Protease Activated Receptors and Arthritis

The catabolic and destructive activity of serine proteases in arthritic joints is well known; however, these enzymes can also signal pain and inflammation in joints. For example, thrombin, trypsin, tryptase, and neutrophil elastase cleave the extracellular N-terminus of a family of G protein-coupled receptors and the remaining tethered ligand sequence then binds to the same receptor to initiate a series of molecular signalling processes. These protease activated receptors (PARs) pervade multiple tissues and cells throughout joints where they have the potential to regulate joint homeostasis. Overall, joint PARs contribute to pain, inflammation, and structural integrity by altering vascular reactivity, nociceptor sensitivity, and tissue remodelling. This review highlights the therapeutic potential of targeting PARs to alleviate the pain and destructive nature of elevated proteases in various arthritic conditions.

Contribution of PAR-1, PAR-4 and GPIbα in intracellular signaling leading to the cleavage of the β3 cytoplasmic domain during thrombin-induced platelet aggregation

Integrin αIIbβ3 plays a pivotal role in platelet aggregation by binding to fibrinogen. The β3 cytoplasmic domain of αIIbβ3 interacts with cytoskeletal and signaling proteins and is cleaved by µ-calpain, a calcium regulated cysteine protease. In the present study, we have investigated in more detail the cleavage of the β3 cytoplasmic domain during platelet aggregation induced by thrombin, TRAP-1 and TRAP-4. Our data show that β3 is cleaved in all three cases. The time course of β3 cleavage and the amount of cleaved β3 depends on the way platelets are activated and on the complete activation of µ-calpain, with a maximum of 90% of cleaved β3 obtained when thrombin is used. Furthermore, our results also show that the cleaved αIIbβ3 is mainly distributed in the Triton soluble fraction, indicating its inability to bind to the cytoskeleton. Interestingly, in the absence of GPIbα or following inhibition of thrombin binding to GPIbα, there is a reduction in the thrombin-induced calcium flux, β3 cleavage and µ-calpain activation. These results suggest that cleavage of the β3 cytoplasmic domain by µ-calpain might be an important step regulating the link between the cytoskeleton and αIIbβ3 during platelet aggregation, and that GPIbα could function as a cofactor for the complete activation of platelets by thrombin.

[Downregulation of proteinase activated receptor 4 inhibits migration of SW620 human colorectal cancer cells]

OBJECTIVE

To establish the human colorectal cancer cell model SW620/PAR4D with inducible suppression of proteinase activated receptor 4 (PAR4) expression, and investigate the role PAR4 plays in the proliferation and migration of cancer cells.

METHODS

A human colorectal cancer cell line with tetracycline-inducible expression regulatory system, namely SW620/Tet-on, was established; inducible expression lentiviral vector with artificial microRNA targeting PAR4, pLVX-Tight-Puro-PAR4-miR, was constructed and transfected into SW620/Tet-on to make an inducible PAR4-suppressed cell model SW620/PAR4D. Western blotting was used to confirm the suppression of PAR4 expression after the doxycycline (DOX) treatment. CCK-8 assay was used to evaluate the impact of suppressed PAR4 expression on cell proliferation, and wound-healing assay was used to analyze the migration of the cells.

RESULTS

The SW620/PAR4D cell model was established successfully. Suppression of PAR4 expression by DOX treatment had no significant impact on the growth/proliferation of SW620/PAR4D cells, but markedly inhibited the cell migration.

CONCLUSION

Suppression of PAR4 expression has no significant effect on the proliferation of SW620 cells, but can inhibit the migration of the cells.

Protease-activated receptor-1 inhibitors: a novel class of antiplatelet agents for the treatment of patients with acute coronary syndrome

The unifying basis of acute coronary syndrome (ACS) is the complication of a vulnerable coronary plaque, an event primarily mediated by platelet activation. Three major pathways are predominantly involved in this process: thromboxane A(2) via the thromboxane A(2) receptor, adenosine diphosphate via the P2Y(12) receptor, and thrombin via the protease-activated receptor (PAR)-1, with the latter being the most potent platelet activator. Despite the effective inhibition of the first two pathways with aspirin and an expanding family of P2Y(12) inhibitors, respectively, the recurrence of ischemic events in patients with ACS remains high. There is also a growing concern regarding the safety profile in terms of bleeding with more powerful antiplatelet agents, which has tempered expectations of newly developed compounds. PAR-1 inhibitors are a novel class of antiplatelet agents that inhibit thrombin-mediated platelet activation. Preliminary data indicate that these compounds have the potential to improve ischemic prognosis without increasing the bleeding risk. In this chapter we will discuss the rationale for developing this novel class of antiplatelet agents and specifically, the two compounds in most advanced clinical development, vorapaxar and atopaxar.

Thrombin regulates the metastatic potential of human rhabdomyosarcoma cells: distinct role of PAR1 and PAR3 signaling

We observed that human rhabdomyosarcoma (RMS) cells highly express a tissue factor that promotes thrombin formation, which indirectly and directly affects RMS progression. First, we found that thrombin activates platelets to generate microvesicles (PMV), which transfer to RMS cells' alpha2beta3 integrin and increase their adhesiveness to endothelial cells. Accordingly, RMS cells covered with PMVs showed higher metastatic potential after i.v. injection into immunodeficient mice. Furthermore, PMVs activate mitogen-activated protein kinase (MAPK)p42/44 and AKT to chemoattract RMS cells. We also found that RMS cells express functional protease-activated receptor-1 (PAR1) and PAR3 and respond to thrombin stimulation by MAPKp42/44 and MAPKp38 phosphorylation. To our surprise, thrombin did not affect RMS proliferation or survival; it inhibited the chemotactic and adhesive properties of RMS cells. However, when PAR1-specific agonist thrombin receptor-activating peptide 6 was used, which does not activate PAR3, selective PAR1 stimulation enhanced RMS proliferation. To learn more on the role of PAR1 and PAR3 antagonism in RMS proliferation and metastasis, we knocked down both receptors by using a short hairpin RNA strategy. We found that although thrombin does not affect growth of PAR1(-/-) cells, it stimulated the proliferation of PAR3(-/-) cells. More importantly, PAR3(-/-) cells, in contrast to PAR1(-/-) ones, formed larger tumors in immunodeficient mice. We conclude that thrombin is a novel underappreciated modulator of RMS metastasis and that we have identified a novel role for PAR3 in thrombin signaling.

[Role of phosphatidylinositol 3-kinase and myosin light chain kinase during the activation of thrombin receptors]

The objective of study was to compare the influences of wortmannin on platelet aggregation and platelet membrane surface glycoproteins GPIb expression after thrombin receptor activation, and to investigate the role of phosphatidylinositol 3-kinase (PI3-K) and myosin light chain kinase (MLCK) in the course of thrombin receptor activation. Peptide SFLLRN (PAR1-AP) and AYPGKF (PAR4-AP) were used for stimulating platelet, and the changes of platelet aggregation and GPIb were analyzed with 100 nmol/L wortmannin (inhibitor of PI3-K) and 10 micromol/L wortmannin (inhibitor of MLCK). The results indicated that the platelet activation was influenced by either concentration of wortmannin in response to PAR stimulation. Platelet aggregation was apparently inhibited by 10 micromol/L wortmannin through both PAR peptides, and was slightly inhibited by 100 nmol/L wortmannin only under PAR1-AP activation. In addition, GPIbalpha internalization was partly inhibited by 100 nmol/L wortmannin in response to PAR1 (p < 0.05 at 1, 2, 5 min) and PAR4 (p < 0.05 at 2, 5, 10 min) activation. Meanwhile, 10 micromol/L wortmannin induced little change for GPIbalpha centralisation in the course of PAR activation, with a delayed restoration of surface GPIbalpha observed under PAR1-AP activation, and no change of GPIbalpha redistribution existed under PAR4-AP activation. It is concluded that the different roles of PI3-K and MLCK exist in the course of thrombin receptor activation. PI3-K accelerates the short course of GPIb centralisation for two PAR signal pathways, while MLCK inhibits the restoration of GPIbalpha in PAR1 pathway.

