Intrahepatic fat, not visceral fat, is linked with metabolic complications of obesity

Comparison of Abdominal Obesity and Fatty Liver and Their Association with Insulin Resistance and Metabolic Syndrome in Chinese Adults

OBJECTIVE

The purpose of this study was to explore the similarities and differences between nonalcoholic fatty liver (FL) disease (NAFLD) and abdominal obesity in their association with insulin resistance and metabolic syndrome (MetS).

METHODS

The Survey on Prevalence in East China for Metabolic Diseases and Risk Factors (SPECT-China) is a population-based survey of Chinese adults. A total of 9,685 participants (median age, 53 years; interquartile range, 44-63) were grouped into four categories by waist circumference (WC) and NAFLD status: NWC/FL(-), NWC/FL(+), HWC/FL(-), and HWC/FL(+) [FL(-), without NAFLD; FL(+), with NAFLD; HWC, higher WC; NWC, normal WC]. Logistic regression models were used to obtain the odds ratios as estimates of the associations between different categories and MetS.

RESULTS

Male participants with HWC/FL(-) had higher homeostatic model assessment for insulin resistance scores than subjects with NWC/FL(+), but female subjects showed the opposite results. Compared with NWC/FL(+), HWC was associated with an increased likelihood of blood pressure disorder and a decreased likelihood of triglyceride and blood glucose disorders in men, but in women, HWC was associated with only a lower likelihood of triglyceride disorder. Despite these differences, patients with NWC/FL(+) and HWC/FL(-) had almost equally serious degrees of metabolic disorders (MetS z score).

CONCLUSIONS

There are significant differences between sexes regarding the association between NAFLD and abdominal obesity status and MetS components.

Whole-body vibration for patients with nonalcoholic fatty liver disease: a 6-month prospective study

Physical exercise has demonstrated benefits for managing nonalcoholic fatty liver disease (NAFLD). However, in daily life maintaining exercise without help may be difficult. A whole-body vibration device (WBV) has been recently introduced as an exercise modality that may be suitable for patients who have difficulty engaging in exercise. We tested WBV in patients with NAFLD and estimated its effectiveness. We studied the effects of a 6-month WBV program on hepatic steatosis and its underlying pathophysiology in 25 patients with NAFLD. Seventeen patients with NAFLD were designated as a control group. After WBV exercise, body weight in the study group decreased by only 2.5% compared with the control group. However, we found significant increases in muscle area (+2.6%) and strength (+20.5%) and decreases in fat mass (-6.8%). The hepatic (-9.9%) and visceral (-6.2%) fat content also significantly decreased (P < 0.05). There was substantial lowering of hepatic stiffness (-15.7%), along with improvements in the levels of inflammatory markers; tumor necrosis factor alpha (-50.9%), adiponectin (+12.0%), ferritin (-33.2%), and high-sensitivity C-reactive protein (-43.0%) (P < 0.05). These results suggest that WBV is an exercise option for patients with NAFLD that is effective, efficient, and convenient.

MicroRNA-26a-CD36 signaling pathway: Pivotal role in lipid accumulation in hepatocytes induced by PM2.5 liposoluble extracts

Exposure to ambient particular matters (PM) has been associated with the development of non-alcoholic fatty liver disease (NAFLD), but the underlying mechanism remains unclear. Given that microRNA (miRNA) is recognized as a key regulator of lipid metabolism and a potential mediator of environmental cues, this study aimed to explore the role of miRNA-mRNA regulation underlying abnormal lipid metabolism triggered by PM_2.5liposoluble extracts. We confirmed that 72-h exposure to liposoluble extracts of PM_2.5 from Nanjing at 25 μg/cm² induced lipid accumulation in HepG2 cells by promoting uptake of free fatty acids (FFAs). Notably, lipid accumulation induced by PM_2.5 liposoluble extracts was associated with decreased expression of miR-26a and consequent upregulation of fatty acid translocase (FAT, also known as CD36). Using gain- and loss-of-function assays, we demonstrated that miR-26a negatively regulated CD36 to mediate lipid accumulation in HepG2 cells. We further confirmed that miR-26a directly acted on the 3' untranslated region (3'UTR) of CD36. Furthermore, overexpression of miR-26a abolished steatosis in HepG2 cells treated with PM_2.5 liposoluble extracts by suppressing CD36. In addition, we demonstrated that PM_2.5 liposoluble extracts caused inflammation in HepG2 cells by raising p65 phosphorylation, thereby fuelling the transition from simple non-alcoholic fatty liver to non-alcoholic steatohepatitis. In conclusion, this study demonstrated a novel mechanism by which miR-26a-CD36 pathway mediated lipid accumulation induced by PM_2.5 liposoluble extracts in hepatocytes. Lipid accumulation and inflammation induced by PM_2.5 liposoluble extracts implied the potential role of PM_2.5 in developing NAFLD.

Non-alcoholic fatty liver disease: causes, diagnosis, cardiometabolic consequences, and treatment strategies

The prevalence of non-alcoholic fatty liver disease (NAFLD) is increasing worldwide. In some patients with NAFLD, isolated steatosis can progress to advanced stages with non-alcoholic steatohepatitis (NASH) and fibrosis, increasing the risk of cirrhosis and hepatocellular carcinoma. Furthermore, NAFLD is believed to be involved in the pathogenesis of common disorders such as type 2 diabetes and cardiovascular disease. In this Review, we highlight novel concepts related to diagnosis, risk prediction, and treatment of NAFLD. First, because NAFLD is a heterogeneous disease, the advanced stages of which seem to be strongly affected by comorbidities such as insulin resistance and type 2 diabetes, early use of reliable, non-invasive diagnostic tools is needed, particularly in patients with insulin resistance or diabetes, to allow the identification of patients at different disease stages. Second, although the strongest genetic risk alleles for NAFLD (ie, the 148Met allele in PNPLA3 and the 167Lys allele in TM6SF2) are associated with increased liver fat content and progression to NASH and cirrhosis, these alleles are also unexpectedly associated with an apparent protection from cardiovascular disease. If consistent across diverse populations, this discordance in NAFLD-related risk prediction between hepatic and extrahepatic disease might need to be accounted for in the management of NAFLD. Third, drug treatments assessed in NAFLD seem to differ with respect to cardiometabolic and antifibrotic efficacy, suggesting the need to better identify and tailor the most appropriate treatment approach, or to use a combination of approaches. These emerging concepts could contribute to the development of a multidisciplinary approach for endocrinologists and hepatologists working together in the management of NAFLD.

Crosstalk between nonalcoholic fatty liver disease and cardiometabolic syndrome

Nonalcoholic fatty liver disease (NAFLD) is a chronic condition characterized by fat accumulation combined with low-grade inflammation in the liver. A large body of clinical and experimental data shows that increased flux of free fatty acids from increased visceral adipose tissue and de novo lipogenesis can lead to NAFLD and insulin resistance. Thus, individuals with obesity, insulin resistance, and dyslipidaemia are at the greatest risk of developing NAFLD. Conversely, NAFLD is a phenotype of cardiometabolic syndrome. Notably, researchers have discovered a close association between NAFLD and impaired glucose metabolism and focused on the role of NAFLD in the development of type 2 diabetes. Moreover, recent studies provide substantial evidence for an association between NAFLD and atherosclerosis and cardiometabolic disorders. Even if NAFLD can progress into severe liver disorders including nonalcoholic steatohepatitis (NASH) and cirrhosis, the majority of subjects with NAFLD die from cardiovascular disease eventually. In this review, we propose a potential pathological link between NAFLD/NASH and cardiometabolic syndrome. The potential factors that can play a pivotal role in this link, such as inflammation, insulin resistance, alteration in lipid metabolism, oxidative stress, genetic predisposition, and gut microbiota are discussed.

