Abstract P1-04-01: X-Linked Inhibitor of Apoptosis Protein (XIAP) Regulates TRAIL-Induced Apoptosis in Inflammatory Breast Cancer Cells

Abstract

Abstract TRAIL, a proapoptotic receptor agonist, is a promising tool for anticancer therapy due to its selective triggering of death receptor-mediated apoptosis in cancer cells while sparing the majority of normal cells. Overexpression of X-linked inhibitor of apoptosis protein (XIAP) has been identified as a major mechanism of TRAIL resistance as a result of its ability to inhibit caspase activity, the effector arm of apoptotic cell death. Previous studies in our laboratory in preclinical models of inflammatory breast cancer (IBC), a highly invasive and lethal subtype, have revealed that apoptotic dysregulation mediated by XIAP is a key factor in IBC pathogenesis and acquired therapeutic resistance to epidermal growth factor receptor (ErbB1/2) targeting agents. Characterization of TRAIL efficacy in IBC cell lines derived from primary tumors revealed that SUM149, a triple negative, EGFR activated cell line, is sensitive to TRAIL-induced apoptosis and exhibits a corresponding decrease in XIAP and an increase in caspase3 activation compared to TRAIL-resistant SUM190, ErbB2 overexpressing cells in which XIAP levels are unchanged by TRAIL treatment. Profile from an apoptotic proteomic array revealed that SUM190 cells have low basal levels of the TRAIL receptors DR4 and DR5 in comparison to SUM149 cells. Stable overexpression of XIAP in the TRAIL-sensitive SUM149 cells resulted in a two fold decrease in sensitivity to TRAIL with decreased caspase3 activation. Further, SUM149wtXIAP cells showed low levels of TRAIL receptor expression similar to the TRAIL-insensitive SUM190 line. Conversely, increased cell death was observed in SUM190 cells in which XIAP was knocked down using a lentiviral shRNA expression construct. In order to examine therapeutic strategies aimed at restoring function to the apoptotic pathways in IBC cells, we used the small molecule inhibitor Embelin to modulate XIAP action. Treatment of SUM149wtXIAP cells with Embelin in combination with TRAIL enhanced cell death compared to treatment with TRAIL or Embelin alone. Assessment of the interaction using a derivation of the Chou-Talalay method revealed a combination index of 0.054 in SUM149wtXIAP cells, which is indicative of strong synergism between the two agents. These results reveal that knockdown or antagonism of XIAP allows TRAIL to induce activation of effector caspases efficiently and establishes the feasibility of use of XIAP inhibitors to enhance therapeutic apoptosis in IBC therapy. Funded by American Cancer Society-RSG-08-290-01-CCE (GRD), predoctoral DOD BCRP award (KMA), and the Duke University Chancellor's Scholarship (JLA). Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P1-04-01.