Abstract
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.
Keywords
HMGB1 prote; ininflammation; organ injuries;
1. Introduction
The high-mobility group box 1 protein (HMGB1) is a ubiquitously expressed nuclear factor and an important mediator of inflammation via receptors of the innate immune system. The inflammatory nature of organ and tissue injury from chemical or physical stress makes HMGB1 a crucial factor in the pathogenesis of acute and chronic diseases and injury. HMGB1, also a universal nucleic acid sensor, is able to detect exogenous nucleic acids in both the extracellular and intracellular spaces,1, 2 activating Toll-like receptors (TLRs). Its interaction with other small molecules such as p53 in a Beclin-1-dependent manner to modulate cell longevity has also been a subject of interest in cancer research,3 although this is beyond the scope of this review. Specifically, we focus here on and summarize the role of HMGB1 in acute and chronic organ injuries.
2. HMGB1 and inflammation
HMGB1 signals for the production of proinflammatory cytokines and chemokines though TLRs and the receptor for advanced glycation end-products (RAGE). Being present both intracellularly and extracellularly, HMGB1 triggers inflammation through direct binding to TLR4, or by formation of complexes with exogenous or endogenous molecules in the cytoplasm or extracellular space.4, 5 The location and post-translational modification state of HMGB1 determine its physiologic action. Under normal conditions, it is responsible for maintenance of the chromosomal architecture, regulating processes in the genome.6 HMGB1 is trafficked into vesicles via a nonclassical pathway, awaiting caspase-1-dependent secretion when immunologic cells become activated.7 In other cells, it is an alarmin, a signal for damaged self.
In the event of infectious or sterile damage, the immune response is triggered primarily by pathogen-associated molecular pattern (PAMP) or damage-associated molecular pattern (DAMP) molecules. These rely on transduction by PAMP/DAMP receptors, resulting in the expression of proinflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ), and interleukin (IL) IL-1β and IL-6.8 The nature of HMGB1-dependent TNF induction and release is different, as it is monophasic in response to endotoxic insult, but biphasic if mediated by HMGB1.9
The primary target of extracellular HMGB1 is TLR4, leading to the translocation of nuclear factor kappa B (NFκB) to the nucleus, and activation of interferon regulatory factor 3 (IRF3) and activator protein 1 (AP-1) to produce the inflammatory cytokine repertoire. It is said that HMGB1 is a late mediator of infectious damage, but an early one of sterile damage. Upon immune cell activation, HMGB1 expression and secretion takes 8–12 hours.10 The positive feedforward nature of immunologic signaling by HMGB1 is intended to prime and amplify the immune response through cytokine induction during stress, infection, or hypoxia.11 The induction of chemokines by immunocytes during this inflammatory process is stromal cell-derived factor-1 (CXCL12) dependent.12 In addition to other TLRs (2 and 9) that can also be activated by HMGB1, RAGE is another of its targets that is able to mediate chemotaxis, cell growth, differentiation, and its own upregulation.13 Despite its strong proinflammatory effect, its role in suppressing inflammation by interacting with sialic acid-binding immunoglobulin-like lectin 12 (Siglec12) in CD24+ immunocytes to prevent NFκB translocation to the nucleus has also been studied.14
3. Inflammation and organ injury
Damage can originate from internal and external events, resulting in mechanical or chemical/molecular stress, and triggering an inflammatory response. Despite initiating recovery and healing, inflammation causes organ injury by processes such as tissue destruction, remodeling, fibrosis, and others. Current literature indicates that HMGB1 plays an important role in mediating organ injury. For example, the marked increase in HMGB1 expression, both locally and/or systemically depending on the disease, has been documented. Damage caused applies to both acute and chronic diseases, and is proportional the amount of HMGB1 secreted.10
4. Current understanding of HMGB1-mediated vital organ injury
4.1. Ischemia and reperfusion injury
Ischemia/reperfusion (I/R) injury is characterized by an inflammatory response initiated by the restoration of blood flow to hypoxic tissue under oxidative stress; prolonged ischemia can cause necrosis. Rather than restoring tissue function, oxidative damage occurs from the induction and accumulation of reactive oxygen species (ROS) from stressed cells. The pathologic effect of HMGB1 in ischemia and I/R is attributed to TLR4/RAGE-dependent ROS production in the solid vital organs.15 HMGB1 is expressed in elevated levels in patients with organ injury or transplantation; hence it is considered a marker of injury.16 Although the pathogenesis of ischemic organ injury is attributed to both TLR4 and RAGE activation, differences in their signaling pathways in different tissues are important.
