Increased insulin sensitivity and hypoinsulinemia in APS knockout mice

STAP-2 facilitates insulin signaling through binding to CAP/c-Cbl and regulates adipocyte differentiation

Signal-transducing adaptor protein-2 (STAP-2) is an adaptor molecule involved in several cellular signaling cascades. Here, we attempted to identify novel STAP-2 interacting molecules, and identified c-Cbl associated protein (CAP) as a binding protein through the C-terminal proline-rich region of STAP-2. Expression of STAP-2 increased the interaction between CAP and c-Cbl, suggesting that STAP-2 bridges these proteins and enhances complex formation. CAP/c-Cbl complex is known to regulate GLUT4 translocation in insulin signaling. STAP-2 overexpressed human hepatocyte Hep3B cells showed enhanced GLUT4 translocation after insulin treatment. Elevated levels of Stap2 mRNA have been observed in 3T3-L1 cells and mouse embryonic fibroblasts (MEFs) during adipocyte differentiation. The differentiation of 3T3-L1 cells into adipocytes was highly promoted by retroviral overexpression of STAP-2. In contrast, STAP-2 knockout (KO) MEFs exhibited suppressed adipogenesis. The increase in body weight with high-fat diet feeding was significantly decreased in STAP-2 KO mice compared to WT animals. These data suggest that the expression of STAP-2 correlates with adipogenesis. Thus, STAP-2 is a novel regulatory molecule that controls insulin signal transduction by forming a c-Cbl/STAP-2/CAP ternary complex.

Microglial NF-κB Signaling Deficiency Protects Against Metabolic Disruptions Caused by Volatile Organic Compound via Modulating the Hypothalamic Transcriptome

Prolonged exposure to benzene, a prevalent volatile organic compound (VOC), at concentrations found in smoke, triggers hyperglycemia, and inflammation in mice. Corroborating this with existing epidemiological data, we show a strong correlation between environmental benzene exposure and metabolic impairments in humans. To uncover the underlying mechanisms, we employed a controlled exposure system and continuous glucose monitoring (CGM), revealing rapid blood glucose surges and disturbances in energy homeostasis in mice. These effects were attributed to alterations in the hypothalamic transcriptome, specifically impacting insulin and immune response genes, leading to hypothalamic insulin resistance and neuroinflammation. Moreover, benzene exposure activated microglial transcription characterized by heightened expression of IKKβ/NF-κB-related genes. Remarkably, selective removal of IKKβ in immune cells or adult microglia in mice alleviated benzene-induced hypothalamic gliosis, and protected against hyperglycemia. In summary, our study uncovers a crucial pathophysiological mechanism, establishing a clear link between airborne toxicant exposure and the onset of metabolic diseases.

Ubiquitin-like processing of TUG proteins as a mechanism to regulate glucose uptake and energy metabolism in fat and muscle

In response to insulin stimulation, fat and muscle cells mobilize GLUT4 glucose transporters to the cell surface to enhance glucose uptake. Ubiquitin-like processing of TUG (Aspscr1, UBXD9) proteins is a central mechanism to regulate this process. Here, recent advances in this area are reviewed. The data support a model in which intact TUG traps insulin-responsive "GLUT4 storage vesicles" at the Golgi matrix by binding vesicle cargoes with its N-terminus and matrix proteins with its C-terminus. Insulin stimulation liberates these vesicles by triggering endoproteolytic cleavage of TUG, mediated by the Usp25m protease. Cleavage occurs in fat and muscle cells, but not in fibroblasts or other cell types. Proteolytic processing of intact TUG generates TUGUL, a ubiquitin-like protein modifier, as the N-terminal cleavage product. In adipocytes, TUGUL modifies a single protein, the KIF5B kinesin motor, which carries GLUT4 and other vesicle cargoes to the cell surface. In muscle, this or another motor may be modified. After cleavage of intact TUG, the TUG C-terminal product is extracted from the Golgi matrix by the p97 (VCP) ATPase. In both muscle and fat, this cleavage product enters the nucleus, binds PPARγ and PGC-1α, and regulates gene expression to promote fatty acid oxidation and thermogenesis. The stability of the TUG C-terminal product is regulated by an Ate1 arginyltransferase-dependent N-degron pathway, which may create a feedback mechanism to control oxidative metabolism. Although it is now clear that TUG processing coordinates glucose uptake with other aspects of physiology and metabolism, many questions remain about how this pathway is regulated and how it is altered in metabolic disease in humans.

β Cell GHS-R Regulates Insulin Secretion and Sensitivity

Growth hormone secretagogue receptor (GHS-R) is widely known to regulate food intake and adiposity, but its role in glucose homeostasis is unclear. In this study, we investigated the expression of GHS-R in mouse pancreatic islets and its role in glycemic regulation. We used Ghsr-IRES-tauGFP mice, with Green Fluorescent Protein (GFP) as a surrogate for GHS-R, to demonstrate the GFP co-localization with insulin and glucagon expression in pancreatic islets, confirming GHS-R expression in β and α cells. We then generated β-cell-specific GHSR-deleted mice with MIP-Cre/ERT and validated that GHS-R suppression was restricted to the pancreatic islets. MIP-Cre/ERT;Ghsr^f/f mice showed normal energy homeostasis with similar body weight, body composition, and indirect calorimetry profile. Interestingly, MIP-Cre/ERT;Ghsr^f/f mice exhibited an impressive phenotype in glucose homeostasis. Compared to controls, MIP-Cre/ERT;Ghsr^f/f mice showed lower fasting blood glucose and insulin; reduced first-phase insulin secretion during a glucose tolerance test (GTT) and glucose-stimulated insulin secretion (GSIS) test in vivo. The isolated pancreatic islets of MIP-Cre/ERT;Ghsr^f/f mice also showed reduced insulin secretion during GSIS ex vivo. Further, MIP-Cre/ERT;Ghsr^f/f mice exhibited improved insulin sensitivity during insulin tolerance tests (ITT). Overall, our results confirmed GHS-R expression in pancreatic β and α cells; GHS-R cell-autonomously regulated GSIS and modulated systemic insulin sensitivity. In conclusion, β cell GHS-R was an important regulator of glucose homeostasis, and GHS-R antagonists may have therapeutic potential for Type 2 Diabetes.

Bioinformatics analysis and genetic polymorphisms in genomic region of the bovine SH2B2 gene and their associations with molecular breeding for body size traits in qinchuan beef cattle

The Src homology 2 B 2 (SH2B2) gene regulate energy balance and body weight at least partially by enhancing Janus kinase-2 (JAK2)-mediated cytokine signaling, including leptin and/or GH signaling. Leptin is an adipose hormone that controls body weight. The objective of the present study is to evaluate the association between body measurement traits and SH2B2 gene polymorphisms as responsible mutations. For this purpose, we selected four single-nucleotide polymorphisms (SNPs) in SH2B2 gene, including two in intron 5 (g.20545A>G, and g.20570G>A, one synonymous SNP g.20693T>C, in exon 6 and one in intron 8 (g.24070C>A, and genotyped them in Qinchuan cattle. SNPs in sample populations were in medium polymorphism level (0.250<PIC<0.500). Association study indicated that the g.20570G>A, g.20693T>C, and g.24070C>A, significantly (P < 0.05) associated with body length (BL) and chest circumference (CC) in Qinchuan cattle. In addition, H4H3 and H5H5 diplotype had highly significantly (P < 0.01) greater body length (BL), rump length (RL), and chest circumference (CC) than H4H2. Our investigation will not only extend the spectrum of genetic variation of bovine SH2B2 gene, but also provide useful information for the marker assisted selection in beef cattle breeding program.

Cullin-RING Ligase 5: Functional characterization and its role in human cancers

Cullin-RING ligase 5 (CRL5) is a multi-protein complex and consists of a scaffold protien cullin 5, a RING protein RBX2 (also known as ROC2 or SAG), adaptor proteins Elongin B/C, and a substrate receptor protein SOCS. Through targeting a variety of substrates for proteasomal degradation or modulating various protein-protein interactions, CRL5 is involved in regulation of many biological processes, such as cytokine signal transduction, inflammation, viral infection, and oncogenesis. As many substrates of CRL5 are well-known oncoproteins or tumor suppressors, abnormal regulation of CRL5 is commonly found in human cancers. In this review, we first briefly introduce each of CRL5 components, and then discuss the biological processes regulated by four members of SOCS-box-containing substrate receptor family through substrate degradation. We next describe how CRL5 is hijacked by a variety of viral proteins to degrade host anti-viral proteins, which facilitates virus infection. We further discuss the regulation of CUL5 and its various roles in human cancers, acting as either a tumor suppressor or an oncoprotein in a context-dependent manner. Finally, we propose novel insights for future perspectives on the validation of cullin5 and other CRL5 components as potential targets, and possible targeting strategies to discover CRL5 inhibitors for anti-cancer and anti-virus therapies.

