Leishmania donovani-induced ceramide as the key mediator of Akt dephosphorylation in murine macrophages: role of protein kinase Czeta and phosphatase

SHIP1 inhibition via 3-alpha-amino-cholestane enhances protection against Leishmania infection

Leishmania major and L. donovani cause cutaneous leishmaniasis and visceral leishmaniasis, respectively. Available chemotherapies suffer from toxicity, drug-resistance or high cost of production prompting the need for the discovery of new anti-leishmanials. Here, we test a novel aminosteriodal compound- 3-alpha-amino-cholestane [3AC] - that shows selective inhibition of SHIP1, an inositol-5'-phosphate-specific phosphatase with potent effects on the immune system. We report that 3AC-sensitive SHIP1 expression increases in Leishmania-infected macrophages. Treatment of BALB/c mice, a Leishmania-susceptible host, with 3AC increased anti-leishmanial, but reduced pro-leishmanial, cytokines' production and reduced the parasite load in both L. major and L. donovani infections. These findings implicate SHIPi as a potential novel immunostimulant with anti-leishmanial function.

Zooming in on common immune evasion mechanisms of pathogens in phagolysosomes: potential broad-spectrum therapeutic targets against infectious diseases

The intracellular viral, bacterial, or parasitic pathogens evade the host immune challenges to propagate and cause fatal diseases. The microbes overpower host immunity at various levels including during entry into host cells, phagosome formation, phagosome maturation, phagosome-lysosome fusion forming phagolysosomes, acidification of phagolysosomes, and at times after escape into the cytosol. Phagolysosome is the final organelle in the phagocyte with sophisticated mechanisms to degrade the pathogens. The immune evasion strategies by the pathogens include the arrest of host cell apoptosis, decrease in reactive oxygen species, the elevation of Th2 anti-inflammatory response, avoidance of autophagy and antigen cross-presentation pathways, and escape from phagolysosomal killing. Since the phagolysosome organelle in relation to infection/cure is seldom discussed in the literature, we summarize here the common host as well as pathogen targets manipulated or utilized by the pathogens established in phagosomes and phagolysosomes, to hijack the host immune system for their benefit. These common molecules or pathways can be broad-spectrum therapeutic targets for drug development for intervention against infectious diseases caused by different intracellular pathogens.

Leishmania intercepts IFN-γR signaling at multiple levels in macrophages

IFN-γ, a type 2 interferon and a cytokine, is critical for both innate and adaptive immunity. IFN-γ binds to the IFN-γRs on the cell membrane of macrophages, signals through JAK1-STAT-1 pathway and induces IFN-γ-stimulated genes (ISGs). As Leishmania amastigotes reside and replicate within macrophages, IFN-γ mediated macrophage activation eventuate in Leishmania elimination. As befits the principle of parasitism, the impaired IFN-γ responsiveness in macrophages ensures Leishmania survival. IFN-γ responsiveness is a function of integrated molecular events at multiple levels in the cells that express IFN-γ receptors. In Leishmania-infected macrophages, reduced IFN-γRα expression, impaired IFN-γRα and IFN-γRβ hetero-dimerization due to altered membrane lipid composition, reduced JAK-1 and STAT-1 phosphorylation but increased STAT-1 degradation and impaired ISGs induction collectively determine the IFN-γ responsiveness and the efficacy of IFN-γ induced antileishmanial function of macrophages. Therefore, parasite load is not only decided by the levels of IFN-γ produced but also by the IFN-γ responsiveness. Indeed, in Leishmania-infected patients, IFN-γ is produced but IFN-γ signalling is downregulated. However, the molecular mechanisms of IFN-γ responsiveness remain unclear. Therefore, we review the current understanding of IFN-γ responsiveness of Leishmania-infected macrophages.

Cancer stem cells and ceramide signaling: the cutting edges of immunotherapy

The multipotent, self renewing "cancer stem cells" (CSCs), a small population within tumor microenvironment facilitates transformed cells to grow and propagate within the body. The CSCs are discovered as resistant to the chemotherapeutic drug with distinct immunological characteristics. In recent years, immunologically targeting CSCs have emerged as an integral part of effective and successful cancer therapy. CSCs notably exhibit dysregulation in conventional sub-cellular sphingolipid metabolism. Recently, ceramide decaying enzymes have been shown to activate alternative ceramide signaling pathways leading to reduction in efficacy of the chemotherapeutic drugs. Therefore, a control over ceramide mediated modulations of CSCs offers an attractive dimension of effective cancer treatment strategy in future. In this review, we focused on the recent findings on broad spectrum of ceramide mediated signaling in CSCs within the tumor niche and their role in potential cancer immunotherapy.

