Non alcoholic fatty liver disease (NAFLD) is the most common liver disorder in the world, and in obesity, type 2 diabetes and related metabolic diseases, its incidence reaches 70-90% . The disease is characterised by the accumulation of triacylglycerol (TG) inside liver cells, and the condition can progress into more serious liver disease, such as non alcoholic steatohepatitis (NASH), liver fibrosis, cirrhosis, and more rarely, liver carcinoma . Although it is known that progression of the disease is more likely to occur in patients with metabolic diseases , the factors involved are not well understood. However, oxidative stress coupled with insulin resistance is believed to play an important role .
Current evidence indicates that insulin resistance causes oxidative stress via increased oxidation in the liver, raised formation of reactive oxygen species, higher levels of hepatic lipid peroxidation, protein oxidation and pro-inflammatory cytokine production and decreased antioxidant capacity in the plasma [3–7]. Since the hepatic steatosis in NAFLD is believed to follow from the development of insulin resistance, oxidative stress is now considered to be one of the major causes of NAFLD [8, 9].
Recent studies have suggested that NAFLD is associated with accelerated atherosclerosis , but the underlying mechanisms are not well understood. Moderately raised plasma levels of homocysteine (Hcy) have been found to be associated with atherosclerosis development, and hyperhomocysteinemia (HHcy) is considered to be an independent risk factor for cardiovascular disease [11, 12]. Hcy is a thiol-containing amino acid involved in the metabolism of methionine in the liver. Methionine from dietary sources is converted to S-adenosyl methionine (SAM) by methionine adenosyl transferase (MAT). SAM is the methyl donor in most biological methylation reactions, and various methyltransferases (eg phosphatidyl-ethanolamine N-methyltransferase (PEMT) and glycine N-methyltransferase (GNMT)) are involved in its usage for the formation of phospholipids, myelin, and other macromolecules . The product of the methyl transfer reactions is S-adenosyl homocysteine (SAH), which is then hydrolyzed to Hcy. Once formed, Hcy may be either remethylated to methionine (methionine cycle) or metabolised to cysteine (Cys) in a two step pathway catalysed by the enzymes cystathionine β-synthase (CBS) and cystathionine γ-lyase (CGL) (the transsulphuration pathway) [12, 13]. HHcy is known to be associated with atherosclerosis and other pathologies, and oxidative stress, due to NADPH oxidase or NO synthase dependent generation of superoxide anion combined with a decrease in antioxidant enzyme activity, is thought to be a major factor in its effects [11, 12]. In particular, when HHcy is due to disorders of the transsulphuration pathway, it associates with a reduced supply of Cys for the formation of the antioxidant glutathione (GSH) . It has been reported that HHcy caused by CBS deficiency leads to disturbances in the regulation of lipid metabolism and fat accumulation in the liver [14, 15]. Moreover, plasma Hcy is elevated in patients with NAFLD, and is a predictor of steatohepatitis .
Our previous work has shown that feeding rats a high fat diet (57% of energy from fat) induces insulin resistance, hypertriglyceridemia, hepatic steatosis and liver damage, which are characteristic of NAFLD and thus provides a suitable model for the early stages of the disease [17, 18]. In the present study, we have used this model to test the hypothesis that high fat diet-induced NAFLD in rats modifies hepatic Hcy metabolism via modulation of the transsuphuration pathway.