Correlation of high density lipoprotein (HDL)-associated sphingosine 1-phosphate with serum levels of HDL-cholesterol and apolipoproteins

Ligand-dependent interactions between SR-B1 and S1PR1 in macrophages and atherosclerotic plaques

HDLs carry sphingosine-1-phosphate (S1P) and stimulate signaling pathways in different cells including macrophages and endothelial cells, involved in atherosclerotic plaque development. HDL signaling via S1P relies on the HDL receptor scavenger receptor class B, type I (SR-B1) and the sphingosine-1-phosphate receptor 1 (S1PR1), which interact when both are heterologously overexpressed in the HEK293 cell line. In this study, we set out to test if SR-B1 and S1PR1 interacted in primary murine macrophages in culture and atherosclerotic plaques. We used knock-in mice that endogenously expressed S1PR1 tagged with eGFP-(S1pr1eGFP/eGFP mice), combined with proximity ligation analysis to demonstrate that HDL stimulates the physical interaction between SR-B1 and S1PR1 in primary macrophages, that this is dependent on HDL-associated S1P and can be blocked by an inhibitor of SR-B1's lipid transfer activity or an antagonist of S1PR1. We also demonstrate that a synthetic S1PR1-selective agonist, SEW2871, stimulates the interaction between SR-B1 and S1PR1 and that this was also blocked by an inhibitor of SR-B1's lipid transport activity. Furthermore, we detected abundant SR-B1/S1PR1 complexes in atherosclerotic plaques of S1pr1eGFP/eGFP mice that also lacked apolipoprotein E. Treatment of mice with the S1PR1 antagonist, Ex26, for 12 h disrupted the SR-B1-S1PR1 interaction in atherosclerotic plaques. These findings demonstrate that SR-B1 and S1PR1 form ligand-dependent complexes both in cultured primary macrophages and within atherosclerotic plaques in mice and provide mechanistic insight into how SR-B1 and S1PR1 participate in mediating HDL signaling to activate atheroprotective responses in macrophages.

Metabolomics Profiling of Vitamin D Status in Relation to Dyslipidemia

Vitamin D deficiency is a global disorder associated with several chronic illnesses including dyslipidemia and metabolic syndrome. The impact of this association with both dyslipidemia and vitamin D deficiency on metabolomics profile is not yet fully understood. This study analyses the metabolomics and lipidomic signatures in relation to vitamin D status and dyslipidemia. Metabolomics data were collected from Qatar Biobank database and categorized into four groups based on vitamin D and dyslipidemia status. Metabolomics multivariate analysis was performed using the orthogonal partial least square discriminate analysis (OPLS-DA) whilst linear models were used to assess the per-metabolite association with each of the four dyslipidemia/vitamin D combination groups. Our results indicate a high prevalence of vitamin D deficiency among the younger age group, while dyslipidemia was more prominent in the older group. A significant alteration of metabolomics profile was observed among the dyslipidemic and vitamin D deficient individuals in comparison with control groups. These modifications reflected changes in some key pathways including ceramides, diacylglycerols, hemosylceramides, lysophospholipids, phosphatidylcholines, phosphatidylethanol amines, and sphingomyelins. Vitamin D deficiency and dyslipidemia have a deep impact on sphingomyelins profile. The modifications were noted at the level of ceramides and are likely to propagate through downstream pathways.

The Relationships between Abnormal Serum Lipid Levels, Depression, and Suicidal Ideation According to Sex

(1) Background: Serum lipid levels affect not only nutritional status but also emotional state. The purpose of this study was to examine the effects of various socio-demographic characteristics, abnormal cholesterol levels, and BMI indicators on depressive symptoms and suicidal ideation in the Korean population. (2) Methods: A total of 23,692 people were surveyed using data from the Korea National Health and Nutrition Examination Survey (KNHNES) 2014, 2016, and 2018. Data from 11,653 patients were analyzed. Age, sex, chronic disease, smoking, alcohol consumption, total cholesterol (HDL, triglycerides), BMI, depression, and suicidal ideation were measured. (3) Results: According to sex, low HDL, high triglycerides, and suicidal ideation were significant, along with low education level, smoking, binge drinking, and high BMI. High triglyceride level was shown to significantly increase the risk of depression in males (OR = 1.535, 95% CI = 1.098–2.147). Factors affecting suicidal ideation in males were age, binge drinking, and depression, while blood lipid factors were not significant. (4) Conclusions: Of the types of serum lipid factors affecting depression and suicidal ideation, high triglycerides were found to be a risk factor for depression in men. Serum lipids can be used as biomarkers to reflect depressive symptoms in men depending on cholesterol level.

Multifaced Roles of HDL in Sepsis and SARS-CoV-2 Infection: Renal Implications

High-density lipoproteins (HDLs) are a class of blood particles, principally involved in mediating reverse cholesterol transport from peripheral tissue to liver. Omics approaches have identified crucial mediators in the HDL proteomic and lipidomic profile, which are involved in distinct pleiotropic functions. Besides their role as cholesterol transporter, HDLs display anti-inflammatory, anti-apoptotic, anti-thrombotic, and anti-infection properties. Experimental and clinical studies have unveiled significant changes in both HDL serum amount and composition that lead to dysregulated host immune response and endothelial dysfunction in the course of sepsis. Most SARS-Coronavirus-2-infected patients admitted to the intensive care unit showed common features of sepsis disease, such as the overwhelmed systemic inflammatory response and the alterations in serum lipid profile. Despite relevant advances, episodes of mild to moderate acute kidney injury (AKI), occurring during systemic inflammatory diseases, are associated with long-term complications, and high risk of mortality. The multi-faceted relationship of kidney dysfunction with dyslipidemia and inflammation encourages to deepen the clarification of the mechanisms connecting these elements. This review analyzes the multifaced roles of HDL in inflammatory diseases, the renal involvement in lipid metabolism, and the novel potential HDL-based therapies.

Current Understanding of the Immunomodulatory Activities of High-Density Lipoproteins

Lipoproteins interact with immune cells, macrophages and endothelial cells - key players of the innate and adaptive immune system. High-density lipoprotein (HDL) particles seem to have evolved as part of the innate immune system since certain HDL subspecies contain combinations of apolipoproteins with immune regulatory functions. HDL is enriched in anti-inflammatory lipids, such as sphingosine-1-phosphate and certain saturated lysophospholipids. HDL reduces inflammation and protects against infection by modulating immune cell function, vasodilation and endothelial barrier function. HDL suppresses immune cell activation at least in part by modulating the cholesterol content in cholesterol/sphingolipid-rich membrane domains (lipid rafts), which play a critical role in the compartmentalization of signaling pathways. Acute infections, inflammation or autoimmune diseases lower HDL cholesterol levels and significantly alter HDL metabolism, composition and function. Such alterations could have a major impact on disease progression and may affect the risk for infections and cardiovascular disease. This review article aims to provide a comprehensive overview of the immune cell modulatory activities of HDL. We focus on newly discovered activities of HDL-associated apolipoproteins, enzymes, lipids, and HDL mimetic peptides.

The Role of Phospholipid Transfer Protein in the Development of Atherosclerosis

PURPOSE OF REVIEW

Phospholipid transfer protein (PLTP), a member of lipid transfer protein family, is an important protein involved in lipid metabolism in the circulation. This article reviews recent PLTP research progresses, involving lipoprotein metabolism and atherogenesis.

