Insulin Induction of SOCS-2 and SOCS-3 mRNA expression in C2C12 Skeletal Muscle Cells Is Mediated by Stat5*

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.

Tyrosine-phosphorylated SOCS3 negatively regulates cellular transformation mediated by the myeloproliferative neoplasm-associated JAK2 V617F mutant

In the majority of myeloproliferative neoplasms (MPNs) patients, a point mutation, V617F has been found in Janus kinase 2 (JAK2) gene, and this JAK2 mutant provoked aberrant signaling pathway. In the current study, we found that suppressor of cytokine signaling proteins 3 (SOCS3) possessed the tumor suppressive activity against the JAK2 V617F mutant-provoked cellular transformation. The knockdown of SOCS3 increased the expression level of the JAK2 V617F mutant, which enhanced the activation of signaling mediators, including signal transducer and activator of transcription 3 and 5 (STAT3, STAT5) and extracellular signal-regulated kinase (ERK), and also increased of the proliferation rate and tumorigenesis activity of Ba/F3 cells expressing the JAK2 V617F mutant and erythropoietin receptor (EpoR). In contrast, the enforced expression of SOCS3 significantly inhibited the JAK2 V617F mutant-induced activation of downstream signaling molecules, cell proliferation, and tumorigenesis by down-regulating the expression level of the JAK2 V617F mutant. SOCS3 interacted with the JAK2V617F mutant through its SH2 domain and was phosphorylated at Tyr-204 and Tyr-221 in its SOCS box by the JAK2V617F mutant. SOCS3 mutants carrying a mutation in the SH2 domain (R71E) and a substitution at Tyr-221 (Y221F) failed to exert inhibitory effects on JAK2V617F mutant-induced cellular transformation and tumorigenesis. Collectively, these results imply that SOCS3 plays a negative role in the JAK2 V617F mutant-induced oncogenic signaling pathway through its SH2 domain and the phosphorylation of Tyr-221 in its SOCS box.

Comprehensive in silico Study of GLUT10: Prediction of Possible Substrate Binding Sites and Interacting Molecules

OBJECTIVES

The Arterial Tortuosity Syndrome (ATS) is an autosomal recessive connective tissue disorder, mainly characterized by tortuosity and stenosis of the arteries with a propensity towards aneurysm formation and dissection. It is caused by mutations in the SLC2A10 gene that encodes the facilitative glucose transporter GLUT10. The molecules transported by and interacting with GLUT10 have still not been unambiguously identified. Hence, the study attempts to identify both the substrate binding site of GLUT10 and the molecules interacting with this site.

METHODS

As High-resolution X-ray crystallographic structure of GLUT10 was not available, 3D homology model of GLUT10 in open conformation was constructed. Further, molecular docking and bioinformatics investigation were employed.

RESULTS AND DISCUSSION

Blind docking of nine reported potential in vitro substrates with this 3D homology model revealed that substrate binding site is possibly made with PRO531, GLU507, GLU437, TRP432, ALA506, LEU519, LEU505, LEU433, GLN525, GLN510, LYS372, LYS373, SER520, SER124, SER533, SER504, SER436 amino acid residues. Virtual screening of all metabolites from the Human Serum Metabolome Database and muscle metabolites from Human Metabolite Database (HMDB) against the GLUT10 revealed possible substrates and interacting molecules for GLUT10, which were found to be involved directly or partially in ATS progression or different arterial disorders. Reported mutation screening revealed that a highly emergent point mutation (c. 1309G>A, p. Glu437Lys) is located in the predicted substrate binding site region.

CONCLUSION

Virtual screening expands the possibility to explore more compounds that can interact with GLUT10 and may aid in understanding the mechanisms leading to ATS.

Inhibitor of IGF1 receptor alleviates the inflammation process in the diabetic kidney mouse model without activating SOCS2

Objective

To explore the anti-inflammatory mechanism of IGF1R inhibitor in diabetic nephropathy.

Methods

C57/BL6 mice were reared with high-fat diet for 8 weeks, then were injected 30 mg/kg streptozotocin intraperitoneally to induce type 2 diabetes. After 8 weeks, the type 2 diabetes nephropathy model was successfully set up the different drugs were administrated to mice with diabetes (insulin 1-2 U/day, benazepril 10 mg/kg per day intragastrically, IGF-1R inhibitor 30 mg/kg per day intragastrically). After 8 weeks drugs administration, all mice were collected the kidney tissue, measured levels of inflammatory factor (F4/80, TLR4and CD68) and fibrosis markers(αSMA, E-cadherin and SR) using immunohistochemistry and in situ hybridization.

Results

The type 2 diabetes nephropathy model was built successfully, which along with increased urinary protein excretion rate and increased inflammatory infiltration, and the correlation was characterized by increased CD68⁺, F4/80⁺ cells and increased TLR4, αSMA, SR expression. IGF-1R inhibitors reversed this changes, but benazepril and insulin were without significant changes. The insulin decreased the expression level of IGF-1, and increased the levels of suppressor of cytokine signaling 2 (SOCS2). Benazepril and IGF-1R inhibitor were no significant changes like insulin.

Conclusion

Inhibition of IGF1R was a more effective choice for inflammation treatment than Ben or Ins in diabetic kidney disease (DKD). The IGF1R inhibitor blocked pathological changes induced by the over-expression of IGF1 in DKD without up-regulating SOCS2 protein levels.

The growth hormone signaling system: Insights into coordinating the anabolic and catabolic actions of growth hormone

Although growth hormone (GH) is a multifunctional factor that coordinates various aspects of feeding, reproduction, osmoregulation, and immune system function, perhaps two of its most studied actions are the regulation of growth and metabolism, particularly lipid metabolism. In this review, we describe the major growth-promoting and lipid metabolic actions of GH and then discuss how the GH system regulates these actions. Numerous intrinsic and extrinsic factors provide information about the metabolic status of the organism and influence the production of release of GH. The actions of GH are mediated by GH receptors (GHR), which are widely distributed among tissues. Teleosts possess multiple forms of GHRs that arose through the evolution of this group. Modulation of tissue responsiveness to GH is regulated by molecular and functional expression of GHRs, and in teleosts GHR subtypes, by various factors that reflect the metabolic and growth status of the organism, including nutritional state. The action of GH is propagated by the linkage of GHRs to several cellular effector systems, including JAK-STAT, ERK, PI3K-Akt, and PKC. The differential activation of these pathways, which is governed by nutrient status, underlies GH stimulation of growth or GH stimulation of lipolysis. Taken together, the multi-functional actions of GH are determined by the distribution and abundance of GHRs (and GHR subtypes in teleosts) as well as by the GHR-effector system linkages.

