The transmembranal and cytoplasmic forms of protein tyrosine phosphatase epsilon physically associate with the adaptor molecule Grb2

Role of Protein Tyrosine Phosphatase Epsilon (PTPε) in Leukotriene D4-Induced CXCL8 Expression

Phosphorylation on tyrosine residues is recognized as an important mechanism for connecting extracellular stimuli to cellular events and defines a variety of physiologic responses downstream of G protein-coupled receptor (GPCR) activation. To date, few protein tyrosine phosphatases (PTPs) have been shown to associate with GPCRs, and little is known about their role in GPCR signaling. To discover potential cysteinyl-leukotriene receptor (CysLT₁R)-interacting proteins, we identified protein tyrosine phosphatase ε (PTPε) in a yeast two-hybrid assay. Since both proteins are closely linked to asthma, we further investigated their association. Using a human embryonic kidney cell line 293 (HEK-293) cell line stably transfected with the receptor (HEK-LT1), as well as human primary monocytes, we found that PTPε colocalized with CysLT₁R in both resting and leukotriene D4 (LTD₄)-stimulated cells. Cotransfection of HEK-LT1 with PTPε had no effect on CysLT₁R expression or LTD₄-induced internalization, but it inhibited LTD₄-induced CXC chemokine 8 (CXCL8) promoter transactivation, protein expression, and secretion. Moreover, reduced phosphorylation of extracellular signal regulated kinase 1/2 (ERK1/2), but not of p38 or c-Jun-N-terminal kinase 1 or 2 mitogen-activated protein kinases (MAPKs), was observed upon LTD₄ stimulation of HEK-LT1 coexpressing cytosolic (cyt-) PTPε, but not receptor (R) PTPε The increased interaction of cyt-PTPε and ERK1/2 after LTD₄ stimulation was shown by coimmunoprecipitation. In addition, enhanced ERK1/2 phosphorylation and CXCL8 secretion were found in LTD₄-stimulated human monocytes transfected with PTPε-specific siRNAs, adding support to a regulatory/inhibitory role of PTPε in CysLT₁R signaling. Given that the prevalence of severe asthma is increasing, the identification of PTPε as a new potential therapeutic target may be of interest.

Phosphorylation of the phosphatase PTPROt at Tyr399 is a molecular switch that controls osteoclast activity and bone mass in vivo

Bone resorption by osteoclasts is essential for bone homeostasis. The kinase Src promotes osteoclast activity and is activated in osteoclasts by the receptor-type tyrosine phosphatase PTPROt. In other contexts, however, PTPROt can inhibit Src activity. Through in vivo and in vitro experiments, we show that PTPROt is bifunctional and can dephosphorylate Src both at its inhibitory residue Tyr⁵²⁷ and its activating residue Tyr⁴¹⁶ Whereas wild-type and PTPROt knockout mice exhibited similar bone masses, mice in which a putative C-terminal phosphorylation site, Tyr³⁹⁹, in endogenous PTPROt was replaced with phenylalanine had increased bone mass and reduced osteoclast activity. Osteoclasts from the knock-in mice also showed reduced Src activity. Experiments in cultured cells and in osteoclasts derived from both mouse strains demonstrated that the absence of phosphorylation at Tyr³⁹⁹ caused PTPROt to dephosphorylate Src at the activating site pTyr⁴¹⁶ In contrast, phosphorylation of PTPROt at Tyr³⁹⁹ enabled PTPROt to recruit Src through Grb2 and to dephosphorylate Src at the inhibitory site Tyr⁵²⁷, thus stimulating Src activity. We conclude that reversible phosphorylation of PTPROt at Tyr³⁹⁹ is a molecular switch that selects between its opposing activities toward Src and maintains a coherent signaling output, and that blocking this phosphorylation event can induce physiological effects in vivo. Because most receptor-type tyrosine phosphatases contain potential phosphorylation sites at their C termini, we propose that preventing phosphorylation at these sites or its consequences may offer an alternative to inhibiting their catalytic activity to achieve therapeutic benefit.

