Molecular basis for resistance to the anticancer drug cisplatin in Dictyostelium

Influenza A virus NS1 induces degradation of sphingosine 1-phosphate lyase to obstruct the host innate immune response

The type I interferon (IFN)-mediated innate immune response is one of the central obstacles influenza A virus (IAV) must overcome in order to successfully replicate within the host. We have previously shown that sphingosine 1-phosphate (S1P) lyase (SPL) enhances IKKϵ-mediated type I IFN responses. Here, we demonstrate that the nonstructural protein 1 (NS1) of IAV counteracts the SPL-mediated antiviral response by inducing degradation of SPL. SPL was ubiquitinated and downregulated upon IAV infection or NS1 expression, whereas NS1-deficient IAV failed to elicit SPL ubiquitination or downregulation. Transiently overexpressed SPL increased phosphorylation of IKKϵ, resulting in enhanced expression of type I IFNs. However, this induction was markedly inhibited by IAV NS1. Collectively, this study reveals a novel strategy employed by IAV to subvert the type I IFN response, providing new insights into the interplay between IAV and host innate immunity.

Identification of antibiotics for use in selection of the chytrid fungi Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans

Global amphibian populations are being decimated by chytridiomycosis, a deadly skin infection caused by the fungal pathogens Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal). Although ongoing efforts are attempting to limit the spread of these infections, targeted treatments are necessary to manage the disease. Currently, no tools for genetic manipulation are available to identify and test specific drug targets in these fungi. To facilitate the development of genetic tools in Bd and Bsal, we have tested five commonly used antibiotics with available resistance genes: Hygromycin, Blasticidin, Puromycin, Zeocin, and Neomycin. We have identified effective concentrations of each for selection in both liquid culture and on solid media. These concentrations are within the range of concentrations used for selecting genetically modified cells from a variety of other eukaryotic species.

Emerging Connections of S1P-Metabolizing Enzymes with Host Defense and Immunity During Virus Infections

The sphingosine 1-phosphate (S1P) metabolic pathway is a dynamic regulator of multiple cellular and disease processes. Identification of the immune regulatory role of the sphingosine analog FTY720 led to the development of the first oral therapy for the treatment of an autoimmune disease, multiple sclerosis. Furthermore, inhibitors of sphingosine kinase (SphK), which mediate S1P synthesis, are being evaluated as a therapeutic option for the treatment of cancer. In conjunction with these captivating discoveries, S1P and S1P-metabolizing enzymes have been revealed to display vital functions during virus infections. For example, S1P lyase, which is known for metabolizing S1P, inhibits influenza virus replication by promoting antiviral type I interferon innate immune responses. In addition, both isoforms of sphingosine kinase have been shown to regulate the replication or pathogenicity of many viruses. Pro- or antiviral activities of S1P-metabolizing enzymes appear to be dependent on diverse virus–host interactions and viral pathogenesis. This review places an emphasis on summarizing the functions of S1P-metabolizing enzymes during virus infections and discusses the opportunities for designing pioneering antiviral drugs by targeting these host enzymes.

Dictyostelium as a Model to Assess Site-Specific ADP-Ribosylation Events

The amoeba Dictyostelium discoideum is a single-cell organism that can undergo a simple developmental program, making it an excellent model to study the molecular mechanisms of cell motility, signal transduction, and cell-type differentiation. A variety of human genes that are absent or show limited conservation in other invertebrate models have been identified in this organism. This includes ADP-ribosyltransferases, also known as poly-ADP-ribose polymerases (PARPs), a family of proteins that catalyze the addition of single or poly-ADP-ribose moieties onto target proteins. The genetic tractability of Dictyostelium and its relatively simple genome structure makes it possible to disrupt PARP gene combinations, in addition to specific ADP-ribosylation sites at endogenous loci. Together, this makes Dictyostelium an attractive model to assess how ADP-ribosylation regulates a variety of cellular processes including DNA repair, transcription, and cell-type specification. Here we describe a range of techniques to study ADP-ribosylation in Dictyostelium, including analysis of ADP-ribosylation events in vitro and in vivo, in addition to approaches to assess the functional roles of this modification in vivo.

Curcumin affects gene expression and reactive oxygen species via a PKA dependent mechanism in Dictyostelium discoideum

Botanicals are widely used as dietary supplements and for the prevention and treatment of disease. Despite a long history of use, there is generally little evidence supporting the efficacy and safety of these preparations. Curcumin has been used to treat a myriad of human diseases and is widely advertised and marketed for its ability to improve health, but there is no clear understanding how curcumin interacts with cells and affects cell physiology. D. discoideum is a simple eukaryotic lead system that allows both tractable genetic and biochemical studies. The studies reported here show novel effects of curcumin on cell proliferation and physiology, and a pleiotropic effect on gene transcription. Transcriptome analysis showed that the effect is two-phased with an early transient effect on the transcription of approximately 5% of the genome, and demonstrates that cells respond to curcumin through a variety of previously unknown molecular pathways. This is followed by later unique transcriptional changes and a protein kinase A dependent decrease in catalase A and three superoxide dismutase enzymes. Although this results in an increase in reactive oxygen species (ROS; superoxide and H2O2), the effects of curcumin on transcription do not appear to be the direct result of oxidation. This study opens the door to future explorations of the effect of curcumin on cell physiology.

