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Biomedical subjects

P Borst

Publications and source records attributed to P Borst.

At least 73 records · Page 4Linked to original sources
PubMed1997

Hepatic secretion of phospholipid vesicles in the mouse critically depends on mdr2 or MDR3 P-glycoprotein expression. Visualization by electron microscopy.

Hepatocellular secretion of bile salts into the biliary space induces phospholipid and cholesterol secretion, but the mechanism for integrated lipid secretion is poorly understood. Knockout mice unable to make the canalicular membrane mdr2 P-glycoprotein exhibit normal rates of bile salt secretion, yet are virtually incapable of secreting biliary phospholipid and cholesterol. As the mdr2 P-glycoprotein is thought to mediate transmembrane movement of phospholipid molecules, this mouse model was used to examine the mechanism for biliary phospholipid secretion. In wild-type mdr2 (+/+) mice, ultrarapid cryofixation of livers in situ revealed abundant unilamellar lipid vesicles within bile canalicular lumina. Although 74% of vesicles were adherent to the external aspect of the canalicular plasma membrane, bilayer exocytosis was not observed. Vesicle numbers in mdr2 (+/-) and (-/-) mice were 55 and 12% of wild-type levels, respectively. In a strain of mdr2 (-/-) mice which had been "rescued" by heterozygous genomic insertion of the MDR3 gene, the human homologue of the murine mdr2 gene, vesicle numbers returned to 95% of wild-type levels. Our findings indicate that biliary phospholipid is secreted as vesicles by a process largely dependent on the action of the murine mdr2 P-glycoprotein or human MDR3 P-glycoprotein. We conclude that mdr2-mediated phospholipid translocation from the internal to external hemileaflet of the canalicular membrane permits exovesiculation of the external hemileaflet, a vesiculation process promoted by the detergent environment of the bile canalicular lumen.

ATP Binding Cassette Transporter, Subfamily B↗
PubMed1997

Analysis of expression of cMOAT (MRP2), MRP3, MRP4, and MRP5, homologues of the multidrug resistance-associated protein gene (MRP1), in human cancer cell lines.

By screening databases of human expressed sequence tags, we have identified three new homologues of MRP1, the gene encoding the multidrug resistance-associated protein, and cMOAT (or MRP2), the canalicular multispecific organic anion transporter gene. We call these new genes MRP3, MRP4, and MRP5. MRP3, like cMOAT, is mainly expressed in the liver. MRP4 is expressed only at very low levels in a few tissues, and MRP5, like MRP1, is expressed in almost every tissue tested. To assess a possible role of these new MRP homologues in multidrug or cisplatin resistance, a large set of resistant cell lines was examined for the (over)expression of MRP1, cMOAT, MRP3, MRP4, and MRP5. We find that even in cells selected for a low level of resistance, several MRP-related genes can be up-regulated simultaneously. However, MRP4 is not overexpressed in any of the cell lines we analyzed; MRP3 and MRP5 are only overexpressed in a few cell lines, and the RNA levels do not seem to correlate with resistance to either doxorubicin or cisplatin. cMOAT is substantially overexpressed in several cell lines, and cMOAT RNA levels correlate with cisplatin but not doxorubicin resistance in a subset of resistant cell lines. Our results emphasize the need for gene-specific blocks in gene expression to define which transporter contributes to resistance in each resistant cell line.

Anion Transport Proteins↗
PubMed1997

Normal viability and altered pharmacokinetics in mice lacking mdr1-type (drug-transporting) P-glycoproteins.

The mdr1-type P-glycoproteins (P-gps) confer multidrug resistance to cancer cells by active extrusion of a wide range of drugs from the cell. To study their physiological roles, we have generated mice genetically deficient in the mdr1b gene [mdr1b (-/-) mice] and in both the mdr1a and mdr1b genes [mdr1a/1b (-/-) mice]. In spite of the host of functions speculatively attributed to the mdrl-type P-gps, we found no physiological abnormalities in either strain. Viability, fertility, and a range of histological, hematological, serum-chemical, and immunological parameters were not abnormal in mdr1a/1b (-/-) mice. The high level of mdrlb P-gp normally present in the pregnant uterus did not protect fetuses from a drug (digoxin) in the bloodstream of the mother, although the protein did reduce drug accumulation in the adrenal gland and ovaries. Pharmacologically, mdr1a/1b (-/-) mice behaved similarly to the previously analyzed mdr1a (-/-) mice, displaying, for instance, increased brain penetration and reduced elimination of digoxin. However, both mdr1a and mdr1b P-gps contributed to the extrusion of rhodamine from hematopoietic progenitor cells, suggesting a potential role for the endogenous mdr1-type P-gps in protection of bone marrow against cytotoxic anticancer drugs. This, and the normal viability of mdr1a/1b (-/-) mice, has implications for the use of P-gp-blocking agents in cancer and other chemotherapy. mdr1a/1b (-/-) mice should provide a useful model system to further test the pharmacological roles of the drug-transporting P-gps and to analyze the specificity and effectivity of P-gp-blocking drugs.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1997

Limited oral bioavailability and active epithelial excretion of paclitaxel (Taxol) caused by P-glycoprotein in the intestine.

