Molecular mechanisms and the role of saturated fatty acids in the progression of non-alcoholic fatty liver disease

Elongation Factor 1A-1 Is a Mediator of Hepatocyte Lipotoxicity Partly through Its Canonical Function in Protein Synthesis

Elongation factor 1A-1 (eEF1A-1) has non-canonical functions in regulation of the actin cytoskeleton and apoptosis. It was previously identified through a promoter-trap screen as a mediator of fatty acid-induced cell death (lipotoxicity), and was found to participate in this process downstream of ER stress. Since ER stress is implicated in the pathogenesis of nonalcoholic fatty liver disease (NAFLD), we investigated the mechanism of action of eEF1A-1 in hepatocyte lipotoxicity. HepG2 cells were exposed to excess fatty acids, followed by assessments of ER stress, subcellular localization of eEF1A-1, and cell death. A specific inhibitor of eEF1A-1 elongation activity, didemnin B, was used to determine whether its function in protein synthesis is involved in lipotoxicity. Within 6 h, eEF1A-1 protein was modestly induced by high palmitate, and partially re-localized from its predominant location at the ER to polymerized actin at the cell periphery. This early induction and subcellular redistribution of eEF1A-1 coincided with the onset of ER stress, and was later followed by cell death. Didemnin B did not prevent the initiation of ER stress by high palmitate, as indicated by eIF2α phosphorylation. However, consistent with sustained inhibition of eEF1A-1-dependent elongation activity, didemnin B prevented the recovery of protein synthesis and increase in GRP78 protein that are normally associated with later phases of the response to ongoing ER stress. This resulted in decreased palmitate-induced cell death. Our data implicate eEF1A-1, and its function in protein synthesis, in hepatocyte lipotoxicity.

Fat-laden macrophages modulate lobular inflammation in nonalcoholic steatohepatitis (NASH)

Nonalcoholic steatohepatitis (NASH) is characterized by extensive hepatic monocyte infiltration and monocyte-derived macrophages have an important role in regulating the disease evolution. However, little is known about the functional changes occurring in liver macrophages during NASH progression. In this study, we investigated phenotypic and functional modifications of hepatic macrophages in experimental NASH induced by feeding C57BL/6 mice with a methionine-choline deficient (MCD) diet up to 8weeks. In mice with steatohepatitis liver F4/80-positive macrophages increased in parallel with the disease progression and formed small clusters of enlarged and vacuolated cells. At immunofluorescence these cells contained lipid vesicles positive for the apoptotic cell marker Annexin V suggesting the phagocytosis of apoptotic bodies derived from dead fat-laden hepatocytes. Flow cytometry revealed that these enlarged macrophages expressed inflammatory monocyte (CD11b, Ly6C, TNF-α) markers. However, as compared to regular size macrophages the enlarged sub-set was characterized by an enhanced production of arginase-1 and of the anti-inflammatory mediators IL-10 and annexin A1. Similar vacuolated macrophages producing annexin A1 were also evident in liver biopsies of NASH patients. In mice with NASH, the accumulation of enlarged F4/80(+) cells paralleled with a decline in the expression of the macrophage M1 activation markers iNOS, IL-12 and CXCL10, while the levels of M2 polarization markers arginase-1 and MGL-1 were unchanged. Interestingly, the lowering of IL-12 expression mainly involved the macrophage sub-set with regular size. We conclude that during the progression of NASH fat accumulation within liver macrophages promotes the production of anti-inflammatory mediators that influence hepatic inflammatory responses.

Sab (Sh3bp5) dependence of JNK mediated inhibition of mitochondrial respiration in palmitic acid induced hepatocyte lipotoxicity

BACKGROUND & AIMS

Sustained c-Jun N-terminal kinase (JNK) activation by saturated fatty acids plays a role in lipotoxicity and the pathogenesis of non-alcoholic steatohepatitis (NASH). We have reported that the interaction of JNK with mitochondrial Sab leads to inhibition of respiration, increased reactive oxygen species (ROS), cell death and hepatotoxicity. We tested whether this pathway underlies palmitic acid (PA)-induced lipotoxicity in hepatocytes.

METHODS

Primary mouse hepatocytes (PMH) from adeno-shlacZ or adeno-shSab treated mice and HuH7 cells were used.

RESULTS

In PMH, PA dose-dependently up to 1mM stimulated oxygen consumption rate (OCR) due to mitochondrial β-oxidation. At ⩾1.5mM, PA gradually reduced OCR, followed by cell death. Inhibition of JNK, caspases or treatment with antioxidant butylated hydroxyanisole (BHA) protected PMH against cell death. Sab knockdown or a membrane permeable Sab blocking peptide prevented PA-induced mitochondrial impairment, but inhibited only the late phase of both JNK activation (beyond 4h) and cell death. In PMH, PA increased p-PERK and its downstream target CHOP, but failed to activate the IRE-1α arm of the UPR. However, Sab silencing did not affect PA-induced PERK activation. Conversely, specific inhibition of PERK prevented JNK activation and cell death, indicating a major role upstream of JNK activation.

CONCLUSIONS

The effect of p-JNK on mitochondria plays a key role in PA-mediated lipotoxicity. The interplay of p-JNK with mitochondrial Sab leads to impaired respiration, ROS production, sustained JNK activation, and apoptosis.

Association between serum alanine aminotransferase level and obesity indices in Korean adolescents

Purpose

To analyze the correlation between serum alanine aminotransferase (ALT) and obesity indices including body mass index (BMI), body fat percentage (BFP), total fat mass (FM), truncal fat mass (TFM), waist circumference (WC), and waist-to-height ratio (WHtR) in Korean adolescents.

Methods

This was a cross-sectional study based on data derived from the 2010-2011 Korean National Health and Nutrition Examination Surveys (KNHANES). Subjects were Korean adolescents aged 10-18 years (871 total; 475 boys and 396 girls) who participated in KNHANES.

Results

In both sexes, BMI, FM, TFM, WC, and WHtR were higher when ALT levels were in the 4th quartile. In boys, there was a significant positive correlation between ALT level and BMI, BFP, FM, TFM, WC, and WHtR (r=0.55, P<0.0001 for BMI; r=0.52, P<0.0001 for BFP; r=0.58, P<0.0001 for FM; r=0.61, P<0.0001 for TFM; and r=0.56, P<0.0001 for WC; r=0.62, P<0.0001 for WHtR), and the correlation coefficient was higher than that in girls.

Conclusion

Our results suggest a significant positive association between serum ALT level and obesity indices in male adolescents.

The Dual Role of Nrf2 in Nonalcoholic Fatty Liver Disease: Regulation of Antioxidant Defenses and Hepatic Lipid Metabolism

Nonalcoholic fatty liver disease (NAFLD) is a progressive liver disease with ever-growing incidence in the industrialized world. It starts with the simple accumulation of lipids in the hepatocyte and can progress to the more severe nonalcoholic steatohepatitis (NASH), which is associated with inflammation, fibrosis, and cirrhosis. There is increasing awareness that reactive oxygen species and electrophiles are implicated in the pathogenesis of NASH. Transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) is a positive regulator of the expression of a battery of genes involved in the protection against oxidative/electrophilic stress. In rodents, Nrf2 is also known to participate in hepatic fatty acid metabolism, as a negative regulator of genes that promote hepatosteatosis. We review relevant evidence in the literature that these two mechanisms may contribute to the protective role of Nrf2 in the development of hepatic steatosis and in the progression to steatohepatitis, particularly in young animals. We propose that age may be a key to explain contradictory findings in the literature. In summary, Nrf2 mediates the crosstalk between lipid metabolism and antioxidant defense mechanisms in experimental models of NAFLD, and the nutritional or pharmacological induction of Nrf2 represents a promising potential new strategy for its prevention and treatment.

