M. Gaggini

Non-Alcoholic Fatty Liver Disease (NAFLD) and Its Connection with Insulin Resistance, Dyslipidemia, Atherosclerosis and Coronary Heart Disease

Non-alcoholic fatty liver disease is marked by hepatic fat accumulation not due to alcohol abuse. Several studies have demonstrated that NAFLD is associated with insulin resistance leading to a resistance in the antilipolytic effect of insulin in the adipose tissue with an increase of free fatty acids (FFAs). The increase of FFAs induces mitochondrial dysfunction and development of lipotoxicity. Moreover, in subjects with NAFLD, ectopic fat also accumulates as cardiac and pancreatic fat. In this review we analyzed the mechanisms that relate NAFLD with metabolic syndrome and dyslipidemia and its association with the development and progression of cardiovascular disease.

The Subtle Balance between Lipolysis and Lipogenesis: A Critical Point in Metabolic Homeostasis.

Excessive accumulation of lipids can lead to lipotoxicity, cell dysfunction and alteration in metabolic pathways, both in adipose tissue and peripheral organs, like liver, heart, pancreas and muscle. This is now a recognized risk factor for the development of metabolic disorders, such as obesity, diabetes, fatty liver disease (NAFLD), cardiovascular diseases (CVD) and hepatocellular carcinoma (HCC). The causes for lipotoxicity are not only a high fat diet but also excessive lipolysis, adipogenesis and adipose tissue insulin resistance. The aims of this review are to investigate the subtle balances that underlie lipolytic, lipogenic and oxidative pathways, to evaluate critical points and the complexities of these processes and to better understand which are the metabolic derangements resulting from their imbalance, such as type 2 diabetes and non alcoholic fatty liver disease.

Ectopic fat: the true culprit linking obesity and cardiovascular disease?

Obesity is a major risk factor for cardiovascular disease and its complications. However, not all fat depots share the same characteristics. Recent studies have found that ectopic rather than subcutaneous fat accumulation is associated with increased cardiometabolic risk. However, ectopic fat accumulation can be seen initially as a protective mechanism against lipotoxicity. Subsequently the adipose tissue becomes dysfunctional, thus inducing systemic metabolic alterations (through release of cytokines) or specific organ dysfunctions. The purpose of this review is to summarise the current available data on the impact of excess adiposity vs ectopic fat in the development of cardio-metabolic diseases.

Not all fats are created equal: adipose vs. ectopic fat, implication in cardiometabolic diseases.

Abstract Adipose tissue is a recognized endocrine organ that acts not only as a fuel storage but also is able to secrete adipokines that can modulate inflammation. Most of the fat is composed of white adipocytes (WAT), although also brown/beige adipocytes (BAT/BeAT) have been found in humans. BAT is located close to the neck but also among WAT in the epicardial fat and perivascular fat. Adipocyte hypertrophy and infiltration of macrophages impair adipose tissue metabolism determining “adiposopathy” (i.e., sick fat) and increasing the risk to develop metabolic and cardiovascular diseases. The purpose of this review was to search and discuss the available literature on the impact of different types of fat and fat distribution on cardiometabolic risk. Visceral fat, but also ectopic fat, either in liver, muscle and heart, can increase the risk to develop insulin resistance, type 2 diabetes and cardiovascular diseases. Results recently published showed that BAT could have an impact on cardiometabolic risk, not only because it is implicated in energy metabolism but also because it can modulate glucose and lipid metabolism. Therapeutical interventions that can increase energy expenditure, successfully change fat distribution and reduce ectopic fat, also through BAT activation, were discussed.

HCC Development Is Associated to Peripheral Insulin Resistance in a Mouse Model of NASH

NAFLD is the most common liver disease worldwide but it is the potential evolution to NASH and eventually to hepatocellular carcinoma (HCC), even in the absence of cirrhosis, that makes NAFLD of such clinical importance. Aim: we aimed to create a mouse model reproducing the pathological spectrum of NAFLD and to investigate the role of possible co-factors in promoting HCC. Methods: mice were treated with a choline-deficient L-amino-acid-defined-diet (CDAA) or its control (CSAA diet) and subjected to a low-dose i.p. injection of CCl4 or vehicle. Insulin resistance was measured by the euglycemic-hyperinsulinemic clamp method. Steatosis, fibrosis and HCC were evaluated by histological and molecular analysis. Results: CDAA-treated mice showed peripheral insulin resistance at 1 month. At 1–3 months, extensive steatosis and fibrosis were observed in CDAA and CDAA+CCl4 groups. At 6 months, equal increase in steatosis and fibrosis was observed between the two groups, together with the appearance of tumor. At 9 months of treatment, the 100% of CDAA+CCl4 treated mice revealed tumor versus 40% of CDAA mice. Insulin-like Growth Factor-2 (IGF-2) and Osteopontin (SPP-1) were increased in CDAA mice versus CSAA. Furthermore, Immunostaining for p-AKT, p-c-Myc and Glypican-3 revealed increased positivity in the tumors. Conclusions: the CDAA model promotes the development of HCC from NAFLD-NASH in the presence of insulin resistance but in the absence of cirrhosis. Since this condition is increasingly recognized in humans, our study provides a model that may help understanding mechanisms of carcinogenesis in NAFLD.

