Sphingosine kinase-dependent activation of endothelial nitric oxide synthase by angiotensin II

Exploring the potential effect of electroacupuncture on cardiovascular function and lipid profiles in spontaneously hypertensive rats

Background

Investigating the effects of electroacupuncture (EA) treatment on cardiovascular function and aortic lipid profiles in spontaneously hypertensive rats (SHR) constitutes the foundational focus of this study. The overarching goal is to comprehensively elucidate the alterations brought about by EA treatment and to assess its potential as an alternative therapy for hypertension.

Methods

Consecutive EA treatments were administered to SHR, and the effects on systolic blood pressure, cardiac function, and hypertension-related neuronal signals were assessed. Aortic lipid profiles in vehicle-treated SHR and EA-treated SHR groups were analyzed using mass spectrometry-based lipid profiling. Additionally, the expression of Cers2 and GNPAT, enzymes involved in the synthesis of specific aortic lipids, was examined.

Results

The study demonstrated that consecutive EA treatments restored systolic blood pressure, improved cardiovascular function, and normalized hypertension-related neuronal signals in SHR. Analysis of the aortic lipid profiles revealed distinct differences between the vehicle-treated SHR group and the EA-treated SHR group. Specifically, EA treatment significantly altered the levels of aortic sphingomyelin and phospholipids, including very long-chain fatty acyl-ceramides and ether phosphatidylcholines. These changes in aortic lipid profiles correlated significantly with systolic blood pressure and cardiac function indicators. Furthermore, EA treatment significantly altered the expression of Cers2 and GNPAT.

Conclusions

The findings suggest that EA may influence cardiovascular functions and aortic lipid profiles in SHR.

The Role of Obesity, Inflammation and Sphingolipids in the Development of an Abdominal Aortic Aneurysm

Abdominal aortic aneurysm (AAA) is a local dilatation of the vessel equal to or exceeding 3 cm. It is a disease with a long preclinical period commonly without any symptoms in its initial stage. Undiagnosed for years, aneurysm often leads to death due to vessel rupture. The basis of AAA pathogenesis is inflammation, which is often associated with the excess of adipose tissue, especially perivascular adipose tissue, which synthesizes adipocytokines that exert a significant influence on the formation of aneurysms. Pro-inflammatory cytokines such as resistin, leptin, and TNFα have been shown to induce changes leading to the formation of aneurysms, while adiponectin is the only known compound that is secreted by adipose tissue and limits the development of aneurysms. However, in obesity, adiponectin levels decline. Moreover, inflammation is associated with an increase in the amount of macrophages infiltrating adipose tissue, which are the source of matrix metalloproteinases (MMP) involved in the degradation of the extracellular matrix, which are an important factor in the formation of aneurysms. In addition, an excess of body fat is associated with altered sphingolipid metabolism. It has been shown that among sphingolipids, there are compounds that play an opposite role in the cell: ceramide is a pro-apoptotic compound that mediates the development of inflammation, while sphingosine-1-phosphate exerts pro-proliferative and anti-inflammatory effects. It has been shown that the increase in the level of ceramide is associated with a decrease in the concentration of adiponectin, an increase in the concentration of TNFα, MMP-9 and reactive oxygen species (which contribute to the apoptosis of vascular smooth muscle cell). The available data indicate a potential relationship between obesity, inflammation and disturbed sphingolipid metabolism with the formation of aneurysms; therefore, the aim of this study was to systematize the current knowledge on the role of these factors in the pathogenesis of abdominal aortic aneurysm.

Antihypertensive activity of oleanolic acid is mediated via downregulation of secretory phospholipase A2 and fatty acid synthase in spontaneously hypertensive rats

Oleanolic acid (OA) is reported to possess antihypertensive activity via the regulation of lipid metabolism; however, the mechanisms underlying lipid regulation by OA are yet to be fully elucidated. The aim of the present study was to evaluate the mechanisms via which OA regulates lipid metabolism in spontaneously hypertensive rats (SHRs) via ultra‑performance liquid chromatography‑quadrupole/Orbitrap‑mass spectrometry (MS)‑based lipidomics analysis. SHRs were treated with OA (1.08 mg/kg) for 4 weeks. The liver tissues were excised, homogenized in dichloromethane and centrifuged, and subsequently the supernatant layer was collected and concentrated under vacuum to dryness. The dichloromethane extract was subjected to MS analysis and database searching, and comparison of standards was performed to identify potential biomarkers. Partial least squares‑discriminant analysis performed on the liver lipidome revealed a total of 14 endogenous metabolites that were significantly changed in the SHR model group (SH group) compared with Wistar Kyoto rats [normal control (NC group)], including glycerophospholipids, sphingolipids and glycerides. Heatmaps revealed that the liver lipid profiles in the OA group were clustered more closely compared with those observed in the NC group, indicating that the antihypertensive effect of OA was mediated via regulation of liver lipid metabolites. It was observed that the protein levels of secretory phospholipase A2 (sPLA2) and fatty acid synthase (FAS) were increased in the SH group compared with the NC group. In addition, the levels of lysophosphatidylcholine and triglycerides in the liver were elevated, whereas the levels of low‑density lipoprotein cholesterol and high‑density lipoprotein cholesterol were reduced in the SH group. Upon treatment with OA, the mRNA and protein levels of PLA2 and FAS were observed to be downregulated. Collectively, the present study indicated that the antihypertensive activity of OA was mediated via downregulation of sPLA2 and FAS in SHRs, and that treatment with OA resulted in significant improvements in blood pressure and associated abnormalities in the lipid metabolites.

Intervention of Uncaria and Its Components on Liver Lipid Metabolism in Spontaneously Hypertensive Rats

Uncaria rhynchophylla (Miq.) Miq. ex Havil is widely used in the treatment of hypertension. The Uncaria extract and its bioactives, rhynchophylline and isorhynchophylline, reduced the blood pressure and fatty content in liver cells. In the present study, the antihypertensive effects of Uncaria ethanol extract (UET), rhynchophylline (RT) and isorhynchophylline (IT) were investigated in spontaneously hypertensive rats (SHR) using UPLC-Q-Orbitrap/MS based lipidomics approach. Histological changes in the liver were evaluated. Cytolysis and fatty degeneration in the liver tissues were observed in the SHR group. Lipid species in WKY, SHR treated with UET, RT, and IT were plotted to obtain the Orthogonal Projections to Latent Structures Discriminant Analysis (OPLS-DA) score plots. Fifty-six endogenous metabolites in the liver such as glycerides, glycerophospholipids, unsaturated fatty acids, and sphingomyelins were selected as potential hypertension associated biomarkers. In order to further explore the metabolite targets of UET for antihypertensive, student's t test and correlation analysis were performed to recognize the pattern recognition and to select the significant metabolites. Similar and prolonged reduction in blood pressure was observed in all SHR groups treated with UET, RT, and IT, while the metabolite profiles were perturbed slightly compared to that of the untreated SHR. Further analysis showed that only a few common components were observed in both RT and IT, which showed similar antihypertensive effect in spite of the distinct metabolic pathways. These results help in understanding the mechanisms of isomeric ingredients in exhibiting the antihypertensive effect but with different targets.

