Galpha i2 enhances in vivo activation of and insulin signaling to GLUT4.

Heterotrimeric G-proteins, including Galpha(i2), have been implicated in modulating glucose disposal and insulin signaling. This cross-talk between G-protein-coupled and tyrosine kinase-coupled signaling pathways is a focal point for the study of integration of cell signaling. Herein we study the role of Galpha(i2) in modulating glucose transport, focusing upon linkages to insulin signaling. Utilizing mice harboring a transgene that directs the expression of a constitutively activated, GTPase-deficient mutant of Galpha(i2) (Q205L) in adipose tissue, skeletal muscle, and liver, we demonstrate that Galpha(i2) regulates the translocation of the insulin-sensitive GLUT4 glucose transporter in skeletal muscle and adipose tissue. The expression of Q205L Galpha(i2) increased glucose transport and translocation of GLUT4 to the plasma membrane in vivo in the absence of insulin stimulation. Adipocytes from the Q205L Galpha(i2) mice displayed enhanced insulin-stimulated glucose transport and GLUT4 translocation to the plasma membrane to levels nearly twice that of those from littermate controls. Phosphatidylinositol 3-kinase and Akt activities were constitutively activated in tissues expressing the Q205L Galpha(i2). Studies of adipocytes from wild-type mice displayed short term activation of phosphatidylinositol 3-kinase, Akt, and GLUT4 translocation in response to activation of Galpha(i2) by lysophosphatidic acid, a response sensitive to pertussis toxin. These data provide an explanation for the marked glucose tolerance of the Q205L Galpha(i2) mice and demonstrate a linkage between Galpha(i2) and GLUT4 translocation.     

Tyrosine dephosphorylation and deactivation of insulin receptor substrate-1 by protein-tyrosine phosphatase 1B. Possible facilitation by the formation of a ternary complex with the Grb2 adaptor protein

Regulation of the steady-state tyrosine phosphorylation of the insulin receptor and its postreceptor substrates are essential determinants of insulin signal transduction. However, little is known regarding the molecular interactions that influence the balance of these processes, especially the phosphorylation state of postinsulin receptor substrates, such as insulin receptor substrate-1 (IRS-1). The specific activity of four candidate protein-tyrosine phosphatases (protein-tyrosine phosphatase 1B (PTP1B), SH2 domain-containing PTPase-2 (SHP-2), leukocyte common antigen-related (LAR), and leukocyte antigen-related phosphatase) (LRP) toward IRS-1 dephosphorylation was studied using recombinant proteins in vitro. PTP1B exhibited the highest specific activity (percentage dephosphorylated per microg per min), and the enzyme activities varied over a range of 5.5 x 10(3). When evaluated as a ratio of activity versus IRS-1 to that versus p-nitrophenyl phosphate, PTP1B remained significantly more active by 3.1-293-fold, respectively. Overlay blots with recombinant Src homology 2 domains of IRS-1 adaptor proteins showed that the loss of IRS-1 binding of Crk, GRB2, SHP-2, and the p85 subunit of phosphatidylinositol 3'-kinase paralleled the rate of overall IRS-1 dephosphorylation. Further studies revealed that the adaptor protein GRB2 strongly promoted the formation of a stable protein complex between tyrosine-phosphorylated IRS-1 and catalytically inactive PTP1B, increasing their co-immunoprecipitation from an equimolar solution by 13.5 +/- 3.3-fold (n = 7; p < 0.01). Inclusion of GRB2 in a reaction mixture of IRS-1 and active PTP1B also increased the overall rate of IRS-1 tyrosine dephosphorylation by 2.7-3.9-fold (p < 0.01). These results provide new insight into novel molecular interactions involving PTP1B and GRB2 that may influence the steady-state capacity of IRS-1 to function as a phosphotyrosine scaffold and possibly affect the balance of postreceptor insulin signaling.     

