Anti-atherogenic actions of high-density lipoprotein through sphingosine 1-phosphate receptors and scavenger receptor class B type I

Potential of Lipoprotein-Based Nanoparticulate Formulations for the Treatment of Eye Diseases

Lipoproteins are naturally occurring nanoparticles and their main physiological function is the promotion of lipid metabolism. They can be prepared in vitro for use as drug carriers, and these reconstituted lipoproteins show similar biological activity to their natural counterparts. Some lipoproteins can cross the blood-retinal barrier and are involved in intraocular lipid metabolism. Drug-loaded lipoproteins can be delivered to the retina for the treatment of posterior eye diseases. In this review, we have discussed the therapeutic applications of lipoproteins for eye diseases and introduced the emerging animal models used for the evaluation of their therapeutic effects.

Endurance training selectively increases high-density lipoprotein-bound sphingosine-1-phosphate in the plasma

Sphingosine-1-phosphate (S1P) is a bioactive lysosphingolipid that is found in relatively high concentration in human plasma. Erythrocytes, endothelial cells, and activated platelets are the main sources of circulating S1P. The majority of plasma S1P is transported bound to high-density lipoprotein (HDL) and albumin. In recent years, HDL-bound S1P attracted much attention due to its cardioprotective and anti-atherogenic properties. We have previously found that endurance-trained athletes are characterized by higher plasma S1P concentration compared to untrained individuals. This finding prompted us to examine the effect of endurance training on S1P metabolism in blood. Thirteen healthy, untrained, male subjects completed an 8-week training program on a rowing ergometer. Three days before the first, and 3 days after the last training session, blood samples were drawn from an antecubital vein. We found that total plasma S1P concentration was increased after the training. Further analysis of different plasma fractions showed that the training selectively elevated HDL-bound S1P. This effect was associated with activation of sphingosine kinase in erythrocytes and platelets and enhanced S1P release from red blood cells. We postulate that increase in HDL-bound S1P level is one of the mechanisms underlying beneficial effects of regular physical activity on cardiovascular diseases.

S1P in HDL promotes interaction between SR-BI and S1PR1 and activates S1PR1-mediated biological functions: calcium flux and S1PR1 internalization

HDL normally transports about 50-70% of plasma sphingosine 1-phosphate (S1P), and the S1P in HDL reportedly mediates several HDL-associated biological effects and signaling pathways. The HDL receptor, SR-BI, as well as the cell surface receptors for S1P (S1PRs) may be involved partially and/or completely in these HDL-induced processes. Here we investigate the nature of the HDL-stimulated interaction between the HDL receptor, SR-BI, and S1PR1 using a protein-fragment complementation assay and confocal microscopy. In both primary rat aortic vascular smooth muscle cells and HEK293 cells, the S1P content in HDL particles increased intracellular calcium concentration, which was mediated by S1PR1. Mechanistic studies performed in HEK293 cells showed that incubation of cells with HDL led to an increase in the physical interaction between the SR-BI and S1PR1 receptors that mainly occurred on the plasma membrane. Model recombinant HDL (rHDL) particles formed in vitro with S1P incorporated into the particle initiated the internalization of S1PR1, whereas rHDL without supplemented S1P did not, suggesting that S1P transported in HDL can selectively activate S1PR1. In conclusion, these data suggest that S1P in HDL stimulates the transient interaction between SR-BI and S1PRs that can activate S1PRs and induce an elevation in intracellular calcium concentration.

The effect of sphingosine-1-phosphate on bone metabolism in humans depends on its plasma/bone marrow gradient

BACKGROUND

Although recent studies provide clinical evidence that sphingosine-1-phosphate (S1P) may primarily affect bone resorption in humans, rather than bone formation or the osteoclast-osteoblast coupling phenomenon, those studies could not determine which bone resorption mechanism is more important, i.e., chemorepulsion of osteoclast precursors via the blood to bone marrow S1P gradient or receptor activator of NF-κB ligand (RANKL) elevation in osteoblasts via local S1P.

AIM

To investigate how S1P mainly contributes to increased bone resorption in humans, we performed this case-control study at a clinical unit in Korea.

METHODS

Blood and bone marrow samples were contemporaneously collected from 70 patients who underwent hip surgery due to either osteoporotic hip fracture (HF) (n = 10) or other causes such as osteoarthritis (n = 60).

