NF-kappaB inhibitors impair platelet activation responses

BACKGROUND

Although platelets are anucleated cells, they express several transcription factors that exert non-genomic functions, including the positive and negative regulation of platelet activation. NF-kappaB is a major transcriptional regulator of genes involved in survival, proliferation and inflammation.

OBJECTIVE

Because platelets play a critical role not only in hemostasis, but also in inflammation and tumor progression, we evaluated the role of NF-kappaB in platelet physiology.

RESULTS

Immunofluorescence, Western blotting and ELISA studies revealed that platelets express IkappaBalpha and NF-kappaB, and that stimulation with thrombin triggers IkappaBalpha phosphorylation and degradation and the binding of platelet NF-kappaB p65 subunit to synthetic oligonucleotides containing the consensus sequence for NF-kappaB. Two specific unrelated inhibitors of NF-kappaB activation, BAY 11-7082 and Ro 106-9920, reduced PAC-1 and fibrinogen binding to integrin alpha(IIb)beta3 and restricted platelet spreading on immobilized fibrinogen. Both inhibitors impaired aggregation mediated by ADP, epinephrine, collagen or thrombin, but not arachidonic acid. ATP release, TXB2 formation, P-selectin expression, ERK phosphorylation and cPLA2 activity stimulated by thrombin were reduced in BAY 11-7082- or Ro 106-9920-treated platelets. Although bleeding time was not affected, ADP-induced platelet aggregation was impaired in mice treated with BAY 11-7082.

CONCLUSIONS

Our results suggest that NF-kappaB may be a novel mediator of platelet responses. The blockade of platelet function by NF-kappaB inhibitors might be relevant in those clinical situations where these drugs are being considered for anti-tumor and/or anti-inflammatory therapy.

Factor VIII is a positive regulator of platelet function

FVIII is an important cofactor in the tenase coagulation factor complex, lack of FVIII causes severe bleeding, whereas high FVIII levels seem to be associated with venous and arterial thromboembolism. Resting platelets do not bind FVIII, but activated platelets bind unactivated FVIII if vWF is not present. We investigated a possible influence of platelet bound FVIII on platelet function itself as it is unclear if there is a direct effect of FVIII on platelet function. The influence of FVIII on platelet function was investigated by flow cytometric analysis of P-selectin expression (CD62P) and PAC-1 binding before and after submaximal stimulation with TRAP-6 (5 microM final concentration), by confocal microscopy and by platelet aggregometry. For flow cytometry and confocal microscopy, washed platelets were incubated with human recombinant FVIII for 5 min at 37 degrees C. Analysis of platelet surface area was measured by computerized image analysis. Treatment with FVIII only caused no changes in P-selectin expression or PAC-1 binding, respectively. Stimulation of platelets with TRAP-6 increased the expression of P-selectin (445%) and PAC-1 binding (934%) as expected. These effects were further increased when platelets were stimulated with TRAP-6 and FVIII (P-selectin 499%, difference not significant; PAC-1 1626%, P < 0.05. Values were expressed in%, related to unstimulated, buffer treated platelets). Platelet spreading on fibrinogen was significantly increased when platelets were treated with FVIII and TRAP-6 compared to TRAP-6 alone (368 vs. 307 average pixel/platelet, P<0.05). In addition platelet aggregation was enhanced when platelets were stimulated with FVIII and TRAP-6 compared to TRAP-6 alone. FVIII can act as a positive regulator of platelet function in TRAP-co-stimulated platelets. We hypothesize that FVIII induced increase in platelet activation might contribute to venous and even arterial thrombus formation in patients with high FVIII levels.

The formation of platelet–leukocyte aggregates varies during the menstrual cycle

Platelet-leukocyte aggregates are considered to play a significant role in blood coagulation and inflammatory processes. We hypothesized that hormonal changes during the menstrual cycle affect the formation of heterotypic aggregates and therefore may constitute cycle-dependent variations of the susceptibility for thromboembolic events and inflammatory disease. We therefore measured platelet-leukocyte interaction by the determination of platelet-leukocyte aggregates (PLA), platelet P-Selectin expression, and platelet fibrinogen receptor activation by PAC-1 binding in 20 healthy women during their menstrual cycle by flow cytometry. The number of platelet-granulocyte aggregates (PGA) and platelet-monocyte aggregates (PMA) was higher at ovulation compared to any other time-point of the menstrual cycle (p = 0.005, p = 0.022, respectively). Likewise, P-Selectin expression peaked on day 14 (p = 0.040). The course of PLA formation during the menstrual cycle followed the course of estrogen levels, strongly suggesting direct effects of estrogen on platelet-leukocyte interaction. The susceptibility to form platelet-leukocyte aggregates that are inducible in vitro by a suboptimal concentration of thrombin receptor activating peptide-6 decreased slightly during the transition from day 1 to 14 (p = 0.040). These data indicate that platelet function varies during particular phases of the normal menstrual cycle.

The hop cassette of the PAC1 receptor confers coupling to Ca2+ elevation required for pituitary adenylate cyclase-activating polypeptide-evoked neurosecretion

We have identified the single PAC1 receptor variant responsible for Ca2+ mobilization from intracellular stores and influx through voltage-gated Ca2+ channels in bovine chromaffin cells and the domain of this receptor variant that confers coupling to [Ca2+]i elevation. This receptor (bPAC1hop) contains a 28-amino acid "hop" insertion in the third intracellular loop, with a full-length 171-amino acid N terminus. Expression of the bPAC1hop receptor in NG108-15 cells, which lack endogenous PAC1 receptors, reconstituted high affinity PACAP binding and PACAP-dependent elevation of both cAMP and intracellular Ca2+ concentrations ([Ca2+]i). Removal of the hop domain and expression of this receptor (bPAC1null) in NG108-15 cells reconstituted high affinity PACAP binding and PACAP-dependent cAMP generation but without a corresponding [Ca2+]i elevation. PC12-G cells express sufficient levels of PAC1 receptors to provide PACAP-saturable coupling to adenylate cyclase and to drive PACAP-dependent differentiation but do not express PAC1 receptors at levels found in postmitotic neuronal and endocrine cells and do not support PACAP-mediated neurosecretion. Expression of bPAC1hop, but not bPAC1(null), at levels comparable with those of bPAC1hop in bovine chromaffin cells resulted in acquisition by PC12-G cells of PACAP-dependent [Ca2+]i increase and extracellular Ca2+ influx. In addition, PC12-G cells expressing bPAC1hop acquired the ability to release [3H]norepinephrine in a Ca2+ influx-dependent manner in response to PACAP. Expression of PACAP receptors in neuroendocrine rather than nonneuroendocrine cells reveals key differences between PAC1hop and PAC1null coupling, indicating an important and previously unrecognized role of the hop cassette in PAC1-mediated Ca2+ signaling in neuroendocrine cells.

Fractionated and acute irradiation induced signaling in a murine tumor

The effect of fractionated doses of Co(60) gamma-irradiation (2 Gy per fraction over 5 days), as is delivered in cancer radiotherapy, was compared with acute doses of 10 and 2 Gy, in a serially transplanted mouse fibrosarcoma grown in Swiss mice. The aspects that were studied included the three major mitogen-activated protein (MAP) kinases, namely p44 MAP kinase, p38 MAP kinase, and stress-activated protein (SAP) kinase, which are known to be involved in determining the cell fate following exposure to ionizing radiation. The response of dual specificity phosphatase PAC1 which is involved in the dephosphorylation of MAP kinases was also looked at. There were significant differences in the response to different dose regimens for all the factors studied. Fractionated irradiation elicited an adaptive response with a sustained activation over 7 days of prosurvival p44 MAP kinase which was balanced by the increased activation of proapoptotic p54 SAP kinase up to 1 day post-irradiation, whereas, phosphorylated p38 MAP kinase showed a decrease at most time points. PAC1 was induced following fractionated irradiation and may be acting as a feed back regulator of p44 MAP kinase. The activation of SAP kinase after fractionated irradiation may be a stress response, whereas, constitutively activated p44 MAP kinase may play an important role in the induction of radioresistance during fractionated radiotherapy of cancer and may serve as a promising target for specific inhibitors to enhance the efficacy of radiotherapy.

Flow cytometric evaluation of platelet activation in chronic autoimmune thrombocytopenia

Platelets from 42 patients (platelet counts median 42x10(9)/L, range 3-223x10(9)/L) with chronic idiopathic autoimmune thrombocytopenia (cAITP) were investigated for P-Selectin expression and PAC-1 binding. The results showed that the levels of P-Selectin positive platelets (n=20) were higher in cAITP than in controls (P<0.0001), and correlated with platelet counts (P=0.04). PAC-1 binding was increased in only six patients, and not correlated with platelet counts. There was no correlation of P-Selectin or PAC-1 with detectable platelet antibodies. Thus, platelets are activated in cAITP, but platelets, characterized by PAC-1 binding, are rare. These are either needed to maintain vascular integrity, or underwent premature sequestration.