Role of Adipose Tissue Insulin Resistance in the Natural History of Type 2 Diabetes: Results From the San Antonio Metabolism Study

In the transition from normal glucose tolerance (NGT) to type 2 diabetes mellitus (T2DM), the role of β-cell dysfunction and peripheral insulin resistance (IR) is well established. However, the impact of dysfunctional adipose tissue has not been fully elucidated. The aim of this study was to evaluate the role of resistance to the antilipolytic effect of insulin (adipose tissue IR [Adipo-IR]) in a large group of subjects with NGT, impaired glucose tolerance (IGT), and T2DM. Three hundred two subjects with varying glucose tolerance received an oral glucose tolerance test (OGTT) and euglycemic insulin clamp. We evaluated Adipo-IR (fasting and mean OGTT plasma free fatty acid [FFA] × insulin concentrations), peripheral IR (1/[Matsuda index] and (M/I)−1 value), and β-cell function (calculated as the ratio of the increment in plasma insulin to glucose [OGTT/IR (ΔI/ΔG ÷ IR)]). Fasting Adipo-IR was increased twofold in obese subjects with NGT and IGT versus lean subjects with NGT (8.0 ± 1.1 and 9.2 ± 0.7 vs. 4.1 ± 0.3, respectively) and threefold in subjects with T2DM (11.9 ± 0.6; P < 0.001). Progressive decline in ΔI/ΔG ÷ IR was associated with a progressive impairment in FFA suppression during OGTT, whereas the rise in mean plasma glucose concentration only became manifest when subjects became overtly diabetic. The progressive decline in β-cell function that begins in individuals with NGT is associated with a progressive increase in FFA and fasting Adipo-IR.

Peripheral insulin resistance predicts liver damage in nondiabetic subjects with nonalcoholic fatty liver disease

Surrogate indexes of insulin resistance and insulin sensitivity are widely used in nonalcoholic fatty liver disease (NAFLD), although they have never been validated in this population. We aimed to validate the available indexes in NAFLD subjects and to test their ability to predict liver damage also in comparison with the NAFLD fibrosis score. Surrogate indexes were validated by the tracer technique (6,6‐D2‐glucose and U‐13C‐glucose) in the basal state and during an oral glucose tolerance test. The best‐performing indexes were used in an independent cohort of 145 nondiabetic NAFLD subjects to identify liver damage (fibrosis and nonalcoholic steatohepatitis). In the validation NAFLD cohort, homeostasis model assessment of insulin resistance, insulin to glucose ratio, and insulin sensitivity index Stumvoll had the best association with hepatic insulin resistance, while peripheral insulin sensitivity was most significantly related to oral glucose insulin sensitivity index (OGIS), insulin sensitivity index Stumvoll, and metabolic clearance rate estimation without demographic parameters. In the independent cohort, only oral glucose tolerance test‐derived indexes were associated with liver damage and OGIS was the best predictor of significant (≥F2) fibrosis (odds ratio = 0.76, 95% confidence interval 0.61‐0.96, P = 0.0233) and of nonalcoholic steatohepatitis (odds ratio = 0.75, 95% confidence interval 0.63‐0.90, P = 0.0021). Both OGIS and NAFLD fibrosis score identified advanced (F3/F4) fibrosis, but OGIS predicted it better than NAFLD fibrosis score (odds ratio = 0.57, 95% confidence interval 0.45‐0.72, P < 0.001) and was also able to discriminate F2 from F3/F4 (P < 0.003). Conclusion: OGIS is associated with peripheral insulin sensitivity in NAFLD and inversely associated with an increased risk of significant/advanced liver damage in nondiabetic subjects with NAFLD. (Hepatology 2016;63:107–116)

Exenatide improves both hepatic and adipose tissue insulin resistance: A dynamic positron emission tomography study

