Microinjection of a protein-tyrosine-phosphatase inhibits insulin action in Xenopus oocytes

Protein tyrosine phosphatase 1B (PTP1B) function, structure, and inhibition strategies to develop antidiabetic drugs

Tyrosine protein phosphatase non-receptor type 1 (PTP1B; also known as protein tyrosine phosphatase 1B) is a member of the protein tyrosine phosphatase (PTP) family and is a soluble enzyme that plays an essential role in different physiological processes, including the regulation of metabolism, specifically in insulin and leptin sensitivity. PTP1B is crucial in the pathogenesis of type 2 diabetes mellitus and obesity. These biological functions have made PTP1B validated as an antidiabetic and anti-obesity, and potentially anticancer, molecular target. Four main approaches aim to inhibit PTP1B: orthosteric, allosteric, bidentate inhibition, and PTPN1 gene silencing. Developing a potent and selective PTP1B inhibitor is still challenging due to the enzyme's ubiquitous expression, subcellular location, and structural properties. This article reviews the main advances in the study of PTP1B since it was first isolated in 1988, as well as recent contextual information related to the PTP family to which this protein belongs. Furthermore, we offer an overview of the role of PTP1B in diabetes and obesity, and the challenges to developing selective, effective, potent, bioavailable, and cell-permeable compounds that can inhibit the enzyme.

Oral sub-chronic treatment with Terminalia phaeocarpa Eichler (Combretaceae) reduces liver PTP1B activity in a murine model of diabetes

ETHNOPHARMACOLOGICAL RELEVANCE

The endemic Brazilian medicinal plants of the genus Terminalia (Combretaceae), popularly known as capitão, comprising the similar species Terminalia phaeocarpa Eichler and Terminalia argentea, are traditionally and indistinguishably used in the country to treat diabetes.

AIM OF THE STUDY

The present work investigated the effect of 28 days of treatment with the crude ethanolic extract (CEE) and its derived ethyl acetate fraction (EAF) from T. phaeocarpa leaves in a mice model of diabetes.

MATERIALS AND METHODS

Streptozotocin-nicotinamide-fructose diabetic model was used to evaluate the antidiabetic activity of 28 days of treatment with the CEE and EAF from the leaves of T. phaeocarpa and metformin as a positive control. Serum levels of total cholesterol, triglycerides, uric acid, ALP, AST, and ALT were measured with specific commercial kits and glucose with a strip glucometer. The thiobarbituric acid method measured the liver MDA level, while a colorimetric assay measured the GSH level and PTP1B activity. A UPLC-DAD profile was obtained to identify the main polyphenolic compound in the EAF.

RESULTS

Treatment with CEE and EAF reduced plasma glucose in diabetic mice. At the end of the treatment, the plasma glucose level was significantly lower in EAF-treated (100 mg/kg) diabetic mice (106.1 ± 13.7 mg/dL) than those treated with 100 mg/kg CEE (175.2 ± 20.9 mg/dL), both significantly lower than untreated diabetic mice (350.4 ± 28.1 mg/dL). The serum levels of total cholesterol, triglycerides, uric acid, ALP, AST, and ALT were significantly reduced in diabetic mice treated with CEE and EAF. In the livers of diabetic mice, the treatment with CEE and EAF reduced MDA levels and the activity of the enzyme PTP1B (96.9 ± 3.7%, 113.8 ± 2.8%, and 134.8 ± 4.6% for CEE-, EAF-treated, and untreated diabetic mice, respectively). Galloylpunicalagin was the main polyphenol observed in the EAF of T. phaeocarpa.

CONCLUSION

The present results demonstrate the significant antidiabetic effect of CEE and EAF of T. phaeocarpa and their reduction on the markers of liver dysfunction in diabetic mice. Moreover, the antidiabetic activity of T. phaeocarpa might be associated with lowering the augmented activity of the PTP1B enzyme in the liver of diabetic mice.

Protein Tyrosine Phosphatases: Mighty oaks from little acorns grow

In October 2020, we were finally able to gather for a celebration of Eddy Fischer's 100th birthday. As with many other events, COVID had disrupted and restricted preparations for the gathering, which ultimately was held via ZOOM. Nevertheless, it was a wonderful opportunity to share a day with Eddy, an exceptional scientist and true renaissance man, and to appreciate his stellar contributions to science. Eddy Fischer, together with Ed Krebs, was responsible for the discovery of reversible protein phosphorylation, which launched the entire field of signal transduction. The importance of this seminal work is now being felt throughout the biotechnology industry with the development of drugs that target protein kinases, which have transformed the treatment of a wide array of cancers. I was privileged to have worked with Eddy both as a postdoc and a junior faculty member, during which time we laid the foundations for our current understanding of the protein tyrosine phosphatase (PTP) family of enzymes and their importance as critical regulators of signal transduction. This tribute to Eddy is based upon the talk I presented at the event, giving a personal perspective on Eddy's influence on my career, our early research efforts together in this area, and how the field has developed since then.

Genetic and Physiological Effects of Insulin-Like Growth Factor-1 (IGF-1) on Human Urate Homeostasis

SIGNIFICANCE STATEMENT

Hyperinsulinemia induces hyperuricemia by activating net renal urate reabsorption in the renal proximal tubule. The basolateral reabsorptive urate transporter GLUT9a appears to be the dominant target for insulin. By contrast, IGF-1 infusion reduces serum urate (SU), through mechanisms unknown. Genetic variants of IGF1R associated with reduced SU have increased IGF-1R expression and interact with genes encoding the GLUT9 and ABCG2 urate transporters, in a sex-specific fashion, which controls the SU level. Activation of IGF-1/IGF-1R signaling in Xenopus oocytes modestly activates GLUT9a and inhibits insulin's stimulatory effect on the transporter, which also activates multiple secretory urate transporters-ABCG2, ABCC4, OAT1, and OAT3. The results collectively suggest that IGF-1 reduces SU by activating secretory urate transporters and inhibiting insulin's action on GLUT9a.

BACKGROUND

Metabolic syndrome and hyperinsulinemia are associated with hyperuricemia. Insulin infusion in healthy volunteers elevates serum urate (SU) by activating net urate reabsorption in the renal proximal tubule, whereas IGF-1 infusion reduces SU by mechanisms unknown. Variation within the IGF1R gene also affects SU levels.

METHODS

Colocalization analyses of a SU genome-wide association studies signal at IGF1R and expression quantitative trait loci signals in cis using COLOC2, RT-PCR, Western blotting, and urate transport assays in transfected HEK 293T cells and in Xenopus laevis oocytes.

