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Z R Vlahcevic

Publications and source records attributed to Z R Vlahcevic.

At least 37 records · Page 2Linked to original sources
PubMed1994

Post-transcriptional regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7 alpha-hydroxylase in rats with subtotal nephrectomy.

Chronic renal failure is associated with hyperlipidemia and atherosclerosis. The mechanism responsible for the observed increase of serum cholesterol in chronic renal disease is not certain. The objective of the present study was to characterize the effect of induced renal failure on 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) and cholesterol 7 alpha-hydroxylase, the two rate determining enzymes of the cholesterol and bile acid biosynthetic pathways, respectively. Studies were carried out in rats with subtotal (75%) nephrectomy, which resulted in a marked elevation of blood urea nitrogen (371 +/- 44% of control, P < 0.001), and was accompanied by significant increases in the levels of serum cholesterol (133 +/- 7%, P < 0.005) and triglycerides (185 +/- 25, P < 0.01). In nephrectomized rats, an increase in the specific activity of HMG-CoA reductase (219 +/- 30% above control levels, P < 0.02) was observed. This increase occurred in the presence of elevated hepatic microsomal cholesterol concentrations (150 +/- 13% of controls, P < 0.01). Surprisingly, the increase in HMG-CoA reductase specific activity was not associated with parallel increases in HMG-CoA reductase steady-state mRNA levels and gene transcriptional activity. These uremic rats also exhibited a marked increase in the specific activity of cholesterol 7 alpha-hydroxylase (240 +/- 559% of controls, P < 0.05). There was no concomitant increase in cholesterol 7 alpha-hydroxylase steady-state mRNA levels or gene transcriptional activity. The factors responsible for the observed increases in HMG-CoA reductase and cholesterol 7 alpha-hydroxylase specific activity in renal failure remain to be determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Effects of different bile salts on steady-state mRNA levels and transcriptional activity of cholesterol 7 alpha-hydroxylase.

Cholesterol 7 alpha-hydroxylase, the rate-limiting enzyme in the bile acid synthesis pathway, is down-regulated by taurocholate by way of negative feedback control at the level of gene transcription. The molecular basis of regulation of cholesterol 7 alpha-hydroxylase by other hydrophobic bile salts and under more physiological conditions is not known. The aim of this study was to investigate the molecular basis of regulation of cholesterol 7 alpha-hydroxylase by several naturally occurring bile salts in rats with intact enterohepatic circulation. Male Sprague-Dawley rats were pair-fed for 14 days normal chow (control), cholestyramine (5% of diet), cholic acid (1%), chenodeoxycholic acid (1%) or deoxycholic acid (0.25%). When rats were killed, livers were harvested and HMG-CoA reductase specific activity and cholesterol 7 alpha-hydroxylase specific activities, steady-state mRNA levels and transcriptional activity were determined and compared with those of control rats fed normal chow. Compared with results in paired controls, cholestyramine feeding led to an approximate threefold increase in HMG-CoA reductase specific activity. Feeding of hydrophobic bile salts profoundly decreased the specific activity of HMG-CoA reductase. Cholestyramine led to a three-fold increase in cholesterol 7 alpha-hydroxylase specific activity, steady-state mRNA levels and gene transcriptional activity. The feeding of cholic (1%), chenodeoxycholic (1%) and deoxycholic acid (0.25%) led to significant decreases in cholesterol 7 alpha-hydroxylase specific activities (62%, 84% and 97%, respectively), steady-state mRNA levels (72%, 29% and 61%, respectively) and transcriptional activities (44%, 43% and 54%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Animal Feed↗
PubMed1994

Regulation of rat hepatic 3 alpha-hydroxysteroid dehydrogenase in vivo and in primary cultures of rat hepatocytes.

