Mohamed Asrih

Inflammation as a potential link between nonalcoholic fatty liver disease and insulin resistance

Nonalcoholic fatty liver disease (NAFLD) has become a major health problem in developed countries. It has affected more than 30% of the general population and is commonly associated with insulin resistance, which is a major risk factor for the development of type 2 diabetes and a central feature of the metabolic syndrome. Furthermore, accumulating evidences reveal that NAFLD as well as insulin resistance is strongly related to inflammation. Cytokines and adipokines play a pivotal role in inflammatory processes. In addition, these inflammatory mediators regulate various functions including metabolic energy balance, inflammation, and immune response. However, their role in modulating ectopic lipids involved in the development of insulin resistance, such as diacylglycerols and ceramides, remains unknown. The aim of this review is first to describe the pathophysiology of insulin resistance in NAFLD. In particular, we discuss the role of ectopic lipid accumulation in the liver. Secondly, we also summarize recent findings emphasizing the role of main inflammatory markers in both NAFLD and insulin resistance and their potential role in modulating hepatic fat content in NAFLD and associated hepatic insulin resistance.

Metabolic syndrome and nonalcoholic fatty liver disease: Is insulin resistance the link?

Metabolic syndrome (MetS) is a disease composed of different risk factors such as obesity, type 2 diabetes or dyslipidemia. The prevalence of this syndrome is increasing worldwide in parallel with the rise in obesity. Nonalcoholic fatty liver disease (NAFLD) is now the most frequent chronic liver disease in western countries, affecting more than 30% of the general population. NAFLD encompasses a spectrum of liver manifestations ranging from simple steatosis to nonalcoholic steatohepatitis (NASH), fibrosis and cirrhosis, which may ultimately progress to hepatocellular carcinoma. There is accumulating evidence supporting an association between NAFLD and MetS. Indeed, NAFLD is recognized as the liver manifestation of MetS. Insulin resistance is increasingly recognized as a key factor linking MetS and NAFLD. Insulin resistance is associated with excessive fat accumulation in ectopic tissues, such as the liver, and increased circulating free fatty acids, which can further promote inflammation and endoplasmic reticulum stress. This in turn aggravates and maintains the insulin resistant state, constituting a vicious cycle. Importantly, evidence shows that most of the patients developing NAFLD present at least one of the MetS traits. This review will define MetS and NAFLD, provide an overview of the common pathophysiological mechanisms linking MetS and NAFLD, and give a perspective regarding treatment of these ever growing metabolic diseases.

Diets and nonalcoholic fatty liver disease: the good and the bad.

Nonalcoholic fatty liver disease (NAFLD) is now described as the hepatic manifestation of the metabolic syndrome and is the most frequent chronic liver disease, affecting about one out of three people in the western world. NAFLD is strongly linked to insulin resistance, which represents a key risk factor for the development of type 2 diabetes. To date, there are no reliable and efficient pharmacotherapies in the treatment of NAFLD. However, obesity, which represents one of the main features of the metabolic syndrome, is strongly associated with NAFLD. Therefore, lifestyle modifications, i.e. weight loss and increased physical activity, are the very first clinical approaches aiming at treating NAFLD. However, although weight loss is beneficial in NAFLD, certain diets known to induce weight loss can actually cause or exacerbate this disease, and therefore induce insulin resistance, such as very low carbohydrate, high fat diets. Moreover, macronutrient diet composition can impact NAFLD without any change in body weight. Indeed, diets rich in fatty acids, particularly saturated, or in refined carbohydrates such as those found in soft drinks, can actually exacerbate NAFLD. The aim of this review is to discuss the role of weight loss and macronutrients modifications, particularly the role of fat and carbohydrate diet composition, in the treatment of NAFLD.

Emerging role of epigenetics and miRNA in diabetic cardiomyopathy.

The prevalence of heart failure independent of coronary artery disease and hypertension is increasing rapidly in diabetic patients. Thus, this pathophysiology has been recognized as a distinct clinical entity termed diabetic cardiomyopathy. Several studies support the notion that diabetes is a threatening insult for the myocardium resulting in functional, cellular, and structural changes manifesting as a cardiac myopathy. Recent data suggested that epigenetics including DNA and histone modifications as well as microRNAs play an important role in the development of cardiac diseases. The role of epigenetics in diabetes is largely recognized; however, its role in diabetes-associated cardiomyopathy remains elusive. Thus, molecular, cellular, and functional modulations in the diabetic cardiomyopathy will be investigated in this review. Moreover, particular attention will be drawn on the epigenetic mechanisms that may play an important role in the pathophysiology of diabetic cardiomyopathy.

