Dietary and genetic disruption of hepatic methionine metabolism induce acid sphingomyelinase to promote steatohepatitis.

Alcoholic (ASH) and nonalcoholic. (NASH).steatohepatitis are advanced.stages.of.fatty.liver.disease.Methionine adenosyltransferase 1A (MAT1A) plays a key role in hepatic methionine metabolism and germline Mat1a deletion in mice promotes NASH. Acid sphingomyelinase (ASMase) triggers hepatocellular apoptosis and liver fibrosis and has been shown to downregulate MAT1A expression in the context of fulminant liver failure. Given the role of ASMase in steatohepatitis development, we investigated the status of ASMase in Mat1a-/- mice and the regulation of ASMase by SAM/SAH. Consistent with its role in NASH, Mat1a-/- mice fed a choline-deficient (CD) diet exhibited macrosteatosis, inflammation, fibrosis and liver injury as well as reduced total and mitochondrial GSH levels. Our data uncovered an increased basal expression and activity of ASMase but not neutral SMase in Mat1a-/- mice, which further increased upon CD feeding. Interestingly, adenovirus-mediated shRNA expression targeting ASMase reduced ASMase activity and protected Mat1a-/- mice against CD diet-induced NASH. Similar results were observed in CD fed Mat1a-/- mice by pharmacological inhibition of ASMase with amitriptyline. Moreover, Mat1a/ASMase double knockout mice were resistant to CD-induced NASH. ASMase knockdown protected wild type mice against NASH induced by feeding a diet deficient in methionine and choline. Furthermore, Mat1a-/- mice developed acute-on-chronic ASH and this outcome was ameliorated by amitriptyline treatment. In vitro data in primary mouse hepatocytes revealed that decreased SAM/SAH ratio increased ASMase mRNA level and activity. MAT1A and ASMase mRNA levels exhibited an inverse correlation in liver samples from patients with ASH and NASH. Thus, disruption of methionine metabolism sensitizes to steatohepatitis by ASMase activation via decreased SAM/SAH. These findings imply that MAT1A deletion and ASMase activation engage in a self-sustained loop of relevance for steatohepatitis.

P2X7 receptor activation impairs antitumour activity of natural killer cells.

BACKGROUND AND PURPOSE: A high number of intratumoural infiltrating natural killer (NK) cells is associated with better survival in several types of cancer, constituting an important first line of defence against tumours. Hypoxia in the core of solid tumours induces cellular stress and ATP release into the extracellular space where it triggers purinergic receptor activation on tumour-associated immune cells. The aim of this study was to assess whether activation of the purinergic receptor P2X7 by extracellular ATP plays a role in the NK cells antitumour activity. EXPERIMENTAL APPROACH: We carried out in vitro experiments using purified human NK cells triggered through P2X7 by extracellular ATP. NK cell killing activity against the tumour target cells K562 was studied by means of NK cytotoxicity assays. Likewise, we designed a subcutaneous solid tumour in vivo mouse model. KEY RESULTS: In this study we found that human NK cells, expressing a functional plasma membrane P2X7, acquired an anergic state after ATP treatment, which impaired their antitumour activity and decreased IFN-γ secretion. This effect was reversed by specific P2X7 antagonists and pretreatment with either IL-2 or IL-15. Furthermore, genetic P2rx7 knockdown resulted in improved control of tumour size by NK cells. In addition, IL-2 therapy restored the ability of NK cells to diminish the size of tumours. CONCLUSIONS AND IMPLICATIONS: Our results show that P2X7 activation represents a new mechanism whereby NK cells may lose antitumour effectiveness, opening the possibility of generating modified NK cells lacking P2X7 but with improved antitumour capacity.

Evolutionary analyses of the gasdermin family suggest conserved roles in infection response despite loss of pore-forming functionality.

