Reverse cholesterol transport (RCT) is the process by which cholesterol (Ch) from peripheral tissues, in the form of high density lipoprotein (HDL), is returned to the liver for elimination from the body. Formation of HDL particles begins when detergent-like apolipoprotein A-I (apoA-I) molecules, secreted by hepatocytes, incorporate phosphatidylcholines (PCs) and Ch from the cell membrane. Phosphatidylcholine Transfer Protein (PC-TP) is a cytosolic protein of unknown function that is highly expressed in liver and in vitro catalyzes the intermembrane transfer of PCs exclusively. Based on it?s substrate specificity, enrichment in the liver and the disproportionately high PC composition of HDL (PCs constitute >80% of HDL lipid but only ~50% cellular lipid), we hypothesize that PC-TP plays a critical role in the process of HDL formation. We postulate that PC-TP supplies PCs to the plasma membrane for incorporation into nascent preBETA-HDL. This hypothesis will be tested by radiolabeling Chinese hamster ovary (CHO) cells that overexpress PC-TP with [3H] cholesterol or [3H] choline and performing lipid efflux experiments, according to standard methods, using apoA-I as the lipid acceptor. The extent to which the effects of PC-TP on lipid secretion are dependent on the synthetic rate of PCs or on the presence of scavenger receptor BI (SR-BI) will be evaluated by utilizing cells that, in addition to PC-TP, also overexpress CTP: choline cytidilyltransferase (CT) and SR-BI respectively. We also propose a role for PC-TP in SR-BI mediated cellular uptake of lipids from HDL. We will discern this role by differentially labeling the protein and lipid components of HDL using [3H] Ch, [3H] dipalmitoyl PC, and [125I] HDL protein for Ch, PCs and protein respectively. Cells that overexpress both PC-TP and SR-BI will be incubated with these radiolabeled particles. Membrane binding will then be measured and rates of lipid uptake determined. We hope our findings will provide new insights into HDL metabolism and help lead to the development of novel strategies for reducing the risk of coronary heart disease.
| Wang, Wen-Jun; Baez, Juan M; Maurer, Rie et al. (2006) Homozygous disruption of Pctp modulates atherosclerosis in apolipoprotein E-deficient mice. J Lipid Res 47:2400-7 |
| Baez, Juan M; Tabas, Ira; Cohen, David E (2005) Decreased lipid efflux and increased susceptibility to cholesterol-induced apoptosis in macrophages lacking phosphatidylcholine transfer protein. Biochem J 388:57-63 |