SOCS2 correlates with malignancy and exerts growth-promoting effects in prostate cancer

Deregulation of cytokine and growth factor signaling due to an altered expression of endogenous regulators is well recognized in prostate cancer (PCa) and other cancers. Suppressor of cytokine signaling 2 (SOCS2) is a key regulator of the GH, IGF, and prolactin signaling pathways that have been implicated in carcinogenesis. In this study, we evaluated the expression patterns and functional significance of SOCS2 in PCa. Protein expression analysis employing tissue microarrays from two independent patient cohorts revealed a significantly enhanced expression in tumor tissue compared with benign tissue as well as association with Gleason score and disease progression. In vitro and in vivo assays uncovered the involvement of SOCS2 in the regulation of cell growth and apoptosis. Functionally, SOCS2 knockdown inhibited PCa cell proliferation and xenograft growth in a CAM assay. Decreased cell growth after SOCS2 downregulation was associated with cell-cycle arrest and apoptosis. In addition, we proved that SOCS2 expression is significantly elevated upon androgenic stimulation in androgen receptor (AR)-positive cell lines, providing a possible mechanistic explanation for high SOCS2 levels in PCa tissue. Consequently, SOCS2 expression correlated with AR expression in the malignant tissue of patients. On the whole, our study linked increased SOCS2 expression in PCa with a pro-proliferative role in vitro and in vivo.

Inputs and outputs of insulin receptor

The insulin receptor (IR) is an important hub in insulin signaling and its activation is tightly regulated. Upon insulin stimulation, IR is activated through autophosphorylation, and consequently phosphorylates several insulin receptor substrate (IRS) proteins, including IRS1-6, Shc and Gab1. Certain adipokines have also been found to activate IR. On the contrary, PTP, Grb and SOCS proteins, which are responsible for the negative regulation of IR, are characterized as IR inhibitors. Additionally, many other proteins have been identified as IR substrates and participate in the insulin signaling pathway. To provide a more comprehensive understanding of the signals mediated through IR, we reviewed the upstream and downstream signal molecules of IR, summarized the positive and negative modulators of IR, and discussed the IR substrates and interacting adaptor proteins. We propose that the molecular events associated with IR should be integrated to obtain a better understanding of the insulin signaling pathway and diabetes.

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.

Suppressors of cytokine signaling (SOCS) and type 2 diabetes

The suppressors of cytokine signaling (SOCS) proteins are originally identified as negative regulators of cytokine-activated Janus kinase/signal transducers and activators of transcription signaling pathway, but increasing evidence reveals that SOCS proteins play an important role in the development of type 2 diabetes involving regulation of the insulin signaling and pancreatic β-cell function, and that SOCS are promising to be the targets for the treatment of type 2 diabetes. In this review, we focus on the emerging role for SOCS and the potential drugs targeting SOCS for type 2 diabetes.

Novel roles for insulin receptor (IR) in adipocytes and skeletal muscle cells via new and unexpected substrates

The insulin signaling pathway regulates whole-body glucose homeostasis by transducing extracellular signals from the insulin receptor (IR) to downstream intracellular targets, thus coordinating a multitude of biological functions. Dysregulation of IR or its signal transduction is associated with insulin resistance, which may culminate in type 2 diabetes. Following initial stimulation of IR, insulin signaling diverges into different pathways, activating multiple substrates that have roles in various metabolic and cellular processes. The integration of multiple pathways arising from IR activation continues to expand as new IR substrates are identified and characterized. Accordingly, our review will focus on roles for IR substrates as they pertain to three primary areas: metabolism/glucose uptake, mitogenesis/growth, and aging/longevity. While IR functions in a seemingly pleiotropic manner in many cell types, through these three main roles in fat and skeletal muscle cells, IR multi-tasks to regulate whole-body glucose homeostasis to impact healthspan and lifespan.

Gene expression in skeletal muscle after an acute intravenous GH bolus in human subjects: identification of a mechanism regulating ANGPTL4

Growth hormone (GH) acutely stimulates lipolysis and fat oxidation, a process that operates postabsorptively and involves activation of the JAK-STAT pathway in the target tissue; no in vivo data exist regarding subsequent GH-regulated gene transcription. We obtained serum samples and muscle biopsies in human subjects before and 2 h after administration of a GH bolus. A significant (~75%) elevation in serum FFA levels was recorded post GH. Microarray identified 79 GH-regulated genes in muscle. With qRT-PCR, we then examined the expression of selected genes in the presence and absence of glucose-induced suppression of lipolysis. Four genes involved in the JAK-STAT5 signaling pathway were regulated by GH, including SOCS1-3 and CISH, in addition to three genes associated with insulin action: NFκB1A, PIK3C2B, and PRKAG2. The gene encoding ANGPTL4, a protein involved in lipolysis and suppression of LPL activity, exhibited the most pronounced upregulation (5.6-fold) after GH, which was abrogated by concomitant suppression of lipolysis. Therefore, the GH-induced stimulation of ANGPTL4 gene expression seems secondary to induction of lipolysis. This new concept implies that abundant supply of circulating FFA decreases the need for alternative triglyceride-derived FFA through distinct inhibition of LPL mediated by increased ANGPTL4 gene expression in human muscle.

miR-135a targets IRS2 and regulates insulin signaling and glucose uptake in the diabetic gastrocnemius skeletal muscle

Although aberrant miRNA signatures are associated with diabetes, yet, the status and role of altered miRNAs in the diabetic skeletal muscle is currently poorly understood. Here, we report that 41 miRNAs are altered in the diabetic gastrocnemius skeletal muscle and of these, miR-135a that is identified as a critical regulator of myogenesis, is significantly up-regulated. IRS2 is predicted as its potential putative target and its levels are down-regulated in the diabetic gastrocnemius skeletal muscle. In C2C12 cells, while miR-135a levels decreased during differentiation, IRS2 levels were up-regulated. miR-135a significantly reduced IRS2 protein levels and its 3'UTR luciferase reporter activity and these were blunted by the miR-135a inhibitor and mutation in the miR-135a binding site. Knock-down of endogenous miR-135a levels increased IRS2 at the mRNA and protein levels. miR-135a also attenuated insulin stimulated phosphorylation and activation of PI3Kp85α and Akt and glucose uptake. miR-135a levels were also found to be elevated in the human diabetic skeletal muscle. In-vivo silencing of miR-135a alleviated hyperglycemia, improved glucose tolerance and significantly restored the levels of IRS2 and p-Akt in the gastrocnemius skeletal muscle of db/db mice without any effect on their hepatic levels. These suggest that miR-135a targets IRS2 levels by binding to its 3'UTR and this interaction regulates skeletal muscle insulin signaling.

