Ceramide-1-phosphate blocks apoptosis through inhibition of acid sphingomyelinase in macrophages

Lysosomes, cholesterol and atherosclerosis

Cholesterol-engorged macrophage foam cells are a critical component of the atherosclerotic lesion. Reducing the sterol deposits in lesions reduces clinical events. Sterol accumulations within lysosomes have proven to be particularly hard to mobilize out of foam cells. Moreover, excess sterol accumulation in lysosomes has untoward effects, including a complete disruption of lysosome function. Recently, we demonstrated that treatment of sterol-engorged macrophages in culture with triglyceride-containing particles can reverse many of the effects of cholesterol on lysosomes and dramatically reduce the sterol burden in these cells. This article describes what is known about lysosomal sterol engorgement, discusses the possible mechanisms by which triglyceride could produce its effects, and evaluates the possible positive and negative effects of reducing the lysosomal cholesterol levels in foam cells.

Ceramide kinase: the first decade

It has been some 20 years since the initial discovery of ceramide 1-phosphate (C1P) and nearly a decade since ceramide kinase (CERK) was cloned. Many studies have shown that C1P is important for membrane biology and for the regulation of membrane-bound proteins, and the CERK enzyme has appeared to be tightly regulated in order to control both ceramide levels and production of C1P. Furthermore, C1P made by CERK has emerged as a genuine signalling entity. However, it represents only part of the C1P pool that is available in the cell, therefore suggesting that alternative unknown C1P-producing mechanisms may also play a role. Recent technological developments for measuring complex sphingolipids in biological samples, together with the availability of Cerk-deficient animals as well as potent CERK inhibitors, have now provided new grounds for investigating C1P biology further. Here, we will review the current understanding of CERK and C1P in terms of biochemistry and functional implications, with particular attention to C1P produced by CERK.

Regulation of phosphatidic Acid metabolism by sphingolipids in the central nervous system

This paper explores the way ceramide, sphingosine, ceramide 1-phosphate, and sphingosine 1-phosphate modulate the generation of second lipid messengers from phosphatidic acid in two experimental models of the central nervous system: in vertebrate rod outer segments prepared from dark-adapted retinas as well as in rod outer segments prepared from light-adapted retinas and in rat cerebral cortex synaptosomes under physiological aging conditions. Particular attention is paid to lipid phosphate phosphatase, diacylglycerol lipase, and monoacylglycerol lipase. Based on the findings reported in this paper, it can be concluded that proteins related to phototransduction phenomena are involved in the effects derived from sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide and that age-related changes occur in the metabolism of phosphatidic acid from cerebral cortex synaptosomes in the presence of either sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide. The present paper demonstrates, in two different models of central nervous system, how sphingolipids influence phosphatidic acid metabolism under different physiological conditions such as light and aging.

The interplay between bioactive sphingolipids and steroid hormones

Steroid hormones regulate various physiological processes including development, reproduction, and metabolism. These regulatory molecules are synthesized from cholesterol in endocrine organs - such as the adrenal glands and gonads - via a multi-step enzymatic process that is catalyzed by the cytochrome P450 superfamily of monooxygenases and hydroxysteroid dehydrogenases. Steroidogenesis is induced by trophic peptide hormones primarily via the activation of a cAMP/protein kinase A (PKA)-dependent pathway. However, other signaling molecules, including cytokines and growth factors, control the steroid hormone biosynthetic pathway. More recently, sphingolipids, including ceramide, sphingosine-1-phosphate, and sphingosine, have been found to modulate steroid hormone secretion at multiple levels. In this review, we provide a brief overview of the mechanisms by which sphingolipids regulate steroidogenesis. In addition, we discuss how steroid hormones control sphingolipid metabolism. Finally, we outline evidence supporting the emerging role of bioactive sphingolipids in various nuclear processes and discuss a role for nuclear sphingolipid metabolism in the control of gene transcription.

Control of metabolism and signaling of simple bioactive sphingolipids: Implications in disease

Simple bioactive sphingolipids include ceramide, sphingosine and their phosphorylated forms sphingosine 1-phosphate and ceramide 1-phosphate. These molecules are crucial regulators of cell functions. In particular, they play important roles in the regulation of angiogenesis, apoptosis, cell proliferation, differentiation, migration, and inflammation. Decoding the mechanisms by which these cellular functions are regulated requires detailed understanding of the signaling pathways that are implicated in these processes. Most importantly, the development of inhibitors of the enzymes involved in their metabolism may be crucial for establishing new therapeutic strategies for treatment of disease.

Ceramide and ceramide 1-phosphate in health and disease

Sphingolipids are essential components of cell membranes, and many of them regulate vital cell functions. In particular, ceramide plays crucial roles in cell signaling processes. Two major actions of ceramides are the promotion of cell cycle arrest and the induction of apoptosis. Phosphorylation of ceramide produces ceramide 1-phosphate (C1P), which has opposite effects to ceramide. C1P is mitogenic and has prosurvival properties. In addition, C1P is an important mediator of inflammatory responses, an action that takes place through stimulation of cytosolic phospholipase A2, and the subsequent release of arachidonic acid and prostaglandin formation. All of the former actions are thought to be mediated by intracellularly generated C1P. However, the recent observation that C1P stimulates macrophage chemotaxis implicates specific plasma membrane receptors that are coupled to Gi proteins. Hence, it can be concluded that C1P has dual actions in cells, as it can act as an intracellular second messenger to promote cell survival, or as an extracellular receptor agonist to stimulate cell migration.

