A rice bran oil diet increases LDL-receptor and HMG-CoA reductase mRNA expressions and insulin sensitivity in rats with streptozotocin/nicotinamide-induced type 2 diabetes

A rice bran oil (RBO) diet can reduce plasma lipids; this was attributed to the specific components, gamma-oryzanol and gamma-tocotrienol, which individually were shown to be hypocholesterolemic; however, the mechanism of their effects on diabetic hyperlipidemia and the development of diabetes is not known. Rats with streptozotocin/nicotinamide-induced type 2 diabetes were divided into control, RO10, and RO15 groups, and fed cholesterol-free diets containing 0, 10, and 15 g RBO with 0, 352, and 528 g gamma-oryzanol and 0, 6.0 and 9.0 mg gamma-tocotrienol/100 g diet for 4 wk. Diabetic rats fed the RBO diet had greater insulin sensitivity (P = 0.02) than rats fed the control diet. Diabetic rats fed the RBO diet also had lower plasma triglyceride (P = 0.003), LDL cholesterol (P = 0.028), and hepatic triglyceride concentrations (P = 0.04), as well as greater fecal neutral sterol and bile acid excretion than those fed the control diet. After 4 wk, there was an approximately 100% (P < 0.001) increase in the abundance of hepatic cholesterol 7alpha-hydroxylase, an 89% (P < 0.001) increase in the hepatic LDL-receptor, and a 50% (P < 0.001) increase in hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA in rats fed the RBO diet compared with those fed the control diet. These findings support the conclusion that a rice bran oil-containing diet can significantly suppress hyperlipidemic and hyperinsulinemic responses in diabetic rats. The high contents of gamma-oryzanol and gamma-tocotrienol in RBO can lead to increased fecal neutral sterol and bile acid excretion, via upregulation of cholesterol synthesis and catabolism.

The hypolipidemic natural product guggulsterone acts as an antagonist of the bile acid receptor

Ayurveda, the ancient Indian system of health care and medicine, has a well-organized materia medica in which plants form a dominant part. A key illustration of the exploitation of this knowledge toward the development of a modern drug is the isolation and characterization of two antihyperlipidemic compounds, Z-, and E-guggulsterone from the tree Commiphora mukul, the exudate of which has been traditionally used for mitigating lipid disorders. Here, we demonstrate that Z-guggulsterone and an analog, 80-574 currently in clinical trials, act as antagonists of the bile acid receptor (BAR), a member of the intracellular receptor superfamily. These compounds antagonize the activity of BAR in vitro, and in cell culture systems on promoters and endogenous target genes. In biochemical assays, they are able to displace coactivator peptides from the receptor in a dose-dependent manner. The mechanism by which they act as BAR antagonists is likely through their inability to recruit coactivator proteins, failure to release corepressor proteins from unliganded receptor, and ability to compete with BAR agonists to block coactivator recruitment. Our data suggest these compounds may mediate at least some of their effects via the BAR.

Regulation of cholesterol 7alpha-hydroxylase gene (CYP7A1) transcription by the liver orphan receptor (LXRalpha)

The cholesterol 7alpha-hydroxylase gene (CYP7A1) plays an important role in regulation of bile acid biosynthesis and cholesterol homeostasis. Oxysterol receptor, LXR, stimulates, whereas the bile acid receptor, FXR, inhibits CYP7A1 transcription. The goal of this study was to investigate the role of LXRalpha on the regulation of rat, human and hamster CYP7A1 transcription in its native promoter and cellular context. Cotransfection with LXRalpha and RXRalpha expression plasmids strongly stimulated rat CYP7A1/luciferase reporter activity in HepG2 cells and oxysterol was not required. However, LXRalpha had much less effect on hamster and no significant effect on human CYP7A1 promoter activity in HepG2 cells. In Chinese hamster ovary cells, cotransfection with LXRalpha stimulated reporter activity by less than 2-fold and addition of 22(R)-hydroxycholesterol caused a small but significant stimulation of rat, human and hamster CYP7A1 promoter activity. At least two direct repeats of AGGTCA-like sequences with 4-base spacing (DR4) and five-base spacing (DR5), in previously identified bile acid response elements of the rat CYP7A1 were able to bind LXRalpha/RXRalpha and confer LXRalpha stimulation. However, LXRalpha did not bind to the corresponding sequences of the human gene and bound weakly to hamster and mouse DR4 sequences. Therefore, rats and mice have the unusual capacity to convert cholesterol to bile acids by LXRalpha-mediated stimulation of CYP7A1 transcription, whereas other species do not respond to cholesterol and develop hypercholesterolemia on a diet high in cholesterol.

