Characterization of an in vitro blood-brain barrier model system for studying drug transport and metabolism

Bovine brain micro vessel endothelial cells have been isolated and grown in culture to monolayers. These endothelial cell monolayers have been characterized morphologically with electron microscopy, histochemically for brain endothelium enzyme markers, alkaline phosphatase and γ-glutamyl trans-peptidase, and by immunofluorescence to detect Factor VIII antigen, an exclusive endothelial antigen. Results of these studies indicate that the cells forming the monolayers are of endothelial origin and possess many features of the in vivo brain endothelium responsible for formation of the blood-brain barrier. This in vitro blood-brain barrier model system was used in experiments to determine the permeability of the cultured monolayer to sucrose, leucine, and propranolol. Leucine rapidly moved across the monolayers of this in vitro system and tended to plateau after approximately 10 min. In contrast, the rates of sucrose and propranolol movement were linear during a 1-hr observation period, with the rate of propranolol movement across the monolayer greater than that of sucrose. The ability to detect differences in the permeability of the monolayers to leucine, propranolol, and sucrose with radioactive tracers suggests that this in vitro model system will be an important tool for the investigation of the role of the blood-brain barrier in the delivery of centrally acting drugs and nutrients.

Effect of bisphenol A on drug efflux in BeWo, a human trophoblast-like cell line

Bisphenol A (BPA) is a monomer of polycarbonate plastics that has estrogenic activities and has been shown to be a substrate for multidrug resistant efflux mechanisms, specifically, P-glycoprotein. Since the natural hormone estrogen reverses multidrug resistance in some cell types, we hypothesized that BPA might have a similar activity in trophoblasts. We have used BeWo cells as an in vitro model for human trophoblasts and calcein AM as a substrate for drug efflux mechanism to characterize BPA interactions with placental P-glycoprotein. We found that chronic exposure of BeWo cells to BPA did not alter intracellular calcein accumulation in a fashion that would be reflective of changes in P-glycoprotein expression. Immunoblots affirmed that BPA had small effects on P-glycoprotein expression. However, BeWo cells acutely exposed to BPA pretreatment were observed to have a significantly decreased calcein accumulation. Addition of cyclosporin A, a P-glycoprotein inhibitor and substrate, completely reversed BPA's effects on calcein accumulation and resulted in a net increase, relative to controls, in calcein accumulation by the BeWo cells. BPA was found not to stimulate P-gp ATPase or alter intracellular esterases mediating calcein release from calcein AM. Therefore, our results suggested that BPA stimulated drug efflux by BeWo cells probably by direct effects on P-glycoprotein.

β-Amyloid-Induced Neurodegeneration and Protection by Structurally Diverse Microtubule-Stabilizing Agents

Deposition of beta-amyloid peptide (Abeta) and hyperphosphorylation of the tau protein are associated with neuronal dysfunction and cell death in Alzheimer's disease. Although the relationship between these two processes is not yet understood, studies have shown that both in vitro and in vivo exposure of neurons to Abeta leads to tau hyperphosphorylation and neuronal dystrophy. We previously reported that the microtubule-stabilizing drug paclitaxel (Taxol) protects primary neurons against toxicity induced by the Abeta(25-35) peptide. The studies in this report were undertaken to characterize the actions of paclitaxel more fully, to assess the effectiveness of structurally diverse microtubulestabilizing agents in protecting neurons, and to determine the time course of the protective effects of the drugs. Primary neurons were exposed to Abeta in the presence or absence of several agents shown to interact with microtubules, and neuronal survival was monitored. Paclitaxel protected neurons against Abeta(1-42) toxicity, and paclitaxel-treated cultures exposed to Abeta showed enhanced survival over Abeta-only cultures for several days. Neuronal apoptosis induced by Abeta was blocked by paclitaxel. Other taxanes and three structurally diverse microtubule-stabilizing compounds also significantly increased survival of Abeta-treated cultures. At concentrations below 100 nM, the drugs that protected the neurons did not produce detectable toxicity when added to the cultures alone. Although multiple mechanisms are likely to contribute to the neuronal cell death induced by oligomeric or fibrillar forms of Abeta, low concentrations of drugs that preserve the integrity of the cytoskeletal network may help neurons survive the toxic cascades initiated by these peptides.

Rapid optimization of the post-column fluorogenic ninhydrin reaction for the HPLC-based determination of bradykinin and related fragments

A flow injection analysis scheme is demonstrated for the rapid optimization of reagent concentrations, flow rates, delay time and temperature using the guanidino moiety specific fluorogenic ninhydrin reaction. Using the amino acid arginine, non-arginine containing peptides, and the arginine-containing peptides, bradykinin and related fragments, specificity is demonstrated. These results serve to extend previous descriptions of the post-column reaction by offering a time efficient approach for the optimization of newly assembled post-column reactors using this chemistry. The reactor is subsequently added to a gradient elution HPLC system with the separation of bradykinin and bradykinin fragments demonstrated. Detection sensitivity in the high femtomole-low picomole mass range was achieved for these substances.

The presence of inducible cytochrome P450 types 1A1 and 1A2 in the BeWo cell line

The activity and inducibility of cytochrome P450 systems (CYP1A1:1A2) of the human placenta were assessed in a representative human trophoblast-like cell line, BeWo. The activity of CYP1A1 and CYP1A2 in microsome preparations from human liver, placenta, primary cultures of human cytotrophoblast, and BeWo cells was measured by O -dealkylation of 7-ethoxyresorufin (EROD) and 7-methoxyresorufin O -demethylation (MROD), respectively. Results indicated high EROD and MROD activity associated with human liver microsomes, sometimes comparable activities in human placenta microsomes prepared from smokers, and relatively low activities in human placenta microsomes from nonsmokers and in the primary cultures of cytotrophoblasts isolated from nonsmokers. Microsomes from BeWo cell monolayers exhibited the lowest EROD and MROD activities relative to all other microsome preparations. However, compared to primary cultures of normal trophoblasts, the EROD activity of the BeWo cells was far more sensitive to typical inducers, 3-methylcholanthrene, 1,2-benzanthracene, and beta-naphthoflavone. EROD activity in BeWo cells was induced approximately 200-fold by 3-methylcholanthrene. Both EROD and MROD activity in BeWo cells was readily induced by 1,2-benzanthracene, 100-fold and 60-fold, respectively. After induction with 1,2-benzanthracene, the CYP1A1 selective inhibitor, alpha-naphthoflavone, and the CYP1A2 selective inhibitor, furafylline, effectively inhibited enzyme activities with IC(50)s of 2.4 microM and 12.8 microM, respectively, in microsomes from both trophoblasts culture systems. These results show that major cytochrome P450 forms present in human placenta are present and inducible in BeWo cells, a potential model for investigation of drug metabolism mechanisms in the human trophoblast.

Contribution of efflux pump activity to the delivery of pulmonary therapeutics

To date, there are few in vitro models of the human lung that have been used to characterize multidrug resistant (MDR) efflux pump activity. It is expected that the presence of these protein transporter molecules, such as P-glycoprotein (Pgp) and the multidrug resistance protein associated protein-1 (MRP1), might play a role in limiting drug absorption through the pulmonary epithelium, as has been reported for other epithelial drug delivery barriers such as the intestine and brain. To date, the exact role of the lung resistance related protein (LRP) in MDR is unclear. In this article, we have summarized the biochemistry, function and in vitro/in vivo modulation of Pgp and MRP1. These topics are discussed in light of pulmonary delivery of therapeutic agents, with particular emphasis being placed on the bronchial region of human airways.

