Back and forth: the regulation and function of transbilayer phospholipid movement in eukaryotic cells

Involvement of mitogen-activated protein kinases in class B scavenger receptor type I-induced phagocytosis of apoptotic cells

Class B scavenger receptor type I (SR-BI) is a multiligand membrane protein expressed in a variety of cell types. This receptor is responsible for the incorporation of lipids from high density lipoprotein (HDL) by steroidogenic cells, as well as for the phosphatidylserine (PS)-mediated phagocytosis of apoptotic cells by some phagocytic cell types, such as testicular Sertoli cells. Although SR-BI directly binds to PS present on the surface of apoptotic cells, as to whether SR-BI transmits signals to induce engulfment has not been clear. In the present study, we examined this issue using a monoclonal antibody that neutralizes SR-BI activity and a chemical known to be an inhibitor of the SR-BI-mediated incorporation of HDL lipids. The chemical compound inhibited the incorporation of HDL lipids and PS-containing liposomes by an SR-BI-expressing culture cell line, with no effect on the binding of these targets. Similarly, the phagocytosis of PS-exposing apoptotic cells by primary cultured rat Sertoli cells was inhibited in the presence of either reagent, not at the recognition but at the engulfment step. The addition of apoptotic cells or PS-containing liposomes caused a temporal increment of the phosphorylation of all three mitogen-activated protein kinases, p38, extracellular-signal-regulated kinase (ERK) and c-Jun amino-terminal kinase (JNK), in Sertoli cells. The increase of phosphorylated p38 and ERK, but not of phosphorylated JNK, was cancelled in the presence of the monoclonal antibody. Furthermore, the level of Sertoli cell phagocytosis of PS-exposing apoptotic cells, as well as that of PS-containing liposomes, was reduced only when the actions of p38 and ERK were simultaneously repressed. In conclusion, these results indicate that SR-BI, when it binds to PS, transmits signals to activate the mitogen-activated protein kinase pathway, which leads to the induction of the engulfment of PS-exposing apoptotic cells by phagocytic cells.

The anti-angiogenic peptide anginex disrupts the cell membrane

Anginex is a synthetic beta-sheet peptide with anti-angiogenic and anti-tumor activity. When added to cultured endothelial cells at concentrations ranging from 2.5 microM to 25 microM, anginex induced cell death, which was reflected by a strong increase of subdiploid cells and fragments, loss of cellular ATP, and LDH release. Cytotoxicity remained the same whether cells were treated with anginex at 4 degrees C or at 37 degrees C. At low temperatures, fluorescein-conjugated anginex accumulated on the endothelial surface, but did not reach into the cytoplasm, indicating that the cell membrane is the primary target for the peptide. Within minutes of treatment, anginex caused endothelial cells to take up propidium iodide and undergo depolarization, both parameters characteristic for permeabilization of the cell membrane. This process was amplified when cells were activated with hydrogen peroxide. Red blood cell membranes were essentially unaffected by anginex. Anginex bound lipid bilayers with high affinity and with a clear preference for anionic over zwitterionic phospholipids. Structural studies by circular dichroism and solid-state nuclear magnetic resonance showed that anginex forms a beta-sheet and adopts a unique and highly ordered conformation upon binding to lipid membranes. This is consistent with lipid micellization or the formation of pore-forming beta-barrels. The data suggest that the cytotoxicity of anginex stems from its ability to target and disrupt the endothelial cell membrane, providing a possible explanation for the angiostatic activity of the peptide.

Localization of phosphatidylserine in boar sperm cell membranes during capacitation and acrosome reaction

One of the essential properties of mammalian, including sperm, plasma membranes is a stable transversal lipid asymmetry with the aminophospholipids, phosphatidylserine (PS) and phosphatidylethanolamine (PE), typically in the inner, cytoplasmic leaflet. The maintenance of this nonrandom lipid distribution is important for the homeostasis of the cell. To clarify the relevance of lipid asymmetry to sperm function, we have studied the localization of PS in boar sperm cell membranes. By using labeled annexin V as a marker for PS and propidium iodide (PI) as a stain for nonviable cells in conjunction with different methods (flow cytometry, fluorescence and electron microscopy), we have assessed the surface exposure of PS in viable cells during sperm genesis, that is, before and during capacitation as well as after acrosome reaction. An approach was set up to address also the presence of PS in the outer acrosome membrane. The results show that PS is localized in the cytoplasmic leaflet of the plasma membrane as well as on the outer acrosome membrane. Our results further indicate the cytoplasmic localization of PS in the postacrosomal region. During capacitation and acrosome reaction of spermatozoa, PS does not become exposed on the outer surface of the viable cells. Only in a subpopulation of PI-positive sperm cells does PS became accessible upon capacitation. The stable cytoplasmic localization of PS in the plasma membrane, as well as in the outer acrosome membrane, is assumed to be essential for a proper genesis of sperm cells during capacitation and acrosome reaction.

Enhancement of enveloped virus entry by phosphatidylserine

Enveloped virus vectors are used in a wide variety of applications. We have discovered that treatment of cultured cells with phosphatidylserine (PS) liposomes can increase virus vector infection by up to 20-fold. This effect does not abrogate virus receptor requirements, is specific to PS compared to other phospholipids, and is limited to enveloped viruses. Furthermore, the enhancement of infection does not occur through increases in virus receptor levels or virus binding, indicating that virus fusion is enhanced. The liposomes are easily generated, store well, and allow enhanced infection with a variety of virus vectors and cell types.

Cholesterol distribution in plasma membranes of beta1 integrin-expressing and beta1 integrin-deficient fibroblasts

The effect of integrin receptors on the level and transmembrane localization of cholesterol molecules was investigated in beta1 integrin-expressing (beta1) and beta1 integrin-deficient (beta1 null) cells. We found that the content of specific raft components-cholesterol, sphingomyelin, and caveolin-was increased in integrin-expressing cells. Integrin presence affected as well the transmembrane distribution of cholesterol-a higher percent was found in the plasma membrane outer monolayer of beta1 compared to beta1 null cells. Sphingomyelin depletion reduced the presence of cholesterol in the outer membrane monolayer of both cell lines, but the differences in cholesterol asymmetry, observed between beta1 and beta1 null cells before sphingomyelinase treatment were preserved. These findings implied that integrin receptors affected the non-random transmembrane distribution of cholesterol. Finally, a higher percent of detergent-resistant membranes was obtained from beta1 integrin-expressing cells, suggesting that the presence of these receptors in the membranes influenced the formation and/or stabilization of lipid raft domains.

The type 4 subfamily of P-type ATPases, putative aminophospholipid translocases with a role in human disease

The maintenance of phospholipid asymmetry in membrane bilayers is a paradigm in cell biology. However, the mechanisms and proteins involved in phospholipid translocation are still poorly understood. Members of the type 4 subfamily of P-type ATPases have been implicated in the translocation of phospholipids from the outer to the inner leaflet of membrane bilayers. In humans, several inherited disorders have been identified which are associated with loci harboring type 4 P-type ATPase genes. Up to now, one inherited disorder, Byler disease or progressive familial intrahepatic cholestasis type 1 (PFIC1), has been directly linked to mutations in a type 4 P-type ATPase gene. How the absence of an aminophospholipid translocase activity relates to this severe disease is, however, still unclear. Studies in the yeast Saccharomyces cerevisiae have recently identified important roles for type 4 P-type ATPases in intracellular membrane- and protein-trafficking events. These processes require an (amino)phospholipid translocase activity to initiate budding or fusion of membrane vesicles from or with other membranes. The studies in yeast have greatly contributed to our cell biological insight in membrane dynamics and intracellular-trafficking events; if this knowledge can be translated to mammalian cells and organs, it will help to elucidate the molecular mechanisms which underlie severe inherited human diseases such as Byler disease.

