Receptor interacting protein kinase 3 is a critical early mediator of acetaminophen-induced hepatocyte necrosis in mice

Mixed lineage kinase domain-like protein MLKL causes necrotic membrane disruption upon phosphorylation by RIP3

Programmed necrotic cell death induced by the tumor necrosis factor alpha (TNF-α) family of cytokines is dependent on a kinase cascade consisting of receptor-interacting kinases RIP1 and RIP3. How these kinase activities cause cells to die by necrosis is not known. The mixed lineage kinase domain-like protein MLKL is a functional RIP3 substrate that binds to RIP3 through its kinase-like domain but lacks kinase activity of its own. RIP3 phosphorylates MLKL at the T357 and S358 sites. Reported here is the development of a monoclonal antibody that specifically recognizes phosphorylated MLKL in cells dying of this pathway and in human liver biopsy samples from patients suffering from drug-induced liver injury. The phosphorylated MLKL forms an oligomer that binds to phosphatidylinositol lipids and cardiolipin. This property allows MLKL to move from the cytosol to the plasma and intracellular membranes, where it directly disrupts membrane integrity, resulting in necrotic death.

Dual role of acetaminophen in promoting hepatoma cell apoptosis and kidney fibroblast proliferation

Acetaminophen (APAP), is a safe analgesic and antipyretic drug at therapeutic dose, and is widely used in the clinic. However, high doses of APAP can induce hepatotoxicity and nephrotoxicity. Most studies have focused on high‑dose APAP‑induced acute liver and kidney injury. So far, few studies have investigated the effects of the therapeutic dose (1/10 of the high dose) or of the low dose (1/100 of the high dose) of APAP on the cells. The aim of this study was to investigate the cellular effects of therapeutic- or low‑dose APAP treatment on hepatoma cells and kidney fibroblasts. As expected, high‑dose APAP treatment inhibited while therapeutic and low‑dose treatment did not inhibit cell survival of kidney tubular epithelial cells. In addition, therapeutic-dose treatment induced an increase in the H2O2 level, activated the caspase‑9/‑3 cascade, and induced cell apoptosis of hepatoma cells. Notably, APAP promoted fibroblast proliferation, even at low doses. This study demonstrates that different cellular effects are exerted upon treatment with different APAP concentrations. Our results indicate that treatment with the therapeutic dose of APAP may exert an antitumor activity on hepatoma, while low‑dose treatment may be harmful for patients with fibrosis, since it may cause proliferation of fibroblasts.

Argininosuccinate synthetase as a plasma biomarker of liver injury after acetaminophen overdose in rodents and humans

CONTEXT

New biomarkers are needed in acetaminophen (APAP) hepatotoxicity. Plasma argininosuccinate synthetase (ASS) is a promising candidate.

OBJECTIVE

Characterize ASS in APAP hepatotoxicity.

METHODS

ASS was measured in plasma from rodents and humans with APAP hepatotoxicity.

RESULTS

In mice, ASS increased before injury, peaked before alanine aminotransferase (ALT) and decreased rapidly. Fischer rats had a greater increase in ASS relative to ALT. Patients with abnormal liver test results had very high ASS compared to controls. ASS appeared to increase early in some patients, and declined rapidly in all.

CONCLUSIONS

ASS may be a useful biomarker of acute cell death in APAP hepatotoxicity.

Role of receptor interacting protein (RIP)1 on apoptosis-inducing factor-mediated necroptosis during acetaminophen-evoked acute liver failure in mice

Acetaminophen (APAP) overdose induces apoptosis-inducing factor (AIF)-dependent necroptosis, but the mechanism remains obscure. The present study investigated the role of receptor interacting protein (RIP)1, a critical mediator of necroptosis, on AIF-dependent necroptosis during APAP-induced acute liver failure. Mice were intraperitoneally injected with APAP (300 mg/kg). As expected, hepatic RIP1 was activated as early as 1 h after APAP, which is earlier than APAP-induced hepatic RIP3 upregulation. APAP-evoked RIP1 activation is associated with hepatic glutathione (GSH) depletion. Either pretreatment or post-treatment with Nec-1, a selective inhibitor of RIP1, significantly alleviated APAP-induced acute liver failure. Moreover, Nec-1 improved the survival and prevented APAP-induced necroptosis, as determined by TdT-mediated dUTP-biotin nick end labeling (TUNEL) assay. Further analysis showed that Nec-1 significantly inhibited APAP-induced hepatic c-Jun N-terminal kinase (JNK) phosphorylation and mitochondrial Bax translocation. In addition, Nec-1 blocked APAP-induced translocation of AIF from the mitochondria to the nucleus. Of interest, no changes were induced by Nec-1 on hepatic CYP2E1 expression. In addition, Nec-1 had little effect on APAP-induced hepatic GSH depletion at early stage. Taken together, these results suggest that RIP1 is involved in APAP-induced necroptosis. Nec-1 is an effective antidote for APAP-induced acute liver failure.

