Roles of phosphatidylinositol 3-kinase and osteopontin in steatosis and aminotransferase release by hepatocytes treated with methionine-choline-deficient medium

Hepatocyte-specific O-GlcNAc transferase downregulation ameliorates nonalcoholic steatohepatitis by improving mitochondrial function

O-GlcNAcylation is a post-translational modification that directly couples the processes of nutrient sensing, metabolism, and signal transduction, affecting protein function and localization, since the O-linked N-acetylglucosamine moiety comes directly from the metabolism of glucose, lipids, and amino acids. De addition and removal of O-GlcNAc of target proteins is mediated by two highly conserved enzymes: O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) and O-GlcNAcase (OGA), respectively. Deregulation of O-GlcNAcylation has been reported to be associated with various human diseases such as cancer, diabetes, and cardiovascular diseases. The contribution of deregulated O-GlcNAcylation to the progression and pathogenesis of NAFLD remains intriguing, and a better understanding of its roles in this pathophysiological context is required to uncover novel avenues for therapeutic intervention. By using a translational approach, our aim is to describe the role of OGT and O-GlcNAcylation in the pathogenesis of NAFLD. We used primary mouse hepatocytes, human hepatic cell lines and in vivo mouse models of steatohepatitis to manipulate O-GlcNAc transferase (OGT). We also studied OGT and O-GlcNAcylation in liver samples from different cohorts of people with NAFLD. O-GlcNAcylation was upregulated in the liver of people and animal models with steatohepatitis. Downregulation of OGT in NAFLD-hepatocytes improved diet-induced liver injury in both in vivo and in vitro models. Proteomics studies revealed that mitochondrial proteins were hyper-O-GlcNAcylated in the liver of mice with steatohepatitis. Inhibition of OGT is able to restore mitochondrial oxidation and decrease hepatic lipid content in in vitro and in vivo models of NAFLD. These results demonstrate that deregulated hyper-O-GlcNAcylation favors NAFLD progression by reducing mitochondrial oxidation and promoting hepatic lipid accumulation.

Targeting Hepatic Glutaminase 1 Ameliorates Non-alcoholic Steatohepatitis by Restoring Very-Low-Density Lipoprotein Triglyceride Assembly

Non-alcoholic steatohepatitis (NASH) is characterized by the accumulation of hepatic fat in an inflammatory/fibrotic background. Herein, we show that the hepatic high-activity glutaminase 1 isoform (GLS1) is overexpressed in NASH. Importantly, GLS1 inhibition reduces lipid content in choline and/or methionine deprivation-induced steatotic mouse primary hepatocytes, in human hepatocyte cell lines, and in NASH mouse livers. We suggest that under these circumstances, defective glutamine fueling of anaplerotic mitochondrial metabolism and concomitant reduction of oxidative stress promotes a reprogramming of serine metabolism, wherein serine is shifted from the generation of the antioxidant glutathione and channeled to provide one-carbon units to regenerate the methionine cycle. The restored methionine cycle can induce phosphatidylcholine synthesis from the phosphatidylethanolamine N-methyltransferase-mediated and CDP-choline pathways as well as by base-exchange reactions between phospholipids, thereby restoring hepatic phosphatidylcholine content and very-low-density lipoprotein export. Overall, we provide evidence that hepatic GLS1 targeting is a valuable therapeutic approach in NASH.

In Utero Dexamethasone Exposure Exacerbates Hepatic Steatosis in Rats That Consume Fructose During Adulthood

Distinct environmental insults might interact with fructose consumption and contribute to the development of metabolic disorders. To address whether in utero glucocorticoid exposure and fructose intake modulate metabolic responses, adult female Wistar rats were exposed to dexamethasone (DEX) during pregnancy, and the offspring were administered fructose at a later time. Briefly, dams received DEX during the third period of pregnancy, while control dams remained untreated. Offspring born to control and DEX-treated mothers were defined as CTL-off and DEX-off, respectively, while untreated animals were designated CTL-off-CTL and DEX-off-CTL. CLT-off and DEX-off treated with 10% fructose in the drinking water for 8 weeks are referred to as CTL-off-FRU and DEX-off-FRU. We found that fructose promoted glucose intolerance and whole-body gluconeogenesis in both CTL-off-FRU and DEX-off-FRU animals. On the other hand, hepatic lipid accumulation was significantly stimulated in DEX-off-FRU rats when compared to the CTL-off-FRU group. The DEX-off-FRU group also displayed impaired very-low-density lipoprotein (VLDL) production and reduced hepatic expression of apoB, mttp, and sec22b. DEX-off-FRU has lower hepatic levels of autophagy markers. Taken together, our results support the unprecedented notion that in utero glucocorticoid exposure exacerbates hepatic steatosis caused by fructose consumption later in life.

