NAFLD: Mechanisms, Treatments, and Biomarkers

Abstract

Nonalcoholic fatty liver disease (NAFLD), recently renamed metabolic-associated fatty liver disease (MAFLD), is one of the most common causes of liver diseases worldwide. NAFLD is growing in parallel with the obesity epidemic. No pharmacological treatment is available to treat NAFLD, specifically. The reason might be that NAFLD is a multi-factorial disease with an incomplete understanding of the mechanisms involved, an absence of accurate and inexpensive imaging tools, and lack of adequate non-invasive biomarkers. NAFLD consists of the accumulation of excess lipids in the liver, causing lipotoxicity that might progress to metabolic-associated steatohepatitis (NASH), liver fibrosis, and hepatocellular carcinoma. The mechanisms for the pathogenesis of NAFLD, current interventions in the management of the disease, and the role of sirtuins as potential targets for treatment are discussed here. In addition, the current diagnostic tools, and the role of non-coding RNAs as emerging diagnostic biomarkers are summarized. The availability of non-invasive biomarkers, and accurate and inexpensive non-invasive diagnosis tools are crucial in the detection of the early signs in the progression of NAFLD. This will expedite clinical trials and the validation of the emerging therapeutic treatments.

Keywords: biomarkers; lipotoxicity; liver; metabolic-associated fatty liver disease (MAFLD); metabolic-associated steatohepatitis (MASH); mitochondria; non-alcoholic fatty liver disease (NAFLD); non-alcoholic steatohepatitis (NASH); reactive oxygen species (ROS); sirtuins.

Figure 1

Pathogenic pathways involved in NAFLD. Top panel: schematic representing the progression of NAFLD, and the factors involved. Hepatic steatosis results from nutrient overload and a sedentary lifestyle. Multiple factors lead to inflammation and NASH, and the progression to fibrosis. Bottom panel: mechanisms for the development and the progression of NAFLD. The pool of FAs in the liver originates from dietary fat, adipose tissue lipolysis, or de novo lipogenesis (DNL) from carbohydrates or other dietary precursors. Inside the liver, FAs are (1) esterified into TG and assembled into very-low-density lipoprotein (VLDL) to be secreted in the circulation, (2) oxidized in the mitochondria (β-oxidation), or (3) stored in lipid droplets (LDs) (<5% of liver weight). LDs undergo lipid hydrolysis (via lipolysis and lipophagy) in fasting conditions to provide FAs for β-oxidation. With chronic nutrient overload and insulin resistance (increased adipose tissue lipolysis) in NAFLD, the input of FAs to the liver exceeds their disposal via VLDL secretion or β-oxidation. Lipotoxicity causes impaired LDs lipolysis and higher lipid accumulation in LDs (hepatic steatosis >5% of liver weight), leading to the endoplasmic reticulum (ER) stress, oxidative stress, and the activation of Kupffer (KCs) to produce inflammatory cytokines and inflammation. In addition, lipotoxicity causes mitochondrial dysfunction and impaired electron transport chain (ETC) function leading to ROS production. In a vicious cycle, ROS causes mitochondrial damage and aggravates NASH. Inflammation and ROS activate hepatic stellate cells (HSCs) to produce excessive extracellular matrix leading to progressive fibrosis.

Figure 2

Role of sirtuin (SIRT) 1 in NAFLD. Left panel: Reduced SIRT1 with nutrient overload leads to the acetylation and inactivation of the peroxisome proliferator-activated receptor (PPAR) gamma-coactivator α (PGC1α). Reduced PGC1α activity downregulates β-oxidation by reducing PPARα expression and decreasing mitochondrial biogenesis, leading to increased lipid accumulation in LDs and hepatic steatosis. In addition, reduced SIRT1 activity decreases the expression of the antioxidant enzymes (SOD2 and catalase), leading to increased cellular ROS. On the other hand, reduced PGC1α activity increases the expression of the transcription factor NF-κB to increase inflammatory cytokines such as TNFα and IL6 and ROS generation. In addition, reduced AMP/ATP ratio with nutrient overload inactivates AMP-activated protein kinase (AMPK), leading to reduced NAD/ATP ratio and reduced SIRT1 and PGC1α activities. AMPK also directly regulates PGC1 via modulation of its phosphorylation. Reduced AMPK also reduces ACC1 phosphorylation and increases its activity leading to increased malonyl CoA and inhibition of β-oxidation. Right panel: Upregulation of SIRT1 by exercise, calorie restriction, and fasting upregulates SIRT1 leading to a healthier liver via opposite effects on B-oxidation, inflammation, and ROS generation than chronic nutrient overload.

Figure 3

Role of sirtuins (SIRT) 3 in NAFLD. Calorie overload and liver lipotoxicity downregulate SIRT3 activity leading to the inactivation of PGC1α. The inactivation of PGC1α downregulates the downstream pathways, including β-oxidation, antioxidant enzymes (superoxide dismutase (SOD)2 and isocitrate dehydrogenase (IDAH)2, the ETC, ketogenesis, biogenesis, and mitophagy, leading to the development of hepatic steatosis, inflammation, and fibrogenesis. Activation of SIRT3 by exercise and calorie restriction activates PGC1α with opposite effects on PGC1 downstream pathways than nutrient overload and lipotoxicity, leading to metabolic health.