Transcriptional regulation of glucose sensors in pancreatic β-cells and liver: an update

Pancreatic β-cells and the liver play a key role in glucose homeostasis. After a meal or in a state of hyperglycemia, glucose is transported into the β-cells or hepatocytes where it is metabolized. In the β-cells, glucose is metabolized to increase the ATP:ADP ratio, resulting in the secretion of insulin stored in the vesicle. In the hepatocytes, glucose is metabolized to CO(2), fatty acids or stored as glycogen. In these cells, solute carrier family 2 (SLC2A2) and glucokinase play a key role in sensing and uptaking glucose. Dysfunction of these proteins results in the hyperglycemia which is one of the characteristics of type 2 diabetes mellitus (T2DM). Thus, studies on the molecular mechanisms of their transcriptional regulations are important in understanding pathogenesis and combating T2DM. In this paper, we will review a recent update on the progress of gene regulation of glucose sensors in the liver and β-cells.

Insulin regulates SOCS2 expression and the mitogenic effect of IGF-1 in mesangial cells

Renal hypertrophy and deposition of extracellular matrix proteins are consistent findings in diabetic nephropathy and these processes can be halted or reversed by euglycemic control. Using DNA microarray analysis of glomerular RNA from control and diabetic rats we found that the expression levels of insulin-like growth factor 1 receptor (IGF-1R) were increased while those of suppressor of cytokine signaling 2 (SOCS2) and STAT5 were decreased. All of these changes were normalized by islet cell transplantation. Overexpression of SOCS2 in rat mesangial cells inhibited IGF-1-induced activation of extracellular signal-regulated kinase, which subsequently reduced type IV collagen and DNA synthesis, an effect due to interaction of SOCS2 with IGF-1R. Inhibition of SOCS2 overexpression by small interfering RNA suppressed IGF-1R-mediated actions by preventing phosphorylation of tyrosine 317 in the p66Shc adaptor protein; however, overexpression of either SOCS1 or SOCS3 did not affect IGF-1R signaling. Insulin directly increased STAT5 and SOCS2 expression in mesangial cells. This study shows that insulin can inhibit the mitogenic action of IGF-1 in mesangial cells by regulating STAT5/SOCS2 expression. Insulin deficiency may contribute to the mesangial expansion found in diabetes through reduced STAT5/SOCS2 expression.

Growth hormone signaling in vivo in human muscle and adipose tissue: impact of insulin, substrate background, and growth hormone receptor blockade

CONTEXT

GH induces insulin resistance in muscle and fat, and in vitro data indicate that this may involve cross-talk between the signaling pathways of the two hormones.

OBJECTIVE

Our objective was to investigate GH and insulin signaling in vivo in human muscle and fat tissue in response to GH, GH receptor blockade, and insulin stimulation.

DESIGN

We conducted two randomized crossover studies.

PARTICIPANTS

Sixteen healthy males participated.

INTERVENTION

GH was administered as a bolus (n = 8) and constant infusion (n = 8). The bolus study included three arms: 1) control (saline), 2) GH (0.5 mg iv), and 3) GH blockade (pegvisomant 30 mg sc), each combined with a hyperinsulinemic glucose clamp. The infusion study included two arms: 1) GH infusion (45 ng/.kg.min, 5.5 h) and 2) saline infusion (5.5 h) combined with a hyperinsulinemic glucose clamp during the final 2.5 h.

MAIN OUTCOME MEASURES

Muscle and fat biopsies were subjected to Western blotting for expression of Stat5/p-Stat5, Akt/p-Akt, and ERK1/2/p-ERK1/2 and to real-time RT-PCR for expression of SOCS1-3 and IGF-I mRNA.

RESULTS

GH significantly reduced insulin sensitivity. The GH bolus as well as GH infusion induced phosphorylation of Stat5 in muscle and fat, and SOCS3 and IGF-I mRNA expression increased after GH infusion. Hyperinsulinemia induced Akt phosphorylation in both tissues, irrespective of GH status. In muscle, ERK1/2 phosphorylation was increased by insulin, but insulin per se did not induce phosphorylation of Stat5.

CONCLUSIONS

GH exposure associated with insulin resistance acutely translates into GH receptor signaling in human muscle and fat without evidence of cross-talk with insulin signaling pathways. The molecular mechanisms subserving GH-induced insulin resistance in humans remain unclarified.

Insulin receptor substrate (IRS)-1 regulates murine embryonic stem (mES) cells self-renewal

Mouse embryonic stem (mES) cells are pluripotent cells that can be propagated in vitro with leukemia inhibitory factor (LIF) and serum. Intracellular signaling by LIF is principally mediated by activation of STAT-3, although additional pathways for self-renewal have been described. Here, we identified a novel role for Insulin receptor substrate-1 (IRS-1) as a critical factor in mES cells self-renewal and differentiation. IRS-1 is expressed and tyrosyl phosphorylated during mES cells self-renewal. Differentiation of mES cells, by LIF withdrawal, is associated with a marked reduction in IRS-1 expression. Targeting of IRS-1 by si-IRS-1 results in a severe reduction of Oct-4 protein expression and alkaline phosphatase activity, markers of undifferentiated mES cells. IRS-1 targeting does not interfere with LIF-induced STAT-3 phosphorylation, but negatively affects protein kinase B (PKB/AKT) and glycogen synthase kinase-3 (GSK-3beta) phosphorylation, which are downstream effectors of the LIF-mediated PI3K signaling cascade. Targeting of IRS-1 also results in a marked down regulation of Id-1 and Id-2 proteins expression, which are important components for self-renewal of ES cells. Conversely, over expression of IRS-1 inhibits mES cell differentiation. Taken together, these results suggest that expression and activity of IRS-1 are critical to the maintenance of the self-renewal program in mES cells.

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.

Regulation of SOCS-3 expression by leptin and its co-localization with insulin receptor in rat skeletal muscle cells

Obesity is a well-defined risk factor for the development of insulin resistance in target tissues, such as skeletal muscle, and thus type 2 diabetes. This may occur due to endocrine effects mediated by adipokines including leptin, the product of the obese (ob) gene, whose circulating levels positively correlate with body mass index. Induction of suppressor of cytokine-3 (SOCS-3) protein expression has been implicated as a possible mechanism of leptin-induced insulin resistance. Here, we show that treatment of rat skeletal muscle cells with leptin activated the SOCS-3 gene promoter and caused a time-dependent increase in both SOCS-3 mRNA and protein content. Confocal microscopy demonstrated increased co-localization of SOCS-3 with insulin receptor in leptin-treated cells and we confirmed a direct interaction between these two proteins by showing increased coimmunoprecipitation of SOCS-3 and insulin receptor after exposure of cells to leptin. However, the expected functional consequences were not observed, as we saw no change in basal or insulin-stimulated glucose uptake and phosphorylation of GSK3beta, Akt (T308 and S473) or ERK1/2. In summary, leptin induced SOCS-3 expression and its association with the insulin receptor in rat skeletal muscle cells but functional significance of this increase was not apparent upon measuring glucose uptake.

