Site-selective dephosphorylation of the platelet-derived growth factor beta-receptor by the receptor-like protein-tyrosine phosphatase DEP-1

The complex world of oligodendroglial differentiation inhibitors

Myelination is a central nervous system (CNS) process wherein oligodendrocyte-axon interactions lead to the establishment of myelin sheaths that stabilize, protect, and electrically insulate axons. In inflammatory demyelinating diseases such as multiple sclerosis (MS), the degeneration and eventual loss of functional myelin sheaths slows and blocks saltatory conduction in axons, which results in clinical impairment. However, remyelination can occur, and lesions can be partially repaired, resulting in clinical remission. The recruitment and activation of resident oligodendrocyte precursor cells (OPCs) play a critical role in the repair process because these cells have the capacity to differentiate into functional myelinating cells. Mature oligodendrocytes, however, are thought to have lost the capacity to develop new myelin sheaths and frequently undergo programmed cell death in MS. The endogenous capacity to generate new oligodendrocytes in MS is limited, and this is predominantly due to the presence of inhibitory components that block OPC differentiation and maturation. Here, we present an overview of recently identified negative regulators of oligodendroglial differentiation and their potential relevance for CNS repair in MS. Because currently available immunomodulatory drugs for MS mainly target inflammatory cascades outside the brain and fail to repair existing lesions, achieving more efficient lesion repair constitutes an important goal for future MS therapies.

Somatostatin receptor biology in neuroendocrine and pituitary tumours: part 1--molecular pathways

Neuroendocrine tumours (NETs) may occur at many sites in the body although the majority occur within the gastroenteropancreatic axis. Non-gastroenteropancreatic NETs encompass phaeochromocytomas and paragangliomas, medullary thyroid carcinoma, anterior pituitary tumour, broncho-pulmonary NETs and parathyroid tumours. Like most endocrine tumours, NETs also express somatostatin (SST) receptors (subtypes 1–5) whose ligand SST is known to inhibit endocrine and exocrine secretions and have anti-tumour effects. In the light of this knowledge, the idea of using SST analogues in the treatment of NETs has become increasingly popular and new studies have centred upon the development of new SST analogues. We attempt to review SST receptor (SSTR) biology primarily in neuroendocrine tissues, focusing on pituitary tumours. A full data search was performed through PubMed over the years 2000–2009 with keywords ‘somatostatin, molecular biology, somatostatin receptors, somatostatin signalling, NET, pituitary’ and all relevant publications have been included, together with selected publications prior to that date. SSTR signalling in non-neuroendocrine solid tumours is beyond the scope of this review. SST is a potent anti-proliferative and anti-secretory agent for some NETs. The successful therapeutic use of SST analogues in the treatment of these tumours depends on a thorough understanding of the diverse effects of SSTR subtypes in different tissues and cell types. Further studies will focus on critical points of SSTR biology such as homo- and heterodimerization of SSTRs and the differences between post-receptor signalling pathways of SSTR subtypes.

Regulation of Src family kinases involved in T cell receptor signaling by protein-tyrosine phosphatase CD148

CD148 is a receptor-like protein-tyrosine phosphatase known to inhibit transduction of mitogenic signals in non-hematopoietic cells. Similarly, in the hematopoietic lineage, CD148 inhibited signal transduction downstream of T cell receptor. However, it also augmented immunoreceptor signaling in B cells and macrophages via dephosphorylating C-terminal tyrosine of Src family kinases (SFK). Accordingly, endogenous CD148 compensated for the loss of the main SFK activator CD45 in murine B cells and macrophages but not in T cells. Hypothetical explanations for the difference between T cells and other leukocyte lineages include the inability of CD148 to dephosphorylate a specific set of SFKs involved in T cell activation or the lack of CD148 expression during critical stages of T cell development. Here we describe striking differences in CD148 expression between human and murine thymocyte subsets, the only unifying feature being the absence of CD148 during the positive selection when the major developmental block occurs under CD45 deficiency. Moreover, we demonstrate that similar to CD45, CD148 has both activating and inhibitory effects on the SFKs involved in TCR signaling. However, in the absence of CD45, activating effects prevail, resulting in functional complementation of CD45 deficiency in human T cell lines. Importantly, this is independent of the tyrosines in the CD148 C-terminal tail, contradicting the recently proposed phosphotyrosine displacement model as a mechanism of SFK activation by CD148. Collectively, our data suggest that differential effects of CD148 in T cells and other leukocyte subsets cannot be explained by the CD148 inability to activate T cell SFKs but rather by its dual inhibitory/activatory function and specific expression pattern.

Inside the human cancer tyrosine phosphatome

Members of the protein tyrosine phosphatase (Ptp) family dephosphorylate target proteins and counter the activities of protein tyrosine kinases that are involved in cellular phosphorylation and signalling. As such, certain PTPs might be tumour suppressors. Indeed, PTPs play an important part in the inhibition or control of growth, but accumulating evidence indicates that some PTPs may exert oncogenic functions. Recent large-scale genetic analyses of various human tumours have highlighted the relevance of PTPs either as putative tumour suppressors or as candidate oncoproteins. Progress in understanding the regulation and function of PTPs has provided insights into which PTPs might be potential therapeutic targets in human cancer.

Loss of the protein-tyrosine phosphatase DEP-1/PTPRJ drives meningioma cell motility

DEP-1/PTPRJ is a transmembrane protein-tyrosine phosphatase which has been proposed as a suppressor of epithelial tumors. We have found loss of heterozygosity (LOH) of the PTPRJ gene and loss of DEP-1 protein expression in a subset of human meningiomas. RNAi-mediated suppression of DEP-1 in DEP-1 positive meningioma cell lines caused enhanced motility and colony formation in semi-solid media. Cells devoid of DEP-1 exhibited enhanced signaling of endogenous platelet-derived growth factor (PDGF) receptors, and reduced paxillin phosphorylation upon seeding. Moreover, DEP-1 loss caused diminished adhesion to different matrices, and impaired cell spreading. DEP-1-deficient meningioma cells exhibited invasive growth in an orthotopic xenotransplantation model in nude mice, indicating that elevated motility translates into a biological phenotype in vivo. We propose that negative regulation of PDGF receptor signaling and positive regulation of adhesion signaling by DEP-1 cooperate in inhibition of meningioma cell motility, and possibly tumor invasiveness. These phenotypes of DEP-1 loss reveal functions of DEP-1 in adherent cells, and may be more generally relevant for tumorigenesis.

Hypoxia enhances platelet-derived growth factor signaling in the pulmonary vasculature by down-regulation of protein tyrosine phosphatases

RATIONALE

Platelet-derived growth factor (PDGF) plays a pivotal role in the pathobiology of pulmonary hypertension (PH) because it promotes pulmonary vascular remodeling. PH is frequently associated with pulmonary hypoxia.

OBJECTIVES

To investigate whether hypoxia alters PDGF β receptor (βPDGFR) signaling in the pulmonary vasculature.

METHODS

The impact of chronic hypoxia on signal transduction by the βPDGFR was measured in human pulmonary arterial smooth muscle cells (hPASMC) in vitro, and in mice with hypoxia-induced PH in vivo.

