Toll-like receptor 2 participates in the response to lung injury in a murine model of pulmonary contusion

Blunt chest trauma resulting in pulmonary contusion with an accompanying acute inflammatory response is a common but poorly understood injury. We report that Toll-like receptor (TLR) 2 participates in the inflammatory response to lung injury. To show this, we use a model of pulmonary contusion in the mouse that is similar to that observed clinically in humans based on histologic, morphologic, and biochemical criteria of acute lung injury. The inflammatory response to pulmonary contusion in our mouse model is characterized by pulmonary edema, neutrophil transepithelial migration, and increased expression of the innate immunity proinflammatory cytokines IL 1beta and IL 6, the adhesion intracellular adhesion molecule 1, and chemokine (CXC motif) ligand 1. Compared with wild-type animals, contused Tlr2(-/-) mice have significantly reduced pulmonary edema and neutrophilia. These findings are associated with decreased levels of circulating chemokine (CXC motif) ligand 1. In contrast, systemic IL 6 levels remain elevated in the TLR2-deficient phenotype. These results show that TLR2 has a primary role in the neutrophil response to acute lung injury. We suggest that an unidentified noninfectious ligand generated by pulmonary contusion acts via TLR2 to generate inflammatory responses.

Assembly of inflammation-related genes for pathway-focused genetic analysis

Recent identifications of associations between novel variants in inflammation-related genes and several common diseases emphasize the need for systematic evaluations of these genes in disease susceptibility. Considering that many genes are involved in the complex inflammation responses and many genetic variants in these genes have the potential to alter the functions and expression of these genes, we assembled a list of key inflammation-related genes to facilitate the identification of genetic associations of diseases with an inflammation-related etiology. We first reviewed various phases of inflammation responses, including the development of immune cells, sensing of danger, influx of cells to sites of insult, activation and functional responses of immune and non-immune cells, and resolution of the immune response. Assisted by the Ingenuity Pathway Analysis, we then identified 17 functional sub-pathways that are involved in one or multiple phases. This organization would greatly increase the chance of detecting gene-gene interactions by hierarchical clustering of genes with their functional closeness in a pathway. Finally, as an example application, we have developed tagging single nucleotide polymorphism (tSNP) arrays for populations of European and African descent to capture all the common variants of these key inflammation-related genes. Assays of these tSNPs have been designed and assembled into two Affymetrix ParAllele customized chips, one each for European (12,011 SNPs) and African (21,542 SNPs) populations. These tSNPs have greater coverage for these inflammation-related genes compared to the existing genome-wide arrays, particularly in the African population. These tSNP arrays can facilitate systematic evaluation of inflammation pathways in disease susceptibility. For additional applications, other genotyping platforms could also be employed. For existing genome-wide association data, this list of key inflammation-related genes and associated subpathways can facilitate comprehensive inflammation pathway- focused association analyses.

Endogenous signals released from necrotic cells augment inflammatory responses to bacterial endotoxin

Stressed cells undergoing necrosis release molecules that acts as endogenous danger signals to alert and activate innate immune cells. Both HMGB1 and HSP70 are induced in activated monocytes/macrophages and also are released from stressed or injured cells. We investigated whether HMGB1 and HSP70 released from necrotic monocytes/macrophages, can act as danger signals to mediate proinflammatory cytokine responses to bacterial endotoxin or lipopolysaccharide (LPS). We show that cell lysate, obtained from necrotic cells directly stimulates the proinflammatory cytokine and chemokine responses in human monocyte/macrophage cell line, THP-1, as revealed by the induction of TNF-alpha, IL-6 and IL-8 mRNA expression and protein production. In the presence of LPS, necrotic cell lysate induced a more robust increase in all three proteins. We found that HMGB1 and HSP70 were indeed present in the necrotic cell lysate and were responsible for the significant induction of the proinflammatory cytokine expression, as neutralization with antibodies against both proteins blocked the increase in the cytokine production seen after incubating LPS-stimulated cells with the necrotic cell lysate. We also found that the newly identified triggering receptor expressed on myeloid cells-1 (TREM-1) was involved in mediating the HMGB1- and HSP70-induced cytokine production. Blocking TREM-1 on THP-1 cells with a recombinant chimera prevented the increase in cytokine production, while simultaneous blocking of TLR4 and TREM-1 completely abolished the proinflammatory response, suggesting that TREM-1 synergizes with TLR4 to mediate the effects of such signals from necrotic cells. In addition, blocking HMGB1 or HSP70 simultaneously with TREM-1 did not decrease the cytokine level further, confirming the involvement of TREM-1 in mediating the effect of HMGB1 and HSP70. Although the interaction of HMGB1 and HSP70 with TREM-1 induced I kappa B alpha and p38 expression, both of which are required for the inflammatory cytokine expression, blockade of TREM-1 did not affect I kappa B alpha expression but markedly reduced p38 activation, as revealed by Western blot analysis. Together, these results demonstrate that HMGB1 and HSP70 released from necrotic cells function as endogenous danger signals to augment the proinflammatory responses in monocytes/macrophage and that TREM-1 relays such signals to the cytokine expression cascade. This mechanism may contribute to the amplification and persistence of the inflammatory response to bacterial infection.