Progress and limitations in the use of in vitro cell cultures to serve as a permeability screen for the blood-brain barrier

A relatively simple, widely applicable, and robust in vitro method of predicting blood-brain barrier (BBB) permeability to central nervous system-acting drugs is an increasing need. A cell-based model offers the potential to account for transcellular and paracellular drug diffusional processes, metabolism, and active transport processes, as well as nondefined interactions between a drug and cellular material that may impact upon a membrane's overall permeability profile. Any in vitro BBB cell model to be utilized for the transendothelial BBB permeability screening of potential central nervous system drugs must display reproducible solute permeability, and a number of other general criteria including: a restrictive paracellular barrier; a physiologically realistic cell architecture; the functional expression of key transporter mechanisms; and allow ease of culture to meet the technical and time constraints of a screening program. This article reviews the range of in vitro cell-based BBB models available, including the primary/low passage bovine and porcine brain endothelial cultures as well as the spectrum of immortalized brain endothelial cell lines that have been established. The article further discusses the benefits and limitations of exploiting such systems as in vitro BBB permeability screens.

Carrier-mediated transport of folic acid in BeWo cell monolayers as a model of the human trophoblast

Using cultured BeWo cells as a model of human trophoblast, we investigated whether carrier-mediated transport of folic acid occurs. BeWo cells, which were derived from human choriocarcinoma, were cultured on a tissue culture plate or in a permeation chamber. When the cells reached confluence, drug uptake or transport experiments were performed. The uptake of [(3)H]folic acid by BeWo cells occurred at a much lower rate at 4 degrees C than at 37 degrees C. The uptake of [(3)H]folic acid was saturable at higher concentrations and inhibited by typical metabolic inhibitors, sodium azide and 2,4-dinitrophenol. The uptake of [(3)H]folic acid was significantly increased with decreasing pH of the incubation buffer and markedly inhibited by 4,4'-diidothiocyanostilbene-2,2'-disulfonic acid (DIDS). Analogs of folic acid, methotrexate and 5-methyltetrahydrofolate, inhibited the uptake of [(3)H]folic acid by BeWo cells. Kinetic analysis using Lineweaver-Burk plots revealed that methotrexate competitively inhibited the uptake of [(3)H]folic acid and folic acid competitively inhibited the uptake of [(3)H]methotrexate. In transport experiments, the permeation of [(3)H]folic acid from the apical-to-basal side was greater than that from the basal-to-apical side, and the transport of [(3)H]folic acid from the apical-to-basal side was inhibited by an excess of folic acid. The findings obtained in the present study confirm the existence of an asymmetric, carrier-mediated transport system for folic acid and its analog, methotrexate, across BeWo cells, a representative of the human trophoblast.

Modulation of P-glycoprotein activity in Calu-3 cells using steroids and beta-ligands

The purpose of this work was to investigate if P-glycoprotein (Pgp) efflux pump activity could be inhibited in the sub-bronchial epithelial cell line, Calu-3, by glucocorticosteroids and beta-ligands. The Pgp modulation efficiency of each compound was determined by its ability to increase the accumulation of the Pgp substrate rhodamine 123 (Rh123) accumulation in these cells. Pgp inhibition was observed at > or =100 microM steroids and beta-ligand. The modulation effectiveness of the beta-ligands increased with increasing hydrophobicity (logP(octanol/aqueous)) whereas an obvious correlation was not obtained with the complete set of steroids tested. Steroidal Pgp substrates did not affect Rh123 accumulation (e.g. aldosterone, dexamethasone, 11beta,17alpha,21-OH progesterone). In contrast, two hydrophobic non-Pgp steroidal substrates (testosterone and progesterone) displayed different effects on Rh123 accumulation, with progesterone being the more potent modulator. The most hydrophobic beta-ligand, propranolol, a known Pgp substrate, gave the largest increase in Rh123 accumulation in this therapeutic class. The beta-ligand modulation efficiency could also be correlated to Pgp structural recognition elements such as hydrogen bonding potential, the presence of a basic nitrogen and planar aromatic ring. No effect on Rh123 accumulation was observed with the formulation additives tested (ethanol, glycerol and palmitoyl carnitine) at concentrations previously reported to be non-toxic to Calu-3 cells.

Investigation of the metabolism of substance P at the blood-brain barrier using capillary electrophoresis with laser-induced fluorescence detection

Substance P (SP) metabolism was investigated upon exposure to a monolayer of bovine brain microvessel endothelial cells (BBMECs), a cell culture model of the blood-brain barrier. SP was incubated with the BBMECs and its metabolism was followed as a function of time over a 5-h period. The resulting samples were derivatized with naphthalene-2,3-dicarboxaldehyde (NDA)/cyanide, separated, and detected using cyclodextrin-modified electrokinetic chromatography with laser-induced fluorescence detection (CDMEKC-LIF). Upon exposure to the BBMEC monolayer, SP rapidly degraded to produce the N-terminal (1-9), (1-4) and (1-7) and C-terminal (2-11) and (3-11) fragments. These results were compared with those in an earlier report from our laboratory, where SP metabolism was investigated in vivo by microdialysis sampling in rat striatum.

Multidrug resistance-associated protein-1 functional activity in Calu-3 cells

The purpose of this work was to determine whether the in vitro bronchiolar epithelial cell model, Calu-3, possesses efflux pump activity by the multidrug resistance-associated protein-1 (MRP1). Reverse transcription-polymerase chain reaction demonstrated MRP1 gene expression in Calu-3 cells. Indirect fluorescence studies showed a basolateral membrane localization of MRP1 compared with P-glycoprotein (Pgp) that was found on the apical side of these cells. An increase in the rate of accumulation of the MRP1 substrate calcein was observed following treatment with the organic anion/MRP1 inhibitor indomethacin, the Pgp inhibitors cyclosporin A (CsA) and vinblastine, as well as conditions of energy depletion. Total calcein efflux was significantly decreased with the MRP1 inhibitors probenecid and indomethacin, while total efflux was unchanged following treatment with CsA. In the latter case, however, intracellular calcein levels postefflux were significantly greater. Probenecid and indomethacin increased calcein net secretion 2.4- and 3.5-fold, respectively. The efflux of etoposide, a known substrate for both Pgp and MRP1, was shown to be mainly Pgp-mediated by using the multidrug-resistant inhibitors quinidine (mixed Pgp/MRP1), CsA (Pgp), and MK571 (MRP1). Together, these data suggest that Calu-3 cells possess MRP1 functional activity that is subordinate to Pgp efflux. We present here kinetic analysis of calcein efflux from Calu-3 cells to support our findings.

Conjugation with L-Glutamate for in vivo brain drug delivery

In vitro studies have shown that conjugation of a model compound [p-di(hydroxyethyl)-amino-D-phenylalanine (D-MOD)] with L-Glu can improve D-MOD permeation through the bovine brain microvessel endothelial cell monolayers (Sakaeda et al., 2000). The transport of this D-MOD-L-Glu conjugate is facilitated by the L-Glu transport system. In this paper, we evaluate the in vivo brain delivery of model compounds (i.e. D-MOD, p-nitro-D-phenylalanine (p-nitro-D-Phe), 5,7-dichlorokynurenic acid (DCKA) and D-kyotorphin) and their L-Glu conjugates. DCKA was also conjugated with L-Asp and L-Gln amino acids. The analgesic activities of D-kyotorphin and its L-Glu conjugate were also evaluated. The results showed that the brain-to-plasma concentration ratio of D-MOD-L-Glu was higher than the D-MOD alone; however, the plasma concentration of both compounds were the same. The plasma concentration of p-nitro-D-Phe-L-Glu conjugate was higher than the parent p-nitro-D-Phe; however, the brain-to-plasma concentration ratio of p-nitro-D-Phe was higher than its conjugate. On the other hand, both DCKA and DCKA conjugates have a low brain-to-plasma concentration ratio due to their inability to cross the blood-brain barrier (BBB). The L-Asp and L-Glu conjugates of DCKA have elevated plasma concentrations relative to DCKA; however, the DCKA-L-Gln conjugate has the same plasma concentration as DCKA. For D-kyotorphin, both the parent and the L-Glu conjugate showed similar analgesic activity. In conclusion, conjugation of a non-permeable drug with L-Glu may improve the drug's brain delivery; however, this improvement may depend on the physicochemical and receptor binding properties of the conjugate.