Microfluidic focal thrombosis model for measuring murine platelet deposition and stability: PAR4 signaling enhances shear-resistance of platelet aggregates

BACKGROUND

Flow chambers allow the ex vivo study of platelet response to defined surfaces at controlled wall shear stresses. However, most assays require 1-10 mL of blood and are poorly suited for murine whole blood experiments.

OBJECTIVE

To measure murine platelet deposition and stability in response to focal zones of prothrombotic stimuli using 100 microL of whole blood and controlled flow exposure.

METHODS

Microfluidic methods were used for patterning acid-soluble collagen in 100 microm x 100 microm patches and creating flow channels with a volume of 150 nL. Within 1 min of collection into PPACK and fluorescent anti-mouse CD41 mAb, whole blood from normal mice or from mice deficient in the integrin alpha(2) subunit was perfused for 5 min over the patterned collagen. Platelet accumulation was measured at venous and arterial wall shear rates. After 5 min, thrombus stability was measured with a 'shear step-up' to 8000 s(-1).

RESULTS

Wild-type murine platelets adhered and aggregated on collagen in a biphasic shear-dependent manner with increased deposition from 100 to 400 s(-1), but decreased deposition at 1000 s(-1). Adhesion to patterned collagen was severely diminished for platelets lacking a functional alpha(2)beta(1) integrin. Those integrin alpha(2)-deficient platelets that did adhere were removed from the surface when challenged to shear step-up. PAR4 agonist (AYPGKF) treatment of the thrombus at 5 min enhanced aggregate stability during the shear step-up.

CONCLUSIONS

PAR4 signaling enhances aggregate stability by mechanisms independent of other thrombin-dependent pathways such as fibrin formation.

Prothrombin/thrombin and the thrombin receptors PAR-1 and PAR-4 in the brain: localization, expression and participation in neurodegenerative diseases

Emerging evidence demonstrates that thrombin exerts physiological and pathological functions in the central nervous system. Both prothrombin and its active form thrombin have been detected locally in the brain. The cellular functions of thrombin are mainly regulated by G protein-coupled protease-activated receptors (PARs). Thrombin can signal via PAR-1, PAR-3 and PAR-4. Some neurological diseases (e.g. Alzheimer's disease or Parkinson's disease) are characterized by increased levels of both active thrombin and PAR-1. This indicates that thrombin and its receptor may be closely involved in the development of neurodegenerative processes. The role of thrombin in brain injury can be either protective or deleterious, depending on the concentration of thrombin. Thrombin at high concentrations exacerbates brain damage. In contrast, low concentrations of thrombin rescue neural cells from death after brain insults. Also thrombin preconditioning has neuroprotective effects. Therefore, thrombin and thrombin receptors represent novel therapeutic targets for treating neurodegenerative diseases.

Protease-activated receptor-4 inhibition protects from multiorgan failure in a murine model of systemic inflammation

Coagulation proteases may act as cell signaling molecules via protease-activated receptor (PAR) cleavage, subsequently affecting cellular and inflammatory responses. Activation of PARs in the setting of systemic inflammation and disseminated intravascular coagulation (DIC) might thus exacerbate the inflammatory response contributing to tissue and organ damage. To investigate the role of PAR-4 in these processes, we subjected mice to a model of systemic inflammation and DIC (Shwartzman reaction) in the absence or presence of a cell-penetrating pepducin antagonist of PAR-4 (P4pal-10). P4pal-10 dose-dependently diminished the severity of endotoxemia and preserved liver, kidney, as well as lung function. Moreover, systemic inflammation and local (neutrophilic) inflammatory responses were attenuated. In vitro migration assays and P4pal-10 treatment in neutropenic mice suggest an essential role for neutrophils in PAR-4-mediated pathology. P4pal-10 treatment of thrombocytopenic mice excluded the involvement of platelets in this phenomenon. These results uncover an important role for PAR-4 in the Shwartzman reaction and suggest that inhibition of PAR-4 signaling in neutrophils could be protective in systemic inflammation and DIC.

Significance of endothelial dysfunction in the pathogenesis of early and delayed radiation enteropathy

This review summarizes the current state of knowledge regarding the role of endothelial dysfunction in the pathogenesis of early and delayed intestinal radiation toxicity and discusses various endothelial-oriented interventions aimed at reducing the risk of radiation enteropathy. Studies published in the biomedical literature during the past four decades and cited in PubMed, as well as clinical and laboratory data from our own research program are reviewed. The risk of injury to normal tissues limits the cancer cure rates that can be achieved with radiation therapy. During treatment of abdominal and pelvic tumors, the intestine is frequently a major dose-limiting factor. Microvascular injury is a prominent feature of both early (inflammatory), as well as delayed (fibroproliferative) radiation injuries in the intestine and in many other normal tissues. Evidence from our and other laboratories suggests that endothelial dysfunction, notably a deficiency of endothelial thrombomodulin, plays a key role in the pathogenesis of these radiation responses. Deficient levels of thrombomodulin cause loss of vascular thromboresistance, excessive activation of cellular thrombin receptors by thrombin, and insufficient activation of protein C, a plasma protein with anticoagulant, anti-inflammatory, and cytoprotective properties. These changes are presumed to be critically involved in many aspects of early intestinal radiation toxicity and may sustain the fibroproliferative processes that lead to delayed intestinal dysfunction, fibrosis, and clinical complications. In conclusion, injury of vascular endothelium is important in the pathogenesis of the intestinal radiation response. Endothelial-oriented interventions are appealing strategies to prevent or treat normal tissue toxicity associated with radiation treatment of cancer.

[Thrombin and tumor metastasis - review]

Thrombin is a multifunctional serine protease that plays a key role in a variety of physiological and pathological conditions. In addition to the role in hemostasis and coagulation, thrombin has other numerous biological activities affecting inflammation, immune responses, tissue repair and wound healing. Apart from its physiological role thrombin activates the oncogenic potential of both normal and malignant cells and leads a metastatic phenotype. It is a potent mitogen for many tumor cells. It potentiates the proliferative response of tumor cells to some growth factors, increases the adhesive properties to the platelets and invasion processes of tumor cells to the extracellular matrix, enhances the metastatic capacity of tumor cells, activates angiogenesis and remodels the microenvironment of the tumor. The cellular biological effects of thrombin are mediated at least in part by a new subfamily of G-protein-coupled receptors designated proteinase-activited receptors (PARs). Thrombin has a bilateral effect on tumor cells:enhanced growth at low concentration, impaired growth/apoptosis at higher concentration. In this papers, the biological function of thrombin, thrombin and tumors, and thrombin receptors etc were reviewed.