RECENT FINDINGS

PLTP activity influences atherogenic and anti-atherogenic lipoprotein levels. Human serum PLTP activity is a risk factor for human cardiovascular disease and is an independent predictor of all-cause mortality. PLTP deficiency reduces VLDL and LDL levels and attenuates atherosclerosis in mouse models, while PLTP overexpression exerts an opposite effect. Both PLTP deficiency and overexpression result in reduction of HDL which has different size, inflammatory index, and lipid composition. Moreover, although both PLTP deficiency and overexpression reduce cholesterol efflux capacity, but this effect has no impact in macrophage reverse cholesterol transport in mice. Furthermore, PLTP activity is related with metabolic syndrome, thrombosis, and inflammation. PLTP could be target for the treatment of dyslipidemia and atherosclerosis, although some potential off-target effects should be noted.

Decreased serum level of sphingosine-1-phosphate: a novel predictor of clinical severity in COVID-19

The severity of coronavirus disease 2019 (COVID-19) is a crucial problem in patient treatment and outcome. The aim of this study is to evaluate circulating level of sphingosine-1-phosphate (S1P) along with severity markers, in COVID-19 patients. One hundred eleven COVID-19 patients and forty-seven healthy subjects were included. The severity of COVID-19 was found significantly associated with anemia, lymphocytopenia, and significant increase of neutrophil-to-lymphocyte ratio, ferritin, fibrinogen, aminotransferases, lactate dehydrogenase (LDH), C-reactive protein (CRP), and D-dimer. Serum S1P level was inversely associated with COVID-19 severity, being significantly correlated with CRP, LDH, ferritin, and D-dimer. The decrease in S1P was strongly associated with the number of erythrocytes, the major source of plasma S1P, and both apolipoprotein M and albumin, the major transporters of blood S1P. Not last, S1P was found to be a relevant predictor of admission to an intensive care unit, and patient's outcome. Circulating S1P emerged as negative biomarker of severity/mortality of COVID-19 patients. Restoring abnormal S1P levels to a normal range may have the potential to be a therapeutic target in patients with COVID-19.

Effects of low-dose unfractionated heparin on early brain injury after subarachnoid hemorrhage in mice

BACKGROUND

Sphingosine kinase (SphK) 1 has been reported as an important signaling node in anti-apoptotic signaling. Heparin is a pleiotropic drug that antagonizes many pathophysiological mechanisms. In this study, we evaluated if heparin prevents early brain injury (EBI) after subarachnoid hemorrhage (SAH) by anti-apoptotic mechanisms including SphK1.

METHODS

SAH was induced by endovascular perforation in mice, which were randomly assigned to sham-operated (n = 23), SAH + vehicle (n = 36), SAH + 10U heparin pretreatment (n = 13), SAH + 30U heparin pretreatment (n = 15), SAH + 10U heparin posttreatment (n = 31), and SAH + 30U heparin posttreatment (n = 23). At 24 hours post-SAH, neurological scores, brain water content and Evans blue extravasation were evaluated. Also, the expression of SphK, phosphorylated Akt, and cleaved caspase-3 was determined by Western blotting, and cell death was examined by terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphate-biotin nick end-labeling staining.

RESULTS

Low-dose heparin posttreatment improved neurobehavioral function, brain edema, blood-brain barrier disruption and cell death in the cortex, associated with an increase in SphK1 and phosphorylated Akt, and a decrease in cleaved caspase-3. High-dose heparin had a tendency for increased SAH severity, which obscured the neuroprotective effects by heparin.

CONCLUSIONS

Low-dose heparin posttreatment may decrease the development of post-SAH EBI through anti-apoptotic mechanisms including sphingosine-related pathway activation.

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.

Impact of Phospholipid Transfer Protein in Lipid Metabolism and Cardiovascular Diseases

PLTP plays an important role in lipoprotein metabolism and cardiovascular disease development in humans; however, the mechanisms are still not completely understood. In mouse models, PLTP deficiency reduces cardiovascular disease, while its overexpression induces it. Therefore, we used mouse models to investigate the involved mechanisms. In this chapter, the recent main progresses in the field of PLTP research are summarized, and our focus is on the relationship between PLTP and lipoprotein metabolism, as well as PLTP and cardiovascular diseases.

Novel Insights into the Role of HDL-Associated Sphingosine-1-Phosphate in Cardiometabolic Diseases

Sphingolipids are key signaling molecules involved in the regulation of cell physiology. These species are found in tissues and in circulation. Although they only constitute a small fraction in lipid composition of circulating lipoproteins, their concentration in plasma and distribution among plasma lipoproteins appears distorted under adverse cardiometabolic conditions such as diabetes mellitus. Sphingosine-1-phosphate (S1P), one of their main representatives, is involved in regulating cardiomyocyte homeostasis in different models of experimental cardiomyopathy. Cardiomyopathy is a common complication of diabetes mellitus and represents a main risk factor for heart failure. Notably, plasma concentration of S1P, particularly high-density lipoprotein (HDL)-bound S1P, may be decreased in patients with diabetes mellitus, and hence, inversely related to cardiac alterations. Despite this, little attention has been given to the circulating levels of either total S1P or HDL-bound S1P as potential biomarkers of diabetic cardiomyopathy. Thus, this review will focus on the potential role of HDL-bound S1P as a circulating biomarker in the diagnosis of main cardiometabolic complications frequently associated with systemic metabolic syndromes with impaired insulin signaling. Given the bioactive nature of these molecules, we also evaluated its potential of HDL-bound S1P-raising strategies for the treatment of cardiometabolic disease.

[The role of sphingolipids in cardiovascular pathologies]

Cardiovascular diseases (CVD) remain the leading cause of death in industrialized countries. One of the most significant risk factors for atherosclerosis is hypercholesterolemia. Its diagnostics is based on routine lipid profile analysis, including the determination of total cholesterol, low and high density lipoprotein cholesterol, and triglycerides. However in recent years, much attention has been paid to the crosstalk between the metabolic pathways of the cholesterol and sphingolipids biosynthesis. Sphingolipids are a group of lipids, containing a molecule of aliphatic alcohol sphingosine. These include sphingomyelins, cerebrosides, gangliosides and ceramides, sphingosines, and sphingosine-1-phosphate (S-1-P). It has been found that catabolism of sphingolipids is associated with catabolism of cholesterol. However, the exact mechanism of this interaction is still unknown. Particular attention as CVD inducer attracts ceramide (Cer). Lipoprotein aggregates isolated from atherosclerotic pluques are enriched with Cer. The level of Cer and sphingosine increases after ischemia reperfusion of the heart, in the infarction zone and in the blood, and also in hypertension. S-1-P exhibits pronounced cardioprotective properties. Its content sharply decreases with ischemia and myocardial infarction. S-1-P presents predominantly in HDL, and influences their multiple functions. Increased levels of Cer and sphingosine and decreased levels of S-1-P formed in the course of coronary heart disease can be an important factor in the development of atherosclerosis. It is proposed to use determination of sphingolipids in blood plasma as markers for early diagnosis of cardiac ischemia and for hypertension in humans. There are intensive studies aimed at correction of metabolism S-1-P. The most successful drugs are those that use S-1-P receptors as a targets, since all of its actions are receptor-mediated.

