Lysophosphatidic acid and sphingosine 1-phosphate biology: the role of lipid phosphate phosphatases

Sphingosine-1-phosphate lyase in immunity and cancer: silencing the siren

Sphingosine-1-phosphate (S1P) is a bioactive lipid that promotes cell survival, proliferation and migration, platelet aggregation, mediates ischemic preconditioning, and is essential for angiogenesis and lymphocyte trafficking. Sphingosine-1-phosphate lyase (SPL) is the enzyme responsible for the irreversible degradation of S1P and is, thus, in a strategic position to regulate these same processes by removing available S1P signaling pools, that is, silencing the siren. In fact, recent studies have implicated SPL in the regulation of immunity, cancer surveillance and other physiological processes. Here, we summarize the current understanding of SPL function and regulation, and discuss how SPL might facilitate cancer chemoprevention and serve as a target for modulation of immune responses in transplantation settings and in the treatment of autoimmune disease.

The formation of TGN-to-plasma-membrane transport carriers

In the trans-Golgi network (TGN), proteins are sorted for transport to the endosomes, plasma membrane, preceding Golgi cisternae, and endoplasmic reticulum. The formation of clathrin-coated vesicles for transport to the endosomes and of COP-I-coated vesicles for retrograde trafficking is fairly well characterized at the molecular level. We describe our current understanding of the TGN-to-cell-surface carriers, with a specific focus on the components involved in membrane fission. Inhibiting the fission machinery promotes growth of transport carriers into large tubules that remain attached to the TGN. Overactivating this machinery, on the other hand, vesiculates the TGN. To understand how membrane fission is regulated by cargo to form transport carriers yet prevents complete vesiculation of the TGN remains a daunting challenge. We discuss these issues with regard to TGN-to-cell-surface transport carriers.

Lysophospholipid receptors: signalling, pharmacology and regulation by lysophospholipid metabolism

The lysophospholipids, sphingosine-1-phosphate (S1P), lysophosphatidic acid (LPA), sphingosylphosphorylcholine (SPC) and lysophosphatidylcholine (LPC), activate diverse groups of G-protein-coupled receptors that are widely expressed and regulate decisive cellular functions. Receptors of the endothelial differentiation gene family are activated by S1P (S1P(1-5)) or LPA (LPA(1-3)); two more distantly related receptors are activated by LPA (LPA(4/5)); the GPR(3/6/12) receptors have a high constitutive activity but are further activated by S1P and/or SPC; and receptors of the OGR1 cluster (OGR1, GPR4, G2A, TDAG8) appear to be activated by SPC, LPC, psychosine and/or protons. G-protein-coupled lysophospholipid receptors regulate cellular Ca(2+) homoeostasis and the cytoskeleton, proliferation and survival, migration and adhesion. They have been implicated in development, regulation of the cardiovascular, immune and nervous systems, inflammation, arteriosclerosis and cancer. The availability of S1P and LPA at their G-protein-coupled receptors is regulated by enzymes that generate or metabolize these lysophospholipids, and localization plays an important role in this process. Besides FTY720, which is phosphorylated by sphingosine kinase-2 and then acts on four of the five S1P receptors of the endothelial differentiation gene family, other compounds have been identified that interact with more ore less selectivity with lysophospholipid receptors.

Essential roles of sphingosine 1-phosphate/S1P1 receptor axis in the migration of neural stem cells toward a site of spinal cord injury

