[Show abstract][Hide abstract] ABSTRACT: Cisplatin is a first-line chemotherapeutic for ovarian cancer, although chemoresistance limits treatment success. Apoptosis, an important determinant of cisplatin sensitivity, occurs via caspase-dependent and -independent mechanisms. Activation of the protein kinase Akt, commonly observed in ovarian tumours, confers resistance to ovarian cancer cells via inhibition of caspase-dependent apoptosis. However, the effect of Akt on cisplatin-induced, caspase-independent apoptosis remains unclear. We show that in chemosensitive ovarian cancer cells, cisplatin induces the mitochondrial release and nuclear translocation of apoptosis-inducing factor (AIF), a mediator of caspase-independent apoptosis, and AIF-dependent apoptosis. Cisplatin failed to induce these effects in the chemoresistant variant cells. Overexpression of AIF sensitised resistant cells to cisplatin-induced apoptosis. Finally, activation of Akt attenuated the cisplatin-induced mitochondrial release and nuclear accumulation of AIF and apoptosis in chemosensitive cells, whereas inhibition of Akt activity facilitated these effects and sensitised chemoresistant cells to AIF-dependent, cisplatin-induced apoptosis. These results suggest that cisplatin-induced apoptosis proceeds, in part, via a caspase-independent mechanism involving AIF, and that Akt activation confers resistance to cisplatin-induced apoptosis by blocking this pathway. These results provide insights into the molecular mechanism of chemoresistance, and suggest that inhibition of Akt activity may represent a novel therapeutic approach to the treatment of cisplatin-resistant ovarian cancer.
Full-text · Article · Mar 2008 · British Journal of Cancer
[Show abstract][Hide abstract] ABSTRACT: Resistance to cisplatin-based chemotherapy is a major cause of treatment failure in human ovarian cancer. Wild-type TP53 status is often, but not always, associated with cisplatin sensitivity, suggesting that additional factors may be involved. Overexpression/activation of the phosphatidylinositol-3-kinase/Akt pathway is commonly observed in ovarian cancer, and Akt activation is a determinant of chemoresistance in ovarian cancer cells, an effect that may be due, in part, to its inhibitory actions on p53-dependent apoptosis. To that end, we examined the role and regulation of p53 in chemosensitive ovarian cancer cells, as well as in their chemoresistant counterparts, and investigated if and how Akt influences this pathway. Cisplatin induced apoptosis in chemosensitive, but not chemoresistant cells, and this was inhibited by downregulation of p53. Cisplatin upregulated PUMA in a p53-dependent manner, and the presence of PUMA was necessary, but not sufficient for cisplatin-induced apoptosis. p53 was phosphorylated on numerous N-terminal residues, including Ser15, Ser20, in response to cisplatin in chemosensitive, but not chemoresistant cells. Furthermore, activation of Akt inhibited the cisplatin-induced upregulation of PUMA, and suppressed cisplatin-induced p53 phosphorylation, while inhibition of Akt increased total and phospho-p53 contents and sensitized p53 wild-type, chemoresistant cells to cisplatin-induced apoptosis. Finally, mutation of Ser15 and/or Ser20, but not of Ser37, to alanine significantly attenuated the ability of p53 to facilitate CDDP-induced apoptosis, and this was independent of PUMA expression. These results support the hypothesis that p53 is a determinant of CDDP sensitivity, and suggest that Akt contributes to chemoresistance, in part, by attenuating p53-mediated PUMA upregulation and phosphorylation of p53, which are essential, but independent determinants of sensitivity to CDDP-induced apoptosis.
Full-text · Article · Feb 2008 · International Journal of Cancer