Tuning the Protein Phosphorylation by Receptor Type Protein Tyrosine Phosphatase Epsilon (PTPRE) in Normal and Cancer Cells

Tyrosine phosphorylation is an important post-translation modification of proteins that is controlled by tyrosine kinases and phosphatases. Disruption of the balance between the activity of tyrosine kinases and phosphatases may result in diseases. Receptor type protein tyrosine phosphatase epsilon (PTPRE) is closely related with receptor type protein tyrosine phosphatase alpha (PTPRA). PTPRE has been studied in osteoclast cells, nerve cells, hematopoietic cells, cancer cells and others, and it has different functions among various tissues. In this review, we summarized the current knowledge about the regulation of PTPRE on cellular signal transduction and its function under normal and pathological conditions.

Regulation of receptor-type protein tyrosine phosphatases by their C-terminal tail domains

Protein tyrosine phosphatases (PTPs) perform specific functions in vivo, despite being vastly outnumbered by their substrates. Because of this and due to the central roles PTPs play in regulating cellular function, PTP activity is regulated by a large variety of molecular mechanisms. We review evidence that indicates that the divergent C-terminal tail sequences (C-terminal domains, CTDs) of receptor-type PTPs (RPTPs) help regulate RPTP function by controlling intermolecular associations in a way that is itself subject to physiological regulation. We propose that the CTD of each RPTP defines an 'interaction code' that helps determine molecules it will interact with under various physiological conditions, thus helping to regulate and diversify PTP function.

Hepatitis C virus infection inhibits a Src-kinase regulatory phosphatase and reduces T cell activation in vivo

Among human RNA viruses, hepatitis C virus (HCV) is unusual in that it causes persistent infection in the majority of infected people. To establish persistence, HCV evades host innate and adaptive immune responses by multiple mechanisms. Recent studies identified virus genome-derived small RNAs (vsRNAs) in HCV-infected cells; however, their biological significance during human HCV infection is unknown. One such vsRNA arising from the hepatitis C virus (HCV) E2 coding region impairs T cell receptor (TCR) signaling by reducing expression of a Src-kinase regulatory phosphatase (PTPRE) in vitro. Since TCR signaling is a critical first step in T cell activation, differentiation, and effector function, its inhibition may contribute towards HCV persistence in vivo. The effect of HCV infection on PTPRE expression in vivo has not been examined. Here, we found that PTPRE levels were significantly reduced in liver tissue and peripheral blood mononuclear cells (PBMCs) obtained from HCV-infected humans compared to uninfected controls. Loss of PTPRE expression impaired antigen-specific TCR signaling, and curative HCV therapy restored PTPRE expression in PBMCs; restoring antigen-specific TCR signaling defects. The extent of PTPRE expression correlated with the amount of sequence complementarity between the HCV E2 vsRNA and the PTPRE 3' UTR target sites. Transfection of a hepatocyte cell line with full-length HCV RNA or with synthetic HCV vsRNA duplexes inhibited PTPRE expression, recapitulating the in vivo observation. Together, these data demonstrate that HCV infection reduces PTPRE expression in the liver and PBMCs of infected humans, and suggest that the HCV E2 vsRNA is a novel viral factor that may contribute towards viral persistence.

Conserved Motifs within Hepatitis C Virus Envelope (E2) RNA and Protein Independently Inhibit T Cell Activation

T cell receptor (TCR) signaling is required for T-cell activation, proliferation, differentiation, and effector function. Hepatitis C virus (HCV) infection is associated with impaired T-cell function leading to persistent viremia, delayed and inconsistent antibody responses, and mild immune dysfunction. Although multiple factors appear to contribute to T-cell dysfunction, a role for HCV particles in this process has not been identified. Here, we show that incubation of primary human CD4+ and CD8+ T-cells with HCV RNA-containing serum, HCV-RNA containing extracellular vesicles (EVs), cell culture derived HCV particles (HCVcc) and HCV envelope pseudotyped retrovirus particles (HCVpp) inhibited TCR-mediated signaling. Since HCVpp’s contain only E1 and E2, we examined the effect of HCV E2 on TCR signaling pathways. HCV E2 expression recapitulated HCV particle-induced TCR inhibition. A highly conserved, 51 nucleotide (nt) RNA sequence was sufficient to inhibit TCR signaling. Cells expressing the HCV E2 coding RNA contained a short, virus-derived RNA predicted to be a Dicer substrate, which targeted a phosphatase involved in Src-kinase signaling (PTPRE). T-cells and hepatocytes containing HCV E2 RNA had reduced PTPRE protein levels. Mutation of 6 nts abolished the predicted Dicer interactions and restored PTPRE expression and proximal TCR signaling. HCV RNA did not inhibit distal TCR signaling induced by PMA and Ionomycin; however, HCV E2 protein inhibited distal TCR signaling. This inhibition required lymphocyte-specific tyrosine kinase (Lck). Lck phosphorylated HCV E2 at a conserved tyrosine (Y613), and phospho-E2 inhibited nuclear translocation of NFAT. Mutation of Y613 restored distal TCR signaling, even in the context of HCVpps. Thus, HCV particles delivered viral RNA and E2 protein to T-cells, and these inhibited proximal and distal TCR signaling respectively. These effects of HCV particles likely aid in establishing infection and contribute to viral persistence.

