Cellular and acellular assays for measuring oxidative stress induced by ambient and laboratory-generated aerosol

Abstract

Exposure to atmospheric particulate matter (PM) is a leading global health risk with various proposed mechanisms of action, including the induction of oxidative stress through PM-initiated production/release of reactive oxygen and nitrogen species (ROS/RNS). This dissertation explores cellular and acellular measurements of PM-induced oxidative stress through systematic laboratory chamber experiments and ambient field studies. A cell-based assay involving murine alveolar macrophages was developed to measure intracellular ROS/RNS produced as a result of aerosol exposure. The area under the dose-response curve was identified as a robust metric to represent ROS/RNS for comparison with different endpoints. A large ambient study with samples collected from urban and rural sites around the greater Atlanta area (n = 104) was conducted using the optimized assay and significant correlations between ROS/RNS and organic constituents were observed for summer samples, highlighting the potential contribution of organic aerosol, particularly summertime photochemically-driven secondary organic aerosol (SOA). To explore these findings, SOA was generated in a series of laboratory experiments from various biogenic (isoprene, α-pinene, β-caryophyllene) and anthropogenic (pentadecane, m-xylene, naphthalene) precursors under different formation conditions (dry vs. humid, NOx, ammonium sulfate vs. iron sulfate seed particles) to probe their effects on PM toxicity. For chemical oxidative potential as measured by dithiothreitol consumption (OP), precursor identity influenced toxicity significantly, with isoprene and naphthalene SOA having the lowest and highest OP, respectively. Both precursor identity and formation conditions influenced ROS/RNS and cytokine (tumor necrosis factor-α and interleukin-6) production. Several response patterns were identified for SOA precursors whose photooxidation products share similar carbon chain length and functionalities. A significant correlation between ROS/RNS levels and aerosol carbon oxidation state was also observed, which may have significant implications as atmospheric aerosol have an atmospheric lifetime of a week, over which oxidation state increases due to photochemical aging, potentially resulting in more toxic aerosol.