Suppressor of cytokine signaling (SOCS)-3 protein interacts with the insulin-like growth factor-I receptor

Insulin-like growth factor (IGF)-I/IGF-binding protein-3 complex: therapeutic efficacy and mechanism of protection against type 1 diabetes

IGF-I regulates islet beta-cell growth, survival, and metabolism and protects against type 1 diabetes (T1D). However, the therapeutic efficacy of free IGF-I may be limited by its biological half-life in vivo. We investigated whether prolongation of its half-life as an IGF-I/IGF binding protein (IGFBP)-3 complex affords increased protection against T1D and whether this occurs by influencing T cell function and/or islet beta-cell growth and survival. Administration of IGF-I either alone or as an IGF-I/IGFBP-3 complex reduced the severity of insulitis and delayed the onset of T1D in nonobese diabetic mice, but IGF-I/IGFBP-3 was significantly more effective. Protection from T1D elicited by IGF-I/IGFBP-3 was mediated by up-regulated CCL4 and down-regulated CCL3 gene expression in pancreatic draining lymph nodes, activation of the phosphatidylinositol 3-kinase and Akt/protein kinase B signaling pathway of beta-cells, reduced beta-cell apoptosis, and stimulation of beta-cell replication. Reduced beta-cell apoptosis resulted from elevated Bcl-2 and Bcl-X(L) activity and diminished caspase-9 activity, indicating a novel role for a mitochondrial-dependent pathway of beta-cell death. Thus, IGF-I/IGFBP-3 affords more efficient protection from insulitis, beta-cell destruction, and T1D than IGF-I, and this complex may represent an efficacious therapeutic treatment for the prevention of T1D.

Tyrosine phosphorylation disrupts elongin interaction and accelerates SOCS3 degradation

The suppressors of cytokine signaling (SOCS) are negative feedback inhibitors of cytokine and growth factor-induced signal transduction. The C-terminal SOCS box region is thought to regulate SOCS protein stability most likely via an elongin C interaction. In the present study, we have found that phosphorylation of SOCS3 at two tyrosine residues in the conserved SOCS box, Tyr204 and Tyr221, can inhibit the SOCS3-elongin C interaction and activate proteasome-mediated SOCS3 degradation. Jak-mediated phosphorylation of SOCS3 decreased SOCS3 protein half-life, and phosphorylation of both Tyr204 and Tyr221 was required to fully destabilize SOCS3. In contrast, a phosphorylation-deficient mutant of SOCS3, Y204F,Y221F, remained stable in the presence of activated Jak2 and receptor tyrosine kinases. SOCS3 stability correlated with the relative amount that bound elongin C, because in vitro phosphorylation of a SOCS3-glutathione S-transferase fusion protein abolished its ability to interact with elongin C. In addition, a SOCS3/SOCS1 chimera that co-precipitates with markedly increased elongin C, was significantly more stable than wild-type SOCS3. The data suggest that interaction with elongin C stabilizes SOCS3 protein expression and that phosphorylation of SOCS box tyrosine residues disrupts the complex and enhances proteasome-mediated degradation of SOCS3.

Molecular mechanisms of insulin receptor substrate protein-mediated modulation of insulin signalling

The insulin receptor substrate (IRS) proteins act as important mediators of insulin action. Their regulation serves to augment the specificity of the insulin signalling cascade. They can be regulated--both positively and negatively--at the level of phosphorylation, and signalling through these proteins can be further modulated through the actions of SOCS (suppressor of cytokine signalling) proteins. Understanding the mechanisms of IRS regulation will provide further insight into the pathophysiology of insulin resistance and type 2 diabetes.

Suppressor of cytokine signaling-3 (SOCS-3), a potential mediator of interleukin-6-dependent insulin resistance in hepatocytes