Ursolic acid inhibits the development of nonalcoholic fatty liver disease by attenuating endoplasmic reticulum stress

Ursolic acid (UA) is a natural pentacyclic triterpenoid compound, which is enriched with many herbs and plants, such as apple, cranberry and olive. UA performs multiple biological activities including anti-oxidation, anti-inflammation, anti-cancer and hepatoprotection. However, the exact mechanism underlying the hepatoprotective activity of UA remains unclear. In this study, the effects of UA on the development of nonalcoholic fatty liver disease (NAFLD) were investigated. In vivo, UA treatment (0.14%, w/w) significantly decreased the liver weight, serum levels of ALT/AST and hepatic steatosis in db/db mice (a type 2 diabetic mouse model). In vitro, UA treatment (10-30 μg ml(-1)) significantly decreased palmitic acid induced intracellular lipid accumulation in L02 cells. Our results suggested that the beneficial effects of UA on NAFLD may be due to its ability to increase lipid β-oxidation and to inhibit the hepatic endoplasmic reticulum (ER) stress. Together, UA may be further considered as a natural compound for NAFLD treatment.

Lowering the dietary omega-6: omega-3 does not hinder nonalcoholic fatty-liver disease development in a murine model

It is hypothesized that a high dietary n-6:n-3 (eg, 10-20:1) is partly responsible for the rise in obesity and related health ailments. However, no tightly controlled studies using high-fat diets differing in the n-6:n-3 have tested this hypothesis. The aim of the study was to determine the role that the dietary n-6:n-3 plays in non-alcoholic fatty-liver disease (NAFLD) and colitis development. We hypothesized that reducing the dietary n-6:n-3 would hinder the development of NAFLD and colitis. Male C57BL/6 J mice were fed high-fat diets, differing in the n-6:n-3 (1:1, 5:1, 10:1, 20:1), for 20 weeks. Gas chromatography-mass spectrometry was used to analyze the hepatic phospholipid arachidonic acid (AA):eicosapentaenoic acid and AA:docosahexaenoic acid. Hepatic metabolism, inflammatory signaling, macrophage polarization, gene expression of inflammatory mediators, oxidative and endoplasmic reticulum stress, and oxidative capacity were assessed as well as colonic inflammatory signaling, and gene expression of inflammatory mediators and tight-junction proteins. Although reducing the dietary n-6:n-3 lowered the hepatic phospholipid AA:eicosapentaenoic acid and AA:docosahexaenoic acid in a dose-dependent manner and mildly influenced inflammatory signaling, it did not significantly attenuate NAFLD development. Furthermore, the onset of NAFLD was not paired to colitis development or changes in tight-junction protein gene expression. In conclusion, reducing the dietary n-6:n-3 did not attenuate NAFLD progression; nor is it likely that colitis, or gut permeability, plays a role in NAFLD initiation in this model.

Lysine-specific demethylase 2 suppresses lipid influx and metabolism in hepatic cells

Cells link environmental fluctuations, such as nutrition, to metabolic remodeling. Epigenetic factors are thought to be involved in such cellular processes, but the molecular basis remains unclear. Here we report that the lysine-specific demethylase 2 (LSD2) suppresses the flux and metabolism of lipids to maintain the energy balance in hepatic cells. Using transcriptome and chromatin immunoprecipitation-sequencing analyses, we revealed that LSD2 represses the genes involved in lipid influx and metabolism through demethylation of histone H3K4. Selective recruitment of LSD2 at lipid metabolism gene loci was mediated in part by a stress-responsive transcription factor, c-Jun. Intriguingly, LSD2 depletion increased the intracellular levels of many lipid metabolites, which was accompanied by an increased susceptibility to toxic cell damage in response to fatty acid exposure. Our data demonstrate that LSD2 maintains metabolic plasticity under fluctuating environment in hepatocytes by mediating the cross talk between the epigenome and metabolism.

Plasma phospholipids, non-esterified plasma polyunsaturated fatty acids and oxylipids are associated with BMI

The obese lipid profile is associated with increased free fatty acids and triacylglycerides. Currently, little is known about the plasma lipid species associated with obesity. In this study, we compared plasma lipid fatty acid (FA) profiles as a function of BMI. Profiling phospholipid (PL) FAs and their respective oxylipids could predict which obese individuals are more likely to suffer from diseases associated with chronic inflammation or oxidative stress. We investigated the relationship between BMI and plasma PL (PPL) FA composition in 126 men using a quantitative gas chromatography analysis. BMI was inversely associated with both PPL nervonic and linoleic acid (LA) but was positively associated with both dihomo-γ-linolenic and palmitoleic acid. Compared to lean individuals, obese participants were more likely to have ω-6 FAs, except arachidonic acid and LA, incorporated into PPLs. Obese participants were less likely to have EPA and DHA incorporated into PPLs compared to lean participants. Non-esterified plasma PUFA and oxylipid analysis showed ω-6 oxylipids were more abundant in the obese plasma pool. These ω-6 oxylipids are associated with increased angiogenesis (i.e. epoxyeicosatrienoates), reactive oxygen species (i.e. 9-hydroxyeicosatetraenoate), and inflammation resolution (i.e. Lipoxin A4). In summary, BMI is directly associated with specific PPL FA and increased ω-6 oxylipids.

Lipidomic-based investigation into the regulatory effect of Schisandrin B on palmitic acid level in non-alcoholic steatotic livers

Schisandrin B (SchB) is one of the most abundant bioactive dibenzocyclooctadiene derivatives found in the fruit of Schisandra chinensis. Here, we investigated the potential therapeutic effects of SchB on non-alcoholic fatty-liver disease (NAFLD). In lipidomic study, ingenuity pathway analysis highlighted palmitate biosynthesis metabolic pathway in the liver samples of SchB-treated high-fat-diet-fed mice. Further experiments showed that the SchB treatment reduced expression and activity of fatty acid synthase, expressions of hepatic mature sterol regulatory element binding protein-1 and tumor necrosis factor-α, and hepatic level of palmitic acid which is known to promote progression of steatosis to steatohepatitis. Furthermore, the treatment also reduced hepatic fibrosis, activated nuclear factor-erythroid-2-related factor-2 which is known to attenuate the progression of NASH-related fibrosis. Interestingly, in fasting mice, a single high-dose SchB induced transient lipolysis and increased the expressions of adipose triglyceride lipase and phospho-hormone sensitive lipase. The treatment also increased plasma cholesterol levels and 3-hydroxy-3-methylglutaryl-CoA reductase activity, reduced the hepatic low-density-lipoprotein receptor expression in these mice. Our data not only suggest SchB is a potential therapeutic agent for NAFLD, but also provided important information for a safe consumption of SchB because SchB overdosed under fasting condition will have adverse effects on lipid metabolism.

Alpha linolenic acid in maternal diet halts the lipid disarray due to saturated fatty acids in the liver of mice offspring at weaning

Background

Alpha linolenic acid (ALA, 18:3) in maternal diets has been shown to attenuate obesity associated insulin resistance (IR) in adult offspring in mice. The objective in the present study was to detect the early effects of maternal dietary saturated fatty acids (SFA) and their partial substitution with ω-3 ALA, docosa hexenoic acid (DHA,22:6) and eicosapentenoic acid 20:5 (EPA,20:5) on the HOMA index, liver lipids and fatty acid desaturases in the offspring at weaning.

Methods

3 month old C57Bl6/J female mice were fed diets containing normal amount of calories but rich in SFA alone or partially replaced with ALA, DHA or EPA before mating, during pregnancy and lactation.

Results

Pregnant mice fed SFA produced offspring with the highest HOMA index, liver lipids and desaturase activities. ALA prevented SFA induced lipid increase but DHA and EPA only reduced it by 42% and 31% respectively. ALA, DHA and EPA decreased HOMA index by 84%, 75% and 83% respectively. ALA, DHA and EPA decreased Δ6 and SCD1 desaturase activities about 30%.

