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Regulation of aquaporin‐2 by hyperosmolality in renal epithelial cells

To investigate the stability, degradation, expression, and targeting of AQP2 by hyperosmolality, stably transfected mIMCD‐3 cells expressing AQP2 (AQP2/IMCD3) were generated. In AQP2/IMCD3 cells, nonglycosylated (ng‐AQP2) and glycosylated (g‐AQP2) forms were detected by immunoblot. The stability of ng‐AQP2 decreased with the lapse of time, whereas that of g‐AQP2 was stable. NaCl, but not urea, destabilized ng‐AQP2. The half‐life of ng‐AQP2 in isotonic conditions was ~5 hours, whereas that in medium supplemented with NaCl was ~1.5 hours. Urea enhanced it compared to isotonic conditions. These findings indicate that the stability of ng‐AQP2 is enhanced by urea, but not NaCl. The stability of ng‐AQP2 was significantly enhanced by the addition of proteasome or lysosome inhibitor. There was no significant effect on the stability of g‐AQP2 by these inhibitors. This result suggests that the degradation of ng‐AQP2 is dependent on proteasome and lysosome degradation pathways. The addition of NaCl or urea to the isotonic medium remarkably increased ng‐AQP2 protein. The maximal accumulation of ng‐AQP2 protein occurred with addition of 175 mM NaCl or 350 mM urea to the isotonic medium. Cell surface biotinylation experiments revealed that hyperosmolality enhanced the apical membrane insertion of ng‐AQP2. The percentage of ng‐AQP2 on the apical membrane by hyperosmotic NaCl and urea were 84% and 86%, respectively. These results indicate that hyperosmolality plays an important role in the stability, degradation, expression, and targeting of ng‐AQP2.