This is coherent with the increased UPR activation observed in the colonic tissue of active IBD patients, sellckchem whereas no increase was seen in the ileal tissue of active CD patients. In the ileum, ER stress is probably dictated by other local factors. The ileum contains a high number of Paneth cells, has an increased number of mucosa-associated E. coli and has a higher metabolic activity compared to the colon. This might contribute to a constitutive triggering of the UPR in the ileal mucosa, which is critical in maintaining homeostasis. The fact that inflammation does not further increase UPR in ileal samples either reflects that the higher basal levels observed can buffer some perturbations or reflect that the ileum is less sensitive to perturbations through inflammation.

This leads us to consider that the colonic mucosa is subject to a lower ER stress, with a significant increase in inflammatory conditions: from low basal levels of UPR, any induction is more uniform and more noticeable in this tissue. In order to determine whether the ileum could still respond to ER stress, paired colonic and ileal samples of five healthy controls were stimulated with tunicamycin, a well-known ER stress inducer [37], [38]. Both colonic and ileal samples revealed higher HSPA5 transcript levels in the tunicamycin stimulated samples. In addition, a higher induction was observed in the ileal samples. This would argue that the ileum rather lives on a higher basal ER stress, but can still induce strongly the gene response.

It also highlights that if the inflammation of the tissue did not significantly increase the UPR, it is not because the tissue is unable to do so. Indeed if the UPR is more activated in basal state, removal of a protective arm (XBP1) would prevent proper re-establishment of homeostasis and could logically result in imbalance. This would be coherent with both our findings and the ones of Kaser [27]. Moreover, in the study of Kaser, XBP1 was deleted exclusively in epithelial cells, pointing toward a defect in epithelial cells in IBD pathogenesis. In our study, we observed that HSPA5 located mainly in intestinal epithelial secretory cells (enteroendocrine cells, paneth cells and goblet cells) which produce large amounts of proteins involved in mucosal defense.

Regarding the activation of the PERK branch, transcript and protein levels of GADD34 in colonic IBD patients were similar to healthy controls, Carfilzomib which would argue against an activation of the PERK pathway. In contrast, western blot analysis showed an increased concentration of pEIF2A in colonic inflammation. If pEIF2A would result from PERK activation, we would expect an induction of GADD34, which is the co-factor of protein phosphatase 1 in the dephosphorylation of pEIF2A [32]. This would represent the canonical PERK pathway, where GADD34 promotes the return to homeostasis of the ER.