ANALYSIS OF THE SELECTIVE UPTAKE OF THE CHOLESTERYL ESTER OF HUMAN INTERMEDIATE DENSITY LIPOPROTEINS BY HEPG2 CELLS

We have recently shown by the analysis of the association to HepG2 cells of human intermediate density lipoproteins (IDL) either labeled in cholesteryl ester (CE) with [H-3]cholesteryl ethers (CEt) or in proteins by iodine-125 that the CE of IDL are selectively taken up by these cells. Our results also revealed that the addition of a sufficient quantity of HDL3 abolishes the binding of IDL to the 'Lipoprotein binding site' (LBS) and also the CE-selective uptake. This suggested that the LBS mediates this uptake (Brissette and Falstrault (1992) Biochim. Biophys. Acta. 1165, 84-92). This study was undertaken to analyze further the mechanism of the selective uptake of the CE of IDL by HepG2 cells to determine if the LBS is directly or indirectly involved. We show that the different labeling had no effect on the binding affinity of IDL to HepG2 cells. To verify that the apolipoprotein moiety of HDL3 was responsible for the abolishment of the CE selective uptake, we have studied the effect of free apoA-I and apoA-II on the association of IDL. Our results demonstrate that apoA-I and apoA-II are approximately 10-times better than HDL3 or apoA-I liposomes in abolishing the selective uptake of the CE from IDL. We also show that this correlates with a more efficient reduction of the binding of I-125-IDL to HepG2 cells by free apoA-I compared to apoA-I associated with lipids. Thus free apoA-I interfere with the binding of IDL to the LBS and free apolipoproteins have a better capacity to saturate the LBS than lipoproteins. Also, we found no evidence for the transfer of CE from the labeled IDL to HDL3 or to apolipoproteins used to abolish the interaction of IDL to the LBS. Thus our results indicate that the LBS is directly responsible for the selective uptake of CE.