Neural stem/progenitor cells (NSPCs) migrate toward a damaged area of the central nervous system (CNS) for the purpose of limiting and/or repairing the damage. Although this migratory property of NSPCs could theoretically be exploited for cell-based therapeutics of CNS diseases, little is known of the mechanisms responsible for migratory responses of NSPCs. Here, we found that sphingosine 1-phosphate (Sph-1-P), a physiological lysophospholipid mediator, had a potent chemoattractant activity for NSPCs, in which, of Sph-1-P receptors, S1P(1) was abundantly expressed. Sph-1-P-induced NSPC migration was inhibited by the pretreatment with pertussis toxin, Y-27632 (a Rho kinase inhibitor), and VPC23019 (a competitive inhibitor of S1P(1) and S1P(3)). Sph-1-P does not act as intracellular mediator or in an autocrine manner, because [(3)H]sphingosine, incorporated into NSPCs, was mainly converted to ceramide and sphingomyeline intracellularly, and the stimulation-dependent formation and extracellular release of Sph-1-P were not observed. Further, Sph-1-P concentration in the spinal cord was significantly increased at 7 days after a contusion injury, due to accumulation of microglia and reactive astrocytes in the injured area. This locally increased Sph-1-P concentration contributed to the migration of in vivo transplanted NSPCs through its receptor S1P(1), given that lentiviral transduction of NSPCs with a short hairpin RNA interference for S1P(1) abolished in vivo NSPC migration toward the injured area. This is the first report to identify a physiological role for a lipid mediator in NSPC migration toward a pathological area of the CNS and further indicates that the Sph-1-P/S1P(1) pathway may have therapeutic potential for CNS injuries.

Inhibitors of sphingolipid metabolism enzymes

Sphingolipids are a family of lipids that play essential roles both as structural cell membrane components and in cell signalling. The cellular contents of the various sphingolipid species are controlled by enzymes involved in their metabolic pathways. In this context, the discovery of small chemical entities able to modify these enzyme activities in a potent and selective way should offer new pharmacological tools and therapeutic agents.

Integrin signalling regulates the nuclear localization and function of the lysophosphatidic acid receptor-1 (LPA1) in mammalian cells

We show that LPA1 (lysophosphatidic acid receptor-1) is constitutively localized in the nucleus of mammalian cells. LPA1 also traffics from cell membranes to the nucleus in response to LPA (lysophosphatidic acid). Several lines of evidence suggest an important role for cell-matrix interaction in regulating the constitutive nuclear localization of LPA1. First, the RGDS peptide, which blocks cell matrix-induced integrin clustering and cytoskeletal rearrangement, reduced the number of cells containing LPA1 in the nucleus. Secondly, a higher proportion of cells contained nuclear LPA1 when adhesion on fibronectin-coated glass was compared with adherence to polylysine-coated glass. Thirdly, pre-treatment of cells with the Rho kinase inhibitor (Y27632) or the myosin light chain kinase inhibitor (ML9) reduced the number of cells containing nuclear LPA1. The addition of LPA and/or Ki16425 (which binds to LPA1) to isolated nuclei containing LPA1 induced the phosphorylation of several proteins with molecular masses of 34, 32, 14 and 11 kDa. These findings demonstrate that trafficking of LPA1 to the nucleus is influenced by cell-matrix interactions and that nuclear LPA1 may be involved in regulating intranuclear protein phosphorylation and signalling.

Thematic review series: systems biology approaches to metabolic and cardiovascular disorders. Lipidomics: a global approach to lipid analysis in biological systems

Lipids are water-insoluble molecules that have a wide variety of functions within cells, including: 1) maintenance of electrochemical gradients; 2) subcellular partitioning; 3) first- and second-messenger cell signaling; 4) energy storage; and 5) protein trafficking and membrane anchoring. The physiological importance of lipids is illustrated by the numerous diseases to which lipid abnormalities contribute, including atherosclerosis, diabetes, obesity, and Alzheimer's disease. Lipidomics, a branch of metabolomics, is a systems-based study of all lipids, the molecules with which they interact, and their function within the cell. Recent advances in soft-ionization mass spectrometry, combined with established separation techniques, have allowed the rapid and sensitive detection of a variety of lipid species with minimal sample preparation. A "lipid profile" from a crude lipid extract is a mass spectrum of the composition and abundance of the lipids it contains, which can be used to monitor changes over time and in response to particular stimuli. Lipidomics, integrated with genomics, proteomics, and metabolomics, will contribute toward understanding how lipids function in a biological system and will provide a powerful tool for elucidating the mechanism of lipid-based disease, for biomarker screening, and for monitoring pharmacologic therapy.

