Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.6.3.44 (P-glycoprotein)
 13,344 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The mouse mdr2 P-glycoprotein (P-gp) and its human MDR3 homologue are present in high concentrations in the canalicular membrane of hepatocytes. Mice lacking this protein are unable to secrete phosphatidylcholine (PC) into bile, suggesting that this P-gp is a PC translocator. We have tested this in fibroblasts from transgenic mice expressing the MDR3 gene under a vimentin promoter. Transgenic and control fibroblasts were incubated with [14C]choline to label PC. When the labeled cells were incubated with a PC transfer protein and acceptor liposomes, transfer of radioactive PC was enhanced in transgenic cells relative to the wild type controls. We conclude that the MDR3 P-glycoprotein is able to promote the transfer of PC from the inner to the outer leaflet of the plasma membrane, supporting the idea that this protein functions as a PC flippase.
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PMID:The human MDR3 P-glycoprotein promotes translocation of phosphatidylcholine through the plasma membrane of fibroblasts from transgenic mice. 795 36

The mouse mdr2 gene (and its human homologue MDR3, also called MDR2) encodes a P-glycoprotein that is present in high concentration in the bile canalicular membrane of hepatocytes. The 129/OlaHsd mice with a homozygous disruption of the mdr2 gene (-/- mice) lack this P-glycoprotein in the canalicular membrane. These mice are unable to secrete phospholipids into bile, showing an essential role for the mdr2 P-glycoprotein in the transport of phosphatidylcholine across the canalicular membrane. The complete absence of phospholipids from bile leads to a hepatic disease, which becomes manifest shortly after birth and shows progression to an end stage in the course of 3 months. The liver pathology is that of a nonsuppurative inflammatory cholangitis with portal inflammation and ductular proliferation, consistent with toxic injury of the biliary system from bile salts unaccompanied by phospholipids. Thus, the mdr2 (-/-) mice can serve as an animal model for studying mechanisms and potential interventions in nonsuppurative inflammatory cholangitis (in a generic sense) in human disease, be it congenital or acquired. When the mice are 4 to 6 months of age, preneoplastic lesions develop in the liver, progressing to metastatic liver cancer in the terminal phase. The mdr2 (-/-) mice therefore also provide a tumor progression model of value for the study of hepatic carcinogenesis. Interestingly, also in this regard, the model mimicks human disease, because chronic inflammation of the biliary system in humans may similarly carry increased cancer risk.
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PMID:Mice with homozygous disruption of the mdr2 P-glycoprotein gene. A novel animal model for studies of nonsuppurative inflammatory cholangitis and hepatocarcinogenesis. 797 54

Monoclonal antibodies (MAbs) recognizing external epitopes of the human MDR1 P-glycoprotein have been used both for the detection of multidrug-resistant cells and as specific inhibitors of P-glycoprotein-mediated multidrug resistance. Using a panel of recently developed transfected or transgenic cell lines containing variants of the human MDR1 and MDR3 P-glycoproteins, we have compared the specificity and binding properties of the previously isolated MAbs MRK16, HYB-241, UIC2 and 4E3, and of the newly isolated MAb 7G4. The removal of 1, 2 or all 3 of the N-glycosylation sites present in the first extracellular loop of MDR1 P-glycoprotein did not significantly affect the binding of these MAbs. In contrast, 20 amino acid deletion in the first extracellular loop of MDR1 P-glycoprotein completely abolished binding of UIC2, whereas the binding of all other MAbs was hardly affected. None of the MAbs tested bound detectably to cell lines containing a high level of the human MDR3 P-glycoprotein. The differences in the binding specificity between UIC2 and the other tested antibodies parallel the reported functional differences in the ability of these antibodies to inhibit P-glycoprotein-mediated drug efflux.
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PMID:Binding properties of monoclonal antibodies recognizing external epitopes of the human MDR1 P-glycoprotein. 810 65

Two types of P-glycoprotein have been found in mammals: the drug-transporting P-glycoproteins and a second type, unable to transport hydrophobic anticancer drugs. The latter is encoded by the human MDR3 (also called MDR2) and the mouse mdr2 genes, and its tissue distribution (bile canalicular membrane of hepatocytes, B cells, heart, and muscle) suggests a specialized metabolic function. We have generated mice homozygous for a disruption of the mdr2 gene. These mice develop a liver disease that appears to be caused by the complete inability of the liver to secrete phospholipid into the bile. Mice heterozygous for the disrupted allele had no detectable liver pathology, but half the level of phospholipid in bile. We conclude that the mdr2 P-glycoprotein has an essential role in the secretion of phosphatidylcholine into bile and hypothesize that it may be a phospholipid transport protein or phospholipid flippase.
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PMID:Homozygous disruption of the murine mdr2 P-glycoprotein gene leads to a complete absence of phospholipid from bile and to liver disease. 810 72

