Determinants of ectopic liver fat in metabolic disease

Metabolic syndrome

The metabolic syndrome (MetS) is a multiplex modifiable risk factor for cardiovascular disease, type 2 diabetes mellitus and other health outcomes, and is a major challenge to clinical practice and public health. The rising global prevalence of MetS, driven by urbanization, sedentary lifestyles and dietary changes, underlines the urgency of addressing this syndrome. We explore the complex underlying mechanisms, including genetic predisposition, insulin resistance, accumulation of dysfunctional adipose tissue and ectopic lipids in abdominal obesity, systemic inflammation and dyslipidaemia, and how they contribute to the clinical manifestations of MetS. Diagnostic approaches vary but commonly focus on abdominal obesity (assessed using waist circumference), hyperglycaemia, dyslipidaemia and hypertension, highlighting the need for population-specific and phenotype-specific diagnostic strategies. Management of MetS prioritizes lifestyle modifications, such as healthy dietary patterns, physical activity and management of excess visceral and ectopic adiposity, as foundational interventions. We also discuss emerging therapies, including new pharmacological treatments and surgical options, providing a forward-looking perspective on MetS research and care. This Primer aims to inform clinicians, researchers and policymakers about MetS complexities, advocating for a cohesive, patient-centred management and prevention strategy. Emphasizing the multifactorial nature of MetS, this Primer calls for integrated public health efforts, personalized care and innovative research to address this escalating health issue.

Pomegranate supplementation alleviates dyslipidemia and the onset of non-alcoholic fatty liver disease in Wistar rats by shifting microbiota and producing urolithin-like microbial metabolites

Non-alcoholic fatty liver disease (NAFLD), obesity and related chronic diseases are major non-communicable diseases with high mortality rates worldwide. While dietary sugars are known to be responsible for insulin resistance and metabolic syndrome (MetS), the underlying pathophysiological effects of sustained fructose consumption require further elucidation. We hypothesize that certain bioactive compounds (i.e. punicalagin and ellagic acid) from dietary pomegranate could counteract the harmful effects of sustained fructose consumption in terms of obesity and liver damage. The present study aimed to elucidate both the molecular mechanisms involved in the pathophysiology associated with fructose intake and the effect of a punicalagin-rich commercial pomegranate dietary supplement (P) used as a nutritional strategy to alleviate fructose-induced metabolic impairments. Thus, nineteen Wistar rats fed on a basal commercial feed were supplemented with either 30% (w/v) fructose in drinking water (F; n = 7) or 30% (w/v) fructose solution plus 0.2% (w/v) P (F + P; n = 6) for 10 weeks. The results were compared to those from a control group fed on the basal diet and provided with drinking water (C; n = 6). Body weight and energy intake were registered weekly. P supplementation decreased fat depots, counteracted the dyslipidemia caused by F and improved markers of liver injury including steatosis. The study of the microbiota by metagenomics and urine by untargeted MS-based metabolomics revealed microbial metabolites from P that may be responsible for these health benefits.

Caloric restriction mitigates age-associated senescence characteristics in subcutaneous adipose tissue-derived stem cells

Adipose tissue regulates metabolic balance, but aging disrupts it, shifting fat from insulin-sensitive subcutaneous to insulin-resistant visceral depots, impacting overall metabolic health. Adipose-derived stem cells (ASCs) are crucial for tissue regeneration, but aging diminishes their stemness and regeneration potential. Our findings reveal that aging is associated with a decrease in subcutaneous adipose tissue mass and an increase in the visceral fat depots mass. Aging is associated with increase in adipose tissue fibrosis but no significant change in adipocyte size was observed with age. Long term caloric restriction failed to prevent fibrotic changes but resulted in significant decrease in adipocytes size. Aged subcutaneous ASCs displayed an increased production of ROS. Using mitochondrial membrane activity as an indicator of stem cell quiescence and senescence, we observed a significant decrease in quiescence ASCs with age exclusively in subcutaneous adipose depot. In addition, aged subcutaneous adipose tissue accumulated more senescent ASCs having defective autophagy activity. However, long-term caloric restriction leads to a reduction in mitochondrial activity in ASCs. Furthermore, caloric restriction prevents the accumulation of senescent cells and helps retain autophagy activity in aging ASCs. These results suggest that caloric restriction and caloric restriction mimetics hold promise as a potential strategy to rejuvenate the stemness of aged ASCs. Further investigations, including in vivo evaluations using controlled interventions in animals and human studies, will be necessary to validate these findings and establish the clinical potential of this well-established approach for enhancing the stemness of aged stem cells.

Advances in body composition: a 100-year journey

Knowledge of human body composition at the dawn of the twentieth century was based largely on cadaver studies and chemical analyses of isolated organs and tissues. Matters soon changed by the nineteen twenties when the Czech anthropologist Jindřich Matiegka introduced an influential new anthropometric method of fractionating body mass into subcutaneous adipose tissue and other major body components. Today, one century later, investigators can not only quantify every major body component in vivo at the atomic, molecular, cellular, tissue-organ, and whole-body organizational levels, but go far beyond to organ and tissue-specific composition and metabolite estimates. These advances are leading to an improved understanding of adiposity structure-function relations, discovery of new obesity phenotypes, and a mechanistic basis of some weight-related pathophysiological processes and adverse clinical outcomes. What factors over the past one hundred years combined to generate these profound new body composition measurement capabilities in living humans? This perspective tracks the origins of these scientific innovations with the aim of providing insights on current methodology gaps and future research needs.

