Variable heparan sulfate proteoglycan binding of apolipoprotein E variants may modulate the expression of type III hyperlipoproteinemia

The initial step in the clearance of apolipoprotein (apo) E-enriched remnant lipoproteins from the plasma appears to be sequestration within the liver mediated by their binding to heparan sulfate proteoglycans (HSPG). The surface-bound remnants are believed to be internalized by their interaction with the low density lipoprotein (LDL) receptor-related protein or by the LDL receptor. Cholesterol-induced rabbit β-very low density lipoproteins (β- VLDL) enriched in human apoE3 display 4-5-fold enhanced binding to cultured cells. The present study attempts to determine whether recessive versus dominant type III hyperlipoproteinemia might be explained, at least in part, by a variable interaction of the mutant forms of apoE with the HSPG and impaired uptake. The β-VLDL + apoE2(Arg¹⁵⁸ → Cys), which is associated with recessive type III hyperlipoproteinemia, bound more poorly than β-VLDL + apoE3 but still possessed significant enhanced binding (~2-2.5-fold compared with β-VLDL without added apoE) to HepG2 and McA-RH7777 cells. In comparison, β-VLDL + apoE(Arg¹⁴² → Cys), β-VLDL + apoE(Arg¹⁴⁵ → Cys), and β-VLDL + apoE-Leiden, which are associated with dominant type III hyperlipoproteinemia, bound more poorly. This same hierarchy of binding and uptake was determined by [¹⁴C]oleate incorporation into cholesteryl esters in LDL receptor-negative cells and by secretion of apoE3 and the variant apoE forms from McA-RH7777 cells. Furthermore, the enhanced binding of the apoE- enriched β-VLDL was almost totally inhibited by heparinase treatment of the cells, and the basal binding activity was inhibited by 80-90% following addition of an LDL receptor antibody capable of blocking receptor-ligand interaction. The β-VLDL enriched in apoE or apoE·dimyristoylphosphatidylcholine complexes bound to isolated HSPG from McA-RH7777 cells or the rat liver to a very similar degree. Likewise, the binding of β-VLDL plus the various forms of apoE to the LDL receptor- related protein on ligand blots paralleled the results of other studies. In conclusion, all of the type III hyperlipoproteinemic apoE variants are defective in displaying enhanced binding to HSPG and in the cellular uptake initiated by HSPG. However, apoE2(Arg¹⁵⁸ → Cys) displayed more activity than the variants associated with the dominant forms of type III hyperlipoproteinemia. The hierarchy of binding and uptake was as follows: apoE3 > apoE2(Arg¹⁵⁸ → Cys) > apoE(Arg¹⁴⁵ → Cys) > apoE(Arg¹⁴² → Cys) ≃ apoE-Leiden (the latter two usually displaying very little, if any, enhanced binding and uptake). Thus, a correlation exists between the mode of expression of type III hyperlipoproteinemia and the binding and uptake of the specific apoE mutation.