The end of RIPK1-RIPK3-MLKL-mediated necroptosis in acetaminophen-induced hepatotoxicity?

Arylacetamide deacetylase regulates hepatic iron homeostasis to protect against carbon tetrachloride-induced ferroptosis

Arylacetamide deacetylase (AADAC) catalyzes the hydrolysis of small molecules containing ester and amide bonds. Recently, it has been reported that AADAC can suppress reactive oxygen species production in cancer cells. This study aimed to elucidate the possibility that AADAC protects against drug-induced liver injury accompanied by oxidative stress and to explore its molecular mechanisms. Intraperitoneal administration of carbon tetrachloride induced significantly more severe liver injury in Aadac knockout (KO) mice (plasma alanine aminotransferase level: 19,381 ± 10,578 U/L) than in wild-type (WT) mice (7219 ± 4729 U/L). More severe liver injury in Aadac KO mice was accompanied by higher hepatic malondialdehyde and antioxidant gene mRNA levels than those in WT mice. The increase in plasma alanine aminotransferase levels in Aadac KO mice was substantially suppressed by pretreatment with the ferroptosis inhibitors deferoxamine or ferrostatin-1, suggesting that Aadac deficiency increases susceptibility to ferroptosis. Immunoprecipitation followed by proteomic analysis revealed that AADAC interacts with ceruloplasmin (CP), which oxidizes ferrous iron to ferric iron. Hepatic CP activity was significantly lower in Aadac KO mice than that in WT mice, resulting in elevated hepatic ferrous iron levels in Aadac KO mice. Overexpression of human AADAC in Huh-7 cells significantly attenuated carbon tetrachloride-induced cytotoxicity by suppressing ferrous iron accumulation, suggesting that AADAC interacts with CP to suppress hepatic ferrous iron accumulation. The hepatoprotective role of Aadac in ferroptosis was also observed in mice with acetaminophen-induced liver injury. This study demonstrates a novel function of AADAC in protecting against ferroptosis induced by hepatotoxicants, carbon tetrachloride and acetaminophen.

Drug-induced oxidative stress actively prevents caspase activation and hepatocyte apoptosis

Cell death is a fundamental process in health and disease. Emerging research shows the existence of numerous distinct cell death modalities with similar and intertwined signaling pathways, but resulting in different cellular outcomes, raising the need to understand the decision-making steps during cell death signaling. Paracetamol (Acetaminophen, APAP)-induced hepatocyte death includes several apoptotic processes but eventually is executed by oncotic necrosis without any caspase activation. Here, we studied this paradoxical form of cell death and revealed that APAP not only fails to activate caspases but also strongly impedes their activation upon classical apoptosis induction, thereby shifting apoptosis to necrosis. While APAP intoxication results in massive drop in mitochondrial respiration, low cellular ATP levels could be excluded as an underlying cause of missing apoptosome formation and caspase activation. In contrast, we identified oxidative stress as a key factor in APAP-induced caspase inhibition. Importantly, caspase inhibition and the associated switch from apoptotic to necrotic cell death was reversible through the administration of antioxidants. Thus, exemplified by APAP-induced cell death, our study stresses that cellular redox status is a critical component in the decision-making between apoptotic and necrotic cell death, as it directly affects caspase activity.

Hepatic interleukin 32 attenuates liver injury through repression of necroptosis in cholestasis

OBJECTIVE

We aimed to evaluate the association between interleukin (IL)-32 and necroptosis in cholestatic liver injury.

METHODS

Levels of necroptosis-related markers in cholestatic and control patients, including the receptor-interacting serine-threonine kinase 3 (RIPK3), receptor-interacting serine-threonine kinase 1 (RIPK1), and mixed lineage kinase domain-like (MLKL) were measured. Animal experiments in C57BL/6J and transgenic mice with IL32β/γ overexpression were also conducted to confirm the effect of IL-32 on necroptosis in cholestasis, which was induced by α-naphthylisothiocyanate (ANIT) and 1% lithocholic acid (LCA). PLC/PRF/5-ASBT and primary mouse hepatocytes were utilized for the investigation of the regulation and mechanism of IL-32 in cholestasis.

RESULTS

In the liver tissues of cholestatic patients, the mRNA and protein expressions of RIPK1, RIPK3, and MLKL were increased and associated with IL-32 expression. In addition, expressions of these indicators in the liver of 1% LCA- and ANIT-induced mouse models were significantly increased, while they were markedly decreased in hIL32βLTg and hIL32γLTg mice. After bile acid stimulation, IL-32 and phosphorylated Akt (p-Akt) expressions significantly elevated in a dose-dependent manner. After treated with tumor necrosis factor (TNF)-α, IL-32 inhibited MLKL expression in primary mouse hepatocytes.

CONCLUSION

IL-32 is negatively associated with necroptosis in cholestatic patients. Moreover, IL-32 is induced by p-Akt and effectively attenuates necroptosis in ANIT- or 1% LCA-induced cholestasis.

