Lipid flippases as key players in plant adaptation to their environment

Lipid flippases (P4 ATPases) are active transporters that catalyse the translocation of lipids between the two sides of the biological membranes in the secretory pathway. This activity modulates biological membrane properties, contributes to vesicle formation, and is the trigger for lipid signalling events, which makes P4 ATPases essential for eukaryotic cell survival. Plant P4 ATPases (also known as aminophospholipid ATPases (ALAs)) are crucial for plant fertility and proper development, and are involved in key adaptive responses to biotic and abiotic stress, including chilling tolerance, heat adaptation, nutrient deficiency responses and pathogen defence. While ALAs present many analogies to mammalian and yeast P4 ATPases, they also show characteristic features as the result of their independent evolution. In this Review, the main properties, roles, regulation and mechanisms of action of ALA proteins are discussed.

Lipid transfer proteins and instructive regulation of lipid kinase activities: Implications for inositol lipid signaling and disease

Cellular membranes are critical platforms for intracellular signaling that involve complex interfaces between lipids and proteins, and a web of interactions between a multitude of lipid metabolic pathways. Membrane lipids impart structural and functional information in this regulatory circuit that encompass biophysical parameters such as membrane thickness and fluidity, as well as chaperoning the interactions of protein binding partners. Phosphatidylinositol and its phosphorylated derivatives, the phosphoinositides, play key roles in intracellular membrane signaling, and these involvements are translated into an impressively diverse set of biological outcomes. The phosphatidylinositol transfer proteins (PITPs) are key regulators of phosphoinositide signaling. Found in a diverse array of organisms from plants, yeast and apicomplexan parasites to mammals, PITPs were initially proposed to be simple transporters of lipids between intracellular membranes. It now appears increasingly unlikely that the soluble versions of these proteins perform such functions within the cell. Rather, these serve to facilitate the activity of intrinsically biologically insufficient inositol lipid kinases and, in so doing, promote diversification of the biological outcomes of phosphoinositide signaling. The central engine for execution of such functions is the lipid exchange cycle that is a fundamental property of PITPs. How PITPs execute lipid exchange remains very poorly understood. Molecular dynamics simulation approaches are now providing the first atomistic insights into how PITPs, and potentially other lipid-exchange/transfer proteins, operate.

Patellin protein family functions in plant development and stress response

The plant patellin (PATL) proteins are yeast Sec14 protein (Sec14p)-like phosphatidylinositol transfer proteins (PITPs), which are widely distributed across the plant kingdom. The model plant Arabidopsis has six PATL members (designated as PATL1-PATL6). Accumulated evidence has indicated the involvement of Arabidopsis PATLs in various biological processes. This mini-review briefly summarizes our current knowledge on individual PATLs regarding their roles in plant development and stress tolerance regulation. The elucidation of PATLs' biological function in plants will provide new insights on plant membrane trafficking and its regulatory roles in either plant growth or environmental stress response signaling networks.

Proteomic Analysis and Functional Characterization of P4-ATPase Phospholipid Flippases from Murine Tissues

P4-ATPases are a subfamily of P-type ATPases that flip phospholipids across membranes to generate lipid asymmetry, a property vital to many cellular processes. Mutations in several P4-ATPases have been linked to severe neurodegenerative and metabolic disorders. Most P4-ATPases associate with one of three accessory subunit isoforms known as CDC50A (TMEM30A), CDC50B (TMEM30B), and CDC50C (TMEM30C). To identify P4-ATPases that associate with CDC50A, in vivo, and determine their tissue distribution, we isolated P4-ATPases-CDC50A complexes from retina, brain, liver, testes, and kidney on a CDC50A immunoaffinity column and identified and quantified P4-ATPases from their tryptic peptides by mass spectrometry. Of the 12 P4-ATPase that associate with CDC50 subunits, 10 P4-ATPases were detected. Four P4-ATPases (ATP8A1, ATP11A, ATP11B, ATP11C) were present in all five tissues. ATP10D was found in low amounts in liver, brain, testes, and kidney, and ATP8A2 was present in significant amounts in retina, brain, and testes. ATP8B1 was detected only in liver, ATP8B3 and ATP10A only in testes, and ATP8B2 primarily in brain. We also show that ATP11A, ATP11B and ATP11C, like ATP8A1 and ATP8A2, selectively flip phosphatidylserine and phosphatidylethanolamine across membranes. These studies provide new insight into the tissue distribution, relative abundance, subunit interactions and substrate specificity of P4-ATPase-CDC50A complexes.

Ebola virus requires a host scramblase for externalization of phosphatidylserine on the surface of viral particles

Cell surface receptors for phosphatidylserine contribute to the entry of Ebola virus (EBOV) particles, indicating that the presence of phosphatidylserine in the envelope of EBOV is important for the internalization of EBOV particles. Phosphatidylserine is typically distributed in the inner layer of the plasma membrane in normal cells. Progeny virions bud from the plasma membrane of infected cells, suggesting that phosphatidylserine is likely flipped to the outer leaflet of the plasma membrane in infected cells for EBOV virions to acquire it. Currently, the intracellular dynamics of phosphatidylserine during EBOV infection are poorly understood. Here, we explored the role of XK-related protein (Xkr) 8, which is a scramblase responsible for exposure of phosphatidylserine in the plasma membrane of apoptotic cells, to understand its significance in phosphatidylserine-dependent entry of EBOV. We found that Xkr8 and transiently expressed EBOV glycoprotein GP often co-localized in intracellular vesicles and the plasma membrane. We also found that co-expression of GP and viral major matrix protein VP40 promoted incorporation of Xkr8 into ebolavirus-like particles (VLPs) and exposure of phosphatidylserine on their surface, although only a limited amount of phosphatidylserine was exposed on the surface of the cells expressing GP and/or VP40. Downregulating Xkr8 or blocking caspase-mediated Xkr8 activation did not affect VLP production, but they reduced the amount of phosphatidylserine on the VLPs and their uptake in recipient cells. Taken together, our findings indicate that Xkr8 is trafficked to budding sites via GP-containing vesicles, is incorporated into VLPs, and then promote the entry of the released EBOV to cells in a phosphatidylserine-dependent manner.

Although Ebola virus causes severe hemorrhagic fever with a high mortality rate, there are no approved therapeutics. The viral entry process is one of the targets for antiviral development. Previous studies suggest that binding of phosphatidylserine, a component of the viral envelop, to the receptors promotes the entry of Ebola virus. Ebola virus is released from the surface membrane of infected cells. However, phosphatidylserine normally distributes in the inner layer of the cell surface membrane, suggesting that phosphatidylserine is likely flipped to the outer leaflet of the membrane in infected cells for Ebola virus to acquire it. Because the mechanism by which phosphatidylserine changes its orientation in Ebola virus-infected cells is poorly understood, we studied and identified a cellular enzyme, XK-related protein 8 (Xkr8), as a responsible factor involved in this process. We demonstrated that the Ebola virus glycoprotein promoted the incorporation of Xkr8 in viral particles, which flips phosphatidylserine on their surface, enhancing their entry to cells. Our findings provide new insights into the mechanism of Ebola virus infection, which may be exploited for the development of therapeutics against Ebola virus infection.

