The SOCS box of suppressor of cytokine signaling-1 is important for inhibition of cytokine action in vivo

Structural basis for c-KIT inhibition by the suppressor of cytokine signaling 6 (SOCS6) ubiquitin ligase

The c-KIT receptor tyrosine kinase mediates the cellular response to stem cell factor (SCF). Whereas c-KIT activity is important for the proliferation of hematopoietic cells, melanocytes and germ cells, uncontrolled c-KIT activity contributes to the growth of diverse human tumors. Suppressor of cytokine signaling 6 (SOCS6) is a member of the SOCS family of E3 ubiquitin ligases that can interact with c-KIT and suppress c-KIT-dependent pathways. Here, we analyzed the molecular mechanisms that determine SOCS6 substrate recognition. Our results show that the SH2 domain of SOCS6 is essential for its interaction with c-KIT pY568. The 1.45-Å crystal structure of SOCS6 SH2 domain bound to the c-KIT substrate peptide (c-KIT residues 564-574) revealed a highly complementary and specific interface giving rise to a high affinity interaction (K(d) = 0.3 μm). Interestingly, the SH2 binding pocket extends to substrate residue position pY+6 and envelopes the c-KIT phosphopeptide with a large BG loop insertion that contributes significantly to substrate interaction. We demonstrate that SOCS6 has ubiquitin ligase activity toward c-KIT and regulates c-KIT protein turnover in cells. Our data support a role of SOCS6 as a feedback inhibitor of SCF-dependent signaling and provides molecular data to account for target specificity within the SOCS family of ubiquitin ligases.

The SPRY domain-containing SOCS box protein SPSB2 targets iNOS for proteasomal degradation

Macrophages lacking SPSB2 have increased NO production and enhanced pathogen-killing capabilities due to decreased ubiquitin-mediated destruction of iNOS.

Inducible nitric oxide (NO) synthase (iNOS; NOS2) produces NO and related reactive nitrogen species, which are critical effectors of the innate host response and are required for the intracellular killing of pathogens such as Mycobacterium tuberculosis and Leishmania major. We have identified SPRY domain–containing SOCS (suppressor of cytokine signaling) box protein 2 (SPSB2) as a novel negative regulator that recruits an E3 ubiquitin ligase complex to polyubiquitinate iNOS, resulting in its proteasomal degradation. SPSB2 interacts with the N-terminal region of iNOS via a binding interface on SPSB2 that has been mapped by nuclear magnetic resonance spectroscopy and mutational analyses. SPSB2-deficient macrophages showed prolonged iNOS expression, resulting in a corresponding increase in NO production and enhanced killing of L. major parasites. These results lay the foundation for the development of small molecule inhibitors that could disrupt the SPSB–iNOS interaction and thus prolong the intracellular lifetime of iNOS, which may be beneficial in chronic and persistent infections.

Mast cell homeostasis and the JAK-STAT pathway

The Janus kinase/signal transducer and activator of transcription (JAK-STAT) pathway mediates important responses in immune cells. Activation of any of the four JAK family members leads to phosphorylation of one or more of seven STAT family members. Phosphorylation of STAT family members leads to their dimerization and translocation into the nucleus, in which they bind specific DNA sequences to activate gene transcription. Regulation of JAKs and STATs therefore has a significant effect on signal transduction and subsequent cellular responses. Mast cells are important mediators of allergic disease and asthma. These cells have the ability to cause profound inflammation and vasodilation upon the release of preformed mediators, as well as subsequent synthesis of new inflammatory mediators. The regulation of mast cells is therefore of intense interest for the treatment of allergic disease. An important regulator of mast cells, STAT5, is activated downstream of the receptors for immunoglobulin E, interleukin-3 and stem cell factor. STAT5 contributes to mast cell homeostasis, by mediating proliferation, survival, and mediator release. Regulators of the JAK-STAT pathway, such as the suppressors of cytokine signaling (SOCS) and protein inhibitor of activated STAT (PIAS) proteins, are required to fine tune the immune response and maintain homeostasis. A better understanding of the role and regulation of JAKs and STATs in mast cells is vital for the development of new therapeutics.

Modulation of T lymphocyte function by the pregnane X receptor

The pregnane X receptor (PXR) is a ligand-activated transcription factor regulating genes central to drug and hormone metabolism in the liver. Previous reports indicated that PXR is expressed in PBMC, but the role of PXR in immune cells remains unknown. In this paper, we report increased PXR expression in mouse and human T lymphocytes upon immune activation. Furthermore, pharmacologic activation of PXR inhibits T lymphocyte proliferation and anergizes T lymphocytes by decreasing the expression of CD25 and IFN-gamma and decreasing phosphorylated NF-kappaB and MEK1/2. Although these effects are preceded by an increase of suppressor of cytokine signaling 1, a master switch for IFN-gamma expression, in a PXR-dependent manner, T-bet expression remains unchanged. Conversely, PXR-deficient mice exhibit an exaggerated T lymphocyte proliferation and increased CD25 expression. Furthermore, PXR-deficient lymphocytes produce more IFN-gamma and less of the anti-inflammatory cytokine IL-10. In summary, these results reveal a novel immune-regulatory role of PXR in T lymphocytes and identify suppressor of cytokine signaling 1 as an early signal in PXR-mediated T lymphocyte suppression.

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.

SOCS-1 binding to tyrosine 441 of IFN-gamma receptor subunit 1 contributes to the attenuation of IFN-gamma signaling in vivo

Suppressor of cytokine signaling (SOCS)-1 is a critical inhibitor of IFN-gamma signal transduction in vivo, but the precise biochemical mechanism of action of SOCS-1 is unclear. Studies in vitro have shown that SOCS-1 binds to Jaks and inhibits their catalytic activity, but recent studies indicate SOCS-1 may act in a similar manner to SOCS-3 by firstly interacting with cytokine receptors and then inhibiting Jak activity. Here, we have generated mice, termed Ifngr1(441F), in which a putative SOCS-1 binding site, tyrosine 441 (Y441), on the IFN-gamma receptor subunit 1 (IFNGR1) is mutated. We confirm that SOCS-1 binds to IFNGR1 in wild-type but not mutant cells. Mutation of Y441 results in impaired negative regulation of IFN-gamma signaling. IFN-gamma-induced STAT1 activation is prolonged in Ifngr1(441F) cells, but not to the extent seen in cells completely lacking SOCS-1, suggesting that SOCS-1 maintains activity to modulate IFN-gamma signaling via other mechanisms. Despite this, we show that hypersensitivity to IFN-gamma results in enhanced innate tumor protection in Ifngr1(441F) mice in vivo, and unregulated expression of an IFN-gamma-dependent chemokine, monokine-induced by IFN-gamma. Collectively, these data indicate that Y441 contributes to the regulation of signaling through IFNGR1 via the recruitment of SOCS-1 to the receptor.

