A labile transcriptional repressor modulates endotoxin tolerance

Inducible nitric oxide synthase is not required in the development of endotoxin tolerance in mice

We investigated the role of inducible nitric oxide synthase (iNOS) in endotoxin tolerance, which was induced in mice genetically deficient of iNOS (iNOS-/-) and in wild-type littermates. In non-tolerant wild-type mice, endotoxin induced high mortality, elevation of plasma levels of nitrite and nitrate, tumor necrosis factor a (TNFalpha), and interleukin 10 (IL-10). These events were preceded by degradation of inhibitors kappaBalpha (IkappaBalpha) and kappaBI (IkappaBbeta), and activation of nuclear factor-kappaB (NF-kappaB) in the lung. Pretreatment of wild-type mice with a sublethal dose of endotoxin prior to lethal endotoxin administration ameliorated lethality and blunted TNFalpha production, whereas IL-10, nitrite, and nitrate production was maintained. These events were associated with reduction of IKBa degradation and NF-kappaB activation in the lung. The kinetics of degradation of IkappaBbeta were also altered. In parallel experiments, nontolerant iNOS-/- mice experienced similar mortality after endotoxin as nontolerant wild-type mice. Plasma levels of nitrite and nitrate were not elevated after lethal endotoxin administration. IL-10 levels were significantly reduced in comparison to nontolerant wild-type mice, whereas TNFalpha levels were similarly increased. These events were preceded by lesser degradation of IkappaBalpha and reduced NF-kappaB activation in the lung. Pretreatment of iNOS-/- mice with a sublethal endotoxin ameliorated lethality. TNFalpha production was significantly reduced, whereas IL-10 production was significantly increased when compared to nontolerant iNOS-/- mice. Degradation of IkappaBalpha and activation of NF-kappaB in the lung were not altered by endotoxin tolerance, whereas kinetics of IkappaBbeta degradation was only delayed. Our data suggests that iNOS is not required for the development of endotoxin tolerance, and that other signal transduction pathways, rather than NF-kappaB, may regulate induction of endotoxin tolerance in the absence of iNOS.

The phosphatidylinositol 3-kinase pathway selectively controls sIL-1RA not interleukin-1beta production in the septic leukocytes

Microbial components such as bacterial endotoxin lipopolysaccharide (LPS) can trigger highly lethal septic shock. The cardinal features of septic leukocytes include the repressed production of inflammatory cytokines, such as interleukin-1 beta (IL-1beta), and elevated production of anti-inflammatory cytokines, such as secretory interleukin-1 receptor antagonist (sIL-1RA). Pro- and anti-inflammatory cytokine gene transcriptions are equally repressed in septic leukocytes due to disruption of the LPS signaling pathway at the level of interleukin-1 receptor-associated kinase. The selective elevation of sIL-1RA protein in septic blood is caused by efficient translation of residual sIL-1RA message. In this study, we report that the LPS-inducible phosphatidylinositol 3-kinase (PI3-kinase)-dependent signaling pathway contributes to the elevated translation of sIL-1RA in septic/LPS-adapted leukocytes. We also observe that this pathway is gene specific and does not affect the production of proinflammatory IL-1beta protein.

Induction of cross-tolerance by lipopolysaccharide and highly purified lipoteichoic acid via different Toll-like receptors independent of paracrine mediators

Exposure of macrophages to LPS induces a state of hyporesponsiveness to subsequent stimulation with LPS termed LPS desensitization or tolerance. To date, it is not known whether similar mechanisms of macrophage refractoriness are induced on contact with components of Gram-positive bacteria. In the present study, we demonstrate that pretreatment with highly purified lipoteichoic acid (LTA) results in suppression of cytokine release on restimulation with LTA in vitro and in vivo in both C3H/HeN and C3H/HeJ mice, but not in macrophages from Toll-like receptor (TLR)-2-deficient mice. Furthermore, desensitization in response to LPS or LTA exposure also inhibits responses to the other stimulus ("cross-tolerance"), suggesting that signaling pathways shared by TLR2 and TLR4 are impaired during tolerance. Finally, we show that LPS- or LTA-induced cross-tolerance is not transferred to hyporesponsive cells cocultured with LPS/LTA-responsive macrophages, showing that soluble mediators do not suffice for tolerance induction in neighboring cells.

