Influence of liver triglycerides on suppression of glucose production by insulin in men

Hepatic glucose metabolism in the steatotic liver

The liver is central in regulating glucose homeostasis, being the major contributor to endogenous glucose production and the greatest reserve of glucose as glycogen. It is both a target and regulator of the action of glucoregulatory hormones. Hepatic metabolic functions are altered in and contribute to the highly prevalent steatotic liver disease (SLD), including metabolic dysfunction-associated SLD (MASLD) and metabolic dysfunction-associated steatohepatitis (MASH). In this Review, we describe the dysregulation of hepatic glucose metabolism in MASLD and MASH and associated metabolic comorbidities, and how advances in techniques and models for the assessment of hepatic glucose fluxes in vivo have led to the identification of the mechanisms related to the alterations in glucose metabolism in MASLD and comorbidities. These fluxes can ultimately increase hepatic glucose production concomitantly with fat accumulation and alterations in the secretion and action of glucoregulatory hormones. No pharmacological treatment has yet been approved for MASLD or MASH, but some antihyperglycaemic drugs approved for treating type 2 diabetes have shown positive effects on hepatic glucose metabolism and hepatosteatosis. A deep understanding of how MASLD affects glucose metabolic fluxes and glucoregulatory hormones might assist in the early identification of at-risk individuals and the use or development of targeted therapies.

Anti-Inflammatory and Therapeutic Effects of a Novel Small-Molecule Inhibitor of Inflammation in a Male C57BL/6J Mouse Model of Obesity-Induced NAFLD/MAFLD

Purpose

Non-alcoholic fatty liver disease (NAFLD), recently renamed metabolic (dysfunction) associated fatty liver disease (MAFLD), is the most common chronic liver disease in the United States. Presently, there is an intense and ongoing effort to identify and develop novel therapeutics for this disease. In this study, we explored the anti-inflammatory activity of a new compound, termed IOI-214, and its therapeutic potential to ameliorate NAFLD/MAFLD in male C57BL/6J mice fed a high fat (HF) diet.

Methods

Murine macrophages and hepatocytes in culture were treated with lipopolysaccharide (LPS) ± IOI-214 or DMSO (vehicle), and RT-qPCR analyses of inflammatory cytokine gene expression were used to assess IOI-214's anti-inflammatory properties in vitro. Male C57BL/6J mice were also placed on a HF diet and treated once daily with IOI-214 or DMSO for 16 weeks. Tissues were collected and analyzed to determine the effects of IOI-214 on HF diet-induced NAFL D/MAFLD. Measurements such as weight, blood glucose, serum cholesterol, liver/serum triglyceride, insulin, and glucose tolerance tests, ELISAs, metabolomics, Western blots, histology, gut microbiome, and serum LPS binding protein analyses were conducted.

Results

IOI-214 inhibited LPS-induced inflammation in macrophages and hepatocytes in culture and abrogated HF diet-induced mesenteric fat accumulation, hepatic inflammation and steatosis/hepatocellular ballooning, as well as fasting hyperglycemia without affecting insulin resistance or fasting insulin, cholesterol or TG levels despite overall obesity in vivo in male C57BL/6J mice. IOI-214 also decreased systemic inflammation in vivo and improved gut microbiota dysbiosis and leaky gut.

Conclusion

Combined, these data indicate that IOI-214 works at multiple levels in parallel to inhibit the inflammation that drives HF diet-induced NAFLD/MAFLD, suggesting that it may have therapeutic potential for NAFLD/MAFLD.

The Extent of Lifestyle-Induced Weight Loss Determines the Risk of Prediabetes and Metabolic Syndrome Recurrence during a 5-Year Follow-Up

