Plasma Membrane-bound G Protein-coupled Bile Acid Receptor Attenuates Liver Ischemia/Reperfusion Injury Via the Inhibition of Toll-like Receptor 4 Signaling in Mice

The plasma membrane-bound G protein-coupled bile acid receptor (TGR5) displays varied levels of expression in different tissues. TGR5-induced liver protection has been demonstrated during several liver diseases, except during ischemia/reperfusion injury (IRI). Male adult wild-type and TGR5 knockout (KO) mice were subjected to liver partial warm ischemia/reperfusion. Hepatic injury was evaluated based on serum alanine aminotransferase and serum aspartate aminotransferase. Liver histological injury and inflammatory cell infiltration were evaluated in tissue sections using liver immunohistochemical analysis. We used quantitative real-time polymerase chain reaction to analyze the liver expression of inflammatory cytokines. The toll-like receptor 4 (TLR4) signaling pathway and its related apoptotic molecules were investigated after reperfusion. Moreover, the effect of TGR5 on inflammation was determined with TGR5(+/+) or TGR5(-/-) primary bone marrow-derived macrophages in vitro. TGR5 significantly attenuated liver damage after IRI. As demonstrated by in vivo experiments, TGR5 significantly reduced the up-regulation of the TLR4-nuclear factor kappa B (NF-kappa B) pathway and inhibited caspase 8 activation after IRI. Later experiments showed that TGR5 KO significantly increased the expression of TLR4-NF-kappa B signaling molecules and promoted hepatocellular apoptosis. In addition, in vitro experiments showed that overexpression of 6alpha-ethyl-23(S)-methylcholic acid (INT-777)-activated TGR5 directly down-regulated tumor necrosis factor a (TNF-alpha) and interleukin (IL) 6 expression but up-regulated IL10 expression in hypoxia/reoxygenation-induced primary TGR5(+/+) macrophages. Moreover, the expression of TLR4-NF-kappa B signaling molecules was significantly inhibited by the activation of TGR5. Importantly, these results were completely reversed in primary TGR5(-/-) macrophages. This work is the first to provide evidence for a TGR5-inhibited inflammatory response in IRI through suppression of the TLR4-NF-kappa B pathway, which may be critical in reducing related inflammatory molecules and modulating innate inflammation.