Biomedical subjects

D M Heuman

Publications and source records attributed to D M Heuman.

At least 19 recordsLinked to original sources

PubMed2005

Comparison of the effects of intravenously and orally administered esomeprazole on acid output in patients with symptoms of gastro-oesophageal reflux disease.

BACKGROUND: Intravenous esomeprazole may be beneficial for patients who cannot take oral medications. AIM: To compare intravenous esomeprazole with oral esomeprazole for effects on maximal acid output during pentagastrin stimulation in patients with gastro-oesophageal reflux disease symptoms. METHODS: In four separate open-label, randomized, two-way crossover studies, adult patients were administered esomeprazole 20 or 40 mg once daily either orally or intravenously (by 15-min infusion or 3-min injection) for 10 days and switched to the other formulation with no washout period. Basal acid output and maximal acid output were measured on days 11, 13 and 21. RESULTS: In the four studies (total of 183 patients), least-squares mean maximal acid output ranged from 3.0 to 4.1 mmol/h after intravenous esomeprazole 40 or 20 mg and from 2.2 to 3.3 mmol/h after oral esomeprazole 20 or 40 mg. Differences between formulations were small and not statistically significant but did not meet the prospectively defined criterion for non-inferiority of the intravenous formulation. Median basal acid output values ranged from 0.04 to 0.27 mmol/h after intravenous administration and from 0.05 to 0.25 mmol/h after oral esomeprazole. CONCLUSIONS: Intravenous esomeprazole is an acceptable alternative to the oral formulation for treatment of up to 10 days of duration.

Administration, Oral↗

PubMed2001

Small intestinal neoplasms.

Small intestinal neoplasms are uncommonly encountered in clinical practice. They may occur sporadically, in association with genetic diseases (e.g., familial adenomatous polyposis coli or Peutz-Jeghers syndrome), or in association with chronic intestinal inflammatory disorders (e.g., Crohn's disease or celiac sprue). Benign small intestinal tumors (e.g., leiomyoma, lipoma, hamartoma, or desmoid tumor) usually are asymptomatic but may present with intussusception. Primary malignancies of the small intestine-including adenocarcinoma, leiomyosarcoma, carcinoid, and lymphoma-may present with intestinal obstruction, jaundice, bleeding, or pain. Extraintestinal neoplasms may involve the intestine via contiguous spread or peritoneal metastasis. Hematogenous metastases to the intestine from an extraintestinal primary are unusual and are most typical of melanoma. Because the small intestine is relatively inaccessible to routine endoscopy, diagnosis of small intestinal neoplasms is often delayed for months after onset of symptoms. When the diagnosis is suspected, enteroclysis is the most useful imaging study. Small bowel endoscopy (enteroscopy) is increasingly widely available and may permit earlier, nonoperative diagnosis.

Biopsy, Needle↗

PubMed2001

Effects of CYP7A1 overexpression on cholesterol and bile acid homeostasis.

The initial and rate-limiting step in the classic pathway of bile acid biosynthesis is 7alpha-hydroxylation of cholesterol, a reaction catalyzed by cholesterol 7alpha-hydroxylase (CYP7A1). The effect of CYP7A1 overexpression on cholesterol homeostasis in human liver cells has not been examined. The specific aim of this study was to determine the effects of overexpression of CYP7A1 on key regulatory steps involved in hepatocellular cholesterol homeostasis, using primary human hepatocytes (PHH) and HepG2 cells. Overexpression of CYP7A1 in HepG2 cells and PHH was accomplished by using a recombinant adenovirus encoding a CYP7A1 cDNA (AdCMV-CYP7A1). CYP7A1 overexpression resulted in a marked activation of the classic pathway of bile acid biosynthesis in both PHH and HepG2 cells. In response, there was decreased HMG-CoA-reductase (HMGR) activity, decreased acyl CoA:cholesterol acyltransferase (ACAT) activity, increased cholesteryl ester hydrolase (CEH) activity, and increased low-density lipoprotein receptor (LDLR) mRNA expression. Changes observed in HMGR, ACAT, and CEH mRNA levels paralleled changes in enzyme specific activities. More specifically, LDLR expression, ACAT activity, and CEH activity appeared responsive to an increase in cholesterol degradation after increased CYP7A1 expression. Conversely, accumulation of the oxysterol 7alpha-hydroxycholesterol in the microsomes after CYP7A1 overexpression was correlated with a decrease in HMGR activity.

