Lack of sphingosine 1-phosphate-degrading enzymes in erythrocytes

Sphingosine-1-phosphate Decreases Erythrocyte Dysfunction Induced by β-Amyloid

Amyloid beta peptides (Aβ) have been identified as the main pathogenic agents in Alzheimer's disease (AD). Soluble Aβ oligomers, rather than monomer or insoluble amyloid fibrils, show red blood cell (RBC) membrane-binding capacity and trigger several morphological and functional alterations in RBCs that can result in impaired oxygen transport and delivery. Since bioactive lipids have been recently proposed as potent protective agents against Aβ toxicity, we investigated the role of sphingosine-1-phosphate (S1P) in signaling pathways involved in the mechanism underlying ATP release in Ab-treated RBCs. In RBCs following different treatments, the ATP, 2,3 DPG and cAMP levels and caspase 3 activity were determined by spectrophotometric and immunoassay. S1P rescued the inhibition of ATP release from RBCs triggered by Ab, through a mechanism involving caspase-3 and restoring 2,3 DPG and cAMP levels within the cell. These findings reveal the molecular basis of S1P protection against Aβ in RBCs and suggest new therapeutic avenues in AD.

Desipramine induces eryptosis in human erythrocytes, an effect blunted by nitric oxide donor sodium nitroprusside and N-acetyl-L-cysteine but enhanced by Calcium depletion

Background: Desipramine a representative of tricyclic antidepressants (TCAs) promotes recovery of depressed patients by inhibition of reuptake of neurotransmitters serotonin (SER) and norepinephrine (NE) in the presynaptic membrane by directly blocking their respective transporters SERT and NET.Aims: To study the effect of desipramine on programmed erythrocyte death (eryptosis) and explore the underlying mechanisms.Methods: Phosphatidylserine (PS) exposure on the cell surface as marker of cell death was estimated from annexin-V-binding, cell volume from forward scatter in flow cytometry. Hemolysis was determined photometrically, and intracellular glutathione [GSH]i from high performance liquid chromatography.Results: Desipramine dose-dependently significantly enhanced the percentage of annexin-V-binding cells and didn´t impact glutathione (GSH) synthesis. Desipramine-induced eryptosis was significantly reversed by pre-treatment of erythrocytes with either nitric oxide (NO) donor sodium nitroprusside (SNP) or N-acetyl-L-cysteine (NAC). The highest inhibitory effect was obtained by using both inhibitors together. Calcium (Ca2+) depletion aggravated desipramine-induced eryptosis. Changing the order of treatment, i.e. desipramine first followed by inhibitors, could not influence the inhibitory effect of SNP or NAC.Conclusion: Antidepressants-caused intoxication can be treated by SNP and NAC, respectively. B) Patients with chronic hypocalcemia should not be treated with tricyclic anti-depressants or their dose should be noticeably reduced.

The Sphingosine 1-Phosphate Axis: an Emerging Therapeutic Opportunity for Endometriosis

Endometriosis is a common condition in women of reproductive age, but its current interventions are unsatisfactory. Recent research discovered a dysregulation of the sphingosine 1-phosphate (S1P) signaling pathway in endometriosis and showed a positive outcome by targeting it. The S1P axis participates in a series of fundamental pathophysiological processes. This narrative review is trying to expound the reported and putative (due to limited reports in this area for now) interactions between the S1P axis and endometriosis in those pathophysiological processes, to provide some perspectives for future research. In short, S1P signaling pathway is highly activated in the endometriotic lesion. The S1P concentration has a surge in the endometriotic cyst fluid and the peritoneal fluid, with the downstream dysregulation of its receptors. The S1P axis plays an essential role in the migration and activation of the immune cells, fibrosis, angiogenesis, pain-related hyperalgesia, and innervation. S1P receptor (S1PR) modulators showed an impressive therapeutic effect by targeting the different S1P receptors in the endometriosis model, and many other conditions resemble endometriosis. And several of them already got approval for clinical application in many diseases, which means a drug repurposing direction and a rapid clinical translation for endometriosis treatments.

Host-Erythrocytic Sphingosine-1-Phosphate Regulates Plasmodium Histone Deacetylase Activity and Exhibits Epigenetic Control over Cell Death and Differentiation

The evolution of resistance to practically all antimalarial drugs poses a challenge to the current malaria elimination and eradication efforts. Given that the epigenome of Plasmodium falciparum governs several crucial parasite functions, pharmaceutical interventions with transmission-blocking potential that target epigenetic molecular markers and regulatory mechanisms are likely to encounter drug resistance. In the malaria parasite, histone deacetylases (HDACs) are essential epigenetic modulators that regulate cellular transcriptional rearrangements, notably the molecular mechanisms underlying parasite proliferation and differentiation. We establish "lipid sequestration" as a mechanism by which sphingolipids, specifically Sphingosine-1-Phosphate (S1P) (a metabolic product of Sphingosine Kinase 1 [SphK-1]), regulate epigenetic reprogramming in the parasite by interacting with, and modulating, the histone-deacetylation activity of PfHDAC-1, thereby regulating Plasmodium pathogenesis. Furthermore, we demonstrate that altering host S1P levels with PF-543, a potent and selective Sphk-1 inhibitor, dysregulates PfHDAC-1 activity, resulting in a significant increase in the global histone acetylation signals and, consequently, transcriptional modulation of genes associated with gametocytogenesis, virulence, and proliferation. Our findings point to a hitherto unrecognized functional role for host S1P-mediated sphingolipid signaling in modulating PfHDAC-1's enzymatic activity and, as a result, the parasite's dynamic genome-wide transcriptional patterns. The epigenetic regulation of parasite proliferation and sexual differentiation offers a novel approach for developing host-targeted therapeutics to combat malaria resistance to conventional regimens. IMPORTANCE Sphingolipid is an 18-carbon amino-alcohol-containing lipid with a sphingosine backbone, which when phosphorylated by sphingosine kinase 1 (SphK-1), generates sphingosine-1-phosphate (S1P), an essential lipid signaling molecule. Dysregulation of S1P function has been observed in a variety of pathologies, including severe malaria. The malaria parasite Plasmodium acquires a host S1P pool for its growth and survival. Here, we describe the molecular attuning of histone deacetylase-1 (PfHDAC-1), a crucial epigenetic modulator that contributes to the establishment of epigenetic chromatin states and parasite survival, in response to S1P binding. Our findings highlight the host lipid-mediated epigenetic regulation of malaria parasite key genes.

Immunosenescence and inflamm-ageing in COVID-19

The destructive effects of coronavirus disease 2019 (COVID-19) on the elderly and people with cardiovascular disease have been proven. New findings shed light on the role of aging pathways on life span and health age. New therapies that focus on aging-related pathways may positively impact the treatment of this acute respiratory infection. Using new therapies that boost the level of the immune system can support the elderly with co-morbidities against the acute form of COVID-19. This article discusses the effect of the aging immune system against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the pathways affecting this severity of infection.