On the molecular mechanism of flippase- and scramblase-mediated phospholipid transport

Phospholipid flippases are key regulators of transbilayer lipid asymmetry in eukaryotic cell membranes, critical to many trafficking and signaling pathways. P4-ATPases, in particular, are responsible for the uphill transport of phospholipids from the exoplasmic to the cytosolic leaflet of the plasma membrane, as well as membranes of the late secretory/endocytic pathways, thereby establishing transbilayer asymmetry. Recent studies combining cell biology and biochemical approaches have improved our understanding of the path taken by lipids through P4-ATPases. Additionally, identification of several protein families catalyzing phospholipid 'scrambling', i.e. disruption of phospholipid asymmetry through energy-independent bi-directional phospholipid transport, as well as the recent report of the structure of such a scramblase, opens the way to a deeper characterization of their mechanism of action. Here, we discuss the molecular nature of the mechanism by which lipids may 'flip' across membranes, with an emphasis on active lipid transport catalyzed by P4-ATPases. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Coordinated Membrane Ballooning and Procoagulant Spreading in Human Platelets

BACKGROUND

Platelets are central to the process of hemostasis, rapidly aggregating at sites of blood vessel injury and acting as coagulation nidus sites. On interaction with the subendothelial matrix, platelets are transformed into balloonlike structures as part of the hemostatic response. It remains unclear, however, how and why platelets generate these structures. We set out to determine the physiological relevance and cellular and molecular mechanisms underlying platelet membrane ballooning.

METHODS AND RESULTS

Using 4-dimensional live-cell imaging and electron microscopy, we show that human platelets adherent to collagen are transformed into phosphatidylserine-exposing balloonlike structures with expansive macro/microvesiculate contact surfaces, by a process that we termed procoagulant spreading. We reveal that ballooning is mechanistically and structurally distinct from membrane blebbing and involves disruption to the platelet microtubule cytoskeleton and inflation through fluid entry. Unlike blebbing, procoagulant ballooning is irreversible and a consequence of Na(+), Cl(-), and water entry. Furthermore, membrane ballooning correlated with microparticle generation. Inhibition of Na(+), Cl(-), or water entry impaired ballooning, procoagulant spreading, and microparticle generation, and it also diminished local thrombin generation. Human Scott syndrome platelets, which lack expression of Ano-6, also showed a marked reduction in membrane ballooning, consistent with a role for chloride entry in the process. Finally, the blockade of water entry by acetazolamide attenuated ballooning in vitro and markedly suppressed thrombus formation in vivo in a mouse model of thrombosis.

CONCLUSIONS

Ballooning and procoagulant spreading of platelets are driven by fluid entry into the cells, and are important for the amplification of localized coagulation in thrombosis.

Ion channel and lipid scramblase activity associated with expression of TMEM16F/ANO6 isoforms

TMEM16F is a membrane protein with possible dual function as an ion channel and a phospholipid scramblase. The properties of ion channels associated with TMEM16F and the link between ion channel and scramblase activity are a matter of debate. We studied the properties of four isoforms of TMEM16F generated by alternative splicing. Upregulation of three TMEM16F isoforms or silencing of endogenous TMEM16F increased and decreased, respectively, both scramblase and channel activities. Introduction of an activating mutation in TMEM16F sequence caused a marked increase in phosphatidylserine scrambling and in ion transport indicating direct involvement of the protein in both functions. TMEM16F, also known as ANO6, is a membrane protein that has been associated with phospholipid scramblase and ion channel activity. However, the characteristics of TMEM16F-dependent channels, particularly the ion selectivity, are a matter of debate. Furthermore, the direct involvement of TMEM16F in phospholipid scrambling has been questioned. We studied the properties of different TMEM16F variants generated by alternative splicing. Using whole-cell patch-clamp recordings, we found that V1, V2 and V5 variants generated membrane currents activated by very high (micromolar) intracellular Ca(2+) concentrations and positive membrane potentials. These variants showed different degrees of Ca(2+) sensitivity and kinetics of activation but similar ion permeability, characterized by a slight selectivity for Cl(-) over Na(+) . A fourth variant (V3) showing a unique carboxy-terminus was devoid of activity, in agreement with its intracellular localization. We also measured scramblase activity using the binding of annexin V to detect phosphatidylserine on the cell surface. V1, V2 and V5 variants were associated with calcium-dependent phosphatidylserine externalization. Interestingly, introduction of an activating mutation, D409G, produced a marked increase in the apparent Ca(2+) sensitivity of TMEM16F-dependent channels. In parallel, this mutation also enhanced the extent of phosphatidylserine externalization that occurred even under resting conditions. These results support the conclusion that TMEM16F proteins are directly involved in dual activity, as a phospholipid scramblase and as an ion channel.

Involvement of C/EBPβ in monocytic differentiation of acute myeloid leukemia cells induced by LW-218, a new synthesized flavonoid

Our previous study investigated the effects of differentiation inducted by flavonoids derived from a Chinese herb. In this study, we found that LW-218, a new synthesized flavonoid, inhibited proliferation and induced differentiation of acute myeloid leukemia cells. The IC50s of LW-218 in HL-60, U937, K562, and NB4 cell lines were all less than 5 μM, suggesting greater capacity than compounds we have reported. LW-218 induced differentiation effects including morphologic changes, NBT reduction, and both of CD11b and CD14 expression. Results of western blots and siRNA transfection revealed that LW-218 increased the LAP/LIP ratio of C/EBPβ which regulated monocytic differentiation of leukemia cells. Meanwhile, these differentiation effects could be attenuated by silencing PLSCR1 via siRNA transfection. In addition, regulation on LAP/LIP ratio, of C/EBPβ was properly mediated by PLSCR1 which was up-regulated by LW-218. All these results suggested that C/EBPβ was involved in regulation of PKCδ/PLSCR1 pathway during flavonoids-induced differentiation. LW-218 was a prospective differentiation inductor of AML cells and was requisite to proceed further investigation.

Role of anoctamins in cancer and apoptosis

Anoctamin 1 (TMEM16A, Ano1) is a recently identified Ca(2+)-activated chloride channel and a member of a large protein family comprising 10 paralogues. Before Ano1 was identified as a chloride channel protein, it was known as the cancer marker DOG1. DOG1/Ano1 is expressed in gastrointestinal stromal tumours (GIST) and particularly in head and neck squamous cell carcinoma, at very high levels never detected in other tissues. It is now emerging that Ano1 is part of the 11q13 locus, amplified in several types of tumour, where it is thought to augment cell proliferation, cell migration and metastasis. Notably, Ano1 is upregulated through histone deacetylase (HDAC), corresponding to the known role of HDAC in HNSCC. As Ano1 does not enhance proliferation in every cell type, its function is perhaps modulated by cell-specific factors, or by the abundance of other anoctamins. Thus Ano6, by regulating Ca(2+)-induced membrane phospholipid scrambling and annexin V binding, supports cellular apoptosis rather than proliferation. Current findings implicate other cellular functions of anoctamins, apart from their role as Ca(2+)-activated Cl(-) channels.

