Dissociation between fatty liver and insulin resistance: the role of adipose triacylglycerol lipase

The effect of weight loss on whole-body and tissue-specific insulin sensitivity and hepatic lipid content and composition: SWEET substudy

OBJECTIVE

This study (1) investigated the effect of weight loss on whole-body and tissue-specific insulin sensitivity and on intrahepatic lipid (IHL) content and composition and (2) investigated the association between weight-loss-induced changes in insulin sensitivity and IHL content in individuals with overweight or obesity.

METHODS

In this secondary analysis of the European SWEET project, 50 adults (age 18-65 years) with overweight or obesity (BMI ≥ 25 kg/m² ) followed a low-energy diet (LED) for 2 months. At baseline and after the LED, body composition (dual-energy x-ray absorptiometry), IHL content and composition (proton magnetic resonance spectroscopy), whole-body insulin sensitivity (Matsuda index), muscle insulin sensitivity index (MISI), and hepatic insulin resistance index (HIRI) were determined (7-point oral glucose tolerance test).

RESULTS

The LED reduced body weight (p < 0.001). This was accompanied by increased Matsuda index and reduced HIRI (both p < 0.001) but no change in MISI (p = 0.260). Weight loss decreased IHL content (mean [SEM], 3.9% [0.7%] vs. 1.6% [0.5%], p < 0.001) and the hepatic saturated fatty acid fraction (41.0% [1.5%] vs. 36.6% [1.9%], p = 0.039). The reduced IHL content was associated with an improvement in HIRI (r = 0.402, p = 0.025).

CONCLUSIONS

Weight loss decreased IHL content and the hepatic saturated fatty acid fraction. The decrease in IHL content was associated with weight-loss-induced improvement in hepatic insulin sensitivity in individuals with overweight or obesity.

TRIM21 ameliorates hepatic glucose and lipid metabolic disorders in type 2 diabetes mellitus by ubiquitination of PEPCK1 and FASN

Hepatic glucose and lipid metabolism disorders promote the development and progression of type 2 diabetes mellitus (T2DM), yet the underlying mechanisms are not fully understood. Here, we identify tripartite motif-containing protein 21 (TRIM21), a class IV TRIM family member, as a pivotal regulator of hepatic metabolism in T2DM for the first time. Bioinformatic analysis suggests that TRIM21 expression is significantly reduced in T2DM patients. Intriguingly, in a mouse model of obese diabetes, TRIM21 expression is predominantly reduced in the liver rather than in other metabolic organs. It is further demonstrated that hepatic overexpression of TRIM21 significantly ameliorates glucose intolerance, insulin resistance, hepatic steatosis, and dyslipidemia in obese diabetic mice. In contrast, the knockdown of TRIM21 promotes glucose intolerance, insulin resistance, and triglyceride accumulation. Mechanistically, both phosphoenolpyruvate carboxykinase 1 (PEPCK1) and fatty acid synthase (FASN) are the hepatic targets of TRIM21. We revealed that TRIM21 promotes the degradation of PEPCK1 and FASN through a direct protein-protein interaction mediated K48-linked ubiquitination. Notably, overexpression of PEPCK1 and FASN essentially abolished the beneficial effects achieved by TRIM21 overexpression in obese diabetic mice. Overall, our data demonstrate that TRIM21 is a novel regulator of hepatic metabolic disorder, and suggest TRIM21 as a promising therapeutic target for T2DM.

Effect of newer antihyperglycemic drugs on liver steatosis indices in patients with diabetes and obesity

AIM

To assess the efficacy of sodium-glucose cotransporter-2 inhibitor (SGLT2i) and glucagon-like peptide-1 receptors agonist (GLP-1RA) therapy on liver steatosis measured by fatty liver index (FLI) and hepatic steatosis index (HSI) at 26 weeks in outpatients with diabetes and obesity.

METHODS

Observational, prospective, multicenter study. Patients with steatosis determined by FLI (values <30 rule out and >60 indicate steatosis) and HIS (values <30 rule out and >36 indicate steatosis) who received combination therapy were included. Patients were stratified into three groups according to the sequential order of treatment. We used robust statistical methods.

RESULTS

In our final report we included 174 patients (58.6% males), mean age 61.9 (10) years. Baseline body mass index, waist circumference and weight were 36.5 (6.8) kg/m², 117.5 (15.1) cm and 99.4 (20.5) kg, respectively. One hundred percent of patients had altered biomarkers of fatty liver scores (FLI 96 [13] and HSI 49.2 [8.5]). At 26 weeks, significant reductions in FLI (-4.5 [95% CI 3.5-5.9] p < .001) and HSI (-2.4 [95% CI 1.6-3.2] p < .001) were found in the total sample and pre-specified treatment and FLI cut-off point subgroups.

