Preferential utilization of free cholesterol from high-density lipoproteins for biliary cholesterol secretion in man.

High- and low-density lipoproteins carrying free cholesterol labeled with 3H or 14C were administered to a patient with a bile fistula. The free cholesterol from high-density lipoproteins was more rapidly incorporated into biliary cholesterol than the free cholesterol from low-density lipoproteins. These findings show that the liver in man selectively utilizes and secretes the free cholesterol from a particular lipoprotein.

Mechanism of secretion of biliary lipids. I. Role of bile canalicular and microsomal membranes in the synthesis and transport of biliary lecithin and cholesterol.

The role of bile canalicular and microsomal membranes in the synthesis and transport of biliary lipids was investigated by using the isolated perfused rat liver model. Labeled lecithin precursors ((3H)-palmitic acid, (14C)linoleic acid, (3H)choline, and 32PO4) and a cholesterol precursor ((3H)mevalonic acid) were administered with and without sodium taurocholate. The incorporation pattern of these labeled precursors into linoleyl and arachidonyl lecithins and cholesterol fractions of microsomes, bile canaliculi, and bile were examined at 30-min intervals up to 90 min. Marker enzymes and electron microscopy indicated that isolated subfractions of plasma membranes were enriched with bile canaliculi (less than 10 percent microsomal contamination). Taurocholate significantly stimulated the incorporation of 32PO4, (3H)choline, (3H)palmitic acid, and (14C)linoleic acid into linoleyl and arachidonyl lecithin with parallel incorporation curves for microsomal and bile canalicular membranes throughout the 90-min study period. During the 30-60-min period, however, these same lecithin fractions in bile significantly exceeded the specific activity of the membrane lecithins. The enzyme CDP-choline diglyceride transferase was virtually absent from canaliculi relative to microsomes, indicating that canaliculi lack the capacity for de novo lecithin synthesis. Incorporation of (3H)mevalonic acid into membranous and biliary cholesterol followed a pattern similar to that for lecithin. These data provide evidence that (a) biliary lecithin and cholesterol are derived from a microsomal subpool regulated by the flux of enterohepatic bile acids, (b) the role of the bile canalicular membranes with respect to biliary lipids is primarily transport rather than synthesis, and (c) lecithin and cholesterol are transported together from microsomes to bile. The findings are consistent with the existence of a cytoplasmic lipid complex within the hepatocyte which is actively involved in the intermembrane transport of biliary lipid.

Multicompartmental analysis of cholesterol metabolism in man. Quantitative kinetic evaluation of precursor sources and turnover of high density lipoprotein cholesterol esters.

The purpose of this study is to delineate the immediate sources and fractional turnover of high density lipoprotein (HDL) esterified cholesterol in man. Various labeled preparations were administered in 11 experiments to six subjects who had either a complete bile fistula (maximally stimulated cholesterol metabolism) or an intact enterohepatic circulation. The administered tracers included [(3)H]mevalonic acid; [(14)C]cholesterol bound to albumin; low density lipoprotein (LDL) free [(3)H] or [(14)C]cholesterol; HDL free [(3)H] or [(14)C]cholesterol; HDL esterified [(3)H]cholesterol; and LDL esterified [(3)H]cholesterol. Blood samples were obtained at frequent intervals for up to 5 d after the administration of tracers. The mass and radioactivity in individual plasma lipoprotein (very low density lipoprotein [VLDL], HDL, and LDL) free and esterified cholesterol were determined. The data were subjected to multicompartmental analysis using the SAAM-27 computer program. The analysis revealed that plasma free cholesterol was not the only immediate source of either a single- or two-compartment HDL ester system. When LDL esters and plasma (HDL) free cholesterol were tested together as sources of one HDL ester compartment, data from all the experiments were readily fit. The fluxes arrived at with the final model indicated that only approximately 20% of the esterified cholesterol in HDL was newly synthesized from plasma (HDL) free cholesterol (2.36 mumol/min); the remaining 80% was from LDL ester (8.92 mumol/min). The presence of a bile fistula had no obvious effect on HDL esterified cholesterol metabolism. The rate of HDL cholesterol ester turnover was 3-12 times/d, indicating that the ester component of the HDL particle is in a very dynamic state.

