Localization of a chemotactic domain in human thrombin

Protease-activated receptors in health and disease

Proteases are signaling molecules that specifically control cellular functions by cleaving protease-activated receptors (PARs). The four known PARs are members of the large family of G protein-coupled receptors. These transmembrane receptors control most physiological and pathological processes and are the target of a large proportion of therapeutic drugs. Signaling proteases include enzymes from the circulation; from immune, inflammatory epithelial, and cancer cells; as well as from commensal and pathogenic bacteria. Advances in our understanding of the structure and function of PARs provide insights into how diverse proteases activate these receptors to regulate physiological and pathological processes in most tissues and organ systems. The realization that proteases and PARs are key mediators of disease, coupled with advances in understanding the atomic level structure of PARs and their mechanisms of signaling in subcellular microdomains, has spurred the development of antagonists, some of which have advanced to the clinic. Herein we review the discovery, structure, and function of this receptor system, highlight the contribution of PARs to homeostatic control, and discuss the potential of PAR antagonists for the treatment of major diseases.

Modeling thrombin generation: plasma composition based approach

Thrombin has multiple functions in blood coagulation and its regulation is central to maintaining the balance between hemorrhage and thrombosis. Empirical and computational methods that capture thrombin generation can provide advancements to current clinical screening of the hemostatic balance at the level of the individual. In any individual, procoagulant and anticoagulant factor levels together act to generate a unique coagulation phenotype (net balance) that is reflective of the sum of its developmental, environmental, genetic, nutritional and pharmacological influences. Defining such thrombin phenotypes may provide a means to track disease progression pre-crisis. In this review we briefly describe thrombin function, methods for assessing thrombin dynamics as a phenotypic marker, computationally derived thrombin phenotypes versus determined clinical phenotypes, the boundaries of normal range thrombin generation using plasma composition based approaches and the feasibility of these approaches for predicting risk.

Protease-activated receptors as therapeutic targets in visceral pain

The protease-activated receptors (PARs) play a pivotal role in inflammatory and nociceptive processes. PARs have raised considerable interest because of their capacity to regulate numerous aspects of viscera physiology and pathophysiology. The present article summarizes research on PARs and proteases as signalling molecules in visceral pain. In particular, experiments in animal models suggest that PAR2 is important for visceral hypersensitivity. Moreover, endogenous PAR2 agonists seem to be released by colonic tissue of patients suffering from irritable bowel syndrome, suggesting a role for this receptor in visceral pain perception. Thus, PARs, together with proteases that activate them, represent exciting targets for therapeutic intervention on visceral pain.

Innate immunity, coagulation and placenta-related adverse pregnancy outcomes

Maternal immunity undergoes subtle adjustment in order to tolerate the semi-allogeneic embryo and maintain the host defense against potential pathogens. Concomitantly, coagulation systems change from an anti-coagulant state to a pro-coagulant state to meet the hemostatic challenge of placentation and delivery. Innate immunity and blood coagulation systems are the first line of defense to protect a host against exogenous challenges, including alloantigens and mechanical insults, and preserve the integrity of an organism. The interactions between coagulation and immune systems have been extensively studied. Immune cells play a pivotal role in the initiation of the coagulation cascade, whereas coagulation proteases display substantial immuno-modulatory effects. Upon exogenous challenges, the immune and coagulation systems are capable of potentiating each other leading to a vicious cycle. Natural killer (NK) cells, macrophages (Mphis) and dendritic cells (DCs) are three major innate immune cells that have been demonstrated to play essential roles in early pregnancy. However, immune maladaptation and hemostatic imbalance have been suggested to be responsible for adverse pregnant outcomes, such as preeclampsia (PE), miscarriage, recurrent spontaneous abortion (RSA) and intrauterine growth restriction (IUGR). In this review, we will summarize the mutual regulation between blood coagulation and innate immune systems as well as their roles in the maintenance of normal pregnancy and in the pathogenesis of adverse pregnancy outcomes.

Proteinases as hormones: targets and mechanisms for proteolytic signaling

Proteinases, such as kallikrein-related peptidases, trypsin and thrombin, can play hormone-like 'messenger roles in vivo. They can regulate cell signaling by cleaving and activating a novel family of G-protein-coupled proteinase-activated receptors (PARs 1-4) by unmasking a tethered receptor-triggering ligand. Short synthetic PAR-derived peptide sequences (PAR-APs) can selectively activate PARs 1, 2 and 4, causing physiological responses in vitro and in vivo. Using the PAR-APs to activate the receptors in vivo, it has been found that PARs, like hormone receptors, can affect the vascular, renal, respiratory, gastrointestinal, musculoskeletal and nervous systems (central and peripheral). PARs trigger responses ranging from vasodilatation to intestinal inflammation, increased cytokine production and increased nociception. These PAR-stimulated responses have been implicated in various disease states, including cancer, atherosclerosis, asthma, arthritis, colitis and Alzheimer's disease. In addition to targeting the PARs, proteinases can also cause hormone-like effects by other signaling mechanisms that may be as important as the activation of PARs. Thus, the PARs themselves, their activating serine proteinases and their signaling pathways can be considered as attractive targets for therapeutic drug development. Further, proteinases can be considered as physiologically relevant 'hormone-like' messengers that can convey signals locally or systemically either via PARs or by other mechanisms.

Proteinases and signalling: pathophysiological and therapeutic implications via PARs and more

Proteinases like thrombin, trypsin and tissue kallikreins are now known to regulate cell signaling by cleaving and activating a novel family of G-protein-coupled proteinase-activated receptors (PARs 1-4) via exposure of a tethered receptor-triggering ligand. On their own, short synthetic PAR-selective PAR-activating peptides (PAR-APs) mimicking the tethered ligand sequences can activate PARs 1, 2 and 4 and cause physiological responses both in vitro and in vivo. Using the PAR-APs as sentinel probes in vivo, it has been found that PAR activation can affect the vascular, renal, respiratory, gastrointestinal, musculoskeletal and nervous systems (both central and peripheral nervous system) and can promote cancer metastasis and invasion. In general, responses triggered by PARs 1, 2 and 4 are in keeping with an innate immune inflammatory response, ranging from vasodilatation to intestinal inflammation, increased cytokine production and increased or decreased nociception. Further, PARs have been implicated in a number of disease states, including cancer and inflammation of the cardiovascular, respiratory, musculoskeletal, gastrointestinal and nervous systems. In addition to activating PARs, proteinases can cause hormone-like effects by other signalling mechanisms, like growth factor receptor activation, that may be as important as the activation of PARs. We, therefore, propose that the PARs themselves, their activating serine proteinases and their associated signalling pathways can be considered as attractive targets for therapeutic drug development. Thus, proteinases in general must now be considered as 'hormone-like' messengers that can signal either via PARs or other mechanisms.

Treatment with anti-factor VIIa in acute pancreatitis in rats: blocking both coagulation and inflammation?

OBJECTIVE

Acute pancreatitis starts as an autodigestive process restricted to the pancreas and progresses to a systemic inflammation via cytokine release into the blood stream. Several inhibitors of the coagulation cascade, including active-site-inactivated factor VIIa, have shown anti-inflammatory properties in other inflammatory models than acute pancreatitis. Free radical scavengers have proven useful in reducing the oxidative damage during hyperinflammatory conditions. The aim of this study was to investigate whether pretreatment with FVIIai would have any effect on the multiple organ dysfunction syndrome (MODS) in severe acute pancreatitis.

MATERIAL AND METHODS

Experimental acute pancreatitis was induced by intraductal infusion of taurodeoxycholate in the pancreatic duct. The animals were pretreated with N-acetyl-cysteine and active-site-inactivated factor VIIa. Neutrophil infiltration in the lungs, ileum and colon was quantified by myeloperoxidase activity. Inflammatory markers, IL-6 and MIP-2, were measured using ELISA.

