Phosphorylation of Tyr342 in the linker region of Syk is critical for Fc epsilon RI signaling in mast cells

The linker region of Syk and ZAP70 tyrosine kinases plays an important role in regulating their function. There are three conserved tyrosines in this linker region; Tyr317 of Syk and its equivalent residue in ZAP70 were previously shown to negatively regulate the function of Syk and ZAP70. Here we studied the roles of the other two tyrosines, Tyr342 and Tyr346 of Syk, in Fc epsilon RI-mediated signaling. Antigen stimulation resulted in Tyr342 phosphorylation in mast cells. Syk with Y342F mutation failed to reconstitute Fc epsilon RI-initiated histamine release. In the Syk Y342F-expressing cells there was dramatically impaired receptor-induced phosphorylation of multiple signaling molecules, including LAT, SLP-76, phospholipase C-gamma2, but not Vav. Compared to wild-type Syk, Y342F Syk had decreased binding to phosphorylated immunoreceptor tyrosine-based activation motifs and reduced kinase activity. Surprisingly, mutation of Tyr346 had much less effect on Fc epsilon RI-dependent mast cell degranulation. An anti-Syk-phospho-346 tyrosine antibody indicated that antigen stimulation induced only a very minor increase in the phosphorylation of this tyrosine. Therefore, Tyr342, but not Tyr346, is critical for regulating Syk in mast cells and the function of these tyrosines in immune receptor signaling appears to be different from what has been previously reported for the equivalent residues of ZAP70.     

Phosphorylation of vinculin in human platelets spreading on a solid surface.

Vinculin is a cytoskeletal protein believed to be involved in linking microfilaments to the cell membrane. It is a substrate for the Ca(2+)- and phospholipid-dependent protein kinase C. We show here that when human platelets attach and spread on a solid surface, the alpha isoforms of vinculin become phosphorylated at serine and/or threonine residues. Phosphorylation is dependent on adhesion to a surface, since suspended, unattached platelets can produce filopodia but no phosphorylation of vinculin. Phosphorylation is also dependent on actin polymerization, as it does not occur when platelets had been pretreated with cytochalasin B. Most likely, protein kinase C is responsible for the phosphorylation of vinculin, since phosphorylation also occurs when platelets are treated with a phorbol ester, which activates protein kinase C, and is blocked by treatment with a staurosporine derivative which inhibits this enzyme. These results suggest that phosphorylation plays a role in anchoring vinculin at sites of microfilament-membrane interaction.     

Distinct Pathways Regulate Syk Protein Activation Downstream of Immune Tyrosine Activation Motif (ITAM) and hemITAM Receptors in Platelets

Tyrosine kinase pathways are known to play an important role in the activation of platelets. In particular, the GPVI and CLEC-2 receptors are known to activate Syk upon tyrosine phosphorylation of an immune tyrosine activation motif (ITAM) and hemITAM, respectively. However, unlike GPVI, the CLEC-2 receptor contains only one tyrosine motif in the intracellular domain. The mechanisms by which this receptor activates Syk are not completely understood. In this study, we identified a novel signaling mechanism in CLEC-2-mediated Syk activation. CLEC-2-mediated, but not GPVI-mediated, platelet activation and Syk phosphorylation were abolished by inhibition of PI3K, which demonstrates that PI3K regulates Syk downstream of CLEC-2. Ibrutinib, a Tec family kinase inhibitor, also completely abolished CLEC-2-mediated aggregation and Syk phosphorylation in human and murine platelets. Furthermore, embryos lacking both Btk and Tec exhibited cutaneous edema associated with blood-filled vessels in a typical lymphatic pattern similar to CLEC-2 or Syk-deficient embryos. Thus, our data show, for the first time, that PI3K and Tec family kinases play a crucial role in the regulation of platelet activation and Syk phosphorylation downstream of the CLEC-2 receptor.     

