Protein-tyrosine Phosphatase 1B Deficiency Protects against Fas-induced Hepatic Failure

Mapping enzyme activity in living systems by real-time mid-infrared photothermal imaging of nitrile chameleons

Simultaneous spatial mapping of the activity of multiple enzymes in a living system can elucidate their functions in health and disease. However, methods based on monitoring fluorescent substrates are limited. Here, we report the development of nitrile (C≡N)-tagged enzyme activity reporters, named nitrile chameleons, for the peak shift between substrate and product. To image these reporters in real time, we developed a laser-scanning mid-infrared photothermal imaging system capable of imaging the enzymatic substrates and products at a resolution of 300 nm. We show that when combined, these tools can map the activity distribution of different enzymes and measure their relative catalytic efficiency in living systems such as cancer cells, Caenorhabditis elegans, and brain tissues, and can be used to directly visualize caspase-phosphatase interactions during apoptosis. Our method is generally applicable to a broad category of enzymes and will enable new analyses of enzymes in their native context.

Targeting Protein Phosphatases for the Treatment of Chronic Liver Disease

There exists a huge number of patients suffering from chronic liver disease worldwide. As a disease with high incidence and mortality worldwide, strengthening the research on the pathogenesis of chronic liver disease and the development of novel drugs is an important issue related to the health of all human beings. Phosphorylation modification of proteins plays a crucial role in cellular signal transduction, and phosphatases are involved in the development of liver diseases. Therefore, this article summarized the important role of protein phosphatases in chronic liver disease with the aim of facilitating the development of drugs targeting protein phosphatases for the treatment of chronic liver disease.

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.

HGF/c-Met Signalling in the Tumor Microenvironment

Recently, it has become clearer that tumor plasticity increases the chance that cancer cells could acquire new mechanisms to escape immune surveillance, become resistant to conventional drugs, and spread to distant sites.Effectively, tumor plasticity drives adaptive response of cancer cells to hypoxia and nutrient deprivation leading to stimulation of neoangionesis or tumor escape. Therefore, tumor plasticity is believed to be a great contributor in recurrence and metastatic dissemination of cancer cells. Importantly, it could be an Achilles' heel of cancer if we could identify molecular mechanisms dictating this phenotype.The reactivation of stem-like signalling pathways is considered a great determinant of tumor plasticity; in addition, a key role has been also attributed to tumor microenvironment (TME). Indeed, it has been proved that cancer cells interact with different cells in the surrounding extracellular matrix (ECM). Interestingly, well-established communication represents a potential allied in maintenance of a plastic phenotype in cancer cells supporting tumor growth and spread. An important signalling pathway mediating cancer cell-TME crosstalk is represented by the HGF/c-Met signalling.Here, we review the role of the HGF/c-Met signalling in tumor-stroma crosstalk focusing on novel findings underlying its role in tumor plasticity, immune escape, and development of adaptive mechanisms.

Hepatic protein-tyrosine phosphatase 1B disruption and pharmacological inhibition attenuate ethanol-induced oxidative stress and ameliorate alcoholic liver disease in mice

Alcoholic liver disease (ALD) is a major health problem and a significant cause of liver-related death. Currently, the mainstay for ALD therapy is alcohol abstinence highlighting the need to develop pharmacotherapeutic approaches. Protein-tyrosine phosphatase 1B (PTP1B) is an established regulator of hepatic functions, but its role in ALD is mostly unexplored. In this study, we used mice with liver-specific PTP1B disruption as well as pharmacological inhibition to investigate the in vivo function of this phosphatase in ALD. We report upregulation of hepatic PTP1B in the chronic plus binge mouse model and, importantly, in liver biopsies of alcoholic hepatitis patients. Also, mice with hepatic PTP1B disruption attenuated ethanol-induced injury, inflammation, and steatosis compared with ethanol-fed control animals. Moreover, PTP1B deficiency was associated with decreased ethanol-induced oxidative stress in vivo and ex vivo. Further, pharmacological modulation of oxidative balance in hepatocytes identified diminished oxidative stress as a contributor to the salutary effects of PTP1B deficiency. Notably, PTP1B pharmacological inhibition elicited beneficial effects and mitigated hepatic injury, inflammation, and steatosis caused by ethanol feeding. In summary, these findings causally link hepatic PTP1B and ALD and define a potential therapeutic target for the management of this disease.

Inhibition of Protein-tyrosine Phosphatase PTP1B and LMPTP Promotes Palmitate/Oleate-challenged HepG2 Cell Survival by Reducing Lipoapoptosis, Improving Mitochondrial Dynamics and Mitigating Oxidative and Endoplasmic Reticulum Stress

Objectives: Non-alcoholic fatty liver disease (NAFLD) is considered a well-known pathology that is determined without using alcohol and has emerged as a growing public health problem. Lipotoxicity is known to promote hepatocyte death, which, in the context of NAFLD, is termed lipoapoptosis. The severity of NAFLD correlates with the degree of hepatocyte lipoapoptosis. Protein–tyrosine phosphatases (PTP) including PTP1B and Low molecular weight PTP (LMPTP), are negative regulators of the insulin signaling pathway and are considered a promising therapeutic target in the treatment of diabetes. In this study, we hypothesized that the inhibition of PTP1B and LMPTP may potentially prevent hepatocyte apoptosis, mitochondrial dysfunction and endoplasmic reticulum (ER) stress onset, following lipotoxicity induced using a free fatty acid (FFA) mixture. Methods: HepG2 cells were cultured in the presence or absence of two PTP inhibitors, namely MSI-1436 and Compound 23, prior to palmitate/oleate overloading. Apoptosis, ER stress, oxidative stress, and mitochondrial dynamics were then evaluated by either MUSE or RT-qPCR analysis. Results: The obtained data demonstrate that the inhibition of PTP1B and LMPTP prevents apoptosis induced by palmitate and oleate in the HepG2 cell line. Moreover, mitochondrial dynamics were positively improved following inhibition of the enzyme, with concomitant oxidative stress reduction and ER stress abrogation. Conclusion: In conclusion, PTP’s inhibitory properties may be a promising therapeutic strategy for the treatment of FFA-induced lipotoxicity in the liver and ultimately in the management of the NAFLD condition.

Impact of global PTP1B deficiency on the gut barrier permeability during NASH in mice

OBJECTIVE

Non-alcoholic steatohepatitis (NASH) is characterized by a robust pro-inflammatory component at both hepatic and systemic levels together with a disease-specific gut microbiome signature. Protein tyrosine phosphatase 1 B (PTP1B) plays distinct roles in non-immune and immune cells, in the latter inhibiting pro-inflammatory signaling cascades. In this study, we have explored the role of PTP1B in the composition of gut microbiota and gut barrier dynamics in methionine and choline-deficient (MCD) diet-induced NASH in mice.

METHODS

Gut features and barrier permeability were characterized in wild-type (PTP1B WT) and PTP1B-deficient knockout (PTP1B KO) mice fed a chow or methionine/choline-deficient (MCD) diet for 4 weeks. The impact of inflammation was studied in intestinal epithelial and enteroendocrine cells. The secretion of GLP-1 was evaluated in primary colonic cultures and plasma of mice.