RESULTS

Tissue infiltration of neutrophils in the lungs, ileum and colon significantly increased during acute pancreatitis as compared to sham operation. These levels were reduced by pretreatment with N-acetylcysteine and active-site-inactivated factor VIIa. Levels of interleukin-6 and macrophage inflammatory protein-2 increased significantly during acute pancreatitis. Pretreatment with NAC and FVIIai reduced these levels.

CONCLUSIONS

Both N-acetylcysteine and active-site-inactivated factor VIIa showed powerful anti-inflammatory properties in experimental acute pancreatitis. As they exert their effects through different physiological mechanisms, they represent potential candidates for future multimodal treatment of acute pancreatitis.

Proteinase-mediated cell signalling: targeting proteinase-activated receptors (PARs) by kallikreins and more

Serine proteinases, like trypsin, can play a hormone-like role by triggering signal transduction pathways in target cells. In many respects these hormone-like actions of proteinases can now be understood in terms of the pharmacodynamics of the G protein-coupled ‘receptor’ responsible for the cellular actions of thrombin (proteinase-activated receptor-1, or PAR1). PAR1, like the other three members of this receptor family (PAR2, PAR3and PAR4), has a unique mechanism of activation involving the proteolytic unmasking of an N-terminally tethered sequence that can activate the receptor. The selective activation of each PAR by short synthetic peptides representing these sequences has demonstrated that PAR1, PAR2and PAR4play important roles in regulating physiological responses ranging from vasoregulation and cell growth to inflammation and nociception. We hypothesise that the tissue kallikreins may regulate signal transduction via the PARs. Although PARs can account for many of their biological actions, kallikreins may also cause effects by mechanisms not involving the PARs. For instance, trypsin activates the insulin receptor and thrombin can act via a mechanism involving its non-catalytic domains. Based on the data we summarise, we propose that the kallikreins, like thrombin and trypsin, must now be considered as important ‘hormonal’ regulators of tissue function.

Physiology and Pathophysiology of Proteinase-Activated Receptors (PARs): Proteinases as Hormone-Like Signal Messengers: PARs and More

Proteinases like thrombin and trypsin, long known for their ability to activate the coagulation cascade or to act as digestive enzymes for many protein targets, are now recognized as hormone-like regulators of cell function. These serine proteinases activate cell signaling by triggering a novel family of G-protein-coupled receptors, termed proteinase-activated receptors (PARs). This article summarizes the unique mechanisms involved in PAR activation and outlines the many different settings in which the PARs act to regulate tissue function. The PARs can be seen to play a role in inflammatory processes in large part via a neurogenic mechanism. Apart from activating PARs to cause their physiological effects in tissues, proteinases can also mediate cell signaling via a number of other mechanisms, including the activation of growth factor receptors, like the one for insulin. Thus, this article also points out the non-PAR mechanisms whereby proteinases can have hormone-like actions in cells and tissues.

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.

Inflammatory response to cardiopulmonary bypass

Inflammation in cardiac surgical patients is produced by complex humoral and cellular interactions with numerous pathways including activation, generation, or expression of thrombin, complement, cytokines, neutrophils, adhesion molecules, mast cells, and multiple inflammatory mediators. Because of the redundancy of the inflammatory cascades, profound amplification occurs to produce multiorgan system dysfunction that can manifest as coagulopathy, respiratory failure, myocardial dysfunction, renal insufficiency, and neurocognitive defects. Coagulation and inflammation are also closely linked through networks of both humoral and cellular components including proteases of the clotting and fibrinolytic cascades, including tissue factor. Vascular endothelial cells also mediate inflammation and the cross talk between coagulation and inflammation. Novel antiinflammatory agents inhibit these processes by several mechanisms such as preventing proteolysis of the protease-activated receptor (aprotinin), inhibiting complement-mediated injury (pexelizumab), or inhibiting contact activation (kallikrein inhibitors). Surgery alone also activates specific hemostatic responses, activation of immune mechanisms, and inflammatory response mediated by the release of various cytokines and chemokines. Novel agents are under investigation to further improve outcomes in cardiac surgical patients.

Monocyte adhesion and transmigration induce tissue factor expression: role of the mitogen-activated protein kinases

The expression of tissue factor (TF) by monocytes that have transmigrated across the endothelium to sites of extravascular inflammation acts both to focus and amplify the inflammatory response. Because clustering of the integrins responsible for endothelial adhesion and transmigration induces tyrosine phosphorylation and activation of the mitogen-activated protein (MAP) kinases, we postulated that transmigration might lead to monocyte activation and TF production. Monocytes were migrated across TNFalpha-primed ECV304 cells grown on fibronectin-coated Transwell chambers in response to FMLP (10(-8) M). After transmigration, monocytes showed a time-dependent increase in surface TF expression and biological procoagulant activity. TF expression was dependent on monocyte adhesion to ECV304 cells. Specifically, TF was not induced by FMLP treatment of suspended monocytes, by migration across fibronectin alone, or by soluble factors induced during migration, whereas monocyte-ECV304 adhesion was sufficient to stimulate TF. Antibodies against CD29 (beta1 integrin), but not against CD18 (beta2 integrin) or CD31 (PECAM-1), inhibited TF expression. Monocyte adhesion to ECV304 cells induced tyrosine phosphorylation of cellular proteins and specifically of the ERK and p38 MAP kinases. Tyrosine kinase inhibition with genistein (10 microg/mL) blocked transmigration, whereas selective ERK inhibition with PD98059 (50 microM) or p38 inhibition with SB203580 (20 microM) did not. However, both ERK and p38 inhibition dose dependently abolished TF expression. These studies suggest that an extravascular focus of infection or inflammation can promote both intravascular thrombosis and extravascular fibrin deposition during the process of adhesion and transmigration across the endothelial barrier. The selective inhibition of the mitogen-activated protein kinases may offer a novel therapeutic means of modulating this inflammatory sequence.

Proteinase-activated receptor-1 agonists attenuate nociception in response to noxious stimuli

Proteinase-activated receptor-1 (PAR-1) is activated by thrombin and can be selectively activated by synthetic peptides (PAR-1-activating peptide: PAR-1-AP) corresponding to the receptor's tethered ligand. PAR-1 being expressed by afferent neurons, we investigated the effects of PAR-1 agonists on nociceptive responses to mechanical and thermal noxious stimuli. Intraplantar injection of selective PAR-1-AP increased nociceptive threshold and withdrawal latency, leading to mechanical and thermal analgesia, while control peptide had no effect. Intraplantar injection of thrombin also showed analgesic properties in response to mechanical, but not to thermal stimulus. Co-injection of PAR-1-AP with carrageenan significantly reduced carrageenan-induced mechanical and thermal hyperalgesia, while thrombin reduced carrageenan-induced mechanical but not thermal hyperalgesia. The fact that thrombin is not a selective agonist for PAR-1 may explain the different effects of thrombin and PAR-1-AP. These results identified analgesic properties for selective PAR-1 agonists that can modulate nociceptive response to noxious stimuli in normal and inflammatory conditions.

Antithrombin activity of an algal polysaccharide

In an effort to reduce the risks of a possible iatrogenic transmission of bovine spongiform encephalitis (BSE) through the use of bovine-derived medicinal products, we patented in the USA in 1999 a polysaccharide from brown algae, endowed with interesting pharmacological activities: (a) concentration-dependent inhibition of thromboplastin or cephalin-kaolin-induced thrombin generation from platelets, (b) concentration-dependent inhibition of thrombin-induced platelet aggregation, (c) thrombin has hypotensive effect, which was blunted and zeroed by our fucansulfate in a dose-dependent way, (d) when aortae are stimulated with thrombin, they become stickier for polymorphonucleated leukocytes (PMNs); our fucansulfate decreased concentration-dependently, PMNs sticking to autologous rabbit aortae, (e) dose-dependent inhibition of thrombin-induced thrombosis. All the above data suggest that our fucansulfate could be a heparin substitute endowed with antithrombotic and anti-inflammatory activities, devoid or the problems caused to heparin by its animal origin, i.e., possible prion protein contamination.

