Activation of signal transduction in platelets by the tyrosine phosphatase inhibitor pervanadate (vanadyl hydroperoxide)

Impaired Glycoprotein VI-Mediated Signaling and Platelet Functional Responses in CD45 Knockout Mice

BACKGROUND AND OBJECTIVE

 CD45 is a receptor protein tyrosine phosphatase present on the surface of all hematopoietic cells except for erythrocytes and platelets. Proteomics studies, however, have demonstrated the presence of a CD45 c-terminal catalytic peptide in platelets. Therefore, we investigated the functional role of this truncated isoform of CD45 in platelets, which contains the c-terminal catalytic domain but lacks the extracellular region.

METHODS AND RESULTS

 We used an antibody specific to the c-terminus of CD45 to confirm the presence of a truncated CD45 isoform in platelets. We also examined ex vivo and in vivo platelet function using CD45 knockout (KO) mice. Aggregation and secretion mediated by the glycoprotein VI (GPVI) receptor was impaired in CD45 KO platelets. Consequently, CD45 KO mice had impaired hemostasis indicated by increased tail bleeding times. Also, using a model of pulmonary embolism we showed that CD45 KO mice had defective in vivo thrombus formation. Next, we investigated whether or not the truncated isoform of CD45 had a role in GPVI signaling. The full-length isoform of CD45 is known to regulate Src family kinase (SFK) activation in lymphocytes. We find a similar role for the truncated isoform of CD45 in platelets. SFK activation was impaired downstream of the GPVI receptor in the CD45 KO murine platelets. Consequently, Syk, PLCγ2, and pleckstrin phosphorylations were also impaired in CD45 KO murine platelets.

CONCLUSION

 We conclude that the truncated CD45 isoform regulates GPVI-mediated signaling and platelet functional responses by regulating SFK activation.

An adventitious interaction of filamin A with RhoGDI2(Tyr153Glu)

Filamin A (FLNA) is an actin filament crosslinking protein with multiple intracellular binding partners. Mechanical force exposes cryptic FLNA binding sites for some of these ligands. To identify new force-dependent binding interactions, we used a fusion construct composed of two FLNA domains, one of which was previously identified as containing a force-dependent binding site as a bait in a yeast two-hybrid system and identified the Rho dissociation inhibitor 2 (RhoGDI2) as a potential interacting partner. A RhoGDI2 truncate with 81 N-terminal amino acid residues and a phosphomimetic mutant, RhoGDI(Tyr153Glu) interacted with the FLNA construct. However, neither wild-type or full-length RhoGDI2 phosphorylated at Y153 interacted with FLNA. Our interpretation of these contradictions is that truncation and/or mutation of RhoGDI2 perturbs its conformation to expose a site that adventitiously binds FLNA and is not a bona-fide interaction. Therefore, previous studies reporting that a RhoGDI(Y153E) mutant suppresses the metastasis of human bladder cancer cells must be reinvestigated in light of artificial interaction of this point mutant with FLNA.

Synergistic effect of signaling from receptors of soluble platelet agonists and outside-in signaling in formation of a stable fibrinogen-integrin αIIbβ3-actin cytoskeleton complex

INTRODUCTION

Thrombus formation in the injured vessel wall is a highly complex process involving various blood-born components that go through specific temporal and spatial changes as observed by intravital videomicroscopy. Platelets bind transiently to the developing thrombus and may either become stably incorporated into or disengage from the thrombus. The aim of the present study was to reveal the processes involved in the formation of a stable thrombus.

METHODS

Platelet-rich plasma and washed platelets were studied by the aggregometer. The aggregate stability was challenged by eptifibatide. Platelet Triton-insoluble fraction was prepared and the actin and αIIb content in the cytoskeleton was analyzed by western blot.

RESULTS

Maximal actin polymerization is achieved 1min after platelet activation while maximal αIIbβ3-actin cytoskeleton association requires 5 to 10min of activation and fibrinogen-mediated platelet-to-platelet bridging. Thus, actin polymerization is dependent on platelet activation and requires neither αIIbβ3 integrin occupation nor platelet aggregation. Formation of a stable aggregate requires platelet activation for more than 1min, complete increase in actin cytoskeleton fraction and partial association of αIIbβ3 with the actin cytoskeleton. However, direct αIIbβ3 activation is not sufficient for cytoskeleton complex formation. Thus, stable αIIbβ3-fibrinogen interaction, representing stable aggregate, is achieved after more than 1min agonist activation, involving inside-out and outside-in signaling but not after direct integrin activation, involving only outside-in signaling.

CONCLUSIONS

Formation of a stable fibrinogen-αIIbβ3-actin cytoskeleton complex is the result of the combined effect of platelet stimulation by soluble agonists, activation of αIIbβ3, fibrinogen binding and platelet-to-platelet bridging.

Quantitative phosphoproteomics reveals the role of protein arginine phosphorylation in the bacterial stress response

Arginine phosphorylation is an emerging protein modification implicated in the general stress response of Gram-positive bacteria. The modification is mediated by the arginine kinase McsB, which phosphorylates and inactivates the heat shock repressor CtsR. In this study, we developed a mass spectrometric approach accounting for the peculiar chemical properties of phosphoarginine. The improved methodology was used to analyze the dynamic changes in the Bacillus subtilis arginine phosphoproteome in response to different stress situations. Quantitative analysis showed that a B. subtilis mutant lacking the YwlE arginine phosphatase accumulated a strikingly large number of arginine phosphorylations (217 sites in 134 proteins), however only a minor fraction of these sites was increasingly modified during heat shock or oxidative stress. The main targets of McsB-mediated arginine phosphorylation comprise central factors of the stress response system including the CtsR and HrcA heat shock repressors, as well as major components of the protein quality control system such as the ClpCP protease and the GroEL chaperonine. These findings highlight the impact of arginine phosphorylation in orchestrating the bacterial stress response.

Protein-tyrosine phosphatases: a new frontier in platelet signal transduction

Platelet activation must be tightly controlled in order to allow platelets to respond rapidly to vascular injury and prevent thrombosis from occurring. Protein-tyrosine phosphorylation is one of the main ways in which activation signals are transmitted in platelets. Although much is known about the protein-tyrosine kinases (PTKs) that initiate and propagate activation signals, relatively little is known about the protein-tyrosine phosphatases (PTPs) that modulate these signals in platelets. PTPs are a family of enzymes that dephosphorylate tyrosine residues in proteins and regulate signals transmitted within cells. PTPs have been implicated in a variety of pathological conditions, including cancer, diabetes and autoimmunity, but their functions in hemostasis and thrombosis remain largely undefined. Exciting new findings from a number of groups have revealed that PTPs are in fact critical regulators of platelet activation and thrombosis. The primary aim of this review is to highlight the unique and important functions of PTPs in regulating platelet activity. Establishing the functions of PTPs in platelets is essential to better understand the molecular basis of thrombosis and may lead to the development of improved antithrombotic therapies.

The effect of vanadium on platelet function

Vanadium pentoxide (V(2)O(5)) inhalation effect on platelet function in mice was explored, as well as the in vitro effect on human platelets. Mouse blood samples were collected and processed for aggregometry and flow cytometry to assess the presence of P-selectin and monocyte-platelet conjugates. Simultaneously, human platelets were processed for aggregometry(.) The mouse results showed platelet aggregation inhibition in platelet-rich-plasma (PRP) at four-week exposure time, and normality returned at eight weeks of exposure, remaining unchanged after the exposure was discontinued after four weeks. This platelet aggregation inhibition effect was reinforced with the in vitro assay. In addition, P-selectin preserved their values during the exposure, until the exposure was discontinued during four weeks, when this activation marker increased. We conclude that vanadium affects platelet function, but further studies are required to evaluate its effect on other components of the hemostatic system.

CD148 enhances platelet responsiveness to collagen by maintaining a pool of active Src family kinases

SUMMARY BACKGROUND

We have previously shown that the receptor-like protein tyrosine phosphatase (PTP) CD148 is essential for initiating glycoprotein VI (GPVI) signaling in platelets. We proposed that CD148 does so by dephosphorylating the C-terminal inhibitory tyrosine of Src family kinases (SFKs). However, this mechanism is complicated by CD148-deficient mouse platelets having a concomitant reduction in GPVI expression.

OBJECTIVES

To investigate the effect of CD148 on GPVI signaling independent of the decrease in GPVI expression and to further establish the molecular basis of the activatory effect of CD148 and downregulation of GPVI.

