Cholestasis is a condition caused by rapidly developing or long-term interruption in the excretion of bile. Increasing evidence shows that the levels of multiple pro-inflammatory cytokines are strongly increased in the cholestatic liver tissue, including IL-1?, IL-6 and tumor necrosis factor ? (TNF-?).
As a key inflammatory mediator upstream of IL-6 and TNF-? signaling cascades, IL-1? is produced primarily by activated macrophages and is involved in diverse acute and chronic liver injury, including liver fibrosis.In the current study, the authors for the first time proved that CDCA, the major toxic bile acid involved in cholestatic liver injury, could dose dependently induce NLRP3 inflammasome activation and secretion of pro-inflammatory cytokine-IL-1? in murine J774A.1 macrophages, bone marrow derived macrophages as well as kupffer cells. Meanwhile, mature IL-1? levels in liver of BDL mouse model also enormously increased. IL-1? was reported to promote fibrogenesis and as an important inflammatory mediator, it can also recruit other inflammatory cells, especially neutrophils, which are actively implicated in tissue inflammation and exacerbate liver damage.
Consistently, the in vivo studies showed that inhibition of IL-1? level with caspase-1 inhibitor dramatically reduced MPO activity in the liver and ameliorated liver fibrosis in BDL mouse model, suggesting an important role of IL1? in triggering tissue inflammation and fibrosis during cholestasis. Thus, the data provided by authors demonstrates a new mechanism that excessive toxic bile acids initiate liver inflammation at least in part through activating NLRP3 inflammasome. The in vitro studies demonstrated that phagocytic uptake was not required for CDCA-induced IL-1? secretion in macrophages, which is different from the mode of NLRP3 activation by various crystals, indicating the involvement of receptor-mediated pathways. The following experiments confirms that CDCA activated NLRP3 inflammasome and induced IL-1? release partially through TGR5. In addition, TGR5 was also observed to exhibit immunomodulatory effects through its abundant expression on monocytes and macrophages. In contrast to results provided by authors, some studies have shown that CDCA exerts an inhibitory effect on inflammatory cytokines production in LPS-stimulated macrophages mainly through TGR5 AC-cAMP dependent pathways.
Based on previous reports, bioactive IL-1? production requires priming signal, which increases pro-IL-1? and NLRP3 level, and then followed by activation signal (2nd signal) which induces NLRP3 inflammasome assembly and activation. Indeed, the authors observed mature IL-1? release from LPS primed macrophages in response to CDCA, therefore, one possible explanation for the discrepancy is that they prime macrophages with LPS first and then use CDCA as second signal activating the inflammasome, whereas different strategy was applied in the others studies. Meanwhile, the results showed that transactivation of EGFR, which is an alternative pathway independent of AC-cAMP, leads to inflammasome activation and IL-1? release, suggesting that TGR5 may regulate inflammatory response through different mechanisms. The activation of NLRP3 inflammasome by CDCA involves the up-regulation of EGFR-ERK/AKT/JNK signaling. NLRP3 inflammasome activation by CDCA requires ROS formation, as evidenced by significantly reduced IL-1? production after NAC treatment.
Several studies have described the induction of ROS by EGFR activation. The present study shows the role of CDCA on inflammasome activation which is also attributed to the induction of ATP release from macrophages, which could trigger K+ efflux through purinergic signalling. P2X7 receptor is involved in CDCA-induces K+ efflux, which is consistent with previous reports showing that ATP can bind to P2X7 receptor and then open the P2X7-associated ion channel pore, which increases K+ conductance(Fig-1).