Bioactive sphingolipids: metabolism and function
2008; Elsevier BV; Volume: 50; Linguagem: Inglês
10.1194/jlr.r800080-jlr200
ISSN1539-7262
Autores Tópico(s)Lysosomal Storage Disorders Research
ResumoSphingolipids (SLs) are essential constituents of eukaryotic cells. Besides playing structural roles in cellular membranes, some metabolites, including ceramide, sphingosine, and sphingosine-1-phosphate, have drawn attention as bioactive signaling molecules involved in the regulation of cell growth, differentiation, senescence, and apoptosis. Understanding the many cell regulatory functions of SL metabolites requires an advanced knowledge of how and where in the cell they are generated, converted, or degraded. This review will provide a short overview of the metabolism, localization, and compartmentalization of SLs. Also, a discussion on bioactive members of the SL family and inducers of SL enzymes that lead to ceramide generation will be presented. Sphingolipids (SLs) are essential constituents of eukaryotic cells. Besides playing structural roles in cellular membranes, some metabolites, including ceramide, sphingosine, and sphingosine-1-phosphate, have drawn attention as bioactive signaling molecules involved in the regulation of cell growth, differentiation, senescence, and apoptosis. Understanding the many cell regulatory functions of SL metabolites requires an advanced knowledge of how and where in the cell they are generated, converted, or degraded. This review will provide a short overview of the metabolism, localization, and compartmentalization of SLs. Also, a discussion on bioactive members of the SL family and inducers of SL enzymes that lead to ceramide generation will be presented. Sphingolipids (SLs) represent a major class of lipids that are ubiquitous constituents of membranes in eukaryotes. First discovered by J. L. W. Thudichum in 1876, for a long time SLs were considered to play primarily structural roles in membrane formation. However, intensive research on SL metabolism and function has revealed members of the SL family, including ceramide (Cer), sphingosine (Sph), Sph-1-phosphate (S1P), and Cer-1-phosphate (C1P), as bioactive molecules playing roles from regulation of signal transduction pathways, through direction of protein sorting to the mediation of cell-to-cell interactions and recognition. SLs have also been reported to dynamically cluster with sterols to form lipid microdomains or rafts, which function as hubs for effective signal transduction and protein sorting (1Simons K. Ikonen E. Functional rafts in cell membranes.Nature. 1997; 387: 569-572Crossref PubMed Scopus (7948) Google Scholar). Various stimuli such as tumor necrosis factor-α (TNF-α), interleukin (IL)-1, Fas ligand, ionizing radiation (2Kolesnick R.N. Fuks A.Haimovitz-Friedman, and Z. The sphingomyelin signal transduction pathway mediates apoptosis for tumor necrosis factor, Fas, and ionizing radiation.Biochem. Cell Biol. 1994; 72: 471-474Crossref PubMed Scopus (141) Google Scholar), phorbol esters (3Becker K.P. Kitatani K. Bielawski J.Idkowiak-Baldys, J. Hannun Y.A. Selective inhibition of juxtanuclear translocation of protein kinase C betaII by a negative feedback mechanism involving ceramide formed from the salvage pathway.J. Biol. Chem. 2005; 280: 2606-2612Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar), heat stress (4Jenkins G.M. Cowart L.A. Signorelli P. Pettus B.J. Chalfant C.E. Hannun Y.A. Acute activation of de novo sphingolipid biosynthesis upon heat shock causes an accumulation of ceramide and subsequent dephosphorylation of SR proteins.J. Biol. Chem. 2002; 277: 42572-42578Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar), oxidative stress (5Goldkorn T. Balaban N. Shannon M. Chea V. Matsukuma K. Gilchrist D. Wang H. Chan C. H2O2 acts on cellular membranes to generate ceramide signaling and initiate apoptosis in tracheobronchial epithelial cells.J. Cell Sci. 1998; 111: 3209-3220Crossref PubMed Google Scholar), and chemotherapeutics (6Bose