Low molecular weight protein-tyrosine phosphatase tyrosine phosphorylation by c-Src during platelet-derived growth factor-induced mitogenesis correlates with its subcellular targeting

Platelet-dependent signaling and Low Molecular Weight Protein Tyrosine Phosphatase expression promote aggressive phenotypic changes in gastrointestinal cancer cells

Over the last decades, some members of the protein tyrosine phosphatase family have emerged as cancer promoters. Among them, the Low Molecular Weight Protein Tyrosine Phosphatase (LMWPTP) has been described to be associated with colorectal cancer liver metastasis and poor prostate cancer prognosis. Of importance in the process of cancer progression and metastasis is the interaction between tumor cells and platelets, as the latter are thought to promote several tumor hallmarks. Here, we examine to what extent LMWPTP expression in tumor cells affects their interaction with platelets. We demonstrate that the gene encoding LMWPTP is overexpressed in upper gastrointestinal (GI) cancer cell as well as colorectal cancer, and subsequently employ cell line models to show that the level of this phosphatase may be further augmented in the presence of platelets. We demonstrate that tumor-platelet interaction promotes GI tumor cell proliferation. Additionally, using know-down/-out models we show that LMWPTP expression in cancer cells contributes to a more efficient interaction with platelets and drives platelet-induced proliferation. These data are the first to demonstrate that phosphatases play a positive role in the tumor-promoting activities of platelets, with LMWPTP emerging as a key player promoting oncogenic phenotypic changes in tumor cells.

The low molecular weight protein tyrosine phosphatase promotes adipogenesis and subcutaneous adipocyte hypertrophy

Obesity is a major contributing factor to the pathogenesis of Type 2 diabetes. Multiple human genetics studies suggest that high activity of the low molecular weight protein tyrosine phosphatase (LMPTP) promotes metabolic syndrome in obesity. We reported that LMPTP is a critical promoter of insulin resistance in obesity by regulating liver insulin receptor signaling and that inhibition of LMPTP reverses obesity-associated diabetes in mice. Since LMPTP is expressed in adipose tissue but little is known about its function, here we examined the role of LMPTP in adipocyte biology. Using conditional knockout mice, we found that selective deletion of LMPTP in adipocytes impaired obesity-induced subcutaneous adipocyte hypertrophy. We assessed the role of LMPTP in adipogenesis in vitro, and found that LMPTP deletion or knockdown substantially impaired differentiation of primary preadipocytes and 3T3-L1 cells into adipocytes, respectively. Inhibition of LMPTP in 3T3-L1 preadipocytes also reduced adipogenesis and expression of proadipogenic transcription factors peroxisome proliferator activated receptor gamma (PPARγ) and CCAAT/enhancer-binding protein alpha. Inhibition of LMPTP increased basal phosphorylation of platelet-derived growth factor receptor alpha (PDGFRα) on activation motif residue Y849 in 3T3-L1, resulting in increased activation of the mitogen-associated protein kinases p38 and c-Jun N-terminal kinase and increased PPARγ phosphorylation on inhibitory residue S82. Analysis of the metabolome of differentiating 3T3-L1 cells suggested that LMPTP inhibition decreased cell glucose utilization while enhancing mitochondrial respiration and nucleotide synthesis. In summary, we report a novel role for LMPTP as a key driver of adipocyte differentiation via control of PDGFRα signaling.

Redox regulation of the actin cytoskeleton and its role in the vascular system

The actin cytoskeleton is critical for form and function of vascular cells, serving mechanical, organizational and signaling roles. Because many cytoskeletal proteins are sensitive to reactive oxygen species, redox regulation has emerged as a pivotal modulator of the actin cytoskeleton and its associated proteins. Here, we summarize work implicating oxidants in altering actin cytoskeletal proteins and focus on how these alterations affect cell migration, proliferation and contraction of vascular cells. Finally, we discuss the role of oxidative modification of the actin cytoskeleton in vivo and highlight its importance for vascular diseases.

Low molecular weight protein tyrosine phosphatase: Multifaceted functions of an evolutionarily conserved enzyme

Originally identified as a low molecular weight acid phosphatase, LMW-PTP is actually a protein tyrosine phosphatase that acts on many phosphotyrosine-containing cellular proteins that are primarily involved in signal transduction. Differences in sequence, structure, and substrate recognition as well as in subcellular localization in different organisms enable LMW-PTP to exert many different functions. In fact, during evolution, the LMW-PTP structure adapted to perform different catalytic actions depending on the organism type. In bacteria, this enzyme is involved in the biosynthesis of group 1 and 4 capsules, but it is also a virulence factor in pathogenic strains. In yeast, LMW-PTPs dephosphorylate immunophilin Fpr3, a peptidyl-prolyl-cis-trans isomerase member of the protein chaperone family. In humans, LMW-PTP is encoded by the ACP1 gene, which is composed of three different alleles, each encoding two active enzymes produced by alternative RNA splicing. In animals, LMW-PTP dephosphorylates a number of growth factor receptors and modulates their signalling processes. The involvement of LMW-PTP in cancer progression and in insulin receptor regulation as well as its actions as a virulence factor in a number of pathogenic bacterial strains may promote the search for potent, selective and bioavailable LMW-PTP inhibitors.

Vanadyl complexes with dansyl-labelled di-picolinic acid ligands: synthesis, phosphatase inhibition activity and cellular uptake studies

Two new vanadyl complexes with a fluorescent label are reported. We show that these complexes inhibit selected phosphatases and are cell permeable.

