[Sphingosine 1-phosphate receptors: from biology to physiopathology]

The role of sphingosine-1-phosphate in autophagy and related disorders

S1P, also referred to as sphingosine-1-phosphate, is a lipid molecule with bioactive properties involved in numerous cellular processes such as cell growth, movement, programmed cell death, self-degradation, cell specialization, aging, and immune system reactions. Autophagy is a meticulously controlled mechanism in which cells repurpose their elements to maintain cellular balance. There are five stages in autophagy: initiation, nucleation, elongation and maturation, fusion, and degradation. New research has provided insight into the complex connection between S1P and autophagy, uncovering their interaction in both normal and abnormal circumstances. Gaining knowledge about the regulatory mechanism of S1P signaling on autophagy can offer a valuable understanding of its function in well-being and illness, potentially leading to innovative therapeutic concepts for diverse ailments. Hence, this review analyzes the essential stages in mammalian autophagy, with a specific emphasis on recent research exploring the control of each stage by S1P. Additionally, it sheds light on the roles of S1P-induced autophagy in various disorders.

Extracellular Lipids in the Lung and Their Role in Pulmonary Fibrosis

Lipids are major actors and regulators of physiological processes within the lung. Initial research has described their critical role in tissue homeostasis and in orchestrating cellular communication to allow respiration. Over the past decades, a growing body of research has also emphasized how lipids and their metabolism may be altered, contributing to the development and progression of chronic lung diseases such as pulmonary fibrosis. In this review, we first describe the current working model of the mechanisms of lung fibrogenesis before introducing lipids and their cellular metabolism. We then summarize the evidence of altered lipid homeostasis during pulmonary fibrosis, focusing on their extracellular forms. Finally, we highlight how lipid targeting may open avenues to develop therapeutic options for patients with lung fibrosis.

Sphingosine-1-Phosphate Metabolism in the Regulation of Obesity/Type 2 Diabetes

Obesity is a pathophysiological condition where excess free fatty acids (FFA) target and promote the dysfunctioning of insulin sensitive tissues and of pancreatic β cells. This leads to the dysregulation of glucose homeostasis, which culminates in the onset of type 2 diabetes (T2D). FFA, which accumulate in these tissues, are metabolized as lipid derivatives such as ceramide, and the ectopic accumulation of the latter has been shown to lead to lipotoxicity. Ceramide is an active lipid that inhibits the insulin signaling pathway as well as inducing pancreatic β cell death. In mammals, ceramide is a key lipid intermediate for sphingolipid metabolism as is sphingosine-1-phosphate (S1P). S1P levels have also been associated with the development of obesity and T2D. In this review, the current knowledge on S1P metabolism in regulating insulin signaling in pancreatic β cell fate and in the regulation of feeding by the hypothalamus in the context of obesity and T2D is summarized. It demonstrates that S1P can display opposite effects on insulin sensitive tissues and pancreatic β cells, which depends on its origin or its degradation pathway.

La sclérose en plaques et les médicaments immuno-modulateurs des récepteurs de la sphingosine 1-phosphate

La sclérose en plaques (SEP) est une maladie du système nerveux central à composante inflammatoire, très invalidante qui atteint généralement de jeunes adultes (20 à 40 ans). Cette maladie se caractérise par la destruction progressive, par les cellules du système immunitaire, de la gaine de myéline des axones, ce qui aboutit à une dégénérescence neuronale. Les lymphocytes T et B sont les acteurs principaux de cette maladie qui peut être rémittente ou progressive. Parmi les médicaments utilisés dans le cadre de son traitement, le fingolimod, un immunosuppresseur dont les cibles sont les récepteurs de la sphingosine 1-phosphate, administré par voie orale, agit en empêchant les lymphocytes de quitter le thymus et les ganglions lymphatiques, et de rejoindre les foyers inflammatoires cérébraux. Une recherche intense pour développer des traitements et des médicaments curatifs est actuellement en cours et d’autres immunosuppresseurs interagissant avec les récepteurs de sphingosine 1-phosphate sont en cours de développement.

