The transport of bile acids in liver cells

Toxicokinetics and tissue distribution of phalloidin in mice

Phalloidin, a bicyclic heptapeptide found in Amanita mushroom, specifically binds to F-actin in the liver causing cholestatic hepatotoxicity. However, the toxicokinetics and tissue distribution properties of phalloidin as well as their underlying mechanisms have to be studied further. The area under the plasma concentration curve (AUC) of phalloidin increased in proportion to the doses (0.2, 0.4, and 0.8 mg/kg for intravenous injection and 2, 5, and 10 mg/kg for oral administration). Phalloidin exhibited dose-independent low volume of distribution (395.6-456.9 mL/kg) and clearance (21.4-25.5 mL/min/kg) and low oral bioavailability (2.4%-3.3%). This could be supported with its low absorptive permeability (0.23 ± 0.05 × 10-6 cm/s) in Caco-2 cells. The tissue-to-plasma AUC ratios of intravenously injected and orally administered phalloidin were the highest in the liver and intestines, respectively, and also high in the kidneys, suggesting that the liver, kidneys, and intestines could be susceptible to phalloidin exposure and that active transport via the hepatic and renal organic anion transporters (OATP1B1, OATP1B3, and OAT3) may contribute to the higher distribution of phalloidin in the liver and kidneys.

Physiological Characterization of the Transporter-Mediated Uptake of the Reversible Male Contraceptive H2-Gamendazole Across the Blood-Testis Barrier

The blood-testis barrier (BTB) is formed by a tight network of Sertoli cells (SCs) to limit the movement of reproductive toxicants from the blood into the male genital tract. Transporters expressed at the basal membranes of SCs also influence the disposition of drugs across the BTB. The reversible, nonhormonal contraceptive, H2-gamendazole (H2-GMZ), is an indazole carboxylic acid analog that accumulates over 10 times more in the testes compared with other organs. However, the mechanism(s) by which H2-GMZ circumvents the BTB are unknown. This study describes the physiologic characteristics of the carrier-mediated process(es) that permit H2-GMZ and other analogs to penetrate SCs. Uptake studies were performed using an immortalized human SC line (hT-SerC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Uptake of H2-GMZ and four analogs followed Michaelis-Menten transport kinetics (one analog exhibited poor penetration). H2-GMZ uptake was strongly inhibited by indomethacin, diclofenac, MK-571, and several analogs. Moreover, H2-GMZ uptake was stimulated by an acidic extracellular pH, reduced at basic pHs, and independent of extracellular Na+, K+, or Cl- levels, which are intrinsic characteristics of OATP-mediated transport. Therefore, the characteristics of H2-GMZ transport suggest that one or more OATPs may be involved. However, endogenous transporter expression in wild-type Chinese hamster ovary (CHO), Madin-Darby canine kidney (MDCK), and human embryonic kidney-293 (HEK-293) cells limited the utility of heterologous transporter expression to identify a specific OATP transporter. Altogether, characterization of the transporters involved in the flux of H2-GMZ provides insight into the selectivity of drug disposition across the human BTB to understand and overcome the pharmacokinetic and pharmacodynamic difficulties presented by this barrier. SIGNIFICANCE STATEMENT: Despite major advancements in female contraceptives, male alternatives, including vasectomy, condom usage, and physical withdrawal, are antiquated and the widespread availability of nonhormonal, reversible chemical contraceptives is nonexistent. Indazole carboxylic acid analogs such as H2-GMZ are promising new reversible, antispermatogenic drugs that are highly effective in rodents. This study characterizes the carrier-mediated processes that permit H2-GMZ and other drugs to enter Sertoli cells and the observations made here will guide the development of drugs that effectively circumvent the BTB.

Organic anion-transporting polypeptides

Organic anion-transporting polypeptides or OATPs are central transporters in the disposition of drugs and other xenobiotics. In addition, they mediate transport of a wide variety of endogenous substrates. The critical role of OATPs in drug disposition has spurred research both in academia and in the pharmaceutical industry. Translational aspects with clinical questions are the focus in academia, while the pharmaceutical industry tries to define and understand the role these transporters play in pharmacotherapy. The present overview summarizes our knowledge on the interaction of food constituents with OATPs and on the OATP transport mechanisms. Further, it gives an update on the available information on the structure-function relationship of the OATPs and, finally, covers the transcriptional and posttranscriptional regulation of OATPs.

Physiological and molecular biochemical mechanisms of bile formation

This review considers the physiological and molecular biochemical mechanisms of bile formation. The composition of bile and structure of a bile canaliculus, biosynthesis and conjugation of bile acids, bile phospholipids, formation of bile micellar structures, and enterohepatic circulation of bile acids are described. In general, the review focuses on the molecular physiology of the transporting systems of the hepatocyte sinusoidal and apical membranes. Knowledge of physiological and biochemical basis of bile formation has implications for understanding the mechanisms of development of pathological processes, associated with diseases of the liver and biliary tract.

The SLC10 carrier family: transport functions and molecular structure

The SLC10 family represents seven genes containing 1-12 exons that encode proteins in humans with sequence lengths of 348-477 amino acids. Although termed solute carriers (SLCs), only three out of seven (i.e. SLC10A1, SLC10A2, and SLC10A6) show sodium-dependent uptake of organic substrates across the cell membrane. These include the uptake of bile salts, sulfated steroids, sulfated thyroidal hormones, and certain statin drugs by SLC10A1 (Na(+)-taurocholate cotransporting polypeptide (NTCP)), the uptake of bile salts by SLC10A2 (apical sodium-dependent bile acid transporter (ASBT)), and uptake of sulfated steroids and sulfated taurolithocholate by SLC10A6 (sodium-dependent organic anion transporter (SOAT)). The other members of the family are orphan carriers not all localized in the cell membrane. The name "bile acid transporter family" arose because the first two SLC10 members (NTCP and ASBT) are carriers for bile salts that establish their enterohepatic circulation. In recent years, information has been obtained on their 2D and 3D membrane topology, structure-transport relationships, and on the ligand and sodium-binding sites. For SLC10A2, the putative 3D morphology was deduced from the crystal structure of a bacterial SLC10A2 analog, ASBT(NM). This information was used in this chapter to calculate the putative 3D structure of NTCP. This review provides first an introduction to recent knowledge about bile acid synthesis and newly found bile acid hormonal functions, and then describes step-by-step each individual member of the family in terms of expression, localization, substrate pattern, as well as protein topology with emphasis on the three functional SLC10 carrier members.