Role of dyslipidemia in preeclampsia-A review of lipidomic analysis of blood, placenta, syncytiotrophoblast microvesicles and umbilical cord artery from women with preeclampsia

Preeclampsia is a complex disorder and the pathogenesis of it is still not fully understood. The most commonly accepted theory of pathogenesis assumes that there occurs impaired trophoblastic invasion and failure in spiral artery remodeling. Nowadays, obesity becomes one of the most important, modifiable risk factors for the development of preeclampsia. Despite research into the condition, predicting which women with risk factors will develop preeclampsia remains problematic. Emerging evidence suggests that dysregulation of maternal and placental lipid metabolism are involved in the pathogenesis of the condition. Hence, researchers are focused on finding a lipid fingerprint, which contains information about the lipid composition and abundance of individual lipids by using new methods in the field of lipidomics. In this review we aimed to discuss the role of dyslipidemia in the pathogenesis of preeclampsia. In addition, on the basis of current research, we attempted to find a specific lipid profile of different tissues in women with preeclampsia.

Signaling pathway involved in the inhibitory effect of FTY720P on the Na+/K+ ATPase in HepG2 cells

The Na+/K+ ATPase modulates the activity of many transporters in the liver, and maintains the ionic constancy of the intracellular milieu, preserving thus normal functioning of hepatocytes. Previous work showed that FTY720P, a sphingosine one phosphate receptor agonist used in the treatment of multiple sclerosis, exerts in HepG2 cells, an inhibitory effect on the activity of the ATPase, mediated via PGE2. This study is an attempt to identify the signaling molecules involved downstream of the prostaglandin. The activity of the ATPase was assayed by measuring the amount of inorganic phosphate liberated in presence and absence of ouabain, a specific inhibitor of the enzyme. The effect of FTY720P and PGE2 disappeared completely in presence of PF-04418948, a blocker of EP2 receptors, RpcAMP, an inhibitor of PKA, PD98059, an inhibitor of ERK, as well as in presence of PTIO, a nitric oxide synthase inhibitor, but was mimicked by butaprost, an EP2 agonist, dbcAMP, a cell permeable cAMP analogue, and SNAP1,a nitric oxide generator. PGE2 and dbcAMP increased the expression of phosphorylated ERK but not total ERK. This increase did not appear however in presence of PTIO, indicating that PKA is upstream of NO. It was concluded that FTY 720P induces PGE2 release which activates NOS leading to NO production and ERK activation. ERK then inhibits directly or indirectly the Na+/K+ ATPase.

Small dense HDLs display potent vasorelaxing activity, reflecting their elevated content of sphingosine-1-phosphate

The functional heterogeneity of HDL is attributed to its diverse bioactive components. We evaluated whether the vasodilatory effects of HDL differed across HDL subpopulations, reflecting their distinct molecular composition. The capacity of five major HDL subfractions to counteract the inhibitory effects of oxidized LDL on acetylcholine-induced vasodilation was tested in a rabbit aortic rings model. NO production, an essential pathway in endothelium-dependent vasorelaxation, was studied in simian vacuolating virus 40-transformed murine endothelial cells (SVECs). Small dense HDL3 subfractions displayed potent vasorelaxing activity (up to +31% vs. baseline, P < 0.05); in contrast, large light HDL2 did not induce aortic-ring relaxation when compared on a total protein basis. HDL3 particles were enriched with sphingosine-1-phosphate (S1P) (up to 3-fold vs. HDL2), with the highest content in HDL3b and -3c that concomitantly revealed the strongest vasorelaxing properties. NO generation was enhanced by HDL3c in SVECs (1.5-fold, P < 0.01), a phenomenon that was blocked by the S1P receptor antagonist, VPC 23019. S1P-enriched reconstituted HDL (rHDL) was a 1.8-fold (P < 0.01) more potent vasorelaxant than control rHDL in aortic rings. Small dense HDL3 particles displayed potent protective effects against oxidative stress-associated endothelium dysfunction, potentially reflecting their elevated content of S1P that might facilitate interaction with S1P receptors and ensuing NO generation.

Vascular transcriptome profiling identifies Sphingosine kinase 1 as a modulator of angiotensin II-induced vascular dysfunction

Vascular dysfunction is an important phenomenon in hypertension. We hypothesized that angiotensin II (AngII) affects transcriptome in the vasculature in a region-specific manner, which may help to identify genes related to vascular dysfunction in AngII-induced hypertension. Mesenteric artery and aortic transcriptome was profiled using Illumina WG-6v2.0 chip in control and AngII infused (490 ng/kg/min) hypertensive mice. Gene set enrichment and leading edge analyses identified Sphingosine kinase 1 (Sphk1) in the highest number of pathways affected by AngII. Sphk1 mRNA, protein and activity were up-regulated in the hypertensive vasculature. Chronic sphingosine-1-phosphate (S1P) infusion resulted in a development of significantly increased vasoconstriction and endothelial dysfunction. AngII-induced hypertension was blunted in Sphk1^-/- mice (systolic BP 167 ± 4.2 vs. 180 ± 3.3 mmHg, p < 0.05), which was associated with decreased aortic and mesenteric vasoconstriction in hypertensive Sphk1^-/- mice. Pharmacological inhibition of S1P synthesis reduced vasoconstriction of mesenteric arteries. While Sphk1 is important in mediating vasoconstriction in hypertension, Sphk1^-/- mice were characterized by enhanced endothelial dysfunction, suggesting a local protective role of Sphk1 in the endothelium. S1P serum level in humans was correlated with endothelial function (arterial tonometry). Thus, vascular transcriptome analysis shows that S1P pathway is critical in the regulation of vascular function in AngII-induced hypertension, although Sphk1 may have opposing roles in the regulation of vasoconstriction and endothelium-dependent vasorelaxation.

