In contrast, the induction of Foxm1b was not affected in ΔIn-FXR mice after liver
damage, indicating the requirement of a cell autonomous mechanism for hepatic FXR to activate Foxm1b and potentially other factors that are involved in regulating cell cycle in liver. Bile acids are potentially toxic and substantial increases in hepatic bile acid levels will induce hepatocyte death.21 We previously demonstrated that FXR was activated by elevated bile acid influx during liver regeneration.5 The importance for a stringent control of bile acid levels is highlighted by a delicate regulation of CYP7a1 find more expression. The identified regulators of CYP7a1 expression include cytokines, growth factors,22–26 and nuclear receptors.27, 28 During liver regeneration, hepatic bile acid levels need to be suppressed rapidly to prevent the toxic effect of increased bile acids in liver, as shown by a dramatic down-regulation of CYP7a1 mRNA levels.5, 7 We previously showed that, in addition to the FXR-SHP axis, hepatocyte growth factor and JNK pathways were involved in suppressing CYP7a1 expression during the acute phases of liver regeneration.7 In the current study, we now further demonstrate that, during liver regeneration/repair, FXR also activates the expression of FGF15 in check details the intestine to suppress
CYP7a1 transcription. Consistently, several reports also suggest that FGF15 secreted from ileum has profound effects on liver metabolism.14, 29, 30, 31 Because we previously
showed that the suppression of CYP7a1 expression and decreased bile acid synthesis was beneficial for liver regeneration, we therefore conclude that FGF15 selleck inhibitor induction after liver damage may also contribute to normal liver regeneration. The most novel observation in this report is the delayed liver regeneration/repair and increased liver injury in ΔIN-FXR mice compared to FXR Fl/Fl control mice after either 70% PH or CCl4 injection. There results identify an unexpected role of intestine FXR in regulating liver regeneration/repair. It is clear that intestine FXR is key to control bile acid levels. Thus, higher levels of bile acids in ΔIN-FXR mice after liver injury may hamper normal liver regeneration/repair. Besides its effect on bile acid levels, the metabolic and mitogenic activities of FGF15 cannot be excluded. Moreover, the hydrophobic bile acid, deoxycholic acid (DCA) is significantly increased in fecal extracts from intestine FXR null mice but not from FXR KO or liver FXR null mice,15 and DCA may cause hepatocyte apoptosis and colon inflammation and necrosis.32, 33, 34 This may also be a protective function of intestine FXR during liver regeneration/repair. We further showed that intestine FXR induced FGF15 expression after liver injury, which in turn suppressed the CYP7a1 transcription and lowered serum bile acid levels.