It is controversial whether lifestyle-induced weight loss (LIWL) intervention provides long-term benefit. Here, we investigated whether the degree of weight loss (WL) in a controlled LIWL intervention study determined the risk of prediabetes and recurrence of metabolic syndrome (MetS) during a 5-year follow-up. Following LIWL, 58 male participants (age 45−55 years) were divided into four quartiles based on initial WL: Q1 (WL 0−8.1%, n = 15), Q2 (WL 8.1−12.8%, n = 14), Q3 (WL 12.8−16.0%, n = 14), and Q4 (WL 16.0−27.5%, n = 15). We analyzed changes in BMI, HDL cholesterol, triglycerides (TGs), blood pressure, and fasting plasma glucose (FPG) at annual follow-up visits. With a weight gain after LIWL between 1.2 (Q2) and 2.5 kg/year (Q4), the reduction in BMI was maintained for 4 (Q2, p = 0.03) or 5 (Q3, p = 0.03; Q4, p < 0.01) years, respectively, and an increase in FPG levels above baseline values was prevented in Q2−Q4. Accordingly, there was no increase in prediabetes incidence after LIWL in participants in Q2 (up to 2 years), Q3 and Q4 (up to 5 years). A sustained reduction in MetS was maintained in Q4 during the 5-year follow-up. The present data indicate that a greater initial LIWL reduces the risk of prediabetes and recurrence of MetS for up to 5 years.

Triglyceride glucose (TyG) index and the progression of liver fibrosis: A cross-sectional study

BACKGROUND

Non-alcoholic fatty liver (NAFLD) is a multi-factorial liver disease and its incidence is globally rising. Little is known about the association between triglyceride glucose (TyG) index and liver fibrosis progression in NAFLD patients.

AIM

To examine the association of liver fibrosis with TyG index among patients with NAFLD in a sample of Iranian adults.

METHODS

The NAFLD fibrosis score and the fibrosis-4 (FIB-4) index were used for the detection of hepatic fibrosis. Multivariable-adjusted odds ratios (ORs) were applied to assess the association of liver fibrosis with TyG index.

RESULTS

The current study included a total of 230 participants with NAFLD and low probability of fibrosis. The TyG index quartiles were higher in patients with higher body mass index (BMI), higher systolic blood pressure (SBP), and less physical activity than in participants with lower BMI, lower SBP and more physical activity, respectively. Moreover, higher serum levels of total cholesterol, triglyceride, LDL-C, aspartate and aminotransferases, and homeostatic model assessment for insulin resistance (HOMA-IR), and lower serum level of HDL-C were observed in patients with higher quartiles of TyG index (all P < 0.01). The severity of NAFLD significantly increased with increment in the quartiles of TyG index. Increased TyG index was positively associated with worsening of NAFLD fibrosis score and FIB-4 index. Based on NAFLD fibrosis score, the multivariable-adjusted ORs (95% CIs) were 1.98 (1.33-2.22), 2.33 (2.09-2.94), and 3.44 (2.63-4.25) in the 2nd, 3rd, and 4th quantiles of TyG index when compared to the 1st quantile of TyG index. A similar trend was observed in the analysis using FIB-4 index.

CONCLUSION

According to the results of the current study, excess TyG index contributes to the development of liver fibrosis.

Phenotypic characterization of a novel type 2 diabetes animal model in a SHANXI MU colony of Chinese hamsters

PURPOSE

Developing animal models for human diseases is critical for studying complex diseases such as type 2 diabetes mellitus (T2DM). Since inbred colonies of Chinese hamsters tend toward spontaneous development of diabetes, we investigated them as a possible model.

METHODS

We regarded individuals with fasting blood glucose (FBG) higher than 6.0 mmol/L and post-prandial blood glucose (PBG) higher than 7.0 mmol/L as diabetic based on the mean and 95% frequency distribution values of FBG and PBG. Diabetic hamsters were characterized based on metabolic profiles, histopathological features, and changes in the expression of genes involved in glucose and lipid metabolism.

RESULTS

Metabolic analyses showed that diabetic hamsters exhibited mild hyperglycemia, hypertriglyceridemia, glucose intolerance, and insulin resistance. Histopathological analysis revealed that cell nuclei migrated inward in skeletal muscle and obvious partial liver lipid deposition and focal necrosis was found. We additionally observed mild injury, atrophy, and occasional vacuolization in islet cells. Changes in the expression of several genes related to glucose and lipid metabolism were observed. Decreased expression of adiponectin and GLUT4 and increased expression of PPARγ, Akt, and leptin was observed in skeletal muscle. Decreased expression of adiponectin with increased expression of PPARγ and leptin was observed in the liver.

CONCLUSIONS

These results indicate that we have established a spontaneous diabetic hamster line that closely mimics human T2DM, which may hold potential for further research on the pathogenesis and treatment of this disease.