Interleukin-6 (IL-6) is one of several pro-inflammatory cytokines implicated in insulin resistance during infection, cachexia, and obesity. We recently demonstrated that IL-6 inhibits insulin signaling in hepatocytes (Senn, J. J., Klover, P. J., Nowak, I. A., and Mooney, R. A. (2002) Diabetes 51, 3391-3399). Members of the suppressors of cytokine signaling (SOCS) family associate with the insulin receptor (IR), and their ectopic expression inhibits IR signaling. Since several SOCS proteins are induced by IL-6, a working hypothesis is that IL-6-dependent insulin resistance is mediated, at least in part, by induction of SOCS protein(s) in insulin target cells. To examine the involvement of SOCS protein(s) in IL-6-dependent inhibition of insulin receptor signaling, HepG2 cells were treated with IL-6 (20 ng/ml) for periods from 1 min to 8 h. IL-6 induced SOCS-3 transcript at 30 min with a maximum effect at 1 h. SOCS-3 protein levels were also markedly elevated at 1 h. Transcript and protein levels returned to near basal levels by 2 h. SOCS-3 induction by IL-6 paralleled IL-6-dependent inhibition of IR signal transduction. Ectopically expressed SOCS-3 associated with the IR and suppressed insulin-dependent receptor autophosphorylation, insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, association of IRS-1 with the p85 subunit of phosphatidylinositol 3-kinase, and activation of Akt. SOCS-3 was also a direct inhibitor of insulin receptor autophosphorylation in vitro. In mice exposed to IL-6 for 60-90 min, hepatic SOCS-3 expression was increased. This was associated with inhibition of hepatic insulin-dependent receptor autophosphorylation and IRS-1 tyrosine phosphorylation. These data suggest that induction of SOCS-3 in liver may be an important mechanism of IL-6-mediated insulin resistance.

A new role for SOCS in insulin action. Suppressor of cytokine signaling

Suppressor of cytokine signaling (SOCS) family proteins were initially identified as inhibiting cytokine signaling pathways through a negative feedback loop involving the inhibition of Janus kinase activity. More recent data suggest that SOCS proteins may also modulate signaling mediated through receptor tyrosine kinases. Krebs and Hilton discuss research implicating SOCS-1 and SOCS-3 as inhibitors of insulin receptor-mediated and insulin-like growth factor-1 receptor-mediated signaling pathways, as well as the increasing evidence that SOCS proteins may act in part by participating in a ubiquitin ligase complex to promote the degradation of target proteins.

Estrogen inhibits GH signaling by suppressing GH-induced JAK2 phosphorylation, an effect mediated by SOCS-2

Oral estrogen administration attenuates the metabolic action of growth hormone (GH) in humans. To investigate the mechanism involved, we studied the effects of estrogen on GH signaling through Janus kinase (JAK)2 and the signal transducers and activators of transcription (STATs) in HEK293 cells stably expressing the GH receptor (293GHR), HuH7 (hepatoma) and T-47D (breast cancer) cells. 293GHR cells were transiently transfected with an estrogen receptor-alpha expression plasmid and luciferase reporters with binding elements for STAT3 and STAT5 or the beta-casein promoter. GH stimulated the reporter activities by four- to sixfold. Cotreatment with 17beta-estradiol (E(2)) resulted in a dose-dependent reduction in the response of all three reporters to GH to a maximum of 49-66% of control at 100 nM (P < 0.05). No reduction was seen when E(2) was added 1-2 h after GH treatment. Similar inhibitory effects were observed in HuH7 and T-47D cells. E(2) suppressed GH-induced JAK2 phosphorylation, an effect attenuated by actinomycin D, suggesting a requirement for gene expression. Next, we investigated the role of the suppressors of cytokine signaling (SOCS) in E(2) inhibition. E(2) increased the mRNA abundance of SOCS-2 but not SOCS-1 and SOCS-3 in HEK293 cells. The inhibitory effect of E(2) was absent in cells lacking SOCS-2 but not in those lacking SOCS-1 and SOCS-3. In conclusion, estrogen inhibits GH signaling, an action mediated by SOCS-2. This paper provides evidence for regulatory interaction between a sex steroid and the GHJAKSTAT pathway, in which SOCS-2 plays a central mechanistic role.

Suppressors of cytokine signaling: Relevance to gastrointestinal function and disease

BACKGROUND & AIMS

The suppressor of cytokine signaling (SOCS) proteins are a family of Src homology 2 domain-containing proteins. Currently, there are 8 members of the SOCS family, of which a number have been implicated strongly in the negative regulation of cytokine signal transduction pathways.

METHODS

This review focuses on recent discoveries about 4 SOCS family members, SOCS-1, -2, and -3, and cytokine-inducible SH2-domain containing (CIS), and provides more limited information about other SOCS family members.