4.1.1. Liver
In the liver, HMGB1 signals for inflammation via TLR4 and calcium-mediated signaling.17, 18 The induction of TNF-α and IL-6 and the upregulation of HMGB1 receptors in both ischemic (however more prominently) and nonischemic lobes have been detected.17 In I/R injury, the primary defense mechanism for oxidative damage-induced cellular damage is elevation of manganese superoxide dismutase.19 IL-1R-associated kinase-M-dependent suppression of TLR4 signaling was also found to dampen inflammation in mice when pretreated with HMGB1.20 The release of HMGB1 and macrophage migration inhibitory factor, a coexpressed DAMP, is indicative of the extent of liver damage.21
4.1.2. Heart
RAGE is the prominent signaling pathway in cardiac I/R injury, signaling for the expression of proinflammatory genes via mitogen-activated protein kinase (MAPK), the JAK-STAT signaling family, and others22, 23; TLR4 is also activated. An array of enzymes and molecules produced in response to a hypoxic insult, such as lactate dehydrogenase, creatine kinase, superoxide dismutase (SOD), and malondialdehyde, can be used as markers to determine the extent of cardiac damage, corresponding to infarct size.24, 25 The inhibition of HMGB1 by, for example, geranylgeranylacetone (a small molecule inhibitor) therefore lowers the expression of these markers.24 After I/R injury, there is an increase in the release of advanced glycation end-products (AGEs) that also increases HMGB1 expression, causing inflammation.23 The acute damage initiated by ischemia or I/R is also thought to be a possible initiator of chronic cardiac inflammatory diseases such as hypertrophy.26 However, in preventing I/R injury, hypoxic cardiac reoxygenation in an initial reperfusion has been shown to decrease oxidative damage as a result of HMGB1 downregulation.27, 28 The protective effects also extend to the respiratory system, in the event of cardiopulmonary bypass being used.27
4.1.3. Kidney
Renal ischemic or I/R injury is significant after transplantation, and the expression of HMGB1 and other injurious molecules plays a crucial role in determining the success or rejection of transplantation.29, 30, 31 The injury is attributed significantly to HMGB1- and TLR4-mediated inflammation that destroys the tubular architecture and induces cell death32; this is mediated at least partly in an alpha-2-adrenoreceptor-dependent manner that suppresses the inflammatory feedforward circuit.33 In acute ischemic injury, there is an increase in TLR4 expression in the proximal tubule endothelial cells near the cortex and outer medulla.34 Renal HMGB1 expression is the key to IL-6 release by infiltrating macrophages that promote inflammatory cytokine production.35 Furthermore, renal endothelial activation of TLR4 induces the expression of NKG2D ligands that renders them susceptible to cytotoxic killing.36
4.1.4. Nervous system
Abundant literature on ischemic and I/R injury in the cerebral and nervous systems suggests that HMGB1 is integral to organ injury. HMGB1 has cytokine-like functions in tissues of the nervous system, and is found in activated microglia and astrocytes, as well as the microvasculature of the infarction core.37 The number of TLRs, especially TLR4, and the signaling strength are proportional to the extent of the damage and the infarct size.38, 39, 40 Similarly to I/R cardiac injury, hypoxic reperfusion techniques have been used in spinal cord reperfusion, such as the use of hydrogen gas.41 Hemostasis in reperfusion is important to postischemic brain injury as large von Willebrand factor multimers have an HMGB1-associated proinflammatory function that is exacerbated by ADAMTS13–/–.42 HMGB1 release from glial cells is partly mediated by the alpha7 nicotinic acetylcholine receptor (α7nAChR), whose expression directly correlates to the number of apoptotic cells in the ischemic penumbra.43 The inhibition of ischemia-induced apoptosis is also seen by the activation of hemeoxygenase-1 via phosphoinositide 3-kinase/Akt signaling pathways and nuclear factor (erythroid-derived 2)-like 2 (Nrf-2) translocation.44 Not only are the nerves susceptible, but the eye is also subject to HMGB1- and RAGE-mediated retinal damage.45 Furthermore, neuronal zymogen matrix metalloproteinase 9 activation in response to HMGB1 release is another key contributor to tissue damage and the release of neurotoxic ligands.46 Glutamate-induced cytotoxicity is another mechanism of damage following acute ischemic damage that can propagate through neural tissue, forming the ischemic penumbra.47
Acute organ injury from ischemia or I/R injury has been investigated in other organs and for quantitative assaying of cellular damage as well, such as in islet cell preparations for xenotransplantation.48 The different tissue types and expression of HMGB1 in ischemia and I/R organ injury are subject to distinct tissue types and signaling mechanisms.