Adeno-Associated Virus-Mediated Knockdown of SLC16A11 Improves Glucose Tolerance and Hepatic Insulin Signaling in High Fat Diet-Fed Mice

BACKGROUND

SLC16A11, a member of the SLC16 family, is associated with lipid metabolism, causing increased intracellular triacylglycerol (TAG) levels. In the current study, our primary goal was to determine if an SLC16A11 knockdown would improve glucose tolerance and hepatic insulin signaling in high fat diet (HFD)-fed mice. Additionally, the mechanism for exercise-improved insulin sensitivity remains unclear, and there is no mechanistic insight into SLC16A11's role in insulin sensitivity under exercise stress. Therefore, we also examined the impact of endurance exercise on the abundance of SLC16A11.

METHODS

C57BL/6 J male mice were fed either regular chow (Control) or HFD for 8 weeks and then injected with adeno-associated virus (AAV). Plasma parameters, tissue lipid contents, glucose tolerance, and expression profiles of hepatic insulin signaling were detected. Also, other mice were divided randomly into sedentary and exercise groups. We assessed hepatic expression of SLC16A11 after 8 weeks of endurance exercise.

RESULTS

1) Hepatic SLC16A11 expression was greater in HFD-fed mice compared to Control mice. 2) AAV-mediated knockdown of SLC16A11 improved glucose tolerance, prevented TAG accumulation in serum and liver, and increased phosphorylation of protein kinase B (Akt) and glycogen synthesis kinase-3β (GSK3β) in HFD-fed mice. 3) Endurance exercise decreased hepatic SLC16A11 expression.

CONCLUSIONS

Inactivation of SLC16A11, which is robustly induced by HFD, improved glucose tolerance and hepatic insulin signaling, independent of body weight, but related to TAG. Additionally, SLC16A11 might mediate the health benefits of endurance exercise.

Silencing of SAA1 inhibits palmitate- or high-fat diet induced insulin resistance through suppression of the NF-κB pathway

Background

Obesity is one of the leading causes of insulin resistance. Accumulating reports have highlighted that serum amyloid A-1 (SAA1) is a potential candidate that is capable of attenuating insulin resistance. Hence, we conducted the current study with aims of investigating our proposed hypothesis that silencing SAA1 could inhibit the progression of obesity-induced insulin resistance through the NF-κB pathway.

Methods

Gene expression microarray analysis was initially performed to screen differentially expressed genes (DEGs) associated with obesity. Palmitate (PA)-induced insulin resistance Huh7 cell models and high-fat diet (HFD)-induced mouse models were established to elucidate the effect of SAA1/Saa1 on insulin resistance. The NF-κB pathway-related expression was subsequently determined through the application of reverse transcription quantitative polymerase chain reaction (RT-qPCR) and Western blot analysis.

Results

Saa1 was identified as an obesity-related gene based on the microarray data of GSE39549. Saa1 was determined to be highly expressed in HFD-induced insulin resistance mouse models. PA-induced Huh7 cells, treated with silenced SAA1 or NF-κB pathway inhibition using BAY 11–7082, displayed a marked decrease in both Saa1 and SOCS3 as well as an elevation in 2DG, IRS1 and the extent of IRS1 phosphorylation. HFD mice treated with silenced Saa1 or inhibited NF-κB pathway exhibited improved fasting blood glucose (FBG) levels as well as fasting plasma insulin (FPI) levels, glucose tolerance and systemic insulin sensitivity. Saa1/SAA1 was determined to show a stimulatory effect on the transport of the NF-κBp65 protein from the cytoplasm to the nucleus both in vivo and in vitro, suggesting that Saa1/SAA1 could activate the NF-κB pathway.

Conclusion

Taken together, our key findings highlight a novel mechanism by which silencing of SAA1 hinders PA or HFD-induced insulin resistance through inhibition of the NF-κB pathway.

Electronic supplementary material

The online version of this article (10.1186/s10020-019-0075-4) contains supplementary material, which is available to authorized users.