MEASUREMENTS AND MAIN RESULTS

Chronic hypoxia significantly enhanced PDGF-BB-dependent proliferation and chemotaxis of hPASMC. Pharmacologic inhibition of PI3 kinase (PI3K) and PLCγ abrogated these events under both normoxia and hypoxia. Although hypoxia did not affect βPDGFR expression, it increased the ligand-induced tyrosine phosphorylation of the receptor, particularly at binding sites for PI3K (Y751) and PLCγ (Y1021). The activated βPDGFR is dephosphorylated by protein tyrosine phosphatases (PTPs). Interestingly, hypoxia decreased expression of numerous PTPs (T cell PTP, density-enhanced phosphatase-1, PTP1B, and SH2 domain-containing phosphatase-2), resulting in reduced PTP activity. Hypoxia-inducible factor (HIF)-1α is involved in this regulation of gene expression, because hypoxia-induced βPDGFR hyperphosphorylation and PTP down-regulation were abolished by HIF-1α siRNA and by the HIF-1α inhibitor 2-methoxyestradiol. βPDGFR hyperphosphorylation and PTP down-regulation were also present in vivo in mice with chronic hypoxia-induced PH.

CONCLUSIONS

Hypoxia reduces expression and activity of βPDGFR-antagonizing PTPs in a HIF-1α-dependent manner, thereby enhancing receptor activation and proliferation and chemotaxis of hPASMC. Because hyperphosphorylation of the βPDGFR and down-regulation of PTPs occur in vivo, this mechanism likely has significant impact on the development and progression of PH and other hypoxia-associated diseases.

12/15-lipoxygenase-derived lipid peroxides control receptor tyrosine kinase signaling through oxidation of protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) are regulated through reversible oxidation of the active-site cysteine. Previous studies have implied soluble reactive oxygen species (ROS), like H(2)O(2), as the mediators of PTP oxidation. The potential role(s) of peroxidized lipids in PTP oxidation have not been described. This study demonstrates that increases in cellular lipid peroxides, induced by disruption of glutathione peroxidase 4, induce cellular PTP oxidation and reduce the activity of PDGF receptor targeting PTPs. These effects were accompanied by site-selective increased PDGF beta-receptor phosphorylation, sensitive to 12/15-lipoxygenase (12/15-LOX) inhibitors, and increased PDGF-induced cytoskeletal rearrangements. Importantly, the 12/15-LOX-derived 15-OOH-eicosatetraenoic acid lipid peroxide was much more effective than H(2)O(2) in induction of in vitro PTP oxidation. Our study thus establishes that lipid peroxides are previously unrecognized inducers of oxidation of PTPs. This identifies a pathway for control of receptor tyrosine kinase signaling, which might also be involved in the etiology of diseases associated with increased lipid peroxidation.

An unbiased screen identifies DEP-1 tumor suppressor as a phosphatase controlling EGFR endocytosis

BACKGROUND

The epidermal growth factor (EGF) stimulates rapid tyrosine phosphorylation of the EGF receptor (EGFR). This event precedes signaling from both the plasma membrane and from endosomes, and it is essential for recruitment of a ubiquitin ligase, CBL, that sorts activated receptors to endosomes and degradation. Because hyperphosphorylation of EGFR is involved in oncogenic pathways, we performed an unbiased screen of small interfering RNA (siRNA) oligonucleotides targeting all human tyrosine phosphatases.

RESULTS

We report the identification of PTPRK and PTPRJ (density-enhanced phosphatase-1 [DEP-1]) as EGFR-targeting phosphatases. DEP-1 is a tumor suppressor that dephosphorylates and thereby stabilizes EGFR by hampering its ability to associate with the CBL-GRB2 ubiquitin ligase complex. DEP-1 silencing enhanced tyrosine phosphorylation of endosomal EGFRs and, accordingly, increased cell proliferation. In line with functional interactions, EGFR and DEP-1 form physical associations, and EGFR phosphorylates a substrate-trapping mutant of DEP-1. Interestingly, the interactions of DEP-1 and EGFR are followed by physical segregation: whereas EGFR undergoes endocytosis, DEP-1 remains confined to the cell surface.

CONCLUSIONS

EGFR and DEP-1 physically interact at the cell surface and maintain bidirectional enzyme-substrate interactions, which are relevant to their respective oncogenic and tumor-suppressive functions. These observations highlight the emerging roles of vesicular trafficking in malignant processes.

Protein tyrosine phosphatases in glioma biology

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

Phosphatome profiling reveals PTPN2, PTPRJ and PTEN as potent negative regulators of PKB/Akt activation in Ras-mutated cancer cells

Oncogenic Ras mutations render the protein constitutively active and promote tumorigenesis via chronic stimulation of effector pathways. In A549 lung adenocarcinoma approx. 50% of the total Ras population is constitutively active, yet these cells display only weak activation of the effectors: ERK1/2 (extracellular-signal-regulated kinase 1/2) and Akt. In order to identify key negative regulators of oncogenic Ras signalling we performed a phosphatome RNAi (RNA interference) screen in A549 cells and ranked their effects on phosphorylation of Ser473 of Akt. As expected, the tumour suppressor PTEN (phosphatase and tensin homologue deleted on chromosome 10) emerged as a leading hit: knockdown elevated Akt activation to 70% of maximal generated by acute EGF (epidermal growth factor) stimulation. Importantly, we identified other phosphatases with similar potencies including PTPN2 (T-cell protein tyrosine phosphatase; also known as TC-PTP) and PTPRJ (protein tyrosine phosphatase receptor type J; also known as DEP-1/CD148). Potentiation of Akt phosphorylation by knockdown of PTEN or PTPRJ was contingent on the presence of oncogenic K-Ras. Our data reveal a synergy between oncogene function and the loss of a tumour suppressor within the same pathway that was necessary for full effector activation since each alone failed to elicit significant Akt phosphorylation. Taken together, these data reveal potent regulators of Akt signalling which contribute to ameliorating the consequences of oncogenic K-Ras activity.

Tumor suppressor density-enhanced phosphatase-1 (DEP-1) inhibits the RAS pathway by direct dephosphorylation of ERK1/2 kinases

Density-enhanced phosphatase-1 (DEP-1) is a trans-membrane receptor protein-tyrosine phosphatase that plays a recognized prominent role as a tumor suppressor. However, the mechanistic details underlying its function are poorly understood because its primary physiological substrate(s) have not been firmly established. To shed light on the mechanisms underlying the anti-proliferative role of this phosphatase, we set out to identify new DEP-1 substrates by a novel approach based on screening of high density peptide arrays. The results of the array experiment were combined with a bioinformatics filter to identify eight potential DEP-1 targets among the proteins annotated in the MAPK pathway. In this study we show that one of these potential targets, the ERK1/2, is indeed a direct DEP-1 substrate in vivo. Pulldown and in vitro dephosphorylation assays confirmed our prediction and demonstrated an overall specificity of DEP-1 in targeting the phosphorylated tyrosine 204 of ERK1/2. After epidermal growth factor stimulation, the phosphorylation of the activation loop of ERK1/2 can be modulated by changing the concentration of DEP-1, without affecting the activity of the upstream kinase MEK. In addition, we show that DEP-1 contains a KIM-like motif to recruit ERK1/2 proteins by a docking mechanism mediated by the common docking domain in ERK1/2. ERK proteins that are mutated in the conserved docking domain become insensitive to DEP-1 de-phosphorylation. Overall this study provides novel insights into the anti-proliferative role of this phosphatase and proposes a new mechanism that may also be relevant for the regulation of density-dependent growth inhibition.

