The phosphoinositide-3 kinase (PI3K)/Akt signal pathway plays a key role in the tumorigenesis of many cancers and in the subsequent development of drug resistance. Using the K562 chronic myelogenous leukemia (CML) cell line and the doxorubicin-resistant derivative lines KD30 and KD225 as models, we observed that enhanced PI3K/Akt activity and the acquisition of chemoresistance correlated unexpectedly with the increased expression and nuclear accumulation of FOXO3a. Moreover, we found that the induction of FOXO3a activity in naïve K562 cells was sufficient to enhance PI3K/Akt activity and to confer resistance to the cytotoxic effects of doxorubicin. Conversely, the knockdown of endogenous FOXO3a expression reduced PI3K/Akt activity and sensitized these cells to doxorubicin. Further chromatin immunoprecipitation and promoter mutation analyses demonstrated that FOXO3a regulates the expression of the PI3K catalytic subunit p110␣ through the activation of a promoter region proximal to a novel untranslated exon upstream from the reported transcription start site of the p110␣ gene PIK3CA. As was the case for FOXO3a, the expression or knockdown of p110␣ was sufficient to amplify or reduce PI3K/Akt activity, respectively. Thus, our results suggest that the chronic activation of FOXO3a by doxorubicin in CML cells can enhance survival through a feedback mechanism that involves enhanced p110␣ expression and hyperactivation of the PI3K/Akt pathway.Chemotherapy is widely used for the treatment of leukemia and other advanced or metastatic cancers. However, its efficacy is often hampered by the development of intrinsic or acquired multidrug resistance (MDR), characterized by simultaneous cross-resistance to anticancer drugs that differ in their chemical structures, modes of action, and cellular targets (35). An understanding of the mechanisms of MDR is important for the development of more effective therapies. At the cellular level, three general mechanisms confer MDR in cancer cells, including decreased hydrophilic drug uptake, increased hydrophobic drug efflux, and enhanced cell survival signals or mechanisms. The first one involves a decrease in the expression or activity of transporters that regulate the uptake of hydrophilic chemotherapeutic drugs, such as folate antagonists, nucleoside analogues, and cisplatin. The second mechanism entails an upregulation of transporters, resulting in an increased energy-dependent efflux of a wide variety of hydrophobic chemotherapeutic agents. Finally, cancer cells often counteract the cytotoxic effects of therapeutic agents by amplifying the activity of proliferation and survival signal pathways, by increasing DNA damage repair, or by altering drug metabolism.Phosphoinositide-3 kinases (PI3Ks) are a family of lipid kinases that serve as mediators of signals generated by many different activated growth factor receptors and adhesion molecules. The class IA PI3Ks are heterodimers composed of a p110 catalytic subunit and a p85 regulatory subunit (25). When activated by growth factors, the p...

Using the doxorubicin-sensitive K562 cell line and the resistant derivative lines KD30 and KD225 as models, we found that acquisition of multidrug resistance (MDR) is associated with enhanced FOXO3a activity and expression of ABCB1 (MDR1), a plasma membrane P-glycoprotein that functions as an efflux pump for various anticancer agents. Furthermore, induction of ABCB1 mRNA expression on doxorubicin treatment of naive K562 cells was also accompanied by increased FOXO3a activity. Analysis of transfected K562, KD30, and KD225 cells in which FOXO3a activity can be induced by 4-hydroxytamoxifen showed that FOXO3a up-regulates ABCB1 expression at protein, mRNA, and gene promoter levels. Conversely, silencing of endogenous FOXO3a expression in KD225 cells inhibited the expression of this transport protein. Promoter analysis and chromatin immunoprecipitation assays showed that FOXO3a regulation of ABCB1 expression involves binding of this transcription factor to the proximal promoter region. Moreover, activation of FOXO3a increased ABCB1 drug efflux potential in KD30 cells, whereas silencing of FOXO3a by siRNA significantly reduced ABCB1 drug efflux ability. Together, these findings suggest a novel mechanism that can contribute towards MDR, involving FOXO3a as sensor for the cytotoxic stress induced by anticancer drugs. Although FOXO3a may initially trigger a program of cell cycle arrest and cell death in response to doxorubicin, sustained FOXO3a activation promotes drug resistance and survival of cells by activating ABCB1 expression. [Mol Cancer Ther 2008;7(3):670 -8]