Metabolic disturbances in non-alcoholic fatty liver disease. Clin Sci (Lond). 2009;116:539-564. [PMID: 19243311 DOI: 10.1042/cs20080253]

Increased serum FGF21 levels in patients with nonalcoholic fatty liver disease

BACKGROUND

The fibroblast growth factor 21 (FGF21) hormonal pathway is a metabolic signalling cascade and has been recently identified as the master hormonal regulator of glucose, lipids and overall energy balance. In this observational, case-control study, we assayed serum levels of FGF21 in patients with nonalcoholic fatty liver disease (NAFLD), a hepatic manifestation of the metabolic syndrome, and examined their association with clinical, biochemical and histological phenotypes.

MATERIALS AND METHODS

Serum levels of FGF21 were assayed by ELISA in 82 patients with biopsy-proven NAFLD and 77 controls. We analysed associations between FGF21 and the characteristics of patients with NAFLD by multiple linear regression analysis.

RESULTS

Levels of FGF21 were significantly higher in patients with NAFLD (median 200 pg mL(-1) ; interquartile range: 87-410 pg mL(-1)) than in healthy controls (median 93 pg mL(-1) ; interquartile range: 70-180 pg mL(-1) , Mann-Whitney U-test, P<0·001). There was a stepwise increase in serum FGF21 levels according to the liver steatosis score (median level in subjects with score 1: 170 pg mL(-1) ; score 2: 220 pg mL(-1) ; score 3: 280 pg mL(-1) , P for trend <0·01). After stepwise linear regression analysis, serum FGF21 levels were the only independent predictor of hepatic steatosis scores in patients with NAFLD (β=0·26; t=2·659, P<0·01).

CONCLUSIONS

Serum FGF21 levels are increased in patients with NAFLD regardless of potential confounders and represent an independent predictor of liver steatosis. These findings support further investigation of this molecule in metabolic liver diseases.

Insulin- and leptin-regulated fatty acid uptake plays a key causal role in hepatic steatosis in mice with intact leptin signaling but not in ob/ob or db/db mice

Hepatic steatosis results from several processes. To assess their relative roles, hepatocellular long-chain fatty acid (LCFA) uptake was assayed in hepatocytes from C57BL/6J control mice, mice with steatosis from a high-fat diet (HFD) or 10%, 14%, or 18% ethanol (EtOH) in drinking water [functioning leptin-signaling groups (FLSGs)], and ob/ob and db/db mice. V(max) for uptake was increased vs. controls (P < 0.001) and correlated significantly with liver weight and triglycerides (TGs) in all FLSG mice but was minimally or not increased in ob/ob and db/db mice, in which liver weights and TGs greatly exceeded projections from regressions in FLSG animals. Coefficients of determination (R(2)) for these FLSG regressions suggest that increased LCFA uptake accounts for ∼80% of the increase in hepatic TGs within these groups, but increased lipogenic gene expression data suggest that enhanced LCFA synthesis is the major contributor in ob/ob and db/db. Got2, Cd36, Slc27a2, and Slc27a5 gene expression ratios were significantly upregulated in the EtOH groups, correlating with sterol regulatory element binding protein 1c (SREBP1c) and V(max), but only Cd36 expression was increased in HFD, ob/ob, and db/db mice. Comparison of V(max) with serum insulin and leptin suggests that both hormones contribute to upregulation of uptake in the FLSG animals. Thus, increased LCFA uptake, reflecting SREBP1c-mediated upregulation of four distinct transporters, is the dominant cause of steatosis in EtOH-fed mice. In ob/ob and db/db mice, increased LCFA synthesis appears more important. In FLSG animals, insulin upregulates hepatocellular LCFA uptake. Leptin appears to upregulate LCFA uptake or to be essential for full expression of upregulation by insulin.

Obstructive sleep apnea syndrome and fatty liver: Association or causal link?

