Critical role for mixed-lineage kinase 3 in acetaminophen-induced hepatotoxicity

The impact of sterile inflammation in acute liver injury

BACKGROUND

The liver has a number of functions in innate immunity. These functions predispose the liver to innate immune-mediated liver injury when inflammation goes unchecked. Significant progress has been made in the last 25 years on sterile inflammatory liver injury in a number of models; however, a great deal of controversy and many questions about the nature of sterile inflammation still exist.

AIM

The goal of this article is to review sterile inflammatory liver injury using both a basic approach to what constitutes the inflammatory injury, and through examination of current models of liver injury and inflammation. This information will be tied to human patient conditions when appropriate.

RELEVANCE FOR PATIENTS

Inflammation is one of the most critical factors for managing in-patient liver disease in a number of scenarios. More information is needed for both scientists and clinicians to develop rational treatments.

Increased hepatic receptor interacting protein kinase 3 expression due to impaired proteasomal functions contributes to alcohol-induced steatosis and liver injury

Chronic alcohol exposure increased hepatic receptor-interacting protein kinase (RIP) 3 expression and necroptosis in the liver but its mechanisms are unclear. In the present study, we demonstrated that chronic alcohol feeding plus binge (Gao-binge) increased RIP3 but not RIP1 protein levels in mouse livers. RIP3 knockout mice had decreased serum alanine amino transferase activity and hepatic steatosis but had no effect on hepatic neutrophil infiltration compared with wild type mice after Gao-binge alcohol treatment. The hepatic mRNA levels of RIP3 did not change between Gao-binge and control mice, suggesting that alcohol-induced hepatic RIP3 proteins are regulated at the posttranslational level. We found that Gao-binge treatment decreased the levels of proteasome subunit alpha type-2 (PSMA2) and proteasome 26S subunit, ATPase 1 (PSMC1) and impaired hepatic proteasome function. Pharmacological or genetic inhibition of proteasome resulted in the accumulation of RIP3 in mouse livers. More importantly, human alcoholics had decreased expression of PSMA2 and PSMC1 but increased protein levels of RIP3 compared with healthy human livers. Moreover, pharmacological inhibition of RIP1 decreased Gao-binge-induced hepatic inflammation, neutrophil infiltration and NF-κB subunit (p65) nuclear translocation but failed to protect against steatosis and liver injury induced by Gao-binge alcohol. In conclusion, results from this study suggest that impaired hepatic proteasome function by alcohol exposure may contribute to hepatic accumulation of RIP3 resulting in necroptosis and steatosis while RIP1 kinase activity is important for alcohol-induced inflammation.

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.

Questions and controversies: the role of necroptosis in liver disease

Acute and chronic liver injury results in hepatocyte death and turnover. If injury becomes chronic, the continuous cell death and turnover leads to chronic inflammation, fibrosis and ultimately cirrhosis and hepatocellular carcinoma. Controlling liver cell death both in acute injury, to rescue the liver from acute liver failure, and in chronic injury, to curb secondary inflammation and fibrosis, is of paramount importance as a therapeutic strategy. Both apoptosis and necrosis occur in the liver, but the occurrence of necroptosis in the liver and its contribution to liver disease is controversial. Necroptosis is a form of regulated necrosis which occurs in certain cell types when caspases (+/-cIAPs) are inhibited through the RIPK1-RIPK3 activation of MLKL. The occurrence of necroptosis in the liver has recently been examined in multiple liver injury models with conflicting results. The aim of this review is to summarize the published data with an emphasis on the controversies and remaining questions in the field.

Differential susceptibility to acetaminophen-induced liver injury in sub-strains of C57BL/6 mice: 6N versus 6J

Mouse models of acetaminophen (APAP) hepatotoxicity are considered relevant for the human pathophysiology. The C57BL/6 strain is most popular because it is the background strain of gene knock-out mice. However, conflicting results in the literature may have been caused by sub-strain mismatches, e.g. C57BL/6J and C57BL/6N. This study was initiated to determine the mechanism behind the sub-strain susceptibility to APAP toxicity. C57BL/6N and C57BL/6J mice were dosed with 200 mg/kg APAP and sacrificed at different time points. C57BL/6N mice developed significantly more liver injury as measured by plasma ALT activities and histology. Although there was no difference in glutathione depletion or cytochrome P450 activity between groups, C57BL/6N had a higher glutathione disulfide-to-glutathione ratio and more APAP protein adducts. C57BL/6N showed more mitochondrial translocation of phospho-JNK and BAX, and more release of mitochondrial intermembrane proteins apoptosis-inducing factor (AIF), second mitochondria-derived activator of caspases (SMAC), which caused more DNA fragmentation. The increased mitochondrial dysfunction was confirmed in vitro as C57BL/6N hepatocytes had a more precipitous drop in JC-1 fluorescence after APAP exposure.

CONCLUSION

C57BL/6N mice are more susceptible to APAP-induced hepatotoxicity, likely due to increased formation of APAP-protein adducts and a subsequent enhancement of mitochondrial dysfunction associated with aggravated nuclear DNA fragmentation.

Myeloid Mixed Lineage Kinase 3 Contributes to Chronic Ethanol-Induced Inflammation and Hepatocyte Injury in Mice

