Regulation of Posttranslational Modifications of HMGB1 During Immune Responses

High-Mobility Group Box 1 (HMGB1) Is Elevated Systemically in Persons with Acute or Chronic Traumatic Spinal Cord Injury

Inflammation in traumatic spinal cord injury (SCI) has been proposed to promote damage acutely and oppose functional recovery chronically. However, we do not yet understand the signals that initiate or prolong inflammation in persons with SCI. High-Mobility Group Box 1 (HMGB1) is a potent systemic inflammatory cytokine-or damage-associated molecular pattern molecule (DAMP)-studied in a variety of clinical settings. It is elevated in pre-clinical models of traumatic spinal cord injury (SCI), where it promotes secondary injury, and strategies that block HMGB1 improve functional recovery. To investigate the potential translational relevance of these observations, we measured HMGB1 in plasma from adults with acute (≤ 1 week post-SCI, n = 16) or chronic (≥ 1 year post-SCI, n = 47) SCI. Plasma from uninjured persons (n = 51) served as controls for comparison. In persons with acute SCI, average HMGB1 levels were significantly elevated within 0-3 days post-injury (6.00 ± 1.8 ng/mL, mean ± standard error of the mean [SEM]) or 4-7 (6.26 ± 1.3 ng/mL, mean ± SEM), compared with controls (1.26 ± 0.24 ng/mL, mean ± SEM; p ≤ 0.001 and p ≤ 0.01, respectively). In persons with chronic SCI who were injured for 15 ± 1.5 years (mean ± SEM), HMGB1 also was significantly elevated, compared with uninjured persons (3.7 ± 0.69 vs. 1.26 ± 0.24 ng/mL, mean ± SEM; p ≤ 0.0001). Together, these data suggest that HMGB1 may be a common, early, and persistent danger signal promoting inflammation in individuals with SCI.

Identification of CD163 as an antiinflammatory receptor for HMGB1-haptoglobin complexes

Secreted by activated cells or passively released by damaged cells, extracellular HMGB1 is a prototypical damage-associated molecular pattern (DAMP) inflammatory mediator. During the course of developing extracorporeal approaches to treating injury and infection, we inadvertently discovered that haptoglobin, the acute phase protein that binds extracellular hemoglobin and targets cellular uptake through CD163, also binds HMGB1. Haptoglobin-HMGB1 complexes elicit the production of antiinflammatory enzymes (heme oxygenase-1) and cytokines (e.g., IL-10) in WT but not in CD163-deficient macrophages. Genetic disruption of haptoglobin or CD163 expression significantly enhances mortality rates in standardized models of intra-abdominal sepsis in mice. Administration of haptoglobin to WT and to haptoglobin gene-deficient animals confers significant protection. These findings reveal a mechanism for haptoglobin modulation of the inflammatory action of HMGB1, with significant implications for developing experimental strategies targeting HMGB1-dependent inflammatory diseases.

The Emerging Role of HMGB1 in Neuropathic Pain: A Potential Therapeutic Target for Neuroinflammation

Neuropathic pain (NPP) is intolerable, persistent, and specific type of long-term pain. It is considered to be a direct consequence of pathological changes affecting the somatosensory system and can be debilitating for affected patients. Despite recent progress and growing interest in understanding the pathogenesis of the disease, NPP still presents a major diagnostic and therapeutic challenge. High mobility group box 1 (HMGB1) mediates inflammatory and immune reactions in nervous system and emerging evidence reveals that HMGB1 plays an essential role in neuroinflammation through receptors such as Toll-like receptors (TLR), receptor for advanced glycation end products (RAGE), C-X-X motif chemokines receptor 4 (CXCR4), and N-methyl-D-aspartate (NMDA) receptor. In this review, we present evidence from studies that address the role of HMGB1 in NPP. First, we review studies aimed at determining the role of HMGB1 in NPP and discuss the possible mechanisms underlying HMGB1-mediated NPP progression where receptors for HMGB1 are involved. Then we review studies that address HMGB1 as a potential therapeutic target for NPP.