In the bile acid biosynthetic pathways of humans and the rat, hepatic 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSDH) catalyzes the stereospecific reduction of the 3-oxo group of bile acid precursors. In addition, 3 alpha-HSDH may serve to shuttle bile acids from sinusoidal to apical (cannalicular) membranes of the rat hepatocyte. The objective of the present study was to define the molecular regulation of rat hepatic 3 alpha-HSDH in response to the key effectors of cholesterol 7 alpha-hydroxylase, the rate-determining enzyme in bile acid biosynthesis. Steady-state 3 alpha-HSDH mRNA levels in primary cultures of rat hepatocytes fell to 16 +/- 1% of whole liver levels after 72 h in culture, indicating that the gene is not spontaneously expressed in isolated hepatocytes. However, the addition of thyroxine (1.0 microM) or dexamethasone (1.0 microM) to the culture medium resulted in steady-state mRNA levels of 34 +/- 4% and 102 +/- 20% of whole liver levels, respectively. Moreover, the combination of thyroxine and dexamethasone (each 1.0 microM) induced mRNA to levels 2-fold higher than whole liver. 3 alpha-HSDH specific activity in cultured hepatocyte cytosol increased from 3.0 +/- 0.7 to 10.4 +/- 1.3 nmol/min per mg protein in no-addition and thyroxine plus dexamethasone-treated cultures, respectively; protein mass underwent similar changes. Whole liver 3 alpha-HSDH mRNA levels decreased in thyroidectomized, adrenalectomized, and hypophysectomized rats, to 60 +/- 6%, 51 +/- 4%, and 29 +/- 5% of sham-operated rats, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

3-Hydroxysteroid Dehydrogenases↗
PubMed1993

Transcriptional regulation of cholesterol 7 alpha-hydroxylase mRNA by conjugated bile acids in primary cultures of rat hepatocytes.

The role of bile acids in the regulation of cholesterol 7 alpha-monooxygenase (EC 1.14.13.17) was characterized using primary cultures of rat hepatocytes supplemented with dexamethasone and thyroxine. Taurocholate and taurodeoxycholate (50 microM) repressed cholesterol 7 alpha-hydroxylase mRNA to 44 +/- 9 and 52 +/- 4%, respectively, of control values. Repression by these natural, relatively hydrophobic bile acids was concentration dependent, with an IC50 of about 50 microM, and time dependent with a t1/2 for repression of 22 h. In contrast, two natural hydrophilic bile acids, tauroursodeoxycholate and taurohyodeoxycholate, had no effect on cholesterol 7 alpha-hydroxylase mRNA levels. Taurochenodeoxycholate and taurolithocholate also had no effect, but these hydrophobic bile acids were rapidly hydroxylated to more hydrophilic bile acids. Hydrophilic bile acid analogues (nor (C23) bile acids and beta-hydroxy epimers) repressed cholesterol 7 alpha-hydroxylase mRNA less potently than their corresponding and more hydrophobic C24 or alpha-hydroxy derivatives. Cholesterol 7 alpha-hydroxylase specific activity was decreased by taurocholate or taurodeoxycholate (50 microM) to 26 +/- 9 and 56 +/- 3% of control, respectively; its transcriptional activity was repressed to 52 +/- 5% of control by taurocholate (50 microM). The addition of cholesterol or the induction of cholesterol biosynthesis did not influence repression of cholesterol 7 alpha-hydroxylase mRNA levels by taurocholate. Based on several lines of evidence, cAMP was not involved in bile acid-induced repression. In rat hepatocytes cultured under conditions in which cholesterol 7 alpha-hydroxylase gene expression is maintained at in vivo levels, hydrophobic bile acids repress this enzyme at the level of gene transcription independently of cholesterol availability.

Animals↗
PubMed1993

Studies of alteration of hepatic cholesterol metabolism in puromycin-induced nephrotic syndrome in rats.

Hypercholesterolemia frequently accompanies the nephrotic syndrome, but the mechanism responsible for elevation of plasma cholesterol is poorly understood. Specifically, the contribution of abnormal hepatic cholesterol metabolism to elevated concentrations of serum cholesterol has never been studied in depth. The objective of the present study was to define the alteration of hepatic cholesterol metabolism in puromycin induced nephrotic syndrome in rats. Studies involved measurements of specific activities of four enzymes participating in the maintenance of hepatic cholesterol metabolism: HMG-CoA-reductase, the rate limiting enzyme of cholesterol synthesis; cholesterol 7 alpha-hydroxylase, the rate limiting enzyme in bile acid synthesis; acyl CoA: cholesterol acyltransferase, the enzyme responsible for esterification of cholesterol; and cholesterol ester hydrolase (CEH), an enzyme which hydrolyzes cholesterol. Multiple injections of puromycin resulted in a production of nephrotic syndrome with massive proteinuria, hypoalbuminemia, hypercholesterolemia, ascites and edema. HMG-CoA-reductase (nmol/hr/mg protein) and cholesterol 7 alpha-hydroxylase activities (nmol/hr/mg protein) in rats with nephrotic syndrome were not statistically significant as compared to control rats (4.0 +/- 0.7 and 2.0 +/- 0.6 vs. 3.3 +/- 0.4 and 1.6 +/- 0.2), respectively. Our results also demonstrate, for the first time, that the normal diurnal rhythm in HMG-CoA reductase activity is no longer present in the nephrotic animals. The activities in the nephrotics in the day was 4.0 nmol/hr/mg and at night, 3.9 nmol/hr/day, compared to the control values of 3.3 nmol/hr/mg in the day and 6.9 nmol/hr/mg at night. ACAT activities were 428 +/- 78 versus 302 +/- 64 pmol/min/mg/protein (P = NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

