EFFECTS OF APOPROTEIN-E ON INTRACELLULAR METABOLISM OF MODEL TRIGLYCERIDE-RICH PARTICLES ARE DISTINCT FROM EFFECTS ON CELL PARTICLE UPTAKE

Apoprotein E (apoE) enhances uptake of triglyceride-rich lipoprotein p articles (TGRP). We questioned whether apoE would also modulate intrac ellular metabolism of TGRP in addition to its effects on particle upta ke. We prepared model TGRP with triolein and cholesteryl oleate (1:1, w/w) as the core lipids, emulsified by egg yolk phosphatidylcholine, a nd containing a non-degradable marker, [H-3]cholesteryl hexadecyl ethe r. Particles were intermediate density lipoprotein-sized as determined by core lipid/phospholipid ratios (2.0-3.0/1) and gel filtration chro matography on Sepharose CL-2B. Emulsions were incubated with J774 macr ophages for 5 min to 6 h at core lipid concentrations of 300-1200 mu g /ml and 0-0.2 mu g recombinant apoE/mg core lipid. Particle uptake was determined by [H-3]cholesteryl ether uptake and fluorescence microsco py in the absence and presence of apoE. Similar uptake of particles wi th and without apoE was achieved by utilizing a 4 times higher particl e concentration in the absence of apoE. At equivalent levels of uptake , particles with apoE lead to one-half of the triglyceride mass accumu lation and twice the triglyceride utilization as compared to particles without apoE. Further, apoE doubles cell cholesteryl ester hydrolysis and to a lesser extent (similar to 30%) increases cholesteryl ester r esynthesis by acyl-CoA cholesterol acyl-transferase. Particles, both w ith and without apoE, reach the lysosomal compartment as determined by colocalization with fluorescein-labeled alpha(2)-macroglobulin. These results suggest that, in addition to its role in enhancing TGRP uptak e, apoE has additional effects on modulating the cellular metabolism o f both triglyceride and cholesteryl ester, after particle internalizat ion.