Proinflammatory activity of hepatic macrophages plays a key role during progression of alcoholic liver disease (ALD). Since mixed lineage kinase 3 (MLK3)-dependent phosphorylation of JNK is involved in the activation of macrophages, we tested the hypothesis that myeloid MLK3 contributes to chronic ethanol-induced inflammatory responses in liver, leading to hepatocyte injury and cell death. Primary cultures of Kupffer cells, as well in vivo chronic ethanol feeding, were used to interrogate the role of MLK3 in the progression of liver injury. Phosphorylation of MLK3 was increased in primary cultures of Kupffer cells isolated from ethanol-fed rats compared to cells from pair-fed rats. Kupffer cells from ethanol-fed rats were more sensitive to LPS-stimulated cytokine production; this sensitization was normalized by pharmacological inhibition of MLK3. Chronic ethanol feeding to mice increased MLK3 phosphorylation robustly in F4/80(+) Kupffer cells, as well as in isolated nonparenchymal cells. MLK3(-/-) mice were protected from chronic ethanol-induced phosphorylation of MLK3 and JNK, as well as multiple indicators of liver injury, including increased ALT/AST, inflammatory cytokines, and induction of RIP3. However, ethanol-induced steatosis and hepatocyte apoptosis were not affected by MLK3. Finally, chimeric mice lacking MLK3 only in myeloid cells were also protected from chronic ethanol-induced phosphorylation of JNK, expression of inflammatory cytokines, and increased ALT/AST. MLK3 expression in myeloid cells contributes to phosphorylation of JNK, increased cytokine production, and hepatocyte injury in response to chronic ethanol. Our data suggest that myeloid MLK3 could be targeted for developing potential therapeutic strategies to suppress liver injury in ALD patients.

c-Jun N-terminal kinase mediates mouse liver injury through a novel Sab (SH3BP5)-dependent pathway leading to inactivation of intramitochondrial Src

Sustained c-Jun N-terminal kinase (JNK) activation has been implicated in many models of cell death and tissue injury. Phosphorylated JNK (p-JNK) interacts with the mitochondrial outer membrane SH3 homology associated BTK binding protein (Sab, or SH3BP5). Using knockdown or liver-specific deletion of Sab, we aimed to elucidate the consequences of this interaction on mitochondrial function in isolated mitochondria and liver injury models in vivo. Respiration in isolated mitochondria was directly inhibited by p-JNK + adenosine triphosphate. Knockdown or liver-specific knockout of Sab abrogated this effect and markedly inhibited sustained JNK activation and liver injury from acetaminophen or tumor necrosis factor/galactosamine. We then elucidated an intramitochondrial pathway in which interaction of JNK and Sab on the outside of the mitochondria released protein tyrosine phosphatase, nonreceptor type 6 (SHP1, or PTPN6) from Sab in the inside of the mitochondrial outer membrane, leading to its activation and transfer to the inner membrane, where it dephosphorylates P-Y419Src (active), which required a platform protein, docking protein 4 (DOK4), on the inner membrane. Knockdown of mitochondrial DOK4 or SHP1 inhibited the inactivation of mitochondrial p-Src and the effect of p-JNK on mitochondria.

CONCLUSIONS

The binding to and phosphorylation of Sab by p-JNK on the outer mitochondrial membrane leads to SHP1-dependent and DOK4-dependent inactivation of p-Src on the inner membrane; inactivation of mitochondrial Src inhibits electron transport and increases reactive oxygen species release, which sustains JNK activation and promotes cell death and organ injury. (Hepatology 2016;63:1987-2003).

3,4-Dihydroxyphenylacetic acid, a microbiota-derived metabolite of quercetin, attenuates acetaminophen (APAP)-induced liver injury through activation of Nrf-2

1. Acetaminophen (APAP) overdose leads to severe hepatotoxicity. 3,4-dihydroxyphenylacetic acid (DOPAC) is a scarcely studied microbiota-derived metabolite of quercetin. The aim of this study was to determine the protective effect of DOPAC against APAP-induced liver injury. 2. Mice were treated intragastrically with DOPAC (10, 20 or 50 mg/kg) for 3 days before APAP (300 mg/kg) injection. APAP alone caused increase in serum aminotransferase levels and changes in hepatic histopathology. APAP also promoted oxidative stress by increasing lipid peroxidation and decreasing anti-oxidant enzyme activities. These events led to hepatocellular necrosis and reduced liver function. DOPAC increased nuclear factor erythroid 2-related factor 2 (Nrf-2) translocation to the nucleus and enhanced the expression of phase II enzymes and anti-oxidant enzymes, and thereby reduced APAP hepatotoxicity and enhanced anti-oxidant ability. 3. Our data provide evidence that DOPAC protected the liver against APAP-induced injury, which is involved in Nrf-2 activation, implying that DOPAC can be considered as a potential natural hepatoprotective agent.

Acetaminophen from liver to brain: New insights into drug pharmacological action and toxicity

Acetaminophen (APAP) is a well-known analgesic and antipyretic drug. It is considered to be safe when administered within its therapeutic range, but in cases of acute intoxication, hepatotoxicity can occur. APAP overdose is the leading cause of acute liver failure in the northern hemisphere. Historically, studies on APAP toxicity have been focused on liver, with alterations in brain function attributed to secondary effects of acute liver failure. However, in the last decade the pharmacological mechanism of APAP as a cannabinoid system modulator has been documented and some articles have reported "in situ" toxicity by APAP in brain tissue at high doses. Paradoxically, low doses of APAP have been reported to produce the opposite, neuroprotective effects. In this paper we present a comprehensive, up-to-date overview of hepatic toxicity as well as a thorough review of both toxic and beneficial effects of APAP in brain.

Receptor interacting protein kinase 1 mediates murine acetaminophen toxicity independent of the necrosome and not through necroptosis

Although necrosis in the acetaminophen (APAP) model is known to be regulated by c-Jun NH2-terminal kinase (JNK) through interaction with mitochondria, the role of necroptosis through receptor-interacting proteins 1 and 3 (RIPK1 and RIPK3) has also been suggested. Our aim was to determine the relationship between these two mechanisms of cell death. To verify the participation of RIPK1, we used antisense knockdown and confirmed protection comparable to the RIPK1 inhibitor, necrostatin, in vivo and in vitro. However, we found no evidence that RIPK3 is expressed in primary mouse hepatocytes under basal conditions or after APAP and RIPK3(-/-) mice were not protected. RIPK3 was exclusively expressed in nonparenchymal cells. RIPK1 knockdown protected RIPK3(-/-) mice to the same extent as wild-type mice, underscoring the independent role of RIPK1. We confirmed that necroptosis is not involved in APAP toxicity by using mixed lineage kinase domain-like protein (MLKL) knockout mice, which were not protected from APAP. Next, we addressed whether there is interplay between RIPK1 and JNK. RIPK1 knockdown decreased the level of JNK activation and translocation to mitochondria and abrogated subsequent translocation of dynamin-related protein 1 (Drp1). Interestingly, APAP induced translocation of RIPK1 to mitochondria, which was unaffected by knockdown of the mitochondrial JNK docking protein, Sh3 homology 3 binding protein 5 (Sab).