Conclusions

SFA feeding to mothers predisposes their offspring to develop IR and liver lipid accumulation already at weaning. ω3 fatty acids reduce IR, ALA halts lipid accumulation whereas DHA and EPA only blunt it.ALA and DHA restore the increased SCD1 to normal. These studies suggest that ω-3 fatty acids have different potencies to preclude lipid accumulation in the offspring partially by affecting pathways associated to SCD1 modulation.

Fluvastatin attenuates hepatic steatosis-induced fibrogenesis in rats through inhibiting paracrine effect of hepatocyte on hepatic stellate cells

Background

Non-alcoholic steatohepatitis (NASH) is associated with hepatic fibrogenesis. Despite well-known cholesterol-lowering action of statins, their mechanisms against NASH-mediated fibrogenesis remain unclear. This study aimed at investigating the in vitro and in vivo anti-fibrotic properties of fluvastatin (Flu).

Methods

Palmitate (PA)-induced changes in intracellular hydrogen peroxide levels in primary rat hepatocytes (PRHs) and human hepatoma cell line (HepG2) were quantified by dichlorofluorescein diacetate (DCF-DA) dye assay, whereas changes in expressions of NADPH oxidase gp91^phox subunit, α-smooth muscle actin (α-SMA), and NFκB p65 nuclear translocation were quantified with Western blotting. Quantitative real-time polymerase chain reaction (q-PCR) was used to investigate mRNA expressions of pro-inflammatory genes (ICAM-1, IL-6, TNF-α). Conditioned medium (CM) from PA-treated PRHs was applied to cultured rat hepatic stellate cell line, HSC-T6, with or without Flu-pretreatment for 2 h. Pro-fibrogenic gene expressions (COL1, TIMP-1, TGF-β1, α-SMA) and protein expression of α-SMA were analyzed. In vivo study using choline-deficient L-amino acid defined (CDAA) diet-induced rat NASH model was performed by randomly assigning Wistar rats (n = 28) to normal controls (n = 4), CDAA diet with vehicles, and CDAA diet with Flu (5 mg/kg or 10 mg/kg) (n = 8 each) through gavage for 4 or 8 weeks. Livers were harvested for histological, Western blot (α-SMA), and q-PCR analyses for expressions of pro-inflammatory (IL-6, iNOS, ICAM-1) and pro-fibrogenic (Col1, α-SMA, TIMP-1) genes.

Results

In vitro, Flu (1–20 μM) inhibited PA-induced free-radical production, gp91^phox expression, and NFκB p65 translocation in HepG2 and PRHs, while CM-induced α-SMA protein expression and pro-fibrogenic gene expressions in HSC-T6 were suppressed in Flu-pretreated cells compared to those without pretreatment. Moreover, α-SMA protein expression was significantly decreased in HSC-T6 cultured with CM from PA-Flu-treated PRHs compared to those cultured with CM from PA-treated PRHs. Flu also reduced steatosis and fibrosis scores, α-SMA protein expression, mRNA expression of pro-inflammatory and pro-fibrogenic genes in livers of CDAA rats.

Conclusions

We demonstrated PA-induced HSC activation through paracrine effect of hepatocyte in vitro that was significantly suppressed by pre-treating HSC with Flu. In vivo, Flu alleviated steatosis-induced HSC activation and hepatic fibrogenesis through mitigating inflammation and oxidative stress, suggesting possible therapeutic role of Flu against NASH.

Clinical features of hepatocellular carcinoma associated with nonalcoholic fatty liver disease: a review of human studies

Most cases of hepatocellular carcinoma (HCC) in Japan develop in the background of chronic liver disease caused by hepatitis C virus (HCV). Recently, however, HCV-associated HCC has been shown to be decreasing, while non-B and non-C HCC (NBNC-HCC), which is negative for HCV and hepatitis B virus infection, has increased. The main cause of NBNC-HCC is alcoholic liver disease, but the recent increase of NBNC-HCC is thought to be due to an increase in patients with nonalcoholic fatty liver disease (NAFLD). Approximately 10% of NAFLD cases involve nonalcoholic steatohepatitis (NASH), and NASH can progress to liver cirrhosis and its associated complications such as HCC. There are no accurate data on the percentage of NASH-related HCC among all-cause HCC in Japan, because no large-scale investigation has been performed. However, the rate is thought to be about 3% of overall HCC, which is lower than that in the United States. The incidence of HCC in patients with NASH-related cirrhosis is thought to be 2% per year, which is lower than that in HCV-related cirrhosis. Risks for NASH-related HCC include advanced hepatic fibrosis, older age, and being male. NAFLD that includes NASH is associated with metabolic syndrome, which includes obesity and diabetes, and metabolic syndrome is a risk factor for HCC. Genetic factors and dietary patterns may also be related to NASH-related HCC. Thus, regular HCC surveillance, as performed for patients with viral chronic liver disease, is required for patients with NAFLD, and diagnostic markers are required for simple, rapid and specific detection of NASH-related HCC.

A Lipidomic Readout of Disease Progression in A Diet-Induced Mouse Model of Nonalcoholic Fatty Liver Disease

Multiple changes in lipid metabolism occur in nonalcoholic fatty liver disease. However, it is not known which of these contribute to disease progression. The objective of this study was to define changes in hepatic lipid composition over time in a diet-induced model of nonalcoholic fatty liver disease to identify changes associated with disease progression. A lipidomic approach was used to quantify individual lipid species with lipid classes of interest including diacylglycerols (DAG), cholesterol, phospholipids, plasmalogens, sphingolipids, and eicosanoids. C57b/S129J mice fed a high-fat, high-cholesterol diet developed fatty liver, inflammation, and ballooning by 16 weeks and extensive fibrosis by week 52. There was a marked increase in monounsaturated fatty acid containing DAGs and cholesterol esters by week 16 which decreased by week 52. The changes in DAG were associated with a 500- to 600-fold increase in phosphatidic acid (< 0.001) and its downstream product phosphatidylglycerol (P <0.01) whereas phosphatidylethanolamine, phosphatidylcholine, and phsophatidylserine all decreased. Disease progression was associated with a significant further decrease in phosphatidylcholine and phosphatidylethanolamine while several lysolecithin species increased. Disease progression was associated with a significant increase in the plasmalogen PC-P 16:0/16:1. Saturated fatty acid (16:0 and 18:0) containing ceramides, sphingosine, sphingosine-1-phosphate, dihydrosphingosine, and dihydrophingosine-1-phosphate increased by week 16 after high-fat high-cholesterol diet. Globotrioseacylceramide (GB3) also increased significantly by week 16 and increased further with disease progression. 12-hydroxyeicosatetranoic acid decreased at week 16 but increased with disease progression. In conclusion, multiple lipids were associated with disease progression and provide clues regarding lipid drivers of nonalcoholic steatohepatitis.

From whole body to cellular models of hepatic triglyceride metabolism: man has got to know his limitations

The liver is a main metabolic organ in the human body and carries out a vital role in lipid metabolism. Nonalcoholic fatty liver disease (NAFLD) is one of the most common liver diseases, encompassing a spectrum of conditions from simple fatty liver (hepatic steatosis) through to cirrhosis. Although obesity is a known risk factor for hepatic steatosis, it remains unclear what factor(s) is/are responsible for the primary event leading to retention of intrahepatocellular fat. Studying hepatic processes and the etiology and progression of disease in vivo in humans is challenging, not least as NAFLD may take years to develop. We present here a review of experimental models and approaches that have been used to assess liver triglyceride metabolism and discuss their usefulness in helping to understand the aetiology and development of NAFLD.

Alimentary regimen in non-alcoholic fatty liver disease: Mediterranean diet

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease worldwide. The mechanisms of the underlying disease development and progression are awaiting clarification. Insulin resistance and obesity-related inflammation status, among other possible genetic, dietary, and lifestyle factors, are thought to play the key role. There is no consensus concerning the pharmacological treatment. However, the dietary nutritional management to achieve weight loss is an essential component of any treatment strategy. On the basis of its components, the literature reports on the effectiveness of the Mediterranean diet in reducing cardiovascular risk and in preventing major chronic diseases, including obesity and diabetes. New evidence supports the idea that the Mediterranean diet, associated with physical activity and cognitive behaviour therapy, may have an important role in the prevention and the treatment of NAFLD.

