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Elanchezhian Rajan, Palsamy Periyasamy, Christian J. Madson, Micheal L. Mulhern, David W. Lynch, Masahiko Ayaki, Thomas W. Hejkal, Toshimichi Shinohara; Age-Related Cataracts: Decrease in Nrf2/Keap1 Mediated Antioxidant Protection by Unfolded Protein Response or Epigenetic Repression. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5289.
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© ARVO (1962-2015); The Authors (2016-present)
Age-related cataracts (ARCs) are tightly associated with age and oxidation of the lens. There are increasing evidences on the aging and cancer studies, which indicate that hypermethylation in the CpG islands of NF-E2-related factor 2 (Nrf2 gene), and its negative regulator, Kelch-like ECH-associated protein 1 (Keap1 gene), suppresses the expression of many antioxidant genes. During oxidative stress, Nrf2 escapes from Keap1-mediated repression and activates transcription of antioxidant responsive element-dependent genes to preserve cellular redox homeostasis. Repression of these genes results in decreased antioxidants to scavenge the reactive oxygen species (ROS). Recent reports show that ARC patients exhibited elevated levels of homocysteine (Hcy), which is known to induce the unfolded protein response (UPR). The transcription of the Nrf2/Keap1 genes is under the control of UPR pathway. We hypothesize that ARCs can be induced by activation of UPR and decreased antioxidant protection due to repression of the Nrf2/Keap1 genes.
Human LECs and rat lenses were cultured in different concentrations of Hcy at 1% atmospheric oxygen. H2-DCFH-DA and EthD staining detects ROS and cell death, respectively. Protein blot analyses were performed with Abs specific to UPR proteins. PCR was used to quantify the transcripts. MS-PCR and DNA sequencing were used to identify the methylated DNA in the ARC lenses.
Higher levels of Hcy induced the UPR in LECs and rat lens organ culture. The ROS production and cell death were also seen in these cells. Catalytic enzymes for the production of ROS were also altered, in contrast, transcription of Nrf2 gene was significantly decreased but Keap1 was increased in these LECs. We also found age-dependent decline of the UPR activation in rat lenses. Bisulfite converted DNA sequence data showed that the CpG islands of Nrf2/Keap1 promoters had methylated DNA in more than 30 capsulotomy specimens of various types of ARCs. DNA methylation repressed the transcription of the Nrf2/Keap1.
ARCs are associated with the UPR and prolonged or higher levels of ROS generate DNA methylation in the Nrf2/Keap1 genes. Lower levels of these factors suppress the antioxidant defense protection against ROS, and resulted in oxidation of the lens.
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