Acinar redistribution and heterogeneity in transport of the organic cation rhodamine B in rat liver

Direct visualization of functional heterogeneity in hepatobiliary metabolism using 6-CFDA as model compound

Hepatobiliary metabolism is one of the major functions of the liver. However, little is known of the relationship between the physiological location of the hepatocytes and their metabolic potential. By the combination of time-lapse multiphoton microscopy and first order kinetic constant image analysis, the hepatocellular metabolic rate of the model compound 6-carboxyfluorescein diacetate (6-CFDA) is quantified at the single cell level. We found that the mouse liver can be divided into three zones, each with distinct metabolic rate constants. The sinusoidal uptake coefficients k₁ of Zones 1, 2, and 3 are respectively 0.239 ± 0.077, 0.295 ± 0.087, and 0.338 ± 0.133 min^-1, the apical excreting coefficients k₂ of Zones 1, 2, and 3 are 0.0117 ± 0.0052, 0.0175 ± 0.0052, and 0.0332 ± 0.0195 min^-1, respectively. Our results show not only the existence of heterogeneities in hepatobiliary metabolism, but they also show that Zone 3 is the main area of metabolism.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Reversible cholestasis induced by experimental partial obstruction of the bile duct

The aim of this investigation was to reproduce in rats a partial stenosis of the common bile duct to analyze early liver functional and morphometric changes. The hepatic transport kinetics of sulfobromophthalein (organic anion) and rhodamine B (organic cation) was also investigated, and compartmental analysis of both compounds was performed. The humoral parameters of liver function indicated a cholestasis after 2 days of surgery, which reverted to reach normal values on the seventh day. Tumor necrosis factor alpha serum levels showed a tendency to increase on the second day of stenosis (7 out of 14 rats) while white blood cells increased on the second day of stenosis, and turned to normal levels on the seventh day. Histological studies showed increased volume of portal areas and ductular proliferation, which did no revert during the time of the study (up to 7 days post-op). Conversely, a moderate fibrosis and leukocyte infiltrates in portal areas predominated on the second day of stenosis, but normalized on the seventh day. Bile flow was considerably diminished on the second day of partial obstruction as compared to controls. The mean recovery in bile of sulfobromophthalein after 1h of being injected was low on the second day of stenosis, but normalized on the seventh day. Conversely, that of rhodamine B was very low in all animals. Sulfobromophthalein kinetics showed that hepatic uptake and canalicular excretion were impaired during the second but normalized on the seventh day of stenosis. However, rhodamine B kinetics showed that this compound was poorly excreted in all groups although canalicular excretion increased on the second day. The results suggested a model of obstructive cholestasis induced by the experimental stenosis of the bile duct which was not only reversible but also implicates the role of hepatic inflammation.

The applicability of rat and human liver slices to the study of mechanisms of hepatic drug uptake

In the present study we investigated the applicability of the liver slice model to study mechanisms of drug uptake. Four model compounds were investigated that enter hepatocytes via entirely different membrane transport mechanisms. Rhodamine B (RB), which enters hepatocytes by passive diffusion, was homogeneously distributed throughout the rat liver slice (250 microm thickness) within 5 min, indicating that the penetration rate into the slice and the diffusion rate into the cells are rapid. In contrast, lucigenin (LU), which is taken up by hepatocytes through adsorptive endocytosis, was detected in the inner cell layers after 15 min. Digoxin uptake into the slice showed a temperature-dependent component and was stereoselectively inhibited by quinine, which is compatible with the involvement of a carrier-mediated uptake mechanism. The neo-glycoalbumin Lactose(27)-Human Serum Albumin (Lact(27)-HSA) and the negatively charged Succinylated-Human Serum Albumin (Suc-HSA) entered the slices and were taken up temperature-dependently into hepatocytes and endothelial cells, respectively. The liver slice preparation is a valuable tool to investigate the mechanisms of cellular uptake of drugs. Moreover, the precision-cut liver slices offer the unique possibility to study both hepatocyte and endothelial cell function in human and rat liver.