RESULTS

We found that a shift in the gut microbiota shape, disruption of gut barrier function, higher levels of serum bile acids, and decreased circulating glucagon-like peptide (GLP)-1 are features during NASH. Surprisingly, despite the pro-inflammatory phenotype of global PTP1B-deficient mice, they were partly protected against the alterations in gut microbiota composition during NASH and presented better gut barrier integrity and less permeability under this pathological condition. These effects concurred with higher colonic mucosal inflammation, decreased serum bile acids, and protection against the decrease in circulating GLP-1 levels during NASH compared with their WT counterparts together with increased expression of GLP-2-sensitive genes in the gut. At the molecular level, stimulation of enteroendocrine STC-1 cells with a pro-inflammatory conditioned medium (CM) from lipopolysaccharide (LPS)-stimulated macrophages triggered pro-inflammatory signaling cascades that were further exacerbated by a PTP1B inhibitor. Likewise, the pro-inflammatory CM induced GLP-1 secretion in primary colonic cultures, an effect augmented by PTP1B inhibition.

CONCLUSION

Altogether our results have unraveled a potential role of PTP1B in the gut-liver axis during NASH, likely mediated by increased sensitivity to GLPs, with potential therapeutic value.

Protein tyrosine phosphatase 1b deficiency protects against hepatic fibrosis by modulating nadph oxidases

Inflammation is typically associated with the development of fibrosis, cirrhosis and hepatocellular carcinoma. The key role of protein tyrosine phosphatase 1B (PTP1B) in inflammatory responses has focused this study in understanding its implication in liver fibrosis. Here we show that hepatic PTP1B mRNA expression increased after bile duct ligation (BDL), while BDL-induced liver fibrosis was markedly reduced in mice lacking Ptpn1 (PTP1B^−/−) as assessed by decreased collagen deposition and α-smooth muscle actin (α-SMA) expression. PTP1B^−/− mice also showed a significant increase in mRNA levels of key markers of monocytes recruitment (Cd68, Adgre1 and Ccl2) compared to their wild-type (PTP1B^+/+) littermates at early stages of injury after BDL. Interestingly, the lack of PTP1B strongly increased the NADPH oxidase (NOX) subunits Nox1/Nox4 ratio and downregulated Cybb expression after BDL, revealing a pro-survival pattern of NADPH oxidase induction in response to liver injury. Chimeric mice generated by transplantation of PTP1B^−/− bone marrow (BM) into irradiated PTP1B^+/+ mice revealed similar hepatic expression profile of NOX subunits than PTP1B^−/− mice while these animals did not show differences in infiltration of myeloid cells at 7 days post-BDL, suggesting that PTP1B deletion in other liver cells is necessary for boosting the early inflammatory response to the BDL. PTP1B^−/− BM transplantation into PTP1B^+/+ mice also led to a blockade of TGF-β and α-SMA induction after BDL. In vitro experiments demonstrated that deficiency of PTP1B in hepatocytes protects against bile acid-induced apoptosis and abrogates hepatic stellate cells (HSC) activation, an effect ameliorated by NOX1 inhibition. In conclusion, our results have revealed that the lack of PTP1B switches NOX expression pattern in response to liver injury after BDL and reduces HSC activation and liver fibrosis.

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PTP1B deficiency in mice ameliorates liver damage induced by cholestasis.

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The increased NOX1/NOX4 ratio in livers from PTP1B^-/- mice was associated with protection against BDL-induced fibrosis.

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The lack of PTP1B exacerbates macrophage recruitment upon BDL which is dispensable for ameliorating cholestatic liver damage.

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Resistance of PTP1B^-/- hepatocytes against bile acid-induced apoptosis protects from HSC activation in a NOX1-dependent manner.

Regulation of platelet-activating factor-induced interleukin-8 expression by protein tyrosine phosphatase 1B

Background

Platelet-activating factor (PAF) is a potent lipid mediator whose involvement in the onset and progression of atherosclerosis is mediated by, among others, the modulation of cytokine expression patterns. The presence of multiple potential protein-tyrosine phosphatase (PTP) 1B substrates in PAF receptor signaling pathways brought us to investigate its involvement in PAF-induced cytokine expression in monocyte-derived dendritic cells (Mo-DCs) and to study the pathways involved in this modulation.

Methods

We used in-vitro-matured human dendritic cells and the HEK-293 cell line in our studies. PTP1B inhibition was though siRNAs and a selective inhibitor. Cytokine expression was studied with RT-PCR, luciferase assays and ELISA. Phosphorylation status of kinases and transcription factors was studied with western blotting.

Results

Here, we report that PTP1B was involved in the modulation of cytokine expression in PAF-stimulated Mo-DCs. A study of the down-regulation of PAF-induced IL-8 expression, by PTP1B, showed modulation of PAF-induced transactivation of the IL-8 promoter which was dependent on the presence of the C/EBPß -binding site. Results also suggested that PTP1B decreased PAF-induced IL-8 production by a glycogen synthase kinase (GSK)-3-dependent pathway via activation of the Src family kinases (SFK). These kinases activated an unidentified pathway at early stimulation times and the PI3K/Akt signaling pathway in a later phase. This change in GSK-3 activity decreased the C/EBPß phosphorylation levels of the threonine 235, a residue whose phosphorylation is known to increase C/EBPß transactivation potential, and consequently modified IL-8 expression.

Conclusion

The negative regulation of GSK-3 activity by PTP1B and the consequent decrease in phosphorylation of the C/EBPß transactivation domain could be an important negative feedback loop by which cells control their cytokine production after PAF stimulation.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0334-6) contains supplementary material, which is available to authorized users.

Dual role of protein tyrosine phosphatase 1B in the progression and reversion of non-alcoholic steatohepatitis

Objectives

Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disease in Western countries. Protein tyrosine phosphatase 1B (PTP1B), a negative modulator of insulin and cytokine signaling, is a therapeutic target for type 2 diabetes and obesity. We investigated the impact of PTP1B deficiency during NAFLD, particularly in non-alcoholic steatohepatitis (NASH).

Methods

NASH features were evaluated in livers from wild-type (PTP1BWT) and PTP1B-deficient (PTP1BKO) mice fed methionine/choline-deficient diet (MCD) for 8 weeks. A recovery model was established by replacing MCD to chow diet (CHD) for 2–7 days. Non-parenchymal liver cells (NPCs) were analyzed by flow cytometry. Oval cells markers were measured in human and mouse livers with NASH, and in oval cells from PTP1BWT and PTP1BKO mice.

Results

PTP1BWT mice fed MCD for 8 weeks exhibited NASH, NPCs infiltration, and elevated Fgf21, Il6 and Il1b mRNAs. These parameters decreased after switching to CHD. PTP1B deficiency accelerated MCD-induced NASH. Conversely, after switching to CHD, PTP1BKO mice rapidly reverted NASH compared to PTP1BWT mice in parallel to the normalization of serum triglycerides (TG) levels. Among NPCs, a drop in cytotoxic natural killer T (NKT) subpopulation was detected in PTP1BKO livers during recovery, and in these conditions M2 macrophage markers were up-regulated. Oval cells markers (EpCAM and cytokeratin 19) significantly increased during NASH only in PTP1B-deficient livers. HGF-mediated signaling and proliferative capacity were enhanced in PTP1BKO oval cells. In NASH patients, oval cells markers were also elevated.

Conclusions

PTP1B elicits a dual role in NASH progression and reversion. Additionally, our results support a new role for PTP1B in oval cell proliferation during NAFLD.

