Upregulation of TGR5 (Gpbar1) in macrophages protects mice from lipopolysaccharide and Listeria monocytogenes-induced liver damage

Introduction:

TGR5 (Gpbar1) is a G protein-coupled receptor responsive to bile acids (BA) expressed in different non-parenchymal cells of the liver, including Kupffer cells [1,2] as well as in monocytes and macrophages of the peripheral blood [3]. Activation of TGR5 in macrophages reduces cytokine and chemokine secretion and suppresses the NLRP3 inflammasome [4 – 7]. Thus, BAs exert a broad range of anti-inflammatory effects in monocytes and macrophages via TGR5-dependent signaling. Aim of our study was to investigate the in vivo relevance of TGR5 in inflammatory liver disease using different mouse models.

Methods:

8 – 12 week old male TGR5 knockout and wildtype mice were injected intraperitoneally (i.p.) with different doses of lipopolysaccharide or infected intravenously (i.v.) with 8 × 104 CFU/mL Listeria monocytogenes (L.m.) and observed for 3 to 7 days. Serum markers of liver damage were determined using a Spotchem biochemical analyzer. Flow cytometry was used to quantify and differentiate immune cells within the liver. Immunohistochemistry was performed on liver tissue.

Results:

Tgr5 mRNA expression was significantly up-regulated in livers from WT mice after LPS injection and L.m. infection as demonstrated by realtime PCR. Similar to Tgr5, mRNA expression of the transcription factor Krüppel-like factor 5 (KLF5) was significantly increased in wildtype animals after LPS or L.m. injection. Furthermore, TGR5 staining in CD11 positive macrophages was increased. In isolated bone marrow-derived macrophages Tgr5 and Klf5 mRNA expression was upregulated following stimulation with either LPS or L.m.. Chromatin immunoprecipitation confirmed binding of KLF5 to the Tgr5 promotor region, which was further supported by cloning and functional characterization of the putative Tgr5 promotor region. LPS and L.m. injection resulted in severe liver injury in TGR5-deficient mice as determined by AST and ALT elevation. Moreover, mortality was significantly increased in TGR5 knockout mice. This phenotype was mirrored by immune cell-specific TGR5 knockout mice.

Conclusion:

Upregulation of TGR5 in immune cells via KLF5 increases anti-inflammatory effects mediated by TGR5 and thus protects wildtype mice from LPS or L.m.-induced injury and mortality. In contrast, TGR5-deficient mice lack this adaptive mechanism resulting in increased mortality after LPS or L.m. injection, underscoring the important role of TGR5 in inflammatory liver disease.

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