Central role of high density lipoprotein in plasma free cholesterol metabolism.

This study was designed to provide direct information on the in vivo metabolism in man of free (unesterified) cholesterol in the major lipoprotein classes. Five human subjects were administered one or two (simultaneous) of the following; [2-(14)C] mevalonic acid, high density lipoprotein (HDL)-free [(14)C] cholesterol, low density lipoprotein (LDL)-free [(14)C] cholesterol, and very low density lipoprotein (VLDL)-free [(3)H]cholesterol. Blood was then obtained at frequent intervals for at least 9 h, and the alpha(HDL) and beta(LDL + VLDL) lipoproteins were quickly separated by heparin-manganese precipitation to prevent ex vivo exchange of free cholesterol. After the administration of [(14)C]mevalonic acid the specific activity (disintegrations per minute/micromole) of free cholesterol in the alpha- and beta-lipoproteins increased for 3 h. During this period the alpha-free cholesterol specific activity was higher than the beta specific activity. After administration of VLDL and LDL labeled with free cholesterol, the alpha-free cholesterol specific activity reached a peak value within 20 min, at which time it was considerably lower than the beta-free cholesterol specific activity. When HDL labeled with free cholesterol was administered, a precursor product relationship was observed between the alpha-free cholesterol (precursor) and beta-free cholesterol (product) specific activities.A multicompartmental model was developed that contained the simplest structure necessary to fit all of the data obtained. The kinetic analysis revealed the presence of extensive exchange (20-85 mumol/min) of free cholesterol between HDL and a tissue pool(s) enriched with newly synthesized free cholesterol. It was found that virtually all (>95%) of the free cholesterol in the beta-lipoproteins (LDL+VLDL) cycles directly through HDL. The free cholesterol in LDL appears to behave in the same fashion as the free cholesterol in VLDL. The results show that there are marked differences in the kinetic behavior of the free cholesterol fractions of alpha- and beta-lipoproteins. There is extensive recycling of free cholesterol between HDL and tissue pools, and between HDL and the beta-lipoproteins; this recycling has been quantitated. The findings support the view that in vivo, the free cholesterol in HDL plays a central role in exchange reactions and in the vascular-tissue cholesterol transport system.

Multicompartmental analysis of cholesterol metabolism in man. Characterization of the hepatic bile acid and biliary cholesterol precursor sites.

