Why are we interested in protease-activated receptor 4 (PAR4)? Currently, there are 4 PARs identified. PAR1 was the first of the family to be described. We think PAR1 is needed to initiate cellular responses to thrombin and activated protein C (APC). In addition, under certain condition where thrombin/APC is not available other proteases can lead to PAR1 activation. In our opinion, PAR1 signaling is mostly important on cells marking a boarder zone like endothelial cells (ECs) and fibroblasts (scar tissue). Importantly, PAR1 can improve activation of PAR4 at low thrombin levels. If PAR4 is alone it needs 10x higher concentration of thrombin. Next on the list is PAR2. PAR2 signaling is initiated by the coagulation proteases FVIIa and FXa as well as serine proteases like trypsin and tryptase. Interestingly, thrombin-activated PAR1 can cross-activate PAR2 when the thrombin-activated N-terminus of PAR1 (tethered ligand) binds the tethered ligand binding pocket of PAR2. This interaction makes PAR2 an indirect thrombin receptor. PAR3 is rather a co-receptor and not a receptor anymore. There are limited studies available showing any PAR3-dependent signaling. However, we think, PAR3 was the initial APC receptor since APC can cleave PAR3. We further think, that APC-activated PAR3 signals via a PAR2-dependent transactivation. Like for PAR1, thrombin can cleave PAR3 and this interaction with thrombin reduces the needed thrombin concentrations to activate PAR4 if PAR4 is in close proximity to PAR3. From the available data we can conclude that the activation of PAR1 and PAR3 leads to activation of PAR2 and PAR4.

Why is PAR4 special? There is a study showing that PAR2 expression leads to increased PAR4 surface expression and therefore increased PAR4 responses. Without PAR2 most of the PAR4 is stuck within the cell due to an ER-retention signal in PAR4. If we combine all this knowledge together, then everything leads to PAR4.

We further think, that PAR4 and PAR2 are potentially the oldest known PARs, then PAR3 and lastly PAR1 evolved. The PAR2-dependent PAR4 surface expression also shows a co-operation between PAR2 and PAR4 which is not known for other PARs. This suggests that PAR4-dependent effects would be more important in cells with increased PAR2 expression. This might explain PAR4’s role in inflammatory responses were PAR2 expression is also elevated.

Publications also suggests that thrombin is not the evolutionary oldest activator of PAR4. With regard to PAR activation, we think that FIII (tissue factor, thromboplastin) is acting similar to PAR3. Alone, FIII has mostly lost all of its receptor function. It seems the receptor is only needed to localize FVIIa and FXa close to PAR2 to initiate FXa-dependent PAR2 activation. That makes FXa one of the oldest PAR2 activators on potentially myeloid cells.