Regulation of protease-activated receptor (PAR) 1 and PAR4 signaling in human platelets by compartmentalized cyclic nucleotide actions

Thrombin potently regulates human platelets by the G protein-coupled receptors protease-activated receptor (PAR) 1 and PAR4. Platelet activation by thrombin and other agonists is broadly inhibited by prostacyclin and nitric oxide acting through adenylyl and guanylyl cyclases to elevate cAMP and cGMP levels, respectively. Using forskolin and YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole] to selectively activate the adenylyl and guanylyl cyclases, respectively, and the membrane-permeable analogs N(6),2'-O-dibutyryladenosine-3'-5'-cAMP (dibutyryl-cAMP) and 8-(4-parachlorophenylthoi)-cGMP (8-pCPT-cGMP), we sought to identify key antiplatelet steps for cyclic nucleotide actions in blocking platelet activation by PAR1 versus PAR4. Platelet aggregation by PAR1 or PAR4 was inhibited with similar EC(50) of 1.2 to 2.1 microM forskolin, 31 to 33 microM YC-1, 57 to 150 microM dibutyryl-cAMP, and 220 to 410 microM 8-pCPT-cGMP. There was a marked left shift in the inhibitory potencies of forskolin and YC-1 for alpha-granule release and glycoprotein IIbIIIa/integrin alphaIIbbeta3 activation (i.e., EC(50) of 1-60 and 40-1300 nM, respectively) that was not observed for dibutyryl-cAMP and 8-pCPT-cGMP (i.e., EC(50) of 200-600 and 40-140 microM, respectively). This inhibition was essentially instantaneous, and measurements of cyclic nucleotide levels and kinase activities support a model of compartmentation involving the cyclic nucleotide effectors and regulators and the key molecular targets for this platelet inhibition. The different sensitivities of PAR1 and PAR4 to inhibition of calcium mobilization and dense granule release identify key antiplatelet steps for cyclic nucleotide actions and are consistent with the signaling models for these receptors. Specifically, PAR4 inhibition depends on the regulation of both calcium mobilization and dense granule release, and PAR1 inhibition depends predominantly on the regulation of dense granule release.

[Thrombin induced tumour growth - pharmacological control]

The central enzyme of blood coagulation, the serine proteinase thrombin, is capable to modify the growth of tumour cells by interaction with protease activated receptors 1 and 4 of the tumour cells. Thrombin is permanently available in tumour micro environment; meizothrombin is generated from prothrombin at a tumour specific activation complex and can influence tumour cell growth via PAR-1 and 7-transdomain protein receptor signalling pathway, too. PEG-coupled direct thrombin inhibitors that possess special pharmacokinetic characteristics and that have been designed for long lasting efficacy in extracellular space, control serine proteinase activity in tumour micro environment and therefore they own a high potential anti-tumour efficacy. In xenographic tumour models this new substance class has shown a significant carcinostatic effect.

Protease-activated receptor-3 (PAR3) regulates PAR1 signaling by receptor dimerization

Thrombin activates endothelial cell signaling by cleaving the protease-activated receptor-1 (PAR1). However, the function of the apparently nonsignaling receptor PAR3 also expressed in endothelial cells is unknown. We demonstrate here the crucial role of PAR3 in potentiating the responsiveness of PAR1 to thrombin. We tested the hypothesis that PAR1/PAR3 heterodimerization and its effect in modifying G protein selectivity was responsible for PAR3 regulation of PAR1 sensitivity. Using bioluminescent resonance energy transfer-2, we showed that PAR1 had comparable dimerization affinity for PAR3 as for itself. We observed increased Galpha(13) coupling between the PAR1/3 heterodimer compared with the PAR1/1 homodimer. Moreover, knockdown of PAR3 moderated the PAR1-activated increase in endothelial permeability. These results demonstrate a role of PAR3 in allosterically regulating PAR1 signaling governing increased endothelial permeability. Because PAR3 is a critical determinant of PAR1 function, targeting of PAR3 may mitigate the effects of PAR1 in activating endothelial responses such as vascular inflammation.

Differential involvement of thrombin receptors in Ca2+ release from two different intracellular stores in human platelets

Physiological agonists increase cytosolic free Ca2+ concentration to regulate a number of cellular processes. The platelet thrombin receptors, PAR (protease-activated receptor) 1 PAR-4 and GPIb-IX-V (glycoprotein Ib-IX-V) have been described as potential contributors of thrombin-induced platelet aggregation. Platelets present two separate Ca2+ stores, the DTS (dense tubular system) and acidic organelles, differentiated by the distinct sensitivity of their respective SERCAs (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPases) to TG (thapsigargin) and TBHQ [2,5-di-(tert-butyl)-1,4-hydroquinone]. However, the involvement of the thrombin receptors in Ca2+ release from each Ca2+ store remains unknown. Depletion of the DTS using ADP, which releases Ca2+ solely from the DTS, in combination with 10 nM TG, to selectively inhibit SERCA2 located on the DTS reduced Ca2+ release evoked by the PAR-1 agonist, SFLLRN, and the PAR-4 agonist, AYPGKF, by 80 and 50% respectively. Desensitization of PAR-1 and PAR-4 or pre-treatment with the PAR-1 and PAR-4 antagonists SCH 79797 and tcY-NH2 reduced Ca2+ mobilization induced by thrombin, and depletion of the DTS after desensitization or blockade of PAR-1 and PAR-4 had no significant effect on Ca2+ release stimulated by thrombin through the GPIb-IX-V receptor. Converse experiments showed that depletion of the acidic stores using TBHQ reduced Ca2+ release evoked by SFLLRN or AYPGKF, by 20 and 50% respectively, and abolished thrombin-stimulated Ca2+ release through the GPIb-IX-V receptor when PAR-1 and PAR-4 had been desensitized or blocked. Our results indicate that thrombin-induced activation of PAR-1 and PAR-4 evokes Ca2+ release from both Ca2+ stores, while activation of GPIb-IX-V by thrombin releases Ca2+ solely from the acidic compartments in human platelets.

Protease-activated receptor-4: a novel mechanism of inflammatory pain modulation

BACKGROUND AND PURPOSE

Protease-activated receptor-4 (PAR(4)), the most recently discovered member of the PARs family, is activated by thrombin, trypsin and cathepsin G, but can also be selectively activated by small synthetic peptides (PAR(4)-activating peptide, PAR(4)-AP). PAR(4) is considered a potent mediator of platelet activation and inflammation. As both PAR(1) and PAR(2) have been implicated in the modulation of nociceptive mechanisms, we investigated the expression of PAR(4) in sensory neurons and the effects of its selective activation on nociception.

EXPERIMENTAL APPROACH AND KEY RESULTS

We demonstrated the expression of PAR(4) in sensory neurons isolated from rat dorsal root ganglia by reverse transcription-polymerase chain reaction and immunofluorescence. We found that PAR(4) colocalized with calcitonin gene-related peptide and substance P. We also showed that a selective PAR(4)-AP was able to inhibit calcium mobilization evoked by KCl and capsaicin in rat sensory neurons. Moreover, the intraplantar injection of a PAR(4)-AP significantly increased nociceptive threshold in response to thermal and mechanical noxious stimuli, while a PAR(4) inactive control peptide had no effect. The anti-nociceptive effects of the PAR(4)-AP were dose-dependent and occurred at doses below the threshold needed to cause inflammation. Finally, co-injection of the PAR(4)-AP with carrageenan significantly reduced the carrageenan-induced inflammatory hyperalgesia and allodynia, but had no effect on inflammatory parameters such as oedema and granulocyte infiltration.

CONCLUSIONS AND IMPLICATIONS

Taken together, these results identified PAR(4) as a novel potential endogenous analgesic factor, which can modulate nociceptive responses in normal and inflammatory conditions.

[Role of adenosine diphosphate in the course of thrombin signal transmission]

OBJECTIVE

To study the effects adenosine diphosphate (ADP) on platelet aggregation and expression of glycoprotein (GP) on the surface of platelet membrane after activation of thrombin receptors, so as to investigate its role in thrombin signal transmission.

METHODS

Peripheral blood samples were collected from from 10 healthy volunteers. Platelets were extracted. The thrombin receptor activating peptides (TRAP), protease-activated receptor 1 activated peptide (PAR1-AP, SFLLRN, 25 micromol/L) and PAR4-AP (AYPGKF, 250 micromol/L) were added into the suspension of platelets respectively to induce platelet aggregation. In apyrase inhibition test apyrase II was added into the suspension of platelets for 2 hours and then PAR1-AP or PAR4-AP was added respectively to observe the the expression of GPIb and P-selectin with flow cytometry.

