Regulation of receptor tyrosine kinase signaling by protein tyrosine phosphatase-1B

PTP1B-dependent regulation of receptor tyrosine kinase signaling by the actin-binding protein Mena

The actin-binding protein Mena regulates RTK signaling after growth factor stimulation in tumor cells by a novel mechanism. The alternatively spliced MenaINV isoform disrupts this attenuation to drive sensitivity to growth factors, resistance to targeted inhibitors, and ultimately tumor invasion and metastasis.

During breast cancer progression, alternative mRNA splicing produces functionally distinct isoforms of Mena, an actin regulator with roles in cell migration and metastasis. Aggressive tumor cell subpopulations express Mena^INV, which promotes tumor cell invasion by potentiating EGF responses. However, the mechanism by which this occurs is unknown. Here we report that Mena associates constitutively with the tyrosine phosphatase PTP1B and mediates a novel negative feedback mechanism that attenuates receptor tyrosine kinase signaling. On EGF stimulation, complexes containing Mena and PTP1B are recruited to the EGFR, causing receptor dephosphorylation and leading to decreased motility responses. Mena also interacts with the 5′ inositol phosphatase SHIP2, which is important for the recruitment of the Mena-PTP1B complex to the EGFR. When Mena^INV is expressed, PTP1B recruitment to the EGFR is impaired, providing a mechanism for growth factor sensitization to EGF, as well as HGF and IGF, and increased resistance to EGFR and Met inhibitors in signaling and motility assays. In sum, we demonstrate that Mena plays an important role in regulating growth factor–induced signaling. Disruption of this attenuation by Mena^INV sensitizes tumor cells to low–growth factor concentrations, thereby increasing the migration and invasion responses that contribute to aggressive, malignant cell phenotypes.

Ubiquitination switches EphA2 vesicular traffic from a continuous safeguard to a finite signalling mode

Autocatalytic phosphorylation of receptor tyrosine kinases (RTKs) enables diverse, context-dependent responses to extracellular signals but comes at the price of autonomous, ligand-independent activation. Using a conformational biosensor that reports on the kinase activity of the cell guidance ephrin receptor type-A (EphA2) in living cells, we observe that autonomous EphA2 activation is suppressed by vesicular recycling and dephosphorylation by protein tyrosine phosphatases 1B (PTP1B) near the pericentriolar recycling endosome. This spatial segregation of catalytically superior PTPs from RTKs at the plasma membrane is essential to preserve ligand responsiveness. Ligand-induced clustering, on the other hand, promotes phosphorylation of a c-Cbl docking site and ubiquitination of the receptor, thereby redirecting it to the late endosome/lysosome. We show that this switch from cyclic to unidirectional receptor trafficking converts a continuous suppressive safeguard mechanism into a transient ligand-responsive signalling mode.

Pathophysiological significance of c-jun N-terminal kinase in acetaminophen hepatotoxicity

BACKGROUND

Acetaminophen (APAP) overdose is the leading cause of acute liver failure in the US. Although substantial progress regarding the mechanisms of APAP hepatotoxicity has been made in the past several decades, therapeutic options are still limited and novel treatments are clearly needed. c-jun N-terminal Kinase (JNK) has emerged as a promising therapeutic target in recent years.

AREAS COVERED

Early studies established the critical role of JNK activation and mitochondrial translocation in APAP hepatotoxicity. However, this concept has also been challenged. Initial studies failed to reproduce the protection of JNK deficiency in APAP toxicity and concerns over off-target effects of JNK inhibitors and even in knock-out mice are increasing. Interestingly, recent studies have even shown that liver injury can be altered with or without effects on JNK activation. The current review addresses these discrepancies and tries to explain or reconcile some of the conflicting results.

EXPERT OPINION

JNK is a potential therapeutic target for APAP poisoning. However, controversies still exist regarding its actual role in APAP hepatotoxicity. Future studies are warranted for more in-depth testing of specific inhibitors in well-defined preclinical models and human hepatocytes before JNK can be considered a relevant therapeutic target for APAP poisoning.

Rho-associated protein kinase inhibitor, Y-27632, significantly enhances cell adhesion and induces a delay in G1 to S phase transition in rabbit corneal endothelial cells

Human corneal endothelial cells are a non-proliferative cell type. As a result of the increase in corneal endothelium disease, increasing numbers of studies have been conducted in order to promote corneal endothelial cell proliferation. The aim of the present study was to investigate the proliferative effects of Rho-associated protein kinase inhibitor, Y-27632, on rabbit corneal endothelial cells (rCECs). Y-27632 (1, 10 or 30 μM) was added at two different time points to two groups of rCECs. The first group received Y-27632 when rCECs were initially plated, and the second following 72 h of cell growth. Cell morphology and cell adhesion ratios were subsequently observed using light microscopy. A cell counting kit was used to measure the number of viable cells that adhered to culture plates. Cell cycle transitions and levels of Annexin V-positive apoptotic cells were detected using flow cytometry. Cells treated with 1 μM Y-27632 and 10 μM Y-27632 retained their cell shape. At a concentration of 30 μM Y-27632, the cell shape became irregular. Cell adhesion ratios, in 1 μM Y-27632 (36.84%), 10 μM Y-27632 (84.21%) and 30 μM Y-27632 (84.21%) were higher than the adhesion ratio in the control group (P<0.01). The optical densities of rCECs treated with 10 μM or 30 μM Y-27632 following 72 h of cell growth was less than that of the control cells (P<0.01), but higher than that of cells which received Y-27632 at the time of plating (P<0.01). Flow cytometry results also demonstrated that there was a delay in G1 to S phase cell cycle progression in rCECs following administration of 10 μM Y-27632 (P<0.01). Cell apoptosis was inhibited when 10 μM Y-27632 was added, at the time of cell plating, as well as when added following 72 h of cell growth (P<0.01). At a concentration of 10 μM Y-27632, there was an improvement in cell adhesion and an inhibition of the cell cycle in rabbit corneal endothelial cells. In conclusion, Y-27632 has different effects on rCECs when administered at varying concentrations and at particular stages of cell growth.

