Conserved Structure and Adjacent Location of the Thrombin Receptor and Protease-Activated Receptor 2 Genes Define a Protease-Activated Receptor Gene Cluster

Molecular basis of protease-activated receptor 1 signaling diversity

Protease-activated receptors (PARs) are a family of highly conserved G protein-coupled receptors (GPCRs) that respond to extracellular proteases via a unique proteolysis-dependent activation mechanism. Protease-activated receptor 1 (PAR1) was the first identified member of the receptor family and plays important roles in hemostasis, inflammation and malignancy. The biology underlying PAR1 signaling by its canonical agonist thrombin is well characterized; however, definition of the mechanistic basis of PAR1 signaling by other proteases, including matrix metalloproteases, activated protein C, plasmin, and activated factors VII and X, remains incompletely understood. In this review, we discuss emerging insights into the molecular bases for "biased" PAR1 signaling, including atypical PAR1 proteolysis, PAR1 heterodimer and coreceptor interactions, PAR1 translocation on the membrane surface, and interactions with different G-proteins and β-arrestins upon receptor activation. Moreover, we consider how these new insights into PAR1 signaling have acted to spur development of novel PAR1-targeted therapeutics that act to inhibit, redirect, or fine-tune PAR1 signaling output to treat cardiovascular and inflammatory disease. Finally, we discuss some of the key unanswered questions relating to PAR1 biology, in particular how differences in PAR1 proteolysis, signaling intermediate coupling, and engagement with coreceptors and GPCRs combine to mediate the diversity of identified PAR1 signaling outputs.

Down-regulation of protease-activated receptor 4 in lung adenocarcinoma is associated with a more aggressive phenotype

The role of protease-activated receptors (PARs) in lung tumors is controversial. Although PAR4 is preferentially expressed in human lung tissues, its possible significance in lung cancer has not been defined. The studies reported herein used a combination of clinical observations and molecular methods. Surgically resected lung adenocarcinomas and associated adjacent normal lung tissues were collected and BEAS-2B and NCI-H157 cell lines were grown in tissue culture. PAR4 expression was evaluated by RT-PCR, RT-qPCR, Western blotting and immunohistochemistry analysis. The results showed that PAR4 mRNA expression was generally decreased in lung adenocarcinoma tissues as compared with matched noncancerous tissues (67.7%) and was associated with poor differentiation (p=0.017) and metastasis (p=0.04). Western blotting and immunohistochemical analysis also showed that PAR4 protein levels were mostly decreased in lung adenocarcinoma tissues (61.3%), and were also associated with poor differentiation (p=0.035) and clinical stage (p=0.027). Moreover, PAR4 expression was decreased in NCI-H157 cells as compared with BEAS-2B cells. In conclusion, PAR4 expression is significantly decreased in lung adenocarcinoma, and down-regulation of PAR4 is associated with a more clinically aggressive phenotype. PAR4 may acts as a tumor suppressor in lung adenocarcinoma.

The roles of proteinase-activated receptors in the vascular physiology and pathophysiology

Proteinase-activated receptors (PARs) belong to a family of G protein-coupled receptors, thus mediating the cellular effects of proteinases. In the vascular system, thrombin and other proteinases in the coagulation-fibrinolysis system are considered to be the physiologically relevant agonists, whereas PARs are among the most important mechanisms mediating the interaction between the coagulation-fibrinolysis system and the vascular wall. Under physiological conditions, PARs are mainly expressed in endothelial cells, and participate in the regulation of vascular tone, mostly by inducing endothelium-dependent relaxation. PARs in endothelial cells are also suggested to contribute to a proinflammatory phenotypic conversion and an increase in the permeability of vascular lesions. In smooth muscle cells, PARs mediate contraction, migration, proliferation, hypertrophy, and production of the extracellular matrix, thereby contributing to the development of vascular lesions and the pathophysiology of such vascular diseases as atherosclerosis. However, the expression of PARs in the smooth muscle of normal arteries is limited. The upregulation of PARs in the smooth muscle is thus considered to be a key step for PARs to participate in the pathogenesis of vascular lesions. Elucidating the molecular mechanism regulating the PARs expression is therefore important to develop new strategies for the prevention and treatment of vascular diseases.

