We established previously that X-linked inhibitor of apoptosis protein (Xiap) is a determinant of cisplatin (CDDP) resistance in human ovarian cancer cells and that down-regulation of Xiap sensitizes cells to CDDP in the presence of wild-type p53. Furthermore, Xiap up-regulates the phosphatidylinositol 3'-kinase/Akt pathway by increasing Akt phosphorylation. However, the precise relationships among Xiap, Akt, and p53 in chemoresistance are unknown. Here we show that both Xiap and Akt can modulate CDDP sensitivity individually but that Xiap requires Akt for its full function. Furthermore, dominant-negative Akt sensitizes ovarian cancer cells to CDDP (10 micro M), an effect that is absent in cells expressing mutant p53 or treated with the p53 inhibitor pifithrin-alpha-hydrobromide (30 micro M) but restored by exogenous wild-type p53. CDDP increased p53, decreased Xiap content, and induced apoptosis in OV2008 cells but not in the resistant counterpart (C13*). However, dominant-negative Akt restored all of these characteristics to C13* cells. Expression of a constitutively active Akt2 prevented CDDP-mediated down-regulation of Xiap and apoptosis in A2780s cells. Akt2-mediated chemoresistance could not be reversed by Xiap down-regulation. These results suggest that whereas Xiap, Akt2, and p53 are important mediators of chemoresistance in ovarian cancer cells, Akt2 may be an important regulator of both Xiap and p53 contents after CDDP challenge. Inhibition of Xiap and/or Akt expression/function may be an effective means of overcoming chemoresistance in ovarian cancer cells expressing either endogenous or reconstituted wild-type p53.