Mechanisms of Oxidative Stress in Human Airway Epithelial Cells Exposed to an Environmental Peroxide

Exposure to airborne fine particulate matter (PM2.5) is a leading cause of morbidity and mortality worldwide. A major progenitor of PM2.5 secondary organic aerosol (SOA) derived from the atmospheric oxidation of isoprene by hydroxy radical, which forms isoprene hydroxy hydroperoxide (ISOPOOH). This reaction is increasing favored as pollution controls have decreased anthropogenic NOX levels in the atmosphere. While the formation of atmospheric ISOPOOH is well understood, relatively little is known about the direct contribution of ISOPOOH to human health effects from exposure to air pollutants. Previous studies have shown that ISOPOOH-generated SOA can induce oxidative stress markers in cultured human airway epithelial cells (HAEC). This dissertation investigates the mechanisms and the molecular initiating events by which exposure to ISOPOOH induces oxidative stress and early adaptive bioenergetic responses in HAEC. The results presented here demonstrate that ISOPOOH exposure leads to intracellular glutathione (GSH) oxidation through a mechanism involving the enzyme glutathione peroxidase 4 (GPX4) and lipid peroxidation in HAEC. Exposure of HAEC supplemented with poly-unsaturated fatty acids showed potentiated ISOPOOH-induced GSH oxidation. Studies presented herein demonstrate that ISOPOOH induces not only proteome-wide thiol oxidation, but also that ISOPOOH specifically induces sulfenylation and enzymatic inhibition of glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Since, the oxidative inactivation of GAPDH is reversible in a physiological context, the oxidation state of GAPDH thiols serve as a molecular switch between cellular bioenergetics and redox homeostasis. Specifically, peroxide-mediated sulfenylation of GAPDH shifts glucose metabolism away from glycolysis and diverts it towards pentose phosphate pathway. The results from this dissertation therefore show that ISOPOOH exposure induces multiple mechanisms of redox toxicity in HAEC. The alterations in cellular homeostasis induced by ISOPOOH exposure evince not only the essentiality and complexity of intracellular redox mechanisms, but also reveal that environmental xenobiotic exposure can alter the intracellular redox landscape. These findings show that ISOPOOH is a potent and unique environmental oxidant that likely contributes to the oxidative burden posed by inhalation of SOA.