In mice, the mdr1a and mdr1b genes encode drug-transporting proteins that can cause multidrug resistance in tumor cells by lowering intracellular drug levels. These P-glycoproteins are also found in various normal tissues such as the intestine. Because mdr1b P-glycoprotein is not detectable in the intestine, mice with a homozygously disrupted mdr1a gene [mdr1a(-/-) mice] do not contain functional P-glycoprotein in this organ. We have used these mdr1a(-/-) mice to study the effect of gut P-glycoprotein on the pharmacokinetics of paclitaxel. The area under the plasma concentration-time curves was 2- and 6-fold higher in mdr1a(-/-) mice than in wild-type (wt) mice after i.v. and oral drug administration, respectively. Consequently, the oral bioavailability in mice receiving 10 mg paclitaxel per kg body weight increased from only 11% in wt mice to 35% in mdr1a(-/-) mice. The cumulative fecal excretion (0-96 hr) was markedly reduced from 40% (after i.v. administration) and 87% (after oral administration) of the administered dose in wt mice to below 3% in mdr1a(-/-) mice. Biliary excretion was not significantly different in wt and mdr1a(-/-) mice. Interestingly, after i.v. drug administration of paclitaxel (10 mg/kg) to mice with a cannulated gall bladder, 11% of the dose was recovered within 90 min in the intestinal contents of wt mice vs. <3% in mdr1a(-/-) mice. We conclude that P-glycoprotein limits the oral uptake of paclitaxel and mediates direct excretion of the drug from the systemic circulation into the intestinal lumen.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1997

[Passive smoking and lung cancer: a new report in perspective].

Recently, a working group of six European scientists published a report on exposure to environmental tobacco smoke and the risk of lung cancer. The report concludes that environmental tobacco smoke is not a primary lung carcinogen. Critical evaluation of the report, however, shows that this conclusion is not justified. Results of recent epidemiologic studies support the earlier conclusion of the American Environmental Protection Agency (EPA) that environmental tobacco smoke does cause lung cancer. Furthermore, the working group fails to present compelling evidence that the results of the epidemiological studies in this field can be explained by bias or confounding, or that the association between environmental tobacco smoke and lung cancer is biologically implausible. Therefore, we see no reason to modify the conclusion of the EPA that passive smoking causes lung cancer.

Environmental Exposure↗
PubMed1997

A mutation in the human canalicular multispecific organic anion transporter gene causes the Dubin-Johnson syndrome.

The human Dubin-Johnson syndrome (DJS) is a rare autosomal recessive liver disorder characterized by chronic conjugated hyperbilirubinemia. Patients have impaired hepatobiliary transport of non-bile salt organic anions. A highly similar phenotype has been described for a mutant Wistar rat strain, the transport-deficient (TR-) rat, which is defective in the canalicular multispecific organic anion transporter (cmoat). This protein mediates adenosine triphosphate-dependent transport of a broad range of endogenous and xenobiotic compounds across the (apical) canalicular membrane of the hepatocyte. The complementary DNA (cDNA) encoding rat cmoat has recently been cloned, and the mutation underlying the defect in TR- rats has been identified. In the present study, we have isolated the human homologue of rat cmoat, human cMOAT, and analyzed the corresponding cDNA from fibroblasts of a DJS patient for mutations. Our results show that a mutation in this gene is the cause of DJS.

Amino Acid Sequence↗
PubMed1997

Do cMOAT (MRP2), other MRP homologues, and LRP play a role in MDR?

