Hepatic fibrosis and cytochrome P450: experimental models of fibrosis compared to AHR knockout mice

Hepatic fibrosis is characterized by abnormal collagen deposition resulting from increased collagen synthesis and decreased collagen degradation. Cytochrome P450 mediates major drug metabolizing enzyme activity in the liver and this activity is reduced in hepatic fibrosis. In this study we assess cytochrome P450 and CYP 1A mRNA in livers of animals that have been induced to hepatic fibrosis using the heterologous serum induced model of fibrosis in rats compared to controls. Fibrosis was confirmed by assessing the collagen in liver sections as quantified by Sirius red/Fast green staining, a quantitative measure of fibrosis as well as by visualization of the hepatic fibrosis. Collagen levels in the liver sections of heterologous serum induced fibrotic rats was increased by 33% compared to controls and a typical fibrotic pattern was seen. Messenger RNA was prepared from heterologous serum induced fibrotic rats and compared to controls. CYP 1A2 was assessed using a specific probe and the CYP 1A2 level was significantly reduced in the heterologous serum induced fibrotic rats compared to controls. These results further suggest that cytochrome P450 is reduced in the presence of hepatic fibrosis. Thus, in three well established experimental models of hepatic fibrosis which had clearly developed hepatic fibrosis (as shown by Sirius red/Fast green staining), cytochrome P450 mediated enzyme activity, or specifically, CYP 1A messenger RNA is decreased. We then investigated a transgenic mouse, deficient in the arylhydrocarbon hydroxylase receptor (AHR), which has undetectable levels of CYP 1A messenger RNA. We quantitated the collagen in liver sections obtained from AHR knockout mice compared to controls, as an indication of the presence of hepatic fibrosis. Collagen concentration was significantly increased by 53% (P<0.0005) in sections from Ahr-/- (knockout) mice compared to wild-type controls. Collagen in livers of the Ahr+/- heterozygous mice was not different from wild-type controls. The increase in collagen concentration in liver sections is an indication of fibrosis in Ahr-/- mice. Collagen protein deposition was also elevated in liver sections from bile duct ligated rats (by 44%) compared to sham operated controls, was elevated in liver sections from heterologous serum induced fibrosis in rats (33%) compared to controls, and was elevated in liver sections from yellow phosphorous induced hepatic fibrosis (74%) compared to vehicle treated controls. In conclusion, these results indicate that cytochrome P450 and specific subtypes of P450, the CYP 1A subgroup, are significantly reduced in three experimental models of hepatic fibrosis when there is evidence of increased collagen deposition in the livers. These results also indicate that mice that are deficient in CYP 1A have elevated levels of hepatic collagen protein, an indication of hepatic fibrosis.