title: Rat reduced-folate carrier-1 is localized basolaterally in MDCK kidney epithelial cells and contributes to the secretory transport of methotrexate and fluoresceinated methotrexate.
abstract: The reduced-folate carrier (Rfc-1), previously also called methotrexate carrier-1 (MTX-1), was recently identified as accounting for approximately 30% of the methotrexate (Mtx) uptake into rat kidney slices. The localization of the carrier and its contribution to secretory or reabsorptive flux of the drug was therefore evaluated in polarized epithelial layers of Madin Darby canine kidney (MDCK) cells. Confocal laser scanning microscopy revealed that the HA-epitope-tagged protein was sorted to the basolateral side. In flux assays, the basolateral-to-apical transport of fluoresceinated methotrexate (FMTX) was two-fold higher than in the apical-to-basolateral direction across rat Rfc-1 transfected, but not mock-transfected, monolayers. The same observation was made for unlabeled Mtx. This secretory transport of FMTX was inhibited by an excess of 1 mM Mtx and was saturable and temperature-dependent. No differences in directional flux were observed for the pure fluorescein label. Removal of sodium resulted in a marked decrease of directional FMTX flux. The pH profile of the active transport component showed a trough around 6.5 and a maximum at acidic pH, as reported for uptake into Rfc-1-expressing cells. Thus, rat Rfc-1 is sorted to the basolateral side in polarized MDCK epithelial cells and mediates the secretion of Mtx, probably in co-operation with efflux proteins, such as multidrug resistance associated proteins, which are also expressed in these cells.
authors.affiliation: Institute of Pharmacology, Pharmacy and Toxicology, Faculty of Veterinary Medicine, University of Leipzig, An den Tierkliniken 15, 04103, Leipzig, Germany.
authors.forename: Carsten
authors.initials: C
authors.lastname: Kneuer
authors.forename: Kerstin U
authors.initials: KU
authors.lastname: Honscha
authors.forename: Walther
authors.initials: W
authors.lastname: Honscha
mesh_terms: D000818:Animals; D000964:Antimetabolites, Antineoplastic / Q000493:pharmacokinetics*; D018501:Antirheumatic Agents / Q000493:pharmacokinetics*; D001692:Biological Transport; D002460:Cell Line; D016764:Cell Polarity; D004285:Dogs; D004847:Epithelial Cells / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007668:Kidney / Q000166:cytology; D026901:Membrane Transport Proteins / Q000378:metabolism*; D008727:Methotrexate / Q000031:analogs & derivatives* / Q000493:pharmacokinetics*; D018613:Microscopy, Confocal; D015778:Minor Histocompatibility Antigens; D051381:Rats; D058978:Reduced Folate Carrier Protein
chemical_list: D000964:Antimetabolites, Antineoplastic; D018501:Antirheumatic Agents; D026901:Membrane Transport Proteins; D015778:Minor Histocompatibility Antigens; D058978:Reduced Folate Carrier Protein; C043408:Slc19a1 protein, rat; C036539:fluoresceinated methotrexate; D008727:Methotrexate