Effects of thyroidectomy and thyroxine treatment on the activity of 12alpha-hydroxylase and of some components of microsomal electron transfer chains in rat liver

Regulation of sterol 12alpha-hydroxylase and cholic acid biosynthesis in the rat

BACKGROUND & AIMS

Sterol 12alpha-hydroxylase (CYP8b1) is required for the biosynthesis of cholic acid (CA) and hence helps determine the ratio of CA to chenodeoxycholic acid (CDCA) in bile. This study examined the in vivo regulation of CYP8b1 in the rat by bile acids, cholesterol, and thyroxine.

METHODS

The specific activities (SAs), messenger RNA (mRNA) levels, and transcriptional activities of CYP8b1 were determined in intact rats and rats with biliary diversion.

RESULTS

CA, CDCA, and deoxycholic acid (DCA), fed as a supplement to the diet, down-regulated CYP8b1 SAs by 99% +/- 0%, 72% +/- 10%, and 98% +/- 1%, respectively. Under these same conditions, mRNA levels decreased by 93% +/- 7%, 60% +/- 11%, and 93% +/- 4%, respectively. Intraduodenal infusion of taurocholate (36 micromol/h. 100 g rat(-1)) decreased SAs and mRNA levels by 63% +/- 8% and 74% +/- 8%, respectively. Ursodeoxycholic acid (UDC) and hyocholic acid (HC) feeding increased CYP8b1 SAs by 119% +/- 21% and 65% +/- 18%, respectively. CA feeding decreased CYP8b1 transcriptional activity by 72%. Complete biliary diversion increased CYP8b1 SAs and mRNA levels by 150% +/- 30% and 287% +/- 51%, respectively. Cholesterol feeding decreased CYP8b1 mRNA by 39% +/- 8%. In intact rats, a single injection of thyroid hormone eliminated CYP8b1 activity.

CONCLUSIONS

CYP8b1 is transcriptionally down-regulated by hydrophobic but not hydrophilic bile acids. Cholesterol feeding and a single thyroid hormone injection repressed CYP8b1 in the face of induction of cholesterol 7alpha-hydroxylase (CYP7a1 by the new nomenclature) SAs. These results suggest that cholesterol, thyroid hormone, and hydrophobic bile acids are important regulators of CYP8b1 and consequently of the bile acid pool composition.

Thyroid hormone suppresses hepatic sterol 12alpha-hydroxylase (CYP8B1) activity and messenger ribonucleic acid in rat liver: failure to define known thyroid hormone response elements in the gene

Sterol 12alpha-hydroxylase (CYP 8B1) is a microsomal cytochrome P450 enzyme involved in bile acid synthesis that is of critical importance for the composition of bile acids formed in the liver. Thyroidectomy of rats caused a more than twofold increase of CYP8B1 and an almost fourfold increase of the corresponding mRNA levels compared to sham-operated rats. Treatment of intact rats with thyroxine caused a 60% reduction of enzyme activity and a 50% reduction of mRNA levels compared to rats injected with saline only. To investigate whether the promoter of the gene contains thyroid hormone response elements, the complete structure of the rat gene was defined. In similarity with the corresponding gene in mouse, rabbit and man, the rat gene was found to lack introns. It had an open reading frame containing 1500 bp corresponding to a protein of 499 amino acid residues. Although thyroid hormone decreased CYP8B1 activity and mRNA in vivo, no hitherto described thyroid hormone response elements were identified 1883 bases upstream of the transcription start site. It is concluded that rat CYP8B1 is regulated by thyroid hormone at the mRNA level. The results are discussed in relation to the structure of the gene coding for the enzyme.

Regulation of bile acid biosynthesis

This article provides a review of the pathways through which cholesterol is degraded to bile acids. Regulation of key enzymes in the bile acid biosynthestic pathways is discussed. The important role of these pathways in the maintenance of cholesterol homeostasis and the possible therapeutic implications for the treatment of hypercholesterolemia are emphasized.

Bile acid synthesis in primary cultures of rat and human hepatocytes

The regulation of hepatic bile acid formation is incompletely understood. Primary cultures of mammalian hepatocytes offer an opportunity to examine putative regulatory factors in relative isolation. Using rat and human hepatocytes in primary culture, we examined bile acid composition and the expression of the rate-limiting enzyme of formation, cholesterol 7alpha-hydroxylase. Control rat hepatocytes showed a declining bile acid production over 4 days, from 156 +/- 24 ng/mL (67% cholic acid) on day 1 to 55 +/- 11 ng/mL (55% cholic acid) on day 4. In addition to cholic acid, chenodeoxycholic acid, alpha-muricholic acid, and beta-muricholic acid were formed. Treatment with triidothyronine (T3) or dexamethasone alone had no significant effect on bile acid production. A combination of T3 and dexamethasone significantly increased the total bile acid production on day 4 (224 +/- 54 ng/mL) and resulted in a marked change in composition to 23% cholic acid and 77% non-12alpha-hydroxylated bile acids. Control rat hepatocytes had a cholesterol 7alpha-hydroxylase activity of 3.3 +/- 0.6 pmol/mg protein/min after 4 days in culture. Cells treated with the combination of dexamethasone and T3 had an activity of 16.4 +/- 3.6 pmol/mg protein/min. The cholesterol 7alpha-hydroxylase messenger RNA (mRNA) levels, determined by solution hybridization after 4 days of culture, showed results similar to those for the activity data; control cells had 5.3 +/- 0.9 cpm/microg total nucleic acids (tNAs). T3 or dexamethasone-treated cells did not differ from control cells, whereas the combination of T3 and dexamethasone increased the mRNA levels to 20.6 +/- 2.8 cpm/microg tNAs. In human hepatocytes, isolated from donor liver, bile acid formation increased from 206 +/- 79 ng/mL on day 2 to 1490 +/- 594 ng/mL on day 6 and then declined slightly. Cholic acid and chenodeoxycholic acid were formed, constituting about 80% and 20%, respectively. The combined addition of T3 and dexamethasone had a tendency to decrease rather than increase bile acid formation. Also, mRNA levels of the cholesterol 7alpha-hydroxylase increased severalfold in the human hepatocytes from day 2 to day 4 and then declined. The addition of T3 or dexamethasone did not effect the mRNA levels in any consistent way. It is noteworthy that the capacity of the cultured human hepatocytes to produce bile acids was higher than that of cultured rat hepatocytes, in spite of the fact that the production of bile acids in rat liver is 3- to 5-fold higher than that in human liver in vivo. It is also evident that while hormonal factors appear to regulate bile acid synthesis in the rat, no evidence for this was found in human hepatocytes. As the composition of bile acids secreted by human hepatocytes in primary culture closely resembles that found in vivo, this represents a useful model for further studies of the synthesis and regulation of bile acids.

Bile acid metabolism in human hyperthyroidism

Decreased levels of serum cholesterol are a well-recognized finding in hyperthyroidism. Since the conversion to bile acids is an important pathway for the elimination of cholesterol, we studied primary bile acid kinetics in seven hyperthyroid patients before and after medical treatment. Pool sizes, fractional turnover and synthesis rates of cholic acid and chenodeoxycholic acid were determined after oral administration of 50 mg [13C]cholic acid and 50 mg [13C]chenodeoxycholic acid. 13C/12C isotope ratios in serum were measured by capillary gas chromatography/electron impact mass spectrometry. Compared with the euthyroid state, serum cholesterol levels were distinctly lower in hyperthyroidism (150 +/- 33 vs. 261 +/- 51 mg per dl, p less than 0.01). Thyroid hormone excess caused a 34% reduction in cholic acid synthesis (5.8 +/- 2.8 vs. 7.9 +/- 4.2 mu moles per kg per day, p less than 0.02), which was associated with a 47% decrease in cholic acid pool size (11.7 +/- 3.4 vs. 22.0 +/- 5.2 mu moles per kg, p less than 0.01). Chenodeoxycholic acid kinetics exhibited no apparent changes. Thus, total primary bile acid synthesis was diminished by 20% in hyperthyroidism. After normalization of thyroid function, the ratio of cholic acid/chenodeoxycholic acid pool size increased in all patients. This was paralleled by a rise in the ratio of concentrations of cholic acid/chenodeoxycholic acid in serum. The depression of cholic acid synthesis in the presence of unaltered subjects is compatible with an inhibition of hepatic 12 alpha-hydroxylation by thyroid hormone. Furthermore, evidence is provided that, in man, the low serum cholesterol levels found during hyperthyroidism are not caused by an increased conversion of cholesterol to bile acid.

Regulation of hydroxylations in biosynthesis of bile acids: modulation of reconstituted 12 alpha-hydroxylase activity by protein fractions from rabbit liver cytosol and microsomes

The activity of purified 12 alpha-hydroxylase from rabbit liver microsomes was modulated by including protein fractions from rabbit liver microsomes and cytosol into the system. The microsomal protein fraction stimulated 12 alpha-hydroxylation two times. The cytosolic fraction inhibited the reaction markedly. The microsomal 12 alpha-hydroxylase stimulatory activity was labile and the cytosolic 12 alpha-hydroxylase inhibitory activity was stable to mild heat treatment. Addition of ATP and MgCl2 or NaF had no effect on the activities of the two protein fractions. The activity of the microsomal stimulatory fraction decreased upon storage but could be reactivated by addition of reduced glutathione to the system.

Alteration of cholesterol metabolism by 4-O-methylascochlorin in rats

The effect of 4-O-methylascochlorin (MAC), an experimental hypocholesterolemic agent, on cholesterol metabolism was investigated in rats in two separate experiments. The administration of MAC for 2 and 6 consecutive weeks at daily doses of 100-135 mg/kg resulted in reduction in serum cholesterol levels of 16% after 2 weeks of treatment in the first experiment, and 13% after 6 weeks in the second experiment in comparison to the corresponding controls. MAC administered at a daily dose of 100 mg/kg for 2 weeks showed a significant increase in the biliary excretion of bile acids and cholesterol in bile-duct cannulated rats with or without the administration of taurocholate. In the second experiment, MAC treatment for 6 weeks produced a marked increase in the fecal output of acidic sterols during a 2 to 6-week period. MAC treatment also further enhanced hepatic cholesterol 7 alpha-hydroxylase in the rats. Therefore, it appears that the mechanism of serum cholesterol lowering due to MAC is related to the enhancement of hepatic bile acid synthesis and the increase in biliary and fecal excretion of bile acids.

Effects of clofibrate on some microsomal hydroxylations involved in the formation and metabolism of bile acids in rat liver

1. The liver microsomal metabolism of [4-14C]cholesterol, endogenous cholesterol, 7 alpha-hydroxy-4-[6 beta-3H]cholesten-3-one, 5-beta-[7 beta-3H]cholestane-3 alpha, 7 alpha-diol and [3H]lithocholic acid was studdied in control and clofibrate (ethyl p-chlorophenoxyisobutyrate)-treated rats. 2. The extent of 7 alpha-hydroxylation of exogenous [414C]cholesterol and endogenous cholesterol, the latter determined with a mass fragmentographic technique, was the same in the two groups of rats. The extent of 12 alpha-hydroxylation of 7 alpha-hydroxy-4-cholesten-3-one and 5 beta-cholestane-3 alpha, 7 alpha-diol was increased by about 60 and 120% respectively by clofibrate treatment. The 26-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha-diol was not significantly affected by clofibrate. The 6 beta-hydroxylation of lithocholic acid was about 80% higher in the clofibrate-treated animals than in the controls. 3. The results are discussed in the context of present knowledge about the liver microsomal hydroxylating system and bile acid formation in patients with hypercholesterolaemia, treated with clofibrate.

Effects of phenobarbital upon bile acid synthesis in two strains of rats

Rats of the Wistar and Sprague-Dawley strains were injected with sodium phenobarbital (100 mg/kg body wt/day) for 8 days. Fecal bile acid excretion was measured on days 6 and 8 of the experiment, and biliary bile acid composition, hepatic microsomal cholesterol, 7alpha-hydroxylase, and 7alpha-hydroxy-4-cholesten-3-one 12alpha-hydroxylase were determined at the end of the study. In the Wistar rat, injection of phenobarbital produced a doubling of fecal bile acid output (controls, 5.3 mg/rat/day; treated rats, 10.6 mg/rat/day) and a two-threefold increase in cholesterol 7alpha-hydroxylase. The fecal bile acid output of Sprague-Dawley rats increased 20% in response to phenobarbital (controls, 9,5 mg/rat/day; treated rats, 11.6 mg/rat/day). The activity of cholesterol 7alpha-hydroxylase remained unchanged. In both strains, phenobarbital treatment produced a decrease in the proportion of cholic acid in total bilary bile acids (controls, 85%; treated groups, 65%). This was associated with a decrease of 7alpha-hydroxy-4-cholesten-3-one 12alpha-hydroxylase activity by ca. 50%. Biliary cholesterol concentrations were reduced in phenobarbital treated rats of both strains, but liver cholesterol concentrations remained unchanged. The drug produced a 25% increase in liver wt, on the average.