The T-cell protein tyrosine phosphatase

Computational Methods in Cooperation with Experimental Approaches to Design Protein Tyrosine Phosphatase 1B Inhibitors in Type 2 Diabetes Drug Design: A Review of the Achievements of This Century

Protein tyrosine phosphatase 1B (PTP1B) dephosphorylates phosphotyrosine residues and is an important regulator of several signaling pathways, such as insulin, leptin, and the ErbB signaling network, among others. Therefore, this enzyme is considered an attractive target to design new drugs against type 2 diabetes, obesity, and cancer. To date, a wide variety of PTP1B inhibitors that have been developed by experimental and computational approaches. In this review, we summarize the achievements with respect to PTP1B inhibitors discovered by applying computer-assisted drug design methodologies (virtual screening, molecular docking, pharmacophore modeling, and quantitative structure–activity relationships (QSAR)) as the principal strategy, in cooperation with experimental approaches, covering articles published from the beginning of the century until the time this review was submitted, with a focus on studies conducted with the aim of discovering new drugs against type 2 diabetes. This review encourages the use of computational techniques and includes helpful information that increases the knowledge generated to date about PTP1B inhibition, with a positive impact on the route toward obtaining a new drug against type 2 diabetes with PTP1B as a molecular target.

PTPN2 improves implant osseointegration in T2DM via inducing the dephosphorylation of ERK

Type 2 diabetes mellitus (T2DM) is considered to compromise implant osseointegration. Protein tyrosine phosphatase non-receptor type 2 (PTPN2) regulates glucose metabolism, systemic inflammation, and bone regeneration. This study aimed to investigate the role of PTPN2 in implant osseointegration in T2DM and explore the potential mechanisms. Streptozotocin-induced diabetic rats received implant surgery, with or without local overexpression of PTPN2 for three months, and implant osseointegration was examined by histological evaluation, micro-CT analysis, pull-out test, and scanning electron microscope. Rat bone marrow stem cells (RBMSCs) were isolated and exposed to high glucose, and osteogenic differentiation was evaluated by alizarin red staining, ALP assay, and Western blot analysis. Overexpression of PTPN2 could improve impaired implant osseointegration in T2DM rats and promote osteogenic differentiation of RBMSCs in high glucose. In addition, p-ERK level in RBMSCs was increased in high glucose and decreased after PTPN2 overexpression. These results suggest that PTPN2 promotes implant osseointegration in T2DM rats and enhances osteogenesis of RBMSCs in high glucose medium via inducing the dephosphorylation of ERK. PTPN2 may be a novel target for the therapy of impaired implant osseointegration in T2DM patients.

Impact statement

Using both in vivo and in vitro approaches, we made important findings that PTPN2 promoted implant osseointegration in T2DM rats and enhanced osteogenesis of RBMSCs in high glucose medium. The positive effects of PTPN2 on osteogenesis are related to the dephosphorylation of ERK and the inhibition of MAPK/ERK pathway. PTPN2 may be a novel target for the therapy of impaired implant osseointegration in T2DM patients.

T cell protein tyrosine phosphatase prevents STAT1 induction of claudin-2 expression in intestinal epithelial cells

T cell protein tyrosine phosphatase (TCPTP) dephosphorylates a number of substrates, including JAK-STAT (signal transducer and activator of transcription) signaling proteins, which are activated by interferon (IFN)-γ, a major proinflammatory cytokine involved in conditions such as inflammatory bowel disease. A critical function of the intestinal epithelium is formation of a selective barrier to luminal contents. The structural units of the epithelium that regulate barrier function are the tight junctions (TJs), and the protein composition of the TJ determines the tightness of the barrier. Claudin-2 is a TJ protein that increases permeability to cations and reduces transepithelial electrical resistance (TER). We previously showed that transient knockdown (KD) of TCPTP permits increased expression of claudin-2 by IFN-γ. Here, we demonstrate that the decreased TER in TCPTP-deficient epithelial cells is alleviated by STAT1 KD. Moreover, increased claudin-2 in TCPTP-deficient cells requires enhanced STAT1 activation and STAT1 binding to the CLDN2 promoter. We also show that mutation of this STAT-binding site prevents elevated CLDN2 promoter activity in TCPTP-deficient epithelial cells. In summary, we demonstrate that TCPTP protects the intestinal epithelial barrier by restricting STAT-induced claudin-2 expression. This is a potential mechanism by which loss-of-function mutations in the gene encoding TCPTP may contribute to barrier defects in chronic intestinal inflammatory disease.

Leveraging Reciprocity to Identify and Characterize Unknown Allosteric Sites in Protein Tyrosine Phosphatases

Drug-like molecules targeting allosteric sites in proteins are of great therapeutic interest; however, identification of potential sites is not trivial. A straightforward approach to identify hidden allosteric sites is demonstrated in protein tyrosine phosphatases (PTP) by creation of single alanine mutations in the catalytic acid loop of PTP1B and VHR. This approach relies on the reciprocal interactions between an allosteric site and its coupled orthosteric site. The resulting NMR chemical shift perturbations (CSPs) of each mutant reveal clusters of distal residues affected by acid loop mutation. In PTP1B and VHR, two new allosteric clusters were identified in each enzyme. Mutations in these allosteric clusters altered phosphatase activity with changes in k_cat/K_M ranging from 30% to nearly 100-fold. This work outlines a simple method for identification of new allosteric sites in PTP, and given the basis of this method in thermodynamics, it is expected to be generally useful in other systems.

Protein Tyrosine Phosphatase N2 Is a Positive Regulator of Lipopolysaccharide Signaling in Raw264.7 Cell through Derepression of Src Tyrosine Kinase

T cell protein tyrosine phosphatase N2 (PTPN2) is a phosphotyrosine-specific nonreceptor phosphatase and is ubiquitously expressed in tissues. Although PTPN2 functions as an important regulator in different signaling pathways, it is still unclear what is specific target protein of PTPN2 and how is regulated in lipopolysaccharide (LPS)-induced inflammatory signaling pathway. Here, we found that PTPN2 deficiency downregulated the expression of LPS-mediated pro-inflammtory cytokine genes. Conversely, overexpression of PTPN2 in Raw264.7 cells enhanced the expression and secretion of those cytokines. The activation of MAPK and NF-κB signaling pathways by LPS was reduced in PTPN2-knockdowned cells and ectopic expression of PTPN2 reversed these effects. Furthermore, we found that PTNP2 directly interacted with Src and removed the inhibitory Tyr527 phosphorylation of Src to enhance the activatory phosphorylation of Tyr416 residue. These results suggested that PTPN2 is a positive regulator of LPS-induced inflammatory response by enhancing the activity of Src through targeting the inhibitory phosphor-tyrosine527 of Src.

Protein tyrosine phosphatase nonreceptor type 2: an important regulator of lnterleukin-6 production in rheumatoid arthritis synovial fibroblasts

OBJECTIVE

To investigate the role of protein tyrosine phosphatase nonreceptor type 2 (PTPN2) in the pathogenesis of rheumatoid arthritis (RA).

