cDNA cloning and tissue-specific expression of the phosphatidylcholine transfer protein gene

Racial differences in human platelet PAR4 reactivity reflect expression of PCTP and miR-376c

Racial differences in the pathophysiology of atherothrombosis are poorly understood. We explored the function and transcriptome of platelets in healthy black (n = 70) and white (n = 84) subjects. PAR4 thrombin receptor induced platelet aggregation and calcium mobilization were significantly greater in black subjects. Numerous differentially expressed (DE) RNAs were associated with both race and PAR4 reactivity, including phosphatidylcholine transfer protein (PCTP), and platelets from blacks expressed higher levels of PC-TP protein. PC-TP inhibition or depletion blocked activation of platelets or megakaryocytic cell lines through PAR4 but not PAR1. MiR-376c levels were DE by race and PAR4 reactivity, and were inversely correlated with PCTP mRNA levels, PC-TP protein levels and PAR4 reactivity. MiR-376c regulated expression of PC-TP in human megakaryocytes. A disproportionately high number of miRNAs DE by race and PAR4 reactivity, including miR-376c, are encoded in the DLK1-DIO3 locus, and were lower in platelets from blacks. These results support PC-TP as a regulator of the racial difference in PAR4-mediated platelet activation, indicate a genomic contribution to platelet function that differs by race, and emphasize a need to consider race effects when developing anti-thrombotic drugs.

PC-TP/StARD2: Of membranes and metabolism

Phosphatidylcholine transfer protein (PC-TP, synonym StARD2) binds phosphatidylcholines, and catalyzes their intermembrane transfer and exchange in vitro. The structure of PC-TP comprises a hydrophobic pocket and a well-defined head group binding site, and its gene expression is regulated by peroxisome proliferator activated receptor-alpha. Recent studies have revealed key regulatory roles for PC-TP in lipid and glucose metabolism. Notably, Pctp(-/-) mice are sensitized to the action of insulin, and exhibit more efficient brown fat-mediated thermogenesis. PC-TP appears to limit access of fatty acids to mitochondria by stimulating the activity of thioesterase superfamily member 2, a newly characterized long-chain fatty acyl-coenzyme A thioesterase. Because PC-TP discriminates between phosphatidylcholines within lipid bilayers, it might function as a sensor that links metabolic regulation to membrane composition.

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.

Regulation of energy substrate utilization and hepatic insulin sensitivity by phosphatidylcholine transfer protein/StarD2

Phosphatidylcholine transfer protein (PC-TP, also known as StarD2) is a highly specific intracellular lipid binding protein with accentuated expression in oxidative tissues. Here we show that decreased plasma concentrations of glucose and free fatty acids in fasting PC-TP-deficient (Pctp(-/-)) mice are attributable to increased hepatic insulin sensitivity. In hyperinsulinemic-euglycemic clamp studies, Pctp(-/-) mice exhibited profound reductions in hepatic glucose production, gluconeogenesis, glycogenolysis, and glucose cycling. These changes were explained in part by the lack of PC-TP expression in liver per se and in part by marked alterations in body fat composition. Reduced respiratory quotients in Pctp(-/-) mice were indicative of preferential fatty acid utilization for energy production in oxidative tissues. In the setting of decreased hepatic fatty acid synthesis, increased clearance rates of dietary triglycerides and increased hepatic triglyceride production rates reflected higher turnover in Pctp(-/-) mice. Collectively, these data support a key biological role for PC-TP in the regulation of energy substrate utilization.

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.

