Mitochondria are present in all mammalian cells except matured red blood cells. Mitochondria consist of several metabolic pathways for glucose, fatty acids, amino acids, and bioenergetic pathways for ATP synthesis, membrane potential, and reactive oxygen production. In the liver, hepatic mitochondria play a key role in hepatic steatosis because mitochondrial metabolism produces acetyl-CoA which is the building block for synthesis of lipids and cholesterol. Mitochondria inner membrane is impermeable of metabolites, reducing equivalents, and small molecules such as phosphate, and sulfate. Thus, mitochondrial shuttles and carriers function as the routes of influx and efflux of these metabolites and molecules across the inner membrane. The signal regulation of these shuttles and mitochondrial enzymes could play a key role in coordinating the mitochondrial metabolism to adapt the cytosolic part of metabolic pathways in liver metabolic stress. Intriguingly, the interaction of mitochondria protein SH3 domain-binding protein 5 (SAB/SH3BP5) and c-Jun N-terminal kinase (JNK) was found as a pivotal role in sustained activation of JNK and phosphorylated-JNK (P-JNK) mediated activation of lipogenic pathway in nutritional excess. Knockout or knockdown of SAB prevented or reversed the hepatic steatosis, inflammation, and fibrosis, and improved metabolic intolerance and energy expenditure. Moreover, blocking the SAB peptide prevents palmitic acid-induced P-JNK interaction with SAB and inhibition of mitochondrial bioenergetics, implying the P-JNK effect on mitochondrial metabolism. This review focuses on the flow of mitochondrial metabolites in metabolic stress conditions and the contribution of mitochondria and mitochondrial stress signals in hepatic steatosis.

Background: Hepatic lipoprotein receptor-related protein 1 (LRP-1) plays a central role in peripheral amyloid beta (A ss) clearance, but its importance in Alzheimer's disease (AD) pathology is understudied. Our previous work showed that intragastric alcohol feeding to C57BL/6 J mice reduced hepatic LRP-1 expression which correlated with significant AD-relevant brain changes. Herein, we examined the role of hepatic LRP-1 in AD pathogenesis in APP/PS1 AD mice using two approaches to modulate hepatic LRP-1, intragastric alcohol feeding to model chronic heavy drinking shown by us to reduce hepatic LRP-1, and hepato-specific LRP-1 silencing. Methods: Eight-month-old male APP/PS1 mice were fed ethanol or control diet intragastrically for 5 weeks (n = 7-11/group). Brain and liver A ss were assessed using immunoassays. Three important mechanisms of brain amyloidosis were investigated: hepatic LRP-1 (major peripheral A ss regulator), blood-brain barrier (BBB) function (vascular A ss regulator), and microglia (major brain A ss regulator) using immunoassays. Spatial LRP-1 gene expression in the periportal versus pericentral hepatic regions was confirmed using NanoString GeoMx Digital Spatial Profiler. Further, hepatic LRP-1 was silenced by injecting LRP-1 microRNA delivered by the adenoassociated virus 8 (AAV8) and the hepato-specific thyroxine-binding globulin (TBG) promoter to 4-month-old male APP/PS1 mice (n = 6). Control male APP/PS1 mice received control AAV8 (n = 6). Spatial memory and locomotion were assessed 12 weeks after LRP-1 silencing using Y-maze and open-field test, respectively, and brain and liver A ss were measured. Results: Alcohol feeding reduced plaque-associated microglia in APP/PS1 mice brains and increased aggregated A ss (p < 0.05) by ELISA and 6E10-positive A ss load by immunostaining (p < 0.05). Increased brain A ss corresponded with a significant downregulation of hepatic LRP-1 (p < 0.01) at the protein and transcript level, primarily in pericentral hepatocytes (zone 3) where alcohol-induced injury occurs. Hepato-specific LRP-1 silencing significantly increased brain A ss and locomotion hyperactivity (p < 0.05) in APP/PS1 mice.