New perspective on SH2B1: An accelerator of cancer progression

SH2B1 is well-known as an adaptor protein, and deletion of SH2B1 results in severe obesity and both leptin and insulin resistance. Some studies have revealed that SH2B1 is involved in the progression of lung cancer, esophageal cancer, gastric cancer, oropharyngeal cancer, and so on. Biological function experiments have proven that SH2B1 can regulate cellular morphology, motility and adhesion by modifying the actin cytoskeletal reorganization, and it can promote cell mitogenesis, transformation, survival and differentiation via different signal pathways by enhancing the kinase activity of several receptor tyrosine kinases. In addition, SH2B1 is an obesity-related gene, and epidemiological surveys suggest a complex relationship between obesity and cancer. Therefore, what is the relationship between SH2B1 and cancer? Herein, we attempt to provide a mini overview of the roles of SH2B1 in cancer.

Fibroblast growth factor 21 is required for beneficial effects of exercise during chronic high-fat feeding

Exercise is an effective therapy against the metabolic syndrome. However, the molecular pathways underlying the advantageous effects of exercise are elusive. Glucagon receptor signaling is essential for exercise benefits, and recent evidence indicates that a downstream effector of glucagon, fibroblast growth factor 21 (FGF21), is implicated in this response. Therefore, we tested the hypothesis that FGF21 action is necessary in mediating metabolic effects of exercise. We utilized acute exhaustive treadmill exercise in Wistar rats to identify a putative, concomitant increase in plasma glucagon and FGF21 with the increase in glucose and lactate following exercise. To test the necessity of FGF21 action in the exercise response, we exposed FGF21 congenitally deficient mice (Fgf21(-/-)) and their wild-type (Wt) littermates to chronic high-fat (HF) feeding and inoperable (sedentary) or operable (exercise) voluntary running wheels. Physiological tests were performed to assess the role of FGF21 in the beneficial effect of exercise on glucose metabolism. Wt and Fgf21(-/-) littermates exhibited similar running behavior, and exercise was effective in suppressing weight and fat mass gain and dyslipidemia independently of genotype. However, exercise failed to positively affect hepatic triglyceride content and glucose tolerance in HF diet-fed Fgf21(-/-) mice. Furthermore, Fgf21(-/-) mice exhibited an impaired adaptation to exercise training, including reduced AMP-activated protein kinase activity in skeletal muscle. This study demonstrates that FGF21 action is necessary to achieve the full metabolic benefits of exercise during chronic HF feeding.

The neuropilin-like protein ESDN regulates insulin signaling and sensitivity

Insulin effects on cell metabolism, growth, and survival are mediated by its binding to, and activation of, insulin receptor. With increasing prevalence of insulin resistance and diabetes there is considerable interest in identifying novel regulators of insulin signal transduction. The transmembrane protein endothelial and smooth muscle cell-derived neuropilin-like protein (ESDN) is a novel regulator of vascular remodeling and angiogenesis. Here, we investigate a potential role of ESDN in insulin signaling, demonstrating that Esdn gene deletion promotes insulin-induced vascular smooth muscle cell proliferation and migration. This is associated with enhanced protein kinase B and mitogen-activated protein kinase activation as well as insulin receptor phosphorylation. Likewise, insulin signaling in the liver, muscle, and adipose tissue is enhanced in Esdn(-/-) mice, and these animals exhibit improved insulin sensitivity and glucose homeostasis in vivo. The effect of ESDN on insulin signaling is traced back to its interaction with insulin receptor, which alters the receptor interaction with regulatory adaptor protein-E3 ubiquitin ligase pairs, adaptor protein with pleckstrin homology and Src homology 2 domain-c-Cbl and growth factor receptor bound protein 10-neuronal precursor cell-expressed developmentally downregulated 4. In conclusion, our findings establish ESDN as an inhibitor of insulin receptor signal transduction through a novel regulatory mechanism. Loss of ESDN potentiates insulin's metabolic and mitotic effects and provides insights into a novel therapeutic avenue.

SH2B1 regulation of energy balance, body weight, and glucose metabolism

The Src homology 2B (SH2B) family members (SH2B1, SH2B2 and SH2B3) are adaptor signaling proteins containing characteristic SH2 and PH domains. SH2B1 (also called SH2-B and PSM) and SH2B2 (also called APS) are able to form homo- or hetero-dimers via their N-terminal dimerization domains. Their C-terminal SH2 domains bind to tyrosyl phosphorylated proteins, including Janus kinase 2 (JAK2), TrkA, insulin receptors, insulin-like growth factor-1 receptors, insulin receptor substrate-1 (IRS1), and IRS2. SH2B1 enhances leptin signaling by both stimulating JAK2 activity and assembling a JAK2/IRS1/2 signaling complex. SH2B1 promotes insulin signaling by both enhancing insulin receptor catalytic activity and protecting against dephosphorylation of IRS proteins. Accordingly, genetic deletion of SH2B1 results in severe leptin resistance, insulin resistance, hyperphagia, obesity, and type 2 diabetes in mice. Neuron-specific overexpression of SH2B1β transgenes protects against diet-induced obesity and insulin resistance. SH2B1 in pancreatic β cells promotes β cell expansion and insulin secretion to counteract insulin resistance in obesity. Moreover, numerous SH2B1 mutations are genetically linked to leptin resistance, insulin resistance, obesity, and type 2 diabetes in humans. Unlike SH2B1, SH2B2 and SH2B3 are not required for the maintenance of normal energy and glucose homeostasis. The metabolic function of the SH2B family is conserved from insects to humans.

A proteolytic pathway that controls glucose uptake in fat and muscle

Insulin regulates glucose uptake by controlling the subcellular location of GLUT4 glucose transporters. GLUT4 is sequestered within fat and muscle cells during low-insulin states, and is translocated to the cell surface upon insulin stimulation. The TUG protein is a functional tether that sequesters GLUT4 at the Golgi matrix. To stimulate glucose uptake, insulin triggers TUG endoproteolytic cleavage. Cleavage accounts for a large proportion of the acute effect of insulin to mobilize GLUT4 to the cell surface. During ongoing insulin exposure, endocytosed GLUT4 recycles to the plasma membrane directly from endosomes, and bypasses a TUG-regulated trafficking step. Insulin acts through the TC10α GTPase and its effector protein, PIST, to stimulate TUG cleavage. This action is coordinated with insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases, and with other signals to direct overall GLUT4 targeting. Data support the idea that the N-terminal TUG cleavage product, TUGUL, functions as a novel ubiquitin-like protein modifier to facilitate GLUT4 movement to the cell surface. The C-terminal TUG cleavage product is extracted from the Golgi matrix, which vacates an "anchoring" site to permit subsequent cycles of GLUT4 retention and release. Together, GLUT4 vesicle translocation and TUG cleavage may coordinate glucose uptake with physiologic effects of other proteins present in the GLUT4-containing vesicles, and with potential additional effects of the TUG C-terminal product. Understanding this TUG pathway for GLUT4 retention and release will shed light on the regulation of glucose uptake and the pathogenesis of type 2 diabetes.

Hepatic SH2B1 and SH2B2 regulate liver lipid metabolism and VLDL secretion in mice

SH2B1 is an SH2 and PH domain-containing adaptor protein. Genetic deletion of SH2B1 results in obesity, type 2 diabetes, and fatty liver diseases in mice. Mutations in SH2B1 are linked to obesity in humans. SH2B1 in the brain controls energy balance and body weight at least in part by enhancing leptin sensitivity in the hypothalamus. SH2B1 in peripheral tissues also regulates glucose and lipid metabolism, presumably by enhancing insulin sensitivity in peripheral metabolically-active tissues. However, the function of SH2B1 in individual peripheral tissues is unknown. Here we generated and metabolically characterized hepatocyte-specific SH2B1 knockout (HKO) mice. Blood glucose and plasma insulin levels, glucose tolerance, and insulin tolerance were similar between HKO, albumin-Cre, and SH2B1^f/f mice fed either a normal chow diet or a high fat diet (HFD). Adult-onset deletion of SH2B1 in the liver either alone or in combination with whole body SH2B2 knockout also did not exacerbate HFD-induced insulin resistance and glucose intolerance. Adult-onset, but not embryonic, deletion of SH2B1 in the liver attenuated HFD-induced hepatic steatosis. In agreement, adult-onset deletion of hepatic SH2B1 decreased the expression of diacylglycerol acyltransferase-2 (DGAT2) and increased the expression of adipose triglyceride lipase (ATGL). Furthermore, deletion of liver SH2B1 in SH2B2 null mice attenuated very low-density lipoprotein (VLDL) secretion. These data indicate that hepatic SH2B1 is not required for the maintenance of normal insulin sensitivity and glucose metabolism; however, it regulates liver triacylglycerol synthesis, lipolysis, and VLDL secretion.

