Acute systemic inflammation up-regulates secretory sphingomyelinase in vivo: a possible link between inflammatory cytokines and atherogenesis

Thrombogenicity of TNF alpha in rheumatoid arthritis defined through biological probes: TNF alpha blockers

Rheumatoid arthritis is a disease at high cardiovascular risk. It has recently been shown that RA patients with more than 10 years disease duration present a risk of myocardial infarction more than three times higher than osteoarthritis controls. The major determinant is thought to be the chronic inflammatory process, driven by some key cytokines among which TNF alpha is thought to play the leading role in the majority of the patients. TNFalpha, therefore, once blocked by specific inhibitors like TNF alpha blockers (Infliximab, Etanercept) should profoundly decrease the cardiovascular risk. However, TNF blockers induce the appearance of autoimmunity though in a small minority of the patients. This autoimmunity is thought to be due to the poor clearance of apoptotic bodies once the systemic inflammation (CRP, SAP) is controlled by the specific blockers, and to the lack of control of some B cell populations producing autoantibodies to specific autoantigens. Among the autoantibodies arising during TNF blockade, anticardiolipin appear to be the most crucial with respect to the cardiovascular risk. The appearance of anticardiolipins at clinically significant levels appears to be driven by two possible mechanisms, one due to common infections of the urinary or upper airways tract during blockade of soluble TNF alpha, the other due to the escape of some autoreactive B cells during blockade of soluble and membranous TNF alpha. Since both autoantibodies related to infections as well as the high levels unrelated to infections, can be well controlled by appropriate therapies, clinicians should pay attention to the biological phenomenon before it becomes a clinical problem.

Proinflammatory cytokine impairment of insulin-like growth factor I-induced protein synthesis in skeletal muscle myoblasts requires ceramide

GH and IGF-I control over 80% of postnatal growth. We recently established that TNFalpha impairs the ability of IGF-I to increase protein synthesis and promote expression of myogenin in myoblasts. Here we extend these results by showing that ceramide, a second messenger in both TNFalpha and IL-1beta receptor signaling pathways, is a key downstream sphingosine-based lipid that leads to IGF-I resistance. A cell-permeable ceramide analog, C2-ceramide, inhibits IGF-I-induced protein synthesis by 65% and blocks the ability of IGF-I to increase expression of two key myogenic factors, myogenin and MyoD. Identical results were obtained with both TNFalpha and IL-1beta (1 ng/ml). Consistent with these data, neutral sphingomyelinase (N-SMase), an enzyme that catalyzes formation of ceramide from sphingomyelin, blocks IGF-I-induced protein synthesis and expression of both myogenin and MyoD. The possibility that cytokine-induced ceramide production is required for disruption of IGF-I biologic activity was confirmed by treating C2C12 myoblasts with inhibitors of all three ceramide-generating pathways. A N-SMase inhibitor, glutathione, as well as an acidic sphingomyelinase (A-SMase) inhibitor, D609, reverse the cytokine inhibition of IGF-I-induced protein synthesis by 80% and 45%, respectively. Likewise, an inhibitor of de novo ceramide synthesis, FB1, causes a 50% inhibition. Similarly, all three inhibitors significantly impair the ability of both TNFalpha and IL-1beta to suppress IGF-I-driven expression of myogenin. These experiments establish that ceramide, derived both from sphingomyelin and de novo synthesis, is a key intermediate by which proinflammatory cytokines impair the ability of IGF-I to promote protein synthesis and expression of critical muscle-specific transcription factors.

Effects of infection and inflammation on lipid and lipoprotein metabolism: mechanisms and consequences to the host

