Inhibition of T cell signaling by mitogen-activated protein kinase-targeted hematopoietic tyrosine phosphatase (HePTP)

A New Paradigm for KIM-PTP Drug Discovery: Identification of Allosteric Sites with Potential for Selective Inhibition Using Virtual Screening and LEI Analysis

The kinase interaction motif protein tyrosine phosphatases (KIM-PTPs), HePTP, PTPSL and STEP, are involved in the negative regulation of mitogen-activated protein kinase (MAPK) signalling pathways and are important therapeutic targets for a number of diseases. We have used VSpipe, a virtual screening pipeline, to identify a ligand cluster distribution that is unique to this subfamily of PTPs. Several clusters map onto KIM-PTP specific sequence motifs in contrast to the cluster distribution obtained for PTP1B, a classic PTP that mapped to general PTP motifs. Importantly, the ligand clusters coincide with previously reported functional and substrate binding sites in KIM-PTPs. Assessment of the KIM-PTP specific clusters, using ligand efficiency index (LEI) plots generated by the VSpipe, ascertained that the binders in these clusters reside in a more drug-like chemical-biological space than those at the active site. LEI analysis showed differences between clusters across all KIM-PTPs, highlighting a distinct and specific profile for each phosphatase. The most druggable cluster sites are unexplored allosteric functional sites unique to each target. Exploiting these sites may facilitate the delivery of inhibitors with improved drug-like properties, with selectivity amongst the KIM-PTPs and over other classical PTPs.

Protein Tyrosine Phosphatases: A new paradigm in an old signaling system?

Protein Tyrosine Phosphatases reverse cellular signals initiated by growth factors receptors and other tyrosine kinases by dephosphorylating phosphotyrosine on target proteins. The activity of these enzymes is crucial for maintaining cell homeostasis, yet these enzymes have been often dismissed as humble house-keeping proteins. Understandably, mutations and changes in expression patterns of Protein Tyrosine Phosphatases are implicated in tumorigenesis and various carcinomas. The conserved nature of their catalytic domains makes drug discovery a challenging pursuit. In this review, we focus on describing the various classes of Protein Tyrosine Phosphatases and their catalytic domains. We also summarize their role in cancer and neurodegenerative diseases using specific members as the model system. Finally, we explain the dichotomy in the biological role of catalytically active vs the pseudoenzyme forms of Protein Tyrosine Phosphatases in the context of their membrane bound receptor forms. This chapter aims to provide a current understanding of these proteins, in the background of their foundational past research.

HePTP promotes migration and invasion in triple-negative breast cancer cells via activation of Wnt/β-catenin signaling

AIM

Cancer metastasis remains a major challenge for the clinical management of breast cancer, especially triple-negative breast cancer (TNBC), and the underlying molecular mechanisms remain largely unknown. The aim of this study was to explore the mechanism of TNBC metastasis.

MAIN METHODS

The expression of protein tyrosine phosphatase, non-receptor type 7 (HePTP) was detected using real time-PCR, western blot. Wound healing assay and transwell matrix assay were used to evaluate the pro-migration and pro-invasion potential of HePTP in vitro. Luciferase activity assay and nuclear extract analysis were used to evaluate Wnt/β-catenin signaling activity.

KEY FINDINGS

We reported that HePTP was overexpressed in TNBC, where it acted to drive migration and invasion of tumor cells. We showed that knockdown of HePTP significantly suppressed metastatic capacity of TNBC cells. Moreover, HePTP promoted cells migration and invasion by dephosphorylating glycogen synthase kinase 3 beta (GSK3β), thereby activating Wnt/β-catenin signaling. Additionally, we demonstrated that overexpression of HePTP in HePTP lowly expressed cells could effectively promote the migration and invasion of breast cancer cells.

SIGNIFICANCE

Our results suggest that HePTP plays a key role in the metastasis of TNBC via activating Wnt/β-catenin signaling. Hence, we propose that HePTP may serve as a novel prognostic marker and a potential therapeutic target for the treatment of TNBC.

Regulation of CD4+ T Cell Signaling and Immunological Synapse by Protein Tyrosine Phosphatases: Molecular Mechanisms in Autoimmunity

T cell activation and effector function is mediated by the formation of a long-lasting interaction established between T cells and antigen-presenting cells (APCs) called immunological synapse (IS). During T cell activation, different signaling molecules as well as the cytoskeleton and the endosomal compartment are polarized to the IS. This molecular dynamics is tightly regulated by phosphorylation networks, which are controlled by protein tyrosine phosphatases (PTPs). While some PTPs are known to be important regulators of adhesion, ligand discrimination or the stimulation threshold, there is still little information about the regulatory role of PTPs in cytoskeleton rearrangements and endosomal compartment dynamics. Besides, spatial and temporal regulation of PTPs and substrates at the IS is only barely known. Consistent with an important role of PTPs in T cell activation, multiple mutations as well as altered expression levels or dynamic behaviors have been associated with autoimmune diseases. However, the precise mechanism for the regulation of T cell activation and effector function by PTPs in health and autoimmunity is not fully understood. Herein, we review the current knowledge about the regulatory role of PTPs in CD4⁺ T cell activation, IS assembly and effector function. The potential molecular mechanisms mediating the action of these enzymes in autoimmune disorders are discussed.

The Extended Family of Protein Tyrosine Phosphatases

In higher eukaryotes, the Tyr phosphorylation status of cellular proteins results from the coordinated action of Protein Tyrosine Kinases (PTKs) and Protein Tyrosine Phosphatases (PTPs). PTPs have emerged as highly regulated enzymes with diverse substrate specificity, and proteins with Tyr-dephosphorylation or Tyr-dephosphorylation-like properties can be clustered as the PTPome. This includes proteins from the PTP superfamily, which display a Cys-based catalytic mechanism, as well as enzymes from other gene families (Asp-based phosphatases, His-based phosphatases) that have converged in protein Tyr-dephosphorylation-related functions by using non-Cys-based catalytic mechanisms. Within the Cys-based members of the PTPome, classical PTPs dephosphorylate specific phosphoTyr (pTyr) residues from protein substrates, whereas VH1-like dual-specificity PTPs dephosphorylate pTyr, pSer, and pThr residues, as well as nonproteinaceous substrates, including phosphoinositides and phosphorylated carbohydrates. In addition, several PTPs have impaired catalytic activity as a result of amino acid substitutions at their active sites, but retain regulatory functions related with pTyr signaling. As a result of their relevant biological activity, many PTPs are linked to human disease, including cancer, neurodevelopmental, and metabolic diseases, making these proteins important drug targets and molecular markers in the clinic. Here, a brief overview on the biochemistry and physiology of the different groups of proteins that belong to the mammalian PTPome is presented.

