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

Unravelling the druggability and immunological roles of the SOCS-family proteins

The Suppressor of Cytokine Signalling (SOCS) protein family play a critical role in cytokine signalling and regulation of the JAK/STAT pathway with functional consequences to the immune response. Members of this family are implicated in multiple different signalling cascades that drive autoimmune diseases and cancer, through their binding to phosphotyrosine modified proteins as well as ubiquitination activity as part of Cullin5 RING E3 ligases. Here we review the SOCS family members CISH and SOCS1-SOCS7, with a focus on their complex role in immunity. The interactome and signalling network of this protein family is discussed, and the intricate mechanisms through which SOCS proteins alter and manage the immune system are assessed. We offer structural insights into how SOCS proteins engage their interacting partners and native substrates at the protein-protein interaction level. We describe how this knowledge has enabled drug discovery efforts on SOCS proteins to date and propose strategies for therapeutic intervention using small molecules, either via direct inhibition or leveraging their E3 ligase activity for targeted protein degradation.

The Neglected Suppressor of Cytokine Signalling (SOCS): SOCS4-7

SOCS proteins are essential for the regulation of oncogenic, anti-pathogenic, and proinflammatory signalling cascades, including the JAK/STAT and NF-kB pathways, where they act as negative feedback regulators. Given their powerful role in a broad spectrum of biological processes, it is surprising that the functions of many SOCS proteins have not been widely explored. While the mechanisms of action of CIS, SOCS1-3 are well-documented, information regarding SOCS4-7 remains limited. However, recent studies have begun to elucidate the regulatory functions of these proteins during infection and disease, such as influenza infection, cancer and diabetes. Therefore, this review aims to describe and discuss studies detailing our current understanding of SOCS4-7, painting a clearer picture of the biological processes these regulatory proteins maintain. Indeed, our review highlights important evidence proving that all SOCS play a role in biological processes that are essential for normal immunological homeostasis, clearance of infection and avoidance of disease. Understanding how SOCS proteins interact with other proteins or how they are dysregulated in disease is likely to provide valuable insights for advancing therapeutic approaches.

Role of SOCS and VHL Proteins in Neuronal Differentiation and Development

The basic helix-loop-helix factors play a central role in neuronal differentiation and nervous system development, which involve the Notch and signal transducer and activator of transcription (STAT)/small mother against decapentaplegic signaling pathways. Neural stem cells differentiate into three nervous system lineages, and the suppressor of cytokine signaling (SOCS) and von Hippel-Lindau (VHL) proteins are involved in this neuronal differentiation. The SOCS and VHL proteins both contain homologous structures comprising the BC-box motif. SOCSs recruit Elongin C, Elongin B, Cullin5(Cul5), and Rbx2, whereas VHL recruits Elongin C, Elongin B, Cul2, and Rbx1. SOCSs form SBC-Cul5/E3 complexes, and VHL forms a VBC-Cul2/E3 complex. These complexes degrade the target protein and suppress its downstream transduction pathway by acting as E3 ligases via the ubiquitin-proteasome system. The Janus kinase (JAK) is the main target protein of the E3 ligase SBC-Cul5, whereas hypoxia-inducible factor is the primary target protein of the E3 ligase VBC-Cul2; nonetheless, VBC-Cul2 also targets the JAK. SOCSs not only act on the ubiquitin-proteasome system but also act directly on JAKs to suppress the Janus kinase-signal transduction and activator of transcription (JAK-STAT) pathway. Both SOCS and VHL are expressed in the nervous system, predominantly in brain neurons in the embryonic stage. Both SOCS and VHL induce neuronal differentiation. SOCS is involved in differentiation into neurons, whereas VHL is involved in differentiation into neurons and oligodendrocytes; both proteins promote neurite outgrowth. It has also been suggested that the inactivation of these proteins may lead to the development of nervous system malignancies and that these proteins may function as tumor suppressors. The mechanism of action of SOCS and VHL involved in neuronal differentiation and nervous system development is thought to be mediated through the inhibition of downstream signaling pathways, JAK-STAT, and hypoxia-inducible factor-vascular endothelial growth factor pathways. In addition, because SOCS and VHL promote nerve regeneration, they are expected to be applied in neuronal regenerative medicine for traumatic brain injury and stroke.

Cytokine-inducible SH2 domain containing protein contributes to regulation of adiposity, food intake, and glucose metabolism

The cytokine-inducible SH2 domain containing protein (CISH) is the founding member of the suppressor of cytokine signaling (SOCS) family of negative feedback regulators and has been shown to be a physiological regulator of signaling in immune cells. This study sought to investigate novel functions for CISH outside of the immune system. Mice deficient in CISH were generated and analyzed using a range of metabolic and other parameters, including in response to a high fat diet and leptin administration. CISH knockout mice possessed decreased body fat and showed resistance to diet-induced obesity. This was associated with reduced food intake, but unaltered energy expenditure and microbiota composition. CISH ablation resulted in reduced basal expression of the orexigenic Agrp gene in the arcuate nucleus (ARC) region of the brain. Cish was basally expressed in the ARC, with evidence of co-expression with the leptin receptor (Lepr) gene in Agrp-positive neurons. CISH-deficient mice also showed enhanced leptin responsiveness, although Cish expression was not itself modulated by leptin. CISH-deficient mice additionally exhibited improved insulin sensitivity on a high-fat diet, but not glucose tolerance despite reduced body weight. These data identify CISH as an important regulator of homeostasis through impacts on appetite control, mediated at least in part by negative regulation of the anorexigenic effects of leptin, and impacts on glucose metabolism.

Mice with gene alterations in the GH and IGF family

Much of our understanding of GH's action stems from animal models and the generation and characterization of genetically altered or modified mice. Manipulation of genes in the GH/IGF1 family in animals started in 1982 when the first GH transgenic mice were produced. Since then, multiple laboratories have altered mouse DNA to globally disrupt Gh, Ghr, and other genes upstream or downstream of GH or its receptor. The ability to stay current with the various genetically manipulated mouse lines within the realm of GH/IGF1 research has been daunting. As such, this review attempts to consolidate and summarize the literature related to the initial characterization of many of the known gene-manipulated mice relating to the actions of GH, PRL and IGF1. We have organized the mouse lines by modifications made to constituents of the GH/IGF1 family either upstream or downstream of GHR or to the GHR itself. Available data on the effect of altered gene expression on growth, GH/IGF1 levels, body composition, reproduction, diabetes, metabolism, cancer, and aging are summarized. For the ease of finding this information, key words are highlighted in bold throughout the main text for each mouse line and this information is summarized in Tables 1, 2, 3 and 4. Most importantly, the collective data derived from and reported for these mice have enhanced our understanding of GH action.

