Hepatic fatty acid partitioning

Hepatic fatty acid and glucose handling in metabolic disease: Potential impact on cardiovascular disease risk

The prevalence of metabolic diseases, including type 2 diabetes mellitus (T2DM) and metabolic dysfunction-associated steatotic liver disease (MASLD) is increasing. Although invariably associated with obesity, the importance of fat deposition in non-adipose tissue organs has yet to be fully explored. Pathological ectopic fat deposition within the liver (known as (MASLD)) has been suggested to underlie the development of T2DM and is now emerging as an independent risk factor for cardiovascular disease (CVD). The process of hepatic de novo lipogenesis (DNL), that is the synthesis of fatty acids from non-lipid precursors (e.g. glucose), has received much attention as it sits at the intersect of hepatic glucose and fatty acid handling. An upregulation of the DNL pathway has been suggested to be central in the development of metabolic diseases (including MASLD, insulin resistance, and T2DM). Here we review the evidence to determine if hepatic DNL may play a role in the development of MASLD and T2DM and therefore underlie an increased risk of CVD.

Defatting of donor transplant livers during normothermic perfusion-a randomised clinical trial: study protocol for the DeFat study

BACKGROUND

Liver disease is the third leading cause of premature death in the UK. Transplantation is the only successful treatment for end-stage liver disease but is limited by a shortage of suitable donor organs. As a result, up to 20% of patients on liver transplant waiting lists die before receiving a transplant. A third of donated livers are not suitable for transplant, often due to steatosis. Hepatic steatosis, which affects 33% of the UK population, is strongly associated with obesity, an increasing problem in the potential donor pool. We have recently tested defatting interventions during normothermic machine perfusion (NMP) in discarded steatotic human livers that were not transplanted. A combination of therapies including forskolin (NKH477) and L-carnitine to defat liver cells and lipoprotein apheresis filtration were investigated. These interventions resulted in functional improvement during perfusion and reduced the intrahepatocellular triglyceride (IHTG) content. We hypothesise that defatting during NMP will allow more steatotic livers to be transplanted with improved outcomes.

METHODS

In the proposed multi-centre clinical trial, we will randomly assign 60 livers from donors with a high-risk of hepatic steatosis to either NMP alone or NMP with defatting interventions. We aim to test the safety and feasibility of the defatting intervention and will explore efficacy by comparing ex-situ and post-reperfusion liver function between the groups. The primary endpoint will be the proportion of livers that achieve predefined functional criteria during perfusion which indicate potential suitability for transplantation. These criteria reflect hepatic metabolism and injury and include lactate clearance, perfusate pH, glucose metabolism, bile composition, vascular flows and transaminase levels. Clinical secondary endpoints will include proportion of livers transplanted in the two arms, graft function; cell-free DNA (cfDNA) at follow-up visits; patient and graft survival; hospital and ITU stay; evidence of ischemia-reperfusion injury (IRI); non-anastomotic biliary strictures and recurrence of steatosis (determined on MRI at 6 months).

DISCUSSION

This study explores ex-situ pharmacological optimisation of steatotic donor livers during NMP. If the intervention proves effective, it will allow the safe transplantation of livers that are currently very likely to be discarded, thereby reducing waiting list deaths.

TRIAL REGISTRATION

ISRCTN ISRCTN14957538. Registered in October 2022.

In Vivo Monitoring of Glycerolipid Metabolism in Animal Nutrition Biomodel-Fed Smart-Farm Eggs

Although many studies have examined the biochemical metabolic pathways by which an egg (egg yolk) lowers blood lipid levels, data on the molecular biological mechanisms that regulate and induce the partitioning of hepatic glycerolipids are missing. The aim of this study was to investigate in vivo monitoring in four study groups using an animal nutrition biomodel fitted with a jugular-vein cannula after egg yolk intake: CON (control group, oral administration of 1.0 g of saline), T1 (oral administration of 1.0 g of pork belly fat), T2 (oral administration of 1.0 g of smart-farm egg yolk), and T3 (oral administration of T1 and T2 alternately every week). The eggs induced significant and reciprocal changes in incorporating 14C lipids into the total glycerolipids and releasing 14CO2, thereby regulating esterification and accelerating oxidation in vivo. The eggs increased phospholipid secretion from the liver into the blood and decreased triacylglycerol secretion by regulating the multiple cleavage of fatty acyl-CoA moieties' fluxes. In conclusion, the results of the current study reveal the novel fact that eggs can lower blood lipids by lowering triacylglycerol secretion in the biochemical metabolic pathway of hepatic glycerolipid partitioning while simultaneously increasing phospholipid secretion and 14CO2 emission.

Cardiometabolic disease in Black African and Caribbean populations: an ethnic divergence in pathophysiology?

In the UK, populations of Black African and Caribbean (BAC) ethnicity suffer higher rates of cardiometabolic disease than White Europeans (WE). Obesity, leading to increased visceral adipose tissue (VAT) and intrahepatic lipid (IHL), has long been associated with cardiometabolic risk, driving insulin resistance and defective fatty acid/lipoprotein metabolism. These defects are compounded by a state of chronic low-grade inflammation, driven by dysfunctional adipose tissue. Emerging evidence has highlighted associations between central complement system components and adipose tissue, fatty acid metabolism and inflammation; it may therefore sit at the intersection of various cardiometabolic disease risk factors. However, increasing evidence suggests an ethnic divergence in pathophysiology, whereby current theories fail to explain the high rates of cardiometabolic disease in BAC populations. Lower fasting and postprandial TAG has been reported in BAC, alongside lower VAT and IHL deposition, which are paradoxical to the high rates of cardiometabolic disease exhibited by this ethnic group. Furthermore, BAC have been shown to exhibit a more anti-inflammatory profile, with lower TNF-α and greater IL-10. In contrast, recent evidence has revealed greater complement activation in BAC compared to WE, suggesting its dysregulation may play a greater role in the high rates of cardiometabolic disease experienced by this population. This review outlines the current theories of how obesity is proposed to drive cardiometabolic disease, before discussing evidence for ethnic differences in disease pathophysiology between BAC and WE populations.

Molecular Mechanism for Hepatic Glycerolipid Partitioning of n-6/n-3 Fatty Acid Ratio in an Obese Animal Biomodels

The n-6/n-3 metabolic pathway associated with hepatic glycerolipid portioning plays a key role in preventing obesity. In this nutrition metabolism study, we used in vivo monitoring techniques with 40 obese male Sprague-Dawley strain rats attached with jugular-vein cannula after obesity was induced by a high-fat diet to determine the molecular mechanism associated with hepatic glycerolipid partitioning involving the n-6/n-3 metabolic pathway. Rats were randomly assigned to four groups (10 animals per group), including one control group (CON, n-6/n-3 of 71:1) and three treatment groups (n-6/n-3 of 4:1, 15:1 and 30:1). They were fed with experimental diets for 60 days. Incorporation rates of [14C]-labeling lipid into glycerolipid in the liver were 28.87−37.03% in treatment groups fed with diets containing an n-6/n-3 ratio of 4:1, 15:1 and 30:1, which were significantly (p < 0.05) lower than that in the CON (40.01%). However, 14CO2 emission % of absorbed dose showed the opposite trend. It was significantly (p < 0.05) higher in a treatment groups (n-6/n-3 of 4:1, 15:1 and 30:1, 30.35−45.08%) than in CON (27.71%). Regarding the metabolic distribution of glycerolipid to blood from livers, phospholipid/total glycerolipid (%) was significantly (p < 0.05) lower in CON at 11.04% than in treatment groups at 18.15% to 25.15%. Moreover, 14CO2/[14C]-total glycerolipid (%) was significantly (p < 0.05) higher in treatment groups at 44.16−78.50% than in CON at 39.50%. Metabolic distribution of fatty acyl moieties flux for oxidation and glycerolipid synthesis in the liver were significantly (p < 0.05) better in order of 4:1 > 15:1 > 30:1 than in the CON. Our data demonstrate that n-6/n-3 of 4:1 could help prevent obesity by controlling the mechanism of hepatic partitioning through oxidation and esterification of glycerolipid in an obese animal biomodel.

