Hormone-like activity of human thrombin

A controlled nucleation and formation rate of self-assembled peptide nanofibers

Self-assembling peptide matrixes are powerful platforms for encouraging tissue regeneration, but are usually formed within seconds and remain relatively static in both structure and function throughout their application. For the first time, we have shown that it is possible to extend the time it takes for peptide self-assembly so as to allow for the dynamic building of a self-assembled system over days, in the presence of an enzyme. Specifically, K₅ and K₁₀ sequences were conjugated, via a thrombin-specific cleavage domain NleTPR/SFL, to prevent the nanofiber formation and form stable nanoparticles composed of (RADA)₄-GG-NleTPR/SFL-K₅ and (RADA)₄-GG-NleTPR/SFL-K₁₀ that act as nucleation sites for reassembling. Upon introduction of thrombin, a model enzyme, this system showed an extremely slow rate of nanofiber formation in a parallel direction that is in sharp contrast to the well-known rapid assembly of (RADA)₄ systems with random networks. These bioresponsive materials may provide a novel platform for utilizing long-term enzymatic profiles to form new nanofibers within an existing matrix over long therapeutic timeframes.

Apixaban decreases brain thrombin activity in a male mouse model of acute ischemic stroke

Factor Xa (FXa) plays a critical role in the coagulation cascade by generation of thrombin. During focal ischemia thrombin levels increase in the brain tissue and cause neural damage. This study examined the hypothesis that administration of the FXa inhibitor, apixaban, following focal ischemic stroke may have therapeutic potential by decreasing brain thrombin activity and infarct volume. Male mice were divided into a treated groups that received different doses of apixaban (2, 20, 100 mg/kg administered I.P.) or saline (controls) immediately after blocking the middle cerebral artery (MCA). Thrombin activity was measured by a fluorescence assay on fresh coronal slices taken from the mice brains 24 hr following the MCA occlusion. Infarct volume was assessed using triphenyltetrazolium chloride staining. A high dose of apixaban (100 mg/kg) significantly decreased thrombin activity levels in the ipsilateral hemisphere compared to the control group (Slice#5, p = .016; Slice#6, p = .016; Slice#7, p = .016; Slice#8, p = .036; by the nonparametric Mann-Whitney test). In addition, treatment with apixaban doses of both 100 mg/kg (32 ± 8% vs. 76 ± 7% in the treatment vs. control groups respectively; p = .005 by the nonparametric Mann-Whitney test) and 20 mg/kg (43 ± 7% vs. 76 ± 7% in the treatment vs. control groups respectively; p = .019 by the nonparametric Mann-Whitney test) decreased infarct volumes in areas surrounding the ischemic core (Slices #3 and #8). No brain hemorrhages were observed either in the treated or control groups. In summary, I.P. administration of high dose of apixaban immediately after MCA occlusion decreases brain thrombin activity and reduces infarct size.

A Linear Temporal Increase in Thrombin Activity and Loss of Its Receptor in Mouse Brain following Ischemic Stroke

BACKGROUND

Brain thrombin activity is increased following acute ischemic stroke and may play a pathogenic role through the protease-activated receptor 1 (PAR1). In order to better assess these factors, we obtained a novel detailed temporal and spatial profile of thrombin activity in a mouse model of permanent middle cerebral artery occlusion (pMCAo).

METHODS

Thrombin activity was measured by fluorescence spectroscopy on coronal slices taken from the ipsilateral and contralateral hemispheres 2, 5, and 24 h following pMCAo (n = 5, 6, 5 mice, respectively). Its spatial distribution was determined by punch samples taken from the ischemic core and penumbra and further confirmed using an enzyme histochemistry technique (n = 4). Levels of PAR1 were determined using western blot.

RESULTS

Two hours following pMCAo, thrombin activity in the stroke core was already significantly higher than the contralateral area (11 ± 5 vs. 2 ± 1 mU/ml). At 5 and 24 h, thrombin activity continued to rise linearly (r = 0.998, p = 0.001) and to expand in the ischemic hemisphere beyond the ischemic core reaching deleterious levels of 271 ± 117 and 123 ± 14 mU/ml (mean ± SEM) in the basal ganglia and ischemic cortex, respectively. The peak elevation of thrombin activity in the ischemic core that was confirmed by fluorescence histochemistry was in good correlation with the infarcts areas. PAR1 levels in the ischemic core decreased as stroke progressed and thrombin activity increased.

CONCLUSION

In conclusion, there is a time- and space-related increase in brain thrombin activity in acute ischemic stroke that is closely related to the progression of brain damage. These results may be useful in the development of therapeutic strategies for ischemic stroke that involve the thrombin-PAR1 pathway in order to prevent secondary thrombin related brain damage.