Suppressor of cytokine signalling 6 is a potential regulator of antimicrobial peptides in the Chinese oak silkworm, Antheraea pernyi

The SOCS/CIS is a family of intracellular proteins distributed widely among living organisms. The members of this family have extensively been studied in mammals and have been shown to regulate various physiological processes. In contrast, the functional roles of SOCS/CIS family proteins are unknown in most invertebrates, including insects. Here, we retrieved a full-length open reading frame (ORF) of SOCS-6 from Chines oak silkworm, Antheraea pernyi (Designated as ApSOCS-6), using the RNA-seq database. The predicted ApSOCS-6 amino acid sequence comprised an N-terminal SH2 domain and a C-terminal SOCS-box domain. It shared the highly conserved structures of the SOCS proteins with other lepidopteran species. ApSOCS-6 mRNA transcript was detected in all the tested tissues of the A. pernyi larvae; however, the highest mRNA levels were found in the larval hemocytes, fat bodies, and integuments. The mRNA transcript levels of ApSOCS-6 were increased in the A. pernyi larval hemocytes and fat bodies after a challenge by the Gram-positive bacteria, M. luteus, Gram-negative bacteria, Escherichia coli, Virus, ApNPV, and Fungus, B. bassiana. After the knockdown of ApSOCS-6, we found a significant increase in bacterial clearance and a decrease in the relative replication of bacteria. To evaluate the possible cause of enhanced antibacterial activity, we measured antimicrobial peptides expression in the fat body of A. pernyi larvae. The production of AMPs was strongly increased in the B. cereus infected larval fat bodies following silencing of ApSOCS-6. Our data indicate that ApSOCS-6 negatively regulates the expression of AMPs in immune tissues in response to the B. cereus challenge.

Class IA PI3K regulatory subunits: p110-independent roles and structures

The phosphatidylinositol 3-kinase (PI3K) pathway is a critical regulator of many cellular processes including cell survival, growth, proliferation and motility. Not surprisingly therefore, the PI3K pathway is one of the most frequently mutated pathways in human cancers. In addition to their canonical role as part of the PI3K holoenzyme, the class IA PI3K regulatory subunits undertake critical functions independent of PI3K. The PI3K regulatory subunits exist in excess over the p110 catalytic subunits and therefore free in the cell. p110-independent p85 is unstable and exists in a monomer-dimer equilibrium. Two conformations of dimeric p85 have been reported that are mediated by N-terminal and C-terminal protein domain interactions, respectively. The role of p110-independent p85 is under investigation and it has been found to perform critical adaptor functions, sequestering or influencing compartmentalisation of key signalling proteins. Free p85 has roles in glucose homeostasis, cellular stress pathways, receptor trafficking and cell migration. As a regulator of fundamental pathways, the amount of p110-independent p85 in the cell is critical. Factors that influence the monomer-dimer equilibrium of p110-independent p85 offer additional control over this system, disruption to which likely results in disease. Here we review the current knowledge of the structure and functions of p110-independent class IA PI3K regulatory subunits.

Molecular characterization, expression pattern and evolution of nine suppressors of cytokine signaling (SOCS) gene in the swamp eel (Monopterus albus)

Suppressors of cytokine signaling (SOCS) family members have negative effects on cytokine signaling pathways involved in immunity, growth and development. Owing to their typical feature, they have been extensively studied in mammalians, but they have not offered systematic studies among teleosts. In the present study, nine SOCS family genes were identified in the swamp eel genome and analyzed regulation mechanisms of SOCS family members in swamp eels. The open reading frames of MaSOCS1a, MaSOCS1b, MaSOCS2, MaSOCS3a, MaSOCS3b, MaSOCS4, MaSOCS5, MaSOCS6 and MaSOCS7 were 663 bp, 603 bp, 717 bp, 618 bp, 645 bp, 1188 bp, 1488 bp, 1611 bp and 1998 bp and encoded 220, 238, 200, 205, 214, 395, 496, 536 and 655 amino acids, respectively. All SOCS proteins have no signal peptides. Multiple alignment revealed that MaSOCS family members possessed a typical conserved SOCS box and SH2 region. Phylogenetic analyses showed that all SOCS proteins were divided into two main clusters. Taken together with the similarity and identity of SOCS protein amino acids, these results indicated that MaSOCS family members shared conserved with other homologous genes, in which MaSOCS7 was more conserved. Further syntenic analysis confirmed the phylogenetic analysis results and annotation of SOCS protein, suggesting that MaSOCS5 shared a common ancestor gene with that of fish and humans. MaSOCS family members were constitutively expressed in a wide range of tissues with different levels. In particular, spleen and head kidneys play an important role in immune-related pathways. After Aeromonas veronii and polyinosinic-polycytidylic acid (poly I:C) challenge in the spleen and head kidney, MaSOCS family members exhibit different expression profiles. These expression patterns indicated that MaSOCS family members could make acute responses after pathogen invasion. Taken together, these results indicate that MaSOCS family members participate in the immune response against pathogens and offer a solid foundation for future studies of SOCS function.

Molecular Cloning and Expression Analysis of Three Suppressors of Cytokine Signaling Genes (SOCS5, SOCS6, SOCS7) in the Mealworm Beetle Tenebrio molitor

Suppressors of cytokine signaling (SOCS) influence cytokine and growth factor signaling by negatively regulating the Janus kinase (JAK)-signal transducers and activators of transcription (STAT) pathway to maintain homeostasis during immune responses. However, functional characterization of SOCS family members in invertebrates is limited. Here, we identified and evaluated three SOCS genes (type I sub-family) in the mealworm beetle Tenebrio molitor. The full-length open reading frames (ORFs) of TmSOCS5, TmSOCS6, and TmSOCS7 comprised of 1389, 897, and 1458 nucleotides, encoding polypeptides of 462, 297, and 485 amino acids, respectively. The SH2 and SOCS box domains of the TmSOCS C-terminal region were highly conserved. Phylogenetic analysis revealed that these SOCS genes were clustered within the type I subfamily that exhibits the highest amino acid identity with Tribolium castaneum SOCS genes. Contrary to TmSOCS7 expression, the expression levels of TmSOCS5 and TmSOCS6 were lower in the larval, pupal, and adult stages. In larvae and adults, the expression levels of TmSOCS5 and TmSOCS6 were highest in the hemocytes and ovaries, respectively. SOCS transcripts were also highly upregulated in the hemocytes of T. molitor larvae within 3–6 h post-infection with the fungus Candida albicans. Collectively, these results provide valuable information regarding the involvement of TmSOCS type-I subfamily in the host immune response of insects.

