Intracellular RIG-I Signaling Regulates TLR4-Independent Endothelial Inflammatory Responses to Endotoxin

Abstract

Sepsis is a systemic inflammatory response to infections associated with organ failure that is the most frequent cause of death in hospitalized patients. Exaggerated endothelial activation, altered blood flow, vascular leakage, and other disturbances synergistically contribute to sepsis-induced organ failure. The underlying signaling events associated with endothelial proinflammatory activation are not well understood, yet they likely consist of molecular pathways that act in an endothelium-specific manner. We found that LPS, a critical factor in the pathogenesis of sepsis, is internalized by endothelial cells, leading to intracellular signaling without the need for priming as found recently in immune cells. By identifying a novel role for retinoic acid-inducible gene-I (RIG-I) as a central regulator of endothelial activation functioning independent of TLR4, we provide evidence that the current paradigm of TLR4 solely being responsible for LPS-mediated endothelial responses is incomplete. RIG-I, as well as the adaptor protein mitochondrial antiviral signaling protein, regulates NF-kB-mediated induction of adhesion molecules and proinflammatory cytokine expression in response to LPS. Our findings provide essential new insights into the proinflammatory signaling pathways in endothelial cells and suggest that combined endothelial-specific inhibition of RIG-I and TLR4 will provide protection from aberrant endothelial responses associated with sepsis.

Endothelial Interferon Regulatory Factor 1 Regulates Lipopolysaccharide-Induced VCAM-1 Expression Independent of NFκB

“…In addition, RIG-I knockdown inhibited LPS-mediated upregulation of VCAM-1 and E-selectin ( Fig. 4 b, c), confirming previous findings [13] , whereas IRF-1 knockdown only inhibited LPS-mediated VCAM-1 induction ( Fig. 3 b, 4 b).…”

“…IRF-1 was shown to regulate basal transcription, as well as dsRNA-mediated upregulation of RIG-I [24] . Recent data from our laboratory showed that in endothelial cells, RIG-I is an intracellular receptor that regulates LPS-induced endothelial activation [13] . Based on these findings we hypothesized that IRF-1 was an upstream regulator of RIG-I- 554 mediated endothelial responses to LPS.…”

“…Recent findings from our laboratory have shown that the RIG-I adaptor protein MAVS also regulates LPSinduced endothelial activation via the NFκB signaling pathway [13] . Knockdown of MAVS partially inhibited VCAM-1 expression ( Fig.…”

“…Recently, we also showed that RIG-I can control VCAM-1 expression in response to LPS challenge [13] . In this study we investigated whether endothelial IRF-1 plays a role in the regulation of LPS-mediated endothelial proinflammatory activation, and explored molecular mechanisms of the regulatory pathways involved.…”

“…NFκB controls the transcription of many proinflammatory genes, including endothelial adhesion molecules, cytoand chemokines [11,12] . Recent work from our laboratory identified retinoic acid inducible gene-I (RIG-I) as an additional regulator of endothelial proinflammatory activation that functions in parallel with TLR4 [13] .…”