Cardioprotective Effects of High-Density Lipoprotein Beyond its Anti-Atherogenic Action

High-density lipoprotein cholesterol (HDL-C) has been identified as a powerful independent negative predictor of cardiovascular disease. The beneficial effect of HDL is largely attributable to its key role in reverse cholesterol transport, whereby excess cholesterol in the peripheral tissues is transported to the liver, reducing the atherosclerotic burden. However, mounting evidence indicates that HDL also has pleiotropic properties, such as anti-inflammatory, anti-oxidative, and vasodilatory properties, which may contribute in reducing the incidence of heart failure. Actually, previous data from clinical and experimental studies have suggested that HDL exerts cardioprotective effects irrespective of the presence/absence of coronary artery disease. This review summarizes the currently available evidence regarding beneficial effects of HDL on the heart beyond its anti-atherogenic property. Understanding the mechanisms of cardiac protection by HDL will provide new insight into the underlying mechanism and therapeutic strategy for heart failure.

Endocytosis of lipoproteins

During their metabolism, all lipoproteins undergo endocytosis, either to be degraded intracellularly, for example in hepatocytes or macrophages, or to be re-secreted, for example in the course of transcytosis by endothelial cells. Moreover, there are several examples of internalized lipoproteins sequestered intracellularly, possibly to exert intracellular functions, for example the cytolysis of trypanosoma. Endocytosis and the subsequent intracellular itinerary of lipoproteins hence are key areas for understanding the regulation of plasma lipid levels as well as the biological functions of lipoproteins. Indeed, the identification of the low-density lipoprotein (LDL)-receptor and the unraveling of its transcriptional regulation led to the elucidation of familial hypercholesterolemia as well as to the development of statins, the most successful therapeutics for lowering of cholesterol levels and risk of atherosclerotic cardiovascular diseases. Novel limiting factors of intracellular trafficking of LDL and the LDL receptor continue to be discovered and to provide drug targets such as PCSK9. Surprisingly, the receptors mediating endocytosis of high-density lipoproteins or lipoprotein(a) are still a matter of controversy or even new discovery. Finally, the receptors and mechanisms, which mediate the uptake of lipoproteins into non-degrading intracellular itineraries for re-secretion (transcytosis, retroendocytosis), storage, or execution of intracellular functions, are largely unknown.

Phospholipid transfer protein: its impact on lipoprotein homeostasis and atherosclerosis

Phospholipid transfer protein (PLTP) is one of the major modulators of lipoprotein metabolism and atherosclerosis development in humans; however, we still do not quite understand the mechanisms. In mouse models, PLTP overexpression induces atherosclerosis, while its deficiency reduces it. Thus, mouse models were used to explore the mechanisms. In this review, I summarize the major progress made in the PLTP research field and emphasize its impact on lipoprotein metabolism and atherosclerosis, as well as its regulation.

Determination of Sphingosine-1-Phosphate in Human Plasma Using Liquid Chromatography Coupled with Q-Tof Mass Spectrometry

Evidence suggests that high-density lipoprotein (HDL) components distinct from cholesterol, such as sphingosine-1-phosphate (S1P), may account for the anti-atherothrombotic effects attributed to this lipoprotein. The current method for the determination of plasma levels of S1P as well as levels associated with HDL particles is still cumbersome an assay method to be worldwide practical. Recently, a simplified protocol based on liquid chromatography-tandem mass spectrometry (LC-MS/MS) for the sensitive and specific quantification of plasma levels of S1P with good accuracy has been reported. This work utilized a triple quadrupole (QqQ)-based LC-MS/MS system. Here we adapt that method for the determination of plasma levels of S1P using a quadrupole time of flight (Q-Tof) based LC-MS system. Calibration curves were linear in the range of 0.05 to 2 µM. The lower limit of quantification (LOQ) was 0.05 µM. The concentration of S1P in human plasma was determined to be 1 ± 0.09 µM (n = 6). The average accuracy over the stated range of the method was found to be 100 ± 5.9% with precision at the LOQ better than 10% when predicting the calibration standards. The concentration of plasma S1P in the prepared samples was stable for 24 h at room temperature. We have demonstrated the quantification of plasma S1P using Q-Tof based LC-MS with very good sensitivity, accuracy, and precision that can used for future studies in this field.

The role of sphingosine-1-phosphate in the tumor microenvironment and its clinical implications

Elucidating the interaction between cancer and non-cancer cells, such as blood vessels, immune cells, and other stromal cells, in the tumor microenvironment is imperative in understanding the mechanisms underlying cancer progression and metastasis, which is expected to lead to the development of new therapeutics. Sphingosine-1-phosphate is a bioactive lipid mediator that promotes cell survival, proliferation, migration, angiogenesis/lymphangiogenesis, and immune responsiveness, which are all factors involved in cancer progression. Sphingosine-1-phosphate is generated inside cancer cells by sphingosine kinases and then exported into the tumor microenvironment. Although sphingosine-1-phosphate is anticipated to play an important role in the tumor microenvironment and cancer progression, determining sphingosine-1-phosphate levels in the tumor microenvironment has been difficult due to a lack of established methods. We have recently developed a method to measure sphingosine-1-phosphate levels in the interstitial fluid that bathes cancer cells in the tumor microenvironment, and reported that high levels of sphingosine-1-phosphate exist in the tumor interstitial fluid. Importantly, sphingosine-1-phosphate can be secreted from cancer cells and non-cancer components such as immune cells and vascular/lymphatic endothelial cells in the tumor microenvironment. Furthermore, sphingosine-1-phosphate affects both cancer and non-cancer cells in the tumor microenvironment promoting cancer progression. Here, we review the roles of sphingosine-1-phosphate in the interaction between cancer and non-cancer cells in tumor microenvironment, and discuss future possibilities for targeted therapies against sphingosine-1-phosphate signaling for cancer patients.

Cellular function and signaling pathways of vascular smooth muscle cells modulated by sphingosine 1-phosphate

Sphingosine 1-phosphate (S1P) plays important roles in cardiovascular pathophysiology. S1P₁ and/or S1P₃, rather than S1P₂ receptors, seem to be predominantly expressed in vascular endothelial cells, while S1P₂ and/or S1P₃, rather than S1P₁ receptors, seem to be predominantly expressed in vascular smooth muscle cells (VSMCs). S1P has multiple actions, such as proliferation, inhibition or stimulation of migration, and vasoconstriction or release of vasoactive mediators. S1P induces an increase of the intracellular Ca²⁺ concentration in many cell types, including VSMCs. Activation of S1P₃ seems to play an important role in Ca²⁺ mobilization. S1P induces cyclooxygenase-2 expression in VSMCs via both S1P₂ and S1P₃ receptors. S1P₂ receptor activation in VSMCs inhibits inducible nitric oxide synthase (iNOS) expression. At the local site of vascular injury, vasoactive mediators such as prostaglandins and NO produced by VSMCs are considered primarily as a defensive and compensatory mechanism for the lack of endothelial function to prevent further pathology. Therefore, selective S1P₂ receptor antagonists may have the potential to be therapeutic agents, in view of their antagonism of iNOS inhibition by S1P. Further progress in studies of the precise mechanisms of S1P may provide useful knowledge for the development of new S1P-related drugs for the treatment of cardiovascular diseases.

