Hyperosmolarity enhanced susceptibility to renal tubular fibrosis by modulating catabolism of type I transforming growth factor‐β receptors |
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Authors: | Tai‐An Chiang Yu‐Lin Yang Ya‐Ying Yang Min‐Hsiu Hu Pei‐Fen Wu Shu‐Fen Liu Ruay‐Ming Huang Tung‐Nan Liao Chien‐Ya Hung Tsung‐Jen Hung Tao‐Chen Lee |
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Affiliation: | 1. Department of Medical Technology, Graduate Institute of Biological Science and Technology, Chung Hwa University of Medical Technology, Tainan, Taiwan;2. Graduate Institute of Biomedical Science, Chung Hwa University of Medical Technology, Tainan, Taiwan;3. Department of Occupational Safety and Hygiene, Tajen University, Pingtung, Taiwan;4. Department of Internal Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan;5. Chest Hospital, Department of Health, Executive Yuan, Tainan, Taiwan;6. Department of Neurosurgery, Kaohsiung Chang Gung Memorial Hospital, Kaohsiung, Taiwan |
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Abstract: | 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‐β receptors and their downstream fibrogenic signal proteins. We show that hyperosmolarity significantly enhances the susceptibility to exogenous transforming growth factor (TGF)‐β1, as mannitol (27.5 mM) significantly enhanced the TGF‐β1‐induced increase in fibronectin levels compared with control experiments (5.5 mM). Specifically, hyperosmolarity induced tyrosine phosphorylation on TGF‐β 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‐β RI in a dose‐ and time‐course dependent manner. These observations may be closely related to decreased catabolism of TGF‐β RI. Hyperosmolarity significantly downregulated the expression of an inhibitory Smad (Smad7), decreased the level of Smurf 1, and reduced ubiquitination of TGF‐β 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‐β 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‐β receptors by retarding proteasomal degradation of TGF‐β RI. This study clarifies the mechanism underlying hyperosmotic‐induced renal fibrosis in renal distal tubule cells. J. Cell. Biochem. 109: 663–671, 2010. © 2010 Wiley‐Liss, Inc. |
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Keywords: | hyperosmolarity TGF‐β Smad receptors Smurf half‐life |
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