NF-kappaB expression in mononuclear cells of patients with sepsis resembles that observed in lipopolysaccharide tolerance

The expression of NF-kappaB was studied in freshly isolated peripheral blood mononuclear cells (PBMC) of patients with severe sepsis and major trauma. The expression of p65p50 heterodimer, the active form of NF-kappaB, was significantly reduced for all patients as compared with control subjects. The p50p50 homodimer, an inhibitory form of NF-kappaB, was reduced in the survivors of sepsis and in patients with trauma. Subsequent in vitro stimulation of PBMC with lipopolysaccharide (LPS) did not induce further NF-kappaB nuclear translocation: the survivors of sepsis and trauma patients showed low expression of both p65p50 and p50p50, whereas nonsurvivors of sepsis showed a predominance of the inactive homodimer and a low p65p50/p50p50 ratio when compared with control subjects. In the later group of patients there was a reverse correlation between plasma IL-10 levels and the p65p50/p50p50 ratio after in vitro LPS stimulation (r = -0.8, p = 0.04). The reduced expression of nuclear NF-kappaB was not due to its inhibition by IkappaBalpha, as very low expression of IkappaBalpha, as well as low levels of p65 and p50 were found in the cytoplasm of PBMC from patients with sepsis and trauma when compared with control subjects. These results demonstrate that upon LPS activation, PBMC of patients with systemic inflammatory response syndrome show patterns of NF-kappaB expression that resemble those reported during LPS tolerance: global down-regulation of NF-kappaB in survivors of sepsis and trauma patients and the presence of large amounts of the inactive homodimer in the nonsurvivors of sepsis.

Characterization of interleukin-1 receptor-associated kinase in normal and endotoxin-tolerant cells

Interleukin-1 receptor-associated kinase (IRAK), a signal transducer for interleukin-1, has also been suggested to participate in the Toll-like receptor-mediated innate immune response to bacterial endotoxin lipopolysaccharide (LPS). Using the human promonocytic THP-1 cell line, we demonstrated that the endogenous IRAK is quickly activated in response to bacterial LPS stimulation, as measured by its in vitro kinase activity toward myelin basic protein. LPS also triggers the association of IRAK with MyD88, the adaptor protein linking IRAK to the Toll-like receptor/interleukin-1beta receptor intracellular domain. Macrophage cells with prolonged LPS treatment become tolerant to additional dose of LPS and no longer express inflammatory cytokines. Endotoxin tolerance is a common phenomenon observed in blood from sepsis patients. We observed for the first time that the quantity of IRAK is greatly reduced in LPS-tolerant THP-1 cells, and its activity no longer responds to further LPS challenge. In addition, IRAK does not associate with MyD88 in the tolerant cells. Furthermore, application of AG126, a putative tyrosine kinase inhibitor, can substantially alleviate the LPS-induced cytokine gene expression and can also decrease IRAK level and activity. Our study indicates that IRAK is essential for LPS-mediated signaling and that cells may develop endotoxin tolerance by down-regulating IRAK.

mRNA and protein stability regulate the differential expression of pro- and anti-inflammatory genes in endotoxin-tolerant THP-1 cells

The products of proinflammatory genes such as interleukin-1beta (IL-1beta) and cyclooxygenase-2 (COX-2) initiate many of the events associated with sepsis. Transcription of these genes is subsequently down-regulated, whereas expression of anti-inflammatory genes such as secretory interleukin-1 receptor antagonist (sIL-1 RA) is maintained. Differential expression is associated with endotoxin tolerance, a cellular phenomenon common to sepsis and characterized by reduced proinflammatory gene expression after repeated exposure to lipopolysaccharide. As a model for endotoxin tolerance, we examined the expression of COX-2 and sIL-1 RA in a human promonocyte cell line, THP-1. We observed a 5-fold decrease in COX-2 protein in endotoxin-tolerant cells relative to control cells. In contrast, sIL-1 RA protein increased 5-fold in control and tolerant cells and remained elevated. Decreased COX-2 production is due to repressed transcription and not enhanced mRNA degradation. In addition, COX-2 protein is turned over rapidly. Transcription of sIL-1 RA is also repressed during tolerance. However, sIL-1 RA mRNA is degraded more slowly than COX-2 mRNA, allowing continued synthesis of sIL-1 RA protein that is very stable. These results indicate that differential expression during endotoxin tolerance occurs by transcriptional repression of COX-2 and by protein and mRNA stabilization of sIL-1 RA.

