Cholesterol feeding increases serum VLDL and hepatic phosphatidate phosphohydrolase in hamsters

Abstract

Hepatic Mg2 + -dependent phosphatidate phosphohydrolase (PAP-I) is involved in the de now synthesis of triacylglycerols, phosphatidylcholine and phosphatidylethanolamine. The biosynthesis of phospholipids takes precedence over that of triacylglycerols when the rate of synthesis of diacylglycerol is low. This ensures that membrane turnover and bile secretion are maintained. PAP-1 is located in the cytosol but for metabolic function it translocates to the endoplasmic reticulum. Such translocation is stimulated by high intrecellular fatty acid concentrations and this is believed to be a major point of regulation of the synthesis of hiacylglycerol and phospholipids Ill.Considerable interest has been centred on the hamster as a model for studying lipoprotein metabolism. This is because cholesterol metabolism in this species resembles that of humans in a number of respects, including the respoase to dietary cholesterol and fat 121. Far less interest has focused on Eriacylglycerol metabolism. although Ontko et al I31 have reported an apparent hypertriglyceridaemia in hamsters, relative to rats. This isassociated w i t h iacreased incorporation of nonesterified fatty acids into triacylglycerol and increased hepatic production of VLDL. In the present study we have looked at the effect of dietary cholesterol on plasma VLDL concentrations and the activity of PAP-1.24 Male Golden Syrian hamsters, 3-4 months old, were fed diets containing O,O.M,O. 12 or 0.24% (wlw) cholesterol, for 28 days. At the end of the trial, fasting blood was collected by cardiac plncture and livers were removed and frozen prior to measuring PAP-1 activity. Lipoproteins were separated from serum by preparative ultracentrifugation and VLDL cholesterol and triacylglycerol was measured enzymatically. Serum nonesterified fatty acids were also measured by an enzymatic method. PAP-I activity was measured by the method of Martin et a1 141 and is defmed as n-ethylmaleimide sensitive, Mg2+ -Stimulated PAP activity. Table 1 summarizes our findings. A strong correlation was found between VLDL triacylglycerol concentrations and dietary cholesterol (r = 0.981). Further correlations were also seen between dietary cholesterol and serum non-esterified fatty acids (r = 0.999) and hepatic PAP-1 (r = 0.971). VLDL triacylglycerol and PAP-I were also highly correlated (r = 0.861).Finally, a strong correlation between VLDL cholesterol and triacylglycerol was also found (r = 0.977).These results suggest that cholesterol feeding induces hypertriglyceridemia in hamsters. This i?mease in serum VLDL triacylglycerol is associated with increases in serum nonesterified fatty acids and hepatic PAP-I activity. The reason for the increase in non-esterified fatty acids is unclear. It may be that cholesterol feeding induces an insulin resistant state and that this results in increased lipolytic activity in adipose tissue. It seems likely that PAP-I activity is increased in response to the elevated fatty acid levels and this results in increased synthesis and secretion of triacylglycerol...