Phosphorylation of the activation loop tyrosines is required for sustained Syk signaling and growth factor-independent B-cell proliferation

The Syk kinase is regarded as a promising target for the treatment of antigen-driven B-cell malignancies, considering its essential role in propagating antigenic stimuli through the B-cell receptor (BCR). In certain common B-cell malignancies Syk is activated even in the absence of BCR engagement, suggesting a wider role for this kinase in lymphomagenesis. In this paper, we have profiled molecular differences between BCR-induced and constitutive Syk activation in terms of phosphorylation of regulatory tyrosine residues, downstream signaling properties and capacity to sustain B-cell proliferation. Analysis of primary chronic lymphocytic leukemia B-cells and diffuse large B-cell lymphoma cell lines revealed that constitutive and BCR-induced Syk activation differ with respect to the phosphorylation status of the regulatory tyrosines at positions 352 and 525/526, with only the first site being phosphorylated in the case of constitutive and both sites in the case of BCR-induced Syk activation. Syk phosphorylated only on Y352 is capable of downstream signaling, as evidenced by experiments with a phosphomimetic mutant in which the activation loop tyrosines (YY525/526) were replaced with phenylalanines. However, phosphorylation at YY525/526 was shown to significantly increase the enzymatic activity of Syk and to be required for sustained PLCγ2, Akt and ERK signaling as well as B-cell transformation. These data demonstrate that constitutively active Syk and Syk activated by BCR crosslinking represent separate stages of Syk activation with distinct signaling properties and transforming capacities.     

Switching between low and high affinity for the Syk tandem SH2 domain by irradiation of azobenzene containing ITAM peptidomimetics

Spleen tyrosine kinase (Syk) plays an essential role in IgE receptor signaling (FcεRI), which leads to mast cell degranulation. Divalent binding of the tandem SH2 domain (tSH2) of Syk to the intracellular ITAM motif of FcεRI activates the kinase domain of Syk, and thereby initiates cell degranulation. The inter SH2 domain distance in Syk tSH2 might be important for Syk kinase activation. In this study, photoswitchable ITAM peptidomimetics containing an azobenzene moiety were synthesized. Irradiation of these constructs changes the distance between the two SH2 binding epitopes and therefore, they may be used as photoswitches. The affinity of the cis‐ and trans‐isomer for tSH2 was assayed with SPR. The ITAM peptidomimetic with the smallest linker displayed the largest difference in affinity between the two isomers (at least 100‐fold), and the affinity of the cis‐isomer was comparable to monovalent binding. The ITAM mimics with larger photoswitchable linkers displayed modest differences. These results indicate that Syk tSH2 is able to adapt the inter SH2 domain distance to ligands larger than native ITAM, but not to smaller ones. Copyright © 2009 European Peptide Society and John Wiley & Sons, Ltd.     

Coordinate interactions of Csk, Src, and Syk kinases with [alpha]IIb[beta]3 initiate integrin signaling to the cytoskeleton

Integrins regulate cell adhesion and motility through tyrosine kinases, but initiation of this process is poorly understood. We find here that Src associates constitutively with integrin alphaIIbbeta3 in platelets. Platelet adhesion to fibrinogen caused a rapid increase in alphaIIbbeta3-associated Src activity, and active Src localized to filopodia and cell edges. Csk, which negatively regulates Src by phosphorylating Tyr-529, was also constitutively associated with alphaIIbbeta3. However, fibrinogen binding caused Csk to dissociate from alphaIIbbeta3, concomitant with dephosphorylation of Src Tyr-529 and phosphorylation of Src activation loop Tyr-418. In contrast to the behavior of Src and Csk, Syk was associated with alphaIIbbeta3 only after fibrinogen binding. Platelets multiply deficient in Src, Hck, Fgr, and Lyn, or normal platelets treated with Src kinase inhibitors failed to spread on fibrinogen. Inhibition of Src kinases blocked Syk activation and inhibited phosphorylation of Syk substrates (Vav1, Vav3, SLP-76) implicated in cytoskeletal regulation. Syk-deficient platelets exhibited Src activation upon adhesion to fibrinogen, but no spreading or phosphorylation of Vav1, Vav3, and SLP-76. These studies establish that platelet spreading on fibrinogen requires sequential activation of Src and Syk in proximity to alphaIIbbeta3, thus providing a paradigm for initiation of integrin signaling to the actin cytoskeleton.     

