Selective association of the tyrosine kinases Src, Fyn, and Lyn with integrin-rich actin cytoskeletons of activated, nonaggregated platelets

ErbB/integrin signaling interactions in regulation of myocardial cell-cell and cell-matrix interactions

Neuregulin (Nrg)/ErbB and integrin signaling pathways are critical for the normal function of the embryonic and adult heart. Both systems activate several downstream signaling pathways, with different physiological outputs: cell survival, fibrosis, excitation-contraction coupling, myofilament structure, cell-cell and cell-matrix interaction. Activation of ErbB2 by Nrg1β in cardiomycytes or its overexpression in cancer cells induces phosphorylation of FAK (Focal Adhesion Kinase) at specific sites with modulation of survival, invasion and cell-cell contacts. FAK is also a critical mediator of integrin receptors, converting extracellular matrix alterations into intracellular signaling. Systemic FAK deletion is lethal and is associated with left ventricular non-compaction whereas cardiac restriction in adult hearts is well tolerated. Nevertheless, these hearts are more susceptible to stress conditions like trans-aortic constriction, hypertrophy, and ischemic injury. As FAK is both downstream and specifically activated by integrins and Nrg-1β, here we will explore the role of FAK in the heart as a protective factor and as possible mediator of the crosstalk between the ErbB and Integrin receptors. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Low levels of hydrogen peroxide stimulate corneal epithelial cell adhesion, migration, and wound healing

PURPOSE

Intracellular reactive oxygen species have been reported to associate with growth factor and integrin signalings in promoting cell adhesion in many cell types. This study is to explore if exogenous H(2)O(2) at low levels can be beneficial to cell adhesion, migration, and wound healing.

METHODS

Primary rabbit corneal epithelial cells treated with 0-70 μM H(2)O(2) were tested for viability by MTT assay, adhesion by centrifugation assay, focal contacts of vinculin and F-actin by immunofluorescence, activated Src(pY416), EGF receptor (pY845), vinculin(pY1065), FAK(pY397), and FAK(pY576) by immunoblotting. Cell migration was examined with 0-50 μM H(2)O(2) using the scratch wound technique. Corneal wound healing of ex vivo pig model and in vivo mouse model was examined using H(2)O(2) with and without antioxidant N-acetylcysteine (NAC).

RESULTS

Compared with the untreated control, H(2)O(2) at 10-50 μM stimulated cell viability and facilitated adhesion and migration with clear induction of vinculin-rich focal adhesions and F-actin-containing stress fibers by increasing activated Src, FAK(pY576), and vinculin(pY1065). H(2)O(2) also increased phosphorylation of EGFR(Y845) parallel to that of activated Src, but both were eliminated by NAC and PP1 (Src inhibitor). Finally, H(2)O(2) induced faster wound healing in cornea both in vitro and in vivo, but the healing was diminished by NAC.

CONCLUSIONS

These findings suggest that H(2)O(2) at low levels promotes cell adhesion, migration, and wound healing in cornea cells or tissue, and the interaction of H(2)O(2) with Src plays a major role.

Synthesis and pharmacological evaluation of novel beta-nitrostyrene derivatives as tyrosine kinase inhibitors with potent antiplatelet activity

Protein tyrosine kinases have been known to be involved in regulation of platelet aggregation, suggesting a potential target for antiplatelet therapy. Our previous study showed that 3,4-methylenedioxy-beta-nitrostyrene (MNS) prevented platelet aggregation caused by various stimulators, and this action was accompanied by inhibition of tyrosine kinases. In the present study, in order to examine the structural determinants required for the actions of MNS and to develop more potent tyrosine kinase inhibitors and antiplatelet agents, a new series of beta-nitrostyrene derivatives were synthesized and pharmacologically characterized. The beta-nitrostyrene derivatives inhibited thrombin- or collagen-induced human platelet aggregation, ATP secretion, GPIIb/IIIa activation and protein tyrosine phosphorylation. In recombinant enzyme assay, some beta-nitrostyrene derivatives also demonstrated potent inhibition of Src and/or Syk kinase activity. Furthermore, there was a good correlation between the inhibitory potency of these compounds on tyrosine kinases and on platelet activation/aggregation. Among them, a benzoyl ester derivative (compound 10) possess up to 8-fold greater potency than MNS and over two orders of magnitude greater potency than genistein or tyrphostin A47 in inhibiting platelet responses to thrombin. Our data suggest that beta-nitrostyrenes may represent a new class of tyrosine kinase inhibitors with potent antiplatelet activity.

mTOR-dependent synthesis of Bcl-3 controls the retraction of fibrin clots by activated human platelets