4.2. Organ injury from chronic and atherosclerotic damage
HMGB1 is also a key contributor mediating organ injury in chronic inflammatory diseases. Recent research has paid close attention to its importance in the pathogenesis of atherosclerosis and other cardiomyopathies, pulmonary diseases such as chronic obstructive pulmonary disease (COPD), and diabetes. The signaling mechanisms of DAMP receptors contribute differently to these diseases.
4.2.1. Atherosclerosis and chronic cardiomyopathies
Atherosclerosis is a chronic cardiovascular disease that arises from plaque formation on the arterial walls that is composed of infiltrated macrophages, oxidized cholesterol, and calcification.49, 50 HMGB1 alteration of endothelial adhesion, particularly the expression of intercellular adhesion molecule 1, vascular cell adhesion molecule 1, and monocyte chemotactic protein 1, which is crucial in inflammocyte recruitment, in addition to the conventional array of proinflammatory cytokines it induces, is responsible for lesion formation and growth.49 Plaque growth is a complex process that is fueled by inflammation significantly caused by HMGB1; it includes entrapment of low-density lipoprotein in the endothelia, monocyte recruitment, and foam cell formation as a result of monocytic internalization of lipids. The growing plaque is stabilized by the secretion of metalloproteinases, forming a fibrous cap, along with other cytokines, ROS, and growth factors.51 The symptoms then manifest once the plaque ruptures from instability, causing thrombosis.
A key characteristic of progressing atherosclerotic lesions is the lipid or necrotic core, which may contain dying macrophages, where intense levels of HMGB1 expression are found.52 Not only is the elevation of HMGB1 expression found in macrophages, but also in the vascular smooth muscle cells of the tunica intima and adventitia, and in the endothelia of vessels that contain atherosclerotic lesions.53, 54 RAGE is thought to be the main HMGB1 receptor responsible for chronic inflammation, as its expression is elevated in all cells involved in atherosclerosis,55 and as elevated AGEs are also thought to contribute greatly to the initial endothelial transformation.56 However, the importance of TLR4 signaling must not be neglected, as its attenuation (e.g., by statins) decreases inflammation and plaque growth to a remarkable extent,57 partly by inhibiting IL-6 secretion.58
HMGB1 is also integral to the pathogenesis of other chronic inflammatory-mediated cardiac diseases. Chronic myocarditis is a chronic inflammatory disorder that involves ventricular remodeling, hypertrophy, and fibrosis that may potentially be mediated by HMGB1 in a mainly RAGE-dependent mechanism similar to those mentioned regarding atherosclerosis26; this also includes ventricular remodeling postmyocardial infarction. Furthermore, HMGB1 is also a marker for different levels of fibrosis in chronic hepatitis.59
4.2.2. Chronic pulmonary disease
Chronic inflammation is involved in the pathogenesis of many pulmonary diseases, such as COPD, oxidative stress, protease–antiprotease imbalance and inflammation, the pathogenic triad of COPD whose features include chronic bronchitis and emphysema and have been shown to involve HMGB1.60 Elevated levels of HMGB1 are found in the peripheral airways of patients with COPD, more so in smokers than nonsmokers; the levels of expressed IL-8 and polymorphonuclear elastase, important for inflammocyte recruitment and airway destruction, were also elevated in a similar manner.61, 62 The deficiency of soluble RAGE to compete with those receptors expressed on pulmonary/inflammatory cells also contributes to the pathogenesis of COPD.63
4.2.3. Diabetes-related diseases
Other chronic diseases to which HMGB1-mediated inflammation may contribute are diabetes-related illnesses. The development of diabetic nephropathy is an inflammatory process, marked by an increase in TLR4 expression on renal tubular cells.64 Like that in the arteries, glomerulosclerosis from tubular fibrosis and injury is caused by the expression of proinflammatory molecules, in particular chemokine (C–C motif) ligand 2 (CCL2), which causes monocyte/macrophage infiltration. In addition, hyperglycemic states can increase TLR2 and TLR4 expression in monocytes.65
