Association of human protein-tyrosine phosphatase kappa with members of the armadillo family

Loss of LGR5 through Therapy-induced Downregulation or Gene Ablation Is Associated with Resistance and Enhanced MET-STAT3 Signaling in Colorectal Cancer Cells

Leucine-rich repeat-containing, G protein-coupled receptor 5 (LGR5) is highly expressed in colorectal cancer and cancer stem cells (CSCs) that play important roles in tumor initiation, progression, and metastasis. Loss of LGR5 has been shown to enhance therapy resistance. However, the molecular mechanisms that mediate this resistance remain elusive. In this study, we demonstrate conversion of LGR5+ colorectal cancer cells to an LGR5- state in response to chemotherapy, LGR5- targeted antibody-drug conjugates (ADCs), or LGR5 gene ablation led to activation of STAT3. Further investigation revealed increased STAT3 activation occurred as a result of increased mesenchymal epithelial transition (MET) factor receptor activity. LGR5 overexpression decreased MET-STAT3 activity and sensitized colorectal cancer cells to therapy. STAT3 inhibition suppressed MET phosphorylation, while constitutively active STAT3 reduced LGR5 levels and increased MET activity, suggesting a potential feedback mechanism. Combination treatment of MET-STAT3 inhibitors with irinotecan or antibody-drug conjugates (ADCs) substantiated synergistic effects in colorectal cancer cells and tumor organoids. In colorectal cancer xenografts, STAT3 inhibition combined with irinotecan enhanced tumor growth suppression and prolonged survival. These findings suggest a mechanism by which drug-resistant LGR5- colorectal cancer cells acquire a survival advantage through activation of MET-STAT3 and provide rationale for new treatment strategies to target colorectal cancer.

Identification and Functional Analysis of a Novel CTNNB1 Mutation in Pediatric Medulloblastoma

Simple Summary

We have analyzed a panel of 88 pediatric medulloblastoma tumors for exon 3 mutations from the CTNNB1 gene and identified eight missense point-mutations and one in-frame deletion. We describe and functionally characterize a novel CTNNB1 in-frame deletion (c.109-111del, pSer37del, ΔS37) found in a pediatric patient with a classic medulloblastoma, WNT-activated grade IV (WHO 2016). To the best of our knowledge, this mutation has not been previously reported in medulloblastoma, and it is uncertain its role in the disease development and progression. Our analysis discloses gain-of-function properties for the new ΔS37 β-catenin variant.

Abstract

Medulloblastoma is the primary malignant tumor of the Central Nervous System (CNS) most common in pediatrics. We present here, the histological, molecular, and functional analysis of a cohort of 88 pediatric medulloblastoma tumor samples. The WNT-activated subgroup comprised 10% of our cohort, and all WNT-activated patients had exon 3 CTNNB1 mutations and were immunostained for nuclear β-catenin. One novel heterozygous CTNNB1 mutation was found, which resulted in the deletion of β-catenin Ser37 residue (ΔS37). The ΔS37 β-catenin variant ectopically expressed in U2OS human osteosarcoma cells displayed higher protein expression levels than wild-type β-catenin, and functional analysis disclosed gain-of-function properties in terms of elevated TCF/LEF transcriptional activity in cells. Our results suggest that the stabilization and nuclear accumulation of ΔS37 β-catenin contributed to early medulloblastoma tumorigenesis.

The Impact of PTPRK and ROS1 Polymorphisms on the Preeclampsia Risk in Han Chinese Women

Objective

Preeclampsia (PE) is a severe complication in pregnancy and a leading cause of maternal and infant mortality. However, the exact underlying etiology of PE remains unknown. Emerging evidence indicates that the cause of PE is associated with genetic factors. Therefore, the aim of this study is to identify susceptibility genes to PE.

Materials and Methods

Human Exome BeadChip assays were conducted using 370 cases and 482 controls and 21 loci were discovered. A further independent set of 958 cases and 1007 controls were recruited for genotyping to determine whether the genes of interest ROS1 and PTPRK are associated with PE. Immunohistochemistry was used for localization. Both qPCR and Western blotting were utilized to investigate the levels of PTPRK in placentas of 20 PE and 20 normal pregnancies.

Results

The allele frequency of PTPRK rs3190930 differed significantly between PE and controls and was particularly significant in severe PE subgroup and early-onset PE subgroup. PTPRK is primarily localized in placental trophoblast cells. The mRNA and protein levels of PTPRK in PE were significantly higher than those in controls.

Conclusion

These results suggest that PTPRK appears to be a previously unrecognized susceptibility gene for PE in Han Chinese women, and its expression is also associated with PE, while ROS1 rs9489124 has no apparent correlation with PE risk.

Protein Tyrosine Phosphatases as Potential Regulators of STAT3 Signaling

The signal transducer and activator of transcription 3 (STAT3) protein is a major transcription factor involved in many cellular processes, such as cell growth and proliferation, differentiation, migration, and cell death or cell apoptosis. It is activated in response to a variety of extracellular stimuli including cytokines and growth factors. The aberrant activation of STAT3 contributes to several human diseases, particularly cancer. Consequently, STAT3-mediated signaling continues to be extensively studied in order to identify potential targets for the development of new and more effective clinical therapeutics. STAT3 activation can be regulated, either positively or negatively, by different posttranslational mechanisms including serine or tyrosine phosphorylation/dephosphorylation, acetylation, or demethylation. One of the major mechanisms that negatively regulates STAT3 activation is dephosphorylation of the tyrosine residue essential for its activation by protein tyrosine phosphatases (PTPs). There are seven PTPs that have been shown to dephosphorylate STAT3 and, thereby, regulate STAT3 signaling: PTP receptor-type D (PTPRD), PTP receptor-type T (PTPRT), PTP receptor-type K (PTPRK), Src homology region 2 (SH-2) domain-containing phosphatase 1(SHP1), SH-2 domain-containing phosphatase 2 (SHP2), MEG2/PTP non-receptor type 9 (PTPN9), and T-cell PTP (TC-PTP)/PTP non-receptor type 2 (PTPN2). These regulators have great potential as targets for the development of more effective therapies against human disease, including cancer.

The shaping, making and baking of a pancreatologist

The European Pancreatic Club Lifetime Achievement award is a distinction awarded for research on the pancreas. It comes with the obligation to submit a review article to the society's journal, Pancreatology. Since the research topics of my group have recently been covered in reviews and book chapters I want to use this opportunity to appraise the stations of my clinical and research education, the projects that I pursued and abandoned, the lessons I have learned from them, and the women and men who influenced my training and development as a physician scientist. Some crossed my path, some become collaborators and friends, and some turned into role models and had a lasting impact on my life.

Analysis of PTPRK polymorphisms in association with risk and age at onset of Alzheimer's disease, cancer risk, and cholesterol

The human receptor-type protein-tyrosine phosphatase kappa (PTPRK) gene is highly expressed in human brain and was previously associated with an increased risk of neuropsychiatric disorders and cancer. This study investigated the association of 52 single nucleotide polymorphisms (SNPs) in PTPRK with the risk and age at onset (AAO) of Alzheimer's disease (AD) in 791 AD patients and 782 controls. Our data analysis showed that five SNPs (top SNP rs4895829 with p = 0.0125) were associated with the risk of AD based on a multiple logistic regression (p < 0.05); while six SNPs (top SNP rs1891150 with p = 8.02 × 10^-6) were associated with AAO by using a multiple linear regression analysis. Interestingly, rs2326681 was associated with both the risk and AAO of AD (p = 4.65 × 10^-2 and 5.18 × 10^-3, respectively). In a replication study, the results from family-based association test - generalized estimating equation (GEE) statistics and Wilcoxon test showed that seven SNPs were associated with the risk of AD (top SNP rs11756545 with p = 1.02 × 10^-2) and 12 SNPs were associated with the AAO (top SNP rs11966128 with p = 1.39 × 10^-4), respectively. One additional sample showed that four SNPs were associated with risk of cancer (top SNP rs1339197 with p = 4.1 × 10^-3), 12 SNPs associated with LDL-cholesterol (top SNP rs4544930 with p = 3.47 × 10^-3), and eight SNPs associated with total cholesterol (top SNP rs1012049 with p = 6.09 × 10^-3). In addition, the AD associated rs4895829 was associated with the gene expression level in the cerebellum (p = 7.3 × 10^-5). The present study is the first study providing evidence of several genetic variants within the PTPRK gene associated with the risk and AAO of AD, risk of cancer, LDL and total cholesterol levels.