Role for pituitary adenylate cyclase activating polypeptide in cystitis-induced plasticity of micturition reflexes

Pituitary adenylate cyclase activating polypeptide (PACAP) peptides are expressed and regulated in sensory afferents of the micturition pathway. Although these studies have implicated PACAP in bladder control, the physiological significance of these observations has not been firmly established. To clarify these issues, the roles of PACAP and PACAP signaling in micturition and cystitis were examined in receptor characterization and physiological assays. PACAP receptors were identified in various tissues of the micturition pathway, including bladder detrusor smooth muscle and urothelium. Bladder smooth muscle expressed heterogeneously PAC(1)null, PAC(1)HOP1, and VPAC(2) receptors; the urothelium was more restricted in expressing preferentially the PAC(1) receptor subtype only. Immunocytochemical studies for PAC(1) receptors were consistent with these tissue distributions. Furthermore, the addition of 50-100 nM PACAP27 or PACAP38 to isolated bladder strips elicited transient contractions and sustained increases in the amplitude of spontaneous phasic contractions. Treatment of the bladder strips with tetrodotoxin (1 muM) did not alter the spontaneous phasic contractions suggesting direct PACAP effects on bladder smooth muscle. PACAP also increased the amplitude of nerve-evoked contractions. By contrast, vasoactive intestinal polypeptide had no direct effects on bladder smooth muscle. In a rat cyclophosphamide (CYP)-induced cystitis paradigm, intrathecal or intravesical administration of PAC(1) receptor antagonist, PACAP6-38, reduced cystitis-induced bladder overactivity. In summary, these studies support roles for PACAP in micturition and suggest that inflammation-induced plasticity in PACAP expression in peripheral and central micturition pathways contribute to bladder dysfunction with cystitis.

Positive regulation of immune cell function and inflammatory responses by phosphatase PAC-1

Mitogen-activated protein kinases facilitate many cellular processes and are essential for immune cell function. Their activity is controlled by kinases and dual-specificity phosphatases. A comprehensive microarray analysis of human leukocytes identified DUSP2 (encoding the phosphatase PAC-1) as one of the most highly induced transcripts in activated immune cells. We generated Dusp2(-/-) mice and found considerably reduced inflammatory responses in the 'K/BxN' model of rheumatoid arthritis. PAC-1 deficiency led to increased activity of Jun kinase (Jnk) but unexpected impairment of the activity of extracellular signal-regulated kinase (Erk) and the kinase p38, reduced activity of the transcription factor Elk1 and a complex of mobilized transcription factor NFAT and the AP-1 transcription factor and decreased effector immune cell function. Thus, PAC-1 is a key positive regulator of inflammatory cell signaling and effector functions, mediated through Jnk and Erk mitogen-activated protein kinase crosstalk.

Significant relationship between platelet activation and intra-media thickness of the carotid artery in patients with ischemic cerebrovascular disease

INTRODUCTION

We have studied the relationship between the ratio of activated platelets and the thickness of intima and media of the carotid artery in ischemic CVD patients in the chronic stage.

METHODS

Platelet activation was assessed by means of flow cytometry of whole blood using activation-dependent monoclonal antibodies (MoAb). Forty-one MRI-proven normative subjects and 55 patients with a history of ischemic CVD were examined. The intima-media thickness of the carotid artery was measured by using B-mode ultrasound in all subjects.

RESULTS

The appearance rates of PAC-1-positive and CD62P-positive platelets (%) were increased in ischemic CVD patients compared with those in controls (p<0.0001, p<0.001, respectively) The patients and controls were divided into those with atherosclerosis (Ath+), defined as intima-media thickness 1.1 mm, and those without (Ath-). There was no significant difference of PAC-1-positive platelets between the Ath- and Ath+ subgroups in either group, but there was increase in Ath- ischemic CVD patients versus Ath- control subjects (p<0.01), and in Ath+ patients versus Ath+ controls (p<0.05). CD62-positive platelets in the Ath+ subgroup were significantly increased versus the Ath- subgroup in both the controls (p<0.001) and ischemic CVD patients (p<0.05), and there was also an increase in Ath- patients versus Ath- controls (p<0.05).

CONCLUSION

Platelet activation markers were increased in patients with ischemic CVD compared with controls. A significant relationship was found between increased CD62-P-positive platelets and carotid artery abnormalities in both controls and ischemic CVD patients, suggesting that platelet activation may be a potential marker for atherosclerosis.

Effect of menopause on platelet activation markers determined by flow cytometry

Pre-menopausal women have a lower risk of cardiovascular disease compared to post-menopausal women. Cardiovascular disease is more age dependent in women than in men. The association of platelet activation and cardiovascular thrombotic events is well established. Standardized techniques were used to evaluate platelet activation markers by flow cytometry, using 3-color analysis (CD 61PerCP, CD 62P, and PAC-1) in 49 post-menopausal (mean +/- SD age, 56.16 +/- 33.51 years) and 42 pre-menopausal (age, 39.38 +/- 7.07 years) women. Results of our study showed a significant increase in CD 62P in post-menopausal women as compared to the pre-menopausal group (2.66 +/- 4.26% vs. 0.52 +/- 2.71%, P < 0.001). Similarly, PAC-1 was significantly increased in post-menopausal women (21.54 +/- 2.48% vs. 3.70 +/- 2.31%, P < 0.001). Furthermore, there was a significant association of CD 62P with serum estradiol in both groups. PAC-1 was significantly associated with age in both groups. The results suggest the role of platelets in the increased incidence of thrombotic events and disease in post-menopausal women.

New Insights into the Catalytic Activation of the MAPK Phosphatase PAC-1 Induced by its Substrate MAPK ERK2 Binding

PAC-1 is an inducible, nuclear-specific, dual-specificity mitogen-activated protein (MAP) kinase phosphatase that has been shown recently to be a transcription target of the human tumor-suppressor protein p53 in signaling apoptosis and growth suppression. However, its substrate specificity and regulation of catalytic activity thus far remain elusive. Here, we report in vitro characterization of PAC-1 phosphatase activity with three distinct MAP kinase subfamilies. We show that the recombinant PAC-1 exists in a virtually inactive state when alone in vitro, and dephosphorylates extracellular signal-regulated kinase 2 (ERK2) but not p38alpha or c-Jun NH(2)-terminal kinase 2 (JNK2). ERK2 dephosphorylation by PAC-1 requires association of its amino-terminal domain with ERK2 that results in catalytic activation of the phosphatase. p38alpha also interacts with but does not activate PAC-1, whereas JNK2 does not bind to or cause catalytic activation by PAC-1. Moreover, our structure-based analysis reveals that individual mutation of the conserved Arg294 and Arg295 that likely comprise the phosphothreonine-binding pocket in PAC-1 to either alanine or lysine results in a nearly complete loss of its phosphatase activity even in the presence of ERK2. These results suggest that Arg294 and Arg295 play an important role in PAC-1 catalytic activation induced by ERK2 binding.

Neonatal platelets from cord blood and peripheral blood

The haemostatic system in neonates is different from that of adults, possibly contributing to an increased incidence of bleeding disorders, such as intracranial hemorrhage. In this study, we analyzed platelets from cord blood and peripheral blood, collected at three time points after delivery from 20 term and 37 preterm neonates as well as blood from 20 healthy adults. Platelet membrane glycoproteins (GP) were quantified and P-selectin expression and PAC-1 binding ability before and after stimulation with TRAP were analyzed by whole blood flow cytometry. We found no significant differences in neonatal platelets from cord blood and peripheral blood within the first 24h of life. Platelets from infants less than 30 weeks of gestation expressed lower levels of GP (33271+/-9381 vs. 44085+/-17287 for GPIIIa, P<0.05) and were less reactive than platelets from term newborns (4.3+/-3.3 vs. 20.1+/-11.8% PAC-1 positive platelets after stimulation with TRAP, P<0.05). A significantly lower level of GPIIb/IIIa expression on platelets from peripheral blood was seen in term newborns as well as preterm infants, compared to adults. There was only a partial enhancement in the degranulation ability (alpha-granules) (13.4+/-12.3 vs. 50.3+/-16.1% P-selectin positive platelets, P<0.05) and no significant increase for PAC-1 binding (13.6+/-10.9 vs. 15.3+/-5.9% PAC-1 positive platelets, P=0.8) during the first 12 days of life. In conclusion, we could demonstrate that neonatal platelet reactivity increases with gestational age.

Highly constrained cyclic (S,S) -CXaaC- peptides as inhibitors of fibrinogen binding to platelets

The Arg-Gly-Asp RGD motif of adhesive proteins is recognized by the activated platelet integrin alpha(IIb)beta3. Binding of fibrinogen (Fg) to activated alpha(IIb)beta3 causes platelet aggregation and thrombus formation. Highly constraint cyclic (S,S) -CXaaC- containing peptides incorporating the (S,S) -CDC- and (S,S) -CRC- motifs were tested for their ability to inhibit platelet aggregation and Fg binding. Our results suggest that the above cyclic scaffolds stabilize a favorable structure for the antiaggregatory activity (IC50-values ranged from 1.7 to 570 microm). The peptides inhibited Fg binding with IC50-values up to 30-fold lower than those determined for the inhibition of the adenosine diphosphate (ADP)-induced platelet aggregation. Importantly, peptides (S,S) PSRCDCR-NH2 (peptide 11) and (S,S) PRCDCK-NH2 (peptide 10) did not inhibit PAC-1 binding to the activated platelets at a concentration in which they completely inhibited Fg binding. Moreover, (S,S) PSRCDCR-NH(2) (peptide 11), one of the more active peptides, inhibited ADP-induced P-selectin exposure. By contrast, peptide (S,S) Ac-RWDCRC-NH2, incorporating the inverse (S,S) -DCRC- sequence (peptide 16), failed to inhibit P-selectin exposure whereas at the same concentration, it effectively inhibited PAC-1 and Fg binding. It is concluded that peptides containing the (S,S) -CDC- as well the (S,S) -CRC- sequences, exhibit a broad range of activities toward platelets, and could be helpful tools for elucidating the structural interaction of Fg with the integrin receptor alpha(IIb)beta3, in its activated form. Furthermore, the (S,S) -RCDC- sequence can be used as a scaffold for developing potent non-RGD-like Fg-binding inhibitors.