The present report has presented the first clear evidence in man for the existence of specific hepatic cholesterol precursor sites associated with the formation and secretion of bile acids and biliary cholesterol. These hepatic compartments derive virtually all their cholesterol from newly synthesized and lipoprotein free cholesterol. The model which is presented was formulated on current concepts of cholesterol metabolism in man and is concerned, at this initial stage, with the elucidation of the bile acid and biliary cholesterol compartments. The complexity of cholesterol metabolism in man necessitated an initial approach that would minimize the number of inputs of cholesterol into the system, allow for the sampling of several cholesterol compartments, and permit the simultaneous labeling of newly synthesized cholesterol and preformed cholesterol. To achieve these objectives, we studied the patient with a total bile fistula. Six patients were administered simultaneously pulse injections of labeled mevalonic acid and [(14)C]cholesterol. The qualitative features of the specific activity time course curves after labeled mevalonic acid revealed no precursor-product relationship between bile acid, biliary cholesterol, and plasma free cholesterol. The peak specific activity of the bile acids was reached in approximately 100 min and was higher than the biliary cholesterol, which was higher than the plasma free cholesterol. The plasma free cholesterol specific activity became higher than the other lipids after 12 h and remained higher throughout the period of study. Similar related observations were made with [(14)C]cholesterol. The data were then subjected to simulation analysis and modeling using the SAAM-27 computer program. Computer least-square fits of the data were obtained after the model was evolved. During the model development, the least number of compartments and transport pathways were introduced consistent with a good fit of the data. Of particular importance was the constraint that the model fit the data obtained from both [(14)C]cholesterol and labeled mevalonic acid. The same parameter values were used to fit the data from both tracers. The fluxes arrived at in the model indicate that 31% and 20%, respectively, of the cholesterol input into the bile acid and biliary cholesterol precursor sites were derived directly from the newly synthesized hepatic cholesterol. The remainder had its origin predominantly from lipoprotein free cholesterol. Plasma esterified cholesterol (as free) made a small contribution (11%) to the bile acid compartment. Similarly, 10% of the biliary cholesterol arose from an unknown hepatic site. The present report has provided the basis for a new procedure for studying in vivo cholesterol metabolism in man. Examination of the derived cholesterol flux rates between the compartments suggests the presence of an important mechanism regulating the partitioning of lipoprotein free cholesterol between the bile acid and biliary cholesterol precursor sites. Aberrations in the proportioning of precursor cholesterol between these sites could be a causative factor precipitating the excessive secretion of biliary cholesterol and the production of lithogenic bile.

Bile acid synthesis in humans.

Metabolic pathways involved in the conversion of cholesterol to cholic and chenodeoxycholic acids have been investigated in bile fistula patients treated with a number of labeled potential bile acid intermediates. The findings of the present report indicate that the human liver cell has the capacity to synthesize both primary bile acids via multiple routes from cholesterol and 7 alpha-hydroxycholesterol. Evidence has been obtained for the existence of a major pathway to chenodeoxycholic acid via the 26-hydroxylation of 7 alpha-hydroxycholest-4-en-3-one. Cholic acid is synthesized preferentially via pathways from 5 beta-cholestane 3 alpha, 7 alpha-diol and a pathway from cholesterol not involving an initial 7 alpha-hydroxylation.

An in vivo evaluation in man of the transfer of esterified cholesterol between lipoproteins and into the liver and bile.

The metabolism of the esterified cholesterol fractions of HDL and LDL has been studied in vivo in man with regard to their ability to serve as precursors (after intrahepatic hydrolysis) for bile acid synthesis and biliary cholesterol secretion. Information was also obtained on the exchange of cholesterol esters between the lipoprotein classes. Fasting subjects were intravenously administered autologous HDL (or LDL) labeled with esterified [3H]cholesterol and free [3H]- and [14C]cholesterol. Following the administration of the labeled lipoproteins, bile and blood were collected at frequent intervals. In each experiment the observed 3H/14C ratios in bile acids, biliary cholesterol, lipoprotein free cholesterol and red blood cell cholesterol were similar and markedly divergent from the lipoprotein esterified cholesterol 3H/14C ratios. Following the administration of labeled HDL, the 3H/14C ratios observed in the esterified cholesterol fractions of VLDL and LDL closely resembled the ratios in HDL indicating that VLDL and LDL received esterified cholesterol by direct transfer from HDL. Following the administration of labeled LDL, the 3H/14C ratios in HDL esterified cholesterol were midway between the ratio in LDL esterified cholesterol and plasma free cholesterol, indicating that HDL esterified cholesterol is derived from more than one source. These sources could be LDL esterified cholesterol and esters formed de novo from plasma free cholesterol. A precursor-product relationship was found between the specific activities of lipoprotein free cholesterol and the bile steroids. Assuming direct entry of lipoprotein free and esterified cholesterol (after hydrolysis) into the bile acid and biliary cholesterol precursor pools, it was calculated that less than 20% of these biliary steroids could be derived from HDL esterified cholesterol. The results support the view that lipoprotein free cholesterol is the major source of bile acids in man. Also, the results suggest that in vivo esterified cholesterol fractions of VLDL and LDL originate from HDL, that some LDL ester is transferred back to HDL, and that the cholesterol liberated form hydrolyzed esters undergoes recirculation into the free cholesterol pool rather than excretion as biliary cholesterol or bile acids.