The discovery of the Multidrug Resistance-associated Protein (MRP or MRP1) as a GS-X pump able to transport both anionic drug conjugates and unmodified anti-cancer drugs out of the cell, has raised the question whether other members of the MRP family might contribute to drug resistance of human tumours. The most extensively studied member of this family is cMOAT, the canalicular Multispecific Organic Anion Transporter. The substrate specificity of this pump was originally defined by an inborn error in rats, lacking this protein. These rats are mildly hyperbilirubinemic, because of their inability to secrete bilirubin glucuronides into their bile. In addition, they have diminished capacity to secrete a variety of other organic anions. Absence of cMOAT in humans results in an analogous inborn error of metabolism, the Dubin-Johnson syndrome. Attempts to determine the effect of cMOAT on the sensitivity of cells to anti-cancer drugs have run into technical problems. Most cells transfected with a cMOAT cDNA construct and overproducing cMOAT seem unable to transport the protein to the cell surface and are not MDR. However, in polarized kidney cell monolayers cMOAT is correctly routed to the apical cell surface and able to transport vinblastine. Hence, overexpression of cMOAT in cancer cells could potentially lead to drug resistance. In studies of cells selected for drug resistance no correlation was found thus far between cMOAT overexpression and MDR, but there was a positive association with cisplatin resistance, raising the possibility that cMOAT might contribute to cisplatin resistance by mediating excretion of cisplatin-glutathione complexes. This remains to be verified by more direct experiments and clinical studies, however. Database searches have yielded four additional MRP family members, MRP3-6. The physiological functions of these putative transporters are not yet known and whether they can contribute to drug resistance needs to be determined. Another putative transporter found in many MDR cells not overproducing P-glycoprotein is the Lung Resistance Protein (LRP), which is the major vault protein. Scheper et al have detected LRP in many MDR cell lines and they have shown that elevated LRP values are a strong and independent predictor of unfavourable outcome for several types of drug-treated human tumours. LRP is a cytoplasmic protein and attempts to demonstrate its involvement in drug transport have failed thus far. The possibility that this protein is only an indicator of resistance caused by upregulation of other proteins, rather than a drug transporter, remains open.

ATP-Binding Cassette Transporters↗
PubMed1997

Genetic dissection of the function of mammalian P-glycoproteins.

Mammalian P-glycoproteins are plasma membrane proteins belonging to the superfamily of ATP-binding cassette transporters. They were discovered as drug pumps in multidrug-resistant cancer cells, but are also present in many normal tissues. Genetic approaches have helped to dissect the physiological functions and mode of action of P-glycoproteins. Disruption of both genes for the drug-transporting P-glycoproteins in mice has no effect on the normal sheltered life of these mice, but renders them hypersensitive to many drugs. P-glycoprotein appears to be especially important in protecting the brain and in limiting uptake of hydrophobic drugs from the gut. Recent experiments with polarized cells support the idea that drug-transporting P-glycoproteins act by flipping drugs from the inner to the outer leaflet of the plasma membrane.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1997

Enhanced oral bioavailability of paclitaxel in mice treated with the P-glycoprotein blocker SDZ PSC 833.

Inhibition of intestinal P-glycoprotein might enhance the absorption of orally administered P-glycoprotein substrate drugs. We show here a 10-fold increased oral bioavailability of paclitaxel in mice treated with the P-glycoprotein blocker SDZ PSC 833. These results encourage further research on the development of a clinically useful oral formulation of paclitaxel.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1997

Increased sensitivity to anticancer drugs and decreased inflammatory response in mice lacking the multidrug resistance-associated protein.

The multidrug resistance-associated protein (MRP) mediates the cellular excretion of many drugs, glutathione S-conjugates (GS-X) of lipophilic xenobiotics and endogenous cysteinyl leukotrienes. Increased MRP levels in tumor cells can cause multidrug resistance (MDR) by decreasing the intracellular drug concentration. The physiological role or roles of MRP remain ill-defined, however. We have generated MRP-deficient mice by using embryonic stem cell technology. Mice homozygous for the mrp mutant allele, mrp-/-, are viable and fertile, but their response to an inflammatory stimulus is impaired. We attribute this defect to a decreased secretion of leukotriene C4 (LTC4) from leukotriene-synthesizing cells. Moreover, the mrp-/- mice are hypersensitive to the anticancer drug etoposide. The phenotype of mrp-/- mice is consistent with a role for MRP as the main LTC4-exporter in leukotriene-synthesizing cells, and as an important drug exporter in drug-sensitive cells. Our results suggest that this ubiquitous GS-X pump is dispensable in mice, making treatment of MDR with MRP-specific reversal agents potentially feasible.

ATP-Binding Cassette Transporters↗
PubMed1997

Gene conversions mediating antigenic variation in Trypanosoma brucei can occur in variant surface glycoprotein expression sites lacking 70-base-pair repeat sequences.