Molecular characterization of nine suppressors of cytokine signaling (SOCS) genes from yellow catfish Pelteobagrus fulvidraco and their changes in mRNA expression to dietary carbohydrate levels

Suppressors of cytokine signaling (SOCS) are important molecules that mediates the regulation of glucose homeostasis. Here, we cloned and characterized the full-length cDNA sequences of nine genes of the SOCS family (SOCS1, 2, 3, 3b, 5, 5b, 6, 7 and CISH) from yellow catfish P. fulvidraco, explored their mRNA abundance across the tissues and their mRNA changes to dietary carbohydrate levels. Structural analysis indicated that the nine members shared conserved functional domains to the orthologues of the mammalian SOCS members, such as SRC homology 2 and the SOCS domains. Their mRNAs were constitutively expressed in various tissues but changed among the tissues. Their mRNA expression in response to dietary carbohydrate levels were explored in the liver, muscle, intestine, testis and ovary. Dietary carbohydrate addition showed significant effects on the mRNA levels of the nine SOCS members. Moreover, their mRNA expressions in response to dietary carbohydrate levels were also tissue-dependent. These indicated that SOCS members potentially mediated the utilization of dietary carbohydrate in yellow catfish.

miR-1260b, mediated by YY1, activates KIT signaling by targeting SOCS6 to regulate cell proliferation and apoptosis in NSCLC

Non-small cell lung cancer (NSCLC) is one of the most common aggressive malignancies. miRNAs have been identified as important biomarkers and regulators of NSCLC. However, the functional contributions of miR-1260b to NSCLC cell proliferation and apoptosis have not been studied. In this study, miR-1260b was upregulated in NSCLC plasma, tissues, and cell lines, and its high expression was correlated with tumor size and progression. Functionally, miR-1260b overexpression promoted cell proliferation and cell cycle, conversely inhibited cell apoptosis and senescence. Mechanically, miR-1260b negatively regulated SOCS6 by directly binding to its 3′-UTR. Furthermore, miR-1260b-mediated suppression of SOCS6 activated KIT signaling. Moreover, YY1 was an upstream regulator of miR-1260b. This study is the first to illustrate that miR-1260b, mediated by YY1, activates KIT signaling by targeting SOCS6 to regulate NSCLC cell proliferation and apoptosis, and is a potential biomarker and therapeutic target for NSCLC. In sum, our work provides new insights into the molecular mechanisms of NSCLC involved in cell proliferation and apoptosis.

Understanding SOCS protein specificity

The development and activity of our immune system are largely controlled by the action of pleiotropic cytokines and growth factors, small secreted proteins, which bind to receptors on the surface of immune cells to initiate an appropriate physiological response. Cytokine signalling is predominantly executed by intracellular proteins known as the Janus kinases (JAKs) and the signal transducers and activators of transcriptions (STATs). Although the 'nuts and bolts' of cytokine-activated pathways have been well established, the nuanced way in which distinct cellular outcomes are achieved and the precise molecular details of the proteins that regulate these pathways are still being elucidated. This is highlighted by the intricate role of the suppressor of cytokine signalling (SOCS) proteins. The SOCS proteins act as negative feedback inhibitors, dampening specific cytokine signals to prevent excessive cellular responses and returning the cell to a homeostatic state. A great deal of study has demonstrated their ability to inhibit these pathways at the receptor complex, either through direct inhibition of JAK activity or by targeting the receptor complex for proteasomal degradation. Detailed analysis of individual SOCS proteins is slowly revealing the complex and highly controlled manner by which they can achieve specificity for distinct substrates. However, for many of the SOCS, a level of detail is still lacking, including confident identification of the full suite of tyrosine phosphorylated targets of their SH2 domain. This review will highlight the general mechanisms which govern SOCS specificity of action and discuss the similarities and differences between selected SOCS proteins, focusing on CIS, SOCS1 and SOCS3. Because of the functional and sequence similarities within the SOCS family, we will also discuss the evidence for functional redundancy.

The involvement of suppressor of cytokine signaling 6 (SOCS6) in immune response of Chinese mitten crab Eriocheir sinensis

Suppressor of cytokine signaling (SOCS) is a family of cytokine-inducible negative regulators of cytokine signaling and it plays a crucial role in various physiological processes. In the present study, the full-length cDNA of a SOCS (designated as EsSOCS6) was cloned from Chinese mitten crab Eriocheir sinensis. The open reading frame of EsSOCS6 cDNA was of 1266 bp, which encoded a polypeptide of 421 amino acid residues. There were two typically conserved SOCS family domains in EsSOCS6, including a central Src homology 2 (SH2) domain and a C-terminal SOCS box. The deduced amino acid sequence of EsSOCS6 shared 72-76% similarity with those of other SOCS6 family members. EsSOCS6 mRNA was constitutively expressed in all the examined tissues with higher expression levels in the immune-related tissues, such as hepatopancreas, hemocytes and gill. The mRNA expression levels of the EsSOCS6 in hemocytes were significantly up-regulated after the stimulations with lipopolysaccharide (LPS), Aeromonas hydrophila and polyinosinic-polycytidylic acid (poly (I:C)). The mRNA expressions of threonine/serine protein kinase (EsAkt) and EsRelish were dramatically declined after EsSOCS6 was interfered by dsRNA. Collectively, these results demonstrated that EsSOCS6 might regulate the activation of the NF-κB signaling pathway and play an important role in the innate immune responses of E. sinensis.

