engleski

Proximal tubule (PT) brush-border membrane (BBM) transporters are redistributed in intracellular vesicles in cadmium-metallothionein (CdMT)-treated rats

Chronic exposure of humans and experimental animals to Cd causes nephrotoxicity manifested by reabsorptive and secretory defects in the PT. The cellular mechanisms of Cd nephrotoxicity are poorly understood. Specific transporters may be directly inhibited by Cd and/or Cd-treatment may cause their loss from the BBM. We recently showed that Cd-treatment in rats inhibits the proton-ATPase (V-ATPase) and endocytosis in the PT cells (Kidney Int. 53:1713-1726, 1998). These data point to vesicle-mediated recycling of BBM transporters as a target for Cd. We tested this hypothesis in an in vivo model of experimental Cd-nephrotoxicity induced by CdMT (0.4 mg/kg b.m., a single dose s.c.) in rats, in which we studied the distribution with time of various BBM transporters by immunofluorescence in tissue sections and by immunoblotting in isolated cortical BBM. By immunocytochemistry, in CdMT-treated rats we observed a time-dependent loss of megalin, V-ATPase, aquaporin-1 (AQP1), sodium-proton exchanger (NHE3), and sodium-phosphate cotransporter (NaPi2) from the BBM and redistribution of these antigens into cytoplasmic vesicles. These data were supported by the decreased density of some (V-ATPase, NaPi2) but not all (AQP1, NHE3) corresponding protein bands in immunoblotting experiments with isolated BBM, and by the diminished yield of BBM isolated from cortical tissue. Translocation of transporters from the BBM into intracellular vesicles was accompanied by a loss of microtubules in PT cells. These processes began 1 hr after CdMT treatment and increased in severity for the next 12 hours. We conclude that the mechanism of Cd toxicity in the PT cells may include a colchicine-like depolymerization of microtubules and impaired vesicle-mediated recycling of some cell membrane proteins. These processes may cause a loss of BBM transporters and shortening and/or loss of BB microvilli, with impaired PT function as the final result.

cadmium; nephrotoxicity; proximal tubule; transport

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