We have newly identified rat riboflavin transporter 2 (rRFT2) and its human orthologue (hRFT2), and carried out detailed functional characterization of rRFT2. The mRNA of rRFT2 was highly expressed in jejunum and ileum. When transiently expressed in human embryonic kidney (HEK) 293 cells, rRFT2 could transport riboflavin efficiently. Riboflavin transport mediated by rRFT2 was Na(+)-independent but moderately pH-sensitive, being more efficient in acidic conditions than in neutral and basic conditions. Kinetic analysis indicated that rRFT2-mediated riboflavin transport was saturable with a Michaelis constant (K(m)) of 0.21 microM. Furthermore, it was specifically and strongly inhibited by lumiflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), and to a lesser extent by amiloride. Such ability to transport riboflavin in a specific manner, together with its high expression in the small intestine, indicates that RFT2 may play a role in the intestinal absorption of riboflavin.

Proton-coupled folate transporter/heme carrier protein 1 (PCFT/HCP1) has recently been identified as a transporter that mediates the translocation of folates across the cellular membrane by a proton-coupled mechanism and suggested to be the possible molecular entity of the carrier-mediated intestinal folate transport system. To further clarify its role in intestinal folate transport, we examined the functional characteristics of rat PCFT/HCP1 (rPCFT/HCP1) expressed in Xenopus laevis oocytes and compared with those of the carrier-mediated folate transport system in the rat small intestine evaluated by using the everted tissue sacs. rPCFT/HCP1 was demonstrated to transport folate and methotrexate more efficiently at lower acidic pH and, as evaluated at pH 5.5, with smaller Michaelis constant ( Km) for the former (2.4 μM) than for the latter (5.7 μM), indicating its characteristic as a proton-coupled folate transporter that favors folate than methotrexate as substrate. rPCFT/HCP1-mediated folate transport was found to be inhibited by several but limited anionic compounds, such as sulfobromophthalein and sulfasalazine. All these characteristics of rPCFT/HCP1 were in agreement with those of carrier-mediated intestinal folate transport system, of which the Km values were 1.2 and 5.8 μM for folate and methotrexate, respectively, in the rat small intestine. Furthermore, the distribution profile of the folate transport system activity along the intestinal tract was in agreement with that of rPCFT/HCP1 mRNA. This study is the first to clone rPCFT/HCP1, and we successfully provided several lines of evidence that indicate its role as the molecular entity of the intestinal folate transport system.

The functional characteristics of human proton coupled folate transporter (hPCFT)/heme carrier protein (HCP) 1 were investigated. hPCFT/HCP1 expressed transiently in human embryonic kidney 293 cells mediated the transport of folate at an acidic extracellular pH of 5.5 in a manner independent of Na ϩ and insensitive to membrane potential, but its transport activity was absent at near-neutral pH. Folate transport mediated by hPCFT/hHCP1 at pH 5.5 was saturable with a K m of 1.67 M and extensively inhibited by reduced folates, such as folinate, 5-methyltetrahydrofolate, and methotrexate (MTX). Sulfobromophthalein and 4,4Ј-diisothiocyanostilbene-2,2Ј-disulfonic acid were also found to be potent inhibitors of hPCFT/hHCP1, but hemin was found to exhibit only minimal inhibitory effect.When expressed stably as a protein fused with green fluorescent protein (GFP-hPCFT/HCP1) in MDCKII cells, GFP-hPCFT/ HCP1 was mainly localized at the apical membrane, and the cellular accumulation of MTX was higher from the apical side than from the basal side. These functional features of hPCFT/ HCP1 are consistent with those of the well characterized carrier-mediated folate transport system in the small intestine, suggesting that hPCFT/HCP1 is responsible for the intestinal absorption of folate and also MTX. We also found that sulfasalazine is a potent inhibitor of hPCFT/HCP1, which would interfere with the intestinal absorption of MTX when coadministered in therapy for rheumatoid arthritis as well as folate.

ABSTRACT:We have cloned and functionally characterized the rat ortholog of multidrug and toxin extrusion type transporter 1 (rMATE1). The mRNA of rMATE1 was strongly expressed in kidney and detectable in the various tissues such as brain, stomach, colon, lung, liver, spleen, skeletal muscle, and prostate. When stably expressed in HEK293 cells, rMATE1 could mediate the transport of tetraethylammonium (TEA) and cimetidine under the condition where the membrane potential was disrupted by a high concentration of potassium ion and intracellular pH was reduced by NH 4 Cl pretreatment. When extracellular pH was changed from 5.5 to 8.5, the transport of TEA by rMATE1 was greatest at pH 7.5. Kinetic analyses showed that the transports of TEA and cimetidine mediated by rMATE1 were both saturable with a K m of 260 ؎ 10 and 3.01 ؎ 0.21 M, respectively. It was found that cimetidine is the most potent inhibitor of rMATE1, and many other organic cations, such as 1-methyl-4-phenylpyridinium, amiloride, imipramine, and quinidine, are also effective as inhibitors. Pretreatment of the cells expressing rMATE1 with p-chloromercuribenzene sulfonate significantly reduced TEA transport, but this effect was totally reversed by subsequent treatment with dithiothreitol. These results indicate that the functional nature of rMATE1 is consistent with that of the hypothetical organic cation/H ؉ antiporter system in the brushborder membrane of the renal tubular epithelial cells. Accordingly, these results suggest that rMATE1 is an electroneutral and multispecific organic cation transporter energized by the trans-proton gradient, and plays a physiological role in renal secretion of organic cations, including clinically used cationic drugs.

ABSTRACT:Cimetidine is known to cause drug-drug interactions (DDIs) with organic cations in the kidney, and a previous clinical study showed that coadministration of cimetidine or probenecid with fexofenadine (FEX) decreased its renal clearance. FEX was taken up into human kidney by human organic anion transporter (hOAT) 3 (SLC22A8), but the mechanism of its luminal efflux has not been clarified. The present study examined the molecular mechanism of these DDIs. Saturable uptake of FEX was observed in human kidney slices, with K m and V max values of 157 ؎ 7 M and 418 ؎ 16 nmol/15 min/g kidney, respectively. Cimetidine only slightly inhibited its uptake even at 100 M, far greater than its clinically relevant concentration, whereas 10 M probenecid markedly inhibited its uptake. As candidate transporters for the luminal efflux of FEX, we focused on human multidrug and toxin extrusions MATE1 (SLC47A1) and MATE2-K (SLC47A2). Saturable uptake of FEX could be observed in human embryonic kidney 293 cells expressing human MATE1 (hMATE1), whereas hMATE2-K-specific uptake of FEX was too small to conduct its further kinetic analysis. The hMATE1-mediated uptake clearance of FEX was inhibited by cimetidine in a concentration-dependent manner, and it was decreased to 60% of the control value in the presence of 3 M cimetidine. Taken together, our results suggest that the DDI of FEX with probenecid can be explained by the inhibition of renal uptake mediated by hOAT3, whereas the DDI with cimetidine is mainly caused by the inhibition of hMATE1-mediated efflux of FEX rather than the inhibition of its renal uptake process.