A novel histochemical method for the visualization of thrombin activity in the nervous system

Although thrombin has an important role in both central and peripheral nerve diseases, characterization of the anatomical distribution of its proteolytic activity has been limited by available methods. This study presents the development, challenges, validation and implementation of a novel histochemical method for visualization of thrombin activity in the nervous system. The method is based on the cleavage of the substrate, Boc-Asp(OBzl)-Pro-Arg-4MβNA by thrombin to liberate free 4-methoxy-2-naphthylamine (4MβNA). In the presence of 5-nitrosalicylaldehyde, free 4MβNA is captured, yielding an insoluble yellow fluorescent precipitate which marks the site of thrombin activity. The sensitivity of the method was determined in vitro using known concentrations of thrombin while the specificity was verified using a highly specific thrombin inhibitor. Using this method we determined the spatial distribution of thrombin activity in mouse brain following transient middle cerebral artery occlusion (tMCAo) and in mouse sciatic nerve following crush injury. Fluorescence microscopy revealed well-defined thrombin activity localized to the right ischemic hemisphere in cortical areas and in the striatum compared to negligible thrombin activity contralaterally. The histochemical localization of thrombin activity following tMCAo was in good correlation with the infarct areas per triphenyltetrazolium chloride staining and to thrombin activity measured biochemically in tissue punches (85 ± 35 and 20 ± 3 mU/ml, in the cortical and striatum areas respectively, compared to 7 ± 2 and 13 ± 2 mU/ml, in the corresponding contralateral areas; mean ± SEM; p<0.05). In addition, 24 h following crush injury, focal areas of highly elevated thrombin activity were detected in teased sciatic fibers. This observation was supported by the biochemical assay and western blot technique. The histochemical method developed in this study can serve as an important tool for studying the role of thrombin in physiological and pathological conditions.

Increased thrombin activity following reperfusion after ischemic stroke alters synaptic transmission in the hippocampus

Thrombin, a key player in thrombogenesis, affects cells in the brain through activation of its receptors. Low levels of thrombin activity are protective while high levels are toxic. We sought to quantify thrombin activity levels and their spatial distribution in brains of mice following reperfusion after ischemic stroke focusing on infarct, peri-infarct and contralateral areas. In order to find out the contribution of brain-derived thrombin, mRNA levels of both prothrombin and factor X were determined. Furthermore, we assessed the effect of thrombin levels that were measured in the ischemic brain on synaptic transmission. We found that in the brains of mice following transient middle cerebral artery occlusion, thrombin activity is elevated throughout the ischemic hemisphere, including in peri-infarct areas (90 ± 33 and 60 ± 18 mU/mL, in the infarct and peri-infarct areas, respectively, compared to 11 ± 3 and 12 ± 5 mU/mL, in the corresponding contralateral areas; mean ± SE; p < 0.05). Brain mRNA levels of prothrombin and, in particular, factor X are up-regulated in the ischemic core. Hippocampal slices treated with thrombin concentrations as found in the ischemic hemisphere show altered synaptic responses. We conclude that high thrombin activity following reperfusion after ischemic stroke may cause synaptic dysfunction. Following transient middle cerebral artery occlusion in mice, thrombin activity is elevated throughout the ischemic hemisphere, including in peri-infarct areas. Brain mRNA levels of prothrombin and factor X are up-regulated in the ischemic core. Thrombin is known to affect synaptic function in a concentration dependent manner and hippocampal slices treated with the concentrations found in the ischemic hemisphere show altered synaptic responses. We conclude that in ischemic stroke, the high brain thrombin activity found after reperfusion may cause synaptic dysfunction.

Quantitative detection of thrombin activity in an ischemic stroke model

Thrombin, a central factor in thrombogenesis, affects cells in the brain through protease activated receptors. Low levels of thrombin activity are neuroprotective while higher levels are deleterious, and we have therefore developed a new method for its direct quantitative measurement in brain slices following stroke. Thrombin activity was measured by a fluorescent substrate on fresh coronal slices taken from the ipsilateral and contralateral hemispheres 24-72 h following permanent right middle cerebral artery occlusion. Prolyl endopeptidase and aminopeptidases were inhibited as a critical step to insure the specificity of the assay for thrombin detection. Infarct volume was assessed using TTC staining. Thrombin activity in the right ischemic hemisphere was significantly higher compared to the contralateral hemisphere (32 ± 6 and 27 ± 10 mU/ml, mean ± SE in the two most affected slices from the ischemic hemisphere vs. 21 ± 6 and 8 ± 2 mU/ml in corresponding contralateral slices; p < 0.05). Thrombin levels in the ischemic and contralateral hemispheres were significantly higher compared to healthy control mice and were above the range known to be protective to brain cells. A significant correlation was found between thrombin activity in the ischemic hemisphere and the infarct volume. Results of studies based on this method may translate into potential thrombin based therapies.

