Prevention of oxidant-induced cell death by lysosomotropic iron chelators

Intralysosomal iron powerfully synergizes oxidant-induced cellular damage. The iron chelator, desferrioxamine (DFO), protects cultured cells against oxidant challenge but pharmacologically effective concentrations of this drug cannot readily be achieved in vivo. DFO localizes almost exclusively within the lysosomes following endocytic uptake, suggesting that truly lysosomotropic chelators might be even more effective. We hypothesized that an amine derivative of α-lipoamide (LM), 5-[1,2] dithiolan-3-yl-pentanoic acid (2-dimethylamino-ethyl)-amide (α-lipoic acid-plus [LAP]; pKa = 8.0), would concentrate via proton trapping within lysosomes, and that the vicinal thiols of the reduced form of this agent would interact with intralysosomal iron, preventing oxidant-mediated cell damage. Using a thiol-reactive fluorochrome, we find that reduced LAP does accumulate within the lysosomes of cultured J774 cells. Furthermore, LAP is approximately 1,000 and 5,000 times more effective than LM and DFO, respectively, in protecting lysosomes against oxidant-induced rupture and in preventing ensuing apoptotic cell death. Suppression of lysosomal accumulation of LAP (by ammonium-mediated lysosomal alkalinization) blocks these protective effects. Electron paramagnetic resonance reveals that the intracellular generation of hydroxyl radical following addition of hydrogen peroxide to J774 cells is totally eliminated by pretreatment with either DFO (1 mM) or LAP (0.2 μM) whereas LM (200 μM) is much less effective.