The influence of dietary fat on liver fat accumulation

Obesity is a known risk factor for the development of non-alcoholic fatty liver disease (NAFLD); however, it has been suggested that dietary fat, both amount and composition, may play a pivotal role in its development, independent of body fatness. Studies that have investigated the role of dietary fat on liver fat accumulation are reasonably sparse. We review here the available work that has investigated the impact of dietary fat: amount, composition and frequency, on liver fat accumulation in human observational and intervention studies. Overall, it would seem that total calorie consumption, rather than dietary fat composition, is an important factor in the development of fatty liver disease in humans.

Mass spectrometry-based metabolite profiling in the mouse liver following exposure to ultraviolet B radiation

Although many studies have been performed on the effects of ultraviolet (UV) radiation on the skin, only a limited number of reports have investigated these effects on non-skin tissue. This study aimed to describe the metabolite changes in the liver of hairless mice following chronic exposure to UVB radiation. We did not observe significant macroscopic changes or alterations in hepatic cholesterol and triglyceride levels in the liver of UVB-irradiated mice, compared with those for normal mice. In this study, we detected hepatic metabolite changes by UVB exposure and identified several amino acids, fatty acids, nucleosides, carbohydrates, phospholipids, lysophospholipids, and taurine-conjugated cholic acids as candidate biomarkers in response to UVB radiation in the mouse liver by using various mass spectrometry (MS)-based metabolite profiling including ultra-performance liquid chromatography-quadrupole time-of-flight (TOF)-MS, gas chromatography-TOF-MS and nanomate LTQ-MS. Glutamine exhibited the most dramatic change with a 5-fold increase in quantity. The results from altering several types of metabolites suggest that chronic UVB irradiation may impact significantly on major hepatic metabolism processes, despite the fact that the liver is not directly exposed to UVB radiation. MS-based metabolomic approach for determining regulatory hepatic metabolites following UV irradiation will provide a better understanding of the relationship between internal organs and UV light.

Pathological features of fatty liver disease

Alcoholic liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD) are significant causes of chronic liver disease worldwide. Both are characterized by histological lesions that can include steatosis, and each can lead to cirrhosis. It might be possible for pathologists to identify lesions and patterns of ALD and NAFLD; we review these lesions and propose methods to distinguish between the disorders. Any form of ALD can lead to end-stage liver disease, according to long-term studies of biopsy specimens and patient outcomes. Although steatosis can be a significant cofactor in progression of established chronic liver disease, or even development of hepatocellular carcinoma, only steatohepatitis indicates the presence of progressive liver disease in patients with NAFLD. Pediatric and adolescent NAFLD differ from adult nonalcoholic steatohepatitis and should be recognized as distinct conditions. Benign and malignant liver tumors have been more frequently reported with the increasing prevalence of obesity and diabetes. Histological scoring systems for ALD and NAFLD have been proposed to monitor efficacy in clinical trials and serve as prognostic factors. We review what we have learned from pathological analyses about the development of these disorders and how this information might be used to detect and treat them.

The saturated fatty acid, palmitic acid, induces anxiety-like behavior in mice

OBJECTIVES

Excess fat in the diet can impact neuropsychiatric functions by negatively affecting cognition, mood and anxiety. We sought to show that the free fatty acid (FFA), palmitic acid, can cause adverse biobehaviors in mice that last beyond an acute elevation in plasma FFAs.

METHODS

Mice were administered palmitic acid or vehicle as a single intraperitoneal (IP) injection. Biobehaviors were profiled 2 and 24 h after palmitic acid treatment. Quantification of dopamine (DA), norepinephrine (NE), serotonin (5-HT) and their major metabolites was performed in cortex, hippocampus and amygdala. FFA concentration was determined in plasma. Relative fold change in mRNA expression of unfolded protein response (UPR)-associated genes was determined in brain regions.

RESULTS

In a dose-dependent fashion, palmitic acid rapidly reduced mouse locomotor activity by a mechanism that did not rely on TLR4, MyD88, IL-1, IL-6 or TNFα but was dependent on fatty acid chain length. Twenty-four hours after palmitic acid administration mice exhibited anxiety-like behavior without impairment in locomotion, food intake, depressive-like behavior or spatial memory. Additionally, the serotonin metabolite 5-HIAA was increased by 33% in the amygdala 24h after palmitic acid treatment.

CONCLUSIONS

Palmitic acid induces anxiety-like behavior in mice while increasing amygdala-based serotonin metabolism. These effects occur at a time point when plasma FFA levels are no longer elevated.

Free fatty acids as modulators of the NLRP3 inflammasome in obesity/type 2 diabetes

Free fatty acids (FFAs) are metabolic intermediates that may be obtained through the diet or synthesized endogenously. In addition to serving as an important source of energy, they produce a variety of both beneficial and detrimental effects. They play essential roles as structural components of all cell membranes and as signaling molecules regulating metabolic pathways through binding to nuclear or membrane receptors. However, under conditions of FFAs overload, they become toxic, inducing ROS production, ER stress, apoptosis and inflammation. SFAs (saturated fatty acids), unlike UFAs (unsaturated fatty acids), have recently been proposed as triggers of the NLRP3 inflammasome, a molecular platform mediating the processing of IL-1β in response to infection and stress conditions. Interestingly, UFAs, especially ω-3 FAs, inhibit NLRP3 inflammasome activation in various settings. We focus on emerging models of NLRP3 inflammasome activation with a special emphasis on the molecular mechanisms by which FFAs modulate the activation of this complex. Taking into consideration the current literature and FFA properties, we discuss the putative involvement of mitochondria and the role of cardiolipin, a mitochondrial phospholipid, proposed to be sensed by NLRP3 after release, exposure and/or oxidation. Finally, we review how this SFA-mediated NLRP3 inflammasome activation contributes to the development of both insulin resistance and deficiency associated with obesity/type 2 diabetes. In this context, we highlight the potential clinical use of ω-3 FAs as anti-inflammatory compounds.

JNK inhibitor SP600125 suppresses free fatty acid-induced apoptosis of hepatic stellate cells

AIM: To elucidate the potential effects of c-Jun amino terminal kinase (JNK) inhibitor SP600125 on apoptosis induced by free fatty acids (FFAs) in hepatic stellate cells and the possible mechanism.

METHODS: MTT assay was used to analyze the inhibitory effect of FFAs on growth of hepatic stellate cells, and flow cytometry was used to observe the apoptotic rate of the cells. The expression of p-JNK and p-c-Jun proteins was detected by Western blot assay.

RESULTS: The growth of hepatic stellate cells was significantly inhibited by FFAs in vitro, and FFAs increased the expression of p-JNK and p-c-Jun. JNK inhibitor SP600125 suppressed FFA-induced up-regulation of p-JNK and p-c-Jun.

CONCLUSION: FFAs can inhibit the proliferation of hepatic stellate cells and induce cell apoptosis, which involves the JNK signaling pathway.

Translational approaches: From fatty liver to non-alcoholic steatohepatitis

Over the past few decades, non-alcoholic fatty liver disease (NAFLD) has become one, if not the most common, cause of chronic liver disease affecting both adults and children. The increasing number of cases at an early age is the most worrying aspect of this pathology, since it provides more time for its evolution. The spectrum of this disease ranges from liver steatosis to steatohepatitis, fibrosis and in some cases, hepatocellular carcinoma. NAFLD may not always be considered a benign disease and hepatologists must be cautious in the presence of fatty liver. This should prompt the use of the available experimental models to understand better the pathogenesis and to develop a rational treatment of a disease that is dangerously increasing. In spite of the growing efforts, the pathogenesis of NAFLD is still poorly understood. In the present article we review the most relevant hypotheses and evidence that account for the progression of NAFLD to non-alcoholic steatohepatitis (NASH) and fibrosis. The available in vitro and in vivo experimental models of NASH are discussed and revised in terms of their validity in translational studies. These studies must be aimed at the discovery of the still unknown triggers or mediators that induce the progression of hepatic inflammation, apoptosis and fibrosis.

