"Only a Life Lived for Others Is Worth Living": Redox Signaling by Oxygenated Phospholipids in Cell Fate Decisions

Inhibition of the skeletal muscle Lands cycle ameliorates weakness induced by physical inactivity

BACKGROUND

Lipid hydroperoxides (LOOH) have been implicated in skeletal muscle atrophy with age and disuse. Lysophosphatidylcholine acyltransferase 3 (LPCAT3), an enzyme of the Lands cycle, conjugates a polyunsaturated fatty acyl chain to a lysophospholipid to form a polyunsaturated fatty acid containing phospholipid (PUFA-PL) molecule, providing substrates for LOOH propagation. Previous studies suggest that inhibition of the Lands cycle is an effective strategy to suppress LOOH. Mice with skeletal muscle-specific tamoxifen-inducible knockout of LPCAT3 (LPCAT3-MKO) were utilized to determine if muscle-specific attenuation of LOOH may alleviate muscle atrophy and weakness with disuse.

METHODS

LPCAT3-MKO and control mice underwent 7 days of sham or hindlimb unloading (HU model) to study muscle mass and force-generating capacity. LOOH was assessed by quantifying 4-hydroxynonenal (4-HNE)-conjugated peptides. Quantitative PCR and lipid mass spectrometry were used to validate LPCAT3 deletion.

RESULTS

Seven days of HU was sufficient to induce muscle atrophy and weakness concomitant to a ~2-fold increase in 4-HNE (P = 0.0069). Deletion of LPCAT3 reversed HU-induced increase in muscle 4-HNE (P = 0.0256). No difference was found in body mass, body composition, or caloric intake between genotypes. The soleus (SOL) and plantaris (PLANT) muscles of the LPCAT3-MKO mice experienced ~15% and ~40% less atrophy than controls, respectively. (P = 0.0011 and P = 0.0265). Type I and IIa SOL myofibers experienced a ~40% decrease in cross sectional area (CSA), which was attenuated to only 15% in the LPCAT3-MKO mice (P = 0.0170 and P = 0.0411, respectively). Strikingly, SOL muscles were fully protected and extensor digitorum longus (EDL) muscles experienced a ~35% protection from HU-induced reduction in force-generating capacity in the LPCAT3-MKO mice compared with controls (P < 0.0001 for both muscles).

CONCLUSIONS

Our findings demonstrate that attenuation of skeletal muscle lipid hydroperoxides is sufficient to restore its function, in particular a protection from reduction in muscle specific force. Our findings suggest muscle lipid peroxidation contributes to atrophy and weakness induced by disuse in mice.

Molecular mechanisms of ROS-modulated cancer chemoresistance and therapeutic strategies

Drug resistance is the main obstacle to achieving a cure in many cancer patients. Reactive oxygen species (ROS) are master regulators of cancer development that act through complex mechanisms. Remarkably, ROS levels and antioxidant content are typically higher in drug-resistant cancer cells than in non-resistant and normal cells, and have been shown to play a central role in modulating drug resistance. Therefore, determining the underlying functions of ROS in the modulation of drug resistance will contribute to develop therapies that sensitize cancer resistant cells by leveraging ROS modulation. In this review, we summarize the notable literature on the sources and regulation of ROS production and highlight the complex roles of ROS in cancer chemoresistance, encompassing transcription factor-mediated chemoresistance, maintenance of cancer stem cells, and their impact on the tumor microenvironment. We also discuss the potential of ROS-targeted therapies in overcoming tumor therapeutic resistance.

The Emerging Role of Ferroptosis in Sepsis, Opportunity or Challenge?

Sepsis is a syndrome in multi-organ dysfunction triggered by a deleterious immunological reaction of the body to a condition caused by infection, surgery, or trauma. Currently, sepsis is thought to be primarily associated with abnormal immune responses resulting in organ microcirculatory disturbances, cellular mitochondrial dysfunction, and induced cell death, although the exact pathogenesis of sepsis is still inconclusive. In recent years, the role of abnormal metabolism of trace nutrients in the pathogenesis of sepsis has been investigated. Ferroptosis is a type of cell death that relies on iron and is characterized by unique morphological, biochemical, and genetic features. Unlike other forms of cell death, such as autophagy, apoptosis, necrosis, and pyroptosis, ferroptosis is primarily driven by lipid peroxidation. Ferroptosis cells may be immunogenic, amplify inflammatory responses, cause more cell death, and ultimately induce multi-organ failure. An increasing number of studies have indicated the significance of ferroptosis in sepsis and its role in reducing inflammation. The effectiveness of sepsis treatment has been demonstrated by the use of drugs that specifically target molecules associated with the ferroptosis pathway, including ferroptosis inhibitors. Nevertheless, there is a scarcity of studies investigating the multi-organ dysfunction caused by ferroptosis in sepsis. This article presents a summary and evaluation of recent progress in the role of ferroptosis through molecularly regulated mechanisms and its potential mechanisms of action in the multi-organ dysfunction associated with sepsis. It also discusses the current challenges and prospects in understanding the connection between sepsis and ferroptosis, and proposes innovative ideas and strategies for the treatment of sepsis.

