Vitamin D modulates biliary fibrosis in ABCB4 deficient mice

Aims: Vitamin D deficiency is common in chronic liver diseases. Especially impaired vitamin D-VDR signalling seems to represent an aggravating factor during biliary-type liver diseases (Chignard et al. 2012). To assess the effect of vitamin D on biliary fibrosis, we treated ATP-binding cassette transporter knockout (Abcb4 ^-/-) mice an established model of biliary fibrosis with 25-hydroxyvitamin D (vitD).

Methods: Abcb4^–/– (n = 60) and wild-type mice (n = 54) were fed a vitD supplemented diet (2400 IE vitD/kg food), a deficient- (100 IE/kg) or a control- (600 IE/kg) diet for 12 weeks. Serum vitD concentrations were measured by chemiluminescence immunoassays. Liver injury was determined by liver enzyme activities (ALT, AP), histopathological stages of fibrosis and hepatic collagen contents. Steady-state mRNA expression levels of Col1a2, Timp1, Tgf-ß and Camp were analyzed by quantitative-RT-PCR.

Results: VitD levels depend on genotype and diet. Abcb4^–/– mice on control diet display lower vitD concentrations as compared to wild-type's (38.4 ± 1.9 vs. 46.8 ± 1.7 ng/ml; p < 0.01), whereas after vitD supplementation Abcb4^–/– animals demonstrate higher levels in comparison to wild-type mice (67.2 ± 2.4 vs. 42.0 ± 2.3 ng/ml; p < 0.001). Fibrosis stages differ significantly between Abcb4^–/– mice fed different diets, while hepatic collagen contents in wild-type's are not affected by diet. Abcb4^–/– mice receiving vitD-deficient diet develop significantly higher fibrosis stages (F-score 2.8 ± 0.1) as compared to knockout mice fed control or vitD-sufficient diets (F-scores 1.6 ± 0.2 and 2.1 ± 0.1). We detected higher (p < 0.05) collagen contents in Abcb4^–/– mice fed the vitD-deficient diet (351 ± 22 µg hydroxyproline/g liver) as compared to mice on control diet (294 ± 15 µg/g); the lowest collagen contents were observed after vitD supplementation (275 ± 14 µg/g). Furthermore, vitD deficient Abcb4^–/– mice reveal reduced (p < 0.05) expression of Timp1 and Tgf-ß in comparison to Abcb4^–/– animals on regular diet.

Conclusions: Liver fibrosis severity in Abcb4^–/– mice depends on vitD status. Our findings indicate that vitD modulate biliary injury and fibrogenesis in vivo. We speculate that adequate vitD intake might have antifibrotic effects in patients with chronic liver diseases.