Suppressed oxidant-induced apoptosis in cadmium adapted alveolar epithelial cells and its potential involvement in cadmium carcinogenesis.

Abstract Apoptosis involves a series of genetically programmed events associated with endonucleolytic cleavage of DNA. This process is triggered by a variety of agents, including oxidants such as hydrogen peroxide (H 2 O 2 ) and it plays a key role in eliminating pre-neoplastic cells from the lung. Failure to do so could favor tumor promotion. The current study demonstrated that alveolar epithelial cells, adapted to cadmium (CdCl 2 ) by repeated in vitro exposure, exhibit lower levels of H 2 O 2 -induced apoptosis than similarly challenged non-adapted cells. An immunologic assay, measuring cytoplasmic histone-associated DNA fragments, indicated maximal apoptosis 24 h after exposure to 400 μM H 2 O 2 . Non-adapted cells showed a 13-fold increase in oxidant-induced apoptosis while Cd-adapted cells had only a 4-fold elevation. A terminal deoxyribonucleotidyl transferase mediated dUTP nick end labeling (TUNEL) method was used to assess the percentage of cells with DNA breaks consistent with apoptosis. Cd-adapted and non-adapted cells that were not exposed to H 2 O 2 did not differ in TUNEL positivity. However, after H 2 O 2 treatment, the percentage of TUNEL positive cells was 4-fold higher in non-adapted cultures than in adapted ones. Suppression of oxidant-induced apoptosis is due, in part, to up-regulation in the gene expression of several resistance factors including metallothioneins (MT-1 and MT-2), glutathione S -transferases (GST-α and GST-π), and γ-glutamylcysteine synthetase catalytic subunit (γ-GCS). These steady-state mRNA changes, determined by Northern blotting, were accompanied by increased levels of MT and γ-GCS protein, GST activity, and glutathione (GSH). Suppressed oxidant-induced apoptosis, resulting at least in part from these response modifications, could leave pre-neoplastic or neoplastic cells alive, favor clonal expansion, and ultimately lead to cancer development.