Papaioannou, A. | Search bioRxiv

bioRxiv2022

IRE1 RNase controls CD95-mediated cell death

Signalling by the Unfolded Protein Response (UPR) or by the Death Receptors (DR) represents cellular stress pathways frequently activated towards pro-tumoral outputs in cancer. Experimental evidence has highlighted functional links between the UPR and the DR TRAIL-R1/2. Herein, we demonstrate that the UPR sensor IRE1 controls the expression of CD95/Fas, another DR, and its cell death-inducing ability. Whereas CD95 is not a general determinant of ER stress-induced cell death, IRE1 RNase activity inhibition increased CD95 expression and exacerbated CD95L-induced cell death in glioblastoma (GB) and Triple-Negative Breast Cancer (TNBC) cell lines. In accordance, CD95 mRNA was identified as a target of Regulated IRE1-Dependent Decay of RNA (RIDD). Moreover, CD95 expression is elevated in TNBC and GB human tumours exhibiting low RIDD activity. Surprisingly, CD95 expression is also lower in XBP1s-low human tumour samples. We show that IRE1 RNase inhibition led to CD95 expression attenuation and reduced CD95-mediated hepatic toxicity in mice. In addition, overexpression of XBP1s increased CD95 expression and sensitized GB and TNBC cells to CD95L-induced cell death. Overall, these results demonstrate the tight IRE1-mediated control of CD95-dependent cell death signals in a dual manner through both RIDD and XBP1s, and they identify a novel, pharmacologically actionable link between IRE1 and CD95 signalling.

cell biology↗

bioRxiv2020

Stress-induced tyrosine phosphorylation of RtcB modulates IRE1 activity and signaling outputs.

Endoplasmic Reticulum (ER) stress is a hallmark of various diseases, which is dealt with through the activation of an adaptive signaling pathway named the Unfolded Protein Response (UPR). This response is mediated by three ER-resident sensors and the most evolutionary conserved, IRE1 signals through its cytosolic kinase and endoribonuclease (RNase) activities. IRE1 RNase activity can either catalyze the initial step of XBP1 mRNA unconventional splicing or degrade a number of RNAs through Regulated IRE1- Dependent Decay (RIDD). The balance between these two activities plays an instrumental role in cells life and death decisions upon ER stress. Until now, the biochemical and biological outputs of IRE1 RNase activity have been well documented, however, the precise mechanisms controlling whether IRE1 signaling is adaptive or pro-death (terminal) remain unclear. This prompted us to further investigate those mechanisms and we hypothesized that XBP1 mRNA splicing and RIDD activity could be co-regulated by the IRE1 RNase regulatory network. We showed that a key nexus in this pathway is the tRNA ligase RtcB which, together with IRE1, is responsible for XBP1 mRNA splicing. We demonstrated that RtcB is tyrosine phosphorylated by c-Abl and dephosphorylated by PTP1B. Moreover, we identified RtcB Y306 as a key residue which, when phosphorylated, perturbs RtcB interaction with IRE1, thereby attenuating XBP1 mRNA splicing and favoring RIDD. Our results demonstrate that the IRE1 RNase regulatory network is dynamically fine-tuned by tyrosine kinases and phosphatases upon various stresses and that the nature of the stress determines cell adaptive or death outputs.

biochemistry↗