CPSF6-mediated XBP1 3'UTR shortening attenuates cisplatin-induced ER stress and elevates chemo-resistance in lung adenocarcinoma.

Alternative polyadenylation (APA) is a widespread mechanism generating RNA molecules with alternative 3' ends. Herein, we discovered that TargetScan includes a novel XBP1 transcript with a longer 3' untranslated region (UTR) (XBP1-UL) than that included in NCBI. XBP1-UL exhibited a lowered level in blood samples from lung adenocarcinoma (LUAD) patients and in those after DDP treatment. Consistently, XBP1-UL was reduced in A549 cells compared to normal BEAS-2B cells, as well as in DDP-treated/resistant A549 cells relative to controls. Moreover, due to decreased usage of the distal polyadenylation site (PAS) in 3'UTR, XBP1-UL level was lowered in A549 cells and decreased further in DDP-resistant A549 (A549/DDP) cells. Importantly, use of the distal PAS (dPAS) and XBP1-UL level were gradually reduced in A549 cells under increasing concentrations of DDP, which was attributed to DDP-induced endoplasmic reticulum (ER) stress. Furthermore, XBP1 transcripts with shorter 3'UTR (XBP1-US) were more stable and presented stronger potentiation on DDP resistance. The choice of proximal PAS (pPAS) was attributed to CPSF6 elevation, which was caused by BRCA1-distrupted R-loop accumulation in CPSF6 5'end. DDP-induced nuclear LINC00221 also facilitated CPSF6-induced pPAS choice in the pre-XBP1 3'end. Finally, we found that unlike the unspliced XBP1 protein (XBP1-u), the spliced form XBP1-s retarded p53 degradation to facilitate DNA damage repair of LUAD cells. The current study provides new insights into tumor progression and DDP resistance in LUAD, which may contribute to improved LUAD treatment.