Cellular metabolism of human plasma intermediate-density lipoprotein (IDL)

The cellular metabolism of human plasma intermediate-density lipoprotein (IDL) was investigated in cultured human skin fibroblasts and hepG-2 cell in the absence and presence of exogenous recombinant or plasmatic apo E-3. IDL (d 1.006-1.019 g/ml) and LDL (d 1.019-1.063 g/ml) were prepared by centrifugation from the plasma of apo E- 3 3 or 4 3 normolipidemic human subjects. Without added apo E-3, IDL binding and cell association are similar or slightly reduced while their degradation is one third to one half. This results in degradation to binding ratios for IDL that are half those for LDL. Exogenous apo E-3 enhances binding, association and degradation of IDL by 50-150%, but the degradation to binding ratio remains low. Exogenous apo E-3 also increased the ability of IDL but not LDL, to down-regulate the incorporation of [¹⁴C]acetate to sterol by the cells. The optimal concentration of apo E-3 is 4 μg protein/10 μg IDL protein and at that concentration appreciable amounts of the apo E are found associated with the lipoprotein. Apo E-2 has no effect on the cellular metabolism of IDL and apo E-3 is not effective in receptor-negative human fibroblasts. Monoclonal antibodies that block apo E binding to B, E (LDL) receptor (1D7) abolish the cellular metabolism of IDL while antibodies against B-100 (4G3) are ineffective. In competitive binding experiments, IDL is slightly more effective than LDL in displacing ¹²⁵I-LDL from receptors in hepG-2 cells and appreciably more effective than LDL when tested against ¹²⁵I-IDL. Apo E-3 increases the capacity of IDL to compete with either ¹²⁵I-LDL or ¹²⁵I-IDL. Addition of apo E-3 also increases the binding affinity of IDL to hepG-2 receptors, with K_d values of 2.50, 0.93 μg protein/ml, respectively. The study demonstrates the essential role that functional apo E molecules play in the interaction of human IDL with cellular receptors. Yet, in spite of presence of apo E in IDL (2-3 molecules /particle) and enrichment of IDL with apo E-3 (to 4-5 molecules /particle) the proteolytic degradation of the lipoprotein by specific cellular receptor is similar to LDL.