Methylglyoxal Activates Chronic ER Stress Mediated Epigenetic Loss of Nrf2/Keap1 Gene Regulation in Diabetic Cataractous Lenses

Abstract

Purpose: Methylglyoxal (MGO) is a highly cytotoxic metabolite produced from glucose metabolism. Earlier studies indicate that MGO is attributed to aging and diabetic complications through formation of advanced glycation end products. But evidence for MGO-induced endoplasmic reticulum (ER) stress and epigenetic loss of Nrf2/Keap1 dependent antioxidant protection is emerging in diabetes. This study investigated the mechanism by which MGO induced the ER stress and by what epigenetic mechanisms MGO inactivated the Nrf2/Keap1 dependent antioxidant protection in lens epithelial cells (LECs) during cataractogenesis.

Methods: Human and Nrf2 knockout (KO) mouse LECs were cultured with MGO to study the ROS production and cell death. MGO was intraperitoneally administered to control and diabetic Nrf2 KO mice. The expression profiles of markers of ER stress, Nrf2/Keap1 dependent antioxidant system and DNA methylation and demethylation enzymes were analyzed by qPCR and Western blotting. ER-Ca²⁺ release was assessed by live cell imaging. Proteasomal degradation was studied by using an inhibitor, MG-132. Promoter DNA methylation status was sequenced by bisulfite genomic DNA sequencing.

Results: We found that MGO stimulates ROS production and cell death in human and Nrf2 KO mouse LECs. MGO also reduced the ER-Ca²⁺ content of human LECs. The mRNA and protein expressions studies confirmed that MGO treatment significantly activates the ER stress-specific genes and suppresses the Nrf2/Keap1 dependent antioxidant genes and DNA methyltransferases. Also, the levels of glyoxalase-1 and DNA demethylation enzyme, methylcytosine dioxygenase were over-expressed by MGO. DNA sequencing results revealed significant demethylated DNA in the Keap1 promoter of diabetic cataractous lenses than clear lenses. Similarly, MGO treated human LECs resulted in significant demethylation of Keap1 promoter and had a notable increase in Keap1 mRNA and protein.

Conclusions: This study revealed a possible mechanistic rationale by which MGO induces ER stress mediated Nrf2/Keap1 dependent antioxidant system failure and altered expressions of DNA methylation and demethylation enzymes. Also, epigenetic modification of Keap1 promoter by MGO stimulates Keap1 expression, which increases proteosomal degradation of Nrf2. So, redox-balance is altered towards lens oxidation and cataract formation.

Keywords: 445 cataract • 498 diabetes • 634 oxidation/oxidative or free radical damage