Protease-activated receptors: contribution to physiology and disease

Redundant signaling mechanisms contribute to the vasodilatory response of the afferent arteriole to proteinase-activated receptor-2

We previously demonstrated that stimulation of proteinase-activated receptor-2 (PAR-2) by SLIGRL-NH2elicits afferent arteriolar vasodilation, in part, by elaborating nitric oxide (NO), suggesting an endothelium-dependent mechanism (Trottier G, Hollenberg M, Wang X, Gui Y, Loutzenhiser K, and Loutzenhiser R. Am J Physiol Renal Physiol 282: F891–F897, 2002). In the present study, we characterized the NO-independent component of this response, using the in vitro perfused hydronephrotic rat kidney. SLIGRL-NH2(10 μmol/l) dilated afferent arterioles preconstricted with ANG II, and the initial transient component of this response was resistant to NO synthase (NOS) and cyclooxygenase inhibition. This NO-independent response was not prevented by treatment with 10 nmol/l charybdotoxin and 1 μmol/l apamin, a manipulation that prevents the endothelium-derived hyperpolarizing factor (EDHF)-like response of the afferent arteriole to acetylcholine, nor was it blocked by the addition of 1 mmol/l tetraethylammonium (TEA) or 50 μmol/l 17-octadecynoic acid, treatments that block the EDHF-like response to bradykinin. To determine whether the PAR-2 response additionally involves the electrogenic Na+-K+-ATPase, responses were evaluated in the presence of 3 mmol/l ouabain. In this setting, SLIGRL-NH2induced a biphasic dilation in control and a transient response after NOS inhibition. The latter was not prevented by charybdotoxin plus apamin or by TEA alone but was abolished by combined treatment with charybdotoxin, apamin, and TEA. This treatment did not prevent the NO-dependent dilation evoked in the absence of NOS inhibition. Our findings indicate a remarkable redundancy in the signaling cascade mediating PAR-2 -induced afferent arteriolar vasodilation, suggesting an importance in settings such as inflamation or ischemia, in which vascular mechanisms might be impaired and the PAR system is thought to be activated.

Blood coagulation and the risk of atherothrombosis: a complex relationship

The principles of Virchov's triad appear to be operational in atherothrombosis or arterial thrombosis: local flow changes and particularly vacular wall damage are the main pathophysiological elements. Furthermore, alterations in arterial blood composition are also involved although the specific role and importance of blood coagulation is an ongoing matter of debate. In this review we provide support for the hypothesis that activated blood coagulation is an essential determinant of the risk of atherothrombotic complications. We distinguish two phases in atherosclerosis: In the first phase, atherosclerosis develops under influence of "classical" risk factors, i.e. both genetic and acquired forces. While fibrinogen/fibrin molecules participate in early plaque lesions, increased activity of systemic coagulation is of no major influence on the risk of arterial thrombosis, except in rare cases where a number of specific procoagulant forces collide. Despite the presence of tissue factor – factor VII complex it is unlikely that all fibrin in the atherosclerotic plaque is the direct result from local clotting activity. The dominant effect of coagulation in this phase is anticoagulant, i.e. thrombin enhances protein C activation through its binding to endothelial thrombomodulin.

The second phase is characterized by advancing atherosclerosis, with greater impact of inflammation as indicated by an elevated level of plasma C-reactive protein, the result of increased production influenced by interleukin-6. Inflammation overwhelms protective anticoagulant forces, which in itself may have become less efficient due to down regulation of thrombomodulin and endothelial cell protein C receptor (EPCR) expression. In this phase, the inflammatory drive leads to recurrent induction of tissue factor and assembly of catalytic complexes on aggregated cells and on microparticles, maintaining a certain level of thrombin production and fibrin formation. In advanced atherosclerosis systemic and vascular wall driven coagulation becomes more important and elevated levels of D-dimer fragments should be interpreted as markers of this hypercoagulability.

Clinical Inhibition of the Seven-Transmembrane Thrombin Receptor (PAR1) by Intravenous Aprotinin During Cardiothoracic Surgery

Background— Protease-activated receptor-1 (PAR1) is the principal thrombin receptor in the vasculature, and antagonists against this receptor are in preclinical trials. Aprotinin, already approved for clinical use to reduce transfusion requirements in cardiopulmonary bypass (CPB) surgery, has been shown to inhibit PAR1 activation in vitro. Here, we exploit CPB as a model for thrombin generation in humans to examine whether aprotinin can inhibit platelet PAR1 activation clinically.

Methods and Results— PAR1 expression and function on platelets was examined in coronary artery bypass grafting (CABG) patients randomized into 2 groups: (1) those receiving saline infusion during CPB (n=17) and (2) those receiving aprotinin (2×10 6 kallikrein inhibitor units [KIU] in pump prime, 2×10 6 KIU loading dose, followed by 0.5×10 6 KIU/h [n=13]). Platelets in the saline group showed loss of PAR1-specific function at 2 hours after CPB, but this was preserved in the aprotinin group ( P <0.001). These effects were most likely targeted at PAR1 receptor cleavage, because (1) the level of thrombin generated during CPB did not vary significantly between groups, (2) expression of SPAN12, which detects only uncleaved PAR1 receptors, was preserved in the aprotinin but not the placebo group ( P <0.05), and (3) supporting evidence in vitro showed reduced thrombin-induced PAR1 cleavage ( P <0.001) and platelet aggregation ( P <0.001) in the presence of aprotinin.