Genomic organization and functional characterization of the promoter for the human suppressor of cytokine signaling 6 gene

In this study, we report the expression and genomic structure of the gene encoding human suppressor of cytokine signaling 6 (SOCS6), and the characterization of the functional promoter region. The human SOCS6 gene, spanning 40 kb on chromosome 18q22.2, is composed of two exons separated by an intron of 35 kb. Two transcripts are ubiquitously expressed, and both encode the full-length open reading frame of SOCS6. A primer extension assay revealed that the major transcription initiation site is located 469 bp upstream the ATG codon. Luciferase promoter analysis demonstrated that the 5'-flanking region is able to drive transcription, and the CpG-rich sequences near the transcription initiation site are important for the TATA-less SOCS6 promoter activity. Analogous to SOCS1 and SOCS3, which are down-regulated in several human cancers, SOCS6 is expressed at lower levels in carcinomas of stomach and colon. We demonstrated that hypermethylation of the SOCS6 promoter is one of the mechanisms for the epigenetic regulation of SOCS6 expression. Firstly, in vitro methylation of the reporter promoter plasmid significantly suppressed the promoter activity. Secondly, SOCS6 expression in vivo was enhanced by treating cells with a methyltransferase inhibitor. The SOCS6 gene from various species shares significant homology in amino acid sequences, transcription factor binding motifs in promoter regions and the two-exon genomic structure, suggesting that the SOCS6 gene is highly conserved.

Intracellular delivery of a cell-penetrating SOCS1 that targets IFN-gamma signaling

Suppressor of cytokine signaling-1 (SOCS1) is an intracellular inhibitor of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway that couples interferon-gamma (IFN-gamma) signaling to the nucleus. Because several inflammatory diseases are associated with uncontrolled IFN-gamma signaling, we engineered a recombinant cell-penetrating SOCS1 (CP-SOCS1) to target this pathway. Here, we show that CP-SOCS1, analogous to endogenous SOCS1, interacted with components of the IFN-gamma signaling complex and functionally attenuated the phosphorylation of STAT1, which resulted in the subsequent inhibition of the production of proinflammatory chemokines and cytokines. Thus, controlled, intracellular delivery of recombinant CP-SOCS1 boosted the anti-inflammatory potential of the cell by restoring the homeostatic balance between pro- and anti-inflammatory signaling. This approach to controlling signal transduction has potential use for therapeutic targeting of signaling pathways associated with inflammatory diseases.

The SOCS box encodes a hierarchy of affinities for Cullin5: implications for ubiquitin ligase formation and cytokine signalling suppression

The SOCS (suppressors of cytokine signalling) family of proteins inhibits the cytokine-induced signalling cascade in part by promoting the ubiquitination of signalling intermediates that are then targeted for proteasomal degradation. This activity relies upon an interaction between the SOCS box domain, the adapter complex elonginBC and a member of the Cullin family, the scaffold protein of an E3 ubiquitin ligase. In this study, we dissected this interaction in vitro using purified components.We found that all eight SOCS proteins bound Cullin5 but required prior recruitment of elonginBC. Neither SOCS nor elonginBC bound Cullin5 when in isolation. Interestingly, the affinity of each SOCS-elonginBC complex for Cullin5 varied by 2 orders of magnitude across the SOCS family. Unexpectedly, the most potent suppressors of signalling, SOCS-1 and SOCS-3, bound most weakly to the E3 ligase scaffold, with affinities 100- and 10-fold lower, respectively, than the rest of the family. The remaining six SOCS proteins all bound Cullin5 with high affinity (K(d) of ~10 nM) due to a slower off-rate and hence a longer halflife of the complex. This difference in affinity may reflect a difference in mode of action as only SOCS-1 and SOCS-3 have been shown to suppress signalling using both SOCS box-dependent and SOCS box-independent mechanisms. This is not the case with the other six SOCS proteins, and our data imply the existence of two distinct subclasses of SOCS proteins with a high affinity for Cullin5, the E3 ligase scaffold, possibly reflecting complete dependence upon ubiquitination for suppression of cytokine signalling.

Chapter 3: Role of SOCS in allergic and innate immune responses

Cytokines are powerful mediators of the immune response that, following initial release by components of the innate system, drive effector functions as well as stimulate the additional arms of the response. Their individual functions are diverse, with stimulatory and inhibitory actions, with the resultant systemic immune response a summation of these actions. The frequently opposing effects of cytokines determine that the blockade of one results in the functional augmentation of the other. Thus, the differential regulation of cytokines profoundly influences the character of the immune response. The suppressor of cytokine signaling proteins are a family of molecules pivotal to this critical regulation. In this review, we will discuss their structural components and functions and our understanding of their impact on the systemic immune response.

Deletion of the SOCS box of suppressor of cytokine signaling 3 (SOCS3) in embryonic stem cells reveals SOCS box-dependent regulation of JAK but not STAT phosphorylation

The mechanism by which Suppressor of Cytokine Signaling-3 (SOCS3) negatively regulates cytokine signaling has been widely investigated using over-expression studies in cell lines and is thought to involve interactions with both the gp130 receptor and JAK1. Here, we compare the endogenous JAK/STAT signaling pathway downstream of Leukemia Inhibitory Factor (LIF) signaling in wild type (WT) Embryonic Stem (ES) cells and in ES cells lacking either the entire Socs3 gene or bearing a truncated form of SOCS3 (SOCS3DeltaSB) lacking the C-terminal SOCS box motif (SOCS3(DeltaSB/DeltaSB)). In SOCS3(DeltaSB/DeltaSB) cells phosphorylated JAK1 accumulated at much higher levels than in WT cells or even cells lacking SOCS3 (SOCS3(-/-)). In contrast enhanced activation of STAT3 and SHP2 was seen in SOCS3(-/-) cells. Size exclusion chromatography of cell extracts showed that in unstimulated cells, JAK1 was exclusively associated with receptors but following cytokine stimulation hyperphosphorylated JAK1 (pJAK1) appeared to dissociate from the receptor complex in a manner independent of SOCS3. In WT and SOCS3(DeltaSB/DeltaSB) cells SOCS3 was associated with pJAK1. The data suggest that dissociation of activated JAK1 from the receptor results in separate targeting of JAK1 for proteasomal degradation through a mechanism dependent on the SOCS3 SOCS box thus preventing further activation of STAT3.

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.

Identification of a nuclear localization signal in suppressor of cytokine signaling 1

Suppressor of cytokine signaling (SOCS) proteins are inducible feedback inhibitors of janus kinase and signal transducer and activators of transcription signaling pathways. In addition, SOCS1 has been identified to regulate stability of nuclear NF-kappaB subunits. However, details about the regulation of the nuclear pool of SOCS1 are unknown. Using different experimental approaches, we observed that SOCS1 but no further SOCS family members localized to the nucleus when expressed in various cell lines. Nuclear transport was confirmed for endogenous SOCS1 in macrophages stimulated with IFN-gamma. Sequence analysis revealed a bipartite nuclear localization signal (NLS) located between the src-homology 2 (SH2) domain and the SOCS box of SOCS1. Deletion of this region, introduction of a series of R/A point mutations, or substitution of this sequence with the respective region of SOCS3 resulted in loss of nuclear localization. Fusion of the SOCS1-NLS to cytokine-inducible SH2 region containing protein (CIS) resulted in nuclear localization of this otherwise cytoplasmic protein. SOCS1 mutants with loss of nuclear localization were still effective in suppressing IFN-alpha-mediated STAT1 tyrosine phosphorylation. However, they showed decreased inhibition of IFN-gamma-mediated induction of CD54. The results identify a hitherto unknown transport of SOCS1 into the nucleus which extends the spectrum of SOCS1 inhibitory activity.