Differential tolerance induction by lipoarabinomannan and lipopolysaccharide in human macrophages

Various bacterial cell wall components have been shown to induce hyporesponsiveness in macrophages (MAC). Here, mycobacterial glycolipids were employed to determine whether they induce a state of 'tolerance/hyporesponsiveness' in MAC in vitro in order to assess whether mycobacterial components negatively affect the immune response to Mycobacterium tuberculosis. Arabinosylated lipoarabinomannan (ARA-LAM) stimulated hyporesponsiveness by reducing TNF-alpha, GM-CSF, G-CSF, IL-10, and IL-6 release similarly to LPS, but caused no changes in IL-8 secretion. Mannose-capped LAM (MAN-LAM) acted in a different way in that TNF-alpha, GM-CSF, and IL-10 were upregulated after restimulation of MAC. Blocking experiments by mannan suggest mannose-receptor involvement in MAN-LAM activation only. Cross-stimulation experiments demonstrated a hierarchy of signaling, with LPS being the most potent stimulator and mediating abrogation of ARA-LAM-stimulated tolerance but not vice versa. MAN-LAM was the least potent stimulator of either MAC activation and induction of hyporesponsiveness. Similarly to LPS, ARA-LAM upregulated CD14 surface expression after restimulation. Recurrent MAN-LAM treatment either downmodulated or did not induce any change in CD14 expression. The role of MAN-LAM regulated cytokine secretion as well as implications regarding M. tuberculosis infection will be discussed.

NF-kappaB1 (p50) is upregulated in lipopolysaccharide tolerance and can block tumor necrosis factor gene expression

Monocytes respond to lipopolysaccharide (LPS) stimulation with a rapid expression of the tumor necrosis factor (TNF) gene. Upon repeated LPS stimulation there is, however, little production of TNF mRNA and protein; i.e., the cells are tolerant to LPS. Analysis of NF-kappaB proteins in gel shift assays demonstrated that the DNA binding activity that is induced by LPS stimulation in tolerant cells consists mainly of p50-p50 homodimers. Since p50 can bind to DNA but lacks a transactivation domain, this may explain the blockade of TNF gene expression. We now show that in the monocytic cell line Mono Mac 6, this inability to respond can be largely ascribed to NF-kappaB, since a reporter construct directed by a trimeric NF-kappaB motif is strongly transactivated by LPS stimulation of naive cells whereas LPS-tolerant cells exhibit only low activity. Also, Western blot analyses of proteins extracted from purified nuclei showed mobilization of threefold-higher levels of p50 protein in tolerant compared to naive cells, while mobilization of p65 was unaltered. Overexpression of p50 in HEK 293 cells resulted in a strong reduction of p65-driven TNF promoter activity at the levels of both luciferase mRNA and protein. These data support the concept that an upregulation of p50 is instrumental in LPS tolerance in human monocytes.

Regulation of an essential innate immune response by the p50 subunit of NF-kappaB

Recognition of bacterial endotoxin (LPS) elicits multiple host responses, including activation of cells of the innate immune system. LPS exposure occurs repeatedly during septicemia, making strict regulation of gene expression necessary. Such regulation might prevent, for example, the continuous production of proinflammatory cytokines such as tumor necrosis factor (TNF), which could lead to severe vascular collapse. Tolerance to LPS is characterized by a diminished production of TNF during prolonged exposure to LPS, and is therefore likely to represent an essential control mechanism during sepsis. In the present study, which uses mice with genetic deletions of the proteins of NF-kappaB complex, we provide data demonstrating that increased expression of the p50 subunit of NF-kappaB directly results in the downregulation of LPS-induced TNF production. This contention is supported by the following observations: (1) tolerance to LPS is not induced in macrophages from p50-/- mice; (2) long-term pretreatment with LPS does not block synthesis of TNF mRNA in p50-/- macrophages (in contrast to wild-type macrophages); (3) ectopic overexpression of p50 reduces transcriptional activation of the murine TNF promoter; and (4) analysis of the four kappaB sites from the murine TNF promoter demonstrates that binding of p50 homodimers to the positively acting kappaB3 element is associated with development of the LPS-tolerant phenotype. Thus, p50 expression plays a key role in the development of LPS tolerance.

