PTPs versus PTKs: the redox side of the coin

The phosphorylation of tyrosine, and to a lesser extent threonine and serine, plays a key role in the regulation of signal transduction during a plethora of eukaryotic cell functions, including cell activation, cell-cycle progression, cytoskeletal rearrangement and cell movement, differentiation, apoptosis and metabolic homeostasis. In vivo, tyrosine phosphorylation is reversible and dynamic; the phosphorylation states are governed by the opposing activities of protein tyrosine kinases (PTKs)2 and protein tyrosine phosphatases (PTPs). Reactive oxygen species (ROS) act as cellular messengers in cellular processes such as mitogenic signal transduction, gene expression, regulation of cell proliferation, senescence and apoptosis. Redox regulated proteins include PTPs and PTKs, although with opposite regulation of enzymatic activity. Transient oxidation of thiols in PTPs leads to their inactivation by the formation of either an intramolecular S-S bridge or a sulfenyl-amide bond. Conversely, oxidation of PTKs leads to their activation, either by direct SH modification or, indirectly, by concomitant inhibition of PTPs that guides to sustained activation of PTKs. This review focuses on the redox regulation of both PTPs and PTKs and the interplay of their specular regulation.     

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

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

Structure and catalytic mechanism of human protein tyrosine phosphatome

Together with protein tyrosine kinases (PTKs), protein tyrosine phosphatases (PTPs) serve as hallmarks in cellular signal transduction by controlling the reversible phosphorylation of their substrates. The human genome is estimated to encode more than 100 PTPs, which can be divided into eleven sub-groups according to their structural and functional characteristics. All the crystal structures of catalytic domains of sub-groups have been elucidated, enabling us to understand their precise catalytic mechanism and to compare their structures across all sub-groups. In this review, I describe the structure and mechanism of catalytic domains of PTPs in the structural context. [BMB Reports 2012; 45(12): 693-699]     

Kinases and glutathione transferases: selective and sensitive targeting

Kinases, representing almost 500 proteins in the human genome, are responsible for catalyzing the phosphorylation reaction of amino acid residues at their targets. As the largest family of kinases, the protein tyrosine kinases (PTKs) have roles in controlling the essential cellular activities, and their deregulation is generally related to pathologic conditions. The recent efforts on identifying their signal transducer or mediator role in cellular signaling revealed the interaction of PTKs with numerous enzymes of different classes, such as Ser/Thr kinases (STKs), glutathione transferases (GSTs), and receptor tyrosine kinases (RTKs). In either regulation or enhancing the signaling, PTKs are determined in close interaction with these enzymes, under specific cellular conditions, such as oxidative stress and inflammation. In this concept, intensive research on thiol metabolizing enzymes recently showed their involvement in the physiologic functions in cellular signaling besides their well known traditional role in antioxidant defense. The shared signaling components between PTK and GST family enzymes will be discussed in depth in this research review to evaluate the results of recent studies important in drug targeting for therapeutic intervention, such as cell viability, migration, differentiation and proliferation.     

The role of protein tyrosine phosphatases in colorectal cancer

Colorectal cancer is one of the most common oncogenic diseases in the Western world. Several cancer associated cellular pathways have been identified, in which protein phosphorylation and dephosphorylation, especially on tyrosine residues, are one of most abundant regulatory mechanisms. The balance between these processes is under tight control by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Aberrant activity of oncogenic PTKs is present in a large portion of human cancers. Because of the counteracting role of PTPs on phosphorylation-based activation of signal pathways, it has long been thought that PTPs must act as tumor suppressors. This dogma is now being challenged, with recent evidence showing that dephosphorylation events induced by some PTPs may actually stimulate tumor formation. As such, PTPs might form a novel attractive target for anticancer therapy. In this review, we summarize the action of different PTPs, the consequences of their altered expression in colorectal cancer, and their potential as target for the treatment of this deadly disease.     

蛋白酪氨酸磷酸酶-PEST在生理调节及相关疾病中的作用

蛋白质分子中酪氨酸残基可逆性的磷酸化是细胞内信号分子传导的基本方式。两类作用相反的酶参与磷酸化的调节:蛋白酪氨酸激酶(protein tyrosinekinase,PTK)和蛋白酪氨酸磷酸酶(protein tyrosine phosphatase,PTP)。含脯氨酸-谷氨酸-丝氨酸-苏氨酸(P-E-S-T)结构域的蛋白酪氨酸磷酸酶(PTP-PEST)属于非受体型酪氨酸磷酸酶类,其本身能与多种蛋白质相互作用,并在细胞迁移、免疫细胞活化和胚胎发育等生理过程中发挥重要作用。本文对PTP-PEST的结构特点、生理功效、介导的信号传导途径和近年来PTP-PEST在疾病中的作用作一综述。     