Regulation of Macrophage, Dendritic Cell, and Microglial Phenotype and Function by the SOCS Proteins

Macrophages are innate immune cells of dynamic phenotype that rapidly respond to external stimuli in the microenvironment by altering their phenotype to respond to and to direct the immune response. The ability to dynamically change phenotype must be carefully regulated to prevent uncontrolled inflammatory responses and subsequently to promote resolution of inflammation. The suppressor of cytokine signaling (SOCS) proteins play a key role in regulating macrophage phenotype. In this review, we summarize research to date from mouse and human studies on the role of the SOCS proteins in determining the phenotype and function of macrophages. We will also touch on the influence of the SOCS on dendritic cell (DC) and microglial phenotype and function. The molecular mechanisms of SOCS function in macrophages and DCs are discussed, along with how dysregulation of SOCS expression or function can lead to alterations in macrophage/DC/microglial phenotype and function and to disease. Regulation of SOCS expression by microRNA is discussed. Novel therapies and unanswered questions with regard to SOCS regulation of monocyte-macrophage phenotype and function are highlighted.

Expression of suppressor of cytokine signaling genes in human elderly and Alzheimer's disease brains and human microglia

Multiple cellular systems exist to prevent uncontrolled inflammation in brain tissues; the suppressor of cytokine signaling (SOCS) proteins have key roles in these processes. SOCS proteins are involved in restricting cellular signaling pathways by enhancing the degradation of activated receptors and removing the stimuli for continued activation. There are eight separate SOCS genes that code for proteins with similar structures and properties. All SOCS proteins can reduce signaling of activated transcription factors Janus kinase (JAK) and signal transducer and activator of transcription (STAT), but they also regulate many other signaling pathways. SOCS-1 and SOCS-3 have particular roles in regulating inflammatory processes. Chronic inflammation is a key feature of the pathology present in Alzheimer's disease (AD)-affected brains resulting from responses to amyloid plaques or neurofibrillary tangles, the pathological hallmarks of AD. The goal of this study was to examine SOCS gene expression in human non-demented (ND) and AD brains and in human brain-derived microglia to determine if AD-related pathology resulted in a deficit of these critical molecules. We demonstrated that SOCS-1, SOCS-2, SOCS-3 and cytokine-inducible SH2 containing protein (CIS) mRNA expression was increased in amyloid beta peptide (Aβ)- and inflammatory-stimulated microglia, while SOCS-6 mRNA expression was decreased by both types of treatments. Using human brain samples from the temporal cortex from ND and AD cases, SOCS-1 through SOCS-7 and CIS mRNA and SOCS-1 through SOCS-7 protein could be detected constitutively in ND and AD human brain samples. Although, the expression of key SOCS genes did not change to a large extent as a result of AD pathology, there were significantly increased levels of SOCS-2, SOCS-3 and CIS mRNA and increased protein levels of SOCS-4 and SOCS-7 in AD brains. In summary, there was no evidence of a deficit of these key inflammatory regulating proteins in aged or AD brains.

SOCS proteins in regulation of receptor tyrosine kinase signaling

Receptor tyrosine kinases (RTKs) are a family of cell surface receptors that play critical roles in signal transduction from extracellular stimuli. Many in this family of kinases are overexpressed or mutated in human malignancies and thus became an attractive drug target for cancer treatment. The signaling mediated by RTKs must be tightly regulated by interacting proteins including protein-tyrosine phosphatases and ubiquitin ligases. The suppressors of cytokine signaling (SOCS) family proteins are well-known negative regulators of cytokine receptors signaling consisting of eight structurally similar proteins, SOCS1-7, and cytokine-inducible SH2-containing protein (CIS). A key feature of this family of proteins is the presence of an SH2 domain and a SOCS box. Recent studies suggest that SOCS proteins also play a role in RTK signaling. Activation of RTK results in transcriptional activation of SOCS-encoding genes. These proteins associate with RTKs through their SH2 domains and subsequently recruit the E3 ubiquitin machinery through the SOCS box, and thereby limit receptor stability by inducing ubiquitination. In a similar fashion, SOCS proteins negatively regulate mitogenic signaling by RTKs. It is also evident that RTKs can sometimes bypass SOCS regulation and SOCS proteins can even potentiate RTKs-mediated mitogenic signaling. Thus, apart from negative regulation of receptor signaling, SOCS proteins may also influence signaling in other ways.

SOCS6 is a selective suppressor of receptor tyrosine kinase signaling

The suppressors of cytokine signaling (SOCS) are well-known negative regulators of cytokine receptor signaling. SOCS6 is one of eight members of the SOCS family of proteins. Similar to other SOCS proteins, SOCS6 consists of an uncharacterized extended N-terminal region followed by an SH2 domain and a SOCS box. Unlike other SOCS proteins, SOCS6 is mainly involved in negative regulation of receptor tyrosine kinase signaling. SOCS6 is widely expressed in many tissues and is found to be downregulated in many cancers including colorectal cancer, gastric cancer, lung cancer, ovarian cancer, stomach cancer, thyroid cancer, hepatocellular carcinoma, and pancreatic cancer. SOCS6 is involved in negative regulation of receptor signaling by increasing degradation mediated by ubiquitination of receptors or substrate proteins and induces apoptosis by targeting mitochondrial proteins. Therefore, SOCS6 turns out as an important regulator of survival signaling and its activity is required for controlling receptor tyrosine kinase signaling.

Suppressor of cytokine signalling (SOCS) proteins as guardians of inflammatory responses critical for regulating insulin sensitivity

Overactivation of immune pathways in obesity is an important cause of insulin resistance and thus new approaches aimed to limit inflammation or its consequences may be effective for treating Type 2 diabetes. The SOCS (suppressors of cytokine signalling) are a family of proteins that play an essential role in mediating inflammatory responses in both immune cells and metabolic organs such as the liver, adipose tissue and skeletal muscle. In the present review we discuss the role of SOCS1 and SOCS3 in controlling immune cells such as macrophages and T-cells and the impact this can have on systemic inflammation and insulin resistance. We also dissect the mechanisms by which SOCS (1-7) regulate insulin signalling in different tissues including their impact on the insulin receptor and insulin receptor substrates. Lastly, we discuss the important findings from SOCS whole-body and tissue-specific null mice, which implicate an important role for these proteins in controlling insulin action and glucose homoeostasis in obesity.

