The anti-apoptotic genes Bcl-X_L and Bcl-2 are over-expressed and contribute to chemoresistance of non-proliferating leukaemic CD34⁺ cells

In acute myeloid leukaemia (AML), cell kinetic quiescence has been postulated to contribute to drug resistance. As the anti-apoptotic genes Bcl-2 and Bcl-X_L have been implicated in cell cycle regulation, we investigated the expression of these genes in non-proliferating (Q) and proliferating (P) AML and normal CD34⁺ progenitor cells. Using reverse transcription polymerase chain reaction, Bcl-X_L and Bcl-2 were overexpressed in Q versus P AML cells, whereas no difference in Bcl-X_S and Bax expression was found. Furthermore, the Bcl-X_L/X_S but not the Bcl-2/Bax ratio was higher in Q AML compared with normal CD34⁺ Q cells (P = 0.001). An inverse correlation between Bcl-2 expression of leukaemic Q cells and their ability to enter the cell cycle was found. Treatment with all-trans retinoic acid (ATRA) reduced Bcl-2 and Bcl-X_L expression in the leukaemic Q cells, and enhanced their chemosensitivity to cytosine arabinoside (ara-C). These findings demonstrate overexpression of the anti-apoptotic proteins Bcl-X_L and Bcl-2 in quiescent CD34⁺ AML cells and suggest their involvement in the chemoresistance. The observed inverse correlation between Bcl-2 and proliferation suggests a role for Bcl-2 in the cell cycle regulation of AML. These findings could be used in the development of therapies that selectively induce apoptosis in quiescent leukaemic progenitor cells.