α-synuclein up-regulation and aggregation during MPP+-induced apoptosis in neuroblastoma cells -: Intermediacy of transferrin receptor iron and hydrogen peroxide

1-Methyl-4-phenylpyridinium (MPP+) is a neurotoxin that causes Parkinson's disease in experimental animals and humans. Despite the fact that intracellular iron was shown to be crucial for MPP+-induced apoptotic cell death, the molecular mechanisms for the iron requirement remain unclear. We investigated the role of transferrin receptor (TfR) and iron in modulating the expression of alpha-synuclein (alpha-syn) in MPP+-induced oxidative stress and apoptosis. Results show that MPP+ inhibits mitochondrial complex-1 and aconitase activities leading to enhanced H2O2 generation, TfR expression and alpha-syn expression/aggregation. Pretreatment with cell-permeable iron chelators, TfR antibody (that inhibits TfR-mediated iron uptake), or transfection with glutathione peroxidase (GPx1) enzyme inhibits intracellular oxidant generation, alpha-syn expression/aggregation, and apoptotic signaling as measured by caspase-3 activation. Cells overexpressing alpha-syn exacerbated MPP+ toxicity, whereas antisense alpha-syn treatment totally abrogated MPP+-induced apoptosis in neuroblastoma cells without affecting oxidant generation. The increased cytotoxic effects of alpha-syn in MPP+-treated cells were attributed to inhibition of mitogen-activated protein kinase and proteasomal function. We conclude that MPP+-induced iron signaling is responsible for intracellular oxidant generation, alpha-syn expression, proteasomal dysfunction, and apoptosis. Relevance to Parkinson's disease is discussed.