Differential roles of integrins alpha2beta1 and alphaIIbbeta3 in collagen and CRP-induced platelet activation

Collagen and collagen-related peptide (CRP) activate platelets by interacting with glycoprotein (GP)VI. In addition, collagen binds to integrin alpha2beta1 and possibly to other receptors. In this study, we have compared the role of integrins alpha2beta1 and alphaIIbbeta3 in platelet activation induced by collagen and CRP. Inhibitors of ADP and thromboxane A2 (TxA2) substantially attenuated collagen-induced platelet aggregation and dense granule release, whereas CRP-induced responses were only partially inhibited. Under these conditions, a proportion of platelets adhered to the collagen fibres resulting in dense granule release and alphaIIbbeta3 activation. This adhesion was substantially mediated by alpha2beta1. The alphaIIbbeta3 antagonist lotrafiban potentiated CRP-induced dense granule release, suggesting that alphaIIbbeta3 outside-in signalling may attenuate GPVI signals. By contrast, lotrafiban inhibited collagen-induced dense granule release. These results emphasise the differential roles of alpha2beta1 and alphaIIbbeta3 in platelet activation induced by collagen and CRP. Further, they show that although ADP and TxA2 greatly facilitate collagen-induced platelet activation, collagen can induce full activation of those platelets to which it binds in the absence of these mediators, via a mechanism that is dependent on adhesion to alpha2beta1.

Effects of integrin alpha IIb(R995A) mutation on receptor affinity and pp125 (FAK) phosphorylation

OBJECTIVE

To investigate the role of cytoplasmic domain of integrin alpha IIb in platelet signal transduction.

METHODS

Binding capacity of integrin alpha IIb(R995A) to antibody platelet activation complex-1 (PAC-1) and pp125 focal adhesion kinase (FAK) phosphorylation of cells were detected by flow cytometry, immune precipitation, and Western blotting.

RESULTS

Without activation, wild-type alpha IIb beta3 Chinese hamster ovary (CHO) cells failed to bind to PAC-1, but mutant chimera alpha IIb(R995A)beta3 CHO cells were able to bind with PAC-1. Furthermore, phosphorylation of pp125 (FAK) in wild-type alpha IIb beta3 CHO cells occured only when cells were adhered to fibrinogen, but could not be detected in bovine serum albumin suspension. However in the mutant chimera group, it could be detected in both conditions.

CONCLUSION

The mutation in integrin alpha IIb(R995A) alters its affinity state as a receptor, thus also mediating cytoplasmic signal transduction leading to the phosphorylation of pp125 (FAK) without ligand binding.

FcgammaRIIb-mediated negative regulation of BCR signalling is associated with the recruitment of the MAPkinase-phosphatase, Pac-1, and the 3'-inositol phosphatase, PTEN

The low-affinity receptor for IgG, FcgammaRIIb, negatively regulates B cell antigen receptor (BCR)-mediated proliferative signalling. FcgammaRIIb has been reported to mediate this inhibition by uncoupling the BCR from the RasMAPkinase pathway. We now show that FcgammaRIIb-mediated negative feedback inhibition also correlates with induction of an Erk-associated phosphatase activity that reflects the rapid association of Erk and the MAPkinase phosphatase, Pac-1, and dephosphorylation and inactivation of ErkMAPkinase. This mechanism of abrogating ongoing ErkMAPkinase signalling therefore provides a rationale for rapid immune-complex-mediated feedback inhibition of active antigen-driven B cell responses. In addition, FcgammaRIIb signalling also induces the recruitment and activation of the 3'-inositol phosphatase, PTEN, which by antagonising PI 3kinase activity and inhibiting BCR-coupling to the anti-apoptotic kinase, Akt, provides an additional mechanism for FcgammaRIIb-mediated negative regulation of BCR-coupling to ErkMAPkinase, cell survival and proliferation.

The Effects of High Molecular Weight Hydroxyethyl Starch Solutions on Platelets

Physicochemical characteristics of hydroxyethyl starch (HES) molecules determine their side effects on hemostasis. Our aim in the present experiments was to test the antiplatelet effect of novel high molecular weight HES. Citrated whole blood was hemodiluted in vitro (0% and 20%) with either HES 550 (Hextend), HES 600 (6%Hetastarch-Baxter), HES 200 (Elohäst), or the solvent of Hextend in its commercially available solution. The availability of glycoprotein IIb-IIIa was assessed on nonstimulated and on agonist-induced platelets using flow cytometry. Glycoprotein IIb-IIIa availability increased significantly after hemodilution with Hextend and its solvent by 23% and 24%, respectively, but decreased in the presence of 6% Hetastarch-Baxter and Elohäst by 18% and 15%, respectively, with no significant difference between the latter two colloids. This study shows that Hextend does not inhibit platelet function as anticipated by its high molecular weight and degree of substitution. The unexpected platelet stimulating effect of Hextend is unique among the currently available HES preparations and may, at least in part, be induced by its solvent containing calcium chloride dihydrate (2.5 mmol/L). The platelet-inhibiting effect of 6%Hetastarch-Baxter was not significantly different from that of medium molecular weight HES 200.

Laminin-induced signaling in tumor cells: the role of the M(r) 67,000 laminin receptor

The expression of the M(r) 67,000 laminin receptor, a nonintegrin laminin receptor, was found to be up-regulated in neoplastic cells and to directly correlate with invasion and metastatic potential. In the present study, we investigated the role of laminin receptor in mediating laminin effects and the involvement of the mitogen-activated protein kinases (MAPK) cascades and dual-specificity phosphatases in laminin signaling in human melanoma cells. Using stable transfection of A375SM melanoma cells, we established lines expressing reduced or elevated laminin receptor. The antisense-transfected cells demonstrated reduced attachment to laminin and reduced invasion through Matrigel-coated filters. In addition, both matrix metalloproteinase-2 (MMP-2) mRNA expression and activity were significantly reduced in the antisense-transfected cells. Antisense-transfected cells showed a reduction in mRNA level of the alpha6B integrin subunit isoform, whereas no change in the mRNA level of the alpha6A isoform was observed. We found that exogenous laminin reduced the phosphorylated (active) form of extracellular signal-regulated kinase, c-Jun NH(2)-terminal protein kinase, and p38 in all of the cells, irrespective of the expression of the laminin receptor. Furthermore, the phosphorylation of extracellular signal-regulated kinase, c-Jun NH(2)-terminal protein kinase, and p38 was significantly higher in the cell lines expressing reduced laminin receptor, regardless of the exposure to exogenous laminin. This increase of MAPK phosphorylation was accompanied by a significant reduction in MKP-1 phosphatase mRNA level and a significant increase in PAC-1 phosphatase mRNA level. In conclusion, our results confirm the involvement of the laminin receptor in different mechanisms related to tumor dissemination and provide first evidence of the involvement of MAPK and dual-specificity phosphatases in its signal transduction pathway.

The PAC-1 dual specificity phosphatase predicts poor outcome in serous ovarian carcinoma

OBJECTIVE

Data regarding signal transduction pathways in human tumors are largely confined to cell line studies to date. We have recently reported on the activation and prognostic role of mitogen-activated protein kinases (MAPK) in ovarian carcinoma in effusions. The objective of the present study was to investigate the expression and clinical role of dual-specificity phosphatases (DUSP), inhibitors of MAPK signaling, in ovarian cancer cells at this site.

METHODS

Thirty-nine fresh frozen malignant effusions from patients diagnosed with serous ovarian carcinoma were studied for mRNA expression of the DUSP MKP-1, MKP-4, MKP-5, and PAC-1 using RT-PCR. DUSP expression was analyzed for possible correlation with patient age, disease stage, tumor grade, histological grade, chemotherapy status, and survival.

RESULTS

MKP-1 and PAC-1 mRNA were found in 36 and 37 effusions, respectively, with expression levels showing considerable variation. MKP-4 and MKP-5 were uniformly absent. MKP-1 showed no association with clinicopathologic parameters. However, PAC-1 expression was significantly higher in effusions obtained before the institution of treatment with both platinum compounds (P = 0.029) and paclitaxel (P = 0.036). In univariate survival analysis, high level of expression of PAC-1 mRNA predicted significantly worse overall survival compared to low expression (mean = 30 vs. 52 months, median = 25 vs. 46 months) (P = 0.007).

