Phosphatidylcholine transfer protein promotes apolipoprotein A-I-mediated lipid efflux in Chinese hamster ovary cells

Trait correlated expression combined with eQTL and ASE analyses identified novel candidate genes affecting intramuscular fat

BACKGROUND

Intramuscular fat (IMF) content is a determining factor for meat taste. The Luchuan pig is a fat-type local breed in southern China that is famous for its desirable meat quality due to high IMF, however, the crossbred offspring of Luchuan sows and Duroc boars displayed within-population variation on meat quality, and the reason remains unknown.

RESULTS

In the present study, we identified 212 IMF-correlated genes (FDR ≤ 0.01) using correlation analysis between gene expression level and the value of IMF content. The IMF-correlated genes were significantly enriched in the processes of lipid metabolism and mitochondrial energy metabolism, as well as the AMPK/PPAR signaling pathway. From the IMF-correlated genes, we identified 99 genes associated with expression quantitative trait locus (eQTL) or allele-specific expression (ASE) signals, including 21 genes identified by both cis-eQTL and ASE analyses and 12 genes identified by trans-eQTL analysis. Genome-wide association study (GWAS) of IMF identified a significant QTL on SSC14 (p-value = 2.51E^-7), and the nearest IMF-correlated gene SFXN4 (r = 0.28, FDR = 4.00E^-4) was proposed as the candidate gene. Furthermore, we highlighted another three novel IMF candidate genes, namely AGT, EMG1, and PCTP, by integrated analysis of GWAS, eQTL, and IMF-gene correlation analysis.

CONCLUSIONS

The AMPK/PPAR signaling pathway together with the processes of lipid and mitochondrial energy metabolism plays a vital role in regulating porcine IMF content. Trait correlated expression combined with eQTL and ASE analysis highlighted a priority list of genes, which compensated for the shortcoming of GWAS, thereby accelerating the mining of causal genes of IMF.

Omics-Driven Systems Interrogation of Metabolic Dysregulation in COVID-19 Pathogenesis

The coronavirus disease 2019 (COVID-19) pandemic presents an unprecedented threat to global public health. Herein, we utilized a combination of targeted and untargeted tandem mass spectrometry to analyze the plasma lipidome and metabolome in mild, moderate, and severe COVID-19 patients and healthy controls. A panel of 10 plasma metabolites effectively distinguished COVID-19 patients from healthy controls (AUC = 0.975). Plasma lipidome of COVID-19 resembled that of monosialodihexosyl ganglioside (GM3)-enriched exosomes, with enhanced levels of sphingomyelins (SMs) and GM3s, and reduced diacylglycerols (DAGs). Systems evaluation of metabolic dysregulation in COVID-19 was performed using multiscale embedded differential correlation network analyses. Using exosomes isolated from the same cohort, we demonstrated that exosomes of COVID-19 patients with elevating disease severity were increasingly enriched in GM3s. Our work suggests that GM3-enriched exosomes may partake in pathological processes related to COVID-19 pathogenesis and presents the largest repository on the plasma lipidome and metabolome distinct to COVID-19.

Phosphatidylcholine transfer protein/StarD2 promotes microvesicular steatosis and liver injury in murine experimental steatohepatitis