CONCLUSION

Our results show a beneficial effect of the combination of GLP-1RAs plus SGLT2is on liver steatosis that goes beyond glucose control, and it is related mainly to weight loss, a decline in biomarkers and reductions in abdominal circumference. For many patients, early detection is essential to improving outcomes in NAFLD and could allow us to select the most efficient treatment options.

NAFLD and Diabetes: Two Sides of the Same Coin? Rationale for Gene-Based Personalized NAFLD Treatment

The prevalence of non-alcoholic fatty liver disease (NAFLD) has been increasing rapidly and at the forefront of worldwide concern. Characterized by excessive fat accumulation in the liver, NAFLD regularly coexists with metabolic disorders, including type 2 diabetes, obesity, and cardiovascular disease. It has been well established that the presence of NAFLD increases the incidence of type 2 diabetes, while diabetes aggravates NAFLD to more severe forms of steatohepatitis, cirrhosis, and hepatocellular carcinoma. However, recent progress on the genotype/phenotype relationships in NAFLD patients indicates the development of NAFLD with a relative conservation of glucose metabolism in individuals with specific gene variants, such as the patatin-like phospholipase domain-containing 3 (PNPLA3) and transmembrane 6 superfamily member 2 protein (TM6SF2) variants. This review will focus on the clinical and pathophysiological connections between NAFLD and type 2 diabetes and will also discuss a disproportionate progression of NAFLD and diabetes, and the different responses to lifestyle and drug intervention in NAFLD patients with specific gene variants that may give insight into personalized treatment for NAFLD.

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.

The TRIB3 Q84R polymorphism, insulin resistance and related metabolic alterations

Insulin resistance is pathogenic for many prevalent disorders including type 2 diabetes mellitus (T2DM), cardiovascular disease (CVD), polycystic ovary syndrome, non-alcoholic fatty liver disease, Alzheimer's and Parkinson's diseases and several cancers. Unravelling molecular abnormalities of insulin resistance may therefore pave the way for tackling such heavy weight on healthcare systems. This review will be focused on studies addressing the role of genetic variability of TRIB3, an inhibitor of insulin signalling at the AKT level on insulin resistance and several related abnormalities. Studies carried out in several cultured cells clearly report that the TRIB3 Q84R missense polymorphism, is a gain-of-function amino acid substitution, with the Arg(84) variant being a stronger inhibitor of insulin-mediated AKT activation as compared with the more frequent Gln(84) variant. Given the key role of AKT in modulating not only insulin signalling but also insulin secretion, it was not surprising that β-cells and human pancreatic islets carrying the Arg(84) variant showed also impaired insulin secretion. Also, of note is that in human vein endothelial cells carrying the Arg(84) variant showed a reduced insulin-induced nitric oxide release, an established early atherosclerotic step. Accordingly with in vitro studies, in vivo studies indicate that TRIB3 Arg(84) is associated with insulin resistance, T2DM and several aspects of atherosclerosis, including overt CVD. In all, several data indicate that the TRIB3 Arg(84) variant plays a role on several aspects of glucose homoeostasis and atherosclerotic processes, thus unravelling new molecular pathogenic mechanisms of highly prevalent disorders such as T2DM and CVD.

Effects of Choline on DNA Methylation and Macronutrient Metabolic Gene Expression in In Vitro Models of Hyperglycemia

Choline is an essential nutrient that plays an important role in lipid metabolism and DNA methylation. Studies in rodents suggest that choline may adversely affect glycemic control, yet studies in humans are lacking. Using the human hepatic and placental cells, HepG2 and BeWo, respectively, we examined the interaction between choline and glucose treatments. In HepG2 cells, choline supplementation (1 mM) increased global DNA methylation and DNA methyltransferase expression in both low-glucose (5 mM) and high-glucose (35 mM) conditions. Choline supplementation increased the expression of peroxisomal acyl-coenzyme A oxidase 1 (ACOX1), which mediates fatty acid β-oxidation, especially in the high-glucose condition. High-glucose exposure increased the transcription of the gluconeogenic gene phosphoenolpyruvate carboxykinase (PEPCK), while choline supplementation mitigated such increase. Compared to HepG2 cells, the placenta-derived BeWo cells were relatively unresponsive to either high-glucose or -choline treatment. In conclusion, choline and glucose interacted to affect macronutrient metabolic genes, yet there was no indication that choline may worsen glycemic control in these in vitro human cell culture models.