A simple method to monitor hepatic gluconeogenesis and triglyceride synthesis following oral sugar tolerance test in obese adolescents

Hepatic energy metabolism is a key element in many metabolic diseases. Hepatic anaplerosis provides carbons for gluconeogenesis (GNG) and triglyceride (TG) synthesis. We aimed to optimize a protocol that measures hepatic anaplerotic contribution for GNG, TG synthesis, and hepatic pentose phosphate pathway (PPP) activity using a single dose of oral [U^-13C₃]glycerol paired with an oral sugar tolerance test (OSTT) in a population with significant insulin resistance. The OSTT (75 g glucose + 25 g fructose) was administered to eight obese adolescents with polycystic ovarian syndrome (PCOS) followed by ingestion of [U-¹³C₃]glycerol at t = 180 or t = 210 min. ¹³C-labeling patterns of serum glucose and TG-glycerol were determined by nuclear magnetic resonance. ¹³C enrichment in plasma TG-glycerol was detectable and stable from 240 to 390 min with the [U-¹³C₃]glycerol drink at t = 180 min(3.65 ± 2.3 to 4.47 ± 1.4%; P > 0.4), but the enrichment was undetectable at 240 min with the glycerol drink at t = 210 min. The relative contribution from anaplerosis was determined at the end of the OSTT [18.5 ±3.4% (t = 180 min) vs. 16.0 ± 3.5% (t = 210 min); P = 0.27]. [U-¹³C₃]glycerol was incorporated into GNG 390 min after the OSTT with an enrichment of 7.5-12.5%. Glucose derived from TCA cycle activity was 0.3-1%, and the PPP activity was 2.8-4.7%. In conclusion, it is possible to obtain relative measurements of hepatic anaplerotic contribution to both GNG and TG esterification following an OSTT in a highly insulin-resistant population using a minimally invasive technique. Tracer administration should be timed to allow enough de novo TG esterification and endogenous glucose release after the sugar drink.

Molecular imaging of diabetes and diabetic complications: Beyond pancreatic β-cell targeting

Diabetes is a chronic non-communicable disease affecting over 400 million people worldwide. Diabetic patients are at a high risk of various complications, such as cardiovascular, renal, and other diseases. The pathogenesis of diabetes (both type 1 and type 2 diabetes) is associated with a functional impairment of pancreatic β-cells. Consequently, most efforts to manage and prevent diabetes have focused on preserving β-cells and their function. Advances in imaging techniques, such as magnetic resonance imaging, magnetic resonance spectroscopy, positron emission tomography, and single-photon-emission computed tomography, have enabled noninvasive and quantitative detection and characterization of the population and function of β-cells in vivo. These advantages aid in defining and monitoring the progress of diabetes and determining the efficacy of anti-diabetic therapies. Beyond β-cell targeting, molecular imaging of biomarkers associated with the development of diabetes, e.g., lymphocyte infiltration, insulitis, and metabolic changes, may also be a promising strategy for early detection of diabetes, monitoring its progression, and occurrence of complications, as well as facilitating exploration of new therapeutic interventions. Moreover, molecular imaging of glucose uptake, production and excretion in specified tissues is critical for understanding the pathogenesis of diabetes. In the current review, we summarize and discuss recent advances in noninvasive imaging technologies for imaging of biomarkers beyond β-cells for early diagnosis of diabetes, investigation of glucose metabolism, and precise diagnosis and monitoring of diabetic complications for better management of diabetic patients.

Upstream signalling of mTORC1 and its hyperactivation in type 2 diabetes (T2D)

Mammalian target of rapamycin complex 1 (mTORC1) plays a major role in cell growth, proliferation, polarity, differentiation, development, and controls transitioning between anabolic and catabolic states of the cell. It collects almost all extracellular and intracellular signals from growth factors, nutrients, and maintains cellular homeostasis, and is involved in several pathological conditions including, neurodegeneration, Type 2 diabetes (T2D), obesity, and cancer. In this review, we summarize current knowledge of upstream signaling of mTORC1 to explain etiology of T2D and hypertriglyceridemia, in which state, the role of telomere attrition is explained. We discuss if chronic inhibition of mTORC1 can reverse adverse effects resulting from hyperactivation. In conclusion, we suggest the regulatory roles of telomerase (TERT) and hexokinase II (HKII) on mTORC1 as possible remedies to treat hyperactivation. The former inhibits mTORC1 under nutrient-rich while the latter under starved condition. We provide an idea of TOS (TOR signaling) motifs that can be used for regulation of mTORC1.

Fatty liver disrupts glycerol metabolism in gluconeogenic and lipogenic pathways in humans

It is a challenge to assess metabolic dysregulation in fatty liver of human patients prior to clinical manifestations. Here, we recruited obese, but otherwise healthy, subjects to examine biochemical processes in the liver with simple triglyceride accumulation using stable isotopes and NMR analysis of metabolic products in blood. Intrahepatic triglycerides were measured using ¹H magnetic resonance spectroscopy, and volunteers received ²H₂O and [U-¹³C₃]glycerol orally, followed by a series of blood draws. NMR analysis of plasma triglycerides and glucose provided detailed information about metabolic pathways in patients with simple hepatic steatosis. Compared with subjects with low hepatic fat, patients with hepatic steatosis were characterized by the following: lower ¹³C enrichments in the glycerol backbones of triglycerides (i.e., TG-[¹³C]glycerol), higher [U-¹³C₃]glycerol metabolism through the tricarboxylic acid (TCA) cycle, delayed gluconeogenesis from [U-¹³C₃]glycerol, and less flexibility in adjusting supporting fluxes of glucose production upon an oral load of glycerol. In summary, simple hepatic steatosis was associated with enhanced [U-¹³C₃]glycerol metabolism through pathways that intersect the TCA cycle and delayed gluconeogenesis from glycerol.

Fatty Acid Metabolic Remodeling During Type 2 Diabetes Remission After Bariatric Surgery

Hypertrophic remodeling of white adipose tissues is associated with overexposure of lean organs to circulating triglycerides (TGs) and nonesterified fatty acids (NEFAs), ultimately leading to insulin resistance. Bariatric surgery promotes type 2 diabetes (T2D) remission through a succession of weight loss-dependent and -independent mechanisms. However, the longitudinal contribution of adipocyte size reduction and fatty acid metabolic handling remain unknown. Here we show that severely obese participants with T2D display hypertriglyceridemia and excessive systemic lipolysis during intravenous lipid overload. Three days after biliopancreatic diversion with duodenal switch (DS), whole-body glycerol turnover was normalized and associated with lower HOMA-insulin resistance index. A mean excess weight loss of 84% was achieved 12 months after DS. The smaller subcutaneous adipocyte size predicted better glycemic control in T2D. TG disposal and acylcarnitine production during lipid overload, along with muscle insulin sensitivity, improved with weight loss. Nevertheless, systemic NEFA fluxes and NEFA spillover remained similar, suggesting that increased NEFA storage capacity per volume of adipose tissue exactly compensated for the decrease in fat mass during weight loss. In conclusion, T2D remission after DS is mainly associated with greater circulating TG disposal, lower systemic lipolysis, and better fatty acid handling by lean tissues.

Inter-relations between 3-hydroxypropionate and propionate metabolism in rat liver: relevance to disorders of propionyl-CoA metabolism

Propionate, 3-hydroxypropionate (3HP), methylcitrate, related compounds, and ammonium accumulate in body fluids of patients with disorders of propionyl-CoA metabolism, such as propionic acidemia. Although liver transplantation alleviates hyperammonemia, high concentrations of propionate, 3HP, and methylcitrate persist in body fluids. We hypothesized that conserved metabolic perturbations occurring in transplanted patients result from the simultaneous presence of propionate and 3HP in body fluids. We investigated the inter-relations of propionate and 3HP metabolism in perfused livers from normal rats using metabolomic and stable isotopic technologies. In the presence of propionate, 3HP, or both, we observed the following metabolic perturbations. First, the citric acid cycle (CAC) is overloaded but does not provide sufficient reducing equivalents to the respiratory chain to maintain the homeostasis of adenine nucleotides. Second, there is major CoA trapping in the propionyl-CoA pathway and a tripling of liver total CoA within 1 h. Third, liver proteolysis is stimulated. Fourth, propionate inhibits the conversion of 3HP to acetyl-CoA and its oxidation in the CAC. Fifth, some propionate and some 3HP are converted to nephrotoxic maleate by different processes. Our data have implications for the clinical management of propionic acidemia. They also emphasize the perturbations of the liver intermediary metabolism induced by supraphysiological, i.e., millimolar, concentrations of labeled propionate used to trace the intermediary metabolism, in particular, inhibition of CAC flux and major decreases in the [ATP]/[ADP] and [ATP]/[AMP] ratios.

