Effects of thrombin on interactions between beta3-integrins and extracellular matrix in platelets and vascular cells

Prothrombin prevents fatal T cell-dependent anemia during chronic virus infection of mice

Thrombin promotes the proliferation and function of CD8+ T cells. To test if thrombin prevents exhaustion and sustains antiviral T cell activity during chronic viral infection, we depleted the thrombin-precursor prothrombin to 10% of normal levels in mice prior to infection with the clone 13 strain of lymphocytic choriomeningitis virus. Unexpectedly, prothrombin insufficiency resulted in 100% mortality after infection that was prevented by depletion of CD8+ T cells, suggesting that reduced availability of prothrombin enhances virus-induced immunopathology. Yet, the number, function, and apparent exhaustion of virus-specific T cells were measurably unaffected by prothrombin depletion. Histological analysis of the lung, heart, liver, kidney, spleen, intestine, and brain did not reveal any evidence of hemorrhage or increased tissue damage in mice with low levels of prothrombin that could explain mortality. Viral loads were also similar in infected mice regardless of prothrombin levels. Instead, infection of prothrombin-depleted mice resulted in a severe, T cell-dependent anemia associated with increased hemolysis. Thus, thrombin plays an unexpected protective role in preventing hemolytic anemia during virus infection, with potential implications for patients who are using direct thrombin inhibitors as an anticoagulant therapy.

The Effect of Activated FXIII, a Transglutaminase, on Vascular Smooth Muscle Cells

Plasma factor XIII (pFXIII) is a heterotetramer of FXIII-A and FXIII-B subunits. The cellular form (cFXIII), a dimer of FXIII-A, is present in a number of cell types. Activated FXIII (FXIIIa), a transglutaminase, plays an important role in clot stabilization, wound healing, angiogenesis and maintenance of pregnancy. It has a direct effect on vascular endothelial cells and fibroblasts, which have been implicated in the development of atherosclerotic plaques. Our aim was to explore the effect of FXIIIa on human aortic smooth muscle cells (HAoSMCs), another major cell type in the atherosclerotic plaque. Osteoblastic transformation induced by Pi and Ca²⁺ failed to elicit the expression of cFXIII in HAoSMCs. EZ4U, CCK-8 and CytoSelect Wound Healing assays were used to investigate cell proliferation and migration. The Sircol Collagen Assay Kit was used to monitor collagen secretion. Thrombospondin-1 (TSP-1) levels were measured by ELISA. Cell-associated TSP-1 was detected by the immunofluorescence technique. The TSP-1 mRNA level was estimated by RT-qPCR. Activated recombinant cFXIII (rFXIIIa) increased cell proliferation and collagen secretion. In parallel, a 67% decrease in TSP-1 concentration in the medium and a 2.5-fold increase in cells were observed. TSP-1 mRNA did not change significantly. These effects of FXIIIa might contribute to the pathogenesis of atherosclerotic plaques.

Pulmonary endothelial NEDD9 and the prothrombotic pathophenotype of acute respiratory distress syndrome due to SARS-CoV-2 infection

The pathobiology of in situ pulmonary thrombosis in acute respiratory distress syndrome (ARDS) due to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection is incompletely characterized. In human pulmonary artery endothelial cells (HPAECs), hypoxia increases neural precursor cell expressed, developmentally downregulated 9 (NEDD9) and induces expression of a prothrombotic NEDD9 peptide (N9P) on the extracellular plasma membrane surface. We hypothesized that the SARS-CoV-2-ARDS pathophenotype involves increased pulmonary endothelial N9P. Paraffin-embedded autopsy lung specimens were acquired from patients with SARS-CoV-2-​​​​​​ARDS (n = 13), ARDS from other causes (n = 10), and organ donor controls (n = 5). Immunofluorescence characterized the expression of N9P, fibrin, and transcription factor 12 (TCF12), a putative binding target of SARS-CoV-2 and known transcriptional regulator of NEDD9. We performed RNA-sequencing on normal HPAECs treated with normoxia or hypoxia (0.2% O2) for 24 h. Immunoprecipitation-liquid chromatography-mass spectrometry (IP-LC-MS) profiled protein-protein interactions involving N9P relevant to thrombus stabilization. Hypoxia increased TCF12 messenger RNA significantly compared to normoxia in HPAECs in vitro (+1.19-fold, p = 0.001; false discovery rate = 0.005), and pulmonary endothelial TCF12 expression was increased threefold in SARS-CoV-2-ARDS versus donor control lungs (p < 0.001). Compared to donor controls, pulmonary endothelial N9P-fibrin colocalization was increased in situ in non-SARS-CoV-2-ARDS and SARS-CoV-2-ARDS decedents (3.7 ± 1.2 vs. 10.3 ± 3.2 and 21.8 ± 4.0 arb. units, p < 0.001). However, total pulmonary endothelial N9P was increased significantly only in SARS-CoV-2-ARDS versus donor controls (15 ± 4.2 vs. 6.3 ± 0.9 arb. units, p < 0.001). In HPAEC plasma membrane isolates, IP-LC-MS identified a novel protein-protein interaction between NEDD9 and the β3-subunit of the αvβ3-integrin, which regulates fibrin anchoring to endothelial cells. In conclusion, lethal SARS-CoV-2-ARDS is associated with increased pulmonary endothelial N9P expression and N9P-fibrin colocalization in situ. Further investigation is needed to determine the pathogenetic and potential therapeutic relevance of N9P to the thrombotic pathophenotype of SARS-CoV-2-ARDS.