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.

Effect of genetic variations within the SH2B2 gene on the growth of Chinese cattle

As an adaptor protein, apart from potentiating Janus kinase 2 (JAK2) activation and promoting the insulin signaling pathway, Src homology 2 B 2 (SH2B2) indirectly takes part in the regulation of glucose uptake through the c-Cb1/CAP/TC10 pathway, which can in turn affect growth. In this study, we identified a 4bp indel mutation and three single nucleotide polymorphisms (SNPs) within the SH2B2 gene in 959 individuals from five Chinese cattle breeds by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and sequencing. Based on the four variations, 12 haplotypes were identified. Additionally, there was a tendency that in every population pair-wise linkages increased progressively from V1-V2 to V3-V4. By association analysis, positive effects of genotypes CC and CT (snp1220 locus), DI (4bp indel locus), and CC (snp21049 locus) on growth traits were obtained. Furthermore, when combined, individuals with the combination CCDITTCC showed the best performance at an early age. These results were suggestive of an association of snp1220, 4bp indel and snp12049 with growth performance in Nanyang cattle, indicating possibly the candidate role of the SH2B2 gene in marker assisted selection in a beef cattle breeding program.

Enhanced fasting glucose turnover in mice with disrupted action of TUG protein in skeletal muscle

Insulin stimulates glucose uptake in 3T3-L1 adipocytes in part by causing endoproteolytic cleavage of TUG (tether containing a ubiquitin regulatory X (UBX) domain for glucose transporter 4 (GLUT4)). Cleavage liberates intracellularly sequestered GLUT4 glucose transporters for translocation to the cell surface. To test the role of this regulation in muscle, we used mice with muscle-specific transgenic expression of a truncated TUG fragment, UBX-Cter. This fragment causes GLUT4 translocation in unstimulated 3T3-L1 adipocytes. We predicted that transgenic mice would have GLUT4 translocation in muscle during fasting. UBX-Cter expression caused depletion of PIST (PDZ domain protein interacting specifically with TC10), which transmits an insulin signal to TUG. Whereas insulin stimulated TUG proteolysis in control muscles, proteolysis was constitutive in transgenic muscles. Fasting transgenic mice had decreased plasma glucose and insulin concentrations compared with controls. Whole-body glucose turnover was increased during fasting but not during hyperinsulinemic clamp studies. In muscles with the greatest UBX-Cter expression, 2-deoxyglucose uptake during fasting was similar to that in control muscles during hyperinsulinemic clamp studies. Fasting transgenic mice had increased muscle glycogen, and GLUT4 targeting to T-tubule fractions was increased 5.7-fold. Whole-body oxygen consumption (VO2), carbon dioxide production (VCO2), and energy expenditure were increased by 12-13%. After 3 weeks on a high fat diet, the decreased fasting plasma glucose in transgenic mice compared with controls was more marked, and increased glucose turnover was not observed; the transgenic mice continued to have an increased metabolic rate. We conclude that insulin stimulates TUG proteolysis to translocate GLUT4 in muscle, that this pathway impacts systemic glucose homeostasis and energy metabolism, and that the effects of activating this pathway are maintained during high fat diet-induced insulin resistance in mice.

Regulation of insulin and type 1 insulin-like growth factor signaling and action by the Grb10/14 and SH2B1/B2 adaptor proteins

The effects of insulin and type 1 insulin-like growth factor (IGF-1) on metabolism, growth and survival are mediated by their association with specific receptor tyrosine kinases, which results in both receptor and substrate phosphorylation. Phosphotyrosine residues on receptors and substrates provide docking sites for signaling proteins containing SH2 (Src homology 2) domains, including molecular adaptors. This review focuses on the regulation of insulin/IGF-1 signaling and action by two adaptor families with a similar domain organization: the growth factor receptor-bound proteins Grb7/10/14 and the SH2B proteins. Both Grb10/14 and SH2B1/B2 associate with the activation loop of insulin/IGF-1 receptors through their SH2 domains, but association of Grb10/14 also involves their unique BPS domain. Consistent with Grb14 binding as a pseudosubstrate to the kinase active site, insulin/IGF-induced activation of receptors and downstream signaling pathways in cultured cells is inhibited by Grb10/14 adaptors, but is potentiated by SH2B1/B2 adaptors. Accordingly, Grb10 and Grb14 knockout mice show improved insulin/IGF sensitivity in vivo, and, for Grb10, overgrowth and increased skeketal muscle and pancreatic β-cell mass. Conversely, SH2B1-depleted mice display insulin and IGF-1 resistance, with peripheral depletion leading to reduced adiposity and neuronal depletion leading to obesity through associated leptin resistance. Grb10/14 and SH2B1 adaptors also modulate insulin/IGF-1 action by interacting with signaling components downstream of receptors and exert several tissue-specific effects. The identification of Grb10/14 and SH2B1 as physiological regulators of insulin signaling and action, together with observations that variants at their gene loci are associated with obesity and/or insulin resistance, highlight them as potential therapeutic targets for these conditions.

Regulation of glucose transporter translocation in health and diabetes

To enhance glucose uptake into muscle and fat cells, insulin stimulates the translocation of GLUT4 glucose transporters from intracellular membranes to the cell surface. This response requires the intersection of insulin signaling and vesicle trafficking pathways, and it is compromised in the setting of overnutrition to cause insulin resistance. Insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases and through the Rho GTPase TC10α to act on other targets. In unstimulated cells, GLUT4 is incorporated into specialized storage vesicles containing IRAP, LRP1, sortilin, and VAMP2, which are sequestered by TUG, Ubc9, and other proteins. Insulin mobilizes these vesicles directly to the plasma membrane, and it modulates the trafficking itinerary so that cargo recycles from endosomes during ongoing insulin exposure. Knowledge of how signaling and trafficking pathways are coordinated will be essential to understanding the pathogenesis of diabetes and the metabolic syndrome and may also inform a wide range of other physiologies.

Molecular basis of signaling specificity of insulin and IGF receptors: neglected corners and recent advances

Insulin and insulin-like growth factor (IGF) receptors utilize common phosphoinositide 3-kinase/Akt and Ras/extracellular signal-regulated kinase signaling pathways to mediate a broad spectrum of "metabolic" and "mitogenic" responses. Specificity of insulin and IGF action in vivo must in part reflect expression of receptors and responsive pathways in different tissues but it is widely assumed that it is also determined by the ligand binding and signaling mechanisms of the receptors. This review focuses on receptor-proximal events in insulin/IGF signaling and examines their contribution to specificity of downstream responses. Insulin and IGF receptors may differ subtly in the efficiency with which they recruit their major substrates (IRS-1 and IRS-2 and Shc) and this could influence effectiveness of signaling to "metabolic" and "mitogenic" responses. Other substrates (Grb2-associated binder, downstream of kinases, SH2Bs, Crk), scaffolds (RACK1, β-arrestins, cytohesins), and pathways (non-receptor tyrosine kinases, phosphoinositide kinases, reactive oxygen species) have been less widely studied. Some of these components appear to be specifically involved in "metabolic" or "mitogenic" signaling but it has not been shown that this reflects receptor-preferential interaction. Very few receptor-specific interactions have been characterized, and their roles in signaling are unclear. Signaling specificity might also be imparted by differences in intracellular trafficking or feedback regulation of receptors, but few studies have directly addressed this possibility. Although published data are not wholly conclusive, no evidence has yet emerged for signaling mechanisms that are specifically engaged by insulin receptors but not IGF receptors or vice versa, and there is only limited evidence for differential activation of signaling mechanisms that are common to both receptors. Cellular context, rather than intrinsic receptor activity, therefore appears to be the major determinant of whether responses to insulin and IGFs are perceived as "metabolic" or "mitogenic."