P-glycoprotein efflux pump expression and activity in Calu-3 cells

The purpose of this work was to determine if the sub-bronchial epithelial cell model, Calu-3, expresses the functionally active P-glycoprotein (Pgp) efflux pump. Calu-3 cells express lower levels of Pgp than both Caco-2 and A549 cells as determined by Western Blot analysis. In Calu-3 cells, accumulation of the Pgp substrates rhodamine 123 (Rh123) and calcein acetoxymethyl ester (calcein-AM) was increased in the presence of the specific Pgp inhibitors cyclosporin A (CsA), vinblastine, and taxol. Significant inhibition of Pgp activity was not observed until after 2 h in both cell lines. The organic anion/multidrug resistance associated protein-1 (MRP1) inhibitors, probenecid and indomethacin, did not affect Rh123 accumulation, whereas an increase in calcein accumulation was observed by both agents. The metabolic inhibitor sodium azide decreased the efflux of Rh123 out of Calu-3 cells to the same degree as CsA, supporting inhibition of an active, efflux pathway. The basolateral-to-apical transport of Rh123 was significantly higher than that in the reverse direction, indicating a secretory pathway of efflux that was inhibited 25-fold by CsA. Basolateral-to-apical transport of Rh123 was inhibited slightly with both MRP1 inhibitors; however, no significant effect of Rh123 net secretion was observed. Mixed inhibitor studies demonstrated that Rh123 efflux was mainly Pgp mediated. These results support an energy-dependent Pgp efflux pump pathway that is sensitive to inhibition with CsA in Calu-3 cells.

Nitric oxide and blood-brain barrier integrity

The blood-brain barrier (BBB) is comprised of the endothelial cells that line the capillaries of the brain. The unique characteristics of this barrier include tight intercellular junctions, a complex glycocalyx, a paucity of pinocytic vesicles, and an absence of fenestra. These properties allow for the selective exchange of substances between the systemic circulation and the extracellular fluid compartment of the brain. It is well established that there are many conditions, including those mediated by nitric oxide (NO), that can lead to an opening of the BBB, eventually leading to vasogenic edema and secondary brain damage. The precise molecular mechanisms mediating NO-induced tissue injury and the breakdown of the BBB are complex and not completely understood. NO is a soluble, easily diffusible gas that is generated by NO synthase. Two of the isoforms of NO synthase are constitutive, calcium-dependent enzymes that modulate many physiological functions, including the regulation of smooth muscle contraction and blood flow. The third isoform is calcium-independent and inducible and can be stimulated by stress, inflammation, and infection. Under these conditions, NO can be generated in large quantities and has detrimental effects on the CNS. NO has been shown to increase permeability of the BBB, allowing substances to enter into the brain passively. This review considers the role of NO and BBB integrity.

Improving the selectivity of HAV-peptides in modulating E-cadherin-E-cadherin interactions in the intercellular junction of MDCK cell monolayers

PURPOSE

The objective of this work is to understand the sequence specificity of HAV peptides and to improve their selectivity in regulating E-cadherin-E-cadherin interactions in the intercellular junctions.

METHODS

Peptide 1 was modified using an alanine scanning method to give peptides 2-6. The ability of these peptides to modulate intercellular junctions was evaluated using Madin-Darby Canine Kidney (MDCK) cell monolayers on Transwell membranes from either the apical (AP) or the basolateral (BL) side. Modulation of the intercellular junctions was measured by the ability to lower the transepithelial electrical resistance (TEER) of MDCK monolayers and by the increase in mannitol flux. Molecular docking experiments were performed to model the binding properties of these peptides to the EC1 domain of E-cadherin.

RESULTS

Peptides 5 (Ac-SHAVAS-NH2) and 6 (Ac-SHAVSA-NH2) were found to be more effective than the parent peptide 1 in decreasing the resistance of the cell monolayer. Furthermore, comparative studies with the control and the weak inhibitor peptide 2 indicate that peptide 5 displayed a significant increase in mannitol flux. Molecular docking of peptides 1, 2 and 5 to the EC1 domain suggests that peptide 5 has the lowest binding energy.

CONCLUSIONS

HAV peptides have the ability to modulate E-cadherin-E-cadherin interactions in the intercellular junctions of the MDCK cell monolayer, thus indirectly increasing the permeability of the tight junctions. This observation indicates that residues flanking the HAV sequence are important in the binding selectivity of HAV peptides to E-cadherin. Molecular docking can further aid in the design of peptides with better selectivity to the EC1 domain of E-cadherin.

Microparticulate uptake mechanisms of in-vitro cell culture models of the respiratory epithelium

The objective of this study was to examine the uptake mechanisms of fluorescent polystyrene microspheres of various diameters and surface chemistry by two human cell lines derived from the respiratory epithelium, A549 and Calu-3. Briefly, A549 and Calu-3 cells were grown to confluence in 12-well cluster plates and the uptake of fluorescent microspheres by the cells was determined at various time points. The amount of microspheres internalized by the cells was determined by correcting for non-specific binding to the cell surface. The data showed that A549 cells appeared to have more phagocytic activity than Calu-3 cells. Albumin-coated microspheres as large as 3 microm diameter can be internalized by A549 cells. The amount of internalization by A549 cells observed for 0.5-microm diameter albumin-coated microspheres was approximately 10-times greater than that observed for 1-microm diameter spheres and approximately 100-times greater than values observed for 2- and 3-microm diameter beads. Transmission electron micrographs confirmed that the microspheres were internalized by the cells. Uptake experiments conducted with Calu-3 cells indicated that albumin-coated microspheres were neither bound nor internalized by the cells. The effect of microsphere surface chemistry on the uptake mechanism indicated that amidine microspheres were internalized more rapidly and to a greater extent by both A549 and Calu-3 cells than carboxylate microspheres and non-coated microspheres. This phenomenon is thought to be attributed to masking of the negative polystyrene core by the positive amidine functional group; this effect was less marked for the carboxylate microspheres. These results suggest that A549 and Calu-3 cells can internalize microspheres and that size and effective charge played an important role in the uptake process.

Characterization of the Calu-3 cell line as a tool to screen pulmonary drug delivery

The objective of this research was to examine the human sub-bronchial gland cell line, Calu-3, and assess its potential as a metabolic and transport model to study drug delivery to the respiratory epithelium. The present studies were conducted using Calu-3 cells grown in Transwells(R) or in multiwell cluster plates. TEER values for Calu-3 monolayers were determined using the World Precision Instrument Voltohmmeter and STX-2 electrode. The results confirmed that Calu-3 cells form tight monolayers and give appreciable TEER values in culture when grown under air-interface conditions. Permeability data for small lipophilic molecules across Calu-3 monolayers suggested that the cell line is a suitable model to examine the transport of low molecular weight substances and xenobiotics. Calu-3 cells were also found to efflux FITC-transferrin (MW 80000) in a polarized manner. The metabolic capacity of Calu-3 cells was also examined. The P4501A1 and P4502B isozymes were determined to be functional, but not inducible, with fluorescent resorufin assays. The data indicated that the Calu-3 cell line may be useful for studying the contributions of bronchial epithelial cells to mechanisms of drug delivery at the respiratory epithelium.