Lipopolysaccharide-Induced Desensitization of junB Gene Expression in a Mouse Macrophage-Like Cell Line, P388D1

Treatment of a mouse macrophage cell line, P388D1, for 1 h with bacterial LPS caused a transient increase in the level of junB mRNA expression. These cells became refractory in terms of the junB gene response to exposure to a second round of LPS or lipid A, but not to PMA. The LPS-induced desensitized state was not due to the shortening of the half-life of junB mRNA, but was suggested, by nuclear run-on analysis, to be caused by reduction of junB gene transcription. Pretreating cells with herbimycin A, a tyrosine kinase inhibitor, substantially inhibited LPS-induced expression of junB mRNA and decreased tyrosine phosphorylation of 38- to 42-kDa proteins, which comigrated with p38 and p42 mitogen-activated protein (MAP) kinases. Parallel to down-regulation of junB mRNA expression, activation of the p38 MAP kinase was markedly reduced in LPS-tolerant cells, whereas activation of p42 MAP kinase was relatively constant. The specific p38 MAP kinase inhibitor, SB202190, potently inhibited LPS-induced junB mRNA expression. These results suggest that the LPS-induced desensitization of junB gene expression occurs at or upstream of the level of gene transcription and may be involved in a defective LPS-induced p38 MAP kinase pathway.

Tolerance to pyrogens

In humans or experimental animals, the repeated confrontation with lipopolysaccharides (LPS) from gram-negative bacteria, but not with muramyl dipeptide (MDP) from gram-positive bacteria, leads to attenuation of almost all pathophysiologic effects mediated by proinflammatory cytokines. Our experiments in guinea pigs and rats demonstrate that attenuation of the febrile response during the development of LPS tolerance is associated with a reduced production of cytokines rather than a decrease in responsiveness to cytokines. Cross-tolerance experiments demonstrate that different stimuli influencing LPS-induced tumor necrosis factor (TNF) release and nitric oxide (NO) synthesis can modify the development of tolerance. On the other hand, the lack of cross-tolerance between LPS and MDP indicates that MDP can activate the cytokine cascade and induce the febrile response in animals tolerant to LPS. This may indicate distinct receptors and signal pathways for LPS and MDP, leading to activation of the cytokine cascade. LPS tolerance has also been demonstrated in ex vivo and in vitro studies. In cultures of monocytes, diminished synthesis of TNF and NO reported after LPS restimulation could be prevented and reversed by interferon and granulocyte-macrophage colony-stimulating factor. These findings add an additional hypothesis in tolerance development.

Increased levels of myocardial IkappaB-alpha protein promote tolerance to endotoxin

Endotoxin [lipopolysaccharide (LPS)] causes tumor necrosis factor-alpha (TNF-alpha)-mediated myocardial contractile depression. Tolerance to the cardiac toxicity of LPS can be induced by a prior exposure to LPS or by pretreatment with glucocorticoids. The mechanisms by which the myocardium acquires tolerance to LPS remain unknown. LPS causes phosphorylation and degradation of inhibitory kappaB-alpha (IkappaB-alpha), releasing nuclear factor-kappaB (NF-kappaB) to activate TNF-alpha gene transcription. We hypothesized that LPS induces supranormal synthesis of myocardial IkappaB-alpha protein and thus renders the myocardium tolerant to subsequent LPS. Rats were challenged with LPS after pretreatment with LPS, dexamethasone, or saline. In saline-pretreated rats, LPS caused a rapid decrease in myocardial IkappaB-alpha protein levels, activation of NF-kappaB, and increased TNF-alpha production. These events were followed by myocardial contractile depression. After the initial decrease in myocardial IkappaB-alpha, IkappaB-alpha protein levels rebounded to a level greater than control levels by 24 h. Dexamethasone pretreatment similarly increased myocardial IkappaB-alpha protein levels. In rats pretreated with either LPS or dexamethasone, myocardial IkappaB-alpha protein levels remained similar to control levels after LPS challenge. The preserved level of myocardial IkappaB-alpha protein was associated with diminished NF-kappaB activation, attenuated myocardial TNF-alpha production, and improved cardiac contractility. We conclude that LPS and dexamethasone upregulate myocardial IkappaB-alpha protein expression and that an increased level of myocardial IkappaB-alpha protein may promote cardiac tolerance to LPS by inhibition of NF-kappaB intranuclear translocation and myocardial TNF-alpha production.

