Biliary fibrosis associated with altered bile composition in a mouse model of erythropoietic protoporphyria

Abstract

Background & Aims: Reduced activity of ferrochelatase in erythropoietic protoporphyria (EPP) results in protoporphyrin (PP) accumulation in erythrocytes and liver. Liver disease may occur in patients with EPP, some of whom develop progressive liver failure that necessitates transplantation. We investigated the mechanisms underlying EPP-associated liver disease in a mouse model of EPP. Methods: Liver histology, indicators of lipid peroxidation, plasma parameters of liver function, and bile composition were studied in mice homozygous ( fch/fch) for a point mutation in the ferrochelatase gene and in heterozygous ( fch/+) and wild-type (+/+) mice. Results: Microscopic examination showed bile duct proliferation and biliary fibrosis with portoportal bridging in fch/fch mice. PP content was 130-fold increased, and thiobarbituric acid–reactive substances (+30%) and conjugated dienes (+75%) were slightly higher in fch/fch than in fch/+ and +/+ livers. Levels of hepatic thiols (−12%) and iron (−52%) were reduced in fch/fch livers. Liver enzymes and plasma bilirubin were markedly increased in the homozygotes. Plasma bile salt levels were 80 times higher in fch/fch than in fch/+ and +/+ mice, probably related to the absence of the Na +-taurocholate cotransporting protein (Ntcp) in fch/fch liver. Paradoxically, bile flow was not impaired and biliary bile salt secretion was 4 times higher in fch/fch mice than in controls. Up-regulation of the intestinal Na +-dependent bile salt transport system in fch/fch mice may enhance efficiency of bile salt reabsorption. The bile salt/lipid ratio and PP content of fch/fch bile were increased 2-fold and 85-fold, respectively, compared with +/+, whereas biliary glutathione was reduced by 90%. Similar effects on bile formation were caused by griseofulvin-induced inhibition of ferrochelatase activity in control mice. Conclusions: Bile formation is strongly affected in mice with impaired ferrochelatase activity. Rather than peroxidative processes, formation of cytotoxic bile with high concentrations of bile salts and PP may cause biliary fibrosis in fch/fch mice by damaging bile duct epithelium. GASTROENTEROLOGY 1999;117:696-705