Multidrug resistance (MDR) genes, which are ATP-binding cassette family genes, encode the cell surface glycoprotein, P-glycoprotein, which functions as an energy-dependent drug efflux pump. Two relevant human genes, PGY1 and PGY3, are located on human chromosome 7, and three relevant mouse genes, mdr1a, mdr1b, and mdr2, are located on mouse chromosome 5. An LMD1 cell line was established after the transfer of a 580-kb yeast artificial chromosome (YAC) clone carrying the human MDR locus into mouse L cells; the cell line was shown to have stably integrated YAC DNA in an apparent intact form. Using LMD1 cells as the parental cell line, five vincristine-resistant sublines, designated LMD1-V50, LMD1-V100, LMD1-V200, LMD1-V500, and LMD1-V1000, were isolated by exposure to increasing concentrations of the drug. LMD1-V50, LMD1-V100, LMD1-V200, LMD1-V500, and LMD1-V1000 showed 3-, 7-, 13-, 45-, and 110-fold higher resistance to the cytotoxic effects of vincristine, respectively, than their parental counterpart, LMD1. Immunofluorescence, Western blot, and Northern blot analyses revealed that the human PGY1 gene or its product was overexpressed, accompanied by gene amplification. The human PGY3 gene was also overexpressed in the LMD1-V20, LMD1-V100, and LMD1-V1000 cell lines. Southern blot and fluorescence in situ hybridization (FISH) analyses demonstrated that although essentially the entire YAC DNA was integrated in mouse genome and amplified, the endogenous mouse mdr genes were not amplified in these drug-resistant cell lines. Similar results were obtained by the analyses of vincristine-resistant cell lines isolated from four independent subclones of LMD1 cells. Thus, in contrast to their mouse counterparts, the integrated human MDR genes retained susceptibility to both gene activation and amplification, during the selection of drug-resistant mouse cell lines. The possibility that transferred YACs may retain regulatory properties observed in the cells of origin, and may have a chromatin structure that favors augmented expression, is discussed.
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PMID:Functional expression of yeast artificial chromosome-human multidrug resistance genes in mouse cells. 859 12

The expression of human MDR1 P-glycoprotein (Pgp) in the capillary endothelial cells of the central nervous system has been demonstrated. The brain capillary endothelial cells maintain the structure and function of the blood-brain barrier. Recently, the human MDR1 Pgp (and its mouse homologue MDR1a Pgp) has been shown to function as an important part of this barrier, pumping out xenobiotics from endothelial cells into the lumen of capillaries resulting in the protection of the brain parenchyma. To examine whether the endothelial cells of the newly formed capillaries during neoangiogenesis within malignant human brain tumors express MDR1 Pgp, 35 adult surgical brain tumor specimens (29 gliomas and 6 tumors metastatic to the brain) were obtained from previously untreated patients and studied by a new immunohistochemical sandwich method developed in our laboratory using the JSB-1 monoclonal antibody. JSB-1 is specific for the Pgp product of the human MDR1 (and not MDR3) gene. This sensitive method allows the detection of Pgp in capillary endothelial cells of normal brain in conventional paraffin sections after formalin fixation. The endothelial cells of the newly formed capillaries in 25 of 29 gliomas (86%) and 3 of 6 metastatic tumors, immunostained positive for MDR1 Pgp. The tumor cells in 7 of 35 cases were also positive for Pgp. In the 35 brain tumor cases investigated, the endothelial cells were Pgp positive in the tumor-brain border and in the brain further from the tumor. Capillary endothelial cells of neovasculature in 137 malignant tumors (non-brain) obtained from previously untreated patients showed no MDR1 Pgp expression. These results demonstrated that MDR1 Pgp is expressed not only in the capillaries of normal brain but also in the majority of the newly formed capillaries of brain tumors. Multidrug resistance of brain tumors may result not only from the expression of resistance markers in neoplastic cells but also from the MDR1 Pgp expression in endothelial cells of tumor capillaries. Pgp in this special localization can exclude chemotherapeutic agents from tumor cells that are located around the capillaries. The therapeutic benefit and selectivity of chemotherapeutic agents in combination with a Pgp-reversing agent should be evaluated.
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PMID:MDR1 P-glycoprotein is expressed by endothelial cells of newly formed capillaries in human gliomas but is not expressed in the neovasculature of other primary tumors. 878 Mar 89

