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.

HDL-mediated mechanisms of protection in cardiovascular disease

Low plasma levels of HDL-cholesterol (HDL-C) represent a strong and independent risk factor for cardiovascular disease. HDL particles display a wide spectrum of atheroprotective activities, which include effluxing cellular cholesterol, diminishing cellular death, decreasing vascular constriction, reducing inflammatory response, protecting from pathological oxidation, combating bacterial infection, lessening platelet activation, regulating gene expression by virtue of microRNAs, and improving glucose metabolism. It remains presently indeterminate as to whether some biological activities of HDL are more relevant for the protection of the endothelium from atherogenesis when compared with others. The multitude of such activities raises the question of a proper assay to assess HDL functionality ex vivo. Together with clear understanding of molecular mechanisms underlying atheroprotective properties of HDL, such assay will provide a basis to resolve the ultimate question of the HDL field to allow the development of efficient HDL-targeting therapies.

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.

eNOS activation by HDL is impaired in genetic CETP deficiency

Mutations in the CETP gene resulting in defective CETP activity have been shown to cause remarkable elevations of plasma HDL-C levels, with the accumulation in plasma of large, buoyant HDL particles enriched in apolipoprotein E. Genetic CETP deficiency thus represents a unique tool to evaluate how structural alterations of HDL impact on HDL atheroprotective functions. Aim of the present study was to assess the ability of HDL obtained from CETP-deficient subjects to protect endothelial cells from the development of endothelial dysfunction. HDL isolated from one homozygous and seven heterozygous carriers of CETP null mutations were evaluated for their ability to down-regulate cytokine-induced cell adhesion molecule expression and to promote NO production in cultured endothelial cells. When compared at the same protein concentration, HDL and HDL3 from carriers proved to be as effective as control HDL and HDL3 in down-regulating cytokine-induced VCAM-1, while carrier HDL2 were more effective than control HDL2 in inhibiting VCAM-1 expression. On the other hand, HDL and HDL fractions from carriers of CETP deficiency were significantly less effective than control HDL and HDL fractions in stimulating NO production, due to a reduced eNOS activating capacity, likely because of a reduced S1P content. In conclusion, the present findings support the notion that genetic CETP deficiency, by affecting HDL particle structure, impacts on HDL vasculoprotective functions. Understanding of these effects might be important for predicting the outcomes of pharmacological CETP inhibition.

Chemical and genetic tools to explore S1P biology

The zwitterionic lysophospholipid Sphingosine 1-Phosphate (S1P) is a pleiotropic mediator of physiology and pathology. The synthesis, transport, and degradation of S1P are tightly regulated to ensure that S1P is present in the proper concentrations in the proper location. The binding of S1P to five G protein-coupled S1P receptors regulates many physiological systems, particularly the immune and vascular systems. Our understanding of the functions of S1P has been aided by the tractability of the system to both chemical and genetic manipulation. Chemical modulators have been generated to affect most of the known components of S1P biology, including agonists of S1P receptors and inhibitors of enzymes regulating S1P production and degradation. Genetic knockouts and manipulations have been similarly engineered to disrupt the functions of individual S1P receptors or enzymes involved in S1P metabolism. This chapter will focus on the development and utilization of these chemical and genetic tools to explore the complex biology surrounding S1P and its receptors, with particular attention paid to the in vivo findings that these tools have allowed for.

High-density lipoproteins and cerebrovascular integrity in Alzheimer's disease

Cerebrovascular dysfunction significantly contributes to the clinical presentation and pathoetiology of Alzheimer's disease (AD). Deposition and aggregation of β-amyloid (Aβ) within vascular smooth muscle cells leads to inflammation, oxidative stress, impaired vasorelaxation, and disruption of blood-brain barrier integrity. Midlife vascular risk factors, such as hypertension, cardiovascular disease, diabetes, and dyslipidemia, increase the relative risk for AD. These comorbidities are all characterized by low and/or dysfunctional high-density lipoproteins (HDL), which itself is a risk factor for AD. HDL performs a wide variety of critical functions in the periphery and CNS. In addition to lipid transport, HDL regulates vascular health via mediating vasorelaxation, inflammation, and oxidative stress and promotes endothelial cell survival and integrity. Here, we summarize clinical and preclinical data examining the involvement of HDL, originating from the circulation and from within the CNS, on AD and hypothesize potential synergistic actions between the two lipoprotein pools.

Sphingosine-1-phosphate as a potential target for the treatment of myocardial infarction

This review focuses on the role of sphingosine-1-phosphate (S1P) signaling in the heart, with particular emphasis on how it could be modulated therapeutically in the context of myocardial infarction (MI). After a brief general description of sphingolipid metabolism and signaling, this review will examine the relationship between S1P and the beneficial effects of high-density lipoprotein (HDL), and finally focus on the known actions of S1P on different mechanisms relevant to MI pathophysiology (cardiomyocyte protection, fibrosis, remodeling, arrhythmia, control of vascular tone and potential repair mechanisms). The potential of particular enzyme isoforms or receptor subtypes for the development of therapeutic agents for MI will also be explored. 