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PTP1B deficiency accelerates MCD-induced NASH.

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The liver inflammatory responses during NASH are enhanced in PTP1B-deficient mice.

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PTP1B deficiency accelerates the reversion of NASH in a recovery dietary model.

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In a DCC model PTP1BKO livers increased oval cells markers and proliferative capacity.

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PTP1B deficiency enhances HGF-mediated signaling and proliferation of oval cells.

Biological roles of hepatocyte growth factor-Met signaling from genetically modified animals

Hepatocyte growth factor (HGF) is produced by stromal and mesenchymal cells, and it stimulates epithelial cell proliferation, motility, morphogenesis and angiogenesis in various organs via tyrosine phosphorylation of its cognate receptor, Met. The HGF-Met signaling pathway contributes in a paracrine manner to the development of epithelial organs, exerts regenerative effects on the epithelium, and promotes the regression of fibrosis in numerous organs. Additionally, the HGF-Met signaling pathway is correlated with the biology of cancer types, neurons and immunity. In vivo analyses using genetic modification have markedly increased the profound understanding of the HGF-Met system in basic biology and its clinical applications. HGF and Met knockout (KO) mice are embryonically lethal. Therefore, amino acids in multifunctional docking sites of Met have been exchanged with specific binding motifs for downstream adaptor molecules in order to investigate the signaling potential of the HGF-Met signaling pathway. Conditional Met KO mice were generated using Cre-loxP methodology and characterization of these mice indicated that the HGF-Met signaling pathway is essential in regeneration, protection, and homeostasis in various tissue types and cells. Furthermore, the results of studies using HGF-overexpressing mice have indicated the therapeutic potential of HGF for various types of disease and injury. In the present review, the phenotypes of Met gene-modified mice are summarized.

Cell death-independent activities of the death receptors CD95, TRAILR1, and TRAILR2

Since their identification more than 20 years ago, the death receptors CD95, TRAILR1, and TRAILR2 have been intensively studied with respect to their cell death-inducing activities. These receptors, however, can also trigger a variety of cell death-independent cellular responses reaching from the activation of proinflammatory gene transcription programs over the stimulation of proliferation and differentiation to induction of cell migration. The cell death-inducing signaling mechanisms of CD95 and the TRAIL death receptors are well understood. In contrast, despite the increasing recognition of the biological and pathophysiological relevance of the cell death-independent activities of CD95, TRAILR1, and TRAILR2, the corresponding signaling mechanisms are less understood and give no fully coherent picture. This review is focused on the cell death-independent activities of CD95 and the TRAIL death receptors and addresses mainly three questions: (a) how are these receptors linked to noncell death pathways at the molecular level, (b) which factors determine the balance of cell death and cell death-independent activities of CD95 and the TRAIL death receptors at the cellular level, and (c) what are the consequences of the cell death-independent functions of these receptors for their role in cancer and inflammatory diseases.

Fucosterol activates the insulin signaling pathway in insulin resistant HepG2 cells via inhibiting PTP1B

Insulin resistance is a characteristic feature of type 2 diabetes mellitus (T2DM) and is characterized by defects in insulin signaling. This study investigated the modulatory effects of fucosterol on the insulin signaling pathway in insulin-resistant HepG2 cells by inhibiting protein tyrosine phosphatase 1B (PTP1B). In addition, molecular docking simulation studies were performed to predict binding energies, the specific binding site of fucosterol to PTP1B, and to identify interacting residues using Autodock 4.2 software. Glucose uptake was determined using a fluorescent D-glucose analogue and the glucose tracer 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino]-2-deoxyglucose, and the signaling pathway was detected by Western blot analysis. We found that fucosterol enhanced insulin-provoked glucose uptake and conjointly decreased PTP1B expression level in insulin-resistant HepG2 cells. Moreover, fucosterol significantly reduced insulin-stimulated serine (Ser307) phosphorylation of insulin receptor substrate 1 (IRS1) and increased phosphorylation of Akt, phosphatidylinositol-3-kinase, and extracellular signal- regulated kinase 1 at concentrations of 12.5, 25, and 50 µM in insulin-resistant HepG2 cells. Fucosterol inhibited caspase-3 activation and nuclear factor kappa B in insulin-resistant hepatocytes. These results suggest that fucosterol stimulates glucose uptake and improves insulin resistance by downregulating expression of PTP1B and activating the insulin signaling pathway. Thus, fucosterol has potential for development as an anti-diabetic agent.

Frequent amplification of PTP1B is associated with poor survival of gastric cancer patients

The protein tyrosine phosphatase 1B (PTP1B), a non-transmembrane protein tyrosine phosphatase, has been implicated in gastric pathogenesis. Several lines of recent evidences have shown that PTP1B is highly amplified in breast and prostate cancers. The aim of this study was to investigate PTP1B amplification in gastric cancer and its association with poor prognosis of gastric cancer patients, and further determine the role of PTP1B in gastric tumorigenesis. Our data demonstrated that PTP1B was significantly up-regulated in gastric cancer tissues as compared with matched normal gastric tissues by using quantitative RT-PCR (qRT-PCR) assay. In addition, copy number analysis showed that PTP1B was amplified in 68/131 (51.9%) gastric cancer cases, whereas no amplification was found in the control subjects. Notably, PTP1B amplification was positively associated with its protein expression, and was significantly related to poor survival of gastric cancer patients. Knocking down PTP1B expression in gastric cancer cells significantly inhibited cell proliferation, colony formation, migration and invasion, and induced cell cycle arrested and apoptosis. Mechanically, PTP1B promotes gastric cancer cell proliferation, survival and invasiveness through modulating Src-related signaling pathways, such as Src/Ras/MAPK and Src/phosphatidylinositol-3-kinase (PI3K)/Akt pathways. Collectively, our data demonstrated frequent overexpression and amplification PTP1B in gastric cancer, and further determined the oncogenic role of PTP1B in gastric carcinogenesis. Importantly, PTP1B amplification predicts poor survival of gastric cancer patients.

Identifying Novel Candidate Genes Related to Apoptosis from a Protein-Protein Interaction Network

Apoptosis is the process of programmed cell death (PCD) that occurs in multicellular organisms. This process of normal cell death is required to maintain the balance of homeostasis. In addition, some diseases, such as obesity, cancer, and neurodegenerative diseases, can be cured through apoptosis, which produces few side effects. An effective comprehension of the mechanisms underlying apoptosis will be helpful to prevent and treat some diseases. The identification of genes related to apoptosis is essential to uncover its underlying mechanisms. In this study, a computational method was proposed to identify novel candidate genes related to apoptosis. First, protein-protein interaction information was used to construct a weighted graph. Second, a shortest path algorithm was applied to the graph to search for new candidate genes. Finally, the obtained genes were filtered by a permutation test. As a result, 26 genes were obtained, and we discuss their likelihood of being novel apoptosis-related genes by collecting evidence from published literature.