PKCζ mediated anti-proliferative effect of C2 ceramide on neutralization of the tumor microenvironment and melanoma regression

Immunotherapy, which has advantages over chemotherapy due to lesser toxicity and higher specificity, is on the rise to treat cancer. Recently, pro-apoptotic glycolipid, ceramide has emerged as a key regulator in cancer immunotherapy. The present study elucidated the potential anti-melanoma efficacy of cell-permeable, exogenous C2 ceramide on cell death and amelioration of tumor microenvironment (TME). We, for the first time, demonstrated that C2 ceramide triggered apoptosis of melanoma cells by augmenting PKCζ along with pro-inflammatory cytokines and signaling factors. C2 ceramide showed a PKCζ-mediated tumor-suppressive role in melanoma without exhibiting hepatotoxicity and nephrotoxicity. Moreover, PKCζ was revealed as one of the key regulators of Akt and ceramide during C2 ceramide-mediated apoptosis. C2 ceramide was effective in repolarization of M2 macrophage phenotype and reduction of angiogenic factors such as VEGF, VEGFR1, VEGFR2, HIF1α. Interestingly, PKCζ knockdown attenuated C2 ceramide-mediated inhibition of melanoma progression. Restoration of the Th1 type TME by C2 ceramide enhanced cytotoxic T cell-mediated killing of melanoma cells. Altogether, the study unraveled that C2 ceramide-induced PKCζ was associated with favorable immune cell functioning in TME leading to melanoma regression. Thus, our findings explored a novel mechanistic insight into C2 ceramide as a promising immunotherapeutic agent in melanoma treatment.

Crosstalk of PD-1 signaling with the SIRT1/FOXO-1 axis during the progression of visceral leishmaniasis

Previously, we documented the role of the programmed death-1 (PD-1, also known as PDCD1) pathway in macrophage apoptosis and the downregulation of this signaling during infection by the intra-macrophage parasite Leishmania donovani However, we also found that, during the late phase of infection, PD-1 expression was significantly increased without activating host cell apoptosis; here we show that inhibition of PD-1 led to markedly decreased parasite survival, along with increased production of TNFα, IL-12, reactive oxygen species (ROS) and nitric oxide (NO). Increased PD-1 led to inactivation of AKT proteins resulting in nuclear sequestration of FOXO-1. Transfecting infected cells with constitutively active FOXO-1 (CA-FOXO) led to increased cell death, thereby suggesting that nuclear FOXO-1 might be inactivated. Infection significantly induced the expression of SIRT1, which inactivated FOXO-1 through deacetylation, and its knockdown led to increased apoptosis. SIRT1 knockdown also significantly decreased parasite survival along with increased production of TNFα, ROS and NO. Administration of the SIRT1 inhibitor sirtinol (10 mg/kg body weight) in infected mice decreased spleen parasite burden and a synergistic effect was found with PD-1 inhibitor. Collectively, our study shows that Leishmania utilizes the SIRT1/FOXO-1 axis for differentially regulating PD-1 signaling and, although they are interconnected, both pathways independently contribute to intracellular parasite survival.This article has an associated First Person interview with the first author of the paper.

TNFα mediated ceramide generation triggers cisplatin induced apoptosis in B16F10 melanoma in a PKCδ independent manner

