Date Published: December 1, 2009
Publisher: Public Library of Science
Author(s): Hiromi Ii, Naoki Yokoyama, Shintaro Yoshida, Kae Tsutsumi, Shinji Hatakeyama, Takashi Sato, Keiichi Ishihara, Satoshi Akiba, Ian Lanza. http://doi.org/10.1371/journal.pone.0008089
Abstract: Hepatic fat deposition with hepatocellular damage, a feature of non-alcoholic fatty liver disease, is mediated by several putative factors including prostaglandins. In the present study, we examined whether group IVA phospholipase A2 (IVA-PLA2), which catalyzes the first step in prostanoid biosynthesis, is involved in the development of fatty liver, using IVA-PLA2-knockout mice. Male wild-type mice on high-fat diets (20% fat and 1.25% cholesterol) developed hepatocellular vacuolation and liver hypertrophy with an increase in the serum levels of liver damage marker aminotransferases when compared with wild-type mice fed normal diets. These high-fat diet-induced alterations were markedly decreased in IVA-PLA2-knockout mice. Hepatic triacylglycerol content was lower in IVA-PLA2-knockout mice than in wild-type mice under normal dietary conditions. Although high-fat diets increased hepatic triacylglycerol content in both genotypes, the degree was lower in IVA-PLA2-knockout mice than in wild-type mice. Under the high-fat dietary conditions, IVA-PLA2-knockout mice had lower epididymal fat pad weight and smaller adipocytes than wild-type mice. The serum level of prostaglandin E2, which has a fat storage effect, was lower in IVA-PLA2-knockout mice than in wild-type mice, irrespective of the kind of diet. In both genotypes, high-fat diets increased serum leptin levels equally between the two groups, but did not affect the serum levels of adiponectin, resistin, free fatty acid, triacylglycerol, glucose, or insulin. Our findings suggest that a deficiency of IVA-PLA2 alleviates fatty liver damage caused by high-fat diets, probably because of the lower generation of IVA-PLA2 metabolites, such as prostaglandin E2. IVA-PLA2 could be a promising therapeutic target for obesity-related diseases including non-alcoholic fatty liver disease.
Partial Text: Phospholipase A2 (PLA2) is an enzyme that catalyzes the hydrolysis of glycerophospholipids at the sn-2 position, generating free fatty acid (FFA) and lysophospholipid. Among the more than twenty isozymes of mammalian PLA2, group IVA PLA2 (IVA-PLA2) is a key enzyme responsible for the release of arachidonic acid, a precursor of prostaglandins (PGs) . Our recent study of IVA-PLA2-knockout mice fed normal chow diets demonstrated that decreases in hepatic triacylglycerol (TG) content and the size of epididymal adipocytes are observed with a lower serum level of PGE2 compared with wild-type mice. This suggests that the circulating level of PGE2 is related to the levels of intracellular TG in the liver and adipose tissues . Previous reports showed that the stimulation of rat hepatocytes with PGE2 and the administration of PGE2 to rats induce increases in TG level in the cells and the liver, respectively , . Similarly, 15-deoxy-PGJ2 endogenously generated in mature 3T3-L1 adipocytes has been shown to be responsible for the storage of TG . Considering these findings, it is possible that IVA-PLA2 mediates fat deposition in the liver and adipose tissues through the generation of PGs.
Microscopic views of the liver of wild-type and IVA-PLA2-knockout mice fed normal or HF diets for 8 or 16 weeks are shown in Figure 1. The results reveal an apparent cytoplasmic vacuolation of hepatocytes around the central vein in wild-type mice fed HF diets for 8 (Figure 1A) or 16 (Figure 1B) weeks compared with wild-type mice fed normal diets for 8 (Figure 1A) or 16 weeks (data not shown). The degree of hepatic vacuolation increased with the period of feeding. In wild-type mice fed HF diets for 16 weeks, a markedly high level of cytoplasmic vacuolation of hepatocytes around the portal vein was observed compared with that in cells around the central vein (Figure 1B). The vacuolated area was not stained with periodic acid-Schiff, which stains glycogen (data not shown). In contrast to wild-type mice fed HF diets, we found that cytoplasmic vacuolation of hepatocytes around the central and portal veins was strikingly suppressed in IVA-PLA2-knockout mice fed HF diets for 8 weeks (Figure 1A). Even after 16 weeks of HF feeding, IVA-PLA2-knockout mice exhibited considerably reduced hepatic vacuolation (Figure 1B). Under normal dietary conditions, slight cytoplasmic vacuolation was observed in hepatocytes of wild-type mice; however, this was not the case for IVA-PLA2-knockout mice (Figure 1A). This observation in mice fed normal diets was consistent with the results shown in our recent paper .
Fatty liver with hepatocellular damage occurs in parallel with the accumulation of hepatic TG, which is associated with abdominal obesity. The present study showed that HF feeding of wild-type mice increased hepatocellular vacuolation, hepatic TG content, and serum aminotransferase levels with increases in adipose tissue mass and the number of hypertrophic adipocytes, indicating that HF diet-fed wild-type mice exhibited fatty liver damage with adipose fat deposition. The amounts of hepatic and adipose fats are regulated by several factors including FFA, insulin, adipokines, and PGs. Among these factors, PGs are generated by the cyclooxygenase pathway of the arachidonic acid cascade, the first step of which is mainly catalyzed by IVA-PLA2. Previous reports showed that PGE2 and 15-deoxy-PGJ2 accelerate the deposition of fats in hepatocytes and adipocytes , , . Furthermore, the administration of PGE2 to rats increases hepatic TG content . In addition to PGs, arachidonic acid metabolites of the 12-lipoxygenase pathway, such as 12-hydroxyeicosatetraenoic acid, are suggested to mediate inflammation occurring with fat deposition in adipose tissues . Considering the role of IVA-PLA2 in the arachidonic acid cascade, it is possible that hepatic fat deposition could be ameliorated when the production of such arachidonic acid metabolites is suppressed by the inhibition of IVA-PLA2. Consistent with our expectations, the present study clearly showed that in IVA-PLA2-knockout mice with lower serum levels of PGE2, a lack of IVA-PLA2 alleviated the HF diet-induced hepatic fat deposition with hepatocellular damage, and reduced adipose accumulation. Our findings suggest that an inhibition of IVA-PLA2 prevents the HF diet-induced development of fatty liver damage. This notion is consistent with a recent observation that the administration of a cyclooxygenase-2 inhibitor to HF diet-fed rats prevents fat deposition in the liver and adipose tissues .