RESULTS

Genetic association at IGF1R with SU is stronger in women and is mediated by control of IGF1R expression. Inheritance of the urate-lowering homozygous genotype at the SLC2A9 locus is associated with a differential effect of IGF1R genotype between men and women. IGF-1, through IGF-1R, stimulated urate uptake in human renal proximal tubule epithelial cells and transfected HEK 293T cells, through activation of IRS1, PI3/Akt, MEK/ERK, and p38 MAPK; urate uptake was inhibited in the presence of uricosuric drugs, specific inhibitors of protein tyrosine kinase, PI3 kinase (PI3K), ERK, and p38 MAPK. In X. laevis oocytes expressing ten individual urate transporters, IGF-1 through endogenous IGF-1R stimulated urate transport mediated by GLUT9, OAT1, OAT3, ABCG2, and ABCC4 and inhibited insulin's stimulatory action on GLUT9a and OAT3. IGF-1 significantly activated Akt and ERK. Specific inhibitors of PI3K, ERK, and PKC significantly affected IGF-1 stimulation of urate transport in oocytes.

CONCLUSIONS

The combined results of infusion, genetics, and transport experiments suggest that IGF-1 reduces SU by activating urate secretory transporters and inhibiting insulin's action.

Pentafluorophosphato‐Phenylalanines: Amphiphilic Phosphotyrosine Mimetics Displaying Fluorine‐Specific Protein Interactions

Phosphotyrosine residues are essential functional switches in health and disease. Thus, phosphotyrosine biomimetics are crucial for the development of chemical tools and drug molecules. We report here the discovery and investigation of pentafluorophosphato amino acids as novel phosphotyrosine biomimetics. A mild acidic pentafluorination protocol was developed and two PF₅‐amino acids were prepared and employed in peptide synthesis. Their structures, reactivities, and fluorine‐specific interactions were studied by NMR and IR spectroscopy, X‐ray diffraction, and in bioactivity assays. The mono‐anionic PF₅ motif displayed an amphiphilic character binding to hydrophobic surfaces, to water molecules, and to protein‐binding sites, exploiting charge and H−F‐bonding interactions. The novel motifs bind 25‐ to 30‐fold stronger to the phosphotyrosine binding site of the protein tyrosine phosphatase PTP1B than the best current biomimetics, as rationalized by computational methods, including molecular dynamics simulations.

The role of protein tyrosine phosphatase 1B (PTP1B) in the pathogenesis of type 2 diabetes mellitus and its complications

Insulin resistance, the most important characteristic of the type 2 diabetes mellitus (T2DM), is mostly caused by impairment in the insulin receptor (IR) signal transduction pathway. Protein tyrosine phosphatase 1B (PTP1B), one of the main negative regulators of the IR signaling pathway, is broadly expressed in various cells and tissues. PTP1B decreases the phosphorylation of the IR resulting in insulin resistance in various tissues. The evidence for the physiological role of PTP1B in regulation of metabolic pathways came from whole-body PTP1B-knockout mice. Whole-body and tissue-specific PTP1B-knockout mice showed improvement in adiposity, insulin resistance, and glucose tolerance. In addition, the key role of PTP1B in the pathogenesis of T2DM and its complications was further investigated in mice models of PTP1B deficient/overexpression. In recent years, targeting PTP1B using PTP1B inhibitors is being considered an attractive target to treat T2DM. PTP1B inhibitors improve the sensitivity of the insulin receptor and have the ability to cure insulin resistance-related diseases. We herein summarized the biological functions of PTP1B in different tissues in vivo and in vitro. We also describe the effectiveness of potent PTP1B inhibitors as pharmaceutical agents to treat T2DM.

Genetic and Physiological Effects of Insulin on Human Urate Homeostasis

Insulin and hyperinsulinemia reduce renal fractional excretion of urate (FeU) and play a key role in the genesis of hyperuricemia and gout, via uncharacterized mechanisms. To explore this association further we studied the effects of genetic variation in insulin-associated pathways on serum urate (SU) levels and the physiological effects of insulin on urate transporters. We found that urate-associated variants in the human insulin (INS), insulin receptor (INSR), and insulin receptor substrate-1 (IRS1) loci associate with the expression of the insulin-like growth factor 2, IRS1, INSR, and ZNF358 genes; additionally, we found genetic interaction between SLC2A9 and the three loci, most evident in women. We also found that insulin stimulates the expression of GLUT9 and increases [14C]-urate uptake in human proximal tubular cells (PTC-05) and HEK293T cells, transport activity that was effectively abrogated by uricosurics or inhibitors of protein tyrosine kinase (PTK), PI3 kinase, MEK/ERK, or p38 MAPK. Heterologous expression of individual urate transporters in Xenopus oocytes revealed that the [14C]-urate transport activities of GLUT9a, GLUT9b, OAT10, OAT3, OAT1, NPT1 and ABCG2 are directly activated by insulin signaling, through PI3 kinase (PI3K)/Akt, MEK/ERK and/or p38 MAPK. Given that the high-capacity urate transporter GLUT9a is the exclusive basolateral exit pathway for reabsorbed urate from the renal proximal tubule into the blood, that insulin stimulates both GLUT9 expression and urate transport activity more than other urate transporters, and that SLC2A9 shows genetic interaction with urate-associated insulin-signaling loci, we postulate that the anti-uricosuric effect of insulin is primarily due to the enhanced expression and activation of GLUT9.

PTP1B negatively regulates STAT1-independent Pseudomonas aeruginosa killing by macrophages

Pseudomonas aeruginosa is the main conditional pathogen of immunodeficiency individuals. The mechanisms governing immune response to P. aeruginosa infection by macrophages remain incompletely defined. Herein, we demonstrate that protein tyrosine phosphatase-1B (PTP1B) is a critical negative regulator of P. aeruginosa infection response by macrophages. PTP1B-deficient macrophages display greatly enhanced bacterial phagocytosis and killing, accompanied by increased lysosome formation during P. aeruginosa infection. We also found that PTP1B repressed nitric oxide (NO) production and nitric oxide synthase (iNOS) induction following P. aeruginosa infection. PTP1B deficiency tended to upregulate the production of TRIF-interferon (IFN) pathway cytokines and chemokines, including IFN-β and interferon γ-inducible protein 10 (CXCL10, IP-10). Unexpectedly, the phosphorylation level of STAT1 was not regulated by PTP1B. In vivo experiments also confirmed that the regulatory function of PTP1B was not dependent on STAT1. These findings demonstrate that STAT1 is dispensable for negative regulation of P. aeruginosa clearance by macrophages.