Saturable stimulation of fatty acid transport through model cytoplasm by soluble binding protein

To better define the role of soluble binding proteins in the cytoplasmic transport of amphipathic molecules, we measured the diffusional mobility of a fluorescent long-chain fatty acid, 12-N-methyl-(7-nitrobenz-2-oxa-1,3-diazol)aminostearate (NBD-stearate), through model cytoplasm as a function of soluble binding protein concentration. Diffusional mobilities were correlated with the partition of the fatty acid between membrane and protein binding sites. Cytoplasm was modeled as a dense suspension of liposomes, and albumin was used as a model binding protein. Albumin saturably increased NBD-stearate mobility through the membrane suspension approximately eightfold. Fatty acid mobility in the absence of albumin was identical to the mobility of the membrane vesicles (1.99 +/- 0.33 x 10(-8) cm(2)/s), whereas the mobility at saturating concentrations was identical to the mobility of albumin (1.65 +/- 0.12 x 10(-7) cm(2)/s). The protein concentration producing half-maximal stimulation of NBD-stearate diffusion (42.8 +/- 0.3 microM) was unexpectedly greater than that required to solubilize half of the NBD-stearate (17.9 +/- 3.0 microM). These results support a proposed mechanism for cytoplasmic transport of small amphipathic molecules in which aqueous diffusion of the protein-bound form of the molecule largely determines the transport rate. However, slow interchange of fatty acid between the binding protein and membranes also appears to influence the transport rate in this model system.

Membrane transport in hepatic clearance of drugs. II: Zonal distribution patterns of concentration-dependent transport and elimination processes

PURPOSE

The objective of the present simulation study was to investigate the effects of hepatic zonal heterogeneity of membrane transporter proteins and intrinsic elimination activities on hepatic clearance (CL) and drug concentration gradient profiles in the sinusoidal blood and hepatocytes.

METHODS

The model used in the simulations assumes an apparent unidirectional carrier-mediated transport and a bidirectional diffusion of substrates in the hepatic sinusoidal membrane as well as a nonlinear intrinsic elimination. Three different distribution patterns of the transporter and the metabolizing enzyme along the sinusoidal flow path were used for the simulations. The effects of changes in the Michaelis-Menten parameters for those nonlinear processes, and in the unbound fractions of the drug in blood and tissue components were investigated.

RESULTS

Significant differences in CL occurred when the distribution patterns of the transporter and/or the metabolizing enzyme activities were altered under nonlinear conditions. The highest CL values were observed when the transporter and the metabolizing enzyme had similar distribution patterns within the liver acinus, while opposite distribution patterns produced the lowest CL values. Tissue concentration profiles were significantly affected by the distribution patterns of the transporter, but the changes in blood concentration profiles were relatively small. Altering protein binding in blood produced significant changes in CL, and blood and tissue concentration gradients, while altering protein binding in tissue affected only drug accumulation patterns within hepatocytes, regardless of the distribution patterns of the transporter or the metabolizing enzyme.

CONCLUSIONS

The present simulations demonstrate that hepatic zonal heterogeneities in the transporter and the metabolizing enzyme activities can significantly influence hepatic clearance and/or drug concentration gradient profiles in the sinusoidal blood and hepatocytes.

