A decade of intense investigation has established a central role for lipoprotein receptors in regulating plasma cholesterol traffic. These receptors bind cholesterol-carrying lipoproteins and transport them into cells by receptor-mediated endocytosis. The receptor for low density lipoprotein (LDL) was the first discovered; it has been purified and its structure characterized. The LDL receptor was described in cultured human fibroblasts; identical receptors are now known to function in liver as well as in extrahepatic tissues. Operationally, the LDL/LDL receptor system can be considered the primary transport mechanism for endogenous cholesterol. Dietary cholesterol, on theother hand, is transported from the intestine in chylomicron remnants, which enter hepatocytes via distinct chylomicron remnant receptors. The chylomicron rem-nant/remnant receptor system is to exogenous cholesterol transport as the LDL/LDL receptor system is to endogenous cholesterol transport. In this article, we review recent developments in this rapidly advancing field.

Two hepatic lipoprotein receptors (Fig. 1). Exogenous (dietary) cholesterol is delivered to the liver in chylomicron remnants (1, 2), which are derived from intestinal chylomicrons through the action of lipopro-tein lipase. The remnantsrapidly enter the liverby receptor-mediated endocytosis after binding to specific remnant receptors (3-5). Endogenous cholesterol transport begins when the liver secretes cholesterol into plasma together with triglycerides in very low density lipoproteins (VLDL). After the triglycerides of VLDL are removed by lipoprotein lipase, the re-