Stable isotope-based flux studies in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease and is associated with the worldwide epidemics of obesity, diabetes and cardiovascular diseases. NAFLD ranges from benign fat accumulation in the liver (steatosis) to non-alcoholic steatohepatitis (NASH), and cirrhosis which can progress to hepatocellular carcinoma and liver failure. Mass spectrometry and magnetic resonance spectroscopy-coupled stable isotope-based flux studies provide new insights into the understanding of NAFLD pathogenesis and the disease progression. This review focuses mainly on the utilization of mass spectrometry-based methods for the understanding of metabolic abnormalities in the different stages of NAFLD. For example, stable isotope-based flux studies demonstrated multi-organ insulin resistance, dysregulated glucose, lipids and lipoprotein metabolism in patients with NAFLD. We also review recent developments in the stable isotope-based technologies for the study of mitochondrial dysfunction, oxidative stress and fibrogenesis in NAFLD. We highlight the limitations of current methodologies, discuss the emerging areas of research in this field, and future directions for the applications of stable isotopes to study NAFLD and its complications.

Liver fat: a relevant target for dietary intervention? Summary of a Unilever workshop

Currently it is estimated that about 1 billion people globally have non-alcoholic fatty liver disease (NAFLD), a condition in which liver fat exceeds 5 % of liver weight in the absence of significant alcohol intake. Due to the central role of the liver in metabolism, the prevalence of NAFLD is increasing in parallel with the prevalence of obesity, insulin resistance and other risk factors of metabolic diseases. However, the contribution of liver fat to the risk of type 2 diabetes mellitus and CVD, relative to other ectopic fat depots and to other risk markers, is unclear. Various studies have suggested that the accumulation of liver fat can be reduced or prevented via dietary changes. However, the amount of liver fat reduction that would be physiologically relevant, and the timeframes and dose-effect relationships for achieving this through different diet-based approaches, are unclear. Also, it is still uncertain whether the changes in liver fat per se or the associated metabolic changes are relevant. Furthermore, the methods available to measure liver fat, or even individual fatty acids, differ in sensitivity and reliability. The present report summarises key messages of presentations from different experts and related discussions from a workshop intended to capture current views and research gaps relating to the points above.

Evidence that supports the prescription of low-carbohydrate high-fat diets: a narrative review

Low-carbohydrate high-fat (LCHF) diets are a highly contentious current topic in nutrition. This narrative review aims to provide clinicians with a broad overview of the effects of LCHF diets on body weight, glycaemic control and cardiovascular risk factors while addressing some common concerns and misconceptions. Blood total cholesterol and LDL-cholesterol concentrations show a variable, highly individual response to LCHF diets, and should be monitored in patients adhering to this diet. In contrast, available evidence from clinical and preclinical studies indicates that LCHF diets consistently improve all other markers of cardiovascular risk-lowering elevated blood glucose, insulin, triglyceride, ApoB and saturated fat (especially palmitoleic acid) concentrations, reducing small dense LDL particle numbers, glycated haemoglobin (HbA_1c) levels, blood pressure and body weight while increasing low HDL-cholesterol concentrations and reversing non-alcoholic fatty liver disease (NAFLD). This particular combination of favourable modifications to all these risk factors is a benefit unique to LCHF diets. These effects are likely due in part to reduced hunger and decreased ad libitum calorie intake common to low-carbohydrate diets, allied to a reduction in hyperinsulinaemia, and reversal of NAFLD. Although LCHF diets may not be suitable for everyone, available evidence shows this eating plan to be a safe and efficacious dietary option to be considered. LCHF diets may also be particularly beneficial in patients with atherogenic dyslipidaemia, insulin resistance, and the frequently associated NAFLD.

Effects of visceral adiposity on glycerol pathways in gluconeogenesis

OBJECTIVE

To determine the feasibility of using oral ¹³C labeled glycerol to assess effects of visceral adiposity on gluconeogenic pathways in obese humans.

RESEARCH DESIGN AND METHODS

Obese (BMI ≥30kg/m²) participants without type 2 diabetes underwent visceral adipose tissue (VAT) assessment and stratification by median VAT into high VAT-fasting (n=3), low VAT-fasting (n=4), and high VAT-refed (n=2) groups. Participants ingested [U-¹³C₃] glycerol and blood samples were subsequently analyzed at multiple time points over 3h by NMR spectroscopy. The fractions of plasma glucose (enrichment) derived from [U-¹³C₃] glycerol via hepatic gluconeogenesis, pentose phosphate pathway (PPP), and tricarboxylic acid (TCA) cycle were assessed using ¹³C NMR analysis of glucose. Mixed linear models were used to compare ¹³C enrichment in glucose between groups.

