Gene expression in human NAFLD

Fatty acid transporters involved in the palmitate and oleate induced insulin resistance in primary rat hepatocytes

AIMS

To determine the presence and possible involvement of FAT/CD36, FABPpm and FATP-2, transporters in (i) fatty acids movement across plasma membrane and (ii) an induction of insulin resistance by palmitic (PA) and oleic (OA) fatty acids in primary hepatocytes.

METHODS

Primary hepatocytes were treated with either PA and OA or combination of activators (AICAR, Insulin) or inhibitors (SSO, phloretin) of FA transport. Expression of FA and glucose transporters as well as insulin signalling proteins was determined using Western blot analyses. Palmitate and glucose transport was measured using radioactive isotopes. Intracellular lipid content [ceramide, diacylglycerols (DG) and triacylglycerols] and FA composition were estimated by GLC.

RESULTS

In primary hepatocytes, adding phloretin diminished insulin, and AICAR stimulated palmitate transport. Both PA and OA fatty acids induced the protein expression of FAT/CD36 and FATP-2 with concomitant: (i) reduction in GLUT-2 protein content, (ii) inhibition of insulin-stimulated glucose uptake, (iii) reduction in insulin-stimulated activation of AKT and GSK, (iv) accumulation of either DG (PA and OA) or ceramide (only PA).

CONCLUSIONS

FA transport into hepatocytes is, at least in part, protein-mediated process, and both PA and OA induce the protein expression of FAT/CD36 and FATP-2. Both saturated (PA) and unsaturated (OA) fatty acids induce insulin resistance in primary hepatocytes, associated with the accumulation of DG and/or ceramide.

Misregulation of PPAR Functioning and Its Pathogenic Consequences Associated with Nonalcoholic Fatty Liver Disease in Human Obesity

Nonalcoholic fatty liver disease in human obesity is characterized by the multifactorial nature of the underlying pathogenic mechanisms, which include misregulation of PPARs signaling. Liver PPAR-α downregulation with parallel PPAR-γ and SREBP-1c up-regulation may trigger major metabolic disturbances between de novo lipogenesis and fatty acid oxidation favouring the former, in association with the onset of steatosis in obesity-induced oxidative stress and related long-chain polyunsaturated fatty acid n-3 (LCPUFA n-3) depletion, insulin resistance, hypoadiponectinemia, and endoplasmic reticulum stress. Considering that antisteatotic strategies targeting PPAR-α revealed that fibrates have poor effectiveness, thiazolidinediones have weight gain limitations, and dual PPAR-α/γ agonists have safety concerns, supplementation with LCPUFA n-3 appears as a promising alternative, which achieves both significant reduction in liver steatosis scores and a positive anti-inflammatory outcome. This latter aspect is of importance as PPAR-α downregulation associated with LCPUFA n-3 depletion may play a role in increasing the DNA binding capacity of proinflammatory factors, NF-κB and AP-1, thus constituting one of the major mechanisms for the progression of steatosis to steatohepatitis.

Dose-dependent effects of calorie restriction on gene expression, metabolism, and tumor progression are partially mediated by insulin-like growth factor-1

The prevalence of obesity, an established risk and progression factor for breast and many other cancer types, remains very high in the United States and throughout the world. Calorie restriction (CR), a reduced-calorie dietary regimen typically involving a 20-40% reduction in calorie consumption, prevents or reverses obesity, and inhibits mammary and other types of cancer in multiple tumor model systems. Unfortunately, the mechanisms underlying the tumor inhibitory effects of CR are poorly understood, and a better understanding of these mechanisms may lead to new intervention targets and strategies for preventing or controlling cancer. We have previously shown that the anticancer effects of CR are associated with decreased systemic levels of insulin-like growth factor-1 (IGF-1), the primary source of which is liver. We have also reported that CR strongly suppresses tumor development and growth in multiple mammary cancer models. To identify CR-responsive genes and pathways, and to further characterize the role of IGF-1 as a mediator of the anticancer effects of CR, we assessed hepatic and mammary gland gene expression, hormone levels and growth of orthotopically transplanted mammary tumors in control and CR mice with and without exogenous IGF-1. C57BL/6 mice were fed either control AIN-76A diet ad libitum (AL), subjected to 20%, 30%, or 40% CR plus placebo timed-release pellets, or subjected to 30% or 40% CR plus timed-release pellets delivering murine IGF-1 (mIGF-1, 20 μg/day). Compared with AL-fed controls, body weights were decreased 14.3% in the 20% CR group, 18.5% in the 30% CR group, and 38% in the 40% CR group; IGF-1 infusion had no effect on body weight. Hepatic transcriptome analyses indicated that compared with 20% CR, 30% CR significantly modulated more than twice the number of genes and 40% CR more than seven times the number of genes. Many of the genes specific to the 40% CR regimen were hepatic stress-related and/or DNA damage-related genes. Exogenous IGF-1 rescued the hepatic expression of several metabolic genes and pathways affected by CR. Exogenous IGF-1 also rescued the expression of several metabolism- and cancer-related genes affected by CR in the mammary gland. Furthermore, exogenous IGF-1 partially reversed the mammary tumor inhibitory effects of 30% CR. We conclude that several genes and pathways, particularly those associated with macronutrient and steroid hormone metabolism, are associated with the anticancer effects of CR, and that reduced IGF-1 levels can account, at least in part, for many of the effects of CR on gene expression and mammary tumor burden.

N-acetylcysteine (NAC) diminishes the severity of PCB 126-induced fatty liver in male rodents

Potent aryl hydrocarbon receptor agonists like PCB 126 (3,3',4,4',5-pentachlorobiphenyl) cause oxidative stress and liver pathology, including fatty liver. Our question was whether dietary supplementation with N-acetylcysteine (NAC), an antioxidant, can prevent these adverse changes. Male Sprague-Dawley rats were fed a standard AIN-93G diet (sufficient in cysteine) or a modified diet supplemented with 1.0% NAC. After one week, rats on each diet were exposed to 0, 1, or 5μmol/kg body weight PCB 126 by i.p. injection (6 rats per group) and euthanized two weeks later. PCB-treatment caused a dose-dependent reduction in growth, feed consumption, relative thymus weight, total glutathione and glutathione disulfide (GSSG), while relative liver weight, glutathione transferase activity and hepatic lipid content were dose-dependently increased with PCB dose. Histologic examination of liver tissue showed PCB 126-induced hepatocellular steatosis with dose dependent increase in lipid deposition and distribution. Dietary NAC resulted in a reduction in hepatocellular lipid in both PCB groups. This effect was confirmed by gravimetric analysis of extracted lipids. Expression of CD36, a scavenger receptor involved in regulating hepatic fatty acid uptake, was reduced with high dose PCB treatment but unaltered in PCB-treated rats on NAC-supplemented diet. These results demonstrate that NAC has a protective effect against hepatic lipid accumulation in rats exposed to PCB 126. The mechanism of this protective effect appears to be independent of NAC as a source of cysteine/precursor of glutathione.

Macrophage-specific transgenic expression of cholesteryl ester hydrolase attenuates hepatic lipid accumulation and also improves glucose tolerance in ob/ob mice

Cellular cholesterol homeostasis is increasingly being recognized as an important determinant of the inflammatory status of macrophages, and a decrease in cellular cholesterol levels polarizes macrophages toward an anti-inflammatory or M2 phenotype. Cholesteryl ester hydrolase (CEH) catalyzes the hydrolysis of stored intracellular cholesteryl esters (CE) and thereby enhances free cholesterol efflux and reduces cellular CE content. We have reported earlier reduced atherosclerosis as well as lesion necrosis and improved insulin sensitivity (due to decreased adipose tissue inflammation) in macrophage-specific CEH transgenic (CEHTg) mice in the LDLR(-/-) background. In the present study, we examined the effects of reduced intracellular accumulation of CE in CEHTg macrophages in an established diabetic mouse model, namely the leptin-deficient ob/ob mouse. Macrophage-specific transgenic expression of CEH improved glucose tolerance in ob/ob-CEHTg mice significantly compared with ob/ob nontransgenic littermates, but with no apparent change in macrophage infiltration into the adipose tissue. However, there was a significant decrease in hepatic lipid accumulation in ob/ob-CEHTg mice. Consistently, decreased [(14)C]acetate incorporation into total lipids and triglycerides was noted in precision-cut liver slices from ob/ob-CEHTg mice. In the primary hepatocyte-macrophage coculture system, macrophages from CEHTg mice significantly reduced the incorporation of [(14)C]acetate into triglycerides in hepatocytes, indicating a direct effect of macrophages on hepatocyte triglyceride biosynthesis. Kupffer cells isolated from ob/ob-CEHTg mice were polarized toward an anti-inflammatory M2 (Ly6C(lo)) phenotype. Taken together, these studies demonstrate that transgenic overexpression of CEH in macrophages polarizes hepatic macrophages (Kupffer cells) to an anti-inflammatory M2 phenotype that attenuates hepatic lipid synthesis and accumulation.