Ablation of sphingosine kinase 2 suppresses fatty liver-associated hepatocellular carcinoma via downregulation of ceramide transfer protein

Hepatocellular carcinoma (HCC) accounts for 90% of primary liver cancer, the third leading cause of cancer-associated death worldwide. With the increasing prevalence of metabolic conditions, non-alcoholic fatty liver disease (NAFLD) is emerging as the fastest-growing HCC risk factor, and it imposes an additional layer of difficulty in HCC management. Dysregulated hepatic lipids are generally believed to constitute a deleterious environment cultivating the development of NAFLD-associated HCC. However, exactly which lipids or lipid regulators drive this process remains elusive. We report herein that sphingosine kinase 2 (SphK2), a key sphingolipid metabolic enzyme, plays a critical role in NAFLD-associated HCC. Ablation of Sphk2 suppressed HCC development in NAFLD livers via inhibition of hepatocyte proliferation both in vivo and in vitro. Mechanistically, SphK2 deficiency led to downregulation of ceramide transfer protein (CERT) that, in turn, decreased the ratio of pro-cancer sphingomyelin (SM) to anti-cancer ceramide. Overexpression of CERT restored hepatocyte proliferation, colony growth and cell cycle progression. In conclusion, the current study demonstrates that SphK2 is an essential lipid regulator in NAFLD-associated HCC, providing experimental evidence to support clinical trials of SphK2 inhibitors as systemic therapies against HCC.

The role of sphingolipids in meibomian gland dysfunction and ocular surface inflammation

Inflammation occurs in response to tissue injury and invasion of microorganisms and is carried out by the innate and adaptive immune systems, which are regulated by numerous chemokines, cytokines, and lipid mediators. There are four major families of bioactive lipid mediators that play an integral role in inflammation - eicosanoids, sphingolipids (SPL), specialized pro-resolving mediators (SPM), and endocannabinoids. SPL have been historically recognized as important structural components of cellular membranes; their roles as bioactive lipids and inflammatory mediators are recent additions. Major SPL metabolites, including sphingomyelin, ceramide, ceramide 1-phosphate (C1P), sphingosine, sphingosine 1-phosphate (S1P), and their respective enzymes have been studied extensively, primarily in cell-culture and animal models, for their roles in cellular signaling and regulating inflammation and apoptosis. Less focus has been given to the involvement of SPL in eye diseases. As such, the aim of this review was to examine relationships between the SPL family and ocular surface diseases, focusing on their role in disease pathophysiology and discussing the potential of therapeutics that disrupt SPL pathways.

A Literature Review of Ozanimod Therapy in Inflammatory Bowel Disease: From Concept to Practical Application

Inflammatory bowel disease (IBD), namely Ulcerative Colitis (UC) and Crohn's Disease (CD), is believed to be due to a dysregulation of the innate immune response. The complexity of the immune cascade offers both a challenge and an opportunity to researchers seeking out new treatments for IBD, as various points along the inflammatory pathways can be targeted for interruption. Sphinogosine-1-phosphate (S1P) is a phospholipid molecule with wide ranging biological effects caused by binding five known S1P receptor subtypes. Ozanimod is a small molecule drug that selectively targets S1P receptors 1 and 5 which play a crucial role in lymphocyte trafficking. In clinical trials for both UC and CD, it has been shown to induce a reversible lymphopenia which correlates with response to therapy. Reported adverse events include infection, anemia, and elevated liver enzymes. Rare instances of bradycardia, heart block, and macular edema were also reported. As a newly available therapy approved for UC patients, we aim to summarize ozanimod's novel mechanism of action, pre-clinical and clinical trial results, and to give context to this newly available drug that gastroenterologists may utilize in their treatment algorithm.

Sphingosine-1-phosphate and Sphingosine-1-phosphate receptors in the cardiovascular system: pharmacology and clinical implications

Sphingosine-1-phosphate (S1P) is a lipid that binds and activates five distinct receptor subtypes, S1P₁, S1P₂, S1P₃, S1P₄, S1P₅, widely expressed in different cells, tissues and organs. In the cardiovascular system these receptors have been extensively studied, but no drug acting on them has been approved so far for treating cardiovascular diseases. In contrast, a number of S1P receptor agonists are approved as immunomodulators, mainly for multiple sclerosis, because of their action on lymphocyte trafficking. This chapter summarizes the available information on S1P receptors in the cardiovascular system and discusses their potential for treating cardiovascular conditions and/or their role on the clinical pharmacology of drugs so far approved for non-cardiovascular conditions. Basic research has recently produced data useful to understand the molecular pharmacology of S1P and S1P receptors, regarding biased agonism, S1P storage, release and vehiculation and chaperoning by lipoproteins, paracrine actions, intracellular non-receptorial S1P actions. On the other hand, the approval of fingolimod and newer generation S1P receptor ligands as immunomodulators, provides information on a number of clinical observations on the impact of these drugs on cardiovascular system which need to be integrated with preclinical data. S1P receptors are potential targets for prevention and treatment of major cardiovascular conditions, including hypertension, myocardial infarction, heart failure and stroke.

Sphingosine 1-phosphate modulation and immune cell trafficking in inflammatory bowel disease

Immune cell trafficking is a critical element of the intestinal immune response, both in homeostasis and in pathological conditions associated with inflammatory bowel disease (IBD). This process involves adhesion molecules, chemoattractants and receptors expressed on immune cell surfaces, blood vessels and stromal intestinal tissue as well as signalling pathways, including those modulated by sphingosine 1-phosphate (S1P). The complex biological processes of leukocyte recruitment, activation, adhesion and migration have been targeted by various monoclonal antibodies (vedolizumab, etrolizumab, ontamalimab). Promising preclinical and clinical data with several oral S1P modulators suggest that inhibition of lymphocyte egress from the lymph nodes to the bloodstream might be a safe and efficacious alternative mechanism for reducing inflammation in immune-mediated disorders, including Crohn's disease and ulcerative colitis. Although various questions remain, including the potential positioning of S1P modulators in treatment algorithms and their long-term safety, this novel class of compounds holds great promise. This Review summarizes the critical mediators and mechanisms involved in immune cell trafficking in IBD and the available evidence for efficacy, safety and pharmacokinetics of S1P receptor modulators in IBD and other immune-mediated disorders. Further, it discusses potential future approaches to incorporate S1P modulators into the treatment of IBD.

The role of sphingosine-1-phosphate in bone remodeling and osteoporosis

Osteoporosis is a systemic bone disease that affects more than 200 million people worldwide and is caused by the disruption of the equilibrium between osteoclastic bone resorption and osteoblastic bone formation. Sphingosine-1-phosphate (S1P) is a natural, bioactive sphingolipid that has been shown to play a major role in cardiovascular and immunological pathologies by regulating biological and cellular processes, including migration, differentiation, proliferation and survival. Recent studies also suggest a central role for S1P in bone diseases, including osteoporosis; however, the effects of S1P, particularly in bone metabolism, remain to be further elucidated. In this review, we summarize the available literature on the role of S1P in bone metabolism with a focus on osteoporosis. On the cellular level, S1P acts as an osteoclast-osteoblast coupling factor to promote osteoblast proliferation and bone formation. Moreover, the recruitment of osteoclast precursors to resorption sites is regulated by the interplay of S1P gradients and S1P receptor expression. From a clinical perspective, increasing evidence suggests that systemically elevated S1P blood levels may serve as an independent risk factor for osteoporosis-related fractures. Taken together, S1P signaling is a potential therapeutic target and may serve as a novel biomarker in patients with systemic bone disease.