A phosphatidylinositol transfer protein integrates phosphoinositide signaling with lipid droplet metabolism to regulate a developmental program of nutrient stress-induced membrane biogenesis

Lipid droplet (LD) utilization is an important cellular activity that regulates energy balance and release of lipid second messengers. Because fatty acids exhibit both beneficial and toxic properties, their release from LDs must be controlled. Here we demonstrate that yeast Sfh3, an unusual Sec14-like phosphatidylinositol transfer protein, is an LD-associated protein that inhibits lipid mobilization from these particles. We further document a complex biochemical diversification of LDs during sporulation in which Sfh3 and select other LD proteins redistribute into discrete LD subpopulations. The data show that Sfh3 modulates the efficiency with which a neutral lipid hydrolase-rich LD subclass is consumed during biogenesis of specialized membrane envelopes that package replicated haploid meiotic genomes. These results present novel insights into the interface between phosphoinositide signaling and developmental regulation of LD metabolism and unveil meiosis-specific aspects of Sfh3 (and phosphoinositide) biology that are invisible to contemporary haploid-centric cell biological, proteomic, and functional genomics approaches.

TMEM16F is a component of a Ca2+-activated Cl- channel but not a volume-sensitive outwardly rectifying Cl- channel

TMEM16 (transmembrane protein 16) proteins, which possess eight putative transmembrane domains with intracellular NH2- and COOH-terminal tails, are thought to comprise a Cl(-) channel family. The function of TMEM16F, a member of the TMEM16 family, has been greatly controversial. In the present study, we performed whole cell patch-clamp recordings to investigate the function of human TMEM16F. In TMEM16F-transfected HEK293T cells but not TMEM16K- and mock-transfected cells, activation of membrane currents with strong outward rectification was found to be induced by application of a Ca(2+) ionophore, ionomycin, or by an increase in the intracellular free Ca(2+) concentration. The free Ca(2+) concentration for half-maximal activation of TMEM16F currents was 9.6 μM, which is distinctly higher than that for TMEM16A/B currents. The outwardly rectifying current-voltage relationship for TMEM16F currents was not changed by an increase in the intracellular Ca(2+) level, in contrast to TMEM16A/B currents. The Ca(2+)-activated TMEM16F currents were anion selective, because replacing Cl(-) with aspartate(-) in the bathing solution without changing cation concentrations caused a positive shift of the reversal potential. The anion selectivity sequence of the TMEM16F channel was I(-) > Br(-) > Cl(-) > F(-) > aspartate(-). Niflumic acid, a Ca(2+)-activated Cl(-) channel blocker, inhibited the TMEM16F-dependent Cl(-) currents. Neither overexpression nor knockdown of TMEM16F affected volume-sensitive outwardly rectifying Cl(-) channel (VSOR) currents activated by osmotic swelling or apoptotic stimulation. These results demonstrate that human TMEM16F is an essential component of a Ca(2+)-activated Cl(-) channel with a Ca(2+) sensitivity that is distinct from that of TMEM16A/B and that it is not related to VSOR activity.

Anoctamins are a family of Ca2+-activated Cl- channels

Anoctamin 1 (Ano1; TMEM16A) and anoctamin 2 (Ano2; TMEM16B) are novel Cl(-) channels transiently activated by an increase in intracellular Ca(2+). These channels are essential for epithelial Cl(-) secretion, smooth muscle peristalsis and olfactory signal transduction. They are central to inherited diseases and cancer and can act as heat sensors. Surprisingly, another member of this protein family, Ano6, operates as a Ca(2+)-activated phospholipid scramblase, and others were reported as intracellular proteins. It is therefore unclear whether anoctamins constitute a family of Ca(2+)-activated Cl(-) channels, or are proteins with heterogeneous functions. Using whole-cell patch clamping we demonstrate that Ano4-10 are all able to produce transient Ca(2+)-activated Cl(-) currents when expressed in HEK293 cells. Although some anoctamins (Ano1, 2, 4, 6, 7) were found to be well expressed in the plasma membrane, others (Ano8, 9, 10) show rather poor membrane expression and were mostly retained in the cytosol. The transient nature of the Cl(-) currents was demonstrated to be independent of intracellular Ca(2+) levels. We show that inactivation of Ano1 currents occurs in the continuous presence of elevated Ca(2+) concentrations, possibly by calmodulin-dependent kinase. The present results demonstrate that anoctamins are a family of Ca(2+)-activated Cl(-) channels, which also induce permeability for cations. They may operate as Cl(-) channels located in the plasma membrane or in intracellular compartments. These results increase our understanding of the physiological significance of anoctamins and their role in disease.

Phospholipid transfer protein is differentially expressed in human arterial and venous placental endothelial cells and enhances cholesterol efflux to fetal HDL

CONTEXT

Phospholipid (PL) transfer protein (PLTP) plays a crucial role in high-density lipoprotein (HDL) metabolism. In the fetal circulation, HDL particles are the main cholesterol carriers and are involved in maternal-fetal cholesterol transfer across human placental endothelial cells (HPEC).

OBJECTIVE

The aim was to investigate local function(s) of PLTP at the fetoplacental endothelium. Because HPEC display morphological and functional diversity when isolated from arteries or veins, we hypothesized that PLTP activity may differ between arterial and venous HPEC.

DESIGN

We determined PLTP mRNA and activity levels from isolated HPEC and investigated PLTP-mediated remodeling of fetal HDL particles and their capacity in mediating cholesterol efflux from HPEC.

RESULTS

Incubation of fetal HDL with active human plasma PLTP resulted in increased particle size (12.6 vs. 13.2 nm, P < 0.05), with a concomitant increase (3.5-fold) in pre-β-mobile HDL particles. Arterial HPEC showed higher Pltp expression levels and secreted PL transfer activity (1.8-fold, P < 0.001) than venous HPEC. In contrast to adult HDL(3), [(3)H]cholesterol efflux to fetal HDL was 21% higher (P < 0.05) from arterial than from venous HPEC. PLTP-facilitated particle conversion increased the cholesterol efflux capacity of fetal HDL to similar extents (55 and 48%, P < 0.001) from arterial and venous HPEC, respectively.

CONCLUSION

PLTP mediates PL transfer and participates in reverse cholesterol transport pathways at the fetoplacental barrier. Enhanced cellular cholesterol efflux from HPEC to fetal HDL remodeled by PLTP supports the idea of a local atheroprotective role of PLTP in the placental vasculature.

Anoctamin 6 is an essential component of the outwardly rectifying chloride channel

Outwardly rectifying chloride channels (ORCC, ICOR) of intermediate single-channel conductance of around 50 pS, are ubiquitously expressed, but have remained a mystery since their description more than 25 y ago. These channels have been shown to be activated on membrane excision and depolarization of the membrane voltage and by cAMP in the presence of the cystic fibrosis transmembrane conductance regulator. We show that anoctamin 6 (Ano6), a member of the recently identified family of putative Cl(-) channels, is the crucial component of ORCC single-channel and whole-cell currents in airway epithelial cells and Jurkat T lymphocytes. Cystic fibrosis transmembrane conductance regulator augmented ORCC produced by Ano6 in A549 airway epithelial cells. Ano6 is activated during membrane depolarization or apoptosis of Jurkat T lymphocytes and epithelial cells, and is inhibited by 5-nitro-2-(3-phenylpropylamino) benzoic acid, 4,4'-diisothio-cyanostilbene-2,2'-disulfonic acid, or AO1. Ano6 belongs to the basic equipment of any cell type, including colonic surface epithelial cells. It forms the essential component of ORCC and seems to have a role for cell shrinkage and programmed cell death.

PLTP regulates STAT3 and NFκB in differentiated THP1 cells and human monocyte-derived macrophages

Phospholipid transfer protein (PLTP) plays an important role in regulation of inflammation. Previously published studies have shown that PLTP binds, transfers and neutralizes bacterial lipopolysaccharides. In the current study we tested the hypothesis that PLTP can also regulate anti-inflammatory pathways in macrophages. Incubation of macrophage-like differentiated THP1 cells and human monocyte-derived macrophages with wild-type PLTP in the presence or absence of tumor necrosis factor alpha (TNFα) or interferon gamma (IFNγ) significantly increased nuclear levels of active signal transducer and activator of transcription 3, pSTAT3(Tyr705) (p<0.01). Similar results were obtained in the presence of a PLTP mutant without lipid transfer activity (PLTP(M159E)), suggesting that PLTP-mediated lipid transfer is not required for activation of the STAT3 pathway. Inhibition of ABCA1 by chemical inhibitor, glyburide, as well as ABCA1 RNA inhibition, reversed the observed PLTP-mediated activation of STAT3. In addition, PLTP reduced nuclear levels of active nuclear factor kappa-B (NFκB) p65 and secretion of pro-inflammatory cytokines in conditioned media of differentiated THP1 cells and human monocyte-derived macrophages. Our data suggest that PLTP has anti-inflammatory capabilities in macrophages.

Ion channels, phosphorylation and mammalian sperm capacitation

Sexually reproducing animals require an orchestrated communication between spermatozoa and the egg to generate a new individual. Capacitation, a maturational complex phenomenon that occurs in the female reproductive tract, renders spermatozoa capable of binding and fusing with the oocyte, and it is a requirement for mammalian fertilization. Capacitation encompasses plasma membrane reorganization, ion permeability regulation, cholesterol loss and changes in the phosphorylation state of many proteins. Novel tools to study sperm ion channels, image intracellular ionic changes and proteins with better spatial and temporal resolution, are unraveling how modifications in sperm ion transport and phosphorylation states lead to capacitation. Recent evidence indicates that two parallel pathways regulate phosphorylation events leading to capacitation, one of them requiring activation of protein kinase A and the second one involving inactivation of ser/thr phosphatases. This review examines the involvement of ion transporters and phosphorylation signaling processes needed for spermatozoa to achieve capacitation. Understanding the molecular mechanisms leading to fertilization is central for societies to deal with rising male infertility rates, to develop safe male gamete-based contraceptives and to preserve biodiversity through better assisted fertilization strategies.

[High-density lipoprotein (HDL) and cholesteryl ester transfer protein (CETP): role in lipid metabolism and clinical meaning]

Large epidemiological studies have consistently shown that plasma levels of high-density lipoprotein (HDL) correlate inversely with cardiovascular risk. The apparent cardioprotective role of HDL has primarily been attributed to its participation in reverse cholesterol transport (RCT) but there is also substantial evidence that supports the concept of HDL and apoA-I preventing oxidative damage, inhibiting systemic inflammation, promoting vascular integrity and preventing thrombosis. Besides conventional therapy to increase HDL like physical exercise, weight loss and dietary changes new strategies to intervene at various steps of its metabolism have been proposed and are in development. One of the most promising approaches is inhibiting cholesteryl ester transfer protein (CETP)which plays a central role in RCT by transferring cholesteryl esters from HDL to apoB containing lipoproteins in exchange for triglycerides. The failure of the CETP inhibitor torcetrapib, however, to cause any benefit on cardiovascular outcomes despite significantly increased HDL levels in several clinical trials casted doubts upon the concept of CETP inhibition. Meanwhile, off target toxicity could be shown for torcetrapib and a new generation of CETP inhibitors stands ready to be tested in large clinical trials. This article describes the formation and remodeling of HDL, how HDL is thought to be beneficial for the vasculature and what options we have today to increase HDL levels with a special focus on CETP inhibition.

Role of phosphatidylcholine biosynthesis in the regulation of lipoprotein homeostasis

PURPOSE OF REVIEW

This review summarizes the role of phosphatidylcholine metabolism in plasma lipoprotein homeostasis.

RECENT FINDINGS

While it was previously known that phosphatidylcholine biosynthesis was required for normal hepatic VLDL secretion, recent studies have shown that both phosphatidylcholine biosynthetic pathways (the cytidine 5'-diphosphocholine and the phosphatidylethanolamine methylation pathways) are required. In addition, a requirement of acyl-coenzyme A synthetase 3, but not acyl-coenzyme A synthetase 1 or 4, for phosphatidylcholine synthesis and VLDL secretion is now documented. ABCA1 has been implicated in the transfer of phosphatidylcholine to apolipoproteinA-1 both during and after secretion of apolipoproteinA-1. Other studies have introduced the concept of reverse phosphatidylcholine transport in which both HDL and LDL supply phosphatidylcholine to the liver. An unexpected finding is that half of the phosphatidylcholine delivered to liver from lipoproteins is converted into triacylglycerol.

SUMMARY

The liver is both a donor of phosphatidylcholine during the assembly and secretion of lipoproteins as well as a recipient of phosphatidylcholine from plasma lipoproteins.

SFH2 regulates fatty acid synthase activity in the yeast Saccharomyces cerevisiae and is critical to prevent saturated fatty acid accumulation in response to haem and oleic acid depletion

The yeast Saccharomyces cerevisiae is a facultative anaerobic organism. Under anaerobiosis, sustained growth relies on the presence of exogenously supplied unsaturated fatty acids and ergosterol that yeast is unable to synthesize in the absence of oxygen or upon haem depletion. In the absence of exogenous supplementation with unsaturated fatty acid, a net accumulation of SFA (saturated fatty acid) is observed that induces significant modification of phospholipid profile [Ferreira, Régnacq, Alimardani, Moreau-Vauzelle and Bergès (2004) Biochem. J. 378, 899–908]. In the present paper, we focus on the role of SFH2/CSR1, a hypoxic gene related to SEC14 and its involvement in lipid metabolism upon haem depletion in the absence of oleic acid supplementation. We observed that inactivation of SFH2 results in enhanced accumulation of SFA and phospholipid metabolism alterations. It results in premature growth arrest and leads to an exacerbated sensitivity to exogenous SFA. This phenotype is suppressed in the presence of exogenous oleic acid, or by a controlled expression of FAS1, one of the two genes encoding FAS. We present several lines of evidence to suggest that Sfh2p and oleic acid regulate SFA synthase in yeast at different levels: whereas oleic acid acts on FAS2 at the transcriptional level, we show that Sfh2p inhibits fatty acid synthase activity in response to haem depletion.

Cholesterol efflux from macrophage foam cells is enhanced by active phospholipid transfer protein through generation of two types of acceptor particles

Phospholipid transfer protein (PLTP) is expressed by macrophage-derived foam cells in human atherosclerotic lesions, suggesting a regulatory role for PLTP in cellular cholesterol homeostasis. However, the exact role of PLTP in the reverse cholesterol transport pathway is not known. PLTP is present in plasma as two forms, a highly active (HA-PLTP) and a lowly active (LA-PLTP) form. In this study we clarify the role of the two forms of PLTP in cholesterol efflux from [3H]cholesterol oleate-acetyl-LDL-loaded THP-1 macrophages. Incubation of HDL in the presence of HA-PLTP resulted in the formation of two types of acceptor particles, prebeta-HDL and large fused HDL. HA-PLTP increased prebeta-HDL formation and caused a 42% increase in [3H]cholesterol efflux to HDL, while LA-PLTP neither formed prebeta-HDL nor increased cholesterol efflux. Removal of the formed prebeta-HDL by immunoprecipitation decreased cholesterol efflux by 47%. Neither HA- nor LA-PLTP enhanced cholesterol efflux to lipid-free apoA-I. Importantly, also the large fused HDL particles formed during incubation of HDL with HA-PLTP acted as efficient cholesterol acceptors. These observations demonstrate that only HA-PLTP increases macrophage cholesterol efflux, via formation of efficient cholesterol acceptors, prebeta-HDL and large fused HDL particles.

