Enhanced expression of pulmonary gamma-glutamylcysteine synthetase heavy subunit in rats exposed to cadmium aerosols.

This investigation sought to determine the effect of cadmium (Cd) aerosol exposure on the pulmonary expression of the heavy subunit (HS) of gamma-glutamylcysteine synthetase (gamma-GCS), the rate-limiting enzyme in de novo synthesis of glutathione (GSH). Using Northern hybridization analysis, we demonstrated that CdO inhalation caused time- and dose-dependent increases in the steady-state levels of gamma-GCS-HS mRNA that were highly correlated with lung Cd burden. Observed increases in gamma-GCS-HS gene expression were maximal 2 h following a single aerosol exposure to Cd and appeared to be triggered by an oxidant stress, characterized by a decline in the reduced to oxidized glutathione ratio. Immunoblotting of proteins in lung extracts from treated and untreated animals produced a single protein band corresponding to a molecular weight of 73 kDa. Elevated levels of gamma-GCS-HS mRNA and gamma-GCS-HS protein in lungs of Cd-exposed animals were also accompanied by higher gamma-GCS enzymatic activity and elevations in glutathione (GSH). Immunohistochemical and in situ hybridization studies were used to identify compartments in the lung where Cd-induced expression of gamma-GCS-HS was localized. The most prominent staining for gamma-GCS-HS protein and gamma-GCS-HS mRNA was observed in the alveolar epithelium of Cd-exposed animals. Quantitative image analysis confirmed a good agreement between relative levels of protein and mRNA transcripts for gamma-GCS-HS. These observations suggest that resistance to Cd toxicity in the lung may reflect the ability of specific lung cells to upregulate gamma-GCS expression and increase de novo GSH synthesis as an adaptive response.