Development of amidine-based sphingosine kinase 1 nanomolar inhibitors and reduction of sphingosine 1-phosphate in human leukemia cells

Pharmacology of the sphingosine-1-phosphate signalling system

The recent success of FTY720 (Fingolimod, Gilenya(®)), which has been approved for the treatment of relapsing-remitting multiple sclerosis and is the first-in-class sphingosine-1-phosphate (S1P) receptor modulating drug, has boosted the interest in further drug development in this area. Several selective S1P1 receptor-modulating drugs are being investigated in clinical trials for the treatment of diverse autoimmune disorders. Sphingosine kinase inhibitors are under development for the treatment of cancer, aberrant angiogenesis and inflammatory diseases; an inhibitor of SK2 with relatively low affinity is being analysed in patients with advanced solid tumours. While an indirect S1P lyase inhibitor has just failed the proof of concept in patients with rheumatoid arthritis, S1P lyase is still a promising target for the treatment of inflammatory and autoimmune diseases. Another approach is the development of S1P-scavenging or -clearing agents, including a monoclonal S1P antibody that has successfully passed phase I clinical trials and will be further developed for age-related macular degeneration.

New perspectives on the role of sphingosine 1-phosphate in cancer

In this chapter, we review the latest developments concerning the role of sphingosine 1-phosphate (S1P) in cancer. Particular focus is paid to the role of sphingosine kinases 1 and 2, S1P lyase and S1P-dependent signalling networks in both solid tumours and haematological cancer. The potential of this S1P-dependent pathophysiology as a therapeutic target for the treatment of cancer is also discussed.

Novel sphingosine-containing analogues selectively inhibit sphingosine kinase (SK) isozymes, induce SK1 proteasomal degradation and reduce DNA synthesis in human pulmonary arterial smooth muscle cells

Sphingosine 1-phosphate (S1P) is involved in hyper-proliferative diseases such as cancer and pulmonary arterial hypertension. We have synthesized inhibitors that are selective for the two isoforms of sphingosine kinase (SK1 and SK2) that catalyze the synthesis of S1P. A thiourea adduct of sphinganine (F02) is selective for SK2 whereas the 1-deoxysphinganines 55-21 and 77-7 are selective for SK1. (2S,3R)-1-Deoxysphinganine (55-21) induced the proteasomal degradation of SK1 in human pulmonary arterial smooth muscle cells and inhibited DNA synthesis, while the more potent SK1 inhibitors PF-543 and VPC96091 failed to inhibit DNA synthesis. These findings indicate that moderate potency inhibitors such as 55-21 are likely to have utility in unraveling the functions of SK1 in inflammatory and hyperproliferative disorders.

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.

Sphingosine kinase: a key to solving the 'French Paradox'?

A host of beneficial effects have been attributed to the red wine polyphenol, resveratrol. Foremost, among these are its anti-cancer properties. Yet, the mechanism by which resveratrol achieves these effects are unknown. In this issue of the BJP, Lim et al. report that resveratrol and its higher order oligomers inhibit sphingosine kinase 1 (SphK1). SphK1 is a key regulator of sphingolipid metabolism and alterations of this key metabolic pathway have been linked to many hyperproliferative diseases. This study identifies a target for the action of resveratrol and its higher order oligomers and opens the door to evaluation of SphK1 as a target for chemo-prevention of cancer.

Structure and catalytic function of sphingosine kinases: analysis by site-directed mutagenesis and enzyme kinetics

Sphingosine kinases 1 and 2 (SK1 and SK2) generate the bioactive lipid mediator sphingosine 1-phosphate and as such play a significant role in cell fate and in human health and disease. Despite significant interest in and examination of the role played by SK enzymes in disease, comparatively little is currently known about the three-dimensional structure and catalytic mechanisms of these enzymes. To date, limited numbers of studies have used site directed mutagenesis and activity determinations to examine the roles of individual SK residues in substrate, calmodulin, and membrane binding, as well as activation via phosphorylation. Assays are currently available that allow for both single and bisubstrate kinetic analysis of mutant proteins that show normal, lowered and enhanced activity as compared to wild type controls. Additional studies will be required to build on this foundation to completely understand SK mediated substrate binding and phosphoryl group transfer. A deeper understanding of the SK catalytic mechanism, as well as SK interactions with potential small molecule inhibitors will be invaluable to the future design and identification of SK activity modulators as research tools and potential therapeutics. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Characterization of isoenzyme-selective inhibitors of human sphingosine kinases