Thyroid hormone receptor beta-deficient mice show complete loss of the normal cholesterol 7alpha-hydroxylase (CYP7A) response to thyroid hormone but display enhanced resistance to dietary cholesterol

Thyroid hormone (T3) influences hepatic cholesterol metabolism, and previous studies have established an important role of this hormone in the regulation of cholesterol 7alpha-hydroxylase (CYP7A), the rate-limiting enzyme in the synthesis of bile acids. To evaluate the respective contribution of thyroid hormone receptors (TR) alpha1 and beta in this regulation, the responses to 2% dietary cholesterol and T3 were studied in TRalpha1 and TRbeta knockout mice under hypo- and hyperthyroid conditions. Our experiments show that the normal stimulation in CYP7A activity and mRNA level by T3 is lost in TRbeta-/- but not in TRalpha1-/-mice, identifying TRbeta as the mediator of T3 action on CYP7A and, consequently, as a major regulator of cholesterol metabolism in vivo. Somewhat unexpectedly, T3-deficient TRbeta-/- mice showed an augmented CYP7A response after challenge with dietary cholesterol, and these animals did not develop hypercholesterolemia to the extent as did wild-type (wt) controls. The latter results lend strong support to the concept that TRs may exert regulatory effects in vivo independent of T3.

Synthesis of 7alpha-hydroxy derivatives of regulatory oxysterols

7alpha-Hydroxy derivatives of oxysterols are of considerable interest because of their possible involvement in regulation of cholesterol metabolism. This paper describes stereoselective syntheses and complete characterization of the 7alpha-hydroxy derivatives of four key oxysterols: 25-hydroxycholesterol, 27-hydroxycholesterol, 24(S)-hydroxycholesterol, and 24(S), 25-epoxycholesterol.

Modulation of cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase activities by steroids and physiological conditions in hamster

Our purpose was to examine the in vitro modulation of liver mitochondrial sterol 27-hydroxylase (S27OHase) and microsomal cholesterol 7alpha-hydroxylase (CH7alphaOHase) activities by certain drugs, sterols, oxysterols and bile acids, and to compare the influence of sex, age, diet and cholestyramine on these activities, in the hamster. In vitro, 7beta-hydroxycholesterol and 5alpha-cholestan-3beta-ol (cholestanol) were strong inhibitors (at 2 microM) of both enzyme activities, while 5beta-cholestan-3alpha-ol (epicoprostanol, 2 microM) and cyclosporin A (20 microM) inhibited S27OHase, but not CH7alphaOHase. These data suggest that a hydroxyl group at the 7alpha position is not required to inhibit CH7alphaOHase and that the presence of an aliphatic CH2-CH-(CH3)2 chain appears to be structurally important for S27OHase activity. Both enzyme activities remained unchanged by hyodeoxycholic acid (40 or 80 microM) while epicoprostanol inhibited only S27OHase and chenodeoxycholic acid only CH7alphaOHase. Adult (9-week old) male or female hamsters displayed similar S27OHase activity but the CH7alphaOHase activity was lower in females than in males, suggesting that the neutral bile acid pathway has a less important role in females. In male hamsters, S27OHase activity did not change with age, while CH7alphaOHase activity significantly increased (one-year vs 9-week old). A semi-purified sucrose-rich (lithogenic) diet significantly lowered both enzyme activities compared to the commercial diet. Cholestyramine induced a stimulation of both enzymes, slightly more vigorously however for the key enzyme involved in the neutral pathway. Taken together, these data indicate that the two enzymes are separately regulated and that certain drugs or steroid compounds can be useful for specifically inhibiting or stimulating the neutral or acidic bile acid pathway.

