Air Pollution Quinones Impair Intestinal Barrier Integrity and Endoplasmic Reticulum in Differentiated Caco-2 Cells

Particulate matter (PM) is a major global health threat, linked to millions of deaths annually. Beyond inhalation, PM components reach the gastrointestinal tract through the mucociliary escalator or contaminated food and water. Among PM's organic fraction, redox-active quinones such as 1,2-naphthoquinone (NQ) and 9,10-phenanthrenequinone (PQ) are key drivers of toxicity, but their effects on intestinal epithelial integrity and associated cellular stress pathways require further investigation. Differentiated human Caco-2 cells were used as a model to assess the effects of NQ and PQ (1 μM), alone or in combination. Oxidative potential was evaluated using acellular dithiothreitol and ascorbic acid assays. Cellular endpoints included intracellular ROS generation, transcriptional activation of endoplasmic reticulum (ER) stress markers, pro-inflammatory cytokine expression, ultrastructural analysis by electron microscopy, junctional protein expression, and functional barrier integrity assessed by transepithelial electrical resistance (TEER). Acellular assays revealed compound-specific redox behaviors and nonadditive interactions depending on the oxidative pathway examined. In Caco-2 cells, quinone exposure induced intracellular ROS production and upregulated ER stress-related genes (GRP78, IRE1, and ATF6), accompanied by increased expression of pro-inflammatory cytokines (IL-1α, IL-6, and TNFα). Ultrastructural analysis demonstrated ER dilation and alterations in intercellular junctions. Notably, differentiated Caco-2 monolayers exhibited a pronounced reduction in TEER following combined quinone exposure, associated with decreased expression of junctional proteins, including Connexin-43 and E-cadherin. These findings indicate that redox-active quinones representative of air pollution organic components impair intestinal barrier integrity, particularly in differentiated epithelial models, in association with oxidative stress, ER stress activation, and junctional protein alterations. The results highlight the vulnerability of the mature intestinal barrier to specific PM constituents and support a role for ER stress-related pathways in pollutant-induced epithelial dysfunction.