Membrane Disruption by Very Long Chain Fatty Acids during Necroptosis

Divergent roles of RIPK3 and MLKL in high-fat diet-induced obesity and MAFLD in mice

Cell death frequently occurs in the pathogenesis of obesity and metabolic dysfunction-associated fatty liver disease (MAFLD). However, the exact contribution of core cell death machinery to disease manifestations remains ill-defined. Here, we show via the direct comparison of mice genetically deficient in the essential necroptotic regulators, receptor-interacting protein kinase-3 (RIPK3) and mixed lineage kinase domain-like (MLKL), as well as mice lacking apoptotic caspase-8 in myeloid cells combined with RIPK3 loss, that RIPK3/caspase-8 signaling regulates macrophage inflammatory responses and drives adipose tissue inflammation and MAFLD upon high-fat diet feeding. In contrast, MLKL, divergent to RIPK3, contributes to both obesity and MAFLD in a manner largely independent of inflammation. We also uncover that MLKL regulates the expression of molecules involved in lipid uptake, transport, and metabolism, and congruent with this, we discover a shift in the hepatic lipidome upon MLKL deletion. Collectively, these findings highlight MLKL as an attractive therapeutic target to combat the growing obesity pandemic and metabolic disease.

Effects of excipients on the interactions of self-emulsifying drug delivery systems with human blood plasma and plasma membranes

Due to its versatility in formulation and manufacturing, self-emulsifying drug delivery systems (SEDDS) can be used to design parenteral formulations. Therefore, it is necessary to understand the effects of excipients on the behavior of SEDDS formulations upon parenteral administration, particularly their interactions with blood plasma and cell membranes. In this study, we prepared three neutrally charged SEDDS formulations composed of medium-chain triglycerides as the oil phase, polyoxyl-35 castor oil (EL35) and polyethylene glycol (15)-hydroxystearate (HS15) as the nonionic surfactants, medium-chain mono- and diglycerides as the co-surfactant, and propylene glycol as the co-solvent. The cationic surfactant, didodecyldimethylammonium bromide (DDA), and the anionic surfactant, sodium deoxycholate (DEO), were added to the neutral SEDDS preconcentrates to obtain cationic and anionic SEDDS, respectively. SEDDS were incubated with human blood plasma and recovered by size exclusion chromatography. Data showed that SEDDS emulsion droplets can bind plasma protein to different extents depending on their surface charge and surfactant used. At pH 7.4, the least protein binding was observed with anionic SEDDS. Positive charges increased protein binding. SEDDS stabilized by HS15 can adsorb more plasma protein and induce more plasma membrane disruption activity than SEDDS stabilized by EL35. These effects were more pronounced with the HS15 + DDA combination. The addition of DDA and DEO to SEDDS increased plasma membrane disruption (PMD) activities, and DDA (1% w/w) was more active than DEO (2% w/w). PMD activities of SEDDS were concentration-dependent and vanished at appropriate dilution ratios.

Intricate interplay between cell metabolism and necroptosis regulation in metabolic dysfunction-associated steatotic liver disease: A narrative review

Metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as non-alcoholic fatty liver disease (NAFLD), encompasses a progressive spectrum of liver conditions, ranging from steatosis to metabolic dysfunction-associated steatohepatitis, characterised by hepatocellular death and inflammation, potentially progressing to cirrhosis and/or liver cancer. In both experimental and human MASLD, necroptosis-a regulated immunogenic necrotic cell death pathway-is triggered, yet its exact role in disease pathogenesis remains unclear. Noteworthy, necroptosis-related signalling pathways are emerging as key players in metabolic reprogramming, including lipid and mitochondrial metabolism. Additionally, metabolic dysregulation is a well-established contributor to MASLD development and progression. This review explores the intricate interplay between cell metabolism and necroptosis regulation and its impact on MASLD pathogenesis. Understanding these cellular events may offer new insights into the complexity of MASLD pathophysiology, potentially uncovering therapeutic opportunities and unforeseen metabolic consequences of targeting necroptosis.

