Discovery, biological evaluation, and structure-activity relationship of amidine based sphingosine kinase inhibitors

Second generation of pyrimidin-quinolone hybrids obtained from virtual screening acting as sphingosine kinase 1 inhibitors and potential anticancer agents

We report here the virtual screening design, synthesis and activity of eight new inhibitors of SphK1. For this study we used a pre-trained Graph Convolutional Network (GCN) combined with docking calculations. This exploratory analysis proposed nine compounds from which eight displayed significant inhibitory effect against sphingosine kinase 1 (SphK1) demonstrating a high level of efficacy for this approach. Four of these compounds also displayed anticancer activity against different tumor cell lines, and three of them (5), (6) and (7) have shown a wide inhibitory action against many of the cancer cell line tested, with GI50 below 5 µM, being (5) the most promising with TGI below 10 µM for the half of cell lines. Our results suggest that the three most promising compounds reported here are the pyrimidine-quinolone hybrids (1) and (6) linked by p-aminophenylsulfanyl and o-aminophenol fragments respectively, and (8) without such aryl linker. We also performed an exhaustive study about the molecular interactions that stabilize the different ligands at the binding site of SphK1. This molecular modeling analysis was carried out by using combined techniques: docking calculations, MD simulations and QTAIM analysis. In this study we also included PF543, as reference compound, in order to better understand the molecular behavior of these ligands at the binding site of SphK1.These results provide useful information for the design of new inhibitors of SphK1 possessing these structural scaffolds.

The emerging roles of sphingosine 1-phosphate and SphK1 in cancer resistance: a promising therapeutic target

Cancer chemoresistance is a problematic dilemma that significantly restrains numerous cancer management protocols. It can promote cancer recurrence, spreading of cancer, and finally, mortality. Accordingly, enhancing the responsiveness of cancer cells towards chemotherapies could be a vital approach to overcoming cancer chemoresistance. Tumour cells express a high level of sphingosine kinase-1 (SphK1), which acts as a protooncogenic factor and is responsible for the synthesis of sphingosine-1 phosphate (S1P). S1P is released through a Human ATP-binding cassette (ABC) transporter to interact with other phosphosphingolipids components in the interstitial fluid in the tumor microenvironment (TME), provoking communication, progression, invasion, and tumor metastasis. Also, S1P is associated with several impacts, including anti-apoptotic behavior, metastasis, mesenchymal transition (EMT), angiogenesis, and chemotherapy resistance. Recent reports addressed high levels of S1P in several carcinomas, including ovarian, prostate, colorectal, breast, and HCC. Therefore, targeting the S1P/SphK signaling pathway is an emerging therapeutic approach to efficiently attenuate chemoresistance. In this review, we comprehensively discussed S1P functions, metabolism, transport, and signaling. Also, through a bioinformatic framework, we pointed out the alterations of SphK1 gene expression within different cancers with their impact on patient survival, and we demonstrated the protein-protein network of SphK1, elaborating its sparse roles. Furthermore, we made emphasis on different machineries of cancer resistance and the tight link with S1P. We evaluated all publicly available SphK1 inhibitors and their inhibition activity using molecular docking and how SphK1 inhibitors reduce the production of S1P and might reduce chemoresistance, an approach that might be vital in the course of cancer treatment and prognosis.

Integrative roles of sphingosine kinase in liver pathophysiology

Bioactive sphingolipids and enzymes that metabolize sphingolipid-related substances have been considered as critical messengers in various signaling pathways. One such enzyme is the crucial lipid kinase, sphingosine kinase (SphK), which mediates the conversion of sphingosine to the potent signaling substance, sphingosine-1-phosphate. Several studies have demonstrated that SphK metabolism is strictly regulated to maintain the homeostatic balance of cells. Here, we summarize the role of SphK in the course of liver disease and illustrate its effects on both physiological and pathological conditions of the liver. SphK has been implicated in a variety of liver diseases, such as steatosis, liver fibrosis, hepatocellular carcinoma, and hepatic failure. This study may advance the understanding of the cellular and molecular foundations of liver disease and establish therapeutic approaches via SphK modulation.

Discovery of In Vivo Active Sphingosine-1-phosphate Transporter (Spns2) Inhibitors

Sphingosine 1-phosphate (S1P) is a pleiotropic signaling molecule that interacts with five G-protein-coupled receptors (S1P1-5) to regulate cellular signaling pathways. S1P export is facilitated by Mfsd2b and spinster homologue 2 (Spns2). While mouse genetic studies suggest that Spns2 functions to maintain lymph S1P, Spns2 inhibitors are necessary to understand its biology and to learn whether Spns2 is a viable drug target. Herein, we report a structure-activity relationship study that identified the first Spns2 inhibitor 16d (SLF1081851). In vitro studies in HeLa cells demonstrated that 16d inhibited S1P release with an IC50 of 1.93 μM. Administration of 16d to mice and rats drove significant decreases in circulating lymphocyte counts and plasma S1P concentrations, recapitulating the phenotype observed in mice made deficient in Spns2. Thus, 16d has the potential for development and use as a probe to investigate Spns2 biology and to determine the potential of Spns2 as a drug target.

Therapeutic Potential of SphK1 Inhibitors Based on Abnormal Expression of SphK1 in Inflammatory Immune Related-Diseases

Sphingosine kinase 1(SphK1) a key enzyme that catalyzes the conversion of sphingosine (Sph) to sphingosine 1-phosphate (S1P), so as to maintain the dynamic balance of sphingolipid-rheostat in cells and participate in cell growth and death, proliferation and migration, vasoconstriction and remodeling, inflammation and metabolism. The normal expression of SphK1 maintains the balance of physiological and pathological states, which is reflected in the regulation of inflammatory factor secretion, immune response in traditional immune cells and non-traditional immune cells, and complex signal transduction. However, abnormal SphK1 expression and activity are found in various inflammatory and immune related-diseases, such as hypertension, atherosclerosis, Alzheimer’s disease, inflammatory bowel disease and rheumatoid arthritis. In view of the therapeutic potential of regulating SphK1 and its signal, the current research is aimed at SphK1 inhibitors, such as SphK1 selective inhibitors and dual SphK1/2 inhibitor, and other compounds with inhibitory potency. This review explores the regulatory role of over-expressed SphK1 in inflammatory and immune related-diseases, and investigate the latest progress of SphK1 inhibitors and the improvement of disease or pathological state.

Targeting sphingosine kinase-1 with the low MW inhibitor SKI-5C suppresses the development of endometriotic lesions in mice

BACKGROUND AND PURPOSE

Limited evidence suggests that the sphingosine-1-phosphate/sphingosine kinase 1 (S1P/SPHK1) signalling pathway is involved in the pathogenesis of endometriosis. Therefore, we analyzed in this study whether the inhibition of SPHK1 and, consequently, decreased levels of S1P affected the vascularization and growth of endometriotic lesions.

EXPERIMENTAL APPROACH

Endometriotic lesions were surgically induced in the peritoneal cavity and the dorsal skinfold chamber of female BALB/c mice. The animals received a daily dose of the SPHK1 inhibitor SKI-5C or vehicle (control). Analyses involved the determination of lesion growth, cyst formation, microvessel density and cell proliferation within peritoneal endometriotic lesions by means of high-resolution ultrasound imaging, caliper measurement, histology and immunohistochemistry. In the dorsal skinfold chamber model the development of newly formed microvascular networks and their microhemodynamic parameters within endometriotic lesions were investigated by means of intravital fluorescence microscopy.