Activation of the low molecular weight protein tyrosine phosphatase in keratinocytes exposed to hyperosmotic stress

Herein, we provide new contribution to the mechanisms involved in keratinocytes response to hyperosmotic shock showing, for the first time, the participation of Low Molecular Weight Protein Tyrosine Phosphatase (LMWPTP) activity in this event. We reported that sorbitol-induced osmotic stress mediates alterations in the phosphorylation of pivotal cytoskeletal proteins, particularly Src and cofilin. Furthermore, an increase in the expression of the phosphorylated form of LMWPTP, which was followed by an augment in its catalytic activity, was observed. Of particular importance, these responses occurred in an intracellular milieu characterized by elevated levels of reduced glutathione (GSH) and increased expression of the antioxidant enzymes glutathione peroxidase and glutathione reductase. Altogether, our results suggest that hyperosmostic stress provides a favorable cellular environment to the activation of LMWPTP, which is associated with increased expression of antioxidant enzymes, high levels of GSH and inhibition of Src kinase. Finally, the real contribution of LMWPTP in the hyperosmotic stress response of keratinocytes was demonstrated through analysis of the effects of ACP1 gene knockdown in stressed and non-stressed cells. LMWPTP knockdown attenuates the effects of sorbitol induced-stress in HaCaT cells, mainly in the status of Src kinase, Rac and STAT5 phosphorylation and activity. These results describe for the first time the participation of LMWPTP in the dynamics of cytoskeleton rearrangement during exposure of human keratinocytes to hyperosmotic shock, which may contribute to cell death.

Crystal structure and putative substrate identification for the Entamoeba histolytica low molecular weight tyrosine phosphatase

Entamoeba histolytica is a eukaryotic intestinal parasite of humans, and is endemic in developing countries. We have characterized the E. histolytica putative low molecular weight protein tyrosine phosphatase (LMW-PTP). The structure for this amebic tyrosine phosphatase was solved, showing the ligand-induced conformational changes necessary for binding of substrate. In amebae, it was expressed at low but detectable levels as detected by immunoprecipitation followed by immunoblotting. A mutant LMW-PTP protein in which the catalytic cysteine in the active site was replaced with a serine lacked phosphatase activity, and was used to identify a number of trapped putative substrate proteins via mass spectrometry analysis. Seven of these putative substrate protein genes were cloned with an epitope tag and overexpressed in amebae. Five of these seven putative substrate proteins were demonstrated to interact specifically with the mutant LMW-PTP. This is the first biochemical study of a small tyrosine phosphatase in Entamoeba, and sets the stage for understanding its role in amebic biology and pathogenesis.

Src-mediated phosphorylation of the tyrosine phosphatase PRL-3 is required for PRL-3 promotion of Rho activation, motility and invasion

The metastasis-associated tyrosine phosphatase PRL-3/PTP4A is upregulated in numerous cancers, but the mechanisms modulating PRL-3 activity other than its expression levels have not been investigated. Here we report evidence for both Src-dependent tyrosine phosphorylation of PRL-3 and Src-mediated regulation of PRL-3 biological activities. We used structural mutants, pharmacological inhibitors and siRNA to demonstrate Src-dependent phosphorylation of endogenous PRL-3 in SW480 colon cancer cells. We also demonstrated that PRL-3 was not tyrosine phosphorylated in SYF mouse embryo fibroblasts deficient in Src, Yes and Fyn unless Src was re-expressed. Further, we show that platelet-derived growth factor (PDGF) can stimulate PRL-3 phosphorylation in a Src-dependent manner. Finally, we show that PRL-3-induced cell motility, Matrigel invasion and activation of the cytoskeleton-regulating small GTPase RhoC were abrogated in the presence of the phosphodeficient PRL-3 mutant Y53F, or by use of a Src inhibitor. Thus, PRL-3 requires the activity of a Src kinase, likely Src itself, to promote these cancer-associated phenotypes. Our data establish a model for the regulation of PRL-3 by Src that supports the possibility of their coordinate roles in signaling pathways promoting invasion and metastasis, and supports simultaneous use of novel molecularly targeted therapeutics directed at these proteins.

Protein tyrosine phosphatases: structure-function relationships

Structural analysis of protein tyrosine phosphatases (PTPs) has expanded considerably in the last several years, producing more than 200 structures in this class of enzymes (from 35 different proteins and their complexes with ligands). The small-medium size of the catalytic domain of approximately 280 residues plus a very compact fold makes it amenable to cloning and overexpression in bacterial systems thus facilitating crystallographic analysis. The low molecular weight PTPs being even smaller, approximately 150 residues, are also perfect targets for NMR analysis. The availability of different structures and complexes of PTPs with substrates and inhibitors has provided a wealth of information with profound effects in the way we understand their biological functions. Developments in mammalian expression technology recently led to the first crystal structure of a receptor-like PTP extracellular region. Altogether, the PTP structural work significantly advanced our knowledge regarding the architecture, regulation and substrate specificity of these enzymes. In this review, we compile the most prominent structural traits that characterize PTPs and their complexes with ligands. We discuss how the data can be used to design further functional experiments and as a basis for drug design given that many PTPs are now considered strategic therapeutic targets for human diseases such as diabetes and cancer.

