Review

NASH: a mitochondrial disease

Hepatic mitochondrial dysfunction is a major cause of fat accumulation in the liver. Individuals with fatty liver conditions have hepatic mitochondrial structural abnormalities and a switch in the side chain composition of the mitochondrial phospholipid, cardiolipin, from poly- to monounsaturated fatty acids. Linoleic acid (LA), an essential dietary fatty acid, is required to remodel nascent cardiolipin (CL) to its tetralinoleoyl cardiolipin (L₄CL, CL with 4 LA side chains) form, which is integral for mitochondrial membrane structure and function to promote fatty acid oxidation. It is unknown, however, whether increasing LA in the diet can increase hepatic L₄CL concentrations and improve mitochondrial respiration in the liver compared with a diet rich in monounsaturated and saturated fatty acids.

The main aim of this study was to test the ability of a diet fortified with LA-rich safflower oil (SO), compared with the one fortified with lard (LD), to increase concentrations of L₄CL and improve mitochondrial respiration in the livers of mice.

Twenty-four (9-wk-old) C57 BL/J6 male mice were fed either the SO or LD diets for ∼100 d, whereas food intake and body weight, fasting glucose, and glucose tolerance tests were performed to determine any changes in glycemic control.

Livers from mice fed SO diet had higher relative concentrations of hepatic L₄CL species compared with LD diet-fed mice (P value = 0.004). Uncoupled mitochondria of mice fed the SO diet, compared with LD diet, had an increased baseline oxygen consumption rate (OCR) and succinate-driven respiration (P values = 0.03 and 0.01). SO diet-fed mice had increased LA content in all phospholipid classes compared with LD-fed mice (P < 0.05).

Our findings reveal that maintaining or increasing hepatic L₄CL may result in increased OCR in uncoupled hepatic mitochondria in healthy mice whereas higher oleate content of CL reduced mitochondrial function shown by lower OCR in uncoupled mitochondria.

Mitochondrial dysfunction affects hepatic lipid homeostasis and promotes ROS generation. Copine7 (CPNE7) belongs to the ubiquitous copine family of calcium-dependent phospholipid binding proteins. CPNE7 has a high calcium ion binding affinity and the capacity to scavenge reactive oxygen species (ROS). A recent study reported that abnormalities in fatty acid and lipid metabolism were linked to the gene variant of CPNE7. Therefore, the purpose of this study is to examine the role of Cpne7 in hepatic lipid metabolism based on mitochondrial function.

Lipid metabolism, mitochondrial function, and ROS production were investigated in high-fat diet (HFD)-fed Cpne7^−/− mice and H₂O₂-damaged HepG2 hepatocytes following CPNE7 silencing or overexpression.

Cpne7 deficiency promoted severe hepatic steatosis in the HFD-induced NAFLD model. More importantly, mitochondrial dysfunction was observed along with an imbalance of mitochondrial dynamics in the livers of HFD-fed Cpne7^−/−mice, resulting in high ROS levels. Similarly, CPNE7-silenced HepG2 hepatocytes showed high ROS levels, mitochondrial dysfunction, and increased lipid contents. On the contrary, CPNE7-overexpressed HepG2 cells showed low ROS levels, enhanced mitochondrial function and decreased lipid contents under H₂O₂-induced oxidative stress.

In the liver, Cpne7 deficiency causes excessive ROS formation and mitochondrial dysfunction, which aggravates lipid metabolism abnormalities. These findings provide evidence that Cpne7 deficiency contributes to the pathogenesis of NAFLD, suggesting Cpne7 as a novel therapeutic target for NAFLD.

Disturbed hepatic energy metabolism contributes to non-alcoholic fatty liver (NAFLD), but the development of changes over time and obesity- or diabetes-related mechanisms remained unclear.

Two-day old male C57BL/6j mice received streptozotocin (STZ) or placebo (PLC) and then high-fat (HFD) or regular chow diet (RCD) from week 4 (W4) to either W8 or W16, yielding control [CTRL = PLC + RCD], diabetes [DIAB = STZ + RCD], obesity [OBES = PLC + HFD] and diabetes-related non-alcoholic steatohepatitis [NASH = STZ + HFD] models. Mitochondrial respiration was measured by high-resolution respirometry and insulin-sensitive glucose metabolism by hyperinsulinemic-euglycemic clamps with stable isotope dilution.