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.

The effects of SOCS-1 on liver endotoxin tolerance development induced by a low dose of lipopolysaccharide are related to dampen NF-kappaB-mediated pathway

BACKGROUND

Endotoxin tolerance is an important mechanism to maintain the homeostasis of liver. It was reported that suppressors of cytokine signalling-1 was a negative regulator of lipopolysaccharide-induced macrophages activation, however, the mechanism underlying endotoxin tolerance and suppressors of cytokine signalling-1 has not been fully elucidated.

AIM

Our aim here is to clarify whether suppressors of cytokine signalling-1 was involved in the mechanisms of endotoxin tolerance in liver through dampening nuclear factor-kappaB-mediated pathway.

METHODS

Endotoxin tolerance models of C57BL/6J mice and isolated Kupffer cells were established by pretreating them with a low dose of lipopolysaccharide to observe the changes of suppressors of cytokine signalling-1 expression during endotoxin tolerance inducement. Moreover, a vector-based short hairpin RNA expression system was used to specifically inhibit suppressors of cytokine signalling-1 expression in RAW264.7 macrophage cells to further explore the role of suppressors of cytokine signalling-1 in endotoxin tolerance inducement. The expression of suppressors of cytokine signalling-1 was analysed by immunohistochemistry, reverse transcription-polymerase chain reaction and Western blotting, respectively. The responses to lipopolysaccharide were assessed by the activation of nuclear factor-kappaB and the production of tumour necrosis factor-alpha, which were analysed by ELISA.

RESULTS

The histopathologic changes in the liver of the non-endotoxin tolerance group were more serious than those of the endotoxin tolerance group. The phagocytic activity of Kupffer cells were depressed and suppressors of cytokine signalling-1 expression in the endotoxin tolerance group obviously increased. Endotoxin tolerance also led to a hyporesponse of Kupffer cells to lipopolysaccharide with less activation of nuclear factor-kappaB, less production of tumour necrosis factor-alpha and more expression of suppressors of cytokine signalling-1 than those of non-endotoxin tolerance group. Moreover, the inhibitive effect was partly refracted in pSOCS-1-short hairpin RNA transfected RAW264.7 cells.

CONCLUSIONS

Endotoxin tolerance induced by lipopolysaccharide pretreatment was accompanied with upregulation of suppressors of cytokine signalling-1 and the silence of suppressors of cytokine signalling-1 by RNA interference obviously attenuated this inhibitive effect, indicating that the absence of suppressors of cytokine signalling-1 caused abnormal enhancement of inflammatory cytokine production and suppressors of cytokine signalling-1 was involved in endotoxin tolerance inducement through dampening nuclear factor-kappaB-mediated pathway. Therefore, suppressors of cytokine signalling-1 may be a new target for the clinical treatment of sepsis.

The SOCS box domain of SOCS3: structure and interaction with the elonginBC-cullin5 ubiquitin ligase

Suppressor of cytokine signalling 3 (SOCS3) is responsible for regulating the cellular response to a variety of cytokines, including interleukin 6 and leukaemia inhibitory factor. Identification of the SOCS box domain led to the hypothesis that SOCS3 can associate with functional E3 ubiquitin ligases and thereby induce the degradation of bound signalling proteins. This model relies upon an interaction between the SOCS box, elonginBC and a cullin protein that forms the E3 ligase scaffold. We have investigated this interaction in vitro using purified components and show that SOCS3 binds to elonginBC and cullin5 with high affinity. The SOCS3-elonginBC interaction was further characterised by determining the solution structure of the SOCS box-elonginBC ternary complex and by deletion and alanine scanning mutagenesis of the SOCS box. These studies revealed that conformational flexibility is a key feature of the SOCS-elonginBC interaction. In particular, the SOCS box is disordered in isolation and only becomes structured upon elonginBC association. The interaction depends upon the first 12 residues of the SOCS box domain and particularly on a deeply buried, conserved leucine. The SOCS box, when bound to elonginBC, binds tightly to cullin5 with 100 nM affinity. Domains upstream of the SOCS box are not required for elonginBC or cullin5 association, indicating that the SOCS box acts as an independent binding domain capable of recruiting elonginBC and cullin5 to promote E3 ligase formation.

Interaction and functional interference of glucocorticoid receptor and SOCS1

Cytokine and glucocorticoid (GC) hormone signaling act in an integrated fashion to control inflammation and immune response. Here we establish a new mode of interaction of these two pathways and propose Suppressor of Cytokine Signaling (SOCS)-1 as an essential player in mediating cross-talk. We observed that glucocorticoid receptor (GR) and SOCS1 form an intracellular complex through an interaction, which required the SH2 domain of SOCS1 and the ligand binding domain of GR. Furthermore, GC stimulation was found to increase the nuclear level of SOCS1. SOCS1 binding to the GR did not require ligand binding of the receptor; however, it was abolished after long term GC stimulation, suggesting a functional role of the interaction for the early phase of GC action. The interaction between GR and SOCS1 appeared to negatively influence the transcription of the two GR-regulated genes, FKBP5 and MKP1, because the GC-dependent expression of these genes was inhibited by the SOCS1 inducer IFNgamma and enhanced in SOCS1-deficient murine embryonic fibroblasts as compared with IFNgamma treated wild-type cells. Our results suggest a prominent role of SOCS1 in the early phase of cross-talk between GR and cytokine signaling.

Cytokine signaling modules in inflammatory responses

Cytokine signaling via a restricted number of Jak-Stat pathways positively and negatively regulates all cell types involved in the initiation, propagation, and resolution of inflammation. Here, we focus on Jak-Stat signaling in three major cell types involved in inflammatory responses: T cells, neutrophils, and macrophages. We summarize how the Jak-Stat pathways in these cells are negatively regulated by the Suppressor of cytokine signaling (Socs) proteins. We emphasize that common Jak-Stat-Socs signaling modules can have diverse developmental, pro- and anti-inflammatory outcomes depending on the cytokine receptor activated and which genes are accessible at a given time in a cell's life. Because multiple components of Jak-Stat-Socs pathways are mutated or closely associated with human inflammatory diseases, and cytokine-based therapies are increasingly deployed to treat inflammation, understanding cytokine signaling will continue to advance our ability to manipulate chronic and acute inflammatory diseases.