Curcumin alleviates experimental colitis in mice by suppressing necroptosis of intestinal epithelial cells

Curcumin, the primary bioactive substance in turmeric, exhibits potential therapeutic effects on ulcerative colitis. However, its mechanism for regulating necroptosis in colitis has not been fully elucidated. In this study, the effect of curcumin on experimental colitis-induced necroptosis of intestinal epithelial cells was investigated, and its molecular mechanism was further explored. We found that curcumin blocked necroptosis in a dose-dependent manner by inhibiting the phosphorylation of RIP3 and MLKL instead of RIP1 in HT-29 cells. Co-Immunoprecipitation assay showed that curcumin weakened the interaction between RIP1 and RIP3, possibly due to the direct binding of curcumin to RIP3 as suggested by drug affinity responsive target stability analysis. In a classical in vivo model of TNF-α and pan-caspase inhibitor-induced necroptosis in C57BL/6 mice, curcumin potently inhibited systemic inflammatory responses initiated by the necroptosis signaling pathway. Then, using a dextran sodium sulfate-induced colitis model in C57BL/6 mice, we found that curcumin inhibited the expression of p-RIP3 in the intestinal epithelium, reduced intestinal epithelial cells loss, improved the function of the intestinal tight junction barrier, and reduced local intestinal inflammation. Collectively, our findings suggest that curcumin is a potent targeted RIP3 inhibitor with anti-necroptotic and anti-inflammatory effects, maintains intestinal barrier function, and effectively alleviates colitis injury.

Necroptosis Plays a Crucial Role in Vascular Injury during DVT and Is Enhanced by IL-17B

Background. This study investigated whether vascular endothelial necroptosis is involved in deep vein thrombosis (DVT) and how IL-17B facilitates necroptosis signaling. Methods. The DVT mouse model was induced by ligation of the IVC. The cross-sectional area of thrombus increases and the thrombus occupied the entire venous lumen at 48 h after ligation. Meanwhile, the increased expression of p-RIP3/RIP3 was most pronounced at 48 h after ligation, and the p-MLKL/MLKL peaked at 72 h. Results. Based on Illumina sequencing and KEGG pathway analyses, the activated RIP3/MLKL is associated with increased IL-17B. With thrombus formation, IL-17B was upregulated and enhanced the expression of RIP3 and MLKL in the IVC wall, as well as their phosphorylation levels (all $P<0.05$ , the comparison group consisted of the control group, DVT group, DVT/IL-17B group, and DVT/anti-IL-17B group). The p-RIP3/RIP3 and p-MLKL/MLKL ratios were reduced by anti-IL-17B. Similarly, the weight and cross-sectional area of the thrombi were increased by IL-17B and decreased by the IL-17B antibody. IL-17B had a smaller effect on thrombosis in knockout mice compared with WT mice. In vitro, the IL-17B protein expression and the level of RIP3 and MLKL phosphorylation increased high in the OGD cells, accompanied by increased expression of IL-6 and TNF-α. IL-17B enhanced the expression of IL-6 and TNF-α but had little effect on the IL-6 and TNF-α after transfected with siRIP3 or siMLKL. Similarly, the plasma IL-17B, IL-6, and TNF-α were significantly increased after thrombosis in WT mice, and enhanced by IL-17B. But IL-17B did not increase the plasma IL-6 and TNF-α in knockout mice. Conclusions. In conclusion, those results suggest that vascular endothelial necroptosis plays a crucial role in vascular injury and IL-17B could enhance the necroptosis pathway.

Aniline exposure activates receptor-interacting serine/threonineprotein kinase 1 and causes necroptosis of AML12 cells

With the increased use of aniline, potential impacts on human health cannot be ignored. The hepatotoxicity of aniline is largely unknown and the underlying mechanism remains unclear. Therefore, the aim of the present study was to investigate the hepatotoxicity of aniline and elucidate the underlying mechanism. AML12 cells were exposed to different concentrations of aniline (0, 5, 10, or 20 mM) to observe changes to reactive oxygen species (ROS) production and the expression patterns of necroptosis-related proteins (RIPK1, RIPK3, and MLKL). The potential mechanism underlying aniline-induced hepatotoxicity was explored by knockout of RIPK1. The results showed that aniline induced cytotoxicity in AML12 cells in a dose-dependent manner in addition to the production of ROS and subsequent necroptosis of AML12 cells. Silencing of RIPK1 reversed upregulation of necroptosis-related proteins in AML12 cells exposed to aniline, demonstrating that aniline-induced ROS production was related to necroptosis of AML12. Moreover, aniline promoted intracellular RIPK1 activation, suggesting that the RIPK1/ROS pathway plays an important role in aniline-induced hepatotoxicity. NAC could quench ROS and inhibit necroptosis. These results provide a scientific basis for future studies of aniline-induced hepatotoxicity for the prevention and treatment of aniline-induced cytotoxicity.

Membrane-delimited signaling and cytosolic action of MG53 preserve hepatocyte integrity during drug-induced liver injury

BACKGROUND & AIMS

Drug-induced liver injury (DILI) remains challenging to treat and is still a leading cause of acute liver failure. MG53 is a muscle-derived tissue-repair protein that circulates in the bloodstream and whose physiological role in protection against DILI has not been examined.

METHODS

Recombinant MG53 protein (rhMG53) was administered exogenously, using mice with deletion of Mg53 or Ripk3. Live-cell imaging, histological, biochemical, and molecular studies were used to investigate the mechanisms that underlie the extracellular and intracellular action of rhMG53 in hepatoprotection.

