Uptake and degradation of human chylomicrons by macrophages in culture. Role of lipoprotein lipase.

Because macrophages secrete lipoprotein lipase (LPL), we sought to determine if LPL activity would influence the metabolism of chylomicrons by macrophages. In initial studies, we showed that normal chylomicrons were a substrate for the macrophage's LPL activity. Uptake of normal chylomicrons occurred in a saturable fashion and was effectively competed for by human chylomicrons, very low density lipoproteins (VLDL), and rabbit beta-VLDL, but not by acetyl-low density lipoprotein (LDL), and only modestly by native LDL. When apoprotein C-II-deficient chylomicrons were incubated with macrophages, no hydrolysis of triglyceride occurred, yet saturable uptake of chylomicron protein and lipid occurred, demonstrating that LPL activity is not a prerequisite for saturable uptake. However, addition of apo C-II led to marked hydrolysis and enhanced uptake of protein and lipid moieties. When albumin was present in the medium, there was approximately equal enhancement of cellular content of triglyceride and cholesteryl ester, despite the fact that chylomicrons are triglyceride-rich. This was due to uptake of a triglyceride-depleted particle produced by LPL, as well as a preferential re-release of triglyceride. These studies suggest potential pathways by which triglyceride-rich lipoproteins could contribute to accumulation of cholesteryl esters in macrophages, even while only small amounts of triglyceride accumulate.