The role of suppressors of cytokine signalling in human neoplasms

Suppressors of cytokine signalling 1-7 (SOCS1-7) and cytokine-inducible SH2-containing protein (CIS) are a group of intracellular proteins that are well known as JAK-STAT and several other signalling pathways negative feedback regulators. More recently several members have been identified as tumour suppressors and dysregulation of their biological roles in controlling cytokine and growth factor signalling may contribute to the development of many solid organ and haematological malignancies. This review explores their biological functions and their possible tumour suppressing role in human neoplasms.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Genome-wide association study identifies Loci and candidate genes for body composition and meat quality traits in Beijing-You chickens

Body composition and meat quality traits are important economic traits of chickens. The development of high-throughput genotyping platforms and relevant statistical methods have enabled genome-wide association studies in chickens. In order to identify molecular markers and candidate genes associated with body composition and meat quality traits, genome-wide association studies were conducted using the Illumina 60 K SNP Beadchip to genotype 724 Beijing-You chickens. For each bird, a total of 16 traits were measured, including carcass weight (CW), eviscerated weight (EW), dressing percentage, breast muscle weight (BrW) and percentage (BrP), thigh muscle weight and percentage, abdominal fat weight and percentage, dry matter and intramuscular fat contents of breast and thigh muscle, ultimate pH, and shear force of the pectoralis major muscle at 100 d of age. The SNPs that were significantly associated with the phenotypic traits were identified using both simple (GLM) and compressed mixed linear (MLM) models. For nine of ten body composition traits studied, SNPs showing genome wide significance (P<2.59E-6) have been identified. A consistent region on chicken (Gallus gallus) chromosome 4 (GGA4), including seven significant SNPs and four candidate genes (LCORL, LAP3, LDB2, TAPT1), were found to be associated with CW and EW. Another 0.65 Mb region on GGA3 for BrW and BrP was identified. After measuring the mRNA content in beast muscle for five genes located in this region, the changes in GJA1 expression were found to be consistent with that of breast muscle weight across development. It is highly possible that GJA1 is a functional gene for breast muscle development in chickens. For meat quality traits, several SNPs reaching suggestive association were identified and possible candidate genes with their functions were discussed.

Implication of inflammatory signaling pathways in obesity-induced insulin resistance

Obesity is characterized by the development of a low-grade chronic inflammatory state in different metabolic tissues including adipose tissue and liver. This inflammation develops in response to an excess of nutrient flux and is now recognized as an important link between obesity and insulin resistance. Several dietary factors like saturated fatty acids and glucose as well as changes in gut microbiota have been proposed as triggers of this metabolic inflammation through the activation of pattern-recognition receptors (PRRs), including Toll-like receptors (TLR), inflammasome, and nucleotide oligomerization domain (NOD). The consequences are the production of pro-inflammatory cytokines and the recruitment of immune cells such as macrophages and T lymphocytes in metabolic tissues. Inflammatory cytokines activate several kinases like IKKβ, mTOR/S6 kinase, and MAP kinases as well as SOCS proteins that interfere with insulin signaling and action in adipocytes and hepatocytes. In this review, we summarize recent studies demonstrating that PRRs and stress kinases are important integrators of metabolic and inflammatory stress signals in metabolic tissues leading to peripheral and central insulin resistance and metabolic dysfunction. We discuss recent data obtained with genetically modified mice and pharmacological approaches suggesting that these inflammatory pathways are potential novel pharmacological targets for the management of obesity-associated insulin resistance.

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.

Inhibition of PI3K binding to activators by serine phosphorylation of PI3K regulatory subunit p85alpha Src homology-2 domains

Class IA PI3Ks are activated by growth factor receptors and generate lipid second messengers that mediate downstream responses including cell growth, cell migration, and cell survival. The p85 regulatory subunit of PI3K contains Src homology-2 (SH2) domains that mediate binding to tyrosine-phosphorylated receptors or adaptor proteins to facilitate localization and activation of PI3K at the plasma membrane. We report here that persistent activation of PKC family members by phorbol ester stimulation in cells leads to phosphorylation of two serine residues at analogous sites on both SH2 domains of p85α (S361 and S652). The modified serine residues are located in the phospho-tyrosine binding pockets of the two SH2 domains, and in the crystal structures the phosphate moieties are predicted to occupy the same space as the phosphate moieties of bound phospho-tyrosine peptides. Consistent with this prediction, phosphorylation at these serine residues or mutation to aspartate inhibits binding of p85α to tyrosine-phosphorylated peptides. We provide evidence that protein kinase D, which is phosphorylated and activated by PKCs, mediates phosphorylation of S652 in the C-terminal SH2 domain. These results reveal cross talk between PKC signaling and PI3K signaling that impairs PI3K pathway activation under conditions of persistent PKC (and protein kinase D) activity.

Impact of SOCS3 overexpression on human skeletal muscle development in vitro

The Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling cascade has been identified as a crucial factor for myogenesis. The STAT3 isoform is essential for satellite cell migration and myogenic differentiation as it mediates the expression of muscle specific myogenic factors. The SOCS (suppressors of cytokine signaling) family of proteins down-regulates STAT activation. Primary human skeletal muscle cells were isolated and cultured to investigate the effect of SOCS3 adenoviral overexpression on myotube maturation. It was demonstrated that STAT3 inhibition did not influence myotube development or survival. Moreover, SOCS3 overexpression enhances the mRNA expression of downstream targets of STAT3, c-FOS and VEGF. These increases were correlated with enhanced mRNA expression of genes associated with muscle maturation and hypertrophy. Thus SOCS3 influences myoblast differentiation and SOCS3 may be significant in regulating the activity of genes previously identified as transcriptionally regulated by STAT3.

Anti-obesity effect of phosphatidylinositol on diet-induced obesity in mice

The aim of this study is to investigate the biodistribution of phosphatidylinositol (PI) after oral administration and its anti-obesity effect. When a suspension of radiolabeled PI was orally administered to mice and the biodistribution was examined, PI radioactivity accumulated in the liver compared to myo-inositol radioactivity at 48 h or later after administration. Then, a PI suspension was orally administered to diet-induced obesity (DIO) mice every 4 days, and the anti-obesity effect of PI was examined. As a result, PI suppressed the body weight increase of DIO mice and significantly reduced the plasma levels of aspartate aminotransferase (AST) and cholesterol. Furthermore, PI regulated the expression of some genes in the liver involved in lipid synthesis and metabolism. The present study demonstrated that PI accumulated in the liver after oral administration and exerted its anti-obesity effect on DIO by regulating the expression of certain genes involved in lipid metabolism in the liver.

