Influence of tauroursodeoxycholic and taurodeoxycholic acids on hepatic metabolism and biliary secretion of phosphatidylcholine in the isolated rat liver

Role of Organic Solute Transporter Alpha/Beta in Hepatotoxic Bile Acid Transport and Drug Interactions

Organic solute transporter (OST) α/β is a key bile acid transporter expressed in various organs, including the liver under cholestatic conditions. However, little is known about the involvement of OSTα/β in bile acid-mediated drug-induced liver injury (DILI), a major safety concern in drug development. This study investigated whether OSTα/β preferentially transports more hepatotoxic, conjugated, primary bile acids and to what extent xenobiotics inhibit this transport. Kinetic studies with OSTα/β-overexpressing cells revealed that OSTα/β preferentially transported bile acids in the following order: taurochenodeoxycholate > glycochenodeoxycholate > taurocholate > glycocholate. The apparent half-maximal inhibitory concentrations for OSTα/β-mediated bile acid (5 µM) transport inhibition by fidaxomicin, troglitazone sulfate, and ethinyl estradiol were: 210, 334, and 1050 µM, respectively, for taurochenodeoxycholate; 97.6, 333, and 337 µM, respectively, for glycochenodeoxycholate; 140, 265, and 527 µM, respectively, for taurocholate; 59.8, 102, and 117 µM, respectively, for glycocholate. The potential role of OSTα/β in hepatocellular glycine-conjugated bile acid accumulation and cholestatic DILI was evaluated using sandwich-cultured human hepatocytes (SCHH). Treatment of SCHH with the farnesoid X receptor agonist chenodeoxycholate (100 µM) resulted in substantial OSTα/β induction, among other proteomic alterations, reducing glycochenodeoxycholate and glycocholate accumulation in cells+bile 4.0- and 4.5-fold, respectively. Treatment of SCHH with troglitazone and fidaxomicin together under cholestatic conditions resulted in increased hepatocellular toxicity compared with either compound alone, suggesting that OSTα/β inhibition may accentuate DILI. In conclusion, this study provides insights into the role of OSTα/β in preferential disposition of bile acids associated with hepatotoxicity, the impact of xenobiotics on OSTα/β-mediated bile acid transport, and the role of this transporter in SCHH and cholestatic DILI.

The role of dietary choline in the beneficial effects of lecithin on the secretion of biliary lipids in rats

Earlier studies showed that dietary soybean lecithin increases biliary lipid secretion, which mainly comes from the contribution of high density lipoprotein (HDL) and hepatic microsomal pools of phosphatidylcholine and cholesterol. In addition, a lecithin diet enhances bile secretion and prevents bile acid-induced cholestasis. This study evaluated the contribution of choline, a component of lecithin, to the observed effect of lecithin on biliary secretory function. Rats were fed either a control diet (CD), a choline diet (ChD) or a lecithin-enriched diet (LD) for 2 weeks. Results showed that like LD, ChD induced an increase in bile flow and bile acid secretion rate when compared with the control diet. However, unlike LD, ChD did not significantly increase biliary phospholipids and cholesterol output. An increase of hydrophilic bile acids (i.e. ursodeoxycholic and muricholic acids) in bile of rats fed choline could explain why the biliary phospholipid and cholesterol secretion was not increased. During taurocholic acid infusion, both experimental diets increased bile flow and the bile acid secretion rate maximum (BASRm). The cholestasis usually observed after the BASRm is reached was inhibited by ChD and LD. Both diets induced a decrease in plasma cholesterol (total and HDL), however, only LD induced statistically significant changes. Analysis of total cholesterol and phospholipid content of microsomes and canalicular membranes indicated no statistically significant difference between control and experimental groups either under basal conditions or after bile acid infusion. Similarly, the phospholipid classes and fatty acid composition of biliary phosphatidylcholine were not altered by feeding ChD and LD. We conclude that choline contributes to the beneficial effect of a lecithin diet on bile secretion. It is postulated that this effect may be attributed to modulation of HDL and an enhancement of the cholesterol and phospholipid pools destined for biliary secretion.