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)

Animals↗
PubMed1993

Role of newly synthesized cholesterol or its metabolites on the regulation of bile acid biosynthesis after short-term biliary diversion in the rat.

Cholesterol 7 alpha-hydroxylase, the rate-limiting enzyme in the bile acid biosynthetic pathway, is thought to be regulated by hydrophobic bile acids through negative feedback control. The role of cholesterol in the regulation of cholesterol 7 alpha-hydroxylase is more controversial, in part because of incomplete understanding of the relationship between the pathways of cholesterol synthesis and degradation. The main objective of this study was to define the interaction between these two pathways in an experimental model in which the supply of newly synthesized cholesterol was interrupted by sustained infusion of mevinolin (lovastatin), an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) or accelerated by a continuous infusion of mevalonate, a cholesterol precursor. The study was carried out in rats subjected to short-term bile fistula. In one set of experiments, rats were treated postoperatively with mevinolin (5 mg/kg loading dose followed by 2 mg/kg/hr infusion), mevalonate (180 mumol/hr infusion) or both for up to 96 hr. In a separate set of experiments, rats were infused intraduodenally with taurocholate (36 mumol/100 gm/hr for up to 96 hr). We determined cholesterol 7 alpha-hydroxylase- and HMG-CoA reductase specific activities at those time intervals, whereas bile acid synthesis rates were determined throughout the study. Compared with rats not subjected to surgery, rats with short-term biliary diversion had increases in cholesterol 7 alpha-hydroxylase activity of 259% and 827% at 48 and 96 hr, respectively. The increase in bile acid biosynthesis was less pronounced. Continuous infusion of mevinolin completely prevented increases in cholesterol 7 alpha-hydroxylase specific activity and bile acid biosynthesis at both time intervals.(ABSTRACT TRUNCATED AT 250 WORDS)

Alkaline Phosphatase↗
PubMed1992

Hormonal regulation of cholesterol 7 alpha-hydroxylase mRNA levels and transcriptional activity in primary rat hepatocyte cultures.

In primary cultures of adult rat hepatocytes the level of cholesterol 7 alpha-hydroxylase steady-state mRNA markedly decreased by 72 h. However, the addition of L-thyroxine (T4) and dexamethasone synergistically returned cholesterol 7 alpha-hydroxylase steady-state mRNA levels near to that of cholestyramine-fed animals. The maximal responses to T4 and dexamethasone in serum-free medium were at 1.0 and 0.1 microM, respectively. The addition of T4 in combination with dexamethasone resulted in an 11-fold increase in transcriptional activity of the cholesterol 7 alpha-hydroxylase gene as compared to no addition controls. The specific activities of cholesterol 7 alpha-hydroxylase in microsomes prepared from cultures treated with dexamethasone and T4 were 1.56 +/- 1.17 nmol/h/mg protein which is similar to that of intact liver (1.70 +/- 0.062 nmol/h/mg protein), but lower than cholestyramine-fed animals. Cholesterol 7 alpha-hydroxylase activity was not detectable (less than 0.020 nmol/h/mg protein) at 72 h in cultures without the addition of both dexamethasone and T4. In the presence of optimal concentrations of dexamethasone and T4, glucagon (0.2 microM), or dibutyryl cAMP (50 microM) decreased (90%) cholesterol 7 alpha-hydroxylase mRNA within 6 h. Transcriptional activity decreased (62%) in 6 h following the addition of glucagon (0.2 microM) to the culture medium. The results reported in this paper suggest an important role for multiple hormones in the regulation of cholesterol 7 alpha-hydroxylase in the liver.

Amino Acid Isomerases↗
PubMed1992

Effect of cholylsarcosine on hepatic cholesterol and bile acid synthesis and bile secretion in rats.

