HMGB1 develops enhanced proinflammatory activity by binding to cytokines

High mobility group box 1 protein (HMGB1), originally characterized as a nuclear DNA-binding protein, has also been described to have an extracellular role when it is involved in cellular activation and proinflammatory responses. In this study, FLAG-tagged HMGB1 was inducibly expressed in the presence of culture media with or without added IL-1beta, IFN-gamma, or TNF-alpha. HMGB1 purified from cells grown in culture media alone only minimally increased cytokine production by MH-S macrophages and had no effect on murine neutrophils. In contrast, HMGB1 isolated from cells cultured in the presence of IL-1beta, IFN-gamma, and TNF-alpha had enhanced proinflammatory activity, resulting in increased production of MIP-2 and TNF-alpha by exposed cells. IL-1beta was bound to HMGB1 isolated from cells cultured with this cytokine, and purified HMGB1 incubated with recombinant IL-1beta acquired proinflammatory activity. Addition of anti-IL-1beta Abs or the IL-1 receptor antagonist to cell cultures blocked the proinflammatory activity of HMGB1 purified from IL-1beta-exposed cells, indicating that such activity was dependent on interaction with the IL-1 receptor. These results demonstrate that HMGB1 acquires proinflammatory activity through binding to proinflammatory mediators, such as IL-1beta.     

Structural basis for the proinflammatory cytokine activity of high mobility group box 1

High mobility group box 1 (HMGB), a ubiquitous DNA-binding protein, has been implicated as a proinflammatory cytokine and late mediator of lethal endotoxemia. HMGB1 is released by activated macrophages. It amplifies and extends the inflammatory response by inducing cytokine release and mediating acute lung injury, anorexia, and the inflammatory response to tissue necrosis. The kinetics of HMGB1 release provide a wide therapeutic window for endotoxemia because extracellular levels of HMGB1 begin to increase 12 to 24 h after exposure to inflammatory stimuli. Here, we demonstrate that a DNA-binding domain of HMGB1, the B box, recapitulates the cytokine activity of full length HMGB1 and efficiently activates macrophages to release tumor necrosis factor (TNF) and other proinflammatory cytokines. Truncation of the B box revealed that the TNF-stimulating activity localizes to 20 amino acids (HMGB1 amino acids 89 to 108). Passive immunization of mice with antibodies raised against B box conferred significant protection against lethal endotoxemia or sepsis, induced by cecal perforation. These results indicate that a proinflammatory domain of HMGB1 maps to the highly conserved DNA-binding B box, making this primary sequence a suitable target in the design of therapeutics.     

Potential effects of HMGB1 on viral replication and virus infection-induced inflammatory responses: A promising therapeutic target for virus infection-induced inflammatory diseases

Inflammatory responses, characterized by the overproduction of numerous proinflammatory mediators by immune cells, is essential to protect the host against invading pathogens. Excessive production of proinflammatory cytokines is a key pathogenic factor accounting for severe tissue injury and disease progression during the infection of multiple viruses, which are therefore termed as “cytokine storm”. High mobility group box 1 (HMGB1), a ubiquitous DNA-binding protein released either over virus-infected cells or activated immune cells, may act as a proinflammatory cytokine with a robust capacity to potentiate inflammatory response and disease severity. Moreover, HMGB1 is a host factor that potentially participates in the regulation of viral replication cycles with complicated mechanisms. Currently, HMGB1 is regarded as a promising therapeutic target against virus infection. Here, we provide an overview of the updated studies on how HMGB1 is differentially manipulated by distinct viruses to regulate viral diseases.     