Disappearance of two major phosphatidylcholines from plasma is predominantly via LCAT and hepatic lipase

Metabolism of 1-stearoyl-2-arachidonyl-phosphatidyl-choline (SAPC), a major phosphatidylcholine (PC) species in rat plasma, was compared with 1-palmitoyl-2-linoleoyl-PC (PLPC) metabolism. High-density lipoproteins containing SAPC and PLPC tracers labeled in the sn-2 fatty acid with 3H and 14C isotopes, respectively, were administered. The rats were depleted of endogenous bile acids and infused via the ileum with individual bile acids that ranged widely in hydrophobicity. The half-lives for SAPC and PLPC in plasma were 48 and 57 min, respectively. Most of the 3H activity that disappeared from plasma at 1 h was found in the liver in 1-palmitoyl-2-arachidonyl-PC, SAPC, and 1-oleoyl-2-arachidonyl-PC, indicating phospholipase A1 hydrolysis of plasma SAPC forming 2-arachidonyl-lysophosphatidylcholine, which was reacylated in the liver. Plasma PLPC also underwent phospholipase A1 hydrolysis, as reported previously. The fraction of 3H dose that accumulated in plasma cholesteryl arachidonate was two- to threefold higher than the fraction of 14C dose in cholesteryl linoleate. Multicompartmental models for SAPC and PLPC were developed that included lysophosphatidylcholines and cholesteryl esters. Bile acids did not influence plasma PC metabolism. Lecithin-cholesterol acyltransferase and phospholipase A1 (hepatic lipase) hydrolysis accounted for > or = 90% of the SAPC and PLPC that disappeared from plasma; SAPC and PLPC are comparable as substrates for hepatic lipase, but SAPC is preferred by lecithin-cholesterol acyltransferase.

Differential effects of chylomicron remnants derived from corn oil or palm oil on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes

The effects of chylomicron remnants derived from corn oil (rich in n-6 polyunsaturated fatty acids) and palm oil (rich in long chain saturated fatty acids) on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes were studied. Incubation of the cells with corn oil remnants led to increased bile acid production, while the secretion of lipid in very low density lipoprotein remained unchanged. In contrast, addition of palm oil remnants to the medium did not affect bile acid synthesis, but resulted in the secretion of cholesterol-rich very low density lipoprotein. These findings show that chylomicron remnants of different fatty acid composition have differential effects on cholesterol metabolism in liver cells, and provide part of the explanation for the hyper- and hypocholesterolaemic effects of saturated and polyunsaturated fatty acids.

Probucol reduces hepatic cholesterol secretion in hyperlipidemic Yoshida rats

In this study, perfused livers from Yoshida rats, either on a normal diet or on a diet with 0.3% probucol, were examined. The analysis of liver lipid content and of bile and lipoprotein secretion changes showed that probucol had a relevant effect on liver lipid biosynthesis. In particular, it reduced the production of triacylglycerols and, to a much greater extent that of cholesterol. In addition, probucol reduced plasma cholesterol concentration by decreasing esterified cholesterol in HDL1 and HDL2 fractions. Furthermore, HDL1 composition of both hepatic neosynthetized and circulating particles was strongly modified by probucol. Finally, probucol did not appear to induce significant differences in lipid bile secretion while phospholipid secretion from perfused livers was increased. These facts suggest that the hypolipidemic action of probucol is not mediated by an increase in bile steroid secretion, but rather by a direct reduction in hepatic lipoprotein cholesterol secretion. This secretion induces a modified plasma profile of HDL particles such that these variations are advantageous in terms of reverse cholesterol transport.

Effect of tauroursodeoxycholic and ursodeoxycholic acid on ethanol-induced cell injuries in the human Hep G2 cell line

BACKGROUND & AIMS

Taurodeoxycholic acid (TUDCA) and ursodeoxycholic acid (UDCA) exert a protective effect in chronic cholestasis. This study reports the effect of TUDCA and UDCA on an in vitro model for ethanol-induced liver damage.

