Inhibition of XIAP, an Anti-Apoptotic Molecular Target Potentiates Radiation-induced Cell Death in Cancer

1989

Radiation therapy plays an important role in the management of majority of cancers. However, resistance to radiotherapy remains a significant hurdle. Radiotherapy induces DNA strand breaks as well as damage to the cell membrane which triggers the apoptotic pathways resulting in activation of caspases. Previous studies from our laboratory and others have identified the role of X-linked inhibitor of apoptosis (XIAP), a member of the IAP family, as a potent caspase inhibitor via its action on both intrinsic and extrinsic apoptotic pathways. Translational upregulation of XIAP has been shown to occur following treatment with acute low dose ionizing radiation correlating with increased resistance to radiation. The purpose of this study was 1) to elaborate the role of XIAP in radiation resistance in prostate cancer cells 2) design and test a molecular-target based strategy using antisense phosphorodiamidate Morpholino oligomers (PMO) to inhibit XIAP in combination with radiotherapy. DU145 androgen independent, Rb-/-, p53-/-, prostate cells were treated with 10 Gy gamma radiation for a period of 6h to 7 days. The cells showed resistance 24 h following radiation. Immunoblot analysis revealed that the observed resistance directly correlated with increased XIAP expression. Cell growth was observed to decrease at later time points with a corresponding decline in XIAP levels in the treated population compared to the control. A 20-mer XIAP AS PMO targeting the start site of XIAP mRNA and a corresponding scrambled control PMO were synthesized. PMO are neutral antisense agents which inhibit RNA translation by an RNAse independent mechanism of action. These agents are currently in clinical trials for cancer and other diseases and have shown excellent efficacy and safety profile. Combination treatment strategies included a) treatment of cells with the XIAP AS or scrambled PMO for 24 h followed by treatment with 10Gy of gamma radiation b) Alternatively, cells were treated with 10Gy of gamma radiation, followed immediately by XIAP AS PMO or scrambled PMO treatment. A significant decrease in cell viability along with specific decrease in XIAP levels was observed in cells pre-treated with XIAP AS PMO followed by radiation compared to radiation alone. On the other hand, radiation treatment followed by XIAP AS PMO did not cause any greater effect on cell viability compared to radiation alone. In summary, the data in this study reveal the role of XIAP expression in radiation resistance and the need for a schedule-dependent combination strategy. Modulation of XIAP using the PMO antisense technology is therefore an attractive mechanism-based therapeutic strategy for prostate cancer.