Contributions of Ca2+ to galectin-1-induced exposure of phosphatidylserine on activated neutrophils

Abstract

Apoptotic cells redistribute phosphatidylserine (PS) to the cell surface by both Ca2+-dependent and -independent mechanisms. Binding of dimeric galectin-1 (dGal-1) to glycoconjugates on N-formyl-Met-Leu-Phe (fMLP)-activated neutrophils exposes PS and facilitates neutrophil phagocytosis by macrophages, yet it does not initiate apoptosis. We asked whether dGal-1 initiated Ca 2+ fluxes that are required to redistribute PS to the surface of activated neutrophils. Prolonged occupancy by dGal-1 was required to maximally mobilize PS to the surfaces of fMLP-activated neutrophils. Like fMLP, dGal-1 rapidly elevated cytosolic Ca2+ levels in Fluo-4-loaded neutrophils. An initial Ca2+ mobilization from intracellular stores was followed by movement of extracellular Ca2+ to the cytosolic compartment, with return to basal Ca2+ levels within 10 min. Chelation of extracellular Ca2+ did not prevent PS mobilization. Chelation of intracellular Ca2+ revealed that fMLP and dGal-1 independently release Ca 2+ from intracellular stores that cooperate to induce optimal redistribution of PS. Ca2+ mobilization by ionomycin did not permit dGal-1 to mobilize PS, indicating that fMLP initiated both Ca 2+-dependent and -independent signals that facilitated dGal-1-induced exposure of PS. dGal-1 elevated cytosolic Ca2+ and mobilized PS through a pathway that required action of Src kinases and phospholipase Cγ. These results demonstrate that transient Ca2+ fluxes contribute to a sustained redistribution of PS on neutrophils activated with fMLP and dGal-1. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.