Cytoskeletal alterations in lipopolysaccharide-induced bovine vascular endothelial cell injury and its prevention by sodium arsenite

Sphingosine-1-phosphate promotes barrier-stabilizing effects in human microvascular endothelial cells via AMPK-dependent mechanisms

Breakdown of the endothelial barrier is a critical step in the development of organ failure in severe inflammatory conditions such as sepsis. Endothelial cells from different tissues show phenotypic variations which are often neglected in endothelial research. Sphingosine-1-phosphate (S1P) and AMP-dependent kinase (AMPK) have been shown to protect the endothelium and phosphorylation of AMPK by S1P was shown in several cell types. However, the role of the S1P-AMPK interrelationship for endothelial barrier stabilization has not been investigated. To assess the role of the S1P-AMPK signalling axis in this context, we established an in vitro model allowing real-time monitoring of endothelial barrier function in human microvascular endothelial cells (HMEC-1) and murine glomerular endothelial cells (GENCs) with the electric cell-substrate impedance sensing (ECIS™) system. Following the disruption of the cell barrier by co-administration of LPS, TNF-α, IL-1ß, IFN-γ, and IL-6, we demonstrated self-recovery of the disrupted barrier in HMEC-1, while the barrier remained compromised in GENCs. Under physiological conditions we observed a rapid phosphorylation of AMPK in HMEC-1 stimulated with S1P, but not in GENCs. Consistently, S1P enhanced the basal endothelial barrier in HMEC-1 exclusively. siRNA-mediated knockdown of AMPK in HMEC-1 led to a less pronounced barrier enhancement. Thus we present evidence for a functional role of AMPK in S1P-mediated barrier stabilization in HMEC-1 and we provide insight into cell-type specific differences of the S1P-AMPK-interrelationship, which might influence the development of interventional strategies targeting endothelial barrier dysfunction.

In vitro treatment of lipopolysaccharide increases invasion of Pasteurella multocida serotype B:2 into bovine aortic endothelial cells

Pasteurella multocida serotype B:2 causes hemorrhagic septicemia in cattle and buffalo. The invasion mechanism of the bacterium when invading the bloodstream is unclear. This study aimed to characterize the effects of immunomodulatory molecules, namely dexamethasone and lipopolysaccharide, on the invasion efficiency of P. multocida serotype B:2 toward bovine aortic endothelial cells (BAECs) and the involvement of actin microfilaments in the invasion mechanism. The results imply that treatment of BAECs with lipopolysaccharide at 100 ng/mL for 24 h significantly increases the intracellular bacteria number per cell (p < 0.01) compared with those in untreated and dexamethasone-treated cells. The lipopolysaccharide-treated cells showed a significant decrease in F-actin expression and an increase in G-actin expression (p < 0.001), indicating actin depolymerization of BAECs. However, no significant differences were detected in the invasion efficiency and actin filament reorganization between the dexamethasone-treated and untreated cells. Transmission electron microscopy showed that P. multocida B:2 resided in a vacuolar compartment of dexamethasone-treated and untreated cells, whereas the bacteria resided in cellular membrane of lipopolysaccharide-treated cells. The results suggest that lipopolysaccharide destabilizes the actin filaments of BAECs, which could facilitate the invasion of P. multocida B:2 into BAECs.

Putting on the brakes: Bacterial impediment of wound healing

The epithelium provides a crucial barrier to infection, and its integrity requires efficient wound healing. Bacterial cells and secretomes from a subset of tested species of bacteria inhibited human and porcine corneal epithelial cell migration in vitro and ex vivo. Secretomes from 95% of Serratia marcescens, 71% of Pseudomonas aeruginosa, 29% of Staphylococcus aureus strains, and other bacterial species inhibited epithelial cell migration. Migration of human foreskin fibroblasts was also inhibited by S. marcescens secretomes indicating that the effect is not cornea specific. Transposon mutagenesis implicated lipopolysaccharide (LPS) core biosynthetic genes as being required to inhibit corneal epithelial cell migration. LPS depletion of S. marcescens secretomes with polymyxin B agarose rendered secretomes unable to inhibit epithelial cell migration. Purified LPS from S. marcescens, but not from Escherichia coli or S. marcescens strains with mutations in the waaG and waaC genes, inhibited epithelial cell migration in vitro and wound healing ex vivo. Together these data suggest that S. marcescens LPS is sufficient for inhibition of epithelial wound healing. This study presents a novel host-pathogen interaction with implications for infections where bacteria impact wound healing and provides evidence that secreted LPS is a key factor in the inhibitory mechanism.