The effect of acute and chronic exercise on hepatic lipid composition

Exercise is recommended for those with, or at risk of nonalcoholic fatty liver disease (NAFLD), owing to beneficial effects on hepatic steatosis and cardiometabolic risk. Whilst exercise training reduces total intrahepatic lipid in people with NAFLD, accumulating evidence indicates that exercise may also modulate hepatic lipid composition. This metabolic influence is important as the profile of saturated (SFA), monounsaturated (MUFA), and polyunsaturated fatty acids (PUFA) dramatically affect the metabolic consequences of hepatic lipid accumulation; with SFA being especially lipotoxic. Relatedly, obesity and NAFLD are associated with hepatic PUFA depletion and elevated SFA. This review summarizes the acute (single bout) and chronic (exercise training) effects of exercise on hepatic lipid composition in rodents (acute studies: n = 3, chronic studies: n = 13) and humans (acute studies: n = 1, chronic studies: n = 3). An increased proportion of hepatic PUFA after acute and chronic exercise is the most consistent finding of this review. Mechanistically, this may relate to an enhanced uptake of adipose-derived PUFA (reflecting habitual diet), particularly in rodents. A relative decrease in the proportion of hepatic MUFA after chronic exercise is also documented repeatedly, particularly in rodent models with elevated hepatic MUFA. This outcome is related to decreased hepatic stearoyl-CoA desaturase-1 activity in some studies. Findings regarding hepatic SFA are less consistent and limited by the absence of metabolic challenge in rodent models. These findings require confirmation in well-controlled interventions in people with NAFLD. These studies will be facilitated by recently validated magnetic resonance spectroscopy techniques, able to precisely quantify hepatic lipid composition in vivo.

Transcriptomic responses are sex-dependent in the skeletal muscle and liver in offspring of obese mice

Infants born to obese mothers are more likely to develop metabolic disease, including glucose intolerance and hepatic steatosis, in adult life. We examined the effects of maternal obesity on the transcriptome of skeletal muscle and liver tissues of the near-term fetus and 3-mo-old offspring in mice born to dams fed a high-fat and -sugar diet. Previously, we have shown that male, but not female, offspring develop glucose intolerance, insulin resistance, and liver steatosis at 3 mo old. Female C57BL6/J mice were fed normal chow or an obesogenic high-calorie diet before mating and throughout pregnancy. RNAseq was performed on the liver and gastrocnemius muscle following collection from fetuses on embryonic day 18.5 (E18.5) as well as from 3-mo-old offspring from obese dams and control dams. Significant genes were generated for each sex, queried for enrichment, and modeled to canonical pathways. RNAseq was corroborated by protein quantification in offspring. The transcriptomic response to maternal obesity in the liver was more marked in males than females. However, in both male and female offspring of obese dams, we found significant enrichment for fatty acid metabolism, mitochondrial transport, and oxidative stress in the liver transcriptomes as well as decreased protein concentrations of electron transport chain members. In skeletal muscle, pathway analysis of gene expression revealed sexual dimorphic patterns, including metabolic processes of fatty acids and glucose, as well as PPAR, AMPK, and PI3K-Akt signaling pathways. Transcriptomic responses to maternal obesity in skeletal muscle were more marked in female offspring than males. Female offspring had greater expression of genes associated with glucose uptake, and protein abundance reflected greater activation of mTOR signaling. Skeletal muscle and livers in mice born to obese dams had sexually dimorphic transcriptomic responses that changed from the fetus to the adult offspring. These data provide insights into mechanisms underpinning metabolic programming in maternal obesity.NEW & NOTEWORTHY Transcriptomic data support that fetuses of obese mothers modulate metabolism in both muscle and liver. These changes were strikingly sexually dimorphic in agreement with published findings that male offspring of obese dams exhibit pronounced metabolic disease earlier. In both males and females, the transcriptomic responses in the fetus were different than those at 3 mo, implicating adaptive mechanisms throughout adulthood.

Determining the temporal, dose, and composition effects of nutritional substrates in an in vitro model of intrahepatocellular triglyceride accumulation

Pathological accumulation of intrahepatic triglyceride underpins the early stages of nonalcoholic fatty liver disease (NAFLD) and can progress to fibrosis, cirrhosis, and cancer of the liver. Studies in humans suggest that consumption of a diet enriched in saturated compared to unsaturated fatty acids (FAs), is more detrimental to liver fat accumulation and metabolism. However, the reasons for the divergence remain unclear and physiologically-relevant cellular models are required. Therefore, the aims of this study were to investigate the effect of modifying media composition, concentration, and treatment frequency of sugars, FAs and insulin on intrahepatocellular triglyceride content and intracellular glucose, FA and circadian function. Huh7 cells were treated with 2% human serum and a combination of sugars and FAs (low fat low sugar [LFLS], high fat low sugar [HFLS], or high fat high sugar [HFHS]) enriched in either unsaturated (OPLA) or saturated (POLA) FAs for 2, 4, or 7 days with a daily or alternating treatment regime. Stable isotope tracers were utilized to investigate basal and/or insulin-responsive changes in hepatocyte metabolism in response to different treatment regimes. Cell viability, media biochemistry, intracellular metabolism, and circadian biology were quantified. The FA composition of the media (OPLA vs. POLA) did not influence cell viability or intracellular triglyceride content in hepatocytes. In contrast, POLA-treated cells had lower FA oxidation and media acetate, and with higher FA concentrations, displayed lower intracellular glycogen content and diminished insulin stimulation of glycogenesis, compared to OPLA-treated cells. The addition of HFHS also had profound effects on circadian oscillation and gene expression. Cells treated daily with HFHS for at least 4 days resulted in a cellular model displaying characteristics of early stage NAFLD seen in humans. Repeated treatment for longer durations (≥7 days) may provide opportunities to investigate lipid and glucose metabolism in more severe stages of NAFLD.

The genetic interactions between non-alcoholic fatty liver disease and cardiovascular diseases

The ongoing debate on whether non-alcoholic fatty liver disease (NAFLD) is an active contributor or an innocent bystander in the development of cardiovascular disease (CVD) has sparked interests in understanding the common mediators between the two biologically distinct entities. This comprehensive review identifies and curates genetic studies of NAFLD overlapping with CVD, and describes the colinear as well as opposing correlations between genetic associations for the two diseases. Here, CVD described in relation to NAFLD are coronary artery disease, cardiomyopathy and atrial fibrillation. Unique findings of this review included certain NAFLD susceptibility genes that possessed cardioprotective properties. Moreover, the complex interactions of genetic and environmental risk factors shed light on the disparity in genetic influence on NAFLD and its incident CVD. This serves to unravel NAFLD-mediated pathways in order to reduce CVD events, and helps identify targeted treatment strategies, develop polygenic risk scores to improve risk prediction and personalise disease prevention.

Role of fatty acid transport protein 4 in metabolic tissues: insights into obesity and fatty liver disease

Fatty acid (FA) metabolism is a series of processes that provide structural substances, signalling molecules and energy. Ample evidence has shown that FA uptake is mediated by plasma membrane transporters including FA transport proteins (FATPs), caveolin-1, fatty-acid translocase (FAT)/CD36, and fatty-acid binding proteins. Unlike other FA transporters, the functions of FATPs have been controversial because they contain both motifs of FA transport and fatty acyl-CoA synthetase (ACS). The widely distributed FATP4 is not a direct FA transporter but plays a predominant function as an ACS. FATP4 deficiency causes ichthyosis premature syndrome in mice and humans associated with suppression of polar lipids but an increase in neutral lipids including triglycerides (TGs). Such a shift has been extensively characterized in enterocyte-, hepatocyte-, and adipocyte-specific Fatp4-deficient mice. The mutants under obese and non-obese fatty livers induced by different diets persistently show an increase in blood non-esterified free fatty acids and glycerol indicating the lipolysis of TGs. This review also focuses on FATP4 role on regulatory networks and factors that modulate FATP4 expression in metabolic tissues including intestine, liver, muscle, and adipose tissues. Metabolic disorders especially regarding blood lipids by FATP4 deficiency in different cell types are herein discussed. Our results may be applicable to not only patients with FATP4 mutations but also represent a model of dysregulated lipid homeostasis, thus providing mechanistic insights into obesity and development of fatty liver disease.