A quantitative evaluation of the conversion of 25-hydroxycholesterol to bile acids in man.

The present study was directed toward providing additional information in man on the nature of a potential alternative pathway to cholic acid not involving an initial 7 alpha-hydroxylation of cholesterol. Two bile fistula patients and one normal subject each received 25-hydroxy[G-3H]cholesterol; [14C]cholic and [14C]chenodeoxycholic acids were also simultaneously administered to one bile fistula patient and normal subject. The labeled 25-hydroxycholesterol was found to be poorly converted to primary bile acids by all three patients; the range of conversion was 9.7 to 18.9%. Cholic acid was favored over chenodeoxycholic acid by a margin of about 1.4/1. It is concluded that a pathway to primary bile acid via the 25-hydroxylation of cholesterol is of minor importance under conditions of normal or accelerated synthesis in man.

Biosynthesis of 5 alpha- and 5 beta-cholanoic acid derivatives during metabolism of [1,1-2H]- and [2,2,2-2H]ethanol in the rat.

Female bile fistula rats were given [1,1-2H]ethanol in a single dose or [2,2,2-2H]ethanol repeatedly for 24 h and incorporation of deuterium into the following bile acids was determined: taurine conjugates of 3 alpha, 7 alpha, 12 alpha-trihydroxy-5(alpha and beta)-cholanoic, 3 alpha, 7 alpha-dihydroxy-5(alpha and beta)-cholanoic and 3 alpha, 6 beta, 7 alpha-trihydroxy-5 beta-cholanoic acids; sulphates of 3(alpha and beta), 7 alpha, 12 alpha-trihydroxy-5 alpha-cholanoic, and 3(alpha and beta), 7 alpha-dihydroxy-5 alpha-cholanoic acids. The kinetics of deuterium incorporation from [2,2,2-2H]ethanol was the same for all bile acids indicating that they were formed from a single pool of cholesterol. The labelling pattern of bile acids formed during metabolism of [1,1-2H]-ethanol indicated that the hydrogen at C-5 was labelled in all bile acids. Taken together with previous results this indicates that 3-oxo-4-cholenoic acid is not an intermediate in the formation of allo bile acids. The results support the view that formation of allo bile acids via a mitochondrial pathway is of little importance in the bile fistula rat.

Effects of ursodeoxycholic acid, analogues of ursodeoxycholic acid and combination of bile acids on bile acid synthesis in cultured rat hepatocytes.

The effect of individual 7 beta-hydroxy bile acids (ursodeoxycholic and ursocholic acid), bile acid analogues of ursodeoxycholic acid, combination of bile acids (taurochenodeoxycholate and taurocholate), and mixtures of bile acids, phospholipids and cholesterol in proportions found in rat bile, on bile acids synthesis was studied in cultured rat hepatocytes. Individual steroids tested included ursodeoxycholate (UDCA), ursocholate (UCA), glycoursodeoxycholate (GUDCA) and tauroursodeoxycholate (TUDCA). Analogues of UDCA (7-methylursodeoxycholate, sarcosylursodeoxycholate and ursooxazoline) and allochenodeoxycholate, a representative of 5 alpha-cholanoic bile acid were also tested in order to determine the specificity of the bile acid biofeedback. Each individual steroid was added to the culture media at concentrations ranging from 10 to 200 microM. Mixtures of taurochenodeoxycholate (TDCA) and taurocholate in concentrations ranging from 150 to 600 microM alone and in combination with phosphatidylcholine (10-125 microM) and cholesterol (3-13 microM) were also tested for their effects on bile acid synthesis. Rates of bile acid synthesis were determined as the conversion of added lipoprotein [4-14C]cholesterol or [2-14C]mevalonate into 14C-labeled bile acids and by GLC quantitation of bile acids secreted into the culture media. Individual bile acids, bile acid analogues, combination of bile acids and mixture of bile acids with phosphatidylcholine and cholesterol failed to inhibit bile acid synthesis in cultured hepatocytes. The addition of UDCA or UCA to the culture medium resulted in a marked increase in the intracellular level of both bile acids, and in the case of UDCA there was a 4-fold increase in beta-muricholate. These results demonstrate effective uptake and metabolism of these bile acids by the rat hepatocytes. UDCA, UCA, TUDCA and GUDCA also failed to inhibit cholesterol-7 alpha-hydroxylase activity in microsomes prepared from cholestyramine-fed rats. The current data confirm and extend our previous observations that, under conditions employed, neither single bile acid nor a mixture of bile acids with or without phosphatidylcholine and cholesterol inhibits bile acid synthesis in primary rat hepatocyte cultures. We postulate that mechanisms other than a direct effect of bile acids on cholesterol-7 alpha-hydroxylase might play a role in the regulation of bile acid synthesis.

