Three distinct classes of receptors for
lipoproteins exist. The best studied is the
LDL receptor, the primary function of which is the delivery of
cholesterol in response to cellular needs. Although originally thought to be specific for
LDL, it clearly recognizes
lipoproteins that contain either
apo B or E. It plays an important role in the catabolism of
LDL and could also be involved in reverse
cholesterol transport. The hepatic remnant receptor, a distinct binding site on liver membranes that recognizes
apo E but not
apo B, appears to function in the clearance of
chylomicrons (and probably VLDL) remnants from the circulation, but also is likely to be important in the recognition of
apo E-containing HDL, and hence is likely to participate in the reverse
cholesterol transport. Finally, there is now evidence for a third group of
lipoprotein receptors that are present on the cell surface of macrophages. They appear to bind
lipoproteins that have been altered chemically or biologically and probably serve a scavenger function. While many of the model systems for studying these macrophage receptors have focused on chemical modifications that are unlikely to occur in vivo, several
lipoproteins that have been shown to interact with these receptors may be naturally occurring or result from biological processes. The discovery of the three receptor classes has resulted in a dramatic increase in the understanding of
lipoprotein physiology and pathophysiology, and future studies should further expand our understanding of the regulation of
lipoprotein metabolism and its relationship to
hyperlipoproteinemia and
atherosclerosis.