METHODS

CD148-deficient mouse platelets were investigated for functional and biochemical defects. The DT40/NFAT-lucifierase reporter assay was used to analyze the effect of CD148 on GPVI signaling. CD148-SFK interactions and dephosphorylation were quantified using biochemical assays.

RESULTS

CD148-deficient mouse platelets exhibited reduced collagen-mediated aggregation, secretion and spreading in association with reduced expression of GPVI and FcR gamma-chain and reduced tyrosine phosphorylation. The phosphorylation status of SFKs suggested a global reduction in SFK activity in resting CD148-deficient platelets. Studies in a cell model confirmed that CD148 inhibits GPVI signaling independent of a change in receptor expression and through a mechanism dependent on tyrosine dephosphorylation. Recombinant CD148 dephosphorylated the inhibitory tyrosines of Fyn, Lyn and Src in vitro, although paradoxically it also dephosphorylated the activation loop of SFKs.

CONCLUSIONS

CD148 plays a critical role in regulating GPVI/FcR gamma-chain expression and maintains a pool of active SFKs in platelets by directly dephosphorylating the C-terminal inhibitory tyrosines of SFKs that is essential for platelet activation.

Involvement of Src tyrosine kinases (SFKs) and of focal adhesion kinase (FAK) in the injurious mechanism in rat primary neuronal cultures exposed to chemical ischemia

Src family of kinases (SFKs) and focal adhesion kinase (FAK) are two important cellular signaling components known to act cooperatively in the transduction of death and survival signals. We investigated the involvement of these proteins in the mechanism of the injurious response in rat primary neuronal cultures exposed to an insult composed of chemical ischemia (poisoning with iodoacetic acid; 100 muM, for 150 min) followed by 1 h of incubation in the regular medium, an insult shown before to be associated with generation of reactive oxygen species and with the depletion of adenosine triphisphate. The exposure of the neuronal cultures to the insult resulted in cell injury, assessed by the increased release of cytoplasmic lactate dehydrogenase (LDH) into the culture media, which could be attenuated markedly by the presence of the antioxidant LY 231617. The insult resulted in the decreased level of phosphorylation of the SFKs members Src, Fyn, and Yes at the Src Y416-equivalent activation sites and of the FAK Y397 activation site, degradation of FAK to a p85 fragment, and disassembling of the FAK-SFKs complexes. The inhibition of SFKs was found to be responsible for part of the insult-induced cell damage manifested in increased LDH release. Pervanadate, an inhibitor of the phosphotyrosine phosphatases (PTPs), abrogated the inactivation of SFKs and attenuated cell injury, indicating that insult-induced activation of PTPs is involved in SFKs inhibition and the ensued damage. The inhibition of SFKs and FAK is probably the cause of the disassembling of SFKs-FAK complexes, a process known to be associated with apoptosis.

Tyrosine phosphatases such as SHP-2 act in a balance with Src-family kinases in stabilization of postsynaptic clusters of acetylcholine receptors

BACKGROUND

Development of neural networks requires that synapses are formed, eliminated and stabilized. At the neuromuscular junction (NMJ), agrin/MuSK signaling, by triggering downstream pathways, causes clustering and phosphorylation of postsynaptic acetylcholine receptors (AChRs). Postnatally, AChR aggregates are stabilized by molecular pathways that are poorly characterized. Gain or loss of function of Src-family kinases (SFKs) disassembles AChR clusters at adult NMJs in vivo, whereas AChR aggregates disperse rapidly upon withdrawal of agrin from cultured src-/-;fyn-/- myotubes. This suggests that a balance between protein tyrosine phosphatases (PTPs) and protein tyrosine kinases (PTKs) such as those of the Src-family may be essential in stabilizing clusters of AChRs.

RESULTS

We have analyzed the role of PTPs in maintenance of AChR aggregates, by adding and then withdrawing agrin from cultured myotubes in the presence of PTP or PTK inhibitors and quantitating remaining AChR clusters. In wild-type myotubes, blocking PTPs with pervanadate caused enhanced disassembly of AChR clusters after agrin withdrawal. When added at the time of agrin withdrawal, SFK inhibitors destabilized AChR aggregates but concomitant addition of pervanadate rescued cluster stability. Likewise in src-/-;fyn-/- myotubes, in which agrin-induced AChR clusters form normally but rapidly disintegrate after agrin withdrawal, pervanadate addition stabilized AChR clusters. The PTP SHP-2, known to be enriched at the NMJ, associated and colocalized with MuSK, and agrin increased this interaction. Specific SHP-2 knockdown by RNA interference reduced the stability of AChR clusters in wild-type myotubes. Similarly, knockdown of SHP-2 in adult mouse soleus muscle by electroporation of RNA interference constructs caused disassembly of pretzel-shaped AChR-rich areas in vivo. Finally, we found that src-/-;fyn-/- myotubes contained elevated levels of SHP-2 protein.

CONCLUSION

Our data are the first to show that the fine balance between PTPs and SFKs is a key aspect in stabilization of postsynaptic AChR clusters. One phosphatase that acts in this equilibrium is SHP-2. Thus, PTPs such as SHP-2 stabilize AChR clusters under normal circumstances, but when these PTPs are not balanced by SFKs, they render clusters unstable.

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.

Regulation of cytosolic PlA2 activity by PP1/PP2A serine/threonine phosphatases in human platelets

Platelet thromboxane A2 (TXA2) synthesis is an important pathway of platelet reactivity. We report that in thrombin-stimulated platelets, PP1/PP2A serine/threonine phosphatases regulate phospholipase A2 (cPLA2) activity, which is required for TXA2 synthesis. Two mechanisms are involved: (a) constitutively active PP1/PP2A regulate cPLA2 phosphorylation, and (b) PP1/PP2A activity mediates agonist-induced increase in cytosolic Ca2+ ([Ca2+]i). Inhibition of PP1/PP2A with okadaic acid (OA) induces cPLA2 phosphorylation but reduces Ca2+ responses: release from intracellular stores and influx through the plasma membrane, particularly that mediated by store-mediated Ca2+ entry (SMCE). A significant correlation (r = 0.64) exists between OA-regulated [Ca2+]i and TXA2 synthesis. Okadaic acid-induced decrease in SMCE and the associated TXA2 synthesis are mediated by a reduction in protein-tyrosine phosphorylation. This reduction is not due to inhibition of tyrosine kinases but rather to an OA-mediated increase in tyrosine phosphatases. This is the first study to report that PP1/PP2A phosphatases are involved in the regulation of the two key elements in eicosanoid synthesis, [Ca2+]i and cPLA2 phosphorylation. Moreover, PP1/PP2A regulation of [Ca2+]i and tyrosine phosphorylation may be important for other calcium-dependent processes and/or signal transduction mechanisms in platelets.

Essential Role of Protein Kinase Cδ in Platelet Signaling, αIIbβ3 Activation, and Thromboxane A2 Release

The protein kinase C (PKC) family is an essential signaling mediator in platelet activation and aggregation. However, the relative importance of the major platelet PKC isoforms and their downstream effectors in platelet signaling and function remain unclear. Using isolated human platelets, we report that PKCdelta, but not PKCalpha or PKCbeta, is required for collagen-induced phospholipase C-dependent signaling, activation of alpha(IIb)beta(3), and platelet aggregation. Analysis of PKCdelta phosphorylation and translocation to the membrane following activation by both collagen and thrombin indicates that it is positively regulated by alpha(IIb)beta(3) outside-in signaling. Moreover, PKCdelta triggers activation of the mitogen-activated protein kinase-kinase (MEK)/extracellular-signal regulated kinase (ERK) and the p38 MAPK signaling. This leads to the subsequent release of thromboxane A(2), which is essential for collagen-induced but not thrombin-induced platelet activation and aggregation. This study adds new insight to the role of PKCs in platelet function, where PKCdelta signaling, via the MEK/ERK and p38 MAPK pathways, is required for the secretion of thromboxane A(2).

von Willebrand factor (VWF)-dependent human platelet activation: porcine VWF utilizes different transmembrane signaling pathways than does thrombin to activate platelets, but both require protein phosphatase function

