Abstract B054: Investigating ER stress responses in pancreatic ductal adenocarcinoma under lipid imbalance

Abstract

Abstract Pancreatic ductal adenocarcinoma (PDAC) is the 4th leading cause of cancer-related death with a five-year survival rate of 10%. Dense desmoplasia results in poor intratumoral perfusion and elevated interstitial fluid pressure, creating regions with severe oxygen and nutrient deprivation. One result is a deficiency in unsaturated fatty acids (uFAs), as the de novo synthesis of uFAs requires oxygen. Our results indicate that PDAC cells experience endoplasmic reticulum (ER) stress and trigger subsequent ER stress responses under uFA limited conditions in vitro. ER stress responses are governed by three major stress sensors: Inositol-requiring enzyme 1𝛼 (IRE1𝛼), PKR-like ER kinase (PERK) and activating transcription factor 6 (ATF6). ER stress responses are also activated in PDAC murine and patient samples. Interestingly, the expression of ER stress markers is much higher in malignant lesions than benign pancreatic intraepithelial neoplasia. In contrast to tumor promoting, we found that PERK, ATF6 and IRE1𝛼 - XBP1 are cytotoxic to PDAC cells in vitro, as their knockout improved viability under stress conditions. Surprisingly, pharmacological inhibitor of XBP1 splicing, B-I09, didn’t recapitulate the results of the genetic approach. While B-I09 has cytoprotective effects in some PDAC lines, it is cytotoxic in others. In addition to inhibiting XBP1 splicing, B-I09 also inhibits regulated IRE1𝛼 dependent decay (RIDD). We speculate that RIDD might have opposite roles to XBP1s, leading to the discrepancy in the cellular phenotypes we observed. Most B-I09 responders are reported to be the basal-like transcriptional subtype. Therefore, we hypothesize that the basal-like subtype is more susceptible to B-I09. So far, we have shown that basal-like patient tumors are significantly enriched in the ER stress response pathways from bulk patient RNA-seq datasets, and we are currently validating this result in the publicly available scRNA-seq datasets from patients which contain basal-like and classical subtypes. We have generated isogenic models with basal-like features, such as epithelial to mesenchymal transition and deltaNP63 overexpression. However, our results suggest that these basal-like features did not sensitize classical PDAC cells to B-I09. Our lab has previously shown that B-I09 is selectively lethal to the MYC-transformed tumors. We are currently testing whether MYC overexpression would sensitize the non-responders to B-I09. It is exciting to see that some of the PDAC lines such as PANC1 and KPC murine line 4662 have reduced viability under B-I09 treatment. However, in our subcutaneous xenograft and allograft experiments, B-I09 only leads to a mild tumor growth suppression, which is unlikely to be therapeutically beneficial by itself. We are currently testing whether XBP1 inhibition could result in an enhanced tumor suppression using OVA antigen stimulated system, as XBP1 inhibition in tumor cells was shown to have tumor-repressing roles via immune system. Citation Format: Yanqing (Christine) Jiang, Xu Han, Nathan Coffey, Mai Wang, John Tobias, Raymond Ng, Carson Poltorack, M. Celeste Simon. Investigating ER stress responses in pancreatic ductal adenocarcinoma under lipid imbalance [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Pancreatic Cancer; 2023 Sep 27-30; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84(2 Suppl):Abstract nr B054.