Role of Toll-Like Receptor 4 in Oxidant-induced Inflammatory Phenotypes in Macrophages

Abstract

A common mechanism by which redox stress may activate inflammatory responses to potentially initiate, propagate and maintain many diseases states has not been characterized. Accumulating evidence suggests that pattern recognition receptors of the innate immune system such as the toll-like receptors (TLRs) are involved. In the present study, we tested the central hypothesis that TLR4 is the link between reactive oxygen/nitrogen species (RONS), oxidative stress and inflammatory phenotypes that mediate diverse disease processes. We first characterized the mediatory role of TLR4 in exogenous oxidant induced nuclear factor-κB (NF-κB) activation in macrophage RAW-Blue cells with stable transfection of NF-κB reporter gene SEAP. Our results show that inhibition of TLR4 significantly attenuated oxidant-induced NF-κB activation, which caused an imbalance in TNF-α and IL-10 production. We used primary peritoneal macrophage (pM) derived from TLR4-wildtype (TLR4-WT) and TLR4-knockout (TLR4-KO) mice to investigate the role of TLR4. Our data show that TLR4 is necessary for RONS-mediated disturbances in redox homeostasis and the production of TNF-α. Our results affirm that exogenous RONS can initiate the production of resolvin D1 by concurrently increasing the expression of its biosynthetic enzymes and its receptor through TLR4 stimulation. Our data show that exogenous RONS-induced TLR4 stimulation plays a critical role in activating both proinflammatory and pro-resolving pathways. Finally, we examined if primed TLR4 would influence the magnitude of responses to exogenous oxidants in pM. Our results indicate that treatment with oxidants alone had a limited effect on prostaglandin E2 (PGE2) synthesis. In contrast, pM sensitized by prior treatment with LPS-EK followed by oxidant stimulation exhibited increased expression of COX-2 and an enhanced PGE2 production only in pM derived from TLR4-WT mice. Thus, we showed a critical role for primed TLR4 in oxidant-induced pro-inflammatory processes. For the first time, we present evidence to support a central mechanism(s) for the intersection between exogenous and endogenous RONS in enhancing inflammatory phenotypes that may initiate, propagate, and maintain multiple disease states.