Anti-Cancer Agents in Medicinal Chemistry (Formerly Current Medicinal Chemistry - Anti-Cancer Agents) - Volume 11, Issue 9, 2011

Although sphingolipids were originally discovered in brain extracts more than one and a quarter centuries ago, our understanding of their roles in physiological and pathophysiological processes only began in the last three decades. Sphingolipids are a diverse group of lipids in which fatty acids are linked via amide bonds to a long-chain base or sphingoid (e.g., sphingosine). The term “sphingo-” was coined by the German biochemist Johann Ludwig Wilhelm Thudichum in 1884 after the Greek mythological creature Sphinx because of its enigmatic nature. Initially considered to be simply the building blocks of biomembranes, the discovery that sphingosine inhibits protein kinase C and induces apoptosis by Yusuf Hannun and colleagues in 1986 has fueled a tremendous increase in research on the roles that sphingolipids play in regulating various cellular functions including cell proliferation, differentiation, apoptosis, cell survival and death, autophagy, and immune response. Accordingly, our knowledge regarding the connections between sphingolipids and human pathophysiology has continued to expand. It is now apparent that dysfunctional sphingolipid metabolism is associated with multiple types of cancer including leukemias. Individual sphingolipid metabolites that correlate with oncogenesis, transformation and metastasis have been identified. There is increasing evidence that an imbalance in the “sphingolipid rheostat” contributes to the pathogenesis and drug resistance of hematological malignancies. In this special series, therefore, we have collected a comprehensive set of reviews on the roles of bioactive sphingolipid metabolites in cancer biology and therapeutics. As will become evident from the superb reviews in this series, the biosynthesis and metabolism of sphingolipids are much more complex and involve a large number of intermediate metabolites. Different sphingolipid species can have distinct biological activities. For example, while sphingosine-1-phosphate (S1P), a phosphorylated product of sphingosine catalyzed by sphingosine kinases, promotes cell growth and survival, its precursors, sphingosine and ceramide, play a role in growth arrest and cell death. Since these metabolites are interconvertible, it has been proposed by Sarah Spiegel and colleagues that it is not their absolute amounts, but rather the balance between these counter-acting sphingolipid metabolites, that controls the cell fate decision. To best appreciate the “sphingolipid rheostat” model, we begin this series with a historical perspective and overview of sphingolipid signaling pathways and its roles in hematopoietic malignancies by Loh et al. The regulation and mechanisms of action of S1P and the kinases responsible for its production in hematological malignancies are thoroughly reviewed by Stevenson et al and Pitson et al. S1P is generated from sphingosine by sphingosine kinases (SphK1 and SphK2). In addition to its intracellular roles in NF-κB activation and epigenetic regulation of gene expression, S1P can be exported outside the cell by ATP-binding cassette transporters, where it signals through five S1P specific G protein-coupled receptors to exert its effects on cancer progression including cell growth, survival, migration, and angiogenesis. A more specific characterization of S1P receptors (S1PRs) and their roles in cancer development is presented in the review by Watters et al. Interestingly, sphingosine kinases and its product S1P also have important roles in regulation of macrophage activation and inflammatory responses, as described by Weigert et al. Moreover, recent findings suggest that the SphK1/S1P signaling regulates the hypoxic response of tumor cells, as reviewed by Cuvillier and Ader. In particular, these reviews also evaluate the potential of pharmacological agents that modulate the SphK/S1P/S1PRs signaling pathway for the treatment of hematological cancers. Sphingolipid metabolism is highly orchestrated by numerous enzymes whose activities are dependent on many factors including intracellular location. Ceramidases are key enzymes that regulate cellular levels of ceramide and sphingosine by catalyzing the cleavage of ceramide into sphingosine and fatty acids. The key roles of ceramidases, particularly acid ceramidase, in cancer initiation, progression and response to radio- and chemotherapy are reviewed by Fabrias et al. Accumulating evidence suggests that inhibition of acid ceramidase may enhance the efficacy of chemotherapeutic drugs in cancer patients. Interestingly, recent studies have revealed a role for sphingolipid metabolites in the regulation of autophagy, a lysosomal catabolic pathway. Autophagy plays vital roles in the quality control of cellular components and cell survival by eliminating damaged materials and supplying nutrients. In this issue, Bedia et al. describe the regulation of autophagy by sphingolipids and its contribution to the response of cancer cells to chemotherapy. With the explosion of information on sphingolipid metabolites in recent years, this is an opportune time to speed efforts to translate our current knowledge about sphingolipid signaling into better cancer prevention and treatment, as discussed by Burns and Luberto. Drug resistance is a major obstacle for the successful treatment of cancer. Accumulating evidence suggests that sphingolipids play an important role in the regulation of multidrug resistance and the action of chemotherapeutic drugs, as reviewed by Spassieva and Bieberich as well as Gouaze-Andersson and Cabot. Ceramide is a central metabolite of the sphingolipid pathway that plays a critical role in cancer cell death induced by chemotherapeutic drugs. Dysregulation of ceramide metabolism is associated with not only malignancy but also multidrug resistance. Indeed, it has recently been shown that CERT, a major regulator of ceramide flux, is increased in drug resistant cancer cells and that suppression of CERT sensitizes cancer cells to anticancer drugs, as reviewed by Scheffer et al. Thus, increasing cellular levels of ceramide by modulating ceramide metabolism or delivering exogenous ceramide (e.g. C6-ceramide) directly into cancer cells has the potential to be a promising therapeutic strategy to overcome multidrug resistance in cancers including hematopoietic malignancies, as discussed by Barth et al.....

