PAR Peptides and Analogs

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Introduction

Protease-activated receptors (PAR) Peptides and analogs are a subfamily of related G protein-coupled receptors that are activated by cleavage of part of their extracellular domain. They belonging to the G-coupled seven-transmembrane domain family, have a unique mode of activation, and are expressed throughout the body. They are highly expressed in platelets, and also on endothelial cells, myocytes and neurons. There are 4 known protease-activated receptors, PAR1, PAR2, PAR3, and PAR4. PAR1 is a thrombin receptor, PAR2 is a receptor for insulin, mast cell fibrinolytic enzyme, coagulation factor and other unknown proteolytic enzymes. PAR2 induces secretion of salivary glands and pancreas. PAR1 and PAR2 have protective effects on gastric mucosa. But the mechanism is different.

Mechanism of action

PARs are activated by the action of serine proteases such as thrombin (acts on PAR1, PAR3 and PAR4) and trypsin (PAR2). These enzymes cleave the N-terminus of the receptor, which in turn acts as a tethered ligand. In the cleaved state, part of the receptor itself acts as the agonist, causing a physiological response. Most of the PAR family act through the actions of G-proteins i (cAMP inhibitory), G_12/13 (Rho and Ras activation) and G_q (calcium signalling) to cause cellular actions.

Serine proteases are signaling molecules that specifically regulate cells by cleaving and triggering members of a new family of proteinase-activated receptors (PARs).  Proteases cleave within the extracellular NH₂-terminus of their receptors to expose a new NH₂-terminus. Specific residues within this tethered ligand domain interact with extracellular domains of the cleaved receptor, resulting in activation. In common with many G protein-coupled receptors, PARs couple to multiple G proteins and thereby activate many parallel mechanisms of signal transduction. PARs are expressed in multiple tissues by a wide variety of cells, where they are involved in several pathophysiological processes, including growth and development, mitogenesis, and inflammation. Because the cleaved receptor is physically coupled to its agonist, efficient mechanisms exist to terminate signaling and prevent uncontrolled stimulation. These include cleavage of the tethered ligand, receptor phosphorylation and uncoupling from G proteins, and endocytosis and lysosomal degradation of activated receptors.

Application of PAR Peptides and Analogs

PAR and analogs plays a key role in physiological and pathophysiological aspects, including inducing coagulation reaction, promoting cell division and proliferation, releasing inflammatory mediators or cytokines to regulate local inflammatory response, gastrointestinal tract and airway smooth muscle, regulate vascular tension, etc.

References

1. Macfarlane, S. R., Seatter, M. J., Kanke, T., Hunter, G. D., Plevin, R. (2001). Proteinase-activated receptors. Pharmacological Reviews, 53(2), 245-282.
2. Chung, A. W., Jurasz, P., Hollenberg, M. D., Radomski, M. W. (2010). Mechanisms of action of proteinase-activated receptor agonists on human platelets. British Journal of Pharmacology, 135(5), 1123-1132.
3. Déry, O., Corvera, C. U., Steinhoff, M., Bunnett, N. W. (1998). Proteinase-activated receptors: novel mechanisms of signaling by serine proteases. American Journal of Physiology, 274(1), 1429-52.