New activities of human platelets continue to emerge. One unexpected response is new synthesis of proteins from previously transcribed RNAs in response to activating signals. We previously reported that activated human platelets synthesize B-cell lymphoma-3 (Bcl-3) under translational control by mammalian target of rapamycin (mTOR). Characterization of the ontogeny and distribution of the mTOR signaling pathway in CD34+ stem cell-derived megakaryocytes now demonstrates that they transfer this regulatory system to developing proplatelets. We also found that Bcl-3 is required for condensation of fibrin by activated platelets, demonstrating functional significance for mTOR-regulated synthesis of the protein. Inhibition of mTOR by rapamycin blocks clot retraction by human platelets. Platelets from wild-type mice synthesize Bcl-3 in response to activation, as do human platelets, and platelets from mice with targeted deletion of Bcl-3 have defective retraction of fibrin in platelet-fibrin clots mimicking treatment of human platelets with rapamycin. In contrast, overexpression of Bcl-3 in a surrogate cell line enhanced clot retraction. These studies identify new features of post-transcriptional gene regulation and signal-dependant protein synthesis in activated platelets that may contribute to thrombus and wound remodeling and suggest that posttranscriptional pathways are targets for molecular intervention in thrombotic disorders.

Thrombin-induced tyrosine phosphorylation of HS1 in human platelets is sequentially catalyzed by Syk and Lyn tyrosine kinases and associated with the cellular migration of the protein

Thrombin stimulation of platelets triggers Tyr phosphorylation of several signaling proteins, most of which remain unidentified. In this study, we demonstrate for the first time that hematopoietic lineage cell-specific protein 1 (HS1) undergoes a transient Tyr phosphorylation in human platelets stimulated with thrombin. The protein is synergistically phosphorylated by Syk and Lyn tyrosine kinases according to a sequential phosphorylation mechanism. By means of specific inhibitors (PP2, SU6656, and piceatannol) and phosphopeptide-specific antibodies, as well as by coimmunoprecipitation and binding competition experiments, we show that Syk acts as the primary kinase that phosphorylates HS1 at Tyr397 and that Syk phosphorylation is required for HS1 interaction with the Lyn SH2 domain. Upon docking to Syk-phosphorylated HS1, Lyn catalyzes the secondary phosphorylation of the protein at Tyr222. Once the secondary Tyr phosphorylation of HS1 is accomplished the protein dissociates from Lyn and undergoes a dephosphorylation process. HS1 Tyr phosphorylation does not occur when thrombin-induced actin assembly is inhibited by cytochalasin D even under conditions in which Syk and Lyn are still active. Immunofluorescence microscopic analysis shows that the agonist promotes HS1 migration to the plasma membrane and that the inhibition of Lyn-mediated secondary phosphorylation of HS1 abrogates the subcellular translocation of the protein. All together these results indicate that HS1 Tyr phosphorylation catalyzed by Syk and Lyn plays a crucial role in the translocation of the protein to the membrane and is involved in the cytoskeleton rearrangement triggered by thrombin in human platelets.

Hydrogen Peroxide Generation Induces pp60 Activation in Human Platelets

Reactive oxygen species, such as H2O2, have been recognized as intracellular messengers involved in several cell functions. Here we report the activation of the tyrosine kinase pp60(src) by H2O2, a mechanism required for the activation of store-mediated Ca2+ entry (SMCE) in human platelets. Treatment of platelets with H2O2 resulted in a time- and concentration-dependent activation of pp60(src). Incubation with GF 109203X, a protein kinase C (PKC) inhibitor, prevented H2O2-induced pp60(src) activation. In contrast, dimethyl-BAPTA loading did not affect this response, suggesting that activation of pp60(src) by H2O2 is independent of increases in [Ca2+](i). Cytochalasin D, an inhibitor of actin polymerization, significantly reduced H2O2-induced pp60(src) activation. We found that platelet stimulation with thapsigargin (TG) plus ionomycin (Iono) or thrombin induced rapid H2O2 production, a mechanism independent of elevations in [Ca2+](i). Treatment of platelets with catalase attenuated TG plus Iono- and thrombin-induced activation of pp60(src). In addition, catalase as well as the pp60(src) inhibitor, PP1, reduced both the activation of SMCE and the coupling between the hTrp1 and the IP(3)R type II without having any effect on the maintenance of SMCE. Consistent with the role of PKC in the activation of pp60(src) by H2O2, the PKC inhibitors GF 109202X and Ro-31-8220 were found to reduced SMCE in platelets. This study suggests that platelet activation with TG plus Iono or thrombin is associated with H2O2 production, which acts as a second messenger by stimulating pp60(src) by a PKC-dependent pathway and is involved in the activation of SMCE in these cells.