Platelet-derived extracellular vesicles are increased in sera of Alzheimer's disease patients, as revealed by Tim4-based assays

Alzheimer's disease (AD) is the most common form of dementia, characterized by the accumulation of β‐amyloid plaques and the formation of neurofibrillary tangles. Extracellular vesicles (EVs) are small vesicles surrounded by a lipid bilayer membrane, which may be involved in the progression of AD. Glycans are essential building blocks of EVs, and we hypothesized that EV glycans may reflect pathological conditions of various diseases. Here, we performed glycan profiling of EVs prepared from sera of three AD patients (APs) compared to three healthy donors (HDs) using lectin microarray. Distinct glycan profiles were observed. Mannose‐binding lectins exhibited significantly higher signals for AP‐derived EVs than HD‐derived EVs. Lectin blotting using mannose‐binding lectin (rPALa) showed a single protein band at ~ 80 kDa exclusively in AP‐derived EVs. LC‐MS/MS analysis identified a protein band precipitated by rPALa as CD61, a marker of platelet‐derived exosomes (P‐Exo). Sandwich assays using Tim4 with specificity for phosphatidylserine on EVs and antibodies against P‐Exo markers (CD61, CD41, CD63, and CD9) revealed that P‐Exo is significantly elevated in sera of APs (n = 16) relative to age‐ and sex‐matched HDs (n = 16). Tim4‐αCD63 showed the highest value for the area under the curve (0.957) for discriminating APs from HDs, which should lead to a better understanding of AD pathology and may facilitate the development of a novel diagnostic method for AD.

Smooth muscle cell and arterial aging: basic and clinical aspects

Arterial aging engages a plethora of key signalling pathways that act in concert to induce vascular smooth muscle cell (VSMC) phenotypic changes leading to vascular degeneration and extracellular matrix degradation responsible for alterations of the mechanical properties of the vascular wall. This review highlights proof-of-concept examples of components of the extracellular matrix, VSMC receptors which connect extracellular and intracellular structures, and signalling pathways regulating changes in mechanotransduction and vascular homeostasis in aging. Furthermore, it provides a new framework for understanding how VSMC stiffness and adhesion to extracellular matrix contribute to arterial stiffness and how interactions with endothelial cells, platelets, and immune cells can regulate vascular aging. The identification of the key players of VSMC changes operating in large and small-sized arteries in response to increased mechanical load may be useful to better elucidate the causes and consequences of vascular aging and associated progression of hypertension, arteriosclerosis, and atherosclerosis.

Antiplatelet agents for cancer treatment: a real perspective or just an echo from the past?

The association between coagulation and cancer development has been observed for centuries. However, the connection between inflammation and malignancy is also well-recognized. The plethora of evidence indicates that among multiple hemostasis components, platelets play major roles in cancer progression by providing surface and granular contents for several interactions as well as behaving like immune cells. Therefore, the anticancer potential of anti-platelet therapy has been intensively investigated for many years. Anti-platelet agents may prevent cancer, decrease tumor growth, and metastatic potential, as well as improve survival of cancer patients. On the other hand, there are suggestions that antiplatelet treatment may promote solid tumor development in a phenomenon described as "cancers follow bleeding." The controversies around antiplatelet agents justify insight into the subject to establish what, if any, role platelet-directed therapy has in the continuum of anticancer management.

Disulfide Bonds as Regulators of Integrin Function in Thrombosis and Hemostasis

SIGNIFICANCE

Disulfide bonds are generally viewed as structure-stabilizing elements in proteins, but some display an alternative functional role as redox switches. Functional disulfide bonds have recently emerged as important regulators of integrin function in thrombosis and hemostasis.

RECENT ADVANCES

Functional disulfide bonds were identified in the β subunit of the major platelet integrin αIIbβ3 and in other integrins involved in thrombus formation that is, αvβ3 and α2β1. Most of these functional bonds are located in the four epidermal growth factor-like domains of the integrins. Redox agents such as glutathione and nitric oxide and enzymatic thiol isomerase activity were shown to regulate the function of these integrins by disulfide bond reduction and thiol/disulfide exchange.