RESULTS

After adjusting for sex, age, BMI, smoking, alcohol, previous fracture, diabetes, and stroke, subjects with osteoporotic HF demonstrated a 3.2-fold higher plasma/bone marrow S1P ratio than those without HF, whereas plasma and bone marrow S1P levels were not significantly different between these groups. Consistently, the risk of osteoporotic HF increased 1.38-fold per increment in the plasma/bone marrow S1P ratio in a multivariate adjustment model. However, the odds ratios for prevalent HF according to the increment in the plasma and bone marrow S1P level were not statistically significant.

CONCLUSION

Our current results using simultaneously collected blood and bone marrow samples suggest that the detrimental effects of S1P on bone metabolism in humans may depend on the S1P gradient between the peripheral blood and bone marrow cavity.

Individual variation of human S1P₁ coding sequence leads to heterogeneity in receptor function and drug interactions

Sphingosine 1-phosphate receptor 1 (S1P₁), an abundantly-expressed G protein-coupled receptor which regulates key vascular and immune responses, is a therapeutic target in autoimmune diseases. Fingolimod/Gilenya (FTY720), an oral medication for relapsing-remitting multiple sclerosis, targets S1P₁ receptors on immune and neural cells to suppress neuroinflammation. However, suppression of endothelial S1P₁ receptors is associated with cardiac and vascular adverse effects. Here we report the genetic variations of the S1P₁ coding region from exon sequencing of >12,000 individuals and their functional consequences. We conducted functional analyses of 14 nonsynonymous single nucleotide polymorphisms (SNPs) of the S1PR1 gene. One SNP mutant (Arg¹²⁰ to Pro) failed to transmit sphingosine 1-phosphate (S1P)-induced intracellular signals such as calcium increase and activation of p44/42 MAPK and Akt. Two other mutants (Ile⁴⁵ to Thr and Gly³⁰⁵ to Cys) showed normal intracellular signals but impaired S1P-induced endocytosis, which made the receptor resistant to FTY720-induced degradation. Another SNP mutant (Arg¹³ to Gly) demonstrated protection from coronary artery disease in a high cardiovascular risk population. Individuals with this mutation showed a significantly lower percentage of multi-vessel coronary obstruction in a risk factor-matched case-control study. This study suggests that individual genetic variations of S1P₁ can influence receptor function and, therefore, infer differential disease risks and interaction with S1P₁-targeted therapeutics.

High density lipoprotein stimulated migration of macrophages depends on the scavenger receptor class B, type I, PDZK1 and Akt1 and is blocked by sphingosine 1 phosphate receptor antagonists

HDL carries biologically active lipids such as sphingosine-1-phosphate (S1P) and stimulates a variety of cell signaling pathways in diverse cell types, which may contribute to its ability to protect against atherosclerosis. HDL and sphingosine-1-phosphate receptor agonists, FTY720 and SEW2871 triggered macrophage migration. HDL-, but not FTY720-stimulated migration was inhibited by an antibody against the HDL receptor, SR-BI, and an inhibitor of SR-BI mediated lipid transfer. HDL and FTY720-stimulated migration was also inhibited in macrophages lacking either SR-BI or PDZK1, an adaptor protein that binds to SR-BI's C-terminal cytoplasmic tail. Migration in response to HDL and S1P receptor agonists was inhibited by treatment of macrophages with sphingosine-1-phosphate receptor type 1 (S1PR1) antagonists and by pertussis toxin. S1PR1 activates signaling pathways including PI3K-Akt, PKC, p38 MAPK, ERK1/2 and Rho kinases. Using selective inhibitors or macrophages from gene targeted mice, we demonstrated the involvement of each of these pathways in HDL-dependent macrophage migration. These data suggest that HDL stimulates the migration of macrophages in a manner that requires the activities of the HDL receptor SR-BI as well as S1PR1 activity.

