N-linked glycosylation plays a crucial role in the secretion of HMGB1

Cytoplasmic HMGB1 promotes the activation of JAK2-STAT3 signaling and PD-L1 expression in breast cancer

BACKGROUND

High-mobility group box 1 (HMGB1) plays various roles depending on its subcellular localization. Extracellular HMGB1 interacts with receptors, such as toll-like receptor 4 and receptor for advanced glycation end products (RAGE), promoting cell proliferation, survival, and migration in cancer cells. It also increases the expression of programmed death-ligand 1 (PD-L1) in cancer cells by binding to RAGE. However, the effect of intracellular HMGB1 on the regulation of immune checkpoints such as PD-L1 has not been well characterized. In this study, we aimed to investigate the effects of intracellular HMGB1 on PD-L1 expression in breast cancer cells.

METHODS

Human and mouse triple-negative breast cancer cells, MDA-MB-231 and 4T1, along with HMGB1-deficient mouse embryonic fibroblast cells, were cultured. HMGB1 overexpression was achieved using a Myc-tagged plasmid, while siHMGB1 constructs were used for gene silencing. Quantitative reverse-transcriptase PCR and western blot analysis were performed to assess gene and protein expressions. Confocal imaging, immunoprecipitation, and proximity ligation assays were used to investigate HMGB1 localization and Janus kinase 2 (JAK2)-signal transducer and activator of transcription 3 (STAT3) interactions. In vivo experiments were performed using tumor-bearing mice treated with STAT3 and HMGB1 inhibitors. Statistical analyses were performed using Student's t-tests, one-way analysis of variance, Pearson's correlation, and Kaplan-Meier survival analysis, with significance set at p < 0.05.

RESULTS

In breast cancer cells, HMGB1 translocation from the nucleus to the cytoplasm increased the JAK2-STAT3 interaction and induced STAT3 phosphorylation, leading to increased STAT3 target signaling, including the epithelial-mesenchymal transition (EMT) phenotype and PD-L1 expression. Inhibition of nucleo-cytoplasmic translocation of HMGB1 decreased STAT3 phosphorylation and PD-L1 expression. Furthermore, HMGB1 enhanced breast cancer cell migration, invasion, and EMT, contributing to tumor growth in an in vivo mouse model that were mitigated by the HMGB1-targeted approach.

CONCLUSIONS

These findings underscore the critical role of intracellular HMGB1 in modulating PD-L1 expression via the JAK2-STAT3 signaling pathways in breast cancer and suggest that targeting HMGB1 translocation is a promising strategy for breast cancer treatment.

Virus and cell specific HMGB1 secretion and subepithelial infiltrate formation in adenovirus keratitis

A highly contagious infection caused by human adenovirus species D (HAdV-D), epidemic keratoconjunctivitis (EKC) results in corneal subepithelial infiltration (SEI) by leukocytes, the hallmark of the infection. To date, the pathogenesis of corneal SEI formation in EKC is unresolved. HMGB1 (high-mobility group box 1 protein) is an alarmin expressed in response to infection and a marker of sepsis. Earlier studies using a different adenovirus species, HAdV-C, showed retention of HMGB1 in the infected cell nucleus by adenovirus protein VII, enabling immune evasion. Here, using HAdV-D we show cell-specific HMGB1 secretion by infected cells, and provide an HAdV-D specific mechanism for SEI formation in EKC. HMGB1 was secreted only upon infection of human corneal epithelial cells, not from other cell types, and only upon infection by HAdV-D types associated with EKC. Acetylated HMGB1 translocation from the nucleus to the cytoplasm, then to the extracellular milieu, was tightly controlled by CRM1 and LAMP1, respectively. Primary stromal cells when stimulated by rHMGB1 expressed proinflammatory chemokines. In a novel 3D culture system in tune with the architecture of the cornea, HMGB1 released by infected corneal epithelial cells induced leukocytic infiltrates either directly and/or indirectly via stimulated stromal cells, which together explains SEI formation in EKC.

When glycobiology meets inflammasome activation: Insights and implications

BACKGROUND

Glycobiology focuses mainly on the study of glycan structures and their biological functions. Glycans not only provide a basic energy supply through the tricarboxylic acid cycle and glycolysis but also serve as important immune regulators during pathogen invasion and homeostasis maintenance. Inflammasomes are critical multiprotein complexes of the immune system that detect both exogenous pathogenic threats and endogenous danger signals to mediate inflammatory responses. Glycobiology has revealed significant insights into the mechanisms of immune responses, particularly in the context of inflammasome activation.

AIM OF REVIEW

This review summarizes the multifaceted relationships between glycobiology and inflammasome activation, highlighting how glycan structures, glycosylation patterns, and glycan-binding proteins influence inflammasome pathways. This review sheds light on novel targets for drug development aimed at modulating inflammatory pathways through the targeting of specific glycan structures.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Glycans directly or indirectly provide prime and activation signals for inflammasomes, glycosylation of inflammasome-related proteins by glycan structures modulates inflammasome activation and downstream inflammation, and the interaction between glycans and lectins also provides regulatory signals for inflammasome activation. This intersection of glycobiology and inflammasome activation presents a unique opportunity to elucidate the molecular mechanisms underlying inflammatory responses and their potential therapeutic implications.

Oligodendrocyte Precursor Cell-Specific HMGB1 Knockout Reduces Immune Cell Infiltration and Demyelination in Experimental Autoimmune Encephalomyelitis Models

Infiltration and activation of peripheral immune cells are critical in the progression of multiple sclerosis and its experimental animal model, experimental autoimmune encephalomyelitis (EAE). This study investigates the role of high mobility group box 1 (HMGB1) in oligodendrocyte precursor cells (OPCs) in modulating pathogenic T cells infiltrating the central nervous system through the blood-brain barrier (BBB) by using OPC-specific HMGB1 knockout (KO) mice. We found that HMGB1 released from OPCs promotes BBB disruption, subsequently allowing increased immune cell infiltration. The migration of CD4+ T cells isolated from EAE-induced mice was enhanced when co-cultured with OPCs compared to oligodendrocytes (OLs). OPC-specific HMGB1 KO mice exhibited lower BBB permeability and reduced immune cell infiltration into the CNS, leading to less damage to the myelin sheath and mitigated EAE progression. CD4+ T cell migration was also reduced when co-cultured with HMGB1 knock-out OPCs. Our findings reveal that HMGB1 secretion from OPCs is crucial for regulating immune cell infiltration and provides insights into the immunomodulatory function of OPCs in autoimmune diseases.

ROS anchor PAMPs-mediated extracellular HMGB1 self-association and its dimerization enhances pro-inflammatory signaling

Many cellular proteins form homo- or hetero-oligomeric complexes through dimerization, and ligand oligomerization is crucial for inducing receptor oligomerization. Intermolecular disulfide bond formation is critical for protein oligomerization that regulates biological functions. HMGB1 is a nuclear protein that acts as a DAMP when secreted. HMGB1 is redox-sensitive, contains three cysteines: Cys23, Cys45, and Cys106, and its function varies depending on the redox state of the extracellular space. However, the homo-dimerization of extracellular HMGB1 and its immunological significance have not been identified. In this study, we investigated the immunological significance of Cys106-mediated HMGB1 homo-dimerization. In the extracellular environment, LPS and LTA induced HMGB1 self-association leading to H2O2 anchoring Cys106-Cys106-mediated HMGB1 intermolecular disulfide bond formation. Despite treatment with H2O2, LPS, or LTA, HMGB1 dimerization was blocked in presence of Cys106 residue mutation, the ROS scavenger NAC, and the thiol-reducing agent DTT. Inflammatory stimulation induced the secretion of monomeric HMGB1 but not dimeric HMGB1. HMGB1 dimerization was promoted by PAMPs and H2O2 in the extracellular environment. Compared to monomeric HMGB1, Cys106-Cys106-linked dimeric HMGB1 significantly enhanced intracellular NF-κB signaling and cytokine production through increased direct binding affinity for TLR2 and TLR4 and effective HMGB1-mediated delivery of PAMPs to their receptors. Therefore, we have demonstrated that dimeric HMGB1 enhances its effect on pro-inflammatory signaling.