CRITICAL ISSUES

Increasing evidence suggests that thiol isomerases such as protein disulfide isomerase (PDI) and Erp57 directly bind to the β3 subunit of αIIbβ3 and αvβ3 and regulate their function during thrombus formation. αIIbβ3 also exhibits an endogenous thiol isomerase activity. The specific functional disulfide bonds identified in the β3 subunit might be the targets for both exogenous and endogenous thiol isomerase activity.

FUTURE DIRECTIONS

Targeting redox sites of integrins or redox agents and enzymes that regulate their function can provide a useful tool for development of anti-thrombotic therapy. Hence, inhibitors of PDI are currently studied for this purpose.

Vascular Smooth Muscle Cells Are Responsible for a Prothrombotic Phenotype of Spontaneously Hypertensive Rat Arteries

Objective—

The hypothesis that hypertension induces a hypercoagulable state arises from the complications associated with hypertension: stroke and myocardial infarction. Here, we determine whether hypertension causes changes in the thrombin-generating capacity of the vascular wall.

Approach and Results—

We used spontaneously hypertensive rats (SHR) compared with Wistar rats. The addition of thoracic aortic rings of SHR to a Wistar or SHR plasma pool resulted in a greater increase in thrombin generation compared with equivalent rings from Wistar. This increase occurred in 12- but not 5-week-old rats and was prevented by an angiotensin II–converting enzyme inhibitor, indicating that established hypertension is required to induce increased thrombin generation within the vessel wall. Whereas no difference was observed for endothelial cells, thrombin formation was higher on aortic smooth muscle cells (SMCs) from SHR than on those from Wistar. Exposure of negatively charged phospholipids was higher on SHR than on Wistar rings, as well as on cultured SMCs. Tissue factor activity was higher in SHR SMCs. Twelve-week-old SHR exhibited accelerated FeCl 3 -induced thrombus formation in carotid arteries, and the resulting occlusive thrombi were disaggregated by blockade of glycoprotein Ibα–von Willebrand factor interactions. SHR SMCs were more sensitive to thrombin-induced proliferation than Wistar SMCs. This effect was totally abolished by a protease-activated receptor 1 inhibitor.

Conclusions—

The prothrombotic phenotype of the SHR vessel wall was due to the ability of SMCs to support greater thrombin generation and resulted in accelerated occlusive thrombus formation after arterial injury, which was sensitive to glycoprotein Ibα–von Willebrand factor inhibitors.

Infections and autoimmunity: role of human cytomegalovirus in autoimmune endothelial cell damage

Molecular mimicry between infectious agents and normal human host cell proteins represents one of the possible mechanisms responsible for autoimmunity. Among infectious agents, human cytomegalovirus (HCMV) is an ideal candidate for involvement in autoimmune disorders because of its lifelong persistence through periods of reactivation and latency and because of the extensive manipulation of innate and adaptive immunity. HCMV has been implicated in the pathogenesis of vascular damage in systemic sclerosis (SSc) and atherosclerosis. Based on our data, which demonstrate a cause-and-effect relationship between HCMV and endothelial cell aggression in SSc and atherosclerosis, we propose that immune responses to particular HCMV proteins may result in autoaggression through a mechanism of molecular mimicry of normally expressed endothelial cell surface molecules.

Impact of moderate blast exposures on thrombin biomarkers assessed by calibrated automated thrombography in rats

Severe blast exposures are frequently complicated with fatal intracranial hemorrhages. However, many more sustain low level blasts without tissue damage detectable by brain imaging. To investigate effects of nonlethal blast on thrombin-related biomarkers, rats were subjected to two different types of head-directed blast: 1) moderate "composite" blast with strong head acceleration or 2) moderate primary blast, without head acceleration. Thrombin generation (TG) ex vivo after blast was studied by calibrated automated thrombography (CAT). In the same blood samples, we assessed maximal concentration of TG (TGmax), start time, peak time, mean time, and concentrations of protein markers for vascular/hemostatic dysfunctions: integrin α/β, soluble endothelial selectin (sE-selectin), soluble intercellular cell adhesion molecule-1 (sICAM-1), and matrix metalloproteinases (MMP)-2, MMP-8, and MMP-13. Blast remarkably affected all TG indices. In animals exposed to "composite" blast, TGmax peaked at 6 h (∼4.5-fold vs. control), sustained at day 1 (∼3.8-fold increase), and declined to a 2-fold increase over control at day 7 post-blast. After primary blast, TGmax also rose to ∼4.2-fold of control at 6 h, dropped to ∼1.7-fold of control at day 1, and then exhibited a slight secondary increase at 2-fold of control at day 7. Other TG indices did not differ significantly between two types of blast exposure. The changes were also observed in other microvascular/inflammatory/hemostatic biomarkers. Integrin α/β and sICAM-1 levels were elevated after both "composite" and primary blast at 6 h, 1 day, and 7 days. sE-selectin exhibited near normal levels after "composite" blast, but increased significantly at 7 days after primary blast; MMP-2, MMP-8, and MMP-13 slightly rose after "composite" blast and significantly increased (∼2-4-fold) after primary blast. In summary, CAT may have a clinical diagnostic utility in combination with selected set of microvascular/inflammatory biomarkers in patients subjected to low/moderate level blast exposures.

