Chronic lymphocytic leukemia (CLL) is a disease in which two-thirds of patients will require therapy, usually with the alkylating agent, chlorambucil (CLB) or the nucleoside analog, fludarabine (FLU). TRAIL is a death receptor ligand that has shown selective cytotoxicity towards a variety of malignancies. CLL cells, however, are relatively resistant to this agent. We have previously demonstrated that CLB- and Flu-induced apoptosis is partially mediated through activation of the TRAIL apoptotic pathway and this is related to the up-regulation of the TRAIL death receptors, DR4 and DR5 (

Oncogene, 22:8356–8369, 2003
). Combining CLB or Flu with TRAIL produced a synergistic apoptotic response in CLL cells. In contrast, the upregulation of DR5 and the synergistic apoptotic response was not observed in normal lymphocytes. We have subsequently demonstrated that the up-regulation of DR5 is mediated by transcription factors, nuclear factor κB (NFκB) and p53, and histone acetylation. Using chromatin immunoprecipitation assays, we have found that the p65 subunit of NFκB and p53 are bound to the DR5 gene in CLL cells following either CLB or Flu treatment. In addition, histone 3 is acetylated following CLB and Flu treatment corresponding to histone acetylase p300 binding to the DR5 gene. Histone deacetylase 1 (HDAC1) also binds to the DR5 gene following CLB or Flu treatment, but generally at later time points. Overall histone acetylation was also found to be increased in CLL cells, as compared to normal lymphocytes. Treatment with HDAC inhibitors, which are being evaluated in clinical trials, resulted in increased binding of p53 and NFκB, but not HDAC1, to the DR5 gene and increased DR5 mRNA levels in cells. Furthermore, enhanced apoptosis was also observed in CLL cells treated with combinations of SAHA (another HDAC inhibitor) and TRAIL. These findings suggest that histone acetylation is important in regulating DR5 expression and that HDAC inhibitors increase DR5 expression mediated by p53 and NFκB. This could provide a mechanism to sensitize CLL cells to TRAIL-induced apoptosis.

Topics:
chronic b-cell leukemias, chronic lymphocytic leukemia, death domain receptors, histone acetylation, influenza, histone deacetylase inhibitors, alkylating agents, cancer, chlorambucil, cytotoxicity

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2005, The American Society of Hematology
2005
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