Regulation of macrophage apolipoprotein E gene expression by cholesterol

The relationship between macrophage cholesterol content and apolipoprotein E (apoE) gene expression was investigated in cultured mouse peritoneal macrophages. Acetylated low density lipoprotein (ALDL) at 25 micrograms/ml led to increased macrophage messenger RNA abundance as detected by Northern blot hybridization. Incubation in a range of ALDL concentrations (0-100 microgram/ml) led to graded increases in macrophage free and esterified cholesterol, and apoE mRNA detected by slot blot hybridization. After a 2-h pulse of ALDL, with or without acyl-CoA:cholesterol acyltransferase inhibition, increased apoE mRNA was not detected until after 6 h. Removal of cholesterol from upregulated macrophages, using human high density lipoprotein, resulted in a return of apoE mRNA levels toward baseline. Nuclear run-off transcription assays demonstrated increased apoE gene transcription in macrophages incubated with ALDL. It is concluded that: a) previously reported changes in macrophage apoE content and secretion after cholesterol enrichment or depletion can be attributed to changes in apoE mRNA levels; and b) cholesterol-induced changes in apoE mRNA are associated with increased apoE gene transcription.