Enhancement of transport of D-melphalan analogue by conjugation with L-glutamate across bovine brain microvessel endothelial cell monolayers

In this paper, the L-glutamate (L-Glu) transport system was targeted to improve the delivery of a model compound, p-di(hydroxyethyl)-amino-D-phenylalanine (D-MOD), through the blood-brain barrier (BBB) in vitro cell culture model. D-MOD is an analogue of an antitumor agent D-melphalan. To target the L-Glu transport system, D-MOD was conjugated to L-Glu to give D-MOD-L-Glu conjugate. D-MOD and D-MOD-L-Glu transport properties were evaluated using the bovine brain microvessel endothelial cell (BBMEC) monolayers. The results suggest that D-MOD-L-Glu conjugate permeates through the BBMEC monolayers more readily than the parent D-MOD. The improvement of transport may be due to the recognition of D-MOD-L-Glu by the L-Glu transport system. The transport mechanism was evaluated using several different experiments including: (a) concentration-dependent studies; (b) temperature-dependent studies; (c) substrate inhibition studies; and (d) metabolic inhibitor studies. The D-MOD-L-Glu transport was inhibited by the change of temperature from 37 degrees C to 4 degrees C. At higher concentrations, the transport of D-MOD-L-Glu reached plateau due to saturation. Furthermore, some amino acids (i.e., L-Glu, L-Asp, D-Asp, and L-Gln) inhibited the transport of D-MOD-L-Glu; presumably the conjugate was competing with these amino acids for the same transport system. Metabolic inhibitors (i.e., 2,4-dinitrophenol and sodium azide) suppressed the transport of the conjugate. However, the conjugate was not transported by monocarboxylic acid, dipeptide and neutral amino acid transporters. In conclusion, the L-Glu transport system can be utilized to facilitate a non-permeable drug across the BBB by conjugating the drug with L-Glu amino acid.

Functional expression of P-glycoprotein in primary cultures of human cytotrophoblasts and BeWo cells

The objective of this study was to investigate the functional expression of the efflux transporter, P-glycoprotein (P-gp), in primary cultures of human cytotrophoblasts and BeWo cell monolayers. Uptake studies with primary cultures of human cytotrophoblasts or BeWo cells were conducted with calcein-AM and vinblastine (P-gp markers) or fluorescein (MRP marker) in the presence of specific P-gp or MRP inhibitors. Results showed that the accumulation of P-gp substrates calcein-AM and vinblastine by BeWo cells or primary cultures of human cytotrophoblasts was significantly enhanced in the presence of a typical P-gp inhibitor, cyclosporin-A, or other inhibitors such as quinidine, verapamil, and dipyridamole. MRP inhibitors had no effect on the accumulation of calcein or fluorescein by BeWo cells. Western blots confirmed the presence of multidrug resistant gene product 1 (MDR1) in both primary cultures of human cytotrophoblasts and BeWo cells. This study demonstrates functional P-gp in term human trophoblasts and further supports the use of primary cultures of human cytotrophoblasts and BeWo cells as in vitro models of the trophoblast to investigate mechanisms regulating drug distribution across the placenta.

Fatty acid transport regulatory proteins in the developing rat placenta and in trophoblast cell culture models

The placenta forms a selective barrier that is able to transport nutrients that are of critical use to the fetus. Delivery of essential fatty acids to the fetus is dependent upon transplacental transport and provides the backbone for the biosynthesis of biological membranes, myelin and various signalling molecules. The primary objective of this research was to elucidate the expression patterns of genes that regulate fatty acid transport across the placenta. Several fatty acid transport regulatory genes have been identified in the rat including; cytoplasmic heart fatty acid binding protein (hFABP), plasma membrane fatty acid binding protein (FABPpm), fatty acid translocase (FAT) and fatty acid transport protein (FATP). In this study, we have elucidated temporal and spatial expression patterns for these genes in the rat placenta and in cell culture models of the rat placenta by Northern blot, RT-PCR, Western blot and/or by in situ hybridization analyses. Expression of hFABP was specific to the labyrinth zone, the main barrier and site of transplacental transport in the rat placenta. In addition, the levels of hFABP expression increased with gestational age, suggesting a growing requirement for fatty acid transport with advancing stages of pregnancy. FABPpm, FAT and FATP are expressed in both the junctional and labyrinth zones of the rat placenta. FAT was predominantly localized to the labyrinth zone by in situ hybridization analysis. The placental cell expression patterns of the genes involved in fatty acid transport were supported by our observations of HRP-1 (labyrinth zone) and Rcho-1 (junctional zone) trophoblast cell culture models. Given their cell surface location, we predict that FABPpm, FAT and FATP potentially participate in placental fatty acid uptake. The predominant expression of hFABP and FAT in the labyrinth zone of the chorioallantoic placenta implicates hFABP and FAT in the transplacental movement of fatty acids from maternal to fetal compartments.

Regeneration and maintenance of bile canalicular networks in collagen-sandwiched hepatocytes

The morphological and cytoskeletal reorganization of collagen-sandwiched rat hepatocytes during the de novo formation of complete canalicular networks was examined by phase, fluorescence and electron microscopy. During the initial stages of membrane repolarization, there was a marked accumulation of both microfilaments and microtubules at the sites of canalicular generation. Microtubule-disrupting agents (colchicine, nocodazole) inhibited the localization of actin filaments at cell margins and the initiation and branching of canalicular networks. After removal of microtubule-disrupting agents, microfilaments relocalized to the canalicular borders and microtubules nucleated along the margins of the bile canaliculi at sites distinct from the peri-canalicular actin networks. Microfilament-perturbing agents (cytochalasin D, phalloidin) did not affect the de novo initiation of bile canaliculi and only slightly impaired the development of canalicular lumina into networks. In established cultures with complete canalicular networks, subsequent treatment with microtubule-disrupting agents did not acutely affect the integrity of preformed canalicular networks. In contrast, treatment with microfilament-perturbing agents caused a marked dilation of most canaliculi. These results illustrate the differential role of the cytoskeleton in the regeneration and maintenance of bile canalicular networks by collagen-sandwiched hepatocytes. Moreover, this study shows the utility of this system as an in vitro model for examining the regulation of cell and membrane polarity.

Carrier-mediated transport of valproic acid in BeWo cells, a human trophoblast cell line

The biochemical mechanisms mediating the rapid distribution of valproic acid across placenta are not precisely known. We have characterized valproic acid transport by the human trophoblast using the human choriocarcinoma cell line, BeWo. The uptake of [14C]valproic acid by BeWo cells was found to be saturable and blocked by pre-exposure to the metabolic inhibitors, sodium azide and 2,4-dinitrophenol. Valproic acid uptake by the BeWo cells was also inhibited by the protonophore, carbonylcyanide p-trifluoromethoxyphenylhydrazone, but not anion exchange inhibitor. Selected monocarboxylic acids inhibited the uptake of [14C]valproic acid by BeWo cells, whereas dicarboxylic acids did not alter the uptake process. Analysis of Lineweaver-Burk plots of valproic acid uptake in the presence of benzoic acid, a marker for the monocarboxylic acid transporter, revealed a competitive process for uptake. In transcellular transport experiments, the permeation of [14C]valproic acid from the apical-to-basal side of the monolayers was significantly greater than the permeation from basal-to-apical side. Additionally, the permeation of [14C]valproic acid from apical-to-basal side was inhibited by monocarboxylic acids and not dicarboxylic acids. The results provide biochemical evidence of a proton-dependent, saturable, and asymmetric transport system, presumed to be a monocarboxylic acid transporter, for valproic acid in a human trophoblast model.