Kaposi's sarcoma-associated herpesvirus induces rapid release of angiopoietin-2 from endothelial cells

Kaposi sarcoma-associated herpesvirus (KSHV) stimulates proliferation, angiogenesis, and inflammation to promote Kaposi sarcoma (KS) tumor growth, which involves various growth factors and cytokines. Previously, we found that KSHV infection of human umbilical vein endothelial cells (HUVECs) induces a transcriptional induction of the proangiogenic and proinflammatory cytokine angiopoietin-2 (Ang-2). Here, we report that KSHV induces rapid release of Ang-2 that is presynthesized and stored in the Weibel-Palade bodies (WPB) of endothelial cells upon binding to its integrin receptors. Blocking viral binding to integrins inhibits Ang-2 release. KSHV binding activates the integrin tyrosine kinase receptor signaling pathways, leading to tyrosine phosphorylation of focal adhesion kinase (FAK), the tyrosine kinase Src, and the Calα2 subunit of the l-type calcium channel to trigger rapid calcium (Ca(2+)) influx. Pretreatment of endothelial cells with specific inhibitors of protein tyrosine kinases inhibits KSHV-induced Ca(2+) influx and Ang-2 release. Inhibition of Ca(2+) mobilization with calcium channel blockers also inhibits Ang-2 release. Thus, the interaction between KSHV and its integrin receptors plays a key role in regulating rapid Ang-2 release from endothelial cells. This finding highlights a novel mechanism of viral induction of angiogenesis and inflammation, which might play important roles in the early event of KS tumor development.

Anti-platelet and Anti-thrombotic Effects of a Poly-ingredient formulation: In vitro and in vivo experimental evidences

OBJECTIVE

The present study was conducted to evaluate the efficacy of Abana^® (a poly-ingredient formulation with natural constituents) on in vitro platelet aggregation and occlusion-induced deep venous thrombosis in rats.

METHODS

Anti-platelet property of Abana^® was evaluated using ADP (Adenosin 5-diphosphate) and adrenaline-induced platelet aggregation models, and anti-thrombotic activity was evaluated against occlusion-induced deep venous thrombosis model in wistar rats.

RESULTS

Under the in vitro conditions, Abana® (250, 500 and 1000 µg/ml) alleviated ADP and adrenaline-induced platelet aggregation in a dose-dependent manner. Abana® (1000 µg/ml) inhibited ADP and adrenaline-induced platelet aggregation by as much as 50.69% and 64.83% respectively. Furthermore, 6 days pre-treatment with Abana® (250 and 500 mg/kg, p.o.) in an in vivo study showed significant and dose-dependent protection against occlusion-induced deep venous thrombosis in rats.

CONCLUSION

These findings suggest that Abana^®, a polyherbal formulation possesses anti-platelet and anti-thrombotic activities in the experimental models of in vitro platelet aggregation and in vivo deep venous thrombosis in rats.

Cyclic stretch-induced thrombin generation by rat vascular smooth muscle cells is mediated by the integrin αvβ3 pathway

AIMS

Vascular smooth muscle cell (VSMC) phenotypic modulation plays a pivotal role in atherothrombotic diseases. Thrombin generation at the surface of VSMCs and activation of integrin mechanotransduction pathways represent potential mechanisms. Here, we examine whether mechanical stretch increases thrombin generation on cultured rat aortic VSMCs.

METHODS AND RESULTS

The integrin α(v)β(3) antagonist peptide (cRGDPV) dose-dependently decreased thrombin generation without stretch. Static stretch (5%, 1 Hz) failed to modify the thrombin-forming capacity of VSMCs, whereas 10% cyclic stretch during 60 and 360 min enhanced integrin α(v)β(3) expression and thrombin generation at the surface of VSMCs by 30% without inducing apoptosis. Cyclic stretch also stimulated Src phosphorylation, cleavage of talin, and binding of prothrombin to VSMCs. Upregulation of α(v)β(3) expression, Src phosphorylation, and enhanced thrombin generation by cyclic stretch were abolished by cRGDPV and silencing RNA (siRNA) against α(v) as well as by selective inhibition of integrin α(v)β(3) inside-out signalling by a talin-siRNA. Complete abolition of stretch-induced VSMC-supported thrombin generation by the RGT peptide, which disrupts the interaction of Src with the β(3) cytoplasmic tail, demonstrates the link between outside-in pathways involving β(3)-Src interaction and thrombin activity dependent on inside-out signalling.