CONCLUSION

RIPK1 participates in APAP-induced necrosis upstream of JNK activation whereas RIPK3 and MLKL are dispensable, indicating that necroptosis does not contribute to APAP-induced necrosis and RIPK1 has a unique, independent role.

Critical review of resveratrol in xenobiotic-induced hepatotoxicity

Use of natural products is increasingly popular. In fact, many patients with liver diseases self-medicate with herbal supplements. Resveratrol (RSV), in particular, is a common natural product that can reduce injury in experimental models of liver disease. Xenobiotic hepatotoxicity is a particularly important area-of-need for therapeutics. Drug-induced liver injury, for example, is the most common cause of acute liver failure (ALF) and ALF-induced deaths in many countries. Importantly, RSV protects against hepatotoxicity in animal models in vivo caused by several drugs and chemicals and may be an effective intervention. Although many mechanisms have been proposed to explain the protection, not all are consistent with other data. Furthermore, RSV suffers from other issues, including limited bioavailability due to extensive hepatic metabolism. The purpose of this article is to summarize recent findings on the protective effects of RSV in xenobiotic-induced liver injury and other forms of liver injury and to provide a critical review of the underlying mechanisms. New mechanisms that are more consistent with data emerging from the toxicology field are suggested. Efforts to move RSV into clinical use are also considered. Overall, RSV is a promising candidate for therapeutic use, but additional studies are needed to better understand its effects.

Mitochondrial protein adducts formation and mitochondrial dysfunction during N-acetyl-m-aminophenol (AMAP)-induced hepatotoxicity in primary human hepatocytes

3'-Hydroxyacetanilide orN-acetyl-meta-aminophenol (AMAP) is generally regarded as a non-hepatotoxic analog of acetaminophen (APAP). Previous studies demonstrated the absence of toxicity after AMAP in mice, hamsters, primary mouse hepatocytes and several cell lines. In contrast, experiments with liver slices suggested that it may be toxic to human hepatocytes; however, the mechanism of toxicity is unclear. To explore this,we treated primary human hepatocytes (PHH) with AMAP or APAP for up to 48 h and measured several parameters to assess metabolism and injury. Although less toxic than APAP, AMAP dose-dependently triggered cell death in PHH as indicated by alanine aminotransferase (ALT) release and propidium iodide (PI) staining. Similar to APAP, AMAP also significantly depleted glutathione (GSH) in PHH and caused mitochondrial damage as indicated by glutamate dehydrogenase (GDH) release and the JC-1 assay. However, unlike APAP, AMAP treatment did not cause relevant c-jun-N-terminal kinase (JNK) activation in the cytosol or phospho-JNK translocation to mitochondria. To compare, AMAP toxicity was assessed in primary mouse hepatocytes (PMH). No cytotoxicity was observed as indicated by the lack of lactate dehydrogenase release and no PI staining. Furthermore, there was no GSH depletion or mitochondrial dysfunction after AMAP treatment in PMH. Immunoblotting for arylated proteins suggested that AMAP treatment caused extensive mitochondrial protein adduct formation in PHH but not in PMH. In conclusion, AMAP is hepatotoxic in PHH and the mechanism involves the formation of mitochondrial protein adducts and mitochondrial dysfunction.

Pathophysiological significance of c-jun N-terminal kinase in acetaminophen hepatotoxicity

BACKGROUND

Acetaminophen (APAP) overdose is the leading cause of acute liver failure in the US. Although substantial progress regarding the mechanisms of APAP hepatotoxicity has been made in the past several decades, therapeutic options are still limited and novel treatments are clearly needed. c-jun N-terminal Kinase (JNK) has emerged as a promising therapeutic target in recent years.

AREAS COVERED

Early studies established the critical role of JNK activation and mitochondrial translocation in APAP hepatotoxicity. However, this concept has also been challenged. Initial studies failed to reproduce the protection of JNK deficiency in APAP toxicity and concerns over off-target effects of JNK inhibitors and even in knock-out mice are increasing. Interestingly, recent studies have even shown that liver injury can be altered with or without effects on JNK activation. The current review addresses these discrepancies and tries to explain or reconcile some of the conflicting results.

EXPERT OPINION

JNK is a potential therapeutic target for APAP poisoning. However, controversies still exist regarding its actual role in APAP hepatotoxicity. Future studies are warranted for more in-depth testing of specific inhibitors in well-defined preclinical models and human hepatocytes before JNK can be considered a relevant therapeutic target for APAP poisoning.

Resveratrol prevents protein nitration and release of endonucleases from mitochondria during acetaminophen hepatotoxicity

Overdose of acetaminophen (APAP) is a common cause of acute liver injury and liver failure. The mechanism involves formation of a reactive metabolite, protein binding, oxidative stress and activation of c-Jun N-terminal kinase (JNK), mitochondrial dysfunction, and nuclear DNA fragmentation caused by endonucleases released from damaged mitochondria. Previous work has shown that the natural product resveratrol (RSV) can protect against APAP hepatotoxicity in mice through prevention of lipid peroxidation and anti-inflammatory effects. However, these earlier studies did not take into consideration several fundamental aspects of the pathophysiology. To address this, we treated C57Bl/6 mice with 300 mg/kg APAP followed by 50 mg/kg RSV 1.5 h later. Our results confirmed that RSV reduced liver injury after APAP overdose in mice. Importantly, RSV did not inhibit reactive metabolite formation and protein bindings, nor did it reduce activation of JNK. However, RSV decreased protein nitration after APAP treatment, possibly through direct scavenging of peroxynitrite. Interestingly, RSV also inhibited release of apoptosis-inducing factor and endonuclease G from mitochondria independent of Bax pore formation and prevented the downstream nuclear DNA fragmentation. Our data show that RSV protects against APAP hepatotoxicity both through antioxidant effects and by preventing mitochondrial release of endonucleases and nuclear DNA damage.

