Role of ceramide in lipopolysaccharide (LPS)-induced signaling. LPS increases ceramide rather than acting as a structural homolog

Functional roles of sphingolipids in immunity and their implication in disease

Sphingolipids, which are components of cellular membranes and organ tissues, can be synthesized or degraded to modulate cellular responses according to environmental cues, and the balance among the different sphingolipids is important for directing immune responses, regardless of whether they originate, as intra- or extracellular immune events. Recent progress in multiomics-based analyses and methodological approaches has revealed that human health and diseases are closely related to the homeostasis of sphingolipid metabolism, and disease-specific alterations in sphingolipids and related enzymes can be prognostic markers of human disease progression. Accumulating human clinical data from genome-wide association studies and preclinical data from disease models provide support for the notion that sphingolipids are the missing pieces that supplement our understanding of immune responses and diseases in which the functions of the involved proteins and nucleotides have been established. In this review, we analyze sphingolipid-related enzymes and reported human diseases to understand the important roles of sphingolipid metabolism. We discuss the defects and alterations in sphingolipid metabolism in human disease, along with functional roles in immune cells. We also introduce several methodological approaches and provide summaries of research on sphingolipid modulators in this review that should be helpful in studying the roles of sphingolipids in preclinical studies for the investigation of experimental and molecular medicines.

Sphingolipid metabolism during Toll-like receptor 4 (TLR4)-mediated macrophage activation

Macrophage activation in response to stimulation of Toll-like receptor 4 (TLR4) provides a paradigm for investigating energy metabolism that regulates the inflammatory response. TLR4-mediated pro-inflammatory macrophage activation is characterized by increased glycolysis and altered mitochondrial metabolism, supported by selective amino acid uptake and/or usage. Fatty acid metabolism remains as a highly complex rewiring that accompanies classical macrophage activation. TLR4 activation leads to de novo synthesis of fatty acids, which flux into sphingolipids, complex lipids that form the building blocks of eukaryotic cell membranes and regulate cell function. Here, we review the importance of TLR4-mediated de novo synthesis of membrane sphingolipids in macrophages. We first highlight fatty acid metabolism during TLR4-driven macrophage immunometabolism. We then focus on the temporal dynamics of sphingolipid biosynthesis and emphasize the modulatory role of some sphingolipid species (i.e. sphingomyelins, ceramides and glycosphingolipids) on the pro-inflammatory and pro-resolution phases of LPS/TLR4 activation in macrophages.

Molecular Mechanisms of Lipid Metabolism Disorders in Infectious Exacerbations of Chronic Obstructive Pulmonary Disease

Exacerbations largely determine the character of the progression and prognosis of chronic obstructive pulmonary disease (COPD). Exacerbations are connected with changes in the microbiological landscape in the bronchi due to a violation of their immune homeostasis. Many metabolic and immune processes involved in COPD progression are associated with bacterial colonization of the bronchi. The objective of this review is the analysis of the molecular mechanisms of lipid metabolism and immune response disorders in the lungs in COPD exacerbations. The complex role of lipid metabolism disorders in the pathogenesis of some infections is only beginning to be understood, however, there are already fewer and fewer doubts even now about its significance both in the pathogenesis of infectious exacerbations of COPD and in general in the progression of the disease. It is shown that the lipid rafts of the plasma membranes of cells are involved in many processes related to the detection of pathogens, signal transduction, the penetration of pathogens into the cell. Smoking disrupts the normally proceeded processes of lipid metabolism in the lungs, which is a part of the COPD pathogenesis.

Adipose tissue and insulin resistance in obese

Currently, obesity has become a global health issue and is referred to as an epidemic. Dysfunctional obese adipose tissue plays a pivotal role in the development of insulin resistance. However, the mechanism of how dysfunctional obese-adipose tissue develops insulin-resistant circumstances remains poorly understood. Therefore, this review attempts to highlight the potential mechanisms behind obesity-associated insulin resistance. Multiple risk factors are directly or indirectly associated with the increased risk of obesity; among them, environmental factors, genetics, aging, gut microbiota, and diets are prominent. Once an individual becomes obese, adipocytes increase in their size; therefore, adipose tissues become larger and dysfunctional, recruit macrophages, and then these polarize to pro-inflammatory states. Enlarged adipose tissues release excess free fatty acids (FFAs), reactive oxygen species (ROS), and pro-inflammatory cytokines. Excess systemic FFAs and dietary lipids enter inside the cells of non-adipose organs such as the liver, muscle, and pancreas, and are deposited as ectopic fat, generating lipotoxicity. Toxic lipids dysregulate cellular organelles, e.g., mitochondria, endoplasmic reticulum, and lysosomes. Dysregulated organelles release excess ROS and pro-inflammation, resulting in systemic inflammation. Long term low-grade systemic inflammation prevents insulin from its action in the insulin signaling pathway, disrupts glucose homeostasis, and results in systemic dysregulation. Overall, long-term obesity and overnutrition develop into insulin resistance and chronic low-grade systemic inflammation through lipotoxicity, creating the circumstances to develop clinical conditions. This review also shows that the liver is the most sensitive organ undergoing insulin impairment faster than other organs, and thus, hepatic insulin resistance is the primary event that leads to the subsequent development of peripheral tissue insulin resistance.

Bioactive lipids, inflammation and chronic diseases

Endogenous bioactive lipids are part of a complex network that modulates a plethora of cellular and molecular processes involved in health and disease, of which inflammation represents one of the most prominent examples. Inflammation serves as a well-conserved defence mechanism, triggered in the event of chemical, mechanical or microbial damage, that is meant to eradicate the source of damage and restore tissue function. However, excessive inflammatory signals, or impairment of pro-resolving/anti-inflammatory pathways leads to chronic inflammation, which is a hallmark of chronic pathologies. All main classes of endogenous bioactive lipids - namely eicosanoids, specialized pro-resolving lipid mediators, lysoglycerophopsholipids and endocannabinoids - have been consistently involved in the chronic inflammation that characterises pathologies such as cancer, diabetes, atherosclerosis, asthma, as well as autoimmune and neurodegenerative disorders and inflammatory bowel diseases. This review gathers the current knowledge concerning the involvement of endogenous bioactive lipids in the pathogenic processes of chronic inflammatory pathologies.

Crosstalk Between the Gut Microbiome and Bioactive Lipids: Therapeutic Targets in Cognitive Frailty

Cognitive frailty is a geriatric condition defined by the coexistence of cognitive impairment and physical frailty. This "composite" aging phenotype is associated with a higher risk of several adverse health-related outcomes, including dementia. In the last decade, cognitive frailty has gained increased attention from the scientific community that has focused on understanding the clinical impact and the physiological and pathological mechanisms of development and on identifying preventive and/or rehabilitative therapeutic interventions. The emergence of gut microbiome in neural signaling increased the interest in targeting the gut-brain axis as a modulation strategy. Multiple studies on gastroenteric, metabolic, and neurodegenerative diseases support the existence of a wide bidirectional communication network of signaling mediators, e.g., bioactive lipids, that can modulate inflammation, gut permeability, microbiota composition, and the gut-brain axis. This crosstalk between the gut-brain axis, microbiome, and bioactive lipids may emerge as the basis of a promising therapeutic strategy to counteract cognitive frailty. In this review, we summarize the evidence in the literature regarding the link between the gut microbiome, brain, and several families of bioactive lipids. In addition, we also explore the applicability of several bioactive lipid members as a potential routes for therapeutic interventions to combat cognitive frailty.

Roles of Ceramides and Other Sphingolipids in Immune Cell Function and Inflammation

Ceramides are bioactive sphingolipids that support the structure of the plasma membrane and mediate numerous cell-signaling events in eukaryotic cells. The finding that ceramides act as second messengers transducing cellular signals has attracted substantial attention in several fields of Biology. Since all cells contain lipid plasma membranes, the impact of various ceramides, ceramide synthases, ceramide metabolites, and other sphingolipids has been implicated in a vast range of cellular functions including, migration, proliferation, response to external stimuli, and death. The roles of lipids in these functions widely differ among the diverse cell types. Herein, we discuss the roles of ceramides and other sphingolipids in mediating the function of various immune cells; particularly dendritic cells, neutrophils, and macrophages. In addition, we highlight the main studies describing effects of ceramides in inflammation, specifically in various inflammatory settings including insulin resistance, graft-versus-host disease, immune suppression in cancer, multiple sclerosis, and inflammatory bowel disease.