Characterization of transport in isolated human hepatocytes. A study with the bile acid taurocholic acid, the uncharged ouabain and the organic cations vecuronium and rocuronium

The uptake and efflux of three categories of substrates were measured in isolated human hepatocytes and compared to those in rat hepatocytes. In addition, the extent to which the in vitro experiments quantitatively reflect liver function in vivo in both species was investigated. The anionic bile acid taurocholic acid was taken up by isolated human hepatocytes at a considerably lower rate than observed in isolated rat hepatocytes. Taurocholic acid uptake both in human hepatocytes and in liver plasma membrane vesicles showed sodium dependency. The uptake rate of taurocholic acid in isolated hepatocytes of both species was quantitatively compatible with the reported liver clearance of the bile acid in vivo. Ouabain uptake rate in isolated human hepatocytes was lower than in rat hepatocytes. This species difference was in accordance with pharmacokinetic studies in vivo on hepatic clearance of ouabain in man and rat. Uptake of vecuronium into human hepatocytes was about a factor of 10 lower than that in rat hepatocytes. Uptake into and efflux from human hepatocytes was comparable for the two short acting muscle relaxants vecuronium and rocuronium. Since distribution to the liver is considered to be a major factor in termination of action of vecuronium and rocuronium these observations were in line with the human pharmacokinetic profiles. In conclusion, the uptake rate of the studied model compounds in human hepatocytes appeared to be lower than that in rat hepatocytes. These observed transport rates reflected the relative hepatic transport rates observed in these species in the intact organism, but the absolute values in both species for some substrates may have been somewhat lower than calculated from in vivo data. It is concluded that transport studies in isolated hepatocytes are suitable for comparative drug transport studies, but are less precise in the prediction of quantitative membrane transport.

Bile canalicular cationic dye secretion as a model for P-glycoprotein mediated transport

This study explores properties of P-glycoprotein dependent membrane transport in rat liver with the use of acridine orange as the substrate. We studied the biliary secretion of the dye, its binding to canalicular membrane P-glycoprotein, and effects of the inhibitor cyclosporin A: acridine orange is excreted into bile together with less hydrophobic and glucuronidated metabolites. Cyclosporin A inhibited both the secretion of acridine orange and of its metabolites. In TR- animals, a rat strain that is deficient of the canalicular multi-specific organic anion transport system, non-metabolized acridine orange is the predominant species in bile and its secretion is also inhibited by cyclosporin A. Binding of acridine orange to liver P-glycoprotein was analyzed by photoaffinity labeling with azidopine, a substrate of P-glycoprotein dependent transport in multi-drug resistant tumor cells. Labeling of the immunoprecipitated P-glycoprotein was inhibited by acridine orange, verapamil, and by cyclosporin A. The results show that biliary secretion of acridine orange is highly analogous to P-glycoprotein mediated membrane drug transport in tumor cells that exhibit multi-drug resistance.

Processing of the phospholipid analogue phosphatidyl(N-sulphorhodamine B sulphonyl)ethanolamine by rat hepatocytes in vitro and in vivo

We have investigated the processing of the non-exchangeable fluorescent phospholipid analogue phosphatidyl(N-sulphorhodamine B sulphonyl)ethanolamine (N-Rh-PE) by rat liver cells. In the hepatocyte couplet system, N-Rh-PE was incorporated into the plasma membrane at 2 degrees C and readily internalized upon warming to 37 degrees C. Fluorescence was initially found to be concentrated in vesicles clustered throughout the cell, but subsequently it started to accumulate in pericanalicular vesicles, tentatively identified as lysosomes, and in the bile canalicular lumen. Analysis of cells and media by t.l.c. revealed the slow formation of at least two metabolites. After intravenous injection into bile-fistula rats of [9,10-3H-oleoyl]N-Rh-PE incorporated in small unilamellar liposomes, the initial rates of elimination from plasma of 3H and rhodamine label were virtually identical. However, biliary secretion of the 3H label (5.5% of dose at 2 h) was much slower than that of the rhodamine label (49.3% at 2 h). The rhodamine label in bile was chloroform-soluble, but not identical to the native molecule, and was resistant to phospholipase A2 and alkaline hydrolysis. To gain insight in the mechanism of the rapid bile secretion of this metabolite, we compared the processing of N-Rh-PE, its deacylated form [glycerophospho(N-sulphorhodamine B sulphonyl)ethanolamine; Gly-N-Rh] and the rhodamine label itself (sulphorhodamine B sulphonyl chloride; SRho). Intravenous injection of chloroform-soluble N-Rh-PE and of methanol/water-soluble Gly-N-Rh complexed with albumin both resulted in rapid bile secretion of chloroform-soluble fluorescent compounds (60.2% and 86.3% respectively at 2 h), which showed behaviour identical to that of the metabolite of liposomal N-Rh-PE on t.l.c. Methanol/water-soluble SRho was also rapidly secreted into bile (89.5% at 2 h) without being metabolized. Bile secretion of the chloroform-soluble metabolite of N-Rh-PE and of SRho was markedly impaired (-31% and -52% respectively) in GY Wistar rats, which express a genetic defect in the hepatobiliary transport of organic anions. Our data show that the rat hepatocyte is capable of modifying the structure of N-Rh-PE, a process which proceeds considerably faster in vivo than in vitro. The chloroform-soluble metabolite is subsequently rapidly removed via the bile. The canalicular organic anion transporting system, which is deficient in GY rats, appears to be involved in the excretion of this apolar product of hepatic metabolism.