Metabolic modulation of acetaminophen-induced hepatotoxicity by osteopontin

Induction of osteopontin (OPN), a well-known pro-inflammatory molecule, has been observed in acetaminophen (APAP)-induced hepatotoxicity. However, the precise cell source for OPN induction and its role during APAP-induced hepatotoxicity has not been fully explored. By employing a hepatotoxic mouse model induced by APAP overdose, we demonstrate that both serum and hepatic OPN levels were significantly elevated in response to APAP treatment. Our in vivo and in vitro studies clearly indicated that the induced expression of hepatic OPN was mainly located in necrosis areas and produced by dying or dead hepatocytes. Functional experiments showed that OPN deficiency protected against the APAP-induced liver injury by inhibiting the toxic APAP metabolism via reducing the expression of the cytochrome P450 family 2 subfamily E member 1 (CYP2E1). Interestingly, this inhibition of CYP2E1 expression did not occur in unfasted Opn^-/- mice, but was significant in fasted Opn^-/- mice and maintained for 2 hours after APAP challenge in fasted Opn^-/- mice. In addition, despite the early protective role of OPN deficiency on APAP-induced hepatotoxicity, OPN deficiency retarded injury resolution by sensitizing hepatocytes to apoptosis and impairing liver regeneration. Finally, we demonstrated that a siRNA-mediated transient hepatic Opn knockdown could sufficiently and significantly protect animals from APAP-induced hepatotoxicity and death. In conclusion, this study clearly defines the cell source of OPN induction in response to APAP treatment, provides a novel insight into the metabolic role of OPN to APAP overdose, and suggests an Opn-targeted therapeutic strategy for the treatment or prevention of APAP-induced hepatotoxicity.

CD18 deficiency improves liver injury in the MCD model of steatohepatitis

Neutrophils and macrophages are important constituents of the hepatic inflammatory infiltrate in non-alcoholic steatohepatitis. These innate immune cells express CD18, an adhesion molecule that facilitates leukocyte activation. In the context of fatty liver, activation of infiltrated leukocytes is believed to enhance hepatocellular injury. The objective of this study was to determine the degree to which activated innate immune cells promote steatohepatitis by comparing hepatic outcomes in wild-type and CD18-mutant mice fed a methionine-choline-deficient (MCD) diet. After 3 weeks of MCD feeding, hepatocyte injury, based on serum ALT elevation, was 40% lower in CD18-mutant than wild-type mice. Leukocyte infiltration into the liver was not impaired in CD18-mutant mice, but leukocyte activation was markedly reduced, as shown by the lack of evidence of oxidant production. Despite having reduced hepatocellular injury, CD18-mutant mice developed significantly more hepatic steatosis than wild-type mice after MCD feeding. This coincided with greater hepatic induction of pro-inflammatory and lipogenic genes as well as a modest reduction in hepatic expression of adipose triglyceride lipase. Overall, the data indicate that CD18 deficiency curbs MCD-mediated liver injury by limiting the activation of innate immune cells in the liver without compromising intrahepatic cytokine activation. Reduced liver injury occurs at the expense of increased hepatic steatosis, which suggests that in addition to damaging hepatocytes, infiltrating leukocytes may influence lipid homeostasis in the liver.

Expression changes of lipid droplets during alcohol-induced liver injury in mice

AIM

To investigate the pathological changes in the process of alcoholic liver injury.

METHODS

Thirty healthy male Kunming mice were randomly divided into a control group (n = 10) and a model group (n = 20). The control group was executed to take the liver tissue at 0 wk. The model group was treated with alcohol of 56 degrees (0.15 ml/20 g•d) for 8 wk, and then executed to take the liver tissue at four and 8 wk. The expression changes of lipid droplets during alcoholic liver injury were assessed by hematoxylin and eosin staining and oil red O staining. The integral optical density of histology samples was analyzed with Image-ProPlus6.0 software.

RESULTS

The content of lipid drops began to increase at 4 wk (20.29 ± 7.07 vs 8.06 ± 2.06, P < 0.01), and significantly increased at 8 wk (34.88 ± 15.33 vs 8.06 ± 2.06, P < 0.01). Compared to that at 4 wk, the amount of expression of lipid droplets showed a rising trend at 8 wk (34.88 ± 15.33 vs 20.29 ± 7.07, P < 0.05).

CONCLUSION

The levels of lipid droplets show a growing trend along with the aggravation of hepatic steatosis during alcoholic liver injury in mice.