Alteration of sphingolipid metabolism as a putative mechanism underlying LPS-induced BBB disruption

Septic encephalopathy with confusion and agitation occurs early during sepsis and contributes to the severity of the disease. A decrease in the sphingosine-1-phosphate (S1P) blood levels has been shown in patients and in animal models of sepsis. The lipid mediator S1P is known to be involved in endothelial barrier function in a context-dependent manner. We utilized lipopolysaccharide (LPS)-injected mice as a model for septic encephalopathy and first performed tracer permeability assays to assess the blood-brain barrier (BBB) breakdown in vivo. At time points corresponding to the BBB breakdown post LPS injection, we aimed to characterize the regulation of the sphingolipid signaling pathway at the BBB during sepsis. We measured sphingolipid concentrations in blood, in mouse brain microvessels (MBMVs), and brain tissue. We also analyzed the expression of S1P receptors, transporters, and metabolizing enzymes in MBMVs and brain tissue. Primary mouse brain microvascular endothelial cells (MBMECs) were isolated to evaluate the effects of LPS on transendothelial electrical resistance (TEER) as a measure of permeability in vitro. We observed a relevant decrease in S1P levels after LPS injection in all three compartments (blood, MBMVs, brain tissue) that was accompanied by an increased expression of the S1P receptor type 1 and of sphingosine kinase 1 on one hand and of the S1P degrading enzymes lipid phosphate phosphatase 1 (LPP1) and S1P phosphatase 1 on the other hand, as well as a down-regulation of sphingosine kinase 2. Application of LPS to a monolayer of primary MBMECs did not alter TEER, but serum from LPS-treated mice lead to a breakdown of the barrier compared to serum from vehicle-treated mice. We observed profound alterations of the sphingolipid metabolism at the BBB after LPS injection that point toward a therapeutic potential of drugs interfering with this pathway as novel approach for the detrimental overwhelming immune response in sepsis. Read the Editorial Highlight for this article on page 115. Cover Image for this Issue: doi. 10.1111/jnc.14161.

Fatty acid transport protein 1 enhances the macrophage inflammatory response by coupling with ceramide and c-Jun N-terminal kinase signaling

Macrophages are important cells that need to be controlled at the site of inflammation. Several factors are involved in chronic inflammation and its timely resolution. Free fatty acids drive the inflammatory response in macrophages and contribute to the vicious cycle of the inflammatory response. However, the identity of the uptake pathways of fatty acids is not fully clear in macrophages and how the inflammatory responses are regulated by the uptake of fatty acids remain poorly understood. We investigated the relationship between fatty acid transport protein (FATP) and the inflammatory response signaling pathway in macrophages as the first report. The FATP family has composed six isoforms, FATP1-6. We found that FATP1 is the most highly expressed isoform in macrophages. Forced expression of FATP1 enhanced production of inflammatory cytokines, such as TNFα and IL-6 concomitant with the increased uptake of fatty acids, increased level of ceramide, and increased phosphorylation of c-Jun N-terminal kinase (JNK). The enhancement by FATP1 was abolished by treatment with a JNK inhibitor, NF-κB inhibitor, or ceramide synthesis inhibitor. siRNA-mediated knockdown of FATP1 strongly inhibited the production of TNFα and IL-6. Similarly, an inhibitor of FATP1 inhibited the production of TNFα and IL-6. Finally, an inhibitor of FATP1 attenuated the production of inflammatory cytokines in bronchoalveolar lavage fluid in an LPS-induced acute lung injury in vivo mouse model. In summary, we propose that FATP1 is an important regulator of inflammatory response signaling in macrophages. Our findings suggest that ceramide-JNK signaling is important to terminate or sustain inflammation.

Phosphorylation of signal transducer and activator of transcription 3 induced by hyperglycemia is different with that induced by lipopolysaccharide or erythropoietin via receptor‑coupled signaling in cardiac cells

The signal transducer and activator of transcription 3 (STAT3) is known to be involved in hypertrophy and fibrosis in cardiac dysfunction. The activation of STAT3 via the phosphorylation of STAT3 is required for the production of functional activity. It has been established that lipopolysaccharide (LPS)‑induced phosphorylation of STAT3 in cardiomyocytes primarily occurs through a direct receptor‑mediated action. This effect is demonstrated to be produced rapidly. STAT3 in cardiac fibrosis of diabetes is induced by high glucose through promotion of the STAT3‑associated signaling pathway. However, the time schedule for STAT3 activation between LPS and high glucose appears to be different. Therefore, the difference in STAT3 activation between LPS and hyperglycemia in cardiomyocytes requires elucidation. The present study investigated the phosphorylation of STAT3 induced by LPS and hyperglycemia in the rat cardiac cell line H9c2. Additionally, phosphorylation of STAT3 induced by erythropoietin (EPO) via receptor activation was compared. Then, the downstream signals for fibrosis, including the connective tissue growth factor (CTGF) and matrix metalloproteinase (MMP)‑9, were determined using western blotting, while the mRNA levels were quantified. LPS induced a rapid elevation of STAT3 phosphorylation in H9c2 cells within 30 min, similar to that produced by EPO. However, LPS or EPO failed to modify the mRNA level of STAT3, and/or the downstream signals for fibrosis. High glucose increased STAT3 phosphorylation to be stable after a long period of incubation. Glucose incubation for 24 h may augment the STAT3 expression in a dose‑dependent manner. Consequently, fibrosis‑associated signals, including CTGF and MMP‑9 protein, were raised in parallel. In the presence of tiron, an antioxidant, these changes by hyperglycemia were markedly reduced, demonstrating the mediation of oxidative stress. Therefore, LPS‑ or EPO‑induced STAT3 phosphorylation is different compared with that caused by high glucose in H9c2 cells. Sustained activation of STAT3 by hyperglycemia may promote the expression of fibrosis‑associated signals, including CTGF and MMP‑9, in H9c2 cells. Therefore, regarding the cardiac dysfunctions associated with diabetes and/or hyperglycemia, the identification of nuclear STAT3 may be more reliable compared with the assay of phosphorylated STAT3 in cardiac cells.

A Comprehensive High-Resolution Targeted Workflow for the Deep Profiling of Sphingolipids

Sphingolipids make up a highly diverse group of biomolecules that not only are membrane components but also are involved in various cellular functions such as signaling and protein sorting. To obtain a quantitative view of the sphingolipidome, sensitive, accurate, and comprehensive methods are needed. Here, we present a targeted reversed-phase liquid chromatography-high-resolution mass spectrometry-based workflow that significantly increases the accuracy of measured sphingolipids by resolving nearly isobaric and isobaric species; this is accomplished by a use of (i) an optimized extraction procedure, (ii) a segmented gradient, and (iii) parallel reaction monitoring of a sphingolipid specific fragmentation pattern. The workflow was benchmarked against an accepted sphingolipid model system, the RAW 264.7 cell line, and 61 sphingolipids were quantified over a dynamic range of 7 orders of magnitude, with detection limits in the low femtomole per milligram of protein level, making this workflow an extremely versatile tool for high-throughput sphingolipidomics.

Dietary and Endogenous Sphingolipid Metabolism in Chronic Inflammation

Chronic inflammation is a common underlying factor in many major metabolic diseases afflicting Western societies. Sphingolipid metabolism is pivotal in the regulation of inflammatory signaling pathways. The regulation of sphingolipid metabolism is in turn influenced by inflammatory pathways. In this review, we provide an overview of sphingolipid metabolism in mammalian cells, including a description of sphingolipid structure, biosynthesis, turnover, and role in inflammatory signaling. Sphingolipid metabolites play distinct and complex roles in inflammatory signaling and will be discussed. We also review studies examining dietary sphingolipids and inflammation, derived from in vitro and rodent models, as well as human clinical trials. Dietary sphingolipids appear to influence inflammation-related chronic diseases through inhibiting intestinal lipid absorption, altering gut microbiota, activation of anti-inflammatory nuclear receptors, and neutralizing responses to inflammatory stimuli. The anti-inflammatory effects observed with consuming dietary sphingolipids are in contrast to the observation that most cellular sphingolipids play roles in augmenting inflammatory signaling. The relationship between dietary sphingolipids and low-grade chronic inflammation in metabolic disorders is complex and appears to depend on sphingolipid structure, digestion, and metabolic state of the organism. Further research is necessary to confirm the reported anti-inflammatory effects of dietary sphingolipids and delineate their impacts on endogenous sphingolipid metabolism.