Infection and inflammation induce the acute-phase response (APR), leading to multiple alterations in lipid and lipoprotein metabolism. Plasma triglyceride levels increase from increased VLDL secretion as a result of adipose tissue lipolysis, increased de novo hepatic fatty acid synthesis, and suppression of fatty acid oxidation. With more severe infection, VLDL clearance decreases secondary to decreased lipoprotein lipase and apolipoprotein E in VLDL. In rodents, hypercholesterolemia occurs attributable to increased hepatic cholesterol synthesis and decreased LDL clearance, conversion of cholesterol to bile acids, and secretion of cholesterol into the bile. Marked alterations in proteins important in HDL metabolism lead to decreased reverse cholesterol transport and increased cholesterol delivery to immune cells. Oxidation of LDL and VLDL increases, whereas HDL becomes a proinflammatory molecule. Lipoproteins become enriched in ceramide, glucosylceramide, and sphingomyelin, enhancing uptake by macrophages. Thus, many of the changes in lipoproteins are proatherogenic. The molecular mechanisms underlying the decrease in many of the proteins during the APR involve coordinated decreases in several nuclear hormone receptors, including peroxisome proliferator-activated receptor, liver X receptor, farnesoid X receptor, and retinoid X receptor. APR-induced alterations initially protect the host from the harmful effects of bacteria, viruses, and parasites. However, if prolonged, these changes in the structure and function of lipoproteins will contribute to atherogenesis.

Modification of LDL with human secretory phospholipase A(2) or sphingomyelinase promotes its arachidonic acid-releasing propensity

Oxidation and lipolytic remodeling of LDL are believed to stimulate LDL entrapment in the arterial wall, expanding the inflammatory response and promoting atherosclerosis. However, the cellular responses and molecular mechanisms underlying the atherogenic effects of lipolytically modified LDL are incompletely understood. Human THP-1 monocytes were prelabeled with [(3)H]arachidonic acid (AA) before incubation with LDL or LDL lipolytically modified by secretory PLA(2) (sPLA(2)) or bacterial sphingomyelinase (SMase). LDL elicited rapid and dose-dependent extracellular release of AA in monocytes. Interestingly, LDL modified by sPLA(2) or SMase displayed a marked increase in AA mobilization relative to native LDL, and this increase correlated with enhanced activity of cytosolic PLA(2) (cPLA(2)) assayed in vitro as well as increased monocyte tumor necrosis factor-alpha secretion. The AA liberation was attenuated by inhibitors toward cPLA(2) and sPLA(2), indicating that both PLA(2) enzymes participate in LDL-induced AA release. In conclusion, these results demonstrate that LDL lipolytically modified by sPLA(2) or SMase potentiates cellular AA release and cPLA(2) activation in human monocytes. From our results, we suggest novel atherogenic properties for LDL modified by sPLA(2) and SMase in AA release and signaling, which could contribute to the inflammatory gene expression observed in atherosclerosis.

Elevation of ceramide in serum lipoproteins during acute phase response in humans and mice: role of serine-palmitoyl transferase

Recent studies have indicated that ceramide generated in the liver is secreted into the bloodstream as component of very-low-density lipoproteins (VLDL) and low-density lipoproteins (LDL). This manuscript investigates the effect of host acute phase response to inflammation on lipoprotein ceramide levels. In humans, two different patterns of responses were found. One group of volunteers experienced transient increases in serum ceramide at 1.5h after LPS administration. Second group showed prolonged increases that reached up to 10-fold above the basal level and continued for up to 24h. Increases in ceramide were found only in VLDL and LDL particles. LPS administration induced similar increases in mice. These increases were accompanied by activation of secreted sphingomyelinase in serum and serine-palmitoyl transferase in liver. ASMase knockout mice retained LPS-induced increases in serum ceramide, thus suggesting that the elevation of VLDL and LDL ceramide content is attributed at least in part to activation of de novo synthesis of ceramide in the liver.

PAF-mediated pulmonary edema: a new role for acid sphingomyelinase and ceramide

Platelet-activating factor (PAF) induces pulmonary edema and has a key role in acute lung injury (ALI). Here we show that PAF induces pulmonary edema through two mechanisms: acid sphingomyelinase (ASM)-dependent production of ceramide, and activation of the cyclooxygenase pathway. Agents that interfere with PAF-induced ceramide synthesis, such as steroids or the xanthogenate D609, attenuate pulmonary edema formation induced by PAF, endotoxin or acid instillation. Our results identify acid sphingomyelinase and ceramide as possible therapeutic targets in acute lung injury.

