Oxytosis/ferroptosis is a type of programmed cell death dependent on iron and characterized by the accumulation of lipid peroxides, and is genetically and biochemically distinct from other forms of regulated cell death such as apoptosis. [1] [2] Oxytosis/ferroptosis is initiated by the failure of the glutathione-dependent antioxidant defenses, resulting in unchecked lipid peroxidation and eventual cell death. [3] Lipophilic antioxidants [4] and iron chelators [5] can prevent ferroptotic cell death. Although the connection between iron and lipid peroxidation has been appreciated for years, [6] it was not until 2012 that Brent Stockwell and Scott J. Dixon coined the term ferroptosis and described several of its key features. [5] Pamela Maher and David Schubert discovered the process in 2001 and called it oxytosis. While they did not describe the involvement of iron at the time, oxytosis and ferroptosis are today thought to be the same cell death mechanism. [1] [7]
Researchers have identified roles in which oxytosis/ferroptosis can contribute to the medical field, such as the development of cancer therapies. [8] Ferroptosis activation plays a regulatory role on growth of tumor cells in the human body. However, the positive effects of oxytosis/ferroptosis could be potentially neutralized by its disruption of metabolic pathways and disruption of homeostasis in the human body. [9] Since oxytosis/ferroptosis is a form of regulated cell death, [10] some of the molecules that regulate oxytosis/ferroptosis are involved in metabolic pathways that regulate cysteine exploitation, glutathione state, nicotinamide adenine dinucleotide phosphate (NADP) function, lipid peroxidation, and iron homeostasis. [9]
The hallmark feature of oxytosis/ferroptosis is the iron-dependent accumulation of oxidatively damaged phospholipids (i.e., lipid peroxides). The implication of Fenton chemistry via iron is crucial for the generation of reactive oxygen species and this feature can be exploited by sequestering iron in lysosomes. [11] Oxidation of phospholipids can occur when free radicals abstract electrons from a lipid molecule (typically affecting polyunsaturated fatty acids), thereby promoting their oxidation. The primary cellular mechanism of protection against oxytosis/ferroptosis is mediated by glutathione peroxidase 4 (GPX4), a glutathione-dependent hydroperoxidase that converts lipid peroxides into non-toxic lipid alcohols. [2] Recently, a second parallel protective pathway was independently discovered by two labs that involves the oxidoreductase FSP1 (also known as AIFM2). [12] [13] Their findings indicate that FSP1 enzymatically reduces non-mitochondrial coenzyme Q10, thereby generating a potent lipophilic antioxidant that suppresses the propagation of lipid peroxides. [12] [13] A similar mechanism for a cofactor moonlighting as a diffusable antioxidant was discovered in the same year for tetrahydrobiopterin (BH4), a product of the rate-limiting enzyme GCH1. [14] [15]
Small molecules such as erastin, sulfasalazine, sorafenib, (1S, 3R)-RSL3, ML162, and ML210 are known inhibitors of tumor cell growth via induction of oxytosis/ferroptosis. These compounds do not trigger apoptosis and therefore do not cause chromatin margination or poly (ADP-ribose) polymerase (PARP) cleavage. Instead, oxytosis/ferroptosis causes changes in mitochondrial phenotype. Iron is also necessary for small-molecule oxytosis/ferroptosis induction; therefore, these compounds can be inhibited by iron chelators. Erastin acts through inhibition of the cystine/glutamate transporter, thus causing decreased intracellular glutathione (GSH) levels. [5] Given that GSH is necessary for GPX4 function, depletion of this cofactor can lead to ferroptotic cell death. [3] Oxytosis/ferroptosis can also be induced through inhibition of GPX4, as is the molecular mechanism of action of RSL3, ML162, and ML210. [16] In some cells, FSP1 compensates for loss of GPX4 activity, and both GPX4 and FSP1 must be inhibited simultaneously to induce oxytosis/ferroptosis.
Replacing natural polyunsaturated fatty acids (PUFA) with deuterated PUFA (dPUFA), which have deuterium in place of the bis-allylic hydrogens, can prevent cell death induced by erastin or RSL3. [17] These deuterated PUFAs effectively inhibit ferroptosis and various chronic degenerative diseases associated with ferroptosis. [18]
Live-cell imaging has been used to observe the morphological changes that cells undergo during oxytosis/ferroptosis. Initially the cell contracts and then begins to swell. Perinuclear lipid assembly is observed immediately before oxytosis/ferroptosis occurs. After the process is complete, lipid droplets are redistributed throughout the cell (see GIF on right side).