Obstructive sleep apnea (OSA) is a complex disorder that consists of upper airway obstruction, chronic intermittent hypoxia and sleep fragmentation. OSA is well known to be associated with hypoxia, insulin resistance and glucose intolerance, and these factors can occur in the presence or absence of obesity and metabolic syndrome. Although it is well established that insulin resistance, glucose intolerance and obesity occur frequently with non-alcoholic fatty liver disease (NAFLD), it is now becoming apparent that hypoxia might also be important in the development of NAFLD, and it is recognized that there is increased risk of NAFLD with OSA. This review discusses the association between OSA, NAFLD and cardiovascular disease, and describes the potential role of hypoxia in the development of NAFLD with OSA.

Nine-year incident diabetes is predicted by fatty liver indices: the French D.E.S.I.R. study

BACKGROUND

Fatty liver is known to be linked with insulin resistance, alcohol intake, diabetes and obesity. Biopsy and even scan-assessed fatty liver are not always feasible in clinical practice. This report evaluates the predictive ability of two recently published markers of fatty liver: the Fatty Liver Index (FLI) and the NAFLD fatty liver score (NAFLD-FLS), for 9-year incident diabetes, in the French general-population cohort: Data from an Epidemiological Study on the Insulin Resistance syndrome (D.E.S.I.R).

METHODS

At baseline, there were 1861 men and 1950 women, non-diabetic, aged 30 to 65 years. Over the follow-up, 203 incident diabetes cases (140 men, 63 women) were identified by diabetes-treatment or fasting plasma glucose > or = 7.0 mmol/l. The FLI includes: BMI, waist circumference, triglycerides and gamma glutamyl transferase, and the NAFLD-FLS: the metabolic syndrome, diabetes, insulin, alanine aminotransferase, and asparate aminotransferase. Logistic regression was used to determine the odds ratios for incident diabetes associated with categories of the fatty liver indices.

RESULTS

In comparison to those with a FLI < 20, the age-adjusted odds ratio (95% confidence interval) for diabetes for a FLI > or = 70 was 9.33 (5.05-17.25) for men and 36.72 (17.12-78.76) for women; these were attenuated to 3.43 (1.61-7.28) and 11.05 (4.09 29.81), after adjusting on baseline glucose, insulin, hypertension, alcohol intake, physical activity, smoking and family antecedents of diabetes; odds ratios increased to 4.71 (1.68-13.16) and 22.77 (6.78-76.44) in those without an excessive alcohol intake. The NAFLD-FLS also predicted incident diabetes, but with odds ratios much lower in women, similar in men.

CONCLUSIONS

These fatty liver indexes are simple clinical tools for evaluating the extent of liver fat and they are predictive of incident diabetes. Physicians should screen for diabetes in patients with fatty liver.

Nine-year incident diabetes is predicted by fatty liver indices: the French D.E.S.I.R. study

BACKGROUND

Fatty liver is known to be linked with insulin resistance, alcohol intake, diabetes and obesity. Biopsy and even scan-assessed fatty liver are not always feasible in clinical practice. This report evaluates the predictive ability of two recently published markers of fatty liver: the Fatty Liver Index (FLI) and the NAFLD fatty liver score (NAFLD-FLS), for 9-year incident diabetes, in the French general-population cohort: Data from an Epidemiological Study on the Insulin Resistance syndrome (D.E.S.I.R).

METHODS

At baseline, there were 1861 men and 1950 women, non-diabetic, aged 30 to 65 years. Over the follow-up, 203 incident diabetes cases (140 men, 63 women) were identified by diabetes-treatment or fasting plasma glucose > or = 7.0 mmol/l. The FLI includes: BMI, waist circumference, triglycerides and gamma glutamyl transferase, and the NAFLD-FLS: the metabolic syndrome, diabetes, insulin, alanine aminotransferase, and asparate aminotransferase. Logistic regression was used to determine the odds ratios for incident diabetes associated with categories of the fatty liver indices.