Phosphorylation and action of the immunomodulator FTY720 inhibits vascular endothelial cell growth factor-induced vascular permeability

FTY720, a potent immunosuppressive agent, is phosphorylated in vivo into FTY720-P, a high affinity agonist for sphingosine 1-phosphate (S1P) receptors. The effects of FTY720 on vascular cells, a major target of S1P action, have not been addressed. We now report the metabolic activation of FTY720 by sphingosine kinase-2 and potent activation of vascular endothelial cell functions in vitro and in vivo by phosphorylated FTY720 (FTY720-P). Incubation of endothelial cells with FTY720 resulted in phosphorylation by sphingosine kinase activity and formation of FTY720-P. Sphingosine kinase-2 effectively phosphorylated FTY720 in the human embryonic kidney 293T heterologous expression system. FTY720-P treatment of endothelial cells stimulated extracellular signal-activated kinase and Akt phosphorylation and adherens junction assembly and promoted cell survival. The effects of FTY720-P were inhibited by pertussis toxin, suggesting the requirement for Gi-coupled S1P receptors. Indeed, transmonolayer permeability induced by vascular endothelial cell growth factor was potently reversed by FTY720-P. Furthermore, oral FTY720 administration in mice potently blocked VEGF-induced vascular permeability in vivo. These findings suggest that FTY720 or its analogs may find utility in the therapeutic regulation of vascular permeability, an important process in angiogenesis, inflammation, and pathological conditions such as sepsis, hypoxia, and solid tumor growth.

A single intravenous dose of endotoxin rapidly alters serum lipoproteins and lipid transfer proteins in normal volunteers

Endotoxemia is associated with rapid and marked declines in serum levels of LDL and HDL by unknown mechanisms. Six normal volunteers received a single, small intravenous (iv) dose of endotoxin (Escherichia coli 0113, 2 ng/kg) or saline in a random order, cross-over design. After endotoxin treatment, volunteers had mild, transient flu-like symptoms and markedly increased serum levels of tumor necrosis factor and its soluble receptors, interleukin-6, cortisol, serum amyloid A, and C-reactive protein. Triglyceride (TG), VLDL-TG, and nonesterified fatty acid increased (peak at 3-4 h), then TG declined (nadir at 9 h), and then cholesterol, LDL cholesterol, apolipoprotein B (apoB), and phospholipid declined (nadirs at 12-24 h). HDL cholesterol and apoA-I levels were not affected, but half of the decrease in phospholipid was HDL phospholipid. Lipopolysaccharide binding protein (LBP) rose 3-fold (peak at 12 h), with smaller and later decreases in the activities of phospholipid transfer protein and cholesteryl ester transfer protein. In conclusion, a decline in LDL was rapidly induced in normal volunteers with a single iv dose of endotoxin. The selective loss of phospholipid from HDL may have been mediated by LBP and, after more intense or prolonged inflammation, could result in increased HDL clearance and reduced HDL levels.

Uptake and metabolism of low density lipoproteins with elevated ceramide content by human microvascular endothelial cells: implications for the regulation of apoptosis

Ceramide is a bioactive molecule involved in cellular responses to stress and inflammation. The major pathway for ceramide accumulation is via agonist-induced activation of cellular sphingomyelinases. It has also been shown that the ceramide level in circulating low density lipoprotein (LDL) increases during systemic inflammation, hence it is of importance to understand whether LDL-derived ceramide also contributes to the cellular ceramide homeostasis and affects cell functions. This article provides evidence of uptake of ceramide-enriched LDL by human microvascular endothelial cells in a receptor-mediated fashion. This uptake can be competed by native LDL, indicating that the LDL-binding receptor may be involved. Following uptake, part of the LDL-derived ceramide is converted to sphingosine, but more importantly, part of it accumulates inside the cells (approximately 1.44 nmol/mg of cell protein). This accumulation of ceramide correlates with an increased incidence of apoptosis. The addition of tumor necrosis factor-alpha further enhances the accumulation of LDL-derived ceramide and the rate of apoptosis. In contrast, inhibitors of receptor-mediated endocytosis block both, the accumulation of LDL-derived ceramide and the concurrent increase in apoptosis. We also show that LDL-delivered ceramide is a more efficient inducer of apoptosis as compared with ethanol-delivered ceramide, the same apoptotic effect being achieved by substantially smaller increases in intracellular ceramide. Taken together, the presented data indicate that increases in lipoprotein ceramide concentration may result in changes in the bioactive properties of circulating lipoproteins such as the enhanced ability to induce apoptosis in the microvascular endothelium.