Control of fatty acid metabolism by leptin in L6 rat myoblasts is regulated by hyperinsulinemia

The development of hypothalamic leptin resistance plays a role in the development of obesity, yet whether peripheral leptin resistance occurs in obesity and diabetes is controversial. Here we investigate whether hyperinsulinemia, as observed during the development of Type 2 diabetes, modifies the effects of leptin on long chain fatty acid metabolism in skeletal muscle cells. We used boron dipyrromethene difluoride (BODIPY)-labeled palmitate to show that leptin (60 nM) caused a time-dependent (0-60 min) increase in fatty acid uptake in L6 myoblasts. Quantitative analysis using 3H-palmitate showed that pre-incubation with insulin (100 nM, 24 h) prevented stimulation of fatty acid uptake by leptin. Insulin pre-treatment also attenuated the ability of leptin to phosphorylate acetyl Co-A carboxylase and increase palmitate oxidation. Suppressor of cytokine-3 (SOCS-3) has been proposed as a possible mediator of insulin-induced leptin resistance. Here we show that treatment of L6 cells with insulin elicited a time-dependent increase in both SOCS-3 mRNA and protein content. In summary, hyperinsulinemia can induce leptin resistance in L6 myoblasts and this may be mediated via a SOCS-3-dependent mechanism.

[Insulin and angiotensin II signaling pathways cross-talk: implications with the association between diabetes mellitus, arterial hypertension and cardiovascular disease]

Insulin (Ins) and angiotensin II (AII) play pivotal roles in the control of two vital and closely related systems: the metabolic and the circulatory, respectively. A failure in the proper action of each of these hormones results, to a variable degree, in the development of two highly prevalent and commonly overlapping diseases--diabetes mellitus (DM) and hypertension (AH). In recent years, a series of studies has revealed a tight connection between the signal transduction pathways that mediate Ins and AII actions in target tissues. This molecular cross-talk occurs at multiple levels and plays an important role in phenomena that range from the action of anti-hypertensive drugs to cardiac hypertrophy and energy acquisition by the heart. At the extracellular level, the angiotensin-converting enzyme controls AII synthesis but also interferes with Ins signaling through the proper regulation of AII and the accumulation of bradykinin. At an early intracellular level, AII, acting through JAK-2/IRS-1/PI3-kinase, JNK and ERK, may induce the serine phosphorylation and inhibition of key elements of the Ins-signaling pathway. Finally, by inducing the expression of the regulatory protein SOCS-3, AII may impose a late control on the Ins signal. This review will focus on the main advances obtained in this field and will discuss the implications of this molecular cross-talk in the common clinical association between DM and AH.

DEVELOPMENTAL PROGRAMMING OF OBESITY AND TYPE 2 DIABETES

Obesity and type 2 diabetes are serious health issues in the developed world and are becoming increasingly important on a global scale. Furthermore, the marked increases in both childhood obesity and type 2 diabetes will translate to further increases in adult obesity, diabetes and associated co-morbidities in the near future; as such it has been ranked as a critical public health threat. It is a widely held view that the primary cause of obesity is the development of an obesogenic environment, due to ease of access to highly calorific food and reduced energy expenditure in work and leisure activities. In addition there is strong evidence for a genetic component to human obesity with the identification of a number of genes associated with human obesity. However, on its own the genetic component of this condition cannot account for the dramatic increase in the prevalence of obesity in recent years. Of relevance and as highlighted by epidemiological and experimental studies, is the relationship between the periconceptual, fetal and early infant phases of life and the subsequent development of adult obesity. The terms “developmental programming” and the “Developmental Origins of Adult Health and Disease” are preferentially used to describe these relationships. Despite initial controversy when these relationships were first suggested, both prospective clinical and experimental studies have clearly shown that the propensity to develop abnormalities of cardiovascular, endocrine and metabolic homeostasis in adulthood are increased when fetal development has been adversely affected. This pathogenesis is not based on genetic defects but on altered gene expression seen as a result of fetal adaptation to an adverse intrauterine environment. The relative role of genetic versus environmental factors and the mechanisms underlying developmental programming remain speculative. It is generally argued that in response to an adverse intrauterine environment, the fetus adapts its physiological development to maximise its immediate chances for survival. Owing to the plasticity of the fetus, these adaptations may include resetting of metabolic homeostasis and endocrine systems and the down-regulation of growth, commonly reflected in an altered birth phenotype. It is thought that whilst these changes in fetal physiology (i.e. the prenatal environment) may be beneficial for short term survivalin uterothey may be maladaptive in postnatal life, contributing to poor health outcomes when offspring are exposed to catch-up growth, diet-induced obesity and other factors. The “predictive adaptive response” hypothesis proposes that the degree of mismatch between the pre- and postnatal environments is a major determinant of subsequent disease. This review will address recent work in animal models and observations in the clinical and epidemiological settings onin uteroadaptations and subsequent development of obesity and type 2 diabetes.

STAT3 signaling is activated in human skeletal muscle following acute resistance exercise

The transcription factor signal transducer and activator of transcription 3 (STAT3) has been identified as a mediator of cytokine signaling and implicated in hypertrophy; however, the importance of this pathway following resistance exercise in human skeletal muscle has not been investigated. In the present study, the phosphorylation and nuclear localization of STAT3, together with STAT3-regulated genes, were measured in the early recovery period following intense resistance exercise. Muscle biopsy samples from healthy subjects (7 males, 23.0 + 0.9 yr) were harvested before and again at 2, 4, and 24 h into recovery following a single bout of maximal leg extension exercise (3 sets, 12 repetitions). Rapid and transient activation of phosphorylated (tyrosine 705) STAT3 was observed at 2 h postexercise. STAT3 phosphorylation paralleled the transient localization of STAT3 to the nucleus, which also peaked at 2 h postexercise. Downstream transcriptional events regulated by STAT3 activation peaked at 2 h postexercise, including early responsive genes c-FOS (800-fold), JUNB (38-fold), and c-MYC (140-fold) at 2 h postexercise. A delayed peak in VEGF (4-fold) was measured 4 h postexercise. Finally, genes associated with modulating STAT3 signaling were also increased following exercise, including the negative regulator SOCS3 (60-fold). Thus, following a single bout of intense resistance exercise, a rapid phosphorylation and nuclear translocation of STAT3 are evident in human skeletal muscle. These data suggest that STAT3 signaling is an important common element and may contribute to the remodeling and adaptation of skeletal muscle following resistance exercise.