RESULTS

A large number of cytokines and growth factors are now known to induce SOCS proteins. In turn, SOCS inhibit the actions of a growing number of cytokines and growth factors in vitro or in vivo. SOCS proteins exert their inhibitory effects at the level of activation of janus kinases (JAKs) or by competing with transcription factors for binding sites on activated cytokine receptors. SOCS proteins also may mediate the ubiquitination and subsequent degradation of the SOCS protein and its bound signaling complex. Genetic modification of SOCS genes in mice has revealed crucial roles in the negative regulation of a number of important physiologic parameters including interferon gamma activity, growth, blood cell production, and placental development.

CONCLUSIONS

Information about SOCS action in gastrointestinal function and disease is only just emerging, but available data indicate a role in growth of gastrointestinal tissues, inflammatory bowel disease, and cancer.

Invited Review: Autocrine/paracrine IGF-I and skeletal muscle adaptation

This brief review presents the basic premises suggesting that insulin-like growth factor I (IGF-I), functioning in an autocrine/paracrine mode, is an important mediator of skeletal muscle adaptation. Key intracellular signaling mechanisms associated with ligation of the primary IGF-I receptor are highlighted to illustrate the mechanisms by which IGF-I may promote muscle hypertrophy. In addition, a number of recent findings are presented that highlight the potential for interactions between IGF-I-related signaling pathways and intracellular signaling mechanisms activated by cytokines or hormonal systems.

Functional inactivation of CXC chemokine receptor 4-mediated responses through SOCS3 up-regulation

Hematopoietic cell growth, differentiation, and chemotactic responses require coordinated action between cytokines and chemokines. Cytokines promote receptor oligomerization, followed by Janus kinase (JAK) kinase activation, signal transducers and transactivators of transcription (STAT) nuclear translocation, and transcription of cytokine-responsive genes. These include genes that encode a family of negative regulators of cytokine signaling, the suppressors of cytokine signaling (SOCS) proteins. After binding their specific receptors, chemokines trigger receptor dimerization and activate the JAK/STAT pathway. We show that SOCS3 overexpression or up-regulation, stimulated by a cytokine such as growth hormone, impairs the response to CXCL12, measured by Ca(2+) flux and chemotaxis in vitro and in vivo. This effect is mediated by SOCS3 binding to the CXC chemokine receptor 4 receptor, blocking JAK/STAT and Galpha(i) pathways, without interfering with cell surface chemokine receptor expression. The data provide clear evidence for signaling cross-talk between cytokine and chemokine responses in building a functional immune system.

Growth enhancement in suppressor of cytokine signaling 2 (SOCS-2)-deficient mice is dependent on signal transducer and activator of transcription 5b (STAT5b)

Mice lacking suppressor of cytokine signaling-2 (SOCS-2) exhibit accelerated postnatal growth resulting in adult mice that are 1.3 to 1.5 times the size of normal mice. In this study we examined the somatotrophic pathway to determine whether the production or actions of GH or IGF-I are altered in these mice. We demonstrated that SOCS-2(-/-) mice do not have elevated GH levels and suffer no major pituitary dysmorphogenesis, and that SOCS-2-deficient embryonic fibroblasts do not have altered IGF-I signaling. Primary hepatocytes from SOCS-2(-/-) mice, however, did have moderately prolonged signal transducer and activator of transcription 5 signaling in response to GH stimulation. Furthermore, the deletion of SOCS-2 from mice also lacking signal transducer and activator of transcription 5b had little effect on growth, suggesting that the action of SOCS-2 may be the regulation of the GH signaling pathway.

Role of growth hormone, insulin-like growth factor-I, and insulin-like growth factor binding proteins in the catabolic response to injury and infection

The erosion of lean body mass resulting from protracted critical illness remains a significant risk factor for increased morbidity and mortality in this patient population. Previous studies have documented the well known impairment in nitrogen balance results from both an increase in muscle protein degradation as well as a decreased rate of both myofibrillar and sacroplasmic protein synthesis. This protein imbalance may be caused by an increased presence or activity of various catabolic agents, such as tumor necrosis factor-alpha, interleukin-1 beta, interleukin-6 or glucocorticoids, or may be mediated via a decreased concentration or responsiveness to various anabolic hormones, such as growth hormone or insulin-like growth factor-I. This review focuses on recent developments pertaining to the importance of alterations in the growth hormone-insulin-like growth factor-I axis as a mechanism for the observed defects in muscle protein balance.

