Cisplatin induces renal expression of P-glycoprotein and canalicular multispecific organic anion transporter.
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The expression of two members of the ATP-binding cassette family of transport proteins, P-glycoprotein (P-gp) and the canalicular multispecific organic anion transporter (cMOAT or Mrp2), was evaluated in renal brush-border membranes (BBM) and various rat tissues after cisplatin treatment. One administration of cisplatin (5 mg/kg) increased P-gp expression by >200-300% in renal BBM and in crude membranes from liver and intestine. The increase in P-gp expression in the kidney was also detected in photolabeling experiments, suggesting the induction of functional P-gp. cMOAT expression was increased by >10-fold in renal BBM after cisplatin administration, although it had no effect on liver cMOAT expression. The increase in the levels of both proteins was maximal at 2 days after cisplatin treatment and lasted for at least 8 days. These results indicate that a single administration of cisplatin induces overexpression of P-gp and cMOAT in specific tissues. This may be of significant relevance to the design of clinical trials using cisplatin as a single chemotherapeutic agent or in combination with other drugs. (+info)
A novel human hepatic organic anion transporting polypeptide (OATP2). Identification of a liver-specific human organic anion transporting polypeptide and identification of rat and human hydroxymethylglutaryl-CoA reductase inhibitor transporters.
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A novel human organic transporter, OATP2, has been identified that transports taurocholic acid, the adrenal androgen dehydroepiandrosterone sulfate, and thyroid hormone, as well as the hydroxymethylglutaryl-CoA reductase inhibitor, pravastatin. OATP2 is expressed exclusively in liver in contrast to all other known transporter subtypes that are found in both hepatic and nonhepatic tissues. OATP2 is considerably diverged from other family members, sharing only 42% sequence identity with the four other subtypes. Furthermore, unlike other subtypes, OATP2 did not transport digoxin or aldosterone. The rat isoform oatp1 was also shown to transport pravastatin, whereas other members of the OATP family, i.e. rat oatp2, human OATP, and the prostaglandin transporter, did not. Cis-inhibition studies indicate that both OATP2 and roatp1 also transport other statins including lovastatin, simvastatin, and atorvastatin. In summary, OATP2 is a novel organic anion transport protein that has overlapping but not identical substrate specificities with each of the other subtypes and, with its liver-specific expression, represents a functionally distinct OATP isoform. Furthermore, the identification of oatp1 and OATP2 as pravastatin transporters suggests that they are responsible for the hepatic uptake of this liver-specific hydroxymethylglutaryl-CoA reductase inhibitor in rat and man. (+info)
Involvement of an organic anion transporter (canalicular multispecific organic anion transporter/multidrug resistance-associated protein 2) in gastrointestinal secretion of glutathione conjugates in rats.
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We investigated the role of cMOAT/MRP2 (canalicular multispecific organic anion transporter/multidrug resistance-associated protein 2) in the intestinal secretion of organic anions by comparing the behavior in Sprague-Dawley (SD) rats and Eisai hyperbilirubinemic rat (EHBR) whose cMOAT/MRP2 is hereditarily defective. After i.v. administration of 1-chloro-2,4-dinitrobenzene (30 micromol/kg), the biliary and intestinal excretion of its glutathione conjugate 2, 4-dinitrophenyl-S-glutathione (DNP-SG), a substrate for cMOAT/MRP2, was significantly reduced in EHBR compared with SD rats. This result also was confirmed by Ussing chamber studies; DNP-SG showed 1.5-fold greater serosal-to-mucosal flux compared with the mucosal-to-serosal flux in SD rats, whereas a similar flux was observed in both directions in EHBR. In addition, metabolic inhibitors reduced the preferential serosal-to-mucosal flux of DNP-SG in SD rats. In everted sac studies, intestinal secretion clearance, defined as the efflux rate of DNP-SG into the mucosal side divided by the area under the curve on the serosal side, was significantly lower in the jejunum of EHBR than that in SD rats. Northern blot analyses demonstrated the highest mRNA level of cMOAT/MRP2 in the jejunum, which is in good agreement with the results of the everted sac studies. These results suggest that cMOAT/MRP2 is involved in the secretion of organic anions in the small intestine. (+info)
Electrophysiologic characterization of an organic anion transporter cloned from winter flounder kidney (fROAT).