Globally, approximately 200 million people are persistently infected with Hepatitis C virus (HCV). Mechanisms by which HCV establishes persistent infection are complex, and several host and viral factors appear to contribute to the ability of HCV to evade immune clearance. T cell activation through the T cell receptor (TCR) is an essential first step in the generation of an adaptive immune response. Although HCV infection is associated with impaired T cell function, the mechanisms for this dysfunction are poorly understood. Here, we demonstrate that HCV particles inhibit T cell activation by interfering with proximal and distal signals that are triggered by activation through the TCR. First, HCV envelope (E2) RNA was processed into a small RNA that targeted a regulatory phosphatase, inhibiting proximal TCR signaling. Second, the lymphocyte specific Src kinase (Lck) phosphorylated HCV E2 at tyrosine 613 (Y613), and phospho-E2 inhibited nuclear translocation of activated NFAT, reducing distal TCR activation signals. The RNA and protein motifs involved are highly conserved among all HCV isolates, and mutation restored TCR signaling. Thus, HCV particles interfere with TCR signaling and impair T cell activation using two distinct mechanisms. This may contribute to HCV persistence and T cell dysfunction during HCV infection.

Adaptor protein GRB2 promotes Src tyrosine kinase activation and podosomal organization by protein-tyrosine phosphatase ϵ in osteoclasts

The non-receptor isoform of protein-tyrosine phosphatase ϵ (cyt-PTPe) supports adhesion of bone-resorbing osteoclasts by activating Src downstream of integrins. Loss of cyt-PTPe reduces Src activity in osteoclasts, reduces resorption of mineralized matrix both in vivo and in cell culture, and induces mild osteopetrosis in young female PTPe KO mice. Activation of Src by cyt-PTPe is dependent upon this phosphatase undergoing phosphorylation at its C-terminal Tyr-638 by partially active Src. To understand how cyt-PTPe activates Src, we screened 73 Src homology 2 (SH2) domains for binding to Tyr(P)-638 of cyt-PTPe. The SH2 domain of GRB2 bound Tyr(P)-638 of cyt-PTPe most prominently, whereas the Src SH2 domain did not bind at all, suggesting that GRB2 may link PTPe with downstream molecules. Further studies indicated that GRB2 is required for activation of Src by cyt-PTPe in osteoclast-like cells (OCLs) in culture. Overexpression of GRB2 in OCLs increased activating phosphorylation of Src at Tyr-416 and of cyt-PTPe at Tyr-638; opposite results were obtained when GRB2 expression was reduced by shRNA or by gene inactivation. Phosphorylation of cyt-PTPe at Tyr-683 and its association with GRB2 are integrin-driven processes in OCLs, and cyt-PTPe undergoes autodephosphorylation at Tyr-683, thus limiting Src activation by integrins. Reduced GRB2 expression also reduced the ability of bone marrow precursors to differentiate into OCLs and reduced the fraction of OCLs in which podosomal adhesion structures assume organization typical of active, resorbing cells. We conclude that GRB2 physically links cyt-PTPe with Src and enables cyt-PTPe to activate Src downstream of activated integrins in OCLs.