Pro- and anti-inflammatory actions of thrombin: a distinct role for proteinase-activated receptor-1 (PAR1)

1. Thrombin has well characterized pro-inflammatory actions that have recently been suggested to occur via activation of its receptor, proteinase-activated receptor-1 (PAR1). 2. In the present study, we have compared the effects of thrombin to those of two peptides that selectively activate the PAR1 receptor, in a rat hindpaw oedema model. We have also examined whether or not thrombin can exert anti-inflammatory activity in this model. 3. Both thrombin and the two PAR1 activating peptides induced significant oedema in the rat hindpaw following subplantar injection. 4. The oedema induced by thrombin was abolished by pre-incubation with hirudin, and was markedly reduced in rats in which mast cells were depleted through treatment with compound 48/80 and in rats pretreated with indomethacin. In contrast, administration of the PAR1 activating peptides produced an oedema response that was not reduced by indomethacin and was only slightly reduced in rats pretreated with compound 48/80. 5. Co-administration of thrombin together with a PAR1 activating receptor resulted in a significantly smaller oedema response than that seen with the PAR1 activating peptide alone. This anti-inflammatory effect of thrombin was abolished by pre-incubation with hirudin. 6. These results demonstrate that the pro-inflammatory effects of thrombin occur through a mast-cell dependent mechanism that is, at least in part, independent of activation of the PAR1 receptor. Moreover, thrombin is able to exert anti-inflammatory effects that are also unrelated to the activation of PAR1.

Hirunorms are true hirudin mimetics. The crystal structure of human alpha-thrombin-hirunorm V complex

A novel class of synthetic, multisite-directed thrombin inhibitors, known as hirunorms, has been described recently. These compounds were designed to mimic the binding mode of hirudin, and they have been proven to be very strong and selective thrombin inhibitors. Here we report the crystal structure of the complex formed by human alpha-thrombin and hirunorm V, a 26-residue polypeptide containing non-natural amino acids, determined at 2.1 A resolution and refined to an R-factor of 0.176. The structure reveals that the inhibitor binding mode is distinctive of a true hirudin mimetic, and it highlights the molecular basis of the high inhibitory potency (Ki is in the picomolar range) and the strong selectivity of hirunorm V. Hirunorm V interacts through the N-terminal tetrapeptide with the thrombin active site in a nonsubstrate mode; at the same time, this inhibitor specifically binds through the C-terminal segment to the fibrinogen recognition exosite. The backbone of the N-terminal tetrapeptide Chg1"-Val2"-2-Nal3"-Thr4" (Chg, cyclohexyl-glycine; 2-Nal, beta-(2-naphthyl)-alanine) forms a short beta-strand parallel to thrombin main-chain residues Ser214-Gly219. The Chg1" side chain fills the S2 subsite, Val2" is located at the entrance of S1, whereas 2-Nal3" side chain occupies the aryl-binding site. Such backbone orientation is very close to that observed for the N-terminal residues of hirudin, and it is similar to that of the synthetic retro-binding peptide BMS-183507, but it is opposite to the proposed binding mode of fibrinogen and of small synthetic substrates. Hirunorm V C-terminal segment binds to the fibrinogen recognition exosite, similarly to what observed for hirudin C-termninal tail and related compounds. The linker polypeptide segment connecting hirunorm V N-and C-terminal regions is not observable in the electron density maps. The crystallographic analysis proves the correctness of the design and it provides a compelling proof on the interaction mechanism for this novel class of high potency multisite-directed synthetic thrombin inhibitors.

Comparison of the structures of the cyclotheonamide A complexes of human alpha-thrombin and bovine beta-trypsin

Thrombin, a trypsin-like serine protease present in blood, plays a central role in the regulation of thrombosis and hemostasis. A cyclic pentapeptide, cyclotheonamide A (CtA), isolated from sponges of the genus Theonella, inhibits thrombin, trypsin, and certain other serine proteases. Enzyme inhibition data for CtA indicate that it is a moderate inhibitor of alpha-thrombin (K(i) = 1.0 nM), but substantially more potent toward trypsin (K(i) = 0.2 nM). The comparative study of the crystal structures of the CtA complexes of alpha-thrombin and beta-trypsin reported here focuses on structure-function relationships in general and the enhanced specificity of trypsin, in particular. The crystal structures of the CtA complexes of thrombin and trypsin were solved and refined at 1.7 and 2.0 A resolution, respectively. The structures show that CtA occupies the active site with the Pro-Arg motif positioned in the S2 and S1 binding sites. The alpha-keto group of CtA is involved in a tetrahedral intermediate hemiketal structure with Ser 195 OG of the catalytic triad and is positioned within bonding distance from, and orthogonal to, the re-face of the carbonyl of the arginine of CtA. As in other productive binding modes of serine proteases, the Ser 214-Gly 216 segment runs in a twisted antiparallel beta-strand manner with respect to the diaminopropionic acid (Dpr)-Arg segment of CtA. The Tyr 60A-Thr 60I insertion loop of thrombin makes a weak aromatic stacking interaction with the v-Tyr of CtA through Trp 60D. The Glu 39 Tyr and Leu 41 Phe substitutions in trypsin produce an enhanced aromatic interaction with D-Phe of CtA, which also leads to different orientations of the side chains of D-Phe and the v-Tyr. The comparison of the CtA complexes of thrombin and trypsin shows that the gross structural features of both in the active site region are the same, whereas the differences observed are mainly due to minor insertions and substitutions. In trypsin, the substitution of Ile 174-Arg 175 by Gly 174-Gln 175 makes the S3 aryl site more polar because the Arg 175 side chain is directed away from thrombin and into the solvent, whereas Gln 175 is not. Because the site is occupied by the Dpr group of CtA, the occupancy of the S3 site is better in trypsin than in thrombin. In trypsin, the D-Phe side chain of CtA fits between Tyr 39 and Phe 41 in a favorable manner, whereas in thrombin, these residues are Glu 39 and Leu 41. The higher degree of specificity for trypsin is most likely the result of these substitutions and the absence of the fairly rigid Tyr 60A-Thr 60I insertion loop of thrombin, which narrows access to the active site and forces less favorable orientations for the D-Phe and v-Tyr residues.

Plasminogen activator inhibitor-1 and vitronectin promote the cellular clearance of thrombin by low density lipoprotein receptor-related proteins 1 and 2

Thrombin is a multifunctional protein that has both proteinase and growth factor-like activities. Its regulation is largely mediated by interaction with a host of inhibitors including antithrombin III (ATIII), heparin cofactor II (HCII), alpha2-macroglobulin (alpha2-M), protease nexin I, and plasminogen activator inhibitor-1 (PAI-1). ATIII, HCII, and alpha2-M are all abundant in blood and can inactivate blood-borne thrombin leading to rapid hepatic clearance of the thrombin-inhibitor complex. PAI-1 alone, a poor solution phase inhibitor of thrombin, can efficiently inhibit thrombin in the presence of native vitronectin (VN). In this study, active thrombin was found to be efficiently endocytosed and degraded by cultured pre-type II pneumocyte cells, and both processes could be blocked by polyclonal antibodies to PAI-1. When the relative efficiency of cellular endocytosis of thrombin in complex with a number of inhibitors was examined, 125I-thrombin-PAI-1 complexes were most efficiently cleared compared to 125I-thrombin in complex with the serpins ATIII, HCII, alpha1-proteinase inhibitor, or d-phenylalanyl-l-prolyl-l-arginine chloromethyl ketone. Low density lipoprotein receptor-related proteins 1 (LRP) and 2 (gp330/megalin) mediate the endocytosis of thrombin-PAI-1, since antagonists of receptor function such as LRP-1 and LRP-2 antibodies and the 39-kDa receptor-associated protein blocked 125I-thrombin-PAI-1 endocytosis and degradation. The LRP-mediated clearance of exogenously added 125I-thrombin by cultured cells was found to be enhanced 5-fold by inclusion of wild-type PAI-1 but by only 2-fold when a mutant form of PAI-1 that is unable to bind VN was included. This wild-type PAI-1 enhancement of 125I-thrombin clearance was found to occur only in the presence of native VN and not with its conformationally altered form. The results highlight a novel mechanism for cellular clearance of thrombin involving native VN promoting the interaction of thrombin and PAI-1 and the subsequent endocytosis of the complex by LRP-1 or LRP-2. This pathway is potentially important for the regulation of the potent biological activities of thrombin, particularly at sites of vascular injury.

