Oxidative stress and gene expression in sepsis
2003; Elsevier BV; Volume: 90; Issue: 2 Linguagem: Inglês
10.1093/bja/aeg034
ISSN1471-6771
AutoresJ. Macdonald, Helen F. Galley, N.R. Webster,
Tópico(s)Vitamin C and Antioxidants Research
ResumoDysregulation of the immuno-inflammatory response, as seen in sepsis, may culminate in host cell and organ damage. Lipopolysaccharide from Gram-negative bacterial cell walls induces gene activation and subsequent inflammatory mediator expression. Gene activation is regulated by a number of transcription factors at the nuclear level, of which nuclear factor κB appears to have a central role. The redox (reduction–oxidation) cellular balance is important for normal cellular function, including transcription factor regulation. In sepsis, a state of severe oxidative stress is encountered, with host endogenous antioxidant defences overcome. This has implications for cellular function and the regulation of gene expression. This review gives an overview of the mechanisms by which transcription factor activation and inflammatory mediator overexpression occur in sepsis, together with the events surrounding the state of oxidative stress encountered and the effects on the host's antioxidant defences. The effect of oxidative stress on transcription factor regulation is considered, together with the role of antioxidant repletion in transcription factor activation and in sepsis in general. Other interventions that may modulate transcription factor activation are also highlighted. Dysregulation of the immuno-inflammatory response, as seen in sepsis, may culminate in host cell and organ damage. Lipopolysaccharide from Gram-negative bacterial cell walls induces gene activation and subsequent inflammatory mediator expression. Gene activation is regulated by a number of transcription factors at the nuclear level, of which nuclear factor κB appears to have a central role. The redox (reduction–oxidation) cellular balance is important for normal cellular function, including transcription factor regulation. In sepsis, a state of severe oxidative stress is encountered, with host endogenous antioxidant defences overcome. This has implications for cellular function and the regulation of gene expression. This review gives an overview of the mechanisms by which transcription factor activation and inflammatory mediator overexpression occur in sepsis, together with the events surrounding the state of oxidative stress encountered and the effects on the host's antioxidant defences. The effect of oxidative stress on transcription factor regulation is considered, together with the role of antioxidant repletion in transcription factor activation and in sepsis in general. Other interventions that may modulate transcription factor activation are also highlighted. Sepsis and its sequelae continue to be the main causes of morbidity and mortality in the intensive care unit (ICU), with an estimated 400 000–500 000 patients developing sepsis in this setting each year in Europe and the USA. Sepsis is part of a spectrum of conditions ranging from the systemic inflammatory response syndrome (SIRS) to septic shock and multiple organ dysfunction syndrome (MODS). The definitions of these conditions have been rationalized, leading to the common language currently used.17Bone RC Balk RA Cerra FB et al.Definitions for sepsis and organ failure and guidelines for the use of innovative therapies in sepsis. The ACCP/SCCM Consensus Conference Committee. American College of Chest Physicians/Society of Critical Care Medicine.Chest. 1992; 101: 1644-1655Abstract Full Text Full Text PDF PubMed Scopus (7529) Google Scholar The mortality associated with these conditions ranges from around 26% in patients with SIRS to around 82% in septic shock,82Salvo I de Cian W Musicco M et al.The Italian SEPSIS study: preliminary results on the incidence and evolution of SIRS, sepsis, severe sepsis and septic shock.Intensive Care Med. 1995; 21: S244-S249Crossref PubMed Scopus (200) Google Scholar and shows no sign of decreasing despite optimal current therapy. Sepsis therefore continues to have significant clinical and financial implications and remains an area that attracts intense research interest. Regulation and coordination of the immuno-inflammatory response by cytokines and other