R. Verheij M. Scotto A.Haimovitz-Friedman, K. Fuks Z. Kolesnick R. Ceramide synthase mediates daunorubicin-induced apoptosis: an alternative mechanism for generating death signals.Cell. 1995; 82: 405-414Abstract Full Text PDF PubMed Scopus (780) Google Scholar) induce the formation of Cer. Understanding the complex regulatory mechanisms of bioactive SLs requires a broad knowledge of how these molecules are generated, degraded, or converted, and how and where they are located or metabolized. Recent molecular advances in identifying enzymes involved in the SL metabolism as well as technical advances in analytical measurements by mass spectrometry have shed light on their roles and regulation and involvement in cellular processes. In this review, we will briefly summarize the current knowledge of these aspects. For more detailed insights, the reader is referred to recent references and reviews (7Hannun Y.A. Obeid L.M. Principles of bioactive lipid signalling: lessons from sphingolipids.Nat. Rev. Mol. Cell Biol. 2008; 9: 139-150Crossref PubMed Scopus (2354) Google Scholar, 8Ogretmen B. Hannun Y.A. Biologically active sphingolipids in cancer pathogenesis and treatment.Nat. Rev. 2004; 4: 604-616Crossref Scopus (992) Google Scholar, 9Zheng W. Kollmeyer J. Symolon H. Momin A. Munter E. Wang E. Kelly S. Allegood J.C. Liu Y. Peng Q. al et 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. 2006; 1758: 1864-1884Crossref PubMed Scopus (436) Google Scholar). The SL metabolic pathway displays an intricate network of reactions resulting in the formation of a multitude of SLs, with Cer [and dihydroceramide (dhCer)] as the center of SL biosynthesis, catabolism, and as precursors of complex SLs. Cer can be produced in at least two distinct ways. First, it can be synthesized through the de novo pathway, and second, through the hydrolysis of complex lipids, especially sphingomyelin (SM) (Fig. 1). The de novo pathway commences with the condensation of serine and palmitoyl-CoA catalyzed by serine palmitoyl transferase to generate 3-keto-dihydrosphingosine. 3-Keto-dihydrosphingosine is subsequently reduced to form dihydrosphingosine (sphinganine), which is then N-acylated by (dh)Cer synthases to produce dhCer or Cer (10Mandon E.C. Ehses I. Rother J. Echten G.van Sandhoff K. Subcellular localization and membrane topology of serine palmitoyltransferase, 3-dehydrosphinganine reductase, and sphinganine N-acyltransferase in mouse liver.J. Biol. Chem. 1992; 267: 11144-11148Abstract Full Text PDF PubMed Google Scholar). Six mammalian genes that encode (dh)Cer synthase have recently been cloned. Each individual so-called longevity-assurance homolog (LASS1-6)/Cer synthase (CerS1-6) isoform shows substrate preference for specific chain length fatty acyl CoAs. CerS1, for example, shows significant preference for C18:0 FA CoA, whereas CerS5 and CerS6 preferably catalyze the acylation of (dh)Sph with myristoyl-, palmitoyl-, and stearoyl-CoA compared with very long-chain FA CoAs (11Pewzner-Jung Y. Futerman S.Ben-Dor, and A.H. When do Lasses (longevity assurance genes) become CerS (ceramide synthases)?: insights into the regulation of ceramide synthesis.J. Biol. Chem. 2006; 281: 25001-25005Abstract Full Text Full Text PDF PubMed Scopus (355) Google Scholar). dhCer is then desaturated by dhCer desaturase, generating a 4,5-trans-double bond to form Cer (12Michel C. van Echten-Deckert G. Conversion of dihydroceramide to ceramide occurs at the cytosolic face of the endoplasmic reticulum.FEBS Lett. 1997; 416: 153-155Crossref PubMed Scopus (79) Google Scholar). In the following SL biosynthetic reactions, Cer is primarily used for the synthesis of SM by transferring a phosphocholine headgroup from phosphatidylcholine through the action of SM synthases, thereby also generating diacylglycerol (13Tafesse F.G. Ternes P. Holthuis J.C. The multigenic sphingomyelin synthase family.J. Biol. Chem. 2006; 281: 29421-29425Abstract Full Text Full Text PDF PubMed Scopus (217) Google Scholar). Cer can also be phosphorylated by Cer kinase, which in turn can be recycled by a C1P phosphatase (14Shinghal R. Scheller R.H. Bajjalieh S.M. Ceramide 1-phosphate phosphatase activity in brain.J. Neurochem. 