Complement-activation fragment C4a mediates effector functions by binding as untethered agonist to protease-activated receptors 1 and 4

Significance How C4a transduces signaling and generates various biological functions has long been unresolved. Using a cell-based reporter assay, we screened C4a against a panel of 168 known and 73 orphan G protein-coupled receptors and now provide evidence that C4a is a ligand for protease-activated receptor (PAR)1 and PAR4. In human endothelial cells, our assessment of ERK activation and calcium mobilization in the presence of various antagonists and inhibitors suggests that C4a-mediated cell activation is mediated through PAR1 and PAR4. Our results demonstrate that C4a is a nontraditional agonist for PAR1 and PAR4 whose direct functional effects indicate a potentially significant direct linkage between the complement, coagulation, and endothelial barrier systems.