Hypogelsolinemia and Decrease in Blood Plasma Sphingosine-1-Phosphate in Patients Diagnosed with Severe Acute Pancreatitis

BACKGROUND

Acute pancreatitis (AP) is a frequent hospitalization cause of patients suffering from gastrointestinal disorders. Gelsolin has an ability to bind bioactive lipids including different sphingolipids engaged in inflammatory response. Importantly, hypogelsolinemia was observed in patients with different states of acute and chronic inflammation.

AIMS

The aim of the present study was to assess the interplay of blood plasma gelsolin and blood plasma sphingosine-1-phosphate (S1P) concentration in patients diagnosed with acute pancreatitis.

MATERIALS AND METHODS

To assess the concentration of gelsolin and S1P, immunoblotting and HPLC technique were employed, respectively. Additionally, the concentrations of amylase, lipase, C-reactive protein (CRP), procalcitonin (PCT) and the number of white blood cells (WBC) and platelet (PLT) were recorded.

RESULTS

We found that both pGSN and S1P concentrations in the plasma of the AP patients were significantly lower (pGSN ~ 15-165 mg/L; S1P ~ 100-360 pmol/mL) when compared to the levels of pGSN and S1P in a control group (pGSN ~ 130-240 mg/L; S1P ~ 260-400 pmol/mL). Additionally, higher concentrations of CRP, WBC, amylase and lipase were associated with low level of gelsolin in the blood of AP patients. No correlations between the level of PCT and PLT with gelsolin concentration were noticed.

CONCLUSION

Plasma gelsolin and S1P levels decrease during severe acute pancreatitis. Simultaneous assessment of pGSN and S1P can be useful in development of more accurate diagnostic strategies for patients with severe acute pancreatitis.

Chemotactic responses of Brugia pahangi infective third-stage larvae to tris(hydroxymethyl)aminomethane-related compounds and amino acids

Since the exogenous compound tris(hydroxymethyl)aminomethane (Tris) showed a potent chemoattractant activity for Brugia pahangi infective third-stage larvae (L3), it was assumed that, in natural infection to a host, filarial L3 can be expected to recognize an endogenous Tris-related compound. In addition, a few amino acids have been identified as water-soluble attractants for second-stage juveniles of Meloidogyne incognita, a plant parasitic nematode. Therefore, the present study assesses the in vitro chemotactic responses of B. pahangi L3 to Tris-related compounds and amino acids using an agar-plate assay. Among Tris-related compounds, 2-amino-1,3-propanediol (APD) and 2-amino-2-methyl-1,3-propanediol (AMPD) exhibited a potent chemoattractant activity for filarial L3 at a level similar to Tris. Furthermore, arginine (Arg) was identified as a potent attractant for filarial L3 among amino acids. In addition, filarial L3 were attracted to Arg, APD and AMPD in mild alkaline conditions rather than acidic conditions. The chemoattractant activity of the three compounds for filarial L3 was observed in concentrations between 6.3 and 200 mm. This is the first report to demonstrate that Arg, APD and AMPD are potent chemoattractants for B. pahangi L3. Endogenous Arg and APD, in particular, may be involved in the regulation of the chemotactic behaviour of filarial L3 in the infection to a host. The present results will help to elucidate the mechanism of filarial skin-penetrating invasion of a host.

Sphingolipids in Hematopoiesis: Exploring Their Role in Lineage Commitment

Sphingolipids, associated enzymes, and the sphingolipid pathway are implicated in complex, multifaceted roles impacting several cell functions, such as cellular homeostasis, apoptosis, cell differentiation, and more through intrinsic and autocrine/paracrine mechanisms. Given this broad range of functions, it comes as no surprise that a large body of evidence points to important functions of sphingolipids in hematopoiesis. As the understanding of the processes that regulate hematopoiesis and of the specific characteristics that define each type of hematopoietic cells is being continuously refined, the understanding of the roles of sphingolipid metabolism in hematopoietic lineage commitment is also evolving. Recent findings indicate that sphingolipid alterations can modulate lineage commitment from stem cells all the way to megakaryocytic, erythroid, myeloid, and lymphoid cells. For instance, recent evidence points to the ability of de novo sphingolipids to regulate the stemness of hematopoietic stem cells while a substantial body of literature implicates various sphingolipids in specialized terminal differentiation, such as thrombopoiesis. This review provides a comprehensive discussion focused on the mechanisms that link sphingolipids to the commitment of hematopoietic cells to the different lineages, also highlighting yet to be resolved questions.

Sphingolipidomics of serum in extremely preterm infants: Association between low sphingosine-1-phosphate levels and severe retinopathy of prematurity

BACKGROUND

Extremely preterm infants are at risk of developing retinopathy of prematurity (ROP) that can cause impaired vision or blindness. Changes in blood lipids have been associated with ROP. This study aimed to monitor longitudinal changes in the serum sphingolipidome of extremely preterm infants and investigate the relationship to development of severe ROP.

METHODS

This is a prospective study that included 47 infants born <28 gestational weeks. Serum samples were collected from cord blood and at postnatal days 1, 7, 14, and 28, and at postmenstrual weeks (PMW) 32, 36, and 40. Serum sphingolipids and phosphatidylcholines were extracted and analyzed by LC-MS/MS. Associations between sphingolipid species and ROP were assessed using mixed models for repeated measures.

RESULTS

The serum concentration of all investigated lipid classes, including ceramide, mono- di- and trihexosylceramide, sphingomyelin, and phosphatidylcholine displayed distinct temporal patterns between birth and PMW40. There were also substantial changes in the lipid species composition within each class. Among the analyzed sphingolipid species, sphingosine-1-phosphate showed the strongest association with severe ROP, and this association was independent of gestational age at birth and weight standard deviation score change.

CONCLUSIONS

The serum phospho- and sphingolipidome undergoes significant remodeling during the first weeks of the preterm infant's life. Low postnatal levels of the signaling lipid sphingosine-1-phosphate are associated with the development of severe ROP.

Sphingosine 1-phosphate and its receptors in ischemia

Sphingosine 1-phosphate (S1P), a metabolite of sphingolipids, is mainly derived from red blood cells (RBCs), platelets and endothelial cells (ECs). It plays important roles in regulating cell survival, vascular integrity and inflammatory responses through its receptors. S1P receptors (S1PRs), including 5 subtypes (S1PR1-5), are G protein-coupled receptors and have been proved to mediate various and complex roles of S1P in atherosclerosis, myocardial infarction (MI) and ischemic stroke by regulating endothelial function and inflammatory response as well as immune cell behavior. This review emphasizes the functions of S1PRs in atherosclerosis and ischemic diseases such as MI and ischemic stroke, enabling mechanistic studies and new S1PRs targeted therapies in atherosclerosis and ischemia in the future.