Mice fed a methionine- and choline-deficient (MCD) diet develop steatohepatitis that recapitulates key features of nonalcoholic steatohepatitis (NASH) in humans. Phosphatidylcholine is the most abundant phospholipid in the surfactant monolayer that coats and stabilizes lipid droplets within cells, and choline is required for its major biosynthetic pathway. Phosphatidylcholine-transfer protein (PC-TP), which exchanges phosphatidylcholines among membranes, is enriched in hepatocytes. PC-TP also regulates fatty acid metabolism through interactions with thioesterase superfamily member 2. We investigated the contribution of PC-TP to steatohepatitis induced by the MCD diet. Pctp^-/- and wild-type control mice were fed the MCD diet for 5 wk and were then euthanized for histopathologic and biochemical analyses, as well as determinations of mRNA and protein expression. Whereas all mice developed steatohepatitis, plasma alanine aminotransferase and aspartate aminotransferase activities were only elevated in wild-type mice, indicating that Pctp^-/- mice were protected from MCD diet-induced hepatocellular injury. Reduced hepatotoxicity due to the MCD diet in the absence of PC-TP expression was further evidenced by decreased activation of c-Jun and reduced plasma concentrations of fibroblast growth factor 21. Despite similar total hepatic concentrations of phosphatidylcholines and other lipids, the relative abundance of microvesicular lipid droplets within hepatocytes was reduced in Pctp^-/- mice. Considering that the formation of larger lipid droplets may serve to protect against lipotoxicity in NASH, our findings suggest a pathogenic role for PC-TP that could be targeted in the management of this condition.NEW & NOTEWORTHY Phosphatidylcholine-transfer protein (PC-TP) is a highly specific phosphatidylcholine-binding protein that we previously showed to regulate hepatocellular nutrient metabolism through its interacting partner thioesterase superfamily member 2 (Them2). This study identifies a pathogenic role for PC-TP, independent of Them2, in the methionine- and choline-deficient diet model of experimental steatohepatitis. Our current observations suggest that PC-TP promotes liver injury by mediating the intermembrane transfer of phosphatidylcholines, thus stabilizing more pathogenic microvesicular lipid droplets.

Novel associations of nonstructural Loci with paraoxonase activity

The high-density-lipoprotein-(HDL-) associated esterase paraoxonase 1 (PON1) is a likely contributor to the antioxidant and antiatherosclerotic capabilities of HDL. Two nonsynonymous mutations in the structural gene, PON1, have been associated with variation in activity levels, but substantial interindividual differences remain unexplained and are greatest for substrates other than the eponymous paraoxon. PON1 activity levels were measured for three substrates-organophosphate paraoxon, arylester phenyl acetate, and lactone dihydrocoumarin-in 767 Mexican American individuals from San Antonio, Texas. Genetic influences on activity levels for each substrate were evaluated by association with approximately one million single nucleotide polymorphism (SNPs) while conditioning on PON1 genotypes. Significant associations were detected at five loci including regions on chromosomes 4 and 17 known to be associated with atherosclerosis and lipoprotein regulation and loci on chromosome 3 that regulate ubiquitous transcription factors. These loci explain 7.8% of variation in PON1 activity with lactone as a substrate, 5.6% with the arylester, and 3.0% with paraoxon. In light of the potential importance of PON1 in preventing cardiovascular disease/events, these novel loci merit further investigation.

Decreased lipid efflux and increased susceptibility to cholesterol-induced apoptosis in macrophages lacking phosphatidylcholine transfer protein

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.

[START domain-containing proteins: a review of their role in lipid transport and exchange]

Fifteen START domain-containing proteins exist in mammals. On the basis of their structural homology, this family is divided into several sub-families consisting mainly of non-vesicular intracellular lipid carriers. With the exception of the Thioesterase-START subfamily, the other subfamilies are represented among invertebrates. The START domain is always located in the C-terminus of the protein. It is a module of about 210 residues that binds lipids, including sterols. Cholesterol, 25-hydroxycholesterol, phosphatidylcholine, phosphatidylethanolamine and ceramides are ligands for STARD1/STARD3-6, STARD5, STARD2/STARD10, STARD10 and STARD11, respectively. The lipids or sterols bound by the remaining 7 START proteins are unknown. The START domain can be regarded as a lipid-exchange and/or a lipid-sensing domain. The START domain consists in a deep lipid-binding pocket--that shields the hydrophic ligand from the external aqueous environment--covered by a lid formed by a C-terminal alpha helix. Within the same subgroup, such as the sterols-carriers subgroup, different START domains have similar biochemical properties; however, their expression profile and their subcellular localization distinguish them and are critical for their different biological functions. START proteins act in a variety of distinct physiological processes, such as lipid transfer between intracellular compartments, lipid metabolism and modulation of signaling events. Mutation or misexpression of START proteins is linked to pathological processes, including genetic disorders, autoimmune diseases and cancers.

