Membrane Transport Mechanism of Aluminum and its Effect on the Inositol Phospholipid Metabolism in the Neuronal System

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Although accumulation of aluminum (Al) in the brain is known to cause neurodegenerative disorders and to be regulated mainly by the blood-brain barrier (BBB), the mechanism for Al accumulation in brain tissue has not been clarified yet. In this study, we investigated what kind of transporter is involved in the transport of Al citrate, which is the major species of Al in the brain, at the BBB using a rat immortalized brain endothelial cell line, RBEC1 cells and human neuroblastoma SH-SY5Y cells, focusing on the glutamate transporter family. The uptake of Al citrate showed temperature- and concentration-dependency, and did not require an inwardly directed Na^+-gradient as a driving force, ruling out the involvement of Na^+-dependent glutamate transporters in its transport. Examination of the expression of a Na^+-independent glutamate transporter, system Xc^-, by RT-PCR, mRNAs for xCT and 4F2hc, as its components, were found in both RBEC1 and SH-SY5Y cells. L-Glutamate aid L-cystine, repr … More esentative ligands for system Xc^-, significantly inhibited the uptake of Al citrate, and loading of them into the cells resulted in stimulation of its uptake in RBEC1 and SH-SY5Y cells, while L-aspartate, which is not a ligand for the system in intact cells, did not have any effect. These results demonstrated that Al citrate is taken up into RBEC1 and SH-SY5Y cells via system Xc^-, and that this system might play an important role in Al citrate transport at the BBB and neurons. We further examined the effect of Al citrate treatment on expression of the transporter and on the susceptibility to oxidative stress of SH-SY5Y cells. When the cells were treated with Al citrate, but not citrate, for 2 weeks, but not a day, the expression of mRNA for xCT and 4F2hc, a chaperon of xCT was decreased, and the effect of the Al citrate treatment on xCT protein was confirmed immunocytochemically. Although the cell viability and glutathione content of the cells were not altered by the treatment, the formation of nitrotyrosine and the number of dead cells among the Al citrate-treated cells increased on exposure to glucose deprivation. These findings demonstrate that Al citrate is a substrate for system Xc^-, and that chronic treatment with Al citrate causes downregulation of xCT and increases the vulnerability of the cells to oxidative stress without a direct effect on the viability or GSH content. Less