Targeting XIAP and ARC (apoptosis repressor with caspase recruitment domain) Overcomes Imatinib Resitance In Blast Crisis CML Cells.

Abstract 3415

The advent of imatinib, a Bcr-Abl tyrosine kinase inhibitor (TKI) has revolutionized the treatment of patients with CML. Development of resistance and limited activity in blast crisis (BC) CML are evolving problems facing this therapy. We found that XIAP, a potent caspase inhibitor, is highly expressed in CML cells, in both, cell lines and patient samples. Treatment with imatinib deceased XIAP levels in imatinib-sensitive KBM5 but much less so in imatinib-resistant KBM5STI571 cells (harboring T315I mutation) suggesting that XIAP expression in CML is regulated at least in part via Bcr-Abl and that targeting XIAP may promote cell death in CML cells by circumventing imatinib resistance. To test this, we treated BC CML cells with XIAP antisense oligonucleotide (ASO) and with SMAC mimetic ABT-10 and found that inhibition of XIAP induced apoptotic cell death with similar efficacy in KBM5 cells and KBM5STI517 cells (EC50=6.3±0.3 μM and 8.4±0.4 μM at 48 hours, respectively for ABT-10). However, we noted that inhibition of XIAP by ASO induced the expression, in both KBM5 and KBM5STI571 cells, of apoptosis repressor with caspase recruitment domain (ARC) in both mRNA and protein levels but not the expression of Bcl-2 protein. ARC is a unique antiapoptotic protein. It acts through inhibiting caspases and antagonizing the activity and function of p53 and Bax. Therefore, its induction may antagonize the effect of XIAP downregulation. Indeed, inhibition of both XIAP and ARC by ASO induced significantly more cell death than inhibiting either protein alone in both KBM5 and KBM5STI cells. Furthermore, we demonstrated that XIAP inhibition induced-apoptosis was enhanced by imatinib in KBM5, but not in KBM5STI cells. Interestingly, inhibition of Bcr-Abl tyrosine kinase by imatinib not only decreased XIAP, but also suppressed ARC levels in KBM5 but had minimal effects on the levels of these proteins in KBM5STI571 cells and enforced expression of the Bcr-Abl p185 fusion protein (in HL-60 cells) greatly increased both XIAP and ARC levels. This induction was inhibited by imatinib suggesting that ARC is also a downstream target of Bcr-Abl tyrosine kinase. Therefore, imatinib enhancing XIAP inhibition induced-apoptosis in KBM5, not KBM5STI cells can be explained at least in part by its ability to decrease XIAP and ARC levels.

In conclusion, XIAP is highly expressed in CML cells and upregulated by Bcr-Abl. Targeting XIAP promotes death of BC and TKI resistant CML cells. Results suggest that XIAP is a potential target in BC and TKI resistant CML cells and that XIAP inhibition-induced apoptosis is enhanced by imatinib in TKI sensitive cells and by ARC inhibition independent of cellular responses to TKIs. Inhibition of XIAP and ARC as a novel therapeutic strategy in CML warrants further investigation.