Ceramide is one of the important cellular components involved in cancer regulation and exerts its pleiotropic role in the protective immune response without exhibiting any adverse effects during malignant neoplasm. Although, the PKCδ-ceramide axis in cancer cells has been an effective target in reduction of cancer, involvement of PKCδ in inducing nephrotoxicity have become a major questionnaire. In the present study, we have elucidated the mechanism by which cisplatin exploits the ceramide to render cancer cell apoptosis leading to the abrogation of malignancy in a PKCδ independent pathway with lesser toxicity. Our study revealed that cisplatin treatment in PKCδ silenced melanoma cells induces ceramide mediated apoptosis. Moreover, cisplatin induced upregulation of the transcription factor IRF1 leading to the induction of the transcriptional activity of the TNFα promoter was evident from the pharmacological inhibition and RNA interference studies. Increased cellular expression of TNFα resulted in an elevated ceramide generation by stimulating acid-sphingomyelinase and cPLA₂. Furthermore, reciprocity in the regulation of sphingosine kinase 1 (Sphk1) and sphingosine kinase 2 (Sphk2) during PKCδ independent ceramide generation was also observed during cisplatin treatment. PKCδ inhibited murine melanoma model showed reduction in nephrotoxicity along with tumor regression by ceramide generation. Altogether, the current study emphasized the unexplored signaling cascade of ceramide generation by cisplatin during PKCδ silenced condition, which is associated with increased TNFα generation. Our findings enlightened the detailed mechanistic insight of ceramide mediated signaling by chemotherapeutic drugs in cancer therapy exploring a new range of targets for cancer treatment strategies.

Defining lipid mediators of insulin resistance - controversies and challenges

Essential elements of all cells, lipids play important roles in energy production, signalling and as structural components. Despite these critical functions, excessive availability and intracellular accumulation of lipid is now recognised as a major factor contributing to many human diseases, including obesity and diabetes. In the context of these metabolic disorders, ectopic deposition of lipid has been proposed to have deleterious effects of insulin action. While this relationship has been recognised for some time now, there is currently no unifying mechanism to explain how lipids precipitate the development of insulin resistance. This review summarises the evidence linking specific lipid molecules to the induction of insulin resistance, describing some of the current controversies and challenges for future studies in this field.

Function of Macrophage and Parasite Phosphatases in Leishmaniasis

The kinetoplastid protozoan parasites belonging to the genus Leishmania are the causative agents of different clinical forms of leishmaniasis, a vector-borne infectious disease with worldwide prevalence. The protective host immune response against Leishmania parasites relies on myeloid cells such as dendritic cells and macrophages in which upon stimulation by cytokines (e.g., interferon-γ) a complex network of signaling pathways is switched on leading to strong antimicrobial activities directed against the intracellular parasite stage. The regulation of these pathways classically depends on post-translational modifications of proteins, with phosphorylation events playing a cardinal role. Leishmania parasites deactivate their phagocytic host cells by inducing specific mammalian phosphatases that are capable to impede signaling. On the other hand, there is now also evidence that Leishmania spp. themselves express phosphatases that might target host cell molecules and thereby facilitate the intracellular survival of the parasite. This review will present an overview on the modulation of host phosphatases by Leishmania parasites as well as on the known families of Leishmania phosphatases and their possible function as virulence factors. A more detailed understanding of the role of phosphatases in Leishmania–host cell interactions might open new avenues for the treatment of non-healing, progressive forms of leishmaniasis.

Parasite excretory-secretory products and their effects on metabolic syndrome

Obesity, one of the main causes of metabolic syndrome (MetS), is an increasingly common health and economic problem worldwide, and one of the major risk factors for developing type 2 diabetes and cardiovascular disease. Chronic, low-grade inflammation is associated with MetS and obesity. A dominant type 2/anti-inflammatory response is required for metabolic homoeostasis within adipose tissue: during obesity, this response is replaced by infiltrating, inflammatory macrophages and T cells. Helminths and certain protozoan parasites are able to manipulate the host immune response towards a TH2 immune phenotype that is beneficial for their survival, and there is emerging data that there is an inverse correlation between the incidence of MetS and helminth infections, suggesting that, as with autoimmune and allergic diseases, helminths may play a protective role against MetS disease. Within this review, we will focus primarily on the excretory-secretory products that the parasites produce to modulate the immune system and discuss their potential use as therapeutics against MetS and its associated pathologies.

Leishmania donovani Utilize Sialic Acids for Binding and Phagocytosis in the Macrophages through Selective Utilization of Siglecs and Impair the Innate Immune Arm

Background

Leishmania donovani, belonging to a unicellular protozoan parasite, display the differential level of linkage-specific sialic acids on their surface. Sialic acids binding immunoglobulin-like lectins (siglecs) are a class of membrane-bound receptors present in the haematopoetic cell lineages interact with the linkage-specific sialic acids. Here we aimed to explore the utilization of sialic acids by Leishmania donovani for siglec-mediated binding, phagocytosis, modulation of innate immune response and signaling pathways for establishment of successful infection in the host.

