DIETARY CHOLESTEROL AS HIGH DENSITY LIPOPROTEIN METABOLISM REGULATOR IN RABBITS

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New Zealand White rabbits develop hypercholesterolemia, hyperlipoproteinemia, and atherosclerosis when fed a diet containing 1% (w/w) cholesterol for 4-8 weeks. A predominant characteristic of this cholesterol-induced hyperlipoproteinemia is the appearance of beta-very low density lipoprotein (beta-VLDL), an atherogenic plasma lipoprotein particle, which is rich in apoliprotein E (apoE) and cholesteryl esters (CE). There is also a dramatic reduction in high density lipoprotein (HDL) levels in the plasma of cholesterol-fed rabbits. HDL particles are anti- atherogenic and are rich in apolipoprotein A-1 (apoA-1) and CE. It is the aim of this research plan to provide a biochemical explanation for the disappearance of HDL particles in cholesterol- fed rabbits. Their disappearance may be due to a decrease in synthesis or to an increase in catabolism or to both. The HDL2 and HDL3 subfractions of the HDL fraction will be separated by polyacrylamide gel electrophoresis and by polyanion precipitation techniques. The gels will be scanned with a densitometer, and the HDL2 and HDL3 peak areas for normal and cholesterolemic rabbits will be compared. In addition, the cholesterol and apoA-1 content of these two HDL subclasses will be measured with an enzymatic assay will determine which of these two HDL subclasses is declining in the cholesterol-fed rabbit. They will also determine if apoA- 1 and cholesterol are disappearing from one or both of these particles at equal rates. Since apoA-1 and cholesterol are disappearing from one or both of these particles at equal rates. Since apoA-1 is the major apolipoprotein in the HDL fraction, its regulation by dietary cholesterol will be used as a model of how dietary cholesterol are disappearing from one or both of these particles at equal rates. Since apoA-1 is the major apolipoprotein in the HDL fraction, its regulation by dietary cholesterol will be used as a model of how dietary cholesterol regulates the rate of HDL synthesis in the liver and small intestine. A monospecific antisera for rabbit apoA-will be used to quantitatively immunoprecipate apoA-1 from labeled hepatocytes and labeled small intestine mucosal cells in order to locate the major site of apoA- 1 synthesis in normal and cholesterolemic rabbits as well as to determine the effect of dietary cholesterol on the rates of apoA- 1 synthesis in these organs. The relative rates of apoA-1 synthesis will be expressed as the percent of total protein synthesized. This research is expected to provide useful biochemical information about the mechanisms by which dietary cholesterol regulates the reduction in HDL levels and the induction of atherosclerosis in rabbits and possibly in man. Our long-term objectives are to determine the biological mechanisms by which dietary cholesterol regulates the metabolism of atherogenic and anti-atherogenic plasma lipoproteins.