Cytotoxin-induced NADPH oxides activation: roles in regulation of cell death

Numerous studies have shown that a variety of cytotoxic agents can activate the NADPH oxidase system and induce redox-dependent regulation of cellular functions. Cytotoxin-induced NADPH oxidase activation may either exert cytoprotective actions (e.g., survival, proliferation, and stress tolerance) or cause cell death. Here we summarize the experimental evidence showing the context-dependent dichotomous effects of NADPH oxidase on cell fate under cytotoxic stress conditions and the potential redox signaling mechanisms underlying this phenomenon. Clearly, it is difficult to create a unified paradigm on the toxicological implications of NADPH oxidase activation in response to cytotoxic stimuli. We suggest that interventional strategies targeting the NADPH oxidase system to prevent the adverse impacts of cytotoxins need to be contemplated in a stimuli- and cell type-specific manner.

Specific dephosphorylation of Janus Kinase 2 by protein tyrosine phosphatases

Many protein kinases are activated through phosphorylation of an activation loop thereby turning on downstream signaling pathways. Activation of JAK2, a nonreceptor tyrosine kinase with an important role in growth factor and cytokine signaling, requires phosphorylation of the 1007 and 1008 tyrosyl residues. Dephosphorylation of these two sites by phosphatases presumably inactivates the enzyme, but the underlying mechanism is not known. In this study, we employed MALDI-TOF/TOF and triple quadrupole mass spectrometers to analyze qualitatively and quantitatively the dephosphorylation process by using synthetic peptides derived from the tandem autophosphorylation sites (Y1007 and Y1008) of human JAK2. We found that tyrosine phosphatases catalyzed the dephosphorylation reaction sequentially, but different enzymes exhibited different selectivity. Protein tyrosine phosphatase 1B caused rapid dephosphorylation of Y1008 followed by Y1007, while SHP1 and SHP2 selectively dephosphorylated Y1008 only, and yet HePTP randomly removed a single phosphate from either Y1007 or Y1008, leaving behind mono-phosphorylated peptides. The specificity of dephosphorylation was further confirmed by molecular modeling. The data reveal multiple modes of JAK2 regulation by tyrosine phosphatases, reflecting a complex, and intricate interplay between protein phosphorylation and dephosphorylation.

Nck1 depletion induces activation of the PI3K/Akt pathway by attenuating PTP1B protein expression

Background

Activation of the PI3K/Akt pathway mediates crucial cellular functions regulated by receptor tyrosine kinases, such as cell growth, proliferation, survival and metabolism. Previously, we reported that the whole-body knockout of the Src homology domain-containing adaptor protein Nck1 improves overall glucose homeostasis and insulin-induced activation of the PI3K/Akt pathway in liver of obese mice. The aim of the current study is to elucidate the mechanism by which Nck1 depletion regulates hepatic insulin signaling.

Results

Here, we demonstrate that Nck1 regulates the activation of the PI3K/Akt pathway in a protein tyrosine phosphatase 1B (PTP1B)-dependent mechanism. Indeed, depletion of Nck1 by siRNA in HepG2 cells enhances PI3K-dependent basal and growth factor-induced Akt activation. In accordance, primary hepatocytes isolated from Nck1^−/− mice also display enhanced Akt activation in response to insulin. Activation of the PI3K/Akt pathway in Nck1-depleted HepG2 cells relies on higher levels of tyrosine-phosphorylated proteins and correlates with decreased PTP1B levels. Interestingly, Nck1 and PTP1B in cells are found in a common molecular complex and their interaction is dependent on the SH3 domains of Nck1. Finally, Nck1 depletion in HepG2 cells neither affects PTP1B gene transcription nor PTP1B protein stability, suggesting that Nck1 modulates PTP1B expression at the translational level.

Conclusion

Our study provides strong evidence supporting that the adaptor protein Nck1 interacts with PTP1B and also regulates PTP1B expression. In this manner, Nck1 plays a role in regulating the PI3K/Akt pathway.

Proteinase-activated receptor 1- and 4-promoted migration of Hep3B hepatocellular carcinoma cells depends on ROS formation and RTK transactivation

PURPOSE

There is growing evidence for a role of proteinase-activated receptors (PARs), a subfamily of G protein-coupled receptors, in cancer. We have previously shown that PAR1 and PAR4 are able to promote the migration of hepatocellular carcinoma (HCC) cells suggesting a function in HCC progression. In this study, we assessed the underlying signalling mechanisms.

METHODS

Using Hep3B liver carcinoma cells, RTK activation was assessed by Western blot employing phospho-RTK specific antibodies, ROS level were estimated by H2DCF-DA using confocal laser scanning microscopy, and measurement of PTP activity was performed in cell lysates using 6,8-difluoro-4-methylumbelliferyl phosphate (DiFMUP) as a substrate.

RESULTS

Thrombin, the PAR1 selective agonist peptide TFLLRN-NH2 (PAR1-AP), and the PAR4 selective agonist peptide, AYPGKF-NH2 (PAR4-AP), induced a significant increase in Hep3B cell migration that could be blocked by inhibitors targeting formation of reactive oxygen species (ROS), or activation of hepatocyte-growth factor receptor (Met), or platelet-derived growth factor receptor (PDGFR), respectively. The involvement of these intracellular effectors in PAR1/4-initiated migratory signalling was further supported by the findings that individual stimulation of Hep3B cells with the PAR1-AP and the PAR4-AP induced an increase in ROS production and the transactivation of Met and PDGFR. In addition, PAR1- and PAR4-mediated inhibition of total PTP activity and specifically PTP1B. ROS inhibition by N-acetyl-L-cysteine prevented the inhibition of PTP1B phosphatase activity induced by PAR1-AP and the PAR4-AP, but had no effect on PAR1/4-mediated activation of Met and PDGFR in Hep3B cells.

CONCLUSIONS

Collectively, our data indicate that PAR1 and PAR4 activate common promigratory signalling pathways in Hep3B liver carcinoma cells including activation of the receptor tyrosine kinases Met and PDGFR, the formation of ROS and the inactivation of PTP1B. However, PAR1/4-triggered Met and PDGFR transactivation seem to be mediated independently from the ROS-PTP1B signalling module.

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.