Regulation of innate immunity by suppressor of cytokine signaling (SOCS) proteins

Innate immunity represents the first line of defense against invading pathogens. Toll-like receptors (TLRs) are important for activation of innate immunity. Moreover, cytokines mediate communication of cells and are necessary to mount an appropriately regulated immune response. However, activation of innate immunity has to be tightly controlled to avoid overshooting immune reactions. Suppressor of cytokine signaling (SOCS) proteins have been identified as inducible feedback inhibitors of cytokine receptors and have been shown to be of crucial importance for the limitation of inflammatory responses. In this review, we describe the role of SOCS proteins in macrophages and dendritic cells (DCs). Based on our own findings, we show that SOCS proteins are directly induced by stimulation of TLRs. However, SOCS proteins do not interfere with direct TLR signaling, but avoid overshooting activation by regulating paracrine IFN-beta signaling. In addition, SOCS proteins in macrophages and DCs regulate the sensitivity towards IFN-gamma and GM-CSF, thereby modulating anti-microbial activity of macrophages and differentiation of DCs. We discuss that SOCS induction can also be used by microbes to evade immune defense, and this is exemplified by the parasite Toxoplasma gondii which induces SOCS1 to inhibit IFN-gamma-mediated macrophage activation. Taken together, the findings indicate that SOCS proteins play an important role in the balanced activation of innate immunity during infectious encounter.

The SOCS box of suppressor of cytokine signaling-3 contributes to the control of G-CSF responsiveness in vivo

Suppressor of cytokine signaling 3 (SOCS3) is a negative regulator of granulocyte-colony stimulating factor (G-CSF) signaling in vivo. SOCS proteins regulate cytokine signaling by binding, via their SH2 domains, to activated cytokine receptors or their associated Janus kinases. In addition, they bind to the elongin B/C ubiquitin ligase complex via the SOCS box. To ascertain the contribution of the SOCS box of SOCS3 to in vivo regulation of G-CSF signaling, we generated mice expressing a truncated SOCS3 protein lacking the C-terminal SOCS box (SOCS3(Delta SB/Delta SB)). SOCS3(Delta SB/Delta SB) mice were viable, had normal steady-state hematopoiesis, and did not develop inflammatory disease. Despite the mild phenotype, STAT3 activation in response to G-CSF signaling was prolonged in SOCS3(Delta SB/Delta SB) bone marrow. SOCS3(Delta SB/Delta SB) bone marrow contained increased numbers of colony-forming cells responsive to G-CSF and IL-6. Treatment of the mice with pharmacologic doses of G-CSF, which mimics emergency granulopoiesis and therapeutic use of G-CSF, revealed that SOCS3(Delta SB/Delta SB) mice were hyperresponsive to G-CSF. Compared with wild-type mice, SOCS3(Delta SB/Delta SB) mice developed a more florid arthritis when tested using an acute disease model. Overall, the results establish a role for the SOCS box of SOCS3 in the in vivo regulation of G-CSF signaling and the response to inflammatory stimuli.

The JAK-STAT signaling pathway: input and output integration

Universal and essential to cytokine receptor signaling, the JAK-STAT pathway is one of the best understood signal transduction cascades. Almost 40 cytokine receptors signal through combinations of four JAK and seven STAT family members, suggesting commonality across the JAK-STAT signaling system. Despite intense study, there remain substantial gaps in understanding how the cascades are activated and regulated. Using the examples of the IL-6 and IL-10 receptors, I will discuss how diverse outcomes in gene expression result from regulatory events that effect the JAK1-STAT3 pathway, common to both receptors. I also consider receptor preferences by different STATs and interpretive problems in the use of STAT-deficient cells and mice. Finally, I consider how the suppressor of cytokine signaling (SOCS) proteins regulate the quality and quantity of STAT signals from cytokine receptors. New data suggests that SOCS proteins introduce additional diversity into the JAK-STAT pathway by adjusting the output of activated STATs that alters downstream gene activation.

Both the Suppressor of Cytokine Signaling 1 (SOCS-1) Kinase Inhibitory Region and SOCS-1 Mimetic Bind to JAK2 Autophosphorylation Site: Implications for the Development of a SOCS-1 Antagonist

Suppressor of cytokine signaling (SOCS)-1 protein modulates signaling by IFN-gamma by binding to the autophosphorylation site of JAK2 and by targeting bound JAK2 to the proteosome for degradation. We have developed a small tyrosine kinase inhibitor peptide (Tkip) that is a SOCS-1 mimetic. Tkip is compared in this study with the kinase inhibitory region (KIR) of SOCS-1 for JAK2 recognition, inhibition of kinase activity, and regulation of IFN-gamma-induced biological activity. Tkip and a peptide corresponding to the KIR of SOCS-1, ((53))DTHFRTFRSHSDYRRI((68)) (SOCS1-KIR), both bound similarly to the autophosphorylation site of JAK2, JAK2(1001-1013). The peptides also bound to JAK2 peptide phosphorylated at Tyr(1007), pJAK2(1001-1013). Dose-response competitions suggest that Tkip and SOCS1-KIR similarly recognize the autophosphorylation site of JAK2, but probably not precisely the same way. Although Tkip inhibited JAK2 autophosphorylation as well as IFN-gamma-induced STAT1-alpha phosphorylation, SOCS1-KIR, like SOCS-1, did not inhibit JAK2 autophosphorylation but inhibited STAT1-alpha activation. Both Tkip and SOCS1-KIR inhibited IFN-gamma activation of Raw 264.7 murine macrophages and inhibited Ag-specific splenocyte proliferation. The fact that SOCS1-KIR binds to pJAK2(1001-1013) suggests that the JAK2 peptide could function as an antagonist of SOCS-1. Thus, pJAK2(1001-1013) enhanced suboptimal IFN-gamma activity, blocked SOCS-1-induced inhibition of STAT3 phosphorylation in IL-6-treated cells, enhanced IFN-gamma activation site promoter activity, and enhanced Ag-specific proliferation. Furthermore, SOCS-1 competed with SOCS1-KIR for pJAK2(1001-1013). Thus, the KIR region of SOCS-1 binds directly to the autophosphorylation site of JAK2 and a peptide corresponding to this site can function as an antagonist of SOCS-1.

Expression of the p60 autolysin enhances NK cell activation and is required for listeria monocytogenes expansion in IFN-gamma-responsive mice

Both peptidoglycan and muropeptides potently modulate inflammatory and innate immune responses. The secreted Listeria monocytogenes p60 autolysin digests peptidoglycan and promotes bacterial infection in vivo. Here, we report that p60 contributes to bacterial subversion of NK cell activation and innate IFN-gamma production. L. monocytogenes deficient for p60 (Deltap60) competed well for expansion in mice doubly deficient for IFNAR1 and IFN-gammaR1 or singly deficient for IFN-gammaR1, but not in wild-type, IFNAR1(-/-), or TLR2(-/-) mice. The restored competitiveness of p60-deficient bacteria suggested a specific role for p60 in bacterial subversion of IFN-gamma-mediated immune responses, since in vivo expansion of three other mutant L. monocytogenes strains (DeltaActA, DeltaNamA, and DeltaPlcB) was not complemented in IFN-gammaR1(-/-) mice. Bacterial expression of p60 was not required to induce socs1, socs3, and il10 expression in infected mouse bone marrow macrophages but did correlate with enhanced production of IL-6, IL-12p70, and most strikingly IFN-gamma. The primary source of p60-dependent innate IFN-gamma was NK cells, whereas bacterial p60 expression did not significantly alter innate IFN-gamma production by T cells. The mechanism for p60-dependent NK cell stimulation was also indirect, given that treatment with purified p60 protein failed to directly activate NK cells for IFN-gamma production. These data suggest that p60 may act on infected cells to indirectly enhance NK cell activation and increase innate IFN-gamma production, which presumably promotes early bacterial expansion through its immunoregulatory effects on bystander cells. Thus, the simultaneous induction of IFN-gamma production and factors that inhibit IFN-gamma signaling may be a common strategy for misdirection of early antibacterial immunity.