Acute viral hepatitis with bridging hepatic necrosis. An overview.

Bridging hepatic necrosis in the setting of acute viral hepatitis (BHN/AVH) represents an enigmatic syndrome inasmuch as its incidence, significance, course, and therapeutic response have not been clearly defined. It has been thought that this histologic finding carries a high risk of early mortality or evolution to chronic active hepatitis and/or cirrhosis. The data are sparse, and largely based on retrospective studies in selected populations. Steroids have not proved to be effective thus far, while drugs used in other forms of serious liver disease (eg, penicillamine, colchicine) have not been tried. A recent prospective study indicates that BHN/AVN may be a far more benign entity than was previously suspected. Further prospective studies are needed to clarify the significance of this lesion as well as the need for and response to medical therapy.

Metabolic fate and hepatocyte toxicity of reverse amide analogs of conjugated ursodeoxycholate in the rat.

Reverse amide analogs of conjugated bile acids were tested for their effects on the viability of cultured primary rat hepatocytes, for their transport and metabolism in the intact rat, and for their susceptibility to hydrolysis by intestinal bacteria. Succinylnorursodeoxycholanylamide (SNUDCN) and its parent C23 amine showed the same general lack of toxicity toward hepatocytes as the normal conjugates of ursodeoxycholic acid, at concentrations up to 500 microM. The 3alpha,7alpha,12alpha-trihydroxy analog and its parent amine were more toxic than the corresponding dihydroxy compounds, although their effects were similar to those observed for the normal conjugates of cholic acid. Following intraduodenal infusion, greater than 80% of administered SNUDCN appeared in the bile of bile fistula rats. Analysis of bile fractions indicated the presence of SNUDCN (81.5 mol% of original amount) and two metabolites, the taurine conjugate of SNUDCN (9.4 mol%) and SNUDCN containing an additional hydroxy group (9.1 mol%). Although SNUDCN underwent an efficient first pass enterohepatic circulation, it displayed a shorter biological half life than taurocholate (T1/2: 8.9 h vs 39.6 h, respectively). The reverse amide analogs were not hydrolyzed by any of a variety of intestinal bacteria known to hydrolyze normal conjugated bile acids. Despite the shorter half-life, the reverse amide analogs may be of potential use in the targeting of therapeutic bile acids to the colon.

Regulation of sterol 27-hydroxylase and an alternative pathway of bile acid biosynthesis in primary cultures of rat hepatocytes.

