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

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 (H2O2) 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 (CdCl2) by repeated in vitro exposure, exhibit lower levels of H2O2-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 mu M H2O2. 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 H2O2 did not differ in TUNEL positivity. However, after H2O2 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-alpha and GST-pi), and gamma-glutamylcysteine synthetase catalytic subunit (gamma-GCS). These steady-state mRNA changes, determined by Northern blotting, were accompanied by increased levels of MT and gamma-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. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.