Molecular Interactions Between Reactive Oxygen Species and Autophagy in Kidney Disease

Abstract

Reactive oxygen species (ROS) are highly reactive signaling molecules that maintain redox homeostasis in mammalian cells. Dysregulation of redox homeostasis under pathological conditions results in excessive generation of ROS, culminating in oxidative stress and the associated oxidative damage of cellular components. ROS and oxidative stress play a vital role in the pathogenesis of acute kidney injury and chronic kidney disease, and it is well documented that increased oxidative stress in patients enhances the progression of renal diseases. Oxidative stress activates autophagy, which facilitates cellular adaptation and diminishes oxidative damage by degrading and recycling intracellular oxidized and damaged macromolecules and dysfunctional organelles. In this review, we report the current understanding of the molecular regulation of autophagy in response to oxidative stress in general and in the pathogenesis of kidney diseases. We summarize how the molecular interactions between ROS and autophagy involve ROS-mediated activation of autophagy and autophagy-mediated reduction of oxidative stress. In particular, we describe how ROS impact various signaling pathways of autophagy, including mTORC1-ULK1, AMPK-mTORC1-ULK1, and Keap1-Nrf2-p62, as well as selective autophagy including mitophagy and pexophagy. Precise elucidation of the molecular mechanisms of interactions between ROS and autophagy in the pathogenesis of renal diseases may identify novel targets for development of drugs for preventing renal injury.

Keywords: AMPK; acute kidney injury; autophagy; chronic kidney disease; diabetic nephropathy; mTORC1; oxidants; reactive oxygen species.

Figure 1

Generation of reactive oxygen species (ROS) in the cell. ROS are generated by enzymatic and non-enzymatic redox reactions during cellular metabolism under normal and pathological conditions. Mitochondria, plasma membrane, peroxisomes, and cytosol first generate the superoxide anion (O₂•−), which becomes the precursor free radical for the generation of other ROS molecules. Cytosolic CuZN superoxide dismutase (SOD) and mitochondrial MnSOD, which are expressed in the kidney, dismutate O₂•− to H₂O₂, which yields highly reactive hydroxyl radicals (OH•) by interaction with reduced transition metal ions (such as Fe and Cu) in a Fenton reaction. In addition to ROS, cells also generate reactive nitrogen species (RNS). The major RNS include nitric oxide (•NO), peroxynitrite (ONOO⁻), and nitrogen dioxide (•NO₂). Nitric oxide (•NO) is produced by three isoforms of nitric oxide synthase (NOS), all of which are expressed in the kidney. ROS produced cause oxidative damage, including DNA damage, lipid and protein oxidation, protein nitration, and mitochondrial dysfunction.

Figure 2

Regulation of autophagy by ROS. (Top left) Under conditions of oxidative stress, ROS oxidize cysteines located in the α and β subunits of AMPK, by activating AMPK and its downstream signaling. Members of the Bcl-2 family suppress autophagy by binding beclin-1 and preventing beclin-1 from forming the PI3 kinase/Vps34 complex of the autophagy pathway. ROS-mediated oxidation of Ask1 results in dissociation of the Bcl-2–beclin-1 interaction, making free beclin-1 available for the formation of PI3 kinase/Vps34 complex and subsequent autophagy activation. (Top right) H₂O₂ oxidizes the cysteine 81 residue of Atg4, which inactivates the protease to facilitate Atg8 in the formation of autophagosome. (Bottom) Oxidative stress also oxidizes mTORC1 and PTEN, which inhibits their activity and promotes autophagy. Suppression of mTORC1 and induction of AMPK promote ULK1 complex (ULK1, Atg13, FIP200, and Atg101) activation at the pre-autophagosomal assembly site (a certain domain of the ER) and initiates the autophagy process.

Figure 3

ROS regulation of Keap1/Nrf2/p62 signaling and its impact on autophagy. (A) Under normal basal conditions, Keap1 regulates Nrf2 and keeps its level low by ubiquitination and proteosomal degradation. (B) P62 has a binding site for Keap 1 and promotes autophagic degradation of Keap1, liberating Nrf2 and enabling Nrf2 to translocate to the nucleus to transactivate target genes to protect against oxidative stress and upregulate autophagy. (C) Under oxidative stress conditions, Keap1 is oxidized by ROS-mediated oxidation of cysteine thiols in the cysteine-rich Keap1-Cullin3-based ubiquitin ligase complex, which dissociates Nrf2 from Keap1 and enables Nrf2 to translocate to the nucleus to transactivate target genes, including antioxidant and autophagy genes.