Low-dose combined exposure of carboxylated black carbon and heavy metal lead induced potentiation of oxidative stress, DNA damage, inflammation, and apoptosis in BEAS-2B cells

Black carbon (BC) and heavy metal lead (Pb), as typical components of atmospheric PM_2.5, have been shown to cause a variety of adverse health effects. However, co-exposure to BC and Pb may induce pulmonary damage by aggravating toxicity via an unknown mechanism. This study aimed to investigate the combined toxicity of carboxylated black carbon (c-BC) and lead acetate (Pb) on human bronchial epithelial cells (BEAS-2B) at the no-observed-adverse-effect level (NOAEL). Cells were exposed to c-BC (6.25 μg/mL) and Pb (4 μg/mL) alone or their combination, and their combined toxicity was investigated by focusing on cell viability, oxidative stress, DNA damage, mitochondrial membrane potential (MMP), apoptosis, and cellular inflammation. Factorial analyses were also used to determine the potential interactions between c-BC and Pb. The results suggested that the combination of c-BC and Pb could significantly increase the production of reactive oxygen species (ROS), malondialdehyde (MDA), and lactate dehydrogenase leakage (LDH) and decrease the activities of glutathione (GSH) and superoxide dismutase (SOD). The excessive oxidative stress could increase the levels of inflammatory cytokine IL-6 and TNF-α, and induce oxidative DNA damage and dissipation of MMP. Moreover, the results also suggested that the combined group could enhance the cellular apoptotic rate and the activation of apoptotic markers like caspase-3, caspase-8, and caspase-9. The factorial analysis further demonstrated that synergistic interaction was responsible for the combined toxicity of c-BC and Pb co-exposure. Most noticeably, the co-exposure of c-BC and Pb could induce some unexpected toxicity, even beyond the known toxicities of the individual compounds in BEAS-2B cells at the NOAEL.

In vitro and in vivo toxic effects and inflammatory responses induced by carboxylated black carbon-lead complex exposure

Black carbon (BC) is a key component of atmospheric fine particulate matter (PM_2.5) and it tends to adsorb various pollutants (e.g., heavy metals and organics) during atmospheric transport. This adsorption leads to the complexity and uncertainty of the source and chemical composition of PM_2.5, making the toxicologic effects and health risks induced by PM2.5 difficult to determine. Here, we used carboxylated black carbon (c-BC) and c-BC-lead complexes (c-BC-Pb) to investigate the in vitro and in vivo toxic effects and inflammatory responses. The physicochemical properties of c-BC and c-BC-Pb complexes were characterized by the transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), and in ductively coupled plasma-atomic emission spectra (ICP-AES). Cytotoxicity in vitro showed that the exposure of human bronchial epithelial cells (BEAS-2B) to low-dose c-BC-Pb particles significantly induced greater toxicity than that of c-BC, suggesting that lead (Pb) might play an important role in induced cytotoxicity after combined exposure to c-BC-Pb particles. The findings were further confirmed by the results in vivo, which indicated that c-BC-Pb particles significantly induced inflammation and lung injury. Based on the results of this experiment, the differences in toxicity can be attributed to the synergistic effect of Pb on the BC particles, which play a synergistic role in vitro and in vivo in the development of toxicity. The c-BC-Pb particles model used in this study may be helpful for the evaluation of cytotoxicity induced by different sources of BC particles or BC-heavy metal complexes and provide a new approach for understanding PM_2.5-induced toxicity and health risks.