Physiological and pathological functions of sphingolipids in pregnancy

Signaling by the bioactive sphingolipid, sphingosine 1-phosphate (S1P), and its precursors are emerging areas in pregnancy research. S1P and ceramide levels increase towards end of gestation, suggesting a physiological role in parturition. However, high levels of circulating S1P and ceramide are correlated with pregnancy disorders such as preeclampsia, gestational diabetes mellitus and intrauterine growth restriction. Expression of placental and decidual enzymes that metabolize S1P and S1P receptors are also dysregulated during pregnancy complications. In this review, we provide an in-depth examination of the signaling mechanism of S1P and ceramide in various reproductive tissues during gestation. These factors determine implantation and early pregnancy success by modulating corpus luteum function from progesterone production to luteolysis through to apoptosis. We also highlight the role of S1P through receptor signaling in inducing decidualization and angiogenesis in the decidua, as well as regulating extravillous trophoblast migration to anchor the placenta into the uterine wall. Recent advances on the role of the S1P:ceramide rheostat in controlling the fate of villous trophoblasts and the role of S1P as a negative regulator of trophoblast syncytialization to a multinucleated placental barrier are discussed. This review also explores the role of S1P in anti-inflammatory and pro-inflammatory signaling, its role as a vasoconstrictor, and the effects of S1P metabolizing enzymes and receptors in pregnancy.

Lysophospholipids in Lung Inflammatory Diseases

The lysophospholipids (LPLs) belong to a group of bioactive lipids that play pivotal roles in several physiological and pathological processes. LPLs are derivatives of phospholipids and consist of a single hydrophobic fatty acid chain, a hydrophilic head, and a phosphate group with or without a large molecule attached. Among the LPLs, lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are the simplest, and have been shown to be involved in lung inflammatory symptoms and diseases such as acute lung injury, asthma, and chronic obstructive pulmonary diseases. G protein-coupled receptors (GPCRs) mediate LPA and S1P signaling. In this chapter, we will discuss on the role of LPA, S1P, their metabolizing enzymes, inhibitors or agonists of their receptors, and their GPCR-mediated signaling in lung inflammatory symptoms and diseases, focusing specially on acute respiratory distress syndrome, asthma, and chronic obstructive pulmonary disease.

Direct uptake of sphingosine-1-phosphate independent of phospholipid phosphatases

Sphingosine-1-phosphate (S1P) is a lipid mediator that is relatively abundant in plasma and plays an important role in the vascular and immune systems. To date, the only known mechanism for removing S1P from plasma has been dephosphorylation by phospholipid phosphatases (PLPPs) on the surface of cells in contact with the plasma. However, there remains a possibility that PLPP-independent dephosphorylation or direct S1P uptake into cells could occur. To examine these possibilities, here we generated triple KO (TKO) HAP1 cells that lacked all PLPPs (PLPP1–3) present in mammals. In the TKO cells, the intracellular metabolism of externally added deuterium-labeled S1P to ceramide was reduced to 17% compared with the WT cells, indicating that most extracellular S1P is dephosphorylated by PLPPs and then taken up into cells. However, this result also reveals the existence of a PLPP-independent S1P uptake pathway. Tracer experiments using [³²P]S1P showed the existence of a direct S1P uptake pathway that functions without prior dephosphorylation. Overexpression of sphingolipid transporter 2 (SPNS2) or of major facilitator superfamily domain containing 2B (MFSD2B), both known S1P efflux transporters, in TKO cells increased the direct uptake of S1P, whereas KO of MFSD2B in TKO cells reduced this uptake. These results suggest that these are channel-type transporters and capable of not only exporting but also importing S1P. Furthermore, we observed that erythroid cells expressing MFSD2B, exhibited high S1P uptake activity. Our findings describing direct S1P uptake may contribute to the elucidation of the molecular mechanisms that regulate plasma S1P concentration.

How could we forget immunometabolism in SARS-CoV2 infection or COVID-19?

SARS-CoV2 infection or COVID-19 has created panic around the world since its first origin in December 2019 in Wuhan city, China. The COVID-19 pandemic has infected more than 46.4 million people, with 1,199,727 deaths. The immune system plays a crucial role in the severity of COVID-19 and the development of pneumonia-induced acute lung injury (ALI) or acute respiratory distress syndrome (ARDS). Along with providing protection, both innate and T cell-based adaptive immune response dysregulate during severe SARS-CoV2 infection. This dysregulation is more pronounced in older population and patients with comorbidities (Diabetes, hypertension, obesity, other pulmonary and autoimmune diseases). However, COVID-19 patients develop protective antibodies (Abs) against SARS-CoV2, but they do not long for last. The induction of the immune response against the pathogen also requires metabolic energy that generates through the process of immunometabolism. The change in the metabolic stage of immune cells from homeostasis to an inflammatory or infectious environment is called immunometabolic reprogramming. The article describes the cellular immunology (macrophages, T cells, B cells, dendritic cells, NK cells and pulmonary epithelial cells (PEC) and vascular endothelial cells) and the associated immune response during COVID-19. Immunometabolism may serve as a cell-specific therapeutic approach to target COVID-19.

Emerging roles of lysophospholipids in health and disease

Lipids are abundant and play essential roles in human health and disease. The main functions of lipids are building blocks for membrane biogenesis. However, lipids are also metabolized to produce signaling molecules. Here, we discuss the emerging roles of circulating lysophospholipids. These lysophospholipids consist of lysoglycerophospholipids and lysosphingolipids. They are both present in cells at low concentration, but their concentrations in extracellular fluids are significantly higher. The biological functions of some of these lysophospholipids have been recently revealed. Remarkably, some of the lysophospholipids play pivotal signaling roles as well as being precursors for membrane biogenesis. Revealing how circulating lysophospholipids are produced, released, transported, and utilized in multi-organ systems is critical to understand their functions. The discovery of enzymes, carriers, transporters, and membrane receptors for these lysophospholipids has shed light on their physiological significance. In this review, we summarize the biological roles of these lysophospholipids via discussing about the proteins regulating their functions. We also discuss about their potential impacts to human health and diseases.

Sphingosine 1-Phosphate in Malaria Pathogenesis and Its Implication in Therapeutic Opportunities

Sphingosine 1-Phosphate (S1P) is a bioactive lipid intermediate in the sphingolipid metabolism, which exist in two pools, intracellular and extracellular, and each pool has a different function. The circulating extracellular pool, specifically the plasma S1P is shown to be important in regulating various physiological processes related to malaria pathogenesis in recent years. Although blood cells (red blood cells and platelets), vascular endothelial cells and hepatocytes are considered as the important sources of plasma S1P, their extent of contribution is still debated. The red blood cells (RBCs) and platelets serve as a major repository of intracellular S1P due to lack, or low activity of S1P degrading enzymes, however, contribution of platelets toward maintaining plasma S1P is shown negligible under normal condition. Substantial evidences suggest platelets loss during falciparum infection as a contributing factor for severe malaria. However, platelets function as a source for plasma S1P in malaria needs to be examined experimentally. RBC being the preferential site for parasite seclusion, and having the ability of trans-cellular S1P transportation to EC upon tight cell-cell contact, might play critical role in differential S1P distribution and parasite growth. In the present review, we have summarized the significance of both the S1P pools in the context of malaria, and how the RBC content of S1P can be channelized in better ways for its possible implication in therapeutic opportunities to control malaria.