Redox control of catalytic activities of membrane-associated protein tyrosine kinases

Protein tyrosine kinases (PTKs) play key roles in starting the signal transduction network for cellular development and functions. A number of both receptor-type and non-receptor-type PTKs, which are normally at a resting state, are initially activated in association with functions of the cell membrane and membrane rafts. Results of recent studies have suggested that these membrane-associated mechanisms for activation of PTKs consist of the two steps that are under redox control. The first step is activation of cell surface receptors through chemical crosslinkage or aggregation of receptors and membrane rafts, which leads to production of reactive oxygen species (ROS) as second messengers of intracellular signal transduction. The second step involves chemical modification of PTKs at the highly conserved cysteine in the MXXCW motif as a global switch for starting the tyrosine phosphorylation-dependent local switch for activation of the catalytic activity of the enzyme.     

SH2 domain-mediated interaction of inhibitory protein tyrosine kinase Csk with protein tyrosine phosphatase-HSCF

The protein tyrosine kinase (PTK) Csk is a potent negative regulator of several signal transduction processes, as a consequence of its exquisite ability to inactivate Src-related PTKs. This function requires not only the kinase domain of Csk, but also its Src homology 3 (SH3) and SH2 regions. We showed previously that the Csk SH3 domain mediates highly specific associations with two members of the PEP family of nonreceptor protein tyrosine phosphatases (PTPs), PEP and PTP-PEST. In comparison, the Csk SH2 domain interacts with several tyrosine phosphorylated molecules, presumed to allow targetting of Csk to sites of Src family kinase activation. Herein, we attempted to understand better the regulation of Csk by identifying ligands for its SH2 domain. Using a modified yeast two-hybrid screen, we uncovered the fact that Csk associates with PTP-HSCF, the third member of the PEP family of PTPs. This association was documented not only in yeast cells but also in a heterologous mammalian cell system and in cytokine-dependent hemopoietic cells. Surprisingly, the Csk-PTP-HSCF interaction was found to be mediated by the Csk SH2 domain and two putative sites of tyrosine phosphorylation in the noncatalytic portion of PTP-HSCF. Transfection experiments indicated that Csk and PTP-HSCF synergized to inhibit signal transduction by Src family kinases and that this cooperativity was dependent on the domains mediating their association. Finally, we obtained evidence that PTP-HSCF inactivated Src-related PTKs by selectively dephosphorylating the positive regulatory tyrosine in their kinase domain. Taken together, these results demonstrate that part of the function of the Csk SH2 domain is to mediate an inducible association with a PTP, thereby engineering a more efficient inhibitory mechanism for Src-related PTKs. Coupled with previously published observations, these data also establish that Csk forms complexes with all three known members of the PEP family.     

ZAP-70: a 70 kd protein-tyrosine kinase that associates with the TCR zeta chain.

Protein-tyrosine kinases (PTKs) play an integral role in T cell activation. Stimulation of the T cell antigen receptor (TCR) results in tyrosine phosphorylation of a number of cellular substrates. One of these is the TCR zeta chain, which can mediate the transduction of extracellular stimuli into cellular effector functions. We have recently identified a 70 kd tyrosine phosphoprotein (ZAP-70) that associates with zeta and undergoes tyrosine phosphorylation following TCR stimulation. Here we report the isolation of a cDNA clone encoding ZAP-70. ZAP-70 represents a novel PTK and is expressed in T and natural killer cells. Moreover, tyrosine phosphorylation and association of ZAP-70 with zeta require the presence of src family PTKs and provide a potential mechanism by which the src family PTKs and ZAP-70 may interact to mediate TCR signal transduction.     

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

Protein tyrosine phosphatase (PTP) is a family of enzymes important for regulating cellular phosphorylation state. The oxidation and consequent inactivation of several PTPs by H₂O₂ are well demonstrated. It is also shown that recovery of enzymatic activity depends on the availability of cellular reductants. Among these redox-regulated PTPs, PTEN, Cdc25 and low molecular weight PTP are known to form a disulfide bond between two cysteines, one in the active site and the other nearby, during oxidation by H₂O₂. The disulfide bond likely confers efficiency in the redox regulation of the PTPs and protects cysteine-sulfenic acid of PTPs from further oxidation. In this review, through a comparative analysis of the oxidation process of Yap1 and PTPs, we propose the mechanism of disulfide bond formation in the PTPs.     

