Interplay between lung and intestine in responses to diesel exhaust particles: Contrasting intestinal effects of cleared and swallowed particles with lung-mediated effects using an in vitro approach

Traffic-borne pollutants can reach the intestine by being swallowed or by translocating across the air-blood barrier into the bloodstream upon inhalation. These distinct exposure ways may cause different mechanistic outcomes. The impacts of combustion-derived particles on secondary organs such as the intestine remain an area of active investigation. In this study, we evaluated the effects of standard reference material diesel exhaust particles (SRM2975 DEPs) on a 3D human intestinal tissue model composed of human intestinal cells (Caco-2 and HT-29) and macrophage-like cells (dTHP-1) using two exposure approaches. The intestinal model was directly exposed to DEPs (5, 20, and 80 μg mL^-1) for 24 h to mimic swallowed particles or indirectly exposed to DEPs via conditioned media from DEP-pretreated human alveolar epithelial A549 cells for 24 h to mimic systemic exposure through the lungs. Following exposure, we measured intestinal tissue integrity, inflammatory reactions, xenobiotic metabolism, and genotoxicity. Direct DEP exposure predominantly induced xenobiotic metabolism, with the upregulation of CYP1A1 gene, which encodes cytochrome P450, but without tissue disruption or inflammatory reactions. In contrast, indirect exposure primarily triggered inflammatory responses, resulting in an increased release of interleukin-8 (IL-8) and tumor necrosis factor-alpha (TNF-α). This suggests that mediators originating from the lungs are likely pivotal contributors to the observed intestinal effects. Changes in CYP1A1 and HMOX1 metabolic gene expressions confirmed in intestinal tissues mediated by mediators. Genes involved in DNA damage and repair pathways were upregulated in both direct DEPs and indirect conditioned CCM exposures, implicating DEPs in the potential induction of genotoxicity in intestinal cells. Our findings reveal that swallowed DEPs could directly affect intestinal tissue via activation of the AhR signaling pathway and DNA damage. While translocated DEPs and lung-derived mediators may cause slight effects on intestinal tissue via the AhR signaling pathway and DNA damage, substantial effects may occur through inflammation.