PTP1B regulates leptin signal transduction in vivo

Mice lacking the protein-tyrosine phosphatase PTP1B are hypersensitive to insulin and resistant to obesity. However, the molecular basis for resistance to obesity has been unclear. Here we show that PTP1B regulates leptin signaling. In transfection studies, PTP1B dephosphorylates the leptin receptor-associated kinase, Jak2. PTP1B is expressed in hypothalamic regions harboring leptin-responsive neurons. Compared to wild-type littermates, PTP1B(-/-) mice have decreased leptin/body fat ratios, leptin hypersensitivity, and enhanced leptin-induced hypothalamic Stat3 tyrosyl phosphorylation. Gold thioglucose treatment, which ablates leptin-responsive hypothalamic neurons, partially overcomes resistance to obesity in PTP1B(-/-) mice. Our data indicate that PTP1B regulates leptin signaling in vivo, likely by targeting Jak2. PTP1B may be a novel target to treat leptin resistance in obesity.     

Protein-tyrosine phosphatase-1B negatively regulates insulin signaling in l6 myocytes and Fao hepatoma cells

Insulin signaling is regulated by tyrosine phosphorylation of the signaling molecules, such as the insulin receptor and insulin receptor substrates (IRSs). Therefore, the balance between protein-tyrosine kinases and protein-tyrosine phosphatase activities is thought to be important in the modulation of insulin signaling in insulin-resistant states. We thus employed the adenovirus-mediated gene transfer technique, and we analyzed the effect of overexpression of a wild-type protein-tyrosine phosphatase-1B (PTP1B) on insulin signaling in both L6 myocytes and Fao cells. In both cells, PTP1B overexpression blocked insulin-stimulated tyrosine phosphorylation of the insulin receptor and IRS-1 by more than 70% and resulted in a significant inhibition of the association between IRS-1 and the p85 subunit of phosphatidylinositol 3-kinase and Akt phosphorylation as well as mitogen-activated protein kinase phosphorylation. Moreover, insulin-stimulated glycogen synthesis was also inhibited by PTP1B overexpression in both cells. These effects were specific for insulin signaling, because platelet-derived growth factor (PDGF)-stimulated PDGF receptor tyrosine phosphorylation and Akt phosphorylation were not inhibited by PTP1B overexpression. The present findings demonstrate that PTP1B negatively regulates insulin signaling in L6 and Fao cells, suggesting that PTP1B plays an important role in insulin resistance in muscle and liver.     

Transgenic overexpression of protein-tyrosine phosphatase 1B in muscle causes insulin resistance, but overexpression with leukocyte antigen-related phosphatase does not additively impair insulin action

Previous studies implicate protein-tyrosine phosphatase 1B (PTP1B) and leukocyte antigen-related phosphatase (LAR) as negative regulators of insulin signaling. The expression and/or activity of PTP1B and LAR are increased in muscle of insulin-resistant rodents and humans. Overexpression of LAR selectively in muscle of transgenic mice causes whole body insulin resistance. To determine whether overexpression of PTP1B also causes insulin resistance, we generated transgenic mice overexpressing human PTP1B selectively in muscle at levels similar to those observed in insulin-resistant humans. Insulin-stimulated insulin receptor (IR) tyrosyl phosphorylation and phosphatidylinositol 3'-kinase activity were impaired by 35% and 40-60% in muscle of PTP1B-overexpressing mice compared with controls. Insulin stimulation of protein kinase C (PKC)lambda/zeta activity, which is required for glucose transport, was impaired in muscle of PTP1B-overexpressing mice compared with controls, showing that PTP1B overexpression impairs activation of these PKC isoforms. Furthermore, hyperinsulinemic-euglycemic clamp studies revealed that whole body glucose disposal and muscle glucose uptake were decreased by 40-50% in PTP1B-overexpressing mice. Overexpression of PTP1B or LAR alone in muscle caused similar impairments in insulin action; however, compound overexpression achieved by crossing PTP1B- and LAR-overexpressing mice was not additive. Antibodies against specific IR phosphotyrosines indicated overlapping sites of action of PTP1B and LAR. Thus, overexpression of PTP1B in vivo impairs insulin sensitivity, suggesting that overexpression of PTP1B in muscle of obese humans and rodents may contribute to their insulin resistance. Lack of additive impairment of insulin signaling by PTP1B and LAR suggests that these PTPs have overlapping actions in causing insulin resistance in vivo.     