Expression of functional type 1 protease-activated thrombin receptors by mouse primary palatal mesenchymal cells in vitro

Development of the primary palate involves a series of processes including cell growth, differentiation, and morphogenesis. To study the molecular and cellular processes during mouse primary palatogenesis, mesenchymal cells were isolated from the primary palate of BALB/cBy embryos (day-11, hour 20). Most of the primary palatal mesenchymal (PPM) cells were morphologically similar to fibroblasts. The population doubling time was about 36 h. At concentrations of 5 and 10 unit/ml, alpha-thrombin significantly stimulated the proliferation of these palatal cells by 2- to 2. 4-fold compared to untreated controls over a 72 hour incubation period. Reverse transcriptase-polymerase chain reaction using primers based on the mouse type 1 protease-activated thrombin receptor (PAR1) detected PAR1 mRNA in the PPM cells, the authenticity of which was confirmed by partial DNA sequencing. Blocking of the alpha-thrombin proteolytic site with the highly specific inhibitor D-phenylalanyl-prolyl-arginyl chloromethyl ketone significantly suppressed the mitogenic effect of thrombin on the PPM cells by 71%. These results suggest that PAR1 is present on PPM cells in the mouse embryo and that serine protease activity is important for the receptor activation.

Gene and locus structure and chromosomal localization of the protease-activated receptor gene family

Protease-activate receptors (PARs) mediate activation of platelets and other cells by thrombin and other proteases. Such protease-triggered signaling events are thought to be critical for hemostasis, thrombosis, and other normal and pathological processes. We report here the structure of the mouse and human PAR3 genes as well as the organization of a PAR gene cluster encompassing the genes encoding PARs 1, 2, and 3. We also report the structure of the mouse and human PAR4 genes, which map to distinct chromosomal locations and encode a new thrombin receptor. PARs 1-4 are all encoded by genes with the same two exon structure. In each case, exon 1 encodes a signal peptide, and exon 2 encodes the mature receptor protein. These are separated by an intron of variable size. The genes encoding PARs 1-3 all map to chromosome 13D2 in mouse and chromosome 5q13 in human. In mouse, all three genes are located within 80 kilobases of each other. The PAR1 gene is located centrally and is flanked upstream by the PAR3 gene and downstream by the PAR2 gene in both species. The proximity of the PAR1 and PAR3 genes suggests the possibility that these genes might share regulatory elements. A comparison of the structures of the PAR amino acid sequences, gene structures, locus organization, and chromosomal locations suggests a working model for PAR gene evolution.

Cloning and characterization of human protease-activated receptor 4

Protease-activated receptors 1-3 (PAR1, PAR2, and PAR3) are members of a unique G protein-coupled receptor family. They are characterized by a tethered peptide ligand at the extracellular amino terminus that is generated by minor proteolysis. A partial cDNA sequence of a fourth member of this family (PAR4) was identified in an expressed sequence tag database, and the full-length cDNA clone has been isolated from a lymphoma Daudi cell cDNA library. The ORF codes for a seven transmembrane domain protein of 385 amino acids with 33% amino acid sequence identity with PAR1, PAR2, and PAR3. A putative protease cleavage site (Arg-47/Gly-48) was identified within the extracellular amino terminus. COS cells transiently transfected with PAR4 resulted in the formation of intracellular inositol triphosphate when treated with either thrombin or trypsin. A PAR4 mutant in which the Arg-47 was replaced with Ala did not respond to thrombin or trypsin. A hexapeptide (GYPGQV) representing the newly exposed tethered ligand from the amino terminus of PAR4 after proteolysis by thrombin activated COS cells transfected with either wild-type or the mutant PAR4. Northern blot showed that PAR4 mRNA was expressed in a number of human tissues, with high levels being present in lung, pancreas, thyroid, testis, and small intestine. By fluorescence in situ hybridization, the human PAR4 gene was mapped to chromosome 19p12.

