With growing accounts of inflammatory diseases such as sepsis, greater understanding the immune system and the mechanisms of cellular immunity have become primary objectives in immunology studies. High mobility group box 1 (HMGB1) is a ubiquitous nuclear protein that is implicated in various aspects of the innate immune system as a damage-associated molecular pattern molecule and a late mediator of inflammation, as well as in principal cellular processes, such as autophagy and apoptosis. HMGB1 functions in the nucleus as a DNA chaperone; however, it exhibits cytokine-like activity when secreted by injurious or infectious stimuli. Extracellular HMGB1 acts through specific receptors to promote activation of the NF-κB signaling pathway, leading to production of cytokines and chemokines. These findings further implicate HMGB1 in lethal inflammatory diseases as a crucial regulator of inflammatory, injurious, and infectious responses. In this paper, we summarize the role of HMGB1 in inflammatory and non-inflammatory states and assess potential therapeutic approaches targeting HMGB1 in inflammatory diseases.

Most extracellular proteins are secreted via the classical endoplasmic reticulum (ER)/Golgi-dependent secretion pathway; however, some proteins, including a few danger-associated molecular patterns (DAMPs), are secreted via non-classical ER/Golgi-independent secretion pathways. The evolutionarily conserved high mobility group box1 (HMGB1) is a ubiquitous nuclear protein that can be released by almost all cell types. HMGB1 lacks signal peptide and utilizes diverse non-canonical secretion mechanisms for its extracellular export. Although the post-translational modifications of HMGB1 were demonstrated, the oxidation of HMGB1 and secretion mechanisms are not highlighted yet. We currently investigated that peroxiredoxins I and II (PrxI/II) induce the intramolecular disulfide bond formation of HMGB1 in the nucleus. Disulfide HMGB1 is preferentially transported out of the nucleus by binding to the nuclear exportin chromosome-region maintenance 1 (CRM1). We determined the kinetics of HMGB1 oxidation in bone marrow-derived macrophage as early as a few minutes after lipopolysaccharide treatment, peaking at 4 h while disulfide HMGB1 accumulation was observed within the cells, starting to secrete in the late time point. We have shown that HMGB1 oxidation status, which is known to determine the biological activity in extracellular HMGB1, is crucial for the secretion of HMGB1 from the nucleus. This review summarizes selected aspects of HMGB1 redox biology relevant to the induction and propagation of inflammatory diseases. We implicate the immunological significance and the need for novel HMGB1 inhibitors through mechanism-based studies.

Nuclear protein HMGB1 is secreted in response to various stimuli and functions as a danger-associated molecular pattern. Extracellular HMGB1 induces inflammation, cytokine production, and immune cell recruitment via activation of various receptors. As HMGB1 does not contain an endoplasmic reticulumtargeting signal peptide, HMGB1 is secreted via the endoplasmic reticulum-Golgi independently via an unconventional secretion pathway. However, the mechanism underlying HMGB1 secretion remains largely unknown. Here, we investigated the role of secretory autophagy machinery and vesicular trafficking in HMGB1 secretion. We observed that HSP90AA1 (heat shock protein 90 alpha family class A member 1), a stress-inducible protein, regulates the translocation of HMGB1 from the nucleus to the cytoplasm and its secretion through direct interaction. Additionally, geldanamycin, an HSP90AA1 inhibitor, reduced HMGB1 secretion. GORASP2/GRASP55 (golgi reassembly stacking protein 2), ARF1 Q71L (ADP ribosylation factor 1), and SAR1A T39N (secretion associated Ras related GTPase 1A), which promoted unconventional protein secretion, increased HMGB1 secretion. HMGB1 secretion was inhibited by an early autophagy inhibitor and diminished in ATG5-deficient cells even when GORASP2 was overexpressed. In contrast, a late autophagy inhibitor increased HMGB1 secretion under the same conditions. The multivesicular body formation inhibitor GW4869 dramatically decreased HMGB1 secretion under HMGB1 secretion-inducing conditions. Thus, we demonstrated that secretory autophagy and multivesicular body formation mediate HMGB1 secretion.