Antitumor effects of a monoclonal antibody that binds anionic phospholipids on the surface of tumor blood vessels in mice

PURPOSE

We recently reported that anionic phospholipids, principally phosphatidylserine, become exposed on the external surface of viable vascular endothelial cells in tumors, possibly in response to oxidative stresses present in the tumor microenvironment. In the present study, we tested the hypothesis that a monoclonal antibody directed against anionic phospholipids might exert antitumor effects by causing vascular damage in tumors.

EXPERIMENTAL DESIGN

A new mouse immunoglobulin G3 monoclonal antibody, 3G4, was raised that binds anionic phospholipids in the presence of serum or beta2-glycoprotein I. The antibody was tested for its ability to localize to tumor vessels and exert antitumor effects in mice.

RESULTS

3G4 recognized anionic phospholipids on the external membrane of H(2)O(2)-treated endothelial cells and in vitro. It localized specifically to tumor vascular endothelium and to necrotic tumor cells after injection into severe combined immunodeficient mice bearing orthotopic MDA-MB-435 tumors. Treatment with 3G4 retarded the growth of four different tumors in mice. It reduced the growth of established orthotopic MDA-MB-231 and MDA-MB-435 human breast tumors in mice by 75% and 65% respectively, large L540 human Hodgkin's tumors by 50%, and small syngeneic Meth A fibrosarcomas by 90%. Histologic examination revealed vascular damage, a reduction in vascular density, and a reduction in tumor plasma volume. Treatment with 3G4 induced the binding of monocytes to tumor endothelium and infiltration of macrophages into MDA-MB-435 and MDA-MB-231 tumors. No toxicity to the mice was observed.

CONCLUSIONS

3G4 localizes specifically to complexes of anionic phospholipids and serum proteins on the surface of vascular endothelial cells in tumors in mice. This results in damage to tumor vasculature and suppression of tumor growth.

A monoclonal antibody that binds anionic phospholipids on tumor blood vessels enhances the antitumor effect of docetaxel on human breast tumors in mice

Anionic phospholipids, principally phosphatidylserine, become exposed on the external surface of viable vascular endothelial cells in tumors, providing an excellent marker for tumor vascular targeting. We recently raised an IgG monoclonal antibody, 3G4, which binds to anionic phospholipids in a beta2-glycoprotein I-dependent manner. It inhibited tumor growth in a variety of rodent tumor models by stimulating antibody-dependent cellular cytotoxicity toward tumor vessels. In the present study, we tested the hypothesis that docetaxel, which is known to have antivascular effects on tumors, might induce exposure of anionic phospholipids on tumor vasculature and, thus, enhance the antitumor activity of 3G4. Treatment of human umbilical vascular endothelial cells with subtoxic concentrations of docetaxel (20 pmol/L) in vitro caused anionic phospholipids to be externalized without inducing apoptosis. Docetaxel treatment of mice increased the percentage of tumor vessels that expose anionic phospholipids from 35% to 60%. No induction of phosphatidylserine was observed on vessels in normal tissues even after systemic treatment with docetaxel. Treatment of mice bearing orthotopic MDA-MB-435 human breast tumors with 3G4 plus docetaxel inhibited tumor growth by 93%. Treatment of mice bearing disseminated MDA-MB-435 tumors with 3G4 plus docetaxel reduced the average number of tumor colonies in the lungs by 93% and half the animals did not develop tumors. In both tumor models, the antitumor effect of the combination was statistically superior (P < 0.01) to that of docetaxel or 3G4 alone. Combination therapy reduced the tumor vessel density and plasma volume in tumors to a greater extent than did the individual drugs. The combination therapy was no more toxic to the mice than was docetaxel alone. These results indicate that, as an adjuvant therapy, 3G4 could enhance the therapeutic efficacy of docetaxel in breast cancer patients.

Phospholipid binding by proteins of the spectrin family: a comparative study

Erythroid and neuronal spectrin (fodrin) are both known to interact strongly with the aminophospholipids that occur in the inner leaflet of plasma membranes. In erythroid spectrin the positions of the binding sites within the constituent (alphaI and betaI) polypeptide chains have been defined, and also the importance of the lipid interaction in regulating the properties of the membrane. Here we report the locations of the corresponding binding sites in the alphaII and betaII chains that make up the fodrin molecule. Of the 10 lipid-binding repeats in the erythroid spectrin chains 5 are conserved in fodrin; one cluster of 3 consecutive structural repeating units in alphaI erythroid spectrin (repeats 8-10) is displaced by one repeat in alphaII fodrin (repeats 9-11). Fodrin also contains one binding site at the N-terminus of the alphaII chain, not present in the erythroid protein. The regions of the two spectrins containing equivalent lipid-binding sites show a much higher degree of sequence identity than corresponding repeats that do not share this property. The evolutionary conservation of the distribution of a large proportion of strong lipid-binding sites in the polypeptide chains of these two proteins of disparate character argues for a specific function of fodrin-phospholipid interactions in the neuron.

In vivo analysis of phagocytosis of apoptotic cells by testicular Sertoli cells

Sertoli cells, a somatic cell type present within the seminiferous tubules of testes, are responsible for the phagocytic elimination of apoptotic spermatogenic cells. We here established an in vivo assay system that enables us to quantitatively analyze Sertoli cell phagocytosis of apoptotic cells in testes of live mice. Apoptotic cells were injected into the seminiferous tubules of spermatogenic cell-depleted mice, and the occurrence of phagocytosis by Sertoli cells was examined by histochemically analyzing testis sections or dispersed testicular cells. We reproducibly observed similar levels of phagocytosis in either examination, and the ratio of Sertoli cells that engulfed injected apoptotic cells was almost the same between the two examinations. These results indicated that a quantitative in vivo assay system was established using the seminiferous tubules of live mice as 'test tubes.' We then determined the requirements for Sertoli cell phagocytosis of apoptotic cells using this assay. For this purpose, apoptotic cells were injected together with various phagocytosis inhibitors, and the extent of phagocytosis by Sertoli cells was determined. The results revealed that Sertoli cells phagocytose apoptotic cells in a manner dependent on class B scavenger receptor type I (SR-BI) of Sertoli cells and phosphatidylserine exposed at the surface of target cells, as previously observed in vitro using primary cultures of dispersed rat testicular cells. Furthermore, the amount of SR-BI in Sertoli cells increased after injection of apoptotic cells into the seminiferous tubules, suggesting a positive feedback regulation of the expression of this phagocytosis receptor.

Hair cells require phosphatidylinositol 4,5-bisphosphate for mechanical transduction and adaptation

After opening in response to mechanical stimuli, hair cell transduction channels adapt with fast and slow mechanisms that each depend on Ca(2+). We demonstrate here that transduction and adaptation require phosphatidylinositol 4,5-bisphosphate (PIP(2)) for normal kinetics. PIP(2) has a striking distribution in hair cells, being excluded from the basal region of hair bundles and apical surfaces of frog saccular hair cells. Localization of a phosphatidylinositol lipid phosphatase, Ptprq, to these PIP(2)-free domains suggests that Ptprq maintains low PIP(2) levels there. Depletion of PIP(2) by inhibition of phosphatidylinositol 4-kinase or sequestration by aminoglycosides reduces the rates of fast and slow adaptation. PIP(2) and other anionic phospholipids bind directly to the IQ domains of myosin-1c, the motor that mediates slow adaptation, permitting a strong interaction with membranes and likely regulating the motor's activity. PIP(2) depletion also causes a loss in transduction current. PIP(2) therefore plays an essential role in hair cell adaptation and transduction.

Flippases and vesicle-mediated protein transport

The best-understood mechanisms for generating transport vesicles in the secretory and endocytic pathways involve the localized assembly of cytosolic coat proteins such as clathrin, coat protein complex (COP)I and COPII onto membranes. These coat proteins can deform membranes by themselves, but accessory proteins might help to generate the tight curvature needed to form a vesicle. Enzymes that pump phospholipid from one leaflet of the bilayer to the other (flippases) can deform membranes by creating an imbalance in the phospholipid number between the two leaflets. Recent studies describe a requirement for the yeast Drs2p family of P-type ATPases in both phospholipid translocation and protein transport in the secretory and endocytic pathways. This indicates that flippases work with coat proteins to form vesicles.