Maternal Vitamin D Deficiency in Mice Sex-Dependently Affects Hepatic Lipid Accumulation in Offspring

SCOPE

Vitamin D deficiency (VDD) is becoming a global issue and low 25-hydroxyvitamin D (25(OH)D) plasma levels have been linked to hepatic steatosis in adulthood. Nevertheless, the impact of maternal VDD on lipid metabolism and hepatic steatosis remains poorly documented, especially under obesogenic condition. The goal of this study is to assess the effects of maternal VDD on hepatic lipid accumulation in adult offspring fed a normal or obesogenic diet.

METHODS AND RESULTS

Several approaches are implemented including histology and lipidomics on the liver in both males and females. No major impact of high-fat (HF) or VDD is observed at histological level in both males and females. Nevertheless, in males born from VDD mice and fed an HF diet, an increase of total lipids and modulation of the relative lipid species distribution characterized by a decrease of triglycerides and increase of phospholipids is observed. In female no major lipid profile is noticed.

CONCLUSION

Maternal VDD combined with a HF diet in male may predispose to hepatic hypertrophia, with a specific lipid profile. Such observations reinforce our knowledge of the impact of maternal VDD on hepatic programming in the offspring.

In praise of pioglitazone: An economically efficacious therapy for type 2 diabetes and other manifestations of the metabolic syndrome

Pioglitazone improves glycaemic control, not only by lowering insulin resistance, but also by improving beta cell function. Because of the improved beta cell function the glycaemic control that occurs with pioglitazone is prolonged. Pioglitazone has positive effects not only on cardiac risk factors and surrogate measures of cardiovascular disease, it also lowers the incidence of cardiac events in patients with diabetes. The recurrence of transient ischaemic attack and ischaemic stroke is also reduced in non-diabetic, insulin-resistant subjects. Utilized at preclinical stages (but not later) of heart failure, pioglitazone improves diastolic function and avoids progression to heart failure. Pioglitazone, through suppression of atrial remodelling, also decreases the incidence of atrial fibrillation. The manifestations of diseases associated with insulin resistance (non-alcoholic steatohepatitis and polycystic ovary disease) are also improved with pioglitazone. Pioglitazone may possibly improve psoriasis and other dermopathies. Pioglitazone is therefore an inexpensive and efficacious drug for the insulin-resistant subject with diabetes that is underutilized because of biases that have evolved from the toxicities of other thiazolidinediones.

Associations of Skeletal Muscle Mass, Muscle Fat Infiltration, Mitochondrial Energetics, and Cardiorespiratory Fitness with Liver Fat Among Older Adults

Background

Muscle mass loss may be associated with liver fat accumulation, yet scientific consensus is lacking and evidence in older adults is scant. It is unclear which muscle characteristics might contribute to this association in older adults.

Methods

We associated comprehensive muscle-related phenotypes including muscle mass normalized to body weight (D 3 -creatine dilution), muscle fat infiltration (MRI), carbohydrate-supported muscle mitochondrial maximal oxidative phosphorylation (respirometry), and cardiorespiratory fitness (VO 2 peak) with liver fat among older adults. Linear regression models adjusted for age, gender, technician (respirometry only), daily minutes of moderate to vigorous physical activity, and prediabetes/diabetes status tested main effects and interactions of each independent variable with waist circumference (high: women-≥88 cm, men-≥102 cm) and gender.

Results

Among older adults aged 75 (IQR 73, 79 years; 59.8% women), muscle mass and liver fat were not associated overall but were positively associated among participants with a high waist circumference (β: 25.2; 95%CI 11.7, 40.4; p =.0002; N=362). Muscle fat infiltration and liver fat were positively associated (β: 15.2; 95%CI 6.8, 24.3; p =.0003; N=378). Carbohydrate-supported maximum oxidative phosphorylation and VO 2 peak (adjusted β: -12.9; 95%CI -20.3, -4.8; p =0.003; N=361) were inversely associated with liver fat; adjustment attenuated the estimate for maximum oxidative phosphorylation although the point estimate remained negative (β: -4.0; 95%CI -11.6, 4.2; p =0.32; N=321).

Conclusions

Skeletal muscle-related characteristics are metabolically relevant factors linked to liver fat in older adults. Future research should confirm our results to determine whether trials targeting mechanisms common to liver and muscle fat accumulation are warranted.

Adipose-targeted SWELL1 deletion exacerbates obesity- and age-related nonalcoholic fatty liver disease

Healthy expansion of adipose tissue is critical for the maintenance of metabolic health, providing an optimized reservoir for energy storage in the form of triacylglycerol-rich lipoproteins. Dysfunctional adipocytes that are unable to efficiently store lipid can result in lipodystrophy and contribute to nonalcoholic fatty liver disease (NAFLD) and metabolic syndrome. Leucine-rich repeat containing protein 8a/SWELL1 functionally encodes the volume-regulated anion channel complex in adipocytes, is induced in early obesity, and is required for normal adipocyte expansion during high-fat feeding. Adipose-specific SWELL1 ablation (Adipo KO) leads to insulin resistance and hyperglycemia during caloric excess, both of which are associated with NAFLD. Here, we show that Adipo-KO mice exhibited impaired adipose depot expansion and excess lipolysis when raised on a variety of high-fat diets, resulting in increased diacylglycerides and hepatic steatosis, thereby driving liver injury. Liver lipidomic analysis revealed increases in oleic acid-containing hepatic triacylglycerides and injurious hepatic diacylglyceride species, with reductions in hepatocyte-protective phospholipids and antiinflammatory free fatty acids. Aged Adipo-KO mice developed hepatic steatosis on a regular chow diet, and Adipo-KO male mice developed spontaneous, aggressive hepatocellular carcinomas (HCCs). These data highlight the importance of adipocyte SWELL1 for healthy adipocyte expansion to protect against NAFLD and HCC in the setting of overnutrition and with aging.