Protein tyrosine phosphatase 1B (PTP1B): A key regulator and therapeutic target in liver diseases

Phosphorylation of tyrosine residues within proteins, which is controlled by the reciprocal action of protein tyrosine kinases and protein tyrosine phosphatases, plays a key role in regulating almost all physiological responses. Therefore, it comes as no surprise that once the balance of tyrosine phosphorylation is disturbed, drastic effects can occur. Protein tyrosine phosphatase 1B (PTP1B), a classical non-transmembrane tyrosine phosphatase, is a pivotal regulator and promising drug target in type 2 diabetes and obesity. Recently it has received renewed attention in liver diseases and represents an intriguing opportunity as a drug target by modulating hepatocyte death and survival, hepatic lipogenesis and so on. Here, the multiple roles of PTP1B in liver diseases will be presented, with respect to liver regeneration, drug-induced liver disease, non-alcoholic fatty liver disease (NAFLD) and hepatocellular carcinoma.

Marine Bromophenol Derivative 3,4-Dibromo-5-(2-bromo-3,4-dihydroxy-6-isopropoxymethyl benzyl)benzene-1,2-diol Protects Hepatocytes from Lipid-Induced Cell Damage and Insulin Resistance via PTP1B Inhibition

3,4-Dibromo-5-(2-bromo-3,4-dihydroxy-6-isopropoxymethyl benzyl)benzene-1,2-diol (HPN) is a bromophenol derivative from the marine red alga Rhodomela confervoides. We have previously found that HPN exerted an anti-hyperglycemic property in db/db mouse model. In the present study, we found that HPN could protect HepG2 cells against palmitate (PA)-induced cell death. Data also showed that HPN inhibited cell death mainly by blocking the cell apoptosis. Further studies demonstrated that HPN (especially at 1.0 μM) significantly restored insulin-stimulated tyrosine phosphorylation of IR and IRS1/2, and inhibited the PTP1B expression level in HepG2 cells. Furthermore, the expression of Akt was activated by HPN, and glucose uptake was significantly increased in PA-treated HepG2 cells. Our results suggest that HPN could protect hepatocytes from lipid-induced cell damage and insulin resistance via PTP1B inhibition. Thus, HPN can be considered to have potential for the development of anti-diabetic agent that could protect both hepatic cell mass and function.

Biology of MET: a double life between normal tissue repair and tumor progression

MNNG HOS transforming gene (MET) is a class IV receptor tyrosine kinase, expressed on the surface of epithelial cells. The interaction with the hepatocyte grow factor (HGF) induces MET dimerization and the activation of multiple intracellular pathways leading to cell proliferation, anti-apoptosis, morphogenic differentiation, motility, invasion, and angiogenesis. Knock out mice have demonstrated that MET is necessary for normal embryogenesis including the formation of striate muscles, liver and trophoblastic structures. The overexpression of MET and HGF are common in solid tumors and contribute to determine their growth. Indeed, MET has been cloned as a transforming gene from a chemically induced human osteosarcoma cell line and therefore is considered a proto-oncogene. Germline MET mutations are characteristic of hereditary papillary kidney cancers and MET amplification is observed in tumors including lung and gastric adenocarcinomas. The inhibition of MET signaling is the target for specific drugs that are raising exciting expectation for medical treatment of cancer.

Highly efficient, in-vivo Fas-mediated apoptosis of B-cell lymphoma by hexameric CTLA4-FasL

Non-Hodgkin lymphomas (NHLs) account for 4% of all malignancies. 5-year survival rate increased to 50% with new treatment modalities, however there is need for new effective treatment for the more aggressive, relapsing forms. Recently, CTLA4-FasL, that can bind to B7 and Fas receptor (Fas), was shown to induce robust apoptosis of cell lines originating from B cell lymphomas expressing both B7 and Fas, by activating pro-apoptotic signals in parallel to abrogating anti-apoptotic ones. The present study focuses on the unique properties of CTLA4-FasL as a potent apoptosis inducer of malignant cells in-vitro and in a xenograft model. CTLA4-FasL was found to naturally form a stable homo-hexamer. CTLA4-FasL induces robust apoptosis of a large variety of malignant cells while relatively sparing non-malignant ones, being more efficient when both receptors (B7 and Fas) are expressed on target cells. Even in non-B7 expressing cells, CTLA4-FasL exhibited better apoptotic activity than its parts, alone or in combination, however, only in B7 expressing cells apoptosis occurs at low concentrations and CTLA4-FasL induces activation of apoptotic signals and reduces anti-apoptotic ones. Importantly, CTLA4-FasL efficiently inhibited the growth of human B cell lineage tumors in a xenograft model, by provoking tumor cells’ apoptosis. Thus, CTLA4-FasL, a natural homo-hexamer protein, induces robust apoptosis of malignant cells, in-vitro and in-vivo. In B-cell lymphoma, its potency stems from the combination of its synergistic effect of activating the caspases while abrogating the anti-apoptotic signaling, with its unique hexameric structure, making CTLA4-FasL a promising candidate for aggressive B cell lymphomas treatment.

Fas-induced apoptosis increases hepatocyte tissue factor procoagulant activity in vitro and in vivo

Hepatocyte (HPC) apoptosis occurs in association with hepatotoxic responses and chronic liver disease, and is coupled to activation of the blood coagulation cascade. HPCs have been shown to express tissue factor (TF), the primary activator of blood coagulation, in a form that lacks procoagulant activity. In this study, we determined the effect of inducing HPC apoptosis on the procoagulant activity of TF. Treatment of primary mouse HPCs with the Fas death receptor agonist (anti-CD95 antibody, Jo2) triggered apoptosis as shown by cleavage of caspase-3, increased caspase-3 proteolytic activity, and cell surface exposure of phosphatidylserine (PS). Jo2-induced apoptosis significantly increased TF-dependent factor Xa generation by HPCs. Moreover, Jo2 treatment was associated with increased levels of microparticle-associated TF procoagulant activity in the culture medium. Pretreatment with a caspase-3 inhibitor significantly reduced Jo2-induced HPC TF activity and prevented the increase in microparticle-associated TF procoagulant activity. Application of the high-affinity PS-binding protein lactadherin inhibited TF-dependent factor Xa generation by Jo2-treated HPCs and dramatically reduced microparticle-associated TF procoagulant activity. Treatment of wild-type mice with a sublethal dose of Jo2 was associated with a robust increase in the activation of coagulation as measured by plasma thrombin-antithrombin (TAT) levels; whereas mice with liver-specific TF deficiency had significantly lower TAT levels. Overall, the results indicate that Fas-initiated, caspase-3-dependent HPC apoptosis increases TF procoagulant activity through a mechanism involving PS externalization. This suggests that activation of liver TF likely contributes to the procoagulant state associated with HPC apoptosis in liver toxicity and disease.

Signalling pathways involved in paracetamol-induced hepatotoxicity: new insights on the role of protein tyrosine phosphatase 1B

Acute hepatic failure secondary to paracetamol poisoning is associated with high mortality. Paracetamol-induced hepatotoxicity causes oxidative stress that triggers signalling pathways and ultimately leads to lethal hepatocyte injury. We will review the signalling pathways activated by paracetamol in the liver emphasizing the role of protein tyrosine phosphatase 1B (PTP1B) in the balance between cell death and survival in hepatocytes. PTP1B has emerged as a key modulator of the antioxidant system mediated by the nuclear factor erythroid-2-related factor 2 (Nrf2) in hepatic cells in response to paracetamol overdose. Also, this phosphatase modulates the classical survival pathways triggered by the activation of the insulin-like growth factor-I (IGF-I) signalling cascade. Therefore, PTP1B is a novel therapeutic target against paracetamol-induced liver failure.