The interaction between von Willebrand factor (VWF) and glycoprotein (GP) Ib results in platelet agglutination and activation of many signaling intermediates. To determine if VWF-dependent platelet activation requires the participation of pivotal transmembrane signaling pathways, we analyzed VWF-dependent platelet activation profiles following inhibition of several transmembrane signaling intermediates. This was accomplished using porcine VWF, which has been shown to interact with human GPIb independently of shear stress or ristocetin. Platelet alpha (CD62) and lysozomal granule release (CD63), microparticle formation, and platelet agglutination/aggregation were evaluated. The ability of signaling inhibitors to prevent VWF-dependent platelet activation was compared to their ability to inhibit thrombin-dependent activation. The results demonstrate that VWF-dependent platelet activation can occur independently of the activities of protein kinase C (PKC), wortmannin-sensitive phosphatidylinositide 3-kinase, and phospholipase C, as well as independently of elevations in the concentration of intracellular calcium. In sharp contrast, these transmembrane signaling intermediates are required for thrombin-dependent platelet activation. In addition, thrombin-dependent but not VWF-dependent platelet activation was associated with elevations in the concentration of intracellular calcium under the conditions used. The family of signaling intermediates which appeared to be pivotal for both thrombin- and VWF-dependent platelet activation were the protein tyrosine phosphatases and the serine/threonine phosphatases. It is concluded that thrombin-dependent platelet activation relies on the activation of several transmembrane signaling pathways, whereas VWF-dependent platelet activation is dependent upon the activity of protein phosphatases. Inhibition of these phosphatases in vivo may provide a novel therapeutic approach for treating VWF-dependent platelet disorders such as thrombotic thrombocytopenic purpura or arterial thrombosis.

Tyrosine phosphatases SHP-1 and SHP-2 are associated with distinct tyrosine-phosphorylated proteins

SHP-1 and SHP-2 are two SH2 domain-containing tyrosine phosphatases. They share significant overall sequence identity but their functions are often opposite. The mechanism underlying this is not well understood. In this study, we have investigated the association of SHP-1 and SHP-2 with tyrosine-phosphorylated proteins in mouse tissues and in cultured cells treated with a potent tyrosine phosphatase inhibitor, pervanadate. Pervanadate was introduced into mice by intravenous injection. It induced robust tyrosine phosphorylation of cellular proteins in a variety of tissues. Both SHP-1 and SHP-2 were phosphorylated on tyrosyl residues upon pervanadate treatment, and they became associated with distinct tyrosine-phosphorylated proteins in different tissues and cells. Among these proteins, PZR and PECAM were identified as major SHP-2-binding proteins while LAIR-1 was shown to be a major SHP-1-binding protein. A number of other proteins are to be identified. We believe that the different binding proteins may determine the distinct physiological functions of SHP-1 and SHP-2. The present study also provides a general method to induce tyrosine phosphorylation of cellular proteins and to study protein-protein interactions involving tyrosine phosphorylation in vivo and in vitro.

Involvement of phospholipase D in oxidative stress-induced necrosis of vascular smooth muscle cells

Phospholipase D (PLD) has been associated with necrosis. However, it is not clear whether PLD plays a causative role in this cellular process. We investigated the role of PLD in oxidative stress-induced necrosis of vascular smooth muscle cells (VSMCs). Pervanadate (hydrogen peroxide plus orthovanadate) but not hydrogen peroxide alone activated PLD in a dose- and time-dependent manner. Exposure of VSMCs to pervanadate resulted in necrosis. Pretreatment with butan-1-ol, a PLD inhibitor, attenuated both pervanadate-induced necrosis and increase of intracellular Ca(2+). Removal of extracellular Ca(2+) inhibited pervanadate-induced necrosis by 50%. These results suggest that PLD activation mediates pervanadate-induced necrosis of VSMCs, which is at least partly due to Ca(2+) toxicity.

Oncogenic mutants of RON and MET receptor tyrosine kinases cause activation of the beta-catenin pathway

beta-Catenin is an oncogenic protein involved in regulation of cell-cell adhesion and gene expression. Accumulation of cellular beta-catenin occurs in many types of human cancers. Four mechanisms are known to cause increases in beta-catenin: mutations of beta-catenin, adenomatous polyposis coli, or axin genes and activation of Wnt signaling. We report a new cause of beta-catenin accumulation involving oncogenic mutants of RON and MET receptor tyrosine kinases (RTKs). Cells transfected with oncogenic RON or MET were characterized by beta-catenin tyrosine phosphorylation and accumulation; constitutive activation of a Tcf transcriptional factor; and increased levels of beta-catenin/Tcf target oncogene proteins c-myc and cyclin D1. Interference with the beta-catenin pathway reduced the transforming potential of mutated RON and MET. Activation of beta-catenin by oncogenic RON and MET constitutes a new pathway, which might lead to cell transformation by these and other mutant growth factor RTKs.

Involvement of MAP kinases in the control of cPLA2 and arachidonic acid release in endothelial cells

Cytosolic Phospholipase A(2) (cPLA(2)) has been implicated in receptor-mediated release of arachidonic acid from membrane phospholipids, the limiting step in prostacyclin and other eicosanoid production. Its activity is controlled by Ca(++) levels and enzymatically regulated phosphorylation. The purpose of this study was to assess the importance of phosphorylation of cPLA(2) in human umbilical vein endothelial cells and to identify the kinases involved. Inhibitors were used to study the pathways leading to phosphorylation and activation of mitogen activated protein kinases (MAP-kinases) and cPLA(2), as well as release of arachidonic acid and prostacyclin production after stimulation with different agonists. We have found that agonists that release arachidonic acid, including histamine, thrombin, AlF(4)(-), and pervanadate, all activate the MAP kinases ERK, p38 and JNK and cause phosphorylation of cPLA(2). Agonist specific differences in the signal transduction pathways included variable contribution of tyrosine phosphorylation, protein kinase C and ERK activity, and different effects of pertussis toxin. Treatment with PD98059 (inhibitor of ERK-activation) or SB203580 (inhibitor of p38) caused partial decrease in arachidonic acid release and cPLA(2) activity. In contrast the nonspecific protein kinase inhibitor staurosporin completely inhibited cPLA(2) activity. We conclude that in endothelial cells arachidonic acid release is largely mediated by cPLA(2) through agonist-specific pathways. The MAP kinases ERK and p38 both have demonstrable but not major effect on agonist stimulated arachidonic acid release and the data suggest that an additional unidentified kinase also has a role.

Fibrin clot retraction by human platelets correlates with alpha(IIb)beta(3) integrin-dependent protein tyrosine dephosphorylation

We have analyzed tyrosine phosphorylation associated with retraction of the fibrin clot by washed platelets in purified fibrinogen. Retraction was dependent on integrin alpha(IIb)beta(3), based on absence of retraction of alpha(IIb)beta(3)-deficient thrombasthenic platelets. However, only a subset of alpha(IIb)beta(3)-blocking antibodies or peptides were able to inhibit retraction, suggesting a differential engagement of alpha(IIb)beta(3) in fibrin clot retraction versus aggregation. Immunoblotting demonstrated a phosphorylated protein pattern comparable with aggregation at early time points. However, as opposed to aggregation, tyrosine phosphorylation decreased rapidly in parallel to retraction (up to 60 min). Dephosphorylation was alpha(IIb)beta(3)-dependent, since it was blocked by alpha(IIb)beta(3)-specific inhibitors and was absent in thrombasthenic platelets. Inhibition of platelet clot retraction by phenyl-arsine oxide and peroxovanadate, suggested a role for tyrosine phosphatases. Cytochalasin D and E (5 microm) blocked fibrin clot retraction and tyrosine dephosphorylation, suggesting regulation by actin cytoskeleton assembly. Tyrosine phosphatase activities were found associated with clot retraction using the "in-gel" tyrosine phosphatase assay; however, none were alpha(IIb)beta(3)-dependent. An 85-kDa protein and to a lesser degree "Src" showed the closest dose-dependent correlation between inhibition of tyrosine dephosphorylation and inhibition of retraction. We thus postulate that alpha(IIb)beta(3) engagement in fibrin clot retraction drives, in an actin cytoskeleton-dependent manner, the interaction of tyrosine phosphatases and of the tyrosine-phosphorylated substrates 85-kDa protein and Src, the dephosphorylation of which regulates the force generation and/or transmission required for full contraction of the fibrin matrix.

