HMGB1 in vascular diseases: Its role in vascular inflammation and atherosclerosis

Emerging roles for triggering receptor expressed on myeloid cells receptor family signaling in inflammatory diseases

Innate immune receptors represent important therapeutic targets for inflammatory disorders. In particular, the Toll-like receptor (TLR) family has emerged as a promoter of chronic inflammation that contributes to obesity, insulin resistance and atherosclerosis. Importantly, triggering receptor expressed on myeloid cells-1 (TREM-1) has been characterized as an 'amplifier' of TLR2 and TLR4 signaling. TREM-1- and TREM-2-dependent signaling, as opposed to TREM-like transcript-1 (TLT-1 or TREML1), are mediated through association with the transmembrane adaptor DNAX activation protein of 12 kDa (DAP12). Recessive inheritance of rare mutations in DAP12 or TREM-2 results in a disorder called polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy, and surprisingly these subjects are not immunocompromised. Recent progress into the roles of TREM/DAP12 signaling is critically reviewed here with a focus on metabolic, cardiovascular and inflammatory diseases. The expanding repertoire of putative ligands for TREM receptors is also discussed.

HMGB1 plays a critical role in vascular inflammation and lesion formation via toll-like receptor 9

OBJECTIVE

Endogenous ligands such as high-mobility group box 1 (HMGB1) and nucleic acids are released by dying cells and bind to Toll-like receptors (TLRs). As TLR9 is involved in both microbial and sterile inflammation by detecting both bacterial and endogenous DNA, we investigated its role in inflammation and lesion formation in a mouse model of vascular injury.

METHODS AND RESULTS

C57BL/6 (WT) and TLR9 KO mice were subjected to wire-mediated vascular injury. Anti-HMGB1 antibody and purified HMGB1 protein were chronically delivered around the injured arteries by gelatin hydrogel, and neointima formation at 4 weeks after injury was evaluated. In addition, the same vascular injury was performed in bone-marrow chimeric mice (WT bone marrow into TLR KO mice; TLR9 KO bone marrow into WT mice). We also evaluated the production of inflammatory cytokines by mouse macrophages in response to HMGB1 and CpG-ODN. In wild-type mice after vascular injury, anti-HMGB1 antibody significantly reduced neointima formation and HMGB1 protein accelerated neointima hyperplasia. HMGB1 failed to accelerate lesion formation in TLR9 KO mice. The bone marrow transplantation study revealed that TLR9 in bone marrow-derived cells played a fundamental role in neointima formation. In vitro, HMGB1 and CpG-ODN synergistically induced the production of inflammatory cytokines by macrophages.

CONCLUSIONS

HMGB1 serves as an endogenous mediator of inflammation and lesion formation via the TLR9 pathway in response to vascular injury. Blockade of HMGB1 and/or TLR9 may represent a novel approach to treating atherosclerosis.

Inhibition of high-mobility group box 1 as therapeutic option in autoimmune disease: lessons from animal models

PURPOSE OF REVIEW

High-mobility group box 1 (HMGB1) is a molecule that has gained much attention in the last couple of years as an important player in innate immune responses and modulating factor in several (auto)immune diseases. Furthermore, advancements have been made in identifying the diverse functions that HMGB1 can play in the body by studying its receptors, pathways and effects. This review will focus on the modulation of HMGB1 in animal models of (auto)immune diseases.

RECENT FINDINGS

In different disease models like sepsis, ischemia-reperfusion and arthritis, HMGB1-blocking therapies have been tested and the disease course was shown to be ameliorated.

SUMMARY

These findings indicate that HMGB1 is an important mediator in innate immunity, inflammation and sterile injury. Furthermore, HMGB1 might be a new therapeutic target in inflammation and autoimmune diseases, which may be translated to the clinic.

HMGB1: a potential target for treatment of benign prostatic hyperplasia

Previous studies have demonstrated an increased incidence of benign prostatic hyperplasia (BPH) in men with prostatitis. In addition to androgens and age, recent studies also pointed to an important role for inflammation in causing and promoting the progression of BPH. Inflammatory infiltrates are frequently observed in prostate tissue specimens, and the degree of inflammation has been correlated with prostate volume and weight. Furthermore, a pro-inflammatory microenvironment is closely related to BPH stromal hyperproliferation and tissue remodeling, although its role in BPH remains unclear. Accumulating evidence indicates that HMGB1 acts as a potent proinflammatory cytokine that contributes to the pathogenesis of many inflammatory and infectious disorders. Experimental studies also reported that HMGB1 promotes cell chemotaxis and proliferation. These observations led us to propose that HMGB1 contributes to the progress of BPH, and that targeting the HMGB1 signaling pathway might be a new strategy to treat prostatic enlargement.