CHIP(-/-)-Mouse Liver: Adiponectin-AMPK-FOXO-Activation Overrides CYP2E1-Elicited JNK1-Activation, Delaying Onset of NASH: Therapeutic Implications

Genetic ablation of C-terminus of Hsc70-interacting protein (CHIP) E3 ubiquitin-ligase impairs hepatic cytochrome P450 CYP2E1 degradation. Consequent CYP2E1 gain of function accelerates reactive O2 species (ROS) production, triggering oxidative/proteotoxic stress associated with sustained activation of c-Jun NH2-terminal kinase (JNK)-signaling cascades, pro-inflammatory effectors/cytokines, insulin resistance, progressive hepatocellular ballooning and microvesicular steatosis. Despite this, little evidence of nonalcoholic fatty liver disease (NAFLD)/nonalcoholic steatohepatitis (NASH) was found in CHIP(-/-)-mice over the first 8-9-months of life. We herein document that this lack of tissue injury is largely due to the concurrent up-regulation and/or activation of the adiponectin-5'-AMP-activated protein kinase (AMPK)-forkhead box O (FOXO)-signaling axis stemming from at the least three synergistic features: Up-regulated expression of adipose tissue adiponectin and its hepatic adipoR1/adipoR2 receptors, stabilization of hepatic AMPKα1-isoform, identified herein for the first time as a CHIP-ubiquitination substrate (unlike its AMPKα2-isoform), as well as nuclear stabilization of FOXOs, well-known CHIP-ubiquitination targets. Such beneficial predominance of the adiponectin-AMPK-FOXO-signaling axis over the sustained JNK-elevation and injurious insulin resistance in CHIP(-/-)-livers apparently counteracts/delays rapid progression of the hepatic microvesicular steatosis to the characteristic macrovesicular steatosis observed in clinical NASH and/or rodent NASH-models.

Quercetin ameliorates dysregulation of lipid metabolism genes via the PI3K/AKT pathway in a diet-induced mouse model of nonalcoholic fatty liver disease

SCOPE

Flavonoids and related compounds seem to have favorable effects on nonalcoholic fatty liver disease (NAFLD) progression, although the exact mechanisms implicated are poorly understood. In this study, we aimed to investigate the effect of the flanovol quercetin on gene expression deregulation involved in the development of NAFLD, as well as the possible implication of phosphatidylinositol 3-kinase (PI3K)/AKT pathway modulation.

METHODS AND RESULTS

We used an in vivo model based on methionine- and choline-deficient (MCD) diet-fed mice and an in vitro model consisting of Huh7 cells incubated with MCD medium. MCD-fed mice showed classical pathophysiological characteristics of nonalcoholic steatohepatitis, associated with altered transcriptional regulation of fatty acid uptake- and trafficking-related gene expression, with increased lipoperoxidation. PI3K/AKT pathway was activated by MCD and triggered gene deregulation causing either activation or inhibition of all studied genes as demonstrated through cell incubation with the PI3K-inhibitor LY294002. Treatment with quercetin reduced AKT phosphorylation, and oxidative/nitrosative stress, inflammation and lipid metabolism-related genes displayed a tendency to normalize in both in vivo and in vitro models.

CONCLUSION

These results place quercetin as a potential therapeutic strategy for preventing NAFLD progression by attenuating gene expression deregulation, at least in part through PI3K/AKT pathway inactivation.

Hepatic Injury in Nonalcoholic Steatohepatitis Contributes to Altered Intestinal Permeability

BACKGROUND & AIMS

Emerging data suggest that changes in intestinal permeability and increased gut microbial translocation contribute to the inflammatory pathway involved in nonalcoholic steatohepatitis (NASH) development. Numerous studies have investigated the association between increased intestinal permeability and NASH. Our meta-analysis of this association investigates the underlying mechanism.

METHODS

A meta-analysis was performed to compare the rates of increased intestinal permeability in patients with NASH and healthy controls. To further address the underlying mechanism of action, we studied changes in intestinal permeability in a diet-induced (methionine-and-choline-deficient; MCD) murine model of NASH. In vitro studies were also performed to investigate the effect of MCD culture medium at the cellular level on hepatocytes, Kupffer cells, and intestinal epithelial cells.

RESULTS

Nonalcoholic fatty liver disease (NAFLD) patients, and in particular those with NASH, are more likely to have increased intestinal permeability compared with healthy controls. We correlate this clinical observation with in vivo data showing mice fed an MCD diet develop intestinal permeability changes after an initial phase of liver injury and tumor necrosis factor-α (TNFα) induction. In vitro studies reveal that MCD medium induces hepatic injury and TNFα production yet has no direct effect on intestinal epithelial cells. Although these data suggest a role for hepatic TNFα in altering intestinal permeability, we found that mice genetically resistant to TNFα-myosin light chain kinase (MLCK)-induced intestinal permeability changes fed an MCD diet still develop increased permeability and liver injury.

