Background: The impact of hypoxia on inflammatory bowel disease (IBD) is controversial with studies showing detrimental and protective effects. Hypoxia regulates autophagy and nucleotide-binding oligomerization domain receptor, pyrin domain containing (NLRP)3, two innate immune mechanisms linked by mutual regulation that have been associated to the development of IBD. We investigated the functional impact of hypoxia on the development of colitis with special emphasis on autophagy and NLRP3 regulation.
Methods: Healthy volunteers and patients with IBD, as well as wild-type, interleukin (IL)-10−/−, NLRP3−/− and IL-10−/− NLRP3−/− double knockout mice were subjected to hypoxia and changes in inflammatory signaling and gene expression were analyzed in colon biopsies using RT-qPCR and Western blotting. The effects of hypoxia on autophagy, NLRP3 regulation and inflammation were further assessed in vitro using the intestinal epithelial cell line HT-29.
Results: Hypoxia significantly reduced tumor necrosis factor α, IL-6 and NLRP3 expression and increased autophagy gene expression in colon biopsies of patients with Crohn's disease. In normoxia, IL-10−/−, but not IL-10−/− Nlrp3−/− mice presented an accumulation of autophagy proteins and an increase in NF-κB activation and inflammatory cytokine expression, which was significantly reduced under hypoxia. In vitro, hypoxia-induced autophagy downregulated NF-κB signaling. Hypoxia also reduced NLRP3 expression, and silencing of NLRP3 activated autophagy following the dephosphorylation of mammalian target of rapamycin (mTOR). Co-immunoprecipitation experiments identified NLRP3 as a novel binding partner of mTOR.
Conclusions: Our results show that hypoxia counteracts inflammation through a novel mechanism involving the downregulation of the binding partner of mTOR NLRP3 and subsequent activation of autophagy.