Variations of Noxa Levels Caused by p38 MAPK Signaling Result In Heterogeneous Sensitivity to ABT-737 In CD40L-Stimulated CLL Cells

Abstract 1825

Chronic lymphocytic leukemia (CLL) is characterized by relentless relapses. It is postulated that this might be due to microenvironment-induced resistance to cytotoxic agents. Previously we have shown that in vitro CD40 stimulation of peripheral blood derived from CLL patients can to a certain extent mimic the lymph node (LN) microenvironment and result in resistance to cytotoxic drugs. This is correlated with CD40-induced changes in apoptosis regulating Bcl-2 family members. The BH3 mimetic drug ABT-737 antagonizes the anti-apoptotic molecules Bcl-XL and Bcl-2, but not Mcl-1 or Bfl-1. At present it is unknown whether patient-specific variations in expression of apoptosis regulating proteins occur and to what extent they correlate with sensitivity or resistance to ABT-737.

In the present investigation we assessed the variability in response to ABT-737 in CD40 stimulated CLL cells. Although in all tested CD40-stimulated CLL patients Mcl-1 and Bfl-1 levels increased, there was a heterogeneous response in cell death to ABT-737 when used as a single agent. Notably, time course analyses showed variations in Noxa levels upon CD40 triggering, which differed between CLL patients. High Noxa/Mcl-1 ratios correlated with sensitivity to ABT-737. This was confirmed by Noxa knockdown as well as Mcl-1 and Bfl-1 overexpression, which resulted in resistance to ABT-737. To determine the signaling pathways involved in Noxa regulation, CLL cells were stimulated with CD40L in the presence of various kinase inhibitors. We observed that Noxa levels were specifically inhibited by the p38 MAP kinase inhibitor SB203580. Preliminary data indicate that after CD40 stimulation, phosphorylation of p38 decreased over time, correlating with a decrease in Noxa levels. Importantly however, a minority of CD40 stimulated CLL samples were sensitive to ABT-737 and showed high levels of phosphorylated p38 and increasing Noxa levels over time.

In conclusion, we demonstrate that following CD40 triggering of CLL cells the p38 MAPK signaling pathway regulates Noxa levels and that patient-specific variations in this pathway contribute to the responsiveness to ABT-737. These findings raise the question whether assessment of p38 MAPK activation status and Noxa levels in vivo could predict the response of CLL patients to ABT-737. Moreover our findings offer possible new targets for the treatment of CLL.