Importance of cytokines, nitric oxide, and apoptosis in the pathological process of necrotizing pancreatitis in rats

OBJECTIVES

Ischemia-reperfusion injury can be involved in the pathophysiology of acute necrotizing pancreatitis. The aim of our study was to determine the production of cytokines, tumor necrosis factor (TNF) and interleukin-6 (IL-6), the activation of the inducible nitric oxide synthase (iNOS), and the development of apoptosis during this pathologic process.

METHODS

Acute pancreatitis was produced in male Wistar rats by injection of 200 microL of 6% taurocholic acid into the main pancreatic duct in combination with the temporary (15 minutes) occlusion of the inferior splenic artery. Six and 24 hours later, the histologic damage was evaluated, and serum amylase, TNF, IL-6 levels, and iNOS and apoptotic activity from pancreatic and pulmonary tissues were determined.

RESULTS

Twenty-four hours after the induction of pancreatitis, the mortality rate was 63%. During this period, the serum TNF and IL-6 levels were permanently high (50 +/- 12 and 58 +/- 10 U/mL and 7083 +/- 1610 and 6790 +/- 850 U/mL after 6 and 24 hours, respectively). The iNOS activity showed an increasing tendency in the pancreas, and a decrease following an initial increase in the lung (from 4.2 +/- 0.6 to 5 +/- 0.4 and from 6.8 +/- 0.6 to 3.8 +/- 0.5 pmol/min/mg protein after 6 and 24 hours, respectively). Histologic examination confirmed severe necrotizing pancreatitis. In the pancreas, the apoptotic activity increased significantly (from 4 +/- 4 to 27 +/- 5/mm at 6 and 24 hours), while in the lungs, following an initial increase it declined during the course of necrotizing pancreatitis (from 49 +/- 4 to 11 +/- 6/mm at 6 and 24 hours).

CONCLUSION

Our results indicate that intraductal taurocholic acid and ischemia-reperfusion provokes severe acute necrotizing pancreatitis with a high mortality rate and leads to systemic inflammatory reaction, which appears to be the consequence of the activation of the cytokine cascade and iNOS. The degree of NO overproduction by iNOS corresponds with the apoptotic process in the pancreas and the lung.

Taurocholate transport by hepatic and intestinal bile acid transporters is independent of FIC1 overexpression in Madin-Darby canine kidney cells

BACKGROUND

Mutations in the human familial intrahepatic cholestasis gene, FIC1, result in progressive familial intrahepatic cholestasis type 1 in children and benign recurrent intrahepatic cholestasis. The present study was performed to determine whether FIC1 transports bile acids and/or influences the activity of apical bile acid transporters.

METHODS

The apical secretion assay utilized transfected Madin-Darby canine kidney (MDCK) cells, which stably express the bile acid uptake protein, Na+/taurocholate co-transporting polypeptide (NTCP). These cells were then transiently transfected with FIC1 and/or the bile salt export pump (BSEP) and were grown on Transwell filters to form a polarized monolayer. [(3)H]Taurocholate was added to the basal medium and the taurocholate secretion was measured in the apical medium. A second assay, apical uptake assays, utilized polarized MDCK-II cells, which were transiently transfected with FIC1, FIC1 mutants and/or the apical sodium-dependent bile acid transporter (ASBT). [(3)H]Taurocholate was added to the apical media and intracellular uptake of taurocholate was measured.

RESULTS

Apical secretion assays: FIC1 expression in MDCK/NTCP cells had no effect on taurocholate secretion compared to controls. In contrast, apical secretion of taurocholate in BSEP-transfected cells was approximately twofold higher than in non-transfected MDCK/NTCP cells (P < 0.01). The BSEP-mediated secretion was unaffected by co-transfection with FIC1. Apical uptake assays: taurocholate uptake in ASBT expressing cells was 6.5-fold higher than in controls and was unaffected by co-transfection of cells with FIC1 or FIC1 mutants.

CONCLUSIONS

These results indicate that FIC1 does not transport taurocholate and, when overexpressed in MDCK cells, had no effect on the function of BSEP or ABST.

Differential expression of cholangiocyte and ileal bile acid transporters following bile acid supplementation and depletion

AIM: We have previously demonstrated that cholangiocytes, the epithelial cells lining intrahepatic bile ducts, encode two functional bile acid transporters via alternative splicing of a single gene to facilitate bile acid vectorial transport. Cholangiocytes possess ASBT, an apical sodium-dependent bile acid transporter to take up bile acids, and t-ASBT, a basolateral alternatively spliced and truncated form of ASBT to efflux bile acids. Though hepatocyte and ileal bile acid transporters are in part regulated by the flux of bile acids, the effect of alterations in bile acid flux on the expression of t-ASBT in terminal ileocytes remains unclear. Thus, we tested the hypothesis that expression of ASBT and t-ASBT in cholangiocytes and ileocytes was regulated by bile acid flux.

METHODS: Expression of ASBT and t-ASBT message and protein in cholangiocytes and ileocytes isolated from pair-fed rats given control (C) and 1% taurocholate (TCA) or 5% cholestyramine (CY) enriched diets, were assessed by both quantitative RNase protection assays and quantitative immunoblotting. The data obtained from each of the control groups were pooled to reflect the changes observed following TCA and CY treatments with respect to the control diets. Cholangiocyte taurocholate uptake was determined using a novel microperfusion technique on intrahepatic bile duct units (IBDUs) derived from C, TCA and CY fed rats.

