NRF2 plays a critical role in mitigating lipid peroxidation and ferroptosis

Abstract

The transcription factor nuclear factor erythroid 2-related factor 2 (NRF2) is a key regulator of the cellular antioxidant response, controlling the expression of genes that counteract oxidative and electrophilic stresses. Many pathological conditions are linked to imbalances in redox homeostasis, illustrating the important role of antioxidant defense systems in preventing the pathogenic effects associated with the accumulation of reactive species. In particular, it is becoming increasingly apparent that the accumulation of lipid peroxides has an important role in driving the pathogenesis of multiple disease states. A key example of this is the recent discovery of a novel form of cell death termed ferroptosis. Ferroptosis is an iron-dependent, lipid peroxidation-driven cell death cascade that has become a key target in the development of anti-cancer therapies, as well as the prevention of neurodegenerative and cardiovascular diseases. In this review, we will provide a brief overview of lipid peroxidation, as well as key components involved in the ferroptotic cascade. We will also highlight the role of the NRF2 signaling pathway in mediating lipid peroxidation and ferroptosis, focusing on established NRF2 target genes that mitigate these pathways, as well as the relevance of the NRF2-lipid peroxidation-ferroptosis axis in disease.

Graphical abstract

Fig. 1

The ferroptotic cascade. Accumulation of free iron is a key initiator of ferroptosis. Free iron exists as part of the labile iron pool (LIP) and can accumulate due to altered iron import/export, decreased iron storage, or breakdown of iron-containing proteins. One recently identified pathway of free iron accumulation is through the autophagic degradation of ferritin, which stores intracellular iron, a process termed ferritinophagy. Nuclear receptor coactivator 4 (NCOA4) has been shown to act as a cargo receptor that binds to the heavy chain of ferritin delivering it to the early-stage autophagosome, resulting in degradation and release of free iron into the cytosol. Free iron can then interact with ROS (specifically hydrogen peroxide to form hydroxyl/peroxyl radicals via the Fenton reaction), which can then abstract a hydrogen atom from poly-unsaturated fatty acids (PUFAs), forming a lipid radical that rapidly reacts with oxygen to generate a lipid peroxide. Normally, lipid peroxides and their degradation products are held in check by GSH-based redox reactions. The xCT antiporter (consisting of two subunits SLC7A11 and SLC3A2) exports glutamine and imports cystine into the cell. Inside the cell, the cystine is reduced to cysteine by cystine reductase and then glutamate-cysteine ligase (GCL) and glutathione synthetase (GSS) add L-glutamate and glycine, respectively, to produce GSH. Many redox enzymes use GSH, including glutathione peroxidase 4 (GPX4), which reduces reactive aldehydes to their alcohol form. If GPX4 or xCT are genetically disrupted or pharmacologically inhibited, lipid peroxides and their degradation products accumulate, and initiate ferroptosis through a poorly understood mechanism involving membrane destabilization, cytoskeletal changes, and altered proteostasis.

Fig. 2

Morphological stages of Ferroptotic cell death. The exact mechanisms underlying the phenotypic changes that occur during ferroptosis remain unclear; however, it is morphologically distinct from other forms of cell death (i.e. apoptosis, necrosis, necroptosis, and autosis). Following treatment with a pro-ferroptotic agent, an initial cell shrinking is followed by condensation of cytoplasmic constituents and a “ballooning” phenotype, which involves the formation of a clear, rounded morphology consisting mainly of empty cytosol. The significant alterations to cell morphology presumably involve plasma membrane thinning/destabilization, significant cytoskeletal rearrangements, and a general disruption of proteostasis. Image represents the final “ballooning” phenotype of SKOV3 (ovarian carcinoma cell line) cells following treatment with 2.5 mM sulfasalazine for 24 h.

Fig. 3

NRF2 target genes are involved in preventing lipid peroxidation and ferroptosis. NRF2 is a master regulator of the antioxidant response and has been shown to regulate the activity of several ferroptosis and lipid peroxidation-related proteins. These targets can be divided into three broad classes: iron/metal metabolism, intermediate metabolism, and GSH synthesis/metabolism. Examples of each class are included, all of which have a verified antioxidant response element (ARE).