Mediators of innate immune recognition of bacteria concentrate in lipid rafts and facilitate lipopolysaccharide-induced cell activation

Bacterial colonization of host cells in the absence of cholesterol

Reports implicating important roles for cholesterol and cholesterol-rich lipid rafts in host-pathogen interactions have largely employed sterol sequestering agents and biosynthesis inhibitors. Because the pleiotropic effects of these compounds can complicate experimental interpretation, we developed a new model system to investigate cholesterol requirements in pathogen infection utilizing DHCR24^−/− mouse embryonic fibroblasts (MEFs). DHCR24^−/− MEFs lack the Δ24 sterol reductase required for the final enzymatic step in cholesterol biosynthesis, and consequently accumulate desmosterol into cellular membranes. Defective lipid raft function by DHCR24^−/− MEFs adapted to growth in cholesterol-free medium was confirmed by showing deficient uptake of cholera-toxin B and impaired signaling by epidermal growth factor. Infection in the absence of cholesterol was then investigated for three intracellular bacterial pathogens: Coxiella burnetii, Salmonella enterica serovar Typhimurium, and Chlamydia trachomatis. Invasion by S. Typhimurium and C. trachomatis was unaltered in DHCR24^−/− MEFs. In contrast, C. burnetii entry was significantly decreased in −cholesterol MEFs, and also in +cholesterol MEFs when lipid raft-associated α_Vβ₃ integrin was blocked, suggesting a role for lipid rafts in C. burnetii uptake. Once internalized, all three pathogens established their respective vacuolar niches and replicated normally. However, the C. burnetii-occupied vacuole within DHCR24^−/− MEFs lacked the CD63-postive material and multilamellar membranes typical of vacuoles formed in wild type cells, indicating cholesterol functions in trafficking of multivesicular bodies to the pathogen vacuole. These data demonstrate that cholesterol is not essential for invasion and intracellular replication by S. Typhimurium and C. trachomatis, but plays a role in C. burnetii-host cell interactions.

Clustered receptors associated with cholesterol-rich microdomains, termed lipid rafts, are thought to provide plasma membrane signaling platforms that bacterial pathogens can subvert to gain entry into host cells. Moreover, cholesterol has been implicated as a critical structural lipid of several pathogen-occupied vacuoles. Cumulative data supporting these models have principally been derived using inhibitors of cholesterol metabolism and various sterol sequestering compounds, agents that can lack specificity and cause unwanted cellular affects. Here, we employed a new system to investigate pathogen reliance on cholesterol for host cell colonization that utilizes mouse embryonic fibroblasts that can synthesize precursor sterols, but not cholesterol. Cells lacking cholesterol displayed strong defects in lipid raft-based signaling. However, no defects were observed in entry, vacuole development, and growth of Salmonella enterica and Chlamydia trachomatis, bacterial pathogens previously shown to rely on cholesterol for optimal host cell parasitism. Entry by Coxiella burnetii, the bacterial cause of human Q fever, was significantly decreased in cholesterol-negative cells as was trafficking of membranous material to the pathogen vacuole. However, subsequent bacterial replication was unaltered. Our results should prompt a reevaluation of the overall importance of cholesterol in bacterial pathogenesis with the described experimental system providing an alternative approach for such studies.

Dietary oil composition differentially modulates intestinal endotoxin transport and postprandial endotoxemia

BACKGROUND

Intestinal derived endotoxin and the subsequent endotoxemia can be considered major predisposing factors for diseases such as atherosclerosis, sepsis, obesity and diabetes. Dietary fat has been shown to increase postprandial endotoxemia. Therefore, the aim of this study was to assess the effects of different dietary oils on intestinal endotoxin transport and postprandial endotoxemia using swine as a model. We hypothesized that oils rich in saturated fatty acids (SFA) would augment, while oils rich in n-3 polyunsaturated fatty acids (PUFA) would attenuate intestinal endotoxin transport and circulating concentrations.

METHODS

Postprandial endotoxemia was measured in twenty four pigs following a porridge meal made with either water (Control), fish oil (FO), vegetable oil (VO) or coconut oil (CO). Blood was collected at 0, 1, 2, 3 and 5 hours postprandial and measured for endotoxin. Furthermore, ex vivo ileum endotoxin transport was assessed using modified Ussing chambers and intestines were treated with either no oil or 12.5% (v/v) VO, FO, cod liver oil (CLO), CO or olive oil (OO). Ex vivo mucosal to serosal endotoxin transport permeability (Papp) was then measured by the addition of fluorescent labeled-lipopolysaccharide.

RESULTS

Postprandial serum endotoxin concentrations were increased after a meal rich in saturated fatty acids and decreased with higher n-3 PUFA intake. Compared to the no oil control, fish oil and CLO which are rich in n-3 fatty acids reduced ex vivo endotoxin Papp by 50% (P < 0.05). Contrarily, saturated fatty acids increased the Papp by 60% (P = 0.008). Olive and vegetable oils did not alter intestinal endotoxin Papp.

CONCLUSION

Overall, these results indicate that saturated and n-3 PUFA differentially regulate intestinal epithelial endotoxin transport. This may be associated with fatty acid regulation of intestinal membrane lipid raft mediated permeability.

Lycopene suppresses proinflammatory response in lipopolysaccharide-stimulated macrophages by inhibiting ROS-induced trafficking of TLR4 to lipid raft-like domains

We recently showed that lycopene inhibited lipopolysaccharide (LPS)-induced productions of nitric oxide (NO) and interleukin-6 (IL-6) in murine RAW264.7 macrophages by mechanisms related to inhibition of ERK and nuclear factor-κB. Since the assembly of Toll-like receptor 4 (TLR4) in lipid rafts is a key element in LPS induced signaling, we investigated whether this process would be influenced by lycopene. We found that pretreatment of RAW264.7 cells with lycopene inhibited LPS-induced recruitment of TLR4 into fractions - enriched with lipid raft marker. By the methods of immunoprecipitation and immunoblotting, we also found that lycopene inhibited the subsequent formation of the complex of TLR4 with its adaptors including myeloid differentiation primary-response protein 88 and TIR domain-containing adaptor-inducing IFN-β. We also found that the lycopene induced inhibition was associated with reduced formation of reactive oxygen species (ROS), which was an upstream mechanism for the effects of lycopene, because treating the cells with the antioxidant N-acetyl-l-cysteine and NADPH oxidase inhibitor diphenyleneiodonium chloride significantly inhibited LPS-induced recruitment of TLR4 into lipid raft-like domains as well as the production of proinflammatory molecule NO and IL-6. Thus, our findings suggest that lycopene may prevent LPS-induced TLR4 assembly into lipid rafts through reducing intracellular ROS level.

Characterization of differential Toll-like receptor responses below the optical diffraction limit

Many membrane receptors are recruited to specific cell surface domains to form nanoscale clusters upon ligand activation. This step appears to be necessary to initiate cell signaling, including pathways in innate immune system activation. However, virulent pathogens such as Yersinia pestis (the causative agent of plague) are known to evade innate immune detection, in contrast to similar microbes (such as Escherichia coli) that elicit a robust response. This disparity has been partly attributed to the structure of lipopolysaccharides (LPS) on the bacterial cell wall, which are recognized by the innate immune receptor TLR4. It is hypothesized that nanoscale differences exist between the spatial clustering of TLR4 upon binding of LPS derived from Y. pestis and E. coli. Although optical imaging can provide exquisite details of the spatial organization of biomolecules, there is a mismatch between the scale at which receptor clustering occurs (<300 nm) and the optical diffraction limit (>400 nm). The last decade has seen the emergence of super-resolution imaging methods that effectively break the optical diffraction barrier to yield truly nanoscale information in intact biological samples. This study reports the first visualizations of TLR4 distributions on intact cells at image resolutions of <30 nm using a novel, dual-color stochastic optical reconstruction microscopy (STORM) technique. This methodology permits distinction between receptors containing bound LPS from those without at the nanoscale. Importantly, it is also shown that LPS derived from immunostimulatory bacteria result in significantly higher LPS-TLR4 cluster sizes and a nearly twofold greater ligand/receptor colocalization as compared to immunoevading LPS.

