Phosphatidylcholine transfer protein and cellular adaptation to cholesterol

Abstract

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.