RESULTS

Either PAR1 and PAR4 induced platelet aggregation. After apyrase II stimulation the PAR4-AP induced platelet aggregation was not influenced and PAR1-AP induced platelet aggregation was partially inhibited with a reversible aggregation curve. Stimulated by PAR1-AP and PAR4-AP the GPIb decreased firstly and then gradually returned to normal. Apyrase VII have not significant influence on the GPIb expression, but accelerated the return of GPIb to the platelet surface after PAR1 stimulation so that the lowest point was accelerated to 2 min, compared to that of the control group (5 min) and there were significantly differences 10 and 30 min later between these 2 groups (all P < 0.05). The P-selectin expression was remarkably increased 2 min after the PAR1-AP and PAR4-AP induction and peaked 2 min later. Apyrase VII did not significantly influence the P-selectin expression in these 2 activation ways.

CONCLUSION

ADP plays an important role in the thrombin signal transmission, especially in the PAR1 pathway.

Protease-activated receptors differentially regulate human platelet activation through a phosphatidic acid-dependent pathway

Pathological conditions such as coronary artery disease are clinically controlled via therapeutic regulation of platelet activity. Thrombin, through protease-activated receptor (PAR) 1 and PAR4, plays a central role in regulation of human platelet function in that it is known to be the most potent activator of human platelets. Currently, direct thrombin inhibitors used to block platelet activation result in unwanted side effects of excessive bleeding. An alternative therapeutic strategy would be to inhibit PAR-mediated intracellular platelet signaling pathways. To elucidate the best target, we are studying differences between the two platelet thrombin receptors, PAR1 and PAR4, in mediating thrombin's action. In this study, we show that platelet activation by PAR1-activating peptide (PAR1-AP) requires a phospholipase D (PLD)-mediated phosphatidic acid (PA) signaling pathway. We show that this PAR1-specific PA-mediated effect is not regulated through differential granule secretion after PAR-induced platelet activation. Perturbation of this signaling pathway via inhibition of lipid phosphate phosphatase-1 (LPP-1) by propranolol or inhibition of the phosphatidylcholine-derived phosphatidic acid (PA) formation by PLD with a primary alcohol significantly attenuated platelet activation by PAR1-AP. Platelet activation by thrombin or PAR4-AP was insensitive to these inhibitors. Furthermore, these inhibitors significantly attenuated activation of Rap1 after stimulation by PAR1-AP but not thrombin or PAR4-AP. Because PA metabolites such as diacylglycerol play an important role in intracellular signaling, identifying crucial differences in PA regulation of PAR-induced platelet activation may lead to a greater understanding of the role of PAR1 versus PAR4 in progression of thrombosis.

Protease-activated receptors in cardiovascular diseases

Thrombosis associated with the pathophysiological activation of platelets and vascular cells has brought thrombin and its receptors to the forefront of cardiovascular medicine. Thrombin signaling through the protease-activated receptors (PARs) has been shown to influence a wide range of physiological responses including platelet activation, intimal hyperplasia, inflammation, and maintenance of vascular tone and barrier function. The thrombin receptors PAR1 and PAR4 can be effectively targeted in animals in which acute or prolonged exposure to thrombin leads to thrombosis and/or restenosis. In the present study, we describe the molecular and pharmacological basis of small-molecule inhibitors that target PAR1. In addition, we discuss a new class of cell-penetrating inhibitors, termed pepducins, that provide insight into previously unidentified roles of PAR1 and PAR4 in protease signaling.

PAR3 acts as a molecular organizer to define the apical domain of chick neuroepithelial cells

Neural progenitors in the vertebrate nervous system are fully polarized epithelial cells, with intercellular junctions at the apical region. These progenitor cells remain within the neuroepithelium throughout neurogenesis, and will ultimately give rise to all the neurons in the mature nervous system. We have addressed the role of the PAR polarity complex in vertebrate neuroepithelial polarity and show that PAR3 functions as the initial scaffold to assemble and organize the PAR complex at the apical region of neuroepithelial cells, coordinating also the recruitment of additional polarity complexes and junction-associated proteins to the same region, while restricting other polarity proteins to the basolateral membrane. We propose that PAR3 acts as a molecular organizer to connect the acquisition of apico-basal polarity with the positioning and formation of junctional structures in neuroepithelial cells, a function of upmost importance for the morphogenesis of embryonic neural tissue and the process of neurogenesis.

Par-4 is a novel mediator of renal tubule cell death in models of ischemia-reperfusion injury

Prostate apoptosis response-4 (Par-4) is a leucine zipper protein linked to apoptotic cell death in prostate cancer and neuronal tissues. The leucine zipper domain of Par-4 (Leu.zip) mediates protein-protein interactions that are essential for sensitization of cells to apoptosis, and overexpression of Leu.zip blocks Par-4 activity in a dominant negative fashion. Ischemia-reperfusion-induced renal injury (IRI) is clinically important because it typically damages renal tubular epithelial cells and glomerular cells, and it is the most common cause of acute renal failure (ARF). We now report that Par-4 is expressed in renal tubule cells and that aberrant expression of Par-4 activity plays a crucial role in activating apoptotic pathways in well-characterized models of renal IRI. Increased levels of Par-4 were observed following chemical ischemia-reperfusion in HK-2 cells in vitro and in mouse renal tubular cells following bilateral clamping of renal pedicles in vivo. Inhibition of Par-4 expression by specific par-4 antisense oligonucleotides largely prevented HK-2 cell apoptosis induced by IRI. Overexpression of Par-4 in these cells exacerbated mitochondrial dysfunction and caspase activation and conferred increased sensitivity to IRI-induced apoptosis. Expression of Leu.zip, a dominant negative regulator of Par-4, largely prevented mitochondrial dysfunction and caspase activation and significantly inhibited IRI-induced apoptosis in HK-2 cells. In addition, transfection of Par-4 increased while transfection of Leu.zip decreased necrosis in HK-2 cells following prolonged IRI. These results identify Par-4 as a novel and early mediator of renal tubule cell injury following IRI and provide a potential target for developing new therapeutic strategies for renal IRI and ARF.

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.

The role of protease-activated receptors on the intracellular calcium ion dynamics of vascular smooth muscles, with special reference to cerebral arterioles

Protease-activated receptors (PARs) mediate cellular responses to various proteases in numerous cell types, including smooth muscles and the endothelium of blood vessels. To clarify whether the stimulation of PARs induces responses in smooth muscle cells of cerebral arterioles, intracellular Ca2+([Ca2+]i) dynamics and nitric oxide (NO) production during PARs stimulation were investigated in the rat cerebral arterioles by real-time confocal microscopy, since [Ca2+]i and NO are both key factors in the maintenance of strain in blood vessels. Testicular arterioles were also investigated for comparison. In smooth muscle cells of small cerebral arterioles (< 50 microm in diameter), thrombin and PAR1-activating peptide (AP) induced an increase in [Ca2+]i and contraction. The response to PAR1 activation was caused by Ca2+ mobilization from intracellular Ca2+ stores. Trypsin and PAR2-AP induced a decrease in [Ca2+]i in the cells which was considered to be mediated by endothelium-derived NO and/or by promoting a Ca2+ sequestration mechanism. PAR3- and 4-AP had little effect. In contrast to small cerebral arterioles, [Ca2+]i dynamics in smooth muscle cells of large cerebral arterioles (< 150 microm in diameter) or testicular arterioles remained unchanged during PARs activation. The effects of PARs activation on the [Ca2+]i dynamics and the contraction/relaxation of cerebral arterioles are also discussed in relation to the role of proteases in the regional tissue circulation of the brain.