Anti-lipoapoptotic effect of Artemisia capillaris extract on free fatty acids-induced HepG2 cells

Background

Artemisia capillaris (AC) has been recognized as one of the promising candidates for hepatoprotective, hypoglycemic, hypolipidemic, antiobesitic and anti-inflammatory therapeutic effectiveness. This study evaluated the inherent mechanism and anti-apoptotic activity of 30% ethanol extract of AC (AC extract) 100 μg/ml on free fatty acids (FFAs)-induced HepG2 cellular steatosis and lipoapoptosis.

Methods

Hepatic steatosis was induced by culturing HepG2 cells with a FFAs mixture (oleic and palmitic acid at the proportion of 2:1) for 24 h, thus ultimately giving rise to lipoapoptosis. Cell viability and lipid accumulation were detected by MTT assay and Oil Red O staining method respectively and Caspase-3, −9, Bax, Bcl-2, p-JNK and PUMA were measured for lipoapoptosis after 24 hours.

Results

AC extract significantly improved the FFAs-induced steatosis without cytotoxicity and Caspase-3, −9, Bax and Bcl-2 were modulated profitably to HepG2 cells after AC treatment. In addition, AC extract inhibited the activation of c-Jun NH₂ terminal kinase (JNK) and PUMA, which mechanism is related to non-alcoholic steatohepatitis (NASH).

Conclusions

Combined together, AC extract exerted an obvious hypolipidemic and anti-apoptotic effect, indicating that AC extract might have potential therapeutic herb against NASH.

Hypoxia, lipids, and cancer: surviving the harsh tumor microenvironment

Solid tumors typically develop hostile microenvironments characterized by irregular vascularization and poor oxygen (O2) and nutrient supply. Whereas normal cells modulate anabolic and catabolic pathways in response to changes in nutrient availability, cancer cells exhibit unregulated growth even under nutrient scarcity. Recent studies have demonstrated that constitutive activation of growth-promoting pathways results in dependence on unsaturated fatty acids for survival under O2 deprivation. In cancer cells, this dependence represents a critical metabolic vulnerability that could be exploited therapeutically. Here we review how this dependence on unsaturated lipids is affected by the microenvironmental conditions faced by cancer cells.

Exendin‑4, a glucagon‑like peptide‑1 receptor agonist, modulates hepatic fatty acid composition and Δ‑5‑desaturase index in a murine model of non‑alcoholic steatohepatitis

Glucagon‑like peptide‑1 (GLP‑1) is involved in the development of non‑alcoholic steatohepatitis (NASH), which is characterized by fatty acid imbalance. The aim of this study was to investigate the effects of the GLP‑1 receptor (GLP‑1R) agonist, exendin‑4 (Ex‑4), on hepatic fatty acid metabolism and its key enzyme, Δ‑5‑desaturase, in a murine model of NASH. NASH was induced in db/db mice fed a methionine‑choline deficient (MCD) diet. Ex‑4 (n=4) or saline [control (CON); n=4] was administered intraperitoneally for 8 weeks. Steatohepatitis activity was evaluated by non‑alcoholic fatty liver disease (NAFLD) activity score. Hepatic fatty acid composition and Δ‑5‑desaturase index were analyzed by gas chromatography. Ex‑4 treatment significantly reduced body weight and the NAFLD activity score. Hepatic concentrations of long‑chain saturated fatty acids (SFAs) were significantly higher in the Ex‑4 group compared to the CON group (23240±955 vs. 31710±8436 µg/g•liver, P<0.05).Ex‑4 significantly reduced hepatic n‑3 polyunsaturated fatty acid (PUFA)/n‑6 PUFA ratio compared to the CON group (13.83±3.15 vs. 8.73±1.95, P<0.05). In addition, the hepatic Δ‑5‑desaturase index was significantly reduced in the Ex‑4 group compared to the CON group (31.1±12.4 vs. 10.5±3.1, P<0.05). In conclusion, the results showed that Ex‑4 improved steatohepatitis in a murine model of NASH. Furthermore, Ex‑4 altered hepatic long‑chain saturated and PUFA composition and reduced the Δ‑5‑desaturase index. Thus, Ex‑4 may improve NASH by regulating hepatic fatty acid metabolism.

Enhanced synthesis of saturated phospholipids is associated with ER stress and lipotoxicity in palmitate treated hepatic cells

High levels of saturated FAs (SFAs) are acutely toxic to a variety of cell types, including hepatocytes, and have been associated with diseases such as type 2 diabetes and nonalcoholic fatty liver disease. SFA accumulation has been previously shown to degrade endoplasmic reticulum (ER) function leading to other manifestations of the lipoapoptotic cascade. We hypothesized that dysfunctional phospholipid (PL) metabolism is an initiating factor in this ER stress response. Treatment of either primary hepatocytes or H4IIEC3 cells with the SFA palmitate resulted in dramatic dilation of the ER membrane, coinciding with other markers of organelle dysfunction. This was accompanied by increased de novo glycerolipid synthesis, significant elevation of dipalmitoyl phosphatidic acid, diacylglycerol, and total PL content in H4IIEC3 cells. Supplementation with oleate (OA) reversed these markers of palmitate (PA)-induced lipotoxicity. OA/PA cotreatment modulated the distribution of PA between lipid classes, increasing the flux toward triacylglycerols while reducing its incorporation into PLs. Similar trends were demonstrated in both primary hepatocytes and the H4IIEC3 hepatoma cell line. Overall, these findings suggest that modifying the FA composition of structural PLs can protect hepatocytes from PA-induced ER stress and associated lipotoxicity.

ER calcium release promotes mitochondrial dysfunction and hepatic cell lipotoxicity in response to palmitate overload

Palmitate overload induces hepatic cell dysfunction characterized by enhanced apoptosis and altered citric acid cycle (CAC) metabolism; however, the mechanism of how this occurs is incompletely understood. We hypothesize that elevated doses of palmitate disrupt intracellular calcium homeostasis resulting in a net flux of calcium from the ER to mitochondria, activating aberrant oxidative metabolism. We treated primary hepatocytes and H4IIEC3 cells with palmitate and calcium chelators to identify the roles of intracellular calcium flux in lipotoxicity. We then applied ¹³C metabolic flux analysis (MFA) to determine the impact of calcium in promoting palmitate-stimulated mitochondrial alterations. Co-treatment with the calcium-specific chelator BAPTA resulted in a suppression of markers for apoptosis and oxygen consumption. Additionally, ¹³C MFA revealed that BAPTA co-treated cells had reduced CAC fluxes compared to cells treated with palmitate alone. Our results demonstrate that palmitate-induced lipoapoptosis is dependent on calcium-stimulated mitochondrial activation, which induces oxidative stress.

Modeling progressive non-alcoholic fatty liver disease in the laboratory mouse

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease in the world and its prevalence is rising. In the absence of disease progression, fatty liver poses minimal risk of detrimental health outcomes. However, advancement to non-alcoholic steatohepatitis (NASH) confers a markedly increased likelihood of developing severe liver pathologies, including fibrosis, cirrhosis, organ failure, and cancer. Although a substantial percentage of NAFLD patients develop NASH, the genetic and molecular mechanisms driving this progression are poorly understood, making it difficult to predict which patients will ultimately develop advanced liver disease. Deficiencies in mechanistic understanding preclude the identification of beneficial prognostic indicators and the development of effective therapies. Mouse models of progressive NAFLD serve as a complementary approach to the direct analysis of human patients. By providing an easily manipulated experimental system that can be rigorously controlled, they facilitate an improved understanding of disease development and progression. In this review, we discuss genetically- and chemically-induced models of NAFLD that progress to NASH, fibrosis, and liver cancer in the context of the major signaling pathways whose disruption has been implicated as a driving force for their development. Additionally, an overview of nutritional models of progressive NAFLD is provided.