Intestinal and hepatic cholesterol carriers in diabetic Psammomys obesus

Insulin resistance and type 2 diabetes (T2D) are characterized by hyperlipidemia. The aim of the present study was to elucidate whether T2D contributes to abnormal cholesterol (CHOL) homeostasis. Experiments were carried out in the small intestine and liver of Psammomys obesus, a model of nutritionally induced T2D. Our results show that diabetic animals exhibited a lower intestinal CHOL uptake, which was associated with a decrease in 1) the gene and protein expression of Niemann-Pick C1 like 1 that plays a pivotal role in CHOL incorporation in the enterocytes; and 2) mRNA of ATP-binding cassette transporters (ABC)A1 that mediates CHOL efflux from intestinal cells to apolipoprotein A-I and high-density lipoprotein. No changes were observed in the other intestinal transporters scavenger receptor-class B type I (SR-BI) and annexin 2. On the other hand, in diabetic animals, a significant mRNA decrease was noticed in intestinal ABCG5 and ABCG8 responsible for the secretion of absorbed CHOL back into the lumen. Furthermore, jejunal PCSK9 protein was diminished and low-density lipoprotein receptor was raised, along with a significant down-regulation in jejunal 3-hydroxy-3-methylglutaryl-coenzyme A reductase in P. obesus with T2D. Finally, among the transcription factors tested, only an increase in liver X receptors alpha and a decrease in peroxisome proliferator-activated receptors delta/beta mRNAs were detected in the intestine. In the liver, there was 1) an augmentation in the protein mass of Niemann-Pick C1 like 1, SR-BI, and annexin 2; 2) an up-regulation of SR-BI mRNA; 3) a fall in ABCG8 protein content as well as in ABCG5 and ABCA1 mRNA; and 4) an augmentation in liver X receptors alpha and peroxisome proliferator-activated receptors beta/delta mRNA, together with a drop in sterol regulatory element binding protein-2 protein. Our findings show that the development in P. obesus with T2D modifies the whole intraenterocyte and hepatocyte machinery responsible for CHOL homeostasis.

Drug uptake systems in liver and kidney: a historic perspective

Drugs and their metabolites are eliminated mainly by excretion into urine and bile. Studies in whole animals, isolated organs, cells, and membrane vesicles led to the conclusion that different transport systems are responsible for the transport of different classes of organic compounds (small, large, anionic, and cationic). In the early 1990s, functional expression cloning resulted in the identification of the first transporters for organic anions and cations. Eventually, all the major transport systems involved in the uptake of these organic compounds were cloned and characterized, and we now know that they belong to the organic anion transporters (OATs) and organic cation transporters (OCTs) of the SLC22A superfamily and the organic anion-transporting polypeptides (OATPs) of the SLCO superfamily of polyspecific drug transporters. Today we can explain, at the molecular level, why small and hydrophilic organic compounds are excreted predominantly through urine whereas large and amphipathic compounds are excreted mainly through bile, and we can start to predict drug-drug interactions in the case of new compounds.

Physiology of bile secretion

The formation of bile depends on the structural and functional integrity of the bile-secretory apparatus and its impairment, in different situations, results in the syndrome of cholestasis. The structural bases that permit bile secretion as well as various aspects related with its composition and flow rate in physiological conditions will first be reviewed. Canalicular bile is produced by polarized hepatocytes that hold transporters in their basolateral (sinusoidal) and apical (canalicular) plasma membrane. This review summarizes recent data on the molecular determinants of this primary bile formation. The major function of the biliary tree is modification of canalicular bile by secretory and reabsorptive processes in bile-duct epithelial cells (cholangiocytes) as bile passes through bile ducts. The mechanisms of fluid and solute transport in cholangiocytes will also be discussed. In contrast to hepatocytes where secretion is constant and poorly controlled, cholangiocyte secretion is regulated by hormones and nerves. A short section dedicated to these regulatory mechanisms of bile secretion has been included. The aim of this revision was to set the bases for other reviews in this series that will be devoted to specific issues related with biliary physiology and pathology.

Atherogenic diet-induced hepatitis is partially dependent on murine TLR4

Diets high in cholesterol and cholate such as the Paigen diet have been used to study atherogenesis, lithogenesis, and proinflammatory microvascular changes induced by nutritional hypercholesterolemia. Although these diets lead to chronic hepatic inflammation and fibrosis, the early inflammatory changes have been poorly characterized. TLR4, a known receptor for LPS, is also a receptor for a variety of endogenous ligands and has been implicated in atheroma formation. Here, we specifically examined the early inflammatory response of the liver to the atherogenic (ATH) diet and the possible contribution of TLR4. Animals fed the high-cholesterol/cholate diet for 3 weeks developed a significant, predominantly mononuclear leukocyte infiltration in the liver, hepatic steatosis, elevated hepatic expression of MCP-1, RANTES, and MIP-2, and increased serum levels of liver enzymes. In TLR4-deleted animals, there was a 30% attenuation in the serum alanine transaminase levels and a 50% reduction in the leukocyte infiltration with a fourfold reduction in chemokine expression. In contrast, hepatic steatosis did not differ from wild-type controls. TLR2 deletion had no effect on diet-induced hepatitis but increased the amount of steatosis. We conclude that the early inflammatory liver injury but not hepatic lipid loading induced by the ATH diet in mice is mediated in part by TLR4.

In vivo relevance of Mrp2-mediated biliary excretion of the Amanita mushroom toxin demethylphalloin

To determine which efflux carriers are involved in hepatic phalloidin elimination, hepatobiliary [(3)H]-demethylphalloin (DMP) excretion was studied in normal Wistar rats and in Mrp2 deficient TR(-) Wistar rats as well as in normal wild-type FVB mice, Mdr1a,b(-/-) knockout mice, and Bcrp1(-/-) knockout mice by in situ bile duct/gallbladder cannulation. A subtoxic dose of 0.03 mg DMP/kg b.w. was used, which did not induce cholestasis in any tested animal. Excretion of DMP into bile was not altered in Mdr1a,b(-/-) mice or in Bcrp1(-/-) mice compared with wild-type FVB mice. Whereas 17.6% of the applied dose was excreted into bile of normal Wistar rats, hepatobiliary excretion decreased to 7.9% in TR(-) rats within 2 h after intravenous application. This decrease was not due to reduced cellular DMP uptake, as shown by normal expression of Oatp1b2 in livers of TR(-) rats and functional DMP uptake into isolated TR(-) rat hepatocytes. Tissue concentrations of phalloidin were also not altered in any of the transgenic mice. Interestingly, the decrease of biliary DMP excretion in the TR(-) rats was not followed by any increase of phalloidin accumulation in the liver but yielded a compensatory excretion of the toxin into urine, indicating that hepatocytes of TR(-) rats expelled phalloidin back into blood circulation.

Guidance values for microcystins in water and cyanobacterial supplement products (blue-green algal supplements): a reasonable or misguided approach?

