Endogenously expressed apolipoprotein E has different effects on cell lipid metabolism as compared to exogenous apolipoprotein E carried on triglyceride-rich particles

Apolipoprotein E (apoE) on model triglyceride-rich particles (TGRP) increas es triglyceride (TG) utilization and cholesteryl eater (CE) hydrolysis, ind ependent of its effect on enhancing particle uptake. We questioned whether, under physiological concentrations, endogenously expressed apoE has simila r effects on cellular lipid metabolism as compared to exogenous apoE. J774 macrophages, which do not express apoE, were engineered to express endogeno us apoE by transfection of human apoE3 cDNA expression constructs (E+) or c ontrol vectors (E-) into the cells. To compare the effects of exogenous apo E; and endogenous apoE on TGRP uptake, cells were incubated with or without apoE associated with H-3-cholesteryl ether-labeled TGRP. Exogenous apoE en hanced TGRP uptake in both E- and E+ cells. E(-)cells displayed significant ly higher TGRP uptake than E+ cells. Sodium chlorate, which inhibits cell p roteoglycan synthesis, markedly diminished differences in TGRP uptake betwe en E- and E+ cells, suggesting that endogenous apoE-proteoglycan interactio n contributes to differences in uptake between the two cell types. Particle uptake by the LDL receptor, by the LDL receptor related protein, or by sca venger receptors were similar between E- and E+ cells indicating that endog enous apoE expression does not have a general effect on endocytic pathways. Exogenous apoE carried on TGRP stimulated TG utilization and CE hydrolysis in both cell types. However, TG utilization and CE hydrolysis were not aff ected by endogenous apoE expression. In conclusion, macrophage expression o f apoE has very different effects on TGRP metabolism than exogenously suppl ied apoE. The fluorescence microscopy results in this study showing that ex ogenous apoE and endogenous apoE were confined in separate cellular compart ments support the hypothesis that these differences resulted from distinct intracellular trafficking pathways followed by exogenous apoE bound to TGRP as compared to endogenous cell-expressed apoE.