Sphingosine 1-phosphate is a key metabolite linking sphingolipids to glycerophospholipids

The sphingolipid metabolite sphingosine 1-phosphate (S1P) is a well-known lipid mediator. As a lipid mediator, S1P must be present in extracellular space and bind to its cell surface receptors (S1P_1–5). However, most S1P, synthesized intracellularly, is metabolized without being released into extracellular space, in other words, without functioning as a lipid mediator in the vast majority of cells except those supplying plasma and lymph S1P such as blood cells and endothelial cells. Instead, intracellular S1P plays an important role as an intermediate of the sole sphingolipid-to-glycerophospholipid metabolic pathway. The degradation of S1P by S1P lyase is the first irreversible reaction (committed step) of this pathway. This metabolic pathway is conserved in eukaryotes from yeast to human, indicating its much older origin than the function of S1P as a lipid mediator, which is found to be present only in vertebrates and chordates. The sphingolipid-to-glycerophospholipid metabolism takes place ubiquitously in mammalian tissues, and its defect causes an aberration of several tissue functions as well as abnormal lipid metabolism. Although this metabolic pathway has been known for over four decades, only recently the precise reactions and enzymes involved in this pathway have been revealed. This review will focus on the recent advances in our understanding of the sphingolipid metabolic pathway via S1P and its physiological and pathological roles. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.     

Physiological and pathological actions of sphingosine 1-phosphate

Sphingosine 1-phosphate (S1P), a product of sphingomyelin (SM) metabolism, occurs widely in nature. Although, originally described as an intracellular second messenger, its role as an extracellular lipid mediator in higher organisms has recently been shown with the discovery of the G protein-coupled receptors (GRCR) for S1P. In mammals, S1P receptors are widely expressed and are thought to regulate important physiological actions, such as immune cell trafficking, vascular development, vascular tone control, cardiac function, and vascular permeability, among others. In addition, S1P may participate in various pathological conditions. For example, S1P has been implicated as an important mediator in autoimmunity, transplant rejection, cancer, angiogenesis, vascular permeability, female infertility, and myocardial infarction. It is important to emphasize that these findings represent an early understanding of the physiological and pathological roles of S1P. The ubiquity of the mediator and its receptors, as well as the evolutionary conservation of S1P metabolism and action, argues that it is a potent and ubiquitous physiological factor in many contexts, and warrant a fuller understanding of its actions at the molecular, cellular and organismal levels.     

Cyclic AMP-lowering mediator of insulin

What appears to be a mediator of insulin action has been successfully produced in rat adipocytes plasma membrane upon its treatment with insulin at concentrations of 50-200 microunits/ml. This apparent mediator, when isolated and presented to adipocyte cells, mimics insulin action in the lowering of hormonally stimulated cAMP levels as well as in stimulating lipogenesis and antilipolysis. The cAMP-lowering activity of such a mediator can be quantitated as insulin-activity equivalents. Insulin at 200 microunits/ml causes, in terms of insulin-activity equivalents, a generation of as much as 50 times more of insulin mediator. The magnitude of amplification is even greater when the amount of insulin bound to its receptor is taken into account in this calculation. The action of insulin in the generation of cAMP-lowering mediator is abolished by the insulin antibody. Inactive insulin analogs do not effectively generate such a mediator activity. On the other hand, while the cAMP-lowering action of insulin shown in the bioassay system is completely inhibited by the insulin antibody, the action of such a mediator is only slightly inhibited by the same antibody. The mediator has a low molecular weight and attributes that resemble a peptide. It can be separated from insulin in a Sephadex G-25 column and has a molecular size smaller than the insulin A-chain but larger than ATP. The molecular weight of this mediator is similar to the insulin mediators isolated by other investigators. In view of the fact that it is small in size and mimics several actions of insulin when used in extracellular situations, its theoretical, as well as practical, implications are substantial.     

Dpb11 coordinates Mec1 kinase activation with cell cycle-regulated Rad9 recruitment

Eukaryotic cells respond to DNA damage by activating checkpoint signalling pathways. Checkpoint signals are transduced by a protein kinase cascade that also requires non-kinase mediator proteins. One such mediator is the Saccharomyces cerevisiae Dpb11 protein, which binds to and activates the apical checkpoint kinase, Mec1. Here, we show that a ternary complex of Dpb11, Mec1 and another key mediator protein Rad9 is required for efficient Rad9 phosphorylation by Mec1 in vitro, and for checkpoint activation in vivo. Phosphorylation of Rad9 by cyclin-dependent kinase (CDK) on two key residues generates a binding site for tandem BRCT repeats of Dpb11, and is thereby required for Rad9 recruitment into the ternary complex. Checkpoint signalling via Dpb11, therefore, does not efficiently occur during G1 phase when CDK is inactive. Thus, Dpb11 coordinates checkpoint signal transduction both temporally and spatially, ensuring the initiator kinase is specifically activated in proximity of one of its critical substrates.     