RESULTS

Mean age, BMI, and baseline glucose were 49years, 40.1kg/m², and 98mg/dl, respectively. Up to 20% of glycerol was metabolized in the TCA cycle prior to gluconeogenesis and PPP activity was minor (<1% of total glucose) in all participants. There was a 21% decrease in ¹³C enrichment in plasma glucose in the high VAT-fasting compared with low VAT-fasting group (p=0.03), suggesting dilution by endogenous glycerol. High VAT-refed participants had 37% less ¹³C enrichment in glucose compared with high VAT-fasting (p=0.02). There was a trend toward lower [1,2-¹³C₂] (via PPP) and [5,6-¹³C₂]/[4,5,6-¹³C₃] (via TCA cycle) glucose in high VAT versus low VAT groups.

CONCLUSIONS

We applied a simple method to detect gluconeogenesis from glycerol in obese humans. Our findings provide preliminary evidence that excess visceral fat disrupts multiple pathways in hepatic gluconeogenesis from glycerol.

Phenotypical heterogeneity linked to adipose tissue dysfunction in patients with Type 2 diabetes

Adipose tissue (AT) inflammation leads to increased free fatty acid (FFA) efflux and ectopic fat deposition, but whether AT dysfunction drives selective fat accumulation in specific sites remains unknown. The aim of the present study was to investigate the correlation between AT dysfunction, hepatic/pancreatic fat fraction (HFF, PFF) and the associated metabolic phenotype in patients with Type 2 diabetes (T2D). Sixty-five consecutive T2D patients were recruited at the Diabetes Centre of Sapienza University, Rome, Italy. The study population underwent clinical examination and blood sampling for routine biochemistry and calculation of insulin secretion [homoeostasis model assessment of insulin secretion (HOMA-β%)] and insulin-resistance [homoeostasis model assessment of insulin resistance (HOMA-IR) and adipose tissue insulin resistance (ADIPO-IR)] indexes. Subcutaneous (SAT) and visceral (VAT) AT area, HFF and PFF were determined by magnetic resonance. Some 55.4% of T2D patients had non-alcoholic fatty liver disease (NAFLD); they were significantly younger and more insulin-resistant than non-NAFLD subjects. ADIPO-IR was the main determinant of HFF independently of age, sex, HOMA-IR, VAT, SAT and predicted severe NAFLD with the area under the receiver operating characteristic curve (AUROC)=0.796 (95% confidence interval: 0.65-0.94, P=0.001). PFF was independently associated with increased total adiposity but did not correlate with AT dysfunction, insulin resistance and secretion or NAFLD. The ADIPO-IR index was capable of predicting NAFLD independently of all confounders, whereas it did not seem to be related to intrapancreatic fat deposition; unlike HFF, higher PFF was not associated with relevant alterations in the metabolic profile. In conclusion, the presence and severity of AT dysfunction may drive ectopic fat accumulation towards specific targets, such as VAT and liver, therefore evaluation of AT dysfunction may contribute to the identification of different risk profiles among T2D patients.

An Oral Load of [13C3]Glycerol and Blood NMR Analysis Detect Fatty Acid Esterification, Pentose Phosphate Pathway, and Glycerol Metabolism through the Tricarboxylic Acid Cycle in Human Liver

Drugs and other interventions for high impact hepatic diseases often target biochemical pathways such as gluconeogenesis, lipogenesis, or the metabolic response to oxidative stress. However, traditional liver function tests do not provide quantitative data about these pathways. In this study, we developed a simple method to evaluate these processes by NMR analysis of plasma metabolites. Healthy subjects ingested [U-(13)C3]glycerol, and blood was drawn at multiple times. Each subject completed three visits under differing nutritional states. High resolution (13)C NMR spectra of plasma triacylglycerols and glucose provided new insights into a number of hepatic processes including fatty acid esterification, the pentose phosphate pathway, and gluconeogenesis through the tricarboxylic acid cycle. Fasting stimulated pentose phosphate pathway activity and metabolism of [U-(13)C3]glycerol in the tricarboxylic acid cycle prior to gluconeogenesis or glyceroneogenesis. Fatty acid esterification was transient in the fasted state but continuous under fed conditions. We conclude that a simple NMR analysis of blood metabolites provides an important biomarker of pentose phosphate pathway activity, triacylglycerol synthesis, and flux through anaplerotic pathways in mitochondria of human liver.

