The Janus face of HMGB1 in heart disease: a necessary update

Abstract

High mobility group box 1 (HMGB1) is a ubiquitous nuclear protein involved in transcription regulation, DNA replication and repair and nucleosome assembly. HMGB1 is passively released by necrotic tissues or actively secreted by stressed cells. Extracellular HMGB1 acts as a damage-associated molecular pattern (DAMPs) molecule and gives rise to several redox forms that by binding to different receptors and interactors promote a variety of cellular responses, including tissue inflammation or regeneration. Inhibition of extracellular HMGB1 in experimental models of myocardial ischemia/reperfusion injury, myocarditis, cardiomyopathies induced by mechanical stress, diabetes, bacterial infection or chemotherapeutic drugs reduces inflammation and is protective. In contrast, administration of HMGB1 after myocardial infarction induced by permanent coronary artery ligation ameliorates cardiac performance by promoting tissue regeneration. HMGB1 decreases contractility and induces hypertrophy and apoptosis in cardiomyocytes, stimulates cardiac fibroblast activities, and promotes cardiac stem cell proliferation and differentiation. Interestingly, maintenance of appropriate nuclear HMGB1 levels protects cardiomyocytes from apoptosis by preventing DNA oxidative stress, and mice with HMGB1cardiomyocyte-specific overexpression are partially protected from cardiac damage. Finally, higher levels of circulating HMGB1 are associated to human heart diseases. Hence, during cardiac injury, HMGB1 elicits both harmful and beneficial responses that may in part depend on the generation and stability of the diverse redox forms, whose specific functions in this context remain mostly unexplored. This review summarizes recent findings on HMGB1 biology and heart dysfunctions and discusses the therapeutic potential of modulating its expression, localization, and oxidative-dependent activities.

Keywords: Alarmin; Biomarker; Inflammation; Oxidative stress; Regeneration.

Fig. 1

Structure and redox modifications of HMGB1. a HMGB1 comprises two DNA-binding domains, named A and B box, and a C-terminal acid tail connected by linker regions. HMGB1 has two lysine-rich nuclear localization sequences, NLS1 and NLS2, localized in the A box and in the linker region between the B box and the acidic tail, respectively. The domains recognized by TLR4 and RAGE are depicted. Three redox forms of HMGB1 depend on the redox conditions of the environment. The intracellular fully reduced HMGB1 (fr-HMGB1) with the three conserved cysteines in the reduced thiol state can be oxidized in the extracellular space to disulfide HMGB1 (ds-HMGB1), characterized by a disulfide bond between C23 and C45, and a thiol C106, that after further oxidation can give rise to the sulfonyl HMGB1 (ox-HMGB1) with cysteines carrying the sulfonyl group. b The non-oxidizable HMGB1 3S mutant. Recombinant 3S has been generated by substitution of cysteines with serine residues (S23–S45–S106)

Fig. 2

Extracellular functions of HMGB1 redox forms. After tissue damage or infection, non-acetylated fr-HMGB1 leaks out from necrotic cells. Acetylated (Ac) fr-HMGB1 is actively secreted by local immunocompetent and infiltrating immune cells upon inflammasome activation by PAMPs, DAMPs or pro-inflammatory stimuli. On the contrary, apoptotic chromatin tightly retains HMGB1. Whether acetylation of HMGB1 affects extracellular activity of HMGB1 is still unknown. Fr-HMGB1 interacts with CXCL12 to activate CXCR4-mediated cell migration, proliferation and differentiation to promote tissue healing and regeneration. HMGB1 also binds to RAGE to induce further production of CXCL12 and migration. MAPKs and NF-κB pathways are involved in these processes. In presence of reactive oxygen species (ROS), fr-HMGB1 is partially oxidized to ds-HMGB1 that binds to the TLR4-MD2 complex to stimulate the release of inflammatory and angiogenic factors through the activation of NF-κB. Further oxidation of ds-HMGB1 to sulfonyl ox-HMGB1 is associated mainly with the resolution of inflammation

Fig. 3

The non-oxidizable 3S mutant interacts directly with CXCR4. Fr-HMGB1 interacts with CXCL12 to promote cell migration and proliferation via CXCR4. A blocking antibody to CXCL12 or the CXCR4/CXCL12 inhibitor AMD3100 as well as the presence of H₂O₂ abolish fr-HMGB1 activities. On the contrary, 3S binds directly to CXCR4 in a CXCL12-independent manner and is more effective that fr-HMGB1 in inducing fibroblast migration mediated by Src activation and myoblast proliferation. It is likely that 3S recognizes a different site in the receptor compared to CXCL12, since neither AMD3100 nor an anti-CXCL12 antibody effectively block 3S-induced migration. Since 3S cannot be converted to oxidized HMGB1 forms, its chemotactic activity lasts in the presence of H₂O₂

Fig. 4

Fr-HMGB1 and 3S exert opposite effects in infarcted hearts. In an experimental model of myocardial infarction induced by permanent coronary ligation, fr-HMGB1 injection reduces the infarcted area and improves cardiac function because is able to promote angiogenesis and differentiation of resident cardiac stem cells (CPCs) into cardiomyocytes. The release of ROS subsequent to the infarction may progressively oxidize fr-HMGB1 to ds-HMGB1 and then to ox-HMGB1, which is important for the regenerative effect of HMGB1. On the contrary, the injection of the non-oxidizable 3S mutant reduces angiogenesis and causes an increase in the infarcted area and collagen deposition, leading to the worsening of cardiac dysfunction

Fig. 5

Effect of HMGB1 blocking during cardiac ischemia/reperfusion (I/R) damage. The cartoon indicates the protective or the detrimental consequences of administering HMGB1 antagonists or recombinant HMGB1 protein at different timing—pre-ischemic or post-ischemic phase—during I/R

Fig. 6

HMGB1 in cardiac dysfunctions: new perspective. Ischemia/reperfusion, cardiotoxic drugs, hyperglycemia, microbial infection, autoimmune responses or mechanical pressure produce oxidative and nitrosative stress that, in turn, induce tissue necrosis with consequent fr-HMGB1 passive release or acetylated fr-HMGB1 secretion from activated cardiac and recruited inflammatory cells. The extracellular fr-HMGB1 undergoes progressive oxidation and yet not specified redox forms (?) may exacerbate inflammation and induce cell apoptosis, cardiomyocytes (CM) hypertrophy and activation of cardiac fibroblasts (CF) to produce Collagen. These cell responses determine the development of cardiac hypertrophy and/or fibrosis and eventually heart failure. Modulation of the oxidative state of HMGB1 could be a strategy to limit inflammation and damage, and favor tissue repair. Furthermore, cardiac injuries lead to an increase in the blood levels of HMGB1 and the different modified forms of the protein may be associated with different disease stages, and could represent selective prognostic biomarkers of the extent of cardiac damage and help in risk stratification