Density-enhanced phosphatase 1 regulates phosphorylation of tight junction proteins and enhances barrier function of epithelial cells

Cell-cell adhesion is a dynamic process that can activate multiple signaling pathways. These signaling pathways can be regulated through reversible tyrosine phosphorylation events. The level of tyrosine phosphorylation of junctional proteins reflects the balance between protein-tyrosine kinase and protein-tyrosine phosphatase activity. The receptor-tyrosine phosphatase DEP-1 (CD148/PTP-eta) has been implicated in cell growth and differentiation as well as in regulating phosphorylation of junctional proteins. However, the role of DEP-1 in regulating tight junction phosphorylation and the integrity of cell-cell junctions is still under investigation. In this study, we used a catalytically dead substrate-trapping mutant of DEP-1 to identify potential substrates at cell-cell junctions. We have shown that in epithelial cells the trapping mutant of DEP-1 interacts with the tight junction proteins occludin and ZO-1 in a tyrosine phosphorylation-dependent manner. In contrast, PTP-PEST, Shp2, and PTPmu did not interact with these proteins, suggesting that the interaction of DEP-1 with occludin and ZO-1 is specific. In addition, occludin and ZO-1 were dephosphorylated by DEP-1 but not these other phosphatases in vitro. Overexpression of DEP-1 increased barrier function as measured by transepithelial electrical resistance and also reduced paracellular flux of fluorescein isothiocyanate-dextran following a calcium switch. Reduced DEP-1 expression by small interfering RNA had a small but significant increase in junction permeability. These data suggest that DEP-1 can modify the phosphorylation state of tight junction proteins and play a role in regulating permeability.

CD148/DEP-1 association with areas of cytoskeletal organisation in macrophages

In macrophages, tyrosine phosphorylation regulates many signalling pathways leading to growth, differentiation, activation, phagocytosis and adhesion. Protein tyrosine phosphatases (PTPs) represent a biochemical counterbalance to the activity of protein tyrosine kinases, thus regulating the dynamic phosphorylation state of a cell. CD148 is a receptor PTP that is highly expressed in macrophages and is further regulated by pro-inflammatory stimuli. CD148 is normally localised to the plasma membrane of macrophages. Following stimulation with CSF-1 or LPS, there was a re-distribution and concentration of CD148 in areas of membrane ruffling. Treatment of macrophages with anti-CD148 monoclonal antibody inhibited CSF-1-induced macrophage spreading, cytoskeletal re-arrangements and chemotaxis, without affecting cell survival. There were no detectable effects on the CSF-1 receptor-akt signalling pathway. These results are consistent with the hypothesis that CD148 is a regulator of macrophage activity.

CD45, CD148, and Lyp/Pep: critical phosphatases regulating Src family kinase signaling networks in immune cells

Reciprocal regulation of tyrosine phosphorylation by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) is central to normal immune cell function. Disruption of the equilibrium between PTK and PTP activity can result in immunodeficiency, autoimmunity, or malignancy. Src family kinases (SFKs) play a central role in both immune cell function and disease due to their proximal position in numerous signal transduction cascades including those emanating from integrin, T and B-cell antigen receptors, Fc, growth factor, and cytokine receptors. Given that tight regulation of SFKs activity is critical for appropriate responses to stimulation of these various signaling pathways, it is perhaps not surprising that multiple PTPs are involved in their regulation. Here, we focus on the role of three phosphatases, CD45, CD148, and LYP/PEP, which are critical regulators of SFKs in hematopoietic cells. We review our current understanding of their structures, expression, functions in different hematopoietic cell subsets, regulation, and putative roles in disease. Finally, we discuss remaining questions that must be addressed if we are to have a clearer understanding of the coordinated regulation of tyrosine phosphorylation and signaling networks in hematopoietic cells and how they could potentially be manipulated therapeutically in disease.

The tyrosine phosphatase CD148 is an essential positive regulator of platelet activation and thrombosis

Platelets play a fundamental role in hemostasis and thrombosis. They are also involved in pathologic conditions resulting from blocked blood vessels, including myocardial infarction and ischemic stroke. Platelet adhesion, activation, and aggregation at sites of vascular injury are regulated by a diverse repertoire of tyrosine kinase–linked and G protein–coupled receptors. Src family kinases (SFKs) play a central role in initiating and propagating signaling from several platelet surface receptors; however, the underlying mechanism of how SFK activity is regulated in platelets remains unclear. CD148 is the only receptor-like protein tyrosine phosphatase identified in platelets to date. In the present study, we show that mutant mice lacking CD148 exhibited a bleeding tendency and defective arterial thrombosis. Basal SFK activity was found to be markedly reduced in CD148-deficient platelets, resulting in a global hyporesponsiveness to agonists that signal through SFKs, including collagen and fibrinogen. G protein–coupled receptor responses to thrombin and other agonists were also marginally reduced. These results highlight CD148 as a global regulator of platelet activation and a novel antithrombotic drug target.

The eighth fibronectin type III domain of protein tyrosine phosphatase receptor J influences the formation of protein complexes and cell localization

Regulation of receptor-type phosphatases can involve the formation of higher-order structures, but the exact role played in this process by protein domains is not well understood. In this study we show the formation of different higher-order structures of the receptor-type phosphatase PTPRJ, detected in HEK293A cells transfected with different PTPRJ expression constructs. In the plasma membrane PTPRJ forms dimers detectable by treatment with the cross-linking reagent BS(3) (bis[sulfosuccinimidyl]suberate). However, other PTPRJ complexes, dependent on the formation of disulfide bonds, are detected by treatment with the oxidant agent H(2)O(2) or by a mutation Asp872Cys, located in the eighth fibronectin type III domain of PTPRJ. A deletion in the eighth fibronectin domain of PTPRJ impairs its dimerization in the plasma membrane and increases the formation of PTPRJ complexes dependent on disulfide bonds that remain trapped in the cytoplasm. The deletion mutant maintains the catalytic activity but is unable to carry out inhibition of proliferation on HeLa cells, achieved by the wild type form, since it does not reach the plasma membrane. Therefore, the intact structure of the eighth fibronectin domain of PTPRJ is critical for its localization in plasma membrane and biological function.

PTPRJ haplotypes and colorectal cancer risk

Recent studies from mouse mapping studies for cancer susceptibility have successfully led to the identification of a handful of susceptibility genes. Ptprj was identified as a strong candidate gene for mouse locus susceptibility to colorectal cancer 1, and one variant, rs1566734, showed evidence of preferential allelic imbalance in human colorectal tumors. Haplotypes in human PTPRJ have also been associated with protective effects for breast cancer risk. To determine if variants or haplotype in PTPRJ confer protective or risk effects for colorectal cancer (CRC), we genotyped rs1566734 and six additional PTPRJ haplotype tagging single nucleotide polymorphisms (SNP) in CRC cases and controls from the Molecular Epidemiology of Colorectal Cancer study. There was no evidence for cancer risk with rs1566734 in 1,897 cases and 1,954 controls with a homozygote odds ratio of 1.09 and 95% confidence interval of 0.85 to 1.39. The 6 tagging SNPs resulted in 6 main haplotypes (frequencies, >1%). None of the six tagSNPs individually showed significant evidence for risk; however, rs1503185 showed a nonsignificant protective effect. One haplotype was overrepresented in cases compared with controls, corresponding to a 34% increase in risk CRC, but there was no significant difference overall in haplotype frequencies between cases and controls (global test P statistic=0.19). From this study, we observe no significant increase in risk for human CRC with variants or haplotypes in PTPRJ. Additional studies are warranted to study possible PTPRJ-interacting loci, which are observed with Scc1 in the mouse models for CRC susceptibility.

