Evaluation of phospholipid transfer protein and cholesteryl ester transfer protein as contributors to the generation of pre beta-high-density lipoproteins.
High-density lipoproteins (HDLs) are considered anti-atherogenic because they mediate peripheral cell cholesterol transport to the liver for excretion and degradation. An important step in this reverse cholesterol-transport pathway is the uptake of cellular cholesterol by a specific subclass of small, lipid-poor apolipoprotein A-I particles designated pre beta-HDL. The two lipid-transfer proteins present in human plasma, cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP), have both been implicated in the formation of pre beta-HDL. In order to investigate the relative contribution of each of these proteins, we used transgenic mouse models. Comparisons were made between human CETP transgenic mice (huCETPtg), human PLTP transgenic mice (huPLTPtg) and mice transgenic for both lipid-transfer proteins (huCETPtg/huPLTPtg). These animals showed elevated plasma levels of CETP activity, PLTP activity or both activities, respectively. We evaluated the generation of pre beta-HDL in mouse plasma by immunoblotting and crossed immuno-electrophoresis. Generation of pre beta-HDL was equal in huCETPtg and wild-type mice. In contrast, in huPLTPtg and huCETPtg/huPLTPtg mice, pre beta-HDL generation was 3-fold higher than in plasma from either wild-type or huCETPtg mice. Our findings demonstrate that, of the two plasma lipid-transfer proteins, PLTP rather than CETP is responsible for the generation of pre beta-HDL. These data support the hypothesis of a role for PLTP in the initial stage of reverse cholesterol transport.