Sensitisation of HL60 human leukaemic cells to cytotoxic drug-induced apoptosis by inhibition of PI3-kinase survival signals

Drug resistance remains a serious limiting factor in the treatment of acute myeloid leukaemia (AML) either at initial presentation or following primar y or subsequent relapses. Using specific kinase inhibitors, this study has investigated the contribution of the Ras/PI3-kinase regulated survival path ways to drug resistance and suppression of apoptosis in a cell line derived from AML (HL60). Inhibition of the Raf/MAP-kinase (ERK) pathway with a spe cific MAP-kinase inhibitor, apigenin did not sensitise HL60 cells to drug-i nduced apoptosis, indicating a lack of involvement in chemoresistance. In c ontrast, the PI3-kinase inhibitors, LY294002 and wortmannin, did induce a s ignificant increase in apoptosis in combination with cytotoxic drugs. The c ontribution of downstream mediators of PI3-kinase, p70S6-kinase and PKB/Akt were then investigated. While inhibition of p70S6-kinase with rapamycin di d not increase drug-induced apoptosis, PI3-kinase inhibition resulted in no table dephosphorylation of PKB, suggesting that the PI3-kinase/PKB survival pathway may play a major role in chemoresistance in AML. This pathway has been reported to mediate heterodimer interactions with the proapoptotic reg ulator, Bad. In contrast to previous studies, we found no evidence of Bad b inding to anti-apoptotic Bcl-2, Bcl-X-L or Mcl-1, or of alterations in Bar heterodimers. This suggests that alternative targets of PI3-kinase/PKB, dis tinct from the Bcl-2 family may be responsible for contributing to survival factor-mediated drug resistance in AML.