Angiotensin II type 1 receptor antagonists in animal models of vascular, cardiac, metabolic and renal disease

We have reviewed the effects of angiotensin II type 1 receptor antagonists (ARBs) in various animal models of hypertension, atherosclerosis, cardiac function, hypertrophy and fibrosis, glucose and lipid metabolism, and renal function and morphology. Those of azilsartan and telmisartan have been included comprehensively whereas those of other ARBs have been included systematically but without intention of completeness. ARBs as a class lower blood pressure in established hypertension and prevent hypertension development in all applicable animal models except those with a markedly suppressed renin-angiotensin system; blood pressure lowering even persists for a considerable time after discontinuation of treatment. This translates into a reduced mortality, particularly in models exhibiting marked hypertension. The retrieved data on vascular, cardiac and renal function and morphology as well as on glucose and lipid metabolism are discussed to address three main questions: 1. Can ARB effects on blood vessels, heart, kidney and metabolic function be explained by blood pressure lowering alone or are they additionally directly related to blockade of the renin-angiotensin system? 2. Are they shared by other inhibitors of the renin-angiotensin system, e.g. angiotensin converting enzyme inhibitors? 3. Are some effects specific for one or more compounds within the ARB class? Taken together these data profile ARBs as a drug class with unique properties that have beneficial effects far beyond those on blood pressure reduction and, in some cases distinct from those of angiotensin converting enzyme inhibitors. The clinical relevance of angiotensin receptor-independent effects of some ARBs remains to be determined.

Inhibition of Sphingosine Kinase 1 Ameliorates Angiotensin II-Induced Hypertension and Inhibits Transmembrane Calcium Entry via Store-Operated Calcium Channel

Angiotensin II (AngII) plays a critical role in the regulation of vascular tone and blood pressure mainly via regulation of Ca(2+) mobilization. Several reports have implicated sphingosine kinase 1 (SK1)/sphingosine 1-phosphate (S1P) in the mobilization of intracellular Ca(2+) through a yet-undefined mechanism. Here we demonstrate that AngII-induces biphasic calcium entry in vascular smooth muscle cells, consisting of an immediate peak due to inositol tris-phosphate-dependent release of intracellular calcium, followed by a sustained transmembrane Ca(2+) influx through store-operated calcium channels (SOCs). Inhibition of SK1 attenuates the second phase of transmembrane Ca(2+) influx, suggesting a role for SK1 in AngII-dependent activation of SOC. Intracellular S1P triggers SOC-dependent Ca(2+) influx independent of S1P receptors, whereas external application of S1P stimulated S1P receptor-dependent Ca(2+) influx that is insensitive to inhibitors of SOCs, suggesting that the SK1/S1P axis regulates store-operated calcium entry via intracellular rather than extracellular actions. Genetic deletion of SK1 significantly inhibits both the acute hypertensive response to AngII in anaesthetized SK1 knockout mice and the sustained hypertensive response to continuous infusion of AngII in conscious animals. Collectively these data implicate SK1 as the missing link that connects the angiotensin AT1A receptor to transmembrane Ca(2+) influx and identify SOCs as a potential intracellular target for SK1.

Metabolic actions of angiotensin II and insulin: a microvascular endothelial balancing act

Metabolic actions of insulin to promote glucose disposal are augmented by nitric oxide (NO)-dependent increases in microvascular blood flow to skeletal muscle. The balance between NO-dependent vasodilator actions and endothelin-1-dependent vasoconstrictor actions of insulin is regulated by phosphatidylinositol 3-kinase-dependent (PI3K)--and mitogen-activated protein kinase (MAPK)-dependent signaling in vascular endothelium, respectively. Angiotensin II acting on AT₂ receptor increases capillary blood flow to increase insulin-mediated glucose disposal. In contrast, AT₁ receptor activation leads to reduced NO bioavailability, impaired insulin signaling, vasoconstriction, and insulin resistance. Insulin-resistant states are characterized by dysregulated local renin-angiotensin-aldosterone system (RAAS). Under insulin-resistant conditions, pathway-specific impairment in PI3K-dependent signaling may cause imbalance between production of NO and secretion of endothelin-1, leading to decreased blood flow, which worsens insulin resistance. Similarly, excess AT₁ receptor activity in the microvasculature may selectively impair vasodilation while simultaneously potentiating the vasoconstrictor actions of insulin. Therapeutic interventions that target pathway-selective impairment in insulin signaling and the imbalance in AT₁ and AT₂ receptor signaling in microvascular endothelium may simultaneously ameliorate endothelial dysfunction and insulin resistance. In the present review, we discuss molecular mechanisms in the endothelium underlying microvascular and metabolic actions of insulin and Angiotensin II, the mechanistic basis for microvascular endothelial dysfunction and insulin resistance in RAAS dysregulated clinical states, and the rationale for therapeutic strategies that restore the balance in vasodilator and constrictor actions of insulin and Angiotensin II in the microvasculature.

Local oxidative stress expansion through endothelial cells--a key role for gap junction intercellular communication

Background

Major circulation pathologies are initiated by oxidative insult expansion from a few injured endothelial cells to distal sites; this possibly involves mechanisms that are important to understanding circulation physiology and designing therapeutic management of myocardial pathologies. We tested the hypothesis that a localized oxidative insult of endothelial cells (ECs) propagates through gap junction inter-cellular communication (GJIC).

Methodology/Principal Findings

Cultures comprising the bEnd.3 cell line, that have been established and recognized as suitable for examining communication among ECs, were used to study the propagation of a localized oxidative insult to remote cells. Spatially confined near infrared illumination of parental or genetically modified bEnd.3 cultures, pretreated with the photosensitizer WST11, generated O₂•⁻ and •OH radicals in the illuminated cells. Time-lapse fluorescence microscopy, utilizing various markers, and other methods, were used to monitor the response of non-illuminated bystander and remote cells. Functional GJIC among ECs was shown to be mandatory for oxidative insult propagation, comprising de-novo generation of reactive oxygen and nitrogen species (ROS and RNS, respectively), activation and nuclear translocation of c-Jun N-terminal kinase, followed by massive apoptosis in all bystander cells adjacent to the primarily injured ECs. The oxidative insult propagated through GJIC for many hours, over hundreds of microns from the primary photogeneration site. This wave is shown to be limited by intracellular ROS scavenging, chemical GJIC inhibition or genetic manipulation of connexin 43 (a key component of GJIC).

Conclusion/Significance

Localized oxidative insults propagate through GJIC between ECs, while stimulating de-novo generation of ROS and RNS in bystander cells, thereby driving the insult's expansion.

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

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

The role of sphingosine kinase 1/sphingosine-1-phosphate pathway in the myogenic tone of posterior cerebral arteries

Aims

The goal of the current study was to determine whether the sphingosine kinase 1 (SK1)/sphingosine-1-phosphate (S1P) pathway is involved in myogenic vasoconstriction under normal physiological conditions. In the present study, we assessed whether endogenous S1P generated by pressure participates in myogenic vasoconstriction and which signaling pathways are involved in SK1/S1P-induced myogenic response under normal physiological conditions.