JH-4 reduces HMGB1-mediated septic responses and improves survival rate in septic mice

Inhibition of high mobility group box 1 (HMGB1) and restoration of endothelial integrity are emerging as attractive therapeutic strategies for the management of severe vascular inflammatory diseases. Recently, we found that JH-4, a synthesized decursin derivative, exhibited a strong anti-Hutchinson-Gilford progeria syndrome by efficiently blocking progerin-lamin A/C binding. In this study, we examined the effects of JH-4 on HMGB1-mediated septic responses and the survival rate in a mouse sepsis model. The anti-inflammatory activities of JH-4 were monitored based on its effects on lipopolysaccharide- or cecal ligation and puncture (CLP)-mediated release of HMGB1. The antiseptic activities of JH-4 were determined by measuring permeability, leukocyte adhesion, migration, and the activation of proinflammatory proteins in HMGB1-activated human umbilical vein endothelial cells and mice. JH-4 inhibited the release of HMGB1 and downregulated HMGB1-dependent inflammatory responses in human endothelial cells. JH-4 also inhibited HMGB1-mediated hyperpermeability and leukocyte migration in mice. In addition, treatment with JH-4 reduced CLP-induced release of HMGB1, sepsis-related mortality, and pulmonary injury in vivo. Our results indicate that JH-4 is a possible therapeutic agent to treat various severe vascular inflammatory diseases via the inhibition of the HMGB1 signaling pathway.     

Further characterization of high mobility group box 1 (HMGB1) as a proinflammatory cytokine: central nervous system effects

High mobility group box 1 (HMGB1), an abundant, highly conserved cellular protein, is widely known as a nuclear DNA-binding protein. HMGB1 has been recently implicated as a proinflammatory cytokine because of its role as a late mediator of endotoxin lethality and ability to stimulate release of proinflammatory cytokines from monocytes. Production of central cytokines is a critical step in the pathway by which endotoxin and peripheral proinflammatory cytokines, including interleukin-1beta (IL-1) and tumor necrosis factor-alpha (TNF), produce sickness behaviors and fever. Intracerebroventricular (ICV) administration of HMGB1 has been shown to increase TNF expression in mouse brain and induce aphagia and taste aversion. Here we show that ICV injections of HMGB1 induce fever and hypothalamic IL-1 in rats. Furthermore, we show that intrathecal administration of HMGB1 produces mechanical allodynia (lowering of the response threshold to calibrated stimuli). Finally, while endotoxin (lipopolysaccharide, LPS) administration elevates IL-1 and TNF mRNA in various brain regions, HMGB1 mRNA is unchanged. It remains possible that HMGB1 protein is released in brain in response to LPS. Nonetheless, these data suggest that HMGB1 may play a role as an endogenous pyrogen and support the concept that HMGB1 has proinflammatory characteristics within the central nervous system.     

Stage-specific secretion of HMGB1 in cartilage regulates endochondral ossification

High mobility group box 1 protein (HMGB1) is a chromatin protein that has a dual function as a nuclear factor and as an extracellular factor. Extracellular HMGB1 released by damaged cells acts as a chemoattractant, as well as a proinflammatory cytokine, suggesting that HMGB1 is tightly connected to the process of tissue organization. However, the role of HMGB1 in bone and cartilage that undergo remodeling during embryogenesis, tissue repair, and disease is largely unknown. We show here that the stage-specific secretion of HMGB1 in cartilage regulates endochondral ossification. We analyzed the skeletal development of Hmgb1(-/-) mice during embryogenesis and found that endochondral ossification is significantly impaired due to the delay of cartilage invasion by osteoclasts, osteoblasts, and blood vessels. Immunohistochemical analysis revealed that HMGB1 protein accumulated in the cytosol of hypertrophic chondrocytes at growth plates, and its extracellular release from the chondrocytes was verified by organ culture. Furthermore, we demonstrated that the chondrocyte-secreted HMGB1 functions as a chemoattractant for osteoclasts and osteoblasts, as well as for endothelial cells, further supporting the conclusion that Hmgb1(-/-) mice are defective in cell invasion. Collectively, these findings suggest that HMGB1 released from differentiating chondrocytes acts, at least in part, as a regulator of endochondral ossification during osteogenesis.     