Draper-mediated and Phosphatidylserine-independent Phagocytosis of Apoptotic Cells by Drosophila Hemocytes/Macrophages

The mechanism of phagocytic elimination of dying cells in Drosophila is poorly understood. This study was undertaken to examine the recognition and engulfment of apoptotic cells by Drosophila hemocytes/macrophages in vitro and in vivo. In the in vitro analysis, l(2)mbn cells (a cell line established from larval hemocytes of a tumorous Drosophila mutant) were used as phagocytes. When l(2)mbn cells were treated with the molting hormone 20-hydroxyecdysone, the cells acquired the ability to phagocytose apoptotic S2 cells, another Drosophila cell line. S2 cells undergoing cycloheximide-induced apoptosis exposed phosphatidylserine on their surface, but their engulfment by l(2)mbn cells did not seem to be mediated by phosphatidylserine. The level of Croquemort, a candidate phagocytosis receptor of Drosophila hemocytes/macrophages, increased in l(2)mbn cells after treatment with 20-hydroxyecdysone, whereas that of Draper, another candidate phagocytosis receptor, remained unchanged. However, apoptotic cell phagocytosis was reduced when the expression of Draper, but not of Croquemort, was inhibited by RNA interference in hormone-treated l(2)mbn cells. We next examined whether Draper is responsible for the phagocytosis of apoptotic cells in vivo using an assay for engulfment based on assessing DNA degradation of apoptotic cells in dICAD mutant embryos (which only occurred after ingestion by the phagocytes). RNA interference-mediated decrease in the level of Draper in embryos of mutant flies was accompanied by a decrease in the number of cells containing fragmented DNA. Furthermore, histochemical analyses of dispersed embryonic cells revealed that the level of phagocytosis of apoptotic cells by hemocytes/macrophages was reduced when Draper expression was inhibited. These results indicate that Drosophila hemocytes/macrophages execute Draper-mediated phagocytosis to eliminate apoptotic cells.

Phosphatidylserine is not the cell surface receptor for vesicular stomatitis virus

The envelope protein from vesicular stomatitis virus (VSV) has become an important tool for gene transfer and gene therapy. It is widely used mainly because of its ability to mediate virus entry into all cell types tested to date. Consistent with the broad tropism of the virus, the receptor for VSV is thought to be a ubiquitous membrane lipid, phosphatidylserine (PS). However, the evidence for this hypothesis is indirect and incomplete. Here, we have examined the potential interaction of VSV and PS at the plasma membrane in more detail. Measurements of cell surface levels of PS show a wide range across cell types from different organisms. We demonstrate that there is no correlation between the cell surface PS levels and VSV infection or binding. We also demonstrate that an excess of annexin V, which binds specifically and tightly to PS, does not inhibit infection or binding by VSV. While the addition of PS to cells does allow increased virus entry, we show that this effect is not specific to the VSV envelope. We conclude that PS is not the cell surface receptor for VSV, although it may be involved in a postbinding step of virus entry.

Drs2p-coupled aminophospholipid translocase activity in yeast Golgi membranes and relationship to in vivo function

Aminophospholipid translocases (APLTs) are defined primarily by their ability to flip fluorescent or spin-labeled derivatives of phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the external leaflet of a membrane bilayer to the cytosolic leaflet and are thought to establish phospholipid asymmetry in biological membranes. The identities of APLTs remain unknown, although candidate proteins include the Drs2p/ATPase II subfamily of P-type ATPases. Drs2p from budding yeast localizes to the trans-Golgi network (TGN), and here we show that this membrane contains an ATP-dependent APLT that flips 7-nitro-2-1,3-benzoxadiazol-4-yl (NBD) PS and PE derivatives from the luminal to the cytosolic leaflet. To assess the contribution of Drs2p to this activity, TGN membranes were prepared from strains harboring WT or temperature-sensitive alleles of DRS2 and null alleles of three other potential APLT genes (DNF1, DNF2, and DNF3). Assay of these membranes indicated that Drs2p was required for the ATP-dependent translocation of NBD-PS, whereas no active translocation of NBD-PE or NBD-phosphatidylcholine was detected. The specificity of Drs2p for NBD-PS suggested that translocation of PS would be required for the function of Drs2p in protein transport from the TGN. However, cho1 yeast strains that are unable to synthesize PS do not phenocopy drs2 but instead transport proteins normally via the secretory pathway. In addition, a drs2 cho1 double mutant retains drs2 transport defects. Therefore, whereas NBD-PS is a preferred substrate for Drs2p in vitro, endogenous PS is not an obligatory substrate in vivo for the role Drs2p plays in protein transport.

A novel aminophospholipid transporter exclusively expressed in spermatozoa is required for membrane lipid asymmetry and normal fertilization

Through the use of a functionally unbiased signal peptide trap screen, we have discovered an ATP-dependent aminophospholipid transporter that is exclusively expressed in the acrosomal region of spermatozoa; it is about 62% similar to the flippase, FIC1. We disrupted the transporter gene and found that the size of litters from male null mice was slightly smaller than found with wild-type males. Sperm morphology and motility were the same between null and wild-type littermates, but agents (merocyanine and annexin) that measure phospholipid packing or phosphatidylserine (PS) in the outer membrane leaflet showed that PS already existed in the outer leaflet of null spermatozoa before sperm capacitation. Fertilization rates were normal when null spermatozoa were added to zona pellucida-free eggs, but in the presence of the extracellular matrix, fewer transporter(-/-) spermatozoa bound tightly or penetrated the zona pellucida (ZP), and fewer underwent acrosome reactions. In vitro fertilization was compromised, especially at early time points or at low sperm concentrations after mixing null spermatozoa and eggs. Thus, a new aminophospholipid transporter expressed exclusively in spermatozoa is critical for normal phospholipid distribution in the bilayer, and for normal binding, penetration, and signaling by the zona pellucida.

Apoptosis-detecting radioligands: current state of the art and future perspectives

This review provides a critical and thorough overview of the radiopharmaceutical development and in vivo evaluation of all apoptosis-detecting radioligands that have emerged so far, along with their possible applications in nuclear medicine. The following SPECT and PET radioligands are discussed: all forms of halogenated Annexin V (i.e. (123)I-labelled, (124)I-labelled, (125)I-labelled, (18)F-labelled), (99m)Tc/(94m)Tc-labelled Annexin V derivatives using different chelators and co-ligands (i.e. BTAP, Hynic, iminothiolane, MAG(3), EDDA, EC, tricarbonyl, SDH) or direct (99m)Tc-labelling, (99m)Tc-labelled Annexin V mutants and (99m)Tc/(18)F-radiopeptide constructs (i.e. AFIM molecules), (111)In-DTPA-PEG-Annexin V, (11)C-Annexin V and (64)Cu-, (67)Ga- and (68)Ga-DOTA-Annexin V. In addition, the potential role and clinical relevance of anti-PS monoclonal antibodies and other alternative apoptosis markers are reviewed, including: anti-Annexin V monoclonal antibodies, radiolabelled caspase inhibitors and substrates and mitochondrial membrane permeability targeting radioligands. Nevertheless, major emphasis is placed on the group of Annexin V-based radioligands, in particular (99m)Tc-Hynic-Annexin V, since this molecule is by far the most extensively investigated and best-characterised apoptosis marker at present. Furthermore, the newly emerging imaging modalities for in vivo detection of programmed cell death, such as MRI, MRS, optical, bioluminescent and ultrasound imaging, are briefly described. Finally, some future perspectives are presented with the aim of promoting the development of potential new strategies in pursuit of the ideal cell death-detecting radioligand.

