Membrane phospholipid synthesis and endoplasmic reticulum function
2008; Elsevier BV; Volume: 50; Linguagem: Inglês
10.1194/jlr.r800049-jlr200
ISSN1539-7262
AutoresPaolo Fagone, Suzanne Jackowski,
Tópico(s)Lipid metabolism and biosynthesis
ResumoThis review presents an overview of mammalian phospholipid synthesis and the cellular locations of the biochemical activities that produce membrane lipid molecular species. The generalized endoplasmic reticulum compartment is a central site for membrane lipid biogenesis, and examples of the emerging relationships between alterations in lipid composition, regulation of membrane lipid biogenesis, and cellular secretory function are discussed. This review presents an overview of mammalian phospholipid synthesis and the cellular locations of the biochemical activities that produce membrane lipid molecular species. The generalized endoplasmic reticulum compartment is a central site for membrane lipid biogenesis, and examples of the emerging relationships between alterations in lipid composition, regulation of membrane lipid biogenesis, and cellular secretory function are discussed. BIOLOGICAL MEMBRANESBiological membranes are composed of lipids and proteins that together form hydrophobic barriers that limit the distribution of aqueous macromolecules and metabolites. Cells use membranes for a number of different purposes, including segregation and protection from the environment, compartmentalization of functions, energy production, storage, protein synthesis and secretion, phagocytosis, movement, and cell-cell interaction. Eukaryotic cells contain ordered infrastructures, called organelles, to organize and carry out complex processes and to enable distinct reactions that require a hydrophobic environment. The level and complexity of compartmentalization varies among organisms and among mammalian cells. Some cells also change in size and organelle complexity after biological stimulation. An example of induced membrane biogenesis occurs in nai¨ve B-lymphocytes that are converted to plasma cells (1Fagone P. Sriburi R. Frank C.Ward-Chapman, M. Wang J. Gunter C. Brewer J.W. Jackowski S. Phospholipid biosynthesis program underlying membrane expansion during B lymphocyte differentiation.J. Biol. Chem. 2007; 282: 7591-7605Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar), and an example of membrane redistribution occurs in macrophages in which the Golgi apparatus is reoriented during transient cytokine synthesis and secretion (2Tian Y. Pate C. Andreolotti A. Wang L. Tuomanen E. Boyd K. Claro E. Jackowski S. Cytokine secretion requires phosphatidylcholine synthesis.J. Cell Biol. 2008; 181: 945-957Crossref PubMed Scopus (48) Google Scholar). The versatility of biological membranes is dependent on their structures and biophysical properties, which are dictated by the types of lipids and proteins that compose the membranes. The functions of membranes require a fluid plasticity that is accomplished through alteration in lipid composition. Lipid composition is diverse, not only among different organisms, but also among different compartments within the same cells and between the two leaflets of the same membrane. Lipid composition is determined through regulation of de novo synthesis at designated cellular sites, selective distribution or trafficking to new sites, and by localized remodeling reactions. Understanding the relationships between the dynamic changes in membrane lipid composition and specific cellular events is our current challenge. This review is focused on membrane phospholipid biogenesis in mammalian cells with a particular emphasis on the role played by the endoplasmic reticulum (ER). The ER, together with the Golgi apparatus, is a major site of de novo bulk membrane lipid synthesis, and recent experiments demonstrate a link between phospholipid synthesis and secretion from this compartment.THE ARCHITECTURE OF THE ERThe ER and Golgi apparatus together constitute the endomembrane compartment in the cytoplasm of eukaryotic cells. The endomembrane compartment is a major site of lipid synthesis, and the ER is where not only lipids are synthesized, but membrane-bound proteins and secretory proteins are also made. The ER is organized into a labyrinthine membrane-bound network of branching tubules and flattened sacs that extends throughout the cytosol. The tubules and sacs interconnect, and their membrane is continuous with the outer nuclear membrane (3Matsuura S. Masuda R. Sakai O. Tashiro Y. Immunoelectron microscopy of the outer membrane of rat hepatocyte nuclear envelopes in relation to the rough endoplasmic reticulum.Cell Struct. Funct. 1983; 8: 1-9Crossref PubMed Scopus (10) Google Scholar). ER and nuclear membranes form a continuous sheet enclosing a single internal space, called the lumen. The ER can be divided into subdomains in relation to their function or location. The nuclear envelope is the domain that separates the genetic material from the cytosol. The ribosomes that synthesize ER-associated proteins are attached to the cytoplasmic aspect of the ER membrane, and these regions are designated as rough ER. The transitional ER is characterized by two domains, namely, a domain associated with ribosomes at a low density and a region that lacks attached ribosomes, called smooth ER. The ER region in close proximity with the mitochondrium is the mitochondrium-associated membrane. Finally, the region in close proximity to the Golgi apparatus, rich in vesicles and tubules, is the ER-Golgi intermediate compartment (ERGIC) (4Lavoie C. Paiement J. Topology of molecular machines of the endoplasmic reticulum: a compilation of proteomics and cytological data.Histochem. Cell Biol. 2008; 129: 117-128Crossref PubMed Scopus (24) Google Scholar). The ERGIC domain represents a continuum of the ER and Golgi apparatus where the lipids and lumenal proteins destined for transport to the cell surface or other organelles are transferred and biochemically modified. The cis-Golgi structure is in close proximity to the ERGIC, and the trans-Golgi network is the site for the formation of budding vesicles that distribute the lumenal protein contents. The ER interacts closely with the cytoskeleton, mostly with microtubules. This interaction allows the ER to maintain its position within the cell and facilitates intracellular trafficking, particularly from the smooth ER (4Lavoie C. Paiement J. Topology of molecular machines of the endoplasmic reticulum: a compilation of proteomics and cytological data.Histochem. Cell Biol. 2008; 129: 117-128Crossref PubMed Scopus (24) Google Scholar).THE ER AND THE GOLGI APPARATUS ARE MAJOR SITES OF MEMBRANE LIPID SYNTHESISPhospholipids, including glycerophospholipids and sphingolipids, constitute the bulk lipid components of all mammalian membranes. Based on the information contained in several previous reviews (5Shindou H. Shimizu T. Acyl-CoA: lysophospholipid acyltransferases.J. Biol. Chem. 2008; 284: 1-5Abstract Full Text Full Text PDF PubMed Scopus (307) Google Scholar, 6Yen C.L. Stone S.J. Koliwad S. Harris C. Farese Jr., R.V. DGAT enzymes and triacylglycerol biosynthesis.J. Lipid Res. 2008; 49: 2283-2301Abstract Full Text Full Text PDF PubMed Scopus (706) Google Scholar, 7Coleman R.A. Lee D.P. Enzymes of triacylglycerol synthesis and their regulation.Prog. Lipid Res. 2004; 43: 134-176Crossref