Distinct receptor-mediated activities in macrophages for natural ceramide-1-phosphate (C1P) and for phospho-ceramide analogue-1 (PCERA-1)

Ceramide-1-phosphate (C1P) is known as a second messenger regulating a multitude of processes including cell growth, apoptosis and inflammation. Exciting recent findings now suggest that C1P can stimulate macrophages migration in an extra-cellular manner via a G protein-coupled receptor (GPCR). Interestingly, a synthetic C1P analog, named phospho-ceramide analogue-1 (PCERA-1), was recently described as a potent in-vivo anti-inflammatory agent, and was suggested to act on macrophages in an extra-cellular manner via a GPCR. Here we summarize and compare the receptor-mediated as well as receptor-independent activities of natural C1P and its synthetic analog. We also provide experimental data in support of distinct C1P and PCERA-1 receptors.

Activation of protein kinase C-alpha is essential for stimulation of cell proliferation by ceramide 1-phosphate

We previously demonstrated that ceramide-1-phosphate (C1P) stimulates fibroblast and macrophage proliferation, but the mechanisms involved in this action have only been partially described. Here we demonstrate that C1P induces translocation of protein kinase C-alpha (PKC-alpha) from the soluble to the membrane fraction of bone marrow-derived macrophages. Translocation of this enzyme was accompanied by its phosphorylation on Ser 657 residue. Activation of PKC-alpha was independent of prior stimulation of phosphatidylinositol-dependent or phosphatidylcholine-dependent phospholipase C activities, but required activation of sphingomyelin synthesis. Inhibition of PKC-alpha activation also blocked C1P-stimulated macrophage proliferation indicating that this enzyme is essential for the mitogenic effect of C1P.

Use of high performance liquid chromatography-electrospray ionization-tandem mass spectrometry for the analysis of ceramide-1-phosphate levels

Ceramide-1-phosphate (C1P) is a bioactive sphingolipid with roles in several biological processes. Currently, high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (HPLC ESI-MS/MS) offers the most efficient method of quantifying C1P. However, the published protocols have several drawbacks causing overestimations and carryovers. Here, the reported overestimation of C1P was shown to be due to incomplete neutralization of base hydrolyzed lipid extracts leading to the hydrolysis of SM to C1P. Actual quantity of C1P in cells (6 pmols/10(6) cells) was much lower than previously reported. Also, the major species of C1P produced by ceramide kinase (CERK) was found to be d(18:1/16:0) with a minority of d(18:1/24:1) and d(18:1/24:0). The artifactual production of C1P from SM was used for generating C1Ps as retention time markers. Elimination of carryovers between samples and a 2-fold enhancement in the signal strength was achieved by heating the chromatographic column to 60 (degrees) C. The role of ceramide transport protein (CERT) in supplying substrate to CERK was also revalidated using this new assay. Finally, our results demonstrate the presence of additional pathway(s) for generation of the C1P subspecies, d(18:1/18:0) C1P, as well as a significant portion of d(18:1/16:0), d(18:1/24:1), and d(18:1/24:0). In conclusion, this study introduces a much improved and validated method for detection of C1P by mass spectrometry and demonstrates specific changes in the C1P subspecies profiles upon downregulation of CERK and CERT.

A TNF- and c-Cbl-dependent FLIP(S)-degradation pathway and its function in Mycobacterium tuberculosis-induced macrophage apoptosis

Apoptosis is central to the interaction between pathogenic mycobacteria and host macrophages. Caspase-8-dependent apoptosis of infected macrophages, which requires activation of the mitogen-activated protein (MAP) kinase p38, lowers the spread of mycobacteria. Here we establish a link between the release of tumor necrosis factor (TNF) and mycobacteria-mediated macrophage apoptosis. TNF activated a pathway involving the kinases ASK1, p38 and c-Abl. This pathway led to phosphorylation of FLIP(S), which facilitated its interaction with the E3 ubiquitin ligase c-Cbl. This interaction triggered proteasomal degradation of FLIP(S), which promoted activation of caspase-8 and apoptosis. Our findings identify a previously unappreciated signaling pathway needed for Mycobacterium tuberculosis-triggered macrophage cell death.

Subcellular localization of ceramide kinase and ceramide kinase-like protein requires interplay of their Pleckstrin Homology domain-containing N-terminal regions together with C-terminal domains

Ceramide kinase (CERK) and the ceramide kinase-like protein (CERKL), two related members of the diacylglycerol kinase family, are ill-defined at the molecular level. In particular, what determines their distinctive subcellular localization is not well understood. Here we show that the Pleckstrin Homology (PH) domain of CERK, which is required for Golgi complex localization, can substitute for the N-terminal region of CERKL and allow for wild-type CERKL localization, which is typified by nucleolar accumulation. This demonstrates that determinants for localization of these two enzymes do not lie solely in their PH domain-containing N-terminal regions. Moreover, we present evidence for a previously unrecognized participation of CERK distal sequences in structural stability, localization and activity of the full-length protein. Progressive deletion of CERK and CERKL from the C-terminus revealed similar sequential organization in both proteins, with nuclear import signals in their N-terminal part, and nuclear export signals in their C-terminal part. Furthermore, mutagenesis of individual cysteine residues of a CERK-specific CXXXCXXC motif severely compromised both exportation of CERK from the nucleus and its association with the Golgi complex. Altogether, this work identifies conserved domains in CERK and CERKL as well as new determinants for their subcellular localization. It further suggests a nucleocytoplasmic shuttling mechanism for both proteins that may be defective in CERKL mutant proteins responsible for retinal degenerative diseases.