Lipopolysaccharide challenge significantly influences lipid metabolism and proteome of white adipose tissue in growing pigs

Background

White adipose tissue is recognized as a highly active organ, which is closely related to a large number of physiological and metabolic processes besides storing triglycerides. However, little is known regarding the response of adipose tissue to acute inflammation. Therefore, in this study we employed growing pigs to investigate the changes of lipid metabolism and proteome in white adipose tissue after lipopolysaccharide (LPS) stimulation as a model for bacterial infection.

Methods

The expression of lipid metabolism and inflammation related genes was determined by quantitative real-time polymerase chain reaction. Label-free proteomics analysis was used to investigate changes of the protein profile in white adipose tissue and western blot was used to verify changes of selected adipokines.

Results

The results indicated that LPS significantly increased the expression of toll-like receptor (TLR) 2/4 pathway-related genes and pro-inflammatory factors. Lipid metabolism related genes, including acetyl-CoA carboxylase 1 (ACACA), fatty acid synthase (FASN), stearoyl-CoA desaturase (SCD), uncoupling protein 2 (UCP2), and 11 β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), were down-regulated and the lipolytic enzyme activity was decreased after LPS injection. Proteome analysis revealed 47 distinct proteins with > 2-fold changes. The down-regulation of two proteins (cAMP-dependent protein kinase type II-alpha regulatory subunit and β-tubulin) has been verified by western blot analysis. In addition, the abundance of two adipokines (adiponectin and zinc-α2-glycoprotein) was significantly increased after LPS injection.

Conclusion

In conclusion, LPS challenge can cause acute inflammation in white adipose tissue. Concurrently, lipid metabolism was significantly suppressed and the abundance of several proteins changed in white adipose tissue. The results provide new clues to understand the adipose dysfunction during inflammation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12944-015-0067-5) contains supplementary material, which is available to authorized users.

The effects of inflammatory cytokines on lymphatic endothelial barrier function

Proper lymphatic function is necessary for the transport of fluids, macromolecules, antigens and immune cells out of the interstitium. The lymphatic endothelium plays important roles in the modulation of lymphatic contractile activity and lymph transport, but it's role as a barrier between the lymph and interstitial compartments is less well understood. Alterations in lymphatic function have long been associated with edema and inflammation although the integrity of the lymphatic endothelial barrier during inflammation is not well-defined. In this paper we evaluated the integrity of the lymphatic barrier in response to inflammatory stimuli commonly associated with increased blood endothelial permeability. We utilized in vitro assays of lymphatic endothelial cell (LEC) monolayer barrier function after treatment with different inflammatory cytokines and signaling molecules including TNF-α, IL-6, IL-1β, IFN-γ and LPS. Moderate increases in an index of monolayer barrier dysfunction were noted with all treatments (20-60 % increase) except IFN-γ which caused a greater than 2.5-fold increase. Cytokine-induced barrier dysfunction was blocked or reduced by the addition of LNAME, except for IL-1β and LPS treatments, suggesting a regulatory role for nitric oxide. The decreased LEC barrier was associated with modulation of both intercellular adhesion and intracellular cytoskeletal activation. Cytokine treatments reduced the expression of VE-cadherin and increased scavenging of β-catenin in the LECs and this was partially reversed by LNAME. Likewise the phosphorylation of myosin light chain 20 at the regulatory serine 19 site, which accompanied the elevated monolayer barrier dysfunction in response to cytokine treatment, was also blunted by LNAME application. This suggests that the lymphatic barrier is regulated during inflammation and that certain inflammatory signals may induce large increases in permeability.