Type II heat-labile enterotoxins: structure, function, and immunomodulatory properties

The heat-labile enterotoxins (HLTs) of Escherichia coli and Vibrio cholerae are classified into two major types on the basis of genetic, biochemical, and immunological properties. Type I and Type II HLT have been intensively studied for their exceptionally strong adjuvant activities. Despite general structural similarities, these molecules, in intact or derivative (non-toxic) forms, display notable differences in their mode of immunomodulatory action. The molecular basis of these differences has remained largely uncharacterized until recently. This review focuses on the Type II HLTs and their immunomodulatory properties which depend largely on interactions with unique gangliosides and Toll-like receptors that are not utilized by the Type I HLTs.

Accumulation of staphylococcal Panton-Valentine leukocidin in the detergent-resistant membrane microdomains on the target cells is essential for its cytotoxicity

The mechanisms for the cytotoxicity of staphylococcal Panton-Valentine leukocidin (PVL), a pore-forming toxin consisting of LukS-PV and LukF-PV, in human immune cells are still unclear. Because LukS-PV binds to ganglioside GM1, a constituent of detergent-resistant membrane microdomains (DRMs) of the plasma membrane, the role of DRMs in PVL cytotoxicity was examined in human polymorphonuclear neutrophils (PMNs), monocytes, HL-60 cells, and THP-1 cells. PVL binding capacities in HL-60 and THP-1 cells were higher than those in PMNs and monocytes; however, the PVL concentration to obtain more than 80% cell lysis in HL-60 cells was 10 times higher than that in PMNs and PVL even at such concentration induced < 10% cell lysis in THP-1 cells. After incubation of PMNs with LukS-PV, more than 90% of LukS-PV bound to the detergent-soluble membranes. Subsequent incubation with LukF-PV at 4 °C induced the accumulation of more than 70% of PVL components and 170- to 220-kDa complex formation in DRMs in an actin-independent manner. However, only 30% of PVL was found, and complex formation was under detectable level in DRMs in HL-60 cells. PVL did not accumulate in DRMs in THP-1 cells. Our observations strongly indicate that PVL accumulation in DRMs is essential for PVL cytotoxicity.

Defining the subcellular sites of innate immune signal transduction

Innate immune activation by microbial detection receptors is a complex process involving at least 100 proteins and multiple signaling pathways. Although there continues to be a need to identify additional regulators of host-microbe interactions, a larger conceptual challenge is our lack of understanding of how the known regulators interact in space and time. This review offers a framework to explain the long appreciated (but poorly understood) observation that innate immune signaling pathways are activated from multiple organelles. Using the Toll-like receptors (TLRs) and the retinoic acid-inducible gene 1 protein (RIG-I)-like receptors (RLRs) as examples, I propose that the receptors do not necessarily define the sites of signaling. Rather, a structurally unrelated class of proteins called 'sorting adaptors' functions in this capacity.

Role of lipid rafts and flagellin in invasion of colonic epithelial cells by Shiga-toxigenic Escherichia coli O113:H21

Shiga-toxigenic Escherichia coli (STEC) O113:H21 strains that lack the locus of enterocyte effacement (LEE) efficiently invade eukaryotic cells in vitro, unlike LEE-positive O157:H7 strains. We used a fliC deletion mutant of the O113:H21 STEC strain 98NK2 (98NK2ΔfliC) to show that invasion of colonic epithelial (HCT-8) cells is heavily dependent on production of flagellin, even though adherence to the cells was actually enhanced in the mutant. Flagellin binds and signals through Toll-like receptor 5 (TLR5), but there was no evidence that either TLR5, the adaptor protein myeloid differentiation primary response gene 88 (MyD88), or the serine kinase interleukin-1 receptor-associated kinase (IRAK) were required for invasion of HCT-8 cells by strain 98NK2, as judged by transfection, RNA knockdown, or inhibitor studies. However, pretreatment of cells with anti-asialo-GM1 significantly decreased 98NK2 invasion (by 40.8%), while neuraminidase treatment (which cleaves terminal sialic acid residues, thus converting GM1 into asialo-GM1) significantly increased invasion (by 70.7%). Pretreatment of HCT-8 cells with either the cholesterol-depleting agent methyl-β-cyclodextrin (MβCD) or the tyrosine kinase inhibitor genistein significantly decreased invasion by 98NK2, indicating a potential role for lipid rafts in the invasion mechanism. Confocal microscopy also showed invading 98NK2 colocalized with lipid raft markers caveolin-1 and GM1. Interestingly, anti-asialo-GM1, neuraminidase, MβCD, and genistein have similar effects on the vestigial level of STEC invasion seen for STEC strain 98NK2ΔfliC, indicating that lipid rafts mediate a common step in flagellin-dependent and flagellin-independent cellular invasion.

Receptor-independent interaction of bacterial lipopolysaccharide with lipid and lymphocyte membranes; the role of cholesterol

Lipopolysaccharide (LPS) is a major constituent of bacterial outer membranes where it makes up the bulk of the outer leaflet and plays a key role as determinant of bacterial interactions with the host. Membrane-free LPS is known to activate T-lymphocytes through interactions with Toll-like receptor 4 via multiprotein complexes. In the present study, we investigate the role of cholesterol and membrane heterogeneities as facilitators of receptor-independent LPS binding and insertion, which underpin bacterial interactions with the host in symbiosis, pathogenesis and cell invasion. We use fluorescence spectroscopy to investigate the interactions of membrane-free LPS from intestinal Gram-negative organisms with cholesterol-containing model membranes and with T-lymphocytes. LPS preparations from Klebsiella pneumoniae and Salmonella enterica were found to bind preferentially to mixed lipid membranes by comparison to pure PC bilayers. The same was observed for LPS from the symbiote Escherichia coli but with an order of magnitude higher dissociation constant. Insertion of LPS into model membranes confirmed the preference for sphimgomyelin/cholesterol-containing systems. LPS insertion into Jurkat T-lymphocyte membranes reveals that they have a significantly greater LPS-binding capacity by comparison to methyl-β-cyclodextrin cholesterol-depleted lymphocyte membranes, albeit at slightly lower binding rates.

New roles of HDL in inflammation and hematopoiesis

High-density lipoprotein (HDL) levels are inversely associated with coronary heart disease due to HDL's ability to transport excess cholesterol in arterial macrophages to the liver for excretion [i.e., reverse cholesterol transport (RCT)]. However, recent advances highlight additional atheroprotective roles for HDL beyond bulk cholesterol removal from cells through RCT. By promoting cellular free cholesterol (FC) efflux, HDL and its apolipoproteins (apoA-I and apoE) decrease plasma membrane FC and lipid raft content in immune and hematopoietic stem cells, decreasing inflammatory and cell proliferation signaling pathways. HDL and apoA-I also dampen inflammatory signaling pathways independent of cellular FC efflux. In addition, HDL lipid and protein cargo provide protection against parasitic and bacterial infection, endothelial damage, and oxidant toxicity. Here, current knowledge is reviewed regarding the role of HDL and its apolipoproteins in regulating cellular cholesterol homeostasis, highlighting recent advances on novel functions and mechanisms by which HDLs regulate inflammation and hematopoiesis.