Adenoviridae↗

PubMed2001

Long-term management of cirrhosis. Appropriate supportive care is both critical and difficult.

Orthotopic liver transplantation has emerged as an important treatment option for patients with advanced liver disease. However, each year the number of new cases of cirrhosis exceeds the number of livers available for transplantation by a factor of 5 to 10. This translates into long waiting lists and restrictive criteria for selecting transplant recipients. Until advances in surgical technique or biotechnology increase the availability of organs for transplantation, the majority of patients with advanced liver disease will have to be managed medically for years--perhaps indefinitely. Early consultation with a liver transplant center can be helpful. The transplant hepatologist and surgeon can help with triage decisions, guide workup, provide advice about patient care, optimize the timing of transplantation, offer specialized diagnostic and therapeutic options, and help the treating physician stay abreast of the continuous changes in this complex field. In the final analysis, however, it is often the skill and diligence of the primary care physician in diagnosing liver disease, identifying and treating correctable causes, optimizing the patient's health and nutrition, and anticipating and preventing catastrophic complications that determine whether the patient lives or dies.

Disease Progression↗

PubMed1999

Cholesterol enhances membrane-damaging properties of model bile by increasing the intervesicular-intermixed micellar concentration of hydrophobic bile salts.

UNLABELLED: Bile salts are potent detergents that, at concentrations attained in bile and intestine, can disrupt cell membranes. Hepatic secretion of vesicles containing lecithin and cholesterol appears to be critical in preventing bile salt damage to hepatobiliary epithelia. We hypothesize that the protective effect of biliary lipids results from lowering of the bile salt intervesicular intermixed micellar bile salt concentration (IMMC) to which epithelial membranes are exposed. We further hypothesize that increases in biliary cholesterol, by reducing association of bile salts with vesicles and mixed micelles, may increase bile toxicity by raising the bile salt IMMC. METHOD: Large unilamellar lecithin vesicles (100 nm) with varying cholesterol:lecithin molar ratios (C:L) of 0, 0.5, and 1 were added to taurochenodeoxycholate (TCDCA), taurocholate (TCA), or taurodeoxycholate (TDCA) in Tris-buffered saline, pH 7.4. Human erythrocyte ghosts (model target membrane), prepared by osmotic hemolysis and resealed with [14C]inulin trapped inside, were added and incubated at 37 degrees C for 30 min and 4 h. Plasma membrane disruption was quantified by [14C]inulin release and bile salt IMMC was determined by ultrafiltration. RESULTS: Membrane disruption started at a concentration of 0.5 mM for TDCA, 1 mM for TCDCA, and 2 mM for TCA and was complete within 4 h at concentrations of 1, 2, and 4 mM, respectively. Addition of 2 mM lecithin to 2 mM TDCA, 4 mM TCDCA, or 5 mM TCA reduced or eliminated membrane leakage and lowered the IMMC. For TDCA and TCDCA, the protective effect of vesicles was entirely attributable to reduction in IMMC; in contrast for TCA, the protective effect exceeded that which would have been expected based solely on reduction of the IMMC. Inclusion of cholesterol attenuated the binding of bile salts to vesicles and raised the IMMC, thereby reducing the protective effect of lecithin over the time course of these studies. Although there was loss of phospholipid and cholesterol from the erythrocyte membranes on addition of bile acids even in the presence of vesicles, the ratio of cholesterol to phospholipid in the erythrocyte membrane did not change. CONCLUSION: Lecithin protects against membrane disruption by hydrophobic bile salts by lowering the IMMC. Cholesterol added to lecithin raises the bile salt IMMC and reduces or eliminates this protective effect. This mechanism of potentiation of bile salt toxicity by cholesterol may be an important contributor to the pathogenesis of gallbladder disease in cholesterol cholelithiasis.

Bile↗

PubMed1998

Lecithin protects against plasma membrane disruption by bile salts.