15-lipoxygenase-mediated modification of HDL3 impairs eNOS activation in human endothelial cells

Caveolae are cholesterol and glycosphingolipids-enriched microdomains of plasma membranes. Caveolin-1 represents the major structural protein of caveolae, that also contain receptors and molecules involved in signal transduction pathways. Caveolae are particularly abundant in endothelial cells, where they play important physiological and pathological roles in regulating endothelial cell functions. Several molecules with relevant functions in endothelial cells are localized in caveolae, including endothelial nitric oxide synthase (eNOS), which regulates the production of nitric oxide, and scavenger receptor class B type I (SR-BI), which plays a key role in the induction of eNOS activity mediated by high density lipoproteins (HDL). HDL have several atheroprotective functions, including a positive effect on endothelial cells, as it is a potent agonist of eNOS through the interaction with SR-BI. However, the oxidative modification of HDL may impair their protective role. In the present study we evaluated the effect of 15-lipoxygenase-mediated modification of HDL3 on the expression and/or activity of some proteins localized in endothelial caveolae and involved in the nitric oxide generation pathway. We found that after modification, HDL3 failed to activate eNOS and to induce NO production, due to both a reduced ability to interact with its own receptor SR-BI and to a reduced expression of SR-BI in cells exposed to modified HDL. These findings suggest that modification of HDL may reduce its endothelial-protective role also by interfering with vasodilatory function of HDL.

High-density lipoprotein mediated cellular cholesterol efflux in acute coronary syndromes

Systemic inflammation at the development of an acute coronary syndrome (ACS) might alter the high-density lipoprotein (HDL) components and function. One of the major functions of HDL particles is their ability to remove cellular cholesterol from macrophages. The aim of the present study was to characterize the HDL efflux capacity in patients with ACS. We analyzed the cholesterol efflux in those ACS (within 72 hours of symptoms [ACS1]) and, again, 3 months later (ACS2). As controls, we used normal subjects without coronary artery disease (CAD) and patients with chronic, stable CAD. The 4 groups were matched for age and HDL cholesterol levels. We used a cell-based efflux system in (3)[H]-cholesterol-labeled J774 macrophages to measure cholesterol efflux from apolipoprotein B-depleted plasma. The present study included 20 patients with ACS. Their mean age was 58 ± 9 years, and the mean HDL cholesterol level was 1.06 ± 0.22 mmol/L (41 ± 9 mg/dl). The ACS1 group showed a marked increase in high-sensitivity C-reactive protein and serum amyloid A, reflecting systemic inflammation. The HDL cholesterol efflux capacity was reduced in ACS1 subjects and remained reduced 12 weeks later and in those with stable CAD. These results suggest that the acute presence of serum amyloid A does not account for the impairment of HDL-mediated cholesterol efflux capacity in the ACS1 group. Little correlation was found between HDL cholesterol and HDL efflux capacity (r = 0.233; p = 0.049), suggesting that HDL cholesterol is a poor marker of HDL function in inflammatory states and CAD. In conclusion, our data support the concept that atherogenic HDL dysfunction and impaired efflux occur in ACS, independent of changes in plasma HDL cholesterol and apolipoprotein A-I levels.

Regulation of endothelial nitric oxide synthase and high-density lipoprotein quality by estradiol in cardiovascular pathology

Estrogens have been recognized, in the last 3 decades, as important hormones in direct and indirect modulation of vascular health. In addition to their direct benefit on cardiovascular health, the presence of esterified estrogen in the lipid core of high-density lipoprotein (HDL) particles indirectly contributes to atheroprotection by significantly improving HDL quality and functionality. Estrogens modulate their physiological activity via genomic and nongenomic mechanisms. Genomic mechanisms are thought to be mediated directly by interaction of the hormone receptor complex with the hormone response elements that regulate gene expression. Nongenomic mechanisms are thought to occur via interaction of the estrogen with membrane-bound receptors, which rapidly activate intracellular signaling without binding of the hormone receptor complex to its hormone response elements. Estradiol in particular mediates early and late endothelial nitric oxide synthase (eNOS) activation via interaction with estrogen receptors through both nongenomic and genomic mechanisms. In the vascular system, the primary endogenous source of nitric oxide (NO) generation is eNOS. Nitric oxide primarily influences blood vessel relaxation, the heart rate, and myocyte contractility. The abnormalities in expression and/or functions of eNOS lead to the development of cardiovascular diseases, both in animals and in humans. Although considerable research efforts have been dedicated to understanding the mechanisms of action of estradiol in regulating cardiac eNOS, more research is needed to fully understand the details of such mechanisms. This review focuses on recent findings from animal and human studies on the regulation of eNOS and HDL quality by estradiol in cardiovascular pathology.

High-Density Lipoprotein

High-density lipoprotein (HDL) is a complex mixture of lipoproteins that is associated with many minor proteins and lipids that influence the function of HDL. Although HDL is a promising marker and potential therapeutic target based on its epidemiological data and the effects of healthy HDL in vitro in endothelial cells and macrophages, as well as based on infusion studies of reconstituted HDL in patients with hypercholesterolemia, it remains still uncertain whether or not HDL cholesterol–raising drugs will improve outcomes. Recent studies suggest that HDL becomes modified in patients with coronary artery disease or acute coronary syndrome because of oxidative processes that result in alterations in its proteome composition (proteome remodelling) leading to HDL dysfunction.

HDL and cholesterol: life after the divorce?