The zinc cluster transcription factor Tac1p regulates PDR16 expression in Candida albicans

The Candida albicans PDR16 gene, encoding a putative phosphatidylinositol transfer protein, is co-induced with the multidrug transporter genes CDR1 and CDR2 in azole-resistant (A(R)) clinical isolates and upon fluphenazine exposure of azole-susceptible (A(S)) cells, suggesting that it is regulated by Tac1p, the transcriptional activator of CDR genes. Deleting TAC1 in an A(R) isolate (5674) overexpressing PDR16, CDR1 and CDR2 decreased the expression of the three genes and fluconazole resistance to levels similar to those detected in the matched A(S) isolate (5457), demonstrating that Tac1p is responsible for PDR16 upregulation in that strain. Deleting TAC1 in the A(S) strain SC5314 abolished CDR2 induction by fluphenazine and decreased that of PDR16 and CDR1, uncovering the participation of an additional factor in the regulation of PDR16 and CDR1 expression. Sequencing of the TAC1 alleles identified one homozygous mutation in strain 5674, an Asn to Asp substitution at position 972 in the C-terminus of Tac1p. Introduction of the Asp(972) allele in a tac1Delta/Delta mutant caused high levels of fluconazole resistance and TAC1, PDR16, CDR1 and CDR2 constitutive induction. These results demonstrate that: (i) Tac1p controls PDR16 expression; (ii) Asn(972) to Asp(972) is a gain-of-function mutation; and (iii) Tac1p is positively autoregulated, directly or indirectly.

Aminophospholipid translocase TAT-1 promotes phosphatidylserine exposure during C. elegans apoptosis

Phospholipids are distributed asymmetrically across the plasma-membrane bilayer of eukaryotic cells: Phosphatidylserine (PS), phosphatidylethanolamine, and phosphoinositides are predominantly restricted to the inner leaflet, whereas phophatidylcholine and sphingolipids are enriched on the outer leaflet [1, 2]. Exposure of PS on the cell surface is a conserved feature of apoptosis and plays an important role in promoting the clearance of apoptotic cells by phagocytosis [3]. However, the molecular mechanism that drives PS exposure remains mysterious. To address this issue, we studied cell-surface changes during apoptosis in the nematode C. elegans. Here, we show that PS exposure can readily be detected on apoptotic C. elegans cells. We generated a transgenic strain expressing a GFP::Annexin V reporter to screen for genes required for this process. Although none of the known engulfment genes was required, RNAi knockdown of the putative aminophospholipid transporter gene tat-1 abrogated PS exposure on apoptotic cells. tat-1(RNAi) also reduced the efficiency of cell-corpse clearance, suggesting that PS exposure acts as an "eat-me" signal in worms. We propose that tat-1 homologs might also play an important role in PS exposure in mammals.

Phospholipid Transfer Protein–Deficient Mice Absorb Less Cholesterol

Objective— Phospholipid transfer protein (PLTP) plays an important role in lipoprotein metabolism and atherosclerosis. PLTP gene knockout (KO) mice show significant reduction of plasma cholesterol levels. Because small intestine is one of the major tissue expressing PLTP, we hypothesize that PLTP deficient small intestine absorbs less cholesterol, thus contributing to the diminishing of cholesterol levels in the plasma.

Methods and Results— We used dual-labeled cholesterol/sitostanol feeding approach to study cholesterol absorption in PLTP KO and WT mice. We found that PLTP KO mice absorb significant less cholesterol than WT mice. Primary enterocytes isolated from PLTP KO enterocytes took up significant less cholesterol. Moreover, we observed that Niemann-Pick C1-like 1 (NPC1L1) mRNA levels were significantly decreased in the small intestine of PLTP KO mice. Next, we studied the secretion of cholesterol by enterocytes. The amounts of cholesterol transported to plasma and liver were significantly reduced in PLTP KO mice, compared with WT animals. Studies with isolated PLTP KO enterocytes revealed that the secretion of cholesterol via chylomicron and intestinal-HDL was significantly reduced. Furthermore, ATP-binding cassette transporters (ABC) A1 mRNA and microsomal triglyceride transfer protein (MTP) activity levels were significantly decreased in PLTP KO small intestine.

Conclusion— These results indicate that PLTP deficiency results in reduced cholesterol uptake as well as secretion by the intestine. We suggest that PLTP could be a useful target to lower plasma cholesterol levels, thus reducing atherosclerosis.

Mobilization of pro-inflammatory lipids in obese Plscr3-deficient mice

Metabolic profiling of mice deficient in phospholipid scramblase 3 reveals a possible molecular link between obesity and inflammation.

Background

The obesity epidemic has prompted the search for candidate genes capable of influencing adipose function. One such candidate, that encoding phospholipid scramblase 3 (PLSCR3), was recently identified, as genetic deletion of it led to lipid accumulation in abdominal fat pads and changes characteristic of metabolic syndrome. Because adipose tissue is increasingly recognized as an endocrine organ, capable of releasing small molecules that modulate disparate physiological processes, we examined the plasma from wild-type, Plscr1-/-, Plscr3-/- and Plscr1&3-/- mice. Using an untargeted comprehensive metabolite profiling approach coupled with targeted gene expression analyses, the perturbed biochemistry and functional redundancy of PLSCR proteins was assessed.

Results

Nineteen metabolites were differentially and similarly regulated in both Plscr3-/- and Plscr1&3-/- animals, of which five were characterized from accurate mass, tandem mass spectrometry data and their correlation to the Metlin database as lysophosphatidylcholine (LPC) species enriched with C16:1, C18:1, C20:3, C20:5 and C22:5 fatty acids. No significant changes in the plasma metabolome were detected upon elimination of PLSCR1, indicating that increases in pro-inflammatory lipids are specifically associated with the obese state of Plscr3-deficient animals. Correspondingly, increases in white adipose lipogenic gene expression confirm a role for PLSCR3 in adipose lipid metabolism.

Conclusion

The untargeted profiling of circulating metabolites suggests no detectable functional redundancies between PLSCR proteins; however, this approach simultaneously identified previously unrecognized lipid metabolites that suggest a novel molecular link between obesity, inflammation and the downstream consequences associated with PLSCR3-deficiency.

Phospholipid scramblases: An overview

Phospholipid scramblases are a group of homologous proteins that are conserved in all eukaryotic organisms. They are believed to be involved in destroying plasma membrane phospholipid asymmetry at critical cellular events like cell activation, injury and apoptosis. However, a detailed mechanism of phospholipid scrambling still awaits a proper understanding. The most studied member of this family, phospholipid scramblase 1 (PLSCR1) (a 37kDa protein), is involved in rapid Ca2+ dependent transbilayer redistribution of plasma membrane phospholipids. Recently the function of PLSCR1 as a phospholipids translocator has been challenged and evidences suggest that PLSCR1 acts as signaling molecule. It has been shown to be involved in protein phosphorylation and as a potential activator of genes in response to interferon and other cytokines. Interferon induced rapid biosynthesis of PLSCR1 targets some of the protein into the nucleus, where it binds to the promoter region of inositol 1,4,5-triphosphate (IP3) receptor type 1 (IP3R1) gene and induces its expression. Palmitoylation of PLSCR1 acts as a switch, controlling its localization either to the PM or inside the nucleus. In the present review, we discuss the current understanding of PLSCR1 in relation to its trafficking, localization and signaling functions.