The regulatory and secretory properties of cholylsarcosine (C-sar), a synthetic conjugated bile acid analogue that resists deconjugation and dehydroxylation, were compared with those of the natural conjugates of cholic acid. After continuous intraduodenal infusion of cholylsarcosine (C-sar), cholyltaurine (C-tau), or cholylglycine (C-gly) at 36 mumol/100 g.h, the infused bile acid in each case became the predominant biliary bile acid. After 48 hours, infusion of C-sar, C-tau, and C-gly suppressed the activity of cholesterol 7 alpha-hydroxylase (C7 alpha H; rate-limiting for bile acid synthesis) by 65%, 78%, and 92%, respectively, compared with biliary fistula controls. After C-sar infusion, levels of C7 alpha H protein, messenger RNA, and transcriptional activity were depressed to the same extent as specific activity, indicating that C-sar, like C-tau, down-regulates C7 alpha H principally at the level of gene transcription. All three bile acids also suppressed activity of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (rate-limiting for cholesterol synthesis). Both short- and long-term, the three cholyl conjugates caused similar increases in bile flow and in biliary secretion of cholesterol and phospholipid. It is concluded that in the rat, cholyl conjugates per se can suppress cholesterol and bile acid biosynthesis without prior conversion to deoxycholate. The effects of C-sar on hepatic cholesterol and bile acid synthesis as well as on induced bile flow and biliary lipid secretion are essentially identical to those of the naturally occurring cholyl conjugates.

Animals↗
PubMed1992

A proposed nomenclature for bile acids.

A proposal is made for a system of nomenclature of the more common unconjugated and conjugated bile acids. Acceptable trivial names for bile acids are tabulated, and guidelines are proposed for using these existing trivial names as roots to create acceptable semi-systematic names for other bile acids, as well as for new natural bile acids that will be discovered in the future. The term alpha-hyocholic is recommended to replace hyocholic, and beta-hyocholic to replace omega-muricholic. The term murideoxycholic acid is recommended for 3 alpha,6 beta-dihydroxy-5 beta-cholan-24-oic acid. Proposals are also made for bile acids with epimeric hydroxy groups, for unsaturated bile acids, and for bile acids with oxo- and/or hydroxy-oxo- substituents on the nucleus and/or on the side chain. For conjugated bile acids, the term "aminoacyl amidates" is recommended to replace "amidates" for bile acids conjugated in N-acyl linkage with amino acids. Nomenclature for other types of conjugates (sulfates, glucuronides, glucosides) is included as well as abbreviations. It is recommended that the historic tradition of naming a newly discovered bile acid after the species from which it was isolated be abandoned, and that in the future such a bile acid should be named using the principles contained in this paper.

Bile Acids and Salts↗
PubMed1992

Bile acid synthesis. VI. Regulation of cholesterol 7 alpha-hydroxylase by taurocholate and mevalonate.

Taurocholate, a relatively hydrophobic bile salt, is a potent down-regulator of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase (C7 alpha H), the rate-determining enzymes of the cholesterol and bile acid biosynthetic pathways, respectively. Inhibition of cholesterol synthesis with a bolus dose of mevinolin (lovastatin) a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, profoundly decreases the specific activity of cholesterol 7 alpha-hydroxylase and rate of bile acid synthesis in rats with complete biliary diversion. It is therefore conceivable that taurocholate may suppress cholesterol 7 alpha-hydroxylase primarily by down-regulating the activity of HMG-CoA reductase. To test this hypothesis, taurocholate was coinfused simultaneously to rats with chronic bile fistula with mevalonate (administered as mevalonolactone), an intermediate in the cholesterol biosynthetic pathway. Mevalonolactone was administered to provide a constant supply of newly synthesized cholesterol to cholesterol 7 alpha-hydroxylase, in order to overcome any inhibitory effect of taurocholate on HMG-Coa reductase. Infusions were started 72 h after biliary diversion, and carried out for an additional 48 h. Complete biliary diversion resulted in an increase in C7 alpha H specific activity (510%), protein mass (550%), steady-state mRNA levels (1430%), and transcriptional activities (330%) as compared to control rats with intact enterohepatic circulations. When rats with biliary diversion were infused intraduodenally with taurocholate, the specific activities of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase activities decreased by 75% (P less than 0.001) and 73% (P less than 0.001), respectively. Cholesterol 7 alpha-hydroxylase mass, mRNA, and transcriptional activity decreased after intraduodenal infusion of taurocholate to levels similar to those of rats with an intact enterohepatic circulation. The combination of constant infusion of mevalonate and taurocholate failed to reverse the inhibitory effects of taurocholate on cholesterol 7 alpha-hydroxylase activity, mRNA levels, and in vitro transcriptional rates. These data provide evidence that taurocholate represses cholesterol 7 alpha-hydroxylase at the level of gene transcription, and not via down-regulation of HMG-CoA reductase. Infusion of mevalonate alone to biliary diverted rats did not alter cholesterol 7 alpha-hydroxylase activity or mRNA levels, while leading to a 57% decrease in C7 alpha H gene transcription. This latter finding suggests that mevalonate or its metabolites may be capable of stabilizing C7 alpha H mRNA levels while down-regulating transcriptional activity.