Cholesterol-lowering effects of NTE-122, a novel acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, on cholesterol diet-fed rats and rabbits

Pharmacological characterization of NTE-122 (trans-1,4-bis[[1-cyclohexyl-3-(4-dimethylamino phenyl)ureido]methyl]cyclohexane), a novel acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor, was performed with both in vitro and in vivo assay systems. NTE-122 inhibited microsomal ACAT activities of various tissues (liver of rabbit and rat, small intestine of rabbit and rat, and aorta of rabbit) and cultured cells (HepG2 and CaCo-2), with IC50 values from 1.2 to 9.6 nM. The inhibition mode of NTE-122 was competitive for HepG2 ACAT. NTE-122 had no effect on other lipid metabolizing enzymes, such as 3-hydroxy-3-methylglutaryl-CoA reductase, acyl-CoA synthetase, cholesterol esterase, lecithin:cholesterol acyltransferase, acyl-CoA:sn-glycerol-3-phosphate acyltransferase and cholesterol 7alpha-hydroxylase up to 10 microM. When NTE-122 was administered to the cholesterol diet-fed rats, serum and liver cholesterol levels were markedly reduced with an ED50 of 0.12 and 0.44 mg/kg/day, respectively. In the cholesterol diet-fed rabbits, NTE-122 significantly lowered plasma and liver cholesterol levels at more than 2 mg/kg/day. These results indicate that NTE-122 is a potent, selective and competitive inhibitor of ACAT, making it a worth while therapeutic agent for hypercholesterolemia and atherosclerosis.

Unexpected inhibition of cholesterol 7 alpha-hydroxylase by cholesterol in New Zealand white and Watanabe heritable hyperlipidemic rabbits

We investigated the effect of cholesterol feeding on plasma cholesterol concentrations, hepatic activities and mRNA levels of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase and hepatic LDL receptor function and mRNA levels in 23 New Zealand White (NZW) and 17 Watanabe heritable hyperlipidemic (WHHL) rabbits. Plasma cholesterol concentrations were 9.9 times greater in WHHL than NZW rabbits and rose significantly in both groups when cholesterol was fed. Baseline liver cholesterol levels were 50% higher but rose only 26% in WHHL as compared with 3.6-fold increase with the cholesterol diet in NZW rabbits. In both rabbit groups, hepatic total HMG-CoA reductase activity was similar and declined > 60% without changing enzyme mRNA levels after cholesterol was fed. In NZW rabbits, cholesterol feeding inhibited LDL receptor function but not mRNA levels. As expected, receptor-mediated LDL binding was reduced in WHHL rabbits. Hepatic cholesterol 7 alpha-hydroxylase activity and mRNA levels were 2.8 and 10.4 times greater in NZW than WHHL rabbits. Unexpectedly, cholesterol 7 alpha-hydroxylase activity was reduced 53% and mRNA levels were reduced 79% in NZW rabbits with 2% cholesterol feeding. These results demonstrate that WHHL as compared with NZW rabbits have markedly elevated plasma and higher liver cholesterol concentrations, less hepatic LDL receptor function, and very low hepatic cholesterol 7 alpha-hydroxylase activity and mRNA levels. Feeding cholesterol to NZW rabbits increased plasma and hepatic concentrations greatly, inhibited LDL receptor-mediated binding, and unexpectedly suppressed cholesterol 7 alpha-hydroxylase activity and mRNA to minimum levels similar to WHHL rabbits. Dietary cholesterol accumulates in the plasma of NZW rabbits, and WHHL rabbits are hypercholesterolemic because reduced LDL receptor function is combined with decreased catabolism of cholesterol to bile acids.

Failure of intravenous infusion of taurocholate to down-regulate cholesterol 7 alpha-hydroxylase in rats with biliary fistulas

BACKGROUND/AIMS

The decrease in cholesterol 7 alpha-hydroxylase induced by intraduodenal infusion of taurocholate in bile fistula rats may be indirect, i.e., mediated through release or absorption of an intestinal factor in response to the presence of bile salts in the intestine. The aim of this study was to determine if negative feedback regulation of cholesterol 7 alpha-hydroxylase can be shown when equimolar concentrations of taurocholate are administered intravenously, thus bypassing the intestine.

METHODS

After 96 hours of biliary diversion, taurocholate (36 mumol.h-1.100 g, rat-1) was infused into the rats either intravenously or intraduodenally for the final 24 hours. Livers were then harvested for analysis of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase specific activity, cholesterol 7 alpha-hydroxylase specific activity, messenger RNA levels, and transcriptional activity.

RESULTS

Intraduodenally administered taurocholate significantly decreased HMG-CoA reductase and cholesterol 7 alpha-hydroxylase specific activity by more than 50% and cholesterol 7 alpha-hydroxylase steady-state messenger RNA levels and transcriptional activity by 50%-75%. In contrast, intravenous administration of taurocholate failed to down-regulate either cholesterol 7 alpha-hydroxylase or HMG-CoA reductase.

