Lipotoxic agents also act profoundly on macrophages and other hepatic non-parenchymal cells.
ReviewLipotoxicity and the gut-liver axis in NASH pathogenesis
Introduction
Non-alcoholic fatty liver disease (NAFLD) is an expanding health problem, with an estimated global prevalence of 25%.1 NAFLD is associated with obesity, insulin resistance or type 2 diabetes and other metabolic abnormalities, collectively termed the metabolic syndrome. NAFLD encompasses a spectrum of conditions including simple steatosis and non-alcoholic steatohepatitis (NASH), characterised by hepatocellular damage, inflammation and fibrosis, which is a significant risk factor for the development of cirrhosis and hepatocellular carcinoma (HCC).2 A critical aspect in the pathogenesis and natural history of NAFLD is the development of hepatic fibrosis, which has been recognised as a factor contributing to liver-related and all-cause mortality.3 As with other chronic liver diseases, fibrosis is the result of a long-standing wound healing process caused by hepatocellular injury. This in turn activates local inflammatory cells, recruits leukocytes from the bloodstream and activates myofibroblasts, mostly derived from transdifferentiation of hepatic stellate cells.4 NAFLD is characterised by the accumulation of different lipid species within hepatocytes. The consequent hepatocellular damage is linked to the accumulation of toxic lipids, resulting from a complex balance involving the liver and several other tissues, including the adipose tissue and the gut. The gut participates in lipotoxicity through the metabolism of nutrients and a number of secretory factors that eventually target the liver.4 Recent investigations have shown that changes in the microbiota have an impact on NAFLs pathogenesis and lipotoxicity, unveiling an additional type of interaction. Herein, we will discuss the major mechanisms leading to the generation of toxic lipid species that impact on the pathogenesis of NASH, and on the signalling mechanisms operating in this context. In addition, we will analyse the major pathways responsible for modulation of the so-called gut-liver axis during NASH, and their interaction with lipotoxic events.
Section snippets
Definition and general mechanisms of lipotoxicity
Lipids are part of the cell structure, and are involved in fundamental functions such as cellular homeostasis, cell-cell communication and regulation of inflammation and immunity. Lipotoxicity is defined as dysregulation of the lipid environment and/or intracellular lipid composition, leading to accumulation of harmful lipids, which may be associated with organelle dysfunction, cell injury, or death. Importantly, lipotoxicity is intimately associated with chronic inflammation
Triglycerides
The hallmark of NAFLD is the accumulation of fat in the hepatocytes, in the form of lipid droplets containing triglycerides. Increased delivery of free fatty acids (FFA) from insulin-resistant adipose tissue, intrahepatic de novo lipogenesis, and dietary fat are the major mechanisms underlying triglyceride accumulation.12 Although triglycerides represent the major lipid class contained in droplets, and abundance of fat is part of the grading system of NAFLD, this form of accumulation is
Molecular effectors of lipotoxicity
Increased availability of harmful lipids eventually leads to cell injury, death, and activation of inflammatory pathways. We will focus on hepatocytes as major targets of lipotoxic pathways, as well as focussing on apoptosis. The contribution of necrosis and necroptosis to NASH is still uncertain.34
Lipotoxicity and non-parenchymal cells
Independently of the actions on hepatocytes, lipotoxic agents profoundly modify the biology of macrophages, with actions that interact with those derived from the gut, discussed later. The fact that Kupffer cell depletion is associated with inhibition of steatosis, indicates that inflammation should be regarded not only as a consequence, but also as a causative agent of lipid accumulation in hepatocytes.
Gut microbiota and NAFLD
Gut microbiota is defined as the complex of microorganisms harboured by each person and is characterised by a high number of genes collectively called the microbiome.76 Normal human gut is colonised by a large number of microorganisms, at least 100 trillion, which maintain symbiotic relationships with the host77 and contribute to various functions such as digestion, vitamin synthesis, and resistance to colonisation of the intestine by pathogens.78 This microbiome is not equally distributed along
Gut hormones and the liver
Glucose kinetics are tightly regulated by the pancreatic hormones, insulin and glucagon. Insulin suppresses hepatic glucose production and stimulates peripheral glucose uptake, while glucagon stimulates gluconeogenesis, glycogenolysis, and net hepatic glucose output.115 Other hormones are also involved in the regulation of glucose kinetics, including catecholamines, cortisol and gastrointestinal hormones, in particular GLP-1 and GIP.116 Incretins are gut hormones that potentiate insulin
Conclusion
The pathogenesis of NAFLD and particularly the development of NASH are still incompletely understood. While several phase III trials are underway, there is still the need to identify novel therapeutic targets based on the pathogenetic mechanisms of the disease. In this review, we have dealt with the initial mechanisms of damage to hepatocytes, i.e. lipotoxicity. The precise understanding of these mechanisms may help to reduce the initial trigger of the hepatic ‘wound healing’ response. An
Financial support
Work on steatohepatitis conducted in Dr. Marra's laboratory is supported by grants from the University of Florence, and the European Union (project EPoS).
Conflict of interest
Dr. Marra reports grants and personal fees from Abbvie, personal fees from Allergan, grants from Alfa-Wassermann, personal fees from AstraZeneca, grants and personal fees from Bayer, personal fees from Gilead, personal fees from Intercept, personal fees from Menarini, personal fees from Novo Nordisk, outside the submitted work.
Dr. Svegliati-Baroni did not declare any conflict of interest.
Please refer to the accompanying ICMJE disclosure forms for further details.
Authors’ contribution
The two authors equally contributed to reference selection, writing and revision of the manuscript.
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