Thrombomodulin and its role in inflammation

The goal is to provide an extensive review of the physiologic role of thrombomodulin (TM) in maintaining vascular homeostasis, with a focus on its anti-inflammatory properties. Data were collected from published research. TM is a transmembrane glycoprotein expressed on the surface of all vascular endothelial cells. Expression of TM is tightly regulated to maintain homeostasis and to ensure a rapid and localized hemostatic and inflammatory response to injury. By virtue of its strategic location, its multidomain structure and complex interactions with thrombin, protein C (PC), thrombin activatable fibrinolysis inhibitor (TAFI), complement components, the Lewis Y antigen, and the cytokine HMGB1, TM exhibits a range of physiologically important anti-inflammatory, anti-coagulant, and anti-fibrinolytic properties. TM is an essential cofactor that impacts on multiple biologic processes. Alterations in expression of TM and its partner proteins may be manifest by inflammatory and thrombotic disorders. Administration of soluble forms of TM holds promise as effective therapies for inflammatory diseases, and infections and malignancies that are complicated by disseminated intravascular coagulation.

Single-pair fluorescence resonance energy transfer (spFRET) for the high sensitivity analysis of low-abundance proteins using aptamers as molecular recognition elements

We have developed a strategy for the detection of single protein molecules, which uses single-pair fluorescence resonance energy transfer (spFRET) as the readout modality and provides exquisite analytical sensitivity and reduced assay turn-around-time by eliminating various sample pre-processing steps. The single-protein detection assay uses two independent aptamer recognition events to form an assembly conducive to intramolecular hybridization of oligonucleotide complements that are tethered to the aptamers. This hybridization brings a donor-acceptor pair within the Förster distance to create a fluorescence signature indicative of the presence of the protein-aptamer(s) association complex. As an example of spFRET, we demonstrate the technique for the analysis of serum thrombin. The assay requires co-association of two distinct epitope-binding aptamers, each of which is labeled with a donor or acceptor fluorescent dye (Cy3 or Cy5, respectively) to produce a FRET response. The FRET response between Cy3 and Cy5 was monitored by single-molecule photon-burst detection, which provides high analytical sensitivity when the number of single-molecule events is plotted versus the target concentration. We are able to identify thrombin with high efficiency based on photon burst events transduced in the Cy5 detection channel. We also demonstrate that the technique can discriminate thrombin molecules from its analogue prothrombin. The analytical sensitivity was >200-fold better than an ensemble measurement.

Proteinase-mediated signaling: proteinase-activated receptors (PARs) and much more

Quite apart from their ability to generate active polypeptides from hormone precursors and to function as digestive enzymes, proteinases are now known to play a hormone-like role by triggering signal transduction pathways in target cells. The best understood example of proteinase-mediated signaling can be seen in the action of thrombin, which in addition to triggering the coagulation cascade, regulates platelet and endothelial cell function via its serine proteinase activity. The discovery of the G-protein-coupled 'receptor' responsible for these cellular actions of thrombin (Proteinase-activated Receptor-1, or PAR(1)) represents one of the more intriguing signal transduction stories elucidated over past decade or so. It is the objective of this brief review to provide an overview of the discovery and molecular pharmacology of the PAR family and to indicate the widespread roles these receptor systems can play in a variety of tissues. Further, the article (1) illustrates the utility of employing receptor-selective PAR-activating peptides to determine the potential physiological roles these receptors play in vivo and (2) describes how these agonists have identified receptors other than the PARs. Finally, the mechanisms other than via the PARs by which proteinases can generate cellular signals are summarized.

PAR1-dependent and independent increases in COX-2 and PGE2 in human colonic myofibroblasts stimulated by thrombin

Subepithelial myofibroblast-derived prostaglandin E(2) (PGE(2)) regulates epithelial chloride secretion in the intestine. Thrombin is elevated in inflammatory conditions of the bowel. Therefore, we sought to determine a role for thrombin in regulating PGE(2) synthesis by colonic myofibroblasts. Incubation of cultured CCD-18Co colonic myofibroblasts with thrombin, the proteinase-activated receptor 1 (PAR(1))-activating peptide (Cit-NH(2)), and peptides corresponding to 2 noncatalytic regions of thrombin (TP367 and TP508) for 18 h increased both cyclooxygenase (COX)-2 expression (immunocytochemistry) and PGE(2) synthesis (enzyme immunoassay). Inhibition of thrombin by D-Phe-Pro-Arg-chloromethylketone (PPACK) did not significantly reduce PGE(2) synthesis, which remained elevated compared with control. We also investigated the basic fibroblast growth factor (bFGF) dependence of thrombin-induced PGE(2) elevations. Recombinant human bFGF concentration dependently increased PGE(2) synthesis, and a bFGF neutralizing antibody inhibited PGE(2) synthesis induced by TP367 and TP508 (approximately 40%) and by thrombin (approximately 20%) (but not Cit-NH(2)). Thrombin, therefore, upregulates COX-2-derived PGE(2) synthesis by both catalytic cleavage of PAR(1) and bFGF-dependent noncatalytic activity. This presents a novel mechanism by which intestinal myofibroblasts might regulate epithelial chloride secretion.