Enhancement of Insulin Responsiveness by Nitric Oxide-mediated Inactivation of Protein-tyrosine Phosphatases

NO synthesis is a prerequisite for proper insulin sensitivity in insulin-targeted tissues; however, the molecular basis for this process remains unclear. Using a gain-of-function model of endothelial nitric-oxide synthase (eNOS)-transfected COS-7 cells, we have shown a critical role of NO in insulin responsiveness, as evidenced by an NO-dependent increase of tyrosine phosphorylation levels of the insulin receptor and its downstream effectors insulin receptor substrate-1 and PKB/AKT. We hypothesized that NO-induced inactivation of endogenous protein-tyrosine phosphatases (PTPs) would enhance insulin receptor-mediated signaling. To test this hypothesis, we devised a new method of the PTP labeling using a cysteine sulfhydryl-reacted probe. Under the acidic conditions employed in this study, the probe recognized the reduced and active forms but not the S-nitrosylated and inactive forms of endogenous PTPs. Our data suggest that phosphatases SHP-1, SHP-2, and PTP1B, but not TC-PTP, are likely S-nitrosylated at the active site cysteine residue concomitantly with a burst of NO production in signaling response to insulin stimulation. These results were further confirmed by phosphatase activity assays. We investigated further the role of NO as a regulator of insulin signaling by RNA interference that ablates endogenous eNOS expression in endothelial MS-1 cells. We have shown that eNOS-dependent NO production is essential for the activation of insulin signaling. Our findings demonstrate that NO mediates enhancement of insulin responsiveness via the inhibition of insulin receptor phosphatases.     

Overexpression of protein-tyrosine phosphatase-1B in adipocytes inhibits insulin-stimulated phosphoinositide 3-kinase activity without altering glucose transport or Akt/Protein kinase B activation

Previous studies suggested that protein-tyrosine phosphatase 1B (PTP1B) antagonizes insulin action by catalyzing dephosphorylation of the insulin receptor (IR) and/or other key proteins in the insulin signaling pathway. In adipose tissue and muscle of obese humans and rodents, PTP1B expression is increased, which led to the hypothesis that PTP1B plays a role in the pathogenesis of insulin resistance. Consistent with this, mice in which the PTP1B gene was disrupted exhibit increased insulin sensitivity. To test whether increased expression of PTP1B in an insulin-sensitive cell type could contribute to insulin resistance, we overexpressed wild-type PTP1B in 3T3L1 adipocytes using adenovirus-mediated gene delivery. PTP1B expression was increased approximately 3-5-fold above endogenous levels at 16 h, approximately 14-fold at 40 h, and approximately 20-fold at 72 h post-transduction. Total protein-tyrosine phosphatase activity was increased by 50% at 16 h, 3-4-fold at 40 h, and 5-6-fold at 72 h post-transduction. Compared with control cells, cells expressing high levels of PTP1B showed a 50-60% decrease in maximally insulin-stimulated tyrosyl phosphorylation of IR and insulin receptor substrate-1 (IRS-1) and phosphoinositide 3-kinase (PI3K) activity associated with IRS-1 or with phosphotyrosine. Akt phosphorylation and activity were unchanged. Phosphorylation of p42 and p44 MAP kinase (MAPK) was reduced approximately 32%. Overexpression of PTP1B had no effect on basal, submaximally or maximally (100 nm) insulin-stimulated glucose transport or on the EC(50) for transport. Our results suggest that: 1) insulin stimulation of glucose transport in adipocytes requires 相似文献   

Structural Determinants of SHP-2 Function and Specificity in Xenopus Mesoderm Induction

SHP-2 is a positive component of many receptor tyrosine kinase signaling pathways. The related protein-tyrosine phosphatase (PTP) SHP-1 usually acts as a negative regulator. The precise domains utilized by SHP-2 to transmit positive signals in vivo and the basis for specificity between SHP-1 and SHP-2 are not clear. In Xenopus, SHP-2 is required for mesoderm induction and completion of gastrulation. We investigated the effects of SHP-2 mutants and SHP-2/SHP-1 chimeras on basic fibroblast growth factor-induced mesoderm induction. Both SH2 domains and the PTP domain are required for normal SHP-2 function in this pathway. The N-terminal SH2 domain is absolutely required, whereas the C-terminal SH2 contributes to wild-type function. The C-terminal tyrosyl phosphorylation sites and proline-rich region are dispensable, arguing against adapter models of SHP-2 function. Although the SH2 domains contribute to SHP-2 specificity, studies of SHP chimeras reveal that substantial specificity resides in the PTP domain. Thus, PTP domains exhibit biologically relevant specificity in vivo, and noncatalytic and catalytic domains of PTPs contribute to specificity in a combinatorial fashion.     

