Stimulation of cholinephosphotransferase activity by phosphatidylcholine transfer protein. Regulation of membrane phospholipid synthesis by a cytosolic protein

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.

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.

Phospholipid transfer proteins and physiological functions

Issues of how cells generate and maintain unique lipid compositions in distinct intracellular membrane systems remain the subject of much study. A ubiquitous class of soluble proteins capable of transporting phospholipid monomers from membrane to membrane across an aqueous milieu has been thought to define part of the mechanism by which lipids are sorted in cells. Progress in the study of these phospholipid transfer proteins (PLTPs) raises questions regarding their physiological functions in cells and the mechanisms by which these proteins execute them. It is now clear that across the eukaryotic kingdom, members of this protein family exert essential roles in the regulation of phospholipid metabolism and central aspects of phospholipid-mediated signaling. Indeed, it is now known that dysfunction of specific PLTPs defines the basis of inherited diseases in mammals, and this list is expected to grow. Phospholipid transfer proteins, their biochemical properties, and the emerging clues regarding their physiological functions are reviewed.

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.

Liver mitochondria, confirmed as intact by complete suppression of succinate uptake and oxidation, possess a carnitine palmitoyltransferase I that is totally inhibited by malonyl CoA

Succinate dehydrogenase activity in mitochondria, which were isolated by centrifuging partially purified mitochondria through 1. 315 M sucrose, was completely suppressed when [14C]succinate uptake was abolished by prior incubation of the mitochondria with carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and valinomycin. The conclusion that these mitochondria were intact was confirmed by the fact that, when these mitochondria were broken by a freeze-thaw cycle followed by sonication, such inhibition was totally abolished. The yield of mitochondria, microsomes, and peroxisomes from the initial homogenate was 17.8, <0.1, and 0%, respectively, indicating that the mitochondria were not only intact but also essentially free of contamination from microsomes and peroxisomes. The overt form of carnitine palmitoyltransferase (CPT I) in these intact and pure mitochondria was totally inhibited by malonyl CoA, indicating that previous reports of incomplete inhibition in mitochondrial preparations resulted from interference from CPT activity in the inner mitochondrial membrane (CPT II), microsomes, or peroxisomes.

Carnitine palmitoyltransferase activities: effects of serum albumin, acyl-CoA binding protein and fatty acid binding protein

The carnitine palmitoyltransferase activity of various subcellular preparations measured with octanoyl-CoA as substrate was markedly increased by bovine serum albumin at low microM concentrations of octanoyl-CoA. However, even a large excess (500 microM) of this acyl-CoA did not inhibit the activity of the mitochondrial outer carnitine palmitoyltransferase, a carnitine palmitoyltransferase isoform that is particularly sensitive to inhibition by low microM concentrations of palmitoyl-CoA. This bovine serum albumin stimulation was independent of the salt activation of the carnitine palmitoyltransferase activity. The effects of acyl-CoA binding protein (ACBP) and the fatty acid binding protein were also examined with palmitoyl-CoA as substrate. The results were in line with the findings of stronger binding of acyl-CoA to ACBP but showed that fatty acid binding protein also binds acyl-CoA esters. Although the effects of these proteins on the outer mitochondrial carnitine palmitoyltransferase activity and its malonyl-CoA inhibition varied with the experimental conditions, they showed that the various carnitine palmitoyltransferase preparations are effectively able to use palmitoyl-CoA bound to ACBP in a near physiological molar ratio of 1:1 as well as that bound to the fatty acid binding protein. It is suggested that the three proteins mentioned above affect the carnitine palmitoyltransferase activities not only by binding of acyl-CoAs, preventing acyl-CoA inhibition, but also by facilitating the removal of the acylcarnitine product from carnitine palmitoyltransferase. These results support the possibility that the acyl-CoA binding ability of acyl-CoA binding protein and of fatty acid binding protein have a role in acyl-CoA metabolism in vivo.