Evaluation of novel antimouse VEGFR2 antibodies as potential antiangiogenic or vascular targeting agents for tumor therapy

We generated a panel of eight rat IgG(2a) monoclonal antibodies with high affinity for mouse VEGFR2 (KDR/Flk-1), the main receptor that mediates the angiogenic effect of VEGF-A. The antibodies (termed RAFL, R at Anti Flk) bound to dividing endothelial cells more strongly than they did to nondividing cells. Most of the RAFL antibodies blocked [(125)I]VEGF(165) binding to VEGFR2. Three of eight antibodies localized to VEGFR2-positive tumor endothelium after intravenous injection into mice bearing orthotopic MDA-MB-231 breast carcinomas, as judged by indirect immunohistochemistry. An average of 60% of vessels in the tumors was stained. The majority (50-80%) of vessels were also stained in a variety of other human and murine tumors growing in mice. The antibodies did not bind detectably to the vascular endothelium in normal heart, lung, liver, and brain cortex, whereas the vascular endothelium in kidney glomerulus and pancreatic islets was stained. Treatment of mice bearing orthotopic MDA-MB-231 tumors with RAFL-1 antibody inhibited tumor growth by an average of 48% and reduced vascular density by 65%, compared to tumors in mice treated with control IgG. Vascular damage was not observed in normal organs, including kidneys and pancreas. These studies demonstrate that anti-VEGFR2 antibodies have potential for vascular targeting and imaging of tumors in vivo.

Scott syndrome, a bleeding disorder caused by defective scrambling of membrane phospholipids

Normal quescent cells maintain membrane lipid asymmetry by ATP-dependent membrane lipid transporters, which shuttle different phospholipids from one leaflet to the other against their respective concentration gradients. When cells are challenged, membrane lipid asymmetry can be perturbed resulting in exposure of phosphatidylserine [PS] at the outer cell surface. Translocation of PS from the inner to outer membrane leaflet of activated blood platelets and platelet-derived microvesicles provides a catalytic surface for interacting coagulation factors. This process is dramatically impaired in Scott syndrome, a rare congenital bleeding disorder, underscoring the indispensible role of PS in hemostasis. This also testifies to a defect of a protein-catalyzed scrambling of membrane phospholipids. The Scott phenotype is not restricted to platelets, but can be demonstrated in other blood cells as well. The functional aberrations observed in Scott syndrome have increased our understanding of transmembrane lipid movements, and may help to identify the molecular elements that promote the collapse of phospholipid asymmetry during cell activation and apoptosis.

Silent cleanup of very early apoptotic cells by macrophages

Apoptotic cells are phagocytosed as soon as they appear in vivo. In this study, we first determined precisely at what stage apoptotic cells are phagocytosed by macrophages, and then examined the subsequent cytokine production. Phagocytosis was confirmed by flow cytometry and confocal laser microscopy, whereas the subsequent response was examined by ELISA and RT-PCR for quantitative and semiquantitative measurement of the protein and mRNA levels of cytokines, respectively. Even the cell populations containing very early apoptotic cells, such as IL-2-dependent CTLL-2 cells cultured in the absence of IL-2 for 4 h and a murine leukemic cell line, P388 cells, treated with etoposide for 5 h, were phagocytosed by macrophages. Although the cell populations containing the very early apoptotic cells used in this study were FITC-Annexin V-negative and did not show a decrease in cell size as compared with untreated cells, they showed a very small increase in phosphatidylserine on the cell surface, as detected with Cy3-Annexin V, and a decrease in mitochondrial membrane potential, indicating that the cell populations had already started the apoptotic process. Phagocytosis of such populations containing very early apoptotic cells was inhibited by phospho-L-serine much more significantly than Arg-Gly-Asp-Ser. In addition, macrophages hardly produced either proinflammatory or anti-inflammatory cytokines after phagocytosis, thus being an almost null response. These results are contrary to the generally accepted concept that the phagocytosis of apoptotic cells leads to the production of anti-inflammatory cytokines, suggesting instead that cells starting to undergo apoptosis are quickly phagocytosed by macrophages without any inflammation in vivo.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

Phosphatidylserine expression and phagocytosis of apoptotic thymocytes during differentiation of monocytic cells

Expression of phosphatidylserine (PS) on the surface of both macrophages and their apoptotic targets is required for efficient phagocytosis. Monocytes, the precursors of macrophages, do not express PS on their surface and do not efficiently phagocytose apoptotic cells. We report here that PS appears on the surface of both human monocytic U937 cells and primary human monocytes as they differentiate in culture and acquire the ability to phagocytose apoptotic thymocytes. Phagocytosis was blocked by pretreating either the apoptotic target or the phagocyte with annexin V to mask PS and was CD14-dependent. Expression of PS, like other events characteristic of differentiating monocytes such as Mac-1 expression, was independent of the agent used to induce differentiation and was insensitive to the addition of caspase inhibitors. These results demonstrate that PS is expressed on monocytes as part of their differentiation program and is independent of apoptosis.

Phosphatidylserine on HIV envelope is a cofactor for infection of monocytic cells

HIV-1 is an enveloped retrovirus that acquires its outer membrane as the virion exits the cell. Because of the association of apoptosis with the progression of AIDS, HIV-1-infected T cells or macrophages might be expected to express elevated levels of surface phosphatidylserine (PS), a hallmark of programmed cell death. Virions produced by these cells would also be predicted to have PS on the surface of their envelopes. In this study, data are presented that support this hypothesis and suggest that PS is required for macrophage infection. The PS-specific protein annexin V was used to enrich for virus particles and to inhibit HIV-1 replication in primary macrophages, but not T cells. HIV-1 replication was also significantly inhibited with vesicles consisting of PS, but not phosphatidylcholine. PS is specifically required for HIV-1 infection because viruses pseudotyped with vesicular stomatitis virus G and amphotropic murine leukemia virus envelopes were not inhibited by PS vesicles or annexin V. These data indicate that PS is an important cofactor for HIV-1 infection of macrophages.

Regional loss of the mitochondrial membrane potential in the hepatocyte is rapidly followed by externalization of phosphatidylserines at that specific site during apoptosis

The spatio-temporal relationship between a decrease in the mitochondrial membrane potential (MMP) and externalization of phosphatidylserines (PS) during induction of apoptosis was investigated in single freshly isolated hepatocytes. Apoptosis was induced in the hepatocytes in three different ways: attack by activated Natural Killer cells, exposure to ATP, or exposure to the inhibitor of protein synthesis cycloheximide. Fluorescence microscopy showed staining of externalized PS at those areas where the staining for MMP was lost whereas in other areas the mitochondria remained intact for longer periods of time, indicating coupling between local loss of MMP and local PS exposure. To discriminate whether the decrease in MMP itself or a decrease in ATP induced PS externalization, hepatocytes were treated with rotenone, which resulted in a rapid collapse of cellular ATP but left the MMP intact for a much longer period. Addition of fructose prevented the decrease of ATP to approximately 30% and also delayed the collapse of the MMP. This indicates that ATP was needed for the maintenance of the MMP probably via reverse action of the ATP synthase. In a subsequent study hepatocytes were incubated with Natural Killer cells for induction of apoptosis followed by addition of rotenone to deplete ATP. Under these conditions the PS staining co-localized with mitochondrial MMP indicating that PS externalization does not require a collapse in MMP. Moreover, exposure of PS was evenly distributed over the whole plasma membrane. In conclusion, we propose that after an apoptotic stimulus some mitochondria start to loose their MMP, which results in cessation of ATP production and perhaps even consumption of ATP. This results in an overall decrease in cellular ATP. ATP-consuming enzyme reactions most distal from still intact mitochondria will be most sensitive to such a decrease. Apparently the translocase that keeps phosphatidylserines inward-oriented is such a sensitive enzyme.