Redox modification of cysteine residues regulates the cytokine activity of high mobility group box-1 (HMGB1)

High mobility group box 1 (HMGB1) is a nuclear protein with extracellular inflammatory cytokine activity. It is released passively during cell injury and necrosis, and secreted actively by immune cells. HMGB1 contains three conserved redox-sensitive cysteine residues: C23 and C45 can form an intramolecular disulfide bond, whereas C106 is unpaired and is essential for the interaction with Toll-Like Receptor (TLR) 4. However, a comprehensive characterization of the dynamic redox states of each cysteine residue and of their impacts on innate immune responses is lacking. Using tandem mass spectrometric analysis, we now have established that the C106 thiol and the C23-C45 disulfide bond are required for HMGB1 to induce nuclear NF-κB translocation and tumor necrosis factor (TNF) production in macrophages. Both irreversible oxidation to sulphonates and complete reduction to thiols of these cysteines inhibited TNF production markedly. In a proof of concept murine model of hepatic necrosis induced by acetaminophen, during inflammation, the predominant form of serum HMGB1 is the active one, containing a C106 thiol group and a disulfide bond between C23 and C45, whereas the inactive form of HMGB1, containing terminally oxidized cysteines, accumulates during inflammation resolution and hepatic regeneration. These results reveal critical posttranslational redox mechanisms that control the proinflammatory activity of HMGB1 and its inactivation during pathogenesis.     

HMGB1 in renal ischemic injury

Factors that initiate cellular damage and trigger the inflammatory response cascade and renal injury are not completely understood after renal ischemia-reperfusion injury (IRI). High-mobility group box-1 protein (HMGB1) is a damage-associated molecular pattern molecule that binds to chromatin, but upon signaling undergoes nuclear-cytoplasmic translocation and release from cells. Immunohistochemical and Western blot analysis identified HMGB1 nuclear-cytoplasmic translocation and release from renal cells (particularly vascular and tubular cells) into the venous circulation after IRI. Time course analysis indicated HMGB1 release into the venous circulation progressively increased parallel to increased renal ischemic duration. Ethyl pyruvate (EP) treatment blocked H(2)O(2) (oxidative stress)-induced HMGB1 release from human umbilical vein endothelial cells in vitro, and in vivo resulted in nuclear retention and significant blunting of HMGB1 release into the circulation after IRI. EP treatment before IRI improved short-term serum creatinine and albuminuria, proinflammatory cyto-/chemokine release, and long-term albuminuria and fibrosis. The renoprotective effect of EP was abolished when exogenous HMGB1 was injected, suggesting EP's therapeutic efficacy is mediated by blocking HMGB1 translocation and release. To determine the independent effects of circulating HMGB1 after injury, exogenous HMGB1 was administered to healthy animals at pathophysiological dose. HMGB1 administration induced a rapid surge in systemic circulating cyto-/chemokines (including TNF-α, eotaxin, G-CSF, IFN-γ, IL-10, IL-1α, IL-6, IP-10, and KC) and led to mobilization of bone marrow CD34+Flk1+ cells into the circulation. Our results indicate that increased ischemic duration causes progressively enhanced HMGB1 release into the circulation triggering damage/repair signaling, an effect inhibited by EP because of its ability to block HMGB1 nuclear-cytoplasmic translocation.     

The pathogenicity,structural and functional exploration of human HMGB1 single nucleotide polymorphisms using in silico study

Abstract

The human HMGB1 gene mutations have a major impact on several immune-related diseases and cancer. The detrimental effect of non-synonymous mutations of HMGB1 has not been investigated yet, hence the present study aims to examine single nucleotide polymorphisms and their implications on the structure-function of human HMGB1. The multifaceted HMGB1 protein acts as pleiotropic cytokine and regulates essential genes for coordinated cellular functions. The mutational effect on HMGB1 was analyzed by sequence-based homology methods, supervised learning methods, and structure-based methods. The study identified 58 non-synonymous mutations in human HMGB1, out of which only 2 mutations; R10T (rs61742222) and F103C (rs61733675) were classified as the SNPs with highest deleterious and disease-causing mutants. The effect of these mutations in structure of HMGB1 was scrutinized and the R10T mutant found to have a distinct structural behaviour in the B-box domain. In addition, R10T mutant predicted that it affects the MoRF function of HMGB1 and it could disrupt the DNA binding or/and protein partner interaction activity by HMGB1. F103C mutation takes place at the TLR binding and cytokine inducing region of HMGB1, hence it could affect the protein binding activity which involves in many cellular signaling. The study identified potent mutations R10T (a cancer-causing somatic mutation) and F103C (a novel mutation) and these mutations either directly or indirectly hinder DNA binding activity and TLR and cytokine binding of HMGB1. These findings will help in understanding the molecular basis of these promising mutations and functional role of human HMGB1 in cancer and immunological diseases.