Glucagon‐like peptide 1 (GLP‐1) receptor agonists (GLP‐1‐RAs) act on multiple tissues, in addition to the pancreas. Recent studies suggest that GLP‐1‐RAs act on liver and adipose tissue to reduce insulin resistance (IR). Thus, we evaluated the acute effects of exenatide (EX) on hepatic (Hep‐IR) and adipose (Adipo‐IR) insulin resistance and glucose uptake. Fifteen male subjects (age = 56 ± 8 years; body mass index = 29 ± 1 kg/m2; A1c = 5.7 ± 0.1%) were studied on two occasions, with a double‐blind subcutaneous injection of EX (5 μg) or placebo (PLC) 30 minutes before a 75‐g oral glucose tolerance test (OGTT). During OGTT, we measured hepatic (HGU) and adipose tissue (ATGU) glucose uptake with [18F]2‐fluoro‐2‐deoxy‐D‐glucose/positron emission tomography, lipolysis (RaGly) with [U‐2H5]‐glycerol, oral glucose absorption (RaO) with [U‐13C6]‐glucose, and hepatic glucose production (EGP) with [6,6‐2H2]‐glucose. Adipo‐IR and Hep‐IR were calculated as (FFA0‐120min) × (Ins0‐120min) and (EGP0‐120min) × (Ins0‐120min), respectively. EX reduced RaO, resulting in reduced plasma glucose and insulin concentration from 0 to 120 minutes postglucose ingestion. EX decreased Hep‐IR (197 ± 28 to 130 ± 37; P = 0.02) and increased HGU of orally administered glucose (23 ± 4 to 232 ± 89 [μmol/min/L]/[μmol/min/kg]; P = 0.003) despite lower insulin (23 ± 5 vs. 41 ± 5 mU/L; P < 0.02). EX enhanced insulin suppression of RaGly by decreasing Adipo‐IR (23 ± 4 to 13 ± 3; P = 0.009). No significant effect of insulin was observed on ATGU (EX = 1.16 ± 0.15 vs. PLC = 1.36 ± 0.13 [μmol/min/L]/[μmol/min/kg]). Conclusion: Acute EX administration (1) improves Hep‐IR, decreases EGP, and enhances HGU and (2) reduces Adipo‐IR, improves the antilipolytic effect of insulin, and reduces plasma free fatty acid levels during OGTT. (Hepatology 2016;64:2028‐2037).

Insulin Resistance and Endothelial Dysfunction: A Mutual Relationship in Cardiometabolic Risk

Cardiometabolic risk comprises a cluster of traditional and emerging factors that are good indicators of a patients overall risk for type 2 diabetes and cardiovascular disease. The insulin resistance, a key feature common to obesity and type 2 diabetes, is associated with impaired vascular response and contributes to increased cardiovascular risk. Abnormal vascular insulin signalling induces endothelial dysfunction, the initial step of atherosclerotic process, characterized by attenuated nitric oxide-mediated vasodilatation and atherogenic response. Insulin resistance and endothelial dysfunction are two pathological conditions that can co-exist, even if their cause-effect relationship is not yet clarified. Multiple signaling pathways shared by insulin resistance and endothelial dysfunction include hyperinsulinemia, glucotoxicity, lipotoxicity, and inflammation. These mechanisms selectively impair PI3K-dependent insulin in vascular endothelium harming endothelial balance and strengthening the evidence of the close association between metabolic and cardiovascular disease. The present review analyzes the close relationship between endothelial dysfunction and insulin resistance and explores the common mechanisms, with clinical considerations and pharmacological strategies.

Exenatide improves both hepatic and adipose tissue insulin resistance: A dynamic PET study

Glucagon‐like peptide 1 (GLP‐1) receptor agonists (GLP‐1‐RAs) act on multiple tissues, in addition to the pancreas. Recent studies suggest that GLP‐1‐RAs act on liver and adipose tissue to reduce insulin resistance (IR). Thus, we evaluated the acute effects of exenatide (EX) on hepatic (Hep‐IR) and adipose (Adipo‐IR) insulin resistance and glucose uptake. Fifteen male subjects (age = 56 ± 8 years; body mass index = 29 ± 1 kg/m2; A1c = 5.7 ± 0.1%) were studied on two occasions, with a double‐blind subcutaneous injection of EX (5 μg) or placebo (PLC) 30 minutes before a 75‐g oral glucose tolerance test (OGTT). During OGTT, we measured hepatic (HGU) and adipose tissue (ATGU) glucose uptake with [18F]2‐fluoro‐2‐deoxy‐D‐glucose/positron emission tomography, lipolysis (RaGly) with [U‐2H5]‐glycerol, oral glucose absorption (RaO) with [U‐13C6]‐glucose, and hepatic glucose production (EGP) with [6,6‐2H2]‐glucose. Adipo‐IR and Hep‐IR were calculated as (FFA0‐120min) × (Ins0‐120min) and (EGP0‐120min) × (Ins0‐120min), respectively. EX reduced RaO, resulting in reduced plasma glucose and insulin concentration from 0 to 120 minutes postglucose ingestion. EX decreased Hep‐IR (197 ± 28 to 130 ± 37; P = 0.02) and increased HGU of orally administered glucose (23 ± 4 to 232 ± 89 [μmol/min/L]/[μmol/min/kg]; P = 0.003) despite lower insulin (23 ± 5 vs. 41 ± 5 mU/L; P < 0.02). EX enhanced insulin suppression of RaGly by decreasing Adipo‐IR (23 ± 4 to 13 ± 3; P = 0.009). No significant effect of insulin was observed on ATGU (EX = 1.16 ± 0.15 vs. PLC = 1.36 ± 0.13 [μmol/min/L]/[μmol/min/kg]). Conclusion: Acute EX administration (1) improves Hep‐IR, decreases EGP, and enhances HGU and (2) reduces Adipo‐IR, improves the antilipolytic effect of insulin, and reduces plasma free fatty acid levels during OGTT. (Hepatology 2016;64:2028‐2037).