This article reviews current scientific knowledge on the toxicity and carcinogenicity of microcystins and compares this to the guidance values proposed for microcystins in water by the World Health Organization, and for blue-green algal food supplements by the Oregon State Department of Health. The basis of the risk assessment underlying these guidance values is viewed as being critical due to overt deficiencies in the data used for its generation: (i) use of one microcystin congener only (microcystin-LR), while the other presently known nearly 80 congeners are largely disregarded, (ii) new knowledge regarding potential neuro and renal toxicity of microcystins in humans and (iii) the inadequacies of assessing realistic microcystin exposures in humans and especially in children via blue-green algal food supplements. In reiterating the state-of-the-art toxicology database on microcystins and in the light of new data on the high degree of toxin contamination of algal food supplements, this review clearly demonstrates the need for improved kinetic data of microcystins in humans and for discussion concerning uncertainty factors, which may result in a lowering of the present guidance values and an increased routine control of water bodies and food supplements for toxin contamination. Similar to the approach taken previously by authorities for dioxin or PCB risk assessment, the use of a toxin equivalent approach to the risk assessment of microcystins is proposed.

Diet induced regulation of genes involved in cholesterol metabolism in rat liver parenchymal and Kupffer cells

BACKGROUND/AIMS

Feeding rodents atherogenic diets enriched in cholesterol or cholic acid changes hepatic cholesterol metabolism. In the present study, the effect of an atherogenic diet enriched in cholesterol and cholic acid on cellular hepatic cholesterol metabolism was studied.

METHODS

Gene and protein expression analysis was performed on parenchymal, endothelial, and Kupffer cells isolated from rats fed a chow or atherogenic diet using quantitative real-time PCR and immunoblotting, respectively.

RESULTS

The atherogenic diet raised the serum cholesterol concentration 11-fold, mostly in the VLDL fraction, and led to heavy lipid loading of rat liver parenchymal and Kupffer cells. Only moderate changes in the expression of genes involved in cholesterol metabolism were observed in parenchymal cells on the diet, while PPAR delta expression was 6.8-fold decreased. Kupffer cells, however, showed a highly adaptive response with a 2- to 9-fold induction of SR-BI, ABCA1, and ABCG5/G8, and an 82-fold induction in CYP7A1 mRNA expression, respectively.

CONCLUSIONS

Heavy lipid loading of parenchymal cells leads to moderate gene expression changes, while Kupffer cells respond in a highly adaptive fashion by stimulating the expression of genes involved in cholesterol metabolism and transport.

The organic anion transporter (OATP) family

In the last decade, many organic anion transporters have been isolated, characterized their distribution and substrates. The recently identified organic anion transporter family OATP (organic anion transporting polypeptide)/LST (liver-specific transporter) family, transport bile acids, hormones as well as eicosanoids, various compounds (BSP, HMG-CoA reductase inhibitor, angiotensin converting enzyme inhibitor, etc.). The isolation of the family revealed that not only hydrophilic compounds, drugs and hormones of lipophilic nature need a membrane transport system to penetrate cell membrane. In this family, the nomenclature becomes very complicated and the physiological role of this family is still unclear except about few organs such as the brain, liver and kidney. Even in such organs, the co-existence of the OATP/LST family and similar substrate specificity hamper the progress and clear characterization identifying the real role of the transporter family. Here, recent progress and an insight of this field are reviewed.

Identification of phalloidin uptake systems of rat and human liver

To determine whether the liver toxin phalloidin is transported into hepatocytes by one of the known bile salt transporters, we expressed the sodium-dependent Na+/taurocholate cotransporting polypeptide (Ntcp) and several sodium-independent bile salt transporters of the organic anion transporting polypeptide (OATP/SLCO) superfamily in Xenopus laevis oocytes and measured uptake of the radiolabeled phalloidin derivative [3H]demethylphalloin. We found that rat Oatp1b2 (previously called Oatp4 (Slc21a10)) as well as human OATP1B1 (previously called OATP-C (SLC21A6)) and OATP1B3 (previously called OATP8 (SLC21A8)) mediate uptake of [3H]demethylphalloin when expressed in X. laevis oocytes. Transport of increasing [3H]demethylphalloin concentrations was saturable with apparent Km values of 5.7 microM (Oatp1b2), 17 microM (OATP1B1) and 7.5 microM (OATP1B3). All other tested Oatps/OATPs as well as the rat liver Ntcp did not transport [3H]demethylphalloin. Therefore, we conclude that rat Oatp1b2 as well as human OATP1B1 and OATP1B3 are responsible for phalloidin uptake into rat and human hepatocytes.

Conjugates of Gadolinium Complexes to Bile Acids as Hepatocyte-Directed Contrast Agents for Magnetic Resonance Imaging

A series of structurally different Gd(III) conjugates incorporating a bile acid moiety have been prepared. Polyaminopolycarboxylic ligands such as diethylenetriaminepentaacetic acid (DTPA) and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetracetic acid (DOTA) have been selected as chelating subunit for the Gd(III) ion. Cholic acid, cholylglycine, and cholyltaurine have been incorporated as the bile acid moieties. In first generation conjugates the Gd(III) complex is linked to the carboxyl group of cholic acid. Second generation conjugates feature the attachment of the Gd(III) complex to the 3 position of the steroidic backbone of the bile acid. Finally, in third generation conjugates the Gd(III) complex is attached to the epsilon nitrogen atom of cholyllysine. The conjugates are eliminated through the biliary route to a various extent (7.5 to 77% in rats) according to their structural features. Among the most promising terms, a second generation conjugate in which the Gd(III) complex is linked to cholic acid through the 3alpha hydroxy group seems to enter hepatocytes using the Na(+)/taurocholate transporter. Noticeably, some of the second generation conjugates are characterized by very high tolerabilities (LD(50) up to 9.5 mmol/kg) after intravenous administration in mice.

Enterohepatic circulation: physiological, pharmacokinetic and clinical implications

Enterohepatic recycling occurs by biliary excretion and intestinal reabsorption of a solute, sometimes with hepatic conjugation and intestinal deconjugation. Cycling is often associated with multiple peaks and a longer apparent half-life in a plasma concentration-time profile. Factors affecting biliary excretion include drug characteristics (chemical structure, polarity and molecular size), transport across sinusoidal plasma membrane and canniculae membranes, biotransformation and possible reabsorption from intrahepatic bile ductules. Intestinal reabsorption to complete the enterohepatic cycle may depend on hydrolysis of a drug conjugate by gut bacteria. Bioavailability is also affected by the extent of intestinal absorption, gut-wall P-glycoprotein efflux and gut-wall metabolism. Recently, there has been a considerable increase in our understanding of the role of transporters, of gene expression of intestinal and hepatic enzymes, and of hepatic zonation. Drugs, disease and genetics may result in induced or inhibited activity of transporters and metabolising enzymes. Reduced expression of one transporter, for example hepatic canalicular multidrug resistance-associated protein (MRP) 2, is often associated with enhanced expression of others, for example the usually quiescent basolateral efflux MRP3, to limit hepatic toxicity. In addition, physiologically relevant pharmacokinetic models, which describe enterohepatic recirculation in terms of its determinants (such as sporadic gall bladder emptying), have been developed. In general, enterohepatic recirculation may prolong the pharmacological effect of certain drugs and drug metabolites. Of particular importance is the potential amplifying effect of enterohepatic variability in defining differences in the bioavailability, apparent volume of distribution and clearance of a given compound. Genetic abnormalities, disease states, orally administered adsorbents and certain coadministered drugs all affect enterohepatic recycling.

