Human plasma lipoproteins regulate apolipoprotein E secretion from a post-Golgi compartment.
The molecular regulation of apolipoprotein E (apoE) synthesis and secretion is incompletely understood. In this study, we have examined the effect of human low density lipoprotein (LDL) on apoE mRNA and protein levels in HepG2 and other eukaryotic cells. Exposing HepG2 cells to LDL for times up to 4 h resulted in an increase in 35S-labeled apoE accumulation in the medium by 2.2-fold, relative to serum free controls (n = 10, p < 0.001), with no changes in apoE mRNA levels. Similar observations have been made in JeG-3 cells and Chinese hamster ovary cells stably transfected with human apoE cDNA constructs. These results indicate that the LDL effect operates at a post-transcriptional level. In pulse-chase experiments, the LDL effect on apoE accumulation in the media was observed when it was added only during the chase even in the presence of cycloheximide, indicating that LDL is functioning at a post-translational level. The use of brefeldin A (BFA), an agent that impedes protein transport from the endoplasmic reticulum to the Golgi apparatus, suggests that the LDL effect occurs in a post-Golgi compartment. The addition of protease inhibitors could not duplicate the effects of LDL on the apoE accumulation in the medium. ApoA-I accumulation in the medium of HepG2 cells, but not albumin, was also significantly increased by 1.9-fold (n = 5, p < 0.001).
Ye
S Q
SQ
Department of Pathology, University of Chicago, Illinois 60637.
Olson
L M
LM
Reardon
C A
CA
Getz
G S
GS
D000418:Albumins / Q000378:metabolism; D000818:Animals; D001057:Apolipoproteins E / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D016466:CHO Cells; D002451:Cell Compartmentation; D002478:Cells, Cultured; D006224:Cricetinae; D003609:Dactinomycin / Q000494:pharmacology; D006056:Golgi Apparatus / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008077:Lipoproteins, LDL / Q000097:blood / Q000502:physiology*; D011233:Precipitin Tests; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000235:genetics; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured
D000418:Albumins; D001057:Apolipoproteins E; D008077:Lipoproteins, LDL; D012333:RNA, Messenger; D003609:Dactinomycin