Chronic ethanol treatment: dolichol and retinol distribution in isolated rat liver cells

The aim of this study was to use chronic ethanol intoxication for 2 and 4 months as a means of studying the distribution of dolichol and retinol in isolated rat liver parenchymal cells, Kupffer cells, sinusoidal endothelial cells, and two subfractions of hepatic stellate cells: Ito 1 and Ito 2. Dolichol and retinol were studied in two batches of rats: on normal nutrition and after a load of vitamin A given 3 d before sacrifice. New observations reported are: (i) on normal nutrition, after 2 months of treatment, dolichol in HC seems to be the first target of chronic ethanol, while retinol is the first target in hepatic stellate cells; (ii) the various types of liver cells are differently affected by chronic ethanol, which highlights the importance of studying each type of sinusoidal cell; (iii) a load of vitamin A given when the damage has already occurred restores dolichol content in HC while retinol decreases; and, (iv) a link between dolichol and vitamin A metabolism might be supposed after the load of vitamin A: the percentage distribution of dolichol with 18 isoprene units (Dolichol -18) increases in all the control cells but decreases after chronic ethanol treatment. A different role of this dolichol and/or a different compartmentalization within the cell need to be further investigated.

Cdc50p, a conserved endosomal membrane protein, controls polarized growth in Saccharomyces cerevisiae

During the cell cycle of the yeast Saccharomyces cerevisiae, the actin cytoskeleton and the growth of cell surface are polarized, mediating bud emergence, bud growth, and cytokinesis. We identified CDC50 as a multicopy suppressor of the myo3 myo5-360 temperature-sensitive mutant, which is defective in organization of cortical actin patches. The cdc50 null mutant showed cold-sensitive cell cycle arrest with a small bud as reported previously. Cortical actin patches and Myo5p, which are normally localized to polarization sites, were depolarized in the cdc50 mutant. Furthermore, actin cables disappeared, and Bni1p and Gic1p, effectors of the Cdc42p small GTPase, were mislocalized in the cdc50 mutant. As predicted by its amino acid sequence, Cdc50p appears to be a transmembrane protein because it was solubilized from the membranes by detergent treatment. Cdc50p colocalized with Vps21p in endosomal compartments and was also localized to the class E compartment in the vps27 mutant. The cdc50 mutant showed defects in a late stage of endocytosis but not in the internalization step. It showed, however, only modest defects in vacuolar protein sorting. Our results indicate that Cdc50p is a novel endosomal protein that regulates polarized cell growth.

Transbilayer phospholipid movement and the clearance of apoptotic cells

When lymphocytes (and other cells) die by apoptosis, they orchestrate their own orderly removal by macrophages, and thereby prevent the inflammation that would otherwise attend cell lysis. As part of their demise, apoptotic cells disrupt the normal asymmetric distribution of phospholipids across their plasma membranes, an asymmetry normally maintained by an aminophospholipid translocase. This disruption of asymmetry, mediated by an activity known as the scramblase, generates ligands on the cell surface that trigger phagocytosis of the dying cell before lysis can occur. This crucial alteration of the plasma membrane is not dependent on caspase-mediated proteolysis, but quite unexpectedly, it is required both on the apoptotic target cell and on the phagocyte that engulfs it. At least in the phagocyte, this rearrangement may depend on the activity of an ABC ATPase, termed ABC1 in mammals and ced-7 in C. elegans.

Energy consumption by phospholipid metabolism in mammalian brain

Until recently, brain phospholipid metabolism was thought to consume only 2% of the ATP consumed by the mammalian brain as a whole. In this paper, however, we calculate that 1.4% of total brain ATP consumption is consumed for the de novo synthesis of ether phospholipids and that another 5% is allocated to the phosphatidylinositide cycle. When added to previous estimates that fatty acid recycling within brain phospholipids and maintenance of membrane lipid asymmetries of acidic phospholipids consume, respectively, 5% and 8% of net brain ATP consumption, it appears that phospholipid metabolism can consume up to 20% of net brain ATP consumption. This new estimate is consistent with recent evidence that phospholipids actively participate in brain signaling and membrane remodeling, among other processes.

Phosphatidylserine is a marker of tumor vasculature and a potential target for cancer imaging and therapy

PURPOSE

(1) To determine whether exposure of phosphatidylserine (PS) occurs on vascular endothelium in solid tumors in mice. (2) To determine whether PS exposure can be induced on viable endothelial cells in tissue culture by conditions present in the tumor microenvironment.

METHODS AND MATERIALS

Externalized PS in vivo was detected by injecting mice with a monoclonal anti-PS antibody and examining frozen sections of tumors and normal tissues for anti-PS antibody bound to vascular endothelium. Apoptotic cells were identified by anti-active caspase-3 antibody or by TUNEL assay. PS exposure on cultured endothelial cells was determined by 125I-annexin V binding.

RESULTS

Anti-PS antibody bound specifically to vascular endothelium in six tumor models. The percentage of PS-positive vessels ranged from 4% to 40% in different tumor types. Vascular endothelium in normal organs was unstained. Very few tumor vessels expressed apoptotic markers. Hypoxia/reoxygenation, acidity, inflammatory cytokines, thrombin, or hydrogen peroxide induced PS exposure on cultured endothelial cells without causing loss of viability.

CONCLUSIONS

Vascular endothelial cells in tumors, but not in normal tissues, externalize PS. PS exposure might be induced by tumor-associated oxidative stress and activating cytokines. PS is an abundant and accessible marker of tumor vasculature and could be used for tumor imaging and therapy.

Phosphatidylserine binding of class B scavenger receptor type I, a phagocytosis receptor of testicular sertoli cells

Testicular Sertoli cells phagocytose apoptotic spermatogenic cells in a manner depending on the membrane phospholipid phosphatidylserine (PS) expressed at the surface of the latter cell type. Our previous studies have indicated that class B scavenger receptor type I (SR-BI) is responsible for the PS-mediated phagocytosis by Sertoli cells. We examined here whether SR-BI binds directly to PS. A cell line acquired the ability to bind to PS-exposing apoptotic cells and to incorporate PS-containing liposomes when it was forced to express SR-BI. Furthermore, the extracellular domain of rat SR-BI fused with human Fc (SRBIecd-Fc) bound to PS with a dissociation equilibrium constant of 2.4 x 10(-7) m in a cell-free solid-phase assay, whereas other phospholipids including phosphatidylethanolamine, phosphatidylinositol, and phosphatidylcholine were poor binding targets. The binding activity was enhanced when CaCl(2) was included in the assay or when SRBIecd-Fc was pre-treated with N-glycanase. A portion of the extracellular domain spanning amino acid positions 33 and 191 (numbered with respect to the amino terminus) fused with Fc (SRBI33-191-Fc) showed activity and phospholipid specificity equivalent to those of SRBIecd-Fc. Finally, SRBI33-191-Fc bound to the surface of apoptotic cells with externalized PS, and the injection of SRBI33-191-Fc into the seminiferous tubules of live mice increased the number of apoptotic spermatogenic cells. These results allowed us to conclude that SR-BI is a phagocytosis-inducing PS receptor of Sertoli cells.