Concerted actions of cholesteryl ester transfer protein and phospholipid transfer protein in type 2 diabetes: effects of apolipoproteins

PURPOSE OF REVIEW

Type 2 diabetes frequently coincides with dyslipidemia, characterized by elevated plasma triglycerides, low high-density lipoprotein cholesterol levels and the presence of small dense low-density lipoprotein particles. Plasma lipid transfer proteins play an essential role in lipoprotein metabolism. It is thus vital to understand their pathophysiology and determine which factors influence their functioning in type 2 diabetes.

RECENT FINDINGS

Cholesteryl ester transfer protein-mediated transfer is increased in diabetic patients and contributes to low plasma high-density lipoprotein cholesterol levels. Apolipoproteins A-I, A-II and E are components of the donor lipoprotein particles that participate in the transfer of cholesteryl esters from high-density lipoprotein to apolipoprotein B-containing lipoproteins. Current evidence for functional roles of apolipoproteins C-I, F and A-IV as modulators of cholesteryl ester transfer is discussed. Phospholipid transfer protein activity is increased in diabetic patients and may contribute to hepatic very low-density lipoprotein synthesis and secretion and vitamin E transfer. Apolipoprotein E could stimulate the phospholipid transfer protein-mediated transfer of surface fragments of triglyceride-rich lipoproteins to high-density lipoprotein, and promote high-density lipoprotein remodelling.

SUMMARY

Both phospholipid and cholesteryl ester transfer proteins are important in very low and high-density lipoprotein metabolism and display concerted actions in patients with type 2 diabetes.

Inducible expression of phospholipid transfer protein (PLTP) in transgenic mice: acute effects of PLTP on lipoprotein metabolism

One main determinant in high-density lipoprotein (HDL) metabolism is phospholipid transfer protein (PLTP), a plasma protein that is associated with HDL. In transgenic mice overexpressing human PLTP we found that elevated plasma PLTP levels dose-dependently increased the susceptibility to diet-induced atherosclerosis. This could be mainly due to the fact that most functions of PLTP are potentially atherogenic, such as decreasing plasma HDL levels. To further elucidate the role of PLTP in lipoprotein metabolism and atherosclerosis we generated a novel transgenic mouse model that allows conditional expression of human PLTP. In this mouse model a human PLTP encoding sequence is controlled by a Tet-On system. Upon induction of PLTP expression, our mouse model showed a strongly increased PLTP activity (from 3.0 +/- 0.6 to 11.4 +/- 2.8 AU, p < 0.001). The increase in PLTP activity resulted in an acute decrease in plasma cholesterol of 33% and a comparable decrease in phospholipids. The decrease in total plasma cholesterol and phospholipids was caused by a 35% decrease in HDL-cholesterol level and a 41% decrease in HDL-phospholipid level. These results demonstrate the feasibility of our mouse model to induce an acute elevation of PLTP activity, which is easily reversible. As a direct consequence of an increase in PLTP activity, HDL-cholesterol and HDL-phospholipid levels strongly decrease. Using this mouse model, it will be possible to study the effects of acute elevation of PLTP activity on lipoprotein metabolism and pre-existing atherosclerosis.

Transcriptional regulation by protein kinase A in Cryptococcus neoformans

A defect in the PKA1 gene encoding the catalytic subunit of cyclic adenosine 5'-monophosphate (cAMP)-dependent protein kinase A (PKA) is known to reduce capsule size and attenuate virulence in the fungal pathogen Cryptococcus neoformans. Conversely, loss of the PKA regulatory subunit encoded by pkr1 results in overproduction of capsule and hypervirulence. We compared the transcriptomes between the pka1 and pkr1 mutants and a wild-type strain, and found that PKA influences transcript levels for genes involved in cell wall synthesis, transport functions such as iron uptake, the tricarboxylic acid cycle, and glycolysis. Among the myriad of transcriptional changes in the mutants, we also identified differential expression of ribosomal protein genes, genes encoding stress and chaperone functions, and genes for secretory pathway components and phospholipid synthesis. The transcriptional influence of PKA on these functions was reminiscent of the linkage between transcription, endoplasmic reticulum stress, and the unfolded protein response in Saccharomyces cerevisiae. Functional analyses confirmed that the PKA mutants have a differential response to temperature stress, caffeine, and lithium, and that secretion inhibitors block capsule production. Importantly, we also found that lithium treatment limits capsule size, thus reinforcing potential connections between this virulence trait and inositol and phospholipid metabolism. In addition, deletion of a PKA-regulated gene, OVA1, revealed an epistatic relationship with pka1 in the control of capsule size and melanin formation. OVA1 encodes a putative phosphatidylethanolamine-binding protein that appears to negatively influence capsule production and melanin accumulation. Overall, these findings support a role for PKA in regulating the delivery of virulence factors such as the capsular polysaccharide to the cell surface and serve to highlight the importance of secretion and phospholipid metabolism as potential targets for anti-cryptococcal therapy.

Lipid flippases and their biological functions

The typically distinct phospholipid composition of the two leaflets of a membrane bilayer is generated and maintained by bi-directional transport (flip-flop) of lipids between the leaflets. Specific membrane proteins, termed lipid flippases, play an essential role in this transport process. Energy-independent flippases allow common phospholipids to equilibrate rapidly between the two monolayers and also play a role in the biosynthesis of a variety of glycoconjugates such as glycosphingolipids, N-glycoproteins, and glycosylphosphatidylinositol (GPI)-anchored proteins. ATP-dependent flippases, including members of a conserved subfamily of P-type ATPases and ATP-binding cassette transporters, mediate the net transfer of specific phospholipids to one leaflet of a membrane and are involved in the creation and maintenance of transbilayer lipid asymmetry of membranes such as the plasma membrane of eukaryotes. Energy-dependent flippases also play a role in the biosynthesis of glycoconjugates such as bacterial lipopolysaccharide. This review summarizes recent progress on the identification and characterization of the various flippases and the demonstration of their biological functions.