Reproductive Endocrinology of Nonalcoholic Fatty Liver Disease

The liver and the reproductive system interact in a multifaceted bidirectional fashion. Sex steroid signaling influences hepatic endobiotic and xenobiotic metabolism and contributes to the pathogenesis of functional and structural disorders of the liver. In turn, liver function affects the reproductive axis via modulating sex steroid metabolism and transport to tissues via sex hormone-binding globulin (SHBG). The liver senses the body's metabolic status and adapts its energy homeostasis in a sex-dependent fashion, a dimorphism signaled by the sex steroid milieu and possibly related to the metabolic costs of reproduction. Sex steroids impact the pathogenesis of nonalcoholic fatty liver disease, including development of hepatic steatosis, fibrosis, and carcinogenesis. Preclinical studies in male rodents demonstrate that androgens protect against hepatic steatosis and insulin resistance both via androgen receptor signaling and, following aromatization to estradiol, estrogen receptor signaling, through regulating genes involved in hepatic lipogenesis and glucose metabolism. In female rodents in contrast to males, androgens promote hepatic steatosis and dysglycemia, whereas estradiol is similarly protective against liver disease. In men, hepatic steatosis is associated with modest reductions in circulating testosterone, in part consequent to a reduction in circulating SHBG. Testosterone treatment has not been demonstrated to improve hepatic steatosis in randomized controlled clinical trials. Consistent with sex-dimorphic preclinical findings, androgens promote hepatic steatosis and dysglycemia in women, whereas endogenous estradiol appears protective in both men and women. In both sexes, androgens promote hepatic fibrosis and the development of hepatocellular carcinoma, whereas estradiol is protective.

Similar Weight Loss Induces Greater Improvements in Insulin Sensitivity and Liver Function among Individuals with NAFLD Compared to Individuals without NAFLD

BACKGROUND

Preliminary evidence suggests that weight loss among obese has differential metabolic effects depending on the presence of non-alcoholic fatty liver disease (NAFLD). We assessed whether NAFLD predisposes to differential changes in liver fat content, liver function, and metabolic parameters upon diet-induced weight loss in a 50-week intervention trial.

METHODS

143 overweight and obese non-smokers underwent a 12-week dietary intervention and a 38-week follow-up. Diet-induced changes in anthropometric measures, circulating biomarkers, and magnetic resonance (MR)-derived liver fat content and adipose tissue volumes were evaluated by mixed linear models stratifying by NAFLD at baseline.

RESULTS

The prevalence of NAFLD at baseline was 52%. Diet-induced weight loss after 12 (NAFLD: 4.8 ± 0.5%, No NAFLD: 5.1 ± 0.5%) and 50 weeks (NAFLD: 3.5 ± 0.7%, No NAFLD: 3.5 ± 0.9%) was similar in both groups, while the decrease in liver fat was significantly greater in the NAFLD group (week 12: 32.9 ± 9.5% vs. 6.3 ± 4.0%; week 50: 23.3 ± 4.4% vs. 5.0 ± 4.2%). Decreases in biomarkers of liver dysfunction (GGT, ALT, AST) and HOMA IR were also significantly greater in the NAFLD group. Other metabolic parameters showed no significant differences.

CONCLUSION

Our data suggest that individuals with NAFLD show greater improvements of liver function and insulin sensitivity after moderate diet-induced weight loss than individuals without NAFLD.

Review article: shared disease mechanisms between non-alcoholic fatty liver disease and metabolic syndrome - translating knowledge from systems biology to the bedside

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is the leading cause of chronic liver disease worldwide. Characterised by abnormal fat accumulation in the liver, NAFLD presents high degree of comorbidity with disorders of the metabolic syndrome, including type 2 diabetes, obesity and cardiovascular disease. These comorbidities have strong negative impact on the natural course of NAFLD and vice versa, whereby the presence of NAFLD substantially modifies the course and prognosis of metabolic syndrome-associated diseases.

AIM

To use systems biology strategies to interrogate disease mechanisms that are common to NAFLD and metabolic syndrome.

METHODS

We mapped shared gene/protein-disease interaction networks, we performed gene-disease enrichment analysis to assess pleiotropy, and we created a gene-drug connectivity network.

RESULTS

We found that a shared network of genes/proteins is overrepresented by immune response-related pathways, post-translational modifications of nuclear receptors, and platelet-related processes, including activation and platelet signalling. Likewise, gene-based disease-enrichment analysis suggested underlying molecular effectors that are shared with major systemic disorders, including diverse autoimmune diseases, kidney, respiratory and nervous system disorders, cancer and infectious diseases. The shared list of genes/proteins was enriched in drug targets for anti-inflammatory therapy, drugs used to treat cardiovascular diseases, antimicrobial agents and phytochemicals, among many other approved pharmaceutical compounds. By leveraging on publicly available OMICs data, we were able to show that shared loci are not necessarily affected by reverse causality.

CONCLUSION

We provide evidence indicating that NAFLD treatment, including severe histological traits, cannot be limited to the use of a single drug, as it rather requires a multi-target therapeutic approach.

L-Ornithine L-Aspartate (LOLA) as a Novel Approach for Therapy of Non-alcoholic Fatty Liver Disease

L-ornithine L-aspartate (LOLA) has been known as an effective ammonia-lowering agent for more than 50 years with good evidence in hepatic encephalopathy. Administration of LOLA removes ammonia via two distinct mechanisms: by synthesis of urea and by the synthesis of glutamine via the enzyme glutamine synthetase. While LOLA has been used in cirrhosis and acute liver injury settings, it is less clear if LOLA could be used in non-alcoholic fatty liver disease (NAFLD). NAFLD and the progressive form non-alcoholic steatohepatitis (NASH) are currently the leading causes of chronic liver disease worldwide, with roughly 25% of the world population affected by NAFLD. Consequences of NASH are end-stage liver disease and cardiovascular morbidity and mortality. As the basis for NAFLD is excess calorie uptake and excess adipose tissue mass, the conservative therapeutic approach is weight loss by intense lifestyle change. However, no pharmacological treatment options are currently approved. LOLA is being investigated as a pharmacological tool to ameliorate liver injury in NAFLD on the basis that it lowers liver ammonia concentrations and supplies anti-oxidative glutamine and glutathione. Indirect hepatoprotective effects currently under investigation could also be beneficial.

Duodenal adipose tissue is associated with obesity in baboons (Papio sp): a novel site of ectopic fat deposition in non-human primates

AIMS

Ectopic fat is a recognized contributor to insulin resistance and metabolic dysfunction, while the role of fat deposition inside intestinal wall tissue remains understudied. We undertook this study to directly quantify and localize intramural fat deposition in duodenal tissue and determine its association with adiposity.

METHODS

Duodenal tissues were collected from aged (21.2 ± 1.3 years, 19.5 ± 3.1 kg, n = 39) female baboons (Papio sp.). Fasted blood was collected for metabolic profiling and abdominal circumference (AC) measurements were taken. Primary tissue samples were collected at the major duodenal papilla at necropsy: one full cross section was processed for hematoxylin and eosin staining and evaluated; a second full cross section was processed for direct chemical lipid analysis on which percentage duodenal fat content was calculated.