Inhibitor of apoptosis signal-regulating kinase 1 protects against acetaminophen-induced liver injury

Metabolic activation and oxidant stress are key events in the pathophysiology of acetaminophen (APAP) hepatotoxicity. The initial mitochondrial oxidative stress triggered by protein adduct formation is amplified by c-jun-N-terminal kinase (JNK), resulting in mitochondrial dysfunction and ultimately cell necrosis. Apoptosis signal-regulating kinase 1 (ASK1) is considered the link between oxidant stress and JNK activation. The objective of the current study was to assess the efficacy and mechanism of action of the small-molecule ASK1 inhibitor GS-459679 in a murine model of APAP hepatotoxicity. APAP (300 mg/kg) caused extensive glutathione depletion, JNK activation and translocation to the mitochondria, oxidant stress and liver injury as indicated by plasma ALT activities and area of necrosis over a 24h observation period. Pretreatment with 30 mg/kg of GS-459679 almost completely prevented JNK activation, oxidant stress and injury without affecting the metabolic activation of APAP. To evaluate the therapeutic potential of GS-459679, mice were treated with APAP and then with the inhibitor. Given 1.5h after APAP, GS-459679 was still protective, which was paralleled by reduced JNK activation and p-JNK translocation to mitochondria. However, GS-459679 treatment was not more effective than N-acetylcysteine, and the combination of GS-459679 and N-acetylcysteine exhibited similar efficacy as N-acetylcysteine monotherapy, suggesting that GS-459769 and N-acetylcysteine affect the same pathway. Importantly, inhibition of ASK1 did not impair liver regeneration as indicated by PCNA staining. In conclusion, the ASK1 inhibitor GS-459679 protected against APAP toxicity by attenuating JNK activation and oxidant stress in mice and may have therapeutic potential for APAP overdose patients.

Chronic Deletion and Acute Knockdown of Parkin Have Differential Responses to Acetaminophen-induced Mitophagy and Liver Injury in Mice

We previously demonstrated that pharmacological induction of autophagy protected against acetaminophen (APAP)-induced liver injury in mice by clearing damaged mitochondria. However, the mechanism for removal of mitochondria by autophagy is unknown. Parkin, an E3 ubiquitin ligase, has been shown to be required for mitophagy induction in cultured mammalian cells following mitochondrial depolarization, but its role in vivo is not clear. The purpose of this study was to investigate the role of Parkin-mediated mitophagy in protection against APAP-induced liver injury. We found that Parkin translocated to mitochondria in mouse livers after APAP treatment followed by mitochondrial protein ubiquitination and mitophagy induction. To our surprise, we found that mitophagy still occurred in Parkin knock-out (KO) mice after APAP treatment based on electron microscopy analysis and Western blot analysis for some mitochondrial proteins, and Parkin KO mice were protected against APAP-induced liver injury compared with wild type mice. Mechanistically, we found that Parkin KO mice had decreased activated c-Jun N-terminal kinase (JNK), increased induction of myeloid leukemia cell differentiation protein (Mcl-1) expression, and increased hepatocyte proliferation after APAP treatment in their livers compared with WT mice. In contrast to chronic deletion of Parkin, acute knockdown of Parkin in mouse livers using adenovirus shRNA reduced mitophagy and Mcl-1 expression but increased JNK activation after APAP administration, which exacerbated APAP-induced liver injury. Therefore, chronic deletion (KO) and acute knockdown of Parkin have differential responses to APAP-induced mitophagy and liver injury in mice.

Pathways involved in acetaminophen hepatotoxicity with specific targets for inhibition/downregulation

Insights on molecular/immunological mechanisms involve in APAP hepatotoxicity and pave way for researchers/clinicians/pharma bodies to identify novel biomarkers, effective bioactive candidates and fruitful therapy against APAP hepatotoxicity.

Serum mitochondrial biomarkers and damage-associated molecular patterns are higher in acetaminophen overdose patients with poor outcome

Acetaminophen (APAP) overdose is a major cause of acute liver failure (ALF). Numerous studies have shown that APAP hepatotoxicity in mice involves mitochondrial dysfunction, and recent data suggest that this is also the case in humans. We have previously shown that glutamate dehydrogenase (GDH), mitochondrial DNA (mtDNA), and nuclear DNA (nDNA) fragments can be measured in circulation of overdose patients as mechanistic biomarkers of mitochondrial damage and damage-associated molecular patterns. In the present study, our aim was to determine whether these biomarkers are higher in serum from nonsurvivors of APAP-induced ALF (AALF), compared to survivors. GDH, mtDNA, and nDNA fragments were measured in serum from AALF patients who did (n = 34) or did not (n = 35) recover. Importantly, all three were significantly increased in patients who died, compared to those who survived (GDH: 450 ± 73 vs. 930 ± 145 U/L; mtDNA: 21 ± 6 vs. 48 ± 13 and 33 ± 10 vs. 43 ± 7 ng/mL for two different genes; nDNA fragments: 148 ± 13 vs. 210 ± 13% of control). Receiver operating characteristic (ROC) curve analyses revealed that nDNA fragments, GDH, and mtDNA were predictive of outcome (area under the curve [AUC], study admission: 0.73, 0.70, and 0.71 or 0.76, respectively, P < 0.05; AUC, time of peak ALT: 0.78, 0.71, and 0.71 or 0.76, respectively, P < 0.05), and the results were similar to those from the Model for End-Stage Liver Disease (MELD; AUC, peak MELD: 0.77; P < 0.05).

CONCLUSIONS

Our data suggest that patients with more mitochondrial damage are less likely to survive, demonstrating that mitochondria are central in the mechanisms of APAP hepatotoxicity in humans. Clinically, serum nDNA fragments, GDH, and mtDNA could be useful as part of a panel of biomarkers to predict patient outcome. (Hepatology 2014;60:1336-1345).