Toll-like receptor 2 is required for LPS-induced Toll-like receptor 4 signaling and inhibition of ion transport in renal thick ascending limb

Previously we demonstrated that basolateral LPS inhibits HCO(3)(-) absorption in the renal medullary thick ascending limb (MTAL) through TLR4-dependent ERK activation. Here we report that the response of the MTAL to basolateral LPS requires TLR2 in addition to TLR4. The basolateral addition of LPS (ultrapure Escherichia coli K12) decreased HCO(3)(-) absorption in isolated, perfused MTALs from wild-type mice but had no effect in MTALs from TLR2(-/-) mice. In contrast, inhibition of HCO(3)(-) absorption by lumen LPS was preserved in TLR2(-/-) MTALs, indicating that TLR2 is involved specifically in mediating the basolateral LPS response. LPS also did not increase ERK phosphorylation in MTALs from TLR2(-/-) mice. TLR2 deficiency had no effect on expression of TLR4, MD-2, or MyD88. However, LPS-induced recruitment of MyD88 to the basolateral membrane was impaired in TLR2(-/-) MTALs. Inhibition of HCO(3)(-) absorption by LPS did not require CD14. Co-immunoprecipitation studies demonstrated an association between TLR4 and TLR2. Inhibition of HCO(3)(-) absorption by TLR2-specific ligands was preserved in MTALs from TLR4(-/-) mice. These results indicate that the effect of basolateral LPS to inhibit HCO(3)(-) absorption in the MTAL through MyD88-dependent ERK activation depends on a novel interaction between TLR4 and TLR2. TLR2 plays a dual role in the induction of intracellular signals that impair MTAL function, both through cooperation with TLR4 to mediate ERK signaling by LPS and through a TLR4-independent signaling pathway activated by Gram-positive bacterial ligands. Regulation of TLR2 expression and its interaction with TLR4 may provide new mechanisms for controlling and therapeutic targeting of TLR4-mediated LPS responses.

Apolipoprotein A-I attenuates palmitate-mediated NF-κB activation by reducing Toll-like receptor-4 recruitment into lipid rafts

While high-density lipoprotein (HDL) is known to protect against a wide range of inflammatory stimuli, its anti-inflammatory mechanisms are not well understood. Furthermore, HDL's protective effects against saturated dietary fats have not been previously described. In this study, we used endothelial cells to demonstrate that while palmitic acid activates NF-κB signaling, apolipoprotein A–I, (apoA-I), the major protein component of HDL, attenuates palmitate-induced NF-κB activation. Further, vascular NF-κB signaling (IL-6, MCP-1, TNF-α) and macrophage markers (CD68, CD11c) induced by 24 weeks of a diabetogenic diet containing cholesterol (DDC) is reduced in human apoA-I overexpressing transgenic C57BL/6 mice compared to age-matched WT controls. Moreover, WT mice on DDC compared to a chow diet display increased gene expression of lipid raft markers such as Caveolin-1 and Flotillin-1, and inflammatory Toll-like receptors (TLRs) (TLR2, TLR4) in the vasculature. However apoA-I transgenic mice on DDC show markedly reduced expression of these genes. Finally, we show that in endothelial cells TLR4 is recruited into lipid rafts in response to palmitate, and that apoA-I prevents palmitate-induced TLR4 trafficking into lipid rafts, thereby blocking NF-κB activation. Thus, apoA-I overexpression might be a useful therapeutic tool against vascular inflammation.

Immunomodulation of microglia by docosahexaenoic acid and eicosapentaenoic acid

PURPOSE OF REVIEW

The omega-3 fatty acids (ω-3 FAs) docosahexaenoic acid and eicosapentaenoic acid are dietary components which have been ascribed many different health benefits. Inflammation is present in, and contributes to, pathological conditions in the central nervous system (CNS). Microglia are the primary cells with immune function in the CNS, and inflammation mediated by activated microglia is present in pathological conditions. In this review, we present and discuss findings on the modulation of microglial activities by ω-3 FAs in vivo as well as in vitro, and propose mechanisms for their effects.

RECENT FINDINGS

The majority of studies show that ω-3 FAs have anti-inflammatory effects on microglia. However, phagocytosis is an activity associated with inflammation and is increased by ω-3 FAs. This can be understood in the light of recent research on the resolution of inflammation. Resolution is induced by proresolving factors, which are metabolites of ω-3 FAs. Proresolving factors are anti-inflammatory and have been shown to increase phagocytosis. Other mechanisms of the anti-inflammatory actions of ω-3 FAs involve the peroxisome proliferator-activated receptor-γ, ω-3 FA incorporation into the cell membrane, and inhibition of ion currents.

SUMMARY

Immunomodulation by ω-3 FAs is mediated by several pathways that are interconnected and is a potential therapy for disorders in the CNS.

Role of sphingomyelin synthesis in pulmonary endothelial cell cytoskeletal activation and endotoxin-induced lung injury

Sphingomyelin (SM), a major sphingolipid in the lipid raft microdomains of the cell membrane, is synthesized by plasma membrane-bound sphingomyelin synthase 2 (SMS2). SMS2 is required for the maintenance of plasma membrane microdomain fluidity and receptor-mediated responses to inflammation in macrophages. However, the exact mechanism of SMS2 activation in endothelial barrier disruption and lung injury is not fully understood. To define the role of SMS activation in lung injury, we hypothesized that the inhibition of SM synthesis may provide protection against acute lung injury (ALI) by preserving endothelial barrier function. Using SMS2-silencing RNA (siRNA) treatment in human pulmonary endothelial cells (HPAECs) and tricyclodecan-9-yl-xanthogenate (D609), a competitive inhibitor of SMS, and phosphatidylcholine-specific phospholipase C in a murine model of bacterial LPS injury, we studied the role of sphingomyelin synthesis in ALI. Results show that pretreating mice with D609 significantly attenuated LPS-induced lung injury, as measured by a significant decrease in wet to dry ratio, bronchoalveolar lavage fluid cell and protein counts, and myeloperoxidase activity in lung tissue. Similarly, LPS-induced endothelial barrier disruption was significantly reduced in HPAECs pretreated with D609 or SMS2 siRNA, as demonstrated by an increase in paracellular integrity on an FITC-dextran assay, by the inhibition of LPS-induced stress fibers, and by the formation of cortical actin rings and lamellipodia at the periphery. These results indicate that D609 attenuates LPS-mediated endothelial barrier dysfunction and lung injury in mice through inhibition of SMS, suggesting a novel and essential role of SMS inhibition in modulating endothelial barrier integrity via actin cytoskeletal activation, with a potential therapeutic role in ALI.

Growth and Development Symposium: Endotoxin, inflammation, and intestinal function in livestock

Endotoxin, also referred to as lipopolysaccharide (LPS), can stimulate localized or systemic inflammation via the activation of pattern recognition receptors. Additionally, endotoxin and inflammation can regulate intestinal epithelial function by altering integrity, nutrient transport, and utilization. The gastrointestinal tract is a large reservoir of both gram-positive and gram-negative bacteria, of which the gram-negative bacteria serve as a source of endotoxin. Luminal endotoxin can enter circulation via two routes: 1) nonspecific paracellular transport through epithelial cell tight junctions, and 2) transcellular transport through lipid raft membrane domains involving receptor-mediated endocytosis. Paracellular transport of endotoxin occurs through dissociation of tight junction protein complexes resulting in reduced intestinal barrier integrity, which can be a result of enteric disease, inflammation, or environmental and metabolic stress. Transcellular transport, via specialized membrane regions rich in glycolipids, sphingolipids, cholesterol, and saturated fatty acids, is a result of raft recruitment of endotoxin-related signaling proteins leading to endotoxin signaling and endocytosis. Both transport routes and sensitivity to endotoxin may be altered by diet and environmental and metabolic stresses. Intestinal-derived endotoxin and inflammation result in suppressed appetite, activation of the immune system, and partitioning of energy and nutrients away from growth toward supporting the immune system requirements. In livestock, this leads to the suppression of growth, particularly suppression of lean tissue accretion. In this paper, we summarize the evidence that intestinal transport of endotoxin and the subsequent inflammation leads to decrease in the production performance of agricultural animals and we present an overview of endotoxin detoxification mechanisms in livestock.