Heart-derived factors and organ cross-talk in settings of health and disease: new knowledge and clinical opportunities for multimorbidity

Cardiovascular disease affects millions of people worldwide and often presents with other conditions including metabolic, renal and neurological disorders. A variety of secreted factors from multiple organs/tissues (proteins, nucleic acids and lipids) have been implicated in facilitating organ cross-talk that may contribute to the development of multimorbidity. Secreted proteins have received the most attention, with the greatest body of research related to factors released from adipose tissue (adipokines), followed by skeletal muscle (myokines). To date, there have been fewer studies on proteins released from the heart (cardiokines) implicated with organ cross-talk. Early evidence for the secretion of cardiac-specific factors facilitating organ cross-talk came in the form of natriuretic peptides which are secreted via the classical endoplasmic reticulum-Golgi pathway. More recently, studies in cardiomyocyte-specific genetic mouse models have revealed cardiac-initiated organ cross-talk. Cardiomyocyte-specific modulation of microRNAs (miR-208a and miR-23-27-24 cluster) and proteins such as the mediator complex subunit 13 (MED13), G-protein-coupled receptor kinase 2 (GRK2), mutant α-myosin heavy-chain (αMHC), ubiquitin-like modifier-activating enzyme (ATG7), oestrogen receptor alpha (ERα) and fibroblast growth factor 21 (FGF21) have resulted in metabolic and renal phenotypes. These studies have implicated a variety of factors which can be secreted via the classical pathway or via non-classical mechanisms including the release of extracellular vesicles. Cross-talk between the heart and the brain has also been described (e.g. via miR-1 and an emerging concept, interoception: detection of internal neural signals). Here we summarize these studies taking into consideration that factors may be secreted in both settings of health and in disease.

Virus and Cell Specific HMGB1 Secretion and Subepithelial Infiltrate Formation in Adenovirus Keratitis

A highly contagious infection caused by human adenovirus species D (HAdV-D), epidemic keratoconjunctivitis (EKC) results in corneal subepithelial infiltration (SEI) by leukocytes, the hallmark of the infection. To date, the pathogenesis of corneal SEI formation in EKC is unresolved. HMGB1 (high-mobility group box 1 protein) is an alarmin expressed in response to infection and a marker of sepsis. Earlier studies using a different adenovirus species, HAdV-C, showed retention of HMGB1 in the infected cell nucleus by adenovirus protein VII, enabling immune evasion. Here, using HAdV-D we show cell-specific HMGB1 secretion by infected cells, and provide an HAdV-D specific mechanism for SEI formation in EKC. HMGB1 was secreted only upon infection of human corneal epithelial cells, not from other cell types, and only upon infection by HAdV-D types associated with EKC. Acetylated HMGB1 translocation from the nucleus to the cytoplasm, then to the extracellular milieu, was tightly controlled by CRM1 and LAMP1, respectively. Primary stromal cells when stimulated by rHMGB1 expressed proinflammatory chemokines. In a novel 3D culture system in tune with the architecture of the cornea, HMGB1 released by infected corneal epithelial cells induced leukocytic infiltrates either directly and/or indirectly via stimulated stromal cells, which together explains SEI formation in EKC.

Emerging Roles of High-mobility Group Box-1 in Liver Disease

High-mobility group box-1 (HMGB1) is an architectural chromosomal protein with various roles depending on its cellular localization. Extracellular HMGB1 functions as a prototypical damage-associated molecular pattern that triggers inflammation and adaptive immune responses, mediated by specific cell surface receptors, including receptors for advanced glycation end products and toll-like receptors. Post-translational modifications of HMGB1 significantly impact various cellular processes that contribute to the pathogenesis of liver diseases. Recent studies have highlighted the close relationship between HMGB1 and the pathogenesis of acute liver injuries, including acetaminophen-induced liver injury, hepatic ischemia-reperfusion injury, and acute liver failure. In chronic liver diseases, HMGB1 plays a role in nonalcoholic fatty liver disease, alcohol-associated liver disease, liver fibrosis, and hepatocellular carcinoma. Targeting HMGB1 as a therapeutic approach, either by inhibiting its release or blocking its extracellular function, is a promising strategy for treating liver diseases. This review aimed to summarize the available evidence on HMGB1's role in liver disease, focusing on its multifaceted signaling pathways, impact on disease progression, and the translation of these findings into clinical interventions.

Targeting HMGB1 and Its Interaction with Receptors: Challenges and Future Directions

High mobility group box 1 (HMGB1) is a nonhistone chromatin protein predominantly located in the nucleus. However, under pathological conditions, HMGB1 can translocate from the nucleus to the cytoplasm and subsequently be released into the extracellular space through both active secretion and passive release mechanisms. The distinct cellular locations of HMGB1 facilitate its interaction with various endogenous and exogenous factors, allowing it to perform diverse functions across a range of diseases. This Perspective provides a comprehensive overview of the structure, release mechanisms, and multifaceted roles of HMGB1 in disease contexts. Furthermore, it introduces the development of both small molecule and macromolecule inhibitors targeting HMGB1 and its interaction with receptors. A detailed analysis of the predicted pockets is also presented, aiming to establish a foundation for the future design and development of HMGB1 inhibitors.

HMGB1, an evolving pleiotropic protein critical for cellular and tissue homeostasis: Role in aging and age-related diseases

Aging is a universal biological process characterized by a progressive, cumulative decline in homeostatic capabilities and physiological functions, which inevitably increases vulnerability to diseases. A number of molecular pathomechanisms and hallmarks of aging have been recognized, yet we miss a thorough understanding of their complex interconnectedness. This review explores the molecular and cellular mechanisms underlying human aging, with a focus on the multiple roles of high mobility group Box 1 protein (HMGB1), the archetypal damage-associated molecular pattern (DAMP) molecule. In the nucleus, this non-histone chromatin-associated protein functions as a DNA chaperone and regulator of gene transcription, influencing DNA structure and gene expression. Moreover, this versatile protein can translocate to the cytoplasm to orchestrate other processes, such as autophagy, or be unconventionally secreted into the extracellular environment, where it acts as a DAMP, combining inflammatory and regenerative properties. Notably, lower expression of HMGB1 within the cell and its heightened extracellular release have been associated with diverse age-associated traits, making it a suitable candidate as a universal biomarker of aging. In this review, we outline the evidence implicating HMGB1 in aging, also in light of an evolutionary perspective on its functional pleiotropy, and propose critical issues that need to be addressed to gauge the value of HMGB1 as a potential biomarker across age-related diseases and therapeutic target to promote healthy longevity.

SIRT2 regulates high mobility group protein B1 nucleoplasmic shuttle and degradation via deacetylation in microglia

High mobility group protein B1 (HMGB1) acts as a pathogenic inflammatory response to mediate ranges of conditions such as epilepsy, septic shock, ischemia, traumatic brain injury, Parkinson's disease, Alzheimer's disease and mass spectrometry. HMGB1 promotes inflammation during sterile and infectious damage and plays a crucial role in disease development. Mobilization from the nucleus to the cytoplasm is the first important step in the release of HMGB1 from activated immune cells. Here, we demonstrated that Sirtuin 2 (SIRT2) physically interacts with and deacetylates HMGB1 at 43 lysine residue at nuclear localization signal locations, strengthening its interaction with HMGB1 and causing HMGB1 to be localized in the cytoplasm. These discoveries are the first to shed light on the SIRT2 nucleoplasmic shuttle, which influences HMGB1 and its degradation, hence revealing novel therapeutic targets and avenues for neuroinflammation treatment.