Critical role of the Src homology 2 (SH2) domain of neuronal SH2B1 in the regulation of body weight and glucose homeostasis in mice

SH2B1 is an SH2 domain-containing adaptor protein that plays a key role in the regulation of energy and glucose metabolism in both rodents and humans. Genetic deletion of SH2B1 in mice results in obesity and type 2 diabetes. Single-nucleotide polymorphisms in the SH2B1 loci and chromosomal deletions of the SH2B1 loci associate with obesity and insulin resistance in humans. In cultured cells, SH2B1 promotes leptin and insulin signaling by binding via its SH2 domain to phosphorylated tyrosines in Janus kinase 2 and the insulin receptor, respectively. Here we generated three lines of mice to analyze the role of the SH2 domain of SH2B1 in the central nervous system. Transgenic mice expressing wild-type, SH2 domain-defective (R555E), or SH2 domain-alone (DeltaN503) forms of SH2B1 specifically in neurons were crossed with SH2B1 knockout mice to generate KO/SH2B1, KO/R555E, or KO/DeltaN503 compound mutant mice. R555E had a replacement of Arg(555) with Glu within the SH2 domain. DeltaN503 contained an intact SH2 domain but lacked amino acids 1-503. Neuron-specific expression of recombinant SH2B1, but not R555E or DeltaN503, corrected hyperphagia, obesity, glucose intolerance, and insulin resistance in SH2B1 null mice. Neuron-specific expression of R555E in wild-type mice promoted obesity and insulin resistance. These results indicate that in addition to the SH2 domain, N-terminal regions of neuronal SH2B1 are also required for the maintenance of normal body weight and glucose metabolism. Additionally, mutations in the SH2 domain of SH2B1 may increase the susceptibility to obesity and type 2 diabetes in a dominant-negative manner.

SH2B regulation of growth, metabolism, and longevity in both insects and mammals

SH2B1 is a key regulator of body weight in mammals. Here, we identified dSH2B as the Drosophila homolog of SH2B1. dSH2B bound to Chico and directly promoted insulin-like signaling. Disruption of dSH2B decreased insulin-like signaling and somatic growth in flies. dSH2B deficiency also increased hemolymph carbohydrate levels, whole-body lipid levels, life span, and resistance to starvation and oxidative stress. Systemic overexpression of dSH2B resulted in opposite phenotypes. dSH2B overexpression in fat body decreased lipid and glucose levels, whereas neuron-specific overexpression of dSH2B decreased oxidative resistance and life span. Genetic deletion of SH2B1 also resulted in growth retardation, obesity, and type 2 diabetes in mice; surprisingly, life span and oxidative resistance were reduced in SH2B1 null mice. These data suggest that dSH2B regulation of insulin-like signaling, growth, and metabolism is conserved in SH2B1, whereas dSH2B regulation of oxidative stress and longevity may be conserved in other SH2B family members.

Regulation of lifespan, metabolism, and stress responses by the Drosophila SH2B protein, Lnk

Drosophila Lnk is the single ancestral orthologue of a highly conserved family of structurally-related intracellular adaptor proteins, the SH2B proteins. As adaptors, they lack catalytic activity but contain several protein–protein interaction domains, thus playing a critical role in signal transduction from receptor tyrosine kinases to form protein networks. Physiological studies of SH2B function in mammals have produced conflicting data. However, a recent study in Drosophila has shown that Lnk is an important regulator of the insulin/insulin-like growth factor (IGF)-1 signaling (IIS) pathway during growth, functioning in parallel to the insulin receptor substrate, Chico. As this pathway also has an evolutionary conserved role in the determination of organism lifespan, we investigated whether Lnk is required for normal lifespan in Drosophila. Phenotypic analysis of mutants for Lnk revealed that loss of Lnk function results in increased lifespan and improved survival under conditions of oxidative stress and starvation. Starvation resistance was found to be associated with increased metabolic stores of carbohydrates and lipids indicative of impaired metabolism. Biochemical and genetic data suggest that Lnk functions in both the IIS and Ras/Mitogen activated protein Kinase (MapK) signaling pathways. Microarray studies support this model, showing transcriptional feedback onto genes in both pathways as well as indicating global changes in both lipid and carbohydrate metabolism. Finally, our data also suggest that Lnk itself may be a direct target of the IIS responsive transcription factor, dFoxo, and that dFoxo may repress Lnk expression. We therefore describe novel functions for a member of the SH2B protein family and provide the first evidence for potential mechanisms of SH2B regulation. Our findings suggest that IIS signaling in Drosophila may require the activity of a second intracellular adaptor, thereby yielding fundamental new insights into the functioning and role of the IIS pathway in ageing and metabolism.

Many human populations are experiencing increased life expectancy, and as populations age the incidence of age-related diseases becomes more prevalent. The identification of single gene mutations that extend lifespan in invertebrate model organisms has revealed that several cellular signaling pathways, including the insulin/insulin-like growth factor (IGF)-1 signaling (IIS) pathway, play a crucial role in modulating the ageing process across multiple species. Thus, studies carried out in yeast, worms, and flies have revealed evolutionarily conserved mechanisms of ageing, which are likely to be relevant to mammals, including humans. A recent study in Drosophila identified the SH2B family adaptor protein, Lnk, as an important regulator of the IIS pathway during organismal growth. In this study, we show that Lnk is also required to determine normal lifespan in Drosophila, as mutations that disrupt Lnk activity result in increased lifespan. In addition, these mutants show improved survival under conditions of stress and metabolic disregulation. Furthermore, we show that the expression of Lnk is regulated by the IIS responsive transcription factor, dFoxo. Our data therefore provide new mechanistic insights into the role of the IIS pathway in ageing.

SH2B1 enhances insulin sensitivity by both stimulating the insulin receptor and inhibiting tyrosine dephosphorylation of insulin receptor substrate proteins

OBJECTIVE

SH2B1 is a SH2 domain-containing adaptor protein expressed in both the central nervous system and peripheral tissues. Neuronal SH2B1 controls body weight; however, the functions of peripheral SH2B1 remain unknown. Here, we studied peripheral SH2B1 regulation of insulin sensitivity and glucose metabolism.

RESEARCH DESIGN AND METHODS

We generated TgKO mice expressing SH2B1 in the brain but not peripheral tissues. Various metabolic parameters and insulin signaling were examined in TgKO mice fed a high-fat diet (HFD). The effect of SH2B1 on the insulin receptor catalytic activity and insulin receptor substrate (IRS)-1/IRS-2 dephosphorylation was examined using in vitro kinase assays and in vitro dephosphorylation assays, respectively. SH2B1 was coexpressed with PTP1B, and insulin receptor-mediated phosphorylation of IRS-1 was examined.

RESULTS

Deletion of peripheral SH2B1 markedly exacerbated HFD-induced hyperglycemia, hyperinsulinemia, and glucose intolerance in TgKO mice. Insulin signaling was dramatically impaired in muscle, liver, and adipose tissue in TgKO mice. Deletion of SH2B1 impaired insulin signaling in primary hepatocytes, whereas SH2B1 overexpression stimulated insulin receptor autophosphorylation and tyrosine phosphorylation of IRSs. Purified SH2B1 stimulated insulin receptor catalytic activity in vitro. The SH2 domain of SH2B1 was both required and sufficient to promote insulin receptor activation. Insulin stimulated the binding of SH2B1 to IRS-1 or IRS-2. This physical interaction inhibited tyrosine dephosphorylation of IRS-1 or IRS-2 and increased the ability of IRS proteins to activate the phosphatidylinositol 3-kinase pathway.

CONCLUSIONS

SH2B1 is an endogenous insulin sensitizer. It directly binds to insulin receptors, IRS-1 and IRS-2, and enhances insulin sensitivity by promoting insulin receptor catalytic activity and by inhibiting tyrosine dephosphorylation of IRS proteins.