Increased prevalence of cardiovascular disease in Type 1 diabetic patients with non-alcoholic fatty liver disease

BACKGROUND AND AIMS

Non-alcoholic fatty liver disease (NAFLD) is associated with an increased prevalence of cardiovascular disease (CVD) in both non-diabetic and Type 2 diabetic individuals. We sought to examine whether NAFLD is associated with prevalent CVD in patients with Type 1 diabetes.

SUBJECTS AND METHODS

We studied 343 (156 men; mean age ~45 yr) consecutive Type 1 diabetic patients with and without NAFLD, which was diagnosed by ultrasonography. The presence of CVD was diagnosed by patient history, chart review, electrocardiogram, and echo-Doppler scanning of carotid and lower limb arteries.

RESULTS

Compared with those without steatosis, patients with ultrasound-diagnosed NAFLD (no.=182) had a remarkably greater age- and sex-adjusted prevalence of coronary (15.4 vs 1.2%, p<0.0001), cerebrovascular (41.7 vs 9.3%, p<0.0001) and peripheral (29.7 vs 6.2%, p<0.0001) vascular disease. A multivariable logistic regression analysis revealed that NAFLD was associated with an ~8-fold higher odds of CVD (composite endpoint), independently of age, sex, body mass index, family history of CVD, smoking status, physical activity, alcohol consumption, diabetes duration, glycated hemoglobin, systolic blood pressure, plasma lipids, estimated glomerular filtration rate, albuminuria, and use of anti-hypertensive, lipid-lowering and anti-platelet medications (adjusted odds ratio 7.6, 95% confidence intervals 3.6-24.0, p<0.001).

CONCLUSIONS

Our results demonstrate that NAFLD is associated with an increased prevalence of asymptomatic/symptomatic CVD in patients with Type 1 diabetes, independently of several established risk factors, including the components of metabolic syndrome.

Androgen levels and metabolic parameters are associated with a genetic variant of F13A1 in women with polycystic ovary syndrome

The polycystic ovary syndrome (PCOS), characterized by hyperandrogenism, is one of the most common hormonal disorders among premenopausal women and is associated with infertility, obesity, and insulin resistance. Accumulating evidence suggests a role of the blood coagulation factor gene F13A1 in obesity (GeneBank ID: NM_000129.3). The aim of this study was to investigate the association of intronic allelic variants of the F13A1 gene with PCOS susceptibility and metabolic parameters in lean and obese PCOS women. In a case-control study, we determined an intronic F13A1 single nucleotide polymorphism (SNP) (dbSNP ID: rs7766109) in 585 PCOS and 171 control women and tested for PCOS susceptibility and associations with anthropometric, metabolic and hormonal parameters. Genotype frequencies of the F13A1 SNP rs7766109 were equivalent in PCOS and control women. In PCOS women, F13A1 gene variants were significantly associated with body mass index (BMI) (p=0.013), systolic blood pressure (p=0.042), insulin response (AUCins) (p=0.015), triglycerides (TG) (p=0.001), and high density lipoprotein cholesterol (HDL) (p=0.012). In the subgroup of obese PCOS women free androgen index (FAI), free testosterone and sex hormone binding globulin (SHBG) as well as glucose measurements showed a significantly different pattern across F13A1 gene variants (p=0.043; p=0.039 and p=0.013, respectively). We report for the first time an association of the F13A1 SNP rs7766109 with BMI, androgens, and insulin resistance in PCOS women. Further studies are needed to confirm our findings and to evaluate whether F13A1 is causally involved in the pathogenesis of PCOS related metabolic and hormonal disturbances.

High fat diet induced hepatic steatosis and insulin resistance: Role of dysregulated ceramide metabolism

AIM

  Non-alcoholic fatty liver disease (NAFLD) is an insulin resistance disease that can progress to cirrhosis and liver failure. We hypothesized that in NAFLD, insulin resistance dysregulates lipid metabolism, increasing production of cytotoxic lipids including ceramides, which exacerbate hepatic insulin resistance and injury.

METHODS

  Long Evans rats were pair-fed low (LFD) or high (HFD) fat diets for 8 weeks. Livers were used to measure lipids, gene expression, insulin receptor binding, integrity of insulin signaling, and pro-inflammatory cytokines. In vitro experiments characterized effects of ceramides on Huh7 cell viability, mitochondrial function, and insulin signaling.

RESULTS

  High fat diet feeding caused NAFLD with peripheral and hepatic insulin resistance, increased hepatic expression of pro-ceramide genes, sphingomyelinase activity, and lipid peroxidation, and increased serum ceramide. Ceramide treatment impaired Huh7 cell viability, mitochondrial function, and insulin signaling.

CONCLUSIONS

  Increased hepatic ceramide generation and release may mediate both hepatic and peripheral insulin resistance in NAFLD.

Plasma sCD36 is associated with markers of atherosclerosis, insulin resistance and fatty liver in a nondiabetic healthy population

OBJECTIVES

Insulin resistance is associated with increased CD36 expression in a number of tissues. Moreover, excess macrophage CD36 may initiate atherosclerotic lesions. The aim of this study was to determine whether plasma soluble CD36 (sCD36) was associated with insulin resistance, fatty liver and carotid atherosclerosis in nondiabetic subjects.

METHODS

In 1296 healthy subjects without diabetes or hypertension recruited from 19 centres in 14 European countries (RISC study), we determined the levels of sCD36, adiponectin, lipids and liver enzymes, insulin sensitivity (M/I) by euglycaemic-hyperinsulinaemic clamp, carotid atherosclerosis as intima-media thickness (IMT) and two estimates of fatty liver, the fatty liver index (FLI) and liver fat percentage (LF%).

RESULTS

IMT, FLI, LF%, presence of the metabolic syndrome, impaired glucose regulation, insulin and triglycerides increased across sCD36 quartiles (Q2-Q4), whereas adiponectin and M/I decreased (P ≤ 0.01). sCD36 was lower in women than in men (P = 0.045). Log sCD36 showed a bimodal distribution, and amongst subjects with sCD36 within the log-normal distribution (log-normal population, n = 1029), sCD36 was increased in subjects with impaired glucose regulation (P = 0.045), metabolic syndrome (P = 0.006) or increased likelihood of fatty liver (P < 0.001). sCD36 correlated significantly with insulin, triglycerides, M/I and FLI (P < 0.05) after adjustment for study centre, gender, age, glucose tolerance status, smoking habits and alcohol consumption. In the log-normal population, these relationships were stronger than in the total study population and, additionally, sCD36 was significantly associated with LF% and IMT (P < 0.05).

CONCLUSIONS

In this cross-sectional study of nondiabetic subjects, sCD36 was significantly associated with indices of insulin resistance, carotid atherosclerosis and fatty liver. Prospective studies are needed to further evaluate the role of sCD36 in the inter-relationship between atherosclerosis, fatty liver and insulin resistance.

Role of cytokines and chemokines in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) includes a variety of histological conditions (ranging from liver steatosis and steatohepatitis, to fibrosis and hepatocarcinoma) that are characterized by an increased fat content within the liver. The accumulation/deposition of fat within the liver is essential for diagnosis of NAFLD and might be associated with alterations in the hepatic and systemic inflammatory state. Although it is still unclear if each histological entity represents a different disease or rather steps of the same disease, inflammatory processes in NAFLD might influence its pathophysiology and prognosis. In particular, non-alcoholic steatohepatitis (the most inflamed condition in NAFLDs, which more frequently evolves towards chronic and serious liver diseases) is characterized by a marked activation of inflammatory cells and the upregulation of several soluble inflammatory mediators. Among several mediators, cytokines and chemokines might play a pivotal active role in NAFLD and are considered as potential therapeutic targets. In this review, we will update evidence from both basic research and clinical studies on the potential role of cytokines and chemokines in the pathophysiology of NAFLD.