The nuclear protein HMGB1 (high mobility group box 1) is secreted by monocytes-macrophages in response to inflammatory stimuli and serves as a danger-associated molecular pattern. Acetylation and phosphorylation of HMGB1 are implicated in the regulation of its nucleocytoplasmic translocation for secretion, although inflammatory stimuli are known to induce H 2 O 2 production. Here we show that H 2 O 2 -induced oxidation of HMGB1, which results in the formation of an intramolecular disulfide bond between Cys 23 and Cys 45 , is necessary and sufficient for its nucleocytoplasmic translocation and secretion. The oxidation is catalyzed by peroxiredoxin I (PrxI) and PrxII, which are first oxidized by H 2 O 2 and then transfer their disulfide oxidation state to HMGB1. The disulfide form of HMGB1 showed higher affinity for nuclear exportin CRM1 compared with the reduced form. Lipopolysaccharide (LPS)–induced HMGB1 secretion was greatly attenuated in macrophages derived from PrxI or PrxII knockout mice, as was the LPS-induced increase in serum HMGB1 levels.

Lipopolysaccharide (LPS) triggers deleterious systemic inflammatory responses when released into the circulation. LPS-binding protein (LBP) in the serum plays an important role in modifying LPS toxicity by facilitating its interaction with LPS signaling receptors, which are expressed on the surface of LPS-responsive cells. We have previously demonstrated that high mobility group box 1 (HMGB1) can bind to and transfer LPS, consequently increasing LPSinduced TNF-a production in human peripheral blood mononuclear cells (PBMCs). We report here on the identification of two LPS-binding domains within HMGB1. Furthermore, using 12 synthetic HMGB1 peptides, we define the LPS-binding regions within each domain. Among them, synthetic peptides HPep1 and HPep6, which are located in the A and B box domains of HMGB1, bind to the polysaccharide and lipid A moieties of LPS respectively. Both HPep1 and HPep6 peptides inhibited binding of LPS to LBP and HMGB1, LBP-mediated LPS transfer to CD14, and cellular uptake of LPS in RAW264.7 cells. These peptides also inhibited LPS-induced TNF-a release in human PBMCs and induced lower levels of TNF-a in the serum in a subclinical endotoxemia mouse model. These results indicate that HMGB1 has two LPS-binding peptide regions that can be utilized to design anti-sepsis or LPS-neutralizing therapeutics.Keywords: Endotoxin shock . High mobility group box 1 . Inflammation . Lipopolysaccharide Supporting Information available online IntroductionLipopolysaccharide (LPS) is the main cause of Gram-negative bacterial sepsis. LPS consists of a lipid A component, a sugar moiety that forms the core, and an O-polysaccharide of variable length [1]. When LPS is introduced into the bloodstream, LPSbinding protein (LBP) recognizes the LPS molecules and catalyzes the movement of LPS from LPS aggregates. LBP transfers LPS to CD14, which in turn transfers LPS to the TLR4-MD2 receptor. Recently, the crystal structure of the TLR4-MD2-LPS complex has been determined [2]. Although there are several proteins that bind LPS, LBP is the first key protein that initiates and amplifies à These authors have contributed equally to this study. Eur. J. Immunol. 2011. 41: 2753-2762 DOI 10.1002 Innate immunity 2753 the LPS-mediated pro-inflammatory process that results in fatal septic shock syndrome. The nuclear protein high-mobility group box 1 protein (HMGB1) is involved in nucleosome stabilization, gene transcription, and neurite outgrowth [3]. HMGB1 can be actively or passively released into the extracellular space through acetylation [4], phosphorylation [5,6], methylation [7], or cell necrosis [8]. HMGB1 can trigger inflammation [8] and is a late mediator of endotoxemia and sepsis in both animal models and humans [9][10][11][12]. Although HMGB1 is a well-known mediator of endotoxemia and a proinflammatory cytokine-like protein in vivo, purified recombinant HMGB1 only has weak in vitro proinflammatory activity, such as the induction of TNF-a production [13,14]. HMGB1 can form highly inflammatory complexes with CpG DNA [15...