The Drosophila SH2B family adaptor Lnk acts in parallel to chico in the insulin signaling pathway

Insulin/insulin-like growth factor signaling (IIS) plays a pivotal role in the regulation of growth at the cellular and the organismal level during animal development. Flies with impaired IIS are developmentally delayed and small due to fewer and smaller cells. In the search for new growth-promoting genes, we identified mutations in the gene encoding Lnk, the single fly member of the SH2B family of adaptor molecules. Flies lacking lnk function are viable but severely reduced in size. Furthermore, lnk mutants display phenotypes reminiscent of reduced IIS, such as developmental delay, female sterility, and accumulation of lipids. Genetic epistasis analysis places lnk downstream of the insulin receptor (InR) and upstream of phosphoinositide 3-kinase (PI3K) in the IIS cascade, at the same level as chico (encoding the single fly insulin receptor substrate [IRS] homolog). Both chico and lnk mutant larvae display a similar reduction in IIS activity as judged by the localization of a PIP(3) reporter and the phosphorylation of protein kinase B (PKB). Furthermore, chico; lnk double mutants are synthetically lethal, suggesting that Chico and Lnk fulfill independent but partially redundant functions in the activation of PI3K upon InR stimulation.

Compartmentalization and regulation of insulin signaling to GLUT4 by the cytoskeleton

One of the early events in the development of Type 2 diabetes appears to be an inhibition of insulin-mediated GLUT4 redistribution to the cell surface in tissues that express GLUT4. Understanding this process, and how it begins to breakdown in the development of insulin resistance is quite important as we face treatment and prevention of metabolic diseases. Over the past few years, and increasing number of laboratories have produced compelling data to demonstrate a role for both the actin and microtubule networks in the regulation of insulin-mediated GLUT4 redistribution to the cell surface. In this review, we explore this process from insulin-signal transduction to fusion of GLUT4 membrane vesicles, focusing on studies that have implicated a role for the cytoskeleton. We see from this body of work that both the actin network and the microtubule cytoskeleton play roles as targets of insulin action and effectors of insulin signaling leading to changes in GLUT4 redistribution to the cell surface and insulin-mediated glucose uptake.

[Sh2b3/Lnk family adaptor proteins in the regulation of lymphohematopoiesis]

Sh2b3/Lnk consisting of an N-terminal proline-rich region, PH-, SH2-domains and a tyrosine phosphorylation site, forms an intracellular adaptor protein family conserved from drosophila to mammals, together with Sh2b1/SH2-B and Sh2b2/APS (adaptor protein with PH and SH2 domains). Lnk negatively regulates lymphopoiesis and early hematopoiesis. The lnk-deficiency results in enhanced production of B cells, and expansion as well as enhanced function of hematopoietic stem cells (HSCs), demonstrating negative regulatory functions of Sh2b3/Lnk in cytokine signaling. Our recent studies also revealed that Sh2b3/Lnk functions in responses controlled by cell adhesion and in crosstalk between integrin- and cytokine-mediated signaling. Importantly, recent genome-wide association studies of the autoimmune type 1 diabetes or celiac disease identified risk variants in the SH2B3/LNK region, indicating possible unrevealed functions mediated by this adaptor molecule. This review summarizes roles of Sh2b3/Lnk in the regulation of B-lymphopoiesis and HSCs expansion and function, and briefly introduces our approach for modulating HSCs function by targeting Sh2b3/Lnk-mediated pathways.

Lnk adaptor protein down-regulates specific Kit-induced signaling pathways in primary mast cells

Stem cell factor (SCF) plays critical roles in proliferation, survival, migration, and function of hematopoietic progenitor and mast cells through binding to Kit receptor. Previous studies have implicated the adaptor protein Lnk as an important negative regulator of SCF signaling. However, the molecular mechanism underlying this regulation is unclear. Here, we showed that the Src homology 2 domain (SH2) of Lnk binds directly and preferentially to phosphorylated tyrosine 567 in Kit juxtamembrane domain. Using Lnk−/− bone marrow mast cells (BMMCs) transduced with different Lnk proteins, we demonstrated that Lnk down-regulates SCF-induced proliferation with attenuation of mitogen-activated protein kinase (MAPK) and c-jun N-terminal kinase signaling. Furthermore, we showed that Lnk−/− BMMCs displayed increased SCF-dependent migration compared with wild-type cells, revealing a novel Lnk-mediated inhibitory function. This correlated with enhanced Rac and p38 MAPK activation. Finally, we found that Lnk domains and carboxy-terminal tyrosine contribute differently to inhibition of in vitro expansion of hematopoietic progenitors. Altogether, our results demonstrate that Lnk, through its binding to Kit tyrosine 567, negatively modulates specific SCF-dependent signaling pathways involved in the proliferation and migration of primary hematopoietic cells.

PSM/SH2B1 splice variants: critical role in src catalytic activation and the resulting STAT3s-mediated mitogenic response

A role of PSM/SH2B1 had been shown in mitogenesis and extending to phenotypic cell transformation, however, the underlying molecular mechanism remained to be established. Here, four alternative PSM splice variants and individual functional protein domains were compared for their role in the regulation of Src activity. We found that elevated cellular levels of PSM variants resulted in phenotypic cell transformation and potentiated cell proliferation and survival in response to serum withdrawal. PSM variant activity presented a consistent signature pattern for any tested response of highest activity observed for gamma, followed by delta, alpha, and beta with decreasing activity. PSM-potentiated cell proliferation was sensitive to Src inhibitor herbimycin and PSM and Src were found in the same immune complex. PSM variants were substrates of the Src Tyr kinase and potentiated Src catalytic activity by increasing the V(max) and decreasing the K(m) for ATP with the signature pattern of variant activity. Dominant-negative PSM peptide mimetics including the SH2 or PH domains inhibited Src catalytic activity as well as Src-mediated phenotypic cell transformation. Activation of major Src substrate STAT3 was similarly potentiated by the PSM variants in a Src-dependent fashion or inhibited by PSM domain-specific peptide mimetics. Expression of a dominant-negative STAT3 mutant blocked PSM variant-mediated phenotypic cell transformation. Our results implicate an essential role of the PSM variants in the activation of the Src kinase and the resulting mitogenic response--extending to phenotypic cell transformation and involving the established Src substrate STAT3.

Insulin signaling pathways and cardiac growth

The development, growth, function and metabolism of the heart are regulated by extracellular growth factors, cytokines and ligands. In this review, the role of insulin and insulin-like growth factors in the regulation of cardiac growth will be discussed. In addition, the role of insulin- and insulin-like growth factor-stimulated intracellular signaling proteins in cardiac growth will be reviewed.

Essential role of PSM/SH2-B variants in insulin receptor catalytic activation and the resulting cellular responses

The positive regulatory role of PSM/SH2-B downstream of various mitogenic receptor tyrosine kinases or gene disruption experiments in mice support a role of PSM in the regulation of insulin action. Here, four alternative PSM splice variants and individual functional domains were compared for their role in the regulation of specific metabolic insulin responses. We found that individual PSM variants in 3T3-L1 adipocytes potentiated insulin-mediated glucose and amino acid transport, glycogenesis, lipogenesis, and key components in the metabolic insulin response including p70 S6 kinase, glycogen synthase, glycogen synthase kinase 3 (GSK3), Akt, Cbl, and IRS-1. Highest activity was consistently observed for PSM alpha, followed by beta, delta, and gamma with decreasing activity. In contrast, dominant-negative peptide mimetics of the PSM Pro-rich, pleckstrin homology (PH), or src homology 2 (SH2) domains inhibited any tested insulin response. Potentiation of the insulin response originated at the insulin receptor (IR) kinase level by PSM variant-specific regulation of the Km (ATP) whereas the Vmax remained unaffected. IR catalytic activation was inhibited by peptide mimetics of the PSM SH2 or dimerization domain (DD). Either peptide should disrupt the complex of a PSM dimer linked to IR via SH2 domains as proposed for PSM activation of tyrosine kinase JAK2. Either peptide abolished downstream insulin responses indistinguishable from PSM siRNA knockdown. Our results implicate an essential role of the PSM variants in the activation of the IR kinase and the resulting metabolic insulin response. PSM variants act as internal IR ligands that in addition to potentiating the insulin response stimulate IR catalytic activation even in the absence of insulin.