Oxysterols exert proinflammatory effects in placental trophoblasts via TLR4-dependent, cholesterol-sensitive activation of NF-κB

Oxidized cholesterol metabolites (oxysterols) promote inflammation in a variety of cell types and are thought to be involved in a number of disease pathologies. Oxysterol concentrations are increased in pregnancy, together with systemic oxidative stress and inflammation. We tested the hypothesis that oxysterols 25-hydroxycholesterol (25-OHC) and 7-ketocholesterol (7-ketoC) promote placental trophoblast inflammation, and determined the mechanisms involved. Treatment of primary trophoblasts in culture with 25-OHC and 7-ketoC increased the production of proinflammatory cytokines (interleukin-6, macrophage inflammatory protein-1β and tumour necrosis factor-α) in a concentration-dependent fashion. Inhibition of TLR4 activation using selective inhibitors of TLR4 complex formation (OxPAPC) or signalling transmission (CLI095) prevented lipopolysaccharide (LPS)- and oxysterol-induced inflammatory cytokine production. Pretreatment of trophoblasts with selective inhibitors of I-kB kinase activity (parthenolide and TPCA-1) reduced oxysterol- and LPS-stimulated inflammatory responses, consistent with the involvement of the nuclear factor kappa B (NF-κB) pathway downstream of TLR4 signalling. Both oxysterols also increased the phosphorylation and nuclear localization of NF-κB subunit p65/RelA. Oxysterols are also known to activate liver X receptors (LXRs) which can inhibit inflammatory signalling, either directly or indirectly via membrane cholesterol reduction. Treatment with the LXR agonist, T0901317, exerted significant anti-inflammatory effects, reducing LPS- and oxysterol-driven cytokine production. Treatment with methyl-β-cyclodextrin to deplete membrane microdomain cholesterol and thereby disrupt TLR4 signalling, similarly abrogated their effects. Together, these findings indicate that although oxysterols likely activate both pro- and anti-inflammatory pathways in the placenta, the predominant effect is the promotion of placental inflammation via TLR4-dependent activation of NF-κB.

Lipid rafts: a signalling platform linking lipoprotein metabolism to atherogenesis

Lipid rafts are microdomains of the plasma membrane which are enriched in cholesterol and sphingolipids. They serve as a platform for signal transduction, in particular during immune and inflammatory responses. As hypercholesterolemia and inflammation are two key elements of atherogenesis, it is conceivable that the cholesterol and cholesterol oxide content of lipid rafts might influence the inflammatory signalling pathways, thus modulating the development of atherosclerosis. In support of this emerging view, lipid rafts have been shown to be involved in several key steps of atherogenesis, such as the oxysterol-mediated apoptosis of vascular cells, the blunted ability of high density lipoproteins (HDL) to exert anti-inflammatory effects, and the exacerbated secretion of pro-inflammatory cytokines by immune cells. Additional studies are now required to address the relative contribution of lipid raft abnormalities to the pathophysiology of atherosclerosis and cardiovascular disease.

HDLs, immunity, and atherosclerosis

PURPOSE OF REVIEW

HDLs possess several physiological activities that may explain their antiatherosclerotic properties. Among them, the most relevant is the ability of HDL to promote the efflux of excess cholesterol from peripheral tissues to the liver for excretion.

RECENT FINDINGS

The ability of HDL to promote cholesterol efflux results also in the modulation of a series of responses in the immune cells involved in atherosclerosis, including monocyte-macrophages, B and T lymphocytes. HDL also acts as a reservoir for a number of biologically active substances that may impact the immune system, and as the HDL composition varies to a large extent during inflammation.

SUMMARY

The understanding of how these interactions take place and how biologically active substances can be delivered to relevant targets during atherogenesis is of great interest and may provide a better understanding for the role of HDL in atherogenesis.

Statin treatment is associated with reduced toll-like receptor 4 immunohistochemical expression on carotid atherosclerotic plaques: a novel effect of statins

Toll-like receptor 4 (TLR4) has been recently implicated in inflammatory pathways involved in carotid plaque destabilization. Given that statins have plaque stabilization and inflammation reduction effects, we investigated whether TLR4 expression on carotid atherosclerotic plaques correlates with statin intake. Carotid atherosclerotic plaques were obtained on 140 patients (preoperative statin intake, n = 70). TLR4 immunohistochemical expression was investigated in endothelial cells (ECs), macrophages (MACs) and smooth muscle cells (SMCs) of carotid atheroma. TLR4 positivity, over-expression and intensity of immunostaining were compared in statin versus no-statin users. The results of this study showed that statin users had a significantly lower expression of TLR4 in ECs (P = 0.02, 0.001, 0.006 for TLR4 positivity, increased intensity and over-expression, respectively). Similarly, TLR4 positivity was less pronounced in carotid plaque MACs of statin users (P = 0.03). No carotid specimen with increased EC TLR4 intensity or over-expression was observed among statin users. The prevalence of any cerebrovascular accident was 61.4% in the 'no statin' versus 18.6% in the 'statin' group (odds ratio for statin use: 0.14, 95% CI: 0.07-0.31, P < 0.001). In conclusion, statin treatment is associated with attenuated TLR4 expression on human carotid atherosclerotic plaques and a reduced risk of carotid-related cerebrovascular events. TLR4 may potentially mediate statins' plaque stabilization effects. Further investigation is necessary.

Anti-atherogenic mechanisms of high density lipoprotein: effects on myeloid cells

In some settings increasing high density lipoprotein (HDL) levels has been associated with a reduction in experimental atherosclerosis. This has been most clearly seen in apolipoprotein A-I (apoA-I) transgenic mice or in animals infused with HDL or its apolipoproteins. A major mechanism by which these treatments are thought to delay progression or cause regression of atherosclerosis is by promoting efflux of cholesterol from macrophage foam cells. In addition, HDL has been described as having anti-inflammatory and other beneficial effects. Some recent research has linked anti-inflammatory effects to cholesterol efflux pathways but likely multiple mechanisms are involved. Macrophage cholesterol efflux may have a role in facilitating emigration of macrophages from lesions during regression. While macrophages can mediate cholesterol efflux by several pathways, studies in knockout mice or cells point to the importance of active efflux mediated by ATP binding cassette transporter (ABC) A1 and G1. In addition to traditional roles in macrophages, these transporters have been implicated in the control of hematopoietic stem cell proliferation, monocytosis and neutrophilia, as well as activation of monocytes and neutrophils. Thus, HDL and cholesterol efflux pathways may have important anti-atherogenic effects at all stages of the myeloid cell/monocyte/dendritic cell/macrophage lifecycle. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Cholangiocyte N-Ras protein mediates lipopolysaccharide-induced interleukin 6 secretion and proliferation

Cholangiocytes, the epithelial cells lining the bile ducts in the liver, are periodically exposed to potentially injurious microbes and/or microbial products. As a result, cholangiocytes actively participate in microbe-associated, hepatic proinflammatory responses. We previously showed that infection of cultured human cholangiocytes with the protozoan parasite, Cryptosporidium parvum, or treatment with gram-negative bacteria-derived LPS, activates NFκB in a myeloid differentiation 88 (MyD88)-dependent manner. Here, we describe a novel signaling pathway initiated by Toll-like receptors (TLRs) involving the small GTPase, Ras, that mediates cholangiocyte proinflammatory cytokine production and induction of cholangiocyte proliferation. Using cultured human cholangiocytes and a Ras activation assay, we found that agonists of plasma membrane TLRs (TLR 1, 2, 4, 5, and 6) rapidly (<10 min) activated N-Ras, but not other p21 Ras isoforms, resulting in the rapid (<15 min) phosphorylation of the downstream Ras effector, ERK1/2. RNA interference-induced depletion of TRAF6, a downstream effector of MyD88 and known activator of MAPK signaling, had no effect on N-Ras activation. Following N-Ras activation the proinflammatory cytokine, IL6, is rapidly secreted. Using a luciferase reporter, we demonstrated that LPS treatment induced IL6 promoter-driven luciferase which was suppressed using MEK/ERK pharmacologic inhibitors (PD98059 or U0126) and RNAi-induced depletion of N-Ras. Finally, we showed that LPS increased cholangiocyte proliferation (1.5-fold), which was inhibited by depletion of N-Ras; TLR agonist-induced proliferation was also inhibited following pretreatment with an IL6 receptor-blocking antibody. Together, our results support a novel signaling axis involving microbial activation of N-Ras likely involved in the cholangiocyte pathogen-induced proinflammatory response.