Carrier-mediated transport of monocarboxylic acids in BeWo cell monolayers as a model of the human trophoblast

The monolayer-forming, human choriocarcinoma cell line, BeWo, was used to study the mechanisms of monocarboxylic acid transport across the human trophoblast. Benzoic acid, acetic acid, and lactic acid were used as markers for monocarboxylic acid carrier-mediated transport. The uptake of benzoic acid by BeWo cells was saturable (K(t) = 0.6 +/- 0.3 mM) at higher concentrations and significantly inhibited by typical metabolic inhibitors, sodium azide and 2, 4-dinitrophenol. A selection of different monocarboxylic acids, including a natural substrate lactic acid, also substantially inhibited the uptake of benzoic acid and acetic acid by BeWo cells, whereas dicarboxylic acids did not affect the uptake of either marker. Monocarboxylic acid uptake was pH-dependent and inhibited by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), a protonophore. Kinetic analysis using Lineweaver-Burk plots revealed that monocarboxylic acids competitively inhibited the uptake of benzoic, lactic, and acetic acid by BeWo cells. In transport experiments, the permeation of benzoic acid from apical-to-basolateral side was greater than the permeation from the basolateral-to-apical side, and the transport of benzoic acid from apical-to-basolateral side was inhibited by monocarboxylic acids. The findings obtained in the present study confirm the existence of an asymmetric, carrier-mediated transport system for monocarboxylic acids across the BeWo cell, a representative of the human trophoblast.

Controlling drug delivery across the placenta

A challenge in modern drug therapy is to develop strategies for safer and more selective targeting of drug delivery in pregnancy. Specifically, approaches are needed that would restrict unnecessary drug exposure to either mother or fetus. There is evidence emerging that indicates the placenta does express natural transport and metabolism processes that function to control drug and nutrient distribution between the mother and fetus. Further, in vitro techniques developed in the past 10 years now provide some of the tools necessary to elucidate transport and metabolism processes typical of the human placenta. As a consequence, pharmaceutical scientists are in a position to contribute significantly to the design and development of drugs for pregnancy.

Nutrient transport across the placenta

The placenta forms a selective barrier that functions to transport nutrients that are of critical use to the fetus. Nutrient transport across the placenta is regulated by many different active transporters found on the surface of both maternal and fetal facing membranes of the placenta. The presence of these transporters in the placenta has been implicated in the facilitation of nutrient diffusion and proper fetal growth. In this review, recent developments concerning nutrient transporters that regulate glucose, amino acid, fatty acid, and nucleoside transplacental movement are discussed.

Transport mechanisms for the antidepressant citalopram in brain microvessel endothelium

Blood-brain barrier transport of the selective serotonin reuptake inhibitor and antidepressant, citalopram, was studied using monolayers of bovine brain microvessel endothelial cells (BMECs). This study provides for the first time, evidence of a transport mechanism for a selective serotonin reuptake inhibitor (SSRI). Carrier-mediated transport, efflux mechanisms, as well as inhibition of metabolizing enzymes of citalopram were investigated. Citalopram transport was saturable and temperature-dependent suggesting that passage of the drug across BMECs was mediated by a carrier mechanism. Since the apical to basolateral and basolateral to apical permeability coefficients were similar and cyclosporin A, a P-glycoprotein inhibitor, does not modify the transport of citalopram, it appeared that no active efflux systems were involved in this transport. Citalopram is only available as a racemic drug and its pharmacological effect resides mainly in the S-(+)-enantiomer. However, the passage of citalopram enantiomers across BMEC monolayers was not stereoselective. Finally, inhibition of the metabolizing enzymes of citalopram and monoamine oxidases did not modify the permeation of citalopram across BMECs. Collectively, our results suggested that citalopram crosses the blood-brain barrier via a non-stereoselective, bidirectional and symmetrical carrier-mediated mechanism without influences of active efflux mechanisms or monoamine oxidases.

Cerebrovascular Biology and the various neural barriers: challenges and future directions

DESPITE MAJOR ADVANCES in neuroscience, potential therapeutic options for the treatment of central nervous system diseases often cannot be optimized secondary to the presence of the blood-brain barrier (BBB). During the next decade of inquiry, it is crucial that basic science and clinical research that is focused on overcoming the BBB, to optimize delivery to the central nervous system, be identified and supported as a priority topic. For this reason, the third international Cerebrovascular Biology and Blood-Brain Barrier Conference was convened in March 1998 in Gleneden Beach, OR. This meeting brought together basic science and clinical researchers from around the world to analyze BBB function and to discuss delivery of effective agents to the central nervous system for treatment of brain disease. This report summarizes the information presented at the meeting and the discussions that ensued. The current state of knowledge, obstacles to further understanding the BBB, and research priorities are identified.

Optimizing candidate selection--a vision in business limited conference. 1-2 December 1998, Basel, Switzerland

The pharmaceutical industry is faced with filtering hundreds of thousands of compounds to identify successful drug candidates. Given these numbers, how does the pharmaceutical industry identify optimal therapeutic agents rapidly, efficiently, economically and successfully, with the ultimate result of the patient receiving the best drug? The conference summarized the present and future requirements for evaluating emerging technologies, integrating that technology into a filter for large and growing numbers of compounds, building and linking diverse knowledge bases, and establishing predictive foundations that will optimize and accelerate drug discovery and development. Specific conference topics focused on organizational and management approaches as well as some of the major technologies and emerging techniques for supporting drug candidate selection and optimization. It is predicted that the pharmaceutical industry will be synthesizing and screening a million or more compounds for multiple therapeutic targets in the near future. Pulling together the resources of current and emerging technology, knowledge, and multidisciplinary teamwork, so that discovery and selection of successful drug candidates from this large pool of compounds can take place rapidly, is a significant challenge. This conference focused on the organizational issues and experimental tools that can provide for a shortening of discovery time, identification of current and future selection techniques and criteria, the linking of technologies and business strategies to reduce risk, and novel processes for optimizing candidates more quickly and efficiently. The conference was directed at industrial scientists involved in all stages along the drug discovery and development interface. This conference was well-attended, with approximately 100 participants.

AT1 receptors mediate angiotensin II uptake and transport by bovine brain microvessel endothelial cells in primary culture

The endothelial lining of the blood-brain barrier tightly controls the distribution of peptide hormones between the central nervous system and the circulation. By using primary cultures of brain microvessel endothelial cells, an in vitro model of the blood-brain barrier, we report here the uptake and transport of the octapeptide angiotensin II by a specific receptor population. With the angiotensin II antagonists losartan (AT1 specific) and PD 123,319 (AT2 specific), we showed that both the uptake and transport of angiotensin II were mediated by the AT1 receptor. Western blot analysis confirmed the existence of the AT1 receptor in our cell-culture model. Rhodamine 123 studies also suggested that both angiotensin II antagonists, but not angiotensin II, were substrates for the P-glycoprotein efflux system, thus restricting the transport of these compounds. These results suggest an AT1 receptor mediates uptake and transport of angiotensin II at the blood-brain barrier and may contribute to the regulation of cerebrovascular levels of the peptide.