The response to thrombin of human neutrophils: evidence for two novel receptors

Human alpha-thrombin was a chemoattractant for human neutrophils yielding a maximal response of similar magnitude to that observed with formyl-Met-Leu-Phe. The observed chemotaxis was not due to stimulation of the proteolytically activated thrombin receptor since: (1) this receptor was not detected by flow cytometry; (2) the inactive thrombin mutant Ser195-->Ala elicited a chemotactic response indistinguishable from that caused by wild-type thrombin; (3) antibodies to the cleavage site of the proteolytically activated receptor did not affect thrombin-induced chemotaxis; (4) a thrombin receptor activating peptide (TRAP) failed to stimulate chemotaxis. These data indicate the existence of a thrombin receptor for neutrophil chemotaxis which is not activated by proteolysis. In addition, although wild-type and ser195-->Ala thrombin did not cause an increase in intracellular Ca2+, a Ca2+ response to TRAP was observed with neutrophils from some donors. The TRAP-induced increase in Ca2+ was reproducible, dose dependent and specific. The use of alanine-substituted peptides demonstrated that the Ca2+ response was due to TRAP stimulation of a receptor other than the proteolytically activated thrombin receptor. Thus, it is necessary to re-evaluate the assumption made in previous studies that responses to TRAP are mediated by the proteolytically activated thrombin receptor.

Thrombin-thrombomodulin interaction: energetics and potential role of water as an allosteric effector

The interaction of rabbit lung thrombomodulin (TM) and C-terminal hirudin 54-65 fragment (Hir54-65) with human alpha-thrombin were investigated by exploiting their competitive inhibition of thrombin-fibrinogen interaction. Measurements of Ki values for TM and Hir54-65 interactions with human alpha-thrombin performed over a temperature range spanning from 10 to 40 degrees C showed a constant enthalpy for both ligands. The enthalpic and entropic contributions to the free energy of binding, however, are different for TM and the hirudin peptide. The calculated values of delta H and delta S, in fact, were -47.3 +/- 2.51 kJ (-11.3 +/- 0.6 kcal)/mol and -42.7 +/- 7.9 J (-10.2 +/- 1.9 cal)/mol.K for the hirudin peptide, while being -22.9 +/- 2.09 kJ (-5.47 +/- 0.5 kcal)/mol and 102.50 +/- 6.69 J (24.5 +/- 1.6 cal)/mol.K respectively for TM binding. These findings indicate that the interaction between thrombin and Hir54-65 is largely driven by the enthalpic contribution, whereas the positive entropy change is the driving force for the formation of the thrombin-TM complex. In other experiments performed in the presence of various concentrations of either sorbitol or sucrose it could be demonstrated that the value of the equilibrium association constant for thrombin-TM interaction increases as a function of the osmotic pressure, while the thrombin-Hir54-65 interaction was not affected by the same conditions. Moreover, control experiments showed that no major conformational changes are produced on TM by osmotic pressures used in the present study. From these experiments it was calculated that roughly 35 water molecules are released into the bulk water upon TM binding. Such a phenomenon, which is likely to be responsible for the entropic change described above, indicates the relevance of hydration processes for the formation of the thrombin-TM adduct.

Regulation of bone metabolism by the kallikrein-kinin system, the coagulation cascade, and the acute-phase reactants

Inflammation-induced localized bone resorption in diseases such as marginal and apical periodontitis, rheumatoid arthritis, and osteomyelitis is due to activation and recruitment of osteoclasts by locally produced cytokines and inflammatory mediators. Thus several interleukins (1, 3, 4, 6, and 11), tumor necrosis factors (alpha, beta), colony-stimulating factors (M and GM), leukemia inhibitory factor, gamma-interferon, and transforming growth factor-beta have effects on bone resorption and bone formation in vivo and in vitro. The kallikrein-kinin system and the coagulation cascade are also activated in inflammation. We have found that peptides produced in the kallikrein-kinin system (bradykinin, kallidin) and thrombin, the end product in the coagulation cascade, can stimulate bone resorption in vitro. The stimulatory effect of bradykinin is linked both to B1 and B2 bradykinin receptors. Both kinins and thrombin stimulate prostaglandin biosynthesis in bone parallel with the bone resorptive effect. The stimulatory effect of bradykinin on bone resorption is completely lost when the prostaglandin response is abolished, whereas thrombin can stimulate bone resorption both via prostaglandin-dependent and independent mechanisms. In addition, bradykinin and thrombin act in concert with interleukin-1 to synergistically stimulate bone resorption and prostaglandin biosynthesis. We also have found that one of the acute-phase reactants, haptoglobin, can stimulate bone resorption in vitro, indicating the possibility of generalized bone loss in chronic inflammatory diseases. Moreover, haptoglobin synergistically potentiates bradykinin-induced and thrombin-induced prostanoid biosynthesis in osteoblasts. These observations indicate that the rate of bone resorption in inflammation-induced bone loss may not be due to a single factor but to the concerted action of several local or systemic factors.

Mouse fibroblasts defective in thrombin mitogenesis possess functional proteolytically activated receptor for thrombin: requirement for a second signaling pathway

Thrombin mitogenesis in fibroblasts requires two distinguishable subsets of signals; one generated by proteolytic cleavage, the other by high-affinity cell surface binding. Characterizing two closely related mouse embryo (ME) cell lines with high numbers of thrombin binding sites, we found that one line, B11-A, responds mitogenically to thrombin, epidermal growth factor (EGF), and serum, whereas the B11-B cell line is responsive to EGF and serum, but not to thrombin. The B11-B defect responsible for loss of thrombin responsiveness is not due to differences in the number of high-affinity binding sites, the affinity of thrombin binding to these sites, or to differences in cell surface expression of proteolytically activated receptors for thrombin (PART). The defect is also not associated with an inability of thrombin to activate PART since thrombin stimulates the cleavage-dependent induction of the proto-oncogene c-fos in both B11-A and B11-B cells. Various combinations of thrombin, synthetic thrombin receptor peptide, TRP-14 (SFFLRNPGENTFEL), platelet-derived growth factor (PDGF), and phorbol 12-myristate 13-acetate (PMA) were used to better define the defect in thrombin-mediated mitogenesis in B11-B cells. Direct activation of protein kinase C with PMA in combination with thrombin did not overcome B11-B nonresponsiveness. However, mitogenic responsiveness was regained in B11-B cells by simultaneous addition of PDGF and either thrombin or TRP-14. Therefore, the B11-B defect may involve a set of signals initiated by nonproteolytic thrombin interactions distinct from those initiated by PART, but related to the downstream signals initiated by the tyrosine kinase-associated growth factors, EGF and PDGF.