Bidirectional Relationships and Disconnects between NAFLD and Features of the Metabolic Syndrome

Non-alcoholic fatty liver disease (NAFLD) represents a wide spectrum of liver disease from simple steatosis, to steatohepatitis, (both with and without liver fibrosis), cirrhosis and end-stage liver failure. NAFLD also increases the risk of hepatocellular carcinoma (HCC) and both HCC and end stage liver disease may markedly increase risk of liver-related mortality. NAFLD is increasing in prevalence and is presently the second most frequent indication for liver transplantation. As NAFLD is frequently associated with insulin resistance, central obesity, dyslipidaemia, hypertension and hyperglycaemia, NAFLD is often considered the hepatic manifestation of the metabolic syndrome. There is growing evidence that this relationship between NAFLD and metabolic syndrome is bidirectional, in that NAFLD can predispose to metabolic syndrome features, which can in turn exacerbate NAFLD or increase the risk of its development in those without a pre-existing diagnosis. Although the relationship between NAFLD and metabolic syndrome is frequently bidirectional, recently there has been much interest in genotype/phenotype relationships where there is a disconnect between the liver disease and metabolic syndrome features. Such potential examples of genotypes that are associated with a dissociation between liver disease and metabolic syndrome are patatin-like phospholipase domain-containing protein-3 (PNPLA3) (I148M) and transmembrane 6 superfamily member 2 protein (TM6SF2) (E167K) genotypes. This review will explore the bidirectional relationship between metabolic syndrome and NAFLD, and will also discuss recent insights from studies of PNPLA3 and TM6SF2 genotypes that may give insight into how and why metabolic syndrome features and liver disease are linked in NAFLD.

[Role of metabolic lipases and lipotoxicity in the development of non-alcoholic steatosis and non-alcoholic steatohepatitis]

Non-alcoholic fatty liver disease (NAFLD) has become the most common liver disease in developed countries, covering a spectrum of pathological conditions ranging from single steatosis to non-alcoholic steatohepatitis, cirrhosis and hepatocellular carcinoma. Its pathogenesis has been often interpreted by the "double-hit" hypothesis, where the lipid accumulation in the liver is followed by proinflammatory mediators inducing inflammation, hepatocellular injury and fibrosis. Nowadays, a more complex model suggests that free fatty acids and their metabolites could be the true lipotoxic agents that contribute to the development of NAFLD and hepatic insulin resistance, suggesting a central role for metabolic lipases in that process.

Nonalcoholic fatty liver disease and type 2 diabetes: common pathophysiologic mechanisms

Nonalcoholic fatty liver disease (NAFLD) is an independent risk factor for advanced liver disease, type 2 diabetes (T2DM), and cardiovascular diseases. The prevalence of NAFLD in the general population is around 30 %, but it is up to three times higher in those with T2DM. Among people with obesity and T2DM, the NAFLD epidemic also is worsening. Therefore, it is important to identify early metabolic alterations and to prevent these diseases and their progression. In this review, we analyze the pathophysiologic mechanisms leading to NAFLD, particularly, those common to T2DM, such as liver and muscle insulin resistance. However, it is mainly adipose tissue insulin resistance that results in increased hepatic de novo lipogenesis, inflammation, and lipotoxicity. Although genetics predispose to NAFLD, an unhealthy lifestyle, including high-fat/high-sugar diets and low physical activity, increases the risk. In addition, alterations in gut microbiota and environmental chemical agents, acting as endocrine disruptors, may play a role.

Role of metabolic lipases and lipolytic metabolites in the pathogenesis of NAFLD

Non-alcoholic fatty liver disease (NAFLD) is the most frequent chronic liver disease in Western countries, ranging from simple steatosis to steatohepatitis, cirrhosis, and hepatocellular cancer. Although the mechanisms underlying disease progression are incompletely understood, lipotoxic events in the liver resulting in inflammation and fibrosis appear to be central. Free fatty acids and their metabolites are potentially lipotoxic mediators triggering liver injury, suggesting a central role for metabolic lipases. These enzymes are major players in lipid partitioning between tissues and within cells, and provide ligands for nuclear receptors (NRs). We discuss the potential role of intracellular lipases and their lipolytic products in NAFLD. Because tissue-specific modulation of lipases is currently impossible, targeting NRs with ligands may open novel therapeutic perspectives.

