Low- and high-density lipoprotein metabolism in HepG2 cells expressing various levels of apolipoprotein E

To determine the importance of hepatic apolipoprotein (apo) E in lipoprotei n metabolism, HepG2 cells were transfected with a constitutive expression v ector (pRc/CMV) containing either the complete or the first 474 base pairs of the human apoE cDNA inserted in an antisense orientation, for apoE gene inactivation, or the full-length human apoE cDNA inserted in a sense orient ation for overexpression of apoE. Stable transformants were obtained that e xpressed 15, 24, 226, nd 287% the apoE level of control HepG2 cells. The me tabolism of low-density lipoprotein (LDL) and high-density lipoprotein-3 (H DL3), two lipoprotein classes following both holoparticle and cholesteryl e sters (CE)-selective uptake pathways, was compared between all these cells. LDL-protein degradation, an indicator of the holoparticle uptake, was grea ter in low apoE expressing cells than in control or high expressing cells, while HDL3-protein degradation paralleled the apoE levels of the cells (r(2 ) = 0.989). LDL- and HDL3-protein association was higher in low apoE expres sing cells compared to control cells. In apposition, LDL- and HDL3-CE assoc iation was not different from control cells in low apoE expressing cells bu t rose in high apoE expressing cells. In consequence, the CE-selective upta ke (CE/protein association ratio) was positively correlated with the level of apoE expression in all cells for both LDL (r(2) = 0.977) and HDL3 (r(2) = 0.998). We also show that, although in normal and low apoE expressor cell s, 92% of LDL- and 80% HDL3-CE hydrolysis is sensitive to chloroquine sugge sting a pathway linked to lysosomes for both Lipoproteins, cells overexpres sing apoE lost 60% of chloroquine-sensitive HDL3-CE hydrolysis without affe cting that of LDL-CE. Thus, the level of apoE expression in HepG2 cells det ermines the fate of LDL and HDL3.