Determination of the tyrosine phosphorylation sites in the T cell transmembrane glycoprotein CD5

Studies of CD5-deficient mice indicate that the transmembrane glycoprotein CD5 negatively regulates antigen receptor-mediated signals in thymocytes, lymph node T cells and B1a cells. CD5 contains four tyrosine residues in its cytoplasmic domain and is phosphorylated on tyrosine residues following antigen receptor ligation. Recently it has been proposed that CD5 function is dependent on the recruitment of the tyrosine phosphatase SHP-1 to tyrosine-phosphorylated CD5 and subsequent dephosphorylation of signaling molecules. In this study we investigated the requirements for, and sites of, CD5 tyrosine phosphorylation. Using a T cell line deficient in the tyrosine kinase p56(lck) and the same cell line reconstituted with this kinase, we show that p56(lck) expression is required for efficient CD5 tyrosine phosphorylation. Using tyrosine-phosphorylated peptides corresponding to CD5 cytoplasmic sequences we also show that the Src homology 2 (SH2) domain of p56(lck) binds prominently to pY429SQP, with 30-fold less affinity to pY463DLQ and not to pY441PAL. A number of murine CD5 Y --> F and deletion mutants were expressed in Jurkat T cells. The Y441F mutant was tyrosine phosphorylated at levels comparable to wild-type, but the Y429F and Y463F mutants were phosphorylated at lower levels. Two deletion mutants, which contain only one tyrosine residue (Y378) located at the interface of the transmembrane and cytoplasmic domains, were not tyrosine phosphorylated, suggesting that Y378 is not readily available for phosphorylation. Taken together these results suggest that both Y429 and Y463 can recruit p56(lck), and that these residues are the only prominent sites for CD5 tyrosine phosphorylation.

Modulation of Kv3 potassium channels expressed in CHO cells by a nitric oxide-activated phosphatase

1.Voltage-gated K+ channels containing Kv3 subunits play specific roles in the repolarization of action potentials. Kv3 channels are expressed in selective populations of CNS neurons and are thought to be important in facilitating sustained and/or repetitive high frequency firing. Regulation of the activity of Kv3 channels by neurotransmitters could have profound effects on the repetitive firing characteristics of those neurons. 2.Kv3 channels are found in several neuronal populations in the CNS that express nitric oxide synthases (NOSs). We therefore investigated whether Kv3 channels are modulated by the signalling gas nitric oxide (NO). 3. We found that Kv3.1 and Kv3.2 currents are potentially suppressed by D-NONOate and other NO donors. The effects of NO on these currents are mediated by the activation of guanylyl cyclase (GC), since they are prevented by Methylene Blue, an inhibitor of GC, and by ODQ, a specific inhibitor of the soluble form of GC. Moreover, application of 8-Br-cGMP, a permeant analogue of cGMP, also blocked Kv3.1 and Kv3.2 currents. 4.KT5283, a cGMP-dependent protein kinase (PKG) blocker, prevented the inhibition of Kv3.1 and Kv3.2 currents by D-NONOate and 8-Br-cGMP. This indicates that activation of PKG as a result of the increase in intracellular cGMP levels produced by D-NONOate or 8-Br-cGMP is necessary for channel block. 5. Although the effects of NO on Kv3.1 and Kv3.2 channels require PKG activity, two observations suggest that they are not mediated by phosphorylation of channel proteins: (a) the reagents affect both Kv3.2 and Kv3.1 channels, although only Kv3.2 proteins have a putative PKA-PKG phosphorylation site, and (b) mutation of the PKA-PKG phosphorylation site in Kv3.2 does not interfere with the effects of NO or cGMP. 6. The inhibitory effects of NO and cGMP on Kv3.1 and Kv3.2 currents appear to be mediated by the activation of serine-threonine phosphatase, since they are blocked by low doses of okadaic acid. Furthermore, direct intracellular application of the catalytic subunit of protein phosphatase 2A inhibited Kv3.2 currents, indicating that activity of PKG-induced phosphatase is necessary and sufficient to inhibit these channels. 7. The results suggest that basal phosphorylation of Kv3 channel proteins is required for proper channel function. Activation of phosphatases via NO or other signals that increase cGMP might be a potent mechanism to regulate Kv3 channel activity in neurons.

Diperoxovanadate alters endothelial cell focal contacts and barrier function: role of tyrosine phosphorylation

Diperoxovanadate (DPV), a potent tyrosine kinase activator and protein tyrosine phosphatase inhibitor, was utilized to explore bovine pulmonary artery endothelial cell barrier regulation. DPV produced dose-dependent decreases in transendothelial electrical resistance (TER) and increases in permeability to albumin, which were preceded by brief increases in TER (peak TER effect at 10-15 min). The significant and sustained DPV-mediated TER reductions were primarily the result of decreased intercellular resistance, rather than decreased resistance between the cell and the extracellular matrix, and were reduced by pretreatment with the tyrosine kinase inhibitor genistein but not by inhibition of p42/p44 mitogen-activating protein kinases. Immunofluorescent analysis after DPV challenge revealed dramatic F-actin polymerization and stress-fiber assembly and increased colocalization of tyrosine phosphoproteins with F-actin in a circumferential pattern at the cell periphery, changes that were abolished by genistein. The phosphorylation of focal adhesion and adherens junction proteins on tyrosine residues was confirmed in immunoprecipitates of focal adhesion kinase and cadherin-associated proteins in which dramatic dose-dependent tyrosine phosphorylation was observed after DPV stimulation. We speculate that DPV enhances endothelial cell monolayer integrity via focal adhesion plaque phosphorylation and produces subsequent monolayer destabilization of adherens junctions initiated by adherens junction protein tyrosine phosphorylation catalyzed by p60(src) or Src-related tyrosine kinases.

Growth cone steering by receptor tyrosine phosphatase delta defines a distinct class of guidance cue

Receptor-type tyrosine phosphatases (RPTPs) are involved in pathfinding decisions by elongating axons, but how they function in these decisions remains unclear. A vertebrate RPTP, PTP-delta, is a neurite-promoting homophilic adhesion molecule; here we demonstrate chemoattraction of CNS growth cones by a locally applied gradient of soluble PTP-delta. The attractive effect of PTP-delta was abolished by inhibition of tyrosine phosphatase activity, but in contrast to other guidance proteins was unaffected by inhibition of cyclic nucleotide activities. Gradients of PTP-delta or of laminin-1 also promoted increases in the speed of growth cone migration, but laminin-1 did not steer growth cones. Our results indicate that PTP-delta is a chemoattractant for vertebrate CNS neurons in vitro and suggest that it represents a distinct class of guidance protein from those previously defined. Further, our data indicate that growth cone attraction is mechanistically distinct from increases in the speed of growth cone movement.

The mechanisms underlying the stimulatory effects of thrombin on myometrial smooth muscle

OBJECTIVE

The mechanisms underlying the stimulation of uterine contractions in the presence of intrauterine hemorrhage have not been well defined. Thrombin, a blood coagulation factor, activates membrane receptors to result in the stimulation of the phosphatidylinositol signaling pathway and the mobilization of cytosolic calcium in platelets. Our studies sought to determine whether thrombin stimulates similar events in myometrial smooth muscle.

STUDY DESIGN

Cytosolic calcium imaging and in vitro contraction studies were performed with rat myometrial tissue.

RESULTS

At a concentration range of 1 to 100 U/mL thrombin produced phasic myometrial contractions, which were comparable in intensity to those produced by oxytocin and prostaglandin F(2)(alpha). Thrombin-induced cytosolic calcium concentration oscillations were similar to those produced by oxytocin. Contractions stimulated by thrombin were significantly suppressed in response to inhibitors of the phosphatidylinositol signaling pathway. These studies also confirmed that membrane receptor-Gq protein coupling events play a more important role than tyrosine kinase-mediated events during thrombin stimulation of myometrial smooth muscle.

CONCLUSION

Thrombin is a potent uterotonic agonist, and its effects in myometrium are mediated by intracellular signaling events comparable to those activated by classic uterotonic agents. The physiologic importance of thrombin appears to be related to its potential role in the stimulation of uterine contractions in the presence of intrauterine hemorrhage.