New role for the protein tyrosine phosphatase DEP-1 in Akt activation and endothelial cell survival

Functional inactivation of the protein tyrosine phosphatase DEP-1 leads to increased endothelial cell proliferation and failure of vessels to remodel and branch. DEP-1 has also been proposed to contribute to the contact inhibition of endothelial cell growth via dephosphorylation of vascular endothelial growth factor receptor 2 (VEGFR2), a mediator of vascular development. However, how DEP-1 regulates VEGF-dependent signaling and biological responses remains ill-defined. We show here that DEP-1 targets tyrosine residues in the VEGFR2 kinase activation loop. Consequently, depletion of DEP-1 results in the increased phosphorylation of all major VEGFR2 autophosphorylation sites, but surprisingly, not in the overall stimulation of VEGF-dependent signaling. The increased phosphorylation of Src on Y529 under these conditions results in impaired Src and Akt activation. This inhibition is similarly observed upon expression of catalytically inactive DEP-1, and coexpression of an active Src-Y529F mutant rescues Akt activation. Reduced Src activity correlates with decreased phosphorylation of Gab1, an adapter protein involved in VEGF-dependent Akt activation. Hypophosphorylated Gab1 is unable to fully associate with phosphatidylinositol 3-kinase, VEGFR2, and VE-cadherin complexes, leading to suboptimal Akt activation and increased cell death. Overall, our results reveal that despite its negative role on global VEGFR2 phosphorylation, DEP-1 is a positive regulator of VEGF-mediated Src and Akt activation and endothelial cell survival.

The structure of the 5'-end of the protein-tyrosine phosphatase PTPRJ mRNA reveals a novel mechanism for translation attenuation

Analysis of the human protein-tyrosine phosphatase (PTP) PTPRJ mRNA detected three in-frame AUGs at the 5′-end (starting at nt +14, +191 and +356) with no intervening stop codons. This tandem AUG arrangement is conserved between humans and the mouse and is unique among the genes of the classical PTPs. Until now it was assumed that the principal open reading frame (ORF) starts at AUG₃₅₆. Our experiments showed that: (i) translation of the mRNA synthesized under the PTPRJ promoter starts predominantly at AUG₁₉₁, leading to the generation of a 55 amino acid sequence preceding the signal peptide; (ii) the longer form is being likewise correctly processed into mature PTPRJ; (iii) the translation of the region between AUG₁₉₁ and AUG₃₅₆ inhibits the overall expression, a feature which depends on the sequence of the encoded peptide. Specifically, a sequence of 13 amino acids containing multiple arginine residues (RRTGWRRRRRRRR) confers the inhibition. In the absence of uORF these previously unrecognized characteristics of the 5′-end of the mRNA present a novel mechanism to suppress, and potentially to regulate translation.

The tyrosine phosphatase CD148 interacts with the p85 regulatory subunit of phosphoinositide 3-kinase

CD148 is a transmembrane tyrosine phosphatase that has been implicated in the regulation of cell growth and transformation. However, the signalling mechanisms of CD148 are incompletely understood. To identify the specific intracellular molecules involved in CD148 signalling, we carried out a modified yeast two-hybrid screening assay. Using the substrate-trapping mutant form of CD148 (CD148 D/A) as bait, we recovered the p85 regulatory subunit of PI3K (phosphoinositide 3-kinase). CD148 D/A, but not catalytically active CD148, interacted with p85 in a phosphorylation-dependent manner in vitro and in intact cells. Growth factor receptor and PI3K activity were also trapped by CD148 D/A via p85 from pervanadate-treated cell lysates. CD148 prominently and specifically dephosphorylated p85 in vitro. Co-expression of CD148 reduced p85 phosphorylation induced by active Src, and attenuated the increases in PI3K activity, yet CD148 did not alter the basal PI3K activity. Finally, CD148 knock-down by siRNA (short interfering RNA) increased PI3K activity on serum stimulation. Taken together, these results demonstrate that CD148 may interact with and dephosphorylate p85 when it is phosphorylated and modulate the magnitude of PI3K activity.

Somatostatin/somatostatin receptor signalling: phosphotyrosine phosphatases

Activation of phosphotyrosine phosphatases (PTPs) by somatostatin receptor (SSTR) represents one of the main intracellular mechanisms involved in the antiproliferative effect of somatostatin (SST) and analogues. Since their molecular cloning, the role of PTPs is emerging as a major regulator of different cell functions including cell proliferation, differentiation, cell to cell interactions, cell matrix adhesion and cell migration. It was demonstrated that PTPs possess high substrate specificity and their activity is tightly regulated. Importantly, different G protein-coupled receptors transduce their biological activities through PTPs. PTPs were identified as down-stream effectors of SSTRs to transduce antiproliferative signals, and so far, three family members (SHP-1, SHP-2 and DEP-1/PTPeta) have been identified as selective SSTR intracellular effectors. Here, the molecular mechanisms leading SSTRs to regulate PTP activity are discussed, focusing on recent data showing a close interplay between PTPs and tyrosine kinases to transduce tumoral cell growth arrest following SST analogs administration.

Structurally distinct phosphatases CD45 and CD148 both regulate B cell and macrophage immunoreceptor signaling

The receptor-type protein tyrosine phosphatase (RPTP) CD148 is thought to have an inhibitory function in signaling and proliferation in nonhematopoietic cells. However, its role in the immune system has not been thoroughly studied. Our analysis of CD148 loss-of-function mice showed that CD148 has a positive regulatory function in B cells and macrophages, similar to the role of CD45 as a positive regulator of Src family kinases (SFKs). Analysis of CD148 and CD45 doubly deficient B cells and macrophages revealed hyperphosphorylation of the C-terminal inhibitory tyrosine of SFKs accompanied by substantial alterations in B and myeloid lineage development and defective immunoreceptor signaling. Because these findings suggest the C-terminal tyrosine of SFKs is a common substrate for both CD148 and CD45 phosphatases and imply a level of redundancy not previously appreciated, a reassessment of the function of CD45 in the B and myeloid lineages based on prior data from the CD45-deficient mouse is warranted.

The effect of imatinib (Glivec) on scleroderma and normal dermal fibroblasts: a preclinical study

BACKGROUND

Scleroderma skin overexpresses the platelet-derived growth factor receptor beta-subunit (PDGFR-beta) in dermal vessels and PDGFR-beta messenger RNA in cultured fibroblasts. Moreover, increased levels of PDGF and stimulatory autoantibodies to PDGFR have been identified in the serum of scleroderma patients.

OBJECTIVE

Imatinib being an inhibitor of tyrosine kinase receptors such as PDGFR, its effect on scleroderma fibroblasts was evaluated in vitro as a preclinical therapeutic step.

METHODS

The effect of imatinib on fibroblasts grown from normal or involved/uninvolved scleroderma skin was studied by Western blot and the methyltetrazolium test. The pattern of distribution of PDGFR-beta in scleroderma versus normal skin was studied by immunohistochemistry.

RESULTS

In vitro, imatinib inhibited the proliferation of normal dermal and scleroderma fibroblasts at least partly via the inhibition of the phosphorylation of PDGFR. PDGFR-beta was expressed in the epidermis and adnexae in 5 lesional scleroderma biopsies and not in controls.

CONCLUSION

This study suggests that imatinib can serve as therapy to limit dermal fibroblast proliferation in scleroderma.

Suppressor of T-cell receptor signalling 1 and 2 differentially regulate endocytosis and signalling of receptor tyrosine kinases

Suppressor of T-cell receptor signalling 1 and 2 (Sts-1 and 2) negatively regulate the endocytosis of receptor tyrosine kinases. The UBA domain of Sts-2 and SH3-dependent Cbl-binding are required for this function. Sts-1 and -2 also possess a PGM domain, which was recently reported to exhibit tyrosine phosphatase activity. Here, we demonstrate that the PGM of Sts-1, but not of Sts-2, dephosphorylates the EGFR at multiple tyrosines thereby terminating its signalling and endocytosis. In contrast to Sts-2 the UBA of Sts-1 did not contribute significantly to receptor stabilization. Thus, although Sts-1 and Sts-2 are structurally highly homologous and both inhibit ligand-induced EGFR degradation, their mechanisms of action differ significantly. As a consequence, Sts-1-containing receptor complexes are inactive, whereas Sts-2-containing complexes are signalling competent.