P-glycoprotein (P-gp), the multidrug resistance (MDR) gene product, is exclusively located on the canalicular membrane of hepatocytes. Recent studies using isolated rat canalicular liver plasma membrane (cLPM) vesicles indicate that daunomycin (DNM) is a substrate for the ATP-dependent P-gp efflux system in the rat liver. The isoforms of P-gp present in cLPM and in cancer cell lines differ in that the major form present in the liver represents the gene product of mdr2 in mice (MDR3 in humans; class III) while the isoform of P-gp in cancer cells is the gene product of mdr1 in mice (MDR1 in humans, class I). The objective of this study was to examine the inhibitory effects of various organic compounds, most of which have been studied previously in MDR cancer cells, on P-gp-mediated [3H]DNM uptake into cLPM. Also, the stereospecificity of P-gp for its substrates was investigated by comparing the inhibitory effects of the enantiomers and the racemic mixtures of verapamil and propranolol. DNM exhibited ATP-dependent active transport into rat liver cLPM with a Km of 26.8 +/- 13.4 microM and a Vmax of 4.9 +/- 0.8 nmol/45 s/mg of protein (n = 4). ADP, AMP, and a nonhydrolyzable ATP analogue did not increase DNM transport over the control value. Thirty-one potential inhibitors were examined; only acridine orange, doxorubicin, verapamil, propranolol, phosphatidylcholine, beta-estradiol glucuronide, and DNM itself showed statistically significant inhibition of [3H]DNM uptake into cLPM. These results suggest that only a limited number of substrates bind to or are transported across the hepatic canalicular membrane via P-gp. Phosphatidylcholine, a substrate for the gene product of the class III P-gp gene, produced significant inhibition of [3H]DNM transport (30.6% at a 10-fold-higher substrate concentration), suggesting that transport may be mediated, at least in part, by this P-gp gene product. There were no statistically significant differences in the inhibitory effects of the enantiomers and racemate of verapamil on [3H]DNM transport into cLPM, but the enantiomers of propranolol exhibited stereospecific inhibition of DNM transport. (R)-(+)-Propranolol produced a statistically significant inhibition of [3H]DNM transport similar to that observed with the racemic mixture, while (S)(-)-propranolol showed no inhibition. These findings suggest that bile canalicular P-gp may exhibit stereospecificity of binding or transport for its substrates.
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PMID:Inhibitors of P-glycoprotein-mediated daunomycin transport in rat liver canalicular membrane vesicles. 887 82

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.
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PMID:MDR1 P-glycoprotein is a lipid translocase of broad specificity, while MDR3 P-glycoprotein specifically translocates phosphatidylcholine. 889 3

P-glycoprotein, the MDR1 gene product which confers multidrug resistance to tumor cells and transports cationic or neutral compounds with high lipophilicity outward from the cells, excretes xenobiotics into the lumen in the kidney, small intestine and liver. The expression of P-glycoprotein is enhanced by stresses including exposure to various xenobiotics, while the activity is regulated to some extent by the phosphorylation. P-glycoprotein and P450 3A have similar substrate specificities and inducers, which may suggest they have complementary roles in the liver. The other subclass, MDR3, which does not show the multidrug resistance, translocates phosphatidyl choline selectively into the outer leaflet of the liver canalicular membrane, and may protect the liver from the detergent effect of bile acids.
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PMID:[The role of P-glycoprotein in the liver]. 915 55

The heterogeneous nature of an adriamycin-selected human MDR squamous lung cell line, DLKP-A, was investigated by isolating and characterising 9 of its clonal subpopulations. The DLKP-A cell line exhibits resistance to the classical MDR drugs, overexpresses P-glycoprotein and displays reduced topoisomerase II amounts. The clonal cell lines exhibit a wide range of resistance extents, with the most resistant clone displaying 9 times the extent of adriamycin resistance observed in the least resistant clone. A number of clones exhibit sensitivity to the concentration of adriamycin in which the parental cell line was selected, possibly indicating cooperation between the more and less resistant cells. Detailed analysis of 4 of the clonal subpopulations revealed broadly similar drug resistance mechanisms. Alterations in expression of the MDR-associated genes MDR1 and Topo IIalpha were observed, with no detectable changes in the expression of MDR3, MRP, GSTpi, Topo IIbeta, Topo I and CYP1A1 noted. However, each clonal cell line displayed a distinct extent of expression of MDR1 and Topo IIalpha and further characterisation of the clones indicated that other modes of drug resistance may exist in at least one of the cell lines. In particular, 2 of the clones (DLKPA6B and DLKPA11B) which have almost identical drug resistance profiles appear to have quite different mechanisms of resistance. The clonal subpopulations possess individual growth rates, amounts of adriamycin accumulation and susceptibility to toxicity-enhancement by MDR-modulating agents. It was possible to generate a cell line with a drug toxicity profile similar to DLKP-A by mixing some of the clonal subpopulations. Our results provide evidence of heterogeneity within an MDR human cell population with respect to resistance and expression of MDR-associated genes.
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PMID:Isolation from a human MDR lung cell line of multiple clonal subpopulations which exhibit significantly different drug resistance. 918 Jan 64


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