Involvement of both caspase-8 and Noxa-activated pathways in endoplasmic reticulum stress-induced apoptosis in triple-negative breast tumor cells

Recent evidences indicate that triple-negative breast cancer (TNBC) cells with a mesenchymal phenotype show a basal activation of the unfolded protein response (UPR) that increases their sensitivity to endoplasmic reticulum (ER) stress although the underlying cell death mechanism remains largely unexplored. Here we show that both caspase-8-dependent and -independent apoptotic mechanisms are activated in TNBC cells undergoing sustained ER stress. Activation of the extrinsic apoptotic pathway by ER stress involves ATF4-dependent upregulation of tumor necrosis factor-related apoptosis-inducing ligand receptor 2 (TRAIL-R2/DR5). In addition, accumulation of BH3-only protein Noxa at the mitochondria further contributes to apoptosis following ER stress in TNBC cells. Accordingly, simultaneous abrogation of both extrinsic and intrinsic apoptotic pathways is required to inhibit ER stress-induced apoptosis in these cells. Importantly, persistent FLICE-inhibitory protein (FLIP) expression plays an adaptive role to prevent early activation of the extrinsic pathway of apoptosis upon ER stress. Overall, our data show that ER stress induces cell death through a pleiotropic mechanism in TNBC cells and suggest that targeting FLIP expression may be an effective approach to sensitize these tumor cells to ER stress-inducing agents.