C2-ceramide enhances sorafenib-induced caspase-dependent apoptosis via PI3K/AKT/mTOR and Erk signaling pathways in HCC cells

Sorafenib as an effective multikinase inhibitor has been approved for the clinical treatment against advanced hepatocellular carcinoma (HCC). HCC treatment requires usually combined therapy because of its complex pathogenesis. Ceramide has been confirmed to induce remarkable apoptosis in human tumor cells and has attracted increasing attention in investigations on combination therapy. In this paper, the anti-HCC effect of sorafenib combined with C2-ceramide was investigated on cell vitality, apoptosis, and migration, and the underlying mechanism was examined using flow cytometry and western blot. Bel7402 cells coincubated with sorafenib and C2-ceramide exhibited lower cell vitality and more irregular cellular morphology and cell cycle arrest. Sorafenib plus C2-ceramide stimulated significantly the production of reactive oxygen species (ROS) and mitochondrial depolarization, which promoted caspases-dependent cell apoptosis as illustrated by related protein expression including caspase 3, caspase 9, Bax, Bcl-2, and cytochrome c. Combination treatment of sorafenib and C2-ceramide inhibited obviously cell growth and proliferation via PI3K/AKT/mTOR and Erk signaling pathways. Furthermore, the combination treatment was proved to inhibit cell migration and epithelial-mesenchymal transition (EMT). These findings indicated that the combination of C2-ceramide and sorafenib provided synergistic inhibitory effects on HCC cells.

Sphingolipids as Mediators in the Crosstalk between Microbiota and Intestinal Cells: Implications for Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) describes different illnesses characterized by chronic inflammation of the gastrointestinal tract. Although the pathogenic mechanisms leading to IBD are poorly understood, immune system disturbances likely underlie its development. Sphingolipids (SLs) have been identified as important players and promising therapeutic targets to control inflammation in IBD. Interestingly, it seems that microorganisms of the normal gut microbiota and probiotics are involved in sphingolipid function. However, there is a great need to investigate the role of SLs as intermediates in the crosstalk between intestinal immunity and microorganisms. This review focuses on recent investigations that describe some mechanisms involved in the regulation of cytokine profiles by SLs. We also describe the importance of gut microbiota in providing signaling molecules that favor the communication between resident bacteria and intestinal cells. This, in turn, modulates the immune response in the bowel and likely in other peripheral organs. The potential of SLs and gut microbiota as targets or therapeutic agents for IBD is also discussed.

Alkaline ceramidase 3 deficiency aggravates colitis and colitis-associated tumorigenesis in mice by hyperactivating the innate immune system

Increasing studies suggest that ceramides differing in acyl chain length and/or degree of unsaturation have distinct roles in mediating biological responses. However, still much remains unclear about regulation and role of distinct ceramide species in the immune response. Here, we demonstrate that alkaline ceramidase 3 (Acer3) mediates the immune response by regulating the levels of C18:1-ceramide in cells of the innate immune system and that Acer3 deficiency aggravates colitis in a murine model by augmenting the expression of pro-inflammatory cytokines in myeloid and colonic epithelial cells (CECs). According to the NCBI Gene Expression Omnibus (GEO) database, ACER3 is downregulated in immune cells in response to lipopolysaccharides (LPS), a potent inducer of the innate immune response. Consistent with these data, we demonstrated that LPS downregulated both Acer3 mRNA levels and its enzymatic activity while elevating C(18:1)-ceramide, a substrate of Acer3, in murine immune cells or CECs. Knocking out Acer3 enhanced the elevation of C(18:1)-ceramide and the expression of pro-inflammatory cytokines in immune cells and CECs in response to LPS challenge. Similar to Acer3 knockout, treatment with C(18:1)-ceramide, but not C18:0-ceramide, potentiated LPS-induced expression of pro-inflammatory cytokines in immune cells. In the mouse model of dextran sulfate sodium-induced colitis, Acer3 deficiency augmented colitis-associated elevation of colonic C(18:1)-ceramide and pro-inflammatory cytokines. Acer3 deficiency aggravated diarrhea, rectal bleeding, weight loss and mortality. Pathological analyses revealed that Acer3 deficiency augmented colonic shortening, immune cell infiltration, colonic epithelial damage and systemic inflammation. Acer3 deficiency also aggravated colonic dysplasia in a mouse model of colitis-associated colorectal cancer. Taken together, these results suggest that Acer3 has an important anti-inflammatory role by suppressing cellular or tissue C(18:1)-ceramide, a potent pro-inflammatory bioactive lipid and that dysregulation of ACER3 and C(18:1)-ceramide may contribute to the pathogenesis of inflammatory diseases including cancer.

The role of sphingolipids in endothelial barrier function

Sphingolipids are a ubiquitous family of essential lipids with an increasingly understood role as biologically active mediators in numerous physiologic and pathologic processes. Two particular sphingolipid species, sphingosine-1-phosphate and ceramide, and their metabolites interact both directly and indirectly with endothelial cells to regulate vascular permeability. Sphingosine-1-phosphate generally augments endothelial integrity while ceramide tends to promote vascular leak, and a tight balance between the two is necessary to maintain normal physiologic function. The mechanisms by which sphingolipids regulate endothelial barrier function are complex and occur through multiple different pathways, and disruptions or imbalances in these pathways have been implicated in a number of specific disease processes. With improved understanding of sphingolipid biology, endothelial function, and the interactions between the two, several targets for therapeutic intervention have emerged and there is immense potential for further advancement in this field.

Arabinoxylan hydrolyzates as immunomodulators in lipopolysaccharide-induced RAW264.7 macrophages

The immunomodulatory effects of arabinoxylan (AX) have been demonstrated before. However, these effects could be structure driven. Thus, we indicate the relationship between fine structural details and immunomodulatory properties of AX.

Tumor suppressive functions of ceramide: evidence and mechanisms

Studies over the past two decades have identified ceramide as a multifunctional central molecule in the sphingolipid biosynthetic pathway. Given its diverse tumor suppressive activities, molecular understanding of ceramide action will produce fundamental insights into processes that limit tumorigenesis and may identify key molecular targets for therapeutic intervention. Ceramide can be activated by a diverse array of stresses such as heat shock, genotoxic damage, oxidative stress and anticancer drugs. Ceramide triggers a variety of tumor suppressive and anti-proliferative cellular programs such as apoptosis, autophagy, senescence, and necroptosis by activating or repressing key effector molecules. Defects in ceramide generation and metabolism in cancer contribute to tumor cell survival and resistance to chemotherapy. The potent and versatile anticancer activity profile of ceramide has motivated drug development efforts to (re-)activate ceramide in established tumors. This review focuses on our current understanding of the tumor suppressive functions of ceramide and highlights the potential downstream targets of ceramide which are involved in its tumor suppressive action.

GPR40/FFA1 and neutral sphingomyelinase are involved in palmitate-boosted inflammatory response of microvascular endothelial cells to LPS

OBJECTIVES

Increased levels of both saturated fatty acids (SFAs) and lipopolysaccharide (LPS) are associated with type 2 diabetes. However, it remains largely unknown how SFAs interact with LPS to regulate inflammatory responses in microvascular endothelial cells (MIC ECs) that are critically involved in atherosclerosis as a diabetic complication. In this study, we compared the effects of LPS, palmitic acid (PA), the most abundant saturated fatty acid, or the combination of LPS and PA on interleukin (IL)-6 expression by MIC ECs and explored the underlying mechanisms.

METHODS

Human cardiac MIC ECs were treated with LPS, PA and LPS plus PA and the regulatory pathways including receptors, signal transduction, transcription and post-transcription, and sphingolipid metabolism for IL-6 expression were investigated.

RESULTS

G protein-coupled receptor (GPR)40 or free fatty acid receptor 1 (FFA1), but not toll-like receptor 4, was involved in PA-stimulated IL-6 expression. PA not only stimulated IL-6 expression by itself, but also remarkably enhanced LPS-stimulated IL-6 expression via a cooperative stimulation on mitogen-activated protein kinase and nuclear factor kappa B signaling pathways, and both transcriptional and post-transcriptional activation. Furthermore, PA induced a robust neutral sphingomyelinase (nSMase)-mediated sphingomyelin hydrolysis that was involved in PA-augmented IL-6 upregulation.

CONCLUSION

PA boosted inflammatory response of microvascular endothelial cells to LPS via GPR40 and nSMase.

