Determinants of the selection of phosphatidylcholine molecular species for secretion into bile in the rat

Specificity and rate of human and mouse liver and plasma phosphatidylcholine synthesis analyzed in vivo

Phosphatidylcholine (PC) synthesis by the direct cytidine diphosphate choline (CDP-choline) pathway in rat liver generates predominantly mono- and di-unsaturated molecular species, while polyunsaturated PC species are synthesized largely by the phosphatidylethanolamine-N-methyltransferase (PEMT) pathway. Although altered PC synthesis has been suggested to contribute to development of hepatocarcinoma and nonalcoholic steatohepatitis, analysis of the specificity of hepatic PC metabolism in human patients has been limited by the lack of sensitive and safe methodologies. Here we incorporated a deuterated methyl-D(9)-labled choline chloride, to quantify biosynthesis fluxes through both of the PC synthetic pathways in vivo in human volunteers and compared these fluxes with those in mice. Rates and molecular specificities of label incorporated into mouse liver and plasma PC were very similar and strongly suggest that label incorporation into human plasma PC can provide a direct measure of hepatic PC synthesis in human subjects. Importantly, we demonstrate for the first time that the PEMT pathway in human liver is selective for polyunsaturated PC species, especially those containing docosahexaenoic acid. Finally, we present a multiple isotopomer distribution analysis approach, based on transfer of deuterated methyl groups to S-adenosylmethionine and subsequent sequential methylations of PE, to quantify absolute flux rates through the PEMT pathway that are applicable to studies of liver dysfunction in clinical studies.

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.

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.

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.

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.

The phosphoprotein StarD10 is overexpressed in breast cancer and cooperates with ErbB receptors in cellular transformation

We have identified that StarD10, a member of the START protein family, is overexpressed in both mouse and human breast tumors. StarD10 was initially discovered on the basis of its cross-reactivity with a phosphoserine-specific antibody in mammary tumors from Neu/ErbB2 transgenic mice and subsequently isolated from SKBR3 human breast carcinoma cells using a multistep biochemical purification strategy. We have shown that StarD10 is capable of binding lipids. StarD10 was found to be overexpressed in 35% of primary breast carcinomas and 64% of human breast cancer cell lines, correlating with their ErbB2/Her2 status. Coexpression of StarD10 with ErbB1/epidermal growth factor receptor in murine fibroblasts enhanced anchorage-independent growth in soft agar, providing evidence for functional cooperation between StarD10 and ErbB receptor signaling. Taken together, these data suggest that overexpression of this lipid-binding protein contributes to breast oncogenesis.

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.

Metabolic alterations by clofibric acid in the formation of molecular species of phosphatidylcholine in rat liver

The mechanism by which p-chlorophenoxyisobutyric acid (clofibric acid) induces striking changes in the proportion of the molecular species of phosphatidylcholine (PC) in rat liver was studied. Treatment of rats with clofibric acid strikingly increased the content of 1-palmitoyl-2-oleoyl (16:0-18:1) PC, but decreased the contents of 1-palmitoyl-2-docosahexaenoyl (16:0-22:6), 1-stearoyl-2-arachidonoyl (18:0-20:4), and 1-stearoyl-2-linoleoyl (18:0-18:2) PC; the drug did not change the content of 1-palmitoyl-2-arachidonoyl (16:0-20:4) PC. The mechanism underlying these changes has been investigated with regard to the in vivo formation of the molecular species of PC by: (i) de novo synthesis, (ii) reacylation, and (iii) methylation of phosphatidylethanolamine (PE). We found that (i) the incorporation of [3H]glycerol, which was injected intravenously, into 16:0-18:1 diacylglycerol (DG) and 16:0-18:1 PC was increased markedly by clofibric acid feeding without changing the substrate specificity of CDP-choline:DG cholinephosphotransferase, (ii) the in vivo formation of 16:0-18:1 and 16:0-20:4 PC from 1-16:0-[3H]glycerophosphocholine (GPC), which was injected intraportally, was increased markedly by clofibric acid feeding, and (iii) the incorporation of [14C]ethanolamine, which was injected intravenously into 16:0-22:6, 18:0-22:6, and 18:0-20:4 PC, was decreased by clofibric acid feeding; the extent of the decrease in 16:0-20:4 PC was less than that of 18:0-20:4 PC. It was concluded, therefore, that (i) clofibric acid selectively increased the content and proportion of 16:0-18:1 PC by enhancing both the CDP-choline pathway and the remodeling of the pre-existing PC molecule, and (ii) the drug kept the content of 16:0-20:4 PC unchanged by stimulating the remodeling of the pre-existing PC molecule, whereas the formation of other more long chain, polyunsaturated molecular species, such as 16:0-22:6, 18:0-22:6, and 18:0-20:4, was decreased owing to the suppression of PE methylation.

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.

Hepatocellular transport and secretion of biliary lipids

Bile is the route for elimination of cholesterol from the body. Recent studies have begun to elucidate hepatocellular, molecular and physical-chemical mechanisms whereby bile salts stimulate biliary secretion of cholesterol together with phospholipids, which are enriched (up to 95%) in phosphatidylcholines. Active translocation of bile salts and phosphatidylcholines across the hepatocyte's canalicular plasma membrane provides the driving force for biliary lipid secretion. This facilitates physical-chemical interactions between detergent-like bile salt molecules and the ectoplasmic leaflet of the canalicular membrane, which result in biliary secretion of cholesterol and phosphatidylcholines as vesicles. Within the hepatocyte, separate molecular pathways function to resupply bile salts, phosphatidylcholines and cholesterol to the canalicular membrane for ongoing biliary lipid secretion.

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.