NASH showed higher steatosis and NAFLD activity already at W8 and liver fibrosis at W16 (all p < 0.01 vs CTRL). Ballooning was increased in DIAB and NASH at W16 (p < 0.01 vs CTRL). At W16, insulin sensitivity was 47%, 58% and 75% lower in DIAB, NASH and OBES (p < 0.001 vs CTRL). Hepatic uncoupled fatty acid oxidation (FAO)-associated respiration was reduced in OBES at W8, but doubled in DIAB and NASH at W16 (p < 0.01 vs CTRL) and correlated with biomarkers of unfolded protein response (UPR), oxidative stress and hepatic expression of certain enzymes (acetyl-CoA carboxylase 2, Acc2; carnitine palmitoyltransferase I, Cpt1a). Tricarboxylic acid cycle (TCA)-driven respiration was lower in OBES at W8 and doubled in DIAB at W16 (p < 0.0001 vs CTRL), which positively correlated with expression of genes related to lipolysis.

Hepatic mitochondria adapt to various metabolic challenges with increasing FAO-driven respiration, which is linked to dysfunctional UPR, systemic oxidative stress, insulin resistance and altered lipid metabolism. In a diabetes model, higher TCA-linked respiration reflected mitochondrial adaptation to greater hepatic lipid turnover.

Funding bodies that contributed to this study were listed in the acknowledgements section.

Nonalcoholic fatty liver disease (NAFLD) is a global health concern associated with significant morbidity and mortality. NAFLD is a spectrum of diseases originating from simple steatosis, progressing through nonalcoholic steatohepatitis (NASH), fibrosis, and cirrhosis that may lead to hepatocellular carcinoma (HCC). The pathogenesis of NAFLD is mediated by the triglyceride accumulation followed by proinflammatory cytokines expression leading to inflammation, oxidative stress, and mitochondrial dysfunction denoted as “two-hit hypothesis”, advancing with a “third hit” of insufficient hepatocyte proliferation, leading to the increase in hepatic progenitor cells contributing to fibrosis and HCC. Wnt/β-catenin signaling is responsible for normal liver development, regeneration, hepatic metabolic zonation, ammonia and drug detoxification, hepatobiliary development, etc., maintaining the overall liver homeostasis. The key regulators of canonical Wnt signaling such as LRP6, Wnt1, Wnt3a, β-catenin, GSK-3β, and APC are abnormally regulated in NAFLD. Many experimental studies have shown the aberrated Wnt/β-catenin signaling during the NAFLD progression and NASH to hepatic fibrosis and HCC. Therefore, in this review, we have emphasized the role of Wnt/β-catenin signaling and its modulators that can potentially aid in the inhibition of NAFLD.

Fatty liver disease (FLD) has a wide histopathology spectrum including simple steatosis, steatohepatitis with or without fibrosis, and cirrhosis that may be complicated by hepatocellular carcinoma (HCC). The two main entities that comprise FLD, alcohol-related liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD) associated to metabolic dysfunction, have both similarities and differences. In this chapter, some of the features that are common to both are considered, followed by a review of the pathogenetic and pathological features of each and discussion on how the two can be distinguished clinically and histopathologically. NAFLD concurrent with other liver disease, NAFLD in other clinical settings and treatment options are also discussed.

Metabolic dysfunction–associated fatty liver disease (MAFLD) is a chronic liver disease that currently lacks approved pharmacological treatment options. The mechanisms and active ingredients of Polygonum cuspidatum (PC) that regulate the mitochondria to relieve MAFLD have not been assessed. Thus, this study was designed to explore the bioactive components of PC extract in regulating mitochondria to alleviate high-fat diet–induced MAFLD using mitochondrial pharmacology and pharmacochemistry. Our results demonstrate that PC protected the mitochondrial ultrastructure and inhibited oxidative stress and energy metabolism disorder in the liver mitochondria. Furthermore, PC-derived components in the liver mitochondria attenuated oxidative stress and restored the energy metabolism of fat emulsion–induced steatosis in L02 cell. Sixteen compounds were identified in the liver–mitochondrial extracts of PC-treated rats. The antisteatotic effects of three identified monomers and anti-MAFLD ability of the monomer group were confirmed. Collectively, our data suggest that the extract of PC can alleviate lipid metabolism disorder in MAFLD by protecting the mitochondrial ultrastructure, reducing oxidative stress injury, and promoting energy metabolism. The sixteen identified compounds were potentially the main effective ingredients of PC in treating MAFLD. Thus, PC shows potential in treating MAFLD and related mitochondrial dysfunction. The proposed strategy to identify the ingredients of herbal medicines based on mitochondrial pharmacology and pharmacochemistry presents a new approach in exploring the pharmacodynamic components of herbal medicines that regulate mitochondria in preventing and treating diseases.