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 μg/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 μg/mg cell protein per 24 h. Preincubation in the presence of malondialdehyde-LDL (100 μg/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 [ 35 S]methionine, followed by fractionation of the 35 S-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 μg levels of apolipoprotein E on a per mg protein basis.