PubMed Scopus (702) Google Scholar, 8Henneberry A.L. Wright M.M. McMaster C.R. The major sites of cellular phospholipid synthesis and molecular determinants of fatty acid and lipid head group specificity.Mol. Biol. Cell. 2002; 13: 3148-3161Crossref PubMed Scopus (161) Google Scholar, 9Vance D.E. Walkey C.J. Cui Z. Phosphatidylethanolamine N-methyltransferase from liver.Biochim. Biophys. Acta. 1997; 1348: 142-150Crossref PubMed Scopus (167) Google Scholar, 10Vance J.E. Phosphatidylserine and phosphatidylethanolamine in mammalian cells: two metabolically related aminophospholipids.J. Lipid Res. 2008; 49: 1377-1387Abstract Full Text Full Text PDF PubMed Scopus (316) Google Scholar, 11Antonsson B. Phosphatidylinositol synthase from mammalian tissues.Biochim. Biophys. Acta. 1997; 1348: 179-186Crossref PubMed Scopus (53) Google Scholar, 12Heacock A.M. Agranoff B.W. CDP-diacylglycerol synthase from mammalian tissues.Biochim. Biophys. Acta. 1997; 1348: 166-172Crossref PubMed Scopus (41) Google Scholar, 13Schlame M. Rua D. Greenberg M.L. The biosynthesis and functional role of cardiolipin.Prog. Lipid Res. 2000; 39: 257-288Crossref PubMed Scopus (655) Google Scholar, 14Kawasaki K. Kuge O. Yamakawa Y. Nishijima M. Purification of phosphatidylglycerophosphate synthase from Chinese hamster ovary cells.Biochem. J. 2001; 354: 9-15Crossref PubMed Scopus (16) Google Scholar, 15Inglis-Broadgate S.L. Ocaka L. Banerjee R. Gaasenbeek M. Chapple J.P. Cheetham M.E. Clark B.J. Hunt D.M. Halford S. Isolation and characterization of murine Cds (CDP-diacylglycerol synthase) 1 and 2.Gene. 2005; 356: 19-31Crossref PubMed Scopus (39) Google Scholar, 16Chen D. Zhang X.Y. Shi Y. Identification and functional characterization of hCLS1, a human cardiolipin synthase localized in mitochondria.Biochem. J. 2006; 398: 169-176Crossref PubMed Scopus (70) Google Scholar, 17Nagan N. Zoeller R.A. Plasmalogens: biosynthesis and functions.Prog. Lipid Res. 2001; 40: 199-229Crossref PubMed Scopus (444) Google Scholar, 18Merrill Jr., A.H. De novo sphingolipid biosynthesis: a necessary, but dangerous, pathway.J. Biol. Chem. 2002; 277: 25843-25846Abstract Full Text Full Text PDF PubMed Scopus (483) Google Scholar, 19Futerman A.H. Riezman H. The ins and outs of sphingolipid synthesis.Trends Cell Biol. 2005; 15: 312-318Abstract Full Text Full Text PDF PubMed Scopus (257) Google Scholar), the phospholipid biosynthetic enzymes that produce membrane lipid products have been assigned to the different organelles in Fig. 1according to where the majority of protein or activity for each has been measured. Enzymes involved in phospholipid degradation or remodeling are not addressed. Enzymes that synthesize unique glycerophospholipids, called plasmalogens, are included. Water soluble intermediates are not shown in the scheme, with the exception of fatty acyl-CoA, which is used as substrate by acyltransferase enzymes for the synthesis of both glycerolipids and sphingolipids in the ER, cardiolipin (CL) and phosphatidylglycerol (DGPGro) in mitochondria, and for the synthesis of plasmalogens (PlmePEtn and PlmePCho) in the peroxisomes. The family of acyltransferases is quite extended, and only a few isoforms are directly involved in the de novo synthesis of membrane lipids, while others are involved in the remodeling of acyl chains of the different lipid classes (5Shindou H. Shimizu T. Acyl-CoA: lysophospholipid acyltransferases.J. Biol. Chem. 2008; 284: 1-5Abstract Full Text Full Text PDF PubMed Scopus (307) Google Scholar, 6Yen C.L. Stone S.J. Koliwad S. Harris C. Farese Jr., R.V. DGAT enzymes and triacylglycerol biosynthesis.J. Lipid Res. 2008; 49: 2283-2301Abstract Full Text Full Text