In vitro evidence that phospholipid secretion into bile may be coordinated intracellularly by the combined actions of bile salts and the specific phosphatidylcholine transfer protein of liver

Using model systems, we explored a potential function of hepatic phosphatidylcholine transfer protein to extract biliary-type phosphatidylcholines from intracellular membranes (e.g., smooth endoplasmic reticulum) and deliver them to canalicular plasma membranes where biliary secretion occurs. We measured transfer rates of parinaroyl phosphatidylcholine, a naturally fluorescent phospholipid, from small unilamellar vesicles composed of sn-1 palmitoyl, sn-2 parinaroyl phosphatidylcholine, and egg yolk phosphatidylcholine (molar ratio 75:25) wherein the fluorophore is self-quenched to small unilamellar vesicles composed of phosphatidylethanolamine, sphingomyelin, phosphatidylserine, phosphatidylinositol, and cholesterol (molar ratios 22:22:10:8:38) representing model microsomal and canalicular plasma membranes, respectively. Following addition of phosphatidylcholine transfer protein (purified from bovine liver), fluorescence intensity increased exponentially indicating net phosphatidylcholine transfer from donor to acceptor vesicles. Submicellar concentrations of a wide hydrophobicity range of common and uncommon taurine and glycine conjugated bile salts species (anionic steroid detergent-like molecules), sodium taurofusidate (a conjugated fungal bile salt analog), and sodium dodecyl sulfate and octylglucoside, anionic and nonionic straight chain detergents, respectively, markedly stimulated phosphatidylcholine transfer protein activity. This 40-115-fold effect was most pronounced for the common bile salts and correlated positively with bile salt hydrophobicity. Thermodynamic analysis of net transfer revealed that the rate-limiting step was extraction of phosphatidylcholine molecules from donor vesicles and that bile salts facilitated their capture by enhancing both phosphatidylcholine transfer protein binding as well as perturbing phospholipid packing in vesicle bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)

CDPcholine:1,2-diacylglycerol cholinephosphotransferase from rat liver microsomes. I. Solubilization and characterization of the partially purified enzyme and the possible existence of an endogenous inhibitor

The solubilization and partial purification of cholinephosphotransferase (CDPcholine:1,2-diacylglycerol cholinephosphotransferase, EC 2.7.8.2) from rat liver microsomes were examined in the presence of ionic (sodium deoxycholate), nonionic (Triton X-100, n-octylglycoside), or zwitter ionic (CHAPS) detergents. Among the four detergents tested, only sodium deoxycholate was found to be an efficient solubilizer of cholinephosphotransferase activity from microsomal membranes, whereas the other three detergents caused irreversible inactivation of the enzyme at the solubilization step. Addition of phospholipids at the solubilization step, or after solubilization of the membrane proteins, could not preserve or reconstitute activity to any extent. The sodium deoxycholate-solubilized activity was partially purified by gel permeation chromatography (Superose 12HR). The partially purified preparation appeared to consist of a large aggregate containing phospholipids; further dissociation of the protein-phospholipid complex caused complete inactivation of the enzyme. The partially purified cholinephosphotransferase showed a specific activity of 100-130 nmol/min/mg protein, which is the highest activity reported to date from any tissue source; this amounts to a 4-fold enrichment of cholinephosphotransferase activity from the original KCl-washed rat liver microsomes. Ethanolaminephosphotransferase (CDPethanolamine:1,2-diacylglycerol ethanolaminephosphotransferase, EC 2.7.8.1) activity was copurified and 6-fold enriched with a total recovery of 60%. During the purification of cholinephosphotransferase activity, a putative endogenous inhibitor of cholinephosphotransferase was also solubilized and was isolated from the microsomal membranes. This heat-labile, nondialyzable inhibitor was shown to act specifically on cholinephosphotransferase and not on ethanolaminephosphotransferase. Further characterization of the inhibitory activity revealed that it may act at the binding step of the cholinephosphotransferase to its lipid substrate, diacylglycerol.