Phosphotyrosine isosteres: past, present and future

Tyrosine phosphorylation is a critical component of signal transduction for multicellular organisms, particularly for pathways that regulate cell proliferation and differentiation. While tyrosine kinase inhibitors have become FDA-approved drugs, inhibitors of the other important components of these signaling pathways have been harder to develop. Specifically, direct phosphotyrosine (pTyr) isosteres have been aggressively pursued as inhibitors of Src homology 2 (SH2) domains and protein tyrosine phosphatases (PTPs). Medicinal chemists have produced many classes of peptide and small molecule inhibitors that mimic pTyr. However, balancing affinity with selectivity and cell penetration has made this an extremely difficult space for developing successful clinical candidates. This review will provide a comprehensive picture of the field of pTyr isosteres, from early beginnings to the current state and trajectory. We will also highlight the major protein targets of these medicinal chemistry efforts, the major classes of peptide and small molecule inhibitors that have been developed, and the handful of compounds which have been tested in clinical trials.

A potent, selective, and orally bioavailable inhibitor of the protein-tyrosine phosphatase PTP1B improves insulin and leptin signaling in animal models

The protein-tyrosine phosphatase PTP1B is a negative regulator of insulin and leptin signaling and a highly validated therapeutic target for diabetes and obesity. Conventional approaches to drug development have produced potent and specific PTP1B inhibitors, but these inhibitors lack oral bioavailability, which limits their potential for drug development. Here, we report that DPM-1001, an analog of the specific PTP1B inhibitor trodusquemine (MSI-1436), is a potent, specific, and orally bioavailable inhibitor of PTP1B. DPM-1001 also chelates copper, which enhanced its potency as a PTP1B inhibitor. DPM-1001 displayed anti-diabetic properties that were associated with enhanced signaling through insulin and leptin receptors in animal models of diet-induced obesity. Therefore, DPM-1001 represents a proof of concept for a new approach to therapeutic intervention in diabetes and obesity. Although the PTPs have been considered undruggable, the findings of this study suggest that allosteric PTP inhibitors may help reinvigorate drug development efforts that focus on this important family of signal-transducing enzymes.

Roux-en-Y Gastric Bypass Improves Hepatic Glucose Metabolism Involving Down-Regulation of Protein Tyrosine Phosphatase 1B in Obese Rats

OBJECTIVE

This study was initiated to investigate the effects of Roux-en-Y gastric bypass (RYGB) surgery on hepatic glucose metabolism and hepatic expression of protein tyrosine phosphatase 1B (PTP1B) in obese rats.

METHODS

Body weight, glucose, intraperitoneal glucose, insulin, and pyruvate tolerance tests were performed pre- and postoperatively, and plasma lipid, insulin and glucagon-like peptide 1 (GLP-1) were measured. The mRNA levels of G6Pase, Pepck, Gsk-3β and Gys-2, and the expression levels of PTP1B mRNA, protein, and other components of the insulin signaling pathway were measured by using RT-PCR and western blotting. The intracellular localization of PTP1B and hepatic glycogen deposition was also observed.

RESULTS

RYGB surgery-treated rats showed persistent weight loss, significantly improved glucose tolerance, pyruvate tolerance, and dyslipidemia, as well as increased insulin sensitivity, hepatic glycogen deposition and increased plasma GLP-1 in obese rats. RT-PCR analyses showed Pepck, G6Pase, and Gsk-3β mRNA to be significantly decreased, and Gys-2 mRNA to be significantly increased in liver tissue in the RYGB group (p < 0.05 vs. high-fat diet (HFD) or HFD + sham group); in addition, the expression of PTP1B were significantly decreased and insulin signaling were improved in the RYGB group (p < 0.05 vs. HFD or HFD + sham group).

CONCLUSION

RYGB can improve hepatic glucose metabolism and down-regulate PTP1B in obese rats. An increased circulating GLP-1 concentration may be correlated with the effects following RYGB in obese rats.

TLR9 expression is required for the development of cigarette smoke-induced emphysema in mice

The expression of Toll-like receptor (TLR)-9, a pathogen recognition receptor that recognizes unmethylated CpG sequences in microbial DNA molecules, is linked to the pathogenesis of several lung diseases. TLR9 expression and signaling was investigated in animal and cell models of chronic obstructive pulmonary disease (COPD). We observed enhanced TLR9 expression in mouse lungs following exposure to cigarette smoke. Tlr9(-/-) mice were resistant to cigarette smoke-induced loss of lung function as determined by mean linear intercept, total lung capacity, lung compliance, and tissue elastance analysis. Tlr9 expression also regulated smoke-mediated immune cell recruitment to the lung; apoptosis; expression of granulocyte-colony stimulating factor (G-CSF), the CXCL5 protein, and matrix metalloproteinase-2 (MMP-2); and protein tyrosine phosphatase 1B (PTP1B) activity in the lung. PTP1B, a phosphatase with anti-inflammatory abilities, was identified as binding to TLR9. In vivo delivery of a TLR9 agonist enhanced TLR9 binding to PTP1B, which inactivated PTP1B. Ptp1b(-/-) mice had elevated lung concentrations of G-CSF, CXCL5, and MMP-2, and tissue expression of type-1 interferon following TLR9 agonist administration, compared with wild-type mice. TLR9 responses were further determined in fully differentiated normal human bronchial epithelial (NHBE) cells isolated from nonsmoker, smoker, and COPD donors, and then cultured at air liquid interface. NHBE cells from smokers and patients with COPD expressed more TLR9 and secreted greater levels of G-CSF, IL-6, CXCL5, IL-1β, and MMP-2 upon TLR9 ligand stimulation compared with cells from nonsmoker donors. Although TLR9 combats infection, our results indicate that TLR9 induction can affect lung function by inactivating PTP1B and upregulating expression of proinflammatory cytokines.

Protein tyrosine phosphatase 1B (PTP1B) is dispensable for IgE-mediated cutaneous reaction in vivo

Mast cells play a critical role in allergic reactions. The cross-linking of FcεRI-bound IgE with multivalent antigen initiates a cascade of signaling events leading to mast cell activation. It has been well-recognized that cross linking of FcεRI mediates tyrosine phosphorylation. However, the mechanism involved in tyrosine dephosphorylation in mast cells is less clear. Here we demonstrated that protein tyrosine phosphatase 1B (PTP1B)-deficient mast cells showed increased IgE-mediated phosphorylation of the signal transducer and activator of transcription 5 (STAT5) and enhanced production of CCL9 (MIP-1γ) and IL-6 in IgE-mediated mast cells activation in vitro. However, IgE-mediated calcium mobilization, β-hexaosaminidase release (degranulation), and phosphorylation of IκB and MAP kinases were not affected by PTP1B deficiency. Furthermore, PTP1B deficient mice showed normal IgE-dependent passive cutaneous anaphylaxis and late phase cutaneous reactions in vivo. Thus, PTP1B specifically regulates IgE-mediated STAT5 pathway, but is redundant in influencing mast cell function in vivo.