Methods and Results

We measured pressure-induced myogenic response, Ca²⁺ concentration, and 20 kDa myosin light chain phosphorylation (MLC₂₀) in rabbit posterior cerebral arteries (PCAs). SK1 was expressed and activated by elevated transmural pressure in rabbit PCAs. Translocation of SK1 by pressure elevation was blocked in the absence of external Ca²⁺ and in the presence of mechanosensitive ion channel and voltage-sensitive Ca²⁺ channel blockers. Pressure-induced myogenic tone was inhibited in rabbit PCAs treated with sphingosine kinase inhibitor (SKI), but was augmented by treatment with NaF, which is an inhibitor of sphingosine-1-phosphate phosphohydrolase. Exogenous S1P further augmented pressure-induced myogenic responses. Pressure induced an increase in Ca²⁺ concentration leading to the development of myogenic tone, which was inhibited by SKI. Exogenous S1P further increased the pressure-induced increased Ca²⁺ concentration and myogenic tone, but SKI had no effect. Pressure- and exogenous S1P-induced myogenic tone was inhibited by pre-treatment with the Rho kinase inhibitor and NADPH oxidase inhibitors. Pressure- and exogenous S1P-induced myogenic tone were inhibited by pre-treatment with S1P receptor blockers, W146 (S1P1), JTE013 (S1P2), and CAY10444 (S1P3). MLC₂₀ phosphorylation was increased when the transmural pressure was raised from 40 to 80 mmHg and exogenous S1P further increased MLC₂₀ phosphorylation. The pressure-induced increase of MLC₂₀ phosphorylation was inhibited by pre-treatment of arteries with SKI.

Conclusions

Our results suggest that the SK1/S1P pathway may play an important role in pressure-induced myogenic responses in rabbit PCAs under normal physiological conditions.

Antihypertensive treatment differentially affects vascular sphingolipid biology in spontaneously hypertensive rats

Background

We have previously shown that essential hypertension in humans and spontaneously hypertensive rats (SHR), is associated with increased levels of ceramide and marked alterations in sphingolipid biology. Pharmacological elevation of ceramide in isolated carotid arteries of SHR leads to vasoconstriction via a calcium-independent phospholipase A₂, cyclooxygenase-1 and thromboxane synthase-dependent release of thromboxane A₂. This phenomenon is almost absent in vessels from normotensive Wistar Kyoto (WKY) rats. Here we investigated whether lowering of blood pressure can reverse elevated ceramide levels and reduce ceramide-mediated contractions in SHR.

Methods and Findings

For this purpose SHR were treated for 4 weeks with the angiotensin II type 1 receptor antagonist losartan or the vasodilator hydralazine. Both drugs decreased blood pressure equally (SBP untreated SHR: 191±7 mmHg, losartan: 125±5 mmHg and hydralazine: 113±14 mmHg). The blood pressure lowering was associated with a 20–25% reduction in vascular ceramide levels and improved endothelial function of isolated carotid arteries in both groups. Interestingly, losartan, but not hydralazine treatment, markedly reduced sphingomyelinase-induced contractions. While both drugs lowered cyclooxygenase-1 expression, only losartan and not hydralazine, reduced the endothelial expression of calcium-independent phospholipase A₂. The latter finding may explain the effect of losartan treatment on sphingomyelinase-induced vascular contraction.

Conclusion

In summary, this study corroborates the importance of sphingolipid biology in blood pressure control and specifically shows that blood pressure lowering reduces vascular ceramide levels in SHR and that losartan treatment, but not blood pressure lowering per se, reduces ceramide-mediated arterial contractions.

Hypertension is associated with marked alterations in sphingolipid biology: a potential role for ceramide

Background

Hypertension is, amongst others, characterized by endothelial dysfunction and vascular remodeling. As sphingolipids have been implicated in both the regulation of vascular contractility and growth, we investigated whether sphingolipid biology is altered in hypertension and whether this is reflected in altered vascular function.

Methods and Findings

In isolated carotid arteries from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats, shifting the ceramide/S1P ratio towards ceramide dominance by administration of a sphingosine kinase inhibitor (dimethylsphingosine) or exogenous application of sphingomyelinase, induced marked endothelium-dependent contractions in SHR vessels (DMS: 1.4±0.4 and SMase: 2.1±0.1 mN/mm; n = 10), that were virtually absent in WKY vessels (DMS: 0.0±0.0 and SMase: 0.6±0.1 mN/mm; n = 9, p<0.05). Imaging mass spectrometry and immunohistochemistry indicated that these contractions were most likely mediated by ceramide and dependent on iPLA₂, cyclooxygenase-1 and thromboxane synthase. Expression levels of these enzymes were higher in SHR vessels. In concurrence, infusion of dimethylsphingosine caused a marked rise in blood pressure in anesthetized SHR (42±4%; n = 7), but not in WKY (−12±10%; n = 6). Lipidomics analysis by mass spectrometry, revealed elevated levels of ceramide in arterial tissue of SHR compared to WKY (691±42 vs. 419±27 pmol, n = 3–5 respectively, p<0.05). These pronounced alterations in SHR sphingolipid biology are also reflected in increased plasma ceramide levels (513±19 pmol WKY vs. 645±25 pmol SHR, n = 6–12, p<0.05). Interestingly, we observed similar increases in ceramide levels (correlating with hypertension grade) in plasma from humans with essential hypertension (185±8 pmol vs. 252±23 pmol; n = 18 normotensive vs. n = 19 hypertensive patients, p<0.05).

Conclusions

Hypertension is associated with marked alterations in vascular sphingolipid biology such as elevated ceramide levels and signaling, that contribute to increased vascular tone.