Does high-density lipoprotein protect vascular function in healthy pregnancy?

The maternal adaptation to pregnancy includes hyperlipidaemia, oxidative stress and chronic inflammation. In non-pregnant individuals, these processes are usually associated with poor vascular function. However, maternal vascular function is enhanced in pregnancy. It is not understood how this is achieved in the face of the adverse metabolic and inflammatory environment. Research into cardiovascular disease demonstrates that plasma HDL (high-density lipoprotein), by merit of its functionality rather than its plasma concentration, exerts protective effects on the vascular endothelium. HDL has vasodilatory, antioxidant, anti-thrombotic and anti-inflammatory effects, and can protect against endothelial cell damage. In pregnancy, the plasma HDL concentration starts to rise at 10 weeks of gestation, peaking at 20 weeks. The initial rise in plasma HDL occurs around the time of the establishment of the feto-placental circulation, a time when the trophoblast plugs in the maternal spiral arteries are released, generating oxidative stress. Thus there is the intriguing possibility that new HDL of improved function is synthesized around the time of the establishment of the feto-placental circulation. In obese pregnancy and, to a greater extent, in pre-eclampsia, plasma HDL levels are significantly decreased and maternal vascular function is reduced. Wire myography studies have shown an association between the plasma content of apolipoprotein AI, the major protein constituent of HDL, and blood vessel relaxation. These observations lead us to hypothesize that HDL concentration, and function, increases in pregnancy in order to protect the maternal vascular endothelium and that in pre-eclampsia this fails to occur.

Therapeutic applications of reconstituted HDL: When structure meets function

Reconstituted forms of HDL (rHDL) are under development for infusion as a therapeutic approach to attenuate atherosclerotic vascular disease and to reduce cardiovascular risk following acute coronary syndrome and ischemic stroke. Currently available rHDL formulations developed for clinical use contain apolipoprotein A-I (apoA-I) and one of the major lipid components of HDL, either phosphatidylcholine or sphingomyelin. Recent data have established that quantitatively minor molecular constituents of HDL particles can strongly influence their anti-atherogenic functionality. Novel rHDL formulations displaying enhanced biological activities, including cellular cholesterol efflux, may therefore offer promising prospects for the development of HDL-based, anti-atherosclerotic therapies. Indeed, recent structural and functional data identify phosphatidylserine as a bioactive component of HDL; the content of phosphatidylserine in HDL particles displays positive correlations with all metrics of their functionality. This review summarizes current knowledge of structure-function relationships in rHDL formulations, with a focus on phosphatidylserine and other negatively-charged phospholipids. Mechanisms potentially underlying the atheroprotective role of these lipids are discussed and their potential for the development of HDL-based therapies highlighted.

HDL-S1P: cardiovascular functions, disease-associated alterations, and therapeutic applications

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid contained in High-density lipoproteins (HDL) and has drawn considerable attention in the lipoprotein field as numerous studies have demonstrated its contribution to several functions inherent to HDL. Some of them are partly and some entirely due to the S1P contained in HDL (HDL-S1P). Despite the presence of over 1000 different lipids in HDL, S1P stands out as it possesses its own cell surface receptors through which it exercises key physiological functions. Most of the S1P in human plasma is associated with HDL, and the amount of HDL-S1P influences the quality and quantity of HDL-dependent functions. The main binding partner of S1P in HDL is apolipoprotein M but others may also exist particularly under conditions of acute S1P elevations. HDL not only exercise functions through their S1P content but have also an impact on genuine S1P signaling by influencing S1P bioactivity and receptor presentation. HDL-S1P content is altered in human diseases such as atherosclerosis, coronary artery disease, myocardial infarction, renal insufficiency and diabetes mellitus. Low HDL-S1P has also been linked to impaired HDL functions associated with these disorders. Although the pathophysiological and molecular reasons for such disease-associated shifts in HDL-S1P are little understood, there have been successful approaches to circumvent their adverse implications by pharmacologically increasing HDL-S1P as means to improve HDL function. This mini-review will cover the current understanding of the contribution of HDL-S1P to physiological HDL function, its alteration in disease and ways for its restoration to correct HDL dysfunction.

Sources, metabolism, and regulation of circulating sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that acts either as an intracellular messenger or as a ligand for its membrane receptors. S1P is a normal constituent of blood, where it is found both in plasma and blood cells. Compared with other cell types, sphingolipid metabolism in erythrocytes and platelets has unique features that allow the erythrocytes and platelets to accumulate S1P. In plasma, S1P is bound mainly to HDLs and albumin. Of note, metabolism and biological activity of S1P is to a large extent affected by the type of its carrier. Plasma S1P is characterized by a short half-life, indicating rapid clearance by degradative enzymes and the presence of high-capacity sources involved in maintaining its high concentration. These sources include blood cells, vascular endothelium, and hepatocytes. However, the extent to which each of these contributes to the plasma pool of S1P is a matter of debate. Circulating S1P plays a significant physiological role. It was found to be the key regulator of lymphocyte trafficking, endothelial barrier function, and vascular tone. The purpose of this review is to summarize the present state of knowledge on the metabolism, transport, and origin of plasma S1P, and to discuss the mechanisms regulating its homeostasis in blood.

Human T-Cell Development and Thymic Egress: An Infectious Disease Perspective

Emigration of mature naïve CD4 SP T cells from the human thymus to the periphery is not fully understood, although elucidation of the mechanisms that govern egress of T cells is crucial to understanding both basic immunology and the immune response in diseases such as HIV infection. Recent work has brought to light the requirement for sphingosine-1-phosphate (S1P) and its receptors in a variety of fields including mature naïve T-cell egress from the thymus of mice. We are examining the expression and function of this novel requisite T-cell egress receptor within the human thymus, characterizing changes observed in the expression and function of this receptor in infectious diseases. To perform this work, we use a variety of humanized murine models reviewed in this article. Future work in the field of T-cell egress, especially as it pertains to S1P receptors, should advance the fields of basic T-cell immunology and immunopathology and open new avenues for exploration into novel therapeutics.

High density lipoprotein/sphingosine-1-phosphate-induced cardioprotection: Role of STAT3 as part of the SAFE pathway

High density lipoprotein (HDL) cholesterol has beneficial effects beyond its atheroprotective function in reverse cholesterol transport, including cardioprotection against ischemia reperfusion (IR) injuries. Two major constituents of HDL, namely the structural protein apolipoprotein AI (apoAI) and the sphingolipid sphingosine-1-phosphate (S1P) appear to contribute to this cardioprotective effect via the activation of intrinsic prosurvival signaling pathways that still remain to be clarified.

 

Recently, a powerful prosurvival signaling pathway, termed the survivor activating factor enhancement (SAFE) pathway, which involves the activation of signal transducer and activator of transcription 3 (STAT3) and tumor necrosis factor α (TNF), has been shown to protect against ischemia-reperfusion injuries.

The present review summarizes the evidence for the roles of HDL and S1P in cardioprotection and discusses the signaling pathways that have been implicated. It thus provides support for our contention that S1P should be considered in potential formulations of reconstituted HDL (reHDL) that may be tested for cardioprotection against coronary artery disease via the activation of the SAFE pathway.