Leptin increases hepatic insulin sensitivity and protein tyrosine phosphatase 1B expression

Leptin has been shown to improve insulin sensitivity and glucose metabolism in obese diabetic ob/ob mice, yet the mechanisms remain poorly defined. We found that 2 d of leptin treatment improved fasting but not postprandial glucose homeostasis, suggesting enhanced hepatic insulin sensitivity. Consistent with this hypothesis, leptin improved in vivo insulin receptor (IR) activation in liver, but not in skeletal muscle or fat. To explore the cellular mechanism by which leptin up-regulates hepatic IR activation, we examined the expression of the protein tyrosine phosphatase PTP1B, recently implicated as an important negative regulator of insulin signaling. Unexpectedly, liver PTP1B protein abundance was increased by leptin to levels similar to lean controls, whereas levels in muscle and fat remained unchanged. The ability of leptin to augment liver IR activation and PTP1B expression was also observed in vitro in human hepatoma cells (HepG2). However, overexpression of PTP1B in HepG2 cells led to diminished insulin-induced IR phosphorylation, supporting the role of PTP1B as a negative regulator of IR activation in hepatocytes. Collectively, our results suggest that leptin acutely improves hepatic insulin sensitivity in vivo with concomitant increases in PTP1B expression possibly serving to counterregulate insulin action and to maintain insulin signaling in proper balance.     

Insulin-stimulated hydrogen peroxide reversibly inhibits protein-tyrosine phosphatase 1b in vivo and enhances the early insulin action cascade

The insulin signaling pathway is activated by tyrosine phosphorylation of the insulin receptor and key post-receptor substrate proteins and balanced by the action of specific protein-tyrosine phosphatases (PTPases). PTPase activity, in turn, is highly regulated in vivo by oxidation/reduction reactions involving the cysteine thiol moiety required for catalysis. Here we show that insulin stimulation generates a burst of intracellular H(2)O(2) in insulin-sensitive hepatoma and adipose cells that is associated with reversible oxidative inhibition of up to 62% of overall cellular PTPase activity, as measured by a novel method using strictly anaerobic conditions. The specific activity of immunoprecipitated PTP1B, a PTPase homolog implicated in the regulation of insulin signaling, was also strongly inhibited by up to 88% following insulin stimulation. Catalase pretreatment abolished the insulin-stimulated production of H(2)O(2) as well as the inhibition of cellular PTPases, including PTP1B, and was associated with reduced insulin-stimulated tyrosine phosphorylation of its receptor and high M(r) insulin receptor substrate (IRS) proteins. These data provide compelling new evidence for a redox signal that enhances the early insulin-stimulated cascade of tyrosine phosphorylation by oxidative inactivation of PTP1B and possibly other tyrosine phosphatases.     

BH3 mimetics derived from Bim-BH3 domain core region show PTP1B inhibitory activity

A series of our previously described BH3 peptide mimetics derived from Bim-BH3 domain core region were found to exhibit weak to potent PTP1B binding affinity and inhibitory activities via target-based drug screening. Among these compounds, a 12-aa Bim-BH3 core sequence peptide conjugated to palmitic acid (SM-6) displayed good PTP1B binding affinity (K_D?=?8.38?nmol/L), inhibitory activity (IC₅₀?=?1.20?μmol/L) and selectivity against other PTPs (TCPTP, LAR, SHP-1 and SHP-2). Furthermore, SM-6 promoted HepG2 cell glucose uptake and inhibited the expression of PTP1B, indicating that SM-6 could improve the insulin resistance effect in the insulin-resistant HepG2 cell model. These results may indicate a new direction for the application of BH3 peptide mimetics and promising PTP1B peptide inhibitors could be designed and developed based on SM-6.     