Gene Expression Profiles of Human Phosphotyrosine Phosphatases Consequent to Th1 Polarisation and Effector Function

Phosphotyrosine phosphatases (PTPs) constitute a complex family of enzymes that control the balance of intracellular phosphorylation levels to allow cell responses while avoiding the development of diseases. Despite the relevance of CD4 T cell polarisation and effector function in human autoimmune diseases, the expression profile of PTPs during T helper polarisation and restimulation at inflammatory sites has not been assessed. Here, a systematic analysis of the expression profile of PTPs has been carried out during Th1-polarising conditions and upon PKC activation and intracellular raise of Ca²⁺ in effector cells. Changes in gene expression levels suggest a previously nonnoted regulatory role of several PTPs in Th1 polarisation and effector function. A substantial change in the spatial compartmentalisation of ERK during T cell responses is proposed based on changes in the dose of cytoplasmic and nuclear MAPK phosphatases. Our study also suggests a regulatory role of autoimmune-related PTPs in controlling T helper polarisation in humans. We expect that those PTPs that regulate T helper polarisation will constitute potential targets for intervening CD4 T cell immune responses in order to generate new therapies for the treatment of autoimmune diseases.

Long-term dynamics of multisite phosphorylation

A systematic framework for exploring the long-term dynamics of a reaction network is applied to a minimal model of ERK regulation that distinguishes both monophosphorylated forms and allows for nonzero enzyme processivity. Bistability and oscillations can be observed at high levels of processivity.

Multisite phosphorylation cycles are ubiquitous in cell regulation systems and are studied at multiple levels of complexity, from molecules to organisms, with the ultimate goal of establishing predictive understanding of the effects of genetic and pharmacological perturbations of protein phosphorylation in vivo. Achieving this goal is essentially impossible without mathematical models, which provide a systematic framework for exploring dynamic interactions of multiple network components. Most of the models studied to date do not discriminate between the distinct partially phosphorylated forms and focus on two limiting reaction regimes, distributive and processive, which differ in the number of enzyme–substrate binding events needed for complete phosphorylation or dephosphorylation. Here we use a minimal model of extracellular signal-related kinase regulation to explore the dynamics of a reaction network that includes all essential phosphorylation forms and arbitrary levels of reaction processivity. In addition to bistability, which has been studied extensively in distributive mechanisms, this network can generate periodic oscillations. Both bistability and oscillations can be realized at high levels of reaction processivity. Our work provides a general framework for systematic analysis of dynamics in multisite phosphorylation systems.

p38 MAP kinases in the heart

p38 kinases are members of the mitogen-activated protein kinases (MAPK) with established contribution to a wide range of signaling pathways and different biological processes. The prototypic p38 MAPK, p38α was originally identified as an essential signaling kinase for inflammatory cytokine production Extensive studies have now revealed that p38s have critical roles in many different tissues far beyond immune regulation and inflammatory responses. In this review, we will focus on the structure and molecular biology of p38s, and their specific roles in heart, especially regarding myocyte proliferation, apoptosis, and hypertrophic responses.

Detection and Differentiation of Threonine- and Tyrosine-Monophosphorylated Forms of ERK1/2 by Capillary Isoelectric Focusing-Immunoassay

The extracellular signal regulated kinases ERK1/2 play important roles in the regulation of diverse cellular functions and have been implicated in several human diseases. In addition to the fully activated, diphosphorylated ERK1/2 protein, monophosphorylated forms of ERK1/2 have been observed, which may have distinct biological functions. We report here on the highly sensitive detection and differentiation of unphosphorylated, threonine-phosphorylated (pT), tyrosine-phosphorylated (pY) and diphosphorylated ERK1 and ERK2 by capillary isoelectric focusing followed by immunological detection (CIEF-immunoassay). Eight different phosphorylated and unphosphorylated forms of ERK1/2 were resolved according to charge. The unequivocal identification and differentiation of ERK1 and ERK2 forms monophosphorylated at either threonine or tyrosine was achieved by competitive blocking with specific phospho-peptides and different phosphorylation-sensitive antibodies. The suitability of the additional pT-ERK1/2 and pY-ERK1/2 differentiation for the time-resolved in-depth study of phospho-form distribution in response to specific stimuli is demonstrated in human neuroblastoma SH-SY5Y and monocytic THP-1 cell lines, and in human peripheral blood mononuclear cells.

Interaction of kinase-interaction-motif protein tyrosine phosphatases with the mitogen-activated protein kinase ERK2

The mitogen-activation protein kinase ERK2 is tightly regulated by multiple phosphatases, including those of the kinase interaction motif (KIM) PTP family (STEP, PTPSL and HePTP). Here, we use small angle X-ray scattering (SAXS) and isothermal titration calorimetry (ITC) to show that the ERK2:STEP complex is compact and that residues outside the canonical KIM motif of STEP contribute to ERK2 binding. Furthermore, we analyzed the interaction of PTPSL with ERK2 showing that residues outside of the canonical KIM motif also contribute to ERK2 binding. The integration of this work with previous studies provides a quantitative and structural map of how the members of a single family of regulators, the KIM-PTPs, differentially interact with their corresponding MAPKs, ERK2 and p38α.

Role of protein tyrosine phosphatase non-receptor type 7 in the regulation of TNF-α production in RAW 264.7 macrophages

Protein tyrosine phosphatases play key roles in a diverse range of cellular processes such as differentiation, cell proliferation, apoptosis, immunological signaling, and cytoskeletal function. Protein tyrosine phosphatase non-receptor type 7 (PTPN7), a member of the phosphatase family, specifically inactivates mitogen-activated protein kinases (MAPKs). Here, we report that PTPN7 acts as a regulator of pro-inflammatory TNF-α production in RAW 264.7 cells that are stimulated with lipopolysaccharide (LPS) that acts as an endotoxin and elicits strong immune responses in animals. Stimulation of RAW 264.7 cells with LPS leads to a transient decrease in the levels of PTPN7 mRNA and protein. The overexpression of PTPN7 inhibits LPS-stimulated production of TNF-α. In addition, small interfering RNA (siRNA) analysis showed that knock-down of PTPN7 in RAW 264.7 cells increased TNF-α production. PTPN7 has a negative regulatory function to extracellular signal regulated kinase 1/2 (ERK1/2) and p38 that increase LPS-induced TNF-α production in macrophages. Thus, our data presents PTPN7 as a negative regulator of TNF-α expression and the inflammatory response in macrophages.

Molecular basis of MAP kinase regulation

Mitogen-activated protein kinases (MAPKs; ERK1/2, p38, JNK, and ERK5) have evolved to transduce environmental and developmental signals (growth factors, stress) into adaptive and programmed responses (differentiation, inflammation, apoptosis). Almost 20 years ago, it was discovered that MAPKs contain a docking site in the C-terminal lobe that binds a conserved 13-16 amino acid sequence known as the D- or KIM-motif (kinase interaction motif). Recent crystal structures of MAPK:KIM-peptide complexes are leading to a precise understanding of how KIM sequences contribute to MAPK selectivity. In addition, new crystal and especially NMR studies are revealing how residues outside the canonical KIM motif interact with specific MAPKs and contribute further to MAPK selectivity and signaling pathway fidelity. In this review, we focus on these recent studies, with an emphasis on the use of NMR spectroscopy, isothermal titration calorimetry and small angle X-ray scattering to investigate these processes.