METHODS

Synovial tissue samples from patients with RA and patients with osteoarthritis (OA) were stained for PTPN2. Synovial fibroblasts were stimulated with tumor necrosis factor (TNF) and interleukin-1β (IL-1β), lipopolysaccharide (LPS), TRAIL, or thapsigargin. The expression of PTPN2 in synovial fibroblasts and peripheral blood mononuclear cells (PBMCs) was analyzed by real-time polymerase chain reaction and Western blotting. Cell death, the release of IL-6 and IL-8, and the induction of autophagy were analyzed after PTPN2 silencing. Methylated DNA immunoprecipitation analysis was used to evaluate DNA methylation-regulated gene expression of PTPN2.

RESULTS

PTPN2 was significantly overexpressed in synovial tissue samples from RA patients compared to OA patients. Patients receiving anti-TNF therapy showed significantly reduced staining for PTPN2 compared with patients treated with nonbiologic agents. PTPN2 expression was higher in RA synovial fibroblasts (RASFs) than in OASFs. This differential expression was not regulated by DNA methylation. PTPN2 was further up-regulated after stimulation with TNF, TNF combined with IL-1β, or LPS. There was no significant difference in basal PTPN2 expression in PBMCs from patients with RA, ankylosing spondylitis, or systemic lupus erythematosus or healthy controls. Most interestingly, PTPN2 silencing in RASFs significantly increased the production of the inflammatory cytokine IL-6 but did not affect levels of IL-8. Moreover, functional analysis showed that high PTPN2 levels contributed to the increased apoptosis resistance of RASFs and increased autophagy.

CONCLUSION

This is the first study of PTPN2 in RASFs showing that PTPN2 regulates IL-6 production, cell death, and autophagy. Our findings indicate that PTPN2 is linked to the pathogenesis of RA via synovial fibroblasts.

Roles of TRAF3 in T cells: many surprises

Tumor necrosis factor receptor (TNFR)-associated factor 3 (TRAF3) is broadly involved in different receptor-mediated signaling pathways. Considerable progress was made recently in understanding the role of TRAF3 in T cell biology. Here we review these new findings about how TRAF3 participates in T cell development and function. The different roles of TRAF3 in distinct immune cells are also compared. That TRAF3 is required for T cell effector functions, and invariant Natural Killer T cell function and development, was unexpected. Another surprising finding is that TRAF3 normally restrains regulatory T cell development. It is now clear that TRAF3 regulates signaling to T cells not only through costimulatory members of the TNFR superfamily, but also through the T cell receptor complex, and cytokine receptors. The diverse roles it plays support the multifaceted nature of this molecule. How TRAF3 mediates integration of different signaling cascades is an important topic for future study.

Protein tyrosine phosphatase non-receptor type 2 and inflammatory bowel disease

Genome wide association studies have associated single nucleotide polymorphisms within the gene locus encoding protein tyrosine phosphatase non-receptor type 2 (PTPN2) with the onset of inflammatory bowel disease (IBD) and other inflammatory disorders. Expression of PTPN2 is enhanced in actively inflamed intestinal tissue featuring a marked up-regulation in intestinal epithelial cells. PTPN2 deficient mice suffer from severe intestinal and systemic inflammation and display aberrant innate and adaptive immune responses. In particular, PTPN2 is involved in the regulation of inflammatory signalling cascades, and critical for protecting intestinal epithelial barrier function, regulating innate and adaptive immune responses, and finally for maintaining intestinal homeostasis. On one hand, dysfunction of PTPN2 has drastic effects on innate host defence mechanisms, including increased secretion of pro-inflammatory cytokines, limited autophagosome formation in response to invading pathogens, and disruption of the intestinal epithelial barrier. On the other hand, PTPN2 function is crucial for controlling adaptive immune functions, by regulating T cell proliferation and differentiation as well as maintaining T cell tolerance. In this way, dysfunction of PTPN2 contributes to the manifestation of IBD. The aim of this review is to present an overview of recent findings on the role of PTPN2 in intestinal homeostasis and the impact of dysfunctional PTPN2 on intestinal inflammation.

Role of protein tyrosine phosphatases in regulating the immune system: implications for chronic intestinal inflammation

Current hypothesis suggests that genetic, immunological, and bacterial factors contribute essentially to the pathogenesis of inflammatory bowel disease. Variations within the gene loci encoding protein tyrosine phosphatases (PTPs) have been associated with the onset of inflammatory bowel disease. PTPs modulate the activity of their substrates by dephosphorylation of tyrosine residues and are critical for the regulation of fundamental cellular signaling processes. Evidence emerges that expression levels of PTPN2, PTPN11, and PTPN22 are altered in actively inflamed intestinal tissue. PTPN2 seems to be critical for protecting intestinal epithelial barrier function, regulating innate and adaptive immune responses and finally for maintaining intestinal homeostasis. These observations have been confirmed in PTPN2 knockout mice in vivo. Those animals are clearly more susceptible to intestinal and systemic inflammation and feature alterations in innate and adaptive immune responses. PTPN22 controls inflammatory signaling in lymphocytes and mononuclear cells resulting in aberrant cytokine secretion pattern and autophagosome formation. PTPN22 deficiency in vivo results in more severe colitis demonstrating the relevance of PTPN22 for intestinal homeostasis in vivo. Of note, loss of PTPN22 promotes mitogen-activated protein kinase-induced cytokine secretion but limits secretion of nuclear factor κB-associated cytokines and autophagy in mononuclear cells. Loss of PTPN11 is also associated with increased colitis severity in vivo. In summary, dysfunction of those PTPs results in aberrant and uncontrolled immune responses that result in chronic inflammatory conditions. This way, it becomes more and more evident that dysfunction of PTPs displays an important factor in the pathogenesis of chronic intestinal inflammation, in particular inflammatory bowel disease.

IBD candidate genes and intestinal barrier regulation

Technological advances in the large scale analysis of human genetics have generated profound insights into possible genetic contributions to chronic diseases including the inflammatory bowel diseases (IBDs), Crohn's disease and ulcerative colitis. To date, 163 distinct genetic risk loci have been associated with either Crohn's disease or ulcerative colitis, with a substantial degree of genetic overlap between these 2 conditions. Although many risk variants show a reproducible correlation with disease, individual gene associations only affect a subset of patients, and the functional contribution(s) of these risk variants to the onset of IBD is largely undetermined. Although studies in twins have demonstrated that the development of IBD is not mediated solely by genetic risk, it is nevertheless important to elucidate the functional consequences of risk variants for gene function in relevant cell types known to regulate key physiological processes that are compromised in IBD. This article will discuss IBD candidate genes that are known to be, or are suspected of being, involved in regulating the intestinal epithelial barrier and several of the physiological processes presided over by this dynamic and versatile layer of cells. This will include assembly and regulation of tight junctions, cell adhesion and polarity, mucus and glycoprotein regulation, bacterial sensing, membrane transport, epithelial differentiation, and restitution.