CONCLUSIONS

Despite the limited size of this cohort, our results present the first evidence supporting a clinical role for PAC-1 in ovarian carcinoma. In view of the improved outcome associated with activation of all three MAPK families, as well as their elevated expression and activation in post-chemotherapy specimens presented in our previous work, they also suggest that PAC-1 is a true negative regulator of MAPK in ovarian carcinoma cells in effusions.

Effects of Valsartan and Valeryl 4-Hydroxy Valsartan on Human Platelets: A Possible Additional Mechanism for Clinical Benefits

Valsartan selectively blocks angiotensin II binding to the AT1 receptor. ince platelet activation plays a key role in the pathogenesis of vascular disease, and because AT1 receptors are present on the platelet surface, we assessed the in vitro effects of valsartan and its metabolite, valeryl 4-hydroxy valsartan (V4HV), on platelets in 30 subjects with multiple risk factors for cardiovascular disease. Platelet characteristics in blood samples pretreated and incubated with 10 nmol to 100 micromol concentrations of valsartan and V4HV were assessed by aggregometry, rapid platelet analyzers, and by flow cytometry. Pretreatment of blood with valsartan and V4HV resulted in inhibition of conventional plasma (ADP, P = 0.0001, valsartan; epinephrine, P = 0.0001, V4HV) and whole blood collagen-induced (P = 0.01, valsartan; P =.0001, V4HV) platelet aggregation. Closure time was delayed (P = 0.02, valsartan; P = 0.03, 4VHV), indicating platelet inhibition in whole blood under high shear conditions. Expression of many surface platelet receptors, namely GP IIb/IIIa antigen, and activity, vitronectin, p-selectin, and LAMP-1 was significantly reduced compared with autologous baseline activity. Intensity of platelet-leukocyte formation and other platelet activation markers remained unchanged. Platelet inhibition was not dose dependent and was more potent for 4VHV than valsartan in the therapeutic range.Valsartan and 4VHV exhibited significant in vitro inhibition of human platelets. Their antiplatelet properties, especially more potent activity of the metabolite appear to be independent of those of other antiplatelet agents. Whether valsartan reduces vascular ischemic events via additional pathways of platelet inhibition in patients with myocardial infarction and ischemic stroke requires further clinical research.

Different levels of platelet activation in preeclamptic, normotensive pregnant, and nonpregnant women

OBJECTIVE

The aims of our study were to determine the basal platelet activation state in women with preeclampsia compared with normotensive pregnant women and nonpregnant women and to investigate the platelet reactivity on in vitro stimulation with adenosine diphosphate or thrombin receptor activation peptide.

STUDY DESIGN

Platelet expression of CD61 (fibrinogen receptor), CD42a (von Willebrand factor receptor), CD62P (P-selectin), CD63 (Glycoprotein 53), and PAC-1 binding (activated fibrinogen receptor) were determined in 20 pairs of women with preeclampsia/normotensive pregnant women and in 12 nonpregnant women, with the use of flow cytometry.

RESULTS

Basal platelet expression of CD61, CD42a and CD62P, and adenosine diphosphate-stimulated CD62P expression were increased in women with preeclampsia compared with normotensive pregnant women. Platelets from women with preeclampsia and normotensive pregnant women differed from platelets from nonpregnant women by expressing higher basal CD63 levels and being more responsive to in vitro agonist stimulation, which was demonstrated by increased expression of CD61, CD62P, and CD63.

CONCLUSION

This study supports the notion that platelets are important in the pathophysiologic condition of preeclampsia.

Effect of synthetic peptides corresponding to residues 313-332 of the alphaIIb subunit on platelet activation and fibrinogen binding to alphaIIbbeta3

The platelet integrin receptor alphaIIbbeta3 plays a critical role in thrombosis and haemostasis by mediating interactions between platelets and several ligands but primarily fibrinogen. It has been shown previously that the YMESRADR KLAEVGRVYLFL (313-332) sequence of the alphaIIb subunit plays an important role in platelet activation, fibrinogen binding and alphaIIbbeta3-mediated outside-in signalling. Furthermore, we recently showed that the 20-residue peptide (20-mer) alphaIIb 313-332, is a potent inhibitor of platelet aggregation and fibrinogen binding to alphaIIbbeta3, interacting with fibrinogen rather than the receptor. In an effort to determine the sequence and the minimum length required for the biological activity of the above 20-mer, we synthesized seven octapeptides, each overlapping by six residues, covering the entire sequence and studied their effect on platelet activation as well as fibrinogen binding to activated platelets. We show for the first time that octapeptides containing the RAD sequence are capable of inhibiting platelet aggregation and secretion as well as fibrinogen binding to the activated alphaIIbbeta3, possibly interacting with the ligand rather than the receptor. This suggests that the RAD sequence, common to all the inhibitory peptides, is critical for their biological activity. However, the presence of the YMES sequence, adjacent to RAD, significantly increases the peptide's biological potency. The development of such inhibitors derived from the 313-332 region of the alphaIIb subunit may be advantageous against the RGD-like antagonists as they could inhibit platelet activation without interacting with alphaIIbbeta3, thus failing to further induce alphaIIbbeta3-mediated outside-in signalling.

VIP/PACAP oppositely affects immature and mature dendritic cell expression of CD80/CD86 and the stimulatory activity for CD4(+) T cells

The neuropeptides vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) released within lymphoid organs from nerve terminals and/or immune cells play a significant, anti-inflammatory role by inhibiting macrophage-induced inflammatory reactions and promoting T helper cell type 2 (Th2) responses. However, dendritic cells (DC) and not macrophages often are the major antigen-presenting cells and link between innate and adaptive immunity. The role of VIP/PACAP in DC development and function is mostly unknown. Here, we report that bone marrow-derived DC express VIP/PACAP receptors and that VIP and PACAP exert a differential effect on immature DC (iDC) and lipopolysaccharide (LPS)-treated DC. In iDC, VIP/PACAP up-regulates CD86 expression and enables them to stimulate T cell proliferation and differentiation into Th2 effectors in vivo and in vitro. In contrast, VIP/PACAP down-regulates CD80/CD86 expression in LPS-stimulated DC and strongly reduces their capacity to stimulate T cell proliferation and secretion of Th1 and Th2 cytokines. The VIP/PACAP effects on iDC and LPS-stimulated DC are mediated primarily through the VIP receptor 1. These results indicate that neuropeptides such as VIP and PACAP can differentially affect the function of iDC and mature DC. In the absence of an ongoing immune response, VIP/PACAP contributes to the initiation of Th2-type immunity, whereas in the presence of a full-blown, inflammatory reaction, VIP/PACAP act as anti-inflammatory agents.

[The relationship between the peripheral blood of CD61, CD63, PAC-1 and the transplant kidney function]

OBJECTIVES

To explore the relationships between the peripheral blood levels of CD61, CD63, PAC-1 and the incidence of acute rejection and tubular necrosis after renal transplantation, and recovery of the graft function.

METHODS

The peripheral blood levels of CD61, CD63, and PAC-1 of 86 patients with uremia in different stages before and after transplantations were analyzed by flow cytometry. The patients were divided into three groups: (1) twenty-nine patients with normal grafts function, (2) hirty with acute rejection and (3) twenty-seven with acute tubular necrosis. The patients with acute rejection were randomly divided into treatment group with anticoagulants and cntrol group.

RESULTS

The peripheral blood levels of CD61, CD63 and PAC-1 significantly increased (P < 0.05) in the patients with acute rejection, in comparison with those with normal grafts function and those with acute tubular necrosis. The peripheral blood levels of CD61, CD63 and PAC-1 in patients with acute rejection in anticoagulants therapy was lower, recovery time of the grafts function was shorter, one-year survival rates of patients and grafts were higher, as compared with those of controls.

CONCLUSIONS

The patients with acute rejection have significantly high peripheral blood levels of CD61, CD63 and PAC-1 before transplantation, however, these values in patients with acute tubular necrosis are not high, this suggesting that acute rejection might relate to platelet activation, while acute tubular necrosis might not relate to it. After anticoagulants therapy in patients with acute rejection, the grafts function might recover faster and their one-year survival rates and grafts might be higher in those with CD61, CD63 and PAC-1 decreasing remarkably.

Characterization of a variant of PAC-1 in large granular lymphocyte leukemia

Phosphatase in activated T cells (PAC-1) is a mitogen-induced early responsive gene. It encodes a 32 kDa tyrosine-threonine dual specificity phosphatase. Constitutive expression of PAC-1 leads to an inhibition of MAP kinase activity in vivo. Such constitutive expression was reported in HTLV-1 infected cell lines. In the present study, we observed the constitutive over-expression of two transcripts related to PAC-1 in large granular lymphocyte (LGL) leukemia. By screening a LGL leukemia cDNA library using the 3' end of a PAC-1 probe, we obtained a clone (clone 8) which retains one and one half introns, excludes two exons, and matches one hundred percent with a DNA sequence on chromosome 2. The deduced amino acid sequence of the predicted protein contains 170 amino acids and is 144 amino acids shorter than PAC-1. When we expressed this protein in Escherichia coli as a GST-fusion protein, a 45 kDa (19 kDa PAC-1 variant+26 kDa GST protein) protein was obtained. The expressed protein was purified to near homogeneity by using a glutathione affinity column. The purified protein did not have any intrinsic phosphatase activity when assayed in vitro. But when this purified protein was added to a phosphatase assay system in combination with a recombinant dual specificity phosphatase, CL100, enhanced phosphatase activity was observed. The significance of the constitutive over-expression and its physiological role of this protein remain to be established in leukemic LGL.