Animals↗
PubMed1991

Effect of hydrophobic bile acids on 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity and mRNA levels in the rat.

We have previously reported that relatively hydrophobic bile acids, decreased hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase (reductase) activity whereas, hydrophilic bile acids had little effect on the enzyme. The purpose of the present study was to determine in more detail the mechanism of down-regulation of hepatic reductase activity by hydrophobic bile salts. Groups of rats were fed bile acids of differing hydrophobicity: ursodeoxycholic, cholic (CA), chenodeoxycholic (CDCA), deoxycholic (DCA), or cholesterol for 14 days. Reductase specific activities and concentrations of reductase protein were determined in hepatic microsomes. Quantitation of "steady state" levels of reductase mRNA was performed using Northern and dot blot hybridization. Reductase gene transcriptional activity (nuclear "run-on") was determined in nuclei isolated from livers of animals fed different bile acids. Hydrophobic bile acids and cholesterol significantly decreased reductase activity: CA (57%), CDCA (77%), DCA (73%), cholesterol (89%), and reductase protein levels as measured by an enzyme-linked immunosorbent assay method were also decreased; CA (27%), CDCA (31%), DCA (42%), and cholesterol (35%). Reductase mRNA levels were also decreased after feeding hydrophobic bile acid: CA (43%), CDCA (47%), DCA (54%), and cholesterol (53%). Ursodeoxycholic, a hydrophilic bile acid, caused a much smaller decrease in reductase activity (18%), protein mass (16%), and mRNA levels (10%). Decreased transcriptional activities were observed in CA- and cholesterol-fed rats. Surprisingly, CDCA- and DCA-fed animals showed transcriptional activities similar to control animals even though steady state mRNA levels were low in CDCA- and DCA-fed animals. We hypothesize a post-transcriptional regulation of reductase mRNA by hydrophobic bile acids.

Animals↗
PubMed1991

High-performance liquid chromatography assay for 3 beta-hydroxy-delta 5-C27-steroid oxidoreductase activity in rat liver microsomes.

A quantitative HPLC method has been developed for measuring 3 beta-hydroxy-delta 5-C27-steroid oxidoreductase activity in rat liver microsomes. Apparent Km values of 23 and 27 microM were estimated for 5-cholestene-3 beta,7 alpha-diol and NAD+, respectively. A temperature optimum of 45 degrees C was estimated using standard assay conditions. It was observed that feeding cholesterol (2%) or cholestyramine (5%) to rats increased (twofold) specific activity. This method should prove useful in studies of the regulation of bile acid synthesis and complement the HPLC assay technique recently developed for measuring cholesterol 7 alpha-hydroxylase activity.

3-Hydroxysteroid Dehydrogenases↗
PubMed1991

Regulation of cholesterol 7 alpha-hydroxylase mRNA and transcriptional activity by taurocholate and cholesterol in the chronic biliary diverted rat.

Cholesterol 7 alpha-hydroxylase catalyzes the rate-limiting step in the bile acid biosynthetic pathway. Regulation of this pathway is thought to occur solely as a result of a negative feedback control mechanism that is dependent upon the flux and composition of bile salts undergoing enterohepatic circulation. We have used the chronic biliary diverted (CBD) rat model to study the mechanism of regulation of cholesterol 7 alpha-hydroxylase by taurocholate. As compared to nonoperated controls, CBD rats exhibited a 4.2-fold increase in cholesterol 7 alpha-hydroxylase-specific activity, a 4.5-fold increase in enzyme mass, a 10-fold increase in steady-state mRNA levels, and a 3.4-fold increase in transcriptional (nuclear "run-on") activity. Intraduodenal infusion of taurocholate at a rate of 36 mumol/100 g/h for 48 h in CBD rats caused a significant (p less than 0.05) decrease (64%) in cholesterol 7 alpha-hydroxylase-specific activity, mass (72%), steady-state mRNA levels (74%), and transcriptional activity (57%) as compared to CBD controls. Cholesterol feeding increased cholesterol 7 alpha-hydroxylase-specific activity (288%), poly(A) RNA levels (291%), and transcriptional activity (220%) as compared to control animals. These results provide convincing evidence that bile salts, either directly or indirectly, down-regulate in vivo transcription of the cholesterol 7 alpha-hydroxylase gene, which is probably the major mechanism regulating the levels of this enzyme. The results of this study also suggest that the promoter for cholesterol 7 alpha-hydroxylase may have both bile salt- and sterol-responsive elements.