Inflammation triggers high mobility group box 1 (HMGB1) secretion in adipose tissue, a potential link to obesity

BACKGROUND

Low grade inflammation is one of the major metabolic disorders in case of obesity due to variable secretion of adipose derived cytokines called adipokines. Recently the nuclear protein HMGB1 was identified as an inflammatory alarmin in obesity associated diseases. However HMGB1 role in adipose tissue inflammation is not yet studied.

OBJECTIVES

The aim of this study was to prove the expression of HMGB1 in human adipose tissue and to assess the levels of expression between normo-weight and obese individuals. Furthermore we determined which type of cells within adipose tissue is involved in HMGB1 production under inflammatory signal.

METHODS

Western-blot was performed on protein lysates from human normo-weight and obese adipose tissue to study the differential HMGB1 expression. Human normo-weight adipose tissue, adipose-derived stromal cells (ASCs) and adipocytes were cultured and stimulated with LPS to induce inflammation. HMGB1, IL-6 and MCP-1 secretion and gene expression were quantified by ELISA and Q-PCR respectively, as well as cell death by LDH assay. HMGB1 translocation during inflammation was tracked down by immunofluorescence in ASCs.

RESULTS

HMGB1 was expressed 2-fold more in adipose tissue from obese compared to normo-weight individuals. LPS led to an up-regulation in HMGB1 secretion and gene expression in ASCs, while no change was noticed in adipocytes. Moreover, this HMGB1 release was not attributable to any cell death. In LPS-stimulated ASCs, HMGB1 translocation from nucleus to cytoplasm was detectable at 12h and the nuclear HMGB1 was completely drained out after 24h of treatment.

CONCLUSION

The expression level studies between adipose tissue from normo-weight and obese individuals together with in vitro results strongly suggest that adipose tissue secretes HMGB1 in response to inflammatory signals which characterized obesity.

HMGB1 induces an inflammatory response in endothelial cells via the RAGE-dependent endoplasmic reticulum stress pathway

The high mobility group 1B protein (HMGB1) mediates chronic inflammatory responses in endothelial cells, which play a critical role in atherosclerosis. However, the underlying mechanism is unknown. The goal of our study was to identify the effects of HMGB1 on the RAGE-induced inflammatory response in endothelial cells and test the possible involvement of the endoplasmic reticulum stress pathway. Our results showed that incubation of endothelial cells with HMGB1 (0.01-1 μg/ml) for 24h induced a dose-dependent activation of endoplasmic reticulum stress transducers, as assessed by PERK and IRE1 protein expression. Moreover, HMGB1 also promoted nuclear translocation of ATF6. HMGB1-mediated ICAM-1 and P-selectin production was dramatically suppressed by PERK siRNA or IRE1 siRNA. However, non-targeting siRNA had no such effects. HMGB1-induced increases in ICAM-1 and P-selectin expression were also inhibited by a specific eIF2α inhibitor (salubrinal) and a specific JNK inhibitor (SP600125). Importantly, a blocking antibody specifically targeted against RAGE (anti-RAGE antibody) decreased ICAM-1, P-selectin and endoplasmic reticulum stress molecule (PERK, eIF2α, IRE1 and JNK) protein expression levels. Collectively, these novel findings suggest that HMGB1 promotes an inflammatory response by inducing the expression of ICAM-1 and P-selectin via RAGE-mediated stimulation of the endoplasmic reticulum stress pathway.

The role of HMGB1 in inflammation-mediated organ injury

HMGB1 is a chromosome-binding protein that also acts as a damage-associated molecular pattern molecule. It has potent proinflammatory effects and is one of key mediators of organ injury. Evidence from research has revealed its involvement in the signaling mechanisms of Toll-like receptors and the receptor for advanced glycation end-products in organ injury. HMGB1-mediated organ injuries are acute damage including ischemic, mechanical, allograft rejection and toxicity, and chronic diseases of the heart, kidneys, lungs, and brain. Strategies against HMGB1 and its associated cellular signal pathways need to be developed and may have preventive and therapeutic potentials in organ injury.

