Proteolytically activated receptor-3. A member of an emerging gene family of protease receptors expressed on vascular endothelial cells and platelets.

Macromolecular assembly and generation of serine proteases on cellular surfaces is critically involved in regulation of hemostatic, inflammatory, or fibrinolytic pathways. The concept that a number of these serine proteases may effect cellular activation and proliferative responses has engendered an emerging paradigm focusing on the molecular mechanisms regulating cellular/protease interactions. Previous data suggest that some of these cellular responses are mediated by a novel class of G protein-coupled proteolytically activated receptors. Proteolytically activated receptor-3 (PAR-3) is the third member of this rapidly emerging gene family, all three of which (PAR-1, PAR-2, PAR-3) are known to co-cluster in the human genome, and are expressed on vascular endothelial cells, cells which critically regulate the hemostatic repertoire. This review will focus on the genetics of these receptors (emphasizing recent advances in the identification and characterization of PAR-3), review known structure/function similarities, and outline potential links in regulation of the hemostatic response by protease generation on the endothelial cell surface.

The human proteinase-activated receptor-3 (PAR-3) gene. Identification within a Par gene cluster and characterization in vascular endothelial cells and platelets.

Proteolytically activated receptors (PARs) represent an emerging subset of seven transmembrane G protein-coupled receptors that mediate cell activation events by receptor cleavage at distinct scissile bonds located within receptor amino termini. Differential genomic blotting using a yeast artificial chromosome known to contain the PAR-1 and PAR-2 genes identified the PAR-3 gene within a PAR gene cluster spanning approximately 100 kilobases at 5q13. The PAR-3 gene is relatively small (approximately 12 kilobases); and, like the PAR-1 and PAR-2 genes, it displays a two-exon structure, with the majority of the coding sequence and the proteolytic cleavage site contained within the larger second exon. Sequence analysis of the 5'-flanking region demonstrates that the promoter is TATA-less, similar to that seen with PAR-1, with the identification of nucleic acid motifs potentially involved in transcriptional gene regulation, including AP-1, GATA, and octameric sequences. PAR-3 transcripts were apparent in human vascular endothelial cells, although at considerably lower levels than those of PAR-1 and not significantly modulated by the endothelial cell stimulus tumor necrosis factor-alpha. Likewise, although PAR-3 mRNA was evident in human platelets, receptor cell surface expression was modest (approximately 10%) compared with that of PAR-1. Thus, although PAR-3 is postulated to represent a second thrombin receptor, its modest endothelial cell and platelet expression suggest that PAR-3 activation by alpha-thrombin is less relevant for physiological responses in these mature cells. Rather, given its disparately greater expression in megakaryocytes (and megakaryocyte-like human erythroleukemia cells), a regulatory role in cellular development (by protease activation) could be postulated.