Acylation of MLKL Impacts Its Function in Necroptosis

Mixed lineage kinase domain-like (MLKL) is a key signaling protein of necroptosis. Upon activation by phosphorylation, MLKL translocates to the plasma membrane and induces membrane permeabilization, which contributes to the necroptosis-associated inflammation. Membrane binding of MLKL is initially initiated by electrostatic interactions between the protein and membrane phospholipids. We previously showed that MLKL and its phosphorylated form (pMLKL) are S-acylated during necroptosis. Here, we characterize the acylation sites of MLKL and identify multiple cysteines that can undergo acylation with an interesting promiscuity at play. Our results show that MLKL and pMLKL undergo acylation at a single cysteine, with C184, C269, and C286 as possible acylation sites. Using all-atom molecular dynamic simulations, we identify differences that the acylation of MLKL causes at the protein and membrane levels. Through investigations of the S-palmitoyltransferases that might acylate pMLKL in necroptosis, we showed that zDHHC21 activity has the strongest effect on pMLKL acylation, inactivation of which profoundly reduced the pMLKL levels in cells and improved membrane integrity. These results suggest that blocking the acylation of pMLKL destabilizes the protein at the membrane interface and causes its degradation, ameliorating the necroptotic activity. At a broader level, our findings shed light on the effect of S-acylation on MLKL functioning in necroptosis and MLKL-membrane interactions mediated by its acylation.

Acylation of MLKL impacts its function in necroptosis

Mixed lineage kinase domain-like (MLKL) is a key signaling protein of necroptosis. Upon activation by phosphorylation, MLKL translocates to the plasma membrane and induces membrane permeabilization which contributes to the necroptosis-associated inflammation. Membrane binding of MLKL is initially initiated by the electrostatic interactions between the protein and membrane phospholipids. We previously showed that MLKL and its phosphorylated form (pMLKL) are S-acylated during necroptosis. Here, we characterize acylation sites of MLKL and identify multiple cysteines that can undergo acylation with an interesting promiscuity at play. Our results show that MLKL and pMLKL undergo acylation at a single cysteine, C184, C269 and C286 are the possible acylation sites. Using all atom molecular dynamic simulations, we identify differences that the acylation of MLKL causes at the protein and membrane level. Through systematic investigations of the S-palmitoyltransferases that might acylate MLKL in necroptosis, we showed that zDHHC21 activity has the strongest effect on pMLKL acylation, inactivation of which profoundly reduced the pMLKL levels in cells and improved membrane integrity. These results suggest that blocking the acylation of pMLKL destabilizes the protein at the membrane interface and causes its degradation, ameliorating necroptotic activity. At a broader level, our findings shed light on the effect of S-acylation on MLKL functioning in necroptosis and MLKL-membrane interactions mediated by its acylation.

Modifications in cellular viability, DNA damage and stress responses inflicted in cancer cells by copper-64 ions

Due to combined therapeutical emissions, a high linear energy transfer Auger-electrons with the longer ranged β^- particles, ⁶⁴Cu-based radiopharmaceuticals raise particular theragnostic interest in cancer, by joined therapeutic and real-time PET imaging properties. The in vitro study aimed to investigate the biological and molecular background of ⁶⁴CuCl₂ therapy by analyzing the damages and stress responses inflicted in various human normal and tumor cell lines. Colon (HT29 and HCT116) and prostate carcinoma (DU145) cell lines, as well as human normal BJ fibroblasts, were treated up to 72 h with 2-40 MBq/mL ⁶⁴CuCl₂. Radioisotope uptake and retention were assessed, and cell viability/death, DNA damage, oxidative stress, and the expression of 84 stress genes were investigated at various time points after [⁶⁴Cu]CuCl₂ addition. All the investigated cells incorporated ⁶⁴Cu ions similarly, independent of their tumoral or normal status, but their fate after exposure to [⁶⁴Cu]CuCl₂ was cell-dependent. The most striking cytotoxic effects of the radioisotope were registered in colon carcinoma HCT116 cells, for which a substantial decrease in the number of metabolically active cells, and an increased DNA damage and oxidative stress were registered. The stress gene expression study highlighted the activation of both death and repair mechanisms in these cells, related to extrinsic apoptosis, necrosis/necroptosis or autophagy, and cell cycle arrest, nucleotide excision repair, antioxidant, and hypoxic responses, respectively. The in vitro study indicated that 40 MBq/mL [⁶⁴Cu]CuCl₂ delivers a therapeutic effect in human colon carcinoma, but its use is limited by harmful, yet lower effects on normal fibroblasts. The exposure of tumor cells to 20 MBq/mL [⁶⁴Cu]CuCl₂, might be used for a softer approach aiming for a lower radiotoxicity in normal fibroblasts as compared to tumor cells. This radioactive concentration was able to induce a persistent decrease in the number of metabolically active cells, accompanied by DNA damage and oxidative stress, associated with significant changes in stress gene expression in HCT116 colon cancer cells.

