Selective localization of recognition complexes for leukotriene B4 and formyl-Met-Leu-Phe within lipid raft microdomains of human polymorphonuclear neutrophils

The Regulation of Neutrophil Migration in Patients with Sepsis: The Complexity of the Molecular Mechanisms and Their Modulation in Sepsis and the Heterogeneity of Sepsis Patients

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. Common causes include gram-negative and gram-positive bacteria as well as fungi. Neutrophils are among the first cells to arrive at an infection site where they function as important effector cells of the innate immune system and as regulators of the host immune response. The regulation of neutrophil migration is therefore important both for the infection-directed host response and for the development of organ dysfunctions in sepsis. Downregulation of CXCR4/CXCL12 stimulates neutrophil migration from the bone marrow. This is followed by transmigration/extravasation across the endothelial cell barrier at the infection site; this process is directed by adhesion molecules and various chemotactic gradients created by chemotactic cytokines, lipid mediators, bacterial peptides, and peptides from damaged cells. These mechanisms of neutrophil migration are modulated by sepsis, leading to reduced neutrophil migration and even reversed migration that contributes to distant organ failure. The sepsis-induced modulation seems to differ between neutrophil subsets. Furthermore, sepsis patients should be regarded as heterogeneous because neutrophil migration will possibly be further modulated by the infecting microorganisms, antimicrobial treatment, patient age/frailty/sex, other diseases (e.g., hematological malignancies and stem cell transplantation), and the metabolic status. The present review describes molecular mechanisms involved in the regulation of neutrophil migration; how these mechanisms are altered during sepsis; and how bacteria/fungi, antimicrobial treatment, and aging/frailty/comorbidity influence the regulation of neutrophil migration.

New Pieces in the Puzzle of uPAR Role in Cell Migration Mechanisms

The urokinase (uPA) receptor (uPAR) plays a key role in cell migration. We previously showed that uPAR-negative HEK-293 cells efficiently migrate toward serum but, after uPAR ectopic expression, migrate only in a uPAR-dependent manner. In fact, migration of uPAR-transfected HEK-293 (uPAR-293) cells is impaired by anti-uPAR antibodies, without recovery of the uPAR-independent migration mechanisms formerly active. Prostate carcinoma PC3 cells, which express high endogenous uPAR levels, migrated only through a uPAR-dependent mechanism; in fact, the silencing of uPAR expression inhibited their migration. We hypothesize a crucial role of the uPAR glycosyl-phosphatidyl-inositol (GPI) tail, which promotes uPAR partitioning to lipid rafts, in uPAR-controlled cell migration. Here, we show that removal of the uPAR GPI-tail, or lipid rafts disruption by methyl-beta-cyclodextrin impairs migration of PC3 cells, incapable of uPAR-independent migration, whereas it restores uPAR-independent migration in uPAR-293 cells. We then show that, in PC3 cells, both uPAR signaling partners, β1 integrins and receptors for formylated peptides (FPRs), partly associate with lipid rafts. Inhibition of their interaction with uPAR impairs this association and impairs cell migration. Interestingly, blocking uPAR association with FPRs also impairs β1 integrin partitioning to lipid rafts, whereas blocking its association with β1 integrins has no effect on FPRs partitioning. On these bases, we propose that uPAR controls cell migration by connecting β1 integrins and FPRs and, through its GPI tail, by driving them into lipid rafts, thus promoting pro-migratory signals. uPAR-mediated partitioning of integrins to lipid rafts is strictly dependent on uPAR association with FPRs.

Arachidonic Acid Induces the Migration of MDA-MB-231 Cells by Activating Raft-associated Leukotriene B4 Receptors

BACKGROUND

The migration of tumor cells is critical in spreading cancers through the lymphatic nodes and circulatory systems. Although arachidonic acid (AA) and its soluble metabolites have been shown to induce the migration of breast and colon cancer cells, the mechanism by which it induces such migration has not been fully understood.

OBJECTIVE

The effect of AA on migratory responses of the MDA-MB-231 cell line (a triple-negative breast cancer cell) was examined and compared with MCF-7 (estrogen-receptor positive) breast cancer cells to elucidate the mechanism of AA-induced migration.