INTRODUCTION: Detergent disruption of epithelial plasma membranes by bile salts may contribute to pathogenesis of cholestasis and gastroesophageal reflux disease. Bile, despite containing high concentrations of bile salts, normally is not toxic to biliary or intestinal epithelia. We hypothesize that lecithin in bile may protect cell membranes from disruption by bile salts. METHODS: We studied the interactions of taurine conjugates of ursodeoxycholate (TUDCA), cholate (TCA), chenodeoxycholate (TCDCA), and deoxycholate (TDCA) with erythrocyte plasma membranes with or without large unilamellar egg lecithin vesicles for various times at 23 degreesC. Release of hemoglobin was quantified spectrophotometrically. The concentration of bile salt monomers and simple micelles in the intermixed micellar aqueous phase (IMMC) was determined by centrifugal ultrafiltration. RESULTS: The degree of hemolysis depended on the hydrophobicity of the bile salts and was progressive over time. Addition of lecithin reduced the hemolytic effects of 20 mM TCA or 2 mM TDCA in a concentration-dependent manner at both 30 min and 4 h. Increasing the concentration of lecithin progressively reduced the IMMC of TDCA. Hemolysis following addition of lecithin to 2 mM TDCA was comparable to hemolysis produced by lecithin-free TDCA solutions when diluted to similar IMMC values. CONCLUSION: We conclude that lecithin reduces plasma membrane disruption by hydrophobic bile salts. This protection may be attributable to association of bile salts with vesicles and mixed micelles, reducing the concentration of bile salt monomers and simple micelles available to interact with cell membranes. Lecithin may play a key role in preventing bile salt injury of biliary and gastrointestinal epithelia.

Bile Acids and Salts↗

PubMed1998

Effect of bile acid composition and manipulation of enterohepatic circulation on leptin gene regulation.

In the rat, the ob gene product, leptin, putatively regulates energy balance via appetite control and energy expenditure. Bile acids in the intestinal lumen are necessary for efficient absorption of dietary lipids and may trigger the release of regulatory peptides. To investigate whether bile acids play a role in leptin gene expression, we altered the bile acid pool and then measured leptin mRNA levels in adipose tissue. Rats fed cholic acid (1% of chow wt/wt) for 2 weeks did not gain weight as rapidly as pair-fed control animals. Despite the lower weight, normalized leptin mRNA levels were 24% greater in cholic acid-fed rats compared with controls. Conversely, cholestyramine, a bile acid sequestrant, in chow (5% wt/wt) resulted in a 26% decline in leptin mRNA. Ligation of the common bile duct or chronic biliary diversion, experimental manipulations that decreased the intestinal concentration of bile salts, decreased leptin gene expression by 30% and 50%, respectively. A fluid and electrolyte (F/E) solution with and without taurocholate (36 micromol/h x 100 g rat[-1]) was then infused for 12 hours into the duodenum in animals with chronic biliary diversion. Taurocholate infusion resulted in a fourfold increase in steady-state adipocyte leptin mRNA levels compared with F/E infusion. Intravenous infusion of taurocholate or incubation of cultured adipocytes with taurocholate had no effect on leptin mRNA levels. We conclude that bile acids upregulate leptin gene expression indirectly, probably via effects on the absorption of dietary lipids or the release of neurohumoral mediators.

Adipocytes↗

PubMed1998

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.

Animals↗

PubMed1997

Quantitative estimations of the contribution of different bile acid pathways to total bile acid synthesis in the rat.

BACKGROUND & AIMS: Cholesterol degradation to bile acids occurs via "classic" or "alternative" bile acid biosynthetic pathways. The aim of this study was to assess the contributions of these two pathways to total bile acid synthesis in vivo. METHODS: Rats with biliary fistulas were infused with squalestatin for 24 and 48 hours; specific activities of cholesterol 7 alpha-hydroxylase (C7 alpha H) and sterol 27-hydroxylase (S27H) and rates of bile acid synthesis were determined. RESULTS: Continuous squalestatin infusion (15 micrograms/h) decreased C7 alpha H specific activities to 4% and 12% of paired biliary fistula controls at 24 and 48 hours, respectively (P < 0.05) without any changes in S27H specific activities (82% and 95% of controls). At 24 hours, bile acid synthesis decreased to 43% (P < 0.05) but returned to 87% at 48 hours (P = NS). Cholic acid synthesis decreased at 24 hours but returned to control levels at 48 hours. Similar changes in C7 alpha H, S27H, and bile acid synthesis were observed in primary rat hepatocytes after addition of squalestatin (1.0 mumol/L). CONCLUSIONS: In the face of persistent suppression of C7 alpha H and the classic pathway, an alternative pathway becomes a main pathway of bile acid synthesis capable of generating cholic and chenodeoxycholic acids. The observed induction of bile acid synthesis via an alternative pathway or pathways represents an important mechanism for maintenance of cholesterol homeostasis in the rat.

Animals↗

PubMed1997

Distribution of mixtures of bile salt taurine conjugates between lecithin-cholesterol vesicles and aqueous media: an empirical model.