Hypoxia-induced miR-210 in epithelial ovarian cancer enhances cancer cell viability via promoting proliferation and inhibiting apoptosis

miR-210 is upregulated in a HIF-1α-dependent way in several types of cancers. In addition, upregulated miR-210 promotes cancer proliferation, via its anti-apoptotic effects. It is blind to the regulation of miR-210 under hypoxia conditions for ovarian cancer cells and to the effect of miR-210 on ovarian cancer growth. In the present study, we determined the expression of miR-210 in epithelial ovarian cancer specimens, and in ovarian cancer cell lines under hypoxia conditions, and determined in detail the effect of miR-210 overexpression on tumor cell proliferation, and the possible mechanisms of tumor growth by miR-210 regulation. It was shown that miR-210 expression is upregulated, in response to hypoxia conditions in epithelial ovarian cancer specimens as well as epithelial ovarian cancer cell lines, with an association to HIF-1α overexpression. Furthermore, upregulated miR-210 promoted tumor growth in vitro via targeting PTPN1 and inhibiting apoptosis. Therefore, our findings shed light on the mechanism of ovarian cancer adaptation to hypoxia.

Protein tyrosine phosphatase 1B inhibition ameliorates palmitate-induced mitochondrial dysfunction and apoptosis in skeletal muscle cells

Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of the insulin signaling pathway and is considered a promising therapeutic target in the treatment of diabetes. However, the role of PTP1B in palmitate-induced mitochondrial dysfunction and apoptosis in skeletal muscle cells has not been studied. Here we investigate the effects of PTP1B modulation on mitochondrial function and apoptosis and elucidate the underlying mechanisms in skeletal muscle cells. PTP1B inhibition significantly reduced palmitate-induced mitochondrial dysfunction and apoptosis in C2C12 cells, as these cells had increased expression levels of PGC-1α, Tfam, and NRF-1; enhanced ATP level and cellular viability; decreased TUNEL-positive cells; and decreased caspase-3 and -9 activity. Alternatively, overexpression of PTP1B resulted in mitochondrial dysfunction and apoptosis in these cells. PTP1B silencing improved mitochondrial dysfunction by an increase in the expression of SIRT1 and a reduction in the phosphorylation of p65 NF-κB. The protection from palmitate-induced apoptosis by PTP1B inhibition was also accompanied by a decrease in protein level of serine palmitoyl transferase, thus resulting in lower ceramide content in muscle cells. Exogenous addition of C2-ceramide to PTP1B-knockdown cells led to a reduced generation of reactive oxygen species (ROS), whereas PTP1B overexpression demonstrated an elevated ROS production in myotubes. In addition, PTP1B inhibition was accompanied by decreased JNK phosphorylation and increased insulin-stimulated Akt (Ser473) phosphorylation, whereas overexpression of PTP1B had the opposite effect. The overexpression of PTP1B also induced the nuclear localization of FOXO-1, but in contrast, suppression of PTP1B reduced palmitate-induced nuclear localization of FOXO-1. In summary, our results indicate that PTP1B modulation results in (1) alterations in mitochondrial function by changes in the activity of SIRT1/NF-κB/PGC-1α pathways and (2) changes in apoptosis that result from either a direct effect of PTP1B on the insulin signaling pathway or an indirect influence on ceramide content, ROS generation, JNK activation, and FOXO-1 nuclear translocation.

Overexpression of PTP1B in human colorectal cancer and its association with tumor progression and prognosis

Protein tyrosine phosphatase 1B (PTP1B) is a non-transmembrane protein tyrosine phosphatase that has been implicated in cancer pathogenesis. However, the expression level and the role of PTP1B in the development and prognosis of colorectal cancer (CRC) remain unclear. In this study, the expression of PTP1B in CRC tissues and matched noncancerous tissues were detected by using immunohistochemistry, real-time PCR and Western blotting. The correlations between PTP1B expression level and clinicopathologic characteristics and patient survival were analyzed. We found that PTP1B expression was significantly higher in CRC tissues compared with matched non-tumour tissues. Statistical analysis showed that the PTP1B expression was correlated with tumor differentiation, tumor invasion, lymph node metastasis, and TNM stage. Patients with higher expressions of PTP1B had the lower survival (P = 0.012). Taken together, our results suggest that PTP1B expression might play a critical role in the progression of CRC and may serve as a valuable prognostic biomarker for CRC.

Protein tyrosine phosphatase 1B modulates GSK3β/Nrf2 and IGFIR signaling pathways in acetaminophen-induced hepatotoxicity

Acute hepatic failure secondary to acetaminophen (APAP) poisoning is associated with high mortality. Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of tyrosine kinase growth factor signaling. In the liver, this pathway confers protection against injury. However, the involvement of PTP1B in the intracellular networks activated by APAP is unknown. We have assessed PTP1B expression in APAP-induced liver failure in humans and its role in the molecular mechanisms that regulate the balance between cell death and survival in human and mouse hepatocytes, as well as in a mouse model of APAP-induced hepatotoxicity. PTP1B expression was increased in human liver tissue removed during liver transplant from patients for APAP overdose. PTP1B was upregulated by APAP in primary human and mouse hepatocytes together with the activation of c-jun (NH2) terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38 MAPK), resulting in cell death. Conversely, Akt phosphorylation and the antiapoptotic Bcl2 family members BclxL and Mcl1 were decreased. PTP1B deficiency in mouse protects hepatocytes against APAP-induced cell death, preventing glutathione depletion, reactive oxygen species (ROS) generation and activation of JNK and p38 MAPK. APAP-treated PTP1B^−/− hepatocytes showed enhanced antioxidant defense through the glycogen synthase kinase 3 (GSK3)β/Src kinase family (SKF) axis, delaying tyrosine phosphorylation of the transcription factor nuclear factor-erythroid 2-related factor (Nrf2) and its nuclear exclusion, ubiquitination and degradation. Insulin-like growth factor-I receptor-mediated signaling decreased in APAP-treated wild-type hepatocytes, but was maintained in PTP1B^−/− cells or in wild-type hepatocytes with reduced PTP1B levels by RNA interference. Likewise, both signaling cascades were modulated in mice, resulting in less severe APAP hepatotoxicity in PTP1B^−/− mice. Our results demonstrated that PTP1B is a central player of the mechanisms triggered by APAP in hepatotoxicity, suggesting a novel therapeutic target against APAP-induced liver failure.

PTP1B expression is an independent positive prognostic factor in human breast cancer

Protein tyrosine phosphatase 1B (PTP1B) is a non-transmembrane protein tyrosine phosphatase that has come into focus as a critical regulator of multiple signaling pathways. The role of PTP1B in breast cancer remains unclear with evidence suggesting that PTP1B can exert both tumor-suppressing and tumor-promoting effects. To better define the role of PTP1B in human breast cancer, and its relationship with HER2, we performed immunohistochemical studies on a large cohort of functionally annotated primary breast cancer specimens. 683 of 1,402 (49 %) evaluable primary breast cancers are positive for PTP1B. There is no statistically significant association between PTP1B expression and age, tumor size, T stage, histologic grade, lymph node status, or histological subtype. Of note, there is no significant association between PTP1B expression and HER2 expression (PTP1B expression 53.1 % in HER2(+) cancers vs. 47.5 % in HER2(-) cancers, p = 0.0985). However, PTP1B expression is significantly associated with estrogen receptor expression (PTP1B expression 50.7 % in ER(+) cancers vs. 43.1 % in ER(-) cancers, p = 0.0137) and intrinsic molecular subtype (PTP1B expression 53.9 % in the luminal B HER2(+) subtype and 37.9 % in the basal-like subtype). Of note, multivariate analyses demonstrate that PTP1B is an independent predictor of improved survival in breast cancer (HR 0.779, p = 0.006). Taken together, we demonstrate in the largest study to date that (1) PTP1B is commonly expressed in breast cancer, (2) there is no association or functional impact of PTP1B expression in HER2(+) breast cancer, and (3) PTP1B expression in breast cancer is associated with significantly improved clinical outcome. Until additional studies are performed, caution should be exercised in using PTP1B inhibitors in human breast cancer.