Mining the function of protein tyrosine phosphatases in health and disease

Protein tyrosine phosphatases (PTPs) play a crucial role in the regulation of human health and it is now clear that PTP dysfunction is causal to a variety of human diseases. Research in the PTP field has accelerated dramatically over the last decade fueled by cutting-edge technologies in genomic and proteomic techniques. This system-wide non-biased approach when applied to the discovery of PTP function has led to the elucidation of new and unanticipated roles for the PTPs. These discoveries, driven by genomic and proteomic approaches, have uncovered novel PTP findings that range from those that describe fundamental cell signaling mechanisms to implications for PTPs as novel therapeutic targets for the treatment of human disease. This review will discuss how new PTP functions have been uncovered through studies that have utilized genomic and proteomic technologies and strategies.

Disruption of protein-tyrosine phosphatase 1B expression in the pancreas affects β-cell function

Protein-tyrosine phosphatase 1B (PTP1B) is a physiological regulator of glucose homeostasis and energy balance. However, the role of PTP1B in pancreatic endocrine function remains largely unknown. To investigate the metabolic role of pancreatic PTP1B, we generated mice with pancreas PTP1B deletion (panc-PTP1B KO). Mice were fed regular chow or a high-fat diet, and metabolic parameters, insulin secretion and glucose tolerance were determined. On regular chow, panc-PTP1B KO and control mice exhibited comparable glucose tolerance whereas aged panc-PTP1B KO exhibited mild glucose intolerance. Furthermore, high-fat feeding promoted earlier impairment of glucose tolerance and attenuated glucose-stimulated insulin secretion in panc-PTP1B KO mice. The secretory defect in glucose-stimulated insulin secretion was recapitulated in primary islets ex vivo, suggesting that the effects were likely cell-autonomous. At the molecular level, PTP1B deficiency in vivo enhanced basal and glucose-stimulated tyrosyl phosphorylation of EphA5 in islets. Consistently, PTP1B overexpression in the glucose-responsive MIN6 β-cell line attenuated EphA5 tyrosyl phosphorylation, and substrate trapping identified EphA5 as a PTP1B substrate. In summary, these studies identify a novel role for PTP1B in pancreatic endocrine function.

Ion channel TRPV1-dependent activation of PTP1B suppresses EGFR-associated intestinal tumorigenesis

The intestinal epithelium has a high rate of turnover, and dysregulation of pathways that regulate regeneration can lead to tumor development; however, the negative regulators of oncogenic events in the intestinal epithelium are not fully understood. Here we identified a feedback loop between the epidermal growth factor receptor (EGFR), a known mediator of proliferation, and the transient receptor potential cation channel, subfamily V, member 1 (TRPV1), in intestinal epithelial cells (IECs). We found that TRPV1 was expressed by IECs and was intrinsically activated upon EGFR stimulation. Subsequently, TRPV1 activation inhibited EGFR-induced epithelial cell proliferation via activation of Ca2+/calpain and resulting activation of protein tyrosine phosphatase 1B (PTP1B). In a murine model of multiple intestinal neoplasia (Apc(Min/+) mice), TRPV1 deficiency increased adenoma formation, and treatment of these animals with an EGFR kinase inhibitor reversed protumorigenic phenotypes, supporting a functional association between TRPV1 and EGFR signaling in IECs. Administration of a TRPV1 agonist suppressed intestinal tumorigenesis in Apc(Min/+) mice, similar to--as well as in conjunction with--a cyclooxygenase-2 (COX-2) inhibitor, which suggests that targeting both TRPV1 and COX-2 has potential as a therapeutic approach for tumor prevention. Our findings implicate TRPV1 as a regulator of growth factor signaling in the intestinal epithelium through activation of PTP1B and subsequent suppression of intestinal tumorigenesis.

Enhanced angiogenesis and increased cardiac perfusion after myocardial infarction in protein tyrosine phosphatase 1B-deficient mice

The protein tyrosine phosphatase 1B (PTP1B) modulates tyrosine kinase receptors, among which is the vascular endothelial growth factor receptor type 2 (VEGFR2), a key component of angiogenesis. Because PTP1B deficiency in mice improves left ventricular (LV) function 2 mo after myocardial infarction (MI), we hypothesized that enhanced angiogenesis early after MI via activated VEGFR2 contributes to this improvement. At 3 d after MI, capillary density was increased at the infarct border of PTP1B(-/-) mice [+7±2% vs. wild-type (WT), P = 0.05]. This was associated with increased extracellular signal-regulated kinase 2 phosphorylation and VEGFR2 activation (i.e., phosphorylated-Src/Src/VEGFR2 and dissociation of endothelial VEGFR2/VE-cadherin), together with higher infiltration of proangiogenic M2 macrophages within unchanged overall infiltration. In vitro, we showed that PTP1B inhibition or silencing using RNA interference increased VEGF-induced migration and proliferation of mouse heart microvascular endothelial cells as well as fibroblast growth factor (FGF)-induced proliferation of rat aortic smooth muscle cells. At 8 d after MI in PTP1B(-/-) mice, increased LV capillary density (+21±3% vs. WT; P<0.05) and an increased number of small diameter arteries (15-50 μm) were likely to participate in increased LV perfusion assessed by magnetic resonance imaging and improved LV compliance, indicating reduced diastolic dysfunction. In conclusion, PTP1B deficiency reduces MI-induced heart failure promptly after ischemia by enhancing angiogenesis, myocardial perfusion, and diastolic function.

Pivotal role of protein tyrosine phosphatase 1B (PTP1B) in the macrophage response to pro-inflammatory and anti-inflammatory challenge

Inhibition of protein tyrosine phosphatase 1B (PTP1B) has been suggested as an attractive target to improve insulin sensitivity in different cell types. In the present work, we have investigated the effect of PTP1B deficiency on the response of human and murine macrophages. Using in vitro and in vivo approaches in mice and silencing PTP1B in human macrophages with specific siRNAs, we have demonstrated that PTP1B deficiency increases the effects of pro-inflammatory stimuli in both human and rodent macrophages at the time that decreases the response to alternative stimulation. Moreover, the absence of PTP1B induces a loss of viability in resting macrophages and mainly after activation through the classic pathway. Analysis of early gene expression in macrophages treated with pro-inflammatory stimuli confirmed this exacerbated inflammatory response in PTP1B-deficient macrophages. Microarray analysis in samples from wild-type and PTP1B-deficient macrophages obtained after 24 h of pro-inflammatory stimulation showed an activation of the p53 pathway, including the excision base repair pathway and the insulin signaling pathway in the absence of PTP1B. In animal models of lipopolysaccharide (LPS) and D-galactosamine challenge as a way to reveal in vivo inflammatory responses, animals lacking PTP1B exhibited a higher rate of death. Moreover, these animals showed an enhanced response to irradiation, in agreement with the data obtained in the microarray analysis. In summary, these results indicate that, although inhibition of PTP1B has potential benefits for the treatment of diabetes, it accentuates pro-inflammatory responses compromising at least macrophage viability.