mediators is essential for host defence. The underlying molecular events are complex and culminate in altered gene expression. Dysregulation of this response may occur in sepsis, leading to excessive or inappropriate release of mediators and ultimately host cell and organ damage. There is convincing evidence of severe oxidative stress in patients with sepsis. As oxygen free radicals and other reactive oxygen species appear to be involved as messengers in cellular signal transduction and gene activation, this has implications for the expression and control of the immuno-inflammatory response in sepsis. Therapeutic intervention with antioxidant therapy to alter signal transduction and mediator production, and hence the course of sepsis, is also a possibility. Under normal physiological conditions, a homeostatic balance exists between the formation of reactive oxidizing/oxygen species and their removal by endogenous antioxidant scavenging compounds.50Gutteridge JMC Mitchell J Redox imbalance in the critically ill.Br Med Bull. 1999; 55: 49-75Crossref PubMed Scopus (175) Google Scholar Oxidative stress occurs when this balance is disrupted by excessive production of reactive oxygen species, including superoxide, hydrogen peroxide and hydroxyl radicals, and/or by inadequate antioxidative defences,49Gutteridge JMC Lipid peroxidation and antioxidants as biomarkers of tissue damage.Clin Chem. 1995; 41: 1819-1828Crossref PubMed Scopus (1547) Google Scholar including superoxide dismutase (SOD), catalase, vitamins C and E, and reduced glutathione (GSH). Both may occur in sepsis. When molecules are oxidized during metabolism, the oxygen molecule itself is reduced to water, giving rise to intermediate reactive oxygen species: O2 → O2.- → H2O2 → .HO → H2O Molecular Superoxide Hydrogen HydroxylWater oxygenanion radicalperoxideradical (. = unpaired electron of a free radical) Superoxide and hydroxyl are described as free radicals because they have an atom or molecule which has one or more unpaired electron(s); this renders the free radical highly reactive and potentially toxic. Superoxide is converted to hydrogen peroxide by the enzyme SOD. In the absence of transition metal ions, hydrogen peroxide is fairly stable. It does, however, allow neutrophils to oxidize chloride ions, via myeloperoxidase, into hypochlorous acid, providing additional cytotoxic activity. Excess hydrogen peroxide is normally converted harmlessly to water by the action of catalase, glutathione peroxidase and other peroxidases. Hydroxyl radicals can be formed by the reaction of superoxide with hydrogen peroxide in the presence of metal ions (usually iron or copper). Hydroxyl free radicals are much more reactive than superoxide anions.100Webster NR Nunn JF Molecular structure of free radicals and their importance in biological reactions.Br J Anaesth. 1988; 60: 98-108Crossref PubMed Scopus (73) Google Scholar Iron-catalysed hydroxyl generation requires that the iron is in its reduced, ferrous form (Fe2+), whereas most iron existing in cells and plasma is in the oxidized form (Fe3+). As well as its involvement with hydrogen peroxide in hydroxyl radical formation, superoxide can also reduce Fe3+ to Fe2+, thereby further promoting hydroxyl production. However, most iron in the plasma exists in a bound form as a protective measure, as it is the free component which is able to participate in biochemical reactions. Biology has used molecules for iron metabolism (haem proteins), storage (ferritin) and transport (transferrin) that lock the iron in a state where free radical production cannot occur. Under normal physiological conditions, the majority of reactive oxygen species are formed during cellular respiration and by activated phagocytic cells, including neutrophils, involved in the inflammatory response. Reactive oxygen species have physiologically essential roles in mitochondrial respiration, prostaglandin production pathways and host defence.100Webster NR Nunn JF Molecular structure of free radicals and their importance in biological reactions.Br J Anaesth. 