6q22.33 microdeletion in a family with intellectual disability, variable major anomalies, and behavioral abnormalities

Interstitial deletions on the long arm of chromosome six have been described for several regions including 6q16, 6q22.1, and 6q21q22.1, and with variable phenotypes such as intellectual disability/developmental delay, growth retardation, major and minor facial anomalies. However, an isolated microdeletion of the sub-band 6q22.33 has not been reported so far and thus, no information about the specific phenotype associated with such a copy number variant is available. Here, we define the clinical picture of an isolated 6q22.33 microdeletion based on the phenotype of six members of one family with loss of approximately 1 Mb in this region. Main clinical features include mild intellectual disability and behavioral abnormalities as well as microcephaly, heart defect, and cleft lip and palate.

Regulation of development and cancer by the R2B subfamily of RPTPs and the implications of proteolysis

The initial cloning of receptor protein tyrosine phosphatases (RPTPs) was met with excitement because of their hypothesized function in counterbalancing receptor tyrosine kinase signaling. In recent years, members of a subfamily of RPTPs with homophilic cell-cell adhesion capabilities, known as the R2B subfamily, have been shown to have functions beyond that of counteracting tyrosine kinase activity, by independently influencing cell signaling in their own right and by regulating cell adhesion. The R2B subfamily is composed of four members: PTPmu (PTPRM), PTPrho (PTPRT), PTPkappa (PTPRK), and PCP-2 (PTPRU). The effects of this small subfamily of RPTPs is far reaching, influencing several developmental processes and cancer. In fact, R2B RPTPs are predicted to be tumor suppressors and are among the most frequently mutated protein tyrosine phosphatases (PTPs) in cancer. Confounding these conclusions are more recent studies suggesting that proteolysis of the full-length R2B RPTPs result in oncogenic extracellular and intracellular protein fragments. This review discusses the current knowledge of the role of R2B RPTPs in development and cancer, with special detail given to the mechanisms and implications that proteolysis has on R2B RPTP function. We also touch upon the concept of exploiting R2B proteolysis to develop cancer imaging tools, and consider the effects of R2B proteolysis on axon guidance, perineural invasion and collective cell migration.

Receptor-type protein tyrosine phosphatase κ directly dephosphorylates CD133 and regulates downstream AKT activation

Although CD133 has been considered to be a molecular marker for cancer stem cells, its functional roles in tumorigenesis remain unclear. We here examined the molecular basis behind CD133-mediated signaling. Knockdown of CD133 resulted in the retardation of xenograft tumor growth of colon cancer-derived HT-29 and LoVo cells accompanied by hypophosphorylation of AKT, which diminished β-catenin/T-cell factor-mediated CD44 expression. As tyrosine residues of CD133 at positions 828 and 852 were phosphorylated in HT-29 and SW480 cells, we further addressed the significance of this phosphorylation in the tumorigenesis of SW480 cells expressing mutant CD133, with substitution of these tyrosine residues by glutamate (CD133-EE) or phenylalanine (CD133-FF). Forced expression of CD133-EE promoted much more aggressive xenograft tumor growth relative to wild-type CD133-expressing cells accompanied by hyperphosphorylation of AKT; however, CD133-FF expression had negligible effects on AKT phosphorylation and xenograft tumor formation. Intriguingly, the tyrosine phosphorylation status of CD133 was closely linked to the growth of SW480-derived spheroids. Using yeast two-hybrid screening, we finally identified receptor-type protein tyrosine phosphatase κ (PTPRK) as a binding partner of CD133. In vitro studies demonstrated that PTPRK associates with the carboxyl-terminal region of CD133 through its intracellular phosphatase domains and also catalyzes dephosphorylation of CD133 at tyrosine-828/tyrosine-852. Silencing of PTPRK elevated the tyrosine phosphorylation of CD133, whereas forced expression of PTPRK reduced its phosphorylation level markedly and abrogated CD133-mediated AKT phosphorylation. Endogenous CD133 expression was also closely associated with higher AKT phosphorylation in primary colon cancer cells, and ectopic expression of CD133 enhanced AKT phosphorylation. Furthermore, lower PTPRK expression significantly correlated with the poor prognosis of colon cancer patients with high expression of CD133. Thus, our present findings strongly indicate that the tyrosine phosphorylation of CD133, which is dephosphorylated by PTPRK, regulates AKT signaling and has a critical role in colon cancer progression.

RPTPα controls epithelial adherens junctions, linking E-cadherin engagement to c-Src-mediated phosphorylation of cortactin

Epithelial junctions are fundamental determinants of tissue organization, subject to regulation by tyrosine phosphorylation. Homophilic binding of E-cadherin activates tyrosine kinases, such as Src, that control junctional integrity. Protein tyrosine phosphatases (PTPs) also contribute to cadherin-based adhesion and signaling, but little is known about their specific identity or functions at epithelial junctions. Here, we report that the receptor PTP RPTPα (human gene name PTPRA) is recruited to epithelial adherens junctions at the time of cell-cell contact, where it is in molecular proximity to E-cadherin. RPTPα is required for appropriate cadherin-dependent adhesion and for cyst architecture in three-dimensional culture. Loss of RPTPα impairs adherens junction integrity, as manifested by defective E-cadherin accumulation and peri-junctional F-actin density. These effects correlate with a role for RPTPα in cellular (c)-Src activation at sites of E-cadherin engagement. Mechanistically, RPTPα is required for appropriate tyrosine phosphorylation of cortactin, a major Src substrate and a cytoskeletal actin organizer. Expression of a phosphomimetic cortactin mutant in RPTPα-depleted cells partially rescues F-actin and E-cadherin accumulation at intercellular contacts. These findings indicate that RPTPα controls cadherin-mediated signaling by linking homophilic E-cadherin engagement to cortactin tyrosine phosphorylation through c-Src.

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.

Receptor type protein tyrosine phosphatases (RPTPs) - roles in signal transduction and human disease

Protein tyrosine phosphorylation is a fundamental regulatory mechanism controlling cell proliferation, differentiation, communication, and adhesion. Disruption of this key regulatory mechanism contributes to a variety of human diseases including cancer, diabetes, and auto-immune diseases. Net protein tyrosine phosphorylation is determined by the dynamic balance of the activity of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Mammals express many distinct PTKs and PTPs. Both of these families can be sub-divided into non-receptor and receptor subtypes. Receptor protein tyrosine kinases (RPTKs) comprise a large family of cell surface proteins that initiate intracellular tyrosine phosphorylation-dependent signal transduction in response to binding of extracellular ligands, such as growth factors and cytokines. Receptor-type protein tyrosine phosphatases (RPTPs) are enzymatic and functional counterparts of RPTKs. RPTPs are a family of integral cell surface proteins that possess intracellular PTP activity, and extracellular domains that have sequence homology to cell adhesion molecules. In comparison to extensively studied RPTKs, much less is known about RPTPs, especially regarding their substrate specificities, regulatory mechanisms, biological functions, and their roles in human diseases. Based on the structure of their extracellular domains, the RPTP family can be grouped into eight sub-families. This article will review one representative member from each RPTP sub-family.