The adaptor TRAF3 restrains the lineage determination of thymic regulatory T cells by modulating signaling via the receptor for IL-2

The number of Foxp3+ regulatory T cells (Treg cells) must be tightly controlled for efficient suppression of autoimmunity with no impairment of normal immune responses. Here we found that the adaptor TRAF3 was intrinsically required for restraining the lineage determination of thymic Treg cells. T cell-specific deficiency in TRAF3 resulted in a two- to threefold greater frequency of Treg cells, due to the more efficient transition of precursors of Treg cells into Foxp3+ Treg cells. TRAF3 dampened interleukin 2 (IL-2) signaling by facilitating recruitment of the tyrosine phosphatase TCPTP to the IL-2 receptor complex, which resulted in dephosphorylation of the signaling molecules Jak1 and Jak3 and negative regulation of signaling via Jak and the transcription factor STAT5. Our results identify a role for TRAF3 as an important negative regulator of signaling via the IL-2 receptor that affects the development of Treg cells.

Pancreatic T cell protein-tyrosine phosphatase deficiency ameliorates cerulein-induced acute pancreatitis

Background

Acute pancreatitis (AP) is a common clinical problem whose incidence has been progressively increasing in recent years. Onset of the disease is trigged by intra-acinar cell activation of digestive enzyme zymogens that induce autodigestion, release of pro-inflammatory cytokines and acinar cell injury. T-cell protein tyrosine phosphatase (TCPTP) is implicated in inflammatory signaling but its significance in AP remains unclear.

Results

In this study we assessed the role of pancreatic TCPTP in cerulein-induced AP. TCPTP expression was increased at the protein and messenger RNA levels in the early phase of AP in mice and rats. To directly determine whether TCPTP may have a causal role in AP we generated mice with pancreatic TCPTP deletion (panc-TCPTP KO) by crossing TCPTP floxed mice with Pdx1-Cre transgenic mice. Amylase and lipase levels were lower in cerulein-treated panc-TCPTP KO mice compared with controls. In addition, pancreatic mRNA and serum concentrations of the inflammatory cytokines TNFα and IL-6 were lower in panc-TCPTP KO mice. At the molecular level, panc-TCPTP KO mice exhibited enhanced cerulein-induced STAT3 Tyr705 phosphorylation accompanied by a decreased cerulein-induced NF-κB inflammatory response, and decreased ER stress and cell death.

Conclusion

These findings revealed a novel role for pancreatic TCPTP in the progression of cerulein-induced AP.

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.

Association between the PTPN2 gene and Crohn's disease: dissection of potential causal variants

BACKGROUND

Although genome-wide association studies (GWAS) and subsequent meta-analyses have confirmed associations between the PTPN2 (protein tyrosine phosphatase, nonreceptor type 2) gene and Crohn's disease (CD), the potential causal variants remain unidentified. We aimed to dissect potential causal CD-associated PTPN2 variants, assess their functional significance, and relate PTPN2 protein expression with inflammation in CD.

METHODS

A 3-stage study was carried out. In stage 1, we genotyped tagging single nucleotide polymorphisms (tag-SNPs) in the PTPN2 gene in a sample of patients with CD (<20 years, n = 556) and controls (n = 602). In stage 2, we resequenced the putative promoter, target exons and introns in the PTPN2 gene, and examined associations with high-frequency variants with CD in the stage 1 cohort. In stage 3 we studied the relationship between PTPN2 protein expression and mucosal inflammation and carried out in silico analyses to study the functional characteristics of the PTPN2 CD-associated SNPs.

RESULTS

In stage 1, we observed associations between 5 intronic SNPs and CD including rs1893217 (P = 2 × 10⁻⁴), the SNP that is in perfect linkage disequilibrium with the lead genome-wide association studies SNP rs2542151. Resequencing revealed 2 known promoter polymorphisms. No associations between these promoter SNPs and CD were evident. In silico analyses revealed that the 5 associated intronic SNPs influenced PTPN2 expression and binding to important transcription factors. PTPN2 protein was overexpressed in inflamed intestinal tissues of patients with CD.

CONCLUSIONS

Our findings suggest that noncoding variation in the PTPN2 gene may represent the causal variations influencing susceptibility for CD.

25-hydroxyvitamin D3 ameliorates periodontitis by modulating the expression of inflammation-associated factors in diabetic mice

Periodontitis is a complication of diabetes mellitus, and the two diseases are highly associated with the dysfunction of inflammatory mediators. 25-hydroxyvitamin D(3) (25(OH)D(3)) plays a pivotal role in inflammatory modulation, but little is known about its effects on the progression of diabetic periodontitis and the underlying mechanism. In this paper, we showed that 25(OH)D(3) ameliorated experimental periodontitis in diabetic mice. The intraperitoneal administration of 25(OH)D(3) to streptozotocin-induced diabetic mice reduced fasting glucose and serum TNF-α levels, leading to decreased alveolar bone loss. Western blot analyses of gingival epithelia showed that vitamin D receptor (VDR) and protein tyrosine phosphatase N2 (PTPN2) were upregulated, while the expression of NF-κB and the phosphorylation of Janus family kinase 1 (JAK1) were attenuated upon 25(OH)D(3) treatment. These data may provide an explanation for the therapeutic benefits and anti-inflammatory effects of 25(OH)D(3). Our findings should have important implications for the clinical therapy of diabetic periodontitis.

Regulation of epithelial barrier function by the inflammatory bowel disease candidate gene, PTPN2

Protein tyrosine phosphatase nonreceptor type 2 (PTPN2) has been identified as an inflammatory bowel disease (IBD) candidate gene. However, the mechanism through which mutations in the PTPN2 gene contribute to the pathogenesis of IBD has not been identified. PTPN2 acts as a negative regulator of signaling induced by the proinflammatory cytokine, interferon-gamma (IFN-γ). IFN-γ is known not only to play an important role in the pathogenesis of Crohn's disease (CD), but also to increase permeability of the intestinal epithelial barrier. We have shown that PTPN2 protects epithelial barrier function by restricting the capacity of IFN-γ to increase epithelial permeability and prevent induction of expression of the pore-forming protein, claudin-2. These data identify an important functional role for PTPN2 as a protector of the intestinal epithelial barrier and provide clues as to how PTPN2 mutations may contribute to the pathophysiology of CD.

