Control of TGFβ signaling by degradation of Smads, TGFβ signal transducers

Smad蛋白是细胞内转化生长因子-β (TGF-β)信号传导的关键分子。在受体诱导激活后，Smad蛋白被磷酸化并易位到细胞核以激活一组选定的靶基因的转录。在这里，我们研究了Smad3的周转，正向调节Smad的TGF-β信号。在稳定状态下，蛋白酶体抑制导致Smad3蛋白稳定。Smad蛋白在TGF-β受体诱导的磷酸化作用下被多泛素化和独立降解。此外，TGF-β增强了Smad3的降解，并通过蛋白酶体抑制积累了磷酸化的Smad3。此外，在TGF-β处理后，在细胞核中观察到磷酸化的Smad3泛素化，而不是SmadS (3SA)，这是一种受体介导的磷酸化无能突变体。这些研究表明，在稳态状态下，Smad3在细胞质中通过泛素蛋白酶体途径进行组成性降解，并在响应TGF-β时被磷酸化并易位到细胞核中，在细胞核中通过泛素蛋白酶体途径进行降解。

Smad proteins are crucial molecules for the intracellular signaling of transforming growth factor-β (TGF-β). Upon receptor-induced activation, Smad proteins are phosphorylated and translocated to the nucleus to activate transcription of a select set of target genes. Here we investigated the turnover of Smad3, positively regulating Smad for TGF-β signaling. In the steady state, proteasome inhibition leads to the stabilization of Smad3 protein. Smad proteins are multi-ubiquitinated and degraded independently of its phosophorylation induced by the TGF-β receptors. Moreover, the degradation of Smad3 was enhanced by the treatment with TGF-β, and phosphorylated Smad3 was accumulated by proteasome inhibition. In addition, ubiquitination of phosphorylated Smad3 but not SmadS (3SA), a receptor-mediated phosphorylation incompetent mutant, was observed in the nucleus after treatment with TGF-β. These studies suggest that at steady state Smad3 is constitutively degraded through ubiquitin proteasome pathway in the cytoplasm and that in response to TGF-β it is phosphorylated and translocates into nuclei, where it is degraded through ubiquitin proteasome pathway.