Apolipoprotein A-I stimulates secretion of apolipoprotein E by foam cell macrophages.
Apolipoprotein A-I (apoA-I) overexpression inhibits atherogenesis in mice, and apolipoprotein E (apoE) secreted by foam cell macrophages may exert antiatherogenic effects within the arterial wall. We hypothesized that interaction between apoA-I and apoE contributed to the antiatherogenic properties of apoA-I, and therefore investigated whether apoA-I stimulated secretion of apoE by foam cell macrophages. Cholesterol enrichment of primary murine and human macrophages increased spontaneous apoE secretion 2-fold, as quantified by Western blot and chemiluminescence detection. Human apoA-I caused a further marked increase of apoE secretion from both murine (3.8-fold, p < 0.01) and human (3.2-fold, p = 0.01) foam cells in a time- and concentration- dependent manner, and this increase was confirmed by immunoprecipitation of [(35)S]methionine-labeled macrophage apoE. The protein synthesis inhibitor cycloheximide, but not the transcription inhibitor actinomycin D, markedly inhibited apoE secretion to apoA-I (73.1 +/- 9.8% inhibition at 4 h) and completely suppressed apoE secretion beyond 4 h. Pretreatment of macrophages with Pronase inhibited initial apoA-I-mediated apoE secretion by 70.5 +/- 6.5% at 2 h, but by 8 h apoA-I-induced apoE secretion was the same in Pronase-pretreated and non-pretreated cells. Non-apolipoprotein-mediated cholesterol efflux induced by trimethyl-beta cyclodextrin did not enhance apoE secretion, whereas phospholipid vesicles inducing the same degree of cholesterol efflux substantially enhanced apoE secretion, and apoA-I and phospholipid vesicles in combination demonstrated additive induction of apoE secretion. We conclude that apoA-I concurrently stimulates apoE secretion and cholesterol efflux from foam cell macrophages and that lipoprotein-derived apoA-I may enhance local secretion and accumulation of apoE in atherosclerotic lesions.
Rees
D
D
Cell Biology, Heart Research Institute, 145 Missenden Road, Camperdown, Sydney, New South Wales 2050, Australia.
Sloane
T
T
Jessup
W
W
Dean
R T
RT
Kritharides
L
L
D000818:Animals; D016632:Apolipoprotein A-I / Q000097:blood / Q000494:pharmacology*; D001057:Apolipoproteins E / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D002478:Cells, Cultured; D002784:Cholesterol / Q000378:metabolism; D003513:Cycloheximide / Q000494:pharmacology; D003609:Dactinomycin / Q000494:pharmacology; D005487:Foam Cells / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008163:Luminescent Measurements; D008715:Methionine / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D013045:Species Specificity; D013462:Sulfur Radioisotopes
D016632:Apolipoprotein A-I; D001057:Apolipoproteins E; D013462:Sulfur Radioisotopes; D003609:Dactinomycin; D002784:Cholesterol; D003513:Cycloheximide; D008715:Methionine