The C-terminus of CIS defines its interaction pattern

Proteins of the SOCS (suppressors of cytokine signalling) family are characterized by a conserved modular structure with pre-SH2 (Src homology 2), SH2 and SOCS-box domains. Several members, including CIS (cytokine-inducible SH2 protein), SOCS1 and SOCS3, are induced rapidly upon cytokine receptor activation and function in a negative-feedback loop, attenuating signalling at the receptor level. We used a recently developed mammalian two-hybrid system [MAPPIT (mammalian protein-protein interaction trap)] to analyse SOCS protein-interaction patterns in intact cells, allowing direct comparison with biological function. We find that, besides the SH2 domain, the C-terminal part of the CIS SOCS-box is required for functional interaction with the cytokine receptor motifs examined, but not with the N-terminal death domain of the TLR (Toll-like receptor) adaptor MyD88. Mutagenesis revealed that one single tyrosine residue at position 253 is a critical binding determinant. In contrast, substrate binding by the highly related SOCS2 protein, and also by SOCS1 and SOCS3, does not require their SOCS-box.

Role of suppressor of cytokine signaling in ocular allergy

PURPOSE OF REVIEW

The goal of this article is to evaluate developments in the knowledge of suppressor of cytokine signaling (SOCS) protein ocular allergy and the potential of SOCS proteins as targets for therapeutic strategies.

RECENT FINDINGS

The family of proteins designated SOCS proteins plays an important role in Th2-mediated allergic responses through the control of the balance between Th1 and Th2 cells. SOCS3 and SOCS5 are predominantly expressed in Th2 and Th1 cells, respectively, and they reciprocally inhibit the Th1 and Th2 differentiation processes. SOCS3 is highly expressed at the disease site of allergic conjunctivitis, and T-cell-specific expression of SOCS3 deteriorates clinical and pathological features of allergic conjunctivitis. Reduction of the expression level or inhibition of function clearly reduces the severity of allergic conjunctivitis. On the other hand, constitutive expression of SOCS5, a specific inhibitor of IL-4 signaling, results in reduced eosinophil infiltration. Moreover, negative regulation of the Th2-mediated response by dominant-negative SOCS3 and SOCS5 reduced the incidence of allergic conjunctivitis in a mouse model.

SUMMARY

The present article summarizes recent findings in terms of a role of SOCS protein as a negative regulator in ocular allergy and its clinical application.

Ankyrin repeat and suppressors of cytokine signaling box protein asb-9 targets creatine kinase B for degradation

The suppressors of cytokine signaling (SOCS) proteins inhibit cytokine action by direct interaction with Janus kinases or activated cytokine receptors. In addition to the N-terminal and Src homology 2 domains that mediate these interactions, SOCS proteins contain a C-terminal SOCS box. DNA data base searches have identified a number of other protein families that possess a SOCS box, of which the ankyrin repeat and SOCS box-containing (Asb) proteins constitute the largest. Although it is known that the SOCS proteins are involved in the negative regulation of cytokine signaling, the biological and biochemical functions of the Asbs are largely undefined. Using a proteomics approach, we demonstrate that creatine kinase B (CKB) interacts with Asb-9 in a specific, SOCS box-independent manner. This interaction increases the polyubiquitylation of CKB and decreases total CKB levels within the cell. The targeting of CKB for degradation by Asb-9 was primarily SOCS box-dependent and suggests that Asb-9 acts as a specific ubiquitin ligase regulating levels of this evolutionarily conserved enzyme.

Suppressor of cytokine signaling 3 (SOCS3) in Th2 cells evokes Th2 cytokines, IgE, and eosinophilia

Atopic dermatitis, allergic rhinitis, and bronchial asthma are allergic immune disorders characterized by a predominance of T helper 2 (Th2) cells, the resulting elevation of allergen-specific immunoglobulin E (IgE), and mast cell- and eosinophil-associated inflammation. The cytokine environment at the site of the initial antigen stimulation determines the direction of helper T-cell differentiation into Th1 or Th2 cells. Therefore, negative regulators of cytokine signaling, suppressors of cytokine signaling (SOCS) proteins, play an important role in Th2-mediated allergic responses through the control of the balance between Th1 and Th2 cells. SOCS3 and SOCS5 are predominantly expressed in Th2 and Th1 cells, respectively, and they reciprocally inhibit the Th1 and Th2 differentiation processes. In this article, we discuss the role of SOCS3 and SOCS5 proteins in atopic asthma and allergic conjunctivitis and explore the potential of SOCS proteins as targets for therapeutic strategies in allergic disorders.

SOCS1: a potent and multifaceted regulator of cytokines and cell-mediated inflammation

Suppressor of cytokine signalling-1 (SOCS1), as the name implies, is a protein that functions as a negative regulator of cytokine signalling. Initially characterized for its ability to inhibit JAK phosphorylation and function, SOCS1 also targets proteins for degradation by the proteosome machinery. The expression of SOCS1 can be regulated at the transcription, translation and protein level. Despite the broad spectrum of cytokines that can induce SOCS1 expression and/or be inhibited by SOCS1 in vitro, the use of genetically modified mice has revealed a more specific role for SOCS1 in vivo including a critical role in the regulation of IFNgamma signalling. In addition, SOCS1 has a complex role in T cell activation, and studies have revealed significant roles for SOCS1 in the regulation of IL-4, IL-12 and IL-15 in vivo. Interestingly, SOCS1 action is not limited to the regulation of the classical JAK/STAT-signalling pathway, because SOCS1 also inhibits cytokines like insulin and toll-like receptor signal transduction, neither of which activates the JAK/STAT pathway. Evidence is emerging for a role for aberrant SOCS1 expression in human disease, particularly in a number of malignancies.

Induction of suppressor of cytokine signaling-1 by Toxoplasma gondii contributes to immune evasion in macrophages by blocking IFN-gamma signaling

Toxoplasma gondii is an intracellular parasite that survives and multiplies in professional phagocytes such as macrophages. Therefore, T. gondii has to cope with the panel of antimicrobial host immune mechanisms, among which IFN-gamma plays a crucial role. We report in this study that in vitro infection of murine macrophages with viable, but not with inactivated, parasites results in inhibition of IFN-gamma signaling within the infected cells. Thus, infection of RAW264.7 macrophages with tachyzoites inhibited IFN-gamma-induced STAT-1 tyrosine phosphorylation, mRNA expression of target genes, and secretion of NO. These effects were dependent on direct contact of the host cells with living parasites and were not due to secreted intermediates. In parallel, we report the induction of suppressor of cytokine signaling-1 (SOCS-1), which is a known feedback inhibitor of IFN-gamma receptor signaling. SOCS-1 was induced directly by viable parasites. SOCS overexpression in macrophages did not affect tachyzoite proliferation per se, yet abolished the inhibitory effects of IFN-gamma on parasite replication. The inhibitory effects of T. gondii on IFN-gamma were diminished in macrophages from SOCS-1-/- mice. The results suggest that induction of SOCS proteins within phagocytes due to infection with T. gondii contributes to the parasite's immune evasion strategies.