Low Molecular Weight Protein Tyrosine Phosphatase and Caveolin-1: Interaction and Isoenzyme-Dependent Regulation

Low molecular weight protein tyrosine phosphatases (LMW-PTPs) are small enzymes that are ubiquitous in many organisms. They are important in biological processes such as cell proliferation, adhesion, migration, and invasiveness. LMW-PTP is expressed in mammalian cells as two isoforms (IF1 and IF2) originating through alternative splicing. We have previously shown that IF2 targets lipid rafts called caveolae and interacts with caveolin-1, their major structural protein. Caveolae are cholesterol- and sphingolipid-rich membrane microdomains that have been implicated in a variety of cellular functions, including signal transduction events. Caveolin-1 contains a scaffolding region that contributes to the binding of the protein to the plasma membrane and mediates protein omo- and etero-oligomerization. Interaction of many signaling molecules with the scaffolding domain sequesters them into caveolae and inhibits or suppresses their activities. Caveolin-interacting proteins usually have a typical sequence motif, also present in all the LMW-PTPs, which is characterized by aromatic or large hydrophobic residues in specific positions. We have examined here the interaction of the LMW-PTP isoforms with caveolin-1 and its molecular mechanism, together with the consequences for their tyrosine phosphatase activities. We found that IF1 and IF2 are both capable of interacting with defined regions of caveolin-1 and that their putative caveolin binding sequence motif is not responsible for the association. The formation of LMW-PTP/caveolin-1 complexes is accompanied by modulation of the enzyme activities, and the inhibitory effect elicited against IF1 is stronger than that against IF2. The caveolin scaffolding domain is directly involved in the observed phenomena.

A novel redox-based switch: LMW-PTP oxidation enhances Grb2 binding and leads to ERK activation

Low molecular weight-PTP has been reported as a redox-sensitive protein during both platelet-derived growth factor and integrin signalling. In response to oxidation the phosphatase undergoes a reversible inactivation, which in turn leads to the increase in tyrosine phosphorylation of its substrates and the properly executed anchorage-dependent proliferation program. Here, we report that an exogenous oxidative stress enhances LMW-PTP tyrosine phosphorylation, through oxidation/inactivation of the enzyme, thus preventing its auto-dephosphorylation activity. In particular, we observed a selective hyper-phosphorylation of Tyr132, that acts as a docking site for the adaptor protein Grb2. The redox-dependent enhancement of Grb2 recruitment to LMW-PTP ultimately leads to an improvement of ERK activation, likely triggering a prosurvival signal against the oxidant environment.

Spectrin interacts with EVL (Enabled/vasodilator-stimulated phosphoprotein-like protein), a protein involved in actin polymerization

BACKGROUND INFORMATION

The alpha- and beta-spectrin chains constitute the filaments of the spectrin-based skeleton, which was first identified in erythrocytes. The discovery of analogous structures at plasma membranes of eukaryotic cells has led to investigations of the role of this spectrin skeleton in many cellular processes. The alphaII-spectrin chain expressed in nucleated cells harbours in its central region several functional motifs, including an SH3 (Src homology 3) domain.

RESULTS

Using yeast two-hybrid screening, we have identified EVL [Enabled/VASP (vasodilator-stimulated phosphoprotein)-like protein] as a new potential partner of the alphaII-spectrin SH3 domain. In the present study, we investigated the interaction of the alphaII-spectrin SH3 domain with EVL and compared this with other proteins related to EVL [Mena (mammalian Enabled) and VASP]. We confirmed the in vitro interaction between EVL and the alphaII-spectrin SH3 domain by GST (glutathione S-transferase) pull-down assays, and showed that the co-expression of EVL with the alphaII-spectrin SH3 domain in COS-7 cells resulted in the partial delocalization of the SH3 domain from cytoplasm to filopodia and lamellipodia, where it was co-localized with EVL. In kidney epithelial and COS-7 cells, we demonstrated the co-immunoprecipitation of the alphaII-spectrin chain with over-expressed EVL. Immunofluorescence studies showed that the over-expression of EVL in COS-7 cells promoted the formation of filopodia and lamellipodia, and the expressed EVL was detected in filopodial tips and the leading edge of lamellipodia. In these cells over-expressing EVL, the alphaII-spectrin membrane labelling lagged behind EVL staining in lamellipodia and filopodia, with co-localization of these two stains in the contact area. In kidney epithelial cell lines, focused co-localization of spectrin with expressed EVL was observed in the membrane of the lateral domain, where the cell-cell contacts are reinforced.

CONCLUSIONS

The possible link between the spectrin-based skeleton and actin via the EVL protein suggests a new way of integrating the spectrin-based skeleton in areas of dynamic actin reorganization.

The extracellular matrix glycoprotein Tenascin-C is expressed by oligodendrocyte precursor cells and required for the regulation of maturation rate, survival and responsiveness to platelet-derived growth factor

Analysis of Tenascin-C (TN-C) knockout mice revealed novel roles for this extracellular matrix (ECM) protein in regulation of the developmental programme of oligodendrocyte precursor cells (OPCs), their maturation into myelinating oligodendrocytes and sensitivity to growth factors. A major component of the ECM of developing nervous tissue, TN-C was expressed in zones of proliferation, migration and morphogenesis. Examination of TN-C knockout mice showed roles for TN-C in control of OPC proliferation and migration towards zones of myelination [E. Garcion et al. (2001) Development, 128, 2485-2496]. Extending our studies of TN-C effects on OPC development we found that OPCs can endogenously express TN-C protein. This expression covered the whole range of possible TN-C isoforms and could be strongly up-regulated by leukaemia inhibitory factor and ciliary neurotrophic factor, cytokines known to modulate OPC proliferation and survival. Comparative analysis of TN-C knockout OPCs with wild-type OPCs reveals an accelerated rate of maturation in the absence of TN-C, with earlier morphological differentiation and precocious expression of myelin basic protein. TN-C knockout OPCs plated on poly-lysine displayed higher levels of apoptosis than wild-type OPCs and there was also an earlier loss of responsiveness to the protective effects of platelet-derived growth factor (PDGF), indicating that TN-C has anti-apoptotic effects that may be associated with PDGF signalling. The existence of mechanisms to compensate for the absence of TN-C in the knockout is indicated by the development of oligodendrocytes derived from TN-C knockout neurospheres. These were present in equivalent proportions to those found in wild-type neurospheres but displayed enhanced myelin membrane formation.