Structure of ceramide-1-phosphate at the air-water solution interface in the absence and presence of Ca2+

Ceramide-1-phosphate, the phosphorylated form of ceramide, gained attention recently due to its diverse intracellular roles, in particular in inflammation mediated by cPLA(2)alpha. However, surprisingly little is known about the physical chemical properties of this lipid and its potential impact on physiological function. For example, the presence of Ca(2+) is indispensable for the interaction of Cer-1-P with the C2 domain of cPLA(2)alpha. We report on the structure and morphology of Cer-1-P in monomolecular layers at the air/water solution interface in the absence and presence of Ca(2+) using diverse biophysical techniques, including synchrotron x-ray reflectivity and grazing angle diffraction, to gain insight into the role and function of Cer-1-P in biomembranes. We show that relatively small changes in pH and the presence of monovalent cations dramatically affect the behavior of Cer-1-P. On pure water Cer-1-P forms a solid monolayer despite the negative charge of the phosphomonoester headgroup. In contrast, pH 7.2 buffer yields a considerably less solid-like monolayer, indicating that charge-charge repulsion becomes important at higher pH. Calcium was found to bind strongly to the headgroup of Cer-1-P even in the presence of a 100-fold larger Na(+) concentration. Analysis of the x-ray reflectivity data allowed us to estimate how much Ca(2+) is bound to the headgroup, approximately 0.5 Ca(2+) and approximately 1.0 Ca(2+) ions per Cer-1-P molecule for the water and buffer subphase respectively. These results can be qualitatively understood based on the molecular structure of Cer-1-P and the electrostatic/hydrogen-bond interactions of its phosphomonoester headgroup. Biological implications of our results are also discussed.

Gene expression of ceramide kinase, galactosyl ceramide synthase and ganglioside GD3 synthase is associated with prognosis in breast cancer

PURPOSE

Sphingolipids are bioactive lipids implicated in apoptosis, cell survival and proliferation. We analyzed the prognostic value of enzymes from sphingolipid metabolism in breast cancer.

METHODS

Differences in expression of ceramide galactosyl transferase (UGT8), ceramide kinase (CERK), and Ganglioside GD3-Synthase (ST8SIA1) in breast cancer cells were investigated by using microarray data of 1,581 tumor samples.

RESULTS

UGT8, CERK, and ST8SIA1 were associated with poor pathohistological grading (P < 0.001). High CERK expression was correlated with ErbB2 status (P = 0.006). Among ER positive breast cancers a significant worse prognosis for patients with tumors showing low ST8SIA1 and UGT8 expression was observed. In the ER negative subgroup those samples with high CERK expression displayed a worse prognosis. In a multivariate analysis only ST8SIA1 and tumor size remained significant.

CONCLUSIONS

Our experiments reveal that expression of enzymes from the sphingolipid metabolism has prognostic implications in breast cancer.

Chain length specificity for activation of cPLA2alpha by C1P: use of the dodecane delivery system to determine lipid-specific effects

Previously, our laboratory demonstrated that ceramide-1-phosphate (C1P) specifically activated group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) in vitro. In this study, we investigated the chain length specificity of this interaction. C1P with an acyl-chain of >or=6 carbons efficiently activated cPLA(2)alpha in vitro, whereas C(2)-C1P, was unable to do so. Delivery of C1P to cells via the newly characterized ethanol/dodecane system demonstrated a lipid-specific activation of cPLA(2)alpha, AA release, and PGE(2) synthesis (EC(50) = 400 nM) when compared to structurally similar lipids. C1P delivered as vesicles in water also induced a lipid-specific increase in AA release. Mass spectrometric analysis demonstrated that C1P delivered via ethanol/dodecane induced a 3-fold increase in endogenous C1P with little metabolism to ceramide. C1P was also more efficiently delivered (>3-fold) to internal membranes by ethanol/dodecane as compared to vesiculated C1P. Using this now established delivery method for lipids, C(2)-C1P was shown to be ineffective in the induction of AA release as compared with C(6)-C1P, C(16)-C1P, and C(18:1) C1P. Here, we demonstrate that C1P requires >or=6 carbon acyl-chain to activate cPLA(2)alpha. Thus, published reports on the biological activity of C(2)-C1P are not via eicosanoid synthesis. Furthermore, this study demonstrates that the alcohol/dodecane system can be used to efficiently deliver exogenous phospholipids to cells for the examination of specific biological effects.

Ceramide 1-phosphate (C1P) promotes cell migration Involvement of a specific C1P receptor

Ceramide 1-phosphate (C1P) is a bioactive sphingolipid that is implicated in the regulation of cell homeostasis and the control of inflammation. It is mitogenic for fibroblasts and macrophages, and has been described as potent inhibitor of apoptosis. Using RAW 264.7 macrophages we have now discovered a new biological activity of C1P: stimulation of cell migration. This novel action can only be observed when C1P is applied exogenously to the cells in culture, and not by increasing the intracellular levels of C1P. This fact led to identify a specific receptor through which C1P stimulates cell migration. The receptor is coupled to G(i) proteins and causes phosphorylation of extracellularly regulated kinases 1 and 2, and protein kinase B (also known as Akt) upon ligation with C1P. Inhibition of either of these pathways completely abolished C1P-stimulated macrophage migration. In addition, C1P stimulated the DNA binding activity of nuclear factor kappa B, and blockade of this transcription factor resulted in complete inhibition of macrophage migration. This newly identified receptor could be an important drug target for treatment of illnesses that are associated to inflammatory processes, or to diseases in which cell migration is a major cause of pathology, as it occurs in metastatic tumors.