Epigenetic silencing of tumor necrosis factor alpha during endotoxin tolerance

Sustained silencing of potentially autotoxic acute proinflammatory genes like tumor necrosis factor alpha (TNFalpha) occurs in circulating leukocytes following the early phase of severe systemic inflammation. Aspects of this gene reprogramming suggest the involvement of epigenetic processes. We used THP-1 human promonocytes, which mimic gene silencing when rendered endotoxin-tolerant in vitro, to test whether TNFalpha proximal promoter nucleosomes and transcription factors adapt to an activation-specific profile by developing characteristic chromatin-based silencing marks. We found increased TNFalpha mRNA levels in endotoxin-responsive cells that was preceded by dissociation of heterochromatin-binding protein 1alpha, demethylation of nucleosomal histone H3 lysine 9 (H3(Lys(9))), increased phosphorylation of the adjacent serine 10 (H3(Ser(10))), and recruitment of NF-kappaB RelA/p65 to the TNFalpha promoter. In contrast, endotoxin-tolerant cells repressed production of TNFalpha mRNA, retained binding of heterochromatin-binding protein 1alpha, sustained methylation of H3(Lys(9)), reduced phosphorylation of H3(Ser(10)), and showed diminished binding of NF-kappaB RelA/p65 to the TNFalpha promoter. Similar levels of NF-kappaB p50 occurred at the TNFalpha promoter in the basal state, during active transcription, and in the silenced phenotype. RelB, which acts as a repressor of TNFalpha transcription, remained bound to the promoter during silencing. These results support an immunodeficiency paradigm where epigenetic changes at the promoter of acute proinflammatory genes mediate their repression during the late phase of severe systemic inflammation.

Gene silencing in severe systemic inflammation

This critical care perspective appraises reprogramming of gene expression in inflammatory diseases as an emerging concept of clinical importance. We emphasize gene reprogramming that "silences" acute proinflammatory genes during severe systemic inflammation, wherein in the systemic inflammatory response syndrome (SIRS) exists as a continuum during severe sepsis, septic shock, and the multiorgan dysfunction and failure phenotypes without infection. In contrast, silencing of acute proinflammatory genes is not apparent in sites of localized inflammatory processes like rheumatoid arthritis. We discuss in three parts the clinical context and the translational basic science associated with gene silencing during the SIRS continuum of severe systemic inflammation: (1) reprogramming of acute proinflammatory genes; (2) a "nuclear factor-kappaB paradox," coupled with RelB expression, that combine to silence genes using an epigenetic (inherited and reversible) signature on the nucleosome; and (3) the potential clinical importance of compartmentalization in gene silencing. Our emergent understanding of these physiologic processes may provide a novel framework for developing treatments.

Induction of RelB Participates in Endotoxin Tolerance

Using a THP-1 human promonocyte model of endotoxin tolerance that simulates the sepsis leukocyte phenotype, we previously showed that tolerant cells remain responsive to LPS endotoxin with degradation of IkappaB in the cytosol and nuclear translocation and accumulation of p50 and p65 NF-kappaB transcription factors. Despite this, endotoxin-inducible NF-kappaB-dependent innate immunity genes, like IL-1beta, remained transcriptionally unresponsive in the tolerant phenotype, similar to the endotoxin tolerance observed in sepsis patients. In this study, we examined this paradox and found that RelB, another member of the NF-kappaB family, is induced during the establishment of tolerance. RelB expression correlated with IL-1beta repression, and sepsis patients showed increased RelB when compared with normal controls. Transient expression of RelB inhibited IL-1beta in endotoxin-responsive cells. In the inverse experiment, small inhibitory RNAs decreased RelB expression in tolerant cells and restored endotoxin induction of IL-1beta. When we examined tolerant cell extracts, we found transcriptionally inactive NF-kappaB p65/RelB heterodimers. Taken together, our findings demonstrate that RelB can repress proinflammatory gene expression, and suggest that RelB expression in sepsis patient blood leukocytes may play a role in the endotoxin-tolerant phenotype.