The role of sphingosine 1-phosphate metabolism in bone and joint pathologies and ectopic calcification

Sphingolipids display important functions in various pathologies such as cancer, obesity, diabetes, cardiovascular or neurodegenerative diseases. Sphingosine, sphingosine 1-phosphate (S1P), and ceramide are the central molecules of sphingolipid metabolism. Sphingosine kinases 1 and 2 (SK1 and SK2) catalyze the conversion of the sphingolipid metabolite sphingosine into S1P. The balance between the levels of S1P and its metabolic precursors ceramide and sphingosine has been considered as a switch that could determine whether a cell proliferates or dies. This balance, also called « sphingolipid rheostat », is mainly under the control of SKs. Several studies have recently pointed out the contribution of SK/S1P metabolic pathway in skeletal development, mineralization and bone homeostasis. Indeed, SK/S1P metabolism participates in different diseases including rheumatoid arthritis, spondyloarthritis, osteoarthritis, osteoporosis, cancer-derived bone metastasis or calcification disorders as vascular calcification. In this review, we will summarize the most important data regarding the implication of SK/S1P axis in bone and joint diseases and ectopic calcification, and discuss the therapeutic potential of targeting SK/S1P metabolism for the treatment of these pathologies.

S1P-S1PR1 Signaling: the "Sphinx" in Osteoimmunology

The fundamental interaction between the immune and skeletal systems, termed as osteoimmunology, has been demonstrated to play indispensable roles in the maintenance of balance between bone resorption and formation. The pleiotropic sphingolipid metabolite, sphingosine 1-phosphate (S1P), together with its cognate receptor, sphingosine-1-phosphate receptor-1 (S1PR1), are known as key players in osteoimmunology due to the regulation on both immune system and bone remodeling. The role of S1P-S1PR1 signaling in bone remodeling can be directly targeting both osteoclastogenesis and osteogenesis. Meanwhile, inflammatory cell function and polarization in both adaptive immune (T cell subsets) and innate immune cells (macrophages) are also regulated by this signaling axis, suggesting that S1P-S1PR1 signaling could aslo indirectly regulate bone remodeling via modulating the immune system. Therefore, it could be likely that S1P-S1PR1 signaling might take part in the maintenance of continuous bone turnover under physiological conditions, while lead to the pathogenesis of bone deformities during inflammation. In this review, we summarized the immunological regulation of S1P-S1PR1 signal axis during bone remodeling with an emphasis on how osteo-immune regulators are affected by inflammation, an issue with relevance to chronical bone disorders such as rheumatoid arthritis, spondyloarthritis and periodontitis.

Neuronal sphingosine kinase 2 subcellular localization is altered in Alzheimer's disease brain

Background

Alzheimer’s disease (AD) is characterized by the accumulation of β-amyloid (Aβ) peptides and hyperphosphorylated tau protein accompanied by neuronal loss. Aβ accumulation has been associated with an impaired sphingosine 1-phosphate (S1P) metabolism. S1P is generated by sphingosine kinases (SphKs), of which there are two isoenzymes SphK1 and SphK2, and degraded by the sphingosine 1-phosphate lyase (SPL). We previously reported, that both a decrease in SphK1 expression and an increase in SPL expression, correlated with amyloid deposits in the entorhinal cortex of AD brains, suggesting a global loss of pro-survival S1P in AD neurons. SphK2 contribution has also been examined in AD yielding to conflicting results that may reflect the complexity of SphK2 regulation. The subcellular localization of SphK2, hence the compartmentalization of generated S1P, is recognized to play a crucial role in dictating either its pro-survival or pro-apoptotic functions. We therefore aimed at studying the expression of SphK2 and notably its subcellular localization in brain tissues from patients with AD.

Results

We report that a decrease in SphK2 protein cytosolic expression correlated with the density of amyloid deposits in a cohort of 25 post-mortem brains. Interestingly, we observed that the equilibrium between cytoplasmic and nuclear SphK2 is disrupted and showed that SphK2 is preferentially localized in the nucleus in AD brain extracts as compared to control extracts, with a marked increase of cleaved SphK2.