The role of sphingosine 1-phosphate and its receptors in cardiovascular diseases

There are many different types of cardiovascular diseases, which impose a huge economic burden due to their extremely high mortality rates, so it is necessary to explore the underlying mechanisms to achieve better supportive and curative care outcomes. Sphingosine 1‐phosphate (S1P) is a bioactive lipid mediator with paracrine and autocrine activities that acts through its cell surface S1P receptors (S1PRs) and intracellular signals. In the circulatory system, S1P is indispensable for both normal and disease conditions; however, there are very different views on its diverse roles, and its specific relevance to cardiovascular pathogenesis remains elusive. Here, we review the synthesis, release and functions of S1P, specifically detail the roles of S1P and S1PRs in some common cardiovascular diseases, and then address several controversial points, finally, we focus on the development of S1P‐based therapeutic approaches in cardiovascular diseases, such as the selective S1PR1 modulator amiselimod (MT‐1303) and the non‐selective S1PR1 and S1PR3 agonist fingolimod, which may provide valuable insights into potential therapeutic strategies for cardiovascular diseases.

Metabolomic and molecular insights into sickle cell disease and innovative therapies

Sickle cell disease (SCD) is an autosomal-recessive hemolytic disorder with high morbidity and mortality. The pathophysiology of SCD is characterized by the polymerization of deoxygenated intracellular sickle hemoglobin, which causes the sickling of erythrocytes. The recent development of metabolomics, the newest member of the "omics" family, has provided a powerful new research strategy to accurately measure functional phenotypes that are the net result of genomic, transcriptomic, and proteomic changes. Metabolomics changes respond faster to external stimuli than any other "ome" and are especially appropriate for surveilling the metabolic profile of erythrocytes. In this review, we summarize recent pioneering research that exploited cutting-edge metabolomics and state-of-the-art isotopically labeled nutrient flux analysis to monitor and trace intracellular metabolism in SCD mice and humans. Genetic, structural, biochemical, and molecular studies in mice and humans demonstrate unrecognized intracellular signaling pathways, including purinergic and sphingolipid signaling networks that promote hypoxic metabolic reprogramming by channeling glucose metabolism to glycolysis via the pentose phosphate pathway. In turn, this hypoxic metabolic reprogramming induces 2,3-bisphosphoglycerate production, deoxygenation of sickle hemoglobin, polymerization, and sickling. Additionally, we review the detrimental role of an impaired Lands' cycle, which contributes to sickling, inflammation, and disease progression. Thus, metabolomic profiling allows us to identify the pathological role of adenosine signaling and S1P-mediated erythrocyte hypoxic metabolic reprogramming and hypoxia-induced impaired Lands' cycle in SCD. These findings further reveal that the inhibition of adenosine and S1P signaling cascade and the restoration of an imbalanced Lands' cycle have potent preclinical efficacy in counteracting sickling, inflammation, and disease progression.

Finding a Way Out: S1P Signaling and Immune Cell Migration

The signaling lipid sphingosine 1-phosphate (S1P) plays critical roles in an immune response. Drugs targeting S1P signaling have been remarkably successful in treatment of multiple sclerosis, and they have shown promise in clinical trials for colitis and psoriasis. One mechanism of these drugs is to block lymphocyte exit from lymph nodes, where lymphocytes are initially activated, into circulation, from which lymphocytes can reach sites of inflammation. Indeed, S1P can be considered a circulation marker, signaling to immune cells to help them find blood and lymphatic vessels, and to endothelial cells to stabilize the vasculature. That said, S1P plays pleiotropic roles in the immune response, and it will be important to build an integrated view of how S1P shapes inflammation. S1P can function so effectively because its distribution is exquisitely tightly controlled. Here we review how S1P gradients regulate immune cell exit from tissues, with particular attention to key outstanding questions in the field.

Sphingosine 1-phosphate and inflammation

AbstractSphingosine 1-phosphate (S1P), a sphingolipid mediator, regulates various cellular functions via high-affinity G protein-coupled receptors, S1P1-5. The S1P-S1P receptor signaling system plays important roles in lymphocyte trafficking and maintenance of vascular integrity, thus contributing to the regulation of complex inflammatory processes. S1P is enriched in blood and lymph while maintained low in intracellular or interstitial fluids, creating a steep S1P gradient that is utilized to facilitate efficient egress of lymphocytes from lymphoid organs. Blockage of the S1P-S1P receptor signaling system results in a marked decrease in circulating lymphocytes because of a failure of lymphocyte egress from lymphoid organs. This provides a basis of immunomodulatory drugs targeting S1P1 receptor such as FTY720, an immunosuppressive drug approved in 2010 as the first oral treatment for relapsing-remitting multiple sclerosis. The S1P-S1P receptor signaling system also plays important roles in maintenance of vascular integrity since it suppresses sprouting angiogenesis and regulates vascular permeability. Dysfunction of the S1P-S1P receptor signaling system results in various vascular defects, such as exaggerated angiogenesis in developing retina and augmented inflammation due to increased permeability. Endothelial-specific deletion of S1P1 receptor in mice fed high-fat diet leads to increased formation of atherosclerotic lesions. This review highlights the importance of the S1P-S1P receptor signaling system in inflammatory processes. We also describe our recent findings regarding a specific S1P chaperone, apolipoprotein M, that anchors to high-density lipoprotein and contributes to shaping the endothelial-protective and anti-inflammatory properties of high-density lipoprotein.

Mechanism of sphingosine 1-phosphate clearance from blood

The interplay of sphingosine 1-phosphate (S1P) synthetic and degradative enzymes as well as S1P exporters creates concentration gradients that are a fundamental to S1P biology. Extracellular S1P levels, such as in blood and lymph, are high relative to cellular S1P. The blood-tissue S1P gradient maintains endothelial integrity while local S1P gradients influence immune cell positioning. Indeed, the importance of S1P gradients was recognized initially when the mechanism of action of an S1P receptor agonist used as a medicine for multiple sclerosis was revealed to be inhibition of T-lymphocytes' recognition of the high S1P in efferent lymph. Furthermore, the increase in erythrocyte S1P in response to hypoxia influences oxygen delivery during high altitude acclimatization. However, understanding of how S1P gradients are maintained is incomplete. For example, S1P is synthesized but is only slowly metabolized by blood yet circulating S1P turns over quickly by an unknown mechanism. Prompted by the counterintuitive observation that blood S1P increases markedly in response to inhibition S1P synthesis (by sphingosine kinase 2 (SphK2)), we studied mice wherein several tissues were made deficient in either SphK2 or S1P degrading enzymes. Our data reveal a mechanism whereby S1P is de-phosphorylated at the hepatocyte surface and the resulting sphingosine is sequestered by SphK phosphorylation and in turn degraded by intracellular S1P lyase. Thus, we identify the liver as the primary site of blood S1P clearance and provide an explanation for the role of SphK2 in this process. Our discovery suggests a general mechanism whereby S1P gradients are shaped.

Potential sphingosine-1-phosphate-related therapeutic targets in the treatment of cerebral ischemia reperfusion injury

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that regulates lymphocyte trafficking, glial cell activation, vasoconstriction, endothelial barrier function, and neuronal death pathways in the brain. Research has increasingly implicated S1P in the pathology of cerebral ischemia reperfusion (IR) injury. As a high-affinity agonist of S1P receptor, fingolimod exhibits excellent neuroprotective effects against ischemic challenge both in vivo and in vitro. By summarizing recent progress on how S1P participates in the development of brain IR injury, this review identifies potential therapeutic targets for the treatment of brain IR injury.