Protein-tyrosine phosphatase 1B substrates and metabolic regulation

Metabolic homeostasis requires integration of complex signaling networks which, when deregulated, contribute to metabolic syndrome and related disorders. Protein-tyrosine phosphatase 1B (PTP1B) has emerged as a key regulator of signaling networks that are implicated in metabolic diseases such as obesity and type 2 diabetes. In this review, we examine mechanisms that regulate PTP1B-substrate interaction, enzymatic activity and experimental approaches to identify PTP1B substrates. We then highlight findings that implicate PTP1B in metabolic regulation. In particular, insulin and leptin signaling are discussed as well as recently identified PTP1B substrates that are involved in endoplasmic reticulum stress response, cell-cell communication, energy balance and vesicle trafficking. In summary, PTP1B exhibits exquisite substrate specificity and is an outstanding pharmaceutical target for obesity and type 2 diabetes.

Androgens alter T-cell immunity by inhibiting T-helper 1 differentiation

The hormonal milieu influences immune tolerance and the immune response against viruses and cancer, but the direct effect of androgens on cellular immunity remains largely uncharacterized. We therefore sought to evaluate the effect of androgens on murine and human T cells in vivo and in vitro. We found that murine androgen deprivation in vivo elicited RNA expression patterns conducive to IFN signaling and T-cell differentiation. Interrogation of mechanism showed that testosterone regulates T-helper 1 (Th1) differentiation by inhibiting IL-12-induced Stat4 phosphorylation: in murine models, we determined that androgen receptor binds a conserved region within the phosphatase, Ptpn1, and consequent up-regulation of Ptpn1 then inhibits IL-12 signaling in CD4 T cells. The clinical relevance of this mechanism, whereby the androgen milieu modulates CD4 T-cell differentiation, was ascertained as we found that androgen deprivation reduced expression of Ptpn1 in CD4 cells from patients undergoing androgen deprivation therapy for prostate cancer. Our findings, which demonstrate a clinically relevant mechanism by which androgens inhibit Th1 differentiation of CD4 T cells, provide rationale for targeting androgens to enhance CD4-mediated immune responses in cancer or, conversely, for modulating androgens to mitigate CD4 responses in disorders of autoimmunity.

The protein tyrosine phosphatase PTP1B is a negative regulator of CD40 and BAFF-R signaling and controls B cell autoimmunity

The protein tyrosine phosphatase PTP1B regulates co-receptor signaling on B cells and thus controls B cell autoimmunity.

Tyrosine phosphorylation of signaling molecules that mediate B cell activation in response to various stimuli is tightly regulated by protein tyrosine phosphatases (PTPs). PTP1B is a ubiquitously expressed tyrosine phosphatase with well-characterized functions in metabolic signaling pathways. We show here that PTP1B negatively regulates CD40, B cell activating factor receptor (BAFF-R), and TLR4 signaling in B cells. Specifically, PTP1B counteracts p38 mitogen-activated protein kinase (MAPK) activation by directly dephosphorylating Tyr¹⁸² of this kinase. Mice with a B cell–specific PTP1B deficiency show increased T cell–dependent immune responses and elevated total serum IgG. Furthermore, aged animals develop systemic autoimmunity with elevated serum anti-dsDNA, spontaneous germinal centers in the spleen, and deposition of IgG immune complexes and C3 in the kidney. In a clinical setting, we observed that B cells of rheumatoid arthritis patients have significantly reduced PTP1B expression. Our data suggest that PTP1B plays an important role in the control of B cell activation and the maintenance of immunological tolerance.

PTP1B and TCPTP--nonredundant phosphatases in insulin signaling and glucose homeostasis

Insulin resistance is a key pathological feature of type 2 diabetes and is characterized by defects in signaling by the insulin receptor (IR) protein tyrosine kinase. The inhibition of protein tyrosine phosphatases (PTPs) that antagonize IR signaling may provide a means for enhancing the insulin response and alleviating insulin resistance. The prototypic phosphotyrosine-specific phosphatase PTP1B dephosphorylates the IR and attenuates insulin signaling in muscle and liver. Mice that are deficient for PTP1B exhibit improved glucose homeostasis in diet and genetic models of insulin resistance and type 2 diabetes. The phosphatase TCPTP shares 72% catalytic domain sequence identity with PTP1B and has also been implicated in IR regulation. Despite their high degree of similarity, PTP1B and TCPTP act together in vitro and in vivo to regulate insulin signaling and glucose homeostasis. This review highlights their capacity to act specifically and nonredundantly in cellular signaling, describes their roles in IR regulation and glucose homeostasis, and discusses their potential as drug targets for the enhancement of IR phosphorylation and insulin sensitivity in type 2 diabetes.

Developmental competence in oocytes and cumulus cells: candidate genes and networks

Common aspects of infertility can be seen across several species. In humans, dairy cows, and mares there is only a 25-35% chance of producing a live offspring after a single insemination, whether natural or artificial. Oocyte quality and subsequent embryo development can be affected by factors such as nutrition, hormonal regulation, and environmental influence. The objective of this study was to identify genes expressed in oocytes and/or cumulus cells, across a diverse range of species, which may be linked to the ability an oocyte has to develop following fertilization. Performing a meta-analysis on previously published microarray data on various models of oocyte and embryo quality allowed for the identification of 56 candidate genes associated with oocyte quality across several species, 4 of which were identified in the cumulus cells that surround the oocyte. Twenty-one potential biomarkers were associated with increased competence and 35 potential biomarkers were associated with decreased competence. The upregulation of Metap2, and the decrease of multiple genes linked to mRNA and protein synthesis in models of competence, highlights the importance of de novo protein synthesis and its regulation for successful oocyte maturation and subsequent development. The negative regulation of Wnt signaling has emerged in human, monkey, bovine, and mouse models of oocyte competence. Atrx expression was linked to decreased competence in both oocytes and cumulus cells. Biological networks and transcription factor regulation associated with increased and decreased competence were also identified. These genes could potentially act as biomarkers of oocyte quality or as pharmacological targets for manipulation in order to improve oocyte developmental potential.