Sphingosine 1-phosphate induced anti-atherogenic and atheroprotective M2 macrophage polarization through IL-4

Sphingosine 1-phosphate (S1P) has been implicated in anti-atherogenic properties of high-density lipoproteins. However, the roles and signaling of S1P in macrophages, the main contributor to atherosclerosis, have not been well studied. Furthermore, pro-inflammatory M1 and anti-inflammatory M2 macrophage phenotypes may influence the development of atherosclerosis. Therefore, we investigated the effects of S1P on macrophage phenotypes, especially on M2 polarization and its signaling in relation to the anti-atherogenic properties of S1P. It was found that S1P induced anti-inflammatory M2 polarization via IL-4 secretion and its signaling, and induced IL-4Rα and IL-2Rγ. In addition, down-stream signalings, such as, stat-6 phosphorylation, SOCS1 induction, and SOCS3 suppression were also observed in macrophages in response to S1P. Furthermore, S1P-induced ERK activation, and the inhibitions of p38 MAPK and JNK were found to be key signals for IL-4 induction. Moreover, the anti-atherogenic effect of S1P in HDL was confirmed by the observation that oxidized LDL-induced lipid accumulation was attenuated in S1P-treated M2 macrophages. Furthermore, the atheroprotective effect of S1P was demonstrated by its anti-apoptotic effect on S1P-treated macrophages. The present study shows that S1P-induced M2 polarization of macrophages could be mediated via IL-4 signaling, and suggests that M2 polarization by S1P is responsible for the anti-atherogenic and atheroprotective properties of high-density lipoproteins in vivo.

High-density lipoprotein 3 and apolipoprotein A-I alleviate platelet storage lesion and release of platelet extracellular vesicles

BACKGROUND

Stored platelet (PLT) concentrates (PLCs) for transfusion develop a PLT storage lesion (PSL), decreasing PLT viability and function with profound lipidomic changes and PLT extracellular vesicle (PL-EV) release. High-density lipoprotein 3 (HDL3 ) improves PLT homeostasis through silencing effects on PLT activation in vivo. This prompted us to investigate HDL3 and apolipoprotein A-I (apoA-I) as PSL-antagonizing agents.

STUDY DESIGN AND METHODS

Healthy donor PLCs were split into low-volume standard PLC storage bags and incubated with native (n)HDL3 or apoA-I from plasma ethanol fractionation (precipitate IV) for 5 days under standard blood banking conditions. Flow cytometry, Born aggregometry, and lipid mass spectrometry were carried out to analyze PL-EV release, PLT aggregation, agonist-induced PLT surface marker expression, and PLT and plasma lipid compositions.

RESULTS

Compared to control, added nHDL3 and apoA-I significantly reduced PL-EV release by up to -62% during 5 days, correlating with the added apoA-I concentration. At the lipid level, nHDL3 and apoA-I antagonized PLT lipid loss (+12%) and decreased cholesteryl ester (CE)/free cholesterol (FC) ratios (-69%), whereas in plasma polyunsaturated/saturated CE ratios increased (+3%) and CE 16:0/20:4 ratios decreased (-5%). Administration of nHDL3 increased PLT bis(monoacylglycero)phosphate/phosphatidylglycerol (+102%) and phosphatidic acid/lysophosphatidic acid (+255%) ratios and improved thrombin receptor-activating peptide 6-induced PLT aggregation (+5%).

CONCLUSION

nHDL3 and apoA-I improve PLT membrane homeostasis and intracellular lipid processing and increase CE efflux, antagonizing PSL-related reduction in PLT viability and function and PL-EV release. We suggest uptake and catabolism of nHDL3 into the PLT open canalicular system. As supplement in PLCs, nHDL3 or apoA-I from Fraction IV of plasma ethanol fractionation have the potential to improve PLC quality to prolong storage.

HDL-cholesterol in coronary artery disease risk: function or structure?

High-density lipoproteins (HDL) are inversely related with coronary artery disease (CAD) and HDL-cholesterol is the only standardized and reproducible parameter available to estimate plasma concentration of these lipoproteins. However, pharmacological interventions intended to increase HDL-cholesterol have not been consistently associated to an effective CAD risk reduction. Among patients with a myocardial infarction, 43 and 44% of men and women, respectively, had normal plasma levels of HDL-cholesterol, whereas genetic studies have failed to show a causal association between HDL-cholesterol and CAD risk. Instead, HDL functionality seems to be the target to be evaluated, but the existing methods are still poorly reproducible and far to be adapted to the clinical laboratory. HDL subclasses rise as a potential alternative for the evaluation of CAD risk; HDL subclasses are a surrogate of intravascular metabolism of these lipoproteins and probably of their functionality. Low levels of large HDL and increased proportions of small particles are the most remarkable features associated to an increased risk of type 2 diabetes mellitus (T2DM) or CAD. However, inflammation and other environmental factors are related with abnormal HDL structure, and, as a consequence, more prospective studies are needed to better support the clinical usefulness of HDL subclasses. New insights from proteome and lipidome profiles of HDL will provide potential HDL-related biomarkers in the coming years.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Liver involvement in sphingosine 1-phosphate dynamism revealed by adenoviral hepatic overexpression of apolipoprotein M

