The effects of dietary n-3 polyunsaturated fatty acids delivered in chylomicron remnants on the transcription of genes regulating synthesis and secretion of very-low-density lipoprotein by the liver: modulation by cellular oxidative state.

The influence of chylomicron remnants enriched in n-3 or n-6 polyunsaturated fatty acids (PUFA) (derived from fish or corn oil, respectively) on the expression of mRNA for four genes involved in the regulation of the synthesis, assembly, and secretion of very-low-density lipoprotein (VLDL) in the liver was investigated in normal rat hepatocytes and after manipulation of the cellular oxidative state by incubation with N-acetyl cysteine (NAC) or CuSO(4). The four genes investigated were those encoding apolipoprotein B (apoB), the microsomal triacylglycerol transfer protein (MTP), and the enzymes acyl coenzyme A:diacylglycerol acyltransferase (DGAT) and acyl coenzyme A:cholesterol acyltransferase 2 (ACAT2), which play a role in the regulation of triacylglycerol and cholesteryl ester synthesis, respectively. mRNA levels for apoB, MTP, and DGAT were unaffected by either fish or corn oil chylomicron remnants, but the amount of ACAT2 mRNA was significantly reduced after incubation of the hepatocytes with fish oil remnants as compared with corn oil remnants or without remnants. These findings indicate that the delivery of dietary n-3 PUFA to hepatocytes in chylomicron remnants downregulates the expression of mRNA for ACAT2, and this may play a role in their inhibition of VLDL secretion. However, when the cells were shifted into a pro-oxidizing or pro-reducing state by pretreatment with CuSO(4) (1 mM) or NAC (5 mM) for 24 hr, levels of mRNA for MTP were increased by about 2- or 4-fold, respectively, by fish oil remnants, whereas corn oil remnants had no significant effect. Fish oil remnants also caused a smaller increase in apoB mRNA in comparison with corn oil remnants in NAC-treated cells (+38%). These changes would be expected to lead to increased VLDL secretion rather than the decrease associated with dietary n-3 PUFA in normal conditions. These findings suggest that relatively minor changes in cellular redox levels can have a major influence on important liver functions such as VLDL synthesis and secretion.

Comparison of the effects of dietary n-3 and n-6 polyunsaturated fatty acids on very-low-density lipoprotein secretion when delivered to hepatocytes in chylomicron remnants.

The effects of chylomicron remnants enriched in n-3 or n-6 polyunsaturated fatty acids (derived from fish or corn oil respectively) on the secretion of very-low-density lipoprotein (VLDL) lipid and apolipoprotein B (apoB) by rat hepatocytes in culture was investigated. Remnants were prepared in vivo from chylomicrons obtained from rats given an oral dose of fish or corn oil and incubated with cultured hepatocytes for up to 16 h. The medium was then removed and the secretion of cholesterol and triacylglycerol into the whole medium or the rho<1.050 g/ml fraction during the following 7-24 h was determined. After exposure of the cells to fish-oil as compared with corn-oil remnants, secretion of both cholesterol and triacylglycerol into the whole medium was decreased by 25-35%, and secretion into the rho<1.050 g/ml fraction was decreased by 20-25%. In addition, the levels of apoB48 found in the rho<1.050 g/ml fraction were significantly lower in cells treated with fish-oil rather than corn-oil remnants, although the levels of apoB100 remained unchanged. The expression of mRNA for apoB, as determined by reverse-transcriptase PCR, however, was not significantly changed after exposure of the cells to both types of remnants. These results demonstrate that the effects of dietary n-3 polyunsaturated fatty acids in depressing hepatic VLDL secretion occur directly when they are delivered to the liver from the intestine in chylomicron remnants, and that the secretion, but not the synthesis, of apoB is targeted.

Very low-density lipoprotein apolipoprotein B-100 turnover in glycogen storage disease type Ia (von Gierke disease).

Mixed hyperlipidaemia is a common finding in glycogen storage disease type Ia (GSD Ia). Although cross-sectional studies have demonstrated increases in intermediate-density lipoproteins (IDLs) and reductions in lipoprotein lipase activity, no studies have investigated the dynamics of apolipoprotein B-100 (apo B) metabolism in GSD Ia. This study investigated apoB turnover in GSD Ia using an exogenous labelling method in one sib from a kinship with established GSD Ia. The study demonstrated normal hepatic secretion of very low-density lipoprotein (VLDL), but hypocatabolism of VLDL, probably due to lack of lipoprotein lipase activity. The production rate of IDL was slightly increased, but the turnover rate of low-density lipoprotein was normal. The findings suggest that, as well as a corn starch diet and dietary fat restriction, treatment of severe mixed hyperlipidaemia in GSD Ia and its attendant risk of pancreatitis should possibly involve fibrates that activate lipoprotein lipase and may enhance the clearance of IDL, rather than omega-3 fatty acids, which principally suppress hepatic secretion of VLDL.