Protein-tyrosine phosphatase 1B (PTP1B) deficiency confers resistance to transforming growth factor-β (TGF-β)-induced suppressor effects in hepatocytes

Transforming growth factor-β (TGF-β) plays a dual role in hepatocytes, mediating both tumor suppressor and promoter effects. The suppressor effects of the cytokine can be negatively regulated by activation of survival signals, mostly dependent on tyrosine kinase activity. The aim of our work was to study the role of the protein-tyrosine phosphatase 1B (PTP1B) on the cellular responses to TGF-β, using for this purpose immortalized neonatal hepatocytes isolated from both PTP1B(+/+) and PTP1B(-/-) mice. We have found that PTP1B deficiency conferred resistance to TGF-β suppressor effects, such as apoptosis and growth inhibition, correlating with lower Smad2/Smad3 activation. Both responses were recovered in the presence of the general tyrosine kinase inhibitor genistein. PTP1B(-/-) cells showed elevated NF-κB activation in response to TGF-β. Knockdown of the NF-κB p65 subunit increased cell response in terms of Smads phosphorylation and apoptosis. Interestingly, these effects were accompanied by inhibition of Smad7 up-regulation. In addition, lack of PTP1B promoted an altered NADPH oxidase (NOX) expression pattern in response to TGF-β, strongly increasing the NOX1/NOX4 ratio, which was reverted by genistein and p65 knockdown. Importantly, NOX1 knockdown inhibited nuclear translocation of p65, promoted Smad phosphorylation, and decreased Smad7 levels. In summary, our results suggest that PTP1B deficiency confers resistance to TGF-β through Smad inhibition, an effect that is mediated by NOX1-dependent NF-κB activation, which in turn, increases the level of the Smad inhibitor Smad7 and participates in a positive feedback loop on NOX1 up-regulation.

Protein-tyrosine phosphatase 1B modulates early endosome fusion and trafficking of Met and epidermal growth factor receptors

The endoplasmic reticulum-localized non-receptor protein-tyrosine phosphatase 1B (PTP1B) is associated with oncogenic, metabolic, and cytokine-related signaling and functionally targets multiple receptor tyrosine kinases (RTKs) for dephosphorylation. Loss of PTP1B activity leads to enhanced ligand-dependent biological activity of the Met RTK among others. Here, we demonstrate that knockdown of PTP1B or expression of a PTP1B trapping aspartic acid-to-alanine substitution (D/A) mutant delayed ligand-induced degradation of the Met and EGF RTKs. Loss of PTP1B function abrogated trafficking of Met and EGF receptor to Rab5- and phosphatidylinositol 3-phosphate (Pl3P)-positive early endosomes and subsequent trafficking through the degradative pathway. Under these conditions, internalization of the Met and EGF receptors was unaltered, suggesting a block at the level of early endosome formation. We show that the N-ethylmaleimide-sensitive factor (NSF), an essential component of the vesicle fusion machinery, was hyperphosphorylated in PTP1B knockdown or PTP1B D/A-expressing cells and was a target for PTP1B. NSF knockdown phenocopied PTP1B knockdown, demonstrating a mechanism through which PTP1B regulates endocytic trafficking. Finally, we show that PTP1B dephosphorylated NSF and that this interaction was required for physiological RTK trafficking and appropriate attenuation of downstream signaling.

IRS2 and PTP1B: Two opposite modulators of hepatic insulin signalling

Type 2 Diabetes mellitus (T2D) is the most common endocrine disorder associated to metabolic syndrome (MS) and occurs when insulin secretion can no compensate peripheral insulin resistance. Among peripheral tissues, the liver controls glucose homeostasis due to its ability to consume and produce glucose. The molecular mechanism underlying hepatic insulin resistance is not completely understood; however, it involves the impairment of the insulin signalling network. Among the critical nodes of hepatic insulin signalling, insulin receptor substrate 2 (IRS2) and protein tyrosine phosphatase 1B (PTP1B) modulate the phosphatidylinositol (PI) 3-kinase/Akt/Foxo1 pathway that controls the suppression of gluconeogenic genes. In this review, we will focus on recent findings regarding the molecular mechanism by which IRS2 and PTP1B elicit opposite effects on carbohydrate metabolism in the liver in response to insulin. Finally, we will discuss the involvement of the critical nodes of insulin signalling in non-alcoholic fatty liver disease (NAFLD) in humans.

Checks and balances: interplay of RTKs and PTPs in cancer progression

In recent years, targeted therapies for receptor tyrosine kinases (RTKs) have shown initial promise in the clinical setting for the treatment of several tumors driven by these oncogenic signaling pathways. Unfortunately, clinical relapse due to acquired resistance to these molecular therapeutics is common. An improved understanding of how tumors bypass the inhibitory effects of RTK-targeted therapies has revealed a rich myriad of possible mechanisms for acquired resistance. Protein tyrosine phosphatases (PTPs) can function as oncogenes or tumor suppressors to either enhance or suppress RTK signaling. Recent studies suggest that the loss or gain of function of PTP's can significantly impinge on RTK signaling during tumor progression. Here we review the interplay between RTKs and PTPs as an emerging mechanism for acquired resistance to RTK-targeted therapies, that may aid in the design of improved therapies to prevent and overcome resistance in treatments for cancer patients.

Protein Tyrosine Phosphatase 1B (PTP1B) deficiency accelerates hepatic regeneration in mice

Protein tyrosine phosphatase 1B (PTP1B) is a key regulator of metabolism and cell growth by its ability to dephosphorylate tyrosine kinase receptors and modulate the intensity of their signaling cascades. Because liver regeneration involves tyrosine phosphorylation-mediated signaling, we investigated the role of PTP1B in this process by performing partial hepatectomy in wild-type (PTP1B(+/+)) and PTP1B-deficient (PTP1B(-/-)) mice. The expression of PCNA and cyclins D1 and E (cell proliferation markers) was enhanced in PTP1B(-/-) regenerating livers, in parallel with 5'-bromo-2'-deoxyuridine incorporation. Phosphorylation of JNK1/2 and STAT3, early triggers of hepatic regeneration in response to TNF-α and IL-6, was accelerated in PTP1B(-/-) mice compared with PTP1B(+/+) mice. These phosphorylations were increased in PTP1B(-/-) hepatocytes or by silencing PTP1B in wild-type cells and decreased further after the addition of recombinant PTP1B. Enhanced EGF- and HGF receptor-mediated signaling was observed in regenerating livers lacking PTP1B and in EGF- or HGF-stimulated PTP1B(-/-) hepatocytes. Moreover, PTP1B(-/-) mice displayed a more rapid increase in intrahepatic lipid accumulation than PTP1B(+/+) control mice. Late responses to partial hepatectomy revealed additional divergences because stress-mediated signaling was attenuated at 24 to 96 hours in PTP1B(-/-) mice compared with PTP1B(+/+) mice. Finally, PTP1B deficiency also improves hepatic regeneration in mice fed a high-fat diet. These results suggest that pharmacological inhibition of PTP1B would improve liver regeneration in patients with acute or chronic liver injury.