Increased cholesterol efflux from cultured fibroblasts to plasma from hypertriglyceridemic type 2 diabetic patients: roles of pre beta-HDL, phospholipid transfer protein and cholesterol esterification

We tested whether hypertriglyceridemia associated with type 2 diabetes mellitus is accompanied by alterations in pre beta-HDL, which are considered to be initial acceptors of cell-derived cholesterol, and by changes in the ability of plasma to promote cellular cholesterol efflux. In 28 hypertriglyceridemic and 56 normotriglyceridemic type 2 diabetic patients, and in 56 control subjects, we determined plasma lipids, HDL cholesterol and phospholipids, plasma pre beta-HDL and pre beta-HDL formation, phospholipid transfer protein (PLTP) activity, plasma cholesterol esterification (EST) and cholesteryl ester transfer (CET) and the ability of plasma to stimulate cholesterol efflux out of cultured human fibroblasts. HDL cholesterol and HDL phospholipids were lower, whereas plasma PLTP activity, EST and CET were higher in hypertriglyceridemic diabetic patients than in the other groups. Pre beta-HDL levels and pre beta-HDL formation were unaltered, although the relative amount of pre beta-HDL (expressed as % of total plasma apo A-I) was increased in hypertriglyeridemic diabetic patients. Cellular cholesterol efflux to plasma from hypertriglyceridemic diabetic patients was increased compared to efflux to normotriglyceridemic diabetic and control plasma, but efflux to normotriglyceridemic diabetic and control plasma did not differ. Multiple linear regression analysis demonstrated that cellular cholesterol efflux to plasma was positively and independently related to pre beta-HDL formation, PLTP activity and EST (multiple r=0.48), but not to the diabetic state. In conclusion, cholesterol efflux from fibroblasts to normotriglyceridemic diabetic plasma is unchanged. Efflux to hypertriglyceridemic diabetic plasma is enhanced, in association with increased plasma PLTP activity and cholesterol esterification. Unaltered pre beta-HDL formation in diabetic hypertriglyceridemia, despite low apo A-I, could contribute to maintenance of cholesterol efflux.

Phospholipid scramblase 1

Phospholipid scramblase 1 (PLSCR1) is a calcium-binding, multiply palmitoylated type II endofacial plasma membrane protein, while unpalmitoylated PLSCR1 protein can import into the nucleus, where it binds to genomic DNA. Although the original work showed that PLSCR1 contributes to the transbilayer movement of phospholipids, the following studies revealed that PLSCR1 expression can be induced by some cytokines such as interferon, epidermal growth factor, and also by leukemic cell differentiation-inducing agents such as all-trans retinoic acid (ATRA) and phorbol 12-myristate 13-acetate (PMA). PLSCR1 was also shown to interact with several protein kinases including c-Abl, c-Src, protein kinase Cdelta as well as some other proteins such as onzin, suggesting the roles of PLSCR1 in cell signaling. Indeed, the current evidence proposes that PLSCR1 contributes to cell proliferation, differentiation, apoptosis, and plays roles in the pathogenesis of cancers, especially leukemia.

R7BP augments the function of RGS7*Gbeta5 complexes by a plasma membrane-targeting mechanism

The RGS7 (R7) family of G protein regulators, Gbeta5, and R7BP form heterotrimeric complexes that potently regulate the kinetics of G protein-coupled receptor signaling. Reversible palmitoylation of R7BP regulates plasma membrane/nuclear shuttling of R7*Gbeta5*R7BP heterotrimers. Here we have investigated mechanisms whereby R7BP controls the function of the R7 family. We show that unpalmitoylated R7BP undergoes nuclear/cytoplasmic shuttling and that a C-terminal polybasic motif proximal to the palmitoylation acceptor sites of R7BP mediates nuclear localization, palmitoylation, and plasma membrane targeting. These results suggest a novel mechanism whereby palmitoyltransferases and nuclear import receptors both utilize the C-terminal domain of R7BP to determine the trafficking fate of R7*Gbeta5*R7BP heterotrimers. Analogous mechanisms may regulate other signaling proteins whose distribution between the plasma membrane and nucleus is controlled by palmitoylation. Lastly, we show that cytoplasmic RGS7*Gbeta5*R7BP heterotrimers and RGS7*Gbeta5 heterodimers are equivalently inefficient regulators of G protein-coupled receptor signaling relative to plasma membrane-bound heterotrimers bearing palmitoylated R7BP. Therefore, R7BP augments the function of the complex by a palmitoylation-regulated plasma membrane-targeting mechanism.

Drosophila melanogaster Scramblases modulate synaptic transmission

Scramblases are a family of single-pass plasma membrane proteins, identified by their purported ability to scramble phospholipids across the two layers of plasma membrane isolated from platelets and red blood cells. However, their true in vivo role has yet to be elucidated. We report the generation and isolation of null mutants of two Scramblases identified in Drosophila melanogaster. We demonstrate that flies lacking either or both of these Scramblases are not compromised in vivo in processes requiring scrambling of phospholipids. Instead, we show that D. melanogaster lacking both Scramblases have more vesicles and display enhanced recruitment from a reserve pool of vesicles and increased neurotransmitter secretion at the larval neuromuscular synapses. These defects are corrected by the introduction of a genomic copy of the Scramb 1 gene. The lack of phenotypes related to failure of scrambling and the neurophysiological analysis lead us to propose that Scramblases play a modulatory role in the process of neurotransmission.

PDR16-mediated azole resistance in Candida albicans

Many Candida albicans azole-resistant (AR) clinical isolates overexpress the CDR1 and CDR2 genes encoding homologous multidrug transporters of the ATP-binding cassette family. We show here that these strains also overexpress the PDR16 gene, the orthologue of Saccharomyces cerevisiae PDR16 encoding a phosphatidylinositol transfer protein of the Sec14p family. It has been reported that S. cerevisiae pdr16Delta mutants are hypersusceptible to azoles, suggesting that C. albicans PDR16 may contribute to azole resistance in these isolates. To address this question, we deleted both alleles of PDR16 in an AR clinical strain overexpressing the three genes, using the mycophenolic acid resistance flipper strategy. Our results show that the homozygous pdr16Delta/pdr16Delta mutant is approximately twofold less resistant to azoles than the parental strain whereas reintroducing a copy of PDR16 in the mutant restored azole resistance, demonstrating that this gene contributes to the AR phenotype of the cells. In addition, overexpression of PDR16 in azole-susceptible (AS) C. albicans and S. cerevisiae strains increased azole resistance by about twofold, indicating that an increased dosage of Pdr16p can confer low levels of azole resistance in the absence of additional molecular alterations. Taken together, these results demonstrate that PDR16 plays a role in C. albicans azole resistance.

TheDrosophilaphosphatidylinositol transfer protein encoded byvibratoris essential to maintain cleavage-furrow ingression in cytokinesis

Cytokinesis requires the coordination of cytoskeletal and plasma membrane dynamics. A role for phosphatidylinositol lipids has been proposed for the successful completion of cytokinesis but this is still poorly characterised. Here, we show mutants of the gene vibrator, previously found to encode the Drosophila phosphatidylinositol transfer protein, produce multinucleate cells indicative of cytokinesis failure in male meiosis. Examination of fixed preparations of mutant spermatocytes showed contractile rings of anillin and actin that were of normal appearance at early stages but were larger and less well organised at later stages of cytokinesis than in wild-type cells. Time-lapse imaging revealed sequential defects in cytokinesis of vibrator spermatocytes. In cells that fail cytokinesis, central spindle formation occurred correctly, but furrow ingression was delayed and the central spindle did not become compressed to the extent seen in wild-type cells. Cells then stalled at this point before the apparent connection between the constricted cytoskeleton and the plasma membrane was lost; the furrow then underwent elastic regression. We discuss these defects in relation to multiple functions of phosphoinositol lipids in regulating actin dynamics and membrane synthesis.