KEY RESULTS

SKI-5C significantly inhibited the development and vascularization of peritoneal endometriotic lesions, as indicated by a reduced growth and cyst formation, a lower microvessel density and a suppressed cell proliferation, when compared to vehicle-treated controls. Endometriotic lesions in dorsal skinfold chambers of SKI-5C-treated animals exhibited a significantly smaller lesion size, lower functional microvessel density, smaller microvessel diameters and a reduced blood perfusion of the newly developing microvascular networks.

CONCLUSIONS AND IMPLICATIONS

SPHK1/S1P signalling promotes the establishment and progression of endometriotic lesions. The inhibition of this pathway suppresses the development of endometriotic lesions, suggesting SPHK1 as a potential novel target for future endometriosis therapy.

Small-Molecule Kinase Inhibitors for the Treatment of Nononcologic Diseases

Great successes have been achieved in developing small-molecule kinase inhibitors as anticancer therapeutic agents. However, kinase deregulation plays essential roles not only in cancer but also in almost all major disease areas. Accumulating evidence has revealed that kinases are promising drug targets for different diseases, including cancer, autoimmune diseases, inflammatory diseases, cardiovascular diseases, central nervous system disorders, viral infections, and malaria. Indeed, the first small-molecule kinase inhibitor for treatment of a nononcologic disease was approved in 2011 by the U.S. FDA. To date, 10 such inhibitors have been approved, and more are in clinical trials for applications other than cancer. This Perspective discusses a number of kinases and their small-molecule inhibitors for the treatment of diseases in nononcologic therapeutic fields. The opportunities and challenges in developing such inhibitors are also highlighted.

Targeting the SphK-S1P-SIPR Pathway as a Potential Therapeutic Approach for COVID-19

The world is currently experiencing the worst health pandemic since the Spanish flu in 1918-the COVID-19 pandemic-caused by the coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This pandemic is the world's third wake-up call this century. In 2003 and 2012, the world experienced two major coronavirus outbreaks, SARS-CoV-1 and Middle East Respiratory syndrome coronavirus (MERS-CoV), causing major respiratory tract infections. At present, there is neither a vaccine nor a cure for COVID-19. The severe COVID-19 symptoms of hyperinflammation, catastrophic damage to the vascular endothelium, thrombotic complications, septic shock, brain damage, acute disseminated encephalomyelitis (ADEM), and acute neurological and psychiatric complications are unprecedented. Many COVID-19 deaths result from the aftermath of hyperinflammatory complications, also referred to as the "cytokine storm syndrome", endotheliitus and blood clotting, all with the potential to cause multiorgan dysfunction. The sphingolipid rheostat plays integral roles in viral replication, activation/modulation of the immune response, and importantly in maintaining vasculature integrity, with sphingosine 1 phosphate (S1P) and its cognate receptors (SIPRs: G-protein-coupled receptors) being key factors in vascular protection against endotheliitus. Hence, modulation of sphingosine kinase (SphK), S1P, and the S1P receptor pathway may provide significant beneficial effects towards counteracting the life-threatening, acute, and chronic complications associated with SARS-CoV-2 infection. This review provides a comprehensive overview of SARS-CoV-2 infection and disease, prospective vaccines, and current treatments. We then discuss the evidence supporting the targeting of SphK/S1P and S1P receptors in the repertoire of COVID-19 therapies to control viral replication and alleviate the known and emerging acute and chronic symptoms of COVID-19. Three clinical trials using FDA-approved sphingolipid-based drugs being repurposed and evaluated to help in alleviating COVID-19 symptoms are discussed.

Analysis of selective target engagement by small-molecule sphingosine kinase inhibitors using the Cellular Thermal Shift Assay (CETSA)

The recently renewed interest in scientific rigor and reproducibility is of critical importance for both scientists developing new targeted small-molecule inhibitors and those employing these molecule in cellular studies, alike. While off-target effects are commonly considered as limitations for any given small-molecule inhibitor, the ability of a given compound to distinguish between enzyme isoforms is often neglected when employing compounds in cellular studies. To call attention to this issue, we have compared the results of an assay for "direct target engagement", the Cellular Thermal Shift Assay (CETSA), to the published isoform selectivity of 12 commercially available sphingosine kinase 1 and 2 (SphK 1 and SphK2) inhibitors. Our results suggest that, at the concentrations commonly employed in cellular assay systems, none of the tested SKIs can be considered isoform selective. Thus, caution and complimentary assay strategies must be employed to fully discern isoform selectivity for the SphKs. Moreover, caution must be employed by the scientific community as a whole when designing experiments that aim to discern the effects of one enzyme isoform versus another to ensure that the concentration ranges used are able to distinguish isoform selectivity.

Design and Development of Novel Urea, Sulfonyltriurea, and Sulfonamide Derivatives as Potential Inhibitors of phingosine Kinase 1

Sphingosine kinase 1 (SphK1) is one of the well-studied drug targets for cancer and inflammatory diseases. Recently discovered small-molecule inhibitors of SphK1 have been recommended in cancer therapeutics; however, selectivity and potency of first-generation inhibitors are great challenge. In search of effective SphK1 inhibitors, a set of small molecules have been designed and synthesized bearing urea, sulfonylurea, sulfonamide, and sulfonyltriurea groups. The binding affinity of these inhibitors was measured by fluorescence-binding assay and isothermal titration calorimetry. Compounds 1, 5, 6, and 7 showed an admirable binding affinity to the SphK1 in the sub-micromolar range and significantly inhibited SphK1 activity with admirable IC50 values. Molecular docking studies revealed that these compounds fit well into the sphingosine binding pocket of SphK1 and formed significant number of hydrogen bonds and van der Waals interactions. These molecules may be exploited as potent and selective inhibitors of SphK1 that could be implicated in cancer therapeutics after the required in vivo validation.

Design of Sphingosine Kinases Inhibitors: Challenges and Recent Developments

BACKGROUND

Sphingosine kinases (SphKs) catalyze the phosphorylation of sphingosine to form the bioactive sphingolipid metabolite sphingosine-1-phosphate (S1P). S1P is an important lipid mediator with a wide range of biological functions; it is also involved in a variety of diseases such as inflammatory diseases, Alzheimer's disease and cancer.

METHODS

This review reports the recent advancement in the research of SphKs inhibitors. Our purpose is also to provide a complete overview useful for underlining the features needed to select a specific pharmacological profile.

DISCUSSION

Two distinct mammalian SphK isoforms have been identified, SphK1 and SphK2. These isoforms are encoded by different genes and exhibit distinct subcellular localizations, biochemical properties and functions. SphK1 and SphK2 inhibition can be useful in different pathological conditions.

CONCLUSION

SphK1 and SphK2 have many common features but different and even opposite biological functions. For this reason, several research groups are interested in understanding the therapeutic usefulness of a selective or non-selective inhibitor of SphKs. Moreover, a compensatory mechanism for the two isoforms has been demonstrated, thus leading to the development of dual inhibitors.