CONCLUSION

These data show that the contribution of cyclic stretch to VSMC-supported thrombin generation is driven by the integrin α(v)β(3) signalling pathway and suggest a role for pulsatility-induced intramural thrombin in VSMC-dependent vascular remodelling.

RGD-dependent binding of TP508 to integrin alphavbeta3 mediates cell adhesion and induction of nitric oxide

TP508, a 23-amino acid RGD-containing synthetic peptide representing residues 508 to 530 of human prothrombin, mitigates the effects of endothelial dysfunction in ischaemic reperfusion injury. The objective of this study was to investigate whether TP508 binds to members of the integrin family of transmembrane receptors leading to nitric oxide synthesis. Immobilised TP508 supported adhesion of endothelial cells and alphavbeta3-expressing human embryonic kidney cells in a dose- and RGD-dependent manner. Soluble TP508 also inhibited cell adhesion to immobilised fibrinogen. The involvement of alphavbeta3 was verified with function-blocking antibodies and surface plasmon resonance studies. Adhesion of the cells to immobilised TP508 resulted in an induction of phosphorylated FAK and ERK1/2. In endothelial cells, TP508 treatment resulted in an induction of nitric oxide that could be inhibited by LM609, an alphavbeta3-specific, function-blocking monoclonal antibody. Finally, TP508 treatment of isolated rat aorta segments enhanced carbachol-induced vasorelaxation. These results suggest that TP508 elicits a potentially therapeutic effect through an RGD-dependent interaction with integrin alphavbeta3.

Enhanced proliferation and migration of vascular smooth muscle cells in response to vascular injury under hyperglycemic conditions is controlled by beta3 integrin signaling

Atheroma formation and restenosis following percutaneous vascular intervention involve the growth and migration of vascular smooth muscle cells (SMCs) into neointimal lesions, in part due to changes in the extracellular matrix. While some clinical studies have suggested that, in comparison to non-diabetics, beta3 integrin inhibition in diabetic patients confers protection from restenosis, little is known regarding the role of beta3 integrin inhibition on SMC responses in this context. To understand the molecular mechanisms underlying integrin-mediated regulation of SMC function in diabetes, we examined SMC responses in diabetic mice deficient in integrin beta3 and observed that the integrin was required for enhanced proliferation, migration and extracellular regulated kinase (ERK) activation. Hyperglycemia-enhanced membrane recruitment and catalytic activity of PKCbeta in an integrin beta3-dependent manner. Hyperglycemia also promoted SMC filopodia formation and cell migration, both of which required alphaVbeta3, PKCbeta, and ERK activity. Furthermore, the integrin-kinase association was regulated by the alphaVbeta3 integrin ligand thrombospondin and the integrin modulator Rap1 under conditions of hyperglycemia. These results suggest that there are differences in SMC responses to vascular injury depending on the presence or absence of hyperglycemia and that SMC response under hyperglycemic conditions is largely mediated through beta3 integrin signaling.

Surfactin C inhibits platelet aggregation

This study was designed to investigate the effect of surfactin C, which is derived from Bacillus subtilis, on platelet aggregation and homotypic leucocyte aggregation. Surfactin C strongly and dose-dependently inhibited platelet aggregation, which was stimulated both by thrombin (0.1 U mL−1), a potent agonist that activates the G protein-coupled protease receptor, and by collagen (5 μg mL−1), a potent ligand that activates αIIbβ3 with IC50 values (concentration inhibiting platelet aggregation by 50%) of 10.9 and 17.0 μM, respectively. Moreover, surfactin C significantly suppressed the intracellular Ca2+ mobilization in thrombin-activated platelets. Surfactin C, however, did not affect various integrin-mediated U937 cell aggregation, implying that the anti-platelet activity of surfactin C was not due to its detergent effect but by its action on the downstream signalling pathway. Therefore, the results suggest that surfactin C may have a beneficial therapeutic effect on aberrant platelet aggregation-mediated cardiovascular diseases.

Lowered albumin extravasation rate in heart but not in other organs in beta3-integrin-deficient mice

AIM

The vascular protein permeability is dependent on the integrity of the vascular wall. The heart capillaries in male mice lacking beta3 integrins have an immature phenotype. Previously, we have demonstrated a role for alphavbeta3 integrins in control of interstitial fluid pressure (Pif) and thereby in the fluid flux during inflammation. We wanted to explore a possible role for alphavbeta3 integrins in controlling capillary protein permeability during control situation and inflammation.

METHODS

We performed double-tracer and microdialysis experiments on beta3-integrin-deficient mice and wild type control mice. We also measured blood pressure and heart rate in the two mice strains.

RESULTS

We found reduced albumin extravasation (during 25 min) in the heart capillaries (0.053 +/- 0.003 vs. 0.087 +/- 0.009 mL g(-1) dw, P < 0.05), and an increased cardiac mass/body weight (5.3 x 10(-3) +/- 0.3 x 10(-3) vs. 3.8 x 10(-3) +/- 0.1 x 10(-3), P < 0.01) in the beta3-integrin-deficient mice (n = 6) compared with the controls (n = 6). Heart rate and blood pressure were the same in mice with and without beta3-integrins. No difference in permeability was found in other tissues studied, or under local inflammation.