Protein tyrosine kinase-mediated pathways in G protein-coupled receptor signaling

Abundant evidence has indicated that protein tyrosine kinases (PTKs) convey signals from G protein-coupled receptors (GPCRs) to regulate cell proliferation, migration, adhesion, and potentialy cellular transformation. Molecular mechanisms by which PTKs regulate such diverse effects in GPCR signaling are not well understood. Recently, an unifying theme has emerged where both growth factors and GPCRs utilize protein tyrosine kinase activity and the highly conserved Ras/MAP kinase pathway to control mitogenic signals. Additionally, PTKs are also involved in the regulation of signal transmission from GPCRs to activation of the JNK/SAPK kinase pathway. Furthermore novel insights in chemokine receptor-activated PTKs and their role in mediating cell functions are discussed in this review.     

Oncogenes,protein tyrosine kinases,and signal transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of thesrc homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs.     

Regulation of insulin signaling through reversible oxidation of the protein-tyrosine phosphatases TC45 and PTP1B

Many studies have illustrated that the production of reactive oxygen species (ROS) is important for optimal tyrosine phosphorylation and signaling in response to diverse stimuli. Protein-tyrosine phosphatases (PTPs), which are important regulators of signal transduction, are exquisitely sensitive to inhibition after generation of ROS, and reversible oxidation is becoming recognized as a general physiological mechanism for regulation of PTP function. Thus, production of ROS facilitates a tyrosine phosphorylation-dependent cellular signaling response by transiently inactivating those PTPs that normally suppress the signal. In this study, we have explored the importance of reversible PTP oxidation in the signaling response to insulin. Using a modified ingel PTP assay, we show that stimulation of cells with insulin resulted in the rapid and transient oxidation and inhibition of two distinct PTPs, which we have identified as PTP1B and TC45, the 45-kDa spliced variant of the T cell protein-tyrosine phosphatase. We investigated further the role of TC45 as a regulator of insulin signaling by combining RNA interference and the use of substrate-trapping mutants. We have shown that TC45 is an inhibitor of insulin signaling, recognizing the beta-subunit of the insulin receptor as a substrate. The data also suggest that this strategy, using ligand-induced oxidation to tag specific PTPs and using interference RNA and substrate-trapping mutants to illustrate their role as regulators of particular signal transduction pathways, may be applied broadly across the PTP family to explore function.     

Identification of a novel mutation in ZAP70 and prenatal diagnosis in a Turkish family with severe combined immunodeficiency disorder

Protein tyrosine kinases (PTKs) play an important role in T cell development and activation. In vitro and in vivo defects, resulting in variable deficiencies in thymic development and in T cell antigen receptor (TCR) signal transduction, in PTKs have been shown. ZAP70, one of those PTKs, is a 70-kDa tyrosine phosphoprotein and associates with the ζ chain and undergoes tyrosine phosphorylation following TCR stimulation. It is expressed in T and natural killer (NK) cells. Several mutations were shown to lead to an autosomal recessive form of severe combined immunodeficiency disease (SCID).     

RAFTK/Pyk2-mediated cellular signalling

Intracellular signal transduction following extracellular ligation by a wide variety of surface molecules involves the activation and tyrosine phosphorylation of protein tyrosine kinases (PTKs). Tyrosine phosphorylation, controlled by the coordinated actions of protein tyrosine phosphatases (PTPs) and tyrosine kinases, is a critical regulatory mechanism for various physiological processes, including cell growth, differentiation, metabolism, cell cycle regulation and cytoskeleton function. The focal adhesion PTK family consists of the focal adhesion kinase (FAK) and the RAFTK/Pyk2 kinase (also known as CAK-beta and CADTK). RAFTK/Pyk2 can be activated by a variety of extracellular signals that elevate intracellular calcium concentration, and by stress signals. RAFTK/Pyk2 is expressed mainly in the central nervous system and in cells derived from hematopoietic lineages, while FAK is widely expressed in various tissues and links transmembrane integrin receptors to intracellular pathways. This review describes the role of RAFTK/Pyk2 in various signalling cascades and details the differential signalling by FAK and RAFTK/Pyk2.     

Tapping the therapeutic potential of protein tyrosine phosphatase 4A with small molecule inhibitors

Protein tyrosine phosphatases (PTPs) are emerging new targets for drug discovery. PTPs and protein tyrosine kinases (PTKs) maintain cellular homeostasis through opposing roles: tyrosine O-dephosphorylation and -phosphorylation, respectively. An imbalance in the phosphorylation equilibrium results in aberrant protein signaling and pathophysiological conditions. PTPs have historically been considered ‘undruggable’, in part due to a lack of evidence defining their relationship to disease causality and a focus on purely competitive inhibitors. However, a better understanding of protein–protein interfaces and shallow active sites has recently renewed interest in the pursuit of allosteric and orthosteric modulators of targets outside the major druggable protein families. While their biological mechanism of action still remains to be clarified, PTP4A1–3 (also referred to as PRL1-3) are validated oncology targets and play an important role in cell proliferation, metastasis, and tumor angiogenesis. In this Digest, recent syntheses and structure-activity relationships (SAR) of small molecule inhibitors (SMIs) of PTP4A1–3 are summarized, and enzyme docking studies of the most potent chemotype are highlighted. In particular, the thienopyridone scaffold has emerged as a potent lead structure to interrogate the function and druggability of this dual-specificity PTP.     

