Docosahexaenoic acid shows no triglyceride-lowering effects but increases the peroxisomal fatty acid oxidation in liver of rats

Abstract

The effect of docosahexaenoic acid (DHA) on mitochondrial and peroxisomal fatty acid oxidation and on key enzymes in triglyceride metabolism was investigated in the liver of rats fed a standard diet, a cholesterol diet, and a pelleted chow diet. Unexpectedly, in all three rat models repeated administration of highly purified DHA (92% pure) at different doses and times, at a dose of 1000 mg/day per kg body weight, resulted in no significant decrease of hepatic and plasma concentration of triglycerides. The serum concentrations of cholesterol and phospholipids showed an increase in a time-dependent manner in rats fed the pelleted chow diet. The hepatic concentration of cholesterol was increased in rats fed the cholesterol diet and pelleted chow diet after administration of DHA compared to palmitic acid. In all rat models, treatment with DHA tended to increase the peroxisomal beta-oxidation. This was accompanied with a significant increase (1.5-fold) of fatty acyl-CoA oxidase activity. The mitochondrial fatty acid oxidation system and carnitine palmitoyl-transferase activity, however, were almost unchanged. Moreover, palmitoyl-CoA synthetase activity was increased, whereas the palmitoyl-CoA hydrolase activity was decreased. Neither microsomal phosphatidate phosphohydrolase activity nor cytosolic phosphatidate phosphohydrolase activity was affected by DHA feeding in the three rat models. Acyl-CoA:1,2-diacylglycerol acyltransferase activity was also unaffected. In contrast to docosahexanoic acid feeding, eicosapentaenoic acid (EPA) administration possessed a hypotriglyceridemic effect and resulted in an increase of mitochondrial and peroxisomal oxidation of fatty acids. Carnitine palmitoyltransferase activity was also stimulated. Phosphatidate phosphohydrolase activity was unaffected whereas diacylglycerol acyltransferase activity was increased by EPA treatment compared with palmitic acid feeding. The results indicate that docosahexaenoic acid, in contrast to eicosapentaenoic acid, does not inhibit the synthesis and secretion of triglycerides in the liver. In addition, the results emphasize the importance that stimulation of peroxisomal beta-oxidation by these n-3 fatty acids is not sufficient to decrease the serum levels of triglycerides. In addition, increased mitochondrial beta-oxidation of fatty acids and thereby decreased availability of nonesterified fatty acids may be a mechanism by which EPA inhibits triglyceride, and subsequently very low density lipoprotein-triglyceride, production. Whether DHA and EPA possess different metabolic properties should be considered.