156 Phosphatase and Tensin Homolog PTEN Is an Essential Regulatory Factor in Toll-Like Receptor 5-Elicited Inflammatory Responses

Abstract

Background. The unfolded protein response (UPR) is a response mechanism to endoplasmic reticulum (ER) stress, which a cell is typically exposed upon protein misfolding. Intestinal epithelial cell (IEC) homeostasis critically depends upon an intact UPR and its perturbation due to deficiency of the key UPR transcription factor X-box binding protein-1 (XBP1) in IECs results in ER stress und spontaneous intestinal inflammation. Rare functional variants of XBP1 are associated with inflammatory bowel disease (IBD) and IBD patients typically exhibit unresolved ER stress in IECs. Long standing inflammation in IBD patients predisposes for colitis-associated cancer and cancer cells typically exhibit ER stress due to increased risk for protein misfolding as a consequence of the tumour microenvironment characterized by low oxygen, nutrient deprivation and pH changes. Aims. We hypothesised XBP1-deficiency and subsequent unresolved ER stress might affect intestinal regeneration, which may impact on tumourigenesis. Methods. NFκB and Stat3 activation was analysed by immunoblot and immunohistochemistry. Specific inhibitors of Stat3 (S3I-201) and NF κB (BAY11-7082), respectively, were administered to Xbp1flox/flox VillinCre mice. Epithelial proliferation was assessed by bromodeoxyuridine (BrdU) staining. Xbp1flox/flox VillinCre mice were analyzed at day 61 of azoxymethane (AOM) followed by three cycles of dextran sodium sulphate (DSS). Results. ER stress due to XBP1-deficiency results in a numerical increase of transit amplifying (TA) cells and consequently in an increased epithelial turn over. Xbp1-/-(IEC) mice activate Stat3, which localizes to TA cells, and its blockade via administration of the Stat3 inhibitor S3I-201 abrogates epithelial hyperregeneration. Xbp1-deletion in IECs leads to activation of NFκB and increased IL-6 secretion, a known activator of Stat3. Blockade of NFκB signaling via administration of the IKK2 inhibitor BAY11-7082 abrogated Stat3 activation in XBP1-deficient mice. NFκB and Stat3 promote colitis-associated cancer and XBP1deficient mice exhibit an increase in tumour burden in a model of colitis-associated cancer characterized by a 10-fold increase in tumour area as well as tumour number. Conclusions. Our data unravels XBP1 as an unexpected tumour suppressor in the intestinal epithelium, thereby opposing the current paradigm that cancer cells require an unimpaired UPR to survive in a stressful tumour microenvironment. Promotion of colitis-associated tumourigenesis is functionally linked to activation of NFκB and Stat3, which are potently engaged in XBP1deficient IECs. Our data show that unresolved ER stress in the intestinal epithelium increases the propensity to develop intestinal tumours.