CONCLUSION

These results show a function for the alphavbeta3 integrin in the regulation of protein permeability, selective for the heart capillaries.

Intensity of platelet beta(3) integrin in patients with hemorrhagic fever with renal syndrome and its correlation with disease severity

beta(3) Integrin has been identified as a cellular receptor for Hantaan virus, which causes hemorrhagic fever with renal syndrome (HFRS). To investigate the relationship between intensity of the platelet membrane beta(3) integrin (CD61) and disease severity, the percentage of CD61-positive platelets and the mean fluorescence intensities (MFI) of platelet CD61 were determined in patients with HFRS by flow cytometry. The intensity levels of CD61 in patients with HFRS were significantly higher than those in the controls and correlated with the clinical phases of the disease. The CD61 intensity at the oliguric phase was inversely correlated with platelet count and serum albumin, and positively correlated with white blood cell count, blood urea nitrogen, serum creatinine, and alanine aminotransferase levels. The results suggest that the intensity levels of platelet CD61 were elevated and associated with clinical phases and disease severity in patients with HFRS, and the intensity of platelet beta(3) integrin in patients with HFRS may be indicative of disease severity.

Thrombin mediates mitogenesis and survival of human endothelial cells through distinct mechanisms

Thrombin has been reported to play a pivotal role in the initiation of angiogenesis by indirectly regulating and organizing a network of angiogenic molecules. In addition, it has been proposed that thrombin can directly activate endothelial cell proliferation. However, in this report it was shown that thrombin is a poor growth factor for human endothelial cells, and its modest mitogenic activity is mediated indirectly by the release of heparin-binding epidermal growth factor, subsequent to proteinase-activated receptor 1 (PAR1) activation. On the other hand, it was demonstrated that thrombin is a potent anti-apoptotic factor for endothelial cells, pointing to a novel role of thrombin in vascular protection. Analysis by annexin V-propidium iodide double staining revealed that thrombin, specifically, promoted survival of serum-starved endothelial cells in a concentration-dependent manner. In contrast to its mitogenic effect, the anti-apoptotic effect of thrombin was largely independent of its catalytic activity and was mediated through interaction with alphanubeta3 and alpha5beta1 integrins, whereas the involvement of PAR1 was limited. These results provide new insights in understanding the role of thrombin in endothelial cell signaling and vascular biology.

The mechanism of anti-platelet activity of davallialactone: involvement of intracellular calcium ions, extracellular signal-regulated kinase 2 and p38 mitogen-activated protein kinase

This study was designed to investigate the effect of davallialactone, which was isolated from the mushroom Inonotus xeranticus, on platelet aggregation induced by collagen, thrombin and ADP. We found that davallialactone dose-dependently inhibited platelet aggregation that was stimulated either by collagen (2.5 microg/ml), a potent ligand of integrin alpha2beta1 and glycoprotein VI, or by thrombin (0.1U/ml), a potent agonist of the protease-activated receptors (PARs) PAR1 and PAR3. In addition, davallialactone inhibited platelet aggregation induced by ADP, an agonist of P2Y receptor. To understand the mechanism of anti-platelet activity, we determined whether davallialactone affected the downstream signaling in collagen-activated platelets. Using the fura-2/AM fluorometric assay, we found that davallialactone dose-dependently inhibited intracellular calcium concentration levels ([Ca2+]i). Moreover, davallialactone inhibited the phosphorylation of extracellular signal-regulated protein kinase (ERK)-2 and p38 mitogen-activated protein kinase (MAPK), in a dose-dependent manner. The tyrosine phosphorylation of 60 and 85kDa proteins, which were activated by collagen, were differentially inhibited by davallialactone. Taken together, these data suggest that davallialactone may have potential anti-platelet aggregation activity via suppression of intracellular downstream signaling pathways.

Osteoclast size heterogeneity in rat long bones is associated with differences in adhesive ligand specificity

Prothrombin (PT) is an RGD-containing bone-residing precursor to the serine protease thrombin (TH), which acts as an agonist for a variety of cellular responses in osteoblasts and osteoclasts. We show here that PT, TH, osteopontin (OPN) and fibronectin (FN) promoted adhesion of isolated neonatal rat long bone osteoclasts. However, the cells that adhered to PT and TH were smaller in size, rounded and contained 3-4 nuclei, in comparison to the cells adhering to OPN and FN, which were larger with extended cytoplasmic processes and 6-7 nuclei. Attachment of the larger osteoclasts to OPN and FN was inhibited by antibodies towards beta 3 and beta 1 integrin subunits, respectively. Whereas an RGD-containing peptide inhibited adhesion of the smaller osteoclasts to PT and TH, this was not seen with the beta 3 or beta 1 antibodies. In contrast, the beta 1 antibody augmented osteoclast adhesion to PT and TH in an RGD-dependent manner. Small osteoclasts were less efficient in resorbing mineralized bovine bone slices, as well as expressed lower mRNA levels of MMP-9 and the cathepsins K and L compared to large osteoclasts. The small osteoclast adhering to PT and TH may represent either an immature, less functional precursor to the large osteoclast or alternatively constitute a distinct osteoclast population with a specific role in bone.