A novel aspect of thrombin in the tissue reaction following central nervous system injury

Thrombin and two different types of control solutions (buffer and albumin) were continuously infused into the rat caudoputamen by an osmotic minipump. Routine histological studies with immunohistochemistry using antibodies for BrdU, GFAP, vimentin and laminin were carried out to assess infiltration of inflammatory cells, formation of edema, cell proliferation, and reactivity of astrocytes and mesenchymal cells. The number of inflammatory cells, number of BrdU positive cells, area and number of vimentin positive astrocytes, and the area of GFAP-positive reactive astrocytes were quantitatively analyzed. In the thrombin group, pale tissue foci due to spongiosis were observed together with infiltration of inflammatory cells, proliferation of mesenchymal cells, and increase of vimentin positive astrocytes which was significantly different from the control groups. The results suggest that thrombin plays an important role in inflammation, brain edema and reactive gliosis following CNS injury.

A player of many parts: the spotlight falls on thrombin's structure

The wealth of structural information now available for thrombin, its precursors, its substrates, and its inhibitors allows a rationalization of its many roles. alpha-thrombin is a rather rigid molecule, binding to its target molecules with little conformational change. Comparison of alpha-thrombin with related trypsin-like serine proteinases reveals an unusually deep and narrow active site cleft, formed by loop insertions characteristic of thrombin. This canyon structure is one of the prime causes for the narrow specificity of thrombin. The observed modularity of thrombin allows a diversity in this specificity; its "mix-and-match" nature is exemplified by its interactions with macromolecules (Fig. 20). The apposition of the active site to a hydrophobic pocket (the apolar binding site) on one side and a basic patch (the fibrinogen recognition exosite) on the other allows for a fine tuning of enzymatic activity, as seen for fibrinogen. Thrombin receptor appears to use the same sites, but in a different way. Protein C seems only able to interact with thrombin if the recognition exosite is occupied by thrombomodulin. These two sites are also optimally used by hirudin, allowing the very tight binding observed; thrombin inhibition is effected by blocking access to the active site. On the other hand, antithrombin III makes little use of the recognition exosite; instead, its interactions are tightened with the help of heparin, which binds to a second basic site (the heparin binding site). Thrombin's modularity is a result of the conjunction of amino acid residues of like properties, such as charge or hydrophobicity. The charge distribution plays a role, not only in the binding of oppositely charged moieties of interacting molecules, but also in selection and preorientation of them. Nonproteolytic cellular properties are attributed to 1) the rigid insertion loop at Tyr60A, and 2) a partially inaccessible RGD sequence. The former can interact with cells in the native form; the latter would appear to be presented only in an (at least partially) unfolded state. The membrane binding properties of prothrombin can be understood from the ordered arrangement of calcium ions on binding to the Gla domain. Kringle F2 binds to thrombin at the heparin binding site through charge complementarity; a conformational change appears to occur on binding. The observed rigidity of the thrombin molecule in its complexes makes thrombin ideal for structure based drug design. Thrombin can be inhibited either at the active site or at the fibrinogen recognition exosite, or both.(ABSTRACT TRUNCATED AT 400 WORDS)

The refined 1.9-A X-ray crystal structure of D-Phe-Pro-Arg chloromethylketone-inhibited human alpha-thrombin: structure analysis, overall structure, electrostatic properties, detailed active-site geometry, and structure-function relationships

Thrombin is a multifunctional serine proteinase that plays a key role in coagulation while exhibiting several other key cellular bioregulatory functions. The X-ray crystal structure of human alpha-thrombin was determined in its complex with the specific thrombin inhibitor D-Phe-Pro-Arg chloromethylketone (PPACK) using Patterson search methods and a search model derived from trypsinlike proteinases of known spatial structure (Bode, W., Mayr, I., Baumann, U., Huber, R., Stone, S.R., & Hofsteenge, J., 1989, EMBO J. 8, 3467-3475). The crystallographic refinement of the PPACK-thrombin model has now been completed at an R value of 0.156 (8 to 1.92 A); in particular, the amino- and the carboxy-termini of the thrombin A-chain are now defined and all side-chain atoms localized; only proline 37 was found to be in a cis-peptidyl conformation. The thrombin B-chain exhibits the characteristic polypeptide fold of trypsinlike serine proteinases; 195 residues occupy topologically equivalent positions with residues in bovine trypsin and 190 with those in bovine chymotrypsin with a root-mean-square (r.m.s.) deviation of 0.8 A for their alpha-carbon atoms. Most of the inserted residues constitute novel surface loops. A chymotrypsinogen numbering is suggested for thrombin based on the topological equivalences. The thrombin A-chain is arranged in a boomeranglike shape against the B-chain globule opposite to the active site; it resembles somewhat the propeptide of chymotrypsin(ogen) and is similarly not involved in substrate and inhibitor binding. Thrombin possesses an exceptionally large proportion of charged residues. The negatively and positively charged residues are not distributed uniformly over the whole molecule, but are clustered to form a sandwichlike electrostatic potential; in particular, two extended patches of mainly positively charged residues occur close to the carboxy-terminal B-chain helix (forming the presumed heparin-binding site) and on the surface of loop segment 70-80 (the fibrin[ogen] secondary binding exosite), respectively; the negatively charged residues are more clustered in the ringlike region between both poles, particularly around the active site. Several of the charged residues are involved in salt bridges; most are on the surface, but 10 charged protein groups form completely buried salt bridges and clusters. These electrostatic interactions play a particularly important role in the intrachain stabilization of the A-chain, in the coherence between the A- and the B-chain, and in the surface structure of the fibrin(ogen) secondary binding exosite (loop segment 67-80).(ABSTRACT TRUNCATED AT 400 WORDS)

Partial characterization of vertebrate prothrombin cDNAs: amplification and sequence analysis of the B chain of thrombin from nine different species

The cDNA sequence of the B chain of thrombin (EC 3.4.21.5) has been determined from nine vertebrate species (rat, mouse, rabbit, chicken, gecko, newt, rainbow trout, sturgeon, and hagfish). The amino acid sequence identities vary from 96.5% (rat vs. mouse) to 62.6% (newt vs. hagfish). Of the 240 amino acids spanned in all the species compared, there is identity at 110 (45.8%) positions. When conservative changes are included, the amino acid similarity increases to 75%. The most conserved portions of the B chain are the active-site residues and adjacent amino acids, the B loop, and the primary substrate-binding region. In addition, the Arg-Gly-Asp motif is conserved in 9 of the 11 species compared, and the chemotactic/growth factor domain is well conserved in all of the 11 species compared. The least conserved regions of the B chain correspond to surface loops, including the putative thrombomodulin-binding sites and one of the hirudin-binding regions. The extent of the amino acid sequence similarity and the conservation of many of the functional/structural motifs suggests that, in addition to their role in blood coagulation, vertebrate thrombins may also play an important role in the general mechanisms of wound repair.

Disseminated Intravascular Coagulation: Pathogenesis and Management

Disseminated intravascular coagulation (DIC) is a clini-copathological syndrome secondary to an underlying disease. Characteristic laboratory abnormalities of DIC should suggest, much like the recognition of fever, anemia, or congestive heart failure, that an inciting disease process must be searched for. The clinical and laboratory consequences of DIC can be ascribed to the unregulated and unbalanced formation of thrombin, the main clot-forming enzyme, and plasmin, the main clot-lysing enzyme. If too much plasmin is formed in relation to thrombin, a hemorrhagic state, which appears in 60 to 75% of patients with deep vein thrombosis, will occur. Alternatively, if too much thrombin is formed in relation to the degree of secondary fibrinolysis, a thrombotic condition, which appears in 25 to 40% of patients with DIC, will become manifest. The diagnosis of DIC is dependent on the presence of an appropriate clinical situation with concurrent laboratory evidence of thrombin and plasmin formation. Thrombin formation, plasmin formation, or both, can be assessed by detection of fibrin monomer, fibrin/fibrinogen degradation products, and D-dimer or E fragment, respectively. Treatment of DIC should initially be addressed to treatment of the primary, underlying condition inciting the disorder. If treatment for DIC is specifically needed, blood product replacement is the first order of therapy. This replacement should be tailored to each patient's specific needs (i.e., platelets, fibrinogen, or plasma proteins). Heparin has a definite but limited use in conditions associated with acral cyanosis and dermal ischemia. Other specific therapies for DIC may be of use in individualized situations.

