Post-translational regulation of macrophage apoprotein E production

We have transfected the murine macrophage cell line, J774, which does not express its endogenous apoE gene, with a constitutively expressed human apoE cDNA in order to study post-transcriptional and post-translational control of apoE production in macrophages. Loading cells with cholesterol using preincubations in acetylated low density lipoprotein, previously shown to enhance macrophage apoE gene transcription and apoE synthesis, did not increase apoE synthesis or secretion in constitutively expressing transfected cells, suggesting that sterol control of macrophage apoE production occurs predominantly at a transcriptional locus. However, incubation in human high density lipoprotein (HDL3) stimulated apoE secretion and appeared to inhibit degradation of newly synthesized apoE. This effect could be entirely reproduced by incubation with phosphatidylcholine vesicles which increased apoE accumulation in the medium by 2-6-fold. Pulse-chase experiments indicated that the effect of HDL3 or phospholipid vesicles was very rapid (occurring within 15 min) and was independent of changes in apoE synthesis. Furthermore, the increased apoE which accumulated in the medium in the presence of phospholipid vesicles or HDL3 was not due to altered rates of reuptake of labeled apoE since this difference was completely preserved in the absence of extracellular calcium. These results indicate that alteration of sterol content does not regulate macrophage apoE production at a translational or post-translational locus but that incubation with HDL3 or phospholipid vesicles can enhance apoprotein E production independent of changes in apoE gene transcription or apoE synthesis. The nature of the signal generated by the phospholipid vesicles which leads to inhibition of intracellular apoE degradation and enhanced apoE secretion will require further investigation.