Different pathways of canalicular secretion of sulfated and non-sulfated fluorescent bile acids: a study in isolated hepatocyte couplets and TR- rats

Tauroursodeoxycholate prevents estradiol 17β-d-glucuronide-induced cholestasis and endocytosis of canalicular transporters by switching off pro-cholestatic signaling pathways

AIMS

Estradiol 17β-d-glucuronide (E217G) induces cholestasis by triggering endocytosis and further intracellular retention of the canalicular transporters Bsep and Mrp2, in a cPKC- and PI3K-dependent manner, respectively. Pregnancy-induced cholestasis has been associated with E217G cholestatic effect, and is routinely treated with ursodeoxycholic acid (UDCA). Since protective mechanisms of UDCA in E217G-induced cholestasis are still unknown, we ascertained here whether its main metabolite, tauroursodeoxycholate (TUDC), can prevent endocytosis of canalicular transporters by counteracting cPKC and PI3K/Akt activation.

MAIN METHODS

Activation of cPKC and PI3K/Akt was evaluated in isolated rat hepatocytes by immunoblotting (assessment of membrane-bound and phosphorylated forms, respectively). Bsep/Mrp2 function was quantified in isolated rat hepatocyte couplets (IRHCs) by assessing the apical accumulation of their fluorescent substrates, CLF and GS-MF, respectively. We also studied, in isolated, perfused rat livers (IPRLs), the status of Bsep and Mrp2 transport function, assessed by the biliary excretion of TC and DNP-SG, respectively, and Bsep/Mrp2 localization by immunofluorescence.

KEY FINDINGS

E217G activated both cPKC- and PI3K/Akt-dependent signaling, and pretreatment with TUDC significantly attenuated these activations. In IRHCs, TUDC prevented the E217G-induced decrease in apical accumulation of CLF and GS-MF, and inhibitors of protein phosphatases failed to counteract this protection. In IPRLs, E217G induced an acute decrease in bile flow and in the biliary excretion of TC and DNP-SG, and this was prevented by TUDC. Immunofluorescence studies revealed that TUDC prevented E217G-induced Bsep/Mrp2 endocytosis.

SIGNIFICANCE

TUDC restores function and localization of Bsep/Mrp2 impaired by E217G, by preventing both cPKC and PI3K/Akt activation in a protein-phosphatase-independent manner.

Intravital Multiphoton Microscopy with Fluorescent Bile Salts in Rats as an In Vivo Biomarker for Hepatobiliary Transport Inhibition

The bile salt export pump (BSEP) is expressed at the canalicular domain of hepatocytes, where it mediates the elimination of monovalent bile salts into the bile. Inhibition of BSEP is considered a susceptibility factor for drug-induced liver injury that often goes undetected during nonclinical testing. Although in vitro assays exist for screening BSEP inhibition, a reliable and specific method for confirming Bsep inhibition in vivo would be a valuable follow up to a BSEP screening strategy, helping to put a translatable context around in vitro inhibition data, incorporating processes such as metabolism, protein binding, and other exposure properties that are lacking in most in vitro BSEP models. Here, we describe studies in which methods of quantitative intravital microscopy were used to identify dose-dependent effects of two known BSEP/Bsep inhibitors, 2-[4-[4-(butylcarbamoyl)-2-[(2,4-dichlorophenyl)sulfonylamino]phenoxy]-3-methoxyphenyl]acetic acid (AMG-009) and bosentan, on hepatocellular transport of the fluorescent bile salts cholylglycyl amidofluorescein and cholyl-lysyl-fluorescein in rats. Results of these studies demonstrate that the intravital microscopy approach is capable of detecting Bsep inhibition at drug doses well below those found to increase serum bile acid levels, and also indicate that basolateral efflux transporters play a significant role in preventing cytosolic accumulation of bile acids under conditions of Bsep inhibition in rats. Studies of this kind can both improve our understanding of exposures needed to inhibit Bsep in vivo and provide unique insights into drug effects in ways that can improve our ability interpret animal studies for the prediction of human drug hepatotoxicity.

