Calpain inhibitors stimulate phagocyte functions via activation of human formyl peptide receptors

Calpain inhibitors induce pertussis toxin (PTx)-sensitive chemotaxis in human neutrophils and monocytes. Here, we show that various calpain inhibitors (PD150606, PD151746, N-acetyl-Leu-Leu-Nle-CHO [ALLN], N-acetyl-Leu-Leu-Met-CHO [ALLM], and calpeptin) and γ-secretase inhibitor I induced PTx-sensitive increase in cytoplasmic free Ca2+ ([Ca2+]i) in human neutrophils and neutrophil migration. HEK-293 cells stably expressing human formyl peptide receptor (hFPR) or hFPR-like 1 (hFPRL1) displayed stimulus-specific increase in [Ca2+]i in response to calpain inhibitors (PD150606, PD151746, ALLN, ALLM, MG-132, and calpeptin), γ-secretase inhibitor I, and N-formyl-Met-Leu-Phe. Parent HEK-293 cells also displayed PTx-sensitive increase in [Ca2+]i in response to calpeptin and γ-secretase inhibitor I, whereas they displayed PTx-resistant increase in [Ca2+]i in response to MG-132. MDL-28170 induced neither an increase in [Ca2+]i in neutrophils and HEK-293 cells nor neutrophil migration. Ionomycin-induced cleavage of talin (a substrate of calpain) in neutrophils was inhibited by all inhibitors used here. These findings suggest that potent calpain inhibitors could stimulate phagocyte functions via activation of hFPR, hFPRL1 and/or other G-protein coupled receptors depending on the inhibitors used.