Platelet activation after stenting with heparin-coated versus noncoated stents

BACKGROUND

In animal models, heparin coating reduces platelet accumulation induced by coronary stenting. However, reduced platelet activation has never been demonstrated in humans. The purpose of the current study was to investigate the effect of heparin coating on platelet activation after coronary artery stenting.

METHODS

In a prospective, randomized, pilot study of 50 consecutive elective patients, platelet activation was analyzed by measuring aggregation and surface receptor expression before and at 2 hours, 24 hours, 5 days and 30 days after implantation of either heparin-coated or noncoated stents.

RESULTS

There was less platelet activation after implantation of a heparin-coated stent, as indicated by reduced expression at 24 hours of active glycoprotein (GP) IIb/IIIa (10.3 +/- 6.1 vs 6.7 +/- 2.1, P =.014), and total GP IIb/IIIa (382 +/- 101 vs 306 +/- 88, P =.01). A trend at 30 days poststenting of lower total (383 +/- 150 vs 296 +/- 86, P =.07) and active GP IIb/IIIa expression (10.3 +/- 6.9 vs 7.5 +/- 2.9, P =.15) was also observed with the heparin-coated stent). Aggregation and stimulated p-selectin did not differ between stent types.

CONCLUSION

Use of a heparin-coated stent in this pilot study of elective patients was associated with primarily less early platelet expression of GP IIb/IIIa. These findings have implications on the risk of subacute thrombosis and deserve further investigation.

Mapping the binding domains of the alpha(IIb) subunit. A study performed on the activated form of the platelet integrin alpha(IIb)beta(3)

alpha(IIb)beta(3), a member of the integrin family of adhesive protein receptors, is the most abundant glycoprotein on platelet plasma-membranes and binds to adhesive proteins via the recognition of short amino acid sequences, for example the ubiquitous RGD motif. However, elucidation of the ligand-binding domains of the receptor remains controversial, mainly owing to the fact that integrins are conformationally labile during purification and storage. In this study, a detailed mapping of the extracellular region of the alpha(IIb) subunit is presented, using overlapping 20-peptides, in order to identify the binding sites of alpha(IIb) potentially involved in the platelet-aggregation event. Regions alpha(IIb) 313-332, alpha(IIb) 265-284 and alpha(IIb) 57-64 of alpha(IIb)beta(3) were identified as putative fibrinogen-binding domains because the corresponding peptides inhibited platelet aggregation and antagonized fibrinogen association, possibly by interacting with this ligand. The latter is further supported by the finding that the above peptides did not interfere with the binding of PAC-1 to the activated form of alpha(IIb)beta(3). Furthermore, alpha(IIb) 313-332 was found to bind to fibrinogen in a solid-phase binding assay. It should be emphasized that all the experiments in this study were carried out on activated platelets and consequently on the activated form of this integrin receptor. We hypothesize that RAD and RAE adhesive motifs, encompassed in alpha(IIb) 313-332, 265-284 and 57-64, are capable of recognizing complementary domains of fibrinogen, thus inhibiting the binding of this ligand to platelets.

Pharmacodynamic characterization of the interaction between the glycoprotein IIb/IIIa inhibitor YM337 and unfractionated heparin and aspirin in humans

AIMS

To investigate the pharmacodynamic interaction of unfractionated heparin (UFH) and acetylic salicylic acid (ASA) on YM337, a monoclonal humanized antibody of the platelet GPIIb/IIIa receptor.

METHODS

In a randomized, placebo-controlled study three treatment groups each with six healthy volunteers received the following medication: group 1, ASA (3 days) + UFH + YM337 (placebo); group 2, ASA (placebo) + UFH (placebo) + YM337; group 3, ASA + UFH + YM337. Assessments were made over 24 h and included bleeding time (BT), ADP (20 microm)- and collagen (5 microg ml-1)-induced platelet aggregation and PAC1 and CD62 expression measured by flow cytometry.

RESULTS

In group 3 BT was prolonged to 35 [median, 16-45 min (1,3 quartile)] after UFH administration, increasing to 45 [median, 42-45 min (1,3 quartile)] after YM infusion (6 h). BT remained elevated to 26 [median, 14-45 min (1,3 quartile)] at 24 h, while groups 1 and 2 returned to normal values. Collagen-induced aggregation was 73% [median, 70-80% (1,3 quartile)] under YM337 alone, 79% [median, 72-80% (1,3 quartile)] under ASA + UFH and reduced only in group 3 to 24% [median, 18-29% (1,3 quartile)]. In both groups receiving active YM337, PAC1 expression showed a reduction to < 20% after 6 h of infusion. CD62 expression was not significantly affected by any treatment.

CONCLUSION

UFH and YM337 have strong synergistic effects on BT, while coadministration of ASA strongly augments inhibitory effects of YM337 on collagen-induced platelet aggregation.

Paraformaldehyde fixation induces a systematic activation of platelets

In whole blood flow cytometric platelet assays sample fixation using paraformaldehyde (PFA) is considered very advantageous to prevent spontaneous activation of platelets in vitro. However, fixation is an important variable in activation assays and its influence on platelets is poorly understood. Using a direct immunofluorescence labelling technique and whole blood flow cytometry, the effect of PFA fixation was investigated for 4 different epitopes on platelet surface each of which mirrors a different aspect of platelet activation, namely P-selectin (CD62P), GP IIbIIa complex (CD41), the fibrinogen binding site of the activated GP IIaIIIb complex (PAC-1) and GP Ib-V-IX complex (CD42b). Platelets fixed with PFA (0.5%) before antibody labelling showed significant (P<0.01) increases in mean fluorescence intensity (MFI) of CD62P (1.10 +/- 0.14 vs. 0.94 +/- 0.12 arbitrary units of fluorescence), CD41 (27.3 +/- 6.3 vs. 15.6 +/- 2.1) and PAC-1 (6.21 +/- 1.25 vs. 0.55 +/- 0.12) when compared to unfixed samples. At the same time, MFI of CD42b was reduced from 28.2 +/- 1.6 to 22.6 +/- 2.3 (P<0.01). When fixation was initiated after antibody labelling, we observed less prominent increases in MFI of CD41 (P<0.05) and PAC-1 (P<0.05) while there was no significant difference for CD62P and rather a moderate rise in CD42b than a decrease (P<0.05). Because these alterations cannot be explained by unspecific effects only, it must be concluded that PFA induces a systematic stimulation of platelets. The lowest in vitro platelet activation was found when antibody labelling was started immediately after blood sampling and when samples were analysed within 10 minutes after being stored without fixation of 4 degrees C in the dark.

Pharmacologic platelet anesthesia by glycoprotein IIb/IIIa complex antagonist and argatroban during in vitro extracorporeal circulation

OBJECTIVE

Contact between blood and the synthetic surfaces of a cardiopulmonary bypass circuit leads to platelet activation, and resultant platelet dysfunction contributes to postoperative bleeding. We compared the effects of various platelet inhibitors on preservation of platelet function during simulated cardiopulmonary bypass circulation.

METHODS

Fresh human blood was recirculated in an in vitro cardiopulmonary bypass model circuit. We measured various platelet activation markers including expressions of PAC-1 and P-selectin, annexin V binding, and microparticle formations by means of whole-blood flow cytometry.

RESULTS

Two types of glycoprotein IIb/IIIa complex antagonists, peptide-mimetic FK633 and abciximab and prostaglandin E(1), significantly prevented platelet loss and the increase in binding of PAC-1, an antibody specific for fibrinogen receptor on activated platelets, during extracorporeal circulation of heparinized blood. These antagonists significantly suppressed but did not abolish P-selectin expression, annexin V binding, and microparticle formation. Anti-von Willebrand factor monoclonal antibody and aurin tricarboxylic acid (an inhibitor of glycoprotein Ib) had no effect on platelet activation during simulated cardiopulmonary bypass circulation. These data suggest that inhibition of fibrinogen binding glycoprotein IIb/IIIa complex is partly effective in attenuating platelet activation in a heparinized cardiopulmonary bypass model circuit. The direct thrombin inhibitor argatroban prevented platelet loss and expression of P-selectin significantly more than did heparin. A combination of FK633 with argatroban as a substitute for heparin further prevented platelet loss and platelet secretion during simulated cardiopulmonary bypass circulation, although the inhibition of microparticle formation was less.

CONCLUSION

The inhibition of both platelet adhesion and thrombin may be effective to preserve platelet number and function during cardiopulmonary bypass circulation.

Relationship between the expression levels of CD61, CD63, and PAC-1 on platelet surface in peripheral blood and the transplanted kidney function

OBJECTIVE

To analyze the relationships between the expression levels of CD61, CD63, and PAC-1 on the platelet surface and the incidences of acute rejection and tubular necrosis as well as the recovery of graft function after renal transplantation.

METHODS

The expression levels of CD61, CD63, and PAC-1 on platelet surfaces were assayed by flow cytometry in 86 patients with different stages of uremia before and after transplantation. Patients were divided into three groups: 29 patients with normal graft function, 30 with acute rejection, and 27 with acute tubular necrosis. Patients with acute rejection were randomly assigned into groups treated with or without anticoagulants.

