Regulation of heavy subunit chain of γ-glutamylcysteine synthetase by tumor necrosis factor-α in lens epithelial cells: role of LEDGF/p75

Abstract

TNF-α induces oxidative stress by generating reactive oxygen species (ROS). This molecule elevates the expression of γ-glutamylcysteine synthetase heavy subunit (γ-GCS-HS). Lens epithelium-derived growth factor (LEDGF)/p75, a transcriptional protein, is inducible by oxidative stress and protects cells from various stresses by upregulating stress-responsive genes. This paper presents evidence that TNF-α elevates the expression of LEDGF and that LEDGF is one of the transactivators of γ-GCS-HS gene. An analysis of the γ-GCS-HS promoter sequence (−819 to +518 nt) revealed the presence of putative sites for LEDGF binding. Gel mobility assay confirmed the binding of LEDGF to the heat shock element (nGAAn) and the stress response element (A/TGGGGA/T) present in γ-GCS-HS promoter. Transactivation experiments showed activation of γ-GCS-HS promoter in cells overexpressing LEDGF or treated with a sublethal dose of TNF-α (20 ng/ml). Downregulation of γ-GCS-HS promoter activity in cells transfected with LEDGF small interfering RNA validated the finding. Notably, cells treated with TNF-α (20 ng/ml) for 24 h had an increased abundance of LEDGF and γ-GCS-HS mRNA and protein. In contrast, cells treated with TNF-α for longer periods or with higher concentrations of TNF-α showed reduced expression of LEDGF and γ-GCS-HS and increased cellular death with higher ROS levels. Cells overexpressing LEDGF revealed elevated GSH levels (10–15%), a condition that may potentially eliminate the insult to cells induced by TNF-α. Thus TNF-α regulation of LEDGF may be physiologically important, as elevated expression of LEDGF increases the expression of endogenous γ-GCS-HS gene, the catalytic subunit of the regulating enzyme in GSH biosynthesis that may constitute a protective mechanism in limiting oxidative stress induced by inflammatory cytokines.