1993; 61: 2279-2285Crossref PubMed Scopus (71) Google Scholar) or glycosylated by glucosyl or galactosyl Cer synthases (15Ichikawa S. Hirabayashi Y. Glucosylceramide synthase and glycosphingolipid synthesis.Trends Cell Biol. 1998; 8: 198-202Abstract Full Text Full Text PDF PubMed Scopus (175) Google Scholar). In the hydrolytic pathway, SM is cleaved by one of several sphingomyelinases (SMases), releasing phosphocholine and Cer. The SMases can be distinguished according to their pH optima and subcellular localization. Several SMases have been characterized: lysosomal acid SMase (aSMase), zinc-dependent secretory SMase, neutral magnesium-dependent SMase (nSMase), and alkaline SMase (16Marchesini N. Hannun Y.A. Acid and neutral sphingomyelinases: roles and mechanisms of regulation.Biochem. Cell Biol. 2004; 82: 27-44Crossref PubMed Scopus (264) Google Scholar). Another important source of Cer is provided by the breakdown of glycosphingolipids through sequential removal of their terminal hydrophilic portions catalyzed by a series of specific hydrolases. The products glucosylceramide (GlcCer) and galactosylceramide are subsequently hydrolyzed by specific β-glucosidases and galactosidases to release Cer (17Tettamanti G. Ganglioside/glycosphingolipid turnover: new concepts.Glycoconj. J. 2004; 20: 301-317Crossref PubMed Scopus (169) Google Scholar). Cer is metabolized by ceramidases (CDases), removing the amide-linked FA to form Sph. Three types of CDases have been classified by their pH optimum and subcellular localization: acid CDase, neutral/alkaline CDase, and alkaline CDase (18Mao C. Obeid L.M. Ceramidases: regulators of cellular responses mediated by ceramide, sphingosine, and sphingosine-1-phosphate.Biochim. Biophys. Acta. 2008; 1781: 424-434Crossref PubMed Scopus (296) Google Scholar). Sph is then available for recycling into SL pathways or for phosphorylation by one of two Sph kinases, SK1 and SK2, to form S1P (19Hait N.C. Oskeritzian C.A. Paugh S.W. Milstien S. Spiegel S. Sphingosine kinases, sphingosine 1-phosphate, apoptosis and diseases.Biochim. Biophys. Acta. 2006; 1758: 2016-2026Crossref PubMed Scopus (400) Google Scholar). In return, S1P phosphatases dephosphorylate S1P regenerating Sph (20Johnson K.R. Johnson K.Y. Becker K.P. Bielawski J. Mao C. Obeid L.M. Role of human sphingosine-1-phosphate phosphatase 1 in the regulation of intra- and extracellular sphingosine-1-phosphate levels and cell viability.J. Biol. Chem. 2003; 278: 34541-34547Abstract Full Text Full Text PDF PubMed Scopus (130) Google Scholar). Finally, S1P lyase can metabolize S1P to release ethanolamine phosphate and hexadecenal (21Bandhuvula P. Saba J.D. Sphingosine-1-phosphate lyase in immunity and cancer: silencing the siren.Trends Mol. Med. 2007; 13: 210-217Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar). Enzymatic reactions in SL metabolism are distributed throughout different cellular compartments (Fig. 2). Understanding the functions of SLs requires insight into the specific subcellular localization of the enzymes involved in the SL pathway. The initial steps of SL de novo synthesis leading to Cer formation take place on the cytosolic surface of the endoplasmic reticulum (ER) and potentially in ER-associated membranes, such as the perinuclear membrane and mitochondria-associated membranes (12Michel C. van Echten-Deckert G. Conversion of dihydroceramide to ceramide occurs at the cytosolic face of the endoplasmic reticulum.FEBS Lett. 1997; 416: 153-155Crossref PubMed Scopus (79) Google Scholar). Synthesis of more complex SL metabolites like SM and GluCer occurs in the Golgi apparatus. Two specific pathways are known for the transport of Cer from the ER to the Golgi. First, SM is formed of Cer provided by the Cer transfer protein, CERT. Second, the synthesis of GluCer is