Neutropenia with impaired immune response to Streptococcus pneumoniae in ceramide kinase-deficient mice

In mammals, ceramide kinase (CerK)-mediated phosphorylation of ceramide is the only known pathway to ceramide-1-phosphate (C1P), a recently identified signaling sphingolipid metabolite. To help delineate the roles of CerK and C1P, we knocked out the gene of CerK in BALB/c mice by homologous recombination. All in vitro as well as cell-based assays indicated that CerK activity is completely abolished in Cerk-/- mice. Labeling with radioactive orthophosphate showed a profound reduction in the levels of de novo C1P formed in Cerk-/- macrophages. Consistently, mass spectrometry analysis revealed a major contribution of CerK to the formation of C16-C1P. However, the significant residual C1P levels in Cerk-/- animals indicate that alternative routes to C1P exist. Furthermore, serum levels of proapoptotic ceramide in these animals were significantly increased while levels of dihydroceramide as the biosynthetic precursor were reduced. Previous literature pointed to a role of CerK or C1P in innate immune cell function. Using a variety of mechanistic and disease models, as well as primary cells, we found that macrophage- and mast cell-dependent readouts are barely affected in the absence of CerK. However, the number of neutrophils was strikingly reduced in blood and spleen of Cerk-/- animals. When tested in a model of fulminant pneumonia, Cerk-/- animals developed a more severe disease, lending support to a defect in neutrophil homeostasis following CerK ablation. These results identify ceramide kinase as a key regulator of C1P, dihydroceramide and ceramide levels, with important implications for neutrophil homeostasis and innate immunity regulation.

Chemical tools to investigate sphingolipid metabolism and functions

Sphingolipids comprise an important group of biomolecules, some of which have been shown to play important roles in the regulation of many cell functions. From a structural standpoint, they all share a long 2-amino-1,3-diol chain, which can be either saturated (sphinganine), hydroxylated at C4 (phytosphingosine), or unsaturated at C4 (sphingosine) as in most mammalian cells. N-acylation of sphingosine leads to ceramide, a key intermediate in sphingolipid metabolism that can be enzymatically modified at the C1-OH position to other biologically important sphingolipids, such as sphingomyelin or glycosphingolipids. In addition, both ceramide and sphingosine can be phosphorylated at C1-OH to give ceramide-1-phosphate and sphingosine-1-phosphate, respectively. To better understand the biological and biophysical roles of sphingolipids, many efforts have been made to design synthetic analogues as chemical tools able to unravel their structure-activity relationships, and to alter their cellular levels. This last approach has been thoroughly studied by the development of specific inhibitors of some key enzymes that play an important role in biosynthesis or metabolism of these intriguing lipids. With the above premises in mind, the aim of this review is to collect, in a systematic way, the recent efforts described in the literature leading to the development of new chemical entities specifically designed to achieve the above goals.

A conserved cysteine motif essential for ceramide kinase function

Ceramide kinase (CerK) is a sphingolipid metabolizing enzyme very sensitive to oxidation; however, the determinants are unknown. We show here that the thiol-modifying agent N-ethyl-maleimide abrogates CerK activity in vitro and in a cell based assay, implying that important cysteine residues are accessible in purified as well as endogenous CerK. We replaced every 22 residues in human CerK, by an alanine, and measured activity in the resulting mutant proteins. This led to identification of a cluster of cysteines, C(347)XXXC(351)XXC(354), essential for CerK function. These findings are discussed based on homology modeling of the catalytic domain of CerK.

Targeting ceramide metabolism with a potent and specific ceramide kinase inhibitor

Ceramide kinase (CerK) produces the bioactive lipid ceramide-1-phosphate (C1P) and appears as a key enzyme for controlling ceramide levels. In this study, we discovered and characterized adamantane-1-carboxylic acid (2-benzoylamino-benzothiazol-6-yl)amide (NVP-231), a potent, specific, and reversible CerK inhibitor that competitively inhibits binding of ceramide to CerK. NVP-231 is active in the low nanomolar range on purified as well as cellular CerK and abrogates phosphorylation of ceramide, resulting in decreased endogenous C1P levels. When combined with another ceramide metabolizing inhibitor, such as tamoxifen, NVP-231 synergistically increased ceramide levels and reduced cell growth. Therefore, NVP-231 represents a novel and promising compound for controlling ceramide metabolism that may provide insight into CerK physiological function.

Involvement of nitric oxide in the promotion of cell survival by ceramide 1-phosphate

Macrophages play vital roles in inflammatory responses, and their number at sites of inflammation is strictly regulated by cell death and division. Here, we demonstrate that production of nitric oxide (NO) is a major mechanism whereby ceramide-1-phosphate (C1P) blocks apoptosis in macrophages. However, NO failed to stimulate macrophage proliferation. The prosurvival effect of C1P was blocked by inhibitors of inducible NO synthase. The antiapoptotic effect of C1P was also blocked by phosphatidylinositol 3-kinase or nuclear factor-kappa B inhibitors. Moreover, NO reversed the inhibitory effect of C1P on acid sphingomyelinase, but the prosurvival effect of C1P was independent of this action.

Membrane organization and ionization behavior of the minor but crucial lipid ceramide-1-phosphate

Ceramide-1-phosphate (Cer-1-P), one of the simplest of all sphingophospholipids, occurs in minor amounts in biological membranes. Yet recent evidence suggests important roles of this lipid as a novel second messenger with crucial tasks in cell survival and inflammatory responses. We present a detailed description of the physical chemistry of this hitherto little explored membrane lipid. At full hydration Cer-1-P forms a highly organized subgel (crystalline) bilayer phase (L(c)) at low temperature, which transforms into a regular gel phase (L(beta)) at approximately 45 degrees C, with the gel to fluid phase transition (L(beta)-L(alpha)) occurring at approximately 65 degrees C. When incorporated at 5 mol % in a phosphatidylcholine bilayer, the pK(a2) of Cer-1-P, 7.39 +/- 0.03, lies within the physiological pH range. Inclusion of phosphatidylethanolamine in the phosphatidylcholine bilayer, at equimolar ratio, dramatically reduces the pK(a2) to 6.64 +/- 0.03. We explain these results in light of the novel electrostatic/hydrogen bond switch model described recently for phosphatidic acid. In mixtures with dielaidoylphosphatidylethanolamine, small concentrations of Cer-1-P cause a large reduction of the lamellar-to-inverted hexagonal phase transition temperature, suggesting that Cer-1-P induces, like phosphatidic acid, negative membrane curvature in these types of lipid mixtures. These properties place Cer-1-P in a class more akin to certain glycerophospholipids (phosphatidylethanolamine, phosphatidic acid) than to any other sphingolipid. In particular, the similarities and differences between ceramide and Cer-1-P may be relevant in explaining some of their physiological roles.