Glutathione, stress responses, and redox signaling in lung inflammation

Changes in the ratio of intracellular reduced and disulfide forms of glutathione (GSH/GSSG) can affect signaling pathways that participate in various physiological responses from cell proliferation to gene expression and apoptosis. It is also now known that many proteins have a highly conserved cysteine (sulfhydryl) sequence in their active/regulatory sites, which are primary targets of oxidative modifications and thus important components of redox signaling. However, the mechanism by which oxidants and GSH/protein-cysteine-thiols actually participate in redox signaling still remains to be elucidated. Initial studies involving the role of cysteine in various proteins have revealed that cysteine-SH may mediate redox signaling via reversible or irreversible oxidative modification to Cys-sulfenate or Cys-sulfinate and Cys-sulfonate species, respectively. Oxidative stress possibly via the modification of cysteine residues activates multiple stress kinase pathways and transcription factors nuclear factor-kappaB and activator protein-1, which differentially regulate the genes for proinflammatory cytokines as well as the protective antioxidant genes. Understanding the redox signaling mechanisms for differential gene regulation may allow for the development of novel pharmacological approaches that preferentially up-regulate key antioxidants genes, which, in turn, reduce or resolve inflammation and injury. This forum article features the current knowledge on the role of GSH in redox signaling, particularly the regulation of transcription factors and downstream signaling in lung inflammation.

Redox regulation of PTEN and protein tyrosine phosphatases in H(2)O(2) mediated cell signaling

Protein tyrosine phosphatase (PTP) is a family of enzymes important for regulating cellular phosphorylation state. The oxidation and consequent inactivation of several PTPs by H(2)O(2) are well demonstrated. It is also shown that recovery of enzymatic activity depends on the availability of cellular reductants. Among these redox-regulated PTPs, PTEN, Cdc25 and low molecular weight PTP are known to form a disulfide bond between two cysteines, one in the active site and the other nearby, during oxidation by H(2)O(2). The disulfide bond likely confers efficiency in the redox regulation of the PTPs and protects cysteine-sulfenic acid of PTPs from further oxidation. In this review, through a comparative analysis of the oxidation process of Yap1 and PTPs, we propose the mechanism of disulfide bond formation in the PTPs.

Low molecular weight phosphotyrosine protein phosphatase from PC12 cells. Purification, some properties and expression during neurogenesis in vitro and in vivo

The purification and partial characterization of low molecular weight phosphotyrosine phosphatase (LMW-PTP) was reported for the first time in PC12 cells. In addition, the expression levels during neuronal phenotype induction by nerve growth factor (NGF) and during neurogenesis in chick embryos were investigated. LMW-PTP was purified to homogeneity and showed a single band of about 18 kDa with sodium dodecyl sulfate polyacrylamide gel electrophoresis. A native molecular mass of 20.1 kDa was determined by gel filtration on Sephadex G-75 column. The LMW-PTP from PC12 cells displays structural and biochemical characteristics similar to the enzyme isolated for normal tissues. It was specifically immunoprecipitated by an affinity purified antibody directed against the bovine liver enzyme. The enzyme is present in the cytosolic and cytoskeletal cell compartment where is tyrosine phosphorylated. Time course expression of LMW-PTP in PC12 cells was investigated after NGF treatment and showed an increase of about 30% in the basal level of LMW-PTP from 0 to 72 h. These changes were related to the appearance in PC12 cells of neuronal processes and to a decrease in cell proliferation. An increase of the LMW-PTP expression was also observed in vivo during chick embryo neurogenesis from 8-day-old embryos to adult chicks. The protein level, assayed by immunoblotting, increases from 14-day-old embryos to the hatched chicks reaching the adult levels within the first week after birth. These data indicate that the neurogenesis process is accompanied by a physiological increment of LMW-PTP expression in vitro and in vivo.

Low Mr phosphotyrosine protein phosphatase associates and dephosphorylates p125 focal adhesion kinase, interfering with cell motility and spreading

Low M(r) phosphotyrosine protein phosphatase interferes in vivo with the activation of several growth factor receptors and is transiently redistributed, following cell stimulation with platelet-derived growth factor, from the cytosol to the cytoskeleton. We demonstrate here that this phosphatase also participates in the regulation of cell spreading and migration, pointing to its involvement in cytoskeleton organization. Low M(r) phosphotyrosine protein phosphatase-overexpressing fibroblasts are, indeed, less spread than controls and display a significantly decreased number of focal adhesions and increased cell motility. Furthermore, p125 focal adhesion kinase is associated to, and dephosphorylated by, low M(r) phosphotyrosine protein phosphatase both in vitro and in vivo. This event is consistent with an altered association of pp60(src) with focal adhesion kinase. The activation of extracellular signal-regulated kinase, another well known event downstream of the focal adhesion kinase, is also affected. On the other hand, cells overexpressing the dominant-negative form of low M(r) phosphotyrosine protein phosphatase exhibit hyperphosphorylated focal adhesion kinase, reduced motility, and an increased number of focal adhesions, which are distributed all over the ventral cell surface. Taken together, the results reported here are in keeping with low M(r) phosphotyrosine protein phosphatase participation in FAK-mediated focal adhesion remodeling.