Acetaminophen overuse is a common cause of acute liver failure (ALF). During ALF, toxins are metabolized by enzymes such as CYP2E1 and transformed into reactive species, leading to oxidative damage and liver failure. Here, we found that oral magnesium (Mg) alleviated acetaminophen-induced ALF through metabolic changes in gut microbiota that inhibit CYP2E1. The gut microbiota from Mg-supplemented humans prevented acetaminophen-induced ALF in mice. Mg exposure modulated Bifidobacterium metabolism and enriched indole-3-carboxylic acid (I3C) levels. Formate C-acetyltransferase (pflB) was identified as a key Bifidobacterium enzyme involved in I3C generation. Accordingly, a Bifidobacterium pflB knockout showed diminished I3C generation and reduced the beneficial effects of Mg. Conversely, treatment with I3C or an engineered bacteria overexpressing Bifidobacterium pflB protected against ALF. Mechanistically, I3C bound and inactivated CYP2E1, thus suppressing formation of harmful reactive intermediates and diminishing hepatocyte oxidative damage. These findings highlight how interactions between Mg and gut microbiota may help combat ALF

Cell death occurs in various circumstances, such as homeostasis, stress response, and defense, via specific pathways and mechanisms that are regulated by specific activator-induced signal transductions. Among them, Jun N-terminal kinases (JNKs) participate in various aspects, and the recent discovery of JNKs and mitochondrial protein SAB interaction in signal regulation of cell death completes our understanding of the mechanism of sustained activation of JNK (P-JNK), which leads to triggering of the machinery of cell death. This understanding will lead the investigators to discover the modulators facilitating or preventing cell death for therapeutic application in acute or chronic diseases and cancer. We discuss here the mechanism and modulators of the JNK-SAB-ROS activation loop, which is the core component of mitochondria-dependent cell death, specifically apoptosis and mitochondrial permeability transition (MPT)-driven necrosis, and which may also contribute to cell death mechanisms of ferroptosis and pyroptosis. The discussion here is based on the results and evidence discovered from liver disease models, but the JNK-SAB-ROS activation loop to sustain JNK activation is universally applicable to various disease models where mitochondria and reactive oxygen species contribute to the mechanism of disease.

AIMS We confirmed that GCLC protein rapidly decreased at the same time P-JNK increased after APAP treatment. Therefore, our aims were to determine if JNK was directly responsible for decreased causing impaired recovery of GSH and if this was an important factor in determining APAP hepatotoxicity. RESULTS Immunoprecipitation of JNK after APAP identified binding to GCLC. Expression of a site directed mutated canonical JNK docking site in GCLC was resistant to degradation and lead to rapid restoration of GSH and inhibited sustained JNK activation. The JNK-resistant GCLC markedly protected against necrosis and ALT elevation. The proteolytic loss of GCLC was abrogated by inhibition of the proteosome, ubiquitination, or calpain. INNOVATION We addressed the aims by preparing mutated Gclc resistant to JNK induced degradation. The results allowed us to identify impaired GSH recovery as an important contributor to early progression of APAP toxicity after the metabolism of APAP and initial GSH depletion had occurred. CONCLUSION Activated JNK interacts directly with GCLC and leads to proteolytic degradation of GCLC. Degradation of GCLC impairs GSH recovery after APAP allowing the continued activation of JNK. Conversely, rapid recovery of GSH inhibits the sustained activation of the MAP Kinase cascade and dampens APAP toxicity by suppressing the continued activation of JNK.

Non-alcoholic fatty liver (NAFL) is the most common chronic liver disease. Activation of mitogen-activated kinases (MAPK) cascade, which leads to c-Jun N-terminal kinase (JNK) activation occurs in the liver in response to the nutritional and metabolic stress. The aberrant activation of MAPKs, especially c-Jun-N-terminal kinases (JNKs), leads to unwanted genetic and epi-genetic modifications in addition to the metabolic stress adaptation in hepatocytes. A mechanism of sustained P-JNK activation was identified in acute and chronic liver diseases, suggesting an important role of aberrant JNK activation in NASH. Therefore, modulation of JNK activation, rather than targeting JNK protein levels, is a plausible therapeutic application for the treatment of chronic liver disease.