RESULTS

The expression levels of CD61, CD63, and PAC-1 on platelet surfaces significantly increased (P <.05) among patients with acute rejection, as compared with those with normal graft function or acute tubular necrosis. Compared with controls, the expression levels of CD61, CD63, and PAC-1 were lower among acute rejection patients who, received anticoagulant therapy. The recovery time for graft function shorter and, the 1-year patients and graft survival rates higher.

CONCLUSIONS

The pretransplant expression levels of CD61, CD63, and PAC-1 on platelet surface were significantly higher among patients with acute rejection, suggesting that this complication rather than acute tubular necrosis may be related to platelet activation. Patients with acute rejection displayed benefit from anticoagulant therapy.

Antisense protein tyrosine phosphatase 1B reverses activation of p38 mitogen-activated protein kinase in liver of ob/ob mice

Phosphorylation of stress-activated kinase p38, a MAPK family member, was increased in liver of ob/ob diabetic mice relative to lean littermates. Treatment of ob/ob mice with protein tyrosine phosphatase 1B (PTP1B) antisense oligonucleotides (ASO) reduced phosphorylation of p38 in liver-to below lean littermate levels-and normalized plasma glucose while reducing plasma insulin. Phosphorylation of ERK, but not JNK, was also decreased in ASO-treated mice. PTP1B ASO decreased TNFalpha protein levels and phosphorylation of the transcription factor cAMP response element binding protein (CREB) in liver, both of which can occur through decreased phosphorylation of p38 and both of which have been implicated in insulin resistance or hyperglycemia. Decreased p38 phosphorylation was not directly due to decreased phosphorylation of the kinases that normally phosphorylate p38-MKK3 and MKK6. Additionally, p38 phosphorylation was not enhanced in liver upon insulin stimulation of ASO-treated ob/ob mice (despite increased activation of other signaling molecules) corroborating that p38 is not directly affected via the insulin receptor. Instead, decreased phosphorylation of p38 may be due to increased expression of MAPK phosphatases, particularly the p38/ERK phosphatase PAC1 (phosphatase of activated cells). This study demonstrates that reduction of PTP1B protein using ASO reduces activation of p38 and its substrates TNFalpha and CREB in liver of diabetic mice, which correlates with decreased hyperglycemia and hyperinsulinemia.

PAC1 phosphatase is a transcription target of p53 in signalling apoptosis and growth suppression

p53 has a role in many cellular processes through the transcriptional regulation of target genes. PAC1 (phosphatase of activated cells 1; also known as dual specificity phosphatase 2, DUSP2) is a dual threonine/tyrosine phosphatase that specifically dephosphorylates and inactivates mitogen-activated protein (MAP) kinases. Here we show that during apoptosis, p53 activates transcription of PAC1 by binding to a palindromic site in the PAC1 promoter. PAC1 transcription is induced in response to serum deprivation and oxidative stress, which results in p53-dependent apoptosis, but not in response to gamma-irradiation, which causes cell cycle arrest. Reduction of PAC1 transcription using small interfering RNA inhibits p53-mediated apoptosis, whereas overexpression of PAC1 increases susceptibility to apoptosis and suppresses tumour formation. Moreover, activation of p53 significantly inhibits MAP kinase activity. We conclude that, under specific stress conditions, p53 regulates transcription of PAC1 through a new p53-binding site, and that PAC1 is necessary and sufficient for p53-mediated apoptosis. Identification of a palindromic motif as a p53-binding site may reveal a novel mechanism whereby p53 regulates its target genes.

Clopidogrel but not aspirin reduces P-selectin expression and formation of platelet-leukocyte aggregates in patients with atherosclerotic vascular disease

Formation of platelet-leukocyte aggregates via the CD62 ligand represents an important mechanism by which leukocytes contribute to thrombotic events. In a cross-sectional study, we investigated platelet-leukocyte aggregate formation and markers indicative for platelet, leukocyte, and endothelial activation (CD62, activated fibrinogin receptor glycoprotein IIb/IIIA [PAC-1], CD11b/CD18 [MAC-1], and soluble intercellular adhesion molecule 1) in 44 patients with atherosclerotic vascular disease and peripheral occlusions receiving clopidogrel (n = 12), aspirin (n = 17), their combination (n = 8), or no treatment (n = 7), as well as in a group of healthy subjects (n = 9). Whole-blood flow cytometry was performed before (baseline) and after stimulation with thrombin receptor-activating peptide or adenosine diphosphate. Both at baseline and after stimulation, untreated patients and those receiving aspirin monotherapy exhibited significantly higher levels of platelet CD62 expression (baseline CD62: untreated, 22% [median]; with aspirin, 16%) and had higher rates of platelet-leukocyte aggregate formation (monocyte-platelet-leukocyte aggregates at baseline: untreated, 27%; with aspirin, 16%) when compared with patients receiving clopidogrel alone (baseline CD62: 10% [P <.05]; monocyte-platelet-leukocyte aggregates: 13% [P <.05]) or combined with aspirin (baseline CD62: 5% [P <.05]; monocyte-platelet-leukocyte aggregates: 7% [P <.05]). Up-regulation of MAC-1 on monocytes after stimulation with thrombin receptor-activating peptide and adenosine diphosphate was significantly lower in patients treated with clopidogrel and aspirin. Plasma levels of soluble intercellular adhesion molecule 1 were significantly lower in the group of healthy subjects (median, 186 ng/mL) when compared with those in untreated patients (median, 352 ng/mL) (P <.05), whereas intercellular adhesion molecule 1 levels in treated patients were similar for any antiplatelet regimen (aspirin, 262 ng/mL; clopidogrel, 274 ng/mL; combination therapy, 273 ng/mL) but significantly lower than those in untreated patients. This is the first report showing that platelet-leukocyte aggregate formation is enhanced in atherosclerotic vascular disease but was found to be reduced in patients receiving clopidogrel.

Activation of platelet gpIIbIIIa by phospholipase C from Clostridium perfringens involves store-operated calcium entry

Clostridium perfringens gas gangrene is characterized by rapid tissue destruction, and amputation remains the single best treatment. Previous studies have demonstrated that tissue destruction follows C. perfringens phospholipase C (PLC)-induced, platelet gpIIbIIIa-mediated formation of occlusive intravascular platelet/leukocyte aggregates. In this study, the intracellular signaling events leading to activation of gpIIbIIIa by PLC were investigated. PLC activated surface expressed gpIIbIIIa and mobilized gpIIbIIIa from internal stores. Chelation of intracellular calcium or inhibition of store-operated calcium entry each blocked PLC-induced activation of gpIIbIIIa, whereas inhibition of protein kinase C was without effect. Thus, PLC initiates an "inside-out" signaling cascade that begins with depletion of internal calcium stores, is sustained by an influx of calcium through store-sensitive channels, and culminates in the functional activation of gpIIbIIIa. These findings suggest that calcium-channel blockade and strategies targeting gpIIbIIIa may prevent vascular occlusion, maintain tissue viability, and provide an alternative to radical amputation for patients with gas gangrene.

Solution structure of the MAPK phosphatase PAC-1 catalytic domain. Insights into substrate-induced enzymatic activation of MKP

Inactivation of mitogen-activated protein kinases (MAPKs) by MAPK phosphatases (MKPs) is accomplished via substrate-induced activation of the latter enzymes; however, the structural basis for the underlying mechanism remains elusive. Here, we report the three-dimensional solution structure of the C-terminal phosphatase domain of the prototypical MKP PAC-1, determined when bound to phosphate. Structural and biochemical analyses reveal unique active site geometry of the enzyme important for binding to phosphorylated threonine and tyrosine of MAPK ERK2. Our study further demonstrates that the dynamic interaction between the N-terminal kinase binding domain and the C-terminal phosphatase domain of an MKP is directly coupled to MAPK-induced conformational change of the phosphatase active site, which is essential for eliciting its full enzymatic activity.

Increased platelet aggregation and activation in peripheral arterial disease

OBJECTIVES

patients with peripheral arterial disease (PAD) have a threefold increase in cardiovascular mortality. Standard antiplatelet treatment may not confer uniform benefit in different patient groups. This study aimed to compare platelet function in patients with lower limb PAD, carotid disease and abdominal aortic aneurysm (AAA) with age- and sex-matched healthy controls.

METHODS

patients with lower limb PAD (n = 20), carotid disease (n = 40), AAA (n = 13) and age/sex matched healthy controls (n= 20) were studied. Whole blood methods to detect spontaneous platelet aggregation (SPA), and adenosine diphosphate (ADP) and collagen-induced aggregation were used. The detection of platelet P-selectin and the PAC-1 antigen by flow cytometry were also used as markers of platelet activation and aggregation.

RESULTS

patients with lower limb PAD or AAA had higher baseline SPA compared to normal controls (p < 0.01). There was significantly higher collagen-induced aggregation in IC patients compared to normal controls (p < 0.01). However, there was no difference in ADP-induced aggregation between lower limb PAD and control patients. There was no difference in PAC-1 binding between control patients and the patients with lower limb PAD, carotid disease or AAA. Patients with carotid disease had a higher expression of P-selectin compared to normal controls (p < 0.05).