based on Cer deriving from vesicular transport. Subsequently, the transport protein FAPP2 delivers GluCer as precursor for glycosphingolipid (GSL) synthesis (22Yamaji T. Kumagai K. Tomishige N. Hanada K. Two sphingolipid transfer proteins, CERT and FAPP2: their roles in sphingolipid metabolism.IUBMB Life. 2008; 60: 511-518Crossref PubMed Scopus (68) Google Scholar). The synthesis of complex GSLs (e.g., gangliosides) occurs in the luminal side of the Golgi. Therefore, GluCer needs to flip from the cytosolic surface to the inside of the Golgi. This mechanism is supported by the ABC transporter, P-glycoprotein (also known as MDR1) (23Lannert H. Gorgas K. Meissner I. Wieland F.T. Jeckel D. Functional organization of the Golgi apparatus in glycosphingolipid biosynthesis. Lactosylceramide and subsequent glycosphingolipids are formed in the lumen of the late Golgi.J. Biol. Chem. 1998; 273: 2939-2946Abstract Full Text Full Text PDF PubMed Scopus (103) Google Scholar). Subsequently, SM and complex GSLs are transported to the plasma membrane via vesicular trafficking. There, SM can be metabolized to Cer, and accordingly other bioactive lipids, either by aSMase on the outer leaflet of the membrane or by nSMase, which resides on the inner leaflet of the bilayer. From the plasma membrane, SLs may be recirculated through the endosomal pathway. Thereby, SM and GluCer are metabolized to Cer in the lysosomal compartment by SMases and glucosidases and Cer is then degraded by acid CDase to form Sph. Due to its ionizable positive charge, the salvaged Sph is able to leave the lysosome and shows adequate solubility in the cytosol to move between membranes, including the ER, where it would be available for recycling (24Riboni L. Bassi R. Caminiti A. Prinetti A. Viani P. Tettamanti G. Metabolic fate of exogenous sphingosine in neuroblastoma neuro2A cells. Dose-dependence and biological effects.Ann. N. Y. Acad. Sci. 1998; 845: 46-56Crossref PubMed Scopus (13) Google Scholar). It is also important to realize that the subcellular localization of enzymes of SL metabolism is a key determinant of site of action of bioactive SLs. SLs are either hydrophobic or amphipathic molecules that have hydrophobic as well as hydrophilic attributes. Hence, it is not surprising that these molecules are mostly integral components of biological membranes and show little movement between membranes, unless acted upon by specific transport mechanisms (such as Cer transfer protein or FAPP2). Bioactive SLs are induced by several agonists, and, in turn, they regulate several downstream targets that mediate their various effects on cell function. It should be noted that the cellular levels of the various bioactive SLs exhibit great differences. Concentrations of Cer, Sph, and S1P differ approximately by an order of magnitude, with Cer presenting the highest and S1P the lowest level. A small change in Cer can therefore drastically increase the levels of Sph or S1P. Within the family of SLs, Cer and S1P are the best explored bioactive components. Cer and Sph are reported to act as tumor-suppressor lipids involved not only in intracellular but also in extracellular processes. Cer signaling has been intimately involved in the regulation of cell growth, differentiation (25Okazaki T. Bell R.M. Hannun Y.A. Sphingomyelin turnover induced by vitamin D3 in HL-60 cells. Role in cell differentiation.J. Biol. Chem. 1989; 264: 19076-19080Abstract Full Text PDF PubMed Google Scholar), senescence (26Venable M.E. Lee J.Y. Smyth M.J. Bielawska A. Obeid L.M. Role of ceramide in cellular senescence.J. Biol. Chem. 1995; 270: 30701-30708Abstract Full Text Full Text PDF PubMed Scopus (371) Google Scholar), necrosis (27Hetz C.A. Hunn M. Rojas P. Torres V. Leyton L. Quest A.F. Caspase-dependent initiation of apoptosis and necrosis by the Fas receptor in lymphoid cells: onset of necrosis is associated with delayed ceramide increase.J. Cell Sci. 2002; 115: 4671-4683Crossref PubMed Scopus (103) Google Scholar), proliferation (28Adam D. Heinrich M. Kabelitz D. Schutze S. Ceramide: does it matter for T cells?.Trends Immunol. 