Phosphorylation of recombinant non-NMDA glutamate receptors on serine and tyrosine residues

Glutamate receptors are the major excitatory neurotransmitter receptors in the central nervous system. A variety of data has recently suggested that protein phosphorylation of glutamate receptors regulates their function. To examine at a molecular level the role of protein phosphorylation in the modification of glutamate receptors, we have transiently expressed the non-NMDA glutamate receptor subunit GluR1 (flop) in human embryonic kidney 293 cells. Using a polyclonal antipeptide antiserum directed specifically against GluR1, we have immunoprecipitated a 106 kDa phosphoprotein corresponding to the GluR1 subunit. Phosphoamino acid analysis and thermolytic peptide mapping demonstrate that this basal phosphorylation occurs exclusively on serine residues in two phosphopeptides. Application of activators of endogenous cAMP-dependent protein kinase or protein kinase C revealed no consistant changes in the phosphorylation of GluR1. However, coexpression of the GluR1 subunit with the well characterized protein tyrosine kinase v-src results in phosphorylation of GluR1 on tyrosine residues, in a single thermolytic phosphopeptide. These results suggest that GluR1 may be a substrate for protein serine/threonine kinases as well as protein tyrosine kinases in the central nervous system.Abbreviations AMPA -amino-3-hydroxy-5-methyl-4-isoxazolepropionate - CNS central nervous system - NMDA N-methyl-D-aspartate; - PAGE polyacrylamide gel electrophoresis - PBS phosphate-buffered saline - PMSF phenylmethylsulfonyl fluoride - SDS sodium dodecyl sulfate - TBS Tris-buffered saline - TPA phorbol 12-myristate-13-acetate Special issue dedicated to Dr. Paul Greengard.     

miR‐129‐2‐3p directly targets SYK gene and associates with the risk of ischaemic stroke in a Chinese population

Spleen tyrosine kinase (SYK) gene has been identified as novel susceptibility locus for ischaemic stroke (IS) previously. However, regulation of SYK gene remains unknown in IS. In this study, we aimed to identify miRNAs that might be involved in the development of IS by targeting SYK gene. miRNAs were firstly screened by bioinformatics predicting tool. The expression levels of SYK gene were detected by qRT‐PCR and western blotting, respectively, after miRNA transfection. Luciferase reporter assay was applied to investigate the direct binding between miRNAs and target gene. miRNA levels were detected by miRNA TaqMan assays in the blood cells of 270 IS patients and 270 control volunteers. Results suggest that SYK gene might be a direct target of miR‐129‐2‐3p. The blood level of miR‐129‐2‐3p was significantly lower in IS patients (P < 0.05), and negatively associated with the risk of IS (adjusted OR: 0.88; 95% CI: 0.80‐0.98; P = 0.021) by multivariable logistic regression analysis. The blood levels of SYK gene were significantly higher in IS patients, and miR‐129‐2‐3p expression was negatively correlated with mean platelet volume. In summary, our study suggests that miR‐129‐2‐3p might be involved in the pathogenesis of IS through interrupting SYK expression and the platelet function, and further investigation is needed to explore the underlying mechanism.     

The immune molecule CD3zeta and its downstream effectors ZAP-70/Syk mediate ephrin signaling in neurons to regulate early neuritogenesis

Recent studies have highlighted the key role of the immune protein CD3ζ in the maturation of neuronal circuits in the CNS. Yet, the upstream signals that might recruit and activate CD3ζ in neurons are still unknown. In this study, we show that CD3ζ functions early in neuronal development and we identify ephrinA1-dependent EphA4 receptor activation as an upstream regulator of CD3ζ. When newly born neurons are still spherical, before neurite extension, we found a transient CD3ζ aggregation at the cell periphery matching the initiation site of the future neurite. This accumulation of CD3ζ correlated with a stimulatory effect on filopodia extension via a Rho-GEF Vav2 pathway and a repression of neurite outgrowth. Conversely, cultured neurons lacking CD3ζ isolated from CD3ζ(-/-) mice showed a decreased number of filopodia and an enhanced neurite number. Stimulation with ephrinA1 induces the translocation of both CD3ζ and its activated effector molecules, ZAP-70/Syk tyrosine kinases, to EphA4 receptor clusters. EphrinA1-induced growth cone collapse was abrogated in CD3ζ(-/-) neurons and was markedly reduced by ZAP-70/Syk inhibition. Moreover, ephrinA1-induced ZAP-70/Syk activation was inhibited in CD3ζ(-/-) neurons. Altogether, our data suggest that CD3ζ mediates the ZAP-70/Syk kinase activation triggered by ephrinA-activated pathway to regulate early neuronal morphogenesis.     