Protein tyrosine phosphatase epsilon affects body weight by downregulating leptin signaling in a phosphorylation-dependent manner

Molecular-level understanding of body weight control is essential for combating obesity. We show that female mice lacking tyrosine phosphatase epsilon (RPTPe) are protected from weight gain induced by high-fat food, ovariectomy, or old age and exhibit increased whole-body energy expenditure and decreased adiposity. RPTPe-deficient mice, in particular males, exhibit improved glucose homeostasis. Female nonobese RPTPe-deficient mice are leptin hypersensitive and exhibit reduced circulating leptin concentrations, suggesting that RPTPe inhibits hypothalamic leptin signaling in vivo. Leptin hypersensitivity persists in aged, ovariectomized, and high-fat-fed RPTPe-deficient mice, indicating that RPTPe helps establish obesity-associated leptin resistance. RPTPe associates with and dephosphorylates JAK2, thereby downregulating leptin receptor signaling. Leptin stimulation induces phosphorylation of hypothalamic RPTPe at its C-terminal Y695, which drives RPTPe to downregulate JAK2. RPTPe is therefore an inhibitor of hypothalamic leptin signaling in vivo, and provides controlled negative-feedback regulation of this pathway following its activation.

Protein tyrosine phosphatase epsilon regulates integrin-mediated podosome stability in osteoclasts by activating Src

The nonreceptor isoform of tyrosine phosphatase epsilon (cyt-PTPe) supports osteoclast adhesion and activity in vivo, leading to increased bone mass in female mice lacking PTPe (EKO mice). The structure and organization of the podosomal adhesion structures of EKO osteoclasts are abnormal; the molecular mechanism behind this is unknown. We show here that EKO podosomes are disorganized, unusually stable, and reorganize poorly in response to physical contact. Phosphorylation and activities of Src, Pyk2, and Rac are decreased and Rho activity is increased in EKO osteoclasts, suggesting that integrin signaling is defective in these cells. Integrin activation regulates cyt-PTPe by inducing Src-dependent phosphorylation of cyt-PTPe at Y638. This phosphorylation event is crucial because wild-type-but not Y638F-cyt-PTPe binds and further activates Src and restores normal stability to podosomes in EKO osteoclasts. Increasing Src activity or inhibiting Rho or its downstream effector Rho kinase in EKO osteoclasts rescues their podosomal stability phenotype, indicating that cyt-PTPe affects podosome stability by functioning upstream of these molecules. We conclude that cyt-PTPe participates in a feedback loop that ensures proper Src activation downstream of integrins, thus linking integrin signaling with Src activation and accurate organization and stability of podosomes in osteoclasts.

PTPepsilon has a critical role in signaling transduction pathways and phosphoprotein network topology in red cells

Protein tyrosine phosphatases (PTPs) are crucial components of cellular signal transduction pathways. Here, we report that red blood cells (RBCs) from mice lacking PTPepsilon (Ptpre(-/-)) exhibit (i) abnormal morphology; (ii) increased Ca(2+)-activated-K(+) channel activity, which was partially blocked by the Src family kinases (SFKs) inhibitor PP1; and (iii) market perturbation of the RBC membrane tyrosine (Tyr-) phosphoproteome, indicating an alteration of RBC signal transduction pathways. Using the signaling network computational analysis of the Tyr-phosphoproteomic data, we identified seven topological clusters. We studied cluster 1 containing Fyn, SFK, and Syk another tyrosine kinase. In Ptpre(-/-)mouse RBCs, the activity of Fyn was increased while Syk kinase activity was decreased compared to wild-type RBCs, validating the network computational analysis, and indicating a novel signaling pathway, which involves Fyn and Syk in regulation of red cell morphology.

Protein-tyrosine phosphatase epsilon regulates Shc signaling in a kinase-specific manner: increasing coherence in tyrosine phosphatase signaling

Individual protein tyrosine kinases and phosphatases target multiple substrates; this may generate conflicting signals, possibly within a single pathway. Protein-tyrosine phosphatase epsilon (PTPepsilon) performs two potentially opposing roles: in Neu-induced mammary tumors, PTPepsilon activates Src downstream of Neu, whereas in other systems PTPepsilon can indirectly down-regulate MAP kinase signaling. We now show that the latter effect is mediated at least in part via the adaptor protein Shc. PTPepsilon binds and dephosphorylates Shc in vivo, reducing the association of Shc with Grb2 and inhibiting downstream ERK activation. PTPepsilon binds Shc in a phosphotyrosine-independent manner mediated by the Shc PTB domain and aided by a sequence of 10 N-terminal residues in PTPepsilon. Surprisingly, PTPepsilon dephosphorylates Shc in a kinase-dependent manner; PTPepsilon targets Shc in the presence of Src but not in the presence of Neu. Using a series of point mutants of Shc and Neu, we show that Neu protects Shc from dephosphorylation by binding the PTB domain of Shc, most likely competing against PTPepsilon for binding the same domain. In agreement, PTPepsilon dephosphorylates Shc in mouse embryo fibroblasts but not in Neu-induced mammary tumor cells. We conclude that in the context of Neu-induced mammary tumor cells, Neu prevents PTPepsilon from targeting Shc and from reducing its promitogenic signal while phosphorylating PTPepsilon and directing it to activate Src in support of mitogenesis. In so doing, Neu contributes to the coherence of the promitogenic role of PTPepsilon in this system.