β2-Adrenergic receptor ablation modulates hepatic lipid accumulation and glucose tolerance in aging mice

Catecholamines acting through β-adrenergic receptors (β(1)-, β(2)-, β(3)-AR subtypes) modulate important biological responses in various tissues. Our previous studies suggest a role for increased hepatic β-AR-mediated signaling during aging as a mediator of hepatic steatosis, liver glucose output, and insulin resistance in rodents. In the current study, we have utilized β(2)-AR knockout (KO) and wildtype (WT) control mice to define further the role of β(2)-AR signaling during aging on lipid and glucose metabolism. Our results demonstrate for the first time that age-related increases in hepatic triglyceride accumulation and body weight are attenuated upon β(2)-AR ablation. Although no differences in plasma triglyceride, non-esterified fatty acids or insulin levels were detected between old WT and KO animals, an age-associated increase in hepatic expression of lipid homeostasis regulator Cidea was significantly reduced in old KO mice. Interestingly, we also observed a shift from reduced glucose tolerance in young adult KO animals to significantly improved glucose tolerance in old KO when compared to age-matched WT mice. These results provide evidence for an important role played by β(2)-ARs in the regulation of lipid and glucose metabolism during aging. The effect of β(2)-AR ablation on caloric intake during aging is currently not known and requires investigation. Future studies are also warranted to delineate the β(2)-AR-mediated mechanisms involved in the control of lipid and glucose homeostasis, especially in the context of a growing aging population.

Hepatic Steatosis as a Marker of Metabolic Dysfunction

Nonalcoholic fatty liver disease (NAFLD) is the liver manifestation of the complex metabolic derangements associated with obesity. NAFLD is characterized by excessive deposition of fat in the liver (steatosis) and develops when hepatic fatty acid availability from plasma and de novo synthesis exceeds hepatic fatty acid disposal by oxidation and triglyceride export. Hepatic steatosis is therefore the biochemical result of an imbalance between complex pathways of lipid metabolism, and is associated with an array of adverse changes in glucose, fatty acid, and lipoprotein metabolism across all tissues of the body. Intrahepatic triglyceride (IHTG) content is therefore a very good marker (and in some cases may be the cause) of the presence and the degree of multiple-organ metabolic dysfunction. These metabolic abnormalities are likely responsible for many cardiometabolic risk factors associated with NAFLD, such as insulin resistance, type 2 diabetes mellitus, and dyslipidemia. Understanding the factors involved in the pathogenesis and pathophysiology of NAFLD will lead to a better understanding of the mechanisms responsible for the metabolic complications of obesity, and hopefully to the discovery of novel effective treatments for their reversal.

The role of short chain fatty acids in appetite regulation and energy homeostasis

Over the last 20 years there has been an increasing interest in the influence of the gastrointestinal tract on appetite regulation. Much of the focus has been on the neuronal and hormonal relationship between the gastrointestinal tract and the brain. There is now mounting evidence that the colonic microbiota and their metabolic activity have a significant role in energy homeostasis. The supply of substrate to the colonic microbiota has a major impact on the microbial population and the metabolites they produce, particularly short chain fatty acids (SCFAs). SCFAs are produced when non-digestible carbohydrates, namely dietary fibres and resistant starch, undergo fermentation by the colonic microbiota. Both the consumption of fermentable carbohydrates and the administration of SCFAs have been reported to result in a wide range of health benefits including improvements in body composition, glucose homeostasis, blood lipid profiles and reduced body weight and colon cancer risk. However, published studies tend to report the effects that fermentable carbohydrates and SCFAs have on specific tissues and metabolic processes, and fail to explain how these local effects translate into systemic effects and the mitigation of disease risk. Moreover, studies tend to investigate SCFAs collectively and neglect to report the effects associated with individual SCFAs. Here, we bring together the recent evidence and suggest an overarching model for the effects of SCFAs on one of their beneficial aspects: appetite regulation and energy homeostasis.