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The two-electrode voltage clamp technique was used to demonstrate translocation of p-aminohippurate (PAH) and related compounds such as loop diuretics in Xenopus laevis oocytes expressing the renal organic anion transporter from winter flounder kidney (fROAT). In fROAT-expressing oocytes, PAH (0.1 mM) induced a depolarization of 4.2 +/- 0.4 mV and at a holding potential of -60 mV an inward current of -22.6 +/- 3.5 nA. PAH-induced current and the current calculated from [3H]-PAH uptake were of similar magnitude. Depolarization, inward current, and current-to-uptake relation indicated exchange of the monovalent PAH with a divalent anion, possibly alpha-ketoglutarate (alpha-KG), causing net efflux of one negative charge. The kinetic analysis of PAH-induced currents revealed that translocation is dependent on membrane potential, saturable with an apparent Km of 58 +/- 8 microM, and sensitive to probenecid and furosemide. In contrast to probenecid and furosemide, the loop diuretics bumetanide, ethacrynic acid, and tienilic acid and the nephrotoxic mycotoxin ochratoxin A elicited inward currents indicating translocation through fROAT. Substrate-dependent currents provide a tool to elucidate the structure/function relationship of the renal organic anion transporter. (+info)
Endothelin B receptor-mediated regulation of ATP-driven drug secretion in renal proximal tubule.
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In the kidney, endothelins (ETs) are important regulators of blood flow, glomerular hemodynamics, and sodium and water homeostasis. They have been implicated in the pathophysiology of acute ischemic renal failure, nephrotoxicity by cyclosporine, cisplatin and radiocontrast agents, and vascular rejection of kidney transplants. Here, we used intact killifish renal proximal tubules, fluorescent substrates for Mrp2 (fluorescein-methotrexate, FL-MTX) and P-glycoprotein (a fluorescent CSA derivative, NBD-CSA), and confocal microscopy to reveal a new role for renal ET: regulation of ATP-driven drug transport in proximal tubule. Subnanomolar to nanomolar concentrations of ET-1 rapidly reduced the cell-to-tubular lumen transport of both fluorescent compounds. These effects were prevented by an ET(B) receptor antagonist but not by an ET(A) receptor antagonist. Immunostaining with an antibody to mammalian ET(B) receptors showed specific localization to the basolateral membrane of the fish tubular epithelial cells. ET-1 effects on transport were blocked by protein kinase C-selective inhibitors, implicating protein kinase C in ET-1 signaling. Finally, the nephrotoxic radiocontrast agent iohexol reduced cell-to-lumen FL-MTX and NBD-CSA transport, and these effects were abolished by an ET(B) receptor antagonist. These are the first results linking ET to the control of xenobiotic transport and the first demonstrating control of renal multidrug resistance-associated protein 2 and P-glycoprotein by a hormone. (+info)
Down-regulation by extracellular ATP of rat hepatocyte organic anion transport is mediated by serine phosphorylation of oatp1.
