Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.6.3.1 (Mg2+-ATPase)
 1,484 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

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

Disruption of the murine mdr2 (multidrug-resistance) gene, which encodes a phosphatidylcholine flippase, leads to a hepatic disorder because of loss of biliary phospholipid secretion. Among the hereditary human cholestasis, a subtype of progressive familial intrahepatic cholestasis with high gamma-glutamyltranspeptidase (GGT) serum activity shares histological, biochemical, and genetic features with mice lacking mdr2 gene expression (mdr2 -/- mice). No MDR3 (human mdr2 homolog) messenger RNA (mRNA) was detected by Northern blotting in the liver of a patient suffering from this form of PFIC, and the biliary phospholipid level in a second patient was substantially decreased. Thus, the absence of the MDR3 P-glycoprotein may be responsible for this type of PFIC, which, as in the murine model, may be due to a toxic effect of bile acids on the biliary epithelium in absence of biliary phospholipids.
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PMID:Defect of multidrug-resistance 3 gene expression in a subtype of progressive familial intrahepatic cholestasis. 866 48

P-glycoprotein (P-gp), encoded by the MDR1 gene, is a plasma membrane transporter which effluxes a large number of structurally nonrelated hydrophobic compounds. The molecular basis of the broad substrate recognition of P-gp is not well understood. Despite the 78% amino acid sequence identity of the MDR1 and MDR2 transporter, MDR2, which has been identified as a phosphatidylcholine transporter, does not transport most MDR1 substrates. The structural and functional differences between MDR1 and MDR2 provide an opportunity to identify the residues essential for the broad substrate spectrum of MDR1. Using an approach involving exchanging homologous segments of MDR1 and MDR2 and site-directed mutagenesis, we have demonstrated that MDR1 residues Q330, V331, and L332 in transmembrane domain 6 are sufficient to allow an MDR2 backbone in the N-terminal half of P-gp to transport several MDR1 substrates, including bisantrene, colchicine, vinblastine, and rhodamine-123. These studies help define some residues important for multidrug transport and indicate the close functional relationship between the multidrug transporter (MDR1) and phosphatidylcholine flippase (MDR2).
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PMID:Studies of human MDR1-MDR2 chimeras demonstrate the functional exchangeability of a major transmembrane segment of the multidrug transporter and phosphatidylcholine flippase. 989 Oct 78

Multidrug resistance (MDR) mediated by P-glycoprotein (MDR1) is clinically significant. Understanding how MDR1 substrate specificity is determined will help to overcome MDR to improve cancer treatment. One potential approach to achieve this goal is to study chimeras of MDR1 and its homolog MDR2 (also called MDR3), which has been identified as a phosphatidylcholine flippase. With an approach involving exchanging homologous segments of MDR1 and MDR2 and site-directed mutagenesis, we previously demonstrated MDR1 residues Q330, V331, and L332 in transmembrane domain 6 (TM6) are essential for multidrug transport activity; substituting these residues allows the N-terminal transmembrane region of MDR2 to support MDR1 activity. To further determine the exchangeability between MDR1 and MDR2, we constructed additional MDR1/MDR2 chimeras. We found that the N-terminal half of MDR1 and MDR2 was mostly exchangeable except for a few residues in TM6. However, this degree of exchangeability was not found in the C-terminal half of MDR1 and MDR2. In addition, with substitution of MDR1 residues 318-332 (TM6) and 937-994 (TM11-12), MDR2 had relatively normal affinity for MDR1 substrates, but it did not have multidrug transporter activity. These results suggest that the inability of MDR2 to transport most MDR1 drugs efficiently may be due to failure of those drugs to stimulate ATPase and activate transport as well as to decreased drug binding.
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PMID:Domain exchangeability between the multidrug transporter (MDR1) and phosphatidylcholine flippase (MDR2). 1053 6