Insight into the impact of dietary saturated fat on tissue-specific cellular processes underlying obesity-related diseases

This study investigated the influence of three high-fat diets (HFDs), differing in the percentage of total calories from saturated fat (SF) (6%, 12%, 24%) but identical in total fat (40%), for a 16-week period in mice on a variety of tissue-specific cellular processes believed to be at the root of obesity-related diseases. Specifically, we examined ectopic lipid accumulation, oxidative capacity [peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) mRNA and protein; mtDNA; Cox IV and cytochrome C protein; citrate synthase activity; and gene expression of fission 1, mitofusin (Mfn) 1 and Mfn2], oxidative stress (4-hydroxy-2-nonenal), endoplasmic reticulum (ER) stress (binding immunoglobulin protein, activating transcription factor 6-p50, p-eukaryotic initiation factor 2 alpha and x-box binding protein 1 spliced protein), inflammatory [p-c-Jun N-terminal kinase (JNK), p-nuclear factor kappa-B, p-p38 mitogen-activated protein kinase) and insulin signaling (p-Akt), and inflammation [tumor necrosis factor-alpha, monocyte chemotactic protein-1, interleukin-6, F4/80, toll-like receptor (TLR)2 and TLR4 gene expression] in various tissues, including the adipose tissue, liver, skeletal muscle and heart. In general, adipose and hepatic tissues were the only tissues which displayed evidence of dysfunction. All HFDs down-regulated adipose, cardiac and hepatic PGC-1α mRNA and hepatic citrate synthase activity, and induced adipose tissue oxidative stress, whereas only the 6%-SF and 12%-SF diet produced hepatic steatosis. However, compared to the 6%-SF and 24%-SF diets, consumption of the 12%-SF diet resulted in the greatest degree of dysregulation (hepatic ER and oxidative stress, JNK activation, increased F4/80 gene expression and down-regulation of adipose tissue Akt signaling). These findings suggest that the saturated fatty acid composition of an HFD can greatly influence the processes responsible for obesity-related diseases - nonalcoholic fatty liver disease, in particular - as well as provide further evidence that the mechanisms at the root of these diseases are diet and tissue sensitive.

Modulation of hepatic steatosis by dietary fatty acids

Non-alcoholic fatty liver disease (NAFLD) describes a range of conditions caused by fat deposition within liver cells. Liver fat content reflects the equilibrium between several metabolic pathways involved in triglyceride synthesis and disposal, such as lipolysis in adipose tissue and de novo lipogenesis, triglyceride esterification, fatty acid oxidation and very-low-density lipoprotein synthesis/secretion in hepatic tissue. In particular, it has been demonstrated that hepatic de novo lipogenesis plays a significant role in NAFLD pathogenesis. It is widely known that the fatty acid composition of the diet influences hepatic lipogenesis along with other metabolic pathways. Therefore, dietary fat may not only be involved in the pathogenesis of hepatic steatosis, but may also prevent and/or reverse hepatic fat accumulation. In this review, major data from the literature about the role of some dietary fats as a potential cause of hepatic fat accumulation or as a potential treatment for NAFLD are described. Moreover, biochemical mechanisms responsible for an increase or decrease in hepatic lipid content are critically analyzed. It is noteworthy that both quantitative and qualitative aspects of dietary fat influence triglyceride deposition in the liver. A high-fat diet or the dietary administration of conjugated linoleic acids induced hepatic steatosis. In contrast, supplementation of the diet with krill oil or pine nut oil helped in the prevention and/or in the treatment of steatotic liver. Quite interesting is the “case” of olive oil, since several studies have often provided different and⁄or conflicting results in animal models.

Palmitate-induced activation of mitochondrial metabolism promotes oxidative stress and apoptosis in H4IIEC3 rat hepatocytes

OBJECTIVE

Hepatic lipotoxicity is characterized by reactive oxygen species (ROS) accumulation, mitochondrial dysfunction, and excessive apoptosis, but the precise sequence of biochemical events leading to oxidative damage and cell death remains unclear. The goal of this study was to delineate the role of mitochondrial metabolism in mediating hepatocyte lipotoxicity.

MATERIALS/METHODS

We treated H4IIEC3 rat hepatoma cells with free fatty acids in combination with antioxidants and mitochondrial inhibitors designed to block key events in the progression toward apoptosis. We then applied (13)C metabolic flux analysis (MFA) to quantify mitochondrial pathway alterations associated with these treatments.

RESULTS

Treatment with palmitate alone led to a doubling in oxygen uptake rate and in most mitochondrial fluxes. Supplementing culture media with the antioxidant N-acetyl-cysteine (NAC) reduced ROS accumulation and caspase activation and partially restored cell viability. However, (13)C MFA revealed that treatment with NAC did not normalize palmitate-induced metabolic alterations, indicating that neither elevated ROS nor downstream apoptotic events contributed to mitochondrial activation. To directly limit mitochondrial metabolism, the complex I inhibitor phenformin was added to cells treated with palmitate. Phenformin addition eliminated abnormal ROS accumulation, prevented the appearance of apoptotic markers, and normalized mitochondrial carbon flow. Further studies revealed that glutamine provided the primary fuel for elevated mitochondrial metabolism in the presence of palmitate, rather than fatty acid beta-oxidation, and that glutamine consumption could be reduced through co-treatment with phenformin but not NAC.

CONCLUSION

Our results indicate that ROS accumulation in palmitate-treated H4IIEC3 cells occurs downstream of altered mitochondrial oxidative metabolism, which is independent of beta-oxidation and precedes apoptosis initiation.

Body composition variables as predictors of NAFLD by ultrasound in obese children and adolescents

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) is a disorder associated with excessive fat accumulation, mainly in the intra-abdominal region. A simple technique to estimate abdominal fat in this region could be useful to assess the presence of NAFLD, in obese subjects who are more vulnerable to this disease. The aim of this cross-sectional study was to verify the reliability of waist circumference and body composition variables to identify the occurrence of NAFLD in obese children and adolescents.

METHODS

Sample was composed of 145 subjects, aged 11 to 17 years. Assessments of waist circumference (WC), trunk fat mass (TFM) and fat mass (FM) by dual-energy X-ray absorptiometry (DXA) and ultrasound for diagnosis of NAFLD and intra-abdominal adipose tissue (IAAT) were used. Correlation between variables was made by Spearman's coefficients; ROC curve parameters (sensitivity, specificity, area under curve) were used to assess the reliability of body composition variables to assess the presence of NAFLD. Statistical significance was set at 5%.

RESULTS

Significant correlations were observed between NAFLD and WC (p = 0.001), TFM (p = 0.002) and IAAT (p = 0.001). The higher values of area under the ROC curve were for WC (AUC = 0.720), TFM (AUC = 0.661) and IAAT (AUC = 0.741).

CONCLUSIONS

Our findings indicated that TFM, IAAT and WC present high potential to identify NAFLD in obese children and adolescents.

Multiple pathways are involved in palmitic acid-induced toxicity

In this study, we identified the toxic mechanism following the accumulation of palmitic acid (PA), a saturated fatty acid, in human Chang liver cells. After PA exposure for 24 h, the mitochondria and the endoplasmic reticulum (ER) became dilated, and lipid droplets and organelles were observed within autophagosomes. Cell viability decreased with an ATP reduction and the G2/M phase arrest. The expression of SOD-2, but not of SOD-1, markedly increased after PA exposure, which also elevated the number of cells generating ROS. PA enhanced the levels of proteins related to apoptosis, necroptosis, autophagy, and ER stress. Moreover, the inhibition of caspases, p53, necroptosis, or ER stress substantially rescued PA-induced cytotoxicity and, similarly, the inhibition of caspases and ER stress counteracted PA-induced changes in the cell cycle. Conversely, the inhibition of necroptosis and p53 signaling accelerated the changes in the cell cycle triggered by PA exposure. Blocking autophagy exacerbated PA-induced cytotoxicity and alterations in the cell cycle and caused disappearance of cellular components. These results suggest that PA induces apoptosis accompanied by autophagy through mitochondrial dysfunction and ER stress, which are triggered by oxidative stress in Chang liver cells and that blocking autophagy accelerates cell damage following PA exposure.