METHODS

Migrations of breast cancer cells were examined with the help of wound-healing assays. AA-induced eicosanoid synthesis was monitored by RP-HPLC. Cellular localizations of lipoxygenase and lipid rafts were assessed by immunoblot and confocal microscopy.

RESULTS

AA treatment stimulated the synthesis of leukotriene B4 (LTB4) and HETE-8, but lowered the levels of prostaglandin E2 (PGE2), prostaglandin D2 (PGD2), and HETE-5 in MDA-MB-231 cells. Further analysis indicated that AA increased the expression of 5-lipoxygenase (5-LOX) in this cell line and inhibiting its expression by small molecule inhibitors lowered the production of LTB4 and reduced migration. In contrast, MCF-7 cells did not show any appreciable changes in eicosanoid synthesis, 5-LOX expression, or cellular migration.

CONCLUSION

Our results suggest that AA treatment activates the BLT1 receptor (present in membrane microdomains) and stimulates the synthesis of LTB4 production, which is likely to be associated with the migration of MDA-MB-231 cells.

Expression and cell distribution of leukotriene B4 receptor 1 in the rat brain cortex after experimental subarachnoid hemorrhage

Convincing evidence supports that nuclear factor kappa B (NF-κB)-meditated inflammation contributes to the adverse prognosis of aneurysmal subarachnoid hemorrhage (SAH), and pathologic neutrophil accumulation after SAH in the brain parenchyma enhances the inflammatory process. Leukotriene B4 (LTB4) is a highly potent lipid chemoattractant of neutrophils, and its biological effects are mediated primarily through the high-affinity LTB4 receptor 1 (BLT1). It is verified that NF-κB-dependent BLT1 mediates LTB4 signaling and LTB4 stimulates NF-κB-dependent inflammation via BLT1. This study aimed to determine the expression and cell distribution of BLT1 in the brain cortex after SAH and investigate the potential relationship between protein expressions of BLT1 and NF-κB. Male Sprague-Dawley rats were randomly assigned into sham group and SAH groups at 6h, 12h and on day 1, day 2 and day 3 (n=6 for each subgroup). SAH groups suffered experimental SAH by injecting 0.3ml autologous blood into the prechiasmatic cistern. BLT1 expression was measured by real-time PCR, western blot, immunohistochemistry and immunofluorescence. Nuclear expression of p65 protein, the major subunit of NF-κB, was also detected by western blot. Our data showed that the expression levels of BLT1 and nuclear p65 protein were both markedly increased after SAH. Moreover, there was a significant positive correlation between BLT1 and nuclear p65 protein expressions in the same specific time course. Double immunofluorescence staining showed that BLT1 were mainly expressed in neurons, microglia and endothelial cells rather than astrocytes after SAH. These results suggest that BLT1 may participate in the NF-κB-mediated inflammatory response after SAH, and there might be important implications for further studies using specific BLT1 antagonists to attenuate the NF-κB-mediated inflammation after SAH.

Isolation of Lipid Rafts from Human Neutrophils by Density Gradient Centrifugation

Neutrophils are present within minutes to the site of aggression in the body making them one of the first cells of the immune system to be in contact with incoming threats. The cell functions of neutrophils are elicited through the engagement of surface receptors, some of which are located in a specific region of the membrane called lipid rafts, a functionally segregated region of the membrane enriched with cholesterol and distinct species of sphingomyelin and glycerophospholipids. Lipid rafts are relatively resistant to detergent extraction and this can be taken advantage of to isolate them from the rest of the cell membrane. This chapter will describe a reliable method to obtain lipid rafts from detergent-resistant membrane fractions of human neutrophils. Cells are lysed in an HEPES solution containing 0.5% Triton X-100, supernatants are mixed with a 42% sucrose solution, which is then overlaid with a 35% and 5% sucrose solution. The gradient is centrifuged for 16 h and the resulting fractions can be further analyzed by immunoblotting or subjected to immunoprecipitation.