Targeting sphingosine kinase 1 for the treatment of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH), characterized by high morbidity and mortality, is a serious hazard to human life. Until now, the long-term survival of the PAH patients is still suboptimal. Recently, sphingosine kinase 1 (SPHK1) has drawn more and more attention due to its essential role in the pulmonary vasoconstriction, remodeling of pulmonary blood vessels and right cardiac lesions in PAH patients, and this enzyme is regarded as a new target for the treatment of PAH. Here, we discussed the multifarious functions of SPHK1 in PAH physiology and pathogenesis. Moreover, the structural features of SPHK1 and binding modes with different inhibitors were summarized. Finally, recent advances in the medicinal chemistry research of SPHK1 inhibitors are presented.

De Novo Fatty Acid Synthesis-Driven Sphingolipid Metabolism Promotes Metastatic Potential of Colorectal Cancer

Metastasis is the most common cause of death in colorectal cancer patients. Fatty acid synthase (FASN) and sphingosine kinase-1 and -2 (SPHK1 and 2) are overexpressed in many cancers, including colorectal cancer. However, the contribution of FASN-mediated upregulation of sphingolipid metabolism to colorectal cancer metastasis and the potential of these pathways as targets for therapeutic intervention remain unknown. This study determined that sphingosine kinases (SPHK) are overexpressed in colorectal cancer as compared with normal mucosa. FASN expression significantly correlated with SPHK2 expression in data sets from The Cancer Genome Atlas (TCGA) and a colorectal cancer tumor microarray. FASN, SPHK1, and SPHK2 colocalized within invadopodia of primary colorectal cancer cells. Moreover, FASN inhibition decreased SPHK2 expression and the levels of dihydrosphingosine 1-phosphate (DH-S1P) and sphingosine 1-phosphate (S1P) in colorectal cancer cells and tumor tissues. Inhibition of FASN using TVB-3664 and sphingolipid metabolism using FTY-720 significantly inhibited the ability of primary colorectal cancer cells to proliferate, migrate, form focal adhesions, and degrade gelatin. Inhibition of the FASN/SPHK/S1P axis was accompanied by decreased activation of p-MET, p-FAK, and p-PAX. S1P treatment rescued FASN-mediated inhibition of these proteins, suggesting that FASN promotes metastatic properties of colorectal cancer cells, in part, through an increased sphingolipid metabolism. These data demonstrate that upregulation of the FASN/SPHK/S1P axis promotes colorectal cancer progression by enhancing proliferation, adhesion, and migration. IMPLICATIONS: This study provides a strong rationale for further investigation of the interconnection of de novo lipogenesis and sphingolipid metabolism that could potentially lead to the identification of new therapeutic targets and strategies for colorectal cancer.

Curtin-Hammett-Driven Intramolecular Cyclization of Heteroenyne-Allenes to Phenanthridine-Fused Quinazoliniminiums

Intramolecular cyclization of the heteroenyne-allene 2-((biphenyl-2-ylimino)methyleneamino)benzonitrile 1 to phenanthridine-fused quinazoliniminium salt PQ in the presence of a Lewis acid at room temperature involves formation of two new bonds: a C-C bond and a C-N bond. In this article, density functional theory (B3LYP and M06-2X) was employed in conjunction with 6-311G* basis set to gain insights into the mechanism of this cyclization reaction. The solvent effects were considered using Polarizable Continuum Model with nitromethane as the solvent. Our calculations show that C-C bond formation precedes the C-N bond formation. Precisely, the mechanism involves initial protonation of 1 at N_α and N_β of the carbodiimide to form rapidly equilibrating conformers of the tautomers 2a,b and 3a,b. The Curtin-Hammett principle is invoked to determine the course of the reaction from these protonated species, which predicts that the intramolecular Friedel-Crafts type N-acylation (C-C bond formation) occurs between the protonated carbodiimide and biphenyl ring of the isomer 3b to form phenanthridinium cation 6b via transition state TS3_b6_b. Once 6b is formed, it converts to its most stable tautomers 8R and 9a. Once again, the Curtin-Hammett principle suggests that intramolecular nucleophilic attack is preferred from the tautomer 8R, where phenanthridine N-atom (N_β) attacks the protonated nitrile group (C-N bond formation) and results in the formation of intermediate 11 via TS8_R11. 11 then tautomerizes to the most stable cation 13. The coordination of the latter with the chloride anion yields the phenanthridine-fused heterocyclic salt PQ with overall release of energy. The pathways involving protonation at the nitrile (N_γ) of 1 were found to be energetically unfavorable and thus insignificant to the mechanism of cyclization.

Endothelial alterations in a canine model of immune thrombocytopenia

Bleeding heterogeneity amongst patients with immune thrombocytopenia (ITP) is poorly understood. Platelets play a role in maintaining endothelial integrity, and variable thrombocytopenia-induced endothelial changes may influence bleeding severity. Platelet-derived endothelial stabilizers and markers of endothelial integrity in ITP are largely underexplored. We hypothesized that, in a canine ITP model, thrombocytopenia would lead to alterations in the endothelial ultrastructure and that the Von Willebrand factor (vWF) would serve as a marker of endothelial injury associated with thrombocytopenia. Thrombocytopenia was induced in healthy dogs with an antiplatelet antibody infusion; control dogs received an isotype control antibody. Cutaneous biopsies were obtained prior to thrombocytopenia induction, at platelet nadir, 24 hours after nadir, and on platelet recovery. Cutaneous capillaries were assessed by electron microscopy for vessel thickness, the number of pinocytotic vesicles, the number of large vacuoles, and the number of gaps between cells. Pinocytotic vesicles are thought to represent an endothelial membrane reserve that can be used for repair of damaged endothelial cells. Plasma samples were assessed for vWF. ITP dogs had significantly decreased pinocytotic vesicle numbers compared to control dogs (P = 0.0357) and the increase in plasma vWF from baseline to 24 hours correlated directly with the endothelial large vacuole score (R = 0.99103; P < 0.0001). This direct correlation between plasma vWF and the number of large vacuoles, representing the vesiculo-vacuolar organelle (VVO), a permeability structure, suggests that circulating vWF could serve as a biomarker for endothelial alterations and potentially a predictor of thrombocytopenic bleeding. Overall, our results indicate that endothelial damage occurs in the canine ITP model and variability in the degree of endothelial damage may account for differences in the bleeding phenotype among patients with ITP.

An integrative study to identify novel scaffolds for sphingosine kinase 1 inhibitors

Sphingosine kinase 1 (SphK1), the enzyme that produces the bioactive sphingolipid metabolite, sphingosine-1-phosphate, is a promising new molecular target for therapeutic intervention in cancer and inflammatory diseases. In view of its importance, the main objective of this work was to find new and more potent inhibitors for this enzyme possessing different structural scaffolds than those of the known inhibitors. Our theoretical and experimental study has allowed us to identify two new structural scaffolds (three new compounds), which could be used as starting structures for the design and then the development of new inhibitors of SphK1. Our study was carried out in different steps: virtual screening, synthesis, bioassays and molecular modelling. From our results, we propose a new dihydrobenzo[b]pyrimido[5,4-f]azepine and two alkyl{3-/4-[1-hydroxy-2-(4-arylpiperazin-1-yl)ethyl]phenyl}carbamates as initial structures for the development of new inhibitors. In addition, our molecular modelling study using QTAIM calculations, allowed us to describe in detail the molecular interactions that stabilize the different Ligand-Receptor complexes. Such analyses indicate that the cationic head of the different compounds must be refined in order to obtain an increase in the binding affinity of these ligands.

