P-glycoprotein of blood brain barrier: cross-reactivity of Mab C219 with a 190 kDa protein in bovine and rat isolated brain capillaries

P-glycoprotein Expression Is Upregulated in a Pre-Clinical Model of Traumatic Brain Injury

Athletes participating in contact sports are at risk for sustaining repeat mild traumatic brain injury (rmTBI). Unfortunately, no pharmacological treatment to lessen the pathophysiology of brain injury has received U.S. Food and Drug Administration (FDA) approval. One hurdle to overcome for potential candidate agents to reach effective therapeutic concentrations in the brain is the blood-brain barrier (BBB). Adenosine triphosphate (ATP)-binding cassette (ABC) transporters, such as P-glycoprotein (Pgp), line the luminal membrane of the brain capillary endothelium facing the vascular space. Although these transporters serve to protect the central nervous system (CNS) from damage by effluxing neurotoxicants before they can reach the brain, they may also limit the accumulation of therapeutic drugs in the brain parenchyma. Thus, increased Pgp expression following brain injury may result in reduced brain availability of therapeutic agents. We therefore questioned if repeat concussive injury increases Pgp expression in the brain. To answer this question, we used a rodent model of repeat mild closed head injury (rmCHI) and examined the messenger RNA (mRN) and protein expression of both isoforms of rodent Pgp (Abcb1a and Abcb1b). Compared with sham-operated controls (n = 5), the mRNA levels of both Abcb1a and Abcb1b were found to be increased in the hippocampus at day 1 (n = 5) and at day 5 (n = 5) post-injury. Using a validated antibody, we show increased immunolabeling for Pgp in the dorsal cortex at day 5 and in the hippocampus at day 1 (n = 5) and at day 5 (n = 5) post-injury compared with sham controls (n = 6). Taken together, these results suggest that increased expression of Pgp after rmCHI may reduce the brain accumulation of therapeutic drugs that are Pgp substrates. It is plausible that including a Pgp inhibitor with a candidate therapeutic agent may be an effective approach to treat the pathophysiology of rmCHI.

P-gp expression in brown trout erythrocytes: evidence of a detoxification mechanism in fish erythrocytes

Blood is a site of physiological transport for a great variety of molecules, including xenobiotics. Blood cells in aquatic vertebrates, such as fish, are directly exposed to aquatic pollution. P-gp are ubiquitous “membrane detoxification proteins” implicated in the cellular efflux of various xenobiotics, such as polycyclic aromatic hydrocarbons (PAHs), which may be pollutants. The existence of this P-gp detoxification system inducible by benzo [a] pyrene (BaP), a highly cytotoxic PAH, was investigated in the nucleated erythrocytes of brown trout. Western blot analysis showed the expression of a 140-kDa P-gp in trout erythrocytes. Primary cultures of erythrocytes exposed to increasing concentrations of BaP showed no evidence of cell toxicity. Yet, in the same BaP-treated erythrocytes, P-gp expression increased significantly in a dose-dependent manner. Brown trout P-gp erythrocytes act as membrane defence mechanism against the pollutant, a property that can be exploited for future biomarker development to monitor water quality.

Expression of P-glycoprotein (ABCB1) and Mrp1 (ABCC1) in adult rat brain: focus on astrocytes

P-glycoprotein (P-gp, ABCB1) and the multidrug resistance-associated protein 1 (Mrp1, ABCC1) are two ATP-driven pumps that mediate the export of organic anions from cells and may confer cellular resistance to many cytotoxic hydrophobic drugs. Immunohistochemistry has shown that P-gp is expressed in rat brain capillary vessels forming the blood-brain barrier (BBB). Mrp1 mRNAs have been detected by RT-PCR in rat brain isolated capillaries. Although many studies have been published in this field, very little information is available on the expression, distribution and physiological functions of the two pumps in rat brain. To characterize the cerebral expression of both P-gp and Mrp1 transporters, we studied immunoreactions of rat brain sections with the two most commonly used antibodies: the monoclonal C219 (anti-P-gp) and the polyclonal 6KQ (anti-Mrp1). Immunological analyses revealed heterogeneity of the P-gp and Mrp1 expressions in rat brain. Indeed, choroidal and ependymal cells expressed Mrp1 rather than P-gp. However, tanycytes lining the third ventricle were strongly immunoreactive with both antibodies, suggesting a particular role for these cells in drug efflux mechanisms. Because of the detection of a 70-kDa component with 6KQ antibodies, immunoreactions obtained in rats were compared with these obtained in wild type and mrp1(-/-) mice. It showed that a positive reaction at the apical surface of the ependymal layer remained obvious, showing that 6KQ antibodies recognize an ependymal molecule, differing from the Mrp1. In addition, a continuous expression of C219-labeled epitopes, similar to endothelial labeling, was detected at the blood-brain barrier, whereas a discontinuous labeling, co-localized with glial fibrillary acidic protein (GFAP) immunostaining, was obtained with 6KQ antibodies. We showed that P-gp was preferentially expressed in the endothelial component and Mrp1 in the astroglial component of the blood-brain barrier. Moreover, Mrp1 was rather expressed than P-gp in parenchyma astrocytes and in glia limitans lining the meninges. These findings provide new insights into the cerebral distribution of two ABC transporters linked to multidrug resistance (MDR).