The role of high mobility group box chromosomal protein 1 in rheumatoid arthritis

High mobility group box chromosomal protein 1 (HMGB1) is a ubiquitous highly conserved single polypeptide in all mammal eukaryotic cells. HMGB1 exists mainly within the nucleus and acts as a DNA chaperone. When passively released from necrotic cells or actively secreted into the extracellular milieu in response to appropriate signal stimulation, HMGB1 binds to related cell signal transduction receptors, such as RAGE, TLR2, TLR4 and TLR9, and becomes a proinflammatory cytokine that participates in the development and progression of many diseases, such as arthritis, acute lung injury, graft rejection immune response, ischaemia reperfusion injury and autoimmune liver damage. Only a small amount of HMGB1 release occurs during apoptosis, which undergoes oxidative modification on Cys106 and delivers tolerogenic signals to suppress immune activity. This review focuses on the important role of HMGB1 in the pathogenesis of RA, mainly manifested as the aberrant expression of HMGB1 in the serum, SF and synovial tissues; overexpression of signal transduction receptors; abnormal regulation of osteoclastogenesis and bone remodelling leading to the destruction of cartilage and bones. Intervention with HMGB1 may ameliorate the pathogenic conditions and attenuate disease progression of RA. Therefore administration of an HMGB1 inhibitor may represent a promising clinical approach for the treatment of RA.

Cerebrospinal fluid high-mobility group box protein 1 in neuromyelitis optica and multiple sclerosis

BACKGROUND

Neuromyelitis optica (NMO) and multiple sclerosis (MS) are two autoimmune inflammatory demyelinating diseases in the central nervous system. Interleukin (IL)-6 and IL-17 may play important roles in the pathogenesis of these diseases. High-mobility group box protein 1 (HMGB1) can activate the nuclear factor light chain enhancer of activated B cells and release cytokines such as IL-6 and IL-17. However, whether cerebrospinal fluid (CSF) HMGB1 levels were altered in NMO and MS patients is still unclear.

OBJECTIVES

It was our aim to measure the CSF HMGB1 concentration in NMO patients and explore their relationship with IL-6, IL-17 and disease activity.

METHODS

CSF HMGB1 was measured by enzyme-linked immunosorbent assay in NMO (n = 22) and MS (n = 18) patients as well as in controls (n = 14).

RESULTS

CSF HMGB1 was notably higher in the NMO group compared with controls (p = 0.007). CSF HMGB1 positively correlated with IL-6 and IL-17 in NMO patients (IL-6, p = 0.034; IL-17, p < 0.001).

CONCLUSIONS

In conclusion, our study suggests that CSF levels of HMGB1 are increased in patients with NMO and reflect the neuroinflammatory process.

Etanercept decreases HMGB1 expression in dorsal root ganglion neuron cells in a rat chronic constriction injury model

In the present study, we examined the effect of etanercept on high mobility group box 1 (HMGB1) expression in dorsal root ganglion (DRG) neuron cells in a rat model of chronic constriction injury (CCI) of the sciatic nerve, with the aim of exploring the molecular mechanism underlying the therapeutic effect of etanercept on sciatica-related nociception and the potential interaction between tumor necrosis factor-α (TNF-α) and HMGB1 in DRG neuron cells. A rat CCI model was employed and the animals were randomly assigned to seven groups (n=20/group): untreated, sham only, sham/saline, sham/etanercept, CCI only, CCI/saline and CCI/etanercept. Our results revealed that compared with the sham/saline and sham/etanercept groups, thermal hyperalgesia and mechanical hyperalgesia, as well as HMGB1 expression at both the mRNA and protein levels in the DRG neuron cells, were induced by CCI, and were significantly inhibited by etanercept. Although etanercept showed no significant effect on the sham group, it significantly reduced the phosphorylated p38 mitogen-activated protein kinase (MAPK) levels induced by CCI in the DRG neuron cells. In conclusion, we demonstrated that etanercept significantly decreased the HMGB1 expression induced by CCI in the DRG neuron cells. This study not only explored the molecular mechanisms underlying the therapeutic effect of etanercept on sciatica-related nociception, but also provided indirect evidence for an interaction between TNF-α and HMGB1 in DRG neuron cells.