Methodology/Principle Findings

We have found enhanced binding of high sialic acids containing virulent strains (AG83^+Sias) with siglec-1 and siglec-5 present on macrophages compared to sialidase treated AG83^+Sias (AG83^-Sias) and low sialic acids-containing avirulent strain (UR6) by flow cytometry. This specific receptor-ligand interaction between sialic acids and siglecs were further confirmed by confocal microscopy. Sialic acids-siglec-1-mediated interaction of AG83^+Sias with macrophages induced enhanced phagocytosis. Additionally, sialic acids-siglec-5 interaction demonstrated reduced ROS, NO generation and Th2 dominant cytokine response upon infection with AG83^+Sias in contrast to AG83^-Sias and UR6. Sialic acids-siglecs binding also facilitated multiplication of intracellular amastigotes. Moreover, AG83^+Sias induced sialic acids-siglec-5-mediated upregulation of host phosphatase SHP-1. Such sialic acids-siglec interaction was responsible for further downregulation of MAPKs (p38, ERK and JNK) and PI3K/Akt pathways followed by the reduced translocation of p65 subunit of NF-κβ to the nucleus from cytosol in the downstream signaling pathways. This sequence of events was reversed in AG83^-Sias and UR6-infected macrophages. Besides, siglec-knockdown macrophages also showed the reversal of AG83^+Sias infection-induced effector functions and downstream signaling events.

Conclusions/Significances

Taken together, this study demonstrated that virulent parasite (AG83^+Sias) establish a unique sialic acids-mediated binding and subsequent phagocytosis in the host cell through the selective exploitation of siglec-1. Additionally, sialic acids-siglec-5 interaction altered the downstream signaling pathways which contributed impairment of immune effector functions of macrophages. To the best of our knowledge, this is a comprehensive report describing sialic acids-siglec interactions and their role in facilitating uptake of the virulent parasite within the host.

Sialic acids are nine carbon sugars present on terminal cell surface glycoproteins and glycolipids. Siglec is a membrane receptor that belongs to an immunoglobulin super family present in almost all the haematopoetic cell lineages. There are 14 different types of siglecs present on human immune cells that take an active part in balancing the magnitude of immunological reactions. In general, these siglecs bind with sialic acids and negatively regulate the immune response. Leishmania contains sialic acids on its surface. Virulent parasites utilize this sugar to bind with macrophages through siglec-1 and siglec-5 compared to low sialic acids containing avirulent parasites. Such sialic acids-siglec-mediated interactions exhibited a suppressed host immune response which helped them to establish successful infection compared to desialylated virulent and avirulent parasites, as well as, siglec-depleted macrophages. Interestingly, interaction between sialic acids and siglec-1 induced enhanced phagocytosis, while sialic acids-siglec-5 interaction upregulated the phosphatase SHP-1. This interaction with the virulent strain exhibited deactivation of various downstream signaling pathways and ultimately controlled translocation of a functional component of transcription factor NF-κβ for regulation of cytokines and other effector molecules in infected macrophages. Thus, the interaction between the parasite and the host cells through sialic acids-siglec binding is clearly a newly identified mechanism by which parasites can establish successful infection by subverting the host’s innate immune response.

Leptin induces the phagocytosis and protective immune response in Leishmania donovani infected THP-1 cell line and human PBMCs

Visceral leishmaniasis (VL) is an infectious disease responsible for several deaths in malnourished children due to impaired cell-mediated immunity, which is accompanied by low circulating leptin levels. The cytokine function of leptin is implicated for several immune regulation activities such as hematopoiesis, angiogenesis, innate and adaptive immunity. Its deficiency associated with polarization of Th2 response, which coincides with VL pathogenesis. To determine the cytokine role of leptin in case of experimental VL, we tested the leptin associated Th1/Th2 type cytokine profile at mRNA level from Leishmania donovani infected human monocytic leukemia cell line (THP-1) and peripheral blood mononuclear cells (PBMCs). We also tested the effect of leptin on macrophages activation (viz. studying the phosphorylation of signaling moieties), phagocytic activity and intracellular reactive oxygen species (ROS) production during infection. We observed that leptin induced Th1 specific response by upregulation of IL-1α, IL-1β, IL-8 and TNF-α in THP-1 and IFN-γ, IL-12 and IL-2 in PBMCs. We also observed the downregulation of Th2 type cytokine i.e. IL-10 in THP-1 and unaltered expression of cytokines i.e. TGF-β, IL-10 and IL-4 in PBMCs. In addition, leptin stimulates the macrophages by inducing phosphorylation of Erk1/2 and Akt which are usually dephosphorylated in L. donovani infection. In concordance, leptin also induces the macrophage phagocytic activity by enhancing the intracellular ROS generation which helps in phagolysosome formation and oxidative killing of the parasite. In compilation, leptin is able to maintain the defensive environment against L. donovani infection through the classical macrophage activity.