Influences of the phosphatidylcholine transfer protein gene variants on the LDL peak particle size

BACKGROUND

The small, dense LDL phenotype is associated with an increased cardiovascular disease risk. A genome-wide scan performed on 236 nuclear families of the Quebec Family Study (QFS) revealed a quantitative trait locus (QTL) affecting LDL peak particle size (LDL-PPD) and density on the 17q21 region. This region contains the phosphatidylcholine transfer protein gene (PCTP). In the liver, phosphatidylcholine transfer protein binds specifically phosphatidylcholine suggesting a role for this protein in the formation of HDL and possibly VLDL phospholipid membranes.

OBJECTIVES

To test the association between two coding polymorphisms (c.29A>C (Glu10Ala) and c.188G>A (Cys63Tyr)) in PCTP gene and the LDL-PPD.

METHODS

LDL-PPD was measured by non-denaturating 2-16% polyacrylamide gradient gel electrophoresis on 623 QFS subjects.

RESULTS

After adjustment for age and sex, carriers of the c.29C allele showed larger LDL-PPD than A/A homozygotes (p<0.05). These results remained significant when LDL-PPD was further adjusted for the effects of BMI and triglyceride levels (p<0.04). We also observed a three-fold lower risk of having the small (LDL-PPD <256A), dense LDL phenotype in subjects carrying the c.29C allele, when compared to A/A homozygotes (OR=0.35 (95% CI: 0.14-0.91; p=0.03)).

CONCLUSION

PCTP gene variants are associated with LDL-PPD.

Interacting proteins dictate function of the minimal START domain phosphatidylcholine transfer protein/StarD2

The Star (steroidogenic acute regulatory protein)-related transfer (START) domain superfamily is characterized by a distinctive lipid-binding motif. START domains typically reside in multidomain proteins, suggesting their function as lipid sensors that trigger biological activities. Phosphatidylcholine transfer protein (PC-TP, also known as StarD2) is an example of a START domain minimal protein that consists only of the lipid-binding motif. PC-TP, which binds phosphatidylcholine exclusively, is expressed during embryonic development and in several tissues of the adult mouse, including liver. Although it catalyzes the intermembrane exchange of phosphatidylcholines in vitro, this activity does not appear to explain the various metabolic alterations observed in mice lacking PC-TP. Here we demonstrate that PC-TP function may be mediated via interacting proteins. Yeast two-hybrid screening using libraries prepared from mouse liver and embryo identified Them2 (thioesterase superfamily member 2) and the homeodomain transcription factor Pax3 (paired box gene 3), respectively, as PC-TP-interacting proteins. These were notable because the START domain superfamily contains multidomain proteins in which the START domain coexists with thioesterase domains in mammals and with homeodomain transcription factors in plants. Interactions were verified in pulldown assays, and colocalization with PC-TP was confirmed within tissues and intracellularly. The acyl-CoA thioesterase activity of purified recombinant Them2 was markedly enhanced by recombinant PC-TP. In tissue culture, PC-TP coactivated the transcriptional activity of Pax3. These findings suggest that PC-TP functions as a phosphatidylcholine-sensing molecule that engages in diverse regulatory activities that depend upon the cellular expression of distinct interacting proteins.

Structure and function of phosphatidylcholine transfer protein (PC-TP)/StarD2

Phosphatidylcholine transfer protein (PC-TP) is a highly specific soluble lipid binding protein that transfers phosphatidylcholine between membranes in vitro. PC-TP is a member of the steroidogenic acute regulatory protein-related transfer (START) domain superfamily. Although its biochemical properties and structure are well characterized, the functions of PC-TP in vivo remain incompletely understood. Studies of mice with homozygous disruption of the Pctp gene have largely refuted the hypothesis that this protein participates in the hepatocellular selection and transport of biliary phospholipids, in the production of lung surfactant, in leukotriene biosynthesis and in cellular phosphatidylcholine metabolism. Nevertheless, Pctp(-/-) mice exhibit interesting defects in lipid homeostasis, the understanding of which should elucidate the biological functions of PC-TP.