Cell death and cell death responses in liver disease: mechanisms and clinical relevance

Hepatocellular death is present in almost all types of human liver disease and is used as a sensitive parameter for the detection of acute and chronic liver disease of viral, toxic, metabolic, or autoimmune origin. Clinical data and animal models suggest that hepatocyte death is the key trigger of liver disease progression, manifested by the subsequent development of inflammation, fibrosis, cirrhosis, and hepatocellular carcinoma. Modes of hepatocellular death differ substantially between liver diseases. Different modes of cell death such as apoptosis, necrosis, and necroptosis trigger specific cell death responses and promote progression of liver disease through distinct mechanisms. In this review, we first discuss molecular mechanisms by which different modes of cell death, damage-associated molecular patterns, and specific cell death responses contribute to the development of liver disease. We then review the clinical relevance of cell death, focusing on biomarkers; the contribution of cell death to drug-induced, viral, and fatty liver disease and liver cancer; and evidence for cell death pathways as therapeutic targets.

New advances in molecular mechanisms and emerging therapeutic targets in alcoholic liver diseases

Alcoholic liver disease is a major health problem in the United States and worldwide. Chronic alcohol consumption can cause steatosis, inflammation, fibrosis, cirrhosis and even liver cancer. Significant progress has been made to understand key events and molecular players for the onset and progression of alcoholic liver disease from both experimental and clinical alcohol studies. No successful treatments are currently available for treating alcoholic liver disease; therefore, development of novel pathophysiological-targeted therapies is urgently needed. This review summarizes the recent progress on animal models used to study alcoholic liver disease and the detrimental factors that contribute to alcoholic liver disease pathogenesis including miRNAs, S-adenosylmethionine, Zinc deficiency, cytosolic lipin-1β, IRF3-mediated apoptosis, RIP3-mediated necrosis and hepcidin. In addition, we summarize emerging adaptive protective effects induced by alcohol to attenuate alcohol-induced liver pathogenesis including FoxO3, IL-22, autophagy and nuclear lipin-1α.

Necroptosis: An emerging type of cell death in liver diseases

Cell death has been extensively evaluated for decades and it is well recognized that pharmacological interventions directed to inhibit cell death can prevent significant cell loss and can thus improve an organ’s physiological function. For long, only apoptosis was considered as a sole form of programmed cell death. Recently necroptosis, a RIP1/RIP3-dependent programmed cell death, has been identified as an apoptotic backup cell death mechanism with necrotic morphology. The evidences of necroptosis and protective effects achieved by blocking necroptosis have been extensively reported in recent past. However, only a few studies reported the evidence of necroptosis and protective effects achieved by inhibiting necroptosis in liver related disease conditions. Although the number of necroptosis initiators is increasing; however, interestingly, it is still unclear that what actually triggers necroptosis in different liver diseases or if there is always a different necroptosis initiator in each specific disease condition followed by specific downstream signaling molecules. Understanding the precise mechanism of necroptosis as well as counteracting other cell death pathways in liver diseases could provide a useful insight towards achieving extensive therapeutic significance. By targeting necroptosis and/or other parallel death pathways, a significant cell loss and thus a decrement in an organ’s physiological function can be prevented.

Acetaminophen-induced Liver Injury: from Animal Models to Humans

Drug-induced liver injury is an important clinical problem and a challenge for drug development. Whereas progress in understanding rare and unpredictable (idiosyncratic) drug hepatotoxicity is severely hampered by the lack of relevant animal models, enormous insight has been gained in the area of predictable hepatotoxins, in particular acetaminophen-induced liver injury, from a broad range of experimental models. Importantly, mechanisms of toxicity obtained with certain experimental systems, such as in vivo mouse models, primary mouse hepatocytes, and metabolically competent cell lines, are being confirmed in translational studies in patients and in primary human hepatocytes. Despite this progress, suboptimal models are still being used and experimental data can be confusing, leading to controversial conclusions. Therefore, this review attempts to discuss mechanisms of drug hepatotoxicity using the most studied drug acetaminophen as an example. We compare the various experimental models that are used to investigate mechanisms of acetaminophen hepatotoxicity, discuss controversial topics in the mechanisms, and assess how these experimental findings can be translated to the clinic. The success with acetaminophen in demonstrating the clinical relevance of experimental findings could serve as an example for the study of other drug toxicities.

Benzyl alcohol attenuates acetaminophen-induced acute liver injury in a Toll-like receptor-4-dependent pattern in mice

Acetaminophen (APAP) toxicity is the most common cause of acute liver failure in industrialized countries. Understanding the mechanisms of APAP-induced liver injury as well as other forms of sterile liver injury is critical to improve the care of patients. Recent studies demonstrate that danger signaling and inflammasome activation play a role in APAP-induced injury. The aim of these investigations was to test the hypothesis that benzyl alcohol (BA) is a therapeutic agent that protects against APAP-induced liver injury by modulation of danger signaling. APAP-induced liver injury was dependent, in part, on Toll-like receptor (TLR)9 and receptor for advanced glycation endproducts (RAGE) signaling. BA limited liver injury over a dose range of 135-540 μg/g body weight or when delivered as a pre-, concurrent, or post-APAP therapeutic. Furthermore, BA abrogated APAP-induced cytokines and chemokines as well as high-mobility group box 1 release. Moreover, BA prevented APAP-induced inflammasome signaling as determined by interleukin (IL)-1β, IL-18, and caspase-1 cleavage in liver tissues. Interestingly, the protective effects of BA on limiting liver injury and inflammasome activation were dependent on TLR4 signaling, but not TLR2 or CD14. Cell-type-specific knockouts of TLR4 were utilized to further determine the protective mechanisms of BA. These studies found that TLR4 expression specifically in myeloid cells (LyzCre-tlr4-/-) were necessary for the protective effects of BA.

CONCLUSION

BA protects against APAP-induced acute liver injury and reduced inflammasome activation in a TLR4-dependent manner. BA may prove to be a useful adjunct in the treatment of APAP and other forms of sterile liver injury.