Receptor protein tyrosine phosphatases and cancer: new insights from structural biology

There is general agreement that many cancers are associated with aberrant phosphotyrosine signaling, which can be caused by the inappropriate activities of tyrosine kinases or tyrosine phosphatases. Furthermore, incorrect activation of signaling pathways has been often linked to changes in adhesion events mediated by cell surface receptors. Among these receptors, receptor protein tyrosine phosphatases (RPTPs) both antagonize tyrosine kinases as well as engage extracellular ligands. A recent wealth of data on this intriguing family indicates that its members can fulfill either tumor suppressing or oncogenic roles. The interpretation of these results at a molecular level has been greatly facilitated by the recent availability of structural information on the extra- and intracellular regions of RPTPs. These structures provide a molecular framework to understand how alterations in extracellular interactions can inactivate RPTPs in cancers or why the overexpression of certain RPTPs may also participate in tumor progression.

The lysyl oxidase propeptide interacts with the receptor-type protein tyrosine phosphatase kappa and inhibits β-catenin transcriptional activity in lung cancer cells

The propeptide region of the lysyl oxidase proenzyme (LOX-PP) has been shown to inhibit Ras signaling in NIH 3T3 and lung cancer cells with activated RAS, but its mechanism of action is poorly understood. Here, a yeast two-hybrid assay of LOX-PP-interacting proteins identified a clone encoding the intracellular phosphatase domains of receptor-type protein tyrosine phosphatase kappa (RPTP-κ), and the interaction of the two proteins in mammalian cells was confirmed. RPTP-κ is proteolytically processed to isoforms that have opposing effects on β-catenin activity. The RPTP-κ transmembrane P subunit interacts with and sequesters β-catenin at the cell membrane, where it can associate with E-cadherin and promote intercellular interactions. At high cell density, further processing of the P subunit yields a phosphatase intracellular portion (PIC) subunit, which chaperones β-catenin to the nucleus, where it can function to activate transcription. Lung cancer cells were found to contain higher PIC levels than untransformed lung epithelial cells. In H1299 lung cancer cells, ectopic LOX-PP expression reduced the nuclear levels of PIC by increasing its turnover in the lysosome, thereby decreasing the nuclear levels and transcriptional activity of β-catenin while increasing β-catenin membrane localization. Thus, LOX-PP is shown to negatively regulate pro-oncogenic β-catenin signaling in lung cancer cells.

The effect of receptor protein tyrosine phosphatase kappa on the change of cell adhesion and proliferation induced by N-acetylglucosaminyltransferase V

N-acetylglucosaminyltransferase V (GnT-V) has been reported to be positively associated with tumor progression, but its mechanism still remains unknown. In the present study, we found that GnT-V overexpression not only changed the glycosylation of receptor protein tyrosine phosphatase kappa (RPTPkappa) but also decreased its protein level. Moreover, GnT-V overexpression decreased cell calcium-independent adhesion and increased the tyrosine phosphorylation level of beta-catenin, in which RPTPkappa played an important role. Since RPTPkappa has an RXKR motif, which is a favored cleavage site for furin, we used furin inhibitor to further explore the effect of RPTPkappa on the change of cell adhesion and beta-catenin signaling induced by GnT-V. Our results showed that preventing RPTPkappa cleavage rescued the above effects of GnT-V, suggesting that furin cleavage could be one of the factors for RPTPkappa to regulate cell adhesion and beta-catenin signaling in GnT-V overexpression cell lines. In addition, the increased tyrosine phosphorylation level of beta-catenin was associated with the increased nuclear level of beta-catenin and downstream signaling molecules such as c-myc and cyclin D1 that were associated with cell proliferation. Our results suggest that GnT-V could decrease human hepatoma SMMC-7721 cell adhesion and promote cell proliferation partially through RPTPkappa.

Receptor type protein tyrosine phosphatase-kappa mediates cross-talk between transforming growth factor-beta and epidermal growth factor receptor signaling pathways in human keratinocytes

Epidermal growth factor receptor (EGFR) signaling pathways promote human keratinocyte survival and proliferation. In contrast, transforming growth factor-beta (TGF-beta) signaling pathways are strongly anti-proliferative. Receptor type protein tyrosine phosphatase-kappa (RPTP-kappa) specifically dephosphorylates EGFR, thereby blocking EGFR-dependent signaling, and inhibiting proliferation. We report here that RPTP-kappa mediates functional integration of EGFR and TGF-beta signaling pathways in human keratinocytes. TGF-beta up-regulates RPTP-kappa mRNA and protein, in a dose and time dependent manner. Induction of RPTP-kappa by TGF-beta significantly decreases basal and EGF-stimulated EGFR tyrosine phosphorylation. shRNA-mediated reduction of TGF-beta-induced RPTP-kappa significantly attenuates the ability of TGF-beta to inhibit proliferation. RPTP-kappa induction is dependent on activation of transcription factors Smad3 and Smad4. Inhibition of TGF-beta receptor kinase completely prevents induction of RPTP-kappa. Chromatin immunoprecipitation assays reveal that TGF-beta stimulates Smad3 and Smad4 binding to RPTP-kappa gene promoter. Smad3/4 binding is localized to an 186-base pair region, which contains a consensus Smad3-binding element. These data describe a novel mechanism of cross-talk between EGFR and TGF-beta pathways, in which RPTP-kappa functions to integrate growth-promoting and growth-inhibiting signaling pathways.

Protein-tyrosine phosphatase-kappa regulates CD4+ T cell development through ERK1/2-mediated signaling

T cells express diverse antigen-specific receptors and are required for eradicating pathogens and transformed cells. T cells expressing CD4 acquire helper effector functions and those expressing CD8 exert cytotoxic activity after antigen recognition. The protein-tyrosine phosphatase, receptor type kappa (PTPRKappa) is mutated in LEC rats, resulting in impaired CD4(+) T cell development in the thymus. However, the molecular mechanism of PTPRK controlling CD4(+) T cell development remains unclear. We demonstrate herein that inhibition of PTPRK by transducing a dominant negative form of the intracellular domain of PTPRK (PTPRK-ICD-DN) in bone marrow-derived stem cells suppresses the development of CD4(+) T cells. The inhibition of PTPRK by PTPRK-ICD-DN or short-hairpin RNA for PTPRK attenuates ERK1/2 phosphorylation in T cells after PMA and ionomycin stimulation. Total thymocytes from LEC rats also showed weaker phosphorylation of ERK1/2 after PMA and ionomycin stimulation than control thymocytes. Furthermore, inhibition of PTPRK by PTPRK-ICD-DN suppressed MEK1/2 and c-Raf phosphorylation, which is required for ERK1/2 phosphorylation. These data indicate that PPTRK positively regulates ERK1/2 phosphorylation, which impacts CD4(+) T cell development.

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.

Transient Gene Expression in Suspension HEK-293 Cells: Application to Large-Scale Protein Production

Recent advances in genomics, proteomics, and structural biology raised the general need for significant amounts of pure recombinant protein (r-protein). Because of the difficulty in obtaining in some cases proper protein folding in bacteria, several methods have been established to obtain large amounts of r-proteins by transgene expression in mammalian cells. We have developed three nonviral DNA transfer protocols for suspension-adapted HEK-293 and CHO cells: (1) a calcium phosphate based method (Ca-Pi), (2) a calcium-mediated method called Calfection, and (3) a polyethylenimine-based method (PEI). The first two methods have already been scaled up to 14 L and 100 L for HEK-293 cells in bioreactors. The third method, entirely serum-free, has been successfully applied to both suspension-adapted CHO and HEK-293 cells. We describe here the application of this technology to the transient expression in suspension cultivated HEK-293 EBNA cells of some out of more than 20 secreted r-proteins, including antibodies, dimeric proteins, and tagged proteins of various complexity. Most of the proteins were expressed from different plasmid vectors within 5-10 days after the availability of the DNA. Transfections were successfully performed from the small scale (1 mL in 12-well microtiter plates) to the 2 L scale. The results reported made it possible to establish an optimized cell culture and transfection protocol that minimizes batch-to-batch variations in protein expression. The work presented here proves the applicability and robustness of transient transfection technology for the expression of a variety of recombinant proteins.