High Mobility Group Box Protein (HMGB1): A Potential Therapeutic Target for Diabetic Encephalopathy

HMGB (high mobility group B) is one of the ubiquitous non-histone nuclear protein superfamilies that make up the HMG (high mobility group) protein group. HMGB1 is involved in a variety of physiological and pathological processes in the human body, including a structural role in the cell nucleus as well as replication, repair, DNA transcription, and assembly of nuclear proteins. It functions as a signaling regulator in the cytoplasm and a pro-inflammatory cytokine in the extracellular environment. Among several studies, HMGB1 protein is also emerging as a crucial factor involved in the development and progression of diabetic encephalopathy (DE) along with other factors such as hyperglycaemia-induced oxidative and nitrosative stress. Diabetes' chronic side effect is DE, which manifests as cognitive and psychoneurological dysfunction. The HMGB1 is released outside to the extracellular medium in diabetes condition through active or passive routes, where it functions as a damage-associated molecular pattern (DAMP) molecule to activate several signaling pathways by interacting with receptors for advanced glycosylation end-products (RAGE)/toll like receptors (TLR). HMGB1 reportedly activates inflammatory pathways, disrupts the blood-brain barrier, causes glutamate toxicity and oxidative stress, and promotes neuroinflammation, contributing to the development of cognitive impairment and neuronal damage which is suggestive of the involvement of HMGB1 in the enhancement of the diabetes-induced encephalopathic condition. Additionally, HMGB1 is reported to induce insulin resistance, further exacerbating the metabolic dysfunction associated with diabetes mellitus (DM). Thus, the present review explores the possible pathways associated with DM-induced hyperactivation of HMGB1 ultimately leading to DE.

Deciphering the roles of the HMGB family in cancer: Insights from subcellular localization dynamics

The high-mobility group box (HMGB) family consists of four DNA-binding proteins that regulate chromatin structure and function. In addition to their intracellular functions, recent studies have revealed their involvement as extracellular damage-associated molecular patterns (DAMPs), contributing to immune responses and tumor development. The HMGB family promotes tumorigenesis by modulating multiple processes including proliferation, metabolic reprogramming, metastasis, immune evasion, and drug resistance. Due to the predominant focus on HMGB1 in the literature, little is known about the remaining members of this family. This review summarizes the structural, distributional, as well as functional similarities and distinctions among members of the HMGB family, followed by a comprehensive exploration of their roles in tumor development. We emphasize the distributional and functional hierarchy of the HMGB family at both the organizational and subcellular levels, with a focus on their relationship with the tumor immune microenvironment (TIME), aiming to prospect potential strategies for anticancer therapy.

Structure and Functions of HMGB3 Protein

HMGB3 protein belongs to the group of HMGB proteins from the superfamily of nuclear proteins with high electrophoretic mobility. HMGB proteins play an active part in almost all cellular processes associated with DNA-repair, replication, recombination, and transcription-and, additionally, can act as cytokines during infectious processes, inflammatory responses, and injuries. Although the structure and functions of HMGB1 and HMGB2 proteins have been intensively studied for decades, very little attention has been paid to HMGB3 until recently. In this review, we summarize the currently available data on the molecular structure, post-translational modifications, and biological functions of HMGB3, as well as the possible role of the ubiquitin-proteasome system-dependent HMGB3 degradation in tumor development.

Autocrine regulation of tumor cell repopulation by Hsp70-HMGB1 alarmin complex

BACKGROUND

Cancer recurrence is regulated by a variety of factors, among which is the material of dying tumor cells; it is suggested that remaining after anti-cancer therapy tumor cells receive a signal from proteins called damage-associated molecular patterns (DAMPs), one of which is heat shock protein 70 (Hsp70).

METHODS

Two models of tumor repopulation were employed, based on minimal population of cancer cells and application of conditioned medium (CM). To deplete the CMs of Hsp70 affinity chromatography on ATP-agarose and immunoprecipitation were used. Cell proliferation and the dynamics of cell growth were measured using MTT assay and xCELLigence technology; cell growth markers were estimated using qPCR and with the aid of ELISA for prostaglandin E detection. Immunoprecipitation followed by mass-spectrometry was employed to identify Hsp70-binding proteins and protein-protein interaction assays were developed to reveal the above protein complexes.

RESULTS

It was found that CM of dying tumor cells contains tumor regrowth-initiating factors and the removal of one of them, Hsp70, caused a reduction in the relapse-activating capacity. The pull out of Hsp70 alone using ATP-agarose had no effect on repopulation, while the immunodepletion of Hsp70 dramatically reduced its repopulation activity. Using proteomic and immunochemical approaches, we showed that Hsp70 in conditioned medium binds and binds another abundant alarmin, the High Mobility Group B1 (HMGB1) protein; the complex is formed in tumor cells treated with anti-cancer drugs, persists in the cytosol and is further released from dying tumor cells. Recurrence-activating power of Hsp70-HMGB1 complex was proved by the enhanced expression of proliferation markers, Ki67, Aurka and MCM-10 as well as by increase of prostaglandin E production and autophagy activation. Accordingly, dissociating the complex with Hsp70 chaperone inhibitors significantly inhibited the pro-growth effects of the above complex, in both in vitro and in vivo tumor relapse models.

CONCLUSIONS

These data led us to suggest that the abundance of the Hsp70-HMGB1 complex in the extracellular matrix may serve as a novel marker of relapse state in cancer patients, while specific targeting of the complex may be promising in the treatment of cancers with a high risk of recurrence.

Adenovirus protein VII binds the A-box of HMGB1 to repress interferon responses

Viruses hijack host proteins to promote infection and dampen host defenses. Adenovirus encodes the multifunctional protein VII that serves both to compact viral genomes inside the virion and disrupt host chromatin. Protein VII binds the abundant nuclear protein high mobility group box 1 (HMGB1) and sequesters HMGB1 in chromatin. HMGB1 is an abundant host nuclear protein that can also be released from infected cells as an alarmin to amplify inflammatory responses. By sequestering HMGB1, protein VII prevents its release, thus inhibiting downstream inflammatory signaling. However, the consequences of this chromatin sequestration on host transcription are unknown. Here, we employ bacterial two-hybrid interaction assays and human cell biological systems to interrogate the mechanism of the protein VII-HMGB1 interaction. HMGB1 contains two DNA binding domains, the A- and B-boxes, that bend DNA to promote transcription factor binding while the C-terminal tail regulates this interaction. We demonstrate that protein VII interacts directly with the A-box of HMGB1, an interaction that is inhibited by the HMGB1 C-terminal tail. By cellular fractionation, we show that protein VII renders A-box containing constructs insoluble, thereby acting to prevent their release from cells. This sequestration is not dependent on HMGB1's ability to bind DNA but does require post-translational modifications on protein VII. Importantly, we demonstrate that protein VII inhibits expression of interferon β, in an HMGB1- dependent manner, but does not affect transcription of downstream interferon- stimulated genes. Together, our results demonstrate that protein VII specifically harnesses HMGB1 through its A-box domain to depress the innate immune response and promote infection.

CHIP induces ubiquitination and degradation of HMGB1 to regulate glycolysis in ovarian endometriosis

Ovarian endometriosis is a common gynecological condition that can cause infertility in women of childbearing age. However, the pathogenesis is still unknown. We demonstrate that the carboxyl terminus of Hsc70-interacting protein (CHIP) is a negative regulator in the development of endometriosis and reduces HMGB1 expression in endometriotic cells. Meanwhile, CHIP interacts with HMGB1 and promotes its ubiquitinated degradation, thereby inhibiting aerobic glycolysis and the progression of endometriosis. Furthermore, the CHIP agonist YL-109 effectively suppresses the growth of ectopic endometrium in endometriosis mouse model, which could be a potential therapeutic approach for endometriosis. In conclusion, our data suggest that CHIP may inhibit the development of endometriosis by suppressing the HMGB1-related glycolysis.

Endothelial Dysfunction, HMGB1, and Dengue: An Enigma to Solve

Dengue is a viral infection caused by dengue virus (DENV), which has a significant impact on public health worldwide. Although most infections are asymptomatic, a series of severe clinical manifestations such as hemorrhage and plasma leakage can occur during the severe presentation of the disease. This suggests that the virus or host immune response may affect the protective function of endothelial barriers, ultimately being considered the most relevant event in severe and fatal dengue pathogenesis. The mechanisms that induce these alterations are diverse. It has been suggested that the high mobility group box 1 protein (HMGB1) may be involved in endothelial dysfunction. This non-histone nuclear protein has different immunomodulatory activities and belongs to the alarmin group. High concentrations of HMGB1 have been detected in patients with several infectious diseases, including dengue, and it could be considered as a biomarker for the early diagnosis of dengue and a predictor of complications of the disease. This review summarizes the main features of dengue infection and describes the known causes associated with endothelial dysfunction, highlighting the involvement and possible relationship between HMGB1 and DENV.