钙离子通道A23187对血小板聚集和蛋白质磷酸化的影响

以 ³²P-Na₂HPO₄标记猪血小板,在阿斯匹林阻断花生四烯酸代谢,Apyrase去除分泌的ADP情况下,以A23187和PMA为血小板激动剂,staurosporine为PKC抑制剂,研究Ca²⁺和蛋白激酶C在血小板聚集中的作用.结果表明,a.A23187在1～20 μmol/L引起血小板聚集,相应地,明显地引起40 ku、20 ku蛋白质磷酸化,且存在剂量和时间效应关系.b.A23187和PMA在血小板聚集和蛋白质磷酸化上都存在着协同效应.c.1 μmol/L staurosporine可大部分抑制20 μmol/L A23187诱导的血小板聚集和20 ku、40 ku蛋白质磷酸化.结果提示,Ca²⁺激活血小板是建立在激活PKC的基础上,Ca²⁺通过激活PKC诱导血小板聚集,这是Ca²⁺激活血小板的主要途径.     

Ligand-based and e-pharmacophore modeling, 3D-QSAR and hierarchical virtual screening to identify dual inhibitors of spleen tyrosine kinase (Syk) and janus kinase 3 (JAK3)

The clinical efficacy of multiple kinase inhibitors has caught the interest of Pharmaceutical and Biotech researchers to develop potential drugs with multi-kinase inhibitory activity for complex diseases. In the present work, we attempted to identify dual inhibitors of spleen tyrosine kinase (Syk) and janus kinase 3 (JAK3), keys players in immune signaling, by developing ideal pharmacophores integrating Ligand-based pharmacophore models (LBPMs) and Structure-based pharmacophore models (SBPMs), thereby projecting the optimum pharmacophoric required for inhibition of both the kinases. The four point LBPM; ADPR.14 suggested the presence of one hydrogen bond acceptor, one hydrogen bond donor, one positive ionizable, and one ring aromatic feature for Syk inhibitory activity and AADH.54 proposed the necessity of two hydrogen bond acceptor, one hydrogen bond donor, and one hydrophobic feature for JAK3 inhibitory activity. To our interest, SBPMs identified additional ring aromatic features required for inhibition of both the kinases. For Syk inhibitory activity, the hydrogen bond acceptor feature indicated by LBPM was devoid of forming hydrogen bonding interaction with the hinge region amino acid residue (Ala451). Thus merging the information revealed by both LBPMs and SBPMs, ideal pharmacophore models i.e. ADPRR.14 (Syk) and AADHR.54 (JAK3) were generated. These models after rigorous statistical validation were used for screening of Asinex database. The systematic virtual screening protocol, including pharmacophore and docking-based screening, ADME property, and MM-GBSA energy calculations, retrieved final 10 hits as dual inhibitors of Syk and JAK3. Final 10 hits thus obtained can aid in the development of potential therapeutic agents for autoimmune disorders. Also the top two hits were evaluated against both the enzymes.     

Differential effects of imatinib on PDGF-induced proliferation and PDGF receptor signaling in human arterial and venous smooth muscle cells

Platelet-derived growth factor (PDGF) has been implicated in smooth muscle cell (SMC) proliferation, a key event in the development of myointimal hyperplasia in vascular grafts. Recent evidence suggests that the PDGF receptor (PDGFR) tyrosine kinase inhibitor, imatinib, can prevent arterial proliferative diseases. Because hyperplasia is far more common at the venous anastomosis than the arterial anastomosis in vascular grafts, we investigated whether imatinib also inhibited venous SMC (VSMC) proliferation, and examined possible differences in its mechanism of action between VSMC and arterial SMC (ASMC). Human ASMC and VSMC were stimulated with PDGF-AB, in the presence or absence of imatinib (0.1-10 microM). Proliferation was assayed using the 5-bromo-2'-deoxyuridine (BrdU) incorporation assay, while PDGFR, Akt and ERK1/2-mitogen activated protein kinase (MAPK) signaling pathways were investigated by immunoblotting. The proliferative response to PDGF at 50 and 100 ng/ml was 32 and 43% greater, respectively, in VSMC than in ASMC. Similarly, PDGF-stimulated proliferation was more sensitive to inhibition by imatinib in VSMC than ASMC (IC(50) = 0.05 microM vs. 0.4 microM; P < 0.01). Imatinib also more effectively inhibited PDGF-induced phosphorylation of PDGFRbeta and Akt in VSMC, compared to ASMC. These data highlight inherent pharmacodynamic differences between VSMC and ASMC in receptor and cell signaling functions and suggest that imatinib therapy may be useful for the prevention of venous stenosis in vascular grafts.     

