Cholesterol that is acquired by most extrahepatic tissues must be returned to the liver for excretion in a process that has been termed reverse cholesterol transport. The concept of reverse cholesterol transport is based mainly on biochemical studies of individual enzymes thought to be involved in reverse cholesterol transport and studies of cholesterol efflux from cultured cells. More recently, proteins thought to be involved in reverse cholesterol transport have been knocked out or overexpressed in mice. However, there is no direct evidence that reverse cholesterol transport has been altered in any of these models. To date, an unambiguous demonstration of the reverse cholesterol transport pathway has not been obtained in whole animals. In preliminary studies we demonstrated the net (mass) movement of cholesterol from individual extrahepatic tissues into reconstituted nascent prebeta-migrating HDL in vivo and showed that this initial step in reverse cholesterol transport can be greatly accelerated. Here we propose to characterize the major steps in reverse cholesterol transport in vivo and to demonstrate sequentially that each step in the reverse cholesterol transport pathway can be accelerated resulting in the net (mass) movement of cholesterol from extrahepatic tissues to the liver for excretion into bile. Finally, we will demonstrate that cholesterol flux through the entire reverse cholesterol transport pathway can be accelerated in vivo resulting in the net movement of cholesterol from individual extrahepatic tissues into feces. These goals are now feasible because of methodological advances that allow us to quantify, for the first time, the major pathways of sterol flux in all tissues of the body in vivo.
The specific aims are: (1) to characterize the initial step in the reverse cholesterol transport pathway in vivo in terms of the effect of enhanced cholesterol efflux on pathways of cholesterol acquisition by extrahepatic tissues, the effect of acceptor particle composition on cholesterol efflux and the effect of diet on cholesterol efflux, (2) to characterize the effects of overexpressing lecithin cholesterol acyl transferase (LCAT) on reverse cholesterol transport in vivo, (3) To determine the metabolic consequences of increasing the flux of HDL cholesteryl ester or LDL cholesterol to the liver and (4) to demonstrate that cholesterol flux through the entire reverse cholesterol transport pathway can be accelerated in vivo resulting in the net movement of cholesterol from extrahepatic tissues into feces both in animals that lack and in animals that possess cholesteryl ester transfer protein (CETP).
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