The housekeeping gene hypoxanthine guanine phosphoribosyltransferase (HPRT) regulates multiple developmental and metabolic pathways of murine embryonic stem cell neuronal differentiation

The mechanisms by which mutations of the purinergic housekeeping gene hypoxanthine guanine phosphoribosyltransferase (HPRT) cause the severe neurodevelopmental Lesch Nyhan Disease (LND) are poorly understood. The best recognized neural consequences of HPRT deficiency are defective basal ganglia expression of the neurotransmitter dopamine (DA) and aberrant DA neuronal function. We have reported that HPRT deficiency leads to dysregulated expression of multiple DA-related developmental functions and cellular signaling defects in a variety of HPRT-deficient cells, including human induced pluripotent stem (iPS) cells. We now describe results of gene expression studies during neuronal differentiation of HPRT-deficient murine ESD3 embryonic stem cells and report that HPRT knockdown causes a marked switch from neuronal to glial gene expression and dysregulates expression of Sox2 and its regulator, genes vital for stem cell pluripotency and for the neuronal/glial cell fate decision. In addition, HPRT deficiency dysregulates many cellular functions controlling cell cycle and proliferation mechanisms, RNA metabolism, DNA replication and repair, replication stress, lysosome function, membrane trafficking, signaling pathway for platelet activation (SPPA) multiple neurotransmission systems and sphingolipid, sulfur and glycan metabolism. We propose that the neural aberrations of HPRT deficiency result from combinatorial effects of these multi-system metabolic errors. Since some of these aberrations are also found in forms of Alzheimer's and Huntington's disease, we predict that some of these systems defects play similar neuropathogenic roles in diverse neurodevelopmental and neurodegenerative diseases in common and may therefore provide new experimental opportunities for clarifying pathogenesis and for devising new potential therapeutic targets in developmental and genetic disease.

The janus head of T cell aging - autoimmunity and immunodeficiency

Immune aging is best known for its immune defects that increase susceptibility to infections and reduce adaptive immune responses to vaccination. In parallel, the aged immune system is prone to autoimmune responses and many autoimmune diseases increase in incidence with age or are even preferentially encountered in the elderly. Why an immune system that suboptimally responds to exogenous antigen fails to maintain tolerance to self-antigens appears to be perplexing. In this review, we will discuss age-associated deviations in the immune repertoire and the regulation of signaling pathways that may shed light on this conundrum.

Protein tyrosine phosphatases: structure, function, and implication in human disease

Protein tyrosine phosphorylation is a key regulatory mechanism in eukaryotic cell physiology. Aberrant expression or function of protein tyrosine kinases and protein tyrosine phosphatases can lead to serious human diseases, including cancer, diabetes, as well as cardiovascular, infectious, autoimmune, and neuropsychiatric disorders. Here, we give an overview of the protein tyrosine phosphatase superfamily with its over 100 members in humans. We review their structure, function, and implications in human diseases, and discuss their potential as novel drug targets, as well as current challenges and possible solutions to developing therapeutics based on these enzymes.

Regulation of TCR signalling by tyrosine phosphatases: from immune homeostasis to autoimmunity

More than half of the known protein tyrosine phosphatases (PTPs) in the human genome are expressed in T cells, and significant progress has been made in elucidating the biology of these enzymes in T-cell development and function. Here we provide a systematic review of the current understanding of the roles of PTPs in T-cell activation, providing insight into their mechanisms of action and regulation in T-cell receptor signalling, the phenotypes of their genetically modified mice, and their possible involvement in T-cell-mediated autoimmune disease. Our projection is that the interest in PTPs as mediators of T-cell homeostasis will continue to rise with further functional analysis of these proteins, and PTPs will be increasingly considered as targets of immunomodulatory therapies.

Specificity of linear motifs that bind to a common mitogen-activated protein kinase docking groove

Mitogen-activated protein kinases (MAPKs) have a docking groove that interacts with linear "docking" motifs in binding partners. To determine the structural basis of binding specificity between MAPKs and docking motifs, we quantitatively analyzed the ability of 15 docking motifs from diverse MAPK partners to bind to c-Jun amino-terminal kinase 1 (JNK1), p38α, and extracellular signal-regulated kinase 2 (ERK2). Classical docking motifs mediated highly specific binding only to JNK1, and only those motifs with a sequence pattern distinct from the classical MAPK binding docking motif consensus differentiated between the topographically similar docking grooves of ERK and p38α. Crystal structures of four complexes of MAPKs with docking peptides, representing JNK-specific, ERK-specific, or ERK- and p38-selective binding modes, revealed that the regions located between consensus positions in the docking motifs showed conformational diversity. Although the consensus positions in the docking motifs served as anchor points that bound to common MAPK surface features and mostly contributed to docking in a nondiscriminatory fashion, the conformation of the intervening region between the anchor points mostly determined specificity. We designed peptides with tailored MAPK binding profiles by rationally changing the length and amino acid composition of intervening regions located between anchor points. These results suggest a coherent structural model for MAPK docking specificity that reveals how short linear motifs binding to a common kinase docking groove can mediate diverse interaction patterns and contribute to correct MAPK partner selection in signaling networks.

Docking interactions of hematopoietic tyrosine phosphatase with MAP kinases ERK2 and p38α

Hematopoietic tyrosine phosphatase (HePTP) regulates orthogonal MAP kinase signaling cascades by dephosphorylating both extracellular signal-regulated kinase (ERK) and p38. HePTP recognizes a docking site (D-recruitment site, DRS) on its targets using a conserved N-terminal sequence motif (D-motif). Using solution nuclear magnetic resonance spectroscopy and isothermal titration calorimetry, we compare, for the first time, the docking interactions of HePTP with ERK2 and p38α. Our results demonstrate that ERK2-HePTP interactions primarily involve the D-motif, while a contiguous region called the kinase specificity motif also plays a key role in p38α-HePTP interactions. D-Motif-DRS interactions for the two kinases, while similar overall, do show some specific differences.

Small molecule tools for functional interrogation of protein tyrosine phosphatases

The importance of protein tyrosine phosphatases (PTPs) in the regulation of cellular signalling is well established. Malfunction of PTP activity is also known to be associated with cancer, metabolic syndromes and autoimmune disorders, as well as neurodegenerative and infectious diseases. However, a detailed understanding of the roles played by the PTPs in normal physiology and in pathogenic conditions has been hampered by the absence of PTP-specific small molecule agents. In addition, the therapeutic benefits of modulating this target class are underexplored as a result of a lack of suitable chemical probes. Potent and specific PTP inhibitors could significantly facilitate functional analysis of the PTPs in complex cellular signal transduction pathways and may constitute valuable therapeutics in the treatment of several human diseases. We highlight the current challenges to and opportunities for developing PTP-specific small molecule agents. We also review available selective small molecule inhibitors developed for a number of PTPs, including PTP1B, TC-PTP, SHP2, lymphoid-specific tyrosine phosphatase, haematopoietic protein tyrosine phosphatase, CD45, PTPβ, PTPγ, PTPRO, Vaccinia H1-related phosphatase, mitogen-activated protein kinase phosphatase-1, mitogen-activated protein kinase phosphatase-3, Cdc25, YopH, mPTPA and mPTPB.