The pathogenesis of pulmonary contusion: an open chest model in the rat

BACKGROUND

Chemokines direct leukocytes to areas of inflammation or injury. In general, CC chemokines (MCP-1, MIP-1alpha, RANTES) are chemoattractants for mononuclear cells and CXC (CINC-1, MIP-2alpha) for polymorphonuclear cells (PMNs). Herein we describe an open chest model of pulmonary contusion (PC) in a rodent (rat) and have identified a possible role for CC and CXC chemokines in the pathogenesis of PC.

METHODS

Sprague-Dawley rats (350 g) underwent thoracotomy. The exposed lung was struck with a piston at 5.2 m/s (150 J/M2). Blood, bronchoalveolar lavage (BAL), and lung tissue were collected at 3 hours and 24 hours after injury. PaO2/FiO2 (P/F) ratio was calculated at 15-minute intervals for 3 hours after contusion. Serum was evaluated for cytokine and chemokine expression using ELISA. Cell count/differential was performed on BAL, and lung tissue was obtained for histologic analysis, protein expression and wet to dry weights. Data are reported as pg/mL +/- SE. Data were analyzed using Student's t test to identify significant differences (p < or = 0.05 significant) between sham and injured animals.

RESULTS

Piston impact caused PC based upon morphologic and histologic criteria. BAL cell count and lung wet to dry weights were increased and P/F ratio was decreased after PC. Systemic levels of IL-ra, MCP-1, and the CXC chemokines MIP-2alpha and CINC-1 were significantly elevated at 3 hours when sham and injured animals were compared. All chemokines were found to be significantly elevated at 24 hours, consistent with the early PMN and subsequent mononuclear infiltration observed in the contused lung. Pulmonary expression of TNF-alpha, IL-1beta, CINC-1, MIP-2alpha, ICAM-1, and elastase were increased and activated systemic neutrophils showed increased CD-11b.

CONCLUSION

A model of PC is described in which innate inflammation is activated locally and systemically. Systemic levels of CC and CXC chemokines are increased after PC. This correlates with elevated PMN CD-11b expression, enhanced pulmonary ICAM-1 expression, and mononuclear and PMN infiltration into the lung and alveolar space. Elevated levels of CC and CXC chemokines are seen after PC and may be involved in the lung's inflammatory response to injury.

Endotoxin Tolerance Disrupts Chromatin Remodeling and NF-κB Transactivation at the IL-1β Promoter

The NF-kappaB family plays a crucial role in the pathogenesis of highly lethal septicemia by modulating transcription of many innate and adaptive immunity genes. Two phases of NF-kappaB activation occur: cytosolic activation and nuclear transactivation. Septicemia with multiorgan failure is associated with chronic activation of cytosolic NF-kappaB with translocation and accumulation of increased levels of nuclear p65 in blood leukocytes. Paradoxically, NF-kappaB-dependent transcription of many proinflammatory genes responding to bacterial LPS endotoxin (LPS) is persistently repressed during septicemia; this phenomenon of LPS tolerance is associated with immunosuppression and poor prognosis. This report suggests an explanation for this paradox. Using an in vitro human leukocyte model and chromatin immunoprecipitation assays, we find that both the cytosolic activation and nuclear transactivation phases of NF-kappaB occur in LPS responsive THP-1 promonocytes with recruitment and binding of NF-kappaB p65 at the IL-1beta promoter. However, transcriptionally repressed LPS-tolerant THP-1 cells do not bind NF-kappaB p65 at the IL-1beta promoter, despite cytosolic activation and accumulation of p65 in the nucleus. In contrast, NF-kappaB p50, which also accumulates in the nucleus, constitutively binds to the IL-1beta promoter NF-kappaB site in both LPS-responsive and LPS-tolerant cells. The level of p65 binding correlates with a binary shift in nucleosome remodeling between histone H3 phosphorylation at serine 10 and methylation of histone H3 at lysine 9. We conclude that LPS tolerance disrupts the transactivating stage of NF-kappaB p65 and altered nucleosome remodeling at the IL-1beta promoter in human leukocytes.