Suppressors of cytokine signaling (SOCS): inhibitors of the JAK/STAT pathway

The suppressors of cytokine signaling (SOCS) are recently identified inhibitors of cytokine and growth factor (GF) signaling that act via the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) pathway. Cytokine-mediated JAK/STAT signaling controls a number of important biologic responses, including immune function, cellular growth, differentiation, and hematopoieses. The SOCS family consists of eight proteins: CIS and SOCS1-SOCS7, which contain a central SH2 domain, a conserved C-terminus referred to as the SOCS box, and a unique N-terminus. The expression of SOCS-1 to -3 and CIS is induced by cytokine or GF stimulation, resulting in the inhibition of JAK/STAT-mediated cytokine signaling by what appears to be a classic negative feedback loop. In this article we review cytokine/GF signaling by the JAK/STAT pathway, discovery of the SOCS family, the regulation of SOCS expression, mechanism(s) of SOCS action, and we summarize some of the biochemical and genetic studies investigating the physiologic role of SOCS in regulating cytokine activity.

SOCS proteins: negative regulators of cytokine signaling

Cytokines regulate the growth and differentiation of cells by binding to cell-surface receptors and activating intracellular signal transduction cascades such as the JAK-STAT pathway. Cytokine signaling is negatively regulated with respect to both magnitude and duration, and it is now clear that the suppressor of cytokine signaling (SOCS) family of proteins (SOCS1-SOCS7 and CIS) contributes significantly to this process. Transcripts encoding CIS, SOCS1, SOCS2, and SOCS3 are upregulated in response to cytokine stimulation, and the corresponding SOCS proteins inhibit cytokine-induced signaling pathways. SOCS proteins therefore form part of a classical negative feedback circuit. SOCS family members modulate signaling by several mechanisms, which include inactivation of the Janus kinases (JAKs), blocking access of the signal transducers and activators of transcription (STATs) to receptor binding sites, and ubiquitination of signaling proteins and their subsequent targeting to the proteasome. Gene targeting has been used to generate mice lacking socs1, socs2, or socs3, in order to elucidate the physiological function of these SOCS family members. The analysis of socs1(-/-) mice has revealed that SOCS1 plays a key role in the negative regulation of interferon-gamma signaling and in T cell differentiation. Socs2(-/-) mice are 30%-40% larger than wild-type mice, demonstrating that SOCS2 is a critical regulator of postnatal growth. Additionally, the study of embryos lacking socs3 has revealed that SOCS3 is an important regulator of fetal liver hematopoiesis. The biological role of other SOCS proteins remains to be determined.

Cytokine-inducible SH2-containing protein suppresses PRL signaling by binding the PRL receptor

Inhibition of PRL hormone signaling by suppressor of cytokine signaling (SOCS)/cytokine-inducible SH2-containing protein (CIS) was investigated in transfected HEK 293 cells. We used the physiologically relevant wild-type beta-casein promoter as a target gene for PRL action. We demonstrate that CIS produces a 70% inhibition of PRL signaling by a mechanism distinct from, and downstream of, the effect of SOCS-1 on JAK2. This inhibition involves association with the PRL receptor (PRLR), resulting in the inhibition of signal transducer and activator of transcription 5 (STAT5) activation. Further, we show that SOCS-3 coimmunoprecipitates with the PRLR. These data suggest that SOCS-3 involves a second pathway for the inhibition of PRL signaling other than JAK2 inhibition. Additional results indicate that SOCS-2 can play a more important potentiator role on PRL signaling, resulting in a restoration of 50% of transcriptional inhibition induced by SOCS-3 and a restoration of 100% of transcriptional inhibition induced by CIS. SOCS-2 was able to block the inhibitory effect of SOCS-1. These results indicate that SOCS-2 seems to be an antagonist of the other SOCS. SOCS-1 binds JAK2 and inhibits its phosphorylation; SOCS-3 does not bind JAK2 but binds the PRLR that may mediate its inhibition of JAK2; and finally, CIS binds the PRLR but inhibits signal transducer and activator of transcription 5 rather than JAK2.