Quantitative time-resolved phosphoproteomic analysis of mast cell signaling

Mast cells play a central role in type I hypersensitivity reactions and allergic disorders such as anaphylaxis and asthma. Activation of mast cells, through a cascade of phosphorylation events, leads to the release of mediators of the early phase allergic response. Understanding the molecular architecture underlying mast cell signaling may provide possibilities for therapeutic intervention in asthma and other allergic diseases. Although many details of mast cell signaling have been described previously, a systematic, quantitative analysis of the global tyrosine phosphorylation events that are triggered by activation of the mast cell receptor is lacking. In many cases, the involvement of particular proteins in mast cell signaling has been established generally, but the precise molecular mechanism of the interaction between known signaling proteins often mediated through phosphorylation is still obscure. Using recently advanced methodologies in mass spectrometry, including automation of phosphopeptide enrichments and detection, we have now substantially characterized, with temporal resolution as short as 10 s, the sites and levels of tyrosine phosphorylation across 10 min of FcepsilonRI-induced mast cell activation. These results reveal a far more extensive array of tyrosine phosphorylation events than previously known, including novel phosphorylation sites on canonical mast cell signaling molecules, as well as unexpected pathway components downstream of FcepsilonRI activation. Furthermore, our results, for the first time in mast cells, reveal the sequence of phosphorylation events for 171 modification sites across 121 proteins in the MCP5 mouse mast cell line and 179 modification sites on 117 proteins in mouse bone marrow-derived mast cells.

Neu-mediated phosphorylation of protein tyrosine phosphatase epsilon is critical for activation of Src in mammary tumor cells

The receptor-type protein tyrosine phosphatase epsilon (RPTPepsilon) activates c-Src in mammary tumor cells induced in vivo by Neu. Tumor cells lacking RPTPepsilon exhibit reduced c-Src activity, appear less transformed morphologically and proliferate slower in vitro and in vivo. Expression of Src rescues most of these phenotypes, indicating that c-Src activity is important for maintaining the transformed phenotype. However, the molecular mechanisms that control activation of c-Src by RPTPepsilon are unknown. We show that Neu induces phosphorylation of RPTPepsilon exclusively at its C-terminal Y695, and that this phosphorylation is required for activation of c-Src by RPTPepsilon. Phosphorylation of RPTPepsilon does not affect its activity toward another substrate, the voltage-gated potassium channel Kv2.1, suggesting that phosphorylation directs RPTPepsilon activity toward c-Src. Phosphorylation of RPTPepsilon reduces its dimerization at the cell membrane, although this does not affect its activity significantly. RPTPepsilon is subject to strong auto- and trans-dephosphorylation, suggesting that dephosphorylation limits the activation of c-Src downstream of Neu. We conclude that an Neu-RPTPepsilon-Src signaling pathway exists in mammary tumor cells, in which phosphorylation of RPTPepsilon by Neu directs RPTPepsilon to activate c-Src. Reversible phosphorylation of RPTPepsilon at Y695 may thus function as a 'molecular switch', which affects the substrate specificity of the phosphatase.