Distinct post-receptor alterations generate gene- and signal-selective adaptation and cross-adaptation of TLR4 and TLR2 in human leukocytes

Gene- and signal-specific adaptation/tolerance of blood leukocytes to lipopolysaccharide endotoxin (LPS) occurs during human and animal septicemia. These phenotypes can be modeled in vitro. LPS-TLR4-adapted human THP-1 promonocytic cells cross-adapt to lipoteichoic acid (LTA)-TLR2-induced IL-1beta/TNF-alpha production, suggesting disruption of a common intracellular signaling event(s). A plausible explanation for homologous adaptation of TLR4 with heterologous adaptation of TLR2 is a persistent inactivation and degradation of IRAK1 following TLR4 activation. LTA stimulation of TLR2 also produces homologous adaptation of TLR2 with inactivation of IRAK1, but there is no detectable degradation of IRAK1. Strikingly, such LTA-adapted cells still respond to LPS stimulation of TLR4 with rapid activation and degradation of IRAK1, and robust IL-1beta/TNF-alpha production. Moreover, cells adapted to either LTA- or LPS-production of IL-1beta/TNF-alpha normally produce soluble interleukin 1 receptor antagonist (sIL-1Ra) anti-inflammatory protein when stimulated by either agonist. We conclude that: (i) disruption of a unique TLR2 signaling component upstream of IRAK1, but downstream of TLR2 sensing, induces homologous adaptation to LTA; (ii) disruption of IRAK1 may induce homologous adaptation of TLR4 to LPS and cross-adaptation of TLR2 to LTA; and (iii) TLR2/TLR4 signaling events that control sIL-1Ra translation do not adapt to LPS or LTA, indicating that TLR4 and TLR2 can still function. We present a hypothetical model of adaptation based on a signalsome, with IRAK1 evolving after IRAK4 to regulate TLR4 adaptation tightly.

Association of an IL-1A 3'UTR polymorphism with end-stage renal disease and IL-1 alpha expression

BACKGROUND

We evaluated polymorphisms in the interleukin-1 alpha 3'-untranslated region (IL-1A 3'[UTR]) for association with type 2 diabetes-associated (DM) and nondiabetic-associated (non-DM) end-stage renal disease (ESRD) in two ethnic groups.

METHODS

IL-1A 3'UTR polymorphisms were identified by alignment of overlapping human expressed sequence tags (ESTs). Sequence ambiguities were experimentally confirmed and variants genotyped to test for association with ESRD in 75 unrelated Caucasians with DM ESRD, 95 unrelated Caucasian controls and, in a parallel study, 92 unrelated African Americans with type 2 DM ESRD, 95 unrelated African Americans with non-DM ESRD, and 86 unrelated African American controls. IL-1A 3' UTR genotype and lipopolysaccharide (LPS)-stimulated IL-1 alpha protein levels were measured in healthy Caucasians (N = 112) and African Americans (N = 101) to evaluate association between genotype and protein level.

RESULTS

A polymorphism in the 3' UTR of the human IL-1A gene was associated with ESRD and IL-1 alpha protein expression. The polymorphism consists of two single nucleotide polymorphisms (SNPs) and an insertion/deletion generating four different haplotypes: TN7TTCAA, AN7TTCAA, TN7TTCAG and an allele deleted for four internal bases, TN7(delTTCA)A. The 4 bp deletion allele, TN7(delTTCA)A, was significantly less common among Caucasian DM ESRD and African American non-DM ESRD patients (recessive model; P = 0.0364 and P = 0.0293, respectively). In vitro, this polymorphism is associated with the amount of IL-1 alpha protein synthesized in LPS-stimulated lymphocytes from healthy subjects (P = 0.0013, additive model), with the TN7(delTTCA)A haplotype associated with higher levels of stimulated IL-1 alpha.

CONCLUSION

The association of the TN7(delTTCA)A haplotype with higher levels of IL-1 alpha expression and reduced risk for ESRD is consistent with involvement of cytokines in risk for developing nephropathy.