Genome-wide CRISPR screen reveals genetic modifiers of Ca 2+ -mediated cell death

Ca 2+ is a fundamental determinant of survival in living cells. Excessive intracellular Ca 2+ causes cellular toxicity and death but the genetic pathways contributing to Ca 2+ induced cell death are incompletely understood. Here, we performed genome-wide CRISPR knock-out screening in human cells challenged with the Ca 2+ ionophore ionomycin and identified genes and pathways essential for cell death after Ca 2+ overload. We discovered 115 protective gene knockouts, 82 of which are non-essential genes and 21 of which belong to the druggable genome. Notably, members of store operated Ca 2+ entry (SOCE), very long-chain fatty acid synthesis, and SWItch/Sucrose Non-Fermentable (SWI/SNF) pathways provided marked protection against Ca 2+ toxicity. These results reveal pathways previously unknown to mediate Ca 2+ -induced cell death and provide a resource for the development of pharmacotherapies against the sequelae of Ca 2+ overload in disease.

Modeling the molecular fingerprint of protein-lipid interactions of MLKL on complex bilayers

Lipids, the structural part of membranes, play important roles in biological functions. However, our understanding of their implication in key cellular processes such as cell division and protein-lipid interaction is just emerging. This is the case for molecular interactions in mechanisms of cell death, where the role of lipids for protein localization and subsequent membrane permeabilization is key. For example, during the last stage of necroptosis, the mixed lineage kinase domain-like (MLKL) protein translocates and, eventually, permeabilizes the plasma membrane (PM). This process results in the leakage of cellular content, inducing an inflammatory response in the microenvironment that is conducive to oncogenesis and metastasis, among other pathologies that exhibit inflammatory activity. This work presents insights from long all-atom molecular dynamics (MD) simulations of complex membrane models for the PM of mammalian cells with an MLKL protein monomer. Our results show that the binding of the protein is initially driven by the electrostatic interactions of positively charged residues. The protein bound conformation modulates lipid recruitment to the binding site, which changes the local lipid environment recruiting PIP lipids and cholesterol, generating a unique fingerprint. These results increase our knowledge of protein-lipid interactions at the membrane interface in the context of molecular mechanisms of the necroptotic pathway, currently under investigation as a potential treatment target in cancer and inflamatory diseases.

Cyclophosphamide Induces Lipid and Metabolite Perturbation in Amniotic Fluid during Rat Embryonic Development

Cyclophosphamide (CP) has been proven to be an embryo-fetal toxic. However, the mechanism responsible for the toxicity of the teratogenic agent has not been fully explored. This study aimed to examine the teratogenicity of CP when administered in the sensitive period of pregnant rats. The effect of CP on the lipid and metabolic profiles of amniotic fluid was evaluated using a UHPLC-Q-Exactive Orbitrap MS-based method. Metabolome analysis was performed using the MS-DIAL software with LipidBlast and NIST. Initially, we identified 636 and 154 lipid compounds in the positive and negative ion modes and 118 metabolites for differential analysis. Mainly 4 types of oxidized lipids in the amniotic fluid were found to accumulate most significantly after CP treatment, including very-long-chain unsaturated fatty acids (VLCUFAs), polyunsaturated fatty acid (PUFA)-containing triglycerides (TGs), oxidized phosphatidylcholine (PC), and sphingomyelin (SM). Tryptophan and some long-chain saturated fatty acids were lowered pronouncedly after CP treatment. These findings suggest that CP may exert teratogenic toxicity on pregnant rats through maternal and fetal oxidative stress. The UHPLC-Q-Exactive Orbitrap MS-based lipidomics approach is worthy of wider application for evaluating the potential toxicity of other agents (toxicants) during embryonic development.