Necrosis-dependent and independent signaling of the RIP kinases in inflammation

It is now widely accepted that some forms of necrosis are controlled by a dedicated signaling pathway triggered by various cell surface and intracellular receptors. This regulated form of necrosis is mediated by the kinase activity of receptor-interacting protein kinase 1 (RIP1/RIPK1) and/or RIP3/RIPK3. A number of studies using the RIP1 kinase inhibitor Necrostatin-1 (Nec-1) and its derivatives, or RIP3-deficient mice demonstrated that RIP1 and RIP3 are involved in various infectious and sterile inflammatory diseases. As a consequence, these specific phenotypes were construed to depend on necrosis. However, emerging evidence indicates that the RIP1 kinase activity and RIP3 can also control apoptosis and inflammatory cytokine production independent of necrosis. Therefore, we may need to re-interpret conclusions drawn based on loss of RIP1 or RIP3 functions in in vivo models. We propose that studies of RIP1 and RIP3 in different inflammatory responses need to consider cell death-dependent and independent mechanisms of the RIP kinases.

The gap junction inhibitor 2-aminoethoxy-diphenyl-borate protects against acetaminophen hepatotoxicity by inhibiting cytochrome P450 enzymes and c-jun N-terminal kinase activation

Acetaminophen (APAP) hepatotoxicity is the leading cause of acute liver failure in the US. Although many aspects of the mechanism are known, recent publications suggest that gap junctions composed of connexin32 function as critical intercellular communication channels which transfer cytotoxic mediators into neighboring hepatocytes and aggravate liver injury. However, these studies did not consider off-target effects of reagents used in these experiments, especially the gap junction inhibitor 2-aminoethoxy-diphenyl-borate (2-APB). In order to assess the mechanisms of protection of 2-APB in vivo, male C56Bl/6 mice were treated with 400 mg/kg APAP to cause extensive liver injury. This injury was prevented when animals were co-treated with 20 mg/kg 2-APB and was attenuated when 2-APB was administered 1.5 h after APAP. However, the protection was completely lost when 2-APB was given 4-6 h after APAP. Measurement of protein adducts and c-jun-N-terminal kinase (JNK) activation indicated that 2-APB reduced both protein binding and JNK activation, which correlated with hepatoprotection. Although some of the protection was due to the solvent dimethyl sulfoxide (DMSO), in vitro experiments clearly demonstrated that 2-APB directly inhibits cytochrome P450 activities. In addition, JNK activation induced by phorone and tert-butylhydroperoxide in vivo was inhibited by 2-APB. The effects against APAP toxicity in vivo were reproduced in primary cultured hepatocytes without use of DMSO and in the absence of functional gap junctions. We conclude that the protective effect of 2-APB was caused by inhibition of metabolic activation of APAP and inhibition of the JNK signaling pathway and not by blocking connexin32-based gap junctions.

Protection against acetaminophen-induced liver injury by allopurinol is dependent on aldehyde oxidase-mediated liver preconditioning

Acetaminophen (APAP) overdose causes severe and occasionally fatal liver injury. Numerous drugs that attenuate APAP toxicity have been described. However these compounds frequently protect by cytochrome P450 inhibition, thereby preventing the initiating step of toxicity. We have previously shown that pretreatment with allopurinol can effectively protect against APAP toxicity, but the mechanism remains unclear. In the current study, C3HeB/FeJ mice were administered allopurinol 18h or 1h prior to an APAP overdose. Administration of allopurinol 18h prior to APAP overdose resulted in an 88% reduction in liver injury (serum ALT) 6h after APAP; however, 1h pretreatment offered no protection. APAP-cysteine adducts and glutathione depletion kinetics were similar with or without allopurinol pretreatment. The phosphorylation and mitochondrial translocation of c-jun-N-terminal-kinase (JNK) have been implicated in the progression of APAP toxicity. In our study we showed equivalent early JNK activation (2h) however late JNK activation (6h) was attenuated in allopurinol treated mice, which suggests that later JNK activation is more critical for the toxicity. Additional mice were administered oxypurinol (primary metabolite of allopurinol) 18h or 1h pre-APAP, but neither treatment protected. This finding implicated an aldehyde oxidase (AO)-mediated metabolism of allopurinol, so mice were treated with hydralazine to inhibit AO prior to allopurinol/APAP administration, which eliminated the protective effects of allopurinol. We evaluated potential targets of AO-mediated preconditioning and found increased hepatic metallothionein 18h post-allopurinol. These data show metabolism of allopurinol occurring independent of P450 isoenzymes preconditions the liver and renders the animal less susceptible to an APAP overdose.

Zonated induction of autophagy and mitochondrial spheroids limits acetaminophen-induced necrosis in the liver

Acetaminophen (APAP) overdose is the most frequent cause of acute liver failure in the US and many western countries. It is well known that APAP induces mitochondrial damage to trigger centrilobular necrosis. Emerging evidence suggests that autophagic removal of damaged mitochondria may protect against APAP-induced liver injury. Electron and confocal microscopy analysis of liver tissues revealed that APAP overdose triggers unique biochemical and pathological zonated changes in the mouse liver, which includes necrosis (zone 1), mitochondrial spheroid formation (zone 2), autophagy (zone 3) and mitochondrial biogenesis (zone 4). In this graphic review, we discuss the role of autophagy/mitophagy in limiting the expansion of necrosis and promoting mitochondrial biogenesis and liver regeneration for the recovery of APAP-induced liver injury. We also discuss possible mechanisms that could be involved in regulating APAP-induced autophagy/mitophagy and the formation of mitochondrial spheroids.