Another form of cell death that occurs in the nervous system is apoptosis, which results in cell breakage into small, apoptotic bodies taken up through phagocytosis. [19] This process occurs continuously within mammalian nervous system processes that begin at fetal development and continue through adult life. Apoptotic death is crucial for the correct population size of neuronal and glial cells. Similarly to oxytosis/ferroptosis, deficiencies in apoptotic processes can result in many health complications, including neurodegeneration.
Within the study of neuronal apoptosis, most research has been conducted on the neurons of the superior cervical ganglion. [20] In order for these neurons to survive and innervate their target tissues, they must have nerve growth factor (NGF). [20] Normally, NGF binds to a tyrosine kinase receptor, TrkA, which activates phosphatidylinositol 3-kinase-Akt (PI3K-Akt) and extracellular signal-regulated kinase (Raf-MEK-ERK) signaling pathways. This occurs during normal development which promotes neuronal growth in the sympathetic nervous system. [20]
During embryonic development, the absence of NGF activates apoptosis by decreasing the activity of the signaling pathways normally activated by NGF. Without NGF, the neurons of the sympathetic nervous system begin to atrophy, glucose uptake rates fall, and the rates of protein synthesis and gene expression slow. [20] Apoptotic death from NGF withdrawal also requires caspase activity. [20] Upon NGF withdrawal, caspase-3 activation occurs through an in-vitro pathway beginning with the release of cytochrome c from the mitochondria. [20] In a surviving sympathetic neuron, the overexpression of anti-apoptotic B-cell CLL/lymphoma 2 (Bcl-2) proteins prevents NGF withdrawal-induced death. However, overexpression of a separate, pro-apoptotic Bcl-2 gene, Bax, stimulates the release of cytochrome c2. Cytochrome c promotes the activation of caspase-9 through the formation of the apoptosome. Once caspase-9 is activated, it can cleave and activate caspase-3 resulting in cell death. Notably, apoptosis does not release intracellular fluid as neurons that are degraded though oxytosis/ferroptosis do. During oxytosis/ferroptosis, neurons release lipid metabolites from inside the cell body. This is a key difference between oxytosis/ferroptosis and apoptosis.
Neural connections are constantly changing within the nervous system. Synaptic connections that are used more often are kept intact and promoted, while synaptic connections that are rarely used are subject to degradation. Elevated levels of synaptic connection loss and degradation of neurons are linked to neurodegenerative diseases. [21] More recently, oxytosis/ferroptosis has been linked to diverse brain diseases, [22] in particular, Alzheimer's disease, amyotrophic lateral sclerosis, and Parkinson's disease. [23] Two new studies show that oxytosis/ferroptosis contributes to neuronal death after intracerebral hemorrhage. [24] [25] Neurons that are degraded through oxytosis/ferroptosis release lipid metabolites from inside the cell body. The lipid metabolites are harmful to surrounding neurons, causing inflammation in the brain. Inflammation is a pathological feature of Alzheimer’s disease and intracerebral hemorrhage.
In a study performed using mice, it was found that the absence of Gpx4 promoted oxytosis/ferroptosis. Foods high in vitamin E promote Gpx4 activity, consequently inhibiting oxytosis/ferroptosis and preventing inflammation in brain regions.[ citation needed ] In the experimental group of mice that were manipulated to have decreased Gpx4 levels, mice were observed to have cognitive impairment and neurodegeneration of hippocampal neurons, again linking oxytosis/ferroptosis to neurodegenerative diseases.
Similarly, presence of transcription factors, specifically ATF4, can influence how readily a neuron undergoes cell death. The presence of ATF4 promotes resistance in cells against oxytosis/ferroptosis.[ citation needed ] However, this resistance can cause other diseases, such as cancer, to progress and become malignant.[ citation needed ] While ATF4 provides resistance oxytosis/ferroptosis, an abundance of ATF4 causes neurodegeneration.[ citation needed ]
Recent studies have suggested that oxytosis/ferroptosis contributes to neuronal cell death after traumatic brain injury. [26]
Preliminary reports suggest that oxytosis/ferroptosis may be a means through which tumor cells can be killed. Oxytosis/ferroptosis has been implicated in several types of cancer, including:
These forms of cancer have been hypothesized to be highly sensitive to oxytosis/ferroptosis induction. An upregulation of iron levels has also been seen to induce oxytosis/ferroptosis in certain types of cancer, such as breast cancer. [8] Breast cancer cells have exhibited vulnerability to oxytosis/ferroptosis via a combination of siramesine and lapatinib. These cells also exhibited an autophagic cycle independent of ferroptotic activity, indicating that the two different forms of cell death could be controlled to activate at specific times following treatment. [27] Furthermore, intratumor bacteria may scavenge iron by producing iron siderophores, which indirectly protect tumor cells from ferroptosis, emphasizing the need for ferroptosis inducers (thiostrepton) for cancer treatment. [28]
Notably, not all cancers are necessarily sensitive to oxytosis/ferroptosis induction. For instance, one study has demonstrated that oxytosis/ferroptosis in polymorphonuclear myeloid-derived suppressor cells in the tumor microenvironment releases oxidized lipids that contribute to immune suppression.
