Biochar Fine Particles Enhance Uptake of Benzo(a)pyrene to Macrophages and Epithelial Cells via Different Mechanisms

Biochars are widely used as soil amendments and are major sources of agricultural dusts. Inhalation of biochar fine particles can increase the exposure of respiratory systems to mutagenic and carcinogenic polycyclic aromatic hydrocarbons (PAHs) that are bound to the materials during pyrolysis of biomass, but the specific mechanisms are elusive. Here, we show that biochar fine particles enhance the intracellular transfer of benzo(a)pyrene (a model PAH) into the J774A.1 macrophage via the "Trojan-horse" mechanism; i.e., the endocytosed benzo(a)pyrene-bearing particles act as a vector to deliver benzo(a)pyrene into the cells. Notably, biochar particles are found to serve as a reservoir that continuously releases benzo(a)pyrene into the lung fluids. Thus, benzo(a)pyrene bound to biochar is available to the neighboring cells such as lung epithelial cells (tested with A549 cells), even without apparent endocytosis. The aryl hydrocarbon receptor activation tests and in vitro bioaccessibility assays further corroborate these findings, in that benzo(a)pyrene carried into the lungs by biochar fine particles is still toxic to lung cells, and the cytotoxicity can be predicted based on the bioaccessibility in lung liquids. This work unearths the cell-specific risk pathways via which biochar fine particles break respiratory defense systems by serving as both contaminant vector and reservoir.