RAGE and TLR4 are responsible not only for glomerulosclerosis, but also for diabetic atherosclerosis and cardiovascular diseases.66, 67 Due to the accumulation of AGEs in diabetic kidney disease, RAGE has been greatly implicated in glomerular, tubular, and vascular dysfunction, especially in the loss of podocyte cells, causing proteinuria.68 HMGB1-dependent RAGE signaling is highly toxic to pancreatic beta islets, resulting in induction of apoptosis and inhibition of insulin secretion. This ultimately results in the perpetuation and exacerbation of type 2 diabetes mellitus.69 Moreover, HMGB1 has been found to be involved in diabetic allodynia by causing astrocytotic activation.70
4.2.4. Chronic kidney diseases
Other chronic kidney diseases that are not related to diabetes are also subject to HMGB1-mediated organ injury. It has been shown that attenuating HMGB1 could effectively treat chronic kidney disease with sepsis in mice, but not sepsis alone, suggesting that it is an important mediator in chronic kidney disease.71 HMGB1-dependent RAGE activation has been shown to be integral to amplifying vascular damage in chronic kidney diseases.68 It was also correlated with indicators of inflammation and malnutrition in end-stage renal patients,72 and correlated directly with renal function atherosclerosis in uremia.73 Chronic damage by HMGB1 includes fibrosis, structural damage, sclerosis, and granulomatous inflammatory lesions that are caused by DAMP receptor signaling.74 TLR2 has been especially attributed to tubular renal matrix accumulation and injury, in addition to TLR4, TLR9, and RAGE signaling.75
4.2.5. Neuroinflammation and neurodegenerative diseases
Evidence from research has supported the role of HMGB1 in neurodegenerative diseases via activation of RAGE in Alzheimer's disease, and TLR4 in multiple sclerosis. Controlled neuroinflammation is important to normal neurologic function, as inflammatory cytokines such as IFN-γ are important in decreasing neuronal oxidative stress by inducing gamma-interferon inducible lysosomal thiol reductase (GILT) and superoxide dismutase 2 (SOD2) activity.76 However, deregulation of neuroinflammation by HMGB1 increases inflammation to toxic levels, and is a cause of pathology.77 Neuronal dysfunction and cognitive deficits are hallmarks of neuroinflammation that are significantly caused by TLR4-dependent TNF-α secretion by activated microglia.77 Hyperexcitability is another hallmark of neuroinflammation that is directly linked to apoptosis. TLR-mediated signaling by IL-1β, a cytokine induced by HMGB1, has been shown to be involved in increasing the hyperexcitation of epileptogenic tissue through modulating post-translational changes in voltage-gated ion channels and transcription of genes affecting neurotransmission and neuroplasticity.78 Mechanistic studies have shown that microglial Mac1 receptors are also able to interact with HMGB1 to express NFκB and NADPH oxidase, producing inflammatory and neurotoxic factors.79 TLR and Mac1 are implicated greatly in the neurodegenerative processes underlying multiple sclerosis, Parkinson's disease, and seizures.
RAGE has also been subjected to vast research in the pathogenesis of Alzheimer's disease as it is able to interact with the neurotoxic beta-amyloid peptide. The ability of HMGB1 and other RAGE ligands to activate and upregulate the expression of RAGE makes it a key contributor to exacerbations of neurotoxicity in Alzheimer's disease.80 RAGE is responsible for transcytosis of beta-amyloid across the blood–brain barrier, and inflammatory signaling resulting in the expression of NFκB and neurotoxic cytokines.81, 82 Its activation has also been implicated as a cause of beta-amyloid aggregation and plaque formation, inducing apoptosis by oxidative stress, in addition to the induction of the proinflammatory milieu in microglia.83 The ability of HMGB1 to interact with TLRs, RAGE, and Mac1 in a neurologic setting makes it a key mediator of neuroinflammation and neuronal injury.
