Regulation of endothelial glutathione by ICAM-1: implications for inflammation

The role of glutathione (GSH) in inflammation is largely discussed from the context of providing reducing equivalents to detoxify reactive oxygen and nitrogen species. Inflammation is now recognized to be an underlying cause of many vascular diseases including atherosclerosis, a disease in which endothelial GSH concentrations are decreased. However, mechanisms that control GSH levels are poorly understood. Key players in the inflammatory process are endothelial adhesion molecules, including intercellular adhesion molecule-1 (ICAM-1). This adhesion molecule is present constitutively and can be induced by a variety of inflammatory stimuli. In this study, using mouse aortic endothelial cells (MAEC) deficient in ICAM-1, we demonstrate a novel interplay between constitutive ICAM-1 and cellular GSH. Deficiency of ICAM-1 was associated with an approximately twofold increase in total GSH content. Inhibiting glutamate-cysteine ligase (GCL), the enzyme that catalyses the rate-limiting step in GSH biosynthesis, prevented the increase in GSH. In addition, the catalytic subunit of GCL was increased (approximate to 1.6-fold) in ICAM-1 deficient relative to wild-type cells, suggesting that constitutive ICAM-1 represses GCL expression. Furthermore, the ratio of reduced (GSH) to oxidized (GSSG) glutathione was also increased suggesting a role for ICAM-1 in modulating cellular redox status. Interestingly, increasing cytosolic GSH in wild-type mouse endothelial cells decreased constitutive ICAM-1, suggesting the presence of an inverse and reciprocal pathway. To test the effects of inducible ICAM-1 on GSH, cells were stimulated with the proinflammatory cytokine TNF-alpha. TNF-alpha stimulated production of ICAM-1, which was however not associated with induction of GSH. In contrast, supplementation of endothelial cells with GSH before TNF-alpha addition, inhibited induction of ICAM-1. These data suggest a novel regulatory pathway between constitutive ICAM-1 and GSH synthesis in the endothelium and are discussed in the context of modulating the inflammatory response.