Emerging role of high density lipoproteins as a player in the immune system

High density lipoproteins (HDL) possess a number of physiological activities. The most studied and, perhaps, better understood is the ability of HDL to promote excess cholesterol efflux from peripheral tissues and transport to the liver for excretion, a mechanism believed to confer protection against atherosclerotic cardiovascular disease. The ability of HDL to modulate cholesterol bioavailability in the lipid rafts, membrane microdomains enriched in glycosphingolipids and cholesterol, is evolutionary conserved and affects the properties of cells involved in the innate and adaptive immune response, tuning inflammatory response and antigen presentation functions in macrophages as well as B and T cell activation. Also sphingosine-1 phosphate (S1P), a major active sphingolipid carried by HDL, is of relevance in the pathogenesis of several immuno-inflammatory disorders through the modulation of macrophage and lymphocyte functions. Furthermore, HDL influence the humoral innate immunity by modulating the activation of the complement system and the expression of pentraxin 3 (PTX3). Finally, in humans, HDL levels and functions are altered in several immune-mediated disorders, such as rheumatoid arthritis, systemic lupus eritematosus, Crohn's disease and multiple sclerosis as well as during inflammatory responses. Altogether these observations suggest that the effects of HDL in immunity could be related, to either the ability of HDL to modulate cholesterol content in immune cell lipid rafts and to their role as reservoir for several biologically active substances that may impact the immune system.

Candida albicans enhances invasion of human gingival epithelial cells and gingival fibroblasts by Porphyromonas gingivalis

Although Candida albicans has been isolated from periodontal pockets, its relationship to periodontitis is unclear. In this study, we investigated the effect of C. albicans on the adhesion and invasion of Ca9-22, a human gingival epithelial cell line, and human gingival fibroblasts by Porphyromonas gingivalis. Heat-killed C. albicans and water-soluble mannoprotein-β-glucan complex from C. albicans (CAWS) did not enhance P. gingivalis adhesion or upregulate the expression of β1 integrin and ICAM-1, which are required for P. gingivalis invasion; both the epithelial cells and fibroblasts expressed dectin-1, which recognizes components of the C. albicans cell wall. However, pretreatment of Ca9-22 cells and human gingival fibroblasts with heat-killed C. albicans or CAWS significantly enhanced P. gingivalis invasion. These results suggest that C. albicans may exacerbate infectious disease by enhancing the invasion of host cells by anaerobic bacteria.

Location, location, location: is membrane partitioning everything when it comes to innate immune activation?

In the last twenty years, the general view of the plasma membrane has changed from a homogeneous arrangement of lipids to a mosaic of microdomains. It is currently thought that islands of highly ordered saturated lipids and cholesterol, which are laterally mobile, exist in the plane of the plasma membrane. Lipid rafts are thought to provide a means to explain the spatial segregation of certain signalling pathways emanating from the cell surface. They seem to provide the necessary microenvironment in order for certain specialised signalling events to take place, such as the innate immune recognition. The innate immune system seems to employ germ-lined encoded receptors, called pattern recognition receptors (PRRs), in order to detect pathogens. One family of such receptors are the Toll-like receptors (TLRs), which are the central “sensing” apparatus of the innate immune system. In recent years, it has become apparent that TLRs are recruited into membrane microdomains in response to ligands. These nanoscale assemblies of sphingolipid, cholesterol, and TLRs stabilize and coalesce, forming signalling platforms, which transduce signals that lead to innate immune activation. In the current paper, we will investigate all past and current literature concerning recruitment of extracellular and intracellular TLRs into lipid rafts and how this membrane organization modulates innate immune responses.

Group X secretory phospholipase A2 enhances TLR4 signaling in macrophages

Secretory phospholipase A(2)s (sPLA(2)) hydrolyze glycerophospholipids to liberate lysophospholipids and free fatty acids. Although group X (GX) sPLA(2) is recognized as the most potent mammalian sPLA(2) in vitro, its precise physiological function(s) remains unclear. We recently reported that GX sPLA(2) suppresses activation of the liver X receptor in macrophages, resulting in reduced expression of liver X receptor-responsive genes including ATP-binding cassette transporters A1 (ABCA1) and G1 (ABCG1), and a consequent decrease in cellular cholesterol efflux and increase in cellular cholesterol content (Shridas et al. 2010. Arterioscler. Thromb. Vasc. Biol. 30: 2014-2021). In this study, we provide evidence that GX sPLA(2) modulates macrophage inflammatory responses by altering cellular cholesterol homeostasis. Transgenic expression or exogenous addition of GX sPLA(2) resulted in a significantly higher induction of TNF-α, IL-6, and cyclooxygenase-2 in J774 macrophage-like cells in response to LPS. This effect required GX sPLA(2) catalytic activity, and was abolished in macrophages that lack either TLR4 or MyD88. The hypersensitivity to LPS in cells overexpressing GX sPLA(2) was reversed when cellular free cholesterol was normalized using cyclodextrin. Consistent with results from gain-of-function studies, peritoneal macrophages from GX sPLA(2)-deficient mice exhibited a significantly dampened response to LPS. Plasma concentrations of inflammatory cytokines were significantly lower in GX sPLA(2)-deficient mice compared with wild-type mice after LPS administration. Thus, GX sPLA(2) amplifies signaling through TLR4 by a mechanism that is dependent on its catalytic activity. Our data indicate this effect is mediated through alterations in plasma membrane free cholesterol and lipid raft content.

Abnormal trafficking and degradation of TLR4 underlie the elevated inflammatory response in cystic fibrosis

Morbidity and mortality in cystic fibrosis (CF) are due not only to abnormal epithelial cell function, but also to an abnormal immune response. We have shown previously that macrophages lacking CF transmembrane conductance regulator (CFTR), the gene mutated in CF, contribute significantly to the hyperinflammatory response observed in CF. In this study, we show that lack of functional CFTR in murine macrophages causes abnormal TLR4 subcellular localization. Upon LPS stimulation, CFTR macrophages have prolonged TLR4 retention in the early endosome and reduced translocation into the lysosomal compartment. This abnormal TLR4 trafficking leads to increased LPS-induced activation of the NF-κB, MAPK, and IFN regulatory factor-3 pathways and decreased TLR4 degradation, which affects downregulation of the proinflammatory state. In addition to primary murine cells, mononuclear cells isolated from CF patients demonstrate similar defects in response to LPS. Moreover, specific inhibition of CFTR function induces abnormal TLR4 trafficking and enhances the inflammatory response of wild-type murine cells to LPS. Thus, functional CFTR in macrophages influences TLR4 spatial and temporal localization and perturbs LPS-mediated signaling in both murine CF models and patients with CF.