Influence of albumin on the net sinusoidal efflux of the organic anion dibromosulfophthalein from rat liver

Besides being excreted into bile, several anionic drugs and anionic drug conjugates are excreted from liver into plasma by a carrier-mediated process (sinusoidal efflux). In the study presented here, we investigated the influence of albumin on the net sinusoidal efflux of an organic anion from isolated perfused liver, with net sinusoidal efflux defined as the resultant of efflux and reuptake. We tested the hypothesis that albumin exerts its effect on net sinusoidal efflux through inhibition of repeat uptake rather than on the sinusoidal efflux process itself. We also studied possible acinar heterogeneity in net sinusoidal efflux rate of the organic anion. Isolated rat livers were preloaded with the nonmetabolizable anionic model compound dibromosulfophthalein in the absence (type I) or presence (type II) of bovine serum albumin. Fluorescence microscopy of dibromosulfophthalein in liver sections showed selective zone 1 loading after type I loading and homogeneous acinar distribution after type II loading. The rate of net sinusoidal efflux and biliary excretion was then studied in single-pass perfusions with various concentrations of bovine serum albumin in the medium. Net sinusoidal efflux of dibromosulfophthalein showed a marked dependence on albumin concentration in both type I and type II experiments. Net efflux rate reached a maximal value at a medium concentration of 300 mumol/L bovine serum albumin and was not increased further at 600 mumol/L. The initial uptake rate of dibromosulfophthalein was not significantly different in the 300 mumol/L and 600 mumol/L bovine serum albumin experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.

Physicochemical determinants in hepatic extraction of small peptides

Although the liver is known to extract amino acids and organic anions by well-characterized transport systems, the factors that regulate the hepatic uptake of small, circulating peptides are poorly understood. We previously reported that cholecystokinin octapeptide, a biologically active form of cholecystokinin, is efficiently cleared by the liver and that uptake depends on its carboxyl-terminal tetrapeptide (Trp-Met-Asp-PheNH2). Here we further define the physicochemical determinants for hepatic clearance of cholecystokinin. A series of 13 tetrapeptides, including eight analogs of the carboxyl-terminal tetrapeptide of cholecystokinin-8 with different charges, hydrophobicity and amino-acid sequences, were prepared by solid-phase synthesis, purified by high-performance liquid chromatography and characterized by amino-acid analysis and mass spectrometry. Radioiodination was performed by oxidative or nonoxidative techniques. Hydrophobicity of individual radiolabeled peptides was calculated using published hydrophobicity data or measured directly by determining their partition between octanol and aqueous triethylammonium acetate. First-pass hepatic extraction of radiolabeled peptides was determined with a nonrecirculating, isolated, perfused rat liver model. First-pass hepatic extraction of injected, labeled peptides varied from 4% to 86% and correlated significantly (r = 0.85; p less than 0.0002) with hydrophobicity. Hydrophobic peptides with positive, neutral or negative charges were avidly extracted (30% to 86%) by the liver; first-pass clearance of hydrophobic peptides with similar charges varied with amino-acid sequence. In contrast, the first-pass hepatic extraction of positively or negatively charged hydrophilic tetrapeptides was negligible (less than 10%). These results suggest that hydrophobicity and amino-acid sequence--but not anionic or cationic nature--are the major determinants of hepatic extraction of cholecystokinin, and perhaps other small, circulating peptides.