For decades, HDL and HDL-cholesterol (HDL-C) levels were viewed as synonymous, and modulation of HDL-C levels by drug therapy held great promise for the prevention and treatment of cardiovascular disease. Nevertheless, recent failures of drugs that raise HDL-C to reduce cardiovascular risk and the now greater understanding of the complexity of HDL composition and biology have prompted researchers in the field to redefine HDL. As such, the focus of HDL has now started to shift away from a cholesterol-centric view toward HDL particle number, subclasses, and other alternative metrics of HDL. Many of the recently discovered functions of HDL are, in fact, not strictly conferred by its ability to promote cholesterol flux but by the other molecules it transports, including a diverse set of proteins, small RNAs, hormones, carotenoids, vitamins, and bioactive lipids. Based on HDL's ability to interact with almost all cells and transport and deliver fat-soluble cargo, HDL has the remarkable capacity to affect a wide variety of endocrine-like systems. In this review, we characterize HDL's unique cargo and address the functional relevance and consequences of HDL transport and delivery of noncholesterol molecules to recipient cells and tissues.

HDLs protect the MIN6 insulinoma cell line against tunicamycin-induced apoptosis without inhibiting ER stress and without restoring ER functionality

HDLs protect pancreatic beta cells against apoptosis induced by several endoplasmic reticulum (ER) stressors, including thapsigargin, cyclopiazonic acid, palmitate and insulin over-expression. This protection is mediated by the capacity of HDLs to maintain proper ER morphology and ER functions such as protein folding and trafficking. Here, we identified a distinct mode of protection exerted by HDLs in beta cells challenged with tunicamycin (TM), a protein glycosylation inhibitor inducing ER stress. HDLs were found to inhibit apoptosis induced by TM in the MIN6 insulinoma cell line and this correlated with the maintenance of a normal ER morphology. Surprisingly however, this protective response was neither associated with a significant ER stress reduction, nor with restoration of protein folding and trafficking in the ER. These data indicate that HDLs can use at least two mechanisms to protect beta cells against ER stressors. One that relies on the maintenance of ER function and one that operates independently of ER function modulation. The capacity of HDLs to activate several anti-apoptotic pathways in beta cells may explain their ability to efficiently protect these cells against a variety of insults.

Second generation S1P pathway modulators: research strategies and clinical developments

Multiple Sclerosis (MS) is a chronic autoimmune disorder affecting the central nervous system (CNS) through demyelination and neurodegeneration. Until recently, major therapeutic treatments have relied on agents requiring injection delivery. In September 2010, fingolimod/FTY720 (Gilenya, Novartis) was approved as the first oral treatment for relapsing forms of MS. Fingolimod causes down-modulation of S1P1 receptors on lymphocytes which prevents the invasion of autoaggressive T cells into the CNS. In astrocytes, down-modulation of S1P1 by the drug reduces astrogliosis, a hallmark of MS, thereby allowing restoration of productive astrocyte communication with other neural cells and the blood brain barrier. Animal data further suggest that the drug directly supports the recovery of nerve conduction and remyelination. In human MS, such mechanisms may explain the significant decrease in the number of inflammatory markers on brain magnetic resonance imaging in recent clinical trials, and the reduction of brain atrophy by the drug. Fingolimod binds to 4 of the 5 known S1P receptor subtypes, and significant efforts were made over the past 5 years to develop next generation S1P receptor modulators and determine the minimal receptor selectivity needed for maximal therapeutic efficacy in MS patients. Other approaches considered were competitive antagonists of the S1P1 receptor, inhibitors of the S1P lyase to prevent S1P degradation, and anti-S1P antibodies. Below we discuss the current status of the field, and the functional properties of the most advanced compounds. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Comprehensive RNA-Seq expression analysis of sensory ganglia with a focus on ion channels and GPCRs in Trigeminal ganglia

The specific functions of sensory systems depend on the tissue-specific expression of genes that code for molecular sensor proteins that are necessary for stimulus detection and membrane signaling. Using the Next Generation Sequencing technique (RNA-Seq), we analyzed the complete transcriptome of the trigeminal ganglia (TG) and dorsal root ganglia (DRG) of adult mice. Focusing on genes with an expression level higher than 1 FPKM (fragments per kilobase of transcript per million mapped reads), we detected the expression of 12984 genes in the TG and 13195 in the DRG. To analyze the specific gene expression patterns of the peripheral neuronal tissues, we compared their gene expression profiles with that of the liver, brain, olfactory epithelium, and skeletal muscle. The transcriptome data of the TG and DRG were scanned for virtually all known G-protein-coupled receptors (GPCRs) as well as for ion channels. The expression profile was ranked with regard to the level and specificity for the TG. In total, we detected 106 non-olfactory GPCRs and 33 ion channels that had not been previously described as expressed in the TG. To validate the RNA-Seq data, in situ hybridization experiments were performed for several of the newly detected transcripts. To identify differences in expression profiles between the sensory ganglia, the RNA-Seq data of the TG and DRG were compared. Among the differentially expressed genes (> 1 FPKM), 65 and 117 were expressed at least 10-fold higher in the TG and DRG, respectively. Our transcriptome analysis allows a comprehensive overview of all ion channels and G protein-coupled receptors that are expressed in trigeminal ganglia and provides additional approaches for the investigation of trigeminal sensing as well as for the physiological and pathophysiological mechanisms of pain.