Partial maintenance of taurocholate uptake by adult rat hepatocytes cultured in a collagen sandwich configuration

PURPOSE

This study was designed to characterize taurocholate uptake properties in primary cultures of rat hepatocytes maintained under different matrix conditions.

METHODS

Hepatocytes isolated from male Wistar rats (230-280 g) were cultured on a simple collagen film, on a substratum of gelled collagen or between two layers of gelled collagen (sandwich configuration). Hepatocyte morphology, taurocholate uptake properties, and expression of the sinusoidal transport protein. Na+/taurocholate-cotransporting polypeptide (Ntcp) were examined in these cultures at day 0 and day 5.

RESULTS

By day 5, monolayer integrity had deteriorated in simple collagen cultures. In contrast, cell morphology was preserved in hepatocytes maintained in a sandwich configuration. At day 5, taurocholate accumulation at 5 min in hepatocytes cultured on a simple collagen film, on a substratum of gelled collagen, and in a sandwich configuration was approximately 13%, 20% and 35% of day-0 levels, respectively, and occurred predominately by a Na+-dependent mechanism. The initial taurocholate uptake rate vs. concentration (1-200 microM) profile was best described by a combined Michaelis-Menten and first-order function. In all cases, the estimated apparent Km values were comparable for day-0 and day-5 hepatocytes (3241 microM). In contrast, the Vmax values of hepatocytes cultured on a simple collagen film, on gelled collagen and in a sandwich configuration were approximately 5, 6 and 14% of the values at day 0, respectively; values for the first-order rate constant were 5-, 3- and 2-fold lower, respectively. Immunoblot analysis indicated that at day 5 Ntcp expression in hepatocytes cultured in a sandwich configuration was greater than in hepatocytes cultured on a simple collagen film.

CONCLUSIONS

A collagen sandwich configuration reestablishes normal morphology and partially restores bile acid uptake properties in primary cultures of rat hepatocytes.

Characterization of the A549 cell line as a type II pulmonary epithelial cell model for drug metabolism

Multiple cell types contribute to the pulmonary barrier including Type I and Type II alveolar epithelium. The objective of this research was to establish and characterize an in vitro model of Type II alveolar epithelium using the A549 human lung adenocarcinoma cell line. A549 cells form confluent monolayers with Type II characteristic morphology and tannic acid staining for typical lamellar bodies. A549 cells possess P450 IA1 and P450 IIB6 as determined by Western blots. Both CYPIA1 and CYPIIB6 P450 isozymes were determined to be functional with the fluorescent resorufin assay. Only the IA1 isozyme was observed to be inducible with selected polycyclic hydrocarbons. Uptake and transport experiments were carried out in cluster plates and in Snapwells. Cationized ferritin, a nonspecific absorbtive marker, was found to be taken up by the cells in a concentration-, time-, and temperature-dependent fashion. Lucifer yellow, a fluid-phase marker, was not internalized by the A549 cells. Transferrin, a representative receptor-mediated endocytic marker, was found to be taken up by the cells in a concentration-dependent and competitive fashion. Transport experiments involving fluorescein-transferrin also showed that A549 monolayers were polarized, with a greater amount of intracellular transferrin being transported out of the basolateral side of the cells. The experimental data agree favorably with literature for primary cultures of Type II pulmonary epithelial cells. These results indicated that the A549 cell line may be useful for the studying the metabolic and macromolecule processing contributions of alveolar Type II cells to mechanisms of drug delivery at the pulmonary epithelium.

Comparison of the effects of potential parenteral vehicles for poorly water soluble anticancer drugs (organic cosolvents and cyclodextrin solutions) on cultured endothelial cells (HUV-EC)

The effect of dilution of parenteral vehicles (organic cosolvent and 0.1 M cyclodextrin solutions) on cultured endothelial cells (HUV-EC) were compared in vitro. Cell morphology was observed by phase contrast light microscopy and cell viability by measuring 3-[4, 5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) reduction or intracellular lactate dehydrogenase (LDH) activity and total protein. Disruption of the HUV-EC monolayer was observed at dilutions of 1 in 20 for the melphalan and PEP cosolvents, 1 in 100 for an investigational drug cosolvent, and 1 in 10 for 0.1 M dimethyl-beta-cyclodextrin. In comparison, 0.1 M SBE7M- and HP-beta-cyclodextrin caused only minor disruption at a 1 in 5 dilution. MTT reduction, intracellular LDH, and total protein were decreased following exposure to 1 in 10 dilution of the melphalan cosolvent. For other test solutions, intracellular LDH activity and total protein were measured, and reductions were observed following exposure to 1 in 10, 20, and 50 dilutions of the investigational drug cosolvent and 1 in 5 dilution of DM-beta-cyclodextrin (0.1 M). At a dilution of 1 in 10, no delayed toxicity was observed for cosolvents or cyclodextrin solutions. Hence, 0.1 M SBE7M- or HP-beta-cyclodextrin formulations may be less damaging to the venous endothelium at the site of injection than organic cosolvent formulations.

Evaluation of the role of P-glycoprotein in ivermectin uptake by primary cultures of bovine brain microvessel endothelial cells

The P-glycoprotein efflux system located on the apical membrane of brain capillary endothelial cells functions as part of the blood-brain barrier. In this study, primary cultures of bovine brain microvessel endothelial cells (BMECs) were investigated for the presence of a P-glycoprotein system and its contribution in regulating ivermectin distribution across the blood-brain barrier. Results of rhodamine 123 uptake studies with cyclosporin A and verapamil as substrates indicated that a functional efflux system was present on BMECs. Immunoblot analysis with the C219 monoclonal antibody to the product of the multidrug resistant member 1(MDR1) gene also confirmed the expression of MDR1 in the BMECs. Unbound ivermectin was shown to significantly increase the uptake of rhodamine 123 in BMECs, however, the drug only modestly enhanced the transcellular passage of rhodamine. The results of these studies affirmed that unbound ivermectin is an inhibitor of the MDR1 efflux system in BMECs.

Gestational and smoking effects on peptidase activity in the placenta

Angiotensin converting enzyme (ACE) and leucine aminopeptidase (LAP) regulate fetally and maternally generated peptides in the placenta. In this study, ACE-like activity was found to be decreased and LAP-like activity increased with increasing days of gestation in rat placental tissues forming the fetal:maternal interface. Membrane-associated ACE-like and LAP-like activities in the placenta of smokers were also found to be significantly higher than their respective activities in placenta of nonsmokers. Our collective findings suggest that gestational and environmentally-induced changes in placental peptidase activities may account for variable peptide hormone and/or therapeutic peptide metabolism in the placenta.

Receptor-mediated angiotensin II transcytosis by brain microvessel endothelial cells

Angiotensin II (Ang II) uptake and transport across monolayers of bovine brain microvessel endothelial cells (BMECs) was demonstrated. Ang II transport was linear up to 2 h, saturable with a K(m) of 1.7 nM, and tended to be polarized with the apical-to-basolateral transport being greater. [3H]Ang II transport was found to be inhibited by excess unlabeled Ang II, by the Ang II analog sarathrin, and by the endocytic inhibitor phenylarsine oxide. Ang II-(2-8) and-(3-8) were shown to significantly increase the transport of Ang II. These results demonstrate for the first time the receptor-mediated transcytosis of Ang II across brain microvessel endothelium.