Gene structure, intracellular localization, and functional roles of sterol carrier protein-2

Since its discovery three decades ago, sterol carrier protein-2 (SCP-2) has remained a fascinating protein whose physiological function in lipid metabolism remains an enigma. Its multiple proposed functions arise from its complex gene structure, post-translational processing, intracellular localization, and ligand specificity. The SCP-2 gene has two initiation sites coding for proteins that share a common 13 kDa SCP-2 C-terminus: (1) One site codes for 58 kDa SCP-x which is partially post-translationally cleaved to 13 kDa SCP-2 and a 45 kDa protein. (2) A second site codes for 15 kDa pro-SCP-2 which is completely post-translationally cleaved to 13 kDa SCP-2. Very little is yet known regarding how the relative proportions of the two transcripts are regulated. Although all three proteins contain a C-terminal SKL peroxisomal targeting sequence, it is unclear why all three proteins are not exclusively localized in peroxisomes. However, the recent demonstration that the SCP-2 N-terminal presequence in pro-SCP-2 dramatically modulated the intracellular targeting coded by the C-terminal peroxisomal targeting sequence may account for the observation that as much as half of total SCP-2 is localized outside the peroxisome. The tertiary and secondary structure of the 13 kDa SCP-2, but not that of 15 kDa pro-SCP-2 and 58 kDa SCP-x, are now resolved. Increasing evidence suggests that the 58 kDa SCP-x and 45 kDa proteins are peroxisomal 3-ketoacyl-CoA-thiolases involved in the oxidation of branched chain fatty acids. Since 15 kDa pro-SCP-2 is post-translationally completely cleaved to 13 kDa SCP-2, relatively little attention has been focused on this protein. Finally, although the 13 kDa SCP-2 is the most studied of these proteins, because it exhibits diversity of its ligand partners (fatty acids, fatty acyl CoAs, cholesterol, phospholipids), new potential physiological function(s) are still being proposed and questions regarding potential compensation by other proteins with overlapping specificity are only beginning to be resolved.

Cloning and functional characterization of the bile acid-sensitive methotrexate carrier from rat liver cells

We have cloned two complementary DNAs (cDNAs), RL-Mtx-1 and RL-Mtx-2, corresponding to the bile acid- sensitive methotrexate carrier from rat liver by direct full-length rapid amplification of cDNA ends polymerase chain reaction (RACE-PCR) using degenerated primers that were deduced from published sequences of tumor cell methotrexate transporters. When expressed in Xenopus laevis oocytes and cosM6 cells, both clones mediate methotrexate and bumetanide transport. RL-Mtx-1 consists of 2,445 bp with an open reading frame of 1,536 bp. The corresponding protein with 512 amino acids has a molecular weight of 58 kd. RL-Mtx-2 (2,654 bp) differs by an additional insert of 203 bp. This insert is located in frame at position 1,196 of the RL-Mtx-1 and contains the typical splice junction sites at the 5' and 3' end, indicating that the RL-Mtx-2 messenger RNA (mRNA) is generated by alternative splicing. The insert contains a stop codon that shortens the RL-Mtx-2 protein to 330 amino acids (38 kd). Both cDNAs contain the binding site sequence for the dioxin/nuclear translocator responsive element (Ah/Arnt-receptor) in conjunction with a barbiturate recognition sequence (Barbie box). Preliminary results show that the Barbie box acts as a negative regulatory element. The two liver cDNA clones show homologies to the published sequences of folate and the reduced folate carriers, but no homology is found to the transport systems for organic anions like the Ntcp1, oatp1, OAT-K1, and OAT1. Expression of the mRNA for the methotrexate carrier is found in liver, kidney, heart, brain, spleen, lung, and skeletal muscle, but not in the testis as revealed by Northern blot analysis. The highest abundance of the mRNA is found in the kidney.

Pathological and biochemical characterization of microcystin-induced hepatopancreas and kidney damage in carp (Cyprinus carpio)

Mass occurrences of cyanobacteria, due to their inherent capacity for toxin production, specifically of microcystins (MC), have been associated with fish kills worldwide. The uptake of MC-LR and the sequence of pathological and associated biochemical changes was investigated in carp (Cyprinus carpio) in vivo over 72 h. Carp were gavaged with a single sublethal bolus dose of toxic Microcystis aeruginosa (PCC 7806) amounting to an equivalent of 400 microg MC-LR/kg body wt. Damage of renal proximal tubular cells and hepatocytes was observed as early as 1 h, followed by pathological changes in the intestinal mucosa at approximately 12 h postdosing. These alterations were characterized in hepatopancreas by a dissociation of hepatocytes, an early onset of apoptotic cell death, and delayed cell lysis. In the renal proximal tubules (P2) observations included increased vacuolation of individual tubular epithelial cells, apoptosis, cell shedding, and finally proteinaceous casts at the cortico-medullary junction. Concurrently with the pathological alterations, MC-immunopositive staining was observed in hepatocytes and the proximal tubular cells; the staining increasing in the hepatopancreas in intensity with increasing time postdosing. The presence of apoptotic cell death was determined using in situ fragment end labeling (ISEL) of the respective tissue sections and agarose gel electrophoresis for detection of DNA-laddering. The analysis of carp tissue extracts (hepatopancreas, kidney, GI tract, skeletal muscle, brain, heart, spleen, and gills) demonstrated MC-LR adducts having molecular weights of 38 kDa (putatively catalytic subunit of protein phosphatases-1 and -2A) and 28 kDa, respectively. An additional band was found to be present at 23 kDa in both hepatopancreas and kidney. The present data demonstrate that, in comparison to the pathological events in salmonids exposed to MC, where a slower development of pathology and primarily necrotic cell death prevails, the pathology in carp develops rapidly and at lower toxin concentrations. This is most likely due to a more efficient uptake of toxin, while the mechanism of cell death is primarily apoptosis.