Ketogenic Diet Impairs FGF21 Signaling and Promotes Differential Inflammatory Responses in the Liver and White Adipose Tissue

Background/Hypothesis Beside its beneficial effects on weight loss, ketogenic diet (KD) causes dyslipidemia, a pro-inflammatory state involved in the development of hepatic steatosis, glucose intolerance and insulin resistance, although the latter is still being debated. Additionally, KD is known to increase fibroblast growth factor 21 (FGF21) plasma levels. However, FGF21 cannot initiate its beneficial actions on metabolism in these conditions. We therefore hypothesized and tested in the present study that KD may impair FGF21 signaling. Methods/Results Using indirect calorimetry, we found that KD-fed mice exhibited higher energy expenditure than regular chow (RC)-fed mice associated with increased Ucp1 levels in white adipose tissue (WAT), along with increased plasma FGF21 levels. We then assessed the effect of KD on FGF21 signaling in both the liver and WAT. We found that Fgfr4 and Klb (β-klotho) were downregulated in the liver, while Fgfr1 was downregulated in WAT of KD-fed mice. Because inflammation could be one of the mechanisms linking KD to impaired FGF21 signaling, we measured the expression levels of inflammatory markers and macrophage accumulation in WAT and liver and found an increased inflammation and macrophage accumulation in the liver, but surprisingly, a reduction of inflammation in WAT.We also showed that KD enhances lipid accumulation in the liver, which may explain hepatic inflammation and impaired Fgfr4 and Klb expression. In contrast, import of lipids from the circulation was significantly reduced in WAT of KD-fed mice, as suggested by a downregulation of Lpl and Cd36. This was further associated with reduced inflammation in WAT. Conclusion Altogether, these results indicate that KD could be beneficial for a given tissue but deleterious for another.

Hepatic insulin resistance and increased hepatic glucose production in mice lacking Fgf21

Fibroblast growth factor 21 (FGF21) is an important regulator of hepatic glucose and lipid metabolism and represents a potential pharmacological agent for the treatment of type 2 diabetes and obesity. Mice fed a ketogenic diet (KD) develop hepatic insulin resistance in association with high levels of FGF21, suggesting a state of FGF21 resistance. To address the role of FGF21 in hepatic insulin resistance, we assessed insulin action in FGF21 whole-body knock-out (FGF21 KO) male mice and their littermate WT controls fed a KD. Here, we report that FGF21 KO mice have hepatic insulin resistance and increased hepatic glucose production associated with an increase in plasma glucagon levels. FGF21 KO mice are also hypometabolic and display increased fat mass compared with their WT littermates. Taken together, these findings support a major role of FGF21 in regulating energy expenditure and hepatic glucose and lipid metabolism, and its potential role as a candidate in the treatment of diseases associated with insulin resistance.

Role of Mitogen-Activated Protein Kinase Pathways in Multifactorial Adverse Cardiac Remodeling Associated with Metabolic Syndrome

Metabolic syndrome has been widely associated with an increased risk for acute cardiovascular events. Emerging evidence supports metabolic syndrome as a condition favoring an adverse cardiac remodeling, which might evolve towards heart dysfunction and failure. This pathological remodeling has been described to result from the cardiac adaptive response to clinical mechanical conditions (such as hypertension, dyslipidemia, and hyperglycemia), soluble inflammatory molecules (such as cytokines and chemokines), as well as hormones (such as insulin), characterizing the pathophysiology of metabolic syndrome. Moreover, these cardiac processes (resulting in cardiac hypertrophy and fibrosis) are also associated with the modulation of intracellular signalling pathways within cardiomyocytes. Amongst the different intracellular kinases, mitogen-activated protein kinases (MAPKs) were shown to be involved in heart damage in metabolic syndrome. However, their role remains controversial. In this paper, we will discuss and update evidence on MAPK-mediated mechanisms underlying cardiac adverse remodeling associated with metabolic syndrome.

Differential regulation of stimulated glucose transport by free fatty acids and PPARα or δ agonists in cardiac myocytes