Discovery of Novel, Orally Bioavailable Pyrimidine Ether-Based Inhibitors of ELOVL1

In our efforts to identify novel small molecule inhibitors for the treatment of adrenoleukodystrophy (ALD), we conducted a high-throughput radiometric screen for inhibitors of elongation of very long chain fatty acid 1 (ELOVL1) enzyme. We developed a series of highly potent, central nervous system (CNS)-penetrant pyrimidine ether-based compounds with favorable pharmacokinetics culminating in compound 22. Compound 22 is a selective inhibitor of ELOVL1, reducing C26:0 VLCFA synthesis in ALD patient fibroblasts and lymphocytes in vitro. Compound 22 reduced C26:0 lysophosphatidyl choline (LPC), a subtype of VLCFA, in the blood of ATP binding cassette transporter D1 (ABCD1) KO mice, a murine model of ALD to near wild-type levels. Compound 22 is a low-molecular-weight, potent ELOVL1 inhibitor that may serve as a useful tool for exploring therapeutic approaches to the treatment of ALD.

Discovery and Optimization of Pyrazole Amides as Inhibitors of ELOVL1

Accumulation of very long chain fatty acids (VLCFAs) due to defects in ATP binding cassette protein D1 (ABCD1) is thought to underlie the pathologies observed in adrenoleukodystrophy (ALD). Pursuing a substrate reduction approach based on the inhibition of elongation of very long chain fatty acid 1 enzyme (ELOVL1), we explored a series of thiazole amides that evolved into compound 27─a highly potent, central nervous system (CNS)-penetrant compound with favorable in vivo pharmacokinetics. Compound 27 selectively inhibits ELOVL1, reducing C26:0 VLCFA synthesis in ALD patient fibroblasts, lymphocytes, and microglia. In mouse models of ALD, compound 27 treatment reduced C26:0 VLCFA concentrations to near-wild-type levels in blood and up to 65% in the brain, a disease-relevant tissue. Preclinical safety findings in the skin, eye, and CNS precluded progression; the origin and relevance of these findings require further study. ELOVL1 inhibition is an effective approach for normalizing VLCFAs in models of ALD.

RIPK3 acts as a lipid metabolism regulator contributing to inflammation and carcinogenesis in non-alcoholic fatty liver disease

OBJECTIVE

Receptor-interacting protein kinase 3 (RIPK3) is a key player in necroptosis execution and an emerging metabolic regulator, whose contribution to non-alcoholic fatty liver disease (NAFLD) is controversial. We aimed to clarify the impact of RIPK3 signalling in the pathogenesis of human and experimental NAFLD.

DESIGN

RIPK3 levels were evaluated in two large independent cohorts of patients with biopsy proven NAFLD diagnosis and correlated with clinical and biochemical parameters. Wild-type (WT) or Ripk3-deficient (Ripk3^-/-) mice were fed a choline-deficient L-amino acid-defined diet (CDAA) or an isocaloric control diet for 32 and 66 weeks.

RESULTS

RIPK3 increased in patients with non-alcoholic steatohepatitis (NASH) in both cohorts, correlating with hepatic inflammation and fibrosis. Accordingly, Ripk3 deficiency ameliorated CDAA-induced inflammation and fibrosis in mice at both 32 and 66 weeks. WT mice on the CDAA diet for 66 weeks developed preneoplastic nodules and displayed increased hepatocellular proliferation, which were reduced in Ripk3^-/- mice. Furthermore, Ripk3 deficiency hampered tumourigenesis. Intriguingly, Ripk3^-/- mice displayed increased body weight gain, while lipidomics showed that deletion of Ripk3 shifted hepatic lipid profiles. Peroxisome proliferator-activated receptor γ (PPARγ) was increased in Ripk3^-/- mice and negatively correlated with hepatic RIPK3 in patients with NAFLD. Mechanistic studies established a functional link between RIPK3 and PPARγ in controlling fat deposition and fibrosis.

