Expression of LDL receptor binding determinants in very low density lipoproteins.

We have used both proteolysis and reconstitution experiments to characterize the determinants for LDL receptor binding of HTG-VLDL. In these studies, we showed that the removal of approximately one mole of apo E per mole of HTG-VLDL (Sf 100-400) and HTG-VLDL (Sf 60-100) by thrombin-specific cleavage results in loss of receptor binding and concomitant loss of suppression of HMG-CoA reductase. This is in direct contrast to the lack of effect thrombin cleavage has on the receptor-mediated uptake of LDL, an apo B-mediated process. We were able to reconstitute receptor binding in thrombin-treated HTG-VLDL (Sf 100-400) by the specific reincorporation of one mole of apo E into the VLDL. The incorporation of one mole of apo E into normal non-suppressive VLDL (Sf 60-400) also enables this lipoprotein to bind to the receptor as effectively as LDL. Trypsin, which destroys apo E-mediated, but not apo B-mediated binding to the LDL receptor, abolishes binding of HTG-VLDL (Sf 100-400) and HTG-VLDL (Sf 60-100), but not that of HTG-VLDL (Sf 20-60), IDL, or LDL to the LDL receptor. Therefore, we conclude that apo E of the appropriate conformation is required for receptor-mediated uptake by the LDL receptor of large TG-rich lipoproteins (Sf greater than 60). This conformation of apo E is probably related to the surface on which it is found (i.e., size of the particle) and the mode of incorporation into the phospholipid surface (i.e., transferred from plasma HDL). In large TG-rich particles, it appears that the intact apo E is necessary for the proper orientation of the molecule on the surface, with the carboxy-terminal one-third needed to anchor the apoprotein to the phospholipid surface. We believe that the binding of apo E to the LDL receptor is a redundant system and is used as a backup system in abnormal pathological states such as hypertriglyceridemia, abetalipoproteinemia, and hypobetalipoproteinemia. In the case of hypertriglyceridemia, where the lipolysis mechanism is overloaded, the abnormal binding of HTG-VLDL (Sf greater than 60) provides an alternate catabolic route for their removal from plasma. In the cases of a beta- and hypobetalipoproteinemia, where the normal particles for cholesterol delivery are either absent or at low levels, apo E-containing particles can serve to deliver cholesterol to cells as has been recently observed in vitro.(ABSTRACT TRUNCATED AT 400 WORDS)