Inhibition of arterial thrombosis by a peptide ligand of the thrombin receptor

Protease-activated receptors in cancer: A systematic review

The traditional view of the role of proteases in tumor growth, progression and metastasis has significantly changed. Apart from their contribution to cancer progression, it is evident that a subclass of proteases, such as thrombin, serves as signal molecules controlling cell functions through the protease-activated receptors (PARs). Among the four types of PAR (PAR1-4; cloned and named in order of their discovery), PAR1, PAR3 and PAR4 are activated by thrombin, unlike PAR2, which is activated by trypsin-like serine proteases. Thrombin has been proven to be a significant factor in both the behavior of cancer in its involvement in hemostasis and blood coagulation. Thrombin is a key supporter of various cellular effects relevant to tumor growth and metastasis, as well as a potent activator of angiogenesis, which is essential for the growth and development of all solid tumor types. This review presents an overview of the role of PAR-mediated thrombin in angiogenesis and cancer, focusing on the ability of PAR1- and PAR4-mediated thrombin to affect tumorigenesis and angiogenesis.

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.

A formylated hexapeptide ligand mimics the ability of Wnt-5a to impair migration of human breast epithelial cells

Loss of Wnt-5a protein expression is associated with shorter recurrence-free survival in breast carcinoma patients and increased motility in mammary cell lines. Based on sequence analysis of Wnt-5a, we identified 14 peptide fragments and investigated their ability to mimic the effects of Wnt-5a on mammary cell adhesion and migration. Two of these peptides significantly increased adhesion and impaired migration in the non-tumorigenic HB2 breast epithelial cell line and in the MDA-MB-468 breast cancer cell line, both of which show little endogenous expression of the Wnt-5a protein. We removed two amino acids at a time from the N terminus of the shorter of these two peptides to identify the shortest peptide that still inhibited migration. The influence on tumor cell adhesion was gradually lost and was no longer detectable when only six amino acids remained. However, formylation of the N-terminal methionine of this hexapeptide restored its effect on adhesion and reduced tumor cell motility via a Frizzled-5 receptor-dependent mechanism, even at a low pH such as encountered in breast tumor tissue. This formylated hexapeptide ligand induced a rapid cytosolic calcium signal, whereas it did not affect the cellular levels of unphosphorylated beta-catenin or active JNK. The novel formyl-Met-Asp-Gly-Cys-Glu-Leu peptide ligand is not only a valuable experimental tool but has also a potential role in antimetastatic treatment of the 50% of human breast cancer patients that have reduced endogenous Wnt-5a protein expression.

Aprotinin Inhibits Protease-Dependent Platelet Aggregation and Thrombosis

BACKGROUND

Hemostatic effects of the protease inhibitor aprotinin in cardiac surgery are well described, and recent evidence suggests an antithrombotic mechanism of aprotinin through inhibition of thrombin-mediated platelet activation. We hypothesized that aprotinin provides hemostasis while reducing vascular thrombosis by attenuating protease-dependent platelet function.

METHODS

Rabbits (3 to 4 kg) underwent carotid artery thrombosis induced by electrical current. Treatment animals (n = 8) received aprotinin by a 100,000-KIU bolus followed by a continuous infusion (25,000 KIU/h). Control animals (n = 8) received crystalloid solution. Thrombus weight and time to thrombotic occlusion were determined. Platelet aggregation was examined in response to protease-dependent (thrombin) and protease-independent (adenosine diphosphate, ADP) platelet agonists. Platelet thrombin protease-activated receptor (PAR) expression was analyzed by Western blot. Ear bleeding time and abdominal incisional bleeding were measured at baseline and serially.