P-glycoprotein and caveolin-1α in endothelium and astrocytes of primate brain

P-glycoprotein (P-gp) and caveolin-1alpha are both involved in membrane transport, and studies in rodent brain show that these proteins are specifically localized at the microvascular endothelium, which forms the blood-brain barrier (BBB). In humans, P-gp is also expressed in astrocytes, especially in pathological tissue. The present study examines the cellular expression of P-gp and caveolin-1alpha in fresh-frozen brain from healthy rhesus monkey using confocal microscopy and polyclonal antibodies against either P-gp or caveolin-1alpha co-labeled for astrocytes or microvascular endothelium. P-gp and caveolin-1alpha are expressed in both astrocytes and endothelium of healthy primate brain. These findings suggest that P-gp and caveolin-1alpha share a broad spectrum of cellular expression and may play a role in drug transport within the brain in addition to the BBB.

The impact of efflux transporters in the brain on the development of drugs for CNS disorders

The development of drugs to treat disorders of the CNS requires consideration of achievable brain concentrations. Factors that influence the brain concentrations of drugs include the rate of transport into the brain across the blood-brain barrier (BBB), metabolic stability of the drug, and active transport out of the brain by efflux mechanisms. To date, three classes of transporter have been implicated in the efflux of drugs from the brain: multidrug resistance transporters, monocarboxylic acid transporters, and organic ion transporters. Each of the three classes comprises multiple transporters, each of which has multiple substrates, and the combined substrate profile of these transporters includes a large number of commonly used drugs. This system of transporters may therefore provide a mechanism through which the penetration of CNS-targeted drugs into the brain is effectively minimised. The action of these efflux transporters at the BBB may be reflected in the clinic as the minimal effectiveness of drugs targeted at CNS disorders, including HIV dementia, epilepsy, CNS-based pain, meningitis and brain cancers. Therefore, modulation of these efflux transporters by design of inhibitors and/or design of compounds that have minimal affinity for these transporters may well enhance the treatment of intractable CNS disorders.

Expression of multidrug-resistance P-glycoprotein (MDR1) in human brain tumors

Multidrug resistance (MDR) is associated with the expression of P-glycoprotein (P-gp), an ATP-dependent transporter which expels anti-cancer drugs from cells. In the present study, MDR1 P-gp was immunodetected by Western blot analysis in 60 human brain tumors, including meningiomas, schwannomas, low-grade gliomas (astrocytomas, pilocytic astrocytomas) and high-grade gliomas (anaplastic astrocytomas, glioblastomas and anaplastic oligodendrogliomas). Most samples from primary tumors expressed P-gp at the same levels as normal brain tissue except for schwannomas, in which levels were reduced by 65%, and meningiomas, in which levels were more than 10-fold higher in 7 of 10 samples. P-gp levels were 70% and 95% lower in brain metastases from melanomas and lung adenocarcinomas, respectively, than in normal brain tissue. These results indicate that the majority of primary brain tumors express MDR1 P-gp and that its high expression levels in meningiomas may be a marker for this type of brain tumor.