PDGF Receptors as Targets in Tumor Treatment

Signaling through platelet-derived growth factor (PDGF) receptors contributes to multiple tumor-associated processes. The recent introduction of clinically useful PDGF inhibitors have the last years validated PDGF receptors in malignant and stromal cells as relevant cancer drug targets. Mutational activation of PDGF receptor signaling in malignant cells has been described in some rare tumor types such as dermatofibrosarcoma protuberans, a subset of GISTs, and some hematologic malignancies. Furthermore, expression of PDGF receptors on pericytes is a common characteristic of solid tumors. The clinical efficacy of novel multikinase inhibitors, such as sunitinib and sorafenib, most likely involves targeting of PDGF receptor-dependent pericytes. Preclinical studies suggest that targeting of stromal PDGF receptors might also constitute a novel strategy to enhance tumor drug uptake. Finally, recent studies have implied both pro- and antimetastatic effects of PDGF receptors on malignant and stromal cells. The studies on the roles of PDGF receptors in cancer signaling are thus presently in a dynamic phase where collaborations between oncologists, pathologists, and tumor biologists are predicted to be highly productive.

Antioxidants relieve phosphatase inhibition and reduce PDGF signaling in cultured VSMCs and in restenosis

OBJECTIVE

Growth factor- and reactive oxygen species (ROS)-induced activation of VSMCs is involved in vascular disease. This study investigates whether inhibitory oxidation of protein tyrosine phosphatases (PTPs) contributes to signaling in VSMCs in vitro and in vivo, and analyzes whether ROS- and growth factor-dependent vascular smooth muscle cell (VSMC) signaling is blunted by antioxidants that are able to activate oxidized PTPs.

METHODS AND RESULTS

Signaling induced by H2O2 and platelet-derived growth factor (PDGF) was analyzed in VSMCs with or without the antioxidants N-acetyl-cysteine (NAC) and tempol. Effects of antioxidants on PDGF-stimulated chemotaxis and proliferation were determined. In vivo effects of antioxidants were analyzed in the rat carotid balloon-injury model, by analyzing neointima formation, cell proliferation, PDGF beta-receptor status, and PTP expression and activity. NAC treatment prevented H2O2-induced PTP inhibition, and reduced H2O2- and ligand-induced PDGF beta-receptor phosphorylation, PDGF-induced proliferation, and chemotaxis of VSMCs. Antioxidants inhibited neointima formation and reduced PDGF receptor phosphorylation in the neointima and also increased PTP activity.

CONCLUSIONS

PTP-inhibition was identified as an intrinsic component of H2O2- and PDGF-induced signaling in cultured VSMCs. The reduction in PDGF beta-receptor phosphorylation in vivo, and the increase in PTP activity, by antioxidants indicate activation of oxidized PTPs as a previously unrecognized mechanism for the antirestenotic effects of antioxidants. The findings thus suggest, in general terms, reactivation of oxidized PTPs as a novel antirestenotic strategy.

Natural compounds as a source of protein tyrosine phosphatase inhibitors: application to the rational design of small-molecule derivatives

Reversible phosphorylation of tyrosine residues is a key regulatory mechanism for numerous cellular events. Protein tyrosine kinases and protein tyrosine phosphatases (PTPs) have a pivotal role in regulating both normal cell physiology and pathophysiology. Accordingly, deregulated activity of both protein tyrosine kinases and PTPs is involved in the development of numerous congenitically inherited and acquired human diseases, prompting obvious pharmaceutical and academic research interest. The development of compound libraries with higher selective PTP inhibitory activity has been bolstered by the realization that many natural products have such activity and thus are interesting biologically lead compounds, which properties are widely exploited. In addition, more rational approaches have focused on the incorporation of phosphotyrosine mimetics into specific peptide templates (peptidomimetic backbones). Additional factors furthering discovery as well as therapeutic application of new bioactive molecules are the integration of functional genomics, cell biology, structural biology, drug design, molecular screening and chemical diversity. Together, all these factors will lead to new avenues to treat clinical disease based on PTP inhibition.

Glutaredoxin modulates platelet-derived growth factor-dependent cell signaling by regulating the redox status of low molecular weight protein-tyrosine phosphatase

Glutaredoxin (GRX) is a glutathione-disulfide oxidoreductase involved in various cellular functions, including the redox-dependent regulation of certain integral proteins. Here we demonstrated that overexpression of GRX suppressed the proliferation of myocardiac H9c2 cells treated with platelet-derived growth factor (PDGF)-BB. After stimulation with PDGF-BB, the phosphorylation of PDGF receptor (PDGFR) beta was suppressed in GRX gene-transfected cells, compared with controls. Conversely, the phosphorylation was enhanced by depletion of GRX by RNA interference. In this study we focused on the role of low molecular weight protein-tyrosine phosphatase (LMW-PTP) in the dephosphorylation of PDGFRbeta via a redox-dependent mechanism. We found that depletion of LMW-PTP using RNA interference enhanced the PDGF-BB-induced phosphorylation of PDGFRbeta, indicating that LMW-PTP works for PDGFRbeta. The enhancement of the phosphorylation of PDGFRbeta was well correlated with inactivation of LMW-PTP by cellular peroxide generated in the cells stimulated with PDGF-BB. In vitro, with hydrogen peroxide treatment, LMW-PTP showed decreased activity with the concomitant formation of dithiothreitol-reducible oligomers. GRX protected LMW-PTP from hydrogen peroxide-induced oxidation and inactivation in concert with glutathione, NADPH, and glutathione disulfide reductase. This strongly suggests that retention of activity of LMW-PTP by enhanced GRX expression suppresses the proliferation of cells treated with PDGF-BB via enhanced dephosphorylation of PDGFRbeta. Thus, GRX plays an important role in PDGF-BB-dependent cell proliferation by regulating the redox state of LMW-PTP.

Genetic ablation of Ptprj, a mouse cancer susceptibility gene, results in normal growth and development and does not predispose to spontaneous tumorigenesis

Ptprj is a ubiquitously expressed murine gene encoding a receptor-type protein tyrosine phosphatase, which has recently been proposed as a candidate gene on the locus Scc1 for colon cancer susceptibility. It has been demonstrated that PTPRJ, the human homologue of Ptprj, is involved in the control of cell growth and adhesion, being furthermore altered in several types of cancer including mammary, thyroid, lung, colon, and pancreatic cancers. To investigate the biological functions of Ptprj, we have generated mice deficient in this receptor protein tyrosine phosphatase. Ptprj-deficient mice are viable, fertile, and show no gross anatomical alterations. Furthermore, neither changes in life span nor spontaneous tumor appearance were observed in Ptprj-null mice. Our results indicate that Ptprj is dispensable for normal growth and development in mice.

The Receptor-Type Protein Tyrosine Phosphatase J Antagonizes the Biochemical and Biological Effects of RET-Derived Oncoproteins

Thyroid cancer is frequently associated with the oncogenic conversion of the RET receptor tyrosine kinase. RET gene rearrangements, which lead to the generation of chimeric RET/papillary thyroid carcinoma (PTC) oncogenes, occur in PTC, whereas RET point mutations occur in familial multiple endocrine neoplasia type 2 (MEN2) and sporadic medullary thyroid carcinomas (MTC). We showed previously that the expression of the receptor-type protein tyrosine phosphatase J (PTPRJ) is suppressed in neoplastically transformed follicular thyroid cells. We now report that PTPRJ coimmunoprecipitates with wild-type RET and with the MEN2A-associated RET(C634R) oncoprotein but not with the RET/PTC1 and RET-MEN2B isoforms. Using mutated forms of PTPRJ and RET-MEN2A, we show that the integrity of the respective catalytic domains is required for the PTPRJ/RET-MEN2A interaction. PTPRJ expression induces dephosphorylation of the RET(C634R) and, probably via an indirect mechanism, RET/PTC1 oncoproteins on two key RET autophosphorylation sites (Tyr1062 and Tyr905). This results in a significant decrease of RET-induced Shc and extracellular signal-regulated kinase 1/2 phosphorylation levels. In line with this finding, adoptive PTPRJ expression reduced the oncogenic activity of RET(C634R) in an in vitro focus formation assay of NIH3T3 cells. As expected from the coimmunoprecipitation results, the RET(M918T) oncoprotein, which is associated to MEN2B and sporadic MTC, was resistant to the dephosphorylating activity of PTPRJ. Taken together, these findings identify RET as a novel substrate of PTPRJ and suggest that PTPRJ expression levels may affect tumor phenotype associated with RET/PTC1 and RET(C634R) mutants. On the other hand, resistance to PTPRJ may be part of the mechanism of RET oncogenic conversion secondary to the M918T mutation.