Increased prostacyclin and PGE2 stimulated cAMP production by macrophages from endotoxin-tolerant rats

Sublethal administration of lipopolysaccharide (LPS) renders rats tolerant to multiple lethal stimuli. Tolerant macrophages exhibit differential alterations in LPS-stimulated cytokine and inflammatory mediator release. Increased cAMP levels stimulated by PGE2 or prostacyclin (PGI2) result in differential effects on LPS-induced cytokine release and protect against the pathophysiological changes of endotoxemia. In the present studies, we sought to determine whether PGE2- and PGI2-stimulated cAMP levels are altered in tolerant macrophages. Incubation of macrophages with cicaprost or 11-deoxy-PGE1 in the presence of phosphodiesterase inhibitors resulted in significantly higher (2.5- to 6.5-fold) cAMP concentrations in tolerant macrophages compared with control. In contrast, isoproterenol-stimulated cAMP levels were not significantly different between control and tolerant cells. Also, incubation of tolerant macrophages with LPS did not result in significantly elevated cAMP levels. Prostacyclin (IP) receptor mRNA levels were significantly increased in tolerant cells compared with controls, whereas [3H]PGE2 binding and PGE2 EP4 receptor mRNA levels were not significantly changed. These studies suggest that LPS tolerance induces selective alterations in eicosanoid regulation of cAMP formation.

Interleukin-1beta expression after inhibition of protein phosphatases in endotoxin-tolerant cells

Endotoxin (lipopolysaccharide [LPS]) is a potent activator of a number of inflammatory genes in blood leukocytes, including interleukin-1 (IL-1). Blood leukocytes isolated from patients with septic shock fail to produce IL-1 in response to LPS, a phenomenon known as endotoxin tolerance. To study the regulation of IL-1 expression in endotoxin-tolerant cells, the protein phosphatase inhibitor okadaic acid was used to examine the effects of protein phosphorylation on IL-1beta gene expression. We found that endotoxin-tolerant cells produced normal levels of IL-1beta when protein phosphatases were inhibited. In the human pro-monocytic cell line THP-1, okadaic acid increased mRNA accumulation and synthesis of IL-1beta protein. Normal and endotoxin-tolerant THP-1 cells accumulated IL-1beta mRNA and protein with similar delayed kinetics. Okadaic acid stabilization of IL-1beta mRNA appears to be the primary mechanism through which endotoxin-tolerant cells accumulate IL-1beta mRNA and protein. Endotoxin-tolerant cells were unable to activate transcription in response to okadaic acid. However, the transcription factor NF-kappaB, which is known to be involved in IL-1beta expression, was translocated to the nucleus in both normal and endotoxin-tolerant cells after treatment with okadaic acid. These studies revealed that protein phosphorylation can affect gene expression on at least two distinct levels, transcription factor activation and mRNA stability. Endotoxin-tolerant cells have decreased transcription activation potential, while IL-1beta mRNA stability remains responsive to protein phosphorylation.

p50 (NF-kappa B1) is upregulated in LPS tolerant P388D1 murine macrophages

Cells of the murine macrophage cell line P388D1 express cell surface CD14 and respond to LPS (lipopolysaccharide) stimulation with the production of TNF (tumor necrosis factor). When the cells are stimulated with LPS a second time then little TNF is produced, i.e. the cells are tolerant. Flow cytometry analysis demonstrates that this tolerance is not due to a downregulation of the CD14 cell surface receptor. Analysis of proteins binding to the -516 NF-kappa B motif of the murine TNF promoter reveals that constitutive p50p50 and LPS stimulation lead to mobilization of a heterodimer consisting of p65/c-rel. In tolerant cells less of the p65/c-rel heterodimer is mobilized but there is a strong upregulation of p50p50. These data show that tolerance to LPS in murine macrophages may involve a predominance of p50 homodimers.

