Lipoprotein clearance mechanisms in LDL receptor-deficient "Apo-B48-only" and '"Apo-B100-only" mice

The role of the low density lipoprotein receptor (LDLR) in the clearance of apo-B48-containing lipoproteins and the role of the LDLR-related protein (LRP) in the removal of apo-B100-containing lipoproteins have not been clearly defined. To address these issues, we characterized LDLR-deficient mice homozygous for an "apo-B48-only" allele, an "apo-B100-only" allele, or a wild-type apo-B allele (Ldlr(-/-)Apob(48/48), Ldlr(-/-)Apob(100/100), and Ldlr(-/-)Apob(+/+), respectively). The plasma apo-B48 and LDL cholesterol levels were higher in Ldlr(-/-)Apob(48/48) mice than in Apob(48/48) mice, indicating that the LDL receptor plays a significant role in the removal of apo-B48-containing lipoproteins. To examine the role of the LRP in the clearance of apo-B100-containing lipoproteins, we blocked hepatic LRP function in Ldlr(-/-)Apob(100/100) mice by adenoviral-mediated expression of the receptor-associated protein (RAP). RAP expression did not change apo-B100 levels in Ldlr(-/-)Apob(100/100) mice. In contrast, RAP expression caused a striking increase in plasma apo-B48 levels in Apob(48/48) and Ldlr(-/-)Apob(48/48) mice. These data imply that LRP is important for the clearance of apo-B48-containing lipoproteins but plays no significant role in the clearance of apo-B100-containing lipoproteins.