Measurement of the cellular oxidative potential and cytotoxicity of ambient pm2.5 – an assessment of toxic pm2.5 chemical components and their sources in the midwest US
Ambient fine particulate matter (particles smaller than 2.5 µm in size or PM2.5 ) is the 4th leading risk factor for global mortality. PM2.5 has a unique toxic mechanism that utilizes both cellular antioxidants and reductants to catalyze the generation of reactive oxygen species (ROS). Excessive accumulation of ROS in the biological system might lead to cellular damage and even apoptosis. The capability to create this imbalance of ROS accumulation in the biological system is called PM2.5 oxidative potential (OP). There are both cellular and acellular methods to measure the OP of ambient PM2.5 . However, despite a commonly wide application of these assays in PM2.5 studies, there are two major questions still unanswered: 1. What is the biological relevance of chemical methods to measure the OP, and 2. What are the chemical components driving the response of both cellular and acellular OP. This dissertation aims to address some of these questions for the associations among PM 2.5 OP, cytotoxicity, and PM2.5 components. The biological relevance of PM2.5 chemical OP assays hasn’t been well established. The first research objective in this dissertation is to link chemical OP assays with cytotoxicity. PM2.5 samples were collected from an urban site in Champaign, IL. The cytotoxicity was assessed by a Chinese hamster ovary (CHO) cells cytotoxicity assay. The PM2.5 OP was measured by three acellular OP assays. The carbonaceous components and concentration of metals in the water-soluble PM2.5 were also determined. Correlation analysis among the PM2.5 OP, cytotoxicity and chemical composition was conducted. All three endpoints in the OP assays correlated significantly with cytotoxicity. The chemical OP assays that measure the •OH generation had the best association with the cytotoxicity. This should be attributed to their capability of capturing the synergistic interactions among organic compounds (i.e., water-soluble organic carbon (WSOC)) and metals (Fe and Cu). To further confirm the interaction of metals (Cu, Fe, and Mn) and organic compounds in cytotoxicity, these two components in different concentration ratios were mixed to expose the cells, and the cytotoxicity results were used to calculate the mixture toxicity index (MTI). MTI is a composite value to quantify the nature of interactions. This study suggested that at environmentally relevant concentrations ratios, the interactions between metals and organic compounds were mostly synergistic. To better understand the PM2.5 OP on a larger spatiotemporal scale, PM2.5 samples from three sites in Illinois (Bondville, Chicago, and Champaign), one site in Indiana (Indianapolis) and one site in Missouri (St. Louis) were collected for a period of one year. A rat alveolar macrophage cell line was employed to measure PM2.5 cellular OP. All sampling sites showed an almost identical seasonal pattern with highest OP levels in summer. Interestingly, the yearly average PM2.5 cellular OP at the rural site (Bondville) was at comparable level to other urban sites. We also measured the concentration of various PM2.5 components to serve as the markers for source apportionment analysis. Results indicate a very different profile of the emission sources for their contribution in PM2.5 mass vs. cellular OP. At the rural site, agricultural activities contributed to more than half of the PM2.5 cellular OP, while in the urban areas, PM2.5 cellular OP was mostly dominated by secondary aerosols. Among the measured components, WSOC and Fe had the strongest correlations with the PM2.5 cellular OP. To further investigate their roles in contributing to PM2.5 cellular OP, water-soluble PM2.5 was separated into metallic and organic fractions. The cellular OP of each fraction was tested by the macrophage assay. The results suggested that most of the PM2.5 cellular OP is driven by water-soluble Fe. Although organic compounds didn’t contribute directly to cellular OP, they enhanced the solubility of Fe through complexation and thus play an important role in the cellular OP. Overall, our results demonstrate a complex interaction among various PM components, particularly organic compounds and metals to originate the oxidative stress and toxicity caused by ambient particles. The chemical methods of OP measurement which are able to incorporate these interactions seem to be more biologically relevant than others.
Cytotoxicity, Chinese hamster ovary (CHO) cells, Oxidative Potential, Reactive oxygen species (ROS) generation, Dithiothreitol, Interactions, Macrophage, Midwest
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign