The Lipid Library

SPHINGOSINE 1-PHOSPHATE

STRUCTURE, OCCURRENCE, BIOSYNTHESIS AND ANALYSIS

1.  Occurrence and Biosynthesis

Sphingosine 1-phosphate is an important cellular metabolite, derived from ceramide that is synthesized de novo or as part of the sphingomyelin cycle (in animal cells). It has also been found in insects, yeasts and plants.

The enzyme ceramidase acts upon ceramides to release sphingosine, and this is phosphorylated by sphingosine kinase, a ubiquitous enzyme in the cytosol and endoplasmic reticulum, to form sphingosine 1-phosphate. The reverse reaction can occur also by the action of sphingosine phosphatases, and the enzymes act in concert to control the cellular concentrations of the metabolite, which are always low.

Recent work suggests that there are in fact two sphingosine kinases, designated Types 1 and 2. They are part of a super-family of enzymes that includes ceramide kinase and diacylglycerol kinase. The type 1 sphingosine kinase is predominantly cytosolic and pro-survival, probably by inhibiting ceramide biosynthesis. The enzyme may also be exported to the cell nucleus. The Type 2 enzyme functions mainly in the endoplasmic reticulum and stimulates apoptosis, but its role is still poorly understood. It appears that the cellular location of sphingosine 1-phosphate production may dictate its functions. Sphingosine 1-phosphate may also be synthesised on the inner leaflet of the plasma membrane, and it is able to cross this via the action of at least two transporter proteins to interact with specific receptors.

In cells, sphingosine 1-phosphate occurs at concentrations in the low nanomolar range. However, in plasma, it can reach a concentration of 0.2 to 0.9 µM, and it is found in association with the lipoproteins, especially the high-density lipoproteins (HDL). It is stored in relatively high concentrations in human platelets and erythrocytes, especially the latter, where there are highly active sphingosine kinases and a lack of the appropriate degradative enzymes.

2.  Biological Functions

Like its precursors, sphingosine 1-phosphate is a potent messenger molecule that perhaps uniquely operates both intra- and inter-cellularly, but with very different functions from ceramides, ceramide 1-phosphate and sphingosine. The balance between these various sphingolipid metabolites is important for health and has sometimes been termed the 'sphingolipid rheostat', although the readiness with which each of them can be interconverted does make it difficult to determine the true function of each. For example, within the cell in contrast to ceramide, sphingosine 1-phosphate promotes cellular division (mitosis) as opposed to cell death (apoptosis), which it inhibits in fact. However, it can also enhance apoptosis in some circumstances. Intracellularly, sphingosine 1-phosphate functions also to regulate calcium mobilization and cell growth in response to a variety of extracellular stimuli.

Like the lysophospholipids, especially lysophosphatidic acid with which it has some structural similarities, sphingosine 1-phosphate exerts many of its extra-cellular effects through acting as a ligand for specific receptors, in this instance five G protein-coupled receptors on cell surfaces (designated S1P₁ to S1P₅). Both sphingosine 1-phosphate and its dihydro analogue bind to them with a high affinity. In mammals, S1P₁, S1P₂, and S1P₃ are found in all tissues, whereas S1P₄ is restricted to lymphoid tissues and lung, and S1P₅ to brain and skin. The ligand-receptor interactions are important for the growth of new blood vessels, vascular maturation, cardiac development and immunity, and for directed cell movement. In addition, sphingosine 1-phosphate and its receptors influence inflammatory processes via the enzymes of the eicosanoid cascade.

Sphingosine 1-phosphate is released into the blood stream upon activation by physiological stimuli, such as growth factors, cytokines, and receptor agonists and antigens. It may have a critical role in platelet aggregation and thrombosis and could potentially aggravate cardiovascular disease. On the other hand, the relatively high concentration of the metabolite in HDL is now believed to have beneficial implications for atherogenesis. There is accumulating evidence that sphingosine 1-phosphate and its receptors regulate heart rate, blood flow in the coronary artery and blood pressure by mechanisms that have yet to be identified. It has been suggested that sphingosine 1-phosphate, together with other lysolipids such as sphingosylphosphorylcholine and lysosulfatide, are responsible for the beneficial clinical effects of HDL by stimulating the production of the potent anti-atherogenic and anti-inflammatory signalling molecule nitric oxide by the vascular endothelium.

In addition, like lysophosphatidic acid (with which it has much in common), it is a marker for certain types of cancer, and there is increasing evidence that its role in cell division or proliferation has an influence on the development of cancers. For example, in contrast to ceramide, it stimulates the growth, survival and migration of tumor cells and it is abundant in malignant tissue. This is currently a topic that is attracting great interest amongst medical researchers, and the potential for therapeutic intervention in sphingosine 1-phosphate metabolism, for example by inhibiting its biosynthesis from ceramide, is under active investigation. Similarly, drugs that antagonize sphingosine 1-phosphate and its receptors are being tested clinically as immuno-suppressants to prevent rejection of kidney grafts and to reduce inflammatory and allergic responses. They are also being tested against some forms of multiple sclerosis. On the other hand sphingosine 1-phosphate is believed to have beneficial effects on wound healing by stimulating the proliferation of new cells that close the wound.