Leptin augments cerebral hemodynamic reserve after three-vessel occlusion: distinct effects on cerebrovascular tone and proliferation in a nonlethal model of hypoperfused rat brain

The adipocytokine leptin has distinct functions regulating vascular tone, inflammation, and collateral artery growth. Arteriogenesis is an inflammatory process and provides a mechanism to overcome the effects of vascular obstruction. We, therefore, tested the effects of leptin in hypoperfused rat brain (three-vessel occlusion). Systemic leptin administration for 1 week after occlusion surgery increased cerebral hemodynamic reserve similar to granulocyte-macrophage colony-stimulating factor (GM-CSF), as indicated by improved CO(2) reactivity (vehicle 0.53%±0.26% versus leptin 1.05%±0.6% per mm Hg arterial pCO(2), P<0.05). Infusion of microspheres under maximal vasodilation failed to show a positive effect of leptin on cerebral perfusion (vehicle 64.9%±4.5% versus leptin 66.3%±7.0%, occluded/nonoccluded hemisphere). Acute treatment with GM-CSF led to a significant increased CO(2) reactivity and cerebral perfusion (79.2%±8.1% versus 64.9%±4.5%, P<0.05). Vasoconstrictive response of isolated rat carotid artery rings, after phenylephrine was attenuated at 24  hours following preincubation with leptin, was unaffected by removal of endothelium but abrogated by coculture with N-(omega)-nitro-L-arginine methylester, pointing toward an inducible nitric oxide synthase-mediated mechanism. In chronic cerebral hypoperfusion, acute leptin treatment restored the hemodynamic reserve of the cerebral vasculature through its effects on vascular tone, while leaving vascular outward remodeling unaffected. Our results, for the first time, reveal a protective role of leptin on vascular function in hemodynamically compromised brain tissue.

Sphingosine-1-phosphate-induced inflammation involves receptor tyrosine kinase transactivation in vascular cells: upregulation in hypertension

Sphingosine-1-phosphate (S1P), a multifunctional phospholipid, regulates vascular cell function. Whether S1P influences vascular inflammatory responses, particularly in hypertension, is unclear. We tested the hypothesis that S1P is a proinflammatory mediator signaling through receptor tyrosine kinase transactivation and that responses are amplified in vascular smooth muscle cells from stroke-prone spontaneously hypertensive rats (SHRSPs), a model in which we demonstrated Edg1 (S1P1 receptor) to be a candidate gene for salt-sensitive hypertension. Vascular smooth muscle cell from Wistar-Kyoto rats and SHRSPs were studied. S1P receptor subtypes, S1P1 and S1P2, were similarly expressed in Wistar-Kyoto rats and SHRSPs. S1P induced phosphorylation of epidermal growth factor receptor and platelet-derived growth factor and activation of p38 mitogen-activated protein kinase and c-Jun N-terminal kinase, with amplified effects in SHRSPs versus Wistar-Kyoto rats. Inhibition of epidermal growth factor receptor and platelet-derived growth factor (with AG1478 and AG1296, respectively) abolished S1P-induced phosphorylation of p38 mitogen-activated protein kinase and c-Jun N-terminal kinase in Wistar-Kyoto rats with variable effects in SHRSPs. Vascular smooth muscle cell inflammation was evaluated by expression of adhesion molecules and functional responses assessed by monocyte adhesion. S1P stimulated expression of intercellular adhesion molecule 1 and vascular cell adhesion protein 1 and promoted monocyte adhesion, particularly in SHRSP cells. S1P-mediated inflammation was blunted by AG1478 and AG1296 in SHRSP cells. VPC23019, a S1P1 receptor antagonist, inhibited S1P-induced mitogen-activated protein kinase phosphorylation, intercellular adhesion molecule 1 and vascular cell adhesion protein 1 expression, and monocyte adhesion. Our data indicate that molecular processes underlying vascular inflammation and cell adhesion in SHRSPs involve S1P/S1P1 receptors and phosphorylation of receptor tyrosine kinases. We identify a novel pathway linking S1P/S1P1 receptors to specific proinflammatory signaling pathways through epidermal growth factor receptor and platelet-derived growth factor transactivation, a process that is upregulated in SHRSPs. Such molecular events may contribute to vascular inflammation in hypertension.

Vessel-specific role of sphingosine kinase 1 in the vasoconstriction of isolated basilar arteries

Sphingosine-1-phosphate (S1P) constricts cerebral arteries through S1P(3) receptor stimulation. Because the activity of the key S1P-synthesizing enzyme, sphingosine kinase (SPK), can be stimulated by agonists of various G protein-coupled receptors, it is likely that S1P also acts as a second messenger for other vasoconstrictors. We investigated the effect of SPK inhibitors and SPK gene deletion on the contractile responses of isolated vessels to vasoactive agonists and KCl-induced depolarization. Basilar and femoral arteries of rat, mounted in a wire myograph, were incubated with dimethylsphingosine (DMS), 2-(p-hydroxyanilino)-4-(p-chlorophenyl) thiazole (Compound 2) or FTY720, and exposed to KCl, 5-hydroxytryptamine (5-HT), S1P or phenylephrine (PE). Vasomotor responses in basilar artery were decreased by DMS, Compound 2 and FTY720, while they were not affected in femoral artery. Basilar arteries from SPK1(-/-) mice exhibited weaker vasoconstriction to both KCl and agonists (S1P and the prostanoid U46619) when compared to either wild type (WT) or SPK2(-/-). In contrast, in mesenteric resistance arteries, neither the contraction to KCl nor the maximum contraction to PE and S1P significantly differed among WT, SPK1(-/-) and SPK2(-/-). Quantitative analysis of SPK mRNA (reverse transcription and real time polymerase chain reaction) in mouse arteries showed 40-80-fold higher SPK1 expression in cerebral arteries than in aorta or mesenteric arteries. SPK1 critically modulates the reactivity of cerebral vasculature to vasoconstrictors. S1P plays a specific role as modulator of cerebral blood flow, potentially acting either directly outside vascular smooth muscle cells on S1P(3) receptors, or indirectly after being generated inside the cell in response to vasoconstrictors.

Sphingosine-1-phosphate-dependent activation of p38 MAPK maintains elevated peripheral resistance in heart failure through increased myogenic vasoconstriction

RATIONALE

Mechanisms underlying vasomotor abnormalities and increased peripheral resistance exacerbating heart failure (HF) are poorly understood.

OBJECTIVE

To explore the role and molecular basis of myogenic responses in HF.

METHODS AND RESULTS

10 weeks old C57Bl6 mice underwent experimental myocardial infarction (MI) or sham surgery. At 1 to 12 weeks postoperative, mice underwent hemodynamic studies, mesenteric, cerebral, and cremaster artery perfusion myography and Western blot. Organ weights and hemodynamics confirmed HF and increased peripheral resistance after MI. Myogenic responses, ie, pressure-induced vasoconstriction, were increased as early as 1 week after MI and remained elevated. Vasoconstrictor responses to phenylephrine were decreased 1 week after MI, but not at 2 to 6 weeks after MI, whereas those to endothelin (ET)-1 and sphingosine-1-phosphate (S1P) were increased at all time points after MI. An antagonist (JTE-013) for the most abundant S1P receptor detected in mesenteric arteries (S1P(2)R) abolished the enhanced myogenic responses of HF, with significantly less effect on controls. Mice with genetic absence of sphingosine-kinases or S1P(2)R (Sphk1(-/-); Sphk1(-/-)/Sphk2(+/-); S1P(2)R(-/-)) did not manifest enhanced myogenic responses after MI. Mesenteric arteries from HF mice exhibited increased phosphorylation of myosin light chain, with deactivation of its phosphatase (MLCP). Among known S1P-responsive regulators of MLCP, GTP-Rho levels were unexpectedly reduced in HF, whereas levels of activated p38 MAPK and ERK1/2 (extracellular signal-regulated kinase 1/2) were increased. Inhibiting p38 MAPK abolished the myogenic responses of animals with HF, with little effect on controls.