Effectiveness of recombinant desulphatohirudin in reducing restenosis after balloon angioplasty of atherosclerotic femoral arteries in rabbits

BACKGROUND

The effectiveness of balloon angioplasty is limited by a restenosis rate of approximately 30%. Recombinant desulphatohirudin (r-hirudin [CGP 39393]) has been found to be highly effective in preventing acute platelet-rich thrombosis after deep arterial injury as compared with heparin.

METHODS AND RESULTS

This study evaluated the effect of intravenous r-hirudin, a selective inhibitor of thrombin, on restenosis after balloon angioplasty in 29 rabbits. Focal femoral atherosclerosis was induced by air desiccation endothelial injury followed by a 2% cholesterol diet for 1 month. At angioplasty (2.5-mm balloon with three 60-second, 10-atm inflations 60 seconds apart), the rabbits received heparin (150 units/kg bolus, n = 16) or r-hirudin (1 mg/kg bolus followed by infusions of 1 mg/kg for the first hour and 0.5 mg/kg for the second hour, n = 13). Angiograms performed before and after angioplasty and before death were analyzed quantitatively by a blinded observer. Rabbits were killed 2 hours (n = 14) or 28 days (n = 15) after angioplasty. Femoral arteries were fixed in situ by perfusion of 10% formaldehyde at 100 mm Hg. The mean luminal diameter of the arteries with successful angioplasty (greater than or equal to 20% increase in luminal diameter) in rabbits treated with heparin (n = 8 arteries) increased from 1.18 +/- 0.29 mm before angioplasty to 1.86 +/- 0.24 mm immediately after angioplasty (p less than 0.001) and decreased to 0.94 +/- 0.69 mm (p = 0.0004) at 28 days after angioplasty. In rabbits treated with r-hirudin (n = 11 arteries), the mean luminal diameter increased from 1.14 +/- 0.17 mm before angioplasty to 1.68 +/- 0.20 mm immediately after angioplasty (p less than 0.001) and decreased to 1.37 +/- 0.47 mm (p = 0.01) at 28 days after angioplasty. The mean reduction in luminal diameter by angiography was less in the r-hirudin-treated group than in the heparin-treated group (0.30 +/- 0.33 versus 0.92 +/- 0.61 mm, p = 0.01). Blinded planimetric analysis of stained histological sections of the femoral arteries also showed less cross-sectional area narrowing by plaque in rabbits treated with r-hirudin compared with those treated with heparin (22 +/- 16% verus 48 +/- 29%, p = 0.01). Both groups had similar numbers of arteries with histological evidence of balloon-induced plaque tear (12 of 13 versus 13 of 15).

CONCLUSIONS

Rabbits receiving r-hirudin at the time of experimental balloon angioplasty had significantly less restenosis by angiography and by quantitative histopathology than rabbits receiving heparin.

The reaction of thrombin with platelet-derived nexin requires a secondary recognition site in addition to the catalytic site

A protease nexin released by activated platelets forms stable complexes with alpha-thrombin. Active-site-blocked thrombin does not form the stable complex, but it inhibits formation of the stable complex by active alpha-thrombin. gamma-Thrombin, which has a damaged substrate recognition site (the anion-binding exosite), did not form the complex and did not inhibit formation of the stable complex by alpha-thrombin. Complex formation was inhibited by the C-terminal dodecapeptide of hirudin, which has been shown to bind to the anion-binding exosite. A monoclonal antibody that blocks reactions of thrombin that involve the anion-binding exosite also inhibited formation of a stable complex of alpha-thrombin and the platelet-derived protease nexin. It is concluded that the anion-binding exosite of thrombin, a site that confers a high degree of specificity for substrates with a complementary site, binds to the platelet nexin prior to reaction of the catalytic site with the serpin.

Linkage between blood coagulation and inflammation: stimulation of neutrophil tissue kallikrein by thrombin

There has been major interest in the potential interaction between blood coagulation and inflammation. Most of the effort has focused on cellular interactions involving platelets and polymorphonuclear leukocytes (PMNS). The recent discovery of tissue kallikrein(TK) activity in PMNs prompted the study of the possible role of thrombin(IIa) in this process. Human PMNs were isolated by density gradient centrifugation. Human IIa was compared with fMLP with respect to chemotaxis and enzyme release. Results from the challenges by IIa and fMLP were compared to a NaCl control using Student's paired t-test. IIa was a potent chemotactic agent for PMNs (p less than or equal to 0.0121) and stimulated the release of TK (p less than or equal to 0.0001) as determined by hydrolysis of S-2266. FMLP significantly stimulated PMN chemotaxis (p less than or equal to 0.0028) but had no effect on TK release. Release of TK was confirmed by Western Blot analysis and 35S-methionine incorporation into a 35 KD protein after IIa challenge. These results demonstrate that IIa is chemotactic for PMNs and can cause release of tissue kallikrein demonstrating a direct role for blood coagulation in the regulation of the inflammatory response.

A thrombin receptor in resident rat peritoneal macrophages

Resident rat peritoneal macrophages possess 6 x 10(2) high-affinity binding sites per cell for bovine thrombin with a Kd of 11 pM, and 7.5 x 10(4) low-affinity sites with a Kd of 5.8 nM. These binding sites are highly specific for thrombin. Half-maximal binding of 125I-labeled bovine thrombin is achieved after 1 min at 37 degrees C, and after 12 min at 4 degrees C. The reversibly bound fraction of the ligand dissociates according to a biexponential time course with the rate constants 0.27 and 0.06 min-1 at 4 degrees C. Part of the tracer remains cell-associated even after prolonged incubation, but all cell-associated radio-activity migrates as intact thrombin upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The bound thrombin is minimally endocytosed as judged by the resistance to pH 3 treatment, and the receptor does not mediate a quantitatively important degradation of the ligand. The binding is not dependent on the catalytic site of thrombin, since irreversibly inactivated thrombin also binds to the receptor. 125I-labeled thrombin covalently cross-linked to its receptor migrates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a Mr 160,000, corresponding to an approximate receptor size of Mr 120,000.

Mitogenic signals for platelet-derived growth factor isoforms in liver fat-storing cells

Platelet-derived growth factor (PDGF), a key mitogen for liver fat-storing cells (FSC), is a dimeric molecule that occurs as homodimers or heterodimers of related polypeptide chains (PDGF-BB, -AB, and -AA). In chronic inflammation of the liver lobule, any of the three dimeric forms of PDGF derived from multiple sources could potentially interact with FSC. We explored the effects of the three different PDGF isoforms on DNA synthesis and early signal transduction pathways potentially related to PDGF mitogenicity in rat liver FSC. PDGF-BB homodimer and -AB heterodimer induced a marked increase in DNA synthesis, whereas the effect of PDGF-AA homodimer was considerably lower. Moreover, the mitogenicity of each isoform proportionally correlated with their effects on phosphoinositide turnover and intracellular Ca2+. Both the PDGF-BB and -AB dimers likely interact with the PDGF-beta-receptor, although PDGF-AB requires at least one alpha-receptor. The low responsiveness to PDGF-AA could not be accounted for by downregulation of the PDGF-alpha-receptor because FSC expressed very low levels of PDGF-A- and B-chain mRNAs and did not secrete detectable amounts of PDGF activity in the conditioned media. In addition, preincubation of FSC with suramin, a potent inhibitor of PDGF binding to its receptor, failed to increase PDGF-AA-induced DNA synthesis. These results are consistent with a predominant expression of PDGF-beta-receptor in liver FSC, that is linked to phospholipase C activation.