Using quantitative intravital multiphoton microscopy to dissect hepatic transport in rats

Hepatic solute transport is a complex process whose disruption is associated with liver disease and drug-induced liver injury. Intravital multiphoton fluorescence excitation microscopy provides the spatial and temporal resolution necessary to characterize hepatic transport at the level of individual hepatocytes in vivo and thus to identify the mechanisms and cellular consequences of cholestasis. Here we present an overview of the use of fluorescence microscopy for studies of hepatic transport in living animals, and describe how we have combined methods of intravital microscopy and digital image analysis to dissect the effects of drugs and pathological conditions on the function of hepatic transporters in vivo.

Quantification of Drug-Induced Inhibition of Canalicular Cholyl-l-Lysyl-Fluorescein Excretion From Hepatocytes by High Content Cell Imaging

We describe the use of a commercially available high content cell imaging algorithm (Cellomics Arrayscan Spot Detector) to quantify biliary excretion of the fluorescent probe substrate cholyl-l-lysyl-fluorescein (CLF) from rat hepatocytes cultured in collagen/matrigel sandwich configuration and to explore inhibition of this process by a variety of test compounds. The method provided robust, reproducible data. Twenty-nine pharmaceuticals inhibited biliary CLF efflux from hepatocytes and a broad range of potencies of inhibition were observed (IC50 values ranged between <1 and 794 µM). Thirteen drugs that inhibited CLF efflux also inhibited hepatocellular uptake of the probe substrate [(3)H]-taurocholate. Although no clear correlation between the potencies of inhibition of the 2 processes was evident, these data highlight the need to consider possible uptake transporter inhibition when interpreting hepatocyte CLF inhibition data. It has been reported that CLF is transported by MRP2. The CLF efflux inhibition data correlated closely with published data on inhibition by the drugs of the bile salt export pump (Bsep), which suggests that the tested drugs inhibit both Bsep and Mrp2. Calculation of the ratios between the maximum human plasma concentrations of the drugs and their CLF efflux inhibition IC50 values raised the possibility that for many, but not all, of them the in vitro effects may be functionally significant in vivo and that Mrp2 inhibition might be a drug-induced liver injury (DILI) risk factor. These data indicate that imaging hepatocyte CLF inhibition is a promising new method for quantification of biliary efflux inhibition by drugs, which could aid assessment of compound-related DILI risk.

Effects of Pringle maneuver and partial hepatectomy on the pharmacokinetics and blood-brain barrier permeability of sodium fluorescein in rats

Liver diseases are known to affect the function of remote organs. The aim of the present study was to investigate the effects of Pringle maneuver, which results in hepatic ischemia-reperfusion (IR) injury, and partial hepatectomy (Hx) on the pharmacokinetics and brain distribution of sodium fluorescein (FL), which is a widely used marker of blood-brain barrier (BBB) permeability. Rats were subjected to Pringle maneuver (total hepatic ischemia) for 20 min with (HxIR) or without (IR) 70% hepatectomy. Sham-operated animals underwent laparotomy only. After 15 min or 8h of reperfusion, a single 25-mg/kg dose of FL was injected intravenously and serial (0-30 min) blood and bile and terminal brain samples were collected. Total and free (ultrafiltration) plasma, total brain homogenate, and bile concentrations of FL and/or its glucuronidated metabolite (FL-Glu) were determined by HPLC. Both IR and HxIR caused significant reductions in the biliary excretions of FL and FL-Glu, resulting in significant increases in the plasma AUC of the marker. Additionally, the free fraction of FL in plasma was significantly increased by HxIR. Although the brain concentrations of FL were increased by almost twofold in both IR and HxIR animals, the brain concentrations corrected by the free FL AUC (and not the total AUC) were similar in both groups at either time points. It is concluded that Pringle maneuver and/or partial hepatectomy substantially alters the hepatobiliary disposition, plasma AUC, plasma free fraction, and brain accumulation of FL without altering the BBB permeability to the marker.