RESULTS

Systemic administration of rhMG53 protein, in mice, can prophylactically and therapeutically treat DILI induced through exposure to acetaminophen, tetracycline, concanavalin A, carbon tetrachloride, or thioacetamide. Circulating MG53 protects hepatocytes from injury through direct interaction with MLKL at the plasma membrane. Extracellular MG53 can enter hepatocytes and act as an E3-ligase to mitigate RIPK3-mediated MLKL phosphorylation and membrane translocation.

CONCLUSIONS

Our data show that the membrane-delimited signaling and cytosolic dual action of MG53 effectively preserves hepatocyte integrity during DILI. rhMG53 may be a potential treatment option for patients with DILI.

LAY SUMMARY

Interventions to treat drug-induced liver injury and halt its progression into liver failure are of great value to society. The present study reveals that muscle-liver cross talk, with MG53 as a messenger, serves an important role in liver cell protection. Thus, MG53 is a potential treatment option for patients with drug-induced liver injury.

An Insight on the Pathways Involved in Crizotinib and Sunitinib Induced Hepatotoxicity in HepG2 Cells and Animal Model

Both crizotinib and sunitinib, novel orally-active multikinase inhibitors, exhibit antitumor activity and extend the survival of patients with a malignant tumor. However, some patients may suffer liver injury that can further limit the clinical use of these drugs, however the mechanisms underlying hepatotoxicity are still to be elucidated. Thus, our study was designed to use HepG2 cells in vitro and the ICR mice model in vivo to investigate the mechanisms of hepatotoxicity induced by crizotinib and sunitinib. Male ICR mice were treated orally with crizotinib (70 mg/kg/day) or sunitinib (7.5 mg/kg/day) for four weeks. The results demonstrated that crizotinib and sunitinib caused cytotoxicity in HepG2 cells and chronic liver injury in mice, which were associated with oxidative stress, apoptosis and/or necrosis. Crizotinib- and sunitinib-induced oxidative stress was accompanied by increasing reactive oxygen species and malondialdehyde levels and decreasing the activity of superoxide dismutase and glutathione peroxidase. Notably, the activation of the Kelch-like ECH-associated protein-1/Nuclear factor erythroid-2 related factor 2 signaling pathway was involved in the process of oxidative stress, and partially protected against oxidative stress. Crizotinib and sunitinib induced apoptosis via the mitochondrial pathway, which was characterized by decreasing Bcl2/Bax ratio to dissipate the mitochondrial membrane potential, and increasing apoptotic markers levels. Moreover, the pan-caspase inhibitor Z-VAD-FMK improved the cell viability and alleviated liver damage, which further indicated the presence of apoptosis. Taken together, this study demonstrated that crizotinib- and sunitinib-caused oxidative stress and apoptosis finally impaired hepatic function, which was strongly supported by the histopathological lesions and markedly increased levels of serum alanine aminotransferase, alkaline phosphatase and lactate dehydrogenase.

The molecular mechanism of acute liver injury and inflammatory response induced by Concanavalin A

Acute liver injury is a common but urgent clinical condition, and its underlying mechanism remains to be further elucidated. Concanavalin A (ConA)-induced liver injury was investigated in the study. Different from the caspase-dependent cell apoptosis in lipopolysaccharide/D-aminogalactose (LPS/D-GalN) induced liver injury, ConA-induced hepatocyte death was independent on caspase. Increased hepatocytic expressions of mixed lineage kinase domain like (MLKL) and receptor-interacting protein kinase 1 (RIPK1), and higher serum concentration of tumor necrosis factor-α (TNF-α) were noticed in mice with ConA-induced liver injury. Inhibition of RIPK1 protein or deletion of MLKL gene could significantly attenuate the acute liver injury and improve mice survival. Besides, the ConA treatment induced severe hepatic inflammation in wide type (WT) mice in comparison with Mlkl-/- mice, suggesting the RIPK1-MLKL-mediated hepatocellular necroptosis might participate in the process of liver injury. Moreover, mitochondrial damage associated molecular patterns (DAMPs) were subsequently released after the hepatocyte death, and further activated the p38 mitogen-activated protein kinase (MAPK) pathway, which could be reduced by deletion or inhibition of Toll-like receptor 9 (TLR9). Taken together, our research revealed that ConA-induced acute liver injury was closely related to TNF-α-mediated cell necroptosis, and inhibiting RIPK1 or deleting MLKL gene could alleviate liver injury in mice. The mitochondrial DNA released by dead hepatocytes further activated neutrophils through TLR9, thus resulting in the exacerbation of liver injury.

Phosphorylation of eIF2α mitigates endoplasmic reticulum stress and hepatocyte necroptosis in acute liver injury

INTRODUCTION AND OBJECTIVES

Necroptosis and endoplasmic reticulum (ER) stress has been implicated in acute and chronic liver injury. Activated eukaryotic initiation factor 2 alpha (eIF2α) attenuates protein synthesis and relieves the load of protein folding in the ER. In this study, we aimed to analyze the impact of eIF2α phosphorylation on hepatocyte necroptosis in acute liver injury.

MATERIALS AND METHODS

Male BALB/c mice were injected with tunicamycin or d-galactosamine, and LO2 cells were incubated with tunicamycin to induce acute liver injury. 4-Phenylbutyric acid (PBA) and salubrinal were used to inhibit ER stress and eIF2α dephosphorylation, respectively. We analyzed the eIF2α phosphorylation, ER stress, and hepatocyte necroptosis in mice and cells model.

