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.     

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.     

Protein tyrosine kinase-substrate interactions

Protein tyrosine kinases (PTKs) are enzymes that catalyze the phosphorylation of tyrosyl residues. They are important in physiological and pathophysiological processes. Protein substrates of PTKs are often difficult to discern, but recently reported methods have helped to identify targets and characterize their structural interactions with kinases. A mechanism-based bisubstrate analog strategy has given X-ray crystallographic insights into how several topical PTKs, including the insulin receptor, Abl and epidermal growth factor receptor, interact with tyrosine-containing peptide substrates. These PTK co-crystal structures reveal both conserved and specialized features of recognition that probably contribute to substrate selection and the individual functions of these key enzymes.     

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.     

ReviewPTPs 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.     

Protein tyrosine kinases: autoregulation and small-molecule inhibition

Receptor and non-receptor protein tyrosine kinases (PTKs) are essential enzymes in cellular signaling processes that regulate cell growth, differentiation, migration and metabolism. The kinase activity of PTKs is tightly controlled through steric, autoregulatory mechanisms, as well as by the action of protein tyrosine phosphatases. Recent structural studies have revealed several modes of autoregulation governing the catalytic state of these enzymes. Aberrant catalytic activity of many PTKs, via mutation or overexpression, plays an important role in numerous pathological conditions, including cancer. Structural studies of the Abl tyrosine kinase domain in complex with the small-molecule inhibitor STI571 provide a molecular basis for understanding the specificity determinants of this highly successful drug used in the treatment of chronic myeloid leukemia.     

Autophosphorylation of the focal adhesion kinase, pp125FAK, directs SH2-dependent binding of pp60src.

The phosphorylation of protein tyrosine kinases (PTKs) on tyrosine residues is a critical regulatory event that modulates catalytic activity and triggers the physical association of PTKs with Src homology 2 (SH2)-containing proteins. The integrin-linked focal adhesion kinase, pp125FAK, exhibits extracellular matrix-dependent phosphorylation on tyrosine and physically associates with two nonreceptor PTKs, pp60src and pp59fyn, via their SH2 domains. Herein, we identify Tyr-397 as the major site of tyrosine phosphorylation on pp125FAK both in vivo and in vitro. Tyrosine 397 is located at the juncture of the N-terminal and catalytic domains, a novel site for PTK autophosphorylation. Mutation of Tyr-397 to a nonphosphorylatable residue dramatically impairs the phosphorylation of pp125FAK on tyrosine in vivo and in vitro. The mutation of Tyr-397 to Phe also inhibits the formation of stable complexes with pp60src in cells expressing Src and FAK397F, suggesting that autophosphorylation of pp125FAK may regulate the association of pp125FAK with Src family kinases in vivo. The identification of Tyr-397 as a major site for FAK autophosphorylation provides one of the first examples of a cellular protein containing a high-affinity binding site for a Src family kinase SH2 domain. This finding has implications for models describing the mechanisms of action of pp125FAK, the regulation of the Src family of PTKs, and signal transduction through the integrins.     

Multiple monoubiquitination of RTKs is sufficient for their endocytosis and degradation

Many cellular proteins are post-translationally modified by the addition of a single ubiquitin or a polyubiquitin chain. Among these are receptor tyrosine kinases (RTKs), which undergo ligand-dependent ubiquitination. The ubiquitination of RTKs has become recognized as an important signal for their endocytosis and degradation in the lysosome; however, it is not clear whether ubiquitination itself is sufficient for this process or simply participates in its regulation. The issue is further complicated by the fact that RTKs are thought to be polyubiquitinated - a modification that is linked to protein degradation by the proteasome. By contrast, monoubiquitination has been associated with diverse proteasome-independent cellular functions including intracellular protein movement. Here we show that the epidermal growth factor and platelet-derived growth factor receptors are not polyubiquitinated but rather are monoubiquitinated at multiple sites after their ligand-induced activation. By using different biochemical and molecular genetics approaches, we show that a single ubiquitin is sufficient for both receptor internalization and degradation. Thus, monoubiquitination is the principal signal responsible for the movement of RTKs from the plasma membrane to the lysosome.     

JAK protein tyrosine kinases: their role in cytokine signalling

Protein tyrosine kinases (PTKs) are integral components of the cellular machinery that mediates the transduction and/or processing of many extra- and intracellular signals. Members of the JAK family of intracellular PTKs (JAK1, JAK2 and TYK2) are characterized by the possession of a PTK-related domain and five additional homology domains, in addition to a classical PTK domain. An important breakthrough in the understanding of JAK kinases function(s) has come from the recent observations that many cytokine receptors compensate for their lack of a PTK domain by utilizing members of the JAK family for signal transduction.     

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.     

Tuberin: a stimulus-regulated tumor suppressor protein controlled by a diverse array of receptor tyrosine kinases and G protein-coupled receptors

Tuberin, a tumor suppressor protein, is involved in various cellular functions including survival, proliferation, and growth. It has emerged as an important effector regulated by receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs). Regulation of tuberin by RTKs and GPCRs is highly complex and dependent on the type of receptors and their associated signaling molecules. Apart from Akt, the first kinase recognized to phosphorylate and inactivate tuberin upon growth factor stimulation, an increasing number of kinases upstream of tuberin have been identified. Furthermore, recruitment of different scaffolding adaptor components to the activated receptors appears to play an important role in the regulation of tuberin activity. More recently, the differential regulation of tuberin by various G protein family members have also been intensively studied, it appears that G proteins can both facilitate (e.g., G(i/o)) as well as inhibit (e.g., G(q)) tuberin phosphorylation. In the present review, we attempt to summarize our emerging understandings of the roles of RTKs, GPCRs, and their cross-talk on the regulation of tuberin.     