Insight into the role of low molecular weight phosphotyrosine phosphatase (LMW-PTP) on platelet-derived growth factor receptor (PDGF-r) signaling. LMW-PTP controls PDGF-r kinase activity through TYR-857 dephosphorylation

Low molecular weight phosphotyrosine phosphatase (LMW-PTP) is an enzyme involved in platelet-derived growth factor-induced mitogenesis and cytoskeleton rearrangement. Our previous results demonstrated that LMW-PTP is able to bind and dephosphorylate activated platelet-derived growth factor receptor (PDGF-r), thus inhibiting cell proliferation. Here we revisit the role of LMW-PTP on activated PDGF-r dephosphorylation. We demonstrate that LMW-PTP preferentially acts on cell surface PDGF-r, excluding the internalized activated receptor pool. Many phosphotyrosine phosphatases act by site-selective dephosphorylation on several sites of PDGF-r, but until now, there has been no evidence of a direct involvement of a specific phosphotyrosine phosphatase in the dephosphorylation of the 857 kinase domain activation tyrosine. Here we report that LMW-PTP affects the kinase activity of the receptor through the binding and dephosphorylation of Tyr-857 and influences many of the signal outputs from the receptor. In particular, we demonstrate a down-regulation of phosphatidylinositol 3-kinase, Src homology phosphatase-2, and phospholipase C-gamma1 binding but not of MAPK activation. In addition, we report a slight action of LMW-PTP on Tyr-716, which directs MAPK activation through Grb2 binding. On the basis of these results, we propose a key role for LMW-PTP in PDGF-r down-regulation through the dephosphorylation of the activation loop Tyr-857, thus determining a general negative regulation of all downstream signals, with the exception of those elicited by internalized receptors.

Tyrosine phosphorylation regulates alpha II spectrin cleavage by calpain

Spectrins, components of the membrane skeleton, are implicated in various cellular functions. Understanding the diversity of these functions requires better characterization of the interacting domains of spectrins, such as the SH3 domain. Yeast two-hybrid screening of a kidney cDNA library revealed that the SH3 domain of alpha II-spectrin binds specifically isoform A of low-molecular-weight phosphotyrosine phosphatase (LMW-PTP). The alpha II-spectrin SH3 domain does not interact with LMW-PTP B or C nor does LMW-PTP A interact with the alpha I-spectrin SH3 domain. The interaction of spectrin with LMW-PTP A led us to look for a tyrosine-phosphorylated residue in alpha II-spectrin. Western blotting showed that alpha II-spectrin is tyrosine phosphorylated in vivo. Using mutagenesis on recombinant peptides, we identified the residue Y1176 located in the calpain cleavage site of alpha II-spectrin, near the SH3 domain, as an in vitro substrate for Src kinase and LMW-PTP A. This Y1176 residue is also an in vivo target for kinases and phosphatases in COS cells. Phosphorylation of this residue decreases spectrin sensitivity to calpain in vitro. Similarly, the presence of phosphatase inhibitors in cell culture is associated with the absence of spectrin cleavage products. This suggests that the Y1176 phosphorylation state could modulate spectrin cleavage by calpain and may play an important role during membrane skeleton remodeling.

Low-molecular-weight protein tyrosine phosphatase is a positive component of the fibroblast growth factor receptor signaling pathway

Low-molecular-weight protein tyrosine phosphatase (LMW-PTP) has been implicated in the regulation of cell growth and actin rearrangement mediated by several receptor tyrosine kinases, including platelet-derived growth factor and epidermal growth factor. Here we identify the Xenopus laevis homolog of LMW-PTP1 (XLPTP1) as an additional positive regulator in the fibroblast growth factor (FGF) signaling pathway during Xenopus development. XLPTP1 has an expression pattern that displays substantial overlap with FGF receptor 1 (FGFR1) during Xenopus development. Using morpholino antisense technology, we show that inhibition of endogenous XLPTP1 expression dramatically restricts anterior and posterior structure development and inhibits mesoderm formation. In ectodermal explants, loss of XLPTP1 expression dramatically blocks the induction of the early mesoderm gene, Xbrachyury (Xbra), by FGF and partially blocks Xbra induction by Activin. Moreover, FGF-induced activation of mitogen-activated protein (MAP) kinase is also inhibited by XLPTP1 morpholino antisense oligonucleotides; however, introduction of RNA encoding XLPTP1 is able to rescue morphological and biochemical effects of antisense inhibition. Inhibition of FGF-induced MAP kinase activity due to loss of XLPTP1 is also rescued by an active Ras, implying that XLPTP1 may act upstream of or parallel to Ras. Finally, XLPTP1 physically associates only with an activated FGFR1, and this interaction requires the presence of SNT1/FRS-2 (FGFR substrate 2). Although LMW-PTP1 has been shown to participate in other receptor systems, the data presented here also reveal XLPTP1 as a new and important component of the FGF signaling pathway.

Low molecular weight protein-tyrosine phosphatase is involved in growth inhibition during cell differentiation

Low molecular weight protein-tyrosine phosphatase (LMW-PTP) is an enzyme involved in mitogenic signaling and cytoskeletal rearrangement after platelet-derived growth factor (PDGF) stimulation. Recently, we demonstrated that LMW-PTP is regulated by a redox mechanism involving the two cysteine residues of the catalytic site, which turn reversibly from reduced to oxidized state after PDGF stimulation. Since recent findings showed a decrease of intracellular reactive oxygen species in contact inhibited cells and a lower tyrosine phosphorylation level in dense cultures in comparison to sparse ones, we studied if the level of endogenous LMW-PTP is regulated by growth inhibition conditions, such as cell confluence and differentiation. Results show that both cell confluence and cell differentiation up-regulate LMW-PTP expression in C2C12 and PC12 cells. We demonstrate that during myogenesis LMW-PTP is regulated at translational level and that the protein accumulates at the plasma membrane. Furthermore, we showed that both myogenesis and cell-cell contact lead to a dramatic decrease of tyrosine phosphorylation level of PDGF receptor. In addition, we observed an increased association of the receptor with LMW-PTP during myogenesis. Herein, we demonstrate that myogenesis decreases the intracellular level of reactive oxygen species, as observed in dense cultures. As a consequence, LMW-PTP turns from oxidized to reduced form during muscle differentiation, increasing its activity in growth inhibition conditions such as differentiation. These data suggest that LMW-PTP plays a crucial role in physiological processes, which require cell growth arrest such as confluence and differentiation.