CONCLUSIONS

this study provides further evidence that platelet hyperactivity is present in patients with PAD despite the use of antiplatelet therapy. Further antiplatelet strategies may be indicated to protect these patients.

Protein kinase C- and calcium-regulated pathways independently synergize with Gi pathways in agonist-induced fibrinogen receptor activation

Platelet fibrinogen receptor activation is a critical step in platelet plug formation. The fibrinogen receptor (integrin alphaIIbbeta3) is activated by agonist-mediated G(q) stimulation and resultant phospholipase C activation. We investigated the role of downstream signalling events from phospholipase C, namely the activation of protein kinase C (PKC) and rise in intracellular calcium, in agonist-induced fibrinogen receptor activation using Ro 31-8220 (a PKC inhibitor) or dimethyl BAPTA [5,5'-dimethyl-bis-(o-aminophenoxy)ethane-N,N,N', N'-tetra-acetic acid], a high-affinity calcium chelator. All the experiments were performed with human platelets treated with aspirin, to avoid positive feedback from thromboxane A2. In the presence of Ro 31-8220, platelet aggregation caused by U46619 was completely inhibited while no effect or partial inhibition was seen with ADP and the thrombin-receptor-activating peptide SFLLRN, respectively. In the presence of intracellular dimethyl BAPTA, ADP- and U46619-induced aggregation and anti-alphaIIbbeta3 antibody PAC-1 binding were completely abolished. However, similar to the effects of Ro 31-8220, dimethyl BAPTA only partially inhibited SFLLRN-induced aggregation, and was accompanied by diminished dense-granule secretion. When either PKC activation or intracellular calcium release was abrogated, aggregation and fibrinogen receptor activation with U46619 or SFLLRN was partially restored by additional selective activation of the G(i) signalling pathway. In contrast, when both PKC activity and intracellular calcium increase were simultaneously inhibited, the complete inhibition of aggregation that occurred in response to either U46619 or SFLLRN could not be restored with concomitant G(i) signalling. We conclude that, while the PKC- and calcium-regulated signalling pathways are capable of inducing activating fibrinogen receptor independently and that each can synergize with G(i) signalling to cause irreversible fibrinogen receptor activation, both pathways act synergistically to effect irreversible fibrinogen receptor activation.

Resistance to aspirin in vitro is associated with increased platelet sensitivity to adenosine diphosphate

BACKGROUND

Platelet activation plays an important role in arterial thrombosis and the widespread use of aspirin has reduced major events by 25% in the secondary prevention of cardiovascular diseases. However, it appears that aspirin antiplatelet effect is not uniform and 8-45% of the population are, in vitro, aspirin resistant, and it is well recognized that platelets can be activated by pathways that are not blocked by aspirin, such as adenosine diphosphate (ADP).

OBJECTIVES

To investigate whether aspirin-resistant patients have a modified sensitivity to ADP-induced platelet activation

MATERIALS AND METHODS

Seventy-two patients were enrolled. Platelet function was measured by the PFA-100(R) analyser; platelet GP IIb-IIIa activation by ADP 10 micro M was assessed by flow cytometry using PAC-1 MoAb.

RESULTS

Using a collagen/epinephrine coated cartridge on the PFA-100(R), the prevalence of aspirin resistance was 29.2% (n=21). For aspirin-resistant patients, the collagen/ADP coated cartridge showed a closure time significantly shorter (p=0.004) compared to the sensitive and control groups. Platelets from aspirin-resistant patients bound PAC-1 significantly more (p=0.03) than the aspirin-sensitive patients and controls when activated with 10 micro M ADP.

CONCLUSIONS

Platelets from aspirin-resistant patients appear to be more sensitive and activable by ADP. This hypersensitivity could provide a possible explanation for the so-called aspirin resistance, and this could justify therapeutic improvement with alternative antiplatelet agents.

[Platelet activation in obstructive sleep apnea syndrome]

The incidences of cardiovascular and cerebrovascular diseases are reportedly higher in patients with obstructive sleep apnea (OSA) than in OSA-free subjects, though the mechanism remains unknown. Recently, the contribution of activated platelets to a number of pathological conditions such as stroke or ischemic heart disease has been suggested. We hypothesized that the expression of activated platelet markers resulting from OSA might be higher than in healthy subjects. By flow cytometry using monoclonal antibodies, we measured two such markers, PAC-1 and CD 62 P, in OSA patients and healthy subjects. Twelve healthy men (age, 52.7 +/- 12.8 y/o; and body mass index (BMI), 22.2 +/- 16.1 kg/m2; mean +/- S.D.) and 20 male patients with OSA (age, 50 +/- 7.96 y/o; BMI, 28.1 +/- 3.3 kg/m2; apnea hypopnea index (AHI), 38.2 +/- 21.2 times/hr; and lowest SpO2, 75.6 +/- 11.3%) were enrolled in this study. PAC-1 expression was significantly higher in OSA patients (65.1 +/- 17.8%) than in healthy subjects (16.8 +/- 7.4%), as was CD 62 P expression (8.5 +/- 8.8% vs. 0.88 +/- 0.57%). The increase in PAC-1 expression was correlated with AHI and the arousal index. These findings suggest that activated platelet markers could be good indicators for untreated OSA.

Platelet alpha-granule secretion and its modification by SC-57101A, a GPIIb/IIIa antagonist

Platelet-agonist interaction results in aggregatory and secretory responses. While the activation of glycoprotein (GP) IIb/IIIa plays an essential role in platelet aggregation, its role in granule secretion is not clear. The present study was performed to examine the effect of 3-[[[[1-[4-(aminoiminomethyl) phenyl]-2-oxo-3S-pyrrolidinyl]amino]carbonyl]amino]-propanoate monohydrochloride salt (SC-57101A), a GPIIb/IIIa antagonist, on platelet alpha-granule secretion responses to collagen, ADP, and thrombin receptor activating peptide (TRAP). Both SC-57101A and prostaglandin E(1) (PGE(1)) inhibited collagen-, ADP-, and TRAP-induced platelet aggregation in a concentration-dependent manner. SC-57101A inhibited the collagen- and ADP-induced release of platelet-derived growth factor (PDGF) and beta-thromboglobulin (beta-TG) from platelets, but not TRAP-induced secretion of these granule contents. On the other hand, PGE(1) inhibited the release of PDGF and beta-TG from platelets activated with all the agonists used. ADP and TRAP elicited P-selectin expression in the absence of platelet aggregation, while collagen produced no such reaction. SC-57101A only moderately inhibited P-selectin expression induced by ADP and had no inhibitory effect on that induced by TRAP. The inhibition of ADP-induced secretion of alpha-granule contents by SC-57101A was abolished when platelets were pretreated with aspirin. These results suggest that GPIIb/IIIa activation plays a minor role, if any, in alpha-granule secretion in human platelets.

Effects of combined therapy with clopidogrel and acetylsalicylic acid on platelet glycoprotein expression and aggregation

SUMMARY

This study aimed to compare the effects of clopidogrel, acetylsalicylic acid (ASA), and the combination of both substances on platelet aggregation and expression of platelet membrane glycoproteins in patients with chronic coronary artery disease. We investigated platelet activation by flow cytometry and by platelet aggregation and disaggregation in 60 patients randomly assigned to 3 treatment groups: ASA, clopidogrel, combination of clopidogrel and ASA, treated for 14 days. Adenosine diphosphate (ADP)-induced expression of P-selectin and of PAC-1 was significantly reduced after 2 wk of clopidogrel but not of ASA treatment. Treatment with clopidogrel reduced the ADP-induced platelet aggregation. The combination of clopidogrel and ASA did not increase the inhibition of platelet activation compared with clopidogrel alone. A significant increase in platelet disaggregation was observed with clopidogrel alone and was more pronounced with the combination of clopidogrel and ASA. ADP-induced platelet degranulation, activation of GPIIb/IIIa receptor, and aggregation in vivo are effectively inhibited by clopidogrel. The significantly increased disaggregation under clopidogrel and ASA suggests that the combined therapy may be superior to the monotherapy in patients with coronary artery disease and a high risk for vascular events.

Increased circulating platelet-leucocyte complexes and platelet activation in patients with antiphospholipid syndrome, systemic lupus erythematosus and rheumatoid arthritis

It is possible that platelet activation may play a pathogenic role in the increased risk of thrombosis associated with antiphospholipid antibodies (APA). In this study, levels of in vivo platelet activation were measured in 20 patients with primary antiphospholipid syndrome (PAPS) and 30 systemic lupus erythematosus (SLE) patients (14 of whom had secondary APS) using sensitive flow cytometry. Soluble P-selectin levels were also assayed. Platelet CD63 expression was significantly higher in PAPS than normal controls (P = 0.007), as well as SLE patients with and without secondary APS (P = 0.03 and P = 0.002 respectively). PAC-1 binding was significantly higher in PAPS than the control group (P = 0.007) and SLE patients without APS (P = 0.015). Platelet-leucocyte complexes were significantly higher in SLE patients than both PAPS and the control group, and platelet-monocyte complexes were significantly increased in PAPS compared with the control group. (Platelet-leucocyte complexes were also significantly higher than controls in 10 rheumatoid arthritis (RA) patients without APA). Soluble P-selectin levels were significantly higher in PAPS and SLE patients than the control group. Platelet CD62p expression, annexin V binding and platelet microparticle numbers were not increased in PAPS or SLE patients. We conclude that there is evidence of increased platelet activation in PAPS and SLE, and this is important to note as it may have potential therapeutic implications with respect to use of antiplatelet agents in these patients.