(Glyco)sphingolipidology: an amazing challenge and opportunity for systems biology

Sphingolipids are found in essentially all eukaryotes and in some prokaryotes and viruses, where they influence cell structure, signaling and interactions with the extracellular environment. Because of the combinatorial nature of their biosynthesis, the sphingolipidome comprises untold thousands of species that encompass bioactive backbones and complex phospho- and glycolipids. Mass spectrometry is able to analyze a growing fraction of the sphingolipidome and is beginning to provide information about localization. Use of these structure specific, quantitative methods is producing insights, and surprises, regarding sphingolipid structure, metabolism, function and disease. Dealing with such large data sets poses special challenges for systems biology, but the intrinsic and elegant interrelationships among these compounds might provide a key to dealing with the complexity of the sphingolipidome.

Sphingolipid metabolizing enzymes as novel therapeutic targets

Pharmacological interference with sphingolipid metabolizing enzymes promises to provide novel ways to modulate cellular pathways relevant in multiple diseases. In this review, we focus on two sphingolipid signaling molecules, sphingosine-1-phosphate (S1P) and ceramide, as they are involved in cell fate decisions (survival vs. apoptosis) and in a wide range of pathophysiological processes. For S1P, we will discuss sphingosine kinases and S1P lyase as the enzymes which are crucial for its production and degradation, respectively, emphasizing the potential therapeutic usefulness of inhibitors of these enzymes. For ceramide, we will concentrate on acid sphingomyelinase, and critically review the substantial literature which implicates this enzyme as a worthwhile target for pharmacological inhibitors. It will become clear that the task to validate these enzymes as drug targets is not finished and many questions regarding the therapeutic usefulness of their inhibitors remain unanswered. Still this approach holds promise for a number of totally new therapies, and, on the way, detailed insight into sphingolipid signaling pathways can be gained.

Multiple roles for sphingolipids in steroid hormone biosynthesis

Steroid hormones are essential regulators of a vast number of physiological processes. The biosynthesis of these chemical messengers occurs in specialized steroidogenic tissues via a multi-step process that is catalyzed by members of the cytochrome P450 superfamily of monooxygenases and hydroxysteroid dehydrogenases. Though numerous signaling mediators, including cytokines and growth factors control steroidogenesis, trophic peptide hormones are the primary regulators of steroid hormone production. These peptide hormones activate a cAMP/cAMP-dependent kinase (PKA) signaling pathway, however, studies have shown that crosstalk between multiple signal transduction pathways and signaling molecules modulates optimal steroidogenic capacity. Sphingolipids such as ceramide, sphingosine, sphingosine-1-phosphate, sphingomyelin, and gangliosides have been shown to control the steroid hormone biosynthetic pathway at multiple levels, including regulating steroidogenic gene expression and activity as well as acting as second messengers in signaling cascades. In this review, we provide an overview of recent studies that have investigated the role of sphingolipids in adrenal, gonadal, and neural steroidogenesis.

Ceramide kinase and ceramide-1-phosphate

It has been over a decade since the sphingolipid ceramide-1-phosphate (C1P) was described. Until recently, only sparse reports on possible biological functions for this lipid have been published. A large number of reports have now surfaced demonstrating distinct biological mechanisms regulated by C1P produced from ceramide kinase (CERK). In the following methods chapter, the methodologies for examining CERK function in vitro and in cells are outlined in detail. The methodologies for examining C1P levels and the use of exogenous C1P on cells to observe lipid specific effects on a particular biology are also detailed.

Ceramide 1-phosphate stimulates macrophage proliferation through activation of the PI3-kinase/PKB, JNK and ERK1/2 pathways

Ceramide 1-phosphate (C1P) was first shown to be mitogenic for fibroblasts, but the mechanisms whereby it stimulated cell proliferation have remained largely unknown. Here we demonstrate that C1P stimulates DNA synthesis and cell division in murine bone marrow-derived macrophages. C1P caused rapid phosphorylation of protein kinase B (PKB, also known as Akt), a downstream target of phosphatidylinositol 3-kinase (PI3-K). Selective inhibition of PI3-K blocked both DNA synthesis and cell growth. C1P induced phosphorylation of GSK-3beta, which is a major target of PKB, and this effect was also abolished by inhibition of PI3-K. In addition, C1P upregulated the expression of cyclin D1 and c-Myc, two major targets of GSK-3beta, which are important regulators of cell proliferation. C1P stimulated the activity of NF-kappaB, and inhibitors of this transcription factor completely blocked macrophage proliferation. Lastly, C1P induced phosphorylation of the mitogen activated protein kinases (MAPK) extracellularly regulated kinases 1 and 2 (ERK1/2), and c-Jun N-terminal kinase (JNK). Inhibition of ERK1/2 and JNK also blocked C1P-induced macrophage proliferation. It can be concluded that C1P stimulates macrophage proliferation through activation of the PI3-K/PKB, ERK and JNK pathways, and that GSK-3beta, c-Myc, cyclin D1, and NF-kappaB are important downstream effectors in this action.