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.

Thrombin chemotactic stimulation of HL-60 cells: studies on thrombin responsiveness as a function of differentiation

Thrombin, a major procoagulant enzyme and growth factor, is also selectively chemotactic for monocytes and macrophages but not for neutrophils. This effect stands in contrast to other well-known chemotactic agents such as fMet-Leu-Phe, C5a fragments, and LTB4, which stimulate directed cell movement in both cell types, and have important physiological implications. The human leukemic cell line HL-60, which is capable of differentiating either along granulocytic or monocytic lineages, was therefore used to explore the development of this selective monocyte/macrophage chemotactic response to thrombin. Esterolytically inactive DIP-alpha-thrombin, as well as the thrombin-derived chemotactic peptide CB67-129, elicits a dose-dependent chemotactic response in HL-60 cells differentiated to monocytelike cells by treatment with 1,25(OH)2D3 (HL-60/mono), whereas no such response is evident in either undifferentiated HL-60 cells or in cells differentiated into granulocytes by treatment with DMSO (HL-60/gran). Similarly, early events which characterize stimulation of inflammatory cells by chemotactic agents are also evident, but only in monocyte-differentiated cells. In HL-60/mono, thrombin selectively stimulates rapid cytosolic Ca2+ elevation as well as rapid cytoskeletal association of cytosolic actin. Following thrombin stimulation, maximal actin association in these cells occurs within 30 sec (declining to basal levels at the end of 5 min), and maximal Ca2+ elevations are also evident within 15-20 sec, suggesting a temporal relationship between these two events. Thus, the events accompanying stimulation of HL-60/mono by thrombin are characteristic of those seen following stimulation of inflammatory cells by chemotaxins, with a major difference being the selectivity of thrombin as a chemotaxin for cells of macrophage/monocytic lineage. The selective chemotactic responsiveness of HL-60/mono to thrombin appears to relate to the development of specific receptors on these cells as part of monocytic differentiation: HL-60/mono (but HL-60/gran nor undifferentiated HL-60) are capable of significant specific 125-I-labeled alpha-thrombin-binding (ka approximately 20 nM), and possess an estimated 400,000 thrombin-binding sites per cell. Our findings further suggest that the thrombin response of HL-60 and particularly the expression of thrombin receptors on these cells may serve as a useful model system for exploring the biology of monocyte/macrophage differentiation.

Thrombin stimulates arachidonate metabolism in murine tumor cells

Thrombin can be formed in the tumor cell microenvironment following activation of the clotting cascade by procoagulants of cancer or host cells. We have tested here the effects of thrombin, either "endogenous" or "exogenous" (see below), on arachidonate mobilization from membrane phospholipids of mouse mammary tumor virus-induced (MMTV) carcinoma cells. These tumor cells exhibit in vitro a tissue type procoagulant activity (130 thromboplastin units/10(4) cells) and are therefore able to induce thrombin formation in a plasmatic milieu. To verify the effect of thrombin formation by tumor cell procoagulant ("endogenous thrombin"), either human or mouse platelet-free plasma (20% in DMEM) was added to the cell layer (prelabelled for 5 hr with a trace amount (0.013 microM) of 3H-arachidonate) and the system was recalcified (15 mM CaCl2). Thin-layer radiochromatography of the culture medium showed a significant release of 3H-labelled arachidonate products PGE2, PGF2 alpha and 6-ketoPGF1 alpha after 1 hr of incubation. To verify the effect of thrombin formation from host sources ("exogenous thrombin"), either bovine or purified human alpha-thrombin (0.1-10 U/ml) was added to the cells for different periods (from 5 min to 20 hr). Exogenous thrombin stimulated arachidonate release and metabolism in a dose-related manner. With short labelling periods (0.013 microM 3H-arachidonate for 30 min-1 hr) thrombin stimulated the release of unmetabolized 3H-arachidonate, but not of 3H-arachidonate metabolites. These processes were inhibited by a specific inhibitor of thrombin enzymatic activity (alpha-NAPAP, 140 microM) and by a cyclo-oxygenase inhibitor (ASA 4mM). Tumor-associated procoagulants may thus contribute not only to fibrin deposition but also to generation of multipotent mediators such as arachidonate metabolites.

Thrombin-induced alterations in endothelial permeability

Figure 15 summarizes the current understanding of mechanisms of endothelial permeability alterations induced with thrombin. If thrombin generation exceeds the antiprotease activity, thrombin results in clotting of fibrinogen and intravascular fibrin accumulation. Pulmonary neutrophil sequestration also occurs after fibrin deposition, and this is related to the degree and duration of fibrin sequestration. Neutrophil activation appears to be an essential requirement for the mediation of the pulmonary vascular injury. Thrombin-induced intravascular coagulation results in the generation of lipid mediators (LTB4 and HETEs), which may be involved in increasing endothelial permeability. The release of thrombin in higher concentrations during lysis of fibrin (sequence; see text) FIGURE 15. Hypothesis showing mechanisms of thrombin-induced increase in endothelial permeability to proteins. Thrombin may have direct effects on endothelial permeability, or thrombin induced fibrinogen clotting, activation of neutrophils, and the release of lipid metabolites that subsequently lead to an increase in endothelial permeability. clots may induce a direct formation of interendothelial "gaps." Therefore, the vascular injury induced by neutrophil activation and the formation of endothelial "gaps" induced directly by thrombin can both increase the endothelial permeability to proteins. Thrombin is an important mediator of increased endothelial permeability to macromolecules, and may participate in the inflammatory response. In this regard, thrombin may be similar to other mediators (such as histamine and serotonin) that have been previously documented to increase macromolecule transport across the endothelium. The implications of free thrombin in increasing endothelial permeability may be greater because thrombin not only has a direct effect on endothelial permeability, but also induces clotting of fibrinogen and the subsequent generation of mediators that activate neutrophils and that in turn can induce endothelial injury.

Hormone-like activity of human thrombin

Recently, we have shown that thrombin is a chemotaxin and growth-promoting agent for cells of the mononuclear phagocytic lineage. These activities are independent of thrombin's enzymatic activity. Unlike other chemotactic factors, thrombin is specific for monocytes and does not attract granulocytes. To further explore the cellular specificity we have used a human leukemia cell line HL-60 that is capable of in vitro differentiation toward either monocytes (HL-60/mono) following incubation with 1,25(OH)2D3, or granulocytes (HL-60/gran) following incubation with DMSO. In contrast to undifferentiated HL-60 cells or HL-60/gran, we find that HL-60/mono respond chemotactically to intact human alpha-thrombin, esterolytically inactive iPR2P-alpha-thrombin, and the thrombin-derived peptide CB67-129, previously shown to contain the thrombin chemotactic exosite. In addition, thrombin induces in HL-60/mono association of actin with the cytoskeleton and causes an increase in levels of free cytosolic Ca2+. These phenomena are well characterized as early events occurring concomitant with directed cell movement associated with exposure to chemotactic agents such as FMLP. Furthermore, in contrast to fibroblasts, both iPR2P-alpha-thrombin and the thrombin chemotactic peptide CB67-129 evoke dose-dependent [3H]TdR incorporation, protein synthesis, and cell replication in growth-arrested J-744 cells, a murine macrophage-like cell line. Limited tryptic digests of CB67-129 lose chemotactic activity but retain full mitogenic activity, demonstrating that as with PDGF, the sites on CB67-129 required for chemotaxis and mitogenesis are clearly dissociable. The mitogenic effects of the CB67-129 digest can be mimicked by a synthetic tetradecapeptide analogue of CB67-129 (residues 367-380) that includes the loop B insertion sequence, previously shown to be critical for thrombin's chemotactic effects. From these data, it is apparent that the loop B insertion is critical for thrombin's nonenzymic biological effects on cells, but additional sites are required for stimulation of cell movement.