Insights into the mechanism for dictating polarity in migrating T-cells

This review is focused on mechanisms of chemokine-induced polarization of T-lymphocytes. Polarization involves, starting from spherical cells, formation of a morphologically and functionally different rear (uropod) and front (leading edge). This polarization is required for efficient random and directed T-cell migration. The addressed topics concern the specific location of cell organelles and of receptors, signaling molecules, and cytoskeletal proteins in chemokine-stimulated polarized T-cells. In chemokine-stimulated, polarized T-cells, specific proteins, signaling molecules and organelles show enrichment either in the rear, the midzone, or the front; different from the random location in spherical resting cells. Possible mechanisms involved in this asymmetric location will be discussed. A major topic is also the functional role of proteins and cell organelles in T-cell polarization and migration. Specifically, the roles of adhesion and chemokine receptors, cytoskeletal proteins, signaling molecules, scaffolding proteins, and membrane microdomains in these processes will be discussed. The polarity which is established during contact formation of T-cells with antigen-presenting cells is not discussed in detail.

Evidence for the presence of functional lipid rafts in immune cells of ectothermic organisms

The role of lipid rafts in non-mammalian leukocytes has been scarcely investigated. We performed biochemical and functional analysis of lipid rafts in fish leukocytes. Fish Flotillin-1 and a fish GM1-like molecule (fGM1-L) were found in low density detergent-resistant membranes (LD-DRM) in goldfish macrophages and catfish B lymphocytes, similarly to mammals. The presence of flotillin-1 and fGM1-L in LD-DRM was sensitive to increased detergent concentrations, and cholesterol extraction. Confocal microscopy analysis of flotillin-1 and fGM1-L in fish leukocytes showed a distinctive punctuated staining pattern, suggestive of pre-existing rafts. Confocal microscopy analysis of macrophages showed that the membrane of phagosomes containing serum-opsonized zymosan was enriched in fGM1-L, and zymosan phagocytosis was reduced after cholesterol extraction. The presence of flotillin-1 and fGM1-L in LD-DRM, the microscopic evidence of flotillin-1 and fGM1-L on fish macrophages and B-cells, and the sensitivity of phagocytosis to cholesterol extraction, indicate that lipid rafts are biochemically and functionally similar in leukocytes from fish and mammals.

LTB4 is a signal-relay molecule during neutrophil chemotaxis

Neutrophil recruitment to inflammation sites purportedly depends on sequential waves of chemoattractants. Current models propose that leukotriene B(4) (LTB(4)), a secondary chemoattractant secreted by neutrophils in response to primary chemoattractants such as formyl peptides, is important in initiating the inflammation process. In this study we demonstrate that LTB(4) plays a central role in neutrophil activation and migration to formyl peptides. We show that LTB(4) production dramatically amplifies formyl peptide-mediated neutrophil polarization and chemotaxis by regulating specific signaling pathways acting upstream of actin polymerization and MyoII phosphorylation. Importantly, by analyzing the migration of neutrophils isolated from wild-type mice and mice lacking the formyl peptide receptor 1, we demonstrate that LTB(4) acts as a signal to relay information from cell to cell over long distances. Together, our findings imply that LTB(4) is a signal-relay molecule that exquisitely regulates neutrophil chemotaxis to formyl peptides, which are produced at the core of inflammation sites.

MT6-MMP is present in lipid rafts and faces inward in living human PMNs but translocates to the cell surface during neutrophil apoptosis

