Blocking of alcohol-induced effects on synthesis and secretion of apo B containing lipoproteins by substituting sardine oil in the diet.

The effect of substitution of fish oil in the diet on the alcohol-induced changes in the metabolism of very low density lipoprotein (VLDL) by primary cultures of rat hepatocytes has been studied. Rats fed groundnut oil diet or sardine oil diet were given alcohol (3 g/kg) for 4 weeks. Substitution of fish oil for groundnut oil in the diet blocked the hypertriglyceridemia and hyperlipoproteinemia caused by alcohol. Enhanced incorporation of [3H]leucine into apo B secreted into the medium and [14C]acetate into lipids associated with secreted VLDL indicated an increased rate of synthesis of apo B containing lipoproteins by hepatocytes from livers of rats receiving alcohol. Fish oil in the diet reduced incorporation of [3H]leucine into apo B and that of [14C]acetate into lipids indicating a lower rate of synthesis of apo B containing lipoproteins. Pulse chase experiments confirmed the above observation. Thus it is suggested that fish oil in the diet prevents hyperlipoproteinemia caused by alcohol possibly by reducing the synthesis and secretion of VLDL by liver.

Synthesis and secretion of lipoproteins by primary cultures of rat hepatocytes.

Synthesis and secretion of VLDL and LDL by primary cultures of rat hepatocytes maintained in serum free medium have been studied. A time-dependent increase found in the [3H]leucine labelled lipoproteins which floated at a density of 1.006 g/ml indicate the secretion of VLDL into the medium. That the hepatocytes also secrete. LDL is shown by floatation of [3H]leucine labelled lipoproteins by sequential centrifugation at a density range of 1.006-1.06 g/ml. Electrophoretic and immunoprecipitation analysis show that about 60% and 65% respectively of 3H-radioactivity is associated with apoB in the two fraction of lipoproteins. At about 12hr 70-75% lipoproteins in the culture medium is in the VLDL density range and 25-30% is in the LDL density range. Conversion of secreted VLDL to LDL has also been shown by incubating hepatocytes with pre-labelled lipoproteins when there is a decrease in the fraction of VLDL range with a corresponding increase in the fraction of the LDL density range. Addition of glycosaminoglycans such as hyaluronic acid, chondroitin sulphate, and heparin into the medium cause significant increase in the synthesis and secretion of [3H]apoB into the medium indicating a possible secretory control of apoB by local reuptake.

Synthesis and secretion of VLDL by rat hepatocytes--modulation by cholesterol and phospholipids.

The synthesis and secretion of apoB, the major protein component of very low density lipoprotein (VLDL) and low density lipoprotein (LDL), were studied using rat hepatocytes maintained in primary culture. Supplementation of hepatocytes with rat serum VLDL and LDL increased the production of apoB while delipidated lipoproteins had no significant effect, suggesting a role for lipids in the production of apoB. Addition of cholesterol to the culture medium also increased the production of apoB in a concentration-dependent manner. Pulse labelling followed by chase in presence of cholesterol indicated enhancement in apoB secretion. Mevinolin which inhibits cholesterol synthesis significantly reduced the secretion of apoB. The presence of phosphatidylcholine and phosphatidylethanolamine in the culture medium also increased the secretion of apoB into the medium. These data suggest that availability of lipids, particularly cholesterol, is an important determinant of apoB synthesis and secretion as VLDL.