Sphingolipid metabolism affects the anticancer effect of cisplatin

Cisplatin, a DNA crosslinking agent, is widely used for the treatment of a variety of solid tumors. Numerous studies have demonstrated that sphingolipid metabolism, which acts as a target for cisplatin treatment, is a highly complex network that consists of sphingolipid signaling molecules and related catalytic enzymes. Ceramide (Cer), which is the central molecule of this network, has been established to induce apoptosis. However, another molecule, sphingosine-1-phosphate (S1P), exerts the opposite function, i.e., serves as a regulator of pro-survival. Other sphingolipid molecules, including dihydroceramide, ceramide-1-phosphate, glucosylceramide (GluCer), and sphingosine (Sph), or sphingolipid catalytic enzymes such as Sph kinase (SphK), Cer synthase (CerS), and S1P lyase, have also attracted considerable attention, particularly Cer, GluCer, SphK, CerS, and S1P lyase, which have been implicated in cisplatin resistance. This review summarizes specific molecules involved in sphingolipid metabolism and related catalytic enzymes affecting the anticancer effect of cisplatin, particularly in relation to induction of apoptosis and drug resistance.

A facile stable-isotope dilution method for determination of sphingosine phosphate lyase activity

A new technique for quantifying sphingosine phosphate lyase activity in biological samples is described. In this procedure, 2-hydrazinoquinoline is used to convert (2E)-hexadecenal into the corresponding hydrazone derivative to improve ionization efficiency and selectivity of detection. Combined utilization of liquid chromatographic separation and multiple reaction monitoring-mass spectrometry allows for simultaneous quantification of the substrate S1P and product (2E)-hexadecenal. Incorporation of (2E)- d5-hexadecenal as an internal standard improves detection accuracy and precision. A simple one-step derivatization procedure eliminates the need for further extractions. Limits of quantification for (2E)-hexadecenal and sphingosine-1-phosphate are 100 and 50fmol, respectively. The assay displays a wide dynamic detection range useful for detection of low basal sphingosine phosphate lyase activity in wild type cells, SPL-overexpressing cell lines, and wild type mouse tissues. Compared to current methods, the capacity for simultaneous detection of sphingosine-1-phosphate and (2E)-hexadecenal greatly improves the accuracy of results and shows excellent sensitivity and specificity for sphingosine phosphate lyase activity detection.

Systems biology of cisplatin resistance: past, present and future

The platinum derivative cis-diamminedichloroplatinum(II), best known as cisplatin, is currently employed for the clinical management of patients affected by testicular, ovarian, head and neck, colorectal, bladder and lung cancers. For a long time, the antineoplastic effects of cisplatin have been fully ascribed to its ability to generate unrepairable DNA lesions, hence inducing either a permanent proliferative arrest known as cellular senescence or the mitochondrial pathway of apoptosis. Accumulating evidence now suggests that the cytostatic and cytotoxic activity of cisplatin involves both a nuclear and a cytoplasmic component. Despite the unresolved issues regarding its mechanism of action, the administration of cisplatin is generally associated with high rates of clinical responses. However, in the vast majority of cases, malignant cells exposed to cisplatin activate a multipronged adaptive response that renders them less susceptible to the antiproliferative and cytotoxic effects of the drug, and eventually resume proliferation. Thus, a large fraction of cisplatin-treated patients is destined to experience therapeutic failure and tumor recurrence. Throughout the last four decades great efforts have been devoted to the characterization of the molecular mechanisms whereby neoplastic cells progressively lose their sensitivity to cisplatin. The advent of high-content and high-throughput screening technologies has accelerated the discovery of cell-intrinsic and cell-extrinsic pathways that may be targeted to prevent or reverse cisplatin resistance in cancer patients. Still, the multifactorial and redundant nature of this phenomenon poses a significant barrier against the identification of effective chemosensitization strategies. Here, we discuss recent systems biology studies aimed at deconvoluting the complex circuitries that underpin cisplatin resistance, and how their findings might drive the development of rational approaches to tackle this clinically relevant problem.

Sphingosine 1-phosphate: a potential molecular target for ovarian cancer therapy?

Sphingosine 1-phosphate (S1P) is an important signaling regulator involved in tumor progression in multiple neoplasms. However, the role of S1P in the pathogenesis of ovarian cancer remains unclear. Herein, we summarize recent advances in understanding the impact of S1P signaling in ovarian cancer progression. S1P, aberrantly produced in ovarian cancer patients, is involved in the regulation of key cellular processes that contribute to ovarian cancer initiation and progression. Moreover, agents that block the S1P signaling pathway inhibit ovarian cancer cell growth or induce apoptosis. Hence, current evidence suggests that S1P may become a potential molecular target for ovarian cancer therapy.

Pharmacogenetics of resistance to Cisplatin and other anticancer drugs and the role of sphingolipid metabolism

Dictyostelium discoideum has proven to be a useful lead genetic system for identifying novel genes and pathways responsible for the regulation of sensitivity to the widely used anticancer drug cisplatin. Resistance to cisplatin is a major factor limiting the efficacy of the drug in treating many types of cancer. Studies using unbiased insertional mutagenesis in D. discoideum have identified the pathway of sphingolipid metabolism as a key regulator in controlling sensitivity to cisplatin. Using the genetic tools including directed homologous recombination and ectopic gene expression available with D. discoideum has shown how pharmacological modulation of this pathway can increase sensitivity to cisplatin, and these results have been extensively translated to, and validated in, human cells. Strategies, experimental conditions, and methods are presented to enable further study of resistance to cisplatin as well as other important drugs.

Inter-organellar communication with mitochondria regulates both the intrinsic and extrinsic pathways of apoptosis

One of our major research interests is to define BCL-2 family function in the cellular decision to induce mitochondrial outer membrane permeabilization and apoptosis. Curiosity in BCL-2 family - mitochondrial interactions led to the identification that the sphingolipid pathway plays a crucial role in BCL-2 family function. For approximately 20 years, sphingolipid biology and apoptotic mechanisms have criss-crossed, but remained distinct, as neither literature could explain the observable interplay between these pathways. We recently identified that two products within the sphingolipid pathway, sphingosine-1-PO₄ and hexadecenal, directly regulate BAK and BAX activation, respectively. Furthermore, our work suggests that mitochondrial communication with heterotypic membrane and/or organelles may be an important regulatory mechanism for apoptotic sensitivity.