Mobilization studies in mice deficient in sphingosine kinase 2 support a crucial role of the plasma level of sphingosine-1-phosphate in the egress of hematopoietic stem progenitor cells

Sphingosine-1-phosphate (S1P) is a bioactive lipid involved in cell signaling and, if released from cells, also plays a crucial role in regulating the trafficking of lympho-hematopoietic cells, including primitive hematopoietic stem/progenitor cells (HSPCs). It has been demonstrated that S1P chemoattracts HSPCs, and its level in peripheral blood creates a gradient directing egress of these cells during mobilization. In this paper we analyzed hematopoiesis in mice deficient in sphingosine kinase 2 (Sphk2-KO mice) and studied the effect of this mutation on plasma S1P levels. We found that Sphk2-KO mice have normal hematopoiesis, and, in contrast to Sphk1-KO mice, the circulating S1P level is highly elevated in these animals and correlates with the fact that HSPCs in Sphk2-KO animals, also in contrast to Sphk1-KO animals, show enhanced mobilization. These results were recapitulated in wild type (WT) animals employing an Sphk2 inhibitor. We also administered an inhibitor of the S1P-degrading enzyme S1P lyase, known as tetrahydroxybutylimidazole (THI), to WT mice and observed that this resulted in an increase in S1P level in PB and enhanced mobilization of HSPCs. In sum, our results support a crucial role for S1P gradients in blood plasma in the mobilization process and indicate that small-molecule inhibitors of Sphk2 and Sgpl1 could be employed as mobilization-facilitating compounds. At the same time, further studies are needed to explain the unexpected effect of Sphk2 inhibition on increasing S1P levels in plasma.

Mammalian sphingosine kinase (SphK) isoenzymes and isoform expression: challenges for SphK as an oncotarget

The various sphingosine kinase (SphK) isoenzymes (isozymes) and isoforms, key players in normal cellular physiology, are strongly implicated in cancer and other diseases. Mutations in SphKs, that may justify abnormal physiological function, have not been recorded. Nonetheless, there is a large and growing body of evidence demonstrating the contribution of gain or loss of function and the imbalance in the SphK/S1P rheostat to a plethora of pathological conditions including cancer, diabetes and inflammatory diseases. SphK is expressed as two isozymes SphK1 and SphK2, transcribed from genes located on different chromosomes and both isozymes catalyze the phosphorylation of sphingosine to S1P. Expression of each SphK isozyme produces alternately spliced isoforms. In recent years the importance of the contribution of SpK1 expression to treatment resistance in cancer has been highlighted and, additionally, differences in treatment outcome appear to also be dependent upon SphK isoform expression. This review focuses on an exciting emerging area of research involving SphKs functions, expression and subcellular localization, highlighting the complexity of targeting SphK in cancer and also comorbid diseases. This review also covers the SphK isoenzymes and isoforms from a historical perspective, from their first discovery in murine species and then in humans, their role(s) in normal cellular function and in disease processes, to advancement of SphK as an oncotarget.

Classification of sphingosine kinase inhibitors using counter propagation artificial neural networks: A systematic route for designing selective SphK inhibitors

Accurate and robust classification models for describing and predicting the activity of 330 chemicals that are sphingosine kinase 1 (SphK1) and/or sphingosine kinase 2 (SphK2) inhibitors were derived. The classification models developed in this work assist in finding selective subspaces in chemical space occupied by particular groups of SphK inhibitors. A combination of a genetic algorithm (GA) and a counter propagation artificial neural network (CPANN) was utilized to select the most efficient subsets of the molecular descriptors. The optimized models in this work reasonably separate active inhibitors of SphK1 from active SphK2 inhibitors. Generally, the CPANN models in this work were used to classify the compounds according to their therapeutic targets and activities. The simplicity of the chosen descriptors and their relative importance sheds some light on the structural features necessary to induce selective inhibitory activity to the studied molecules. The areas under the receiver operating characteristic (ROC) curves for the GA-CPANN models in this work were 0.934 and 0.922 for active SphK1 and SphK2 inhibitors, respectively. Generally, the results in this work suggest some important molecular features and pharmacophores that could help medicinal chemists develop selective and potent SphK inhibitors.

Sphingosine kinase inhibitors: a review of patent literature (2006-2015)

INTRODUCTION

Sphingosine kinase (SphK1 & SphK2) is the sole source of the pleiotropic lipid mediator, sphingosine-1-phosphate (S1P). S1P has been implicated in a variety of diseases such as cancer, Alzheimer's disease, sickle cell disease and fibrosis and thus the biosynthetic route to S1P is a logical target for drug discovery. Areas covered: In this review, the authors consider the SphK inhibitor patent literature from 2006-2016 Q1 with the emphasis on composition of matter utility patents. The Espacenet database was queried with the search term 'sphingosine AND kinase' to identify relevant literature. Expert opinion: Early inhibitor discovery focused on SphK1 with a bias towards oncology indications. Structurally, the reported inhibitors occupy the sphingosine 'J-shaped' binding pocket. The lack of cytotoxicity with improved SphK1 inhibitors raises doubt about the enzyme as an oncology target. SphK2 inhibitors are featured in more recent patent applications. Interestingly, both SphK1 and SphK2 inhibition and gene 'knockout' share opposing effects on circulating S1P levels: SphK1 inhibition/gene ablation decreases, while SphK2 inhibition/gene ablation increases, blood S1P. As understanding of S1P's physiological roles increases and more drug-like SphK inhibitors emerge, inhibiting one or both SphK isotypes could provide unique strategies for treating disease.

Therapeutic Strategies and Pharmacological Tools Influencing S1P Signaling and Metabolism

During the last two decades the study of the sphingolipid anabolic, catabolic, and signaling pathways has attracted enormous interest. Especially the introduction of fingolimod into market as first p.o. therapeutic for the treatment of multiple sclerosis has boosted this effect. Although the complex regulation of sphingosine-1-phosphate (S1P) and other catabolic and anabolic sphingosine-related compounds is not fully understood, the influence on different (patho)physiological states from inflammation to cytotoxicity as well as the availability of versatile pharmacological tools that represent new approaches to study these states are described. Here, we have summarized various aspects concerning the many faces of sphingolipid function modulation by different pharmacological tools up to clinical candidates. Due to the immense heterogeneity of physiological or pharmacological actions and complex cross regulations, it is difficult to predict their role in upcoming therapeutic approaches. Currently, inflammatory, immunological, and/or antitumor aspects are discussed.

Data correlations between gender, cytomegalovirus infection and T cells, NK cells, and soluble immune mediators in elderly humans

We describe a cohort of 50 elderly subjects, age at least 70 years. We present gender-specific findings in T lymphocyte markers and soluble immune mediators. We show the correlation between cytomegalovirus infection status with CD56(dim) NK cell responses to a variety of stimuli and with CD56(bright)/CD56(dim) NK cell ratio. We also present the correlation of retinol binding protein (RBP)-4 plasma levels with NK cell responses and we explore the relationship between gender and adiponectin, 25(OH)D (vitamin D), and RBP4 in affecting CD56(dim) NK cell responses. These data are discussed in Al-Attar et al. (2016) [1].