In man, hepatic mitochondrial sterol 27-hydroxylase and microsomal cholesterol 7alpha-hydroxylase initiate distinct pathways of bile acid biosynthesis from cholesterol, the "acidic" and "neutral" pathways, respectively. A similar acidic pathway in the rat has been hypothesized, but its quantitative importance and ability to be regulated at the level of sterol 27-hydroxylase are uncertain. In this study, we explored the molecular regulation of sterol 27-hydroxylase and the acidic pathway of bile acid biosynthesis in primary cultures of adult rat hepatocytes. mRNA and protein turnover rates were approximately 10-fold slower for sterol 27-hydroxylase than for cholesterol 7alpha-hydroxylase. Sterol 27-hydroxylase mRNA was not spontaneously expressed in culture. The sole requirement for preserving sterol 27-hydroxylase mRNA at the level of freshly isolated hepatocytes (0 h) after 72 h was the addition of dexamethasone (0.1 microM; > 7-fold induction). Sterol 27-hydroxylase mRNA, mass and specific activity were not affected by thyroxine (1.0 microM), dibutyryl-cAMP (5O microM), nor squalestatin 1 (15O nM-1.0 microM), an inhibitor of cholesterol biosynthesis. Taurocholate (50 microM), however, repressed sterol 27-hydroxylase mRNA levels by 55%. Sterol 27-hydroxylase specific activity in isolated mitochondria was increased > 10-fold by the addition of 2-hydroxypropyl-beta-cyclodextrin. Under culture conditions designed to maximally repress cholesterol 7alpha-hydroxylase and bile acid synthesis from the neutral pathway but maintain sterol 27-hydroxylase mRNA and activity near 0 h levels, bile acid synthesis from [14C]cholesterol remained relatively high and consisted of beta-muricholate, the product of chenodeoxycholate in the rat. We conclude that rat liver harbors a quantitatively important alternative pathway of bile acid biosynthesis and that its initiating enzyme, sterol 27-hydroxylase, may be slowly regulated by glucocorticoids and bile acids.

Down-regulation of the rat hepatic sterol 27-hydroxylase gene by bile acids in transfected primary hepatocytes: possible role of hepatic nuclear factor 1alpha.

In vitro and in vivo studies have shown that the sterol 27-hydroxylase (CYP27) gene is transcriptionally repressed by hydrophobic bile acids. The molecular mechanism(s) of repression of CYP27 by bile acids is unknown. To identify the bile acid responsive element (BARE) and transcription factor(s) that mediate the repression of CYP27 by bile acids, constructs of the CYP27 5'-flanking DNA were linked to either the CAT or luciferase reporter gene and transiently transfected into primary rat hepatocytes. Taurocholate (TCA), taurodeoxycholate (TDCA) and taurochenodeoxycholate (TCDCA) significantly reduced CAT activities of the -840/+23, -329/+23, and -195/+23 mCAT constructs. A -76/+23 construct showed no regulation by bile acids. When a DNA fragment (-110/-86) from this region was cloned in front of an SV 40 promoter it showed down-regulation by TDCA. 'Super'-electrophoretic mobility shift assays (EMSA) indicated that both HNF1alpha and C/EBP bind to the -110 to -86 bp DNA fragment. Recombinant rat HNF1alpha and C/EBPalpha competitively bound to this DNA fragment. 'Super'-EMSA showed that TDCA addition to hepatocytes in culture decreased HNF1alpha, but not C/EBP, binding to the -110/-86 bp DNA fragment. A four base pair substitution mutation (-103 to -99) in this sequence eliminated TCA and TDCA regulation of the (-840/+23) construct. The substitution mutation also eliminated (>95%) HNF1alpha, but not C/EBP, binding to this DNA fragment. We conclude that bile acids repress CYP27 transcription through a putative BARE located between -110 and -86 bp of the CYP27 promoter. The data suggest that bile acids repress CYP27 transcriptional activity by decreasing HNF1alpha binding to the CYP27 promoter.

Receptor-mediated stimulation of taurocholate efflux from the rat hepatocyte and the ex vivo perfused rat liver.