Dok1 mediates high-fat diet-induced adipocyte hypertrophy and obesity through modulation of PPAR-gamma phosphorylation

Insulin receptor substrate (IRS)-1 and IRS-2 have dominant roles in the action of insulin, but other substrates of the insulin receptor kinase, such as Gab1, c-Cbl, SH2-B and APS, are also of physiological relevance. Although the protein downstream of tyrosine kinases-1 (Dok1) is known to function as a multisite adapter molecule in insulin signaling, its role in energy homeostasis has remained unclear. Here we show that Dok1 regulates adiposity. Expression of Dok1 in white adipose tissue was markedly increased in mice fed a high-fat diet, whereas adipocytes lacking this adapter were smaller and showed a reduced hypertrophic response to this dietary manipulation. Dok1-deficient mice were leaner and showed improved glucose tolerance and insulin sensitivity compared with wild-type mice. Embryonic fibroblasts from Dok1-deficient mice were impaired in adipogenic differentiation, and this defect was accompanied by an increased activity of the protein kinase ERK and a consequent increase in the phosphorylation of peroxisome proliferator-activated receptor (PPAR)-gamma on Ser112. Mutation of this negative regulatory site for the transactivation activity of PPAR-gamma blocked development of the lean phenotype caused by Dok1 ablation. These results indicate that Dok1 promotes adipocyte hypertrophy by counteracting the inhibitory effect of ERK on PPAR-gamma and may thus confer predisposition to diet-induced obesity.

The association between the SH2-containing inositol polyphosphate 5-Phosphatase 2 (SHIP2) and the adaptor protein APS has an impact on biochemical properties of both partners

SHIP2 (SH2-containing inositol polyphosphate 5-phosphatase 2) is a phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P(3)) 5-phosphatase containing various motifs susceptible to mediate protein-protein interaction. In cell models, SHIP2 negatively regulates insulin signalling through its catalytic PtdIns(3,4,5)P(3) 5-phosphatase activity. We have previously reported that SHIP2 interacts with the c-Cbl associated protein (CAP) and c-Cbl, proteins implicated in the insulin cellular response regulating the small G protein TC10. The first steps of the TC10 pathway are the recruitment and tyrosine phosphorylation by the insulin receptor of the adaptor protein with Pleckstrin Homology and Src Homology 2 domains (APS). Herein, we show that SHIP2 can directly interact with APS in 3T3-L1 adipocytes and in transfected CHO-IR cells (Chinese hamster ovary cells stably transfected with the insulin receptor). Upon insulin stimulation, APS and SHIP2 are recruited to cell membranes as seen by immunofluorescence studies, which is consistent with their interaction. We also observed that SHIP2 negatively regulates APS insulin-induced tyrosine phosphorylation and consequently inhibits APS association with c-Cbl. APS, which specifically interacts with SHIP2, but not PTEN, in turn, increases the PtdIns(3,4,5)P(3) 5-phosphatase activity of SHIP2 in an inositol phosphatase assay. Co-transfection of SHIP2 and APS in CHO-IR cells further increases the inhibitory effect of SHIP2 on Akt insulin-induced phosphorylation. Therefore, the interaction between APS and SHIP2 provides to both proteins potential negative regulatory mechanisms to act on the insulin cascade.

Deficiency of Cbl-b gene enhances infiltration and activation of macrophages in adipose tissue and causes peripheral insulin resistance in mice

OBJECTIVE

c-Cbl plays an important role in whole-body fuel homeostasis by regulating insulin action. In the present study, we examined the role of Cbl-b, another member of the Cbl family, in insulin action.

RESEARCH DESIGN AND METHODS

C57BL/6 (Cbl-b(+/+)) or Cbl-b-deficient (Cbl-b(-/-)) mice were subjected to insulin and glucose tolerance tests and a hyperinsulinemic-euglycemic clamp test. Infiltration of macrophages into white adipose tissue (WAT) was assessed by immunohistochemistry and flow cytometry. We examined macrophage activation using co-cultures of 3T3-L1 adipocytes and peritoneal macrophages.

RESULTS

Elderly Cbl-b(-/-) mice developed glucose intolerance and peripheral insulin resistance; serum insulin concentrations after a glucose challenge were always higher in elderly Cbl-b(-/-) mice than age-matched Cbl-b(+/+) mice. Deficiency of the Cbl-b gene significantly decreased the uptake of 2-deoxyglucose into WAT and glucose infusion rate, whereas fatty liver was apparent in elderly Cbl-b(-/-) mice. Cbl-b deficiency was associated with infiltration of macrophages into the WAT and expression of cytokines, such as tumor necrosis factor-alpha, interleukin-6, and monocyte chemoattractant protein (MCP)-1. Co-culture of Cbl-b(-/-) macrophages with 3T3-L1 adipocytes induced leptin expression and dephosphorylation of insulin receptor substrate 1, leading to impaired glucose uptake in adipocytes. Furthermore, Vav1, a key factor in macrophage activation, was highly phosphorylated in peritoneal Cbl-b(-/-) macrophages compared with Cbl-b(+/+) macrophages. Treatment with a neutralizing anti-MCP-1 antibody improved peripheral insulin resistance and macrophage infiltration into WAT in elderly Cbl-b(-/-) mice.

CONCLUSIONS

Cbl-b is a negative regulator of macrophage infiltration and activation, and macrophage activation by Cbl-b deficiency contributes to the peripheral insulin resistance and glucose intolerance via cytokines secreted from macrophages.

SH2B1 enhances leptin signaling by both Janus kinase 2 Tyr813 phosphorylation-dependent and -independent mechanisms

Leptin controls body weight by activating its long form receptor (LEPRb). LEPRb binds to Janus kinase 2 (JAK2), a cytoplasmic tyrosine kinase that mediates leptin signaling. We previously reported that genetic deletion of SH2B1 (previously known as SH2-B), a JAK2-binding protein, results in severe leptin-resistant and obese phenotypes, indicating that SH2B1 is a key endogenous positive regulator of leptin sensitivity. Here we show that SH2B1 regulates leptin signaling by multiple mechanisms. In the absence of leptin, SH2B1 constitutively bound, via its non-SH2 domain region(s), to non-tyrosyl-phosphorylated JAK2, and inhibited JAK2. Leptin stimulated JAK2 phosphorylation on Tyr(813), which subsequently bound to the SH2 domain of SH2B1. Binding of the SH2 domain of SH2B1 to phospho-Tyr(813) in JAK2 enhanced leptin induction of JAK2 activity. JAK2 was required for leptin-stimulated phosphorylation of insulin receptor substrate 1 (IRS1), an upstream activator of the phosphatidylinositol 3-kinase pathway. Overexpression of SH2B1 enhanced both JAK2- and JAK2(Y813F)-mediated tyrosine phosphorylation of IRS1 in response to leptin, even though SH2B1 did not enhance JAK2(Y813F) activation. Leptin promoted the interaction of SH2B1 with IRS1. These data suggest that constitutive SH2B1-JAK2 interaction, mediated by the non-SH2 domain region(s) of SH2B1 and the non-Tyr(813) region(s) in JAK2, increases the local concentration of SH2B1 close to JAK2 and inhibits JAK2 activity. Leptin-stimulated SH2B1-JAK2 interaction, mediated by the SH2 domain of SH2B1 and phospho-Tyr(813) in JAK2, promotes JAK2 activation, thus globally enhancing leptin signaling. SH2B1-IRS1 interaction facilitates IRS1 phosphorylation by recruiting IRS1 to JAK2 and/or by protecting IRS1 from dephosphorylation, thus specifically enhancing leptin stimulation of the phosphatidylinositol 3-kinase pathway.

Mice with a disruption of the imprinted Grb10 gene exhibit altered body composition, glucose homeostasis, and insulin signaling during postnatal life

The Grb10 adapter protein is capable of interacting with a variety of receptor tyrosine kinases, including, notably, the insulin receptor. Biochemical and cell culture experiments have indicated that Grb10 might act as an inhibitor of insulin signaling. We have used mice with a disruption of the Grb10 gene (Grb10Delta2-4 mice) to assess whether Grb10 might influence insulin signaling and glucose homeostasis in vivo. Adult Grb10Delta2-4 mice were found to have improved whole-body glucose tolerance and insulin sensitivity, as well as increased muscle mass and reduced adiposity. Tissue-specific changes in insulin receptor tyrosine phosphorylation were consistent with a model in which Grb10, like the closely related Grb14 adapter protein, prevents specific protein tyrosine phosphatases from accessing phosphorylated tyrosines within the kinase activation loop. Furthermore, insulin-induced IRS-1 tyrosine phosphorylation was enhanced in Grb10Delta2-4 mutant animals, supporting a role for Grb10 in attenuation of signal transmission from the insulin receptor to IRS-1. We have previously shown that Grb10 strongly influences growth of the fetus and placenta. Thus, Grb10 forms a link between fetal growth and glucose-regulated metabolism in postnatal life and is a candidate for involvement in the process of fetal programming of adult metabolic health.