Improved glucose homeostasis in mice with muscle-specific deletion of protein-tyrosine phosphatase 1B

Obesity and type 2 diabetes are characterized by insulin resistance. Mice lacking the protein-tyrosine phosphatase PTP1B in all tissues are hypersensitive to insulin but also have diminished fat stores. Because adiposity affects insulin sensitivity, the extent to which PTP1B directly regulates glucose homeostasis has been unclear. We report that mice lacking PTP1B only in muscle have body weight and adiposity comparable to those of controls on either chow or a high-fat diet (HFD). Muscle triglycerides and serum adipokines are also affected similarly by HFD in both groups. Nevertheless, muscle-specific PTP1B(-/-) mice exhibit increased muscle glucose uptake, improved systemic insulin sensitivity, and enhanced glucose tolerance. These findings correlate with and are most likely caused by increased phosphorylation of the insulin receptor and its downstream signaling components. Thus, muscle PTP1B plays a major role in regulating insulin action and glucose homeostasis, independent of adiposity. In addition, rosiglitazone treatment of HFD-fed control and muscle-specific PTP1B(-/-) mice revealed that rosiglitazone acts additively with PTP1B deletion. Therefore, combining PTP1B inhibition with thiazolidinediones should be more effective than either alone for treating insulin-resistant states.     

Insulin and insulin-like growth factor I receptors utilize different G protein signaling components

We examined the role of heterotrimeric G protein signaling components in insulin and insulin-like growth factor I (IGF-I) action. In HIRcB cells and in 3T3L1 adipocytes, treatment with the Galpha(i) inhibitor (pertussis toxin) or microinjection of the Gbetagamma inhibitor (glutathione S-transferase-betaARK) inhibited IGF-I and lysophosphatidic acid-stimulated mitogenesis but had no effect on epidermal growth factor (EGF) or insulin action. In basal state, Galpha(i) and Gbeta were associated with the IGF-I receptor (IGF-IR), and after ligand stimulation the association of IGF-IR with Galpha(i) increased concomitantly with a decrease in Gbeta association. No association of Galpha(i) was found with either the insulin or EGF receptor. Microinjection of anti-beta-arrestin-1 antibody specifically inhibited IGF-I mitogenic action but had no effect on EGF or insulin action. beta-Arrestin-1 was associated with the receptors for IGF-I, insulin, and EGF in a ligand-dependent manner. We demonstrated that Galpha(i), betagamma subunits, and beta-arrestin-1 all play a critical role in IGF-I mitogenic signaling. In contrast, neither metabolic, such as GLUT4 translocation, nor mitogenic signaling by insulin is dependent on these protein components. These results suggest that insulin receptors and IGF-IRs can function as G protein-coupled receptors and engage different G protein partners for downstream signaling.     

Bromophenols as inhibitors of protein tyrosine phosphatase 1B with antidiabetic properties

A series of bromophenol derivatives were synthesized and evaluated as protein tyrosine phosphatase 1B (PTP1B) inhibitors in vitro and in vivo based on bromophenol 4e (IC(50)=2.42 μmol/L), which was isolated from red algae Rhodomela confervoides. The results showed that all of the synthesized compounds displayed weak to good PTP1B inhibition at tested concentration. Among them, highly brominated compound 4g exhibited promising inhibitory activity against PTP1B with IC(50) 0.68 μmol/L, which was approximately fourfold more potent than lead compound 4e. Further, compound 4g demonstrated high selectivity against other PTPs (TCPTP, LAR, SHP-1 and SHP-2). More importantly, in vivo antidiabetic activities investigations of compound 4g also demonstrated inspiring results.     

The reciprocal role of Egr-1 and Sp family proteins in regulation of the PTP1B promoter in response to the p210 Bcr-Abl oncoprotein-tyrosine kinase

Protein-tyrosine phosphatase 1B (PTP1B) is an important regulator of protein-tyrosine kinase-dependent signaling pathways. Changes in expression and activity of PTP1B have been associated with various human diseases; however, the mechanisms by which PTP1B expression is regulated have yet to be characterized. Previously, we have shown that the expression of PTP1B is enhanced by p210 Bcr-Abl and that PTP1B is a specific antagonist of transformation induced by this oncoprotein protein-tyrosine kinase. Here we have characterized the PTP1B promoter and demonstrate that a motif with features of a stress-response element acts as a p210 Bcr-Abl-responsive sequence, termed PRS. We have shown that three C(2)H(2) zinc finger proteins, namely Sp1, Sp3, and Egr-1, bind to PRS. Whereas binding of either Sp1 or Sp3 induced promoter function, Egr-1 repressed Sp3-mediated PTP1B promoter activation. The binding of Egr-1 to PRS is suppressed by p210 Bcr-Abl due to the inhibition of Egr-1 expression, resulting in the enhancement of PTP1B promoter activity. Our data indicate that Egr-1 and Sp family proteins play a reciprocal role in the control of expression from the PTP1B promoter.     