OBJECTIVES

Sphingosine 1-phosphate (S1P) is a vasoprotective lipid mediator that is mainly carried on HDL in the circulation and several anti-atherosclerotic properties of HDL is considered to be ascribed to S1P. Since S1P riding on HDL was recently shown to bind to apolipoprotein M (apoM), which is derived from liver, we analyzed the possible involvement of liver in S1P metabolism.

METHODS AND RESULTS

Using adenoviruses, we overexpressed apoM in HepG2 cells and mice livers and found that both the medium/plasma and cell/liver S1P contents increased. Among lipoprotein subclasses, S1P contents increased mainly in HDL fractions. On the other hand, hepatectomy resulted in the reduction of plasma S1P levels in mice. The incubation of S1P in the conditional medium of apoM-overexpressing HepG2 cells interfered with S1P degradation. Furthermore, adenoviral hepatic overexpression of apoM resulted in increase in the S1P level of plasma but not of blood cells, while combination of hepatic apoM overexpression and intraperitoneal administration of C₁₇-sphingosine resulted in the increase in the C₁₇-S1P level both in livers and in plasma, but again not in blood cells.

CONCLUSIONS

Livers are involved in S1P dynamism, and it was suggested that apoM, produced from livers, increases circulating plasma S1P by augmenting the S1P output from livers and modifies extracellular S1P metabolism.

ApoA-IV promotes the biogenesis of apoA-IV-containing HDL particles with the participation of ABCA1 and LCAT

The objective of this study was to establish the role of apoA-IV, ABCA1, and LCAT in the biogenesis of apoA-IV-containing HDL (HDL-A-IV) using different mouse models. Adenovirus-mediated gene transfer of apoA-IV in apoA-I(-/-) mice did not change plasma lipid levels. ApoA-IV floated in the HDL2/HDL3 region, promoted the formation of spherical HDL particles as determined by electron microscopy, and generated mostly α- and a few pre-β-like HDL subpopulations. Gene transfer of apoA-IV in apoA-I(-/-) × apoE(-/-) mice increased plasma cholesterol and triglyceride levels, and 80% of the protein was distributed in the VLDL/IDL/LDL region. This treatment likewise generated α- and pre-β-like HDL subpopulations. Spherical and α-migrating HDL particles were not detectable following gene transfer of apoA-IV in ABCA1(-/-) or LCAT(-/-) mice. Coexpression of apoA-IV and LCAT in apoA-I(-/-) mice restored the formation of HDL-A-IV. Lipid-free apoA-IV and reconstituted HDL-A-IV promoted ABCA1 and scavenger receptor BI (SR-BI)-mediated cholesterol efflux, respectively, as efficiently as apoA-I and apoE. Our findings are consistent with a novel function of apoA-IV in the biogenesis of discrete HDL-A-IV particles with the participation of ABCA1 and LCAT, and may explain previously reported anti-inflammatory and atheroprotective properties of apoA-IV.

HDL drug carriers for targeted therapy

Plasma concentrations of high-density lipoprotein cholesterol (HDL-C) are strongly and inversely associated with cardiovascular risk. HDL is not a simple lipid transporter, but possesses multiple anti-atherosclerosis activities because it contains special proteins, signaling lipid, and microRNAs. Natural or recombinant HDLs have emerged as potential carriers for delivering a drug to a specified target. However, HDL function also depends on enzymes that alter its structure and composition, as well as cellular receptors and membrane micro-domains that facilitate interactions with the microenvironment. In this review, four mechanisms predicted to enhance functions or targeted therapy of HDL in vivo are discussed. The first involves caveolae-mediated recruitment of HDL signal to bind their receptors. The second involves scavenger receptor class B type I (SR-BI) mediating anchoring and fluidity for signal-lipid of HDL. The third involves lecithin-cholesterol acyltransferase (LCAT) concentrating the signaling lipid at the surface of the HDL particle. The fourth involves microRNAs (miRNAs) being delivered in the blood to special targets by HDL. Exploitation of these four mechanisms will promote HDL to carry targeted drugs and increase HDL's clinical value.