Background and Aims The hepatic mitogen‐activated protein kinase (MAPK) cascade leading to c‐Jun N‐terminal kinase (JNK) activation has been implicated in the pathogenesis of nonalcoholic fatty liver (NAFL)/NASH. In acute hepatotoxicity, we previously identified a pivotal role for mitochondrial SH3BP5 (SAB; SH3 homology associated BTK binding protein) as a target of JNK, which sustains its activation through promotion of reactive oxygen species production. Therefore, we assessed the role of hepatic SAB in experimental NASH and metabolic syndrome. Approach and Results In mice fed high‐fat, high‐calorie, high‐fructose (HFHC) diet, SAB expression progressively increased through a sustained JNK/activating transcription factor 2 (ATF2) activation loop. Inducible deletion of hepatic SAB markedly decreased sustained JNK activation and improved systemic energy expenditure at 8 weeks followed by decreased body fat at 16 weeks of HFHC diet. After 30 weeks, mice treated with control–antisense oligonucleotide (control‐ASO) developed steatohepatitis and fibrosis, which was prevented by Sab‐ASO treatment. Phosphorylated JNK (p‐JNK) and phosphorylated ATF2 (p‐ATF2) were markedly attenuated by Sab‐ASO treatment. After 52 weeks of HFHC feeding, control N‐acetylgalactosamine antisense oligonucleotide (GalNAc‐Ctl‐ASO) treated mice fed the HFHC diet exhibited progression of steatohepatitis and fibrosis, but GalNAc‐Sab‐ASO treatment from weeks 40 to 52 reversed these findings while decreasing hepatic SAB, p‐ATF2, and p‐JNK to chow‐fed levels. Conclusions Hepatic SAB expression increases in HFHC diet–fed mice. Deletion or knockdown of SAB inhibited sustained JNK activation and steatohepatitis, fibrosis, and systemic metabolic effects, suggesting that induction of hepatocyte Sab is an important driver of the interplay between the liver and the systemic metabolic consequences of overfeeding. In established NASH, hepatocyte‐targeted GalNAc‐Sab‐ASO treatment reversed steatohepatitis and fibrosis.

