Carmen Bedia

Disruption of sphingosine 1-phosphate lyase confers resistance to chemotherapy and promotes oncogenesis through Bcl-2/Bcl-xL upregulation.

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid metabolite involved in cancer development through stimulation of cell survival, proliferation, migration, and angiogenesis. Irreversible degradation of S1P is catalyzed by S1P lyase (SPL). The human SGPL1 gene that encodes SPL maps to a region often mutated in cancers. To investigate the effect of SPL deficiency on cell survival and transformation, the susceptibility to anticancer drugs of fibroblasts generated from SPL-deficient mouse embryos (Sgpl1(-/-)) was compared with that of cells from heterozygous (Sgpl1(+/-)) or wild-type (Sgpl1(+/+)) embryos. First, loss of SPL caused resistance to the toxic effects of etoposide and doxorubicin. Interestingly, heterozygosity for the Sgpl1 gene resulted in partial resistance to apoptosis. Secondly, doxorubicin-induced apoptotic signaling was strongly inhibited in Sgpl1(-/-) cells (phosphatidylserine externalization, caspase activation, and cytochrome c release). This was accompanied by a strong increase in Bcl-2 and Bcl-xL protein content. Whereas correction of SPL deficiency in Sgpl1(-/-) cells led to downregulation of antiapoptotic proteins, Bcl-2 and Bcl-xL small interfering RNA-mediated knockdown in SPL-deficient cells resulted in increased sensitivity to doxorubicin, suggesting that Bcl-2 upregulation mediates SPL protective effects. Moreover, SPL deficiency led to increased cell proliferation, anchorage-independent cell growth, and formation of tumors in nude mice. Finally, transcriptomic studies showed that SPL expression is downregulated in human melanoma cell lines. Thus, by affecting S1P metabolism and the expression of Bcl-2 members, the loss of SPL enhances cell resistance to anticancer regimens and results in an increased ability of cells to acquire a transformed phenotype and become malignant.

Synthesis and Biological Activity of a Novel Inhibitor of Dihydroceramide Desaturase

A novel mechanism‐based dihydroceramide desaturase inhibitor (XM462) in which the substrate C5 methylene group is replaced by a sulfur atom is reported. Dihydroceramide desaturase inhibition occurred both in vitro and in cultured cells with IC50 values of 8.2 and 0.78 μM, respectively, at a substrate concentration of 10 μM. In vitro experiments showed that XM462 produced a mixed‐type inhibition (Ki=2 μM, α=0.83). LC‐MS analyses showed that accumulation of endogenous dihydroceramides occurred in cells upon treatment with XM462 in serum‐free medium, whereas ceramides built up in controls. In addition, XM462 was found to be metabolised to its 1‐glucosyl and 1‐phosphocholine derivatives, and to the products of N‐deacylation and reacylation with palmitoyl and stearoyl groups. In Jurkat A3 cells cultured in serum‐free medium, viability, as the percentage of trypan blue unstained cells in total cells, was reduced upon XM462 treatment (5 μM, 24 h), but not in controls. The interest of this compound is discussed.

Synthesis of a Novel Ceramide Analogue and its Use in a High-Throughput Fluorogenic Assay for Ceramidases

Several investigations have shown that acid ceramidase inhibitors are potential antiproliferative and cytostatic drugs for cancer chemotherapy. The combinatorial chemistry approach for the discovery of acid ceramidase inhibitors requires the availability of a high‐throughput enzyme assay. The synthesis of a novel fluorogenic ceramidase substrate, and its processing both in vitro and in cultured cells in a microtiter plate layout, are reported in this article. This coumarinic substrate was hydrolyzed in vitro (rat liver lysosomes) with Km and Vmax values of 113 μM and 3.6 pmol min−1 mg−1, respectively. Similarly, hydrolysis occurred in intact cultured cells that overexpressed acidic ceramidase. The assay was validated for the identification and characterization of acidic ceramidase inhibitors by using several α‐ketoamide ceramide analogues, whose inhibitory activity had been previously described.