Up-regulation of proteinase-activated receptor 1 expression in astrocytes during HIV encephalitis

Proteinase-activated receptor 1 (PAR-1) is a G protein-coupled receptor that is activated by thrombin and is implicated in the pathogenesis of inflammation. Although PAR-1 is expressed on immunocompetent cells within the brain such as astrocytes, little is known about its role in the pathogenesis of inflammatory brain diseases. Herein, we investigated PAR-1 regulation of brain inflammation by stimulating human astrocytic cells with thrombin or the selective PAR-1-activating peptide. Activated cells expressed significantly increased levels of IL-1 beta, inducible NO synthase, and PAR-1 mRNA. Moreover, supernatants of these same cells were neurotoxic, which was inhibited by an N-methyl-D-aspartate receptor antagonist. Striatal implantation of the PAR-1-activating peptide significantly induced brain inflammation and neurobehavioral deficits in mice compared with mice implanted with the control peptide or saline. Since HIV-related neurological disease is predicated on brain inflammation and neuronal injury, the expression of PAR-1 in HIV encephalitis (HIVE) was investigated. Immunohistochemical analysis revealed that PAR-1 and (pro)-thrombin protein expression was low in control brains, but intense immunoreactivity was observed on astrocytes in HIVE brains. Similarly, PAR-1 and thrombin mRNA levels were significantly increased in HIVE brains compared with control and multiple sclerosis brains. These data indicated that activation and up-regulation of PAR-1 probably contribute to brain inflammation and neuronal damage during HIV-1 infection, thus providing new therapeutic targets for the treatment of HIV-related neurodegeneration.

Thrombin promotes fibroblast proliferation during the early stages of experimental radiation pneumonitis

Huang, L., Ogushi, F., Tani, K., Ogawa, H., Kawano, T., Endo, T., Izumi, K., Ueno, J., Nishitani, H. and Sone, S. Thrombin Promotes Fibroblast Proliferation during the Early Stages of Experimental Radiation Pneumonitis. Radiat. Res. 156, 45-52 (2001). To clarify the role of thrombin in the pathogenesis of radiation-induced pneumonitis, we measured the thrombin activity and fibroblast growth-inducing activity in bronchoalveolar lavage fluid obtained from the irradiated lungs of rats at 1, 2, 4, 8 and 18 weeks after irradiation. Thrombin activity was not detected in the bronchoalveolar lavage fluid from unirradiated rats, but the bronchoalveolar lavage fluid from irradiated rats showed significantly increased thrombin activity which reached a maximum at 4 weeks after treatment. Higher fibroblast growth-inducing activity was detected in the bronchoalveolar lavage fluid from irradiated rats at 4 and 18 weeks than in fluid from unirradiated rats. Bronchoalveolar lavage fluid from irradiated rats that were pretreated with the thrombin inhibitors antithrombin III and argatroban showed significantly inhibited fibroblast growth-inducing activity and thrombin activity at 4 weeks. However, these thrombin inhibitors did not inhibit fibroblast growth-inducing activity in bronchoalveolar lavage fluid from irradiated rats at 18 weeks. Purified rat thrombin similarly induced proliferation of fibroblasts derived from irradiated and unirradiated rats. These findings suggest that thrombin may play an important role as a fibroblast growth-inducing factor during the early stages of radiation pneumonitis.

Thrombin stimulates fibroblast procollagen production via proteolytic activation of protease-activated receptor 1

Thrombin is a multifunctional serine protease that has a crucial role in blood coagulation. It is also a potent mesenchymal cell mitogen and chemoattractant and might therefore have an important role in the recruitment and local proliferation of mesenchymal cells at sites of tissue injury. We hypothesized that thrombin might also affect the deposition of connective tissue proteins at these sites by directly stimulating fibroblast procollagen production. To address this hypothesis, the effect of thrombin on procollagen production and gene expression by human foetal lung fibroblasts was assessed over 48 h. Thrombin stimulated procollagen production at concentrations of 1 nM and above, with maximal increases of between 60% and 117% at 10 nM thrombin. These effects of thrombin were, at least in part, due to increased steady-state levels of alpha1(I) procollagen mRNA. They could furthermore be reproduced with thrombin receptor-activating peptides for the protease-activated receptor 1 (PAR-1) and were completely abolished when thrombin was rendered proteolytically inactive with the specific inhibitors d-Phe-Pro-ArgCH2Cl and hirudin, indicating that thrombin is mediating these effects via the proteolytic activation of PAR-1. These results suggest that thrombin might influence the deposition of connective tissue proteins during normal wound healing and the development of tissue fibrosis by stimulating fibroblast procollagen production.