Role of mitochondria in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) affects about 30% of the general population in the United States and includes a spectrum of disease that includes simple steatosis, non-alcoholic steatohepatitis (NASH), fibrosis and cirrhosis. Significant insight has been gained into our understanding of the pathogenesis of NALFD; however the key metabolic aberrations underlying lipid accumulation in hepatocytes and the progression of NAFLD remain to be elucidated. Accumulating and emerging evidence indicate that hepatic mitochondria play a critical role in the development and pathogenesis of steatosis and NAFLD. Here, we review studies that document a link between the pathogenesis of NAFLD and hepatic mitochondrial dysfunction with particular focus on new insights into the role of impaired fatty acid oxidation, the transcription factor peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), and sirtuins in development and progression of NAFLD.

Resolution of fatty liver and risk of incident diabetes

CONTEXT

Fatty liver is associated with an increased risk of type 2 diabetes, but whether an increased risk remains in people in whom fatty liver resolves over time is not known.

OBJECTIVE

The objective of the study was to assess the risk of incident diabetes at a 5-year follow-up in people in whom: 1) new fatty liver developed; 2) existing fatty liver resolved, and 3) fatty liver severity worsened over 5 years.

DESIGN AND METHODS

A total of 13,218 people without diabetes at baseline from a Korean occupational cohort were examined at baseline and after 5 years, using a retrospective study design. Fatty liver status was assessed at baseline and follow-up as absent, mild, or moderate/severe using standard ultrasound criteria. Adjusted odds ratios (aORs) and 95% confidence intervals (CIs) for incident diabetes at follow-up were estimated after controlling for multiple potential confounders.

RESULTS

Two hundred thirty-four people developed incident diabetes. Over 5 years, fatty liver resolved in 828, developed in 1640, and progressed from mild to moderate/severe in 324 people. Resolution of fatty liver was not associated with a risk of incident diabetes [aOR 0.95 (95% CIs 0.46, 1.96), P = .89]. Development of new fatty liver was associated with incident diabetes [aOR 2.49 (95% CI 1.49, 4.14), P < .001]. In individuals in whom severity of fatty liver worsened over 5 years (from mild to moderate/severe), there was a marked increase in the risk of incident diabetes [aOR 6.13 (2.56, 95% CI 14.68) P < .001 (compared with the risk in people with resolution of fatty liver)].

CONCLUSION

Change in fatty liver status over time is associated with markedly variable risks of incident diabetes.

Liver and diabetes. A vicious circle

The complex and bi-directional relationship linking the liver and diabetes has recently gained intense new interest. This critical review of the published work aims to highlight the most recent basic and clinical data underlying the development of type 2 diabetes, in those with non-alcoholic fatty liver disease. Moreover, the potentially detrimental effects of type 2 diabetes in liver injury are also discussed in each of the two sections of the present paper. Fatty liver and diabetes share insulin resistance as their chief pathogenic determinant. The roles of the hypothalamus, the intestinal microbiome, white adipose tissue and inflammation are discussed in detail. Molecular insights into hepatocyte insulin resistance as the initiator of systemic insulin resistance are also presented with full coverage of the danger of fatty acids. Lipotoxicity, apoptosis, lipoautophagy, endoplasmic reticular stress response and recent developments in genetics are discussed. Closing the circle, special emphasis is given to biochemical pathways and clinical evidence supporting the role of type 2 diabetes as a risk factor for the development of progressive liver disease, including non-alcoholic steatohepatitis, cirrhosis and primary liver cancer. In conclusion, data support non-alcoholic fatty liver disease as a risk factor for the development of type 2 diabetes which is, in turn, a major contributor to progressive liver disease. This pathway leading from fatty liver to type 2 diabetes and back from the latter to the progressive liver disease is a vicious circle.

The mammalian tribbles homolog TRIB3, glucose homeostasis, and cardiovascular diseases

Insulin signaling plays a physiological role in traditional insulin target tissues controlling glucose homeostasis as well as in pancreatic β-cells and in the endothelium. Insulin signaling abnormalities may, therefore, be pathogenic for insulin resistance, impaired insulin secretion, endothelial dysfunction, and eventually, type 2 diabetes mellitus (T2DM) and cardiovascular disease. Tribbles homolog 3 (TRIB3) is a 45-kDa pseudokinase binding to and inhibiting Akt, a key mediator of insulin signaling. Akt-mediated effects of TRIB3 in the liver, pancreatic β-cells, and skeletal muscle result in impaired glucose homeostasis. TRIB3 effects are also modulated by its direct interaction with other signaling molecules. In humans, TRIB3 overactivity, due to TRIB3 overexpression or to Q84R genetic polymorphism, with R84 being a gain-of-function variant, may be involved in shaping the risk of insulin resistance, T2DM, and cardiovascular disease. TRIB3 overexpression has been observed in the liver, adipose tissue, skeletal muscle, and pancreatic β-cells of individuals with insulin resistance and/or T2DM. The R84 variant has also proved to be associated with insulin resistance, T2DM, and cardiovascular disease. TRIB3 direct effects on the endothelium might also play a role in increasing the risk of atherosclerosis, as indicated by studies on human endothelial cells carrying the R84 variant that are dysfunctional in terms of Akt activation, NO production, and other proatherogenic changes. In conclusion, studies on TRIB3 have unraveled new molecular mechanisms underlying metabolic and cardiovascular abnormalities. Additional investigations are needed to verify whether such acquired knowledge will be relevant for improving care delivery to patients with metabolic and cardiovascular alterations.