CONCLUSIONS

Passage of taurocholate through the intestine strongly potentiates negative feedback regulation of cholesterol 7 alpha-hydroxylase. A putative intestinal factor, released or absorbed in the presence of bile acids in the intestinal lumen, may play a role in the regulation of bile acid synthesis.

Effect of pravastatin, a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, on hepatic cholesterol 7 alpha-hydroxylase, acyl-coenzyme A: cholesterol acyltransferase, and bile lipid secretion in the hamster with intact enterohepatic circulation

The effects of administration of pravastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, on hepatic cholesterol 7 alpha-hydroxylase and acyl-coenzyme A: cholesterol acyltransferase (ACAT) activities and bile lipid secretion were investigated in Syrian golden hamsters. Continuous administration of pravastatin induced no significant changes in hepatic cholesterol content, ACAT and cholesterol 7 alpha-hydroxylase activities, or bile lipid and acid composition. Abrupt withdrawal of pravastatin induced increases in hepatic cholesterol content and ACAT activity and no change in hepatic cholesterol 7 alpha-hydroxylase activity, and increased cholesterol saturation in bile. Hepatic cholesterol 7 alpha-hydroxylase activity paralleled hepatic mRNA levels of this enzyme. These results suggest that a change in hepatic cholesterol metabolism induced by continuous administration of pravastatin maintains a constant net balance of hepatic cholesterol content. In addition, the drug has no deleterious influence on metabolism of bile lipids and acids and related enzymes, except for a transient increase in cholesterol saturation in bile induced by an inappropriate increase in hepatic cholesterol content and a lack of response of cholesterol 7 alpha-hydroxylase activity to changes in hepatic cholesterol content upon abrupt withdrawal of pravastatin.

Effect of estradiol and progesterone on cholesterol 7 alpha-hydroxylase activity in rats subjected to different feeding conditions

The regulation of cholesterol 7 alpha-hydroxylase activity by estradiol and progesterone was investigated in liver microsomes isolated from rats fed standard diet, either ad libitum or fasted for 24 h, and diet containing the bile acid sequesterant cholestyramine. Differential effects were observed when the direct action of estradiol and progesterone on microsome preparations was examined. Cholesterol 7 alpha-hydroxylase activity was inhibited by progesterone in a dose-dependent way to almost complete abolition; similar patterns of declines were found in the three feeding groups under study. In contrast, the addition of 5 microM estradiol induced small and selective 7 alpha-hydroxylase increases in fasting and cholestryamine-fed animals, then activity declined to control values and consistent decreases were found from 20 microM. The administration of estradiol (50 micrograms) or progesterone (100 micrograms) for 21 days resulted in depressed cholesterol 7 alpha-hydroxylase activity in rats with high bile acid synthesis basal rate due to cholestyramine feeding. In rats receiving a standard diet, either ad libitum or after 24 h fasting, the hormonal effects did not reach significance. Declines in the content of free cholesterol were provoked by progesterone, not by estradiol, in liver microsomes prepared from all feeding groups. No changes in cholesterol 7 alpha-hydroxylase activity and microsomal free cholesterol were observed after administration of the sex hormones for 3 days. Rapid and transient inhibitions in 7 alpha-hydroxylase activity were found after the single injection of progesterone to fed animals. Estradiol, on the contrary, was unable to alter rapidly the hepatic 7 alpha-hydroxylase capacity.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of ursodeoxycholic acid on hepatic cholesterol metabolism

Oral administration of ursodeoxycholic acid (UDCA) renders bile unsaturated with cholesterol by reducing the hepatic output of cholesterol. Theoretically, several mechanisms may be of importance. In the present overview, the effect of treatment with UDCA on hepatic cholesterol metabolism is evaluated, in particular the influence on hepatic cholesterol synthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity, bile acid synthesis, 7 alpha-hydroxylation of cholesterol, and esterification of cholesterol--acyl coenzyme A: cholesterol acetyltransferase (ACAT) activity. It is apparent that UDCA treatment does not inhibit the hepatic HMG CoA reductase activity. Neither is ACAT activity or the cholesteryl ester content changed by UDCA. The catabolism of cholesterol to bile acids is unaffected or slightly increased during administration of UDCA. It is concluded that a stimulated degradation of cholesterol to bile acids may partly explain the decrease in hepatic secretion of cholesterol obtained during UDCA administration. It is suggested that the reduction in cholesterol absorption from the intestine seen during UDCA therapy may also be of importance.