Identification of SH2B2beta as an inhibitor for SH2B1- and SH2B2alpha-promoted Janus kinase-2 activation and insulin signaling

The SH2B family has three members (SH2B1, SH2B2, and SH2B3) that contain conserved dimerization (DD), pleckstrin homology, and SH2 domains. The DD domain mediates the formation of homo- and heterodimers between members of the SH2B family. The SH2 domain of SH2B1 (previously named SH2-B) or SH2B2 (previously named APS) binds to phosphorylated tyrosines in a variety of tyrosine kinases, including Janus kinase-2 (JAK2) and the insulin receptor, thereby promoting the activation of JAK2 or the insulin receptor, respectively. JAK2 binds to various members of the cytokine receptor family, including receptors for GH and leptin, to mediate cytokine responses. In mice, SH2B1 regulates energy and glucose homeostasis by enhancing leptin and insulin sensitivity. In this work, we identify SH2B2beta as a new isoform of SH2B2 (designated as SH2B2alpha) derived from the SH2B2 gene by alternative mRNA splicing. SH2B2beta has a DD and pleckstrin homology domain but lacks a SH2 domain. SH2B2beta bound to both SH2B1 and SH2B2alpha, as demonstrated by both the interaction of glutathione S-transferase-SH2B2beta fusion protein with SH2B1 or SH2B2alpha in vitro and coimmunoprecipitation of SH2B2beta with SH2B1 or SH2B2alpha in intact cells. SH2B2beta markedly attenuated the ability of SH2B1 to promote JAK2 activation and subsequent tyrosine phosphorylation of insulin receptor substrate-1 by JAK2. SH2B2beta also significantly inhibited SH2B1- or SH2B2alpha-promoted insulin signaling, including insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1. These data suggest that SH2B2beta is an endogenous inhibitor of SH2B1 and/or SH2B2alpha, negatively regulating insulin signaling and/or JAK2-mediated cellular responses.

The GLUT4 glucose transporter

Few physiological parameters are more tightly and acutely regulated in humans than blood glucose concentration. The major cellular mechanism that diminishes blood glucose when carbohydrates are ingested is insulin-stimulated glucose transport into skeletal muscle. Skeletal muscle both stores glucose as glycogen and oxidizes it to produce energy following the transport step. The principal glucose transporter protein that mediates this uptake is GLUT4, which plays a key role in regulating whole body glucose homeostasis. This review focuses on recent advances on the biology of GLUT4.

PSM/SH2-B distributes selected mitogenic receptor signals to distinct components in the PI3-kinase and MAP kinase signaling pathways

The Pro-rich, PH, and SH2 domain containing mitogenic signaling adapter PSM/SH2-B has been implicated as a cellular partner of various mitogenic receptor tyrosine kinases and related signaling mechanisms. Here, we report in a direct comparison of three peptide hormones, that PSM participates in the assembly of distinct mitogenic signaling complexes in response to insulin or IGF-I when compared to PDGF in cultured normal fibroblasts. The complex formed in response to insulin or IGF-I involves the respective peptide hormone receptor and presumably the established components leading to MAP kinase activation. However, our data suggest an alternative link from the PDGF receptor via PSM directly to MEK1/2 and consequently also to p44/42 activation, possibly through a scaffold protein. At least two PSM domains participate, the SH2 domain anticipated to link PSM to the respective receptor and the Pro-rich region in an association with an unidentified downstream component resulting in direct MEK1/2 and p44/42 regulation. The PDGF receptor signaling complex formed in response to PDGF involves PI 3-kinase in addition to the same components and interactions as described for insulin or IGF-I. PSM associates with PI 3-kinase via p85 and in addition the PSM PH domain participates in the regulation of PI 3-kinase activity, presumably through membrane interaction. In contrast, the PSM Pro-rich region appears to participate only in the MAP kinase signal. Both pathways contribute to the mitogenic response as shown by cell proliferation, survival, and focus formation. PSM regulates p38 MAP kinase activity in a pathway unrelated to the mitogenic response.

Bone marrow-specific Cap gene deletion protects against high-fat diet-induced insulin resistance

Cbl-associated protein (Cap) is a member of a phosphatidylinositol 3-kinase-independent pathway for insulin-stimulated translocation of the glucose transporter GLUT4. Despite this positive role of Cap in glucose uptake, here we show that deletion of the gene encoding Cap (official gene name: Sorbs1) protects against high-fat diet (HFD)-induced insulin resistance in mice while also having an opposite, insulin-sensitizing effect, accompanied by reduced tissue markers of inflammation. Given the emerging role of chronic inflammation in insulin resistance and the macrophage in initiating this inflammatory process, we considered that Sorbs1 deletion from macrophages may have resulted in the observed protection from HFD-induced insulin resistance. Using bone marrow transplantation to generate functional Sorbs1-null macrophages, we show that the insulin-sensitive phenotype can be transferred to wild-type mice by transplantation of Sorbs1-null bone marrow. These studies show that macrophages are an important cell type in the induction of insulin resistance and that Cap has a modulatory role in this function.

APS-mediated ubiquitination of the insulin receptor enhances its internalization, but does not induce its degradation

APS, a tyrosine kinase adaptor protein with pleckstrin homology and Src homology 2 domains, is rapidly and strongly tyrosine-phosphorylated by insulin receptor kinase upon insulin stimulation. We have previously shown that APS knockout mice have increased insulin sensitivity, and that this enhancement is possibly due to increased insulin-response on adipose tissues. However, the function of APS in insulin signaling has so far been controversial. Here, we report that APS enhanced ligand-dependent multi-ubiquitination of the insulin receptor (IR) in CHO cells overexpressing the IR. APS-mediated ubiquitination of the IR induced enhancement of the IR internalization, but did not affect the IR degradation. This finding shows one of the pleiotropic functions of APS in insulin signaling.

The Cbl family proteins: ring leaders in regulation of cell signaling

The proto-oncogenic protein c-Cbl was discovered as the cellular form of v-Cbl, a retroviral transforming protein. This was followed over the years by important discoveries, which identified c-Cbl and other Cbl-family proteins as key players in several signaling pathways. c-Cbl has donned the role of a multivalent adaptor protein, capable of interacting with a plethora of proteins, and has been shown to positively influence certain biological processes. The identity of c-Cbl as an E3 ubiquitin ligase unveiled the existence of an important negative regulatory pathway involved in maintaining homeostasis in protein tyrosine kinase (PTK) signaling. Recent years have also seen the emergence of novel regulators of Cbl, which have provided further insights into the complexity of Cbl-influenced pathways. This review will endeavor to provide a summary of current studies focused on the effects of Cbl proteins on various biological processes and the mechanism of these effects. The major sections of the review are as follows: Structure and genomic organization of Cbl proteins; Phosphorylation of Cbl; Interactions of Cbl; Localization of Cbl; Mechanism of effects of Cbl: (a) Ubiquitylation-dependent events: This section elucidates the mechanism of Cbl-mediated downregulation of EGFR and details the PTK and non-PTKs targeted by Cbl. In addition, it addresses the functional requirements for E3 Ubiquitin ligase activity of Cbl and negative regulation of Cbl-mediated downregulation of PTKs, (b) Adaptor functions: This section discusses the mechanisms of adaptor functions of Cbl in mitogen-activated protein kinase (MAPK) activation, insulin signaling, regulation of Ras-related protein 1 (Rap1), PI-3' kinase signaling, and regulation of Rho-family GTPases and cytoskeleton; Biological functions: This section gives an account of the diverse biological functions of Cbl and includes the role of Cbl in transformation, T-cell signaling and thymus development, B-cell signaling, mast-cell degranulation, macrophage functions, bone development, neurite growth, platelet activation, muscle degeneration, and bacterial invasion; Conclusions and perspectives.