Investigation of DNA repair pathway activity

DNA is constantly being damaged from endogenous and exogenous sources and efficient repair of different types of DNA lesions is essential for the survival of the organism. Dictyostelium is highly resistant to DNA damage and its genome sequence has revealed the presence of multiple repair pathways conserved with vertebrates but lost in other genetically tractable invertebrate models. As such, Dictyostelium is a powerful model organism to study selected human DNA repair pathways and may provide insights into the molecular basis of how cells become resistant to DNA damage. Here we describe a range of assays used to study DNA repair in Dictyostelium. Genes required for repair of DNA damage can be identified and analyzed by comparing the ability of control or mutant cells to survive exposure to genotoxic agents that induce different types of DNA lesion. We also describe assays that assess the presence of markers for DNA repair within chromatin either in the form of posttranslational modification of proteins at sites of damage or the recruitment of repair factors to DNA lesions. Finally, we also describe more direct assays to assess repair of DNA double-strand breaks by either homologous recombination or non-homologous end joining.

Determination of sphingosine-1-phosphate lyase activity by gas chromatography coupled to electron impact mass spectrometry

Sphingosine-1-phosphate lyase (SGPL1) is the last enzyme in the catabolism of sphingolipids. It catalyzes the retroaldolic cleavage of long chain base phosphates into phosphoethanolamine and a fatty aldehyde. In this article we report on an easy and sensitive procedure to determine SPL activity. The assays uses C17-sphinganine-1-phosphate as substrate and the aldehyde product, pentadecanal, is quantified as its pentafluorobenzyloxime derivative by GC/MS. Derivatization of pentadecanal is performed as a one-step reaction, and the oxime product is directly injected for GC/MS analysis without any further purification. Acquisition in selected ion monitoring mode allows very high sensitivity, with a limit of detection of 281fmol. The assay is linear with both protein concentration and incubation time up to 20μg and 40min, respectively. The K(m) value obtained (6μM) is similar to that for the natural substrate sphingosine-1-phosphate. Using this method, FTY720 and deoxypyridoxine phosphate inhibited SPL with similar potencies to those reported.

The role of ADP-ribosylation in regulating DNA double-strand break repair

ADP-ribosylation is the post translational modification of proteins catalysed by ADP-ribosyltransferases (ARTs). ADP-ribosylation has been implicated in a wide variety of cellular processes including cell growth and differentiation, apoptosis and transcriptional regulation. Perhaps the best characterised role, however, is in DNA repair and genome stability where ADP-ribosylation promotes resolution of DNA single strand breaks. Although ADP-ribosylation also occurs at DNA double strand breaks (DSBs), which ARTs catalyse this reaction and the molecular basis of how this modification regulates their repair remains a matter of debate. Here we review recent advances in our understanding of how ADP-ribosylation regulates DSB repair. Specifically, we highlight studies using the genetic model organism Dictyostelium, in addition to vertebrate cells that identify a third ART that accelerates DSB repair by non-homologous end-joining through promoting the interaction of repair factors with DNA lesions. The implications of these data with regards to how ADP-ribosylation regulates DNA repair and genome stability are discussed.

Investigating the effect of emetic compounds on chemotaxis in Dictyostelium identifies a non-sentient model for bitter and hot tastant research

Novel chemical entities (NCEs) may be investigated for emetic liability in a range of unpleasant experiments involving retching, vomiting or conditioned taste aversion/food avoidance in sentient animals. We have used a range of compounds with known emetic /aversive properties to examine the possibility of using the social amoeba, Dictyostelium discoideum, for research into identifying and understanding emetic liability, and hence reduce adverse animal experimentation in this area. Twenty eight emetic or taste aversive compounds were employed to investigate the acute (10 min) effect of compounds on Dictyostelium cell behaviour (shape, speed and direction of movement) in a shallow chemotaxic gradient (Dunn chamber). Compound concentrations were chosen based on those previously reported to be emetic or aversive in in vivo studies and results were recorded and quantified by automated image analysis. Dictyostelium cell motility was rapidly and strongly inhibited by four structurally distinct tastants (three bitter tasting compounds--denatonium benzoate, quinine hydrochloride, phenylthiourea, and the pungent constituent of chilli peppers--capsaicin). In addition, stomach irritants (copper chloride and copper sulphate), and a phosphodiesterase IV inhibitor also rapidly blocked movement. A concentration-dependant relationship was established for five of these compounds, showing potency of inhibition as capsaicin (IC(50) = 11.9 ± 4.0 µM) > quinine hydrochloride (IC(50) = 44.3 ± 6.8 µM) > denatonium benzoate (IC(50) = 129 ± 4 µM) > phenylthiourea (IC(50) = 366 ± 5 µM) > copper sulphate (IC(50) = 1433 ± 3 µM). In contrast, 21 compounds within the cytotoxic and receptor agonist/antagonist classes did not affect cell behaviour. Further analysis of bitter and pungent compounds showed that the effect on cell behaviour was reversible and not cytotoxic, suggesting an uncharacterised molecular mechanism of action for these compounds. These results therefore demonstrate that Dictyostelium has potential as a non-sentient model in the analysis of the molecular effects of tastants, although it has limited utility in identification of emetic agents in general.

The professional phagocyte Dictyostelium discoideum as a model host for bacterial pathogens

The use of simple hosts such as Dictyostelium discoideum in the study of host pathogen interactions offers a number of advantages and has steadily increased in recent years. Infection-specific genes can often only be studied in a very limited way in man and even in the mouse model their analysis is usually expensive, time consuming and technically challenging or sometimes even impossible. In contrast, their functional analysis in D. discoideum and other simple model organisms is often easier, faster and cheaper. Because host-pathogen interactions necessarily involve two organisms, it is desirable to be able to genetically manipulate both the pathogen and its host. Particularly suited are those hosts, like D. discoideum, whose genome sequence is known and annotated and for which excellent genetic and cell biological tools are available in order to dissect the complex crosstalk between host and pathogen. The review focusses on host-pathogen interactions of D. discoideum with Legionella pneumophila, mycobacteria, and Salmonella typhimurium which replicate intracellularly.