Conclusions— This study demonstrates that platelet PAR1 activation by thrombin can be inhibited by aprotinin. Our results extend the clinical mechanism of action of aprotinin and provide the first proof of principle that PAR1 can be inhibited clinically. This has implications beyond cardiac surgery for the development of therapeutic PAR1 blockade.

Effect of invertebrate serine proteinase inhibitors on carrageenan-induced pleural exudation and bradykinin release

The carrageenan model of pleurisy is described as temporal plasma exudation (1-5 h) with extensive neutrophil infiltration and release of proteinases into the pleural cavity. The aim of this work was to study the effects of serine proteinase inhibitors on the inflammatory process induced by administration of carrageenan to the rat pleural cavity and on release of kinins in pleural exudate. Pleurisy was induced by injecting carrageenan and serine proteinase inhibitors simultaneously into the pleural cavity. The proteinase inhibitors used were: aprotinin, a plasma kallikrein inhibitor; recombinant leech derived tryptase inhibitor-2PL (LDTI-2PL), a plasmin inhibitor; Boophilus microplus trypsin inhibitors (BmTIs); trypsin; plasma kallikrein; plasmin and neutrophil elastase inhibitors; and a synthetic neutrophil elastase inhibitor (EIsynt). Administration of carrageenan with LDTI-2PL and BmTIs induced a marked increase in exudation (143% and 201%) and leukocyte migration (288% and 408%), respectively, when compared to the control group. Pleural exudate from LDTI-2PL and BmTIs plus carrageenan-treated rats showed a significant increase in plasma kallikrein-like activity, measured by chromogenic substrate hydrolysis. The specific inhibition of enzymatic activity with aprotinin confirmed that 50% of S2302 hydrolysis was produced by plasma kallikrein-like enzymes. Kinin release was increased by 97% and 103% in exudates from LDTI-2PL and BmTIs plus carrageenan-treated rats, respectively. Considering that the plasmin inhibitors LDTI-2PL and BmTIs increased exudation, leukocyte migration and bradykinin release, our results suggest an anti-inflammatory role for plasmin in the pleurisy model.

Activation of trigeminal nociceptive neurons by parotid PAR-2 activation in rats

To clarify involvement of protease-activated receptor-2 (PAR-2) in parotid pain, we examined whether PAR-2 activation in the parotid gland could activate trigeminal nociceptive neurons in anesthetized rats, by analyzing immunoreactive Fos as a nociceptive marker. Either the PAR-2 agonist SLIGRL-NH2 or capsaicin, injected into the parotid duct, caused expression of Fos in the trigeminal subnucleus caudalis, although the PAR-2-inactive reversed peptide had no such effect. The Fos expression caused by PAR-2 activation was inhibited by ablation of capsaicin-sensitive sensory neurons. Intraductal SLIGRL-NH2 did not increase vascular permeability in the parotid gland. Our data thus reveal that activation of PAR-2 in the parotid gland can cause activation of trigeminal nociceptive neurons via capsaicin-sensitive sensory nerves most probably by a non-inflammatory mechanism.

Proteinase-activated receptor-4: evaluation of tethered ligand-derived peptides as probes for receptor function and as inflammatory agonists in vivo

1. We evaluated the ability of a number of peptides based on the tethered ligand sequences of human, rat and murine proteinase-activated receptor-4 (PAR(4)), to serve as receptor-activating probes or antagonists for bioassays carried out in vitro and for in vivo models of inflammation. 2. In a rat PAR(4)-dependent platelet aggregation assay, the relative potencies of the active sequences (AYPGKF-NH(2)>GYPGKF-NH(2)>GYPGFK-NH(2)>GFPGKP-NH(2)) were consistent with an activation of PAR(4). 3. In the aggregation assay, the reverse or partial reverse-sequence peptides (VQGPYG-NH(2), YAPGKF-NH(2) and FKGPYA-NH(2)) were inactive, while trans-cinnamoyl (Tc)-YPGKF-NH(2), Tc-APGKF-NH(2) and N-palmitoyl-SGRRYGHALR-NH(2) (pepducin P4pal-10) were antagonists. 4. However, in an endothelium-dependent NO-mediated rat aorta (RA) relaxation assay and in a gastric longitudinal muscle (LM) contraction assay, these antagonist peptides were agonists as were most other peptides, with distinct orders of potencies that differed for both the RA and LM assays and from the platelet assay. 5. We conclude that PAR(4)-derived tethered ligand peptide agonists can act at receptors other than PAR(4) and that a judicious choice of ligands is required to probe for PAR(4) function in bioassay systems and in particular for in vivo models. 6. By selecting from these peptides the ones most reliably reflecting PAR(4) activation (AYPGKF-NH(2) as a standard agonist; YAPGKF-NH(2) as a PAR(4)-inactive standard), we were able to establish an inflammatory role for the PAR(4)-activating peptides acting via a non-neurogenic mechanism in a rat paw oedema model.

