Protective Effect of Nrf2 and Catalase in Maternal Diabetes-Induced Perinatal Hypertension and Kidney Disease

A small but significant amount of reactive oxygen metabolites (ROMs) is generated in the course of metabolism by mitochondria and numerous cellular oxygenases, oxidases, and peroxidases. Under normal conditions, ROMs produced in the course of metabolism and signal transduction processes are contained by the antioxidant system, which consists of numerous endogenous enzymes, substrates, and scavenger molecules as well dietary antioxidants. Individual components of the antioxidant system serve specific functions and work in concert to protect against tissue injury. Nuclear factor erythroid 2p45-related factor 2 (Nrf2) regulates constitutive expression and coordinated induction of numerous genes encoding antioxidant and phase-2 detoxifying enzymes and related proteins, such as superoxide dismutases (SODs), catalase, UDP-glucuronosyltransferase, NAD(P)H:quinone oxidoreductase-1 (NQO1), heme oxygenase-1, glutamate cysteine ligase, glutathione S-transferase, glutathione peroxidase, and thioredoxin (1). Nrf2 is kept as an inactive complex in the cytoplasm by a repressor molecule, Keap1 (Kelch-like ECH-associated protein 1). Oxidative or covalent modification of thiols in the cysteine residues of Keap1 by ROM or phosphorylation of threonine or serine residues of Nrf2 by upstream kinases results in the release and migration of Nrf2 to the nucleus where it binds to the antioxidant response elements in the promoter regions of the target genes (2,3) to promote transcription. Regulation of cellular antioxidant and antiinflammatory machinery by Nrf2 is critical in defense against oxidative stress. In fact, Nrf2 disruption in mice attenuates or abrogates the induction of genes encoding antioxidant molecules in response to oxidative stress. In addition, ablation of the Nrf2 gene causes a lupus-like autoimmune nephritis, intensifies cyclosporine-induced tubulointerstial fibrosis, and exacerbates diabetes-induced oxidative stress, inflammation, and nephropathy in experimental animals (4–6). The primary ROM generated in the cell is superoxide anion [O2 ], which is the byproduct of the single electron reduction of molecular oxygen [O2 + e 2 → O2 ]. Superoxide is normally converted to hydrogen peroxide [O2 2 + 2H → H2O2] by the SOD family of enzymes located in the cytoplasm(CuZn SOD), mitochondria (MnSOD), and plasma membrane (extracellular SOD). Compared with superoxide, hydrogen peroxide is much more stable and less cytotoxic. It serves as the principal activator of redoxsensitive signal transduction pathways, transcription factors (including nuclear factor-kB [NF-kB] and activator protein-1), and growth factors. Hydrogen peroxide is normally converted to water by catalase [2H2O2 → 2 H2O + O2] and glutathione peroxidase [H2O2 + GSH → 2H2O + SGGS], where GSH is glutathione and SG-GS is the oxidized GSH. However, it can serve as the substrate for nearly uncontainable and highly cytotoxic oxidants such as hydroxyl radical ($OH) in presence of catalytically active iron [H2O2 + Fe 2+ → $OH + OH + Fe] or other transition metals and to hypochlorous acid (HOCl, commonly known as bleach) in presence of myeloperoxidase, which is abundantly expressed in granulocytes, monocytes, and macrophages [H2O2 + Cl 2 → HOCl]. Unlike superoxide and hydrogen peroxide, which are readily contained by the above enzymes, cells have no enzymes to neutralize hydroxyl radical or hypochlorous acid, which once formed freely damage tissues by attacking and denaturing nucleic acids, proteins, and lipids. Therefore, conditions that lead to impaired containment of hydrogen peroxide or facilitate its conversion to hydroxyl radical or hypochlorous acid can lead to cell damage and dysfunction. In this context, glycated proteins, which are produced in the hyperglycemic states, avidly bind iron and other transition metals, forming complexes in which the transition metals retain their catalytic activities (7). Conversely iron and other transition metals facilitate glycation of protein (8). In fact plasma nontransferrinbound iron level is elevated in diabetic patients and has been implicated in the pathogenesis of the kidney and vascular complications (9). The imbalance between the rate of ROM production and the antioxidant capacity leads to oxidative stress in which the uncontained ROMs cause tissue injury and cytotoxicity by attacking, denaturing, and modifying structural and functional molecules and activating redox-sensitive transcription factors and signal transduction pathways. These events lead to necrosis, apoptosis, inflammation, fibrosis, and other disorders that participate in the pathogenesis and progression of many acute, chronic, and degenerative disorders. Oxidative stress is caused by either increased ROM production, impaired antioxidant defense capacity, or both. Exposure of the embryo to sustained elevation of glucose during the early stages of gestation in humans and experimental animals can result in cardiovascular, neurological, skeletal, and urogenital birth defects and partial or total renal agenesis (10). In fact elevated maternal glucose concentration has been shown to result in impaired renal From the Division of Nephrology and Hypertension, Deparment of Medicine, University of California, Irvine, Irvine, California. Corresponding author: Nosratola D. Vaziri, [email protected]. DOI: 10.2337/db12-0764 2012 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/licenses/by -nc-nd/3.0/ for details. See accompanying original article, p. 2565.