Epigenetic modifications of Nrf2-mediated glutamate-cysteine ligase: implications for the development of diabetic retinopathy and the metabolic memory phenomenon associated with its continued progression

Diabetes increases oxidative stress in the retina and decreases the levels of the intracellular antioxidant glutathione (GSH). The transcriptional factor Nrf2 regulates the expression of Gclc, the enzyme important in the biosynthesis of GSH, and in diabetes the binding of Nrf2 at the antioxidant response element region 4 (ARE4) is decreased. Our aim was to investigate the role of epigenetic modifications in the decreased Nrf2 binding at Gclc-ARE4 in the development of diabetic retinopathy and in the metabolic memory associated with its continued progression. The effect of hyperglycemia on H3K4 methylation in Nrf2 binding at Gclc-ARE4 was investigated by chromatin immunoprecipitation in the rat retina and was confirmed in retinal endothelial cells in which histone demethylase (LSD1) was manipulated. The role of histone methylation at Gclc-ARE4 in the metabolic memory was examined in rats maintained under poor control for 3 months followed by good control (GC) for 3 months. Although H3K4me2 at Gclc-ARE4 was increased in diabetes, H3K4me3 and H3K4me1 were decreased. LSD1 siRNA abrogated the glucose-induced decrease in H3K4me1 at Gclc-ARE4 and ameliorated decreases in Nrf2 binding at Gclc-ARE4 and Gclc transcripts. Reestablishment of GC failed to provide any benefits to histone methylation, and Nrf2 binding activity remained compromised. Thus, in diabetic retinopathy, histone methylation at Gclc-ARE4 plays an important role in regulating the Nrf2-Gclc-GSH cascade. Targeting histone methylation could help inhibit/slow down this blinding disease.