Methylglyoxal induces endoplasmic reticulum stress and DNA demethylation in the Keap1 promoter of human lens epithelial cells and age-related cataracts

Abstract

Age-related cataracts are a leading cause of blindness. Previously, we have demonstrated the association of unfolded protein response with various cataractogenic stressors. However, DNA methylation alterations leading to suppression of lenticular antioxidant protection remains unclear. Here, we report the methylglyoxal-mediated sequential events responsible for Keap1 promoter DNA demethylation in human lens epithelial cells, because Keap1 is a negative regulatory protein that regulates the Nrf2 antioxidant protein. Methylglyoxal induces the ER stress and activates the unfolded protein response leading to overproduction of ROS prior to human lens epithelial cells death. Methylglyoxal also suppresses the Nrf2 and DNA methyltransferases but activates the DNA demethylation pathway enzyme, TET1. Bisulfite genomic DNA sequencing confirms the methylglyoxal-mediated Keap1 promoter DNA demethylation leading to over-expression of Keap1 mRNA and protein. Similarly, bisulfite genomic DNA sequencing of human clear lenses (n=15) slowly lose 5-methylcytosine in the Keap1 promoter throughout life, at a rate of 1% per year. By contrast, diabetic cataractous lenses (n=21) lose an average of 90% of the 5-methylcytosine regardless of the age. Over-expressed Keap1 protein is responsible for decreasing the Nrf2 by proteasomal degradation, thereby suppressing the Nrf2 dependent stress protection. This study demonstrates for the first time about the associations of unfolded protein response activation, Nrf2 dependent antioxidant system failure and loss of Keap1 promoter methylation because of altered active and passive DNA demethylation pathway enzymes in human lens epithelial cells by methylglyoxal. As an outcome, cellular redox balance is altered towards lens oxidation and cataract formation.

Keap1–Nrf2 regulated redox signaling in utero: Priming of disease susceptibility in offspring

“…Similar findings of reduced DJ-1 expression have been reported in Type-II diabetic mouse models or cell lines [123,124], with a marked correlation between altered lipid metabolism and DJ-1 expression in both rodents and humans [125,126]. In most age-related diabetic pathologies (as well as other Nrf2-related disease pathologies such as cancer), alterations in Nrf2 activation have been strongly linked with Keap1 promoter demethylation [127][128][129][130] as opposed to enhanced DJ-1 methylation [122].…”

Possible mechanisms of development of carcinoma of breast in patients with early-onset cataract

“…Methylglyoxal is implicated in cataract development by inducing endoplasmic reticulum stress in human lens epithelial cells, and activating an unfolded protein response leading to overproduction of ROS and altering the cellular redox balance toward lens oxidation. 23 In vitro studies have also confirmed that glyphosate stimulates proliferation of human breast cancer cells when present in concentrations of parts per trillion. 24 This effect is specific to hormone-dependent cell lines, and is mediated by the ability of glyphosate to act as an estrogenic agent.…”

The SUR2B subunit of rat vascular KATPchannel is targeted by miR-9a-3p induced by prolonged exposure to methylglyoxal

“…Accumulating evidence suggests that aberrant DNA methylation of tumor suppressor genes occurs commonly in cancer, and events for miR hypermethylation/hypomethylation are found to play a role in human metastasis (24,25). Indeed, MGO-mediated DNA demethylation has been found to alter the cellular redox balance in human cataract formation (32). Similar mechanisms may work in the VSM cells in reactive carbonyl stress.…”