Atheroprotective role of high-density lipoprotein (HDL)-associated sphingosine-1-phosphate (S1P)

Numerous epidemiological studies documented an inverse relationship between plasma high-density lipoprotein (HDL) cholesterol levels and the extent of atherosclerotic disease. However, clinical interventions targeting HDL cholesterol failed to show clinical benefits with respect to cardiovascular risk reduction, suggesting that HDL components distinct from cholesterol may account for anti-atherogenic effects attributed to this lipoprotein. Sphingosine-1-phosphate (S1P)-a lysosphingolipid exerting its biological activity via binding to specific G protein-coupled receptors and regulating a wide array of biological responses in a variety of different organs and tissues including the cardiovascular system-has been identified as an integral constituent of HDL particles. In the present review, we discuss current evidence from epidemiological studies, experimental approaches in vitro, and animal models of atherosclerosis, suggesting that S1P contributes to atheroprotective effects exerted by HDL particles.

Higher level of plasma bioactive molecule sphingosine 1-phosphate in women is associated with estrogen

Both sphingosine 1-phosphate (S1P) and estrogen have been documented to play endothelial protective roles. However, it remains unclear whether estrogen could regulate the anabolism of the bioactive molecule S1P and the underlying mechanisms. In this study, 108 healthy participants were separated into three age groups, and their plasma S1P levels were analyzed by liquid chromatography tandem mass spectrometry. Results showed that the plasma S1P levels were significantly higher in women than those in men within the age of 16-55years old and higher in pre-menopausal than post-menopausal women. The experiment in C57 BL/6 mice confirmed the gender difference of plasma S1P level. In vitro study demonstrated that after the stimulation of 17β-estradiol (E2), S1P levels both in EA.hy926 cells and the culture media were increased about 9 and 3 times, respectively; the mRNA expression, the protein level and the activity of sphingosine kinase (SphK) 1, not SphK2, were markedly increased; the mRNA and protein expression of ATP-binding cassette transporter (ABC) C1, G2 and S1P transporter spinster homolog 2 (Spns2) were significantly elevated; furthermore, the mRNA and protein expressions of S1P receptors (S1PRs) 1-2 were increased in a time-dependent manner. This study suggests that E2 markedly improves S1P synthesis by activating SphK1 and induces S1P export via activating ABCC1, G2 and Spns2 from endothelium system, which may consequently lead to the gender difference of plasma S1P in adult human and mouse. The results of this study suggest that E2 may exert its vasculoprotective function by activation of the SphK1-S1P-S1PR signaling axis.

Sphingosine 1-phosphate increases an intracellular Ca(2+) concentration via S1P3 receptor in cultured vascular smooth muscle cells

OBJECTIVE

We investigated the effect of sphingosine 1-phosphate (S1P) on intracellular Ca(2+) dynamics in rat vascular smooth muscle cells (VSMCs).

METHODS

Intracellular Ca(2+) concentration ([Ca(2+) ]i) was determined using a fluorescence dye fura-2/AM. Small interfering RNAs (siRNA) were transfected into VSMCs to deplete the expression of S1P2 and S1P3 receptors.

KEY FINDINGS

S1P induced a rapid and transient elevation in [Ca(2+) ]i, which was maximal 1 min after the stimulation, followed by a sustained increase. When extracellular Ca(2+) was removed, a decrease in resting level and a small and transient increase in [Ca(2+) ]i by S1P stimulation were observed. siRNA targeted for the S1P3 receptor almost completely inhibited the S1P-induced increase in [Ca(2+) ]i. The rapid and transient increase in [Ca(2+) ]i was significantly inhibited by diltiazem at a high concentration. Pertussis toxin and a phospholipase C (PLC) inhibitor inhibited the S1P-induced increase in [Ca(2+) ]i regardless of the presence of extracellular Ca(2+) . Furthermore, S1P activated store-operated and receptor-operated Ca(2+) entry.

CONCLUSIONS

These results suggest that S1P increases [Ca(2+) ]i via the S1P3 receptor by inducing an influx of extracellular Ca(2+) partially through the voltage-dependent Ca(2+) channels, as well as by mobilizing Ca(2+) from its intracellular stores. S1P3 receptor-coupled Gi/o protein and PLC activation mediate the mechanisms.

Apolipoprotein M: bridging HDL and endothelial function

PURPOSE OF REVIEW

The review will address the potential roles of apolipoprotein M (apoM) as a carrier protein and modulator of sphingosine-1-phosphate (S1P) bioactivity.

RECENT FINDINGS

Recombinant apoM can bind small lipids such as retinoic acid, oxidized phospholipids, and S1P. Thus, the effects of apoM may be pleiotrophic. The S1P binding ability of apoM has biological impact. ApoM-bound S1P can activate S1P1 receptors on endothelial cells and deficiency of apoM abolishes the presence of S1P in HDL. In mice, the lack of apoM causes dysfunctional endothelial barrier function in the lungs. In humans, sepsis that is characterized by impaired endothelial function is associated with low plasma apoM.

SUMMARY

Plasma apoM is mainly bound to HDL. The roles of apoM in atherosclerosis and lipoprotein metabolism have been given much attention. New in the field is the discovery of apoM as a chaperone for S1P. S1P is a bioactive lipid with effects on angiogenesis, lymphocyte trafficking, endothelial cell migration, and inflammation. A drug targeting the S1P-system (fingolimod) is now used for treatment of multiple sclerosis. It improves the blood-brain barrier and inhibits migration of lymphocytes into the brain. Further exploration of the apoM/S1P axis may uncover its potential as a biomarker and target for new treatments.

Unraveling the complexities of the HDL lipidome

Plasma high density lipoproteins (HDL) are small, dense, protein-rich particles compared with other lipoprotein classes; roughly half of total HDL mass is accounted for by lipid components. Phospholipids predominate in the HDL lipidome, accounting for 40-60% of total lipid, with lesser proportions of cholesteryl esters (30-40%), triglycerides (5-12%), and free cholesterol (5-10%). Lipidomic approaches have provided initial insights into the HDL lipidome with identification of over 200 individual molecular lipids species in normolipidemic HDL. Plasma HDL particles, however, reveal high levels of structural, compositional, and functional heterogeneity. Establishing direct relationships between HDL structure, composition, and atheroprotective functions bears the potential to identify clinically relevant HDL subpopulations. Furthermore, development of HDL-based therapies designed to target beneficial subspecies within the circulating HDL pool can be facilitated using this approach. HDL lipidomics can equally contribute to the identification of biomarkers of both normal and deficient HDL functionality, which may prove useful as biomarkers of cardiovascular risk. However, numerous technical issues remain to be addressed in order to make such developments possible. With all technical questions resolved, quantitative analysis of the molecular components of the HDL lipidome will contribute to expand our knowledge of cardiovascular and metabolic diseases.

HDL protects against ischemia reperfusion injury by preserving mitochondrial integrity

OBJECTIVE

High density lipoproteins (HDL) protect against ischemia reperfusion injury (IRI). However the precise mechanisms are not clearly understood. The novel intrinsic prosurvival signaling pathway named survivor activating factor enhancement (SAFE) path involves the activation of tumor necrosis factor (TNF) alpha and signal transducer and activator of transcription 3 (STAT3). SAFE plays a crucial role in cardioprotection against IRI. We propose that HDL protect against IRI via activation of the SAFE pathway and modulation of the mitochondrial permeability transition pore (mPTP) opening.