Pathogenesis of type 2 diabetes in South Asians

The risk of developing type 2 diabetes mellitus (T2DM) is exceptionally high among both native and migrant South Asians. T2DM occurs more often and at a younger age and lower BMI, and the risk of coronary artery and cerebrovascular disease, and renal complications is higher for South Asians compared with people of White Caucasian descent. The high prevalence of T2DM and its related complications in South Asians, which comprise one-fifth of the total world's population, poses a major health and socioeconomic burden. The underlying cause of this excess risk, however, is still not completely understood. Therefore, gaining insight into the pathogenesis of T2DM in South Asians is of great importance. The predominant mechanism, in this ethnicity seems to be insulin resistance (IR) rather than an impaired β-cell function. In this systematic review, we describe several possible mechanisms that may underlie or contribute to the increased IR observed in South Asians.

Deleterious effect of glycation on the ability of HDL to counteract the inhibitory effect of oxidized LDL on endothelium-dependent vasorelaxation

BACKGROUND

Contrary to high-density lipoprotein (HDL) from normolipidaemic and normoglycaemic subjects, HDL from diabetic patients loses its ability to reverse the inhibition of vasorelaxation induced by oxidized low-density lipoprotein (LDL). The aim of this study was to analyze the role of glycation, a major abnormality observed in diabetes, on the impairment of the vasorelaxant effect of HDL.

METHODS

HDL from healthy subjects was glycated in vitro by incubation in glucose 200 mmol/L for 3 days. Vasoreactivity was evaluated by the relaxation response to acetylcholine of rabbit aorta rings pre-contracted with noradrenaline, before and after 2 h incubation with or without different lipoprotein fractions (Krebs buffer, oxidized LDL, normal or glycated HDL alone and with oxidized LDL).

RESULT

The fructosamine/apolipoprotein AI ratio was significantly increased in glycated HDL compared with native HDL (53.63 ± 7.91 vs 18.51 ± 4.10 µmol/g; p < 0.05). Oxidized LDL inhibited endothelium-dependent vasodilation compared with Krebs buffer [maximal relaxation (Emax) = 53.15 ± 6.50 vs 98.67 ± 2.07%, p < 0.001]. Native HDL was able to counteract the oxidized LDL-induced inhibition of vasorelaxation (Emax = 76.93 ± 5.41 vs 53.15 ± 6.50%, p < 0.001). On the other hand, glycated HDL had no effect on oxidized LDL-induced inhibition of endothelium vasorelaxation compared with incubation with oxidized LDL alone (Emax = 52.98 ± 2.07 vs 53.15 ± 6.50%, not significant).

CONCLUSION

Glycation of HDL induces the loss of the ability of HDL to counteract the inhibitory effect of oxidized LDL on endothelium-dependent vasorelaxation, this is likely contributing to the impairment of antiatherogenic properties of HDL in diabetic patients.

Discovery, design and synthesis of a selective S1P(3) receptor allosteric agonist

Potent and selective S1P3 receptor (S1P3-R) agonists may represent important proof-of-principle tools used to clarify the receptor biological function and assess the therapeutic potential of the S1P3-R in cardiovascular, inflammatory and pulmonary diseases. N,N-Dicyclohexyl-5-propylisoxazole-3-carboxamide was identified by a high-throughput screening of MLSMR library as a promising S1P3-R agonist. Rational chemical modifications of the hit allowed the identification of N,N-dicyclohexyl-5-cyclopropylisoxazole-3-carboxamide, a S1P3-R agonist endowed with submicromolar activity and exquisite selectivity over the remaining S1P1,2,4,5-R family members. A combination of ligand competition, site-directed mutagenesis and molecular modeling studies showed that the N,N-dicyclohexyl-5-cyclopropylisoxazole-3-carboxamide is an allosteric agonist and binds to the S1P3-R in a manner that does not disrupt the S1P3-R-S1P binding. The lead molecule herein disclosed constitutes a valuable pharmacological tool to explore the molecular basis of the receptor function, and provides the bases for further rational design of more potent and drug-like S1P3-R allosteric agonists.

HDL-C: clinical equipoise and vascular endothelial function

Serum levels of HDL cholesterol represent a strong, and coherent cardiovascular risk marker seen across all populations, with higher levels of HDL cholesterol being associated with decreased incidence of coronary artery disease. The cardiovascular protective effects of HDL particles are attributed, in great part, to the ability of HDL particles to promote cellular cholesterol efflux from lipid-laden macrophages within the atherosclerotic plaque. HDL also has pleiotropic effects that protect the vascular wall, at least in vitro. These effects include potent anti-inflammatory and antioxidant properties and the modulation of vascular endothelial function. The mechanisms by which HDL exert their function on the vascular endothelium is dependent on HDL particle size, protein (proteome) and lipid (lipidome). The cooperative binding of HDL via SR-BI and G-coupled S1PR1-5 receptors mediates phosphorylation of endothelial nitric oxide synthase at residue 1177 through AKT signaling, preventing uncoupling of NADPH oxidation and nitric oxide synthesis and increasing endothelial nitric oxide synthase abundance. Furthermore, HDL can modulate the activation of NF-κB and the expression of cell adhesion molecules, an early step in endothelial dysfunction. In the present review the authors will focus on the controversies surrounding HDL, clinical treatments and vascular endothelial functions of HDL.