CONCLUSION

Hepatic RIPK3 correlates with NAFLD severity in humans and mice, playing a key role in managing liver metabolism, damage, inflammation, fibrosis and carcinogenesis. Targeting RIPK3 and its intricate signalling arises as a novel promising approach to treat NASH and arrest disease progression.

A Not-So-Ancient Grease History: Click Chemistry and Protein Lipid Modifications

Protein lipid modification involves the attachment of hydrophobic groups to proteins via ester, thioester, amide, or thioether linkages. In this review, the specific click chemical reactions that have been employed to study protein lipid modification and their use for specific labeling applications are first described. This is followed by an introduction to the different types of protein lipid modifications that occur in biology. Next, the roles of click chemistry in elucidating specific biological features including the identification of lipid-modified proteins, studies of their regulation, and their role in diseases are presented. A description of the use of protein-lipid modifying enzymes for specific labeling applications including protein immobilization, fluorescent labeling, nanostructure assembly, and the construction of protein-drug conjugates is presented next. Concluding remarks and future directions are presented in the final section.

The structural basis of fatty acid elongation by the ELOVL elongases

Very long chain fatty acids (VLCFAs) are essential building blocks for the synthesis of ceramides and sphingolipids. The first step in the fatty acid elongation cycle is catalyzed by the 3-keto acyl-coenzyme A (CoA) synthases (in mammals, ELOVL elongases). Although ELOVLs are implicated in common diseases, including insulin resistance, hepatic steatosis and Parkinson's, their underlying molecular mechanisms are unknown. Here we report the structure of the human ELOVL7 elongase, which comprises an inverted transmembrane barrel surrounding a 35-Å long tunnel containing a covalently attached product analogue. The structure reveals the substrate-binding sites in the narrow tunnel and an active site deep in the membrane. We demonstrate that chain elongation proceeds via an acyl-enzyme intermediate involving the second histidine in the canonical HxxHH motif. The unusual substrate-binding arrangement and chemistry suggest mechanisms for selective ELOVL inhibition, relevant for diseases where VLCFAs accumulate, such as X-linked adrenoleukodystrophy.

Human Cytomegalovirus Uses a Host Stress Response To Balance the Elongation of Saturated/Monounsaturated and Polyunsaturated Very-Long-Chain Fatty Acids

Stress and virus infection regulate lipid metabolism. Human cytomegalovirus (HCMV) infection induces fatty acid (FA) elongation and increases the abundance of lipids with very-long-chain FA (VLCFA) tails. While reprogramming of metabolism can be stress related, the role of stress in HCMV reprogramming of lipid metabolism is poorly understood. In this study, we engineered cells to knock out protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) in the ER stress pathway and measured lipid changes using lipidomics to determine if PERK is needed for lipid changes associated with HCMV infection. In HCMV-infected cells, PERK promotes increases in the levels of phospholipids with saturated FA (SFA) and monounsaturated FA (MUFA) VLCFA tails. Further, PERK enhances FA elongase 7 (ELOVL7) protein levels, which elongates SFA and MUFA VLCFAs. Additionally, we found that increases in the elongation of polyunsaturated fatty acids (PUFAs) associated with HCMV infection were independent of PERK and that lipids with PUFA tails accumulated in HCMV-infected PERK knockout cells. Additionally, the protein levels of ELOVL5, which elongates PUFAs, are increased by HCMV infection through a PERK-independent mechanism. These observations show that PERK differentially regulates ELOVL7 and ELOVL5, creating a balance between the synthesis of lipids with SFA/MUFA tails and PUFA tails. Additionally, we found that PERK was necessary for virus replication and the infectivity of released viral progeny. Overall, our findings indicate that PERK—and, more broadly, ER stress—may be necessary for the membrane biogenesis needed to generate infectious HCMV virions.