In vivo radioprotection of mouse brain endothelial cells by Hoechst 33342

Radiation-induced loss of mouse brain endothelial cells has been examined in mice given an intravenous injection of the DNA-binding radioprotector Hoechst 33342 (80 mg kg-1). At the time of irradiation, 10 min after injection, Hoechst fluorescence in the brain was confined to the endothelial cells. Endothelial cell density was measured using a histochemical fluorescence technique that had been used previously to monitor post-irradiation changes in endothelial cell density in rat brain, in which it was shown that a sensitive subpopulation comprising about 15% of the endothelial cells was lost within 24 h of radiation exposure. The present study shows a similar dose-response for the control mice, with depletion of the sensitive subpopulation to 85% being almost complete after a dose of 2.5 Gy gamma-rays. However, in mice irradiated 10 min after Hoechst 33342 administration, doses between 12 Gy and 20 Gy were required to ablate these cells. The kinetics of cell loss and the rather large dose modification factor suggests that Hoechst 33342 may be suppressing an apoptotic response in this subpopulation. Whatever the mechanism involved, Hoechst 33342 clearly provides substantial protection against early radiation-induced endothelial cell loss. Further studies are necessary to determine the extent to which this initial protection translates into an improved long-term survival of the "protected" cells and, especially, to see whether this endothelial cell protection can ameliorate the later consequences of central nervous system irradiation, namely necrosis and paralysis.

Astrocytes increase the functional expression of P-glycoprotein in an in vitro model of the blood-brain barrier

PURPOSE

To investigate the influence of astrocytes on P-glycoprotein (Pgp) expression and intracellular accumulation of Pgp substrates, separate from their net transcellular transport across the blood-brain barrier (BBB).

METHODS

An in vitro BBB model was used, comprising of brain capillary endothelial cells (BCEC) monolayers or BCEC co-cultured with astrocytes.

RESULTS

BCEC+astrocyte co-cultures seemed to express a higher level of Pgp compared to BCEC monolayers. Inhibition of Pgp results in an increased intracellular accumulation of Pgp substrates in both BCEC monolayers and BCEC+astrocyte co-cultures, and increased the sensitivity for vinblastine mediated disruption of the in vitro BBB (called the vinblastine exclusion assay). BCEC monolayers were more sensitive to vinblastine mediated disruption compared to BCEC+astrocyte co-cultures. In the latter, but not in BCEC monolayers, an inhibitable polar transport of Pgp substrates was only found from the brain to the blood side of the filter.

CONCLUSIONS

Astrocytes increase the functional expression of Pgp in our in vitro BBB model. These results also illustrate that an important role for Pgp on the BBB is to protect the barrier against intracellular accumulation of cytotoxic BBB disrupting compounds.

Brain microvascular P-glycoprotein and a revised model of multidrug resistance in brain

1. P-Glycoprotein is a 170-kDa transmembrane glycoprotein active efflux system that confers multidrug resistance in tumors, as well as normal tissues including brain. 2. The classical model of multidrug resistance in brain places the expression of P-glycoprotein at the luminal membrane of the brain microvascular endothelial cell. However, recent studies have been performed with human brain microvessels and double-labeling confocal microscopy using (a) the MRK16 antibody to human P-glycoprotein, (b) an antiserum to glial fibrillary acidic protein (GFAP), an astrocyte foot process marker, or (c) an antiserum to the GLUT1 glucose transporter, a brain endothelial plasma membrane marker. These results provide evidence for a revised model of P-glycoprotein function at the brain microvasculature. In human brain capillaries, there is colocalization of immunoreactive P-glycoprotein with astrocytic GFAP but not with endothelial GLUT1 glucose transporter. 3. In the revised model of multidrug resistance in brain, P-glycoprotein is hypothesized to function at the plasma membrane of astrocyte foot processes. These astrocyte foot processes invest the brain microvascular endothelium but are located behind the blood-brain barrier in vivo, which is formed by the brain capillary endothelial plasma membrane. 4. In the classical model, an inhibition of endothelial P-glycoprotein would result in both an increase in the blood-brain barrier permeability to a given drug substrate of P-glycoprotein and an increase in the brain volume of distribution (VD) of the drug. However, in the revised model of P-glycoprotein function in brain, which positions this protein transporter at the astrocyte foot process, an inhibition of P-glycoprotein would result in no increase in blood-brain barrier permeability, per se, but only an increase in the VD in brain of P-glycoprotein substrates.