Oxidized high-density lipoprotein inhibits platelet activation and aggregation via scavenger receptor BI

Numerous studies have reported the presence of oxidatively modified high-density lipoprotein (OxHDL) within the intima of atheromatous plaques as well as in plasma; however, its role in the pathogenesis of thrombotic disease is not established. We now report that OxHDL, but not native HDL, is a potent inhibitor of platelet activation and aggregation induced by physiologic agonists. This antithrombotic effect was concentration and time dependent and positively correlated with the degree of lipoprotein oxidation. Oxidized lipoproteins are known ligands for scavenger receptors type B, CD36 and scavenger receptor B type I (SR-BI), both of which are expressed on platelets. Studies using murine CD36(-/-) or SR-BI(-/-) platelets demonstrated that the antithrombotic activity of OxHDL depends on platelet SR-BI but not CD36. Binding to SR-BI was required since preincubation of human and murine platelets with anti-SR-BI blocking antibody abrogated the inhibitory effect of OxHDL. Agonist-induced aggregation of platelets from endothelial nitric oxide synthase (eNOS)(-/-), Akt-1(-/-), and Akt-2(-/-) mice was inhibited by OxHDL to the same degree as platelets from wild-type (WT) mice, indicating that the OxHDL effect is mediated by a pathway different from the eNOS/Akt pathway. These novel findings suggest that contrary to the prothrombotic activity of oxidized low-density lipoprotein (OxLDL), HDL upon oxidation acquires antithrombotic activity that depends on platelet SR-BI.

The expression patterns of RGS transcripts in platelets

Regulators of G protein signalling (RGS) are involved in the negative regulation of cell activation processes and are involved in the pathophysiology of cardiovascular diseases. To get some further evidence for a role of RGS proteins in platelets, we determined the expression profile of RGS-specific mRNA in rat platelets using reverse transcription-polymerase chain reaction (RT-PCR) with a poly dT18 primer and transcript-specific primers. We found that RGS2, RGS3, RGS5, RGS6, RGS10, RGS14, RGS16 and RGS18, Leukemia-associated Rho-GEF factor (LARG), and Galpha interacting protein (GAIP) were differentially expressed in platelets. The highest expression rate was found for RGS18 (about 1.3 fold when compared to GAPDH), followed by LARG, RGS6, RGS10 and RGS16 (0.7 to 0.95), whereas expression rates for RGS2, RGS3, RGS5, RGS14, and GAIP were in a range of 0.1 to 0.3. Our results suggest that G-protein-coupled receptor-mediated signalling in platelet may be regulated mainly by RGS 18, 16, 10, 6, and LARG.

RGD-dependent binding of procathepsin X to integrin alphavbeta3 mediates cell-adhesive properties

Secreted lysosomal cysteine proteases (cathepsins) are involved in degradation and remodeling of the extracellular matrix, thus contributing to cell adhesion and migration. Among the eleven human lysosomal cysteine proteases, only procathepsin X contains an RGD motif located in a highly exposed region of the propeptide, which may allow binding of the proenzyme to RGD-recognizing integrins. Here, we have tested procathepsin X for cell-adhesive properties and found that it supports integrin alpha(v)beta(3)-dependent attachment and spreading of human umbilical vein endothelial cells. Using site-directed mutants of procathepsin X, we proved that this effect is mediated by the RGD sequence within the proregion of the protease. Endogenous procathepsin X is transported to the plasma membrane, accumulates in vesicles at lamellipodia of the human umbilical vein endothelial cell, and is partly associated with the cell surface, as shown by immunofluorescence. In addition, procathepsin X is partly co-localized with integrin beta(3), as detected by immunogold electron microscopy. A direct interaction between endogenous procathepsin X and alpha(v)beta(3) was demonstrated by co-immunoprecipitation. Moreover, surface plasmon resonance analysis revealed significant and RGD-dependent binding of procathepsin X to integrin alpha(v)beta(3). Our results provide for the first time evidence that the extracellular function of cathepsin X may include binding to integrins thereby modulating the attachment of migrating cells to ECM components.