Directing traffic in neural cells: determinants of receptor tyrosine kinase localization and cellular responses

The trafficking of receptor tyrosine kinases (RTKs) to distinct subcellular locations is essential for the specificity and fidelity of signal transduction and biological responses. This is particularly important in the PNS and CNS in which RTKs mediate key events in the development and maintenance of neurons and glia through a wide range of neural processes, including survival, proliferation, differentiation, neurite outgrowth, and synaptogenesis. The mechanisms that regulate the targeting of RTKs to their subcellular destinations for appropriate signal transduction, however, are still elusive. In this review, we discuss evidence for the spatial organization of signaling machinery into distinct subcellular compartments, as well as the role for ligand specificity, receptor sorting signals, and lipid raft microdomains in RTK targeting and the resultant cellular responses in neural cells.     

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.     

Galectins as modulators of receptor tyrosine kinases signaling in health and disease

Receptor tyrosine kinases (RTKs) constitute a large group of cell surface proteins that mediate communication of cells with extracellular environment. RTKs recognize external signals and transfer information to the cell interior, modulating key cellular activities, like metabolism, proliferation, motility, or death. To ensure balanced stream of signals the activity of RTKs is tightly regulated by numerous mechanisms, including receptor expression and degradation, ligand specificity and availability, engagement of co-receptors, cellular trafficking of the receptors or their post-translational modifications. One of the most widespread post-translational modifications of RTKs is glycosylation of their extracellular domains. The sugar chains attached to RTKs form a new layer of information, so called glyco-code that is read by galectins, carbohydrate binding proteins. Galectins are family of fifteen lectins implicated in immune response, inflammation, cell division, motility and death. The versatility of cellular activities attributed to galectins is a result of their high abundance and diversity of their cellular targets. A various sugar specificity of galectins and the differential ability of galectin family members to form oligomers affect the spatial distribution and the function of their cellular targets. Importantly, galectins and RTKs are tightly linked to the development, progression and metastasis of various cancers. A growing number of studies points on the close cooperation between RTKs and galectins in eliciting specific cellular responses. This review focuses on the identified complexes between galectins and RTK members and discusses their relevance for the cell physiology both in healthy tissues and in cancer.     

Spatio‐temporally precise activation of engineered receptor tyrosine kinases by light

Receptor tyrosine kinases (RTKs) are a large family of cell surface receptors that sense growth factors and hormones and regulate a variety of cell behaviours in health and disease. Contactless activation of RTKs with spatial and temporal precision is currently not feasible. Here, we generated RTKs that are insensitive to endogenous ligands but can be selectively activated by low‐intensity blue light. We screened light‐oxygen‐voltage (LOV)‐sensing domains for their ability to activate RTKs by light‐activated dimerization. Incorporation of LOV domains found in aureochrome photoreceptors of stramenopiles resulted in robust activation of the fibroblast growth factor receptor 1 (FGFR1), epidermal growth factor receptor (EGFR) and rearranged during transfection (RET). In human cancer and endothelial cells, light induced cellular signalling with spatial and temporal precision. Furthermore, light faithfully mimicked complex mitogenic and morphogenic cell behaviour induced by growth factors. RTKs under optical control (Opto‐RTKs) provide a powerful optogenetic approach to actuate cellular signals and manipulate cell behaviour.     

Receptor tyrosine kinases: genetic evidence for their role in Drosophila and mouse development

Receptor tyrosine kinases (RTKs) and their ligands are important components of the signalling pathways by which cells interact. This review summarizes a growing body of genetic evidence showing that many developmentally important mutations in Drosophila and the mouse are in the genes that encode RTKs or their ligands, indicating that these molecules play central roles during both invertebrate and vertebrate development.     

Paths of FGFR-driven tumorigenesis

Fibroblast growth factor receptors (FGFRs) comprise a subfamily of receptor tyrosine kinases (RTKs) that are master regulators of a broad spectrum of cellular and developmental processes, including apoptosis, proliferation, migration, and angiogenesis. Due to their broad impact, FGFRs and other RTKs are highly regulated and normally only basally active. Deregulation of FGFR signaling by activating mutations or ligand/receptor overexpression could allow these receptors to become constitutively active, leading to cancer development, including both hematopoietic and solid tumors, such as breast, bladder, and prostate carcinomas. In this review, we focus on potential modes of FGFR-mediated tumorigenesis, in particular, the role of FGFR1 during prostate cancer progression.     

Protein tyrosine phosphatases as novel targets in breast cancer therapy

Breast cancer is linked to hyperactivation of protein tyrosine kinases (PTKs), and recent studies have unveiled that selective tyrosine dephosphorylation by protein tyrosine phosphatases (PTPs) of specific substrates, including PTKs, may activate or inactivate oncogenic pathways in human breast cancer cell growth-related processes. Here, we review the current knowledge on the involvement of PTPs in breast cancer, as major regulators of breast cancer therapy-targeted PTKs, such as HER1/EGFR, HER2/Neu, and Src. The functional interplay between PTKs and PTK-activating or -inactivating PTPs, and its implications in novel breast cancer therapies based on targeting of specific PTPs, are discussed.     

Receptor tyrosine kinase signalling: not so complex after all?

Receptor tyrosine kinases (RTKs) transmit intercellular signals that control many cellular events including proliferation, differentiation and cell survival. Ligand-bound RTKs regulate a complex network of intracellular signalling pathways. However, activation of just one of these pathways, which involves Ras and MAP kinase, is both necessary and sufficient to mediate the diverse developmental effects of several invertebrate RTKs. This article discusses these findings, which suggest that RTK-induced activation of MAP kinase in invertebrates acts as a simple developmental switch in multiple cell types, and considers the evidence that the Ras-MAP-kinase pathway also plays a similar role in vertebrates.