Evidence for Reversible Tyrosine Protein Phosphorylation in the Okadaic Acid-Producing Marine Dinoflagellate Prorocentrum lima

ABSTRACT. The protist Prorocentrum lima , a primary producer of the tumour promoter okadaic acid, is a member of the dinoflagellate class of marine microorganisms. Herein, we have identified and characterized a protein tyrosine kinase (designated PLIK 1A) in P. lima that autophosphorylates almost exclusively on tyrosine residues. PLIK 1A was shown to have an approximate molecular mass of 38 kDa by SDS-PAGE and a native molecular mass within the range of 47–55 kDa by Superdex-75 gel filtration. Phosphoamino acid analysis of autophosphorylated PLIK 1A revealed the presence of phosphotyrosine and autophosphorylated PLJK 1A reacted with monoclonal anti-phosphotyrosine antibodies in a Western immunoblot. In addition, two protein tyrosine phosphatases were identified in P. lima that had apparent molecular masses within the ranges of 150–168 kDa and 73–82 kDa as determined by Superdex-200 gel filtration. These P. lima phosphatases, termed PLPTP-I and PLPTP-II, efficiently dephosphorylated tyrosine phosphorylated myelin basic protein. owever, only PLPTP-I was capable of dephosphorylating the tyrosine phosphorylated substrate angiotensin. Both PLPTP-I and PLPTP-II were able to dephosphorylate tyrosine autophosphorylated PLIK 1A. These data provide the first evidence for reversible tyrosine protein phosphorylation in P. lima by protein tyrosine kinases and phosphatases     

Phosphorylation control of the ubiquitin ligase Cbl is conserved in choanoflagellates

Cbl proteins are E3 ubiquitin ligases specialized for the regulation of tyrosine kinases by ubiquitylation. Human Cbl proteins are activated by tyrosine phosphorylation, thus setting up a feedback loop whereby the activation of tyrosine kinases triggers their own degradation. Cbl proteins are targeted to their substrates by a phosphotyrosine‐binding SH2 domain. Choanoflagellates, unicellular eukaryotes that are closely related to metazoans, also contain Cbl. The tyrosine kinase complement of choanoflagellates is distinct from that of metazoans, and it is unclear if choanoflagellate Cbl is regulated similarly to metazoan Cbl. Here, we performed structure‐function studies on Cbl from the choanoflagellate species Salpingoeca rosetta and found that it undergoes phosphorylation‐dependent activation. We show that S. rosetta Cbl can be phosphorylated by S. rosetta Src kinase, and that it can ubiquitylate S. rosetta Src. We also compared the substrate selectivity of human and S. rosetta Cbl by measuring ubiquitylation of Src constructs in which Cbl‐recruitment sites are placed in different contexts with respect to the kinase domain. Our results indicate that for both human and S. rosetta Cbl, ubiquitylation depends on proximity and accessibility, rather than being targeted toward specific lysine residues. Our results point to an ancient interplay between phosphotyrosine and ubiquitin signaling in the metazoan lineage.     

5-aza-CdR induces the demethylation of Syk promoter in nasopharyngeal carcinoma cell

The present study employed 5-aza-2′-deoxycytidine (5-aza-CdR) to treat nasopharyngeal carcinoma cell line CNE-1, CNE-2 and non-cancerous human nasopharyngeal epithelial cell line NP-69 to understand the effects on spleen tyrosine kinase (Syk) gene promoter methylation. The results showed that the methylation level of Syk gene is negatively associated with the differentiation level of the cell lines, and the 5-aza-CdR treatment decreased the methylation level in nasopharyngeal carcinoma cell lines. Additionally, the drug sensitivity of low-differentiated cell line was significantly higher than the high-differentiated cell line. In conclusion, the Syk gene promoter methylation reflects the cell differentiation status, and 5-aza-CdR treatment could induce the demethylation of Syk gene promoter.     

Shedding of syndecan-1 and -4 ectodomains is regulated by multiple signaling pathways and mediated by a TIMP-3-sensitive metalloproteinase

