Identification of neutrophil granule protein cathepsin G as a novel chemotactic agonist for the G protein-coupled formyl peptide receptor

The antimicrobial and proinflammatory neutrophil granule protein cathepsin G (CaG) has been reported as a chemoattractant for human phagocytic leukocytes by using a putative G protein coupled receptor. In an effort to identify potential CaG receptor(s), we found that CaG-induced phagocyte migration was specifically attenuated by the bacterial chemotactic peptide fMLP, suggesting these two chemoattractants might share a receptor. In fact, CaG chemoattracts rat basophilic leukemia cells (RBL cells) expressing the high affinity human fMLP receptor FPR, but not parental RBL cells or cells transfected with other chemoattractant receptors. In addition, a specific FPR Ab and a defined FPR antagonist, cyclosporin H, abolished the chemotactic response of phagocytes and FPR-transfected cells to CaG. Furthermore, CaG down-regulated the cell surface expression of FPR in association with receptor internalization. Unlike fMLP, CaG did not induce potent Ca2(+) flux and was a relatively weaker activator of MAPKs through FPR. Yet CaG activated an atypical protein kinase C,isozyme, protein kinase Q, which was essential for FPR to mediate the chemotactic activity of CaG. Thus, our studies identify CaG as a novel, host-derived chemotactic agonist for FPR and expand the functional scope of this receptor in inflammatory and immune responses.