The kinase c-Src and the phosphatase TC45 coordinately regulate c-Fos tyrosine phosphorylation and c-Fos phospholipid synthesis activation capacity

Our previous work showed that in T98G cells, a human glioblastoma multiforme-derived cell line, the association of c-Fos to the endoplasmic reticulum (ER) and consequently, the capacity of c-Fos to activate phospholipid synthesis, is regulated by the phosphorylation state of tyrosine (tyr) residues #10 and #30 of c-Fos. The small amount of c-Fos present in quiescent cells is tyr-phosphorylated, is dissociated from the ER membranes and does not activate phospholipid synthesis. However, on induction of the cell to re-enter growth, c-Fos expression is rapidly induced, it is found dephosphorylated, associated to ER membranes and activating phospholipid synthesis (Portal et al., 2007). Herein, using in vivo and in vitro experimental strategies, we show that the kinase c-Src is capable of phosphorylating tyr residues of c-Fos whereas the phosphatase TC45 T-cell protein-tyr phosphatase (TC-PTP) dephosphorylates them, thus enabling c-Fos/ER association and activation of phospholipid synthesis. Results also suggest that the regulation of the phosphorylation/dephosphorylation cycle of c-Fos occurs at the TC-PTP level: induction of cells to re-enter growth promotes the translocation of TC45 from a nuclear to a cytoplasmic location concomitant with its activation. Activated TC45 in its turn promotes dephosphorylation of pre-formed c-Fos, enabling cells to rapidly activate phospholipid synthesis to respond to its growth demands.

Assessing sequence-specific DNA binding and transcriptional activity of STAT1 transcription factor

Continuous nucleocytoplasmic shuttling of signal transducer and activator of transcription (STAT) proteins is a key to understand their function as cytokine-responsive transcription factors. STATs enter the nucleus both by carrier-dependent and carrier-independent transport pathways, and it was previously shown that STAT1 exits the nucleus only after its prior enzymatic dephosphorylation by nuclear phosphatases. The identification of different transport pathways for unphosphorylated and tyrosine-phosphorylated STAT dimers was made possible by a combination of a diverse set of experimental approaches in the field of molecular biology. In the following, we will summarize some of the techniques that have been successfully used to decipher molecular mechanisms engaged in STAT1 dynamics.

Loss, mutation and deregulation of L3MBTL4 in breast cancers

Background

Many alterations are involved in mammary oncogenesis, including amplifications of oncogenes and losses of tumor suppressor genes (TSG). Losses may affect almost all chromosome arms and many TSGs remain to be identified.

Results

We studied 307 primary breast tumors and 47 breast cancer cell lines by high resolution array comparative genomic hybridization (aCGH). We identified a region on 18p11.31 lost in about 20% of the tumors and 40% of the cell lines. The minimal common region of loss (Chr18:6,366,938-6,375,929 bp) targeted the L3MBTL4 gene. This gene was also targeted by breakage in one tumor and in two cell lines. We studied the exon sequence of L3MBTL4 in 180 primary tumor samples and 47 cell lines and found six missense and one nonsense heterozygous mutations. Compared with normal breast tissue, L3MBTL4 mRNA expression was downregulated in 73% of the tumors notably in luminal, ERBB2 and normal-like subtypes. Losses of the 18p11 region were associated with low L3MBTL4 expression level. Integrated analysis combining genome and gene expression profiles of the same tumors pointed to 14 other potential 18p TSG candidates. Downregulated expression of ZFP161, PPP4R1 and YES1 was correlated with luminal B molecular subtype. Low ZFP161 gene expression was associated with adverse clinical outcome.

Conclusion

We have identified L3MBTL4 as a potential TSG of chromosome arm 18p. The gene is targeted by deletion, breakage and mutations and its mRNA is downregulated in breast tumors. Additional 18p TSG candidates might explain the aggressive phenotype associated with the loss of 18p in breast tumors.

Protein tyrosine phosphatases in glioma biology

Gliomas are a diverse group of brain tumors of glial origin. Most are characterized by diffuse infiltrative growth in the surrounding brain. In combination with their refractive nature to chemotherapy this makes it almost impossible to cure patients using combinations of conventional therapeutic strategies. The drastically increased knowledge about the molecular underpinnings of gliomas during the last decade has elicited high expectations for a more rational and effective therapy for these tumors. Most studies on the molecular pathways involved in glioma biology thus far had a strong focus on growth factor receptor protein tyrosine kinase (PTK) and phosphatidylinositol phosphatase signaling pathways. Except for the tumor suppressor PTEN, much less attention has been paid to the PTK counterparts, the protein tyrosine phosphatase (PTP) superfamily, in gliomas. PTPs are instrumental in the reversible phosphorylation of tyrosine residues and have emerged as important regulators of signaling pathways that are linked to various developmental and disease-related processes. Here, we provide an overview of the current knowledge on PTP involvement in gliomagenesis. So far, the data point to the potential implication of receptor-type (RPTPδ, DEP1, RPTPμ, RPTPζ) and intracellular (PTP1B, TCPTP, SHP2, PTPN13) classical PTPs, dual-specific PTPs (MKP-1, VHP, PRL-3, KAP, PTEN) and the CDC25B and CDC25C PTPs in glioma biology. Like PTKs, these PTPs may represent promising targets for the development of novel diagnostic and therapeutic strategies in the treatment of high-grade gliomas.

Protection of epithelial barrier function by the Crohn's disease associated gene protein tyrosine phosphatase n2

BACKGROUND & AIMS

Protein tyrosine phosphatase N2 (PTPN2) has been identified as a Crohn's disease (CD) candidate gene. However, a role for PTPN2 in the pathogenesis of CD has not been identified. Increased permeability of the intestinal epithelium is believed to contribute prominently to CD. The aim of this study was to determine a possible role for PTPN2 in CD pathogenesis.

METHODS

Intestinal epithelial cell (IEC) lines T(84) and HT29cl.19a were used in all studies. Protein analysis was performed by Western blotting, and protein knockdown was induced by small interfering RNA. Primary samples were from control and CD patients.

RESULTS

Here, we demonstrate increased PTPN2 expression in CD intestinal biopsy specimens and that the proinflammatory cytokine interferon (IFN)-gamma increases PTPN2 expression and activity in IEC. Moreover, IFN-gamma-induced STAT1 and STAT3 phosphorylation in IEC is enhanced by PTPN2 knockdown. The cellular energy sensor adenosine monophosphate-activated protein kinase partially regulates the IFN-gamma-induced effects on PTPN2. Additionally, PTPN2 knockdown potentiates IFN-gamma-induced increases in epithelial permeability, accompanied by elevated expression of the pore-forming protein claudin-2.

CONCLUSIONS

PTPN2 is activated by IFN-gamma and limits IFN-gamma-induced signalling and consequent barrier defects. These data suggest a functional role for PTPN2 in maintaining the intestinal epithelial barrier and in the pathophysiology of CD.