PDF PubMed Scopus (706) Google Scholar). The acronyms for the different lipids are those proposed by the Lipid Maps project (20Fahy E. Subramaniam S. Brown H.A. Glass C.K. Merrill Jr., A.H. Murphy R.C. Raetz C.R. Russell D.W. Seyama Y. Shaw W. al et A comprehensive classification system for lipids.J. Lipid Res. 2005; 46: 839-861Abstract Full Text Full Text PDF PubMed Scopus (1086) Google Scholar), and the corresponding nomenclature for the lipid biosynthetic enzymes are defined in Table 1. Different isoforms of the lipid biosynthetic enzymes are not pointed out unless they are known to have alternate substrate specificities. Rather, the goal is to obtain a holistic view of the overall process of membrane lipid biogenesis.TABLE 1Enzyme and lipid abbreviationsEnzymes (Symbol and Name)Lipids (Symbol and Name)AGPAT, 1-acyl-sn-glycerol-3-phosphate O-acyltransferaseAcylGnP, 1-acyl-glyceronephosphateAGNPR, acyl/alkylglycerone-phosphate reductaseAlkylGnP, 1-alkyl-glyceronephosphateAGNPS, alkylglycerone-phosphate synthaseAlkylGP, 1-alkyl-glycerophosphateCDS, phosphatidate cytidylyltransferaseCDP-DG, CDP-diacylglycerolCEPT, diacylglycerol choline/ethanolaminephosphotransferaseCer, N-acylsphingosine (ceramide)CERT, ceramide transfer proteinCerPCho, ceramide phosphocholine (sphingomyelin)CGT, N-acylsphingosine galactosyltransferaseCL, diacylglycerophosphoglycerophosphodiradylglycerolCPT, diacylglycerol cholinephosphotransferaseDG, diacylglycerolCLS, cardiolipin synthasedhCer, dihydroceramideCRD, ceramidaseGalCer, galactosylceramideDGAT, ciacylglycerol O-acyltransferaseGlcCer, glucosylceramideDHCD, dihydroceramide δ(4)-desaturaseDGP, diacylglycerophosphateEPT, ethanolaminephosphostransferaseDGPCho, diacylglycerophosphocholineGCS, ceramide glucosyltransferaseDGPEtn, diacylglycerophosphoethanolamineGNPAT, glycerone-phosphate O-acyltransferaseDGPGro, diacylglycerophosphoglycerolGPAT, glycerol-3-phosphate O-acyltransferaseDGPGroP, diacylglycerophosphoglycerophosphateKDSR, 3-ketosphinganine reductaseDGPIns, diacylglycerophosphoinositolLCS, polypeptide N-acetylgalactosaminyltransferaseGPSer, diacylglycerophosphoserinePAP, phosphatidic acid phosphatasekSphn, 3-ketosphinganinePED, plasmanylethanolamine desaturaseLacCer, lactosylceramidePEMT, phosphatidylethanolamine N-methyltransferaseMGP, monoacylglycerophosphatePGP, phosphatidylglycerophosphatasePlmaH, 1-alkyl,2-acylglycerolPGS, CDP-diacylglycerol-glycerol-3-phosphate 3-phosphatidyltransferasePlmaP, 1-alkyl,2-acyl-glycerophosphatePIS, CDP-diacylglycerol-inositol 3-phosphatidyltransferasePlmaPEtn, 1-alkyl,2-acylglycerophosphosphoethanolamine (plasmanylethanolamine)PSD, phostatidylserine decarboxylasePlmePCho, 1Z-Alkenyl-2-acyl-glycerophosphocholine (plasmenylcholine)PSS1, phosphatidylserine synthase 1PlmePEtn, 1Z-Alkenyl,2-acylglycerophosphosphoethanolamine (plasmenylethanolamine)PSS2, phosphatidylserine synthase 2Sph, sphingosineSGMS, ceramide cholinephosphotransferaseSphn, sphinganineSNAT, sphingosine N-acyltransferaseSphnP, sphinganine-1-phosphateSPK, sphinganine kinaseSphP, sphingosine-1-phosphateSPP, sphingosine-1-phosphate phosphataseTG, triacylglycerolSPT, serine C-palmitoyltransferase Open table in a new tab GPAT and AGPAT activities begin the process of glycerolipid synthesis by attaching the fatty acyl moieties to the 1-position and then the 2-position of glycerol-3-phosphate, respectively. GPAT and AGPAT activities are associated with the ER and the mitochondria, providing the diacylglycerol phosphate (DGP) precursor for phospholipids in both locations. In the ER compartment, the DGP is dephosphorylated by the phosphatidic acid phosphatase enzymes to yield diacylglycerol (DG), which is