Acid Ceramidase Expression Modulates the Sensitivity of A375 Melanoma Cells to Dacarbazine

Dacarbazine (DTIC) is the treatment of choice for metastatic melanoma, but its response in patients remains very poor. Ceramide has been shown to be a death effector and to play an important role in regulating cancer cell growth upon chemotherapy. Among ceramidases, the enzymes that catabolize ceramide, acid ceramidase (aCDase) has been implicated in cancer progression. Here we show that DTIC elicits a time- and dose-dependent decrease of aCDase activity and an increase of intracellular ceramide levels in human A375 melanoma cells. The loss of enzyme activity occurred as a consequence of reactive oxygen species-dependent activation of cathepsin B-mediated degradation of aCDase. These events preceded autophagic features and loss of cell viability. Down-regulation of acid but not neutral or alkaline ceramidase 2 resulted in elevated levels of ceramide and sensitization to the toxic effects of DTIC. Conversely, inducible overexpression of acid but not neutral ceramidase reduced ceramide levels and conferred resistance to DTIC. In conclusion, we report that increased levels of ceramide, due to enhanced degradation of aCDase, are in part responsible for the cell death effects of DTIC. These results suggest that down-regulation of aCDase alone or in combination with DTIC may represent a useful tool in the treatment of metastatic melanoma.

Legionella pneumophila S1P-lyase targets host sphingolipid metabolism and restrains autophagy.

Significance Legionella pneumophila is the causative agent of Legionnaires’ disease. It translocates a large repertoire of effectors into the host cell through a specialized secretion system to subvert cellular defenses. A key characteristic of this pathogen is that the majority of its effectors are encoded by eukaryotic-like genes acquired through horizontal gene transfer. We determined the crystal structure of one of these effectors, sphingosine-1 phosphate lyase (LpSpl), and show that it has high similarity with its eukaryotic homologue. We demonstrate that LpSpl possesses lyase activity and that it disrupts sphingolipid metabolism in the host cells. LpSpl plays a critical and previously unknown role in decreasing autophagy and is a unique virulence factor facilitating intracellular replication of L. pneumophila. Autophagy is an essential component of innate immunity, enabling the detection and elimination of intracellular pathogens. Legionella pneumophila, an intracellular pathogen that can cause a severe pneumonia in humans, is able to modulate autophagy through the action of effector proteins that are translocated into the host cell by the pathogen’s Dot/Icm type IV secretion system. Many of these effectors share structural and sequence similarity with eukaryotic proteins. Indeed, phylogenetic analyses have indicated their acquisition by horizontal gene transfer from a eukaryotic host. Here we report that L. pneumophila translocates the effector protein sphingosine-1 phosphate lyase (LpSpl) to target the host sphingosine biosynthesis and to curtail autophagy. Our structural characterization of LpSpl and its comparison with human SPL reveals high structural conservation, thus supporting prior phylogenetic analysis. We show that LpSpl possesses S1P lyase activity that was abrogated by mutation of the catalytic site residues. L. pneumophila triggers the reduction of several sphingolipids critical for macrophage function in an LpSpl-dependent and -independent manner. LpSpl activity alone was sufficient to prevent an increase in sphingosine levels in infected host cells and to inhibit autophagy during macrophage infection. LpSpl was required for efficient infection of A/J mice, highlighting an important virulence role for this effector. Thus, we have uncovered a previously unidentified mechanism used by intracellular pathogens to inhibit autophagy, namely the disruption of host sphingolipid biosynthesis.

Regulation of Autophagy by Sphingolipids

Autophagy is an evolutionary conserved process by which cells recycle intracellular materials to maintain homeostasis in different cellular contexts. Under basal conditions it prevents accumulation of damaged proteins and organelles; during starvation, autophagy provides cells with sufficient nutrients to survive. Sphingolipids are a family of bioactive molecules modulating vital cellular functions such as apoptosis, cell cycle arrest or proliferation. Besides these functions, some sphingolipids like ceramide, sphingosine- 1-phosphate or gangliosides have been described to promote autophagy in several cancer cell lines. Current evidence supports the notion that induction of autophagic cell death can halt tumorigenesis. Of interest, some chemotherapeutic agents used for the treatment of hematological malignancies trigger the production of endogenous sphingolipids with pro-autophagic effects. In this review we describe the regulation and functions of the sphingolipid-induced autophagy and the tight relationship with the cancer cell response to current chemotherapeutic regimens.