CONCLUSIONS

Our clinical and experimental results strengthen the association between intestinal permeability increases and NASH and also suggest that an early phase of hepatic injury and inflammation contributes to altered intestinal permeability in a fashion independent of TNFα and MLCK.

Repression of the nuclear receptor small heterodimer partner by steatotic drugs and in advanced nonalcoholic fatty liver disease

The small heterodimer partner (SHP) (NR0B2) is an atypical nuclear receptor that lacks a DNA-binding domain. It interacts with and inhibits many transcription factors, affecting key metabolic processes, including bile acid, cholesterol, fatty acid, and drug metabolism. Our aim was to determine the influence of steatotic drugs and nonalcoholic fatty liver disease (NAFLD) on SHP expression and investigate the potential mechanisms. SHP was found to be repressed by steatotic drugs (valproate, doxycycline, tetracycline, and cyclosporin A) in cultured hepatic cells and the livers of different animal models of NAFLD: iatrogenic (tetracycline-treated rats), genetic (glycine N-methyltransferase-deficient mice), and nutritional (mice fed a methionine- and choline-deficient diet). Among the different transcription factors investigated, CCAAT-enhancer-binding protein α (C/EBPα) showed the strongest dominant-repressive effect on SHP expression in HepG2 and human hepatocytes. Reporter assays revealed that the inhibitory effect of C/EBPα and steatotic drugs colocalize between -340 and -509 base pair of the SHP promoter, and mutation of a predicted C/EBPα response element at -473 base pair abolished SHP repression by both C/EBPα and drugs. Moreover, inhibition of major stress signaling pathways demonstrated that the mitogen-activated protein kinase kinase 1/2 pathway activates, while the phosphatidylinositol 3 kinase pathway represses SHP in a C/EBP-dependent manner. We conclude that SHP is downregulated by several steatotic drugs and in advanced NAFLD. These conditions can activate signals that target C/EBPα and consequently repress SHP, thus favoring the progression and severity of NAFLD.

Osteopontin is an important mediator of alcoholic liver disease via hepatic stellate cell activation

AIM: To investigate over-expression of Osteopontin (OPN) pathway expression and mechanisms of action in human alcoholic liver disease (ALD), in vivo and in vitro acute alcohol models.

METHODS: OPN pathway was evaluated in livers from patients with progressive stages of human ALD and serum from drinkers with and without liver cirrhosis. In vitro stellate LX2 cells exposed to acute alcohol and in vivo in acute alcoholic steatosis mouse models were also investigated for OPN pathway expression and function. WT and OPN^-/- mice were administered an acute dose of alcohol and extent of liver injury was examined by histopathology and liver biochemistry after 16-24 h. The causative role of OPN was studied in OPN knockout animals and in vitro in stellate LX2 cells, utilizing siRNA, aptamer and neutralizing antibodies to block OPN and OPN pathway. OPN pathway expression and downstream functional consequences were measured for signaling by Western blotting, plasmin activation by spectrophotometric assays and cell migration by confocal imaging and quantitation.

RESULTS: OPN expression positively correlated with disease severity in patients with progressive stages of ALD. In vivo, associated with alcoholic steatosis, a single dose of acute alcohol significantly increased hepatic OPN mRNA and protein, and a cleaved OPN form in a dose dependent manner. OPN mRNA and secreted OPN also increased in parallel with activation of LX2 stellate cells within 4 h of a single dose of alcohol. Expression of OPN receptors, αvβ3-integrin and CD44, increased in human ALD, and in vivo and in vitro with alcohol administration. This was accompanied by downstream phosphorylation of Akt and Erk, increased mRNA expression of several fibrogenesis, fibrinolysis and extracellular matrix pathway genes, plasmin activation and hepatic stellate cell (HSC) migration. Inhibition of OPN and OPN-receptor mediated signaling partially inhibited alcohol-induced HSC activation, plasmin activity and cell migration.

CONCLUSION: OPN is a key mediator of the alcohol-induced effects on hepatic stellate cell functions and liver fibrogenesis.

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.