Polymorphonuclear neutrophils (PMNs) are the first line of defense against invading organisms in humans; in addition, PMNs contribute to the linking of innate and adaptive immunity. To fulfill their biological behavior, PMNs utilize an arsenal of proteolytic enzymes, including members of the matrix metalloproteinase family of zinc-dependent endopeptidases. PMNs express high levels of MT6-MMP (MMP-25), a glycosyl-phosphatidylinositol-anchored MMP, that belongs to the subfamily of membrane-anchored matrix metalloproteinases. Due to the paucity of information on MT6-MMP in primary cells, we set to investigate the localization and potential function of MT6-MMP in human PMNs. We found that MT6-MMP is present in the membrane, granules and nuclear/endoplasmic reticulum/Golgi fractions of PMNs where it is displayed as a disulfide-linked homodimer of 120 kDa. Stimulation of PMNs resulted in secretion of active MT6-MMP into the supernatants. Membrane-bound MT6-MMP, conversely, is located in the lipid rafts of resting PMNs and stimulation does not alter this location. In addition, TIMP-2, a natural inhibitor of MT6-MMP, does not co-localize with it in the lipid rafts. Interestingly, living PMNs do not display MT6-MMP on the cell surface. However, induction of apoptosis induces MT6-MMP relocation on PMNs' cell surface. Our studies suggest that metalloproteinases may play a role in respiratory burst and IL-8 secretion, but not chemotaxis or granulocyte macrophage colony-stimulating factor-induced survival. Collectively, these results provide new insights on the role of MT6-MMP in the physiology of human PMNs.

Migrating human neutrophils exhibit dynamic spatiotemporal variation in membrane lipid organization

Highly ordered sphingolipid-enriched lipid raft microdomains (LRMs) within plasma membranes purportedly function as specialized signaling platforms. Leukocyte migration is believed to entail LRM redistribution, but progress in studying LRMs in situ during cell movement has been limited. By using an improved method for imaging the spectral shift of the environmentally sensitive probe, laurdan (expressed as a generalized polarization function), the plasma membrane order (i.e., tight packing of membrane bilayer lipids) of human polymorphonuclear neutrophils (PMNs) was mapped in real time during migration. Morphologically polarized PMNs exhibited prominent LRM clusters at the uropod, where in every instance membrane order was found to oscillate with mean periodicities of 37.0 ± 1.46 and 149.9 ± 9.0 seconds (P < 0.01). LRM aggregates were also demonstrated in punctate and clustered distributions of nonpolarized cells and transiently at the lamellipodia of polarized PMNs. Cellular polarization was not accompanied by an overall increase in membrane order. LRM disorganization with methyl-β-cyclodextrin had small negative effects on cell velocity, but it abrogated directionally biased migration toward chemotactic gradients of FMLP or leukotriene B(4). LRMs disruption also caused redistribution of Rac 1/2 GTPase and GM3 ganglioside away from the lamellipodium, as well as extension of multiple pseudopods simultaneously or in rapid succession, rather than formation of a defined leading edge. Thus, we demonstrate that the plasma membrane order of migrating PMNs changes dynamically, with prominent oscillations consistently seen at the uropod. These findings solidify the existence of rapidly reorganizing LRMs in situ and support a role for LRMs in chemotaxin responsiveness.

Lipid raft-mediated regulation of G-protein coupled receptor signaling by ligands which influence receptor dimerization: a computational study

G-protein coupled receptors (GPCRs) are the largest family of cell surface receptors; they activate heterotrimeric G-proteins in response to ligand stimulation. Although many GPCRs have been shown to form homo- and/or heterodimers on the cell membrane, the purpose of this dimerization is not known. Recent research has shown that receptor dimerization may have a role in organization of receptors on the cell surface. In addition, microdomains on the cell membrane termed lipid rafts have been shown to play a role in GPCR localization. Using a combination of stochastic (Monte Carlo) and deterministic modeling, we propose a novel mechanism for lipid raft partitioning of GPCRs based on reversible dimerization of receptors and then demonstrate that such localization can affect GPCR signaling. Modeling results are consistent with a variety of experimental data indicating that lipid rafts have a role in amplification or attenuation of G-protein signaling. Thus our work suggests a new mechanism by which dimerization-inducing or inhibiting characteristics of ligands can influence GPCR signaling by controlling receptor organization on the cell membrane.