METHODS AND RESULTS

Isolated mouse hearts were subjected to global ischemia (35 min) followed by reperfusion (45 min). HDL were given during the first 7 min of reperfusion. In control hearts, the post-reperfusion infarct size was 41.3 ± 2.3%. Addition of HDL during reperfusion reduced the infarct size in a dose-dependent manner (HDL 200 μg protein/ml: 25.5 ± 1.6%, p < 0.001 vs. control). This protective effect was absent in TNF deficient mice (TNF-KO) or cardiomyocyte-STAT3 deficient mice (STAT3-KO). Similarly, HDL, given as a preconditioning stimulus, improved cell survival and inhibited mPTP opening in isolated cardiomyocytes subjected to simulated ischemia. These protective responses were inhibited in cardiomyocytes from TNF-KO and STAT3-KO mice.

CONCLUSION

Our data demonstrate that HDL protect against IRI by inhibition of mPTP opening, an effect mediated via activation of the SAFE pathway.

Class II phosphoinositide 3-kinases contribute to endothelial cells morphogenesis

The question of whether the distinct isoforms of the family of enzymes phosphoinositide 3-kinases (PI3Ks) play redundant roles within a cell or whether they control distinct cellular processes or distinct steps within the same cellular process has gained considerable importance in the recent years due to the development of inhibitors able to selectively target individual isoforms. It is important to understand whether inhibition of one PI3K can result in compensatory effect from other isoform(s) and therefore whether strategies aimed at simultaneously blocking more than one PI3K may be needed. In this study we investigated the relative contribution of distinct PI3K isoforms to endothelial cells (EC) functions specifically regulated by the sphingolipid sphingosine-1-phosphate (S1P) and by high density lipoproteins (HDL), the major carrier of S1P in human plasma. Here we show that a co-ordinated action of different PI3Ks is required to tightly regulate remodelling of EC on Matrigel, a process dependent on cell proliferation, apoptosis and migration. The contribution of each isoform to this process appears to be distinct, with the class II enzyme PI3K-C2β and the class IB isoform p110γ mainly regulating the S1P- and HDL-dependent EC migration and PI3K-C2α primarily controlling EC survival. Data further indicate that PI3K-C2β and p110γ control distinct steps involved in cell migration supporting the hypothesis that different PI3Ks regulate distinct cellular processes.

Cardiovascular effects of sphingosine-1-phosphate (S1P)

Sphingosine-1-phosphate (S1P) regulates important functions in cardiac and vascular homeostasis. It has been implied to play causal roles in the pathogenesis of many cardiovascular disorders such as coronary artery disease, atherosclerosis, myocardial infarction, and heart failure. The majority of S1P in plasma is associated with high-density lipoproteins (HDL), and their S1P content has been shown to be responsible, at least in part, for several of the beneficial effects of HDL on cardiovascular risk. The attractiveness of S1P-based drugs for potential cardiovascular applications is increasing in the wake of the clinical approval of FTY720, but answers to important questions on the effects of S1P in cardiovascular biology and medicine must still be found. This chapter focuses on the current understanding of the role of S1P and its receptors in cardiovascular physiology, pathology, and disease.

Sphingolipid metabolism and atherosclerosis

Atherosclerosis is the major cause of mortality in the developed countries. Although presently known risk factors have some predictive value for the disease, a major part of the variability in this process remains unexplained. It is extremely important to find new approaches for better understanding of the disease and for treating it. Exploration of the sphingolipid metabolism is one of these approaches. Sphingolipids are a large class of lipids with structural and signaling functions. Recent researches indicated that these lipids play important roles in the development of atherosclerosis. In this chapter, we summarized the major findings in the field.

Sphingosine 1-phosphate distribution in human plasma: associations with lipid profiles

The physiological significance of sphingosine 1-phosphate (S1P) transport in blood has been debated. We have recently reported a comprehensive sphingolipid profile in human plasma and lipoprotein particles (VLDL, LDL, and HDL) using HPLC-MS/MS (Hammad et al., 2010). We now determined the relative concentrations of sphingolipids including S1P in the plasma subfraction containing lipoproteins compared to those in the remaining plasma proteins. Sphingomyelin and ceramide were predominantly recovered in the lipoprotein-containing fraction. Total plasma S1P concentration was positively correlated with S1P concentration in the protein-containing fraction, but not with S1P concentration in the lipoprotein-containing fraction. The percentage of S1P transported in plasma lipoproteins was positively correlated with HDL cholesterol (HDL-C) concentration; however, S1P transport in lipoproteins was not limited by the concentration of HDL-C in the individual subject. Thus, different plasma pools of S1P may have different contributions to S1P signaling in health and disease.

Contrary effects of sphingosine-1-phosphate on expression of α-smooth muscle actin in transforming growth factor β1-stimulated lung fibroblasts

Transforming growth factor-β1 (TGFβ1) plays a pivotal role in fibrosis in various organs including the lung. Following pulmonary injury, TGFβ1 stimulates conversion of fibroblasts to myofibroblasts that are mainly characterized by up-regulation of α-smooth muscle actin (αSMA) expression, and the resulting excess production of extracellular matrix proteins causes fibrosis with loss of alveolar function. The present study was undertaken to define the role of the sphingosine-1-phosphate (S1P) pathway in TGFβ1-induced expression of αSMA in human fetal lung fibroblasts, HFL1 cells. Analysis of mRNA revealed the existence of S1P(1), S1P(2), and S1P(3) receptor mRNAs. Treatment with TGFβ1 increased sphingosine kinase (SphK) activity and S1P(3) receptor mRNA at 24h after stimulation, and pharmacological data showed the involvement of sphingomyelinase, SphK, and S1P(3) receptor in the TGFβ1-induced up-regulation of αSMA with and without serum. Treatment with pertussis toxin and S1P(1) receptor antagonist W146 enhanced αSMA expression by TGFβ1/serum, and S1P decreased and increased αSMA levels with and without serum, respectively. TGFβ1 increased cyclooxygenase-2 expression in a manner dependent on serum and the sphingomyelinase/SphK pathway, and the response was decreased by pertussis toxin. Prostaglandin E(2), formed by TGFβ1/serum stimulation, decreased the TGFβ1-induced expression of αSMA via EP prostanoid receptor. These data suggest that S1P formed by TGFβ1 stimulation has diverse effects on the expression of αSMA, inhibition via the S1P(1) receptor-mediated and serum-dependent expression of cyclooxygenase-2 and the resulting formation of prostaglandin E(2), and stimulation via the S1P(3) receptor in a serum-independent manner.