CONCLUSIONS

Rho-independent p38 MAPK-mediated deactivation of MLCP underlies S1P/S1P(2)R-regulated increases in myogenic vasoconstriction observed in HF. Therapeutic targeting of these findings in HF models deserves study.

Sphingolipids and the orchestration of endothelium-derived vasoactive factors: when endothelial function demands greasing

Vasomotor tone is regulated by a complex interplay of a variety of extrinsic neurohumoral and intrinsic factors. It is the endothelium that has a major influence on smooth muscle cell tone via the release of intrinsic vasoactive factors and is therefore an important regulator of vasomotor tone. Sphingolipids are an emerging class of lipid mediators with important physiological properties. In the last two decades it has not only become increasingly clear that sphingolipid signaling plays a pivotal role in immune function, but also its role in the vascular system is now becoming more recognized. In this mini-review we will highlight the possible cross-talk between sphingolipids and intrinsic vasoactive factors released by the endothelium. Via this cross-talk sphingolipids can orchestrate vasomotor tone and may therefore also be involved in the pathophysiology of disease states associated with endothelial dysfunction.

Exogenous and endogenous ceramides elicit volume-sensitive chloride current in ventricular myocytes

AIMS

Because ceramide accumulates in several forms of cardiovascular disease and ceramide-induced apoptosis may involve the volume-sensitive Cl(-) current, I(Cl,swell), we assessed whether ceramide activates I(Cl,swell).

METHODS AND RESULTS

I(Cl,swell) was measured in rabbit ventricular myocytes by whole-cell patch clamp after isolating anion currents. Exogenous C(2)-ceramide (C(2)-Cer), a membrane-permeant short-chain ceramide, elicited an outwardly rectifying Cl(-) current in both physiological and symmetrical Cl(-) solutions that was fully inhibited by DCPIB, a specific I(Cl,swell) blocker. In contrast, the metabolically inactive C(2)-Cer analogue C(2)-dihydroceramide (C(2)-H(2)Cer) failed to activate Cl(-) current. Bacterial sphingomyelinase (SMase), which generates endogenous long-chain ceramides as was confirmed by tandem mass spectrometry, also elicited an outwardly rectifying Cl(-) current that was inhibited by DCPIB and tamoxifen, another I(Cl,swell) blocker. Bacterial SMase-induced current was partially reversed by osmotic shrinkage and fully suppressed by ebselen, a scavenger of reactive oxygen species. Outward rectification with physiological and symmetrical Cl(-) gradients, block by DCPIB and tamoxifen, and volume sensitivity are characteristics that identify I(Cl,swell). Insensitivity to C(2)-H(2)Cer and block by ebselen suggest involvement of ceramide signalling rather than direct lipid-channel interaction.

CONCLUSION

Exogenous and endogenous ceramide elicited I(Cl,swell) in ventricular myocytes. This may contribute to persistent activation of I(Cl,swell) and aspects of altered myocyte function in cardiovascular diseases associated with by ceramide accumulation.

Sphingosine 1-phosphate increases glucose uptake through trans-activation of insulin receptor

Sphingosine 1-phosphate (S1P) is a bioactive lipid that acts through a family of G-protein-coupled receptors. Herein, we report evidence of a novel redox-based cross-talk between S1P and insulin signaling pathways. In skeletal muscle cells S1P, through engagement of its S1P(2) receptor, is found to produce a transient burst of reactive oxygen species through a calcium-dependent activation of the small GTPase Rac1. S1P-induced redox-signaling is sensed by protein tyrosine phosphatase-1B, the main negative regulator of insulin receptor phosphorylation, which undergoes oxidation and enzymatic inhibition. This redox-based inhibition of the phosphatase provokes a ligand-independent trans-phosphorylation of insulin receptor and a strong increase in glucose uptake. Our results propose a new role of S1P, recognizing the lipid as an insulin-mimetic cue and pointing at reactive oxygen species as critical regulators of the cross-talk between S1P and insulin pathways. Any possible implication of S1P-directed insulin signaling in diabetes and insulin resistance remains to be established.

Thymic progenitor homing and lymphocyte homeostasis are linked via S1P-controlled expression of thymic P-selectin/CCL25

Thymic T cell progenitor (TCP) importation is a periodic, gated event that is dependent on the expression of functional P-selectin ligands on TCPs. Occupancy of intrathymic TCP niches is believed to negatively regulate TCP importation, but the nature of this feedback mechanism is not yet resolved. We show that P-selectin and CCL25 are periodically expressed in the thymus and are essential parts of the thymic gate-keeping mechanism. Periodicity of thymic TCP receptivity and the size of the earliest intrathymic TCP pool were dependent on the presence of functional P-selectin ligand on TCPs. Furthermore, we show that the numbers of peripheral blood lymphocytes directly affected thymic P-selectin expression and TCP receptivity. We identified sphingosine-1-phosphate (S1P) as one feedback signal that could mediate influence of the peripheral lymphocyte pool on thymic TCP receptivity. Our findings suggest a model whereby thymic TCP importation is controlled by both early thymic niche occupancy and the peripheral lymphocyte pool via S1P.