Changes in arterial expression of fibrinolytic system proteins in atherogenesis

Plasminogen activators (PAs) and their inhibitor, plasminogen activator inhibitor type-1 (PAI-1), have been implicated in modulation of luminal fibrinolysis and mural proteolysis contributing to atherogenesis. Expression of PAs/PAI-1 (normalized to extracted tissue protein) was delineated by assays of conditioned media and of extracts from walls of human arterial segments in culture. Arterial specimens (n = 39 from 26 subjects) were divided into four groups: normal (n = 14), fatty streak (n = 6), moderate atherosclerosis (mural thickening with < 70% lumen obstruction, n = 5), and severe atherosclerosis (mural thickening with > 70% lumen obstruction, n = 14). Paired samples from the same individual comprising a normal arterial segment and an atherosclerotic segment were evaluated also. A fourfold molar excess in PAI-1:t-PA was seen in conditioned media from samples with any evidence of atherosclerosis compared with normal specimens (normal 21 +/- 4, diseased 82 +/- 21, P < or = .05). Compared with normal pairs, the tissue content of PAI-1 (ng) was increased in fatty streak lesions (n = 3, normal 35 +/- 12, fatty streak 50 +/- 8, P < or = .05); stable to decreased in moderate atherosclerosis (n = 3, normal 34 +/- 3, moderate 22 +/- 7, P = .16); and increased in severe atherosclerosis (n = 6, normal 48 +/- 9, severe 85 +/- 19, P < or = .05). The tissue content of PAs (ng), though not increased in fatty streak lesions, was elevated in moderately and severely atherosclerotic segments (normal 0.7 +/- 0.2, moderate 1.6 +/- 0.1; normal 0.8 +/- 0.3, severe 2.1 +/- 0.3, P < or = .05 for each comparison). Atherogenesis is associated with decreased luminal fibrinolytic capacity that may exacerbate thrombosis. Decreased mural proteolysis in early atherogenesis may exacerbate matrix accumulation. Increased mural proteolysis later is associated with, and may potentiate, smooth muscle cell migration and proliferation.

Determinants of coronary vascular disease in patients with type II diabetes mellitus and their therapeutic implications

Cardiovascular morbidity and mortality are increased 4- to 6-fold in patients with type II diabetes. The high prevalence is multifactorial and reflects in part the adverse influence of covariate, cardiac risk factors such as hypertension and hyperlipidemia. Type II diabetes is characterized by insulin resistance, hyperinsulinemia, and altered carbohydrate and lipid metabolism resulting in hyperglycemia, increased concentrations in blood of very low-density and low-density lipoproteins, and decreased blood high-density lipoproteins. Abnormalities seen predispose to vasculopathy through lipid deposition into vessel walls associated with monocyte infiltration, vascular smooth muscle cell proliferation, arterial mural fibrosis, and thrombosis. Conventional therapy for cardiovascular disease such as angioplasty and bypass surgery are of only limited efficacy. Thus, retardation of progression of atherosclerosis is essential. In addition to focusing on co-existent cardiac risk factors such as hypertension, therapy for patients with type II diabetes should reduce or reverse insulin resistance, improve metabolic control, and, ideally, do so without exacerbating hyperinsulinemia. Diet and exercise are central, and novel orally active hyperglycemic agents such as the biguanides and the thiazolidinediones that sensitize diverse tissues to insulin offer particular promise.

The involvement of a novel mechanism distinct from the thrombin receptor in the vasocontraction induced by trypsin

1. The vasocontracting effect of a serine protease trypsin and its mechanisms were investigated by monitoring the isometric tension in endothelium-denuded rings of rabbit thoracic aortae and its effects on intracellular free Ca2+ concentrations ([Ca2+]i) in dispersed rabbit vascular smooth muscle cells with a Ca2+ indicator fura-2. The actions of trypsin were compared with those of thrombin. 2. Both thrombin and trypsin reversibly contracted aortic rings without endothelium in a concentration-dependent manner. The vasocontraction induced by trypsin was well correlated with the protease activity of trypsin actually added to the tissue baths containing the aortic rings and was completely blocked by soybean trypsin inhibitor and phenylmethylsulphonyl fluoride (PMSF), a serine protease inhibitor. 3. The trypsin-induced contractions of the aortic rings were not the result of irreversible damage to vascular smooth muscle cells, since the contractile responses induced by noradrenaline or 30 mM KCl were unaffected by pretreatment with trypsin. 4. The contractions induced by either thrombin or trypsin were reduced to about 30% of control responses after removal of extracellular Ca2+, indicating that most of the contraction is dependent on extracellular Ca2+. By contrast, the contractions induced by either of the proteases were reduced by an antagonist of L-type voltage-operated Ca2+ channels, nifedipine, to about 70% of control responses, indicating that both nifedipine-sensitive and -resistant Ca2+ channels are involved in these contractions. 5. In the aortic rings precontracted by a maximally effective concentration of thrombin, the second application of thrombin virtually failed to induce contractions but trypsin could still induce contractions amounting to 10% of control values by it's protease activity. 6. After the first application of a maximal concentration of thrombin, the second application of thrombin could not induce an increase in [Ca2+]i, but an application of trypsin could still induce an increase in [Ca2+]i in dispersed rabbit vascular smooth muscle cells. 7. These data suggest that in addition to activation of a thrombin receptor, trypsin can contract rabbit aortae by a proteinase-activated receptor 2 or a novel mechanism.