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.

C/EBPα down-regulation is associated with reduced hepatic cellular viability during hypoxia in vitro and in vivo

C/EBPα transcription factor is a key regulator in liver biology and was preliminarily shown to be down-regulated in hypoxic primary rat hepatocytes. The aim of this study was to explore the possible association between C/EBPα expression level and hepatocyte viability in both the in-vitro cultured hypoxic rat primary hepatocytes and two models of acute liver hypoxia induced by carbon tetrachloride or Fas antibody. C/EBPα mRNA was significantly down-regulated under hypoxic conditions both in vitro and in vivo, which was paralleled by a similar decrease in hepatocyte viability and partially reversed by 3D matrix and dexamethasone. These results suggested that C/EBPα down-regulation may be one mechanism of reduced hepatocyte viability in these settings.

Inhibition of PTP1B restores IRS1-mediated hepatic insulin signaling in IRS2-deficient mice

OBJECTIVE

Mice with complete deletion of insulin receptor substrate 2 (IRS2) develop hyperglycemia, impaired hepatic insulin signaling, and elevated gluconeogenesis, whereas mice deficient for protein tyrosine phosphatase (PTP)1B display an opposing hepatic phenotype characterized by increased sensitivity to insulin. To define the relationship between these two signaling pathways in the regulation of liver metabolism, we used genetic and pharmacological approaches to study the effects of inhibiting PTP1B on hepatic insulin signaling and expression of gluconeogenic enzymes in IRS2(-/-) mice.

RESEARCH DESIGN AND METHODS

We analyzed glucose homeostasis and insulin signaling in liver and isolated hepatocytes from IRS2(-/-) and IRS2(-/-)/PTP1B(-/-) mice. Additionally, hepatic insulin signaling was assessed in control and IRS2(-/-) mice treated with resveratrol, an antioxidant present in red wine.

RESULTS

In livers of hyperglycemic IRS2(-/-) mice, the expression levels of PTP1B and its association with the insulin receptor (IR) were increased. The absence of PTP1B in the double-mutant mice restored hepatic IRS1-mediated phosphatidylinositol (PI) 3-kinase/Akt/Foxo1 signaling. Moreover, resveratrol treatment of hyperglycemic IRS2(-/-) mice decreased hepatic PTP1B mRNA and inhibited PTP1B activity, thereby restoring IRS1-mediated PI 3-kinase/Akt/Foxo1 signaling and peripheral insulin sensitivity.

CONCLUSIONS

By regulating the phosphorylation state of IR, PTB1B determines sensitivity to insulin in liver and exerts a unique role in the interplay between IRS1 and IRS2 in the modulation of hepatic insulin action.

Beneficial effects of PTP1B deficiency on brown adipocyte differentiation and protection against apoptosis induced by pro- and anti-inflammatory stimuli

Insulin is an inducer of brown fat adipogenesis through the activation of a signalling network that involves positive/negative modulators. Given the importance of brown adipose tissue (BAT) for basal thermogenic energy expenditure, we investigated the role of PTP1B in the acquisition of terminal differentiated phenotype and in the apoptotic responses of brown adipocytes. Immortalized brown preadipocytes lacking (PTP1B(-/-)) or expressing (PTP1B(+/+)) PTP1B have been generated. PTP1B deficiency accelerated a full program of brown adipogenesis including induction of transcription factors, coactivators, adipogenic markers and signalling molecules. Fully differentiated PTP1B(-/-) brown adipocytes were resistant to tumor necrosis factor (TNFalpha)-induced apoptosis as these cells were protected against caspase-8 activation, FLIP degradation, Bid cleavage and caspase-3 activation compared to wild-type controls. These events were recovered by PTP1B rescue. Survival signalling including phosphorylation of IRS-1 and Akt/PKB and BclxL expression were decreased in TNFalpha-treated PTP1B(-/-) cells but not in the wild-type. Similarly, PTP1B(-/-) brown adipocytes were protected against resveratrol-induced apoptosis. Phosphorylation of Akt/PKB and Foxo1 phosphorylation/acetylation decreased exclusively in resveratrol-treated wild-type cells, leading to nuclear localization of Foxo1 and up-regulation of Bim. Thus, PTP1B inhibition could be of benefit against obesity by counteracting TNFalpha-induced brown fat atrophy, and combined with resveratrol might improve low-grade inflammation.

The two faces of PTP1B in cancer

PTP1B is a classical non-transmembrane protein tyrosine phosphatase that plays a key role in metabolic signaling and is a promising drug target for type 2 diabetes and obesity. Accumulating evidence also indicates that PTP1B is involved in cancer, but contrasting findings suggest that it can exert both tumor suppressing and tumor promoting effects depending on the substrate involved and the cellular context. In this review, we will discuss the diverse mechanisms by which PTP1B may influence tumorigenesis as well as recent in vivo data on the impact of PTP1B deficiency in murine cancer models. Together, these results highlight not only the great potential of PTP1B inhibitors in cancer therapy but also the need for a better understanding of PTP1B function prior to use of these compounds in human patients.

Crosstalk in Met receptor oncogenesis

The Met receptor tyrosine kinase (RTK) regulates several distinct biological processes, including cell scatter, cell invasion, cell survival and epithelial remodeling. MET is genetically altered through several mechanisms in multiple human cancers; these events are causally related to cancer initiation and progression, identifying Met as a potential therapeutic target. Recent evidence highlights additional roles for Met in cancer through crosstalk with other receptors and cell surface proteins. In this review, we discuss recent progress in our understanding of mechanisms of interaction between Met, the epidermal growth factor receptor family and other cell surface protein families, and how these contribute to signal crosstalk, oncogenesis and drug resistance.

Targeting MET as a strategy to overcome crosstalk-related resistance to EGFR inhibitors

The hepatocyte growth factor (HGF)-mesenchymal-epithelial transition factor (MET) pathway has a key role in carcinogenesis; it is implicated in proliferation, inhibition of apoptosis, angiogenesis, migration, invasiveness, and metastasis. All of these molecular events are driven through membrane and intracellular coplayers and several downstream effector proteins. MET has been shown to cross react with epithelial growth factor receptor (EGFR) proteins and possibly substitutes their activity, thus conferring resistance to EGFR-targeting drugs. Therefore, identification of MET inhibitors might lead to new treatments for MET-triggered neoplasia and improve the sensitivity of molecularly targeted antineoplastic compounds that are currently in use. In this Review, we outline current data regarding the HGF-MET pathway during carcinogenesis and the strategies for therapeutic targeting of this pathway. We also discuss the rationale and future perspectives of the combinatorial blockade of HGF-MET and EGFR signalling cascades in cancer treatment.