The Genetics of PTPN1 and Obesity: Insights from Mouse Models of Tissue-Specific PTP1B Deficiency

The protein tyrosine phosphatase PTP1B is a negative regulator of both insulin and leptin signaling and is involved in the control of glucose homeostasis and energy expenditure. Due to its prominent role in regulating metabolism, PTP1B is a promising therapeutic target for the treatment of human obesity and type 2 diabetes. The PTP1B protein is encoded by the PTPN1 gene on human chromosome 20q13, a region that shows linkage with insulin resistance, type 2 diabetes, and obesity in human populations. In this paper, we summarize the genetics of the PTPN1 locus and associations with metabolic disease. In addition, we discuss the tissue-specific functions of PTP1B as gleaned from genetic mouse models.

A new highly efficient substrate-trapping mutant of protein tyrosine phosphatase 1B (PTP1B) reveals full autoactivation of the insulin receptor precursor

PTP1B is a protein tyrosine-phosphatase located on the cytosolic side of the endoplasmic reticulum that plays an important role in the regulation of the insulin receptor (IR). Replacement of the conserved Asp-181 by alanine is known to convert PTP1B into a substrate-trapping protein that binds to but cannot dephosphorylate its substrates. In this work, we have studied the effect of an additional mutation (Y46F) on the substrate-trapping efficiency of PTP1B-D181A. We observed that this mutation converts PTP1B-D181A into a highly efficient substrate-trapping mutant, resulting in much higher recovery of tyrosine-phosphorylated proteins coimmunoprecipitated with PTP1B. Bioluminescence resonance energy transfer (BRET) experiments were also performed to compare the dynamics of interaction of the IR with these mutants. Basal BRET, which mainly reflects the interaction of PTP1B with the IR precursor during its biosynthesis in the endoplasmic reticulum, was markedly increased with the PTP1B-D181A-Y46F mutant. In contrast, insulin-induced BRET was markedly reduced with PTP1B-D181A-Y46F. I(125) insulin binding experiments indicated that PTP1B-D181-Y46F reduced the expression of IR at the plasma membrane. Reduced expression at the cell surface was associated with higher amounts of the uncleaved IR precursor in the cell. Moreover, we observed that substantial amounts of the uncleaved IR precursor reached the Tris-phosphorylated, fully activated form in an insulin independent fashion. These results support the notion that PTP1B plays a crucial role in the control of the activity of the IR precursor during its biosynthesis. In addition, this new substrate-trapping mutant may be a valuable tool for the identification of new PTP1B substrates.

PTP1B: a double agent in metabolism and oncogenesis

PTP1B, a non-transmembrane protein tyrosine phosphatase that has long been studied as a negative regulator of insulin and leptin signaling, has received renewed attention as an unexpected positive factor in tumorigenesis. Here, we highlight how views of this enzyme have evolved from regarding it as a simple metabolic off-switch to a more complex view of PTP1B as an enzyme that can play both negative and positive roles in diverse signaling pathways. These dual characteristics make PTP1B a particularly attractive therapeutic target for diabetes, obesity, and perhaps breast cancer.

Molecular dynamics simulation study of PTP1B with allosteric inhibitor and its application in receptor based pharmacophore modeling

Allosteric inhibition of protein tyrosine phosphatase 1B (PTP1B), has paved a new path to design specific inhibitors for PTP1B, which is an important drug target for the treatment of type II diabetes and obesity. The PTP1B1-282-allosteric inhibitor complex crystal structure lacks alpha7 (287-298) and moreover there is no available 3D structure of PTP1B1-298 in open form. As the interaction between alpha7 and alpha6-alpha3 helices plays a crucial role in allosteric inhibition, alpha7 was modeled to the PTP1B1-282 in open form complexed with an allosteric inhibitor (compound-2) and a 5 ns MD simulation was performed to investigate the relative orientation of the alpha7-alpha6-alpha3 helices. The simulation conformational space was statistically sampled by clustering analyses. This approach was helpful to reveal certain clues on PTP1B allosteric inhibition. The simulation was also utilized in the generation of receptor based pharmacophore models to include the conformational flexibility of the protein-inhibitor complex. Three cluster representative structures of the highly populated clusters were selected for pharmacophore model generation. The three pharmacophore models were subsequently utilized for screening databases to retrieve molecules containing the features that complement the allosteric site. The retrieved hits were filtered based on certain drug-like properties and molecular docking simulations were performed in two different conformations of protein. Thus, performing MD simulation with alpha7 to investigate the changes at the allosteric site, then developing receptor based pharmacophore models and finally docking the retrieved hits into two distinct conformations will be a reliable methodology in identifying PTP1B allosteric inhibitors.

Inhibition of the protein tyrosine phosphatase PTP1B: potential therapy for obesity, insulin resistance and type-2 diabetes mellitus

The global epidemic of obesity and type-2 diabetes mellitus (T2DM) has highlighted the need for new therapeutic approaches. The association of insulin resistance with these disorders and the knowledge that insulin receptor signaling is mediated by tyrosine (Tyr) phosphorylation have generated great interest in the regulation of the balance between Tyr phosphorylation and dephosphorylation. Several protein Tyr phosphatases (PTPs) have been implicated in the regulation of insulin action, with the most convincing data for PTP1B. Murine models targeting PTP1B, PTP1B(-/-)mice, demonstrate enhanced insulin sensitivity without the weight gain seen with other insulin sensitizers such as peroxisome proliferator-activated receptor gamma (PPARgamma) agonists, probably due to a second action of PTP1B as a negative regulator of leptin signaling. Despite intensive efforts and recent progress, a safe, selective and efficacious PTP1B inhibitor has yet to be identified.

Tyrosine phosphatases and their possible interplay with tyrosine kinases

Protein tyrosine phosphatases represent a new family of intracellular and receptor-linked enzymes. They are totally specific toward tyrosyl residues in proteins, and, with specific activities 10-1000-fold greater than those of the protein tyrosine kinases, they can be expected to tightly control the level of phosphotyrosine within the cell. Most transmembrane forms contain two conserved intracellular catalytic domains, as displayed by the leukocyte common antigen CD45, but highly variable external segments. Some are related to the neuronal cell adhesion molecules (NCAMs) or fasciclin II and others contain fibronectin III repeats; this suggests that these enzymes might be involved in cell-cell interaction. The intercellular enzymes appear to contain a highly conserved catalytic core linked to a regulatory segment. Deletion of the regulatory domain alters both substrate specificity and cellular localization. Likewise, overexpression of the full-length and truncated enzymes affects cell cycle progression and actin filament stability, respectively. The interplay between tyrosine kinases and phosphatases is considered. A hypothesis is presented suggesting that in some systems phosphatases might act synergistically with the kinases and elicit a physiological response, irrespective of the state of phosphorylation of the target protein.