A comparative analysis of cytokine production and tolerance induction by bacterial lipopeptides, lipopolysaccharides and Staphyloccous aureus in human monocytes

Monocytes (MO) and macrophages (MAC) are important producers of cytokines involved in the pathophysiology of bacterial sepsis. Most studies concentrate on the effects of bacterial lipopolysaccharides (LPS) regarding the induction of cytokine gene expression and secretion in MO/MAC. Here we report that besides LPS, the synthetic lipoprotein analogue lipopeptide N-palmitoyl-S-(2,3-bis(palmitoyl)-(2RS)-propyl)-(R)-cysteinyl-alanyl- glycine (Pam3-Cys-Ala-Gly), another component of the outer membrane of Gram-negative bacteria, as well as heat-killed Staphyloccocus aureus (S. aureus/SAC) are potent stimuli for cytokines in human MO. For all three investigated stimuli we found an individual pattern of cytokine induction: LPS was most potent in inducing interleukin-6 (IL-6) synthesis, whereas for tumour necrosis factor-alpha (TNF-alpha) secretion SAC was the best stimulus. Comparable amounts of IL-8 were induced by either LPS or Pam3-Cys-Ala-Gly, with SAC being less effective even at higher concentrations. The addition of serum led to an increase in LPS-, SAC- and Pam3-Cys-Ala-Gly-stimulated TNF-alpha secretion, indicating that the presence of serum is critical not just for LPS stimulation. Furthermore, as is known for LPS, Pam3-Cys-Ala-Gly and SAC rendered MO refractory to a second bacterial stimulus. Pam3-Cys-Ala-Gly and SAC induced tolerance for itself, but LPS could partially overcome this effect. As the CD14 molecule is discussed as a common receptor for different bacterial components, we investigated whether the TNF-alpha response of MO could be blocked by anti-CD14 antibodies. MY4, a CD14 antibody, selectively blocked the TNF-alpha secretion induced by LPS but not by Pam3-Cys-Ala-Gly or SAC. In summary, we conclude that besides LPS, lipopeptide Pam3-Cys-Ala-Gly and SAC are potent stimuli for human MO, while the mechanisms of activation seem to be partially different from LPS.

Lack of cross tolerance between LPS and muramyl dipeptide in induction of circulating TNF-alpha and IL-6 in guinea pigs

In guinea pigs, lipopolysaccharide (LPS) from gram-negative bacteria and muramyl dipeptide (MDP) from gram-positive bacteria are potent inducers of systemic production of proinflammatory cytokines and fever. However, there is a striking difference between these two bacterial pyrogens in so far as repeated administration of LPS, but not of MDP, in short-term intervals induces tolerance by a progressive downregulation of the systemic cytokine network. In the present study, we investigated MDP-induced fever and the systemic release of tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) in LPS-tolerant guinea pigs in comparison with naive animals. Endotoxin tolerance was induced by repeated intramuscular injections of 20 microg/kg LPS at intervals of 3 days. In response to the last of five injections with LPS, systemic production of TNF-alpha and IL-6 as well as the development of a febrile response was abrogated almost completely. Those guinea pigs that had developed an LPS tolerance could, however, produce the same amounts of TNF-alpha and IL-6 as naive animals in response to a challenge with MDP. Also, MDP-induced fever was identical in LPS-tolerant and naive guinea pigs. These results provide evidence for a lack of cross tolerance between LPS and MDP in induction of circulating cytokines and fever in guinea pigs.

Protein-tyrosine kinase activation is required for lipopolysaccharide induction of interleukin 1beta and NFkappaB activation, but not NFkappaB nuclear translocation