Pim3 negatively regulates glucose-stimulated insulin secretion

Pancreatic β-cell response to glucose stimulation is governed by tightly regulated signaling pathways which have not been fully characterized. A screen for novel signaling intermediates identified Pim3 as a glucose-responsive gene in the β cell, and here, we characterize its role in the regulation of β-cell function. Pim3 expression in the β-cell was first observed through microarray analysis on glucose-stimulated murine insulinoma (MIN6) cells where expression was strongly and transiently induced. In the pancreas, Pim3 expression exhibited similar dynamics and was restricted to the β cell. Perturbation of Pim3 function resulted in enhanced glucose-stimulated insulin secretion, both in MIN6 cells and in isolated islets from Pim3-/- mice, where the augmentation was specifically seen in the second phase of secretion. Consequently, Pim3-/- mice displayed an increased glucose tolerance in vivo. Interestingly, Pim3-/- mice also exhibited increased insulin sensitivity. Glucose stimulation of isolated Pim3-/- islets resulted in increased phosphorylation of ERK1/2, a kinase involved in regulating β-cell response to glucose. Pim3 was also found to physically interact with SOCS6 and SOCS6 levels were strongly reduced in Pim3-/- islets. Overexpression of SOCS6 inhibited glucose-induced ERK1/2 activation, strongly suggesting that Pim3 regulates ERK1/2 activity through SOCS6. These data reveal that Pim3 is a novel glucose-responsive gene in the β cell that negatively regulates insulin secretion by inhibiting the activation of ERK1/2, and through its effect on insulin sensitivity, has potentially a more global function in glucose homeostasis.

SOCS-6 negatively regulates T cell activation through targeting p56lck to proteasomal degradation

The T cell-specific tyrosine kinase, p56(lck), plays crucial roles in T cell receptor (TCR)-mediated T cell activation. Here, we report that SOCS-6 (suppressor of cytokine signaling-6) is a negative regulator of p56(lck). SOCS-6 was identified as a protein binding to the kinase domain of p56(lck) through yeast two-hybrid screening. SOCS-6 bound specifically to p56(lck) (F505), which mimics the active form of p56(lck), but not to wild type p56(lck). In Jurkat T cells, SOCS-6 binding to p56(lck) was detected 1-2 h after TCR stimulation. Confocal microscopy showed that upon APC-T cell conjugation, SOCS-6 was recruited to the immunological synapse and colocalized with the active form of p56(lck). SOCS-6 promoted p56(lck) ubiquitination and its subsequent targeting to the proteasome. Moreover, SOCS-6 overexpression led to repression of TCR-dependent interleukin-2 promoter activity. These results establish that SOCS-6 acts as a negative regulator of T cell activation by promoting ubiquitin-dependent proteolysis.

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.

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.

Insulin-like growth factor-I receptor signal transduction and the Janus Kinase/Signal Transducer and Activator of Transcription (JAK-STAT) pathway

The insulin-like growth factor IGF-I is an important fetal and postnatal growth factor, which is also involved in tissue homeostasis via regulation of proliferation, differentiation, and cell survival. To understand the role of IGF-I in the pathophysiology of a variety of disorders, including growth disorders, cancer, and neurodegenerative diseases, a detailed knowledge of IGF-I signal transduction is required. This knowledge may also contribute to the development of new therapies directed at the IGF-I receptor or other signaling molecules. In this review, we will address IGF-I receptor signaling through the JAK/STAT pathway in IGF-I signaling and the role of cytokine-induced inhibitors of signaling (CIS) and suppressors of cytokine signaling (SOCS). It appears that, in addition to the canonical IGF-I signaling pathways through extracellular-regulated kinase (ERK) and phosphatidylinositol-3 kinase (PI3K)-Akt, IGF-I also signals through the JAK/STAT pathway. Activation of this pathway may lead to induction of SOCS molecules, well-known feedback inhibitors of the JAK/STAT pathway, which also suppress of IGF-I-induced JAK/STAT signaling. Furthermore, other IGF-I-induced signaling pathways may also be modulated by SOCS. It is conceivable that the effect of these classical inhibitors of cytokine signaling directly affect IGF-I receptor signaling, because they are able to associate to the intracellular part of the IGF-I receptor. These observations indicate that CIS and SOCS molecules are key to cross-talk between IGF-I receptor signaling and signaling through receptors belonging to the hematopoietic/cytokine receptor superfamily. Theoretically, dysregulation of CIS or SOCS may affect IGF-I-mediated effects on body growth, cell differentiation, proliferation, and cell survival.

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.

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.

Suppressors of cytokine-signaling proteins induce insulin resistance in the retina and promote survival of retinal cells

OBJECTIVE

Suppressors of cytokine signaling (SOCS) are implicated in the etiology of diabetes, obesity, and metabolic syndrome. Here, we show that some SOCS members are induced, while others are constitutively expressed, in retina and examine whether persistent elevation of SOCS levels in retina by chronic inflammation or cellular stress predisposes to developing insulin resistance in retina, a condition implicated in diabetic retinopathy.

RESEARCH DESIGN AND METHODS

SOCS-mediated insulin resistance and neuroprotection in retina were investigated in 1) an experimental uveitis model, 2) SOCS1 transgenic rats, 3) insulin-deficient diabetic rats, 4) retinal cells depleted of SOCS6 or overexpressing SOCS1/SOCS3, and 5) oxidative stress and light-induced retinal degeneration models.

RESULTS

We show that constitutive expression of SOCS6 protein in retinal neurons may improve glucose metabolism, while elevated SOCS1/SOCS3 expression during uveitis induces insulin resistance in neuroretina. SOCS-mediated insulin resistance, as indicated by its inhibition of basally active phosphoinositide 3-kinase/AKT signaling in retina, is validated in retina-specific SOCS1 transgenic rats and retinal cells overexpressing SOCS1/SOCS3. We further show that the SOCS3 level is elevated in retina by oxidative stress, metabolic stress of insulin-deficient diabetes, or light-induced retinal damage and protects ganglion cells from apoptosis, suggesting that upregulation of SOCS3 may be a common physiologic response of neuroretinal cells to cellular stress.

CONCLUSIONS

Our data suggest two-sided roles of SOCS proteins in retina. Whereas SOCS proteins may improve glucose metabolism, mitigate deleterious effects of inflammation, and promote neuroprotection, persistent SOCS3 expression caused by chronic inflammation or cellular stress can induce insulin resistance and inhibit neurotrophic factors, such as ciliary neurotrophic factor, leukemia inhibitory factor, and insulin, that are essential for retinal cell survival.

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.