Capacitation induces cyclic adenosine 3',5'-monophosphate-dependent, but apoptosis-unrelated, exposure of aminophospholipids at the apical head plasma membrane of boar sperm cells

The capacitating agent bicarbonate/CO(2) has been shown to induce profound changes in the architecture and dynamics within the sperm's plasma membrane lipid bilayer via a cAMP-dependent protein phosphorylation signaling pathway. Here we have investigated the effect of bicarbonate on surface exposure of endogenous aminophospholipids in boar spermatozoa, detecting phosphatidylserine (PS) with fluorescein-conjugated annexin V and phosphatidylethanolamine (PE) with fluorescein-conjugated streptavidin/biotinylated Ro-09-0198. Flow cytometric analyses revealed that incubation with 15 mM bicarbonate induced 30%-70% of live acrosome-intact cells to expose PE very rapidly; this exposure was closely related to a decrease in lipid packing order as detected by enhanced binding of merocyanine 540. PS exposure was detectable in the same proportion of cells, though its expression was slower. Confocal microscopy revealed that exposure of aminophospholipids in intact cells was restricted to the anterior acrosomal region of the head plasma membrane. Aminophospholipid exposure, merocyanine stainability, and a subsequent migration of cholesterol to the apical region of the head plasma membrane, were all under the control of the cAMP-dependent protein phosphorylation pathway. The close coupling of decreased lipid packing order with exposure of PE led us to conclude that bicarbonate was inducing phospholipid scrambling (i.e., collapse of asymmetric transverse distribution), and that the scrambling was a prerequisite for cholesterol relocation. There was no evidence whatever that the bicarbonate-induced scrambling was an apoptotic process. It was not accompanied by major loss of viability or by DNA degeneration or by loss of mitochondrial function, and it could not be blocked by the broad-specificity caspase inhibitors zVAD-fmk and BocD-fmk. In the absence of bicarbonate, scrambling could not be induced by the apoptotic agents UV, staurosporine, or cycloheximide. Bicarbonate-induced phospholipid scrambling thus appears to be an important and early physiological event in the capacitation process.

Role of phosphatidylserine exposure and sugar chain desialylation at the surface of influenza virus-infected cells in efficient phagocytosis by macrophages

HeLa cells infected with influenza A virus undergo typical caspase-dependent apoptosis and are efficiently phagocytosed by mouse peritoneal macrophages in a manner mediated by the membrane phospholipid phosphatidylserine, which is translocated to the surface of virus-infected cells during apoptosis. However, the extent of phagocytosis is not always parallel with the level of phosphatidylserine externalization. Here we examined the involvement of influenza virus neuraminidase (NA) in efficient phagocytosis of virus-infected cells. HeLa cells infected with an influenza virus strain expressing temperature-sensitive NA underwent apoptosis and produced viral proteins, including the defective NA, at a non-permissive temperature to almost the same extent as cells infected with the wild-type virus. The cells were, however, phagocytosed by macrophages with reduced efficiency. In addition, phagocytosis of cells infected with the wild-type virus was severely inhibited when the cells had been maintained in the presence of the NA inhibitor zanamivir. On the other hand, the binding of sialic acid-recognizing lectins to the cell surface declined after infection with the wild-type virus. The decrease in the extent of lectin binding was greatly attenuated when cells were infected with the mutant virus or when wild-type virus-infected cells were maintained in the presence of zanamivir. These results indicate that sugar chains are desialylated by NA at the surface of virus-infected cells. We conclude that the presence of both phosphatidylserine and asialoglycomoieties on the cell surface is required for efficient phagocytosis of influenza virus-infected cells by macrophages.

Involvement of the mitochondrial death pathway in chemopreventive benzyl isothiocyanate-induced apoptosis

In the present study, we studied the molecular mechanism underlying cell death induced by a cancer chemoprotective compound benzyl isothiocyanate (BITC). The cytotoxic effect of BITC was examined in rat liver epithelial RL34 cells. Apoptosis was induced when the cells were treated with 20 mum BITC, characterized by the appearance of phosphatidylserine on the outer surface of the plasma membrane and caspase-3 activation, whereas no caspase activation and propidium iodide incorporation into cell were detected with 50 mum BITC that induced necrosis. The mitochondrial death pathway was suggested to be involved in BITC-induced apoptosis because the treatment of cells with BITC-induced caspase-9-dependent apoptosis and mitochondrial transmembrane potential (Delta Psi m) alteration. We demonstrated here for the first time that BITC directly modifies mitochondrial functions, including inhibition of respiration, mitochondrial swelling, and release of cytochrome c. Moreover, glutathione depletion by diethyl maleate significantly accelerated BITC-triggered apoptosis, suggesting the involvement of a redox-dependent mechanism. This was also implicated by the observations that intracellular accumulation of reactive oxygen species, including superoxide (O(2)) and hydroperoxides (HPOs), was indeed detected in the cells treated with BITC and that the intracellular HPO level was significantly attenuated by pretreatment with N-acetylcysteine. The treatment with a pharmacological scavenger of O(2), Tiron, also diminished the HPO formation by approximately 80%, suggesting that most of the HPOs were H(2)O(2) derived from the dismutation of O(2). These results suggest that BITC induces apoptosis through a mitochondrial redox-sensitive mechanism.

Identification of a functional role for lipid asymmetry in biological membranes: Phosphatidylserine-skeletal protein interactions modulate membrane stability

Asymmetric distribution of phospholipids is ubiquitous in the plasma membranes of many eukaryotic cells. The majority of the aminophospholipids are located in the inner leaflet whereas the cholinephospholipids are localized predominantly in the outer leaflet. Several functional roles for asymmetric phospholipid distribution in plasma membranes have been suggested. Disruption of lipid asymmetry creates a procoagulant surface on platelets and serves as a trigger for macrophage recognition of apoptotic cells. Furthermore, the dynamic process of phospholipid translocation regulates important cellular events such as membrane budding and endocytosis. In the present study, we used the red cell membrane as the model system to explore the contribution of phospholipid asymmetry to the maintenance of membrane mechanical properties. We prepared two different types of membranes in terms of their phospholipid distribution, one in which phospholipids were scrambled and the other in which the asymmetric distribution of phospholipids was maintained and quantitated their mechanical properties. We documented that maintenance of asymmetric distribution of phospholipids resulted in improved membrane mechanical stability. The greater difficulty in extracting the spectrin-actin complex at low-ionic strength from the membranes with asymmetric phospholipid distribution further suggested the involvement of interactions between aminophospholipids in the inner leaflet and skeletal proteins in modulating mechanical stability of the red cell membrane. These findings have enabled us to document a functional role of lipid asymmetry in regulating membrane material properties.

Difference in the way of macrophage recognition of target cells depending on their apoptotic states

Dying cells are selectively eliminated from the organism by phagocytosis. Previous studies suggested the existence of some other phagocytosis marker(s) that function together with phosphatidylserine, the best-characterized phagocytosis marker. We obtained here a monoclonal antibody named PH2 that inhibited macrophage phagocytosis of late apoptotic or necrotic cells, but not of early apoptotic cells. On the other hand, phagocytosis of cells at any time during the process of apoptosis was inhibitable by phosphatidylserine-containing liposomes. Inhibition occurred even when target cells were preincubated with PH2 and separated from unbound antibodies. Moreover, PH2 bound to apoptotic cells at late stages more efficiently than to those at early stages, and it did not bind to normal cells unless their plasma membrane was permeabilized. These results suggest that the putative PH2 antigen is a novel phagocytosis marker that translocates to the cell surface at late stages of apoptosis, resulting in maximal recognition and engulfment by macrophages.

Apoptosis and syncytial fusion in human placental trophoblast and skeletal muscle

Skeletal muscle fibers and placental villous trophoblast are the main representatives of syncytia in the human. Both syncytia are derived from fusion of mononucleated stem cells, show a high degree of differentiation, and have lost their generative potency. Consequently, for their growth both depend on fusion of additional stem cells. There is evidence that syncytial fusion is directly or indirectly related to apoptotic events: As early as in the differentiated stages of the mononucleated stem cells, initiation stages of the apoptosis cascade have been observed. After syncytial fusion progression of the cascade is retarded or blocked by a variety of mechanisms. In this review we emphasize the links between apoptosis cascade, differentiation pathways and syncytial fusion. It needs to be elucidated whether these processes simply take place in parallel, both temporally and spatially, or whether there are causal connections between apoptosis cascade and syncytial fusion. Based on recent data obtained for placental villous trophoblast, it is tempting to speculate that early molecular mechanisms of the apoptosis cascade are involved in differentiation and syncytial fusion. Data obtained in skeletal muscles support this assumption and reveal a considerable degree of homology in genesis, maintenance and turnover of both tissues.