In human monocytes, interleukin 1beta protein production and steady state mRNA levels are increased in response to lipopolysaccharide, predominantly as a result of increased transcription of the interleukin 1beta gene. Expression of interleukin 1beta and other cytokines, such as interleukin 6 and tumor necrosis factor alpha, has been shown to be dependent on the activation of the transcription factor, NFkappaB. Since recent studies have shown that lipopolysaccharide-induced tyrosine kinase activation is not required for NFkappaB nuclear translocation, we sought to determine whether NFkappaB translocated in the absence of tyrosine kinase activity was active in stimulating transcription. We have found that, in the human pro-monocytic cell line, THP-1, the lipopolysaccharide-induced expression of interleukin 1beta is dependent on tyrosine kinase activation. Tyrosine kinases are not required for lipopolysaccharide-mediated nuclear translocation of NFkappaB. However, in the absence of tyrosine kinase activity, the ability of NFkappaB to stimulate transcription is impaired. This inhibition of transcription is specific for NFkappaB; in the absence of tyrosine kinase activity, AP-1-dependent transcription is enhanced. These results suggest that, while lipopolysaccharide-induced expression of inflammatory mediators requires tyrosine kinase activity, tyrosine kinase activity is not obligatory for lipopolysaccharide signal transduction.

A20 blocks endothelial cell activation through a NF-kappaB-dependent mechanism

The A20 gene product is a novel zinc finger protein originally described as a tumor necrosis factor alpha (TNF)-inducible early response gene in human umbilical vein endothelial cells (HUVEC). Its described function is to block TNF-induced apoptosis in fibroblasts and B lymphocytes, but more recently it has also been shown to play a role in lymphoid cell maturation. The mechanism of action of A20 is unknown. The aim of our study was to assess the effect of A20 upon endothelial cell activation. By transfecting bovine aortic endothelial cells (BAEC) with A20 as well as reporter constructs consisting of the promoters of genes known to be up-regulated during endothelial cell activation, i.e. E-selectin, interleukin (IL)-8, tissue factor (TF), and inhibitor of nuclear factor kappaBalpha (IkappaBalpha), we demonstrate that A20 expression inhibits gene up-regulation associated with TNF, lipopolysaccharide (LPS), phorbol 12-myristate 13-acetate (PMA), and hydrogen peroxide (H2O2)-induced endothelial cell (EC) activation. The mechanism of action of A20 is in part, or totally, due to the blockade of nuclear factor kappaB (NF-kappaB), as shown by its ability to suppress the activity of a NF-kappaB reporter. This effect is specific, as A20 does not block a noninducible, constitutively expressed reporter, Rous sarcoma virus-luciferase (RSV-LUC); nor does it block the c-Tat-inducible, NF-kappaB-independent reporter, human immunodeficiency virus-chloramphenicol acetyltransferase (HIV-CAT). How A20 blocks NF-kappaB is unclear, although we demonstrate that it does not affect p65 (RelA)-mediated gene transactivation. The inhibition of endothelial cell activation by A20 is a novel function for A20.

Alterations in macrophage G proteins are associated with endotoxin tolerance

Previous studies have suggested that endotoxin tolerance induces macrophage desensitization to endotoxin through altered guanine nucleotide regulatory (G) protein function. In the present study the binding characteristics of the nonhydrolyzable GTP analogue GTP gamma [35S] to macrophage membranes from endotoxin tolerant and control rats were determined. Membranes were prepared from peritoneal macrophages harvested from rats 72 h after two sequential daily doses of vehicle or Salmonella enteritidis endotoxin (100 micrograms/kg on day 1 and 500 micrograms/kg on day 2). GTP gamma [35S] bound to a single class of sites that were saturable and displaceable in control and endotoxin tolerant macrophage membranes. The maximum specific binding of GTP gamma [35S] was significantly (P < 0.01) decreased in membranes from tolerant rats compared to control (Bmax = 39 +/- 7 pmol/mg protein in control vs. 11 +/- 2 pmol/mg protein in endotoxin tolerant; n = 5). There were no significant differences in the Kd values. To determine whether the reduced GTP gamma S binding was due to decreases in G proteins, macrophage membrane G protein content was determined by western blotting with specific antisera to Gi1,2 alpha, Gi3 alpha, Gs alpha, and the beta subunit of G. Scanning densitometric analysis demonstrated differential decreases in tolerant macrophage membrane G proteins. Gi3 alpha was reduced the most to 48 +/- 8% of controls (n = 3), and this reduction was significant compared to those of other G proteins. Gi1,2 alpha and G beta were reduced to 73 +/- 5% (n = 3) and 65 +/- 4% (n = 3) of control values, respectively. Gs alpha(L) and Gs alpha(H) were reduced to 61 +/- 5% (n = 3) and 68 +/- 3% (n = 3) of control, respectively. These results demonstrate that endotoxin tolerant macrophages exhibit decreased membrane GTP binding capacity and differential reductions in the content of specific G proteins. The cellular mechanisms leading to such alterations in G proteins and their functional significance in the acquisition of endotoxin tolerance merit further investigation.

