ACAT activity in freshly isolated human mononuclear cell homogenates from hyperlipidemic subjects

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) catalyzes the esterification of cholesterol in human mononuclear cells (MNC). In order to assess the relationship between lipid levels and ACAT activity in circulating MNC, we measured the rate of [14C]oleoyl-CoA incorporation into cholesterol ester in freshly isolated MNC homogenates from hyperlipidemic subjects. Baseline, off-treatment results obtained in 14 hypertriglyceridemic subjects (eight type IV and six type III) and seven subjects with familial hypercholesterolemia (FH) due to the same deletion of greater than 10 kb on the low-density lipoprotein (LDL)-receptor gene were compared with values determined in 12 healthy normolipidemic subjects. The rate of cholesterol esterification was 45 +/- 28 pmol/5 min/mg cell protein in healthy normolipidemic controls. This rate was significantly higher in type IV subjects (84 +/- 52 pmol/5 min/mg cell protein, P less than .05) and FH subjects (67 +/- 25 pmol/5 min/mg cell protein, P less than .05). The values were more dispersed in type III subjects; the mean value for the group (72 +/- 46 pmol/5 min/mg cell protein) was not statistically different from the control. Hypertriglyceridemic patients were then treated with 6 g/d of omega-3 fatty acids. This resulted in a significant reduction in plasma total triglycerides and very-low-density lipoprotein (VLDL)-cholesterol in both type III subjects (-57% and -51%, P less than .05) and type IV subjects (-62% and -62%, P less than .01). The reduction in VLDL concentration was associated with a significantly lower ACAT activity in MNC homogenates from type IV subjects (from 84 +/- 52 to 60 +/- 36 pmol/5 min/mg cell protein, P less than .05), but not from type III hypertriglyceridemic subjects (from 72 +/- 46 to 73 +/- 36 pmol/5 min/mg cell protein). In conclusion, we found that cholesterol esterification in human MNC is elevated in hyperlipidemic subjects and can be decreased with normalization of lipid levels. However, ACAT activity changes occurring with treatment are heterogeneous among hyperlipidemic subjects, suggesting that factors other than plasma lipid level reduction affect ACAT activity in vivo.