The use of bioluminescence resonance energy transfer for the study of therapeutic targets: application to tyrosine kinase receptors

During recent years, the bioluminescence resonance energy transfer (BRET) methodology has emerged as a powerful technique for the study of protein-protein interactions. This review focuses on recent work demonstrating the power of BRET for the study of tyrosine kinase receptors, using insulin and IGF-1 receptors as models. The authors show that BRET can be used to monitor ligand-induced conformational changes within homodimeric insulin and IGF-1 receptors, as well as heterodimeric insulin/IGF-1 hybrid receptors. BRET can also be used to study, in real time and in living cells, the interaction of tyrosine kinase receptors with cellular partners negatively or positively involved in the regulation of intracellular signalling (protein tyrosine phosphatases, molecular adaptors). In addition, BRET can be used to develop high-throughput screening assays for the search of molecules with therapeutic interest and could, therefore, constitute a valuable tool for laboratories involved in drug discovery.

Protein tyrosine phosphatase function: the substrate perspective

It is now well established that the members of the PTP (protein tyrosine phosphatase) superfamily play critical roles in fundamental biological processes. Although there has been much progress in defining the function of PTPs, the task of identifying substrates for these enzymes still presents a challenge. Many PTPs have yet to have their physiological substrates identified. The focus of this review will be on the current state of knowledge of PTP substrates and the approaches used to identify them. We propose experimental criteria that should be satisfied in order to rigorously assign PTP substrates as bona fide. Finally, the progress that has been made in defining the biological roles of PTPs through the identification of their substrates will be discussed.

Cellular Inhibition of Protein Tyrosine Phosphatase 1B by Uncharged Thioxothiazolidinone Derivatives

As important regulators of cellular signal transduction, members of the protein tyrosine phosphatase (PTP) family are considered to be promising drug targets. However, to date, the most effective in vitro PTP inhibitors have tended to be highly charged, thus limiting cellular permeability. Here, we have identified an uncharged thioxothiazolidinone derivative (compound 1), as a competitive inhibitor of a subset of PTPs. Compound 1 effectively inhibited protein tyrosine phosphatase 1B (PTP1B) in two cell-based systems: it sensitized wild-type, but not PTP1B-null fibroblasts to insulin stimulation and prevented PTP1B-dependent dephosphorylation of the FLT3-ITD receptor tyrosine kinase. We have also tested a series of derivatives in vitro against PTP1B and proposed a model of the PTP1B-inhibitor interaction. These compounds should be useful in the elucidation of cellular PTP function and could represent a starting point for development of therapeutic PTP inhibitors.

Involvement of the small protein tyrosine phosphatases TC-PTP and PTP1B in signal transduction and diseases: from diabetes, obesity to cell cycle, and cancer

As in other fields of biomedical research, the use of gene-targeted mice by homologous recombination in embryonic stem cells has provided important findings on the function of several members of the protein tyrosine phosphatase (PTP) family. For instance, the phenotypic characterization of knockout mice has been critical in understanding the sites of action of the related PTPs protein tyrosine phosphatase 1B (PTP1B) and T-cell-PTP (TC-PTP). By their increased insulin sensitivity and insulin receptor hyperphosphorylation, PTP1B null mice demonstrated a clear function for this enzyme as a negative regulator of insulin signaling. As well, TC-PTP has also been recently involved in insulin signaling in vitro. Importantly, the high identity in their amino acid sequences suggests that they must be examined simultaneously as targets of drug development. Indeed, they possess different as well as overlapping substrates, which suggest complementary and overlapping roles of both TC-PTP and PTP1B. Here, we review the function of PTP1B and TC-PTP in diabetes, obesity, and processes related to cancer.

Cytoplasmic protein tyrosine phosphatases, regulation and function: the roles of PTP1B and TC-PTP

PTP1B and TC-PTP are closely related protein tyrosine phosphatases, sharing 74% homology in their catalytic domain. However, their cellular localization, function, and regulation are found to be different. Their substrate specificity has implicated these enzymes in various signaling pathways, regulating metabolism, proliferation and cytokine signaling. For instance, PTP1B has been shown to regulate the activation of cytokine receptors through the dephosphorylation of specific members of the JAK family, namely JAK2 and TYK2, whereas TC-PTP is involved in the modulation of cytokine signaling via JAK1 and JAK3 molecules. Gene-targeting approaches will help us to unravel the physiological functions of these enzymes.

Interaction of PTPB with the insulin receptor precursor during its biosynthesis in the endoplasmic reticulum

PTP1B is a protein tyrosine-phosphatase predominantly located on the cystosolic surface of the endoplasmic reticulum. This tyrosine-phosphatase plays a major role in the regulation of the activity of the insulin receptor (IR). We have studied the interaction of the IR with PTP1B in living cells using bioluminescence resonance energy transfer (BRET). The IR was fused to Renilla luciferase and a substrate-trapping mutant of PTP1B was fused to the yellow variant of the green fluorescent protein (YFP). When the two partners interacted, an energy transfer occurred between the luciferase and the YFP, and a fluorescent signal, emitted by the YFP, could be detected. The interaction of the IR with PTP1B could be monitored in real time for more than 30 min. Insulin rapidly and dose-dependently stimulated this interaction. The basal (insulin-independent) interaction of IR with PTP1B was much lower with a soluble form than with the endoplasmic reticulum-targeted form of PTP1B, indicating that this basal interaction mainly occurred in the endoplasmic reticulum. In the basal state, PTP1B and the IR indeed co-localized in the endoplasmic reticulum, as demonstrated by confocal microscopy and cell fractionation experiments. Moreover, inhibition of IR processing with tunicamycin indicated that the basal interaction of PTP1B with IR occurred during biosynthesis of the IR precursor in the endoplasmic reticulum. These results strongly suggest that PTP1B not only dephosphorylates the insulin receptor that has been activated by insulin, but also regulates the insulin receptor precursor during its biosynthesis. Localisation of PTP1B to the endoplasmic reticulum may be important to prevent insulin-independent autonomous activity of the immature insulin receptor precursor.

