Involvement of MyD88 in zinc oxide nanoparticle-induced lung inflammation

Zinc oxide nanoparticles (ZnONP) have great potential for medical applications. However, ZnONP is reported to induce acute lung inflammation, which limits its application in humans. We designed in vivo and in vitro studies to clarify ZnONP inflammation and its associated molecular signals. ZnONP with a single dose of 80 μg/30 μl was instilled into the tracheas of mice sacrificed at days 2, 7, 14, and 28 after instillation. Bronchoalveolar lavage fluid showed increased neutrophils and macrophages after treatment. Lung pathology showed a mixed inflammatory infiltrate of neutrophils, lymphocytes, and macrophages primarily in the bronchioles and peribronchiolar areas. Proinflammatory gene expression of TNF-α, IL-6, CXCL1, and MCP-1 was increased at day 2 and decreased after 7 days. The lung pathology resolved at day 28, without fibrosis. It remains unclear whether this acute lung inflammation was caused by ZnONP themselves or Zn(2+) iron released from the nanoparticles. In vitro studies confirming the results of in vivo studies showed increased expression of proinflammatory genes in both MLE12 cells (mouse lung epithelial cells) and RAW264.7 cells (mouse macrophages) with either ZnONP or Zn(NO₃)₂ treatment; notably, increased levels of proinflammatory genes were obviously higher in cells treated with ZnONP than in cells treated with Zn(NO₃)₂ at the same molarity dose. TNF-α and MCP-1 were induced only in MLE12 cells. MyD88, an adaptor protein for most Toll-like receptors (TLR) signaling pathways, initiated the ZnONP or Zn(NO₃)₂-induced lung inflammation. Silencing MyD88 expression with siRNA significantly reduced ZnONP or Zn(NO₃)₂-induced proinflammatory gene expression in MLE12 and RAW264.7 cells. Single-dose exposure to ZnONP produced the short-term lung inflammation via a MyD88-dependent TLR pathway. These data suggest that although both ZnONP and zinc ion might participate in the inflammatory reactions, ZnONP more effectively induced MyD88-dependent proinflammatory cytokines than zinc ion in lung epithelial cells.