NAFLD: Mechanisms, Treatments, and Biomarkers

Nonalcoholic fatty liver disease (NAFLD), recently renamed metabolic-associated fatty liver disease (MAFLD), is one of the most common causes of liver diseases worldwide. NAFLD is growing in parallel with the obesity epidemic. No pharmacological treatment is available to treat NAFLD, specifically. The reason might be that NAFLD is a multi-factorial disease with an incomplete understanding of the mechanisms involved, an absence of accurate and inexpensive imaging tools, and lack of adequate non-invasive biomarkers. NAFLD consists of the accumulation of excess lipids in the liver, causing lipotoxicity that might progress to metabolic-associated steatohepatitis (NASH), liver fibrosis, and hepatocellular carcinoma. The mechanisms for the pathogenesis of NAFLD, current interventions in the management of the disease, and the role of sirtuins as potential targets for treatment are discussed here. In addition, the current diagnostic tools, and the role of non-coding RNAs as emerging diagnostic biomarkers are summarized. The availability of non-invasive biomarkers, and accurate and inexpensive non-invasive diagnosis tools are crucial in the detection of the early signs in the progression of NAFLD. This will expedite clinical trials and the validation of the emerging therapeutic treatments.

Dysregulation of hepatic metabolism with obesity: factors influencing glucose and lipid metabolism

The liver is a key metabolic organ that undertakes a multitude of physiological processes over the course of a day, including intrahepatic lipid and glucose metabolism which plays a key role in the regulation of systemic lipid and glucose concentrations. It serves as an intermediary organ between exogenous (dietary) and endogenous energy supply to extrahepatic organs. Thus, perturbations in hepatic metabolism can impact widely on metabolic disease risk. For example, the accumulation of intra-hepatocellular TAG (IHTG), for which adiposity is almost invariably a causative factor may result in dysregulation of metabolic pathways. Accumulation of IHTG is likely due to an imbalance between fatty acid delivery, synthesis and removal (via oxidation or export as TAG) from the liver; insulin plays a key role in all of these processes.

Nonalcoholic fatty liver disease shows significant sex dimorphism

Nonalcoholic fatty liver disease (NAFLD), which has been renamed metabolic dysfunction-associated fatty liver disease, is a growing global medical problem. The incidence of NAFLD and its associated end-stage liver disease is increasing each year, and many research advancements have been achieved to date. This review focuses on the current knowledge of the sex differences in NAFLD and does not elaborate on areas without differences. Studies have revealed significant sex differences in the prevalence, influencing factors, pathophysiology, complications and therapies of NAFLD. Men have a higher incidence than women. Compared with women, men exhibit increased visceral fat deposition, are more susceptible to leptin resistance, lack estrogen receptors, and tend to synthesize fatty acids into fat storage. Male patients will experience more severe hepatic fibrosis and a higher incidence of liver cancer. However, once NAFLD occurs, women show a faster progression of liver fibrosis, higher levels of liver cell damage and inflammation and are less likely to undergo liver transplantation than men. In general, men have more risk factors and more severe pathophysiological reactions than women, whereas the development of NAFLD is faster in women, and the treatments for women are more limited than those for men. Thus, whether sex differences should be considered in the individualized prevention and treatment of NAFLD in the future is worth considering.

Dietary carbohydrates and fats in nonalcoholic fatty liver disease

The global prevalence of nonalcoholic fatty liver disease (NAFLD) has dramatically increased in parallel with the epidemic of obesity. Controversy has emerged around dietary guidelines recommending low-fat-high-carbohydrate diets and the roles of dietary macronutrients in the pathogenesis of metabolic disease. In this Review, the topical questions of whether and how dietary fats and carbohydrates, including free sugars, differentially influence the accumulation of liver fat (specifically, intrahepatic triglyceride (IHTG) content) are addressed. Focusing on evidence from humans, we examine data from stable isotope studies elucidating how macronutrients regulate IHTG synthesis and disposal, alter pools of bioactive lipids and influence insulin sensitivity. In addition, we review cross-sectional studies on dietary habits of patients with NAFLD and randomized controlled trials on the effects of altering dietary macronutrients on IHTG. Perhaps surprisingly, evidence to date shows no differential effects between free sugars, with both glucose and fructose increasing IHTG in the context of excess energy. Moreover, saturated fat raises IHTG more than polyunsaturated or monounsaturated fats, with adverse effects on insulin sensitivity, which are probably mediated in part by increased ceramide synthesis. Taken together, the data support the use of diets that have a reduced content of free sugars, refined carbohydrates and saturated fat in the treatment of NAFLD.

Roles of vitamin A in the regulation of fatty acid synthesis

Dietary macronutrients and micronutrients play important roles in human health. On the other hand, the excessive energy derived from food is stored in the form of triacylglycerol. A variety of dietary and hormonal factors affect this process through the regulation of the activities and expression levels of those key player enzymes involved in fatty acid biosynthesis such as acetyl-CoA carboxylase, fatty acid synthase, fatty acid elongases, and desaturases. As a micronutrient, vitamin A is essential for the health of humans. Recently, vitamin A has been shown to play a role in the regulation of glucose and lipid metabolism. This review summarizes recent research progresses about the roles of vitamin A in fatty acid synthesis. It focuses on the effects of vitamin A on the activities and expression levels of mRNA and proteins of key enzymes for fatty acid synthesis in vitro and in vivo. It appears that vitamin A status and its signaling pathway regulate the expression levels of enzymes involved in fatty acid synthesis. Future research directions are also discussed.

Managing NAFLD in Type 2 Diabetes: The Effect of Lifestyle Interventions, a Narrative Review

The prevalence of non-alcoholic fatty liver disease (NAFLD) and type 2 diabetes (T2D) is increasing. As a strong association between these two diseases exist, it is unsurprising that the number of patients with coexisting NAFLD and T2D is also increasing. These patients display a deleterious metabolic profile (e.g. hypertriglyceridemia), and increased mortality rates relative to those with only NAFLD or T2D in isolation; therefore, effective treatment strategies are required. Here we review the available intervention studies that have investigated the effects of changes in lifestyle (diet and exercise/physical activity) on NAFLD in patients with both NAFLD and T2D. On the basis of the available evidence, it appears that the addition of any kind of exercise (i.e. resistance, aerobic, or high-intensity intermittent exercise) is beneficial for patients with both NAFLD and T2D. These effects appear to occur independently of changes in body weight. Hypocaloric diets leading to weight loss are also effective in improving metabolic parameters in patients with both NAFLD and T2D, with data indicating that ~ 7–10% weight loss is required in order to observe beneficial effects. It is unclear if multidisciplinary interventions incorporating changes in both diet and physical activity levels are a more effective treatment strategy in this population than diet or exercise interventions in isolation. In conclusion, it is clear that lifestyle interventions are an effective treatment strategy in patients with both NAFLD and T2D, although further research is required to optimise these interventions and determine their scalability.

Effects of Simvastatin on the Metabolism of Fatty Acids in Combined Secondary Prevention of Coronary Heart Disease: Dosage and Gender Differences between the Effects

BACKGROUND

Statins are currently used for secondary prevention of Coronary Heart Disease (CHD), as the lipid-lowering therapy with them is proven safe and effective.

OBJECTIVE

The purpose of this research is to investigate the dose-dependent effect of statins used for secondary prevention of coronary heart disease, as well as mechanisms of quantitative and qualitative changes in lipoproteins, fatty acids and cholesterol in the blood and tissues of people of both sexes.

METHODS

In a clinical trial (n=125, of which 89 patients belong to group 1 and 36 to group 2) and an experiment on laboratory animals (n = 100), simvastatin reduced the total level of fatty acids in blood plasma, when given in the amount that was within the therapeutic dose range.

RESULTS

This effect was achieved through a drug-induced improvement in the capacity of hepatic cells to absorb Low-density (LDL) and Very-low-density (VLDL) lipoproteins.

CONCLUSION

Considering the formation of saturated fatty acids, statin performed better in males. With Omega-3 polyunsaturated fatty acids involved, changes in lipoproteins, cholesterol and fatty acids (liver and myocardium) were similar to those caused by small doses of a statin drug. Effects of the combination of bisoprolol and acetylsalicylic acid were completely different from those caused by the use of statin.