Role of lipid microdomains in TLR-mediated signalling

Over the last twenty years, evidence has been provided that the plasma membrane is partitioned with microdomains, laterally mobile in the bilayer, providing the necessary microenvironment to specific membrane proteins for signalling pathways to be initiated. We discuss here the importance of such microdomains for Toll-like receptors (TLR) localization and function. First, lipid microdomains favour recruitment and clustering of the TLR machinery partners, i.e. receptors and co-receptors previously identified to be required for ligand recognition and signal transmission. Further, the presence of the so-called Cholesterol Recognition Amino-Acid Consensus (CRAC) sequences in the intracellular juxtamembrane domain of several Toll-like receptors suggests a direct role of cholesterol in the activation process. This article is part of a Special Issue entitled: Lipid-protein interactions.

Co-operation of TLR4 and raft proteins in LPS-induced pro-inflammatory signaling

Toll-like receptor 4 (TLR4) is activated by lipopolysaccharide (LPS), a component of Gram-negative bacteria to induce production of pro-inflammatory mediators aiming at eradication of the bacteria. Dysregulation of the host responses to LPS can lead to a systemic inflammatory condition named sepsis. In a typical scenario, activation of TLR4 is preceded by binding of LPS to CD14 protein anchored in cholesterol- and sphingolipid-rich microdomains of the plasma membrane called rafts. CD14 then transfers the LPS to the TLR4/MD-2 complex which dimerizes and triggers MyD88- and TRIF-dependent production of pro-inflammatory cytokines and type I interferons. The TRIF-dependent signaling is linked with endocytosis of the activated TLR4, which is controlled by CD14. In addition to CD14, other raft proteins like Lyn tyrosine kinase of the Src family, acid sphingomyelinase, CD44, Hsp70, and CD36 participate in the TLR4 signaling triggered by LPS and non-microbial endogenous ligands. In this review, we summarize the current state of the knowledge on the involvement of rafts in TLR4 signaling, with an emphasis on how the raft proteins regulate the TLR4 signaling pathways. CD14-bearing rafts, and possibly CD36-rich rafts, are believed to be preferred sites of the assembly of a multimolecular complex which mediates the endocytosis of activated TLR4.

TLR2, TLR4 and CD14 recognize venom-associated molecular patterns from Tityus serrulatus to induce macrophage-derived inflammatory mediators

Scorpion sting-induced human envenomation provokes an intense inflammatory reaction. However, the mechanisms behind the recognition of scorpion venom and the induction of mediator release in mammalian cells are unknown. We demonstrated that TLR2, TLR4 and CD14 receptors sense Tityus serrulatus venom (TsV) and its major component, toxin 1 (Ts1), to mediate cytokine and lipid mediator production. Additionally, we demonstrated that TsV induces TLR2- and TLR4/MyD88-dependent NF-κB activation and TLR4-dependent and TLR2/MyD88-independent c-Jun activation. Similar to TsV, Ts1 induces MyD88-dependent NF-κB phosphorylation via TLR2 and TLR4 receptors, while c-Jun activation is dependent on neither TLR2 nor TLR4/MyD88. Therefore, we propose the term venom-associated molecular pattern (VAMP) to refer to molecules that are introduced into the host by stings and are recognized by PRRs, resulting in inflammation.

Acid sphingomyelinase plays a key role in palmitic acid-amplified inflammatory signaling triggered by lipopolysaccharide at low concentrations in macrophages

Periodontal disease is more prevalent and severe in patients with diabetes than in nondiabetic patients. In addition to diabetes, a large number of studies have demonstrated an association between obesity and chronic periodontal disease. However, the underlying mechanisms have not been well understood. Since plasma free fatty acids (FAs) are elevated in obese patients and saturated FAs such as palmitic acid (PA) have been shown to increase host inflammatory response, we sought to find out how PA interacts with lipopolysaccharide (LPS), an important pathological factor involved in periodontal disease, to enhance inflammation. We found that whereas low concentration of LPS (1 ng/ml) stimulated interleukin (IL)-6 expression in RAW 264.7 macrophages, PA further augmented it fourfold. Besides IL-6, PA amplified the stimulatory effect of LPS on a large amount of Toll-like receptor (TLR)4-mediated expression of proinflammatory signaling molecules such as IL-1 receptor-associated kinase-like 2 and proinflammatory molecules, including monocyte chemotactic protein-1 and colony-stimulating factor. We also observed that PA augmented TLR4 but not TLR2 signal, and the augmentation was mediated by nuclear factor-κB (NF-κB) pathways. To further elucidate the regulatory mechanism whereby PA amplifies LPS signal, our studies showed that PA and LPS synergistically increased hydrolysis of sphingomyelin by stimulating acid sphingomyelinase (ASMase) activity, which contributed to a marked increase in ceramide production and IL-6 upregulation. Taken together, this study has demonstrated that PA markedly augments TLR4-mediated proinflammatory signaling triggered by low concentration of LPS in macrophages, and ASMase plays a key role in the augmentation.

A combined omics study on activated macrophages--enhanced role of STATs in apoptosis, immunity and lipid metabolism

BACKGROUND

Macrophage activation by lipopolysaccharide and adenosine triphosphate (ATP) has been studied extensively because this model system mimics the physiological context of bacterial infection and subsequent inflammatory responses. Previous studies on macrophages elucidated the biological roles of caspase-1 in post-translational activation of interleukin-1β and interleukin-18 in inflammation and apoptosis. However, the results from these studies focused only on a small number of factors. To better understand the host response, we have performed a high-throughput study of Kdo2-lipid A (KLA)-primed macrophages stimulated with ATP.

RESULTS

The study suggests that treating mouse bone marrow-derived macrophages with KLA and ATP produces 'synergistic' effects that are not seen with treatment of KLA or ATP alone. The synergistic regulation of genes related to immunity, apoptosis and lipid metabolism is observed in a time-dependent manner. The synergistic effects are produced by nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) and activator protein (AP)-1 through regulation of their target cytokines. The synergistically regulated cytokines then activate signal transducer and activator of transcription (STAT) factors that result in enhanced immunity, apoptosis and lipid metabolism; STAT1 enhances immunity by promoting anti-microbial factors; and STAT3 contributes to downregulation of cell cycle and upregulation of apoptosis. STAT1 and STAT3 also regulate glycerolipid and eicosanoid metabolism, respectively. Further, western blot analysis for STAT1 and STAT3 showed that the changes in transcriptomic levels were consistent with their proteomic levels. In summary, this study shows the synergistic interaction between the toll-like receptor and purinergic receptor signaling during macrophage activation on bacterial infection.

AVAILABILITY

Time-course data of transcriptomics and lipidomics can be queried or downloaded from http://www.lipidmaps.org.

CONTACT

shankar@ucsd.edu.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

LPS-treated macrophage cytokines repress surfactant protein-B in lung epithelial cells

In the mouse lung, Escherichia coli LPS can decrease surfactant protein-B (SFTPB) mRNA and protein concentrations. LPS also regulates the expression, synthesis, and concentrations of a variety of gene and metabolic products that inhibit SFTPB gene expression. The purpose of the present study was to determine whether LPS acts directly or indirectly on pulmonary epithelial cells to trigger signaling pathways that inhibit SFTPB expression, and whether the transcription factor CCAAT/enhancer binding protein (C/EBP)-β (CEBPB) is a downstream inhibitory effector. To investigate the mechanism of SFTPB repression, the human pulmonary epithelial cell lines NCI-H441 (H441) and NCI-H820 (H820) and the mouse macrophage-like cell line RAW264.7 were treated with LPS. Whereas LPS did not decrease SFTPB transcripts in H441 or H820 cells, the conditioned medium of LPS-treated RAW264.7 cells decreased SFTPB transcripts in H441 and H820 cells, and inhibited SFTPB promoter activity in H441 cells. In the presence of neutralizing anti-tumor necrosis factor (TNF) antibodies, the conditioned medium of LPS-treated RAW264.7 cells did not inhibit SFTPB promoter activity. In H441 cells treated with recombinant TNF protein, SFTPB transcripts decreased, whereas CEBPB transcripts increased and the transient coexpression of CEBPB decreased SFTPB promoter activity. Further, CEBPB short, interfering RNA increased basal SFTPB transcripts and countered the decrease of SFTPB transcripts by TNF. Together, these findings suggest that macrophages participate in the repression of SFTPB expression by LPS, and that macrophage-released cytokines (including TNF) regulate the transcription factor CEBPB, which can function as a downstream transcriptional repressor of SFTPB gene expression in pulmonary epithelial cells.