Cell and molecular biology of the novel protein tyrosine-phosphatase-interacting protein 51

This chapter examines the current state of knowledge about the expression profile, as well as biochemical properties and biological functions of the evolutionarily conserved protein PTPIP51. PTPIP51 is apparently expressed in splice variants and shows a particularly high expression in epithelia, skeletal muscle, placenta, and germ cells, as well as during mammalian development and in cancer. PTPIP51 is an in vitro substrate of Src- and protein kinase A, the PTP1B/TCPTP protein tyrosine phosphatases and interacts with 14-3-3 proteins, the Nuf2 kinetochore protein, the ninein-interacting CGI-99 protein, diacylglycerol kinase alpha, and also with itself forming dimers and trimers. Although the precise cellular function remains to be elucidated, the current data implicate PTPIP51 in signaling cascades mediating proliferation, differentiation, apoptosis, and motility.

Regulation of the Met receptor-tyrosine kinase by the protein-tyrosine phosphatase 1B and T-cell phosphatase

The non-receptor protein-tyrosine phosphatases (PTPs) 1B and T-cell phosphatase (TCPTP) have been implicated as negative regulators of multiple signaling pathways including receptor-tyrosine kinases. We have identified PTP1B and TCPTP as negative regulators of the hepatocyte growth factor receptor, the Met receptor-tyrosine kinase. In vivo, loss of PTP1B or TCPTP enhances hepatocyte growth factor-mediated phosphorylation of Met. Using substrate trapping mutants of PTP1B or TCPTP, we have demonstrated that both phosphatases interact with Met and that these interactions require phosphorylation of twin tyrosines (Tyr-1234/1235) in the activation loop of the Met kinase domain. Using confocal microscopy, we show that trapping mutants of both PTP1B and the endoplasmic reticulum-targeted TCPTP isoform, TC48, colocalize with Met and that activation of Met enables the nuclear-localized isoform of TCPTP, TC45, to exit the nucleus. Using small interfering RNA against PTP1B and TCPTP, we demonstrate that phosphorylation of Tyr-1234/1235 in the activation loop of the Met receptor is elevated in the absence of either PTP1B or TCPTP and further elevated upon loss of both phosphatases. This enhanced phosphorylation of Met corresponds to enhanced biological activity and cellular invasion. Our data demonstrate that PTP1B and TCPTP play distinct and non-redundant roles in the regulation of the Met receptor-tyrosine kinase.

Association of protein tyrosine phosphatases (PTPs)-1B with c-Met receptor and modulation of corneal epithelial wound healing

PURPOSE

The purpose of this study was to investigate the expression and activity of protein tyrosine phosphatases (PTPs) in epithelium during corneal wound healing and to investigate how PTPs regulate activation of the c-Met receptor and the receptor's proximal signaling.

METHODS

Rabbit corneas were injured by gentle scraping of the surface, leaving the limbal epithelium intact, and epithelium was collected at 1, 2, 3, and 7 days after injury. In organ culture models, epithelium was removed and corneas were incubated with hepatocyte growth factor (HGF), with or without the PTP inhibitor bpV(phen), and the PI-3K inhibitors wortmannin and LY294002. Human corneal epithelial (HCE) cells were stimulated with HGF with or without bpV(phen). Total cell lysates and cytosolic and membrane fractions were analyzed by Western blot. PTP activities were measured with specific substrates. PTP1B and SHP-2 genes were knocked down by interference RNA (siRNA).

RESULTS

PTP activity and expression increased during wound healing. The most abundant were SHP-2, PTP1B, and PTEN. HGF activated the c-Met receptor in HCE cells up to 30 minutes and was downregulated by 2 hours. Inhibition of PTPs increased HGF-promoted wound healing, HGF-activated phosphorylation of c-Met, and its downstream signal PI-3K/Akt, but not ERK1/2 or p70S6K. PTP1B and SHP-2 were bound to the c-Met. Part of the c-Met was colocalized in the endoplasmic reticulum with PTP1B. PTP1B phosphorylation increased when the c-Met receptor was deactivated, and gene knockdown of PTP1B increased c-Met activation. SHP-2 phosphorylation and binding to c-Met was higher during receptor activation, and SHP-2 gene silencing decreased receptor phosphorylation.

CONCLUSIONS

Inhibition of PTP activity mimics the effect of HGF by activating the PI-3K/Akt signal involved in wound healing. PTP1B and SHP-2 are bound to the c-Met receptor to control its activity. Although the binding of PTP1B increases when there is a decrease in c-Met activation and acts as a negative regulator of the receptor, the increased binding and phosphorylation of SHP-2 coincide with maximal stimulation of c-Met, acting as a positive regulator.

Regulation of CD95/APO-1/Fas-induced apoptosis by protein phosphatases

Triggering the CD95/APO-1/Fas receptor by CD95-L induces the assembly of the death-inducing signaling complex (DISC), which permits initiator caspases activation and progression of a signaling cascade that culminates in cellular apoptosis. Despite the CD95 receptor does not exhibit any kinase activity by itself, phosphorylation/dephosphorylation events seem important to regulate many aspects of CD95-mediated apoptosis. Here, we try to highlight particularly the importance of protein phosphatases in the modulation of the CD95 system.

Hepatocyte-specific c-Met deletion disrupts redox homeostasis and sensitizes to Fas-mediated apoptosis

The hepatocyte growth factor and its receptor c-Met direct a pleiotropic signal transduction pathway that controls cell survival. We previously demonstrated that mice lacking c-Met (Met-KO) in hepatocytes were hypersensitive to Fas-induced liver injury. In this study, we used primary hepatocytes isolated from Met-KO and control (Cre-Ctrl) mice to address more directly the protective effects of c-Met signaling. Loss of c-Met function increased sensitivity to Fas-mediated apoptosis. Hepatocyte growth factor suppressed apoptosis in Cre-Ctrl but not Met-KO hepatocytes concurrently with up-regulation of NF-kappaB and major antiapoptotic proteins Bcl-2 and Bcl-xL. Intriguingly, Met-KO hepatocytes exhibited intrinsic activation of NF-kappaBas well as Bcl-2 and Bcl-xL. Furthermore, unchallenged Met-KO cells displayed oxidative stress as evidenced by overproduction of reactive oxygen species, which was associated with greater NADPH and Rac1 activities, was blocked by the known NADPH oxidase inhibitors, and was paralleled by increased lipid peroxidation and reduced glutathione (GSH) content. N-Acetylcysteine, an antioxidant and GSH precursor, significantly reduced Jo2-induced cell death. Conversely, the GSH-depleting agent buthionine sulfoximine completely abolished the protective effects of N-acetylcysteine in Met-KO hepatocytes. In conclusion, genetic inactivation of c-Met in mouse hepatocytes caused defects in redox regulation, which may account for the increased sensitivity to Fas-induced apoptosis and adaptive up-regulation of NF-kappaB survival signaling. These data provide evidence that intact c-Met signaling is a critical factor in the protection against excessive generation of endogenous reactive oxygen species.

The Met tyrosine kinase receptor in development and cancer

Met is a tyrosine kinase receptor, encoded by an oncogene, whose crucial role has been elucidated during the last two decades. The complex biological program triggered by Met has been dissected and its biological relevance in both physiology and pathology has been proven. Met supports a morphogenetic program, known as invasive growth, taking place both during embryogenesis and adulthood. In tumors Met is often aberrantly activated, giving rise to the pathological counterpart of the invasive growth program: cancer progression towards metastasis. Several approaches have been recently developed to interfere with the tumorigenic and metastatic processes triggered by Met.