Inhibition of histone deacetylation enhances endotoxin-stimulated transcription but does not reverse endotoxin tolerance

Covalent modification of histones and the subsequent remodeling of chromatin have emerged as important mechanisms in regulating gene expression. In particular, recent identification of the enzyme families responsible for the steady-state balance of histone acetylation has served to redefine our understanding of these modifications as fundamental biochemical processes regulating transcription. Current evidence suggests that histone acetylation correlates positively with gene activation, while histone deacetylation acts to repress transcription. In this study, we examined the role of histone modification in the inflammatory response to endotoxin. We focused on the endotoxin-stimulated expression of the interleukin-1beta promoter and tested the hypotheses that persistent histone deacetylation was responsible for the decreased expression of this promoter observed after prolonged exposure to endotoxin, a manifestation of a phenomenon known as endotoxin tolerance. We found that histone deacetylase inhibitors enhanced endotoxin-stimulated transcription; however, deacetylation inhibitors could neither block the development of tolerance nor restore endotoxin sensitivity in a tolerant cell. Deacetylase inhibitors could not restore LPS-mediated transcription in tolerant cells. These results show that histone acetylation/deacetylation regulates, at least in part, the endotoxin-induced expression of inflammatory genes and that repressed transcription observed in endotoxin tolerance is not caused by enhanced activity of histone deacetylases.

Endotoxin-adapted septic shock leukocytes selectively alter production of sIL-1RA and IL-1beta

During septic shock, circulating levels of anti-inflammatory mediators are increased relative to those of pro-inflammatory. The reduced capacity of septic shock blood leukocytes in expressing pro-inflammatory genes in response to bacterial lipopolysaccharide endotoxin (LPS) may contribute to reductions in these mediators, but the reasons for persistent increases in circulating anti-inflammatory mediators are unknown. We determined whether septic shock leukocytes that have adapted to LPS induction of the IL-1beta gene could continue to express sIL-1RA in response to LPS. Septic shock whole-blood leukocytes and neutrophils (PMNs) selectively maintained production of sIL-1RA after treatment with LPS while limiting that of IL-1beta. Repressed transcription of IL-1beta and rapid decay of IL-1beta mRNA in septic shock neutrophils correlated with reductions in levels of IL-1beta after stimulation with LPS. Transcription of sIL-1RA mRNA was also suppressed, but the ability of LPS to stimulate events that lead to efficient translation of a stable sIL-1RA mRNA appeared responsible for maintaining sIL-1RA production. We conclude that LPS adaptation of septic shock leukocytes selectively influences signaling pathways that regulate transcription, mRNA processing, and translation, leading to changes in the balance of production of pro- and anti-inflammatory mediators.

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.

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.

Endotoxin inducible transcription is repressed in endotoxin tolerant cells

Stimulation of the human promonocytic cell line, THP-1, with endotoxin results in a rapid and transient increase in interleukin 1beta expression. Endotoxin pretreatment of THP-1 cells results in tolerance, characterized by decreased levels of endotoxin-induced interleukin 1beta expression due to decreased transcription of the interleukin 1beta gene. We hypothesized that tolerant cells could not activate transcription factors necessary to express the interleukin 1beta gene. This hypothesis was tested in tolerant THP-1 cells by using stable and transiently transfected reporter genes containing the interleukin 1beta promoter. We found decreased endotoxin-induced transcription of all reporter genes tested; however, individual transcription factors, such as NFkappaB, retain normal, CD14-dependent, nuclear translocation and DNA binding. Tolerance is specific for endotoxin, because phorbol ester is still able to activate transcription of the endogenous interleukin 1beta gene and transfected reporter genes. A constitutively active reporter gene that is not inducible by endotoxin is unaffected. We further show that nuclear extracts of tolerant cells show transcription inhibitor activity that is specific for promoter sequences of the interleukin 1beta gene. These results support a mechanism of endotoxin tolerance that is independent of transcription factor DNA binding and appears to be associated with the inability of DNA-bound transcription factors to activate transcription, perhaps through the activity of a repressor.

