Cholesterol accumulation in macrophages that have migrated in the subintimal space leads to foam cell formation, which is believed to be one of the initiating events in atherosclerosis. In this study we investigated the effect of cholesterol feeding on peritoneal monocyte/macrophage cholesterol content and peritoneal cavity lipoprotein composition in rats. A cholesterol (2%) and cholic acid (1%) diet caused significant hypercholesterolemia in plasma, and at the same time the cholesterol content of peritoneal monocytes/macrophages was increased. At day 7, the cellular cholesteryl ester content had risen to 30.1-mu-g/mg cellular protein from a baseline value of 9.2-mu-g/mg. The unesterified cholesterol content also increased by 56%. At this time, acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity was doubled, whereas neutral and acidic cholesteryl ester hydrolase activities were unchanged. Reversal to the regular chow diet after 7 days of the cholesterol-enriched diet normalized plasma cholesterol levels as well as peritoneal monocyte/macrophage cholesteryl ester content. ACAT activity also decreased toward normal levels. Analysis of the d < 1.21 g/ml peritoneal lipoproteins isolated by ultracentrifugation revealed the presence, in both normal and hypercholesterolemic rats, of apolipoprotein A-I-rich lipid complexes with pre-beta mobility on agarose gel electrophoresis. The size of the peritoneal lipoproteins was smaller than that of plasmatic high density lipoproteins, and their chemical composition was also different from that of the major plasma lipoproteins. The cholesteryl ester content of peritoneal lipoproteins increased after feeding of the cholesterol-enriched diet. In conclusion, our results show that cholesterol feeding leads to rapid accumulation of cholesteryl esters in monocytes/macrophages. As soon as plasma cholesterol levels are returned to normal, cellular cholesterol content is also normalized. This finding is in accordance with the concept that cholesterol homeostasis in monocytes/macrophages is influenced by the level of circulating lipoproteins. Lipoproteins isolated from the peritoneal cavity may be the same as the pre-beta-migrating apolipoprotein A-I lipoproteins identified in plasma that are involved in the early phases of reverse cholesterol transport.