PAR3 is essential for cyst-mediated epicardial development by establishing apical cortical domains

Epithelial cysts are one of the fundamental architectures for mammalian organogenesis. Although in vitro studies using cultured epithelial cells have revealed proteins required for cyst formation, the mechanisms that orchestrate the functions of these proteins in vivo remain to be clarified. We show that the targeted disruption of the mouse Par3 gene results in midgestational embryonic lethality with defective epicardial development. The epicardium is mainly derived from epicardial cysts and essential for cardiomyocyte proliferation during cardiac morphogenesis. PAR3-deficient epicardial progenitor (EPP) cells do not form cell cysts and show defects in the establishment of apical cortical domains, but not in basolateral domains. In PAR3-deficient EPP cells, the localizations of aPKC, PAR6β and ezrin to the apical cortical domains are disturbed. By contrast, ZO1 andα4/β1 integrins normally localize to cell-cell junctions and basal domains, respectively. Our observations indicate that EPP cell cyst formation requires PAR3 to interpret the polarity cues from cell-cell and cell-extracellular matrix interactions so that each EPP cell establishes apical cortical domains. These results also provide a clear example of the proper organization of epithelial tissues through the regulation of individual cell polarity.

Involvement of Gi/o in the PAR-4-induced NO production in endothelial cells

We investigated the involvement of G(i/o) protein in NO production following the activation of proteinase-activated receptor-4 (PAR-4) in cultured bovine aortic endothelial cells. AYPGKF-NH(2) (PAR-4 activating peptide), thrombin, and ionomycin induced a concentration-dependent NO production, with the maximal production seen at 30 microM, 0.1U/ml, and 1 microM, respectively. Ionomycin elevated [Ca(2+)](i) in a concentration-dependent manner. However, AYPGKF-NH(2) and thrombin induced no [Ca(2+)](i) elevation. The loading of cells with BAPTA almost completely inhibited both the NO production and [Ca(2+)](i) elevation induced by 1 microM ionomycin, while it had no significant effect on the AYPGKF-NH(2)-induced NO production. Treatment with pertussis toxin inhibited the AYPGKF-NH(2)-induced NO production, while it had no effect on the ionomycin-induced NO production. Our findings thus demonstrate, for the first time, that PAR-4 activation induced NO production in a manner mostly independent of the Ca(2+) signal and also that G(i/o) is involved in such NO production in vascular endothelial cells.

Neutrophils and the kallikrein-kinin system in proteinase-activated receptor 4-mediated inflammation in rodents

1 We evaluated a potential role for proteinase-activated receptor 4 (PAR(4)) in a rodent paw inflammation model, with a focus on two main features of inflammation: (1) oedema and (2) granulocyte recruitment. 2 A PAR(4) antagonist (Pepducin P4pal-10; palmitoyl-SGRRYGHALR-NH(2)) reduced both the oedema and granulocyte recruitment induced by a localized administration of carrageenan in the rat hind paw, pointing to a key role for PAR(4) in this inflammation model. 3 Further, intraplantar injection in the mouse hind paw of a PAR(4) agonist (AYPGKF-NH(2)), but not its standard PAR(4)-inactive peptide control (YAPGKF-NH(2)), caused an inflammatory reaction characterized by oedema (increased paw thickness) and granulocyte recruitment (increased paw myeloperoxidase activity). The PAR(4) agonist-induced effects were inhibited in mice pretreated with pepducin P4pal10. 4 These PAR(4) agonist-mediated effects were not affected by pretreatment with inhibitors of either NO production or prostaglandin release (L-NAME and indomethacin, respectively). 5 However, selective immuno-depletion of neutrophils significantly reduced PAR(4) agonist-induced oedema formation. 6 Moreover, AYPGKF-NH(2)-induced oedema was also reduced by pretreatment with either a kinin B(2) receptor antagonist (icatibant) or a tissue or plasma kallikrein inhibitor (FE999024 and FE999026, respectively), but not with a kinin B(1) receptor antagonist (SSR240612). 7 We conclude: (1) that PAR(4) plays an important role in the inflammatory response as it mediates some of the hallmarks of inflammation and (2) that PAR(4)-mediated oedema is dependent on the recruitment of neutrophils and components of the kallikrein-kinin system.

[Thrombin and angiogenesis of cancer--review]

Thrombin is the most important factor in hemostasis. In recent years, it has been found that thrombin is a potent mitogen capable of inducing cellular functions. Therefore, it is proved to be of importance in promoting the growth, metastasis and angiogenesis of cancer. Anticoagulant therapy not only reduce the characteristic hypercoagulability of cancer, but also inhibits growth and metastasis of cancer, and alters the fundamental biology of cancer. In this paper thrombin and its receptor, relationship of thrombin and its receptor with cancer growth, metastasis and angiogenesis, the mechanisms of thrombin influence on cancer angiogenesis, as well as application prospects on anti-angiogenesis and anti-coagulation therapy were reviewed.

Chronic morphine up-regulates G alpha12 and cytoskeletal proteins in Chinese hamster ovary cells expressing the cloned mu opioid receptor

A growing body of literature indicates that chronic morphine exposure alters the expression and function of cytoskeletal proteins in addition to the well established interactions between mu opioid receptors and G proteins. In the present study, we hypothesized that chronic morphine alters the expression and functional effects of G alpha12, a G protein that regulates downstream cytoskeletal proteins via its control of RhoA. Our results showed that chronic morphine treatment decreased the expression of G alpha i2 (64%) and G alpha i3 (60%), had no effect of G alpha o, and increased G alpha12 (66%) expression in Chinese hamster ovary (CHO) cells expressing the cloned human mu opioid receptors (hMOR-CHO cells) but not in cells expressing a mutant mu opioid receptor that do not develop morphine tolerance and dependence (T394A-CHO cells). Morphine treatment had no significant effect on PAR-1 thrombin receptor-activated G protein activity, as measured by thrombin-stimulated guanosine 5'-O-(3-[35S]thio)triphosphate binding. Chronic morphine treatment significantly enhanced thrombin-stimulated RhoA activity and thrombin-stimulated expression of alpha-actinin, a cytoskeletal anchoring protein, in hMOR-CHO cells. Proteomic analysis of two-dimensional gel spots prepared from hMOR-CHO cells showed that morphine treatment affected the expression of a number of proteins associated with morphological changes. Up-regulation of G alpha12 and alpha-actinin by chronic morphine was also observed in mouse brain. Viewed collectively, these findings indicate, for the first time, that chronic morphine enhances the G alpha12-associated signaling system, which is involved in regulating cellular morphology and growth, supporting other findings that chronic morphine may alter cellular morphology, in addition to cellular function.

PAR1-type thrombin receptor stimulates migration and matrix adhesion of human colon carcinoma cells by a PKCepsilon-dependent mechanism

The proteinase-activated receptor1 (PAR1) was characterized as a functional receptor for thrombin in cells from different tumor entities. In colon carcinoma, its function has to be defined. In this study we demonstrate that the PAR1-selective agonist peptide TFLLRN induced activation of protein kinase C isoenzymes alpha and epsilon in human HT-29 colon carcinoma cells expressing PAR1 endogeneously. On the cellular level, TFLLRN and thrombin prompted HT-29 cell migration and matrix adhesion by a PKCepsilon-dependent mechanism as concluded because of the inhibition of PAR1-mediated effects by the PKC inhibitor bisindolylmaleimide I and the PKCepsilon translocation inhibitory peptide EAVSLKPT but not by the PKC inhibitor Gö 6976. In addition, blockade of PAR1 by RWJ 56110, a selective PAR1 antagonist, fully abolished the effect of thrombin on HT-29 cell migration and adhesion. Therefore, PAR1 seems to be the responsible receptor for thrombin-induced migration and adhesion of human colon carcinoma cells including PKCepsilon as an essential signal transducer.

[Functions of thrombin receptors in the reversible distribution of platelet surface glycoprotein I balpha in activated platelets]

OBJECTIVE

To detect the redistribution of platelet surface glycoprotein (GP)Ib alpha and cytoskeleton reorganization in the course of thrombin receptor activation, and investigate the mechanism of GPIb alpha re-translocation and the role of thrombin receptors in platelet signal transduction.

