Arcinoma, and the distributions and localization of human (h)OAT1, hOAT3, and hOCT2 proteins were investigated by immunohistochemical analyses in the human kidney. The expression level of hOAT3 mRNA was the highest among the organic ion transporter family, followed by that of hOAT1 mRNA. The hOCT2 mRNA level was the highest in the human OCT family, and the level of hOCTN2 mRNA was higher than that of hOCTN1. hOCT1 mRNA showed the lowest level of expression in organic ion transporter family. hOAT1, hOAT3, and hOCT2 proteins were detected in crude membranes from the kidney of all patients by Western blot analyses, whereas hOCT1 protein could not be detected. Immunohistochemical analyses showed that both hOAT1 and hOAT3 were localized to the basolateral membrane of the proximal tubules in the cortex, and hOCT2 was localized to the basolateral membrane of the proximal tubules in both the cortex and medullary ray. Immunohistochemical analyses of serial sections indicated that hOAT1, hOAT3, and hOCT2 were coexpressed in a portion of the proximal tubules. These results suggest that hOAT1, hOAT3, and hOCT2 play predominant roles in the transport of organic ions across the basolateral membrane of human proximal tubules....

A cDNA coding a new H⁺/organic cation antiporter, human kidney-specific multidrug and toxin extrusion 2 (hMATE2-K), has been isolated from the human kidney. The hMATE2-K cDNA had an open reading frame that encodes a 566–amino acid protein, which shows 94, 82, 52, and 52% identity with the hMATE2, hMATE2-B, hMATE1, and rat MATE1, respectively. Reverse transcriptase–PCR revealed that hMATE2-K mRNA but not hMATE2 was expressed predominantly in the kidney, and hMATE2-B was ubiquitously found in all tissues examined except the kidney. The immunohistochemical analyses revealed that the hMATE2-K as well as the hMATE1 was localized at the brush border membranes of the proximal tubules. HEK293 cells that were transiently transfected with the hMATE2-K cDNA but not hMATE2-B exhibited the H⁺ gradient–dependent antiport of tetraethylammonium (TEA). Transfection of hMATE2-B had no affect on the hMATE2-K–mediated transport of TEA. hMATE2-K also transported cimetidine, 1-methyl-4-phenylpyridinium (MPP), procainamide, metformin, and N¹-methylnicotinamide. Kinetic analyses demonstrated that the Michaelis-Menten constants for the hMATE2-K–mediated transport of TEA, MPP, cimetidine, metformin, and procainamide were 0.83 mM, 93.5 μM, 0.37 mM, 1.05 mM, and 4.10 mM, respectively. Ammonium chloride–induced intracellular acidification significantly stimulated the hMATE2-K–dependent transport of organic cations such as TEA, MPP, procainamide, metformin, N¹-methylnicotinamide, creatinine, guanidine, quinidine, quinine, thiamine, and verapamil. These results indicate that hMATE2-K is a new human kidney–specific H⁺/organic cation antiporter that is responsible for the tubular secretion of cationic drugs across the brush border membranes.

We have examined the role of the human organic cation transporters [hOCTs and human novel organic cation transporter (hOCTN); SLC22A1–5] and apical multidrug and toxin extrusion (hMATE) in the cellular accumulation and cytotoxicity of platinum agents using the human embryonic kidney (HEK) 293 cells transiently transfected with the transporter cDNAs. Both the cytotoxicity and accumulation of cisplatin were enhanced by the expression of hOCT2 and weakly by hOCT1, and those of oxaliplatin were also enhanced by the expression of hOCT2 and weakly by hOCT3. The hOCT-mediated uptake of tetraethylammonium (TEA) was markedly decreased in the presence of cisplatin in a concentration-dependent manner. However, oxaliplatin showed almost no influence on the TEA uptakes in the HEK293 cells expressing hOCT1, hOCT2, and hOCT3. The hMATE1 and hMATE2-K, but not hOCTN1 and OCTN2, mediated the cellular accumulation of cisplatin and oxaliplatin without a marked release of lactate dehydrogenase. Oxaliplatin, but not cisplatin, markedly decreased the hMATE2-K-mediated TEA uptake. However, the inhibitory effect of cisplatin and oxaliplatin against the hMATE1-mediated TEA uptake was similar. The release of lactate dehydrogenase and the cellular accumulation of carboplatin and nedaplatin were not found in the HEK293 cells transiently expressing these seven organic cation transporters. These results indicate that cisplatin is a relatively good substrate of hOCT1, hOCT2, and hMATE1, and oxaliplatin is of hOCT2, hOCT3, hMATE1, and hMATE2-K. These transporters could play predominant roles in the tissue distribution and anticancer effects and/or adverse effects of platinum agent-based chemotherapy.