A method for the generation of standardized qualitative dynamical systems of regulatory networks

BACKGROUND

Modeling of molecular networks is necessary to understand their dynamical properties. While a wealth of information on molecular connectivity is available, there are still relatively few data regarding the precise stoichiometry and kinetics of the biochemical reactions underlying most molecular networks. This imbalance has limited the development of dynamical models of biological networks to a small number of well-characterized systems. To overcome this problem, we wanted to develop a methodology that would systematically create dynamical models of regulatory networks where the flow of information is known but the biochemical reactions are not. There are already diverse methodologies for modeling regulatory networks, but we aimed to create a method that could be completely standardized, i.e. independent of the network under study, so as to use it systematically.

RESULTS

We developed a set of equations that can be used to translate the graph of any regulatory network into a continuous dynamical system. Furthermore, it is also possible to locate its stable steady states. The method is based on the construction of two dynamical systems for a given network, one discrete and one continuous. The stable steady states of the discrete system can be found analytically, so they are used to locate the stable steady states of the continuous system numerically. To provide an example of the applicability of the method, we used it to model the regulatory network controlling T helper cell differentiation.

CONCLUSION

The proposed equations have a form that permit any regulatory network to be translated into a continuous dynamical system, and also find its steady stable states. We showed that by applying the method to the T helper regulatory network it is possible to find its known states of activation, which correspond the molecular profiles observed in the precursor and effector cell types.

A method for the generation of standardized qualitative dynamical systems of regulatory networks

Background

Modeling of molecular networks is necessary to understand their dynamical properties. While a wealth of information on molecular connectivity is available, there are still relatively few data regarding the precise stoichiometry and kinetics of the biochemical reactions underlying most molecular networks. This imbalance has limited the development of dynamical models of biological networks to a small number of well-characterized systems. To overcome this problem, we wanted to develop a methodology that would systematically create dynamical models of regulatory networks where the flow of information is known but the biochemical reactions are not. There are already diverse methodologies for modeling regulatory networks, but we aimed to create a method that could be completely standardized, i.e. independent of the network under study, so as to use it systematically.

Results

We developed a set of equations that can be used to translate the graph of any regulatory network into a continuous dynamical system. Furthermore, it is also possible to locate its stable steady states. The method is based on the construction of two dynamical systems for a given network, one discrete and one continuous. The stable steady states of the discrete system can be found analytically, so they are used to locate the stable steady states of the continuous system numerically. To provide an example of the applicability of the method, we used it to model the regulatory network controlling T helper cell differentiation.

Conclusion

The proposed equations have a form that permit any regulatory network to be translated into a continuous dynamical system, and also find its steady stable states. We showed that by applying the method to the T helper regulatory network it is possible to find its known states of activation, which correspond the molecular profiles observed in the precursor and effector cell types.

Immunity by ubiquitylation: a reversible process of modification

Conjugation of ubiquitin to a protein substrate provides a tag that either marks the labelled protein for degradation or modulates its function.

The process of ubiquitylation, which is catalysed by coordinated enzymatic reactions that require enzymes known as E1, E2 and E3, has an important role in the modulation of immune responses.

Immune tolerance is induced in the thymus and the periphery through diverse mechanisms, and E3 ligases are involved in thymic antigen presentation, T-cell anergy and follicular B helper T-cell development.

The immunological defect in mice with a disrupted itchy (Itch) locus results from a defect in degradation of the transcription factor JUNB. This process is tightly regulated by upstream protein kinases that modulate the activity of the E3 ligase ITCH rather than directly affect JUNB, as commonly thought.

Nuclear factor-κB (NF-κB) signalling is crucial for both innate and adaptive immunity and is regulated by K48 (Lys48)-linked polyubiquitylation (which targets inhibitor of NF-κB (IκB) for proteasomal-dependent degradation), K63-linked polyubiquitylation (which activates IκB kinase, IKK) and A20-mediated de-ubiquitylation.

E3 ligases also regulate other cytokine-induced cellular responses, such as transforming-growth-factor-β-mediated signalling and interferon (IFN)-triggered gene expression.

The ubiquitin-like molecule ISG15 (IFN-stimulated protein of 15 kDa) participates in IFN-mediated signalling, and defects in de-ISGylation result in resistance to viral infection.

The conjugation of ubiquitin, a 76-amino-acid peptide, to a protein substrate provides a tag that either marks the labelled protein for degradation or modulates its function. The process of protein ubiquitylation — which is catalysed by coordinated enzymatic reactions that are mediated by enzymes known as E1, E2 and E3 — has an important role in the modulation of immune responses. Importantly, protein ubiquitylation is a reversible process, and removal of ubiquitin molecules is mediated by de-ubiquitylating enzymes: for example, A20, which has been implicated in the regulation of immune responses. In addition, the conjugation of ubiquitin-like molecules, such as ISG15 (interferon-stimulated protein of 15 kDa), to proteins is also involved in immune regulation. This Review covers recent progress in our understanding of protein ubiquitylation in the immune system.

SLIM trims STATs: ubiquitin E3 ligases provide insights for specificity in the regulation of cytokine signaling

The Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway has evolved to serve highly specialized functions in the regulation of hematopoiesis, cell metabolism, and immune responses. The duration, strength, and specificity of cytokine signaling are controlled by several mechanisms, including the ubiquitin-proteasome pathway, which modulates the turnover of cytokine receptors and activated JAKs. The specificity of the ubiquitin pathway is achieved through various E3 ligase complexes that recognize and interact with distinct target proteins, often in a phosphorylation-dependent manner. Intriguing new information about the ubiquitin pathway came with the identification of an E3 ubiquitin ligase, SLIM, that specifically interacts with activated STAT1 and STAT4 and induces their ubiquitination and degradation. These findings, together with the evidence from paramyxoviruses about the role of ubiquitination as a highly specific STAT inhibition mechanism, highlight the role of E3 ubiquitin ligases as specificity determinants in the regulation of STAT activation, and open the field for investigation of additional E3s that target other STAT proteins.

The role of suppressors of cytokine signalling in thymopoiesis and T cell activation

Cytokines play an essential role in mediating interactions between cells of the immune system. Suppressors of cytokine signalling proteins act to negatively regulate these cytokine signals, thereby exerting control over the expression of cytokine responsive genes. Various lines of experimental evidence suggest that two closely related members of the this family, suppressor of cytokine signalling 1 and 3, are important in the processes of T cell development, activation and homeostasis. This review outlines the principles underlying these processes and relates these to the potentially important roles played by suppressor of cytokine signalling 1 and 3.

