My previous work described a receptor-independent pathway by which LDL is metabolized by hepatocytes. Uptake via this pathway displays several features which contrast with high-affinity receptor-mediated uptake of LDL: endogenous cholesterol synthesis is not regulated; much of the degradation of LDL apoprotein appears to be extralysosomal; there is a delay in the catabolism of internalized lipoprotein. In vivo studies which I have performed show that the liver is responsible for two-thirds of LDL catabolism both in normal animals (pigs rats, rabbits) and in animals genetically deficient in LDL receptors (WHHL rabbits). In normal animals we estimated that one-third of hepatic uptake is receptor-independent (low affinity). The studies described in this proposal are aimed at studying the pathways of LDL protein and cholesterol taken up by cells via high-affinity and low-affinity pathways. To elucidate the mechanisms underlying these differences I will employ some novel tracer techniques to follow the intracellular transport of LDL apoprotein and cholesterol. The questions which will be addressed are: 1) What is the intracellular route of LDL apoprotein and cholesterol in hepatocytes and adrenal cells? 2) Do metabolic perturbations (such as ACTH activation in adrenal cells) regulate intracellular transport of LDL apoprotein and/or cholesterol? 3) Is there a difference in the metabolic products of cholesterol entering via high-affinity pathways as contrasted from those derived from cholesterol entering via low-affinitity pathways? The experimental systems I will employ are cultured rat hepatocytes and cultured rat adrenal cells. Since these cells metabolize cholesterol to bile acids and corticosteroids respectively, they will provide """"""""benchmarks"""""""" with which to follow intracellular cholesterol transport. It is expected that these studies will augment our understanding of the complex traffic of lipoproteins through cells.
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