The obesity and fatty liver are reduced by plant-derived Pediococcus pentosaceus LP28 in high fat diet-induced obese mice

We evaluated the effect of an oral administration of a plant-derived lactic acid bacterium, Pediococcus pentosaceus LP28 (LP28), on metabolic syndrome by using high fat diet-induced obese mice. The obese mice were divided into 2 groups and fed either a high fat or regular diet for 8 weeks. Each group was further divided into 3 groups, which took LP28, another plant-derived Lactobacillus plantarum SN13T (SN13T) or no lactic acid bacteria (LAB). The lean control mice were fed a regular diet without inducing obesity prior to the experiment. LP28 reduced body weight gain and liver lipid contents (triglyceride and cholesterol), in mice fed a high fat diet for 8 weeks (40%, 54%, and 70% less than those of the control group without LAB, and P = 0.018, P<0.001, and P = 0.021, respectively), whereas SN13T and the heat treated LP28 at 121°C for 15 min were ineffective. Abdominal visceral fat in the high fat diet mice fed with LP28 was also lower than that without LAB by 44%, although it was not significant but borderline (P = 0.076). The sizes of the adipocytes and the lipid droplets in the livers were obviously decreased. A real-time PCR analyses showed that lipid metabolism-related genes, such as CD36 (P = 0.013), SCD1 encoding stearoyl-CoA desaturase 1 (not significant but borderline, P = 0.066), and PPARγ encoding peroxisome proliferator-activated receptor gamma (P = 0.039), were down-regulated by taking LP28 continuously, when compared with those of the control group. In conclusion, LP28 may be a useful LAB strain for the prevention and reduction of the metabolic syndrome.

A novel murine model for non-alcoholic steatohepatitis developed by combination of a high-fat diet and oxidized low-density lipoprotein

Non-alcoholic steatohepatitis (NASH) is the hepatic manifestation of metabolic syndrome that is characterized by steatosis, inflammation, and fibrosis, and may progress to cirrhosis and carcinoma. To investigate its pathogenic processes, we established a novel murine model for NASH by combination of a high-fat diet (HFD) and oxidized low-density lipoprotein (oxLDL). Mice that received HFD for 23 weeks showed hepatic steatosis, slight fibrosis, and a high level of lipid peroxidation compared with a regular diet (RD)-fed mice. Hepatic injury and elevated tumor necrosis factor (TNF)-α mRNA expression were also detected in these mice. Moreover, oxLDL administration to HFD-fed mice during weeks 21-23 not only aggravated hepatic steatosis, fibrosis, and lipid metabolism, but also resulted in intense inflammation, including severe hepatic injury and inflammatory cell infiltration, which are the typical histological features of NASH. Inflammation was accompanied by increased gene expression of TNF-α and interleukin (IL)-6. Additionally, the livers of RD-fed animals treated with oxLDL during weeks 21-23 were characterized by foamy macrophages and inflammatory cell infiltration along with an elevated IL-6 mRNA level. These results suggest that an increased oxidative state, including HFD-induced intracellular lipid peroxidation and its extracellular source from oxLDL, is the actual trigger for hepatic inflammation in which liver injury is mediated by TNF-α and inflammatory cell accumulation is dependent on IL-6. HFD and oxLDL also induced insulin resistance in mice; additionally, oxLDL downregulated insulin secretion. In this model, CD36 overexpression was observed in the hepatocytes of HFD-fed mice and those treated with HFD and oxLDL, and in the hepatic macrophages of RD-fed mice immediately after oxLDL treatment. In vitro experiments indicated a rapid and transient elevation of CD36 on macrophage plasma membrane in response to oxLDL. Our findings demonstrate that CD36 expressed on hepatocytes and hepatic macrophages mediates the pathophysiology of NASH.

Postprandial metabolism of meal triglyceride in humans

The intake of dietary fat above energy needs has contributed to the growing rates of obesity worldwide. The concept of disease development occurring in the fed state now has much support and dysregulation of substrate flux may occur due to poor handling of dietary fat in the immediate postprandial period. The present paper will review recent observations implicating cephalic phase events in the control of enterocyte lipid transport, the impact of varying the composition of meals on subsequent fat metabolism, and the means by which dietary lipid carried in chylomicrons can lead to elevated postprandial non-esterified fatty acid concentrations. This discussion is followed by an evaluation of the data on quantitative meal fat oxidation at the whole body level and an examination of dietary fat clearance to peripheral tissues - with particular attention paid to skeletal muscle and liver given the role of ectopic lipid deposition in insulin resistance. Estimates derived from data of dietary-TG clearance show good agreement with clearance to the liver equaling 8-12% of meal fat in lean subjects and this number appears higher (10-16%) in subjects with diabetes and fatty liver disease. Finally, we discuss new methods with which to study dietary fatty acid partitioning in vivo. Future research is needed to include a more comprehensive understanding of 1) the potential for differential oxidation of saturated versus unsaturated fatty acids which might lead to meaningful energy deficit and whether this parameter varies based on insulin sensitivity, 2) whether compartmentalization exists for diet-derived fatty acids within tissues vs. intracellular pools, and 3) the role of reduced peripheral fatty acid clearance in the development of fatty liver disease. Further advancements in the quantitation of dietary fat absorption and disposal will be central to the development of therapies designed to treat diet-induced obesity. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

WAT is a functional adipocyte?

In vertebrates, adipose tissue is the main storage site for lipids within specialized lipid-laden mature adipocytes. While many species have evolved cells capable of lipid storage, the adipocyte represents a unique specialized cell involved in fuel storage, endocrine, nervous and immune function. However, the adipocytes are not the only cell type in mammals that can accumulate lipid droplets. The ectopic accumulation of lipid in non-adipose tissues including the liver, skeletal muscle, bone, pancreas, and heart in combination with its excessive accumulation in adipose tissue contributes to metabolic disease. Determining the lipid processing components that are necessary and sufficiently for lipid accumulation in adipose and non-adipose tissues, in addition to endocrine function, will lead to a clearer definition of an adipocyte.

Identification by proteomic analysis of early post-mortem markers involved in the variability in fat loss during cooking of mule duck "foie gras"

Fat loss during cooking of duck "foie gras" is the main quality issue for both processors and consumers. Despite the efforts of the processing industry to control fat loss, the variability of fatty liver cooking yield remains high and uncontrolled. To better understand the biological basis of this phenomenon, a proteomic study was conducted. To analyze the protein fraction soluble at low ionic strength (LIS), we used bidimensional electrophoresis and mass spectrometry for the identification of spots of interest. To analyze the protein fraction not soluble at low ionic strength (NS), we used the shotgun strategy. The analysis of data acquired from both protein fractions suggested that at the time of slaughter, livers with low fat loss during cooking were still in anabolic processes with regard to energy metabolism and protein synthesis, whereas livers with high fat loss during cooking developed cell protection mechanisms. The variability in the technological yield observed in processing plants could be explained by a different physiological stage of liver steatosis.

Gene expression in livers of BALB/C and C57BL/6J mice fed a high-fat diet

We previously demonstrated that high-fat diet (HFD)-induced hepatic lipid accumulation is more severe in BALB/c mice than in C57BL/6J (B6) mice. To understand the changes in liver metabolism, we studied blood chemistry, gene expression, and histopathological changes of the liver in nine-week HFD-fed BALB/c and B6 mice and one- or four-week HFD-fed BALB/c mice. Serum total cholesterol and triglyceride levels were significantly increased in all HFD-fed groups, and one- and four-week HFD-fed BALB/c groups, respectively. Histopathology revealed that vacuolation of hepatocytes was severe in nine-week HFD-fed BALB/c mice, although it was less severe in the other groups. Microarray analysis of mRNA expression of nine-week HFD-fed BALB/c mice showed up-regulation of genes involved in fatty acid uptake and biosynthesis, such as Cd36, Acaca, Acly, and Fasn. Some changes were observed in the one- and four-week HFD-fed BALB/c groups and the nine-week HFD-fed B6 group, however these changes in mRNA expression were not so marked. In conclusion, the fatty accumulation observed in BALB/c mice may be caused, at least in part, by up-regulation of fatty acid uptake and biosynthesis. Cd36, Acaca, Acly and Fasn may be involved in these metabolic processes.