Plasma ceramide and lysophosphatidylcholine inversely correlate with mortality in sepsis patients

Recent data indicate that ceramide (Cer) and lysophosphatidylcholine (LPC) regulate immune cell functions. Since these bioactive lipids are generated in blood plasma by inflammatory lipases, we hypothesized that they may be involved in the process of acute systemic sepsis. In order to provide support for this hypothesis, we analyzed the plasma levels of Cer and LPC by quantitative tandem mass spectrometry in 102 sepsis patients starting with the day at which the sepsis criteria were fulfilled for the first time, as well as on day 4 and day 11. The values were compared with 56 healthy controls and correlated with sepsis-related mortality within 30 days of study entry. Most Cer species were increased in sepsis patients, while all LPC species were markedly decreased. In addition, we determined the molar ratios with their precursor molecules sphingomyelin (SPM) and phosphatidylcholine (PC), which reflect the enzymatic reactions responsible for their formation. Species-specific as well as total Cer-SPM ratios were increased, whereas LPC-PC ratios were decreased in sepsis patients. The increased Cer-SPM ratios as well as the decreased LPC-PC ratios showed a strong predictive power for sepsis-related mortality. Together with existing data from in vitro experiments and animal models, the results provide the first ex vivo indication for the role of Cer and lysophospholipids in systemic inflammation in humans.

Inactivation of Kex2p diminishes the virulence of Candida albicans

Deletion of the kexin gene (KEX2) in Candida albicans has a pleiotropic effect on phenotype and virulence due partly to a defect in the expression of two major virulence factors: the secretion of active aspartyl proteinases and the formation of hyphae. kex2/kex2 mutants are highly attenuated in a mouse systemic infection model and persist within cultured macrophages for at least 24 h without causing damage. Pathology is modest, with little disruption of kidney matrix. The infecting mutant cells are largely confined to glomeruli, and are aberrant in morphology. The complex phenotype of the deletion mutants reflects a role for kexin in a wide range of cellular processes. Taking advantage of the specificity of Kex2p cleavage, an algorithm we developed to scan the 9168 open reading frames in Assembly 6 of the C. albicans genome identified 147 potential substrates of Kex2p. These include all previously identified substrates, including eight secreted aspartyl proteinases, the exoglucanase Xog1p, the immunodominant antigen Mp65, and the adhesin Hwp1p. Other putative Kex2p substrates identified include several adhesins, cell wall proteins, and hydrolases previously not implicated in pathogenesis. Kexins also process fungal mating pheromones; a modification of the algorithm identified a putative mating pheromone with structural similarities to Saccharomyces cerevisiae alpha-factor.

Alveolar sphingolipids generated in response to TNF-alpha modifies surfactant biophysical activity

Sphingolipids represent a diverse group of bioactive lipid species that are generated intracellularly in response to tumor necrosis factor-alpha (TNF-alpha) and are implicated as potential mediators of acute lung injury. The purpose of these studies was to determine whether there was an extracellular, TNF-alpha-regulated pool of sphingolipids in the alveolus that modulates the surface tension lowering capacity of pulmonary surfactant. Intratracheal instillation of TNF-alpha in adult rats led to a twofold increase in the amount of surfactant-associated ceramide and tended to decrease levels of sphingomyelin without significantly altering sphingosine or sphinganine content. TNF-alpha induction of alveolar ceramide was associated with nearly an 80% increase in acid sphingomyelinase activity recovered in cell-free alveolar lavage. Ceramide administered in a dose-dependent manner potently antagonized the surface tension lowering effects of natural surfactant in vitro. Intratracheal TNF-alpha and ceramide treatment of rats significantly increased lung permeability, as was evidenced by extravasation of Evans blue dye into alveolar lavage and lung tissue. Thus these studies are the first to demonstrate the existence of a cytokine-regulated alveolar pool of sphingomyelin hydrolysis products that impairs the biophysical properties of the alveolar surfactant film. The results also suggest the presence of a secretory alveolar sphingomylinase that is TNF-alpha responsive and mediates effects of the cytokine on alveolar sphingolipid metabolism.