Nonalcoholic Fatty Liver Disease in Adults: Current Concepts in Etiology, Outcomes, and Management

Nonalcoholic fatty liver disease (NAFLD) is a spectrum of disease, extending from simple steatosis to inflammation and fibrosis with a significant risk for the development of cirrhosis. It is highly prevalent and is associated with significant adverse outcomes both through liver-specific morbidity and mortality but, perhaps more important, through adverse cardiovascular and metabolic outcomes. It is closely associated with type 2 diabetes and obesity, and both of these conditions drive progressive disease toward the more advanced stages. The mechanisms that govern hepatic lipid accumulation and the predisposition to inflammation and fibrosis are still not fully understood but reflect a complex interplay between metabolic target tissues including adipose and skeletal muscle, and immune and inflammatory cells. The ability to make an accurate assessment of disease stage (that relates to clinical outcome) can also be challenging. While liver biopsy is still regarded as the gold-standard investigative tool, there is an extensive literature on the search for novel noninvasive biomarkers and imaging modalities that aim to accurately reflect the stage of underlying disease. Finally, although no therapies are currently licensed for the treatment of NAFLD, there are interventions that appear to have proven efficacy in randomized controlled trials as well as an extensive emerging therapeutic landscape of new agents that target many of the fundamental pathophysiological processes that drive NAFLD. It is highly likely that over the next few years, new treatments with a specific license for the treatment of NAFLD will become available.

In vivo monitoring of hepatic glycolipid distribution of n-6 ∕ n-3 in jugular-vein-cannulated rats as a nutritional research model for monogastric animal

The metabolic distribution via blood from liver of glycerolipids by omega-6 to omega-3 fatty acid (n-6 / n-3) ratio in monogastric animal nutrition is very important. In vivo monitoring technique using jugular-vein-cannulated rats as a nutritional model for monogastric animal can yield important insights into animal nutrition. This study was conducted to determine the effect of different n-6 / n-3 ratios (71:1, 4:1, 15:1, 30:1) on metabolic distribution of glycerolipids newly synthesized and secreted in the liver of the rats and explore the mechanism involved. Regarding 14CO2 released from oxidation of glycerolipid metabolism, it was the highest (22.5 %) in groups with a n-6 / n-3 ratio of 4:1 (P<0.05). The control group showed the highest total glycerolipid level, followed by the 30:1, 15:1, and 4:1 groups in order (P<0.05). When secreted triacylglycerol level of each group was compared with that of the control group, the 4:1, 15:1, and 30:1 groups were decreased by 36.3 %, 20.9 %, and 13.3 %, respectively (P<0.05). Regarding the distribution of phospholipid against total glycerolipid compared to the control group, the 4:1, 15:1, and 30:1 groups were 1.38, 1.29, and 1.17 times higher, respectively (P<0.05). In the comparison of 14CO2 emission against total glycerolipid compared with the control group, the 4:1, 15:1, and 30:1 groups were 1.61, 1.52, and 1.29 times higher, respectively (P<0.05). These results demonstrate that a dietary n-6 / n-3 fatty acid ratio of 4:1 could significantly decrease harmful lipid levels in the blood by controlling the mechanism of metabolic distribution via blood from triglyceride and phospholipid newly synthesized in the liver of cannulated rat.

Overeating Saturated Fat Promotes Fatty Liver and Ceramides Compared With Polyunsaturated Fat: A Randomized Trial

CONTEXT

Saturated fatty acid (SFA) vs polyunsaturated fatty acid (PUFA) may promote nonalcoholic fatty liver disease by yet unclear mechanisms.

OBJECTIVE

To investigate if overeating SFA- and PUFA-enriched diets lead to differential liver fat accumulation in overweight and obese humans.

DESIGN

Double-blind randomized trial (LIPOGAIN-2). Overfeeding SFA vs PUFA for 8 weeks, followed by 4 weeks of caloric restriction.

SETTING

General community.

PARTICIPANTS

Men and women who are overweight or have obesity (n = 61).

INTERVENTION

Muffins, high in either palm (SFA) or sunflower oil (PUFA), were added to the habitual diet.

MAIN OUTCOME MEASURES

Lean tissue mass (not reported here). Secondary and exploratory outcomes included liver and ectopic fat depots.

RESULTS

By design, body weight gain was similar in SFA (2.31 ± 1.38 kg) and PUFA (2.01 ± 1.90 kg) groups, P = 0.50. SFA markedly induced liver fat content (50% relative increase) along with liver enzymes and atherogenic serum lipids. In contrast, despite similar weight gain, PUFA did not increase liver fat or liver enzymes or cause any adverse effects on blood lipids. SFA had no differential effect on the accumulation of visceral fat, pancreas fat, or total body fat compared with PUFA. SFA consistently increased, whereas PUFA reduced circulating ceramides, changes that were moderately associated with liver fat changes and proposed markers of hepatic lipogenesis. The adverse metabolic effects of SFA were reversed by calorie restriction.

CONCLUSIONS

SFA markedly induces liver fat and serum ceramides, whereas dietary PUFA prevents liver fat accumulation and reduces ceramides and hyperlipidemia during excess energy intake and weight gain in overweight individuals.

The regulation of hepatic fatty acid synthesis and partitioning: the effect of nutritional state

Nonalcoholic fatty liver disease (NAFLD) is an increasing global public health burden. NAFLD is strongly associated with type 2 diabetes mellitus, obesity and cardiovascular disease and begins with intrahepatic triacylglycerol accumulation. Under healthy conditions, the liver regulates lipid metabolism to meet systemic energy needs in the fed and fasted states. The processes of fatty acid uptake, fatty acid synthesis and the intracellular partitioning of fatty acids into storage, oxidation and secretion pathways are tightly regulated. When one or more of these processes becomes dysregulated, excess lipid accumulation can occur. Although genetic and environmental factors have been implicated in the development of NAFLD, it remains unclear why an imbalance in these pathways begins. The regulation of fatty acid partitioning occurs at several points, including during triacylglycerol synthesis, lipid droplet formation and lipolysis. These processes are influenced by enzyme function, intake of dietary fats and sugars and whole-body metabolism, and are further affected by the presence of obesity or insulin resistance. Insight into how the liver controls fatty acid metabolism in health and how these processes might be affected in disease would offer the potential for new therapeutic treatments for NAFLD to be developed.

Fat and Sugar-A Dangerous Duet. A Comparative Review on Metabolic Remodeling in Rodent Models of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is a common disease in Western society and ranges from steatosis to steatohepatitis to end-stage liver disease such as cirrhosis and hepatocellular carcinoma. The molecular mechanisms that are involved in the progression of steatosis to more severe liver damage in patients are not fully understood. A deeper investigation of NAFLD pathogenesis is possible due to the many different animal models developed recently. In this review, we present a comparative overview of the most common dietary NAFLD rodent models with respect to their metabolic phenotype and morphological manifestation. Moreover, we describe similarities and controversies concerning the effect of NAFLD-inducing diets on mitochondria as well as mitochondria-derived oxidative stress in the progression of NAFLD.

Gene expressions of de novo hepatic lipogenesis in feline hepatic lipidosis

Objectives

The aim of this study was to evaluate if de novo hepatic lipid synthesis contributes to fatty acid overload in the liver of cats with feline hepatic lipidosis (FHL).

Methods

Lipogenic gene expression of peroxisome proliferator-activated receptor-alpha (PPAR-α), peroxisome proliferator-activated receptor-gamma (PPAR-γ), fatty acid synthase (FASN) and sterol regulatory element-binding factor (SREBF1) were evaluated using quantitative RT-PCR in liver tissue of six cats with FHL and compared with the liver tissue of eight healthy cats.

Results

In liver tissue, PPAR-α, PPAR-γ and FASN mRNA expression levels were not significantly different (P >0.12, P >0.89 and P >0.5, respectively) in the FHL group compared with the control group. SREBF1 gene expression was downregulated around 10-fold in the FHL group vs the control group (P = 0.039).

Conclusions and relevance

The downregulation of SREBF1 in the liver tissue of cats with FHL does not support the hypothesis that de novo lipogenesis in the liver is an important pathway of fatty acid accumulation in FHL.