Genetics of inflammatory bowel disease: implications for disease pathogenesis and natural history

Epidemiological data, detailed molecular studies and recent genome-wide association studies strongly suggest that ulcerative colitis (UC) and Crohn's disease (CD) are related polygenic diseases that share some susceptibility loci, but differ at others. To date, there are more than 50 confirmed inflammatory bowel disease genes/loci, a number that is widely anticipated to at least double in the next 2 years. Germline variation in IL23R, IL12B, JAK2 and STAT3 is associated with inflammatory bowel disease susceptibility, consistent with the newly described role for IL23 signaling and Th17 cells in disease pathogenesis. Several genes involved in different aspects of bacterial handling are defective only in CD, including NOD2 and the autophagy genes ATG16L1 and IRGM. IL10 and ECM1 are associated with UC, while inherited variation at the HLA region is related to an inflammatory colonic phenotype. The application of genome-wide association studies to inflammatory bowel disease has been successful in defining the genetic architecture of CD and UC and in delivering genuinely novel and important insights into disease pathogenesis. This has unearthed a plethora of attractive targets for the development of future therapeutics. Insights into the natural history of these complex diseases will follow and may enable appropriate patient selection for early aggressive therapy with the view to modifying the disease course.

Protein tyrosine phosphatases expression during development of mouse superior colliculus

Protein tyrosine phosphatases (PTPs) are key regulators of different processes during development of the central nervous system. However, expression patterns and potential roles of PTPs in the developing superior colliculus remain poorly investigated. In this study, a degenerate primer-based reverse transcription-polymerase chain reaction (RT-PCR) approach was used to isolate seven different intracellular PTPs and nine different receptor-type PTPs (RPTPs) from embryonic E15 mouse superior colliculus. Subsequently, the expression patterns of 11 PTPs (TC-PTP, PTP1C, PTP1D, PTP-MEG2, PTP-PEST, RPTPJ, RPTPε, RPTPRR, RPTPσ, RPTPκ and RPTPγ) were further analyzed in detail in superior colliculus from embryonic E13 to postnatal P20 stages by quantitative real-time RT-PCR, Western blotting and immunohistochemistry. Each of the 11 PTPs exhibits distinct spatiotemporal regulation of mRNAs and proteins in the developing superior colliculus suggesting their versatile roles in genesis of neuronal and glial cells and retinocollicular topographic mapping. At E13, additional double-immunohistochemical analysis revealed the expression of PTPs in collicular nestin-positive neural progenitor cells and RC-2-immunoreactive radial glia cells, indicating the potential functional importance of PTPs in neurogenesis and gliogenesis.

Common polymorphic transcript variation in human disease

Most human genes are thought to express different transcript isoforms in different cell types; however, the full extent and functional consequences of polymorphic transcript variation (PTV), which differ between individuals within the same cell type, are unknown. Here we show that PTV is widespread in B-cells from two human populations. Tens of thousands of exons were found to be polymorphically expressed in a heritable fashion, and over 1000 of these showed strong correlations with single nucleotide polymorphism (SNP) genotypes in cis. The SNPs associated with PTV display signs of having been subject to recent positive selection in humans, and they are also highly enriched for SNPs implicated by recent genome-wide association studies of four autoimmune diseases. From this disease-association overlap, we infer that PTV is the likely mechanism by which eight common polymorphisms contribute to disease risk. A catalog of PTV will be a valuable resource for interpreting results from future disease-association studies and understanding the spectrum of phenotypic differences among humans.

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.

The protein tyrosine phosphatase TCPTP controls VEGFR2 signalling

Vascular endothelial growth factor (VEGF) is a major angiogenic factor that triggers formation of new vessels under physiological and pathological conditions. However, the mechanisms that limit the VEGF responses in target cells and hence prevent excessive and harmful angiogenesis are not well understood. Here, our objective was to study whether T-cell protein tyrosine phosphatase (TCPTP, also known as PTN2), which we found to be expressed in human endothelial cells, could alter VEGF signalling by controlling phosphorylation of VEGFR2. We show that a TCPTP substrate-trapping mutant interacts with VEGFR2. Moreover, TCPTP dephosphorylates VEGFR2 in a phosphosite-specific manner, inhibits its kinase activity and prevents its internalization from the cell surface. We found that TCPTP activity is induced upon integrin-mediated binding of endothelial cells to collagen matrix. TCPTP activation was also induced by using cell-permeable peptides from the cytoplasmic tail of the collagen-binding integrin alpha1. Controlled activation of TCPTP results in inhibition of VEGF-triggered endothelial cell proliferation, angiogenic sprouting, chemokinesis and chemotaxis. We conclude that matrix-controlled TCPTP phosphatase activity can inhibit VEGFR2 signalling, and the growth, migration and differentiation of human endothelial cells.

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.

The expanding universe of inflammatory bowel disease genetics

PURPOSE OF REVIEW

Genetic factors play an important role in the pathogenesis of inflammatory bowel disease. In this review, we will provide an update on the rapid advances in the discovery of inflammatory bowel disease, primarily Crohn's disease, associated genes.

RECENT FINDINGS

Seven recently published Crohn's disease genome-wide association studies have confirmed prior findings related to the nucleotide-binding oligomerization domain 2 (NOD2) gene and the IBD5 locus. In addition, 10 novel loci have been identified and well replicated.

SUMMARY

Several promising associations between Crohn's disease and gene variants have been identified and replicated, the two most widely replicated being variants in the IL23R and ATG16L1 genes. These findings highlight and further support the importance of the immune system and its interactions with the intestinal microflora in the pathogenesis of inflammatory bowel disease.

Protein tyrosine phosphatases: regulatory mechanisms

Protein-tyrosine phosphatases are tightly controlled by various mechanisms, ranging from differential expression in specific cell types to restricted subcellular localization, limited proteolysis, post-translational modifications affecting intrinsic catalytic activity, ligand binding and dimerization. Here, we review the regulatory mechanisms found to control the classical protein-tyrosine phosphatases.

Modulation of the eukaryotic initiation factor 2 alpha-subunit kinase PERK by tyrosine phosphorylation

The endoplasmic reticulum (ER)-resident protein kinase PERK attenuates protein synthesis in response to ER stress through the phosphorylation of translation initiation factor eIF2alpha at serine 51. ER stress induces PERK autophosphorylation at several serine/threonine residues, a process that is required for kinase activation and phosphorylation of eIF2alpha. Herein, we demonstrate that PERK also possesses tyrosine kinase activity. Specifically, we show that PERK is capable of autophosphorylating on tyrosine residues in vitro and in vivo. We further show that tyrosine 615, which is embedded in a highly conserved region of the kinase domain of PERK, is essential for autocatalytic activity. That is, mutation of Tyr-615 to phenylalanine compromises the autophosphorylation capacity of PERK and the phosphorylation of eIF2alpha in vitro and in vivo. The Y615F mutation also impairs the ability of PERK to induce translation of ATF4. Immunoblot analyses with a phosphospecific antibody confirm the phosphorylation of PERK at Tyr-615 both in vitro and in vivo. Thus, our data classify PERK as a dual specificity kinase whose regulation by tyrosine phosphorylation contributes to its optimal activation in response to ER stress.