Molecular insights into the multifaceted functions and therapeutic targeting of high mobility group box 1 in metabolic diseases

HMGB1 is a ubiquitously expressed protein localized in nucleus, cytoplasm, as well as secreted into extracellular space. Nuclear HMGB1 binds to DNAs and RNAs, regulating genomic stability and transcription. Cytoplasmic HMGB1 regulates autophagy through binding to core autophagy regulators. Secreted extracellular HMGB1 functions as a ligand to various receptors (RAGE and TLRs, etc.), regulating multiple signalling pathways, such as MAPK, PI3K and NF-κB signallings. Trafficking and localization of HMGB1 across cellular compartments could be regulated by its posttranslational modifications, which fine-tune its functions in metabolic diseases, inflammation and cancers. The current review examines the up-to-date findings pertaining to the biological functions of HMGB1, with focus on its posttranslational modifications and roles in downstream signalling pathways involved in metabolic diseases. This review also discusses the feasibility of targeting HMGB1 as a potential pharmacological intervention for metabolic diseases.

The spectrum of neurodevelopmental, neuromuscular and neurodegenerative disorders due to defective autophagy

Primary dysfunction of autophagy due to Mendelian defects affecting core components of the autophagy machinery or closely related proteins have recently emerged as an important cause of genetic disease. This novel group of human disorders may present throughout life and comprises severe early-onset neurodevelopmental and more common adult-onset neurodegenerative disorders. Early-onset (or congenital) disorders of autophagy often share a recognizable "clinical signature," including variable combinations of neurological, neuromuscular and multisystem manifestations. Structural CNS abnormalities, cerebellar involvement, spasticity and peripheral nerve pathology are prominent neurological features, indicating a specific vulnerability of certain neuronal populations to autophagic disturbance. A typically biphasic disease course of late-onset neurodegeneration occurring on the background of a neurodevelopmental disorder further supports a role of autophagy in both neuronal development and maintenance. Additionally, an associated myopathy has been characterized in several conditions. The differential diagnosis comprises a wide range of other multisystem disorders, including mitochondrial, glycogen and lysosomal storage disorders, as well as ciliopathies, glycosylation and vesicular trafficking defects. The clinical overlap between the congenital disorders of autophagy and these conditions reflects the multiple roles of the proteins and/or emerging molecular connections between the pathways implicated and suggests an exciting area for future research. Therapy development for congenital disorders of autophagy is still in its infancy but may result in the identification of molecules that target autophagy more specifically than currently available compounds. The close connection with adult-onset neurodegenerative disorders highlights the relevance of research into rare early-onset neurodevelopmental conditions for much more common, age-related human diseases.Abbreviations: AC: anterior commissure; AD: Alzheimer disease; ALR: autophagic lysosomal reformation; ALS: amyotrophic lateral sclerosis; AMBRA1: autophagy and beclin 1 regulator 1; AMPK: AMP-activated protein kinase; ASD: autism spectrum disorder; ATG: autophagy related; BIN1: bridging integrator 1; BPAN: beta-propeller protein associated neurodegeneration; CC: corpus callosum; CHMP2B: charged multivesicular body protein 2B; CHS: Chediak-Higashi syndrome; CMA: chaperone-mediated autophagy; CMT: Charcot-Marie-Tooth disease; CNM: centronuclear myopathy; CNS: central nervous system; DNM2: dynamin 2; DPR: dipeptide repeat protein; DVL3: disheveled segment polarity protein 3; EPG5: ectopic P-granules autophagy protein 5 homolog; ER: endoplasmic reticulum; ESCRT: homotypic fusion and protein sorting complex; FIG4: FIG4 phosphoinositide 5-phosphatase; FTD: frontotemporal dementia; GBA: glucocerebrosidase; GD: Gaucher disease; GRN: progranulin; GSD: glycogen storage disorder; HC: hippocampal commissure; HD: Huntington disease; HOPS: homotypic fusion and protein sorting complex; HSPP: hereditary spastic paraparesis; LAMP2A: lysosomal associated membrane protein 2A; MEAX: X-linked myopathy with excessive autophagy; mHTT: mutant huntingtin; MSS: Marinesco-Sjoegren syndrome; MTM1: myotubularin 1; MTOR: mechanistic target of rapamycin kinase; NBIA: neurodegeneration with brain iron accumulation; NCL: neuronal ceroid lipofuscinosis; NPC1: Niemann-Pick disease type 1; PD: Parkinson disease; PtdIns3P: phosphatidylinositol-3-phosphate; RAB3GAP1: RAB3 GTPase activating protein catalytic subunit 1; RAB3GAP2: RAB3 GTPase activating non-catalytic protein subunit 2; RB1: RB1-inducible coiled-coil protein 1; RHEB: ras homolog, mTORC1 binding; SCAR20: SNX14-related ataxia; SENDA: static encephalopathy of childhood with neurodegeneration in adulthood; SNX14: sorting nexin 14; SPG11: SPG11 vesicle trafficking associated, spatacsin; SQSTM1: sequestosome 1; TBC1D20: TBC1 domain family member 20; TECPR2: tectonin beta-propeller repeat containing 2; TSC1: TSC complex subunit 1; TSC2: TSC complex subunit 2; UBQLN2: ubiquilin 2; VCP: valosin-containing protein; VMA21: vacuolar ATPase assembly factor VMA21; WDFY3/ALFY: WD repeat and FYVE domain containing protein 3; WDR45: WD repeat domain 45; WDR47: WD repeat domain 47; WMS: Warburg Micro syndrome; XLMTM: X-linked myotubular myopathy; ZFYVE26: zinc finger FYVE-type containing 26.

The mechanism of HMGB1 secretion and release

High mobility group box 1 (HMGB1) is a nonhistone nuclear protein that has multiple functions according to its subcellular location. In the nucleus, HMGB1 is a DNA chaperone that maintains the structure and function of chromosomes. In the cytoplasm, HMGB1 can promote autophagy by binding to BECN1 protein. After its active secretion or passive release, extracellular HMGB1 usually acts as a damage-associated molecular pattern (DAMP) molecule, regulating inflammation and immune responses through different receptors or direct uptake. The secretion and release of HMGB1 is fine-tuned by a variety of factors, including its posttranslational modification (e.g., acetylation, ADP-ribosylation, phosphorylation, and methylation) and the molecular machinery of cell death (e.g., apoptosis, pyroptosis, necroptosis, alkaliptosis, and ferroptosis). In this minireview, we introduce the basic structure and function of HMGB1 and focus on the regulatory mechanism of HMGB1 secretion and release. Understanding these topics may help us develop new HMGB1-targeted drugs for various conditions, especially inflammatory diseases and tissue damage.

A nuclear protein that gets released after cell death or is actively secreted by immune cells offers a promising therapeutic target for treating diseases linked to excessive inflammation. Daolin Tang from the University of Texas Southwestern Medical Center in Dallas, USA, and colleagues review how cellular stresses can trigger the accumulation of HMGB1, a type of alarm signal protein that promotes the recruitment and activation of inflammation-promoting immune cells. The researchers discuss various mechanisms that drive both passive and active release of HMGB1 into the space around cells. These processes, which include enzymatic modifications of the HMGB1 protein, cell–cell interactions and molecular pathways of cell death, could be targeted by drugs to lessen tissue damage and inflammatory disease caused by HMGB1-induced immune responses

Etiological Value of Sterile Inflammation in Preeclampsia: Is It a Non-Infectious Pregnancy Complication?