RESULTS

Tunicamycin or d-galactosamine significantly induced ER stress and necroptosis, as well as eIF2α phosphorylation, in mice and LO2 cells (p<0.05). ER stress aggravated tunicamycin-induced hepatocyte necroptosis in mice and LO2 cells (p<0.05). Elevated eIF2α phosphorylation significantly mitigated hepatocyte ER stress (p<0.05) and hepatocyte necroptosis in mice (34.37±3.39% vs 22.53±2.18%; p<0.05) and LO2 cells (1±0.11 vs 0.33±0.05; p<0.05). Interestingly, tumor necrosis factor receptor (TNFR) 1 protein levels were not completely synchronized with necroptosis. TNFR1 expression was reduced in d-galactosamine-treated mice (p<0.05) and cells incubated with tunicamycin for 12 and 24h (p<0.05). ER stress partially restored TNFR1 expression and increased necroptosis in tunicamycin-incubated cells (p<0.05).

CONCLUSIONS

These results imply that ER stress can mediate hepatocyte necroptosis independent of TNFR1 signaling and elevated eIF2α phosphorylation can mitigate ER stress during acute liver injury.

Anwulignan Alleviates Carbon Tetrachloride-Induced Acute Liver Injury in Mice

Acute liver injury is a serious threat to human health. Complementary therapy including a traditional Chinese herb has been used for the prevention and treatment of liver injuries. Schisandrae sphenantherae fructus (Schisandra) is the mature dry fruit of Schisandra sphenanthera Rehd. et Wils. Wuzhi capsule, a preparation containing Schisandra and its main component anwulignan, is used to treat hepatitis and hepatic insufficiency caused by viruses and drugs in the clinic. However, to date, there has been little study to reveal the effect of anwulignan in the protection of the liver. Therefore, in this study, we hypothesized that anwulignan could protect carbon tetrachloride (CCl4)-induced acute liver injury in mice. Anwulignan was shown to reduce significantly the liver index, decrease liver histopathological injury, decrease the serum level of aspartate aminotransferase and alanine aminotransferase, increase the activities of superoxide dismutase (SOD) and glutathione peroxidase, reduce liver malondialdehyde content, and downregulate the expression levels of interleukin (IL)-6, IL-1β, and tumor necrosis factor-α in the liver tissue, as well as the protein expression levels of receptor-interacting serine/threonine-protein kinase 1 (RIPK1), RIPK3, and phosphorylated mixed lineage kinase domain-like protein. All these results suggest that anwulignan can alleviate the CCl4-induced acute liver injury in mice, which may be related to its antioxidant, anti-inflammation, and inhibition of liver cell necroptosis effects.

Ferroptosis driven by radical oxidation of n-6 polyunsaturated fatty acids mediates acetaminophen-induced acute liver failure

Acetaminophen (APAP) overdose is a common cause of drug-induced acute liver failure. Although hepatocyte cell death is considered to be the critical event in APAP-induced hepatotoxicity, the underlying mechanism remains unclear. Ferroptosis is a newly discovered type of cell death that is caused by a loss of cellular redox homeostasis. As glutathione (GSH) depletion triggers APAP-induced hepatotoxicity, we investigated the role of ferroptosis in a murine model of APAP-induced acute liver failure. APAP-induced hepatotoxicity (evaluated in terms of ALT, AST, and the histopathological score), lipid peroxidation (4-HNE and MDA), and upregulation of the ferroptosis maker PTGS2 mRNA were markedly prevented by the ferroptosis-specific inhibitor ferrostatin-1 (Fer-1). Fer-1 treatment also completely prevented mortality induced by high-dose APAP. Similarly, APAP-induced hepatotoxicity and lipid peroxidation were prevented by the iron chelator deferoxamine. Using mass spectrometry, we found that lipid peroxides derived from n-6 fatty acids, mainly arachidonic acid, were elevated by APAP, and that auto-oxidation is the predominant mechanism of APAP-derived lipid oxidation. APAP-induced hepatotoxicity was also prevented by genetic inhibition of acyl-CoA synthetase long-chain family member 4 or α-tocopherol supplementation. We found that ferroptosis is responsible for APAP-induced hepatocyte cell death. Our findings provide new insights into the mechanism of APAP-induced hepatotoxicity and suggest that ferroptosis is a potential therapeutic target for APAP-induced acute liver failure.

Emerging and established modes of cell death during acetaminophen-induced liver injury

Acetaminophen (APAP)-induced liver injury is an important clinical and toxicological problem. Understanding the mechanisms and modes of cell death are vital for the development of therapeutic interventions. The histological and clinical features of APAP hepatotoxicity including cell and organelle swelling, karyolysis, and extensive cell contents release lead to the characterization of the cell death as oncotic necrosis. However, the more recent identification of detailed signaling mechanisms of mitochondrial dysfunction, the amplification mechanisms of mitochondrial oxidant stress and peroxynitrite formation by a mitogen-activated protein kinase cascade, mechanisms of the mitochondrial permeability transition pore opening and nuclear DNA fragmentation as well as the characterization of the sterile inflammatory response suggested that the mode of cell death is better termed programmed necrosis. Additional features like mitochondrial Bax translocation and cytochrome c release, mobilization of lysosomal iron and the activation of receptor-interacting protein kinases and the inflammasome raised the question whether other emerging modes of cell death such as apoptosis, necroptosis, ferroptosis and pyroptosis could also play a role. The current review summarizes the key mechanisms of APAP-induced liver injury and compares these with key features of the newly described modes of cell death. Based on the preponderance of experimental and clinical evidence, the mode of APAP-induced cell death should be termed programmed necrosis; despite some overlap with other modes of cell death, APAP hepatotoxicity does not fulfill the characteristics of either apoptosis, necroptosis, ferroptosis, pyroptosis or autophagic cell death.