Visualization of Differential Cardiolipin Profiles in Murine Retinal Cell Layers by High-Resolution MALDI Mass Spectrometry Imaging

The precise fatty acyl chain configuration of cardiolipin (CL), a tetrameric mitochondrial-specific membrane lipid, exhibits dependence on cell and tissue types. A powerful method to map CL profiles in tissue sections in a spatially resolved manner is matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI). To build on and explore this potential, we employed a quadrupole time-of-flight mass spectrometer along with optimized sample preparation protocols. We imaged the CL profiles of individual murine retinal cell layers at a pixel size of 10 μm. In combination with tandem MS, we obtained detailed insights into the CL composition of individual retinal cell layers. In particular, we found differential expression of the polyunsaturated fatty acids (PUFA) linoleic, arachidonic, and docosahexaenoic acids. PUFAs are prone to peroxidation and hence regarded as critical factors in development and progression of retinal pathologies, such as age-related macular degeneration (AMD). The ability of MALDI-MSI to provide cues on the CL composition in neuronal tissue with close to single-cell resolution can provide important insights into retinal physiology in health and disease.

Inhibition of skeletal muscle Lands cycle ameliorates weakness induced by physical inactivity

Background

Lipid hydroperoxides (LOOH) have been implicated in skeletal muscle atrophy with age and disuse. Lysophosphatidylcholine acyltransferase 3 (LPCAT3), an enzyme of Lands cycle, conjugates a polyunsaturated fatty acyl chain to a lysophospholipid (PUFA-PL) molecule, providing substrates for LOOH propagation. Previous studies suggest that inhibition of Lands cycle is an effective strategy to suppress LOOH. Mice with skeletal muscle-specific tamoxifen-inducible knockout of LPCAT3 (LPCAT3-MKO) were utilized to determine if muscle-specific attenuation of LOOH may alleviate muscle atrophy and weakness with disuse.

Methods

LPCAT3-MKO and control mice underwent 7 days of sham or hindlimb unloading (HU model) to study muscle mass and force-generating capacity. LOOH was assessed by quantifying 4-hydroxynonenal (4-HNE)-conjugated peptides. Quantitative PCR and lipid mass spectrometry were used to validate LPCAT3 deletion.

Results

7 days of HU was sufficient to induce muscle atrophy and weakness concomitant to an increase in 4-HNE. Deletion of LPCAT3 reversed HU-induced increase in muscle 4HNE. No difference was found in body mass, body composition, or caloric intake between genotypes. The soleus (SOL) and plantaris (PLANT) muscles of the LPCAT3-MKO mice were partially protected from atrophy compared to controls, concomitant to attenuated decrease in cross-sectional areas in type I and IIa fibers. Strikingly, SOL and extensor digitorum longus (EDL) were robustly protected from HU-induced reduction in force-generating capacity in the LPCAT3-MKO mice compared to controls.

Conclusion

Our findings demonstrate that attenuation of muscle LOOH is sufficient to restore skeletal muscle function, in particular a protection from reduction in muscle specific force. Thus, muscle LOOH contributes to atrophy and weakness induced by HU in mice.

Defense systems to avoid ferroptosis caused by lipid peroxidation-mediated membrane damage

The presence of hydrogen peroxide along with ferrous iron produces hydroxyl radicals that preferably oxidize polyunsaturated fatty acids (PUFA) to alkyl radicals (L•). The reaction of L• with an oxygen molecule produces lipid peroxyl radical (LOO•) that collectively trigger chain reactions, which results in the accumulation of lipid peroxidation products (LOOH). Oxygenase enzymes, such as lipoxygenase, also stimulate the peroxidation of PUFA. The production of phospholipid hydroperoxides (P-LOOH) can result in the destruction of the architecture of cell membranes and ultimate cell death. This iron-dependent regulated cell death is generally referred to as ferroptosis. Radical scavengers, which include tocopherol and nitric oxide (•NO), react with lipid radicals and terminate the chain reaction. When tocopherol reductively detoxifies lipid radicals, the resultant tocopherol radicals are recycled via reduction by coenzyme Q or ascorbate. CoQ radicals are reduced back by the anti-ferroptotic enzyme FSP1. •NO reacts with lipid radicals and produces less reactive nitroso compounds. The resulting P-LOOH is reductively detoxified by the action of glutathione peroxidase 4 (GPX4) or peroxiredoxin 6 (PRDX6). The hydrolytic removal of LOOH from P-LOOH by calcium-independent phospholipase A2 leads the preservation of membrane structure. While the expression of such protective genes or the presence of these anti-oxidant compounds serve to maintain a healthy condition, tumor cells employ them to make themselves resistant to anti-tumor treatments. Thus, these defense mechanisms against ferroptosis are protective in ordinary cells but are also potential targets for cancer treatment.

The role of mitochondrial dynamics in cerebral ischemia-reperfusion injury

Stroke is one of the leading causes of death and long-term disability worldwide. More than 80 % of strokes are ischemic, caused by an occlusion of cerebral arteries. Without question, restoration of blood supply as soon as possible is the first therapeutic strategy. Nonetheless paradoxically, reperfusion can further aggravate the injury through a series of reactions known as cerebral ischemia-reperfusion injury (CIRI). Mitochondria play a vital role in promoting nerve survival and neurological function recovery and mitochondrial dysfunction is considered one of the characteristics of CIRI. Neurons often die due to oxidative stress and an imbalance in energy metabolism following CIRI, and there is a strong association with mitochondrial dysfunction. Altered mitochondrial dynamics is the first reaction of mitochondrial stress. Mitochondrial dynamics refers to the maintenance of the integrity, distribution, and size of mitochondria as well as their ability to resist external stimuli through a continuous cycle of mitochondrial fission and fusion. Therefore, improving mitochondrial dynamics is a vital means of treating CIRI. This review discusses the relationship between mitochondria and CIRI and emphasizes improving mitochondrial dynamics as a potential therapeutic approach to improve the prognosis of CIRI.