Understanding of sterile inflammation and its associated biological triggers and diseases is still at the elementary stage. This becomes more warranted in cases where infections are not associated with the pathology. Detrimental effects of bacterial and viral infections on the immune responses at the maternal-fetal interface as well as pregnancy outcomes have been well documented. However, an infection-induced etiology is not thought to be a major contributing component to severe pregnancy complications such as preeclampsia (PE) and gestational diabetes. How is then an inflammatory signal thought to be associated with these pregnancy complications? It is not clear what type of inflammation is involved in the onset of PE-like features. We opine that sterile inflammation regulated by the inflammasome-gasdermins-caspase-1 axis is a contributory factor to the onset of PE. We hypothesize that increased production and release of damage-associated molecular patterns (DAMPs) or Alarmins such as high-mobility group box1 (HMGB1), cell-free fetal DNA, uric acid, the NOD-like receptor pyrin-containing receptor 3 (NLRP3) inflammasome, IL-1β and IL-18 occur in the PE placenta. Some of these molecules have already been observed in the placenta from women with PE. Mechanistically, emerging evidence has demonstrated that excessive placental endoplasmic reticulum (ER) stress, impaired autophagy and gasdermine D (GSDMD)-mediated intrinsic pyroptosis are key events that contribute to systemic sterile inflammation in patients with PE, especially early-onset PE (e-PE). In this review, we highlight the advances on the roles of sterile inflammation and inflammatory signaling cascades involving ER stress, autophagy deficiency and pyroptosis in PE pathophysiology. Deciphering the mechanisms underlying these inflammatory pathways may provide potential diagnostic biomarkers and facilitate the development of therapeutic strategies to treat this devastating disease.

Predicted N-terminal N-linked glycosylation sites may underlie membrane protein expression patterns in Saccharomyces cerevisiae

N-linked glycosylation is one type of posttranslational modification that proteins undergo during expression. The following describes the effects of N-linked glycosylation on high-level membrane protein expression in yeast with an emphasis on Saccharomyces cerevisiae. N-linked glycosylation is highlighted here as an important consideration when preparing membrane protein gene constructs for expression in S. cerevisiae, which continues to be used as a workhorse in both research and industrial applications. Non-native N-linked glycosylation commonly occurs during the heterologous expression of mammalian proteins in many yeast species which can have important immunological consequences when used in the production of biotherapeutic proteins or peptides. Further, non-native N-linked glycosylation can lead to improper protein folding and premature degradation, which can impede high-level expression yields and hinder downstream analysis. Multiple strategies are presented in this article, which suggest different methods that can be implemented to circumvent the unwanted consequences of N-linked glycosylation during the expression process. These considerations may have long-term benefits for high-level protein production in S. cerevisiae across a broad spectrum of expression targets with special emphasis placed on G-protein coupled receptors, one of the largest families of membrane proteins.

Reactive oxygen species induce Cys106-mediated anti-parallel HMGB1 dimerization that protects against DNA damage

Oxidative stress can induce covalent disulfide bond formation between protein-protein thiol groups and generate hydroxyl free radicals that damage DNA. HMGB1 is a DNA chaperone and damage-associated molecular pattern molecule. As a redox-sensitive protein, HMGB1 contains three cysteine residues: Cys23, Cys45, and Cys106. In this study, we focused on the relationship between HMGB1 dimerization and DNA stabilization under oxidative stress conditions. HMGB1 dimerization was positively modulated by CuCl₂ and H₂O₂. Mutation of the Cys106 residue blocked dimer formation. Treatment of HEK293T cells with CuCl₂ and H₂O₂ enhanced the oxidative self-dimerization of HMGB1, whereas this dimerization was inhibited in mutant HMGB1^C106A cells. Furthermore, we performed a bimolecular fluorescence complementation assay to visualize Cys106 oxidation-induced HMGB1 dimerization in live cells exposed to oxidative stress and were able to reproduce the dimerization effect of HMGB1 in fluorescence resonance energy transfer analysis. Interestingly, dimerized HMGB1 bound to DNA with higher affinity than monomeric HMGB1. Dimerized HMGB1 protected DNA from damage due to hydroxyl free radicals and prevented cell death. In conclusion, dimerized HMGB1 may play a regulatory role in DNA stabilization under oxidative stress.

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Accumulation of excessive ROS induces DNA damage, causing cell death.

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HMGB1 dimerizes in the presence of excessive ROS and binds DNA with high affinity.

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Binding of dimerized HMGB1 (Di-HMGB1) protects DNA from ROS action.

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We prepared an in vitro HMGB1 dimerization due to excessive ROS.

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Di-HMGB1 protected against DNA damage induced by radiation exposure.

Role of HMGB1 in Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN), one of major dose-limiting side effects of first-line chemotherapeutic agents such as paclitaxel, oxaliplatin, vincristine, and bortezomib is resistant to most of existing medicines. The molecular mechanisms of CIPN have not been fully understood. High mobility group box 1 (HMGB1), a nuclear protein, is a damage-associated molecular pattern protein now considered to function as a pro-nociceptive mediator once released to the extracellular space. Most interestingly, HMGB1 plays a key role in the development of CIPN. Soluble thrombomodulin (TMα), known to degrade HMGB1 in a thrombin-dependent manner, prevents CIPN in rodents treated with paclitaxel, oxaliplatin, or vincristine and in patients with colorectal cancer undergoing oxaliplatin-based chemotherapy. In this review, we describe the role of HMGB1 and its upstream/downstream mechanisms in the development of CIPN and show drug candidates that inhibit the HMGB1 pathway, possibly useful for prevention of CIPN.

A conserved aromatic moiety in the ectodomain is a key determinant for structural integrity and protein trafficking of TNFR superfamily

Extracellular trafficking of tumor necrosis factor receptor superfamily (TNFRSF) is tightly regulated, disruption of which triggers various autoinflammatory disorders, including TNF receptor-associated periodic syndrome (TRAPS). Here, we provide thus far unraveled molecular basis of noncysteine mutations in TNFR1 ectodomain where loss of an aromatic moiety in cysteine-rich domain (CRD) 2 results in TRAPS disease-associated phenotype. Our study characterized that a missense mutation on phenylalanine residue located in CRD2 (TNFR1F60V ) causes a delay in TNFR1 transport to cell membrane, leading to sustained receptor responsiveness and downstream NF-κB activation, characteristic of clinical manifestation of a prolonged fever. By creating and characterizing identical mutations on structurally conserved ectodomains of osteoprotegerin (OPG) and decoy receptor 3, other two secreted forms of TNFRSF, we further identified that a conserved aromatic residue at the A1 submodule of CRD2 (A1CRD2) confers structural integrity of ectodomain where aromatic sidechain deletion increases thermal instability, interfering with efficient posttranslational modification and subsequent receptor secretion. Interestingly, our functional analyses indicated that this particular noncysteine mutation is not associated with either protein misfolding or loss of function. Finally, by using a synthetic agonist, we demonstrated gain-of-function of the trafficking defect, suggesting the possibility of rescuing affected pathology in related disorders. Given the structural and topological similarities present in the ectodomains of TNFRSF members, our findings provide mechanistic insights of defects in subcellular trafficking of TNF receptors, reported in various TNFRSF-associated diseases.

Downregulation of lncRNA FIRRE relieved the neuropathic pain of female mice by suppressing HMGB1 expression

Long non-coding RNAs are novel regulators in neuropathic pain. In this study, we aimed to explore the role and the mechanism of lncRNA FIRRE in regulating the secretion of microglial cells-derived proinflammatory cytokines in neuropathic pain. The female mouse model of neuropathic pain was established by bilateral chronic constriction injury (CCI) surgery. The mouse primary microglial cells were induced by lipopolysaccharide (LPS). The interaction between FIRRE and high mobility group box 1 (HMGB1) was assessed by RNA immunoprecipitation, RNA pull-down, and ubiquitination assays. FIRRE expression was upregulated in the spinal cord tissue of female CCI mice and LPS-induced microglial cells. The concentrations of IL-1β, TNF-α, and IL-6 from LPS-induced microglial cells were reduced by FIRRE knockdown. FIRRE bound to HMGB1 and negatively regulated its protein level. The ubiquitination degradation of HMGB1 was promoted by FIRRE silence. The HMGB1 over-expression reversed the inhibitory effect of FIRRE silence on the secretion of IL-1β, TNF-α, and IL-6 from LPS-induced microglial cells. The in vivo experiment showed that FIRRE knockdown alleviated neuropathic pain of CCI female mice. Our findings indicated that lncRNA FIRRE downregulation inhibits the secretion of microglial cells-derived proinflammatory cytokines by decreasing HMGB1 expression, thereby relieving neuropathic pain of female mice.