incorporated into phosphatidylcholine (DGPCho) and phosphatidylethanolamine (DGPEtn). The GPAT and AGPAT association with the mitochondria suggests that these activities provide the DGP precursor for the synthesis of phosphatidylglycerol (DGPGro) and CL located at the same site.DGPCho is the most abundant glycerophospholipid species in mammalian cells, and it is synthesized in the ER and Golgi apparatus. Two biosynthetic pathways are available for DGPCho synthesis and are located in different endomembrane domains. The Kennedy pathway is the predominant route to DGPCho in most cells, and the final step is catalyzed by the bifunctional CEPT, which is located in the ER, or alternatively by the CPT, which is located in the Golgi apparatus (8Henneberry A.L. Wright M.M. McMaster C.R. The major sites of cellular phospholipid synthesis and molecular determinants of fatty acid and lipid head group specificity.Mol. Biol. Cell. 2002; 13: 3148-3161Crossref PubMed Scopus (161) Google Scholar, 9Vance D.E. Walkey C.J. Cui Z. Phosphatidylethanolamine N-methyltransferase from liver.Biochim. Biophys. Acta. 1997; 1348: 142-150Crossref PubMed Scopus (167) Google Scholar). Both the CEPT and the CPT use DG to form DGPCho. The PEMT activity, which converts DGPEtn to DGPCho, is restricted to the mitochondrium-associated membrane.DGPEtn is the second most abundant glycerophospholipid species, and its de novo synthesis can be catalyzed by the CEPT, located in the ER, or the EPT, a recently described isoform with strict specificity for DGPEtn production (21Horibata Y. Hirabayashi Y. Identification and characterization of a human ethanolaminephosphotransferase1 (hEPT1).J. Lipid Res. 2006; 48: 503-508Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar). An EPT activity has been described that is associated with peroxisomes (17Nagan N. Zoeller R.A. Plasmalogens: biosynthesis and functions.Prog. Lipid Res. 2001; 40: 199-229Crossref PubMed Scopus (444) Google Scholar). DGPEtn can also arise from head-group exchange with phosphatidylserine (DGPSer) in the ER as mediated by PSS2 or in the mitochondria by DGPSer decarboxylation, mediated by PSD (10Vance J.E. Phosphatidylserine and phosphatidylethanolamine in mammalian cells: two metabolically related aminophospholipids.J. Lipid Res. 2008; 49: 1377-1387Abstract Full Text Full Text PDF PubMed Scopus (316) Google Scholar). Differently from DGPCho and DGPEtn, DGPSer is synthesized in the ER through head-group exchange of already-made DGPCho and DGPEtn as catalyzed by PSS1 and PSS2, respectively (10Vance J.E. Phosphatidylserine and phosphatidylethanolamine in mammalian cells: two metabolically related aminophospholipids.J. Lipid Res. 2008; 49: 1377-1387Abstract Full Text Full Text PDF PubMed Scopus (316) Google Scholar). Triacylglycerol has no structural role but serves primarily as a storage lipid and is also synthesized in the ER (7Coleman R.A. Lee D.P. Enzymes of triacylglycerol synthesis and their regulation.Prog. Lipid Res. 2004; 43: 134-176Crossref PubMed Scopus (702) Google Scholar). Phosphatidylinositol (DGPIns) is synthesized in the ER by the PIS, and, apart from phosphatidylinositol-4-phosphate, its conversion into the highly phosphorylated forms, which play critical roles in signaling and membrane vesicle trafficking, occurs outside the ER (11Antonsson B. Phosphatidylinositol synthase from mammalian tissues.Biochim. Biophys. Acta. 1997; 1348: 179-186Crossref PubMed Scopus (53) Google Scholar, 12Heacock A.M. Agranoff B.W. CDP-diacylglycerol synthase from mammalian tissues.Biochim. Biophys. Acta. 1997; 1348: 166-172Crossref PubMed Scopus (41) Google Scholar, 15Inglis-Broadgate S.L. Ocaka L. Banerjee R. Gaasenbeek M. Chapple J.P. Cheetham M.E. Clark B.J. Hunt D.M. Halford S. Isolation and characterization of murine Cds (CDP-diacylglycerol synthase) 1 and 2.Gene. 2005; 356: 19-31Crossref PubMed Scopus (39) Google Scholar, 22Di Paolo G. Camilli P.De Phosphoinositides in cell regulation and membrane dynamics.Nature. 