Fractional efflux and net change in cellular cholesterol content mediated by sera from mice expressing both human apolipoprotein AI and human lecithin : cholesterol acyltransferase genes

Human lecithin:cholesterol acyltransferase (LCAT) is a key enzyme in the me tabolism of cholesterol and is postulated to participate in the physiologic al process called reverse cholesterol transport. We have used transgenic mi ce (Tgm) expressing either both human apolipoprotein AI (apo AI) and human LCAT genes or only the human apo AI gene (HuAILCAT or HuAI Tgm, respectivel y) to assess the consequences of LCAT overexpression on serum lipid and lip oprotein profiles and on the ability of each serum to promote bidirectional flux of cholesterol between serum and Fu5AH hepatoma cells. Mean serum LCA T activity of HuAILCAT Tgm was 2-fold increased compared to the HuAI group (48 +/- 9 vs. 24 +/- 5 nmol/ml per h, P < 0.01 for HuAILCAT and HuAI Tgm, r espectively) and the cholesterol esterification rates were not significantl y different between the two groups of animals (66 +/- 11 vs. 74 +/- 18 nmol /ml per h for HuAILCAT and HuAI Tgm, respectively). HuAILCAT Tgm exhibited higher total cholesterol serum values (2.3-fold) due to an increase in both HDL-cholesterol(1.9-fold) and non-HDL-cholesterol (3-fold). The HDL partic les from HuAILCAT Tgm were relatively phospholipid depleted and cholesterol enriched compared to HuAI mice. When cells were incubated for six hours wi th the mouse serum, the fractional efflux of radiolabeled cholesterol was s lightly increased with the HuAILCAT Tgm (1.2-fold) but the increase in intr acellular cholesterol content was also 2-fold higher than with the HuAI Tgm . Fu5AH can be viewed as a model for the evaluation of bidirectional flux o f cholesterol in SR-BI-rich cells. In this model LCAT overexpression in mic e, by increasing both HDL and non-HDL-cholesterol, mostly enhances the upta ke of cholesterol by the cells, which would be of benefit for the last step of reverse cholesterol transport in hepatocytes. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.