FTY720 prevents progression of renal fibrosis by inhibiting renal microvasculature endothelial dysfunction in a rat model of chronic kidney disease

Recent studies have shown that chronic endothelial dysfunction can impair multiple aspects of renal physiology and, in turn, contribute to renal fibrosis. Sphingosine 1-phosphate (S1P) has been highlighted as an endothelial barrier-stabilizing mediator. The aim of our study was to investigate the effect of FTY720, an S1P analog, on the progression of renal fibrosis by inhibiting renal microvasculature endothelial dysfunction in a rat model of chronic kidney disease. Thirty male Sprague-Dawley rats were used in this study. Seven days after surgery, we placed the animals into three groups: sham surgery; 5/6 nephrectomized (Nx) rats; and 5/6Nx + FTY720 (1 mg/kg/day). All of the animals were sacrificed 12 weeks after surgery. We obtained and analyzed blood and kidney tissue samples from all of the groups. Glomerular capillary density and peritubular capillary (PTC) density were determined by CD31 immunostaining. The expression of transforming growth factor beta 1 (TGF-β1), collagen IV, fibronectin, endothelial nitric oxide synthase (eNOS) and vascular endothelial growth factor (VEGF) were analyzed by immunohistochemistry, reverse transcription-polymerase chain reaction and western blotting. The 5/6Nx group exhibited increased blood urea nitrogen and serum creatinine, visible renal histological changes, pro-fibrotic molecule (TGF-β1) and production of extracellular matrix proteins such as collagen IV and fibronectin and decreased glomerular and PTC density, compared to the sham controls (P < 0.01). We observed that treatment with FTY720 reduced these abnormalities. Furthermore, the level of NO, the expression levels of eNOS and VEGF were downregulated in the kidney tissue in 5/6Nx rats, FTY720 treatment significantly attenuated this decrease. FTY720 prevents the progression of renal fibrosis by inhibiting renal microvasculature endothelial dysfunction in a rat model of chronic kidney disease.

HDL quality or cholesterol cargo: what really matters--spotlight on sphingosine-1-phosphate-rich HDL

PURPOSE OF REVIEW

The absolute level of HDL cholesterol (HDL-C) may not be the only criterion contributing to their antiatherothrombotic effects. This review focuses on evidence in support of the concept that HDL-bound sphingosine-1-phosphate (S1P) plays a role in different HDL atheroprotective properties and may represent a potential target for therapeutic interventions.

RECENT FINDINGS

Recent large randomized clinical trials testing the hypothesis of raising HDL-C with niacin and dalcetrapib in statin-treated patients failed to improve cardiovascular outcomes. Emerging evidence suggests that many of the cardioprotective functions of HDL, such as vasodilation, angiogenesis and endothelial barrier function, protection against ischemia/reperfusion injury, and inhibition of atherosclerosis, may be attributable to its S1P cargo. HDL-associated S1P may represent a future therapeutic target.

SUMMARY

HDL functionality is affected by its composition and there is evidence to suggest S1P plays a role in some of HDL's functions and atheroprotective properties.

Sphingolipids in acute lung injury

Acute lung injury is a life-threatening disease that is characterized by pulmonary inflammation, loss of barrier functions, and hypoxemia. Sphingolipids are critically involved in the disease process that they can both expedite and extenuate: They expedite inflammation by promoting chemotaxis (neutral sphingomyelinase), increased endothelial permeability (acid sphingomyelinase, S1P3-receptors), increased epithelial permeability (S1P2- and S1P3-receptors), and delaying neutrophil apoptosis (neutral sphingomyelinase, S1P1-receptors). They extenuate inflammation by attenuating chemotaxis (S1P) and by stabilizing the endothelial and the epithelial barrier (S1P1-receptor). This chapter discusses the multiple roles and therapeutic options that sphingolipids offer with respect to acute lung injury.

High density lipoproteins and endothelial functions: mechanistic insights and alterations in cardiovascular disease

Prospective population studies in the primary prevention setting have shown that reduced plasma levels of HDL cholesterol are associated with an increased risk of coronary disease and myocardial infarction. Experimental and translational studies have further revealed several potential anti-atherogenic effects of HDL, including protective effects on endothelial cell functions. HDL has been suggested to protect endothelial cell functions by prevention of oxidation of LDL and its adverse endothelial effects. Moreover, HDL from healthy subjects can directly stimulate endothelial cell production of nitric oxide and anti-inflammatory, anti-apoptotic, and anti-thrombotic effects as well as endothelial repair processes. However, several recent clinical trials using HDL cholesterol-raising agents, such as torcetrapib, dalcetrapib, and niacin, did not demonstrate a significant reduction of cardiovascular events in patients with coronary disease. Of note, growing evidence suggests that the vascular effects of HDL can be highly heterogeneous and vasoprotective properties of HDL are altered in patients with coronary disease. Characterization of underlying mechanisms and understanding of the clinical relevance of this "HDL dysfunction" is currently an active field of cardiovascular research. Notably, in some recent studies no clear association of higher HDL cholesterol levels with a reduced risk of cardiovascular events was observed in patients with already established coronary disease. A greater understanding of mechanisms of action of HDL and its altered vascular effects is therefore critical within the context of HDL-targeted therapies. In this review, we will address different effects of HDL on endothelial cell functions potentially relevant to atherosclerotic vascular disease and explore molecular mechanisms leading to "dysfunctional HDL".