BACKGROUND: Gasdermins are ancient (>500million-years-ago) proteins, constituting a family of pore-forming proteins that allow the release of intracellular content including proinflammatory cytokines. Despite their importance in the immune response, and although gasdermin and gasdermin-like genes have been identified across a wide range of animal and non-animal species, there is limited information about the evolutionary history of the gasdermin family, and their functional roles after infection. In this study, we assess the lytic functions of different gasdermins across Metazoa species, and use a mouse model of sepsis to evaluate the expression of the different gasdermins during infection. RESULTS: We show that the majority of gasdermin family members from distantly related animal clades are pore-forming, in line with the function of the ancestral proto-gasdermin and gasdermin-like proteins of Bacteria. We demonstrate the first expansion of this family occurred through a duplication of the ancestral gasdermin gene which formed gasdermin E and pejvakin prior to the divergence of cartilaginous fish and bony fish ~475 mya. We show that pejvakin from cartilaginous fish and mammals lost the pore-forming functionality and thus its role in cell lysis. We describe that the pore-forming gasdermin A formed ~320 mya as a duplication of gasdermin E prior to the divergence of the Sauropsida clade (the ancestral lineage of reptiles, turtles, and birds) and the Synapsid clade (the ancestral lineage of mammals). We then demonstrate that the gasdermin A gene duplicated to form the rest of the gasdermin family including gasdermins B, C, and D: pore-forming proteins that present a high variation of the exons in the linker sequence, which in turn allows for diverse activation pathways. Finally, we describe expression of murine gasdermin family members in different tissues in a mouse sepsis model, indicating function during infection response. CONCLUSIONS: In this study we explored the evolutionary history of the gasdermin proteins in animals and demonstrated that the pore-formation functionality has been conserved from the ancient proto-gasdermin protein. We also showed that one gasdermin family member, pejvakin, lost its pore-forming functionality, but that all gasdermin family members, including pejvakin, likely retained a role in inflammation and the physiological response to infection.

Increased expression of the ATP-gated P2X7 receptor reduces responsiveness to anti-convulsants during status epilepticus in mice.

BACKGROUND AND PURPOSE: Refractory status epilepticus is a clinical emergency associated with high mortality and morbidity. Increasing evidence suggests neuroinflammation contributes to the development of drug-refractoriness during status epilepticus. Here, we have determined the contribution of the ATP-gated P2X7 receptor, previously linked to inflammation and increased hyperexcitability, to drug-refractory status epilepticus and its therapeutic potential. EXPERIMENTAL APPROACH: Status epilepticus was induced via a unilateral microinjection of kainic acid into the amygdala in adult mice. Severity of status epilepticus was compared in animals with overexpressing or knock-out of the P2X7 receptor, after inflammatory priming by pre-injection of bacterial lipopolysaccharide (LPS) and in mice treated with P2X7 receptor-targeting and anti-inflammatory drugs. KEY RESULTS: Mice overexpressing P2X7 receptors were unresponsive to several anticonvulsants (lorazepam, midazolam, phenytoin and carbamazepine) during status epilepticus. P2X7 receptor expression increased in microglia during status epilepticus, at times when responses to anticonvulsants were reduced. Overexpression of P2X7 receptors induced a pro-inflammatory phenotype in microglia during status epilepticus and the anti-inflammatory drug minocycline restored normal responses to anticonvulsants in mice overexpressing P2X7 receptors. Pretreatment of wild-type mice with LPS increased P2X7 receptor levels in the brain and reduced responsiveness to anticonvulsants during status epilepticus, which was overcome by either genetic deletion of P2X7 receptors or treatment with the P2X7 receptor antagonists, AFC-5128 or ITH15004. CONCLUSION AND IMPLICATIONS: Our results demonstrate that P2X7 receptor-induced pro-inflammatory effects contribute to resistance to pharmacotherapy during status epilepticus. Therapies targeting P2X7 receptors could be novel adjunctive treatments for drug-refractory status epilepticus.

Sensing low intracellular potassium by NLRP3 results in a stable open structure that promotes inflammasome activation.

The NLRP3 inflammasome is activated by a wide range of stimuli and drives diverse inflammatory diseases. The decrease of intracellular K+ concentration is a minimal upstream signal to most of the NLRP3 activation models. Here, we found that cellular K+ efflux induces a stable structural change in the inactive NLRP3, promoting an open conformation as a step preceding activation. This conformational change is facilitated by the specific NLRP3 FISNA domain and a unique flexible linker sequence between the PYD and FISNA domains. This linker also facilitates the ensemble of NLRP3PYD into a seed structure for ASC oligomerization. The introduction of the NLRP3 PYD-linker-FISNA sequence into NLRP6 resulted in a chimeric receptor able to be activated by K+ efflux­specific NLRP3 activators and promoted an in vivo inflammatory response to uric acid crystals. Our results establish that the amino-terminal sequence between PYD and NACHT domain of NLRP3 is key for inflammasome activation.