SOCS-2 interferes with myotube formation and potentiates osteoblast differentiation through upregulation of JunB in C2C12 cells

Suppressor of cytokine signaling (SOCS)-2 regulates normal postnatal growth and its deficiency in mice causes gigantism with increased bone length and proportional enlargement in skeletal muscles. Using C2C12 mesenchymal precursor cell line as a model, we investigated a possible role of SOCS-2 in the differentiation process of mesenchymal precursors. Stable transfection of SOCS-2 into C2C12 cells resulted in the acceleration of proliferation and survival, and inhibition of spontaneous myotube formation. In addition, SOCS-2 potentiated bone morphogenic protein (BMP)-induced transdifferentiation of C2C12 cells into osteoblast phenotypes. These effects of SOCS-2 on C2C12 cells differed strikingly from that of SOCS-1, another member of SOCS family, and its mechanisms were evaluated. SOCS-2 did not alter BMP-induced phosphorylation and nuclear accumulation of Smad1, nor the expression of inhibitory-Smads mRNA. However, SOCS-2 enhanced BMP-induced transcriptional activation of the Smad-responsive reporter gene, suggesting that the action of SOCS-2 is exerted at the transcriptional level. Interestingly, SOCS-2 overexpression in C2C12 cells increased the endogenous JunB protein, one of the key transcriptional factors in the control of BMP/Smad signaling responsiveness. In addition, the proteasome inhibitor enhanced JunB protein expression in C2C12 cells. Moreover, we found that SOCS-2 reduced JunB ubiquitination in COS-7 cells. Although SOCS-2 is a modulator of growth hormone (GH) signaling, the upregulation of JunB by SOCS-2 did not require GH signaling. Taken together, these results suggest that SOCS-2 positively regulates endogenous JunB protein expression in C2C12 cells through inhibition of JunB destabilization by the ubiquitin-proteasome pathway, and thereby regulates the cell fate of mesenchymal precursors.

The Genes Induced by Signal Transducer and Activators of Transcription (STAT)3 and STAT5 in Mammary Epithelial Cells Define the Roles of these STATs in Mammary Development

Prolactin and leukemia inhibitory factor (LIF) have different roles in the adult mammary gland, which are mediated in part by the signal transducers and activators of transcription (STAT)5 and STAT3. In vivo studies have shown that STAT5 contributes to prolactin-dependent lobuloalveolar development and lactation whereas STAT3 mediates LIF-dependent epithelial apoptosis during postlactational involution. To understand the molecular basis of these STAT-dependent pathways, we demonstrate the ligand-independent effects of STAT5 and STAT3 in mammary epithelial cells in vitro and also identify the genes regulated by these related transcription factors. Thus, using conditionally active STAT3- or STAT5a-GyraseB fusion proteins, we observed that enforced and specific dimerization of STAT3 induced apoptosis whereas STAT5 induced differentiation of mammary epithelial cells. Furthermore, STAT5 attenuated apoptosis mediated by LIF, the physiological inducer of STAT3. Microarray analysis of STAT3- and STAT5-induced genes using this system demonstrated a marked specificity, which reflected their different physiological effects in vitro and in vivo. STAT5-specific gene targets included the milk protein genes alpha-casein and kallikrein-8 and the survival factors prosaposin and Grb10. STAT3-specific genes included the apoptosis regulators CCAAT enhancer binding protein-delta, phosphatidylinositol 3-kinase-regulatory subunits, purine nucleoside phosphorylase, and c-fos. These data illustrate that specific activation of STAT3 and STAT5 alone is sufficient to induce and suppress apoptosis, respectively, and that these transcription factors elicit their actions by inducing distinct subsets of target genes in mammary epithelial cells.

The multi-faceted cross-talk between the insulin and angiotensin II signaling systems

Insulin and angiotensin II are hormones that play pivotal roles in the control of two vital and closely related systems, the metabolic and the circulatory systems, respectively. A failure in the proper action of each of these hormones results, to a variable degree, in the development of two highly prevalent and commonly overlapping diseases-diabetes mellitus and hypertension. In recent years, a series of studies has revealed a tight connection between the signal transduction pathways that mediate insulin and angiotensin II actions in target tissues. This molecular cross-talk occurs at multiple levels and plays an important role in phenomena that range from the action of anti-hypertensive drugs to cardiac hypertrophy and energy acquisition by the heart. At the extracellular level, the angiotensin-converting enzyme controls angiotensin II synthesis but also interferes with insulin signaling through the proper regulation of angiotensin II and through the accumulation of bradykinin. At an early intracellular level, angiotensin II, acting through JAK-2/IRS-1/PI3-kinase, JNK and ERK, may induce the serine phosphorylation and inhibition of key elements of the insulin-signaling pathway. Finally, by inducing the expression of the regulatory protein SOCS-3, angiotensin II may impose a late control on the insulin signal. This review will focus on the main advances obtained in this field and will discuss the implications of this molecular cross-talk in the common clinical association between diabetes mellitus and hypertension.

Insulin enhances growth hormone induction of the MEK/ERK signaling pathway

Growth hormone (GH) plays an important role in growth and metabolism by signaling via at least three major pathways, including STATs, ERK1/2, and phosphatidylinositol 3-kinase/Akt. Physiological concentrations of insulin promote growth probably by modulating liver GH receptor (GHR) levels in vivo, but the possible effects of insulin on GH-induced post-GHR signaling have yet to be studied. We hypothesized that short-term insulin, similar to the fluctuations that occur following feeding, affects GH-induced post-GHR signaling. Our present studies suggest that, in rat H4IIE hepatoma cells, insulin (4 h or less) selectively enhanced GH-induced phosphorylation of MEK1/2 and ERK1/2, but not GH-induced activation of STAT5 and Akt. Although insulin pretreatment altered GH-induced formation of Shc.Grb2.SOS complex, it did not significantly affect GH-induced activation of other signaling intermediates upstream of MEK/ERK, including JAK2, Ras, and Raf-1. Immunofluorescent staining indicated that insulin pretreatment facilitated GH-induced cell membrane translocation of MEK1/2. Insulin pretreatment also increased the amount of MEK association with its scaffolding protein, KSR. In summary, short-term insulin treatment of cultured, liver-derived cells selectively sensitized GH-induced MEK/ERK phosphorylation independent of JAK2, Ras, and Raf-1, but likely resulted from increased cell membrane translocation of MEK1/2. These findings suggest that insulin may be necessary for sensitization of cells to GH-induced ERK1/2 activation and provides a potential cellular mechanism by which insulin promotes growth.