A simple fluorogenic method for determination of acid ceramidase activity and diagnosis of Farber disease

Acid ceramidase (aCDase) is one of several enzymes responsible for ceramide degradation within mammalian cells. As such, aCDase regulates the intracellular levels of the bioactive lipid ceramide. An inherited deficiency of aCDase activity results in Farber disease (FD), also called lipogranulomatosis, which is characterized by ceramide accumulation in the tissues of patients. Diagnosis of FD is confirmed by demonstration of a deficient aCDase activity and the subsequent storage of ceramide. Existing methods include extremely complex assays, many of them using radiolabeled compounds. Therefore, the aCDase assay and the in vitro enzymatic diagnosis of FD are still performed in only a very limited number of specialized laboratories. Here, the new fluorogenic substrate Rbm14-12 was synthesized and characterized as a new tool to determine aCDase activity. The resulting optimized assay was performed in 96-well plates, and different fibroblast and lymphoid cell lines derived from FD patients and controls were tested to measure aCDase activity. As a result, the activity in cells of FD patients was found to be very low or even null. This new fluorogenic method offers a very easy and rapid way for specific and accurate determination of aCDase activity and, consequently, for diagnosis of FD.

Cytotoxicity and acid ceramidase inhibitory activity of 2-substituted aminoethanol amides.

The acid ceramidase (AC) inhibitory activity of octanoylamides, p-tert-butylbenzamides and pivaloylamides of several 2-substituted aminoethanols is reported. All the aminoethanol amides bearing a hexadecyl substituent (C16), as well as (S)-N-(1-(hexadecylthio)-3-hydroxypropan-2-yl)pivaloylamide (SC16-tb) were inhibitory in cell lysates overexpressing AC, while all other compounds were not inhibitors. Kinetic experiments with (R,E)-N-(1-hydroxyoctadec-3-en-2-yl)pivaloylamide (E-tb) and SC16-tb showed that inhibition was competitive, with K(i) values of 34 and 94.0 microM, respectively. None of the compounds inhibited neutral ceramidase. Compounds E-tb and E-c7 (the octanoylamide of the unsaturated base E), which elicited a dose-response inhibition with IC(50) values around 15 microM, were the only AC inhibitors in intact cells. Both compounds were toxic to A549 cells with LD(50) values nearly 40 microM. Flow cytometry studies with E-tb evidenced that this compound induced a concentration-dependent cell cycle arrest at G(1) and a 20-25% apoptosis/late apoptosis/necrosis after a 24-h incubation at 50 microM. In agreement with its activity as acidic ceramidase inhibitor, this effect was accompanied with an increase in the amounts of C14, C16 and C18 ceramides (LC-MS analyses), which suggested that these lipids may be responsible for the cytotoxic activity of E-tb.

Lipidomic data analysis: Tutorial, practical guidelines and applications

Lipids are a broad group of biomolecules involved in diverse critical biological roles such as cellular membrane structure, energy storage or cell signaling and homeostasis. Lipidomics is the -omics science that pursues the comprehensive characterization of lipids present in a biological sample. Different analytical strategies such as nuclear magnetic resonance or mass spectrometry with or without previous chromatographic separation are currently used to analyze the lipid composition of a sample. However, current analytical techniques provide a vast amount of data which complicates the interpretation of results without the use of advanced data analysis tools. The choice of the appropriate chemometric method is essential to extract valuable information from the crude data as well as to interpret the lipidomic results in the biological context studied. The present work summarizes the diverse methods of analysis than can be used to study lipidomic data, from statistical inference tests to more sophisticated multivariate analysis methods. In addition to the theoretical description of the methods, application of various methods to a particular lipidomic data set as well as literature examples are presented.

Synthesis of a fluorogenic analogue of sphingosine-1-phosphate and its use to determine sphingosine-1-phosphate lyase activity

Illuminating an ER enzyme: We report on the design and synthesis of a fluorogenic chemical sensor (1) to measure sphingosine‐1‐phosphate lyase activity in high‐throughput screening formats, as well as its validation using lyase knockout (Sgpl1−/−) cells.