p21(Cip1/Waf1/Sdi1) protects against hyperoxia by maintaining expression of Bcl-X(L)

p21(Cip1/WAF1/Sdi1) is a major transcriptional target of p53 that promotes survival of cells exposed to continuous oxidative stress caused by hyperoxia. Because p21 can protect against genotoxic stress by reducing p53-dependent transcription of the proapoptotic proteins PUMA and Bax, the current study uses genetically modified lines of HCT116 colon carcinoma cells to investigate whether p21-mediated protection against hyperoxia involves attenuation of the p53 apoptotic pathway. Hyperoxia stimulated p53-dependent expression of p21 and Bax. Genetic ablation of p21 increased cell death, and loss of Bax or PUMA increased cell survival. Unlike damage caused by adriamycin, whereby p21 sensitivity could be rescued by removal of p53, PUMA, or Bax, increased sensitivity of p21-deficient cells to hyperoxia could not be rescued by additional loss of these genes. Instead, expression of the antiapoptotic protein Bcl-X(L) declined in p21-deficient cells exposed to hyperoxia, but when genetically restored, increased their survival. Conversely, siRNA knockdown of Bcl-X(L) in parental HCT116 cells increased hyperoxia-induced cell death. These findings reveal that p21-mediated protection against hyperoxia does not involve attenuation of p53-dependent apoptosis, but rather functions to maintain Bcl-X(L) expression during periods of persistent oxidative stress.