Effects of short-term portacaval anastomosis on the peripheral and brain disposition of the blood-brain barrier permeability marker sodium fluorescein in rats

Contradictory results have been reported with regard to the effects of various models of hepatic encephalopathy on the blood-brain barrier (BBB) permeability, which may be due partly to the use of brain concentrations of BBB markers without attention to their peripheral pharmacokinetics. The purpose of the current study was to investigate the effects of short-term portacaval anastomosis (PCA), a type B model of hepatic encephalopathy, on the peripheral pharmacokinetics and brain distribution of sodium fluorescein (FL), which is a small molecule marker of BBB passive permeability. A single 25mg/kg dose of FL was administered intravenously to 10-day PCA and sham-operated rats, and serial blood and bile (0-30min) and terminal (30min) brain samples were collected, and the concentrations of FL and its glucuronidated metabolite (FL-Glu) were measured by HPLC. Additionally, the free fractions of FL (fu) in all the plasma samples were determined, and the effects of bile salts on fu were investigated in vitro. Passive permeability of BBB to FL was estimated by brain uptake clearance (Kin) based on both the brain concentrations of FL and plasma concentrations of free (unbound) FL. PCA caused a 26% increase in the fu of FL in plasma, which was due to competition of bile acids with FL for binding to plasma proteins. Additionally, PCA reduced the biliary excretion of FL-Glu by 55%. However, free Kin values (µl/min/g brain) for the sham (0.265±0.034) and PCA (0.228±0.038) rats were not significantly different. It is concluded that whereas 10-day PCA alters the peripheral pharmacokinetics of FL, it does not significantly affect the BBB permeability to the marker.

Primary hepatocyte cultures as prominent in vitro tools to study hepatic drug transporters

Before any drug can be placed on the market, drug efficacy and safety must be ensured through rigorous testing. Animal models are used for this purpose, though currently increasing attention goes to the use of alternative in vitro systems. In particular, liver-based testing platforms that allow the prediction of pharmacokinetic (PK) and pharmacotoxicological properties during the early phase of drug development are of interest. They also enable the screening of potential effects on hepatic drug transporters. The latter are known to affect drug metabolism and disposition, thereby possibly underlying drug-drug interactions, which, in turn, may result in liver toxicity. Clearly, stable in vivo-like functional expression of drug transporters in hepatic in vitro settings is a prerequisite to be applicable in routine PK and pharmacotoxicological testing. In the first part of the article, an updated overview of hepatic drug transporters is provided, followed by a state-of-the-art review of drug-transporter production and activity in primary hepatocyte cultures (PHCs), being the gold-standard in vitro system. Specific focus is hereby put on strategies to maintain long-term functional expression, in casu of drug transporters, in these systems. In the second part, the use of PHCs to assess hepatobiliary transport and transporter-mediated interactions is outlined.

Prevention of estradiol 17beta-D-glucuronide-induced canalicular transporter internalization by hormonal modulation of cAMP in rat hepatocytes

In estradiol 17β-d-glucuronide (E17G)-induced cholestasis, the canalicular hepatocellular transporters bile salt export pump (Abcb11) and multidrug-resistance associated protein 2 (Abcc2) undergo endocytic internalization. cAMP stimulates the trafficking of transporter-containing vesicles to the apical membrane and is able to prevent internalization of these transporters in estrogen-induced cholestasis. Hepatocyte levels of cAMP are regulated by hormones such as glucagon and adrenaline (via the β2 receptor). We analyzed the effects of glucagon and salbutamol (a β2 adrenergic agonist) on function and localization of Abcb11 and Abcc2 in isolated rat hepatocyte couplets exposed to E17G and compared the mechanistic bases of their effects. Glucagon and salbutamol partially prevented the impairment in Abcb11 and Abcc2 transport capacity. E17G also induced endocytic internalization of Abcb11 and Abcc2, which partially colocalized with the endosomal marker Rab11a. This effect was completely prevented by salbutamol, whereas some transporter-containing vesicles remained internalized and mainly colocalizing with Rab11a in the perinuclear region after incubation with glucagon. Glucagon prevention was dependent on cAMP-dependent protein kinase (PKA) and independent of exchange proteins activated directly by cAMP (Epac) and microtubules. In contrast, salbutamol prevention was PKA independent and Epac/MEK and microtubule dependent. Anticholestatic effects of glucagon and salbutamol were additive in nature. Our results show that increases in cAMP could activate different anticholestatic signaling pathways, depending on the hormonal mediator involved.