METHODS

Hep G2 cells were incubated for 24 hours with 80 mmol/L ethanol in the presence or absence of 50 mumol/L TUDCA or UDCA. Cells were also pretreated with 80 mmol/L EtOH and then exposed to 50 mumol/L bile acids. Cytotoxicity was assessed by the metabolism of formazan (3-(4,5-dimethyl-thiazol-2-yl)-2, 5-diphenyl tetrazolium bromide and sodium 3,3'-(phenylamino) carbonyl-3,4-tetrazolium-bis (4-methoxy-6-nitro) benzene sulfonic acid hydrase and by the release into the culture medium of different enzymes (aspartate aminotransferase, glutamate dehydrogenase, gamma-glutamyl transferase, and lactate dehydrogenase).

RESULTS

The incubation of Hep G2 with EtOH significantly (P < 0.001) increased cytotoxicity. Both TUDCA or UDCA reduced cytoxicity to a similar extent (P < 0.001). Cells pretreated with EtOH and then added with TUDCA or UDCA responded differently because TUDCA was significantly more effective (P < 0.05) than an equimolar dose of UDCA in reversing the damage. Electron microscopic examination revealed that TUDCA and UDCA were both able to prevent mitochondrial damage and to reduce steatosis induced by EtOH.

CONCLUSIONS

Low doses of TUDCA and UDCA protect Hep G2 cells from EtOH-induced cytotoxicity. However, TUDCA seems to be more effective than UDCA in reversing the damage.

Age-related variations in plasma and liver lipids of Yoshida rats: a comparison with Wistar rats

Lipoprotein and liver lipids of spontaneously hyperlipidemic Yoshida rats were compared with those of normolipidemic Wistar animals for studying their age- and strain-related differences. Both strains showed an age-related increase in the total plasma cholesterol concentration. However, the Yoshida strain had a higher content of cholesteryl esters and triglycerides than the Wistar strain in both young and adult animals (2- and 8-month-old animals, respectively). The free cholesterol content was also higher, but only in the 8-month-old animals. Both strains showed an age-related increase in the proportion of HDL1 and a symmetrical decrease in both the HDL2 and HDL3 subfractions, but the variations were more evident in the Yoshida strain. The study of strain-related differences suggested that the spontaneous hypertriglyceridemia of the Yoshida strain was not only related to the higher amount and proportion of the VLDL fraction, but also to the higher content of triglycerides in the LDL fraction. The livers of Yoshida rats accumulated more triglycerides (with an age-related progression) than those of Wistar rats. The major lipid classes in the liver of Yoshida rats contained a significantly higher proportion of monounsaturated fatty acyls. Furthermore, this proportion showed an age-related increase in all the lipid classes, but in cholesteryl esters. This suggested that liver desaturases had a relevant role in the development of hyperlipidemia, and of its age-related variations, in the Yoshida strain.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes in blood and liver lipids of male wistar rats

The results of this study indicate that the age-dependent plasma cholesterol increase observed in male Wistar rats is correlated with changes in both the distribution of high-density lipoprotein fractions and the storage of hepatic cholesterol. Specifically, the lipoprotein distribution showed a significant increase in the proportion of HDL(1) and a symmetrical decrease in both the HDL(2) and HDL(3) fractions during the 3 month to 18 month age period. There were no significant changes in the very-low density and low-density lipoprotein fractions. The chemical composition of lipoproteins showed many age-related variations, especially in the proportion of cholesteryl ester and in the distribution of HDL subfractions. A study of fatty acyl composition of the major lipid classes showed that, within cholesteryl ester found in liver, there was an increase in the proportion of saturated fatty acids. Polyunsaturated fatty acids increased in the cholesteryl esters found in high-density lipoproteins of older rats. These observations suggest that the age-dependent accumulation of body cholesterol occurs by a reduced catabolism of HDL(1) fraction, and modifications in plasma and liver lipids.