CD14 release induced by P2X7 receptor restricts inflammation and increases survival during sepsis.

P2X7 receptor activation induces the release of different cellular proteins, such as CD14, a glycosylphosphatidylinositol (GPI)-anchored protein to the plasma membrane important for LPS signaling via TLR4. Circulating CD14 has been found at elevated levels in sepsis, but the exact mechanism of CD14 release in sepsis has not been established. Here, we show for first time that P2X7 receptor induces the release of CD14 in extracellular vesicles, resulting in a net reduction in macrophage plasma membrane CD14 that functionally affects LPS, but not monophosphoryl lipid A, pro-inflammatory cytokine production. Also, we found that during a murine model of sepsis, P2X7 receptor activity is important for maintaining elevated levels of CD14 in biological fluids and a decrease in its activity results in higher bacterial load and exacerbated organ damage, ultimately leading to premature deaths. Our data reveal that P2X7 is a key receptor for helping to clear sepsis because it maintains elevated concentrations of circulating CD14 during infection.

When the immune system detects an infection, it often launches an inflammatory response to fight off the disease. This defense mechanism is activated by a cascade of signaling molecules that can aggravate inflammation, causing it to damage the body's own tissues and organs. This life-threatening reaction is referred to as sepsis, and kills around 11 million people each year. New approaches are therefore needed to help alleviate the damage caused by this condition. The inflammatory response is often triggered by proteins called receptors, which sit on the surface of immune cells. When these receptors are activated, they induce cells to secrete proteins that travel around the body and activate immune cells that can eliminate the infection. In 2016, a group of researchers showed that a receptor called P2X7 stimulates the release of a signaling molecule called CD14. Patients with sepsis often have elevated amounts of CD14 in their bloodstream. Yet, it remained unclear what causes this rise in CD14 and what role this molecule plays in the development of sepsis. Now, Alarcón-Vila et al. ­ including some of the researchers involved in the 2016 study ­ have investigated the role of P2X7 in mice undergoing sepsis. This was done by puncturing the mice's intestines, causing bacteria to leak out and initiate an over-active immune response. Alarcón-Vila et al. found that mice lacking the P2X7 receptor had less CD14 and struggled to eliminate the bacterial infection from their system. This increase in bacteria caused excessive damage to the mice's organs, ultimately leading to premature death. These findings suggest that P2X7 plays an important role in preventing the onset of sepsis by helping maintain high levels of CD14 following infection. This result could help to identify new therapies that reduce the mortality rates of septic infections.

P2X7 receptor induces mitochondrial failure in monocytes and compromises NLRP3 inflammasome activation during sepsis.

Sepsis is characterized by a systemic inflammatory response followed by immunosuppression of the host. Metabolic defects and mitochondrial failure are common in immunocompromised patients with sepsis. The NLRP3 inflammasome is important for establishing an inflammatory response after activation by the purinergic P2X7 receptor. Here, we study a cohort of individuals with intra-abdominal origin sepsis and show that patient monocytes have impaired NLRP3 activation by the P2X7 receptor. Furthermore, most sepsis-related deaths are among patients whose NLRP3 activation is profoundly altered. In monocytes from sepsis patients, the P2X7 receptor is associated with mitochondrial dysfunction. Furthermore, activation of the P2X7 receptor results in mitochondrial damage, which in turn inhibits NLRP3 activation by HIF-1α. We show that mortality increases in a mouse model of sepsis when the P2X7 receptor is activated in vivo. These data reveal a molecular mechanism initiated by the P2X7 receptor that contributes to NLRP3 impairment during infection.

Endoplasmic Reticulum Stress-Induced Upregulation of STARD1 Promotes Acetaminophen-Induced Acute Liver Failure.