Changes in the localization of the rat canalicular conjugate export pump Mrp2 in phalloidin-induced cholestasis

Administration of phalloidin, one of the toxic peptides of the mushroom Amanita phalloides, leads to rapid and sustained cholestasis in rats. Although attributed to the interaction of phalloidin with microfilaments, the events leading to cholestasis are incompletely understood. The adenosine triphosphate (ATP)-dependent, apical conjugate export pump, termed multidrug resistance protein 2 (Mrp2) or canalicular multispecific organic anion transporter, is the major driving force for bile salt-independent bile flow. We investigated the role of Mrp2 in phalloidin-induced cholestasis. Bile flow decreased to 53% and 31% of control at 15 and 30 minutes after phalloidin (0.5 mg/kg), respectively. Mrp2-mediated [3H]leukotriene excretion into bile during the initial 45 minutes was reduced to 44% of control when [3H]LTC4 was injected 15 minutes after phalloidin treatment. Mrp2 was progressively lost from the hepatocyte canalicular membrane and detected predominantly on intracellular membrane structures together with other canalicular proteins including P-glycoproteins, ecto-ATPase, and dipeptidyl-peptidase IV. By contrast, structures involved in intercellular adhesion (zonula occludens, zonula adhaerens, and desmosomes) as well as intermediate filaments of the cytokeratin type appeared largely unaffected within 30 minutes after phalloidin. In line with the immunofluorescence analysis, immunoblots indicated a loss of Mrp2 and P-glycoproteins from the canalicular membrane and a 3- and 4.6-fold increase of these transport proteins in the microsomal fraction, respectively. Our results indicate that phalloidin induces marked alterations of the hepatocyte canalicular architecture and a loss of Mrp2 together with other proteins from the canalicular membrane. The resulting cholestasis can therefore be explained in part by the loss of export pumps, including Mrp2, from the canalicular membrane.

Substrate specificity of the rat liver Na(+)-bile salt cotransporter in Xenopus laevis oocytes and in CHO cells

It has been proposed that the hepatocellular Na(+)-dependent bile salt uptake system exhibits a broad substrate specificity in intact hepatocytes. In contrast, recent expression studies in mammalian cell lines have suggested that the cloned rat liver Na(+)-taurocholate cotransporting polypeptide (Ntcp) may transport only taurocholate. To characterize its substrate specificity Ntcp was stably transfected into Chinese hamster ovary (CHO) cells. These cells exhibited saturable Na(+)-dependent uptake of [3H]taurocholate [Michaelis constant (K(m)) of approximately 34 microM] that was strongly inhibited by all major bile salts, estrone 3-sulfate, bumetanide, and cyclosporin A. Ntcp cRNA-injected Xenopus laevis oocytes and the transfected CHO cells exhibited saturable Na(+)-dependent uptake of [3H]taurochenodeoxycholate (Km of approximately 5 microM), [3H]tauroursodeoxycholate (Km of approximately 14 microM), and [14C]glycocholate (Km of approximately 27 microM). After induction of gene expression by sodium butyrate, Na(+)-dependent transport of [3H]estrone 3-sulfate (Km of approximately 27 microM) could also be detected in the transfected CHO cells. However, there was no detectable Na(+)-dependent uptake of [3H]bumetanide or [3H]cyclosporin A. These results show that the cloned Ntcp can mediate Na(+)-dependent uptake of all physiological bile salts as well as of the steroid conjugate estrone 3-sulfate. Hence, Ntcp is a multispecific transporter with preference for bile salts and other anionic steroidal compounds.

Toluene-induced elevation of serum bile acids: relationship to bile acid transport

Raised concentrations of serum bile acids (SBA) following occupational exposure to a number of halogenated aliphatic hydrocarbon solvents and after in vivo exposure of experimental animals to these substances have been reported in several studies in recent years. However, the widely used nonchlorinated aromatic hydrocarbon solvent, toluene, has not been critically examined for its effect on serum bile acids. Accordingly, the effect of in vivo treatment with toluene on SBA and its direct in vitro effects on the transport of bile acids by isolated rat hepatocytes were investigated in this study. In vivo treatment with toluene (2.3 mmol/kg body weight, ip, on each of 3 consecutive days) resulted in a significant rise in the serum concentrations of total and some individual bile acids while other parameters of hepatobiliary function were unaltered. Administration of a higher dose of solvent (9.2 mmol/kg body weight, i.p.) resulted in a further increase in total SBA levels together with a significant rise in serum activities of some liver enzymes. In vitro application of noncytotoxic doses of toluene in the vapor phase to hepatocytes isolated from untreated rats resulted in a significant inhibition of the initial rate-(V0)-of uptake of cholic acid (CA). Similarly, accumulation of CA and taurocholic acid (TC) over an extended incubation time by hepatocytes exposed to toluene was significantly inhibited. Kinetic analysis revealed a noncompetitive inhibition of CA uptake as suggested by a decline in Vmax and an unaltered K(m). In contrast, the initial rate of efflux of these substates and their continuous efflux from preloaded cells were unaffected by exposure to toluene. Thus, toluene exposure inhibited the transport and accumulation of bile acids by hepatocytes in a manner largely similar to that of halogenated solvents, and this inhibition could explain the raised SBA concentrations following in vivo exposure to this solvent. These findings are consistent with and provide mechanistic data to support previous studies where increased SBA levels (in the absence of any evidence of liver injury as measured by liver enzyme tests) were reported in workers following occupational exposure to this solvent. Additionally, in full agreement with our previous investigations in which SBA levels were found to be a sensitive biological marker of exposure to halogenated aliphatic hydrocarbon solvents, the data support a similar role for SBA on exposure to toluene as well.

Identification of a locus for progressive familial intrahepatic cholestasis PFIC2 on chromosome 2q24

Progressive familial intrahepatic cholestasis (PFIC; OMIM 211600) is the second most common familial cholestatic syndrome presenting in infancy. A locus has previously been mapped to chromosome 18q21-22 in the original Byler pedigree. This chromosomal region also harbors the locus for benign recurrent intrahepatic cholestasis (BRIC) a related phenotype. Linkage analysis in six consanguineous PFIC pedigrees from the Middle East has previously excluded linkage to chromosome 18q21-22, indicating the existence of locus heterogeneity within the PFIC phenotype. By use of homozygosity mapping and a genome scan in these pedigrees, a locus designated "PFIC2" has been mapped to chromosome 2q24. A maximum LOD score of 8.5 was obtained in the interval between marker loci D2S306 and D2S124, with all families linked.

High affinity uptake by isolated rat hepatocytes of a linear pseudo-hexapeptide, ditekiren

The hepatic elimination of many oligopeptides is both rapid and extensive, and often limits their potential as therapeutic agents. The linear, hydrophobic pseudo-hexapeptide ditekiren, a renin inhibitor, is one such example. The mechanism(s) involved in its hepatic clearance are largely unknown; accordingly, the characteristics of ditekiren's transport into isolated rat hepatocytes was investigated. In addition to a concentration-independent, linear process, uptake also involved a carrier-mediated component (Km = 0.2 +/- 0.05 microM; Vmax = 11.6 +/- 0.6 pmol (mg protein)[-1] min[-1]). Phenobarbital pretreatment in vivo resulted in marked induction of such transport. Negative results from cis-inhibition studies with substrates and/or inhibitors of well-established hepatic transport systems, e.g., sodium-dependent bile acid, sodium-independent multispecific bile acid and cation carriers, ruled out their involvement in ditekiren's uptake. By contrast, a number of cyclic and linear oligopeptides inhibited the uptake process to varying extents and in the case of EMD-59121, the most inhibitory compound, the interaction was competitive in nature. Collectively, these data suggest the presence of a novel high affinity, low capacity transporter in rat hepatocytes with specific affinity for ditekiren and possibly other oligopeptides.