Suppressor of cytokine signaling 3 (SOCS-3) protects beta -cells against interleukin-1beta - and interferon-gamma -mediated toxicity

Suppressor of cytokine signaling 3 (SOCS-3) is a negative feedback regulator of IFN-gamma signaling, shown up-regulated in mouse bone marrow cells by the proinflammatory cytokines interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha), and IFN-gamma. IL-1beta and IFN-gamma alone, or potentiated by TNF-alpha, are cytotoxic to the insulin producing pancreatic beta-cells and beta-cell lines in vitro and suggested to contribute to the specific beta-cell destruction in Type-1 diabetes mellitus (T1DM). Using a doxycycline-inducible SOCS-3 expression system in the rat beta-cell line INS-1, we demonstrate that the toxic effect of both IL-1beta or IFN-gamma at concentrations that reduced the viability by 50% over 3 days, was fully preventable when SOCS-3 expression was turned on in the cells. At cytokine concentrations or combinations more toxic to the cells, SOCS-3 overexpression yielded a partial protection. Whereas SOCS-3-mediated inhibition of IFN-gamma signaling is described in other cell systems, SOCS-3 mediated inhibition of IL-1beta signaling has not previously been described. In addition we show that SOCS-3 prevention of IL-1beta-induced toxicity is accompanied by inhibited transcription of the inducible nitric oxide synthase (iNOS) by 80%, resulting in 60% decreased formation of the toxic nitric oxide (NO). Analysis of isolated native rat islets exposed to IL-1beta revealed a naturally occurring but delayed up-regulated SOCS-3 transcription. Influencing SOCS-3 expression thus represents an approach for affecting cytokine-induced signal transduction at a proximal step in the signal cascade, potentially useful in future therapies aimed at reducing the destructive potential of beta-cell cytotoxic cytokines in T1DM, as well as other cytokine-dependent diseases.

Suppressors of cytokine signaling-1 and -6 associate with and inhibit the insulin receptor. A potential mechanism for cytokine-mediated insulin resistance

Insulin resistance contributes to a number of metabolic disorders, including type II diabetes, hypertension, and atherosclerosis. Cytokines, such as tumor necrosis factor-alpha, interleukin-1 beta, and interleukin-6, and hormones, such as growth hormone, are known to cause insulin resistance, but the mechanisms by which they inhibit the cellular response to insulin have not been elucidated. One mechanism by which these agents could cause insulin resistance is by inducing the expression of cellular proteins that inhibit insulin receptor (IR) signaling. Suppressors of cytokine signaling (SOCS) proteins are negative regulators of cytokine signaling pathways, the expression of which is regulated by certain cytokines. SOCS proteins are therefore attractive candidates as mediators of cytokine-induced insulin resistance. We have found that SOCS-1 and SOCS-6 interact with the IR when expressed in human hepatoma cells (HepG2) or in rat hepatoma cells overexpressing the human IR. In SOCS-1-expressing cells, insulin treatment increases the extent of interaction with the IR, whereas in SOCS-6-expressing cells the association with the IR appears to require insulin treatment. SOCS-1 and SOCS-6 do not inhibit insulin-dependent IR autophosphorylation, but both proteins inhibit insulin-dependent activation of ERK1/2 and protein kinase B in vivo and IR-directed phosphorylation of IRS-1 in vitro. These results suggest that SOCS proteins may be inhibitors of IR signaling and could mediate cytokine-induced insulin resistance and contribute to the pathogenesis of type II diabetes.

Suppressor of cytokine signaling 1 interacts with the macrophage colony-stimulating factor receptor and negatively regulates its proliferation signal

Macrophage colony-stimulating factor receptor (M-CSF-R) is a tyrosine kinase that regulates proliferation, differentiation, and cell survival during monocytic lineage development. Upon activation, M-CSF-R dimerizes and autophosphorylates on specific tyrosines, creating binding sites for several cytoplasmic SH2-containing signaling molecules that relay and modulate the M-CSF signal. Here we show that M-CSF-R interacts with suppressor of cytokine signaling 1 (Socs1), a negative regulator of various cytokine and growth factor signaling pathways. Using the yeast two-hybrid system, in vitro glutathione S-transferase-M-CSF-R pull-down, and in vivo coimmunoprecipitation experiments, we demonstrated a direct interaction between the SH2 domain of Socs1 and phosphorylated tyrosines 697 or 721 of the M-CSF-R kinase insert region. Moreover, Socs1 is tyrosine-phosphorylated in response to M-CSF. Ectopic expression of Socs1 in FDC-P1/MAC and EML hematopoietic cell lines decreased their growth rates in the presence of limiting concentrations of M-CSF. However, Socs1 expression did not totally suppress long term cell growth in the presence of saturating M-CSF concentrations, in contrast to other cytokines such as stem cell factor and interleukin 3. Taken together, these results suggest that Socs1 is an M-CSF-R-binding partner involved in negative regulation of proliferation signaling and that it differentially affects cytokine receptor signals.