Insulin mediator stimulation of pyruvate dehydrogenase phosphatases.

A two stage assay for detecting insulin mediator based upon its stimulation of soluble pyruvate dehydrogenase (PDH) phosphatase to activate soluble pyruvate dehydrogenase complex (PDC) has been developed. This coupled assay determines the activation of PDC by monitoring production of [14C]CO2 from [1-14C]pyruvic acid. In addition to being more sensitive than the rat liver mitoplast assay previously used, it allows for the separation and investigation of the effects of mediator on the PDH phosphatases individually. It has been previously shown that the insulin mediator stimulates the most abundant PDH phosphatase, the divalent cation dependent PDH phosphatase, by decreasing the phosphatase's metal requirement (1). A metal independent PDH phosphatase has been found in bovine heart mitochondria. This phosphatase is not immunoprecipitated by antiphosphatase 2A antibody, it is not inhibited by okadaic acid, and it is not stimulated by spermine. However, it is stimulated (more than threefold) by insulin mediator prepared from isolated rat liver membranes. It is inhibited by Mg-ATP, with half-maximal inhibition at 0.3 mM; however, this inhibition is overcome by the insulin mediator.     

Modulation of adaptive immune responses by sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) has long been recognized as a mediator of a variety of cell functions. A growing body of evidence has accumulated demonstrating its role in cell migration and as a mediator of growth factor-induced events. In recent years, it has become apparent that S1P also mediates many cytokine and chemokine functions. Cells of the immune system function and migrate in response to a complex network of cytokines and chemokines, and the outcome is determined by the interplay of the effects of these molecules on the target cell. S1P may be a bona fide component of these networks and influence the responses of cells to these immune modulators.     

PGI2: a potential mediator of inflammation.

PGI2, but not its metabolite 6oxoPGF1alpha, is equivalent in potency to PGE1 as a potentiator of carrageenan, histamine and bradykinin-induced rat paw oedemas. PGI2 must, therefore, be considered as a potential mediator of inflammatory processes.     

Understanding the connection between platelet-activating factor,a UV-induced lipid mediator of inflammation,immune suppression and skin cancer

Lipid mediators of inflammation play important roles in several diseases including skin cancer, the most prevalent type of cancer found in the industrialized world. Ultraviolet (UV) radiation is a complete carcinogen and is the primary cause of skin cancer. UV radiation is also a potent immunosuppressive agent, and UV-induced immunosuppression is a well-known risk factor for skin cancer induction. An essential mediator in this process is the glyercophosphocholine 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine commonly referred to as platelet-activating factor (PAF). PAF is produced by keratinocytes in response to diverse stimuli and exerts its biological effects by binding to a single specific G-protein-coupled receptor (PAF-R) expressed on a variety of cells. This review will attempt to describe how this lipid mediator is involved in transmitting the immunosuppressive signal from the skin to the immune system, starting from its production by keratinocytes, to its role in activating mast cell migration in vivo, and to the mechanisms involved that ultimately lead to immune suppression. Recent findings related to its role in regulating DNA repair and activating epigenetic mechanisms, further pinpoint the importance of this bioactive lipid, which may serve as a critical molecular mediator that links the environment (UVB radiation) to the immune system and the epigenome.     

A proteolytic mechanism for the action of insulin via oligopeptide mediator formation

Evidence is presented that the chemical mediator of insulin action is a peptide(s) and most likely glycopeptide(s). The mediator is formed proteolytically because 1) protease inhibitors inhibit insulin action and 2) trypsin mimicks insulin action via mediator formation. Trypsin mediator does not faithfully reproduce the action of insulin mediator, which indicates that the sites of proteolytic cleavage by insulin and trypsin differ. A coordinated multivalent proteolytic mechanism by which insulin acts to trigger an external membrane-bound protease to cleave mediator from a membrane glycoprotein precursor is presented.     

Plasma lipoproteins behave as carriers of extracellular sphingosine 1-phosphate: is this an atherogenic mediator or an anti-atherogenic mediator?

Sphingosine 1-phosphate (S1P) concentration in plasma and serum has been estimated to be within 200-900 nM. Among plasma and serum components, S1P is concentrated in lipoprotein fractions with a rank order of high-density lipoprotein (HDL)>low-density lipoprotein (LDL)>very low-density lipoprotein (VLDL)>lipoprotein-deficient plasma (LPDP) when expressed as the per unit amount of protein. It is well known that LDL, especially oxidized LDL, is closely correlated and HDL is inversely correlated, with the risk of cardiovascular disease, such as atherosclerosis. Evidence was presented that a part of HDL-induced actions previously reported are mediated by the lipoprotein-associated S1P. Furthermore, S1P content in LDL was markedly decreased during its oxidation. This paper will discuss whether S1P is an atherogenic mediator or an anti-atherogenic mediator.     