2002; 23: 1-4Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar), and apoptosis (29Obeid L.M. Linardic C.M. Karolak L.A. Hannun Y.A. Programmed cell death induced by ceramide.Science. 1993; 259: 1769-1771Crossref PubMed Scopus (1585) Google Scholar). Cer functions, at least in part, by activating protein phosphatases PP1A and PP2A, which exhibit specificity for the D-erythro stereoisomer in vitro (30Chalfant C.E. Kishikawa K. Mumby M.C. Kamibayashi C. Bielawska A. Hannun Y.A. Long chain ceramides activate protein phosphatase-1 and protein phosphatase-2A. Activation is stereospecific and regulated by phosphatidic acid.J. Biol. Chem. 1999; 274: 20313-20317Abstract Full Text Full Text PDF PubMed Scopus (272) Google Scholar). It has also been shown that Cer may regulate protein kinase C (PKC) ζ (31Wang G. Silva J. Krishnamurthy K. Tran E. Condie B.G. Bieberich E. Direct binding to ceramide activates protein kinase Czeta before the formation of a pro-apoptotic complex with PAR-4 in differentiating stem cells.J. Biol. Chem. 2005; 280: 26415-26424Abstract Full Text Full Text PDF PubMed Scopus (128) Google Scholar), raf-1 (32Blazquez C. Guzman I.Galve-Roperh, and M. De novo-synthesized ceramide signals apoptosis in astrocytes via extracellular signal-regulated kinase.FASEB J. 2000; 14: 2315-2322Crossref PubMed Scopus (140) Google Scholar), and the kinase-suppressor of Ras (33Ruvolo P.P. Intracellular signal transduction pathways activated by ceramide and its metabolites.Pharmacol. Res. 2003; 47: 383-392Crossref PubMed Scopus (291) Google Scholar), significantly changing the level of phosphorylation of various key substrates. Another binding target for Cer is the cellular protease cathepsin D, which may mediate the actions of lysosomally generated Cer (34Heinrich M. Wickel M. Sandberg W.Schneider-Brachert, C. Gahr J. Schwandner R. Weber T. Saftig P. Peters C. Brunner J. al et Cathepsin D targeted by acid sphingomyelinase-derived ceramide.EMBO J. 1999; 18: 5252-5263Crossref PubMed Scopus (301) Google Scholar). Sph has been connected with cellular processes such as inducing cell cycle arrest and apoptosis by modulation of protein kinases and other signaling pathways. It has roles in regulating the actin cytoskeleton and endocytosis and has been shown to inhibit PKC (35Smith E.R. Merrill A.H. Obeid L.M. Hannun Y.A. Effects of sphingosine and other sphingolipids on protein kinase C.Methods Enzymol. 2000; 312: 361-373Crossref PubMed Google Scholar). Kinase targets for sphingoid bases including pkh1p and pkh2p have been found in yeast, indicating functions in regulating endocytosis, cell cycle arrest, and protein synthesis (36Cowart L.A. Hannun Y.A. Selective substrate supply in the regulation of yeast de novo sphingolipid synthesis.J. Biol. Chem. 2007; 282: 12330-12340Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar). S1P can be seen as a tumor-promoting lipid involved in the regulation of proliferation, cell growth, cell survival, cell migration, inflammation, angiogenesis, vasculogenesis, and resistance to apoptotic cell death, and therefore shows antagonizing effects to those of Cer (37Spiegel S. Milstien S. Sphingosine-1-phosphate: an enigmatic signalling lipid.Nat. Rev. Mol. Cell Biol. 2003; 4: 397-407Crossref PubMed Scopus (1724) Google Scholar). S1P binds to G-protein-coupled receptors (S1P receptors) on the cell surface, thus regulating diverse G proteins and subsequently various intracellular signaling pathways (38Taha T.A. Argraves K.M. Obeid L.M. Sphingosine-1-phosphate receptors: receptor specificity versus functional redundancy.Biochim. Biophys. Acta. 2004; 1682: 48-55Crossref PubMed Scopus (163) Google Scholar). More recent studies have broadened the attention to additional bioactive SLs such as C1P, GlcCer, lyso-SM, and dhCer. Metabolism and biological function of S1P and C1P have recently been reviewed (39Kihara A. Mitsutake S. Mizutani