Arterial stiffness evaluation by cardio-ankle vascular index in hypertension and diabetes mellitus subjects

Arterial stiffness is an independent predictor for vascular diseases. Cardio-ankle vascular index (CAVI) is a new index of arterial stiffness. In the present study, we investigated the possible risk factors involving CAVI in hypertension and diabetes mellitus (DM) subjects. One thousand sixty-three subjects (M/F 533/530) from Shougang Corporation Examination Center were divided into four groups: healthy group (n = 639); hypertension group (n = 312); DM group (n = 58); and hypertension with DM group (n = 54). CAVI was measured by VS-1000 apparatus. Our results showed that CAVI was significantly higher in hypertension subjects with DM than in healthy and hypertension group, respectively (8.59 ± 1.08 vs 7.23 ± 1.10; 8.59 ± 1.08 vs 7.94 ± 1.33; both P < .05). CAVI was positively correlated with age, systolic blood pressure, diastolic blood pressure, pulse pressure, fasting plasma glucose, HbA1c, uric acid, total cholesterol, triglycerides in the entire group (r = 0.633, 0.280, 0.172, 0.269, 0.209, 0.254, 0.176, 0.129, 0.175; all P < .05, respectively). There was negatively correlation between CAVI and high-density lipoprotein cholesterol in the entire group (r = -0.167; P < .05). Multivariate analysis showed that age, body mass index, HbA1c, and high-density lipoprotein cholesterol were independent associating factors of CAVI in all subjects (β = 0.699; P < .001, β = -0.189; P = .001, β = 0.144; P = .015, β = -0.136; P = .019, respectively). Our present study suggested that CAVI was significantly higher in hypertension subjects with DM compared with healthy and hypertension groups.

Heart function in magnetic resonance imaging and the mesenteric artery reactivity in rats receiving lead-contaminated drinking water

The aim of this study was to evaluate the effect of lead (Pb)-contaminated drinking water on magnetic resonance imaging (MRI)-estimated cardiac function, vascular reactivity, and serum lipids in rats. For 3 months, male Wistar rats, aged 4–6 weeks, were given drinking water with the addition of lead acetate at a concentration of 100 ppm Pb (10 rats) or water free from Pb (8 control rats). The cardiac MRI was performed at rest and under β-adrenergic stimulation on a 4.7 T scanner using electrocardiogram-triggered gradient echo (FLASH) cine sequence. After 1–2 weeks of the MRI test, experiments were performed ex vivo. After stabilization of perfusion pressure (PP), norepinephrine at doses from 0.01 to 5.0 μg was dissolved in Krebs solution, injected in a volume of 100 μl, and next infused at a concentration of 0.5 μg/ml into the isolated mesenteric artery. In this manner, preconstricted mesenteric bed was used to determine PP changes induced by acetylcholine, given at doses from 0.05 to 5.0 μg, before and during the infusion of nitric oxide synthase inhibitor (1.0 μg/ml). At the end, dobutamine (5 mg), followed by potassium chloride (10.5 mg), was injected. Lipid levels were determined enzymatically, blood Pb level was measured by the atomic absorption spectrophotometer. This study showed that Pb impairs the left ventricular systolic and diastolic function. Pb-induced changes in response to resistance of vessels to vasoactive agents may be secondary to the reduced left ventricular ejection fraction. The high-density lipoprotein subfraction 2 (HDL2) is involved in the cardiovascular effect of Pb.

An ocular view of the IGF-IGFBP system

IGFs and their binding proteins have been shown to exhibit both protective and deleterious effects in ocular disease. Recent studies have characterized the expression patterns of different IGFBPs in retinal layers and within the vitreous. IGFBP-3 has roles in vascular protection stimulating proliferation, migration, and differentiation of vascular progenitor cells to sites of injury. IGFBP-3 increases pericyte ensheathment and shows anti-inflammatory effects by reducing microglia activation in diabetes. IGFBP-5 has recently been linked to mediating fibrosis in proliferative vitreoretinopathy but also reduces neovascularization. Thus, the regulatory balance between IGF and IGFBPs can have profound impact on target tissues. This review discusses recent findings of IGF and IGFBP expression in the eye with relevance to different retinopathies.

Assessing the functional properties of high-density lipoproteins: an emerging concept in cardiovascular research

Although plasma concentrations of high-density lipoprotein (HDL) cholesterol correlate inversely with the incidence of atherosclerotic cardiovascular disease, results from recent epidemiological, genetic and pharmacological intervention studies resulted in a shift of concept. Rather than HDL cholesterol mass levels, the functionality of HDL particles is increasingly regarded as potentially clinically important. This review provides an overview of four key functional properties of HDL, namely cholesterol efflux and reverse cholesterol transport; antioxidative activities; anti-inflammatory activities; and the ability of HDL to increase vascular nitric oxide production resulting in vasorelaxation. Currently available assays are put into context with different HDL isolation procedures yielding compositional heterogeneity of the particle. Gathered knowledge on the impact of different disease states on HDL function is discussed together with potential underlying causative factors modulating HDL functionalities. In addition, a perspective is provided regarding how a better understanding of the determinants of (dys)functional HDL might impact clinical practice and the future design of rational and specific therapeutic approaches targeting atherosclerotic cardiovascular disease.

Endothelial lipase is a critical determinant of high-density lipoprotein-stimulated sphingosine 1-phosphate-dependent signaling in vascular endothelium

OBJECTIVE

In addition to an extensively characterized role of high-density lipoprotein (HDL) in reverse cholesterol transport, bioactive lipids bound to HDL can also exert diverse vascular effects. Despite this, integration of HDL action in the vasculature with pathways that metabolize HDL and release bioactive lipids has been much less explored. The effects of HDL on endothelial cells are mediated in part by HDL-associated sphingosine 1-phosphate (S1P), which binds to S1P1 receptors and promotes activation of endothelial NO synthase (eNOS) and the kinase Akt. In these studies, we characterized the role of endothelial lipase (EL) in the control of endothelial signaling and biology, including those mediated by HDL-associated S1P.

