# Cellular Effects of Silver Nanoparticle Suspensions on Lung Epithelial Cells and Macrophages
## 4 Discussion
Although the two types of AgNPs induced similar cellular responses, the strength of the responses depended on the type of AgNP suspension. The AgNPs used in this study decreased mitochondrial activity and increased intracellular oxidative stress and IL-8 expression. However, whether these cellular effects are associated with intracellular silver ion release from AgNPs requires further study. In A549 cells, the intracellular silver concentration was significantly higher in AgNP-A- and AgNP-B-treated cells than in untreated cells. The AgNP-induced increase in IL-8 and HO-1 expression was higher in dTHP-1 cells than in A549 cells. The gene expression levels of IL-8 in AgNP-A- and AgNP-B-treated dTHP-1 cells were 8.6- and 1.8-times higher than those in A549 cells, respectively. The gene expression levels of HO-1 in AgNP-A- and AgNP-B-treated dTHP-1 cells were 4.5- and 1.4-times higher than those in A549 cells, respectively. Furthermore, the gene expression of MT2A in AgNP-A- and AgNP-B-treated A549 cells was 3.1- and 6.1-times higher, respectively, than that in dTHP-1 cells. MT2A gene expression was remarkably enhanced in A549 cells following exposure to AgNP suspensions. In other words, the increase in IL-8 and HO-1 gene expression was dTHP-1 cells > A549 cells, whereas the increase in MT2A gene expression was A549 cells > dTHP-1 cells. It should be noted that MTA2 binds to and detoxifies heavy metals such as cadmium. Thus, it is possible that the silver ions released from intracellular AgNPs were inactivated by MT2A. Therefore, intracellular silver concentration is an important factor in the efficacy of AgNPs in inducing IL-8 and HO-1 expression. The increase in intracellular silver concentration may be caused by the cellular uptake and subsequent intracellular dissolution of AgNPs. However, the reason why intracellular AgNP-B were undetectable is unclear. One possibility is that intracellular AgNP-B were rapid and completely dissolved. Although the solubility of AgNP-B under acidic conditions was lower than that of AgNP-A, AgNP-B contained a nitrogen-containing compound as a binder. Thus, the actual mass of AgNPs in the AgNP-B suspension may be smaller. However, we do not yet have any evidence. In fact, the findings of MT2A expression and intracellular silver levels suggested that intracellular AgNP-B were more soluble than AgNP-A. Therefore, binder compounds may have affected the intracellular dissolution of AgNP-B. The cellular uptake and intracellular dissolution of AgNPs are certainly causes of their cellular effects. On the other hand, intracellular dissolution is not the sole cause of the cellular effect caused by AgNPs. Other properties of AgNPs, such as surface activity and figure, may also be important. Understanding of these properties of AgNPs is important for the utilization of AgNPs (Figure 6). In conclusion, AgNPs induced IL-8 and HO-1 expression, but cytotoxicity was weak. Moreover, increased intracellular silver concentrations contributed to the elevation of IL-8 and HO-1 expression. The increase of intracellular Ag\({}^{2+}\) level caused by silver nanoparticle suspensions may be important for these cellular responses. This research received no external funding. Not applicable. All data are presented in this article in the form of figures and tables. The authors declare no conflict of interest.