RESULTS

In comparison to those with a FLI < 20, the age-adjusted odds ratio (95% confidence interval) for diabetes for a FLI > or = 70 was 9.33 (5.05-17.25) for men and 36.72 (17.12-78.76) for women; these were attenuated to 3.43 (1.61-7.28) and 11.05 (4.09 29.81), after adjusting on baseline glucose, insulin, hypertension, alcohol intake, physical activity, smoking and family antecedents of diabetes; odds ratios increased to 4.71 (1.68-13.16) and 22.77 (6.78-76.44) in those without an excessive alcohol intake. The NAFLD-FLS also predicted incident diabetes, but with odds ratios much lower in women, similar in men.

CONCLUSIONS

These fatty liver indexes are simple clinical tools for evaluating the extent of liver fat and they are predictive of incident diabetes. Physicians should screen for diabetes in patients with fatty liver.

Nitration of specific tyrosines in FoF1 ATP synthase and activity loss in aging

It has been reported that C-nitration of proteins occurs under nitrative/oxidative stress; however, its role in pathophysiological situations is not fully understood. In this study, we determined that nitration of Tyr(345) and Tyr(368) in the beta-subunit of the mitochondrial F(o)F(1)-ATPase is a major target for nitrative stress in rat liver under in vivo conditions. The chemical characteristics of these Tyr make them suitable for a facilitated nitration (solvent accessibility, consensus sequence, and pK(a)). Moreover, beta-subunit nitration increased significantly with the age of the rats (from 4 to 80 weeks old) and correlated with decreased ATP hydrolysis and synthesis rates. Although its affinity for ATP binding was unchanged, maximal ATPase activity decreased between young and old rats by a factor of two. These changes directly impacted the available ATP concentration in vivo, and it was expected that they would affect multiple cellular ATP-dependent processes. For instance, at least 50% of available [ATP] in the liver of older rats would have to be committed to sustain maximal Na(+)-K(+)-ATPase activity, whereas only 30% would be required for young rats. If this requirement was not fulfilled, the osmoregulation and Na(+)-nutrient cotransport in liver of older rats would be compromised. On the basis of our studies, we propose that targeted nitration of the beta-subunit is an early marker for nitrative stress and aging.

The role of adiponectin in the pathogenesis and treatment of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is recognized as the most common type of chronic liver disease in Western countries and the leading cause of cryptogenic cirrhosis. Insulin resistance (IR) is a key factor in the pathogenesis of NAFLD, the latter being considered as the hepatic component of IR or metabolic syndrome (MetS). Although the pathogenesis of NAFLD is not fully elucidated, a complex interaction between adipokines and cytokines produced by adipocytes and/or inflammatory cells infiltrating adipose tissue appears to play a crucial role in MetS and NAFLD. Adiponectin is the most abundant and adipose-specific adipokine. In the liver, adiponectin acts through the activation of 5-AMP-activated protein kinase and peroxisome proliferator-activated receptor-alpha pathways and inhibition of toll-like receptor-4 mediated signalling. There is an evidence that adiponectin decreases hepatic and systematic IR and attenuates liver inflammation and fibrosis. Adiponectin generally predicts steatosis grade and severity of NAFLD, but it remains to be addressed to what extent this is a direct effect or related to the presence of more severe IR. Although there is no proven pharmacotherapy for the treatment of NAFLD, recent therapeutic strategies have focused on the indirect upregulation of adiponectin through the administration of various therapeutic agents and/or lifestyle modifications. Weight loss, through diet, lifestyle changes and/or medications including orlistat, sibutramine, rimonabant or bariatric surgery, increase adiponectin and may improve liver histology. Insulin sensitizers, including pioglitazone and rosiglitazone, and lipid-lowering agents, including statins and fibrates, also upregulate adiponectin and ameliorate liver histology. The wider use of new treatment approaches appears to signal the dawn of a new era in the management of NAFLD. In this adiponectin-focused review, the pathogenetic role and the potential therapeutic benefits of adiponectin in NAFLD are systematically analysed.