Functional Diversity of Non-Histone Chromosomal Protein HmgB1

The functioning of DNA in the cell nucleus is ensured by a multitude of proteins, whose interactions with DNA as well as with other proteins lead to the formation of a complicated, organized, and quite dynamic system known as chromatin. This review is devoted to the description of properties and structure of the progenitors of the most abundant non-histone protein of the HMGB family-the HmgB1 protein. The proteins of the HMGB family are also known as "architectural factors" of chromatin, which play an important role in gene expression, transcription, DNA replication, and repair. However, as soon as HmgB1 goes outside the nucleus, it acquires completely different functions, post-translational modifications, and change of its redox state. Despite a lot of evidence of the functional activity of HmgB1, there are still many issues to be solved related to the mechanisms of the influence of HmgB1 on the development and treatment of different diseases-from oncological and cardiovascular diseases to pathologies during pregnancy and childbirth. Here, we describe molecular structure of the HmgB1 protein and discuss general mechanisms of its interactions with other proteins and DNA in cell.

Secretory autophagy machinery and vesicular trafficking are involved in HMGB1 secretion

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 reticulum-targeting 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.

High mobility group box 1 (HMGB1): a pivotal regulator of hematopoietic malignancies

High mobility group box 1 (HMGB1) is a nonhistone chromatin-associated protein that has been widely reported to play a pivotal role in the pathogenesis of hematopoietic malignancies. As a representative damage-associated molecular pattern (DAMP), HMGB1 normally exists inside cells but can be secreted into the extracellular environment through passive or active release. Extracellular HMGB1 binds with several different receptors and interactors to mediate the proliferation, differentiation, mobilization, and senescence of hematopoietic stem cells (HSCs). HMGB1 is also involved in the formation of the inflammatory bone marrow (BM) microenvironment by activating proinflammatory signaling pathways. Moreover, HMGB1-dependent autophagy induces chemotherapy resistance in leukemia and multiple myeloma. In this review, we systematically summarize the emerging roles of HMGB1 in carcinogenesis, progression, prognosis, and potential clinical applications in different hematopoietic malignancies. In summary, targeting the regulation of HMGB1 activity in HSCs and the BM microenvironment is highly beneficial in the diagnosis and treatment of various hematopoietic malignancies.

N-glycosylation of High Mobility Group Box 1 protein (HMGB1) modulates the interaction with glycyrrhizin: A molecular modeling study

High Mobility Group Box 1 protein (HMGB1) is an abundant protein with multiple functions in cells, acting as a DNA chaperone and damage-associated molecular pattern molecule. It represents an attractive target for the treatment of inflammatory diseases and cancers. The plant natural product glycyrrhizin (GLR) is a well-characterized ligand of HMGB1 and a drug used to treat diverse liver and skin diseases. The drug is known to bind to each of the two adjacent HMG boxes of the non-glycosylated protein. In cells, HMGB1 is N-glycosylated at three asparagine residues located in boxes A and B, and these N-glycans are essential for the nucleocytoplasmic transport of the protein. But the impact of the N-glycans on drug binding is unknown. Here we have investigated the effect of the N-glycosylation of HMGB1 on its interaction with GLR using molecular modelling, after incorporation of three N-glycans on a Human HMGB1 structure (PDB code 2YRQ). Sialylated bi-antennary N-glycans were introduced on the protein and exposed in a folded or an extended conformation for the drug binding study. The docking of the drug was performed using both 18α- and 18β-epimers of GLR and the conformations and potential energy of interaction (ΔE) of the different drug-protein complexes were compared. The N-glycans do not shield the drug binding sites on boxes A and B but can modulate the drug-protein interaction, via both direct and indirect effects. The calculations indicate that binding of 18α/β-GLR to the HMG box is generally reduced when the protein is N-glycosylated vs. the non-glycosylated protein. In particular, the N-glycans in an extended configuration significantly weaken the binding of GLR to box-B. The effects of the N-glycans are mostly indirect, but in one case a direct contact with the drug, via a carbohydrate-carbohydrate interaction, was observed with 18β-GLR bound to Box-B of glycosylated HMGB1. For the first time, it is shown (at least in silico) that N-glycosylation, one of the many post-translational modifications of HMGB1, can affect drug binding.

Immunological Significance of HMGB1 Post-Translational Modification and Redox Biology

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.

Autophagy-based unconventional secretion of HMGB1 by keratinocytes plays a pivotal role in psoriatic skin inflammation

The precise mechanism through which macroautophagy/autophagy affects psoriasis is poorly understood. Here, we found that keratinocyte (KC) autophagy, which was positively correlated with psoriatic severity in patients and mouse models and could be inhibited by mitogen-activated protein kinase (MAPK) family inactivation. The impairment of autophagic flux alleviated psoriasisform inflammation. We also found that an autophagy-based unconventional secretory pathway (autosecretion) dependent on ATG5 (autophagy related 5) and GORASP2 (golgi reassembly stacking protein 2) promoted psoriasiform KC inflammation. Moreover, the alarmin HMGB1 (high mobility group box 1) was more effective than other autosecretory proteins in regulating psoriasiform cutaneous inflammation. HMGB1 neutralization in autophagy-efficient KCs eliminated the differences in psoriasiform inflammation between Krt14^+/+-Atg5^f/f KCs and Krt14^Cre/+-atg5^f/f KCs, and conversely, recombinant HMGB1 almost completely restored psoriasiform inflammation in Krt14^Cre/+-atg5^f/f KCs in vivo. These results suggest that HMGB1-associated autosecretion plays a pivotal role in cutaneous inflammation. Finally, we demonstrated that Krt14^Cre/+-hmgb1^f/f mice displayed attenuated psoriatic inflammation due to the essential crosstalk between KC-specific HMGB1-associated autosecretion and γδT cells. Thus, this study uncovered a novel autophagy mechanism in psoriasis pathogenesis, and the findings imply the clinical significance of investigating and treating psoriasis.Abbreviations: 3-MA: 3-methyladenine; ACTB: actin beta; AGER: advanced glycosylation end-product specific receptor; Anti-HMGB1: anti-HMGB1 neutralizing antibody; Anti-IL18: anti-IL18 neutralizing antibody; Anti-IL1B: anti-IL1B neutralizing antibody; ATG5: autophagy related 5; BAF: bafilomycin A₁; BECN1: beclin 1; CASP1: caspase 1; CCL: C-C motif chemokine ligand; CsA: cyclosporine A; ctrl shRNA: lentivirus harboring shRNA against control; CXCL: C-X-C motif chemokine ligand; DCs: dendritic cells; DMEM: dulbecco's modified Eagle's medium; ELISA: enzyme-linked immunosorbent assay; EM: electron microscopy; FBS: fetal bovine serum; GORASP2 shRNA: lentivirus harboring shRNA against GORASP2; GORASP2/GRASP55: golgi reassembly stacking protein 2; GR1: a composite epitope between LY6 (lymphocyte antigen 6 complex) locus C1 and LY6 locus G6D antigens; H&E: hematoxylin and eosin; HMGB1: high mobility group box 1; HMGB1 shRNA: lentivirus harboring shRNA against HMGB1; IFNG/IFN-γ: interferon gamma; IL17A: interleukin 17A; IL18: interleukin 18; IL1A/IL-1α: interleukin 1 alpha; IL1B/IL-1β: interleukin 1 beta; IL22/IL-22: interleukin 22; IL23A: interleukin 23 subunit alpha; IL23R: interleukin 23 receptor; IMQ: imiquimod; ITGAM/CD11B: integrin subunit alpha M; ITGAX/CD11C: integrin subunit alpha X; IVL: involucrin; KC: keratinocyte; KD: knockdown; KO: knockout; Krt14^+/+-Atg5^f/f mice: mice bearing an Atg5 flox allele, in which exon 3 of the Atg5 gene is flanked by two loxP sites; Krt14^+/+-Hmgb1^f/f: mice bearing an Hmgb1 flox allele, in which exon 2 to 4 of the Hmgb1 gene is flanked by two loxP sites; Krt14^Cre/+-atg5^f/f mice: keratinocyte-specific atg5 knockout mice generated by mating Atg5-floxed mice with mice expressing Cre recombinase under the control of the promoter of Krt4; Krt14^Cre/+-hmgb1^f/f mice: keratinocyte-specific hmgb1 knockout mice generated by mating Hmgb1-floxed mice with mice expressing Cre recombinase under the control of the promoter of Krt14; Krt14-Vegfa mice: mice expressing 164-amino acid Vegfa splice variant recombinase under the control of promoter of Krt14; LAMP1: lysosomal associated membrane protein 1; LDH: lactate dehydrogenase; LORICRIN: loricrin cornified envelope precursor protein; M5: TNF, IL1A, IL17A, IL22 and OSM in combination; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MAPK: mitogen-activated protein kinase; MKI67: marker of proliferation Ki-67; MTT: thiazolyl blue tetrazolium bromide; NFKB/NF-κB: nuclear factor kappa B; NHEKs: primary normal human epidermal keratinocytes; NS: not significant; OSM: oncostatin M; PASI: psoriasis area and severity index; PtdIns3K: class III phosphatidylinositol 3-kinase; qRT-PCR: quantitative RT-PCR; RELA/p65: RELA proto-oncogene, NF-kB subunit; rHMGB1: recombinant HMGB1; rIL18: recombinant interleukin 18; rIL1B: recombinant interleukin 1 beta; S100A: S100 calcium binding protein A; SQSTM1/p62: sequestosome 1; T17: IL17A-producing T; TCR: T-cell receptor; tcrd KO mice: tcrd (T cell receptor delta chain) knockout mice, which show deficient receptor expression in all adult lymphoid and epithelial organs; TLR: toll-like receptor; TNF/TNF-α: tumor necrosis factor; WOR: wortmannin; WT: wild-type; γδT17 cells: IL17A-producing γδ T cells.