The pattern of enhancement of Src kinase activity on platelet-derived growth factor stimulation of glioblastoma cells is affected by the integrin engaged

Enhanced expression of both integrin alpha v beta 3 and platelet-derived growth factor receptor (PDGFr) has been described in glioblastoma tumors. We therefore explored the possibility that integrin alpha v beta 3 cooperates with PDGFr to promote cell migration in glioblastoma cells, and extended the study to identify the Src family members that are activated on PDGF stimulation. Glioblastoma cells utilize integrins alpha v beta 3 and alpha v beta 5 to mediate vitronectin attachment. We found that physiologic PDGF stimulation (83 pm, 10 min) of vitronectin-adherent cells promoted the specific recruitment of integrin alpha v beta 3-containing focal adhesions to the cell cortex and alpha v beta 3-mediated cell motility. Analysis of PDGFr immunoprecipitates indicated an association of the PDGFr beta with integrin alpha v beta 3, but not integrin alpha v beta 5. Cells plated onto collagen or laminin, which engage different integrins, exhibited significantly less migration on PDGF stimulation, indicating a cooperation of alpha v beta 3 and the PDGFr beta in glioblastoma cells that promotes migration. Further analysis of the cells plated onto vitronectin indicated that PDGF stimulation caused an increase in Src kinase activity, which was associated with integrin alpha v beta 3. In the vitronectin-adherent cells, Lyn was associated preferentially with alpha v beta 3 both in the presence and absence of PDGF stimulation. In contrast, Fyn was associated with both alpha v beta 3 and alpha v beta 5. Moreover, PDGF stimulation increased the activity of Lyn, but not Fyn, in vitronectin-adherent cells, and the activity of Fyn, but not Lyn, in laminin-adherent cells. Using cells attached to mAb anti-alpha v beta 3 or mAb anti-integrin alpha 6, we confirmed the activation of specific members of the Src kinase family with PDGF stimulation. Down-regulation of Lyn expression by siRNA significantly inhibited the cell migration mediated by integrin alpha v beta 3 in PDGF-stimulated cells, demonstrating the PDGFr beta cooperates with integrin alpha v beta 3 in promoting the motility of vitronectin-adherent glioblastoma cells through a Lyn kinase-mediated pathway. Notably, the data indicate that engagement of different integrins alters the identity of the Src family members that are activated on stimulation with PDGF.

Evidence for secretion-like coupling involving pp60src in the activation and maintenance of store-mediated Ca2+ entry in mouse pancreatic acinar cells

Store-mediated Ca2+ entry (SMCE) is one of the main pathways for Ca2+ influx in non-excitable cells. Recent studies favour a secretion-like coupling mechanism to explain SMCE, where Ca2+ entry is mediated by an interaction of the endoplasmic reticulum (ER) with the plasma membrane (PM) and is modulated by the actin cytoskeleton. To explore this possibility further we have now investigated the role of the actin cytoskeleton in the activation and maintenance of SMCE in pancreatic acinar cells, a more specialized secretory cell type which might be an ideal cellular model to investigate further the properties of the secretion-like coupling model. In these cells, the cytoskeletal disrupters cytochalasin D and latrunculin A inhibited both the activation and maintenance of SMCE. In addition, stabilization of a cortical actin barrier by jasplakinolide prevented the activation, but not the maintenance, of SMCE, suggesting that, as for secretion, the actin cytoskeleton plays a double role in SMCE as a negative modulator of the interaction between the ER and PM, but is also required for this mechanism, since the cytoskeleton disrupters impaired Ca2+ entry. Finally, depletion of the intracellular Ca2+ stores induces cytoskeletal association and activation of pp60(src), which is independent on Ca2+ entry. pp60(src) activation requires the integrity of the actin cytoskeleton and participates in the initial phase of the activation of SMCE in pancreatic acinar cells.

Actin-rich spherical extrusion induced in okadaic acid-treated K562 cells by crosslinking of membrane microdomains

Interconnection between surface microdomains and the actin cytoskeleton is vital to various cellular activities. We studied the responses of okadaic acid (OKA)-treated K562 leukemia cells to crosslinking of membrane microdomains. Although OKA alone induced clustering of surface-bound F-actin, addition of a biotinylated poly(ethylene glycol) derivative of cholesterol (bPEG-Chol) and subsequent binding of streptavidin (SA) further induced accumulation of the clusters, resulting in the formation of a spherical cell extrusion. This extrusion was also induced by direct crosslinking of a raft marker, CD59, and ganglioside GM1. In addition, we found that knockout of the gene encoding Fyn kinase inhibited formation of the spherical extrusion in murine T-cells. In bPEG-Chol/SA-treated cells, CD59, ganglioside GM1, and clathrin/AP-2 were all accumulated on the surface of the actin-rich extrusion, whereas dynamin and transferrin receptors were unaffected. Intermediate filaments, mitochondria, and other vesicles also accumulated. These results suggest that crosslinking of membrane domains exaggerates the linkage between actin and a defined set of membrane proteins in OKA-treated cells.