Tyrosine dephosphorylation, but not phosphorylation, of p130Cas is dependent on integrin alpha IIb beta 3-mediated aggregation in platelets: implication of p130Cas involvement in pathways unrelated to cytoskeletal reorganization

The newly described adapter molecule p130 Crk-associated substrate (Cas) has been reported to contribute to cytoskeletal organization through assembly of actin filaments and to be pivotal in embryonic development and in oncogene-mediated transformation. We characterized the regulation of Cas tyrosine phosphorylation in highly differentiated, anucleate platelets. Phospholipase C-activating receptor agonists, including collagen, thrombin receptor-activating peptide (TRAP), and U46619 (a thromboxane A2 analogue), and A23187 (a Ca2+ ionophore) induced rapid Cas tyrosine phosphorylation in platelets. 12-O-Tetradecanoylphorbol 13-acetate and 1-oleoyl-2-acetyl-sn-glycerol, protein kinase C (PKC) activators, also induced Cas tyrosine phosphorylation, albeit sluggishly. Cas tyrosine phosphorylation induced by collagen or TRAP was transient in aggregating platelets; Cas became dephosphorylated in a manner dependent on integrin alpha IIb beta 3-mediated aggregation. While BAPTA-AM (an intracellular Ca2+ chelator) inhibited Cas phosphorylation induced by collagen or TRAP, Ro31-8220 (a PKC inhibitor) rather prolonged it. Under the conditions, this PKC inhibitor suppressed platelet aggregation but not intracellular Ca2+ mobilization. In contrast to Cas involvement in focal adhesions in other cells, platelet Cas phosphorylation preceded the activation of focal adhesion kinase (FAK), and blockage of alpha IIb beta 3-mediated platelet aggregation with a GRGDS peptide resulted in prolongation of stimulation-dependent Cas tyrosine phosphorylation but in suppression of FAK tyrosine phosphorylation. Furthermore, TRAP-induced Cas phosphorylation was insensitive to cytochalasin D, an actin polymerization inhibitor. The failure of FAK to associate with Cas in immunoprecipitation studies also suggests that Cas tyrosine phosphorylation is independent of FAK activation. Of the signaling molecules investigated in this study, Src seemed to associate with Cas. Finally, Cas existed mainly in cytosol and membrane cytoskeleton fractions in the resting state, and remained unchanged during platelet aggregation, when FAK translocated to the cytoskeletal fraction. Our findings on platelet Cas suggest that (i) rapid Cas tyrosine phosphorylation occurs following phosphoinositide turnover by receptor-mediated agonists and may be mediated by intracellular Ca2+ mobilization; (ii) PKC activation, by itself, may elicit sluggish Cas phosphorylation; (iii) Cas tyrosine dephosphorylation, but not phosphorylation, is dependent on integrin alpha IIb beta 3-mediated aggregation; and (iv) Cas is not involved in cytoskeletal reorganization. Anucleate platelets seem to provide a unique model system to fully elucidate the functional role(s) of Cas.

Tyrosine phosphorylation of focal adhesion kinase and paxillin regulates the signaling mechanism of the rapid nongenomic action of dexamethasone on actin cytoskeleton

We have previously shown that dexamethasone (DEX) stimulates rapid polymerization of actin and stabilization of microfilaments in human endometrial adenocarcinoma cells. As the content of total cellular actin and the concentration of the actin transcript did not change, we concluded that polymerization of actin by glucocorticoids involves nongenomic mechanisms. However, the signaling events by which the latter is achieved remain unknown. In the present study we evaluated whether tyrosine phosphorylation is required for the rapid, nongenomic DEX effect on actin assembly. In cells preincubated with the tyrosine kinase inhibitors, genistein or erbstatin analogue (EA), before adding DEX the G-/total actin ratio remained unchanged, whereas DEX in the absence of both inhibitors reduced the ratio by 25%. In addition, when cells were preincubated with the protein tyrosine phosphatase inhibitor pervanadate and subsequently incubated with DEX, the G-/total actin ratio was dramatically reduced by 65%. Furthermore, DEX increased transiently the levels of tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin within 2 to 15 min, without a change in their expression levels. Pervanadate mimicked this effect of DEX and enhanced tyrosine phosphorylation of both proteins. In addition, when cells were exposed to the anticytoskeletal agent cytochalasin B, the basal levels of tyrosine phosphorylation of both proteins were reduced. This effect was reversed by DEX, indicating that actin cytoskeleton integrity is required for the effect of DEX on tyrosine phosphorylation of FAK and paxillin. Finally, we documented enhanced expression of the Ras-related GTP-binding protein Rho-B after long-term (12- and 24-hr) treatment with DEX, whereas Rho-B levels remained unchanged after short-term (3- and 6-hr) treatment. Our observations demonstrate a novel mechanism through which the rapid nongenomic effect of DEX on actin assembly requires tyrosine phosphorylation of the cytoskeleton-associated proteins FAK and paxillin. We also propose that the DEX-induced actin polymerization may constitute a mechanism for transduction of signals resulting in tyrosine phosphorylation of FAK and paxillin. Moreover, the enhanced Rho-B levels observed after long-term treatment with DEX imply a mechanism for the well-described, long-term effects of glucocorticoids on actin cytoskeleton.

Regulation of endothelial cell myosin light chain kinase by Rho, cortactin, and p60(src)

Inflammatory diseases of the lung are characterized by increases in vascular permeability and enhanced leukocyte infiltration, reflecting compromise of the endothelial cell (EC) barrier. We examined potential molecular mechanisms that underlie these alterations and assessed the effects of diperoxovanadate (DPV), a potent tyrosine kinase activator and phosphatase inhibitor, on EC contractile events. Confocal immunofluorescent microscopy confirmed dramatic increases in stress-fiber formation and colocalization of EC myosin light chain (MLC) kinase (MLCK) with the actin cytoskeleton, findings consistent with activation of the endothelial contractile apparatus. DPV produced significant time-dependent increases in MLC phosphorylation that were significantly attenuated but not abolished by EC MLCK inhibition with KT-5926. Pretreatment with the Rho GTPase-inhibitory C3 exotoxin completely abolished DPV-induced MLC phosphorylation, consistent with Rho-mediated MLC phosphatase inhibition and novel regulation of EC MLCK activity. Immunoprecipitation of EC MLCK after DPV challenge revealed dramatic time-dependent tyrosine phosphorylation of the kinase in association with increased MLCK activity and a stable association of MLCK with the p85 actin-binding protein cortactin and p60(src). Translocation of immunoreactive cortactin from the cytosol to the cytoskeleton was noted after DPV in concert with cortactin tyrosine phosphorylation. These studies indicate that DPV activates the endothelial contractile apparatus in a Rho GTPase-dependent fashion and suggests that p60(src)-induced tyrosine phosphorylation of MLCK and cortactin may be important features of contractile complex assembly.

TNF-alpha induces actin cytoskeleton reorganization in glomerular epithelial cells involving tyrosine phosphorylation of paxillin and focal adhesion kinase

Glomerular permeability for macromolecules depends partially on proper attachment of the glomerular epithelial cells (GEC) to the glomerular basement membrane (GBM). The latter requires integrity of the actin cytoskeleton, which in turn is regulated by specific actin-associated proteins. Since several glomerulopathies characterized by heavy proteinuria are associated with increased glomerular tumor necrosis factor alpha (TNF-alpha) expression, we studied the interaction of TNF-alpha with the actin cytoskeleton of cultured rat GEC. Incubation of GEC with 10 ng/ml TNF-alpha for variable time periods ranging from 15 min to 24 hr demonstrated a marked accentuation and redistribution of actin microfilaments, as shown by direct fluorescence analysis and confocal laser scanning microscopy. Quantitative biochemical determination of the G/total-actin ratio confirmed the above observations. Indeed, this ratio was significantly reduced, indicating substantial polymerization of G-actin and formation of F-actin. Concurrently, TNF-alpha rapidly induced tyrosine phosphorylation of both paxillin and focal adhesion kinase, without affecting the expression levels of these two proteins. In addition, tyrosine phosphorylation of vinculin became evident, indicating involvement of this focal adhesion marker in the observed actin reorganization. Inhibition of tyrosine phosphorylation by genistein prevented the reorganization of the actin cytoskeleton by TNF-alpha. We conclude that TNF-alpha induces substantial reorganization of actin cytoskeleton and focal adhesions. These effects occur simultaneously, with a prompt TNF-alpha-induced tyrosine phosphorylation of paxillin and focal adhesion kinase, indicating that these proteins, known to regulate actin polymerization and formation of focal adhesions, may be directly involved in the mechanism controlling the observed actin redistribution. These findings suggest that the observed TNF-alpha-actin cytoskeleton interactions may relate to the pathogenesis of glomerulopathies with heavy proteinuria, in which increased glomerular expression of TNF-alpha is associated with disturbances in the attachment of podocytes to the GBM.