Estimation of fat content in soft tissues using dual frequency ultrasound-A phantom study

This paper presents an initial investigation into the use of dual frequency pulse-echo ultrasound, second order ultrasound field (SURF) imaging, to measure the fat content of soft tissues. The SURF imaging method was used to measure the non-linear bulk elasticity (NBE) of several fatty phantoms that were created by mixing different mass fractions of soybean oil uniformly into agar phantoms. The median of the measured NBE within the estimation region was found to increase linearly with fat mass fraction (R² = 0.99), from 1.7 GPa^-1 at 9.6% fat to 2.52 GPa^-1 at 63.6% fat, thus, showing promise as a sensitive parameter for fat content measurement. Comparisons to mixture laws in earlier literature are made, and the most important error sources that need to be considered for the in vivo applications of the method are discussed.

Amelioration of obesity-related metabolic disorders via supplementation of Caulerpa lentillifera in rats fed with a high-fat and high-cholesterol diet

Dietary modification, including functional foods, could reduce comorbidities due to obesity. An increase in serum glucose and lipids is often seen in obesity. Furthermore, obesity is also characterized by a decrease in antioxidant capacity (i.e., decrease in superoxide dismutase/SOD) and downregulation of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α). It has been well established that PGC-1α is important to regulate mitochondrial biogenesis. Sea grapes (Caulerpa lentillifera) are known as a traditional food in many Asia-Pacific countries. Recent evidence suggests that sea grapes have many beneficial properties as functional foods and may have potential therapeutic functions. We investigated the effect of sea grapes (C. lentillifera) on serum glucose, lipids, PGC-1α, and protein levels of SOD in the liver of Rattus norvegicus, which is induced with a high-fat and high-cholesterol diet. A total of four groups were made, each containing ten male Rattus norvegicus; group A received a standard dry pellet diet as control, group B received cholesterol- and fat-enriched diets (CFED), groups C and D received CFED and 150 and 450 mg/kg body weight (BW) of sea grape extract, respectively, for 4 weeks. Serum glucose and cholesterol were assessed using a blood auto-analyzer. Serum PGC-1α was measured using ELISA. SOD levels were calculated using the superoxide dismutase assay kit by Sigma-Aldrich with blood taken from liver tissue. In this study, sea grape extracts improved total cholesterol levels better than the CFED and normal groups. The efficacy of total cholesterol improvement was similar between the two doses of sea grape extract. Furthermore, sea grape extract increased PCG-1α levels, especially with the dose of 150 mg/kg BW. Blood glucose was also lower in the groups of sea grape extract. Interestingly, the groups treated with sea grapes extract exhibited higher levels of liver SOD compared to the normal and CFED groups. To conclude, sea grapes (C. lentillifera) have promising potential for anti-hyperglycemia and anti-hypercholesterolemia, and for reducing oxidative stress, and providing various health benefits for metabolic disorders.

Clinical Significance of Pancreatic Fat in Children: A Single-Center Experience

OBJECTIVES

Recently, interest in pancreatic fat has increased, and fatty pancreas is considered to be related to nonalcoholic fatty liver disease (NAFLD) and metabolic syndrome. We aimed to evaluate the prevalence of echogenic pancreas in children and its related factors.

METHODS

We retrospectively analyzed the data of patients aged 5 to 18 years who had undergone abdominal sonography between January 2020 and December 2020. Patients with chronic or pancreatic diseases were excluded.

RESULTS

Of 102 patients, 27 (26.5%) had echogenic pancreas and 55 (53.9%) had NAFLD. Among the 55 patients with NAFLD, 18 (32.7%) had an echogenic pancreas. Patients with echogenic pancreas had significantly higher fasting glucose, low-density lipoprotein cholesterol, and triglyceride levels than those without echogenic pancreas. The proportion of NAFLD and obesity was higher in the echogenic group; however, only the proportion of obese subjects showed a significant difference. In multivariate analysis, family history of diabetes mellitus (DM) and/or dyslipidemia and presence of DM and/or dyslipidemia were factors related to the presence of echogenic pancreas.

CONCLUSIONS

Echogenic pancreas is relatively common in children. Echogenic pancreas, typically observed in patients with a family history of DM and/or dyslipidemia, was strongly associated with metabolic syndrome, even in the absence of fatty liver.

Distinct impacts of fat and fructose on the liver, muscle, and adipose tissue metabolome: An integrated view

Objective

In the last years, changes in dietary habits have contributed to the increasing prevalence of metabolic disorders, such as non-alcoholic fatty liver disease (NAFLD) and type 2 diabetes mellitus (T2DM). The differential burden of lipids and fructose on distinct organs needs to be unveiled. Herein, we hypothesized that high-fat and high-fructose diets differentially affect the metabolome of insulin-sensitive organs such as the liver, muscle, and different adipose tissue depots.

Methods

We have studied the impact of 12 weeks of a control (11.50% calories from fat, 26.93% from protein, and 61.57% from carbohydrates), high-fat/sucrose (HFat), or high-fructose (HFruct) feeding on C57Bl/6J male mice. Besides glucose homeostasis, we analyzed the hepatic levels of glucose and lipid-metabolism-related genes and the metabolome of the liver, the muscle, and white (WAT) and brown adipose tissue (BAT) depots.

Results

HFat diet led to a more profound impact on hepatic glucose and lipid metabolism than HFruct, with mice presenting glucose intolerance, increased saturated fatty acids, and no glycogen pool, yet both HFat and HFruct presented hepatic insulin resistance. HFat diet promoted a decrease in glucose and lactate pools in the muscle and an increase in glutamate levels. While HFat had alterations in BAT metabolites that indicate increased thermogenesis, HFruct led to an increase in betaine, a protective metabolite against fructose-induced inflammation.

Conclusions

Our data illustrate that HFat and HFruct have a negative but distinct impact on the metabolome of the liver, muscle, WAT, and BAT.

Esaxerenone, a selective mineralocorticoid receptor blocker, improves insulin sensitivity in mice consuming high-fat diet

BACKGROUND

Esaxerenone is a novel, non-steroidal selective mineralocorticoid receptor (MR) blocker. MR activation plays a crucial role in the development of cardiovascular and metabolic diseases. In this study, we investigated the effects of esaxerenone on various metabolic parameters in mice.

MATERIALS AND METHODS

Esaxerenone (3 mg/kg/day) was orally administered to high-fat diet (HFD)-fed male C57BL/6 mice. Mice fed a normal diet (ND) served as controls. Glucose and insulin tolerance, plasma lipid levels, and transaminase levels were assessed as metabolic parameters. Macrophage accumulation in the adipose tissue was evaluated using histological analysis. 3T3-L1 adipocytes, HepG2 cells, and C2C12 myotubes were used for in vitro experiments. Gene expression and insulin signaling were examined using quantitative RT-PCR and western blotting, respectively.

RESULTS

HFD successfully induced insulin resistance compared with that in ND. Esaxerenone ameliorated insulin resistance (P < 0.05) without altering other metabolic parameters, such as the lipid profile. Esaxerenone administration tended to decrease plasma transaminase levels compared with those in the non-treated group. In the adipose tissue, esaxerenone decreased macrophage accumulation (P < 0.05) and increased the expression levels of adiponectin and PPARγ. Aldosterone significantly decreased the expression levels of PPARγ and adiponectin in 3T3-L1 adipocytes. Furthermore, aldosterone attenuated insulin-induced Akt phosphorylation in 3T3-L1 adipocytes, HepG2 cells, and C2C12 myotubes in a dose-dependent manner (P < 0.01). These effects were ameliorated by pretreatment with esaxerenone.

CONCLUSION

Esaxerenone ameliorated insulin resistance in HFD-fed mice. Reduction of inflammation and improvement in insulin signaling may underlie the beneficial effects of esaxerenone.

ANGPTL8 promotes adipogenic differentiation of mesenchymal stem cells: potential role in ectopic lipid deposition

BACKGROUND

Ectopic lipid deposition plays a promoting role in many chronic metabolic diseases. Abnormal adipogenic differentiation of mesenchymal stem cells (MSCs) is an important cause of lipid deposition in organs. Studies have shown that serum angiopoietin-like protein 8 (ANGPTL8) levels are increased in patients with many chronic metabolic diseases (such as type 2 diabetes, obesity, and hepatic steatosis), while the role of ANGPTL8 in ectopic lipid accumulation has not been reported.

METHODS

We used the Gene Expression Omnibus (GEO) database to analyze the expression of ANGPTL8 in subcutaneous adipose tissue of obese patients and qPCR to analyze the expression of ANGPTL8 in the liver of high-fat diet (HFD)-induced obese mice. To explore the potential roles of ANGPTL8 in the progression of ectopic lipid deposition, ANGPTL8 knockout (KO) mice were constructed, and obesity models were induced by diet and ovariectomy (OVX). We analyzed lipid deposition (TG) in the liver, kidney, and heart tissues of different groups of mice by Oil Red O, Sudan black B staining, and the single reagent GPO-PAP method. We isolated and characterized MSCs to analyze the regulatory effect of ANGPTL8 on Wnt/β-Catenin, a key pathway in adipogenic differentiation. Finally, we used the pathway activator LiCl and a GSK3β inhibitor (i.e., CHIR99021) to analyze the regulatory mechanism of this pathway by ANGPTL8.

RESULTS

ANGPTL8 is highly expressed in the subcutaneous adipose tissue of obese patients and the liver of HFD-induced obese mice. Both normal chow diet (NCD)- and HFD-treated ANGPTL8 KO male mice gained significantly less weight than wild-type (WT) male mice and reduced ectopic lipid deposition in organs. However, the female mice of ANGPTL8 KO, especially the HFD group, did not show differences in body weight or ectopic lipid deposition because HFD could induce estrogen overexpression and then downregulate ANGPTL8 expression, thereby counteracting the reduction in HFD-induced ectopic lipid deposition by ANGPTL8 deletion, and this result was also further proven by the OVX model. Mechanistic studies demonstrated that ANGPTL8 could promote the differentiation of MSCs into adipocytes by inhibiting the Wnt/β-Catenin pathway and upregulating PPARγ and c/EBPα mRNA expression.

CONCLUSIONS

ANGPTL8 promotes the differentiation of MSCs into adipocytes, suggesting that ANGPTL8 may be a new target for the prevention and treatment of ectopic lipid deposition in males.