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.

Assessment of sphingosine-1-phosphate activity in biological samples by receptor internalization and adherens junction formation

Sphingosine-1-phosphate (S1P) is a bioactive lipid mediator involved in many biological actions, including vascular homeostasis and immune cell trafficking. S1P activity is mediated by specific G protein-coupled receptors, leading to multiple physiological responses including adherens junction formation in endothelial cells. Here, we describe bioassays for rapidly assessing S1P activity in biological fluids based on ligand-induced receptor internalization in transfected HEK293 cells and consequent adherens junction formation of vascular endothelial cells.

Exercise increases plasma levels of sphingoid base-1 phosphates in humans

AIM

In recent years, blood sphingolipids attracted much attention and have been implicated in both pathophysiology and prevention of cardiovascular diseases and insulin resistance. However, factors affecting concentration and metabolism of sphingolipids in blood remain poorly recognized. We have previously found that exercise alters skeletal muscle sphingolipid metabolism. This finding prompted us to examine whether physical activity induces similar effects in blood.

METHODS

Twenty healthy male patients were assigned to either untrained (UT, n = 10) or endurance trained (ET, n = 10) group. The patients performed either a 30 (UT group) or 60 (ET group) min exercise on a cycloergometer at a workload corresponding to 70% of VO(2max) . Blood samples were taken just before exercise, after 30 and 60 (ET group only) min of pedalling and following a 30-min rest.

RESULTS

ET patients were characterized by higher basal plasma sphingosine-1-phosphate (S1P) concentration and decreased content of sphingosine, S1P, sphinganine-1-phosphate (SA1P) and ceramide in erythrocytes. In ET group, plasma concentrations of all measured sphingolipids remained stable both during and after exercise. On the other hand, in UT patients, the post-exercise levels of S1P and SA1P were markedly higher compared with the baseline values and this effect was accompanied by decreased erythrocyte ceramide content.

CONCLUSION

It is likely that single bout of exercise and endurance training enhances production and release of S1P by erythrocytes. We speculate that exercise-induced increase in plasma S1P concentration might be one of the mechanisms underlying beneficial effects of physical activity on cardiovascular health and insulin sensitivity.

Sphingosine 1-phosphate in coagulation and inflammation

Sphingosine 1-phosphate (S1P) is a lipid mediator produced from sphingomyelin by the sequential enzymatic actions of sphingomyelinase, ceramidase, and sphingosine kinase. Five subtypes of cell surface G-protein-coupled receptors, S1P(1-5), mediate the actions of S1P in various organs systems, most notably cardiovascular, immune, and central nervous systems. S1P is enriched in blood and lymph but is present at much lower concentrations in interstitial fluids of tissues. This vascular S1P gradient is important for the regulation of trafficking of various immune cells. FTY720, which was recently approved for the treatment of relapsing-remitting multiple sclerosis, potently sequesters lymphocytes into lymph nodes by functionally antagonizing the activity of the S1P(1) receptor. S1P also plays critical roles in the vascular barrier integrity, thereby regulating inflammation, tumor metastasis, angiogenesis, and atherosclerosis. Recent studies have also revealed the involvement of S1P signaling in coagulation and in tumor necrosis factor α-mediated signaling. This review highlights the importance of S1P signaling in these inflammatory processes as well as the contribution of each receptor subtype, which exhibits both cooperative and redundant functions.

HDL and sphingosine-1-phosphate activate stat3 in prostate cancer DU145 cells via ERK1/2 and S1P receptors, and promote cell migration and invasion

BACKGROUND

Androgen deprivation therapy in men with prostate cancer leads to a significant increase of high density lipoprotein (HDL), but the effect of HDL on prostate cancer is unknown. Recently, HDL, which transports sphingosine-1-phosphate (S1P), was reported to activate signal transducer and activator of transcription 3 (Stat3) in cardiomyocytes. In this study, we examined the effect of HDL and S1P on Stat3 activation in prostate cancer cells and the involvement of S1P receptors in this process in three prostate cancer cell lines (PC-3, LNCaP, and DU145).