Shaping the landscape: metabolic regulation of S1P gradients

Sphingosine-1-phosphate (S1P) is a lipid that functions as a metabolic intermediate and a cellular signaling molecule. These roles are integrated when compartments with differing extracellular S1P concentrations are formed that serve to regulate functions within the immune and vascular systems, as well as during pathologic conditions. Gradients of S1P concentration are achieved by the organization of cells with specialized expression of S1P metabolic pathways within tissues. S1P concentration gradients underpin the ability of S1P signaling to regulate in vivo physiology. This review will discuss the mechanisms that are necessary for the formation and maintenance of S1P gradients, with the aim of understanding how a simple lipid controls complex physiology. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Fenofibrate increases high-density lipoprotein and sphingosine 1 phosphate concentrations limiting abdominal aortic aneurysm progression in a mouse model

There are currently no acceptable treatments to limit progression of abdominal aortic aneurysm (AAA). Increased serum concentrations of high-density lipoprotein (HDL) are associated with reduced risk of developing an AAA. The present study aimed to assess the effects of fenofibrate on aortic dilatation in a mouse model of AAA. Male low-density lipoprotein receptor-deficient (Ldlr(-/-)) mice were maintained on a high-fat diet for 3 weeks followed by 6 weeks of oral administration of vehicle or fenofibrate. From 14 to 18 weeks of age, all mice were infused with angiotensin II (AngII). At 18 weeks of age, blood and aortas were collected for assessment of serum lipoproteins, aortic pathology, aortic Akt1 and endothelial nitric oxide synthase (eNOS) activities, immune cell infiltration, eNOS and inducible NOS (iNOS) expression, sphingosine 1 phosphate (S1P) receptor status, and apoptosis. Mice receiving fenofibrate had reduced suprarenal aortic diameter, reduced aortic arch Sudan IV staining, higher serum HDL levels, increased serum S1P concentrations, and increased aortic Akt1 and eNOS activities compared with control mice. Macrophages, T lymphocytes, and apoptotic cells were less evident and eNOS, iNOS, and S1P receptors 1 and 3 were up-regulated in aortas from mice receiving fenofibrate. The present findings suggest that fenofibrate antagonizes AngII-induced AAA and atherosclerosis by up-regulating serum HDL and S1P levels, with associated activation of NO-producing enzymes and reduction of aortic inflammation.

Effect of sphingosine 1-phosphate (S1P) receptor agonists FTY720 and CYM5442 on atherosclerosis development in LDL receptor deficient (LDL-R⁻/⁻) mice

OBJECTIVES

Sphingosine 1-phosphate (S1P)--a lysosphingolipid present in HDL--exerts atheroprotective effects in vitro, while FTY720, a non-selective S1P mimetic inhibits atherosclerosis in LDL receptor-deficient (LDL-R⁻/⁻) mice under conditions of severe hypercholesterolemia. We here examined the effect of FTY720 and a selective S1P receptor type 1 agonist CYM5442 on atherosclerosis in moderately hypercholesterolemic LDL-R⁻/⁻ mice.

METHODS AND RESULTS

LDL-R⁻/⁻ mice fed Western diet (0.25% cholesterol) were given FTY720 (0.4 mg/kg/day) or CYM5442 (2.0 mg/kg/day) for 18 weeks. FTY720 but not CYM5422 persistently lowered blood lymphocytes, depleted CD4⁺ and CD8⁺ T cells in spleen and lymph nodes, and reduced splenocyte IL-2 secretion. However, both compounds reduced the activity of splenic and peritoneal macrophages as inferred from the down-regulated CD68 and MHC-II expression in CD11b⁺ cells and the reduced IL-6 secretion in response to LPS, respectively. CYM5442 and FTY720 reduced weight gain, white adipose tissue depots and fasting glucose suggesting improvement of metabolic control, but failed to influence atherosclerosis in LDL-R⁻/⁻ mice.

CONCLUSION

Despite down-regulating macrophage function and--in case of FTY720--altering lymphocyte distribution CYM5442 and FTY720 fail to affect atherosclerosis in moderately hypercholesterolemic LDL-R⁻/⁻ mice. We hypothesize that S1P mimetics exert atheroprotective effects only under conditions of increased cholesterol burden exacerbating vascular inflammation.

Low-density lipoprotein induced expression of connective tissue growth factor via transactivation of sphingosine 1-phosphate receptors in mesangial cells

The pro-fibrotic connective tissue growth factor (CTGF) has been linked to the development and progression of diabetic vascular and renal disease. We recently reported that low-density lipoproteins (LDL) induced expression of CTGF in aortic endothelial cells. However, the molecular mechanisms are not fully defined. Here, we have studied the mechanism by which LDL regulates CTGF expression in renal mesangial cells. In these cells, treatment with pertussis toxin abolished LDL-stimulated activation of ERK1/2 and c-Jun N-terminal kinase (JNK), indicating the involvement of heterotrimeric G proteins in LDL signaling. Treatment with LDL promoted activation and translocation of endogenous sphingosine kinase 1 (SK1) from the cytosol to the plasma membrane concomitant with production of sphingosine-1-phosphate (S1P). Pretreating cells with SK inhibitor, dimethylsphinogsine or down-regulation of SK1 and SK2 revealed that LDL-dependent activation of ERK1/2 and JNK is mediated by SK1. Using a green fluorescent protein-tagged S1P₁ receptor as a biological sensor for the generation of physiologically relevant S1P levels, we found that LDL induced S1P receptor activation. Pretreating cells with S1P₁/S1P₃ receptor antagonist VPC23019 significantly inhibited activation of ERK1/2 and JNK by LDL, suggesting that LDL elicits G protein-dependent activation of ERK1/2 and JNK by stimulating SK1-dependent transactivation of S1P receptors. Furthermore, S1P stimulation induced expression of CTGF in a dose-dependent manner that was markedly inhibited by blocking the ERK1/2 and JNK signaling pathways. LDL-induced CTGF expression was pertussis toxin sensitive and inhibited by dimethylsphinogsine down-regulation of SK1 and VPC23019 treatment. Our data suggest that SK1-dependent S1P receptor transactivation is upstream of ERK1/2 and JNK and that all three steps are required for LDL-regulated expression of CTGF in mesangial cells.

Sphingosine kinase inhibition exerts both pro- and anti-atherogenic effects in low-density lipoprotein receptor-deficient (LDL-R(-/-)) mice

Sphingosine 1-phosphate (S1P), a lysosphingolipid associated with high-density lipoprotein (HDL), contributes to the anti-atherogenic potential attributed to this lipoprotein. This study examined whether a reduction of S1P plasma levels affects atherosclerosis in a murine model of disease. LDL-R(-/-)mice on Western diet were given ABC294640, an inhibitor of sphingosine kinase (SphK) for 16 weeks. ABC294640 decreased plasma S1P by approximately 30%. However, ABC294640 failed to affect atherosclerotic lesion formation. Plasma triglycerides were reduced whereas total and HDL-cholesterol remained unchanged in course of ABC294640 treatment. ABC294640 increased plasma interleukin (IL)-12p70 and RANTES concentration as well as IL-12p70, RANTES and interferon (IFN)-γ production by peritoneal cells and this was paralleled by enhanced activity of peritoneal and spleen dendritic cells as evidenced by up-regulation of CD86 and MHC-II on CD11c(+) cells. As a consequence, increased T-cell activation was noted in ABC294640-treated mice as indicated by enhanced CD4(+) splenocyte proliferation, IFN-γ and IL-2 production, and CD69 expression. Concomitantly, however, ABC294640 treatment redistributed CD4(+) and CD8(+) cells from blood to lymphatic organs and reduced T-cell number within atherosclerotic lesions. In addition, plasma sVCAM-1, sICAM-1, and MCP-1 levels as well as in vivo leukocyte adhesion and CCL19-induced T-cell penetration into peritoneum were lower in ABC294640-treated animals. In vitro experiments demonstrated reduced VCAM-1 and ICAM-1 expression and lymphocyte adhesion to endothelial cells exposed to ABC294640. In conclusion, treatment with SphK inhibitor leads to both pro- and anti-atherogenic effects in LDL-R(-/-) mice. As a consequence, SphK inhibition fails to affect atherosclerosis despite significant S1P reduction in plasma.