4.2.6. Mechanical stress and systemic inflammation
Mechanical or shear/traumatic stress is the basis of organ damage from accidents or from surgery. In clinical settings where the patient is subjected to significant physical trauma, the systemic inflammatory response syndrome is often present, which can lead to multiple organ failure/damage; the expression of IL-6 and TNF-α has been shown to be induced by HMGB1, followed by increased RAGE expression at the site of injury.84 The risk for developing sepsis from multiple organ dysfunction can be assessed by single-nucleotide polymorphisms present in the HMGB1 gene.85 The plasma levels of HMGB1 are correlated to mast cell activation, but it is still unknown whether this is due to less organ damage in mast cell-deficient mice, or to regulation of HMGB1 release by mast cells.86
In surgical procedures of the liver that involve compression or clamping, organ injury can arise not only from ischemic damage, but from trauma as well. Neutrophil elastase-mediated damage is directly linked with the chemotactic properties of HMGB1 signaling.87 Experimental data suggest that mechanical stretch in the alveolar epithelial cells stimulates the expression of HMGB1 and the activity of MKK6, an MAPK known to regulate apoptosis.88, 89 The predictive value of HMGB1 levels for lung injury, sepsis, and multiple organ injury is also shown in postoperative thoracic esophagectomy patients, as those with complications had higher postoperative serum HMGB1 levels.90 The pathogenesis of injury of different organs arising from systemic inflammatory responses seems to be due to the feedforward hyperstimulation of TLR4- and RAGE-mediated mechanisms that were discussed previously for HMGB1.
4.3. Acute toxicity
Other forms of acute organ injury that are not related to ischemic damage are also influenced by HMGB1. Allograft rejection and drug/nondrug-related toxicity are two major areas that have been reported frequently in recent literature.
4.3.1. Allograft rejection
The role of HMGB1 as an alarmin in allograft rejection and the success of organ transplantation has been investigated. The secretion of IL-6 from dendritic cells that induces the expression of IL-17 in alloreactive T-cells has been shown in early-phase acute cardiac allograft rejection, usually followed by T helper type 1 cell responses that produce IFN-γ, provoking graft destruction.91 Nonchronic fibrosing conditions are the result of TLR2/4 signaling in both chronic and acute transplant settings, directly and significantly affecting renal allograft function.92 It has also been shown that chronic vasculopathies result from HMGB1-dependent RAGE signaling that produces vascular endothelial growth factor.93 Both HMGB1 and its cytokine IL-1β are responsible for orchestrating the strength of rejection reactions.94 Being involved in the exacerbation of allograft rejections of many organs, such as the liver, kidney, heart, islet cells, and vessels, HMGB1 can be used as a marker for transplant success before and after organ harvest.30, 95, 96
4.3.2. Acute toxicity and lesions
Acute toxicity has also been reported to damage organs through HMGB1 release. In acetaminophen-induced hepatotoxicity, HMGB1 is a marker for apoptotic and necrotic cell death that is used to assess survival.97 Cytoplasmic translocation of HMGB1 occurs in stressed hepatocytes, and HMGB1 is also released from dead cells.98 More importantly, the extent of acute damage is determined by hepatocyte expression of CXCR3 crucial for immunocyte infiltration.99 Although recent research has focused on the liver in acute toxicity, acute inflammatory lesions generally signal via TLR4, implying that HMGB1 is a key player in the disease process.100
5. HMGB1 and the prevention of organ damage
Besides conventional inhibition of HMGB1 or its receptors by antibodies, much of the current literature tries to elucidate different mechanisms of HMGB1 release and signaling, revealing different methods of inhibiting its release, or the damage that is caused. These methods include splenectomies, small-molecule inhibitors, anti-HMGB1 antibodies, and competitive agents that decrease DAMP receptor activation. HMGB1 preconditioning has been shown to decrease the inflammatory response by activating the natural signal-dampening mechanism.20 Techniques of reperfusion, such as hypoxic reoxygenation and thermoregulation, have also been shown to decrease inflammation.17, 28
As for xenobiotics, statins and ethyl pyruvate inhibit the cytokine response and decrease HMGB1 release, respectively.58, 101 Small inhibitors like glycyrrhizin, geranylgeranylacetone and dexmedetomidine, neutrophil elastase inhibitors, and lysosomal thiol reductase inhibitors, have recently been studied and shown to inhibit HMGB1-mediated organ injury.33, 43, 76, 87, 102 The current discovery of receptors involved in HMGB1-mediated organ injury has also led to alpha-2-adrenergic receptor inhibition by dexmedetomidine and electroacupuncture for α7nAChR attenuation to decrease inflammation.33, 43 Furthermore, using soluble RAGE to compete for HMGB1-dependent RAGE activation has also been shown to decrease organ injury.63 The study of HMGB1 and its signaling pathways continues to increase the possibilities in drug design and therapy to prevent and/or treat organ injury.
6. Conclusions
HMGB1 is a key mediator of inflammation that results in both acute and chronic organ injury. Although vital to a healthy immune system and initiating healing, its multireceptor interaction with TLRs and RAGE, and its feedforward mechanistic nature, is the key to its role in the pathogenesis of diseases, resulting in damage to the vital organs.