Binding of SH2-B family members within a potential negative regulatory region maintains JAK2 in an active state

The tyrosine kinase Janus kinase 2 (JAK2) transduces signaling for the majority of known cytokine receptor family members and is constitutively activated in some cancers. Here we examine the mechanisms by which the adapter proteins SH2-Bbeta and APS regulate the activity of JAK2. We show that like SH2-Bbeta, APS binds JAK2 at multiple sites and that binding to phosphotyrosine 813 is essential for APS to increase active JAK2 and to be phosphorylated by JAK2. Binding of APS to a phosphotyrosine 813-independent site inhibits JAK2. Both APS and SH2-Bbeta increase JAK2 activity independent of their N-terminal dimerization domains. SH2-Bbeta-induced increases in JAK2 dimerization require only the SH2 domain and only one SH2-Bbeta to be bound to a JAK2 dimer. JAK2 mutations and truncations revealed that amino acids 809 to 811 in JAK2 are a critical component of a larger regulatory region within JAK2, most likely including amino acids within the JAK homology 1 (JH1) and JH2 domains and possibly the FERM domain. Together, our data suggest that SH2-Bbeta and APS do not activate JAK2 as a consequence of their own dimerization, recruitment of an activator of JAK2, or direct competition with a JAK2 inhibitor for binding to JAK2. Rather, they most likely induce or stabilize an active conformation of JAK2.

The human and mouse complement of SH2 domain proteins-establishing the boundaries of phosphotyrosine signaling

SH2 domains are interaction modules uniquely dedicated to the recognition of phosphotyrosine sites and are embedded in proteins that couple protein-tyrosine kinases to intracellular signaling pathways. Here, we report a comprehensive bioinformatics, structural, and functional view of the human and mouse complement of SH2 domain proteins. This information delimits the set of SH2-containing effectors available for PTK signaling and will facilitate the systems-level analysis of pTyr-dependent protein-protein interactions and PTK-mediated signal transduction. The domain-based architecture of SH2-containing proteins is of more general relevance for understanding the large family of protein interaction domains and the modular organization of the majority of human proteins.

Endothelin-1 impairs glucose transporter trafficking via a membrane-based mechanism

Endothelin-1 (ET-1) disrupts insulin-regulated glucose transporter GLUT4 trafficking. Since the negative consequence of chronic ET-1 exposure appears to be independent of signal disturbance along the insulin receptor substrate-1/phosphatidylinositol (PI) 3-kinase (PI3K)/Akt-2 pathway of insulin action, we tested if ET-1 altered GLUT4 regulation engaged by osmotic shock, a PI3K-independent stimulus that mimics insulin action. Regulation of GLUT4 by hyperosmotic stress was impaired by ET-1. Because of the mutual disruption of both insulin- and hyperosmolarity-stimulated GLUT4 translocation, we tested whether shared signaling and/or key phosphatidylinositol 4,5-bisphosphate (PIP2)-regulated cytoskeletal events of GLUT4 trafficking were targets of ET-1. Both insulin and hyperosmotic stress signaling to Cbl were impaired by ET-1. Also, plasma membrane PIP2 and cortical actin levels were reduced in cells exposed to ET-1. Exogenous PIP2, but not PI 3,4,5-bisphosphate, restored actin structure, Cbl activation, and GLUT4 translocation. These data show that ET-1-induced PIP2/actin disruption impairs GLUT4 trafficking elicited by insulin and hyperosmolarity. In addition to showing for the first time the important role of PIP2-regulated cytoskeletal events in GLUT4 regulation by stimuli other than insulin, these studies reveal a novel function of PIP2/actin structure in signal transduction.

Enigma interacts with adaptor protein with PH and SH2 domains to control insulin-induced actin cytoskeleton remodeling and glucose transporter 4 translocation

APS (adaptor protein with PH and SH2 domains) initiates a phosphatidylinositol 3-kinase-independent pathway involved in insulin-stimulated glucose transport. We recently identified Enigma, a PDZ and LIM domain-containing protein, as a partner of APS and showed that APS-Enigma complex plays a critical role in actin cytoskeleton organization in fibroblastic cells. Because actin rearrangement is important for insulin-induced glucose transporter 4 (Glut 4) translocation, we studied the potential involvement of Enigma in insulin-induced glucose transport in 3T3-L1 adipocytes. Enigma mRNA was expressed in differentiated adipocytes and APS and Enigma were colocalized with cortical actin. Expression of an APS mutant unable to bind Enigma increased the insulin-induced Glut 4 translocation to the plasma membrane. By contrast, overexpression of Enigma inhibited insulin-stimulated glucose transport and Glut 4 translocation without alterations in proximal insulin signaling. This inhibitory effect was prevented with the deletion of the LIM domains of Enigma. Using time-lapse fluorescent microscopy of green fluorescent protein-actin, we demonstrated that the overexpression of Enigma altered insulin-induced actin rearrangements, whereas the expression of Enigma without its LIM domains was without effect. A physiological link between increased expression of Enigma and an alteration in insulin-induced glucose uptake was suggested by the increase in Enigma mRNA expression in adipose tissue of diabetic obese patients. Taken together, these data strongly suggest that the interaction between APS and Enigma is involved in insulin-induced Glut 4 translocation by regulating cortical actin remodeling and raise the possibility that modification of APS/Enigma ratio could participate in the alteration of insulin-induced glucose uptake in adipose tissue.

Insulin receptor signals regulating GLUT4 translocation and actin dynamics

In skeletal muscle and adipose tissue, insulin-stimulated glucose uptake is dependent upon translocation of the insulin-responsive glucose transporter GLUT4 from intracellular storage compartments to the plasma membrane. This insulin-induced redistribution of GLUT4 protein is achieved through a series of highly organized membrane trafficking events, orchestrated by insulin receptor signals. Recently, several key molecules linking insulin receptor signals and membrane trafficking have been identified, and emerging evidence supports the importance of subcellular compartmentalization of signaling components at the right time and in the right place. In addition, the translocation of GLUT4 in adipocytes requires insulin stimulation of dynamic actin remodeling at the inner surface of the plasma membrane (cortical actin) and in the perinuclear region. This results from at least two independent insulin receptor signals, one leading to the activation of phosphatidylinositol (PI) 3-kinase and the other to the activation of the Rho family small GTP-binding protein TC10. Thus, both spatial and temporal regulations of actin dynamics, both beneath the plasma membrane and around endomembranes, by insulin receptor signals are also involved in the process of GLUT4 translocation.

Cardiac metabolism in mice: tracer method developments and in vivo application revealing profound metabolic inflexibility in diabetes

Studies of cardiac fuel metabolism in mice have been almost exclusively conducted ex vivo. The major aim of this study was to assess in vivo plasma FFA and glucose utilization by the hearts of healthy control (db/+) and diabetic (db/db) mice, based on cardiac uptake of (R)-2-[9,10-(3)H]bromopalmitate ([3H]R-BrP) and 2-deoxy-D-[U-14C]glucose tracers. To obtain quantitative information about the evaluation of cardiac FFA utilization with [3H]R-BrP, simultaneous comparisons of [3H]R-BrP and [14C]palmitate ([14C]P) uptake were first made in isolated perfused working hearts from db/+ mice. It was found that [3H]R-BrP uptake was closely correlated with [14C]P oxidation (r2 = 0.94, P < 0.001). Then, methods for in vivo application of [3H]R-BrP and [14C]2-DG previously developed for application in the rat were specially adapted for use in the mouse. The method yields indexes of cardiac FFA utilization (R(f)*) and clearance (K(f)*), as well as glucose utilization (R(g)'). Finally, in the main part of the study, the ability of the heart to switch between FFA and glucose fuels (metabolic flexibility) was investigated by studying anesthetized, 8-h-fasted control and db/db mice in either the basal state or during glucose infusion. In control mice, glucose infusion raised plasma levels of glucose and insulin, raised R(g)' (+58%), and lowered plasma FFA level (-48%), K(f)* (-45%), and R(f)* (-70%). This apparent reciprocal regulation of glucose and FFA utilization by control hearts illustrates metabolic flexibility for substrate use. By contrast, in the db/db mice, glucose infusion raised glucose levels with no apparent influence on cardiac FFA or glucose utilization. In conclusion, tracer methodology for assessing in vivo tissue-specific plasma FFA and glucose utilization has been adapted for use in mice and reveals a profound loss of metabolic flexibility in the diabetic db/db heart, suggesting a fixed level of FFA oxidation in fasted and glucose-infused states.

Regulation of small GTP-binding proteins by insulin

Several members of the extensive family of small GTP-binding proteins are regulated by insulin, and have been implicated in insulin action on glucose uptake. These proteins are themselves negatively regulated by a series of specific GAPs (GTPase-activating proteins). Interestingly, there is increasing evidence to suggest that PKB (protein kinase B)-dependent phosphorylation of some GAPs may relieve this negative regulation and so lead to the activation of the target small GTP-binding protein. We review recent evidence that this may be the case, and place specific emphasis on the role of these pathways in insulin-stimulated glucose uptake.