The dual-specificity protein phosphatase MkpB, homologous to mammalian MKP phosphatases, is required for D. discoideum post-aggregative development and cisplatin response

Dual-specificity protein phosphatases participate in signal transduction pathways inactivating mitogen-activated protein kinases (MAP kinases). These signaling pathways are of critical importance in the regulation of numerous biological processes, including cell proliferation, differentiation and development. The social ameba Dictyostelium discoideum harbors 14 genes coding for proteins containing regions very similar to the dual-specificity protein phosphatase domain. One of these genes, mkpB, additionally codes for a region similar to the Rhodanase domain, characteristic of animal MAP kinase-phosphatases, in its N-terminal region. Cells that over-express this gene show increased protein phosphatase activity. mkpB is expressed in D. discoideum ameba at growth but it is greatly induced at 12h of multicellular development. Although it is expressed in all the cells of developmental structures, mkpB mRNA is enriched in cells with a distribution typical of anterior-like cells. Cells that express a catalytically inactive mutant of MkpB grow and aggregate like wild-type cells but show a greatly impaired post-aggregative development. In addition, the expression of cell-type specific genes is very delayed, indicating that this protein plays an important role in cell differentiation and development. Cells expressing the MkpB catalytically inactive mutant show increased sensitivity to cisplatin, while cells over-expressing wild type MkpB, or MkpA, proteins or mutated in the MAP kinase erkB gene are more resistant to this chemotherapeutic drug, as also shown in human tumor cells.

Dictyostelium finds new roles to model

Any established or aspiring model organism must justify itself using two criteria: does the model organism offer experimental advantages not offered by competing systems? And will any discoveries made using the model be of wider relevance? This review addresses these issues for the social amoeba Dictyostelium and highlights some of the organisms more recent applications. These cover a remarkably wide gamut, ranging from sociobiological to medical research with much else in between.

Lead genetic studies in Dictyostelium discoideum and translational studies in human cells demonstrate that sphingolipids are key regulators of sensitivity to cisplatin and other anticancer drugs

A Dictyostelium discoideum mutant with a disruption in the sphingosine-1-phosphate (S-1-P) lyase gene was obtained in an unbiased genetic analysis, using random insertional mutagenesis, for mutants with increased resistance to the widely used cancer chemotherapeutic drug cisplatin. This finding opened the way to extensive studies in both D. discoideum and human cells on the role and mechanism of action of the bioactive sphingolipids S-1-P and ceramide in regulating the response to chemotherapeutic drugs. These studies showed that the levels of activities of the sphingolipid metabolizing enzymes S-1-P lyase, sphingosine kinase and ceramide synthase, affect whether a cell dies or lives in the presence of specific drugs. The demonstration that multiple enzymes of this biochemical pathway were involved in regulating drug sensitivity provided new opportunities to test whether pharmacological intervention might increase sensitivity. Thus it is of considerable clinical significance that pharmacological inhibition of sphingosine kinase synergistically sensitizes cells to cisplatin, both in D. discoideum and human cells. Linkage to the p38 MAP kinase and protein kinase C (PKC) signaling pathways has been demonstrated. This work demonstrates the utility of D. discoideum as a lead genetic system to interrogate molecular mechanisms controlling the sensitivity of tumor cells to chemotherapeutic agents and for determining novel ways of increasing efficacy. The D. discoideum system could be easily adapted to a high throughput screen for novel chemotherapeutic agents.

S1P metabolism in cancer and other pathological conditions

Nearly two decades ago, the sphingolipid metabolite sphingosine 1-phosphate was discovered to function as a lipid mediator and regulator of cell proliferation. Since that time, sphingosine 1-phosphate has been shown to mediate a diverse array of fundamental biological processes including cell proliferation, migration, invasion, angiogenesis, vascular maturation and lymphocyte trafficking. Sphingosine 1-phosphate acts primarily via signaling through five ubiquitously expressed G protein-coupled receptors. Intracellular sphingosine 1-phosphate molecules are transported extracellularly and gain access to cognate receptors for autocrine and paracrine signaling and for signaling at distant sites reached through blood and lymphatic circulation systems. Intracellular pools of sphingosine 1-phosphate available for signaling are tightly regulated primarily by three enzymes: sphinosine kinase, S1P lyase and S1P phosphatase. Alterations in sphingosine 1-phosphate as well as the enzymes involved in its synthesis and catabolism have been observed in many types of malignancy. These enzymes are being evaluated for their role in mediating cancer formation and progression, as well as their potential to serve as targets of anti-cancer therapeutics. In this review, the impact of sphingosine 1-phosphate, its cognate receptors, and the enzymes of sphingosine 1-phosphate metabolism on cell survival, apoptosis, autophagy, cellular transformation, invasion, angiogenesis and hypoxia in relation to cancer biology and treatment are discussed.

Lyase to live by: sphingosine phosphate lyase as a therapeutic target

BACKGROUND

Sphingosine 1-phosphate (S1P) is a bioactive lipid that regulates cell proliferation, survival and migration and plays an essential role in angiogenesis and lymphocyte trafficking. S1P levels in the circulation and tissues are tightly regulated for proper cell functioning, and dysregulation of this system may contribute to the pathophysiology of certain human diseases. Sphingosine phosphate lyase (SPL) irreversibly degrades S1P and thereby acts as a gatekeeper that regulates S1P signaling by modulating intracellular S1P levels and the chemical S1P gradient that exists between lymphoid organs and circulating blood and lymph. However, SPL also generates biochemical products that may be relevant in human disease. SPL has been directly implicated in various physiological and pathological processes, including cell stress responses, cancer, immunity, hematopoietic function, muscle homeostasis, inflammation and development.

