Phosphorylated and hypoacetylated mutant p53 enhances cisplatin-induced apoptosis through caspase-9 pathway in the absence of transcriptional activation or translation.

It is not completely understood how certain epithelial cells harboring mutant p53 have better response to chemotherapy. We investigate the mechanism of cisplatin-induced apoptosis in two resistant cell lines (parental TCCSUP and R273L mutant p53 transfectant) and two sensitive cell lines (V143A and N247I mutant p53 transfectants). Activation of caspase 9 was demonstrated by Western blotting, and specific inhibitor for caspase 9 could inhibit apoptosis. Inhibitors for caspases 1, 2, 6, and 8 had no effect on apoptosis. Transcriptional repression of Bcl-2 occurred during apoptosis and could be reversed by the treatment of histone deacetylase inhibitor trichostatin A (TSA). The expression of Noxa, p53 inducible ribonucleotide reductase subunit 2 (p53R2), and p53 inducible death domain (PIDD) gene were not elevated with treatment of cisplatin (CDDP). Surface trafficking of Fas or Fas-L was not observed. Ser15 of wild-type p53 and mutant p53 was phosphorylated in response to cisplatin. Acetylation of wild-type p53 increased, while acetylation of mutant p53 decreased during cisplatin treatment. Both transcriptional inhibitor actinomycin D and translational inhibitor cycloheximide did not inhibit apoptosis. These results indicated that phosphorylated and hypoacetylated mutant p53 could enhance cisplatin-induced apoptosis through activation of caspase 9 independent of transcriptional activation and translation.