Analysis of global and absorption, distribution, metabolism, and elimination gene expression in the progressive stages of human nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by a series of pathological changes that range from simple fatty liver to nonalcoholic steatohepatitis (NASH). The objective of this study is to describe changes in global gene expression associated with the progression of human NAFLD. This study is focused on the expression levels of genes responsible for the absorption, distribution, metabolism, and elimination (ADME) of drugs. Differential gene expression between three clinically defined pathological groups-normal, steatosis, and NASH-was analyzed. Genome-wide mRNA levels in samples of human liver tissue were assayed with Affymetrix GeneChip Human 1.0ST arrays. A total of 11,633 genes exhibited altered expression out of 33,252 genes at a 5% false discovery rate. Most gene expression changes occurred in the progression from steatosis to NASH. Principal component analysis revealed that hepatic disease status was the major determinant of differential ADME gene expression rather than age or sex of sample donors. Among the 515 drug transporters and 258 drug-metabolizing enzymes (DMEs) examined, uptake transporters but not efflux transporters or DMEs were significantly over-represented in the number of genes down-regulated. These results suggest that uptake transporter genes are coordinately targeted for down-regulation at the global level during the pathological development of NASH and that these patients may have decreased drug uptake capacity. This coordinated regulation of uptake transporter genes is indicative of a hepatoprotective mechanism acting to prevent accumulation of toxic intermediates in disease-compromised hepatocytes.

The emerging roles of fatty acid translocase/CD36 and the aryl hydrocarbon receptor in fatty liver disease

The fatty acid translocase (FAT)/CD36 belongs to the class B scavenger receptor family. In addition to the known functions of CD36 in the uptake of oxidized low-density lipoprotein by macrophages and uptake of fatty acids by adipose tissues, skeletal muscle and heart, emerging evidence has pointed to an equally important function of CD36 in the uptake of fatty acids in the liver and the pathogenesis of fatty liver disease. Recent reports have also suggested CD36 as a shared transcriptional target of several ligand-sensing and lipogenic transcriptional factors, such as the aryl hydrocarbon receptor, and several nuclear hormone receptors, such as pregnane X receptor, liver X receptor and peroxisome proliferator activated receptor γ. Non-alcoholic fatty liver disease is common and medically significant, because it is closely related to metabolic syndrome and has a potential to progress into the more harmful non-alcoholic steatohepatitis. It is hoped that CD36 and their transcriptional regulators can represent novel therapeutic targets for the prevention and management of fatty liver disease.

The in utero programming effect of increased maternal androgens and a direct fetal intervention on liver and metabolic function in adult sheep

Epigenetic changes in response to external stimuli are fast emerging as common underlying causes for the pre-disposition to adult disease. Prenatal androgenization is one such model that results in reproductive and metabolic features that are present in conditions such as polycystic ovary syndrome (PCOS). We examined the effect of prenatal androgens on liver function and metabolism of adult sheep. As non-alcoholic fatty liver disease is increased in PCOS we hypothesized that this, and other important liver pathways including metabolic function, insulin–like growth factor (IGF) and steroid receptivity, would be affected. Pregnant ewes received vehicle control (C; n = 5) or testosterone propionate (TP; n = 9) twice weekly (100 mg; i.m) from d62–102 (gestation 147 days). In a novel treatment paradigm, a second cohort received a direct C (n = 4) or TP (20 mg; n = 7) fetal injection at d62 and d82. In adults, maternal TP exposure resulted in increased insulin secretion to glucose load (P<0.05) and the histological presence of fatty liver (P<0.05) independent of central obesity. Additionally, hepatic androgen receptor (AR; P<0.05), glucocorticoid receptor (GR; P<0.05), UDP- glucose ceramide glucosyltransferase (UGCG; P<0.05) and IGF1 (P<0.01) expression were upregulated. The direct fetal intervention (C and TP) led to early fatty liver changes in all animals without differential changes in insulin secretion. Furthermore, hepatic phosphoenolpyruvate carboxykinase (PEPCK) was up-regulated in the fetal controls (P<0.05) and this was opposed by fetal TP (P<0.05). Hepatic estrogen receptor (ERα; P<0.05) and mitogen activated protein kinase kinase 4 (MAP2K4; P<0.05) were increased following fetal TP exposure. Adult liver metabolism and signaling can be altered by early exposure to sex steroids implicating epigenetic regulation of metabolic disturbances that are common in PCOS.

Dietary cholesterol exacerbates hepatic steatosis and inflammation in obese LDL receptor-deficient mice

Non-alcoholic fatty liver disease (NAFLD), the hepatic manifestation of the metabolic syndrome, can progress to steatohepatitis (NASH) and advanced liver disease. Mechanisms that underlie this progression remain poorly understood, partly due to lack of good animal models that resemble human NASH. We previously showed that several metabolic syndrome features that develop in LDL receptor-deficient (LDLR-/-) mice fed a diabetogenic diet are worsened by dietary cholesterol. To test whether dietary cholesterol can alter the hepatic phenotype in the metabolic syndrome, we fed LDLR-/- mice a high-fat, high-carbohydrate diabetogenic diet (DD) without or with added cholesterol (DDC). Both groups of mice developed obesity and insulin resistance. Hyperinsulinemia, dyslipidemia, hepatic triglyceride, and alanine aminotransferase (ALT) elevations were greater with DDC. Livers of DD-fed mice showed histological changes resembling NAFLD, including steatosis and modest fibrotic changes; however, DDC-fed animals developed micro- and macrovesicular steatosis, inflammatory cell foci, and fibrosis resembling human NASH. Dietary cholesterol also exacerbated hepatic macrophage infiltration, apoptosis, and oxidative stress. Thus, LDLR-/- mice fed diabetogenic diets may be useful models for studying human NASH. Dietary cholesterol appears to confer a second "hit" that results in a distinct hepatic phenotype characterized by increased inflammation and oxidative stress.

Non-additive hepatic gene expression elicited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) co-treatment in C57BL/6 mice

Interactions between environmental contaminants can lead to non-additive effects that may affect the toxicity and risk assessment of a mixture. Comprehensive time course and dose-response studies with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), non-dioxin-like 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) and their mixture were performed in immature, ovariectomized C57BL/6 mice. Mice were gavaged once with 30 μg/kg TCDD, 300 mg/kg PCB153, a mixture of 30 μg/kg TCDD with 300 mg/kg PCB153 (MIX) or sesame oil vehicle for 4,12, 24,72 or 168 h. In the 24h dose-response study, animals were gavaged with TCDD (0.3,1, 3, 6, 10, 15, 30, 45 μg/kg), PCB153 (3,10, 30, 60, 100, 150, 300, 450 mg/kg), MIX (0.3+3, 1+10, 3+30, 6+60, 10+100, 15+150, 30+300, 45 μg/kg TCDD+450 mg/kg PCB153, respectively) or vehicle. All three treatments significantly increased relative liver weights (RLW), with MIX eliciting significantly greater increases compared to TCDD and PCB153 alone. Histologically, MIX induced hepatocellular hypertrophy, vacuolization, inflammation, hyperplasia and necrosis, a combination of TCDD and PCB153 responses. Complementary lipid analyses identified significant increases in hepatic triglycerides in MIX and TCDD samples, while PCB153 had no effect on lipids. Hepatic PCB153 levels were also significantly increased with TCDD co-treatment. Microarray analysis identified 167 TCDD, 185 PCB153 and 388 MIX unique differentially expressed genes. Statistical modeling of quantitative real-time PCR analysis of Pla2g12a, Serpinb6a, Nqo1, Srxn1, and Dysf verified non-additive expression following MIX treatment compared to TCDD and PCB153 alone. In summary, TCDD and PCB153 co-treatment elicited specific non-additive gene expression effects that are consistent with RLW increases, histopathology, and hepatic lipid accumulation.

Mechanisms of lipotoxicity in NAFLD and clinical implications

With the epidemic of childhood obesity, nonalcoholic fatty liver disease (NAFLD) has become the most common cause of chronic liver disease in pediatrics. NAFLD is strongly associated with insulin resistance and increased level of serum free fatty acids (FFAs). FFAs have direct hepatotoxicity through the induction of an endoplasmic reticulum stress response and subsequently activation of the mitochondrial pathway of cell death. FFAs may also result in lysosomal dysfunction and alter death receptor gene expression. Lipoapoptosis is a key pathogenic process in NAFLD, and correlates with progressive inflammation, and fibrosis. Accumulation of triglyceride in the liver results from uptake and esterification of FFAs by the hepatocyte, and is less likely to be hepatotoxic per se. To date, there are no proven effective therapies that halt NAFLD progression or unfortunately improve prognosis in children. The cellular mechanisms of lipotoxicity are complex but provide potential therapeutic targets for NAFLD. In this review we discuss several potential therapeutic opportunities in detail including inhibition of apoptosis, c-Jun-N-terminal kinase, and endoplasmic reticulum stress pathways.

