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 Bcl-2/Bax ratios in chronic lymphocy tic leukaemia (B-CLL) correlate with ill vitro apoptosis and clinical resis tance, We have now monitored the in vitro viability of B-CLL cells in relat ion to Bcl-2 and Bax expression over a 48 h time course following exposure to chlorambucil. The results showed that Bar up-regulation was essential fo r chlorambucil-induced apoptosis in B-CLL cells and a 3-fold increase in ex pression within 4 h of exposure to drug was typically observed in sensitive cells: resistant cells failed to up-regulate Bar at all. In contrast, the constitutively high levels of Bcl-2 found in B-CLL cells were found to be d own-regulated in apoptotic cells but the mean Bcl-2 expression in viable ce lls was increased, probably as a result of the loss of lower Bcl-2-expressi ng cells into the apoptotic compartment. Taken together, these data add fur ther weight to the suggestion that Bcl-2/Bax ratios map be pivotal in deter mining the fate of B-CLL cells, Furthermore, the Bcl-2/Bax ratios found in apoptotic B lymphocytes were remarkably similar in the treated, untreated a nd normal control cells, which suggests that there is a universal Bcl-2/Bax ratio threshold for cell survival and cell death.