See Article on Page 1787 Potential conflict of interest: Nothing to report. Oxidative stress refers to the consequences of exposure to reactive oxygen species. In many disease conditions, interruption along the mitochondrial electron transport chain (ETC) leads to build‐up of electrons, which are transferred to O2. Reactive oxygen species (ROS) then can oxidize any chemical constituent of cells. This causes thiol‐disulfide redox stress or imbalance, which activates or inhibits many enzymes and signal transduction pathways. Countering oxidative/redox stress is antioxidant defense, which detoxifies reactive oxygen. The transcription factor, nuclear erythroid 2 p45‐related factor 2 (Nrf2), regulates the expression of many enzymes of defense. Nrf2 is regulated by two mechanisms.(1) Keap1 is an ubiquitin ligase adaptor. The Neh2 domain of Nrf2 binds to Keap1 in the cytoplasm, allowing an E3‐ubiquitylation complex to facilitate proteasomal degradation of Nrf2.(1) Following redox stress or binding of toxic metabolites, Keap1 releases Nrf2, which translocates to the nucleus and activates transcription of ARE‐driven genes. However, Nrf2 degradation can occur in a Keap1‐independent fashion through the binding of β‐TrCP. β‐TrCP is redox‐independent and is activated by phosphorylation of Nrf2 in the Neh6 domain (such as by glycogen synthase kinase 3β [GSK‐3β]).(1) β‐TrCP binds an E3‐ubiquitylation complex, which promotes proteasomal degradation. Acetaminophen (APAP) hepatotoxicity is an excellent model for examining the consequences of oxidative stress/redox imbalance. A small portion of an APAP dose is converted to N‐acetyl‐p‐quinone imine (NAPQI), a reactive metabolite that is selectively detoxified by glutathione (GSH). At higher doses, sufficient NAPQI is generated to deplete GSH in cytoplasm and mitochondria; the remaining NAPQI is free to covalently bind to protein thiols. In mitochondria, the combination of GSH depletion and covalent binding exposes mitochondria and the cytoplasm to ROS. This early stage of APAP toxicity occurs very rapidly in the mouse model, and the effect reaches maximum with an hour. ROS species released into the cytoplasm activate the redox‐sensitive mitogen‐activated protein kinase (MAPK) kinase cascade through ASK1 or MLK3, leading to c‐Jun N‐terminal kinase (JNK) activation (P‐JNK). The latter binds to and phosphorylates the mitochondrial outer membrane protein, SAB (Sh3bp5). Analogous to a receptor, SAB then activates an intramitochondrial signal transduction pathway, leading to impairment of the ETC and enhanced ROS production and release, and continued JNK activation in a sustained cycle.(2) The continued production of mitochondrial ROS ultimately leads to complete collapse of mitochondrial function and lytic necrosis.(3) However, there is a time window between APAP metabolism and the onset of necrosis when the situation may be reversible. This is well‐demonstrated in the mouse model by the efficacy of delayed administration of N‐acetylcysteine (NAC) or JNK inhibitor up to 1.5‐2 hours after APAP administration but not beyond.(4,5) NAC is a precursor for de novo synthesis of GSH, which very early on enhances detoxification of NAPQI, but in the window period NAC accelerates GSH recovery and detoxifies ROS, interrupting sustained activation of the MAPK pathway (Fig. 1). Therefore, this raises the question of the potential contribution of activation of antioxidant defense mediated by Nrf2 in dampening oxidative stress in the interval between early APAP metabolism, covalent binding and initial GSH depletion, and the commitment to cell death.Fig. 1: Interplay between JNK and Nrf2 degradation in APAP hepatotoxicity. APAP induces mitochondrial ROS production, activating JNK that further amplifies the ROS production, sustaining JNK activation. Oxidative stress leads to release of Nrf2 from KEAP1 in the cytoplasm and P‐JNK mediates Nrf2 degradation in the nucleus. The latter inhibits dampening of oxidative stress. The key issue is the rapidity of these responses in the interval phase between APAP metabolism and onset and progression of cell death. Abbreviations: KD, knockdown; MPT, mitochondrial permeability transition; NAC, N‐acetylcysteine.Chen et al.(6) have now shown convincingly that Nrf2 can be added to the large number of proteins that are phosphorylated by JNK. In an elegant series of experiments in A549 cells that lack functional Keap1, they identified the JNK phosphorylation site (serine335) in the Neh6 region of Nrf2, distinct from the Keap1 binding region in Neh2,(1) permitting β‐TrCP‐E3 ubiquitylation complex to bind to Nrf2 followed by proteasomal degradation. Thus, two counteracting mechanisms may occur in toxic stress: redox modification of Keap1, which favors Nrf2 activation of the gene programs for defense, and P‐JNK‐mediated degradation of Nrf2 that impairs defense. Chen et al.(6) demonstrated that inhibitors or knockdown of JNK prevented APAP hepatotoxicity and stabilized Nrf2. The key question is whether these two outcomes of JNK inhibition are causally linked. In approaching this question, it is very important to distinguish pre‐existent effects of Nrf2, as exemplified by the previously identified effect of Nrf2 knockout versus the effect of APAP on the response of Nrf2 from its basal condition, leading to increased transcripts and increased proteins of the antioxidant gene repertoire with sufficient rapidity to dampen the oxidative stress in the window period after APAP treatment. Chen et al.(6) showed that while the Nrf2 level actually increased, ARE‐dependent gene messenger RNAs (mRNAs) were suppressed by 6 hours, although actually up‐regulated in the first 3 hours after APAP. At 6 hours, the authors observed that despite the decline in mRNAs, the respective proteins they examined in the Nrf2 repertoire remained unaffected. However, nontranscriptional effects of P‐JNK on many targets are known to occur very rapidly and promote necrosis, which is already well‐established in the first 6 hours. By the time peak necrosis had occurred (24 hours), the expression of antioxidant proteins had declined. Therefore, it is conceivable that in the interval between 6 hours after APAP, when injury was approximately half maximum until peak injury, the declining expression of Nrf2‐responsive genes might contribute to the progression to peak injury. However, this remains unproven. Alternatively, because the antioxidant response usually accompanies cell proliferation, the late decline in Nrf2‐regulated genes might impair recovery. It should be noted that Nrf2 regulates expression of many antioxidant and detoxification genes, only some of which were examined by Chen et al.(5) Thus, it remains possible that selective decrease of antioxidative proteins due to faster protein turnover in the absence of continued synthesis may contribute in the early phase. Proteins of potential importance include GSH peroxidase, peroxiredoxins, thioredoxin, thioredoxin reductase, sulforedoxin, ferritin, heme oxygenase, MKP‐1, and GSH synthetic enzymes. The authors did examine the GCLC protein, which was not significantly changed at 6 hours after APAP. This is a key enzyme in de novo GSH synthesis and mediates GSH recovery in this model. Others have found increased GCLC mRNA up to 6 hours after APAP, but rapid decline in GCLC protein after APAP,(7) presumably contributing to delayed GSH recovery, which impairs GSH dampening of oxidative stress. Chen et al.(6) demonstrated that JNK inhibitor did not protect Nrf2−/− mice from APAP. However, Nrf2−/− mice are highly susceptible to APAP toxicity and have altered basal defense, which may overwhelm the ability of JNK inhibitors to protect.(8) Strikingly, Nrf2−/− mice expressing mutated JNK phosphorylation site were markedly resistant to APAP toxicity. It should be noted that 70% mortality at 6 hours was seen after APAP (300 mg/kg) in the knockout mice, but no mortality at 6 hours after expression of either wild‐type or JNK‐resistant Nrf2. As proof of principle, the results of these experiments are quite striking. However, Nrf2−/− mice are very different from wild‐type mice, so extrapolation is not straightforward. In summary, the paper by Chen et al.(6) provides an important advance, namely the demonstration that Nrf2 is a target substrate of JNK, which leads to degradation of Nrf2. The impact of this on acute APAP toxicity requires more work with respect to its contribution to various stages of injury onset and progression in the rapid sequence of events during the time course of acute APAP injury. Nevertheless, the implications of JNK‐mediated Nrf2 degradation have much broader implications in the pathophysiology of diseases. The determinants of the balance between Nrf2 activation by release from Keap1‐mediated degradation versus activation of JNK‐mediated Nrf2 degradation appear to be a pivotal aspect of the response to oxidative stress.