Sphingosine-1-phosphate (S1P) is a bioactive lipid with diverse functions including the promotion of cell survival, proliferation and migration, as well as the regulation of angiogenesis, inflammation, immunity, vascular permeability and nuclear mechanisms that control gene transcription. S1P is derived from metabolism of ceramide, which itself has diverse and generally growth-inhibitory effects through its impact on downstream targets involved in regulation of apoptosis, senescence and cell cycle progression. Regulation of ceramide, S1P and the biochemical steps that modulate the balance and interconversion of these two lipids are major determinants of cell fate, a concept referred to as the “sphingolipid rheostat.” There is abundant evidence that the sphingolipid rheostat plays a role in the origination, progression and drug resistance patterns of hematopoietic malignancies. The pathway has also been exploited to circumvent the problem of chemotherapy resistance in leukemia and lymphoma. Given the broad effects of sphingolipids, targeting multiple steps in the metabolic pathway may provide possible therapeutic avenues. However, new observations have revealed that sphingolipid signaling effects are more complex than previously recognized, requiring a revision of the sphingolipid rheostat model. Here, we summarize recent insights regarding the sphingolipid metabolic pathway and its role in hematopoietic malignancies.

Sphingosine-1-phosphate (S1P) is a pleiotropic bioactive lipid mediator that regulates several processes important for hematologic cancer progression. S1P is generated by two sphingosine kinases, SphK1 and SphK2, and is exported outside the cell, where it activates specific cell surface S1P G-protein coupled receptors in autocrine/paracrine manner, coined “inside-out signaling”. In this review, we highlight the importance of SphK1 and inside-out signaling by S1P in hematologic malignancy. We also summarize the results of studies targeting the SphK1/S1P/S1P receptor axis and the effects of the S1P receptor modulator, FTY720, in hematologic malignancy.

The sphingolipids ceramide, sphingosine and sphingosine 1-phosphate have emerged as important signaling molecules that regulate a number of important cellular processes. Sphingosine 1-phosphate enhances cell survival and proliferation, and also regulates angiogenesis, cell invasion, and differentiation via both its cell surface G protein-coupled receptors and recently identified intracellular effectors. In contrast, ceramide and sphingosine elicit growth arrest and apoptosis through direct modulation of a number of intracellular targets. The cellular balance of these sphingolipids contributes to the determination of cell fate, and it is now clear that disruption in this ‘sphingolipid rheostat’ contributes to the development, progression and chemotherapeutic resistance of both hematological malignancies and solid tumors. The sphingosine kinases are central regulators of this pathway since they not only increase sphingosine 1-phosphate and assist in reduction of ceramide and sphingosine, but are also regulated at multiple levels by external stimuli. Thus, targeting the regulation of the sphingosine kinases may be a viable therapeutic strategy for a diverse array of cancers. Here, we describe the current knowledge of sphingosine kinase regulation, effects of current and potential chemotherapeutic agents on this system, and discuss the implications of this for the treatment of hematological malignancies and other cancers.