The N-glycosylation of Equine Tetherin Affects Antiviral Activity by Regulating Its Subcellular Localization

Tetherin is an interferon-inducible type II transmembrane glycoprotein which inhibits the release of viruses, including retroviruses, through a “physical tethering” model. However, the role that the glycosylation of tetherin plays in its antiviral activity remains controversial. In this study, we found that mutation of N-glycosylation sites resulted in an attenuation of the antiviral activity of equine tetherin (eqTHN), as well as a reduction in the expression of eqTHN at the plasma membrane (PM). In addition, eqTHN N-glycosylation mutants colocalize obviously with ER, CD63, LAMP1 and endosomes, while WT eqTHN do not. Furthermore, we also found that N-glycosylation impacts the transport of eqTHN in the cell not by affecting the endocytosis, but rather by influencing the anterograde trafficking of the protein. These results suggest that the N-glycosylation of eqTHN is important for the antiviral activity of the protein through regulating its normal subcellular localization. This finding will enhance our understanding of the function of this important restriction factor.

Complex N-glycan promotes CD133 mono-ubiquitination and secretion

CD133 is a widely used cell surface marker of cancer stem cells that plays an important role in tumor initiation and metastasis. Increasing evidence shows that CD133 is secreted to the extracellular space. However, the underlying mechanisms of CD133 secretion remain largely unknown. In this study, we report that secreted CD133 has a complex-type N-glycosylation and is modified by beta1,6GlcNAc N-glycan. We found that inhibition of CD133 complex-type N-glycosylation by swainsonine does not affect the membrane localization of CD133, but significantly reduces CD133 secretion and promotes its accumulation in early endosomes. Moreover, swainsonine reduces CD133 secretion by reducing its mono-ubiquitination and inhibiting the interaction between CD133 and Tsg101. These findings reveal a new mechanism of glycosylation-dependent secretion of CD133.

Regulation of Acetylation in High Mobility Group Protein B1 Cytosol Translocation

High mobility group protein B1 (HMGB1) is a nonhistone that mainly binds to nucleus DNA. As an important late inflammatory transmitter, extracellular HMGB1 is involved in the inflammatory immune response, tumor growth, infiltration, and metastasis. HMGB1 is actively released by activated inflammatory cells or passively released by necrotic cells. Then the released extracellular HMGB1 further induces monocytes/macrophages, neutrophils, and dendritic cells to secrete inflammatory cytokines. Therefore, HMGB1 can not only act as a proinflammatory factor to directly involve in tissue damage, but also acts as an inflammatory medium to aggravate the inflammatory cascade reaction. Studies have shown that the post-translational modification (PTM) participated in the process of HMGB1 cytosol translocation and extracellular release. The acetylation modification is the most common PTM for localization sequence of HMGB1, and the affinity of HMGB1 to DNA depends on the degree of acetylation for HMGB1. The acetylation can weaken the binding of HMGB1 to DNA, which means less HMGB1 cytosol translocation and extracellular release. This article reviews the acetylation regulation mechanisms of cytosol translocation and extracellular release of HMGB1 and provides a therapeutic strategy for controlling HMGB1-induced inflammatory responses in the future.

HMGB1 and repair: focus on the heart

High-mobility group box 1 (HMGB1) is one of the most abundant proteins in eukaryotes and the best characterized damage-associated molecular pattern (DAMP). The biological activities of HMGB1 depend on its subcellular location, context and post-translational modifications. Inside the nucleus, HMGB1 is engaged in many DNA events such as DNA repair, transcription regulation and genome stability; in the cytoplasm, its main function is to regulate the autophagic flux while in the extracellular environment, it possesses more complicated functions and it is involved in a large variety of different processes such as inflammation, migration, invasion, proliferation, differentiation and tissue regeneration. Due to this pleiotropy, the role of HMGB1 has been vastly investigated in various pathological diseases and a large number of studies have explored its function in cardiovascular pathologies. However, in this contest, the precise mechanism of action of HMGB1 and its therapeutic potential are still very controversial since is debated whether HMGB1 is involved in tissue damage or plays a role in tissue repair and regeneration. The main focus of this review is to provide an overview of the effects of HMGB1 in different ischemic heart diseases and to discuss its functions in these pathological conditions.

Identification and Expression Analysis of an Atypical Alkaline Phosphatase in Emiliania huxleyi

Emiliania huxleyi, a cosmopolitan coccolithophore in the modern ocean, plays an important role in the carbon cycle and local climate feedback as it can form extensive blooms, calcify, and produce dimethylsulfoniopropionate (DMSP) leading to the generation of dimethyl sulfide (DMS) which affects climate when oxidized in the atmosphere. It is known to be able to utilize dissolved organic phosphorus (DOP) by expressing a specific type of alkaline phosphatase (EHAP1) under phosphorus-limited conditions. In this study, we identified a new alkaline phosphatase (EH-PhoA^aty) in this species, which we found belongs to the newly classified PhoA^aty family. The expression of this atypical phosphatase was up-regulated under P-depleted conditions at both the transcriptional and translational levels, suggesting that E. huxleyi is able to express this AP to cope with phosphorus limitation. Comparative analysis revealed different transcriptional expression dynamics between eh-PhoA^aty and ehap1, although both genes exhibited inducible expression under phosphate deficiency. In addition, after AP activity was eliminated by using EDTA to chelate metal ions, we found that AP activity was recovered with the supplement of Ca²⁺ and Zn²⁺, indicative of the adoption of Ca²⁺ as the cofactor under Zn-P co-limited conditions, likely a result of adaptation to oceanic environments where Zn²⁺ is often limiting.

High-Mobility Group Box-1 and Liver Disease

High‐mobility group box‐1 (HMGB1) is a ubiquitous protein. While initially thought to be simply an architectural protein due to its DNA‐binding ability, evidence from the last decade suggests that HMGB1 is a key protein participating in the pathogenesis of acute liver injury and chronic liver disease. When it is passively released or actively secreted after injury, HMGB1 acts as a damage‐associated molecular pattern that communicates injury and inflammation to neighboring cells by the receptor for advanced glycation end products or toll‐like receptor 4, among others. In the setting of acute liver injury, HMGB1 participates in ischemia/reperfusion, sepsis, and drug‐induced liver injury. In the context of chronic liver disease, it has been implicated in alcoholic liver disease, liver fibrosis, nonalcoholic steatohepatitis, and hepatocellular carcinoma. Recently, specific posttranslational modifications have been identified that could condition the effects of the protein in the liver. Here, we provide a detailed review of how HMGB1 signaling participates in acute liver injury and chronic liver disease.