Animals↗
PubMed1991

Conjugates of ursodeoxycholate protect against cytotoxicity of more hydrophobic bile salts: in vitro studies in rat hepatocytes and human erythrocytes.

Intraduodenal infusion of hydrophobic bile salts to bile-fistula rats leads within hours to severe hepatocellular necrosis and cholestasis; simultaneous administration of conjugates of ursodeoxycholate, either intraduodenally or intravenously, reduces or prevents liver injury. To evaluate the short-term protective effects of ursodeoxycholate at the cellular level, we incubated primary monolayer cultures of adult rat hepatocytes or freshly isolated washed human erythrocytes for 1 to 240 min with varying defined concentrations of different bile salts in the presence or absence of ursodeoxycholate. Cytolysis was quantified by measuring the release into the medium of cytosolic lactate dehydrogenase (hepatocytes) or hemoglobin (erythrocytes). In both systems, cytolysis increased sigmoidally with increasing bile salt concentration, and the relative toxicity of different bile salts proceeded in the following order: tauroursodeoxycholate was less toxic than taurocholate, which was less toxic than taurodeoxycholate. Taurochenodeoxycholate was more toxic to erythrocytes than taurodeoxycholate; the two were equally toxic to rat hepatocytes. Unconjugated bile salts were more toxic than their conjugates. The addition of tauroursodeoxycholate to taurochenodeoxycholate or taurodeoxycholate led to time-dependent and concentration-dependent reduction or elimination of the toxicity of the more hydrophobic component. Protection was evident within minutes. With respect to hemolysis, at pH 8.5 glyco was less protective than tauroursodeoxycholate, and free ursodeoxycholate was only minimally protective. We conclude that the hepatocytotoxicity of hydrophobic bile salts at millimolar concentrations is markedly reduced in the presence of tauroursodeoxycholate. Conjugates of ursodeoxycholate also prevented disruption of erythrocytes by bile salts, suggesting that protection does not depend on liver-specific pathways of bile salt uptake, compartmentation, transport or metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

Effects of perturbations in hepatic free and esterified cholesterol pools on bile acid synthesis, cholesterol 7 alpha-hydroxylase, HMG-CoA reductase, acyl-CoA:cholesterol acyltransferase and cytosolic cholesteryl ester hydrolase.

Effects of expansion of the hepatic free cholesterol pool on bile acid and cholesterol metabolism and homeostasis were examined in rats fed cholesterol in high-fat diets or treated with oleyl-p-(n-decyl)-benzenesulfonate (ODS) or progesterone. Cholesterol feeding for 10-16 days, which increased free (33%) and esterified (6-fold) cholesterol, had no effect on cholate synthesis, total bile acid synthesis, or cholate turnover, whereas these activities were increased 60-80% by ODS and progesterone, which produced only small increases (19%) in free cholesterol. Cholesterol feeding reduced beta-hydroxy-beta-methylglutaryl (HMG)-CoA reductase (72%) and cholesteryl ester hydrolase (48%) and increased acyl-CoA:cholesterol acyltransferase (184%), whereas ODS and progesterone reversed these compensatory responses in cholesterol-fed rats. Cholesterol 7 alpha-hydroxylase was changed no more than 22% by any treatment. A bolus of ODS elevated biliary cholesterol output 41% and shifted biliary bile acid synthesis and composition toward 12-deoxy bile acids. These effects were not seen in ODS-fed or progesterone-treated rats, in which cholesteryl ester stores were depleted. It is concluded that effects of free cholesterol on bile acid synthesis and biliary cholesterol are probably mediated by specific precursor or regulatory pools which can be independently regulated and which represent a relatively small fraction of hepatic free cholesterol.

Animals↗