African trypanosomes undergo antigenic variation of their variant surface glycoprotein (VSG) coat to avoid immune system-mediated killing by their mammalian host. An important mechanism for switching the expressed VSG gene is the duplicative transposition of a silent VSG gene into one of the telomeric VSG expression sites of the trypanosome, resulting in the replacement of the previously expressed VSG gene. This process appears to be a gene conversion reaction, and it has been postulated that sequences within the expression site may act to initiate and direct the reaction. All bloodstream form expression sites contain huge arrays (many kilobase pairs) of 70-bp repeat sequences that act as the 5' boundary of gene conversion reactions involving most silent VSG genes. For this reason, the 70-bp repeats seemed a likely candidate to be involved in the initiation of switching. Here, we show that deletion of the 70-bp repeats from the active expression site does not affect duplicative transposition of VSG genes from silent expression sites. We conclude that the 70-bp repeats do not appear to function as indispensable initiation sites for duplicative transposition and are unlikely to be the recognition sequence for a sequence-specific enzyme which initiates recombination-based VSG switching.

Animals↗
PubMed1997

Mechanisms of antigenic variation in African trypanosomes.

African trypanosomes can escape destruction by the immune system of their mammalian host by antigenic variation of the trypanosome surface coat. This coat is mainly composed of a single protein species, the Variant Surface Glycoprotein or VSG. The genes for VSGs are expressed in a polycistronic telomeric expression site together with at least eight expression site-associated genes (ESAGs). Trypanosomes may switch coat either by replacing the VSG gene in the active expression site by a different one, or by activating another expression site with concomitant silencing of the previously active one. Here we review our present knowledge of antigenic variation in Trypanosome brucei. We focus on four questions: How do trypanosomes switch from one VSG gene expression site to another one? What is the role of the novel base J in silencing expression sites? What is the functional significance of the antigenic variation of the heterodimeric transferrin receptor encoded by two ESAG genes? Why do trypanosomes have multiple expression sites at all?

Africa↗
PubMed1996

MDR1 P-glycoprotein is a lipid translocase of broad specificity, while MDR3 P-glycoprotein specifically translocates phosphatidylcholine.

The human MDR1 P-glycoprotein (Pgp) extrudes a variety of drugs across the plasma membrane. The homologous MDR3 Pgp is required for phosphatidylcholine secretion into bile. After stable transfection of epithelial LLC-PK1 cells, MDR1 and MDR3 Pgp were localized in the apical membrane. At 15 degrees C, newly synthesized short-chain analogs of various membrane lipids were recovered in the apical albumin-containing medium of MDR1 cells but not control cells. MDR inhibitors and energy depletion reduced apical release. MDR3 cells exclusively released a short-chain phosphatidylcholine. Since no vesicular secretion occurs at 15 degrees C, the short-chain lipids must have been translocated by the Pgps across the plasma membrane before extraction into the medium by the lipid-acceptor albumin.

ATP Binding Cassette Transporter, Subfamily B↗
PubMed1996

Peripheral neuropathy in mice transgenic for a human MDR3 P-glycoprotein mini-gene.

We have generated mice transgenic for a human MDR3 mini-gene, under control of a hamster vimentin promoter. Expression of the MDR3 transgene was found in mesenchymal tissues, peripheral nerves, and the eye lens. These MDR3 transgenic mice have a slowed motor nerve conduction and dysmyelination of their peripheral nerves. An extensive dysmyelination in some transgenic strains results in a severe peripheral neuropathy with paresis of the hind legs. How expression of the MDR3 transgene causes these abnormalities is unknown. The MDR3 gene encodes a large glycosylated plasma membrane protein with multiple transmembrane spanning domains, which are involved in the translocation of the phospholipid phosphatidylcholine through the hepatocyte canalicular membrane. The ability of the MDR3 P-glycoprotein to alter phsopholipid distribution in the plasma membrane of Schwann cells may cause the damage. It is also possible, however, that the presence of a large glycoprotein in the cell membrane may be sufficient to severely disturb myelination of peripheral nerves.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1996

Transport of the glutathione conjugate of ethacrynic acid by the human multidrug resistance protein MRP.

The multidrug resistance protein MRP has been shown to mediate the transport of glutathione S-conjugates across membranes. In this study we demonstrate that the glutathione S-conjugate of the diuretic drug ethacrynic acid, which is an efficient inhibitor of glutathione S-transferases, is a high-affinity substrate and inhibitor of the glutathione S-conjugate pump associated with MRP. This implies that ethacrynic acid may modulate drug resistance of tumor cells not only by inhibiting glutathione S-transferase activity, but also by inhibiting the export of drug conjugates from the cell by MRP.