OxPAPC stabilizes liquid-ordered domains in biomimetic membranes

Long-chain polyunsaturated fatty acids (PUFAs) are prone to nonenzymatic oxidation in response to differing environmental stressors and endogenous cellular sources. There is increasing evidence that phospholipids containing oxidized PUFA acyl chains control the inflammatory response. However, the underlying mechanism(s) of action by which oxidized PUFAs exert their functional effects remain unclear. Herein, we tested the hypothesis that replacement of 1-palmitoyl-2-arachidonyl-phosphatidylcholine (PAPC) with oxidized 1-palmitoyl-2-arachidonyl-phosphatidylcholine (oxPAPC) regulates membrane architecture. Specifically, with solid-state 2H NMR of biomimetic membranes, we investigated how substituting oxPAPC for PAPC modulates the molecular organization of liquid-ordered (Lo) domains. 2H NMR spectra for bilayer mixtures of 1,2-dipalmitoylphosphatidylcholine-d62 (an analog of DPPC deuterated throughout sn-1 and -2 chains) and cholesterol to which PAPC or oxPAPC was added revealed that replacing PAPC with oxPAPC disrupted molecular organization, indicating that oxPAPC does not mix favorably in a tightly packed Lo phase. Furthermore, unlike PAPC, adding oxPAPC stabilized 1,2-dipalmitoylphosphatidylcholine-d6-rich/cholesterol-rich Lo domains formed in mixtures with 1,2-dioleoylphosphatidylcholine while decreasing the molecular order within 1,2-dioleoylphosphatidylcholine-rich liquid-disordered regions of the membrane. Collectively, these results suggest a mechanism in which oxPAPC stabilizes Lo domains-by disordering the surrounding liquid-disordered region. Changes in the structure, and thereby functionality, of Lo domains may underly regulation of plasma membrane-based inflammatory signaling by oxPAPC.

Sepsis, pyruvate, and mitochondria energy supply chain shortage

Balancing high energy-consuming danger resistance and low energy supply of disease tolerance is a universal survival principle that often fails during sepsis. Our research supports the concept that sepsis phosphorylates and deactivates mitochondrial pyruvate dehydrogenase complex control over the tricarboxylic cycle and the electron transport chain. StimulatIng mitochondrial energetics in septic mice and human sepsis cell models can be achieved by inhibiting pyruvate dehydrogenase kinases with the pyruvate structural analog dichloroacetate. Stimulating the pyruvate dehydrogenase complex by dichloroacetate reverses a disruption in the tricarboxylic cycle that induces itaconate, a key mediator of the disease tolerance pathway. Dichloroacetate treatment increases mitochondrial respiration and ATP synthesis, decreases oxidant stress, overcomes metabolic paralysis, regenerates tissue, organ, and innate and adaptive immune cells, and doubles the survival rate in a murine model of sepsis.

Cardiolipin Alterations during Obesity: Exploring Therapeutic Opportunities

Cardiolipin is a specific phospholipid of the mitochondrial inner membrane that participates in many aspects of its organization and function, hence promoting proper mitochondrial ATP production. Here, we review recent data that have investigated alterations of cardiolipin in different tissues in the context of obesity and the related metabolic syndrome. Data relating perturbations of cardiolipin content or composition are accumulating and suggest their involvement in mitochondrial dysfunction in tissues from obese patients. Conversely, cardiolipin modulation is a promising field of investigation in a search for strategies for obesity management. Several ways to restore cardiolipin content, composition or integrity are emerging and may contribute to the improvement of mitochondrial function in tissues facing excessive fat storage. Inversely, reduction of mitochondrial efficiency in a controlled way may increase energy expenditure and help fight against obesity and in this perspective, several options aim at targeting cardiolipin to achieve a mild reduction of mitochondrial coupling. Far from being just a victim of the deleterious consequences of obesity, cardiolipin may ultimately prove to be a possible weapon to fight against obesity in the future.

Redox Pioneer: Professor Valerian Kagan

Professor Valerian Kagan (PhD, 1972, MV Lomonosov Moscow State University; DSci, 1981, USSR, Academy of Sciences, Moscow) is recognized as a Redox Pioneer because he has published 4 articles in the field of redox biology that have been cited >1000 times and 138 articles in this field have been cited between 100 and 924 times. The central and most important impact of Dr. Kagan's research is in the field of redox lipidomics-a term coined for the first time by Dr. Kagan in 2004-and consequently the definition of signaling pathways by oxidatively modified phospholipids; this acquires further significance considering that oxygenated phospholipids play multifunctional roles as essential signals coordinating metabolism and physiology. Some examples are the selective oxidation of cardiolipin (CL) by a cytochrome c peroxidase activity leading to the activation of the intrinsic apoptotic pathway; the hydroperoxy-arachidonoyl/adrenoyl phosphatidylethanolamine (PE) species, driven by 15-lipoxygenases (15-LOX), as death signals leading to ferroptotic cell death; the regulation of ferroptosis by iNOS/NO^• in pro-inflammatory conditions by a novel mechanism (realized via interactions of 15-LOX reaction intermediates formed from arachidonoyl phosphatidylethanolamine [PE] species) and Ca²⁺-independent phospholipase A2 (iPLA₂β; via elimination of peroxidized PE); the involvement of oxygenated (phospho)lipids in immunosuppression by myeloid cells in the tumor microenvironment; hydrolysis of peroxidized CL by Ca²⁺-independent phospholipase A2 (iPLA₂γ) leading to pro- and anti-inflammatory signals and lipid mediators. Kagan continues his investigations to decipher the roles of enzyme-linked oxygenated phospholipids. Antioxid. Redox Signal. 36, 813-823.