Saturated and unsaturated fat induce hepatic insulin resistance independently of TLR-4 signaling and ceramide synthesis in vivo

Hepatic insulin resistance is a principal component of type 2 diabetes, but the cellular and molecular mechanisms responsible for its pathogenesis remain unknown. Recent studies have suggested that saturated fatty acids induce hepatic insulin resistance through activation of the toll-like receptor 4 (TLR-4) receptor in the liver, which in turn transcriptionally activates hepatic ceramide synthesis leading to inhibition of insulin signaling. In this study, we demonstrate that TLR-4 receptor signaling is not directly required for saturated or unsaturated fat-induced hepatic insulin resistance in both TLR-4 antisense oligonucleotide treated and TLR-4 knockout mice, and that ceramide accumulation is not dependent on TLR-4 signaling or a primary event in hepatic steatosis and impairment of insulin signaling. Further, we show that both saturated and unsaturated fats lead to hepatic accumulation of diacylglycerols, activation of PKCε, and impairment of insulin-stimulated IRS-2 signaling. These data demonstrate that saturated fat-induced insulin resistance is independent of TLR-4 activation and ceramides.

Bacterial infections and ceramide

Ceramide is released from sphingomyelin primarily by the activity of acid, neutral, or alkaline sphingomyelinases or is synthesized de novo. Several bacteria, viruses, and even parasites infect mammalian cells by exploiting the acid sphingomyelinase or the neutral sphingomyelinase-ceramide system, or both. Sphingomyelinases and ceramide have been shown to be crucially involved in the internalization of pathogens, the induction of apoptosis in infected cells, the intracellular activation of signaling pathways, and the release of cytokines. The diverse functions of ceramide in infections suggest that the sphingomyelinase-ceramide system is a key player in the host response to many pathogens.

AICAR inhibits ceramide biosynthesis in skeletal muscle

BACKGROUND

The worldwide prevalence of obesity has lead to increased efforts to find therapies to treat obesity-related pathologies. Ceramide is a well-established mediator of several health problems that arise from adipose tissue expansion. The purpose of this study was to determine whether AICAR, an AMPK-activating drug, selectively reduces skeletal muscle ceramide synthesis.

METHODS

Murine myotubes and rats were challenged with palmitate and high-fat diet, respectively, to induce ceramide accrual, in the absence or presence of AICAR. Transcript levels of the rate-limiting enzyme in ceramide biosynthesis, serine palmitoyltransferase 2 (SPT2) were measured, in addition to lipid analysis. Student's t-test and ANOVA were used to assess the association between outcomes and groups.

RESULTS

Palmitate alone induced an increase in serine palmitoyltransferase 2 (SPT2) expression and an elevation of ceramide levels in myotubes. Co-incubation with palmitate and AICAR prevented both effects. However, ceramide and SPT2 increased with the addition of compound C, an AMPK inhibitor. In rats fed a high-fat diet (HFD), soleus SPT2 expression increased compared with normal chow-fed littermates. Moreover, rats on HFD that received daily AICAR injections had lower SPT2 levels and reduced muscle ceramide content compared with those on HFD only.

CONCLUSIONS

These results suggest that AICAR reduces ceramide synthesis by targeting SPT2 transcription, likely via AMPK activation as AMPK inhibition prevented the AICAR-induced improvements. Given the role of skeletal muscle ceramide in insulin resistance, it is tempting to speculate that interventions that activate AMPK may lead to long-term ceramide reduction and improved metabolic function.

Lipidomic profiling in Crohn's disease: abnormalities in phosphatidylinositols, with preservation of ceramide, phosphatidylcholine and phosphatidylserine composition

Crohn's disease is a chronic inflammatory condition largely affecting the terminal ileum and large bowel. A contributing cause is the failure of an adequate acute inflammatory response as a result of impaired secretion of pro-inflammatory cytokines by macrophages. This defective secretion arises from aberrant vesicle trafficking, misdirecting the cytokines to lysosomal degradation. Aberrant intestinal permeability is also well-established in Crohn's disease. Both the disordered vesicle trafficking and increased bowel permeability could result from abnormal lipid composition. We thus measured the sphingo- and phospholipid composition of macrophages, using mass spectrometry and stable isotope labelling approaches. Stimulation of macrophages with heat-killed Escherichia coli resulted in three main changes; a significant reduction in the amount of individual ceramide species, an altered composition of phosphatidylcholine, and an increased rate of phosphatidylcholine synthesis in macrophages. These changes were observed in macrophages from both healthy control individuals and patients with Crohn's disease. The only difference detected between control and Crohn's disease macrophages was a reduced proportion of newly-synthesised phosphatidylinositol 16:0/18:1 over a defined time period. Shotgun lipidomics analysis of macroscopically non-inflamed ileal biopsies showed a significant decrease in this same lipid species with overall preservation of sphingolipid, phospholipid and cholesterol composition.

The role of acid sphingomyelinase and caspase 5 in hypoxia-induced HuR cleavage and subsequent apoptosis in hepatocytes

A previous data showed that the hypoxia mimetic compound CoCl(2) induced cleavage of HuR and subsequent apoptosis in human oral cancer cells. We also previously demonstrated that exposure of NT-2 human neuronal precursor cells to hypoxia resulted in changes in sphingolipid levels and apoptosis. Since it is known that CoCl(2) induces cleavage of HuR, we investigated whether there is a link between HuR cleavage and the observed sphingolipid changes in cells exposed to hypoxia, and whether this link is associated with the induction of apoptosis. Exposure of hepatocytes to direct hypoxia by means of a hypoxic chamber resulted in acid sphingomyelinase activation and ceramide elevation. The elevation in ceramide levels was associated with activation of caspase 5 and the subsequent cleavage of HuR and apoptotic cell death. These data raise the possibility that acid sphingomyelinase and caspase 5 are each potential targets for treating hypoxia (ischemia)-induced liver injury.

IKK NBD peptide inhibits LPS induced pulmonary inflammation and alters sphingolipid metabolism in a murine model

Airway epithelial NF-κB is a key regulator of host defence in bacterial infections and has recently evolved as a target for therapeutical approaches. Evidence is accumulating that ceramide, generated by acid sphingomyelinase (aSMase), and sphingosine-1-phosphate (S1-P) are important mediators in host defence as well as in pathologic processes of acute lung injury. Little is known about the regulatory mechanisms of pulmonary sphingolipid metabolism in bacterial infections of the lung. The objective of this study was to evaluate the influence of NF-κB on sphingolipid metabolism in Pseudomonas aeruginosa LPS-induced pulmonary inflammation. In a murine acute lung injury model with intranasal Pseudomonas aeruginosa LPS we investigated TNF-α, KC (murine IL-8), IL-6, MCP-1 and neutrophilic infiltration next to aSMase activity and ceramide and S1-P lung tissue concentrations. Airway epithelial NF-κB was inhibited by topically applied IKK NBD, a cell penetrating NEMO binding peptide. This treatment resulted in significantly reduced inflammation and suppression of aSMase activity along with decreased ceramide and S1-P tissue concentrations down to levels observed in healthy animals. In conclusion our results confirm that changes in sphingolipid metabolim due to Pseudomonas aeruginosa LPS inhalation are regulated by NF-κB translocation. This confirms the critical role of airway epithelial NF-κB pathway for the inflammatory response to bacterial pathogens and underlines the impact of sphingolipids in inflammatory host defence mechanisms.

Exogenous ceramide-1-phosphate reduces lipopolysaccharide (LPS)-mediated cytokine expression

Toll-like receptor 4 (TLR4) is a component of the innate immune system that recognizes a diverse group of molecular structures, such as lipopolysaccharide (LPS) from Gram-negative bacteria. TLR4 signaling ultimately leads to activation of the transcription factor, nuclear factor κB (NF-κB), and the production of cytokines. Ceramide is a bioactive sphingolipid that has been suggested to regulate TLR4-induced NF-κB signaling, although reports on the role of ceramide in TLR4 activation conflict. We investigated the possibility that ceramide metabolites, such as ceramide-1-phosphate (C-1-P), may explain these discrepancies. We now report that exogenous C-1-P, but not ceramide, reduces NF-κB-mediated gene transcription in HEK 293 cells stably transfected with human TLR4, CD14, and MD-2. We demonstrate that inhibition of NF-κB by exogenous C-1-P is dose-dependent and specific to TLR4 in a reporter assay. We further demonstrate a requirement for both the phosphate moiety and the sphingoid backbone to inhibit LPS-activated NF-κB transcription. Specifically, C-1-P prevents the degradation of IκB, the phosphorylation of the p65 subunit of NF-κB, and LPS-stimulated MAPK activation. The functional consequence of C-1-P inhibition of NF-κB is a reduction in LPS-mediated cytokine release from HEK 293 TLR4-expressing cells and human peripheral blood mononuclear cells. Taken together, these data demonstrate that C-1-P may function as an anti-inflammatory lipid mediator of immune response.