Overexpressed PKCδ downregulates the expression of PKCα in B16F10 melanoma: induction of apoptosis by PKCδ via ceramide generation

In the present study, we observed a marked variation in the expression of PKCα and PKCδ isotypes in B16F10 melanoma tumor cells compared to the normal melanocytes. Interestingly, the tumor instructed expression or genetically manipulated overexpression of PKCα isotype resulted in enhanced G1 to S transition. This in turn promoted cellular proliferation by activating PLD1 expression and subsequent AKT phosphorylation, which eventually resulted in suppressed ceramide generation and apoptosis. On the other hand, B16F10 melanoma tumors preferentially blocked the expression of PKCδ isotype, which otherwise could exhibit antagonistic effects on PKCα-PLD1-AKT signaling and rendered B16F10 cells more sensitive to apoptosis via generating ceramide and subsequently triggering caspase pathway. Hence our data suggested a reciprocal PKC signaling operational in B16F10 melanoma cells, which regulates ceramide generation and provide important clues to target melanoma cancer by manipulating the PKCδ-ceramide axis.

Host-lipidome as a potential target of protozoan parasites

Host-lipidome caters parasite interaction by acting as first line of recognition, attachment on the cell surface, intracellular trafficking, and survival of the parasite inside the host cell. Here, we summarize how protozoan parasites exploit host-lipidome by suppressing, augmenting, engulfing, remodeling and metabolizing lipids to achieve successful parasitism inside the host.

Immunopathogenesis of non-healing American cutaneous leishmaniasis and progressive visceral leishmaniasis

The outcomes of Leishmania infection are determined by host immune and nutrition status, parasite species, and co-infection with other pathogens. While subclinical infection and self-healing cutaneous leishmaniasis (CL) are common, uncontrolled parasite replication can lead to non-healing local lesions or visceral leishmaniasis (VL). It is known that infection control requires Th1-differentiation cytokines (IL-12, IL-18, and IL-27) and Th1 cell and macrophage activation. However, there is no generalized consensus for the mechanisms of host susceptibility. The recent studies on regulatory T cells and IL-17-producing cells help explain the effector T cell responses that occur independently of the known Th1/Th2 cell signaling pathways. This review focuses on the immunopathogenesis of non-healing American CL and progressive VL. We summarize recent evidence from human and animal studies that reveals the mechanisms of dysregulated, hyper-responses to Leishmania braziliensis, as well as the presence of disease-promoting or the absence of protective responses to Leishmania amazonensis and Leishmania donovani. We highlight immune-mediated parasite growth and immunopathogenesis, with an emphasis on the putative roles of IL-17 and its related cytokines as well as arginase. A better understanding of the quality and regulation of innate immunity and T cell responses triggered by Leishmania will aid in the rational control of pathology and the infection.

Curative effect of 18β-glycyrrhetinic acid in experimental visceral leishmaniasis depends on phosphatase-dependent modulation of cellular MAP kinases