Investigation of protein-tyrosine phosphatase 1B function by quantitative proteomics

Because of their antagonistic catalytic functions, protein-tyrosine phosphatases (PTPs) and protein-tyrosine kinases act together to control phosphotyrosine-mediated signaling processes in mammalian cells. However, unlike for protein-tyrosine kinases, little is known about the cellular substrate specificity of many PTPs because of the lack of appropriate methods for the systematic and detailed analysis of cellular PTP function. Even for the most intensely studied, prototypic family member PTP1B many of its physiological functions cannot be explained by its known substrates. To gain better insights into cellular PTP1B function, we used quantitative MS to monitor alterations in the global tyrosine phosphorylation of PTP1B-deficient mouse embryonic fibroblasts in comparison with their wild-type counterparts. In total, we quantified 124 proteins containing 301 phosphotyrosine sites under basal, epidermal growth factor-, or platelet-derived growth factor-stimulated conditions. A subset of 18 proteins was found to harbor hyperphosphorylated phosphotyrosine sites in knock-out cells and was functionally linked to PTP1B. Among these proteins, regulators of cell motility and adhesion are overrepresented, such as cortactin, lipoma-preferred partner, ZO-1, or p120ctn. In addition, regulators of proliferation like p62DOK or p120RasGAP also showed increased cellular tyrosine phosphorylation. Physical interactions of these proteins with PTP1B were further demonstrated by using phosphatase-inactive substrate-trapping mutants in a parallel MS-based analysis. Our results correlate well with the described phenotype of PTP1B-deficient fibroblasts that is characterized by an increase in motility and reduced cell proliferation. The presented study provides a broad overview about phosphotyrosine signaling processes in mouse fibroblasts and, supported by the identification of various new potential substrate proteins, indicates a central role of PTP1B within cellular signaling networks. Importantly the MS-based strategies described here are entirely generic and can be used to address the poorly understood aspects of cellular PTP function.

The receptor phosphatase HmLAR2 collaborates with focal adhesion proteins in filopodial tips to control growth cone morphology

Receptor protein tyrosine phosphatases (RPTPs) have been shown to play key roles in regulating axon guidance and synaptogenesis. HmLAR2, one of two closely related LAR-like RPTPs in the embryonic leech, is expressed in a few central neurons and in a unique segmentally-iterated peripheral cell, the comb cell (CC). Here we show that tagged HmLAR2-EGFP has a punctate pattern of expression in the growth cones of the CC, particularly at the tips of extending filopodia. Moreover, although expression of the wild-type EGFP-tagged receptor does not affect CC growth cone morphology, expression of a putative dominant-negative mutant of the receptor, CS-HmLAR2, leads to the enlargement of the growth cones, a shortening of filopodia, and errors in cellular tiling. RNAi of several candidate substrate signaling proteins, Lena (leech Ena/Vasp), beta-integrin and paxillin, but not beta-catenin, phenocopies particular aspects of the effects of HmLAR2 RNAi. For paxillin, which co-localizes with HmLAR2 at growth cone puncta, knock-down led to a reduction in the number of such puncta. Together, our data suggests that HmLAR2 regulates the morphology of the growth cone by controlling F-actin polymerization and focal adhesion complexes.

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.

Receptor protein tyrosine phosphatases are expressed by cycling retinal progenitor cells and involved in neuronal development of mouse retina

Receptor protein tyrosine phosphatases (RPTPs) appear to coordinate many aspects of neural development, including cell proliferation, migration and differentiation. Here we investigated potential roles of RPTPs in the developing mouse retina. Using a degenerate oligonucleotide-based reverse transcription polymerase chain reaction approach, we identified 11 different RPTPs in the retina at embryonic stage 13 (E13). Subsequently, the expression patterns of RPTPkappa, RPTPJ, RPTPRR, RPTPsigma, RPTPepsilon and RPTPgamma in the retina from embryonic stages to adult were analyzed in detail using quantitative real-time-PCR, in situ hybridization, immunohistochemistry and Western blotting. At E13, all six RPTPs are expressed in actively cycling retinal progenitor cells and postmitotic newborn retinal neurons. With ongoing maturation, RPTPkappa, RPTPJ, RPTPRR, RPTPsigma, RPTPepsilon and RPTPgamma display a different spatiotemporal regulation of mRNAs and proteins in the pre- and postnatal retina. Finally, in adulthood these six RPTPs localize to distinct cellular compartments of multiple retinal neurons. Additional studies in RPTPgamma(-/-) and RPTPbeta/zeta(-/-) (also known as PTPRZ1, RPTPbeta or RPTPzeta) mice at postnatal stage P1 reveal no apparent differences in retinal laminar organization or in the expression pattern of specific retinal cell-type markers when compared with wild type. However, in RPTPbeta/zeta(-/-) retinas, immunoreactivity of vimentin, a marker of Müller glial cells, is selectively reduced and the morphology of vimentin-immunoreactive radial processes of Müller cells is considerably disturbed. Our results suggest distinct roles of RPTPs in cell proliferation and establishing phenotypes of different retinal cells during retinogenesis as well as later in the maintenance of mature retina.

Outside-in signaling through integrins and cadherins: a central mechanism to control epidermal growth and differentiation?

The process of epidermal renewal persists throughout the entire life of an organism. It begins when a keratinocyte progenitor leaves the stem cell compartment, undergoes a limited number of mitotic divisions, exits the cell cycle, and commits to terminal differentiation. At the end of this phase, the postmitotic keratinocytes detach from the basement membrane to build up the overlaying stratified epithelium. Although highly coordinated, this sequence of events is endowed with a remarkable versatility, which enables the quiescent keratinocyte to reintegrate into the cell cycle and become migratory when necessary, for example after wounding. It is this versatility that represents the Achilles heel of epithelial cells allowing for the development of severe pathologies. Over the past decade, compelling evidence has been provided that epithelial cancer cells achieve uncontrolled proliferation following hijacking of a "survival program" with PI3K/Akt and a "proliferation program" with growth factor receptor signaling at its core. Recent insights into adhesion receptor signaling now propose that integrins, but also cadherins, can centrally control these programs. It is suggested that the two types of adhesion receptors act as sensors to transmit extracellular stimuli in an outside-in mode, to inversely modulate epidermal growth factor receptor signaling and ensure cell survival. Hence, cell-matrix and cell-cell adhesion receptors likely play a more powerful and wide-ranging role than initially anticipated. This Perspective article discusses the relevance of this emerging field for epidermal growth and differentiation, which can be of importance for severe pathologies such as tumorigenesis and invasive metastasis, as well as psoriasis and Pemphigus vulgaris.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

Structure of a tyrosine phosphatase adhesive interaction reveals a spacer-clamp mechanism

Cell-cell contacts are fundamental to multicellular organisms and are subject to exquisite levels of control. Human RPTPmu is a type IIB receptor protein tyrosine phosphatase that both forms an adhesive contact itself and is involved in regulating adhesion by dephosphorylating components of cadherin-catenin complexes. Here we describe a 3.1 angstrom crystal structure of the RPTPmu ectodomain that forms a homophilic trans (antiparallel) dimer with an extended and rigid architecture, matching the dimensions of adherens junctions. Cell surface expression of deletion constructs induces intercellular spacings that correlate with the ectodomain length. These data suggest that the RPTPmu ectodomain acts as a distance gauge and plays a key regulatory function, locking the phosphatase to its appropriate functional location.