The Amino Terminal Lectin-Like Domain of Thrombomodulin Is Required for Constitutive Endocytosis

Thrombomodulin (TM) is a multidomain protein that serves as a cofactor in a major natural anticoagulant system. To further characterize the structure-function of TM, we have transfected COS cells with different truncated forms of TM. In the first form, COS cells expressing TM that lacks the putative signal peptide (17 residues); the lectin-like, hydrophobic N-terminal domain (226 residues); and 12 residues of the first epidermal growth factor (EGF )-like repeat (COSdel.238 cells) were found to function normally with respect to TM transport to the cell surface and thrombin-dependent protein C activation. However, in contrast to wild-type TM, as visually studied by immunofluorescence and immunogold electron microscopy, the COSdel.238 cells did not constitutively internalize anti-TM–TM or thrombin-TM complexes. To identify the region responsible for mediating the endocytic process, deletant forms of TM lacking either the lectin-like region (residues 2-155) or the hydrophobic region of the N-terminal domain (residues 161-202) were expressed in COS cells (COSdel.2-155 and COSdel.161-202, respectively). Protein C cofactor activity was maintained in both cells. Although the COSdel.161-202 cells behaved similarly to wild-type TM-transfected cells, visual studies showed a lack of constitutive internalization of thrombin-TM or anti-TM–TM complexes in the COSdel.2-155 cells. We conclude that the lectin-like domain of human TM serves to regulate cell surface expression of TM via the endocytic route and therefore may also play a major physiologic role in controlling intracellular and extracellular accumulation of thrombin in a variety of biologic systems.

Synergistic actions of a thrombin-derived synthetic peptide and a thrombin receptor-activating peptide in stimulating fibroblast mitogenesis

We measured the ability of the thrombin receptor activating peptide, SFLLR-NH2 (P5A) to stimulate 3H-thymidine incorporation in hamster CCL-39 fibroblasts either alone or in combination with the thrombin-derived polypeptides, YPPWNKNFTENDLL (TDP-1) and AGYKPDEGKRGDACEGDSGGPFV (TDP-2). In the presence (but not absence) of the amino peptidase inhibitor amastatin (10 microM), P5A alone (7.5 to 100 microM) caused a 1.5- to 2-fold stimulation of thymidine incorporation above basal, even though this inhibitor did not abrogate the degradation of P5A by other peptidases present in the assay medium. Neither TDP-1 nor TDP-2 alone had any effect on thymidine incorporation. However, TDP-1 (30 to 90 microM) considerably augmented P5A-mediated thymidine incorporation at low P5A concentrations (7.5 to 30 microM), shifting the P5A concentration-effect curve to the left. TDP-2 was inactive in this regard. The EC50 for this potentiating action of TDP-1 was approximately 40 microM. Further, thrombin, rendered proteolytically inactive by a low-molecular-weight bifunctional inhibitor, hirutonin-6, also acted synergistically with P5A to stimulate CCL-39 cell thymidine incorporation. We hypothesize that thrombin can cause its cellular effects, such as thymidine incorporation, not only via the proteolytic activation of its G-protein-coupled receptor, but also via the concurrent and synergistic interaction of its TDP-1 peptide domain with a separate cell surface docking site.

Elaboration of type-1 plasminogen activator inhibitor from adipocytes. A potential pathogenetic link between obesity and cardiovascular disease

BACKGROUND

Obesity is known to predispose to attenuated fibrinolysis attributable to increased concentrations in plasma of type-1 plasminogen activator inhibitor (PAI-1), the primary physiological inhibitor of endogenous fibrinolysis. PAI-1 is present in neointimal vascular smooth muscle cells and lipid-laden macrophages.