Adiponectin inhibits leptin-induced oncogenic signalling in oesophageal cancer cells by activation of PTP1B

Obesity is characterised by hyperleptinaemia and hypoadiponectinaemia and these metabolic abnormalities may contribute to the progression of several obesity-associated cancers including oesophageal adenocarcinoma (OAC). We have examined the effects of leptin and adiponectin on OE33 OAC cells. Leptin stimulated proliferation, invasion and migration and inhibited apoptosis in a STAT3-dependant manner. Leptin-stimulated MMP-2 secretion in a partly STAT3-dependent manner and MMP-9 secretion via a STAT3-independent pathway. Adiponectin inhibited leptin-induced proliferation, migration, invasion, MMP secretion and reduced the anti-apoptotic effects: these effects of adiponectin were ameliorated by both a non-specific tyrosine phosphatase inhibitor and a specific PTP1B inhibitor. Adiponectin reduced leptin-stimulated JAK2 activation and STAT3 transcriptional activity in a PTP1B-sensitive manner and adiponectin increased both PTP1B protein and activity. We conclude that adiponectin restrains leptin-induced signalling and pro-carcinogenic behaviour by inhibiting the early events in leptin-induced signal transduction by activating PTP1B. Relative adiponectin deficiency in obesity may contribute to the promotion of OAC.

Targeting the PDGF signaling pathway in tumor treatment

Platelet-derived growth factor (PDGF) isoforms and PDGF receptors have important functions in the regulation of growth and survival of certain cell types during embryonal development and e.g. tissue repair in the adult. Overactivity of PDGF receptor signaling, by overexpression or mutational events, may drive tumor cell growth. In addition, pericytes of the vasculature and fibroblasts and myofibroblasts of the stroma of solid tumors express PDGF receptors, and PDGF stimulation of such cells promotes tumorigenesis. Inhibition of PDGF receptor signaling has proven to useful for the treatment of patients with certain rare tumors. Whether treatment with PDGF/PDGF receptor antagonists will be beneficial for more common malignancies is the subject for ongoing studies.

EGF receptor trafficking: consequences for signaling and cancer

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EGF receptor endocytic traffic can regulate signaling and cell survival.

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Signaling from activated EGFR occurs at the endosome as well as the cell surface.

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Endocytosis can have positive and negative effects on signaling and tumorigenesis.

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EGFR traffic promoted by antineoplastic therapy is important in tumor resistance.

The ligand-stimulated epidermal growth factor receptor (EGFR) has been extensively studied in the analysis of molecular mechanisms regulating endocytic traffic and the role of that traffic in signal transduction. Although such studies have largely focused on mitogenic signaling and dysregulated traffic in tumorigenesis, there is growing interest in the potential role of EGFR traffic in cell survival and the consequent response to cancer therapy. Here we review recent advances in our understanding of molecular mechanisms regulating ligand-stimulated EGFR activation, internalization, and post-endocytic sorting. The role of EGFR overexpression/mutation and new modulators of EGFR traffic in cancer and the response to cancer therapeutics are also discussed. Finally, we speculate on the relationship between EGFR traffic and cell survival.

Phosphatase regulation of intercellular junctions

Intercellular junctions represent the key contact points and sites of communication between neighboring cells. Assembly of these junctions is absolutely essential for the structural integrity of cell monolayers, tissues and organs. Disruption of junctions can have severe consequences such as diarrhea, edema and sepsis, and contribute to the development of chronic inflammatory diseases. Cell junctions are not static structures, but rather they represent highly dynamic micro-domains that respond to signals from the intracellular and extracellular environments to modify their composition and function. This review article will focus on the regulation of tight junctions and adherens junctions by phosphatase enzymes that play an essential role in preserving and modulating the properties of intercellular junction proteins.

Analysis of PTP1B sumoylation

Signaling enzymes are typically regulated by a variety of post-translational modifications. One such enzyme, protein tyrosine phosphatase (PTP) 1B, which plays an important role in metabolic and growth signaling, has recently been shown to be inhibited by posttranslational modifications by ubiquitin-related proteins of the SUMO family. We have adapted several approaches to analyzing the sites and effects of sumoylation of PTP1B in cells and in vitro. The principal outline of the assays should be applicable to other enzymes as well.

Regulation of the SNARE-interacting protein Munc18c tyrosine phosphorylation in adipocytes by protein-tyrosine phosphatase 1B

BACKGROUND

Protein-tyrosine phosphatase 1B (PTP1B) is a physiological regulator of insulin signaling and adiposity and is a drug target for the treatment of obesity and diabetes. The molecular mechanisms underlying PTP1B metabolic actions require additional investigation.

RESULTS

Herein, we identify Munc18c as a novel PTP1B substrate in adipocytes and in vivo. We demonstrate nutritional regulation of Munc18c in adipose tissue revealing decreased expression upon high fat feeding. In addition, PTP1B deficiency leads to elevated Munc18c tyrosine phosphorylation and dissociation from syntaxin4. At the molecular level, we identify Munc18c Tyr218/219 and Tyr521 as key residues that mediate Munc18c interaction with PTP1B. Further, we uncover an essential role of Munc18c total tyrosine phosphorylation in general, and Tyr218/219 and Tyr521 in particular, in regulating its interactions and glucose uptake in adipocytes.

CONCLUSION

In conclusion, our findings identify PTP1B as the first known tyrosine phosphatase for Munc18c and a regulator of its phosphorylation and function in adipocytes.