The influence of dietary fatty acids on liver fat content and metabolism

Non-alcoholic fatty liver disease encompasses a spectrum of conditions from hepatic steatosis through to cirrhosis; obesity is a known risk factor. The liver plays a major role in regulating fatty acid metabolism and perturbations in intrahepatic processes have potential to impact on metabolic health. It remains unclear why intra-hepatocellular fat starts to accumulate, but it likely involves an imbalance between fatty acid delivery to the liver, fatty acid synthesis and oxidation within the liver and TAG export from the liver. As man spends the majority of the day in a postprandial rather than postabsorptive state, dietary fatty acid intake should be taken into consideration when investigating why intra-hepatic fat starts to accumulate. This review will discuss the impact of the quantity and quality of dietary fatty acids on liver fat accumulation and metabolism, along with some of the potential mechanisms involved. Studies investigating the role of dietary fat in liver fat accumulation, although surprisingly limited, have clearly demonstrated that it is total energy intake, rather than fat intake per se, that is a key mediator of liver fat content; hyperenergetic diets increase liver fat whilst hypoenergetic diets decrease liver fat content irrespective of total fat content. Moreover, there is now, albeit limited evidence emerging to suggest the composition of dietary fat may also play a role in liver fat accumulation, with diets enriched in saturated fat appearing to increase liver fat content to a greater extent when compared with diets enriched in unsaturated fats.

NAFLD risk alleles in PNPLA3, TM6SF2, GCKR and LYPLAL1 show divergent metabolic effects

Fatty liver has been associated with unfavourable metabolic changes in circulation. To provide insights in fatty liver-related metabolic deviations, we compared metabolic association profile of fatty liver versus metabolic association profiles of genotypes increasing the risk of non-alcoholic fatty liver disease (NAFLD). The cross-sectional associations of ultrasound-ascertained fatty liver with 123 metabolic measures were determined in 1810 (N_{fatty liver} = 338) individuals aged 34–49 years from The Cardiovascular Risk in Young Finns Study. The association profiles of NAFLD-risk alleles in PNPLA3, TM6SF2, GCKR, and LYPLAL1 with the corresponding metabolic measures were obtained from a publicly available metabolomics GWAS including up to 24 925 Europeans. The risk alleles showed different metabolic effects: PNPLA3 rs738409-G, the strongest genetic NAFLD risk factor, did not associate with metabolic changes. Metabolic effects of GCKR rs1260326-T were comparable in many respects to the fatty liver associations. Metabolic effects of LYPLAL1 rs12137855-C were similar, but statistically less robust, to the effects of GCKR rs1260326-T. TM6SF2 rs58542926-T displayed opposite metabolic effects when compared with the fatty liver associations. The metabolic effects of the risk alleles highlight heterogeneity of the molecular pathways leading to fatty liver and suggest that the fatty liver-related changes in the circulating lipids and metabolites may vary depending on the underlying pathophysiological mechanism. Despite the robust cross-sectional associations on population level, the present results showing neutral or cardioprotective metabolic effects for some of the NAFLD risk alleles advocate that hepatic lipid accumulation by itself may not increase the level of circulating lipids or other metabolites.

Hepatic fatty acid synthesis and partitioning: the effect of metabolic and nutritional state

When we consume dietary fat, a series of complex metabolic processes ensures that fatty acids are absorbed, transported around the body and used/stored appropriately. The liver is a central metabolic organ within the human body and has a major role in regulating fat and carbohydrate metabolism. Studying hepatic metabolism in human subjects is challenging; the use of stable isotope tracers and measurement of particles or molecules secreted by the liver such as VLDL-TAG and 3-hydroxybutyrate offers the best insight into postprandial hepatic fatty acid metabolism in human subjects. Diet derived fatty acids are taken up by the liver and mix with fatty acids coming from the lipolysis of adipose tissue, and those already present in the liver (cytosolic TAG) and fatty acids synthesised de novo within the liver from non-lipid precursors (known as de novo lipogenesis). Fatty acids are removed from the liver by secretion as VLDL-TAG and oxidation. Perturbations in these processes have the potential to impact on metabolic health. Whether fatty acids are partitioned towards oxidation or esterification pathways appears to be dependent on a number of metabolic factors; not least ambient insulin concentrations. Moreover, along with the phenotype and lifestyle factors (e.g. habitual diet) of an individual, it is becoming apparent that the composition of the diet (macronutrient and fatty acid composition) may play pivotal roles in determining if intra-hepatic fat accumulates, although what remains to be elucidated is the influence these nutrients have on intra-hepatic fatty acid synthesis and partitioning.

Sex Differences in Hepatic De Novo Lipogenesis with Acute Fructose Feeding

Dietary free sugars have received much attention over the past few years. Much of the focus has been on the effect of fructose on hepatic de novo lipogenesis (DNL). Therefore the aim of the present study was to investigate the effects of meals high and low in fructose on postprandial hepatic DNL and fatty acid partitioning and dietary fatty acid oxidation. Sixteen healthy adults (eight men, eight women) participated in this randomised cross-over study; study days were separated by a 4-week wash-out period. Hepatic DNL and dietary fatty acid oxidation were assessed using stable-isotope tracer methodology. Consumption of the high fructose meal significantly increased postprandial hepatic DNL to a greater extent than consumption of the low fructose meal and this effect was evident in women but not men. Despite an increase in hepatic DNL, there was no change in dietary fatty acid oxidation. Taken together, our data show that women are more responsive to ingestion of higher amounts of fructose than men and if continued over time this may lead to changes in hepatic fatty acid partitioning and eventually liver fat content.

Ectopic Fat Accumulation in Distinct Insulin Resistant Phenotypes; Targets for Personalized Nutritional Interventions

Cardiometabolic diseases are one of the leading causes for disability and mortality in the Western world. The prevalence of these chronic diseases is expected to rise even further in the next decades. Insulin resistance (IR) and related metabolic disturbances are linked to ectopic fat deposition, which is the storage of excess lipids in metabolic organs such as liver and muscle. Notably, a vicious circle exists between IR and ectopic fat, together increasing the risk for the development of cardiometabolic diseases. Nutrition is a key-determining factor for both IR and ectopic fat deposition. The macronutrient composition of the diet may impact metabolic processes related to ectopic fat accumulation and IR. Interestingly, however, the metabolic phenotype of an individual may determine the response to a certain diet. Therefore, population-based nutritional interventions may not always lead to the most optimal (cardiometabolic) outcomes at the individual level, and differences in the metabolic phenotype may underlie conflicting findings related to IR and ectopic fat in dietary intervention studies. Detailed metabolic phenotyping will help to better understand the complex relationship between diet and metabolic regulation, and to optimize intervention outcomes. A subgroup-based approach that integrates, among others, tissue-specific IR, cardiometabolic parameters, anthropometrics, gut microbiota, age, sex, ethnicity, and psychological factors may thereby increase the efficacy of dietary interventions. Nevertheless, the implementation of more personalized nutrition may be complex, costly, and time consuming. Future studies are urgently warranted to obtain insight into a more personalized approach to nutritional interventions, taking into account the metabolic phenotype to ultimately improve insulin sensitivity and reduce the risk for cardiometabolic diseases.

Studying non-alcoholic fatty liver disease: the ins and outs of in vivo, ex vivo and in vitro human models

The prevalence of non-alcoholic fatty liver disease (NAFLD) is increasing. Determining the pathogenesis and pathophysiology of human NAFLD will allow for evidence-based prevention strategies, and more targeted mechanistic investigations. Various in vivo, ex situ and in vitro models may be utilised to study NAFLD; but all come with their own specific caveats. Here, we review the human-based models and discuss their advantages and limitations in regards to studying the development and progression of NAFLD. Overall, in vivo whole-body human studies are advantageous in that they allow for investigation within the physiological setting, however, limited accessibility to the liver makes direct investigations challenging. Non-invasive imaging techniques are able to somewhat overcome this challenge, whilst the use of stable-isotope tracers enables mechanistic insight to be obtained. Recent technological advances (i.e. normothermic machine perfusion) have opened new opportunities to investigate whole-organ metabolism, thus ex situ livers can be investigated directly. Therefore, investigations that cannot be performed in vivo in humans have the potential to be undertaken. In vitro models offer the ability to perform investigations at a cellular level, aiding in elucidating the molecular mechanisms of NAFLD. However, a number of current models do not closely resemble the human condition and work is ongoing to optimise culturing parameters in order to recapitulate this. In summary, no single model currently provides insight into the development, pathophysiology and progression across the NAFLD spectrum, each experimental model has limitations, which need to be taken into consideration to ensure appropriate conclusions and extrapolation of findings are made.