Sphingosine kinases (SKs) are promising new therapeutic targets for cancer because they regulate the balance between pro-apoptotic ceramides and mitogenic sphingosine-1-phosphate. The functions of the two SK isoenzymes, SK1 and SK2, are not redundant, with genetic ablation of SK2 having more pronounced anticancer effects than removal of SK1. Although several small molecule inhibitors of SKs have been described in the literature, detailed characterization of their molecular and cellular pharmacology, particularly their activities against human SK1 and SK2, have not been completed. Computational modeling of the putative active sites of SK1 and SK2 suggests structural differences that might allow isozyme-selective inhibitors. Therefore, we characterized several SK-inhibitory compounds which revealed differential inhibitory effects on SK1 and SK2 as follows: SKI-II and ABC294735 are SK1/2-dual inhibitors; CB5468139 is a SK1-selective inhibitor; and ABC294640 is a SK2-selective inhibitor. We examined the effects of the SK inhibitors on several biochemical and phenotypic processes in A498 kidney adenocarcinoma cells. The SK2-selective inhibitor ABC294640 demonstrated the most pronounced effects on SK1 and SK2 mRNA expression, decrease of S1P levels, elevation of ceramide levels, cell cycle arrest, and inhibition of proliferation, migration and invasion. ABC294640 also down-regulated the expression or activation of several signaling proteins, including STAT3, AKT, ERK, p21, p53 and FAK. These effects were equivalent or superior to responses to the SK1/2-dual inhibitors. Overall, these results suggest that inhibition of SK2 results in stronger anticancer effects than does inhibition of SK1 or both SK1 and SK2.

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.

Modulation of cellular S1P levels with a novel, potent and specific inhibitor of sphingosine kinase-1

SphK (sphingosine kinase) is the major source of the bioactive lipid and GPCR (G-protein-coupled receptor) agonist S1P (sphingosine 1-phosphate). S1P promotes cell growth, survival and migration, and is a key regulator of lymphocyte trafficking. Inhibition of S1P signalling has been proposed as a strategy for treatment of inflammatory diseases and cancer. In the present paper we describe the discovery and characterization of PF-543, a novel cell-permeant inhibitor of SphK1. PF-543 inhibits SphK1 with a Ki of 3.6 nM, is sphingosine-competitive and is more than 100-fold selective for SphK1 over the SphK2 isoform. In 1483 head and neck carcinoma cells, which are characterized by high levels of SphK1 expression and an unusually high rate of S1P production, PF-543 decreased the level of endogenous S1P 10-fold with a proportional increase in the level of sphingosine. In contrast with past reports that show that the growth of many cancer cell lines is SphK1-dependent, specific inhibition of SphK1 had no effect on the proliferation and survival of 1483 cells, despite a dramatic change in the cellular S1P/sphingosine ratio. PF-543 was effective as a potent inhibitor of S1P formation in whole blood, indicating that the SphK1 isoform of sphingosine kinase is the major source of S1P in human blood. PF-543 is the most potent inhibitor of SphK1 described to date and it will be useful for dissecting specific roles of SphK1-driven S1P signalling.

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.

Sphingosine-1-phosphate signaling and its role in disease

The bioactive sphingolipid metabolite sphingosine-1-phosphate (S1P) is now recognized as a critical regulator of many physiological and pathophysiological processes, including cancer, atherosclerosis, diabetes and osteoporosis. S1P is produced in cells by two sphingosine kinase isoenzymes, SphK1 and SphK2. Many cells secrete S1P, which can then act in an autocrine or paracrine manner. Most of the known actions of S1P are mediated by a family of five specific G protein-coupled receptors. More recently, it was shown that S1P also has important intracellular targets involved in inflammation, cancer and Alzheimer's disease. This suggests that S1P actions are much more complex than previously thought, with important ramifications for development of therapeutics. This review highlights recent advances in our understanding of the mechanisms of action of S1P and its roles in disease.

Sphingosine kinase inhibitors and cancer: seeking the golden sword of Hercules

There is considerable evidence that sphingosine kinases play a key role in cancer progression, which might involve positive selection of cancer cells that have been provided with a survival and growth advantage as a consequence of overexpression of the enzyme. Therefore, inhibitors of sphingosine kinase represent a novel class of compounds that have potential as anticancer agents. Poor inhibitor potency is a major issue that has precluded successful translation of these compounds into the clinic. However, recent discoveries have shown that sphingosine kinase 1 is an allosteric enzyme and that some inhibitors offer improved effectiveness by inducing proteasomal degradation of the enzyme or having nanomolar potency. Herein, we provide a perspective about these recent developments and highlight the importance of translating basic pharmacologic and biochemical findings on sphingosine kinase into new drug discovery programs for treatment of cancer.