Metastasis-associated PRL-3 induces EGFR activation and addiction in cancer cells

Metastasis-associated phosphatase of regenerating liver-3 (PRL-3) has pleiotropic effects in driving cancer progression, yet the signaling mechanisms of PRL-3 are still not fully understood. Here, we provide evidence for PRL-3-induced hyperactivation of EGFR and its downstream signaling cascades in multiple human cancer cell lines. Mechanistically, PRL-3-induced activation of EGFR was attributed primarily to transcriptional downregulation of protein tyrosine phosphatase 1B (PTP1B), an inhibitory phosphatase for EGFR. Functionally, PRL-3-induced hyperactivation of EGFR correlated with increased cell growth, promigratory characteristics, and tumorigenicity. Moreover, PRL-3 induced cellular addiction to EGFR signaling, as evidenced by the pronounced reversion of these oncogenic attributes upon EGFR-specific inhibition. Of clinical significance, we verified elevated PRL-3 expression as a predictive marker for favorable therapeutic response in a heterogeneous colorectal cancer (CRC) patient cohort treated with the clinically approved anti-EGFR antibody cetuximab. The identification of PRL-3-driven EGFR hyperactivation and consequential addiction to EGFR signaling opens new avenues for inhibiting PRL-3-driven cancer progression. We propose that elevated PRL-3 expression is an important clinical predictive biomarker for favorable anti-EGFR cancer therapy.

Selective activation of oxidized PTP1B by the thioredoxin system modulates PDGF-β receptor tyrosine kinase signaling

The inhibitory reversible oxidation of protein tyrosine phosphatases (PTPs) is an important regulatory mechanism in growth factor signaling. Studies on PTP oxidation have focused on pathways that increase or decrease reactive oxygen species levels and thereby affect PTP oxidation. The processes involved in reactivation of oxidized PTPs remain largely unknown. Here the role of the thioredoxin (Trx) system in reactivation of oxidized PTPs was analyzed using a combination of in vitro and cell-based assays. Cells lacking the major Trx reductase TrxR1 (Txnrd1(-/-)) displayed increased oxidation of PTP1B, whereas SHP2 oxidation was unchanged. Furthermore, in vivo-oxidized PTP1B was reduced by exogenously added Trx system components, whereas SHP2 oxidation remained unchanged. Trx1 reduced oxidized PTP1B in vitro but failed to reactivate oxidized SHP2. Interestingly, the alternative TrxR1 substrate TRP14 also reactivated oxidized PTP1B, but not SHP2. Txnrd1-depleted cells displayed increased phosphorylation of PDGF-β receptor, and an enhanced mitogenic response, after PDGF-BB stimulation. The TrxR inhibitor auranofin also increased PDGF-β receptor phosphorylation. This effect was not observed in cells specifically lacking PTP1B. Together these results demonstrate that the Trx system, including both Trx1 and TRP14, impacts differentially on the oxidation of individual PTPs, with a preference of PTP1B over SHP2 activation. The studies demonstrate a previously unrecognized pathway for selective redox-regulated control of receptor tyrosine kinase signaling.

Protein tyrosine phosphatase 1B regulates pyruvate kinase M2 tyrosine phosphorylation

Protein-tyrosine phosphatase 1B (PTP1B) is a physiological regulator of glucose homeostasis and adiposity and is a drug target for the treatment of obesity and diabetes. Here we identify pyruvate kinase M2 (PKM2) as a novel PTP1B substrate in adipocytes. PTP1B deficiency leads to increased PKM2 total tyrosine and Tyr(105) phosphorylation in cultured adipocytes and in vivo. Substrate trapping and mutagenesis studies identify PKM2 Tyr-105 and Tyr-148 as key sites that mediate PTP1B-PKM2 interaction. In addition, in vitro analyses illustrate a direct effect of Tyr-105 phosphorylation on PKM2 activity in adipocytes. Importantly, PTP1B pharmacological inhibition increased PKM2 Tyr-105 phosphorylation and decreased PKM2 activity. Moreover, PKM2 Tyr-105 phosphorylation is regulated nutritionally, decreasing in adipose tissue depots after high-fat feeding. Further, decreased PKM2 Tyr-105 phosphorylation correlates with the development of glucose intolerance and insulin resistance in rodents, non-human primates, and humans. Together, these findings identify PKM2 as a novel substrate of PTP1B and provide new insights into the regulation of adipose PKM2 activity.

Protein tyrosine phosphatases as novel targets in breast cancer therapy

Breast cancer is linked to hyperactivation of protein tyrosine kinases (PTKs), and recent studies have unveiled that selective tyrosine dephosphorylation by protein tyrosine phosphatases (PTPs) of specific substrates, including PTKs, may activate or inactivate oncogenic pathways in human breast cancer cell growth-related processes. Here, we review the current knowledge on the involvement of PTPs in breast cancer, as major regulators of breast cancer therapy-targeted PTKs, such as HER1/EGFR, HER2/Neu, and Src. The functional interplay between PTKs and PTK-activating or -inactivating PTPs, and its implications in novel breast cancer therapies based on targeting of specific PTPs, are discussed.

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.

Redox regulation of protein kinases

Protein kinases represent one of the largest families of genes found in eukaryotes. Kinases mediate distinct cellular processes ranging from proliferation, differentiation, survival, and apoptosis. Ligand-mediated activation of receptor kinases can lead to the production of endogenous hydrogen peroxide (H₂O₂) by membrane-bound NADPH oxidases. In turn, H₂O₂ can be utilized as a secondary messenger in signal transduction pathways. This review presents an overview of the molecular mechanisms involved in redox regulation of protein kinases and its effects on signaling cascades. In the first half, we will focus primarily on receptor tyrosine kinases (RTKs), whereas the latter will concentrate on downstream non-receptor kinases involved in relaying stimulant response. Select examples from the literature are used to highlight the functional role of H₂O₂ regarding kinase activity, as well as the components involved in H₂O₂ production and regulation during cellular signaling. In addition, studies demonstrating direct modulation of protein kinases by H₂O₂ through cysteine oxidation will be emphasized. Identification of these redox-sensitive residues may help uncover signaling mechanisms conserved within kinase subfamilies. In some cases, these residues can even be exploited as targets for the development of new therapeutics. Continued efforts in this field will further basic understanding of kinase redox regulation, and delineate the mechanisms involved in physiological and pathological H₂O₂ responses.