Over the last few decades, intestinal microbial communities have been considered to play a vital role in host liver health. Acute liver injury (ALI) is the manifestation of sudden hepatic injury and arises from a variety of causes. The studies of dysbiosis in gut microbiota provide new insight into the pathogenesis of ALI. However, the relationship of gut microbiota and ALI is not well understood, and the contribution of gut microbiota to ALI has not been well characterized. In this chapter, we integrate several major pathogenic factors in ALI with the role of gut microbiota to stress the significance of gut microbiota in prevention and treatment of ALI.

Acetaminophen (APAP) overdose-induced hepatotoxicity is the leading cause of drug-induced liver injury worldwide. The related injury pathogenesis is mainly focused on the liver. Here, the authors report that gut barrier disruption may also be involved in APAP hepatotoxicity. APAP administration led to gut leakiness and colonic epithelial chemokine (C-C motif) ligand 7 (CCL7) up-regulation. Intestinal epithelial cell (IEC)-specific CCL7 transgenic mice (CCL7(t9IEC) mice) showed markedly increased myosin Light chain kinase phosphorylation, and elevated gut permeability and bacterial translocation into the liver compared to wild-type mice. Global transcriptome analysis revealed that the expression of hepatic proinflammatory genes was enhanced in CCL7(t9IEC) mice compared with wild-type animals. Moreover, CCL7 overexpression in intestinal epithelial cells significantly augmented APAP-induced acute liver injury. These data provide new evidence that dysfunction of CCL7-mediated gut barrier integrity may be an important contributor to APAP-induced hepatotoxicity.

