Emerging functional cross-talk between the Keap1-Nrf2 system and mitochondria

Abstract

Nuclear factor erythroid-derived 2-related factor 2 (Nrf2) was originally identified as a positive regulator of drug detoxifying enzyme gene expression during exposure to environmental electrophiles. Currently, Nrf2 is known to regulate the expression of hundreds of cytoprotective genes to counteract endogenously or exogenously generated oxidative stress. Furthermore, when activated in human tumors by somatic mutations, Nrf2 confers growth advantages and chemoresistance by regulating genes involved in various processes such as the pentose phosphate pathway and nucleotide synthesis in addition to antioxidant proteins. Interestingly, increasing evidence shows that Nrf2 is associated with mitochondrial biogenesis during environmental stresses in certain tissues such as the heart. Furthermore, SKN-1, a functional homolog of Nrf2 in C. elegans, is activated by mitochondrial reactive oxygen species and extends life span by promoting mitochondrial homeostasis (i.e., mitohormesis). Similarly, Nrf2 activation was recently observed in the heart of surfeit locus protein 1 (Surf1) -/- mice in which cellular respiration was decreased due to cytochrome c oxidase defects. In this review, we critically examine the relationship between Nrf2 and mitochondria and argue that the Nrf2 stress pathway intimately communicates with mitochondria to maintain cellular homeostasis during oxidative stress.

Keywords: NRF-1; Nrf2; heme oxygenase-1; mitochondria; p62.

Fig. 1

Keap1-Nrf2 stress response system. (A) Schematic domain structures of Nrf2 and Nrf2-related proteins. Nrf2 possesses six evolutionarily conserved domains called Neh (Nrf2 ECH homology) 1–6 (only Neh1 and Neh2 are shown in the figure). The Neh2 domain is a regulatory domain of Nrf2 and contains ETGE and DLG motifs. Both motifs are conserved in Nrf1, which is another member of the CNC protein family, and Cnc-C, a Drosophila Nrf2 homolog. (B) Nrf2 heterodimerizes with small Maf proteins and binds to antioxidant-responsive elements (AREs). (C) Schematic domain structures of Keap1. Keap1 shows structural similarity to the Drosophila protein Kelch and has two canonical protein interaction domains, BTB (bric-a-brac, tramtrack, broad complex) and Kelch (also called double glycine-repeat, DGR). CTR, C-terminal region; IVR, intervening region; NTR, N-terminal region. (D) Schematic representation of the Nrf2-activation mechanism. Together, the DGR and CTR of Keap1 comprise a six-bladed β-propeller structure shown as DC (DGR, CTR). Keap1 homodimerizes via its BTB domain and binds to the ETGE and DLG motifs of the Nrf2 Neh2 domain. The ETGE and DLG motifs are high- and low-affinity binding sites, respectively, for Keap1. The lysine residues (K) that are ubiquitinated localize to one side of the intervening α-helix, enhancing Keap1-mediated ubiquitination. During oxidative stress, reactive cysteines in Keap1 are oxidized, leading to conformational changes in Keap1. As a result, only binding via the low-affinity site is disrupted or the Cul3 interaction with Keap1 is disrupted leading to the inhibition of Nrf2 ubiquitination. Ub; ubiquitin.

Fig. 2

Keap1 interaction with mitochondria-related proteins. Arrows in the figure indicate activation. For example, p62 and prothymosin α activate Nrf2 by competitively binding to Keap1 via their ETGE-related motifs. See text for details.

Fig. 3

Hypothetical mitochondrial retrograde signals that activate Nrf2. In specific cell types such as cardiomyocytes, HO-1-mediated CO production activates Nrf2 via the generation of H₂O₂ from the mitochondria. GSK3β is known to inhibit Nrf2 via β-TrCP-mediated degradation or Fyn-mediated nuclear export.^(68,69) H₂O₂ activates the PI3K-AKT pathway and inhibits GSK3β to activate Nrf2. COX; cytochrome c oxidase, GSK3β; glycogen synthase kinase-3β.

Fig. 4

Mitochondria-permissive signal for the Nrf2-HO-1 cascade. PINK1 directly phosphorylates Parkin and TRAP1, subsequently inducing the transfer of Parkin to the mitochondria and preventing mitochondrial oxidative damage. PI3K-AKT, ERK1/2 and DJ-1 signaling are indispensable for oxidative stress-induced Nrf2 activation. See text for details.