Biochemical and genetic studies of the apoprotein E secreted by mouse macrophages and human monocytes

Monolayers of mouse peritoneal macrophages secrete large amounts of apoliprotein E (apo-E) after the cells have been loaded with cholesterol by incubation with modified lipoproteins. In the current studies, we characterize some of the biochemical morphological, and functional properties of macrophage-secreted apo-E, and we also show that a similar protein is secreted by human blood monocytes. Apo-E was secreted by mouse macrophages in the form of lipoprotein particles with a mean density of 1.075 g/ml. By electron microscopy, these lipoproteins appeared as lamellar discs. The ³⁵S-labeled apo-E secreted by macrophages was able to bind to low density lipoprotein receptors on human fibroblasts and was taken up and degraded with high affinity. The ³⁵S-labeled apo-E secreted by human monocytes was examined by two-dimensional isoelectric focusing-sodium dodecyl sulfate polyacrylamide gel electrophoresis. The secreted protein was more acidic than the apo-E of plasma very low density lipoprotein from the same individual. Treatment with neuramidase converted the monocyte-produced apo-E to a form that resembled plasma apo-E, suggesting that the secreted product had more sialic acid residues than the circulating form. Monocytes from individuals with different alleles at the genetic locus specifying apo-E secreted a form of ³⁵S-labeled apo-E that was electrophoretically similar to their plasma apo-E after neuraminidase treatment, suggesting that the monocyte-secreted apo-E and the plasma apo-E are products of the same gene. The finding that cholesterol-loaded macrophages and monocytes synthesize and secrete apo-E is discussed in relation to the in vivo process of 'reverse cholesterol transport.'