The molecular determinants regulating redox signaling in diabetic endothelial cells

Abstract

Oxidation and reduction are vital for keeping life through several prime mechanisms, including respiration, metabolism, and other energy supplies. Mitochondria are considered the cell's powerhouse and use nutrients to produce redox potential and generate ATP and H₂O through the process of oxidative phosphorylation by operating electron transfer and proton pumping. Simultaneously, mitochondria also produce oxygen free radicals, called superoxide (O₂ ^-), non-enzymatically, which interacts with other moieties and generate reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂), peroxynitrite (ONOO-), and hydroxyl radical (OH^-). These reactive oxygen species modify nucleic acids, proteins, and carbohydrates and ultimately cause damage to organs. The nutrient-sensing kinases, such as AMPK and mTOR, function as a key regulator of cellular ROS levels, as loss of AMPK or aberrant activation of mTOR signaling causes ROS production and compromises the cell's oxidant status, resulting in various cellular injuries. The increased ROS not only directly damages DNA, proteins, and lipids but also alters cellular signaling pathways, such as the activation of MAPK or PI3K, the accumulation of HIF-1α in the nucleus, and NFkB-mediated transcription of pro-inflammatory cytokines. These factors cause mesenchymal activation in renal endothelial cells. Here, we discuss the biology of redox signaling that underlies the pathophysiology of diabetic renal endothelial cells.

Keywords: AMPK; EndMT; diabetes; endothelial cells; endothelial dysfunctions; mTORC1.

FIGURE 1

An overview of redox signaling. In oxidative phosphorylation, mitochondria use oxygen as an electron acceptor, producing superoxide (O₂·−). Superoxide is then converted into hydrogen peroxide (H₂O₂), which can either be further reduced to water through the action of catalase (CAT), glutathione peroxidase (GPx), and peroxiredoxins (PRx) or transformed into harmful reactive oxygen species (ROS). Hydroxyl radicals (·OH⁻) can cause defects in the lipid oxidation and lead to DNA damage. Under physiological conditions, protein thiol exist in the thiolate anion (S−) state, making them more susceptible to oxidation by H₂O₂. H₂O₂ oxidizes the thiolate anion, forming sulfenic acid, which can react with other thiols to generate disulfide bonds. In high concentrations, H₂O₂ can further oxidize sulfenic acid into sulfinic acid, potentially resulting in oxidative stress.

FIGURE 2

Redox signaling in diabetes mellitus. Diabetes mellitus leads to a significant increase in hydrogen peroxide concentrations in the body. Excess hydrogen peroxide can result in heightened oxidative stress, vascular inflammation, mitochondrial dysfunction, DNA damage, and issues with lipid peroxidation. These problems arise due to the phosphorylation of proteins such as ERK, p38, c-Jun, and mTORC1, which disrupt key signaling pathways by altering the levels of various cytokines, including ICAM, VEGF, MCP1, and IL-6.

FIGURE 3

Redox signaling in diabetic endothelium. Under diabetic condition, the increased dimerization of TGFβR1 and TGFβR2 leads to the phosphorylation of Smad2 and Smad3, producing ROS activating NF-kB. Phosphorylated Smad2 and Smad3 along with NF-kB, then enter the nucleus. Additionally, in diabetes, elevated Wnt signaling and hypoxia contribute to increased levels of β-catenin, accumulation of HIF1α, and expression of Snail1 in the nucleus. The combined effects of NF-kB, Smad2, Smad3, HIF1α, Snail1, and β-catenin in the nucleus drive the transcription of mesenchymal and inflammatory genes. This results in mesenchymal cellular reprogramming in endothelial cells or EndMT under diabetic conditions.

FIGURE 4

In normal, quiescent endothelial cells, high levels of AMPK expression are associated with low mTORC1 activity, reduced glycolysis, and increased fatty acid oxidation (FAO). This suggests that AMPK is a key kinase that promotes FAO and helps maintain mitochondrial integrity, which controls inflammation at lower levels and a reduced endothelial-to-mesenchymal transition (EndMT). Conversely, in diabetic endothelial cells, AMPK activity is diminished while mTORC1 activity is enhanced. This shift leads to increased glycolysis and decreased FAO, resulting in mitochondrial damage and leakage of mitochondrial DNA into the cytosol. These combined effects activate the cGAS-STING pathway, allowing the p65 and p50 subunits of NF-κB to enter the nucleus. This process is primarily responsible for the transcription of inflammatory genes, contributing to inflammation in endothelial cells and promoting the pro-EndMT signal.

FIGURE 5

A hypothetical schematic diagram illustrates the role of Glucocorticoid receptor (GR), Sirtuin3 (SIRT3), and fibroblast growth factor receptor 1 (FGFR1) deficiency in the redox signaling and metabolic shift of myofibroblasts in diabetic endothelial cells. It is theorized that pro-EndMT signals disrupt cellular homeostasis and decrease the activity of SIRT3, GRs, and FGFR1. This disruption leads to an increased presence of reactive oxidative species. The defects in redox signaling in diabetic endothelial cells enhance a variety of inflammatory pathways, including MAPK, NF-kB, PI3K and HIF1α.