1988; 60: 98-108Crossref PubMed Scopus (73) Google Scholar The four-electron reduction of oxygen occurs in the mitochondrial electron transport system of all aerobically respiring cells. The enzyme catalysing this reaction (cytochrome c oxidase) contains the transition metals iron and copper in its active site. These ions can be paramagnetic and contain stable unpaired electrons. By using the unpaired electrons in these transition metals to control the oxygen reactions, mitochondria prevent the unwanted release of oxygen-derived free radicals. In sepsis, there are several potential sources of reactive oxygen species, including the mitochondrial respiratory electron transport chain, xanthine oxidase activation as a result of ischaemia and reperfusion, the respiratory burst associated with neutrophil activation, and arachidonic acid metabolism. Activated neutrophils produce superoxide as a cytotoxic agent as part of the respiratory burst via the action of membrane-bound NADPH oxidase on molecular oxygen. Neutrophils also produce the free radical nitric oxide (NO.), which can react with superoxide to produce peroxynitrite, itself a powerful oxidant, which may decompose to form the hydroxyl radical. Under ischaemic conditions followed by subsequent reperfusion, the enzyme xanthine oxidase catalyses the formation of uric acid with the coproduction of superoxide. Superoxide release results in the recruitment and activation of neutrophils and their adherence to endothelial cells, which stimulates the formation of xanthine oxidase in the endothelium, with further superoxide production. During oxidative stress, damage mediated by reactive oxygen species can occur. Oxidation of DNA and proteins may take place, along with membrane damage, because of lipid peroxidation, leading to alterations in membrane permeability, modification of protein structure and functional changes.105Zimmerman JJ Defining the role of oxyradicals in the pathogenesis of sepsis.Crit Care Med. 1995; 23: 616-617Crossref PubMed Scopus (65) Google Scholar Oxidative damage to the mitochondrial membrane can also occur, resulting in membrane depolarization and the uncoupling of oxidative phosphorylation, with altered cellular respiration.70Nathan AT Singer M The oxygen trail: tissue oxygenation.Br Med Bull. 1999; 55: 96-108Crossref PubMed Scopus (65) Google Scholar This can ultimately lead to mitochondrial damage, with release of cytochrome c, activation of caspases and apoptosis (programmed cell death). Antioxidants are central to the redox balance in the human body. They do not act in isolation, but synergistically. Primary antioxidants prevent oxygen radical formation, whether by removing free radical precursors or by inhibiting catalysts, e.g. glutathione peroxidase and catalase. Secondary antioxidants react with reactive oxygen species which have already been formed, either to remove or inhibit them, e.g. vitamins C and E. Endogenous antioxidant defences exist at a number of locations, namely intracellularly, on the cell membrane and extracellularly (reviewed by Gutteridge and Mitchell).50Gutteridge JMC Mitchell J Redox imbalance in the critically ill.Br Med Bull. 1999; 55: 49-75Crossref PubMed Scopus (175) Google Scholar The SOD enzymes are a family of metalloenzymes which rapidly promote the conversion of superoxide to hydrogen peroxide. Three forms of SOD are recognized to be important: copper–zinc SOD (cytoplasm), manganese SOD (mitochondria) and extracellular SOD (extracellular matrix). Catalase and glutathione peroxidase, a selenium-containing enzyme which requires the presence of reduced GSH for its action, catalyse the conversion of hydrogen peroxide to water. Reduced GSH (l-γ-glutamyl-l-cysteinyl-glycine) contains a thiol (sulphydryl) group. The intracellular antioxidants and the role of reduced GSH (including synthesis and recycling) are shown in Figure 1. GSH also has direct antioxidant activity, through donation of hydrogen ions, to repair damaged DNA. Oxidative stress and modulation of GSH/GSSG (GSSG=oxidized GSH) levels also up-regulate gene expression of several other antioxidant proteins, such as manganese SOD, glutathione peroxidase, thioredoxin and metallothionein. The hydrophobic lipid interior of membranes requires a different spectrum of antioxidants. Fat-soluble vitamin E (α-tocopherol) is the most important antioxidant in this environment. β-Carotene, lycopene and co-enzyme Q have also been implicated as membrane antioxidants. Lipid-soluble antioxidants are important in preventing membrane polyunsaturated