Ligands and signaling through receptor-type tyrosine phosphatases

Virtually every aspect of cellular proliferation and differentiation is regulated by changes in tyrosine phosphorylation. Tyrosine phosphorylation, in turn, is controlled by the opposing activities of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). PTKs are often transmembrane proteins (receptor PTKs) whose enzymatic activities and signaling functions are tightly regulated by the binding of specific ligands. A variety of transmembrane PTPs has also been identified; these proteins are called receptor PTPs (RPTPs), but in most cases their roles as receptors are very poorly understood. This review discusses the evidence that RPTPs are actually receptors for extrinsic ligands, and the extent to which interactions with putative ligands are known or suspected to cause changes in enzymatic activity. Finally, some of the RPTP substrates believed to be physiologically important are described. The evidence gathered to date suggests that models derived from studies of receptor PTKs may be too simple to account for the diversity and complexity of mechanisms through which ligand binding controls RPTP function.     

The "Gab" in signal transduction

Tyrosine phosphorylation plays an important role in controlling cellular growth, differentiation and function. Abnormal regulation of tyrosine phosphorylation can result in human diseases such as cancer. A major challenge of signal transduction research is to determine how the initial activation of protein-tyrosine kinases (PTKs) by extracellular stimuli triggers multiple downstream signaling cascades, which ultimately elicit diverse cellular responses. Recent studies reveal that members of the Gab/Dos subfamily of scaffolding adaptor proteins (hereafter, "Gab proteins") play a crucial role in transmitting key signals that control cell growth, differentiation and function from multiple receptors. Here, we review the structure, mechanism of action and function of these interesting molecules in normal biology and disease.     

Tyrosine phosphatases as regulators of skeletal development and metabolism

The protein tyrosine kinases (PTK) and the protein tyrosine phosphatases (PTPs) are enzymes which play an integral role in tyrosine phosphorylation-dependent signaling cascades. By catalyzing the phosphorylation and dephosphorylation of cellular proteins, these enzymes direct the steady-state levels of specific phosphoproteins and ultimately dictate the functional state of all cells. The importance of this type of signaling in the skeleton is accepted but poorly understood. The contribution of the PTKs to signaling events in bone has been well studied but, in contrast, the regulation by PTPs is poorly defined. The recent identification of 107 genes within the human genome which encode members of the PTP superfamily emphasizes the need to consider the importance of these proteins in skeletal tissue. In this prospective, we will summarize the present state of our knowledge regarding the function of this enzyme superfamily, illustrating its relevance to the development and maintenance of the skeleton and highlighting future directions that should improve our understanding of these critical signaling molecules.     

Heat stress induces tyrosine phosphorylation/activation of kinase Fa/GSK-3α (a human carcinoma dedifferentiation modulator) in A431 cells

Exposure of A431 cells to a rapid temperature increase from 37° to 46°C could induce an increased expression (∼200% of control) and tyrosine phosphorylation/activation (∼300% of control) of protein kinase FA/glycogen synthase kinase-3α (kinase FA/GSK-3α) in a time-dependent manner, as demonstrated by an anti-kinase FA/GSK-3α immunoprecipitate kinase assay and by immunoblotting analysis with anti-kinase FA/GSK-3α and anti-phosphotyrosine antibodies. The heat induction on the increased expression of kinase FA/GSK-3α could be blocked by actinomycin D but not by genistein. In contrast, the heat induction on tyrosine phosphorylation/activation of kinase FA/GSK-3α could be blocked by genistein or protein tyrosine phosphatase, indicating that heat stress induces a dual control mechanism, namely, protein expression and subsequent tyrosine phosphorylation to cause cellular activation of kinase FA/GSK-3α. Taken together, the results provide initial evidence that kinase FA/GSK-3α represents a newly described heat stress–inducible protein subjected to tyrosine phosphorylation/activation, representing a new mode of signal transduction for the regulation of this human carcinoma dedifferentiation modulator and a new mode of heat induction on cascade activation of a protein kinase. J. Cell. Biochem. 66:16–26, 1997. © 1997 Wiley-Liss, Inc.