Anticancer treatment with ifosfamide but not with its structural isomer cyclophosphamide is associated with development of renal Fanconi syndrome leading to diminished growth in children and bone problems in adults. Since both cytotoxics share the same principal metabolites, we investigated whether a specific renal uptake of ifosfamide is the basis for this differential effect. First we studied the interaction of these cytotoxics using cells transfected with organic anion or cation transporters and freshly isolated murine and human proximal tubules with appropriate tracers. Next we determined changes in membrane voltage in proximal tubular cells to understand their differentiated nephrotoxicity. Ifosfamide but not cyclophosphamide was significantly transported into cells expressing human organic cation transporter 2 (hOCT2) while both did not interact with organic anion transporters. This points toward a specific interaction of ifosfamide with hOCT2, which is the main OCT isoform in human kidney. In isolated human proximal tubules ifosfamide also interacted with organic cation transport. This interaction was also seen in isolated mouse proximal tubules; however, it was absent in tubules from OCT-deficient mice, illustrating the biological importance of this selective transport. Ifosfamide decreased the viability of cells expressing hOCT2, but not that of control cells. Coadministration of cimetidine, a known competitive substrate of hOCT2, completely prevented this ifosfamide-induced toxicity. Finally, ifosfamide but not cyclophosphamide depolarized proximal tubular cells. We propose that the nephrotoxicity of ifosfamide is due to its selective uptake by hOCT2 into renal proximal tubular cells, and that coadministration of cimetidine may be used to prevent ifosfamide-induced nephrotoxicity.
The organic cation transporter 3 (OCT3) is a widely expressed transporter for endogenous and exogenous organic cations. Of particular interest is OCT3 expression and function in the brain, where it plays a role in serotonin clearance and influences mood and behavior. Protein kinase signaling mediates rapid modulation of cerebral processes, but little is known about acute regulation of OCT3 by protein kinases. Therefore, we cloned mouse OCT3 (mOCT3) and generated a human embryonic kidney cell line stably expressing the transporter to study transport characteristics, acute regulation by protein kinases, and interaction with psychotropic drugs. Uptake measurement was performed using the fluorescent cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (ASP(+), 1 μM) as a substrate. The translational value of these findings was determined by comparing results obtained with cloned mouse and human OCT3. mOCT3-mediated transport is membrane potential dependent and pH independent. ASP(+) uptake by mOCT3 and human OCT3 (hOCT3) was efficiently inhibited by 1-methyl-4-phenylpyridinium, tetrapentylammonium (TPA(+)), corticosterone, serotonin, and histamine and by the drugs ketamine, fluoxetine, and diazepam. The half maximal inhibitory concentrations of mOCT3 and hOCT3 for TPA(+), serotonin, diazepam, and ketamine are significantly different. Diazepam is a non-transported inhibitor. Furthermore, the activities of mOCT3 and hOCT3 are acutely regulated by the p56 (lck) tyrosine kinase by decreasing their V max. Studies with freshly isolated renal proximal tubules from mOCT1/2(-/-) mice, in which mOCT3 is the only OCT present, confirmed this regulation pathway. Only the activity of hOCT3 is regulated by calmodulin. These findings suggest that even though many transport properties of mOCT3 and hOCT3 are similar, there are also species-specific aspects of OCT3 function.
CD63 is a ubiquitously expressed member of the tetraspanin superfamily. Using a mating-based split-ubiquitin-yeast 2-hybrid system, pull-down experiments, total internal reflection fluorescence microscopy, Förster resonance energy transfer, and biotinylation assays, we found that CD63 interacts with human organic cation transporter 2 (hOCT2), which transports endogenous and exogenous substrates, such as neurotransmitters and drugs in several epithelial cells. CD63 overexpression affects cellular localization of hOCT2 expressed in human embryonic kidney (HEK)293 cells. Studies with CD63-knockout mice indicate that in renal proximal tubules, CD63 determines the insertion of the mouse ortholog of the transporter into the proper membrane domain and mediates transporter regulation by trafficking processes. In polarized Madin-Darby kidney canine kidney (MDCK) epithelial cells, CD63 and hOCT2 colocalize with the small GTPase Rab4, which controls the rapid recycling from sorting endosomes back to the cell surface. Suitable negative and positive control experiments were performed for each experimental approach. Empty vector transfected cells and wild-type mice were used as control. CD63 seems to play a role in the recycling of hOCT2 from endosomes to the basolateral membrane in polarized epithelia. These data indicate that CD63 has a previously uncharacterized function in regulating trafficking of specific membrane proteins in polarized cells.-Schulze, U., Brast, S., Grabner, A., Albiker, C., Snieder, B., Holle, S., Schlatter, E., Schröter, R., Pavenstädt, H., Herrmann, E., Lambert, C., Spoden, G. A., Florin, L., Saftig, P., Ciarimboli, G. Tetraspanin CD63 controls basolateral sorting of organic cation transporter 2 in renal proximal tubules.
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