Nonreceptor Protein-Tyrosine Phosphatases in Immune Cell Signaling

Tyrosyl phosphorylation plays a critical role in multiple signaling pathways regulating innate and acquired immunity. Although tyrosyl phosphorylation is a reversible process, we know much more about the functions of protein-tyrosine kinases (PTKs) than about protein-tyrosine phosphatases (PTPs). Genome sequencing efforts have revealed a large and diverse superfamily of PTPs, which can be subdivided into receptor-like (RPTPs) and nonreceptor (NRPTPs). The role of the RPTP CD45 in immune cell signaling is well known, but those of most other PTPs remain poorly understood. Here, we review the mechanism of action, regulation, and physiological functions of NRPTPs in immune cell signaling. Such an analysis indicates that PTPs are as important as PTKs in regulating the immune system.

Dampening of IFN-gamma-inducible gene expression in human choriocarcinoma cells is due to phosphatase-mediated inhibition of the JAK/STAT-1 pathway

Trophoblast cells (TBCs) form the blastocyst-derived component of the placenta and play essential roles in fetal maintenance. The proinflammatory cytokine IFN-gamma plays a central role in activating cellular immunity, controlling cell proliferation, and inducing apoptosis. IFN-gamma is secreted by uterine NK cells in the placenta during pregnancy and in mice is required for proper formation of the decidual layer and remodeling of the uterine vasculature. Despite the presence of IFN-gamma in the placenta, TBCs do not express either MHC class Ia or class II Ags, and are resistant to IFN-gamma-mediated apoptosis. In this study, we demonstrate that IFN-gamma-induced expression of multiple genes is significantly reduced in human trophoblast-derived choriocarcinoma cells relative to HeLa epithelial or fibroblast cells. These results prompted us to investigate the integrity of the JAK/STAT-1 pathway in these cells. Choriocarcinoma cells and HeLa cells express comparable levels of the IFN-gamma receptor. However, tyrosine phosphorylation of JAK-2 is compromised in IFN-gamma-treated choriocarcinoma cells. Moreover, phosphorylation of STAT-1 at tyrosine 701 is substantially reduced in both IFN-gamma-treated human choriocarcinoma and primary TBCs compared with HeLa cells or primary foreskin fibroblasts. A corresponding reduction of both IFN regulatory factor 1 mRNA and protein expression was observed in IFN-gamma-treated TBCs. Treatment of choriocarcinoma cells with the tyrosine phosphatase inhibitor pervanadate significantly enhanced IFN-gamma-inducible JAK and STAT-1 tyrosine phosphorylation and select IFN-gamma-inducible gene expression. We propose that phosphatase-mediated suppression of IFN-gamma signaling in TBCs contributes to fetal maintenance by inhibiting expression of genes that could be detrimental to successful pregnancy.

PTP1B and TC-PTP: novel roles in immune-cell signaling

It has recently been demonstrated that the protein tyrosine phosphatase (PTP) PTP1B and the T-cell PTP (TC-PTP) target several substrates involved in immune cell signaling. Recent data have furthered the view of these 2 PTP members as key regulators of the immune response. This review will focus on the substrate specificities of PTP1B and TC-PTP and their roles in immune cell signaling, and will discuss some new data implicating PTP1B and TC-PTP in myeloid development.

T-cell protein tyrosine phosphatase, distinctively expressed in activated-B-cell-like diffuse large B-cell lymphomas, is the nuclear phosphatase of STAT6

Diffuse large B-cell lymphomas (DLBCLs) consist of clinically distinct subtypes: germinal center B-cell (GCB)-like and activated-B-cell (ABC)-like tumors, characterized by long and short survival, respectively. We reported distinct interleukin 4 (IL-4) responsiveness and STAT6 signaling in these DLBCL subtypes. Increased nuclear dephosphorylation of phospho-STAT6 (pSTAT6) was observed in ABC-like tumors, which exhibited a different expression profile of protein tyrosine phosphatases (PTPs). Among the differentially expressed PTPs, only T-cell PTP (TCPTP) localizes to the nucleus. Herein, we report that the elevated expression of TCPTP in ABC- versus GCB-like DLBCL tumors is not due to the distinct ontogeny of these neoplasms but rather may be an acquired feature of the tumors. Moreover, we report that STAT6 may serve as a physiological nuclear substrate for TCPTP. We demonstrate interactions between endogenous TCPTP and STAT6 and delineate the domains responsible for the interaction. Overexpression of TCPTP ameliorates IL-4-induced STAT6 phosphorylation and associated gene transcription, whereas knockdown of endogenous TCPTP results in increased IL-4-induced STAT6 signaling. Moreover, we report that TCPTP protein levels may be increased in response to IL-4 and that TCPTP may serve in a negative feedback loop for the suppression of IL-4-induced signaling. Taken together, these results identify TCPTP as a physiological regulator of STAT6 phosphorylation and suggest that specific increases in TCPTP expression in ABC-like DLBCLs may contribute to the different biological characteristics of these tumors.

The Catalytic Activity of the Eukaryotic Initiation Factor-2α Kinase PKR Is Required to Negatively Regulate Stat1 and Stat3 via Activation of the T-cell Protein-tyrosine Phosphatase

Tyrosine phosphorylation of the transcription factors Stat1 and Stat3 is required for them to dimerize, translocate to the nucleus, and induce gene transcription. Nuclear Stat1 and Stat3 are dephosphorylated and deactivated by the T-cell protein-tyrosine phosphatase (TC-PTP), which facilitates the return of both proteins to the cytoplasm. The protein kinase PKR plays an important role in translational control through the modulation of eukaryotic initiation factor-2alpha phosphorylation. Previous data have implicated PKR in cell signaling via regulation of Stat1 and Stat3, but the molecular mechanisms underlying these events have remained elusive. Using PKR(-/-) mouse embryonic fibroblasts and a conditionally active form of human PKR, we demonstrate herein that tyrosine (but not serine) phosphorylation of either Stat1 or Stat3 is impaired in cells with activated kinase. This reduction in Stat1 and Stat3 tyrosine phosphorylation by active PKR proceeds through TC-PTP, which is a substrate of the eukaryotic initiation factor-2alpha kinase both in vitro and in vivo. TC-PTP phosphorylation alone is insufficient to increase its in vivo phosphatase activity unless accompanied by the inhibition of protein synthesis as a result of PKR activation. These data reveal a novel function of PKR as a negative regulator of Stat1 and Stat3 with important implications in cell signaling.