Characterization and tissue-specific expression of phosphatidylcholine transfer protein gene from amphioxus Branchiostoma belcheri

An amphioxus cDNA, encoding phosphatidylcholine transfer protein (AmphiPCTP), was identified for the first time from the gut cDNA library of Branchiostoma belcheri. It contains a 660-bp open reading frame corresponding to a deduced protein of 219 amino acids. Phylogenetic tree analysis showed that AmphiPCTP clustered with PCTP subgroup of PCTP subfamily containing steroidogenic acute regulatory protein (StAR)-related lipid transfer (START) domains. AmphiPCTP had an exon-intron organization similar to that of human and rat PCTP genes in terms of both exon number and sequence homology of each exon, suggesting that PCTP has probably maintained a similar function in both amphioxus and mammalian species. Both in situ hybridization histochemistry and whole-mount in situ hybridization revealed a tissue-specific expression pattern of AmphiPCTP with the high levels in the hepatic caecum and primitive gut, including the region where the hepatic caecum will form later during development. This apparently agrees with the hypothesis that amphioxus hepatic caecum is equivalent to vertebrate liver. These results suggest a conserved role of PCTPs in amphioxus as well as mammalian species.

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.

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.

Impaired response of biliary lipid secretion to a lithogenic diet in phosphatidylcholine transfer protein-deficient mice

Phosphatidylcholine transfer protein (PC-TP) is a cytosolic lipid transfer protein that is highly expressed in liver and catalyzes intermembrane transfer of phosphatidylcholines in vitro. To explore a role for PC-TP in the hepatocellular trafficking of biliary phosphatidylcholines, we characterized biliary lipid secretion using Pctp(-/-) and wild-type littermate control mice with C57BL/6J and FVB/NJ genetic backgrounds, which express PC-TP at relatively high and low levels in liver, respectively. Eight-week-old male Pctp(-/-) and wild-type mice were fed a chow diet or a lithogenic diet, which served to upregulate biliary lipid secretion. In chow-fed mice, the absence of PC-TP did not reduce biliary phospholipid secretion or alter the phospholipid composition of biles. However, the responses in secretion of biliary phospholipids, cholesterol, and bile salts to the lithogenic diet were impaired in Pctp(-/-) mice from both genetic backgrounds. Alterations in biliary lipid secretion could not be attributed to transcriptional regulation of the expression of canalicular membrane lipid transporters, but possibly to a defect in their trafficking to the canalicular membrane. These findings support a role for PC-TP in the response of biliary lipid secretion to a lithogenic diet, but not specifically in the hepatocellular transport and secretion of phosphatidylcholines.

GTT1/StarD7, a Novel Phosphatidylcholine Transfer Protein-like Highly Expressed in Gestational Trophoblastic Tumour:

We report the cDNA cloning and characterization of GTT1/StarD7, a novel gestational trophoblastic tumour gene, initially identified by its up-regulated expression in the choriocarcinoma JEG-3 cell line with respect to their nonmalignant counterpart, complete hydatidiform mole and normal trophoblastic tissue. Using the differential display fragment as a probe we screened placenta and HeLa cDNA libraries and isolated a clone carrying a 3315 bp insert (accession number AF270647). This cDNA encodes a protein of 295 amino acid residues with a molecular weight of approximately 34.7 kDa and a pI of 5.79. Computer-mediated homology search revealed that the deduced amino acid sequence had similarity to phosphatidylcholine transfer protein (PCTP) with a conserved StAR-related lipid transfer (START) domain extending between the amino acids 66 to 250. The GTT1 gene contains at least 9 exons spread nearly 30 kb on chromosome 2p12-2p11.2. Northern blot assays of total RNA derived from normal early placenta (NEP), complete hydatidiform mole (CHM) and JEG-3 cell line revealed a 3.5 kb mRNA expressed exclusively in the JEG-3 cell line. However, semiquantitative RT-PCR analysis performed with the same RNA samples demonstrated GTT1 expression throughout all of them with the highest level in JEG-3 cell line. Examination of GTT1 mRNA expression by semiquantitative RT-PCR assays in a series of tumour cell lines indicated wide-spread GTT1 expression with predominance in both choriocarcinoma JEG-3 and JAR cells, colorectal adenocarcinoma HT29 and hepatocellular carcinoma HepG2 cells. In conclusion, the highly GTT1 expression profile in JEG-3 and JAR cell lines and its lipid binding domain suggest that GTT1 may play an important role in the phospholipid-mediated signalling of trophoblastic tumour cellular events.

