Genetic predisposition in NAFLD and NASH: impact on severity of liver disease and response to treatment

Abstract

Liver fat deposition related to systemic insulin resistance defines non-alcoholic fatty liver disease (NAFLD) which, when associated with oxidative hepatocellular damage, inflammation, and activation of fibrogenesis, i.e. non-alcoholic steatohepatitis (NASH), can progress towards cirrhosis and hepatocellular carcinoma. Due to the epidemic of obesity, NAFLD is now the most frequent liver disease and the leading cause of altered liver enzymes in Western countries. Epidemiological, familial, and twin studies provide evidence for an element of heritability of NAFLD. Genetic modifiers of disease severity and progression have been identified through genome-wide association studies. These include the Patatin-like phosholipase domain-containing 3 (PNPLA3) gene variant I148M as a major determinant of inter-individual and ethnicity-related differences in hepatic fat content independent of insulin resistance and serum lipid concentration. Association studies confirm that the I148M polymorphism is also a strong modifier of NASH and progressive hepatic injury. Furthermore, a few large multicentre case-control studies have demonstrated a role for genetic variants implicated in insulin signalling, oxidative stress, and fibrogenesis in the progression of NAFLD towards fibrosing NASH, and confirm that hepatocellular fat accumulation and insulin resistance are key operative mechanisms closely involved in the progression of liver damage. It is now important to explore the molecular mechanisms underlying these associations between gene variants and progressive liver disease, and to evaluate their impact on the response to available therapies. It is hoped that this knowledge will offer further insights into pathogenesis, suggest novel therapeutic targets, and could help guide physicians towards individualised therapy that improves clinical outcome.

Fig. (1)

Mechanisms involved in the pathogenesis of NASH. NAFLD is characterized by the hepatic fat accumulation resulting from an unbalance between triglycerides acquisition and removal. Most of free fatty acids (FFAs) that are stored as triglycerides during hepatic steatosis derive from peripheral lipolysis related to adipose tissue insulin resistance, followed by de novo lipogenesis induced by hyperinsulinemia, and diet. In the liver, FFAs can be catabolized through β-oxidation, re-esterification to triglycerides and stored as lipid droplets, or exported as very low density lipoproteins (VLDL). Impaired ability to secrete lipoproteins and decreased β-oxidation due to mitochondrial damage (expecially in the presence of NASH) may play a role in hepatic fat accumulation. Long-term injury arising from i) hepatocellular triglycerides storage and lipotoxicity, ii) hepatocellular oxidative stress secondary to free radical produced during β- and omega- oxidation of FFAs, iii) inflammation triggered by endotoxin, iv) cytokines release, v) and endoplasmic reticulum (ER) stress lead in the end to inflammation, perpetuation of cellular damage, and activation of fibrogenesis.

Fig. (2)

Inflammation in NAFLD. Obesity and NAFLD are directly associated with activation of inflammatory pathways. Hypertrophic adipocytes release chemokines and proinflammatory cytokines including TNFα, IL-6, resistin and MCP-1. Chemokines recruit macrophages, especially in visceral adipose tissue. Adipose tissue macrophages produce inflammatory cytokines such as TNFα, IL-6 and IL-1β. These inflammatory changes in adipose tissue induce adipocytokine dysregulation: a decrease in insulin sensitizing and anti-inflammatory adipocytokines as adiponectin, and an increase in pro-inflammatory cytokines such as TNFα, interleukins and resistin. Extracellular free fatty acids (FFAs) as well as bacterial endotoxins activate Kupffer cells by engaging the Toll-like receptor 4 (TLR4). Upon TLR ligation, MyD88, an adaptor molecule, is recruited to transmit the signals that activate NF-kB and JNK. Activated Kupffer cells produce inflammatory cytokines such as TNFα and IL-1β, chemokines such as MCP-1 and ROS leading to liver damage. Acute loss of hepatocytes triggers a compensatory proliferative response in surviving hepatocytes. However, in chronic fatty liver many hepatocytes have sustained oxidative damage that inhibits progression to the cycle and regeneration. Moreover fatty hepatocytes have reduced proliferative capacity. Damaged hepatocytes release several factors including ROS, cytokines, chemokines that recruit inflammatory cells into the liver. Once in the liver, these inflammatory cells release cytotoxic factors that increase hepatocytes death. Other hepatocytes-derived factors activate hepatic stellate cells (HSC), which produce more extracellular matrix (ECM) leading to matrix accumulation and fibrogenesis. HSCs also release fibrogenic cytokines with autocrine and paracrine effects, including TGF-β1, and over-express tissue inhibitors of metalloproteinase, which promote ECM accumulation by inhibiting matrix degradation.

Fig. (3)

Heritable components of non-alcoholic fatty liver disease (NAFLD) and liver indices associated with steatosis. Average values of studies reported in the manuscript are presented. HFF: hepatic fat fraction; ALT: alanine transaminase; GGT: gamma-glutamyl transferase.

Fig. (4)

The I148M PNPLA3 mutation and progressive liver disease. A simplified working model showing the influence of the I148M PNPLA3 variant on the progression of chronic liver disease.

Fig. (5)

The insulin signalling pathway in hepatocytes. Insulin (I) binding to the extracellular subunits of insulin receptor (InsR) leads to activation of tyrosine kinase in the intracellular domain, adenosine triphosphate (ATP) binding and finally receptor autophosphorylation. ENPP1 is a membrane glycoprotein that interact with InsR inhibiting its kinase activity. InsR autophosphorylation is followed by phosphorylation of the insulin-receptor substrates (IRS), activation of phosphoinositide 3-kinase (PI3-kinase) and subsequent phosphorylation of Akt/PKB (protein kinase B), which are involved in mediating the metabolic effect of insulin. FOXO1 is a transcription factor that in the absence of insulin induces gluconeogenesis, lipoprotein export, and apoptosis. Insulin-mediated Akt phosphorylation of FOXO1 leads to its nuclear exclusion, ubiquitination, and subsequent proteasomal degradation.