Endothelial cell PTP1B regulates leukocyte recruitment during allergic inflammation

Pulmonary eosinophilia is a consistent hallmark of allergic lung inflammation. Infiltration of eosinophils into ovalbumin (OVA)-challenged lungs is dependent on the adhesion molecule vascular cell adhesion molecule-1 (VCAM-1) on endothelial cells. Ligation of VCAM-1 activates endothelial cell protein tyrosine phosphatase 1B (PTP1B), which is required for VCAM-1-dependent leukocyte migration in vitro. To examine whether nonhematopoietic PTP1B modulates eosinophil recruitment in vivo, mice deficient in PTP1B were irradiated and received wild-type hematopoietic cells to generate chimeric PTP1B-/- mice. In response to OVA challenge, the chimeric PTP1B-/- mice had reduced eosinophilia in the lung tissue and bronchoalveolar lavage, indicating a role for PTP1B in nonhematopoietic cells during leukocyte recruitment. To determine whether endothelial cell PTP1B modulates eosinophil recruitment, mice with an inducible endothelial cell-specific PTP1B deletion (iePTP1B mice) were generated and the PTP1B deletion was induced after antigen sensitization before antigen challenge. In response to OVA challenge, the iePTP1B mice with the endothelial cell PTP1B deletion had an increased accumulation of eosinophils bound to the luminal surface of the endothelium in the lung vasculature and had a decrease in leukocyte recruitment into the lung tissue. In the iePTP1B mice, expression of adhesion molecules, cytokines, or chemokines that regulate leukocyte recruitment during inflammation was not altered, consistent with other studies that deletion of endothelial adhesion molecule signals does not alter lung cytokines and chemokines. In summary, these data suggest that VCAM-1 activation of PTP1B in the endothelium is necessary for eosinophil recruitment during allergic inflammation. Moreover, these studies provide a basis for targeting VCAM-1-dependent signaling pathways in allergy therapies.

Necrotic renal epithelial cell inhibits renal interstitial fibroblast activation: role of protein tyrosine phosphatase 1B

Our recent studies showed that contents of necrotic renal proximal tubular cells (RPTC) from 2 × 10(6) cells/ml directly induced death of cultured renal interstitial fibroblasts. However, it remains unknown whether nonlethal number of necrotic RPTC would also alter the fate of renal interstitial fibroblasts. To address this issue, renal interstitial fibroblasts (NRK-49F) were exposed to necrotic RPTC supernatant (RPTC-Sup) obtained from 2 × 10(4) to 5 × 10(5) cells/ml. These concentrations of RPTC did not induce cell death, but led to inactivation of renal fibroblasts as indicated by reduced expression of α-smooth muscle actin and fibronectin, two hallmarks of activated fibroblasts. Concurrently, the same doses of necrotic RPTC-Sup suppressed phosphorylation of epidermal growth factor receptor (EGFR) and signal transducers and activators of transcription-3 (STAT3) in a time- and dose-dependent manner, but did not affect phosphorylation of platelet-derived growth factor receptor-β, AKT, and extracellular signal-regulated kinase 1/2. The presence of sodium orthovanadate, a general protein tyrosine phosphatase (PTP) inhibitor or TCS-401 (a selective PTP1B inhibitor), abrogated those effects of RPTC-Sup, whereas coincubation with the EGFR inhibitor (Gefitinib) or silencing of EGFR with siRNA preserved the ability of RPTC-Sup in suppressing renal fibroblast activation and STAT3 phosphorylation. Moreover, RPTC-Sup treatment induced PTP1B phosphorylation and its interaction with EGFR. Collectively, these results indicate that nonlethal necrotic RPTC-Sup can induce inactivation of renal interstitial fibroblasts, which occurs through a mechanism involved in PTP1B-mediated inhibition of EGFR signaling.

Computational analysis of the regulation of EGFR by protein tyrosine phosphatases

The tyrosine phosphorylated epidermal growth factor receptor (EGFR) initiates numerous cell signaling pathways. Although EGFR phosphorylation levels are ultimately determined by the balance of receptor kinase and protein tyrosine phosphatase (PTP) activities, the kinetics of EGFR dephosphorylation are not well understood. Previous models of EGFR signaling have generally neglected PTP activity or computed PTP activity by considering data that do not fully reveal the kinetics and compartmentalization of EGFR dephosphorylation. We developed a compartmentalized, mechanistic model to elucidate the kinetics of EGFR dephosphorylation and the coupling of this process to phosphorylation-dependent EGFR endocytosis. Model regression against data from HeLa cells for EGFR phosphorylation response to EGFR activation, PTP inhibition, and EGFR kinase inhibition led to the conclusion that EGFR dephosphorylation occurs at the plasma membrane and in the cell interior with a timescale that is smaller than that for ligand-mediated EGFR endocytosis. The model further predicted that sufficiently rapid dephosphorylation of EGFR at the plasma membrane could potentially impede EGFR endocytosis, consistent with recent experimental findings. Overall, our results suggest that PTPs regulate multiple receptor-level phenomena via their action at the plasma membrane and cell interior and point to new possibilities for targeting PTPs for modulation of EGFR dynamics.

Redox regulation of epidermal growth factor receptor signaling through cysteine oxidation

Epidermal growth factor receptor (EGFR) exemplifies the family of receptor tyrosine kinases that mediate numerous cellular processes, including growth, proliferation, and differentiation. Moreover, gene amplification and EGFR mutations have been identified in a number of human malignancies, making this receptor an important target for the development of anticancer drugs. In addition to ligand-dependent activation and concomitant tyrosine phosphorylation, EGFR stimulation results in the localized generation of H(2)O(2) by NADPH-dependent oxidases. In turn, H(2)O(2) functions as a secondary messenger to regulate intracellular signaling cascades, largely through the modification of specific cysteine residues within redox-sensitive protein targets, including Cys797 in the EGFR active site. In this review, we highlight recent advances in our understanding of the mechanisms that underlie redox regulation of EGFR signaling and how these discoveries may form the basis for the development of new therapeutic strategies for targeting this and other H(2)O(2)-modulated pathways.

The molecular details of WPD-loop movement differ in the protein-tyrosine phosphatases YopH and PTP1B

The movement of a conserved protein loop (the WPD-loop) is important in catalysis by protein tyrosine phosphatases (PTPs). Using kinetics, isotope effects, and X-ray crystallography, the different effects arising from mutation of the conserved tryptophan in the WPD-loop were compared in two PTPs, the human PTP1B, and the bacterial YopH from Yersinia. Mutation of the conserved tryptophan in the WPD-loop to phenylalanine has a negligible effect on k(cat) in PTP1B and full loop movement is maintained. In contrast, the corresponding mutation in YopH reduces k(cat) by two orders of magnitude and the WPD loop locks in an intermediate position, disabling general acid catalysis. During loop movement the indole moiety of the WPD-loop tryptophan moves in opposite directions in the two enzymes. Comparisons of mammalian and bacterial PTPs reveal differences in the residues forming the hydrophobic pocket surrounding the conserved tryptophan. Thus, although WPD-loop movement is a conserved feature in PTPs, differences exist in the molecular details, and in the tolerance to mutation, in PTP1B compared to YopH. Despite high structural similarity of the active sites in both WPD-loop open and closed conformations, differences are identified in the molecular details associated with loop movement in PTPs from different organisms.

