Noncanonical farnesoid X receptor signaling inhibits apoptosis and impedes liver fibrosis.

BACKGROUND

Hepatocyte is particularly vulnerable to apoptosis, a hallmark of many liver diseases. Although pro-apoptotic mechanisms have been extensively explored, less is known about the hepatocyte-specific anti-apoptotic molecular events and it lacks effective approach to combat hepatocyte apoptosis. We investigated the anti-apoptotic effect and mechanism of farnesoid X receptor (FXR), and strategies of how to target FXR for inhibiting apoptosis implicated in liver fibrosis.

METHODS

Sensitivity to apoptosis was compared between wild type and Fxr mice and in cultured cells. Cell-based and cell-free assays were employed to identify the binding protein of FXR and to uncover the mechanism of its anti-apoptotic effect. Overexpression of FXR by adenovirus-FXR was employed to determine its anti-fibrotic effect in CCl-treated mice. Specimens from fibrotic patients were collected to validate the relevance of FXR on apoptosis/fibrosis.

FINDINGS

FXR deficiency sensitizes hepatocytes to death receptors (DRs)-engaged apoptosis. FXR overexpression, but not FXR ligands, inhibits apoptosis both in vitro and in vivo. Apoptotic stimuli lead to drastic reduction of FXR protein levels, a prerequisite for DRs-engaged apoptosis. Mechanistically, FXR interacts with caspase 8 (CASP8) in the cytoplasm, thus preventing the formation of death-inducing signaling complex (DISC) and activation of CASP8. Adenovirus-FXR transfection impedes liver fibrosis in CCl-treated mice. Specimens from fibrotic patients are characterized with reduced FXR expression and compromised FXR/CASP8 colocalization.

INTERPRETATION

FXR represents an intrinsic apoptosis inhibitor in hepatocytes and can be targeted via restoring its expression or strengthening FXR/CASP8 interaction for inhibiting hepatocytes apoptosis in liver fibrosis. FUND: National Natural Science Foundation of China.