Stimulation of glucose transport in response to insulin or metabolic stress is an important determinant of cardiac myocyte function and survival, particularly during ischemia-reperfusion episodes. The impact of dyslipidemia and its consequence PPAR activation on stimulated glucose transport in cardiac myocytes remains unknown. Isolated adult rat cardiac myocytes were chronically exposed to free fatty acids (FFA) or PPAR agonists. Insulin- (ISGT) and oligomycin-stimulated glucose transport (OSGT) and related cell signaling were analyzed. Exposure of cardiac myocytes to FFA reduced both ISGT and OSGT. Exposure to either PPARα or PPARδ agonists, but not to a PPARγ agonist, reduced ISGT but not OSGT and increased fatty acid oxidation (FAO). The reduction in ISGT was associated with impaired insulin signaling and, in the case of PPAR stimulation, overexpression of SOCS-3, a protein known to hinder proximal insulin signaling. In contrast, the reduction of OSGT could not be explained by a reduced activity of the cellular energy-sensing system, as assessed from the maintained phosphorylation state of AMPK. Inhibition of FAO at the level of mitochondrial acylcarnitine uptake restored OSGT but not ISGT. Seemingly paradoxically, further stimulation of FAO with PPARα or PPARδ agonists also restored OSGT but not ISGT. Together, these results suggest that inhibition of OSGT occurs downstream of energy gauging and is caused by some intermediate(s) of fatty acid oxidation, which does not appear to be acylcarnitines. The results indicate that the mechanisms underlying FFA-mediated inhibition of ISGT and OSGT differ remarkably.

Dual effect of the heart-targeting cytokine cardiotrophin-1 on glucose transport in cardiomyocytes

Cardiotrophin-1 (CT-1) is a heart-targeting cytokine that is increased in the metabolic syndrome due to overexpression in the adipocytes. The effects of CT-1 on cardiomyocyte substrate metabolism remain unknown. We therefore determined the effects of CT-1 on basal and stimulated glucose transport in cardiomyocytes exposed to a low dose (1nM) or a high dose (10nM). Dose-response curves for insulin showed that 1nM CT-1 reduced insulin responsiveness, while 10nM CT-1 increased insulin responsiveness. In either condition insulin sensitivity was unaffected. Similarly 1nM CT-1 reduced the stimulation of glucose transport in response to metabolic stress, induced by the mitochondrial poison oligomycin, while 10nM CT-1 increased this response. Reduction of stimulated glucose transport by 1nM CT-1 was associated with overexpression of SOCS-3, a protein known to hinder proximal insulin signaling, and increased phosphorylation of STAT5. In cardiomyocytes exposed to 1nM CT-1 there was also reduced phosphorylation of Akt and AS160 in response to insulin, and of AMPK in response to oligomycin. Insulin-stimulated glucose transport and signaling were restored by inhibition of STAT5 activity. On the other hand in cardiomyocytes exposed to 10nM CT-1 there was increased phosphorylation of the AS160 and Akt in response to insulin. Most importantly, basal and oligomycin-stimulated phosphorylation of AMPK was markedly increased in cardiomyocytes exposed to 10nM CT-1. The enhancement of basal and stimulated-glucose transport was abolished in cardiomyocytes treated with the calmodulin-dependent kinase II (CaMKII) inhibitor KN93, and so was AMPK phosphorylation. This suggests that activation of CaMKII mediates activation of AMPK by a high dose of CT-1 independently of metabolic stress. Our results point to a role for CT-1 in the regulation of myocardial glucose metabolism and implicate entirely separate mechanisms in the inhibitory or stimulatory effects of CT-1 on glucose transport at low or high concentrations respectively.

Role of ERK1/2 activation in microtubule stabilization and glucose transport in cardiomyocytes

We previously demonstrated that microtubule disruption impairs stimulation of glucose uptake in cardiomyocytes and that 9-cis retinoic acid (9cRA) treatment preserved both microtubule integrity and stimulated glucose transport. Herein we investigated whether 1) activation of the extracellular signal-regulated kinases (ERK1/2) is responsible for microtubule destabilization and 2) ERK1/2 inactivation may explain the positive effects of 9cRA on glucose uptake and microtubule stabilization. Adult rat cardiomyocytes in primary culture showed increased basal ERK1/2 phosphorylation. Cardiomyocytes exposed to inhibitors of the ERK1/2 kinase mitogen/extracellular signal-regulated kinase (MEK) 1/2 had preserved microtubular scaffold, including microtubule-organizing centers (MTOC), together with increased insulin and metabolic stress-stimulated glucose transport as well as signaling, thus replicating the effects of 9cRA treatment. Although 9cRA treatment did not significantly reduce global ERK1/2 activation, it markedly reduced perinuclear-activated ERK1/2 at the location of MTOC. 9cRA also triggered relocation of the ERK1/2 phosphatase mitogen-activated protein kinase phosphatase-3 from the cytosol to the nucleus. These results indicate that, in cardiomyocytes, microtubule destabilization, leading to impaired stimulation of glucose transport, is mediated by ERK1/2 activation, impacting on the MTOC. 9cRA acid restores stimulated glucose transport indirectly through compartmentalized inactivation of ERK1/2.