The syndecan family of four transmembrane heparan sulfate proteoglycans binds a variety of soluble and insoluble extracellular effectors. Syndecan extracellular domains (ectodomains) can be shed intact by proteolytic cleavage of their core proteins, yielding soluble proteoglycans that retain the binding properties of their cell surface precursors. Shedding is accelerated by PMA activation of protein kinase C, and by ligand activation of the thrombin (G-protein-coupled) and EGF (protein tyrosine kinase) receptors (Subramanian, S.V., M.L. Fitzgerald, and M. Bernfield. 1997. J. Biol. Chem. 272:14713-14720). Syndecan-1 and -4 ectodomains are found in acute dermal wound fluids, where they regulate growth factor activity (Kato, M., H. Wang, V. Kainulainen, M.L. Fitzgerald, S. Ledbetter, D.M. Ornitz, and M. Bernfield. 1998. Nat. Med. 4:691-697) and proteolytic balance (Kainulainen, V., H. Wang, C. Schick, and M. Bernfield. 1998. J. Biol. Chem. 273:11563-11569). However, little is known about how syndecan ectodomain shedding is regulated.To elucidate the mechanisms that regulate syndecan shedding, we analyzed several features of the process that sheds the syndecan-1 and -4 ectodomains. We find that shedding accelerated by various physiologic agents involves activation of distinct intracellular signaling pathways; and the proteolytic activity responsible for cleavage of syndecan core proteins, which is associated with the cell surface, can act on unstimulated adjacent cells, and is specifically inhibited by TIMP-3, a matrix-associated metalloproteinase inhibitor. In addition, we find that the syndecan-1 core protein is cleaved on the cell surface at a juxtamembrane site; and the proteolytic activity responsible for accelerated shedding differs from that involved in constitutive shedding of the syndecan ectodomains. These results demonstrate the existence of highly regulated mechanisms that can rapidly convert syndecans from cell surface receptors or coreceptors to soluble heparan sulfate proteoglycan effectors. Because the shed ectodomains are found and function in vivo, regulation of syndecan ectodomain shedding by physiological mediators indicates that shedding is a response to specific developmental and pathophysiological cues.     

Evidence of both extra- and intracellular cysteine targets of protein modification for activation of RET kinase

By use of a specifically sulfhydryl group-reactive chemical, 1,4-butanediyl-bismethanethiosulfonate (BMTS), we studied the localization of oxidative stress-responsive target cysteines for activation of a receptor-type protein tyrosine kinase, c-RET. The chemical, which reacted with RET proteins on the cell surface for sulfhydryl-linked aggregation, induced autophosphorylation and activation of RET kinase. When extracellular domain-deleted RET mutant (RET-PTC-1) cells were exposed to BMTS, neither the molecular status nor the activity of the enzyme was affected, suggesting that the target cysteines of BMTS to which cells were exposed for reaction are located in the cysteine-rich region of the extracellular domain of RET kinase. Despite this result, the exposure of a subcellular form of c-RET or RET-PTC-1 kinase isolated by immunoprecipitation to BMTS did induce activation of the enzyme. These results suggest that cysteines in both the extracellular and the intracellular domains of RET can work as target sites of accessible BMTS and possibly other oxidative elements for structural modification and activation of RET kinase.     

Altered membrane fluidity and signal transduction in the platelets from patients of thrombotic stroke

Several earlier studies have implicated platelet activation with the pathogenesis of thrombotic stroke. In this report we have studied the changes in membrane physical microenvironment and signal transduction in the platelets obtained from the patients with thrombotic stroke. Aggregation induced by the synthetic agonist thrombin receptor-activating peptide was significantly enhanced (p < 0.001) in the platelets obtained from the patients. Steady-state fluorescence anisotropy measurements using diphenylhexatriene reflected a significant increase in membrane microviscosity from 3.315 (± 0.103) in the control to 4.600 (± 0.119) in the stroke. Proteins of relative mobilities of 131, 100, 47 and 38 kDa were found to remain phosphorylated on tyrosine in the resting platelets obtained from thrombotic stroke patients while they were not phosphorylated in the control counterparts. Besides, calpain, a calcium dependent thiol protease present in the platelets, was found to remain active in this disease as reflected from the proteolysis of calpain substrates. Taken together, these data indicated abnormal circulating platelets in the patients of thrombotic stroke, which could contribute to the etiopathogenesis of this disease.     

PDGF receptor signaling networks in normal and cancer cells

For about four decades, platelet-derived growth factors (PDGF) and their receptors have been the subject of intense research, revealing their roles in embryo development and human diseases. Drugs such as imatinib, which selectively inhibit the tyrosine kinase activity of these receptors, have been approved for the treatment of cancers such as gastrointestinal stromal tumors and chronic eosinophilic leukemia. Today, the interest in these factors is still increasing in relationship with new potential clinical applications in cancer, stroke, fibrosis and infectious diseases. This review focuses on the mechanisms of PDGF receptor signaling, with an emphasis on pathways that are important for disease development. Of particular interest, recent studies revealed significant differences between normal and cancer cells regarding signal transduction by these growth factors.