Homozygous disruption of Pctp modulates atherosclerosis in apolipoprotein E-deficient mice

Phosphatidylcholine transfer protein (PC-TP) is a cytosolic phospholipid binding protein and a member of the steroidogenic acute regulatory-related transfer domain superfamily. Its tissue distribution includes liver and macrophages. PC-TP regulates hepatic lipid metabolism, and its absence in cholesterol-loaded macrophages is associated with reduced ATP binding cassette transporter A1-mediated lipid efflux and increased susceptibility to apoptosis induced by unesterified cholesterol. To explore a role for PC-TP in atherosclerosis, we prepared PC-TP-deficient/apolipoprotein E-deficient (Pctp(-/-)/Apoe(-/-)) mice and littermate Apoe(-/-) controls. At 16 weeks, atherosclerosis was increased in chow-fed male, but not female, Pctp(-/-)/Apoe(-/-) mice. This effect was associated with increases in plasma lipid concentrations. By contrast, no differences in atherosclerosis were observed between male or female Pctp(-/-)/Apoe(-/-) mice and Apoe(-/-) controls fed a Western-type diet for 16 weeks. At 24 weeks, atherosclerosis in chow-fed male Pctp(-/-)/Apoe(-/-) mice tended to be reduced in proportion to plasma cholesterol. The attenuation of atherosclerosis in female Pctp(-/-)/Apoe(-/-) mice fed chow or the Western-type diet for 24 weeks was not attributable to changes in plasma cholesterol or triglyceride concentrations. These findings suggest that PC-TP modulates the development of atherosclerosis, in part by regulating plasma lipid concentrations.

A polymorphism in New Zealand inbred mouse strains that inactivates phosphatidylcholine transfer protein

New Zealand obese (NZO/HlLt) male mice develop polygenic diabetes and altered phosphatidylcholine metabolism. The gene encoding phosphatidylcholine transfer protein (PC-TP) is sited within the support interval for Nidd3, a recessive NZO-derived locus on Chromosome 11 identified by prior segregation analysis between NZO/HlLt and NON/Lt. Sequence analysis revealed that the NZO-derived PC-TP contained a non-synonymous point mutation that resulted in an Arg120His substitution, which was shared by the related NZB/BlNJ and NZW/LacJ mouse strains. Consistent with the structure-based predictions, functional studies demonstrated that Arg120His PC-TP was inactive, suggesting that this mutation contributes to the deficiencies in phosphatidylcholine metabolism observed in NZO mice.

Phospholipid transfer proteins in perspective

Since their discovery and subsequent purification from mammalian tissues more than 30 years ago an impressive number of studies have been carried out to characterize and elucidate the biological functions of phosphatidylcholine transfer protein (PC-TP), phosphatidylinositol transfer protein (PI-TP) and non-specific lipid transfer protein, more commonly known as sterol carrier protein 2 (SCP-2). Here I will present information to show that these soluble, low-molecular weight proteins constitute domain structures in StArR-related lipid transfer (START) proteins (i.e. PC-TP), in retinal degeneration protein, type B (RdgB)-related PI-TPs (e.g. Dm RdgB, Nir2, Nir3) and in peroxisomal beta-oxidation enzyme-related SCP-2 (i.e. 3-oxoacyl-CoA thiolase, also denoted as SCP-X and the 80-kDa D-bifunctional protein). Further I will summarize the most recent studies pertaining to the physiological function of these soluble phospholipid transfer proteins in metazoa.

Phosphatidylcholine transfer protein regulates size and hepatic uptake of high-density lipoproteins

Phosphatidylcholine transfer protein (PC-TP) is a steroidogenic acute regulatory-related transfer domain protein that is enriched in liver cytosol and binds phosphatidylcholines with high specificity. In tissue culture systems, PC-TP promotes ATP-binding cassette protein A1-mediated efflux of cholesterol and phosphatidylcholine molecules as nascent pre-beta-high-density lipoprotein (HDL) particles. Here, we explored a role for PC-TP in HDL metabolism in vivo utilizing 8-wk-old male Pctp(-/-) and wild-type littermate C57BL/6J mice that were fed for 7 days with either chow or a high-fat/high-cholesterol diet. In chow-fed mice, neither plasma cholesterol concentrations nor the concentrations and compositions of plasma phospholipids were influenced by PC-TP expression. However, in Pctp(-/-) mice, there was an accumulation of small alpha-migrating HDL particles. This occurred without changes in hepatic expression of ATP-binding cassette protein A1 or in proteins that regulate the intravascular metabolism and clearance of HDL particles. In Pctp(-/-) mice fed the high-fat/high-cholesterol diet, HDL particle sizes were normalized, whereas plasma cholesterol and phospholipid concentrations were increased compared with wild-type mice. In the absence of upregulation of hepatic ATP-binding cassette protein A1, reduced HDL uptake from plasma into livers of Pctp(-/-) mice contributed to higher plasma lipid concentrations. These data indicate that PC-TP is not essential for the enrichment of HDL with phosphatidylcholines but that it does modulate particle size and rates of hepatic clearance.

