Which apolipoprotein serves as the remnant receptor ligand recognizing chylomicron and VLDL remnants for hepatic uptake?

• A. Apolipoprotein C1
• B. Apolipoprotein C2
• C. Apolipoprotein C3
• D. Apolipoprotein E

Answer: (D)

Apolipoprotein E serves as the remnant receptor ligand that recognizes chylomicron and VLDL remnants for hepatic uptake. After triglyceride-rich lipoproteins are processed by lipoprotein lipase, the resulting remnants contain apoE on their surface. Hepatocytes express receptors from the LDL receptor family (including LDL receptor and LDL receptor–related protein), which bind apoE with high affinity. This binding triggers endocytosis and clearance of the remnants from the bloodstream, keeping circulating remnants in check and delivering lipids to the liver.

Other apolipoproteins on the surface modulate lipoprotein metabolism in different ways. ApoC-II acts as a cofactor that activates lipoprotein lipase to hydrolyze triglycerides, promoting lipolysis. ApoC-III can inhibit lipoprotein lipase activity and, in some contexts, slow hepatic clearance, but they are not the primary ligands for remnant uptake. ApoC-I and others have regulatory roles as well, but apoE is the key ligand for receptor-mediated hepatic uptake of remnants.