Sympathetic and cardiac baroreflex function in panic disorder

BACKGROUND

Recent reports have demonstrated increased cardiac risk, and an association with essential hypertension in patients with panic disorder. The cause is not known, but possibly involves sympathetic nervous activation. In this study, we evaluated the arterial baroreflex control of vascular sympathetic nervous outflow and cardiac baroreflex function in panic disorder patients.

METHODS AND RESULTS

We studied nine patients suffering from panic disorder and ten healthy subjects. Microneurographic recording of muscle sympathetic nerve activity (MSNA) was made with simultaneous recording of blood pressure (BP) and electrocardiogram (ECG). The relationship between MSNA and spontaneous diastolic BP (DBP) changes was assessed at rest and was defined as the arterial baroreflex control of MSNA. Cardiac baroreflex function was assessed using the sequence method. Anxiety was assessed using Spielberger's anxiety state and trait inventory. The slopes of the relationship between MSNA and DBP were more negative (steeper) in the panic disorder group compared with the control subjects (-5.97 +/- 0.45 versus -3.06 +/- 0.43 bursts/100 heart beats per mmHg, P < 0.001). Panic disorder patients had significantly higher state and trait anxiety scores. The slope of the relationship between MSNA and diastolic BP was significantly related to the trait anxiety of the subjects. There was no difference between the cardiac baroreflex sensitivity between the two groups.

CONCLUSION

Patients with panic disorder exhibit enhanced reflex gain of the arterial baroreflex control of MSNA but no change in the cardiac baroreflex. While any clinical significance this observation might have in relation to increased cardiac risk in panic disorder, or to concordance with essential hypertension, remains to be elucidated, increased reactivity of vasoconstricting sympathetic nerves may be a trait characteristic in this cohort.

Accelerated atherogenesis in autoimmune rheumatic diseases

The observation that systemic inflammatory rheumatic diseases such as rheumatoid arthritis (RA) are associated with a significantly increased rate of cardiovascular disease, which often occurs at a younger age than in the normal population, is particularly important given the increasing interest in the role of inflammation in atherogenesis in the general population. This review examines the accumulating evidence for accelerated atherogenesis of RA and updates the hypothesis that vasculitis plays a major role in this. Endothelial dysfunction (ECD), widely regarded as initial lesion in atherogenesis, has been shown to occur commonly in primary vasculitis. This ECD is a diffuse event, demonstrable in more than one vascular bed. It is not simply due to scarring in the vessel wall, related to the focal inflammation of the underlying vasculitis, since it may be reversed by suppression of the immune inflammation. However, the mechanisms for this ECD differ from that of the primary vasculitis. Preliminary evidence suggests that inflammatory mediators such as CRP, TNF, or sphingolipids may be involved. The diffuse ECD of vasculitis may have important consequences for both the progression of the primary disease and for cardiovascular events. A model for the role of vasculitis-induced ECD in the accelerated atherogenesis of rheumatic diseases is presented. These concepts are discussed together with the messages they suggest for 'idiopathic' atherosclerosis in the general population.

Sphingomyelinase and ceramide analogs induce vasoconstriction and leukocyte-endothelial interactions in cerebral venules in the intact rat brain: Insight into mechanisms and possible relation to brain injury and stroke