Sphingosine-1-phosphate receptor signalling in the heart

The five known members of the sphingosine-1-phosphate (S1P) receptor family exhibit diverse tissue expression profiles and couple to distinct G-protein-mediated signalling pathways. S1P1, S1P2, and S1P3 receptors are all present in the heart, but the ratio of these subtypes differs for various cardiac cells. The goal of this review is to summarize data concerning which S1P receptor subtypes regulate cardiac physiology and pathophysiology, which G-proteins and signalling pathways they couple to, and in which cell types they are expressed. The available information is based on studies using a lamentably limited set of pharmacological agonists/antagonists, but is complemented by work with S1P receptor subtype-specific knockout mice and sphingosine kinase knockout mice. In cardiac myocytes, the S1P1 receptor subtype is the predominant subtype expressed, and the activation of this receptor inhibits cAMP formation and antagonizes adrenergic receptor-mediated contractility. The S1P3 receptor, while expressed at lower levels, mediates the bradycardic effect of S1P agonists. Studies using knockout mice indicate that S1P2 and S1P3 receptors play a major role in mediating cardioprotection from ischaemia/reperfusion injury in vivo. S1P receptors are also involved in remodelling, proliferation, and differentiation of cardiac fibroblasts, a cell type in which the S1P3 receptor predominates. Receptors for S1P are also present in endothelial and smooth muscle cells where they mediate peripheral vascular tone and endothelial responses, but the role of this regulatory system in the cardiac vasculature is unknown. Further understanding of the contributions of each cell and receptor subtype to cardiac function and pathophysiology should expedite consideration of the endogenous S1P signalling pathway as a therapeutic target for cardiovascular disease.

Activation of sphingosine kinase by muscarinic receptors enhances NO-mediated and attenuates EDHF-mediated vasorelaxation

Local formation of the sphingomyelin metabolite sphingosine-1-phosphate (S1P) within the vascular wall has been shown to modulate vascular reactivity. In this study we investigated whether sphingosine kinase, the enzyme responsible for S1P synthesis, plays a role in muscarinic receptor-mediated NO production and vascular relaxation in different blood vessel types. For this purpose, sphingosine kinase translocation and sphingolipid-dependent NO-production after muscarinic receptor stimulation were assessed in an endothelial cell line. Furthermore, we used the sphingosine kinase inhibitor N,N-dimethylsphingosine (DMS) to investigate the role of sphingosine kinase in the relaxant responses to the muscarinic agonist methacholine (MCh) in isolated rat aorta and mesenteric arteries. Activation of M(3)-receptors in an endothelial cell line induced a fast translocation of YFP-tagged sphingosine kinase-1 from the cytosol to the plasma membrane. Concomitant NO-production in this cell line was partially inhibited by DMS. Accordingly, in rat aorta the relaxant responses to MCh were attenuated in the presence of DMS, while the responses to the NO-donor sodium nitroprusside were unaltered. In contrast, DMS enhanced the relaxant responses to MCh in mesenteric artery preparations. This effect could also be observed in the presence of NO synthase and cyclooxygenase inhibitors, indicating that sphingosine kinase inhibition specifically enhanced endothelium-derived hyperpolarizing factor-mediated (i.e. non-NO and non-prostacyclin-dependent) relaxation. We conclude that sphingosine kinase differentially regulates vascular tone in different vessel types, enhancing NO-dependent vasorelaxation but counteracting EDHF-dependent vasorelaxation. This observation enhances our understanding of the complex mechanisms by which sphingolipids regulate vascular homeostasis. Moreover, a disturbed regulation of sphingolipid metabolism in the vascular wall may therefore play a role in the aetiology/pathology of disease states characterized by endothelial dysfunction.

The role of sphingosine kinase and sphingosine-1-phosphate in the regulation of endothelial cell biology

Sphingolipids, in particular sphingosine kinase (SphK) and its product sphingosine-1-phosphate (S1P), are now recognized to play an important role in regulating many critical processes in endothelial cells. Activation of SphK1 is essential in mediating the endothelial proinflammatory effects of inflammatory cytokines such as tumor necrosis factor (TNF). In addition, S1P regulates the survival and proliferation of endothelial cells, as well as their ability to undergo cell migration, all essential components of angiogenesis. Thus the inflammatory and angiogenic potential of the endothelium is in part regulated by intracellular components including the activity of SphK1 and levels of S1P. Herein a review of the sphingomyelin pathway with a particular focus on its relevance to endothelial cell biology is presented.

Sphingolipid signalling in the cardiovascular system: good, bad or both?

Sphingolipids are biologically active lipids that play important roles in various cellular processes and the sphingomyelin metabolites ceramide, sphingosine and sphingosine-1-phosphate can act as signalling molecules in most cell types. With the recent development of the immunosuppressant drug FTY720 (Fingolimod) which after phosphorylation in vivo acts as a sphingosine-1-phosphate receptor agonist, research on the role of sphingolipids in the immune and other organ systems was triggered enormously. Since it was reported that FTY720 induced a modest, but significant transient decrease in heart rate in animals and humans, the question was raised which pharmacological properties of drugs targeting sphingolipid signalling will affect cardiovascular function in vivo. The answer to this question will most likely also indicate what type of drug could be used to treat cardiovascular disease. The latter is becoming increasingly important because of the increasing population carrying characteristics of the metabolic syndrome. This syndrome is, amongst others, characterized by obesity, hypertension, atherosclerosis and diabetes. As such, individuals with this syndrome are at increased risk of heart disease. Now numerous studies have investigated sphingolipid effects in the cardiovascular system, can we speculate whether certain sphingolipids under specific conditions are good, bad or maybe both? In this review we will give a brief overview of the pathophysiological role of sphingolipids in cardiovascular disease. In addition, we will try to answer how drugs that target sphingolipid signalling will potentially influence cardiovascular function and whether these drugs would be useful to treat cardiovascular disease.

Regulation of G protein-coupled receptor signalling: focus on the cardiovascular system and regulator of G protein signalling proteins

G protein-coupled receptors (GPCRs) are involved in many biological processes. Therefore, GPCR function is tightly controlled both at receptor level and at the level of signalling components. Well-known mechanisms by which GPCR function can be regulated comprise desensitization/resensitization processes and GPCR up- and downregulation. GPCR function can also be regulated by several proteins that directly interact with the receptor and thereby modulate receptor activity. An additional mechanism by which receptor signalling is regulated involves an emerging class of proteins, the so-called regulators of G protein signalling (RGS). In this review we will describe some of these control mechanisms in more detail with some specific examples in the cardiovascular system. In addition, we will provide an overview on RGS proteins and the involvement of RGS proteins in cardiovascular function.

Angiotensin II signal transduction through the AT1 receptor: novel insights into mechanisms and pathophysiology

The intracellular signal transduction of AngII (angiotensin II) has been implicated in cardiovascular diseases, such as hypertension, atherosclerosis and restenosis after injury. AT(1) receptor (AngII type-1 receptor), a G-protein-coupled receptor, mediates most of the physiological and pathophysiological actions of AngII, and this receptor is predominantly expressed in cardiovascular cells, such as VSMCs (vascular smooth muscle cells). AngII activates various signalling molecules, including G-protein-derived second messengers, protein kinases and small G-proteins (Ras, Rho, Rac etc), through the AT(1) receptor leading to vascular remodelling. Growth factor receptors, such as EGFR (epidermal growth factor receptor), have been demonstrated to be 'trans'-activated by the AT(1) receptor in VSMCs to mediate growth and migration. Rho and its effector Rho-kinase/ROCK are also implicated in the pathological cellular actions of AngII in VSMCs. Less is known about the endothelial AngII signalling; however, recent studies suggest the endothelial AngII signalling positively, as well as negatively, regulates the NO (nitric oxide) signalling pathway and, thereby, modulates endothelial dysfunction. Moreover, selective AT(1)-receptor-interacting proteins have recently been identified that potentially regulate AngII signal transduction and their pathogenic functions in the target organs. In this review, we focus our discussion on the recent findings and concepts that suggest the existence of the above-mentioned novel signalling mechanisms whereby AngII mediates the formation of cardiovascular diseases.