Peroxisomes form intralumenal vesicles with roles in fatty acid catabolism and protein compartmentalization in Arabidopsis

Peroxisomes are vital organelles that compartmentalize critical metabolic reactions, such as the breakdown of fats, in eukaryotic cells. Although peroxisomes typically are considered to consist of a single membrane enclosing a protein lumen, more complex peroxisomal membrane structure has occasionally been observed in yeast, mammals, and plants. However, technical challenges have limited the recognition and understanding of this complexity. Here we exploit the unusually large size of Arabidopsis peroxisomes to demonstrate that peroxisomes have extensive internal membranes. These internal vesicles accumulate over time, use ESCRT (endosomal sorting complexes required for transport) machinery for formation, and appear to derive from the outer peroxisomal membrane. Moreover, these vesicles can harbor distinct proteins and do not form normally when fatty acid β-oxidation, a core function of peroxisomes, is impaired. Our findings suggest a mechanism for lipid mobilization that circumvents challenges in processing insoluble metabolites. This revision of the classical view of peroxisomes as single-membrane organelles has implications for all aspects of peroxisome biogenesis and function and may help address fundamental questions in peroxisome evolution.

Deciphering the modifiers for phenotypic variability of X-linked adrenoleukodystrophy

X-linked adrenoleukodystrophy (X-ALD), an inborn error of peroxisomal β-oxidation, is caused by defects in the ATP Binding Cassette Subfamily D Member 1 (ABCD1) gene. X-ALD patients may be asymptomatic or present with several clinical phenotypes varying from severe to mild, severe cerebral adrenoleuko-dystrophy to mild adrenomyeloneuropathy (AMN). Although most female heterozygotes present with AMN-like symptoms after 60 years of age, occasional cases of females with the cerebral form have been reported. Phenotypic variability has been described within the same kindreds and even among monozygotic twins. There is no association between the nature of ABCD1 mutation and the clinical phenotypes, and the molecular basis of phenotypic variability in X-ALD is yet to be resolved. Various genetic, epigenetic, and environmental influences are speculated to modify the disease onset and severity. In this review, we summarize the observations made in various studies investigating the potential modifying factors regulating the clinical manifestation of X-ALD, which could help understand the pathogenesis of the disease and develop suitable therapeutic strategies.

PPARα exacerbates necroptosis, leading to increased mortality in postinfluenza bacterial superinfection

Patients infected with influenza are at high risk of secondary bacterial infection, which is a major proximate cause of morbidity and mortality. We have shown that in mice, prior infection with influenza results in increased inflammation and mortality upon Staphylococcus aureus infection, recapitulating the human disease. Lipidomic profiling of the lungs of superinfected mice revealed an increase in CYP450 metabolites during lethal superinfection. These lipids are endogenous ligands for the nuclear receptor PPARα, and we demonstrate that Ppara ^-/- mice are less susceptible to superinfection than wild-type mice. PPARα is an inhibitor of NFκB activation, and transcriptional profiling of cells isolated by bronchoalveolar lavage confirmed that influenza infection inhibits NFκB, thereby dampening proinflammatory and prosurvival signals. Furthermore, network analysis indicated an increase in necrotic cell death in the lungs of superinfected mice compared to mice infected with S. aureus alone. Consistent with this, we observed reduced NFκB-mediated inflammation and cell survival signaling in cells isolated from the lungs of superinfected mice. The kinase RIPK3 is required to induce necrotic cell death and is strongly induced in cells isolated from the lungs of superinfected mice compared to mice infected with S. aureus alone. Genetic and pharmacological perturbations demonstrated that PPARα mediates RIPK3-dependent necroptosis and that this pathway plays a central role in mortality following superinfection. Thus, we have identified a molecular circuit in which infection with influenza induces CYP450 metabolites that activate PPARα, leading to increased necrotic cell death in the lung which correlates with the excess mortality observed in superinfection.