Permeability properties of monolayers of the human trophoblast cell line BeWo

The BeWo cell line (b30 clone) has been examined as a potential in vitro system to study transplacental transport. At the light and electron microscope level, the cells were observed to form confluent monolayers on polycarbonate filters in approximately 5 days and morphologically resembled the typical human trophoblast. BeWo monolayers developed a modest transepithelial electrical resistance and a molecular size-dependent permeability to hydrophilic passive diffusion markers, fluorescein, and selected fluorescein-labeled dextrans. Linoleic acid permeation across BeWo monolayers was asymmetric, saturable, and inhibited by low temperature and excess competing fatty acid. Forskolin and 8-bromoadenosine 3',5'-cyclic monophosphate treatments stimulated morphological changes in BeWo cultures and enhanced the asymmetric passage of linoleic acid across the BeWo monolayers while having minimal effects on passive permeability, affirming that the differentiation state of the cells can influence membrane transporters and transmonolayer permeability. The basic permeability properties of the BeWo monolayers suggest that the cells grown on permeable supports may be examined as a convenient in vitro system to evaluate some transplacental transport mechanisms.

Permeability and metabolic properties of a trophoblast cell line (HRP-1) derived from normal rat placenta

The HRP-1 cell line is derived from normal rat placenta and appears morphologically similar to and retains characteristic expression of cellular markers of labyrinthine trophoblast cells. In this study, monolayers of HRP-1 cells grown on permeable supports were evaluated as a potential in vitro system to study trophoblast transport and metabolism. The cell line was shown to express and retain functional activity of the predominant placental cytochrome P450 isozyme, CYP1A1. Additionally, the HRP-1 cells retain functional activity of angiotensin I converting enzyme and carboxypeptidase N-like enzyme, peptidases characteristic of the trophoblast. The permeation of several hydrophilic, inert markers across the HRP-1 monolayers was observed to be dependent on effective molecular size and to be passive in nature. Functional asymmetry of the HRP-1 cells was illustrated by the predominant permeation of linoleic acid in the apical-to-basolateral direction across the monolayers. Transferrin passage across HRP-1 monolayers was concentration-dependent, was bidirectional, and could be inhibited by unlabeled transferrin, features typical of the trophoblast transport system for transferrin. Collectively, these properties suggest that the HRP-1 cell line may provide a useful tool for evaluating some of the permeability and metabolic properties of the trophoblast.

Modulation of cellular adhesion in bovine brain microvessel endothelial cells by a decapeptide

The importance of cell adhesion molecules in maintaining the cellular integrity of the endothelial layer is well recognized, yet their exact participation in regulating the blood-brain barrier (BBB) is poorly understood. Both Ca(2+)-dependent and Ca(2+)-independent cell adhesion molecules are found in endothelial cells. In this study, we used immunofluorescence, ELISA, Western blot and cell adhesion assay to identify a Ca(2+)-dependent cell adhesion molecule, E-cadherin, in bovine brain microvessel endothelial cells (BBMECs). Monoclonal anti-E-cadherin antibody specifically interacted with cultured BBMECs and decorated the cellular junctions with a series of punctate fluorescence spots as seen by indirect immunofluorescence using a confocal microscope. The intensity of these fluorescence spots increased after brief treatment with hIFN-gamma or CPT-cAMP. In the cellular extract of BBMECs, a 120 kDa protein was immunoprecipitated with anti-E-cadherin antibody. BBMECs did not react with anti-N-cadherin antibody, but recognized the FITC-labeled LRAHAVDVNG-NH2, a decapeptide generated from the EC-1 domain of N-cadherin, which decorated the lateral margins of the cells with fluorescence spots. A concentration-dependent binding of this decapeptide was also observed in the flow cytometry assay. BBMECs dissociated with trypsin plus Ca2+ were able to reaggregate only in the presence of Ca2+. However, such cell-cell aggregations of BBMECs were prevented by the presence of either anti-E-cadherin antibody or the decapeptide in the assay medium. These results confirm that BBMECs possess a distinct Ca(2+)-dependent cell adhesion mechanism that can be modulated by the decapeptide. This modulation of cell-cell adhesion in BBMECs by the decapeptide is thought-provoking for creating channels for paracellular drug delivery across the BBB.

Passive diffusion of weak organic electrolytes across Caco-2 cell monolayers: uncoupling the contributions of hydrodynamic, transcellular, and paracellular barriers

A systematic approach was used to demonstrate the quantitative interplay of pH, pKa, lipophilicity, charged and uncharged molecular species, molecular size, aqueous diffusivity, and stirring in passive transport across the aqueous boundary layer, microporous filter support, and transcellular and paracellular barriers in Caco-2 cell monolayers. The relationship of permeability of the aqueous boundary layer and hydrodynamic stirring was elucidated from transmonolayer fluxes of testosterone. Adrenergic receptor antagonists including propranolol (PPL), alprenolol (APL), pindolol (PDL), and atenolol (ATL) represented the model series of structurally similar weak bases with pKa values between 8.8 and 9.65. Although intrinsically lipophilic, their apparent log PC (n-octanol/water) at pH 7.4 and 6.5 ranged from -2.6 to 1.3. Effective permeability coefficients (Pe) correlated with log PC at both pH 7.4 and 6.5 showing a single sigmoidal-like curve: PPL > APL > PDL > or = ATL. The Pe approached a minimum plateau value established by the protonated ATL for the paracellular route (pore radius of 12 A) by molecular size-restricted diffusion within a negative electrostatic field of force. The Pe of the weak bases was delineated into component permeability coefficients of the aqueous boundary layer and porous filter support, the intrinsic permeabilities of charged and uncharged species for the transcellular and paracellular routes, and the extent to which the routes were utilized at each pH. This study emphasized a generally applicable approach to quantitatively analyze passive transport data on weak organic electrolytes and neutral molecules generated using cell culture monolayers.

21-aminosteroid and 2-(aminomethyl)chromans inhibition of arachidonic acid-induced lipid peroxidation and permeability enhancement in bovine brain microvessel endothelial cell monolayers

Selected 21-aminosteroids (U74500A, U74006F, and U74389G) and a 2-(aminomethyl)chromans (U78517F) were tested for their efficacy in preventing arachidonate-induced lipid peroxidation and permeability alterations in brain microvessel endothelial cells (BMECs). The 21-aminosteroids and 2-(aminomethyl)chromans were effective in varying degrees in inhibiting (U74500A = U78517F > U74006F = U74389G) concentration- and time-dependent arachidonate-induced thiobarbituric acid reactive substances (TBARS) production by BMECs. Arachidonate produced a corresponding concentration-dependent increase in BMEC monolayer permeability to the membrane impermeant marker, sucrose. Pretreatment of BMEC monolayers with either the 21-aminosteroids or the 2-(aminomethyl)chromans completely blocked the arachidonate-induced increase in permeability to sucrose. Our results demonstrated that these membrane-associating antioxidants were particularly effective in preventing both arachidonic acid-induced lipid peroxidation and permeability changes in BMEC monolayers. However, concentrations of some antioxidants that only partially inhibited TBARS production, completely inhibited the arachidonic acid-induced enhancement in BMEC monolayer permeability. Therefore, arachidonic acid-induced effects on BMEC permeability were likely due in part to both lipid peroxidation and direct or indirect effects of the fatty acid on membrane integrity. This study provides further support for the application of primary cultures of BMECs as a useful in vitro system to evaluate mechanisms through which mediators of disease or injury states compromise blood-brain barrier integrity.