Urocortin increased LPS-induced endothelial permeability by regulating the cadherin-catenin complex via corticotrophin-releasing hormone receptor 2

Urocortin (Ucn1), a member of corticotrophin-releasing hormone (CRH) family, has been reported to be upregulated in inflammatory diseases and function as an autocrine or paracrine inflammatory mediator. Growing evidence shows that Ucn1 increases the endothelial permeability in inflammatory conditions; however, the detailed mechanisms are not clear. In the present study, we investigated the mechanisms of increased endothelial permeability by Ucn1 in human umbilical vein endothelial cells (HUVECs) exposed to lipopolysaccharide (LPS). Pretreatment of HUVECs with Ucn1 increased the endothelial cell permeability, which was augmented by LPS synergistically. Significant downregulation of VE-cadherin expression was also observed. Moreover, Ucn1 increased phosphorylation of protein kinase D (PKD) and heat shock protein 27 (HSP27) in a time- and CRHR(2) -dependent manner. Inhibition of PKD and HSP27 drastically attenuated Ucn1-induced downregulation of VE-cadherin expression. Further investigations demonstrated that Ucn1 phosphorylated β-catenin at Ser552 to disrupt the cadherin-catenin complex and hence promote the disassociation of β-catenin and VE-cadherin. Disassociation of β-catenin and VE-cadherin resulted in decreased VE-cadherin expression while on the contrary β-catenin was increased, which may due to the inactivation of GSK-3β. Increased β-catenin translocated into the nucleus and subsequently bound to TCF/LEF site, contributing to the elevated expression of vascular endothelial growth factor (VEGF). The above effects of Ucn1 were completely reversed by CRHR(2) receptor blocker, antisauvagine-30. Taken together, our data suggest that Ucn1 increase LPS-induced endothelial permeability by disrupting the VE-cadherin-β-catenin complex via activation of CRHR(2) and PKD-HSP27 signaling pathway.

Silencing of C5a receptor gene with siRNA for protection from Gram-negative bacterial lipopolysaccharide-induced vascular permeability

Endothelial barrier dysfunction leading to increased permeability and vascular leakage is an underlying cause of several pathological conditions. Whereas these changes have been shown to be associated with activation of the complement system, leading to the release of C5a and interaction of C5a-C5a receptor (C5aR), the role of C5aR in endothelial cells remain(s) ill-defined. Here, we report an essential role of C5aR in endothelial cell injury and vascular permeability through silencing of the C5aR gene using siRNA. In the cultured mouse dermal microvascular endothelial cells (MEMECs) monolayer transfected with C5aR-siRNA, endotoxin-induced cell injury by evaluated as transendothelial flux, cell detachment, and cytoskeletal disorganization was inhibited. Upregulation of vascular cell adhesion molecule-1 (VCAM-1) was also suppressed. Studies exploring the underlying mechanism of siRNA-mediated suppression in VCAM-1 expression were related to reduction of NF-kappaB activation and nuclear localization of both p50 and p65. The effect was associated with inhibition in activation of protein kinase Cdelta(PKC-delta) and induction of PKC-mediated mitogen-activated protein kinase phosphatases-1 (MKP-1) leading to the increased activity of p42/p44 mitogen-activated protein (MAP) kinase cascade. In the model of mice administrated with C5aR-siRNA, endotoxin-induced plasma leakage was inhibited in local abdominal skin. Systemic administration of endotoxin to mice resulted in increased microvascular permeability in multiple organs was reduced. These studies demonstrate that the C5aR responsible for vascular endothelial cell injury and plasma permeability is an important factor, and that blockade of C5aR may be useful therapeutic targets for the prevention of vascular permeability in pathogenic condition.

Functionalized solid lipid nanoparticles for transendothelial delivery

The objectives of this study were to synthesize and characterize functionalized solid lipid nanoparticles (fSLN) to investigate their interaction with endothelial cell monolayers and to evaluate their transendothelial transport capabilities. fSLN bearing tetramethylrhodamine-isothiocyanate-labeled bovine serum albumin (TRITC-BSA) and Coumarin 6 were prepared using a single-step phase-inversion process that afforded concurrent surface modification with a variety of macromolecules such as polystyrene sulfonate (PSS), poly-L-lysine (PLL), heparin (Hep), polyacrylic acid (PAA), polyvinyl alcohol, and polyethylene glycol (PEG). TRITC-BSA/Coumarin 6 encapsulated in fSLN with composite surface functionality (PSS-PLL and PSS-PLL-Hep) were also investigated. Size and surface charge of fSLN were analyzed using dynamic light scattering and transmission electron microscopy. Transport across bovine aortic endothelial cell (BAEC) monolayers was assessed spectrophotometrically using a transwell assay, and fSLN localization at the level of the cell and permeable support was analyzed using fluorescence microscopy. fSLN with tunable size and surface functionality were successfully produced, and had significant effects on cell localization and transport. Specifically, fSLN with PSS-PLL-Hep composite surface functionalization was capable of translocating 53.2 +/- 8.7 mug of TRITC-BSA within 4 h, with fSLN-PEG, fSLN-PAA, and fSLN-PSS exhibiting near-complete apical, paracellular, and cytosolic localization, respectively. Coumarin 6 was released by fSLN as indicated by dye labeling of BAEC membranes. We have developed a rapid process for the production of fSLN bearing low- and high-molecular-weight payloads of varying physicochemical properties. These findings have impications for drug delivery and bioimaging applications, since due to tunable surface chemistry, fSLN internalization and/or translocation across intact endothelial cell monolayers is possible.