Phosphorylation of the thromboxane receptor alpha, the predominant isoform expressed in human platelets

A single gene encodes the human thromboxane receptor (TP), of which there are two identified splice variants, alpha and beta. Both isoforms are rapidly phosphorylated in response to thromboxane agonists when overexpressed in human embryonic kidney 293 cells; this phenomenon is only slightly altered by inhibitors of protein kinase C. Pharmacological studies have defined two classes of TP in human platelets; sites that bind the agonist I-BOP with high affinity support platelet shape change. Low affinity sites, which irreversibly bind the antagonist GR 32191, transduce platelet activation and aggregation. Isoform-specific antibodies permitted detection of TPalpha, but not TPbeta, from human platelets, although mRNA for both isoforms is present. A broad protein band of 50-60 kDa, reflecting the glycosylated receptor, was phosphorylated upon activation of platelets for 2 min with I-BOP. This was a rapid ( approximately 30 s) and transient (maximum, 2-4 min) event and was inhibited by TP antagonists. Both arachidonic acid and low concentrations of collagen stimulated TPalpha phosphorylation, which was blocked by cyclooxygenase inhibition or TP antagonism. Blockade of the low affinity TP sites with GR 32191 prevented I-BOP-induced TPalpha phosphorylation. This coincided with agonist-induced platelet aggregation and activation but not shape change. Also, activation of these sites with the isoprostane iPF2alpha-III induced platelet shape change but not TPalpha phosphorylation. Heterologous TP phosphorylation was observed in aspirin-treated platelets exposed to thrombin, high concentrations of collagen, and the calcium ionophore A 23187. Both homologous and heterologous agonist-induced phosphorylation of endogenous TPalpha was blocked by protein kinase C inhibitors. TPalpha was the only isoform detectably translated in human platelets. This appeared to correspond to the activation of the low affinity site defined by the antagonist GR 32191 and not activated by the high affinity agonist, iPF2alpha-III. Protein kinase C played a more important role in agonist-induced phosphorylation of native TPalpha in human platelets than in human embryonic kidney 293 cells overexpressing recombinant TPalpha.

Arachidonic acid metabolites mediate angiotensin II-induced NADH/NADPH oxidase activity and hypertrophy in vascular smooth muscle cells

Previously, we showed that angiotensin II stimulation of the NADH/NADPH oxidase is involved in hypertrophy of cultured vascular smooth muscle cells (VSMC). Here, we examine the pathways leading to oxidase activation, and demonstrate that arachidonic acid metabolites mediate hypertrophy by activating the p22phox-based NADH/NADPH oxidase. Angiotensin II stimulates phospholipase A2, releasing arachidonic acid, which stimulates oxidase activity in vitro. When arachidonic acid metabolism is blocked with 5,8,11,14-eicosatetraynoic acid (ETYA) or nordihydroguaiaretic acid (NDGA), oxidase activity decreases by 80 +/- 10%. In VSMC transfected with antisense p22phox to attenuate NADH/NADPH oxidase expression, arachidonic acid is unable to stimulate NADH/NADPH-dependent superoxide production. In these cells, or in cells in which NADH/NADPH oxidase activity is inhibited by diphenylene iodonium, angiotensin II-induced [3H]leucine incorporation is also inhibited. Attenuation of oxidase activation by inhibiting arachidonic acid metabolism with ETYA, NDGA, baicalein, or SKF-525A also inhibits angiotensin II-stimulated protein synthesis (74 +/- 2% and 34 +/- 1%, respectively). Thus, endogenous noncyclooxygenase arachidonic acid metabolites mediate angiotensin II-stimulated protein synthesis in cultured VSMC by activating the NADH/NADPH oxidase, providing mechanistic evidence for redox control of VSMC hypertrophy.

Involvement of tyrosine phosphorylation and protein kinase C in the activation of phospholipase D by H2O2 in Swiss 3T3 fibroblasts

We have investigated the mechanisms involved in H2O2-mediated phospholipase D (PLD) activation in Swiss 3T3 fibroblasts. In the presence of vanadate, H2O2 induced tyrosine phosphorylation of PLD as well as the platelet-derived growth (PDGF) factor receptor, protein kinase Calpha (PKCalpha), and a 62-kDa protein in rat brain PLD1 (rPLD1) immune complexes. PDGF also induced tyrosine phosphorylation of PLD, but this was abolished by catalase, indicating that it was mediated by H2O2 generation. Interestingly, PLD was found to be constitutively associated with the PDGF receptor and PKCalpha. Stimulation by H2O2 showed a concentration- and time-dependent tyrosine phosphorylation of the proteins in rPLD1 immunoprecipitates and activation of PLD in the cells. Pretreatment of the cells with the protein-tyrosine kinase inhibitors genistein and herbimycin A resulted in a concentration-dependent inhibition of H2O2-induced tyrosine phosphorylation and PLD activation. Activation of PLD by H2O2 was also inhibited dose-dependently by the PKC inhibitors Ro 31-8220 and calphostin C. Down-regulation of PKC by prolonged treatment with 4beta-phorbol 12-myristate 13-acetate also abolished H2O2-stimulated PLD activity. H2O2 or vanadate alone did not induce tyrosine phosphorylation of proteins in the rPLD1 immune complex or PLD activation. Reduction of intracellular H2O2 levels by pretreatment of the cells with catalase dramatically abrogated tyrosine phosphorylation of proteins in the rPLD1 immune complex and PLD activation, suggesting the potential role of intracellular H2O2 in H2O2-mediated PLD signaling. Taken together, these results suggest that both protein-tyrosine kinase(s) and protein kinase C participate in H2O2-induced PLD activation in Swiss 3T3 cells.

Transduction pathways involved in rapid hormone receptor regulation in the mammary epithelium

Previous studies have shown that the envelope protein of the mouse mammary tumor virus (MMTV) rapidly upregulates prolactin (PRL) receptors by shifting them from internal pools to the cell surface and downregulates epidermal growth factor (EGF) receptors by inducing their internalization and degradation. This study shows that the effect on PRL receptors is mediated by the nitric oxide (NO)/cGMP pathway, since it can be mimicked by an NO donor or 8-bromo-cGMP and can be blocked by an NO synthase inhibitor. In contrast, the effect on EGF receptors is mediated by tyrosine phosphorylation and phosphatidylinositol 3-kinase (PI3K), since it can be blocked by either a tyrosine kinase inhibitor or by a PI3K inhibitor. Both of these pathways can be activated by a calcium ionophore and inhibited by calcium chelation. Therefore, it appears that the mouse mammary tumor virus envelope protein, like other retroviral envelope proteins, initially elevates cytoplasmic calcium, which can then stimulate both the NO/cGMP and the tyrosine phosphorylation/PI3K pathways, leading to PRL receptor upregulation and EGF receptor downregulation, respectively.

Role of NADH/NADPH oxidase-derived H2O2 in angiotensin II-induced vascular hypertrophy

Recent evidence suggests that oxidative mechanisms may be involved in vascular smooth muscle cell (VSMC) hypertrophy. We previously showed that angiotensin II (Ang II) increases superoxide production by activating an NADH/NADPH oxidase, which contributes to hypertrophy. In this study, we determined whether Ang II stimulation of this oxidase results in H2O2 production by studying the effects of Ang II on intracellular H2O2 generation, intracellular superoxide dismutase and catalase activity, and hypertrophy. Ang II (100 nmol/L) significantly increased intracellular H2O2 levels at 4 hours. Neither superoxide dismutase activity nor catalase activity was affected by Ang II; the SOD present in VSMCs is sufficient to metabolize Ang II-stimulated superoxide to H2O2, which accumulates more rapidly than it is degraded by catalase. This increase in H2O2 was inhibited by extracellular catalase, diphenylene iodonium, an inhibitor of the NADH/NADPH oxidase, and the AT1 receptor blocker losartan. In VSMCs stably transfected with antisense p22phox, a critical component of the NADH/NADPH oxidase in which oxidase activity was markedly reduced, Ang II-induced production of H2O2 was almost completely inhibited, confirming that the source of Ang II-induced H2O2 was the NADH/NADPH oxidase. Using a novel cell line that stably overexpresses catalase, we showed that this increased H2O2 is a critical step in VSMC hypertrophy, a hallmark of many vascular diseases. Inhibition of intracellular superoxide dismutase by diethylthiocarbamate (1 mmol/L) also resulted in attenuation of Ang II-induced hypertrophy (62+/-2% inhibition). These data indicate that AT1 receptor-mediated production of superoxide generated by the NADH/NADPH oxidase is followed by an increase in intracellular H2O2, suggesting a specific role for these oxygen species and scavenging systems in modifying the intracellular redox state in vascular growth.