Apoptosis is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemical events lead to characteristic cell changes (morphology) and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, DNA fragmentation, and mRNA decay. The average adult human loses 50 to 70 billion cells each day due to apoptosis. For the average human child between 8 and 14 years old, each day the approximate loss is 20 to 30 billion cells.
Programmed cell death is the death of a cell as a result of events inside of a cell, such as apoptosis or autophagy. PCD is carried out in a biological process, which usually confers advantage during an organism's lifecycle. For example, the differentiation of fingers and toes in a developing human embryo occurs because cells between the fingers apoptose; the result is that the digits are separate. PCD serves fundamental functions during both plant and animal tissue development.
In chemistry and biology, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (O2), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (O2H), superoxide (O2-), hydroxyl radical (OH.), and singlet oxygen. ROS are pervasive because they are readily produced from O2, which is abundant. ROS are important in many ways, both beneficial and otherwise. ROS function as signals, that turn on and off biological functions. They are intermediates in the redox behavior of O2, which is central to fuel cells. ROS are central to the photodegradation of organic pollutants in the atmosphere. Most often however, ROS are discussed in a biological context, ranging from their effects on aging and their role in causing dangerous genetic mutations.
Neurotrophins are a family of proteins that induce the survival, development, and function of neurons.
4-Hydroxynonenal, or 4-hydroxy-2E-nonenal or 4-hydroxy-2-nonenal or 4-HNE or HNE,, is an α,β-unsaturated hydroxyalkenal that is produced by lipid peroxidation in cells. 4-HNE is the primary α,β-unsaturated hydroxyalkenal formed in this process. It is a colorless oil. It is found throughout animal tissues, and in higher quantities during oxidative stress due to the increase in the lipid peroxidation chain reaction, due to the increase in stress events. 4-HNE has been hypothesized to play a key role in cell signal transduction, in a variety of pathways from cell cycle events to cellular adhesion.
Fas ligand is a type-II transmembrane protein expressed on various types of cells, including cytotoxic T lymphocytes, monocytes, neutrophils, breast epithelial cells, vascular endothelial cells and natural killer (NK) cells. It binds with its receptor, called FAS receptor and plays a crucial role in the regulation of the immune system and in induction of apoptosis, a programmed cell death.
A neurodegenerative disease is caused by the progressive loss of neurons, in the process known as neurodegeneration. Neuronal damage may also ultimately result in their death. Neurodegenerative diseases include amyotrophic lateral sclerosis, multiple sclerosis, Parkinson's disease, Alzheimer's disease, Huntington's disease, multiple system atrophy, tauopathies, and prion diseases. Neurodegeneration can be found in the brain at many different levels of neuronal circuitry, ranging from molecular to systemic.Because there is no known way to reverse the progressive degeneration of neurons, these diseases are considered to be incurable; however research has shown that the two major contributing factors to neurodegeneration are oxidative stress and inflammation. Biomedical research has revealed many similarities between these diseases at the subcellular level, including atypical protein assemblies and induced cell death. These similarities suggest that therapeutic advances against one neurodegenerative disease might ameliorate other diseases as well.
The p75 neurotrophin receptor (p75NTR) was first identified in 1973 as the low-affinity nerve growth factor receptor (LNGFR) before discovery that p75NTR bound other neurotrophins equally well as nerve growth factor. p75NTR is a neurotrophic factor receptor. Neurotrophic factor receptors bind Neurotrophins including Nerve growth factor, Neurotrophin-3, Brain-derived neurotrophic factor, and Neurotrophin-4. All neurotrophins bind to p75NTR. This also includes the immature pro-neurotrophin forms. Neurotrophic factor receptors, including p75NTR, are responsible for ensuring a proper density to target ratio of developing neurons, refining broader maps in development into precise connections. p75NTR is involved in pathways that promote neuronal survival and neuronal death.
