Gefitinib reverses TRAIL resistance in human bladder cancer cell lines via inhibition of AKT-mediated X-linked inhibitor of apoptosis protein expression

In a previous study, we found that the small-molecule epidermal growth factor receptor (EGFR) inhibitor gefitinib (ZD1839, Iressa) blocked cell proliferation at biologically relevant concentrations in approximately one third (6 of 17) of human bladder cancer cell lines examined. Here, we studied the effects of gefitinib on apoptosis in a representative subset of the same panel of cells. The drug had modest effects on DNA fragmentation as a single agent at concentrations that produced strong growth inhibition (< or =1 micromol/L) and also failed to promote apoptosis induced by conventional chemotherapeutic agents (gemcitabine and paclitaxel). However, gefitinib did interact with recombinant human tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) to induce high levels of apoptosis in gefitinib-responsive but not gefitinib-unresponsive lines. The molecular mechanisms involved down-regulation of active AKT and X-linked inhibitor of apoptosis protein (XIAP) expression and were mimicked by chemical inhibitors of the phosphatidylinositol 3-kinase/AKT pathway but not of the mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase/ERK pathway. Furthermore, direct small interfering RNA-mediated knockdown of AKT resulted in down-regulation of XIAP and TRAIL sensitization, and knockdown of XIAP itself was sufficient to reverse TRAIL resistance. Together, our results show that EGFR pathway activation limits TRAIL-induced apoptosis via an AKT- and XIAP-dependent mechanism in EGFR-dependent human bladder cancer cells, providing the conceptual framework for a further evaluation of the combination in relevant preclinical in vivo models.