The peptide hormone, arginine-vasopressin[( Arg8]vasopressin, AVP), stimulates efflux of the bile salts taurocholate and glycocholate from the rat hepatocyte in suspension via its association with the V1 receptor on the hepatic cell membrane. At a concentration ratio of 5:1 (antagonist to hormone), the V1 vasopressin antagonist, (dCH2)5Tyr(Me)AVP, inhibits the vasopressin induced efflux of taurocholate by approximately 82%, and of glycocholate, by approximately 85%. In contrast, the V2 antagonist (d(CH2)5[D-Ile2,Ala4]AVP, does not interfere with the stimulation of taurocholate and glycocholate efflux by vasopressin. In the isolated perfused rat liver, vasopressin (5 X 10(-10) M) causes an immediate increase of 55 +/- 12% over baseline in [14C]taurocholate secretion and a corresponding increase in bile flow. A more gradual and prolonged increase in [14C]taurocholate secretion, reflecting an increased biliary concentration of [14C]taurocholate, is observed beginning 6 min after vasopressin, reaching a plateau of 23 +/- 12% over baseline by 14 min and returning to baseline by 30 min. The mean rate of 14C secretion during the 30 min following administration of vasopressin (non-steady state) is increased by 14.3 +/- 6.4% over pre-infusion steady-state baseline (P less than 0.05). Prior administration of the V1 receptor antagonist d(CH2)5Tyr(Me)AVP attenuates these effects of vasopressin. The combination of these in vitro and in vivo findings suggest that vasopressin may play a role in regulating bile salt efflux. Furthermore, these studies in the isolated hepatocyte and the intact liver may provide a unique approach for defining biochemical changes associated with bile salt transport from the hepatic cell.

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.

Hepatic encephalopathy. Metabolic consequence of cirrhosis often is reversible.

Hepatic encephalopathy is a well-recognized clinical complication of chronic liver disease. About 30% of patients with cirrhosis die in hepatic coma. Hepatic encephalopathy can occur in patients with fulminant liver disease without evidence of portal-systemic shunting. These patients have increased intracranial pressure and brain edema with a deleterious clinical course and poor prognosis unless liver transplantation is available. The pathogenesis of portal-systemic hepatic encephalopathy probably is multifactorial, although the predominant causative agent appears to be ammonia. The molecular basis of neurotoxicity of ammonia or other agents implicated in the condition is poorly understood. Therapy includes timely recognition and correction of precipitating factors. Once the condition is manifested, standard therapy is acute administration of lactulose, a disaccharide that is undigested in the small intestine. Its beneficial action is not fully understood. The use of oral antibiotics and BCAAs is of some benefit in patients who do not respond to lactulose. Limitation of protein in the diet may be useful for short periods but is not recommended for long-term use because of potential worsening of already poor nutrition. Several experimental therapies based on potential pathogenetic mechanisms have not resulted in improved outcomes over standard therapy with lactulose. However, future research will likely focus on the correction of alterations in neurotransmission. It is hoped that newer therapies will provide protection from the putative neurotoxins that cause secondary defects in neurotransmission.

Regulation of cholesterol 7 alpha-hydroxylase by different effectors.

Cholesterol degradation to bile acids represents 50% of total elimination of cholesterol from the body each day. Cholesterol 7 alpha-hydroxylase and sterol 27-hydroxylase catalyze initial steps in the neutral and acidic pathways, respectively. Both enzymes were recently cloned and sequenced, and hence the molecular basis of their regulation could be studied. In the rat, cholesterol 7 alpha-hydroxylase is regulated by three classes of effector molecules: a) hydrophobic (but not hydrophilic) bile acids, b) cholesterol, and c) hormones (glucocorticoids plus thyroxine, glucagon and insulin). Most studies presented so far indicate that regulation of cholesterol 7 alpha-hydroxylase probably occurs at the level of gene transcription. Sterol 27-hydroxylase, a mitochondrial P-450 enzyme, appears to be regulated by hydrophobic bile acids, but to a lesser extent than cholesterol 7 alpha-hydroxylase. The contribution of this acidic pathway to total bile acid synthesis is not known but it appears to be more significant than previously thought.