Sphingosine 1-phosphate and lysophosphatidic acid are involved similarly in the regulation of the proliferation, survival, differentiation and migration of many types of stem cells.

In plants, sphinganine, sphingosine and phytosphingosine are phosphorylated by kinases in a similar way to form the appropriate 1-phosphate derivatives. Much less appears to be known about the function of these metabolites, although there is evidence that they may be involved in such diverse processes as defense mechanisms, pathogenesis, calcium mobilization, membrane stability, and especially the response to drought or heat stress. Interestingly, sphing-4-enine-1-phosphate appears to be the molecular species with much the highest biological activity, although sphing-4-enine per se is a rather minor component of plant long-chain bases, i.e. the double bond in position 4 is essential for biological activity.

3.  Catabolism

Long-chain bases can be regenerated from sphingosine 1-phosphate by the action of phosphatases. However, in animals and plants, production of sphingosine 1-phosphate (and homologues, etc) is also a key step in the catabolism of long-chain bases. The molecule is cleaved by the enzyme sphingosine 1-phosphate lyase to yield trans-2-hexadecenal, which can be further catabolized or reduced to the long-chain alcohol and incorporated into ether lipids. The reaction with sphingosine 1-phosphate lyase reduces the cellular levels of sphingosine and ceramide and is ultimately the means by which all sphingolipids are removed from cells.

The ethanolamine phosphate that is the other product of the reaction can be utilized for biosynthesis of phosphatidylethanolamine. This reaction is a further important link between sphingolipid metabolism and that of the glycerophospholipids. It may be especially relevant to the metabolism of dietary sphingolipids, since the activities of all the required enzymes are high in the intestines.

4.  Analysis

Analysis of sphingosine 1-phosphate does present some problems because of its high polarity and relatively low hydrophobicity. However, methods are available for quantitative extraction from tissues, and modern electrospray-ionization mass spectrometry techniques for detection and quantification afford high sensitivity and specificity.

Recommended Reading

• Auge, N, Negre-Salvayre, A. Salvayre, R. and Levade, T. Sphingomyelin metabolites in vascular cell signaling and atherogenesis. Prog. Lipid Res., 39, 207-229 (2000).
• Hait, N.C., Oskeritzian, C.A., Paugh, S.W., Milstien, S. and Spiegel, S. Sphingosine kinases, sphingosine 1-phosphate, apoptosis and diseases. Biochim. Biophys. Acta, 1758, 2016-2026 (2006).
• Huwiler, A. and Zangemeister-Wittke, U. Targeting the conversion of ceramide to sphingosine 1-phosphate as a novel strategy for cancer therapy. Crit. Rev. Oncol. Hematol., 63, 150-159 (2007).
• Ishii, I., Fukushima, N., Ye, X. and Chun, J. Lysophospholipid receptors: signaling and biology. Annu. Rev. Biochem., 73, 321-354 (2004).
• Lynch, D.V. and Dunn, T.M. An introduction to plant sphingolipids and a review of recent advances in understanding their metabolism and function. New Phytologist, 161, 677-702 (2004).
• Maceyka, M., Sankala, H., Hait, N.C., Le Stunff, H., Liu, H., Toman, R., Collier, C., Zhang, M., Satin, L.S., Merrill, A.H., Milstien, S. and Spiegel, S. SphK1 and SphK2, sphingosine kinase isoenzymes with opposing functions in sphingolipid metabolism. J. Biol. Chem., 280, 37118-37129 (2005).
• Nofer, J.R. High-density lipoprotein, sphingosine 1-phosphate, and atherosclerosis. J. Clin. Lipidology, 2, 4-11 (2008).
• Sperling, P., Warnecke, D. and Heinz, E. Plant sphingolipids. In: Lipid Metabolism and Membrane Biogenesis. pp. 337-381 (ed. G. Daum, Springer-Verlag, Heidelberg) (2004).
• Spiegel, S. and Milstien, S. Sphingosine-1-phosphate: an enigmatic signalling lipid.. Nature Reviews, Molecular Cell Biology, 4, 397-407 (2003).
• Zheng, W., Kollmeyer, J., Symolon, H., Momin, A., Munter, E., Wang, E., Kelly, S., Allegood, J.C., Liu, Y., Peng, Q., Ramaraju, H., Sullards, M.C., Cabot, M. and Merrill, A.H. Ceramides and other bioactive sphingolipid backbones in health and disease: lipidomic analysis, metabolism and roles in membrane structure, dynamics, signaling and autophagy. Biochim. Biophys. Acta, 1758, 1864-1884 (2006).

Updated: 5/3/2008 Credits/disclaimer	Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland