Cholestasis is associated with induction of antimicrobial active reduced human β-defensin 1

Background & Aims: Despite a multitude of microbial challenges, knowledge about innate antimicrobial defense of the liver is limited. We systematically investigated expression and regulation of hepatic antibacterial peptides and post-translational modification of human beta defensin-1 (hBD-1).

Methods: Different defensins including hBD-1 and its activating factor thioredoxin-1 (TXN) were analyzed in healthy and cholestatic human liver samples by qPCR and immunostaining. Regulation of hBD-1 expression was studied in different in vitro cell systems and in vivo using bile duct ligated mice (BDL mice). Regulation pathways of bilirubin and bile acids were studied using siRNA-mediated knock-down of potential nuclear receptors.

Results: We found major expression of hBD-1 and TXN – a known activator of hBD-1 antimicrobial activity – in hepatic cells whereas other β-defensins were minimally expressed. Using a specific antibody for the reduced active form of hBD-1 we found strong active peptide expression in co-localization with TXN in human hepatocytes. hBD-1 was upregulated in cholestasis using two independent clinical cohorts. In cholestatic mice hepatic AMP expression (mBD-1 and Hamp) was also enhanced. Bilirubin and bile acids were able to induce hBD-1 in hepatic cell cultures in vitro. Treatment with siRNA and/or agonists demonstrated that the farnesoid X receptor (FXR) mediates basal expression of hBD-1, whereas both CAR and FXR seem to be responsible for the induction of hBD-1 by bilirubin.

Conclusions: hBD-1 is the major defensin in the liver and is further induced during cholestasis through bilirubin and bile acids, mediated by CAR and especially FXR. Reduction of the peptide by TXN post-translationally modifies and activates hBD-1.