OBJECTIVE/METHODS

This review summarizes the current know-ledge of SPL structure, function and regulation, its involvement in various disease states and currently available small molecules known to modulate SPL activity.

RESULTS/CONCLUSION

This review provides evidence that SPL is a potential target for pharmacological manipulation for the treatment of malignant, autoimmune, inflammatory and other diseases.

Dysregulation of purine nucleotide biosynthesis pathways modulates cisplatin cytotoxicity in Saccharomyces cerevisiae

We found previously that inactivation of the FCY2 gene, encoding a purine-cytosine permease, or the HPT1 gene, encoding the hypoxanthine guanine phosphoribosyl transferase, enhances cisplatin resistance in yeast cells. Here, we report that in addition to fcy2Delta and hpt1Delta mutants in the salvage pathway of purine nucleotide biosynthesis, mutants in the de novo pathway that disable the feedback inhibition of AMP and GMP biosynthesis also enhanced cisplatin resistance. An activity-enhancing mutant of the ADE4 gene, which constitutively synthesizes AMP and excretes hypoxanthine, and a GMP kinase mutant (guk1), which accumulates GMP and feedback inhibits Hpt1 function, both enhanced resistance to cisplatin. In addition, overexpression of the ADE4 gene in wild-type cells, which increases de novo synthesis of purine nucleotides, also resulted in elevated cisplatin resistance. Cisplatin cytotoxicity in wild-type cells was abolished by low concentration of extracellular purines (adenine, hypoxanthine, and guanine) but not cytosine. Inhibition of cytotoxicity by exogenous adenine was accompanied by a reduction of DNA-bound cisplatin in wild-type cells. As a membrane permease, Fcy2 may mediate limited cisplatin transport because cisplatin accumulation in whole cells was slightly affected in the fcy2Delta mutant. However, the fcy2Delta mutant had a greater effect on the amount of DNA-bound cisplatin, which decreased to 50 to 60% of that in the wild-type cells. Taken together, our results indicate that dysregulation of the purine nucleotide biosynthesis pathways and the addition of exogenous purines can modulate cisplatin cytotoxicity in Saccharomyces cerevisiae.

Sphingosine-1-phosphate lyase in development and disease: sphingolipid metabolism takes flight

Sphingosine-1-phosphate lyase (SPL) is a highly conserved enzyme that catalyses the final step of sphingolipid degradation, namely the irreversible cleavage of the carbon chain at positions 2-3 of a long-chain base phosphate (LCBP), thereby yielding a long-chain aldehyde and phosphoethanolamine. LCBPs are potent signaling molecules involved in cell proliferation, survival, migration, cell-cell interactions and cell stress responses. Therefore, tight regulation of LCBP signaling is required for proper cell function, and perturbations of this system can lead to alterations in biological processes including development, reproduction and physiology. SPL is a key enzyme in regulating the intracellular and circulating levels of LCBPs and is, therefore, gaining attention as a putative target for pharmacological intervention. This review provides an overview of our current understanding of SPL structure and function, mechanisms involved in SPL regulation and the role of SPL in development and disease.

Analysis of autophagic and necrotic cell death in Dictyostelium

Non-apoptotic cell death types can be conveniently studied in Dictyostelium discoideum, an exceptionally favorable model not only because of its well-known genetic and experimental advantages, but also because in Dictyostelium there is no apoptosis machinery that could interfere with non-apoptotic cell death. We show here how to conveniently demonstrate, assess, and study these non-apoptotic cell death types. These can be generated by use of modifications of the monolayer technique of Rob Kay et al., and either wild-type HMX44A Dictyostelium cells, leading to autophagic cell death, or the corresponding atg1- autophagy gene mutant cells, leading to necrotic cell death. Methods to follow these non-apoptotic cell death types qualitatively and quantitatively will be reported.

(Dihydro)ceramide synthase 1 regulated sensitivity to cisplatin is associated with the activation of p38 mitogen-activated protein kinase and is abrogated by sphingosine kinase 1

Resistance to chemotherapeutic drugs often limits their clinical efficacy. Previous studies have implicated the bioactive sphingolipid sphingosine-1-phosphate (S-1-P) in regulating sensitivity to cisplatin [cis-diamminedichloroplatinum(II)] and showed that modulating the S-1-P lyase can alter cisplatin sensitivity. Here, we show that the members of the sphingosine kinase (SphK1 and SphK2) and dihydroceramide synthase (LASS1/CerS1, LASS4/CerS4, and LASS5/CerS5) enzyme families each have a unique role in regulating sensitivity to cisplatin and other drugs. Thus, expression of SphK1 decreases sensitivity to cisplatin, carboplatin, doxorubicin, and vincristine, whereas expression of SphK2 increases sensitivity. Expression of LASS1/CerS1 increases the sensitivity to all the drugs tested, whereas LASS5/CerS5 only increases sensitivity to doxorubicin and vincristine. LASS4/CerS4 expression has no effect on the sensitivity to any drug tested. Reflecting this, we show that the activation of the p38 mitogen-activated protein (MAP) kinase is increased only by LASS1/CerS1, and not by LASS4/CerS4 or LASS5/CerS5. Cisplatin was shown to cause a specific translocation of LASS1/CerS1, but not LASS4/CerS4 or LASS5/CerS5, from the endoplasmic reticulum (ER) to the Golgi apparatus. Supporting the hypothesis that this translocation is mechanistically involved in the response to cisplatin, we showed that expression of SphK1, but not SphK2, abrogates both the increased cisplatin sensitivity in cells stably expressing LASS1/CerS and the translocation of the LASS1/CerS1. The data suggest that the enzymes of the sphingolipid metabolic pathway can be manipulated to improve sensitivity to the widely used drug cisplatin.