Bicarbonate stimulated phospholipid scrambling induces cholesterol redistribution and enables cholesterol depletion in the sperm plasma membrane

Mammalian sperm cells are activated prior to fertilization by high bicarbonate levels, which facilitate lipoprotein-mediated cholesterol efflux. The role of bicarbonate and cholesterol acceptors on the cholesterol organization in the sperm plasma membrane was tested. Bicarbonate induced an albumin-independent change in lipid architecture that was detectable by an increase in merocyanine staining (due to protein kinase A-mediated phospholipid scrambling). The response was limited to a subpopulation of viable sperm cells that were sorted from the non-responding subpopulation by flow cytometry. The responding cells had reduced cholesterol levels (30% reduction) compared with non-responding cells. The subpopulation differences were caused by variable efficiencies in epididymal maturation as judged by cell morphology. Membrane cholesterol organization was observed with filipin, which labeled the entire sperm surface of non-stimulated and non-responding cells, but labeled only the apical surface area of bicarbonate-responding cells. Addition of albumin caused cholesterol efflux, but only in bicarbonate-responding cells that exhibited virtually no filipin labeling in the sperm head area. Albumin had no effect on other lipid components, and no affinity for cholesterol in the absence of bicarbonate. Therefore, bicarbonate induces first a lateral redistribution in the low cholesterol containing spermatozoa, which in turn facilitates cholesterol extraction by albumin. A model is proposed in which phospholipid scrambling induces the formation of an apical membrane raft in the sperm head surface that enables albumin mediated efflux of cholesterol.

Phosphatidylserine, a death knell

Virtually every cell in the body restricts phosphatidylserine (PS) to the inner leaflet of the plasma membrane by energy-dependent transport from the outer to the inner leaflet of the bilayer. Apoptotic cells of all types rapidly randomize the asymmetric distribution, bringing PS to the surface where it serves as a signal for phagocytosis. A myriad of phagocyte receptors have been implicated in the recognition of apoptotic cells, among them a PS receptor, yet few ligands other than PS have been identified on the apoptotic cell surface. Since apoptosis and the associated exposure of PS on the cell surface is probably over 600 million years old, it is not surprising that evolution has appropriated aspects of this process for specialized purposes such as blood coagulation, membrane fusion and erythrocyte differentiation. Failure to efficiently remove apoptotic cells may contribute to inflammatory responses and autoimmune diseases resulting from chronic, inappropriate exposure of PS.

Energy-dependent flip of fluorescence-labeled phospholipids is regulated by nutrient starvation and transcription factors, PDR1 and PDR3

The yeast Saccharomyces cerevisiae readily accumulates short-chain, fluorescent 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled phosphatidylcholine and phosphatidylethanolamine at the nuclear envelope/endoplasmic reticulum and mitochondria. The net intracellular accumulation reflects the sum of their inwardly and outwardly directed transbilayer translocation across the plasma membrane (flip and flop, respectively). The rate of flop is negligible in energy-depleted cells as well as at low temperature (2 degrees C). Although flip is reduced at 2 degrees C, it can still be measured by flow cytometry, allowing the rate of flip, independent of flop, to be characterized at this temperature. Flip requires the energy of the plasma membrane proton electrochemical gradient and is down-regulated as cells pass through the diauxic shift and enter stationary phase. Furthermore, drug-resistant, gain-of-function mutations in the transcription factors, PDR1 and PDR3, result in a dramatic down-regulation of flip in addition to their already established up-regulation of flop. These results imply that down-regulation of the NBD-phospholipid flip pathway is a physiological response to environmental stress.

The central role of phosphatidylserine in the phagocytosis of apoptotic thymocytes

Apoptotic thymocytes inactivate the aminophospholipid translocase, which transports phosphatidylserine (PS) to the inner leaflet of the plasma membrane, and activate the scramblase, which randomizes phospholipids across the membrane and brings PS to the cell surface. Although different macrophages use at least two different systems to recognize and engulf apoptotic thymocytes, both systems recognize PS on the apoptotic target. Thymocytes treated with Ca2+ and ionophore to inactivate the translocase and activate the scramblase immediately expose PS on their surface and are immediately recognized and phagocytosed. These targets, on which PS has been artificially exposed, are recognized by the PS exposed on their surface. However, they apparently also engage the vitronectic receptor, a lectin-like receptor and CD14. All of these receptors are implicated in the phagocytosis of apoptotic thymocytes, suggesting that loss of asymmetry and/or exposure of PS is sufficient to generate the ligands recognized by those receptors. The role of PS is not confined to the target cell surface, however. PS is constitutively exposed on the surface of macrophages and is as necessary for apoptotic cell engulfment as is recognition of PS on the target cell surface.

Physiological and nutritional regulation of enzymes of triacylglycerol synthesis

Although triacylglycerol stores play the critical role in an organism's ability to withstand fuel deprivation and are strongly associated with such disorders as diabetes, obesity, and atherosclerotic heart disease, information concerning the enzymes of triacylglycerol synthesis, their regulation by hormones, nutrients, and physiological conditions, their mechanisms of action, and the roles of specific isoforms has been limited by a lack of cloned cDNAs and purified proteins. Fortunately, molecular tools for several key enzymes in the synthetic pathway are becoming available. This review summarizes recent studies of these enzymes, their regulation under varying physiological conditions, their purported roles in synthesis of triacylglycerol and related glycerolipids, the possible functions of different isoenzymes, and the evidence for specialized cellular pools of triacylglycerol and glycerolipid intermediates.

Stimulated nonspecific transport of phospholipids results in elevated external appearance of phosphatidylserine in ras-transformed fibroblasts

The content of phosphatidylserine (PS) was found to be increased three times in the plasma membrane outer leaflet of ras-transformed fibroblasts compared to their nontransformed counterparts. In an attempt to determine the mechanisms responsible for the enhanced external appearance of PS, we investigated the activities of aminophospholipid translocase and the nonspecific lipid scramblase. Both transport systems could separately or in combination contribute to PS accumulation in the extracellular leaflet. Aminophospholipid transfer was assessed by measuring the rate of NBD-PS internalization, and scramblase activity was estimated from the internalization of NBD-PC. The results showed that the aminophospholipid transport was inhibited and the nonspecific transport was stimulated in ras-transformed cells. To assess which of these two transport systems was related to elevation of PS external appearance, each of them was submitted to reversible alterations and the content of PS was measured simultaneously. Aminophospholipid translocase activity was inhibited by pyridyldithioethylamine treatment and reversed by reduction with dithiothreitol. Scramblase activity was modulated by a calcium repletion-depletion procedure. Calcium depletion was performed by cell incubation with BAPTA-AM and EGTA as Ca2+ intracellular and extracellular chelators. Restoration of the intracellular Ca2+ was achieved by cell incubation with Ca2+ and Ca2+-ionophore A23187. The results showed that the changes in PS outer appearance did not correlate with the uptake of NBD-PS but were closely related to NBD-PC internalization, suggesting that the nonspecific bidirectional lipid transfer was the major transport system translocating PS to the outer leaflet in ras-transformed cells.

Secretory and cytosolic phospholipase A(2)regulate the long-term cytokine-induced eicosanoid production in human keratinocytes

The involvement of cytosolic phospholipase A(2)(cPLA(2)) and secretory non-pancreatic PLA(2)(npPLA(2)) in release of arachidonic acid (AA) preceding eicosanoid formation in the human keratinocyte cell line HaCaT was examined. Interleukin 1beta (IL-1beta) and tumour necrosis factor-alpha (TNF), phorbol myristate acetate (PMA) and calcium ionophore A(23187)increased the extracellular AA release, and stimulated eicosanoid synthesis as determined by HPLC analysis. The main metabolites after stimulation with IL-1beta, PMA or A(23187)were PGE(2), an unidentified PG and LTB(4), while TNF stimulated HETE-production. Both cPLA(2)and npPLA(2)message and enzyme activity were detected in unstimulated HaCaT cells. IL-1beta, PMA and TNF increased both cPLA(2)enzyme activity and expression, but did not lead to any increase in npPLA(2)expression or activity. The selective npPLA(2)inhibitors LY311727 and 12-epi-scalaradial, or the cPLA(2)inhibitor arachidonyl trifluoro methyl ketone (AACOCF(3)) reduced IL-1beta-induced eicosanoid production in a concentration dependent manner. The results presented strongly suggest that both cPLA(2)and npPLA(2)contribute to the long-term generation of AA preceding eicosanoid production in differentiated, human keratinocytes. Inhibitors against npPLA2 or cPLA2 enzymes should be useful in treating inflammatory skin diseases, such as psoriasis.