APPROACH AND RESULTS

HDL-induced angiogenesis in aortic rings from EL-deficient (EL(-/-)) mice was markedly decreased compared with wild-type controls. In cultured endothelial cells, small interfering RNA-mediated knockdown of EL abrogated HDL-promoted endothelial cell migration and tube formation. Small interfering RNA-mediated EL knockdown also attenuated HDL-induced phosphorylation of eNOS(1179) and Akt(473). S1P stimulation restored HDL-induced endothelial migration and Akt/eNOS phosphorylation that had been blocked by small interfering RNA-mediated EL knockdown. HDL-induced endothelial cell migration and Akt/eNOS phosphorylation were completely inhibited by the S1P1 antagonist W146 but not by the S1P3 antagonist CAY10444.

CONCLUSIONS

EL is a critical determinant of the effects of HDL on S1P-mediated vascular responses and acts on HDL to promote activation of S1P1, leading to Akt/eNOS phosphorylation and subsequent endothelial migration and angiogenesis. The role of EL in HDL-associated S1P effects provides new insights into EL action, the responses seen through EL and HDL interaction, and S1P signaling.

Regulation of signal transduction by HDL

High density lipoprotein (HDL) cholesterol has direct effects on numerous cell types that influence cardiovascular and metabolic health. These include endothelial cells, vascular smooth-muscle cells, leukocytes, platelets, adipocytes, skeletal muscle myocytes, and pancreatic β cells. The effects of HDL or apoA-I, its major apolipoprotein, occur through the modulation of intracellular calcium, oxygen-derived free-radical production, numerous kinases, and enzymes, including endothelial nitric-oxide synthase (eNOS). ApoA-I and HDL also influence gene expression, particularly genes encoding mediators of inflammation in vascular cells. In many paradigms, the change in intracellular signaling occurs as a result of cholesterol efflux, with the cholesterol acceptor methyl-β-cyclodextrin often invoking responses identical to HDL or apoA-I. The ABC transporters ABCA1 and ABCG1 and scavenger receptor class B, type I (SR-BI) frequently participate in the cellular responses. Structure-function relationships are emerging for signal initiation by ABCA1 and SR-BI, with plasma membrane cholesterol binding by the C-terminal transmembrane domain of SR-BI uniquely enabling it to serve as a sensor of changes in membrane cholesterol. Further investigation of the processes underlying HDL and apoA-I modulation of intracellular signaling will potentially reveal new prophylactic and therapeutic strategies to optimize both cardiovascular and metabolic health.

Uraemic dyslipidaemia revisited: role of high-density lipoprotein

Chronic kidney disease (CKD) is accompanied by strong cardiovascular risk. In a rather rigid picture of lipoprotein biology, low-density lipoprotein (LDL) is referred to as 'bad cholesterol', while high-density lipoprotein (HDL) is referred to as 'good cholesterol'. However, recent experimental evidence suggests that HDL may be rendered dysfunctional regarding its effects on the vasculature under various clinical conditions such as CKD. Indeed, HDL from the blood of CKD patients has been shown to transform into a particle which promotes endothelial dysfunction, endothelial proinflammatory activation and, thereby, sets the conditions for the development of atherosclerotic disease. Notably, pharmaceutical interventions to raise serum HDL-cholesterol levels have not been proven beneficial so far. Collectively, these findings indicate that HDL cholesterol levels do not represent a valuable marker for indicating the vascular properties of HDL.

Templated high density lipoprotein nanoparticles as potential therapies and for molecular delivery

High density lipoproteins (HDLs) are dynamic natural nanoparticles best known for their role in cholesterol transport and the inverse correlation that exists between blood HDL levels and the risk of developing coronary heart disease. In addition, enhanced HDL-cholesterol uptake has been demonstrated in several human cancers. As such, the use of HDL as a therapeutic and as a vehicle for systemic delivery of drugs and as imaging agents is increasingly important. HDLs exist on a continuum from the secreted HDL-scaffolding protein, apolipoprotein A-1 (Apo A1), to complex, spherical "mature" HDLs. Aspects of HDL particles including their size, shape, and surface chemical composition are being recognized as critical to their diverse biological functions. Here we review HDL biology; strategies for synthesizing HDLs; data supporting the clinical use and benefit of directly administered HDL; a rationale for developing synthetic methods for spherical, mature HDLs; and, the potential to employ HDLs as therapies, imaging agents, and drug delivery vehicles. Importantly, methods that utilize nanoparticle templates to control synthetic HDL size, shape, and surface chemistry are highlighted.

Sphingosine 1-phosphate receptor agonists: a patent review (2010-2012)

INTRODUCTION

The sphingosine-1-phosphate (S1P)-driven signaling regulates fundamental biological functions, including cell proliferation, angiogenesis, endothelial cell chemotaxis, immune cell trafficking and mitogenesis. A large body of research has been focusing on the development of immunosuppressive S1P1 receptor (S1P1-R) agonist molecules. The S1P(1,3-5)-R pan-agonist fingolimod (FTY720) has been approved by the FDA for the treatment of relapsing-remitting multiple sclerosis. However, FTY720 is now contraindicated in patients with compromised cardiac function. Although the main adverse effect bradycardia has been linked to the S1P3-R activation, cardiovascular liabilities persist with more selective S1P1-R agonists that have entered clinical trials. In contrast to the S1P1-R, the therapeutic application of the S1P(2-5)-Rs remains poorly explored.