Regulation and traffic of ceramide 1-phosphate produced by ceramide kinase: comparative analysis to glucosylceramide and sphingomyelin

Ceramide 1-phosphate (C1P) has been characterized as a sphingolipid that participates in cell signaling. Although C1P synthesis is thought to occur via phosphorylation of ceramide by ceramide kinase (CerK), the processes that regulate C1P formation and fate remain largely unknown. In this study we analyzed bone marrow-derived macrophages (BMDM) from CerK-null mice (Cerk(-/-)) and found significant levels of C1P, suggesting that previously unrecognized pathways may also lead to C1P formation. After these experiments we used an overexpression system, BMDM from Cerk(-/-) mice, and short-chain fluorescent ceramides to trace CerK-dependent formation of C1P. Because the ceramide analogs can also be converted to glucosylceramide (GlcCer) and sphingomyelin (SM), they allowed us to directly compare all three metabolites. We found that C1P produced by CerK is turned over rapidly when serum is removed or upon calcium chelation, whereas GlcCer and SM are stable under these conditions. We further demonstrated that ceramide must be transported to the Golgi complex to be phosphorylated by CerK. Inhibition of the ceramide transfer protein slowed down SM formation without decreasing C1P, suggesting an alternate route of ceramide transport. Other experiments indicated that, like GlcCer and SM, C1P traffics along the secretory pathway to reach the plasma membrane. Furthermore, in BMDM C1P was secreted more readily than was GlcCer or SM. Altogether, our results indicate that CerK is essential to C1P formation via phosphorylation of Cer, providing the first insights into mechanisms underlying ceramide access to CerK and C1P trafficking as well as clarifying C1P as a signaling entity.

Oxidative stress, lipid rafts, and macrophage reprogramming

Oxidant stress, induced under a variety of conditions, is known to lead to the molecular reprogramming of the tissue-fixed macrophage. This reprogramming is associated with an altered response to subsequent inflammatory stimuli, such as lipopolysaccharide (LPS), leading to enhanced liberation of proinflammatory chemokines and cytokines. Due to this altered response, dysregulated immunity ensues, leading to the development of clinical syndromes such as multiple organ dysfunction syndrome (MODS). Although the mechanisms responsible for this altered macrophage activity by oxidant stress remains complex and poorly elucidated, it appears, based on recent research, that early and direct alterations within lipid rafts are responsible. This early and direct interaction with lipid rafts by oxidants leads to the mobilization of annexin VI from lipid raft constructs, leading to the release of calcium. This increased cytosolic concentration of this secondary messenger, in turn, results in the activation of calcium-dependent kinases, leading to further alterations in lipid raft lipids and eventually lipid raft proteins. Due to these lipid raft compositional changes, preassembly of receptor complexes occur, leading to enhanced proinflammatory activation. Within this review, the complexity of oxidant-induced reprogramming within the tissue fixed macrophage as currently understood is explained.

Membrane regulation of the stress response from prokaryotic models to mammalian cells

"Membrane regulation" of stress responses in various systems is widely studied. In poikilotherms, membrane rigidification could be the first reaction to cold perception: reducing membrane fluidity of membranes at physiological temperatures is coupled with enhanced cold inducibility of a number of genes, including desaturases (see J.L. Harwood's article in this Proceedings volume). A similar role of changes in membrane physical state in heat (oxidative stress, etc.) sensing- and signaling gained support recently from prokaryotes to mammalian cells. Stress-induced remodeling of membrane lipids could influence generation, transduction, and deactivation of stress signals, either through global effects on the fluidity of the membrane matrix, or by specific interactions of boundary (or raft) lipids with receptor proteins, lipases, ion channels, etc. Our data point to membranes not only as targets of stress, but also as sensors in activating a stress response.

Translational aspects of sphingolipid metabolism

Sphingolipids, a major class of lipids in cell membranes, play diverse roles in biological processes. As bioactive and structural molecules, they have signaling activities and biophysical properties that are essential for regulating various cellular, tissue and systemic functions. Moreover, sphingolipids are receiving increasing attention as contributors to the pathogenesis of several human disorders, including, cancer, inflammation and neurological, immune and metabolic disorders. Small-molecule inhibitors and monoclonal antibodies that target sphingolipid metabolism recently enabled giant strides toward treatment of malignant and autoimmune disorders. Here, we review the emerging roles of sphingolipids in disease pathogenesis and the attendant possibilities for sphingolipid-based therapeutics.

Mechanisms by which fatty acids regulate leucocyte function

Fatty acids (FAs) have been shown to alter leucocyte function and thus to modulate inflammatory and immune responses. In this review, the effects of FAs on several aspects of lymphocyte, neutrophil and macrophage function are discussed. The mechanisms by which FAs modulate the production of lipid mediators, activity of intracellular signalling pathways, activity of lipid-raft-associated proteins, binding to TLRs (Toll-like receptors), control of gene expression, activation of transcription factors, induction of cell death and production of reactive oxygen and nitrogen species are described in this review. The rationale for the use of specific FAs to treat patients with impaired immune function is explained. Substantial improvement in the therapeutic usage of FAs or FA derivatives may be possible based on an improvement in the understanding of the precise molecular mechanisms of action with respect to the different leucocyte types and outcome with respect to the inflammatory responses.

Direct extracellular interaction between the early secreted antigen ESAT-6 of Mycobacterium tuberculosis and TLR2 inhibits TLR signaling in macrophages

Expression of early secreted antigenic target protein 6 (ESAT-6) by Mycobacterium tuberculosis is associated with lower innate immune responses to infection. Here we show that ESAT-6 inhibited activation of transcription factor NF-kappaB and interferon-regulatory factors (IRFs) after Toll-like receptor (TLR) signaling; inhibition of TLR signaling by ESAT-6 required the kinase Akt. Direct binding of ESAT-6 to TLR2 activated Akt and prevented interaction between the adaptor MyD88 and 'downstream' kinase IRAK4, thus abrogating NF-kappaB activation. The six carboxy-terminal amino acid residues of ESAT-6 were required and sufficient for the TLR2-mediated inhibitory effect. A critical function for the carboxy-terminal peptide of ESAT-6 in restricting MyD88-dependent TLR signaling emphasizes the possibility that mimetic inhibitory peptides could be used to restrict innate immune responses in situations in which prolonged TLR signaling has deleterious effects.