Low M(r) protein tyrosine phosphatase inhibits growth and migration of vascular smooth muscle cells induced by platelet-derived growth factor

Vascular smooth muscle cell (VSMC) migration and growth are positively regulated by protein tyrosine phosphorylation. Therefore, a dephosphorylation process controlled by protein tyrosine phosphatases (PTPs) must also be critical. The present study identified six cytoplasmic PTPs expressed in VSMCs: low M(r) protein tyrosine phosphatase (LMW-PTP), SHP-2, PTP36, PTP2, PTP1B, and FAP1. We further examined the functions of LMW-PTP in VSMCs using the adenovirus-mediated gene transfer of recombinant LMW-PTP. PDGF-induced activation of p38, but not of ERK MAP kinase, was blocked by LMW-PTP. LMW-PTP as well as the p38 inhibitor SB203580 inhibited DNA synthesis and cell migration upon PDGF stimulation. LMW-PTP dephosphorylated activated PDGF receptors in NIH3T3 cells, but not in VSMCs. Thus, LMW-PTP negatively regulates PDGF functions by inhibiting the p38 pathway in VSMCs although its substrate is unclear. These findings strongly demonstrate that PTPs are important as negative regulators for VSMC growth and migration, processes that are closely related to the progression of atherosclerosis.

Two vicinal cysteines confer a peculiar redox regulation to low molecular weight protein tyrosine phosphatase in response to platelet-derived growth factor receptor stimulation

Low molecular weight protein tyrosine phosphatase (LMW-PTP) is an enzyme involved in platelet-derived growth factor (PDGF)-induced mitogenesis and cytoskeleton rearrangement because it is able to bind and dephosphorylate the activated receptor. LMW-PTP presents two cysteines in positions 12 and 17, both belonging to the catalytic pocket; this is a unique feature of LMW-PTP among all protein tyrosine phosphatases. Our previous results demonstrated that in vitro LMW-PTP is oxidized by either H(2)O(2) or nitric oxide with the formation of a disulfide bond between Cys-12 and Cys-17. This oxidation leads to reversible enzyme inactivation because treatment with reductants permits catalytic activity rescue. In the present study we investigated the in vivo inactivation of LMW-PTP by either extracellularly or intracellularly generated H(2)O(2), evaluating its action directly on its natural substrate, PDGF receptor. LMW-PTP is oxidized and inactivated by exogenous oxidative stress and recovers its activity after oxidant removal. LMW-PTP is oxidized also during PDGF signaling, very likely upon PDGF-induced H(2)O(2) production, and recovers its activity within 40 min. Our results strongly suggest that reversibility of in vivo LMW-PTP oxidation is glutathione-dependent. In addition, we propose an intriguing and peculiar role of Cys-17 in the formation of a S-S intramolecular bond, which protects the catalytic Cys-12 from further and irreversible oxidation. On the basis of our results we propose that the presence of an additional cysteine near the catalytic cysteine could confer to LMW-PTP the ability to rapidly recover its activity and finely regulate PDGF receptor activation during both extracellularly and intracellularly generated oxidative stress.

18 kDa protein tyrosine phosphatase in the ocular lens

Protein tyrosyl phosphorylation and dephosphorylation play essential roles in regulating cellular events such as proliferation and differentiation, and their involvement in the lens development and transparency is also suggested. The level of tyrosine phosphorylation in a given protein is regulated by the opposing actions of protein-tyrosine kinases (Tyr kinases) and protein-tyrosine phosphatases (TyrPases). Recent studies have revealed that some Tyr kinases, such as platelet-derived growth factor receptor and fibroblast growth factor receptor, are present in the lens, however, little is known about TyrPases in the lens. In this study, we found a 18 kDa protein tyrosine phosphatase (18 kDa TyrPase) predominantly present in the ocular lens of various animals. We purified the phosphatase from the lens of chick embryo and characterized its activity.Phosphatase activity was determined in chick embryo, mouse, rabbit and bovine lenses using p -nitrophenyl phosphate (p NPP) as substrate. All lenses examined dephosphorylated p NPP under acidic conditions, and a large portion of the activity resided in a low molecular weight protein, ca. 18 kDa, following high-resolution gel permeation column chromatography. The brain and liver showed high dephosphorylation activities, but most of their activity was present in high molecular weight fractions, unlike that in the lens. The 18 kDa phosphatase was purified from the lens of 17 day old chick embryos to near-homogeneity with two-step rapid chromatography. This phosphatase showed strict substrate specificity for phosphotyrosine and phosphotyrosyl peptides, suggesting that it was a kind of protein tyrosine phosphatases (TyrPases). Several known inhibitors of TyrPases, such as SH blockers, vanadate and phenylarsine oxide, strongly inhibited the enzyme activity. The molecular weight, substrate specificity, and responses to various inhibitors and activators coincide well with those reported for the low molecular weight protein tyrosine phosphatase (LMW-TyrPase), belonging to the TyrPase superfamily. These results suggest that the 18 kDa phosphatase found in the lens is a LMW-TyrPase. The 18 kDa TyrPase is the predominant phosphatase in the ocular lens. It may be involved in regulation of lens cell proliferation, differentiation and/or lens transparency.

