Regulation of Toll-Like Receptor 4 Expression in the Lung Following Hemorrhagic Shock and Lipopolysaccharide

Abstract

The Toll-like receptor 4 (TLR4) has recently been shown to function as the major upstream sensor for LPS. In this study, a rodent model of lung injury following resuscitated hemorrhagic shock was used to examine the regulation of TLR4 gene and protein expression in vivo and in vitro. Intratracheal LPS alone induced a rapid reduction in whole lung TLR4 mRNA, an effect which is also observed in recovered alveolar macrophages. This effect appeared to be due to a lowering of TLR4 mRNA stability by ∼69%. By contrast, while shock/resuscitation alone had no effect on TLR4 mRNA levels, it markedly altered the response to LPS. Specifically, antecedent shock prevented the LPS-induced reduction in TLR4 mRNA levels. This reversal was explained by the ability of prior resuscitated shock both to prevent the destabilization of TLR4 mRNA by LPS and also to augment LPS-stimulated TLR4 gene transcription compared with LPS alone. Oxidant stress related to shock/resuscitation appeared to contribute to the regulation of TLR4 mRNA, because supplementation of the resuscitation fluid with the antioxidant N-acetylcysteine reversed the ability of shock/resuscitation to preserve TLR4 mRNA levels following LPS. TLR4 protein levels in whole lung mirrored the changes seen for TLR4 mRNA. Considered in aggregate, these data suggest that levels of tlr4 expression are controlled both transcriptionally as well as posttranscriptionally through altered mRNA stability and that antecedent shock/resuscitation, a form of global ischemia/reperfusion, might influence regulation of this gene.

Treatment of hemorrhagic shock with hypertonic saline solution modulates the inflammatory response to live bacteria in lungs

“…Regarding the lack of alterations in TLR4 among our experimental groups, our findings differ from those of another report showing that intratracheal LPS injection induced a reduction in lung tissue and alveolar macrophage levels of TLR4 mRNA, while previous shock and resuscitation prevented a TLR4 decrease when lungs were exposed to LPS (27). Different experimental conditions may account for these discrepancies, the most important being the fact that our model relies on infection with live bacteria, which implies that a much more complex process is under way and may even involve the participation of other TLRs (10).…”

Corticotropin-Releasing Factor and the Urocortins Induce the Expression of TLR4 in Macrophages via Activation of the Transcription Factors PU.1 and AP-1

“…The levels of TLR4 mRNA expression in macrophages are regulated at the transcriptional as well as at a posttranscriptional level by altering its stability (25,41). Our data suggest that the effect of CRF peptides most probably occurs at the transcriptional level The Tlr4 gene is regulated at the transcriptional level through activation of transcription factors of the Ets family and primarily PU.1 (25), as well as AP-1 (26).…”

A Critical Role for TLR4 in the Pathogenesis of Necrotizing Enterocolitis by Modulating Intestinal Injury and Repair

“…Such molecules, including fibronectin (80), heat shock proteins (81), and high-mobility group box 1 protein (82), may activate TLR4 to cause to the activation of the host immune system and the release of proinflammatory cytokines. Such activation of TLR4 in response to endogenous molecules during stress may explain the observation that the severity of various noninfectious models of critical illness are dependent on the activation of TLR4, including hemorrhagic shock (83)(84)(85)(86) and ischemia reperfusion injury (87)(88). The relative contribution of endogenous vs exogenous molecules in the activation of TLR4 in the pathogenesis of NEC remains of great scientific interest with respect to unraveling the complex origins of NEC.…”