Necroptosis signaling in liver diseases: An update

The apoptosis alternate cell death pathways are extensively studied in recent years and their significance has been well recognized. With identification of newer cell death pathways, the therapeutic opportunities to modulate cell death have indeed further extended. Necroptosis, among other apoptosis alternate pathways, has been immensely studied recently in different hepatic disease models. Receptor-interacting protein 1 (RIPK1), RIPK3 and mixed lineage kinase domain like (MLKL) seemed to be the key players to mediate necroptosis pathway. Initially, necroptosis seemed to be following the typical pathway. But recently diverse pathways and outcomes have been observed. With recent studies reporting diverse outcomes, the necroptosis signalling has become a lot more interesting and intricate. The typical RIPK1 signalling followed by RIPK3 and MLKL might not always be strictly followed. Although, necroptosis signalling has been intensively investigated in various disease conditions; however, there is still a need to further elaborate and understand the unique scaffolding and kinase properties and other signalling interactions of necroptosis signalling molecules.

Microcystin-LR promotes necroptosis in primary mouse hepatocytes by overproducing reactive oxygen species

Microcystin-LR (MC-LR) is a type of cyclic heptapeptide toxin produced by cyanobacteria during bloom events. MC-LR-induced cell death is critically involved in its potent specific hepatotoxicity. Many studies have demonstrated that prototypical apoptosis as a form of programmed cell death after MC-LR is associated with liver injury. However, whether another form of programmed cell death exists and the underlying mechanism have not been reported. Here, we demonstrate that MC-LR can induce necroptosis via ROS overactivation in primary mouse hepatocytes. Various potential pathways of programmed cell death induced by MC-LR were evaluated by annexin V/PI dual staining for flow cytometric analysis, image-based PI staining analysis and western blot analysis. Cell viability was determined by the CCK8 assay. Rupture of the plasma membrane was indicated by lactate dehydrogenase release. ROS was evaluated with the carboxy-H2DCFDA fluorescent probe. It was found that in MC-LR-treated cells, as the plasma membrane was damaged, annexin V/PI-stained double-positive cells were significantly induced and PI-stained nuclei were more diffuse. Western blot analysis showed that MC-LR treatment significantly upregulated the expression of necroptotic and apoptotic proteins. Mechanistically, MC-LR induced ROS overproduction by dysregulating the expression and activity of the pro-oxidants SOD1, MAOA, and NOX4 and the antioxidant GPX1. These results indicate the presence of a novel mechanism for MC-LR-mediated liver injury and present a novel target in the treatment of MC-LR-exposed patients.

Furosine, a Maillard Reaction Product, Triggers Necroptosis in Hepatocytes by Regulating the RIPK1/RIPK3/MLKL Pathway

As one of the typical Maillard reaction products, furosine has been widely reported in a variety of heat-processed food. Though furosine was shown to be toxic on organs, its toxicity mechanism is still unclear. The present study aimed to investigate the toxicity mechanism of furosine in liver tissue. An intragastric gavage mice model (42-day administration, 0.1/0.25/0.5 g/kg of furosine per day) and a mice primary hepatocyte model were employed to investigate the toxicity mechanism of furosine on mice liver tissue. A metabonomics analysis of mice liver, serum, and red blood cells (RBC) was performed. The special metabolic mediator of furosine, lysophosphatidylcholine 18:0 (LPC (18:0)) was identified. Then, the effect of the upstream gene phospholipase A2 gamma (PLA2-3) on LPC (18:0), as well as the effect of furosine (100 mg/L) on the receptor-interacting serine/threonine-protein kinase (RIPK)1/RIPK3/mixed lineage kinase domain-like protein (MLKL) pathway and inflammatory factors, was determined in liver tissue and primary hepatocytes. PLA2-3 was found to regulate the level of LPC (18:0) and activate the expression of RIPK1, RIPK3, P-MLKL, and of the inflammatory factors including tumor necrosis factor α (TNF-α) and interleukin (IL-1β), both in liver tissue and in primary hepatocytes. Upon treatment with furosine, the upstream sensor PLA2-3 activated the RIPK1/RIPK3/MLKL necroptosis pathway and caused inflammation by regulating the expression of LPC (18:0), which further caused liver damage.

