Chlorambucil resistance in B-cell chronic lymphocytic leukaemia is mediated through failed Bax induction and selection of high Bcl-2-expressing subclones.

Our previous data have shown that high Bct-2/ Bax ratios in chronic lymphocytic leukaemia (B-CLL) correlate with in vitro apoptosis and clinical resistance. We have now monitored the in vitro viability of B-CLL cells in relation to Bcl-2 and Bax expression over a 48 h time course following exposure to chlorambucil. The results showed that Bax up-regulation was essential for chlorambucil-induced apoptosis in B-CLL cells and a 3-fold increase in expression within 4 h of exposure to drug was typically observed in sensitive cells; resistant cells failed to up-regulate Bax at all. In contrast, the constitutively high levels of Bcl-2 found in B-CLL cells were found to be down-regulated in apoptotic cells but the mean Bcl-2 expression in viable cells was increased, probably as a result of the loss of lower Bcl-2-expressing cells into the apoptotic compartment. Taken together, these data add further weight to the suggestion that Bcl-2/Bax ratios may be pivotal in determining the fate of B-CLL cells. Furthermore, the Bcl-2/Bax ratios found in apoptotic B lymphocytes were remarkably similar in the treated, untreated and normal control cells, which suggests that there is a universal Bcl-2/Bax ratio threshold for cell survival and cell death.