Disease relevance of PAR4

• Here we show that at the relatively high concentration of agonist most likely found at the site of a local thrombus, dual inhibition of the P2Y12 receptor and calcium mobilization result in a complete inhibition of PAR4-induced aggregation, while having no effect on either thrombin or PAR1-mediated platelet aggregation [1].
• Adenocarcinomas were significantly more positive than SCC for PAR-1 (17 vs. 8 cases) and PAR-4 (10 vs. 5 cases) [2].

High impact information on PAR4

• Infusion of the anti-PAR4 pepducin into mice extended bleeding time and protected against systemic platelet activation, consistent with the phenotype of PAR4-deficient mice [3].
• The Caenorhabditis elegans and Drosophila melanogaster LKB1 homologs, termed PAR4 and dLKB1, respectively, regulate cell polarity [4].
• PAR1 and PAR4 agonists triggered responses in wild-type but not in Par1-/- and Par4-/- endothelial cells, respectively [5].
• Because adenosine diphosphate and thromboxane A(2) have been shown to cause platelet aggregation by concomitant signaling through G(q) and G(i) pathways, we investigated whether coactivation of G(q) and G(i) signaling pathways is the general mechanism by which PAR1 and PAR4 agonists also activate platelet fibrinogen receptor (alphaIIbbeta3) [6].
• Thrombin activates platelets mainly through protease-activated receptor 1 (PAR1), PAR4, and glycoprotein Ib [6].

Biological context of PAR4

• Human platelets express two thrombin receptors, PAR1 and PAR4, both of which signal platelet activation [1].
• In transfected fibroblasts, PAR4 triggered substantially more phosphoinositide hydrolysis per activated receptor than PAR1 and was shut off more slowly than PAR1 [7].
• Thus PAR4 signaling is shut off less rapidly than PAR1, probably due to differences in receptor phosphorylation [7].
• Thus, the strong signal emanating from PAR4 during platelet aggregation would provide an explanation for the mild bleeding diathesis of HPS [8].
• 3D models of thrombin complexed with PAR1, PAR3, and PAR4 are constructed and account for the perturbations documented by the mutagenesis studies [9].

Anatomical context of PAR4

• 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 [5].
• Cathepsin G triggered calcium mobilization in PAR4-transfected fibroblasts, PAR4-expressing Xenopus oocytes, and washed human platelets [10].
• Expression of constitutively active GFP-Rap1b (V12) had no effect on unstimulated megakaryocytes, but it greatly augmented fibrinogen binding to alpha(IIb)beta(3) induced by a PAR4 thrombin receptor agonist (p < 0.01) [11].
• RT-PCR and Western blot analyses identified PAR4 expression in human pulmonary artery endothelial cells and in human microvascular endothelial cells from lung [12].
• Activation of endothelial PAR1 and PAR2 causes nitric oxide-mediated arterial smooth muscle relaxation in a number of species and PAR4 activation causes similar responses in isolated rat aorta [13].

Associations of PAR4 with chemical compounds

• The N-terminal arginine on RPPGF binds to the P2 position or proline46 on PAR4 to block thrombin cleavage [14].
• Inhibition of PAR4 signaling mediates ethanol-induced attenuation of platelet function in vitro [15].
• Trypsin, thrombin and PAR4 AP released prostaglandin E2 [16].
• Based on the individual kinetic parameters calculated here, bradykinin is classified as a weak competitive inhibitor against hydrolysis of S-2238 and of a PAR4-like peptide [17].
• Adventitially, trypsin, thrombin and PAR4 AP (but not PAR2 AP) relaxed carbachol-toned airways after <3 min [16].

Regulatory relationships of PAR4

• We studied the actions of receptor-activating peptide analogues (PAR4APs), modeled on the proteolytically-revealed tethered ligand sequence of murine proteinase-activated receptor-4 (PAR4), in a rat platelet aggregation assay [18].

Other interactions of PAR4

• The IHC method of streptavidin biotin peroxidase on paraffin tissue sections was used to detect bcl2 and p53 oncoproteins and PAR4 pro-apoptotic protein expression in surgical specimens [19].
• Platelets derived from mer-deficient mice had decreased platelet aggregation in responses to low concentrations of collagen, U46619, and PAR4 thrombin receptor agonist peptide in vitro [20].
• In the present study, we have tested the hypothesis that thrombin-induced endostatin release from rat platelets is mediated via proteinase-activated receptor-4 (PAR4) [21].
• The reverse human PAR4 sequence (VQGPYG-NH2, YG-NH2) and the PAR1AP SFLLR-NH2, did not cause aggregation [18].

Analytical, diagnostic and therapeutic context of PAR4

• Minimal expression of PAR4 was detected by RT-PCR, but not by Northern analysis [22].
• HPLC kinetic results on hydrolysis of PAR4 peptides (38-51 and 38-62) reveal that extending the sequence from the active site toward the exosite does not promote further binding interactions with thrombin [23].
• 2. mRNAs for both, PAR-1 and PAR-4, were detected in the SMC by using reverse transcriptase polymerase chain reaction (RT - PCR) [24].
• Molecular cloning of the rat proteinase-activated receptor 4 (PAR4) [25].

References