METHODS

The thrombin receptor activating peptide (PAR1-AP, TRAP) was used for stimulating platelet at different time points (0 - 60 min), then the platelet surface GPIb alpha and P-selectin were examined with flow cytometry, and the alterations of GPIb alpha, actin and myosin were analyzed in cytoskeleton by Western blot and GPIb alpha immunoprecipitation. Cytochalasin D and/or Apyrase VII were used for investigating their inhibitory effect on platelet activation.

RESULTS

An increase of P-selectin and reversible internalization of GPIb alpha were observed within platelets upon TRAP activation, and transient changes of actin, myosin and GPIb alpha/myosin, GPIb alpha/actin association were also found in this course. These changes were apparently blocked by cytochalasin D, which inhibited the incorporation of GPIb alpha, actin and myosin into cytoskeleton. Apyrase VII had a weak effect on GPIb alpha internalization, although it accelerated the return of GPIb alpha to platelet surface. In addition, Apyrase VII also quickened the GPIb alpha disappearance in cytoskeleton and the dissociation of GPIb/myosin or GPIb/actin during activation.

CONCLUSION

Thrombin receptor activation takes part in platelet signal transduction, inducing a reversible redistribution of GPIb alpha. This process is related to cytoskeleton reorganisation and ADP.

The role of protease activation of inflammation in allergic respiratory diseases

Extracellular endogenous proteases, as well as exogenous proteases from mites and molds, react with cell-surface receptors in the airways to generate leukocyte infiltration and to amplify the response to allergens. Stimulation leads to increased intracellular Ca ++ and gene transcription. The most thoroughly investigated receptors, protease-activated receptors (PARs), are 7-transmembrane proteins coupled to G proteins. PARs are widely distributed on the cells of the airways, where they contribute to the inflammation characteristic of allergic diseases. PAR stimulation of epithelial cells opens tight junctions, causes desquamation, and produces cytokines, chemokines, and growth factors. They degranulate eosinophils and mast cells. Proteases contract bronchial smooth muscle and cause it to proliferate. PARs also promote maturation, proliferation, and collagen production of fibroblast precursors and mature fibroblasts. PAR-2, apparently the most important of the 4 PARs that have been characterized, is increased on the epithelium of patients with asthma. Trypsin, a product of injured epithelial cells, and mast cell tryptase are potent activators of PAR-2. Mast cell chymase activates PAR-1. Proteases from mites and molds appear to act through similar receptors. They amplify IgE production to allergens, degranulate eosinophils, and can generate inflammation, even in the absence of IgE. Proteases produced by Aspergillus species to support its growth are presumably responsible for the exuberant IgE, IgG, and granulomatous response of allergic bronchopulmonary aspergillosis. Similar proteases from molds germinating on the respiratory mucosa have been recently been implicated in the pathogenesis of chronic hyperplastic rhinitis and polyps and, by extension, of intrinsic asthma. Finally, proteases from mites and fungi growing in damp, water-damaged buildings might be the basis for the increased prevalence in these buildings of rhinitis, asthma, and other respiratory diseases. Future research promises to promote our understanding of the pathogenesis of allergic respiratory diseases and point the way to new therapies.

How important is preventing platelet activation in the setting of percutaneous coronary intervention?

The benefit of antithrombotic therapies is exerted primarily through inhibition of platelet activation and not solely through platelet aggregation. Inhibition of platelet activation can influence localized inflammatory processes as well as long-term clinical outcome. The combination of aspirin, adequate platelet inhibition with P2Y12-receptor inhibitors and thrombin inhibition can synergistically inhibit platelet activation, and the use of this combination may have important clinical implications for treatment of patients undergoing percutaneous coronary intervention.

Multiple platelet defects identified by flow cytometry at diagnosis in acute myeloid leukaemia

Summary Previous findings of megakaryocytic hypogranulation and dysmegakaryocytopoietic features in acute myeloid leukaemia (AML) strongly indicate defects in platelet production. The bleeding tendency of these patients may result from dysregulated platelet production, resulting in thrombocytopenia as well as qualitative platelet defects. The present study examined platelet function at diagnosis in 50 AML patients by whole blood flow cytometry. Following in vitro platelet agonist stimulation, platelet activation markers were analysed and compared with 20 healthy individuals. To detect recent in vivo platelet activation, plasma soluble P-selectin (sP-selectin) was measured. Flow cytometric analysis of platelet activation markers demonstrated reduced CD62P [35.6 vs. 118.5 x 10(3) molecules of equivalent soluble fluorochrome (MESF); P < 0.0001], CD63 (11.3 vs. 50.7 x 10(3) MESF; P < 0.0001), and PAC-1 (41.5 vs. 90.5%; P = 0.0001) while reductions in CD42b were abnormal (45.6 vs. 70%; P < 0.0001). sP-selectin levels were similar in patients and healthy controls (0.04 vs. 0.27 fg/platelet; P = 0.84). The presented data indicate that AML pathogenesis may result in multiple platelet defects, involving adhesion, aggregation, and secretion and demonstrate that flow cytometry is a feasible method for platelet function analysis in patients with thrombocytopenia.

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.

Impaired hemostasis and protection against thrombosis in protease-activated receptor 4-deficient mice is due to lack of thrombin signaling in platelets

Platelets from protease-activated receptor 4 (PAR4)-deficient mice are unresponsive to thrombin, and Par4-/- mice have prolonged bleeding times and are protected against thrombosis. However, in addition to its role in platelets, PAR4 contributes to thrombin signaling in cells in the blood vessel wall that might participate in hemostasis and thrombosis, such as endothelial cells. To determine whether the hemostatic and thrombotic phenotypes of Par4-/- mice were due to loss of PAR4 function in hematopoietic vs. other cell types, tail bleed times and thromboplastin-induced pulmonary embolism were examined in lethally irradiated mice reconstituted with Par4+/+ or Par4-/- bone marrow. In Par4+/+ and Par4-/- mice reconstituted with Par4+/+ marrow, the median tail bleed times were 2.0 and 1.7 min, respectively, vs. > 10 min for both Par4+/+ and Par4-/- mice reconstituted with Par4-/- marrow. In the pulmonary embolism model, Par4+/+ and Par4-/- mice reconstituted with Par4+/+ marrow survived a median of 3.7 and 2.8 min, respectively, after administration of thromboplastin, vs. > 20 min for both Par4+/+ and Par4-/- mice reconstituted with Par4-/- marrow. Further, the phenotype of mice reconstituted with Par4-/- marrow was almost as dramatic as that seen in Nf-e2-/- mice, which lack platelets. These data strongly suggest that increased tail bleed times and protection against thrombosis in Par4-/- mice are accounted for by lack of PAR4 function in platelets, emphasize the importance of thrombin signaling in platelets among the multiple pathways and cell types that govern hemostasis and thrombosis.

Evidence for functionally active protease-activated receptor-3 (PAR-3) in human vascular smooth muscle cells

The present study investigates whether vascular smooth muscle cells of the human saphenous vein (SMC) express a functionally active protease-activated receptor-3 (PAR-3). PAR-3 mRNA was detected by RT-PCR. In the presence of thrombin, a rapid and transient increase in PAR-3 mRNA was observed. Stimulation of SMC with thrombin or the synthetic PAR-3-activating peptide, TFRGAP, resulted in transient mobilization of intracellular calcium. After a preceding challenge with thrombin, the calcium signal to TFRGAP was abolished, suggesting cleavage and subsequent desensitization of PAR-3 by thrombin. Activation of PAR-3 by TFRGAP elicited a time-dependent activation of the extracellular-signal-regulated kinase (ERK)-1/2 with a maximum response 10-20 min after stimulation. At 200 microM, TFRGAP increased [3H]-thymidine incorporation into cellular DNA about two-fold. These data indicate that PAR-3 is expressed in human SMC and triggers intracellular signaling. Thus, in the SMC PAR-3 might contribute to thrombin-induced responses.