Osteopontin deficiency aggravates hepatic injury induced by ischemia-reperfusion in mice

Osteopontin (OPN) is a multifunctional protein involved in hepatic steatosis, inflammation, fibrosis and cancer progression. However, its role in hepatic injury induced by ischemia–reperfusion (I–R) has not yet been investigated. We show here that hepatic warm ischemia for 45 min followed by reperfusion for 4 h induced the upregulation of the hepatic and systemic level of OPN in mice. Plasma aspartate aminotransferase and alanine aminotransferase levels were strongly increased in Opn^−/− mice compared with wild-type (Wt) mice after I–R, and histological analysis of the liver revealed a significantly higher incidence of necrosis of hepatocytes. In addition, the expression levels of inducible nitric oxide synthase (iNOS), tumor necrosis factor-α (TNFα), interleukin 6 (IL6) and interferon-γ were strongly upregulated in Opn^−/− mice versus Wt mice after I–R. One explanation for these responses could be the vulnerability of the OPN-deficient hepatocyte. Indeed, the downregulation of OPN in primary and AML12 hepatocytes decreased cell viability in the basal state and sensitized AML12 hepatocytes to cell death induced by oxygen–glucose deprivation and TNFα. Further, the downregulation of OPN in AML12 hepatocytes caused a strong decrease in the expression of anti-apoptotic Bcl2 and in the ATP level. The hepatic expression of Bcl2 also decreased in Opn^−/− mice versus Wt mice livers after I–R. Another explanation could be the regulation of the macrophage activity by OPN. In RAW macrophages, the downregulation of OPN enhanced iNOS expression in the basal state and sensitized macrophages to inflammatory signals, as evaluated by the upregulation of iNOS, TNFα and IL6 in response to lipopolysaccharide. In conclusion, OPN partially protects from hepatic injury and inflammation induced in this experimental model of liver I–R. This could be due to its ability to partially prevent death of hepatocytes and to limit the production of toxic iNOS-derived NO by macrophages.

Propranolol, a β-adrenoceptor antagonist, worsens liver injury in a model of non-alcoholic steatohepatitis

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β-Blocker propranolol worsens liver injury in model of non-alcoholic steatohepatitis.

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Mechanism of hepatic injury is via activation of apoptotic pathway in hepatocytes.

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β-Blockers should be avoided or used with extreme caution in patients with NASH.

Prazosin an α1-adrenoceptor (AR) antagonist has been shown to reduce liver injury in a mouse model of non-alcoholic steatohepatitis (NASH) and is suggested as a potential treatment of NASH especially given its concomitant anti-fibrotic properties. The effect however, of β-AR blockade in non-cirrhotic NASH is unknown and is as such investigated here. In the presence of the β-blocker propranolol (PRL), mice fed normal chow or a half methionine and choline deficient diet, supplemented with ethionine (HMCDE), to induce NASH, showed significantly enhanced liver injury, as evidenced by higher hepatic necrosis scores and elevated serum aminotransferases (ALT). Mechanistically, we showed that murine hepatocytes express α and β adrenoceptors; that PRL directly induces hepatocyte injury and death as evidenced by increased release of lactate dehydrogenase, FASL and TNF-α from hepatocytes in the presence of PRL; and that PRL activated the apoptotic pathway in primary hepatocyte cultures, as indicated by upregulation of Fas receptor and caspase-8 proteins. The β-AR antagonist PRL therefore appears to enhance liver injury through induction of hepatocyte death via the death pathway. Further studies are now required to extrapolate these findings to humans but meanwhile, β-AR antagonists should be avoided or used with caution in patients with non-cirrhotic NASH as they may worsen liver injury.

The dual role of osteopontin in acetaminophen hepatotoxicity

AIM

Osteopontin (OPN), a multifunctional protein, has been reported to be protoxicant in acetaminophen hepatotoxicity. In this study, the mechanisms underlying the detrimental role of OPN in acetaminophen toxicity were explored.

METHODS

Male C57BL/6 (wild-type, WT) and OPN(-/-) mice were administered with acetaminophen (500 mg/kg, ip). After the treatment, serum transaminase (ALT), as well as OPN expression, histology changes, oxidative stress and inflammation response in liver tissue were studied. Freshly isolated hepatocytes of WT and OPN(-/-) mice were prepared.

RESULTS

Acetaminophen administration significantly increased OPN protein level in livers of WT mice. OPN expression was mainly localized in hepatic macrophages 6 h after the administration. In OPN(-/-) mice, acetaminophen-induced serum ALT release was reduced, but the centrilobular hepatic necrosis was increased. In OPN(-/-) mice, the expression of CYP2E1 and CYP1A2 in livers was significantly increased; GSH depletion and lipid peroxidation in livers were enhanced. On the other hand, OPN(-/-) mice exhibited less macrophage and neutrophil infiltration and reduced expression of proinflammatory cytokines TNF-α and IL-1α in livers. An anti-OPN neutralizing antibody significantly reduced acetaminophen-induced serum ALT level and inflammatory infiltration in livers of WT mice.