Farnesoid X receptor (FXR) is a bile acid-activated transcription factor that is a member of the nuclear hormone receptor superfamily. Fxr-null mice exhibit a phenotype similar to Byler disease, an inherited cholestatic liver disorder. In the liver, activation of FXR induces transcription of transporter genes involved in promoting bile acid clearance and represses genes involved in bile acid biosynthesis. We investigated whether the synthetic FXR agonist GW4064 could protect against cholestatic liver damage in rat models of extrahepatic and intrahepatic cholestasis. In the bile duct-ligation and alpha-naphthylisothiocyanate models of cholestasis, GW4064 treatment resulted in significant reductions in serum alanine aminotransferase, aspartate aminotransferase, and lactate dehydrogenase, as well as other markers of liver damage. Rats that received GW4064 treatment also had decreased incidence and extent of necrosis, decreased inflammatory cell infiltration, and decreased bile duct proliferation. Analysis of gene expression in livers from GW4064-treated cholestatic rats revealed decreased expression of bile acid biosynthetic genes and increased expression of genes involved in bile acid transport, including the phospholipid flippase MDR2. The hepatoprotection seen in these animal models by the synthetic FXR agonist suggests FXR agonists may be useful in the treatment of cholestatic liver disease.
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PMID:Hepatoprotection by the farnesoid X receptor agonist GW4064 in rat models of intra- and extrahepatic cholestasis. 1523 10

Bile is a complex mixture that includes bile salts, the membrane phospholipid phosphatidylcholine (PC), cholesterol and various endobiotic and xenobiotic toxins, each of which is secreted across the canalicular membrane of the hepatocyte by different ATP-binding cassette (ABC) transporters. The bile salts are essential for the emulsification of dietary fat and lipophilic vitamins. They are synthesized from cholesterol in the hepatocyte and their secretion by the bile salt export pump (BSEP or ABCB11) drives bile flow and is the starting point for the enterohepatic cycle. The detergent nature of bile salts that is key to their physiological role also means that they are inherently cytotoxic, and failure to secrete bile (intraheptic cholestasis) can precipitate severe liver disease and mortality. Such progressive familial intrahepatic cholestasis (PFIC) comes in three types of autosomal recessive disease. PFIC2 is caused by mutation to ABCB11. PFIC3 is caused by mutation of a closely related ABC transporter, ABCB4, which flops PC into the outerleaflet of the canalicular membrane. The flopped PC is extracted by the bile salts in the canaliculus to form a mixed micelle that reduces bile salt detergent activity. The third protein that is essential for bile flow from the hepatocyte is a member of a different class of transporter protein, a P-type ATPase, ATP8B1. Mutation of ATP8B1 causes PFIC1, but ATP8B1 does not transport a component of bile into the canaliculus. Data from different laboratories, published this year, suggests two different roles for ATP8B1 in the hepatocyte: a lipid flippase, that counterbalances the deleterious effects of ABCB4 on barrier function of the canalicular membrane; and an anchor of the actin cytoskeleton necessary to form the microvilli of the brush border. These latest discoveries are described, along with a spectrum of cholestatic disorders whose aetiologies lie in these and other transporters of the canalicular membrane.
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PMID:Canalicular ABC transporters and liver disease. 2198 74

Type IV P-type ATPases (P4-ATPases) are believed to translocate aminophospholipids from the exoplasmic to the cytoplasmic leaflets of cellular membranes. The yeast P4-ATPases, Drs2p and Dnf1p/Dnf2p, flip nitrobenzoxadiazole-labeled phosphatidylserine at the Golgi complex and nitrobenzoxadiazole-labeled phosphatidylcholine (PC) at the plasma membrane, respectively. However, the flippase activities and substrate specificities of mammalian P4-ATPases remain incompletely characterized. In this study, we established an assay for phospholipid flippase activities of plasma membrane-localized P4-ATPases using human cell lines stably expressing ATP8B1, ATP8B2, ATP11A, and ATP11C. We found that ATP11A and ATP11C have flippase activities toward phosphatidylserine and phosphatidylethanolamine but not PC or sphingomyelin. By contrast, ATPase-deficient mutants of ATP11A and ATP11C did not exhibit any flippase activity, indicating that these enzymes catalyze flipping in an ATPase-dependent manner. Furthermore, ATP8B1 and ATP8B2 exhibited preferential flippase activities toward PC. Some ATP8B1 mutants found in patients of progressive familial intrahepatic cholestasis type 1 (PFIC1), a severe liver disease caused by impaired bile flow, failed to translocate PC despite their delivery to the plasma membrane. Moreover, incorporation of PC mediated by ATP8B1 can be reversed by simultaneous expression of ABCB4, a PC floppase mutated in PFIC3 patients. Our findings elucidate the flippase activities and substrate specificities of plasma membrane-localized human P4-ATPases and suggest that phenotypes of some PFIC1 patients result from impairment of the PC flippase activity of ATP8B1.
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PMID:Phospholipid flippase activities and substrate specificities of human type IV P-type ATPases localized to the plasma membrane. 2782 76