DEP-1 protein tyrosine phosphatase inhibits proliferation and migration of colon carcinoma cells and is upregulated by protective nutrients

The transmembrane protein-tyrosine phosphatase (PTP) DEP-1 (density-enhanced phosphatase) is a candidate tumor suppressor in the colon epithelium. We have explored the function of DEP-1 in colon epithelial cells by inducible re-expression in a DEP-1-deficient human colon cancer cell line. Density-enhanced phosphatase-1 re-expression led to profound inhibition of cell proliferation and cell migration, and was associated with cytoskeletal rearrangements. These effects were dependent on the PTP activity of DEP-1 as they were not observed with cells expressing the catalytically inactive DEP-1 C1239S variant. shRNA-mediated suppression of DEP-1 in a colon epithelial cell line with high endogenous DEP-1 levels enhanced proliferation, further supporting the antiproliferative function of DEP-1. Nutrients, which are considered to be chemoprotective with respect to colon cancer development, including butyrate, green tea and apple polyphenols, had the capacity to elevate transcription of endogenous DEP-1 mRNA and expression of DEP-1 protein. Upregulation of DEP-1 expression, and in turn inhibition of cell growth and migration may present a previously unrecognized mechanism of chemoprevention by nutrients.

A monoclonal antibody against CD148, a receptor-like tyrosine phosphatase, inhibits endothelial-cell growth and angiogenesis

Angiogenesis contributes to a wide range of neoplastic, ischemic, and inflammatory disorders. Definition of the intrinsic molecular controls in angiogenic vessel growth promises novel therapeutic approaches for angiogenesis-related diseases. CD148 (also named DEP-1/PTP eta) is a receptor-like protein tyrosine phosphatase that is abundantly expressed in vascular endothelial cells. To explore a role of CD148 in endothelial vessel formation, we generated a monoclonal antibody, Ab1, against the ectodomain sequence of CD148 and examined its effects on endothelial-cell growth and vessel formation. Here we report that a bivalent, but not a monovalent, form of the Ab1 antibody inhibits endothelial-cell growth and blocks angiogenesis in mouse cornea in vivo. We further demonstrate that (1) bivalent Ab1 arrests cell-cycle progression of CD148-transfected CHO cells at G(0)/G(1) phase, (2) coexpression of catalytically inactive CD148 mutants attenuates the Ab1-cell growth inhibition, and (3) bivalent Ab1 suppresses phosphorylation of ERK1/2 kinases and Met tyrosine kinase as activated CD148 does, with an increase in CD148-associated tyrosine phosphatase activity. Taken together, these findings demonstrate that Ab1-induced ectodomain oligomerization arrests endothelial-cell growth through catalytic activity of the CD148 cytoplasmic domain. The present study defines CD148 as a valuable molecular target for antiangiogenesis therapy.

Protein-tyrosine phosphatases and cancer

Tyrosine phosphorylation is an important signalling mechanism in eukaryotic cells. In cancer, oncogenic activation of tyrosine kinases is a common feature, and novel anticancer drugs have been introduced that target these enzymes. Tyrosine phosphorylation is also controlled by protein-tyrosine phosphatases (PTPs). Recent evidence has shown that PTPs can function as tumour suppressors. In addition, some PTPs, including SHP2, positively regulate the signalling of growth-factor receptors, and can be oncogenic. An improved understanding of how these enzymes function and how they are regulated might aid the development of new anticancer agents.

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.

A novel substrate of receptor tyrosine phosphatase PTPRO is required for nerve growth factor-induced process outgrowth

The receptor protein tyrosine phosphatase PTPRO may be involved in axon guidance both as a ligand and as a neuronal receptor. We have begun to characterize signaling by PTPRO as a receptor by screening for proteins interacting with the intracellular domain of PTPRO. In a yeast-two hybrid screen, we identified a novel class of protein, which we named neuronal pentraxin with chromo domain (NPCD), as a PTPRO-interacting protein. We have shown recently that NPCD has multiple cytoplasmic isoforms as a result of alternative splicing and that these proteins are present in many neurons, mainly associated with the inner side of the plasma membrane. Through additional two-hybrid experiments, cotransfection and reciprocal coprecipitation, glutathione S-transferase pulldown, and immunoprecipitation in vivo, we confirm that NPCD isoforms interact with the catalytic phosphatase domain of PTPRO. We also find that at least one NPCD isoform is tyrosine phosphorylated in vivo and can serve as a substrate for PTPRO in vitro. Analysis of PTPRO knock-out mice demonstrates that normal localization of NPCD at the plasma membrane requires PTPRO expression, suggesting a physiological role for the NPCD/PTPRO interaction. NPCD is likely to be relevant to axon growth and/or guidance, because RNA interference mediated knock-down of NPCD expression in pheochromocytoma cells inhibits NGF-induced neuronal process outgrowth without affecting NGF-dependent survival or initial NGF signaling.

The rat tyrosine phosphatase η increases cell adhesion by activating c-Src through dephosphorylation of its inhibitory phosphotyrosine residue

The expression of the receptor protein tyrosine phosphatase r-PTPeta is drastically reduced in rat and human malignant thyroid cells, whereas its restoration reverts the neoplastic phenotype of retrovirally transformed rat thyroid cells. Moreover, reduced levels and loss of heterozygosity of DEP-1, the human homolog of r-PTPeta, have been found in many human neoplasias. Here, we report that the r-PTPeta protein binds to c-Src in living cells and dephosphorylates the c-Src inhibitory tyrosine phosphorylation site (Tyr 529), thereby increasing c-Src tyrosine kinase activity in malignant rat thyroid cells stably transfected with r-PTPeta. Tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin was enhanced in r-PTPeta-expressing cells. This was associated with increased adhesion of malignant r-PTPeta-transfected thyroid cells vs both untransfected cells and cells stably transfected with an inactive r-PTPeta mutant. Treatment of rat thyroid cells with the c-Src inhibitor PP2 decreased cell adhesion to a higher extent in r-PTPeta-transfected cells than in mock-transfected or stably transfected cells with the inactive r-PTPeta mutant, indicating that r-PTPeta regulates cell-substratum adhesion by activating c-Src. Interestingly, the extent of both c-Src dephosphorylation at Tyr 529, FAK and paxillin phosphorylation, and the increased cell adhesion were associated with the degree of r-PTPeta expression.

