Scientific Report 2005

Immunology

Reverse Cholesterol Transport and High-Density Lipoproteins

L.K. Curtiss, C. Flood, N.J. Hime, A.E. Mullick, R.J. Petrovan, D.T. Valenta

In order to be removed from the body, cholesterol must be dissolved into or converted to bile acids in the liver. This biliary excretion pathway is fed by the transport of cholesterol from peripheral tissues and is referred to as reverse cholesterol transport. An early step in reverse cholesterol transport is the transfer of peripheral cell-free cholesterol to plasma high-density lipoproteins (HDLs). The HDLs serve as transport vehicles for excess cellular cholesterol through the plasma compartment to the liver. The major protein in HDL is apolipoprotein AI. Importantly, the transfer of cellular cholesterol to HDL does not occur in plasma. It occurs in extracellular spaces such as the subendothelial space or intima of a vessel within an atherosclerotic lesion.

We are investigating how apolipoprotein AI promotes the efficient transfer of excess cholesterol from peripheral cells (i.e., intimal macrophage foam cells) to HDL. We have published convincing in vivo data that apolipoprotein E and apolipoprotein AI participate in the efflux of free cholesterol from atherosclerotic lesions. We found that both plasma-derived apolipoprotein AI and macrophage-produced apolipoprotein E participate in the efficient efflux of free cholesterol from aortas containing atherosclerotic lesions in atherosclerosis-prone mice. The in vivo specificity for apolipoprotein AI in efflux of cholesterol from macrophages in atherosclerotic lesions is a direct function of the ability of the apolipoprotein to dissociate from spherical HDL and to form transiently stable, lipid-poor apolipoprotein AI, which can accept macrophage ATP binding cassette A1–transported free cholesterol and phospholipid. We are studying this phenomenon in mice that lack receptors for low-density lipoprotein.

Mice deficient in receptors for low-density lipoprotein are severely hyperlipidemic, and detectable atherosclerotic lesions develop in the aorta and aortic sinus within weeks if the animals are fed a high-fat diet. We are focusing on the role of multiple macrophage nuclear liver X receptor–inducible genes in reverse cholesterol transport. We are determining the role played by phospholipid transfer protein (PLTP) in the generation of lipid-poor or lipid-free apolipoprotein AI in vivo and in vitro. To examine the role of macrophage-derived PLTP in cholesterol metabolism and atherosclerosis, we performed bone marrow transplantations in mice deficient in receptors for low-density lipoprotein; the mice were lethally irradiated and were reconstituted with either wild-type or PLTP-deficient bone marrow cells. The transplanted animals were fed a high-fat diet for 16 weeks to induce atherosclerosis. We found that macrophage PLTP deficiency led to increases in the total plasma levels of cholesterol and in the extent of atherosclerotic lesions, suggesting an atheroprotective role of macrophage-derived PLTP in the intima.

We are also determining the role of cholesteryl ester transfer protein (CETP) in this same process in vivo and in vitro. Both PLTP and CETP are expressed by macrophages, can generate lipid-poor apolipoprotein AI from spherical HDL, are present in atherosclerotic lesions, and are induced by ligation of liver X receptors.

Finally, we are studying the role played by the lipoprotein triglyceride hydrolases lipoprotein lipase and hepatic lipase in the generation of lipid-poor apolipoprotein AI in vitro. Both of these lipases also are expressed by macrophages, are present in atherosclerotic lesions, and are upregulated by ligation of nuclear liver X receptors. Macrophage-derived lipoprotein lipase and hepatic lipase promote the formation of lipid-poor apolipoprotein AI from mature HDL. This remodeling of HDL facilitates a reduction in the size of HDL particles to provide free apolipoprotein AI substrate for PLTP and CETP lipid transfer. Both lipid transfer proteins and neutral triglyceride lipases participate in HDL remodeling and in macrophage-mediated efflux of cholesterol from atherosclerotic lesions. Therefore a number of nuclear liver X receptor–inducible gene products are expressed by macrophages in response to the accumulation of cholesteryl ester and participate in reverse cholesterol transport to prevent the formation of foam cells.

Publications

Bradshaw, G., Gutierrez, A., Miyake, J.H., Davis, K.R., Li, A.C., Glass, C.K., Curtiss, L.K., Davis, R.A. Facilitated replacement of Kupffer cells expressing a paraoxonase-1 transgene is essential for ameliorating atherosclerosis in mice. Proc. Natl. Acad. Sci. U. S. A. 102:11029, 2005.

Jahangiri, A., Rader, D.J., Marchadier, D., Curtiss, L.K., Bonnet, D.J., Rye, K.-A. Evidence that endothelial lipase remodels high density lipoproteins without mediating the dissociation of apolipoprotein A-I. J. Lipid Res. 25:896, 2005.

Mullick, A.E., Tobias, P.S., Curtiss, L.K. Modulation of atherosclerosis in mice by Toll-like receptor 2. J. Clin. Invest., in press.

Schneider, M., Witztum, J.L., Young, S.G., Ludwig, E.H., Miller, E., Tsimikas, S., Curtiss, L.K., Marcovina, S.M., Taylor, J.M., Lawn, R.W., Innerarity, T.L., Pitas, R.E. High level lipoprotein (a) expression in transgenic mice: evidence for oxidized phospholipid in lipoprotein (a) but not in low density lipoprotein. J. Lipid Res. 46:769, 2005.

Tobias, P., Curtiss, L.K. Paying the price for pathogen protection: Toll receptors in atherogenesis. J. Lipid Res. 46:404, 2005.

Wiedmer, T., Zhao, J., Li, L., Zhou, Q., Hevener, A., Olefsky, J.M., Curtiss, L.K., Corr, M., Witztum, J.L. Adiposity, dyslipidemia and insulin resistance in mice with targeted deletion of phospholipid scramblase 3 (PLSCR3). Proc. Natl. Acad. Sci.
U. S. A. 101:13296, 2004.