SOCS3 promotes apoptosis of mammary differentiated cells

Growth and function of the mammary gland is regulated by cytokines and modulated by suppressor of cytokine signalling (SOCS) proteins. In vitro experiments demonstrated that SOCS3 can inhibit PRL induction of milk protein gene expression and STAT5 activation. We explored the SOCS3 expression pattern during mouse mammary development and its regulation by PRL and GH in wild-type and STAT5a-null mammary tissue. Our results suggest that, in vivo, PRL stimulates SOCS3 expression in stromal adipocytes, independently of STAT5a stimulation. In mammary epithelial cells, SOCS3 expression appears to be related to STAT3 activation. Together, our results are consistent with a role of SOCS3 in the mammary gland by promoting apoptosis of differentiated cells (adipocytes during gestation and epithelial cells during involution).

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.

Insulin signaling in the central nervous system: a critical role in metabolic homeostasis and disease from C. elegans to humans

Insulin and its signaling systems are implicated in both central and peripheral mechanisms governing the ingestion, distribution, metabolism, and storage of nutrients in organisms ranging from worms to humans. Input from the environment regarding the availability and type of nutrients is sensed and integrated with humoral information (provided in part by insulin) regarding the sufficiency of body fat stores. In response to these afferent inputs, neuronal pathways are activated that influence energy flux and nutrient metabolism in the body and ensure reproductive competency. Growing evidence supports the hypothesis that reduced central nervous system insulin signaling from either defective secretion or action contributes to the pathogenesis of common metabolic disorders, including diabetes and obesity, and may therefore help to explain the close association between these two disorders. These considerations implicate insulin action in the brain, an organ previously considered to be insulin independent, as a key determinant of both glucose and energy homeostasis.

Suppressor of cytokine signaling-3 Provides a novel interface in the cross-talk between angiotensin II and insulin signaling systems

Angiotensin II inhibits insulin-induced activation of phosphatidylinositol 3-kinase through a mechanism, at least in part, dependent on serine phosphorylation of the insulin receptor and insulin receptor substrates (IRS)-1/2. Recent evidence shows that suppressor of cytokine signaling-3 (SOCS-3) is induced by insulin and angiotensin II and participates in the negative control of further stimulation of each of these signaling systems independently. In the present study, we evaluated the interaction of angiotensin II-induced SOCS-3 with the insulin signaling pathway in the heart of living rats. A single iv dose of angiotensin II promotes a significant increase of SOCS-3 in heart, an effect that lasts up to 180 min. Once induced, SOCS-3 interacts with the insulin receptor, JAK-2, IRS-1, and IRS-2. The inhibition of SOCS-3 expression by a phosphorthioate-modified antisense oligonucleotide partially restores angiotensin II-induced inhibition of insulin-induced insulin receptor, IRS-1 and IRS-2 tyrosine phosphorylation, and IRS-1 and IRS-2 association with p85-phosphatidylinositol 3-kinase and [Ser473] phosphorylation of Akt. Moreover, the inhibition of SOCS-3 expression partially reverses angiotensin II-induced inhibition of insulin-stimulated glucose transporter-4 translocation to the cell membrane. These results are reproduced in isolated cardiomyocytes. Thus, SOCS-3 participates, as a late event, in the negative cross-talk between angiotensin II and insulin, producing an inhibitory effect on insulin-induced glucose transporter-4 translocation.

24p3 in differentiation of myeloid cells

24p3 is a secreted lipocalin that has been variously related to apoptosis, proliferation, and the neutrophil lineage of blood cells. We have investigated the expression of 24p3 mRNA and protein in myeloid cell lines induced to differentiate by insulin-like growth factor 1 (IGF-1) and the granulocytic-colony simulating factor (G-CSF). Both these growth factors, which cause myeloid cells to differentiate into granulocytes, induced a marked increase in the expression of both 24p3 protein and mRNA. The mRNA especially appeared early after the cells were induced with either IGF-1 or G-CSF, at a time when the cells were still proliferating and are morphologically undifferentiated. 24p3 can be considered an early marker of granulocytic differentiation.

Distinct IL-2 receptor signaling pattern in CD4+CD25+ regulatory T cells

Despite expression of the high-affinity IL-2R, CD4(+)CD25(+) regulatory T cells (Tregs) are hypoproliferative upon IL-2R stimulation in vitro. However the mechanisms by which CD4(+)CD25(+) T cells respond to IL-2 signals are undefined. In this report, we examine the cellular and molecular responses of CD4(+)CD25(+) Tregs to IL-2. IL-2R stimulation results in a G(1) cell cycle arrest, cellular enlargement and increased cellular survival of CD4(+)CD25(+) T cells. We find a distinct pattern of IL-2R signaling in which the Janus kinase/STAT pathway remains intact, whereas IL-2 does not activate downstream targets of phosphatidylinositol 3-kinase. Negative regulation of phosphatidylinositol 3-kinase signaling and IL-2-mediated proliferation of CD4(+)CD25(+) T cells is inversely associated with expression of the phosphatase and tensin homologue deleted on chromosome 10, PTEN.

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.

Role of pescadillo and upstream binding factor in the proliferation and differentiation of murine myeloid cells

Pescadillo (PES1) and the upstream binding factor (UBF1) play a role in ribosome biogenesis, which regulates cell size, an important component of cell proliferation. We have investigated the effects of PES1 and UBF1 on the growth and differentiation of cell lines derived from 32D cells, an interleukin-3 (IL-3)-dependent murine myeloid cell line. Parental 32D cells and 32D IGF-IR cells (expressing increased levels of the type 1 insulin-like growth factor I [IGF-I] receptor [IGF-IR]) do not express insulin receptor substrate 1 (IRS-1) or IRS-2. 32D IGF-IR cells differentiate when the cells are shifted from IL-3 to IGF-I. Ectopic expression of IRS-1 inhibits differentiation and transforms 32D IGF-IR cells into a tumor-forming cell line. We found that PES1 and UBF1 increased cell size and/or altered the cell cycle distribution of 32D-derived cells but failed to make them IL-3 independent. PES1 and UBF1 also failed to inhibit the differentiation program initiated by the activation of the IGF-IR, which is blocked by IRS-1. 32D IGF-IR cells expressing PES1 or UBF1 differentiate into granulocytes like their parental cells. In contrast, PES1 and UBF1 can transform mouse embryo fibroblasts that have high levels of endogenous IRS-1 and are not prone to differentiation. Our results provide a model for one of the theories of myeloid leukemia, in which both a stimulus of proliferation and a block of differentiation are required for leukemia development.