Inhibition of hematopoietic protein tyrosine phosphatase augments and prolongs ERK1/2 and p38 activation

The hematopoietic protein tyrosine phosphatase (HePTP) is implicated in the development of blood cancers through its ability to negatively regulate the mitogen-activated protein kinases (MAPKs) ERK1/2 and p38. Small-molecule modulators of HePTP activity may become valuable in treating hematopoietic malignancies such as T cell acute lymphoblastic leukemia (T-ALL) and acute myelogenous leukemia (AML). Moreover, such compounds will further elucidate the regulation of MAPKs in hematopoietic cells. Although transient activation of MAPKs is crucial for growth and proliferation, prolonged activation of these important signaling molecules induces differentiation, cell cycle arrest, cell senescence, and apoptosis. Specific HePTP inhibitors may promote the latter and thereby may halt the growth of cancer cells. Here, we report the development of a small molecule that augments ERK1/2 and p38 activation in human T cells, specifically by inhibiting HePTP. Structure-activity relationship analysis, in silico docking studies, and mutagenesis experiments reveal how the inhibitor achieves selectivity for HePTP over related phosphatases by interacting with unique amino acid residues in the periphery of the highly conserved catalytic pocket. Importantly, we utilize this compound to show that pharmacological inhibition of HePTP not only augments but also prolongs activation of ERK1/2 and, especially, p38. Moreover, we present similar effects in leukocytes from mice intraperitoneally injected with the inhibitor at doses as low as 3 mg/kg. Our results warrant future studies with this probe compound that may establish HePTP as a new drug target for acute leukemic conditions.

Structural basis of p38α regulation by hematopoietic tyrosine phosphatase

MAP kinases regulate essential cellular events, including cell growth, differentiation and inflammation. The solution structure of a complete MAPK-MAPK-regulatory protein complex, p38α-HePTP, was determined, enabling a comprehensive investigation of the molecular basis of specificity and fidelity in MAPK regulation. Structure determination was achieved by combining NMR spectroscopy and small-angle X-ray scattering data with a new ensemble calculation-refinement procedure. We identified 25 residues outside of the HePTP kinase interaction motif necessary for p38α recognition. The complex adopts an extended conformation in solution and rarely samples the conformation necessary for kinase deactivation. Complex formation also does not affect the N-terminal lobe, the activation loop of p38α or the catalytic domain of HePTP. Together, these results show how the downstream tyrosine phosphatase HePTP regulates p38α and provide for fundamentally new insights into MAPK regulation and specificity.

Resting and active states of the ERK2:HePTP complex

The MAP kinase ERK2 (ERK2, extracellular signal-regulated kinase 2) is regulated by numerous phosphatases that tightly control its activity. For example, the hematopoietic tyrosine phosphatase (HePTP) negatively regulates T cell activation in lymphocytes via ERK2 dephosphorylation. However, only very limited structural information is available for these biologically important complexes. Here, we use small-angle X-ray scattering combined with EROS ensemble refinement to characterize the structures of the resting and active states of ERK2:HePTP complexes. Our data show that the resting state ERK2:HePTP complex adopts a highly extended, dynamic conformation that becomes compact and ordered in the active state complex. This work experimentally demonstrates that these complexes undergo significant dynamic structural changes in solution and provides the first structural insight into an active state MAPK complex.

Mitogen-activated protein kinase (MAPK) phosphatase 3-mediated cross-talk between MAPKs ERK2 and p38alpha

MAPK phosphatase 3 (MKP3) is highly specific for ERK1/2 inactivation via dephosphorylation of both phosphotyrosine and phosphothreonine critical for enzymatic activation. Here, we show that MKP3 is able to effectively dephosphorylate the phosphotyrosine, but not phosphothreonine, in the activation loop of p38α in vitro and in intact cells. The catalytic constant of the MKP3 reaction for p38α is comparable with that for ERK2. Remarkably, MKP3, ERK2, and phosphorylated p38α can form a stable ternary complex in solution, and the phosphatase activity of MKP3 toward p38α substrate is allosterically regulated by ERK2-MKP3 interaction. This suggests that MKP3 not only controls the activities of ERK2 and p38α but also mediates cross-talk between these two MAPK pathways. The crystal structure of bisphosphorylated p38α has been determined at 2.1 Å resolution. Comparisons between the phosphorylated MAPK structures reveal the molecular basis of MKP3 substrate specificity.

Inhibition of the Hematopoietic Protein Tyrosine Phosphatase by Phenoxyacetic Acids

Protein tyrosine phosphatases (PTPs) have only recently become the focus of attention in the search for novel drug targets despite the fact that they play vital roles in numerous cellular processes and are implicated in many human diseases. The hematopoietic protein tyrosine phosphatase (HePTP) is often found dysregulated in preleukemic myelodysplastic syndrome (MDS), as well as in acute myelogenous leukemia (AML). Physiological substrates of HePTP include the mitogen-activated protein kinases (MAPKs) ERK1/2 and p38. Specific modulators of HePTP catalytic activity will be useful for elucidating mechanisms of MAPK regulation in hematopietic cells, and may also provide treatments for hematopoietic malignancies such as AML. Here we report the discovery of phenoxyacetic acids as inhibitors of HePTP. Structure-activity relationship (SAR) analysis and in silico docking studies reveal the molecular basis of HePTP inhibition by these compounds. We also show that these compounds are able to penetrate cell membranes and inhibit HePTP in human T lymphocytes.

NADPH oxidase-mediated redox signaling: roles in cellular stress response, stress tolerance, and tissue repair

NADPH oxidase (Nox) has a dedicated function of generating reactive oxygen species (ROS). Accumulating evidence suggests that Nox has an important role in signal transduction in cellular stress responses. We have reviewed the current evidence showing that the Nox system can be activated by a collection of chemical, physical, and biological cellular stresses. In many circumstances, Nox activation fits to the cellular stress response paradigm, in that (1) the response can be initiated by various forms of cellular stresses; (2) Nox-derived ROS may activate mitogen-activated protein kinases (extracellular signal-regulated kinase, p38) and c-Jun NH(2)-terminal kinase, which are the core of the cell stress-response signaling network; and (3) Nox is involved in the development of stress cross-tolerance. Activation of the cell survival pathway by Nox may promote cell adaptation to stresses, whereas Nox may also convey signals toward apoptosis in irreversibly injured cells. At later stage after injury, Nox is involved in tissue repair by modulating cell proliferation, angiogenesis, and fibrosis. We suggest that Nox may have an integral role in cell stress responses and the subsequent tissue repair process. Understanding Nox-mediated redox signaling mechanisms may be of prominent significance at the crossroads of directing cellular responses to stress, aiming at either enhancing the stress resistance (in such situations as preventing ischemia-reperfusion injuries and accelerating wound healing) or sensitizing the stress-induced cytotoxicity for proliferative diseases such as cancer. Therefore, an optimal outcome of interventions on Nox will only be achieved when this is dealt with in a timely and disease-and stage-specific manner.

