no effect on axonal or dendritic growth, indicating that AKT phosphorylation is necessary but not sufficient for neurite outgrowth in hippocampal neurons.

Neuroendocrine hormones such as growth hormone and prolactin are integral members of the immunological cytokine network

Neuroendocrine hormones such as growth hormone (GH) and prolactin (PRL) have been demonstrated to accelerate the recovery of the immune response after chemotherapy and bone marrow transplantation and to enhance the restoration of immunity in individuals infected with HIV and in normal individuals with compromised immune systems associated with aging. As the mechanism of action of these hormones has been elucidated, it has become clear that they are integral members of the immunological cytokine/chemokine network and share regulatory mechanisms with a wide variety of cytokines and chemokines. The members of this cytokine network induce and can be regulated by members of the suppressor of cytokine signaling (SOCS) family of intracellular proteins. In order to take advantage of the potential beneficial effects of hormones such as GH or PRL, it is essential to take into consideration the overall cytokine network and the regulatory effects of SOCS proteins.

SOCS proteins causing trouble in insulin action

First discovered as inhibitors of cytokine signalling, the suppressor of cytokine signalling (SOCS) proteins have appeared, over recent years, as potent repressors of other signalling pathways including the one induced by insulin. SOCS-1 and SOCS-3 have been extensively studied both in vitro and in vivo in the context of insulin action. It has been shown that these two SOCS members are able to inhibit the insulin signalling pathway by three different mechanisms: (1) inhibition of tyrosine phosphorylation of insulin receptor substrate (IRS) proteins because of competition at the docking site on the insulin receptor (IR), (2) induction of the proteasomal degradation of the IRS and (3) inhibition of the IR kinase. A key feature of the SOCS proteins is that they are induced regulators. Indeed, expression of SOCS proteins is virtually absent in basal conditions, but is rapidly and robustly induced in response to several stimuli such as hormones, cytokines and growth factors. A significant correlation between SOCS-3 expression and insulin resistance has been demonstrated in vivo. Interestingly, the level of SOCS-3 expression is strikingly enhanced in insulin-sensitive tissues from both patients and animal models with type 2 diabetes and insulin resistance. While it remains to be established whether the increased expression of SOCS is a cause or a consequence of insulin resistance, a large body of observations supports a role for SOCS proteins in the disease process found in states with insulin resistance.

Insulin receptor substrate 4 couples the leptin receptor to multiple signaling pathways

Leptin is an adipokine that regulates food intake and energy expenditure by activating its hypothalamic leptin receptor (LR). Members of the insulin receptor substrate (IRS) family serve as adaptor proteins in the signaling pathways of several cytokines and hormones and a role for IRS2 in central leptin physiology is well established. Using mammalian protein-protein interaction trap (MAPPIT), a cytokine receptor-based two-hybrid method, in the N38 hypothalamic cell line, we here demonstrate that also IRS4 interacts with the LR. This recruitment is leptin dependent and requires phosphorylation of the Y1077 motif of the LR. Domain mapping of IRS4 revealed the critical role of the pleckstrin homology domain for full interaction. In line with its function as an adaptor protein, IRS4 interacted with the regulatory p85 subunit of the phosphatidylinositol 3-kinase, phospholipase Cgamma, and the suppressor of cytokine signaling (SOCS) family members SOCS2, SOCS6, and SOCS7 and thus can modulate LR signaling.

Mammalian protein-protein interaction trap (MAPPIT) analysis of STAT5, CIS, and SOCS2 interactions with the growth hormone receptor

Binding of GH to its receptor induces rapid phosphorylation of conserved tyrosine motifs that function as recruitment sites for downstream signaling molecules. Using mammalian protein-protein interaction trap (MAPPIT), a mammalian two-hybrid method, we mapped the binding sites in the GH receptor for signal transducer and activator of transcription 5 (STAT5) a and b and for the negative regulators of cytokine signaling cytokine-inducible Src-homology 2 (SH2)-containing protein (CIS) and suppressor of cytokine signaling 2 (SOCS2). Y534, Y566, and Y627 are the major recruitment sites for STAT5. A non-overlapping recruitment pattern is observed for SOCS2 and CIS with positions Y487 and Y595 as major binding sites, ruling out SOCS-mediated inhibition of STAT5 activation by competition for shared binding sites. More detailed analysis revealed that CIS binding to the Y595, but not to the Y487 motif, depends on both its SH2 domain and the C-terminal part of its SOCS box, with a critical role for the CIS Y253 residue. This functional divergence of the two CIS/SOCS2 recruitment sites is also observed upon substitution of the Y+1 residue by leucine, turning the Y487, but not the Y595 motif into a functional STAT5 recruitment site.

Suppressor of cytokine signaling 3 controls lysosomal routing of G-CSF receptor

The hematopoietic system provides an attractive model for studying growth factor-controlled expansion and differentiation of cells in relation to receptor routing and its consequences for signal transduction. Suppressor of cytokine signaling (SOCS) proteins regulate receptor signaling partly via their ubiquitin ligase (E3)-recruiting SOCS box domain. Whether SOCS proteins affect signaling through modulating intracellular trafficking of receptors is unknown. Here, we show that a juxtamembrane lysine residue (K632) of the granulocyte colony-stimulating factor receptor (G-CSFR) plays a key role in receptor routing and demonstrate that the effects of SOCS3 on G-CSF signaling to a major extent depend on this lysine. Mutation of K632 causes accumulation of G-CSFR in early endosomes and leads to sustained activation of signal transducer and activator of transcription 5 and ERK, but not protein kinase B. Myeloid progenitors expressing G-CSFR mutants lacking K632 show a perturbed proliferation/differentiation balance in response to G-CSF. This is the first demonstration of SOCS-mediated ubiquitination and routing of a cytokine receptor and its impact on maintaining an appropriate signaling output.

The role of STATs in lung carcinogenesis: an emerging target for novel therapeutics

The signal transducer and activator of transcription (STAT) proteins are a family of latent cytoplasmic transcription factors, which form dimers when activated by cytokine receptors, tyrosine kinase growth factor receptors as well as non-receptor tyrosine kinases. Dimeric STATs translocate to the nucleus, where they bind to specific DNA-response elements in the promoters of target genes, thereby inducing unique gene expression programs often in association with other transcription regulatory proteins. The functional consequence of different STAT proteins activation varies, as their target genes play diverse roles in normal cellular/tissue functions, including growth, apoptosis, differentiation and angiogenesis. Certain activated STATs have been implicated in human carcinogenesis, albeit only few studies have focused into their role in lung tumours. Converging evidence unravels their molecular interplays and complex multipartite regulation, rendering some of them appealing targets for lung cancer treatment with new developing strategies.