Regulation of 12-hydroxyeicosatetraenoic acid synthesis by acetyl-LDL in mouse peritoneal macrophages

The mechanism for the regulation of 12-hydroxyeicosatetraenoic acid (12-HETE) production by cholesterol-rich macrophages was investigated. beta-VLDL and acetyl-LDL, lipoproteins which result in cholesterol accumulation in macrophages, stimulated 12-HETE secretion. Lipoproteins which do not induce cholesterol accumulation, such as low- and high-density lipoproteins, did not. Cell-free homogenates from cholesterol-rich macrophages had significantly more 12-lipoxygenase activity than homogenates from unmodified cells. Preincubating homogenates prepared from unmodified macrophages with acetyl-LDL, LDL or multilamellar liposomes containing total lipids from acetyl-LDL but not apoproteins significantly increased 12-lipoxygenase activity. This stimulatory effect was caused by the phospholipid moiety of the lipoprotein. 12-HETE synthesis was not increased in macrophages enriched 6-fold in unesterified cholesterol. Acetyl-LDL stimulated 12-HETE synthesis in macrophages in which cholesteryl ester accumulation was prevented by inhibiting acylcoenzyme A:cholesterol acyltransferase activity. When binding of acetyl-LDL to its receptor was decreased by increasing concentrations of dextran sulfate, or when lysosomal metabolism of the lipoprotein was prevented by chloroquine, 12-HETE production significantly decreased. Moreover, the combination of inhibiting acetyl-LDL binding and degradation completely blocked the stimulation of 12-HETE synthesis by acetyl-LDL. The data indicate that acetyl-LDL must enter the macrophage and be partially degraded to regulate 12-HETE synthesis. The regulation is independent of cholesterol accumulation but is related to the entering lipoprotein phospholipid.