Abbreviations AGER Advanced glycosylation end product-specific receptor

CXCL Chemokine (C-X-C motif) ligand

dbSNP The single nucleotide polymorphism database

HMGB1 High mobility group box 1

LINCS LINear Constraint Solver

MDS Molecular dynamics simulation

MoRF Molecular recognition features

NPT Number of particle, Pressure and Temperature

NVT Number of particle, Volume and Temperature

nsSNP Non-synonymous SNP

PBC Partial boundary condition

PCA Principal component analysis

PME Partial mesh Ewald

RMSD Root mean square deviation

RMSF Root mean square fluctuation

SNP Single nucleotide polymorphism

SPC Single-point charge

TLR Toll-like receptor

UTR Un-translated Region

Communicated by Ramaswamy H. Sarma     

Barrier protective effects of rosmarinic acid on HMGB1‐induced inflammatory responses in vitro and in vivo

High mobility group box 1 (HMGB1) protein is a crucial cytokine that mediates response to infection, injury, and inflammation. Rosmarinic acid (RA) is an important component of the leaves of Perilla frutescens and has neuroprotective, anti‐microbial, anti‐oxidant, and anti‐cancer effects but little is known of its effects on HMGB1‐mediated inflammatory response. Here, we investigated this issue by monitoring the effects of RA on the lipopolysaccharide (LPS) or cecal ligation and puncture (CLP)‐mediated release of HMGB1 and HMGB1‐mediated modulation of inflammatory responses. RA potently inhibited the release of HMGB1 and down‐regulated HMGB1‐dependent inflammatory responses in human endothelial cells. RA also inhibited HMGB1‐mediated hyperpermeability and leukocyte migration in mice. Furthermore, RA reduced CLP‐induced HMGB1 release and sepsis‐related mortality. Given these results, RA should be viewed as a candidate therapeutic agent for the treatment of various inflammatory diseases via inhibition of the HMGB1 signaling pathway. J. Cell. Physiol. © 2012 Wiley Periodicals, Inc.     

High mobility group box protein 1 (HMGB1)-partner molecule complexes enhance cytokine production by signaling through the partner molecule receptor

The nuclear protein high mobility group box protein 1 (HMGB1) promotes inflammation upon extracellular release. HMGB1 induces proinflammatory cytokine production in macrophages via Toll-like receptor (TLR)-4 signaling in a redox-dependent fashion. Independent of its redox state and endogenous cytokine-inducing ability, HMGB1 can form highly immunostimulatory complexes by interaction with certain proinflammatory mediators. Such complexes have the ability to enhance the induced immune response up to 100-fold, compared with induction by the ligand alone. To clarify the mechanisms for these strong synergistic effects, we studied receptor requirements. Interleukin (IL)-6 production was assessed in supernatants from cultured peritoneal macrophages from mice each deficient in one of the HMGB1 receptors (receptor for advanced glycation end products [RAGE], TLR2 or TLR4) or from wild-type controls. The cultures were stimulated with the TLR4 ligand lipopolysaccaride (LPS), the TLR2 ligand Pam₃CysSerLys₄ (Pam₃CSK₄), noninflammatory HMGB1 or each TLR ligand in complex with noninflammatory HMGB1. The activity of the HMGB1-TLR ligand complexes relied on engagement of the same receptor as for the noncomplexed TLR ligand, since HMGB1-LPS complexes used TLR4 and HMGB1-Pam₃CSK₄ complexes used TLR2. Deletion of any of the intracellular adaptor molecules used by TLR2 (myeloid differentiation factor-88 [MyD88], TIR domain–containing adaptor protein [TIRAP]) or TLR4 (MyD88, TIRAP, TIR domain–containing adaptor-inducing interferon-β [TRIF], TRIF-related adaptor molecule [TRAM]) had similar effects on HMGB1 complex activation compared with noncomplexed LPS or Pam₃CSK₄. This result implies that the enhancing effects of HMGB1-partner molecule complexes are not regulated by the induction of additional signaling cascades. Elucidating HMGB1 receptor usage in processes where HMGB1 acts alone or in complex with other molecules is essential for the understanding of basic HMGB1 biology and for designing HMGB1-targeted therapies.     