Mechanisms of acetaminophen-induced cell death in primary human hepatocytes

Acetaminophen (APAP) overdose is the most prevalent cause of drug-induced liver injury in western countries. Numerous studies have been conducted to investigate the mechanisms of injury after APAP overdose in various animal models; however, the importance of these mechanisms for humans remains unclear. Here we investigated APAP hepatotoxicity using freshly isolated primary human hepatocytes (PHH) from either donor livers or liver resections. PHH were exposed to 5mM, 10mM or 20mM APAP over a period of 48 h and multiple parameters were assessed. APAP dose-dependently induced significant hepatocyte necrosis starting from 24h, which correlated with the clinical onset of human liver injury after APAP overdose. Interestingly, cellular glutathione was depleted rapidly during the first 3h. APAP also resulted in early formation of APAP-protein adducts (measured in whole cell lysate and in mitochondria) and mitochondrial dysfunction, indicated by the loss of mitochondrial membrane potential after 12h. Furthermore, APAP time-dependently triggered c-Jun N-terminal kinase (JNK) activation in the cytosol and translocation of phospho-JNK to the mitochondria. Both co-treatment and post-treatment (3h) with the JNK inhibitor SP600125 reduced JNK activation and significantly attenuated cell death at 24h and 48h after APAP. The clinical antidote N-acetylcysteine offered almost complete protection even if administered 6h after APAP and a partial protection when given at 15 h.

CONCLUSION

These data highlight important mechanistic events in APAP toxicity in PHH and indicate a critical role of JNK in the progression of injury after APAP in humans. The JNK pathway may represent a therapeutic target in the clinic.

Mechanistic biomarkers in acetaminophen-induced hepatotoxicity and acute liver failure: from preclinical models to patients

INTRODUCTION

Drug hepatotoxicity is a major clinical issue. Acetaminophen (APAP) overdose is especially common. Serum biomarkers used to follow patient progress reflect either liver injury or function, but focus on biomarkers that can provide insight into the basic mechanisms of hepatotoxicity is increasing and enabling us to translate mechanisms of toxicity from animal models into humans.

AREAS COVERED

We review recent advances in mechanistic serum biomarker research in drug hepatotoxicity. Specifically, biomarkers for reactive drug intermediates, mitochondrial dysfunction, nuclear DNA damage, mode of cell death and inflammation are discussed, as well as microRNAs. Emphasis is placed on APAP-induced liver injury.

EXPERT OPINION

Several serum biomarkers of reactive drug intermediates, mitochondrial damage, nuclear DNA damage, apoptosis and necrosis and inflammation have been described. These studies have provided evidence that mitochondrial damage is critical in APAP hepatotoxicity in humans, while apoptosis has only a minor role, and inflammation is important for recovery and regeneration after APAP overdose. Additionally, mechanistic serum biomarkers have been shown to predict outcome as well as, or better than, some clinical scores. In the future, such biomarkers will help determine the need for liver transplantation and, with improved understanding of the human pathophysiology, identify novel therapeutic targets.

Protein kinase C (PKC) participates in acetaminophen hepatotoxicity through c-jun-N-terminal kinase (JNK)-dependent and -independent signaling pathways

This study examines the role of protein kinase C (PKC) and AMP-activated kinase (AMPK) in acetaminophen (APAP) hepatotoxicity. Treatment of primary mouse hepatocytes with broad-spectrum PKC inhibitors (Ro-31-8245, Go6983), protected against APAP cytotoxicity despite sustained c-jun-N-terminal kinase (JNK) activation. Broad-spectrum PKC inhibitor treatment enhanced p-AMPK levels and AMPK regulated survival-energy pathways including autophagy. AMPK inhibition by compound C or activation using an AMPK activator oppositely modulated APAP cytotoxicity, suggesting that p-AMPK and AMPK regulated energy survival pathways, particularly autophagy, play a critical role in APAP cytotoxicity. Ro-31-8245 treatment in mice up-regulated p-AMPK levels, increased autophagy (i.e., increased LC3-II formation, p62 degradation), and protected against APAP-induced liver injury, even in the presence of sustained JNK activation and translocation to mitochondria. In contrast, treatment of hepatocytes with a classical PKC inhibitor (Go6976) protected against APAP by inhibiting JNK activation. Knockdown of PKC-α using antisense (ASO) in mice also protected against APAP-induced liver injury by inhibiting JNK activation. APAP treatment resulted in PKC-α translocation to mitochondria and phosphorylation of mitochondrial PKC substrates. JNK 1 and 2 silencing in vivo decreased APAP-induced PKC-α translocation to mitochondria, suggesting PKC-α and JNK interplay in a feed-forward mechanism to mediate APAP-induced liver injury.

CONCLUSION

PKC-α and other PKC(s) regulate death (JNK) and survival (AMPK) proteins, to modulate APAP-induced liver injury.

Mixed lineage kinase 3 deficient mice are protected against the high fat high carbohydrate diet-induced steatohepatitis

BACKGROUND & AIMS

C-Jun N-terminal kinase (JNK) activation is pivotal in the development of nonalcoholic steatohepatitis (NASH). Mixed lineage kinase 3 (MLK) 3 is one of the mitogen activated protein kinase kinase kinase (MAP3K) that mediates JNK activation in the liver. Despite this concept, the role of MLK3 in modulating liver injury during nutrient excess has not been explored. Our aim was to determine if MLK3 deficient mice were protected against high fat high carbohydrate (HFHC) diet-induced NASH.

METHODS

We employed eight-week-old Mlk3(-/-) male C57BL/6J mice, and wild type (WT) mice C57BL/6J as controls. Mice were fed a HFHC or a chow diet adlib for 16 weeks.