Extracellular heat shock proteins, cellular export vesicles, and the Stress Observation System: a form of communication during injury, infection, and cell damage. It is never known how far a controversial finding will go! Dedicated to Ferruccio Ritossa

Heat shock proteins (hsp) have been found to play a fundamental role in the recovery from multiple stress conditions and to offer protection from subsequent insults. The function of hsp during stress goes beyond their intracellular localization and chaperone role as they have been detected outside cells activating signaling pathways. Extracellular hsp are likely to act as indicators of the stress conditions, priming other cells, particularly of the immune system, to avoid the propagation of the insult. Some extracellular hsp, for instance Hsp70, are associated with export vesicles, displaying a robust activation of macrophages. We have coined the term Stress Observation System (SOS) for the mechanism for sensing extracellular hsp, which we propose is a form of cellular communication during stress conditions. An enigmatic and still poorly understood process is the mechanism for the release of hsp, which do not contain any consensus secretory signal. The export of hsp appears to be a very complex phenomenon encompassing different alternative pathways. Moreover, extracellular hsp may not come in a single flavor, but rather in a variety of physical conditions. This review addresses some of our current knowledge about the release and function of extracellular hsp, in particular those associated with vesicles.

Rapid decrease of CD16 (FcγRIII) expression on heat-shocked neutrophils and their recognition by macrophages

Accumulation of neutrophils in the site of inflammation is a typical mechanism of innate immunity. The accumulated neutrophils are exposed to stressogenic factors usually associated with inflammation. Here, we studied response of human peripheral blood neutrophils subjected to short, febrile-range heat stress. We show that 90 min heat stress slowed down the spontaneous apoptosis of neutrophils. In the absence of typical markers of apoptosis the heat-shocked neutrophils induced antiinflammatory effect in human monocyte-derived macrophages (hMDMs), yet without being engulfed. Importantly, the expression of FcγRIII (CD16) was sharply reduced. Surprisingly, concentration of the soluble CD16 did not change in heat-shocked neutrophil supernates indicating that the reduction of the cell surface CD16 was achieved mainly by inhibition of fresh CD16 delivery. Inhibitors of 90 kDa heat shock protein (HSP90), a molecular chaperone found in membrane platforms together with CD16 and CD11b, significantly increased the observed effects caused by heat shock. The presented data suggest a novel systemic aspect of increased temperature which relies on immediate modification by heat of a neutrophil molecular pattern. This effect precedes cell death and may be beneficial in the initial phase of inflammation providing a nonphlogistic signal to macrophages before it comes from apoptotic cells.

IMPDHII protein inhibits Toll-like receptor 2-mediated activation of NF-kappaB

Toll-like receptor 2 (TLR2) plays an essential role in innate immunity by the recognition of a large variety of pathogen-associated molecular patterns. It induces its recruitment to lipid rafts induces the formation of a membranous activation cluster necessary to enhance, amplify, and control downstream signaling. However, the exact composition of the TLR2-mediated molecular complex is unknown. We performed a proteomic analysis in lipopeptide-stimulated THP1 and found IMPDHII protein rapidly recruited to lipid raft. Whereas IMPDHII is essential for lymphocyte proliferation, its biologic function within innate immune signal pathways has not been established yet. We report here that IMPDHII plays an important role in the negative regulation of TLR2 signaling by modulating PI3K activity. Indeed, IMPDHII increases the phosphatase activity of SHP1, which participates to the inactivation of PI3K.

Regulation of Hsp27 and Hsp70 expression in human and mouse skin construct models by caveolae following exposure to the model sulfur mustard vesicant, 2-chloroethyl ethyl sulfide

Dermal exposure to the vesicant sulfur mustard causes marked inflammation and tissue damage. Basal keratinocytes appear to be a major target of sulfur mustard. In the present studies, mechanisms mediating skin toxicity were examined using a mouse skin construct model and a full-thickness human skin equivalent (EpiDerm-FT™). In both systems, administration of the model sulfur mustard vesicant, 2-chloroethyl ethyl sulfide (CEES, 100-1000μM) at the air surface induced mRNA and protein expression of heat shock proteins 27 and 70 (Hsp27 and Hsp70). CEES treatment also resulted in increased expression of caveolin-1, the major structural component of caveolae. Immunohistochemistry revealed that Hsp27, Hsp70 and caveolin-1 were localized in basal and suprabasal layers of the epidermis. Caveolin-1 was also detected in fibroblasts in the dermal component of the full thickness human skin equivalent. Western blot analysis of caveolar membrane fractions isolated by sucrose density centrifugation demonstrated that Hsp27 and Hsp70 were localized in caveolae. Treatment of mouse keratinocytes with filipin III or methyl-β-cyclodextrin, which disrupt caveolar structure, markedly suppressed CEES-induced Hsp27 and Hsp70 mRNA and protein expression. CEES treatment is known to activate JNK and p38 MAP kinases; in mouse keratinocytes, inhibition of these enzymes suppressed CEES-induced expression of Hsp27 and Hsp70. These data suggest that MAP kinases regulate Hsp 27 and Hsp70; moreover, caveolae-mediated regulation of heat shock protein expression may be important in the pathophysiology of vesicant-induced skin toxicity.

Apical expression of human full-length hCEACAM1-4L protein renders the Madin Darby Canine Kidney cells responsive to lipopolysaccharide leading to TLR4-dependent Erk1/2 and p38 MAPK signalling

CEACAM1 expressed by granulocytes and epithelial cells is recognized as a membrane-associated receptor by some Gram-negative pathogens. Here we report a previously unsuspected role of human CEACAM1-4L (hCEACAM1-4L) in polarized epithelial cells. We find that in contrast with non-transfected cells, Madin Darby Canine Kidney strain II (MDCK) engineered for the apical expression of the long cytoplasmic chain protein hCEACAM1-4L showed a serum-independent increase in the phosphorylation of the extracellular signal-regulated kinase 1/2 (Erk1/2) and p38 mitogen-activated protein kinases (MAPKs) after treatment with lipopolysaccharide (LPS) of wild-type, diffusely adhering Afa/Dr Escherichia coli (Afa/Dr DAEC) strain IH11128. Aggregates of FITC-LPS bind the apical domain of MDCK-hCEACAM1-4L cells colocalizing with the apically expressed hCEACAM1-4L protein and do not bind MDCK-pCEP cells, and surface plasmon resonance analysis shows that LPS binds to the extracellular domain of the CEACAM1-4L protein. We showed that cell polarization and lipid rafts positively control the LPS-IH11128-induced phosphorylation of Erk1/2 in MDCK-hCEACAM1-4L cells. Structure-function analysis using mutated hCEACAM1-4L protein shows that the cytoplasmic domain of the protein is needed for LPS-induced MAPK signalling, and that phosphorylation of Tyr-residues is not increased in association with MAPK signalling. The hCEACAM1-4L-dependent Erk1/2 phosphorylation develops in the presence of lipid A and does not develop in the presence of penta-acylated LPS. Finally, small interfering RNA (siRNA) silencing of canine TLR4 abolishes the hCEACAM1-4L-dependent, LPS-induced phosphorylation of Erk1/2. Collectively, our results support the notion that the apically expressed, full-length hCEACAM1-4L protein functions as a novel LPS-conveying molecule at the mucosal surface of polarized epithelial cells for subsequent MD-2/TLR4 receptor-dependent MAPK Erk1/2 and p38 signalling.

Titanium particles activate toll-like receptor 4 independently of lipid rafts in RAW264.7 murine macrophages

Adherent pathogen-associated molecular patterns (PAMPs) act through toll-like receptor 2 (TLR2) and TLR4 to increase the biological activity of orthopedic wear particles in cell culture and animal models of implant loosening. This study tested whether this is dependent on TLR association with lipid rafts as reported for the response to soluble TLR ligands. For this purpose, RAW264.7 murine macrophages were activated by exposure to titanium particles with adherent PAMPs, soluble lipopolysaccharide (LPS), soluble lipotecichoic acid (LTA), or heat-killed bacteria that had been extensively washed to remove soluble PAMPs. Lipid rafts were isolated by two independent methods and the location of TLR4 and TLR2 was analyzed by Western blotting. The cognate TLRs associated with lipid rafts when the macrophages were activated with soluble LPS and LTA but not after stimulation with either titanium particles with adherent PAMPs or heat-killed bacteria. The lipid raft disruptor, methyl-β-cyclodextrin, dose-dependently inhibited TNF-α release in response to LPS but had no affect on TNF-α release in response to titanium particles with adherent PAMPs. We conclude, therefore, that titanium particles with adherent PAMPs and heat-killed bacteria activate TLR2 and TLR4 in macrophages without inducing either TLR to associate with lipid rafts. These results have important implications for the mechanisms of orthopedic implant loosening as well the mechanisms for TLR activation in other inflammatory situations.