Antioxidant Synergy of Mitochondrial Phospholipase PNPLA8/iPLA2γ with Fatty Acid-Conducting SLC25 Gene Family Transporters

Patatin-like phospholipase domain-containing protein PNPLA8, also termed Ca²⁺-independent phospholipase A2γ (iPLA2γ), is addressed to the mitochondrial matrix (or peroxisomes), where it may manifest its unique activity to cleave phospholipid side-chains from both sn-1 and sn-2 positions, consequently releasing either saturated or unsaturated fatty acids (FAs), including oxidized FAs. Moreover, iPLA2γ is directly stimulated by H₂O₂ and, hence, is activated by redox signaling or oxidative stress. This redox activation permits the antioxidant synergy with mitochondrial uncoupling proteins (UCPs) or other SLC25 mitochondrial carrier family members by FA-mediated protonophoretic activity, termed mild uncoupling, that leads to diminishing of mitochondrial superoxide formation. This mechanism allows for the maintenance of the steady-state redox status of the cell. Besides the antioxidant role, we review the relations of iPLA2γ to lipid peroxidation since iPLA2γ is alternatively activated by cardiolipin hydroperoxides and hypothetically by structural alterations of lipid bilayer due to lipid peroxidation. Other iPLA2γ roles include the remodeling of mitochondrial (or peroxisomal) membranes and the generation of specific lipid second messengers. Thus, for example, during FA β-oxidation in pancreatic β-cells, H₂O₂-activated iPLA2γ supplies the GPR40 metabotropic FA receptor to amplify FA-stimulated insulin secretion. Cytoprotective roles of iPLA2γ in the heart and brain are also discussed.

Ferroptosis and cardiovascular disease: role of free radical-induced lipid peroxidation

Cardiovascular disease (CVD), including heart attack, stroke, heart failure, arrhythmia, and other congenital heart diseases remain the leading cause of morbidity and mortality worldwide. The leading cause of deaths in CVD is attributed to myocardial infarction due to the rupture of atherosclerotic plaque. Atherosclerosis refers a condition when restricted or even blockage of blood flow occurs due to the narrowing of blood vessels as a result of the buildup of plaques composed of oxidized lipids. It is well-established that free radical oxidation of polyunsaturated fatty acids (PUFAs) in lipoproteins or cell membranes, termed lipid peroxidation (LPO), plays a significant role in atherosclerosis. LPO products are involved in immune responses and cell deaths in this process, in which previous evidence supports the role of programmed cell death (apoptosis) and necrosis. Ferroptosis is a newly identified form of regulated cell death characterized by the iron-dependent accumulation of lipid hydroperoxides to lethal levels, which exhibits distinct features from apoptosis, necrosis and autophagy in morphology, biochemistry and genetics. Emerging evidence appears to demonstrate that ferroptosis is also involved in CVD. In this review, we summarize the recent progress on ferroptosis in CVD and atherosclerosis, highlighting the role of free radical LPO. The evidence underlying the ferroptosis and challenges in the field will also be critically discussed.

Crosstalk Between Autophagy and Ferroptosis and Its Putative Role in Ischemic Stroke

Autophagy is a conserved process to maintains homeostasis via the degradation of toxic cell contents, which can either promote cell survival or accelerate cellular demise. Ferroptosis is a recently discovered iron-dependent cell death pathway associated with the accumulation of lethal reactive lipid species. In the past few years, an increasing number of studies have suggested the crosstalk between autophagy and ferroptosis. Ischemic stroke is a complex brain disease regulated by several cell death pathways, including autophagy and ferroptosis. However, the potential links between autophagy and ferroptosis in ischemic stroke have not yet been explored. In this review, we briefly overview the mechanisms of ferroptosis and autophagy, as well as their possible connections in ischemic stroke. The elucidation of crosstalk between different cell death pathways may provide insight into new future ischemic stroke therapies.

Metabolic Dysfunction in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome Not Due to Anti-mitochondrial Antibodies

Metabolic profiling studies have recently indicated dysfunctional mitochondria in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). This includes an impaired function of pyruvate dehydrogenase complex (PDC), possibly driven by serum factor(s), which leads to inadequate adenosine triphosphate generation and excessive lactate accumulation. A reminiscent energy blockade is likely to occur in primary biliary cholangitis (PBC), caused by anti-PDC autoantibodies, as recently proposed. PBC is associated with fatigue and post-exertional malaise, also signifying ME/CFS. We herein have investigated whether ME/CFS patients have autoreactive antibodies that could interfere with mitochondrial function. We found that only 1 of 161 examined ME/CFS patients was positive for anti-PDC, while all PBC patients (15/15) presented significant IgM, IgG, and IgA anti-PDC reactivity, as previously shown. None of fibromyalgia patients (0/14), multiple sclerosis patients (0/29), and healthy blood donors (0/44) controls showed reactivities. Anti-mitochondrial autoantibodies (inner and outer membrane) were negative in ME/CFS cohort. Anti-cardiolipin antibody levels in patients did not differ significantly from healthy blood donors. In conclusion, the impaired mitochondrial/metabolic dysfunction, observed in ME/CFS, cannot be explained by presence of circulating autoantibodies against the tested mitochondrial epitopes.