Impaired mobilization of hematopoietic stem/progenitor cells in C5-deficient mice supports the pivotal involvement of innate immunity in this process and reveals novel promobilization effects of granulocytes

We reported that complement cascade (CC) becomes activated in bone marrow (BM) during granulocyte colony-stimulating factor (G-CSF) mobilization of hematopoietic stem/progenitor cells (HSPCs) and showed that, although third CC component (C3)-deficient mice are easy mobilizers, fifth CC component (C5)-deficient mice mobilize very poorly. To explain this, we postulated that activation/cleavage of CC releases C3a and C5a anaphylatoxins that differently regulate mobilization. Accordingly, C3a, by enhancing responsiveness of HSPCs to decreasing concentrations of stromal-derived growth factor-1 (SDF-1) in BM, prevents mobilization and promotes their BM retention. Therefore, in this study, we focused on the mobilization-enhancing role of C5a. We found that C5a receptor (C5aR) is not expressed on the surface of HSPCs, and that C5a-mediated promobilization effects are mediated by stimulation of granulocytes. Overall, our data support the following model. First C5aR(+) granulocytes are chemoattracted by plasma C5 cleavage fragments, being the first wave of cells leaving BM. This facilitates a subsequent egress of HSPCs. In the next step, after leaving BM, granulocytes undergo degranulation in response to plasma C5a and secrete some cationic peptides (cathelicidin, beta-defensin) that, as shown here for the first time, highly enhance the responsiveness of HSPCs to plasma SDF-1 gradient. In conclusion, our data reveal the underappreciated central role of innate immunity in mobilization, in which C5 cleavage fragments through granulocytes orchestrate this process.

FcgammaRI ligation leads to a complex with BLT1 in lipid rafts that enhances rat lung macrophage antimicrobial functions

Leukotriene (LT) B(4) is generated in response to engagement of the Fc gamma receptor (Fc gamma R) and potently contributes to Fc gamma R-mediated antimicrobial functions in pulmonary alveolar macrophages. In this study, we report that the LTB(4) receptor leukotriene B(4) receptor 1 (BLT1) redistributes from nonlipid raft (LR) to LR membrane microdomains upon immunoglobulin G-red blood cell, but not LTB(4), challenge. Cholesterol depletion to disrupt LRs abolished LTB(4)-induced enhancement of phagocytosis, microbicidal activity, and signaling. The dependence on LR integrity for BLT1 signaling correlated with formation of a complex consisting of BLT1, its primary coupled G protein G alpha i3, Src kinase, and Fc gamma RI within LRs. This association was dependent on Src-mediated phosphorylation of BLT1. These data identify a novel form of regulation in which engagement of a macrophage immunoreceptor recruits a stimulatory G protein-coupled receptor into a LR microdomain with resultant enhanced antimicrobial signaling.

A ceramide-binding C1 domain mediates kinase suppressor of ras membrane translocation

Genetic and biochemical data support Kinase Suppressor of Ras 1 (KSR1) as a positive regulator of the Ras-Raf-MAPK pathway, functioning as a kinase and/or scaffold to regulate c-Raf-1 activation. Membrane translocation mediated by the KSR1 CA3 domain, which is homologous to the atypical PKC C1 lipid-binding domain, is a critical step of KSR1-mediated c-Raf-1 activation. In this study, we used an ELISA to characterize the KSR1 CA3 domain as a lipid-binding moiety. Purified GST-KSR1-CA3 protein effectively binds ceramide but not other lipids including 1,2-diacylglyceol, dihydroceramide, ganglioside GM1, sphingomyelin and phosphatidylcholine. Upon epidermal growth factor stimulation of COS-7 cells, KSR1 translocates into and is activated within glycosphingolipid-enriched plasma membrane platforms. Pharmacologic inhibition of ceramide generation attenuates KSR1 translocation and KSR1 kinase activation in COS-7 cells. Disruption of two cysteines, which are indispensable for maintaining ternary structure of all C1 domains and their lipid binding capability, mitigates ceramide-binding capacity of purified GST-KSR1-CA3 protein, and inhibits full length KSR1 membrane translocation and kinase activation. These studies provide evidence for a mechanism by which the second messenger ceramide can target proteins to subcellular compartments in the process of transmembrane signal transduction.