Antibody C219 recognizes an alpha-helical epitope on P-glycoprotein

The ABC transporter, P-glycoprotein, is an integral membrane protein that mediates the ATP-driven efflux of drugs from multidrug-resistant cancer and HIV-infected cells. Anti-P-glycoprotein antibody C219 binds to both of the ATP-binding regions of P-glycoprotein and has been shown to inhibit its ATPase activity and drug binding capacity. C219 has been widely used in a clinical setting as a tumor marker, but recent observations of cross-reactivity with other proteins, including the c-erbB2 protein in breast cancer cells, impose potential limitations in detecting P-glycoprotein. We have determined the crystal structure at a resolution of 2.4 A of the variable fragment of C219 in complex with an epitope peptide derived from the nucleotide binding domain of P-glycoprotein. The 14-residue peptide adopts an amphipathic alpha-helical conformation, a secondary structure not previously observed in structures of antibody-peptide complexes. Together with available biochemical data, the crystal structure of the C219-peptide complex indicates the molecular basis of the cross-reactivity of C219 with non-multidrug resistance-associated proteins. Alignment of the C219 epitope with the recent crystal structure of the ATP-binding subunit of histidine permease suggests a structural basis for the inhibition of the ATP and drug binding capacity of P-glycoprotein by C219. The results provide a rationale for the development of C219 mutants with improved specificity and affinity that could be useful in antibody-based P-glycoprotein detection and therapy in multidrug resistant cancers.

Identification of a multidrug resistance-like system in Tetrahymena pyriformis: evidence for a new detoxication mechanism in freshwater ciliates

The freshwater ciliate Tetrahymena pyriformis is an ubiquitous organism that is present in all aquatic ecosystems. This protozoan showed a clear resistance against some polycyclic aromatic hydrocarbons which can be attributed to an efflux pump probably of the multidrug resistance (MDR) type. Immunocytochemical detection showed a positive stain of ciliate cells using the monoclonal antibodies 4E3, raised against P-glycoprotein (P-gp). The kinetics of P-gp expression were studied for control cultures and cultures treated with 15 microM benzo(a)pyrene. Western blot analysis using the Ab1, anti-P-gp polyclonal antibodies indicates the presence of two bands of 66 and 96 kDa of which the intensity increased with time in benzo(a)pyrene-treated ciliates. Uptake experiments with target compounds for the MDR pump, namely adriamycin, rhodamine 123 and two polycyclic aromatic hydrocarbons, benzo(a)pyrene and 7,12-dimethylbenzanthracene, were carried out by flow cytometry, in the presence or absence of cyclosporin (an inhibitor of the multidrug resistant pump). The data indicate that the accumulation of these compounds by ciliate cells is significantly enhanced in the presence of cyclosporin. This suggests that Tetrahymena is provided with a P-gp-like system that is functionally active in a way similar to that of the mammalian P-gp.

A single chain Fv fragment of P-glycoprotein-specific monoclonal antibody C219. Design, expression, and crystal structure at 2.4 A resolution

A construct encoding a single chain variable fragment of the anti-P-glycoprotein monoclonal antibody C219 was made by combining the coding sequences for the heavy and light chain variable domains with a sequence encoding the flexible linker (GGGGS)3, an OmpA signal sequence, a c-myc identification tag, and a five-histidine purification tag. The construct was expressed in Escherichia coli and purified from the periplasmic fraction using a nickel chelate column and ion exchange chromatography. Three-step Western blot analysis showed that the construct retains binding affinity for P-glycoprotein. Crystals of 1.0 x 0.2 x 0.2 mm were grown in 100 mM citrate, pH 4.5, 21% polyethylene glycol 6000 in the presence of low concentrations of subtilisin, resulting in proteolytic removal of the linker and purification tags. The structure was solved to a resolution of 2.4 A with an R factor of 20.6, an Rfree of 28.5, and good stereochemistry. This result could lead to a clinically useful product based on antibody C219 for the diagnosis of P-glycoprotein-mediated multidrug resistance. The molecule will also be useful in biophysical studies of functional domains of P-glycoprotein, as well as studies of the intact molecule.

Cross-reactivity of C219 anti-p170(mdr-1) antibody with p185(c-erbB2) in breast cancer cells: cautions on evaluating p170(mdr-1)

BACKGROUND

Increased expression of the multidrug resistance gene (MDR-1)-encoded P-glycoprotein (p170[mdr-1]) is a major cause of tumor cell multidrug resistance. p170(mdr-1) functions as a drug-efflux pump to reduce the cellular accumulation of specific drugs. MDA-MB-435 human breast cancer cells that have been transfected with oncogene c-erbB2 complementary DNA (435.eb cells) express high levels of the transmembrane glycoprotein p185(c-erbB2) and exhibit increased resistance to the chemotherapeutic agent paclitaxel via p170(mdr-1)-independent mechanisms. We have recently discovered that the widely used monoclonal antibody C219, which is specific for p170(mdr-1), may cross-react with p185(c-erbB2) in 435.eb cells. In this study, we have investigated the nature of this cross-reactivity.