Vascular effects of sphingolipids

The sphingomyelin metabolites ceramide, sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC) are emerging modulators of vascular tone. While ceramide appears to act primarily intracellularly, S1P and SPC appear to mainly work via specific receptors, although those for SPC have not yet been defined unequivocally. Each of the sphingomyelin metabolites can induce both vasoconstriction and vasodilatation and, in some cases--ceramide on the one hand, and S1P and SPC on the other hand--have opposite effects on vascular tone. The differences in effects between vessels may relate to the relative roles of endothelial and smooth muscle cells in mediating them, as well as to the distinct expression patterns of S1P receptors among vascular beds and among endothelial and smooth muscle cells. Recent evidence suggests that vascular tone is not only modulated by sphingomyelin metabolites which are exogenously added or reach the vessel wall via the bloodstream but also by those formed locally by cells in the vessel wall. Such local formation can be induced by known vasoactive agents such as angiotensin II and may serve a signalling function.

CONCLUSION

We conclude that sphingomyelin metabolites are important endogenous modulators of vascular function, which may contribute to the pathophysiology of some diseases and be targets for therapeutic interventions.

Intracellular generation of sphingosine 1-phosphate in human lung endothelial cells: role of lipid phosphate phosphatase-1 and sphingosine kinase 1

Sphingosine 1-phosphate (S1P) regulates diverse cellular functions through extracellular ligation to S1P receptors, and it also functions as an intracellular second messenger. Human pulmonary artery endothelial cells (HPAECs) effectively utilized exogenous S1P to generate intracellular S1P. We, therefore, examined the role of lipid phosphate phosphatase (LPP)-1 and sphingosine kinase1 (SphK1) in converting exogenous S1P to intracellular S1P. Exposure of (32)P-labeled HPAECs to S1P or sphingosine (Sph) increased the intracellular accumulation of [(32)P]S1P in a dose- and time-dependent manner. The S1P formed in the cells was not released into the medium. The exogenously added S1P did not stimulate the sphingomyelinase pathway; however, added [(3)H]S1P was hydrolyzed to [(3)H]Sph in HPAECs, and this was blocked by XY-14, an inhibitor of LPPs. HPAECs expressed LPP1-3, and overexpression of LPP-1 enhanced the hydrolysis of exogenous [(3)H]S1P to [(3)H]Sph and increased intracellular S1P production by 2-3-fold compared with vector control cells. Down-regulation of LPP-1 by siRNA decreased intracellular S1P production from extracellular S1P but had no effect on the phosphorylation of Sph to S1P. Knockdown of SphK1, but not SphK2, by siRNA attenuated the intracellular generation of S1P. Overexpression of wild type SphK1, but not SphK2 wild type, increased the accumulation of intracellular S1P after exposure to extracellular S1P. These studies provide the first direct evidence for a novel pathway of intracellular S1P generation. This involves the conversion of extracellular S1P to Sph by LPP-1, which facilitates Sph uptake, followed by the intracellular conversion of Sph to S1P by SphK1.

Regulation and functional roles of sphingosine kinases

Sphingosine kinases (SphKs) catalyze the phosphorylation of sphingosine to sphingosine-1-phosphate (S1P). Together with other sphingolipid metabolizing enzymes, SphKs regulate the balance of the lipid mediators, ceramide, sphingosine, and S1P. The ubiquitous mediator S1P regulates cellular functions such as proliferation and survival, cytoskeleton architecture and Ca(2+) homoeostasis, migration, and adhesion by activating specific high-affinity G-protein-coupled receptors or by acting intracellularly. In mammals, two isoforms of SphK have been identified. They are activated by G-protein-coupled receptors, receptor tyrosine kinases, immunoglobulin receptors, cytokines, and other stimuli. The molecular mechanisms by which SphK1 and SphK2 are specifically regulated are complex and only partially understood. Although SphK1 and SphK2 appear to have opposing roles, promoting cell growth and apoptosis, respectively, they can obviously also substitute for each other, as mice deficient in either SphK1 or SphK2 had no obvious abnormalities, whereas double-knockout animals were embryonic lethal. In this review, our understanding of structure, regulation, and functional roles of SphKs is updated and discussed with regard to their implication in pathophysiological and disease states.

Activation of endothelial nitric oxide synthase by the angiotensin II type 1 receptor

Enhanced angiotensin II (AngII) action has been implicated in endothelial dysfunction that is characterized as decreased nitric oxide availability. Although endothelial cells have been reported to express AngII type 1 (AT1) receptors, the exact role of AT1 in regulating endothelial NO synthase (eNOS) activity remains unclear. We investigated the possible regulation of eNOS through AT1 in bovine aortic endothelial cells (BAECs) and its functional significance in rat aortic vascular smooth muscle cells (VSMCs). In BAECs infected with adenovirus encoding AT1 and in VSMCs infected with adenovirus encoding eNOS, AngII rapidly stimulated phosphorylation of eNOS at Ser1179. This was accompanied with increased cGMP production. These effects were blocked by an AT1 antagonist. The cGMP production was abolished by a NOS inhibitor as well. To explore the importance of eNOS phosphorylation, VSMCs were also infected with adenovirus encoding S1179A-eNOS. AngII did not stimulate cGMP production in VSMCs expressing S1179A. However, S1179A was able to enhance basal NO production as confirmed with cGMP production and enhanced vasodilator-stimulated phosphoprotein phosphorylation. Interestingly, S1179A prevented the hypertrophic response similar to wild type in VSMCs. From these data, we conclude that the AngII/AT1 system positively couples to eNOS via Ser1179 phosphorylation in ECs and VSMCs if eNOS and AT1 coexist. However, basal level NO production may be sufficient for prevention of AngII-induced hypertrophy by eNOS expression. These data demonstrate a novel molecular mechanism of eNOS regulation and function and thus provide useful information for eNOS gene therapy under endothelial dysfunction.