Correlation analysis between gene expression profile of rat liver tissues and high-fat emulsion-induced nonalcoholic fatty liver

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) is caused by fat metabolism disorders and thereby abnormal or excessive accumulation of fat in hepatocytes, and characterized by steatosis, inflammation, fibrosis, apoptosis or necrosis.

AIM

This study was carried out to explore the correlation between gene expression profiles of rat livers and the occurrence and progression of NAFLD at the transcriptional level.

METHODS

A rat model of nonalcoholic steatohepatitis (NASH) was established by feeding male rats with high-fat emulsion via gavage, and Rat Genome 230 2.0 Array was used to detect gene expression profiles of liver tissues obtained from male rats following 0, 2, 4, and 6 weeks of high-fat emulsion feeding. Methods of bioinformatics and systems biology were applied to analyze the correlation between gene expression changes and physiological activities involved in NAFLD.

RESULTS

In total, 93 function-known genes, including 36 up-regulated and 57 down-regulated, differed significantly in expression compared to those of control rats, and 18 physiological activities were closely related to NAFLD. Especially, the activity of cell differentiation was decreased during the whole process of NAFLD, and the activities of inflammation response, stimulus response, cell migration and adhesion were attenuated in the second, fourth and sixth week, respectively. In the fourth and sixth weeks, lipid metabolism and cell apoptosis were augmented, and the former might be associated with the enhanced expression of plin, acsl6, scd2, elovl3, etc.

CONCLUSION

These data provide useful information on the global gene expression changes due to high-fat emulsion feeding and bring important insights into the mechanisms of NAFLD.

Weight loss amelioration of non-alcoholic steatohepatitis linked to shifts in hepatic ceramide expression and serum ceramide levels

AIM

  Non-alcoholic steatohepatitis (NASH) is associated with increased hepatic insulin resistance. Ceramides and other toxic sphingolipids promote inflammation, lipotoxicity and insulin resistance; however, the role of ceramides in the pathogenesis of NASH has not been determined. This study characterizes expression of ceramide-related genes in human livers with NASH and examines the effects of weight loss on NASH and pro-ceramide gene expression in liver.

METHODS

  Liver biopsies were obtained to assess the histopathological status of non-alcoholic fatty liver disease/NASH prior to and following completion of a 1-year course of implementing either lifestyle changes or a standard enrichment protocol designed to encourage weight loss. Liver biopsy samples were used to measure pro-ceramide gene expression by quantitative reverse transcriptase polymerase chain reaction analysis (qRT-PCR), and serum was used to measure ceramide immunoreactivity.

RESULTS

  At baseline, serine palmitoyltransferase (SPTLC)2 (P = 0.02) and ceramide synthase (CER)1 (P = 0.001) mRNA transcripts were less abundantly expressed in livers with NASH relative to normal controls. After weight loss (average 9.3%), SPTLC1 (P = 0.005) and uridine diphosphate glucose ceramide glucosyltransferase (UGCG) (P = 0.001) expression significantly declined while CER1 increased (P = 0.001) among subjects randomized to the lifestyle change subgroup. Reductions in calorie and fat consumption were significantly correlated with changes in ceramide-related gene expression. Finally, both net and relative reductions in serum ceramide levels were significantly greater in the lifestyles compared with the standard enrichment (control) protocol group (both P < 0.005).

CONCLUSION

  NASH is associated with increased insulin resistance and altered ceramide gene expression in liver. Weight loss-mediated reversal of NASH is associated with reduced pro-ceramide gene expression in liver.

Induction of vesicular steatosis by amiodarone and tetracycline is associated with up-regulation of lipogenic genes in HepaRG cells

Drug-induced liver injury occurs in general after several weeks and is often unpredictable. It is characterized by a large spectrum of lesions that includes steatosis and phospholipidosis. Many drugs such as amiodarone and tetracycline have been reported to cause phospholipidosis and/or steatosis. In this study, acute and chronic hepatic effects of these two drugs were investigated using well-differentiated human hepatoma HepaRG cells. Accumulation of typical lipid droplets, labeled with Oil Red O, was observed in hepatocyte-like HepaRG cells after repeat exposure to either drug. Amiodarone caused the formation of additional intracytoplasmic vesicles that did not stain in all HepaRG cells. At the electron microscopic level, these vesicles appeared as typical lamellar bodies and were associated with an increase of phosphatidylethanolamine and phosphatidylcholine. A dose-dependent induction of triglycerides (TG) was observed after repeat exposure to either amiodarone or tetracycline. Several genes known to be related to lipogenesis were induced after treatment by these two drugs. By contrast, opposite deregulation of some of these genes (FASN, SCD1, and THSRP) was observed in fat HepaRG cells induced by oleic acid overload, supporting the conclusion that different mechanisms were involved in the induction of steatosis by drugs and oleic acid. Moreover, several genes related to lipid droplet formation (ADFP, PLIN4) were up-regulated after exposure to both drugs and oleic acid.

CONCLUSION

Our results show that amiodarone causes phospholipidosis after short-term treatment and, like tetracycline, induces vesicular steatosis after repeat exposure in HepaRG cells. These data represent the first demonstration that drugs can induce vesicular steatosis in vitro and show a direct relationship between TG accumulation and enhanced expression of lipogenic genes.

The role of lipid droplets in metabolic disease in rodents and humans

Lipid droplets (LDs) are intracellular organelles that store neutral lipids within cells. Over the last two decades there has been a dramatic growth in our understanding of LD biology and, in parallel, our understanding of the role of LDs in health and disease. In its simplest form, the LD regulates the storage and hydrolysis of neutral lipids, including triacylglycerol and/or cholesterol esters. It is becoming increasingly evident that alterations in the regulation of LD physiology and metabolism influence the risk of developing metabolic diseases such as diabetes. In this review we provide an update on the role of LD-associated proteins and LDs in metabolic disease.

Drug metabolism alterations in nonalcoholic fatty liver disease

Drug-metabolizing enzymes play a vital role in the elimination of the majority of therapeutic drugs. The major organ involved in drug metabolism is the liver. Chronic liver diseases have been identified as a potential source of significant interindividual variation in metabolism. Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease in the United States, affecting between 60 and 90 million Americans, yet the vast majority of NAFLD patients are undiagnosed. NAFLD encompasses a spectrum of pathologies, ranging from steatosis to nonalcoholic steatohepatitis and fibrosis. Numerous animal studies have investigated the effects of NAFLD on hepatic gene expression, observing significant alterations in mRNA, protein, and activity levels. Information on the effects of NAFLD in human patients is limited, though several significant investigations have recently been published. Significant alterations in the activity of drug-metabolizing enzymes may affect the clearance of therapeutic drugs, with the potential to result in adverse drug reactions. With the enormous prevalence of NAFLD, it is conceivable that every drug currently on the market is being given to patients with NAFLD. The current review is intended to present the results from both animal models and human patients, summarizing the observed alterations in the expression and activity of the phase I and II drug-metabolizing enzymes.

Soluble CD36- a marker of the (pathophysiological) role of CD36 in the metabolic syndrome?

CD36 is a class B scavenger receptor observed in many cell types and tissues throughout the body. Recent literature has implicated CD36 in the pathogenesis of metabolic dysregulation such as found in obesity, insulin resistance, and atherosclerosis. Genetic variation at the CD36 loci have been associated with obesity and lipid components of the metabolic syndrome, with risk of heart disease and type 2 diabetes. Recently, non-cell bound CD36 was identified in human plasma and was termed soluble CD36 (sCD36). In this review we will describe the functions of CD36 in tissues and address the role of sCD36 in the context of the metabolic syndrome. We will also highlight recent findings from human genetic studies looking at the CD36 locus in relation to metabolic profile in the general population. Finally, we present a model in which insulin resistance, oxLDL, low-grade inflammation and liver steatosis may contribute to elevated levels of sCD36.

The mechanistic basis for the induction of hepatic steatosis by xenobiotics

INTRODUCTION

Hepatic steatosis is the histological observation of numerous lipid inclusions due to an excess accumulation of triacylglycerols. They are a concern with new therapeutic candidates because they signify altered lipid metabolism that can progress to more serious liver toxicity.

AREAS COVERED

This article is based on an article search using the PubMed database from 1987 to 2011 and confirms associations for several previously marketed drugs with four basic hepatocellular mechanisms. The article also describes how these mechanisms are controlled by master regulators of lipid metabolism, which include gene transcription factors, nuclear receptors, hormonal signaling, energy sensing proteins, endoplasmic reticulum stress signaling and certain key metabolic intermediates.