Structural Requirements and Docking Analysis of Amidine-Based Sphingosine Kinase 1 Inhibitors Containing Oxadiazoles

Sphingosine 1-phosphate (S1P) is a potent growth-signaling lipid that has been implicated in cancer progression, inflammation, sickle cell disease, and fibrosis. Two sphingosine kinases (SphK1 and 2) are the source of S1P; thus, inhibitors of the SphKs have potential as targeted cancer therapies and will help to clarify the roles of S1P and the SphKs in other hyperproliferative diseases. Recently, we reported a series of amidine-based inhibitors with high selectivity for SphK1 and potency in the nanomolar range. However, these inhibitors display a short half-life. With the goal of increasing metabolic stability and maintaining efficacy, we designed an analogous series of molecules containing oxadiazole moieties. Generation of a library of molecules resulted in the identification of the most selective inhibitor of SphK1 reported to date (705-fold selectivity over SphK2), and we found that potency and selectivity vary significantly depending on the particular oxadiazole isomer employed. The best inhibitors were subjected to in silico molecular dynamics docking analysis, which revealed key insights into the binding of amidine-based inhibitors by SphK1. Herein, the design, synthesis, biological evaluation, and docking analysis of these molecules are described.

The effect of sex on immune cells in healthy aging: Elderly women have more robust natural killer lymphocytes than do elderly men

Immune gender differences have been reported, but are little studied in elderly humans. We compared monocyte and lymphocyte subsets, along with soluble immune mediators in healthy men and women over the age of 70. We also measured natural killer (NK) lymphocyte cytotoxic granule exocytosis, chemokine synthesis, and cytokine synthesis in response to a variety of stimuli. Elderly women had significantly more circulating B cells than men, whereas men had more CD4 central memory T cells and higher monocyte levels. Plasma adiponectin levels were higher in women, plasma retinol-binding protein 4 levels were higher in men, but there were no significant gender differences in C-reactive protein, IL-15, or sphingosine-1-phosphate. Women had a higher ratio of immature CD56(bright) NK cells to mature CD56(dim) NK cells, indicating a gender difference in NK cell maturation in the elderly. Comparing sexes, female mature NK cells had more vigorous cytotoxic granule responses to K562 leukemia cells and IFN-γ responses to NKp46 crosslinking. Moreover, female NK cells were more likely to produce MIP-1β in response to a variety of stimuli. These data show that gender influences NK cell activity in elderly humans.

Assessing the ligand selectivity of sphingosine kinases using molecular dynamics and MM-PBSA binding free energy calculations

The dynamic balance of sphingolipids plays a crucial role in diverse biological processes such as mitogenesis, cell migration and angiogenesis. Sphingosine kinases (SKs) including SK1 and SK2 phosphorylate sphingosine to sphingosine 1-phosphate (S1P), and control the critical balance. SK1 overexpression was reported to increase cell survival and proliferation. Although several SK1 selective inhibitors have been reported, detailed analysis toward their selectivity to understand the molecular mechanism has not been performed to our knowledge. Herein, the crystal structure of SK1 and a homology model of SK2 were used to dock five inhibitors (1, 2, 3, 4 and 5). Protein-ligand complexes were then subjected to a molecular dynamics study and MM-PBSA binding free energy calculations. By analyzing the binding model of these inhibitors, we found that residues ILE170, PHE188 and THR192 in SK1 significantly contribute a favorable binding energy to the selectivity.

Pharmacological Elevation of Circulating Bioactive Phosphosphingolipids Enhances Myocardial Recovery After Acute Infarction

Acute myocardial infarction (AMI) triggers mobilization of bone marrow (BM)-derived stem/progenitor cells (BMSPCs) through poorly understood processes. Recently, we postulated a major role for bioactive lipids such as sphingosine-1 phosphate (S1P) in mobilization of BMSPCs into the peripheral blood (PB). We hypothesized that elevating S1P levels after AMI could augment BMSPC mobilization and enhance cardiac recovery after AMI. After AMI, elevating bioactive lipid levels was achieved by treating mice with the S1P lyase inhibitor tetrahydroxybutylimidazole (THI) for 3 days (starting at day 4 after AMI) to differentiate between stem cell mobilization and the known effects of S1P on myocardial ischemic pre- and postconditioning. Cardiac function was assessed using echocardiography, and myocardial scar size evolution was examined using cardiac magnetic resonance imaging. PB S1P and BMSPCs peaked at 5 days after AMI and returned to baseline levels within 10 days (p < .05 for 5 days vs. baseline). Elevated S1P paralleled a significant increase in circulating BMSPCs (p < .05 vs. controls). We observed a greater than twofold increase in plasma S1P and circulating BMSPCs after THI treatment. Mechanistically, enhanced BMSPC mobilization was associated with significant increases in angiogenesis, BM cell homing, cardiomyocytes, and c-Kit cell proliferation in THI-treated mice. Mice treated with THI demonstrated better recovery of cardiac functional parameters and a reduction in scar size. Pharmacological elevation of plasma bioactive lipids after AMI could contribute to BMSPC mobilization and could represent an attractive strategy for enhancing myocardial recovery and improving BMSC targeting.

SIGNIFICANCE

Acute myocardial infarction (AMI) initiates innate immune and reparatory mechanisms through which bone marrow-derived stem/progenitor cells (BMSPCs) are mobilized toward the ischemic myocardium and contribute to myocardial regeneration. Although it is clear that the magnitude of BMSPC mobilization after AMI correlates with cardiac recovery, the molecular events driving BMSPC mobilization and homing are poorly understood. The present study confirms the role of bioactive lipids in BMSPC mobilization after AMI and proposes a new strategy that improves cardiac recovery. Inhibiting sphingosine-1 phosphate (S1P) lyase (SPL) allows for the augmentation of the plasma levels of S1P and stem cell mobilization. These findings demonstrate that early transient SPL inhibition after MI correlates with increased stem cell mobilization and their homing to the infarct border zones. Augmenting BMSPC mobilization correlated with the formation of new blood vessels and cardiomyocytes and c-Kit cell proliferation. These novel findings on the cellular level were associated with functional cardiac recovery, reduced adverse remodeling, and a decrease in scar size. Taken together, these data indicate that pharmacological elevation of bioactive lipid levels can be beneficial in the early phase after cardiac ischemic injury. These findings provide the first evidence that a carefully timed transient pharmacological upregulation of bioactive lipids after AMI could be therapeutic, because it results in significant cardiac structural and functional improvements.

Discovery of I-BRD9, a Selective Cell Active Chemical Probe for Bromodomain Containing Protein 9 Inhibition

Acetylation of histone lysine residues is one of the most well-studied post-translational modifications of chromatin, selectively recognized by bromodomain "reader" modules. Inhibitors of the bromodomain and extra terminal domain (BET) family of bromodomains have shown profound anticancer and anti-inflammatory properties, generating much interest in targeting other bromodomain-containing proteins for disease treatment. Herein, we report the discovery of I-BRD9, the first selective cellular chemical probe for bromodomain-containing protein 9 (BRD9). I-BRD9 was identified through structure-based design, leading to greater than 700-fold selectivity over the BET family and 200-fold over the highly homologous bromodomain-containing protein 7 (BRD7). I-BRD9 was used to identify genes regulated by BRD9 in Kasumi-1 cells involved in oncology and immune response pathways and to the best of our knowledge, represents the first selective tool compound available to elucidate the cellular phenotype of BRD9 bromodomain inhibition.