Human Kininogens Regulate Thrombin Binding to Platelets Through the Glycoprotein Ib-IX-V Complex

We and others have shown that both high and low molecular mass kininogens are able to inhibit the thrombin-induced aggregation of gel-filtered platelets, indicating that the locus for inhibition resides in the heavy chain. The inhibitory site is present in domain 3, confined to the C-terminal portion of the region encoded by exon 7 (K270-G292), and the minimal effective sequence is a heptapeptide (L271-A277; Kunapuli et al, J Biol Chem 271:11228, 1996). Kininogens inhibit thrombin binding to platelets and thus inhibit thrombin-induced aggregation. The molecular mechanism by which kininogens inhibit thrombin-induced aggregation of platelets is unknown. Thrombin has previously been shown to bind to two receptors on the platelet surface, glycoprotein (GP) Ib-IX-V complex and the hepta-spanning transmembrane receptor coupled to G protein(s). We now show that, unlike its effect on normal platelets, kininogen (2 μmol/L) did not inhibit the thrombin-induced aggregation of Bernard-Soulier platelets, which lack the GP Ib-IX-V complex, suggesting that kininogen interacts either directly or indirectly with that complex and restricts access by thrombin to this receptor. We further show that both recombinant K270-G292 polypeptide and the synthetic peptide L271-A277 derived from high molecular mass kininogen lower thrombin binding to platelets in a manner similar to monoclonal antibodies to or ligands (von Willebrand factor and echicetin) of GP Ib-IX. The anti–GP Ib-IX-V complex antibodies, TM-60 and SZ 2, can inhibit 125I-high molecular mass kininogen binding to platelets. Conversely, kininogen could block the binding of biotinylated TM-60 or of 125I-SZ 2. Kininogen inhibited the binding of biotinylated thrombin bound to a mouse fibroblast cell line transfected with the GP Ib-IX-V complex. These results indicated that kininogen binds to the GP Ib-IX-V complex modulating thrombin binding to platelets and the consequent platelet aggregation. Kininogen can thus serve as an important regulator of the early stages of platelet stimulation by thrombin.

Human Kininogens Regulate Thrombin Binding to Platelets Through the Glycoprotein Ib-IX-V Complex

We and others have shown that both high and low molecular mass kininogens are able to inhibit the thrombin-induced aggregation of gel-filtered platelets, indicating that the locus for inhibition resides in the heavy chain. The inhibitory site is present in domain 3, confined to the C-terminal portion of the region encoded by exon 7 (K270-G292), and the minimal effective sequence is a heptapeptide (L271-A277; Kunapuli et al, J Biol Chem 271:11228, 1996). Kininogens inhibit thrombin binding to platelets and thus inhibit thrombin-induced aggregation. The molecular mechanism by which kininogens inhibit thrombin-induced aggregation of platelets is unknown. Thrombin has previously been shown to bind to two receptors on the platelet surface, glycoprotein (GP) Ib-IX-V complex and the hepta-spanning transmembrane receptor coupled to G protein(s). We now show that, unlike its effect on normal platelets, kininogen (2 μmol/L) did not inhibit the thrombin-induced aggregation of Bernard-Soulier platelets, which lack the GP Ib-IX-V complex, suggesting that kininogen interacts either directly or indirectly with that complex and restricts access by thrombin to this receptor. We further show that both recombinant K270-G292 polypeptide and the synthetic peptide L271-A277 derived from high molecular mass kininogen lower thrombin binding to platelets in a manner similar to monoclonal antibodies to or ligands (von Willebrand factor and echicetin) of GP Ib-IX. The anti–GP Ib-IX-V complex antibodies, TM-60 and SZ 2, can inhibit 125I-high molecular mass kininogen binding to platelets. Conversely, kininogen could block the binding of biotinylated TM-60 or of 125I-SZ 2. Kininogen inhibited the binding of biotinylated thrombin bound to a mouse fibroblast cell line transfected with the GP Ib-IX-V complex. These results indicated that kininogen binds to the GP Ib-IX-V complex modulating thrombin binding to platelets and the consequent platelet aggregation. Kininogen can thus serve as an important regulator of the early stages of platelet stimulation by thrombin.