Expression of the small tyrosine phosphatase (Stp1) in Saccharomyces cerevisiae: a study on protein tyrosine phosphorylation

Small tyrosine phoshatase 1 (Stp1) is a Schizosaccharomyces pombe low-molecular-mass phosphotyrosine-phosphatase 50% identical to Saccharomyces cerevisiae Ltp1. In order to investigate the role of Stp1 in yeast, a mutant was generated having the characteristic of a dominant negative molecule. Changes in protein tyrosine phosphorylation in S. cerevisiae proteome in response to Stp1 or its dominant negative mutant expression were analyzed by high-resolution two-dimensional (2-D) electrophoresis. The most remarkable result is the modification by phosphorylation on tyrosine of several proteins involved in carbohydrate metabolism. Twelve proteins were identified on the basis of their positions in the anti-phosphotyrosine immunoblot of the 2-D electrophoresis. Ten of these present tyrosyl residues that are within the consensus sequence for protein kinase CK2 (casein kinase-2). These data open the possibility for the identification of Stp1 substrates in yeast and provide hints about the nature of tyrosine phosphorylating agents in yeast and in other organisms where bona fide tyrosine kinases are lacking.

Low molecular weight protein-tyrosine phosphatase controls the rate and the strength of NIH-3T3 cells adhesion through its phosphorylation on tyrosine 131 or 132

The low molecular weight protein-tyrosine phosphatase (LMW-PTP) is an enzyme involved in platelet-derived growth factor (PDGF)-induced mitogenesis and cytoskeleton rearrangement. Our previous results demonstrated that LMW-PTP is able to bind and dephosphorylate activated PDGF receptor, thus inhibiting cell proliferation. Recently we have shown that LMW-PTP is specifically phosphorylated by c-Src in a cytoskeleton-associated fraction in response to PDGF, and this phosphorylation increases LMW-PTP activity about 20-fold. LMW-PTP strongly influences cell adhesion, spreading, and chemotaxis induced by PDGF stimulation, by regulating the phosphorylation level of p190Rho-GAP, a protein that is able to regulate Rho activity and hence cytoskeleton rearrangement. In the present study we investigate the physiological role of the two LMW-PTP tyrosine phosphorylation sites, using LMW-PTP mutants on tyrosine 131 or 132. We demonstrate that each tyrosine residue is involved in specific LMW-PTP functions. Both of them are phosphorylated during PDGF signaling. Phosphorylation on tyrosine 131 influences mitogenesis, dephosphorylating activated PDGF-R and cytoskeleton rearrangement, acting on p190RhoGAP. Phosphorylation on tyrosine 132 leads to an increase in the strength of cell substrate adhesion, down-regulating matrix metalloproteases expression, through the inhibition of Grb2/MAPK pathway. In conclusion, LMW-PTP tyrosine phosphorylation on both Tyr(131) or Tyr(132) cooperate to determine a faster and stronger adhesion to extracellular matrix, although these two events may diverge in timing and relative amount.

Anandamide and 2-arachidonoylglycerol inhibit fatty acid amide hydrolase by activating the lipoxygenase pathway of the arachidonate cascade

Treatment of intact human neuroblastoma CHP100 cells with anandamide (arachidonoylethanolamide, AEA) or 2-arachidonoylglycerol (2-AG) inhibits intracellular fatty acid amide hydrolase (FAAH). This effect was not associated with covalent modifications of FAAH, since specific inhibitors of farnesyltransferase, kinases, phosphatases, glycosyltransferase or nitric oxide synthase were ineffective. Electrophoretic analysis of (33)P-labelled proteins, Western blot with anti-phosphotyrosine antibodies, and glycan analysis of cellular proteins confirmed the absence of covalent modifications of FAAH. The inhibition by AEA was paralleled by an increased arachidonate release, which was not observed upon treatment of cells with linoleoylethanolamide, palmitoylethanolamide, or oleoylethanolamide. Moreover, cell treatment with AEA or 2-AG increased the activity of cyclooxygenase and 5-lipoxygenase, and the hydro(pero)xides generated from arachidonate by lipoxygenase were shown to inhibit FAAH, with inhibition constants in the low micromolar range. Consistently, inhibitors of 5-lipoxygenase, but not those of cyclooxygenase, significantly counteracted the inhibition of FAAH by AEA or 2-AG.

LMW-PTP exerts a differential regulation on PDGF- and insulin-mediated signaling

Low-molecular-weight protein tyrosine phosphatase (LMW-PTP) is able to specifically bind and dephosphorylate activated PDGF and insulin receptors, modulating the onset of mitogenic process. LMW-PTP is present in two distinct intracellular locations. While the cytosolic LMW-PTP pool interacts directly with activated insulin or PDGF receptors, the cytoskeleton-associated LMW-PTP is tyrosine phosphorylated upon PDGF stimulation and is involved in cytoskeleton rearrangement acting on p190Rho-GAP. We investigated the differential role of LMW-PTP in PDGF- or insulin-induced mitogenesis and cytoskeleton rearrangement. Dominant negative LMW-PTP influences both PDGF- and insulin-induced mitogenesis with a different extent and it induces a decrease in cellular adhesion and chemotaxis after PDGF but not insulin treatment. PDGF but not insulin stimulation leads to tyrosine phosphorylation of LMW-PTP. We propose that the differential effect of LMW-PTP on PDGF and insulin signaling is mainly due to the fact that during insulin signaling LMW-PTP does not become phosphorylated and thus does not act on its cytoskeleton-associated substrate/s.