Unsaturated fatty acids, desaturases, and human health

With the increasing concern for health and nutrition, dietary fat has attracted considerable attention. The composition of fatty acids in a diet is important since they are associated with major diseases, such as cancers, diabetes, and cardiovascular disease. The biosynthesis of unsaturated fatty acids (UFA) requires the expression of dietary fat-associated genes, such as SCD, FADS1, FADS2, and FADS3, which encode a variety of desaturases, to catalyze the addition of a double bond in a fatty acid chain. Recent studies using new molecular techniques and genomics, as well as clinical trials have shown that these genes and UFA are closely related to physiological conditions and chronic diseases; it was found that the existence of alternative transcripts of the desaturase genes and desaturase isoforms might affect human health and lipid metabolism in different ways. In this review, we provide an overview of UFA and desaturases associated with human health and nutrition. Moreover, recent findings of UFA, desaturases, and their associated genes in human systems are discussed. Consequently, this review may help elucidate the complicated physiology of UFA in human health and diseases.

Circulating levels of sirtuin 4, a potential marker of oxidative metabolism, related to coronary artery disease in obese patients suffering from NAFLD, with normal or slightly increased liver enzymes

The present study shows low circulating levels of SIRT4 in obese patients with nonalcoholic fatty liver disease mirroring its reduced mitochondrial expression in an attempt to increase the fat oxidative capacity and then the mitochondrial function in liver and in muscle. SIRT4 modulates the metabolism of free fatty acids reducing their high circulating levels but, unfortunately, increasing ROS production. Great concentration of free fatty acids, released by adipose tissue, coupled with oxidative stress, directly results in endothelial dysfunction, early atherosclerosis, and coronary artery disease risk factor.

The endoplasmic reticulum coat protein II transport machinery coordinates cellular lipid secretion and cholesterol biosynthesis

Triglycerides and cholesterol are essential for life in most organisms. Triglycerides serve as the principal energy storage depot and, where vascular systems exist, as a means of energy transport. Cholesterol is essential for the functional integrity of all cellular membrane systems. The endoplasmic reticulum is the site of secretory lipoprotein production and de novo cholesterol synthesis, yet little is known about how these activities are coordinated with each other or with the activity of the COPII machinery, which transports endoplasmic reticulum cargo to the Golgi. The Sar1B component of this machinery is mutated in chylomicron retention disorder, indicating that this Sar1 isoform secures delivery of dietary lipids into the circulation. However, it is not known why some patients with chylomicron retention disorder develop hepatic steatosis, despite impaired intestinal fat malabsorption, and why very severe hypocholesterolemia develops in this condition. Here, we show that Sar1B also promotes hepatic apolipoprotein (apo) B lipoprotein secretion and that this promoting activity is coordinated with the processes regulating apoB expression and the transfer of triglycerides/cholesterol moieties onto this large lipid transport protein. We also show that although Sar1A antagonizes the lipoprotein secretion-promoting activity of Sar1B, both isoforms modulate the expression of genes encoding cholesterol biosynthetic enzymes and the synthesis of cholesterol de novo. These results not only establish that Sar1B promotes the secretion of hepatic lipids but also adds regulation of cholesterol synthesis to Sar1B's repertoire of transport functions.

The effects of α-lipoic acid on liver oxidative stress and free fatty acid composition in methionine-choline deficient diet-induced NAFLD

Development of nonalcoholic fatty liver disease (NAFLD) occurs through initial steatosis and subsequent oxidative stress. The aim of this study was to examine the effects of α-lipoic acid (LA) on methionine-choline deficient (MCD) diet-induced NAFLD in mice. Male C57BL/6 mice (n=21) were divided into three groups (n=7 per group): (1) control fed with standard chow, (2) MCD2 group--fed with MCD diet for 2 weeks, and (3) MCD2+LA group--2 weeks on MCD receiving LA i.p. 100 mg/kg/day. After the treatment, liver samples were taken for pathohistology, oxidative stress parameters, antioxidative enzymes, and liver free fatty acid (FFA) composition. Mild microvesicular hepatic steatosis was found in MCD2 group, while it was reduced to single fat droplets evident in MCD2+LA group. Lipid peroxidation and nitrosative stress were increased by MCD diet, while LA administration induced a decrease in liver malondialdehyde and nitrates+nitrites level. Similary, LA improved liver antioxidative capacity by increasing total superoxide dismutase (tSOD), manganese SOD (MnSOD), and copper/zinc-SOD (Cu/ZnSOD) activity as well as glutathione (GSH) content. Liver FFA profile has shown a significant decrease in saturated acids, arachidonic, and docosahexaenoic acid (DHA), while LA treatment increased their proportions. It can be concluded that LA ameliorates lipid peroxidation and nitrosative stress in MCD diet-induced hepatic steatosis through an increase in SOD activity and GSH level. In addition, LA increases the proportion of palmitic, stearic, arachidonic, and DHA in the fatty liver. An increase in DHA may be a potential mechanism of anti-inflammatory and antioxidant effects of LA in MCD diet-induced NAFLD.

Hepatic fatty acid trafficking: multiple forks in the road

The liver plays a unique, central role in regulating lipid metabolism. In addition to influencing hepatic function and disease, changes in specific pathways of fatty acid (FA) metabolism have wide-ranging effects on the metabolism of other nutrients, extra-hepatic physiology, and the development of metabolic diseases. The high prevalence of nonalcoholic fatty liver disease (NAFLD) has led to increased efforts to characterize the underlying biology of hepatic energy metabolism and FA trafficking that leads to disease development. Recent advances have uncovered novel roles of metabolic pathways and specific enzymes in generating lipids important for cellular processes such as signal transduction and transcriptional activation. These studies have also advanced our understanding of key branch points involving FA partitioning between metabolic pathways and have identified new roles for lipid droplets in these events. This review covers recent advances in our understanding of FA trafficking and its regulation. An emphasis will be placed on branch points in these pathways and how alterations in FA trafficking contribute to NAFLD and related comorbidities.

The adaptive endoplasmic reticulum stress response to lipotoxicity in progressive human nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) may progress from simple steatosis to severe, nonalcoholic steatohepatitis (NASH) in 7%-14% of the U.S. population through a second "hit" in the form of increased oxidative stress and inflammation. Endoplasmic reticulum (ER) stress signaling and the unfolded protein response (UPR) are triggered when high levels of lipids and misfolded proteins alter ER homeostasis creating a lipotoxic environment within NAFLD livers. The objective of this study was to determine the coordinate regulation of ER stress-associated genes in the progressive stages of human NAFLD. Human liver samples categorized as normal, steatosis, NASH (Fatty), and NASH (Not Fatty) were analyzed by individual Affymetrix GeneChip Human 1.0 ST microarrays, immunoblots, and immunohistochemistry. A gene set enrichment analysis was performed on autophagy, apoptosis, lipogenesis, and ER stress/UPR gene categories. An enrichment of downregulated genes in the ER stress-associated lipogenesis and ER stress/UPR gene categories was observed in NASH. Conversely, an enrichment of upregulated ER stress-associated genes for autophagy and apoptosis gene categories was observed in NASH. Protein expression of the adaptive liver response protein STC2 and the transcription factor X-box binding protein 1 spliced (XBP-1s) were significantly elevated among NASH samples, whereas other downstream ER stress proteins including CHOP, ATF4, and phosphorylated JNK and eIF2α were not significantly changed in disease progression. Increased nuclear accumulation of total XBP-1 protein was observed in steatosis and NASH livers. The findings reveal the presence of a coordinated, adaptive transcriptional response to hepatic ER stress in human NAFLD.