METHODS

Discordial reconstituted(r) HDL containing POPC, apoA-1, and S1P were prepared by the cholate dialysis method. The phosphorylations of Stat3, ERK1/2, and Akt were detected by Western blotting. Cell migration and invasion were determined by wound-healing assay and matrigel invasion chamber assay.

RESULTS

HDL increased serine 727 phosphorylation of Stat3, but not tyrosine 705 only in DU145 cells. S1P and rHDL-S1P also induced the phosphorylation, but not rHDL without S1P. They also induced DU145 cells migration and invasion. PD98059, a MEK inhibitor, and pertussis toxin, a Gi inhibitor, attenuated HDL-, S1P-, and rHDL-S1P-induced Stat3 phosphorylation, whereas LY294002, a PI3K inhibitor, had no effect. Concerning S1P receptors, S1P1 expression was much lower than S1P2 and S1P3 in DU145 cells. Both JTE013, a S1P2 antagonist, and VPC23019, a S1P1/S1P3 antagonist, attenuated HDL-, S1P-, and rHDL-S1P-induced Stat3 phosphorylations and cell migrations.

CONCLUSIONS

These results suggest that the change in HDL plasma levels by androgen deprivation therapy may alter prostate cancer growth and metastasis.

Negatively cooperative binding of high-density lipoprotein to the HDL receptor SR-BI

Scavenger receptor class B, type I (SR-BI), is a high-density lipoprotein (HDL) receptor, which also binds low-density lipoprotein (LDL), and mediates the cellular selective uptake of cholesteryl esters from lipoproteins. SR-BI also is a coreceptor for hepatitis C virus and a signaling receptor that regulates cell metabolism. Many investigators have reported that lipoproteins bind to SR-BI via a single class of independent (not interacting), high-affinity binding sites (one site model). We have reinvestigated the ligand concentration dependence of (125)I-HDL binding to SR-BI and SR-BI-mediated specific uptake of [(3)H]CE from [(3)H]CE-HDL using an expanded range of ligand concentrations (<1 μg of protein/mL, lower than previously reported). Scatchard and nonlinear least-squares model fitting analyses of the binding and uptake data were both inconsistent with a single class of independent binding sites binding univalent lipoprotein ligands. The data are best fit by models in which SR-BI has either two independent classes of binding sites or one class of sites exhibiting negative cooperativity due to either classic allostery or ensemble effects ("lattice model"). Similar results were observed for LDL. Application of the "infinite dilution" dissociation rate method established that the binding of (125)I-HDL to SR-BI at 4 °C exhibits negative cooperativity. The unexpected complexity of the interactions of lipoproteins with SR-BI should be taken into account when interpreting the results of experiments that explore the mechanism(s) by which SR-BI mediates ligand binding, lipid transport, and cell signaling.

Negatively cooperative binding of high-density lipoprotein to the HDL receptor SR-BI

Scavenger receptor class B, type I (SR-BI), is a high-density lipoprotein (HDL) receptor, which also binds low-density lipoprotein (LDL), and mediates the cellular selective uptake of cholesteryl esters from lipoproteins. SR-BI also is a coreceptor for hepatitis C virus and a signaling receptor that regulates cell metabolism. Many investigators have reported that lipoproteins bind to SR-BI via a single class of independent (not interacting), high-affinity binding sites (one site model). We have reinvestigated the ligand concentration dependence of (125)I-HDL binding to SR-BI and SR-BI-mediated specific uptake of [(3)H]CE from [(3)H]CE-HDL using an expanded range of ligand concentrations (<1 μg of protein/mL, lower than previously reported). Scatchard and nonlinear least-squares model fitting analyses of the binding and uptake data were both inconsistent with a single class of independent binding sites binding univalent lipoprotein ligands. The data are best fit by models in which SR-BI has either two independent classes of binding sites or one class of sites exhibiting negative cooperativity due to either classic allostery or ensemble effects ("lattice model"). Similar results were observed for LDL. Application of the "infinite dilution" dissociation rate method established that the binding of (125)I-HDL to SR-BI at 4 °C exhibits negative cooperativity. The unexpected complexity of the interactions of lipoproteins with SR-BI should be taken into account when interpreting the results of experiments that explore the mechanism(s) by which SR-BI mediates ligand binding, lipid transport, and cell signaling.