The low M(r) protein-tyrosine phosphatase is involved in Rho-mediated cytoskeleton rearrangement after integrin and platelet-derived growth factor stimulation

The low molecular weight protein-tyrosine phosphatase (LMW-PTP) is an enzyme that is involved in the early events of platelet-derived growth factor (PDGF) receptor signal transduction. In fact, LMW-PTP is able to specifically bind and dephosphorylate activated PDGF receptor, thus modulating PDGF-induced mitogenesis. In particular, LMW-PTP is involved in pathways that regulate the transcription of the immediately early genes myc and fos in response to growth factor stimulation. Recently, we have found that LMW-PTP exists constitutively in cytosolic and cytoskeleton-associated localization and that, after PDGF stimulation, c-Src is able to bind and phosphorylate LMW-PTP only in the cytoskeleton-associated fraction. As a consequence of its phosphorylation, LMW-PTP increases its catalytic activity about 20-fold. In this study, our interest was to investigate the role of LMW-PTP phosphorylation in cellular response to PDGF stimulation. To address this issue, we have transfected in NIH-3T3 cells a mutant form of LMW-PTP in which the c-Src phosphorylation sites (Tyr(131) and Tyr(132)) were mutated to alanine. We have established that LMW-PTP phosphorylation by c-Src after PDGF treatment strongly influences both cell adhesion and migration. In addition, we have discovered a new LMW-PTP substrate localized in the cytoskeleton that becomes tyrosine-phosphorylated after PDGF treatment: p190Rho-GAP. Hence, LMW-PTP plays multiple roles in PDGF receptor-mediated mitogenesis, since it can bind and dephosphorylate PDGF receptor, and, at the same time, the cytoskeleton-associated LMW-PTP, through the regulation of the p190Rho-GAP phosphorylation state, controls the cytoskeleton rearrangement in response to PDGF stimulation.

Activation of EphA2 kinase suppresses integrin function and causes focal-adhesion-kinase dephosphorylation

Interactions between receptor tyrosine kinases of the Eph family and their ligands, ephrins, are implicated in establishment of organ boundaries and repulsive guidance of cell migration during development, but the mechanisms by which this is achieved are unclear. Here we show that activation of endogenous EphA2 kinase induces an inactive conformation of integrins and inhibits cell spreading, migration and integrin-mediated adhesion. Moreover, EphA2 is constitutively associated with focal-adhesion kinase (FAK) in resting cells. Within one minute after stimulation of EphA2 with its ligand, ephrin-A1, the protein tyrosine phosphatase SHP2 is recruited to EphA2; this is followed by dephosphorylation of FAK and paxillin, and dissociation of the FAK-EphA2 complex. We conclude that Eph kinases mediate some of their functions by negatively regulating integrins and FAK.

Phosphospecific antibodies reveal temporal regulation of platelet-derived growth factor beta receptor signaling

The platelet-derived growth factor beta receptor (betaPDGFR) is a receptor tyrosine kinase involved in multiple aspects of cell growth and differentiation. Upon activation, betaPDGFR is phosphorylated at up to nine different tyrosine residues. Phosphorylation of the receptor results in at least two different outcomes: recruitment of signaling molecules and activation of intrinsic receptor kinase activity. In order to evaluate the phosphorylation state of the receptor, phosphospecific antibodies were generated against peptides encompassing betaPDGFR phospho-Y751, phospho-Y771, or phospho-Y857. When phosphorylated, these sites enable the receptor to recruit signaling molecules PI3K or RasGAP, or enhance the receptor's kinase activity, respectively. We found that receptors phosphorylated at Y751, Y771, and Y857 display distinct temporal and spatial distribution by immunofluorescence. Subsequent biochemical studies revealed that receptor function corresponding to each of the phosphorylated sites was regulated as a function of time. Within the first 10 min, PDGF enhanced the receptor's kinase activity and initiated recruitment of PI3K and RasGAP. After prolonged exposure to PDGF, PI3K binding persisted to approximately 85% of the amount bound at 10 min, whereas binding of RasGAP and the exogenous kinase activity of the receptor diminished to less than 15% of the levels displayed at 10 min. We conclude that the phosphorylation state of the receptor, as well as its signaling capacity, is dynamic and changes as cells are continuously exposed to PDGF.

Protein tyrosine phosphatases as negative regulators of mitogenic signaling

The regulation of tyrosine phosphorylation represents a key mechanism governing cell proliferation. In fibroblasts, inputs from both growth factor and extracellular matrix receptors are required for cell division. Triggering such receptors induces a wave of tyrosine phosphorylation on key signaling molecules, culminating in the activation of cyclin-dependent kinases and cell cycle progression. In general, protein tyrosine kinases stimulate, while protein tyrosine phosphatases inhibit, such cell proliferation pathways. The role of protein tyrosine kinases in mitogenesis has been extensively studied, but the identity and targets of the protein tyrosine phosphatases that regulate cell growth are not well described. In this review, I will survey recent advances in the identification and regulation of protein tyrosine phosphatases that downregulate cell proliferation.

The low Mr phosphotyrosine protein phosphatase behaves differently when phosphorylated at Tyr131 or Tyr132 by Src kinase

The low molecular weight phosphotyrosine protein phosphatase (LMW-PTP) is phosphorylated by Src and Src-related kinases both in vitro and in vivo; in Jurkat cells, and in NIH-3T3 cells, it becomes tyrosine-phosphorylated upon stimulation by PDGF. In this study we show that pp60Src phosphorylates in vitro the enzyme at two tyrosine residues, Tyr131 and Tyr132, previously indicated as the main phosphorylation sites of the enzyme, whereas phosphorylation by the PDGF-R kinase is much less effective and not specific. The effects of LMW-PTP phosphorylation at each tyrosine residue were investigated by using Tyr131 and Tyr132 mutants. We found that the phosphorylation at either residue has differing effects on the enzyme behaviour: Tyr131 phosphorylation is followed by a strong (about 25-fold) increase of the enzyme specific activity, whereas phosphorylation at Tyr132 leads to Grb2 recruitment. These differing effects are discussed on the light of the enzyme structure.