Conclusions

Our results suggest that a shift in the subcellular localization of the S1P generating SphK2 may compromise the well established pro-survival cytosolic S1P by favoring the production of nuclear S1P associated with adverse effects in AD pathogenesis.

Electronic supplementary material

The online version of this article (10.1186/s40478-018-0527-z) contains supplementary material, which is available to authorized users.

Essential role for SphK1/S1P signaling to regulate hypoxia-inducible factor 2α expression and activity in cancer

The sphingosine kinase-1/sphingosine 1-phosphate (SphK1/S1P) signaling pathway has been reported to modulate the expression of the canonical transcription factor hypoxia-inducible HIF-1α in multiple cell lineages. HIF-2α is also frequently overexpressed in solid tumors but its role has been mostly studied in clear cell renal cell carcinoma (ccRCC), the most common form of kidney cancer, where HIF-2α has been established as a driver of a more aggressive disease. In this study, the role of SphK1/S1P signaling with regard to HIF-2α was investigated in various cancer cell models including ccRCC cells. Under hypoxic conditions or in ccRCC lacking a functional von Hippel-Lindau (VHL) gene and expressing high levels of HIF-2α, SphK1 activity controls HIF-2α expression and transcriptional activity through a phospholipase D (PLD)-driven mechanism. SphK1 silencing promotes a VHL-independent HIF-2α loss of expression and activity and reduces cell proliferation in ccRCC. Importantly, downregulation of SphK1 is associated with impaired Akt and mTOR signaling in ccRCC. Taking advantage of a monoclonal antibody neutralizing extracellular S1P, we show that inhibition of S1P extracellular signaling blocks HIF-2α accumulation in ccRCC cell lines, an effect mimicked when the S1P transporter Spns2 or the S1P receptor 1 (S1P₁) is silenced. Here, we report the first evidence that the SphK1/S1P signaling pathway regulates the transcription factor hypoxia-inducible HIF-2α in diverse cancer cell lineages notably ccRCC, where HIF-2α has been established as a driver of a more aggressive disease. These findings demonstrate that SphK1/S1P signaling may act as a canonical regulator of HIF-2α expression in ccRCC, giving support to its inhibition as a therapeutic strategy that could contribute to reduce HIF-2 activity in ccRCC.

Sphingosine-1-Phosphate Induces Dose-Dependent Chemotaxis or Fugetaxis of T-ALL Blasts through S1P1 Activation

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid involved in several physiological processes including cell migration and differentiation. S1P signaling is mediated through five G protein-coupled receptors (S1P1-S1P5). S1P1 is crucial to the exit of T-lymphocytes from the thymus and peripheral lymphoid organs through a gradient of S1P. We have previously observed that T-ALL and T-LBL blasts express S1P1. Herein we analyzed the role of S1P receptors in the migratory pattern of human T-cell neoplastic blasts. S1P-triggered cell migration was directly related to S1P1 expression. T-ALL blasts expressing low levels of S1P1 mRNA (HPB-ALL) did not migrate toward S1P, whereas those expressing higher levels of S1P1 (MOLT-4, JURKAT and CEM) did migrate. The S1P ligand induced T-ALL cells chemotaxis in concentrations up to 500 nM and induced fugetaxis in higher concentrations (1000-10000 nM) through interactions with S1P1. When S1P1 was specifically blocked by the W146 compound, S1P-induced migration at lower concentrations was reduced, whereas higher concentrations induced cell migration. Furthermore, we observed that S1P/S1P1 interactions induced ERK and AKT phosphorylation, and modulation of Rac1 activity. Responding T-ALL blasts also expressed S1P3 mRNA but blockage of this receptor did not modify migratory responses. Our results indicate that S1P is involved in the migration of T-ALL/LBL blasts, which is dependent on S1P1 expression. Moreover, S1P concentrations in the given microenvironment might induce dose-dependent chemotaxis or fugetaxis of T-ALL blasts.

