Cisplatin-induced Alterations of Na+-dependent Phosphate Uptake in Renal Epithelial Cells

Cisplatin treatment increases the excretion of inorganic phosphate in vivo. However, the mechanism by which cisplatin reduces phosphate uptake through renal proximal tubular cells has not yet been elucidated. We examined the effect of cisplatin on Na+-dependent phosphate uptake in opossum kidney (OK) cells, an established proximal tubular cell line. Cells were exposed to cisplatin for an appropriate time period and phosphate uptake was measured using [32P]-phosphate. Changes in the number of phosphate transporter in membranes were evaluated by kinetic analysis, [14C]phosphonoformic acid binding, and Western blot analysis. Cisplatin inhibited phosphate uptake in a time- and dose-dependent manner, and also the Na+-dependent uptake without altering Na+-independent uptake. The cisplatin inhibition was not affected by the hydrogen peroxide scavenger catalase, but completely prevented by the hydroxyl radical scavenger dimethylthiourea. Antioxidants were ineffective in preventing the cisplatin-induced inhibition of phosphate uptake. Kinetic analysis indicated that cisplatin decreased Vmax of Na+-dependent phosphate uptake without any change in the Km value. Na+-dependent phosphonoformic acid binding was decreased by cisplatin treatment. Western blot analysis showed that cisplatin caused degradation of Na+-dependent phosphate transporter protein. Taken together, these data suggest that cisplatin inhibits phosphate transport in renal proximal tubular cells through the reduction in the number of functional phosphate transport units. Such effects of cisplatin are mediated by production of hydroxyl radicals.

Functional characteristics of neutral amino acid transporter in opossum kidney (OK) cells

The characteristics of Na+/-dependent cycloleucine uptake was investigated in OK cells with regard to substrate specificity and regulation by protein kinase C (PKC). Inhibition studies with different synthetic and natural amino acids showed a broad spectrum affinity to neutral amino acids regardless of their different side chains including branched or aromatic, indicating that the Na+/-dependent cycloleucine uptake in OK cells is mediated by System B-o or System B degree -like transporter rather than the classical System A or ASC. Phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate, but not 4 alpha-PMA elicited a time-dependent biphasic stimulation of Na+/-dependent cycloleucine uptake, which produced early transient peak at 30 min and late sustained peak at 180 min. Both the early and late stimulations by PMA were due to an increase in Vmax and not due to a change in Km. PKC inhibitors blocked both the early and late stimulation by PMA, while protein synthesis inhibitors blocked the late stimulation only. These results suggest the existence and regulation by PKC of System B degree or System B degree -like broad spectrum transport system for neutral amino acids in OK cells.