Effects of bile acid feeding on hepatic deoxycholate 7 alpha-hydroxylase activity in the hamster

Differential regulation of bile acid and cholesterol metabolism by the farnesoid X receptor in Ldlr -/- mice versus hamsters

Modulating bile acid synthesis has long been considered a good strategy by which to improve cholesterol homeostasis in humans. The farnesoid X receptor (FXR), the key regulator of bile acid synthesis, was, therefore, identified as an interesting target for drug discovery. We compared the effect of four, structurally unrelated, synthetic FXR agonists in two fat-fed rodent species and observed that the three most potent and selective agonists decreased plasma cholesterol in LDL receptor-deficient (Ldlr (-/-)) mice, but none did so in hamsters. Detailed investigation revealed increases in the expression of small heterodimer partner (Shp) in their livers and of intestinal fibroblast growth factor 15 or 19 (Fgf15/19) in mice only. Cyp7a1 expression and fecal bile acid (BA) excretion were strongly reduced in mice and hamsters by all four FXR agonists, whereas bile acid pool sizes were reduced in both species by all but the X-Ceptor compound in hamsters. In Ldlr (-/-) mice, the predominant bile acid changed from cholate to the more hydrophilic β-muricholate due to a strong repression of Cyp8b1 and increase in Cyp3a11 expression. However, FXR agonists caused only minor changes in the expression of Cyp8b1 and in bile acid profiles in hamsters. In summary, FXR agonist-induced decreases in bile acid pool size and lipophilicity and in cholesterol absorption and synthesis could explain the decreased plasma cholesterol in Ldlr (-/-) mice. In hamsters, FXR agonists reduced bile acid pool size to a smaller extent with minor changes in bile acid profile and reductions in sterol absorption, and consequently, plasma cholesterol was unchanged.

Protective effects of ursodeoxycholic acid on chenodeoxycholic acid-induced liver injury in hamsters

AIM: To investigate the effects of ursodeoxycholic acid (UDCA) on chenodeoxycholic acid (CDCA)-induced liver injury in hamsters, and to elucidate a correlation between liver injury and bile acid profiles in the liver.

METHODS: Liver injury was induced in hamsters by administration of 0.5% (w/w) CDCA in their feed for 7 d. UDCA (50 mg/kg and 150 mg/kg) was administered for the last 3 d of the experiment.

RESULTS: At the end of the experiment, serum alanine aminotransferase (ALT) increased more than 10 times and the presence of liver injury was confirmed histologically. Marked increase in bile acids was observed in the liver. The amount of total bile acids increased approximately three-fold and was accompanied by the increase in hydrophobic bile acids, CDCA and lithocholic acid (LCA). UDCA (50 mg/kg and 150 mg/kg) improved liver histology, with a significant decrease (679.3 ± 77.5 U/L vs 333.6 ± 50.4 U/L and 254.3 ± 35.5 U/L, respectively, P < 0.01) in serum ALT level. UDCA decreased the concentrations of the hydrophobic bile acids, and as a result, a decrease in the total bile acid level in the liver was achieved.

CONCLUSION: The results show that UDCA improves oral CDCA-induced liver damage in hamsters. The protective effects of UDCA appear to result from a decrease in the concentration of hydrophobic bile acids, CDCA and LCA, which accumulate and show the cytotoxicity in the liver.

Short-term hepatic effects of depleted uranium on xenobiotic and bile acid metabolizing cytochrome P450 enzymes in the rat

The toxicity of uranium has been demonstrated in different organs, including the kidneys, skeleton, central nervous system, and liver. However, few works have investigated the biological effects of uranium contamination on important metabolic function in the liver. In vivo studies were conducted to evaluate its effects on cytochrome P450 (CYP) enzymes involved in the metabolism of cholesterol and xenobiotics in the rat liver. The effects of depleted uranium (DU) contamination on Sprague-Dawley were measured at 1 and 3 days after exposure. Biochemical indicators characterizing liver and kidney functions were measured in the plasma. The DU affected bile acid CYP activity: 7alpha-hydroxycholesterol plasma level decreased by 52% at day 3 whereas microsomal CYP7A1 activity in the liver did not change significantly and mitochondrial CYP27A1 activity quintupled at day 1. Gene expression of the nuclear receptors related to lipid metabolism (FXR and LXR) also changed, while PPARalpha mRNA levels did not. The increased mRNA levels of the xenobiotic-metabolizing CYP3A enzyme at day 3 may be caused by feedback up-regulation due to the decreased CYP3A activity at day 1. CAR mRNA levels, which tripled on day 1, may be involved in this up-regulation, while mRNA levels of PXR did not change. These results indicate that high levels of depleted uranium, acting through modulation of the CYP enzymes and some of their nuclear receptors, affect the hepatic metabolism of bile acids and xenobiotics.

Negative feedback regulation of the ileal bile acid transport system in rodents

BACKGROUND

Active transport of conjugated bile acids by ileal enterocytes is a key mechanism for conservation of the bile acid pool. Experiments were performed to determine whether such transport is regulated by substrate load.

METHODS

Using anesthetized biliary fistula guinea pigs or rats, the ileum was perfused with ursodeoxycholyltaurine at a concentration causing maximal ileal transport of this bile acid; absorption was assessed by biliary recovery. Before ileal perfusion, animals ingested one of three diets: chow, chow with added conjugated bile acid, or chow with added cholestyramine.

RESULTS

In the guinea pig, ingestion of a taurocholate-enriched diet resulted in a 75% decrease in the absorption rate of ursodeoxycholyltaurine. Similar results were obtained with cholylsarcosine (a deconjugation-dehydroxylation resistant analogue) or with chenodeoxycholylglycine, the endogenous bile acid of the guinea pig. In contrast, cholestyramine ingestion caused an increase in ursodeoxycholyltaurine absorption. In the rat, cholyltaurine or cholylsarcosine ingestion also caused decreased ileal transport. In the guinea pig, maximal down-regulation of active ileal bile acid transport occurred after 2-3 days of bile acid feeding; up-regulation required 3-4 days.

CONCLUSIONS

Bile acid metabolism is regulated by feedback inhibition of active ileal transport in addition to the well-established feedback inhibition of bile acid biosynthesis in the liver. Together, these two regulatory mechanisms ensure constancy of bile acid secretion.