Clofibrate-induced relocation of phosphatidylcholine transfer protein to mitochondria in endothelial cells

The phosphatidylcholine transfer protein (PC-TP) is a specific transporter of phosphatidylcholine (PC) between membranes. To get more insight into its physiological function, we have studied the localization of PC-TP by microinjection of fluorescently labeled PC-TP in foetal bovine heart endothelial (FBHE) cells and by expression of an enhanced yellow fluorescent protein-PC-TP fusion protein in FBHE cells, human umbilical vein endothelial cells, and HepG2 cells. Analysis by confocal laser scanning microscopy showed that PC-TP was evenly distributed throughout the cytosol with an apparently elevated level in nuclei. By measuring the fluorescence recovery after bleaching it was established that PC-TP is highly mobile throughout the cell, with its transport into the nucleus being hindered by the nuclear envelope. Given the proposed function of PC-TP in lipid metabolism, we have tested a number of compounds (phorbol ester, bombesin, A23187, thrombin, dibutyryl cyclic AMP, oleate, clofibrate, platelet-derived growth factor, epidermal growth factor, and hydrogen peroxide) for their ability to affect intracellular PC-TP distribution. Only clofibrate (100 microM) was found to have an effect, with PC-TP moving to mitochondria within 5 min of stimulation. This relocation did not occur with PC-TP(S110A), lacking the putative protein kinase C (PKC)-dependent phosphorylation site, and was restricted to the primary endothelial cells. Relocation did not occur in HepG2 cells, possibly due to the fact that clofibrate does not induce PKC activation in these cells.

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

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-beta-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-beta-HDL particle formation due to the activity of ATP-binding cassette A1.

The binding of phosphatidylcholine to the phosphatidylcholine transfer protein: affinity and role in folding

Bovine liver phosphatidylcholine transfer protein (PC-TP) has been expressed in Escherichia coli and purified to homogeneity from the cytosol fraction at a yield of 0.45 mg PC-TP per 10 mg total cytosolic protein. In addition, active PC-TP was obtained from inclusion bodies. An essential factor in the activation of PC-TP was phosphatidylcholine (PC) present in the folding buffer. PC-TP from the cytosol contains phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) with a preference for the di-monounsaturated species over the saturated species as determined by fast atom bombardment mass spectrometry (FAB-MS). By incubation with microsomal membranes the endogenous PE and PG were replaced by PC. Relative to the microsomal PC species composition, PC-TP bound preferentially C16:0/C20:4-PC and C16:0/C18:2-PC (twofold enriched) whereas the major microsomal species C18:0/C18:1-PC and C18:0/C18:2-PC were distinctly less bound. PC-TP is structurally homologous to the lipid-binding domain of the steroidogenic acute regulatory protein (Nat. Struct. Biol. 7 (2000) 408). Replacement of Lys(55) present in one of the beta-strands forming the lipid-binding site, with an isoleucine residue yielded an inactive protein. This suggests that Lys(55) be involved in the binding of the PC molecule.

Molecular cloning and characterization of phosphatidylcholine transfer protein-like protein gene expressed in murine haploid germ cells

We have isolated a cDNA clone specifically expressed in spermiogenesis from a subtracted cDNA library of mouse testis. The cDNA consisted of 1392 nucleotides and had an open reading frame of 873 nucleotides encoding a protein of 291 amino acid residues. Computer-mediated homology search revealed that the nucleotide sequence was unique but the deduced amino acid sequence had similarity to mouse phosphatidylcholine transfer protein (PCTP). We named this newly isolated gene PCTP-like protein. Northern blot analysis revealed a 1.4-kilobase mRNA expressed in the testis, kidney, liver, and intestine with the highest level in the testis. Messenger RNA expression in the testis was detected first on Day 23 in postnatal development and then increased up to adulthood. The protein, having a molecular weight of approximately 40 000, was encoded by the mRNA and was detected at the tail of the elongated spermatids and sperm by immunohistochemical staining.

