Abstract
Increased activation of the epidermal growth factor receptor (EGFR) is frequently observed in tumors, and inhibition of the signaling pathways originated in the EGFR normally renders tumor cells more sensitive to apoptotic stimuli. However, we show that inhibition of EGFR signaling in non-transformed breast epithelial cells by EGF deprivation or gefitinib, an inhibitor of EGFR tyrosine kinase, causes the upregulation of the long isoform of caspase-8 inhibitor FLICE-inhibitory protein (FLIP(L)) and makes these cells more resistant to the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). We demonstrate that the extracellular signal-regulated kinase (ERK)1/2 pathway plays a pivotal role in the regulation of FLIP(L) levels and sensitivity to TRAIL-induced apoptosis by EGF. Upregulation of FLIP(L) upon EGF deprivation correlates with a decrease in c-Myc levels and c-Myc knockdown by siRNA induces FLIP(L) expression. FLIP(L) upregulation and resistance to TRAIL in EGF-deprived cells are reversed following activation of an estrogen activatable form of c-Myc (c-Myc-ER). Finally, constitutive activation of the ERK1/2 pathway in HER2/ERBB2-transformed cells prevents EGF deprivation-induced FLIP(L) upregulation and TRAIL resistance. Collectively, our results suggest that a regulated ERK1/2 pathway is crucial to control FLIP(L) levels and sensitivity to TRAIL in non-transformed cells, and this mechanism may explain the increased sensitivity of tumor cells to TRAIL, in which the ERK1/2 pathway is frequently deregulated.