Proteinase-mediated signaling: Proteinase-activated receptors (PARs) and much more

Abstract

Quite apart from their ability to generate active polypeptides from hormone precursors and to function as digestive enzymes, proteinases are now known to play a hormone-like role by triggering signal transduction pathways in target cells. The best understood example of proteinase-mediated signaling can be seen in the action of thrombin, which in addition to triggering the coagulation cascade, regulates platelet and endothelial cell function via its serine proteinase activity. The discovery of the G-protein-coupled ‘receptor’ responsible for these cellular actions of thrombin (Proteinase-activated Receptor-1, or PAR₁) represents one of the more intriguing signal transduction stories elucidated over past decade or so. It is the objective of this brief review to provide an overview of the discovery and molecular pharmacology of the PAR family and to indicate the widespread roles these receptor systems can play in a variety of tissues. Further, the article (1) illustrates the utility of employing receptor-selective PAR-activating peptides to determine the potential physiological roles these receptors play in vivo and (2) describes how these agonists have identified receptors other than the PARs. Finally, the mechanisms other than via the PARs by which proteinases can generate cellular signals are summarized.

Introduction

The discovery of the G-protein coupled proteinase-activated receptor (PAR) family, activated by the proteolytic unmasking of a receptor-tethered ligand (Fig. 1), represents one of the more intriguing signal transduction stories elucidated over the past decade or so. A search for the cell surface receptor responsible for thrombin signaling led two groups independently to clone a G-protein coupled receptor responsible for thrombin-mediated calcium signaling and platelet aggregation Rasmussen et al., 1991, Vu et al., 1991. The unique feature of the first receptor discovered to be responsible for these actions of thrombin (now termed proteinase activated receptor-1 or PAR₁), relates to the cleavage by thrombin at arginine 41 of the amino-terminal domain of the human receptor, to reveal a sequence: SFLLRNPN…., that remaining tethered, then binds to and activates the receptor, as depicted in Fig. 1A. A second novel feature discovered about this receptor family by the Coughlin laboratory, as illustrated in Fig. 1B, was that without proteolytic cleavage, the receptor can nonetheless be activated by short peptides (PAR-activating peptides, or PAR-APs) with sequences modeled on the revealed ‘tethered ligand’ (Vu et al., 1991). Cloning of the PAR₁ receptor for thrombin has led ultimately to the discovery of three other members of this novel receptor family, PARs 2, 3, and 4 (summarized by Coughlin, 2000, Dery et al., 1998, Macfarlane et al., 2001, Hollenberg and Compton, 2002). Work on the pharmacology of the PAR-activating peptides has led to the synthesis of PAR-selective receptor-activating agonists. By using the peptide, TFLLR-amide for activating PAR₁ and SLIGRL-amide to activate PAR₂ it has been possible to evaluate the impact of activating these PARs in cell, tissue or intact animals to determine the potential actions that the PAR-activating enzymes might have. Use of the PAR-APs eliminates the confounding effects that the proteinases could potentially have by mechanisms other than via the PARs. One enigma that remains relates to the third member of the family, PAR₃. Unlike other family members, PAR₃ does not appear to signal in response to its cognate activating peptide, but rather appears to function as a co-factor for the activation of PAR₄ Nakanishi-Matsui et al., 2000, Sambrano et al., 2001. However, the independent differential expression of PAR₃ and PAR₄ in different tissues raises the possibility that PAR₃ may play a functional role that has yet to be determined.