ABCC2/Abcc2: a multispecific transporter with dominant excretory functions

ABCC2/Abcc2 (MRP2/Mrp2) is expressed at major physiological barriers, such as the canalicular membrane of liver cells, kidney proximal tubule epithelial cells, enterocytes of the small and large intestine, and syncytiotrophoblast of the placenta. ABCC2/Abcc2 always localizes in the apical membranes. Although ABCC2/Abcc2 transports a variety of amphiphilic anions that belong to different classes of molecules, such as endogenous compounds (e.g., bilirubin-glucuronides), drugs, toxic chemicals, nutraceuticals, and their conjugates, it displays a preference for phase II conjugates. Phenotypically, the most obvious consequence of mutations in ABCC2 that lead to Dubin-Johnson syndrome is conjugate hyperbilirubinemia. ABCC2/Abcc2 harbors multiple binding sites and displays complex transport kinetics.

Sandwich-cultured hepatocytes: an in vitro model to evaluate hepatobiliary transporter-based drug interactions and hepatotoxicity

Sandwich-cultured hepatocytes (SCH) are a powerful in vitro tool that can be utilized to study hepatobiliary drug transport, species differences in drug transport, transport protein regulation, drug-drug interactions, and hepatotoxicity. This review provides an up-to-date summary of the SCH model, including a brief history of, and introduction to, the use of SCH, as well as methodology to evaluate hepatobiliary drug disposition. A summary of the literature that has utilized this model to examine the interplay between drug-metabolizing enzymes and transport proteins, drug-drug interactions at the transport level, and hepatotoxicity as a result of altered hepatic transport also is provided.

Liver fibrosis in biliary atresia

Biliary atresia (BA) is the most common cholestatic liver disorder requiring liver transplantation in children. Hepatic fibrosis is not only a universal and prominent feature of BA, it is also the most important predictor of outcome following portoenterostomy (PE). Without PE, the progression of hepatic fibrosis is quite dramatic, such that liver cirrhosis is established within a few weeks after birth. Etiologies and molecular networks underpinning such an expeditious fibrogenic process have not been well established. However, immune and nonimmune factors implicated in the pathogenesis of BA, and the resultant cholestasis and oxidative stress, appear to be the main triggers of hepatic fibrosis in BA. Owing to a lack of validated noninvasive tools to monitor liver fibrosis, current prognostic models of BA entail clinical and biochemical variables reflecting liver dysfunction rather than hepatic fibrogenesis. Further work is necessary to validate the results of preliminary studies indicating a good relationship between liver fibrosis determined by transient elastography and other clinical and routinely performed biochemical parameters in pediatric patients. Although a prime candidate for a number of antifibrotic therapies on the horizon, owing to poor understanding of molecular mechanisms, a clear framework of antifibrotic targets has not been outlined in BA. Similarly, specific antifibrotic therapies have not yet been incorporated in clinical practice, limiting these measures to prompt diagnosis and PE operation, prevention and treatment of cholangitis and optimal nutritional support including the administration of fat-soluble vitamins.

Hepatic transport mechanisms of cholyl-L-lysyl-fluorescein

Cholyl-L-lysyl-fluorescein (CLF) is a fluorescent bile salt derivative that is being developed as an agent for determining in vivo liver function. However, the mechanisms of uptake and excretion by hepatocytes have not been rigorously studied. We have directly assessed the transport capacity of various hepatobiliary transporters for CLF. Uptake experiments were performed in Chinese hamster ovary cells transfected with human NTCP, OATP1B1, OATP1B3, and OATP2B1. Conversely, excretory systems were tested with plasma membrane vesicles from Sf21 insect cells expressing human ABCB11, ABCC2, ABCC3, and ABCG2. In addition, plasma clearance and biliary excretion of CLF were examined in wild-type, Abcc2(-/-), and Abcc3(-/-) mice. Human Na(+)-dependent taurocholic-cotransporting polypeptide (NTCP) and ATP-binding cassette B11 (ABCB11) were incapable of transporting CLF. In contrast, high-affinity transport of CLF was observed for organic anion-transporting polypeptide 1B3 (OATP1B3), ABCC2, and ABCC3 with K(m) values of 4.6 +/- 2.7, 3.3 +/- 2.0, and 3.7 +/- 1.0 microM, respectively. In Abcc2(-/-) mice biliary excretion of CLF was strongly reduced compared with wild-type mice. This resulted in a much higher hepatic retention of CLF in Abcc2(-/-) versus wild-type mice: 64 versus 1% of the administered dose (2 h after administration). In mice intestinal uptake of CLF was negligible compared with that of taurocholate. Our conclusion is that human NTCP and ABCB11 are incapable of transporting CLF, whereas OATP1B3 and ABCC2/Abcc2 most likely mediate hepatic uptake and biliary excretion of CLF, respectively. CLF can be transported back into the blood by ABCC3. Enterohepatic circulation of CLF is minimal. This renders CLF suitable as an agent for assessing in vivo liver function.