Sphingosine 1-phosphate (S1P) is a bioactive lipid with diverse biological functions, including cell proliferation, differentiation, angiogenesis, chemotaxis, and migration. Many of the activities of S1P are mediated through five closely related G-protein-coupled receptors of the sphingosine-1-phosphate receptor family (S1PR) which play a crucial role in sphingolipid metabolism. Each of these receptors appears to be tissue specific and to have demonstrated roles in the regulation of cell proliferation and survival in various cancer types. Further analysis of the function that S1PRs serve in hematological malignancies offers a great potential for the discovery of novel and selective therapeutic agents targeting these receptors. This review focuses on the characterization of S1PRs and their roles in cancer development in various signaling pathways mediated through specific G coupled protein. In particular, pharmacological agents targeting these S1PRs will be discussed and their potential will be examined.

The sphingolipid sphingosine-1-phosphate (S1P) is an important regulator of immune cell functions in vivo. Besides recruiting lymphocytes to blood and lymph, it may promote immune cell survival and proliferation, but also interferes with their activation. Hereby, S1P may act as an intracellular second messenger or cofactor or, upon being secreted from cells, may bind to and activate a family of specific G-protein-coupled receptors (S1PR1-5). Extracellular versus intracellular S1P hereby might trigger synergistic/identical or fundamentally distinct responses. Furthermore, engagement of different S1PRs is connected to different functional outcome. This complexity is exemplified by the influence of S1P on the inflammatory potential of macrophages, shaping their role in inflammatory pathologies such as atherosclerosis and cancer. Here, we summarize the recent progress in understanding the impact of S1P signaling in macrophage biology, discuss its impact in solid as well as ‘wet’ tumors and elaborate potential options to interfere with S1P signaling in the context of cancer.

Ceramidases are ubiquitous amidohydrolases that catalyze the cleavage of ceramides into sphingosine and fatty acids. This reaction exerts a cytoprotective role in physiological conditions, while altered ceramidase activities favour a number of human diseases. Among these diseases, several reports point to important roles of ceramidases, mainly the acid ceramidase, in the initiation and progression of cancer, and the response of tumors to radio- or chemotherapy. Multiple reports confirm the interest of acid ceramidase inhibitors as anticancer drugs, either alone or in combination with other therapies. Sphingolipid metabolism plays a role in hematological malignancies and appears as an interesting target for therapeutic intervention. Although the use of ceramidase inhibitors in chemotherapy of hematologic cancers has not been widely investigated, a number of indirect evidence suggest that inhibition of specific ceramidases could potentiate the effect of drugs in clinical use to treat hematologic malignancies and may afford strategies to combat relapses. The arsenal of ceramidase inhibitors so far available is wide and hopefully, upcoming research will assess the feasibility of this approach.

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

Hypoxia, defined as reduced tissue oxygen concentration, is a characteristic of solid tumors and is an indicator of unfavorable diagnosis in patients. At the cellular level, the adaptation to hypoxia is under the control of two related transcription factors, HIF-1α and HIF-2α (Hypoxia-Inducible Factor), which activate expression of genes promoting angiogenesis, metastasis, increased tumor growth and resistance to treatments. A role for HIF-1α and HIF-2α is also emerging in hematologic malignancies such as lymphoma and l eukemia. Recent studies have identified the sphingosine kinase 1/sphingosine 1-phosphate (SphK1/S1P) signaling pathway - which elicits various cellular processes including cell proliferation, cell survival or angiogenesis - as a new regulator of HIF-1α or HIF-2α activity. This review will consider how targeting the SphK1/S1P signaling could represent an attractive strategy for therapeutic intervention in cancer.

Since the discovery and initial characterizations of sphingolipids (SLs) in 1884, extensive research has established that these molecules not only are structural components of eukaryotic membranes but they are also critical bioactive lipids involved in fundamental cellular processes such as proliferation, differentiation, apoptosis, inflammation, migration, and autophagy. Altered SL metabolism has been observed in many pathological conditions including hematological malignancies. Thus, targeting the SL pathway to induce lipid changes to counteract specific pathologies is currently being pursued as a promising, novel therapeutic intervention. In this review, we discuss the general characteristics of the SL pathway, illustrating those features relevant to the understanding of the role of SLs in leukemia, and we address novel SL-targeting therapeutic approaches.