fatty acids from undergoing lipid peroxidation, which leads to loss of membrane integrity. Reactive oxygen species may also be present in the extracellular compartment, especially as a result of neutrophil activation. The plasma and red cell components of blood both act as antioxidants; red cells have a copper–zinc SOD-dependent pathway for the inactivation of superoxide, and catalase and glutathione peroxidase for dealing with hydrogen peroxide. A number of metal-binding plasma proteins function as valuable antioxidants in addition to their transport roles, including apotransferrin, lactoferrin and caeruloplasmin. Albumin is also effective via its oxidizable thiol group, which permits radical scavenging, and the binding of reactive transition metal ions. A number of important smaller molecules are present in the plasma, which act as secondary antioxidants. These include vitamin E, vitamin C (ascorbic acid), uric acid and bilirubin. Ascorbic acid interacts with superoxide to form dehydroascorbic acid. Vitamin C may also reduce Fe3+ to Fe2+, which can then be involved in iron-catalysed hydroxyl generation, thereby implicating vitamin C as both a pro-oxidant and an antioxidant. There now exists a considerable body of evidence for redox imbalance and oxidative stress in human sepsis, demonstrating increased markers of oxidative damage, direct evidence of free radical production using electron paramagnetic resonance analysis, xanthine oxidase activation, increased redox reactive iron, abnormal handling of exogenous antioxidants, and low concentrations of individual endogenous antioxidants. Early work by Takeda and colleagues94Takeda K Shimada Y Amano M et al.Plasma lipid peroxides and alpha-tocopherol in critically ill patients.Crit Care Med. 1984; 12: 957-959Crossref PubMed Scopus (100) Google Scholar found reduced plasma α-tocopherol levels accompanied by increased plasma thiobarbituric acid-reactive substance (TBARS) levels in critically ill patients compared with controls, suggesting increased lipid peroxidation. Goode and colleagues45Goode HF Cowley HC Walker BE Howdle PD Webster NR Decreased antioxidant status and increased lipid peroxidation in patients with septic shock and secondary organ dysfunction.Crit Care Med. 1995; 23: 646-651Crossref PubMed Scopus (402) Google Scholar investigated antioxidant status in patients with septic shock. They reported reduced plasma concentrations of retinol (vitamin A), tocopherol (vitamin E), β-carotene and lycopene in these patients compared with healthy controls. They also found increased plasma TBARS in patients who developed three or more dysfunctional secondary organs, suggesting increased lipid peroxidation. Borrelli and colleagues18Borrelli E Roux-Lombard P Grau GE et al.Plasma concentrations of cytokines, their soluble receptors, and antioxidant vitamins can predict the development of multiple organ failure in patients at risk.Crit Care Med. 1996; 24: 392-397Crossref PubMed Scopus (265) Google Scholar documented that plasma vitamin C was significantly decreased in ICU patients who developed multiple organ failure compared with those who did not; plasma concentrations of vitamin E, copper and zinc, however, did not differ between the two groups. Galley and colleagues reported increased redox reactive iron concentrations in patients with sepsis or septic shock, coupled with lowered plasma levels of vitamin C40Galley HF Webster NR Elevated serum bleomycin-detectable iron concentrations in patients with sepsis syndrome.Intensive Care Med. 1996; 22: 226-229Crossref PubMed Scopus (29) Google Scholar, 41Galley HF Davies MJ Webster NR Ascorbyl radical formation in patients with sepsis: effect of ascorbate loading.Free Radic Biol Med. 1996; 20: 139-143Crossref PubMed Scopus (177) Google Scholar and elevated lipid peroxides. 