Low and high density lipoprotein metabolism in primary cultures of hepaticcells from normal and apolipoprotein E knockout mice

Apolipoprotein E (apoE) plays a major role in lipoprotein metabolism by med iating the binding of apoE-containing lipoproteins to receptors. The role o f hepatic apoE in the catabolism of apoE-free lipoproteins such as low dens ity lipoprotein (LDL) and high density lipoprotein-3 (HDL3) is however, unc lear. We analyzed the importance of hepatic apoE by comparing human LDL and HDL3 metabolism in primary cultures of hepatic cells from control C57BL/6J and apoE knockout (KO) mice. Binding analysis showed that the maximal bind ing capacity (B-max) of LDL, but not of HDL3, is increased by twofold in th e absence of apoE synthesis/secretion. Compared to control hepatic cells, L DL and HDL3 holoparticle uptake by apoE KO hepatic cells, as monitored by p rotein degradation, is reduced by 54 and 77%, respectively. Cleavage of hep aran sulfate proteoglycans (HSPG) by treatment with heparinase I reduces LD L association by 21% in control hepatic cells. Thus, HSPG alone or a hepati c apoE-HSPG complex is partially involved in LDL association with mouse hep atic cells. In apoE KO, but not in normal hepatic cells, the same treatment increases LDL uptake/degradation by 2.4-fold suggesting that in normal hep atic cells, hepatic apoE increases LDL degradation by masking apoB-100 bind ing sites on proteoglycans. Cholesteryl ester (CE) association and CE selec tive uptake (CE/protein association ratio) from LDL and HDL3 by mouse hepat ic cells were not affected by the absence of apoE expression. We also show that 69 and 72% of LDL-CE hydrolysis in control and apoE KO hepatic cells, respectively, is sensitive to chloroquine revealing the importance of a pat hway linked to lysosomes. In contrast, HDL3-CE hydrolysis is only mediated by a nonlysosomal pathway in both control and apoE KO hepatic cells. Overal l, our results indicate that hepatic apoE increases the holoparticle uptake pathway of LDL and HDL3 by mouse hepatic cells, that HSPG devoid of apoE f avors LDL binding/association but impairs LDL uptake/degradation and that a poE plays no significant role in CE selective uptake from either human LDL or HDL3 lipoproteins.