Significant increase of Kuppfer cells associated with loss of Na+,K+-ATPase activity in rat hepatic allograft rejection

BACKGROUND

Cholestasis is a complication that occurs during the rejection of liver transplants. The aim of this study was to investigate the association of activated Kupffer cells (KCs) and Na+,K+-ATPase activity for taurocholate cotransport and bile canalicular (BC) Mg++-ATPase activity for hepatobiliary excretion in rat liver allograft.

METHODS

Quantitative analyses of KC number and size in relationship to enzyme activity of Na+,K+-ATPase and of BC Mg++-ATPase were conducted in rejected liver after allogenic transplantation and after prevention of rejection using cyclosporine.

RESULTS

The animals were examined on the 10th postoperative day. In the rejection group, the number of KCs significantly increased more than fourfold in comparison with the number of KCs in the control livers. Some KCs were found in the sinusoids, but the majority were located in the space of Disse. Na+,K+-ATPase activity vanished from the basolateral plasma membrane, whereas BC Mg++-ATPase activity was restored in the apical domain. With immunosuppression, KCs showed the same behavior as in the control group, and activity of both ATPases was observed as strong electron-dense precipitates in basolateral and apical plasma membrane domains.

CONCLUSIONS

In this study, we demonstrate that activated KCs migrate into the donor liver and release cytokines, which leads to the loss of Na+,K+-ATPase activity in the rejection group. BC Mg++-ATPase activity was not influenced by these mediators of activated macrophages. Since Na+,K+-ATPase is the cotransporter for hepatocyte taurocholate uptake, these data may contribute to understanding the mechanisms for cholestasis during hepatic allograft rejection.

In vitro interference with hepatocellular uptake of bile acids by xylene

Occupational exposure to a mixture of two widely used aromatic hydrocarbon solvents, xylene and toluene, has been associated with a significant rise in the concentrations of serum bile acids (SBA). We have recently shown that toluene interferes with the transport of bile acids by hepatocytes and this could explain elevated SBA after occupational exposure or following in vivo administration of this compound to experimental animals. However, it is not known if xylene, like its monomethylated homologue, toluene, could interfere with the processes of bile acid transport by hepatocytes. Therefore, the present studies were undertaken to examine this possibility. Direct addition of a non-cytotoxic dose (2.5 microliters/2.8 x 10(6) cells) of xylene (in vapour phase) to hepatocytes isolated from untreated rats significantly inhibited the initial rates (determined from slope of the lines in the linear range (20-80 s)) of uptake (V0) of 10 microM cholic acid (CA) and-taurocholic acid (TC) by 37 and 48%, respectively (P < 0.05). Similarly, accumulation of these substrates by hepatocytes over an extended incubation time up to 30 min was significantly inhibited to the same extent by xylene exposure. This inhibitory effect was found to be reversible when sufficient time was allowed for the cells to recover. In contrast, the initial rates (V0) of efflux (determined from slope of the lines in the linear range (1-5 min)) of these bile acids (25 microM) and their continuous efflux (up to 30 min) from preloaded cells incubated with a similar dose of xylene were not (except for the 1 min time point) significantly different from those of controls. In conclusion, xylene interferes with the transport of bile acids by hepatocytes in a manner largely similar to that of its monomethylated homologue, toluene. These findings extend our previous observations on aliphatic and aromatic hydrocarbon solvents and provide mechanistic data at a cellular level to support a causal role for xylene (as well as toluene) in raised SBA levels of exposed individuals.

Multispecific amphipathic substrate transport by an organic anion transporter of human liver

BACKGROUND

Hepatic uptake of differently charged amphipathic endo- and xenobiotics is thought to occur via distinct carrier-mediated transport systems. Alternatively, a single rat organic anion transporting polypeptide (oatp) has recently been demonstrated to mediate hepatocellular uptake of differently charged amphipathic substrates.

AIM

To investigate whether a cloned human liver organic anion transporting polypeptide (OATP) also can mediate charge- and class-independent hepatocellular uptake of amphipathic substrates.

METHODS

Xenopus laevis oocytes were injected with OATP-cRNA. Sodium-independent uptake of estrone-3-sulfate, ouabain and the organic cation N-(4,4-azo-n-pentyl)-21-ajmalinium was compared in OATP-expressing and uninjected (or water injected) control oocytes.

RESULTS

Our results indicate that OATP, in addition to bromosulfophthalein and bile salts, can also transport anionic estrone-3-sulfate (Km approximately 59 microM), neutral ouabain (K(m) approximately 5.5 mM) and cationic N-(4,4-azo-n-pentyl)-21-ajmalinium. For each of these compounds, OATP-mediated uptake was cis-inhibited by the OATP substrate taurochenodeoxycholate and the transport activities correlated well with the amounts of cRNA injected.

CONCLUSION

Similar to the rat liver oatp, the human liver OATP can also mediate multispecific and charge-independent uptake of lipophilic amphipathic organic compounds. Thus, OATP may play an important role in the first pass clearance of drugs and other xenobiotics by the human liver.

Locus heterogeneity in progressive familial intrahepatic cholestasis

Progressive familial intrahepatic cholestasis (PFIC or Byler disease) is a rare autosomal recessive form of severe and fatal cholestatic liver disease. A locus for PFIC has recently been mapped to chromosome 18q21-q22 in the original Byler pedigree. This region harbours the locus for a related phenotype, benign recurrent intrahepatic cholestasis (BRIC), suggesting that these traits are allelic. Linkage analysis was undertaken in five consanguineous PFIC pedigrees from Saudi Arabia using marker loci (D18S69, D18S41, D18S64, D18S38, D18S42, D18S55, D18S68, and D18S61) which span the Byler disease/BRIC region on 18q21-q22. In this family set the disease locus was excluded from this region, showing that locus heterogeneity exists for the PFIC phenotype.