Sphingolipid metabolism, oxidant signaling, and contractile function of skeletal muscle

SIGNIFICANCE

Sphingolipids are a class of bioactive lipids that regulate diverse cell functions. Ceramide, sphingosine, and sphingosine-1-phosphate accumulate in tissues such as liver, brain, and lung under conditions of cellular stress, including oxidative stress. The activity of some sphingolipid metabolizing enzymes, chiefly the sphingomyelinases, is stimulated during inflammation and in response to oxidative stress. Ceramide, the sphingomyelinase product, as well as the ceramide metabolite, sphingosine-1-phosphate, can induce the generation of more reactive oxygen species, propagating further inflammation.

RECENT ADVANCES

This review article summarizes information on sphingolipid biochemistry and signaling pertinent to skeletal muscle and describes the potential influence of sphingolipids on contractile function.

CRITICAL ISSUES

It encompasses topics related to (1) the pathways for complex sphingolipid biosynthesis and degradation, emphasizing sphingolipid regulation in various muscle fiber types and subcellular compartments; (2) the emerging evidence that implicates ceramide, sphingosine, and sphingosine-1-phosphate as regulators of muscle oxidant activity, and (3) sphingolipid effects on contractile function and fatigue.

FUTURE DIRECTIONS

We propose that prolonged inflammatory conditions alter ceramide, sphingosine, and sphingosine-1-phosphate levels in skeletal muscle and that these changes promote the weakness, premature fatigue, and cachexia that plague individuals with heart failure, cancer, diabetes, and other chronic inflammatory diseases.

ASA404: a tumor vascular-disrupting agent with broad potential for cancer therapy

ASA404 (5,6-dimethylxanthenone-4-acetic acid) was developed as an analogue of flavone acetic acid and found to induce hemorrhagic necrosis of experimental tumors. ASA404 simultaneously targets at least two cell types - vascular endothelial cells and macrophages - within the tumor microenvironment. In murine tumors, ASA404 induces coordinated decreases in tumor blood flow, increases in vascular permeability and increases in vascular endothelial apoptosis, all occurring within 1 h of administration. Over a slightly longer time scale, ASA404 induces an increase in tumor concentrations of TNF and a number of other cytokines. Phase I clinical trials confirmed its vascular effects in humans and Phase II trials demonstrated its activity in combination with the cytotoxic agents carboplatin and paclitaxel. While the molecular target of its action is not yet identified, current results suggest that ASA404 has the potential to augment the antitumor effects of other agents in cancer treatment. Studies of changes in tumor tissue following treatment with ASA404 either alone or combined and other agents will provide new insights into the dynamics of the tumor microenvironment.

DC-SIGN mediated sphingomyelinase-activation and ceramide generation is essential for enhancement of viral uptake in dendritic cells

As pattern recognition receptor on dendritic cells (DCs), DC-SIGN binds carbohydrate structures on its pathogen ligands and essentially determines host pathogen interactions because it both skews T cell responses and enhances pathogen uptake for cis infection and/or T cell trans-infection. How these processes are initiated at the plasma membrane level is poorly understood. We now show that DC-SIGN ligation on DCs by antibodies, mannan or measles virus (MV) causes rapid activation of neutral and acid sphingomyelinases followed by accumulation of ceramides in the outer membrane leaflet. SMase activation is important in promoting DC-SIGN signaling, but also for enhancement of MV uptake into DCs. DC-SIGN-dependent SMase activation induces efficient, transient recruitment of CD150, which functions both as MV uptake receptor and microbial sensor, from an intracellular Lamp-1+ storage compartment shared with acid sphingomyelinase (ASM) within a few minutes. Subsequently, CD150 is displayed at the cell surface and co-clusters with DC-SIGN. Thus, DC-SIGN ligation initiates SMase-dependent formation of ceramide-enriched membrane microdomains which promote vertical segregation of CD150 from intracellular storage compartments along with ASM. Given the ability to promote receptor and signalosome co-segration into (or exclusion from) ceramide enriched microdomains which provide a favorable environment for membrane fusion, DC-SIGN-dependent SMase activation may be of general importance for modes and efficiency of pathogen uptake into DCs, and their routing to specific compartments, but also for modulating T cell responses.

Dendritic cells (DCs) bear receptors specialized on recognition of patterns specific to pathogens (such as carbohydrates), which can either promote functional activation of these cells (such as TLRs), which renders them capable of efficiently presenting antigens to T cells, or, as DC-SIGN, endocytic uptake as essential for loading MHC molecules. Viruses such as HIV and measles virus (MV) exploit DC-SIGN for both their uptake into DCs and modulation of TLR signaling, yet how this is mechanistically exerted is poorly understood. We now show that DC-SIGN activates sphingomyelinases (SMases) which convert their sphingomyelin substrate into ceramides, thereby catalysing the formation of membrane platforms able to recruit and concentrate receptors and associated signaling components. We found DC-SIGN-dependent SMase activation as essential for DC-SIGN and thereby modulation of TLR signaling, but also for enhancement of MV uptake. This is mediated by a fast, transient recruitment of its entry receptor, CD150, from an intracellular storage compartment to the cell surface where it co-clusters in ceramide enriched platforms with DC-SIGN. The ability to segregate viral receptors into (or exclude them from) membrane microdomains, which, based on their biophysical properties, facilitate membrane fusion, proposes DC-SIGN-mediated SMAse activation as a central regulator of pathogen uptake into DCs.

Kdo2-lipid A, a TLR4-specific agonist, induces de novo sphingolipid biosynthesis in RAW264.7 macrophages, which is essential for induction of autophagy

Activation of RAW264.7 cells with a lipopolysaccharide specific for the TLR4 receptor, Kdo₂-lipid A (KLA), causes a large increase in cellular sphingolipids, from 1.5 to 2.6 × 10⁹ molecules per cell in 24 h, based on the sum of subspecies analyzed by “lipidomic” mass spectrometry. Thus, this study asked the following question. What is the cause of this increase and is there a cell function connected with it? The sphingolipids arise primarily from de novo biosynthesis based on [U-¹³C]palmitate labeling, inhibition by ISP1 (myriocin), and an apparent induction of many steps of the pathway (according to the distribution of metabolites and microarray analysis), with the exception of ceramide, which is also produced from pre-existing sources. Nonetheless, the activated RAW264.7 cells have a higher number of sphingolipids per cell because KLA inhibits cell division; thus, the cells are larger and contain increased numbers of membrane vacuoles termed autophagosomes, which were detected by the protein marker GFP-LC3. Indeed, de novo biosynthesis of sphingolipids performs an essential structural and/or signaling function in autophagy because autophagosome formation was eliminated by ISP1 in KLA-stimulated RAW264.7 cells (and mutation of serine palmitoyltransferase in CHO-LYB cells); furthermore, an anti-ceramide antibody co-localizes with autophagosomes in activated RAW264.7 cells versus the Golgi in unstimulated or ISP1-inhibited cells. These findings establish that KLA induces profound changes in sphingolipid metabolism and content in this macrophage-like cell line, apparently to produce sphingolipids that are necessary for formation of autophagosomes, which are thought to play important roles in the mechanisms of innate immunity.

The effects of TGF-β1 on the expression of type IV collagenases in mouse peritoneal macrophages

Transforming growth factor-β (TGF-β) is a pleiotropic cytokine that plays a critical role in modulating immune response and inflammation. We have investigated the effects of TGF-β1 on the expression of type IV collagenases, matrix metalloproteinase (MMP)-2 and MMP-9, in mouse peritoneal macrophages. TGF-β1 alone enhanced the secretion of MMP-9, while it blocked lipopolysaccharide (LPS)-stimulated MMP-9 production. We have shown that this biphasic effect of TGF-β1 is exerted at the mRNA level of the MMP-9 gene. Although TGF-β1 increased both basal and LPS-induced MMP-2 production at the protein and mRNA levels, the extent of the increase was smaller in LPS-activated macrophages than in control macrophages. The expression of type I and type II receptors for TGF-β was significantly decreased upon activation, suggesting that the lesser effect of TGF-β1 in activated macrophages may result from the decreased expression of TGF-β receptors. In addition, the expression of endogenous TGF-β1 mRNA was decreased significantly in activated macrophages. These findings suggest that activated macrophages not only produce less TGF-β1, but also respond less well to TGF-β to provide for inflammatory response.