CONCLUSION

OPN plays a dual role in acetaminophen toxicity: OPN in hepatocytes inhibits acetaminophen metabolism, while OPN in macrophages enhances acetaminophen toxicity via recruitment of inflammatory cells and production of proinflammatory cytokines.

Activation of peroxisome proliferator activated receptor alpha ameliorates ethanol induced steatohepatitis in mice

BACKGROUND

Peroxisome proliferator activated receptor alpha (PPARα) regulates lipids metabolism and inhibits inflammatory response. However, the role of PPARα in alcoholic liver disease is largely unknown. We aim to elucidate the effect and the molecular basis of PPARα in ethanol induced hepatic injury in mice.

RESULTS

C57BL/6J mice fed with 4% ethanol-containing Lieber-DeCarli liquid diet for 12 weeks exhibited hepatocyte steatosis, necrosis and inflammatory infiltration, accompanied with elevated serum alanine aminotransferase (ALT) and aspartic transaminase (AST) levels, decreased hepatic expression of PPARα, lipids oxidation promoting genes and anti-inflammatory factors, as well as enhanced hepatic expression of fatty acids synthesis promoting genes and pro-inflammatory cytokines. Induction of PPARα by PPARα agonist WY14643 treatment for 2 weeks ameliorated the severity of liver injury and restored expression of genes altered by ethanol treatment. However, administration of PPARα antagonist GW6471 for 2 weeks promoted the inflammatory response.

CONCLUSIONS

The present study provided the evidence for the protective role of PPARα in ameliorating ethanol induced liver injury through modulation of the genes related to lipid metabolism and inflammatory response.

Activation of peroxisome proliferator activated receptor alpha ameliorates ethanol induced steatohepatitis in mice

BACKGROUND

Peroxisome proliferator activated receptor alpha (PPARα) regulates lipids metabolism and inhibits inflammatory response. However, the role of PPARα in alcoholic liver disease is largely unknown. We aim to elucidate the effect and the molecular basis of PPARα in ethanol induced hepatic injury in mice.

RESULTS

C57BL/6J mice fed with 4% ethanol-containing Lieber-DeCarli liquid diet for 12 weeks exhibited hepatocyte steatosis, necrosis and inflammatory infiltration, accompanied with elevated serum alanine aminotransferase (ALT) and aspartic transaminase (AST) levels, decreased hepatic expression of PPARα, lipids oxidation promoting genes and anti-inflammatory factors, as well as enhanced hepatic expression of fatty acids synthesis promoting genes and pro-inflammatory cytokines. Induction of PPARα by PPARα agonist WY14643 treatment for 2 weeks ameliorated the severity of liver injury and restored expression of genes altered by ethanol treatment. However, administration of PPARα antagonist GW6471 for 2 weeks promoted the inflammatory response.

CONCLUSIONS

The present study provided the evidence for the protective role of PPARα in ameliorating ethanol induced liver injury through modulation of the genes related to lipid metabolism and inflammatory response.

Pathogenesis of alcohol-induced liver disease: classical concepts and recent advances

Alcoholic liver disease (ALD) is a primary consequence of heavy and prolonged drinking. ALD contributes to the bulk of liver disease burden worldwide. Progression of ALD is a multifactorial and multistep process that includes many genetic and environmental risk factors. The molecular pathogenesis of ALD involves alcohol metabolism and secondary mechanisms such as oxidative stress, endotoxin, cytokines and immune regulators. The histopathological manifestation of ALD occurs as an outcome of complex but controlled interactions between hepatic cell types. Hepatic stellate cells (HSCs) are the key drivers of fibrogenesis, but transformation of hepatocytes to myofibroblastoids also implicate parenchymal cells as playing an active role in hepatic fibrogenesis. Recent discoveries indicate that lipogenesis during the early stages of ALD is a risk for advancement to cirrhosis. Other recently identified novel molecules and physiological/cell signaling pathways include fibrinolysis, osteopontin, transforming growth factor-β-SMAD and hedgehog signaling, and involvement of novel cytokines in hepatic fibrogenesis. The observation that ALD and non-alcoholic steatohepatitis share common pathways and genetic polymorphisms suggests operation of parallel pathogenic mechanisms. Future research involving genomics, epigenomics, deep sequencing and non-coding regulatory elements holds promise to identify novel diagnostic and therapeutic targets for ALD. There is also a need for adequate animal models to study pathogenic mechanisms at the molecular level and targeted therapy.