Human neutrophils express the prostaglandin G/H synthase 2 gene when stimulated with bacterial lipopolysaccharide

Human blood neutrophils (PMN) rapidly release arachidonic acid (AA) from cellular phospholipids when stimulated in vitro with a variety of inflammatory agonists. Free AA is then metabolized via 5'-lipoxygenase to produce bioactive mediators such as leukotriene B4 and 5-hydroxyeicosatetraenoate. Arachidonic acid can also be metabolized via the cyclooxygenase or prostaglandin G/H synthase (PGHS) pathway to form prostaglandins and thromboxane. We show here that human blood PMN express the PGHS 2 gene when stimulated with bacterial lipopolysaccharide (LPS). PGHS 2 mRNA increases within 30 min after LPS stimulation and PGHS 2 immunoreactive protein is detectable by 5 h. Although PGHS 1 mRNA is detectable in PMN, no immunoreactive protein is observed in either resting or LPS-stimulated cells. Following stimulation with LPS and expression of PGHS 2, PMN increase secretion of prostaglandin E2. This phenotypic change in PMN could be an important mechanism for regulating inflammation.

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.

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.

Elevated levels of shed type II IL-1 receptor in sepsis. Potential role for type II receptor in regulation of IL-1 responses

Two types of cellular IL-1Rs have been characterized and cloned from both human and murine sources. The type II IL-1R has a very short cytoplasmic domain and does not seem to participate in IL-1 signaling. We demonstrate that type II IL-1Rs are released from the surface of neutrophils in response to treatment with TNF or endotoxin. In addition, serum from patients with sepsis syndrome contains elevated levels of soluble type II IL-1Rs. Neutrophils isolated from patients with sepsis have greatly enhanced expression of type II IL-1R mRNA and cell surface receptors and are therefore a likely source for the shed receptors in serum. Of the three forms of IL-1, soluble type II IL-1R binds IL-1 beta with highest affinity and also selectively inhibits IL-1 beta activity. We propose that increased cell surface expression and rapid release of preformed type II IL-1R from neutrophils, as a soluble IL-1 beta binding protein, represents a mechanism that has evolved for regulating IL-1 activity in sepsis.

Elevated levels of shed type II IL-1 receptor in sepsis. Potential role for type II receptor in regulation of IL-1 responses

Two types of cellular IL-1Rs have been characterized and cloned from both human and murine sources. The type II IL-1R has a very short cytoplasmic domain and does not seem to participate in IL-1 signaling. We demonstrate that type II IL-1Rs are released from the surface of neutrophils in response to treatment with TNF or endotoxin. In addition, serum from patients with sepsis syndrome contains elevated levels of soluble type II IL-1Rs. Neutrophils isolated from patients with sepsis have greatly enhanced expression of type II IL-1R mRNA and cell surface receptors and are therefore a likely source for the shed receptors in serum. Of the three forms of IL-1, soluble type II IL-1R binds IL-1 beta with highest affinity and also selectively inhibits IL-1 beta activity. We propose that increased cell surface expression and rapid release of preformed type II IL-1R from neutrophils, as a soluble IL-1 beta binding protein, represents a mechanism that has evolved for regulating IL-1 activity in sepsis.

A labile transcriptional repressor modulates endotoxin tolerance

Tolerance to bacterial lipopolysaccharide (LPS, endotoxin) is an adaptive cellular process whereby exposure to endotoxin induces a subsequent hyporesponsive state characterized by decreased levels of LPS-induced cytokine mRNA and protein. We demonstrate, in a human promonocytic cell line, THP-1, that endotoxin tolerance is manifested by decreased LPS-induced interleukin 1 beta (IL-1 beta) transcription. Inhibition of protein synthesis reverses the tolerant phenotype by inducing transcription of IL-1 beta in the absence of a second stimulus. These results indicate that a labile protein contributes to the endotoxin-tolerant phenotype, and that this factor acts in a dominant repressive manner to inhibit the activity of existing transcription factors. We provide further data that cellular expression of I kappa B-alpha correlates with downregulated IL-1 beta gene expression during endotoxin tolerance, implicating I kappa B-alpha as a potential candidate for the labile repressor identified herein.