Mass Spectrometry-based Structural Analysis and Systems Immunoproteomics Strategies for Deciphering the Host Response to Endotoxin

One cause of sepsis is systemic maladaptive immune response of the host to bacteria and specifically, to Gram-negative bacterial outer-membrane glycolipid lipopolysaccharide (LPS). On the host myeloid cell surface, proinflammatory LPS activates the innate immune system via Toll-like receptor-4/myeloid differentiation factor-2 complex. Intracellularly, LPS is also sensed by the noncanonical inflammasome through caspase-11 in mice and 4/5 in humans. The minimal functional determinant for innate immune activation is the membrane anchor of LPS called lipid A. Even subtle modifications to the lipid A scaffold can enable, diminish, or abolish immune activation. Bacteria are known to modify their LPS structure during environmental stress and infection of hosts to alter cellular immune phenotypes. In this review, we describe how mass spectrometry-based structural analysis of endotoxin helped uncover major determinations of molecular pathogenesis. Through characterization of LPS modifications, we now better understand resistance to antibiotics and cationic antimicrobial peptides, as well as how the environment impacts overall endotoxin structure. In addition, mass spectrometry-based systems immunoproteomics approaches can assist in elucidating the immune response against LPS. Many regulatory proteins have been characterized through proteomics and global/targeted analysis of protein modifications, enabling the discovery and characterization of novel endotoxin-mediated protein translational modifications.

Quantification of Serum High Mobility Group Box 1 by Liquid Chromatography/High-Resolution Mass Spectrometry: Implications for Its Role in Immunity, Inflammation, and Cancer

High mobility group box 1 (HMGB1) is a non-histone chromosomal protein, which can be secreted through a variety of pathways and bind to pattern recognition receptors to release pro-inflammatory cytokines. Previous studies have suggested that HMGB1 is upregulated in numerous inflammatory diseases and that it could be a biomarker for such diseases. However, these studies used immunoassay-based methods to analyze serum HMGB1. Autoantibodies to HMGB1 in serum are found in healthy control subjects as well as in patients with different diseases. HMGB1 also binds to haptoglobin, a highly abundant plasma protein. This means that antibodies used in immunoassays must compete with binding of HMGB1 to endogenous serum HMGB1 autoantibodies and haptoglobin. To overcome these potential problems, we developed and validated a specific and sensitive assay based on stable isotope dilution and immunopurification to quantify HMGB1 in plasma and serum using two-dimensional nano-ultra-high-performance liquid chromatography parallel reaction monitoring/high-resolution mass spectrometry. Using this assay, we found that serum HMGB1 in 24 healthy control subjects (6.0 ± 2.1 ng/mL) was above the mean concentration reported for 18 different diseases (5.4 ± 2.8 ng/mL) where the analyses were conducted with immunoassay methodology. In light of our finding, the role of HMGB1 in these diseases will have to be re-evaluated. The concentration of HMGB1 in citrated and EDTA-treated plasma from the same healthy control subjects was below the limit of detection of our assay (1 ng/mL), confirming that HMGB1 in serum arises when blood is allowed to clot. This means that future studies on the role of HMGB1 in vivo should be conducted on plasma rather than serum.

Alarmins: Feel the Stress

Over the last decade, danger-associated molecular pattern molecules, or alarmins, have been recognized as signaling mediators of sterile inflammatory responses after trauma and injury. In contrast with the accepted passive release models suggested by the "danger hypothesis," it was recently shown that alarmins can also directly sense and report damage by signaling to the environment when released from live cells undergoing physiological stress, even without loss of subcellular compartmentalization. In this article, we review the involvement of alarmins such as IL-1α, IL-33, IL-16, and high-mobility group box 1 in cellular and physiological stress, and suggest a novel activity of these molecules as central initiators of sterile inflammation in response to nonlethal stress, a function we denote "stressorins." We highlight the role of posttranslational modifications of stressorins as key regulators of their activity and propose that targeted inhibition of stressorins or their modifiers could serve as attractive new anti-inflammatory treatments for a broad range of diseases.

Ethyl pyruvate inhibits the acetylation and release of HMGB1 via effects on SIRT1/STAT signaling in LPS-activated RAW264.7 cells and peritoneal macrophages

High mobility group box 1 (HMGB1), a cytokine present in the late phase of sepsis, may be a potential target for the treatment of sepsis. For HMGB1 to be actively secreted from macrophages during infections, it must be post-translationally modified. Although ethyl pyruvate (EP), a simple aliphatic ester derived from pyruvic acid, has been shown to inhibit the release of HMGB1 in lipopolysaccharide (LPS)-treated RAW 264.7 cells, the underlying mechanism(s) are not yet clear. We investigated the hypothesis that the upregulation of SIRT1 by EP might promote the deacetylation of HMGB1, which reduces HMGB1 release in LPS-activated macrophages. Our results show that EP induced the expression of the SIRT1 protein in RAW264.7 cells and that it significantly inhibited the LPS-induced acetylation of HMGB1. Transfection with a SIRT1-overexpressing vector resulted in a significant decrease in the acetylation of HMGB1 in LPS-activated RAW264.7 cells relative to control cells. The genetic ablation or the pharmacological inhibition of SIRT1 by sirtinol increased LPS-induced HMGB1 acetylation. Moreover, EP inhibited the acetylation of HMGB1 in peritoneal macrophages treated with LPS. Interestingly, EP significantly reduced the LPS-induced phosphorylation of STAT1, which was significantly reversed by siSIRT1 transfection in RAW264.7 cells, indicating that SIRT1 negatively regulates the phosphorylation of STAT1. Overall, the results show that EP promotes the deacetylation of HMGB1 via the inhibition of STAT1 phosphorylation through the upregulation of SIRT1, which reduces HMGB1 release in LPS-activated RAW264.7 cells. In conclusion, EP might be useful in the treatment of diseases that target HMGB1, such as sepsis.

Down-regulation of N-acetylglucosamine-1-phosphate transferase (WecA) enhanced the sensitivity of Mycobacterium smegmatis against rifampin

AIMS

To construct a conditional N-acetylglucosamine-1-phosphate transferase (WecA) knockdown strain of Mycobacterium smegmatis and to investigate the biological effect of WecA on mycobacterial growth, morphology and susceptibilities against anti-tuberculosis drugs.

METHODS AND RESULTS

Mycobacterium smegmatis wecA knockdown strain was constructed by using a tetracycline-inducible expression vector pMind and the expression of WecA was regulated by antisense RNA. The results of growth curves and the colony formation unit curves showed that the growth rate of WecA down-regulation strain was decreased and the amount of live bacterial cells dropped. In addition, the wecA knockdown strain exhibited dramatically morphological alterations through scanning electron microscopy observation. The susceptibility of WecA low-expression strain to anti-tuberculosis drugs was detected by using a rapid resazurin microtitre assay as well as a traditional agar dilution method. Notably, the wecA knockdown strain was more sensitive to rifampin, compared with the wecA normal-expression strain. In addition, the sensitivity of wild type Myco. smegmatis mc(2) 155 strain against rifampin was also enhanced in the presence of a low concentration of tunicamycin, a natural WecA inhibitor.

CONCLUSIONS

Down-regulation of WecA enhanced the sensitivity of Myco. smegmatis against rifampin.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results provided a possibility of combined application of rifampin together with tunicamycin or other WecA inhibitors, which could be a new approach for the treatment of tuberculosis.

Aggressiveness Niche: Can It Be the Foster Ground for Cancer Metastasis Precursors?

The relationship between tumor initiation and tumor progression can follow a linear projection in which all tumor cells are equally endowed with the ability to progress into metastasis. Alternatively, not all tumor cells are equal genetically and/or epigenetically, and only few cells are induced to become metastatic tumor cells. The location of these cells within the tumor can also impact the fate of these cells. The most inner core of a tumor where an elevated pressure of adverse conditions forms, such as necrosis-induced inflammation and hypoxia-induced immunosuppressive environment, seems to be the most fertile ground to generate such tumor cells with metastatic potential. Here we will call this necrotic/hypoxic core the “aggressiveness niche” and will present data to support its involvement in generating these metastatic precursors. Within this niche, interaction of hypoxia-surviving cells with the inflammatory microenvironment influenced by newly recruited mesenchymal stromal cells (MSCs), tumor-associated macrophages (TAMs), and other types of cells and the establishment of bidirectional interactions between them elevate the aggressiveness of these tumor cells. Additionally, immune evasion properties induced in these cells most likely contribute in the formation and maintenance of such aggressiveness niche.