Modulation of insulin action

Insulin is a key hormone regulating the control of metabolism and the maintenance of normoglycaemia and normolipidaemia. Insulin acts by binding to its cell surface receptor, thus activating the receptor's intrinsic tyrosine kinase activity, resulting in receptor autophosphorylation and phosphorylation of several substrates. Tyrosine phosphorylated residues on the receptor itself and on subsequently bound receptor substrates provide docking sites for downstream signalling molecules, including adapters, protein serine/threonine kinases, phosphoinositide kinases and exchange factors. Collectively, those molecules orchestrate the numerous insulin-mediated physiological responses. A clear picture is emerging of the way in which insulin elicits several intracellular signalling pathways to mediate its physiologic functions. A further challenge, being pursued by several laboratories, is to understand the molecular mechanisms that underlie insulin action at the peripheral level, deregulation of which ultimately leads to hyperglycaemia and Type 2 diabetes. We review how circulating factors such as insulin itself, TNF-alpha, interleukins, fatty acids and glycation products influence insulin action through insulin signalling molecules themselves or through other pathways ultimately impinging on the insulin-signalling pathway. Understanding how the mechanism by which molecular insulin action is modulated by these factors will potentially provide new targets for pharmacological agents, to enable the control of altered glucose and lipid metabolism and diabetes.

Suppressor of cytokine signaling 3 is induced by angiotensin II in heart and isolated cardiomyocytes, and participates in desensitization

Angiotensin II (Ang II) exerts a potent growth stimulus on the heart and vascular wall. Activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) intracellular signaling pathway by Ang II mediates at least some of the mitogenic responses to this hormone. In other signaling systems that use the JAK/STAT pathway, proteins of the suppressor of cytokine signaling (SOCS) family participate in signal regulation. In the present study it is demonstrated that SOCS3 is constitutively expressed at a low level in rat heart and neonatal rat ventricular myocytes. Ang II at a physiological concentration enhances the expression of SOCS3 mRNA and protein, mainly via AT1 receptors. After induction, SOCS3 associates with JAK2 and impairs further activation of the JAK2/STAT1 pathway. Pretreatment of rats with a specific phosphorthioate antisense oligonucleotide to SOCS3, reverses the desensitization to angiotensin signaling, as detected by a fall in c-Jun expression after repetitive infusions of the hormone. Thus, SOCS3 is induced by Ang II in rat heart and neonatal rat ventricular myocytes and participates in the modulation of the signal generated by this hormone.

Global analysis of gene expression patterns during disuse atrophy in rat skeletal muscle

Muscular inactivity leads to atrophy, weakness, and decreased fatigue resistance. In order to provide a window into the dynamic processes that underlie muscle atrophy, we performed global gene expression analysis of rat soleus muscles using Affymetrix GeneChips at 1, 4, 7 and 14 days of hindlimb unloading. Expression of 309 known genes was significantly changed by at least 2-fold (212 upregulated, 97 downregulated). K-means clustering was used to divide these genes into co-regulated clusters based on the similarity of temporal expression patterns. This allowed the development of a timeline of the atrophy process with respect to the behaviour of genes in a broad array of functional categories. Regulatory genes were often upregulated early, in either a transient or sustained manner, but they also populated clusters with later patterns of activation, suggesting different phases of molecular adaptations. Other early events were the activation of ubiquitination genes and downregulation of protein chaperones. In comparison, clusters representing slightly delayed activation patterns included genes involved in fast contraction, glycolysis, translational inhibition, oxidative stress, protein degradation, and amino acid catabolism. Downregulated genes exhibited fewer unique expression patterns and included structural and regulatory genes of the extracellular matrix and cytoskeleton, and genes that define a slow-oxidative phenotype. Other novel findings include the tight co-activation of proteasome subunit and ubiquitination genes, differential regulation of serine proteases and serine protease inhibitors, and the identification of transcriptional, signalling, growth and cell cycle genes that probably play a role in the atrophy process. The present work has uncovered temporal patterns of gene expression that highlight the molecular processes that underlie muscle atrophy and provide new avenues for study.

Signaling differences from the A and B isoforms of the insulin receptor (IR) in 32D cells in the presence or absence of IR substrate-1

The A isoform of the insulin receptor (IR) is frequently overexpressed in cancer cells and is activated by IGF-II as well as by insulin, whereas the B isoform is predominant in differentiated tissues and responds poorly to IGF-II. The IR substrate-1 (IRS-1), a docking protein for the IR, is known to send a mitogenic signal and to be a powerful inhibitor of cell differentiation. We have investigated the biological effects of the two IR isoforms in parental 32D hemopoietic cells, which do not express IRS-1, and in 32D-derived cells in which IRS-1 is ectopically expressed. The effects of the two isoforms on cell survival, differentiation markers and nuclear translocation of IRS-1 were compared. The results confirm that the A isoform responds to IGF-II and preferentially sends mitogenic, antiapoptotic signals, whereas the B form, poorly responsive to IGF-II, tends to send differentiation signals.

Dual mechanism of signal transducer and activator of transcription 5 activation by the insulin receptor

Insulin stimulates signal transducer and activator of transcription 5 (Stat5) activation in insulin receptor (IR)-overexpressing cell lines and in insulin target tissues of mice. Stat5b and insulin receptor substrate 1 (IRS-1) interact with the same autophosphorylation site in the IR [phosphotyrosine (pY) 972] in yeast two-hybrid assays, and the IR phosphorylates Stat5b in vitro. These data suggest that Stat5 proteins might be recruited to, and phosphorylated by, the activated IR in vivo. Nevertheless, insulin activates Janus kinases (JAKs) in IR-overexpressing cell lines and in insulin target tissues. To determine whether Stat5 proteins must be recruited to the pY972LSA motif in the IR for insulin-stimulated activation in mammalian cells, we generated and tested a series of IR mutants. The L973R/A975D mutation abolishes the ability of the IR to induce Stat5 activation, whereas IRS-1 phosphorylation is unaffected. In contrast, the N969A/P970A mutation in the IR has no effect on Stat5 activation but significantly reduces IRS-1 phosphorylation. In coimmunoprecipitation assays, insulin-stimulated Stat5 activation correlates with Stat5 recruitment to the IR. We also find that insulin stimulates tyrosine phosphorylation of JAKs that are constitutively associated with the IR. Expression of dominant-negative (DN) JAKs, the JAK inhibitor suppressor of cytokine signaling 1, or pretreatment with the JAK inhibitor, AG490, reduces, but does not eliminate, insulin-induced Stat5 activation. Expression of the appropriate pair of DN JAKs in each of the singly JAK-deficient cell lines further establishes a component of insulin-stimulated Stat5 activation that is JAK independent. This likely represents phosphorylation of Stat5 proteins by the IR, as we find that IR kinase domain phosphorylates Stat5b in vitro on Y699 as efficiently as JAK2. Increasing the concentration of Stat5 proteins in cells favors the direct phosphorylation of Stat5 by the IR kinase where the DN-JAK inhibition of insulin-stimulated Stat5 activation becomes insignificant. At physiological levels of Stat5 however, we propose that JAKs and the IR both contribute to the insulin-induced phosphorylation of Stat5.