Cloning, tissue-specific expression, gene structure and chromosomal localization of human phosphatidylcholine transfer protein

Phosphatidylcholine transfer protein (PC-TP) is a cytosolic protein that catalyzes intermembrane transfer of phosphatidylcholines in vitro. We have cloned a cDNA encoding the human ortholog of PC-TP and have determined its tissue-specific expression as well as genomic organization. Radiation hybrid mapping localized the human gene, PCTP, to chromosome 17q21-22 and PCR-based single strand conformation polymorphism analysis of an interspecific backcross assigned mouse Pctp to the region of syntenic conservation on chromosome 11.

Mice without phosphatidylcholine transfer protein have no defects in the secretion of phosphatidylcholine into bile or into lung airspaces

Phosphatidylcholine transfer protein (Pc-tp) is a highly specific carrier of phosphatidylcholine (PC) without known function. Proposed functions include the supply of PC required for secretion into bile or lung air space (surfactant) and the facilitation of enzymatic reactions involving PC synthesis or breakdown. To test these functions, we generated knock-out mice unable to make Pc-tp. Remarkably, these mice are normal and have no defect in any of the postulated Pc-tp functions analyzed. The lipid content and composition of the bile, as well as lung surfactant secretion and composition, of Pc-tp (-/-) mice, is normal. The lack of a Pc-tp contribution to biliary lipid secretion is in agreement with our finding that Pc-tp is down-regulated in adult mouse liver: whereas Pc-tp is abundant in the liver of mouse pups, Pc-tp levels decrease > 10-fold around 2 wk after birth, when bile formation starts. In adult mice, Pc-tp levels are high only in epididymis, testis, kidney, and bone marrow-derived mast cells. Absence of Pc-tp in bone marrow-derived mast cells does not affect their lipid composition or PC synthesis and degradation. We discuss how PC might reach the canalicular membrane of the hepatocyte for secretion into the bile, if not by Pc-tp.

Cloning and gene structure of rat phosphatidylcholine transfer protein, Pctp

Phosphatidylcholine transfer protein (PC-TP) is a cytosolic lipid transfer protein that promotes intermembrane transfer of phosphatidylcholines but no other phospholipids. Although its physiological function remains unknown, phosphatidylcholine transfer protein is enriched in liver and evidence from model systems suggests a role in hepatocellular selection and transport of biliary phospholipids. To facilitate in vivo studies, a cDNA encoding rat PC-TP was cloned by library screening and 5'-rapid amplification of cDNA ends. Genomic cloning demonstrated the rat Pctp gene spans 10. 8kb and is comprised of six exons. The putative transcription initiation site was identified 50bp upstream of the translation initiation site. Nucleotide sequence analysis of the 5'-flanking region revealed a CAAT- but no TATA-box. Transient transfection of a series of 5'-deleted Pctp-promoter-firefly luciferase constructs into Reuber H35 rat hepatoma cells, which express Pctp mRNA, and Gunn rat fibroblasts, which do not, suggest that cis-acting elements in a 637bp promoter region contribute to enhanced expression of PC-TP in liver.

Phosphatidylcholine transfer protein from bovine liver contains highly unsaturated phosphatidylcholine species

The phosphatidylcholine transfer protein (PC-TP) from bovine liver contains one molecule of non-covalently bound PC. In order to gain more insight into the physiological function of PC-TP, PC was extracted from bovine liver PC-TP and its molecular species composition identified by fast atom bombardment mass spectrometry. The prevailing molecular species were C18:0/C18:1-, C18:0/C18:2-, C18:0/C20:4-, C18:0/20:5- and C18:0/C22:5-PC accounting for 85% of the PC species present. This molecular species composition is not representative for what is present in bovine liver where these species account for 43% of the total PC content [Montfoort et al. (1971) Biochim. Biophys. Acta 231, 335-342]. Another striking observation is that PC species carrying a palmitoyl chain at the sn-1 position are nearly absent, despite these species being abundantly present in bovine liver. This study suggests that PC-TP could play a role in the metabolism of highly unsaturated, stearoyl-containing PC species.