2006; 443: 651-657Crossref PubMed Scopus (2040) Google Scholar). CL is present only in the mitochondria, where it is absolutely required for energy production, and its synthesis is restricted to the inner mitochondrial membrane (13Schlame M. Rua D. Greenberg M.L. The biosynthesis and functional role of cardiolipin.Prog. Lipid Res. 2000; 39: 257-288Crossref PubMed Scopus (655) Google Scholar, 14Kawasaki K. Kuge O. Yamakawa Y. Nishijima M. Purification of phosphatidylglycerophosphate synthase from Chinese hamster ovary cells.Biochem. J. 2001; 354: 9-15Crossref PubMed Scopus (16) Google Scholar, 16Chen D. Zhang X.Y. Shi Y. Identification and functional characterization of hCLS1, a human cardiolipin synthase localized in mitochondria.Biochem. J. 2006; 398: 169-176Crossref PubMed Scopus (70) Google Scholar). Plasmalogens, which are vinyl-ether linked at the 1-position of the glycerophospholipid, are an important class of lipids, and they contribute almost 18% to the total lipid mass in humans. Among plasmalogens, plasmenylethanolamine (PlmePEtn) is the most abundant and it is also the precursor to plasmenylcholine (PlmePCho). Plasmalogen synthesis occurs in the peroxisomes, where the key enzyme GNPAT initiates formation using a fatty alcohol to yield AlkylGnP (17Nagan N. Zoeller R.A. Plasmalogens: biosynthesis and functions.Prog. Lipid Res. 2001; 40: 199-229Crossref PubMed Scopus (444) Google Scholar).Sphingolipid synthesis spans from the ER, where it begins, to the Golgi complex, where it ends (18Merrill Jr., A.H. De novo sphingolipid biosynthesis: a necessary, but dangerous, pathway.J. Biol. Chem. 2002; 277: 25843-25846Abstract Full Text Full Text PDF PubMed Scopus (483) Google Scholar, 19Futerman A.H. Riezman H. The ins and outs of sphingolipid synthesis.Trends Cell Biol. 2005; 15: 312-318Abstract Full Text Full Text PDF PubMed Scopus (257) Google Scholar). Synthesis of the sphingosine and ceramide (Cer) intermediates occurs in the ER. Cer is then transferred to the Golgi apparatus in two manners, and each mode determines whether Cer is converted into either sphingomyelin (CerPCho) or glucosylceramide (GlcCer) and lactosylceramide (LacCer). Lipids such as DGPCho and DGPIns can be synthesized in the nuclear matrix apart from the nuclear envelope, but information about the exact location of the nuclear enzymes is limited (23Hunt A.N. Dynamic lipidomics of the nucleus.J. Cell. Biochem. 2006; 97: 244-251Crossref PubMed Scopus (42) Google Scholar).Although different lipids are synthesized in different organelles, they are widely distributed within the cell and the membrane composition of the different organelles does not necessarily reflect their lipid biosynthetic capacity. DGPCho is synthesized in the endomembrane compartment and in the nuclear compartment in immortalized cells, but it is present everywhere in the cell. DGPSer and CerPCho are synthesized in the ER and Golgi, but they are highly abundant in the plasma membrane. PlmePEtn is synthesized in the peroxisomes but does not accumulate as it is primarily secreted. Within the same membrane, lipids are transported to or segregated into one of the two leaflets of the membrane by virtue of their chemical structure or by the action of enzymes called flippases, whose function is to favor or force the movement of specific lipids between the two leaflets of the membrane (24Holthuis J.C. Levine T.P. Lipid traffic: floppy drives and a superhighway.Nat. Rev. Mol. Cell Biol. 2005; 6: 209-220Crossref PubMed Scopus (408) Google Scholar, 25van Meer G. Voelker D.R. Feigenson G.W. Membrane lipids: where they are and how they behave.Nat. Rev. Mol. Cell Biol. 2008; 9: 112-124Crossref PubMed Scopus (4352) Google Scholar). The transport of lipids to different membranes can occur through the vesicular pathways, which allow the transport of membrane to even distant cellular locations or by lipid-transfer proteins, a process that is particularly active and fast within membrane contact sites (MCS), where membrane regions from different organelles come in close proximity (within 10 nm) to one another (24Holthuis J.C. Levine T.P. Lipid traffic: floppy drives and a superhighway.Nat. Rev. Mol. Cell Biol. 2005; 6: 209-220Crossref PubMed Scopus (408) Google Scholar). For example, the ER is known to generate MCS structures with mitochondria, plasma membrane, the Golgi apparatus, endosomes, and other organelles. The means by which DGPCho and DGPEtn are transported to the peroxisomes is still unknown, and MCS structures as well as vesicles may be responsible for mediating the process.REGULATION OF MEMBRANE PHOSPHOLIPID SYNTHESIS AT THE ERA number of proteins that play a determining role in membrane phospholipid biogenesis are localized to the endomembrane compartment. The rate-limiting enzyme in DGPCho synthesis is the phosphocholine cytidylyltransferase (CCT) (26Jackowski S. Fagone P. CTP: Phosphocholine cytidylyltransferase: paving the way from gene to membrane.J. Biol. Chem. 2005; 280: 853-856Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar). The CCT is predominantly located in the nucleus of immortalized cells, but in primary cells, the CCT is almost exclusively found outside the nucleus and in association with the endomembrane compartment (1Fagone P. Sriburi R. Frank C.Ward-Chapman, M. Wang J. Gunter C. Brewer J.W. Jackowski S. Phospholipid biosynthesis program underlying membrane expansion during B lymphocyte differentiation.J. Biol. Chem. 2007; 282: 7591-7605Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar, 2Tian Y. Pate C. Andreolotti A. Wang L. Tuomanen E. Boyd K. Claro E. Jackowski S. Cytokine secretion requires phosphatidylcholine synthesis.J. Cell Biol. 2008; 181: 945-957Crossref PubMed Scopus (48) Google Scholar, 27Ridsdale R. Tseu I. Wang J. Post M. CTP:phosphocholine cytidylyltransferase alpha is a cytosolic protein in pulmonary epithelial cells and tissues.J. Biol. Chem. 2001; 276: 49148-49155Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar, 28Tian Y. Zhou R. Rehg J.E. Jackowski S. Role of phosphocholine cytidylyltransferase a in lung development.Mol. Cell. Biol. 2007; 27: 975-982Crossref PubMed Scopus (51) Google Scholar, 29Houweling M. Cui Z. Anfuso C.D. Chen M.Bussière, M.H. Vance D.E. CTP:phosphocholine cytidylyltransferase is both a nuclear and cytoplasmic protein in primary hepatocytes.Eur. J. Cell Biol. 1996; 69: 55-63PubMed Google Scholar). CCT is not included in Fig. 1 because its substrates and products are water soluble, but the enzyme controls the rate of DGPCho formation through its peripheral association with the cytoplasmic aspect of the endomembrane compartment (26Jackowski S. Fagone P. CTP: Phosphocholine cytidylyltransferase: paving the way from gene to membrane.J. Biol. Chem. 2005; 280: 853-856Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar). The CCT activity responds to changes in membrane lipid composition and governs whether more or less DGPCho is made as a function of the proximal accumulation of membrane lipid metabolic products (30Attard G.S. Templer