Dual role of the MAPK/Erk pathway in the regulation of TRAIL sensitivity in human breast epithelial cells

Abstract

Increased activation of the epidermal growth factor receptor (EGFR) is frequently observed in tumors, and inhibition of the signalling pathways originated in the EGFR normally renders tumor cells more sensitive to different apoptotic stimuli, including TRAIL. However, despite all the available evidences in tumor cells, the regulation of TRAIL sensitivity by EGFR signalling in non-tumor cells remains to be investigated in detail. We have recently demonstrated that inhibition of EGFR signalling in various non-transformed breast epithelial cells by long-term EGF deprivation or gefitinib treatment, causes the up-regulation of the long isoform of caspase-8 inhibitor FLICE-inhibitory protein (FLIPL) and makes these cells more resistant to TRAIL. Hereby, we have examined the role of signalling pathways downstream of EGF receptor in the regulation of TRAIL-induced apoptosis in human breast epithelial cells. We show that the prolonged inhibition of the MAPK/Erk pathway prior to TRAIL addition leads to the up-regulation of FLIP and a marked inhibition of TRAIL-induced apoptosis. In contrast, simultaneous treatment with the MAPK/Erk inhibitor U0216 and TRAIL synergistically induces apoptosis in breast epithelial cells. Moreover, inhibition of both the MAPK/Erk pathway and Akt further sensitizes breast epithelial cells to TRAIL-induced DISC formation, caspase-8 activation and apoptosis. Interestingly, TRAIL promotes the early activation of the MAPK/Erk pathway in a caspase-8-dependent manner. Collectively, our results suggest a dual role of the MAPK/Erk pathway in the regulation of TRAIL-induced apoptosis in EGF-dependent human breast epithelial cells. Firstly, activation of the MAPK/Erk pathway by EGF maintains low levels of FLIP in cells and makes these cells sensitive to TRAIL. On the other hand, the early activation of the MAPK/Erk pathway by TRAIL is an obstacle to the formation of the TRAIL DISC and activation of apoptosis by TRAIL. We are currently investigating the mechanism underlying the feedback inhibition of early TRAIL apoptotic signalling by the MAPK/Erk pathway.