Give lipids a START: the StAR-related lipid transfer (START) domain in mammals

The steroidogenic acute regulatory protein (StAR)-related lipid transfer (START) domain is a protein module of approximately 210 residues that binds lipids, including sterols. Fifteen mammalian proteins, STARD1-STARD15, possess a START domain and these can be grouped into six subfamilies. Cholesterol, 25-hydroxycholesterol, phosphatidylcholine, phosphatidylethanolamine and ceramides are ligands for STARD1/STARD3/STARD5, STARD5, STARD2/STARD10, STARD10 and STARD11, respectively. The lipids or sterols bound by the remaining 9 START proteins are unknown. Recent studies show that the C-terminal end of the domain plays a fundamental role, forming a lid over a deep lipid-binding pocket that shields the ligand from the external environment. The START domain can be regarded as a lipid-exchange and/or a lipid-sensing domain. Mammalian START proteins have diverse expression patterns and can be found free in the cytoplasm, attached to membranes or in the nucleus. They appear to function in a variety of distinct physiological processes, such as lipid transfer between intracellular compartments, lipid metabolism and modulation of signaling events. Mutation or misexpression of START proteins is linked to pathological processes, including genetic disorders, autoimmune disease and cancer.

Altered hepatic cholesterol metabolism compensates for disruption of phosphatidylcholine transfer protein in mice

Phosphatidylcholine transfer protein (PC-TP) is a member of the steroidogenic acute regulatory transfer protein-related domain superfamily and is enriched in liver. To explore a role for PC-TP in hepatic cholesterol metabolism, Pctp-/- and wild-type C57BL/6J mice were fed a standard chow diet or a high-fat, high-cholesterol lithogenic diet. In chow-fed Pctp-/- mice, acyl CoA:cholesterol acyltransferase (Acat) activity was markedly increased, 3-hydroxy-3-methylglutaryl-CoA reductase activity was unchanged, and cholesterol 7alpha-hydroxylase activity was reduced. Consistent with increased Acat activity, esterified cholesterol concentrations in livers of Pctp-/- mice were increased, whereas unesterified cholesterol concentrations were reduced. Hepatic phospholipid concentrations were also decreased in the absence of PC-TP and consequently, unesterified cholesterol-to-phospholipid ratios in liver remained unchanged. The lithogenic diet downregulated 3-hydroxy-3-methylglutaryl-CoA reductase in wild-type and Pctp-/- mice, whereas Acat was increased only in wild-type mice. In response to the lithogenic diet, a greater reduction in cholesterol 7alpha-hydroxylase activity in Pctp-/- mice could be attributed to increased size and hydrophobicity of the bile salt pool. Despite higher hepatic phospholipid concentrations, the unesterified cholesterol-to-phospholipid ratio increased. The lack of Acat upregulation suggests that, in the setting of the dietary challenge, the capacity for esterification to defend against hepatic accumulation of unesterified cholesterol was exceeded in the absence of PC-TP expression. We speculate that regulation of cholesterol homeostasis is a physiological function of PC-TP in liver, which can be overcome with a cholesterol-rich lithogenic diet.

HDL 3 Induces Cyclooxygenase-2 Expression and Prostacyclin Release in Human Endothelial Cells Via a p38 MAPK/CRE-Dependent Pathway: Effects on COX-2/PGI-Synthase Coupling

Objective— In endothelial cells, cyclooxygenase-1 (COX-1) and COX-2 both contribute to prostacyclin production. Recent findings suggest that COX-2 contributes significantly to systemic prostacyclin synthesis in humans; whether COX-2 inhibition is related to an increased cardiovascular risk is undergoing debate. HDLs have been shown to increase prostacyclin synthesis, thus in the present study we investigated the molecular mechanisms involved in this effect in endothelial cells.