Bile salts are surfactants that partition into phospholipid bilayers. When liposomes or membranes are exposed to mixed solutions of bile salts, the more hydrophobic bile salt species associate preferentially with the lipid bilayer. As a consequence, in the aqueous phase, the free monomeric concentration of bile salt declines and the more hydrophilic species become relatively enriched. Above a critical saturating concentration of lecithin-associated bile salt, a phase transition occurs with loss of membrane integrity and formation of mixed micelles. In this paper we present a quantitative model which, for mixed solutions of bile salt taurine conjugates, predicts the distribution of bile salt monomers between large unilamellar vesicles composed of lecithin and cholesterol and the aqueous phase. The model is based on association isotherms for individual bile salts, determined by an ultrafiltration method with empirical curve fitting, and is critically dependent upon the observation that association coefficients of each bile salt are a function of the total bound bile salt/lecithin mole ratio. Given the concentrations of individual bile salts, lecithin and cholesterol, the model permits calculation of the membrane-bound bile salt/lecithin ratio and the concentration of each bile salt remaining free as soluble monomer in the aqueous phase, as well as the overall hydrophilic-hydrophobic balance (hydrophobicity index) of the bile salts remaining free in aqueous solution. Distribution data determined empirically for a variety of mixtures of bile salt taurine conjugates and large unilamellar vesicles of varying cholesterol:lecithin ratio agree closely with predictions. This model may be of value in predicting the physical, biological and toxic properties of mixed bile salt solutions.

Animals↗

PubMed1997

Hormonal regulation of cholesterol 7alpha-hydroxylase specific activity, mRNA levels, and transcriptional activity in vivo in the rat.

In primary cultures of rat hepatocytes, transcription of the cholesterol 7alpha-hydroxylase gene is induced synergistically by glucocorticoid and thyroid hormones. The objective of the present study was to evaluate the role of endogenous glucocorticoid and thyroid hormones in the maintenance of cholesterol 7alpha-hydroxylase gene expression in vivo. Male Sprague-Dawley rats underwent adrenalectomy (A), thyroidectomy (T), adrenalectomy + thyroidectomy (A + T), hypophysectomy (H), or sham surgery (paired controls). Ten days post surgery, livers were harvested and choles terol 7alpha-hydroxylase specific activity, steady-state mRNA levels, and transcriptional activity were determined. Serum corticosterone levels were <2% of paired controls in A, A + T, and H rats. Free thyroxine index was <32% of paired controls in rats with T and H. When compared to sham-operated controls, A + T and H led to decreases in cholesterol 7alpha-hydroxylase specific activities of 44 +/- 8% and 57 +/- 3%, respectively (P < 0.03 and < 0.05). Similar changes were observed in cholesterol 7alpha-hydroxylase steady-state mRNA levels, which decreased by 43 +/- 10% (P < 0.001) and 56 +/- 19% (P < 0.05), respectively. Cholesterol 7alpha-hydroxylase transcriptional activity in A + T and H rats decreased by 34 +/- 11% (P < 0.01) and 61 +/- 4% (P < 0.001), respectively. The observed decreases were greater after H than after A + T, suggesting the possibility that another pituitary hormone plays a role in regulation of cholesterol 7alpha-hydroxylase. Thyroidectomy alone led to a decrease in cholesterol 7alpha-hydroxylase specific activity of 37 +/- 7% (P < 0.05) and a trend toward decreased steady-state mRNA levels (21 +/- 12%; P = ns). Adrenalectomy did not significantly decrease cholesterol 7alpha-hydroxylase specific activity or mRNA levels. Neither thyroidectomy nor adrenalectomy alone affected transcriptional activity. We conclude that under physiologic circumstances, full expression of the cholesterol 7alpha-hydroxylase gene requires synergistic action of glucocorticoids and thyroid hormone.

Adrenalectomy↗

PubMed1996

Hep G2 cells: a model for studies on regulation of human cholesterol 7alpha-hydroxylase at the molecular level.