Anti-vascular permeability of the cleaved reactive center loop within the carboxyl-terminal domain of C1 inhibitor

C1 inhibitor (C1INH), a member of the serine proteinase inhibitor (serpin) family, functions as an inhibitor of the complement and contact systems. Cleavage of the reactive center loop (RCL) within the carboxyl-terminal domain of C1INH (iC1INH), lacking of serpin function, induces a conformational change in the molecule. Our previous data demonstrated that active, intact C1INH prevents vascular permeability induced by gram-negative bacterial lipopolysaccharide (LPS). In this study, we investigate the role of RCL-cleaved, inactive C1INH (iC1INH) in vascular endothelial activation. In the cultured primary human umbilical vein endothelial cell (HUVEC) monolayer, iC1INH blocked LPS-induced cell injury by evaluated as transendothelial flux, cell detachment, and cytoskeletal disorganization. LPS-induced upregulation of vascular cell adhesion molecule-1 (VCAM-1) could be suppressed by treatment with iC1INH. Studies exploring the underlying mechanism of iC1INH-mediated suppression in VCAM-1 expression were related to reduction of NF-kappaB activation and nuclear translocation in an I kappa B alpha-dependent manner. The inhibitory effect was associated with stabilization of the NF-kappaB inhibitor I kappa B and reduction of inhibitor I kappa B kinase activity. In the model of endotoxin-induced mice, increased plasma leakage in local abdominal skin in response to LPS was reversed by treatment with iC1INH. Furthermore, systemic administration of LPS to mice resulted in increased microvascular permeability in multiple organs, which was reduced by iC1INH. These data provide evidence that iC1INH has an anti-vascular permeability independent on the serpin function.

Lipopolysaccharide prevents apoptosis induced by brefeldin A, an endoplasmic reticulum stress agent, in RAW 264.7 cells

The effect of lipopolysaccharide (LPS) on the cell death induced by endoplasmic reticulum (ER) stress agents in RAW 264.7 cells was studied. LPS prevented the cell death by brefeldin A, but not thapsigargin and tunicamycin. CpG DNA as well as LPS prevented brefeldin A-induced cell death whereas tumor necrosis factor-alpha or interferon-gamma did not. Brefeldin A-induced cell death was mediated with apoptotic cell death and it was significantly inhibited by LPS. LPS abolished the activation of ER stress-related caspases, such as caspases 1, 3, and 4. LPS prevented brefeldin A-induced morphological changes in RAW 264.7 cells. Further, LPS prevented brefeldin A-induced Golgi dispersion. Therefore, LPS was suggested to diminish the stress of ER/Golgi complexes induced by brefeldin A and inhibit apoptosis. The preventive action of LPS on brefeldin A-induced apoptosis is discussed.

C1 inhibitor prevents Gram-negative bacterial lipopolysaccharide-induced vascular permeability

Gram-negative bacterial endotoxemia may lead to the pathological increase of vascular permeability with systemic vascular collapse, a vascular leak syndrome, multiple organ failure (MOF), and/or shock. Previous studies demonstrated that C1 inhibitor (C1INH) protects mice from lipopolysaccharide (LPS)-induced lethal septic shock via a direct interaction with LPS. Here, we report that C1INH blocked the LPS-induced increase in transendothelial flux through an endothelial monolayer. In addition, LPS-mediated detachment of cultured endothelial cells was prevented with C1INH. C1INH also inhibited LPS-induced endothelial cell apoptosis as demonstrated by suppression of DNA fragmentation and annexin V expression. As illustrated by laser scanning confocal microscopy, C1INH completely blocked the binding of fluorescein isothiocyanate (FITC)-LPS to human umbilical vein endothelial cells (HUVECs). C1INH protected from localized LPS-induced increased plasma leakage in C57BL/6J mice and in C1INH-deficient mice. Local vascular permeability in response to LPS was increased to a greater extent in C1INH-deficient mice compared with wild-type littermate controls and was reversed by treatment with C1INH. Systemic administration of LPS to mice resulted in increased vascular permeability, which was reduced by C1INH. Therefore, these studies demonstrate that C1INH, in addition to its role in suppression of LPS-mediated macrophage activation, may play an important role in the prevention of LPS-mediated increased vascular permeability, endothelial cell injury, and multiple organ failure.