RESULTS

Duodenal fat content obtained by direct tissue quantification showed considerable variability (11.95 ± 6.93%) and was correlated with AC (r = 0.60, p < 0.001), weight (r = 0.38, p = 0.02), leptin (r = 0.63, p < 0.001), adiponectin (r = - 0.32, p < 0.05), and triglyceride (r = 0.41, p = 0.01). The relationship between duodenal fat content and leptin remained after adjusting for body weight and abdominal circumference. Intramural adipocytes were found in duodenal sections from all animals and were localized to the submucosa. Consistent with the variation in tissue fat content, the submucosal adipocytes were non-uniformly distributed in clusters of varying size. Duodenal adipocytes were larger in obese vs. lean animals (106.9 vs. 66.7 µm², p = 0.02).

CONCLUSIONS

Fat accumulation inside the duodenal wall is strongly associated with adiposity and adiposity related circulating biomarkers in baboons. Duodenal tissue fat represents a novel and potentially metabolically active site of ectopic fat deposition.

Does nonalcoholic fatty liver disease cause cardiovascular disease? Current knowledge and gaps

Non-alcoholic fatty liver disease (NAFLD) is highly prevalent and includes a spectrum of abnormalities ranging from steatosis to cirrhosis. In this review, we address recent evidence and limitations of studies that evaluated the association of NAFLD with atherosclerotic cardiovascular disease. NAFLD is considered an ectopic fat deposit associated with metabolic (insulin resistance, hyperglycemia and dyslipidemia), inflammatory, coagulation and blood pressure disturbances. Prospective studies have associated NAFLD presence and severity, particularly steatohepatitis and fibrosis, with an increased risk of cardiovascular disease. However, these studies are limited by heterogeneity concerning NAFLD diagnostic criteria and disease severity stratification, as well as by the presence of confounding factors. In addition, genetic variants predisposing to NAFLD, such as the PNPLA3 I148M mutation, were not consistently associated with an increased risk of cardiovascular events. Therefore, currently, it is not possible to prove a causal relation between NAFLD and cardiovascular disease. Furthermore, there is presently no evidence that NAFLD diagnosis can be used as a tool to improve cardiovascular risk stratification and modify treatment. Specific treatments for NAFLD are being developed and must be tested prospectively in adequately designed trials to determine the potential of reducing both hepatic and cardiovascular diseases and to prove whether NAFLD is indeed a cause of atherosclerosis.

Effects of liver function, insulin resistance and inflammatory factors on vascular endothelial dilation function and prognosis of coronary heart disease patients complicated with NAFLD

Effects of liver function, insulin resistance (IR) and inflammatory factors on vascular endothelial dilation function and prognosis of coronary heart disease (CHD) patients complicated with non-alcoholic fatty liver disease (NAFLD) were investigated. A total of 80 patients with CHD treated in Jinan Central Hospital from October 2016 to July 2017 were randomly enrolled and divided into the NAFLD group (n=41) and the simple CHD group (n=39). The IR, the vascular endothelial dilation function and the related inflammatory factors were also detected, followed by statistical analysis. The level of high-sensitivity C-reactive protein (hs-CRP), serum total bilirubin and alanine aminotransferase (ALT) levels and body mass index in the NAFLD group was decreased and the expression of tumor necrosis factor-α was increased compared with that in the simple CHD group (P<0.05). There was a linearly positive correlation between ALT and brachial artery diameter in the NAFLD group (r=0.311, P<0.05). There was a correlation between homeostasis model assessment-IR (HOMA-IR) and HOMA-β indexes and brachial artery diameter, hs-CRP and brachial artery diameter in both groups (r=−0.128, r=0.219, P<0.05). The HOMA indexes in the NAFLD group were increased compared with those in the simple CHD group (P<0.01). There were significant differences in the intima-media thickness, number of carotid plaques and detection rate of carotid plaques (P<0.05). The risk of cardiovascular events within 10 years in the NAFLD group was increased compared with that in the simple CHD group. The differences of incidence of cardiovascular diseases (CVD)s were statistically significant (P<0.05). Therefore, The changes in liver function indexes, IR and related inflammatory factors in CHD patients complicated with NAFLD significantly affect the vascular endothelial dilation function, which also have some effects on the occurrence of CVDs.

Systems Genomics of Thigh Adipose Tissue From Asian Indian Type-2 Diabetics Revealed Distinct Protein Interaction Hubs

We performed a systematic analysis of genes implicated in thigh subcutaneous adipose tissue of Asian Indian Type 2 Diabetes Mellitus (AIT2DM) and created a phenome-interactome network. This analysis was performed on 60 subjects specific to limb thigh fat by integrating phenotypic traits and similarity scores associated with AIT2DM. Using a phenotypic attribute, a contextual neighbor was identified across all the traits, viz. body mass index (BMI) statistics, adipocyte size, lipid parameters, homeostatic model assessment- insulin resistance (HOMA-IR), HOMA-ß. In this work, we have attempted to characterize transcription signatures using the phenome-interactome maps where each of the traits under study including the intermediary phenotypes has a distinct set of genes forming the hubs. Furthermore, we have identified various clinical, biochemical, and radiological parameters which show significant correlation with distinct hubs. We observed a number of novel pathways and genes including those that are non-coding RNAs implicated in AIT2DM.We showed that they appear to be associated with pathways, viz. tyrosine kinase JAK2, NOTCH thereby recruiting signaling molecules such as STAT5 and Src family kinases on the cell surface regulated them and our analyses comprising significant hubs suggest that thigh subcutaneous adipose tissue plays a role in pathophysiology of AIT2DM.

Application of Stable Isotope Labels for Metabolomics in Studies in Fatty Liver Disease

The progression of nonalcoholic fatty liver disease (NAFLD) increases the risks of cirrhosis and cardiovascular disease. Marked alteration of both cytosolic and mitochondrial metabolism, and in combination with insulin resistance, increases hepatic glucose production. Utilization of stable isotope tracers to study liver metabolism offers deep insight into rearrangements of metabolic pathways and substrate-product relationships under the conditions leading to fatty liver and induced by diseases, drugs, toxins, or genetic manipulations. Isotope tracing untargeted metabolomics (ITUM) recently emerged as a powerful platform in which the label can be tracked in an untargeted fashion, revealing the penetration of substrates into metabolic pathways, even at low abundance. Here, we describe a protocol that can be utilized to study the changes in utilization of any labeled substrate toward a wide range of metabolites either in isolated liver cells or whole liver tissue under conditions mimicking various stages of fatty liver disease. Furthermore, a routine protocol for extraction, separation, and mass spectrometric detection of isotopically labeled metabolites in an untargeted or targeted fashion. An informatic approach to analyze stable isotope untargeted metabolomic datasets is also described.