The tyrosine phosphatase PTPRJ/DEP-1 genotype affects thyroid carcinogenesis

We recently isolated the r-PTPeta gene, which encodes a receptor-type tyrosine phosphatase protein that suppresses the neoplastic phenotype of retrovirally transformed rat thyroid cells. The human homologue gene PTPRJ/DEP-1 is deleted in various tumors. Moreover, the Gln276Pro polymorphism, located in the extracellular region of the gene, seems to play a critical role in susceptibility to some human neoplasias. Here we report the loss of heterozygosity (LOH) of PTPRJ in 11/76 (14.5%) informative thyroid tumors (including adenomas and carcinomas). We also looked for the Gln276Pro, Arg326Gln and Asp872Glu polymorphisms in exons 5, 6 and 13 of PTPRJ in 88 patients with thyroid tumors and in 54 healthy individuals. We found that the PTPRJ genotypes homozygous for the Gln276Pro and Arg326Gln polymorphisms, and the Asp872 allele were more frequent in thyroid carcinoma patients than in healthy individuals (P=0.032). In addition, PTPRJ LOH was more frequent in thyroid carcinomas of heterozygotes for Gln276Pro and Arg326Gln compared with homozygotes (P=0.006). This suggests that the presence of hemizygosity for these polymorphisms in the tumor facilitates tumor progression. These results indicate that the genotypic profile of PTPRJ affects susceptibility to thyroid carcinomas, and that allelic loss of this gene is involved in thyroid carcinogenesis.

The Phosphotyrosine Phosphatase η Mediates Somatostatin Inhibition of Glioma Proliferation via the Dephosphorylation of ERK1/2

Somatostatin (SST) controls the proliferation of a variety of cell types. Its effects are mediated by five G protein-coupled receptors (SSTR1-SSTR5), variably expressed in normal and cancer tissues. SST inhibition of cell proliferation can be exploited by both direct and indirect mechanisms: the main direct pathway involves the modulation of phosphotyrosine phosphatase (PTP) activity. Here we show that SST cytostatic activity is mediated by the activation of a receptor-like PTP, named PTPeta. The role of this PTP in the antiproliferative activity of SST in five glioma cell lines (C6, U87MG, U373MG, DBTRG05MG, and CAS1) and in four postsurgical human glioblastoma specimens, has been studied. SST inhibited growth only in C6 and U87MG that express PTPeta. In C6 cells, SST antiproliferative effects were reverted by pretreatment with pertussis toxin and vanadate, indicating the involvement of G proteins and PTPs. The role of PTPeta in the SST inhibitory effects was demonstrated by testing the PTPeta activity: it was increased by SST treatment and paralleled by inhibition of ERK1/2 activation. Since basic fibroblast growth factor-dependent MEK phosphorylation was not affected by SST, we propose a direct effect of SST-activated PTPeta on ERK1/2 phosphorylation. Finally, the SSTR mRNAs were identified in all of the 36 gliomas analyzed, whereas PTPeta expression was found in 33% of cases. Culturing four gliomas, a precise correlation between the expression of PTPeta and the SST antiproliferative effects was identified. In conclusion, in glioma cells, SST antiproliferative activity requires the expression and activation of PTPeta, which directly dephosphorylates ERK1/2.

On the cross-regulation of protein tyrosine phosphatases and receptor tyrosine kinases in intracellular signaling

Intracellular signaling proteins are very often regulated by site-specific phosphorylation. For example, growth factor receptors in eukaryotic cells contain intrinsic tyrosine kinase activity and use inter- and intra-molecular interactions to recruit and orient potential protein substrates for phosphorylation. Equally important in determining the magnitude and kinetics of such a response is protein dephosphorylation, catalysed by phosphatase enzymes. A growing body of evidence indicates that certain protein tyrosine phosphatases (PTPs), like tyrosine kinases, are affected by intermolecular interactions that alter the specific activity or localization of their catalytic domains. Using a detailed kinetic modeling framework, we theoretically explore the regulation of PTPs through their association with receptor tyrosine kinases, as noted for the Src homology 2-domain-containing PTPs, SHP-1 and -2. Receptor-PTP binding, in turn, is expected to influence the phosphorylation pattern of those receptors and proteins they associate with, and we show how PTPs might serve to co- or counter-regulate parallel pathways in a signaling network.

The Expression of the Phosphotyrosine Phosphatase DEP-1/PTPη Dictates the Responsivity of Glioma Cells to Somatostatin Inhibition of Cell Proliferation

Here we characterize the intracellular effectors of the antiproliferative activity of somatostatin in glioma cell lines and post-surgical specimens. The responsiveness to somatostatin correlated with the expression of the phosphotyrosine phosphatase DEP-1/PTPeta, identified in C6 and U87MG cells, in which somatostatin inhibited cell growth. The expression of a dominant negative mutant of DEP-1/PTPeta in C6 cells abolished somatostatin effects, confirming the involvement of this phosphotyrosine phosphatase in such effects. Somatostatin treatment increased the activity of DEP-1/PTPeta and inhibited ERK1/2 activation. Conversely, basic fibroblast growth factor-dependent MEK phosphorylation was not affected, suggesting a direct effect on ERK1/2. In vitro experiments showed that PTPeta was able to interact and dephosphorylate ERK1/2 activated by basic fibroblast growth factor. Furthermore, by transfecting PTPeta in the somatostatin-unresponsive, DEP-1/PTPeta-deficient U373MG cells, the somatostatin-dependent control of cell proliferation was recovered. Finally we evaluated the requirement for DEP-1/PTPeta in somatostatin inhibition of cell proliferation in post-surgical specimens derived from different grade human gliomas. Although all of the glioma analyzed expressed somatostatin receptor mRNA, DEP-1/PTPeta expression was limited to 8 of 22 of the tumors. Culturing seven gliomas, a correlation between the expression of DEP-1/PTPeta and the somatostatin antiproliferative effects was identified. In conclusion we propose that the expression and activation of DEP-1/PTPeta is required for somatostatin inhibition of glioma proliferation.

Site-selective regulation of platelet-derived growth factor beta receptor tyrosine phosphorylation by T-cell protein tyrosine phosphatase

The platelet-derived growth factor (PDGF) beta receptor mediates mitogenic and chemotactic signals. Like other tyrosine kinase receptors, the PDGF beta receptor is negatively regulated by protein tyrosine phosphatases (PTPs). To explore whether T-cell PTP (TC-PTP) negatively regulates the PDGF beta receptor, we compared PDGF beta receptor tyrosine phosphorylation in wild-type and TC-PTP knockout (ko) mouse embryos. PDGF beta receptors were hyperphosphorylated in TC-PTP ko embryos. Fivefold-higher ligand-induced receptor phosphorylation was observed in TC-PTP ko mouse embryo fibroblasts (MEFs) as well. Reexpression of TC-PTP partly abolished this difference. As determined with site-specific phosphotyrosine antibodies, the extent of hyperphosphorylation varied among different autophosphorylation sites. The phospholipase Cgamma1 binding site Y1021, previously implicated in chemotaxis, displayed the largest increase in phosphorylation. The increase in Y1021 phosphorylation was accompanied by increased phospholipase Cgamma1 activity and migratory hyperresponsiveness to PDGF. PDGF beta receptor tyrosine phosphorylation in PTP-1B ko MEFs but not in PTPepsilon ko MEFs was also higher than that in control cells. This increase occurred with a site distribution different from that seen after TC-PTP depletion. PDGF-induced migration was not increased in PTP-1B ko cells. In summary, our findings identify TC-PTP as a previously unrecognized negative regulator of PDGF beta receptor signaling and support the general notion that PTPs display site selectivity in their action on tyrosine kinase receptors.