Cloning and functional characterization of a novel rat organic anion transporter mediating basolateral uptake of methotrexate in the kidney

We have cloned a cDNA coding for a novel member of organic anion transporter, designated OAT-K1, expressed specifically in the kidney of rats. The rat OAT-K1 cDNA (2788 base pairs) had an open reading frame encoding for a 669-amino acid protein (calculated molecular mass of 74 kDa) which shows 72% identity with the cloned rat liver organic anion transporter, oatp. Northern hybridization and reverse transcription-coupled polymerase chain reaction revealed that the rat OAT-K1 messenger RNA transcript is expressed predominantly in the kidney. By use of stable LLC-PK1 cell monolayers transfected with the rat OAT-K1 cDNA, the transporter was suggested to mediate basolateral uptake of methotrexate, an anionic anticancer drug, but not taurocholate, p-aminohippurate, prostaglandin E2, and leukotriene C4. The methotrexate transport by rat OAT-K1 was unaffected by the presence of Na+ or Cl- gradient. The methotrexate accumulation by the OAT-K1-expressing cells showed saturability with the apparent Km value of 1.0 microM. Folate, sulfobromophthalein, and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) inhibited the methotrexate accumulation markedly. These findings suggest that the rat OAT-K1 is localized in the basolateral membranes of renal tubules, where it mediates renal clearance of methotrexate from the blood.

The peptide-based thrombin inhibitor CRC 220 is a new substrate of the basolateral rat liver organic anion-transporting polypeptide

The peptidomimetic thrombin inhibitor CRC 220, 4-methoxy-2,3,6-trimethylphenylsulfonyl-L-aspartyl-D-4-amidinop henylalanyl- piperidide, is taken up into isolated rat hepatocytes through active, carrier-mediated transport. This uptake is inhibited by bile acids. Functional expression in Xenopus laevis oocytes was performed to identify the transport system responsible for the hepatocellular CRC 220 uptake. Injection of poly(A)+RNA in X. laevis oocytes resulted in a two- to three-times higher uptake of CRC 220, compared with uninjected or water-injected control oocytes. Taurocholate (200 mumol/L) inhibited this uptake completely. No uptake of the peptidomimetic thrombin inhibitor was observed, when X. laevis oocytes were injected with complementary RNA (cRNA) encoding either the cloned rat liver Na(+)-dependent taurocholate transporter Ntcp, the renal oligopeptide carrier rhaPT or the intestinal oligopeptide transporter PepT1. However, after injection of cRNA of the cloned rat liver Na(+)-independent organic anion transporting polypeptide oatp, a specific and saturable CRC 220 uptake was observed (Michaelis-Menten constant 29.5 mumol/L). Cis-inhibition with known oatp-substrates, e.g., 20 mumol/L Bromsulphalein (BSP), 2007 mumol/L taurocholate and 2007 mumol/L cholate, occurred in oatp-expressing X. laevis oocytes, whereas substrates of the two peptide carriers as well as dipeptide- and single-amino acid constituents of the thrombin inhibitor itself lacked any significant inhibitory effects. These data show that the modified dipeptide CRC 220 is a highly selective substrate of the organic anion transporting polypeptide oatp in the basolateral plasma membrane of rat hepatocytes.

Effect of antisense oligonucleotides on the expression of hepatocellular bile acid and organic anion uptake systems in Xenopus laevis oocytes

A Na(+)-dependent bile acid (Na+/taurocholate co-transporting polypeptide; Ntcp) and a Na(+)-independent bromosulphophthalein (BSP)/bile acid uptake system (organic-anion-transporting polypeptide; oatp) have been cloned from rat liver by using functional expression cloning in Xenopus laevis oocytes. To evaluate the extent to which these cloned transporters could account for overall hepatic bile acid and BSP uptake, we used antisense oligonucleotides to inhibit the expression of Ntcp and oatp in Xenopus laevis oocytes injected with total rat liver mRNA. An Ntcp-specific antisense oligonucleotide co-injected with total rat liver mRNA blocked the expression of Na(+)-dependent taurocholate uptake by approx. 95%. In contrast, an oatp-specific antisense oligonucleotide when co-injected with total rat liver mRNA had no effect on the expression of Na(+)-dependent taurocholate uptake, but it blocked Na(+)-independent uptake of taurocholate by approx. 80% and of BSP by 50%. Assuming similar expression of hepatocellular bile acid and organic anion transporters in Xenopus laevis oocytes, these results indicate that Ntcp and oatp respectively represent the major, if not the only, Na(+)-dependent and Na(+)-independent taurocholate uptake systems in rat liver. By contrast, the cloned oatp accounts for only half of BSP transport, suggesting that there must be additional, non-bile acid transporting organic anion uptake systems in rat liver.

Bumetanide is not transported by the Ntcp or by the oatp: evidence for a third organic anion transporter in rat liver cells

The loop diuretic bumetanide which inhibits hepatic bile acid uptake competitively according to its transport kinetics has been proposed to serve as a substrate of a multispecific bile acid transport system in liver parenchymal cells. However, when the in vitro transcripts of two cloned hepatic bile acid uptake carriers, the Ntcp (Na+/taurocholate cotransporting polypeptide) and the oatp (organic anion transporting polypeptide), was expressed for three days in Xenopus laevis oocytes [3H]bumetanide uptake was not increased although bile acid uptake was stimulated. The data presented show that bumetanide is taken up by a third organic anion transport system which is different from the cloned bile acid transporters.

Current concepts of hepatic uptake, intracellular transport and biliary secretion of bile acids: physiological basis and pathophysiological changes in cholestatic liver dysfunction

Hepatic sinusoidal uptake of bile acids is mediated by defined carrier proteins against unfavourable concentration and electrical gradients. Putative carrier proteins have been identified using bile acid photoaffinity labels and more recently using immunological probes, such as monoclonal antibodies. At the sinusoidal domain, proteins with molecular weights of 49 and 54 kDa have been shown to be carriers for bile acid transport. The 49 kDa protein has been associated with the Na(+)-dependent uptake of conjugated bile acids, while the 54 kDa carrier has been involved in the Na(+)-independent bile acid uptake process. Within the hepatocyte, cytosolic proteins, such as the glutathione S-transferase (also designated the Y protein), the Y binders and the fatty acid binding proteins, are able to bind bile acids and possibly facilitate their movement to the canalicular domain. At the canalicular domain a 100 kDa carrier protein has been isolated and it has been shown by several laboratories that this particular protein is concerned with canalicular bile acid transport. The system is ATP-dependent and follows Michaelis-Menten kinetics. Interference with bile acid transport has been demonstrated by several chemicals. The mechanisms by which these chemicals inhibit bile acid transport may explain the apparent cholestatic properties observed in patients and experimental animals treated with these agents. Several studies have shown that Na+/K(+)-ATPase activity is markedly decreased in cholestasis induced by ethinyloestradiol, taurolithocholate and chlorpromazine. However, other types of interference have been described and the cholestatic effects may be the result of several mechanisms. Cholestasis is associated with several adaptive changes that may be responsible for the accumulation of bile acids and other cholephilic compounds in the blood of these patients. It may be speculated that the nature of these changes is to protect liver parenchymal cells from an accumulation of bile acids to toxic levels. However, more detailed quantitative experiments are necessary to answer questions with regard to the significance of these changes and the effect of various hepatobiliary disorders in modifying these mechanisms. It is expected that the mechanisms by which bile acid transport is regulated and efforts to understand the molecular basis for these processes will be among the areas of future research.