Interaction of the insulin receptor with the receptor-like protein tyrosine phosphatases PTPalpha and PTPepsilon in living cells

The interactions between the insulin receptor and the two highly homologous receptor-like protein tyrosine phosphatases (PTPase) PTPalpha and PTPepsilon were studied in living cells by using bioluminescence resonance energy transfer. In human embryonic kidney 293 cells expressing the insulin receptor fused to luciferase and substrate-trapping mutants of PTPalpha or PTPepsilon fused to the fluorescent protein Topaz, insulin induces an increase in resonance energy transfer that could be followed in real time in living cells. Insulin effect could be detected at very early time points and was maximal less than 2 min after insulin addition. Bioluminescence resonance energy-transfer saturation experiments indicate that insulin does not stimulate the recruitment of protein tyrosine phosphatase molecules to the insulin receptor but rather induces conformational changes within preassociated insulin receptor/protein tyrosine phosphatase complexes. Physical preassociation of the insulin receptor with these protein tyrosine phosphatases at the plasma membrane, in the absence of insulin, was also demonstrated by chemical cross-linking with a non-cell-permeable agent. These data provide the first evidence that PTPalpha and PTPepsilon associate with the insulin receptor in the basal state and suggest that these protein tyrosine phosphatases may constitute important negative regulators of the insulin receptor tyrosine kinase activity by acting rapidly at the plasma membrane level.

Metformin (Glucophage) inhibits tyrosine phosphatase activity to stimulate the insulin receptor tyrosine kinase

Metformin is a commonly used anti-diabetic but whether its mechanism involves action on the insulin receptor or on downstream events is still controversial. With a time course that was slow compared with insulin action, metformin increased tyrosine phosphorylation of the regulatory domain of the insulin receptor (specifically, tyrosine residues 1150 and 1151). In a direct action, therapeutic levels of metformin stimulated the tyrosine kinase activity of the soluble intracellular portion of the beta subunit of the human insulin receptor toward a substrate derived from the insulin receptor regulatory domain. However, metformin did not alter the order of substrate phosphorylation by the insulin receptor kinase. Using a Xenopus oocyte preparation, we simultaneously recorded tyrosine kinase and phosphatase activities that regulate the insulin receptor by measuring the tyrosine phosphorylation and dephosphorylation of peptides derived from the regulatory domain of the human insulin receptor. In an indirect stimulation of the insulin receptor, metformin inhibited endogenous tyrosine phosphatases and purified human protein tyrosine phosphatase 1B that dephosphorylate and inhibit the insulin receptor kinase. Thus, there was evidence that metformin acted directly upon the insulin receptor and indirectly through inhibition of tyrosine phosphatases.

Aging-related attenuation of EGF receptor signaling is mediated in part by increased protein tyrosine phosphatase activity

As fibroblasts near senescence, their responsiveness to external signals diminishes. This well-documented phenomenon likely underlies physiological deterioration and limited tissue regeneration in aging individuals. Understanding the underlying molecular mechanisms would provide opportunities to ameliorate these situations. A key stimulus for human dermal fibroblasts are ligands for the epidermal growth factor receptor (EGFR). We have shown earlier that EGFR expression decreases by about half in near senescent fibroblasts (Shiraha et al., 2000, J. Biol. Chem. 275 (25), 19343-19351). However, as the cell responses are nearly absent near senescence, other aging-related signal attenuation changes must also occur. Herein, we show that EGFR signaling as determined by receptor autophosphorylation is diminished over 80%, with a corresponding decrease in the phosphorylation of the immediate postreceptor adaptor Shc. Interestingly, we found that this was due at least in part to increased dephosphorylation of EGFR. The global cell phosphotyrosine phosphatase activity increased some threefold in near senescent cells. An initial survey of EGFR-associated protein tyrosine phosphatases (PTPases) showed that SHP-1 (PTPIC, HCP, SHPTP-1) and PTPIB levels are increased in parallel in these cells. Concomitantly, we also discovered an increase in expression of receptor protein tyrosine phosphatase alpha (RPTPalpha). Last, inhibition of protein tyrosine phosphatases by sodium orthovanadate in near senescent cells resulted in increased EGFR phosphorylation. These data support a model in which, near senescence, dermal fibroblasts become resistant to EGFR-mediated stimuli by a combination of receptor downregulation and increased signal attenuation.

PTP1B: from the sidelines to the front lines!

Although initially viewed as housekeeping enzymes, research over the last 15 years has revealed that the protein tyrosine phosphatases (PTPs) are critical regulators of tyrosine phosphorylation-dependent signaling events and may represent novel targets for therapeutic intervention in a variety of human diseases. In this review I will describe some of the key advances in the characterization of the structure, regulation and function of the prototypic PTP, PTP1B, and illustrate how our understanding of the properties of this enzyme has revealed principles that apply to the PTP family as a whole.

Looking for an insulin pill? Use the BRET methodology!

Insulin exerts its biological effects through a plasma membrane receptor that possesses a tyrosine-kinase activity. This tyrosine-kinase activity depends on the autophosphorylation of the receptor on tyrosine residues and on its dephosphorylation by protein tyrosine-phosphatases. The discovery of pharmacological agents that specifically stimulate the autophosphorylation of the insulin receptor or inhibit its dephosphorylation will be of great importance for the treatment of insulin resistant or insulin deficient patients. Bioluminescence Resonance Energy Transfer (BRET) has developed in recent years as a new technique to study protein-protein interactions. In the BRET technique, one partner is fused to Renilla luciferase, whereas the other partner is fused to a fluorescent protein (e.g. YFP, Yellow Fluorescent Protein). The luciferase is excited by addition of its substrate, coelenterazine. If the two partners interact, resonance energy transfer occurs between the luciferase and the YFP, and a fluorescent signal, emitted by the YFP, can be detected. Our work indicates that this methodology could be an important tool for the search of molecules that activate insulin receptor autophosphorylation or that inhibit its dephosphorylation. Indeed, we first showed that the activation of the insulin receptor by different ligands can be monitored using a chimeric receptor with one B-subunit fused to Renilla luciferase and the other B-subunit fused to YFP. The conformational changes induced by different ligands could be detected as an energy transfer (BRET signal) between the luciferase and the YFP, that reflects the activation state of the receptor. This methodology allows for rapid analysis of the effects of agonists on insulin receptor activity and may therefore be used in high-throughput screening for the discovery of molecules with insulin-like properties. More recently, we demonstrated that the BRET methodology could also be used to monitor the interaction of the insulin receptor with protein tyrosine-phosphatase 1B, one of the main tyrosine-phosphatase that controls its activity. HEK cells were co-transfected with the insulin receptor fused to Renilla luciferase and a substrate-trapping mutant of PTP1B (PTP1B-D181A) fused to YFP. Insulin-induced BRET signal could be followed in real time for more than 30 min. Therefore, this methodology can also be used in high-throughput screening for the search of molecules that will specifically disrupt the interaction between the insulin receptor and PTP1B.