ATP-Binding Cassette Transporters↗
PubMed1996

Altered pharmacokinetics of vinblastine in Mdr1a P-glycoprotein-deficient Mice.

BACKGROUND: P-glycoprotein (Pgp) is a membrane protein that acts as an extrusion pump for many cytotoxic drugs. Pgp is expressed in normal tissues, and its (over)expression in tumor cells contributes to their drug resistance. Human Pgp is encoded by the MDR1 gene, In mice, two Pgps (encoded by the mdr1a and mdr1b genes) appear to perform the same function as the single human protein. The simultaneous use of cytotoxic drugs and agents that block Pgp function has raised questions of safety, since a blockade of Pgp in normal tissues could alter drug pharmacokinetics and change the spectrum of toxic side effects. Analysis of the consequences of Pgp blockade has been facilitated by the generation of mice with disrupted mdr1a genes [mdr1a(-/-)]. PURPOSE: We studied the plasma pharmaco-kinetics, tissue distribution, and excretion of the cytotoxic drug vinblastine (VBL) and its metabolites in mdr1a (-/-) mice and in wild-type [mdr1a(+/+)] mice. METHODS: VBL was administered to mice in bolus doses of either 1 or 6 mg/kg body weight by intravenous injection. VBL and its metabolites were quantified in tissue specimens, plasma, feces, and urine by use of high-performance liquid chromatography. Liquid scintillation counting was used to measure radioactivity in specimens from animals that had received [3H]VBL. Pharmacokinetic parameters were calculated by use of noncompartmental methods. Only two-sided P values are reported. RESULTS: The half-life (t1/2) of VBL during its terminal phase of elimination was longer in mdr1a (-/-) mice than in wild-type mice. The t1/2 values with a 1-mg/kg dose were 3.6 hours +/- 0.3 hour (mean +/- standard error) and 2.1 hours +/- 0.3 hour, respectively (P < .05); with a 6-mg/kg dose, the values were 8.6 hours +/- 1.8 hours and 4.2 hours +/- 0.2 hour, respectively (P = .058). Fecal excretion of nonmetabolized VBL was reduced from 20%-25% of the administered dose (either 1 or 6 mg/kg) in wild-type mice to 9.3% (1-mg/kg dose) or 3.4% (6-mg/kg dose) in mdr1a(-/-) mice (both P < .05); the cumulative urinary excretion of VBL was low (< 6% of the administered dose) and not substantially different in the two types of mice. The metabolism of VBL to hydrophilic compounds, a primary mechanism involved in its elimination, was not altered in mdr1a(-/-) mice. The brains of mdr1a(-/-) mice accumulated substantially more VBL than the brains of wild-type mice. In mdr1a(-/-) mice, a few other tissues, such as the heart and the liver, accumulated increased amounts of VBL, but the relative levels of accumulation were lower than those found in the brain. CONCLUSIONS: Mice lacking the Pgp encoded by the mdr1a gene exhibit reduced fecal excretion of VBL, leading to a prolonged elimination t1/2 for this drug. Intact mdr1a function appears to protect the brain against high plasma levels of VBL, but most other tissues are not similarly protected. IMPLICATIONS: Enhanced drug accumulation in nonmalignant tissues after Pgp blockade should be carefully considered in future clinical trials of Pgp modulation.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1996

The telomeric GGGTTA repeats of Trypanosoma brucei contain the hypermodified base J in both strands.

We have previously shown that nuclear DNA of bloodstream from Trypanosoma brucei contains a novel base beta-glucosyl-hydroxymethyluracil, called J. Base J is enriched in minichromosome fractions but not in the minichromosome internal repeats, suggesting the association of J with telomeric DNA. To test whether J is present in the long telomeric (GGGTTA)n repeat arrays, which are 2-26 kb in T.brucei, we have purified these arrays both by hybrid selection and by isolating 2-26 kb fragments from DNA digested with multiple restriction enzymes. We find that in purified telomeric repeats approximately 13% of T is replaced by J, compared to 0.8% in total DNA, and we estimate that approximately 50% of the total J is in these repeats. Highly purified complementary strands of the repeats were obtained by alkaline CsCl equilibrium centrifugation. In the (TAACCC)n strand 14% of T was replaced by J. In the (GGGTTA)n strand approximately 36% of the second T was replaced by J; the first T was not detectably replaced. Modified bases have not been found in telomeric repeats before. How the bulky base J affects telomere function and structure in bloodstream form trypanosomes remains to be determined.

Animals↗