REGULATION OF HEPATIC LDL TRANSPORT IN VIVO

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The objectives of the original proposal were to develop methods for studying the regulation of hepatic LDL transport in vivo and then to apply this methodology to the study of dietary and drug manipulations that are known to alter LDL levels. During the past two years we have demonstrated that the liver is the main site of LDL catabolism, largely via a receptor-dependent pathways that is saturable and regulatable. We have also defined the kinetics of receptor-dependent and -independent LDL transport in the various organs of rats and hamsters in vivo. Finally, we have shown that dietary cholesterol can down-regulate hepatic receptor-dependent LDL transport and that the degree of down-regulation is greatly influenced by the type of triglyceride present in the diet. During the proposed two-year extension, I will attempt to define more precisely how dietary cholesterol and triglycerides regulate receptor-dependent LDL transport in the liver. In addition, studies will be undertaken to determine if these dietary lipids affect the rate of VLDL and LDL production, and if so, how? Animals will be fed diets containing various amounts of cholesterol plus or minus polyunsaturated, monounsaturated or saturated triglyceride. Rates of hepatic receptor-dependent LDL transport will be quantitated in vivo and compared with levels of LDL receptor mRNA and LDL receptor protein (assessed by ligand blotting) measured in the same animals. The effect of these diets on the lipid composition of hepatic membranes and on the transport of other ligands taken up by receptors will also be determined. These studies will indicate if the regulation of hepatic LDL receptors by dietary lipid is mediated at the transcriptional level or through effects on the lipid composition of plasma membranes which, in turn, could effect receptor binding, internalization or recycling. Since the effect of dietary triglycerides seems to require the presence of at least some cholesterol in the diet, cholesterol absorption and bile acid synthesis will be measured in animals fed the various triglycerides to see if the effect of triglycerides is simply to alter gross cholesterol balance across the liver. The effect of dietary lipids on VLDL and LDL production will also be determined and an attempt will be made to determine how LDL production is regulated. Finally, the effects of various drugs on diet induced hyperlipidemia will be studied. These studies will provide information that will be valuable in the development of regimens aimed at lowering plasms LDL levels in man.