Characterization of Heparin Affin Regulatory Peptide Signaling in Human Endothelial Cells

Heparin affin regulatory peptide (HARP) is an 18-kDa secreted growth factor that has a high affinity for heparin and a potent role on tumor growth and angiogenesis. We have previously reported that HARP is mitogenic for different types of endothelial cells and also affects cell migration and differentiation (12). In this study we examined the signaling pathways involved in the migration and tube formation on matrigel of human umbilical vein endothelial cells (HUVEC) induced by HARP. We report for the first time that receptor-type protein-tyrosine phosphatase beta/zeta (RPTPbeta/zeta), which is a receptor for HARP in neuronal cell types, is also expressed in HUVEC. We also document that HARP signaling through RPTPbeta/zeta leads to activation of Src kinase, focal adhesion kinase, phosphatidylinositol 3-kinase, and Erk1/2. Sodium orthovanadate, chondroitin sulfate-C, PP1, wortmannin, LY294002, and U0126 inhibit HARP-mediated signaling and HUVEC migration and tube formation. In addition, RPTPbeta/zeta suppression using small interfering RNA technology interrupts intracellular signals and HUVEC migration and tube formation induced by HARP. These results establish the role of RPTPbeta/zeta as a receptor of HARP in HUVEC and elucidate the HARP signaling pathway in endothelial cells.

Tyrosine phosphatase epsilonM stimulates migration and survival of porcine aortic endothelial cells by activating c-Src

The cell growth, survival, and migration of vascular endothelial cells (ECs) are positively regulated by several protein tyrosine kinase receptors. Therefore, protein tyrosine phosphatases (PTPs) must also be important for these processes. The present study found that transmembranal PTPepsilonM, but not cytoplasmic PTPepsilonC, is expressed in porcine ECs and in rat smooth muscle cells, both of which were prepared from the aorta. The overexpression of wild-type PTPepsilonM promoted cell survival and migration in porcine aortic ECs even in medium without and with 1% serum, respectively. A catalytically inactive, substrate-trapping mutant of PTPepsilonM, respectively, did not affect and conversely suppressed cell survival and migration. Interestingly, the forced expression of wild-type PTPepsilonC reduced cell viability in contrast to PTPepsilonM in ECs lacking endogenous PTPepsilonC, indicating the biological significance of selective expression of PTPepsilon isoforms in the vasculature. PTPepsilonM activated c-Src kinase probably by directly dephosphorylating phospho-Tyr527, a negative regulatory site of c-Src. The increases in cell survival and migration induced by overexpressed PTPepsilonM were suppressed by the c-Src inhibitor SU6656. Considering the behaviors of vascular ECs in the pathogenesis of atherosclerosis, these data suggest that PTPepsilonM negatively regulates the development of this disease by activating c-Src.

Phosphorylation-dependent regulation of Kv2.1 Channel activity at tyrosine 124 by Src and by protein-tyrosine phosphatase epsilon

Voltage-gated potassium (Kv) channels are a complex and heterogeneous family of proteins that play major roles in brain and cardiac excitability. Although Kv channels are activated by changes in cell membrane potential, tyrosine phosphorylation of channel subunits can modulate the extent of channel activation by depolarization. We have previously shown that dephosphorylation of Kv2.1 by the nonreceptor-type tyrosine phosphatase PTPepsilon (cyt-PTPepsilon) down-regulates channel activity and counters its phosphorylation and up-regulation by Src or Fyn. In the present study, we identify tyrosine 124 within the T1 cytosolic domain of Kv2.1 as a target site for the activities of Src and cyt-PTPepsilon. Tyr(124) is phosphorylated by Src in vitro; in whole cells, Y124F Kv2.1 is significantly less phosphorylated by Src and loses most of its ability to bind the D245A substrate-trapping mutant of cyt-PTPepsilon. Phosphorylation of Tyr(124) is critical for Src-mediated up-regulation of Kv2.1 channel activity, since Y124F Kv2.1-mediated K(+) currents are only marginally up-regulated by Src, in contrast with a 3-fold up-regulation of wild-type Kv2.1 channels by the kinase. Other properties of Kv2.1, such as expression levels, subcellular localization, and voltage dependence of channel activation, are unchanged in Y124F Kv2.1, indicating that the effects of the Y124F mutation are specific. Together, these results indicate that Tyr(124) is a significant site at which the mutually antagonistic activities of Src and cyt-PTPepsilon affect Kv2.1 phosphorylation and activity.