Regulation of hepatic gene expression by saturated fatty acids

Diets rich in saturated fatty acids have long been associated with increased plasma cholesterol concentrations and hence increased risk of cardiovascular disease. More recently, they have also been suggested to promote the development of non-alcoholic fatty liver disease. While there is now considerable evidence to suggest that polyunsaturated fatty acids exert many of their effects through regulating the activity of transcription factors, including peroxisome proliferator activated receptors, sterol regulatory binding proteins (SREBPs) and liver X receptor, our understanding of how saturated fatty acids act is still limited. Here we review the potential mechanisms whereby saturated fatty acids modulate hepatic lipid metabolism thereby impacting on the synthesis, storage and secretion of lipids. Evidence is presented that their effects are, at least partly, mediated through modulation of the activity of the SREBP family of transcription factors.

Fatty liver: role of inflammation and fatty acid nutrition

Nonalcoholic fatty liver disease (NAFLD) refers to a wide spectrum of liver damage, ranging from simple steatosis to nonalcoholic steatohepatitis (NASH), advanced fibrosis, and cirrhosis. NAFLD is strongly associated with insulin resistance and is defined by accumulation of liver fat >5% per liver weight in the presence of <10g of daily alcohol consumption. The exact prevalence of NAFLD is uncertain because of the absence of simple noninvasive diagnostic tests to facilitate an estimate of prevalence but in subgroups of people such as those with type 2 diabetes, the prevalence may be as high as 70%. NASH is an important subgroup within the spectrum of NAFLD that progresses over time with worsening fibrosis and cirrhosis, and NASH is associated with increased risk for cardiovascular disease. It is, therefore, important to understand the pathogenesis of NASH specifically, to develop strategies for interventions to treat this condition. The purpose of this review is to discuss the roles of inflammation, fatty acids and fatty acids in nutrition, in the pathogenesis and potential treatment of NAFLD.

Inhibition of inducible nitric oxide synthase prevents mitochondrial damage and improves survival of steatotic partial liver grafts

BACKGROUND

Steatotic liver grafts are excluded for partial liver transplantation because of increased risk of primary nonfunction. Mechanisms underlying the failure of fatty partial liver grafts (FPG) remain unknown. This study investigated whether inducible nitric oxide synthase (iNOS) plays a role in failure of FPG.

METHODS

Fatty livers were induced by feeding rats a high-fat high-fructose diet for 2 weeks. Hepatic triglyceride was approximately 9-fold higher in rats fed the high-fat high-fructose diet than those fed a low-fat low-fructose diet. Lean and fatty liver explants were reduced in size ex vivo to approximately one third, stored in the University of Wisconsin cold storage solution for 2 hr, and implanted.

RESULTS

Posttransplantational hepatic iNOS expression and reactive nitrogen species (RNS) formation (nitrite and nitrate levels and 3-nitrotyrosine adducts) increased more profoundly in FPG than in lean partial grafts (LPG). Serum alanine aminotransferase and bilirubin were 2- and 5.5-fold higher after transplantation of FPG than LPG. 5-Bromo-2'-deoxyuridine incorporation was 25% in LPG but only 5% in FPG, and graft weight increased by 64% in LPG while remaining unchanged in FPG. All rats that received FPG died, whereas all those receiving LPG survived. N-(1-naphtyl)ethylendiamine dihydrochloride (5 microM), a specific iNOS inhibitor, largely blunted the production of RNS, prevented the increase of alanine aminotransferase and bilirubin, restored liver regeneration, and improved survival of FPG. Mitochondrial cytochrome c oxidase-IV, ATP synthase-beta, and NADH dehydrogenase-3 decreased markedly in FPG, and these effects were blocked by N-(1-naphtyl)ethylendiamine dihydrochloride.