Negative regulation of growth hormone receptor signaling

GH has been of significant scientific interest for decades because of its capacity to dramatically change physiological growth parameters. Furthermore, GH interacts with a range of other hormonal pathways and is an established pharmacological agent for which novel therapeutical applications can be foreseen. It is easy to see the requirement for a number of postreceptor mechanisms to regulate and control target tissue sensitivity to this versatile hormone. In recent years, some of the components that take part in the down-regulatory mechanism targeting the activated GH receptor (GHR) have been defined, and the physiological significance of some of these key components has begun to be characterized. Down-regulation of the GHR is achieved through a complex mechanism that involves rapid ubiquitin-dependent endocytosis of the receptor, the action of tyrosine phosphatases, and the degradation by the proteasome. The suppressors of cytokine signaling (SOCS) protein family, particularly SOCS2, plays an important role in regulating GH actions. The aim of this review is to summarize collected knowledge, including very recent findings, regarding the intracellular mechanisms responsible for the GHR signaling down-regulation. Insights into these mechanisms can be of relevance to several aspects of GH research. It can help to understand growth-related disease conditions, to explain GH resistance, and may be used to develop pharmaceuticals that enhance some the beneficial actions of endogenously secreted GH in a tissue-specific manner.

Intracellular protein therapy with SOCS3 inhibits inflammation and apoptosis

Suppressor of cytokine signaling (SOCS) 3 attenuates proinflammatory signaling mediated by the signal transducer and activator of transcription (STAT) family of proteins. But acute inflammation can occur after exposure to pathogen-derived inducers staphylococcal enterotoxin B (SEB) and lipopolysaccharide (LPS), or the lectin concanavalin A (ConA), suggesting that physiologic levels of SOCS3 are insufficient to stem proinflammatory signaling under pathogenic circumstances. To test this hypothesis, we developed recombinant cell-penetrating forms of SOCS3 (CP-SOCS3) for intracellular delivery to counteract SEB-, LPS- and ConA-induced inflammation. We found that CP-SOCS3 was distributed in multiple organs within 2 h and persisted for at least 8 h in leukocytes and lymphocytes. CP-SOCS3 protected animals from lethal effects of SEB and LPS by reducing production of inflammatory cytokines and attenuating liver apoptosis and hemorrhagic necrosis. It also reduced ConA-induced liver apoptosis. Thus, replenishing the intracellular stores of SOCS3 with CP-SOCS3 effectively suppresses the devastating effects of acute inflammation.

Negative regulation of cytokine signaling and immune responses by SOCS proteins

Immune and inflammatory systems are controlled by multiple cytokines, including interleukins and interferons. Many of these cytokines exert their biological functions through JAKs (Janus tyrosine kinases) and STAT (signal transduction and activators of transcription) transcription factors. CIS (cytokine-inducible SH2 (Src homology 2) protein) and SOCS (suppressor of cytokine signaling) are a family of intracellular proteins, several of which have emerged as key physiological regulators of cytokine-mediated homeostasis, including innate and adaptive immunity. In this review we focus on the molecular mechanism of the action of CIS/SOCS family proteins and their roles in immune regulation and inflammatory diseases including rheumatoid arthritis.

SOCS2 negatively regulates growth hormone action in vitro and in vivo

Mice deficient in SOCS2 display an excessive growth phenotype characterized by a 30-50% increase in mature body size. Here we show that the SOCS2-/- phenotype is dependent upon the presence of endogenous growth hormone (GH) and that treatment with exogenous GH induced excessive growth in mice lacking both endogenous GH and SOCS2. This was reflected in terms of overall body weight, body and bone lengths, and the weight of internal organs and tissues. A heightened response to GH was also measured by examining GH-responsive genes expressed in the liver after exogenous GH administration. To further understand the link between SOCS2 and the GH-signaling cascade, we investigated the nature of these interactions using structure/function and biochemical interaction studies. Analysis of the 3 structural motifs of the SOCS2 molecule revealed that each plays a crucial role in SOCS2 function, with the conserved SOCS-box motif being essential for all inhibitory function. SOCS2 was found to bind 2 phosphorylated tyrosines on the GH receptor, and mutational analysis of these amino acids showed that both were essential for SOCS2 function. Together, the data provide clear evidence that SOCS2 is a negative regulator of GH signaling.

Regulation of the immune system by SOCS family adaptor proteins

Signal transduction via cytokine receptors is regulated by several mechanisms that control initiation, magnitude and duration of the signaling pathways. Cytokine-induced SOCS family adaptors function as feedback inhibitors of cytokine receptor signaling by inhibiting the JAK-STAT signal transduction pathway. Specific gene-targeted mice have unveiled critical, non-overlapping functions for SOCS1 and SOCS3 in lymphocyte development and homeostasis, and in the regulation of macrophage and dendritic cell functions. In this review, we will discuss the structure of SOCS proteins, mechanisms by which they control the JAK-STAT pathway and their role in immune regulation.

SOCS-1 localizes to the microtubule organizing complex-associated 20S proteasome

The regulation of cytokine signaling is critical for controlling cellular proliferation and activation during an immune response. SOCS-1 is a potent inhibitor of Jak kinase activity and of signaling initiated by several cytokines. SOCS-1 protein levels are tightly regulated, and recent data suggest that SOCS-1 may regulate the protein levels of some signaling proteins by the ubiquitin proteasome pathway; however, the cellular mechanism by which SOCS-1 directs proteins for degradation is unknown. In this report, SOCS-1 is found to colocalize and biochemically copurify with the microtubule organizing complex (MTOC) and its associated 20S proteasome. The SOCS-1 SH2 domain is required for the localization of SOCS-1 to the MTOC. Overexpression of SOCS-1 targets Jak1 in an SH2-dependent manner to a perinuclear distribution resembling the MTOC-associated 20S proteasome. Analysis of MTOCs fractionated from SOCS-1-deficient cells demonstrates that SOCS-1 may function redundantly to regulate the localization of Jak1 to the MTOC. Nocodazole inhibits the protein turnover of SOCS-1, demonstrating that the minus-end transport of SOCS-1 to the MTOC-associated 20S proteasome is required to regulate SOCS-1 protein levels. These data link SOCS-1 directly with the proteasome pathway and suggest another function for the SH2 domain of SOCS-1 in the regulation of Jak/STAT signaling.

The role of suppressors of cytokine signaling (SOCS) proteins in regulation of the immune response

Cytokines are an integral component of the adaptive and innate immune responses. The signaling pathways triggered by the engagement of cytokines with their specific cell surface receptors have been extensively studied and have provided a profound understanding of the intracellular machinery that translates exposure of cells to cytokine to a coordinated biological response. It has also become clear that cells have evolved sophisticated mechanisms to prevent excessive responses to cytokines. In this review we focus on the suppressors of cytokine signaling (SOCS) family of cytoplasmic proteins that completes a negative feedback loop to attenuate signal transduction from cytokines that act through the janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway. SOCS proteins inhibit components of the cytokine signaling cascade via direct binding or by preventing access to the signaling complex. The SOCS proteins also appear to target signal transducers for proteasomal destruction. Analyses of genetically modified mice in which SOCS proteins are overexpressed or deleted have established that this family of negative regulators has indispensable roles in regulating cytokine responses in cells of the immune system as well as other tissues. Emerging evidence also suggests that disruption of SOCS expression or activity is associated with several immune and inflammatory diseases, raising the prospect that manipulation of SOCS activity may provide a novel future therapeutic strategy in the management of immunological disorders.