METHODS

Immunoprecipitation experiments involving the use of breast cancer cells that express different levels of p185(c-erbB2) were performed, and C219 was used for western blot analysis of immunoprecipitated proteins. Immunohistochemical analyses were performed on acetone-fixed slides of human breast cancer cells. Peptide sequence comparisons and enzyme-linked immunosorbent assays were performed to determine the molecular basis of C219 cross-reactivity with p185(c-erbB2).

RESULTS

The cross-reactivity of C219 with p185(c-erbB2) was demonstrated by both western blot and immunohistochemical analyses. Peptide sequence comparisons revealed that C219 recognizes an epitope in p170(mdr-1) (C219 epitope) that shares sequence homology with p185(c-erbB2). Enzyme-linked immunosorbent assays demonstrated that C219 recognizes synthetic peptides derived from both the C219 epitope in p170(mdr-1) and the C219 epitope-homologous region in p185(c-erbB2).

CONCLUSIONS

The anti-p170(mdr-1) monoclonal antibody C219 cross-reacts with p185(c-erbB2) through a peptide sequence in p185(c-erbB2) that is homologous to the C219 epitope in p170(mdr-1).

P-glycoprotein is strongly expressed in the luminal membranes of the endothelium of blood vessels in the brain

Luminal membranes of the vascular endothelium were isolated from brain, heart and lungs by modification of their density. The presence of P-glycoprotein (P-gp) was detected by Western blotting in luminal membranes from the endothelium of the three tissues. Strong enrichment in brain capillary luminal membranes, compared with brain capillaries (17-fold) and whole membranes (400-500-fold), indicates that P-gp is mainly located on the luminal side of the brain endothelium. Western blotting was also performed with antibodies directed against GLUT1, glial fibrillary acidic protein, adaptin, IP3R-3, integrins alphav and collagen IV as controls to determine whether the preparations were contaminated by other membranes. Strong enrichment of GLUT1 in brain capillary luminal membranes (9.9-fold) showed that the preparation consisted mainly of endothelial cell plasma membranes. Poor enrichment of glial fibrillary acidic protein (1.4-fold) and adaptin (2.4-fold) and a decreased level of IP3R-3, integrins alphav and collagen IV excludes the possibility of major contamination by astrocytes or internal and anti-luminal membranes. High levels of P-gp in the luminal membranes of brain capillary endothelial cells suggests that it may play an important role in limiting the access of anti-cancer drugs to the brain.

Use of rhodamine 123 to examine the functional activity of P-glycoprotein in primary cultured brain microvessel endothelial cell monolayers

The fluorescent dye, rhodamine 123, was used to evaluate the functional activity of the P-glycoprotein (P-gp) efflux transport system in primary cultured bovine brain microvessel endothelial cell (BBMEC) monolayers. Rhodamine 123 accumulation was increased significantly in BBMEC monolayers treated with the P-gp modifying agent, cyclosporin A (CSA). Rhodamine 123 accumulation was also increased by other P-gp modifying agents. The rank effectiveness of these agents in increasing rhodamine 123 accumulation in BBMEC monolayers was CSA = dipyridamole > verapamil = quinidine. The maximal increase in rhodamine 123 accumulation in CSA treated. BBMEC monolayers was approximately 3 fold greater than in control monolayers and was qualitatively similar to that observed with 3H-vincristine. Comparison of functional activity with the biochemical expression of P-gp in BBMEC monolayers and in an established tumor cell line that over-expresses P-gp indicate that functional activity may be a more descriptive measure of the importance of this drug efflux system than protein expression. Furthermore, these studies suggest that accumulation of rhodamine 123 in BBMEC monolayers can be used to quantitatively examine P-gp activity in the blood-brain barrier.

