Abstract
Purpose :
Diabetic patients have elevated homocysteine, and animal models have shown that homocysteine accelerates and exacerbates mitochondrial damage and the development of diabetic retinopathy. Elevated homocysteine disrupts a delicate balance between methyl donor S-adenosylmethionine (SAM) and S-adenosylhomocysteine, and SAM is a critical component of DNA methylation, one of the major epigenetic modifications. Free radicals are reduced to peroxides by glutathione peroxidase (Gpx), using intracellular antioxidant, glutathione (GSH), as a cofactor, and in diabetes both Gpx and GSH are downregulated. Our aim was to investigate the effect of homocysteine in Gpx1 downregulation in diabetic retinopathy.
Methods :
Human retinal endothelial cells incubated in 20mM D-glucose containing 100μM homocysteine for 96 hours were used to analyze DNA methylation machinery (SAM levels by competitive ELISA and Dnmt activity by quantifying methylated DNA), Gpx1 transcripts (qRT-PCR) and activity (glutathione reductase coupled reaction), and mitochondrial GSH (enzymatic recycling method) and ROS (MitoSOX red). 5-methyl cytosine levels were quantified at Gpx1 promoter by an ELISA-based method in genomic DNA, and confirmed by methylation-specific PCR. Specific role of SAM in Gpx1 promoter DNA methylation was validated using si-RNA of methionine adenosyltransferase 1A (Mat1a). The results were confirmed in the retina from streptozotocin-induced diabetic mice with hyperhomocysteinemia.
Results :
: High glucose decreased Gpx (activity and mRNA) and mitochondrial GSH levels, and increased SAM levels and Gpx1 promoter DNA methylation. Homocysteine, in high glucose medium, further decreased GSH and Gpx, and increased Gpx1 promoter DNA methylation and mitochondrial ROS. Consistent with in vitro results, compared to wildtype, hyperhomocysteinemic diabetic mice had significant increase in retinal Gpx1 promoter DNA methylation and decrease in Gpx1 and GSH.
Conclusions :
Elevated homocysteine in diabetes hypermethylates retinal Gpx1 promoter DNA, downregulating its transcription and decreasing the GSH pool. This subsequently elevates ROS and culminates in the development of diabetic retinopathy. Hence, modulation of hyperhomocysteinemia presents a prospective avenue for therapeutic intervention targeting mitochondrial dysfunction in diabetic retinopathy.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.