42Galley HF Howdle PD Walker BE Webster NR The effects of intravenous antioxidants in patients with septic shock.Free Radic Biol Med. 1997; 23: 768-774Crossref PubMed Scopus (186) Google Scholar Later work has, however, disputed the presence of redox reactive iron in the plasma of patients with septic shock.69Mumby S Margarson M Quinlan GJ Evans TW Gutteridge JMC Is bleomycin-detectable iron present in the plasma of patients with septic shock?.Intensive Care Med. 1997; 23: 635-639Crossref PubMed Scopus (14) Google Scholar, 101Weinberg ED Is bleomycin-detectable iron present in the plasma of patients with sepsis syndrome?.Intensive Care Med. 1997; 23: 613-614Crossref PubMed Scopus (3) Google Scholar Cowley and colleagues27Cowley HC Bacon PJ Goode HF Webster NR Jones JG Menon DK Plasma antioxidant potential in severe sepsis: a comparison of survivors and nonsurvivors.Crit Care Med. 1996; 24: 1179-1183Crossref PubMed Scopus (204) Google Scholar described decreased total antioxidant potential in patients with sepsis and secondary organ dysfunction, associated with non-survival. However, a subsequent study reported that although total antioxidant capacity was decreased in patients with sepsis, it was increased in patients with septic shock, which was attributed to high bilirubin levels.75Pascual C Karzai W Meier-Hellmann A et al.Total plasma antioxidant capacity is not always decreased in sepsis.Crit Care Med. 1998; 26: 705-709Crossref PubMed Scopus (73) Google Scholar Increased xanthine oxidase activity has been reported in patients with sepsis or SIRS in both the adult39Galley HF Davies MJ Webster NR Xanthine oxidase activity and free radical generation in patients with sepsis syndrome.Crit Care Med. 1996; 24: 1649-1653Crossref PubMed Scopus (142) Google Scholar and paediatric10Batra S Kumar R Seema Kapoor AK Ray G Alterations in antioxidant status during neonatal sepsis.Ann Trop Paediatr. 2000; 20: 27-33Crossref PubMed Scopus (83) Google Scholar, 71Németh I Boda D Xanthine oxidase activity and blood glutathione redox ratio in infants and children with septic shock syndrome.Intensive Care Med. 2001; 27: 216-221Crossref PubMed Scopus (43) Google Scholar populations. Galley and colleagues39Galley HF Davies MJ Webster NR Xanthine oxidase activity and free radical generation in patients with sepsis syndrome.Crit Care Med. 1996; 24: 1649-1653Crossref PubMed Scopus (142) Google Scholar found xanthine oxidase activation and high free radical concentrations in septic patients compared with both healthy volunteers and non-infected patients. Batra and colleagues10Batra S Kumar R Seema Kapoor AK Ray G Alterations in antioxidant status during neonatal sepsis.Ann Trop Paediatr. 2000; 20: 27-33Crossref PubMed Scopus (83) Google Scholar found an increase in xanthine oxidase, SOD and glutathione peroxidase activity in neonates with sepsis, suggesting increased production of reactive oxygen species in this population. However, malondialdehyde levels (a marker of lipid peroxidation) were also increased, suggesting that the elevations of these antioxidant enzymes were not so effective as to prevent cellular damage. GSH metabolism is altered in sepsis. Rapid depletion of intracellular GSH in human and animal endothelial and epithelial cells occurs in response to tumour necrosis factor α (TNF-α) in vitro because of oxidation of GSH to GSSG, followed by rebound increases in GSH synthesis as a result of up-regulation of the enzyme γ-glutamylcysteine synthetase (γGCS). GSH turnover is increased early in sepsis in rats, with increased GSH synthesis in a number of tissues (especially the liver), but with lower blood GSH concentrations.64Malmezat T Breuillé D Capitan P Patureau Mirand P Obled C Glutathione turnover is increased during the acute phase of sepsis in rats.J Nutr. 2000; 130: 1239-1246Crossref PubMed Scopus (159) Google Scholar In a rat lipopolysaccharide (LPS) endotoxic shock model, oxidative stress was apparent, with decreased plasma antioxidant capacity, potentiated by depletion of liver GSH.21Carbonell LF Nadal JA Llanos MC Hernández I Nava E Díaz J Depletion of liver glutathione potentiates the oxidative stress and decreases nitric oxide synthesis in a rat endotoxin shock model.Crit Care Med. 2000; 28: 2002-2006Crossref PubMed Scopus (57) Google Scholar In children with sepsis, whole blood GSH concentrations and synthesis rates were found to be decreased,63Lyons J Rauh-Pfeiffer A Ming-Yu Y et al.Cysteine metabolism and whole blood glutathione synthesis in septic pediatric patients.Crit Care Med. 2001; 29: 870-877Crossref PubMed Scopus (110) Google Scholar while blood GSH redox ratios (GSSG:GSH) were found to be increased,71Németh I Boda D Xanthine oxidase activity and blood glutathione redox ratio in infants and children with septic shock