Breakdown of an Ironclad Defense System: The Critical Role of NRF2 in Mediating Ferroptosis

Abstract

Ferroptosis is a non-apoptotic mode of regulated cell death that is iron and lipid peroxidation dependent. As new mechanistic insight into ferroptotic effectors and how they are regulated in different disease contexts is uncovered, our understanding of the physiological and pathological relevance of this mode of cell death continues to grow. Along these lines, a host of pharmacological modulators of this pathway have been identified, targeting proteins involved in iron homeostasis; the generation and reduction of lipid peroxides; or cystine import and glutathione metabolism. Also, of note, many components of the ferroptosis cascade are target genes of the transcription factor nuclear factor erythroid 2-related factor 2 (NRF2), indicating its critical role in mediating the ferroptotic response. In this review, we discuss the in vitro, in vivo, and clinical evidence of ferroptosis in disease, including a brief discussion of targeting upstream mediators of this cascade, including NRF2, to treat ferroptosis-driven diseases.

Keywords: NRF2; cancer; cardiovascular disease; cell death; diabetes; ferritinophagy; ferroptosis; iron; lipid peroxidation; neurodegeneration.

Figure 1:. NRF2 regulates genes involved in preventing ferroptosis.

Under homeostatic conditions, NRF2 is ubiquitylated and targeted for proteasomal degradation by a KEAP1-CUL3-RBX1 E3 ubiquitin ligase complex. Under conditions of oxidative/electrophilic stress, or as a result of mutations in KEAP1, CUL3, or NRF2 itself, NRF2 is no longer degraded, allowing for nuclear translocation and activation of antioxidant response element (ARE)-containing genes. NRF2 transcriptional targets are involved in mediating iron/metal metabolism, the catabolism/detoxification of reactive intermediates, and glutathione synthesis and metabolism, all of which play a key role in preventing initiation of ferroptosis.

Figure 2:. Iron metabolism and ferritinophagy and their regulation by NRF2.

Ferric iron (Fe³⁺) in the blood is bound and transported by transferrin (Ferretti et al.). Fe³⁺-TR is bound by transferrin receptor 1 (TFRC) and endocytosed. In the endosome, the acidic pH promotes dissociation of TFRC and Fe³⁺, which is then reduced to Fe²⁺ by the metalloreductase STEAP3. Fe²⁺ is then transported to the cytosol by divalent metal transporter 1 (DMT1), contributing to the labile iron pool (LIP). Fe²⁺ can be exported out of cells by ferroportin (FPN1), incorporated into iron-containing proteins, or bound and stored by ferritin as Fe³⁺. Ferritinophagy is the autophagy-dependent degradation of ferritin via nuclear receptor coactivator 4 (NCOA4). Following ferritin degradation, newly freed Fe³⁺ is reduced to Fe²⁺ by STEAP3 and exported from the lysosome into the cytosol by DMT1 to again become part of the LIP, where it can subsequently react with ROS via the Fenton reaction. Importantly, a number of key iron metabolism, storage, and transport, as well as key autophagy/ferritinophagy initiation genes are transcriptionally regulated by NRF2.

Figure 3:. Ferroptosis and its role in disease.

Ferroptosis has been shown to play a role in neurodegeneration, cardiovascular disease, diabetes, and cancer, among others. In cancer, the therapeutic goal is to activate ferroptosis to kill tumor cells that are resistant to other modes of cell death. In neurodegeneration, type II diabetes, and cardiovascular disease, the therapeutic goal is to prevent ferroptosis-induced cell loss to slow onset or mitigate disease progression. Inducers and inhibitors of ferroptosis that have been shown to affect experimental models of each disease in vitro and in vivo, or patients in clinic, are also included.

Figure 4:. Modulation of NRF2 to target ferroptosis-driven diseases.

NRF2 regulates the expression of enzymes responsible for glutathione synthesis (GLC and GSS), as well as preventing lipid peroxidation and reducing oxidized CoQ10, a key membrane antioxidant (GPX4 and FSP1). Lipid peroxides are key initiators of ferroptosis, which has been shown to be involved in numerous diseases, including cancer, type II diabetes, cardiovascular disease, and neurodegeneration.