Subcellular organelle lipidomics in TLR-4-activated macrophages

Lipids orchestrate biological processes by acting remotely as signaling molecules or locally as membrane components that modulate protein function. Detailed insight into lipid function requires knowledge of the subcellular localization of individual lipids. We report an analysis of the subcellular lipidome of the mammalian macrophage, a cell type that plays key roles in inflammation, immune responses, and phagocytosis. Nuclei, mitochondria, endoplasmic reticulum (ER), plasmalemma, and cytoplasm were isolated from RAW 264.7 macrophages in basal and activated states. Subsequent lipidomic analyses of major membrane lipid categories identified 229 individual/isobaric species, including 163 glycerophospholipids, 48 sphingolipids, 13 sterols, and 5 prenols. Major subcellular compartments exhibited substantially divergent glycerophospholipid profiles. Activation of macrophages by the Toll-like receptor 4-specific lipopolysaccharide Kdo(2)-lipid A caused significant remodeling of the subcellular lipidome. Some changes in lipid composition occurred in all compartments (e.g., increases in the levels of ceramides and the cholesterol precursors desmosterol and lanosterol). Other changes were manifest in specific organelles. For example, oxidized sterols increased and unsaturated cardiolipins decreased in mitochondria, whereas unsaturated ether-linked phosphatidylethanolamines decreased in the ER. We speculate that these changes may reflect mitochondrial oxidative stress and the release of arachidonic acid from the ER in response to cell activation.

Studies on the role of acid sphingomyelinase and ceramide in the regulation of tumor necrosis factor alpha (TNFalpha)-converting enzyme activity and TNFalpha secretion in macrophages

Acid sphingomyelinase (ASMase) has been proposed to mediate lipopolysaccharide (LPS) signaling in various cell types. This study shows that ASMase is a negative regulator of LPS-induced tumor necrosis factor alpha (TNFalpha) secretion in macrophages. ASMase-deficient (asm(-/-)) mice and isolated peritoneal macrophages produce severalfold more TNFalpha than their wild-type (asm(+/+)) counterparts when stimulated with LPS, whereas the addition of exogenous ceramides or sphingomyelinase reduces the differences. The underlying mechanism for these effects is not transcriptional but post-translational. The TNFalpha-converting enzyme (TACE) catalyzes the maturation of the 26-kDa precursor (pro-TNFalpha) to an active 17-kDa form (soluble (s)TNFalpha). In mouse peritoneal macrophages, the activity of TACE was the rate-limiting factor regulating TNFalpha production. A substantial portion of the translated pro-TNFalpha was not processed to sTNFalpha; instead, it was rapidly internalized and degraded in the lysosomes. TACE activity was 2-3-fold higher in asm(-/-) macrophages as compared with asm(+/+) macrophages and was suppressed when cells were treated with exogenous ceramide and sphingomyelinase. Indirect immunofluorescence analyses revealed distinct TNFalpha-positive structures in the close vicinity of the plasma membrane in asm(-/-) but not in asm(+/+) macrophages. asm(-/-) cells also had a higher number of early endosomal antigen 1-positive early endosomes. Experiments that involved inhibitors of TACE, endocytosis, and lysosomal proteolysis suggest that in the asm(-/-) cells a significant portion of pro-TNFalpha was sequestered within the early endosomes, and instead of undergoing lysosomal proteolysis, it was recycled to the plasma membrane and processed to sTNFalpha.

A ceramide-1-phosphate analogue, PCERA-1, simultaneously suppresses tumour necrosis factor-alpha and induces interleukin-10 production in activated macrophages

Tight regulation of the production of the key pro-inflammatory cytokine tumour necrosis factor-alpha (TNF-alpha) is essential for the prevention of chronic inflammatory diseases. In vivo administration of a synthetic phospholipid, named hereafter phospho-ceramide analogue-1 (PCERA-1), was previously found to suppress lipopolysaccharide (LPS)-induced TNF-alpha blood levels. We therefore investigated the in vitro anti-inflammatory effects of PCERA-1. Here, we show that extracellular PCERA-1 potently suppresses production of the pro-inflammatory cytokine TNF-alpha in RAW264.7 macrophages, and in addition, independently and reciprocally regulates the production of the anti-inflammatory cytokine interleukin-10 (IL-10). Specificity is demonstrated by the inability of the phospholipids ceramide-1-phosphate (C1P), sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) to perform these activities. Similar TNF-alpha suppression and IL-10 induction by PCERA-1 were observed in macrophages when activated by Toll-like receptor 4 (TLR4), TLR2 and TLR7 agonists. Regulation of cytokine production is demonstrated at the mRNA and protein levels. Finally, we show that, while PCERA-1 does not block activation of nuclear factor (NF)-kappaB and mitogen-activated protein kinases by LPS, it elevates the intracellular cAMP level. In conclusion, the anti-inflammatory activity of PCERA-1 seems to be mediated by a cell membrane receptor, upstream of cAMP production, and eventually TNF-alpha suppression and IL-10 induction. Thus, identification of the PCERA-1 receptor may provide new pharmacological means to block inflammation.

How Mycobacterium tuberculosis subverts host immune responses

Mycobacterium tuberculosis is the causative agent of pulmonary tuberculosis which has infected one third of the mankind and causes 2-3 million deaths worldwide each year. The persistence of the infection ensues from the ability of M. tuberculosis to subvert host immune responses in favor of survival and growth of mycobacteria in macrophages. The mechanisms by which M. tuberculosis manipulates the host immune system have only recently come to light. These activities are attributed to lipoarabinomannans (LAM) and their precursors lipomannans (LM), two predominant glycolipids of M. tuberculosis cell wall. LM are able to skew anti-mycobacterial immune responses into un-protective ones, while LAM evoke immunosupression upon binding to macrophage and dendritic cell receptors specialized in binding to "self" host components. A newly emerging idea implicates plasma membrane rafts in LM and LAM signaling. Depending on acylation patterns, the glycolipids may either directly incorporate into the raft membrane via mannosylphosphatidylinositol anchors or interact with raft-associated proteins to affect the assembly of receptor signaling complexes.

Induction of proinflammatory cytokines by long-chain saturated fatty acids in human macrophages

Increased circulating free fatty acids in subjects with type 2 diabetes may contribute to activation of macrophages, and thus the development of atherosclerosis. In this study, we investigated the effect of the saturated fatty acids (SFA) palmitate, stearate, myristate and laurate, and the unsaturated fatty acid linoleate, on the production of proinflammatory cytokines in phorbol ester-differentiated THP-1 cells, a model of human macrophages. Palmitate induced secretion and mRNA expression of TNF-alpha, IL-8 and IL-1 beta, and enhanced lipopolysaccharide (LPS)-induced IL-1 beta secretion. Proinflammatory cytokine secretion was also induced by stearate, but not by the shorter chain SFA, myristate and laurate, or linoleate. Triacsin C abolished the palmitate-induced cytokine secretion, suggesting that palmitate activation to palmitoyl-CoA is required for its effect. Palmitate-induced cytokine secretion was decreased by knockdown of serine palmitoyltransferase and mimicked by C(2)-ceramide, indicating that ceramide is involved in palmitate-induced cytokine secretion. Palmitate phosphorylated p38 and JNK kinases, and blocking of these kinases with specific inhibitors diminished the palmitate-induced cytokine secretion. Palmitate also activated the AP-1 (c-Jun) transcription factor. Knockdown of MyD88 reduced the palmitate-induced IL-8, but not TNF-alpha or IL-1 beta secretion. In conclusion, our data suggest that the long-chain SFA induce proinflammatory cytokines in human macrophages via pathways involving de novo ceramide synthesis. This might contribute to the activation of macrophages in atherosclerotic plaques, especially in type 2 diabetes.

Ceramide inhibits LPS-induced production of IL-5, IL-10, and IL-13 from mast cells

Mast cells are central regulators of allergic inflammation through production of various chemical mediators and cytokines. Bacterial infection occasionally worsens allergic inflammation. Although the exact mechanism of this phenomenon remains unclear, we have previously reported that LPS stimulates mast cells to produce not only pro-inflammatory cytokines, such as IL-6 and TNF-alpha, but also Th2-type cytokines, such as IL-5 and IL-13, and a regulatory cytokine, IL-10. In the present study, we have studied the effect of ceramide on LPS-mediated cytokine production from mast cells, as ceramide modulates various cellular functions in many cell types. Administration of cell-permeable C8 ceramide reduced production of IL-5, IL-10, and IL-13 from LPS-stimulated mouse bone marrow-derived mast cells (BMMCs) apparently through transcriptional inhibition, but did not affect IL-6 or TNF-alpha production. Consistently, LPS-stimulated production of IL-5, IL-10, and IL-13 from BMMCs is significantly enhanced in the presence of fumonisin B1, a de novo ceramide synthesis inhibitor. Interestingly, the same C8 ceramide treatment showed opposite effects on cytokine production from LPS-stimulated macrophages, reducing IL-6 and TNF-alpha while not affecting IL-10 production. C8 ceramide pretreatment significantly reduced LPS-induced Akt phosphorylation in BMMCs, but not in macrophages. Furthermore, pretreatment of BMMCs by wortmannin, a specific inhibitor of PI3 kinase, inhibited LPS-stimulated expression of IL-5, IL-10, and IL-13, but not that of TNF-alpha or IL-6. Thus, ceramide appeared to down-regulate LPS-stimulated production of IL-5, IL-10, and IL-13 from mast cells by inhibiting PI3 kinase-Akt pathway in a cell type-specific manner.