Sec14p-like proteins regulate phosphoinositide homoeostasis and intracellular protein and lipid trafficking in yeast

The major PI (phosphatidylinositol)/PC (phosphatidylcholine)-transfer protein in yeast, Sec14p, co-ordinates lipid metabolism with protein transport from the Golgi complex. Yeast also express five additional gene products that share 24–65% primary sequence identity with Sec14p. These Sec14p-like proteins are termed SFH (Sec Fourteen Homologue) proteins, and overexpression of certain individual SFH gene products rescues sec14-1ts-associated growth and secretory defects. SFH proteins are atypical in that these stimulate the transfer of PI, but not PC, between distinct membrane bilayer systems in vitro. Further analysis reveals that SFH proteins functionally interact with the Stt4p phosphoinositide 4-kinase to stimulate PtdIns(4,5)P2 synthesis which in turn activates phospholipase D. Finally, genetic analyses indicate that Sfh5p interfaces with the function of specific subunits of the exocyst complex as well as the yeast SNAP-25 (25 kDa synaptosome-associated protein) homologue, Sec9p. Our current view is that Sfh5p regulates PtdIns(4,5)P2 homoeostasis at the plasma membrane, and that Sec9p responds to that regulation. Thus SFH proteins individually regulate specific aspects of lipid metabolism that couple, with exquisite specificity, with key cellular functions.

Phosphatidylinositol-transfer protein and its homologues in yeast

Yeast Sec14p acts as a phosphatidylinositol/phosphatidylcholine-transfer protein in vitro. In vivo, it is essential in promoting Golgi secretory function. Products of five genes named SFH1–SFH5 (Sec Fourteen Homologues 1–5) exhibit significant sequence homology to Sec14p and together they form the Sec14p family of lipid-transfer proteins. It is a diverse group of proteins with distinct subcellular localizations and varied physiological functions related to lipid metabolism and membrane trafficking.

Antileukemic roles of human phospholipid scramblase 1 gene, evidence from inducible PLSCR1-expressing leukemic cells

Phospholipid scramblase 1 (PLSCR1) is a multiply palmitoylated protein which is localized in either the cell membrane or nucleus depending on its palmitoylated state. The increasing evidence showed the biological roles of PLSCR1 in cell signaling, maturation and apoptosis. To investigate the functions of PLSCR1 in leukemic cells, we generated an inducible PLSCR1-expressing cell line using myeloid leukemic U937 cells. In this cell line, PLSCR1 was tightly regulated and induced upon tetracycline withdrawal. Our results showed that inducible PLSCR1 expression arrested the proliferation of U937 cells at G1 phase. Meanwhile, PLSCR1-overexpressing U937 cells also underwent granulocyte-like differentiation with increased sensitivity to etoposide-induced apoptosis. Furthermore, we also found that PLSCR1 induction increased cyclin-dependent kinase inhibitors p27(Kip1) and p21(Cip1) proteins, together with downregulation of S phase kinase-associated protein 2 (SKP2), an F-box subunit of the ubiquitin-ligase complex that targets proteins for degradation. Additionally, PLSCR1 induction significantly decreased c-Myc protein and antiapoptotic Bcl-2 protein. Although the exact mechanism by which PLSCR1 regulates these cellular events and gene expression remains unresolved, our results suggest that PLSCR1 plays the antagonistic role regarding leukemia development. These data will shed new insights into understanding the biochemical and biological functions of PLSCR1 protein.

Absence of endogenous phospholipid transfer protein impairs ABCA1-dependent efflux of cholesterol from macrophage foam cells

In vitro experiments have demonstrated that exogenous phospholipid transfer protein (PLTP), i.e. purified PLTP added to macrophage cultures, influences ABCA1-mediated cholesterol efflux from macrophages to HDL. To investigate whether PLTP produced by the macrophages (i.e., endogenous PLTP) is also part of this process, we used peritoneal macrophages derived from PLTP-knockout (KO) and wild-type (WT) mice. The macrophages were transformed to foam cells by cholesterol loading, and this resulted in the upregulation of ABCA1. Such macrophage foam cells from PLTP-KO mice released less cholesterol to lipid-free apolipoprotein A-I (apoA-I) and to HDL than did the corresponding WT foam cells. Also, when plasma from either WT or PLTP-KO mice was used as an acceptor, cholesterol efflux from PLTP-KO foam cells was less efficient than that from WT foam cells. After cAMP treatment, which upregulated the expression of ABCA1, cholesterol efflux from PLTP-KO foam cells to apoA-I increased markedly and reached a level similar to that observed in cAMP-treated WT foam cells, restoring the decreased cholesterol efflux associated with PLTP deficiency. These results indicate that endogenous PLTP produced by macrophages contributes to the optimal function of the ABCA1-mediated cholesterol efflux-promoting machinery in these cells. Whether macrophage PLTP acts at the plasma membrane or intracellularly or shuttles between these compartments needs further study.

Sulphydryl modifications alter scramblase activity in murine sickle cell disease

Increased oxidant stress has been suggested to play a role in the process of phosphatidylserine (PS) externalization in the red blood cells of sickle cell patients. Inhibition of the ATP-driven translocation from outer to inner monolayer (flippase) by sulphydryl modification has been established. The present study showed that phospholipid scrambling was also sensitive to protein sulphydryl modification. Treatment with N-ethylmaleimide lead to enhanced PS exposure and a lower Ca(++) requirement for scrambling. In contrast, pyridyldithioethylamine treatment inhibited PS exposure. Red blood cells from a murine model for sickle cell disease exhibited a reduced response to both reagents, suggestive of previous sulphydryl modifications to the protein(s) involved in phospholipid scrambling. We conclude that sulphydryl modifications to both scramblase and flippase underlie the enhanced formation of PS-exposing cells in sickle cell disease.

Mutational analysis of the Lem3p-Dnf1p putative phospholipid-translocating P-type ATPase reveals novel regulatory roles for Lem3p and a carboxyl-terminal region of Dnf1p independent of the phospholipid-translocating activity of Dnf1p in yeast

Lem3p-Dnf1p is a putative aminophospholipid translocase (APLT) complex that is localized to the plasma membrane; Lem3p is required for Dnf1p localization to the plasma membrane. We have identified lem3 mutations, which did not affect formation or localization of the Lem3p-Dnf1p complex, but caused a synthetic growth defect with the null mutation of CDC50, a structurally and functionally redundant homologue of LEM3. Interestingly, these lem3 mutants exhibited nearly normal levels of NBD-labeled phospholipid internalization across the plasma membrane, suggesting that Lem3p may have other functions in addition to regulation of the putative APLT activity of Dnf1p at the plasma membrane. Similarly, deletion of the COOH-terminal cytoplasmic region of Dnf1p affected neither the localization nor the APLT activity of Dnf1p at the plasma membrane, but caused a growth defect in the cdc50Delta background. Our results suggest that the Lem3p-Dnf1p complex may play a role distinct from its plasma membrane APLT activity when it substitutes for the Cdc50p-Drs2p complex, its redundant partner in the endosomal/trans-Golgi network compartments.

Phosphatidylserine and signal transduction: who needs whom?

The plasma membrane, long considered a simple barrier between the extracellular and intracellular compartments, is now thought to play a pivotal role in many physiological processes that regulate the communication of cells with their environment. On one hand, the plasma membrane directly participates in intracellular signaling; on the other hand, changes in membrane structure contribute to the transcellular transfer of biological information. Among the membrane constituents, phosphatidylserine is a major actor implicated in these effects. Evidence now exists for a role for phosphatidylserine redistribution in modulating the activities of several membrane proteins during signaling in nonapoptotic T lymphocytes.