Role of autophagy in protection afforded by hypoxic preconditioning against MPP⁺-induced neurotoxicity in SH-SY5Y cells

A sublethal preconditioning has been proposed as a neuroprotective strategy against several CNS neurodegenerative diseases. In this study, the involvement of autophagy in the protection provided by hypoxic preconditioning against 1-methyl-4-phenylpyridinium (MPP⁺)-induced neurotoxicity was studied in SH-SY5Y neuroblastoma cells. In contrast to the cytotoxicity of 0.1% oxygen, 1% oxygen hypoxia for 24h did not cause significant cell death. A transient increase in LC3-II level, a biomarker of autophagy, was demonstrated during hypoxic treatment. At the same time, 8-h hypoxia increased fluorescence due to monodansylcadaverine, a specific dye for autophagosomes, in the treated cells. Co-incubation with bafilomycin A1 (10 nM) further increased hypoxia-induced LC3-II levels but 3-methyladenine (3-MA; 10mM) reduced the elevation in LC3-II levels induced by 8-h hypoxia. Moreover, 8-h hypoxia increased free radical formation and nuclear HIF-1α level. Glutathione was found to diminish hypoxia-induced LC3-II elevation. In contrast to the elevated LC3-II level, 8-h hypoxia significantly decreased mitochondrial mass. Furthermore, a rebound elevation in mitochondrial mass was observed under 8-h hypoxia and subsequent 12-h normoxia. Prior hypoxia attenuated the MPP⁺-induced elevation in LC3-II levels and cell death. Moreover, hypoxic pretreatment inhibited MPP⁺-induced activation of caspase-3 and DNA fragmentation. Co-incubation with 3-MA during hypoxia prevented the protection afforded by hypoxic preconditioning against MPP⁺-induced increases in LC3-II levels and neurotoxicity. Taken together, our results suggest that sublethal hypoxia induces autophagy that is mediated by oxidative stress. Furthermore, autophagy may be involved in the protection provided by hypoxic preconditioning against MPP⁺-induced neurotoxicity, indicating a neuroprotective role of autophagy in hypoxic preconditioning.