The cardiovascular actions of protease-activated receptors

Protease-activated receptors (PARs) comprise a family of G protein-coupled receptors with a unique proteolytic activation mechanism. PARs are activated by thrombin or other coagulation or inflammatory proteases formed at sites of tissue injury. PARs play a particularly important role in the pathogenesis of clinical disorders characterized by chronic inflammation or smoldering activation of the coagulation cascade. Individual PARs have been linked to the regulation of a broad range of cellular functions. Recent studies identify PAR family members in the vasculature (including within atherosclerotic lesions) and in the heart. Here, PAR-triggered responses contribute to vasoregulation and influence cardiac electrical and mechanical activity. PAR activation also is linked to structural remodeling of the vasculature and the myocardium. This review focuses on the cardiovascular actions of PARs that play a role in normal cardiovascular physiology and that are likely to contribute to cardiovascular diseases.

Proteinase-activated receptor 2 activation modulates guinea-pig mesenteric lymphatic vessel pacemaker potential and contractile activity

Lymphatic vessels rhythmically constrict to avoid fluid and protein accumulation in the interstitial space. This activity is critical during inflammation to prevent excessive oedema. Lymphatic pumping is intrinsic to the smooth muscle in the vessel wall and is due to the spontaneous occurrence of action potentials, the pacemaker of which is proposed to be spontaneous transient depolarizations (STDs). This function is highly susceptible to the fluid load and modulated by chemical agents, amongst which inflammatory mediators are important players. Activation of proteinase-activated receptors (PARs) has been involved in inflammation and affects vascular smooth muscle tone. The present study aims to investigate the role of PAR2, a member of the PAR family, in lymphatic vessel pumping. RT-PCR experiments revealed that PAR2 message is present in lymphatic vessels of the guinea-pig mesentery. Agonists of PAR2 such as trypsin and the activating peptide, SLIGRL-NH2, caused a decrease in the contractile activity of intraluminally perfused lymphatic vessels. Moreover, intracellular microelectrode recordings from isolated vessels revealed that PAR2 activation hyperpolarized the lymphatic smooth muscle membrane potential and altered STD amplitude and frequency. The decreases in constriction frequency and STD activity as well as the hyperpolarization were dependent on a functional endothelium, not affected by NO synthase or guanylyl-cyclase inhibition, but mimicked by PGE2 and iloprost and blocked by indomethacin (10 microM) and glibenclamide (1 microM). These results show that PAR2 activation alters guinea-pig lymphatic vessel contractile and electrical activity via the production of endothelium-derived cyclo-oxygenase metabolites.

Protease-Activated Receptor-2 (PAR-2)-Related Peptides Induce Tear Secretion in Rats: Involvement of PAR-2 and Non-PAR-2 Mechanisms

Protease-activated receptor-2 (PAR-2) plays an extensive role in the regulation of digestive exocrine secretion. The present study examined whether PAR-2-related peptides could modulate tear secretion in rats and analyzed the underlying mechanisms. SLIGRL-NH(2), a PAR-2-activating peptide (PAR-2-AP) derived from mouse/rat PAR-2, when administered i.v. in combination with amastatin, an aminopeptidase inhibitor, evoked tear secretion, whereas LRGILS-NH(2), a PAR-2-inactive reversed peptide, had no such effect. In contrast, LSIGRL-NH(2), a partially reversed peptide known to be inactive with PAR-2, caused tear secretion equivalent to the effect of SLIGRL-NH(2). SLIGKV-NH(2), a human-derived PAR-2-AP, also induced significant tear secretion though to a lesser extent, whereas neither VKGILS-NH(2), a reversed peptide, nor LSIGKV-NH(2), a partially reversed peptide, produced any secretion. In desensitization experiments, after the first dose of SLIGRL-NH(2), the second dose of SLIGRL-NH(2) produced no tear secretion, whereas the response to LSIGRL-NH(2) was only partially inhibited by preadministration of SLIGRL-NH(2). Preadministration of LSIGRL-NH(2) abolished the response to subsequently administered LSIGRL-NH(2) but not SLIGRL-NH(2). The tear secretion induced by LSIGRL-NH(2) but not by PAR-2-APs was blocked by atropine or hexamethonium. Mast cell depletion due to repeated doses of compound 48/80 did not alter the effect of SLIGRL-NH(2) or LSIGRL-NH(2). Finally, IGRL-NH(2), a possible core structure of LSIGRL-NH(2), triggered tear secretion in an atropine-reversible manner. Our findings suggest that the PAR-2-APs SLIGRL-NH(2) and SLIGKV-NH(2) cause tear secretion, most likely via PAR-2 and that LSIGRL-NH(2), a PAR-2-inactive peptide, and IGRL-NH(2), its key structure, trigger tear secretion by stimulating parasympathetic nerves via an unidentified target molecule.