ESeroS-GS modulates lipopolysaccharide-induced macrophage activation by impairing the assembly of TLR-4 complexes in lipid rafts

The binding of lipopolysaccharides (LPS) to macrophages results in inflammatory responses. In extreme cases it can lead to endotoxic shock, often resulting in death. A broad range of antioxidants, including tocopherols, can reduce LPS activity in vitro and in vivo. To elucidate the underlying mechanisms of their action, we investigated the effect of the sodium salt of γ-L-glutamyl-S-[2-[[[3,4-dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecyl)-2H-1-benzopyran-6-yl]oxy]carbonyl]-3-[[2-(1H-indol-3-yl)ethyl]amino]-3-oxopropyl]-L-cysteinylglycine (ESeroS-GS), a novel α-tocopherol derivative, on LPS-induced inflammation in vitro and in vivo. ESeroS-GS reduced the transcription of TNF-α, IL-1β, IL-6 and iNOS genes in a dose-dependent manner in RAW264.7 macrophages, and inhibited the release of these inflammatory factors. In addition, ESeroS-GS inhibited LPS-induced mortality in a mouse sepsis model. Electrophoretic mobility shift assays (EMSA) and reporter gene assays revealed that ESeroS-GS down-regulated the transcriptional activity of NF-κB. By analyzing the partitioning of CD14 and Toll-like receptor 4 (TLR-4) in cell membrane microdomains, we found that ESeroS-GS attenuates the binding of LPS to RAW264.7 cells via interfering with the relocation of CD14 and TLR-4 to lipid rafts, blocking the activation of interleukin-1 receptor-associated kinase 1 (IRAK-1), and inhibiting the consequent phosphorylation of TAK1 and IKKα/β, which together account for the suppression of NF-κB activation. Taken together, our data suggest that ESeroS-GS can modulate LPS signaling in macrophages by impairing TLR-4 complex assembly via a lipid raft dependent mechanism. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Role of lipid rafts in innate immunity and phagocytosis of polystyrene latex microspheres

Understanding of the association of phagocytosis of polymers with signaling of innate immunity of macrophages is the major purpose of this study. Polymer conjugates have been utilized for clinical therapy of cancer and infections, such as Mycobacterium tuberculosis, as effective vectors of drug-delivery systems. They are incorporated through phagocytosis into macrophages and activate innate immunity signaling, which plays a crucial role in its therapeutic and side effects. Macrophage phagocytosis of polystyrene latex microspheres was examined and assayed by treatment of macrophages with the cholesterol depletor methyl-β-cyclodextrin (MβCD) or the sphingolipid depletor n-octyl-β-D-glucopyranoside (OGP). Expressions of various mRNAs during phagocytosis were quantified by real-time PCR. Phagocytosis of polystyrene latex microspheres by various macrophages, such as murine monocyte-derived macrophage J774, rat alveolar macrophage NR8383, and murine Kupffer cell KC13-2, was suppressed by treatment with MβCD or OGP in a concentration-dependent manner. The expression of mRNAs of TNFα, IL-1β, IL-6 and CXCL10 genes induced by lipopolysaccharide (LPS) was not suppressed by treatment with MβCD in J774 cells. Moreover, genes that were induced by LPS were up-regulated even in the absence of LPS by the phagocytosis of polymer conjugates, but such up-regulations were not suppressed by the treatment with MβCD. It was shown that lipid rafts play a significant role in incorporation of polymer conjugates through phagocytosis of macrophages, but their association with signal transduction in innate immunity is very limited.

Sphingolipids and cardiovascular diseases: lipoprotein metabolism, atherosclerosis and cardiomyopathy

Heart disease is widely believed to develop from two pathological processes. Circulating lipoproteins containing the nondegradable lipid, cholesterol, accumulate within the arterial wall and perhaps are oxidized to more toxic lipids. Both lipid accumulation and vascular reaction to the lipids lead to the gradual thickening of the vascular wall. A second major process that in some circumstances is a primary event is the development of a local inflammatory reaction. This might be a reaction to vessel wall injury that accompanies infections, immune disease, and perhaps diabetes and renal failure. In this chapter, we will focus on the relationship between de novo synthesis of sphingolipids and lipid metabolism, atherosclerosis, and cardiomyopathy.

Sphingomyelin synthase 2 (SMS2) deficiency attenuates LPS-induced lung injury

Sphingomyelin synthase (SMS) catalyzes the synthesis of sphingomyelin (SM) and is required for maintenance of plasma membrane microdomain fluidity. Of the two isoforms of mammalian SMS, SMS1 is mostly present in the trans-Golgi apparatus, whereas SMS2 is predominantly found at the plasma membrane. SMS2 has a role in receptor mediated response to inflammation in macrophages, however, the role of SMS2 in vascular permeability, pulmonary edema, and lung injury have not been investigated. To define the role of SMS activation in lung injury, we utilized a lipopolysaccharide (LPS)-induced lung edema model. SMS activity was measured and correlated with the severity of lung injury. Within 4 h of LPS treatment, SMS activity was increased significantly and remained upregulated up to 24 h. Comparison of LPS-induced lung injury in SMS2 knockout (SMS2(-/-)) and wild-type littermate control mice showed that inflammation, cytokine induction, and lung injury were significantly inhibited in SMS2(-/-) mice. Our results suggest that a deficiency of SMS2 can diminish the extent of pulmonary edema and lung injury. Furthermore, we show that depletion of SMS2 was sufficient to decrease MAP kinase-JNK activation, severity of LPS-induced pulmonary neutrophil influx, and inflammation, suggesting a novel role of SMS2 activation in lung injury.

Deciphering the complexity of Toll-like receptor signaling

Toll-like receptors (TLRs) are essential players in the innate immune response to invading pathogens. Although extensive research efforts have provided a considerable wealth of information on how TLRs function, substantial gaps in our knowledge still prevent the definition of a complete picture of TLR signaling. However, several recent studies describe additional layers of complexity in the regulation of TLR ligand recognition, adaptor recruitment, posttranslational modifications of signaling proteins, and the newly described, autonomous role of the TLR4 co-receptor CD14. In this review, by using it as model system for the whole TLR family, we attempt to provide a complete description of the signal transduction pathways triggered by TLR4, with a particular emphasis on the molecular and cell biological aspects regulating its function. Finally, we discuss a recently reported model of CD14-dependent signaling and highlight its biological implications.

Emodin suppresses lipopolysaccharide-induced pro-inflammatory responses and NF-κB activation by disrupting lipid rafts in CD14-negative endothelial cells

BACKGROUND AND PURPOSE

Emodin [1,3,8-trihydroxy-6-methylanthraquinone] has been reported to exhibit vascular anti-inflammatory properties. However, the corresponding mechanisms are not well understood. The present study was designed to explore the molecular target(s) of emodin in modifying lipopolysaccharide (LPS)-associated signal transduction pathways in endothelial cells.

EXPERIMENTAL APPROACH

Cultured primary human umbilical vein endothelial cells (HUVECs; passages 3-5) were pre-incubated with emodin (1-50 µg·mL(-1) ). LPS-induced expression of pro-inflammatory cytokines [interleukin (IL)-1β, IL-6] and chemokines (IL-8; CCL2/MCP-1) were determined by reverse transcription-PCR and elisa. Nuclear factor-κB (NF-κB) activation, inhibitor of κB (IκB)α degradation and Toll-like receptor-4 (TLR-4) were detected by immunocytochemistry and Western blotting. Cholesterol depletion [by methyl β-cyclodextrin (MBCD), a specific cholesterol binding agent] and cholesterol replenishment were further used to investigate the roles of lipid rafts in activation of HUVECs.