Visualizing active-site dynamics in single crystals of HePTP: opening of the WPD loop involves coordinated movement of the E loop

Phosphotyrosine hydrolysis by protein tyrosine phosphatases (PTPs) involves substrate binding by the PTP loop and closure over the active site by the WPD loop. The E loop, located immediately adjacent to the PTP and WPD loops, is conserved among human PTPs in both sequence and structure, yet the role of this loop in substrate binding and catalysis is comparatively unexplored. Hematopoietic PTP (HePTP) is a member of the kinase interaction motif (KIM) PTP family. Compared to other PTPs, KIM-PTPs have E loops that are unique in both sequence and structure. In order to understand the role of the E loop in the transition between the closed state and the open state of HePTP, we identified a novel crystal form of HePTP that allowed the closed-state-to-open-state transition to be observed within a single crystal form. These structures, which include the first structure of the HePTP open state, show that the WPD loop adopts an 'atypically open' conformation and, importantly, that ligands can be exchanged at the active site, which is critical for HePTP inhibitor development. These structures also show that tetrahedral oxyanions bind at a novel secondary site and function to coordinate the PTP, WPD, and E loops. Finally, using both structural and kinetic data, we reveal a novel role for E-loop residue Lys182 in enhancing HePTP catalytic activity through its interaction with Asp236 of the WPD loop, providing the first evidence for the coordinated dynamics of the WPD and E loops in the catalytic cycle, which, as we show, is relevant to multiple PTP families.

T cell adhesion primes antigen receptor-induced calcium responses through a transient rise in adenosine 3',5'-cyclic monophosphate

It is well established that sustained increases in cyclic AMP (cAMP) such as those triggered by forskolin inhibit T cell activation. We describe here an unexpected phenomenon: in T cells, a transient cAMP increase triggered by the interaction with a dendritic cell strongly potentiates T cell receptor (TCR) signaling. We discovered this effect by examining the molecular basis of the adhesion-dependent sensitization of T cells. T cell adhesion caused extracellular-signal-regulated kinase (ERK) activation, which was necessary for the sensitization process. T cell sensitization could be mimicked in suspended cells by the uncaging of caged cAMP upon ultraviolet illumination. Calcium responses occurring in T cells upon interaction with dendritic cells were strongly inhibited when protein kinase A activation was blocked. Thus, whereas sustained cAMP increases are well known to inhibit TCR signaling, transient cAMP increases occurring physiologically upon formation of an immunological synapse facilitate antigen detection.

Structural basis of substrate recognition by hematopoietic tyrosine phosphatase

Hematopoietic tyrosine phosphatase (HePTP) is one of three members of the kinase interaction motif (KIM) phosphatase family which also includes STEP and PCPTP1. The KIM-PTPs are characterized by a 15 residue sequence, the KIM, which confers specific high-affinity binding to their only known substrates, the MAP kinases Erk and p38, an interaction which is critical for their ability to regulate processes such as T cell differentiation (HePTP) and neuronal signaling (STEP). The KIM-PTPs are also characterized by a unique set of residues in their PTP substrate binding loops, where 4 of the 13 residues are differentially conserved among the KIM-PTPs as compared to more than 30 other class I PTPs. One of these residues, T106 in HePTP, is either an aspartate or asparagine in nearly every other PTP. Using multiple techniques, we investigate the role of these KIM-PTP specific residues in order to elucidate the molecular basis of substrate recognition by HePTP. First, we used NMR spectroscopy to show that Erk2-derived peptides interact specifically with HePTP at the active site. Next, to reveal the molecular details of this interaction, we solved the high-resolution three-dimensional structures of two distinct HePTP-Erk2 peptide complexes. Strikingly, we were only able to obtain crystals of these transient complexes using a KIM-PTP specific substrate-trapping mutant, in which the KIM-PTP specific residue T106 was mutated to an aspartic acid (T106D). The introduced aspartate side chain facilitates the coordination of the bound peptides, thereby stabilizing the active dephosphorylation complex. These structures establish the essential role of HePTP T106 in restricting HePTP specificity to only those substrates which are able to interact with KIM-PTPs via the KIM (e.g., Erk2, p38). Finally, we describe how this interaction of the KIM is sufficient for overcoming the otherwise weak interaction at the active site of KIM-PTPs.

Sequence-specific 1H, 13C and 15N backbone resonance assignments of the 34 kDa catalytic domain of human PTPN7

PTPN7 is a protein tyrosine phosphatase responsible for inactivation of MAPK in leukocytes. Here we report the backbone resonance assignments of the 34 kDa phosphatase domain of human PTPN7, which is amplified in myeloid malignancies and deleted in lymphoproliferative disorders.

Enzymatic activity and substrate specificity of mitogen-activated protein kinase p38alpha in different phosphorylation states

The mitogen-activated protein (MAP) kinases are essential signaling molecules that mediate many cellular effects of growth factors, cytokines, and stress stimuli. Full activation of the MAP kinases requires dual phosphorylation of the Thr and Tyr residues in the TXY motif of the activation loop by MAP kinase kinases. Down-regulation of MAP kinase activity can be initiated by multiple serine/threonine phosphatases, tyrosine-specific phosphatases, and dual specificity phosphatases (MAP kinase phosphatases). This would inevitably lead to the formation of monophosphorylated MAP kinases. However, the biological functions of these monophosphorylated MAP kinases are currently not clear. In this study, we have prepared MAP kinase p38alpha, a member of the MAP kinase family, in all phosphorylated forms and characterized their biochemical properties. Our results indicated the following: (i) p38alpha phosphorylated at both Thr-180 and Tyr-182 was 10-20-fold more active than p38alpha phosphorylated at Thr-180 only, whereas p38alpha phosphorylated at Tyr-182 alone was inactive; (ii) the dual-specific MKP5, the tyrosine-specific hematopoietic protein-tyrosine phosphatase, and the serine/threonine-specific PP2Calpha are all highly specific for the dephosphorylation of p38alpha, and the dephosphorylation rates were significantly affected by different phosphorylated states of p38alpha; (iii) the N-terminal domain of MPK5 has no effect on enzyme catalysis, whereas deletion of the MAP kinase-binding domain in MKP5 leads to a 370-fold decrease in k(cat)/K(m) for the dephosphorylation of p38alpha. This study has thus revealed the quantitative contributions of phosphorylation of Thr, Tyr, or both to the activation of p38alpha and to the substrate specificity for various phosphatases.

