Proteolytic cleavage of annexin 1 by human leukocyte elastase.

Annexin 1 has been shown to participate through its unique N-terminal domain in the recruitment and activation of leukocytes at sites of inflammation. Peptides derived from this domain are true mimetics of the annexin 1 action in all inflammation models tested and most likely serve as the active entities generated at sites of inflammation. To elucidate mechanisms underlying peptide generation we used isolated blood leukocytes and endothelial cell monolayers. We show that following endothelial adhesion, annexin 1 was externalized from leukocytes and rapidly cleaved. Addition of purified annexin 1 to degranulating leukocytes resulted in the truncation of annexin 1, which seemed to depend on the proteolytic activity of human leukocyte elastase (HLE). The capacity of elastase to proteolytically cleave annexin 1 was confirmed using both purified annexin 1 and HLE. The identification of annexin 1 as a substrate for HLE supports the model in which annexin 1 participates in regulating leukocyte emigration into inflamed tissue through N-terminal peptides generated at inflammatory sites.

An annexin 1 N-terminal peptide activates leukocytes by triggering different members of the formyl peptide receptor family.

The human N-formyl peptide receptor (FPR) is a key modulator of chemotaxis directing granulocytes toward sites of bacterial infections. FPR is the founding member of a subfamily of G protein-coupled receptors thought to function in inflammatory processes. The other two members, FPR-like (FPRL)1 and FPRL2, have a greatly reduced affinity for bacterial peptides or do not bind them at all, with FPRL2 being considered an orphan receptor so far. In this study we show that a peptide derived from the N-terminal domain of the anti-inflammatory protein annexin 1 (lipocortin 1) can activate all three FPR family members at similar concentrations. The annexin 1 peptide initiates chemotactic responses in human monocytes that express all three FPR family members and also desensitizes the cells toward subsequent stimulation with bacterial peptide agonists. Experiments using HEK 293 cells stably expressing a single FPR family member reveal that all three receptors can be activated and desensitized by the N-terminal annexin 1 peptide. These observations identify the annexin 1 peptide as the first endogenous ligand of FPRL2 and indicate that annexin 1 participates in regulating leukocyte emigration into inflamed tissue by activating and desensitizing different receptors of the FPR family.