Ca(2+)-dependent protein kinase C isoforms are critical to estradiol 17beta-D-glucuronide-induced cholestasis in the rat

The endogenous estradiol metabolite estradiol 17beta-D-glucuronide (E(2)17G) induces an acute cholestasis in rat liver coincident with retrieval of the canalicular transporters bile salt export pump (Bsep, Abcc11) and multidrug resistance-associated protein 2 (Mrp2, Abcc2) and their associated loss of function. We assessed the participation of Ca(2+)-dependent protein kinase C isoforms (cPKC) in the cholestatic manifestations of E(2)17G in perfused rat liver (PRL) and in isolated rat hepatocyte couplets (IRHCs). In PRL, E(2)17G (2 mumol/liver; intraportal, single injection) maximally decreased bile flow, total glutathione, and [(3)H] taurocholate excretion by 61%, 62%, and 79%, respectively; incorporation of the specific cPKC inhibitor Gö6976 (500 nM) in the perfusate almost totally prevented these decreases. In dose-response studies using IRHC, E(2)17G (3.75-800 muM) decreased the canalicular vacuolar accumulation of the Bsep substrate cholyl-lysylfluorescein with an IC50 of 54.9 +/- 7.9 muM. Gö6976 (1 muM) increased the IC50 to 178.4 +/- 23.1 muM, and similarly prevented the decrease in the canalicular vacuolar accumulation of the Mrp2 substrate, glutathione methylfluorescein. Prevention of these changes by Gö6976 coincided with complete protection against E(2)17G-induced retrieval of Bsep and Mrp2 from the canalicular membrane, as detected both in the PRL and IRHC. E(2)17G also increased paracellular permeability in IRHC, which was only partially prevented by Gö6976. The cPKC isoform PKCalpha, but not the Ca(2+)-independent PKC isoform, PKCepsilon, translocated to the plasma membrane after E(2)17G administration in primary cultured rat hepatocytes; Gö6976 completely prevented this translocation, thus indicating specific activation of cPKC. This is consistent with increased autophosphorylation of cPKC by E(2)17G, as detected via western blotting.

CONCLUSION

Our findings support a central role for cPKC isoforms in E(2)17G-induced cholestasis, by inducing both transporter retrieval from the canalicular membrane and opening of the paracellular route.

Oxidative Stress Induces Internalization of the Bile Salt Export Pump, Bsep, and Bile Salt Secretory Failure in Isolated Rat Hepatocyte Couplets: A Role for Protein Kinase C and Prevention by Protein Kinase A

We have shown that Ca2+-mediated protein kinase C (PKC) activation induces impairment of bile salt secretory function and F-actin redistribution in hepatocyte couplets. Because oxidative stress induces Ca2+ elevation, we tested here whether PKC inhibition or protein kinase A (PKA) activation, which often counteracts PKC-dependent effects, can prevent and reverse these alterations. The pro-oxidant compounds tert-butylhydroperoxide (tBOOH, 100 microM) and 2,3-dimethoxy-1,4-naphthoquinone (30 microM), reduced by -41% and -29%, respectively, the percentage of couplets accumulating the fluorescent bile salt analog, cholyl-lysylfluorescein in their canalicular vacuoles (p < 0.01). tBOOH-induced bile salt secretory failure was accompanied by internalization of the canalicular bile salt export pump (Bsep), and disarrangement of cytoskeletal F-actin. All these deleterious effects were fully prevented by the intracellular Ca2+ chelator BAPTA/AM (20 microM), the pan-specific PKC inhibitors H7 (100 microM) and staurosporine (1 microM), the inhibitor of Ca2+-dependent PKCs, Gö6976 (2 microM), and the PKA activator dibutyryl-cAMP (500 microM). H7, Gö6976, and dibutyryl-cAMP not only prevented but also fully reversed the decrease in the cholyl-lysyl-fluorescein accumulation. In conclusion, these results suggest that low levels of oxidative stress impair bile salt secretion by internalizing Bsep through a Ca2+-dependent, PKC-mediated mechanism, and that inhibition of PKC, or activation of PKA, prevents and reverses these effects. Alterations in actin organization may be a causal factor.