RESULTS: In cholangiocytes, both ASBT and t-ASBT message RNA and protein were significantly decreased in response to TCA feeding compared to C diet. In contrast, message and protein of both bile acid transporters significantly increased following CY feeding compared to C diet. In the ileum, TCA feeding significantly up-regulated both ASBT and t-ASBT message and protein compared to C diet, while CY feeding significantly down-regulated message and protein of both bile acid transporters compared to C diet. As anticipated from alterations in cholangiocyte ASBT expression, the uptake of taurocholate in microperfused IBDUs derived from rats on TCA diet decreased 2.7-fold, whereas it increased 1.7-fold in those on CY diet compared to C diet fed groups.

CONCLUSION: These data demonstrate that expression of ASBT and t-ASBT in cholangiocytes is regulated by a negative feedback loop while the expression of these transporters in terminal ileum is modified via positive feedback. Thus, while transcriptional regulatory mechanisms in response to alterations in bile acid pool size are operative in both cholangiocytes and ileocytes, each cell type responds differently to bile acid supplementation and depletion.

Effect of simulated intestinal fluid on drug permeability estimation across Caco-2 monolayers

Presently, the Caco-2 cell culture model is widely used during drug discovery and development as a predictive tool for the oral absorption of drug candidates. For transport experiments in the Caco-2 system, HBSS-like buffered salt solutions are commonly used, although different shortcomings have been associated with the use of these buffers. In this paper, we investigated the effect of using fasted state simulated intestinal fluid (FaSSIF) as potential biorelevant medium for the drug permeability estimation across Caco-2 monolayers. The transport characteristics of 19 model compounds were determined in the Caco-2 cell culture model in the presence of FaSSIF as compared to classic transport medium. A sigmoidal relation was obtained when the estimated P(app), s of the apical to basolateral transport were plotted versus the reported values of the fraction absorbed in man. Although no effect of FaSSIF as compared to classic transport medium (TM) was observed on the total predictability of the model, an impact was demonstrated (1) on the bi-directional transport of actively transported drugs (including talinolol, digoxin and doxorubicin), (2) on recovery and (3) on the solubility and permeability estimation of poorly water-soluble drugs. The observed differences may be attributed to a P-gp inhibitory effect of sodium taurocholate (NaTC), micellar encapsulation by the NaTC/lecithin mixed micelles and/or an increase of the solubility of lipophilic drugs. As the experimental conditions should mimic the physiological in vivo conditions, the use of FaSSIF as medium during Caco-2 experiments may improve the biorelevance of the model.

Ontogenetic development of rat intestinal bile acid transport requires thyroxine but not corticosterone

Absorption of bile acids by the distal ileum is an essential component of the enterohepatic circulation. In neonatal rats, the appearance of the apical sodium-dependent bile acid transporter (ASBT) at 17 d of age coincides with increases in serum corticosterone and thyroxine. We tested the hypothesis that these hormones modulate ASBT expression during ileal development. Taurocholate uptake into the isolated ileum of normal 20-d-old pups exhibited saturable (K(m) = 0.52 mM, J(max) = 0.34 pmol mg/min) and nonsaturable (K(diff) = 0.015 min(-1)) components and was two to five times greater than uptake in the proximal intestine. Hypothyroid or euthyroid pups received daily thyroxine injections starting at 6 d of age. At 12 d of age, serum concentrations of thyroxine, ileal abundance of ASBT mRNA, and ileal rates of taurocholate uptake were low in hypothyroid pups that received an injection of vehicle (HT-) or thyroxine (HT+) and in euthyroid pups that received an injection of vehicle (ET-) or thyroxine (ET+). At 20 and 26 d, ileal ASBT mRNA abundance and taurocholate uptake rate remained low in HT- pups but increased dramatically in ET- and ET+ pups, paralleling the increase in serum thyroxine. Restoration of normal plasma thyroxine in HT- pups by thyroxine injections (HT+) restored normal ASBT development. Sodium-glucose co-transporter activity and mRNA expression were independent of serum thyroxine levels. Corticosterone levels were significantly lower in pups that were adrenalectomized at 10 d of age. ASBT mRNA abundance and taurocholate uptake rate increased markedly with age but were the same in adrenalectomized, sham-operated, and nonoperated pups. Hence, endogenous thyroxine but not corticosterone regulates the developmentally timed appearance of ASBT.

CbsT2 from Lactobacillus johnsonii 100-100 Is a Transport Protein of the Major Facilitator Superfamily That Facilitates Bile Acid Antiport

We previously identified two conjugated bile acid transporters, CbsT1 and CbsT2, in Lactobacillus johnsonii 100-100 and Lactobacillus acidophilus KS-13 that are gene duplicates encoded in tandem with a conjugated bile salt hydrolase (BSH) [Elkins and Savage, J. Bacteriol. 180:4344-4349, 1998; Elkins et al., Microbiology 147: 3403-3412, 2001]. CbsT2 from 100-100 was shown to increase taurocholic acid (TCA) uptake in Escherichia coli; however, higher levels were achieved when an extracellular factor (EF) from 100-100 was present in the assay medium (spent medium from 100-100, pH 4.2). We continued this study here to determine the role of EF in this transport system. Kinetic studies revealed that the previously observed CbsT2- and EF-mediated TCA accumulation is rapid (< 15 s) but not saturable, suggesting that EF is limiting. In addition, uptake of TCA by E. coli expressing CbsT2 was insensitive to ionophores, 2,4-dinitrophenol and carbonyl cyanide m-chlorophenylhydrazone, and thus, is independent of the proton motive force. Since BSH converts [24-(14)C]TCA to [24-(14)C]cholic acid (CA), we measured net radiolabel uptake in E. coli cells expressing transporter(s) and BSH. Interestingly, such cells accumulated less 14C radiolabel (by approximately half) than cells expressing CbsT2 alone. These data can be explained if CA diffuses out of E. coli through the transporter(s). We, therefore, added exogenous, unlabeled CA to EF-spent media, which under our assay conditions, performed similarly to EF+ culture supernatant in TCA and CA uptake assays. Thus, unlabeled CA (a protonated, neutral lipophile) can partition directly into E. coli cells especially at low pH. These findings were validated in uptake assays with [3H]TCA, which yields [3H]taurine (a hydrophilic moiety) upon hydrolysis by the BSH. Amounts of cell-associated 3H radiolabel remained similar in cells expressing CbsT2 and BSH versus cells expressing only CbsT2, both of which were higher than in cells expressing BSH alone. Our data support a hypothesis that these transporters, which comprise a new subfamily of the major facilitator superfamily, facilitate antiport of TCA and CA.

Cross-talk between protein kinases Czeta and B in cyclic AMP-mediated sodium taurocholate co-transporting polypeptide translocation in hepatocytes

Cyclic AMP stimulates taurocholate (TC) uptake and sodium taurocholate co-transporting polypeptide (Ntcp) translocation in hepatocytes via the phosphoinositide-3 kinase (PI3K) signaling pathway. The aim of the present study was to determine whether protein kinase (PK) Czeta, one of the downstream mediators of the PI3K signaling pathway, is involved in cAMP-mediated stimulation of TC uptake. Studies were conducted in isolated rat hepatocytes and in HuH-7 cells stably transfected with rat liver Ntcp (HuH-Ntcp cells). Studies in hepatocytes showed that cAMP activates PKCzeta in a PI3K-dependent manner without inducing translocation of PKCzeta to the plasma membrane. Inhibition of cAMP-induced PKCzeta activity by myristoylated PKC (zeta/lambda) pseudosubstrate, a specific inhibitor of PKCzeta, and Gö 6850, a PKC inhibitor, resulted in inhibition of cAMP-induced increases in TC uptake and Ntcp translocation. Studies in HuH-Ntcp cells showed that inhibition of cAMP-induced PKCzeta activation by dominant-negative (DN) PKCzeta resulted in inhibition of cAMP-induced increases in TC uptake and Ntcp translocation. DN PKCzeta also inhibited wild-type PKCzeta-induced increases in PKCzeta activity, TC uptake, and Ntcp translocation. Myristoylated PKC (zeta/lambda) pseudosubstrate and DN PKCzeta also inhibited cAMP-induced activation of PKB in hepatocytes and HuH-Ntcp cells, respectively. Neither DN PKB nor constitutively active PKB affected cAMP-induced activation of PKCzeta, and wild-type PKCzeta did not activate PKB. Taken together, these results suggest that cAMP-induced activation of PKB is dependent on cAMP-induced stimulation of PKCzeta. It is proposed that cAMP-induced Ntcp translocation involves the activation of the PI3K/PKCzeta signaling pathway followed by the activation of the PI3K/PKB signaling pathway.

Molecular cloning and functional characterization of the bovine (Bos taurus) organic anion transporting polypeptide Oatp1a2 (Slco1a2)

We describe the cloning, functional characterization and tissue localization of a novel membrane transporter of the OATP/Oatp-gene family obtained from liver and kidney of cattle (Bos taurus). The carrier protein exhibits highest sequence identity to the human OATP1A2 (previously called OATP-A) and is, therefore, named bovine Oatp1a2. Bovine Oatp1a2 received the gene symbol Slco1a2 that is identical to the SLC classification of human OATP1A2 (SLCO1A2, previously called SLC21A3) and is likely an orthologue of the human gene. Two different full-length bOatp1a2 cDNAs of 2316-bp and 3504-bp were obtained and encoded for a 666 amino acid membrane protein, which contains twelve putative transmembrane spanning domains. Bovine Oatp1a2 expression was detected in liver, kidney, brain and adrenal gland. Uptake studies in cRNA-injected oocytes demonstrated that bOatp1a2 transports estrone-3-sulfate and taurocholate, with K(m) values of 9.6 microM and 51 microM, respectively, and estradiol-17beta-glucuronide. However, the structurally-related heart glycosides ouabain (1 microM) and digoxin (1 microM) are neither transported by bovine Oatp1a2 nor by human OATP1A2. We conclude that based on the tested substrates bovine Oatp1a2 shows functional homology to human OATP1A2.

Regulation of renal tubular bile acid transport in the early phase of an obstructive cholestasis in the rat

BACKGROUND/AIMS

In obstructive liver diseases, urinary excretion of bile acids is markedly enhanced. The mechanism of this effect is not entirely clear. The aim of the present study was to assess the glomerular and tubular factors involved in the renal handling of bile acids during the early phase of an obstructive cholestasis induced by a 24-hour bile duct ligation in rats.

METHODS

In addition to conventional clearance experiments, taurocholate transport was studied on proximal tubular cells freshly isolated from rat kidneys. The expression of the taurocholate transport protein was determined in these cells by immunoblotting.

RESULTS

Glomerular filtration rate and arterial blood pressure were not significantly affected by the cholestasis induced. The (3)H-taurocholate and (3)H-cholate clearances significantly increased whereas the fractional tubular reabsorption rates of these bile acids significantly fell. Probenecid did not affect the (3)H-taurocholate and (3)H-cholate clearances. By employing S0960, a specific inhibitor of the sodium-dependent bile salt transporter ASBT, it could be shown that this transporter mediates (3)H-taurocholate uptake into the proximal tubular cells. The bile duct ligation caused a significant decrease of the V(max) value of the taurocholate transporter indicating a downregulation of this transporter in cholestasis. The downregulation occurred without a change of the ASBT protein content of the proximal tubular cells.

CONCLUSION

The results demonstrate that there is a functional adaptive downregulation of renal tubular bile acid transport enhancing renal clearance of bile acids during the early phase of an obstructive cholestasis.

A mouse genetic model for familial cholestasis caused by ATP8B1 mutations reveals perturbed bile salt homeostasis but no impairment in bile secretion

Mutations in ATP8B1, a broadly expressed P-type ATPase, result, through unknown mechanisms, in disorders of bile secretion. These disorders vary in severity from mild and episodic to progressive with liver failure. We generated Atp8b1G308V/G308V mutant mice, which carry a mutation orthologous to that present in homozygous form in patients from the Amish index kindred for severe ATP8B1 disease. In contrast to human patients, Atp8b1(G308V/G308V) mice had unimpaired bile secretion and no liver damage, but showed mild abnormalities including depressed weight at weaning and elevated serum bile salt levels. We challenged the hepatobiliary metabolism of Atp8b1G308V/G308V mice by administering exogenous bile salts. Upon bile salt feeding, Atp8b1G308V/G308V mice, but not wild-types, demonstrated serum bile salt accumulation, hepatic injury and expansion of the systemic bile salt pool. Unexpectedly, this failure of bile salt homeostasis occurred in the absence of any defect in hepatic bile secretion. Upon infusion of a hydrophobic bile salt, wild-type mice developed cholestasis while Atp8b1G308V/G308V mice maintained high biliary output and more extensively rehydroxylated the infused bile salt. Increased bile salt hydroxylation, which reduces bile salt toxicity, may explain the milder phenotype in Atp8b1G308V/G308V mice compared with humans with the equivalent mutation. These results demonstrate the key role of Atp8b1 in bile salt homeostasis and highlight the importance of bile salt hydroxylation in the prevention of cholestasis. The mouse phenotype reveals that loss of Atp8b1 disrupts bile salt homeostasis without impairment of canalicular bile secretion; in humans this process is likely to be obscured by early onset of severe liver disease.

Alteration of the enterohepatic recirculation of bile acids in rats after exposure to ionizing radiation

The aim of this work was to study acute alterations of the enterohepatic recirculation (EHR) of bile acids 3 days after an 8-Gy radiation exposure in vivo in the rat by a washout technique. Using this technique in association with HPLC analysis, the EHR of the major individual bile acids was determined in control and irradiated animals. Ex vivo ileal taurocholate absorption was also studied in Ussing chambers. Major hepatic enzyme activities involved in bile acid synthesis were also measured. Measurements of bile acid intestinal content and intestinal absorption efficiency calculation from washout showed reduced intestinal absorption with significant differences from one bile acid to another: absorption of taurocholate and tauromuricholate was decreased, whereas absorption of the more hydrophobic taurochenodeoxycholate was increased, suggesting that intestinal passive diffusion was enhanced, whereas ileal active transport might be reduced. Basal hepatic secretion was increased only for taurocholate, in accordance with the marked increase of CYP8B1 activity in the liver. The results are clearly demonstrate that concomitantly with radiation-induced intestinal bile acid malabsorption, hepatic bile acid synthesis and secretion are also changed. A current working model for pathophysiological changes in enterohepatic recycling after irradiation is thus proposed.Key words: irradiation, bile acids, intestine, liver, enterohepatic recycling.

Organic anion transport in choroid plexus from wild-type and organic anion transporter 3 (Slc22a8)-null mice

The choroid plexus actively transports endogenous, xenobiotic, and therapeutic compounds from cerebrospinal fluid to blood, thereby limiting their exposure to the central nervous system (CNS). Establishing the mechanisms responsible for this transport is critical to our understanding of basic choroid plexus physiology and will likely impact drug targeting to the CNS. We recently generated an organic anion transporter 3- (Oat3)-null mouse, which exhibited loss of PAH, estrone sulfate, and taurocholate transport in kidney and of fluorescein (FL) transport in choroid plexus. Here, we measured the uptake of four Oat3 substrates by choroid plexus from wild-type and Oat3-null mice to establish 1) the contribution of Oat3 to the apical uptake of each substrate and 2) the Na dependence of transport by Oat3 in the intact tissue. Mediated transport of PAH and FL was essentially abolished in tissue from Oat3-null mice. In contrast, only a 33% reduction in estrone sulfate uptake was observed in tissue from Oat3-null mice and, surprisingly, no reduction in taurocholate uptake could be detected. For PAH, FL, and estrone sulfate, all Oat3-mediated transport was Na dependent. However, estrone sulfate and taurocholate also exhibited additional mediated and Na-dependent components of uptake that were not attributed to Oat3, demonstrating the complexity of organic anion transport in this tissue and the need for further examination of expressed transporters and their energetics.

Inhibition of apolipoprotein B secretion by taurocholate is controlled by the N-terminal end of the protein in rat hepatoma McArdle-RH7777 cells

Bile salts (BS) inhibit the secretion of apolipoprotein B (apoB) and triacylglycerol (TG) in primary rat, mouse and human hepatocytes and in mice in vivo. We investigated whether lipidation of apoB into a lipoprotein particle is required for this inhibitory action of BS. The sodium/taurocholate co-transporting polypeptide (Ntcp) was co-expressed in McArdle-RH7777 (McA-RH7777) cells stably expressing the full-length human apoB100 (h-apoB100, secreted as TG-rich lipoprotein particles) or carboxyl-truncated human apoB18 (h-apoB18, secreted in lipid-free form). The doubly transfected cell lines (h-apoB/r-Ntcp) effectively accumulated taurocholic acid (TC). TC incubation decreased the secretion of endogenous rat apoB100 (-50%) and h-apoB18 (-35%), but did not affect secretion of rat apoA-I. Pulse-chase experiments (35S-methionine) indicated that the impaired secretion of radiolabeled h-apoB18 and h-apoB100 was associated with accelerated intracellular degradation. The calpain protease inhibitor N-acetyl-leucyl-leucyl-norleucinal (ALLN) partially inhibited intracellular apoB degradation but did not affect the amount of either h-apoB18 or h-apoB100 secreted into the medium, indicating that inhibition of apoB secretion by TC is not due to calpain-dependent proteasomal degradation. We conclude that TC does not inhibit apoB secretion by interference with its lipidation, but rather involves a mechanism dependent on the N-terminal end of apoB.

Evaluation of hepatotoxic potential of drugs by inhibition of bile-acid transport in cultured primary human hepatocytes and intact rats

Inhibition of canalicular bile acid efflux by medications is associated with clinical liver toxicity, sometimes in the absence of major liver effects in experimental species. To predict the hepatotoxic potential of compounds in vitro and in vivo, we investigated the effect of clinical cholestatic agents on [3H]taurocholic acid transport in regular and collagen-sandwich cultured human hepatocytes. Hepatocytes established a well-developed canalicular network with bile acid accumulating in the canalicular lumen within 15 min of addition to cells. Removing Ca2+ and Mg2+ from the incubation buffer destroyed canalicular junctions, resulting in bile acid efflux into the incubation buffer. Canalicular transport was calculated based on the difference between the amount of bile acid effluxed into the Ca/Mg2+-free and regular buffers with linear efflux up to 10 min. Hepatocytes cultured in the nonsandwich configuration also transported taurocholic acid, but at 50% the rate in sandwiched cultures. Cyclosporin A, bosentan, CI-1034, glyburide, erythromycin estolate, and troleandomycin inhibited efflux in a concentration-dependent manner. In contrast, new generation macrolide antibiotics with lower incidence of clinical hepatotoxicity were much less potent inhibitors of efflux. An in vivo study was conducted whereby glyburide or CI-1034, administered iv to male rats, produced a 2.4-fold increase in rat total serum bile acids. A synergistic 6.8-fold increase in serum total bile acids was found when both drugs were delivered together. These results provide methods to evaluate inhibitory effects of potentially cholestatic compounds on bile-acid transport, and to rank compounds according to their hepatotoxic potential.

Intestinal absorption of the bile acid analogue 75Se-homocholic acid-taurine is increased in primary biliary cirrhosis, and reverts to normal during ursodeoxycholic acid administration

BACKGROUND

Whether ileal absorption of bile acid is up or downregulated in chronic cholestasis is still debated, and most evidence has come from animal studies.

AIMS

To compare ileal bile acid absorption in patients with primary biliary cirrhosis (PBC) and in healthy control subjects, and to assess the effect of ursodeoxycholic acid (UDCA).

PATIENTS

We studied 14 PBC patients before and during (n=11) UDCA administration, 14 healthy control subjects, and 14 Crohn's disease patients (as disease controls).

METHODS

We used cholescintigraphy to measure retention in the enterohepatic circulation over five successive days of the bile acid analogue (75)Se-homocholic acid-taurine ((75)SeHCAT) as an index of ileal bile acid absorption. Results were expressed as (75)SeHCAT fractional turnover rate (FTR) and t(1/2)12.

RESULTS

(75)SeHCAT FTR was 0.19 (0.11)/day, 0.34 (0.11)/day (p<0.001), and 0.83 (0.32)/day in PBC patients, healthy controls (p<0.0001), and Crohn's patients (p<0.001), respectively, which increased to 0.36 (0.16)/day in PBC patients during UDCA treatment (p<0.005). (75)SeHCAT t(1/2)12 was 4.8 (2.1) days in PBC patients, 2.2 (0.5) days (p<0.001) in healthy controls, and 1.0 (0.5) days (p<0.001) in Crohn's disease patients. (75)SeHCAT t(1/2)12 decreased to 2.2 (0.93) days (p< 0.001) in PBC patients during UDCA treatment.

CONCLUSIONS

Our results support the concept that ileal bile acid absorption is upregulated in PBC patients, and that this effect may contribute towards damaging the cholestatic liver. This upregulation of bile acid absorption is abolished by UDCA.

Characterization of the role of polar amino acid residues within predicted transmembrane helix 17 in determining the substrate specificity of multidrug resistance protein 3

Human multidrug resistance protein (MRP) 3 is the most closely related homologue of MRP1. Like MRP1, MRP3 confers resistance to etoposide (VP-16) and actively transports 17 beta-estradiol 17-(beta-D-glucuronide) (E(2)17 beta G), cysteinyl leukotriene 4 (LTC(4)), and methotrexate, although with generally lower affinity. Unlike MRP1, MRP3 also transports monovalent bile salts. We have previously demonstrated that hydrogen-bonding residues predicted to be in the inner-leaflet spanning segment of transmembrane (TM) 17 of MRP1 are important for drug resistance and E(2)17 beta G transport. We have now examined the importance of the hydrogen-bonding potential of residues in TM17 of MRP3 on both substrate specificity and overall activity. Mutation S1229A reduced only methotrexate transport. Mutations S1231A and N1241A decreased resistance to VP-16 and transport of E(2)17 beta G and methotrexate but not taurocholate. Mutation Q1235A also reduced resistance to VP-16 and transport of E(2)17beta G but increased taurocholate transport without affecting transport of methotrexate. Mutations Y1232F and S1233A reduced resistance to VP-16 and the transport of all three substrates tested. In contrast, mutation T1237A markedly increased VP-16 resistance and transport of all substrates. On the basis of the substrates analyzed, residues Ser(1229), Ser(1231), Gln(1235), and Asn(1241) play an important role in determining the specificity of MRP3, while mutation of Tyr(1232), Ser(1233), and Thr(1237) affects overall activity. Unlike MRP1, the involvement of polar residues in determining substrate specificity extends throughout the TM helix. Furthermore, elimination of the hydrogen-bonding potential of a single amino acid, Thr(1237), markedly enhanced the ability of the protein to confer drug resistance and to transport all substrates examined.

Mechanisms by which cAMP increases bile acid secretion in rat liver and canalicular membrane vesicles

Bile acid secretion induced by cAMP and taurocholate is associated with recruitment of several ATP binding cassette (ABC) transporters to the canalicular membrane. Taurocholate-mediated bile acid secretion and recruitment of ABC transporters are phosphatidylinositol 3-kinase (PI3K) dependent and require an intact microtubular apparatus. We examined mechanisms involved in cAMP-mediated bile acid secretion. Bile acid secretion induced by perfusion of rat liver with dibutyryl cAMP was blocked by colchicine and wortmannin, a PI3K inhibitor. Canalicular membrane vesicles isolated from cAMP-treated rats manifested increased ATP-dependent transport of taurocholate and PI3K activity that were reduced by prior in vivo administration of colchicine or wortmannin. Addition of a PI3K lipid product, phosphoinositide 3,4-bisphosphate, but not its isomer, phosphoinositide 4,5-bisphosphate, restored ATP-dependent taurocholate in these vesicles. Addition of a decapeptide that activates PI3K to canalicular membrane vesicles increased ATP-dependent transport above baseline activity. In contrast to effects induced by taurocholate, cAMP-stimulated intracellular trafficking of the canalicular ABC transporters was unaffected by wortmannin, and recruitment of multidrug resistance protein 2, but not bile salt excretory protein (bsep), was partially decreased by colchicine. These studies indicate that trafficking of bsep and other canalicular ABC transporters to the canalicular membrane in response to cAMP is independent of PI3K activity. In addition, PI3K lipid products are required for activation of bsep in the canalicular membrane. These observations prompt revision of current concepts regarding the role of cAMP and PI3K in intracellular trafficking, regulation of canalicular bsep, and bile acid secretion.

N-glycosylation controls functional activity of Oatp1, an organic anion transporter

Rat Oatp1 (Slc21a1) is an organic anion-transporting polypeptide believed to be an anion exchanger. To characterize its mechanism of transport, Oatp1 was expressed in Saccharomyces cerevisiae under control of the GAL1 promoter. Protein was present at high levels in isolated S. cerevisiae secretory vesicles but had minimal posttranslational modifications and failed to exhibit taurocholate transport activity. Apparent molecular mass (M) of Oatp1 in yeast was similar to that of unmodified protein, approximately 62 kDa, whereas in liver plasma membranes Oatp1 has an M of approximately 85 kDa. To assess whether underglycosylation of Oatp1 in yeast suppressed functional activity, Oatp1 was expressed in Xenopus laevis oocytes with and without tunicamycin, a glycosylation inhibitor. With tunicamycin, M of Oatp1 decreased from approximately 72 to approximately 62 kDa and transport activity was nearly abolished. Mutations to four predicted N-glycosylation sites on Oatp1 (Asn to Asp at positions 62, 124, 135, and 492) revealed a cumulative effect on function of Oatp1, leading to total loss of taurocholate transport activity when all glycosylation sites were removed. M of the quadruple mutant was approximately 62 kDa, confirming that these asparagine residues are sites of glycosylation in Oatp1. Relatively little of the quadruple mutant was able to reach the plasma membrane, and most remained in unidentified intracellular compartments. In contrast, two of the triple mutants tested (N62/124/135D and N124/135/492D) were present in the plasma membrane fraction yet exhibited minimal transport activity. These results demonstrate that both membrane targeting and functional activity of Oatp1 are controlled by the extent of N-glycosylation.