The outer membrane protein Tp92 of Treponema pallidum induces human mononuclear cell death and IL-8 secretion

Treponema pallidum is the pathogen that causes syphilis, a sexually transmitted disease; however, the pathogenic mechanism of this organism remains unclear. Tp92 is the only T. pallidum outer membrane protein that has structural features similar to the outer membrane proteins of other Gram-negative bacteria, but the exact functions of this protein remain unknown. In the present study, we demonstrated that the recombinant Tp92 protein can induce human mononuclear cell death. Tp92 mediated the human monocytic cell line derived from an acute monicytic leukemia patient (THP-1) cell death by recognizing CD14 and/or TLR2 on cell surfaces. After the stimulation of THP-1 cells by the Tp92 protein, Tp92 may induce atypical pyroptosis of THP-1 cells via the pro-caspase-1 pathway. Meanwhile, this protein caused the apoptosis of THP-1 cells via the receptor-interacting protein kinase 1/caspase-8/aspase-3 pathway. Tp92 reduced the number of monocytes among peripheral blood mononuclear cells. Interestingly, further research showed that Tp92 failed to increase the tumour necrosis factor-α, interleukin (IL)-1β, IL-6, IL-10, IL-18 and monocyte chemotactic protein 1 (MCP)-1 levels but slightly elevated the IL-8 levels via the Nuclear Factor (NF)-κB pathway in THP-1 cells. The data suggest that Tp92 recognizes CD14 and TLR2, transfers the signal to a downstream pathway, and activates NF-κB to mediate the production of IL-8. This mechanism may help T. pallidum escape recognition and elimination by the host innate immune system.

Curcumol induces RIPK1/RIPK3 complex-dependent necroptosis via JNK1/2-ROS signaling in hepatic stellate cells

It is generally recognized that hepatic fibrogenesis is an end result of increased extracellular matrix (ECM) production from the activation and proliferation of hepatic stellate cells (HSCs). An in-depth understanding of the mechanisms of HSC necroptosis might provide a new therapeutic strategy for prevention and treatment of hepatic fibrosis. In this study, we attempted to investigate the effect of curcumol on necroptosis in HSCs, and further to explore the molecular mechanisms. We found that curcumol ameliorated the carbon tetrachloride (CCl₄)-induced mice liver fibrosis and suppressed HSC proliferation and activation, which was associated with regulating HSC necroptosis through increasing the phosphorylation of receptor-interacting protein kinase 1 (RIPK1), receptor-interacting protein kinase 3 (RIPK3). Moreover, curcumol promoted the migration of RIPK1 and RIPK3 into necrosome in HSCs. RIPK3 depletion impaired the anti-fibrotic effect of curcumol. Importantly, we showed that curcumol-induced RIPK3 up-regulation significantly increased mitochondrial reactive oxygen species (ROS) production and mitochondrial depolarization. ROS scavenger, N-acetyl-L-cysteine (NAC) impaired RIPK3-mediated necroptosis. In addition, our study also identified that the activation of c-Jun N-terminal kinase1/2 (JNK1/2) was regulated by RIPK3, which mediated curcumol-induced ROS production. Down-regulation of RIPK3 expression, using siRIPK3, markedly abrogated JNK1/2 expression. The use of specific JNK1/2 inhibitor (SP600125) resulted in the suppression of curcumol-induced ROS production and mitochondrial depolarization, which in turn, contributed to the inhibition of curcumol-triggered necroptosis. In summary, our study results reveal the molecular mechanism of curcumol-induced HSC necroptosis, and suggest a potential clinical use of curcumol-targeted RIPK1/RIPK3 complex-dependent necroptosis via JNK1/2-ROS signaling for the treatment of hepatic fibrosis.

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Curcumol exerted anti-hepatic fibrogenesis effects in CCl₄-treated mice.

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Curcumol inhibited the activation of hepatic stellate cell in vitro.

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Curcumol promoted the generation of RIPK1/RIPK3-complex to induce hepatic stellate cell necroptosis.

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Curcumol modulated RIPK3/JNK/ROS signaling axis.

Role and mechanisms of autophagy in acetaminophen-induced liver injury

Acetaminophen (APAP) overdose is the most frequent cause of acute liver failure in the USA and many other countries. Although the metabolism and pathogenesis of APAP has been extensively investigated for decades, the mechanisms by which APAP induces liver injury are incompletely known, which hampers the development of effective therapeutic approaches to tackle this important clinical problem. Autophagy is a highly conserved intracellular degradation pathway, which aims at recycling cellular components and damaged organelles in response to adverse environmental conditions and stresses as a survival mechanism. There is accumulating evidence indicating that autophagy is activated in response to APAP overdose in specific liver zone areas, and pharmacological activation of autophagy protects against APAP-induced liver injury. Increasing evidence also suggests that hepatic autophagy is impaired in nonalcoholic fatty livers (NAFLD), and NAFLD patients are more susceptible to APAP-induced liver injury. Here, we summarized the current progress on the role and mechanisms of autophagy in protecting against APAP-induced liver injury.

