Abstract
Particulate matter (PM) is emitted during the combustion of fuels and wastes. PM exposure exacerbates pulmonary diseases, and the mechanism may involve oxidative stress. At lower combustion temperatures such as occurs in the cool zone of a flame, aromatic compounds chemisorb to the surface of metal-oxide-containing PM, resulting in the formation of surface-stabilized environmentally persistent free radicals (EPFR). Prior studies showed that PM-containing EPFR redox cycle to produce reactive oxygen species (ROS), and after inhalation, EPFR induce pulmonary inflammation and oxidative stress. Our objective was to elucidate mechanisms linking EPFR-induced oxidant injury with increased cytokine production by pulmonary epithelial cells. We thus treated human bronchial epithelial cells with EPFR at sub-toxic doses and measured ROS and cytokine production. To assess aryl hydrocarbon receptor (AhR) activity, cells were transfected with a luciferase reporter for xenobiotic response element activation. To test whether cytokine production was dependent upon AhR activation or oxidative stress, some cells were co-treated with an antioxidant or an AhR antagonist. EPFR increased IL-6 release in an ROS and AhR- and oxidant-dependent manner. Moreover, EPFR induced an AhR activation that was dependent upon oxidant production, since antioxidant co-treatment blocked AhR activation. On the other hand, EPFR treatment increased a cellular ROS production that was at least partially attenuated by AhR knockdown using siRNA. While AhR activation was correlated with an increased expression of oxidant-producing enzymes like cytochrome P450 CYP1A1, it is possible that AhR activation is both a cause and effect of EPFR-induced ROS. Finally, lipid oxidation products also induced AhR activation. ROS-dependent AhR activation may be a mechanism for altered epithelial cell responses after EPFR exposure, potentially via formation of bioactive lipid or protein oxidation products.
Highlights
While thermal processing of organic wastes is commonly used at Superfund sites in the U.S and at similar locations across the world, the unintended health consequences associated with the combustion of pollutants have not yet been fully identified [1]
While not as dramatic an increase as that observed for environmentally persistent free radicals (EPFR) exposure, BEAS-2Bs exposed to CuO/SiO particles increased reactive oxygen species (ROS) production; this could be inhibited by both α-T and 3, 4-DMF pre-treatments (Fig 1C)
From the compilation of studies conducted here, we propose a mechanism wherein EPFR induce ROS production and oxidative stress, culminating in the oxidation of biomolecules to yield small molecule metabolites capable of activating the aryl hydrocarbon receptor (AhR)
Summary
While thermal processing of organic wastes is commonly used at Superfund sites in the U.S and at similar locations across the world, the unintended health consequences associated with the combustion of pollutants have not yet been fully identified [1]. Epidemiologic studies have identified a strong association between increased PM concentrations and both cardiovascular and respiratory events [2]. Increases in PM exposure correlated with the development of asthma [3,4,5], and exposure to fine PM (PM2.5) at concentrations approved by the EPA for even a short interval decreased lung function [6]. PM2.5 exposure was linked with increases in hospital admissions for cardiovascular events and chronic obstructive pulmonary disease, and on average it is estimated that PM2.5 exposure is responsible for approximately 200,000 deaths each year in the U.S [9]. Numerous studies have clearly documented the health consequences of PM exposure, the underlying mechanisms driving the development of these diseases after inhalation exposure have yet to be elucidated
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