Similar Metabolic Health in Overweight/Obese Individuals With Contrasting Metabolic Flexibility to an Oral Glucose Tolerance Test

Background: Low metabolic flexibility (MetF) may be an underlying factor for metabolic health impairment. Individuals with low MetF are thus expected to have worse metabolic health than subjects with high MetF. Therefore, we aimed to compare metabolic health in individuals with contrasting MetF to an oral glucose tolerance test (OGTT).

Methods: In individuals with excess body weight, we measured MetF as the change in respiratory quotient (RQ) from fasting to 1 h after ingestion of a 75-g glucose load (i.e., OGTT). Individuals were then grouped into low and high MetF (Low-MetF n = 12; High-MetF n = 13). The groups had similar body mass index, body fat, sex, age, and maximum oxygen uptake. Metabolic health markers (clinical markers, insulin sensitivity/resistance, abdominal fat, and intrahepatic fat) were compared between groups.

Results: Fasting glucose, triglycerides (TG), and high-density lipoprotein (HDL) were similar between groups. So were insulin sensitivity/resistance, visceral, and intrahepatic fat. Nevertheless, High-MetF individuals had higher diastolic blood pressure, a larger drop in TG concentration during the OGTT, and a borderline significant (P = 0.05) higher Subcutaneous Adipose Tissue (SAT). Further, compared to Low-MetF, High-MetF individuals had an about 2-fold steeper slope for the relationship between SAT and fat mass index.

Conclusion: Individuals with contrasting MetF to an OGTT had similar metabolic health. Yet High-MetF appears related to enhanced circulating TG clearance and enlarged subcutaneous fat.

The mitochondrial dicarboxylate carrier prevents hepatic lipotoxicity by inhibiting white adipocyte lipolysis

BACKGROUND & AIMS

We have previously reported that the mitochondrial dicarboxylate carrier (mDIC [SLC25A10]) is predominantly expressed in the white adipose tissue (WAT) and subject to regulation by metabolic cues. However, the specific physiological functions of mDIC and the reasons for its abundant presence in adipocytes are poorly understood.

METHODS

To systemically investigate the impact of mDIC function in adipocytes in vivo, we generated loss- and gain-of-function mouse models, selectively eliminating or overexpressing mDIC in mature adipocytes, respectively.

RESULTS

In in vitro differentiated white adipocytes, mDIC is responsible for succinate transport from the mitochondrial matrix to the cytosol, from where succinate can act on the succinate receptor SUCNR1 and inhibit lipolysis by dampening the cAMP- phosphorylated hormone-sensitive lipase (pHSL) pathway. We eliminated mDIC expression in adipocytes in a doxycycline (dox)-inducible manner (mDIC^iKO) and demonstrated that such a deletion results in enhanced adipocyte lipolysis and promotes high-fat diet (HFD)-induced adipocyte dysfunction, liver lipotoxicity, and systemic insulin resistance. Conversely, in a mouse model with dox-inducible, adipocyte-specific overexpression of mDIC (mDIC^iOE), we observed suppression of adipocyte lipolysis both in vivo and ex vivo. mDIC^iOE mice are potently protected from liver lipotoxicity upon HFD feeding. Furthermore, they show resistance to HFD-induced weight gain and adipose tissue expansion with concomitant improvements in glucose tolerance and insulin sensitivity. Beyond our data in rodents, we found that human WAT SLC25A10 mRNA levels are positively correlated with insulin sensitivity and negatively correlated with intrahepatic triglyceride levels, suggesting a critical role of mDIC in regulating overall metabolic homeostasis in humans as well.

CONCLUSIONS

In summary, we highlight that mDIC plays an essential role in governing adipocyte lipolysis and preventing liver lipotoxicity in response to a HFD.

LAY SUMMARY

Dysfunctional fat tissue plays an important role in the development of fatty liver disease and liver injury. Our present study identifies a mitochondrial transporter, mDIC, which tightly controls the release of free fatty acids from adipocytes to the liver through the export of succinate from mitochondria. We believe this mDIC-succinate axis could be targeted for the treatment of fatty liver disease.

Metabolic-associated fatty liver disease and lipoprotein metabolism

BACKGROUND

Non-alcoholic fatty liver disease, or as recently proposed 'metabolic-associated fatty liver disease' (MAFLD), is characterized by pathological accumulation of triglycerides and other lipids in hepatocytes. This common disease can progress from simple steatosis to steatohepatitis, and eventually end-stage liver diseases. MAFLD is closely related to disturbances in systemic energy metabolism, including insulin resistance and atherogenic dyslipidemia.

SCOPE OF REVIEW

The liver is the central organ in lipid metabolism by secreting very low density lipoproteins (VLDL) and, on the other hand, by internalizing fatty acids and lipoproteins. This review article discusses recent research addressing hepatic lipid synthesis, VLDL production, and lipoprotein internalization as well as the lipid exchange between adipose tissue and the liver in the context of MAFLD.