Immunohistochemical analyses of phosphatases in childhood B-cell lymphoma: lower expression of PTEN and HePTP and higher number of positive cells for nuclear SHP2 in B-cell lymphoma cases compared to controls

Although many pediatric B-cell lymphoma patients are being cured today, much is still unknown about the pathogenesis of this disease. Protein tyrosine phosphatases are involved in the control of survival, growth, and differentiation of cells. The authors have analyzed 26 pediatric B-cell lymphoma cases for the expression of a panel of phosphatases and report a statistically significant lower expression intensity of PTEN and HePTP and higher nuclear SHP2 expression in B-cell lymphoma cases compared to lymphoid tissue. Knowledge about the expression of key regulatory proteins in pediatric B-cell lymphomas is necessary for revealing the complex molecular background of this disease.

Protein tyrosine phosphatases in autoimmunity

Protein tyrosine phosphatases (PTPs) are important regulators of many cellular functions and a growing number of PTPs have been implicated in human disease conditions, such as developmental defects, neoplastic disorders, and immunodeficiency. Here, we review the involvement of PTPs in human autoimmunity. The leading examples include the allelic variant of the lymphoid tyrosine phosphatase (PTPN22), which is associated with multiple autoimmune diseases, and mutations that affect the exon-intron splicing of CD45 (PTPRC). We also find it likely that additional PTPs are involved in susceptibility to autoimmune and inflammatory diseases. Finally, we discuss the possibility that PTPs regulating the immune system may serve as therapeutic targets.

TCR-induced downregulation of protein tyrosine phosphatase PEST augments secondary T cell responses

We report that the protein tyrosine phosphatase PTP-PEST is expressed in resting human and mouse CD4(+) and CD8(+) T cells, but not in Jurkat T leukemia cells, and that PTP-PEST protein, but not mRNA, was dramatically downregulated in CD4(+) and CD8(+) primary human T cells upon T cell activation. This was also true in mouse CD4(+) T cells, but less striking in mouse CD8(+) T cells. PTP-PEST reintroduced into Jurkat at levels similar to those in primary human T cells, was a potent inhibitor of TCR-induced transactivation of reporter genes driven by NFAT/AP-1 and NF-kappaB elements and by the entire IL-2 gene promoter. Introduction of PTP-PEST into previously activated primary human T cells also reduced subsequent IL-2 production by these cells in response to TCR and CD28 stimulation. The inhibitory effect of PTP-PEST was associated with dephosphorylation the Lck kinase at its activation loop site (Y394), reduced early TCR-induced tyrosine phosphorylation, reduced ZAP-70 phosphorylation and inhibition of MAP kinase activation. We propose that PTP-PEST tempers T cell activation by dephosphorylating TCR-proximal signaling molecules, such as Lck, and that down-regulation of PTP-PEST may be a reason for the increased response to TCR triggering of previously activated T cells.

Anoikis effector Bit1 negatively regulates Erk activity

Bcl-2 inhibitor of transcription (Bit1) is a mitochondrial protein that functions as a peptidyl-tRNA hydrolase, but, when released into the cytoplasm, it elicits apoptosis. The proapoptotic function is uniquely counteracted by integrin-mediated cell attachment. We generated a conditional KO mouse of the Bit1 gene by using the Cre-LoxP recombination system. Bit1-null mice were born alive but with some developmental abnormalities. They developed a runting syndrome after birth and died within the first 2 weeks. Cultured fibroblasts from the Bit1-null embryos [mouse embryo fibroblasts (MEFs)] were more resistant to cell death induced by loss of attachment to extracellular matrix (anoikis) than cells from the wild-type or heterozygous littermates. MEFs and tissues from Bit1 KO mice displayed a marked increase in Erk phosphorylation. Knocking down Bit1 expression in cultured cells resulted in increased Erk activation, and partially knocking down Erk reversed the increased anoikis resistance of Bit1 knockdown. The enhanced Erk activation was associated with decreased Erk phosphatase activity. These studies establish the physiological significance of Bit1 activity and begin to delineate a Bit1 signaling pathway that acts through Erk regulation.

Several dual specificity phosphatases coordinate to control the magnitude and duration of JNK activation in signaling response to oxidative stress

Mitogen-activated protein kinases (MAPKs) are important mediators that integrate signaling from upstream pathways in response to various environmental cues. In order to control appropriate gene expression through phosphorylation of transcription factors, the activity of MAPKs must be tightly regulated by the actions coordinated between protein kinases and phosphatases. In this study, we explore the underlying mechanism through which the oxidative stress-activated c-Jun N-terminal kinases (JNKs), members of MAPKs, are regulated by dual specificity phosphatases (DUSPs). DUSPs are a group of enzymes that belong to the superfamily of protein-tyrosine phosphatases. They are able to recognize phospho-Ser/Thr and phospho-Tyr residues in substrates. Using quantitative real time PCR, we found that stimulation of human embryonic kidney 293T cells with H(2)O(2) or xanthine/xanthine oxidase led to inducible expression of multiple DUSPs. We used RNA interference to characterize the functional role of these DUSPs and found rapid and transient induction of DUSP1 and DUSP10 to be essential for determining the appropriate magnitude of JNK activation in response to oxidative stress. The transcription factor ATF2, which is phosphorylated and activated by JNK, is a critical mediator for inducible expression of DUSP1 and DUSP10 in this signaling pathway. We further demonstrated that DUSP4 and DUSP16, both showing significant late phase induction, dephosphorylate JNK effectively, causing the down-regulation of the signaling cascade. Thus, this study provides new insights into the role of several DUSPs that coordinate with each other to control the magnitude and duration of JNK activity in response to oxidative stress.

Regulation of MAP kinases by MAP kinase phosphatases

MAP kinase phosphatases (MKPs) catalyze dephosphorylation of activated MAP kinase (MAPK) molecules and deactivate them. Therefore, MKPs play an important role in determining the magnitude and duration of MAPK activities. MKPs constitute a structurally distinct family of dual-specificity phosphatases. The MKP family members share the sequence homology and the preference for MAPK molecules, but they are different in substrate specificity among MAPK molecules, tissue distribution, subcellular localization and inducibility by extracellular stimuli. Our understanding of their protein structure, substrate recognition mechanisms, and regulatory mechanisms of the enzymatic activity has greatly increased over the past few years. Furthermore, although there are a number of MKPs, that have similar substrate specificities, non-redundant roles of MKPs have begun to be identified. Here we focus on recent findings regarding regulation and function of the MKP family members as physiological regulators of MAPK signaling.