RESULTS

Thrombus weight was decreased by aprotinin (6.1 +/- 1.1 mg versus 10.8 +/- 1.5 mg, aprotinin versus control, p < 0.05). Time to thrombotic occlusion was prolonged in the aprotinin group (17.4 +/- 1.0 minutes versus 8.3 +/- 1.3 minutes, p < 0.001). Rabbit platelet expression of thrombin PARs was demonstrated by Western blot analysis, and was not altered by aprotinin therapy. Platelet aggregation due to thrombin was decreased by aprotinin therapy (59.2% +/- 3.0% versus 95.8% +/- 1.5%, p < 0.001), whereas protease-independent, ADP-induced platelet aggregation was unchanged with aprotinin. Incisional bleeding was not different between groups. In the aprotinin group, bleeding time was unchanged at baseline and then reduced for the duration of the experiment (35.0 +/- 4.7 seconds versus 76.8 +/- 6.4 seconds, p < 0.05).

CONCLUSIONS

While providing hemostatic effects, aprotinin attenuates vascular thrombosis in part by inhibition of PAR activation, resulting in the prevention of thrombin-induced platelet aggregation.

Development of CoMFA, advance CoMFA and CoMSIA models in pyrroloquinazolines as thrombin receptor antagonist

3D QSAR studies namely CoMFA, advance CoMFA and CoMSIA have been carried out on a series of pyrroloquinazolines for their thrombin receptor antagonistic activity. The predicted activities by highly significant CoMFA (q2=0.66 and CoMSIA (q2=0.67) models were in good accordance with observed activities and the models may be useful for optimization of thrombin receptor antagonistic activity.

Role of protease-activated receptors in airway function: a target for therapeutic intervention?

Protease-activated receptors (PARs) are G-protein-coupled, seven transmembrane domain receptors that act as cellular enzyme sensors. These receptors are activated by the proteolytic cleavage at the amino terminus, enabling interaction between the newly formed "tethered ligand" and the second extracellular loop of the receptor to confer cellular signalling. PARs can also be activated by small peptides that mimic the tethered ligand. In the respiratory tract, PARs may be regulated by endogenous proteases, such as airway trypsin and mast cell tryptase, as well as exogenous proteases, including inhaled aeroallergens such as those from house dust mite faecal pellets. Immunoreactive PARs have been identified in multiple cell types of the respiratory tract, and PAR activation has been reported to stimulate cellular mitogenesis and to promote tissue inflammation. Activation of PARs concurrently stimulates the release of bronchorelaxant and anti-inflammatory mediators, which may serve to induce cytoprotection and to minimise tissue trauma associated with severe chronic airways inflammation. Furthermore, airway inflammatory responses are associated with increased epithelial PAR expression and elevated concentrations of PAR-activating, and PAR-inactivating, proteases in the extracellular space. On this basis, PARs are likely to play a regulatory role in airway homeostasis, and may participate in respiratory inflammatory disorders, such as asthma and chronic obstructive pulmonary disease. Further studies focussing on the effects of newly developed PAR agonists and antagonists in appropriate models of airway inflammation will permit better insight into the role of PARs in respiratory pathophysiology and their potential as therapeutic targets.

Blockade of the thrombin receptor protease-activated receptor-1 with a small-molecule antagonist prevents thrombus formation and vascular occlusion in nonhuman primates