Evidence for P-glycoprotein-modulated penetration of morphine-6-glucuronide into brain capillary endothelium

1. Morphine-6-glucuronide is one of the major metabolites of morphine. The potent analgesic action of this compound together with its potential lower apparent toxicity in man, when compared with morphine, indicated its clinical importance. 2. Primary cultures of porcine brain capillary endothelial cells were used to study brain penetration of morphine-6-glucuronide. Biochemical characterization of the cell cultures revealed a marked enrichment in enzymatic activity of alkaline phosphatase (56 fold) and angiotensin converting enzyme (230 fold) as compared to whole brain tissue. By immunostaining the presence of vimentin, factor VIII, the tight junction associated protein ZO-1, and P-glycoprotein was shown. Functional characterization revealed that the carrier system responsible for transport of neutral amino acids was intact. 3. Uptake and transport of morphine-6-glucuronide was marginal and in the range of the extracellular marker sucrose. However, uptake of morphine-6-glucuronide was enhanced significantly (P < 0.0001) in presence of the inhibitors of P-glycoprotein, verapamil or vincristine. The finding that morphine-6-glucuronide may serve as a substrate for P-glycoprotein was confirmed in multidrug-resistant P388 tumour cells. 4. We conclude that penetration of the blood-brain barrier by morphine-6-glucuronide may depend on the expression of the product of the multidrug-resistance (MDR) gene in brain capillary endothelial cells.

P-glycoprotein-associated chloride currents revealed by specific block by an anti-P-glycoprotein antibody

The relationships between P-glycoprotein (PGP) expression and plasma membrane ion currents activated by cell swelling were studied in several cell lines by use of the whole cell configuration of the patch-clamp technique. Swelling-activated Cl- currents (ICls) had similar characteristics independently of whether PGP was expressed. Addition of the anti-PGP monoclonal antibody C219 or its Fab fragment to the pipette solution prevented ICls in cells expressing functional PGP (assessed by immunoblots, immunofluorescence, and transport of rhodamine 123) but not in cells lacking PGP expression. A peptide analogue of the C219 epitope abolished the effect of C219. Other anti-PGP antibodies and mouse immunoglobulin G were ineffective. C219 did not alter swelling-activated cation currents. Inasmuch as ICls is present in cells that do not express PGP and C219 has no effect on ICls in these cells, we conclude that PGP is not required for the ICls phenotype. However, when expressed in the plasma membrane, PGP is involved, directly or indirectly, in ICls but not in swelling-activated K+ currents.

Interaction of drugs with P-glycoprotein in brain capillaries

P-glycoprotein (P-gp) is expressed at high levels in a variety of non-cancerous tissues such as the endothelial cells of the blood-brain barrier (BBB) capillaries. These thin capillaries tightly regulate the movement of substrates from the circulating blood into the brain. P-gp may be involved in the exclusion of various drugs from the capillary endothelial cells, blocking their entry into the brain. However, interactions of drugs with P-gp expressed in brain capillaries remain to be characterized. We have performed photoaffinity labeling studies using [125I]arylazidoprazosin (IAAP) to evaluate the inhibitory efficiency of various compounds. Cyclosporin A (CsA) and its derivative PSC 833 (PSC) were the most effective inhibitors of IAAP binding among the drugs tested. The magnitude of inhibition was: PSC > CsA > quinidine > vinblastine > verapamil < actinomycin D > colchicine > reserpine > bilirubin > doxorubicin > progesterone. Cremophor El, the vehicle used to administer CsA and PSC intravenously, was also able to inhibit IAAP photolabeling of P-gp. Labeling experiments were also performed using a photoactivatable [3H]CsA derivative. Photolabeling of P-gp with this compound was abolished almost completely by CsA and PSC. In vivo studies were also performed by treating rats with CsA [10 mg/(kg.day) for 10 days]. Following this treatment, no alteration in the level of P-gp expression in brain capillaries was observed. These results suggest that, at the proper dosage, administration of CsA to cancer patients could help to enhance the response of brain tumors to chemotherapeutic agents without modifying the intrinsic level of P-gp expression in this tissue.

Comparisons of P-glycoprotein expression in isolated rat brain microvessels and in primary cultures of endothelial cells derived from microvasculature of rat brain, epididymal fat pad and from aorta

In vivo expression of P-glycoprotein in isolated rat brain microvessels is compared with that in vitro in primary cultures of brain endothelial cells. More P-glycoprotein is detected by Western immunoblotting in microvessels than in cultured endothelium. RT-PCR with isoform-specific primers and immunoblotting with a mdr1b-specific antibody reveals only mdr1a in vivo but both mdr1a and mdr1b in vitro. Thus mdr1a decreases whereas mdr1b increases during culture. P-Glycoprotein activity is evident in vitro, with resistance modulators, e.g. verapamil, producing increases in intracellular [3H]vincristine accumulation. Endothelial cells cultured from epididymal fat pad microvasculature and aorta contain little or no P-glycoprotein. Here, resistance modulators are less effective.