Identification of three new members of the phospholipid scramblase gene family

Phospholipid (PL) scramblase is a 35 kDa protein that is thought to mediate Ca2+-induced bidirectional transbilayer movement of plasma membrane phospholipids in activated, injured, or apoptotic cells. We recently reported the molecular cloning of a PL scramblase of human (HuPLSCR1) and mouse origin, respectively. In the present study, the gene for HuPLSCR1 was cloned from a human genomic library. The gene size is 29.7 kb and includes nine exons. Analysis of the 5' flanking genomic sequence with luciferase reporter constructs located the promoter to a region spanning from -95 to +60 of the first (untranslated) exon. Furthermore, we report the molecular cloning of three additional novel cDNAs encoding proteins with high homology to HuPLSCR1. The predicted open reading frames encode proteins with 59% (HuPLSCR2; 224 aa), 47% (HuPLSCR3; 295 aa) and 46% (HuPLSCR4; 329 aa) identity, respectively, to HuPLSCR1. All members of the PLSCR gene family conserve those residues contained in the segment of the PLSCR1 polypeptide that was previously shown to bind Ca2+. With the exception of HuPLSCR2, these proteins also each contain multiple PXXP motifs and a PPXY motif located near the N-terminus, implying the potential for interaction with SH3 or WW domain-containing proteins, respectively. HuPLSCR1, 2, and 4 were found to be closely clustered on chromosome 3 (3q23), whereas HuPLSCR3 is located on chromosome 17. Northern blots revealed that the expression of HuPLSCR2 is restricted to testis, whereas HuPLSCR1, 3 and 4 are expressed in most of the 16 tissues examined. Notable exceptions were HuPLSCR4, which was not detected in peripheral blood lymphocytes, and HuPLSCR1 and HuPLSCR3, which were not detected in brain.

Identification and functional expression of four isoforms of ATPase II, the putative aminophospholipid translocase. Effect of isoform variation on the ATPase activity and phospholipid specificity

ATPase II, a vanadate-sensitive and phosphatidylserine-dependent Mg(2+)-ATPase, is a member of a subfamily of P-type ATPase and is presumably responsible for aminophospholipid translocation activity in eukaryotic cells. The aminophospholipid translocation activity plays an important physiological role in the maintenance of membrane phospholipid asymmetry that is observed in the plasma membrane as well as the membranes of certain cellular organelles. While the preparations of ATPase II from different sources share common fundamental properties, such as substrate specificity, inhibitor spectrum, and phospholipid dependence, they are divergent in several characteristics. These include specific ATPase activity and phospholipid selectivity. We report here the identification of four isoforms of ATPase II in bovine brain. These isoforms are formed by a combination of two major variations in their primary sequences and show that the structural variation of these isoforms has functional significance in both ATPase activity and phosholipid selectivity. Furthermore, studies with the phosphoenzyme intermediate of ATPase II and its recombinant isoforms revealed that phosphatidylserine is essential for the dephosphorylation of the intermediate. Without phosphatidylserine, ATPase II would be accumulated as phosphoenzyme in the presence of ATP, resulting in the interruption of its catalytic cycle.

Surface expression of phosphatidylserine on macrophages is required for phagocytosis of apoptotic thymocytes

Cells generally maintain an asymmetric distribution of phospholipids across the plasma membrane bilayer, restricting the phospholipid, phosphatidylserine (PS), to the inner leaflet of the plasma membrane. When cells undergo apoptosis, this asymmetric transbilayer distribution is lost, bringing PS to the surface where it acts as a signal for engulfment by phagocytes. The fluorescent dye merocyanine 540 specifically stains the plasma membrane of apoptotic cells which have lost their asymmetric distribution of phospholipids. However, it also stains non-apoptotic macrophages, suggesting that phospholipid asymmetry may not be maintained in these cells, and thus that they may express PS on their surface. Here, the PS-binding protein, annexin V, was used to show that in fact normal macrophages do express PS on their surface. Furthermore, pre-treating macrophages with annexin V was found to inhibit phagocytosis of apoptotic thymocytes and thymocytes on which PS expression was artificially induced, but did not inhibit phagocytosis of latex beads or Fc receptor-mediated phagocytosis of opsonized erythrocytes. These results indicate that PS is constitutively expressed on the surface of macrophages and is functionally significant for the phagocytosis of PS-expressing target cells.

Identification and purification of aminophospholipid flippases

Transbilayer phospholipid asymmetry is a common structural feature of most biological membranes. This organization of lipids is generated and maintained by a number of phospholipid transporters that vary in lipid specificity, energy requirements and direction of transport. These transporters can be divided into three classes: (1) bidirectional, non-energy dependent 'scramblases', and energy-dependent transporters that move lipids (2) toward ('flippases') or (3) away from ('floppases') the cytofacial surface of the membrane. One of the more elusive members of this family is the plasma membrane aminophospholipid flippase, which selectively transports phosphatidylserine from the external to the cytofacial monolayer of the plasma membrane. This review summarizes the characteristics of aminophospholipid flippase activity in intact cells and describes current strategies to identify and isolate this protein. The biochemical characteristics of candidate flippases are critically compared and their potential role in flippase activity is evaluated.

Phosphatidylserine-dependent adhesion of T cells to endothelial cells

Phosphatidylserine (PS) was exposed at the surface of human umbilical vein endothelial cells (HUVECs) and cultured cell lines by agonists that increase cytosolic Ca(2+), and factors governing the adhesion of T cells to the treated cells were investigated. Thrombin, ionophore A23187 and the Ca(2+)-ATPase inhibitor 2, 5-di-tert-butyl-1,4-benzohydroquinone each induced a PS-dependent adhesion of Jurkat T cells. A23187, which was the most effective agonist in releasing PS-bearing microvesicles, was the least effective in inducing the PS-dependent adhesion of Jurkat cells. Treatment of ECV304 and EA.hy926 cells with EGTA, followed by a return to normal medium, resulted in an influx of Ca(2+) and an increase in adhering Jurkat cells. Oxidised low-density lipoprotein induced a procoagulant response in cultured ECV304 cells and increased the number of adhering Jurkat cells, but adhesion was not inhibited by pretreating ECV304 cells with annexin V. PS was not significantly exposed on untreated Jurkat cells, as determined by flow cytometry with annexin V-FITC. However, after adhesion to thrombin-treated ECV304 cells for 10 min followed by detachment in 1 mM EDTA, there was a marked exposure of PS on the Jurkat cells. Binding of annexin V-FITC to the detached cells was inhibited by pretreating them with unlabelled annexin V. Contact with thrombin-treated ECV304 cells thus induced the exposure of PS on Jurkat cells and, as Jurkat cells were unable to adhere to thrombin-treated ECV304 cells in the presence of EGTA, the adhesion of the two cell types may involve a Ca(2+) bridge between PS on both cell surfaces. The number of T cells from normal, human peripheral blood that adhered to ECV304 cells was not increased by treating the latter with thrombin. However, findings made with several T cell lines were generally, but not completely, consistent with the possibility that adhesion to surface PS on endothelial cells may be a feature of T cells that express both CD4(+) and CD8(+) antigens. Possible implications for PS-dependent adhesion of T cells to endothelial cells in metastasis, and early in atherogenesis, are discussed.