Almost all classes of bioactive lipids such as cholesterol and cholesterol derivatives, phospholipids and lysophospholipids, eicosanoids, and sphingolipids are critically involved in tumorigenesis. However, a systematic analysis of the distinct tumorigenic functions of lipids is rare. As a general principle, lipids either act directly by binding to receptors and other cell signaling proteins in growth control, or indirectly by regulating membrane organization such as the formation of membrane microdomains (lipid rafts) that modulate receptor or other membrane protein function. Lipid rafts are known to be formed by cholesterol and the sphingolipids or ceramide derivatives sphingomyelin and glucosylceramide (cholesterol-sphingomyelin-glucosylceramide or CSG rafts). In this review, we discuss the interconnection of sphingolipids with cholesterol and its derivatives in breast cancer drug resistance. Bile acids are cholesterol derivatives that are first synthesized in the liver (primary bile acids) and then metabolized by intestinal bacteria giving rise to secondary bile acids. They activate farnesoid X receptor (FXR), which inhibits cholesterol conversion to primary bile acids and induces the expression of drug resistance proteins. We introduce a novel model by which bile acid-mediated activation of FXR may promote the formation of CSG lipid rafts that trans-activate drug resistance proteins in breast cancer. Since breast cancer stem cells express high levels of drug resistance proteins, our model predicts that serum bile acids promote breast cancer stem cell survival and metastasis. Our model also predicts that FXR antagonists in combination with sphingolipid biosynthesis inhibitors may be promising candidates for novel drugs in lipid therapy of breast cancer.

Drug resistance represents a serious barrier to the successful treatment of hematological malignancies. In leukemias, resistance mechanisms that involve membrane-resident proteins belonging to the ABC (ATP-binding cassette) transporter protein family are of particular interest, wherein enhanced expression is often associated with poor prognosis and frequent in relapsed or refractory disease. These proteins reduce the intracellular concentration of antitumor agents, greatly diminishing clinical efficacy. Research in this area has been directed at the design of agents, “pump antagonists”, to overcome the effluxing capacity of drug transporters; however, this direction has had limited clinical success. An allied function of ABC transporters like P-glycoprotein (P-gp) is glycolipid trafficking, an area that has not been explored from a therapeutic standpoint. In this capacity, it turns out that glycolipid synthesis can be attenuated by pump antagonists; this is perhaps an adventitious property of P-gp. Recent research in the area of lipid metabolism, specifically ceramide and glycolipids, has provided insight into the function of glycosphingolipids in multidrug resistance and in the action of chemotherapy. This review is intended to bring together those aspects of glycosphingolipid metabolism that might be leveraged to enhance the therapeutic performance of ceramide and to discuss how ABC transporters like P-gp might be targeted to potentiate and magnify ceramide-driven proapoptotic cascades.

Sphingolipids are important structural components of membranes, and play an equally important role in basic cellular processes as second messengers. Recently, sphingolipids are receiving increasing attention in cancer research. Ceramide is the central molecule that regulates sphingolipid metabolism forming the basic structural backbone of sphingolipids and the precursor of all complex sphingolipids. It is been proposed to be an important regulator of tumor cell death following exposure to stress stimuli. The increase or decrease of ceramide levels leading to change in sensitivity of cancer cells to stress stimuli provides support for a central role of ceramide signaling in cell death. In this review, we have focused on ceramide transfer protein (CERT) as a major regulator of ceramide flux in the cell.

The bioactive sphingolipid, ceramide, has garnered major interest as a principle regulator of cellular stress, proliferation, senescence, and death. Of particular interest to cancer biologists and clinical oncologist, dysregulated ceramide metabolism has been documented in both solid and non-solid malignancies. Moreover, most anticancer chemotherapeutics stimulate ceramide accumulation through increased ceramide synthesis or through the inhibition of ceramide catabolism. In fact, neutralization of ceramide via glycosylation or phosphorylation in malignant cells has been linked to multidrug chemoresistance. New therapeutic strategies to overcome chemoresistance focus on increasing endogenous ceramide levels by stimulating ceramide synthesis, by inhibiting ceramide neutralization, or by the direct delivery of exogenous ceramide. This review will discuss new therapeutic strategies designed specifically to modulate ceramide metabolism, as well as nanoscale delivery systems engineered to selectively deliver ceramide to cancerous cells and tissues.