A role of tyrosine phosphatase in acetylcholine receptor cluster dispersal and formation

Innervation of the skeletal muscle involves local signaling, leading to acetylcholine receptor (AChR) clustering, and global signaling, manifested by the dispersal of preexisting AChR clusters (hot spots). Receptor tyrosine kinase (RTK) activation has been shown to mediate AChR clustering. In this study, the role of tyrosine phosphatase (PTPase) in the dispersal of hot spots was examined. Hot spot dispersal in cultured Xenopus muscle cells was initiated immediately upon the presentation of growth factor-coated beads that induce both AChR cluster formation and dispersal. Whereas the density of AChRs decreased with time, the fine structure of the hot spot remained relatively constant. Although AChR, rapsyn, and phosphotyrosine disappeared, a large part of the original hot spot-associated cytoskeleton remained. This suggests that the dispersal involves the removal of a key linkage between the receptor and its cytoskeletal infrastructure. The rate of hot spot dispersal is inversely related to its distance from the site of synaptic stimulation, implicating the diffusible nature of the signal. PTPase inhibitors, such as pervanadate or phenylarsine oxide, inhibited hot spot dispersal. In addition, they also affected the formation of new clusters in such a way that AChR microclusters extended beyond the boundary set by the clustering stimuli. Furthermore, by introducing a constitutively active PTPase into cultured muscle cells, hot spots were dispersed in a stimulus- independent fashion. This effect of exogenous PTPase was also blocked by pervanadate. These results implicate a role of PTPase in AChR cluster dispersal and formation. In addition to RTK activation, synaptic stimulation may also activate PTPase which acts globally to destabilize preexisting AChR hot spots and locally to facilitate AChR clustering in a spatially discrete manner by countering the action of RTKs.

Effects of FK506-binding protein 12 and FK506 on autophosphorylation of epidermal growth factor receptor

FK506-binding proteins and cyclophilins are intracellular proteins that express peptidylproline cis-trans-isomerase (PPIase) activity. The effects of FK506-binding protein 12 (FKBP12) and the cyclophilins 18 and 23 on autophosphorylation of the epidermal growth factor (EGF) receptor prepared from plasma membranes of the human epidermoid cell line A431 have been investigated. Whereas FKBP12 inhibited EGF receptor tyrosine kinase activity in a concentration-dependent manner, the cyclophilins did not affect autophosphorylation. In contrast to the wild-type enzyme, several variants of FKBP12 with greatly reduced PPIase activity were unable to suppress EGF receptor tyrosine kinase significantly. Pervanadate an inhibitor of protein tyrosine phosphatases, abolished the effect of FKBP12 on EGF receptor autophosphorylation. Finally, FK506 and rapamycin, which are known to block the PPIase activity of FKBP12, induced a significant stimulation of EGF receptor autophosphorylation in intact A431 cells suggesting suppression of EGF receptor autophosphorylation by intracellular FKBP12 in vivo. Taken together the data point to an inhibitory function of FKBP12 in EGF receptor signaling, possibly induced by stimulation of a protein tyrosine phosphatase coupled to the EGF receptor. Both PPIase activity and substrate specificity of FKBP12 seem to be indispensable for this effect.

Properties of pervanadate and permolybdate. Connexin43, phosphatase inhibition, and thiol reactivity as model systems

Pervanadate and permolybdate are irreversible protein-tyrosine phosphatase inhibitors, with IC50 values of 0.3 and 20 microM, respectively, in intact cells. Maximal inhibition was obtained within 1 min at higher concentrations of the compounds. They induced prominent changes in the phosphorylation status of the gap junction protein, connexin43. These effects were utilized as model systems to assess the stability and inactivation of the compounds. Although the concentrated stock solutions were relatively stable, the diluted compounds were unstable. The biological activity had decreased to 20-30% after 6 h of incubation in a phosphate buffer, 1 h in phosphate buffer with 10% fetal calf serum, and 1-3 minutes in culture medium. Thiols reacted rapidly with the compounds and inactivated them (initial reaction rates with cysteine: permolybdate > pervanadate > H2O2). Catalase inactivated the compounds, and permolybdate was the more sensitive. The cells inactivated permolybdate faster than pervanadate. Cellular inactivation of permolybdate, and to a lesser degree pervanadate, appeared to be partly dependent on catalase and thiols. However, a general decrease in cellular thiols was not the mediator of the biological effects of pervanadate or permolybdate. Mathematical modeling of the thiol reactivity suggested that monoperoxovanadate at maximum could possess 20% of the biological activity of diperoxovanadate.

Phosphatidic acid elicits calcium mobilization and actin polymerization through a tyrosine kinase-dependent process in human neutrophils: a mechanism for induction of chemotaxis

Phospholipids mediate important effects as extracellular messengers in diverse biological systems. We investigated the effects of phosphatidic acid, a biologically active phospholipid potentially involved in the inflammatory process, on calcium mobilization and actin polymerization in human neutrophils and correlated these effects with induction of chemotactic migration. Intermediate-chain length phosphatidic acid (DiC10-PA) induced a biphasic increase in intracellular Ca2+ characterized by a rapid rise commencing immediately upon addition of stimulus followed by a secondary increase which, unlike the initial response, was eliminated by chelation of extracellular Ca2+. Neither of these responses were induced by C10-lysophosphatidic acid or diacylglycerol. The tyrosine kinase inhibitor herbimycin-A (5-10 microg/ml) completely blunted the initial but not the delayed response effected by DiC10-PA. Long-chain phosphatidic acid (DiC18:1) induced only an initial rapid increase in intracellular Ca2+ and this response was similarly markedly attenuated by herbimycin-A. Among several physiologically relevant phospholipids, only phosphatidic acid was able to induce Ca2+ mobilization; phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol -- used individually or in mixed liposomes -- were without effect. Phosphatidic acid conferred calcium-mobilizing activity upon inactive liposome preparations and phosphatidic acid-enriched cellular plasma membranes possessed similar calcium-mobilizing activity. Both DiC10-PA and DiC18:1-PA induced actin polymerization in neutrophils at rates which mirrored the influence of each agent on Ca2+ mobilization. Herbimycin-A blunted the initial increase in actin polymerization effected by phosphatidic acid but had no effect on the delayed, EGTA-sensitive phase. DiC10-PA and DiC18:1-PA also induced neutrophil migration along a concentration gradient. Phospholipids that failed to induce a calcium transient, including phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol, likewise failed to induce either actin polymerization or chemotactic migration. Unlike chemotaxis induced by zymosan-activated human serum, phosphatidate-induced chemotaxis was strongly inhibited by pretreatment of cells with herbimycin-A. Consistent with these observations, phosphatidic acid induced the tyrosine phosphorylation of several proteins as early as 10 s after stimulation. Phosphorylation of two distinct proteins with approximate molecular sizes of 72 and 82 kDa was inhibited by levels of herbimycin A used to effectively inhibit calcium mobilization, actin polymerization and chemotaxis. Thus, in neutrophilic leukocytes, extracellular phosphatidic acid induces a unique tyrosine kinase-based signalling pathway that results in calcium mobilization and actin polymerization. These processes may promote directed cellular migration as a consequence of the interaction of phosphatidic acid with neutrophil plasma membranes.

Comparison between the loss of platelet membrane asymmetry, microvesiculation and the tyrosine phosphorylation of proteins

Platelet activation by agents such as the Ca2+-ionophore A23187 or Ca2+-ATPase inhibitors leads to the generation of a procoagulant surface and the formation of microparticles. These responses are late events of platelet activation and readily detected by flow cytometry using annexin V-FITC as an aminophospholipid probe. One Ca2+-ATPase inhibitor, 2,5-di-(tertbutyl)-1,4-benzohydroquinone induced aminophospholipid exposure without microparticle formation. Previous work has shown that microparticle formation is strictly linked to the activation of calpain, a thiol-protease that modifies the platelet cytoskeleton and some signal transduction enzymes. We now report how the detection of platelet tyrosine phosphorylation by western-blotting clearly shows that when platelet activation and aminophospholipid exposure are accompanied by microparticle formation there is a decrease in the tyrosine phosphorylation of proteins.