Acid sphingomyelinase is required for lipid Raft TLR4 complex formation

BACKGROUND

Lipid rafts, composed of sphingolipids, are critical to Toll-like receptor 4 (TLR4) assembly during lipopolysaccharide (LPS) exposure as a result of phosphokinase C (PKC)-zeta activation. However, the mechanism responsible for these events remains unknown.

PURPOSE

We determined whether LPS-induced TLR4 assembly and activation are dependent on the sphingolipid metabolite ceramide, produced by acid sphingomyelinase following the initial binding of LPS to CD14.

METHODS

Cultured THP-1 cells were stimulated with LPS, exogenous C(2) ceramide, or both. Selected cells were pretreated with the acid sphingomyelinase inhibitor imipramine or CD14 neutralizing antibody.

RESULTS

Exposure to LPS led to activation of acid sphingomyelinase, production of ceramide, phosphorylation of PKCzeta, and assembly of the TLR4 receptor within lipid rafts. This was followed by activation of the MAPK family of products and the liberation of tumor necrosis factor-alpha. Pretreatment with imipramine or CD14 blockade was associated with attenuation of all of these LPS-induced events. Simultaneous treatment with C(2) ceramide and LPS reversed all the inhibitory effects induced by imipramine, but not those associated with CD14 blockade.

CONCLUSION

Assembly and activation of the TLR4 receptor following LPS binding to CD14 requires the production of ceramide by acid sphingomyelinase.

Effect of ceramide on the contractility of pregnant rat uterus

Ceramide and other sphingolipid mediators have emerged as a novel class of lipid second messengers in cell signaling. We assessed the effect of C(2)-ceramide (a membrane permeable analog of ceramide) on spontaneous and agonist-induced contractile responses of uterus, isolated from 19-day pregnant rats. Ceramide (3, 10 microM) moderately, but significantly inhibited the amplitude of spontaneous rhythmic contractions. However, a variable effect was seen on agonist-induced contractions. While 5-HT-induced contractions were markedly inhibited at 3 and 10 microM ceramide, oxytocin and carboprost (a PGF(2)alpha analogue)-induced contractions were not affected by the sphingolipid. Ceramide (10 microM) also markedly inhibited CaCl(2)-induced contractions elicited in K(+)-depolarized tissues. Further, in rabbit portal vein myocytes, which display robust L-type calcium channel current, ceramide inhibited the I(Ba) in a dose-dependent manner. Therefore, it is suggested that the inhibitory effect of ceramide on uterine contractility may involve a decrease in the influx of Ca(2+) through voltage-dependent L-type Ca(2+) channels, such that contractile responses that are primarily dependent on extracellular Ca(2+), like rhythmic and serotonin contractions, were inhibited by ceramide. Further study is required to establish the role of endogenous ceramide and other sphingolipids in regulating uterine tone during gestation and at term.

Inhibition of lipopolysaccharide-induced release of interleukin-8 from intestinal epithelial cells by SMA, a novel inhibitor of sphingomyelinase and its therapeutic effect on dextran sulphate sodium-induced colitis in mice

Lipopolysaccharide (LPS) and inflammatory cytokines cause activation of sphingomyelinases (SMases) and subsequent hydrolysis of sphingomyelin (SM) to produce a lipid messenger ceramide. The use of SMase inhibitors may offer new therapies for the treatment of the LPS- and cytokines-related inflammatory bowel disease (IBD). We synthesized a series of difluoromethylene analogues of SM (SMAs). Here, we show that LPS efficiently increases the release of IL-8 from HT-29 intestinal epithelial cells by activating both neutral SMase and nuclear factor (NF)-kappaB in the cells. The addition of SMA-7 suppressed neutral SMase-catalyzed ceramide production, NF-kappaB activation, and IL-8 release from HT-29 cells caused by LPS. The results suggest that activation of neutral SMase is an underlying mechanism of LPS-induced release of IL-8 from the intestinal epithelial cells. Ceramide production following LPS-induced SM hydrolysis may trigger the activation of NF-kappaB in nuclei. Oral administration of SMA-7 (60 mg/kg) to mice with 2% dextran sulfate sodium (DSS) in their drinking water, for 21 consecutive days, reduced significantly the severity of colonic injury. This finding suggests a central role for SMase/ceramide signaling in the pathology of DSS-induced colitis in mice. The therapeutic effect of SMA-7 observed in mice may involve the suppression of IL-8 production from intestinal epithelial cells by LPS or other inflammatory cytokines.

Acid sphingomyelinase inhibition suppresses lipopolysaccharide-mediated release of inflammatory cytokines from macrophages and protects against disease pathology in dextran sulphate sodium-induced colitis in mice

Lipopolysaccharide (LPS) and inflammatory cytokines cause activation of sphingomyelinases (SMases) and subsequent hydrolysis of sphingomyelin (SM) to produce a lipid messenger ceramide. The design of SMase inhibitors may offer new therapies for the treatment of LPS- and cytokine-related inflammatory bowel disease. We synthesized a series of difluoromethylene analogues of SM (SMAs). We report here the effects of the most potent SMase inhibitor, SMA-7, on the LPS-mediated release of tumour necrosis factor-alpha, interleukin-1beta and interleukin-6 from THP-1 macrophages and the pathology of dextran sulphate sodium (DSS)-induced colitis in mice. SMA-7 suppressed the LPS-induced cytokine release and nuclear factor-kappaB activation. LPS stimulation caused a four-fold increase in acid SMase activation, but little increase in neutral SMase activity. The presence of 10 microm SMA-7 caused acid SMase to remain at the control levels and reduced the formation of ceramide. HT-29 cells had significantly decreased cell viability when incubated with media from LPS-stimulated THP-1 macrophages. However, incubating the colon cells in media from both SMA-7 and LPS-treated macrophages caused little decrease in viability, suggesting that ceramide has a role in the LPS-stimulated signalling that releases cytotoxic factors against colon cells. Oral administration of SMA-7 to mice with 2% DSS in the drinking water, for 10 or 21 consecutive days, reduced significantly the cytokine levels in the colon and the severity of colonic injury. These findings suggest a central role for acid SMase/ceramide signalling in the pathology of DSS-induced colitis in mice, indicating a possible preventive or therapeutic role for SMase inhibitor in inflammatory bowel disease.

Ceramide as a TLR4 agonist; a putative signalling intermediate between sphingolipid receptors for microbial ligands and TLR4

Mucosal Toll-like receptors (TLRs) respond to pathogens, but remain inert to the indigenous flora, suggesting that the TLRs can receive pathogen-specific signals. For example, TLR4 signalling is activated in CD14-negative epithelial cells by P-fimbriated, uropathogenic Escherichia coli, but not by lipopolysaccharide. The fimbriae use glycosphingolipids as recognition receptors and there is release of ceramide, which is the membrane-anchoring domain of the receptors. In this study, ceramide was identified as a TLR4 agonist and as a putative signalling intermediate between the glycosphingolipid recognition receptors and TLR4. Exogenous ceramide activated a TLR4-dependent epithelial cell response, as shown by exposing stably transfected TLR4-positive or -negative human embryonal kidney cells to C2 and C6 ceramide. A similar, TLR4-dependent response occurred after deliberate release of endogenous long-chained ceramide with sphingomyelinase. Microbial ligands with glycosphingolipid specificity (P fimbriae or the B subunit of Shiga toxin) were shown to increase the levels of ceramide and to trigger a TLR4-dependent response in epithelial cells. The results show that ceramide activates TLR4 signalling and suggest that this mechanism might allow pathogens to elicit mucosal TLR4 responses by perturbing sphingolipid receptors for virulence ligands like P fimbriae.