c-Abl inhibition decreases Mcl-1 expression in CLL cells and induces apoptosis. (A) Western blot analysis of Mcl-1 expression in CLL cells treated with imatinib. CLL cells were cultured for 24 hours in the presence or absence of 20μM imatinib and/or 50μM z-VAD. Lysates were probed by Western blot for the indicated proteins. (B) Graphical representation of the data in panel A. Mcl-1 expression is quantitated relative to β-actin and normalized between patient samples to the levels of Mcl-1 in freshly thawed CLL cells. The data are presented as the mean ± SEM using the malignant cells from 4 different patient samples. (C) Western blot analysis of Mcl-1 expression of CLL cells transfected with control (c) and c-Abl–specific (−Abl) siRNA. Cell viability (% viable) of the transfected cells, as determined by trypan blue, and percentage knockdown (% KD) of c-Abl and Mcl-1 are indicated. (D) Quantitative RT-PCR analysis of Mcl-1 mRNA levels in CLL cells treated with imatinib. CLL cells were cultured for 24 hours in the presence or absence of 20μM imatinib. Mcl-1 mRNA levels were measured relative to those of RPL27 (a housekeeping gene). The results are presented as mean ± SEM using the malignant cells from 6 different patient samples. Statistical significance in all parts of this figure was determined using a Student t test.