Tauroursodeoxycholic acid protects rat hepatocytes from bile acid-induced apoptosis via activation of survival pathways

Ursodeoxycholic acid (UDCA) is used in the treatment of cholestatic liver diseases, but its mechanism of action is not yet well defined. The aim of this study was to explore the protective mechanisms of the taurine-conjugate of UDCA (tauroursodeoxycholic acid [TUDCA]) against glycochenodeoxycholic acid (GCDCA)-induced apoptosis in primary cultures of rat hepatocytes. Hepatocytes were exposed to GCDCA, TUDCA, the glyco-conjugate of UDCA (GUDCA), and TCDCA. The phosphatidylinositol-3 kinase pathway (PI3K) and nuclear factor-kappaB were inhibited using LY 294002 and adenoviral overexpression of dominant-negative IkappaB, respectively. The role of p38 and extracellular signal-regulated protein kinase mitogen-activated protein kinase (MAPK) pathways were investigated using the inhibitors SB 203580 and U0 126 and Western blot analysis. Transcription was blocked by actinomycin-D. Apoptosis was determined by measuring caspase-3, -9, and -8 activity using fluorimetric enzyme detection, Western blot analysis, immunocytochemistry, and nuclear morphological analysis. Our results demonstrated that uptake of GCDCA is needed for apoptosis induction. TUDCA, but not TCDCA and GUDCA, rapidly inhibited, but did not delay, apoptosis at all time points tested. However, the protective effect of TUDCA was independent of its inhibition of caspase-8. Up to 6 hours of preincubation with TUDCA before addition of GCDCA clearly decreased GCDCA-induced apoptosis. At up to 1.5 hours after exposure with GCDCA, the addition of TUDCA was still protective. This protection was dependent on activation of p38, ERK MAPK, and PI3K pathways, but independent of competition on the cell membrane, NF-kappaB activation, and transcription. In conclusion, TUDCA contributes to the protection against GCDCA-induced mitochondria-controlled apoptosis by activating survival pathways.

Hepatobiliary elimination of bile acid-modified oligodeoxynucleotides in Wistar and TR- rats: evidence for mrp2 as carrier for oligodeoxynucleotides

As therapeutic antisense tools, oligonucleotides (ODNs) must enter cells to bind to their target structures. ODNs distribute in nearly each tissue with relatively high concentrations in kidney and liver from where excretion into urine and bile occurs. To investigate mechanisms involved in hepatic ODN transport, normal mixed backbone phosphodiester/phosphorothioate ODNs (n-ODN) and two different bile acid-conjugated mixed backbone ODNs (1BA-ODN and 2BA-ODN) were applied to two different rat strains, normal Wistar rats and Wistar TR- rats. In normal Wistar rats, concentration-dependent hepatobiliary elimination of the ODNs was observed with a remarkable increase of excretion of the cholic acid BA-ODN conjugates. In contrast to normal Wistar rats, n-ODN excretion into bile by TR- rats, a mutant Wistar rat strain lacking a functional multidrug resistance-associated protein 2 (mrp2) at the canalicular membrane, was strongly diminished, whereas these rats excreted an ODN conjugated with two cholic acid molecules (2BA-ODN) into bile. Concomitant application of substrates transported by mrp2 such as bromosulfophthalein (BSP) or the synthetic chlorogenic acid derivative S 3025 significantly reduced the biliary appearance of normal ODN and 2BA-ODN in Wistar rats and also in TR- rats. To inhibit the expression of cRNA derived from the Na+ -dependent taurocholate cotransporting polypeptide (Ntcp), antisense ODNs were constructed which fully retained the antisense properties when coupled with two bile acid molecules. The results indicate that ODNs are secreted via the mrp2 into bile. In the absence of mrp2, further excretory transport systems with affinity for bile acids seem to be relevant for their excretion. The results further indicate that bile acid tagged ODNs are useful tools for liver specific antisense therapy.