Children With Metabolically Healthy Obesity: A Review

Children with "metabolically healthy obesity" (MHO) are a distinct subgroup of youth with obesity, who are less prone to the clustering of cardiometabolic risk factors. Although this phenotype, frequently defined by the absence of metabolic syndrome components or insulin resistance, was first described during the early 1980s, a consensus-based definition of pediatric MHO was introduced only recently, in 2018. The purpose of this review was to concisely summarize current knowledge regarding the MHO phenomenon in youth. The prevalence of MHO in children varies from 3 to 87%, depending on the definition used and the parameters evaluated, as well as the ethnicity and the pubertal status of the sample. The most consistent predictors of MHO in youth include younger age, lower body mass index, lower waist circumference, and lower body fat measurements. Various hypotheses have been proposed to elucidate the underlying factors maintaining the favorable MHO phenotype. While preserved insulin sensitivity and lack of inflammation were previously considered to be the main etiological factors, the most recent findings have implicated adipokine levels, the number of inflammatory immune cells in the adipose tissue, and the reduction of visceral adiposity due to adipose tissue expandability. Physical activity and genetic factors also contribute to the MHO phenotype. Obesity constitutes a continuum-increased risk for cardiometabolic complications, which is less evident in children with MHO. However, some findings have highlighted the emergence of hepatic steatosis, increased carotid intima-media thickness and inflammatory biomarkers in the MHO group compared to peers without obesity. Screening should be directed at those more likely to develop clustering of cardiometabolic risk factors. Lifestyle modifications should include behavioral changes focusing on sleep duration, screen time, diet, physical activity, and tobacco smoke exposure. Weight loss has also been associated with the improvement of insulin sensitivity and inflammation. Further investigative efforts are needed in order to elucidate the mechanisms which protect against the clustering of cardiometabolic risk factors in pediatric obesity, to provide more efficient, targeted treatment approaches for children with obesity, and to identify the protective factors preserving the MHO profile, avoiding the crossover of MHO to the phenotype with metabolically unhealthy obesity.

Changes in IGFBP-2 levels following a one-year lifestyle modification program are independently related to improvements in plasma apo B and LDL apo B levels

BACKGROUND AND AIMS

Recent transversal studies have associated insulin-like growth factor binding protein (IGFBP)-2 levels with glucose tolerance and parameters of the lipoprotein-lipid profile. Here, we aimed at determining the longitudinal effects of a one-year lifestyle modification program on IGFBP-2 levels and to identify specific metabolic improvements impacted by the changes in IGFBP-2.

METHODS

99 middle-aged Caucasian men were involved in a lifestyle modification program consisting in personalized healthy eating and physical activity counseling, combined to elicit a daily 500 kcal deficit. Anthropometric and metabolic parameters as well as circulating IGFBP-2 levels were measured before and after one year of the lifestyle modification program.

RESULTS

The intervention triggered positive changes in many metabolic parameters and a 43% (p < 0.0001) increase of IGFBP-2 levels. Subjects with the most substantial increases in IGFBP-2 also experienced the most important metabolic improvements. Changes in IGFBP-2 levels (both absolute and relative) were correlated with markers of body fat distribution and lipoprotein-lipid profile, and independently associated with changes in LDL apolipoprotein (apo) B but not VLDL apo B concentrations. Further analyses showed that for similar changes in BMI, waist circumference and visceral adipose tissue volume, large changes in IGFBP-2 levels were required to observe improvements in LDL apo B levels.

CONCLUSIONS

The 1-year lifestyle modification program was associated with increased IGFBP-2 concentrations. Increases in IGFBP-2 levels were closely associated with reduced LDL apo B concentrations and independently of the modifications in fat mass and insulin sensitivity. Further mechanistic studies are required to assess the effects of IGFBP-2 levels on LDL metabolism.

Contribution of Adipose Tissue Inflammation to the Development of Type 2 Diabetes Mellitus

The objective of this comprehensive review is to summarize and discuss the available evidence of how adipose tissue inflammation affects insulin sensitivity and glucose tolerance. Low-grade, chronic adipose tissue inflammation is characterized by infiltration of macrophages and other immune cell populations into adipose tissue, and a shift toward more proinflammatory subtypes of leukocytes. The infiltration of proinflammatory cells in adipose tissue is associated with an increased production of key chemokines such as C-C motif chemokine ligand 2, proinflammatory cytokines including tumor necrosis factor α and interleukins 1β and 6 as well as reduced expression of the key insulin-sensitizing adipokine, adiponectin. In both rodent models and humans, adipose tissue inflammation is consistently associated with excess fat mass and insulin resistance. In humans, associations with insulin resistance are stronger and more consistent for inflammation in visceral as opposed to subcutaneous fat. Further, genetic alterations in mouse models of obesity that reduce adipose tissue inflammation are-almost without exception-associated with improved insulin sensitivity. However, a dissociation between adipose tissue inflammation and insulin resistance can be observed in very few rodent models of obesity as well as in humans following bariatric surgery- or low-calorie-diet-induced weight loss, illustrating that the etiology of insulin resistance is multifactorial. Taken together, adipose tissue inflammation is a key factor in the development of insulin resistance and type 2 diabetes in obesity, along with other factors that likely include inflammation and fat accumulation in other metabolically active tissues. © 2019 American Physiological Society. Compr Physiol 9:1-58, 2019.

Can obesity-induced inflammation in skeletal muscle and intramuscular adipose tissue accurately detect liver fibrosis?

OBJECTIVES

Obesity is characterized by a chronic, low grade, systemic inflammation. However, little is known about the role of skeletal muscle, which represents an active metabolic organ whose activities need to be determined. The purpose of our study was to detect relationships between skeletal muscle and adipose tissue inflammation with nonalcoholic fatty liver disease (NAFLD) and diabetes, as well as to explore associations with clinicopathological parameters.

METHODS

Our study population consisted of 50 morbidly obese patients undergoing planned bariatric surgery. Biopsies were taken from visceral adipose tissue (VAT), subcutaneous adipose tissue (SAT), skeletal muscle (SM), extramyocellular adipose tissue (EMAT) and liver. The expression of CD68 and CD3 was assessed by immunohistochemistry.

RESULTS

Our findings suggest a complex inter- and intra-tissue co-expression network that links obesity-induced inflammation in adipose depots and skeletal muscle with NAFLD. A novel finding is the intricate cross-talk between SM, EMAT and the liver and the probable correlation between SM, EMAT inflammation and the presence of liver fibrosis.

CONCLUSIONS

Although the mechanisms of obesity-induced inflammation and its association with NAFLD and liver fibrosis are incompletely understood, our findings indicate an extensive and complex tissue network that needs to be further investigated.

Advances in Pediatric Fatty Liver Disease: Pathogenesis, Diagnosis, and Treatment

Pediatric nonalcoholic fatty liver disease is an increasingly prevalent disease, but its pathophysiology is not fully elucidated, diagnosis is difficult and invasive, and therapeutic options are limited. This article addresses the recent advancements made in understanding the pathophysiology of nonalcoholic fatty liver disease, the development of less invasive diagnostic modalities, and emerging therapeutic options, including ongoing clinical trials in children.

