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 could 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 was shown to 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 site of PAR2. This interaction makes PAR2 an indirect thrombin receptor. PAR3 could be seen as co-receptor and not as a receptor anymore. There are limited studies available showing any PAR3-dependent signaling. However, it could be that PAR3 was the initial APC receptor since APC cleavage was suggested by others. It could be that an APC-activated PAR3 signals via a PAR2-dependent transactivation. Like for PAR1, interaction of thrombin with PAR3 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 interaction of PAR1 or PAR3 with thrombin leads to activation of PAR2 or 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, PAR4 can stay stuck within the cell due to an ER-retention signal in PAR4. If we combine all this knowledge together, then everything leads to PAR4.
The PAR2-dependent PAR4 surface expression also shows a co-operation between PAR2 and PAR4 which is currently not known for other PARs. This suggests that PAR4-dependent effects would be more important in cells with increased PAR2 expression (except for platelets). This might explain PAR4’s role in inflammatory responses were cellular PAR2 expression is also elevated.
With regard to PAR activation, we think that TF (tissue factor, FIII, thromboplastin) is acting similar to PAR3. Alone, TF 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 FVIIa or FVIIa/FXa-dependent PAR2 activation. That makes FVIIa and FXa one of the oldest PAR2 activators on potentially myeloid (monocytes/macrophages) cells.
Other projects in the lab are the role of thrombin signaling in chemotherapy-induced heart failure. In addition, we are interested in further investigating the role of PARs in inflammatory responses.
May 2024
