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

Abstract

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.

Proteotoxic Stress and Cell Death in Cancer Cells

“…However, proteotoxic stress is a broad and complex pro-death insult; additionally, the extrinsic pathway is involved [95]. This role was suggested by early studies reporting the upregulation of TNFRSF10B/DR5, the TRAIL receptor, in response to ER-stressors/PERK activation, UPS inhibitors, as well as the influence of caspase-8 inhibitors on proteotoxic stress-induced cell death [39,[95][96][97][98][99][100][101]. More recently, it has been proposed that UPR not only upregulates DR5 expression but misfolded proteins can directly engage with DR5 in the ER-Golgi intermediate compartment to drive the assembly of DR5 in complexes competent for caspase-8 activation ( Figure 1).…”

Stress-induced TRAILR2 expression overcomes TRAIL resistance in cancer cell spheroids

“…Regarding the reasons underlying the accumulation of TRAILR2 in cells close to spheroid cores it is therefore conceivable that a prolonged and extensive oxygen and nutrient deprivation results in robust induction of ER stress and in the activation of the unfolded protein response. Interestingly, besides elevated cell surface amounts of TRAILR2, this TRAIL receptor can also accumulate intracellularly in response to ER stress and thereby contribute to ligand independent but caspase-8-dependent apoptosis [ 32 , 35 – 37 ] Correspondingly, directly inducing persistent ER stress by thapsigargin promotes TRAILR2 accumulation and apoptotic cell death [ 36 ]. Even though cells deprived of oxygen and nutrient within the centers of spheroids or micrometastases ultimately will die, it could be speculated that elevated TRAILR2 expression close to spheroid cores contributes to this being an ordered cell death process.…”

Stress Management: Death Receptor Signalling and Cross-Talks with the Unfolded Protein Response in Cancer

“…Several regulators of DR signalling are also logically shown to control ER-stress-induced death. In line with its role in controlling caspase-8 activation, cFLIP can limit ER-stress-induced apoptosis (e.g., in TNBC cell lines [259]). It is worth noting that the activation of the p65 unit of NF-κB, can counteract the ER-stress-induced expression of CHOP and could thus constitute a manner for some cells to circumvent ER-stress-induced apoptosis [291].…”

“…Similar to its role in CD95L/TRAIL-induced apoptosis, the engagement of the mitochondrial pathway through Bid is also required in certain cells, like Hct116, to mediate ER-stress-induced TRAIL-R2-dependent cell death [258]. In TNBC cell lines, blocking both TRAIL-R2 and the intrinsic pathway is required to prevent TG-induced apoptosis [259].…”