Recent GWAS have identified SNPs at a human chromosom1 locus associated with coronary artery disease risk and LDL cholesterol levels. The SNPs are also associated with altered expression of hepatic sortilin-1 (SORT1), which encodes a protein thought to be involved in apoB trafficking and degradation. Here, we investigated the regulation of Sort1 expression in mouse models of obesity. Sort1 expression was markedly repressed in both genetic (ob/ob) and high-fat diet models of obesity; restoration of hepatic sortilin-1 levels resulted in reduced triglyceride and apoB secretion. Mouse models of obesity also exhibit increased hepatic activity of mammalian target of rapamycin complex 1 (mTORC1) and ER stress, and we found that administration of the mTOR inhibitor rapamycin to ob/ob mice reduced ER stress and increased hepatic sortilin-1 levels. Conversely, genetically increased hepatic mTORC1 activity was associated with repressed Sort1 and increased apoB secretion. Treating WT mice with the ER stressor tunicamycin led to marked repression of hepatic sortilin-1 expression, while administration of the chemical chaperone PBA to ob/ob mice led to amelioration of ER stress, increased sortilin-1 expression, and reduced apoB and triglyceride secretion. Moreover, the ER stress target Atf3 acted at the SORT1 promoter region as a transcriptional repressor, whereas knockdown of Atf3 mRNA in ob/ob mice led to increased hepatic sortilin-1 levels and decreased apoB and triglyceride secretion. Thus, in mouse models of obesity, induction of mTORC1 and ER stress led to repression of hepatic Sort1 and increased VLDL secretion via Atf3. This pathway may contribute to dyslipidemia in metabolic disease. IntroductionIncreased hepatic VLDL secretion is a characteristic feature of type 2 diabetes and obesity, leading to dyslipidemia characterized by increased triglycerides (TGs) and apoB levels, increased VLDL and LDL cholesterol, reduced HDL, and increased atherosclerosis (1-4). apoB is the major apolipoprotein of VLDL and LDL, which transport TGs and cholesterol from the liver into the bloodstream (5). The regulation of hepatic apoB secretion occurs primarily on a posttranscriptional level and appears to involve intracellular protein degradation both in the ER and post-Golgi (6, 7). The mechanisms underlying the dysregulation of VLDL apoB secretion in obesity are poorly understood.Recently, human GWAS have identified several novel genetic loci associated with LDL cholesterol levels and coronary artery disease risk. SNPs at a widely replicated chromosome 1p13 locus have been associated with myocardial infarction (MI) and with total and LDL cholesterol levels (8-10). The minor alleles at this locus are present in approximately 20%-35% of individuals of mixed European descent, and homozygosity for the minor alleles, as opposed to homozygosity for the major alleles, is associated with a 20%-40%

Phosphatidylcholine transfer protein (PC-TP) is a cytosolic protein of unknown function that catalyzes intermembrane transfer of phosphatidylcholines in vitro.Using stably transfected CHO cells, we explored the influence of PC-TP on apolipoprotein A-I-and high density lipoprotein 3 (HDL 3 )-mediated lipid efflux. In proportion to its cellular level of expression, PC-TP accelerated apolipoprotein A-I-mediated phospholipid and cholesterol efflux as pre-␤-HDL particles. PC-TP increased rates of efflux of both lipids by >2-fold but did not affect mRNA levels or the activity of ATP-binding cassette A1, a plasma membrane protein that regulates apolipoprotein A-I-mediated lipid efflux. Overexpression of PC-TP was associated with only slight increases in HDL 3 -mediated phospholipid efflux and no changes in cholesterol efflux. In scavenger receptor BI-overexpressing cells, PC-TP expression minimally influenced apolipoprotein A-I-or HDL 3 -mediated lipid efflux. PC-TP did not affect cellular phospholipid compositions, phosphatidylcholine contents, or phosphatidylcholine synthetic rates. These findings suggest that a physiological function of PC-TP is to replenish the plasma membrane with phosphatidylcholines that are removed during pre-␤-HDL particle formation due to the activity of ATP-binding cassette A1.

Macrophages are the predominant cellular component of atherosclerotic lesions, where they scavenge oxidatively modified lipoproteins while defending themselves against cholesterol-induced cytotoxicity by adaptive mechanisms that depend in part on the synthesis, distribution and efflux of phosphatidylcholines. PC-TP (phosphatidylcholine transfer protein) is a START (steroidogenic acute regulatory protein-related lipid transfer) domain protein that catalyses the intermembrane transfer of phosphatidylcholines and promotes apolipoprotein AI-mediated lipid efflux when overexpressed in the cytosol of Chinese-hamster ovary cells. To explore a role for PC-TP in the adaptive responses of macrophages to cholesterol loading, we utilized peritoneal macrophages from mice with homozygous disruption of the gene encoding PC-TP (Pctp(-/-)) and wild-type littermate controls. PC-TP was abundantly expressed in macrophages from wild-type but not Pctp(-/-) mice. In cholesteryl ester-loaded macrophages from Pctp(-/-) mice, the apolipoprotein AI-mediated efflux of phospholipids and cholesterol was decreased. This could be attributed to proportional decreases in the expression levels of ATP-binding cassette A1. Also, in response to free cholesterol loading, the absence of PC-TP from macrophages was associated with marked increases in apoptotic cell death. These findings suggest that PC-TP in macrophages may serve an atheroprotective role by defending against cholesterol-induced cytotoxicity.

Phosphatidylcholine transfer protein (PC-TP) is a cytosolic phospholipid binding protein and a member of the steroidogenic acute regulatory-related transfer domain superfamily.

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