Reactive oxygen species (ROS) as pleiotropic physiological signalling agents

'Reactive oxygen species' (ROS) is an umbrella term for an array of derivatives of molecular oxygen that occur as a normal attribute of aerobic life. Elevated formation of the different ROS leads to molecular damage, denoted as 'oxidative distress'. Here we focus on ROS at physiological levels and their central role in redox signalling via different post-translational modifications, denoted as 'oxidative eustress'. Two species, hydrogen peroxide (H₂O₂) and the superoxide anion radical (O₂^·-), are key redox signalling agents generated under the control of growth factors and cytokines by more than 40 enzymes, prominently including NADPH oxidases and the mitochondrial electron transport chain. At the low physiological levels in the nanomolar range, H₂O₂ is the major agent signalling through specific protein targets, which engage in metabolic regulation and stress responses to support cellular adaptation to a changing environment and stress. In addition, several other reactive species are involved in redox signalling, for instance nitric oxide, hydrogen sulfide and oxidized lipids. Recent methodological advances permit the assessment of molecular interactions of specific ROS molecules with specific targets in redox signalling pathways. Accordingly, major advances have occurred in understanding the role of these oxidants in physiology and disease, including the nervous, cardiovascular and immune systems, skeletal muscle and metabolic regulation as well as ageing and cancer. In the past, unspecific elimination of ROS by use of low molecular mass antioxidant compounds was not successful in counteracting disease initiation and progression in clinical trials. However, controlling specific ROS-mediated signalling pathways by selective targeting offers a perspective for a future of more refined redox medicine. This includes enzymatic defence systems such as those controlled by the stress-response transcription factors NRF2 and nuclear factor-κB, the role of trace elements such as selenium, the use of redox drugs and the modulation of environmental factors collectively known as the exposome (for example, nutrition, lifestyle and irradiation).

Lipidomics and RNA sequencing reveal a novel subpopulation of nanovesicle within extracellular matrix biomaterials

Biomaterials composed of extracellular matrix (ECM) provide both mechanical support and a reservoir of constructive signaling molecules that promote functional tissue repair. Recently, matrix-bound nanovesicles (MBVs) have been reported as an integral component of ECM bioscaffolds. Although liquid-phase extracellular vesicles (EVs) have been the subject of intense investigation, their similarity to MBV is limited to size and shape. Liquid chromatography-mass spectrometry (LC-MS)-based lipidomics and redox lipidomics were used to conduct a detailed comparison of liquid-phase EV and MBV phospholipids. Combined with comprehensive RNA sequencing and bioinformatic analysis of the intravesicular cargo, we show that MBVs are a distinct and unique subpopulation of EV and a distinguishing feature of ECM-based biomaterials. The results begin to identify the differential biologic activities mediated by EV that are secreted by tissue-resident cells and deposited within the ECM.

Redox Epiphospholipidome in Programmed Cell Death Signaling: Catalytic Mechanisms and Regulation

A huge diversification of phospholipids, forming the aqueous interfaces of all biomembranes, cannot be accommodated within a simple concept of their role as membrane building blocks. Indeed, a number of signaling functions of (phospho)lipid molecules has been discovered. Among these signaling lipids, a particular group of oxygenated polyunsaturated fatty acids (PUFA), so called lipid mediators, has been thoroughly investigated over several decades. This group includes oxygenated octadecanoids, eicosanoids, and docosanoids and includes several hundreds of individual species. Oxygenation of PUFA can occur when they are esterified into major classes of phospholipids. Initially, these events have been associated with non-specific oxidative injury of biomembranes. An alternative concept is that these post-synthetically oxidatively modified phospholipids and their adducts with proteins are a part of a redox epiphospholipidome that represents a rich and versatile language for intra- and inter-cellular communications. The redox epiphospholipidome may include hundreds of thousands of individual molecular species acting as meaningful biological signals. This review describes the signaling role of oxygenated phospholipids in programs of regulated cell death. Although phospholipid peroxidation has been associated with almost all known cell death programs, we chose to discuss enzymatic pathways activated during apoptosis and ferroptosis and leading to peroxidation of two phospholipid classes, cardiolipins (CLs) and phosphatidylethanolamines (PEs). This is based on the available LC-MS identification and quantitative information on the respective peroxidation products of CLs and PEs. We focused on molecular mechanisms through which two proteins, a mitochondrial hemoprotein cytochrome c (cyt c), and non-heme Fe lipoxygenase (LOX), change their catalytic properties to fulfill new functions of generating oxygenated CL and PE species. Given the high selectivity and specificity of CL and PE peroxidation we argue that enzymatic reactions catalyzed by cyt c/CL complexes and 15-lipoxygenase/phosphatidylethanolamine binding protein 1 (15LOX/PEBP1) complexes dominate, at least during the initiation stage of peroxidation, in apoptosis and ferroptosis. We contrast cell-autonomous nature of CLox signaling in apoptosis correlating with its anti-inflammatory functions vs. non-cell-autonomous ferroptotic signaling facilitating pro-inflammatory (necro-inflammatory) responses. Finally, we propose that small molecule mechanism-based regulators of enzymatic phospholipid peroxidation may lead to highly specific anti-apoptotic and anti-ferroptotic therapeutic modalities.