R.H. Smith W.S. Hunt A.N. Jackowski S. Modulation of CTP:phosphocholine cytidylyltransferase by membrane curvature elastic stress.Proc. Natl. Acad. Sci. USA. 2000; 97: 9032-9036Crossref PubMed Scopus (224) Google Scholar).The ER stress response is a complex signal transduction pathway emanating from the ER membrane that is activated by the perturbation of normal ER metabolism. Lumenal proteins that are properly folded and assembled can leave the ER. Those that are misfolded or incompletely assembled remain in the ER where they disrupt ER homeostasis and cause ER stress. Thus, the ER stress response is oftentimes called the unfolded protein response (UPR) (31Schroder M. Kaufman R.J. The mammalian unfolded protein response.Annu. Rev. Biochem. 2005; 74: 739-789Crossref PubMed Scopus (2403) Google Scholar). The UPR relieves ER stress by repressing translation, increasing expression of ER chaperones and folding enzymes, and enhancing ER-associated degradation. The UPR has been most extensively studied as it relates to protein quality control in the ER, but membrane lipid metabolism appears to be equally important. Inhibition of DGPCho synthesis leads to DGPCho depletion, activation of some components of the ER stress response, and cell death in cultured fibroblasts (32Van Der Sanden M.H. Houweling M. Golde L.M. van Vaandrager A.B. Inhibition of phosphatidylcholine synthesis induces expression of the endoplasmic reticulum stress and apoptosis-related protein CCAAT/enhancer-binding protein-homologous protein (CHOP/GADD153).Biochem. J. 2003; 369: 643-650Crossref PubMed Google Scholar). Alteration of the ER lipid composition by accumulation of free cholesterol initiates ER stress in macrophages (33Devries-Seimon T. Li Y. Yao P.M. Stone E. Wang Y. Davis R.J. Flavell R. Tabas I. Cholesterol-induced macrophage apoptosis requires ER stress pathways and engagement of the type A scavenger receptor.J. Cell Biol. 2005; 171: 61-73Crossref PubMed Scopus (285) Google Scholar, 34Feng B. Yao P.M. Li Y. Devlin C.M. Zhang D. Harding H.P. Sweeney M. Rong J.X. Kuriakose G. Fisher E.A. al et The endoplasmic reticulum is the site of cholesterol-induced cytotoxicity in macrophages.Nat. Cell Biol. 2003; 5: 781-792Crossref PubMed Scopus (705) Google Scholar), and deficient DGPCho synthesis renders macrophages more sensitive to free cholesterol overload (35Zhang D. Tang W. Yao P.M. Yang C. Xie B. Jackowski S. Tabas I. Macrophages deficient in CTP:phosphocholine cytidylyltransferase-a are viable under normal culture conditions but are highly susceptible to free cholesterol-induced death. Molecular genetic evidence that the induction of phosphatidylcholine biosynthesis in free cholesterol-loaded macrophages is an adaptive response.J. Biol. Chem. 2000; 275: 35368-35376Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar). Thus, disruption of membrane lipid homeostasis is a trigger, either directly or indirectly, and mechanisms to reestablish ER lipid composition are components of the response.Upon induction of the UPR, the mRNA encoding the X-box binding protein 1 (XBP-1) is spliced to form XBP-1(S), which encodes a transcription factor (36Yamamoto K. Sato T. Matsui T. Sato M. Okada T. Yoshida H. Harada A. Mori K. Transcriptional induction of mammalian ER quality control proteins is mediated by single or combined action of ATF6a and XBP1.Dev. Cell. 2007; 13: 365-376Abstract Full Text Full Text PDF PubMed Scopus (766) Google Scholar). The site of splicing is at the ER membrane, where a transmembrane kinase/endonuclease called IRE1α modifies XBP1 transcripts upon accumulation of misfolded lumenal proteins. 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