Identification and characterization of novel rat and human gonad-specific organic anion transporters

We have isolated three novel organic anion transporter cDNAs designated rat GST-1 (gonad-specific transporter), rat GST-2, and human GST, expressed at high levels in the testis. Rat GST-1, GST-2, and human GST consist of 748, 702, and 719 amino acids, respectively, and all molecules possess the 12 predicted transmembrane domains, which is a common structure of organic anion transporters. Northern blot analyses and in situ hybridization revealed that both of the rat molecules are highly expressed in the testis, especially in Sertoli cells, spermatogonia, and Leydig cells. Weak signals are also detected in the epididymis and ovary in adult rat. The exclusive expression of human GST mRNA in the testis was confirmed by RT-PCR. The pharmacological experiments of Xenopus laevis oocytes injected with the respective rat GST-1- and GST-2-cRNAs revealed that both rat GST-1 and GST-2 transport taurocholic acid, dehydroepiandrosterone sulfate, and T4 with Michaelis-Menten kinetics (taurocholic acid, Km = 8.9 and 2.5 microm, dehydroepiandrosterone sulfate, Km = 25.5 and 21.microm, and T4, Km = 6.4 and 5.8 for rat GST-1 and GST-2, respectively). T3 was also transported by rat GST-1 and GST-2. These data suggest that rat GST-1 and GST-2 might be one of the molecular entities responsible for transporting dehydroepiandrosterone sulfate and thyroid hormones involved in the regulation of sex steroid transportation and spermatogenesis in the gonad.

Inhibition of cholesterol crystallization under bilirubin deconjugation: partial characterization of mechanisms whereby infected bile accelerates pigment stone formation

Pigment gallstones have been reported to be closely associated with biliary tract infection. We previously reported that addition of unconjugated bilirubin (UCB), which is deconjugated by beta-glucuronidase in infected bile, could enhance cholesterol crystal formation in supersaturated model bile (MB). The present study evaluated the effect of beta-glucuronidase on the processes of pigment gallstone formation and cholesterol crystallization. Supersaturated MB (taurocholate/lecithin/cholesterol at 71:18:11, a total lipid concentration of 10.0 g/dl and a cholesterol saturation index (CSI) of 2.0) and native rat bile were mixed at a ratio of 3:1. Then, mixed bile was incubated with or without beta-glucuronidase and changes of the following parameters were investigated over time: (1) the UCB/total bilirubin ratio; (2) cholesterol crystal formation; (3) the precipitate weight and the cholesterol concentration in the precipitate and supernatant; and (4) the lipid distribution of vesicles in the supernatant. Compared with beta-glucuronidase-free bile, (1) beta-glucuronidase-containing bile showed a significant increase of the UCB/total bilirubin ratio, (2) as well as a significantly longer nucleation time (96+/-17.0 vs. 114+/-20.0) and fewer cholesterol crystals. (3) The precipitate weight and the cholesterol concentration in the precipitate were significantly increased, while the cholesterol concentration in supernatant was decreased. (4) When mixed bile was incubated with beta-glucuronidase, the cholesterol concentration in the vesicles was lower than in bile without beta-glucuronidase. The precipitate weight and the cholesterol concentration in the precipitate was increased by incubation with beta-glucuronidase, while cholesterol concentration was decreased in the supernatant (especially in the vesicles). This means that bile vesicles were more stable and it was more difficult for cholesterol crystals to form. Thus, the presence of beta-glucuronidase may inhibit the formation of pure cholesterol stones even in the presence of cholesterol supersaturation.

Retention of transporter activities in cryopreserved, isolated rat hepatocytes

The success of cryopreservation of isolated hepatocytes with existing methodologies is assessed with respect to the retentivity of cell integrity/viability (defined by trypan blue) and metabolic activities upon thawing in comparison to those of freshly prepared cells. But the ability of the cryopreserved cells to transport xenobiotics relative to that of freshly prepared cells has not been investigated. In this study, we optimized our previous methodology for cryopreservation and evaluated the metabolism and transport of thawed hepatocytes. Half of the freshly, isolated rat hepatocytes prepared by collagenase perfusion were immediately used for studies of transport of [(14)C]taurocholate, [(3)H]estrone sulfate and [(3)H]estradiol 17beta-D-glucuronide (1 microM) and metabolism of 7-hydroxy-4-(trifluoromethyl)-coumarin (100 microM), (3,4-difluorobenzyloxy)-5,5-dimethyl-4-(4-methylsulfonylphenyl)-(5H)-furan-2-one (250 microM), bufuralol (100 microM), and tolbutamide (100 microM), probes for UDP-glucuronyl transferase (UGT) and CYP3A, CYP2D, and CYP2C, respectively. The remaining half was cryopreserved using an optimized, programmed-freezing protocol, which was developed to minimize the prolonged release of latent heat during freezing. With the exception of the UGT probe, no significant difference (P > 0.05) was found in both metabolism and transport with freshly isolated versus cryopreserved hepatocytes upon thawing. In conclusion, we have demonstrated for the first time that thawed rat hepatocytes cryopreserved by a programmed-freezing protocol retain drug transport activities.

