Hyperosmolarity enhanced susceptibility to renal tubular fibrosis by modulating catabolism of type I transforming growth factor-beta receptors.

Hyperosmolarity plays an essential role in the pathogenesis of diabetic tubular fibrosis. However, the mechanism of the involvement of hyperosmolarity remains unclear. In this study, mannitol was used to evaluate the effects of hyperosmolarity on a renal distal tubule cell line (MDCK). We investigated transforming growth factor-beta receptors and their downstream fibrogenic signal proteins. We show that hyperosmolarity significantly enhances the susceptibility to exogenous transforming growth factor (TGF)-beta1, as mannitol (27.5 mM) significantly enhanced the TGF-beta1-induced increase in fibronectin levels compared with control experiments (5.5 mM). Specifically, hyperosmolarity induced tyrosine phosphorylation on TGF-beta RII at 336 residues in a time (0-24 h) and dose (5.5-38.5 mM) dependent manner. In addition, hyperosmolarity increased the level of TGF-beta RI in a dose- and time-course dependent manner. These observations may be closely related to decreased catabolism of TGF-beta RI. Hyperosmolarity significantly downregulated the expression of an inhibitory Smad (Smad7), decreased the level of Smurf 1, and reduced ubiquitination of TGF-beta RI. In addition, through the use of cycloheximide and the proteasome inhibitor MG132, we showed that hyperosmolarity significantly increased the half-life and inhibited the protein level of TGF-beta RI by polyubiquitination and proteasomal degradation. Taken together, our data suggest that hyperosmolarity enhances cellular susceptibility to renal tubular fibrosis by activating the Smad7 pathway and increasing the stability of type I TGF-beta receptors by retarding proteasomal degradation of TGF-beta RI. This study clarifies the mechanism underlying hyperosmotic-induced renal fibrosis in renal distal tubule cells.