GSH Transport in Immortalized Mouse Brain Endothelial Cells

Abstract

We have previously shown GSH transport across the blood-brain barrier in vivo and expression of transport in Xenopus laevis oocytes injected with bovine brain capillary mRNA. In the present study, we have used MBEC-4, an immortalized mouse brain endothelial cell line, to establish the presence of Na⁺-dependent and Na⁺-independent GSH transport and have localized the Na⁺-dependent transporter using domain-enriched plasma membrane vesicles. In cells depleted of GSH with buthionine sulfoximine, a significant increase of intracellular GSH could be demonstrated only in the presence of Na⁺. Partial but significant Na⁺ dependency of [³⁵S]GSH uptake was observed for two GSH concentrations in MBEC-4 cells in which γ-glutamyltranspeptidase and γ-glutamylcysteine synthetase were inhibited to ensure absence of breakdown and resynthesis of GSH. Uniqueness of Na⁺-dependent uptake in MBEC-4 cells was confirmed with parallel uptake studies with Cos-7 cells that did not show this activity. Molecular form of uptake was varified as predominantly GSH, and very little conversion of [³⁵S]cysteine to GSH occurred under the same incubation conditions. Poly(A)⁺ RNA from MBEC expressed GSH uptake with significant (∼40-70%) Na⁺ dependency, whereas uptake expressed by poly(A)⁺ RNA from HepG2 and Cos-1 cells was Na⁺ independent. Plasma membrane vesicles from MBEC were separated into three fractions (30, 34, and 38% sucrose, by wt) by density gradient centrifugation. Na⁺-dependent glucose transport, reported to be localized to the abluminal membrane, was found to be associated with the 38% fraction (abluminal). Na⁺-dependent GSH transport was present in the 30% fraction, which was identified as the apical (luminal) membrane by localization of P-glycoprotein 170 by western blot analysis. Localization of Na⁺-dependent GSH transport to the luminal membrane and its ability to drive up intracellular GSH may find application in the delivery of supplemented GSH to the brain in vivo.