METHODS AND RESULTS

The present study was designed to determine whether PAI-1 expression occurs in adipose tissue as well, thereby potentially contributing to increased cardiovascular risk associated with obesity. 3T3-L1 preadipocytes were differentiated into adipocytes by exposing them to isobutylxanthine (0.5 mmol/L) and dexamethasone (0.25 mumol/L) over 7 days and incubated for 24 hours with transforming growth factor-beta (TGF-beta), known to augment PAI-1 synthesis in several cell types and to be released from platelets when they are activated. TGF-beta increased PAI-1 activity in the conditioned media of the 3T3-L1-derived cells in a concentration-dependent fashion without significantly affecting cell proliferation. Western blotting and immunoprecipitation of 35S-labeled PAI-1 showed that the increased PAI-1 activity paralleled increased PAI-1 protein. Northern blotting showed that increased PAI-1 mRNA preceded increased accumulation of PAI-1 activity and protein in the conditioned media. Furthermore, TGF-beta (10 ng/g body wt) administered in vivo increased PAI-1 activity in mouse plasma and PAI-1 mRNA expression in mouse adipose tissue.

CONCLUSIONS

Increased plasma PAI-1 activity in obese human subjects may result from PAI-1 release from an increased mass of adipose tissue, particularly in association with thrombosis and elaboration of TGF-beta from platelet alpha-granules into the circulation. The increased PAI-1 may exacerbate vascular disease by shifting the balance between thrombosis and thrombolysis toward thrombosis and consequently exposing luminal surfaces of vessels to mitogens associated with microthrombi over protracted intervals.

Thrombin regulates expression of monocyte chemoattractant protein-1 in vascular smooth muscle cells

Thrombin, a serine protease generated at sites of vascular injury, plays a role in the pathogenesis of atherosclerosis and restenosis after angioplasty. Adherence of monocytes to the endothelium and migration into the subendothelial space is an important early event in the pathogenesis of atherosclerosis. Monocyte chemoattractant protein 1 (MCP-1) may be an important mediator of monocyte recruitment to the tissue in this and other diseases. We have characterized the expression of MCP-1 in vascular smooth muscle cells (VSMCs) isolated from human renal artery and studied its regulation by thrombin. Serum-deprived cells release monocyte chemotactic activity that is neutralized (80%) by an MCP-1 antibody. The antibody recognized a 13- and 15-kD protein in smooth muscle cell-conditioned medium. Thrombin stimulates MCP-1 gene expression in a concentration- and time-dependent manner. An increase over basal levels was observed with concentrations of thrombin as low as 0.05 U/mL. The maximal effect occurred at 5 U/mL. The stimulatory effect was detected within 1 hour, reached a maximum at 3 hours, and was still present at 8 to 24 hours after the addition of thrombin. A concentration- and time-dependent effect of thrombin on MCP-1 gene expression was also found in rat VSMCs. The thrombin protease inhibitor hirudin blocked thrombin-induced MCP-1 expression. Thrombin stimulated the release of MCP-1 protein in conditioned medium of human VSMCs as measured by radioimmunoassay and chemotactic assay. Thrombin also increased monocyte chemotactic activity in short-term organ cultures of rat aortic rings and in first passage cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct thrombin inhibitors in cardiovascular medicine

Currently used antithrombotics such as heparin have a number of potential limitations that may be overcome by the new class of agents that directly inhibit thrombin. These agents variously block the active catalytic and/or the anion binding exosites of the thrombin molecule and are potent and specific inhibitors of thrombin's many biological actions, as demonstrated by in vitro and animal models of thrombosis. Preliminary data indicate that the direct antithrombins are safe and efficacious in humans, and their use in acute coronary syndromes and coronary angioplasty in place of heparin has yielded promising early results. Phase III trials in these clinical settings are currently under way. Newer antithrombotics that inhibit thrombin generation and thrombin activity at various strategic points within the coagulation cascade are also in the early stages of development.

Thrombin stimulates inositol phosphate formation, intracellular calcium fluxes and DNA synthesis in cultured fetal human non-pigmented ciliary epithelial cells

Thrombin at concentrations as low as 20 pM (0.002 U ml-1) was found to stimulate inositol phosphate levels in cultured human non-pigmented ciliary epithelial cells. Several other proteases, including trypsin and plasmin, had little or no effect, of several protease inhibitors tested, only those with specificity for thrombin blocked the effect. Studies with active site-blocked thrombin suggested that the esterolytic active site of thrombin is required for inositol phosphate stimulation, while gamma-thrombin, which has reduced binding affinity to fibrinogen also showed reduced effectiveness in stimulating inositol phosphates. In the presence of 10 mM LiCl, thrombin stimulated inositol monophosphate, inositol bisphosphate and inositol trisphosphate formation, with a prolonged rise of the first and transient early rises in the latter two species. Thrombin also elevated intracellular Ca2+ levels as measured with the fluorescent calcium probe, indo-1-AM. This elevation could be blocked by prior addition to cells of the thrombin inhibitor, hirudin, and was dependent upon extracellular Ca2+ for the maintenance of an elevated level in the presence of thrombin. Incorporation of thymidine into DNA in confluent cultures was also stimulated by thrombin, with a four-fold increase in incorporation at 35 hr in thrombin-treated cells compared to controls. The half-maximal concentration for this process was 0.25 U ml-1. Pretreatment with 100 ng ml-1 pertussis toxin greatly reduced the thrombin effect, which is consistent with a role for a G-protein in stimulation of DNA synthesis by thrombin.