The relative ligand binding preference of the murine ileal lipid binding protein

The ileal lipid binding protein (ILBP), a member of the intracellular lipid binding protein family, is a 14-kDa protein that has bile and fatty acids as possible physiological ligands. The ligand binding specificity of this protein is not well characterized. Therefore, we studied the lipid binding activity of purified recombinant murine ILBP (mILBP) in vitro. These studies demonstrated by direct analysis the interaction of mILBP with naturally occurring bile and fatty acids. The rank order of binding preference for fatty acids, or unconjugated and conjugated bile acids, was assessed. Among fatty acids, mILBP preferred species that had longer chain length and increased saturation, similar to other members of the intracellular lipid binding protein family. Among the bile acids, mILBP showed the greatest preference for conjugated species that contained a doubly hydroxylated steroid moiety. The results demonstrate that mILBP exhibits a preference for certain species of bile and fatty acids.

Transport of taurocholate by mutants of negatively charged amino acids, cysteines, and threonines of the rat liver sodium-dependent taurocholate cotransporting polypeptide Ntcp

The relevance of functional amino acids for taurocholate transport by the sodium-dependent taurocholate cotransporting polypeptide Ntcp was determined by site-directed mutagenesis. cRNA from 28 single-points mutants of the rat liver Ntcp clone was expressed in Xenopus laevis oocytes. Mutations were generated in five conserved negatively charged amino acids (aspartates and glutamates) which were present in nine members of the SBAT-family, in two nonconserved negatively charged amino acids, in all eight Ntcp-cysteines, and in two threonines from a protein kinase C consensus region of the Ntcp C-terminus. Functional amino acids were Asp115, Glu257, and Cys266, which were found to be essential for the maintenance of taurocholic acid transport. Asp115 is located in the large intracellular loop III, whereas Glu257 and Cys266 are located in the large extracellular loop VI. Four mutations of threonines from the C-terminus of the Ntcp by alanines or tyrosines showed no effects on sodium-dependent taurocholate transport. Introduction of the FLAG(R) motif into several transport negative point mutations demonstrated that all mutated proteins besides one were present within the cell membrane of the oocytes and provided proof that an insertion defect has not caused transport deficiency by these Ntcp mutants. The latter was observed only with the transport negative mutant Asp24Asn. In conclusion, loop amino acids are required for sodium-dependent substrate translocation by the Ntcp.

Insights into the bile acid transportation system: the human ileal lipid-binding protein-cholyltaurine complex and its comparison with homologous structures

Bile acids are generated in vivo from cholesterol in the liver, and they undergo an enterohepatic circulation involving the small intestine, liver, and kidney. To understand the molecular mechanism of this transportation, it is essential to gain insight into the three-dimensional (3D) structures of proteins involved in the bile acid recycling in free and complexed form and to compare them with homologous members of this protein family. Here we report the solution structure of the human ileal lipid-binding protein (ILBP) in free form and in complex with cholyltaurine. Both structures are compared with a previously published structure of the porcine ILBP-cholylglycine complex and with related lipid-binding proteins. Protein structures were determined in solution by using two-dimensional (2D)- and 3D-homo and heteronuclear NMR techniques, leading to an almost complete resonance assignment and a significant number of distance constraints for distance geometry and restrained molecular dynamics simulations. The identification of several intermolecular distance constraints unambiguously determines the cholyltaurine-binding site. The bile acid is deeply buried within ILBP with its flexible side-chain situated close to the fatty acid portal as entry region into the inner ILBP core. This binding mode differs significantly from the orientation of cholylglycine in porcine ILBP. A detailed analysis using the GRID/CPCA strategy reveals differences in favorable interactions between protein-binding sites and potential ligands. This characterization will allow for the rational design of potential inhibitors for this relevant system.

Substitution of Trp1242 of TM17 alters substrate specificity of human multidrug resistance protein 3

Multidrug resistance protein 3 (MRP3) is an ATP-dependent transporter of 17beta-estradiol 17beta(d-glucuronide) (E(2)17betaG), leukotriene C(4) (LTC(4)), methotrexate, and the bile salts taurocholate and glycocholate. In the present study, the role of a highly conserved Trp residue at position 1242 on MRP3 transport function was examined by expressing wild-type MRP3 and Ala-, Cys-, Phe-, Tyr-, and Pro-substituted mutants in human embryonic kidney 293T cells. Four MRP3-Trp(1242) mutants showed significantly increased E(2)17betaG uptake, whereas transport by the Pro mutant was undetectable. Similarly, the Pro mutant did not transport LTC(4). By comparison, LTC(4) transport by the Ala, Cys, Phe, and Tyr mutants was reduced by approximately 35%. The Ala, Cys, Phe, and Tyr mutants all showed greatly reduced methotrexate and leucovorin transport, except the Tyr mutant, which transported leucovorin at levels comparable with wild-type MRP3. In contrast, the MRP3-Trp(1242) substitutions did not significantly affect taurocholate transport or taurocholate and glycocholate inhibition of E(2)17betaG uptake. Thus Trp(1242) substitutions markedly alter the substrate specificity of MRP3 but leave bile salt binding and transport intact.