LiverSex Computational Model: Sexual Aspects in Hepatic Metabolism and Abnormalities

The liver is to date the best example of a sexually dimorphic non-reproductive organ. Over 1,000 genes are differentially expressed between sexes indicating that female and male livers are two metabolically distinct organs. The spectrum of liver diseases is broad and is usually prevalent in one or the other sex, with different contributing genetic and environmental factors. It is thus difficult to predict individual's disease outcomes and treatment options. Systems approaches including mathematical modeling can aid importantly in understanding the multifactorial liver disease etiology leading toward tailored diagnostics, prognostics and therapy. The currently established computational models of hepatic metabolism that have proven to be essential for understanding of non-alcoholic fatty liver disease (NAFLD) and hepatocellular carcinoma (HCC) are limited to the description of gender-independent response or reflect solely the response of the males. Herein we present LiverSex, the first sex-based multi-tissue and multi-level liver metabolic computational model. The model was constructed based on in silico liver model SteatoNet and the object-oriented modeling. The crucial factor in adaptation of liver metabolism to the sex is the inclusion of estrogen and androgen receptor responses to respective hormones and the link to sex-differences in growth hormone release. The model was extensively validated on literature data and experimental data obtained from wild type C57BL/6 mice fed with regular chow and western diet. These experimental results show extensive sex-dependent changes and could not be reproduced in silico with the uniform model SteatoNet. LiverSex represents the first large-scale liver metabolic model, which allows a detailed insight into the sex-dependent complex liver pathologies, and how the genetic and environmental factors interact with the sex in disease appearance and progression. We used the model to identify the most important sex-dependent metabolic pathways, which are involved in accumulation of triglycerides representing initial steps of NAFLD. We identified PGC1A, PPARα, FXR, and LXR as regulatory factors that could become important in sex-dependent personalized treatment of NAFLD.

Influence of dietary macronutrients on liver fat accumulation and metabolism

The liver is a principal metabolic organ within the human body and has a major role in regulating carbohydrate, fat, and protein metabolism. With increasing rates of obesity, the prevalence of non-alcoholic fatty liver disease (NAFLD) is growing. It remains unclear why NAFLD, which is now defined as the hepatic manifestation of the metabolic syndrome, develops but lifestyle factors such as diet (ie, total calorie and specific nutrient intakes), appear to play a key role. Here we review the available observational and intervention studies that have investigated the influence of dietary macronutrients on liver fat content. Findings from observational studies are conflicting with some reporting that relative to healthy controls, patients with NAFLD consume diets higher in total fat/saturated fatty acids, whilst others find they consume diets higher in carbohydrates/sugars. From the limited number of intervention studies that have been undertaken, a consistent finding is a hypercaloric diet, regardless of whether the excess calories have been provided either as fat, sugar, or both, increases liver fat content. In contrast, a hypocaloric diet decreases liver fat content. Findings from both hyper- and hypo-caloric feeding studies provide some suggestion that macronutrient composition may also play a role in regulating liver fat content and this is supported by data from isocaloric feeding studies; fatty acid composition and/or carbohydrate content/type appear to influence whether there is accrual of liver fat or not. The mechanisms by which specific macronutrients, when consumed as part of an isocaloric diet, cause a change in liver fat remain to be fully elucidated.

Insights into the Hexose Liver Metabolism-Glucose versus Fructose

High-fructose intake in healthy men is associated with characteristics of metabolic syndrome. Extensive knowledge exists about the differences between hepatic fructose and glucose metabolism and fructose-specific mechanisms favoring the development of metabolic disturbances. Nevertheless, the causal relationship between fructose consumption and metabolic alterations is still debated. Multiple effects of fructose on hepatic metabolism are attributed to the fact that the liver represents the major sink of fructose. Fructose, as a lipogenic substrate and potent inducer of lipogenic enzyme expression, enhances fatty acid synthesis. Consequently, increased hepatic diacylglycerols (DAG) are thought to directly interfere with insulin signaling. However, independently of this effect, fructose may also counteract insulin-mediated effects on liver metabolism by a range of mechanisms. It may drive gluconeogenesis not only as a gluconeogenic substrate, but also as a potent inducer of carbohydrate responsive element binding protein (ChREBP), which induces the expression of lipogenic enzymes as well as gluconeogenic enzymes. It remains a challenge to determine the relative contributions of the impact of fructose on hepatic transcriptome, proteome and allosterome changes and consequently on the regulation of plasma glucose metabolism/homeostasis. Mathematical models exist modeling hepatic glucose metabolism. Future models should not only consider the hepatic adjustments of enzyme abundances and activities in response to changing plasma glucose and insulin/glucagon concentrations, but also to varying fructose concentrations for defining the role of fructose in the hepatic control of plasma glucose homeostasis.

Reduction in liver fat by dietary MUFA in type 2 diabetes is helped by enhanced hepatic fat oxidation

AIMS/HYPOTHESIS

The aim of this work was to investigate hepatic lipid metabolic processes possibly involved in the reduction of liver fat content (LF) observed in patients with type 2 diabetes after an isoenergetic diet enriched in monounsaturated fatty acids (MUFAs).

METHODS

This is an ancillary analysis of a published study. In a parallel-group design, 30 men and eight women, aged 35-70 years, with type 2 diabetes and whose blood glucose was controlled satisfactorily (HbA_1c < 7.5% [58 mmol/mol]) by diet or diet plus metformin, were randomised by MINIM software to follow either a high-carbohydrate/high-fibre/low-glycaemic index diet (CHO/fibre diet, n = 20) or a high-MUFA diet (MUFA diet, n = 18) for 8 weeks. The assigned diets were known for the participants and blinded for people doing measurements. Before and after intervention, LF was measured by ¹H-MRS (primary outcome) and indirect indices of de novo lipogenesis (DNL) (serum triacylglycerol palmitic:linoleic acid ratio), stearoyl-CoA desaturase activity (SCD-1) (serum triacylglycerol palmitoleic:palmitic acid ratio) and hepatic β-oxidation of fatty acids (β-hydroxybutyrate plasma concentrations) were measured.

RESULTS

LF was reduced by 30% after the MUFA diet, as already reported. Postprandial β-hydroxybutyrate incremental AUC (iAUC) was significantly less suppressed after the MUFA diet (n = 16) (-2504 ± 4488 μmol/l × 360 min vs baseline -9021 ± 6489 μmol/l × 360 min) while it was unchanged after the CHO/fibre diet (n = 17) (-8168 ± 9827 μmol/l × 360 min vs baseline -7206 ± 10,005 μmol/l × 360 min, p = 0.962) (mean ± SD, p = 0.043). In the participants assigned to the MUFA diet, the change in postprandial β-hydroxybutyrate iAUC was inversely associated with the change in LF (r = -0.642, p = 0.010). DNL and SCD-1 indirect indices did not change significantly after either of the dietary interventions.

CONCLUSIONS/INTERPRETATION

Postprandial hepatic oxidation of fatty acids is a metabolic process possibly involved in the reduction of LF by a MUFA-rich diet in patients with type 2 diabetes.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01025856 FUNDING : The study was funded by Ministero Istruzione Università e Ricerca and Italian Minister of Health.

Independent effects of diet and exercise training on fat oxidation in non-alcoholic fatty liver disease

AIM

To investigate the independent effects of 6-mo of dietary energy restriction or exercise training on whole-body and hepatic fat oxidation of patients with non-alcoholic fatty liver disease (NAFLD).

METHODS

Participants were randomised into either circuit exercise training (EX; n = 13; 3 h/wk without changes in dietary habits), or dietary energy restriction (ER) without changes in structured physical activity (ER; n = 8). Respiratory quotient (RQ) and whole-body fat oxidation rates (Fat_ox) were determined by indirect calorimetry under basal, insulin-stimulated and exercise conditions. Severity of disease and steatosis was determined by liver histology; hepatic Fat_ox was estimated from plasma β-hydroxybutyrate concentrations; cardiorespiratory fitness was expressed as VO_2peak. Complete-case analysis was performed (EX: n = 10; ER: n = 6).