Secrets and lyase: Control of sphingosine 1-phosphate distribution

The signaling lipid sphingosine 1-phosphate (S1P) plays key roles in many physiological processes. In the immune system, S1P's best-described function is to draw cells out of tissues into circulation. Here, we will review models of S1P distribution in the thymus, lymph nodes, spleen, and nonlymphoid tissues. These models have been challenging to construct, because of the lack of tools to map lipid gradients. Nonetheless, evidence to date suggests that S1P distribution is exquisitely tightly controlled, and that concentrations of signaling-available S1P cannot be predicted by standard rules of thumb. The fine regulation of S1P gradients may explain how S1P can simultaneously direct multiple cell movements both between tissues and circulation and within tissues. It may also make it feasible to develop drugs that enable spatially specific modulation of S1P signaling.

Sphingolipids in Alzheimer's disease, how can we target them?

Altered levels of sphingolipids and their metabolites in the brain, and the related downstream effects on neuronal homeostasis and the immune system, provide a framework for understanding mechanisms in neurodegenerative disorders and for developing new intervention strategies. In this review we will discuss: the metabolites of sphingolipids that function as second messengers; and functional aberrations of the pathway resulting in Alzheimer's disease (AD) pathophysiology. Focusing on the central product of the sphingolipid pathway ceramide, we describ approaches to pharmacologically decrease ceramide levels in the brain and we argue on how the sphingolipid pathway may represent a new framework for developing novel intervention strategies in AD. We also highlight the possible use of clinical and non-clinical drugs to modulate the sphingolipid pathway and sphingolipid-related biological cascades.

Hemoglobin: Structure, Function and Allostery

This chapter reviews how allosteric (heterotrophic) effectors and natural mutations impact hemoglobin (Hb) primary physiological function of oxygen binding and transport. First, an introduction about the structure of Hb is provided, including the ensemble of tense and relaxed Hb states and the dynamic equilibrium of Hb multistate. This is followed by a brief review of Hb variants with altered Hb structure and oxygen binding properties. Finally, a review of different endogenous and exogenous allosteric effectors of Hb is presented with particular emphasis on the atomic interactions of synthetic ligands with altered allosteric function of Hb that could potentially be harnessed for the treatment of diseases.

S1P/S1P Receptor Signaling in Neuromuscolar Disorders

The bioactive sphingolipid metabolite, sphingosine 1-phosphate (S1P), and the signaling pathways triggered by its binding to specific G protein-coupled receptors play a critical regulatory role in many pathophysiological processes, including skeletal muscle and nervous system degeneration. The signaling transduced by S1P binding appears to be much more complex than previously thought, with important implications for clinical applications and for personalized medicine. In particular, the understanding of S1P/S1P receptor signaling functions in specific compartmentalized locations of the cell is worthy of being better investigated, because in various circumstances it might be crucial for the development or/and the progression of neuromuscular diseases, such as Charcot-Marie-Tooth disease, myasthenia gravis, and Duchenne muscular dystrophy.

Sphingosine 1-Phosphate (S1P)/ S1P Receptor Signaling and Mechanotransduction: Implications for Intrinsic Tissue Repair/Regeneration

Tissue damage, irrespective from the underlying etiology, destroys tissue structure and, eventually, function. In attempt to achieve a morpho-functional recover of the damaged tissue, reparative/regenerative processes start in those tissues endowed with regenerative potential, mainly mediated by activated resident stem cells. These cells reside in a specialized niche that includes different components, cells and surrounding extracellular matrix (ECM), which, reciprocally interacting with stem cells, direct their cell behavior. Evidence suggests that ECM stiffness represents an instructive signal for the activation of stem cells sensing it by various mechanosensors, able to transduce mechanical cues into gene/protein expression responses. The actin cytoskeleton network dynamic acts as key mechanotransducer of ECM signal. The identification of signaling pathways influencing stem cell mechanobiology may offer therapeutic perspectives in the regenerative medicine field. Sphingosine 1-phosphate (S1P)/S1P receptor (S1PR) signaling, acting as modulator of ECM, ECM-cytoskeleton linking proteins and cytoskeleton dynamics appears a promising candidate. This review focuses on the current knowledge on the contribution of S1P/S1PR signaling in the control of mechanotransduction in stem/progenitor cells. The potential contribution of S1P/S1PR signaling in the mechanobiology of skeletal muscle stem cells will be argued based on the intriguing findings on S1P/S1PR action in this mechanically dynamic tissue.

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.

The role of sphingosine-1-phosphate in skeletal muscle: Physiology, mechanisms, and clinical perspectives

Sphingolipids were discovered more than a century ago and were simply considered as a class of cell membrane lipids for a long time. However, after the discovery of several intracellular functions and their role in the control of many physiological and pathophysiological conditions, these molecules have gained much attention. For instance, the sphingosine-1-phosphate (S1P) is a circulating bioactive sphingolipid capable of triggering strong intracellular reactions through the family of S1P receptors (S1PRs) spread in several cell types and tissues. Recently, the role of S1P in the control of skeletal muscle metabolism, atrophy, regeneration, and metabolic disorders has been widely investigated. In this review, we summarized the knowledge of S1P and its effects in skeletal muscle metabolism, highlighting the role of S1P/S1PRs axis in skeletal muscle regeneration, fatigue, ceramide accumulation, and insulin resistance. Finally, we discussed the physical exercise role in S1P/S1PRs signaling in skeletal muscle cells, and how this nonpharmacological strategy may be prospective for future investigations due to its ability to increase S1P levels.

Cortical Actin Dynamics in Endothelial Permeability

The pulmonary endothelial cell forms a critical semi-permeable barrier between the vascular and interstitial space. As part of the blood-gas barrier in the lung, the endothelium plays a key role in normal physiologic function and pathologic disease. Changes in endothelial cell shape, defined by its plasma membrane, determine barrier integrity. A number of key cytoskeletal regulatory and effector proteins including non-muscle myosin light chain kinase, cortactin, and Arp 2/3 mediate actin rearrangements to form cortical and membrane associated structures in response to barrier enhancing stimuli. These actin formations support and interact with junctional complexes and exert forces to protrude the lipid membrane to and close gaps between individual cells. The current knowledge of these cytoskeletal processes and regulatory proteins are the subject of this review. In addition, we explore novel advancements in cellular imaging that are poised to shed light on the complex nature of pulmonary endothelial permeability.

A Circadian Rhythm in both Complement Cascade (ComC) Activation and Sphingosine-1-Phosphate (S1P) Levels in Human Peripheral Blood Supports a Role for the ComC-S1P Axis in Circadian Changes in the Number of Stem Cells Circulating in Peripheral Blood

The number of hematopoietic stem/progenitor cells (HSPCs) circulating in peripheral blood (PB) is regulated by a circadian rhythm, and more HSPCs circulate in PB in the morning hours than at night. Different mechanisms have been proposed that might regulate this process, including changes in tonus of β-adrenergic innervation of bone marrow (BM) tissue. Our group reported that in mice circadian changes in the number of HSPCs circulating in PB correlates with diurnal activation of the complement cascade (ComC) and that the mice deficient in C5 component of ComC (C5-KO mice) do not show circadian changes in the number of circulating HSPCs in PB. We also reported the existence of a gradient between PB and BM of a bioactive phosphosphingolipid, sphingosine-1-phosphate (S1P), which is a major PB chemottractant for BM-residing HSPCs. Based on these observations, we investigated activation of the ComC and the level of S1P in the PB of 66 healthy volunteers. We found that both ComC activation and the S1P level undergo changes in a circadian cycle. While the ComC becomes highly activated during deep sleep at 2 am, S1P becomes activated later, and its highest level is observed at 8 am, which precedes circadian egress of HSPCs from BM into PB. In sum, circadian activation of the ComC-S1P axis releases HSPCs from BM into PB.

