TRB3 interacts with SMAD3 promoting tumor cell migration and invasion

Tribbles homolog 3 (TRB3, also known as TRIB3, NIPK and SKIP3), a human homolog of Drosophila Tribbles, has been found to interact with a variety of signaling molecules to regulate diverse cellular functions. Here, we report that TRB3 is a novel SMAD3-interacting protein. Expression of exogenous TRB3 enhanced the transcriptional activity of SMAD3, whereas knocking down endogenous TRB3 reduced the transcriptional activity of SMAD3. The kinase-like domain (KD) of TRB3 was responsible for the interaction with SMAD3 and the regulation of SMAD3-mediated transcriptional activity. In addition, TGF-β1 stimulation or overexpression of SMAD3 enhanced the TRB3 promoter activity and expression, suggesting that there is a positive feedback loop between TRB3 and TGF-β-SMAD3 signaling. Mechanistically, TRB3 was found to trigger the degradation of SMAD ubiquitin regulatory factor 2 (Smurf2), which resulted in a decrease in the degradation of SMAD2 and phosphorylated SMAD3. Moreover, TRB3-SMAD3 interaction promoted the nuclear localization of SMAD3 because of the interaction of TRB3 with the MH2 domain of SMAD3. These effects of TRB3 were responsible for potentiating the SMAD3-mediated activity. Furthermore, knockdown of endogenous TRB3 expression inhibited the migration and invasion of tumor cells in vitro, which were associated with an increase in the expression of E-cadherin and a decrease in the expression of Twist-1 and Snail, two master regulators of epithelial-to-mesenchymal transition, suggesting a crucial role for TRB3 in maintaining the mesenchymal status of tumor cells. These results demonstrate that TRB3 acts as a novel SMAD3-interacting protein to participate in the positive regulation of TGF-β-SMAD-mediated cellular biological functions.