We earlier showed that 18β-glycyrrhetinic acid (GRA), a pentacyclic triterpenoid from licorice root, could completely cure visceral leishmaniasis in BALB/c mouse model. This was associated with induction of nitric oxide and proinflammatory cytokine production through the up regulation of NF-κB. In the present study we tried to decipher the underlying cellular mechanisms of the curative effect of GRA. Analysis of MAP kinase pathways revealed that GRA caused strong activation of p38 and to a lesser extent, ERK in bone marrow-derived macrophages (BMDM). Almost complete abrogation of GRA-induced cytokine production in presence of specific inhibitors of p38 and ERK1/2 confirmed the involvement of these MAP kinases in GRA-mediated responses. GRA induced mitogen- and stress-activated protein kinase (MSK1) activity in a time-dependent manner suggested that GRA-mediated NF-κB transactivation is mediated by p38, ERK and MSK1 pathway. As kinase/phosphatase balance plays an important role in modulating infection, the effect of GRA on MAPK directed phosphatases (MKP) was studied. GRA markedly reduced the expression and activities of three phosphatases, MKP1, MKP3 and protein phosphatase 2A (PP2A) along with a substantial reduction of p38 and ERK dephosphorylation in infected BMDM. Similarly in the in vivo situation, GRA treatment of L. donovani-infected BALB/c mice caused marked reduction of spleen parasite burden associated with concomitant decrease of individual phosphatase levels. However, activation of kinases also played an important role as the protective effect of GRA was significantly abrogated by pharmacological inhibition of p38 and ERK pathway. Curative effect of GRA may, therefore, be associated with restoration of proper cellular kinase/phosphatase balance, rather than modulation of either kinases or phosphatases.

Leishmania repression of host translation through mTOR cleavage is required for parasite survival and infection

The protozoan parasite Leishmania alters the activity of its host cell, the macrophage. However, little is known about the effect of Leishmania infection on host protein synthesis. Here, we show that the Leishmania protease GP63 cleaves the mammalian/mechanistic target of rapamycin (mTOR), a serine/threonine kinase that regulates the translational repressor 4E-BP1. mTOR cleavage results in the inhibition of mTOR complex 1 (mTORC1) and concomitant activation of 4E-BP1 to promote Leishmania proliferation. Consistent with these results, pharmacological activation of 4E-BPs with rapamycin, results in a dramatic increase in parasite replication. In contrast, genetic deletion of 4E-BP1/2 reduces parasite load in macrophages ex vivo and decreases susceptibility to cutaneous leishmaniasis in vivo. The parasite resistant phenotype of 4E-BP1/2 double-knockout mice involves an enhanced type I IFN response. This study demonstrates that Leishmania evolved a survival mechanism by activating 4E-BPs, which serve as major targets for host translational control.

Sphingolipid degradation by Leishmania major is required for its resistance to acidic pH in the mammalian host

Leishmania parasites alternate between flagellated promastigotes in sand flies and nonflagellated amastigotes in mammals, causing a spectrum of serious diseases. To survive, they must resist the harsh conditions in phagocytes (including acidic pH, elevated temperature, and increased oxidative/nitrosative stress) and evade the immune response. Recent studies have highlighted the importance of sphingolipid (SL) metabolism in Leishmania virulence. In particular, we have generated a Leishmania major iscl(-) mutant which is deficient in SL degradation but grows normally as promastigotes in culture. Importantly, iscl(-) mutants cannot induce pathology in either immunocompetent or immunodeficient mice yet are able to persist at low levels. In this study, we investigated how the degradation of SLs might contribute to Leishmania infection. First, unlike wild-type (WT) L. major, iscl(-) mutants do not trigger polarized T cell responses in mice. Second, like WT parasites, iscl(-) mutants possess the ability to downregulate macrophage activation by suppressing the production of interleukin-12 (IL-12) and nitric oxide. Third, during the stationary phase, iscl(-) promastigotes were extremely vulnerable to acidic pH but not to other adverse conditions, such as elevated temperature and oxidative/nitrosative stress. In addition, inhibition of phagosomal acidification significantly improved iscl(-) survival in murine macrophages. Together, these findings indicate that SL degradation by Leishmania is essential for its adaption to the acidic environment in phagolysosomes but is not required for the suppression of host cell activation. Finally, our studies with iscl(-) mutant-infected mice suggest that having viable, persistent parasites is not sufficient to provide immunity against virulent Leishmania challenge.

Evasion of Host Defence by Leishmania donovani: Subversion of Signaling Pathways

Protozoan parasites of the genus Leishmania are responsible for causing a variety of human diseases known as leishmaniasis, which range from self-healing skin lesions to severe infection of visceral organs that are often fatal if left untreated. Leishmania donovani (L. donovani), the causative agent of visceral leishmaniasis, exemplifys a devious organism that has developed the ability to invade and replicate within host macrophage. In fact, the parasite has evolved strategies to interfere with a broad range of signaling processes in macrophage that includes Protein Kinase C, the JAK2/STAT1 cascade, and the MAP Kinase pathway. This paper focuses on how L. donovani modulates these signaling pathways that favour its survival and persistence in host cells.