Investigation of protein-tyrosine phosphatases by in-gel assays

Renaturation permits the detection of protein-tyrosine phosphatase (PTP) activities subsequent to separation by SDS-PAGE in the presence of the (32)P-labeled poly(Glu(4)Tyr) as a macromolecular substrate. An efficient corresponding method has been developed by Burridge and Nelson [Anal. Biochem. 232 (1995) 56]. We describe here the modification of the basic method, its extension to two-dimensional gel electrophoresis, and applications to identify PTPs in signaling complexes and reversibly oxidized PTPs. Particular attention is given to the preparation of samples, to interpretation of the results as well as to advantages and limitations of the technique. Immunodepletion and the use of cells from knockout animals have been successful in the identification of individual PTPs. Readily detectable in cell lysates are PTP-PEST, SHP-2, SHP-1, PTP1B, and T-cell PTP. A much greater complexity of activity bands is, to a large extent, due to the generation of active fragments of PTPs. In-gel detection of PTPs can be employed to monitor cell fractionations and potential PTP activity changes.     

Differential regulation of endoplasmic reticulum stress by protein tyrosine phosphatase 1B and T cell protein tyrosine phosphatase

Protein-tyrosine phosphatase 1B (PTP1B) and T cell protein-tyrosine phosphatase (TCPTP) are closely related intracellular phosphatases implicated in the control of glucose homeostasis. PTP1B and TCPTP can function coordinately to regulate protein tyrosine kinase signaling, and PTP1B has been implicated previously in the regulation of endoplasmic reticulum (ER) stress. In this study, we assessed the roles of PTP1B and TCPTP in regulating ER stress in the endocrine pancreas. PTP1B and TCPTP expression was determined in pancreases from chow and high fat fed mice and the impact of PTP1B and TCPTP over- or underexpression on palmitate- or tunicamycin-induced ER stress signaling assessed in MIN6 insulinoma β cells. PTP1B expression was increased, and TCPTP expression decreased in pancreases of mice fed a high fat diet, as well as in MIN6 cells treated with palmitate. PTP1B overexpression or TCPTP knockdown in MIN6 cells mitigated palmitate- or tunicamycin-induced PERK/eIF2α ER stress signaling, whereas PTP1B deficiency enhanced ER stress. Moreover, PTP1B deficiency increased ER stress-induced cell death, whereas TCPTP deficiency protected MIN6 cells from ER stress-induced death. ER stress coincided with the inhibition of Src family kinases (SFKs), which was exacerbated by PTP1B overexpression and largely prevented by TCPTP knockdown. Pharmacological inhibition of SFKs ameliorated the protective effect of TCPTP deficiency on ER stress-induced cell death. These results demonstrate that PTP1B and TCPTP play nonredundant roles in modulating ER stress in pancreatic β cells and suggest that changes in PTP1B and TCPTP expression may serve as an adaptive response for the mitigation of chronic ER stress.     

SHP-1 requires inhibitory co-receptors to down-modulate B cell antigen receptor-mediated phosphorylation of cellular substrates