Global transcriptional responses to cisplatin in Dictyostelium discoideum identify potential drug targets

Dictyostelium discoideum is a useful model for studying mechanisms of cisplatin drug sensitivity. Our previous findings, that mutations in sphingolipid metabolism genes confer cisplatin resistance in D. discoideum and in human cells, raised interest in the resistance mechanisms and their implications for cisplatin chemotherapy. Here we used expression microarrays to monitor physiological changes and to identify pathways that are affected by cisplatin treatment of D. discoideum. We found >400 genes whose regulation was altered by cisplatin treatment of wild-type cells, including groups of genes that participate in cell proliferation and in nucleotide and protein metabolism, showing that the cisplatin response is orderly and multifaceted. Transcriptional profiling of two isogenic cisplatin-resistant mutants, impaired in different sphingolipid metabolism steps, showed that the effect of cisplatin treatment was greater than the effect of the mutations, indicating that cisplatin resistance in the mutants is due to specific abilities to overcome the drug effects rather than to general drug insensitivity. Nevertheless, the mutants exhibited significantly different responses to cisplatin compared with the parent, and >200 genes accounted for that difference. Mutations in five cisplatin response genes (sgkB, csbA, acbA, smlA, and atg8) resulted in altered drug sensitivity, implicating novel pathways in cisplatin response. Our data illustrate how modeling complex cellular responses to drugs in genetically stable and tractable systems can uncover new targets with the potential for improving chemotherapy.

QSAR of genotoxic active benzazoles

Previously synthesized 2,5-disubstituted benzoxazole and benzimidazole derivatives, were tested for their genotoxic activity in the Bacillus subtilis rec- assay. The results revealed that 5-methyl-2-(p-aminobenzyl)benzoxazole exhibited the highest genotoxic response, which was comparable to 4-nitroquinoline 1-oxide (4-NQO), the reference agent of classical positive mutagen. Among the other tested compounds, four showed a genotoxic activity. A QSAR study revealed that structural parameters IY(C(2)H(4)) and IY(CH(2)O), indicating the bridge elements between the phenyl moiety and the fused ring system at position 2 and the quantum chemical parameter (DeltaE ), showing the difference between HOMO and LUMO energies, were found significant for enhancing the genotoxic activity in these compounds. In addition, the substituent effects on positions R and R(1) were found important for the activity as well as holding a substituent possessing a maximum length with a minimum width property on position R(1) like alkyl groups. On the other hand, substituting position R with an electron donating group instead of electron withdrawing group increased the genotoxic activity.

Sphingosine-1-phosphate lyase in immunity and cancer: silencing the siren

Sphingosine-1-phosphate (S1P) is a bioactive lipid that promotes cell survival, proliferation and migration, platelet aggregation, mediates ischemic preconditioning, and is essential for angiogenesis and lymphocyte trafficking. Sphingosine-1-phosphate lyase (SPL) is the enzyme responsible for the irreversible degradation of S1P and is, thus, in a strategic position to regulate these same processes by removing available S1P signaling pools, that is, silencing the siren. In fact, recent studies have implicated SPL in the regulation of immunity, cancer surveillance and other physiological processes. Here, we summarize the current understanding of SPL function and regulation, and discuss how SPL might facilitate cancer chemoprevention and serve as a target for modulation of immune responses in transplantation settings and in the treatment of autoimmune disease.

Sensitive cell viability assay for use in drug screens and for studying the mechanism of action of drugs in Dictyostelium discoideum

In order to increase the effectiveness of Dictyostelium discoideum as a lead genetic model for drug discovery, a luminescence-based assay has been adapted and standardized for sensitive and rapid cell viability measurements. The applicability of the assay was demonstrated by measuring the cytotoxicity of several drugs in wild-type and mutant cells. The robustness and ease of the assay demonstrate that it can be used in high-throughput applications such as drug or mutant screens. Conclusions from these studies are applicable to evaluating cell viability assays in other systems as well.

Sphingosine-1-phosphate lyase potentiates apoptosis via p53- and p38-dependent pathways and is down-regulated in colon cancer

Sphingolipid metabolites such as sphingosine-1-phosphate (S1P) and ceramide modulate apoptosis during development and in response to stress. In general, ceramide promotes apoptosis, whereas S1P stimulates cell proliferation and protects against apoptosis. S1P is irreversibly degraded by the enzyme S1P lyase (SPL). In this study, we show a crucial role for SPL in mediating cellular responses to stress. SPL expression in HEK293 cells potentiated apoptosis in response to stressful stimuli including DNA damage. This effect seemed to be independent of ceramide generation but required SPL enzymatic activity and the actions of p38 MAP kinase, p53, p53-inducible death domain protein (PIDD), and caspase-2 as shown by molecular and chemical inhibition of each of these targets. Further, SPL expression led to constitutive activation of p38. Endogenous SPL expression was induced by DNA damage in WT cells, whereas SPL knockdown diminished apoptotic responses. Importantly, SPL expression was significantly down-regulated in human colon cancer tissues in comparison with normal adjacent tissues, as determined by quantitative real-time PCR (Q-PCR) and immunohistochemical analysis. Down-regulation of S1P phosphatases was also observed, suggesting that colon cancer cells manifest a block in S1P catabolism. In addition, SPL expression and activity were down-regulated in adenomatous lesions of the Min mouse model of intestinal tumorigenesis. Taken together, these results indicate that endogenous SPL may play a physiological role in stress-induced apoptosis and provide an example of altered SPL expression in a human tumor. Our findings suggest that genetic or epigenetic changes affecting intestinal S1P metabolism may correlate with and potentially contribute to carcinogenesis.