The nuclear localization of SOCS6 requires the N-terminal region and negatively regulates Stat3 protein levels

We determined that endogenous- and overexpressed- SOCS6 was localized in both the nucleus and cytoplasm. The localization of SOCS6 depended on amino acids 1-210 in the N-terminal region of the protein, which contains an unidentified domain. GFP-tagged SOCS6 or the N-terminal region, was exclusively localized and widely distributed throughout the entire nucleus, whereas the C-terminal region displayed a nuclear omission pattern. We also demonstrated that the SOCS6 protein could decrease the levels of the Stat3 protein in the nucleus, and that its negative regulation of the Stat3 protein level was dependent on its C-terminal region. These observations suggest that SOCS6 is composed of at least two functional domains required for its biological role in localizing and degrading Stat3 in the nucleus.

Increased SOCS6 stability with PMA requires its N-terminal region and the Erk pathway via Pkcδ activation

We investigated stability of the ectopically expressed the SOCS6 protein in HEK293T cells with PMA, which activates protein kinase C (PKC). The treatment of PMA could largely increase SOCS6 stability in HEK293T cells. But, we did not observe increased protein levels of SOCS3 or Erk1 with PMA. This result suggests that the increased stability of SOCS6 with PMA did not generally occur in other proteins. The stability of SOCS6 depended on the N-terminal region containing an unidentified domain. We then studied the role of signal pathways in SOCS6 stability with PMA. We found that both Erk and Pkcdelta activation were required for the increased SOCS6 stability by PMA. The Erk activation by PMA appeared to be downstream from the Pkcdelta activation. The increased SOCS6 stability and Erk activation by PMA were both conserved in another cell line, MCF7. In addition, we demonstrated that PMA, insulin, and PDGF increased both the stability of endogenous-expressed SOCS6 and Erk activation in MDA-MB231 cells. These observations suggest that Erk activation may be correlated in the cells with high expression of SOCS6.

Functional Cross-modulation between SOCS Proteins Can Stimulate Cytokine Signaling

SOCS (suppressors of cytokine signaling) proteins are negative regulators of cytokine signaling that function primarily at the receptor level. Remarkably, in vitro and in vivo observations revealed both inhibitory and stimulatory effects of SOCS2 on growth hormone signaling, suggesting an additional regulatory level. In this study, we examined the possibility of direct cross-modulation between SOCS proteins and found that SOCS2 could interfere with the inhibitory actions of other SOCS proteins in growth hormone, interferon, and leptin signaling. This SOCS2 effect was SOCS box-dependent, required recruitment of the elongin BC complex, and coincided with degradation of target SOCS proteins. Detailed mammalian protein-protein interaction trap (MAPPIT) analysis indicated that SOCS2 can interact with all members of the SOCS family. SOCS2 may thus function as a molecular bridge between a ubiquitin-protein isopeptide ligase complex and SOCS proteins, targeting them for proteasomal turnover. We furthermore extended these observations to SOCS6 and SOCS7. Our findings point to a unique regulatory role for SOCS2, SOCS6, and SOCS7 within the SOCS family and provide an explanation for the unexpected phenotypes observed in SOCS2 and SOCS6 transgenic mice.

Attenuation of leptin and insulin signaling by SOCS proteins

Leptin and insulin are key hormones involved in the regulation of energy balance and glucose homeostasis. Development of resistance to the action of these hormones, which can occur with age, obesity and inflammation, appears to have a prime role in the pathogenesis of obesity and type 2 diabetes. Specific members of the suppressor of cytokine signaling (SOCS) family of proteins are now thought to have a role in the development of leptin and insulin resistance owing to their ability to inhibit leptin and insulin signaling pathways. In the case of leptin, current evidence suggests that SOCS3 appears to be of particular importance in the development of leptin resistance, whereas the ability to diminish insulin action has been described for several SOCS proteins (SOCS1, SOCS3, SOCS6 and SOCS7).

Exercise increases SOCS-3 expression in rat skeletal muscle: potential relationship to IL-6 expression

Suppressor of cytokine signalling-3 (SOCS-3) has been implicated in the onset of insulin resistance in non-muscle tissue. Thus, we examined the effects of exercise training on SOCS-3 expression and the potential role of SOCS-3 in muscle. Female Sprague-Dawley rats (5-8 months) were treadmill trained for 12 weeks and the muscles were removed 24 h after the last bout of exercise. Exercise training increased SOCS-3 mRNA expression by 80% and 154% in the plantaris and soleus muscle, respectively. To mimic the effects of increased SOCS-3 expression, SOCS-3 cDNA was cotransfected with a NF-kappa B (NF-kappaB) luciferase construct into cultured C2C12 myotubes. SOCS-3 overexpression increased NF-kappaB transcriptional activity by 27-fold. The proximal region of the IL-6 gene promoter contains a NF-kappaB consensus site, which contributes to increased IL-6 expression in various tissues. SOCS-3 cDNA was cotransfected into cultured C2C12 myotubes with either the IL-6 luciferase construct or a mutated NF-kappaB IL-6 luciferase construct. SOCS-3 overexpression increased IL-6 transcriptional activity by 15-fold, however, when the NF-kappaB site was mutated SOCS-3 failed to increase IL-6 transcriptional activity. We subsequently found that IL-6 mRNA expression was elevated in the plantaris and soleus muscles of the trained animals compared to the sedentary animals. Finally, exercise induced a significant reduction in IkappaBalpha and increased phosphorylation of Ikappakappa suggesting that NF-kappaB activation was elevated after exercise training. These data suggest that training-induced elevations in SOCS-3 expression in skeletal muscle may contribute to the exercise-induced increase in IL-6 expression through alterations in the mechanisms that mediate NF-kappaB activity.

The E3 ubiquitin ligase HOIL-1 induces the polyubiquitination and degradation of SOCS6 associated proteins

The suppressor of cytokine signaling (SOCS) proteins are thought to exert their function through the recruitment of interacting-proteins to the ubiquitin/proteasome degradation pathway. All SOCS proteins bind an Elongin BC E3 ubiquitin ligase complex through the common Socs-box. Here, we show that haem-oxidized IRP2 ubiquitin ligase-1 (HOIL-1), another E3 ubiquitin ligase, interacts with SOCS6. The Ubl domain of HOIL-1 and the SH2 and Socs-box domains of SOCS6 are required for the interaction. HOIL-1 expression stabilizes SOCS6 and induces the ubiquitination and degradation of proteins associated with SOCS6. These data suggest that SOCS proteins may interact with different E3 ubiquitin ligases in addition to a common Elongin BC E3 complex.