Exercise as 'precision medicine' for insulin resistance and its progression to type 2 diabetes: a research review

Type 2 diabetes and obesity epidemics are in effect in the United States and the two pathologies are linked. In accordance with the growing appreciation that ‘exercise is medicine,’ it is intuitive to suggest that exercise can play an important role in the prevention and/or treatment of these conditions. However, if exercise is to truly be considered as a viable alternative to conventional healthcare prevention/treatment strategies involving pharmaceuticals, it must be prescribed with similar scrutiny. Indeed, it seems reasonable to posit that the recent initiative calling for ‘precision medicine’ in the US standard healthcare system should also be applied in the exercise setting. In this narrative review, we consider a number of explanations that have been forwarded regarding the pathological progression to type 2 diabetes both with and without the concurrent influence of overweight/obesity. Our goal is to provide insight regarding exercise strategies that might be useful as ‘precision medicine’ to prevent/treat this disease. Although the etiology of type 2 diabetes is complex and cause/consequence characteristics of associated dysfunctions have been debated, it is well established that impaired insulin action plays a critical early role. Consequently, an exercise strategy to prevent/treat this disease should be geared toward improving insulin sensitivity both from an acute and chronic standpoint. However, research suggests that a chronic improvement in insulin sensitivity only manifests when weight loss accompanies an exercise intervention. This has resonance because ectopic fat accumulation appears to represent a central component of disease progression regardless of whether obesity is also part of the equation. The cause/consequence characteristics of the relationship between insulin resistance, pathological fat deposition and/or mobilsation, elevated and/or poorly-distributed lipid within myocytes and an impaired capacity to use lipid as fuel remains to be clarified as does the role of muscle mitochondria in the metabolic decline. Until these issues are resolved, a multidimensional exercise strategy (e.g., aerobic exercise at a range of intensities and resistance training for muscular hypertrophy) could provide the best alternative for prevention/treatment.

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.

Long-term effects of increased protein intake after weight loss on intrahepatic lipid content and implications for insulin sensitivity: a PREVIEW study

The aim of this study was to assess the effects of a weight maintenance period comprising two diets differing in protein intake, after weight loss, on intrahepatic lipid content and implications for insulin sensitivity. A total of 25 participants [body mass index (BMI): 31.1 (3.5 kg/m²; intrahepatic lipid (IHL): 8.7 (8.3%; fasting glucose: 6.4 (0.6 mmol/l; homeostatic model assessment for insulin resistance (HOMA-IR): 3.7 (1.6; Matsuda index: 3.4 (2.9] started an 8-wk low-energy diet followed by a 2-yr weight maintenance period with either high protein or medium protein dietary guidelines. At baseline, after 6 mo, and after 2 yr, IHL, visceral adipose tissue (VAT), and subcutaneous adipose tissue (SAT) were determined by magnetic resonance spectroscopy/imaging. Glucose and insulin concentrations, determined during an oral glucose challenge, were used to assess the HOMA-IR and Matsuda insulin sensitivity index (ISI). Protein intake was measured with 24-h urinary nitrogen excretion. Protein intake, BMI, IHL, VAT, SAT, HOMA-IR, and ISI did not change differently between the groups during the intervention. In the whole group, BMI, IHL, VAT, SAT, HOMA-IR, and ISI were favorably changed at 6 mo and 2 yr compared with baseline ( P < 0.05). Mixed-model analysis showed that independent of BMI, protein intake (g/d) at 6 mo was inversely related to IHL (coefficient: -0.04; P < 0.05) and VAT (coefficient: -0.01; P < 0.05). Overall, IHL was positively related to HOMA-IR (coefficient: 0.10; P < 0.01) and inversely related to ISI (coefficient: -0.17; P < 0.01), independent of BMI. A 2-yr medium- to high-protein energy-restricted diet reduced IHL and VAT. Independently of changes in BMI, IHL was inversely related to insulin sensitivity.

Hepatic Insulin Clearance in Regulation of Systemic Insulin Concentrations-Role of Carbohydrate and Energy Availability

Hyperinsulinemia is the hallmark of insulin resistance in obesity, and the relative importance of insulin clearance, insulin resistance, and insulin hypersecretion has been widely debated. On the basis of recent experimental evidence, we summarize existing evidence to suggest hepatic insulin clearance as a major and immediate regulator of systemic insulin concentrations responding within days to altered dietary energy and, in particular, carbohydrate intake. Hepatic insulin clearance seems to be closely associated with opposite alterations in hepatic lipid content and glucose production, providing a potential mechanistic link to hepatic insulin sensitivity. The molecular regulation of insulin clearance in the liver is likely to involve changes in insulin binding and receptor internalization in response to the dietary alterations, the molecular mechanisms of which await further research.

Determinants of ectopic liver fat in metabolic disease

Common obesity-associated hepatic steatosis (nonalcoholic fatty liver disease (NAFLD)) and insulin resistance are mainly caused by dysfunctional adipose tissue. This adipose tissue dysfunction leads to increased delivery of NEFA and glycerol to the liver that (i) drives hepatic gluconeogenesis and (ii) facilitates the accumulation of lipids and insulin signaling inhibiting lipid intermediates. Dysfunctional adipose tissue can be caused by impaired lipid storage (overflow hypothesis, characterized by large visceral adipocytes) or increased lipolysis (due to impaired postprandial suppression of lipolysis in inflamed, insulin-resistant adipocytes). In line with the adipose tissue expandability hypothesis the amount and distribution of adipose tissue correlate with its dysfunction and thus with liver fat. This relationship is however modified by endocrine effects on lipid storage and lipolysis as well as dietary effects on hepatic lipogenesis and lipid oxidation. The association between body composition characteristics like visceral obesity or fat cell size and ectopic liver fat is modified by these influences. Phenotyping obesity according to metabolic risk should integrate body composition characteristics, endocrine parameters and information on diet.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Hepatic Steatosis Is Associated With Adverse Molecular Signatures in Subjects Without Diabetes

BACKGROUND AND AIMS

Exaggerated hepatic triglyceride accumulation (i.e., hepatic steatosis) represents a strong risk factor for type 2 diabetes mellitus and cardiovascular disease. Despite the clear association of hepatic steatosis with impaired insulin signaling, the precise molecular mechanisms involved are still under debate. We combined data from several metabolomics techniques to gain a comprehensive picture of molecular alterations related to the presence of hepatic steatosis in a diabetes-free sample (N = 769) of the population-based Study of Health in Pomerania.

METHODS

Liver fat content (LFC) was assessed using MRI. Metabolome measurements of plasma and urine samples were done by mass spectrometry and nuclear magnetic resonance spectroscopy. Linear regression analyses were used to detect significant associations with either LFC or markers of hepatic damage. Possible mediations through insulin resistance, hypertriglyceridemia, and inflammation were tested. A predictive molecular signature of hepatic steatosis was established using regularized logistic regression.

RESULTS

The LFC-associated atherogenic lipid profile, tightly connected to shifts in the phospholipid content, and a prediabetic amino acid cluster were mediated by insulin resistance. Molecular surrogates of oxidative stress and multiple associations with urine metabolites (e.g., indicating altered cortisol metabolism or phase II detoxification products) were unaffected in mediation analyses. Incorporation of urine metabolites slightly improved classification of hepatic steatosis.

CONCLUSIONS

Comprehensive metabolic profiling allowed us to reveal molecular patterns accompanying hepatic steatosis independent of the known hallmarks. Novel biomarkers from urine (e.g., cortisol glucuronide) are worthwhile for follow-up in patients suffering from more severe liver impairment compared with our merely healthy population-based sample.

