Plasma apolipoprotein secretion by human monocyte-derived macrophages

Apolipoprotein E has been demonstrated to be a major secretory protein of human monocyte macrophages. The synthesis of the other plasma apolipoproteins by these cells has not been documented. Human monocyte macrophages cultured for 17-76 days were preincubated for 24 h in RPMI 1640/0.2% bovine serum albumin with or without malondialdehyde-LDL (100 micrograms/ml), followed by an additional 24 h incubation in RPMI 1640/0.2% bovine serum albumin. The media from the two incubation periods were analyzed for apolipoproteins A-I, B, C-II, C-III and E by specific radioimmunoassays. No apolipoprotein B mass was detected with a specific radioimmunoassay capable of detecting 10 ng apolipoprotein B. No apolipoproteins A-I, C-II or C-III mass was detected, even though the radioimmunoassays for these apolipoproteins were as sensitive as that for apolipoprotein E (detection limit of 0.2 ng). In contrast, significant levels of macrophage-secreted apolipoprotein E were quantified. Baseline apolipoprotein E production ranged from 0.64 to 2.82 micrograms/mg cell protein per 24 h. Preincubation in the presence of malondialdehyde-LDL (100 micrograms/ml) stimulated a 1.6-3.0-fold increase in apolipoprotein E secretion. The identification of the immunoreactive material as apolipoprotein E was confirmed by labelling the cells with [35S]methionine, followed by fractionation of the 35S-labelled secretory products by anti-apolipoprotein E affinity chromatography and SDS-gel electrophoresis. We thus report the absence of synthesis of apolipoproteins A-I, B, C-II and C-III by cultured human monocyte macrophages. These cells, however, can synthesize microgram levels of apolipoprotein E on a per mg protein basis.