Diesel exhaust particles alter cAMP dynamics in human bronchial epithelial cells

Abstract

Diesel exhaust particles (DEP) are one of the main causes of air pollution and are associated with lung disorders. DEP exposure may promote several pathological processes, including epithelial‐to‐mesenchymal transition (EMT), inflammation and oxidative stress, which contribute to a decline in lung function. Cyclic AMP (cAMP) has been reported to alleviate lung dysfunction. The impact of DEP on cAMP signaling is largely unknown. Human bronchial epithelial (BEAS‐2B) cells were exposed to different concentrations (≤ 300 µg/ml) for up to 72 h. Markers of oxidative stress, inflammation, EMT, components of mitochondrial bioenergetics and cAMP signaling were evaluated. DEP increased gene expression of Nrf1, thioredoxin, HO‐1 and SOD‐2 (oxidative stress markers), and interleukin (IL)‐6 and IL‐8 (inflammatory markers), as well as the EMT marker transforming growth factor‐β1 (both mRNA and protein). Epithelial cell markers (E‐cadherin, ZO‐1) were reduced by DEP, whereas mesenchymal markers (β‐catenin, collagen‐1) were increased. DEP reduced both basal and ATP‐linked respiration as well as the glycolytic capacity of mitochondria. Concomitantly, DEP profoundly altered mitochondrial morphology and increased the gene expression of A‐kinase anchoring protein (AKAP) 1 (a known mitochondrial scaffold protein). Furthermore, DEP significantly affected Epac1 mRNA levels and appeared to increase Epac1 protein, but did not alter the expression of phospho‐PKA substrates. Interestingly, DEP increased gene expression levels of several AC family members (AC3, AC6 and AC7), the β2‐adrenoceptor (β2‐AR) and prostanoid E receptor subtype 4 (EP4). Importantly, real‐time measurements of cAMP demonstrated that DEP strongly reduced cAMP induced by forskolin (AC activator), fenoterol (β2‐AR agonist) and a stable EP4 agonist, implicating that DEP might limit the therapeutic effects of β2‐AR agonists used to treat lung disorders. Our findings suggest that DEP promotes oxidative stress, inflammation, and EMT. These profound alterations in cellular behavior were accompanied by changes in mitochondrial dysfunction and real‐time cAMP dynamics, which may contribute to lung dysfunction induced by air pollution exposure.