Lipid alterations in chronic liver disease and liver cancer

The human body stores energy as fat and carbohydrates. The neutral storage of FAs in the healthy liver is in the form of triglycerides (TGs), which are 3 FAs attached to a glycerol moiety, and sterol esters (SEs), in which FA is esterified to sterol. Neutral lipids (SEs and TGs) are stored in lipid droplets, and in a healthy liver, these lipids should not exceed 5%. FAs stored in TGs and SEs can be utilised at any time during liver homeostasis to generate energy (ATP) via fatty acid oxidation (FAO) or be transported to other organs in very-low-density lipoprotein.

Glycerophospholipids, sphingolipids, and cholesterol are major building blocks of the cellular membrane (Fig. 1). Glycerophospholipids include phosphatidylcholine (PC), phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and phosphatidic acid. PC accounts for more than 50% of the phospholipids in most eukaryotic membranes. The second most abundant lipid in the mammalian membrane is cholesterol, which accounts for 30% of lipids, and increases the lipid-packing density to maintain a high membrane fluidity. Lastly, sphingomyelin (SM) is the most abundant sphingolipid in mammalian cells and this lipid plays a crucial role in the formation of sterol-enriched ordered membrane domains and in cell-cell signalling.

Lipids act as first (extracellular) and second (intracellular) messengers in signal transduction and molecular recognition processes (reviewed in^,). As such, membrane glycerolipids and sphingolipids transduce signals through hydrolysis to generate bioactive molecules: ceramides and sphingosine-1-phosphate (S1P), while steroids (oxysterols, bile acids [BAs], steroid hormones) and FAs interact directly with receptors,^{,  ,  ,  ,}  such as CD36 (FA translocase) (Fig. 1).

HBV and HCV infections are important risk factors for liver cancer development. HBV is the main aetiology for HCC in most regions of Asia, Africa, and South America. HCV is the predominant cause in Western Europe, North America, and Japan.

Hepatic steatosis is often associated with both HBV and HCV infections,^,, as well as being observed in HBx (HBx protein is crucial in HBV tumorigenesis) transgenic^, and HCV transgenic mouse models. Hence, viral hepatitis leads to prominent changes in both the serum and hepatic (tumour and tumour-adjacent compartments) lipidomes (Table 1). Indeed, the blood FA composition is significantly altered in HBV- and HCV-infected patients. As such, serum levels of saturated FAs (SFAs) and monounsaturated FAs (MUFAs) are significantly increased^{,  ,}  in HBV-positive patients, and increase in parallel with disease severity.^, Concurrently, the class of polyunsaturated FA (PUFAs) is depleted.^{,  ,}  This change in FA composition is also observed in mouse HBV-positive liver tumours. Moreover, PUFAs are necessary for HCV particle replication as knockdown of fatty acid desaturase 2 (FADS2) - the first step in PUFA synthesis – impairs HCV virus particle production. Similarly, FAs are involved in stabilisation of the HBx protein. HBx can also increase the cholesterol levels in HCC cells, both in vitro and in vivo. Conversely, in both patients and HCV transgenic mice, cholesterol,^, TG, and lysophosphatidylcholine (LPC)^, of longer FA chains have been shown to be significantly depleted.

Excessive alcohol consumption is the main aetiological factor for liver cancer development in Central and Eastern Europe. In patients with alcohol-related liver disease (ALD), FAs that accumulate in the liver are predominantly released from the adipose tissue. Ethanol increases the uptake of FAs by the liver in vivo and in vitro leading to intrahepatic accumulation of TGs. Abstinence has been shown to reduce serum FA and LPC levels, while TGs stay elevated in patients with ALD (Table 1). Moreover, in several models, FA synthesis pathways are significantly upregulated in mice fed alcohol ad libitum in their drinking water.^{,  ,} 

NAFLD, ranging from steatosis to its progressive form NASH, is the most common liver disease in the developed world, and is an important risk factor for liver cancer development. NAFLD is associated with prominent changes in both the hepatic and serum lipidomes at the onset of steatosis, but as the disease progresses to NASH, only certain TGs^{,  ,}  and steroids change progressively. However, NAFLD-HCC is reflected by a complete rearrangement of the serum lipidome (Table 1). Patients with NAFLD have significantly increased levels of FAs, TGs, ceramides (Cer), and BAs, while phospholipids in the blood are depleted.^,,, In NAFLD, the most important metabolic dysregulation is a result of high lipolysis and non-esterified FAs released into the bloodstream. Hepatic FA profiles in patients with NAFLD are severely deregulated.^, Specifically, SFAs and PUFAs are significantly increased in NAFLD compared to normal livers. Conversely, in murine models, a higher consumption of n-6 FAs leads to the onset of NASH by inducing mitochondrial dysfunction and altered apoptosis. Furthermore, palmitic acid and linoleic acid (LA) have been found to modulate the immune response in murine models of NASH. Palmitic acid and LA stimulate neutrophils as well as macrophages to express and secrete inflammatory proteins (for example, interleukin-6, interleukin-10, chemokine (C-C motif) ligand 2, interferon-γ, and tumour necrosis factor). LA also upregulates carnitine palmitoyltransferase (CPT) leading to increased apoptosis of CD4⁺ T cells. This LA-mediated loss of intrahepatic CD4⁺ T cells, but not CD8⁺ T lymphocytes, results in HCC progression. On a background of NASH, CD8⁺ T cells promote the incidence of murine HCC because of impaired tumour surveillance and increased tissue damage by lymphocytes.

NAFLD is characterised by a significant increase of TGs in the circulation^,, and liver.^, These are TGs with longer carbon chains and fewer double bonds.^,, Among the lipids increased in both the blood and livers of NAFLD patients are Cer,^,, with an increase in dihydroceramides^, that are basic markers of de novo Cer synthesis. As such, murine models have shown a decrease in hepatic steatosis when levels of liver Cer are lowered by an increase in acid ceramidase activity or deletion of dihydroceramide desaturase 1¹⁰⁹. Moreover, several studies have shown that BA levels are increased in the liver,^, plasma^,, and faeces of patients with NASH. Elevated plasma levels of glycocholate, taurocholate, and taurochenodeoxycholate^, are associated with progressive liver deterioration and dysfunction. Furthermore, increased levels of cholic, chenodeoxycholic, and deoxycholic acids are present in liver tissue, leading to altered expression and activity of genes involved in BA, lipid and carbohydrate metabolism, energy expenditure, and inflammation. Meanwhile, PCs and LPCs (particularly classes that contain PUFAs) are depleted in livers and blood obtained from patients with NAFLD and NASH. Interestingly, sphingolipids, phospholipids, and TGs are putative biomarkers of NAFLD progression.^, Furthermore, cholesterol promotes NAFLD development.