Structure-function relationships of thrombin based on the computer-generated three-dimensional model of the B chain of bovine thrombin

The advent of sophisticated computer graphics systems that permit the representation of macromolecular structure has made it possible to examine protein structure in detail. We have used one aspect of this technology to develop a model of thrombin. The model is based on structural and functional similarities this enzyme exhibits with respect to proteins found in the family of serine proteinases. This review has covered interpretations of the structure of the model based on analyses of data that had been collected before and after the model was developed. On one hand, the conceptualization of primary and secondary features in the model of the active site of thrombin has for the most part been preceded by data from experiments on the interaction of thrombin with naturally occurring substrates and inhibitors. The features of the model explain these data adequately. On the other hand, the model has been more recently used in an interactive way to derive information about the bioregulatory aspects of thrombin. The realization that the amino-terminus portion of the cyanogen-bromide fragment was probably not part of the chemotactic activity, because it was probably internalized in the native protein, has suggested that synthetic analogs should focus more on the carboxyterminus of the peptide. It is hoped that in the future the model will continue to serve more in this function and that it can be used to explore further other aspects about the structural and functional relationships of this enzyme.

Thrombin-induced chemotaxis and aggregation of neutrophils

Thrombin-induced neutrophil chemotaxis and aggregation were studied using cells isolated from either human or sheep blood. Sheep neutrophils (10(8) cells/ml) exhibited maximum chemotactic migration towards 10(-8)M human alpha-thrombin, 10(-8)M gamma-thrombin (which lacks the fibrinogen site), and 10(-12)MD-Phe-Pro-Arg-CH2-alpha-thrombin (catalytically inactive thrombin). Chemotactic responses of the same magnitude were obtained with human neutrophils (10(8) cells/ml). The chemotactic responses to thrombin were comparable to those obtained with diluted (1:200 v/v) zymosan activated serum (ZAS) and 10(-11)M FMLP. Premixing of the thrombin forms with hirudin in 1:1 stoichiometric amounts abolished the chemotaxis but not chemokinesis Aggregatory responses of human and sheep neutrophils were comparable for ZAS, alpha-thrombin, and gamma-thrombin. The responses of both human and sheep neutrophils to D-Phe-Pro-Arg-CH2-alpha-thrombin were attenuated, indicating that the proteolytic site may be involved in the aggregatory response. The results suggest that thrombin-induced neutrophil chemotaxis and aggregation are mediated by different mechanisms, since chemotaxis is a catalytically independent response whereas aggregation is an active site independent response.

Thrombin-induced increase in albumin permeability across the endothelium

We studied the effect of thrombin on albumin permeability across the endothelial monolayer in vitro. Bovine pulmonary artery endothelial cells were grown on micropore membranes. Morphologic analysis confirmed the presence of a confluent monolayer with interendothelial junctions. Albumin permeability was measured by the clearance of 125I-albumin across the endothelial monolayer. The control 125I-albumin clearance was 0.273 +/- 0.02 microliter/min. The native enzyme, alpha-thrombin (10(-6) to 10(-10) M), added to the luminal side of the endothelium produced concentration-dependent increases in albumin clearance (maximum clearance of 0.586 +/- 0.08 microliter/min at 10(-6) M). Gamma (gamma) thrombin (10(-6) M and 10(-8) M), which lacks the fibrinogen recognition site, also produced a concentration-dependent increase in albumin clearance similar to that observed with alpha-thrombin. Moreover, the two proteolytically inactive forms of the native enzyme, i-Pr2 P-alpha-thrombin and D-Phe-Pro-Arg-CH2-alpha-thrombin, increased the 125I-albumin clearance (0.610 +/- 0.09 microliter/min and 0.609 +/- 0.02 microliter/min for i-Pr2 P-alpha-thrombin and D-Phe-Pro-Arg-CH2-alpha-thrombin at 10(-6) M, respectively). Since the modified forms of thrombin lack the fibrinogen recognition and active serine protease sites, the results indicate that neither site is required for increased albumin permeability. The increase in albumin clearance with alpha-thrombin was not secondary to endothelial cell lysis because lactate dehydrogenase concentration in the medium following thrombin was not significantly different from baseline values. There was also no morphological evidence of cell lysis. Moreover, the increase in 125I-albumin clearance induced by alpha-thrombin was reversible by washing thrombin from the endothelium. The basis for the increased albumin permeability following the addition of alpha-thrombin appears to be a reversible change in endothelial cell shape with formation of intercellular gaps.

Identification of a thrombin sequence with growth factor activity on macrophages

In contrast to fibroblasts, the exposure of G0/G1-arrested J774 cells, a murine macrophage-like tumor cell line, with either active or esterolytically inactive diisopropyl phosphorofluoridate-conjugated alpha-thrombin (the enzymatically active form of thrombin, EC 3.4.21.5) results in a mitogenic response as measured by increased [3H]thymidine incorporation. This response to thrombin is optimal at 10 nM and is specifically blocked by hirudin, a high-affinity thrombin inhibitor. When prethrombin 1 [a single-chain prothrombin derivative lacking fragment 1, resulting from the action of thrombin on prothrombin] is cleaved with cyanogen bromide, a fragment (peptide CB67-129) is produced that, like the parent thrombin molecule, is mitogenic for J774 cells but not for fibroblasts. Limited tryptic digests of this fragment retain the ability to stimulate macrophages--a function that can be mimicked by a synthetic tetradecapeptide homologue of CB67-129 (representing residues 367-380 of the human thrombin B chain sequence) but not by any of a series of well-known growth promoters, including platelet-derived growth factor, epidermal growth factor, nerve growth factor, and fibroblast epidermal growth factor, nerve growth factor, and fibroblast growth factor. The mitogenic effects of this peptide are not limited to J774 cells but can be expressed in other macrophage-like tumor cell lines, including P388D1, RAW, and PU5. In addition to increased [3H]thymidine incorporation, the synthetic B chain peptide stimulates cell proliferation as evidenced by a dose-dependent increase in total protein per culture well and cell number. We conclude that the thrombin molecule contains a macrophage growth factor domain that is separate and distinct from its active center. Thus, thrombin, in addition to its major role in hemostasis and thrombosis, may also have important functions in such basic processes as the inflammatory response and monocytopoiesis.

Growth-promoting effects of esterolytically inactive thrombin on macrophages

It has been recognized for many years that alpha-thrombin, like other better known mitogens (eg, PDGF, EGF, etc) is capable of initiating proliferation in quiescent cells belonging to the fibroblast family. However, unlike these other peptides, thrombin is a serine protease whose function as a growth stimulator for fibroblasts is intimately linked to its esterolytic activity. Thus, while native alpha-thrombin is capable of evoking DNA synthesis in G0/G1-arrested cells, neither enzymatically inactive thrombin (eg, iPR2P-alpha-thrombin) nor partially degraded thrombin (eg, gamma-thrombin) shares in this capability. Data from our laboratory have shown that thrombin is chemotactic for peripheral blood monocytes and for cells belonging to the monocyte/macrophage family and that this activity is not dependent upon thrombin's enzymatic properties. Our recent findings demonstrate that thrombin also serves as a growth factor for these cells, and this mitogenic capability is independent of esterolytic function and resides in the same region of the molecule as that responsible for chemotaxis. Additionally, by means of techniques such as computer modeling and peptide synthesis, we have now been able to delineate a distinct mitogenic subsite within this chemotactic thrombin sequence. Thus, the sequence in the thrombin B chain that mediates chemotaxis represents a true cell interactive exosite additionally capable of stimulating growth and possibly other biological functions in cells of macrophage/monocyte lineage.