Caspase-9 is an enzyme that in humans is encoded by the CASP9 gene. It is an initiator caspase, critical to the apoptotic pathway found in many tissues. Caspase-9 homologs have been identified in all mammals for which they are known to exist, such as Mus musculus and Pan troglodytes.
Caspase-8 is a caspase protein, encoded by the CASP8 gene. It most likely acts upon caspase-3. CASP8 orthologs have been identified in numerous mammals for which complete genome data are available. These unique orthologs are also present in birds.
Glutathione peroxidase 4, also known as GPX4, is an enzyme that in humans is encoded by the GPX4 gene. GPX4 is a phospholipid hydroperoxidase that protects cells against membrane lipid peroxidation.
DNA damage-inducible transcript 3, also known as C/EBP homologous protein (CHOP), is a pro-apoptotic transcription factor that is encoded by the DDIT3 gene. It is a member of the CCAAT/enhancer-binding protein (C/EBP) family of DNA-binding transcription factors. The protein functions as a dominant-negative inhibitor by forming heterodimers with other C/EBP members, preventing their DNA binding activity. The protein is implicated in adipogenesis and erythropoiesis and has an important role in the cell's stress response.
Apoptosis-inducing factor 2 (AIFM2), also known as ferroptosis suppressor protein 1 (FSP1), apoptosis-inducing factor-homologous mitochondrion-associated inducer of death (AMID), is a protein that in humans is encoded by the AIFM2 gene, also known as p53-responsive gene 3 (PRG3), on chromosome 10.
Neurotrophic factor receptors or neurotrophin receptors are a group of growth factor receptors which specifically bind to neurotrophins.
Necroptosis is a programmed form of necrosis, or inflammatory cell death. Conventionally, necrosis is associated with unprogrammed cell death resulting from cellular damage or infiltration by pathogens, in contrast to orderly, programmed cell death via apoptosis. The discovery of necroptosis showed that cells can execute necrosis in a programmed fashion and that apoptosis is not always the preferred form of cell death. Furthermore, the immunogenic nature of necroptosis favors its participation in certain circumstances, such as aiding in defence against pathogens by the immune system. Necroptosis is well defined as a viral defense mechanism, allowing the cell to undergo "cellular suicide" in a caspase-independent fashion in the presence of viral caspase inhibitors to restrict virus replication. In addition to being a response to disease, necroptosis has also been characterized as a component of inflammatory diseases such as Crohn's disease, pancreatitis, and myocardial infarction.
In cellular biology, dependence receptors are proteins that mediate programmed cell death by monitoring the absence of certain trophic factors that otherwise serve as ligands (interactors) for the dependence receptors. A trophic ligand is a molecule whose protein binding stimulates cell growth, differentiation, and/or survival. Cells depend for their survival on stimulation that is mediated by various receptors and sensors, and integrated via signaling within the cell and between cells. The withdrawal of such trophic support leads to a form of cellular suicide.
Immunogenic cell death is any type of cell death eliciting an immune response. Both accidental cell death and regulated cell death can result in immune response. Immunogenic cell death contrasts to forms of cell death that do not elicit any response or even mediate immune tolerance.
Paraptosis is a type of programmed cell death, morphologically distinct from apoptosis and necrosis. The defining features of paraptosis are cytoplasmic vacuolation, independent of caspase activation and inhibition, and lack of apoptotic morphology. Paraptosis lacks several of the hallmark characteristics of apoptosis, such as membrane blebbing, chromatin condensation, and nuclear fragmentation. Like apoptosis and other types of programmed cell death, the cell is involved in causing its own death, and gene expression is required. This is in contrast to necrosis, which is non-programmed cell death that results from injury to the cell.
Alzheimer's disease (AD) is a neurodegenerative condition characterized by two hallmarks: senile plaques and the neurofibrillary tangle. Senile plaques are extracellular aggregations of amyloid-b (Aβ) protein. Neurofibrillary tangles are collections of hyperphosphorylated tau protein associated with microtubules found within neurons. Senile plaques and neurofibrillary tangles are widespread throughout brain tissue and mirror other pathological changes associated with AD.
Erastin is a small molecule capable of initiating ferroptotic cell death. Erastin binds and activates voltage-dependent anion channels (VDAC) by reversing tubulin's inhibition on VDAC2 and VDAC3, and functionally inhibits the cystine-glutamate antiporter system Xc−. Cells treated with erastin are deprived of cysteine and are unable to synthesize the antioxidant glutathione. Depletion of glutathione eventually leads to excessive lipid peroxidation and cell death.