Stimulation of human formyl peptide receptors by calpain inhibitors: Homology modeling of receptors and ligand docking simulation

Calpain inhibitors, including peptide aldehydes (N-acetyl-Leu-Leu-Nle-CHO and N-acetyl-Leu-Leu-Met-CHO) and α-mercapto-acrylic acid derivatives (PD150606 and PD151746), have been shown to stimulate phagocyte functions via activation of human formyl peptide receptor (hFPR) and/or hFPR-like 1 (hFPRL1). Using the homology modeling of the receptors and the ligand docking simulation, here we show that these calpain inhibitors could bind to the putative N-formyl-Met-Leu-Phe (fMLF) binding site on hFPR and/or hFPRL1. The studies with HEK-293 cells stably expressing hFPR or hFPRL1 showed that the concentrations of calpain inhibitors required to induce an increase in cytoplasmic free Ca2+ ([Ca2+]i) was much higher (>100 folds) than those of fMLF and Trp-Lys-Tyr-Met-Val-D-Met (WKYMVm). HEK-293 cells expressing hFPR or hFPRL1 with the mutated fMLF binding site never exhibited the [Ca2+]i response to calpain inhibitors. When the optimal concentrations of each stimulus were used, pretreatment of cells with fMLF or WKYMVm abolished an increase in [Ca2+]i induced by calpain inhibitors as well as the same stimulus, whereas pretreatment of cells with calpain inhibitors significantly suppressed, but never abolished, the [Ca2+]i response induced by fMLF or WKYMVm, suggesting that the binding affinity of the inhibitors to the putative fMLF binding site may be lower than that of fMLF or WKYMVm.