Structure-activity relationship studies and in vivo activity of guanidine-based sphingosine kinase inhibitors: discovery of SphK1- and SphK2-selective inhibitors

Sphingosine 1-phosphate (S1P) is a pleiotropic signaling molecule that acts as a ligand for five G-protein coupled receptors (S1P1-5) whose downstream effects are implicated in a variety of important pathologies including sickle cell disease, cancer, inflammation, and fibrosis. The synthesis of S1P is catalyzed by sphingosine kinase (SphK) isoforms 1 and 2, and hence, inhibitors of this phosphorylation step are pivotal in understanding the physiological functions of SphKs. To date, SphK1 and 2 inhibitors with the potency, selectivity, and in vivo stability necessary to determine the potential of these kinases as therapeutic targets are lacking. Herein, we report the design, synthesis, and structure-activity relationship studies of guanidine-based SphK inhibitors bearing an oxadiazole ring in the scaffold. Our studies demonstrate that SLP120701, a SphK2-selective inhibitor (Ki = 1 μM), decreases S1P levels in histiocytic lymphoma (U937) cells. Surprisingly, homologation with a single methylene unit between the oxadiazole and heterocyclic ring afforded a SphK1-selective inhibitor in SLP7111228 (Ki = 48 nM), which also decreased S1P levels in cultured U937 cells. In vivo application of both compounds, however, resulted in contrasting effect in circulating levels of S1P. Administration of SLP7111228 depressed blood S1P levels while SLP120701 increased levels of S1P. Taken together, these compounds provide an in vivo chemical toolkit to interrogate the effect of increasing or decreasing S1P levels and whether such a maneuver can have implications in disease states.

Granule-mediated release of sphingosine-1-phosphate by activated platelets

Sphingosine-1-phosphate (S1P) is an intracellularly generated bioactive lipid essential for development, vascular integrity, and immunity. These functions are mediated by S1P-selective cell surface G-protein coupled receptors. S1P signaling therefore requires extracellular release of this lipid. Several cell types release S1P and evidence for both plasma membrane transporter-mediated and vesicle-dependent secretion has been presented. Platelets are an important source of S1P and can release it in response to agonists generated at sites of vascular injury. S1P release from agonist-stimulated platelets was measured in the presence of a carrier molecule (albumin) using HPLC-MS/MS. The kinetics and agonist-dependence of S1P release were similar to that of other granule cargo e.g. platelet factor IV (PF4). Agonist-stimulated S1P release was defective in platelets from Unc13d(Jinx) (Munc13-4 null) mice demonstrating a critical role for regulated membrane fusion in this process. Consistent with this observation, platelets efficiently converted fluorescent NBD-sphingosine to its phosphorylated derivative which accumulated in granules. Fractionation of platelet organelles revealed the presence of S1P in both the plasma membrane and in α-granules. Resting platelets contained a second pool of constitutively releasable S1P that was more rapidly labeled by exogenously added sphingosine. Our studies indicate that platelets contain two pools of S1P that are released extracellularly: a readily-exchangeable, metabolically active pool of S1P, perhaps in the plasma membrane, and a granular pool that requires platelet activation and regulated exocytosis for release.

Importance of sphingosine kinase (SphK) as a target in developing cancer therapeutics and recent developments in the synthesis of novel SphK inhibitors

Sphingosine kinase (SphK) is an oncogenic lipid kinase that regulates the sphingolipid metabolic pathway that has been shown to play a role in numerous hyperproliferative/inflammatory diseases. The SphK isoforms (SphK1 and SphK2) catalyze the conversion of the proapoptotic substrate d-erythrosphingosine to the promitogenic/migratory product sphingosine 1-phosphate (S1P). Accumulation of S1P has been linked to the development/progression of cancer and various other diseases including, but not limited to, asthma, inflammatory bowel disease, rheumatoid arthritis, and diabetic nephropathy. SphK therefore represents a potential new target for developing novel therapeutics for cancer and other diseases. This finding has stimulated the development and evaluation of numerous SphK inhibitors over the past decade or so. In this review, we highlight the recent advancement in the field of SphK inhibitors including SphK1 and SphK2 specific inhibitors. Both sphingolipid based and nolipidic small molecule inhibitors and their importance in treatment of cancer and other diseases are discussed.

Structure-activity relationships and molecular modeling of sphingosine kinase inhibitors

The design, synthesis, and evaluation of the potency of new isoform-selective inhibitors of sphingosine kinases 1 and 2 (SK1 and SK2), the enzyme that catalyzes the phosphorylation of d-erythro-sphingosine to produce the key signaling lipid, sphingosine 1-phosphate, are described. Recently, we reported that 1-(4-octylphenethyl)piperidin-4-ol (RB-005) is a selective inhibitor of SK1. Here we report the synthesis of 43 new analogues of RB-005, in which the lipophilic tail, polar headgroup, and linker region were modified to extend the structure–activity relationship profile for this lead compound, which we explain using modeling studies with the recently published crystal structure of SK1. We provide a basis for the key residues targeted by our profiled series and provide further evidence for the ability to discriminate between the two isoforms using pharmacological intervention.

Structure guided design of a series of sphingosine kinase (SphK) inhibitors

Sphingosine-1-phosphate (S1P) signaling plays a vital role in mitogenesis, cell migration and angiogenesis. Sphingosine kinases (SphKs) catalyze a key step in sphingomyelin metabolism that leads to the production of S1P. There are two isoforms of SphK and observations made with SphK deficient mice show the two isoforms can compensate for each other's loss. Thus, inhibition of both isoforms is likely required to block SphK dependent angiogenesis. A structure based approach was used to design and synthesize a series of SphK inhibitors resulting in the identification of the first potent inhibitors of both isoforms of human SphK. Additionally, to our knowledge, this series of inhibitors contains the only sufficiently potent inhibitors of murine SphK1 with suitable physico-chemical properties to pharmacologically interrogate the role of SphK1 in rodent models and to reproduce the phenotype of SphK1 (-/-) mice.

Bioactive lipids and cationic antimicrobial peptides as new potential regulators for trafficking of bone marrow-derived stem cells in patients with acute myocardial infarction

Acute myocardial infarction (AMI) triggers mobilization of stem cells from bone marrow (BM) into peripheral blood (PB). Based on our observation that the bioactive sphingophospholipids, sphingosine-1 phosphate (S1P), and ceramide-1 phosphate (C1P) regulate trafficking of hematopoietic stem cells (HSCs), we explored whether they also direct trafficking of non-hematopoietic stem cells (non-HSCs). We detected a 3-6-fold increase in circulating CD34+, CD133+, and CXCR4+ lineage-negative (Lin-)/CD45- cells that are enriched in non-HSCs [including endothelial progenitors (EPCs) and very small embryonic-like stem cells (VSELs)] in PB from AMI patients (P<0.05 vs. controls). Concurrently, we measured a ∼3-fold increase in S1P and C1P levels in plasma from AMI patients. At the same time, plasma obtained at hospital admission and 6 h after AMI strongly chemoattracted human BM-derived CD34+/Lin- and CXCR4+/Lin- cells in Transwell chemotaxis assays. This effect of plasma was blunted after depletion of S1P level by charcoal stripping and was further inhibited by the specific S1P1 receptor antagonist such as W146 and VPC23019. We also noted that the expression of S1P receptor 1 (S1P1), which is dominant in naïve BM, is reduced after the exposure to S1P at concentrations similar to the plasma S1P levels in patients with AMI, thus influencing the role of S1P in homing to the injured myocardium. Therefore, we examined mechanisms, other than bioactive lipids, that may contribute to the homing of BM non-HSCs to the infarcted myocardium. Hypoxic cardiac tissue increases the expression of cathelicidin and β-2 defensin, which could explain why PB cells isolated from patients with AMI migrated more efficiently to a low, yet physiological, gradient of stromal-derived factor-1 in Transwell migration assays. Together, these observations suggest that while elevated S1P and C1P levels early in the course of AMI may trigger mobilization of non-HSCs into PB, cathelicidin and β-2 defensin could play an important role in their homing to damaged myocardium.