Characterization, expression and functional aspects of a novel protein tyrosine phosphatase epsilon isoform

This report describes the identification and characterization of a novel cytoplasmic isoform of human protein tyrosine phosphatase epsilon (PTPepsilon). The novel isoform, denoted cyt-PTPepsilonPD1, displays only the N-terminal catalytic, active phosphatase domain 1 (PD1) which is common in all known PTPepsilon isoforms. In addition, it contains a unique 132-residue long C-terminal end with no known motifs or homology to other characterized proteins. RNAse protection assay on isolated leucocyte subpopulations and selected cell lines demonstrated highest expression of cyt-PTPepsilonPD1 in monocytes. The mRNA-encoding cyt-PTPepsilonPD1 is detected as distinct transcript(s) by Northern blot analysis and is a result of alternative splicing. cyt-PTPepsilonPD1 shows similar cellular localization in transfected cells, both in the cytoplasm and nucleus, as has been previously described for cytoplasmic PTPepsilon isoform. Our previous data suggest that the expression of cytoplasmic PTPepsilon inhibits the mitogen-activated protein kinase cascade through the extracellular signal-regulated kinase 1 and 2 pathway. A similar functional role is also presented here for cyt-PTPepsilonPD1, supporting our previous data suggesting that the catalytic first PD of PTPepsilon is responsible for this inhibition.

Expression of human protein tyrosine phosphatase epsilon in leucocytes: a potential ERK pathway-regulating phosphatase

The expression of protein tyrosine phosphatase epsilon (PTPepsilon) was studied in human tissues and blood cells. High mRNA expression was observed in peripheral blood leucocytes, particularly in monocytes and granulocytes which revealed at least four distinct transcripts. In lymphocytes, PTPepsilon expression was induced after 12-O-tetradecanoylphorbol-13-acetate (TPA) or antigen-receptor stimulation, indicating that PTPepsilon plays a role in the events taking place after antigen engagement. Previously, PTPepsilon has been shown to be involved in regulating voltage-gated potassium channel activity, insulin-receptor signalling and Janus kinase-signal transducers and activators of transcription (STAT) signalling. Transfection of cells with different PTPepsilon constructs and activator protein-1 reporter gene indicates that the catalytic activity of PTPepsilon is involved in the regulation of the mitogen-activated protein kinase cascade. In particular, the extracellular signal-regulated kinases (ERK1/2) were shown to be inhibited in both phosphorylation status and enzymatic activity after overexpression of PTPepsilon. Thus, PTPepsilon emerges as a phosphatase with a potential to regulate the ERK1/2 pathway either directly or indirectly through its catalytic activity.

PTP-epsilon, a tyrosine phosphatase expressed in endothelium, negatively regulates endothelial cell proliferation

The vascular endothelium is a dynamic interface between the blood vessel and circulating factors and, as such, plays a critical role in vascular events like inflammation, angiogenesis, and hemostasis. Whereas specific protein tyrosine kinases have been identified in these processes, less is known about their protein tyrosine phosphatase (PTP) counterparts. We utilized a RT-PCR/differential hybridization assay to identify PTP-epsilon as a highly abundant endothelial cell PTP. PTP-epsilon mRNA expression is growth factor responsive, suggesting a role for this enzyme in endothelial cell proliferation. Overexpression of PTP-epsilon decreases proliferation by 60% in human umbilical vein endothelial cells (HUVEC) but not in smooth muscle cells or fibroblasts. In contrast, overexpression of PTP-epsilon (D284A), a catalytically inactive mutant, has no significant effect on HUVEC proliferation. These data provide the first functional characterization of PTP-epsilon in endothelial cells and identify a novel pathway that negatively regulates endothelial cell growth. Such a pathway may have important implications in vascular development and angiogenesis.

Comparative study of protein tyrosine phosphatase-epsilon isoforms: membrane localization confers specificity in cellular signalling