Ceramide kinase uses ceramide provided by ceramide transport protein: localization to organelles of eicosanoid synthesis

Ceramide kinase (CERK) is a critical mediator of eicosanoid synthesis, and its product, ceramide-1-phosphate (C1P), is required for the production of prostaglandins in response to several inflammatory agonists. In this study, mass spectrometry analysis disclosed that the main forms of C1P in cells were C(16:0) C1P and C(18:0) C1P, suggesting that CERK uses ceramide transported to the trans-Golgi apparatus by ceramide transport protein (CERT). To this end, downregulation of CERT by RNA interference technology dramatically reduced the levels of newly synthesized C1P (kinase-derived) as well as significantly reduced the total mass levels of C1P in cells. Confocal microscopy, subcellular fractionation, and surface plasmon resonance analysis were used to further localize CERK to the trans-Golgi network, placing the generation of C1P in the proper intracellular location for the recruitment of cytosolic phospholipase A(2)alpha. In conclusion, these results demonstrate that CERK localizes to areas of eicosanoid synthesis and uses a ceramide "pool" transported in an active manner via CERT.

Enhanced ceramide-induced apoptosis in ceramide kinase overexpressing cells

We evaluated how increased levels of ceramide kinase (CerK) would impact the growth of COS-1 fibroblasts and RBL-2H3 basophils. The low CerK activity in these cells was strongly up-regulated upon recombinant expression of CerK. CerK-overexpressing COS-1 cells depended on higher concentrations of serum for their growth and displayed many filipodia. The two CerK-overexpressing cell lines were more sensitive to C2-ceramide-mediated apoptosis, and this correlated with the production of C2-ceramide-1-phosphate by CerK. This study indicates that ceramide kinase may participate in the control of cell growth, and establishes a novel assay that will be valuable for testing ceramide kinase inhibitors.

DNA polymerase beta catalytic efficiency mirrors the Asn279-dCTP H-bonding strength

Ternary complexes of wild type or mutant form of human DNA polymerase beta (pol beta) bound to DNA and dCTP substrates were studied by molecular dynamics (MD) simulations. The occurrences of contact configurations (CC) of structurally important atom pairs were sampled along the MD trajectories, and converted into free-energy differences, DeltaG(CC). DeltaG(CC) values were correlated with the experimental binding and catalytic free energies for the wild type pol beta and its Arg183Ala, Tyr271Ala, Asp276Val, Lys280Gly, Arg283Ala, and Glu295Ala mutants. The correlation coefficients show that the strength of the H-bond between dCTP and Asn279 is a strong predictor of the mutation-induced changes in the catalytic efficiency of pol beta. This finding is consistent with the view that enzyme preorganization plays a major role in controlling DNA polymerase specific activity.

Sphingolipid Metabolism in Systemic Inflammation

The inflammatory response - induced and regulated by a variety of mediators such as cytokines, prostaglandins, and reactive oxygen species (ROS) - is the localized host’s response of the tissue to injury, irritation, or infection. In a very similar and stereotyped sequence, the mediators are thought to induce an acute phase response orchestrated by an array of substances produced locally or near the source or origin of the inflammatory response. Despite its basically protective function, the response can become inappropriate in intensity or duration damaging host tissues or interfering with normal metabolism. Thus, inflammation is the cause and/or consequence of a diversity of diseases and plays a major role in the development of remote organ failure. Better knowledge of the underlying mechanisms of these processes is, therefore, a fundamental pre-requisite fostering the molecular understanding of novel therapeutic targets or diagnostic variables.

Sphingolipid Metabolism in Systemic Inflammation

The inflammatory response — induced and regulated by a variety of mediators such as cytokines, prostaglandins, and reactive oxygen species (ROS) — is the localized host’s response of the tissue to injury, irritation, or infection. In a very similar and stereotyped sequence, the mediators are thought to induce an acute phase response orchestrated by an array of substances produced locally or near the source or origin of the inflammatory response. Despite its basically protective function, the response can become inappropriate in intensity or duration damaging host tissues or interfering with normal metabolism. Thus, inflammation is the cause and/or consequence of a diversity of diseases and plays a major role in the development of remote organ failure. Better knowledge of the underlying mechanisms of these processes is, therefore, a fundamental pre-requisite fostering the molecular understanding of novel therapeutic targets or diagnostic variables.

A critical beta6-beta7 loop in the pleckstrin homology domain of ceramide kinase

CerK (ceramide kinase) produces ceramide 1-phosphate, a sphingophospholipid with recognized signalling properties. It localizes to the Golgi complex and fractionates essentially between detergent-soluble and -insoluble fractions; however, the determinants are unknown. Here, we made a detailed mutagenesis study of the N-terminal PH domain (pleckstrin homology domain) of CerK, based on modelling, and identified key positively charged amino acid residues within an unusual motif in the loop interconnecting beta-strands 6 and 7. These residues are critical for CerK membrane association and polyphosphoinositide binding and activity. Their mutagenesis results in increased thermolability, sensitivity to proteolysis, reduced apparent molecular mass as well as propensity of the recombinant mutant protein to aggregate, indicating that this loop impacts the overall conformation of the CerK protein. This is in contrast with most PH domains whose function strongly relies on charges located in the beta1-beta2 loop.