PTEN and SHIP2 phosphoinositide phosphatases as negative regulators of insulin signalling

Insulin resistance in peripheral tissues is the primary cause responsible for onset of type II diabetes mellitus. Recently, the genetic and biochemical dissection of intracellular signalling pathways transducing the metabolic and mitogenic effects of insulin has contributed to the understanding of the molecular causes of this insulin resistance. In particular, important efforts have been developed to comprehend the role of negative regulators of insulin signalling, since they might represent future therapeutical targets to reduce insulin resistance in peripheral tissues. Herein, we will briefly review major intracellular signalling pathways activated by insulin and how they are negatively regulated by distinct mechanisms. In particular, the role of PTEN and SHIP2, two phosphoinositide phosphatases recently implicated as negative modulators of insulin signalling, is in focus. Current knowledge on the role of PTEN and SHIP2 in insulin resistance, type II diabetes and related disorders will also be discussed.

Deletion of SOCS7 leads to enhanced insulin action and enlarged islets of Langerhans

NIDDM is characterized by progressive insulin resistance and the failure of insulin-producing pancreatic beta cells to compensate for this resistance. Hyperinsulinemia, inflammation, and prolonged activation of the insulin receptor (INSR) have been shown to induce insulin resistance by decreasing INSR substrate (IRS) protein levels. Here we describe a role for SOCS7 in regulating insulin signaling. Socs7-deficient mice exhibited lower glucose levels and prolonged hypoglycemia during an insulin tolerance test and increased glucose clearance in a glucose tolerance test. Six-month-old Socs7-deficient mice exhibited increased growth of pancreatic islets with mildly increased fasting insulin levels and hypoglycemia. These defects correlated with increased IRS protein levels and enhanced insulin action in cells lacking SOCS7. Additionally, SOCS7 associated with the INSR and IRS1--molecules that are essential for normal regulation of insulin action. These data suggest that SOCS7 is a potent regulator of glucose homeostasis and insulin signaling.

The p85 regulatory subunit of phosphoinositide 3-kinase down-regulates IRS-1 signaling via the formation of a sequestration complex

Phosphoinositide (PI) 3-kinase is required for most insulin and insulin-like growth factor (IGF) 1–dependent cellular responses. The p85 regulatory subunit of PI 3-kinase is required to mediate the insulin-dependent recruitment of PI 3-kinase to the plasma membrane, yet mice with reduced p85 expression have increased insulin sensitivity. To further understand the role of p85, we examined IGF-1–dependent translocation of p85α by using a green fluorescence protein (GFP)–tagged p85α (EGFP–p85α). In response to IGF-1, but not to PDGF signaling, EGFP–p85α translocates to discrete foci in the cell. These foci contain the insulin receptor substrate (IRS) 1 adaptor molecule, and their formation requires the binding of p85 to IRS-1. Surprisingly, monomeric p85 is preferentially localized to these foci compared with the p85–p110 dimer, and these foci are not sites of phosphatidylinositol-3,4,5-trisphosphate production. Ultrastructural analysis reveals that p85–IRS-1 foci are cytosolic protein complexes devoid of membrane. These results suggest a mechanism of signal down-regulation of IRS-1 that is mediated by monomeric p85 through the formation of a sequestration complex between p85 and IRS-1.

Calorie restriction increases the ratio of phosphatidylinositol 3-kinase catalytic to regulatory subunits in rat skeletal muscle

Calorie restriction [CR; 60% of ad libitum (AL) intake] improves insulin-stimulated glucose transport, concomitant with enhanced phosphorylation of Akt. The mechanism(s) for the CR-induced increase in Akt phosphorylation of insulin-stimulated muscle is unknown. The purpose of this study was to determine whether CR increased the ratio of catalytic to regulatory subunits favoring enhanced phosphatidylinositol (PI) 3-kinase signaling, which may contribute to increases in Akt phosphorylation and glucose transport in insulin-stimulated muscles. We measured the PI 3-kinase regulatory (p85alpha/beta, p50alpha, and p55alpha) and catalytic (p110) subunits abundance in skeletal muscle from male F344B/N rats after 8 wk of AL or CR treatment. In CR compared with AL muscles, regulatory isoforms, p50alpha and p55alpha abundance were approximately 40% lower (P < 0.01) with unchanged p85alpha/beta levels. There was no diet-related change in catalytic subunit abundance. Despite lower IRS-1 levels ( approximately 35%) for CR vs. AL, IRS-1-p110 association in insulin-stimulated muscles was significantly (P < 0.05) enhanced by approximately 50%. Downstream of PI 3-kinase, CR compared with AL significantly enhanced Akt serine phosphorylation by 1.5-fold higher (P = 0.01) and 3-O-methylglucose transport by approximately 20% in muscles incubated with insulin. The increased ratio of PI 3-kinase catalytic to regulatory subunits favors enhanced insulin signaling, which likely contributes to greater Akt phosphorylation and improved insulin sensitivity associated with CR in skeletal muscle.

Insulin Receptor Substrate 2 Plays Diverse Cell-specific Roles in the Regulation of Glucose Transport

The insulin receptor substrate 2 (IRS-2) protein is one of the major insulin-signaling substrates. In the present study, we investigated the role of IRS-2 in skin epidermal keratinocytes and dermal fibroblasts. Although skin is not a classical insulin target tissue, we have previously demonstrated that insulin, via the insulin receptor, is essential for normal skin cell physiology. To identify the role of IRS-2 in skin cells, we studied cells isolated from IRS-2 knock-out (KO) mice. Whereas proliferation and differentiation were not affected in the IRS-2 KO cells, a striking effect was observed on glucose transport. In IRS-2 KO keratinocytes, the lack of IRS-2 resulted in a dramatic increase in basal and insulin-stimulated glucose transport. The increase in glucose transport was associated with an increase in total phosphatidylinositol (PI) 3-kinase and Akt activation. In contrast, fibroblasts lacking IRS-2 exhibited a significant decrease in basal and insulin-induced glucose transport. We identified the point of divergence, leading to these differences between keratinocytes and fibroblasts, at the IRS-PI 3-kinase association step. In epidermal keratinocytes, PI 3-kinase is associated with and activated by only the IRS-1 protein. On the other hand, in dermal fibroblasts, PI 3-kinase is exclusively associated with and activated by the IRS-2 protein. These observations suggest that IRS-2 functions as a negative or positive regulator of glucose transport in a cell-specific manner. Our results also show that IRS-2 function depends on its cell-specific association with PI 3-kinase.