Down-regulation of the TGF-beta target gene, PTPRK, by the Epstein-Barr virus encoded EBNA1 contributes to the growth and survival of Hodgkin lymphoma cells

The Epstein-Barr virus (EBV) contributes to the growth and survival of Hodgkin lymphoma (HL) cells. Here we report that down-regulation of the transforming growth factor-beta (TGF-beta) target gene, protein tyrosine phosphatase receptor kappa (PTPRK), followed EBV infection of HL cells and was also more frequently observed in the Hodgkin and Reed-Sternberg (HRS) cells of EBV-positive compared with EBV-negative primary HL. The viability and proliferation of EBV-positive HL cells was decreased by overexpression of PTPRK, but increased following the knockdown of PTPRK expression in EBV-negative HL cells, demonstrating that PTPRK is a functional tumor suppressor in HL. EBV suppressed the TGF-beta-mediated activation of PTPRK expression, suggesting disruption of TGF-beta signaling upstream of PTPRK. This was confirmed when we showed that the Epstein-Barr nuclear antigen-1 (EBNA1) decreased Smad2 protein levels and that this was responsible for PTPRK down-regulation. EBNA1 decreased the half-life of Smad2 but did not interact with Smad2. By down-regulating Smad2 protein expression, EBNA1 apparently disables TGF-beta signaling, which subsequently decreases transcription of the PTPRK tumor suppressor. We speculate that loss of the phosphatase function of PTPRK may activate as-yet-unidentified growth-promoting protein tyrosine kinases, which in turn contribute to the pathogenesis of EBV-positive HL.

Transcriptomal profiling of site-specific Ras signals

Ras proteins are distributed in distinct plasma-membrane microdomains and endomembranes. The biochemical signals generated by Ras therein differ qualitatively and quantitatively, but the extent to which this spatial variability impacts on the genetic program switched-on by Ras is unknown. We have used microarray technology to identify the transcriptional targets of localization-specific Ras subsignals in NIH3T3 cells expressing H-RasV12 selectively tethered to distinct cellular microenvironments. We report that the transcriptomes resulting from site-specific Ras activation show a significant overlap. However, distinct genetic signatures can also be found for each of the Ras subsignals. Our analyses unveil 121 genes uniquely regulated by Ras signals emanating from plasma-membrane microdomains. Interestingly, not a single gene is specifically controlled by lipid raft-anchored Ras. Furthermore, only 9 genes are exclusive for Ras signals from endomembranes. Also, we have identified 31 genes common to the site-specific Ras subsignals capable of inducing cellular transformation. Among these are the genes coding for Vitamin D receptor and for p120-GAP and we have assessed their impact in Ras-induced transformation. Overall, this report reveals the complexity and variability of the different genetic programs orchestrated by Ras from its main sublocalizations.

The homeodomain transcription factors Cdx1 and Cdx2 induce E-cadherin adhesion activity by reducing beta- and p120-catenin tyrosine phosphorylation

The homeodomain transcription factors Cdx1 and Cdx2 are regulators of intestine-specific gene expression. They also regulate intestinal cell differentiation and proliferation; however, these effects are poorly understood. Previously, we have shown that expression of Cdx1 or Cdx2 in human Colo 205 cells induces a mature colonocyte morphology characterized by the induction of a polarized, columnar shape with apical microvilli and strong cell-cell adhesion. To elucidate the mechanism underlying this phenomenon, we investigated the adherens junction complex. Cdx1 or Cdx2 expression reduced Colo 205 cell migration and invasion in vitro, suggesting a physiologically significant change in cadherin function. However, Cdx expression did not significantly effect E-cadherin, alpha-, beta-, or gamma-catenin, or p120-catenin protein levels. Additionally, no alteration in their intracellular distribution was observed. Cdx expression did not alter the coprecipitation of beta-catenin with E-cadherin; however, it did reduce p120-catenin-E-cadherin coprecipitation. Tyrosine phosphorylation of beta- and p120-catenin is known to disrupt E-cadherin-mediated cell adhesion and is associated with robust p120-catenin/E-cadherin interactions. We specifically investigated beta- and p120-catenin for tyrosine phosphorylation and found that it was significantly diminished by Cdx1 or Cdx2 expression. We restored beta- and p120-catenin tyrosine phosphorylation in Cdx2-expressing cells by knocking down the expression of protein tyrosine phosphatase 1B and noted a significant decline in cell-cell adhesion. We conclude that Cdx expression in Colo 205 cells induces E-cadherin-dependent cell-cell adhesion by reducing beta- and p120-catenin tyrosine phosphorylation. Ascertaining the mechanism for this novel Cdx effect may improve our understanding of the regulation of cell-cell adhesion in the colonic epithelium.

The role of protein tyrosine phosphorylation in the cell-cell interactions, junctional permeability and cell cycle control in post-confluent bovine corneal endothelial cells

Cell-cell interaction, junctional permeability and contact inhibition are important mechanisms that allow corneal endothelial cells to maintain stable corneal hydration status and also keep these cells in non-proliferative status. Protein tyrosine phosphatases (PTPs) are well known to play an important role in regulating cell-cell contacts, growth and differentiation. Inhibition of PTPs activity by a general PTP inhibitor has been proved to trigger post-confluent rat corneal endothelial cells to reenter cell cycles. In this study, we aimed to evaluate whether protein tyrosine phosphorylation is involved in cell-cell interactions, junctional permeability and cell cycle control in post-confluent, contact inhibited bovine corneal endothelial cells. Confluent cultures of bovine corneal endothelial cells were treated with different concentrations of general phosphatase inhibitor, sodium orthovanadate (vanadate) for several different durations. Protein tyrosine phosphorylation was confirmed by Western blot analysis. Immunocytochemistry was used to evaluate the effect of vanadate on adherens-type junctional proteins by staining of p120, N-cadherin and alpha-catenin. Paracelluar permeability was evaluated by transepithelial electric resistance. The effect of vanadate on cell cycle progression was confirmed by immunostaining of Ki67. Western blot analysis was used to evaluate the expression level of cell-cycle-associated proteins, including PCNA, cyclin D1, cyclin E and cyclin A. Time-dependent effects of vanadate on protein tyrosine phosphorylation were confirmed by Western blot analysis. ICC demonstrated the effect of vanadate on the disruption of p120, N-cadherin and alpha-catenin. Time- and dose-effects of vanadate on the severity of p120 disruption were also found. TER demonstrated the time- and dose-effect of vanadte on paracellular permeability. Although cell-cell junctions can be broken through by vanadate, no significant increase of Ki67 positive cells was found among the control group and all groups with different concentrations/durations of vanadate treatment. Western blot also showed no change of PCNA, cyclin D1, cyclin E and cyclin A after treatment with vanadate. In conclusion, protein tyrosine phosphatase inhibition can induce time-dependent release of cell-cell contacts and increase transepithelial permeability in post-confluent cultures of bovine corneal endothelial cells. However, such phenomenon is not enough to promoted cell cycle progression as seen in rat corneal endothelial cells.

Not so simple: the complexity of phosphotyrosine signaling at cadherin adhesive contacts

Cadherin cell-cell adhesion critically determines tissue organization and integrity in many organs of the body. Cadherin function influences patterning and morphogenesis while cadherin dysfunction contributes to disease, notably tumor invasion and metastasis. Cell signaling events are intimately linked with cadherin function; it is increasingly apparent that not only do cellular signals regulate cadherin function, but cadherins can also, in turn, modulate cell signaling itself. In this review, we discuss the complex interrelationship between phosphotyrosine-based cell signaling and cadherin adhesion. We focus on the interplay of events that occur at the cell surface and address three issues: the diverse mechanisms that activate phosphotyrosine signaling at cadherin cell-cell contacts, the functional impact of such signaling for cadherin adhesion, and the emerging capacity for cadherins to regulate growth factor signaling.

Reverse interactomics: decoding protein–protein interactions with combinatorial peptide libraries

Identification of binding partners is the crucial first step towards understanding the biological function of a protein. Many protein-protein interactions occur via modular domains that recognize short peptide motifs in their target proteins. Here we describe a chemical/bioinformatics approach for predicting the binding partners of modular domains. The optimal binding motif(s) of a protein domain is identified by screening a combinatorial peptide library. The resulting consensus sequence is used to search protein and genomic databases for potential binding proteins, which are subsequently confirmed (or disproved) by conventional protein binding assays (e.g. pull-down and co-immunoprecipitation).