Leishmania interferes with host cell signaling to devise a survival strategy

The protozoan parasite Leishmania spp. exists as extracellular promastigotes in its vector whereas it resides and replicates as amastigotes within the macrophages of its mammalian host. As a survival strategy, Leishmania modulates macrophage functions directly or indirectly. The direct interference includes prevention of oxidative burst and the effector functions that lead to its elimination. The indirect effects include the antigen presentation and modulation of T cell functions in such a way that the effector T cells help the parasite survive by macrophage deactivation. Most of these direct and indirect effects are regulated by host cell receptor signaling that occurs through cycles of phosphorylation and dephosphorylation in cascades of kinases and phosphatases. This review highlights how Leishmania selectively manipulates the different signaling pathways to ensure its survival.

Inhibition of Akt/GSK3β signalling pathway by Legionella pneumophila is involved in induction of T-cell apoptosis

Legionella pneumophila is the causative agent of human Legionnaires' disease. L. pneumophila has been shown to induce apoptosis of T-cells and this may be important pathologically and clinically. The present study has determined the molecular mechanisms underlying L. pneumophila-induced apoptosis, which were unclear. Wild-type L. pneumophila and flagellin-deficient Legionella, but not L. pneumophila lacking a functional type IV secretion system Dot/Icm, replicated in T-cells. However, apoptosis was efficiently induced in T-cells only by wild-type L. pneumophila, and not flagellin-deficient or Dot/Icm-deficient Legionella. Induction of apoptosis involved activation of the initiator caspase 9 and effector caspase 3. Infection with L. pneumophila inhibited phosphorylation of Akt (also known as protein kinase B) and the Akt substrate GSK3β (glycogen synthase kinase 3β), and reduced the levels of β-catenin, a transcriptional activator regulated by GSK3β. It also caused the activation of the pro-apoptotic protein Bax and inhibited the expression of the anti-apoptotic protein XIAP (X-linked inhibitor of apoptosis) via inhibition of the Akt pathway. In conclusion, L. pneumophila induces mitochondria-mediated T-cell apoptosis through inhibition of the Akt/GSK3β signalling pathway.

Cancer treatment strategies targeting sphingolipid metabolism

Ceramide and sphingosine-1-phosphate are related sphingolipid metabolites that can be generated through a de novo biosynthetic route or derived from the recycling of membrane sphingomyelin. Both these lipids regulate cellular responses to stress, with generally opposing effects. Sphingosine-1-phosphate functions as a growth and survival factor, acting as a ligand for a family of G protein-coupled receptors, whereas ceramide activates intrinsic and extrinsic apoptotic pathways through receptor-independent mechanisms. A growing body of evidence has implicated ceramide, sphingosine-1-phosphate and the genes involved in their synthesis, catabolism and signaling in various aspects of oncogenesis, cancer progression and drug- and radiation resistance. This may be explained in part by the finding that both lipids impinge upon the PI3K/ AKT pathway, which represses apoptosis and autophagy. In addition, sphingolipids influence cell cycle progression, telomerase function, cell migration and stem cell biology. Considering the central role of ceramide in mediating physiological as well as pharmacologically stimulated apoptosis, ceramide can be considered a tumor-suppressor lipid. In contrast, sphingosine-1-phosphate can be considered a tumor-promoting lipid, and the enzyme responsible for its synthesis functions as an oncogene. Not surprisingly, genetic mutations that result in reduced ceramide generation, increased sphingosine-1-phosphate synthesis or which reduce steady state ceramide levels and increase sphingosine-1-phosphate levels have been identified as mechanisms of tumor progression and drug resistance in cancer cells. Pharmacological tools for modulating sphingolipid pathways are being developed and represent novel therapeutic strategies for the treatment of cancer.

