Phosphatidylcholine transfer protein and cellular adaptation to cholesterol
Baez, Juan Manuel
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Cellular secretion of cholesterol into blood as high density lipoproteins is critically dependent on the efflux of phosphatidylcholines. Phosphatidylcholine transfer protein (PC-TP) is a steroidogenic acute regulatory (StAR) protein-related lipid transfer (START) domain that catalyzes the intermembrane transfer of phosphatidylcholines in vitro. Whereas other START domain-containing proteins appear to play critical roles in cellular cholesterol metabolism, the biological function of PC-TP is unknown. We employed Chinese hamster ovary (CHO) cells to test the hypothesis that PC-TP plays a critical role in HDL formation. We found that heterologous expression of PC-TP in CHO cells leads to dose-dependent increases in the apparent-V max observed for apolipoprotein A-I-mediated efflux of both phospholipids and cholesterol. PC-TP expression stimulated the incorporation of phospholipids and cholesterol specifically into HDL-like particles displaying pre-beta mobility on agarose gels. Expression of PC-TP was also not associated with changes in phosphatidylcholine mass, biosynthetic rates or molecular species distribution. Collectively, these data suggest that PC-TP promotes delivery of phosphatidylcholines from the endoplasmic reticulum to the plasma membrane where ATP-binding cassette transporter A1 (ABCA1) catalyzes their incorporation along with cholesterol into nascent pre-beta-HDL. In separate experiments, we used macrophages to further explore a role for PC-TP in cellular cholesterol metabolism. While accumulating cholesterol in the form of oxidized low density lipoproteins, macrophages defend against cholesterol-related cytotoxicity in part by upregulating cholesterol efflux in the form of pre-beta-HDL particles. Using Pctp knockout (Pctp-KO) mice we showed reduced efflux of phospholipids and cholesterol from cholesterol-laden macrophages that lack PC-TP. However, unlike in CHO cells in which PC-TP overexpression did not influence expression of ABCA1, the impairment in efflux from macrophages was associated with similar reductions in cellular ABCA1 protein concentration. This suggests that PC-TP may stabilize ABCA1 at the cell surface by protecting against degradation. When subjected to a prolonged cholesterol challenge, macrophages derived from Pctp-KO mice displayed substantially more apoptotic and necrotic cell death, highlighting the function for PC-TP in protection against cholesterol induced toxicity. Collectively, these studies strongly support a role for PC-TP in the delivery of phosphatidylcholines to ABCA1 at the plasma membrane for incorporation into pre-beta-HDL particles. Because impairments in HDL formation may promote atherosclerosis, this research suggests a key protective role for PC-TP against cardiovascular diseases.