Biosynthesis of leukotriene B4 in human polymorphonuclear leukocytes: regulation by cholesterol and other lipids

Leukotriene B4 (LTB4) is one of the most potent chemotactic compounds produced in macrophages and neutrophils. LTB4 is a product of arachidonic acid oxygenation by 5-lipoxygenase pathway. We present here the data on regulation of LT synthesis in human polymorphonuclear leukocytes by cholesterol, cholesterol sulfate and cholesterol phosphate. The addition of Pseudomonas aeruginosa lipopolysaccharides (LPS) with lipid vesicles containing phosphatidylcholine or phosphatidylcholine/cholesterol (70:30) showed that omitting cholesterol abolished the effect of LPS on LT synthesis. We show here the capacity of cholesterol sulfate, the most abundant sulfated sterol in human blood, to suppress LT production in human neutrophils and to neutralize the effect of P. aeruginosa LPS on LT synthesis. We suggest that sulfated lipids serve as specific endogenous regulators of LT synthesis in neutrophils, and anti-inflammatory therapy may be based on modification of cholesterol level and its conversion to anionic derivatives.

A benzodiazepines derived compound, 4-(3-chlorophenyl)-1,3-dihydronaphtho [2,3-b][1,4]diazepin-2-one (ND700C), inhibits fMLP-induced superoxide anion release by activating protein phosphatase 2A in human neutrophils

We studied the mechanism underlying the inhibitory effect of a benzodiazepines derivative, 4-(3-chlorophenyl)-1,3-dihydronaphtho [2,3-b][1,4]diazepin-2-one (ND700C), on superoxide anion production induced by formly-methionyl-leucyl-phenylalanine (fMLP) in human neutrophils. ND700C inhibited the fMLP-induced superoxide anion production and cathepsin G release in a concentration-dependent manner with respective IC50 values of 5.0+/-0.5 and 8.7+/-0.8muM. In addition, ND700C was found to suppress fMLP-induced intracellular calcium mobilization and the phosphorylation of ERK and Akt. In another study, ND700C was observed to cause a rapid increase in intracellular cAMP level by up to threefold. Furthermore, when H89 was used to inhibit cAMP-dependent protein kinase A (PKA), we discovered that ND700C's suppressive effects on calcium mobilization, phosphorylation, and superoxide anion production were abrogated. ND700C demonstrated additive effect on the PGE1-induced increase in cAMP. However, this additive effect was not demonstrated with the IBMX-induced rise in cAMP. Our results indicated that ND700C did not directly inhibit the activity of phosphodiesterase 4. In another set of experiments, calyculin A and okadaic acid (both protein phosphatase 2A inhibitors) were found to reverse ND700C's positive effect on cAMP level. This observation suggested the involvement of protein phosphatase 2A in ND700C's cAMP-elevating mechanism. We found that the activity of protein phosphatase 2A was activated by ND700C. Furthermore, protein phosphatase 2A was co-immunoprecipitated with phosphodiesterase 4 after ND700C treatment in human neutrophils.

CONCLUSION

ND700C inhibited fMLP-induced superoxide anion production through a PKA-dependent pathway. ND700C increased cAMP by activating protein phosphatase 2A, which subsequently inhibited phosphodiesterase 4.

Formyl peptide-receptor like-1 requires lipid raft and extracellular signal-regulated protein kinase to activate inhibitor-κB kinase in human U87 astrocytoma cells