Mitochondria regulation in ferroptosis

Ferroptosis is recognized as a new form of regulated cell death which is initiated by severe lipid peroxidation relying on reactive oxygen species (ROS) generation and iron overload. This iron-dependent cell death manifests evident morphological, biochemical and genetic differences from other forms of regulated cell death, such as apoptosis, autophagy, necrosis and pyroptosis. Ferroptosis was primarily characterized by condensed mitochondrial membrane densities and smaller volume than normal mitochondria, as well as the diminished or vanished of mitochondria crista and outer membrane ruptured. Mitochondria take the center role in iron metabolism, as well as substance and energy metabolism as it's the major organelle in iron utilization, catabolic and anabolic pathways. Interference of key regulators of mitochondrial lipid metabolism (e.g., ASCF2 and CS), iron homeostasis (e.g., ferritin, mitoferrin1/2 and NEET proteins), glutamine metabolism and other signaling pathways make a difference to ferroptotic sensitivity. Targeted induction of ferroptosis was also considered as a potential therapeutic strategy to some oxidative stress diseases, including neurodegenerative disorders, ischemia-reperfusion injury, traumatic spinal cord injury. However, the pertinence between mitochondria and ferroptosis is still in dispute. Here we systematic elucidate the morphological characteristics and metabolic regulation of mitochondria in the regulation of ferroptosis.

Regulation of Respiration and Apoptosis by Cytochrome c Threonine 58 Phosphorylation

Cytochrome c (Cytc) is a multifunctional protein, acting as an electron carrier in the electron transport chain (ETC), where it shuttles electrons from bc₁ complex to cytochrome c oxidase (COX), and as a trigger of type II apoptosis when released from the mitochondria. We previously showed that Cytc is regulated in a highly tissue-specific manner: Cytc isolated from heart, liver, and kidney is phosphorylated on Y97, Y48, and T28, respectively. Here, we have analyzed the effect of a new Cytc phosphorylation site, threonine 58, which we mapped in rat kidney Cytc by mass spectrometry. We generated and overexpressed wild-type, phosphomimetic T58E, and two controls, T58A and T58I Cytc; the latter replacement is found in human and testis-specific Cytc. In vitro, COX activity, caspase-3 activity, and heme degradation in the presence of H₂O₂ were decreased with phosphomimetic Cytc compared to wild-type. Cytc-knockout cells expressing T58E or T58I Cytc showed a reduction in intact cell respiration, mitochondrial membrane potential (∆Ψ_m), ROS production, and apoptotic activity compared to wild-type. We propose that, under physiological conditions, Cytc is phosphorylated, which controls mitochondrial respiration and apoptosis. Under conditions of stress Cytc phosphorylations are lost leading to maximal respiration rates, ∆Ψ_m hyperpolarization, ROS production, and apoptosis.

Contribution of Oxidative Stress and Impaired Biogenesis of Pancreatic β-Cells to Type 2 Diabetes

Significance: Type 2 diabetes development involves multiple changes in β-cells, related to the oxidative stress and impaired redox signaling, beginning frequently by sustained overfeeding due to the resulting lipotoxicity and glucotoxicity. Uncovering relationships among the dysregulated metabolism, impaired β-cell "well-being," biogenesis, or cross talk with peripheral insulin resistance is required for elucidation of type 2 diabetes etiology. Recent Advances: It has been recognized that the oxidative stress, lipotoxicity, and glucotoxicity cannot be separated from numerous other cell pathology events, such as the attempted compensation of β-cell for the increased insulin demand and dynamics of β-cell biogenesis and its "reversal" at dedifferentiation, that is, from the concomitantly decreasing islet β-cell mass (also due to transdifferentiation) and low-grade islet or systemic inflammation. Critical Issues: At prediabetes, the compensation responses of β-cells, attempting to delay the pathology progression-when exaggerated-set a new state, in which a self-checking redox signaling related to the expression of Ins gene expression is impaired. The resulting altered redox signaling, diminished insulin secretion responses to various secretagogues including glucose, may lead to excretion of cytokines or chemokines by β-cells or excretion of endosomes. They could substantiate putative stress signals to the periphery. Subsequent changes and lasting glucolipotoxicity promote islet inflammatory responses and further pathology spiral. Future Directions: Should bring an understanding of the β-cell self-checking and related redox signaling, including the putative stress signal to periphery. Strategies to cure or prevent type 2 diabetes could be based on the substitution of the "wrong" signal by the "correct" self-checking signal.

Redox (phospho)lipidomics of signaling in inflammation and programmed cell death

In addition to the known prominent role of polyunsaturated (phospho)lipids as structural blocks of biomembranes, there is an emerging understanding of another important function of these molecules as a highly diversified signaling language utilized for intra- and extracellular communications. Technological developments in high-resolution mass spectrometry facilitated the development of a new branch of metabolomics, redox lipidomics. Analysis of lipid peroxidation reactions has already identified specific enzymatic mechanisms responsible for the biosynthesis of several unique signals in response to inflammation and regulated cell death programs. Obtaining comprehensive information about millions of signals encoded by oxidized phospholipids, represented by thousands of interactive reactions and pleiotropic (patho)physiological effects, is a daunting task. However, there is still reasonable hope that significant discoveries, of at least some of the important contributors to the overall overwhelmingly complex network of interactions triggered by inflammation, will lead to the discovery of new small molecule regulators and therapeutic modalities. For example, suppression of the production of AA-derived pro-inflammatory mediators, HXA3 and LTB4, by an iPLA2 γ inhibitor, R-BEL, mitigated injury associated with the activation of pro-inflammatory processes in animals exposed to whole-body irradiation. Further, technological developments promise to make redox lipidomics a powerful approach in the arsenal of diagnostic and therapeutic instruments for personalized medicine of inflammatory diseases and conditions.