Towards a molecular understanding of human diseases using Dictyostelium discoideum

The social amoeba Dictyostelium discoideum is increasingly being used as a simple model for the investigation of problems that are relevant to human health. This article focuses on several recent examples of Dictyostelium-based biomedical research, including the analysis of immune-cell disease and chemotaxis, centrosomal abnormalities and lissencephaly, bacterial intracellular pathogenesis, and mechanisms of neuroprotective and anti-cancer drug action. The combination of cellular, genetic and molecular biology techniques that are available in Dictyostelium often makes the analysis of these problems more amenable to study in this system than in mammalian cell culture. Findings that have been made in these areas using Dictyostelium have driven research in mammalian systems and have established Dictyostelium as a powerful model for human-disease analysis.

Sphingosine-1-phosphate lyase regulates sensitivity of human cells to select chemotherapy drugs in a p38-dependent manner

Resistance to cisplatin is a common problem that limits its usefulness in cancer therapy. Molecular genetic studies in the model organism Dictyostelium discoideum have established that modulation of sphingosine kinase or sphingosine-1-phosphate (S-1-P) lyase, by disruption or overexpression, results in altered cellular sensitivity to this widely used drug. Parallel changes in sensitivity were observed for the related compound carboplatin but not for other chemotherapy drugs tested. Sensitivity to cisplatin could also be potentiated pharmacologically with dimethylsphingosine, a sphingosine kinase inhibitor. We now have validated these studies in cultured human cell lines. HEK293 or A549 lung cancer cells expressing human S-1-P lyase (hSPL) show an increase in sensitivity to cisplatin and carboplatin as predicted from the earlier model studies. The hSPL-overexpressing cells were also more sensitive to doxorubicin but not to vincristine or chlorambucil. Studies using inhibitors to specific mitogen-activated protein kinases (MAPK) show that the increased cisplatin sensitivity in the hSPL-overexpressing cells is mediated by p38 and to a lesser extent by c-Jun NH2-terminal kinase MAPKs. p38 is not involved in vincristine or chlorambucil cytotoxicity. Measurements of MAPK phosphorylation and enzyme activity as well as small interfering RNA inhibition studies show that the response to the drug is accompanied by up-regulation of p38 and c-Jun NH2-terminal kinase and the lack of extracellular signal-regulated kinase up-regulation. These studies confirm an earlier model proposing a mechanism for the drug specificity observed in the studies with D. discoideum and support the idea that the sphingosine kinases and S-1-P lyase are potential targets for improving the efficacy of cisplatin therapy for human tumors.

dictyBase, the model organism database for Dictyostelium discoideum

dictyBase (http://dictybase.org) is the model organism database (MOD) for the social amoeba Dictyostelium discoideum. The unique biology and phylogenetic position of Dictyostelium offer a great opportunity to gain knowledge of processes not characterized in other organisms. The recent completion of the 34 MB genome sequence, together with the sizable scientific literature using Dictyostelium as a research organism, provided the necessary tools to create a well-annotated genome. dictyBase has leveraged software developed by the Saccharomyces Genome Database and the Generic Model Organism Database project. This has reduced the time required to develop a full-featured MOD and greatly facilitated our ability to focus on annotation and providing new functionality. We hope that manual curation of the Dictyostelium genome will facilitate the annotation of other genomes.

Dictyostelium discoideum to human cells: pharmacogenetic studies demonstrate a role for sphingolipids in chemoresistance

Resistance to chemotherapy is a major obstacle for the treatment of cancer and a subject of extensive research. Numerous mechanisms of drug resistance have been proposed, and they differ for different drugs. Nevertheless, it is clear that our understanding of this important problem is still incomplete, and that new targets for modulating therapy still await discovery. The attractive biology and the availability of powerful molecular techniques have made the cellular slime mold Dictyostelium discoideum, a powerful non-mammalian model for drug target discovery, and the problem of drug resistance. To understand the molecular basis of chemoresistance to the widely used drug cisplatin, both genetic and pharmacological approaches have been applied to this versatile experimental system. These studies have resulted in the identification of novel molecular pathways which can be used to increase the efficacy of cisplatin, and brought attention to the role of sphingolipids in mediating the cellular response to chemotherapeutic drugs. In the following review, we will describe the history and utility of D. discoideum in pharmacogenetics, and discuss recent studies which focus attention on the role of sphingolipids in chemotherapy and chemoresistance.

Overexpression of sphingosine-1-phosphate lyase or inhibition of sphingosine kinase in Dictyostelium discoideum results in a selective increase in sensitivity to platinum-based chemotherapy drugs

The efficacy of the chemotherapy drug cisplatin is often limited due to resistance of the tumors to the drug, and increasing the potency of cisplatin without increasing its concentration could prove beneficial. A previously characterized Dictyostelium discoideum mutant with increased resistance to cisplatin was defective in the gene encoding sphingosine-1-phosphate (S-1-P) lyase, which catalyzes the breakdown of S-1-P, an important regulatory molecule in cell function and development and in the regulation of cell fate. We hypothesized that the increased resistance to cisplatin was due to an elevation of S-1-P and predicted that lowering levels of S-1-P should increase sensitivity to the drug. We generated three strains that stably overexpress different levels of the S-1-P lyase. The overexpressor strains have reduced growth rate and, confirming the hypothesis, showed an expression-dependent increase in sensitivity to cisplatin. Consistently, treating the cells with D-erythro-N,N,-dimethylsphingosine, a known inhibitor of sphingosine kinase, increased the sensitivity of mutant and parent cells to cisplatin, while addition of exogenous S-1-P or 8-Br-cyclic AMP made the cells more resistant to cisplatin. The increased sensitivity of the overexpressors to cisplatin was also observed with the cisplatin analog carboplatin. In contrast, the response to doxorubicin, 5-flurouracil, or etoposide was unaffected, indicating that the involvement of the sphingolipid metabolic pathway in modulating the response to cisplatin is not part of a global genotoxic stress response. The augmented sensitivity to cisplatin appears to be the result of an intracellular signaling function of S-1-P, because D. discoideum does not appear to have endothelial differentiation growth (EDG/S1P) receptors. Overall, the results show that modulation of the sphingolipid pathway at multiple points can result in increased sensitivity to cisplatin and has the potential for increasing the clinical usefulness of this important drug.

