EAAT and Xc− Exchanger Inhibition Depletes Glutathione in the Transformed Human Lens Epithelial Cell Line SRA 01/04

Abstract

ABSTRACTPurpose: Maintaining the high glutathione (GSH; tripeptide of glutamate, cysteine and glycine) levels in the lens cortex promotes lens health. The role of glutamate/aspartate (Glu/Asp) transporters and the cystine (Cys)/Glu exchanger (Xc− exchanger) in maintaining GSH in transformed human lens epithelial cells (SRA 01/04) was investigated.Methods: Detection and differentiation of excitatory amino acid transporters (EAAT1-5) and the Xc− exchanger was performed by the uptake of radiolabeled l-Glu, d-Asp and l-Cys in the presence and absence of Na+, substrate-specific inhibition studies and Western-blot analysis. Reductions in GSH levels post-inhibition of Xc− exchanger and EAAT activities by substrate inhibitors demonstrated the roles of EAAT and Xc− exchanger in maintaining GSH.Results: Glu and d-Asp uptake in HLEC was Na+-dependent. Strong inhibition by substrate-specific Glu/Asp uptake inhibitors and weak inhibition by kainic acid (KA) was consistent with Na+-dependent EAAT1/3/4/5 activity and weak EAAT2 activity, respectively. Na+-independency and Glu inhibition of Cys uptake were consistent with Xc− exchanger activity, but inhibition of Na+-dependent Cys uptake by N-acetylcysteine suggests Cys uptake by EAAT3. EAAT1-5 and xCT (Xc− exchanger light chain) immunoreactive peptides were detected by Western-blot analysis of HLEC lysates. EAAT and Xc− exchanger inhibition by substrate antagonists depleted GSH concentrations by 15–28% (p’s ≤ 0.02), while GSH synthesis inhibition by buthionine sulfoximine depleted GSH by 33% (p = 0.008).Conclusion: Inhibition of Glu and Cys uptake by EAAT and Xc− exchanger antagonists depletes GSH in human lens epithelial cells. These in vitro results support pivotal roles for EAAT and Xc− exchanger activities in maintaining GSH and protection against oxidative stress in cortical lens epithelium.