Leucine enkephalin effects on paracellular and transcellular permeation pathways across brain microvessel endothelial cell monolayers

Leucine enkephalin (YGGFL) effects on markers for transcellular and paracellular permeation across the blood-brain barrier (BBB) were investigated in vitro with bovine brain microvessel endothelial cell (BMEC) monolayers in primary culture. Intact YGGFL, but not metabolites of YGGFL, stimulated BMEC uptake of lucifer yellow (LY), a marker for fluid-phase endocytosis, in a concentration-dependent manner. However, D-[Ala2]-leucine enkephalin (YAGFL), a YGGFL analogue that altered BMEC monolayer permeability, had no effect on LY uptake. In part, these results suggested that YGGFL's effects on fluid-phase uptake might not relate directly to enhanced BMEC transcellular permeability in the presence of the peptide. The measurement of the fluorescence anisotropy of membrane-bound diphenyl-hexatriene probes did not show substantial peptide-induced changes in membrane lipid packing order (i.e., membrane fluidity) and indicated a limited role for membrane perturbations in YGGFL-induced changes in BMEC monolayer permeability. Conversely, the apparent permeability coefficients showed size-dependent YGGL-induced alterations for passage of membrane-impermeant substances across BMEC monolayers. The apparent permeability coefficients of low-molecular-weight (low-mol-wt) molecules (mannitol, sucrose, and fluorescein) were increased on exposure to YGGFL. The apparent permeability coefficients for high-mol-wt molecules, FITC dextran conjugates (4, 20, and 71.6 Kd), were not affected by exposure to YGGFL. Transmission electron micrographs of lanthanum (Stoke's radius, 10 A) exclusion from BMEC intercellular junctions supported these observations. Collectively, results from this study suggest that YGGFL enhanced BMEC permeability either by altering paracellular openings or through formation of a small pore in the monolayers to allow preferential penetration of low-mol-wt or small molecular size (< 10 A) substances.

Quantitative approaches to delineate paracellular diffusion in cultured epithelial cell monolayers

When using cultured cell monolayers to determine the mechanism of transcellular diffusion of molecules, it may be important to identify the fraction that moves through the paracellular route or passively diffuses through tight junctions. We characterized the apparent diameter of the junctional pore in a variety of epithelial cell monolayers (Caco-2, MDCK, alveolar). Using hydrophilic extracellular permeants varying in molecular radii and charge (neutral, anionic, cationic, zwitterionic), rate-determining steps and factors of the paracellular route were quantitatively delineated by the model for molecular size-restricted diffusion within a negative electrostatic field of force. Protonated amines permeated the pores faster than their neutral images while organic anions were slower. With increasing molecular size the influence of charge diminished. This approach was used to quantify the relationship between permeant radius and transepithelial electrical resistance and to analyze changes in junctional pore size as a function of pharmacological perturbation, such as in the use of absorption promoters or adjuvants.

Changes in brain microvessel endothelial cell monolayer permeability induced by adrenergic drugs

Brain microvessel endothelial cell monolayers have been shown to be a suitable blood-brain barrier in vitro system to study adrenergic regulation of permeability. We tested adrenergic drugs on bovine brain microvessel endothelial cell monolayer permeability to a biomembrane impermeant molecule, sodium fluorescein. Endogenous catecholamines noradrenaline and adrenaline were tested as well as the alpha-adrenoceptor agonist phenylephrine, the beta-adrenoceptor agonist clenbuterol and the alpha-adrenoceptor antagonist prazosin. Results showed an alpha-adrenoceptor mediated increase and a beta-adrenoceptor mediated decrease in monolayer permeability. Both alpha- and beta-adrenoceptor mediated changes in permeability were abolished by inhibiting fluid-phase pinocytosis, either by vincristine or by avoiding bovine brain microvessel endothelial cell's energy utilization. The reverse transport (i.e., from brain to blood side) was also influenced by adrenergic drugs; alpha- or beta-adrenoceptor stimulation induced a permeability-reducing effect. We conclude that alpha-adrenoceptor stimulation increases bovine brain microvessel endothelial cell monolayer permeability and that beta-adrenoceptor stimulation has the opposite effect. Reverse transport results obtained with beta-adrenoceptor stimulation seem controversial and deserve further study. These results also support in vivo findings that demonstrated adrenergic influences on blood brain barrier permeability.

Formation of extensive canalicular networks by rat hepatocytes cultured in collagen-sandwich configuration

Rat primary hepatocytes were cultured under different extracellular matrix configurations and evaluated for the acquisition and maintenance of structural and functional cell polarity. De novo repolarization of the plasma membrane was variable in rate and extent in hepatocyte cultures maintained on a conventional single layer of either gelled or ungelled collagen. However, cultures maintained in a collagen-sandwich configuration initiated uniform formation of a contiguous anastomosing network of bile canaliculi throughout the entire culture. Localization of apical membrane markers demonstrated normal distribution at the canalicular membrane. A marked rearrangement of the intracellular microfilaments to the cell periphery was observed and coincided with the development of the bile canaliculi. Acquisition of normal bile canalicular function and integrity was observed within 3-4 days postoverlay as indicated by the concentration and retention of carboxyfluorescein within the canalicular network. These results demonstrate that cultures of hepatocytes maintained in a sandwich configuration may serve as a more reliable and representative model in which to study the physiology of hepatic function as well as the morphogenesis of polarized membrane domains in vitro.

Biochemical characteristics of primary and passaged cultures of primate brain microvessel endothelial cells

Rhesus macaque monkey brain microvessel endothelial cells (BMECs) were isolated and grown in culture in an effort to establish an appropriate primate in vitro model of the endothelial component of the blood-brain barrier. The presence of Factor VIII antigen, alkaline phosphatase, gamma-glutamyl transpeptidase, lactate dehydrogenase, total protein, and the passive permeability properties was documented for both primary and passaged cultures. Primate BMECs were shown to exhibit similar morphological and biochemical properties described for other BMEC culture systems derived from other species. In addition, the passaged primate BMECs were particularly notable for the changes in enzyme activities and total protein that parallel age-dependent changes in brain capillary endothelia. This study provides further support for the possible application of BMEC culture systems in investigations of blood-brain barrier functions under normal, aging, and diseased conditions.

Effect of some penetration enhancers on epithelial membrane lipid domains: evidence from fluorescence spectroscopy studies

The effect of the penetration enhancers Azone, oleic acid, 1-dodecanol, dodecyl N,N-dimethylaminoacetate (DDAA), and dodecyl N,N-dimethylaminoisopropionate (DDAIP) on epithelial membrane lipids was examined using human buccal cell membranes as a model for epithelial lipid bilayer. Buccal epithelial cells (BEC) were labeled with 1,6-diphenyl-1,3,5-hexatriene (DPH), 1-(4-(trimethylammonio)phenyl)-6- phenyl-1,3,5-hexatriene (TMA-DPH), and 8-anilino-1-naphthalene sulphonic acid (ANS) fluorophores to characterize enhancer-induced changes in the hydrophobic core, in the superficial polar head region, and on the exterior surface, respectively, with fluorescence anisotropy and fluorescence lifetimes. All the enhancers studied were found to decrease the BEC membrane lipid packing order in a concentration-dependent and time-dependent manner in the deep bilayer region, as shown by a 37-66% decrease in anisotropy. Oleic acid was also found to disrupt membrane lipids strongly in the polar head region, causing at least a 34% decrease in anisotropy values. Azone and DDAA were shown to alter molecular movement on the surface of the bilayers (24 and 19% decrease in anisotropy, respectively). The results suggest that interaction with membrane lipid domains is an important, but not the only, mode of action for the penetration enhancers studied.