Y. Igarashi Y. Metabolism and biological functions of two phosphorylated sphingolipids, sphingosine 1-phosphate and ceramide 1-phosphate.Prog. Lipid Res. 2007; 46: 126-144Crossref PubMed Scopus (135) Google Scholar). C1P has been implicated in playing roles in inflammation and vesicular trafficking (40Chalfant C.E. Spiegel S. Sphingosine 1-phosphate and ceramide 1-phosphate: expanding roles in cell signaling.J. Cell Sci. 2005; 118: 4605-4612Crossref PubMed Scopus (348) Google Scholar). It mediates the activation of phospholipase A2 and the release of arachidonic acid in response to interleukin-β (41Pettus B.J. Bielawska A. Subramanian P. Wijesinghe D.S. Maceyka M. Leslie C.C. Evans J.H. Freiberg J. Roddy P. Hannun Y.A. al et Ceramide 1-phosphate is a direct activator of cytosolic phospholipase A2.J. Biol. Chem. 2004; 279: 11320-11326Abstract Full Text Full Text PDF PubMed Scopus (295) Google Scholar). GlcCer has be shown to be involved in post-Golgi trafficking and in drug resistance (42Radin N.S. Shayman J.A. Inokuchi J. Metabolic effects of inhibiting glucosylceramide synthesis with PDMP and other substances.Adv. Lipid Res. 1993; 26: 183-213PubMed Google Scholar). In recent years, substantial evidence has accumulated demonstrating that Cer generation pathways such as the SMase and de novo pathway are activated by inducers of apoptosis or growth arrest. Thus, endogenous Cer is produced in response to various stimuli such as cytokines, heat stress (4Jenkins G.M. Cowart L.A. Signorelli P. Pettus B.J. Chalfant C.E. Hannun Y.A. Acute activation of de novo sphingolipid biosynthesis upon heat shock causes an accumulation of ceramide and subsequent dephosphorylation of SR proteins.J. Biol. Chem. 2002; 277: 42572-42578Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar), UV radiation, hypoxia/reperfusion (43Hernandez O.M. Discher D.J. Bishopric N.H. Webster K.A. Rapid activation of neutral sphingomyelinase by hypoxia-reoxygenation of cardiac myocytes.Circ. Res. 2000; 86: 198-204Crossref PubMed Scopus (107) Google Scholar), lipopolysaccharides (44Haimovitz-Friedman A. Bayoumy C.Cordon-Cardo, S. Garzotto M. McLoughlin M. Gallily R. III C.K. Edwards Schuchman E.H. Fuks Z. Kolesnick R. Lipopolysaccharide induces disseminated endothelial apoptosis requiring ceramide generation.J. Exp. Med. 1997; 186: 1831-1841Crossref PubMed Scopus (379) Google Scholar), chemotherapeutic agents, and other miscellaneous agents (6Bose R. Verheij M. Scotto A.Haimovitz-Friedman, K. Fuks Z. Kolesnick R. Ceramide synthase mediates daunorubicin-induced apoptosis: an alternative mechanism for generating death signals.Cell. 1995; 82: 405-414Abstract Full Text PDF PubMed Scopus (780) Google Scholar). TNF-α and IL-1 have been shown to act on the de novo pathway as well as on nSMase (2Kolesnick R.N. Fuks A.Haimovitz-Friedman, and Z. The sphingomyelin signal transduction pathway mediates apoptosis for tumor necrosis factor, Fas, and ionizing radiation.Biochem. Cell Biol. 1994; 72: 471-474Crossref PubMed Scopus (141) Google Scholar). Furthermore, Cer has recently been linked to Alzheimer's disease and neurodegeneration, possibly mediating the cytotoxic response to amyloid-β peptide (45Lee J.T. Xu J. Lee J.M. Ku G. Han X. Yang D.I. Chen S. Hsu C.Y. Amyloid-beta peptide induces oligodendrocyte death by activating the neutral sphingomyelinase-ceramide pathway.J. Cell Biol. 2004; 164: 123-131Crossref PubMed Scopus (209) Google Scholar). aSMase, on the other hand, is induced by various stress stimuli, such as UV and ionizing radiation, ligation of death receptors, and chemotherapeutic agents such as platinum, paclitaxel, and histone deacetylase inhibitors. Recent studies also implicate an activation of aSMase by reactive oxygen species and nitrosative stress (46Castillo S.S. Levy M. Thaikoottathil J.V. Goldkorn T. Reactive nitrogen and oxygen species activate different sphingomyelinases to induce apoptosis in airway epithelial cells.Exp. Cell Res. 2007; 313: 2680-2686Crossref PubMed Scopus (82) Google