On the mechanism of the hypolipidemic effect of sulfur-substituted hexadecanedioic acid (3-thiadicarboxylic acid) in normolipidemic rats

The mechanism behind the hypolipidemic effect of the sulfur-substituted non-beta-oxidizable fatty acid analogue 1,10 bis(carboxymethylthio)decane, also known as 3-thiadicarboxylic acid, was studied in normolipidemic rats. Treatment with 3-thiadicarboxylic acid markedly decreased plasma levels of free fatty acids, triglycerides, and cholesterol. This was accompanied by a corresponding reduction in very low density lipoprotein (VLDL)-triglyceride and low density lipoprotein (LDL)-cholesterol levels (by 46% and 42%, respectively), whereas the decrease in high density lipoprotein (HDL)-cholesterol levels was less pronounced (16%). However, the composition of the various plasma lipoprotein fractions was essentially unchanged. Fatty acid oxidation in both mitochondria and peroxisomes was stimulated in parallel; the activities of ATP:citrate lyase and fatty acid synthase, two key enzymes in fatty acid synthesis, were inhibited. Hepatic triglyceride biosynthesis was retarded, as indicated by a decrease in the liver triglyceride content along with a 30% reduction of hepatic VLDL-triglyceride secretion. This was accompanied by a 50% inhibition of phosphatidate phosphohydrolase. The activities of plasma lipoprotein lipase as well as hepatic lipase were somewhat higher (18%) in treated animals, suggesting a slight increase in the clearance potential of triglyceride-rich lipoproteins. The cholesterol-lowering effect was accompanied by a considerable reduction (75%) in HMG-CoA reductase activity and a less pronounced inhibition of cholesterol 7 alpha-hydroxylase (52%), and acyl-CoA:cholesterol acyltransferase (25%) activities. The present data suggest that the hypotriglyceridemic and hypocholesterolemic properties of sulfur-substituted fatty acid analogues are primarily due to effects on triglyceride and cholesterol synthesis.

Cholesterol 7 alpha-hydroxylase: evidence for transcriptional regulation by cholesterol or metabolic products of cholesterol in the rat

Cholesterol 7 alpha-hydroxylase, the rate-determining enzyme in the bile acid biosynthesis pathway, is regulated in a negative feedback manner by hydrophobic bile salts returning to the liver via the portal circulation. The role of cholesterol in the regulation of cholesterol 7 alpha-hydroxylase and the interrelationship between the cholesterol and bile acid biosynthesis pathways remain controversial. The objective of the present study was to define the role of cholesterol in the regulation of cholesterol 7 alpha-hydroxylase and determine the molecular level of its control. In order to avoid intestinal or intravenous administration of cholesterol, we manipulated the flow of cholesterol within the hepatocytes by decreasing cholesterol synthesis with lovastatin in bile fistula rats (bile acid synthesis is up-regulated), or by increasing cholesterol supply by administering mevalonate, a precursor of cholesterol, to rats with intact enterohepatic circulation (bile acid synthesis is normal). In the first series of studies, lovastatin was administered as a single intravenous bolus (10 mg/kg) to rats with chronic bile fistula and to rats with intact enterohepatic circulation (cholesterol and bile acid synthesis is normal). Three hours after lovastatin administration, cholesterol 7 alpha-hydroxylase specific activity, enzyme mass, mRNA, and gene transcriptional activity were decreased by 35%, 32%, 56%, and 34%, respectively, in rats with chronic bile fistula. In rats with intact enterohepatic circulation, lovastatin administration resulted in a similar decrease (34%) of cholesterol 7 alpha-hydroxylase specific activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Gallstone formation in cholestanol-fed mice