VEGF inhibits tumor cell invasion and mesenchymal transition through a MET/VEGFR2 complex

Inhibition of VEGF signaling leads to a proinvasive phenotype in mouse models of glioblastoma multiforme (GBM) and in a subset of GBM patients treated with bevacizumab. Here, we demonstrate that vascular endothelial growth factor (VEGF) directly and negatively regulates tumor cell invasion through enhanced recruitment of the protein tyrosine phosphatase 1B (PTP1B) to a MET/VEGFR2 heterocomplex, thereby suppressing HGF-dependent MET phosphorylation and tumor cell migration. Consequently, VEGF blockade restores and increases MET activity in GBM cells in a hypoxia-independent manner, while inducing a program reminiscent of epithelial-to-mesenchymal transition highlighted by a T-cadherin to N-cadherin switch and enhanced mesenchymal features. Inhibition of MET in GBM mouse models blocks mesenchymal transition and invasion provoked by VEGF ablation, resulting in substantial survival benefit.

Pre- and postnatal calorie restriction perturbs early hypothalamic neuropeptide and energy balance

Energy balance is regulated by circulating leptin concentrations and hypothalamic leptin receptor (ObRb) signaling via STAT3 but is inhibited by SOCS3 and PTP1B. Leptin signaling enhances anorexigenic neuropeptides and receptor (POMC, MC3-R, MC4-R) activation while suppressing orexigenic neuropeptides (NPY, AgRP). We investigated in a sex-specific manner the early (PN2) and late (PN21) postnatal hypothalamic mechanisms in response to intrauterine (IUGR), postnatal (PNGR), and combined (IPGR) calorie and growth restriction. At PN2, both male and female IUGR were hypoleptinemic, but hypothalamic leptin signaling in females was activated as seen by enhanced STAT3. In addition, increased SOCS3 and PTP1B supported early initiation of leptin resistance in females that led to elevated AgRP but diminished MC3-R and MC4-R. In contrast, males demonstrated leptin sensitivity seen as a reduction in PTP1B and MC3-R and MC4-R with no effect on neuropeptide expression. At PN21, with adequate postnatal caloric intake, a sex-specific dichotomy in leptin concentrations was seen in IUGR, with euleptinemia in males indicative of persisting leptin sensitivity and hyperleptinemia in females consistent with leptin resistance, both with normal hypothalamic ObRb signaling, neuropeptides, and energy balance. In contrast, superimposition of PNGR upon IUGR (IPGR) led to diminished leptin concentrations with enhanced PTP1B and an imbalance in arcuate nuclear NPY/AgRP and POMC expression that favored exponential hyperphagia and diminished energy expenditure postweaning. We conclude that IUGR results in sex-specific leptin resistance observed mainly in females, whereas PNGR and IPGR abolish this sex-specificity, setting the stage for acquiring obesity after weaning.

Regulation of signaling at regions of cell-cell contact by endoplasmic reticulum-bound protein-tyrosine phosphatase 1B

Protein-tyrosine phosphatase 1B (PTP1B) is a ubiquitously expressed PTP that is anchored to the endoplasmic reticulum (ER). PTP1B dephosphorylates activated receptor tyrosine kinases after endocytosis, as they transit past the ER. However, PTP1B also can access some plasma membrane (PM)-bound substrates at points of cell-cell contact. To explore how PTP1B interacts with such substrates, we utilized quantitative cellular imaging approaches and mathematical modeling of protein mobility. We find that the ER network comes in close proximity to the PM at apparently specialized regions of cell-cell contact, enabling PTP1B to engage substrate(s) at these sites. Studies using PTP1B mutants show that the ER anchor plays an important role in restricting its interactions with PM substrates mainly to regions of cell-cell contact. In addition, treatment with PTP1B inhibitor leads to increased tyrosine phosphorylation of EphA2, a PTP1B substrate, specifically at regions of cell-cell contact. Collectively, our results identify PM-proximal sub-regions of the ER as important sites of cellular signaling regulation by PTP1B.

Protein tyrosine phosphatase 1B deficiency potentiates PERK/eIF2α signaling in brown adipocytes

Background

Protein-tyrosine phosphatase 1B (PTP1B) is a physiological regulator of glucose homeostasis and body mass, and has been implicated in endoplasmic reticulum (ER) stress. Herein, we assess the role of PTP1B in ER stress in brown adipocytes, which are key regulators of thermogenesis and metabolic response.

Methodology/Principal Findings

To determine the role of PTP1B in ER stress, we utilized brown adipose tissue (BAT) from mice with adipose-specific PTP1B deletion, and brown adipocytes deficient in PTP1B and reconstituted with PTP1B wild type (WT) or the substrate-trapping PTP1B D181A (D/A) mutant. PTP1B deficiency led to upregulation of PERK-eIF2α phosphorylation and IRE1α-XBP1 sub-arms of the unfolded protein response. In addition, PTP1B deficiency sensitized differentiated brown adipocytes to chemical-induced ER stress. Moreover, PERK activation and tyrosine phosphorylation were increased in BAT and adipocytes lacking PTP1B. Increased PERK activity resulted in the induction of eIF2α phosphorylation at Ser51 and better translatability of ATF4 mRNA in response to ER stress. At the molecular level, we demonstrate direct interaction between PTP1B and PERK and identify PERK Tyr615 as a mediator of this association.

Conclusions

Collectively, the data demonstrate that PTP1B is a physiologically-relevant modulator of ER stress in brown adipocytes and that PTP1B deficiency modulates PERK-eIF2α phosphorylation and protein synthesis.