RESULTS

Hepatic JNK activating phosphorylation was readily absent in the Mlk3(-/-) mice fed the HFHC diet, but not in WT mice. This inhibition of JNK activation was hepatoprotective. Despite a comparable increase in weight gain, hepatic steatosis by histological examination and hepatic triglyceride quantification was reduced in HFHC diet-fed Mlk3(-/-) mice compared with WT mice. In addition, compared with the WT mice, HFHC diet-fed Mlk3(-/-) mice had significantly attenuated liver injury as manifested by reduced ALT levels, hepatocyte apoptosis, markers of hepatic inflammation and indices of hepatic fibrogenesis.

CONCLUSION

Our results suggest that loss of MLK3 in mice is protective against HFHC diet-induced NASH, in a weight-independent fashion, through attenuation of JNK activation. MLK3 is a potential therapeutic target for the treatment of human NASH.

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.

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

Acetaminophen (APAP) overdose is a major cause of hepatotoxicity and acute liver failure in the U.S., but the pathophysiology is incompletely understood. Despite evidence for apoptotic signaling, hepatic cell death after APAP is generally considered necrotic in mice and in humans. Recent findings suggest that the receptor interacting protein kinase 3 (RIP3) acts as a switch from apoptosis to necrosis (programmed necrosis). Thus, the aim of the current investigation was to determine if RIP3 is involved in APAP-induced liver cell death. APAP (200-300 mg/kg) caused glutathione depletion and protein adduct formation, oxidant stress, mitochondrial release of apoptosis inducing factor, and nuclear DNA fragmentation resulting in centrilobular necrosis in C57Bl/6J mice. Inhibiting RIP3 protein induction with antisense morpholinos in wild-type animals or using RIP3-deficient mice had no effect on protein adduct formation but attenuated all other parameters, including necrotic cell death, at 6 hours after APAP. In addition, cultured hepatocytes from RIP3-deficient mice showed reduced injury compared to wild-type cells after 24 hours. Interestingly, APAP-induced mitochondrial translocation of dynamin-related protein 1 (Drp1), the initiator of mitochondrial fission, was inhibited by reduced RIP3 protein expression and the Drp1 inhibitor MDIVI reduced APAP-induced cell death at 24 hours. All of these protective effects were lost after 24 hours in vivo or 48 hours in vitro.

CONCLUSION

RIP3 is an early mediator of APAP hepatotoxicity, involving modulation of mitochondrial dysfunction and oxidant stress. Controlling RIP3 expression could be a promising new approach to reduce APAP-induced liver injury, but requires complementary strategies to control mitochondrial dysfunction for long-term protection.

Mechanisms of drug-induced liver injury

Drug-induced liver injury (DILI) represents a broad spectrum of liver manifestations. However, the most common manifestation is hepatocyte death following drug intake. DILI can be predictable and dose dependent with a notable example of acetaminophen toxicity. Idiosyncratic DILI occurs in an unpredictable fashion at low frequencies, implying that environmental and genetic factors alter the susceptibility of individuals to the insult (drugs).

Circulating acylcarnitines as biomarkers of mitochondrial dysfunction after acetaminophen overdose in mice and humans

Acetaminophen (APAP) is a widely used analgesic. However, APAP overdose is hepatotoxic and is the primary cause of acute liver failure in the developed world. The mechanism of APAP-induced liver injury begins with protein binding and involves mitochondrial dysfunction and oxidative stress. Recent efforts to discover blood biomarkers of mitochondrial damage have identified increased plasma glutamate dehydrogenase activity and mitochondrial DNA concentration in APAP overdose patients. However, a problem with these markers is that they are too large to be released from cells without cell death or loss of membrane integrity. Metabolomic studies are more likely to reveal biomarkers that are useful at early time points, before injury begins. Similar to earlier work, our metabolomic studies revealed that acylcarnitines are elevated in serum from mice after treatment with toxic doses of APAP. Importantly, a comparison with furosemide demonstrated that increased serum acylcarnitines are specific for mitochondrial dysfunction. However, when we measured these compounds in plasma from humans with liver injury after APAP overdose, we could not detect any significant differences from control groups. Further experiments with mice showed that N-acetylcysteine, the antidote for APAP overdose in humans, can reduce acylcarnitine levels in serum. Altogether, our data do not support the clinical measurement of acylcarnitines in blood after APAP overdose due to the standard N-acetylcysteine treatment in patients, but strongly suggest that acylcarnitines would be useful mechanistic biomarkers in other forms of liver injury involving mitochondrial dysfunction.

Hepatitis C virus NS5A inhibits mixed lineage kinase 3 to block apoptosis

Hepatitis C virus (HCV) infection results in the activation of numerous stress responses including oxidative stress, with the potential to induce an apoptotic state. Previously we have shown that HCV attenuates the stress-induced, p38MAPK-mediated up-regulation of the K(+) channel Kv2.1, to maintain the survival of infected cells in the face of cellular stress. We demonstrated that this effect was mediated by HCV non-structural 5A (NS5A) protein, which impaired p38MAPK activity through a polyproline motif-dependent interaction, resulting in reduction of phosphorylation activation of Kv2.1. In this study, we investigated the host cell proteins targeted by NS5A to mediate Kv2.1 inhibition. We screened a phage-display library expressing the entire complement of human SH3 domains for novel NS5A-host cell interactions. This analysis identified mixed lineage kinase 3 (MLK3) as a putative NS5A interacting partner. MLK3 is a serine/threonine protein kinase that is a member of the MAPK kinase kinase (MAP3K) family and activates p38MAPK. An NS5A-MLK3 interaction was confirmed by co-immunoprecipitation and Western blot analysis. We further demonstrate a novel role of MLK3 in the modulation of Kv2.1 activity, whereby MLK3 overexpression leads to the up-regulation of channel activity. Accordingly, coexpression of NS5A suppressed this stimulation. Additionally we demonstrate that overexpression of MLK3 induced apoptosis, which was also counteracted by NS5A. We conclude that NS5A targets MLK3 with multiple downstream consequences for both apoptosis and K(+) homeostasis.