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Recent studies implicate a role in hepatocyte organic anion transport of a plasma membrane protein that has been termed oatp1 (organic anion transport protein 1). Little is known regarding mechanisms by which its transport activity is modulated in vivo. In previous studies (Campbell, C. G., Spray, D. C., and Wolkoff, A. W. (1993) J. Biol. Chem. 268, 15399-15404), we demonstrated that hepatocyte uptake of sulfobromophthalein was down-regulated by extracellular ATP. We have now found that extracellular ATP reduces the V(max) for transport of sulfobromophthalein by rat hepatocytes; K(m) remains unaltered. Reduced transport also results from incubation of hepatocytes with the phosphatase inhibitors okadaic acid and calyculin A. Immunoprecipitation of biotinylated cell surface proteins indicates that oatp1 remains on the cell surface after exposure of cells to ATP or phosphatase inhibitor, suggesting that loss of transport activity is not caused by transporter internalization. Exposure of (32)P-loaded hepatocytes to extracellular ATP results in serine phosphorylation of oatp1 with the appearance of a single major tryptic phosphopeptide; oatp1 from control cells is not phosphorylated. This phosphopeptide comigrates with one of four phosphopeptides resulting from incubation of cells with okadaic acid. These studies indicate that the phosphorylation state of oatp1 must be an important consideration when assessing alterations of its functional expression in pathobiological states. (+info)
Nitrite reductase mutants as an approach to understanding nitrate assimilation in Chlamydomonas reinhardtii.
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We constructed mutant strains lacking the nitrite reductase (NR) gene in Chlamydomonas reinhardtii. Two types of NR mutants were obtained, which either have or lack the high-affinity nitrate transporter (Nrt2;1, Nrt2;2, and Nar2) genes. None of these mutants overexpressed nitrate assimilation gene transcripts nor NR activity in nitrogen-free medium, in contrast to NR mutants. This finding confirms the previous role proposed for NR on its own regulation (autoregulation) and on the other genes for nitrate assimilation in C. reinhardtii. In addition, the NR mutants were used to study nitrate transporters from nitrite excretion. At high CO(2), only strains carrying the above high-affinity nitrate transporter genes excreted stoichiometric amounts of nitrite from 100 microM nitrate in the medium. A double mutant, deficient in both the high-affinity nitrate transporter genes and NR, excreted nitrite at high CO(2) only when nitrate was present at mM concentrations. This suggests that there exists a low-affinity nitrate transporter that might correspond to the nitrate/nitrite transport system III. Moreover, under low CO(2) conditions, the double mutant excreted nitrite from nitrate at micromolar concentrations by a transporter with the properties of the nitrate/nitrite transport system IV. (+info)
Characterization of the transport properties of organic anion transporting polypeptide 1 (oatp1) and Na(+)/taurocholate cotransporting polypeptide (Ntcp): comparative studies on the inhibitory effect of their possible substrates in hepatocytes and cDNA-transfected COS-7 cells.
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In the present study, we compared the inhibitory effects of organic anions (including bile acids) on the uptake of taurocholate (TC) and estradiol 17beta-D-glucuronide (E(2)17betaG), typical substrates for sodium taurocholate cotransporting polypeptide (Ntcp) and organic anion transporting polypeptide (oatp1), respectively, using primary cultured rat hepatocytes and Ntcp- or oatp1-transfected COS-7 cells. The Na(+)-dependent uptake of TC was inhibited by nine bile acids and five nonbile acid organic anions in a concentration-dependent manner, and their inhibitory effects were similar in both primary cultured rat hepatocytes and Ntcp-transfected COS-7 cells. BQ-123 (1 microM) and indomethacin (10 microM), both of which exhibit no Ntcp-mediated transport, significantly inhibited the Na(+)-dependent uptake of TC mediated by Ntcp. In addition, the Na(+)-independent uptake of E(2)17betaG was inhibited by 15 organic anions in a concentration-dependent manner, and their inhibitory effects were similar between primary cultured rat hepatocytes and oatp1-transfected COS-7 cells. BQ-123 (1 microM), pravastatin (1 microM), and indomethacin (10 microM), all of which do not undergo oatp1-mediated transport, significantly inhibited the Na(+)-independent uptake of E(2)17betaG mediated by oatp1. These results are consistent with the hypothesis that the hepatic uptake of TC and E(2)17betaG is predominantly mediated by Ntcp and oatp1, respectively. In addition, it was clearly demonstrated that we cannot refer to the substrate specificity of transporters based on inhibition studies. (+info)