A Janus Tale of Two Active High Mobility Group Box 1 (HMGB1) Redox States

High mobility group box 1 (HMGB1), the prototypic damage–associated molecular pattern molecule, is released at sites of inflammation and/or tissue damage. There, it promotes cytokine production and chemokine production/cell migration. New work shows that the redox status of HMGB1 distinguishes its cytokine-inducing and chemokine activity. Reduced all-thiol-HMGB1 has sole chemokine activity, whereas disulfide-HMGB1 has only cytokine activity, and oxidized, denatured HMGB1 has neither. Autophagy (programmed cell survival) and apoptosis (programmed cell death) have been implicated in controlling both innate and adaptive immune functions. Reduced HMGB1 protein promotes autophagy, whereas oxidized HMGB1 promotes apoptosis. Thus, the differential activity of HMGB1 in immunity, inflammation and cell death depends on the cellular redox status within tissues.High mobility group box 1 (HMGB1), a nonhistone nuclear factor, acts extracellularly as a damage-associated molecular pattern (DAMP) molecule to modulate inflammation, promoting autophagy and innate immune responses (1–5). HMGB1 has compartment-specific functions: nuclear, intracellular (but extranuclear) and extracellular. Its extracellular functions can now be divided further into cytokine-like or cytokine-inducing, chemokinelike and proangiogenic. Signaling pathways that induce variations on the posttranslational modification, such as phosphorylation and acetylation, have been implicated in the regulation of HMGB1 release. Importantly, HMGB1 contains three cysteines, each of which is susceptible to redox modification (6,7). The redox state of these cysteines is important for the proinflammatory cytokine-stimulating and proautophagic activity of HMGB1 (8–10). Autophagy (literally “self-eating”), a lysosome-mediated catabolic process, contributes to maintenance of intracellular homeostasis and promotes cell survival in response to environmental stress (11–13).Treatment with reduced but not oxidized HMGB1 protein increases autophagy in cancer cells (9). In contrast, oxidized HMGB1 protein activates the caspase-dependent apoptotic cell death pathway (9). Venereau et al.(14) described a new role for redox control of both the cytokine-inducing and chemokine activity of HMGB1 in the setting of sterile inflammation, regulating leukocyte recruitment and their ability to secrete inflammatory cytokines (Figure 1).Open in a separate windowFigure 1Redox control of HMGB1 activity. To act as a DAMP/danger signal and inflammatory mediator, HMGB1 is transported extracellularly by two principal means: active secretion from living inflammatory cells (for example, macrophages) or passive release from necrotic cells. The activities of extracellular HMGB1 are redox dependent. All-thiol-HMGB1 promotes chemokine production and leukocyte recruitment. Disulfide-HMGB1, originating from infiltrating leukocytes, promotes release of proinflammatory cytokines and thus participates in the inflammatory response. Reactive oxygen species produced by leukocytes induces the terminal oxidation of HMGB1, which is inactivated during resolution of inflammation.Structurally, HMGB1 is composed of three domains: two positively charged proximal DNA-binding domains (A box and B box) and a negatively charged carboxyl terminus. Three cysteines are encoded within the molecule: two vicinal cysteines in box A (C23 and C45) and a single one in box B (C106). C23 and C45 can form an intermolecular disulfide bond, whereas C106 is unpaired. Therefore, three different redox forms HMGB1 (all-thiol-HMGB1, disulfide-HMGB1 and oxidized HMGB1) were derived from bacterial expression systems (14). In addition, by using tryptic digests and liquid chromatography tandem mass spectrometric analysis, Venereau et al. observed that recombinant HMGB1 can be reversibly oxidized and reduced in the presence of electron donors (for example, dithiothreitol) or acceptors (oxygen) (14).Next, Venereau et al. assessed whether individual redox forms of HMGB1 have a differential role in cytokine-stimulating and chemoattractant activities (14). They found that disulfide-HMGB1 induced activation of the nuclear factor (NF)-κB pathway and production of proinflammatory cytokines (for example, tumor necrosis factors-α, interleukin [IL]-6 and IL-8) in fibroblasts and macrophages. Interestingly, all-thiol-HMGB1 failed to induce a proinflammatory response. In contrast, all-thiol-HMGB1, but