Unmasking of LPA1 receptor-mediated migration response to lysophosphatidic acid by interleukin-1β-induced attenuation of Rho signaling pathways in rat astrocytes

Action mechanism of lipopolysaccharide (LPS), interleukin-1β (IL-1β), and lysophosphatidic acid (LPA) to regulate motility, an important process of astrogliosis, was investigated in rat astrocytes. While LPA exerted no significant effect on the cell migration, the prior treatment of the cells with LPS or IL-1β resulted in the appearance of migration activity in response to LPA. The LPS induction of the migration response to LPA was associated with the production of IL-1β precursor protein and inhibited by the IL-1 receptor antagonist. The IL-1β treatment also allowed LPA to activate Rac1. The LPA-induced Rac1 activation and migration were inhibited by pertussis toxin, a small interfering RNA specific to LPA(1) receptors, and LPA(1) receptor antagonists, including Ki16425. However, the IL-1β treatment had no appreciable effect on LPA(1) receptor mRNA expression and LPA-induced activation of ERK, Akt, and proliferation. The induction of the migration response to LPA by IL-1β was inhibited by a constitutively active RhoA. Moreover, LPA significantly activated RhoA through the LPA(1) receptor in the control cells but not in the IL-1β-treated cells. These results suggest that IL-1β inhibits the LPA(1) receptor-mediated Rho signaling through the IL-1 receptor, thereby disclosing the LPA(1) receptor-mediated G(i) protein/Rac/migration pathway.

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.

Potentially modifiable factors affecting the progression of autosomal dominant polycystic kidney disease

BACKGROUND AND OBJECTIVES

The Consortium for Radiologic Imaging Studies of Polycystic Kidney Disease (CRISP) was created to identify markers of disease progression in patients with autosomal dominant polycystic kidney disease (ADPKD).

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Linear mixed models were utilized to model effects of baseline parameters on changes in natural-log (ln)-transformed total kidney volume (TKV) and iothalamate clearance (GFR) across time in CRISP participants (creatinine clearance at entry >70 ml/min). Stepwise selection was used to obtain a final main effect model.

RESULTS

TKV increased from year to year, whereas GFR uncorrected for body surface area (BSA) decreased only at year 6. Higher lnTKV and urine sodium excretion (U(Na)V), lower serum HDL-cholesterol, and younger age at baseline associated with greater lnTKV growth from baseline to year 3 and to year 6. Higher lnTKV at baseline associated with greater GFR decline from year 1 to year 3 and to year 6. Higher BSA and 24-hour urine osmolality at baseline associated with greater GFR decline from year 1 to year 6. Higher U(Na)V and lower serum HDL-cholesterol at baseline associated with greater GFR decline from year 1 to year 6 by univariate analysis only. Associations seen during year 1 to year 6 (not seen during year 1 to year 3) reflect the time lag between structural and functional disease progression.

CONCLUSIONS

Serum HDL-cholesterol, U(Na)V, and 24-hour urine osmolality likely affect ADPKD progression. To what extent their modification may influence the clinical course of ADPKD remains to be determined.

High-density lipoprotein induces proliferation and migration of human prostate androgen-independent cancer cells by an ABCA1-dependent mechanism

Androgen deprivation therapy for prostate cancer leads to a significant increase of high-density lipoprotein (HDL), which is generally viewed as beneficial, particularly for cardiovascular disease, but the effect of HDL on prostate cancer is unknown. In this study, we investigated the effect of HDL on prostate cancer cell proliferation, migration, intracellular cholesterol levels, and the role of cholesterol transporters, namely ABCA1, ABCG1, and SR-BI in these processes. HDL induced cell proliferation and migration of the androgen-independent PC-3 and DU145 cells by a mechanism involving extracellular signal-regulated kinase (ERK) 1/2 and Akt, but had no effect on the androgen-dependent LNCaP cell, which did not express ABCA1 unlike the other cell lines. Treatment with HDL did not significantly alter the cholesterol content of the cell lines. Knockdown of ABCA1 but not ABCG1 or SR-BI by small interfering RNA (siRNA) inhibited HDL-induced cell proliferation, migration, and ERK1/2 and Akt signal transduction in PC-3 cells. Moreover, after treatment of LNCaP cells with charcoal-stripped fetal bovine serum, ABCA1 was induced ∼10-fold, enabling HDL to induce ERK1/2 activation, whereas small interfering RNA knockdown of ABCA1 inhibited HDL-induced ERK1/2 activation. Simvastatin, which inhibited ABCA1 expression in PC-3 and DU145 cells, attenuated HDL-induced PC-3 and DU145 cell proliferation, migration, and ERK1/2 and Akt phosphorylation. In human prostate biopsy samples, ABCA1 mRNA expression was ∼2-fold higher in the androgen deprivation therapy group than in subjects with benign prostatic hyperplasia or pretreatment prostate cancer groups. In summary, these results suggest that HDL by an ABCA1-dependent mechanism can mediate signal transduction, leading to increased proliferation and migration of prostate cancer cells.