Colchicine induces membrane-associated activation of matrix metalloproteinase-2 in osteosarcoma cells in an S100A4-independent manner

Like the metastasis-associated protein S100A4, matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) are important in physiological and pathological conditions. Previously, we showed that S100A4 is involved in the regulation of MMPs and TIMPs, and in the present work we have investigated whether the anti-inflammatory and microtubule-disrupting drug colchicine has an effect on the expression of these proteins in osteosarcoma cell lines (OHS) with high and low levels of S100A4. Colchicine treatment of the various OHS cells resulted in an increased expression of MT1-MMP and TIMP-2 mRNA, and a corresponding increase of these two proteins in isolated cell membranes. Colchicine-treated cells produced more of the activated form of MMP-2 than control cells. However, the drug did not affect the amount of MMP-2 and TIMP-1 mRNA or protein, and it reduced the S100A4 mRNA expression. Isolated cell membranes from the colchicine-treated cells were more effective in activating exogenous proMMP-2 than membranes from control cells, and inhibitory studies indicated that it was the colchicine-induced increase in MT1-MMP that caused the increased activation of endogenous MMP-2. A peptide inhibitor of nuclear factor kappaB nuclear translocation, SN50, blocked the colchicine-induced activation of proMMP-2 and reduced the synthesis of MMP-2 in colchicine-treated cells, but not in control cells. It can be concluded that colchicine modulates the expression of MT1-MMP and TIMP-2 and hence the activation of proMMP-2 independently of the S100A4 level in osteosarcoma cells.

Difference in uptake and toxicity of trivalent and pentavalent inorganic arsenic in rat heart microvessel endothelial cells

Intake of inorganic arsenic is known to cause vascular diseases as well as skin lesions and cancer in humans. We investigated the differences in cytotoxicity, uptake rate of arsenic, and gene expression of antioxidative enzymes between arsenite (As(3+))- and arsenate (As(5+))-exposed rat heart microvessel endothelial cells. As(3+) was more cytotoxic than As(5+), and LC(50) values were calculated to be 36 and 220 micro M, respectively. As(3+) (1-25 micro M) increased mRNA levels of antioxidant enzymes such as heme oxygenase-1 (HO-1), thioredoxin peroxidase 2, NADPH dehydrogenase, and glutathione S-transferase P subunit. HO-1 mRNA levels showed the most remarkable increase in response to As(3+). cDNA microarray analysis indicated that there was no prominent difference in arsenic-induced transcriptional changes between As(3+)- and As(5+)-exposed cells, when the cells were exposed to one-fourth the LC(50) concentration of arsenic (9 and 55 micro M for As(3+) and As(5+), respectively). N-acetyl- l-cysteine (NAC) reduced both the cytotoxicity of inorganic arsenic and the HO-1 mRNA level, and buthionine sulfoximine enhanced cytotoxicity of inorganic arsenic. As(3+) was taken up by the endothelial cells 6-7 times faster than As(5+), and the presence of NAC in the culture medium did not change the uptake rate of As(3+). These results suggest that the effects of NAC on arsenic-induced cytotoxicity and oxidative stress were due to the antioxidative role of non-protein thiols and not to chelation of arsenic in the culture medium. The difference in cellular uptake of arsenic between As(3+) and As(5+) appeared not to be due to the ionic charge on arsenic (at physiological pH, trivalent arsenic is neutral whereas pentavalent arsenic is negatively charged). These results suggest that the higher toxicity of As(3+) compared with that of As(5+) is probably due to the faster uptake of As(3+) by endothelial cells, and inorganic arsenic exerts its toxicity at least in part via intracellular oxidative stress.