Apoptosis in prostate cancer: progressive and therapeutic implications (Review)

Prostate cancer is the most common non-cutaneous malignancy in American men and the second greatest cause of cancer-related death. Development of effective therapeutic modalities for the treatment of this cancer relies heavily on understanding the molecular alterations that result in the initiation and progression of the tumorigenic process. Increasing evidence indicates that impaired ability to undergo apoptosis plays an important role in the evolution from androgen-dependent to androgen-independent prostate cancer. In this review, we address recent progress toward the central objectives of understanding the molecular events that contribute to prostate cancer progression. We focus on some key regulatory molecules, including the pro-apoptotic regulators p53, PTEN, caspases and Par-4, and the anti-apoptotic molecules Bcl-2, NF-kappaB and Akt, to discuss their roles in prostate cancer progression and their therapeutic implications in human prostate carcinoma.

Contribution of protease-activated receptors 1 and 4 and glycoprotein Ib-IX-V in the G(i)-independent activation of platelet Rap1B by thrombin

Thrombin activates human platelets through three different membrane receptors, the protease-activated receptors PAR-1 and PAR-4 and the glycoprotein Ib (GPIb)-IX-V complex. We investigated the contribution of these three receptors to thrombin-induced activation of the small GTPase Rap1B. We found that, similarly to thrombin, selective stimulation of either PAR-1 or PAR-4 by specific activating peptides caused accumulation of GTP-bound Rap1B in a dose-dependent manner. By contrast, in PAR-1- and PAR-4-desensitized platelets, thrombin failed to activate Rap1B. Thrombin, PAR-1-, or PAR-4-activating peptides also induced the increase of intracellular Ca(2+) concentration and the release of serotonin in a dose-dependent manner. We found that activation of Rap1B by selected doses of agonists able to elicit comparable intracellular Ca(2+) increase and serotonin release was differently dependent on secreted ADP. In the presence of the ADP scavengers apyrase or phosphocreatine-phosphocreatine kinase, activation of Rap1B induced by stimulation of either PAR-1 or PAR-4 was totally inhibited. By contrast, thrombin-induced activation of Rap1B was only minimally affected by neutralization of secreted ADP. Concomitant stimulation of both PAR-1 and PAR-4 in the presence of ADP scavengers still resulted in a strongly reduced activation of Rap1B. A similar effect was also observed upon blockade of the P2Y12 receptor for ADP, as well as in P2Y12 receptor-deficient human platelets, but not after blockade of the P2Y1 receptor. Activation of Rap1B induced by thrombin was not affected by preincubation of platelets with the anti-GPIbalpha monoclonal antibody AK2 in the absence of ADP scavengers or a P2Y12 antagonist but was totally abolished when secreted ADP was neutralized or after blockade of the P2Y12 receptor. Similarly, cleavage of the extracellular portion of GPIbalpha by the cobra venom mocarhagin totally prevented Rap1B activation induced by thrombin in the presence of apyrase and in P2Y12 receptor-deficient platelets. By contrast, inhibition of MAP kinases or p160ROCK, which have been shown to be activated upon thrombin binding to GPIb-IX-V, did not affect agonist-induced activation of Rap1B in the presence of ADP scavengers. These results indicate that although both PAR-1 and PAR-4 signal Rap1B activation, the ability of thrombin to activate this GTPase independently of secreted ADP involves costimulation of both receptors as well as binding to GPIb-IX-V.

Role of protease-activated receptors in the vascular system

Thrombin is one of the key molecules involved in the development of vascular diseases. Thrombin does not only serve as a coagulation factor, but it also exerts cellular effects by activating protease (proteinase)-activated receptors (PARs), a family of seven-transmembrane G protein-coupled receptors. This study focused on the role of PARs in the vascular system. Among the four members so far identified, PAR-1 and PAR-2 were found to play an important role in the vascular system, while the functional roles of PAR-3 and PAR-4 appear to be mostly limited to platelets. The endothelial cells play a primary role in mediating the vascular effects of PARs under physiological conditions, while PARs of the smooth muscle cells can be induced under pathological conditions, and therefore play a more pathophysiological role. PAR-1 and PAR-2 mediate various vascular effects including regulation of vascular tone, proliferation and hypertrophy of smooth muscle and angiogenesis. Since proteases are activated under pathological conditions such as hemorrhage, tissue damage, and inflammation, PARs are suggested to play a critical role in the development of functional and structural abnormality in the vascular lesion. Understanding the functional role of PARs in the vascular system can thus help in the development of new strategies for the prevention and therapy of vascular diseases.

Role of Thrombin in Angiogenesis and Tumor Progression

Clinical, laboratory, histopathological, and pharmacological evidence support the notion that the coagulation system, which is activated in most cancer patients, plays an important role in tumor biology. Our laboratory has provided evidence that thrombin activates angiogenesis, a process which is essential in tumor growth and metastasis. This event is independent of fibrin formation. At the cellular level many actions of thrombin can contribute to activation of angiogenesis: (1). Thrombin decreases the ability of endothelial cells to attach to basement membrane proteins. (2). Thrombin greatly potentiates vascular endothelial growth factor- (VEGF-) induced endothelial cell proliferation. This potentiation is accompanied by up-regulation of the expression of VEGF receptors (kinase insert domain-containing receptor [KDR] and fms-like tyrosine kinase [Flt-1]). (3). Thrombin increases the mRNA and protein levels of alpha (v)beta (3) integrin and serves as a ligand to this receptor. Furthermore, thrombin increases the secretion of VEGF and enhances the expression and protein synthesis of matrix metalloprotease-9 and alpha (v)beta (3) integrin in human prostate cancer PC-3 cells. These results could explain the angiogenic and tumor-promoting effect of thrombin and provide the basis for development of thrombin receptor mimetics or antagonists for therapeutic application.

Thrombin and phenotypic modulation of the endothelium

Thrombin signaling in the endothelium is linked to multiple phenotypic changes, including alterations in permeability, vasomotor tone, and leukocyte trafficking. The thrombin signal is transduced, at least in part, at the level of gene transcription. In this review, we focus on the role of thrombin signaling and transcriptional networks in mediating downstream gene expression and endothelial phenotype. In addition, we report the results of DNA microarrays in control and thrombin-treated endothelial cells. We conclude that (1) thrombin induces the upregulation and downregulation of multiple genes in the endothelium, (2) thrombin-mediated gene expression involves a multitude of transcription factors, and (3) future breakthroughs in the field will depend on a better understanding of the spatial and temporal dynamics of these transcriptional networks.

Proinflammatory role of proteinase-activated receptor-2 in humans and mice during cutaneous inflammation in vivo

Proteinase-activated receptor-2 belongs to a new subfamily of G-protein-coupled receptors. Its precise role during inflammation and the underlying mechanisms is still unclear. Our study establishes that PAR-2 plays a direct proinflammatory role during cutaneous inflammation in mice and humans in vivo. In a model of experimentally induced allergic (ACD) and toxic (ICD) contact dermatitis (CD) we show that ear swelling responses, plasma extravasation, and leucocyte adherence were significantly attenuated in PAR-2 null mutant (PAR-2-/-) mice compared with wild-type (PAR-2+/+) mice, especially at early stages. The proinflammatory effects by PAR-2 activation were significantly diminished using nitric oxide-synthase inhibitors, while NF-kappaB and neuropeptides appear to play a minor role in these mechanisms. PAR-2-mediated up-regulation of E-selectin and cell adhesion molecule ICAM-1; enhanced plasma extravasation was observed in humans and mice and of interleukin-6 in mice in vivo. Thus, PAR-2 may be a beneficial therapeutic target for the treatment of inflammatory skin diseases.