Tolerance to the anti-metastatic effect of lipopolysaccharide against liver metastasis in mice

We describe the involvement of endotoxin tolerance in the refractoriness of its anti-metastatic effect against murine syngeneic tumors. Three i.v. administrations of LPS at intervals of 4 days after tumor inoculation inhibited liver metastasis of L5178Y-ML25 cells, whereas 3 consecutive i.v. administrations of LPS showed only a slight suppressive effect. Multiple i.v. administrations of LPS, synthetic lipid A, its synthetic derivative DT-5461, Staphylococcus aureus (S. aureus) BioParticles or Staphylococcal enterotoxin B (SEB) on days 1, 5 and 9 after tumor inoculation inhibited liver metastasis of T-lymphoma cells in normal mice. The anti-metastatic effects of LPS, synthetic lipid A or DT-5461 but not S. aureus BioParticles or SEB were diminished in mice injected with LPS at daily intervals for 7 days before tumor inoculation. Mice receiving 3 consecutive i.v. administrations of LPS at daily intervals exhibited suppression of LPS-induced production of endogenous tumor necrosis factor-alpha (TNF-alpha), tumoricidal activity of macrophages, and natural-killer (NK) activity of splenocytes when compared with those of normal mice. Macrophages from mice receiving consecutive daily i.v. administrations of LPS for 3 days showed reduction of LPS-induced tyrosine phosphorylation of several intracellular proteins, including p42(mapk) /ERK2 when compared with that of the cells obtained from normal mice. These data suggest that the LPS-induced anergic state of monocytes/macrophages plays a crucial role in endotoxin tolerance with respect to the metastasis of T lymphoma in the liver.

Low-dose lipopolysaccharide (LPS) pretreatment of mouse macrophages modulates LPS-dependent interleukin-6 production in vitro

Lipopolysaccharide (LPS) can induce mouse macrophages to produce a number of cytokines and other inflammatory mediators. Our laboratory previously reported that LPS-dependent macrophage-derived tumor necrosis factor alpha (TNF-alpha) production could be significantly potentiated by pretreatment with LPS at substimulatory LPS priming doses. The observed potentiation was shown to be coincident with a down-regulation of LPS-dependent nitric oxide (NO) production (X. Zhang and D. C. Morrison, J. Exp. Med. 177: 511-516, 1993). In order to determine whether these LPS reprogramming effects in mouse macrophages were selective for these two macrophage-derived mediators, we have examined the effects of LPS pretreatment on LPS-dependent interleukin 6 (IL-6) production. Thioglycolate-elicited mouse peritoneal macrophages were pretreated with various subthreshold stimulatory concentrations of LPS for 6 h, washed three times, and then stimulated with an effective stimulatory concentration of smooth LPS for 18 h. In confirmation of earlier studies, pretreatment of mouse macrophages with substimulatory doses of LPS inhibited the subsequent LPS-dependent NO production. This down-regulation was accompanied by a coordinate up-regulation of LPS-dependent IL-6 production, similar to what was shown earlier for TNF-alpha production. These priming effects with the substimulatory dose of smooth LPS are shown to be independent of doses of LPS used for subsequent activation and are not restricted to specific LPS stimulation. Moreover, the enhancement of the IL-6 response by LPS pretreatment is still observed in the presence of neutralizing antibody to TNF-alpha. These findings, therefore, provide further support for the conclusion that LPS-dependent macrophage reprogramming is likely to involve common regulatory pathways that control the secretion of both IL-6 and TNF-alpha.

The nonspecific nature of endotoxin tolerance

Re-exposure of organisms or cells to endotoxin after a previous challenge is not accompanied by the profound metabolic changes that are induced by the first encounter with endotoxin. Endotoxin tolerance is not specific to the action of lipopolysaccharide, and crossreactivity with other exogenous stimuli occurs. Various cytokines can mimic the effects of endotoxin in vivo and/or in vitro.