To study the influence of subcellular localization as a determinant of signal transduction specificity, we assessed the effects of wild-type transmembrane and cytoplasmic protein tyrosine phosphatase (PTP) epsilon on tyrosine kinase signalling in baby hamster kidney (BHK) cells overexpressing the insulin receptor (BHK-IR). The efficiency by which differently localized PTPepsilon and PTPalpha variants attenuated insulin-induced cell rounding and detachment was determined in a functional clonal-selection assay and in stable cell lines. Compared with the corresponding receptor-type PTPs, the cytoplasmic PTPs (cytPTPs) were considerably less efficient in generating insulin-resistant clones, and exceptionally high compensatory expression levels were required to counteract phosphotyrosine-based signal transduction. Targeting of cytPTPepsilon to the plasma membrane via the Lck-tyrosine kinase dual acylation motif restored high rescue efficiency and abolished the need for high cytPTPepsilon levels. Consistent with these results, expression levels and subcellular localization of PTPepsilon were also found to determine the phosphorylation level of cellular proteins including focal adhesion kinase (FAK). Furthermore, PTPepsilon stabilized binding of phosphorylated FAK to Src, suggesting this complex as a possible mediator of the PTPepsilon inhibitory response to insulin-induced cell rounding and detachment in BHK-IR cells. Taken together, the present localization-function study indicates that transcriptional control of the subcellular localization of PTPepsilon may provide a molecular mechanism that determines PTPepsilon substrate selectivity and isoform-specific function.

Generation of novel cytoplasmic forms of protein tyrosine phosphatase epsilon by proteolytic processing and translational control

Two protein forms of tyrosine phosphatase epsilon (PTPepsilon) are known - receptor-like (tm-PTPepsilon) and non receptor-like (cyt-PTPepsilon), with each form possessing unique tissue-specific expression patterns, subcellular localization, and physiological functions. We describe two additional forms of PTPepsilon protein - p67 and p65. p67 is produced by initiation of translation at an internal initiation codon of PTPepsilon mRNA molecules, while p65 is produced by specific proteolytic cleavage of larger PTPepsilon proteins. Cleavage is inhibited by MG132, but is proteasome-independent. In contrast with full-length tm-PTPepsilon and cyt-PTPepsilon, p67 and p65 are exclusively cytoplasmic, are not phosphorylated by Neu, and do not associate with Grb2 in unstimulated cells. p67 and p65 are catalytically active and can reduce Src-mediated phosphorylation of the Kv2.1 voltage-gated potassium channel, albeit with reduced efficiency which most likely results from their cytoplasmic localization. We also show that full-length cyt-PTPepsilon protein can be found at the cell membrane and in the nucleus and that it is the first 27 residues of cyt-PTPepsilon which determine this localization. p67 and p65 provide mechanisms for removing PTPepsilon activity from the cell membrane, possibly serving to down-regulate PTPepsilon activity there. PTPepsilon emerges as a family of four related proteins whose expression, subcellular localization and most likely physiological roles are subject to complex regulation at the transcriptional, translational and post-translational levels.

Hypomyelination and increased activity of voltage-gated K(+) channels in mice lacking protein tyrosine phosphatase epsilon

Protein tyrosine phosphatase epsilon (PTP epsilon) is strongly expressed in the nervous system; however, little is known about its physiological role. We report that mice lacking PTP epsilon exhibit hypomyelination of sciatic nerve axons at an early post-natal age. This occurs together with increased activity of delayed- rectifier, voltage-gated potassium (Kv) channels and with hyperphosphorylation of Kv1.5 and Kv2.1 Kv channel alpha-subunits in sciatic nerve tissue and in primary Schwann cells. PTP epsilon markedly reduces Kv1.5 or Kv2.1 current amplitudes in Xenopus oocytes. Kv2.1 associates with a substrate-trapping mutant of PTP epsilon, and PTP epsilon profoundly reduces Src- or Fyn-stimulated Kv2.1 currents and tyrosine phosphorylation in transfected HEK 293 cells. In all, PTP epsilon antagonizes activation of Kv channels by tyrosine kinases in vivo, and affects Schwann cell function during a critical period of Schwann cell growth and myelination.

Emerging issues in receptor protein tyrosine phosphatase function: lifting fog or simply shifting?

Transmembrane (receptor) tyrosine phosphatases are intimately involved in responses to cell-cell and cell-matrix contact. Several important issues regarding the targets and regulation of this protein family are now emerging. For example, these phosphatases exhibit complex interactions with signaling pathways involving SRC family kinases, which result from their ability to control phosphorylation of both activating and inhibitory sites in these kinases and possibly also their substrates. Similarly, integrin signaling illustrates how phosphorylation of a single protein, or the activity of a pathway, can be controlled by multiple tyrosine phosphatases, attesting to the intricate integration of these enzymes in cellular regulation. Lastly, we are starting to appreciate the roles of intracellular topology, tyrosine phosphorylation and oligomerization among the many mechanisms regulating tyrosine phosphatase activity.