N-cadherin is an in vivo substrate for protein tyrosine phosphatase sigma (PTPsigma) and participates in PTPsigma-mediated inhibition of axon growth

Protein tyrosine phosphatase sigma (PTPsigma) belongs to the LAR family of receptor tyrosine phosphatases and was previously shown to negatively regulate axon growth. The substrate for PTPsigma and the effector(s) mediating this inhibitory effect were unknown. Here we report the identification of N-cadherin as an in vivo substrate for PTPsigma. Using brain lysates from PTPsigma knockout mice, in combination with substrate trapping, we identified a hyper-tyrosine-phosphorylated protein of approximately 120 kDa in the knockout animals (relative to sibling controls), which was identified by mass spectrometry and immunoblotting as N-cadherin. beta-Catenin also precipitated in the complex and was also a substrate for PTPsigma. Dorsal root ganglion (DRG) neurons, which highly express endogenous N-cadherin and PTPsigma, exhibited a faster growth rate in the knockout mice than in the sibling controls when grown on laminin or N-cadherin substrata. However, when N-cadherin function was disrupted by an inhibitory peptide or lowering calcium concentrations, the differential growth rate between the knockout and sibling control mice was greatly diminished. These results suggest that the elevated tyrosine phosphorylation of N-cadherin in the PTPsigma(-/-) mice likely disrupted N-cadherin function, resulting in accelerated DRG nerve growth. We conclude that N-cadherin is a physiological substrate for PTPsigma and that N-cadherin (and likely beta-catenin) participates in PTPsigma-mediated inhibition of axon growth.

Intracellular substrates of brain-enriched receptor protein tyrosine phosphatase rho (RPTPrho/PTPRT)

Receptor protein tyrosine phosphatase rho (RPTPrho/PTPRT) is a transmembrane protein that is highly expressed in the developing and adult central nervous system. It is a member of the RPTP R2B subfamily, which includes PTPkappa, PTPmu and PCP-2. Glutathione-S-transferase (GST) pulldown assays were used to show that RPTPrho interacts with several adherens junctional proteins in brain, including E-cadherin, N-cadherin, VE-cadherin (cadherin-5), desmoglein, alpha, beta and gamma catenin, p120(ctn) and alpha-actinin. With the exception of E-cadherin and alpha-actinin, binding was considerably reduced at high sodium concentrations. Furthermore, immunoprecipitation phosphatase assays indicated that E-cadherin, and to a far lesser extent p120(ctn), were tyrosine dephosphorylated by a recombinant RPTPrho intracellular fragment, and thus, were likely to be primary substrates for RPTPrho. The interaction of RPTPrho with adherens junctional components suggests that this phosphatase may transduce extracellular signals to the actin cytoskeleton and thereby play a role in regulating cadherin-mediated cell adhesion in the central nervous system.

Furin-, ADAM 10-, and gamma-secretase-mediated cleavage of a receptor tyrosine phosphatase and regulation of beta-catenin's transcriptional activity

Several receptor protein tyrosine phosphatases (RPTPs) are cell adhesion molecules involved in homophilic interactions, suggesting that RPTP outside-in signaling is coupled to cell contact formation. However, little is known about the mechanisms by which cell density regulates RPTP function. We show that the MAM family prototype RPTPkappa is cleaved by three proteases: furin, ADAM 10, and gamma-secretase. Cell density promotes ADAM 10-mediated cleavage and shedding of RPTPkappa. This is followed by gamma-secretase-dependent intramembrane proteolysis of the remaining transmembrane part to release the phosphatase intracellular portion (PIC) from the membrane, thereby allowing its translocation to the nucleus. When cells were treated with leptomycin B, a nuclear export inhibitor, PIC accumulated in nuclear bodies. PIC is an active protein tyrosine phosphatase that binds to and dephosphorylates beta-catenin, an RPTPkappa substrate. The expression of RPTPkappa suppresses beta-catenin's transcriptional activity, whereas the expression of PIC increases it. Notably, this increase required the phosphatase activity of PIC. Thus, both isoforms have acquired opposing roles in the regulation of beta-catenin signaling. We also found that RPTPmu, another MAM family member, undergoes gamma-secretase-dependent processing. Our results identify intramembrane proteolysis as a regulatory switch in RPTPkappa signaling and implicate PIC in the activation of beta-catenin-mediated transcription.

The crystal structure of human receptor protein tyrosine phosphatase kappa phosphatase domain 1

The receptor-type protein tyrosine phosphatases (RPTPs) are integral membrane proteins composed of extracellular adhesion molecule-like domains, a single transmembrane domain, and a cytoplasmic domain. The cytoplasmic domain consists of tandem PTP domains, of which the D1 domain is enzymatically active. RPTPkappa is a member of the R2A/IIb subfamily of RPTPs along with RPTPmu, RPTPrho, and RPTPlambda. Here, we have determined the crystal structure of catalytically active, monomeric D1 domain of RPTPkappa at 1.9 A. Structural comparison with other PTP family members indicates an overall classical PTP architecture of twisted mixed beta-sheets flanked by alpha-helices, in which the catalytically important WPD loop is in an unhindered open conformation. Though the residues forming the dimeric interface in the RPTPmu structure are all conserved, they are not involved in the protein-protein interaction in RPTPkappa. The N-terminal beta-strand, formed by betax association with betay, is conserved only in RPTPs but not in cytosolic PTPs, and this feature is conserved in the RPTPkappa structure forming a beta-strand. Analytical ultracentrifugation studies show that the presence of reducing agents and higher ionic strength are necessary to maintain RPTPkappa as a monomer. In this family the crystal structure of catalytically active RPTPmu D1 was solved as a dimer, but the dimerization was proposed to be a consequence of crystallization since the protein was monomeric in solution. In agreement, we show that RPTPkappa is monomeric in solution and crystal structure.

Protein-tyrosine phosphatase PCP-2 inhibits beta-catenin signaling and increases E-cadherin-dependent cell adhesion

beta-Catenin is a key molecule involved in both cell adhesion and Wnt signaling pathway. However, the exact relationship between these two roles has not been clearly elucidated. Tyrosine phosphorylation of beta-catenin was shown to decrease its binding to E-cadherin, leading to decreased cell adhesion and increased beta-catenin signaling. We have previously shown that receptor-like protein-tyrosine phosphatase PCP-2 localizes to the adherens junctions and directly binds and dephosphorylates beta-catenin, suggesting that PCP-2 might regulate the balance between signaling and adhesive beta-catenin. Here we demonstrate that PCP-2 can inhibit both the wild-type and constitutively active forms of beta-catenin in activating target genes such as c-myc. The phosphatase activity of PCP-2 is required for this effect since loss of catalytic activity attenuates its inhibitory effect on beta-catenin activation. Expression of PCP-2 in SW480 colon cancer cells can lead to stabilization of cytosolic pools of beta-catenin perhaps, by virtue of their physical interaction. PCP-2 expression also leads to increased membrane-bound E-cadherin and greater stabilization of adherens junctions by dephosphorylation of beta-catenin, which could further sequester cytosolic beta-catenin and thus inhibit beta-catenin mediated nuclear signaling. Furthermore, SW480 cells stably expressing PCP-2 have a reduced ability to proliferate and migrate. Thus, PCP-2 may play an important role in the maintenance of epithelial integrity, and a loss of its regulatory function may be an alternative mechanism for activating beta-catenin signaling.