not disulfide-HMGB1, had chemoattractant activity in fibroblasts. These findings prompted them to determine whether HMGB1 inhibitors, such as box A and monoclonal antibody PDH1.1, block the chemoattractant and/or cytokine-inducing activities of HMGB1. Unexpectedly, these inhibitors prevented cell migration but not cytokine production, although they are widely used as HMGB1-targeting agents in experimental inflammatory diseases.Reactive oxygen species oxidize the HMGB1 released from dying cells, thereby neutralizing its stimulatory activity and promoting tolerance in immune cells (15,16). In addition, oxidation of C106 or lack of a disulfide bridge between C23 and C45 then causes HMGB1 to lose its proinflammatory effects in macrophages (8). Venereau et al. found that terminal oxidation by hydrogen peroxide results in the loss of both the cytokine-stimulating and chemoattractant activities of HMBG1. Moreover, the authors found that the three HMGB1 cysteine residues were required for the cytokine-stimulating activity but not for the chemoattractant activity of HMGB1. Cysteine mutant HMGB1 promotes fibroblast migration, but not cytokine expression in macrophages (14). Collectively, these findings establish a crucial role for redox in the regulation of HMGB1 activity in inflammation and migration.What is the redox state of HMGB1 in the pathogenesis of individual diseases? The redox state of HMGB1 from the human acute monocytic leukemia cell line THP-1 was measured in the presence or absence of lipopolysaccharide (LPS) and necrotic medium in vitro. Intracellular HMGB1 was all-thiol-HMGB1, whereas secreted HMGB1 contained both all-thiol- and disulfide-HMGB1 (14). Furthermore, disulfide-HMGB1 was present later and time-dependently increased in cardiotoxin-injured muscles in vivo, confirming that the redox state of HMGB1 is altered during tissue damage and inflammation. HMGB1 protein with all three cysteines mutated to serine are resistant to oxidation and induce leukocyte recruitment without inducing cytokine production (14). The activities of HMGB1 are thus redox-dependent and can be modified within the injured tissues after HMGB1 release. Therefore, release of dynamic redox-regulated HMGB1 contributes to the orderly orchestrated recruitment of leukocytes, activation of cytokine release and subsequent resolution of inflammation.Several issues remain unresolved regarding the redox control of HMGB1 activity. First, HMGB1 is specifically recognized by several cell surface receptors (2), including Toll-like receptor (TLR)-4 and the receptor for advanced glycation end products (RAGE), but most recently was joined by T-cell immunoglobulin and mucin domain 3 (TIM-3) (17). Initial studies suggest that reduced C106 is necessary for the binding of HMGB1 to one of its receptors, TLR4, to stimulate cytokine release (8). HMGB1-induced recruitment of inflammatory cells depends on forming a complex with CXCL12 and signaling via CXCR4 (18). Moreover, RAGE is required for reduced HMGB1-mediated autophagy, but not oxidized HMGB1-induced apoptosis (9). All-thiol-HMGB1, but not disulfide-HMGB1, binds CXCL12 (14). The influence of HMGB1 receptors (for example, RAGE, TLR4, TLR2, CD24, TIM-3 and triggering receptor expressed on myeloid cells 1 [TREM1]) on biological activities of individual redox forms of HMGB1 remains to be carefully investigated. Second, HMGB1 forms highly inflammatory complexes with DNA, lipoteichoic acid, LPS, IL-1β, chemokine (C-X-C motif) ligand 12 (CXCL12)/ stromal cell–derived factor-1 (SDF-1) and nucleosomes (19). There is great interest in determining whether the individual redox forms of HMGB1 have varying affinity profiles active in inflammation and immunity. Third, HMGB1 has multiple intracellular and extracellular functions in health and disease, including cancer (1,2,6,20). Additional studies will be needed to determine whether redox is required for other functions of HMGB1, such as regeneration and cellular differentiation as well as the complex interactions between autophagy and immunity (5). One additional unanswered question is where and how the formation of the disulfide takes place and whether there is an enzyme specific for regulating this. This is important, knowing that the nuclear form is mostly all thiol. Finally, the development and performance of a simple, sensitive method for the detection of individual HMGB1 redox state isoforms in clinical specimens remains to be accomplished.     