Silymarin prevents acetaminophen-induced hepatotoxicity in mice

Acetaminophen or paracetamol (APAP) overdose is a common cause of liver injury. Silymarin (SLM) is a hepatoprotective agent widely used for treating liver injury of different origin. In order to evaluate the possible beneficial effects of SLM, Balb/c mice were pretreated with SLM (100 mg/kg b.wt. per os) once daily for three days. Two hours after the last SLM dose, the mice were administered APAP (300 mg/kg b.wt. i.p.) and killed 6 (T₆), 12 (T₁₂) and 24 (T₂₄) hours later. SLM-treated mice exhibited a significant reduction in APAP-induced liver injury, assessed according to AST and ALT release and histological examination. SLM treatment significantly reduced superoxide production, as indicated by lower GSSG content, lower HO-1 induction, alleviated nitrosative stress, decreased p-JNK activation and direct measurement of mitochondrial superoxide production in vitro. SLM did not affect the APAP-induced decrease in CYP2E1 activity and expression during the first 12 hrs. Neutrophil infiltration and enhanced expression of inflammatory markers were first detected at T₁₂ in both groups. Inflammation progressed in the APAP group at T₂₄ but became attenuated in SLM-treated animals. Histological examination suggests that necrosis the dominant cell death pathway in APAP intoxication, which is partially preventable by SLM pretreatment. We demonstrate that SLM significantly protects against APAP-induced liver damage through the scavenger activity of SLM and the reduction of superoxide and peroxynitrite content. Neutrophil-induced damage is probably secondary to necrosis development.

Immature mice are more susceptible than adult mice to acetaminophen-induced acute liver injury

Acetaminophen (APAP) overdose induces acute liver injury. The aim of the present study was to analyze the difference of susceptibility between immature and adult mice to APAP-induced acute liver injury. Weanling immature and adult mice were injected with APAP (300 mg/kg). As expected, immature mice were more susceptible than adult mice to APAP-induced acute liver injury. APAP-evoked hepatic c-Jun N-terminal kinase phosphorylation was stronger in immature mice than in adult mice. Hepatic receptor-interacting protein (RIP)1 was obviously activated at APAP-exposed immature and adult mice. Interestingly, hepatic RIP3 activation was more obvious in APAP-treated immature mice than adult mice. Although there was no difference on hepatic GSH metabolic enzymes between immature and adult mice, immature mice were more susceptible than adult mice to APAP-induced hepatic GSH depletion. Of interest, immature mice expressed a much higher level of hepatic Cyp2e1 and Cyp3a11 mRNAs than adult mice. Correspondingly, immature mice expressed a higher level of hepatic CYP2E1, the key drug metabolic enzyme that metabolized APAP into the reactive metabolite NAPQI. These results suggest that a higher level of hepatic drug metabolic enzymes in immature mice than adult mice might contribute to the difference of susceptibility to APAP-induced acute liver injury.

Mechanisms of acetaminophen hepatotoxicity and their translation to the human pathophysiology

Acetaminophen (APAP) overdose is the most common cause of acute liver failure in the United States and mechanisms of liver injury induced by APAP overdose have been the focus of extensive investigation. Studies in the mouse model, which closely reproduces the human condition, have shown that hepatotoxicity is initiated by formation of a reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI), which depletes cellular glutathione and forms protein adducts on mitochondrial proteins. This leads to mitochondrial oxidative and nitrosative stress, accompanied by activation of c-jun N-terminal kinase (JNK) and its translocation to the mitochondria. This then amplifies the mitochondrial oxidant stress, resulting in translocation of Bax and dynamin related protein 1 (Drp1) to the mitochondria, which induces mitochondrial fission, and ultimately induction of the mitochondrial membrane permeability transition (MPT). The induction of MPT triggers release of intermembrane proteins such as apoptosis inducing factor (AIF) and endonuclease G into the cytosol and their translocation to the nucleus, causing nuclear DNA fragmentation and activation of regulated necrosis. Though these cascades of events were primarily identified in the mouse model, studies on human hepatocytes and analysis of circulating biomarkers from patients after APAP overdose, indicate that a number of mechanistic events are identical in mice and humans. Circulating biomarkers also seem to be useful in predicting the course of liver injury after APAP overdose in humans and hold promise for significant clinical use in the near future.

Activation of necroptosis in human and experimental cholestasis

Cholestasis encompasses liver injury and inflammation. Necroptosis, a necrotic cell death pathway regulated by receptor-interacting protein (RIP) 3, may mediate cell death and inflammation in the liver. We aimed to investigate the role of necroptosis in mediating deleterious processes associated with cholestatic liver disease. Hallmarks of necroptosis were evaluated in liver biopsies of primary biliary cholangitis (PBC) patients and in wild-type and RIP3-deficient (RIP3^−/−) mice subjected to common bile duct ligation (BDL). The functional link between RIP3, heme oxygenase-1 (HO-1) and antioxidant response was investigated in vivo after BDL and in vitro. We demonstrate increased RIP3 expression and mixed lineage kinase domain-like protein (MLKL) phosphorylation in liver samples of human PBC patients, coincident with thioflavin T labeling, suggesting activation of necroptosis. BDL resulted in evident hallmarks of necroptosis, concomitant with progressive bile duct hyperplasia, multifocal necrosis, fibrosis and inflammation. MLKL phosphorylation was increased and insoluble aggregates of RIP3, MLKL and RIP1 formed in BLD liver tissue samples. Furthermore, RIP3 deficiency blocked BDL-induced necroinflammation at 3 and 14 days post-BDL. Serum hepatic enzymes, fibrogenic liver gene expression and oxidative stress decreased in RIP3^−/− mice at 3 days after BDL. However, at 14 days, cholestasis aggravated and fibrosis was not halted. RIP3 deficiency further associated with increased hepatic expression of HO-1 and accumulation of iron in BDL mice. The functional link between HO-1 activity and bile acid toxicity was established in RIP3-deficient primary hepatocytes. Necroptosis is triggered in PBC patients and mediates hepatic necroinflammation in BDL-induced acute cholestasis. Targeting necroptosis may represent a therapeutic strategy for acute cholestasis, although complementary approaches may be required to control progression of chronic cholestatic liver disease.

