The pregnane X receptor (PXR) modulates NLRP3 inflammasome activation – linking the environment with innate immune signaling

Abstract

BackgroundThe pregnane X receptor (PXR) is a xenobiotic sensing nuclear receptor that is activated by a diverse array of substances, including environmental toxins, pharmaceutical compounds and metabolites released from the intestinal microbiota. While the PXR's prototypical role is to regulate the expression of the drug metabolizing/detoxifying genes in the liver and intestinal epithelium, we and others have reported that it plays a role in regulating inflammatory signaling. More specifically, the PXR can inhibit NFκB‐dependent inflammatory signaling and regulates the function/expression of innate immune receptors, including TLR4 and NLRP3. In the current study, we sought to characterize the function of the PXR in macrophages. Given the interactions between the PXR and innate immune receptors described in other non‐immune cell types, we hypothesized that its activation would modulate NLRP3 inflammasome activation and the resulting processing/release of IL‐1β.AimsTo evaluate the effect of PXR activation of NLRP3 inflammasome activation and to determine the mechanism(s) whereby the PXR modulates inflammasome activity.MethodsMouse peritoneal macrophages and PMA‐differentiated THP‐1 cells were used to assess NLRP3 inflammasome activation. Due to species‐specific ligand interactions with the PXR, in mouse studies we used pregnenolone 16α‐carbonitrile (PCN), whereas in THP‐1 experiments, we used the human PXR agonists, rifaximin and SR12813. To test if PXR activation modulated NLRP3 inflammasome activation, LPS‐primed macrophages or PMA‐differentiated THP‐1 were pretreated for one hour with PXR agonists and then challenged with ATP (5mM), a known NLRP3 inflammasome activator. Caspase‐1 cleavage and IL‐1β secretion were measured to assess inflammasome activation. In some experiments, extracellular ATP was measured following PXR agonist treatment.ResultsWhile PXR activation had no effect on ATP‐induced NLRP3 inflammasome activation, each PXR agonist alone stimulated the cleavage of caspase‐1 and the secretion of IL‐1ββ, reminiscent of inflammasome activation. In PXR−/− macrophages, these responses were absent, supporting a direct role of the PXR in these responses. Deletion of NLRP3 in mouse macrophages and THP‐1 cells attenuated PXR agonist‐induced caspase‐1 cleavage and IL‐1β secretion, suggesting that PXR activation triggers NLRP3 inflammasome activation. Furthermore, PXR driven responses were attenuated following caspase‐1 inhibition. Mechanistically, NLRP3 activation by the PXR did not involve ROS production, nor was it sensitive to chelation of intracellular calcium. However, treating cells with apyrase, to catabolize extracellular ATP, or selective inhibition of the P2X7 receptor attenuated PXR agonist‐induced caspase‐1 activation and IL‐1β secretion. Interestingly, we subsequently found that PXR activation led to a rapid (within 15 seconds) increase in extracellular ATP, reaching levels previous described in NLRP3 inflammasome activation models. This response was absent in PXR−/− cells, supporting a direct role of the PXR in ATP release.ConclusionsTaken together, our data suggest that activation of the xenobiotic sensing PXR triggers the release of ATP, which in turn causes NLRP3 inflammasome activation in macrophages. These findings suggest a novel mechanism whereby non‐microbial/non‐viral agents of environmental origin can stimulate innate immune responses and contribute to inflammatory disease.Support or Funding InformationCrohn's and Colitis CanadaNSERCLloyd Sutherland Investigatorship in IBD/GI researchThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.