Effects of ceramide-1-phosphate on cultured cells: dependence on dodecane in the vehicle

Ceramide-1-phosphate (C1P), the product of ceramide kinase, is a sphingophospholipid with recently recognized signaling properties. In particular, it was reported to be mitogenic and capable of direct stimulation of cytosolic phospholipase A(2alpha). Much of the present knowledge has relied on the use of C1P of various acyl chain lengths, together with diverse protocols to deliver it to cultured cells. A mixture of ethanol (or methanol) with dodecane, as the vehicle, has become popular. However, the contribution of this solvent to the observed effects of C1P has not been documented. Here, we show that addition of C1P in ethanol-dodecane to culture medium leads to irreversible cytotoxic effects. These culminate in mitochondrial swelling, vacuole formation, and cell death. Not only the toxicity of C1P, but also its ability to trigger prostaglandin E2 release, is fully dependent upon addition of a premade C1P-dodecane mixture. Furthermore, we show that these effects are not restricted to C1P. They result from the capacity of dodecane to interact with phospholipids; hence, they go undetected with a vehicle control. This study should raise awareness about the use of dodecane for phospholipid delivery and, in turn, help in unraveling C1P signaling, which is still poorly understood.

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.

Ceramides and other bioactive sphingolipid backbones in health and disease: lipidomic analysis, metabolism and roles in membrane structure, dynamics, signaling and autophagy

Sphingolipids are comprised of a backbone sphingoid base that may be phosphorylated, acylated, glycosylated, bridged to various headgroups through phosphodiester linkages, or otherwise modified. Organisms usually contain large numbers of sphingolipid subspecies and knowledge about the types and amounts is imperative because they influence membrane structure, interactions with the extracellular matrix and neighboring cells, vesicular traffic and the formation of specialized structures such as phagosomes and autophagosomes, as well as participate in intracellular and extracellular signaling. Fortunately, "sphingolipidomic" analysis is becoming feasible (at least for important subsets such as all of the backbone "signaling" subspecies: ceramides, ceramide 1-phosphates, sphingoid bases, sphingoid base 1-phosphates, inter alia) using mass spectrometry, and these profiles are revealing many surprises, such as that under certain conditions cells contain significant amounts of "unusual" species: N-mono-, di-, and tri-methyl-sphingoid bases (including N,N-dimethylsphingosine); 3-ketodihydroceramides; N-acetyl-sphingoid bases (C2-ceramides); and dihydroceramides, in the latter case, in very high proportions when cells are treated with the anticancer drug fenretinide (4-hydroxyphenylretinamide). The elevation of DHceramides by fenretinide is befuddling because the 4,5-trans-double bond of ceramide has been thought to be required for biological activity; however, DHceramides induce autophagy and may be important in the regulation of this important cellular process. The complexity of the sphingolipidome is hard to imagine, but one hopes that, when partnered with other systems biology approaches, the causes and consequences of the complexity will explain how these intriguing compounds are involved in almost every aspect of cell behavior and the malfunctions of many diseases.

Identification of a nuclear localization signal in the retinitis pigmentosa-mutated RP26 protein, ceramide kinase-like protein

Retinitis pigmentosa (RP) is a genetically heterogeneous disease characterized by degeneration of the retina. A mutation in a new ceramide kinase (CERK) homologous gene, named CERK-like protein (CERKL), was found to cause autosomal recessive retinitis pigmentosa (RP26). Here, we show a point mutation of one of two putative nuclear localization signal (NLS) sequences inhibited the nuclear localization of the protein. Furthermore, the tetra-GFP-tagged NLS, which cannot passively enter the nucleus, was observed not only in the nucleus but also in the nucleolus. Our results provide the first evidence of the active nuclear import of CERKL and suggest that the identified NLS might be responsible for nucleolar retention of the protein. As recent studies have shown other RP-related proteins are localized in the nucleus or the nucleolus, our identification of NLS in CERKL suggests that CERKL likely plays important roles for retinal functions in the nucleus and the nucleolus.

Ceramide 1-phosphate/ceramide, a switch between life and death

Ceramide is a well-characterized sphingolipid metabolite and second messenger that participates in numerous biological processes. In addition to serving as a precursor to complex sphingolipids, ceramide is a potent signaling molecule capable of regulating vital cellular functions. Perhaps its major role in signal transduction is to induce cell cycle arrest, and promote apoptosis. In contrast, little is known about the metabolic or signaling pathways that are regulated by the phosphorylated form of ceramide. It was first demonstrated that ceramide-1-phosphate (C1P) had mitogenic properties, and more recently it has been described as potent inhibitor of apoptosis and inducer of cell survival. C1P and ceramide are antagonistic molecules that can be interconverted in cells by kinase and phosphatase activities. An appropriate balance between the levels of these two metabolites seems to be crucial for cell and tissue homeostasis. Switching this balance towards accumulation of one or the other may result in metabolic dysfunction, or disease. Therefore, the activity of the enzymes that are involved in C1P and ceramide metabolism must be efficiently coordinated to ensure normal cell functioning.

The interaction between the pleckstrin homology domain of ceramide kinase and phosphatidylinositol 4,5-bisphosphate regulates the plasma membrane targeting and ceramide 1-phosphate levels

Ceramide kinase (CERK) converts ceramide to ceramide-1-phosphate (C1P), which has recently emerged as a new bioactive molecule capable of regulating diverse cellular functions. The N-terminus of the CERK protein encompasses a sequence motif known as a pleckstrin homology (PH) domain. Although the PH domain was previously demonstrated to be an important domain for the subcellular localization of CERK, the precise properties of this domain remained unclear. In this study, we reveal that the PH domain of CERK exhibits high affinity for phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), among other lipids. Furthermore, in COS7 cells, GFP-fused CERK translocated rapidly from the cytoplasm to the plasma membrane in response to hyper-osmotic stress, which is known to increase the intracellular PI(4,5)P(2) levels, whereas a PH domain deletion mutant did not. Additionally, in [(32)P]orthophosphate-labeled COS7 cells, the translocation of CERK to the plasma membrane induced a 2.8-fold increase in C1P levels. The study presented here provides insight into the crucial role of the CERK-PH domain in plasma membrane targeting, through its binding to PI(4,5)P(2), and subsequent induction of C1P production in the vicinity of the membrane.