The present study examines the feedback control governing human cholesterol 7alpha-hydroxylase mRNA expression in the human hepatoblastoma cell line, Hep G2. Glycochenodeoxycholate (GCDC) and glycodeoxycholate, hydrophobic bile salts, decreased cholesterol 7alpha-hydroxylase mRNA levels and bile acid synthesis in a concentration-dependent (76 +/- 8%, P<0.001, and 48 +/- 3%, P<0.01, respectively) and time-dependent manner. Cholesterol 7alpha-hydroxylase mRNA levels were repressed with a half-maximal inhibitory concentration of <12.5 microM by GCDC and a half-life of 30 min by 100 microM of the bile acid. The addition of actinomycin D (10 microgram/ml) alone or in combination with GCDC (100 microM) led to similar concentration-and time-dependent suppression of cholesterol 7alpha-hydroxylase mRNA. Glycocholate (100 microM), not internalized based on lack of uptake of a fluorescent cholate analogue, had no effect on cholesterol 7alpha-hydroxylase mRNA or total bile acid synthesis. In cultures transfected with a rat cholesterol 7alpha-hydroxylase promoter construct, reporter gene activity was decreased (31%, P<0.01) by GCDC (100 microM). Hep G2 cells maintain the intracellular machinery to express and rapidly regulate human cholesterol 7alpha-hydroxylase by hydrophobic bile acids. These data suggest that Hep G2 cells will support functional studies of the human cholesterol 7alpha-hydroxylase gene.

Animals↗

PubMed1996

Transcriptional regulation of hepatic sterol 27-hydroxylase by bile acids.

The study objective was to determine whether and to what extent sterol 27-hydroxylase, the initial step in the "acidic" pathway of bile acid biosynthesis, is regulated by bile acids. Rats were fed diets supplemented with cholestyramine (CT, 5%), cholate (CA, 1%), chenodeoxycholate (CDCA, 1%), or deoxycholate (DCA, 0.25%). When compared with paired controls, sterol 27-hydroxylase and cholesterol 7 alpha-hydroxylase specific activities increased after CT administration by 188 +/- 20% (P < 0.05) and 415 +/- 36% (P < 0.01), respectively. Similarly, mRNA levels increased by 159 +/- 14% (P < 0.05) and 311 +/- 106% (P < 0.05), respectively. Feeding CA, CDCA, or DCA decreased sterol 27-hydroxylase specific activity to 57 +/- 6, 61 +/- 8, and 74 +/- 8% of controls, respectively (P < 0.05). By comparison, the specific activity of cholesterol 7 alpha-hydroxylase decreased to 46 +/- 7 , 32 +/- 10, and 26 +/- 8% (P = 0.001). mRNA levels and transcriptional activities for sterol 27-hydroxylase and cholesterol 7 alpha-hydroxylase transcriptional activity were changed to the same extent as the specific activities after CT or bile acid feeding. We conclude that sterol 27-hydroxylase and cholesterol 7 alpha-hydroxylase are subject to negative feedback regulation by hydrophobic bile acids at the level of transcription. However, the responses of sterol 27-hydroxylase to manipulation of the bile acid pool are less prominent than those of cholesterol 7 alpha-hydroxylase. During the diurnal cycle the specific activities of sterol 27-hydroxylase and cholesterol 7 alpha-hydroxylase changed in tandem, suggesting that both may be under control of glucocorticoids.

Animals↗

PubMed1996

Adsorption of mixtures of bile salt taurine conjugates to lecithin-cholesterol membranes: implications for bile salt toxicity and cytoprotection.

Tauroursodeoxycholate (TUDC), a relatively hydrophilic bile salt, reduces disruption of cholesterol-rich membranes by more hydrophobic bile salts such as taurocholate (TC), taurochenodeoxycholate (TCDC), or taurodeoxycholate (TDC). We examined the interactions of these bile salts in adsorption to large unilamellar vesicles to determine whether TUDC may stabilize membranes by preventing adsorption of more toxic bile salts. Fractional adsorption was quantified by rapid ultrafiltration. Adsorption coefficient Ai was defined for each bile salt i as ([bound i]/[free i])/[lecithin]. Affinity of different bile salts for lecithin vesicles varied with their relative hydrophobicity, increasing in the order TUDC < TC << TCDC < or = TDC. Ai of each bile salt fell with its accumulation on membranes, reaching a minimum at bound bile salt/lecithin mole ratio (B:L) between 0.05 and 0.1, then increasing with formation of higher-affinity mixed micelles. Inclusion of cholesterol in vesicles reduced Ai of all bile salts. In heterologous binding studies at submicellar concentrations, Ai of each bile salt varied with total B:L but was independent of the specific bile salts present on the membrane. Addition of TUDC to TDC reduced binding of TDC to membranes only slightly and lowered the threshold TDC concentration associated with transition to mixed micelles. However, above this threshold, TUDC markedly altered the adsorption of TDC to lecithin-containing phases. We conclude that TUDC does not directly stabilize membranes; rather, reduced permeabilization and dissolution of cholesterol-rich membranes after addition of TUDC to TDC may result from effects on the formation and structure of simple and mixed micelles.