MAJOR CONCLUSIONS

Liver steatosis in MAFLD is triggered by excessive hepatic triglyceride synthesis utilizing fatty acids derived from white adipose tissue (WAT), de novo lipogenesis (DNL) and endocytosed remnants of triglyceride-rich lipoproteins. In consequence of high hepatic lipid content, VLDL secretion is enhanced, which is the primary cause of complex dyslipidemia typical for subjects with MAFLD. Interventions reducing VLDL secretory capacity attenuate dyslipidemia while they exacerbate MAFLD, indicating that the balance of lipid storage versus secretion in hepatocytes is a critical parameter determining disease outcome. Proof of concept studies have shown that promoting lipid storage and energy combustion in adipose tissues reduces hepatic lipid load and thus ameliorates MAFLD. Moreover, hepatocellular triglyceride synthesis from DNL and WAT-derived fatty acids can be targeted to treat MAFLD. However, more research is needed to understand how individual transporters, enzymes, and their isoforms affect steatosis and dyslipidemia in vivo, and whether these two aspects of MAFLD can be selectively treated. Processing of cholesterol-enriched lipoproteins appears less important for steatosis. It may, however, modulate inflammation and consequently MAFLD progression.

Relationship Between Non-Alcoholic Fatty Liver Disease and Abdominal and Pericardial Adipose Tissue in Middle-Aged and Elderly Subjects

Objective

The present study aimed to explore the relationship between non-alcoholic fatty liver disease (NAFLD) and abdominal and pericardial adipose tissue in middle-aged and elderly subjects.

Methods

Between July 2019 and July 2020, 471 subjects attending the Health Care Medical Department of Peking Union Medical College Hospital for a medical examination were enrolled in the study. The volume and distribution of abdominal adipose tissue together with the volume of pericardial adipose tissue were calculated according to the results of the abdominal computed tomography. The differences between subjects with NAFLD and the normal population were analyzed.

Results

The volume of pericardial adipose tissue, abdominal visceral and subcutaneous adipose tissue, the total volume of abdominal adipose tissue, and volume of pelvic visceral adipose tissue were all significantly increased in subjects with NAFLD. For every 100 cm³ increase in the volume of abdominal visceral adipose tissue, the incidence of developing NAFLD increased by 9.4%. According to the results of the receiver operating curve, the cut-off point of abdominal visceral adipose tissue for the diagnosis of NAFLD was 2691.1 cm³.

Conclusion

Overall, the risk of NAFLD increases significantly with the increase in the volume of adipose tissue.

Adipotropic effects of heavy metals and their potential role in obesity

Epidemiological studies demonstrated an association between heavy metal exposure and the incidence of obesity and metabolic syndrome. However, the particular effects of metal toxicity on adipose tissue functioning are unclear. Therefore, recent findings of direct influence of heavy metals (mercury, cadmium, and lead) and metalloid (arsenic) on adipose tissue physiology are discussed while considering existing gaps and contradictions. Here, we provide a literature review addressing adipose tissue as a potential target of heavy metal toxicity. Experimental in vivo studies demonstrated a significant influence of mercury, cadmium, lead, and arsenic exposure on body adiposity. In turn, in vitro experiments revealed both up- and downregulation of adipogenesis associated with aberrant expression of key adipogenic pathways, namely CCAAT/enhancer-binding protein (C/EBP) and peroxisome proliferator-activated receptor gamma (PPARγ). Comparison of the existing studies on the basis of dose and route of exposure demonstrated that the effects of heavy metal exposure on adipose tissue may be dose-dependent, varying from increased adipogenesis at low-dose exposure to inhibition of adipose tissue differentiation at higher doses. However, direct dose-response data are available in a single study only for arsenic. Nonetheless, both types of these effects, irrespective of their directionality, contribute significantly to metabolic disturbances due to dysregulated adipogenesis. Particularly, inhibition of adipocyte differentiation is known to reduce lipid-storage capacity of adipose tissue, leading to ectopic lipid accumulation. In contrast, metal-associated stimulation of adipogenesis may result in increased adipose tissue accumulation and obesity. However, further studies are required to reveal the particular dose- and species-dependent effects of heavy metal exposure on adipogenesis and adipose tissue functioning.

Buriti pulp oil did not improve high-fat diet-induced metabolic disorders in c57bl/6 mice

Metabolic syndrome (MetS) and obesity are growing in many parts of the world, becoming public health problems. It is proposed that foods with functional properties can assist in the treatment of these diseases. Crude buriti pulp oil (BPO) is a food traditionally consumed by residents in the Pantanal, Cerrado and Brazilian Amazon. It is rich in oleic acid, tocopherols and carotenoids, emerging as a potential functional food. Thus, this study aimed to evaluate the effect of the supplementation of BPO on metabolic disorders caused by a high-fat diet. Four groups of C57BL6 mice were used, a lean group with AIN-93M diet and control oil supplementation, an obese group with a high-fat diet and control oil supplementation, and two obese groups with a high-fat diet and BPO supplementation in the amounts of 50 and 100 mg/kg. BPO worsened the metabolic state caused by the high-fat diet, worsening risk factors associated with MetS, as the abdominal circumference and retroperitoneal fat, serum levels of total cholesterol, uric acid, alanine transaminase, glucose and triglycerides, and renal fat, in addition to changes in glycaemic control and oxidative stress markers. C57BL/6 mice fed with a high-fat diet and supplemented with BPO presented a worsening in metabolic risk factors associated with MetS.