Caspase Inhibition Prevents Tumor Necrosis Factor-α-Induced Apoptosis and Promotes Necrotic Cell Death in Mouse Hepatocytes in Vivo and in Vitro

How different cell death modes and cell survival pathways cross talk remains elusive. We determined the interrelation of apoptosis, necrosis, and autophagy in tumor necrosis factor (TNF)-α/actinomycin D (ActD) and lipopolysaccharide/D-galactosamine (GalN)-induced hepatotoxicity in vitro and in vivo. We found that TNF-α/ActD-induced apoptosis was completely blocked by a general caspase inhibitor ZVAD-fmk at 24 hours but hepatocytes still died by necrosis at 48 hours. Inhibition of caspases also protected mice against lipopolysaccharide/GalN-induced apoptosis and liver injury at the early time point, but this protection was diminished after prolonged treatment by switching apoptosis to necrosis. Inhibition of receptor-interacting protein kinase (RIP)1 by necrostatin 1 partially inhibited TNF-α/ZVAD-induced necrosis in primary hepatocytes. Pharmacologic inhibition of autophagy or genetic deletion of Atg5 in hepatocytes did not protect against TNF-α/ActD/ZVAD-induced necrosis. Moreover, pharmacologic inhibition of RIP1 or genetic deletion of RIP3 failed to protect and even exacerbated liver injury after mice were treated with lipopolysaccharide/GalN and a pan-caspase inhibitor. In conclusion, our results suggest that different cell death mode and cell survival pathways are closely integrated during TNF-α-induced liver injury when both caspases and NF-κB are blocked. Moreover, results from our study also raised concerns about the safety of currently ongoing clinical trials that use caspase inhibitors.

Knockdown of RIPK1 Markedly Exacerbates Murine Immune-Mediated Liver Injury through Massive Apoptosis of Hepatocytes, Independent of Necroptosis and Inhibition of NF-κB

Receptor-interacting protein kinase (RIPK)1 has an essential role in the signaling pathways triggered by death receptors through activation of NF-κB and regulation of caspase-dependent apoptosis and RIPK3/mixed lineage kinase domain-like protein (MLKL)-mediated necroptosis. We examined the effect of RIPK1 antisense knockdown on immune-mediated liver injury in C57BL/6 mice caused by α-galactosylceramide (αGalCer), a specific activator for invariant NKT cells. We found that knockdown of RIPK1 markedly exacerbated αGalCer-mediated liver injury and induced lethality. This was associated with increased hepatic inflammation and massive apoptotic death of hepatocytes, as indicated by TUNEL staining and caspase-3 activation. Pretreatment with zVAD.fmk, a pan-caspase inhibitor, or neutralizing Abs against TNF, almost completely protected against the exacerbated liver injury and lethality. Primary hepatocytes isolated from RIPK1-knockdown mice were sensitized to TNF-induced cell death that was completely inhibited by adding zVAD.fmk. The exacerbated liver injury was not due to impaired hepatic NF-κB activation in terms of IκBα phosphorylation and degradation in in vivo and in vitro studies. Lack of RIPK1 kinase activity by pretreatment with necrostatin-1, a RIPK1 kinase inhibitor, or in the RIPK1 kinase-dead knock-in (RIPK1^D138N) mice did not exacerbate αGalCer-mediated liver injury. Furthermore, RIPK3-knockout and MLKL-knockout mice behaved similarly as wild-type control mice in response to αGalCer, with or without knockdown of RIPK1, excluding a switch to RIPK3/MLKL-mediated necroptosis. Our findings reveal a critical kinase-independent platform role for RIPK1 in protecting against TNF/caspase-dependent apoptosis of hepatocytes in immune-mediated liver injury.

An outline of necrosome triggers

Necroptosis was initially identified as a backup cell death program when apoptosis is blocked. However, it is now recognized as a cellular defense mechanism against infections and is presumed to be a detrimental factor in several pathologies driven by cell death. Necroptosis is a prototypic form of regulated necrosis that depends on activation of the necrosome, which is a protein complex in which receptor interacting protein kinase (RIPK) 3 is activated. The RIP homotypic interaction motif (RHIM) is the core domain that regulates activation of the necrosome. To date, three RHIM-containing proteins have been reported to activate the kinase activity of RIPK3 within the necrosome: RIPK1, Toll/IL-1 receptor domain-containing adaptor inducing IFN-β (TRIF), and DNA-dependent activator of interferon regulatory factors (DAI). Here, we review and discuss commonalities and differences of the increasing number of activators of the necrosome. Since the discovery that activation of mixed lineage kinase domain-like (MLKL) by RIPK3 kinase activity is crucial in necroptosis, interest has increased in monitoring and therapeutically targeting their activation. The availability of new phospho-specific antibodies, pharmacologic inhibitors, and transgenic models will allow us to further document the role of necroptosis in degenerative, inflammatory and infectious diseases.