The many faces of HMGB1: molecular structure-functional activity in inflammation, apoptosis, and chemotaxis

Review of recent advances on post‐translational modifications that determine the role of HMGB1 as a cytokine mediator. HMGB1 is a ubiquitous nuclear protein present in almost all cell types. In addition to its intracellular functions, HMGB1 can be extracellularly released, where it mediates activation of innate immune responses, including chemotaxis and cytokine release. HMGB1 contains three conserved redox‐sensitive cysteines (C23, C45, and C106); modification of these cysteines determines the bioactivity of extracellular HMGB1. Firstly, the cytokine‐stimulating activity of HMGB1 requires C23 and C45 to be in a disulfide linkage, at the same time that C106 must remain in its reduced form as a thiol. This distinctive molecular conformation enables HMGB1 to bind and signal via the TLR4/MD‐2 complex to induce cytokine release in macrophages. Secondly, for HMGB1 to act as a chemotactic mediator, all three cysteines must be in the reduced form. This all‐thiol HMGB1 exerts its chemotactic activity to initiate inflammation by forming a heterocomplex with CXCL12; that complex binds exclusively to CXCR4 to initiate chemotaxis. Thirdly, binding of the HMGB1 to CXCR4 or to TLR4 is completely prevented by all‐cysteine oxidation. Also, the initial post‐translational redox modifications of HMGB1 are reversible processes, enabling HMGB1 to shift from acting as a chemotactic factor to acting as a cytokine and vice versa. Lastly, post‐translational acetylation of key lysine residues within NLSs of HMGB1 affects HMGB1 to promote inflammation; hyperacetylation of HMGB1 shifts its equilibrium from a predominant nuclear location toward a cytosolic and subsequent extracellular presence. Hence, post‐translational modifications of HMGB1 determine its role in inflammation and immunity.