BACKGROUND & AIMS: Acetaminophen (APAP) overdose is a major cause of acute liver failure (ALF). Mitochondrial SH3BP5 (also called SAB) and phosphorylation of c-Jun N-terminal kinase (JNK) mediate the hepatotoxic effects of APAP. We investigated the involvement of steroidogenic acute regulatory protein (STARD1), a mitochondrial cholesterol transporter, in this process and sensitization by valproic acid (VPA), which depletes glutathione and stimulates steroidogenesis. METHODS: Nonfasted C57BL/6J mice (control) and mice with liver-specific deletion of STARD1 (Stard1ΔHep), SAB (SabΔHep), or JNK1 and JNK2 (Jnk1+2ΔHep) were given VPA with or without APAP. Liver tissues were collected and analyzed by histology and immunohistochemistry and for APAP metabolism, endoplasmic reticulum (ER) stress, and mitochondrial function. Adult human hepatocytes were transplanted into Fah-/-/Rag2-/-/Il2rg-/-/NOD (FRGN) mice to create mice with humanized livers. RESULTS: Administration of VPA before administration of APAP increased the severity of liver damage in control mice. The combination of VPA and APAP increased expression of CYP2E1, formation of NAPQI-protein adducts, and depletion of glutathione from liver tissues of control mice, resulting in ER stress and the upregulation of STARD1. Livers from control mice given VPA and APAP accumulated cholesterol in the mitochondria and had sustained mitochondrial depletion of glutathione and mitochondrial dysfunction. Inhibition of ER stress, by administration of tauroursodeoxycholic acid to control mice, prevented upregulation of STARD1 in liver and protected the mice from hepatoxicity following administration of VPA and APAP. Administration of N-acetylcysteine to control mice prevented VPA- and APAP-induced ER stress and liver injury. Stard1ΔHep mice were resistant to induction of ALF by VPA and APAP, despite increased mitochondrial levels of glutathione and phosphorylated JNK; we made similar observations in fasted Stard1ΔHep mice given APAP alone. SabΔHep mice or Jnk1+2ΔHep mice did not develop ALF following administration of VPA and APAP. The ability of VPA to increase the severity of APAP-induced liver damage was observed in FRGN mice with humanized liver. CONCLUSIONS: In studies of mice, we found that upregulation of STARD1 following ER stress mediates APAP hepatoxicity via SH3BP5 and phosphorylation of JNK1 and JNK2.

Purinergic receptors and the inflammatory response mediated by lipids.

The inflammatory response is regulated by the production of different extracellular mediators, including lipids and extracellular nucleotides. In the extracellular environment, intermediate lipids activate specific G-protein-coupled receptors (GPCRs) in target cells and promote cell recruitment and activation. Extracellular nucleotides activate two types of receptors, the ionotropic purinergic P2X and the metabotropic purinergic P2Y receptors, inducing the release of cytokines and promoting cell recruitment. Several P2X receptors are associated with an increase in the production of immunoactive lipids mediators, which in turn are able to interfere with the activation of different P2Y receptors, establishing a tight signalling link between purinergic receptors and lipid mediators. In this review, we summarise recent studies indicating signalling crosstalk between purinergic P2X and P2Y receptor activation and lipid mediators with a focus on inflammatory diseases. Novel concepts arising from this crosstalk would result in the development of combinatorial therapies targeting lipid synthesis together with individual P2 receptors for the management of inflammatory diseases.

Mitochondrial GSH replenishment as a potential therapeutic approach for Niemann Pick type C disease.

Niemann Pick type C (NPC) disease is a progressive lysosomal storage disorder caused by mutations in genes encoding NPC1/NPC2 proteins, characterized by neurological defects, hepatosplenomegaly and premature death. While the primary biochemical feature of NPC disease is the intracellular accumulation of cholesterol and gangliosides, predominantly in endolysosomes, mitochondrial cholesterol accumulation has also been reported. As accumulation of cholesterol in mitochondria is known to impair the transport of GSH into mitochondria, resulting in mitochondrial GSH (mGSH) depletion, we investigated the impact of mGSH recovery in NPC disease. We show that GSH ethyl ester (GSH-EE), but not N-acetylcysteine (NAC), restored the mGSH pool in liver and brain of Npc1-/- mice and in fibroblasts from NPC patients, while both GSH-EE and NAC increased total GSH levels. GSH-EE but not NAC increased the median survival and maximal life span of Npc1-/- mice. Moreover, intraperitoneal therapy with GSH-EE protected against oxidative stress and oxidant-induced cell death, restored calbindin levels in cerebellar Purkinje cells and reversed locomotor impairment in Npc1-/- mice. High-resolution respirometry analyses revealed that GSH-EE improved oxidative phosphorylation, coupled respiration and maximal electron transfer in cerebellum of Npc1-/- mice. Lipidomic analyses showed that GSH-EE treatment had not effect in the profile of most sphingolipids in liver and brain, except for some particular species in brain of Npc1-/- mice. These findings indicate that the specific replenishment of mGSH may be a potential promising therapy for NPC disease, worth exploring alone or in combination with other options.

