NRF2, a Transcription Factor for Stress Response and Beyond

Abstract

Nuclear factor erythroid 2-related factor 2 (NRF2) is a transcription factor that regulates the cellular defense against toxic and oxidative insults through the expression of genes involved in oxidative stress response and drug detoxification. NRF2 activation renders cells resistant to chemical carcinogens and inflammatory challenges. In addition to antioxidant responses, NRF2 is involved in many other cellular processes, including metabolism and inflammation, and its functions are beyond the originally envisioned. NRF2 activity is tightly regulated through a complex transcriptional and post-translational network that enables it to orchestrate the cell's response and adaptation to various pathological stressors for the homeostasis maintenance. Elevated or decreased NRF2 activity by pharmacological and genetic manipulations of NRF2 activation is associated with many metabolism- or inflammation-related diseases. Emerging evidence shows that NRF2 lies at the center of a complex regulatory network and establishes NRF2 as a truly pleiotropic transcription factor. Here we summarize the complex regulatory network of NRF2 activity and its roles in metabolic reprogramming, unfolded protein response, proteostasis, autophagy, mitochondrial biogenesis, inflammation, and immunity.

Keywords: NRF2; UPR; autophagy; inflammation; metabolism; oxidative stress; proteostasis; transcription factor.

Figure 1

The architecture of Nuclear factor erythroid 2-related factor 2 (NRF2), Kelch-like-ECH-associated protein 1 (KEAP1), and β-transducin repeat-containing protein (βTrCP). (A) NRF2 contains seven conserved NRF2-ECH homology NRF2-ECH homology (Neh) domains, Neh1-Neh7. Neh1 contains a basic leucine zipper (bZip) motif, where the basic region is responsible for DNA binding and the Zip dimerizes with other binding partners such as sMAFs. Neh2 contains ETGE and DLG motifs, which are required for the interaction with KEAP1 and subsequent KEAP1-mediated proteasomal degradation. Neh3, 4 and 5 domains are transactivation domains of NRF2. Neh4 and 5 domains also interact with HRD1 that mediates NRF2 degradation. Neh6 contains two βTrCP degrons DSGIS and DSAPGS that are responsible for the β-TrCP mediated proteasomal degradation. (B) KEAP1 contains five domains, amino terminal region (NTR), a broad complex, tramtrack, bric-a-brac (BTB) domain, an intervening region (IVR), six Kelch domains, and the C-terminal region (CTR). The Kelch domain and CTR mediate the interaction with NRF2, p62, DPP3, WTX, and PALB2 that contains ETGE motifs. The BTB domain homodimerizes with KEAP1 and contributes to the interaction of IVR with Cul3/RBX1 complex. Several functional important cysteine residues (C151, C226, C273 and C278) that sense reactive oxygen species (ROS) and electrophiles and modulate KEAP1-NRF2 interaction. (C) βTrCP has three domains, dimerization domain (D) that forms homo- and heterodimers between βTrCP1 and βTrCP2, the F-box that recruits SKP1 for the binding of CUL1/RBX1 complex, and the WD40 repeat domain that binds βTrCP degrons DSGIS and DSAPGS in NRF2. βTrCP, β-transducing repeat-containing protein; CUL3, Cullin3; RBX1, RING-box protein; WD40, WD Repeat protein 40; RXRα, retinoic X receptor alpha; DPP3, dipeptidyl peptidase 3; WTX, Wilms tumor gene on X chromosome; PALB2, Partner and Localizer of BRCA2; GSK3, Glycogen synthase kinase-3; SKP1, S-phase kinase-associated protein-1.

Figure 2

Regulation of NRF2 activity. Under basal conditions, the amount of NRF2 is low due to its continuous sequestration by KEAP1 and subsequent proteasomal degradation. Under stressed condition, the cellular NRF2 amount is temporarily or constitutively increased upon exposure to toxicants and ROS, oncogenic signaling, genetic mutations, autophagy disruption, or metabolic alteration, which disrupt the KEAP1-NRF2 complex and lead to activation of NRF2. Activated NRF2 accumulates in the nucleus, where it interacts with other transcription factors and cofactors to regulate transcription of its target genes, which encoding proteins involved in the antioxidants, detoxification, metabolism, and inflammation. NRF2 also regulates its own NFE2L2 mRNA transcription. Activity of NRF2 is modulated at multiple levels, including transcriptional regulation (NF-κB, AhR-ARNT, ATF4, and other transcription factors, cofactors), post-transcriptional regulation (miRNA, RBPs, alternative splicing), post-translational regulation (ERK, JNK, PKC, CK2, PERK, GSK3, p38), and regulation of NRF2 stability (KEAP1, βTrCP, HRD1, WDR23, CRIF1). AhR, aryl hydrocarbon receptor; ARNT, AHR nuclear translocator; NF-κB, nuclear factor-κB; PI3K, phopshoinositide 3-kinase; PKC, protein kinase C; ERG, extracellular signal-regulated protein kinases; JNK, c-jun N-terminal kinase; PERK, protein kinase R (PKR)-like endoplasmic reticulum kinase; CK2, casein kinase 2; WDR23, WD40-repeat protein 23; CRIF1, CR6-interacting Factor 1; ATF4, activating transcription factor.

