The role of p21CIP1/WAF1 in growth of epithelial cells exposed to hyperoxia

Previous studies have shown that hyperoxia inhibits proliferation and increases the expression of the tumor suppressor p53 and its downstream target, the cyclin-dependent kinase inhibitor p21(CIP1/WAF1), which inhibits proliferation in the G(1) phase of the cell cycle. To determine whether growth arrest was mediated through activation of the p21-dependent G(1) checkpoint, the kinetics of cell cycle movement during exposure to 95% O-2 were assessed in the Mv1Lu and A549 pulmonary adenocarcinoma cell lines. Cell counts, 5-bromo-2'-deoxyuridine incorporation, and cell cycle analyses revealed that growth arrest of both cell lines occurred in S phase, with A549 cells also showing evidence of a G(1) arrest. Hyperoxia increased p21 in A549 but not in Mv1Lu cells, consistent with the activation of the p21-dependent G(1) checkpoint. The ability of p21 to exert the G(1) arrest was confirmed by showing that hyperoxia inhibited proliferation of HCT 116 colon carcinoma cells predominantly in G(1), whereas an isogenic line lacking p21 arrested in S phase. The cell cycle arrest in S phase appears to be a p21-independent process caused by a gradual reduction in the rate of DNA strand elongation. Our data reveal that hyperoxia inhibits proliferation in G(1) and S phase and demonstrate that p53 and p21 retain their ability to affect G(1) checkpoint control during exposure to elevated O-2 levels.