Stimulation of inositol phosphate formation in cultured human retinal pigment epithelium

Several hormones, neurotransmitters, and neuropeptides were screened for the ability to stimulate inositol phosphate formation in cultured human retinal epithelial (RPE) cells. Carbachol, vasopressin and thrombin were found to be effective. Treatment of RPE cells with all three agents produced increases in inositol monophosphate, inositol bisphosphate and inositol trisphosphate in the presence of 10 mM LiCl. Carbachol stimulated a 4-fold increase in the total of inositol phosphates at 1 mM. Studies with cholinergic antagonists showed a rank order of 4 DAMP greater than QNX greater than pirenzepine greater than methoctramine, suggesting the presence of M3 muscarinic receptors. Vasopressin gave a 2.5-fold stimulation at 10 microM. Agonists of vasopressin were also tested and gave differential responses. Studies using a V1 agonist (PIOVP) and a V2 agonist (DAVP) showed DAVP matching the level of stimulation elicited by vasopressin whereas treatment with PIOVP only reached 50% of the vasopressin response. These data suggested the presence of V2 receptors in the RPE cells. Several proteases were tested for their ability to stimulate RPE inositol phosphates. Thrombin caused a 7-fold increase in inositol phosphate formation at 1 U/ml, whereas trypsin and plasmin elicited smaller responses (approximately 2-fold). The thrombin effect was blocked by the thrombin-specific inhibitor, hirudin, but not by other protease inhibitors. Several mediators of inflammation such as bradykinin, histamine and serotonin were also tested, and they were ineffective in stimulating inositol phosphate turnover in the RPE cells.

A comparison of the growth promoting properties of ascitic fluids, cyst fluids and peritoneal fluids from patients with ovarian tumours

The growth promoting properties of ascitic fluids, cyst fluids and peritoneal fluids from patients with ovarian malignancy, benign ovarian tumours and non-tumour related gynaecological conditions have been investigated using an ovarian carcinoma cell line (OAW 42), mesothelial cells (58MC) and rat kidney cells (NRK-49F). Colony stimulating activity (CSA) for tumour cells and transforming activity (TA) for mesothelial cells were weakly correlated, but whereas elevated TA was tumour-associated, CSA was not. However, TA was not cancer-associated and, although the difference between the mean TA values of benign and malignant cyst fluids was of borderline significance, some benign cyst fluids from cystadenomas showed high TA values. Higher levels of TA in the cystadenomas showed a significant correlation with the menopausal status of the patient and higher levels of TA in the malignant cyst fluid/peritoneal fluid groups were associated with more advanced disease. Results indicated that some fluids contained TGF-beta-like activity, but there was no direct evidence for the presence of TGF-alpha/EGF-like activity in the fluids. Heparin inhibited clonogenic growth of tumour cells but not mesothelial cells. The reduced CSA which was observed after treatment of fluids with both heparin and thrombin implicated coagulation factors in the manifestation of CSA. It was concluded that CSA in the fluids was due, at least partly, to fibrin coagulation, and TA was due to unknown growth factor(s) which may include TGF-beta-like activity. The results are discussed in the context of the aetiology of ovarian carcinoma, and the possible clinical significance of TA.

Localization of the structural domain responsible for the chemotactic properties of thrombin on polymorphonuclear leukocytes

Human alpha thrombin at 1.1.10(-5) M is chemotactic for human polymorphonuclear leukocytes. This thrombin property disappears when the alpha thrombin (1.1.10(-5) M) hirudin (1.32.10(-5) M) mixture is realized. The same result is obtained when the thrombin at 1.1. 10(-5) M is inhibited by antithrombin III in a ratio of 1 mol of thrombin for 4.5 mol of antithrombin III. The hirudin and the antithrombin III appear therefore to mask, by their binding the structural domain responsible for the chemotactic properties of thrombin on polymorphonuclear leukocytes.

Coagulation impact on chemotactic activity generation for polymorphonuclear leukocytes

Chemotactic technique in agarose gel has exposed the attractive properties of human alpha thrombin with respect to human polymorphonuclear leukocytes. The observed chemotaxis is maximal between 1.4.10-5 M and 1.6.10-5 M but extends from 2.8.10-6 M to 2.2.10-5 M. Human prothrombin, in an identical concentration zone as that studied for thrombin shows no chemotactic activity on the polymorphonuclear leukocytes. During coagulation the formed alpha thrombin attracts the polymorphonuclear leukocytes to it's formation site.