The tyrosine phosphatase DEP-1 induces cytoskeletal rearrangements, aberrant cell-substratum interactions and a reduction in cell proliferation

The receptor protein tyrosine phosphatase density-enhanced phosphatase-1 (DEP-1) has been implicated in aberrant cancer cell growth and immune cell function, however, its function within cells has yet to be properly elucidated. To investigate the cellular function of DEP-1, stable cell lines inducibly expressing DEP-1 were generated. Induction of DEP-1 expression was found to decrease PDGF-stimulated tyrosine phosphorylation of a number of cellular proteins including the PDGF receptor, and to inhibit growth factor-stimulated phosphorylation of components of the MAPK pathway, indicating that DEP-1 antagonised PDGF receptor signalling. This was supported by data showing that DEP-1 expression resulted in a reduction in cell proliferation. DEP-1-expressing cells had fewer actin-containing microfilament bundles, reduced vinculin and paxillin-containing adhesion plaques, and were defective in interactions with fibronectin. Defective cell-substratum adhesion correlated with lack of activation of FAK in DEP-1-expressing cells. Time-lapse interference reflection microscopy of live cells revealed that although small focal contacts at the leading edge were generated in DEP-1-expressing cells, they failed to mature into stable focal adhesions, as found in control cells. Further motility analysis revealed that DEP-1-expressing cells retained limited random motility, but showed no chemotaxis towards a gradient of PDGF. In addition, cell-cell contacts were disrupted, with a change in the localisation of cadherin from discrete areas of cell-cell contact to large areas of membrane interaction, and there was a parallel redistribution of beta-catenin. These results demonstrate that DEP-1 is a negative regulator of cell proliferation, cell-substratum contacts, motility and chemotaxis in fibroblasts.

Redox regulation of protein tyrosine phosphatases during receptor tyrosine kinase signal transduction

In addition to protein phosphorylation, redox-dependent post-translational modification of proteins is emerging as a key signaling system that has been conserved throughout evolution and that influences many aspects of cellular homeostasis. Both systems exemplify dynamic regulation of protein function by reversible modification, which, in turn, regulates many cellular processes such as cell proliferation, differentiation and apoptosis. In this article we focus on the interplay between phosphorylation- and redox-dependent signaling at the level of phosphotyrosine phosphatase-mediated regulation of receptor tyrosine kinases (RTKs). We propose that signal transduction by oxygen species through reversible phosphotyrosine phosphatase inhibition, represents a widespread and conserved component of the biochemical machinery that is triggered by RTKs.

The protein-tyrosine phosphatase DEP-1 modulates growth factor-stimulated cell migration and cell-matrix adhesion

Density-enhanced protein-tyrosine phosphatase-1 (DEP-1 also CD148) is a transmembrane molecule with a single intracellular PTP domain. It has recently been proposed to function as a tumor suppressor. We have previously shown that DEP-1 dephosphorylates the activated platelet-derived growth factor (PDGF) beta-receptor in a site-selective manner (Kovalenko et al. (2000). J. Biol. Chem. 275, 16219-16226). We analysed cell lines with inducible DEP-1 expression for cellular functions of DEP-1. Several aspects of PDGFbeta-receptor signaling were negatively affected by DEP-1 expression. These include PDGF-stimulated activation of inositol trisphosphate formation, Erk1/2, p21Ras, and Src. Activation of receptor-associated phosphoinositide-3 kinase activity and of Akt/PKB were weakly attenuated at early time points of stimulation. Inhibition of PDGF-stimulated signaling depended on DEP-1 catalytic activity. Importantly, DEP-1 inhibited PDGF-stimulated cell migration. The catalytically inactive DEP-1 C1239S variant enhanced cell migration and PDGF-stimulated Erk1/2 activation, suggesting a dominant negative interference with endogenous DEP-1. In contrast to cell migration, cell-substrate adhesion was promoted by active DEP-1 and delayed or suppressed by DEP-1 C1239S, correlating with positive effects of DEP-1 on adhesion-stimulated Src kinase. We propose that negative regulation of growth-factor stimulated cell migration and promotion of cell-matrix adhesion may be related to the function of DEP-1 as tumor suppressor.

Evaluating function of transmembrane protein tyrosine phosphatase CD148 in lymphocyte biology

The transmembrane protein tyrosine phosphatase CD148 is expressed on numerous cell types, including most cells of the hematopoietic lineage. CD148 has been shown to regulate density-dependent inhibition of cell growth as well as cellular differentiation in nonhematopoietic cells and has been shown to regulate signal transduction processes in several nonlymphoid hematopoietic cell types. Analysis of CD148 expression on lymphoid cells has demonstrated that CD148 is expressed at low levels on T cells and that it is upregulated in response to activation. Several groups have observed that CD148 negatively regulates T cell activation in response to crosslinking of the T cell antigen receptor, suggesting that it may play a role in feedback inhibition of the T cell immune response. In the B cell compartment, CD 148 expression appears to be restricted to the memory subpopulation, raising the possibility that it serves a unique function in these cells, which has yet to be determined. Recent studies have shown that CD148 interacts with the PDZ domain-containing protein syntenin, raising the possibility that its function or its localization with substrates in T and B cells may be controlled through this or a related interaction with another PDZ domain protein.

Identification of protein tyrosine phosphatases associating with the PDGF receptor

Protein tyrosine phosphatase (PTP) in-gel assays were used to explore association of PTPs with the platelet-derived growth factor beta-receptor (PDGFbetaR). Five PTP activity bands of approximately 120, approximately 70, approximately 60, approximately 53, and approximately 45 kDa could be detected in PDGFbetaR immunoprecipitates and were identified by immunodepletion experiments as PTP-PEST, SHP-2, an active fragment of SHP-2, PTP-1B, and T-cell PTP, respectively. The PTP pattern that was obtained was similar in PDGFbetaR immunoprecipitates from HEK 293 cells overexpressing the human PDGFbetaR and from murine fibroblasts. Association of PTP-1B with the PDGFbetaR was stabilized by pretreatment of the cells with hydrogen peroxide. The epidermal growth factor receptor (EGFR) immunoprecipitated from fibroblasts, and c-Kit isolated from CHRF myeloid cells, were associated with partially overlapping but quantitatively different patterns of PTPs. PTP-PEST was the predominant PTP in EGFR immunoprecipitates, and SHP-1 appeared in c-Kit immunoprecipitates. We propose that the differential association of PTPs with different RTKs is related to their respective contributions to regulation of RTK signaling.

Hepatocyte growth factor receptor tyrosine kinase met is a substrate of the receptor protein-tyrosine phosphatase DEP-1

The receptor protein-tyrosine phosphatase (PTP) DEP-1 (CD148/PTP-eta) has been implicated in the regulation of cell growth, differentiation, and transformation, and most recently has been identified as a potential tumor suppressor gene mutated in colon, lung, and breast cancers. We have generated constructs comprising the cytoplasmic segment of DEP-1 fused to the maltose-binding protein to identify potential substrates and thereby suggest a physiological function for DEP-1. We have shown that the substrate-trapping mutant form of DEP-1 interacted with a small subset of tyrosine-phosphorylated proteins from lysates of the human breast tumor cell lines MDA-MB-231, T-47D, and T-47D/Met and have identified the hepatocyte growth factor/scatter factor receptor Met, the adapter protein Gab1, and the junctional component p120 catenin as potential substrates. Following ligand stimulation, phosphorylation of specific tyrosyl residues in Met induces mitogenic, motogenic, and morphogenic responses. When co-expressed in 293 cells, the full-length substrate-trapping mutant form of DEP-1 formed a stable complex with the chimeric receptor colony stimulating factor 1 (CSF)-Met and wild type DEP-1 dephosphorylated CSF-Met. Furthermore, we observed that DEP-1 preferentially dephosphorylated a Gab1 binding site (Tyr(1349)) and a COOH-terminal tyrosine implicated in morphogenesis (Tyr(1365)), whereas tyrosine residues in the activation loop of Met (Tyr(1230), Tyr(1234), and Tyr(1235)) were not preferred targets of the PTP. The ability of DEP-1 preferentially to dephosphorylate particular tyrosine residues that are required for Met-induced signaling suggests that DEP-1 may function in controlling the specificity of signals induced by this PTK, rather than as a simple "off-switch" to counteract PTK activity.