Differential phosphorylation of NG2 proteoglycan by ERK and PKCalpha helps balance cell proliferation and migration

Two distinct Thr phosphorylation events within the cytoplasmic domain of the NG2 proteoglycan help regulate the cellular balance between proliferation and motility. Protein kinase Calpha mediates the phosphorylation of NG2 at Thr2256, resulting in enhanced cell motility. Extracellular signal-regulated kinase phosphorylates NG2 at Thr2314, stimulating cell proliferation. The effects of NG2 phosphorylation on proliferation and motility are dependent on beta1-integrin activation. Differential cell surface localization of the two distinctly phosphorylated forms of NG2 may be the mechanism by which the NG2-beta1-integrin interaction promotes proliferation in one case and motility in the other. NG2 phosphorylated at Thr2314 colocalizes with beta1-integrin on microprotrusions from the apical cell surface. In contrast, NG2 phosphorylated at Thr2256 colocalizes with beta1-integrin on lamellipodia at the leading edges of cells. Thus, phosphorylation and the resulting site of NG2-integrin localization may determine the specific downstream effects of integrin signaling.

The striatal-enriched protein tyrosine phosphatase gates long-term potentiation and fear memory in the lateral amygdala

BACKGROUND

Formation of long-term memories is critically dependent on extracellular-regulated kinase (ERK) signaling. Activation of the ERK pathway by the sequential recruitment of mitogen-activated protein kinases is well understood. In contrast, the proteins that inactivate this pathway are not as well characterized.

METHODS

Here we tested the hypothesis that the brain-specific striatal-enriched protein tyrosine phosphatase (STEP) plays a key role in neuroplasticity and fear memory formation by its ability to regulate ERK1/2 activation.

RESULTS

STEP co-localizes with the ERKs within neurons of the lateral amygdala. A substrate-trapping STEP protein binds to the ERKs and prevents their nuclear translocation after glutamate stimulation in primary cell cultures. Administration of TAT-STEP into the lateral amygdala (LA) disrupts long-term potentiation (LTP) and selectively disrupts fear memory consolidation. Fear conditioning induces a biphasic activation of ERK1/2 in the LA with an initial activation within 5 minutes of training, a return to baseline levels by 15 minutes, and an increase again at 1 hour. In addition, fear conditioning results in the de novo translation of STEP. Inhibitors of ERK1/2 activation or of protein translation block the synthesis of STEP within the LA after fear conditioning.

CONCLUSIONS

Together, these data imply a role for STEP in experience-dependent plasticity and suggest that STEP modulates the activation of ERK1/2 during amygdala-dependent memory formation. The regulation of emotional memory by modulating STEP activity may represent a target for the treatment of psychiatric disorders such as posttraumatic stress disorder (PTSD), panic, and anxiety disorders.

Light-inducible and clock-controlled expression of MAP kinase phosphatase 1 in mouse central pacemaker neurons

MAP kinase phosphatase 1 (MKP1) is a negative regulator for the mitogen-activated protein kinase (MAPK)-mediated signal transduction, a key pathway that leads to the regulated expression of circadian clock genes. Here the authors analyzed mkp1 expression by in situ hybridization and found that mkp1 is a light-inducible and clock-controlled gene expressed in the central pacemaker neurons of the hypothalamic SCN. Interestingly, mkp1 presents a marked similarity to the clock core gene per1 in terms of the gene expression profiles as well as the gene promoter organization. Both mkp1 and per1 are subject to bimodal regulation in the SCN: the external light-dependent acute up-regulation and the functional clock-dependent circadian oscillation. Consistent with this, the authors show that mkp1 gene has a per1-like promoter that contains 2 functionally distinct elements: cAMP-responsive element (CRE) and E-box. CRE sites present in the mkp1 promoter constitute the functional binding sites for the CRE binding protein (CREB), which serves as an important regulator that mediates the light-induced signaling cascades in the SCN neurons. Furthermore, the authors show that the E-box present in the mkp1 promoter is necessary and sufficient for transcriptional control exerted by circadian clock core regulators that include a positive complex CLOCK/BMAL1 and a negative factor CRY1. The authors' studies on mkp1 have identified for the first time a gene encoding a phosphatase that functions in light-dependent and time-of-day-dependent manners in the mammalian central clock structure SCN.

Nonreceptor Protein-Tyrosine Phosphatases in Immune Cell Signaling

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

Regulation of the ring finger E3 ligase Siah2 by p38 MAPK

The RING finger ubiquitin ligase Siah2 controls the stability of various substrates involved in stress and hypoxia responses, including the PHD3, which controls the stability of HIF-1alpha. In the present study we determined the role of Siah2 phosphorylation in the regulation of its activity toward PHD3. We show that Siah2 is subject to phosphorylation by p38 MAPK, which increases Siah2-mediated degradation of PHD3. Consistent with these findings, MKK3/MKK6 double-deficient cells, which cannot activate p38 kinases, exhibit impaired Siah2-dependent degradation of PHD3. Phosphopeptide mapping identified T24 and S29 as the primary phospho-acceptor sites. Phospho-mutant forms of Siah2 (S29A or T24A/S29A) exhibit impaired degradation of PHD3, particularly after hypoxia. Conversely, a phospho-mimic form of Siah2 (T24E/S29D) exhibits stronger degradation of PHD3, compared with wild type Siah2. Whereas phospho-mutant Siah2 exhibits weaker association with PHD3, phospho-mimic Siah2 associates as well as wild type and is localized within the perinuclear region, suggesting that phosphorylation of Siah2 affects its subcellular localization and, consequently, the degree of its association with PHD3. In all, our findings reveal the phosphorylation of Siah2 by p38 and the implications of such phosphorylation for Siah2 activity toward PHD3.

MAPK-specific tyrosine phosphatases: new targets for drug discovery?

Protein tyrosine phosphatases (PTPs) have key roles in a diverse range of cellular processes, and their dysregulation is associated with several human diseases. Many PTPs are recognized as potential drug targets; however, inhibitor development has focused only on a small number of enzymes, most notably PTP1B for type II diabetes and obesity, and MKP1 and CDC25 for cancer. The future challenge of selective-inhibitor development for PTPs will be significantly facilitated by the recent rapid progress in the structural biology of the 'PTPome'. In this article, we focus on the family of mitogen-activated protein kinase (MAPK)-specific tyrosine phosphatases--PTPN5 [also called striatal-enriched phosphatase (STEP)], PTPN7 (also called hematopoietic PTP) and PTPRR (also called PC12 PTP or STEP-like PTP)--and discuss approaches for achieving selectivity for the MAPK-PTPs at the molecular level using recently determined high-resolution X-ray crystal structures. We believe that the development of specific inhibitors would provide a valuable set of experimental pharmacological tools for investigating the physiological role of these phosphatases and exploring their emerging role in human disease.

Synaptic plasticity: one STEP at a time

Striatal enriched tyrosine phosphatase (STEP) has recently been identified as a crucial player in the regulation of synaptic function. It is restricted to neurons within the CNS and acts by downregulating the activity of MAP kinases, the tyrosine kinase Fyn and NMDA receptors. By modulating these substrates, STEP acts on several parallel pathways that impact upon the progression of synaptic plasticity. Here, we review recent advances that demonstrate the importance of STEP in normal cognitive function, and its possible involvement in cognitive disorders such as Alzheimer's disease.

