Toll-Like Receptor 4 and High-Mobility Group Box 1 Are Critical Mediators of Tissue Injury and Survival in a Mouse Model for Heatstroke

Abstract

The molecular mechanisms that initiate the inflammatory response in heatstroke and their relation with tissue injury and lethality are not fully elucidated. We examined whether endogenous ligands released by damaged/stressed cells such as high-mobility group box 1 (HMGB1) signaling through Toll-like receptor 4 (TLR4) may play a pathogenic role in heatstroke. Mutant TLR4-defective (C3H/HeJ) and wild type (C3H/HeOuJ) mice were subjected to heat stress in an environmental chamber pre-warmed at 43.5°C until their core temperature reached 42.7°C, which was taken as the onset of heatstroke. The animals were then allowed to recover passively at ambient temperature. A sham-heated group served as a control. Mutant mice displayed more histological liver damage and higher mortality compared with wild type mice (73% vs. 27%, respectively, P<0.001). Compared to wild type mice, mutant mice exhibited earlier plasma release of markers of systemic inflammation such as HMGB1 (206±105 vs. 63±21 ng/ml; P = 0.0018 and 209±100 vs. 46±32 ng/ml; P<0.0001), IL-6 (144±40 vs. 46±20 pg/ml; P<0.001 and 184±21 vs. 84±54 pg/ml; P = 0.04), and IL-1β (27±4 vs. 1.7±2.3 pg/ml; P<0.0001 at 1 hour). Both strains of mice displayed early release of HMGB1 into the circulation upstream of IL-1β and IL-6 responses which remained elevated up to 24 h. Specific inhibition of HMGB1 activity with DNA-binding A Box (600 µg/mouse) protected the mutant mice against the lethal effect of heat stress (60% A Box vs. 18% GST protein, P = 0.04). These findings suggest a protective role for the TLR4 in the host response to severe heat stress. They also suggest that HMGB1 is an early mediator of inflammation, tissue injury and lethality in heatstroke in the presence of defective TLR4 signaling.

The pro-inflammatory role of high-mobility group box 1 protein (HMGB-1) in photoreceptors and retinal explants exposed to elevated pressure

“…Those findings promote recent findings indicating the independence of TLR-4 signaling in HMGB-1-affected tissue and related apoptosis in non-neuronal tissue. 70 The present study further shows that regulation of HMGB-1 in retinal explants occurs under conditions of elevated pressure. The upregulation of RAGE suggests that elevated pressure switches the retinal environment into a pro-inflammatory one.…”

Heat Stroke

“…C3H/HeJ mice displayed more profound and sustained hypothermia, more rapid induction of circulating IL-1β, IL-6, TNFα, HMGB1 levels, more severe liver damage, and increased classic heat stroke mortality than wildtype mice during 72 h of recovery (90). Purified A box protein (a specific antagonist of HMGB1) protected C3H/HeJ mice from lethality when provided immediately prior to heat exposure (90). Given that TLR4 polymorphisms exist in humans, this may be one (of several) genetic factors that predispose to mortality during the systemic inflammatory response to heat stroke (16,108).…”

“…In the 1960s, a spontaneous mutation in the TLR4 gene was discovered in C3H/HeJ mice, which has been an important animal model to determine the role of TLR4 in endotoxin and heat stroke responsiveness. C3H/HeJ mice displayed more profound and sustained hypothermia, more rapid induction of circulating IL-1β, IL-6, TNFα, HMGB1 levels, more severe liver damage, and increased classic heat stroke mortality than wildtype mice during 72 h of recovery (90). Purified A box protein (a specific antagonist of HMGB1) protected C3H/HeJ mice from lethality when provided immediately prior to heat exposure (90).…”

Invited review: Inflammation during the transition to lactation: New adventures with an old flame

“…These proteins serve as molecular chaperones and are upregulated to promote normal protein folding during heat stress; when released extracellularly, though, they also serve as ligands for the PRR TLR2 and TLR4, which can drive inflammatory responses (Galloway et al, 2008). In fact, TLR4 plays a critical protective role in survival after heatstroke in mice (Dehbi et al, 2012), indicating that some aspect of the heat stress response does activate this receptor. Heat stress also may contribute to inflammation indirectly by promoting gut leakiness, as demonstrated in monogastric species (Pearce et al, 2013).…”