Evidence for a Superoxide Permeability Pathway in Endosomal Membranes

ABSTRACT The compartmentalized production of superoxide (·O2 −) by endosomal NADPH oxidase is important in the redox-dependent activation of NF-κB following interleukin 1β (IL-1β) stimulation. It remains unclear how ·O2 − produced within endosomes facilitates redox-dependent signaling events in the cytoplasm. We evaluated ·O2 − movement out of IL-1β-stimulated endosomes and whether SOD1 at the endosomal surface mediates redox-signaling events required for NF-κB activation. The relative outward permeability of NADPH-dependent ·O2 − from fractionated endosomes was assessed using membrane-permeable (luminol and lucigenin) and -impermeable (isoluminol) luminescent probes for ·O2 −. In these studies, ∼60% of ·O2 − efflux out of endosomes was inhibited by treatment with either of two anion channel blockers, 4′-diisothiocyano-2,2′-disulfonic acid stilbene (DIDS) or niflumic acid (NFA). Furthermore, radioisotopic electrodiffusion flux assays on endomembrane proteoliposomes suggested that ·O2 − and Cl− are transported through the same DIDS-sensitive channel(s). Rab5-based immunoaffinity isolation of IL-1β-stimulated early endosomes demonstrated SOD1 recruitment to endosomes harboring the IL-1 receptor. Finally, SOD1-deficient cells were found to be defective in their ability to activate NF-κB following IL-1β stimulation. Together, these results suggest that ·O2 − exits endosomes through a DIDS-sensitive chloride channel(s) and that SOD1-mediated dismutation of ·O2 − at the endosomal surface may produce the localized H2O2 required for redox-activation of NF-κB.