Subcutaneous adipose tissue imaging of human obesity reveals two types of adipocyte membranes: Insulin-responsive and -nonresponsive

In adipose tissue, resistance to insulin's ability to increase glucose uptake can be induced by multiple factors, including obesity. Impaired insulin action may take place at different spatial loci at the cellular or subcellular level. To begin to understand the spatial response to insulin in human subcutaneous adipose tissue (hSAT), we developed a quantitative imaging method for activation of a major signaling node in the glucoregulatory insulin signaling pathway. After treatment with insulin or control media, biopsied tissues were immunostained for Akt phosphorylation at Thr-308/9 (pAkt) and then imaged by confocal fluorescence microscopy automated to collect a large grid of high resolution fields. In hSAT from 40 men and women with obesity, substantial heterogeneity of pAkt densities in adipocyte membranes were quantified in each image mosaic using a spatial unit of at least twice the size of the point spread function. Statistical analysis of the distribution of pAkt spatial units was best fit as the weighted sum of two separate distributions, corresponding to either a low or high pAkt density. A "high pAkt fraction" metric was calculated from the fraction of high pAkt distributed units over the total units. Importantly, upon insulin stimulation, tissues from the same biopsy showed either a minimal or a substantial change in the high pAkt fraction. Further supporting a two-state response to insulin stimulation, subjects with similar insulin sensitivity indices are also segregated into either of two clusters identified by the amount of membrane-localized pAkt.

The NOAEL Metformin Dose Is Ineffective against Metabolic Disruption Induced by Chronic Cadmium Exposure in Wistar Rats

Previous studies have proposed that cadmium (Cd) is a metabolic disruptor, which is associated with insulin resistance, metabolic syndrome, and diabetes. This metal is not considered by international agencies for the study of metabolic diseases. In this study, we investigate the effect of metformin on Cd-exposed Wistar rats at a lowest-observed-adverse-effect level (LOAEL) dose (32.5 ppm) in drinking water. Metabolic complications in the rats exposed to Cd were dysglycemia, insulin resistance, dyslipidemia, dyslipoproteinemia, and imbalance in triglyceride and glycogen storage in the liver, muscle, heart, kidney, and adipose tissue. Meanwhile, rats treated orally with a No-observable-adverse-effect level (NOAEL) dose of metformin (200 mg/kg/day) showed mild improvement on serum lipids, but not on glucose tolerance; in tissues, glycogen storage was improved, but lipid storage was ineffective. In conclusion, metformin as a first-line pharmacological therapy must take into consideration the origin and duration of metabolic disruption, because in this work the NOAEL dose of metformin (200 mg/kg/day) showed a limited efficiency in the metabolic disruption caused by chronic Cd exposure.

Molecular mechanisms of hepatic lipid accumulation in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is currently the world's most common liver disease, estimated to affect up to one-fourth of the population. Hallmarked by hepatic steatosis, NAFLD is associated with a multitude of detrimental effects and increased mortality. This narrative review investigates the molecular mechanisms of hepatic steatosis in NAFLD, focusing on the four major pathways contributing to lipid homeostasis in the liver. Hepatic steatosis is a consequence of lipid acquisition exceeding lipid disposal, i.e., the uptake of fatty acids and de novo lipogenesis surpassing fatty acid oxidation and export. In NAFLD, hepatic uptake and de novo lipogenesis are increased, while a compensatory enhancement of fatty acid oxidation is insufficient in normalizing lipid levels and may even promote cellular damage and disease progression by inducing oxidative stress, especially with compromised mitochondrial function and increased oxidation in peroxisomes and cytochromes. While lipid export initially increases, it plateaus and may even decrease with disease progression, sustaining the accumulation of lipids. Fueled by lipo-apoptosis, hepatic steatosis leads to systemic metabolic disarray that adversely affects multiple organs, placing abnormal lipid metabolism associated with NAFLD in close relation to many of the current life-style-related diseases.

Visceral adipose tissue tracks more closely with metabolic dysfunction than intrahepatic triglyceride in lean Asians without diabetes

Increased visceral adipose tissue (VAT) and intrahepatic triglyceride (IHTG) are important risk factors for the development of type 2 diabetes in subjects with obesity. The relative contribution of these ectopic fat depots to cardiometabolic risk differs between populations, depends on the degree of obesity and the level of cardiorespiratory fitness, and is difficult to dissect because VAT and IHTG typically covary. The aim of this study was to evaluate the effect of an isolated increase in VAT or IHTG on insulin sensitivity and insulin secretion in apparently healthy normal-weight Asian subjects. Total body fat (dual X-ray absorptiometry), VAT and IHTG (magnetic resonance), insulin sensitivity (4-h hyperinsulinemic-euglycemic clamp), beta cell responsivity and insulin secretion rate (3-h mixed meal with mathematical modeling), and cardiorespiratory fitness (maximal oxygen consumption [V̇o2max]) were evaluated in groups of lean subjects with low or high VAT (687 ± 94 vs. 1,279 ± 197 ml, matched for IHTG; n = 13 each) and low or high IHTG (1.7 ± 0.3 vs. 6.7 ± 2.0%, matched for VAT; n = 15 each). All groups were matched for age, sex, total body fat, and V̇o2max. High-VAT subjects had ~25% lower insulin sensitivity, ~20%–40% greater beta cell responsivity and insulin secretion rate, ~35% greater fasting triglyceride concentration, and ~40% lower adiponectin concentration than low-VAT subjects (all P < 0.05). No differences were observed between low-IHTG and high-IHTG subjects. Accumulation of excess fat in the intra-abdominal area is more strongly associated with metabolic dysfunction than accumulation of liver fat in lean Asians without diabetes.

NEW & NOTEWORTHY It is not known whether metabolic abnormalities in Asians without obesity track more closely with visceral or liver fat. We found an isolated increase in visceral fat was associated with reduced insulin sensitivity, greater insulin secretion, greater triglyceride, and lower adiponectin concentrations; no differences were observed with an isolated increase in liver fat. These results suggest that visceral fat is a better correlate of metabolic dysfunction than liver fat in Asians without obesity.

Ectopic Lipid Deposition Is Associated With Insulin Resistance in Postmenopausal Women

CONTEXT

Menopause is associated with an increased incidence of insulin resistance and diabetes.

OBJECTIVE

The aim of this study was to explore the lipid deposition in liver and skeletal muscle and investigate the association with insulin sensitivity in postmenopausal and premenopausal women.

DESIGN AND SETTING

Single-center cross-sectional study of 55 healthy women between 45 and 60 years of age. We measured lipid deposition in the liver with magnetic resonance spectroscopy, intramuscular and intra-abdominal lipid deposition with MRI, body composition with a dual-energy X-ray absorptiometry scan, and insulin sensitivity with the composite Matsuda Index.

OUTCOME MEASURES

We studied the association between fat distribution, ectopic lipid deposition, and insulin sensitivity in pre- and postmenopausal women.

RESULTS

Postmenopausal women had an increased lipid deposition in the liver [0.68% (0.44 to 0.99) vs 0.49% (0.38 to 0.64), P = 0.01] and skeletal muscle [3% (2 to 4) vs 2% (1 to 3), P = 0.001] and had a 28% lower Matsuda insulin sensitivity index during an oral glucose tolerance test (6.31 ± 3.48 vs 8.78 ± 4.67, P = 0.05) compared with premenopausal women. Total fat mass and leg fat mass were stronger predictors of ectopic lipid deposition, and visceral fat mass was a stronger predictor of both ectopic lipid deposition and insulin resistance in postmenopausal women compared with premenopausal women.

CONCLUSIONS

For a given subcutaneous and visceral fat depot size, postmenopausal women show increased ectopic lipid deposition and insulin resistance compared with premenopausal women. It is suggested that lipid deposition in liver and skeletal muscle may represent important mechanistic links between the changes in fat depots and the increased incidence of insulin resistance seen after menopause.