Formyl peptide-receptor like-1 (FPRL-1) may possess critical roles in Alzheimer's diseases, chemotaxis and release of neurotoxins, possibly through its regulation of nuclear factor-kappaB (NFkappaB). Here we illustrate that activation of FPRL-1 in human U87 astrocytoma or Chinese hamster ovary cells stably expressing the receptor resulted in the phosphorylations of inhibitor-kappaB kinase (IKK), an onset kinase for NFkappaB signaling cascade. FPRL-1 selective hexapeptide Trp-Lys-Tyr-Met-Val-Met (WKYMVM) promoted IKK phosphorylations in time- and dose-dependent manners while pre-treatment of pertussis toxin abrogated the Galpha(i/o)-dependent stimulations. The FPRL-1-mediated IKK phosphorylation required extracellular signal-regulated protein kinase (ERK), phosphatidylinositol 3-kinase and cellular Src (c-Src), but not c-Jun N-terminal kinase and p38 mitogen-activated protein kinase. Despite its ability to mobilize Ca(2+), WKYMVM did not require Ca(2+) for the modulation of IKK phosphorylation. Activation of FPRL-1 also induced NFkappaB-driven luciferase expression. Interestingly, cholesterol depletion from plasma membrane by methyl-beta-cyclodextrin abolished the FPRL-1-stimulated IKK phosphorylation, denoting the important role of lipid raft integrity in the FPRL-1 to IKK signaling. Furthermore, we demonstrated that in U87 cells, several signaling intermediates in the FPRL-1-IKK pathway including Galpha(i2), c-Src and ERK were constitutively localized at the raft microdomains. WKYMVM administration not only resulted in higher amount of ERK recruitment to the raft region, but also specifically stimulated raft-associated c-Src and ERK phosphorylations. Taken together, these results demonstrate that FPRL-1 is capable of activating NFkappaB signaling through IKK phosphorylation and this may serve as a useful therapeutical target for FPRL-1-related diseases.

(2R,3R)-2-(3',4'-dihydroxybenzyl)-3-(3'',4''-dimethoxybenzyl)butyrolactone suppresses fMLP-induced superoxide production by inhibiting fMLP-receptor binding in human neutrophils

This study investigated the mechanism underlying the inhibiting effect of (2R,3R)-2-(3',4'-dihydroxybenzyl)-3-(3'',4''-dimethoxybenzyl) butyrolactone (PP-6), a lignan from Piper philippinum, on superoxide anion production induced by the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMLP) in human neutrophils. Human neutrophils were stimulated with fMLP (1 microM), PMA (100 nM) or leukotriene B(4) (LTB(4); 1 microM) and induced superoxide anion release. PP-6 specifically inhibited fMLP-induced superoxide anion production in a concentration-dependent manner with an IC(50) value of 0.3+/-0.1 microM. Intracellular signaling caused by fMLP, PMA or LTB(4) were evaluated. PP-6 specifically inhibited fMLP-induced intracellular calcium mobilization and ERK (p42/p44), Akt and p38 phosphorylation. Moreover, PP-6 specifically inhibited fMLP-induced Mac-1 expression without affecting this caused by LTB(4) or PMA. PP-6 did not increase cAMP level in human neutrophils. PP-6 did not inhibit superoxide anion production by NaF (20 mM), a direct activator of G-protein, the target of the inhibitory action of PP-6 appears to be a component of the signal transduction pathway upstream of G-protein. PP-6 inhibited FITC-fMLP binding to neutrophils in a concentration-dependent manner with an IC(50) of 1.5+/-0.2 microM. PP-6 did not bring a parallel shift in the concentration response of fMLP-induced superoxide anion. Additionally, the inhibiting effect of PP-6 on fMLP-induced superoxide anion was reversed when PP-6 was washed out. These experimental results suggest that PP-6 exerts non-competitive and reversible antagonistic effect on fMLP receptor.

Lipid mediators in membrane rafts are important determinants of human health and disease

The new field of membrane rafts has provided fresh insight and a novel framework in which to understand the interaction, relation, and organization of lipids and proteins within cell membranes. This review will examine our current understanding of membrane rafts and their role in human health. In addition, the effect of various lipids, including dietary lipids, on membrane raft structure and function will be discussed. Membrane rafts are found in all cells and are characterized by their high concentration of cholesterol, sphingolipids, and saturated fatty acids. These lipids impart lateral segregation of membrane proteins, thus facilitating the spatial organization and regulation of membrane proteins involved in many cellular processes, such as cell proliferation, apoptosis, and cell signaling. Therefore, membrane rafts are shedding new light on the origins of metabolic disturbances and diseases such as cancer, insulin resistance, inflammation, cardiovascular disease, and Alzheimer's disease, which will be further discussed in this review.