Signaling through the B cell antigen receptor (BCR) is negatively regulated by the SH2 domain-containing protein-tyrosine phosphatase SHP-1, which requires association with tyrosine-phosphorylated proteins for activation. Upon BCR ligation, SHP-1 has been shown to associate with the BCR, the cytoplasmic protein-tyrosine kinases Lyn and Syk, and the inhibitory co-receptors CD22 and CD72. How SHP-1 is activated by BCR ligation and regulates BCR signaling is, however, not fully understood. Here we demonstrate that, in the BCR-expressing myeloma line J558L mu 3, CD72 expression reduces the BCR ligation-induced phosphorylation of the BCR component Ig alpha/Ig beta and its cytoplasmic effectors Syk and SLP-65. Substrate phosphorylation was restored by expression of dominant negative mutants of SHP-1, whereas the SHP-1 mutants failed to enhance phosphorylation of the cellular substrates in the absence of CD72. This indicates that SHP-1 is efficiently activated by CD72 but not by other pathways in J558L mu m3 cells and that inhibition of SHP-1 specifically activated by CD72 reverses CD72-induced dephosphorylation of cellular substrates in these cells. Taken together, BCR-induced SHP-1 activation is likely to require inhibitory co-receptors such as CD72, and SHP-1 appears to mediate the negative regulatory effect of CD72 on BCR signaling by dephosphorylating Ig alpha/Ig beta and its downstream signaling molecules Syk and SLP-65.     

Insulin resistance associated to obesity: the link TNF-alpha

Adipose tissue secretes proteins which may influence insulin sensitivity. Among them, tumour necrosis factor (TNF)-alpha has been proposed as a link between obesity and insulin resistance because TNF-alpha is overexpressed in adipose tissue from obese animals and humans, and obese mice lacking either TNF-alpha or its receptor show protection against developing insulin resistance. The activation of proinflammatory pathways after exposure to TNF-alpha induces a state of insulin resistance in terms of glucose uptake in myocytes and adipocytes that impair insulin signalling at the level of the insulin receptor substrate (IRS) proteins. The mechanism found in brown adipocytes involves Ser phosphorylation of IRS-2 mediated by TNF-alpha activation of MAPKs. The Ser307 residue in IRS-1 has been identified as a site for the inhibitory effects of TNF-alpha in myotubes, with p38 mitogen-activated protein kinase (MAPK) and inhibitor kB kinase being involved in the phosphorylation of this residue. Moreover, up-regulation of protein-tyrosine phosphatase (PTP)1B expression was recently found in cells and animals treated with TNF-alpha. PTP1B acts as a physiological negative regulator of insulin signalling by dephosphorylating the phosphotyrosine residues of the insulin receptor and IRS-1, and PTP1B expression is increased in peripheral tissues from obese and diabetic humans and rodents. Accordingly, down-regulation of PTP1B activity by treatment with pharmacological agonists of nuclear receptors restores insulin sensitivity in the presence of TNF-alpha. Furthermore, mice and cells deficient in PTP1B are protected against insulin resistance induced by this cytokine. In conclusion, the absence or inhibition of PTP1B in insulin-target tissues could confer protection against insulin resistance induced by cytokines.     

Essential role of protein tyrosine phosphatase 1B in obesity-induced inflammation and peripheral insulin resistance during aging

Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of insulin signaling and a therapeutic target for type 2 diabetes (T2DM). In this study, we have evaluated the role of PTP1B in the development of aging-associated obesity, inflammation, and peripheral insulin resistance by assessing metabolic parameters at 3 and 16 months in PTP1B(-/-) mice maintained on mixed genetic background (C57Bl/6J × 129Sv/J). Whereas fat mass and adipocyte size were increased in wild-type control mice at 16 months, these parameters did not change with aging in PTP1B(-/-) mice. Increased levels of pro-inflammatory cytokines, crown-like structures, and hypoxia-inducible factor (HIF)-1α were observed only in adipose tissue from 16-month-old wild-type mice. Similarly, islet hyperplasia and hyperinsulinemia were observed in wild-type mice with aging-associated obesity, but not in PTP1B(-/-) animals. Leanness in 16-month-old PTP1B(-/-) mice was associated with increased energy expenditure. Whole-body insulin sensitivity decreased in 16-month-old control mice; however, studies with the hyperinsulinemic-euglycemic clamp revealed that PTP1B deficiency prevented this obesity-related decreased peripheral insulin sensitivity. At a molecular level, PTP1B expression and enzymatic activity were up-regulated in liver and muscle of 16-month-old wild-type mice as were the activation of stress kinases and the expression of p53. Conversely, insulin receptor-mediated Akt/Foxo1 signaling was attenuated in these aged control mice. Collectively, these data implicate PTP1B in the development of inflammation and insulin resistance associated with obesity during aging and suggest that inhibition of this phosphatase by therapeutic strategies might protect against age-dependent T2DM.