RESULTS

Hepatic steatosis and NAFLD activity score decreased with ER but not with EX. β-hydroxybutyrate concentrations increased significantly in response to ER (0.08 ± 0.02 mmol/L vs 0.12 ± 0.04 mmol/L, P = 0.03) but remained unchanged in response to EX (0.10 ± 0.03 mmol/L vs 0.11 ± 0.07 mmol/L, P = 0.39). Basal RQ decreased (P = 0.05) in response to EX, while this change was not significant after ER (P = 0.38). VO_2peak (P < 0.001) and maximal Fat_ox during aerobic exercise (P = 0.03) improved with EX but not with ER (P > 0.05). The increase in β-hydroxybutyrate concentrations was correlated with the reduction in hepatic steatosis (r = -0.56, P = 0.04).

CONCLUSION

ER and EX lead to specific benefits on fat metabolism of patients with NAFLD. Increased hepatic Fat_ox in response to ER could be one mechanism through which the ER group achieved reduction in steatosis.

Fasting Plasma Insulin Concentrations Are Associated With Changes in Hepatic Fatty Acid Synthesis and Partitioning Prior to Changes in Liver Fat Content in Healthy Adults

Resistance to the action of insulin affects fatty acid delivery to the liver, fatty acid synthesis and oxidation within the liver, and triglyceride export from the liver. To understand the metabolic consequences of hepatic fatty acid synthesis, partitioning, oxidation, and net liver fat content in the fasted and postprandial states, we used stable-isotope tracer methodologies to study healthy men and women with varying degrees of insulin resistance before and after consumption of a mixed meal. Subjects were classified as being normoinsulinemic (NI) (fasting plasma insulin <11.2 mU/L, n = 18) or hyperinsulinemic (HI) (fasting plasma insulin >11.2 mU/L, n = 19). Liver fat content was similar between HI and NI individuals, despite HI subjects having marginally more visceral fat. However, de novo lipogenesis was higher and fatty acid oxidation was lower in HI individuals compared with NI subjects. These data suggest that metabolic pathways promoting fat accumulation are enhanced in HI but, paradoxically, without any significant effect on liver fat content when observed in healthy people. This is likely to be explained by increased triglyceride secretion as observed by hypertriglyceridemia.

Sex-Specific Differences in Hepatic Fat Oxidation and Synthesis May Explain the Higher Propensity for NAFLD in Men

CONTEXT AND OBJECTIVE

In most populations a greater proportion of men have hepatic steatosis than women. Sex-specific differences in hepatic dietary fatty acid (FA) metabolism have not been well characterized. We compared fasting and postprandial hepatic FA synthesis (de novo lipogenesis [DNL]) and oxidation in men and women.

PARTICIPANTS AND METHODS

Fasting and postprandial hepatic FA metabolism was studied in 22 healthy men (n = 11) and women with similar age, body mass index, and liver fat content using metabolic substrates labeled with stable-isotope tracers ((2)H2O and [U(13)C]palmitate). Dietary FA oxidation was assessed by appearance of (13)C into plasma 3-hydroxybutyrate and breath CO2 as markers of liver and whole-body FA oxidation, respectively.

RESULTS

Despite similar liver fat content, fasting and postprandial plasma triacylglycerol (TG) concentrations were significantly (P < .05) higher in men compared with women. The appearance of (13)C from dietary FA into plasma 3-hydroxybutyrate and breath CO2 was greater (P < .05) in women compared with men. Although the contribution of DNL into very low-density lipoprotein (VLDL)-TG was similar (∼ 10%) in the fasting state, there was a divergence in pattern over the course of the study, with men maintaining a higher contribution of DNL to VLDL-TG than women (P = .006 time x sex interaction).

CONCLUSIONS

The combination of lower dietary FA oxidation and a prolonged increase in DNL observed in men may represent partitioning of FA into esterification and storage pathways within the liver, leading to greater VLDL-TG production, and predispose to the sex difference in hepatic steatosis.

Characterization of lipid metabolism in a novel immortalized human hepatocyte cell line

The development of hepatocyte cell models that represent fatty acid partitioning within the human liver would be beneficial for the study of the development and progression of nonalcoholic fatty liver disease (NAFLD). We sought to develop and characterize a novel human liver cell line (LIV0APOLY) to establish a model of lipid accumulation using a physiological mixture of fatty acids under low- and high-glucose conditions. LIV0APOLY cells were compared with a well-established cell line (HepG2) and, where possible, primary human hepatocytes. LIV0APOLY cells were found to proliferate and express some mature liver markers and were wild type for the PNPLA3 (rs738409) gene, whereas HepG2 cells carried the Ile(148)Met variant that is positively associated with liver fat content. Intracellular triglyceride content was higher in HepG2 than in LIV0APOLY cells; exposure to high glucose and/or exogenous fatty acids increased intracellular triglyceride in both cell lines. Triglyceride concentrations in media were higher from LIV0APOLY compared with HepG2 cells. Culturing LIV0APOLY cells in high glucose increased a marker of endoplasmic reticulum stress and attenuated insulin-stimulated Akt phosphorylation whereas low glucose and exogenous fatty acids increased AMPK phosphorylation. Although LIV0APOLY cells and primary hepatocytes stored similar amounts of exogenous fatty acids as triglyceride, more exogenous fatty acids were partitioned toward oxidation in the LIV0APOLY cells than in primary hepatocytes. LIV0APOLY cells offer the potential to be a renewable cellular model for studying the effects of exogenous metabolic substrates on fatty acid partitioning; however, their usefulness as a model of lipoprotein metabolism needs to be further explored.

New insights into the pathophysiology of dyslipidemia in type 2 diabetes

Cardiovascular disease (CVD) remains the leading cause of morbidity and mortality for patients with type 2 diabetes, despite recent significant advances in management strategies to lessen CVD risk factors. A major cause is the atherogenic dyslipidemia, which consists of elevated plasma concentrations of both fasting and postprandial triglyceride-rich lipoproteins (TRLs), small dense low-density lipoprotein (LDL) and low high-density lipoprotein (HDL) cholesterol. The different components of diabetic dyslipidemia are not isolated abnormalities but closely linked to each other metabolically. The underlying disturbances are hepatic overproduction and delayed clearance of TRLs. Recent results have unequivocally shown that triglyceride-rich lipoproteins and their remnants are atherogenic. To develop novel strategies for the prevention and treatment of dyslipidaemia, it is essential to understand the pathophysiology of dyslipoproteinaemia in humans. Here, we review recent advances in our understanding of the pathophysiology of diabetic dyslipidemia.

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

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

The influence of dietary fat on liver fat accumulation

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

Hepatic lipogenesis and a marker of hepatic lipid oxidation, predict postprandial responses of triglyceride-rich lipoproteins

OBJECTIVE

Postprandial hypertriglyceridemia is an important risk factor for cardiovascular disease. The mechanisms are still unclear. Here it was tested if hepatic de novo lipogenesis (DNL) and lipid oxidation influence the postprandial responses of triglyceride-rich lipoproteins (TRL) in humans.

METHODS

The contribution of hepatic DNL to hepatic TRL production was analyzed in 67 men and women with a moderate range of BMI after a fat-rich meal. Also, lipase activities, liver fat, and 3-OH-butyrate were quantitated as an indicator of β-oxidation. Lipoproteins and metabolic markers were measured in fasting and postprandial blood samples.

RESULTS

Postprandial DNL correlates with postprandial TG and apolipoprotein (apo) C-III responses in plasma and with TG, apoB48 and apoB100 responses in TRLs and their larger remnant particles. Fasting and 8-h postprandial DNL was inversely related to 3-OH-butyrate but not to liver fat content. Fasting apoC-III and 3-OH-butyrate, but not liver fat, independently predicted fasting DNL.

CONCLUSIONS

The fasting and 8-h postprandial rate of DNL was inversely associated with the hepatic lipid oxidation in humans. DNL contributes significantly to the TG content in TRLs but not to the amount of liver fat, suggesting that an imbalance between DNL and fat oxidation contributes to postprandial atherogenic dyslipidemia.

