Silica nanoparticles inhibit responses to ATP in human airway epithelial 16HBE cells

Besides its role in gas exchange, the airway epithelium exerts protective responses against environmental insults. Epithelial cells and sensory neurons lining up the airway epithelium can sense the presence of and the damage induced by various foreign substances and react by triggering protective reflexes, such as enhanced mucociliary clearance, initiation of cough and induction of inflammation. As a result of nano-technological development, the exposure of the general population to airborne nanomaterials has increased in the past decades, raising concerns about the toxicity of such particulates. Because of their low cost, easy production and versatility, silica nanoparticles (SiNPs) are widely used in multiple manufacturing applications as anti-caking, densifying and hydrophobic agents and are among the most highly produced nanomaterials worldwide. SiNPs affect the function of the airway epithelium, but the biochemical pathways targeted by these particles remain largely unknown. SiNPs have been repeatedly shown to induce release of damage-associated molecular pattern ATP in exposed cells. We have investigated the effects of SiNPs on the responses of 16HBE14o- cultured human bronchial epithelial (16HBE) cells to extracellular ATP, using measurements of intracellular Ca2+ concentration. Upon stimulation with extracellular ATP, these cells displayed a concentration-dependent increase in intracellular Ca2+, which was mediated by release from intracellular stores. SiNPs induced a dose-dependent Ca2+ increase and inhibited the Ca2+ responses to ATP. The latter effect occurred within minutes of application and at low µg/ml concentrations, which are significantly faster and more potent than those previously reported for the induction of cellular toxicity and pro-inflammatory responses. SiNPs-induced inhibition is independent from the increase in intracellular Ca2+ they produce, is largely irreversible and occurs via a non-competitive mechanism. These findings suggest that SiNPs reduce the ability of epithelial cells to support defensive responses via ATP-dependent signaling.