A filarial nematode-secreted phosphorylcholine-containing glycoprotein uncouples the B cell antigen receptor from extracellular signal-regulated kinase-mitogen-activated protein kinase by promoting the surface Ig-mediated recruitment of Src homology 2 domain-containing tyrosine phosphatase-1 and Pac-1 mitogen-activated kinase-phosphatase

Unraveling the molecular mechanisms by which filarial nematodes, major human pathogens in the tropics, evade the host immune system remains an elusive goal. We have previously shown that excretory-secretory product-62 (ES-62), a homologue of phosphorylcholine-containing molecules that are secreted by human parasites and which is active in rodent models of filarial infection, is able to polyclonally activate certain protein tyrosine kinase and mitogen-activating protein kinase signal transduction elements in B lymphocytes. Such activation mediates desensitization of subsequent B cell Ag receptor (BCR) ligation-induced activation of extracellular signal-regulated kinase-mitogen-activated protein (ErkMAP) kinase and ultimately B cell proliferation. We now show that the desensitization is due to ES-62 targeting two major regulatory sites of B cell activation. Firstly, pre-exposure to ES-62 primes subsequent BCR-mediated recruitment of SHP-1 tyrosine phosphatase to abolish recruitment of the RasErkMAP kinase cascade via the Igalphabeta-ShcGrb2Sos adaptor complex interactions. Secondly, any ongoing ErkMAP kinase signaling in ES-62-primed B cells is terminated by the MAP kinase phosphatase, Pac-1 that is activated consequently to challenge via the BCR.

Biological action of nitric oxide donor compounds on platelets from patients with sickle cell disease

Several lines of evidence point to the potential role of nitric oxide (NO) in the pathophysiology, as well as in the therapy, of sickle cell disease (SCD). In this study, we compared the effects of NO on platelets from normal individuals and from patients with SCD. Three NO donors were used to deliver NO to platelets: sodium 2-(N, N-diethylamino)-diazenolate-2-oxide (DEANO), S-nitrosocysteine (CysNO) and sodium trioxdintrate (OXINO or Angeli's salt). ADP-induced platelet aggregation, CD62P expression, PAC-1 binding and calcium elevation were evaluated in paired studies of normal and SCD subjects. DEANO significantly reduced aggregation in SCD platelets compared with normal platelets. DEANO similarly reduced the extent of CD62P expression in SCD platelets. All NO donors reduced PAC-1 binding, but there were no significant differences between platelets from normal or SCD subjects. Calcium elevation, as induced by ADP, was not altered by the presence of NO donors. However, when platelets were stimulated with thrombin, there was an increased initial response of SCD platelets compared with normal platelets. Taken together, these data suggest that the mode of NO delivery to platelets may produce various physiological responses and the optimization of NO delivery may contribute to reducing platelet aggregation in sickle cell disease.

Antiplatelet effects of clopidogrel compared with aspirin after myocardial infarction: enhanced inhibitory effects of combination therapy

OBJECTIVES

We sought to compare the inhibitory effects of the combination of two doses of aspirin plus clopidogrel with either drug alone on platelet aggregation and activation.

BACKGROUND

Enhanced platelet inhibitory effects of clopidogrel by aspirin on platelet aggregation and activation are suggested by experimental studies but have not been shown in humans.

METHODS

The effects of clopidogrel 75 mg or aspirin 100 (300) mg on platelet aggregation and activation by flow cytometry after stimulation with various agonists were determined in 30 patients with a past history of myocardial infarction.

RESULTS

Clopidogrel alone or in combination with aspirin markedly inhibited adenosine diphosphate (ADP)-mediated platelet aggregation compared with monotherapy with aspirin (24.6 +/- 3.3% or 26.6 +/- 2.7% vs. 44.7 +/- 2.9%; p < 0.001). Combined treatment significantly inhibited collagen-induced aggregation compared with aspirin and clopidogrel (16.4 +/- 2.4%, 36.5 +/- 4.2% and 59.3 +/- 5.1%, respectively;, p < 0.001) and resulted in considerable inhibition of aggregation induced by thrombin receptor agonist peptide (TRAP, p < 0.03). Clopidogrel with or without aspirin significantly suppressed expression of platelet activation markers CD 62p, CD 63 and PAC-1 after stimulation with ADP or thrombin (p < 0.001). In addition, the combined treatment was more effective than either agent alone after activation with low dose thrombin (p < 0.05). Both doses of aspirin equally potentiated the platelet inhibitory effects of clopidogrel. CONCLUSIONS In this prospective clinical ex vivo platelet study, clopidogrel was more effective than aspirin in inhibiting ADP-mediated platelet aggregation and activation. Clopidogrel in combination with aspirin showed synergistic inhibitory effects after stimulation with collagen and thrombin compared with monotherapies. Thus, this dual antiplatelet treatment strategy deserves further evaluation in clinical trials for secondary prevention of acute myocardial infarction or unstable angina.

Cocoa and wine polyphenols modulate platelet activation and function

There is speculation that dietary polyphenols can provide cardioprotective effects due to direct antioxidant or antithrombotic mechanisms. We report in vitro and postingestion ex vivo effects of cocoa procyanidins, a procyanidin-rich cocoa beverage and dealcoholized red wine (DRW) on human platelet activation. In a series of in vitro studies, cocoa procyanidin trimers, pentamers or DRW (3 and 10 micromol/L) were incubated with citrated peripheral whole blood in the presence and absence of platelet agonists. Platelet activation was detected using fluorescent-labeled monoclonal antibodies recognizing the fibrinogen binding conformation of GPIIb-IIIa (referred to herein as PAC-1 binding) and the activation-dependent platelet epitope CD62P (P-selectin). The percentage of CD42a-positive platelets coexpressing PAC-1 binding and/or CD62P was determined by multiparameter flow cytometry. Procyanidin trimers, pentamers and DRW added to whole blood in vitro increased PAC-1 binding and P-selectin expression. In contrast, procyanidin trimers, pentamers and DRW inhibited the platelet activation in response to epinephrine. The effects on platelet activation of cocoa beverage and DRW consumption were also studied in healthy subjects. Citrated blood was obtained before and 2 and 6 h after the ingestion of a cocoa beverage, a caffeine-containing beverage, DRW or water. Platelet activation was measured by flow cytometry. The consumption of DRW did not affect the expression of activation-dependent platelet antigens, either unstimulated or after ex vivo activation with epinephrine. However, the consumption of DRW increased PAC-1 binding in response to 100 micromol/L ADP ex vivo. Cocoa consumption reduced platelet response to agonists ex vivo. The ingestion of water had no effect on platelet activation, whereas a caffeine-containing beverage augmented the response of platelets to epinephrine. In summary, select cocoa procyanidins and DRW added to whole blood in vitro increased expression of platelet activation markers in unstimulated platelets but suppressed the platelet activation response to epinephrine. In contrast, cocoa consumption suppressed unstimulated and stimulated platelet activation in whole blood. This suppressive effect observed on platelet reactivity may explain in part the reported cardioprotective effects of dietary polyphenols.

Platelet activation as a potential mechanism of GP IIb/IIIa inhibitor-induced thrombocytopenia

The blockade of the platelet integrin glycoprotein (GP) IIb/IIIa has proved to be an effective antiplatelet therapy. Profound thrombocytopenia has repeatedly been described as an adverse effect in patients treated with GP IIb/IIIa inhibitors, but its mechanism has not been elucidated yet. With use of flow cytometry, the activation status of platelets was monitored in 26 patients presenting with acute myocardial infarction who were treated with the GP IIb/IIIa inhibitor abciximab alone or in combination with the fibrinolytic agent reteplase. Fibrinogen and PAC-1 (a GP IIb/IIIa activation-specific monoclonal antibody) binding, as well as P-selectin expression on unstimulated platelets were constant in 25 patients throughout a follow-up of 7 days. In 1 patient (D.F.), the percentage of platelet-binding fibrinogen increased from 2.2% to 17.8%, for PAC-1 from 2.8% to 13.2%, and for P-selectin expression from 10.2% to 58.3% 10 minutes after the start of treatment. Furthermore, D.F. had a decrease in single platelet count in ethylenediaminetetraacetic acid-, citrate-, and heparin-anticoagulated and native blood. Blood films revealed platelet aggregates. In vitro testing of D.F.'s blood 2 and 4 weeks after initial admission demonstrated a reinduction of fibrinogen and PAC-1 binding to platelets, an increase of P-selectin expression, and formation of platelet aggregates following exposition of platelets to abciximab in vitro. In summary, this report describes the induction of platelet activation by a GP IIb/IIIa inhibitor in vivo and reinduction in vitro in direct association with thrombocytopenia. Platelet activation by GP IIb/IIIa inhibitors may be one potential mechanism for GP IIb/IIIa inhibitor-induced thrombocytopenia.