Sodium-dependent methotrexate carrier-1 is expressed in rat kidney: cloning and functional characterization

Alternative transcripts of rat slc19a1: Cloning, genomic organisation, tissue specific promoters and alternative splicing

Recently, the rat genome project revealed the genomic sequence of slc19a1, coding for the methotrexate carrier-1, identical to the reduced folate carrier-1 of humans, on rat chromosome 20. At the same time, we have cloned and analysed the complete or partial cDNAs of now at least six different transcripts from rat liver and kidneys. Alignment with the genomic sequence revealed seven exons. The first two non-coding exons, exon I and Ia were used alternatively in kidneys and liver, respectively, suggesting usage of alternative promoters. Three minor mRNA forms resulted from absent splicing of intron III, a shortened exon III (exon IIIa), and a shortened exon IV (exon IVa). The minor transcripts were predicted to result in translation products with 7 or 6 instead of 12 transmembrane domains (TMDs) and a peptide mass of 38, 39 and 40 kDa instead of 58 kDa.

Rat reduced-folate carrier-1 is localized basolaterally in MDCK kidney epithelial cells and contributes to the secretory transport of methotrexate and fluoresceinated methotrexate

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.

Directional trans-epithelial transport of organic anions in porcine LLC-PK1 cells that co-express human OATP1B1 (OATP-C) and MRP2

The transcellular transport of many compounds, which cannot readily cross the lipid bilayer, is mediated by drug uptake and efflux transporters. Human OATP1B1 and MRP2 have been implicated in the hepato-biliary transport of many endogenous and exogenous compounds. Here, we have established epithelial porcine kidney LLC-PK1 derived cell lines, that express both transporters in a polarized fashion, as a model to predict hepato-biliary transport. Immunological identification of OATP1B1 in the recombinant cell lines was greatly facilitated by its C-terminal tagging with a peptide sequence derived from hemagglutinin (HA) avoiding the generation of OATP1B1 specific antibodies. Importantly, the tag did not interfere with the functionality of the transporter. Compared to LLC-PK1 cells and cells which expressed only OATP1B1, the cell line that co-expressed MRP2 and OATP1B1 displayed high directional basolateral-to-apical transport of 17 beta-estradiol-17 beta-glucuronide and estrone-3-sulfate. Dehydroepiandrosterone sulfate already displayed a significant basolateral-to-apical transport in the parental cell line, which was further stimulated upon expression of both transporters. Transcellular flux of all steroid conjugates in the opposite direction (apical-to-basolateral) was much lower. By employing this cellular model we were able to demonstrate for the first time that OATP1B1 together with MRP2 mediates the trans-cellular transport of rifampicin. It is anticipated that the models established herein will greatly facilitate the identification of transporters involved in the disposition of novel drug candidates.

The H(+)-dependent reduced folate carrier 1 of humans and the sodium-dependent methotrexate carrier-1 of the rat are orthologs

Previously, two different carrier systems for uptake of reduced folates and the antifolate methotrexate (Mtx) were described: the pH-dependent folate sensitive reduced folate carrier 1 (RFC1) from human, hamster and mouse and a sodium-dependent and folate insensitive Mtx carrier-1 (MTX-1) from rat. It was found that all critical residues of the homologous amino acid sequence were identical. RFC1- as well as MTX-1-mediated uptake of a marker substrate into suitable human and rat cell lines increased with proton concentration, was sodium-dependent at neutral pH, and inhibited by folate at acidic pH. It is concluded that RFC1 and MTX-1 are orthologs.