Plasma and liver acetaminophen-protein adduct levels in mice after acetaminophen treatment: dose-response, mechanisms, and clinical implications

At therapeutic doses, acetaminophen (APAP) is a safe and effective analgesic. However, overdose of APAP is the principal cause of acute liver failure in the West. Binding of the reactive metabolite of APAP (NAPQI) to proteins is thought to be the initiating event in the mechanism of hepatotoxicity. Early work suggested that APAP-protein binding could not occur without glutathione (GSH) depletion, and likely only at toxic doses. Moreover, it was found that protein-derived APAP-cysteine could only be detected in serum after the onset of liver injury. On this basis, it was recently proposed that serum APAP-cysteine could be used as diagnostic marker of APAP overdose. However, comprehensive dose-response and time course studies have not yet been done. Furthermore, the effects of co-morbidities on this parameter have not been investigated. We treated groups of mice with APAP at multiple doses and measured liver GSH and both liver and plasma APAP-protein adducts at various timepoints. Our results show that protein binding can occur without much loss of GSH. Importantly, the data confirm earlier work that showed that protein-derived APAP-cysteine can appear in plasma without liver injury. Experiments performed in vitro suggest that this may involve multiple mechanisms, including secretion of adducted proteins and diffusion of NAPQI directly into plasma. Induction of liver necrosis through ischemia-reperfusion significantly increased the plasma concentration of protein-derived APAP-cysteine after a subtoxic dose of APAP. While our data generally support the measurement of serum APAP-protein adducts in the clinic, caution is suggested in the interpretation of this parameter.

Novel phosphorylation and ubiquitination sites regulate reactive oxygen species-dependent degradation of anti-apoptotic c-FLIP protein

The cytosolic protein c-FLIP (cellular Fas-associated death domain-like interleukin 1β-converting enzyme inhibitory protein) is an inhibitor of death receptor-mediated apoptosis that is up-regulated in a variety of cancers, contributing to apoptosis resistance. Several compounds found to restore sensitivity of cancer cells to TRAIL, a TNF family death ligand with promising therapeutic potential, act by targeting c-FLIP ubiquitination and degradation by the proteasome. The generation of reactive oxygen species (ROS) has been implicated in c-FLIP protein degradation. However, the mechanism by which ROS post-transcriptionally regulate c-FLIP protein levels is not well understood. We show here that treatment of prostate cancer PPC-1 cells with the superoxide generators menadione, paraquat, or buthionine sulfoximine down-regulates c-FLIP long (c-FLIP(L)) protein levels, which is prevented by the proteasome inhibitor MG132. Furthermore, pretreatment of PPC-1 cells with a ROS scavenger prevented ubiquitination and loss of c-FLIP(L) protein induced by menadione or paraquat. We identified lysine 167 as a novel ubiquitination site of c-FLIP(L) important for ROS-dependent degradation. We also identified threonine 166 as a novel phosphorylation site and demonstrate that Thr-166 phosphorylation is required for ROS-induced Lys-167 ubiquitination. The mutation of either Thr-166 or Lys-167 was sufficient to stabilize c-FLIP protein levels in PPC-1, HEK293T, and HeLa cancer cells treated with menadione or paraquat. Accordingly, expression of c-FLIP T166A or K167R mutants protected cells from ROS-mediated sensitization to TRAIL-induced cell death. Our findings reveal novel ROS-dependent post-translational modifications of the c-FLIP protein that regulate its stability, thus impacting sensitivity of cancer cells to TRAIL.

Metabolism and disposition of acetaminophen: recent advances in relation to hepatotoxicity and diagnosis

Acetaminophen (APAP) is one of the most widely used drugs. Though safe at therapeutic doses, overdose causes mitochondrial dysfunction and centrilobular necrosis in the liver. The first studies of APAP metabolism and activation were published more than 40 years ago. Most of the drug is eliminated by glucuronidation and sulfation. These reactions are catalyzed by UDP-glucuronosyltransferases (UGT1A1 and 1A6) and sulfotransferases (SULT1A1, 1A3/4, and 1E1), respectively. However, some is converted by CYP2E1 and other cytochrome P450 enzymes to a reactive intermediate that can bind to sulfhydryl groups. The metabolite can deplete liver glutathione (GSH) and modify cellular proteins. GSH binding occurs spontaneously, but may also involve GSH-S-transferases. Protein binding leads to oxidative stress and mitochondrial damage. The glucuronide, sulfate, and GSH conjugates are excreted by transporters in the canalicular (Mrp2 and Bcrp) and basolateral (Mrp3 and Mrp4) hepatocyte membranes. Conditions that interfere with metabolism and metabolic activation can alter the hepatotoxicity of the drug. Recent data providing novel insights into these processes, particularly in humans, are reviewed in the context of earlier work, and the effects of altered metabolism and reactive metabolite formation are discussed. Recent advances in the diagnostic use of serum adducts are covered.

Regulation of drug-induced liver injury by signal transduction pathways: critical role of mitochondria

Drugs that cause liver injury often 'stress' mitochondria and activate signal transduction pathways important in determining cell survival or death. In most cases, hepatocytes adapt to the drug-induced stress by activating adaptive signaling pathways, such as mitochondrial adaptive responses and nuclear factor erythroid 2-related factor 2 (Nrf-2), a transcription factor that upregulates antioxidant defenses. Owing to adaptation, drugs alone rarely cause liver injury, with acetaminophen (APAP) being the notable exception. Drug-induced liver injury (DILI) usually involves other extrinsic factors, such as the adaptive immune system, that cause 'stressed' hepatocytes to become injured, leading to idiosyncratic DILI, the rare and unpredictable adverse drug reaction in the liver. Hepatocyte injury, due to drug and extrinsic insult, causes a second wave of signaling changes associated with adaptation, cell death, and repair. If the stress and injury reach a critical threshold, then death signaling pathways such as c-Jun N-terminal kinase (JNK) become dominant and hepatocytes enter a failsafe mode to undergo self-destruction. DILI can be seen as an active process involving recruitment of death signaling pathways that mediate cell death rather than a passive process due to overwhelming biochemical injury. In this review, we highlight the role of signal transduction pathways, which frequently involve mitochondria, in the development of DILI.