Lysophospholipids control integrin-dependent adhesion in splenic B cells through G(i) and G(12)/G(13) family G-proteins but not through G(q)/G(11)

Integrin-mediated adhesion is a crucial step in lymphocyte extravasation and homing. We show here that not only the chemokines CXCL12 and CXCL13 but also the lysophospholipids sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) enhance adhesion of murine follicular and marginal zone B cells to ICAM-1 in vitro. This process involves clustering of integrin LFA-1 and is blocked by pertussis toxin, suggesting that G(i) family G-proteins are involved. In addition, lysophospholipid-induced adhesion on ICAM-1 depends on Rho and Rhokinase, indicative of an involvement of G(12)/G(13), possibly also G(q)/G(11) family G-proteins. We used G(12)/G(13)- or G(q)/G(11)-deficient B cells to study the role of these G-protein families in lysophospholipid-induced adhesion and found that the pro-adhesive effects of LPA and S1P are completely abrogated in G(12)/G(13)-deficient marginal zone B cells, reduced in G(12)/G(13)-deficient follicular B cells, and normal in G(q)/G(11)-deficient B cells. We also show that loss of lysophospholipid-induced adhesion results in disinhibition of migration in response to the follicular chemokine CXCL13, which might contribute to the abnormal localization of splenic B cell populations observed in B cell-specific G(12)/G(13)-deficient mice in vivo. Taken together, this study shows that lysophospholipids regulate integrin-mediated adhesion of splenic B cells to ICAM-1 through G(i) and G(12)/G(13) family G-proteins but not through G(q)/G(11).

Cell vesiculation and immunopathology: implications in cerebral malaria

Microparticles are plasma membrane fragments that are generated and released under physiological conditions. They are also released when tissue and/or systemic homeostasis is disrupted. These microparticles display different physiological features of the cells from which they originate. They are detected in some pathological conditions, but rarely suspected of participating in the disease's pathogenesis. In the present review, we summarise data about the production of the microparticles, their biological significance and potential role during microorganism-driven processes, especially in cerebral malaria.

Identification of the thrombin light chain a as the single best mass for differentiation of gastric cancer patients from individuals with dyspepsia by proteome analysis

Gastric cancer mortality is second only to lung cancer, and its prognosis is dismal. Using surface-enhanced laser desorption/ionization-time-of-flight mass spectrometry, we previously identified a single best mass, which could separate gastric cancer from patients without cancer, with a sensitivity of 89.9% and a specificity of 90%. Using protein liquid chromatography systems with various chromatography media and MS/MS analysis, we were able to identify thrombin light chain A, a proteolytic fragment of prothrombin, as the single best mass for early detection of gastric cancer patients. These findings indicate that disturbances in the coagulation-system are early events in gastric cancer biology and that a decrease or loss of thrombin light chain A, which we termed negative serum protein profiling, may contribute to the diagnosis of cancer patients.

Thrombin functions through its RGD sequence in a non-canonical conformation

Previous studies have suggested that thrombin interacts with integrins in endothelial cells through its RGD (Arg-187, Gly-188, Asp-189) sequence. All existing crystal structures of thrombin show that most of this sequence is buried under the 220-loop and therefore interaction via RGD implies either partial unfolding of the enzyme or its proteolytic digestion. Here, we demonstrate that surface-absorbed thrombin promotes attachment and migration of endothelial cells through interaction with alpha(v)beta(3) and alpha(5)beta(1) integrins. Using site-directed mutants of thrombin we prove that this effect is mediated by the RGD sequence and does not require catalytic activity. The effect is abrogated when residues of the RGD sequence are mutated to Ala and is not observed with proteases like trypsin and tissue-type plasminogen activator, unless the RGD sequence is introduced at position 187-189. The potent inhibitor hirudin does not abrogate the effect, suggesting that thrombin functions through its RGD sequence in a non-canonical conformation. A 1.9-Angstroms resolution crystal structure of free thrombin grown in the presence of high salt (400 mm KCl) shows two molecules in the asymmetric unit, one of which assumes an unprecedented conformation with the autolysis loop shifted 20 Angstroms away from its canonical position, the 220-loop entirely disordered, and the RGD sequence exposed to the solvent.

Cellular microparticles: a disseminated storage pool of bioactive vascular effectors

PURPOSE OF REVIEW

Microparticles (MP) or microvesicles are fragments shed from the plasma membrane of stimulated or apoptotic cells. Having long been considered inert debris reflecting cellular activation or damage, MP are now acknowledged as cellular effectors involved in cell-cell crosstalk. This review focuses on procoagulant MP circulating in the vascular compartment, their role in hemostasis and thrombosis, and possible impact in vascular functions.

RECENT FINDINGS

Microparticles can be viewed as a "storage pool" by themselves, disseminating blood-borne tissue factor activity and procoagulant phospholipids. Increasing evidences of integrated loops involving dynamic exchanges and transfer events through multiple MP-cell interactions are summarized.

SUMMARY

Microparticles can be considered true targets in the pharmacological control of thrombosis. Another challenging issue is to take advantage of their procoagulant potential for the management of hemophilia.