Methods and Results— HDL 3 (30 μg/mL) induced COX-2 expression in a time- and dose-dependent manner. COX-2 was found mainly in the perinuclear area where it co-localizes with PGI synthase. Transient transfection experiments showed that CRE is required for HDL-induced COX-2 transcription, and we demonstrated that p38 MAPK activation by HDL 3 is involved in COX-2 mRNA transcription and stabilization. As a consequence of COX-2-induction by HDL 3 prostacyclin production increased, incubation with a COX-2 selective inhibitor blocked this effect. Moreover, HDL 3 increased caveolin-1 phosphorylation, thus promoting PGI-synthase shuttling from the membrane to the perinuclear area.

Conclusion— We conclude that in endothelial cells, HDL modulates COX-2/PGI-S activity via both p38 MAPK-dependent COX-2 mRNA stability and transcription and both caveolin-1–dependent PGI-synthase shuttling and COX-2 coupling. The understanding of these mechanisms may provide new insights into the antiatherogenic role of HDL.

Phospholipid transfer protein is regulated by liver X receptors in vivo

Liver X receptors (LXR) belong to the nuclear receptor superfamily that can regulate important lipid metabolic pathways. The plasma phospholipid transfer protein (PLTP) is known to mediate transfer of phospholipids from triglyceride-rich lipoproteins to high density lipoprotein (HDL) and plays a critical role in HDL metabolism. We report here that a specific LXR agonist, T0901317, elevated HDL cholesterol and phospholipid in C57/BL6 mice and generated enlarged HDL particles that were enriched in cholesterol, ApoAI, ApoE, and phospholipid. The appearance of these HDL particles upon oral dosing of T0901317 in C57/BL6 mice was closely correlated with the increased plasma PLTP activity and liver PLTP mRNA levels. Nuclear run-on assay indicated that the effect of LXR agonist on PLTP expression was at the transcriptional level. In mouse peritoneal macrophage cells, PLTP expression was also up-regulated by the LXR/RXR (retinoid X receptor) heterodimer. However, cholesterol efflux in mouse peritoneal macrophage cells from PLTP-deficient mice (PLTP0) was not significantly different from wild type animals. Although in PLTP-deficient mice, the induction of HDL cholesterol as well as HDL particle size increase persisted, the extent of the induction was greatly attenuated. We conclude that PLTP is a direct target gene of LXRs in vivo and plays an important role in LXR agonist-mediated HDL cholesterol and size increase in mice.

Relevance of Niemann-Pick type C1 protein expression in controlling plasma cholesterol and biliary lipid secretion in mice

Receptor-mediated endocytosis is one of the major mechanisms for uptake of lipoprotein cholesterol in the liver. Because Niemann-Pick C1 (NPC1) protein is a key component in the intracellular distribution of cholesterol obtained from lipoproteins by the endocytic pathway, it may play a critical role in controlling plasma lipoprotein cholesterol and its biliary secretion. A murine model of Niemann-Pick type C disease (NPC), the NPC1-deficient [NPC1 (-/-)] mouse, was used to evaluate the relevance of hepatic NPC1 expression in regulating plasma lipoprotein cholesterol profile and biliary lipid secretion under chow and high-cholesterol diets. Total plasma cholesterol concentrations were increased in NPC1 (-/-) mice compared with wild-type mice when both mouse strains were fed chow or high-cholesterol diets. The increased plasma cholesterol levels found in NPC1 (-/-) mice were mostly due to elevated cholesterol content in larger and more heterogeneous HDL particles. On the chow diet, biliary lipid secretion was not impaired by NPC1 deficiency. Furthermore, chow-fed NPC1 (-/-) mice showed a small, but significant, increase in biliary cholesterol secretion. On the high-cholesterol diet, wild-type mice increased biliary cholesterol output, whereas NPC1 (-/-) mice did not. Finally, hepatic NPC1 overexpression by adenovirus-mediated gene transfer increased biliary cholesterol secretion by 100% to 150% in both wild-type mice and cholesterol-fed NPC1 (-/-) mice. In conclusion, hepatic NPC1 expression is an important factor for regulating plasma HDL cholesterol levels and biliary cholesterol secretion in mice.