That cholestatic conditions are accompanied by an enhanced susceptibility to bacterial infection in human and animal models is a known phenomenon. This correlates with the observation that bile acids have suppressive effects on cells of innate and adaptive immunity. The present study provides evidence that in human macrophages, bile acids inhibit the LPS-induced expression of proinflammatory cytokines without affecting the expression of the anti-inflammatory cytokine IL-10. This results in a macrophage phenotype that is characterized by an increased IL-10/IL-12 ratio. Correspondingly, bile acids suppress basal phagocytic activity of human macrophages. These effects of bile acids can be mimicked by cAMP, which is presumably induced TGR5-dependently. The data provided further suggest that in primary human macrophages, modulation of the macrophage response toward LPS by bile acids involves activation of CREB, disturbed nuclear translocation of NF-κB, and PKA-dependent enhancement of LPS-induced cFos expression. The increase in cFos expression is paralleled by an enhanced formation of a protein complex comprising cFos and the p65 subunit of NF-κB. In summary, the data provided suggest that in human macrophages, bile acids induce an anti-inflammatory phenotype characterized by an increased IL-10/IL-12 ratio via activation of PKA and thereby, prevent their activation as classically activated macrophages. This bile acid-induced modulation of macrophage function may also be responsible for the experimentally and clinically observed anti-inflammatory and immunosuppressive effects of bile acids.

Interleukin-6 (IL-6) is a major regulator of the acute phase reaction in the liver and is thought to mediate protective effects in response to hepatotoxins. In this study, the influence of bile acids on IL-6 signal transduction was analyzed. It was shown that hydrophobic bile acids such as glycochenodeoxycholate (GCDC) inhibited IL-6 -induced tyrosine phosphorylation of signal transducer and activator of transcription (STAT) 3 in hepatocytes and in perfused rat liver. This inhibition was accompanied by GCDC-mediated downregulation of glycoprotein (gp) 130 expression, whereas gp130 and suppressor of cytokine signaling 3 messenger RNA and gp80 protein levels remained unaffected. The GCDC-induced downregulation of gp130 protein expression was insensitive to inhibition of proteasomal or lysosomal protein degradation but turned out to be sensitive to inhibition of caspase-3 or caspase-8 activity. Accordingly, treatment of cell extracts with active recombinant caspase-3 led to a decay of immunoreactive gp130. Moreover, activation of caspases by CD95 ligand or hyperosmotic stress also resulted in a downregulation of gp130 levels. This indicates that caspase activation antagonizes IL-6 signaling by decay of gp130 levels. However, caspase inhibition did not prevent GCDC-dependent inhibition of IL-6 -induced STAT3 activation, which turned out to be at least partially sensitive to suppression of p38 MAPK activation. In conclusion, hydrophobic bile acids compromise IL-6 signaling through both a caspasemediated downregulation of gp130 and a p38 MAPK -dependent inhibition of STAT3 phosphorylation. This may contribute to bile acid-induced hepatotoxicity in cholestasis through counteracting the known hepatoprotective effects of

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Apart from viral conditions, host factors such as elevated bile acid concentrations are determinants of successful interferon-α (IFN-α) treatment in patients with chronic hepatitis C or B. The present study demonstrates that hydrophobic bile acids inhibit Jak1- and Tyk2-phosphorylation, which lead to blockade of STAT1-mediated IFN-α-signaling in the sodium-taurocholate cotransporting peptide (NTCP)-transfected human hepatoma cell line HepG2, resulting in a decreased mRNA and protein expression of IFN-stimulated genes such as myxovirus resistance protein A (MxA) or dsRNA-activated protein kinase (PKR). In addition, hyperosmotic stress leads to an inhibition of IFN-α-induced Jak1- and Tyk2-phosphorylation, and STAT1/STAT2-phosphorylation and gene expression. This inhibitory effect of hydrophobic bile acids or hyperosmolarity is not due to caspase-mediated cleavage or lysosomal degradation of the cognate receptors or to the generation of oxidative stress, activation of p38- or Erk-mediated MAPK pathways or phosphatase activity. Preincubation with the organic osmolyte betaine blocked the inhibitory effect of bile acids or hyperosmolarity on MxA protein expression, but had no effect on transcript levels or activation of STAT1, suggesting that betaine mediates its effects on MxA expression at a translational or post-translational level. Our findings could provide a rationale for betaine use in cholestatic HBV/HCV patients undergoing interferon therapy.