Cellular responses of human bronchial epithelial cells following short-term exposure to oxidative particulate matter

Adverse health effects associated with fine particulate matter (PM_2.5) in urban areas can occur even at PM_2.5 concentrations below current regulatory limits - a situation increasingly observed in high-income countries. However, the underlying biological mechanisms remain poorly understood. In this study, we investigated the molecular and cellular responses in human bronchial epithelial cells exposed to low PM_2.5 using a novel methodology. We first identified specific meteorological conditions that favor low PM_2.5 mass concentrations (<10 μg m^-3) combined with high traffic-related aerosol emissions, which we found to correspond to a highly oxidant atmosphere. Under these conditions, PM_2.5 samples were collected in the urban background of Rome. The cells were exposed in vitro using a novel methodological approach based on a direct filter-contact model. Our focus was on associating measurable aerosol properties with gene expression pathways related to oxidative stress, inflammation, and their epigenetic modulation through microRNAs. Our findings indicate that exposure to fresh traffic-related aerosols under low PM_2.5 concentrations elicited a biphasic gene expression response. The initial response involved the activation of genes such as NRF2, NF-κB, CAT1, SOD1, HIF-1α, and HMOX1; while a secondary response involved TNF-α and GPX4. A strong association was observed between these biological effects and black carbon metrics related to fossil fuel, implicating fresh traffic emissions as key contributors. Additionally, a significant modulation of air pollution-associated microRNAs was observed, even at early times of exposure, suggesting an epigenetic dimension to the cellular stress response. These findings have important implications for future air quality regulations. We provide mechanistic insights into oxidative and epigenetic responses underlying PM_2.5 induced biological effects at low PM_2.5 levels, emphasizing that neither PM_2.5 mass concentration nor its oxidative potential - two metrics currently considered by legislation - are sufficient on their own to explain the observed effects.