The role of acinar zone 3 hepatocytes in bile formation: influence of bromobenzene treatment on bile formation in the rat

The role of zone 3 hepatocytes in bile formation was determined when they were selectively destroyed by 3.8 mmol/kg b.w. of bromobenzene injected i.p. for 48 h, as compared to appropriate controls. Bromobenzene treatment resulted in 29 +/- 4.4% hepatic lobule necrosis localized in the zone 3 hepatocytes. Although bile flow and bile salt-independent flow were not affected, this treatment was associated with a significant reduction in bile salt, and phospholipid secretion. The bile salt pool and bile salt synthesis were also significantly decreased. These results suggest that necrosis of zone 3 hepatocytes induced by bromobenzene reduced bile acid synthesis which decreased bile salt pool and affected bile salt and phospholipid secretion rates. However, necrosis of zone 3 hepatocytes did not affect bile flow or the bile salt-independent flow, suggesting that hepatocytes of zones 1 and 2 maintained the normal bile salt-independent flow when zone 3 hepatocytes were damaged.

Carrier-mediated transport in the hepatic distribution and elimination of drugs, with special reference to the category of organic cations

Carrier-mediated transport of drugs occurs in various tissues in the body and may largely affect the rate of distribution and elimination. Saturable translocation mechanisms allowing competitive interactions have been identified in the kidneys (tubular secretion), mucosal cells in the gut (intestinal absorption and secretion), choroid plexus (removal of drug from the cerebrospinal fluid), and liver (hepatobiliary excretion). Drugs with quaternary and tertiary amine groups represent the large category of organic cations that can be transported via such mechanisms. The hepatic and to a lesser extent the intestinal cation carrier systems preferentially recognize relatively large molecular weight amphipathic compounds. In the case of multivalent cationic drugs, efficient transport only occurs if large hydrophobic ring structures provide a sufficient lipophilicity-hydrophilicity balance within the drug molecule. At least two separate carrier systems for hepatic uptake of organic cations have been identified through kinetic and photoaffinity labeling studies. In addition absorptive endocytosis may play a role that along with proton-antiport systems and membrane potential driven transport may lead to intracellular sequestration in lysosomes and mitochondria. Concentration gradients of inorganic ions may represent the driving forces for hepatic uptake and biliary excretion of drugs. Recent studies that aim to the identification of potential membrane carrier proteins indicate multiple carriers for organic anions, cations, and uncharged compounds with molecular weights around 50,000 Da. They may represent a family of closely related proteins exhibiting overlapping substrate specificity or, alternatively, an aspecific transport system that mediates translocation of various forms of drugs coupled with inorganic ions. Consequently, extensive pharmacokinetic interactions can be anticipated at the level of uptake and secretion of drugs regardless of their charge.

