Proteomic identification of macrophage migration-inhibitory factor upon exposure to TiO₂ particles

Inhalation of particulate matter aggravates respiratory symptoms in patients with chronic airway diseases, but the mechanisms underlying this response remain poorly understood. We used a proteornics approach to examine this phenomenon. Treatment of epithelial cells with BSA-coated titanium dioxide (TiO₂) particles altered 20 protein spots on the two-dimensional gel, and these were then analyzed by nano-LC-MS/ MS. These proteins included defense-related, cell-activating, and cytoskeletal proteins implicated in the response to oxidative stress. The proteins were classified into four groups according to the time course of their expression patterns. For validation, RT-PCR was performed on extracts of in vitro TiO₂-treated cells, and lung issues from TiO₂-treated rats were analyzed by immunohistochemical staining and enzyme immunoassay. TiO₂ treatment was found to increase the amount of mRNA for macrophage migration-inhibitory factor (MIF). MIF was expressed primarily in epithelium and was elevated in lung tissues and bronchoalveolar lavage fluids of TiO₂-treated rats as compared with sham-treated rats. Carbon black and diesel exhaust particles also induced expression of MIF protein in the epithelial cells.