c-Abl kinase inhibitors overcome CD40-mediated drug resistance in CLL: implications for therapeutic targeting of chemoresistant niches

In lymph node (LN) proliferation centers in chronic lymphocytic leukemia (CLL), the environment protects from apoptotic and cytotoxic triggers. Here, we aimed to define the molecular basis for the increased drug resistance and searched for novel strategies to circumvent it. The situation in CLL LN could be mimicked by prolonged in vitro CD40 stimulation, which resulted in up-regulation of antiapoptotic Bcl-xL, A1/Bfl-1, and Mcl-1 proteins, and afforded resistance to various classes of drugs (fludarabine, bortezomib, roscovitine). CD40 stimulation also caused ERK-dependent reduction of Bim-EL protein, but ERK inhibition did not prevent drug resistance. Drugs combined with sublethal doses of the BH3-mimetic ABT-737 displayed partial and variable effects per individual CD40-stimulated CLL. The antiapoptotic profile of CD40-triggered CLL resembled BCR-Abl-dependent changes seen in chronic myeloid leukemia (CML), which prompted application of c-Abl inhibitors imatinib or dasatinib. Both compounds, but especially dasatinib, prevented the entire antiapoptotic CD40 program in CLL cells, and restored drug sensitivity. These effects also occurred in CLL samples with dysfunctional p53. Importantly, ex vivo CLL LN samples also displayed strong ERK activation together with high Bcl-xL and Mcl-1 but low Bim levels. These data indicate that CLL cells in chemoresistant niches may be sensitive to therapeutic strategies that include c-Abl inhibitors. (Blood. 2008; 112: 5141-5149

Differential Noxa/Mcl-1 balance in peripheral versus lymph node chronic lymphocytic leukemia cells correlates with survival capacity

The gradual accumulation of chronic lymphocytic leukemia (B-CLL) cells is presumed to derive from proliferation centers in lymph nodes and bone marrow. To what extent these cells possess the purported antiapoptotic phenotype of peripheral B-CLL cells is unknown. Recently, we have described that, in B-CLL samples from peripheral blood, aberrant apoptosis gene expression was not limited to protective changes but also included increased levels of proapoptotic BH3-only member Noxa. Here, we compare apoptosis gene profiles from peripheral blood B-CLL (n=15) with lymph node B-CLL (>90% CD5+/CD19+/CD23+ lymphocytes with Ki67+ centers; n=9). Apart from expected differences in Survivin and Bcl-xL, a prominent distinction with peripheral B-CLL cells was the decreased averaged level of Noxa in lymph nodes. Mcl-1 protein expression showed a reverse trend. Noxa expression could be reduced also in vitro by CD40 stimulation of peripheral blood B-CLL. Direct manipulation of Noxa protein levels was achieved by proteasome inhibition in B-CLL and via RNAi in model cell lines. In each instance, cell viability was directly linked with Noxa levels. These data indicate that suppression of Noxa in the lymph node environment contributes to the persistence of B-CLL at these sites and suggest that therapeutic targeting of Noxa might be beneficia