In SilicoStructure-Based Design of a Potent and Selective Small Peptide Inhibitor of Protein Tyrosine Phosphatase 1B, A Novel Therapeutic Target for Obesity and Type 2 Diabetes Mellitus: A Computer Modeling Approach

Protein Tyrosine Phosphatase 1B (PTP1B) has been shown to be a negative regulator of insulin signaling by dephosphorylating key tyrosine residues within the regulatory domain of the beta-subunit of the insulin receptor. Recent gene knockout studies in mice have shown the mice to have increased insulin sensitivity and improved glucose tolerance. Furthermore, these mice also exhibited a resistance to diet induced obesity. Inhibitors of PTP1B would have the potential of enhancing insulin action by prolonging the phosphorylated state of the insulin receptor. In addition, recent clinical studies have shown that the haplotype ACTTCAG0 of the PTPN1 gene, which encodes PTP1B, is a major risk contributor to type 2 diabetes mellitus (T2DM). Thus, there is compelling evidence that small molecule inhibitors of PTP1B may be effective in treating insulin resistance at an early stage, thereby leading to a prevention strategy for T2DM and obesity. Based on the crystal structure of the complex of PTP1B with a known inhibitor, we have identified a tetrapeptide inhibitor with the sequence WKPD. Docking calculations indicate that this peptide is as potent as the existing inhibitors. Moreover, the peptide is also found to be selective for PTP1B with a greatly reduced potency against other biologically important protein tyrosine phosphatases such as PTP-LAR, Calcineurin, and the highly homologous T-Cell Protein Tyrosine Phosphatase (TCPTP). Thus the designed tetrapeptide is a suitable lead compound for the development of new drugs against type 2 diabetes and obesity.

Tyrosine phosphorylation acts as a molecular switch to full-scale activation of the eIF2alpha RNA-dependent protein kinase

Phosphorylation of the alpha-subunit of translation eukaryotic initiation factor-2 (eIF2) leads to the inhibition of protein synthesis in response to diverse conditions of stress. Serine/threonine RNA-dependent protein kinase (PKR) is an eIF2alpha kinase family member induced by type I IFN and activated in response to dsRNA or virus infection. Herein, we demonstrate that human PKR is a dual specificity kinase phosphorylated at Y101, Y162 and Y293 in vitro and in vivo. Site-specific tyrosine phosphorylation is essential for efficient dsRNA-binding, dimerization, kinase activation and eIF2alpha phosphorylation of PKR. Biologically, tyrosine phosphorylation of PKR mediates the antiviral and antiproliferative properties of the kinase through its ability to control translation. Our data demonstrate an important role of tyrosine phosphorylation in biochemical and biological processes caused or mediated by the activation of the eIF2alpha kinase PKR.

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

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

Coordinated regulation of insulin signaling by the protein tyrosine phosphatases PTP1B and TCPTP

The protein tyrosine phosphatase PTP1B is a negative regulator of insulin signaling and a therapeutic target for type 2 diabetes. Our previous studies have shown that the closely related tyrosine phosphatase TCPTP might also contribute to the regulation of insulin receptor (IR) signaling in vivo (S. Galic, M. Klingler-Hoffmann, M. T. Fodero-Tavoletti, M. A. Puryer, T. C. Meng, N. K. Tonks, and T. Tiganis, Mol. Cell. Biol. 23:2096-2108, 2003). Here we show that PTP1B and TCPTP function in a coordinated and temporally distinct manner to achieve an overall regulation of IR phosphorylation and signaling. Whereas insulin-induced phosphatidylinositol 3-kinase/Akt signaling was prolonged in both TCPTP-/- and PTP1B-/- immortalized mouse embryo fibroblasts (MEFs), mitogen-activated protein kinase ERK1/2 signaling was elevated only in PTP1B-null MEFs. By using phosphorylation-specific antibodies, we demonstrate that both IR beta-subunit Y1162/Y1163 and Y972 phosphorylation are elevated in PTP1B-/- MEFs, whereas Y972 phosphorylation was elevated and Y1162/Y1163 phosphorylation was sustained in TCPTP-/- MEFs, indicating that PTP1B and TCPTP differentially contribute to the regulation of IR phosphorylation and signaling. Consistent with this, suppression of TCPTP protein levels by RNA interference in PTP1B-/- MEFs resulted in no change in ERK1/2 signaling but caused prolonged Akt activation and Y1162/Y1163 phosphorylation. These results demonstrate that PTP1B and TCPTP are not redundant in insulin signaling and that they act to control both common as well as distinct insulin signaling pathways in the same cell.

Selective regulation of tumor necrosis factor-induced Erk signaling by Src family kinases and the T cell protein tyrosine phosphatase

The proinflammatory cytokine tumor necrosis factor (TNF) modulates cellular responses through the mitogen-activated protein kinase (MAPK) and nuclear factor-kappaB (NF-kappaB) signaling pathways, but the molecular mechanisms underlying MAPK activation are unknown. T cell protein tyrosine phosphatase (TCPTP) is essential for hematopoietic development and negatively regulates inflammatory responses. Using TCPTP-deficient fibroblasts, we show here that TCPTP regulates TNF-induced MAPK but not NF-kappaB signaling. TCPTP interacted with the adaptor protein TRAF2, and dephosphorylated and inactivated Src tyrosine kinases to suppress downstream signaling through extracellular signal-regulated kinases and production of interleukin 6. These results link TCPTP and Src tyrosine kinases to the selective regulation of TNF-induced MAPK signaling and identify a previously unknown mechanism for modulating inflammatory responses mediated by TNF.

The T-cell protein tyrosine phosphatase is phosphorylated on Ser-304 by cyclin-dependent protein kinases in mitosis

Two alternatively spliced forms of the human protein tyrosine phosphatase TCPTP (T-cell protein tyrosine phosphatase) exist: a 48 kDa form that is targeted to the endoplasmic reticulum (TC48) and a shorter 45 kDa form that is targeted to the nucleus (TC45). In this study we have identified Ser-304 (Phe301-Asp-His-Ser304-Pro-Asn-Lys307) as a major TCPTP phosphory-lation site and demonstrate that TC45, but not TC48, is phosphorylated on this site in vivo. Phosphorylation of TC45 on Ser-304 was cell cycle-dependent, and increased as cells progressed from G2 into mitosis, but subsided upon mitotic exit. Ser-304 phosphorylation was increased when cells were arrested in mitosis by microtubule poisons such as nocodazole, but remained unaltered when cells were arrested at the G2/M checkpoint by adriamycin. Phosphorylation of Ser-304 did not alter significantly the phosphatase activity or the protein stability of TC45, and had no apparent effect on TC45 localization. Ser-304 phosphorylation was ablated when cells were treated with the CDK (cyclin-dependent protein kinase) inhibitors roscovitine or SU9516, but remained unaltered when ERK1/2 activation was inhibited with the MEK (mitogen-activated protein kinase/extracellular-signal-regulated kinase kinase) inhibitor PD98059. In addition, recombinant CDKs, but not the Polo-like kinase Plk1, phosphorylated Ser-304 in vitro. Our studies identify Ser-304 as a major phosphorylation site in human TCPTP, and the TC45 variant as a novel mitotic CDK substrate.

Isoxazole carboxylic acids as protein tyrosine phosphatase 1B (PTP1B) inhibitors

Guided by X-ray crystallography, we have extended the structure-activity relationship (SAR) study on an isoxazole carboxylic acid-based PTP1B inhibitor (1) and more potent and equally selective (>20-fold selectivity over the highly homologous T-cell PTPase, TCPTP) PTP1B inhibitors were identified. Inhibitor 7 demonstrated good cellular activity against PTP1B in COS 7 cells.