Crystal structures and inhibitor identification for PTPN5, PTPRR and PTPN7: a family of human MAPK-specific protein tyrosine phosphatases

Protein tyrosine phosphatases PTPN5, PTPRR and PTPN7 comprise a family of phosphatases that specifically inactivate MAPKs (mitogen-activated protein kinases). We have determined high-resolution structures of all of the human family members, screened them against a library of 24000 compounds and identified two classes of inhibitors, cyclopenta[c]quinolinecarboxylic acids and 2,5-dimethylpyrrolyl benzoic acids. Comparative structural analysis revealed significant differences within this conserved family that could be explored for the design of selective inhibitors. PTPN5 crystallized, in two distinct crystal forms, with a sulphate ion in close proximity to the active site and the WPD (Trp-Pro-Asp) loop in a unique conformation, not seen in other PTPs, ending in a 3(10)-helix. In the PTPN7 structure, the WPD loop was in the closed conformation and part of the KIM (kinase-interaction motif) was visible, which forms an N-terminal aliphatic helix with the phosphorylation site Thr66 in an accessible position. The WPD loop of PTPRR was open; however, in contrast with the structure of its mouse homologue, PTPSL, a salt bridge between the conserved lysine and aspartate residues, which has been postulated to confer a more rigid loop structure, thereby modulating activity in PTPSL, does not form in PTPRR. One of the identified inhibitor scaffolds, cyclopenta[c]quinoline, was docked successfully into PTPRR, suggesting several possibilities for hit expansion. The determined structures together with the established SAR (structure-activity relationship) propose new avenues for the development of selective inhibitors that may have therapeutic potential for treating neurodegenerative diseases in the case of PTPRR or acute myeloblastic leukaemia targeting PTPN7.

Protein phosphatases in MAPK signalling: we keep learning from yeast

Because of their key role in cell signalling, a rigorous regulation of mitogen-activated protein kinases (MAPKs) is essential in eukaryotic physiology. Whereas the use of binding motifs and scaffold proteins guarantees the selective activation of a specific MAPK pathway, activating kinases and downregulating phosphatases control the appropriate intensity and timing of MAPK activation. Tyrosine, serine/threonine and dual-specificity phosphatases co-ordinately dephosphorylate and thereby inactivate MAPKs. In budding yeast, enzymes that belong to these three types of phosphatases have been shown to counteract the MAPKs that govern the cellular response to varied extracellular stimuli. Studies carried out with these yeast phosphatases have expanded our knowledge of essential key aspects of the biology of these negative regulators, such as their function, the mechanisms that operate in their modulation by MAPK pathways and their binding to MAPK substrates. Furthermore, yeast MAPK phosphatases have been shown to play additional and essential roles in MAPK-mediated signalling, controlling MAPK localization or cross-talk among pathways. This review stresses the importance of these negative regulators in eukaryotic signalling by discussing the recent developments and perspectives in the study of yeast MAPK phosphatases.

Lipid raft targeting of hematopoietic protein tyrosine phosphatase by protein kinase C theta-mediated phosphorylation

Protein kinase C theta (PKC theta) is unique among PKC isozymes in its translocation to the center of the immune synapse in T cells and its unique downstream signaling. Here we show that the hematopoietic protein tyrosine phosphatase (HePTP) also accumulates in the immune synapse in a PKC theta-dependent manner upon antigen recognition by T cells and is phosphorylated by PKC theta at Ser-225, which is required for lipid raft translocation. Immune synapse translocation was completely absent in antigen-specific T cells from PKC theta-/- mice. In intact T cells, HePTP-S225A enhanced T-cell receptor (TCR)-induced NFAT/AP-1 transactivation, while the acidic substitution mutant was as efficient as wild-type HePTP. We conclude that HePTP is phosphorylated in the immune synapse by PKC theta and thereby targeted to lipid rafts to temper TCR signaling. This represents a novel mechanism for the active immune synapse recruitment and activation of a phosphatase in TCR signaling.

Structural basis of docking interactions between ERK2 and MAP kinase phosphatase 3

Mitogen-activated protein (MAP) kinases are central components of signal transduction pathways for cell proliferation, stress responses, and differentiation. Signaling efficiency and specificity are modulated in large part by docking interactions between individual MAP kinase and the kinase interaction motif (KIM), (R/K)(2-3)-X(1-6)-Phi(A)-X-Phi(B), in its cognate kinases, phosphatases, scaffolding proteins, and substrates. We have determined the crystal structure of extracellular signal-regulated protein kinase 2 bound to the KIM peptide from MAP kinase phosphatase 3, an extracellular signal-regulated protein kinase 2-specific phosphatase. The structure reveals that the KIM docking site, situated in a noncatalytic region opposite of the kinase catalytic pocket, is comprised of a highly acidic patch and a hydrophobic groove, which engage the basic and Phi(A)-X-Phi(B) residues, respectively, in the KIM sequence. The specific docking interactions observed in the structure consolidate all known biochemical data. In addition, structural comparison indicates that the KIM docking site is conserved in all MAP kinases. The results establish a structural model for understanding how MAP kinases interact with their regulators and substrates and provide new insights into how MAP kinase docking specificity can be achieved.

Beta-agonists modulate T-cell functions via direct actions on type 1 and type 2 cells

Although the beta2-adrenergic receptor (beta2AR) is the most extensively characterized G-protein-coupled receptor (GPCR), the effects of beta-agonists on T-cell subtype function remain poorly understood. In contrast to studies suggesting lack of beta2AR expression on type 2 T cells, we demonstrate that type 2 interleukin-13+ (IL-13+) T cells (CD4+ or CD8+) in human peripheral blood lymphocytes (PBLs) can respond directly to beta-agonist, with effects including induction of protein kinase A (PKA) activity and associated inhibition of CD3-stimulated CD25 expression; CD3-stimulated IL-13, interferon-gamma (IFN-gamma), and IL-2 production; and p38 mitogen-activated protein kinase (MAPK) phosphorylation. PGE2 was more efficacious than beta-agonist in activating PKA and inhibiting cytokine production. beta-agonist and PGE2 also inhibited phorbol myristate acetate (PMA) + calcimycin-stimulated IFN-gamma and IL-2 (but not IL-13) production, suggesting that upstream CD3-initiated signaling is not the sole locus of PKA actions. Differential regulation of PMA-stimulated p38, p42/p44, and NF-kappaB explained the capacity of PGE2 and beta-agonist to inhibit IFN-gamma but not IL-13 production. The inhibition of CD3 + CD28-stimulated IL-13 production by both beta-agonist and PGE2 was reversed at low agonist concentrations, resulting in enhanced IL-13, but not IFN-gamma or IL-2, production. These findings identify direct effects of beta2AR activation on T-cell subtypes and suggest a complex role for GPCRs and PKA activity in modulating T-cell functions.