Scholar). Interestingly, activation of aSMase in response to phorbol esters and to UV radiation has been shown to lead to an increase in Cer formation through the salvage (or recycling) pathway, i.e., by inducing formation of Cer, then Sph, which is recycled back to Cer (3Becker K.P. Kitatani K. Bielawski J.Idkowiak-Baldys, J. Hannun Y.A. Selective inhibition of juxtanuclear translocation of protein kinase C betaII by a negative feedback mechanism involving ceramide formed from the salvage pathway.J. Biol. Chem. 2005; 280: 2606-2612Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar). SK is activated by various growth factors such as epidermal growth factor, platelet-derived growth factor, and proinflammatory cytokines such as TNF-α and IL-1. However, this reaction is part of a complex signaling cascade dependent on upstream regulators such as PKC (47Johnson K.R. Becker K.P. Facchinetti M.M. Hannun Y.A. Obeid L.M. PKC-dependent activation of sphingosine kinase 1 and translocation to the plasma membrane. Extracellular release of sphingosine-1-phosphate induced by phorbol 12-myristate 13-acetate (PMA).J. Biol. Chem. 2002; 277: 35257-35262Abstract Full Text Full Text PDF PubMed Scopus (261) Google Scholar), phospholipase D (48Melendez A. Floto R.A. Gillooly D.J. Harnett M.M. Allen J.M. FcgammaRI coupling to phospholipase D initiates sphingosine kinase-mediated calcium mobilization and vesicular trafficking.J. Biol. Chem. 1998; 273: 9393-9402Abstract Full Text Full Text PDF PubMed Scopus (165) Google Scholar), and the extracellular signal-regulated kinase mitogen-activated protein kinases. The change in the level of the product S1P subsequently regulates cell viability and inflammatory responses (19Hait N.C. Oskeritzian C.A. Paugh S.W. Milstien S. Spiegel S. Sphingosine kinases, sphingosine 1-phosphate, apoptosis and diseases.Biochim. Biophys. Acta. 2006; 1758: 2016-2026Crossref PubMed Scopus (400) Google Scholar). For more detailed insights into SL pathway activators, the reader is referred to recent reviews (49El Alwani M. Wu B.X. Obeid L.M. Hannun Y.A. Bioactive sphingolipids in the modulation of the inflammatory response.Pharmacol. Ther. 2006; 112: 171-183Crossref PubMed Scopus (132) Google Scholar, 50Levade T. Auge N. Veldman R.J. Cuvillier O. Salvayre A.Negre-Salvayre, and R. Sphingolipid mediators in cardiovascular cell biology and pathology.Circ. Res. 2001; 89: 957-968Crossref PubMed Scopus (144) Google Scholar). In recent years, there is more and more evidence that SLs function as key components in modulating cell responses and act as signaling/regulatory molecules. To begin to understand and integrate SL function and regulation, one has to look not only into individual regulated pathways (such as SK1 or aSMase) but also into the complex network of SL metabolism, as one bioactive SL may be metabolically transformed into another bioactive molecule, often with counter functions (e.g., Cer and S1P). Importantly, one should also consider the place of action of bioactive SLs by determining the compartmentalization of both the involved enzymes and their substrates and products. Additional challenges are the experimental difficulties in studying lipid metabolism and function. The last years yielded many useful analytical and molecular tools that now enable extensive research in metabolic, topologic, structural, and functional aspects of Cer generations. More and more enzymes of SL metabolism have now been cloned and characterized, and several SL specific antibodies have been generated, which provides the possibility and the tools to study the topology of SL enzyme actions. Increasingly, animal models and knockouts in various enzymes of SL metabolism are revealing important and critical functions for SLs. The application of mass spectrometry now allows specific qualitative and quantitative analysis of endogenous SL metabolites in experimental systems and in response to agonist stimulation.
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