BACKGROUND & AIMS: Acetaminophen (APAP) overdose is a major cause of acute liver failure (ALF). Mitochondrial SH3BP5 (also called SAB) and phosphorylation of c-Jun N-terminal kinase (JNK) mediate the hepatotoxic effects of APAP. We investigated the involvement of steroidogenic acute regulatory protein (STARD1), a mitochondrial cholesterol transporter, in this process and sensitization by valproic acid (VPA), which depletes glutathione and stimulates steroidogenesis. METHODS: Nonfasted C57BL/6J mice (control) and mice with liver-specific deletion of STARD1 (Stard1(Delta Hep)), SAB (Sab(Delta Hep)), or JNK1 and JNK2 (Jnk1+2(Delta Hep)) were given VPA with or without APAP. Liver tissues were collected and analyzed by histology and immunohistochemistry and for APAP metabolism, endoplasmic reticulum (ER) stress, and mitochondrial function. Adult human hepatocytes were transplanted into Fah(-/-)/Rag2(-/-)/Il2rg(-/-)/NOD (FRGN) mice to create mice with humanized livers. RESULTS: Administration of VPA before administration of APAP increased the severity of liver damage in control mice. The combination of VPA and APAP increased expression of CYP2E1, formation of NAPQI-protein adducts, and depletion of glutathione from liver tissues of control mice, resulting in ER stress and the upregulation of STARD1. Livers from control mice given VPA and APAP accumulated cholesterol in the mitochondria and had sustained mitochondrial depletion of glutathione and mitochondrial dysfunction. Inhibition of ER stress, by administration of tauroursodeoxycholic acid to control mice, prevented upregulation of STARD1 in liver and protected the mice from hepatoxicity following administration of VPA and APAP. Administration of N-acetylcysteine to control mice prevented VPA- and APAP-induced ER stress and liver injury. Stard1(Delta Hep) mice were resistant to induction of ALF by VPA and APAP, despite increased mitochondrial levels of glutathione and phosphorylated JNK; we made similar observations in fasted Stard1(Delta Hep) mice given APAP alone. Sab(Delta Hep) mice or Jnk1+2(Delta Hep) mice did not develop ALF following administration of VPA and APAP. The ability of VPA to increase the severity of APAP-induced liver damage was observed in FRGN mice with humanized liver. CONCLUSIONS: In studies of mice, we found that upregulation of STARD1 following ER stress mediates APAP hepatoxicity via SH3BP5 and phosphorylation of JNK1 and JNK2.

Despite tremendous research advancements in nonalcoholic fatty liver disease (NAFLD), our understanding of sex differences in NAFLD remains insufficient. This review summarizes the current knowledge on sex differences in NAFLD, identifies gaps, and discusses important considerations for future research. The prevalence and severity of NAFLD are higher in men than in women during the reproductive age. However, after menopause, NAFLD occurs at a higher rate in women, suggesting that estrogen is protective. Sex differences also exist for the major risk factors of NAFLD. In general, animal models of NAFLD recapitulate the sex differences observed in patients, with more severe steatosis and steatohepatitis, more proinflammatory/profibrotic cytokines, and a higher incidence of hepatic tumors in male than female subjects. Based on computer modeling, female and male livers are metabolically distinct with unique regulators modulating sex-specific metabolic outcomes. Analysis of the literature reveals that most published clinical and epidemiological studies fail to examine sex differences appropriately. Considering the paucity of data on sex differences and the knowledge that regulators of pathways relevant to current therapeutic targets for NAFLD differ by sex, clinical trials should be designed to test drug efficacy and safety according to sex, age, reproductive stage (i.e., menopause), and synthetic hormone use. Conclusion: Sex differences do exist in the prevalence, risk factors, fibrosis, and clinical outcomes of NAFLD, suggesting that, while not yet incorporated, sex will probably be considered in future practice guidelines; adequate consideration of sex differences, sex hormones/menopausal status, age, and other reproductive information in clinical investigation and gene association studies of NAFLD are needed to fill current gaps and implement precision medicine for patients with NAFLD.

SAB is an outer membrane docking protein for JNK mediated impaired mitochondrial function. Deletion of Sab in hepatocytes inhibits sustained JNK activation and cell death. Current work demonstrated that increasing SAB enhanced the severity of APAP liver injury. Female mice were resistant to liver injury and exhibited markedly decreased hepatic SAB protein expression versus males. The mechanism of SAB repression involved a pathway from ERα to p53 expression which induced miR34a-5p. miR34a-5p targeted the Sab mRNA coding region, repressing SAB expression. Fulvestrant or p53 knockdown decreased miR34a-5p and increased SAB in females leading to increased injury from APAP and TNF/galactosamine. In contrast, ERα agonist increased p53 and miR34a-5p which decreased SAB expression and hepatotoxicity in males. Hepatocyte-specific deletion of miR34a also increased severity of liver injury in females, which was prevented by GalNAc-ASO knockdown of Sab. Similar to mice, premenopausal human females also expressed high hepatic p53 and low SAB levels while age-matched males expressed low p53 and high SAB levels, but there was no sex difference of SAB expression in postmenopause. In conclusion, the level of SAB expression determined the severity of JNK dependent liver injury. Females expressed low hepatic SAB protein levels due to an ERα-p53-miR34a pathway which repressed SAB expression, accounting for resistance to liver injury.

Michal Pyzik, Timo Rath, Timothy T. Kuo, Sanda Win, Kristi Baker, Jonathan J. Hubbard, Rosa Grenha, Amit Gandhi, Thomas D. Krämer, Adam R. Mezo, Zachary S. Taylor, Kevin McDonnell, Vicki Nienaber, Jan Terje Andersen, Atsushi Mizoguchi, Laurence Blumberg, Shalaka Purohit, Susan D. Jones,Greg Christianson, Wayne I. Lencer, Inger Sandlie, Neil Kaplowitz, Derry C. Roopenian, and Richard S. Blumberg To whom correspondence should be addressed. Email: rblumberg@bwh.harvard.edu Edited by Lawrence Steinman, Stanford University School of Medicine, Stanford, CA, and approved February 23, 2017 (received for review November 3, 2016)

c-Jun-N-terminal kinase (JNK) activity plays a critical role in modulating cell death, which depends on the level and duration of JNK activation. The kinase cascade from MAPkinase kinase kinase (MAP3K) to MAPkinase kinase (MAP2K) to MAPKinase (MAPK) can be regulated by a number of direct and indirect post-transcriptional modifications, including acetylation, ubiquitination, phosphorylation, and their reversals. Recently, a JNK-mitochondrial SH3-domain binding protein 5 (SH3BP5/SAB)-ROS activation loop has been elucidated, which is required to sustain JNK activity. Importantly, the level of SAB expression in the outer membrane of mitochondria is a major determinant of the set-point for sustained JNK activation. SAB is a docking protein and substrate for JNK, leading to an intramitochondrial signal transduction pathway, which impairs electron transport and promotes reactive oxygen species (ROS) release to sustain the MAPK cascade.

The c‐Jun‐N‐terminal‐kinase (JNK) family is highly conserved across species such as Drosophila, C. elegans , zebrafish and mammals, and plays a central role in hepatic physiologic and pathophysiologic responses. These responses range from cell death to cell proliferation and carcinogenesis, as well as metabolism and survival, depending on the specific context and duration of activation of the JNK signaling pathway. Recently, several investigators identified the key molecules in the JNK activation loop which include apoptosis signal‐regulating kinase (ASK1) and SH3‐domain binding protein 5 (Sab) and their involvement in acute or chronic liver disease models. Thus, regulating JNK activation through modulating the JNK activation loop may represent an important new strategy in the prevention and treatment of acute and chronic liver diseases. In this review, we will discuss the molecular pathophysiology of the JNK activation loop and its role in the pathogenesis of liver diseases. (H epatology 2018;67:2013‐2024).