syndrome.Intensive Care Med. 2001; 27: 216-221Crossref PubMed Scopus (43) Google Scholar suggesting increased oxidative stress. Glutathione peroxidase is a selenium-containing enzyme (selenoenzyme) and selenium depletion is therefore likely to be crucial in antioxidant defences secondary to reduced glutathione peroxidase activity. Selenium itself inhibits transcription and proinflammatory gene expression.51Handel ML Watts CKW deFazio A Day RO Sutherland RL Inhibition of AP-1 binding and transcription by gold and selenium involving conserved cysteine residues in Jun and Fos.Proc Natl Acad Sci USA. 1995; 92: 4497-4501Crossref PubMed Scopus (130) Google Scholar Selenium excess (toxicity), however, has also been linked to oxidative stress.91Stewart MS Spallholz JE Neldner KH Pence BC Selenium compounds have disparate abilities to impose oxidative stress and induce apoptosis.Free Radic Biol Med. 1999; 26: 42-48Crossref PubMed Scopus (289) Google Scholar Forceville and colleagues35Forceville X Vitoux D Gauzit R Combes A Lahilaire P Chappuis P Selenium, systemic immune response syndrome, sepsis, and outcome in critically ill patients.Crit Care Med. 1998; 26: 1536-1544Crossref PubMed Scopus (290) Google Scholar reported early and prolonged decreases in plasma selenium concentrations in patients with SIRS, associated with a three-fold increase in morbidity and mortality. Activation of the immune and inflammatory systems occurs in response to both infectious and non-infectious stimuli. In sepsis, Gram-negative and, increasingly, Gram-positive organisms are important causative microbes. Infection initially results in stimulation of the innate (non-specific) immune response, mediated mainly via circulating and tissue inflammatory cells, such as monocytes/macrophages and neutrophils. These cells normally exist in a non-activated state but are rapidly activated in response to bacteria, their products or inflammatory mediators, to become highly active phagocytes. LPS (endotoxin) is the principle component of the cell wall of Gram-negative bacteria, and exotoxins are from Gram-positive bacteria. The molecular mechanisms by which LPS induces gene activation, and hence inflammatory mediator expression, have been reviewed recently.48Guha M Mackman N LPS induction of gene expression in human monocytes.Cell Signal. 2001; 13: 85-94Crossref PubMed Scopus (1970) Google Scholar Briefly, LPS initially binds to the acute-phase LPS-binding protein (LBP) in the plasma, the level of which appears to rise in response to the insult. Bound LPS is subsequently delivered to the monocyte (and neutrophil) CD14 surface receptor. LPS then interacts with the transmembrane signal transduction receptor Toll-like receptor 4 (TLR4), which exists in complex with the accessory protein MD-2. TLR2 has been implicated as the receptor for Gram-positive exotoxin.34Faure E Thomas L Xu H Medvedev AE Equils O Arditi M Bacterial lipopolysaccharide and IFN-γ induce Toll-like receptor 2 and Toll-like receptor 4 expression in human endothelial cells: role of NF-κB activation.J Immunol. 2001; 166: 2018-2024Crossref PubMed Scopus (401) Google Scholar This complexing and binding of LPS subsequently activates a number of intracellular signalling pathways, including the IκB kinase (IKK)–nuclear factor κB (NFκB) pathway and three mitogen-activated protein kinase (MAPK) pathways. These pathways phosphorylate and activate various transcription factors (see below), including NFκB/Rel proteins, activator protein 1 (AP-1) and nuclear factor–interleukin 6 (NF-IL-6), thereby allowing rapid gene induction and the expression of inflammatory mediators, including cytokines, chemokines, lipid mediators, inducible nitric oxide synthase (type II NOS), enzymes and adhesion molecules. Cytokines are low molecular weight soluble proteins which are synthesized and secreted directly in response to inflammatory stimuli. As well as initiating the immuno-inflammatory response, they also coordinate and modulate the nature, amplitude and duration of this response. Cytokines have a variety of target cells, with their specific actions dependent upon the stimulus, the cell type and the presence of other inflammatory mediators and receptors.43Galley HF Webster NR The immuno-inflammatory cascade.Br J Anaesth. 