Lysyl oxidase promotes actin-dependent neutrophil activation and cytotoxicity in diabetes: Implications for diabetic retinopathy

Activated neutrophils contribute to retinal endothelial cell (EC) death and capillary degeneration associated with early diabetic retinopathy (DR). However, the factors and mechanisms driving neutrophil activation and cytotoxicity in diabetes remain insufficiently understood. Here we show that lysyl oxidase (LOX), a collagen crosslinking and matrix stiffening enzyme that increases retinal EC susceptibility to activated neutrophils, simultaneously activates neutrophils in its alternate soluble form. Specifically, treatment of diabetic mice with LOX inhibitor {beta}-aminopropionitrile (BAPN) prevented the diabetes-induced increase in neutrophil activation (extracellular release of neutrophil elastase and superoxide) and cytotoxicity towards co-cultured mouse retinal ECs. Mouse neutrophils and differentiated (neutrophil-like) human HL-60 cells treated with recombinant LOX alone exhibited similar activation and cytotoxicity. Mechanistically, this LOX-induced neutrophil activation was associated with biphasic F-actin remodeling, with the initial and rapid (<15 min) F-actin depolymerization followed by a significant increase in F-actin polymerization and polarization. Preventing the initial F-actin depolymerization blocked LOX-induced neutrophil activation and cytotoxicity towards co-cultured retinal ECs. Finally, we show that this biphasic F-actin remodeling is essential for LOX-induced membrane clustering of azurophilic granule marker CD63 and NADPH organizer p47 that are associated with extracellular release of neutrophil elastase and superoxide, respectively. By revealing a causal and previously unrecognized link between LOX and actin-dependent neutrophil activation in diabetes, these findings provide fresh mechanistic insights into the proinflammatory role of LOX in early DR that goes beyond its canonical matrix-stiffening effects.