This study was designed to test the hypothesis that the sphingomyelin-ceramide signaling pathway may be important in proinflammatory-like responses in the intact brain. Effects of neutral sphingomyelinase (N-SMase), ceramide analogs, phosphorylcholine and ceramide metabolites were studied on rat brain cerebral (cortical) venule lumen sizes, leukocyte rolling, velocity and endothelial cell wall adhesion, microvessel permeability, microvessel rupture and focal hemorrhages using in vivo high resolution TV microscopy. Perivascular and close intra-arterial administration of N-SMase, C(2)-, C(8)-, and C(16)-ceramide, but not either phosphorylcholine, C(6)-ceramide, nervonic (C(24):1) ceramide, lignoceric (C(24):0) ceramide, C(8)-ceramide-1-phosphate, glucosylceramide or 1-0-acylceramide, resulted in potent, concentration-dependent constriction (and spasm) of cortical venules, followed by increased leukocyte rolling, decreased leukocyte velocities, increased leukocyte-endothelial wall adhesion, increased venular wall permeability, postcapillary venule rupture and, often, micro-hemorrhaging at high concentrations; angiotensin II, serotonin and PGF(2alpha) didn't demonstrate these characteristics. Pretreatment with either one of three different antioxidants, including inhibitors of NF-kappaB activation, or two different Ca(2+) channel blockers either prevented or attenuated the adverse venular effects of N-SMase and the ceramides. Likewise, pretreatment with either a PKCalpha-beta antagonist or a MAP kinase antagonist also attenuated the adverse venular effects. These results suggest that N-SMase and several ceramides can result in potent venular cerebrovasospasm, leukocyte-endothelial chemoattraction, and microvessel wall permeability changes in the intact rat brain. These proinflammatory-like actions suggest that N-SMase and ceramides could produce brain-vascular damage by reperfusion injury triggering lipid peroxidation, release of reactive oxygen species and activation of diverse signaling pathways: PKCalpha-beta isozymes, MAP kinase and NF-kappaB.

Elevated sphingomyelinase and hypercytokinemia in hemophagocytic lymphohistiocytosis

PURPOSE

Ceramide generated from sphingomyelinase activation has been reported to play a role in cytokine-mediated events. Secretory sphingomyelinase (S-SMase), a product of the acid sphingomyelinase gene, has been found to be derived from many cell types and to exist in human serum. The purpose of the current study was to investigate the serum level of S-SMase.

PATIENTS AND METHODS

Three patients with hypercytokinemia caused by hemophagocytic lymphohistiocytosis (HLH) were studied. Serum samples were collected from the three patients with HLH, patients with a deficiency of acid sphingomyelinase, or type B Niemann-Pick disease, and normal controls. The S-SMase activities were determined using 14C-sphingomyelin as a substrate.

RESULTS

The serum S-SMase activities were increased 10-to 20-fold in the sera from the three patients with HLH (P < 0.0001). The high S-SMase activity was decreased to the normal level along with the clinical improvement of HLH.

CONCLUSIONS

Increased release of S-SMase from cells into the circulation is observed in patients with active HLH. The authors suggest that S-SMase is related to the pathophysiology of hypercytokinemia.

Genetic alterations of IL-1 receptor antagonist in mice affect plasma cholesterol level and foam cell lesion size

Inflammatory cytokines have been linked to atherosclerosis by using cell culture models and acute inflammation in animals. The goal of this study was to examine lipoprotein levels and early atherosclerosis in chronic animal models of altered IL-1 physiology by using mice with deficient or excess IL-1 receptor antagonist (IL-1ra). IL-1ra knockout C57BL/6J mice fed a cholesterol/cholate diet for 3 mo had a 3-fold decrease in non-high-density lipoprotein cholesterol and a trend toward increased foam-cell lesion area compared to wild-type littermate controls. IL-1ra transgenic/low-density lipoprotein receptor (LDLR) knockout mice fed a cholesterol-saturated fat diet for 10 wk showed a 40% increase in non-high-density lipoprotein cholesterol, consistent with the IL-1ra knockout data, although there was no change in lesion size. When these IL1-ra overexpressing transgenic mice on the LDLR knockout background were fed a high-cholesterol/high-fat diet containing cholate, however, a statistically significant 40% decrease in lesion area was observed compared to LDLR knockout mice lacking the transgene. By immunohistochemistry, IL-1ra was present in C57BL/6J and LDLR knockout aortae, absent in IL-1ra knockout aortae, and present at high levels in LDLR knockout/IL-1ra transgene aortae. In summary, IL-1ra tended to increase plasma lipoprotein levels and, when fed a cholate-containing diet, decrease foam-cell lesion size. These data demonstrate that in selected models of murine atherosclerosis, chronic IL-1ra depletion or overexpression has potentially important effects on lipoprotein metabolism and foam-cell lesion development.