The role of reverse cholesterol transport in animals and humans and relationship to atherosclerosis

Abstract

Reverse cholesterol transport (RCT) is a term used to describe the efflux of excess cellular cholesterol from peripheral tissues and its return to the liver for excretion in the bile and ultimately the feces. It is believed to be a critical mechanism by which HDL exert a protective effect on the development of atherosclerosis. In this paradigm, cholesterol is effluxed from arterial macrophages to extracellular HDL-based acceptors through the action of transporters such as ABCA1 and ABCG1. After efflux to HDL, cholesterol may be esterified in the plasma by the enzyme lecithin:cholesterol acyltransferase and is ultimately transported from HDL to the liver, either directly via the scavenger receptor BI or after transfer to apolipoprotein B-containing lipoproteins by the cholesteryl ester transfer protein. Methods for assessing the integrated rate of macrophage RCT in animals have provided insights into the molecular regulation of the process and suggest that the dynamic rate of macrophage RCT is more strongly associated with atherosclerosis than the steady-state plasma concentration of HDL cholesterol. Promotion of macrophage RCT is a potential therapeutic approach to preventing or regressing atherosclerotic vascular disease, but robust measures of RCT in humans will be needed in order to confidently advance RCT-promoting therapies in clinical development.

Fig. 1.

Reverse cholesterol transport. The liver secretes lipid-poor apoA-I which quickly acquires cholesterol via the hepatocyte ABCA1 transporter. Lipid-poor apoA-I also promotes the efflux of cholesterol from macrophages via ABCA1. Mature HDL is formed by the action of LCAT and promotes cholesterol efflux from macrophages via the ABCG1 transporter, as well as from other peripheral tissues by processes not fully defined. Mature HDL can transfer its cholesterol to the liver directly via SR-BI or indirectly via CETP-mediated transfer to apoB-containing lipoproteins, with subsequent uptake by the liver via the LDL receptor. Hepatic cholesterol can be excreted into the bile directly as cholesterol or after conversion to bile acids and unless reabsorbed by the intestine is ultimately excreted in the feces.