Figure 3

NRF2 regulates metabolism. NRF2 upregulates expression of genes involved in the glycolysis, glycogen metabolism, pentose phosphate pathway, one carbon metabolism, amino acid metabolism, nucleotide biosynthesis, glutaminolysis, fatty acid synthesis, heme metabolism, and glutathione synthesis and utilization (colored in red). NRF2 downregulates the expression of genes involved in the gluconeogenesis (colored in blue). Enzyme reactions that produce NADPH are indicated and colored in green. Enzyme abbreviations: ELOVL7, fatty acid elongase 7; FADS1, fatty acid desaturase 1; G6PD, glucose-6-phosphate dehydrogenase; GCLC, glutamate-cysteine ligase, catalytic subunit; GCLM, glutamate-cysteine ligase, modifier subunit; GLS2, glutaminase 2; GPT2, glutamic pyruvate transaminase; GSS, glutathione synthetase; IDH1, isocitrate dehydrogenase 1; ME1, malic enzyme 1; MTFHD2, methylenetetrahydrofolate dehydrogenase 2; PGD, 6-phosphogluconate dehydrogenase; PHGDH, phosphoglycerate dehydrogenase; PPAT, phosphoribosyl pyrophosphate amidotransferase; TALDO, transaldolase; TKT, transketolase; TXN, thioredoxin; SLC7A11, Solute Carrier Family 7 Member 11; SLC1A5, solute carrier family 1, member 5; HK1, hexokinase 1; GPI1 glucose phosphate isomerase 1; PFK2, 6-phosphofructo-2-kinase; ALDA, aldolase A, fructose-bisphosphate; Eno1,4, enolase 1 and enolase 4; PKM, pyruvate kinase, muscle; GR, glutathione reductase; GPx, glutathione peroxidase; SOD1, superoxide dismutase 1; CAT, catalase; HO-1, heme oxygenase-1; FPN1; ferroportin 1; BLVR, biliverdin reductase; FTH, ferritin heavy chain; FTL, ferritin light chain; PDM, phosphoglucomutase; GBE1, glycogen branching enzyme. Metabolite abbreviations: G-6-P, glucose 6-phosphate; G-1-P, glucose 1-phosphate; F-6-P, fructose 6-phosphate; F-1,6-BP, fructose 1,6-bisphosphate; F-2,6-BP, fructose 2,6-bisphosphate; GA-3-P, glyceraldehyde 3-phosphate; 1,3-PG, 1,3-phosphoglycerate; 3-PG, 3-phosphoglycerate; 2-PG, 2-phosphoglycerate; PEP, phosphoenolpyruvate; UDP-G, uracil-diphosphate glucose; 6-P-GL, 6-phosphogluconolactone; 6-PG, 6-phosphogluconate; R-5-P, ribulose 5-phosphate; 5-PRA, phosphoribosylamine; PRPP, 5-phosphoribosyl-α-1-pyrophosphate; THF, tetrahydrofolate; CH2-THF, 5,10-methylene-tetrahydrofolate; CHO-THF, 10-formyl- tetrahydrofolate; IMP, inosine monophosphate; AMP, adenosine monophosphate; GMP, guanosine monophosphate. Gly, glycine; Ala, alanine; Cys, cysteine; Glu, glutamate; Gln, glutamine; aKG, a-ketoglutarate; Y-Glu-Cys, gamma-glutamyl cysteine; Acetyl-CoA, acetyl-coenzyme A; NADPH, Nicotinamide adenine dinucleotide phosphate, reduced.

Figure 4

NRF2 regulates inflammation and immunity. The activation of NRF2 may alter the differentiation, expansion, and survival of the immune cells as well as the cytokine release. NRF2 activation shifts Th1/Th2 balance in disease models by impairing Th1-driven response and skews them towards Th2 differentiation. NRF2-mediated antioxidant defense in Treg cells leads to their expansion and survival. Nfe2l2-deficiency leads to elevated oxidative damage that exacerbates the differentiation of Th17 cells. Nfe2l2 deletion in dendritic cells augments expression of MHC class II and the cells surface expression of co-stimulatory molecules CD86 and CD80 to influence the behavior of Th cells. Increased NRF2 expression in leukocytes such as macrophages inhibits the expression of pro-inflammatory genes through down-regulation of the NF-κB pathway. NRF2 activation in MDSCs regulates metabolism that leads to expansion of suppressive MDSCs. However, Nfe2l2-deletion in MDSCs have higher levels of intracellular ROS that suppress CTLs proliferation and induces T-cell anergy.