p53 expression overcomes p21^WAF1/CIP1-mediated G₁ arrest and induces apoptosis in human cancer cells

The p21^WAF1/CIP1 gene, which encodes a cyclin-dependent kinase inhibitor, may be critical for tumor suppressor gene p53-induced cell cycle arrest. The p53 gene is known to regulate G₁ checkpoint, which can either induce G₁ arrest or initiate apoptosis. To directly examine the role of p21^WAF1/CIP1 in the control of p53 function, we have introduced human p21^WAF1/CIP1 gene into a p53-deficient human non-small cell lung cancer cell line H1299 using a p21^WAF1/CIP1-expressing adenoviral vector (AdCMVp21). Infection with AdCMVp21 resulted in high levels of p21^WAF1/CIP1 expression and significantly suppressed the growth of H1299 cells through the G₁ arrest of the cell cycle. In contrast, transient expression of the wild-type p53 gene by a recombinant adenoviral vector (AdCMVp53) in H1299 cells induced apoptotic cell death and resulted in a rapid loss of cell viability. We then examined the effects of combined infection with AdCMVp21 and AdCMVp53 on H1299 cells to explore the dominant function of these molecules. Interestingly, introduction of exogenous p53 overcame p21^WAF1/CIP1-mediated cell cycle arrest at G₁ and induced apoptosis, although viral-transduced p21^WAF1/CIP1 expression level was unaffected. These observations suggest that p53 expression converts a p21^WAF1/CIP1-mediated growth arrest into apoptosis. The result was repeated with two additional human colon adenocarcinoma cell lines with the different p53 status, mutant p53-expressing DLD-1 and wild-type p53-expressing LoVo, suggesting that this phemonenon is a general event among human cancer cells. Thus, p53-mediated apoptotic pathway is dominant over the growth arrest pathway, indicating that p53 may be an essential upstream mediator of p21^WAF1/CIP1 in the regulation of a cell process leading either to growth arrest or to apoptotic suicide.