The role of phospholipid transfer protein and cholesteryl ester transfer protein in reverse cholesterol transport

In atherosclerosis, cholesterol accumulates in the vessel wall, mainly in the form of low-density lipoprotein (LDL). Macrophages of the vessel wall scavenge cholesterol, which leads to formation of lipid-laden foam cells. Several pathways and transporter proteins are involved in the regulation of cholesterol balance in macrophages. High plasma levels of high-density lipoprotein (HDL) protect against atherosclerosis, as HDL particles are able to accept cellular cholesterol and transport it to the liver for excretion in a process called reverse cholesterol transport. Phospholipid transfer protein (PLTP) remodels HDL particles in the circulation, generating preβ-HDL and large fused HDL particles. In addition, PLTP maintains plasma HDL levels by facilitating the transfer of post-lipolytic surface remnants of triglyceride-rich lipoproteins to HDL. Reportedly, PLTP has both antiatherogenic and proatherogenic properties. Most of the cholesteryl ester transfer protein (CETP) in plasma is bound to HDL particles and CETP is also involved in the remodeling of HDL particles. CETP enhances the heteroexchange of cholesteryl esters in HDL particles for triglycerides in LDL and very low-density lipoprotein (VLDL). The role of CETP in the development of atherosclerosis is controversial. The aim of this thesis project was to study the importance of endogenous PLTP in the removal of cholesterol from macrophage foam cells by using macrophages derived from PLTP-deficient mice, determine the effect of macrophage-derived PLTP on the development of atherosclerosis by using bone marrow transplantation, and clarify the role of the two forms of PLTP, active and inactive, in the removal of cholesterol from foam cells. In addition, the ability of CETP to protect HDL against the action of chymase in CETP-HDL complexes was studied. Furthermore, cholesterol efflux from macrophages derived from lowand high-HDL subjects was studied. Finally, cholesterol efflux potential of sera obtained from the study subjects was compared. In this thesis project it was demonstrated that the absence of PLTP in macrophages derived from PLTP-deficient mice decreased cholesterol efflux mediated by ATP-binding cassette transporter A1 (ABCA1). The bone marrow transplantation studies demonstrated that selective deficiency of PLTP in macrophages decreased the size of atherosclerotic lesions and caused major changes in serum lipoprotein levels and PLTP activity. It was further demonstrated that the active form of PLTP can enhance cholesterol efflux from macrophage foam cells. Both preβ-HDL and large fused HDL particles enriched with apoE and phospholipids were effective in cholesterol efflux. Besides PLTP, also CETP may enhance the reverse cholesterol transport process, as association of CETP with reconstituted HDL (rHDL) particles prevented chymase-dependent proteolysis of discoidal rHDL and preserved their cholesterol efflux potential. Finally, monocyte macrophages isolated from either lowor high-HDL subjects displayed similar cholesterol efflux to exogenous acceptors. However, serum from high-HDL subjects promoted more efficient cholesterol efflux than did serum from low-HDL subjects. The observed difference was most probably due to differences in the distribution of HDL subpopulations in low-HDL and high-HDL subjects. The results of this thesis project indicate that both PLTP and CETP can remodel HDL as a cholesterol acceptor, which affects the removal of cholesterol from foam cells present in the vessel wall.