Bioactive lipids S1P and C1P are prometastatic factors in human rhabdomyosarcoma, and their tissue levels increase in response to radio/chemotherapy

Evidence suggests that bioactive lipids may regulate pathophysiologic functions such as cancer cell metastasis. Therefore, we determined that the bioactive lipid chemoattractants sphingosine-1-phosphate (S1P) and ceramide-1-phosphate (C1P) strongly enhanced the in vitro motility and adhesion of human rhabdomyosarcoma (RMS) cells. Importantly, this effect was observed at physiologic concentrations for both bioactive lipids, which are present in biologic fluids, and were much stronger than the effects observed in response to known RMS prometastatic factors such as stromal derived factors-1 (SDF-1/CXCL12) or hepatocyte growth factor/scatter factor (HGF/SF). We also present novel evidence that the levels of S1P and C1P were increased in several organs after γ-irradiation or chemotherapy, which indicates an unwanted prometastatic environment related to treatment. Critically, we found that the metastasis of RMS cells in response to S1P can be effectively inhibited in vivo with the S1P-specific binder NOX-S93 that is based on a high-affinity Spiegelmer. These data indicate that bioactive lipids play a vital role in dissemination of RMS and contribute to the unwanted side effects of radio/chemotherapy by creating a prometastatic microenvironment.

Sphingosine kinase type 2 inhibition elevates circulating sphingosine 1-phosphate

S1P (sphingosine 1-phosphate) is a pleiotropic lipid mediator involved in numerous cellular and physiological functions. Of note among these are cell survival and migration, as well as lymphocyte trafficking. S1P, which exerts its effects via five GPCRs (G-protein-coupled receptors) (S1P1-S1P5), is formed by the action of two SphKs (sphingosine kinases). Although SphK1 is the more intensively studied isotype, SphK2 is unique in it nuclear localization and has been reported to oppose some of the actions ascribed to SphK1. Although several scaffolds of SphK1 inhibitors have been described, there is a scarcity of selective SphK2 inhibitors that are necessary to evaluate the downstream effects of inhibition of this isotype. In the present paper we report a cationic amphiphilic small molecule that is a selective SphK2 inhibitor. In the course of characterizing this compound in wild-type and SphK-null mice, we discovered that administration of the inhibitor to wild-type mice resulted in a rapid increase in blood S1P, which is in contrast with our SphK1 inhibitor that drives circulating S1P levels down. Using a cohort of F2 hybrid mice, we confirmed, compared with wild-type mice, that circulating S1P levels were higher in SphK2-null mice and lower in SphK1-null mice. Thus both SphK1 and SphK2 inhibitors recapitulate the blood S1P levels observed in the corresponding null mice. Moreover, circulating S1P levels mirror SphK2 inhibitor levels, providing a convenient biomarker of target engagement.

Lack of lipid phosphate phosphatase-3 in embryonic stem cells compromises neuronal differentiation and neurite outgrowth

BACKGROUND

Bioactive lipids such as lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) have been recently described as important regulators of pluripotency and differentiation of embryonic stem (ES) cells and neural progenitors. Due to the early lethality of LPP3, an enzyme that regulates the levels and biological activities of the aforementioned lipids, it has been difficult to assess its participation in early neural differentiation and neuritogenesis.

RESULTS

We find that Ppap2b(-/-) (Lpp3(-/-) ) ES cells differentiated in vitro into spinal neurons show a considerable reduction in the amount of neural precursors and young neurons formed. In addition, differentiated Lpp3(-/-) neurons exhibit impaired neurite outgrowth. Surprisingly, when Lpp3(-/-) ES cells were differentiated, an unexpected appearance of smooth muscle actin-positive cells was observed, an event that was partially dependent upon phosphorylated sphingosines.

CONCLUSIONS

Our data show that LPP3 plays a fundamental role during spinal neuron differentiation from ES and that it also participates in regulating neurite and axon outgrowth.

Targeting the sphingosine kinase/sphingosine 1-phosphate pathway in disease: review of sphingosine kinase inhibitors

Sphingosine 1-phosphate (S1P) is an important bioactive sphingolipid metabolite that has been implicated in numerous physiological and cellular processes. Not only does S1P play a structural role in cells by defining the components of the plasma membrane, but in the last 20 years it has been implicated in various significant cell signaling pathways and physiological processes: for example, cell migration, survival and proliferation, cellular architecture, cell-cell contacts and adhesions, vascular development, atherosclerosis, acute pulmonary injury and respiratory distress, inflammation and immunity, and tumorogenesis and metastasis [1,2]. Given the wide variety of cellular and physiological processes in which S1P is involved, it is immediately obvious why the mechanisms governing S1P synthesis and degradation, and the manner in which these processes are regulated, are necessary to understand. In gaining more knowledge about regulation of the sphingosine kinase (SK)/S1P pathway, many potential therapeutic targets may be revealed. This review explores the roles of the SK/S1P pathway in disease, summarizes available SK enzyme inhibitors and examines their potential as therapeutic agents. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Sphingosine 1-phosphate (S1P) regulates glucose-stimulated insulin secretion in pancreatic beta cells

Recent studies suggest that sphingolipid metabolism is altered during type 2 diabetes. Increased levels of the sphingolipid ceramide are associated with insulin resistance. However, a role for sphingolipids in pancreatic beta cell function, or insulin production, and release remains to be established. Our studies in MIN6 cells and mouse pancreatic islets demonstrate that glucose stimulates an intracellular rise in the sphingolipid, sphingosine 1-phosphate (S1P), whereas the levels of ceramide and sphingomyelin remain unchanged. The increase in S1P levels by glucose is due to activation of sphingosine kinase 2 (SphK2). Interestingly, rises in S1P correlate with increased glucose-stimulated insulin secretion (GSIS). Decreasing S1P levels by treatment of MIN6 cells or primary islets with the sphingosine kinase inhibitor reduces GSIS. Moreover, knockdown of SphK2 alone results in decreased GSIS, whereas knockdown of the S1P phosphatase, Sgpp1, leads to a rise in GSIS. Treatment of mice with the sphingosine kinase inhibitor impairs glucose disposal due to decreased plasma insulin levels. Altogether, our data suggest that glucose activates SphK2 in pancreatic beta cells leading to a rise in S1P levels, which is important for GSIS.