Unraveling the Role of Leptin in Liver Function and Its Relationship with Liver Diseases

Since its discovery twenty-five years ago, the fat-derived hormone leptin has provided a revolutionary framework for studying the physiological role of adipose tissue as an endocrine organ. Leptin exerts pleiotropic effects on many metabolic pathways and is tightly connected with the liver, the major player in systemic metabolism. As a consequence, understanding the metabolic and hormonal interplay between the liver and adipose tissue could provide us with new therapeutic targets for some chronic liver diseases, an increasing problem worldwide. In this review, we assess relevant literature regarding the main metabolic effects of leptin on the liver, by direct regulation or through the central nervous system (CNS). We draw special attention to the contribution of leptin to the non-alcoholic fatty liver disease (NAFLD) pathogenesis and its progression to more advanced stages of the disease as non-alcoholic steatohepatitis (NASH). Likewise, we describe the contribution of leptin to the liver regeneration process after partial hepatectomy, the mainstay of treatment for certain hepatic malignant tumors.

Aldh1l2 knockout mouse metabolomics links the loss of the mitochondrial folate enzyme to deregulation of a lipid metabolism observed in rare human disorder

Background

Mitochondrial folate enzyme ALDH1L2 (aldehyde dehydrogenase 1 family member L2) converts 10-formyltetrahydrofolate to tetrahydrofolate and CO₂ simultaneously producing NADPH. We have recently reported that the lack of the enzyme due to compound heterozygous mutations was associated with neuro-ichthyotic syndrome in a male patient. Here, we address the role of ALDH1L2 in cellular metabolism and highlight the mechanism by which the enzyme regulates lipid oxidation.

Methods

We generated Aldh1l2 knockout (KO) mouse model, characterized its phenotype, tissue histology, and levels of reduced folate pools and applied untargeted metabolomics to determine metabolic changes in the liver, pancreas, and plasma caused by the enzyme loss. We have also used NanoString Mouse Inflammation V2 Code Set to analyze inflammatory gene expression and evaluate the role of ALDH1L2 in the regulation of inflammatory pathways.

Results

Both male and female Aldh1l2 KO mice were viable and did not show an apparent phenotype. However, H&E and Oil Red O staining revealed the accumulation of lipid vesicles localized between the central veins and portal triads in the liver of Aldh1l2^-/- male mice indicating abnormal lipid metabolism. The metabolomic analysis showed vastly changed metabotypes in the liver and plasma in these mice suggesting channeling of fatty acids away from β-oxidation. Specifically, drastically increased plasma acylcarnitine and acylglycine conjugates were indicative of impaired β-oxidation in the liver. Our metabolomics data further showed that mechanistically, the regulation of lipid metabolism by ALDH1L2 is linked to coenzyme A biosynthesis through the following steps. ALDH1L2 enables sufficient NADPH production in mitochondria to maintain high levels of glutathione, which in turn is required to support high levels of cysteine, the coenzyme A precursor. As the final outcome, the deregulation of lipid metabolism due to ALDH1L2 loss led to decreased ATP levels in mitochondria.

Conclusions

The ALDH1L2 function is important for CoA-dependent pathways including β-oxidation, TCA cycle, and bile acid biosynthesis. The role of ALDH1L2 in the lipid metabolism explains why the loss of this enzyme is associated with neuro-cutaneous diseases. On a broader scale, our study links folate metabolism to the regulation of lipid homeostasis and the energy balance in the cell.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40246-020-00291-3.

Hepatic Carbohydrate Response Element Binding Protein Activation Limits Nonalcoholic Fatty Liver Disease Development in a Mouse Model for Glycogen Storage Disease Type 1a

BACKGROUND AND AIMS

Glycogen storage disease (GSD) type 1a is an inborn error of metabolism caused by defective glucose-6-phosphatase catalytic subunit (G6PC) activity. Patients with GSD 1a exhibit severe hepatomegaly due to glycogen and triglyceride (TG) accumulation in the liver. We have shown that the activity of carbohydrate response element binding protein (ChREBP), a key regulator of glycolysis and de novo lipogenesis, is increased in GSD 1a. In the current study, we assessed the contribution of ChREBP to nonalcoholic fatty liver disease (NAFLD) development in a mouse model for hepatic GSD 1a.

APPROACH AND RESULTS

Liver-specific G6pc-knockout (L-G6pc^-/- ) mice were treated with adeno-associated viruses (AAVs) 2 or 8 directed against short hairpin ChREBP to normalize hepatic ChREBP activity to levels observed in wild-type mice receiving AAV8-scrambled short hairpin RNA (shSCR). Hepatic ChREBP knockdown markedly increased liver weight and hepatocyte size in L-G6pc^-/- mice. This was associated with hepatic accumulation of G6P, glycogen, and lipids, whereas the expression of glycolytic and lipogenic genes was reduced. Enzyme activities, flux measurements, hepatic metabolite analysis and very low density lipoprotein (VLDL)-TG secretion assays revealed that hepatic ChREBP knockdown reduced downstream glycolysis and de novo lipogenesis but also strongly suppressed hepatic VLDL lipidation, hence promoting the storage of "old fat." Interestingly, enhanced VLDL-TG secretion in shSCR-treated L-G6pc^-/- mice associated with a ChREBP-dependent induction of the VLDL lipidation proteins microsomal TG transfer protein and transmembrane 6 superfamily member 2 (TM6SF2), the latter being confirmed by ChIP-qPCR.

CONCLUSIONS

Attenuation of hepatic ChREBP induction in GSD 1a liver aggravates hepatomegaly because of further accumulation of glycogen and lipids as a result of reduced glycolysis and suppressed VLDL-TG secretion. TM6SF2, critical for VLDL formation, was identified as a ChREBP target in mouse liver. Altogether, our data show that enhanced ChREBP activity limits NAFLD development in GSD 1a by balancing hepatic TG production and secretion.