We examined the effect of cholestanol (5 alpha-dihydrocholesterol) on cholesterol and bile acid metabolism in BALB/c mice. After feeding 1% cholestanol in the diet for 14 months, gallstones composed of 55% cholesterol and 45% cholestanol developed in 20% of the mice and were associated with mucosal inflammation and serosal vessel thickening of the gallbladder. Cholestanol concentrations increased 42-fold in the serum (0.17 versus 0.004 mg/ml) and 18-fold in the liver (0.55 versus 0.03 mg/g) as compared with control mice, whereas cholesterol declined 20 and 26% in serum and liver, respectively. Hepatic microsomal HMG-CoA reductase activity, reflecting cholesterol synthesis, rose 51% (from 7.2 to 10.9 pmol/mg/min). In contrast, hepatic microsomal cholesterol 7 alpha-hydroxylase activity, the rate-determining enzyme for bile acid synthesis, was severely depressed as compared with control mice (0.9 versus 2.2 pmol/mg/min). Discontinuing cholestanol from the diet for 1 month reduced the elevated serum and liver cholestanol concentrations and restored hepatic HMG-CoA reductase and cholesterol 7 alpha-hydroxylase activities to normal. These results demonstrate that cholestanol is absorbed, replaces cholesterol in serum and liver, causes increased cholesterol synthesis, but inhibits bile acid synthesis. The combination of increased cholesterol synthesis with decreased bile acid formation promotes gallstone formation in cholestanol-fed mice.

Differing effects of cholesterol and taurocholate on steady state hepatic HMG-CoA reductase and cholesterol 7 alpha-hydroxylase activities and mRNA levels in the rat

We investigated the effects of cholesterol, cholestyramine, and taurocholate feeding on steady state specific activities and mRNA levels of hepatic 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase and cholesterol 7 alpha-hydroxylase in the rat. Interruption of the enterohepatic circulation of bile acids (cholestyramine feeding) increased total HMG-CoA reductase activity 5-fold. Cholesterol and taurocholate administration suppressed total microsomal HMG-CoA reductase activities 87% and 65%, respectively. HMG-CoA reductase mRNA levels increased 3-fold with cholestyramine, did not decrease significantly with cholesterol feeding, but were markedly decreased after taurocholate treatment. Cholesterol 7 alpha-hydroxylase activity increased 4-fold with cholestyramine and 29% during cholesterol feeding, but decreased 64% with taurocholate. Cholesterol 7 alpha-hydroxylase mRNA levels rose 150% and 50% with cholestyramine and cholesterol feeding, respectively, but decreased 73% with taurocholate. The administration of cholesterol together with taurocholate prevented the decline in cholesterol 7 alpha-hydroxylase mRNA levels, but inhibition of enzyme activity persisted (-76%). Hepatic microsomal cholesterol concentrations increased 2-fold with cholesterol feeding but did not change with taurocholate or cholestyramine treatment. These results demonstrate that mRNA levels of HMG-CoA reductase are controlled by the hepatic taurocholate flux, whereas mRNA levels of cholesterol 7 alpha-hydroxylase are controlled by the cholesterol substrate supply. These end products, cholesterol and bile acids, exert post-transcriptional regulation on HMG-CoA reductase and cholesterol 7 alpha-hydroxylase, respectively.

On the mechanism of stimulation of cholesterol 7 alpha-hydroxylase by dietary cholesterol

In agreement with previous work, treatment of rats with cholesterol, 2% in diet, stimulated the cholesterol 7 alpha-hydroxylase activity more than 2-fold. With less than 1% in diet, no significant effect was obtained. Intravenous infusion of cholesterol-enriched Intralipid had no stimulatory effect. In accordance with some recent work by other groups, it was shown that the stimulation of the cholesterol 7 alpha-hydroxylase by dietary cholesterol was associated with elevated levels of mRNA corresponding to the enzyme. Most of the stimulation of the activity induced by dietary cholesterol could not be prevented by lymphatic drainage. Feeding lymph fistulated rats with 2% cholesterol in diet stimulated the cholesterol 7 alpha-hydroxylase almost 2-fold, indicating that under the conditions employed, a major part of the cholesterol-induced stimulation of the activity was due to factor(s) unrelated to the flux of cholesterol from the intestine to the liver. There was a good correlation between the amount of cholesterol excreted in faeces and the activity of the cholesterol 7 alpha-hydroxylase. The half-life of intraperitoneally administered labelled cholic acid was significantly shorter in rats treated with 2% cholesterol in diet (t1/2 = 1.2 +/- 0.1 days) than in control rats (t1/2 = 1.9 +/- 0.18 days). A notable finding was that the weight of faeces was considerably higher in rats fed cholesterol than in the controls. It is hypothesized that a high dietary load of cholesterol causes increased binding of bile acids in the intestine and increased loss of bile acids in faeces. This leads to a reduced suppression of the cholesterol 7 alpha-hydroxylase by the bile acids. The results support the contention that the flux of bile acids rather than the flux of cholesterol from the intestine is the major direct regulator of bile acid biosynthesis.