Prolonged blood glucose reduction in mrp-2 deficient rats (GY/TR(-)) by the glucose-6-phosphate translocase inhibitor S 3025

Chlorogenic acid derivatives are potent inhibitors of hepatic glucose production by inhibition of the glucose-6-phosphate translocase component of the hepatic glucose-6-phosphatase system. The pharmacological proof of concept was clearly demonstrated during i.v. infusion of potent derivatives (S 4048, S 3483) in rats. However, the blood glucose lowering effect of S 4048 after bolus i.v. injection lasted only 60-90 min. Plasma clearance of S 4048 was very high, and the parent compound was rapidly and efficiently excreted into the bile of Wistar and GY/TR(-) rats, indicating that mrp-2 was not involved in this hepatobiliary elimination process. About 72% of the total administered radioactivity appeared in the bile within 20 min after i.v. bolus injection of the radiolabeled analogue [(3)H]S 1743 in a Wistar rat. However, in GY/TR(-) rats the dicarboxylic analogue of S 4048, S 3025, was cleared from the plasma less rapidly than its parent compound and its biliary elimination was comparatively low. In contrast, S 3025 exhibited comparable pharmacokinetics and biliary elimination profile as S 4048 in Wistar rats, suggesting that biliary elimination of S 3025 is facilitated by mrp-2, functionally absent in GY/TR(-) rats. Targeting to mrp-2 resulted in a significantly prolonged reduction of blood glucose levels in GY/TR(-) rats after i.v. bolus administration of S 3025.

Effect of tauroursodeoxycholate and S-adenosyl-L-methionine on 17beta-estradiol glucuronide-induced cholestasis

BACKGROUND/AIMS

S-adenosyl-L-methionine (SAMe) and tauroursodeoxycholate (TUDC) exert an additive ameliorating effect on taurolithocholate (TLC)-induced cholestasis. The aims were to investigate the protective effect of SAMe on 17beta-estradiol-glucuronide (17betaEG) cholestasis and to find out whether SAMe and TUDC may exert an additive, ameliorating effect.

METHODS

Hepatocyte couplet function was assessed by canalicular vacuolar accumulation (cVA) of cholyllysylfluorescein (CLF). Cells were co-treated with 17betaEG and SAMe, TUDC, or both (protection study), or treated with 17betaEG and then with SAMe, TUDC or both (reversion study) before CLF uptake. Couplets were also co-treated with SAMe and dehydroepiandrosterone (DHEA), a competitive substrate for the sulfotransferase involved in 17betaEG detoxification. The effects of 17betaEG, SAMe and TUDC were also examined on intracellular distribution of F-actin.

RESULTS

Both SAMe and TUDC significantly protected against, and reversed, 17betaEG-induced cholestasis, but their effects were not additive. DHEA abolished the protective effect of SAMe. 17BetaEG did not affect the uptake of CLF into hepatocytes at the concentrations used, and also, it did not affect the intracellular distribution of F-actin.

CONCLUSIONS

17BetaEG does not affect the uptake of CLF into hepatocytes. SAMe and TUDC protect and reverse 17betaEG-induced cholestasis, but without an additive effect. Protection by SAMe may involve facilitating the sulfation of 17betaEG.

Visualization of the transport of primary and secondary bile acids across liver tissue in rats: in vivo study with fluorescent bile acids

BACKGROUND/AIMS

Lysyl fluorescein conjugated bile acid analogues (LFCBAA) closely parallel their natural counterparts. To assess LFCBAA as a tool for the visualization of bile acid transport within liver tissue.

METHODS

Wistar rats were administered physiological concentrations of the primary bile acid analogue cholyllysyl fluoroscein (CLF) and of the secondary bile acid analogue lithocholyllysyl fluorescein (LLF) and serial liver biopsies were taken at fixed intervals. Both compounds were also injected retrogradely into the biliary tree. Frozen sections were examined by fluorescence microscopy.

RESULTS

Both CLF and LLF were rapidly taken up from sinusoidal blood but differed significantly in their hepatic handling. CLF was rapidly transported into bile, whereas LLF transport was slower and produced significantly more bile duct fluorescence. LLF clearance showed a lobular gradient with last remaining bile acid being confined largely to zone 3. Both compounds were avidly taken up by cholangiocytes after injection intravenously or retrogradely into the biliary tree.

CONCLUSIONS

Visualization of LFCBAA by fluorescence microscopy may yield further information regarding hepatobiliary bile acid localization during studies of physiological and pathological mechanisms involved in transport of bile acids. The presence of both compounds within cholangiocytes strongly suggests that they may undergo a degree of chole-hepatic recirculation.