Bile acids inhibit interleukin-6 signaling via gp130 receptor-dependent and -independent pathways in rat liver

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 caspase-mediated 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 IL-6.