Pathophysiological significance of hepatokine overproduction in type 2 diabetes

Currently, many studies draw attention to novel secretory factors, such as adipokines or myokines, derived from the tissues that were not originally recognized as endocrine organs. The liver may contribute to the onset of various kinds of pathologies of type 2 diabetes by way of the production of secretory proteins "hepatokines." Using the comprehensive gene expression analyses in human livers, we have rediscovered selenoprotein P and LECT2 as hepatokines involved in the onset of dysregulated glucose metabolism. Overproduction of selenoprotein P, previously reported as a transport protein of selenium, induces insulin resistance and hyperglycemia in type 2 diabetic condition. Selenoprotein P also contributes to vascular complications of type 2 diabetes directly by inducing VEGF resistance in vascular endothelial cells. Notably, selenoprotein P impairs health-promoting effects of exercise by inhibiting ROS/AMPK/PGC-1α pathway in the skeletal muscle through its receptor LRP1. Overproduction of LECT2, previously reported as a neutrophil chemotactic protein, links obesity to insulin resistance in the skeletal muscle. Further studies would develop novel diagnostic or therapeutic procedures targeting hepatokines to combat over-nutrition-related diseases such as type 2 diabetes.

Prediction of carotid intima-media thickness in obese patients with low prevalence of comorbidities by serum copper bioavailability

BACKGROUND AND AIM

Western societies, with growing prevalence, suffer from various metabolic diseases like obesity and hepatic steatosis, better defined as non-alcoholic fatty liver disease, or cardiovascular (CV) diseases that are strictly linked to each other. The association of their occurrence with the altered homeostasis of metals is an intriguing issue. Copper in particular was identified as key player in various metabolic derangements. On these bases, we aimed at investigating the possible association of serum copper levels with an indicator of early CV risk as the intima-media thickness (IMT) of carotid artery and its predictive value in a selected population of obese patients.

METHODS

We performed a cross-sectional study recruiting 100 obese patients characterized by a low prevalence of comorbidities. Ultrasound investigation for hepatic steatosis and IMT evaluation were performed. Serum samples were collected and then analyzed through atomic absorption spectrometry to evaluate their copper content. Possible correlations between copper bioavailability and biochemical, clinical, and anthropometric characteristics of patients were sought.

RESULTS

Age negatively predicted copper serum levels of patients (P = 0.009). However, the most interesting finding is the negative prediction of IMT by the copper serum levels (t = -2.23, P = 0.028, least absolute deviations regression). Factor analysis confirmed the aforementioned inverse correlation and highlighted the strong inverse correlation between smoking and copper serum levels.

CONCLUSION

Our data show that an altered copper bioavailability predicts early atherosclerosis as main CV risk in obese patients with hepatic steatosis detected by ultrasound, shedding some light in this pathological scenario.

Relevance of human fat distribution on lipid and lipoprotein metabolism and cardiovascular disease risk

PURPOSE OF REVIEW

Upper body abdominal and lower body gluteofemoral fat depot masses display opposing associations with plasma lipid and lipoprotein and cardiovascular disease (CVD) risk profiles. We review developments on adipose tissue fatty acid metabolism in the context of body fat distribution and how that might be related to adverse lipid and lipoprotein profiles and CVD risk.

RECENT FINDINGS

Recent data have confirmed the paradoxical relationship of upper abdominal and lower body gluteofemoral adiposity and CVD risk. Mechanistically, this is likely to reflect the different ways fat depots handle lipid storage and release, which impacts directly and indirectly on lipid and lipoprotein metabolism. The upper body enhances immediate fat storage pathway with rapid uptake of dietary-derived fatty acids, whereas the lower body fat depot has a reduced lipid turnover accommodating a slower fat redistribution. Body fat distribution and the fat depots' ability to undergo appropriate expansion when fat storage is required, rather than overall body fatness, appear as the important determinant of metabolic health.

SUMMARY

A focus on fat distribution in overweight people, preferably using precise imaging methods, rather than quantifying total body fatness, is likely to provide the medical community with better tools to stratify and treat patients with obesity-related complications.

ACC2 Deletion Enhances IMCL Reduction Along With Acetyl-CoA Metabolism and Improves Insulin Sensitivity in Male Mice

Intramyocellular lipid (IMCL) accumulation in skeletal muscle greatly contributes to lipid-induced insulin resistance. Because acetyl-coenzyme A (CoA) carboxylase (ACC) 2 negatively modulates mitochondrial fatty acid oxidation (FAO) in skeletal muscle, ACC2 inhibition is expected to reduce IMCL via elevation of FAO and to attenuate insulin resistance. However, the concept of substrate competition suggests that enhanced FAO results in reduced glucose use because of an excessive acetyl-CoA pool in mitochondria. To identify how ACC2-regulated FAO affects IMCL accumulation and glucose metabolism, we generated ACC2 knockout (ACC2-/-) mice and investigated skeletal muscle metabolites associated with fatty acid and glucose metabolism, as well as whole-body glucose metabolism. ACC2-/- mice displayed higher capacity of glucose disposal at the whole-body levels. In skeletal muscle, ACC2-/- mice exhibited enhanced acylcarnitine formation and reduced IMCL levels without alteration in glycolytic intermediate levels. Notably, these changes were accompanied by decreased acetyl-CoA content and enhanced mitochondrial pathways related to acetyl-CoA metabolism, such as the acetylcarnitine production and tricarboxylic acid cycle. Furthermore, ACC2-/- mice exhibited lower levels of IMCL and acetyl-CoA even under HFD conditions and showed protection against HFD-induced insulin resistance. Our findings suggest that ACC2 deletion leads to IMCL reduction without suppressing glucose use via an elevation in acetyl-CoA metabolism even under HFD conditions and offer new mechanistic insight into the therapeutic potential of ACC2 inhibition on insulin resistance.

A simple clinical model predicts incident hepatic steatosis in a community-based cohort: The Framingham Heart Study

BACKGROUND AND AIMS

The factors associated with incident hepatic steatosis are not definitively known. We sought to determine factors associated with incident hepatic steatosis, as measured on computed tomography, in the community.

METHODS

We studied Framingham Heart Study participants without heavy alcohol use or baseline hepatic steatosis who underwent computed tomography scans between 2002-2005 (baseline) and 2008-2011 (follow-up). We performed a stepwise logistic regression procedure to determine the predictors associated with incident hepatic steatosis.

RESULTS

We included 685 participants (mean age: 45.0 ± 6.2 years, 46.8% women). The incidence of hepatic steatosis in our sample was 17.1% over a mean 6.3 years of follow-up. Participants who developed hepatic steatosis had more adverse cardiometabolic profiles at baseline compared to those free of hepatic steatosis at follow-up. Multivariable stepwise regression analysis showed that a simple clinical model including age, sex, body mass index, alcohol consumption and triglycerides was predictive of incident hepatic steatosis (C statistic = 0.791, 95% CI: 0.748-0.834). A complex clinical model, which included visceral adipose tissue volume and liver phantom ratio added to the simple clinical model, and had improved discrimination for predicting incident hepatic steatosis (C statistic = 0.826, 95% CI: 0.786-0.866, P < .0001).

CONCLUSIONS

The combination of demographic, clinical and imaging characteristics at baseline was predictive of incident hepatic steatosis. The use of our predictive model may help identify those at increased risk for developing hepatic steatosis who may benefit from risk factor modification although further investigation is warranted.