KEY RESULTS

Emodin inhibited, concentration-dependently, the expression of LPS-induced pro-inflammatory cytokines (IL-1β, IL-6) and chemokines (IL-8, CCL2) and, in parallel, inhibited NF-κB activation and IκBα degradation in HUVECs. However, emodin did not inhibit the NF-κB activation and IκBα degradation induced by IL-1β. The cholesterol binding agent, MBCD, inhibited LPS-induced NF-κB activation in passaged HUVECs [which also lack the LPS receptor, membrane CD14 (mCD14)], showing that lipid rafts played a key role in LPS signalling in mCD14-negative HUVECs. Moreover, emodin disrupted the formation of lipid rafts in cell membranes by depleting cholesterol.

CONCLUSIONS AND IMPLICATIONS

Lipid rafts were crucial in facilitating inflammatory responses of mCD14-negative HUVECs to LPS. Emodin disrupted lipid rafts through depleting cholesterol and, consequently, inhibited inflammatory responses in endothelial cells.

UNC93B1 is essential for TLR11 activation and IL-12-dependent host resistance to Toxoplasma gondii

Toll-like receptor (TLR) activation relies on biochemical recognition of microbial molecules and localization of the TLR within specific cellular compartments. Cell surface TLRs largely recognize bacterial membrane components, and intracellular TLRs are exclusively involved in sensing nucleic acids. Here we show that TLR11, an innate sensor for the Toxoplasma protein profilin, is an intracellular receptor that resides in the endoplasmic reticulum. The 12 membrane-spanning endoplasmic reticulum-resident protein UNC93B1 interacts directly with TLR11 and regulates the activation of dendritic cells in response to Toxoplasma gondii profilin and parasitic infection in vivo. A deficiency in functional UNC93B1 protein abolished TLR11-dependent IL-12 secretion by dendritic cells, attenuated Th1 responses against T. gondii, and dramatically enhanced susceptibility to the parasite. Our results reveal that the association with UNC93B1 and the intracellular localization of TLRs are not unique features of nucleic acid-sensing TLRs but is also essential for TLR11-dependent recognition of T. gondii profilin and for host protection against this parasite.

In vitro characterization of the microglial inflammatory response to Streptococcus suis, an important emerging zoonotic agent of meningitis

Streptococcus suis is an important swine and human pathogen responsible for septicemia and meningitis. In vivo research in mice suggested that in the brain, microglia might be involved in activating the inflammatory response against S. suis. The aim of this study was to better understand the interactions between S. suis and microglia. Murine microglial cells were infected with a virulent wild-type strain of S. suis. Two isogenic mutants deficient at either capsular polysaccharide (CPS) or hemolysin production were also included. CPS contributed to S. suis resistance to phagocytosis and regulated the inflammatory response by hiding proinflammatory components from the bacterial cell wall, while the absence of hemolysin, a potential cytotoxic factor, did not have a major impact on S. suis interactions with microglia. Wild-type S. suis induced enhanced expression of Toll-like receptor 2 by microglial cells, as well as phosphotyrosine, protein kinase C, and different mitogen-activated protein kinase signaling events. However, cells infected with the CPS-deficient mutant showed overall stronger and more sustained phosphorylation profiles. CPS also modulated inducible nitric oxide synthase expression and further nitric oxide production from S. suis-infected microglia. Finally, S. suis-induced NF-κB translocation was faster for cells stimulated with the CPS-deficient mutant, suggesting that bacterial cell wall components are potent inducers of NF-κB. These results contribute to increase the knowledge of mechanisms underlying S. suis inflammation in the brain and will be useful in designing more efficient anti-inflammatory strategies for meningitis.

Macrophage ABCA1 reduces MyD88-dependent Toll-like receptor trafficking to lipid rafts by reduction of lipid raft cholesterol

We previously showed that macrophages from macrophage-specific ATP-binding cassette transporter A1 (ABCA1) knockout (Abca1(-M/-M)) mice had an enhanced proinflammatory response to the Toll-like receptor (TLR) 4 agonist, lipopolysaccharide (LPS), compared with wild-type (WT) mice. In the present study, we demonstrate a direct association between free cholesterol (FC), lipid raft content, and hyper-responsiveness of macrophages to LPS in WT mice. Abca1(-M/-M) macrophages were also hyper-responsive to specific agonists to TLR2, TLR7, and TLR9, but not TLR3, compared with WT macrophages. We hypothesized that ABCA1 regulates macrophage responsiveness to TLR agonists by modulation of lipid raft cholesterol and TLR mobilization to lipid rafts. We demonstrated that Abca1(-M/-M) vs. WT macrophages contained 23% more FC in isolated lipid rafts. Further, mass spectrometric analysis suggested raft phospholipid composition was unchanged. Although cell surface expression of TLR4 was similar between Abca1(-M/-M) and WT macrophages, significantly more TLR4 was distributed in membrane lipid rafts in Abca1(-M/-M) macrophages. Abca1(-M/-M) macrophages also exhibited increased trafficking of the predominantly intracellular TLR9 into lipid rafts in response to TLR9-specific agonist (CpG). Collectively, our data suggest that macrophage ABCA1 dampens inflammation by reducing MyD88-dependent TLRs trafficking to lipid rafts by selective reduction of FC content in lipid rafts.

Myeloid differentiation primary response protein 88 couples reverse cholesterol transport to inflammation

Crosstalk exists in mammalian cells between cholesterol trafficking and innate immune signaling. Apolipoprotein A-I (apoA-I), a serum apolipoprotein that induces antiatherogenic efflux of macrophage cholesterol, is widely described as anti-inflammatory because it neutralizes bacterial lipopolysaccharide. Conversely, lipopolysaccharide-induced inflammation is proatherogenic. However, whether innate immunity plays an endogenous, physiological role in host cholesterol homeostasis in the absence of infection is undetermined. We report that apoA-I signals in the macrophage through Toll-like receptor (TLR)2, TLR4, and CD14, utilizing myeloid differentiation primary response protein 88 (MyD88)-dependent and -independent pathways, to activate nuclear factor-kappaB and induce cytokines. MyD88 plays a critical role in reverse cholesterol transport in vitro and in vivo, in part through promoting ATP-binding cassette A1 transporter upregulation. Taken together, this work identifies apoA-I as an endogenous stimulus of innate immunity that couples cholesterol trafficking to inflammation through MyD88 and identifies innate immunity as a physiologic signal in cholesterol homeostasis.

Atorvastatin affects TLR4 clustering via lipid raft modulation

Statins, HMG-CoA reductase inhibitors, are used widely in the treatment of hypercholesterolemia. Apart from lowering lipid levels, statins have been shown to have anti-inflammatory effects. Previously we showed that atorvastatin inhibits NF-kappaB activation, dose and time dependently, in LPS-TLR4 signaling pathway. In this study, we investigated the anti-inflammatory mechanism of atorvastatin via Toll-like receptor 4 (TLR4) in murine pro-B cell lines transfected with TLR4. Co-treatment of LPS-stimulated cells with both atorvastatin and mevalonate rescued NF-kappaB activation and TLR4 blockade demonstrated that atorvastatin does not exert its inhibitory effect via TLR4 receptor-ligand binding mechanism. Further investigation into the anti-inflammatory mechanism has shown that atorvastatin causes an impairment of TLR4 recruitment into the lipid raft thereby affecting anti-inflammatory responses. In contrast, mevalonate repaired lipid raft function leading to TLR4 clustering in the lipid raft. Together, these data suggest that atorvastatin exerts its anti-inflammatory effect via lipid raft modification. This novel finding offers another insight into the pleiotropic effects of atorvastatin and may be applicable to other pattern recognition receptors that utilize membrane lipid raft as a platform for signal transduction.