v-Abl Signaling Disrupts SOCS-1 Function in Transformed Pre-B Cells

The v-Abl oncogene activates Jak-Stat signaling during transformation of pre-B cells in mice. Disrupting Jak activation by deleting the Jak binding domain of v-Abl or by expressing a dominant-negative Jak1 decreases v-Abl transformation efficiency. As SOCS-1 is a known potent inhibitor of Jak kinases, the mechanism by which v-Abl bypasses SOCS-1 regulation to constitutively activate Jak kinases was investigated. SOCS-1 is expressed in v-Abl-transformed cells but is unable to inhibit v-Abl-mediated Jak-Stat signaling. In v-Abl transformants, SOCS-1 can inhibit cytokine signals, but it is more efficient at doing so when the cells are treated with STI571, an Abl kinase inhibitor. Downstream effects of v-Abl signaling include phosphorylation of SOCS-1 on nontyrosine residues, disruption of the interaction between SOCS-1 and the Elongin BC complex, and inhibition of SOCS-1-mediated proteasomal targeting of activated Jaks. These findings reveal a mechanism by which Jak-dependent oncogenes may bypass SOCS-1 inhibition.

Insulin Induces SOCS-6 Expression and Its Binding to the p85 Monomer of Phosphoinositide 3-Kinase, Resulting in Improvement in Glucose Metabolism

The suppressors of cytokine signaling (SOCS) family is thought to act largely as a negative regulator of signaling by cytokines and some growth factors. Surprisingly, the SOCS-6 transgenics had no significant defects in the cytokine signaling and hematopoietic system but displayed significant improvements in glucose metabolism. Insulin stimulation of Akt/protein kinase B was also potentiated. Biochemical analysis showed that, after insulin stimulation, SOCS-6 interacted with the monomeric p85 subunit of class-Ia phosphoinositide (PI) 3-kinase but not with p85/p110 dimers. Furthermore, SOCS-6 expression is transiently increased by serum and insulin in normal fibroblasts. However, both the mRNA and protein of SOCS-6 were rapidly degraded after induction by insulin. The degradation of the SOCS-6 protein was partially inhibited by a proteasome inhibitor, suggesting a proteasome-mediated degradation mechanism. In contrast, SOCS-6-associated p85 was not degraded and could be recruited to the newly synthesized SOCS-6 molecules in the presence of insulin, suggesting that SOCS-6 expression and its interaction with p85, but not the degradation, is regulated by insulin. The phenotype of SOCS-6 transgenic mice bears a striking resemblance to p85 knock-out mouse models in which glucose metabolism stimulated by insulin is significantly improved despite reduced activation of PI 3-kinase. This suggests that monomeric p85 might play a physiologically important role in attenuating signaling through PI 3-kinase-dependent pathways in unstimulated cells. Therefore, our results indicate that SOCS-6 may provide a dynamically regulated mechanism by which insulin can transiently overcome the negative effects that p85 monomers have on signaling via PI 3-kinase-dependent signaling pathways.

Distinct activities of suppressor of cytokine signaling (SOCS) proteins and involvement of the SOCS box in controlling G-CSF signaling

Granulocyte-colony stimulating factor (G-CSF) induces proliferation of myeloid progenitor cells and controls their differentiation into mature neutrophils. Signal transducer and activator of transcription (STAT) proteins STAT3 and STAT5 are activated by G-CSF and play distinct roles in neutrophil development. Suppressor of cytokine signaling (SOCS) proteins are induced by STATs and inhibit signaling through various negative-feedback mechanisms. SOCS proteins can compete with docking of signaling substrates to receptors, interfere with Janus tyrosine kinase activity, and target proteins for proteasomal degradation. The latter process is mediated through the conserved C-terminal SOCS box. We determined the role of various SOCS proteins in controlling G-CSF responses and investigated the involvement of the SOCS box therein. We show that SOCS1 and SOCS3, but not CIS and SOCS2, inhibited G-CSF-induced STAT activation in human embryo kidney 293 cells. In myeloid 32D cells, SOCS1 and SOCS3 are induced by G-CSF. However, relative to interleukin-3-containing cultures, during G-CSF-induced neutrophilic differentiation, SOCS3 expression was further elevated, while SOCS1 levels remained constant. SOCS box deletion mutants of SOCS1 and SOCS3 were severely hampered in their abilities to inhibit STAT activation and to efficiently suppress colony formation by primary myeloid progenitors in response to G-CSF. These data demonstrate the importance of the SOCS box for the inhibitory effects of SOCS proteins on G-CSF signaling and show that among the different SOCS family members, SOCS3 is the major negative regulator of G-CSF responses during neutrophilic differentiation.

Alternate interferon signaling pathways

More than a half a century ago, interferons (IFN) were identified as antiviral cytokines. Since that discovery, IFN have been in the forefront of basic and clinical cytokine research. The pleiotropic nature of these cytokines continues to engage a large number of investigators to define their actions further. IFN paved the way for discovery of Janus tyrosine kinase (JAK)-signal transducing activators of transcription (STAT) pathways. A number of important tumor suppressive pathways are controlled by IFN. Several infectious pathogens counteract IFN-induced signaling pathways. Recent studies indicate that IFN activate several new protein kinases, including the MAP kinase family, and downstream transcription factors. This review not only details the established IFN signaling paradigms but also provides insights into emerging alternate signaling pathways and mechanisms of pathogen-induced signaling interference.

Negative regulation of cytokine signaling by CIS/SOCS family proteins and their roles in inflammatory diseases

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.

Altered mRNA abundance of ASB15 and four other genes in skeletal muscle following administration of beta-adrenergic receptor agonists

Beta-adrenergic receptor agonists (BA) stimulate skeletal muscle growth. However, downstream signaling pathways that facilitate this effect remain poorly defined. Objectives of this study were to identify genes differentially expressed after administration of a novel BA and to evaluate the expression of one of those genes in additional models of skeletal muscle growth. Differentially expressed gene fragments were identified through differential display of skeletal muscle biopsies from five steers 24 h after administration of the BA. Five gene fragments designated DD53, DD143, DD163, DD209, and DD214 were identified. Tissue distribution of these genes was evaluated by RT-PCR. While DD53, DD163, DD209, and DD214 were expressed across tissues, DD143 mRNA expression was most abundant in skeletal muscle. DD143, later identified as bovine ASB15, was evaluated in rats following administration of anabolic compounds. Thirteen 7-wk-old female rats were randomly assigned to each of four treatment groups including: control, clenbuterol, trenbolone acetate (TBA), and growth hormone (GH). Changes in rat Asb-15 mRNA were measured at 30 min, 12 h, and 24 h following intraperitoneal injections of each compound. Clenbuterol treatment decreased Asb-15 mRNA in skeletal muscle at 12 and 24 h (P < 0.01) and also decreased mRNA in lung at 12 h (P < 0.05). TBA and GH treatments did not alter Asb-15 mRNA in any of the tissues evaluated (P > 0.10). These results are the first to associate an Asb gene family member with muscle growth or BA administration and suggest a potential role for ASB15 in beta-agonist-induced skeletal muscle hypertrophy.