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

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

Plasma levels of sphingosine-1-phosphate and apolipoprotein M in patients with monogenic disorders of HDL metabolism

BACKGROUND

Apolipoprotein M (apoM) has been identified as a specific sphingosine-1-phosphate (S1P) binding protein of HDL.

OBJECTIVES AND METHODS

To investigate the in vivo effects of disturbed apoM or HDL metabolism we quantified S1P and apoM in plasmas of wild-type, apoM-knock-out, and apoM transgenic mice as well as 50 patients with seven different monogenic disorders of HDL metabolism and their 51 unaffected relatives.

RESULTS

Compared to wild type mice, S1P plasma levels in apoM knock-out and apoM transgenic mice were decreased by 30% and increased by 270%, respectively. Compared to family controls, S1P and apoM levels in apoB-depleted plasma were significantly decreased by in average 34% and 12%, respectively, in heterozygous carriers of mutations in APOA1, LCAT or ABCA1, and by 70% and 48%, respectively, in carriers of two defective alleles in LCAT or ABCA1. Heterozygous mutations in CETP, SCARB1, LIPC, or LIPG did not significantly affect S1P or apoM concentrations. Albumin-corrected molar S1P-to-apoM ratios varied from 0.12 to 0.8 (median 0.3) and were not affected by any mutation. S1P levels in apoB-depleted plasma correlated significantly with HDL-cholesterol and less so with apoM both if apoA-I plasma concentrations were below the median.

CONCLUSION

In the context of previous data, our findings can be explained by the existence of a specific apoM and S1P containing HDL subclass which contains a considerable molar excess of apoM over S1P and is critically determined by apoA-I up to a threshold concentration around the median found in a Caucasian population.

Sphingosine-1-Phosphate-Specific G Protein-Coupled Receptors as Novel Therapeutic Targets for Atherosclerosis

Atherosclerosis is a chronic inflammatory process involving complex interactions of modified lipoproteins, monocyte-derived macrophages or foam cells, lymphocytes, endothelial cells (ECs), and vascular smooth muscle cells. Sphingosine-1-phosphate (S1P), a biologically active blood-borne lipid mediator, exerts pleiotropic effects such as cell proliferation, migration and cell-cell adhesion in a variety of cell types via five members of S1P-specific high-affinity G protein-coupled receptors (S1P₁-S1P₅). Among them, S1P₁, S1P₂ and S1P₃ are major receptor subtypes which are widely expressed in various tissues. Available evidence suggest that S1P and HDL-bound S1P exert atheroprotective effects including inhibition of leukocyte adhesion and stimulation of endothelial nitric oxide synthase (eNOS) in endothelial cells (ECs) through the activation of G_i signaling pathway via S1P₃ and probably S1P₁, although there is still controversy. FTY720, the phosphorylation product of which is a high-affinity agonist for all S1P receptors except S1P₂ and act as an immunosuppressant by downregulating S1P₁ on lymphocytes, inhibits atherosclerosis in LDL receptor-null mice and apoE-null mice through the inhibition of lymphocyte and macrophage functions and probably stimulation of EC functions, without influencing plasma lipid concentrations. In contrast to S1P₁ and S1P₃, S1P₂ facilitates atherosclerosis by activating G_12/13-Rho-Rho kinase (ROCK) in apoE-null mice. S1P₂ mediates transmigration of monocytes into the arterial intima, oxidized LDL accumulation and cytokine secretion in monocyte-derived macrophages, and eNOS inhibition and cytokine secretion in ECs through Rac inhibition, NF-κB activation and 3′-specific phosphoinositide phosphatase (PTEN) stimulation downstream of G_12/13-Rho-ROCK. Systemic long-term administration of a selective S1P₂-blocker remarkably inhibits atherosclerosis without overt toxicity. Thus, multiple S1P receptors positively and negatively regulate atherosclerosis through multitudes of mechanisms. Considering the essential and multi-faceted role of S1P₂ in atherogenesis and the impact of S1P₂ inactivation on atherosclerosis, S1P₂ is a particularly promising therapeutic target for atherosclerosis.

Sphingosine-1-phosphate receptor 3 promotes recruitment of monocyte/macrophages in inflammation and atherosclerosis

RATIONALE

The role of sphingosine-1-phosphate (S1P) and its receptors in the pathogenesis of atherosclerosis has not been investigated.

OBJECTIVE

We hypothesized that the S1P receptor 3 (S1P(3)) plays a causal role in the pathogenesis of atherosclerosis.

METHODS AND RESULTS

We examined atherosclerotic lesion development in mice deficient for S1P(3) and apolipoprotein (Apo)E. Although S1P(3) deficiency did not affect lesion size after 25 or 45 weeks of normal chow diet, it resulted in a dramatic reduction of the monocyte/macrophage content in lesions of S1P(3)(-/-)/ApoE(-/-) double knockout mice. To search for putative defects in monocyte/macrophage recruitment, we examined macrophage-driven inflammation during thioglycollate-induced peritonitis. Elicited peritoneal macrophages were reduced in S1P(3)-deficient mice and expressed lower levels of tumor necrosis factor-α and monocyte chemoattractant protein-1. Bone marrow-derived S1P(3)-deficient macrophages produced less MCP-1 in response to lipopolysaccharide stimulation. In vitro, S1P was chemotactic for wild-type but not S1P(3)-deficient peritoneal macrophages. In vivo, S1P concentration increased rapidly in the peritoneal cavity after initiation of peritonitis. Treatment with the S1P analog FTY720 attenuated macrophage recruitment to the peritoneum. Studies in bone marrow chimeras showed that S1P(3) in both hematopoietic and nonhematopoietic cells contributed to monocyte/macrophage accumulation in atherosclerotic lesions. Finally, S1P(3) deficiency increased the smooth muscle cell content of atherosclerotic lesions and enhanced neointima formation after carotid ligation arguing for an antiproliferative/antimigratory role of S1P(3) in the arterial injury response.

CONCLUSIONS

Our data suggest that S1P(3) mediates the chemotactic effect of S1P in macrophages in vitro and in vivo and plays a causal role in atherosclerosis by promoting inflammatory monocyte/macrophage recruitment and altering smooth muscle cell behavior.

Role of sphingosine 1-phosphate in anti-atherogenic actions of high-density lipoprotein

The reverse cholesterol transport mediated by high-density lipoprotein (HDL) is an important mechanism for maintaining body cholesterol, and hence, the crucial anti-atherogenic action of the lipoprotein. Recent studies, however, have shown that HDL exerts a variety of anti-inflammatory and anti-atherogenic actions independently of cholesterol metabolism. The present review provides an overview of the roles of sphingosine 1-phosphate (S1P)/S1P receptor and apolipoprotein A-I/scavenger receptor class B type I systems in the anti-atherogenic HDL actions. In addition, the physiological significance of the existence of S1P in the HDL particles is discussed.