Defining a role for sphingosine kinase 1 in p53-dependent tumors

p53 is a crucial tumor suppressor that is mutated or deleted in a majority of cancers. Exactly how p53 prevents tumor progression has proved elusive for many years; however, this information is crucial to define targets for chemotherapeutic development that can effectively restore p53 function. Bioactive sphingolipids have recently emerged as important regulators of proliferative, apoptotic and senescent cellular processes. In this study, we demonstrate that the enzyme sphingosine kinase 1 (SK1), a critical enzyme in the regulation of the key bioactive sphingolipids ceramide, sphingosine and sphingosine-1-phosphate (S1P), serves as a key downstream target for p53 action. Our results show that SK1 is proteolysed in response to genotoxic stress in a p53-dependent manner. p53 null mice display elevation of SK1 levels and a tumor-promoting dysregulation of bioactive sphingolipids in which the anti-growth sphingolipid ceramide is decreased and the pro-growth sphingolipid S1P is increased. Importantly, deletion of SK1 in p53 null mice completely abrogated thymic lymphomas in these mice and prolonged their life span by ~30%. Deletion of SK1 also significantly attenuated the formation of other cancers in p53 heterozygote mice. The mechanism of p53 tumor suppression by loss of SK1 is mediated by elevations of sphingosine and ceramide, which in turn were accompanied by increased expression of cell cycle inhibitors and tumor cell senescence. Thus, targeting SK1 may restore sphingolipid homeostasis in p53-dependent tumors and provide insights into novel therapeutic approaches to cancer.

Sphingosine kinase type 1 inhibition reveals rapid turnover of circulating sphingosine 1-phosphate

S1P (sphingosine 1-phosphate) is a signalling molecule involved in a host of cellular and physiological functions, most notably cell survival and migration. S1P, which signals via a set of five G-protein-coupled receptors (S1P1-S1P5), is formed by the action of two SphKs (sphingosine kinases) from Sph (sphingosine). Interfering RNA strategies and SphK1 (sphingosine kinase type 1)-null (Sphk1-/-) mouse studies implicate SphK1 in multiple signalling cascades, yet there is a paucity of potent and selective SphK1 inhibitors necessary to evaluate the effects of rapid onset inhibition of this enzyme. We have identified a set of submicromolar amidine-based SphK1 inhibitors and report using a pair of these compounds to probe the cellular and physiological functions of SphK1. In so doing, we demonstrate that our inhibitors effectively lower S1P levels in cell-based assays, but we have been unable to correlate SphK1 inhibition with changes in cell survival. However, SphK1 inhibition did diminish EGF (epidermal growth factor)-driven increases in S1P levels and Akt (also known as protein kinase B)/ERK (extracellular-signal-regulated kinase) phosphorylation. Finally, administration of the SphK1 inhibitor to wild-type, but not Sphk1-/-, mice resulted in a rapid decrease in blood S1P levels indicating that circulating S1P is rapidly turned over.

Design, synthesis and biological activity of sphingosine kinase 2 selective inhibitors

Sphingosine kinase (SphK) has emerged as an attractive target for cancer therapeutics due to its role in cell survival. SphK phosphorylates sphingosine to form sphingosine 1-phosphate (S1P), which has been implicated in cancer growth and survival. SphK exists as two different isotypes, namely SphK1 and SphK2, which play different roles inside the cell. In this report, we describe SphK inhibitors based on the immunomodulatory drug, FTY720, which is phosphorylated by SphK2 to generate a S1P mimic. Structural modification of FTY720 provided a template for synthesizing new inhibitors. A diversity-oriented synthesis generated a library of SphK inhibitors with a novel scaffold and headgroup. We have discovered subtype selective inhibitors with K(i)'s in the low micromolar range. This is the first report describing quaternary ammonium salts as SphK inhibitors.

Pharmacological relevance and potential of sphingosine 1-phosphate in the vascular system

Sphingosine-1-phosphate (S1P) was identified as a crucial molecule for regulating immune responses, inflammatory processes as well as influencing the cardiovascular system. S1P mediates differentiation, proliferation and migration during vascular development and homoeostasis. S1P is a naturally occurring lipid metabolite and is present in human blood in nanomolar concentrations. S1P is not only involved in physiological but also in pathophysiological processes. Therefore, this complex signalling system is potentially interesting for pharmacological intervention. Modulation of the system might influence inflammatory, angiogenic or vasoregulatory processes. S1P activates G-protein coupled receptors, namely S1P(1-5) , whereas only S1P(1-3) is present in vascular cells. S1P can also act as an intracellular signalling molecule. This review highlights the pharmacological potential of S1P signalling in the vascular system by giving an overview of S1P-mediated processes in endothelial cells (ECs) and vascular smooth muscle cells (VSMCs). After a short summary of S1P metabolism and signalling pathways, the role of S1P in EC and VSMC proliferation and migration, the cause of relaxation and constriction of arterial blood vessels, the protective functions on endothelial apoptosis, as well as the regulatory function in leukocyte adhesion and inflammatory responses are summarized. This is followed by a detailed description of currently known pharmacological agonists and antagonists as new tools for mediating S1P signalling in the vasculature. The variety of effects influenced by S1P provides plenty of therapeutic targets currently under investigation for potential pharmacological intervention.

Z-formamidoximes in molecular folding and macrocycles

The formamidoxime configurational Z isomer coupled with the pyridylbiscarboxamide conformational codon were used to fold planar, curved structures. When embedded into macrocycles, this folded motif promotes dimerization through π-π stacking and hydrogen-bonding and the formation of tubules akin to molecular channels in the solid state.

Plasma levels of sphingosine 1-phosphate are strongly correlated with haematocrit, but variably restored by red blood cell transfusions

Anaemia and RBC (red blood cell) transfusion may be associated with worse clinical outcomes, especially with longer blood storage duration prior to transfusion. The mechanisms underlying these harmful effects are unknown. RBCs have been proposed to buffer plasma S1P (sphingosine 1-phosphate), a lysophospholipid essential for the maintenance of endothelial integrity and important in the regulation of haematopoietic cell trafficking. The present study examined the effect of anaemia, RBC transfusion and RBC storage duration on plasma S1P levels. Plasma S1P from 30 individuals demonstrated a linear correlation with Hct (haematocrit; R²=0.51, P<0.001) with no evidence for a plateau at Hct values as low as 19%. RBC transfusion in 23 anaemic patients with baseline mean Hct of 22.2±0.34% (value is the mean±S.D.) increased Hct to 28.3±0.6% at 72 h. Despite an Hct increase, RBC transfusion failed to elevate plasma S1P consistently. A trend towards an inverse correlation was observed between RBC storage duration and the post-transfusion increase in plasma S1P. After 30 days of storage, RBC S1P decreased to 19% of that observed in fresh (3–7-day-old) RBC segments. RBC membranes contain low levels of both S1P phosphatase and S1P lyase activities that may account for the decline in S1P levels with storage. Our results support a role for RBCs in buffering plasma S1P and identify a disturbance in the capacity after transfusion. Changes in S1P content may contribute to an RBC storage lesion. Further studies should investigate the clinical significance of alterations in circulating S1P levels and the potential value of enriching stored RBCs with S1P.

Alkoxyamine-derived formamidines: configurational control and molecular folding

N,N'-Disubstituted formamidines, and amidines in general, have very rich configurational, conformational, and tautomeric diversities. As part of an effort to incorporate alkoxyamine-derived formamidine units into foldamers, the first evidence for the isolation of the up-to-now unknown E isomer, the conditions for its exclusive formation, its stability and self-assembly properties, and its configurational isomerization to its much more common Z counterpart are reported. Considering the distinctly different H-bonding patterns displayed by both E and Z isomers, such configurational control may find applications in self-assembly, molecular recognition, and biomimetic systems.