Inhibitors of sphingosine-1-phosphate metabolism (sphingosine kinases and sphingosine-1-phosphate lyase)

Sphingolipids (SLs) are essential structural and signaling molecules of eukaryotic cells. Among them, sphingosine 1 phosphate (S1P) is a recognized promoter of cell survival, also involved, inter alia, in inflammation and tumorigenesis processes. The knowledge and modulation of the enzymes implicated in the biosynthesis and degradation of S1P are capital to control the intracellular levels of this lipid and, ultimately, to determine the cell fate. Starting with a general overview of the main metabolic pathways involved in SL metabolism, this review is mainly focused on the description of the most relevant findings concerning the development of modulators of S1P, namely inhibitors of the enzymes regulating S1P synthesis (sphingosine kinases) and degradation (sphingosine 1 phosphate phosphatase and lyase). In addition, a brief overview of the most significant agonists and antagonists at the S1P receptors is also addressed.

microRNA-148a inhibits hepatocellular carcinoma cell invasion by targeting sphingosine-1-phosphate receptor 1

microRNA (miR)-148a has been shown to act as an important suppressor in numerous human malignancies and is markedly downregulated in hepatocellular carcinoma; however, the role of miR-148a in the regulation of hepatocellular carcinoma cell invasion, as well as the underlying mechanism, has never been studied. In the present study, the expression level of miR-148a was found to be significantly decreased in hepatocellular carcinoma tissues and HepG2 cells when compared with that in the normal adjacent tissues. Furthermore, a novel target of miR-148a was found, sphingosine-1-phosphate receptor 1 (S1PR1), whose expression was negatively regulated by miR-148a at a post-transcriptional level in hepatocellular carcinoma HepG2 cells. Upregulation of miR-148a by transfection with miR-148a mimics notably suppressed HepG2 cell invasion, similar to the effect of the SIPR1 downregulation induced by SIPR1-specific small interfering RNA, while the restoration of S1PR1 expression reversed the inhibitory effect of miR-148a upregulation on HepG2 cell invasion. Accordingly, the current study suggests that miR-148a plays an inhibitory role in the regulation of hepatocellular carcinoma cell invasion by directly targeting S1PR1.

Sphingosine kinase 1 and sphingosine-1-phosphate in oxidative stress evoked by 1-methyl-4-phenylpyridinium (MPP+) in human dopaminergic neuronal cells

Sphingosine kinases (Sphk1/2) are crucial enzymes in regulation of the biostat between sphingosine-1-phosphate (S1P) and ceramide and play an important role in the pathogenesis/pathomechanism of Alzheimer's disease (AD). These enzymes synthesise S1P, which regulates neurotransmission, synaptic function and neuron cell proliferation, by activating five G protein-coupled receptors (S1P1-5). However, S1P synthesised by Sphk2 could be involved in amyloid β (Aβ) release by stimulation of Aβ precursor protein degradation. The significance of this bioactive sphingolipid in the pathogenesis of Parkinson's disease (PD) is unknown. The aim of our study was to investigate the expression level of Sphk1 and its role in human dopaminergic neuronal cell (SH-SY5Y) viability under oxidative stress, evoked by 1-methyl-4-phenylpyridinium (MPP+). Moreover, the mechanism of S1P action on the death signalling pathway in these experimental conditions was evaluated. Our study indicated marked downregulation of Sphk1 expression in this cellular PD model. Inhibition of Sphk1 decreased SH-SY5Y cell viability and concomitantly enhanced the reactive oxygen species (ROS) level. It was found that exogenous S1P (1 μM) exerted the neuroprotective effect by activation of Sphk1 and S1P1 receptor gene expression. Moreover, S1P downregulated Bax and harakiri, death protein 5 (Hrk/DP5) expression and enhanced cell viability in MPP+-treated cells. The neuroprotective mechanism of S1P is mainly dependent on S1P1 receptor signalling, which was indicated by using specific agonists and antagonists of S1P1 receptor. The results show that S1P and S1P1 receptor agonists protected a significant population of neuronal cells against death.