Protease-activated receptors 1 and 4 mediate thrombin signaling in endothelial cells

Defining the relative importance of protease-activated receptors (PARs) for thrombin signaling in mouse endothelial cells is critical for a basic understanding of thrombin signaling in these cells and for the rational use of knockout mice to probe the roles of thrombin's actions on endothelial cells in vivo. We examined thrombin- and PAR agonist-induced increases in cytoplasmic calcium, phosphoinositide hydrolysis, extracellular signal-regulated kinase (ERK) phosphorylation, and gene expression in endothelial cells from wild-type and PAR-deficient mice. PAR1 and PAR4 agonists triggered responses in wild-type but not in Par1-/- and Par4-/- endothelial cells, respectively. Calcium imaging confirmed that a substantial fraction of individual endothelial cells responded to both agonists. Compared with wild-type cells, Par1-/- endothelial cells showed markedly decreased responses to low concentrations of thrombin, and cells that lacked both PAR1 and PAR4 showed no responses to even high concentrations of thrombin. Similar results were obtained when endothelial-dependent vasorelaxation of freshly isolated mouse aorta was used as an index of signaling in native endothelial cells. Thus PAR1 is the major thrombin receptor in mouse endothelial cells, but PAR4 also contributes. These receptors serve at least partially redundant roles in endothelial cells in vitro and in vivo and together are necessary for the thrombin responses measured.

[Mechanism of action of protease-activated receptors 1 and 4 in platelet activation]

This study was designed to compare the effects of protease-activated receptor 1 (PAR-1) and protease-activated receptor 4 (PAR-4) to the expression of platelet surface GPIbalpha and cytoskeleton reorganization, then to investigate the role of PARs in platelet signal transmission. PAR1 (25 micromol/L) and PAR4 (250 micromol/L) were used to stimulate platelet at different time points (0 - 60 minutes), and the platelet surface GPIbalpha, actin and myosin and P-selectin were detected with flow cytometry, the alteration of GPIbalpha, actin and myosin in cytoskeleton was compared by Western blot, the membrane cytoskeleton followed GPIbalpha immunoprecipitation was analyzed. The results showed that an increase of P-selectin and reversible decrease of GPIbalpha expression were obtained after platelet activation by PAR1 o r PAR4, and a different kinetics of redistribution of GPIbalpha was found for the two peptides all over the time course (P < 0.05). PAR1 acted more potently and rapidly than PAR4, but the effect of PAR4 persisted longer in the course of platelet activation. Meanwhile, there was a transient change of actin, myosin and GPIbalpha in cytoskeleton proteins. Similar redistribution was also found in GPIbalpha/myosin and GPIbalpha/actin association. It is concluded that PAR1 and PAR4 possess an important role in platelet signal transmission. Either of the receptors can mediate platelet activation and GPIbalpha redistribution, which is correlated with cytoskeleton reorganization. PAR1 acts more rapidly, and effect of PAR4 persists longer.

Rapid tau aggregation and delayed hippocampal neuronal death induced by persistent thrombin signaling

Tau hyperphosphorylation, leading to self-aggregation, is widely held to underlie the neurofibrillary degeneration found in Alzheimer's disease (AD) and other tauopathies. However, it is unclear exactly what environmental factors may trigger this pathogenetic tau hyperphosphorylation. From several perspectives, the coagulation serine protease, thrombin, has been implicated in AD and activates several different protein kinase pathways but has not previously been shown how it may contribute to AD pathogenesis. Here we report that nanomolar thrombin induced rapid tau hyperphosphorylation and aggregation in murine hippocampal neurons via protease-activated receptors, which was followed by delayed synaptophysin reduction and apoptotic neuronal death. Mechanistic study revealed that a persistent thrombin signaling via protease-activated receptor 4 and prolonged downstream p44/42 mitogenactivated protein kinase activation are at least in part responsible. These results pathogenetically linked thrombin to subpopulations of AD and other tauopathies associated with cerebrovascular damage. Such knowledge may be instrumental in transforming therapeutic paradigms.

Thrombin and platelet activation

The accumulation of thrombin at sites of vascular injury provides one of the chief means for recruiting platelets into a growing hemostatic plug. Studies completed over the past 10 years show that platelet responses to thrombin are mediated by a subset of G protein-coupled receptors known as protease-activated receptors. These receptors are activated on cleavage by thrombin, initiating the intracellular signaling events needed to transform mobile, nonadhesive platelets into cells that can participate in the growth of an immobile hemostatic plug. How this is accomplished is the subject of this review.

PAR-2 modulates pepsinogen secretion from gastric-isolated chief cells

In the present study, we investigated whether activation of protease-activated receptor type 2 (PAR-2) with SLIGRL (SL)NH2, a short mimetic agonistic peptide, directly stimulates pepsinogen secretion from gastric-isolated, pepsinogen-secreting (chief) cells. Immunostaining of gastric-dispersed chief cells with a specific anti-PAR-2 antibody demonstrated expression of PAR-2 receptors on membrane and cytoplasm. SL-NH2 and trypsin potently stimulated pepsinogen secretion (EC50 = 0.3 nM) and caused Ca2+ mobilization (EC50 = 0.6 nM). In contrast to SL-NH2, the scramble peptide LSIGRL-NH2 failed to stimulate pepsinogen release. Exposure to SL-NH2 also resulted in ERK1/2 phosphorylation and activation. Exposure of chief cells to phosphotyrosine kinase inhibitors and 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one, a selective MEK inhibitor, significantly reduced secretion induced by SL-NH2. Pepsinogen secretion induced by SL-NH2 was desensitized by pretreating the cells with the mimetic peptide and trypsin, and exposure to SL-NH2 abrogates pepsinogen secretion induced by carbachol and CCK-8, but not secretion induced by secretin and vasointestinal peptide. Exposure to Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2 (substance P) but not to calcitonin gene-related peptide increased pepsinogen release. The neurokinin-1 receptor antagonist, N-acetyl-l-tryptophan 3,5-bis(trifluoromethyl)benzyl ester, inhibited substance P-stimulated pepsinogen secretion, whereas it did not affect secretion induced by SL-NH2. Collectively, these data indicate that PAR-2 is expressed on gastric chief cells and that its activation causes a Ca2+-ERK-dependent stimulation of pepsinogen secretion.

The role of PAR4 in thrombin-induced thromboxane production in human platelets

There are two protease-activated receptors (PARs), PAR1 and PAR4, in human platelets. It has been suggested that PAR1 mediates platelet responses to low concentrations of thrombin, whereas PAR4 mediates signaling only at high concentrations. In the present study, we used a selective PAR4 blocker, YD-3, to investigate the role of PAR4 in thrombin-induced thromboxane formation in human platelets. YD-3 completely prevented thromboxane production by either a low concentration of thrombin (0.1 U/ml) or the PAR4 agonist peptide GYPGKF. In contrast, YD-3 did not affect thromboxane production caused by the PAR1 agonist peptide SFLLRN, collagen or arachidonic acid. YD-3 also decreased [(3) H]arachidonic acid release from thrombin-stimulated platelets. Moreover, desensitization of platelets with GYPGKF prevented low thrombin-induced thromboxane formation. The decreased thromboxane production by YD-3 is linked to inhibition of calcium influx in thrombin-stimulated platelets. These results suggest that PAR4 plays an important role in the regulation of thromboxane formation in platelets responding to thrombin through prolonged elevation of [Ca(2+)](i) and activation of phospholipase A(2). These data also indicate that PAR4 can be activated by relatively low concentrations of thrombin in human platelets. The selective inhibition of thrombin-induced thromboxane production by YD-3 may be of therapeutic benefit for thrombotic diseases.