High Mobility Group Box Protein 1 (HMGB1): The Prototypical Endogenous Danger Molecule (1)
The detection of extracellular HMGB1 as a delayed mediator after LPS stimulation. The exposure of cultured human monocytes to HMGB1 stimulated the release of multiple proinflammatory cytokines including tumor necrosis factor (TNF), interleukin (IL)-1, IL-6, IL-8 and macrophage inflammatory protein (MIP)-1. HMGB1-induced TNF release is biphasic with a delayed second wave, whereas LPS-mediated TNF release only occurs in a monophasic early mode. HMGB1 is involved in DNA-containing complex– mediated immune responses via TLR9. Extracellular HMGB1 carrying nucleic acids binds to the receptor for advanced glycation end products (RAGE) and gets internalized via dynamin-dependent endocytosis, which enables the transported nucleic acids to interact with intracellular receptors to mediate interferon and cytokine responsesHMGB proteins play an essential role as universal sentinels in nucleic acid–activated innate immune responses. Intestinal epithelial cells were recently identified as the main source for the detrimental HMGB1 release in an experimental model of hemorrhagic-induced systemic inflammation. Increased extracellular HMGB1 expression has also been observed in several addi- tional sterile injury models, including collagen-induced arthritis or during the spontaneous development of arthritis in mice.
HMGB1 contains 215 amino acids (aa) with two folded DNA binding motifs called box A (aa 9–79) and box B (aa 95–163) and an acidic C tail (aa 186–215). HMGB1 has two nuclear localization sequences (NLSs) located in box A (aa 28–44) and between box B and the C tail (aa 179–185). HMGB1 receptor usage and subse- quent biological activities depend on the redox state of each of its three cysteines (C23, C45 and C106).
Fully reduced HMGB1, which expresses three cysteine thiol residues, exerts chemotactic activity by forming a hetero-complex with CXCL12, which binds to the CXCL12-reciprocal receptor CXCR4 and initiates chemotaxis in a synergistic fashion compared with CXCL12 alone. The cytokine- stimulating activity of HMGB1 requires C23 and C45 to form a disulfide link, whereas C106 must express a thiol group. The fully reduced form of HMGB1 with or without CXCL12 can not activate the TLR4/MD-2 signaling pathway, and the disulfide HMGB1 can not activate the CXCL12/CXCR4 pathway.
The initial step is to translocate nuclear HMGB1 to the cytoplasm, which depends on JAK–STAT signaling that will generate hyperacetylation of critical lysine residues located in the two NLS sites. Proinflammatory cell death (pyroptosis) allows cytoplasmic HMGB1 to reach the extracellular space or alternatively via exocytosis of secretory lysosomes that deliver HMGB1 outside cells. Inflammasome- induced HMGB1 release leads to extra- cellular, hyperacetylated HMGB1, which is thus a novel biomarker for pyroptotic cell death. Necrotic as well as apoptotic cell deaths do not generate hyperacetylated HMGB1. The redox state of HMGB1 released after pyroptosis is generally in the disulfide form, after necrosis in the fully reduced or disulfide forms and after apoptosis in the fully oxidized form.
At least 11 different HMGB1 receptors have been described. TLR4/MD-2 is a mandatory HMGB1 receptor complex for cytokine production in macrophages and the interaction requires the disulfide HMGB1 redox isoform, which binds the TLR4 coreceptor MD-2 with nanomolar avidity, just like LPS but at another MD-2 site. RAGE binding site to be located in the HMGB1 sequence 150–183. An additional RAGE-binding epitope in HMGB1 lo- cated in the box A domain (binding re- gion sequence 23–50). This observation suggests the possibility that HMGB1 may mediate different biological functions when interacting with RAGE depending on which of the two RAGE- binding epiotopes is involved.
