Ferroptosis is a recently described form of programmed cell death with distinct morphological and biochemical characteristics. It does not result in the chromatin condensation observed during apoptosis or the loss of plasma membrane integrity that occurs during necrosis. Rather, it is morphologically characterized by the presence of condensed mitochondria, reduction in mitochondrial cristae, and increased membrane densities.
Ferratin & Iron Homeostasis
Ferroptosis is an iron-dependent cell death that results in an increase in lipid peroxides. As such, it is largely regulated by pathways that contribute to iron homeostasis and oxidative stress. Iron homeostasis is controlled, in part, by ferratin, which is a large protein complex that is responsible for the transport of iron in its non-toxic form. Ferratin levels can be regulated by a selective autophagy process targeting ferratin, termed ferritinophagy. This pathway is mediated by a selective cargo receptor for ferratin called nuclear receptor coactivator 4 (NCOA4). Ferroptosis has been shown to be inhibited with the use of some common iron chelators, such as deferoamine.
Defense against Oxidative Stress
Pathways regulating cellular defense against oxidative stress are critical to mitigate ferroptosis. Small-molecule lipophilic free radical scavengers, like ferrostatin-1 and liproxstatin-1, can inhibit ferroptosis. The glutathione pathway, in particular, has been identified as a key antioxidant defense pathway. A central player in this process is the metabolic protein glutathione peroxidase 4 (GPX4), which converts GSH into oxidized glutathione (GSSH), thus protecting cells against ferroptosis by limiting cytotoxic lipid peroxidation. The glutathione peroxidase pathway is further regulated by System Xc-, an amino acid antiporter consisting of a heterodimer of xCT/SLC7A11 and SLC3A2 (4F2hc/CD98), which is critical for glutathione (GSH) synthesis. Importantly, one way to induce ferroptosis is to use the compound erastin that directly inhibits the function of System Xc-.
Regulation of genes involved in oxidative stress, including GPX4, are largely controlled by the transcription factor NRF2. This serves as a critical defense against ferroptosis. Under normal conditions, expression of NRF2 is inhibited through interaction with KEAP1, part of a ubiquitin E3 ligase complex that leads to NRF2 proteasomal degradation. Oxidative stress leads to conformation changes in KEAP1 that disrupts this interaction, resulting in stabilization of NRF2. This process is further regulated through the autophagy pathway in which the autophagy cargo receptor p62/SQSTM1 can competitively inhibit the KEAP1-NRF2 complex leading to upregulation of NRF2.
Tools for Studying Ferroptosis
Tools to monitor ferroptosis may involve multiple approaches, including pharmacological sensitivity (such as iron chelators, antioxidants), changes in expression of targets (GPX4, SCL7A11, ferratin, NRF2, etc.), monitoring reactive oxygens and lipid peroxidation, and glutathione assays.
To find out more about the mechanisms, morphology, and target proteins involved in many types of cell death, download the Researcher's Guide to Mechanisms of Cell Death.
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