Adsorption↗

PubMed1995

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.

Animals↗

PubMed1995

Bile salt-membrane interactions and the physico-chemical mechanisms of bile salt toxicity.

We present evidence that ursodeoxycholate prevents toxicity of more hydrophobic bile salts by inhibiting micellar solubilization of membrane lipids. Using both centrifugal ultrafiltration and gel filtration methods we studied leakage of inulin from vesicles composed of egg phosphatidylcholine and cholesterol. We observed that the addition of tauroursodeoxycholate to taurodeoxycholate reduced leakage of inulin from large unilamelar vesicles compared to that seen with taurodeoxycholate alone. This protective effect was observed only at high membrane cholesterol:phospholipid ratios (> or = 0.5). By gel filtration we found that fractional leakage of inulin from vesicles was identical to fractional phospholipid solubilization, indicating that release of inulin from vesicles results from membrane dissolution rather than from increased permeability of otherwise intact membranes. Addition of tauroursodeoxycholate to taurodeoxycholate was found to suppress the dissolution of phospholipid from cholesterol-rich vesicles. Bile salts were found to absorb to vesicles with an affinity proportional to their relative hydrophobicity, as estimated by reverse phase HPLC. Adsorption affinity decreased progressively with increasing membrane cholesterol content. Different bile salts displaced each other from membranes in proportion to their respective binding, affinities. Tauroursodeoxycholate, which absorbed to membranes with low affinity, displaced taurodeoxycholate from vesicles only weakly. Based on these findings we postulate that bile salts may damage the liver through solubilization of canalicular membrane lipids. Ursodeoxycholate may protect the liver by inhibiting dissolution of the cholesterol-rich canalicular membrane by more hydrophobic endogenous bile salts. Biliary secretion of vesicles rich in phosphatidylcholine may buffer the intermicellar concentration of bile acids at levels below those required to disrupt the cholesterol-rich canalicular membrane; thus biliary vesicle secretion may have evolved as a mechanism to protect the biliary epithelium from injury by luminal bile salts.

Bile Acids and Salts↗

PubMed1994

Ursodeoxycholate conjugates protect against disruption of cholesterol-rich membranes by bile salts.

BACKGROUND/AIMS: Ursodeoxycholic acid attenuates hepatocellular injury in cholestatic disorders, possibly by counteracting membrane disruptive effects of endogenous bile salts. The possible physicochemical basis of this protective effect was explored by using model membranes composed of egg phosphatidylcholine and cholesterol. METHODS: Large unilamellar vesicles containing trapped 3H inulin were prepared by extrusion and gel filtration. Vesicle disruption (release of trapped inulin) was quantified using rapid centrifugal ultrafiltration. RESULTS: Disruption of membranes increased with bile salt concentration, hydrophobicity, and increasing ionic strength. Disruption decreased with a decreasing bile salt/phospholipid ratio or an increasing cholesterol/phospholipid ratio. Vesicle disruption by taurodeoxycholate (3 alpha, 12 alpha-dihydroxy-5 beta-cholanoyl taurine) was reduced in a concentration-dependent manner by addition of tauroursodeoxycholate (3 alpha,7 beta-dihydroxy-5 beta-cholanoyl taurine) (TUDC) when the cholesterol/phospholipid ratio was > or = 0.5, but TUDC was not protective at a cholesterol/phospholipid ratio < or = 0.2. Glycoursodeoxycholate (3 alpha,7 beta-dihydroxy-5 beta-cholanoyl glycine) was somewhat less protective than TUDC, and unconjugated ursodeoxycholate (3 alpha,7 beta-dihydroxy-5 beta-cholanoate) (UDC) had little effect. Taurine conjugates of several other hydrophilic bile salts were also protective, but protection was not strictly proportional to hydrophilicity. CONCLUSIONS: Conjugates of UDC and other hydrophilic bile salts can reduce disruption of cholesterol-rich model membranes by more toxic bile salts via a purely physicochemical mechanism. UDC conjugates in vivo may protect the cholestatic liver by preventing bile salt disruption of the cholesterol-rich canalicular membrane.

Bile Acids and Salts↗

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↗