Effect of ethinylestradiol and epomediol on bile flow and biliary lipid composition in rat

Epomediol (1,3,3-trimethyl-2-oxabicyclo(2.2.2.)octan-6,7-endo,endo-diol) (EPO) is a terpenoid compound shown to reverse 17 alpha-ethinylestradiol (EE)-induced cholestasis in rat. The effect is related to the restoration of normal liver plasma membrane fluidity values. To further characterize the effect of EPO, bile flow and biliary lipid composition were measured in rats treated either with EE or EE associated with EPO. EE significantly reduced the bile flow; this reduction was prevented by concomitant treatment with EPO with an increase in the bile salt secretion rate. EPO alone showed a choleretic effect. The biliary secretion rate of cholesterol was also significantly reduced by EE while being comparable to controls in EE-EPO-treated animals. Phospholipid (PL) biliary excretion was significantly (P less than 0.002) increased by EE either alone or combined with EPO. After EE treatment, the biliary PL composition showed a reduction in phosphatidylcholine (PC) concentration with a parallel increase in lyso-phosphatidylcholine (LPC) when compared to control animals (PC:LPC ratio 5.0 +/- 2.5 vs 26.8 +/- 9.9, mean +/- SD, P less than 0.005). EPO administration to EE-treated rats restored the biliary PC:LPC ratio to control values (27.6 +/- 10.6). EPO alone did not show any appreciable effect as compared to both control and EE-EPO treated animals. As increased concentrations of LPC have been reported to induce an alteration in the function of membrane lipids and membrane-associated proteins, such as regulatory enzymes for bile acid, cholesterol and phospholipid metabolism, these results suggest that the protective effect of EPO in EE-induced cholestasis may be related to the reversal of the alterations in membrane lipid composition and function induced by EE.

Impaired hepatic handling and processing of lysophosphatidylcholine in rats with liver cirrhosis

Lysophosphatidylcholine is a major metabolic product in the plasma and cellular turnover of phospholipids, with well-known membrane-toxic and proinflammatory properties. Because the liver plays a key role in plasma lysophosphatidylcholine removal and biotransformation and because virtually nothing is known of these processes in a diseased organ, the hepatobiliary metabolism of lysophosphatidylcholine was investigated in rats with carbon tetrachloride-induced liver cirrhosis. Twelve adult male Wistar rats with histologically confirmed cirrhosis and 8 control animals were fitted with jugular and biliary catheters and allowed to recover. The animals were kept under constant IV infusion of taurocholate (1 mumol/min). Two microcuries of sn-1[14C]palmitoyl-lysophosphatidylcholine was administered as a single bolus. The fate of the injected radioactivity, including removal from plasma, uptake, and subcellular location in the liver and molecular and aggregative forms, was studied by combined chromatographic and radiochemical methods. Major findings were (a) that lysophosphatidylcholine has a prolonged permanence in plasma of cirrhotic rats, due both to decreased hepatic clearance and to depressed conversion into phosphatidylcholine; (b) that the rate of lysophosphatidylcholine acylation is much slower in the cirrhotic than in the normal liver, both at the microsomal and at the cytosolic level; (c) that cytosolic lysophosphatidylcholine in the cirrhotic liver, but not in the normal liver, is predominantly non-protein bound; (d) that the strict molecular selectivity of lysophosphatidylcholine acylation observed in controls is partially lost in cirrhosis; and (e) that a consistent fraction of lysophosphatidylcholine is converted into triacylglycerols in cirrhotics but not in controls. These findings show a profound derangment of lysophosphatidylcholine handling and processing in the cirrhotic liver, which is of potential pathogenetic significance.