Phosphatidic acid and diacylglycerol synergize in a cell-free system for activation of NADPH oxidase from human neutrophils

NADPH oxidase, the respiratory burst enzyme of human neutrophils, is a multi-component complex that is assembled and activated during stimulation of the cells by inflammatory or phagocytic stimuli. The signal mechanisms leading to activation of the enzyme are unclear, but it is likely that phospholipases are involved. Recent work has shown that phosphatidic acid, the initial product of phospholipase D activation, is a weak activator of NADPH oxidase in a cell-free system. We now show that diacylglycerol enhances the cell-free activation of NADPH oxidase activation by phosphatidic acid. 1,2-Didecanoyl phosphatidic acid (10:0-PA) and 1,2-dioctanoylglycerol (8:0-DG) each increased levels of NADPH oxidase activity in mixtures of membrane and cytosolic fractions about 2-fold. The combination of both lipids increased NADPH oxidase activity approximately 12-fold, indicative of a synergistic response. Fatty acid and neutral lipid metabolites of 10:0-PA or 8:0-DG were ineffective, suggesting activation is directly mediated by phosphatidic acid and diacylglycerol. Activation was time- and concentration-dependent with maximum activation at 30-60 min and a sharp peak of maximal activity at 10 microM 10:0-PA and 30 microM 8:0-DG. In lipid specificity studies, activity of PA or DG decreased with increasing acyl chain length but was restored by introducing unsaturation in the acyl chain. Natural forms of PA stimulated levels of activity comparable to that seen with 10:0-PA. Synthetic and natural phosphatidylserines, but not other phospholipids, could replace phosphatidic acid in the synergistic response. These studies provide direct evidence for a synergistic interaction between phosphatidic acid and diacylglycerol in mediating a cellular function: the assembly and activation of NADPH oxidase. Our results support the concept that the generation of second messenger lipids by phospholipase D is a key step in activation of the respiratory burst enzyme.

Phospholipases and activation of the NADPH oxidase

The signal transductional mechanisms regulating the activation of NADPH oxidase, the respiratory burst enzyme in phagocytic cells, are not completely understood. Receptors for most physiologic stimuli trigger the activation of various phospholipases, including phospholipases A2, C, and D. The lipid mediators formed (arachidonic acid, 1,2-diacylglycerol, and phosphatidic acid) have been implicated as second messengers in the induction of the respiratory burst. In intact cells, we have correlated phospholipase D activation and the production of phosphatidic acid with the activation of NADPH oxidase, using the drug propranolol. Phosphatidic acid activated NADPH oxidase in a cell-free system, but the level of activation was low. 1,2-Diacylglycerol markedly enhanced NADPH oxidase activation by phosphatidic acid. The synergistic effect required the diacyl species, since mono- or tri-acylglycerols were ineffective. Phosphatidic acid could be replaced by either lysophosphatidic acid or phosphatidylserine, but not by phosphatidylcholine, phosphatidylethanolamine, or phosphatidylinositol, suggesting specificity for an anionic phospholipid. Since other cell-free activators of NADPH oxidase (arachidonic acid, sodium dodecyl sulfate) are also anionic amphiphiles, phosphatidic acid may directly interact with an enzyme component(s). The targets for phosphatidic acid and diacylglycerol in the cell-free system are currently under investigation. These results emphasize the critical importance of phospholipases, particularly phospholipase D, in the regulation of the respiratory burst.

Lipopolysaccharide-induced expression of prostaglandin H synthase-2 in alveolar macrophages is inhibited by dexamethasone but not by aspirin

Alveolar macrophages cultured with lipopolysaccharide release markedly increased amounts of prostanoids upon subsequent stimulation, an effect that is due to induction of prostaglandin H synthase-2 (J. Biol. Chem., (1992), 267, 14545-14550, and Biochem. Biophys. Res. Comm., (1992), 187, 1123-1127). The effects of dexamethasone and aspirin on this enhanced formation of thromboxane by stimulated lipopolysaccharide-primed alveolar macrophages were investigated. Under conditions of maximum inhibition, dexamethasone and aspirin decreased the formation of thromboxane by approximately 50% and 80%, respectively. Expression of lipopolysaccharide-induced prostaglandin H synthase-2 in dexamethasone-treated macrophages was similarly inhibited by about 50%, as determined by Northern blot and immunoprecipitation. In contrast, levels of lipopolysaccharide-induced prostaglandin H synthase-2 mRNA and protein were not reduced in aspirin-treated macrophages. We conclude that inhibition of prostaglandin H synthase-2 expression represents a mechanism by which dexamethasone, but not aspirin, may inhibit prostanoid formation by alveolar macrophages.