PKC{delta} is activated in a dietary model of steatohepatitis and regulates endoplasmic reticulum stress and cell death

Hepatic steatosis can progress to the clinical condition of non-alcoholic steatohepatitis (NASH), which is a precursor of more serious liver diseases. The novel PKC isoforms δ and ε are activated by lipid metabolites and have been implicated in lipid-induced hepatic disease. Using a methionine- and choline-deficient (MCD) dietary model of NASH, we addressed the question of whether hepatic PKCδ and PKCε are activated. With progression from steatosis to steatohepatitis, there was activation and increased PKCδ protein content coincident with hepatic endoplasmic reticulum (ER) stress parameters. To examine whether similar changes could be induced in vitro, McA-RH 7777 (McA) hepatoma cells were used. We observed that McA cells stored triglyceride and released alanine aminotransferase (ALT) when treated with MCD medium in the presence of fatty acids. Further, MCD medium with palmitic acid, but not oleic or linoleic acids, maximally activated PKCδ and stimulated ER stress. In PKCδ knockdown McA cells, MCD/fatty acid medium-induced ALT release and ER stress induction were completely blocked, but triglyceride storage was not. In addition, a reduction in the uptake of propidium iodide and the number of apoptotic nuclei and a significant increase in cell viability and DNA content were observed in PKCδ knockdown McA cells incubated in MCD medium with palmitic acid. Our studies show that PKCδ activation and protein levels are elevated in an animal model of steatohepatitis, which was recapitulated in a cell model, supporting the conclusion that PKCδ plays a role in ALT release, the ER stress signal, and cell death.

Glucagon-like peptide-1 receptor is present on human hepatocytes and has a direct role in decreasing hepatic steatosis in vitro by modulating elements of the insulin signaling pathway

Glucagon-like peptide 1 (GLP-1) is a naturally occurring peptide secreted by the L cells of the small intestine. GLP-1 functions as an incretin and stimulates glucose-mediated insulin production by pancreatic beta cells. In this study, we demonstrate that exendin-4/GLP-1 has a cognate receptor on human hepatocytes and that exendin-4 has a direct effect on the reduction of hepatic steatosis in the absence of insulin. Both glucagon-like peptide 1 receptor (GLP/R) messenger RNA and protein were detected on primary human hepatocytes, and receptor was internalized in the presence of GLP-1. Exendin-4 increased the phosphorylation of 3-phosphoinositide-dependent kinase-1 (PDK-1), AKT, and protein kinase C zeta (PKC-zeta) in HepG2 and Huh7 cells. Small interfering RNA against GLP-1R abolished the effects on PDK-1 and PKC-zeta. Treatment with exendin-4 quantitatively reduced triglyceride stores compared with control-treated cells.

CONCLUSION

This is the first report that the G protein-coupled receptor GLP-1R is present on human hepatocytes. Furthermore, it appears that exendin-4 has the same beneficial effects in vitro as those seen in our previously published in vivo study in ob/ob mice, directly reducing hepatocyte steatosis. Future use for human nonalcoholic fatty liver disease, either in combination with dietary manipulation or other pharmacotherapy, may be a significant advance in treatment of this common form of liver disease.

Autophagy regulates lipid metabolism

The intracellular storage and utilization of lipids are critical to maintain cellular energy homeostasis. During nutrient deprivation, cellular lipids stored as triglycerides in lipid droplets are hydrolysed into fatty acids for energy. A second cellular response to starvation is the induction of autophagy, which delivers intracellular proteins and organelles sequestered in double-membrane vesicles (autophagosomes) to lysosomes for degradation and use as an energy source. Lipolysis and autophagy share similarities in regulation and function but are not known to be interrelated. Here we show a previously unknown function for autophagy in regulating intracellular lipid stores (macrolipophagy). Lipid droplets and autophagic components associated during nutrient deprivation, and inhibition of autophagy in cultured hepatocytes and mouse liver increased triglyceride storage in lipid droplets. This study identifies a critical function for autophagy in lipid metabolism that could have important implications for human diseases with lipid over-accumulation such as those that comprise the metabolic syndrome.

Amino acids: metabolism, functions, and nutrition

Recent years have witnessed the discovery that amino acids (AA) are not only cell signaling molecules but are also regulators of gene expression and the protein phosphorylation cascade. Additionally, AA are key precursors for syntheses of hormones and low-molecular weight nitrogenous substances with each having enormous biological importance. Physiological concentrations of AA and their metabolites (e.g., nitric oxide, polyamines, glutathione, taurine, thyroid hormones, and serotonin) are required for the functions. However, elevated levels of AA and their products (e.g., ammonia, homocysteine, and asymmetric dimethylarginine) are pathogenic factors for neurological disorders, oxidative stress, and cardiovascular disease. Thus, an optimal balance among AA in the diet and circulation is crucial for whole body homeostasis. There is growing recognition that besides their role as building blocks of proteins and polypeptides, some AA regulate key metabolic pathways that are necessary for maintenance, growth, reproduction, and immunity. They are called functional AA, which include arginine, cysteine, glutamine, leucine, proline, and tryptophan. Dietary supplementation with one or a mixture of these AA may be beneficial for (1) ameliorating health problems at various stages of the life cycle (e.g., fetal growth restriction, neonatal morbidity and mortality, weaning-associated intestinal dysfunction and wasting syndrome, obesity, diabetes, cardiovascular disease, the metabolic syndrome, and infertility); (2) optimizing efficiency of metabolic transformations to enhance muscle growth, milk production, egg and meat quality and athletic performance, while preventing excess fat deposition and reducing adiposity. Thus, AA have important functions in both nutrition and health.