Degradation of host sphingomyelin is essential for Leishmania virulence

In eukaryotes, sphingolipids (SLs) are important membrane components and powerful signaling molecules. In Leishmania, the major group of SLs is inositol phosphorylceramide (IPC), which is common in yeast and Trypanosomatids but absent in mammals. In contrast, sphingomyelin is not synthesized by Leishmania but is abundant in mammals. In the promastigote stage in vitro, Leishmania use SL metabolism as a major pathway to produce ethanolamine (EtN), a metabolite essential for survival and differentiation from non-virulent procyclics to highly virulent metacyclics. To further probe SL metabolism, we identified a gene encoding a putative neutral sphingomyelinase (SMase) and/or IPC hydrolase (IPCase), designated ISCL (Inositol phosphoSphingolipid phospholipase C-Like). Despite the lack of sphingomyelin synthesis, L. major promastigotes exhibited a potent SMase activity which was abolished upon deletion of ISCL, and increased following over-expression by episomal complementation. ISCL-dependent activity with sphingomyelin was about 20 fold greater than that seen with IPC. Null mutants of ISCL (iscl(-)) showed modest accumulation of IPC, but grew and differentiated normally in vitro. Interestingly, iscl(-) mutants did not induce lesion pathology in the susceptible BALB/c mice, yet persisted indefinitely at low levels at the site of infection. Notably, the acute virulence of iscl(-) was completely restored by the expression of ISCL or heterologous mammalian or fungal SMases, but not by fungal proteins exhibiting only IPCase activity. Together, these findings strongly suggest that degradation of host-derived sphingomyelin plays a pivotal role in the proliferation of Leishmania in mammalian hosts and the manifestation of acute disease pathology.

Protein Ser/Thr phosphatases of parasitic protozoa

Protein phosphorylation is an important mechanism implicated in physiology of any organism, including parasitic protozoa. Enzymes that control protein phosphorylation (kinases and phosphatases) are considered promising targets for drug development. This review attempts to provide the first account of the current understanding of the structure, regulation and biological functions of protein Ser/Thr phosphatases in unicellular parasites. We have examined the complements of phosphatases ("phosphatomes") of the PPP and PPM families in several species of Apicomplexa (including malaria parasite Plasmodium), as well as Giardia lamblia, Entamoeba histolytica, Trichomonas vaginalis and a microsporidium Encephalitozoon cuniculi. Apicomplexans have homologues (in most cases represented by single isoforms) of all human PPP subfamilies. Some apicomplexan PPP phosphatases have no orthologues in their vertebrate hosts, including a previously unrecognised group of pseudo-phosphatases with putative Ca(2+)-binding domains, which we designate as EFPP. We also describe the presence of previously undetected Zn finger motifs in PPEF phosphatases from kinetoplastids, and a likely case of convergent evolution of tetratricopeptide repeat domain-containing phosphatases in G. lamblia. Among the parasites examined, E. cuniculi has the smallest Ser/Thr phosphatome (5 PPP and no PPM), while T. vaginalis shows the largest expansion of the PPP family (169 predicted phosphatases). Most protozoan PPM phosphatases cluster separately from human sequences. The structural peculiarities or absence of human orthologues of a number of protozoan protein Ser/Thr phosphatases makes them potentially suitable targets for chemotherapy and thus warrants their functional assessment.

Targeting of PKCzeta and PKB to caveolin-enriched microdomains represents a crucial step underpinning the disruption in PKB-directed signalling by ceramide

Elevated ceramide concentrations in adipocytes and skeletal muscle impair PKB (protein kinase B; also known as Akt)-directed insulin signalling to key hormonal end points. An important feature of this inhibition involves the ceramide-induced activation of atypical PKCzeta (protein kinase C-zeta), which associates with and negatively regulates PKB. In the present study, we demonstrate that this inhibition is critically dependent on the targeting and subsequent retention of PKCzeta-PKB within CEM (caveolin-enriched microdomains), which is facilitated by kinase interactions with caveolin. Ceramide also recruits PTEN (phosphatase and tensin homologue detected on chromosome 10), a 3'-phosphoinositide phosphatase, thereby creating a repressive membrane microenvironment from which PKB cannot signal. Disrupting the structural integrity of caveolae by cholesterol depletion prevented caveolar targeting of PKCzeta and PKB and suppressed kinase-caveolin association, but, importantly, also ameliorated ceramide-induced inhibition of PKB. Consistent with this, adipocytes from caveolin-1-/- mice, which lack functional caveolae, exhibit greater resistance to ceramide compared with caveolin-1+/+ adipocytes. We conclude that the recruitment and retention of PKB within CEM contribute significantly to ceramide-induced inhibition of PKB-directed signalling.