Greater Microbial Translocation and Vulnerability to Metabolic Disease in Healthy Aged Female Monkeys

Monkeys demonstrate gastrointestinal barrier dysfunction (leaky gut) as evidenced by higher biomarkers of microbial translocation (MT) and inflammation with ageing despite equivalent health status, and lifelong diet and environmental conditions. We evaluated colonic structural, microbiomic and functional changes in old female vervet monkeys (Chlorocebus aethiops sabeus) and how age-related leaky gut alters responses to Western diet. We additionally assessed serum bovine immunoglobulin therapy to lower MT burden. MT was increased in old monkeys despite comparable histological appearance of the ascending colon. Microbiome profiles from 16S sequencing did not show large differences by age grouping, but there was evidence for higher mucosal bacterial loads using qPCR. Innate immune responses were increased in old monkeys consistent with higher MT burdens. Western diet challenge led to elevations in glycemic and hepatic biochemistry values only in old monkeys, and immunoglobulin therapy was not effective in reducing MT markers or improving metabolic health. We interpret these findings to suggest that ageing may lead to lower control over colonization at the mucosal surface, and reduced clearance of pathogens resulting in MT and inflammation. Leaky gut in ageing, which is not readily rescued by innate immune support with immunoglobulin, primes the liver for negative consequences of high fat, high sugar diets.

Adult weight change in relation to visceral fat and liver fat at middle age: The Netherlands epidemiology of obesity study

OBJECTIVE

We aimed to investigate the associations between weight change during adulthood and the amount of abdominal subcutaneous fat, visceral fat, and liver fat at middle age.

METHODS

The Netherlands Epidemiology of Obesity (NEO) study is a population-based cohort of 6671 middle-aged men and women. We calculated the percentage of weight change during adulthood based on body weight at middle age and recalled body weight at age 20. Abdominal subcutaneous and visceral adipose tissue were assessed by magnetic resonance imaging (MRI), in addition to hepatic triglyceride content by ¹H-MR spectroscopy in a random subgroup (maximum of n = 2580). With multivariable linear regression analysis, we examined the associations between categories of adult weight change, body mass index (BMI) at age 20 and measures of abdominal adiposity at middle age, adjusted for age, sex, ethnicity, lifestyle factors, menopausal status, parity, use of medication and total body fat at middle age.

RESULTS

In 2399 participants (54% women), individuals who gained more than 50% of body weight during adulthood had 1.96 (95% CI: 1.64; 2.33) times more visceral adipose tissue at middle age and 2.39 (95% CI: 1.70, 3.36) times more hepatic triglyceride content than weight maintainers (weight change between -5% and 5%). Associations with abdominal subcutaneous adipose tissue were weaker: participants who gained more than 50% of their body weight had 1.54 (95% CI: 1.38, 1.72) times more abdominal subcutaneous adipose tissue compared with weight maintainers.

CONCLUSIONS

In this population-based study, adult weight gain was associated with relatively more visceral adipose tissue and hepatic triglyceride content at middle age than abdominal subcutaneous adipose tissue. Overall, our study suggests that weight maintenance during adulthood plays an important role in limiting excess visceral adipose tissue and hepatic triglyceride content at middle age.

Hyperlipidemia-induced hepassocin in the liver contributes to insulin resistance in skeletal muscle

Hepassocin (HPS) has recently been identified as a novel hepatokine that causes hepatic steatosis. However, the role of HPS in the development of insulin resistance in skeletal muscle under obesity remains unclear. The effect of hyperlipidemia on hepatic HPS expression was evaluated in primary hepatocytes and liver of mice. HPS-mediated signal pathways were explored using small interfering (si) RNAs of specific genes or inhibitors. We found that treatment of primary hepatocytes with palmitate could induce HPS expression through C/EBPβ-mediated transcriptional activation. Furthermore, increased HPS expression was observed in the liver of high fat diet (HFD)-fed or tunicamycin-treated mice. Pretreatment with 4-phenylbutyrate (4-BPA) (an endoplasmic reticulum (ER) stress inhibitor) and suppression of p38 by siRNA abrogated the effect of palmitate on HPS expression in primary hepatocytes. Treatment of differentiated C2C12 cells with recombinant HPS caused c-Jun N-terminal kinase (JNK) phosphorylation and impairment of insulin sensitivity in a dose-dependent manner. siRNA-mediated suppression of JNK reduced the effect of HPS on insulin signaling. Furthermore, the suppression of epidermal growth factor receptor (EGFR) by siRNA mitigated both HPS-induced JNK phosphorylation and insulin resistance. In addition, HPS did not affect inflammation and ER stress in differentiated C2C12 cells. In conclusion, we elucidated that ER stress induced by palmitate could increase the expression of HPS in hepatocytes and further contribute to the development of insulin resistance in skeletal muscle via EGFR/JNK-mediated pathway. Taken together, we suggest that HPS could be a therapeutic target for obesity-linked insulin resistance.

Conversion of Sugar to Fat: Is Hepatic de Novo Lipogenesis Leading to Metabolic Syndrome and Associated Chronic Diseases?

Epidemiologic studies suggest a link between excess sugar consumption and obesity, fatty liver disease, metabolic syndrome, and type 2 diabetes mellitus. One important pathway that may link these metabolic diseases to sugar consumption is hepatic conversion of sugar to fat, a process known as de novo lipogenesis (DNL). Mechanistic studies have shown that diets high in simple sugars increase both DNL and liver fat. Importantly, removal of sugar from diets of children with obesity for only 9 days consistently reduced DNL and liver fat and improved glucose and lipid metabolism. Although the sugar and beverage industries continue to question the scientific evidence linking high-sugar diets to metabolic diseases, major health organizations now make evidence-based recommendations to limit consumption of simple sugars to no more than 5% to 10% of daily intake. Clear recommendation about moderating sugar intake to patients may be an important nonpharmacologic tool to include in clinical practice.

Hepatic steatosis risk is partly driven by increased de novo lipogenesis following carbohydrate consumption

Background

Diet is a major contributor to metabolic disease risk, but there is controversy as to whether increased incidences of diseases such as non-alcoholic fatty liver disease arise from consumption of saturated fats or free sugars. Here, we investigate whether a sub-set of triacylglycerols (TAGs) were associated with hepatic steatosis and whether they arise from de novo lipogenesis (DNL) from the consumption of carbohydrates.

Results

We conduct direct infusion mass spectrometry of lipids in plasma to study the association between specific TAGs and hepatic steatosis assessed by ultrasound and fatty liver index in volunteers from the UK-based Fenland Study and evaluate clustering of TAGs in the National Survey of Health and Development UK cohort. We find that TAGs containing saturated and monounsaturated fatty acids with 16–18 carbons are specifically associated with hepatic steatosis. These TAGs are additionally associated with higher consumption of carbohydrate and saturated fat, hepatic steatosis, and variations in the gene for protein phosphatase 1, regulatory subunit 3b (PPP1R3B), which in part regulates glycogen synthesis. DNL is measured in hyperphagic ob/ob mice, mice on a western diet (high in fat and free sugar) and in healthy humans using stable isotope techniques following high carbohydrate meals, demonstrating the rate of DNL correlates with increased synthesis of this cluster of TAGs. Furthermore, these TAGs are increased in plasma from patients with biopsy-confirmed steatosis.

Conclusion

A subset of TAGs is associated with hepatic steatosis, even when correcting for common confounding factors. We suggest that hepatic steatosis risk in western populations is in part driven by increased DNL following carbohydrate rich meals in addition to the consumption of saturated fat.

Electronic supplementary material

The online version of this article (10.1186/s13059-018-1439-8) contains supplementary material, which is available to authorized users.