EXPERT OPINION

Drug-induced hepatic steatosis is typically not detectable by conventional means other than invasive histological examinations. By understanding the basic mechanisms, key regulators and energy signaling systems of the liver, the investigator is better equipped to avoid xenobiotics with steatogenic potential in the drug discovery or early development process. There are now a number of methods for detecting this potential, specifically gene expression or metabolomic profiling and pathway analysis or mechanism-based in vitro systems.

Systematic review of performance of non-invasive biomarkers in the evaluation of non-alcoholic fatty liver disease

This systematic review evaluates the many studies carried out to discover and evaluate non-invasive markers of non-alcoholic fatty liver disease (NAFLD). Many different strategies and methods have been used in this task, from the discovery of new markers by global 'shotgun' studies to hypothesis-driven approaches, to the development of algorithm tests based on routinely available clinical and biochemical parameters. We examined the various different approaches, summarising the findings in an attempt to give an overview of the field of non-invasive markers in NAFLD, encompassing markers of steatosis, necro-inflammation and fibrosis. The body of literature surrounding this topic is complex and varied, encompassing not only different methodologies but also different patient characteristics, different disease definitions, as well as different end points. This reflects the heterogeneity of NAFLD, which, however, introduces considerably difficulty when trying to draw a conclusion between studies. We have divided this review into three main chapters based on the characteristics of the studies. The Genomics/Proteomics chapter reviews studies using a non-hypothesis-driven approach to biomarker discovery. Thereafter, we evaluate studies of association - studies that target-specific markers, comparing levels between disease and control groups. Finally, we examine the algorithm tests - mathematical systems developed on the basis of previously described markers and assessed, usually, by receiver operator curve analysis. While radiological examination and investigations offer important diagnostic information, such studies are not discussed in this review - the body of literature surrounding blood and anthropological markers is complex and varied, demanding close attention.

Overexpression of CD36 and acyl-CoA synthetases FATP2, FATP4 and ACSL1 increases fatty acid uptake in human hepatoma cells

BACKGROUND

Understanding the mechanisms of long chain fatty acid (LCFA) uptake in hepatic cells is of high medical importance to treat and to prevent fatty liver disease (FLD). ACSs (Acyl-CoA synthetases) are a family of enzymes that catalyze the esterification of fatty acids (FA) with CoA. Recent studies suggest that ACS enzymes drive the uptake of LCFA indirectly by their enzymatic activity and could promote special metabolic pathways dependent on their localization.The only protein located at the plasma membrane which has consistently been shown to enhance FA uptake is CD36.

AIMS

The current study investigated whether ACSs and CD36 could regulate hepatic LCFA uptake.

METHODS AND RESULTS

FATP2 and FATP4 were both localized to the ER of HuH7 and HepG2 cells as shown by double immunofluorescence in comparison to marker proteins. ACSL1 was located at mitochondria in both cell lines. Overexpression of FATP2, FATP4 and ACSL1 highly increased ACS activity as well as the uptake of [3H]-oleic acid and fluorescent Bodipy-C12 (B12) fatty acid. Quantitative FACS analysis showed a correlation between ACS expression levels and B12 uptake. FATP2 had the highest effect on B12 uptake of all proteins tested. CD36 was mainly localized at the plasma membrane. Whereas [3H]-oleic acid uptake was increased after overexpression, CD36 had no effect on B12 uptake.

CONCLUSION

Uptake of LCFA into hepatoma cells can be regulated by the expression levels of intracellular enzymes. We propose that ACS enzymes drive FA uptake indirectly by esterification. Therefore these molecules are potential targets for treatment of nonalcoholic fatty liver disease (NAFLD) or steatohepatitis (NASH).

Adipose tissue and ceramide biosynthesis in the pathogenesis of obesity

Although obesity is a complex metabolic disorder often associated with insulin resistance, hyperinsulinemia and Type 2 diabetes, as well as with accelerated atherosclerosis, the molecular changes in obesity that promote these disorders are not completely understood. Several mechanisms have been proposed to explain how increased adipose tissue mass affects whole body insulin resistance and cardiovascular risk. One theory is that increased adipose derived inflammatory cytokines induces a chronic inflammatory state that not only increases cardiovascular risk, but also antagonizes insulin signaling and mitochondrial function and thereby impair glucose hemostasis. Another suggests that lipid accumulation in nonadipose tissues not suited for fat storage leads to the buildup of bioactive lipids that inhibit insulin signaling and metabolism. Recent evidence demonstrates that sphingolipid metabolism is dysregulated in obesity and specific sphingolipids may provide a common pathway that link excess nutrients and inflammation to increased metabolic and cardiovascular risk. This chapter will focus primarily on the expression and regulation of adipose and plasma ceramide biosynthesis in obesity and, its potential contribution to the pathogenesis of obesity and the metabolic syndrome.

Hepatic gene networks in morbidly obese patients with nonalcoholic fatty liver disease

BACKGROUND

Genetic factors alter the risk for nonalcoholic fatty liver disease (NAFLD). We sought to identify NAFLD-associated genes and elucidate gene networks and pathways involved in the pathogenesis of NAFLD.

METHODS

Quantitative global hepatic gene expression analysis was performed on 53 morbidly obese Caucasian subjects undergoing bariatric surgery (27 with NAFLD and 26 controls). After standardization of data, gene expression profiles were compared between patients with NAFLD and controls. The set of genes that significantly correlated with NAFLD was further analyzed by hierarchical clustering and ingenuity pathways analyses.

RESULTS

There were 25,643 quantitative transcripts, of which 108 were significantly associated with NAFLD (p < 0.001). Canonical pathway analysis in the NAFLD-associated gene clusters showed that the hepatic fibrosis signaling was the most significant pathway in the up-regulated NAFLD gene cluster containing three (COL1A1, IL10, IGFBP3) significantly altered genes, whereas the endoplasmic reticulum stress and protein ubiquitination pathways were the most significant pathways in the down-regulated NAFLD gene cluster, with the first pathway containing one (HSPA5) and the second containing two (HSPA5, USP25) significantly altered genes. The four primary gene networks associated with NAFLD were involved in cell death, immunological disease, cellular movement, and lipid metabolism with several significantly altered "hub" genes in these networks.

CONCLUSIONS

This study reveals the canonical pathways and gene networks associated with NAFLD in morbidly obese Caucasians. The application of gene network analysis highlights the transcriptional relationships among NAFLD-associated genes and allows identification of hub genes that may represent high-priority candidates for NAFLD.

Monocyte chemoattractant protein-1 deficiency does not affect steatosis or inflammation in livers of mice fed a methionine-choline-deficient diet

Monocyte chemoattractant protein-1 (MCP-1, Ccl2) expression is increased in livers of patients with nonalcoholic steatohepatitis and in murine models of steatohepatitis. Several studies in rodents indicate that MCP-1 contributes to liver steatosis induced by feeding a high-fat diet. However, the extent of MCP-1 involvement in the widely utilized methionine-choline-deficient (MCD) diet model of steatohepatitis has not been determined. We tested the hypothesis that MCP-1 contributes to steatohepatitis in mice fed the MCD diet. MCP-1-deficient mice on a C57Bl/6J background and age-matched C57Bl/6J mice were fed either MCD diet or control diet for 4 weeks. MCP-1 deficiency did not affect steatohepatitis, as indicated by liver histopathology, nor did it affect serum alanine aminotransferase activity, hepatic triglyceride levels, hepatic inflammatory gene induction, or macrophage accumulation in mice fed the MCD diet. MCP-1 deficiency reduced the expression of the profibrogenic genes, pro-collagen 1a1, connective tissue growth factor, and transforming growth factor-β, in mice fed the MCD diet. MCP-1 deficiency significantly reduced collagen deposition and α-smooth muscle actin protein levels in the livers of mice fed the MCD diet. The results indicate that MCP-1 does not contribute to liver steatosis or inflammation in the MCD diet model of steatohepatitis. Rather, the data suggest that MCP-1 contributes to fibrosis in mice fed the MCD diet, independent of effects on steatosis and inflammation.