The critical role of LIGHT in promoting intestinal inflammation and Crohn's disease

Crohn's disease (CD) is a type of inflammatory bowel disease associated with increased Th1 cytokines and unique pathological features. However, its pathogenesis has not been fully understood. Previous studies showed that homologous to lymphotoxin, exhibits inducible expression, competes with herpesvirus glycoprotein D for HVEM on T cells (LIGHT) transgenic (Tg) mice develop autoimmunity including intestinal inflammation with a variable time course. In this study, we establish an experimental model for CD by adoptive transfer of Tg mesenteric lymph node cells into RAG(-/-) mice. The recipients of Tg lymphocytes rapidly develop a disease strikingly similar to the key pathologic features and cytokine characterization observed in CD. We demonstrate that, as a costimulatory molecule, LIGHT preferentially drives Th1 responses. LIGHT-mediated intestinal disease is dependent on both of its identified signaling receptors, lymphotoxin beta receptor and herpes virus entry mediator, because LIGHT Tg mesenteric lymph node cells do not cause intestinal inflammation when transferred into the lymphotoxin beta receptor-deficient mice, and herpes virus entry mediator on donor T cells is required for the full development of disease. Furthermore, we demonstrated that up-regulation of LIGHT is associated with active CD. These data establish a new mouse model resembling CD and suggest that up-regulation of LIGHT may be an important mediator of CD pathogenesis.     

APPL1: role in adiponectin signaling and beyond

Adiponectin, an adipokine secreted by the white adipose tissue, plays an important role in regulating glucose and lipid metabolism and controlling energy homeostasis in insulin-sensitive tissues. A decrease in the circulating level of adiponectin has been linked to insulin resistance, type 2 diabetes, atherosclerosis, and metabolic syndrome. Adiponectin exerts its effects through two membrane receptors, AdipoR1 and AdipoR2. APPL1 is the first identified protein that interacts directly with adiponectin receptors. APPL1 is an adaptor protein with multiple functional domains, the Bin1/amphiphysin/rvs167, pleckstrin homology, and phosphotyrosine binding domains. The PTB domain of APPL1 interacts directly with the intracellular region of adiponectin receptors. Through this interaction, APPL1 mediates adiponectin signaling and its effects on metabolism. APPL1 also functions in insulin-signaling pathway and is an important mediator of adiponectin-dependent insulin sensitization in skeletal muscle. Adiponectin signaling through APPL1 is necessary to exert its anti-inflammatory and cytoprotective effects on endothelial cells. APPL1 also acts as a mediator of other signaling pathways by interacting directly with membrane receptors or signaling proteins, thereby playing critical roles in cell proliferation, apoptosis, cell survival, endosomal trafficking, and chromatin remodeling. This review focuses mainly on our current understanding of adiponectin signaling in various tissues, the role of APPL1 in mediating adiponectin signaling, and also its role in the cross-talk between adiponectin/insulin-signaling pathways.     

The role of apoptosis signal-regulating kinase 1 in lymphotoxin-beta receptor-mediated cell death

LIGHT (homologous to lymphotoxins, shows inducible expression, and competes with herpes simplex virus glycoprotein D for herpesvirus entry mediator, a receptor expressed by T lymphocytes) is a member of the tumor necrosis factor superfamily that can interact with lymphotoxin-beta receptor (LTbetaR), herpes virus entry mediator, and decoy receptor (DcR3). In our previous study, we showed that LIGHT is able to induce cell death via the non-death domain containing receptor LTbetaR to activate both caspase-dependent and caspase-independent pathway. In this study, a LIGHT mutein, LIGHT-R228E, was shown to exhibit similar binding specificity as wild type LIGHT to LTbetaR, but lose the ability to interact with herpes virus entry mediator. By using both LIGHT-R228E and agonistic anti-LTbetaR monoclonal antibody, we found that signaling triggered by LTbetaR alone is sufficient to activate both caspase-dependent and caspase-independent pathways. Cross-linking of LTbetaR is able to recruit TRAF3 and TRAF5 to activate ASK1, whereas its activity is inhibited by free radical scavenger carboxyfullerenes. The activation of ASK1 is independent of caspase-3 activation, and kinase-inactive ASK1-KE mutant can inhibit LTbetaR-mediated cell death. This suggests that ASK1 is one of the factors involved in the caspase-independent pathway of LTbetaR-induced cell death.