CONCLUSION

Thus, hepatic steatosis causes failure of partial liver grafts, most likely by increasing RNS that leads to mitochondrial damage and dysfunction.

Coronary flow reserve is impaired in patients with nonalcoholic fatty liver disease: association with liver fibrosis

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) is associated with an increased risk of cardiovascular disease. Coronary flow reserve (CFR) is widely used to examine the integrity of coronary microvascular circulation. We evaluated the prevalence of impaired CFR in patients with biopsy-proven NAFLD. We also investigated the independent clinical, biochemical, and liver histology predictors of CFR in the setting of NAFLD.

METHODS

Fifty-nine consecutive patients with NAFLD and 77 age- and gender-matched controls were evaluated. CFR recordings were performed by transthoracic Doppler harmonic echocardiography. CFR>or=2.0 was considered normal.

RESULTS

CFR was significantly lower in patients with NAFLD than in controls (2.11+/-0.45 vs. 2.52+/-0.62, P<0.001). An impaired CFR (i.e. <2) was found in 25 NAFLD patients (42.4%) whereas all controls had normal CFR values (P<0.001). A stepwise linear regression analysis in NAFLD patients identified liver fibrosis scores as the only independent predictor of CFR values (beta=-0.60; t=-2.44, P=0.021).

CONCLUSION

Our findings indicate that in patients with biopsy-proven NAFLD: (a) an abnormal CFR is found in approximately 42.4% of cases, and (b) liver fibrosis scores are an independent predictor of depressed CFR.

Advances in the prevention and treatment of fatty liver disease

In recent years, lifestyle changes have resulted in a rising prevalence rate of fatty liver disease (FLD). Even worse, there are some misunderstandings concerning the prevention and treatment of FLD. Therefore, it is necessary to improve our understanding of FLD and explore and summarize effective countermeasures for prevention and treatment of FLD. In this article, we will review the recent advances in research of FLD from five aspects: epidemiology, pathogenesis, relationship with metabolic syndrome, misunderstandings concerning prevention and treatment and countermeasures, and highlight the necessity and urgency for prevention and treatment of FLD.

Hypoxia and non-alcoholic fatty liver disease

NAFLD (non-alcoholic fatty liver disease) represents a spectrum of fatty liver diseases associated with an increased risk of Type 2 diabetes and cardiovascular disease. The spectrum of fatty liver diseases comprises simple steatosis, steatosis with inflammation [i.e. NASH (non-alcoholic steatohepatitis)], fatty liver disease with inflammation and fibrosis (severe NASH) and cirrhosis. The molecular mechanisms contributing to NASH are the subject of considerable investigation, as a better understanding of the pathogenesis of NASH will lead to novel therapies for a condition that hitherto remains difficult to treat. In the present issue of Clinical Science, Piguet and co-workers have investigated the effects of hypoxia in the PTEN (phosphatase and tensin homologue deleted on chromosome 10)-deficient mouse, a mouse model that develops NAFLD. The authors show that a short period (7 days) of exposure to hypoxia aggravates the NAFLD phenotype, causing changes in the liver that are in keeping with NASH with increased lipogenesis and inflammation.

Role of biological rhythms in gastrointestinal health and disease

The molecular basis for biological rhythms is formed by clock genes. Clock genes are functional in the liver, within gastrointestinal epithelial cells and neurons of the enteric nervous system. These observations suggest a possible role for clock genes in various circadian functions of the liver and the gastrointestinal tract through the modulation of organ specific clock-controlled genes. Consequently, disruptions in circadian rhythmicity may lead to adverse health consequences. This review will focus on the current understanding of the role of circadian rhythms in the pathogenesis of gastrointestinal- and hepatic disease such as obesity, non-alcoholic fatty liver disease, alcoholic fatty liver disease and alterations in colonic motility.