Lysosomal Cholesterol Accumulation Sensitizes To Acetaminophen Hepatotoxicity by Impairing Mitophagy.

The role of lysosomes in acetaminophen (APAP) hepatotoxicity is poorly understood. Here, we investigated the impact of genetic and drug-induced lysosomal cholesterol (LC) accumulation in APAP hepatotoxicity. Acid sphingomyelinase (ASMase)(-/-) mice exhibit LC accumulation and higher mortality after APAP overdose compared to ASMase(+/+) littermates. ASMase(-/-) hepatocytes display lower threshold for APAP-induced cell death and defective fusion of mitochondria-containing autophagosomes with lysosomes, which decreased mitochondrial quality control. LC accumulation in ASMase(+/+) hepatocytes caused by U18666A reproduces the susceptibility of ASMase(-/-) hepatocytes to APAP and the impairment in the formation of mitochondria-containing autolysosomes. LC extraction by 25-hydroxycholesterol increased APAP-mediated mitophagy and protected ASMase(-/-) mice and hepatocytes against APAP hepatotoxicity, effects that were reversed by chloroquine to disrupt autophagy. The regulation of LC by U18666A or 25-hydroxycholesterol did not affect total cellular sphingomyelin content or its lysosomal distribution. Of relevance, amitriptyline-induced ASMase inhibition in human hepatocytes caused LC accumulation, impaired mitophagy and increased susceptibility to APAP. Similar results were observed upon glucocerebrosidase inhibition by conduritol ß-epoxide, a cellular model of Gaucher disease. These findings indicate that LC accumulation determines susceptibility to APAP hepatotoxicity by modulating mitophagy, and imply that genetic or drug-mediated ASMase disruption sensitizes to APAP-induced liver injury.

Myristic acid potentiates palmitic acid-induced lipotoxicity and steatohepatitis associated with lipodystrophy by sustaning de novo ceramide synthesis.

Palmitic acid (PA) induces hepatocyte apoptosis and fuels de novo ceramide synthesis in the endoplasmic reticulum (ER). Myristic acid (MA), a free fatty acid highly abundant in copra/palmist oils, is a predictor of nonalcoholic steatohepatitis (NASH) and stimulates ceramide synthesis. Here we investigated the synergism between MA and PA in ceramide synthesis, ER stress, lipotoxicity and NASH. Unlike PA, MA is not lipotoxic but potentiated PA-mediated lipoapoptosis, ER stress, caspase-3 activation and cytochrome c release in primary mouse hepatocytes (PMH). Moreover, MA kinetically sustained PA-induced total ceramide content by stimulating dehydroceramide desaturase and switched the ceramide profile from decreased to increased ceramide 14:0/ceramide16:0, without changing medium and long-chain ceramide species. PMH were more sensitive to equimolar ceramide14:0/ceramide16:0 exposure, which mimics the outcome of PA plus MA treatment on ceramide homeostasis, than to either ceramide alone. Treatment with myriocin to inhibit ceramide synthesis and tauroursodeoxycholic acid to prevent ER stress ameliorated PA plus MA induced apoptosis, similar to the protection afforded by the antioxidant BHA, the pan-caspase inhibitor z-VAD-Fmk and JNK inhibition. Moreover, ruthenium red protected PMH against PA and MA-induced cell death. Recapitulating in vitro findings, mice fed a diet enriched in PA plus MA exhibited lipodystrophy, hepatosplenomegaly, increased liver ceramide content and cholesterol levels, ER stress, liver damage, inflammation and fibrosis compared to mice fed diets enriched in PA or MA alone. The deleterious effects of PA plus MA-enriched diet were largely prevented by in vivo myriocin treatment. These findings indicate a causal link between ceramide synthesis and ER stress in lipotoxicity, and imply that the consumption of diets enriched in MA and PA can cause NASH associated with lipodystrophy.