Relationship of visceral adipose tissue with surrogate insulin resistance and liver markers in individuals with metabolic syndrome chronic complications

BACKGROUND

Visceral adipose tissue (VAT) has a hazardous influence on systemic inflammation, insulin resistance and an adverse metabolic profile, which increases the risk of developing non-alcoholic fatty liver disease (NAFLD) and chronic complications of diabetes. In our study we aimed to evaluate the association of VAT and the triglyceride glucose (TyG) as a proxy of insulin resistance surrogated with metabolic and liver risk factors among subjects diagnosed with metabolic syndrome (MetS).

METHODS

A cross-sectional study was performed including 326 participants with MetS (55-75 years) from the PREDIMED-Plus study. Liver-status markers, VAT and TyG were assessed. Participants were stratified by tertiles according to VAT (n = 254) and TyG (n = 326). A receiver operating characteristic curve was used to analyse the efficiency of TyG for VAT.

RESULTS

Subjects with greater visceral fat depots showed worse lipid profile, higher homeostatic model assessment for insulin resistance (HOMA-IR), TyG, alanine transaminase (ALT), fibroblast growth factor-21 (FGF-21), fatty liver index (FLI) and hepatic steatosis index (HSI) compared with participants in the first tertile. The multi-adjusted linear-regression analyses indicated that individuals in the third tertile of TyG (>9.1-10.7) had a positive association with HOMA-IR [β = 3.07 (95% confidence interval (CI) 2.28-3.86; p trend < 0.001)], ALT [β = 7.43 (95% CI 2.23-12.63; p trend = 0.005)], gamma glutamyl transferase (GGT) [β = 14.12 (95% CI 3.64-24.61; p trend = 0.008)], FGF-21 [β = 190.69 (95% CI 93.13-288.25; p trend < 0.001)], FLI [β = 18.65 (95% CI 14.97-22.23; p trend < 0.001)] and HSI [β = 3.46 (95% CI, 2.23-4.68; p trend < 0.001)] versus participants from the first tertile. Interestingly, the TyG showed the largest area under the receiver operating curve (AUC) for women (AUC = 0.713; 95% CI 0.62-0.79) compared with men (AUC = 0.570; 95% CI 0.48-0.66).

CONCLUSIONS

A disrupted VAT enlargement and impairment of TyG are strongly associated with liver status and cardiometabolic risk factors linked with NAFLD in individuals diagnosed with MetS. Moreover, the TyG could be used as a suitable and reliable marker estimator of VAT.

Ethnic differences in hepatic, pancreatic, muscular and visceral fat deposition in healthy men of white European and black west African ethnicity

AIMS

We aimed to assess ethnic differences in visceral adipose tissue (VAT), intrahepatic (IHL), intrapancreatic (IPL) and intramyocellular lipids (IMCL) between healthy white European (WE) and black west African (BWA) men.

METHODS

23 WE and 20 BWA men underwent Dixon-magnetic resonance imaging to quantify VAT, IHL and IPL; and proton-magnetic resonance spectroscopy to quantify IMCL. Insulin sensitivity and beta-cell function were determined using homeostasis model assessment (HOMA-2).

RESULTS

BWA men exhibited significantly lower VAT (P = 0.021) and IHL (P = 0.044) than WE men, but comparable IPL (P = 0.92) and IMCL (P = 0.87). VAT was associated with IPL in both ethnicities (WE: P < 0.001; BWA: P = 0.001) but the relationship with IHL differed by ethnicity (P_interaction = 0.018) and was only significant in WE men (WE: P < 0.001; BWA: P = 0.36). All ectopic fat depots inversely associated with insulin sensitivity and positively associated with beta-cell function in WE but not BWA men.

CONCLUSIONS

Lower VAT and IHL, and their lack of interrelation, in BWA men suggests ethnic differences exist in the mechanisms of ectopic fat deposition. The lack of association between ectopic fat with insulin sensitivity and beta-cell function in BWA men may indicate a lesser role for ectopic fat in the development of type 2 diabetes mellitus in black populations.

Obesity Tissue: Composition, Energy Expenditure, and Energy Content in Adult Humans

OBJECTIVE

Chronic positive energy balance leads to obesity, and the "excess" weight is usually described as consisting solely of adipose tissue (AT) or its two components, fat and fat-free mass (nonfat cell mass, extracellular fluid). This study aimed to clarify the nature of "obesity" tissue.

METHODS

A total of 333 adults had AT, skin, skeletal muscle, bone, heart, liver, kidney, spleen, brain, and residual mass measured or derived using magnetic resonance imaging and dual-energy x-ray absorptiometry. First, associations between these components and AT were examined by developing multiple regression models. Next, obesity-tissue composition was developed by deriving mean component mass differences between participant groups with normal weight (BMI < 25 kg/m² ) and those with obesity (BMI > 29.9 kg/m² ); respective resting energy expenditures and metabolizable energy and protein contents were calculated.

RESULTS

AT significantly predicted organ-tissue mass in 17 of 18 multiple regression models. In addition to AT and skeletal muscle, the following associations were found: skin, liver, and bone were main contributors to obesity-tissue composition; liver, kidneys, and heart to resting energy expenditure; and skin, liver, and bone to metabolizable energy and protein contents. A pronounced sexual dimorphism was present in all three models.

CONCLUSIONS

Obesity is characterized not only by excess AT but by increases in the masses of other "companion" organs and tissues and their related metabolic properties.