Evidence of endoplasmic reticulum stress and liver inflammation in the American mink Neovison vison with benign hepatic steatosis

We investigated the presence of inflammatory signs in the progression of fatty liver disease induced by fasting. Sixty standard black American mink (Neovison vison) were fasted for 0, 1, 3, 5, or 7 days and one group for 7 days followed by re-feeding for 28 days. Liver sections were evaluated histologically and liver mRNA levels indicating endoplasmic reticulum (ER) stress, adipogenic transformation, and inflammation were assessed by quantitative real-time PCR. After 3 days of fasting, the mink had developed moderate liver steatosis. Increased hyaluronan reactivity in lymphocytic foci but no Mallory-Denk bodies were seen in livers of the mink fasted for 5-7 days. Up-regulation of glucose-regulated protein, 78 kDa was observed on day 7 indicating ER stress, especially in the females. Liver lipoprotein lipase and monocyte chemoattractant protein 1 mRNA levels increased in response to 5-7 days of food deprivation, while tumor necrosis factor α (TNF-α) was the highest in the mink fasted for 5 days. The expression of the genes of interest, except for TNF-α, correlated with each other and with the liver fat content. The mRNA levels were found to change more rapidly below n-3/n-6 polyunsaturated fatty acid ratio threshold of 0.15. Following re-feeding, hepatocyte morphology and mRNA abundance returned to pre-fasting levels. Within the studied timeframe, evidence for ER stress, adipogenic transformation, and liver inflammation suggested incipient transition from steatosis to steatohepatitis with potential for development of more severe liver disease. This may present a possibility to influence disease progression before histologically observable steatohepatitis.

The storage stability and concentration of acetoacetate differs between blood fractions

BACKGROUND

Plasma concentrations of 3-hydroxybutyrate (3HB) are measured more often than acetoacetate (AcAc) which may be due to the reported storage instability of AcAc. The aims of the study were to compare the storage stability of AcAc in different blood fractions over time (90days) when stored at -80°C and to determine the postprandial concentration of AcAc in whole blood, plasma and red blood cells.

METHODS

Blood was collected from fasting subjects (n=5): whole blood, plasma and red blood cells were isolated and deproteinised in perchloric acid, and supernatants were stored at -80°C until analysis. Postprandial concentrations of AcAc in whole blood, plasma and red blood cells were determined at regular intervals over 420min, after subjects (n=23) had consumed a mixed test meal.

RESULTS

Storing deproteinised plasma at -80°C resulted in no significant change in AcAc concentration over 60days. In contrast, whole blood AcAc concentrations significantly decreased by 51% (p=0.018) within 30days. The concentration of AcAc in fasting and postprandial plasma was notably higher than that of whole blood and red blood cells.

DISCUSSION

Our data demonstrates that plasma for AcAc analysis can be stored for longer than previously suggested provided that plasma is deproteinised and stored at -80°C.

The development and functions of oenocytes

Oenocytes have intrigued insect physiologists since the nineteenth century. Many years of careful but mostly descriptive research on these cells highlights their diverse sizes, numbers, and anatomical distributions across Insecta. Contemporary molecular genetic studies in Drosophila melanogaster and Tribolium castaneum support the hypothesis that oenocytes are of ectodermal origin. They also suggest that, in both short and long germ-band species, oenocytes are induced from a Spalt major/Engrailed ectodermal zone by MAPK signaling. Recent glimpses into some of the physiological functions of oenocytes indicate that they involve fatty acid and hydrocarbon metabolism. Genetic studies in D. melanogaster have shown that larval oenocytes synthesize very-long-chain fatty acids required for tracheal waterproofing and that adult oenocytes produce cuticular hydrocarbons required for desiccation resistance and pheromonal communication. Exciting areas of future research include the evolution of oenocytes and their cross talk with other tissues involved in lipid metabolism such as the fat body.

Circadian rhythms in liver physiology and liver diseases

In mammals, circadian rhythms function to coordinate a diverse panel of physiological processes with environmental conditions such as food and light. As the driving force for circadian rhythmicity, the molecular clock is a self-sustained transcription-translational feedback loop system consisting of transcription factors, epigenetic modulators, kinases/phosphatases, and ubiquitin E3 ligases. The molecular clock exists not only in the suprachiasmatic nuclei of the hypothalamus but also in the peripheral tissues to regulate cellular and physiological function in a tissue-specific manner. The circadian clock system in the liver plays important roles in regulating metabolism and energy homeostasis. Clock gene mutant animals display impaired glucose and lipid metabolism and are susceptible to diet-induced obesity and metabolic dysfunction, providing strong evidence for the connection between the circadian clock and metabolic homeostasis. Circadian-controlled hepatic metabolism is partially achieved by controlling the expression and/or activity of key metabolic enzymes, transcription factors, signaling molecules, and transporters. Reciprocally, intracellular metabolites modulate the molecular clock activity in response to the energy status. Although still at the early stage, circadian clock dysfunction has been implicated in common chronic liver diseases. Circadian dysregulation of lipid metabolism, detoxification, reactive oxygen species (ROS) production, and cell-cycle control might contribute to the onset and progression of liver steatosis, fibrosis, and even carcinogenesis. In summary, these findings call for a comprehensive study of the function and mechanisms of hepatic circadian clock to gain better understanding of liver physiology and diseases.

Whole-body substrate metabolism is associated with disease severity in patients with non-alcoholic fatty liver disease

OBJECTIVES

In non-alcoholic fatty liver disease (NAFLD), hepatic steatosis is intricately linked with a number of metabolic alterations. We studied substrate utilisation in NAFLD during basal, insulin-stimulated and exercise conditions, and correlated these outcomes with disease severity.

METHODS

20 patients with NAFLD (mean ± SD body mass index (BMI) 34.1 ± 6.7 kg/m(2)) and 15 healthy controls (BMI 23.4 ± 2.7 kg/m(2)) were assessed. Respiratory quotient (RQ), whole-body fat (Fat ox) and carbohydrate (CHO ox) oxidation rates were determined by indirect calorimetry in three conditions: basal (resting and fasted), insulin-stimulated (hyperinsulinaemic-euglycaemic clamp) and exercise (cycling at an intensity to elicit maximal Fat ox). Severity of disease and steatosis were determined by liver histology, hepatic Fat ox from plasma β-hydroxybutyrate concentrations, aerobic fitness expressed as VO2 peak, and visceral adipose tissue (VAT) measured by computed tomography.

RESULTS

Within the overweight/obese NAFLD cohort, basal RQ correlated positively with steatosis (r=0.57, p=0.01) and was higher (indicating smaller contribution of Fat ox to energy expenditure) in patients with NAFLD activity score (NAS) ≥ 5 vs <5 (p=0.008). Both results were independent of VAT, % body fat and BMI. Compared with the lean control group, patients with NAFLD had lower basal whole-body Fat ox (1.2 ± 0.3 vs 1.5 ± 0.4 mg/kg FFM/min, p=0.024) and lower basal hepatic Fat ox (ie, β-hydroxybutyrate, p=0.004). During exercise, they achieved lower maximal Fat ox (2.5 ± 1.4 vs. 5.8 ± 3.7 mg/kg FFM/min, p=0.002) and lower VO2 peak (p<0.001) than controls. Fat ox during exercise was not associated with disease severity (p=0.79).

CONCLUSIONS

Overweight/obese patients with NAFLD had reduced hepatic Fat ox and reduced whole-body Fat ox under basal and exercise conditions. There was an inverse relationship between ability to oxidise fat in basal conditions and histological features of NAFLD including severity of steatosis and NAS.

Insulin resistance in type 1 diabetes: what is 'double diabetes' and what are the risks?

In this review, we explore the concept of 'double diabetes', a combination of type 1 diabetes with features of insulin resistance and type 2 diabetes. After considering whether double diabetes is a useful concept, we discuss potential mechanisms of increased insulin resistance in type 1 diabetes before examining the extent to which double diabetes might increase the risk of cardiovascular disease (CVD). We then go on to consider the proposal that weight gain from intensive insulin regimens may be associated with increased CV risk factors in some patients with type 1 diabetes, and explore the complex relationships between weight gain, insulin resistance, glycaemic control and CV outcome. Important comparisons and contrasts between type 1 diabetes and type 2 diabetes are highlighted in terms of hepatic fat, fat partitioning and lipid profile, and how these may differ between type 1 diabetic patients with and without double diabetes. In so doing, we hope this work will stimulate much-needed research in this area and an improvement in clinical practice.