"Redox lipidomics technology: Looking for a needle in a haystack"

Aerobic life is based on numerous metabolic oxidation reactions as well as biosynthesis of oxygenated signaling compounds. Among the latter are the myriads of oxygenated lipids including a well-studied group of polyunsaturated fatty acids (PUFA) - octadecanoids, eicosanoids, and docosanoids. During the last two decades, remarkable progress in liquid-chromatography-mass spectrometry has led to significant progress in the characterization of oxygenated PUFA-containing phospholipids, thus designating the emergence of a new field of lipidomics, redox lipidomics. Although non-enzymatic free radical reactions of lipid peroxidation have been mostly associated with the aberrant metabolism typical of acute injury or chronic degenerative processes, newly accumulated evidence suggests that enzymatically catalyzed (phospho)lipid oxygenation reactions are essential mechanisms of many physiological pathways. In this review, we discuss a variety of contemporary protocols applicable for identification and quantitative characterization of different classes of peroxidized (phospho)lipids. We describe applications of different types of LCMS for analysis of peroxidized (phospho)lipids, particularly cardiolipins and phosphatidylethanolalmines, in two important types of programmed cell death - apoptosis and ferroptosis. We discuss the role of peroxidized phosphatidylserines in phagocytotic signaling. We exemplify the participation of peroxidized neutral lipids, particularly tri-acylglycerides, in immuno-suppressive signaling in cancer. We also consider new approaches to exploring the spatial distribution of phospholipids in the context of their oxidizability by MS imaging, including the latest achievements in high resolution imaging techniques. We present innovative approaches to the interpretation of LC-MS data, including audio-representation analysis. Overall, we emphasize the role of redox lipidomics as a communication language, unprecedented in diversity and richness, through the analysis of peroxidized (phospho)lipids.

Secondary-Ion Mass Spectrometry Images Cardiolipins and Phosphatidylethanolamines at the Subcellular Level

Millions of diverse molecules constituting the lipidome act as important signals within cells. Of these, cardiolipin (CL) and phosphatidylethanolamine (PE) participate in apoptosis and ferroptosis, respectively. Their subcellular distribution is largely unknown. Imaging mass spectrometry is capable of deciphering the spatial distribution of multiple lipids at subcellular levels. Here we report the development of a unique 70 keV gas-cluster ion beam that consists of (CO₂ )_n ⁺ (n>10 000) projectiles. Coupled with direct current beam buncher-time-of-flight secondary-ion mass spectrometry, it is optimized for sensitivity towards high-mass species (up to m/z 3000) at high spatial resolution (1 μm). In combination with immunohistochemistry, phospholipids, including PE and CL, have been assessed in subcellular compartments of mouse hippocampal neuronal cells and rat brain tissue.

BFL1 modulates apoptosis at the membrane level through a bifunctional and multimodal mechanism showing key differences with BCLXL

BFL1 is a relatively understudied member of the BCL2 protein family which has been implicated in the pathogenesis and chemoresistance of a variety of human cancers, including hematological malignancies and solid tumours. BFL1 is generally considered to have an antiapoptotic function, although its precise mode of action remains unclear. By quantitatively analyzing BFL1 action in synthetic membrane models and in cells, we found that BFL1 inhibits apoptosis through three distinct mechanisms which are similar but not identical to those of BCLXL, the paradigmatic antiapoptotic BCL2 family protein. Strikingly, alterations in lipid composition during apoptosis activate a prodeath function of BFL1 that is based on noncanonical oligomerization of the protein and breaching of the permeability barrier of the outer mitochondrial membrane (OMM). This lipid-triggered prodeath function of BFL1 is absent in BCLXL and also differs from that of the apoptotic effector BAX, which sets it apart from other BCL2 family members. Our findings support a new model in which BFL1 modulates apoptosis through a bifunctional and multimodal mode of action that is distinctly regulated by OMM lipids compared to BCLXL.

Hydrogen Peroxide and Redox Regulation of Developments

Reactive oxygen species (ROS), which were originally classified as exclusively deleterious compounds, have gained increasing interest in the recent years given their action as bona fide signalling molecules. The main target of ROS action is the reversible oxidation of cysteines, leading to the formation of disulfide bonds, which modulate protein conformation and activity. ROS, endowed with signalling properties, are mainly produced by NADPH oxidases (NOXs) at the plasma membrane, but their action also involves a complex machinery of multiple redox-sensitive protein families that differ in their subcellular localization and their activity. Given that the levels and distribution of ROS are highly dynamic, in part due to their limited stability, the development of various fluorescent ROS sensors, some of which are quantitative (ratiometric), represents a clear breakthrough in the field and have been adapted to both ex vivo and in vivo applications. The physiological implication of ROS signalling will be presented mainly in the frame of morphogenetic processes, embryogenesis, regeneration, and stem cell differentiation. Gain and loss of function, as well as pharmacological strategies, have demonstrated the wide but specific requirement of ROS signalling at multiple stages of these processes and its intricate relationship with other well-known signalling pathways.

Redox Dichotomy in Cell Fate Decision: Evasive Mechanism or Achilles Heel?