1996; 77: 11-16Crossref PubMed Scopus (78) Google Scholar The key characteristics which cytokines exhibit are redundancy, pleiotropy, synergy and antagonism. Recently, a number of other mechanisms by which LPS induces gene transcription and inflammatory mediator expression have been described, including mechanisms operating TREM-1 (triggering receptor expressed on myeloid cells 1),19Bouchon A Dietrich J Colonna M Cutting edge: inflammatory responses can be triggered by TREM-1, a novel receptor expressed on neutrophils and monocytes.J Immunol. 2000; 164: 4991-4995Crossref PubMed Scopus (894) Google Scholar, 20Bouchon A Facchetti F Weigand MA Colonna M TREM-1 amplifies inflammation and is a crucial mediator of septic shock.Nature. 2001; 410: 1103-1107Crossref PubMed Scopus (841) Google Scholar macrophage transmembrane potassium channels,15Blunck R Scheel O Müller M Brandenburg K Seitzer U Seydel U New insights into endotoxin-induced activation of macrophages: involvement of a K+ channel in transmembrane signaling.J Immunol. 2001; 166: 1009-1015Crossref PubMed Scopus (125) Google Scholar and the intracellular cytoplasmic proteins Nod1 and Nod2.53Inohara N Ogura Y Chen FF Muto A Nuñez G Human Nod1 confers responsiveness to bacterial lipopolysaccharides.J Biol Chem. 2001; 276: 2551-2554Crossref PubMed Scopus (451) Google Scholar An outline of the proposed molecular mechanisms by which LPS stimulates the host inflammatory response is seen in Figure 2. The synthesis of messenger RNA (mRNA) from template DNA is called gene transcription, and occurs in the cell nucleus. After modification, mRNA is transported to the cytoplasm, where it is translated into a protein, such as a cytokine or an enzyme. Gene transcription is controlled by various transcription factors, which are DNA-binding proteins, and several transcription factors can control the production of one protein. Translocation of a transcription factor to the cell nucleus and subsequent binding to a target gene(s) results in protein synthesis. Although a number of transcription factors may be linked to the altered gene activation seen in sepsis, including AP-1 and NF-IL-6, the one that has been described in most detail is NFκB (a family of proteins belonging to the Rel family). Substantial in vitro and in vivo evidence suggests a pivotal role for NFκB in sepsis and SIRS. NFκB is a ubiquitous transcription factor which is crucial for normal immune system function, regulating the activation of genes necessary to provide rapid and appropriate responses. However, inappropriate, increased and/or prolonged activation of NFκB, resulting in the overexpression of mediator proteins, may account for the deleterious effects seen in sepsis. NFκB is a dimer consisting of two Rel subunits, and there are five known mammalian NFκB-Rel proteins. The classical NFκB dimer contains the proteins p50 (NFκB1) and p65 (Rel A). However, the subunit composition of NFκB can vary, with differing effects on gene regulation depending upon the specific combinations.2Abraham E NF-κB activation.Crit Care Med. 2000; 28: N100-N104Crossref PubMed Scopus (207) Google Scholar NFκB exists in the cytoplasm in an inactive form, complexed with an inhibitory protein from the IκB family, which includes IκBα, IκBβ and IκBɛ. The IκB proteins mask a nuclear localization signal on NFκB proteins, thereby preventing the translocation of NFκB to the nucleus. NFκB can be activated in cells by a number of inflammatory stimuli in addition to LPS, including cytokines [such as TNF-α, interleukin (IL) 1β, 11 and 17], reactive oxidant species (especially hydrogen peroxide), protein kinase C activators, viruses, UV light and ionizing radiation.8Barnes PJ Nuclear factor-κB.Int J Biochem Cell Biol. 1997; 29: 867-870Crossref PubMed Scopus (294) Google Scholar NFκB activation is achieved via phosphorylation and degradation of the inhibitory IκB protein through the action of specific kinases, the NFκB-inducible kinases (NIK), IKK-1 and IKK-2.2Abraham E NF-κB activation.Crit Care Med. 2000; 28: N100-N104Crossref PubMed Scopus (207) Google Scholar Degradation of IκB is ultimately accomplished by attachment of ubiquitin residues and proteolysis, unmasking the nuclear localization signal, thus allowing translocation of NFκB to the cell nucleu
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