Cis-platin is an effective anti-neoplastic agent, but it is also highly nephrotoxic. Here, we clearly identify the human organic cation transporter 2 (hOCT2) as the critical transporter for cis-platin nephrotoxicity in isolated human proximal tubules and offer a potential mechanism for reducing nephrotoxicity in clinical practice. Interaction of cis-platin with hOCT2 in kidney or hOCT1 in liver was investigated with the fluorescent cation 4-[4-(dimethyl-amino)styril]-methylpyridinium in stably transfected HEK293 cells and for the first time in tissues physiologically expressing these transporters, human proximal tubules, and human hepatocyte couplets. Cis-platin (100 micromol/L) inhibited transport via hOCT2-HEK293 but not hOCT1-HEK293. In human proximal tubules cis-platin competed with basolateral organic cation transport, whereas it had no effect in tubules from a diabetic kidney or in hepatocytes. In hOCT2-HEK293 cells treated for 15 hours, incubation with cis-platin induced apoptosis, which was completely suppressed by contemporaneous incubation with the hOCT2 substrate cimetidine (100 micromol/L). These findings demonstrate that uptake of cis-platin is mediated by hOCT2 in renal proximal tubules, explaining its organ-specific toxicity. A combination of cis-platin with other substrates that compete for hOCT2 offers an effective option to decrease nephrotoxicity in the clinical setting.

The kidney, and more specifically the proximal tubule, is the main site of elimination of cationic endogenous metabolites and xenobiotics. Although numerous studies exist on renal organic cation transport of rat and rabbit, no information is available from humans. Therefore, we examined organic cation transport and its regulation across the basolateral membrane of isolated human proximal tubules. mRNA for the cation transporters hOCT1 and hOCT2 as well as hOCTN1 and hOCTN2 was detected in these tubules. ؉ uptake by 29 ؎ 3% (n ‫؍‬ 10). hANP (10 nM) or 8-bromo-cGMP (100 M) also decreased ASP ؉ uptake by 17 ؎ 3 (n ‫؍‬ 9) or 32 ؎ 5% (n ‫؍‬ 10), respectively. We show for the first time that organic cation transport across the basolateral membrane of isolated human proximal tubules, most likely mediated via hOCT2, is electrogenic and regulated by protein kinase C, the cAMP-and the cGMP-dependent protein kinases.The proximal tubule is the site of secretion and reabsorption of endogenous metabolites and xenobiotics in the kidney. Many of these substances are organic cations. As several drugs are among these organic cations, specific knowledge about properties of organic cation transport in the human proximal tubule is of great importance. The first organic cation transporter was cloned from rat (rOCT1) in 1994 (1). The first human organic cation transporters (hOCT1 and hOCT2) were cloned 3 years later (2, 3), and three other members of this family (hOCTN1, hOCTN2, and hOCT3, originally named EMT) followed (4 -6). Previous functional studies (7, 8) on organic cation transport in the proximal tubule of the rat showed the differences between transport across the luminal or basolateral membrane. After cloning of the transporters data from functional investigations of these transporters together with those obtained by microperfusion experiments and transport studies with membrane vesicles (8 -12) led to a model of organic cation transport in the proximal tubule. OCTN1 was shown to be a H ϩ /organic cation exchanger (4, 13), whereas OCT1 has been characterized as a functionally different potential driven uniporter. Immunohistochemistry localized rOCT1 and rOCT2 to the basolateral membrane of proximal tubules (14 -16). The OCTN1 is most likely located in the luminal membrane of proximal tubules (17, 18). Thus, organic cation transporters expressed in the luminal and basolateral membranes of the proximal tubule are molecularly and functionally very different members of this protein family.After cloning of the first transporters, studies were performed to gain information on properties of these organic cation transporters expressed in Xenopus laevis oocytes or cell lines (19 -21). From these studies we have information on their electrogeneity, substrate specificities, and inhibitors with K m and K i values for the distinct cation transporters from rat and man. The homologous transporters of these two species differ in K m and K i values for the investigated cations (19 -24). Properties found for the cloned rOCT1 and rOCT2 do not...

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