Reduction of protein-tyrosine phosphatase-1B increases insulin signaling in FAO hepatoma cells

Protein-tyrosine phosphatase-1B (PTP1B) has been implicated as a negative regulator of insulin signaling. PTP1B dephosphorylates the insulin receptor and insulin receptor substrates (IRS-1/2), inhibiting the insulin-signaling pathway. PTP1B has been reported to be elevated in diabetes and insulin-resistant states. Conversely, PTP1B null mice have increased insulin sensitivity. To further investigate the effect of PTP1B reduction on insulin signaling, FAO rat hepatoma cells were transfected, by electroporation, with a specific PTP1B antisense oligonucleotide (ASO), or a control oligonucleotide. The PTP1B ASO caused a 50-70% reduction in PTP1B protein expression as measured by Western blot analysis. Upon insulin stimulation, an increase in the phosphorylation of the insulin receptor and insulin receptor substrates was observed, without any change in protein expression levels. Reduction of PTP1B expression in FAO cells also caused an increase in insulin-stimulated phosphorylation of PKB and GSK3, without any change in protein expression. These results demonstrate that reduction of PTP1B can modulate key insulin signaling events downstream of the insulin receptor.

Protein-tyrosine-phosphatase 1B activation is regulated developmentally in muscle of neonatal pigs

The high activity of the insulin-signaling pathway contributes to the enhanced feeding-induced stimulation of translation initiation in skeletal muscle of neonatal pigs. Protein-tyrosine-phosphatase 1B (PTP1B) is a negative regulator of the tyrosine phosphorylation of the insulin receptor (IR) and insulin receptor substrate 1 (IRS-1). The activity of PTP1B is determined mainly by its association with IR and Grb2. We examined the level of PTP1B activity, PTP1B protein abundance, PTP1B tyrosine phosphorylation, and the association of PTP1B with IR and Grb2 in skeletal muscle and liver of fasted and fed 7- and 26-day-old pigs. PTP1B activity in skeletal muscle was lower (P < 0.05) in 7- compared with 26-day-old pigs but in liver was similar in the two age groups. PTP1B abundances were similar in muscle but lower (P < 0.05) in liver of 7- compared with 26-day-old pigs. PTP1B tyrosine phosphorylation in muscle was lower (P < 0.05) in 7- than in 26-day-old pigs. The associations of PTP1B with IR and with Grb2 were lower (P < 0.05) at 7 than at 26 days of age in muscle, but there were no age effects in liver. Finally, in both age groups, fasting did not have any effect on these parameters. These results indicate that basal PTP1B activation is developmentally regulated in skeletal muscle of neonatal pigs, consistent with the developmental changes in the activation of the insulin-signaling pathway reported previously. Reduced PTP1B activation in neonatal muscle likely contributes to the enhanced insulin sensitivity of skeletal muscle in neonatal pigs.

Role of 17beta-estradiol administration on insulin sensitivity in the rat: implications for the insulin receptor

The role of 17beta-estradiol in the early steps of insulin action is only partially known, although its effect on glucose homeostasis has been reported. In this paper, we attempt to prove the influence of 17beta-estradiol on the insulin receptor of ovariectomized rats treated with different hormonal doses. Our results show that high doses of estradiol impair insulin sensitivity while low doses improve it. We think that these results are the consequence of changes at a molecular level, because high doses of estradiol produced lower expression of the insulin receptor gene, lower content of this receptor in target tissues, and lower phosphorylation of insulin receptor in these tissues. However, low doses of estradiol seem to produce just the opposite. The possible existence of consensus response elements in the insulin receptor gene promoter to estradiol could be controlling the expression of this gene, this control being dose and timing dependent. Moreover, we cannot discard a possible effect of estradiol on the activity of protein tyrosine phosphatases, and therefore, on the activity of the insulin receptor. These new findings improve knowledge about the possible risk for insulin resistance in women taking oral contraceptives or receiving hormonal replacement therapy around the menopause, but could also open the door towards the possible utilization of 17beta-estradiol in some diabetes cases.

Role of SRC homology 2-containing tyrosine phosphatase 2 on proliferation of rat smooth muscle cells

OBJECTIVE

Src homology 2-containing phosphotyrosine phosphatase 2 (SHP2) is ubiquitously expressed and believed to function as part of a positive signaling pathway mediating growth factor-induced protein tyrosine phosphorylation. Proliferation of aortic vascular smooth muscle cells (SMCs) is an important contributor to atherosclerosis. We examined the effect of SHP2 expression on SMC proliferative activity.

METHODS AND RESULTS

SHP2 was abundant in cultured aortic SMCs, and SHP2 staining was markedly increased in the thickened aortic intima in rats with balloon-induced injury. We obtained several SMC clones by using geneticin screening. Endogenous SHP2 expression varied among individual clones. Significant positive relationships were observed between SHP2 expression and bromodeoxyuridine uptake in SMCs stimulated by FBS, platelet-derived growth factor, or insulin-like growth factor-1. In SMCs transiently transfected with SHP2, FBS stimulation significantly increased bromodeoxyuridine uptake beyond the uptake by control SMCs.

CONCLUSIONS

Increased SHP2 expression in SMCs may accelerate aortic atherosclerosis by increasing cell growth.

Coordinated action of protein tyrosine phosphatases in insulin signal transduction

Insulin is the principal regulatory hormone involved in the tight regulation of fuel metabolism. In response to blood glucose levels, it is secreted by the beta cells of the pancreas and exerts its effects by binding to cell surface receptors that are present on virtually all cell types and tissues. In humans, perturbations in insulin function and/or secretion lead to diabetes mellitus, a severe disorder primarily characterized by an inability to maintain blood glucose homeostasis. Furthermore, it is estimated that 90-95% of diabetic patients exhibit resistance to insulin action. Thus an understanding of insulin signal transduction and insulin resistance at the molecular level is crucial to the understanding of the pathogenesis of this disease. The insulin receptor (IR) is a transmembrane tyrosine kinase that becomes activated upon ligand binding. Consequently, the receptor and its downstream substrates become tyrosine phosphorylated. This activates a series of intracellular signaling cascades which coordinately initiate the appropriate biological response. One important mechanism by which insulin signaling is regulated involves the protein tyrosine phosphatases (PTPs), which may either act on the IR itself and/or its substrates. Two well characterized examples include leuckocyte antigen related (LAR) and protein tyrosine phosphatase-1B (PTP-1B). The present review will discuss the current knowledge of these two and other potential PTPs involved in the insulin signaling pathway.