Increased susceptibility to activation and increased uptake of low density lipoprotein by cholesterol-loaded macrophages.

Inflammation is associated with macrophage activation, and this process has been shown to occur during atherogenesis. Macrophages (J774A.1) that were activated with either lipopolysaccharide (LPS), zymosan, or phorbol ester demonstrated a 30-35% increased uptake and degradation of low density lipoprotein (LDL) in comparison with nonactivated cells. This phenomenon was also shown for LDL cellular binding, and it resulted in macrophage cholesterol accumulation, as evidenced by cholesterol mass determination and flow cell cytometric analysis. Enhanced uptake of LDL was also obtained with two other types of macrophages: mouse peritoneal macrophages and human monocyte-derived macrophages. In LPS-stimulated macrophages, high density lipoprotein-mediated cholesterol efflux was not different from that shown in nonstimulated cells. Cellular cholesterol synthesis, however, was increased by 25% in the activated macrophages. Macrophage activation, measured as cellular procoagulant activity, was higher in cholesterol-loaded than in nonloaded cells. On stimulation of cholesterol-loaded macrophages, cellular uptake and degradation of LDL were increased by 3.3-fold in comparison with nonactivated cholesterol-loaded cells. Human monocyte-derived macrophages from hypercholesterolemic patients were found to contain 52% more cholesterol mass than macrophages derived from normal healthy donors. These cells demonstrated increased activation (twofold) in response to LPS stimulation and also showed 25% enhanced cellular degradation of LDL. We conclude that activation of macrophages during atherogenesis can lead to foam cell formation, and this mechanism is probably operative in hypercholesterolemic patients.