A novel mechanism for imatinib mesylate (STI571) resistance in CML cell line KT-1: role of TC-PTP in modulating signals downstream from the BCR-ABL fusion protein

OBJECTIVE

Acquired resistance to imatinib mesylate (STI571) in chronic myelogenous leukemia (CML) patients has become a serious clinical problem. We previously established STI571-resistant sublines (designated KTR cells) from the CML cell line KT-1. T cell protein tyrosine phosphatase (TC-PTP) was markedly downregulated in all KTR cells compared to parental KT-1 cells. Therefore, we examined whether the suppression of TC-PTP expression might contribute to imatinib mesylate-resistance in KTR cells.

MATERIALS AND METHODS

We transduced the nuclear isoform of TC-PTP (TC45) and catalytically inactive TC45-D182A cDNAs into KTR cells by retroviral gene transfer. Subsequently, we analyzed the sensitivity to imatinib mesylate and the status of signaling pathways in the transduced cells.

RESULTS

The overall levels of STAT5 phosphorylation were significantly higher in KTR cells as compared to KT-1 cells, but reconstitution of TC-PTP in KTR cells resulted in a dramatic decrease of STAT5 phosphorylation. Furthermore, STAT5 phosphorylation was ablated by imatinib mesylate in KT-1 cells but remained elevated in KTR cells. In contrast, we observed no difference in BCR-ABL or JAK2 phosphorylation and no difference in activation of other signaling pathways. Importantly, reconstitution of TC-PTP in KTR cells to levels found in parental KT-1 cells restored their sensitivity to imatinib mesylate as monitored by reduced proliferation and increased apoptosis.

CONCLUSIONS

We have demonstrated that forced expression of TC-PTP in imatinib mesylate-resistant KTR cells can restore sensitivity to imatinib mesylate. Our studies indicate that loss of TC-PTP may represent a novel mechanism by which CML cells can acquire imatinib mesylate-resistance.

Heterologous expression of a mammalian protein tyrosine phosphatase gene in Leishmania: effect on differentiation

Leishmania is a protozoan pathogen which is transmitted to humans through the bite of an infected sandfly. This infection results in a spectrum of diseases throughout the developing world, collectively known as leishmaniasis. During its life cycle, Leishmania differentiates from the promastigote stage in the sandfly vector into the amastigote stage in the mammalian host where it multiplies exclusively in macrophage phagolysosomes. Although differentiation of Leishmania is essential for its survival and pathogenesis in the mammalian host, this process is poorly understood. In higher eukaryotic cells, protein tyrosine phosphorylation plays a central role in cell proliferation, differentiation and overall function. We have therefore investigated the role of protein tyrosine phosphorylation in Leishmania differentiation by undertaking complementary approaches to mediate protein tyrosine dephosphorylation in vivo. In the present study, L. donovani were engineered to express a mammalian protein tyrosine phosphatase, or were treated with inhibitors of protein tyrosine kinases, and the resulting phenotype was examined. Both approaches resulted in a partial differentiation from promastigotes to amastigotes including the expression of the amastigote specific A2 protein, morphological change and increased virulence. These data provide support for the involvement of tyrosine phosphorylation in the differentiation of Leishmania.

Protein tyrosyl phosphatases in T cell activation: implication for human immunodeficiency virus transcriptional activity

The protein tyrosine phosphatases (PTPs) superfamily is a large group of enzymes showing a wide diversity of structure and biological functions. Their implication in the regulation of signal transduction processes is critical for homeostasis and efficient cellular activation. Disturbance of the delicate balance between protein tyrosine kinase and protein tyrosine phosphatase activities is at the heart of a large number of diseases. Control of cellular activation is especially important for human immunodeficiency virus type 1 (HIV-1) since this retrovirus requires activated T cells in order to replicate efficiently. Identification of PTPs implicated in signaling pathways leading to upregulation of HIV-1 gene transcription therefore contributes to the general understanding of cellular factors needed for strong HIV-1 replication and progression to AIDS. The use of bisperoxovanadium compounds as potent, specific, and highly purified PTP inhibitors releases HIV-1 from PTP control and strongly increases HIV-1 gene expression. These inhibitors can thus be used to study signal transduction mechanisms regulated by PTP activity that are important for HIV-1 replication and provide new and interesting therapeutic avenues for the efficient control of this debilitating retroviral infection.

T-cell protein tyrosine phosphatase deletion results in progressive systemic inflammatory disease

The deregulation of the immune response is a critical component in inflammatory disease. Recent in vitro data show that T-cell protein tyrosine phosphatase (TC-PTP) is a negative regulator of cytokine signaling. Furthermore, tc-ptp(-/-) mice display immune defects and die within 5 weeks of birth. We report here that tc-ptp(-/-) mice develop progressive systemic inflammatory disease as shown by chronic myocarditis, gastritis, nephritis, and sialadenitis as well as elevated serum interferon-gamma. The widespread mononuclear cellular infiltrates correlate with exaggerated interferon-gamma, tumor necrosis factor-alpha, interleukin-12, and nitric oxide production in vivo. Macrophages grown from tc-ptp(-/-) mice are inherently hypersensitive to lipopolysaccharide, which can also be detected in vivo as an increased susceptibility to endotoxic shock. These results identify T-cell protein tyrosine phosphatase as a key modulator of inflammatory signals and macrophage function.

STAT proteins: from normal control of cellular events to tumorigenesis

Signal transducers and activators of transcription (STAT) proteins comprise a family of transcription factors latent in the cytoplasm that participate in normal cellular events, such as differentiation, proliferation, cell survival, apoptosis, and angiogenesis following cytokine, growth factor, and hormone signaling. STATs are activated by tyrosine phosphorylation, which is normally a transient and tightly regulates process. Nevertheless, several constitutively activated STATs have been observed in a wide number of human cancer cell lines and primary tumors, including blood malignancies and solid neoplasias. STATs can be divided into two groups according to their specific functions. One is made up of STAT2, STAT4, and STAT6, which are activated by a small number of cytokines and play a distinct role in the development of T-cells and in IFNgamma signaling. The other group includes STAT1, STAT3, and STAT5, activated in different tissues by means of a series of ligands and involved in IFN signaling, development of the mammary gland, response to GH, and embriogenesis. This latter group of STATS plays an important role in controlling cell-cycle progression and apoptosis and thus contributes to oncogenesis. Although an increased expression of STAT1 has been observed in many human neoplasias, this molecule can be considered a potential tumor suppressor, since it plays an important role in growth arrest and in promoting apoptosis. On the other hand, STAT3 and 5 are considered as oncogenes, since they bring about the activation of cyclin D1, c-Myc, and bcl-xl expression, and are involved in promoting cell-cycle progression, cellular transformation, and in preventing apoptosis.