Differential hepatic processing and biliary secretion of head-group and acyl chains of liposomal phosphatidylcholines

To investigate the contribution of plasma-derived phosphatidylcholine (PC) to bile PC, the hepatic processing and biliary secretion of liposome-associated PC was studied in rats. For this purpose, small unilamellar vesicles (SUV), containing trace amounts of [2-palmitoyl-9,10-3H]dipalmitoylphosphatidylcholine ([palmitoyl-3H]DPPC), [choline-14C]-dipalmitoylphosphatidylcholine ([choline-14C]DPPC), di[14C]palmitoylphosphatidylcholine ([14C]DPPC) or di[1-14C]-oleoylphosphatidylcholine ([14C]DOPC), were administered intravenously to unanaesthetized rats, equipped with permanent catheters in heart and bile duct. Biliary secretion of the 14C-head-group label of DPPC was very slow (0.3% of injected dose in 4 h), whereas the [3H]palmitoyl label was secreted at a much higher rate (16% in 4 h), but only after substantial catabolism of the acyl chain. To study the latter process in more detail, we compared hepatic metabolism and biliary secretion of [1-14C]acyl-labelled DPPC and DOPC. In rats with an 8-day bile drainage, degradation products of the oleoyl chain were utilized for synthesis of bile acids, which were subsequently secreted into the bile (2% in 6 h). A much smaller fraction (0.6% in 6 h) was secreted as PC and lyso-PC. When bile drainage was started immediately after SUV injection, i.e. a situation with a low hepatic bile acid synthesis rate and a high phospholipid secretion, the secretion of [14C]DOPC-derived radioactivity in the form of bile acids was decreased (0.2% in 6 h), and that as (lyso-)PC increased (1.5% in 6 h). Biliary secretion of DPPC palmitoyl chains in bile-diverted rats was much less than that of the oleoyl chains, and occurred predominantly as PC and lyso-PC (0.6%, compared with 0.4% as bile acids in 6 h). Breath analyses demonstrated that a considerable fraction of both acyl chains was oxidized to CO2 and expired: 25.1% of the administered label for oleoyl chains and 13.4% for palmitoyl chains respectively in a 4 h period. The results of this study indicate that liposomal PC is only minimally secreted into bile via a direct pathway; the bulk is extensively degraded in the liver. Resulting products are partly secreted into bile, as bile acid or as resynthesized PC. There appears to be a quantitative difference in the metabolism of oleoyl and palmitoyl acyl chains.

Spontaneous formation of pigmentary precipitates in bile salt-depleted rat bile and its prevention by micelle-forming bile salts

During studies on the effect of bile salt-pool depletion in the bile-fistula rat (adult male Sprague-Dawley), the spontaneous formation of an orange-brown precipitate was noted. The nature of this phenomenon and its relationship to BS and calcium concentration was investigated in depth. Bile from 18 animals was collected in the dark into transparent tubes containing sodium azide, ascorbic acid, and glucaro-1,4-lactone. The tubes were flushed with nitrogen, sealed, and incubated at 37 degrees C. The pigmentary precipitate formed in all the bile salt-depleted (less than 3-5 mM) bile samples (i.e., those collected after 5-7 h of external biliary drainage), but not in bile salt-rich biles. It appeared within 30-240 min after collection, both in bile samples collected at room temperature and at 37 degrees C, initially as a pale flocculation and then slowly sedimenting to form, after centrifugation, a solid, dark-orange pellet. There were no pH changes during incubation, and bile cultures were negative. Under polarizing microscopy, the precipitate appeared amorphous, and there was no evidence of birefringence. High-performance liquid chromatography showed that unconjugated bilirubin was the prevalent pigmentary component, but significant amounts of monoconjugated bilirubin also coprecipitated. Lipid chemistry showed the presence of lecithin (80.1% of total lipids), which was rich in palmitoyl and linoleoyl fatty acids, and of fatty acids (predominantly palmitic and oleic). Infrared spectroscopy and x-ray diffraction showed the presence of calcium bilirubinate and palmitate. In-vivo replenishment of the bile salt pool by intravenous infusion of either taurocholate or taurochenodeoxycholate (1 mumol/min) completely prevented the pigmentary precipitation. In vitro experiments showed inhibition of the precipitate formation by the addition of individual bile salt in concentrations approximating their critical micellar concentration. Precipitate formation was hastened by the addition of calcium chloride (4-12 mM), but only in bile salt-depleted biles. As the composition of the precipitate closely resembles that of human brown-pigment stones and sludge, these findings may provide new insights into an understanding of the pathogenesis of pigment gallstone disease.