Bioactive Sphingolipids, Complement Cascade, and Free Hemoglobin Levels in Stable Coronary Artery Disease and Acute Myocardial Infarction

BACKGROUND

Acute myocardial infarction (AMI) and coronary artery bypass graft (CABG) surgery are associated with a pathogen-free inflammatory response (sterile inflammation). Complement cascade (CC) and bioactive sphingolipids (BS) are postulated to be involved in this process.

AIM

The aim of this study was to evaluate plasma levels of CC cleavage fragments (C3a, C5a, and C5b9), sphingosine (SP), sphingosine-1-phosphate (S1P), and free hemoglobin (fHb) in AMI patients treated with primary percutaneous coronary intervention (pPCI) and stable coronary artery disease (SCAD) undergoing CABG.

PATIENTS AND METHODS

The study enrolled 37 subjects (27 male) including 22 AMI patients, 7 CABG patients, and 8 healthy individuals as the control group (CTRL). In the AMI group, blood samples were collected at 5 time points (admission to hospital, 6, 12, 24, and 48 hours post pPCI) and 4 time points in the CABG group (6, 12, 24, and 48 hours post operation). SP and S1P concentrations were measured by high-performance liquid chromatography (HPLC). Analysis of C3a, C5a, and C5b9 levels was carried out using high-sensitivity ELISA and free hemoglobin by spectrophotometry.

RESULTS

The plasma levels of CC cleavage fragments (C3a and C5b9) were significantly higher, while those of SP and S1P were lower in patients undergoing CABG surgery in comparison to the AMI group. In both groups, levels of CC factors showed no significant changes within 48 hours of follow-up. Conversely, SP and S1P levels gradually decreased throughout 48 hours in the AMI group but remained stable after CABG. Moreover, the fHb concentration was significantly higher after 24 and 48 hours post pPCI compared to the corresponding postoperative time points. Additionally, the fHb concentrations increased between 12 and 48 hours after PCI in patients with AMI.

CONCLUSIONS

Inflammatory response after AMI and CABG differed regarding the release of sphingolipids, free hemoglobin, and complement cascade cleavage fragments.

Nuclear lipid mediators: Role of nuclear sphingolipids and sphingosine-1-phosphate signaling in epigenetic regulation of inflammation and gene expression

Phospholipids, sphingolipids, and cholesterol are integral components of eukaryotic cell organelles, including the nucleus. Recent evidence shows characteristic features of nuclear lipid composition and signaling, which are distinct from that of the cytoplasm and plasma membrane. While the nuclear phosphoinositol lipid signaling in cell cycle regulation and differentiation has been well described, there is a paucity on the role of nuclear sphingolipids and sphingolipid signaling in different physiological and pathophysiological human conditions. In this prospective, we describe the role of sphingolipids and specifically focus on the sphingoid bases, such as sphingosine, ceramide, and sphingosine-1-phosphate (S1P) generation and catabolism in nuclear signaling and function. Particularly, S1P generated in the nucleus by phosphorylation of SPHK2 modulates HDAC activity either by direct binding or through activation of nuclear reactive oxygen species and regulates cell cycle and pro-inflammatory gene expression. Potential implication of association of SPHK2 with the co-repressor complexes and generation of S1P in the nucleus on chromatin remodeling under normal and pathological conditions is discussed. A better understanding of sphingolipid signaling in the nucleus will facilitate the design and development of new and novel therapeutic approaches to modulate expression of pro-inflammatory and cell cycle dependent genes in human pathologies such as cancer, bacterial lung infection, neurodegeneration, and cystic fibrosis.

Sphingolipids and Redox Signaling in Renal Regulation and Chronic Kidney Diseases

Significance: Sphingolipids play critical roles in the membrane biology and intracellular signaling events that influence cellular behavior and function. Our review focuses on the cellular mechanisms and functional relevance of the cross talk between sphingolipids and redox signaling, which may be critically implicated in the pathogenesis of different renal diseases. Recent Advances: Reactive oxygen species (ROS) and sphingolipids can regulate cellular redox homeostasis through the regulation of NADPH oxidase, mitochondrial integrity, nitric oxide synthase (NOS), and antioxidant enzymes. Over the last two decades, there have been significant advancements in the field of sphingolipid research, and it was in 2010 for the first time that sphingolipid receptor modulator was exploited as a therapeutic in humans. The cross talk of sphingolipids with redox signaling pathways becomes an important mechanism in the development of many different diseases such as renal diseases. Critical Issues: The critical issues to be addressed in this review are how sphingolipids interact with the redox signaling pathway to regulate renal function and even result in chronic kidney diseases. Ceramide, sphingosine, and sphingosine-1-phosphate (S1P) as main signaling sphingolipids are discussed in more detail. Future Directions: Although sphingolipids and ROS may mediate or modulate cellular responses to physiological and pathological stimuli, more translational studies and mechanistic pursuit in a tissue- or cell-specific way are needed to enhance our understanding of this important topic and to develop effective therapeutic strategies to treat diseases associated with redox signaling and sphingolipid cross talk. Antioxid. Redox Signal. 28, 1008-1026.

Sphingosine-1-phosphate receptor 1 mediates elevated IL-6 signaling to promote chronic inflammation and multitissue damage in sickle cell disease

Sphingosine-1-phosphate (S1P) is a biolipid involved in chronic inflammation in several inflammatory disorders. Recent studies revealed that elevated S1P contributes to sickling in sickle cell disease (SCD), a devastating hemolytic, genetic disorder associated with severe chronic inflammation and tissue damage. We evaluated the effect of elevated S1P in chronic inflammation and tissue damage in SCD and underlying mechanisms. First, we demonstrated that interfering with S1P receptor signaling by FTY720, a U.S. Food and Drug Administration-approved drug, significantly reduced systemic, local inflammation and tissue damage without antisickling effects. These findings led us to discover that S1P receptor activation leads to substantial elevated local and systemic IL-6 levels in SCD mice. Genetic deletion of IL-6 in SCD mice significantly reduced local and systemic inflammation, tissue damage, and kidney dysfunction. At the cellular level, we determined that elevated IL-6 is a key cytokine functioning downstream of elevated S1P, which contributes to increased S1P receptor 1 ( S1pr1) gene expression in the macrophages of several tissues in SCD mice. Mechanistically, we revealed that S1P-S1PR1 signaling reciprocally up-regulated IL-6 gene expression in primary mouse macrophages in a JAK2-dependent manner. Altogether, we revealed that elevated S1P, coupled with macrophage S1PR1 reciprocally inducing IL-6 expression, is a key signaling network functioning as a malicious, positive, feed-forward loop to sustain inflammation and promote tissue damage in SCD. Our findings immediately highlight novel therapeutic possibilities.-Zhao, S., Adebiyi, M. G., Zhang, Y., Couturier, J. P., Fan, X., Zhang, H., Kellems, R. E., Lewis, D. E., Xia, Y. Sphingosine-1-phosphate receptor 1 mediates elevated IL-6 signaling to promote chronic inflammation and multitissue damage in sickle cell disease.