Separation of hepatocytes of different acinar zones by flow cytometry

Hepatocytes in the proximal (zone 1) and distal (zone 3) regions of the liver acinus are selectively stained by perfusion of the isolated rat liver with 0.2-20 microM acridine orange (AO). After 10-60 min of anterograde perfusion, AO fluorescence is visible in zone 1 cells, whereas retrograde perfusion stains cells of zone 3. In this paper, we describe a technique to isolate a mixed population of fluorescent and nonfluorescent hepatocytes (cells from all acinar zones, which do not loose the zone specific AO labeling) and to separate these cells according to their zonal origin by fluorescence activated cell sorting. The zonal populations obtained were either fluorescent or nonfluorescent (purity greater than 95%). Separated cell fractions differed in their enzyme content (5' nucleotidase, succinate-dehydrogenase, beta-glucuronidase). An unidentified AO metabolite, which is not found in bile after retrograde perfusion (not formed in zone 3 cells), is also absent after retrograde perfusion in sorted fluorescent cells (zone 3 cells), indicating zonal purity of sorted cells.

Covalent and noncovalent protein binding of drugs: implications for hepatic clearance, storage, and cell-specific drug delivery

This review deals with the mechanisms by which the liver disposes of drugs that are covalently or noncovalently associated with proteins. Many drugs bind to plasma proteins such as albumin (mainly anionic compounds) and alpha 1-acid glycoprotein (cationic compounds). Nevertheless, the liver is able to clear such drugs efficiently from the circulation because of intrahepatic dissociation of the drug-protein complex. This clearance may involve spontaneous dissociation because of progressive removal of the unbound drug during liver passage, a process that can be rate limiting in hepatic uptake. Alternatively, the porous endothelial lining of the hepatic sinusoids may allow extensive surface interactions of the drug-protein complexes with hepatocytes, leading to facilitation of drug dissociation. Binding to plasma proteins and intracellular proteins in the cytoplasm or cell organelles is an important factor determining the hepatic storage and elimination rate of drugs. Drugs noncovalently associated with glycosylated proteins, which can be endocytosed by various liver cells, are not coendocytosed with such proteins. However, covalently bound drugs can be internalized by receptor-mediated endocytosis, which permits specific targeting to hepatocytes, endothelial cells, Kupffer cells, and lipocytes by coupling to different glycoproteins that are recognized on the basis of their terminal sugar. The endocytosed drug-carrier complex is routed into endosomes and lysosomes, where the active drug is liberated by cleavage of acid-sensitive linkages or proteolytic degradation of peptide linkers. This concept has been applied to antineoplastic, antiparasitic, and antiviral drugs.

Heterogeneous acinar localization of the asialoglycoprotein internalization system in rat hepatocytes

Desialylated glycoprotein is rapidly cleared from plasma by a receptor-mediated endocytic mechanism located on hepatocytes. We studied the hepatic acinar distribution of this asialoglycoprotein transport system with the ligand 125I-asialoorosomucoid using rat liver perfused in either antegrade or retrograde direction in combination with quantitative light microscopic autoradiography. Grain distribution along the acinus appeared dependent on the perfusion direction. A rather shallow zone 1 to zone 3 gradient was observed if livers were perfused in the normal direction. However, a statistically significantly steeper zone 3 to zone 1 gradient was detected in retrograde perfusions. Kinetic analysis of perfusate clearance profiles yielded a hepatic clearance of 21.6 +/- 1.3 ml per min in antegradely perfused liver. Hepatic extraction was calculated to be 60.1 +/- 7.4%. Biliary secretion of radioactivity amounted to 1.89 +/- 0.18% of the dose within 1 hr after injection and consisted of intact material (1.39 +/- 0.25%) and radioactive low-molecular-weight degradation products (0.52 +/- 0.08%), of which more than 90% could be accounted for by 125I-. Apart from a minor difference regarding biliary secretion of an unidentified glycopeptide (less than 0.1% of the injected dose), transport data for the retrogradely perfused livers were identical to those obtained with livers perfused in antegrade direction, emphasizing the functional equivalence of both groups of livers. The autoradiographic data indicate that zone 3 hepatocytes take up 125I-asialoorosomucoid more avidly than zone 1 cells. The kinetic and biochemical data indicate that further processing in the hepatocytes is virtually similar in the two zones.(ABSTRACT TRUNCATED AT 250 WORDS)