Selective hepatic enrichment of polyunsaturated phosphatidylcholines after intravenous administration of dimethylethanolamine in the rat

The content of polyunsaturated phosphatidylcholines (PCs) is one of the parameters which regulate membrane functions. Polyunsaturated PCs are preferentially synthesized in the liver by the microsomal enzyme phosphatidylethanolamine N-methyltransferase. The activity of this enzyme may be stimulated in vitro in isolated rat hepatocytes by supplementation with dimethylethanolamine (DME), the polar head group of the precursor of PC along this pathway. The aim of this study was to evaluate in vivo the effect of an intravenous infusion of DME in the rat on the hepatic phospholipid composition. Bile fistula rats were intravenously infused for 15 h with sodium taurocholate (1 mumol/kg per min), with or without the addition of 0.3 mg/kg per min of [14C]DME. The concentration per gram of wet liver of individual phospholipid classes, PC molecular species and of total triacylglycerols, as well as the distribution of radioactivity in liver phospholipids, in rat tissues and body fluids were analyzed. A significant (P less than 0.01) enrichment in PC was found in the liver of DME-infused rats with respect to controls. No differences in the other phospholipid classes were found. DME-infused rats showed a significant (P less than 0.01) decrease in the hepatic concentration of triacylglycerols. At HPLC analysis, the enrichment in PC in DME-infused rats was found to be selectively due to three molecular species (i.e., sn-1 stearoyl/sn-2 arachidonoyl, sn-1 stearoyl/sn-2 linoleoyl, sn-1 stearoyl/sn-2 docosahexanoyl molecular species). In agreement with quantitative data, more than 70% of hepatic radioactivity was recovered in polyunsaturated PC species, with the highest specific activity in the sn-1 stearoyl PCs. The specific activity of hepatic PC approximates that of phosphatidyldimethylethanolamine. This finding together with the effective incorporation of DME in PC suggests that this amino base is methylated after its incorporation into phosphatidyldimethylethanolamine, throughout the stimulation of hepatic N-methyltransferase activity. The selective hepatic enrichment with polyunsaturated PC species after DME infusion may offer a new experimental tool for studying hepatic membrane metabolism.

Transport, utilization and biliary secretion of lysophosphatidylcholine in the rat liver

The hepatic uptake, transport and utilization of plasma lysophosphatidylcholine (lysoPC) and its contribution to biliary lipid secretion have been investigated in bile-fistula rats. The animals were given a single intravenous dose of sn-1-[1-14C]palmitoyl-lysoPC, under constant intravenous sodium taurocholate infusion (1 mumol/min), and the fate of the label was followed in blood, bile and liver for up to 3 h. The livers were excised at given time points, extracted and/or homogenized to determine the lipid distribution and subcellular location of radioactivity. LysoPC was rapidly cleared from plasma, though a consistent fraction of the label persisted in plasma over the experimental time-period in the form of either lysoPC or PC. Recovery of radioactivity in the liver varied from 15.6% after 5 min to 19.5% after 3 h. Hepatic lysoPC underwent rapid microsomal acylation to form specific PC molecular species (mainly 16:0-20:4 and, to a lesser extent, 16:0-18:2 and 16:0-16:1). Ultrafiltration, dialysis and gel-chromatographic analyses of cytosolic fractions (post 105,000 X g supernatants) indicated that lysoPC is transported to the site of acylation mostly as a macromolecular aggregate with an approx. Mr of 14,400. Small amounts of radioactivity were secreted into bile over 3 h (20% in the form of lysoPC and the remainder as 16:0-18:2 and 16:0-20:4 PC species). Plasma lysoPC, taken up by the liver, is mostly transported by a cytosolic carrier with a molecular weight close to fatty-acid-binding proteins; it then enters a distinct acylation pathway, selective for some polyunsaturated-PC species and does not contribute significantly to biliary secretion, either directly, or through its products.