Combined influence of insulin resistance, overweight/obesity, and fatty liver as risk factors for type 2 diabetes

OBJECTIVE

There is dissociation between insulin resistance, overweight/obesity, and fatty liver as risk factors for type 2 diabetes, suggesting that different mechanisms are involved. Our aim was to 1) quantify risk of incident diabetes at follow-up with different combinations of these risk factors at baseline and 2) determine whether each is an independent risk factor for diabetes.

RESEARCH DESIGN AND METHODS

We examined 12,853 subjects without diabetes from a South Korean occupational cohort, and insulin resistance (IR) (homeostasis model assessment-IR ≥75th centile, ≥2.0), fatty liver (defined by standard ultrasound criteria), and overweight/obesity (BMI ≥25 kg/m(2)) identified at baseline. Odds ratios (ORs) and 95% confidence intervals (CIs) for incident diabetes at 5-year follow-up were estimated using logistic regression.

RESULTS

We identified 223 incident cases of diabetes from which 26 subjects had none of the three risk factors, 37 had one, 56 had two, and 104 had three. In the fully adjusted model, the OR and CI for diabetes were 3.92 (2.86-5.37) for IR, 1.62 (1.17-2.24) for overweight/obesity, and 2.42 (1.74-3.36) for fatty liver. The OR for the presence of all three factors in a fully adjusted model was 14.13 (8.99-22.21).

CONCLUSIONS

The clustering of IR, overweight/obesity, and fatty liver is common and markedly increases the odds of developing type 2 diabetes, but these factors also have effects independently of each other and of confounding factors. The data suggest that treatment for each factor is needed to decrease risk of type 2 diabetes.

Ripened dairy products differentially affect hepatic lipid content and adipose tissue oxidative stress markers in obese and type 2 diabetic mice

Growing evidence suggests that the consumption of dairy products may contribute to a reduced incidence of cardiovascular risk factors, such as obesity, dyslipidemia, and type 2 diabetes. The fatty acid composition in milk fat, the duration of ripening, and the complexity of the food matrices are important factors that may interfere with the physiological impact. In this study, we treated genetic obese and type 2 diabetic mice (db/db) for 4 weeks with different dairy (cheese-based) products, differing by the duration of ripening (0, 15, or 35 days). We found that 35 days ripened product significantly improved glucose tolerance, an effect associated with a decreased adipose tissue lipid peroxide markers (TBARS and NAPDH-oxidase mRNA expression), without affecting body weight, food intake, and fat mass. Both fermented matrices significantly decreased the hepatic lipid content, without modifying plasma triglycerides or plasma total cholesterol. These data suggest that dairy products issued from longer ripening positively impact glucose tolerance, hepatic steatosis, and adipose tissue oxidative stress. Further investigations are warranted to decipher the interactions between milk products fermentation, lipids, and host metabolism.

Nonalcoholic Fatty liver disease: focus on lipoprotein and lipid deregulation

Obesity with associated comorbidities is currently a worldwide epidemic and among the most challenging health conditions in the 21st century. A major metabolic consequence of obesity is insulin resistance which underlies the pathogenesis of the metabolic syndrome. Nonalcoholic fatty liver disease (NAFLD) is the hepatic manifestation of obesity and metabolic syndrome. It comprises a disease spectrum ranging from simple steatosis (fatty liver), through nonalcoholic steatohepatitis (NASH) to fibrosis, and ultimately liver cirrhosis. Abnormality in lipid and lipoprotein metabolism accompanied by chronic inflammation is the central pathway for the development of metabolic syndrome-related diseases, such as atherosclerosis, cardiovascular disease (CVD), and NAFLD. This paper focuses on pathogenic aspect of lipid and lipoprotein metabolism in NAFLD and the relevant mouse models of this complex multifactorial disease.