Sepiapterin attenuates 1-methyl-4-phenylpyridinium-induced apoptosis in neuroblastoma cells transfected with neuronal NOS: Role of tetrahydrobiopterin, nitric oxide, and proteasome activation

In this study, we investigated the molecular mechanism of toxicity of 1-methyl-4-phenylpyridinium (MPP⁺), an ultimate toxic metabolite of a mitochondrial neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, that causes parkinsonism in experimental animals and humans. Using wild-type and human neuronal nitric oxide synthase (nNOS) stably transfected neuroblastoma cells (SH-SY5Y), we showed that nNOS overexpression in SH-SY5Y cells greatly enhanced proteasome activity and mitigated MPP⁺-induced apoptosis. During MPP⁺-induced oxidative stress, intracellular BH₄ levels decreased, resulting in nNOS "uncoupling" (i.e., switching from nitric oxide to superoxide generation). Increasing the intracellular BH ₄ levels by sepiapterin supplementation restored the nNOS activity, inhibited superoxide formation, increased proteasome activity, decreased protein ubiquitination, and attenuated apoptosis in MPP⁺-treated cells. Implications of BH₄ depletion in dopaminergic cells and sepiapterin supplementation to augment the striatal nNOS activity in the pathogenesis mechanism and treatment of Parkinson disease are discussed.