Protective effect of glial cells against lipopolysaccharide-mediated blood-brain barrier injury

Numerous infections of the central nervous system are characterized by altered blood-brain barrier (BBB) functions leading to brain damage. To study the mechanisms that cause BBB disruption in these pathologies, we used an in vitro BBB model consisting of a coculture of brain capillary endothelial cells and glial cells. When these endothelial cells were submitted alone to lipopolysaccharide (LPS), added in the luminal compartment, a huge increase in the paracellular permeability of the monolayer was observed. As glial cells surrounding the brain capillaries are of prime importance in specifying at least some cellular properties, we investigated whether glial cells would be able to modulate this endothelial cell response to LPS. When endothelial cells were incubated with LPS added luminally, in the presence of glial cells, LPS surprisingly had no effect on the endothelial cell monolayer permeability, suggesting a protective effect of glial cells on the LPS-mediated injury. As in our experiments, the endotoxin does not interact with the glial cell population. This protective effect suggests a close communication between cerebral endothelial cells and brain parenchymal cells. In our coculture model, the glial cell population is a mixture of astrocytes, oligodendrocytes, and microglial cells. Further experiments performed with purified astrocytes showed that microglial cells or oligodendrocytes, or both, are essential for the complete protection of the endothelial cell monolayer integrity. All these results are direct evidence for a modulatory effect of glial cells on brain capillary endothelial cell response in the pathogenesis of endotoxemia.

Gastric nitric oxide synthase expression during endotoxemia: implications in mucosal defense in rats

BACKGROUND & AIMS

This study was performed to examine expression of gastric nitric oxide synthase (NOS) isoforms during endotoxemia in rats and to assess their role(s) in gastric injury from bile and ethanol.

METHODS

Lipopolysaccharide (LPS) enhanced the expression and activity of inducible nitric oxide synthase in gastric mucosa in a dose- and time-dependent manner.

RESULTS

Endothelial nitric oxide synthase and neural nitric oxide synthase expression did not significantly change, but constitutive nitric oxide synthase activity decreased over time. LPS alone caused injury to the gastric mucosa and disrupted F-actin filaments in the same cells with enhanced immunostaining for inducible nitric oxide synthase. LPS also exacerbated gastric injury from the mild irritants 5 mmol/L acidified taurocholate and 20% ethanol as did local intra-arterial infusion of the nitric oxide donor S-nitroso-N-acetyl-penicillamine. The selective inducible nitric oxide synthase inhibitor aminoguanidine negated LPS-induced exacerbation of gastric injury from these irritants. The nonselective NOS inhibitor N(G)-nitro-L-arginine methyl ester augmented the deleterious effects of LPS, an effect reversed by L-arginine but not D-arginine. Aminoguanidine, but not N(G)-nitro-L-arginine methyl ester, negated LPS-induced accumulation of gastric luminal nitrates.

CONCLUSIONS

These data suggest that increased inducible NOS activity and decreased constitutive nitric oxide synthase activity are primarily responsible for exacerbating gastric injury from luminal irritants during endotoxemia. Moreover, septic patients may be more susceptible to gastric injury from bile during gastrointestinal ileus.

The inhibitory action of sodium arsenite on lipopolysaccharide-induced nitric oxide production in RAW 267.4 macrophage cells: a role of Raf-1 in lipopolysaccharide signaling

The effect of sodium arsenite (SA) on LPS-induced NO production in RAW 267.4 murine macrophage cells was studied. SA pretreatment of LPS-stimulated RAW cells resulted in a striking reduction in NO production. No significant difference in LPS binding was observed between RAW cells pretreated with SA and control untreated RAW cells, suggesting that SA might impair the intracellular signal pathway for NO production. SA inhibited LPS-induced NF-kappaB activation by preventing loss of IkappaB-alpha and -beta. Furthermore, SA blocked phosphorylation of extracellular signal-regulated kinase 1/2 (Erk1/2), but not phosphorylation of p38 and c-Jun N-terminal kinase. SA treatment resulted in the disappearance of Raf-1, suggesting that it might cause the inhibition of the Erk1/2 mitogen-activated protein (MAP) kinase pathway. The SA-mediated loss of Raf-1 also abolished LPS-induced NF-kappaB activation as well as the Erk1/2 pathway. The dominant negative mutant of MAP kinase kinase 1 inhibited both NO production and NF-kappaB activation in LPS-stimulated RAW cells. Taken together, these results indicate that the inhibitory action of SA on NO production in LPS-stimulated macrophages might be due to abrogation of inducible NO synthase induction, and it might be closely related to inactivation of the NF-kappaB and Erk1/2 MAP kinase pathways through loss of Raf-1.