NFE2L2 and ferroptosis resistance in cancer therapy

Abstract

NFE2-like basic leucine zipper transcription factor 2 (NFE2L2, also known as NRF2), is a key transcription factor in the cellular defense against oxidative stress, playing a crucial role in cancer cell survival and resistance to therapies. This review outlines the current knowledge on the link between NFE2L2 and ferroptosis - a form of regulated cell death characterized by iron-dependent lipid peroxidation - within cancer cells. While NFE2L2 activation can protect normal cells from oxidative damage, its overexpression in cancer cells contributes to drug resistance by upregulating antioxidant defenses and inhibiting ferroptosis. We delve into the molecular pathways of ferroptosis, highlighting the involvement of NFE2L2 and its target genes, such as NQO1, HMOX1, FTH1, FTL, HERC2, SLC40A1, ABCB6, FECH, PIR, MT1G, SLC7A11, GCL, GSS, GSR, GPX4, AIFM2, MGST1, ALDH1A1, ALDH3A1, and G6PD, in ferroptosis resistance. Understanding the delicate balance between NFE2L2's protective and deleterious roles could pave the way for novel therapeutic strategies targeting NFE2L2 to enhance the efficacy of ferroptosis inducers in cancer therapy.

Keywords: Cancer therapy; NFE2L2; drug resistance; ferroptosis; oxidative stress.

Figure 1

The KEAP1-NFE2L2 signaling pathway. NFE2L2 is primarily regulated by the KEAP1-CUL3-RBX1 E3 ubiquitin ligase complex, which, under normal conditions, promotes the ubiquitination and proteasomal degradation of NFE2L2, keeping its cellular levels low. However, in response to oxidative stress or electrophilic agents, key cysteine residues on KEAP1 are modified, preventing NFE2L2 ubiquitination. As a result, NFE2L2 stabilizes and accumulates in the cytoplasm before translocating to the nucleus. There, it dimerizes with sMAF proteins and binds to AREs in the promoters of target genes. These genes regulate a variety of crucial cellular processes, including antioxidant defense, detoxification, cell proliferation and death, autophagy, metabolism, mitochondrial function, and iron homeostasis. Through these pathways, NFE2L2-mediated gene expression plays an essential role in protecting cells from oxidative damage, maintaining redox balance, and ensuring overall cellular homeostasis. KEAP1: Kelch-like ECH-associated protein 1; NFE2L2: NFE2-like basic leucine zipper transcription factor 2; CUL3: cullin 3; RBX1: ring-box 1; sMAF: small musculoaponeurotic fibrosarcoma; AREs: antioxidant response elements.

Figure 2

NFE2L2-dependent genes in ferroptosis. Ferroptosis, a form of oxidative cell death, can be triggered by increased iron accumulation or the inhibition of antioxidant systems. Fe³⁺ is taken up by TFRC and exported by SLC40A1. Inside the cell, Fe³⁺ is reduced to Fe²⁺, with excess iron stored in ferritin (composed of FTH1 and FTL) or sequestered by MT1G. Conversely, autophagic degradation of ferritin via NCOA4 can elevate the labile iron pool, a process regulated by HERC2-mediated degradation of NCOA4 through the ubiquitin-proteasome system. HMOX1 also contributes to the labile iron pool by catalyzing heme degradation. Iron is utilized in mitochondria for heme synthesis via FECH and ABCB6 or transported to the nucleus by PIR, which suppresses autophagy by limiting the cytosolic translocation of HMGB1. Excess iron can lead to oxidative stress and ROS production via the Fenton reaction, which can be countered by NQO1, HMOX1, or G6PD. ACSL4 is a key regulator of ferroptosis, producing PUFAs for subsequent ALOX-mediated lipid peroxidation. MGST1 inhibits ALOX activity through direct interaction, while MUFAs may competitively inhibit PUFA lipid peroxidation. Multiple mechanisms inhibit excessive lipid peroxidation, with GPX4 playing a central role by using GSH to neutralize lipid peroxides. GSH levels are positively regulated by the system xc^--mediated cystine uptake and subsequent cysteine production, with SLC7A11 and SLC3A2 as key components. GCL (composed of GCLC and GCLM) is the rate-limiting enzyme in GSH synthesis, and GSSG can be reduced back to GSH by GSR. AIFM2 provides a GPX4-independent anti-ferroptotic pathway through various mechanisms, including the production of CoQ10 or vitamin K, or activation of ESCRT-III membrane repair pathways. 4-HNE, an end product of lipid peroxidation, is detoxified by the ALDH family. NFE2L2-dependent genes are highlighted in green, while positive regulators of ferroptosis are shown in red. NFE2L2: NFE2-like basic leucine zipper transcription factor 2; TFRC: transferrin receptor; SLC40A1: solute carrier family 40 member 1; FTH1: ferritin heavy chain 1; FTL: ferritin light chain; MT1G: metallothionein 1G; NCOA4: nuclear receptor coactivator 4; HERC2: HECT and RLD domain containing E3 ubiquitin protein ligase 2; HMOX1: heme oxygenase 1; FECH: ferrochelatase; ABCB6: ATP-binding cassette subfamily B member 6; PIR: pirin; HMGB1: high mobility group box 1; ROS: reactive oxygen species; NQO1: quinone oxidoreductase-1; G6PD: glucose-6-phosphate dehydrogenase; ACSL4: acyl-CoA synthetase long-chain family member 4; PUFAs: polyunsaturated fatty acids; ALOX: lipoxygenase; MGST1: microsomal glutathione S-transferase 1; MUFAs: monounsaturated fatty acids; GPX4: glutathione peroxidase 4; GSH: glutathione; GCL: glutamate-cysteine ligase; GCLC: glutamate-cysteine ligase catalytic subunit; CGLM: glutamate-cysteine ligase modifier subunit; GSSG: oxidized glutathione; GSR: glutathione reductase; AIFM2: apoptosis-inducing factor mitochondria-associated 2; CoQ10: coenzyme Q10; ESCRT-III: endosomal sorting complex required for transport-III; 4-HNE: 4-hydroxynonenal; ALDH: aldehyde dehydrogenase.

Figure 3

Effects of autophagy impairment on NFE2L2 activation in ferroptosis. (A) Under physiological conditions, NFE2L2 is continuously ubiquitinated by the KEAP1-CUL3 E3 ubiquitin ligase complex, resulting in its proteasomal degradation; (B) When autophagy is impaired, accumulated SQSTM1 interacts with KEAP1, leading to the proteasomal degradation of KEAP1. This interaction sequesters KEAP1 away from NFE2L2, preventing the ubiquitination and degradation of NFE2L2. Consequently, NFE2L2 is stabilized and translocates to the nucleus, where it dimerizes with sMAF proteins. This complex binds to AREs to mediate the transcription of genes involved in iron metabolism, GSH metabolism, and ROS detoxification. NFE2L2: NFE2-like basic leucine zipper transcription factor 2; KEAP1: Kelch-like ECH-associated protein 1; CUL3: cullin 3; SQSTM1: sequestosome 1; sMAF: small musculoaponeurotic fibrosarcoma; AREs: antioxidant response elements; GSH: glutathione; ROS: reactive oxygen species.