AREAS COVERED

This review provides the patent literature from 2010 to date on S1P-R agonist molecules and their relevant biological properties.

EXPERT OPINION

Limited progress has been made on agonists at S1P(4,5)-R subtypes, with some families of S1P5-R agonists showing promising results in animal models of age-related cognitive disorders. A discrete number of reviewed molecules are S1P1-R agonists with a promising clinical outlook in transplantation, inflammation, cancer and autoimmune settings. Further preclinical and clinical studies will determine whether the new developed S1P1-R agonists do indeed improve the safety profile of FTY720.

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.

Abnormal high-density lipoprotein induces endothelial dysfunction via activation of Toll-like receptor-2

Endothelial injury and dysfunction (ED) represent a link between cardiovascular risk factors promoting hypertension and atherosclerosis, the leading cause of death in Western populations. High-density lipoprotein (HDL) is considered antiatherogenic and known to prevent ED. Using HDL from children and adults with chronic kidney dysfunction (HDL(CKD)), a population with high cardiovascular risk, we have demonstrated that HDL(CKD) in contrast to HDL(Healthy) promoted endothelial superoxide production, substantially reduced nitric oxide (NO) bioavailability, and subsequently increased arterial blood pressure (ABP). We have identified symmetric dimethylarginine (SDMA) in HDL(CKD) that causes transformation from physiological HDL into an abnormal lipoprotein inducing ED. Furthermore, we report that HDL(CKD) reduced endothelial NO availability via toll-like receptor-2 (TLR-2), leading to impaired endothelial repair, increased proinflammatory activation, and ABP. These data demonstrate how SDMA can modify the HDL particle to mimic a damage-associated molecular pattern that activates TLR-2 via a TLR-1- or TLR-6-coreceptor-independent pathway, linking abnormal HDL to innate immunity, ED, and hypertension.

The apolipoprotein m-sphingosine-1-phosphate axis: biological relevance in lipoprotein metabolism, lipid disorders and atherosclerosis

Apolipoprotein M (apoM) is a plasma apolipoprotein that mainly associates with high-density lipoproteins. Hence, most studies on apoM so far have investigated its effect on and association with lipid metabolism and atherosclerosis. The insight into apoM biology recently took a major turn. ApoM was identified as a carrier of the bioactive lipid sphingosine-1-phosphate (S1P). S1P activates five different G-protein-coupled receptors, known as the S1P-receptors 1–5 and, hence, affects a wide range of biological processes, such as lymphocyte trafficking, angiogenesis, wound repair and even virus suppression and cancer. The ability of apoM to bind S1P is due to a lipophilic binding pocket within the lipocalin structure of the apoM molecule. Mice overexpressing apoM have increased plasma S1P concentrations, whereas apoM-deficient mice have decreased S1P levels. ApoM-S1P is able to activate the S1P-receptor-1, affecting the function of endothelial cells, and apoM-deficient mice display impaired endothelial permeability in the lung. This review will focus on the putative biological roles of the new apoM–S1P axis in relation to lipoprotein metabolism, lipid disorders and atherosclerosis.

Altered activation of endothelial anti- and proapoptotic pathways by high-density lipoprotein from patients with coronary artery disease: role of high-density lipoprotein-proteome remodeling

BACKGROUND

Endothelial dysfunction and injury are thought to play an important role in the progression of coronary artery disease (CAD). High-density lipoprotein from healthy subjects (HDL(Healthy)) has been proposed to exert endothelial antiapoptotic effects that may represent an important antiatherogenic property of the lipoprotein. The present study therefore aimed to compare effects of HDL(CAD) and HDL(Healthy) on the activation of endothelial anti- and proapoptotic pathways and to determine which changes of the lipoprotein are relevant for these processes.

METHODS AND RESULTS

HDL was isolated from patients with stable CAD (HDL(sCAD)), an acute coronary syndrome (HDL(ACS)), and healthy subjects. HDL(Healthy) induced expression of the endothelial antiapoptotic Bcl-2 protein Bcl-xL and reduced endothelial cell apoptosis in vitro and in apolipoprotein E-deficient mice in vivo. In contrast, HDL(sCAD) and HDL(ACS) did not inhibit endothelial apoptosis, failed to activate endothelial Bcl-xL, and stimulated endothelial proapoptotic pathways, in particular, p38-mitogen-activated protein kinase-mediated activation of the proapoptotic Bcl-2 protein tBid. Endothelial antiapoptotic effects of HDL(Healthy) were observed after inhibition of endothelial nitric oxide synthase and after delipidation, but not completely mimicked by apolipoprotein A-I or reconstituted HDL, suggesting an important role of the HDL proteome. HDL proteomics analyses and subsequent validations and functional characterizations suggested a reduced clusterin and increased apolipoprotein C-III content of HDL(sCAD) and HDL(ACS) as mechanisms leading to altered effects on endothelial apoptosis.

CONCLUSIONS

The present study demonstrates for the first time that HDL(CAD) does not activate endothelial antiapoptotic pathways, but rather stimulates potential endothelial proapoptotic pathways. HDL-proteome remodeling plays an important role for these altered functional properties of HDL. These findings provide novel insights into mechanisms leading to altered vascular effects of HDL in coronary disease.

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.