Role of stearoyl-CoA desaturase-1 in skeletal muscle function and metabolism

Stearoyl-CoA desaturase-1 (SCD1) converts saturated fatty acids (SFA) into monounsaturated fatty acids and is necessary for proper liver, adipose tissue, and skeletal muscle lipid metabolism. While there is a wealth of information regarding SCD1 expression in the liver, research on its effect in skeletal muscle is scarce. Furthermore, the majority of information about its role is derived from global knockout mice, which are known to be hypermetabolic and fail to accumulate SCD1's substrate, SFA. We now know that SCD1 expression is important in regulating lipid bilayer fluidity, increasing triglyceride formation, and enabling lipogenesis and may protect against SFA-induced lipotoxicity. Exercise has been shown to increase SCD1 expression, which may contribute to an increase in intramyocellular triglyceride at the expense of free fatty acids and diacylglycerol. This review is intended to define the role of SCD1 in skeletal muscle and discuss the potential benefits of its activity in the context of lipid metabolism, insulin sensitivity, exercise training, and obesity.

tert-Butylhydroquinone (tBHQ) protects hepatocytes against lipotoxicity via inducing autophagy independently of Nrf2 activation

Saturated fatty acids (SFAs) induce hepatocyte cell death, wherein oxidative stress is mechanistically involved. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is a master transcriptional regulator of cellular antioxidant defense enzymes. Therefore, Nrf2 activation is regarded as an effective strategy against oxidative stress-triggered cellular damage. In this study, tert-butylhydroquinone (tBHQ), a widely used Nrf2 activator, was initially employed to investigate the potential protective role of Nrf2 activation in SFA-induced hepatoxicity. As expected, SFA-induced hepatocyte cell death was prevented by tBHQ in both AML-12 mouse hepatocytes and HepG2 human hepatoma cells. However, the protective effect of tBHQ is Nrf2-independent, because the siRNA-mediated Nrf2 silencing did not abrogate tBHQ-conferred protection. Alternatively, our results revealed that autophagy activation was critically involved in the protective effect of tBHQ on lipotoxicity. tBHQ induced autophagy activation and autophagy inhibitors abolished tBHQ's protection. The induction of autophagy by tBHQ exposure was demonstrated by the increased accumulation of LC3 puncta, LC3-II conversion, and autophagic flux (LC3-II conversion in the presence of proteolysis inhibitors). Subsequent mechanistic investigation discovered that tBHQ exposure activated AMP-activated protein kinase (AMPK) and siRNA-mediated AMPK gene silencing abolished tBHQ-induced autophagy activation, indicating that AMPK is critically involved in tBHQ-triggered autophagy induction. Furthermore, our study provided evidence that tBHQ-induced autophagy activation is required for its Nrf2-activating property. Collectively, our data uncover a novel mechanism for tBHQ in protecting hepatocytes against SFA-induced lipotoxicity. tBHQ-triggered autophagy induction contributes not only to its hepatoprotective effect, but also to its Nrf2-activating property.

Silibinin ameliorates steatosis and insulin resistance during non-alcoholic fatty liver disease development partly through targeting IRS-1/PI3K/Akt pathway

Silibinin (SIL) is a well-studied hepato-protective agent against a spectrum of liver diseases. However, the role of SIL in non-alcoholic fatty liver disease (NAFLD) induced insulin resistance and underlying signaling is not fully characterized. In this study, Sprague-Dawley (SD) rats were fed with high-fat diet to develop NAFLD with or without an SIL co-treatment. NAFLD rats showed typical NAFLD symptoms including histological changes, insulin resistance, and glucose metabolism dysfunction. SIL co-treatment significantly ameliorated these pathological features partly through restoring the IRS-1/PI3K/Akt pathway. In addition, BRL-3A and HepG2 cells were incubated with palmitic acid (PA) to induce steatosis. SIL co-treatment in cells also reduced lipid accumulation, recovered cell viability, and down-regulated the protein expression of resistin, the marker for insulin resistance. Specific blocker of PI3K abolished the ameliorative effects of SIL on cellular steatosis. In conclusion, SIL alleviated steatosis and insulin resistance both in vivo and in vitro partly through regulating the IRS-1/PI3K/Akt pathway.

Decoding cell death signals in liver inflammation

Inflammation can be either beneficial or detrimental to the liver, depending on multiple factors. Mild (i.e., limited in intensity and destined to resolve) inflammatory responses have indeed been shown to exert consistent hepatoprotective effects, contributing to tissue repair and promoting the re-establishment of homeostasis. Conversely, excessive (i.e., disproportionate in intensity and permanent) inflammation may induce a massive loss of hepatocytes and hence exacerbate the severity of various hepatic conditions, including ischemia-reperfusion injury, systemic metabolic alterations (e.g., obesity, diabetes, non-alcoholic fatty liver disorders), alcoholic hepatitis, intoxication by xenobiotics and infection, de facto being associated with irreversible liver damage, fibrosis, and carcinogenesis. Both liver-resident cells (e.g., Kupffer cells, hepatic stellate cells, sinusoidal endothelial cells) and cells that are recruited in response to injury (e.g., monocytes, macrophages, dendritic cells, natural killer cells) emit pro-inflammatory signals including - but not limited to - cytokines, chemokines, lipid messengers, and reactive oxygen species that contribute to the apoptotic or necrotic demise of hepatocytes. In turn, dying hepatocytes release damage-associated molecular patterns that-upon binding to evolutionary conserved pattern recognition receptors-activate cells of the innate immune system to further stimulate inflammatory responses, hence establishing a highly hepatotoxic feedforward cycle of inflammation and cell death. In this review, we discuss the cellular and molecular mechanisms that account for the most deleterious effect of hepatic inflammation at the cellular level, that is, the initiation of a massive cell death response among hepatocytes.

Mechanisms of liver involvement in systemic disease

The liver may be injured during the course of many systemic diseases. The mechanisms of injury can be broadly divided into four pathways: vascular, toxic, immune, and hormonal. Vascular obstruction may be an early event but is also the late common pathway from all mechanisms. Despite the large number of possible initiating factors, the end results are few, including death of hepatocytes or cholangiocytes, leading to the stereotyped syndromes of acute liver failure, non-cirrhotic portal hypertension, or cirrhosis. This small number of outcomes is a reflection of the few anatomic patterns that can be generated by microvascular obstruction. Vascular obstruction may occur by thrombosis, inflammation, or congestive injury. The innate immunity pathway is activated by endotoxin and other agents, leading to inflammatory infiltration, release of cytokines and reactive oxygen species, and necrosis. The adaptive immune pathway involves the generation of antibodies and antigen-specific cell-mediated attack on hepatic cells. Hormonal effects are principally involved when overnutrition leads to hyperinsulinemia followed by hepatocellular necrosis.

Obesity-associated oxidative stress: strategies finalized to improve redox state

Obesity represents a major risk factor for a plethora of severe diseases, including diabetes, cardiovascular disease, non-alcoholic fatty liver disease, and cancer. It is often accompanied by an increased risk of mortality and, in the case of non-fatal health problems, the quality of life is impaired because of associated conditions, including sleep apnea, respiratory problems, osteoarthritis, and infertility. Recent evidence suggests that oxidative stress may be the mechanistic link between obesity and related complications. In obese patients, antioxidant defenses are lower than normal weight counterparts and their levels inversely correlate with central adiposity; obesity is also characterized by enhanced levels of reactive oxygen or nitrogen species. Inadequacy of antioxidant defenses probably relies on different factors: obese individuals may have a lower intake of antioxidant- and phytochemical-rich foods, such as fruits, vegetables, and legumes; otherwise, consumption of antioxidant nutrients is normal, but obese individuals may have an increased utilization of these molecules, likewise to that reported in diabetic patients and smokers. Also inadequate physical activity may account for a decreased antioxidant state. In this review, we describe current concepts in the meaning of obesity as a state of chronic oxidative stress and the potential interventions to improve redox balance.