High-mobility group box 1 (HMGB1), a well-known danger-associated molecular pattern molecule, acts as a pro-inflammatory molecule when secreted by activated immune cells or released after necrotic cell damage. HMGB1 binds to immunogenic bacterial components and augments septic inflammation. In this study, we show how HMGB1 mediates complement activation, promoting sterile inflammation. We show that HMGB1 activates the classical pathway of complement system in an antibody-independent manner after binding to C1q. The C3a complement activation product in human plasma and C5b-9 membrane attack complexes on cell membrane surface are detected after the addition of HMGB1. In an acetaminophen (APAP)-induced hepatotoxicity model, APAP injection reduced HMGB1 levels and elevated C3 levels in C1q-deficient mouse serum samples, compared to that in wild-type (WT) mice. APAP-induced C3 consumption was inhibited by sRAGE treatment in WT mice. Moreover, in a mouse model of brain ischemia–reperfusion injury based on middle cerebral arterial occlusion, C5b-9 complexes were deposited on vessels where HMGB1 was accumulated, an effect that was suppressed upon HMGB1 neutralization. We propose that the HMGB1 released after cell necrosis and in ischemic condition can trigger the classical pathway of complement activation to exacerbate sterile inflammation.

HMGB1 protein is a delayed mediator of sepsis that is secreted to the extracellular milieu in response to various stimulants, inducing a pro-inflammatory response. HMGB1 is devoid of an endoplasmic reticulum (ER)-targeting signal peptide; hence, the mechanism of extracellular secretion is not completely understood, although HMGB1 is secreted after being subjected to post-translational modifications. Here, we identified the role of N-glycosylation of HMGB1 in extracellular secretion. We found two consensus (N37 and N134) and one non-consensus (N135) residues that were Nglycosylated in HMGB1 by performing liquid chromatography tandem mass spectrometry (LC-MS/MS) and analyzing for Nglycan composition and structure. Inhibition of N-glycosylation with tunicamycin resulted in a molecular shift of HMGB1 as assessed by gel electrophoresis. Non-glycosylated double mutant (N→Q) HMGB1 proteins (HMGB1 N37Q/N134Q and HMGB1 N37Q/N135Q ) showed localization to the nuclei, strong binding to DNA, weak binding to the nuclear export protein CRM1 and rapid degradation by ubiquitylation. These mutant proteins had reduced secretion even after acetylation, phosphorylation, oxidation and exposure to pro-inflammatory stimuli. Taken together, we propose that HMGB1 is N-glycosylated, and that this is important for its DNA interaction and is a prerequisite for its nucleocytoplasmic transport and extracellular secretion.

Lipoteichoic acid (LTA) is a component of the cell wall of Gram-positive bacteria and induces a toll-like receptor 2 (TLR2)-mediated inflammatory response upon initial binding to lipopolysaccharide-binding protein (LBP) and subsequent transfer to CD14. In this study, we identified a novel role for the nuclear protein high-mobility group box 1 (HMGB1) in LTA-mediated inflammation. Results of ELISA, surface plasmon resonance and native PAGE electrophoretic mobility shift analyses indicated that HMGB1 binds to LTA in a concentration-dependent manner and that this binding is inhibited by LBP. Native PAGE, fluorescence-based transfer and confocal imaging analyses indicated that HMGB1 catalytically disaggregates LTA and transfers LTA to CD14. NF-κB p65 nuclear transmigration, degradation of IκBa and reporter assay results demonstrated that NF-κB activity in HEK293-hTLR2/6 cells is significantly upregulated by a mixture of LTA and soluble CD14 in the presence of HMGB1. Furthermore, the production of TNF-a and IL-6 in J774A.1 and RAW264.7 cells increased significantly following treatment with a mixture of LTA and HMGB1 compared with treatment with LTA or HMGB1 alone. Thus, we propose that HMGB1 plays an important role in LTA-mediated inflammation by binding to and transferring LTA to CD14, which is subsequently transferred to TLR2 to induce an inflammatory response.