Transport of dehydroepiandrosterone and dehydroepiandrosterone sulphate into rat hepatocytes

The purpose of the present study was to characterize the transport of dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulphate (DHEAS) into hepatocytes at physiological and pharmacological concentrations. Hepatocytes were isolated from female Sprague-Dawley rats by collagenase perfusion. Uptake of [3H]DHEA and [3H]DHEAS at increasing concentrations (3.5 nM-100 microM) was measured by the rapid filtration technique at 30 s intervals up to 120 s. The uptake of DHEAS by hepatocytes was saturable (Km = 17.0 microM; Vmax = 3.7 nmol/min/mg cell protein). In contrast, a specific saturable transport system for DHEA could not be detected in rat hepatocytes. It is suggested that DHEA enters the cell by diffusion. The uptake of DHEAS could be inhibited by antimycin A, carbonylcyanide-m-chlorophenylhydrazone, and dinitrophenol (inhibitors of the mitochondrial respiratory chain), by dinitrofluorobenzene and p-hydroxymercuribenzoate (NH2- and SH-blockers, respectively), and by monensin (Na(+)-specific ionophore). No inhibition was seen in the presence of ouabain (inhibitor of Na(+)-K(+)-ATPase) and phalloidin (inhibitor of cholate transport and actin-blocker). Interestingly, DHEAS uptake was inhibited by bile acids (cholate, taurocholate and glycocholate). Conversely, [3H]cholate uptake was strongly inhibited by DHEAS, which indicates a competition for the same carrier. Replacement of sodium ion with choline markedly decreased uptake velocity at pharmacological DHEAS concentrations. The results suggest that DHEAS uptake is a saturable, energy-dependent, carrier-mediated, partially Na(+)-dependent process, and that DHEAS may be taken up via the multispecific bile acid transport system.

The toxicity of the mutagen 'MX' and its analogue, mucochloric acid, to rainbow trout hepatocytes and gill epithelial cells and to Daphnia magna

The cytotoxicity of the, in Salmonella, potent mutagenic compound, 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) and its structural analogue 3,4-dichloro-5-hydroxy-2[5H]-furanone (mucochloric acid, MCA), was studied in freshly isolated rainbow trout hepatocytes and gill epithelial cells by determining 86Rb-leakage and decrease in fluorescence intensity in calcein AM-loaded cells. The acute toxicity of the compounds to Daphnia magna was studied by determining the concentration causing immobilization of the organism. MX proved to be more toxic than MCA both in the cellular assays and in the acute toxicity test with D. magna. MX was more toxic to hepatocytes than to gill epithelial cells. The uptake of [14C]MX was also much more efficient in hepatocytes than in gill epithelial cells. The uptake of [14C]MX in hepatocytes was not inhibited by taurocholic acid in excess, indicating that MX is not taken up by the carrier complex responsible for the uptake of taurocholate in the hepatocytes. Both the acute toxicity to D. magna and cytotoxicity of MX and MCA was rather low (EC50 values > 0.1 mM) and we conclude that it is very unlikely that MX and MCA at concentrations occurring in recipients receiving chlorination effluents from pulp mills or chlorinated domestic sewage, as regards their acute toxicity, implies a risk for aquatic animals.

Alterations in microtubules, intermediate filaments, and microfilaments induced by microcystin-LR in cultured cells

Microcystin-LR (MCLR) is a cyanobacterial hepatotoxin that inhibits intracellular serine/threonine protein phosphatases causing disruption of actin microfilaments (MFs) and intermediate filaments (IFs) in hepatocytes. This study compared the effects of MCLR on the organization of MFs, IFs, and microtubules (MTs) in hepatocytes and nonhepatocyte cell lines and determined the sequence of toxin-induced changes in these cytoskeletal components. Rat renal epithelial cells and fibroblasts were incubated with MCLR at 100 or 200 microM for 6-18 hr. Rat hepatocytes in primary culture were exposed to the toxin at 1 or 10 microM for 2-64 min. Cells were fixed and incubated with primary antibodies against beta-tubulin, actin, and vimentin or cytokeratin IFs, followed by gold-labeled secondary antibodies with silver enhancement of the gold probe. The fraction of fibroblasts and hepatocytes with altered cytoskeletal morphology was evaluated as a function of MCLR dose and exposure time to assess the sequence of changes in cytoskeletal components. Changes in fibroblasts and some hepatocytes were characterized initially by disorganization of IFs, followed rapidly by disorganization of MTs, with the progressive collapse of both cytoskeletal components around cell nuclei. Many hepatocytes exhibited MT changes prior to effects on IF structure. Alterations in MFs occurred later and included initial aggregation of actin under the plasma membrane, followed by condensation into rosette-like structures and eventual complete collapse into a dense perinuclear bundle. The similarity of effects among different cell types suggests a common mechanism of action, but the independent kinetics of IF and MT disruption in hepatocytes suggests that there may be at least 2 sites of phosphorylation that lead to cytoskeletal alterations.

Comparative pathology of microcystin-LR in cultured hepatocytes, fibroblasts, and renal epithelial cells

The cyanobacterial toxin microcystin-LR (MCLR) is a potent inhibitor of protein phosphatases 1 and 2A, and is selectively toxic to the liver in vivo and to isolated hepatocytes in vitro. This selectivity is believed to be due to toxin uptake via bile acid carriers. We investigated at the light and ultrastructural levels the effects of high concentrations of MCLR and long incubation times to determine in vitro whether fibroblasts and kidney cells (non-target cells) respond in the same manner as do hepatocytes (target cells) at low concentrations and short incubation times. Cultured rat skin fibroblasts (ATCC 1213) and rat kidney epithelial cells (ATCC 1571) were incubated with with MCLR at 133 microM for 1-24 hr. Lesions in these cells were compared with those in cultured hepatocytes incubated MCLR at 13.3 microM from 1 to 32 min. Lesions in hepatocytes, kidney cells, and fibroblasts were noted at 4 min, 1 hr, and 8 hr, respectively, after initial exposure to MCLR. Lesions in all three cell types progressed and included plasma membrane blebbing, loss of cell-to-cell contact, clumping and rounding of cells, cytoplasmic vacuolization, and redistribution of cytoplasmic organelles. Loss of microvilli, whorling of rough endoplasmic reticulum, dense staining and dilated cristae in mitochondria, and pinching off of membrane blebs were noted only in hepatocytes. Nuclear changes typical of apoptosis were observed only in fibroblasts and kidney cells. Similarities in responses of different cell types to MCLR exposure probably reflect a common biochemical mechanism of action, i.e., inhibition of protein phosphatases 1 and 2A as described by others.(ABSTRACT TRUNCATED AT 250 WORDS)