Increased hepatic expression is a major determinant of serum alanine aminotransferase elevation in mice with nonalcoholic steatohepatitis

BACKGROUND

Serum alanine aminotransferase (ALT) is a biomarker for hepatitis of various aetiologies including fatty liver disease. Increased serum ALT is thought to be related to its increased release from dying hepatocytes.

AIM

We sought to understand the mechanisms by which serum ALT is elevated in a mouse model of experimental fatty liver disease where hepatocyte death is minimal.

METHODS

To induce fatty liver disease, female A/J mice were fed a methionine-choline deficient (MCD) diet for up to 12 weeks. Serum and liver ALT expression and hepatic inflammation, necrosis and apoptosis were assessed and expressed relative to their expressions in control-diet-fed mice.

RESULTS

Feeding mice the MCD diet produced hepatic steatosis with minimal hepatic inflammation or necrosis. Liver cell apoptosis was not significantly increased by MCD diet treatment. Conversely, serum ALT activity was approximately four-fold increased at 12 weeks of diet treatment, and ALT protein expressions in serum were correspondingly increased: ALT1 1.7-fold and ALT2 1.9-fold at 12 weeks. The expressions of ALT1 and ALT2 protein in liver increased over 2-12 weeks of MCD treatment. At 12 weeks, liver ALT1 protein was 2.27+/-0.31-fold increased and ALT2 protein 4.72+/-0.48-fold increased relative to their expressions in the mice fed a diet replete with methionine and choline. Liver ALT mRNA expressions were correspondingly increased: ALT1 mRNA 2.58-fold and ALT2 mRNA 4.97-fold at 12 weeks. Linear regression analysis showed a strong correlation between serum and liver tissue expressions for both ALT1 and ALT2.

CONCLUSIONS

These findings suggest that induction of hepatic expression significantly contributes to increased serum ALT in this model of experimental fatty liver disease, whereas cell death appears not to.

Phyllanthus urinaria ameliorates the severity of nutritional steatohepatitis both in vitro and in vivo

Hepatic oxidative stress plays a critical role in metabolic forms of steatohepatitis. Phyllanthus urinaria, an herbal medicine, has been reported to have potential antioxidant properties. We tested the effects of P. urinaria on nutritional steatohepatitis both in vitro and in vivo. Immortalized normal hepatocytes (AML-12) or primary hepatocytes were exposed to control, the methionine-and-choline-deficient (MCD) culture medium, in the presence or absence of P. urinaria for 24 hours. Hepatocyte triglyceride, release of alanine aminotransferase, lipoperoxides, and reactive oxygen species production were determined. Age-matched C57BL/6 and db/db mice were fed control or MCD diet for 10 days with or without P. urinaria. Hepatic steatosis, necroinflammation, triglycerides, and lipid peroxide levels were determined. Hepatic expression of inflammatory factors and lipid regulatory mediators were assayed. P. urinaria reduced steatosis and alanine aminotransferase (ALT) levels in culture of hepatocytes in a dose-dependent manner. Phyllanthus prevented MCD-induced hepatic fat accumulation and steatohepatitis in mice. This effect was associated with repressed levels of hepatic lipid peroxides, reduced expression of cytochrome P450-2E1, pro-inflammatory tumor necrosis factor alpha, interleukin-6, dampened activation of inflammatory c-Jun N-terminal kinase (JNK) and nuclear factor kappa B (NF-kappaB), increased expression of lipolytic cytochrome P450 (Cyp4a10), and suppressed transcriptional activity of lipogenic CCAAT/enhancer binding protein beta (C/EBPbeta). Hepatic acyl co-enzyme A oxidase that regulated hepatic beta-oxidation of fatty acid and other lipid regulators were not affected by P. urinaria. In conclusion, P. urinaria effectively alleviated the steatohepatitis induced by the MCD, probably through dampening oxidative stress, ameliorating inflammation, and decreasing lipid accumulation.