In the present study, we demonstrated that levels of vanin-1 mRNA in mice on a high-fat diet increase within a day and that vanin-1 mRNA expression is more prominent around lipid droplet formations in the liver. Although vanin-1 expression did not change with the age of the mice with liver steatosis or grade of liver steatosis, the appearance of vanin-1 upregulation before the onset of liver steatosis suggests a definite role of vanin-1 in the progression of fatty liver disease.
Our findings indicate that serum FFAs derived from a high-fat diet may act as regulators of vanin-1 mRNA expression. Normally, high plasma FFA levels may either increase hepatic FFA uptake or at least maintain a normal rate of hepatic FFA uptake, despite increased hepatic FFA levels.(23)
Therefore, in our study model, the vanin-1 mRNA expression in hepatocytes may have been upregulated by the increased rate of hepatic FFA uptake caused by elevated plasma FFA levels.
In general, FFAs are continuously metabolized and oxidized in hepatocytes. Oxidative products generated during these processes give rise to oxidative stress.(24,25)
Vanin-1 gene expression is reportedly upregulated by oxidative stress caused by H2
and γ-irradiation through 2 antioxidant response-like elements in thymic-sorted cells.(12)
Interestingly, we observed that following the administration of high-fat diet, the increase in vanin-1 mRNA levels (seen as early as Day 1 after high-fat diet) preceded the onset of lipid accumulation, the hallmark of NAFLD (seen at 2 weeks after high-fat diet). Oleic acid, in particular, at concentrations of
0.01 mM was able to significantly upregulate vanin-1 mRNA expression in vitro
when compared with control cultures (Fig. B); however, lipid droplets were not observed at oleic acid concentrations of 0.01 and 0.1 mM. Hence, it is possible that oxidative stress produced through beta oxidation of FFAs in the hepatocytes contributes to the induction of vanin-1 gene, and marks an early stage of NAFLD. The pantetheinase activity of vanin-1 hydrolyzes pantetheine, an intermediate metabolite of coenzyme A, into pantothenic acid (vitamin B5) and cysteamine, a potent antioxidant.(26)
Pantothenic acid, pantothenol, and other derivatives are known to protect cells and whole organs against peroxidative damage by increasing the content of cellular glutathione.(27)
Therefore, the upregulation of vanin-1, leading to increased levels of cysteamine, may be an adaptive mechanism against hepatic oxidative stress induced by high-fat diet.
Previously, it has been reported that hepatic vanin-1 mRNA was upregulated in hepatic steatosis.(7–9)
Further, PPARα is a major coordinator of fatty acid oxidation in the liver.(28)
FFAs increase PPARα mRNA expression, and its activation regulates several key genes involved in fatty acid uptake and β-oxidation.(29,30)
We have previously shown that a high-fat diet induces the mRNA expression of ADRP,(31)
which is a reliable lipid droplet marker in fatty liver condition.(32)
Furthermore, it has been reported that vanin-1 mRNA is upregulated by PPARα activation.(33–36)
In the present study, we observed that PPARα, but not vanin-1, is upregulated by all types of fatty acids. Hence, it is appealing to speculate that in addition to PPARα, vanin-1 is regulated by another upstream factor, whose expression/activity is controlled by the uptake of FFAs, and which may play a role in lipid metabolism in the liver.
NAFLD is characterized by fatty infiltration of the liver in the absence of alcoholic consumption, and currently affects approximately 30% of the adults and 10% of the children in the United States. It ranges from simple steatosis to NASH, which can progress to end-stage liver disease.(24,25)
The development of NAFLD is closely associated with obesity and type 2 diabetes.(6)
Previous reports have suggested hyperinsulinemia and inappropriately high amounts of free fatty acids as a cause of hepatic steatosis.(37)
However, a greater understanding of the pathophysiological changes is needed to develop more effective therapies for NAFLD.(6)
We therefore believe that our mouse model for high-fat diet-induced hepatic steatosis would be useful to study the pathophysiology and regulatory factors involved in the progression of steatosis to NASH. Because NASH is thought to be a mitochondrial-dysfunction disease, which is caused by the overproduction of reactive oxygen species that in turn triggers lipid peroxidation,(38)
it is quite likely that upregulation of vanin-1 plays a role in disease progression.
Our findings reveal that the upregulation of vanin-1 precedes lipid accumulation and is differentially mediated by various types of FFAs in the fatty liver model, presenting vanin-1 as a novel player in the pathogenesis of NAFLD. Further experiments using vanin-1 knockout mice would be necessary to establish the function of vanin-1 in the progression of NAFLD.