Thrombin-Mediated Direct Activation of Proteinase-Activated Receptor-2: Another Target for Thrombin Signaling

Thrombin is known to signal to cells by cleaving/activating a G-protein-coupled family of proteinase-activated receptors (PARs). The signaling mechanism involves the proteolytic unmasking of an N-terminal receptor sequence that acts as a tethered receptor-activating ligand. To date, the recognized targets of thrombin cleavage and activation for signaling are PAR1 and PAR4, in which thrombin cleaves at a conserved target arginine to reveal a tethered ligand. PAR2, which like PAR1 is also cleaved at an N-terminal arginine to unmask its tethered ligand, is generally regarded as a target for trypsin but not for thrombin signaling. We now show that thrombin, at concentrations that can be achieved at sites of acute injury or in a tumor microenvironment, can directly activate PAR2 vaso-relaxation and signaling, stimulating calcium and mitogen-activated protein kinase responses along with triggering beta-arrestin recruitment. Thus, PAR2 can be added alongside PAR1 and PAR4 to the targets, whereby thrombin can affect tissue function.