Genome-wide screen identifies genes whose inactivation confer resistance to cisplatin in Saccharomyces cerevisiae

To identify novel genes that mediate cellular resistance to cisplatin, we have screened the collection of Saccharomyces cerevisiae deletion strains. We have found reproducibly 22 genes/open reading frames (ORF), which when deleted, confer resistance to cisplatin at a concentration that is lethal to wild-type cells. Complementation of individual deletion strains with the corresponding wild-type gene abolished cisplatin resistance, confirming that specific gene deletions caused the resistance. Twenty of the genes/ORFs identified have not been previously linked to cisplatin resistance and belong to several distinct functional groups. Major functional groups encode proteins involved in nucleotide metabolism, mRNA catabolism, RNA-polymerase-II-dependent gene regulation and vacuolar transport systems. In addition, proteins that function in ubiquitination, sphingolipid biogenesis, cyclic AMP-dependent signaling, DNA repair, and genome stability are also associated with cisplatin resistance. More than half of the identified genes are known to have sequences or functional homology to mammalian counterparts. Some deletion strains are cross-resistant to selected cytotoxic agents whereas hypersensitive to others. The sensitivity of certain resistant strains to other cytotoxic agents suggests that our findings may point to particular drug combinations that can overcome resistance caused by inactivation of specific genes.

Manifestations of multicellularity: Dictyostelium reports in

The recent release of the Dictyostelium genome sequence is important because Dictyostelium has become a much-favoured model system for cell and developmental biologists. The sequence has revealed a remarkably high total number of approximately 12 500 genes, only a thousand fewer than are encoded by Drosophila. Previous protein-sequence comparisons suggested that Dictyostelium is evolutionarily closer to animals and fungi than to plants, and the global protein sequence comparison, now made possible by the genome sequence, confirms this. This review focuses on several classes of proteins that are shared by Dictyostelium and animals: a highly sophisticated array of microfilament components, a large family of G-protein-coupled receptors and a diverse set of SH2 domain-containing proteins. The presence of these proteins strengthens the case for a relatively close relationship with animals and extends the range of problems that can be addressed using Dictyostelium as a model organism.

Sphingosine kinase regulates the sensitivity of Dictyostelium discoideum cells to the anticancer drug cisplatin

The drug cisplatin is widely used to treat a number of tumor types. However, resistance to the drug, which remains poorly understood, limits its usefulness. Previous work using Dictyostelium discoideum as a model for studying drug resistance showed that mutants lacking sphingosine-1-phosphate (S-1-P) lyase, the enzyme that degrades S-1-P, had increased resistance to cisplatin, whereas mutants overexpressing the enzyme were more sensitive to the drug. S-1-P is synthesized from sphingosine and ATP by the enzyme sphingosine kinase. We have identified two sphingosine kinase genes in D. discoideum--sgkA and sgkB--that are homologous to those of other species. The biochemical properties of the SgkA and SgkB enzymes suggest that they are the equivalent of the human Sphk1 and Sphk2 enzymes, respectively. Disruption of the kinases by homologous recombination (both single and double mutants) or overexpression of the sgkA gene resulted in altered growth rates and altered response to cisplatin. The null mutants showed increased sensitivity to cisplatin, whereas mutants overexpressing the sphingosine kinase resulted in increased resistance compared to the parental cells. The results indicate that both the SgkA and the SgkB enzymes function in regulating cisplatin sensitivity. The increase in sensitivity of the sphingosine kinase-null mutants was reversed by the addition of S-1-P, and the increased resistance of the sphingosine kinase overexpressor mutant was reversed by the inhibitor N,N-dimethylsphingosine. Parallel changes in sensitivity of the null mutants are seen with the platinum-based drug carboplatin but not with doxorubicin, 5-fluorouracil, and etoposide. This pattern of specificity is similar to that observed with the S-1-P lyase mutants and should be useful in designing therapeutic schemes involving more than one drug. This study identifies the sphingosine kinases as new drug targets for modulating the sensitivity to platinum-based drugs.

Sphingosine-1-phosphate plays a role in the suppression of lateral pseudopod formation during Dictyostelium discoideum cell migration and chemotaxis

Sphingosine-1-phosphate (S-1-P) is a bioactive lipid that plays a role in diverse biological processes. It functions both as an extracellular ligand through a family of high-affinity G-protein-coupled receptors, and intracellularly as a second messenger. A growing body of evidence has implicated S-1-P in controlling cell movement and chemotaxis in cultured mammalian cells. Mutant D. discoideum cells, in which the gene encoding the S-1-P lyase had been specifically disrupted by homologous recombination, previously were shown to be defective in pseudopod formation, suggesting that a resulting defect might exist in motility and/or chemotaxis. To test this prediction, we analyzed the behavior of mutant cells in buffer, and in both spatial and temporal gradients of the chemoattractant cAMP, using computer-assisted 2-D and 3-D motion analysis systems. Under all conditions, S-1-P lyase null mutants were unable to suppress lateral pseudopod formation like wild-type control cells. This resulted in a reduction in velocity in buffer and spatial gradients of cAMP. Mutant cells exhibited positive chemotaxis in spatial gradients of cAMP, but did so with lowered efficiency, again because of their inability to suppress lateral pseudopod formation. Mutant cells responded normally to simulated temporal waves of cAMP but mimicked the temporal dynamics of natural chemotactic waves. The effect must be intracellular since no homologs of the S-1-P receptors have been identified in the Dictyostelium genome. The defects in the S-1-P lyase null mutants were similar to those seen in mutants lacking the genes for myosin IA, myosin IB, and clathrin, indicating that S-1-P signaling may play a role in modulating the activity or organization of these cytoskeletal elements in the regulation of lateral pseudopod formation.