Mechanism and role of high density lipoprotein-induced activation of AMP-activated protein kinase in endothelial cells

The upstream signaling pathway leading to the activation of AMP-activated protein kinase (AMPK) by high density lipoprotein (HDL) and the role of AMPK in HDL-induced antiatherogenic actions were investigated. Experiments using genetic and pharmacological tools showed that HDL-induced activation of AMPK is dependent on both sphingosine 1-phosphate receptors and scavenger receptor class B type I through calcium/calmodulin-dependent protein kinase kinase and, for scavenger receptor class B type I system, additionally serine-threonine kinase LKB1 in human umbilical vein endothelial cells. HDL-induced activation of Akt and endothelial NO synthase, stimulation of migration, and inhibition of monocyte adhesion and adhesion molecule expression were dependent on AMPK activation. The inhibitory role of AMPK in the adhesion molecule expression and monocyte adhesion on endothelium of mouse aorta was confirmed in vivo and ex vivo. On the other hand, stimulation of ERK and proliferation were hardly affected by AMPK knockdown but completely inhibited by an N17Ras, whereas the dominant-negative Ras was ineffective for AMPK activation. In conclusion, dual HDL receptor systems differentially regulate AMPK activity through calcium/calmodulin-dependent protein kinase kinase and/or LKB1. Several HDL-induced antiatherogenic actions are regulated by AMPK, but proliferation-related actions are regulated by Ras rather than AMPK.

Signaling pathways involved in DNA synthesis and migration in response to lysophosphatidic acid and low-density lipoprotein in coronary artery smooth muscle cells

Low-density lipoprotein (LDL) and lysophosphatidic acid (LPA), one of the lipid components of lipoprotein, induced the DNA synthesis of coronary artery smooth muscle cells (CASMCs). The LDL- and LPA-induced DNA synthesis was markedly inhibited by the LPA receptor antagonist Ki16425, pertussis toxin, small interfering RNAs targeted for LPA1 receptors, and a potent calcineurin inhibitor cyclosporine A. It has been reported that LDL and LPA induced a migration response in a manner sensitive to Ki16425, pertussis toxin, and a LPA1 receptor-specific small interfering RNA. However, cyclosporine A was ineffective in inhibiting the migration response. Instead, an epidermal growth factor (EGF) receptor tyrosine kinase inhibitor markedly suppressed the migration response to LDL and LPA without having any significant effect on DNA synthesis. Thus, the LDL-induced stimulation of DNA synthesis and migration in CASMCs is mediated by its component LPA through LPA1 receptors and G(i/o)-proteins. Ca2+/calcineurin pathways and transactivation of EGF receptors mediate LPA1-receptor-induced DNA synthesis and migration, respectively.

Export and functions of sphingosine-1-phosphate

The sphingolipid metabolite, sphingosine-1-phosphate (S1P), has emerged as a critical player in a number of fundamental biological processes and is important in cancer, angiogenesis, wound healing, cardiovascular function, atherosclerosis, immunity and asthma, among others. Activation of sphingosine kinases, enzymes that catalyze the phosphorylation of sphingosine to S1P, by a variety of agonists, including growth factors, cytokines, hormones, and antigen, increases intracellular S1P. Many of the biological effects of S1P are mediated by its binding to five specific G protein-coupled receptors located on the cell surface in an autocrine and/or paracrine manner. Therefore, understanding the mechanism by which intracellularly generated S1P is released out of cells is both interesting and important. In this review, we will discuss how S1P is formed and released. We will focus particularly on the current knowledge of how the S1P gradient between tissues and blood is maintained, and the role of ABC transporters in S1P release.