The Role of HMGB1 in the Pathogenesis of Inflammatory and Autoimmune Diseases

High-mobility group box 1 (HMGB1) protein is a highly abundant protein that can promote the pathogenesis of inflammatory and autoimmune diseases once it is in an extracellular location. This translocation can occur with immune cell activation as well as cell death, with the conditions for release associated with the expression of different isoforms. These isoforms result from post-translational modifications, with the redox states of three cysteines at positions 23, 45 and 106 critical for activity. Depending on the redox states of these residues, HMGB1 can induce cytokine production via toll-like receptor 4 (TLR4) or promote chemotaxis by binding the chemokine CXCL12 for stimulation via CXCR4. Fully oxidized HMGB1 is inactive. During the course of inflammatory disease, HMGB1 can therefore play a dynamic role depending on its redox state. As a mechanism to generate alarmins, cell death is an important source of HMGB1, although each major cell death form (necrosis, apoptosis, pyroptosis and NETosis) can lead to different isoforms of HMGB1 and variable levels of association of HMGB1 with nucleosomes. The association of HMGB1 with nucleosomes may contribute to the pathogenesis of systemic lupus erythematosus by producing nuclear material whose immunological properties are enhanced by the presence of an alarmin. Since HMGB1 levels in blood or tissue are elevated in many inflammatory and autoimmune diseases, this molecule can serve as a unique biomarker as well as represent a target of novel therapies to block its various activities.     

HMGB1: endogenous danger signaling

While foreign pathogens and their products have long been known to activate the innate immune system, the recent recognition of a group of endogenous molecules that serve a similar function has provided a framework for understanding the overlap between the inflammatory responses activated by pathogens and injury. These endogenous molecules, termed alarmins, are normal cell constituents that can be released into the extracellular milieu during states of cellular stress or damage and subsequently activate the immune system. One nuclear protein, High mobility group box-1 (HMGB1), has received particular attention as fulfilling the functions of an alarmin by being involved in both infectious and non-infectious inflammatory conditions. Once released, HMGB1 signals through various receptors to activate immune cells involved in the immune process. Although initial studies demonstrated HMGB1 as a late mediator of sepsis, recent findings indicate HMGB1 to have an important role in models of non-infectious inflammation, such as autoimmunity, cancer, trauma, and ischemia reperfusion injury. Furthermore, in contrast to its pro-inflammatory functions, there is evidence that HMGB1 also has restorative effects leading to tissue repair and regeneration. The complex functions of HMGB1 as an archetypical alarmin are outlined here to review our current understanding of a molecule that holds the potential for treatment in many important human conditions.