HMGB1 is a critical molecule in the pathogenesis of Gram-negative sepsis

Abstract

Gram-negative sepsis is a severe clinical syndrome associated with significant morbidity and mortality. Lipopolysaccharide (LPS), expressed on Gram-negative bacteria, is a potent pro-inflammatory toxin that induces inflammation and coagulation via two separate receptor systems. One is Toll-like receptor 4 (TLR4), expressed on cell surfaces and in endosomes, and the other is the cytosolic receptor caspase-11 (caspases-4 and -5 in humans). Extracellular LPS binds to high mobility group box 1 (HMGB1) protein, a cytokine-like molecule. The HMGB1-LPS complex is transported via receptor for advanced glycated end products (RAGE)-endocytosis to the endolysosomal system to reach the cytosolic LPS receptor caspase-11 to induce HMGB1 release, inflammation, and coagulation that may cause multi-organ failure. The insight that LPS needs HMGB1 assistance to generate severe inflammation has led to successful therapeutic results in preclinical Gram-negative sepsis studies targeting HMGB1. However, to date, no clinical studies have been performed based on this strategy. HMGB1 is also actively released by peripheral sensory nerves and this mechanism is fundamental for the initiation and propagation of inflammation during tissue injury. Homeostasis is achieved when other neurons actively restrict the inflammatory response via monitoring by the central nervous system and the vagus nerve through the cholinergic anti-inflammatory pathway. The neuronal control in Gram-negative sepsis needs further studies since a deeper understanding of the interplay between HMGB1 and acetylcholine may have beneficial therapeutic implications. Herein, we review the synergistic overlapping mechanisms of LPS and HMGB1 and discuss future treatment opportunities in Gram-negative sepsis.

Keywords: Caspase-11; High mobility group box 1 (HMGB1); Lipopolysaccharide (LPS); Receptor for advanced glycated end products (RAGE); Sepsis; Toll-like receptor 4 (TLR4).

Figure 1

LPS binds to MD-2 and activates the TLR4/MD-2 receptor complex via two separate intracellular signal pathways that mediate HMGB1 release. Pro-inflammatory cytokines are formed when the adaptor molecule TIRAP gets associated with TLR4 and the MyD88 that activates the NF-κB signaling pathway. Interferon-β is produced when TRAM associates with TRIF. HMGB1: High mobility group box 1; LPS: Lipopolysaccharide; MD-2: myeloid differentiation factor 2; MyD88: Myeloid differentiation factor 88; NF-κB: Nuclear factor-kappa B; TIRAP:  Toll/interleukin-1 receptor(TIR) domain-containing adapter protein; TLR4: Toll-like receptor 4; TRAM: TRIF-related adaptor molecule; TRIF: TIR-domain-containing adapter-inducing interferon-β.

Figure 2

LPS needs HMGB1 to trigger fulminant inflammation. The initial event in LPS toxicity is due to extracellular LPS activation of cell surface TLR4, which generates the release of many pro-inflammatory molecules including HMGB1. HMGB1 has two LPS-binding sites and thus forms an extracellular HMGB1–LPS complex that gets endocytosed via RAGE to finally reach the cytosol culminating in activation of the intracellular LPS sensor caspase-11 (caspases-4 and −5 in humans) that instigates inflammation and coagulation. HMGB1: High mobility group box 1; IL: interleukin; LPS: Lipopolysaccharide; RAGE: Receptor for advanced glycated end products; TLR4: Toll-like receptor 4.

Figure 3

HMGB1 structure and location of binding sites for the HMGB1-receptors TLR4 and RAGE, heparin, and LPS. HMGB1: High mobility group box 1; LPS: Lipopolysaccharide; RAGE: Receptor for advanced glycated end products; TLR4: Toll-like receptor 4.

Figure 4

Intracellular HMGB1 localization in macrophages before (A) and after (B) 24 h-LPS activation. HMGB1, visualized with green immunofluorescent staining is predominately localized in the nucleus in resting human monocytes, while actively translocated from the nucleus to the cytoplasm after LPS activation. HMGB1: High mobility group box 1; LPS: Lipopolysaccharide.

Figure 5

Stimulated sensory neurons actively secrete HMGB1 in an antidromic fashion by molecular mechanisms that remain elusive HMGB1: High mobility group box 1.