Regulation of cell adhesion by protein-tyrosine phosphatases: II. Cell-cell adhesion

Cell-cell adhesion is critical to the development and maintenance of multicellular organisms. The stability of many adhesions is regulated by protein tyrosine phosphorylation of cell adhesion molecules and their associated components, with high levels of phosphorylation promoting disassembly. The level of tyrosine phosphorylation reflects the balance between protein-tyrosine kinase and protein-tyrosine phosphatase activity. Many protein-tyrosine phosphatases associate with the cadherin-catenin complex, directly regulating the phosphorylation of these proteins, thereby affecting their interactions and the integrity of cell-cell junctions. Tyrosine phosphatases can also affect cell-cell adhesions indirectly by regulating the signaling pathways that control the activities of Rho family G proteins. In addition, receptor-type tyrosine phosphatases can mediate outside-in signaling through both ligand binding and dimerization of their extracellular domains. This review will discuss the role of protein-tyrosine phosphatases in cell-cell interactions, with an emphasis on cadherin-mediated adhesions.

TGFbeta-induced downregulation of E-cadherin-based cell-cell adhesion depends on PI3-kinase and PTEN

Transforming growth factor beta (TGFbeta) has profound growth-suppressive effects on normal epithelial cells, but supports metastasis formation in many tumour types. In most epithelial tumour cells TGFbeta(1) treatment results in epithelial dedifferentiation with reduced cell aggregation and enhanced cellular migration. Here we show that the epithelial dedifferentiation, accompanied by dissociation of the E-cadherin adhesion complex, induced by TGFbeta(1) depended on phosphatidylinositol 3-kinase (PI3-kinase) and the phosphatase PTEN as analysed in PANC-1 and Smad4-deficient BxPC-3 pancreatic carcinoma cells. TGFbeta(1) treatment enhanced tyrosine phosphorylation of alpha- and beta-catenin, which resulted in dissociation of the E-cadherin/catenin complex from the actin cytoskeleton and reduced cell-cell adhesion. The PI3-kinase and PTEN were found associated with the E-cadherin/catenin complex via beta-catenin. TGFbeta(1) treatment reduced the amount of PTEN bound to beta-catenin and markedly increased the tyrosine phosphorylation of beta-catenin. By contrast, forced expression of PTEN clearly reduced the TGFbeta(1)-induced phosphorylation of beta-catenin. The TGFbeta(1)-induced beta-catenin phosphorylation was also dependent on PI3-kinase and Ras activity. The described effects of TGFbeta(1) were independent of Smad4, which is homozygous deleted in BxPC-3 cells. Collectively, these data show that the TGFbeta(1)-induced destabilisation of E-cadherin-mediated cell-cell adhesion involves phosphorylation of beta-catenin, which is regulated by E-cadherin adhesion complex-associated PI3-kinase and PTEN.

Regulation of cell adhesion in the Drosophila embryo by phosphorylation of the cadherin-catenin-complex

Cell-culture studies indicate that tyrosine phosphorylation of the cadherin-catenin-complex (CCC) is one of the post-translational mechanism regulating E-cadherin-mediated cell adhesion. In this investigation, controlled application of a tyrosine phosphatase inhibitor (orthovanadate) and tyrosine kinase inhibitor (tyrphostin) to early Drosophila embryos, followed by biochemical assays and phenotypic analysis, has been utilized to address the mechanism by which tyrosine phosphorylation regulates E-cadherin-mediated cell adhesion in vivo. Our data suggest that, in the Drosophila embryo, beta-catenin (Drosophila homolog Armadillo) is the primary tyrosine-phosphorylated protein in the CCC. The increase in tyrosine phosphorylation correlates with a loss of epithelial integrity and adherens junctions in the ectoderm of early embryos. Late application of the phosphatase inhibitor does not have this effect, presumably because of the formation of septate junctions in late embryos. Co-immunoprecipitation assays have demonstrated that tyrosine hyper-phosphorylation does not cause the dissociation of Drosophila (D)E-cadherin and alpha-catenin or Armadillo, suggesting that abrogation in adhesion is most likely attributable to the detachment of actin-associated proteins from the CCC. Finally, although the Drosophila epidermal growth factor receptor (EGFR), a receptor tyrosine kinase, is linked to the CCC and shows genetic interactions with DE-cadherin, we find that a constitutively active Drosophila EGFR construct does not cause any detectable changes in the level of tyrosine phosphorylation of Armadillo or destabilization of the CCC.

The regulation of cadherin-mediated adhesion by tyrosine phosphorylation/dephosphorylation of beta-catenin

The formation of stable cell-cell adhesions by type I cadherins depends on the association of their cytoplasmic domain with beta-catenin, and of beta-catenin with alpha-catenin. The binding of beta-catenin to these partners is regulated by phosphorylation of at least three critical tyrosine residues. Each of these residues is targeted by one or more specific kinases: Y142 by Fyn, Fer and cMet; Y489 by Abl; and Y654 by Src and the epidermal growth factor receptor. Developmental and physiological signals have been identified that initiate the specific phosphorylation and dephosphorylation of these residues, regulating cadherin function during neurite outgrowth, permeability of airway epithelium and synapse remodeling, and possibly initiating epithelial cell migration during development and metastasis.

Receptor-type protein-tyrosine phosphatase-kappa regulates epidermal growth factor receptor function

Epidermal growth factor receptor (EGFR), the prototypic receptor protein tyrosine kinase, is a major regulator of growth and survival for many epithelial cell types. We report here that receptor-type protein-tyrosine phosphatase-kappa (RPTP-kappa) dephosphorylates EGFR and thereby regulates its function in human keratinocytes. Protein-tyrosine phosphatase (PTP) inhibitors induced EGFR tyrosine phosphorylation in intact primary human keratinocytes and cell-free membrane preparations. Five highly expressed RPTPs (RPTP-beta, delta, kappa, mu, and xi) were functionally analyzed in a Chinese hamster ovary (CHO) cell-based expression system. Full-length human EGFR expressed in CHO cells, which lack endogenous EGFR, displayed high basal (i.e. in the absence of ligand) tyrosine phosphorylation. Co-expression of RPTP-kappa, but not other RPTPs, specifically reduced basal EGFR tyrosine phosphorylation. RPTP-kappa also reduced epidermal growth factor-dependent EGFR tyrosine phosphorylation in CHO cells. Purified RPTP-kappa preferentially dephosphorylated EGFR tyrosines 1068 and 1173 in vitro. Overexpression of wild-type or catalytically inactive RPTP-kappa reduced or enhanced, respectively, basal and EGF-induced EGFR tyrosine phosphorylation in human keratinocytes. Furthermore, siRNA-mediated knockdown of RPTP-kappa increased basal and EGF-stimulated EGFR tyrosine phosphorylation and augmented downstream Erk activation in human keratinocytes. RPTP-kappa levels increased in keratinocytes as cells reached confluency, and overexpression of RPTP-kappa in subconfluent keratinocytes reduced keratinocyte proliferation. Taken together, the above data indicate that RPTP-kappa is a key regulator of EGFR tyrosine phosphorylation and function in human keratinocytes.

Extracellular cleavage of E-cadherin by leukocyte elastase during acute experimental pancreatitis in rats

BACKGROUND & AIMS

Cadherins play an important role in cell-cell contact formation at adherens junctions. During the course of acute pancreatitis, adherens junctions are known to dissociate-a requirement for the interstitial accumulation of fluid and inflammatory cells-but the underlying mechanism is unknown.

METHODS

Acute pancreatitis was induced in rats by supramaximal cerulein infusion. The pancreas and lungs were either homogenized for protein analysis or fixed for morphology. Protein sequencing was used to identify proteolytic cleavage sites and freshly prepared acini for ex vivo studies with recombinant proteases. Results were confirmed in vivo by treating experimental pancreatitis animals with specific protease inhibitors.

RESULTS

A 15-kilodalton smaller variant of E-cadherin was detected in the pancreas within 60 minutes of pancreatitis, was found to be the product of E-cadherin cleavage at amino acid 394 in the extracellular domain that controls cell-contact formation, and was consistent with E-cadherin cleavage by leukocyte elastase. Employing cell culture and ex vivo acini leukocyte elastase was confirmed to cleave E-cadherin at the identified position, followed by dissociation of cell contacts and the internalization of cleaved E-cadherin to the cytosol. Inhibition of leukocyte elastase in vivo prevented E-cadherin cleavage during pancreatitis and reduced leukocyte transmigration into the pancreas.