Plasma Sphingolipids in Acute Pancreatitis

Acute pancreatitis (AP) is a prevalent gastrointestinal disorder associated with systemic inflammatory response syndrome and, in the case of severe AP, a mortality rate ranging from 36% to 50%. Standard clinical treatment of AP includes intensive hydration, analgesia, and management of complications. Unfortunately, the direct treatment of AP at the level of its molecular pathomechanism has not yet been established. Recent studies indicate that the sphingolipid signaling pathway may be one of the important factors contributing to the development of inflammation in pancreatic diseases. In the current study, we sought to investigate this promising route. We examined the plasma sphingolipid profile of 44 patients with acute pancreatitis, dividing them into three groups: mild, moderate and severe AP. Samples were collected from these groups at days 1, 3 and 7 following their hospital admission. We demonstrated significant changes in blood plasma sphingolipids in relation to the time course of AP. We also found an inhibition of de novo ceramide synthesis in mild and moderate AP. However, the most important and novel finding was a significant elevation in sphingosine-1-phosphate (S1P) (a downstream metabolite of ceramide) in mild AP, as well as a dramatic reduction in the lipid molecule content in the early stage (days 1 and 3) of severe AP. This strongly indicates that plasma S1P could serve as a prognostic marker of AP severity.

S1P Lyase Regulation of Thymic Egress and Oncogenic Inflammatory Signaling

Sphingosine-1-phosphate (S1P) is a potent lipid signaling molecule that regulates pleiotropic biological functions including cell migration, survival, angiogenesis, immune cell trafficking, inflammation, and carcinogenesis. It acts as a ligand for a family of cell surface receptors. S1P concentrations are high in blood and lymph but low in tissues, especially the thymus and lymphoid organs. S1P chemotactic gradients are essential for lymphocyte egress and other aspects of physiological cell trafficking. S1P is irreversibly degraded by S1P lyase (SPL). SPL regulates lymphocyte trafficking, inflammation and other physiological and pathological processes. For example, SPL located in thymic dendritic cells acts as a metabolic gatekeeper that controls the normal egress of mature T lymphocytes from the thymus into the circulation, whereas SPL deficiency in gut epithelial cells promotes colitis and colitis-associated carcinogenesis (CAC). Recently, we identified a complex syndrome comprised of nephrosis, adrenal insufficiency, and immunological defects caused by inherited mutations in human SGPL1, the gene encoding SPL. In the present article, we review current evidence supporting the role of SPL in thymic egress, inflammation, and cancer. Lastly, we summarize recent progress in understanding other SPL functions, its role in inherited disease, and SPL targeting for therapeutic purposes.

Translational Aspects of Sphingolipid Metabolism in Renal Disorders

Sphingolipids, long thought to be passive components of biological membranes with merely a structural role, have proved throughout the past decade to be major players in the pathogenesis of many human diseases. The study and characterization of several genetic disorders like Fabry’s and Tay Sachs, where sphingolipid metabolism is disrupted, leading to a systemic array of clinical symptoms, have indeed helped elucidate and appreciate the importance of sphingolipids and their metabolites as active signaling molecules. In addition to being involved in dynamic cellular processes like apoptosis, senescence and differentiation, sphingolipids are implicated in critical physiological functions such as immune responses and pathophysiological conditions like inflammation and insulin resistance. Interestingly, the kidneys are among the most sensitive organ systems to sphingolipid alterations, rendering these molecules and the enzymes involved in their metabolism, promising therapeutic targets for numerous nephropathic complications that stand behind podocyte injury and renal failure.

Altered Leukocyte Sphingolipid Pathway in Breast Cancer

Sphingolipid metabolism pathway is essential in membrane homeostasis, and its dysfunction has been associated with favorable tumor microenvironment, disease progression, and chemotherapy resistance. Its major components have key functions on survival and proliferation, with opposing effects. We have profiled the components of the sphingolipid pathway on leukocytes of breast cancer (BC) patients undergoing chemotherapy treatment and without, including the five sphingosine 1-phosphate (S1P) receptors, the major functional genes, and cytokines, in order to better understand the S1P signaling in the immune cells of these patients. To the best of our knowledge, this is the first characterization of the sphingolipid pathway in whole blood of BC patients. Skewed gene profiles favoring high SPHK1 expression toward S1P production during BC development was observed, which was reversed by chemotherapy treatment, and reached similar levels to those found in healthy donors. Such levels were also correlated with high levels of TNF-α. Our data revealed an important role of the sphingolipid pathway in immune cells in BC with skewed signaling of S1P receptors, which favored cancer development even under chemotherapy, and may probably be a trigger of cancer resistance. Thus, these molecules must be considered as a target pathway for combined BC therapeutics.

The balance between Gαi-Cdc42/Rac and Gα12/13-RhoA pathways determines endothelial barrier regulation by sphingosine-1-phosphate

The bioactive sphingosine-1-phosphatephosphate (S1P) is present in plasma, bound to carrier proteins, and involved in many physiological processes, including angiogenesis, inflammatory responses, and vascular stabilization. S1P can bind to several G-protein-coupled receptors (GPCRs) activating a number of different signaling networks. At present, the dynamics and relative importance of signaling events activated immediately downstream of GPCR activation are unclear. To examine these, we used a set of fluorescence resonance energy transfer-based biosensors for different RhoGTPases (Rac1, RhoA/B/C, and Cdc42) as well as for heterotrimeric G-proteins in a series of live-cell imaging experiments in primary human endothelial cells. These experiments were accompanied by biochemical GTPase activity assays and transendothelial resistance measurements. We show that S1P promotes cell spreading and endothelial barrier function through S1PR1-Gαi-Rac1 and S1PR1-Gαi-Cdc42 pathways. In parallel, a S1PR2-Gα12/13-RhoA pathway is activated that can induce cell contraction and loss of barrier function, but only if Gαi-mediated signaling is suppressed. Our results suggest that Gαq activity is not involved in S1P-mediated regulation of barrier integrity. Moreover, we show that early activation of RhoA by S1P inactivates Rac1 but not Cdc42, and vice versa. Together, our data show that the rapid S1P-induced increase in endothelial integrity is mediated by a S1PR1-Gαi-Cdc42 pathway.

A Novel Perspective on the ApoM-S1P Axis, Highlighting the Metabolism of ApoM and Its Role in Liver Fibrosis and Neuroinflammation

Hepatocytes, renal proximal tubule cells as well as the highly specialized endothelium of the blood brain barrier (BBB) express and secrete apolipoprotein M (apoM). ApoM is a typical lipocalin containing a hydrophobic binding pocket predominantly carrying Sphingosine-1-Phosphate (S1P). The small signaling molecule S1P is associated with several physiological as well as pathological pathways whereas the role of apoM is less explored. Hepatic apoM acts as a chaperone to transport S1P through the circulation and kidney derived apoM seems to play a role in S1P recovery to prevent urinal loss. Finally, polarized endothelial cells constituting the lining of the BBB express apoM and secrete the protein to the brain as well as to the blood compartment. The review will provide novel insights on apoM and S1P, and its role in hepatic fibrosis, neuroinflammation and BBB integrity.