Insulin and insulin-like growth factor-1 regulate tau phosphorylation in cultured human neurons

Hyperphosphorylated tau is the major component of paired helical filaments in neurofibrillary lesions associated with Alzheimer's disease. Hyperphosphorylation reduces the affinity of tau for microtubules and is thought to be a critical event in the pathogenesis of this disease. Recently, glycogen-synthase kinase-3 has been shown to phosphorylate tau in vitro and in non-neuronal cells transfected with tau. The activity of glycogen-synthase kinase-3 can be down-regulated in response to insulin or insulin-like growth factor-1 through the activation of the phosphatidylinositol 3-kinase pathway. We therefore hypothesize that insulin or insulin-like growth factor-1 may affect tau phosphorylation through the inhibition of glycogen-synthase kinase-3 in neurons. Using cultured human neuronal NT2N cells, we demonstrate that glycogen-synthase kinase-3 phosphorylates tau and reduces its affinity for microtubules and that insulin and insulin-like growth factor-1 stimulation reduces tau phosphorylation and promotes tau binding to microtubules. We further demonstrate that these effects of insulin and insulin-like growth factor-1 are mediated through the inhibition of glycogen-synthase kinase-3 via the phosphatidylinositol 3-kinase/protein kinase B signaling pathway.

A characterization of permolybdate and its effect on cellular tyrosine phosphorylation, gap junctional intercellular communication and phosphorylation status of the gap junction protein, connexin43

Biological and analytical characterizations of permolybdate (a mixture of H2O2 and molybdate) were done. Molybdate (10 mM) and molybdenum(V) chloride (3 mM) did not affect gap junctional intercellular communication (GJIC), phosphorylation status of connexin43 (Cx43) or cellular tyrosine phosphorylation in early passage hamster embryonic cells (mainly fibroblast-like). High concentrations of H2O2 (3-10 mM) affected some of the parameters. Acidified permolybdate was clearly more stable than the unadjusted permolybdate. The maximum biological potency of acidified permolybdate was found at a molar ratio of 2:1 (H2O2:molybdate). The mixtures of molybdenum(V) chloride and H2O2 gave a maximum effect at 4:1 molar ratio (H2O2:molybdenum(V)). This can be explained by decomposition of H2O2 and by the generation of less biologically active compounds. Spectrophotometric analyses of the mixtures corroborated the biological results. The Mo(V) electron spin resonance spectrum disappeared upon addition of H2O2 to Mo(V) solutions, and no spectrum appeared when H2O2 was mixed with Mo(VI). Thus, permolybdate is probably diperoxomolybdate, a Mo(VI) compound. Regardless of the parent metal salt, the H2O2/metal salt mixtures showed concentration-dependent biphasic responses with an initial decrease in GJIC followed by an increase. A dissociation between alteration in Cx43 phosphorylation status and GJIC was obtained under certain conditions. The biological activities of permolybdate were only partially mimicked by phenylarsine oxide, an alternative protein tyrosine phosphatase inhibitor.

Differential pre- and postsynaptic modulation of chemical transmission in the squid giant synapse by tyrosine phosphorylation

To assess the role of tyrosine phosphorylation/dephosphorylation balance in synaptic transmission, a set of studies was implemented at the squid giant synapse. Presynaptic induction of tyrosine phosphorylation, following administration of the tyrosine phosphatase inhibitor pervanadate, produced a sizable increase in presynaptic calcium current and a concomitant and paradoxical decrement of the postsynaptic potential amplitude. Presynaptic microinjection of an active protein tyrosine kinase dramatically increased calcium currents and incremented postsynaptic potential amplitude. By contrast, the same procedure at the postsynaptic terminal reduced the size of the postsynaptic potential. This differential effect may be prodromic to long-term plasticity, as postsynaptic sensitivity is momentarily deemphasized, whereas presynaptic second messenger cascades triggered by increased calcium currents are accentuated.

Thymocyte activation induces the association of phosphatidylinositol 3-kinase and pp120 with CD5

CD5 is a glycoprotein expressed on thymocytes, T cells, and a subset of B cells. Antibody-mediated cross-linking studies or studies on CD5 knockout mice implicate CD5 as a co-stimulatory or negative regulatory molecule. CD5 is rapidly phosphorylated on tyrosine (Y) residues following Tcell activation. Y429 and Y441 occur in an imperfect immunoreceptor tyrosine-based activation motif (ITAM)-like sequence. We investigated whether phosphatidylinositol (PI) 3-kinase, which binds to tyrosine-phosphorylated ITAM, interacts with CD5 following T cell activation. PI 3-kinase activity and the regulatory p85 subunit of PI 3-kinase associated with CD5 in pervanadate-stimulated, but not in unstimulated thymocytes. Cellular p85 as well as the recombinant Src homology 2 (SH2) domains of p85 bound a tyrosine-phosphorylated peptide encompassing Y463 with approximately threefold greater affinity than a doubly tyrosine-phosphorylated Y429-Y441 peptide. Binding of the C-SH2 domain to the Y463 phosphopeptide, together with preferential binding of the N-SH2 domain to the Y429-Y441 phosphopeptide, suggests a bivalent interaction. A 120-kDa phosphoprotein (pp120) associated with CD5 and specifically with the Y429-Y441 phosphopeptide in stimulated thymocytes. We conclude that stimulation of thymocytes with pervanadate induces the recruitment of PI 3-kinase and pp120 to CD5.

Inhibition by ajoene of protein tyrosine phosphatase activity in human platelets

The effects of ajoene (a potent antithrombotic agent obtained from garlic) on the tyrosine phosphorylation status of human platelet proteins were investigated by immunoblotting-based experiments using an anti-phosphotyrosine antibody. Incubation of platelets with ajoene enhanced the phosphorylation of at least four proteins (estimated MWs 76, 80, 84 and 120 kDa), both in resting platelets and in platelets subsequently stimulated with thrombin (0.1 U/ml). This effect was both dose- and incubation-time-dependent. High concentrations of ajoene (50 microM) or long periods of incubation (10 min) led to nonselective 'hyperphosphorylation' of numerous proteins. The effects of ajoene on protein tyrosine phosphatase (PTP) activity in platelet lysates were also investigated, PTP activity was inhibited when platelets were incubated with ajoene before lysis, but not when ajoene was added to lysates of platelets which had not been pre-exposed to ajoene.

A role for tyrosine phosphorylation in generation of inositol phosphates and prostacyclin production in endothelial cells

We have examined the effects of the protein tyrosine phosphatase inhibitor pervanadate on activation of signal transduction in human umbilical vein endothelial cells. Endothelial cells responded to pervanadate treatment by increasing tyrosine phosphorylation of cellular proteins, including phospholipase C (PLC) gamma 1, generating inositol phosphates (IPs), releasing arachidonic acid, and producing prostacyclin (prostaglandin [PG] I2). The dose and time responses for these events were similar. Tyrosine phosphorylation and formation of IPs in response to pervanadate were reduced by both staurosporine and genistein. Short-term incubation with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate, which inhibits thrombin-induced IP generation, did not affect the IP response to pervanadate. To investigate the possible involvement of tyrosine phosphorylation in thrombin or histamine-induced IP generation and PGI2 production, we examined the effects of costimulation with pervanadate and either thrombin or histamine. These responses proved to be different. While the tyrosine phosphorylation of PLC gamma 1 was enhanced after cotreatment with thrombin and pervanadate compared with pervanadate alone, costimulation with pervanadate and histamine resulted in no more tyrosine phosphorylation of PLC gamma 1 than after pervanadate alone. Similarly, while cotreatment with pervanadate and thrombin caused synergistic increase in IP generation, costimulation with pervanadate and histamine resulted in an additive response. However, PGI2 responses to costimulation of pervanadate with either thrombin or histamine were both synergistic. Furthermore, stimulation with histamine, thrombin, or pervanadate all caused tyrosine phosphorylation of a mitogen-activated protein kinase (ERK1/p44). The results suggest that a tyrosine phosphorylation-dependent mechanism has a role in the phosphoinositide signal transduction pathway of human endothelial cells. Moreover, thrombin- but not histamine-induced generation of IPs appears to be partly caused by tyrosine phosphorylation of PLC gamma 1.