CONCLUSIONS

These data provide evidence that polymorphonuclear leukocyte elastase is involved in, and required for, the dissociation of cell-cell contacts at adherens junctions, the extracellular cleavage of E-cadherin, and, ultimately, the transmigration of leukocytes into the epithelial tissue during the initial phase of experimental pancreatitis.

G1 checkpoint failure and increased tumor susceptibility in mice lacking the novel p53 target Ptprv

In response to DNA damage, p53 activates a G1 cell cycle checkpoint, in part through induction of the cyclin-dependent kinase inhibitor p21(Waf1/Cip1). Here we report the identification of a new direct p53 target, Ptprv (or ESP), encoding a transmembrane tyrosine phosphatase. Ptprv transcription is dramatically and preferentially increased in cultured cells undergoing p53-dependent cell cycle arrest, but not in cells undergoing p53-mediated apoptosis. This observation was further confirmed in vivo using a Ptprv null-reporter mouse line. A p53-responsive element is present in the Ptprv promoter and p53 is recruited to this site in vivo. Importantly, while p53-dependent apoptosis is intact in mice lacking Ptprv, Ptprv-null fibroblasts and epithelial cells of the small intestine are defective in G1 checkpoint control. Thus, Ptprv is a new direct p53 target and a key mediator of p53-induced cell cycle arrest. Finally, Ptprv loss enhances the formation of epidermal papillomas after exposure to chemical carcinogens, suggesting that Ptprv acts to suppress tumor formation in vivo.

Transforming growth factor {beta} (TGF-{beta})-Smad target gene protein tyrosine phosphatase receptor type kappa is required for TGF-{beta} function

Transforming growth factor beta (TGF-beta) inhibits proliferation and promotes cell migration. In TGF-beta-treated MCF10A mammary epithelial cells overexpressing HER2 and by chromatin immunoprecipitation, we identified novel Smad targets including protein tyrosine phosphatase receptor type kappa (PTPRK). TGF-beta up-regulated PTPRK mRNA and RPTPkappa (receptor type protein tyrosine phosphatase kappa, the protein product encoded by the PTPRK gene) protein in tumor and nontumor mammary cells; HER2 overexpression down-regulated its expression. RNA interference (RNAi) of PTPRK accelerated cell cycle progression, enhanced response to epidermal growth factor (EGF), and abrogated TGF-beta-mediated antimitogenesis. Endogenous RPTPkappa associated with EGF receptor and HER2, resulting in suppression of basal and ErbB ligand-induced proliferation and receptor phosphorylation. In MCF10A/HER2 cells, TGF-beta enhanced cell motility, FAK phosphorylation, F-actin assembly, and focal adhesion formation and inhibited RhoA activity. These responses were abolished when RPTPkappa was eliminated by RNA interference (RNAi). In cells expressing RPTPkappa RNAi, phosphorylation of Src at Tyr527 was increased and (activating) phosphorylation of Src at Tyr416 was reduced. These data suggest that (i) RPTPkappa positively regulates Src; (ii) HER2 signaling and TGF-beta-induced RPTPkappa converge at Src, providing an adequate input for activation of FAK and increased cell motility and adhesion; and (iii) RPTPkappa is required for both the antiproliferative and the promigratory effects of TGF-beta.

Structural basis of interaction between protein tyrosine phosphatase PCP-2 and beta-catenin

PCP-2 is a member of receptor-like protein tyrosine phosphatase of the MAM domain family. To investigate which part of PCP-2 was involved in its interaction with beta-catenin, we constructed various deletion mutants of PCP-2. These PCP-2 mutants and wild-type PCP-2 were co-transfected into BHK-21 cells with beta-catenin individually. An in vivo binding assay revealed that the expression of wild-type PCP-2, PCP-2 deltaC1C2 (deleted PCP-2 without both PTP domains) and PCP-2 deltaC2 (deleted PCP-2 without the second PTP domain) could be immunoprecipitated by anti-catenin antibody in every co-transfection, but PCP-2 EXT (deleted PCP-2 without the juxtamembrane region and both PTP domains) was missing, which implied that PCP-2 and beta-catenin could associate directly and the juxtamembrane region in PCP-2 was sufficient for the process.

Receptor protein tyrosine phosphatase micro regulates the paracellular pathway in human lung microvascular endothelia

The pulmonary vascular endothelial paracellular pathway and zonula adherens (ZA) integrity are regulated, in part, through protein tyrosine phosphorylation. ZA-associated protein tyrosine phosphatase (PTP)s are thought to counterregulate tyrosine phosphorylation events within the ZA multiprotein complex. One such receptor PTP, PTPmu, is highly expressed in lung tissue and is almost exclusively restricted to the endothelium. We therefore studied whether PTPmu, in pulmonary vascular endothelia, associates with and/or regulates both the tyrosine phosphorylation state of vascular endothelial (VE)-cadherin and the paracellular pathway. PTPmu was expressed in postconfluent human pulmonary artery and lung microvascular endothelial cells (ECs) where it was almost exclusively restricted to EC-EC boundaries. In human lung microvascular ECs, knockdown of PTPmu through RNA interference dramatically impaired barrier function. In immortalized human microvascular ECs, overexpression of wild-type PTPmu enhanced barrier function. PTPmu-VE-cadherin interactions were demonstrated through reciprocal co-immunoprecipitation assays and co-localization with double-label fluorescence microscopy. When glutathione S-transferase-PTPmu was incubated with purified recombinant VE-cadherin, and when glutathione S-transferase-VE-cadherin was incubated with purified recombinant PTPmu, PTPmu directly bound to VE-cadherin. Overexpression of wild-type PTPmu decreased tyrosine phosphorylation of VE-cadherin. Therefore, PTPmu is expressed in human pulmonary vascular endothelia where it directly binds to VE-cadherin and regulates both the tyrosine phosphorylation state of VE-cadherin and barrier integrity.

Vascular endothelial-cadherin tyrosine phosphorylation in angiogenic and quiescent adult tissues

Vascular endothelial-cadherin (VE-cadherin) plays a key role in angiogenesis and in vascular permeability. The regulation of its biological activity may be a central mechanism in normal or pathological angiogenesis. VE-cadherin has been shown to be phosphorylated on tyrosine in vitro under various conditions, including stimulation by VEGF. In the present study, we addressed the question of the existence of a tyrosine phosphorylated form of VE-cadherin in vivo, in correlation with the quiescent versus angiogenic state of adult tissues. Phosphorylated VE-cadherin was detected in mouse lung, uterus, and ovary but not in other tissues unless mice were injected with peroxovanadate to block protein phosphatases. Remarkably, VE-cadherin tyrosine phosphorylation was dramatically increased in uterus and ovary, and not in other organs, during PMSG/hCG-induced angiogenesis. In parallel, we observed an increased association of VE-cadherin with Flk1 (VEGF receptor 2) during hormonal angiogenesis. Additionally, Src kinase was constitutively associated with VE-cadherin in both quiescent and angiogenic tissues and increased phosphorylation of VE-cadherin-associated Src was detected in uterus and ovary after hormonal treatment. Src-VE-cadherin association was detected in cultured endothelial cells, independent of VE-cadherin phosphorylation state and Src activation level. In this model, Src inhibition impaired VEGF-induced VE-cadherin phosphorylation, indicating that VE-cadherin phosphorylation was dependent on Src activation. We conclude that VE-cadherin is a substrate for tyrosine kinases in vivo and that its phosphorylation, together with that of associated Src, is increased by angiogenic stimulation. Physical association between Flk1, Src, and VE-cadherin may thus provide an efficient mechanism for amplification and perpetuation of VEGF-stimulated angiogenic processes.