Src protein kinase pp60c-src influences adhesion stabilization of HT-29 colon carcinoma cells to extracellular matrix components under dynamic conditions of laminar flow

Tumor cell adhesion to extracellular matrix (ECM) and its stabilization are important determinants in metastasis formation, and they are mediated, in part, by integrins and regulated by a variety of protein kinases. Protein tyrosine kinase pp60c-src is found in adhesion-dependent focal adhesion plaques where it may regulate different integrin-mediated signaling cascades. Using human HT-29 colon carcinoma cells stably transfected with pp60c-src--specific antisense oligonucleotides (HT-29AS15), we investigated the role of pp60c-src in integrin-mediated adhesion and its stabilization to ECM components collagen I or IV under static and laminar fluid flow conditions. Under static adhesion conditions transfection of pp60c-src antisense oligonucleotides did not modify adhesive properties. Phosphorylation of focal adhesion kinase (FAK) and paxillin induced by static adhesion to collagen I or IV were similar in HT-29P and HT-29AS15 cells. However, using hydrodynamic conditions in a laminar flow chamber we found a slight reduction in early adhesion events and an even greater difference in adhesion stabilization rates (ASRs). The transfected cells showed a significant reduction in their ability to withstand shear forces and stabilize adhesive bounds. These changes correlated with the cellular expression levels of pp60c-src. Our results suggest that pp60c-src may be involved in stabilization of dynamic HT-29 cell adhesion to ECM components, and this kinase appears to be part of a mechanosensory protein complex during integrin-mediated cell adhesion.

Effects of reactive oxygen species on actin filament polymerisation and amylase secretion in mouse pancreatic acinar cells

The present study investigates the effect of reactive oxygen species (ROS) on actin filament reorganisation and its relevance to exocytosis in pancreatic acinar cells. Treatment of pancreatic acini with cholecystokinin (CCK-8) induced spatial and temporal changes in actin filament reorganisation with an initial depolymerisation of the apical actin barrier followed by an increase in the actin filament content in the subapical area leading to amylase release. Hydrogen peroxide (H(2)O(2)) increased actin filament content and potentiated the polymerizing effects of CCK-8 in these cells but abolished the disruption of the apical actin layer and amylase release induced by CCK-8. Similar to CCK-8, ROS generated by the oxidation of hypoxanthine (HX) with xanthine oxidase (XOD) induced an initial decrease in actin filaments located under the apical membrane followed by a smaller increase in the content of actin filaments in the subapical area. XOD-generated ROS are able to increase amylase release in pancreatic acini although combination with CCK-8 leads to abnormal exocytosis. We provide evidence that indicates that CCK-8- and ROS-induced actin reorganisation is entirely dependent on Ca(2+) mobilisation and independent of PKC activation. The regulation of the actin cytoskeleton by ROS might be involved in radical-induced cell injury in pancreatic acinar cells.

Fibrinogen binding to the integrin alpha(IIb)beta(3) modulates store-mediated calcium entry in human platelets

Effects of the occupation of integrin alpha(IIb)beta(3) by fibrinogen on Ca(++) signaling in fura-2-loaded human platelets were investigated. Adding fibrinogen to washed platelet suspensions inhibited increases in cytosolic [Ca(++)] concentrations ([Ca(++)](i)) evoked by adenosine diphosphate (ADP) and thrombin in a concentration-dependent manner in the presence of external Ca(++) but not in the absence of external Ca(++) or in the presence of the nonselective cation channel blocker SKF96365, indicating selective inhibition of Ca(++) entry. Fibrinogen also inhibited store-mediated Ca(++) entry (SMCE) activated after Ca(++) store depletion using thapsigargin. The inhibitory effect of fibrinogen was reversed if fibrinogen binding to alpha(IIb)beta(3) was blocked using RDGS or abciximab and was absent in platelets from patients homozygous for Glanzmann thrombasthenia. Fibrinogen was without effect on SMCE once activated. Activation of SMCE in platelets occurs through conformational coupling between the intracellular stores and the plasma membrane and requires remodeling of the actin cytoskeleton. Fibrinogen inhibited actin polymerization evoked by ADP or thapsigargin in control cells and in cells loaded with the Ca(++) chelator dimethyl BAPTA. It also inhibited the translocation of the tyrosine kinase p60(src) to the cytoskeleton. These results indicate that the binding of fibrinogen to integrin alpha(IIb)beta(3) inhibits the activation of SMCE in platelets by a mechanism that may involve modulation of the reorganization of the actin cytoskeleton and the cytoskeletal association of p60(src). This action may be important in intrinsic negative feedback to prevent the further activation of platelets subjected.