Cellular redox state is a consequence of the balance between the rates of reactive oxygen species and/or reactive nitrogen species, and their dissipation via enzymatic and nonenzymatic redox buffering systems. While low levels of oscillation are associated with normal cellular metabolism, stimuli that favor a significant shift in the redox microenvironment, through either the increased production and/or compromise of the antioxidant defenses, induce overt oxidative stress. This change in the redox set point triggers a host of cellular responses ranging from modifications in cellular macromolecules, organelle morphology and physiology, amplified cell-to-cell and intracellular signaling, and changes in genome, epigenome, and proteome. The consequence of this dysregulated cellular homeostasis is therefore manifested in the form of a plethora of pathological states such as inflammation, diabetes mellitus, neurodegenerative disorders, atherosclerosis, and cancer. On the backdrop of these observations, this Forum attempts at reviewing the current understanding of how a prooxidant intracellular milieu favors cell survival while overt oxidative stress results in death execution, and the translation of these biological effects in human disease states, in particular cancer. The far-reaching biochemical, biological, and clinical ramifications of an altered redox environment are also discussed from the standpoint of strategic therapeutic design against refractory and aggressive cancers. It is tempting to conjecture if the inherent or acquired redox heterogeneity, at least in the case of cancer, has evolved as an "evasive mechanism," or presents itself as the "Achilles heel" for therapeutic exploitation.

Regulation of lipid peroxidation and ferroptosis in diverse species

This review by Conrad et al. reviews the functions and regulation of lipid peroxidation, ferroptosis, and the antioxidant network in diverse species, including humans, other mammals and vertebrates, plants, invertebrates, yeast, bacteria, and archaea, and discusses the potential evolutionary roles of lipid peroxidation and ferroptosis.

Lipid peroxidation is the process by which oxygen combines with lipids to generate lipid hydroperoxides via intermediate formation of peroxyl radicals. Vitamin E and coenzyme Q₁₀ react with peroxyl radicals to yield peroxides, and then these oxidized lipid species can be detoxified by glutathione and glutathione peroxidase 4 (GPX4) and other components of the cellular antioxidant defense network. Ferroptosis is a form of regulated nonapoptotic cell death involving overwhelming iron-dependent lipid peroxidation. Here, we review the functions and regulation of lipid peroxidation, ferroptosis, and the antioxidant network in diverse species, including humans, other mammals and vertebrates, plants, invertebrates, yeast, bacteria, and archaea. We also discuss the potential evolutionary roles of lipid peroxidation and ferroptosis.

Fatty Acid-Stimulated Insulin Secretion vs. Lipotoxicity

Fatty acid (FA)-stimulated insulin secretion (FASIS) is reviewed here in contrast to type 2 diabetes etiology, resulting from FA overload, oxidative stress, intermediate hyperinsulinemia, and inflammation, all converging into insulin resistance. Focusing on pancreatic islet β-cells, we compare the physiological FA roles with the pathological ones. Considering FAs not as mere amplifiers of glucose-stimulated insulin secretion (GSIS), but as parallel insulin granule exocytosis inductors, partly independent of the K_ATP channel closure, we describe the FA initiating roles in the prediabetic state that is induced by retardations in the glycerol-3-phosphate (glucose)-promoted glycerol/FA cycle and by the impaired GPR40/FFA1 (free FA1) receptor pathway, specifically in its amplification by the redox-activated mitochondrial phospholipase, iPLA2γ. Also, excessive dietary FAs stimulate intestine enterocyte incretin secretion, further elevating GSIS, even at low glucose levels, thus contributing to diabetic hyperinsulinemia. With overnutrition and obesity, the FA overload causes impaired GSIS by metabolic dysbalance, paralleled by oxidative and metabolic stress, endoplasmic reticulum stress and numerous pro-apoptotic signaling, all leading to decreased β-cell survival. Lipotoxicity is exerted by saturated FAs, whereas ω-3 polyunsaturated FAs frequently exert antilipotoxic effects. FA-facilitated inflammation upon the recruitment of excess M1 macrophages into islets (over resolving M2 type), amplified by cytokine and chemokine secretion by β-cells, leads to an inevitable failure of pancreatic β-cells.

Overexpression of Pyruvate Dehydrogenase E1α Subunit Inhibits Warburg Effect and Induces Cell Apoptosis Through Mitochondria-Mediated Pathway in Hepatocellular Carcinoma

Most cancers rely disproportionately on glycolysis for energy even in the presence of an adequate oxygen supply, a condition known as “aerobic glycolysis,” or the “Warburg effect.” Pyruvate dehydrogenase E1α subunit (PDHA1) is one of the main factors for the metabolic switch from oxidative phosphorylation (OXPHOS) to aerobic glycolysis and has been suggested to be closely associated with tumorigenesis. Here we observed that the PDHA1 protein was reduced in hepatocellular carcinoma (HCC) specimens by immunohistochemistry and Western blot, which was significantly associated with poor overall survival. To further analyze the function of PDHA1 in cancer cells, PDHA1 was upregulated in the HCC cell lines SMMC-7721 and HepG2. The results demonstrated that overexpression of the PDHA1 gene inhibited aerobic glycolysis with lower lactate via increased PDH activity; meanwhile, mitochondrial OXPHOS was enhanced accompanied with higher ATP and lower glucose consumption. We also found that apoptosis was promoted and intrinsic pathway proteins were increased in PDHA1-overexpressing cells. Collectively, our data indicate that reduced PDHA1 protein expression is associated with the poor clinical outcome of HCC. Upregulated PDHA1 gene expression can inhibit the Warburg effect and enhance the mitochondria-mediated apoptosis pathway.