Proteinases and signalling: pathophysiological and therapeutic implications via PARs and more

Abstract

Proteinases like thrombin, trypsin and tissue kallikreins are now known to regulate cell signaling by cleaving and activating a novel family of G-protein-coupled proteinase-activated receptors (PARs 1-4) via exposure of a tethered receptor-triggering ligand. On their own, short synthetic PAR-selective PAR-activating peptides (PAR-APs) mimicking the tethered ligand sequences can activate PARs 1, 2 and 4 and cause physiological responses both in vitro and in vivo. Using the PAR-APs as sentinel probes in vivo, it has been found that PAR activation can affect the vascular, renal, respiratory, gastrointestinal, musculoskeletal and nervous systems (both central and peripheral nervous system) and can promote cancer metastasis and invasion. In general, responses triggered by PARs 1, 2 and 4 are in keeping with an innate immune inflammatory response, ranging from vasodilatation to intestinal inflammation, increased cytokine production and increased or decreased nociception. Further, PARs have been implicated in a number of disease states, including cancer and inflammation of the cardiovascular, respiratory, musculoskeletal, gastrointestinal and nervous systems. In addition to activating PARs, proteinases can cause hormone-like effects by other signalling mechanisms, like growth factor receptor activation, that may be as important as the activation of PARs. We, therefore, propose that the PARs themselves, their activating serine proteinases and their associated signalling pathways can be considered as attractive targets for therapeutic drug development. Thus, proteinases in general must now be considered as 'hormone-like' messengers that can signal either via PARs or other mechanisms.

Figure 1

Mechanisms of PAR activation. (a) Activation of PAR signalling by proteinase-mediated cleavage of receptor N-terminus to reveal tethered ligand (TL). (b) Activation of PAR signalling by exogenous application of synthetic PAR agonist peptide (AP) without the need for proteolytic revealing of the TL. PAR, proteinase-activated receptor.

Figure 2

Proteinases targeting PAR₁ and PAR₂. Proteinases can activate PARs by cleaving the receptor to reveal the tethered ligand (TL) (↓) or disarm them by cleaving at other sites (↑), making the TL unavailable to the receptor. Disarmed receptors remain responsive to AP-stimulated activation. hk, human kallekrien; TF, tissue factor; PAR, proteinase-activated receptor; PR3, proteinase 3.

Figure 3

Signalling by proteinases: PARs and more. The scheme illustrates the main mechanisms that proteinases can use to regulate cell signalling in addition to targeting the PARs. The potential proteolytic targets for proteinase-mediated signalling include growth factor receptors, such as the one for insulin that can be activated by trypsin (Cuatrecasas, 1971; Shoelson et al., 1988), membrane-tethered agonists like heparin-binding EGF that can be released by metalloproteinase action to activate the EGF receptor (Prenzel et al., 1999), and either processing of peptide agonist precursors, like angiotensinogen, or inactivation of peptide agonists like neurokinins by membrane-tethered proteinases. Extracellular matrix–integrin interactions that regulate intracellular signalling are also shown as potential targets for enzymes, like the matrix metalloproteinases. Not shown is the ability of a proteinase like thrombin to regulate cell function via its non-catalytic domains (Bar-Shavit et al., 1984, 1986; Glenn et al., 1988). EGF, epidermal growth factor; PAR, proteinase-activated receptor.