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.

Suppressor of cytokine signaling-3 is recruited to the activated granulocyte-colony stimulating factor receptor and modulates its signal transduction

G-CSF is a polypeptide growth factor used in treatment following chemotherapy. G-CSF regulates granulopoiesis and acts on its target cells by inducing homodimerization of the G-CSFR, thereby activating intracellular signaling cascades. The G-CSFR encompasses four tyrosine motifs on its cytoplasmic tail that have been shown to recruit a number of regulatory proteins. Suppressor of cytokine signaling 3 (SOCS-3), also referred to as cytokine-inducible Src homolgy 2-containing protein 3, is a member of a recently discovered family of feedback inhibitors that have been shown to inhibit the Janus kinase/STAT pathway. In this study, we demonstrate that human SOCS-3 is rapidly induced by G-CSF in polymorphonuclear neutrophils as well as in the myeloid precursor cell line U937 and that SOCS-3 negatively regulates G-CSFR-mediated STAT activation. Most importantly, we show that SOCS-3 is recruited to the G-CSFR in a phosphorylation-dependent manner and we identify phosphotyrosine (pY)729 as the major recruitment site for SOCS-3. Furthermore, we demonstrate that SOCS-3 directly binds to this pY motif. Surface plasmon resonance analysis reveals a dissociation constant (K(D)) for this interaction of around 2.8 microM. These findings strongly suggest that the recruitment of SOCS-3 to pY729 is important for the modulation of G-CSFR-mediated signal transduction by SOCS-3.

Prostaglandin-E2 enhances EPO-mediated STAT5 transcriptional activity by serine phosphorylation of CREB

Erythroid colony formation in response to erythropoietin (EPO) stimulation is enhanced by costimulating the cells with prostaglandin-E2 (PGE2). The present study further analyzed the underlying mechanisms and demonstrated that EPO-mediated STAT5 transactivation in the erythroid AS-E2 cell line was enhanced 6-fold by PGE2 (10 microM), without affecting the STAT5 tyrosine phosphorylation or STAT5-DNA binding. Moreover, the PGE2-enhancing effect was independent of STAT5 serine phosphorylation. In AS-E2 cells STAT5 is constitutively phosphorylated on Ser780 (STAT5A) and EPO-dependently phosphorylated on Ser726/731 (STAT5A/STAT5B), but overexpression of STAT5 serine mutants did not affect STAT5 transactivation. In addition, PGE2 did not affect STAT5 serine phosphorylation. Instead, the stimulatory effect of PGE2 on STAT5 signaling could be mimicked by dibutyryl-cyclic adenosine monophosphate (cAMP) and the phosphodiesterase inhibitor IBMX, suggesting that the effect was mediated by cAMP. Activation of the cAMP pathway resulted in cAMP-response element binding protein (CREB) phosphorylation, which was sustained in the presence of EPO plus PGE2 and transient on EPO stimulation alone. The costimulatory effect of PGE2 on EPO-mediated STAT5 transactivation was inhibited by overexpression of serine-dead CREB or protein kinase A (PKA) inhibitor (PKI), in contrast to EPO-mediated transactivation, which was PKA independent. Furthermore, CREB-binding protein (CBP)/p300 was shown to be involved in EPO-mediated STAT5 transactivation, and a CBP mutant with increased affinity for CREB resulted in an additional enhancement of the PGE2 effect. Finally, we demonstrated that the STAT5 target genes Bcl-X, SOCS2, and SOCS3 were up-regulated by costimulation with PGE2. In summary, these studies demonstrate that PGE2 enhancement of EPO-induced STAT5 transactivation is mediated by the cAMP/PKA/CREB pathway.

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.

Suppressors of cytokine signalling (SOCS) in the immune system

The suppressors of cytokine signalling (SOCS) are a family of intracellular proteins, several of which have emerged as key physiological regulators of cytokine responses, including those that regulate the immune system. The SOCS proteins seem to regulate signal transduction by combining direct inhibitory interactions with cytokine receptors and signalling proteins with a generic mechanism of targeting associated proteins for degradation. Evidence is emerging for the involvement of SOCS proteins in diseases of the human immune system, which raises the possibility that therapeutic strategies that are based on the manipulation of SOCS activity might be of clinical benefit.

SOCS-3 inhibits insulin signaling and is up-regulated in response to tumor necrosis factor-alpha in the adipose tissue of obese mice

SOCS (suppressor of cytokine signaling) proteins are inhibitors of cytokine signaling involved in negative feedback loops. We have recently shown that insulin increases SOCS-3 mRNA expression in 3T3-L1 adipocytes. When expressed, SOCS-3 binds to phosphorylated Tyr(960) of the insulin receptor and prevents Stat 5B activation by insulin. Here we show that in COS-7 cells SOCS-3 decreases insulin-induced insulin receptor substrate 1 (IRS-1) tyrosine phosphorylation and its association with p85, a regulatory subunit of phosphatidylinositol-3 kinase. This mechanism points to a function of SOCS-3 in insulin resistance. Interestingly, SOCS-3 expression was found to be increased in the adipose tissue of obese mice, but not in the liver and muscle of these animals. Two polypeptides known to be elevated during obesity, insulin and tumor necrosis factor-alpha (TNF-alpha), induce SOCS-3 mRNA expression in mice. Insulin induces a transient expression of SOCS-3 in the liver, muscle, and the white adipose tissue (WAT). Strikingly, TNF-alpha induced a sustained SOCS-3 expression, essentially in the WAT. Moreover, transgenic ob/ob mice lacking both TNF receptors have a pronounced decrease in SOCS-3 expression in the WAT compared with ob/ob mice, providing genetic evidence for a function of this cytokine in obesity-induced SOCS-3 expression. As SOCS-3 appears as a TNF-alpha target gene that is elevated during obesity, and as SOCS-3 antagonizes insulin-induced IRS-1 tyrosine phosphorylation, we suggest that it is a player in the development of insulin resistance.