Although it is well recognized that human platelet responses to alpha-thrombin are mediated by the protease-activated receptors PAR-1 and PAR-4, their role and relative importance in platelet-dependent human disease has not yet been elucidated. Because the expression profile of PARs in platelets from nonprimates differs from humans, we used cynomolgus monkeys to evaluate the role of PAR-1 in thrombosis. Based on reverse transcription-polymerase chain reaction, PAR expression in platelets from cynomolgus monkeys consisted primarily of PAR-1 and PAR-4, thereby mirroring the profile of human platelets. We probed the role of PAR-1 in a primate model of vascular injury-induced thrombosis with the selective PAR-1 antagonist (alpha S)-N-[(1S)-3-amino-1-[[(phenylmethyl)amino]carbonyl]propyl]-alpha-[[[[[1-(2,6-dichlorophenyl)methyl]-3-(1-pyrrolidinylmethyl)-1H-indazol-6-yl]amino]carbonyl]amino]-3,4-difluorobenzenepropanamide (RWJ-58259). After pretreatment with RWJ-58259 or vehicle, both carotid arteries of anesthetized monkeys were electrolytically injured and blood flow was monitored for 60 min. Time to occlusion was significantly extended after RWJ-58259 administration (27 +/- 3 to 53 +/- 8 min; p < 0.048). Vessels from three of the five treated animals remained patent. Ex vivo platelet aggregation measurements indicated complete PAR-1 inhibition, as well as an operational PAR-4 response. Immunohistochemical staining of mural thrombi with antibodies to the platelet marker CD61 and fibrinogen indicated that RWJ-58259 significantly reduced thrombus platelet deposition. Drug treatment had no effect on key hematological or coagulation parameters. Our results provide direct evidence that PAR-1 is the primary receptor that mediates alpha-thrombin's prothrombotic actions in primates and suggest that PAR-1 antagonists may have potential for the treatment of thrombotic disorders in humans.

Discovery of a nonpeptidic small molecule antagonist of the human platelet thrombin receptor (PAR-1)

The synthesis and biological evaluation of a series of nonpeptidic small molecule antagonists of the human platelet thrombin receptor (PAR-1) are described. Optimization of the 5-amino-3-arylisoxazole lead resulted in an approximate 100-fold increase in potency. The most potent of these compounds (54) inhibits platelet activation with IC(50)s of 90 nM against the thrombin receptor agonist peptide (TRAP) and 510 nM against thrombin as the agonist. Further, antagonist 54 fully blocks platelet aggregation stimulated by 1 nM thrombin for 10 min.

Discovery and initial structure-activity relationships of trisubstituted ureas as thrombin receptor (PAR-1) antagonists

Thrombin is the most potent agonist of platelet activation, and its effects are predominantly mediated by platelet thrombin receptors. Therefore, antagonists of the thrombin receptor have potential utility for the treatment of thrombotic disorders. Screening of combinatorial libraries revealed 2 to be a potent antagonist of the thrombin receptor. Modifications of this structure produced 11k, which inhibits thrombin receptor stimulated secretion and aggregation of platelets.

A peptide analogue of thrombin receptor-activating peptide inhibits thrombin and thrombin-receptor-activating peptide-induced vascular smooth muscle cell proliferation

The serine protease thrombin, in addition to its pivotal role in the coagulation cascade, plays an important role in the development of atherosclerosis and restenosis by inducing smooth cell proliferation. Thrombin exerts its cellular effects mainly by cleaving its own receptor, leaving a new NH2-terminus that can act as a tethered ligand to activate the thrombin receptor. Peptides derived from the new NH2-terminus are able to fully activate thrombin receptor and mimic cellular effects of thrombin. Peptides with structural similarities to the tethered ligand have been tested for their ability to prevent thrombin- and tethered ligand-induced platelet aggregation and thrombus formation. We synthesized a peptide with multiple alanine substitutions in both critical and noncritical residues of tethered ligand that specifically inhibited platelet aggregation induced by thrombin and thrombin receptor-activating peptide and prevented thrombus formation in a rabbit thrombosis model. In the present study we demonstrate that this peptide inhibited only thrombin- and tethered ligand-induced human vascular smooth muscle cell proliferation as determined by (3H)-thymidine incorporation and has no effect on platelet-derived growth factor and serum-induced smooth muscle cell proliferation. The inhibitory effect of this peptide is dependent on the concentration of the antagonist used and length of preincubation time. The possible mechanism by which this peptide exerts its inhibitory effect may by desensitizing the thrombin receptor. The results of the present study suggest that apart from being antithrombotic, tethered ligand antagonist peptides can also act as antiatherosclerotic or antirestenotic agents.