OBJECTIVETo investigate reversal effect of histone deacetylase inhibitor LBH589 alone or in combination with proteasome inhibitor bortezomib on drug resistance in acute myeloid leukemia (AML) and its mechanism.

METHODSEx vivo cultures of HL-60/ADM cells and fresh refractory AML cells were treated with LBH589, bortezomib or their combination at varying concentrations. Proliferation capacity, apoptosis rate and reversal of drug resistance were evaluated by MTT assay, dual staining of Hoechst 33342 and Annexin VFITC/PI by flow cytometry, and adriamycin uptake rate with proliferation inhibition, respectively. The change of signal pathway at protein level was analyzed by Western blot.

RESULTSSynergistic cytotoxicity was observed in the combination treatment with LBH589 and bortezomib against HL-60/ADM cells, as well as the fresh AML cells, the most powerful synergy being observed at 21 nmol/L LBH589 plus 12 nmol/L bortezomib, with CI values of 0.531 and 0.498, respectively by Calcusyn software analysis. Moreover, the accumulation of adriamycin in HL-60/ADM cells was increased more in combination treatment [(64.81 +/- 3.69)%] than in either LBH589 [(28.96 +/- 2.52)%] or bortezomib [(37.29 +/- 3.71)%] alone (P < 0.05), and so did the uptake rate of adriamycin being (64.81 +/- 3.69)%, (28.96 +/- 2.52)% and (37.29 +/- 3.71)% respectively (P < 0.05). The combination treatment induced multiple apoptotic molecules co-action and intracellular drug accumulation contributed to the synergistic cytotoxicity, including caspase activation, PARP cleavage, XIAP downregulation, p53-dependent suppression of Bcl-2 and MRP1 expression via the inhibition of phosphoinositide 3-kinase (PI3K)/Akt/nuclear factor-kappaB (NF-kappaB) signaling pathway.

CONCLUSIONSCombination treatment of drug resistant AML cells with LBH589 and bortezomib produces a synergistic effect of in creating sensitivity to chemotherapy. The mechanism may be mainly resulted from inhibition of PI3K/ Akt/NF-kappaB signaling pathway.