Common CD36 SNPs reduce protein expression and may contribute to a protective atherogenic profile

Membrane CD36 functions in the uptake of fatty acids (FAs), oxidized lipoproteins and in signal transduction after binding these ligands. In rodents, CD36 is implicated in abnormal lipid metabolism, inflammation and atherosclerosis. In humans, CD36 variants have been identified to influence free FA and high-density lipoprotein (HDL) levels and to associate with the risk of the metabolic syndrome, coronary artery disease and stroke. In this study, 15 common lipid-associated CD36 single nucleotide polymorphisms (SNPs) were evaluated for the impact on monocyte CD36 expression (protein and transcript) in 104 African Americans. In a subset of subjects, the SNPs were tested for association with monocyte surface CD36 (n=65) and platelet total CD36 (n=57). The relationship between CD36 expression and serum HDL and very low-density lipoproteins (VLDLs) levels was also examined. After a permutation-based correction for multiple tests, four SNPs (rs1761667, rs3211909, rs3211913, rs3211938) influenced monocyte CD36 protein and two (rs3211909, rs3211938) platelet CD36. The effect of the HDL-associated SNPs on CD36 expression inversely related to the impact on serum HDL and potential causality was supported by Mendelian randomization analysis. Consistent with this, monocyte CD36 protein negatively correlated with total HDL and HDL subfractions. In contrast, positive correlations were documented between monocyte CD36 and VLDL lipid, particle number and apolipoprotein B. In conclusion, CD36 variants that reduce protein expression appear to promote a protective metabolic profile. The SNPs in this study may have predictive potential on CD36 expression and disease susceptibility in African Americans. Further studies are warranted to validate and determine whether these findings are population specific.

Activation of the aryl hydrocarbon receptor induces hepatic steatosis via the upregulation of fatty acid transport

The aryl hydrocarbon receptor (AHR) is a basic helix-loop-helix/Per-ARNT-Sim domain transcription factor, which is activated by various xenobiotic ligands. AHR is known to be abundant in liver tissue and to be associated with hepatic steatosis. However, it has not yet been elucidated how the activation of AHR promotes hepatic steatosis. The aim of this study is to clarify the role of AHR in hepatic steatosis. The intraperitoneal injection of 3-methylcholanthrene (3MC), a potent AHR ligand, into C57BL/6J mice significantly increased the levels of triglycerides and six long-chain monounsaturated fatty acids in the livers of mice, resulting in hepatic microvesicular steatosis. 3MC significantly enhanced the expression level of fatty acid translocase (FAT), a factor regulating the uptake of long-chain fatty acids into hepatocytes, in the liver. In an in vitro experiment using human hepatoma HepG2 cells, 3MC increased the expression level of FAT, and the downregulation of AHR by AHR siRNA led to the suppression of 3MC-induced FAT expression. In addition, the mRNA level of peroxisome proliferator-activated receptor (PPAR) α, an upstream factor of FAT, was increased in the livers of 3MC-treated mice. Taking together, AHR activation induces hepatic microvesicular steatosis by increasing the expression level of FAT.

Eicosapentaenoic acid improves hepatic steatosis independent of PPARα activation through inhibition of SREBP-1 maturation in mice

Eicosapentaenoic acid (EPA) in fish oil is known to improve hepatic steatosis. However, it remains unclear whether such action of EPA is actually caused by peroxisome proliferator-activated receptor α (PPARα) activation. To explore the contribution of PPARα to the effects of EPA itself, male wild-type and Ppara-null mice were fed a saturated fat diet for 16 weeks, and highly (>98%)-purified EPA was administered in the last 12 weeks. Furthermore, the changes caused by EPA treatment were compared to those elicited by fenofibrate (FF), a typical PPARα activator. A saturated fat diet caused macrovesicular steatosis in both genotypes. However, EPA ameliorated steatosis only in wild-type mice without PPARα activation, which was evidently different from numerous previous observations. Instead, EPA inhibited maturation of sterol-responsive element-binding protein (SREBP)-1 in the presence of PPARα through down-regulation of SREBP cleavage-activating protein and site-1 protease. Additionally, EPA suppressed fatty acid uptake and promoted hydrolysis of intrahepatic triglycerides in a PPARα-independent manner. These effects were distinct from those of fenofibrate. Although fenofibrate induced NAPDH oxidase and acyl-coenzyme A oxidase and significantly increased hepatic lipid peroxides, EPA caused PPARα-dependent induction of superoxide dismutases, probably contributing to a decrease in the lipid peroxides. These results firstly demonstrate detailed mechanisms of steatosis-ameliorating effects of EPA without PPARα activation and ensuing augmentation of hepatic oxidative stress.

A novel role for the dioxin receptor in fatty acid metabolism and hepatic steatosis

BACKGROUND & AIMS

The aryl hydrocarbon receptor (AhR) also known as the dioxin receptor or xenobiotic receptor is a member of the basic helix-loop-helix/period AhR nuclear translocator single minded family. The goal of this study was to determine the endobiotic role of AhR in hepatic steatosis.

METHODS

Wild-type, constitutively activated AhR transgenic, AhR null and CD36/fatty acid translocase null mice were used to investigate the role of AhR in steatosis and the involvement of CD36 in the steatotic effect of AhR. The promoters of the mouse and human CD36 genes were cloned and their regulation by AhR was analyzed.

RESULTS

Activation of AhR induced spontaneous hepatic steatosis characterized by the accumulation of triglycerides. The steatotic effect of AhR likely is owing to the combined up-regulation of CD36 and fatty acid transport proteins, suppression of fatty acid oxidation, inhibition of hepatic export of triglycerides, increase in peripheral fat mobilization, and increased hepatic oxidative stress. Promoter analysis established CD36 as a novel transcriptional target of AhR. Activation of AhR in liver cells induced CD36 gene expression and enhanced fatty acid uptake. The steatotic effect of an AhR agonist was inhibited in CD36-/- mice.

CONCLUSIONS

Our study reveals a novel link between AhR-induced steatosis and the expression of CD36. Industrial or military exposures to dioxin and related compounds have been linked to increased prevalence of fatty liver in human beings. Results from this study may help to establish AhR and its target CD36 as novel therapeutic and preventive targets for fatty liver disease.

Use of genome-wide expression data to mine the "Gray Zone" of GWA studies leads to novel candidate obesity genes

To get beyond the “low-hanging fruits” so far identified by genome-wide association (GWA) studies, new methods must be developed in order to discover the numerous remaining genes that estimates of heritability indicate should be contributing to complex human phenotypes, such as obesity. Here we describe a novel integrative method for complex disease gene identification utilizing both genome-wide transcript profiling of adipose tissue samples and consequent analysis of genome-wide association data generated in large SNP scans. We infer causality of genes with obesity by employing a unique set of monozygotic twin pairs discordant for BMI (n = 13 pairs, age 24–28 years, 15.4 kg mean weight difference) and contrast the transcript profiles with those from a larger sample of non-related adult individuals (N = 77). Using this approach, we were able to identify 27 genes with possibly causal roles in determining the degree of human adiposity. Testing for association of SNP variants in these 27 genes in the population samples of the large ENGAGE consortium (N = 21,000) revealed a significant deviation of P-values from the expected (P = 4×10⁻⁴). A total of 13 genes contained SNPs nominally associated with BMI. The top finding was blood coagulation factor F13A1 identified as a novel obesity gene also replicated in a second GWA set of ∼2,000 individuals. This study presents a new approach to utilizing gene expression studies for informing choice of candidate genes for complex human phenotypes, such as obesity.

Obesity has a strong genetic component and an estimated 45%–85% of the variation in adult relative weight is genetically determined. Many genes have recently been identified in genome-wide association studies. The individual effects of the identified genes, however, have been very modest, and their identification required very large sample sizes. New approaches are therefore needed to uncover further genetic variants that contribute to the development of obesity and related conditions. Much can be learned from studying the expression of genes in adipose tissue of obese and non-obese subjects, but it is very difficult to distinguish which genes' expression differences represent reactions to obesity from those related to causal processes. We studied monozygotic twin pairs discordant for obesity and contrasted the gene expression profiles of obese and lean co-twins (controlling for genetic variation) to those from unrelated individuals to try to discern the cause-and-effect relationships of the identified changes in gene expression in fat. Testing the identified genes in 21,000 individuals identified numerous new genes with possible roles in the development of obesity. Among the top findings was a gene involved in blood coagulation (Factor XIIIA1), possibly linking obesity with known complications including deep vein thrombosis, heart attack, and stroke.

