In vivo expression of metallothionein in rat alveolar macrophages and type II epithelial cells following repeated cadmium aerosol exposures.

This study examined the ability of alveolar macrophages and alveolar type II epithelial cells to accumulate cadmium (Cd) and to express metallothionein (MT). Lung cells were isolated from Lewis rats repeatedly exposed to a Cd aerosol (1.6 mg Cd/m3). Intracellular Cd concentration rose following Cd exposure and showed an increasing rend as a function of exposure number. Alveolar macrophages accrued approximately four times more Cd than type II epithelial cells similarly exposed. Macrophages and type II cells responded to the presence of intracellular Cd by increasing MT protein levels. MT concentration was highly correlated with intracellular Cd. Immunocytochemical studies revealed that not all macrophages and type II cells from Cd-exposed animals were immunopositive for MT and that the intensity of immunostaining varied within each cell population. Although a greater percentage of macrophages were immunopositive for MT than type II cells, a greater proportion of type II cells showed moderate and dark MT staining patterns. Oligonucleotide probes, shown to distinguish between MT-1 and MT-2 mRNA isoforms, were used to test for cell-specific differences in MT isoform gene expression. The basal level of MT-1 mRNA was greater in macrophages than in type II cells. Following Cd administration, the level of MT-1 mRNA and MT-2 mRNA increased in each cell class but the response to Cd was three times greater in alveolar macrophages. Neither macrophages nor type II cells expressed MT mRNA isoforms in equal proportions. Macrophages expressed more MT-1 mRNA when exposed to air and more MT-2 mRNA in response to Cd exposure. Type II cells, on the other hand, expressed more MT-2 mRNA than MT-1 mRNA regardless of whether the cells were exposed to air or Cd. In conclusion, the present study has demonstrated that (1) alveolar macrophages and type II cells respond to in vivo Cd exposure by increasing MT protein and mRNA levels; (2) MT expression is greater in macrophages than in type II cells and correlates well with intracellular Cd concentration; and (3) the MT-2 mRNA to MT-1 mRNA ratio is cell and treatment specific.