Sumoylated protein tyrosine phosphatase 1B localizes to the inner nuclear membrane and regulates the tyrosine phosphorylation of emerin

Protein tyrosine phosphatase (PTP)1B is an abundant non-transmembrane enzyme that plays a major role in regulating insulin and leptin signaling. Recently, we reported that PTP1B is inhibited by sumoylation, and that sumoylated PTP1B accumulates in a perinuclear distribution, consistent with its known localization in the endoplasmic reticulum (ER) and the contiguous outer nuclear membrane. Here, we report that, in addition to its localization at the ER, PTP1B also is found at the inner nuclear membrane, where it is heavily sumoylated. We also find that PTP1B interacts with emerin, an inner nuclear membrane protein that is known to be tyrosine phosphorylated, and that PTP1B expression levels are inversely correlated with tyrosine phosphorylation levels of emerin. PTP1B sumoylation greatly increases as cells approach mitosis, corresponding to the stage where tyrosine phosphorylation of emerin is maximal. In addition, expression of a non-sumoylatable mutant of PTP1B greatly reduced levels of emerin tyrosine phosphorylation. These results suggest that PTP1B regulates the tyrosine phosphorylation of a key inner nuclear membrane protein in a sumoylation- and cell-cycle-dependent manner.

Protein tyrosine phosphatase 1B: a novel molecular target for retinal degenerative diseases

Protein tyrosine phosphatase 1B (PTP1B) is considered as a major negative regulator of insulin receptor (IR) signaling. IR signaling in retina has been demonstrated to be neuroprotective. Photoreceptor specific deletion of PTP1B results in enhanced retinal IR-mediated neuroprotection indicating the importance of PTP1B as a negative regulator in the retina. Elevated levels of retinal PTP1B activity has been observed in mice lacking retinal pigment epithelium (Rpe65^-/-), a mouse model of leber congenital amaurosis (LCA-type 2), retinitis pigmentosa and diabetic retinopathy animal models. This enhanced PTP1B activity could down regulate the IR signaling which may contribute to the death of photoreceptor neurons and ultimately lead to retinal degenerations. The potential therapeutic agents that specifically reduce or inhibit the PTP1B activity could be beneficial in protecting or delaying the photoreceptor cell death in the retinal degenerative diseases.

Protection against LPS-induced pulmonary edema through the attenuation of protein tyrosine phosphatase-1B oxidation

One hallmark of acute lung injury is the disruption of the pulmonary endothelial barrier. Such disruption correlates with increased endothelial permeability, partly through the disruption of cell-cell contacts. Protein tyrosine phosphatases (PTPs) are known to affect the stability of both cell-extracellular matrix adhesions and intercellular adherens junctions (AJs). However, evidence for the role of select PTPs in regulating endothelial permeability is limited. Our investigations noted that the inhibition of PTP1B in cultured pulmonary endothelial cells (ECs), as well as in the vasculature of intact murine lungs via the transient overexpression of a catalytically inactive PTP1B, decreased the baseline resistance of cultured EC monolayers and increased the formation of edema in murine lungs, respectively. In addition, we observed that the overexpression of wild-type PTP1B enhanced basal barrier function in vitro. Immunohistochemical analyses of pulmonary ECs and the coimmunoprecipitation of murine lung homogenates demonstrated the association of PTP1B with the AJ proteins β-catenin, p120-catenin, and VE-cadherin both in vitro and ex vivo. Using LPS in a model of sepsis-induced acute lung injury, we showed that reactive oxygen species were generated in response to LPS, which correlated with enhanced PTP1B oxidation, inhibited phosphatase activity, and attenuation of the interactions between PTP1B and β-catenin, as well as enhanced β-catenin tyrosine phosphorylation. Finally, the overexpression of a cytosolic PTP1B fragment, shown to be resistant to nicotinamide adenine dinucleotide phosphate-reduced oxidase-4 (Nox4)-mediated oxidation, significantly attenuated LPS-induced endothelial barrier dysfunction and the formation of lung edema, and preserved the associations of PTP1B with AJ protein components, independent of PTP1B phosphatase activity. We conclude that PTP1B plays an important role in maintaining the pulmonary endothelial barrier, and PTP1B oxidation appears to contribute to sepsis-induced pulmonary vascular dysfunction, possibly through the disruption of AJs.

Effect of oxidative stress on protein tyrosine phosphatase 1B in scleroderma dermal fibroblasts

OBJECTIVE

Platelet-derived growth factor (PDGF) and its receptor, PDGFR, promote fibrosis in systemic sclerosis (SSc; scleroderma) dermal fibroblasts, and such cells in scleroderma skin lesions produce excessive reactive oxygen species (ROS). PDGFR is phosphorylated upon PDGF stimulation, and is dephosphorylated by protein tyrosine phosphatases (PTPs), including PTP1B. This study was undertaken to determine whether the thiol-sensitive PTP1B is affected by ROS in SSc dermal fibroblasts, thereby enhancing the phosphorylation of PDGFR and synthesis of type I collagen. This study also sought to investigate the effect of a thiol antioxidant, N-acetylcysteine (NAC), in SSc.

METHODS

Fibroblasts were isolated from the skin of patients with diffuse SSc and normal healthy donors for cell culture experiments and immunofluorescence analyses. A phosphate release assay was used to determine the activity of PTP1B.

RESULTS

Levels of ROS and type I collagen were significantly higher and amounts of free thiol were significantly lower in SSc fibroblasts compared to normal fibroblasts. After stimulation with PDGF, not only were PDGFR and ERK-1/2 phosphorylated to a greater extent, but also the ability to produce PTP1B was hampered in SSc fibroblasts. The activity of PTP1B was significantly inactivated in SSc fibroblasts as a result of cysteine oxidation by the raised levels of ROS, which was confirmed by the oxidation of multiple PTPs, including PTP1B, in SSc fibroblasts. Decreased expression of PTP1B in normal fibroblasts led to increased expression of type I collagen. Treatment of the cells with NAC restored the activity of PTP1B, improved the profile of PDGFR phosphorylation, decreased the numbers of tyrosine-phosphorylated proteins and levels of type I collagen, and scavenged ROS in SSc fibroblasts.

CONCLUSION

This study describes a new mechanism by which ROS may promote a profibrotic phenotype in SSc fibroblasts through the oxidative inactivation of PTP1B, leading to pronounced activation of PDGFR. The study also presents a novel molecular mechanism by which NAC may act on ROS and PTP1B to provide therapeutic benefit in SSc.