Non-alcoholic fatty liver disease from pathogenesis to management: an update

Non-alcoholic fatty liver disease (NAFLD), the most common chronic liver disease in the Western world, is tightly associated with obesity and metabolic syndrome. NAFLD entails an increased cardiometabolic and liver-related risk, the latter regarding almost exclusively non-alcoholic steatohepatitis (NASH), the progressive form of NAFLD. Pathogenetic models encompass altered hepatic lipid partitioning and adipokine action, increased oxidative stress, free fatty acid lipotoxicity. On this basis, lifestyle-, drug- or surgically induced weight loss, insulin sensitizers, antioxidants, lipid-lowering drugs have been evaluated in NAFLD/NASH. Most trials are small, of short duration, nonrandomized, without histological end points, thus limiting assessment of long-term safety and efficacy of proposed treatments. All NAFLD patients should be evaluated for their metabolic, cardiovascular and liver-related risk. Liver biopsy remains the gold standard for staging NAFLD, but non-invasive methods are under intense development. Weight loss through lifestyle intervention is the initial approach, because of established efficacy on NAFLD-associated cardiometabolic abnormalities, and to emerging benefits on necroinflammation and overall disease activity in NASH. Bariatric surgery warrants further evaluation before it can be routinely considered in morbidly obese NASH. Larger- and longer-duration randomized trials assessing safety and benefits of drugs on patient-oriented outcomes are needed before pharmacological treatment can be routinely recommended for NASH.

A human-type nonalcoholic steatohepatitis model with advanced fibrosis in rabbits

Nonalcoholic steatohepatitis (NASH) progresses to liver fibrosis and cirrhosis, which can lead to life-threatening liver failure and the development of hepatocellular carcinoma. The aim of the present study was to create a rabbit model of NASH with advanced fibrosis (almost cirrhosis) by feeding the animals a diet supplemented with 0.75% cholesterol and 12% corn oil. After 9 months of feeding with this diet, the rabbits showed high total cholesterol levels in serum and liver tissues in the absence of insulin resistance. The livers became whitish and nodular. In addition, the number of rabbit macrophage antigen-positive cells and the expression of mRNAs for inflammatory cytokines showed a significant increase. Moreover, fibrotic septa composed of collagens and alpha-smooth muscle actin-positive cells were found between the central and portal veins, indicating alteration of the parenchymal architecture. There was also a marked increase of mRNAs for transforming growth factor-beta1 and collagen 1A1. Comprehensive analysis of protein and gene expression revealed an imbalance of the antioxidant system and methionine metabolism. We also found that ezetimibe attenuated steatohepatitis in this model. In conclusion, the present rabbit model of NASH features advanced fibrosis that is close to cirrhosis and may be useful for analyzing the molecular mechanisms of human NASH. Ezetimibe blunted the development of NASH in this model, suggesting its potential clinical usefulness for human steatohepatitis.

PPAR/RXR Regulation of Fatty Acid Metabolism and Fatty Acid omega-Hydroxylase (CYP4) Isozymes: Implications for Prevention of Lipotoxicity in Fatty Liver Disease

Fatty liver disease is a common lipid metabolism disorder influenced by the combination of individual genetic makeup, drug exposure, and life-style choices that are frequently associated with metabolic syndrome, which encompasses obesity, dyslipidemia, hypertension, hypertriglyceridemia, and insulin resistant diabetes. Common to obesity related dyslipidemia is the excessive storage of hepatic fatty acids (steatosis), due to a decrease in mitochondria β-oxidation with an increase in both peroxisomal β-oxidation, and microsomal ω-oxidation of fatty acids through peroxisome proliferator activated receptors (PPARs). How steatosis increases PPARα activated gene expression of fatty acid transport proteins, peroxisomal and mitochondrial fatty acid β-oxidation and ω-oxidation of fatty acids genes regardless of whether dietary fatty acids are polyunsaturated (PUFA), monounsaturated (MUFA), or saturated (SFA) may be determined by the interplay of PPARs and HNF4α with the fatty acid transport proteins L-FABP and ACBP. In hepatic steatosis and steatohepatitis, the ω-oxidation cytochrome P450 CYP4A gene expression is increased even with reduced hepatic levels of PPARα. Although numerous studies have suggested the role ethanol-inducible CYP2E1 in contributing to increased oxidative stress, Cyp2e1-null mice still develop steatohepatitis with a dramatic increase in CYP4A gene expression. This strongly implies that CYP4A fatty acid ω-hydroxylase P450s may play an important role in the development of steatohepatitis. In this review and tutorial, we briefly describe how fatty acids are partitioned by fatty acid transport proteins to either anabolic or catabolic pathways regulated by PPARs, and we explore how medium-chain fatty acid (MCFA) CYP4A and long-chain fatty acid (LCFA) CYP4Fω-hydroxylase genes are regulated in fatty liver. We finally propose a hypothesis that increased CYP4A expression with a decrease in CYP4F genes may promote the progression of steatosis to steatohepatitis.

Preliminary report: hepatic fat and inflammation in type 2 diabetes mellitus

Although the association between inflammation and hepatic fat is fairly established, it remains unclear whether this association is independent of general measures of obesity and standard cardiovascular risk factors. Therefore, the aim of this study was to investigate the contribution of hepatic steatosis as an independent predictor of chronic inflammation in 281 subjects with type 2 diabetes mellitus. Reduced hepatic steatosis significantly (P < .01) correlated with C-reactive protein (r = -0.16) and adiponectin (r = 0.23). The association of hepatic steatosis with both C-reactive protein and adiponectin remained significant after adjustment for age, ethnicity, body mass index (or waist circumference), triglycerides, high-density lipoprotein, and total cholesterol. These data support the concept that accumulation of hepatic fat is related to enhanced inflammation in type 2 diabetes mellitus independent of general measures of obesity and standard cardiovascular risk factors.

Obesity and nonalcoholic fatty liver disease: biochemical, metabolic, and clinical implications

Obesity is associated with an increased risk of nonalcoholic fatty liver disease (NAFLD). Steatosis, the hallmark feature of NAFLD, occurs when the rate of hepatic fatty acid uptake from plasma and de novo fatty acid synthesis is greater than the rate of fatty acid oxidation and export (as triglyceride within very low-density lipoprotein). Therefore, an excessive amount of intrahepatic triglyceride (IHTG) represents an imbalance between complex interactions of metabolic events. The presence of steatosis is associated with a constellation of adverse alterations in glucose, fatty acid, and lipoprotein metabolism. It is likely that abnormalities in fatty acid metabolism, in conjunction with adipose tissue, hepatic, and systemic inflammation, are key factors involved in the development of insulin resistance, dyslipidemia, and other cardiometabolic risk factors associated with NAFLD. However, it is not clear whether NAFLD causes metabolic dysfunction or whether metabolic dysfunction is responsible for IHTG accumulation, or possibly both. Understanding the precise factors involved in the pathogenesis and pathophysiology of NAFLD will provide important insights into the mechanisms responsible for the cardiometabolic complications of obesity.

Patomechanisms of hepatic steatosis

Az elhízás és hozzá társuló kórállapotok, így az inzulinrezisztenciát kísérő nem alkoholos zsírmájbetegség a világ egészségügyi rendszereire jelentős terhet rónak. A betegség kialakulásának első lépése a májban történő trigliceridlerakódást kísérő elzsírosodás, amely később gyulladáshoz, illetve májzsugorhoz és májrákhoz is vezethet. A steatosis hátterében a májba történő nagy mennyiségű zsírsavbeáramlás és a szervben megfigyelhető fokozott lipogenesis, valamint a zsírsav-oxidációs és -szekréciós utak relatív csökkenése áll. Jelen összefoglalónkban ezen molekuláris mechanizmusokra világítunk rá. Ezek megértése fontos adalékul szolgálhat az elhízás és az inzulinrezisztencia mellett kialakuló májbetegség, illetve a következményes kardiometabolikus következmények hátterének tisztázásához.

Mechanisms and clinical implications of hepatocyte lipoapoptosis

Nonalcoholic fatty liver disease (NAFLD) is characterized by insulin resistance, elevated serum levels of free fatty acids (FFAs) and fatty infiltration of the liver. Accumulation of triglycerides in the hepatocyte results from the uptake and esterification of circulating FFAs by the liver. Contrary to current theory, hepatic steatosis appears to be a detoxification process, as FFAs are directly cytotoxic for the hepatocyte and inhibition of triglyceride formation enhances FFAs toxicity. Hepatocyte apoptosis is a key feature of NAFLD and correlates with disease severity. Since FFA-induced toxicity, or lipoapoptosis, represents a mechanism for the pathogenesis of NAFLD, this article will highlight the cellular pathways contributing to hepatocyte lipoapoptosis. To date, there is no proven effective therapy for patients with NAFLD and insights into the molecular mediators of lipoapoptosis should help promote effective therapeutic strategies for this disease.