The effect of hypoxia on lipid phosphate receptor and sphingosine kinase expression and mitogen-activated protein kinase signaling in human pulmonary smooth muscle cells

Both acute and chronic hypoxia had no effect on S1P(1), S1P(3) or LPA(1) receptor transcript expression in human pulmonary smooth muscle cells. However, acute hypoxia increased sphingosine kinase SK1/2 and LPP1 mRNA transcript levels, while chronic hypoxia increased SK1 mRNA transcript alone. Acute hypoxia had no effect on S1P-, PDGF- or phorbol ester (PMA)-stimulated activation of ERK-1/2, but increased the ability of S1P to activate p38 MAPK. Chronic hypoxia increased the ability of S1P to stimulate the phosphorylation of ERK-1/2. Therefore, we have demonstrated for the first time that hypoxia can lead to marked changes in the expression of genes involved in S1P production and may modify post S1P receptor signal transduction pathways.

Sphingosine-1-phosphate inhibits cell migration and endothelial to mesenchymal cell transformation during cardiac development

Sphingosine-1-phosphate (S1P) is a biologically active sphingolipid metabolite that exerts important effects on numerous cellular events via cell surface receptors, S1P(1-5). S1P influences differentiation, proliferation, and migration during vascular development. However, the effects of S1P signaling on early cardiac development are not well understood. To address this issue, we examined the expression of S1P regulatory enzymes and S1P receptors during cardiac development. We observed that enzymes that regulate S1P levels, sphingosine kinase and sphingosine-1-phosphate phosphatase, are expressed in the developing heart. In addition, RT-PCR revealed that four of the five known S1P receptors (S1P(1-4)) are also expressed in the developing heart. Next, effects of altered S1P levels on whole embryo and atrioventricular (AV) canal cultures were investigated. We demonstrate that inactivation of the S1P producing enzyme, sphingosine kinase, leads to cell death in cardiac tissue which is rescued by exogenous S1P treatment. Other experiments reveal that increased S1P concentration prevents alterations in cell morphology that are required for cell migration. This effect results in reduced cell migration and inhibited mesenchymal cell formation in AV canal cushion tissue. These data indicate that S1P, locally maintained within a specific concentration range, is an important and necessary component of early heart development.

Lipid phosphate phosphatases and lipid phosphate signalling

Mammalian LPPs (lipid phosphate phosphatases) are integral membrane proteins that belong to a superfamily of lipid phosphatases/phosphotransferases. They have broad substrate specificity in vitro, dephosphorylating PA (phosphatidic acid), S1P (sphingosine 1-phosphate), LPA (lysophosphatidic acid) etc. Their physiological role may include the attenuation of S1P- and LPA-stimulated signalling by virtue of an ecto-activity (i.e. dephosphorylation of extracellular S1P and LPA), thereby limiting the activation of LPA- and S1P-specific G-protein-coupled receptors at the cell surface. However, our recent work suggests that an intracellular action of LPP2 and LPP3 may account for the reduced agonist-stimulated p42/p44 mitogen-activated protein kinase activation of HEK-293 (human embryonic kidney 293) cells. This may involve a reduction in the basal levels of PA and S1P respectively and the presence of an early apoptotic phenotype under conditions of stress (serum deprivation). Additionally, we describe a model whereby LPP2, but not LPP3, may be functionally linked to the phospholipase D1-derived PA-dependent recruitment of sphingosine kinase 1 to the perinuclear compartment. We also consider the potential regulatory mechanisms for LPPs, which may involve oligomerization. Lastly, we highlight many aspects of the LPP biology that remain to be fully defined.