SMAD4 suppresses AURKA-induced metastatic phenotypes via degradation of AURKA in a TGFβ-independent manner

SMAD4 has been suggested to inhibit the activity of theWNT/β-catenin signaling pathway in cancer.However, the mechanism by which SMAD4 antagonizes WNT/β-catenin signaling in cancer remains largely unknown. Aurora A kinase (AURKA), which is frequently overexpressed in cancer, increases the transcriptional activity of β-catenin/T-cell factor (TCF) complex by stabilizing β-catenin through the inhibition ofGSK-3β. Here, SMAD4modulated AURKA in a TGFβ-independent manner. Overexpression of SMAD4 significantly suppressed AURKA function, including colony formation,migration, and invasion of cell lines. In addition,SMAD4 bound toAURKAinduced degradation of AURKA by the proteasome. A luciferase activity assay revealed that the transcriptional activity of the β-catenin/TCF complex was elevated by AURKA, but decreased by SMAD4 overexpression. Moreover, target gene analysis showed that SMAD4 abrogated the AURKA-mediated increase of β-catenin target genes. However, this inhibitory effect of SMAD4 was abolished by overexpression of AURKA or silencing of AURKA in SMAD4-overexpressed cells. Meanwhile, the SMAD4-mediated repression of AURKA and β-catenin was independent of TGFβ signaling because blockage ofTGFbR1 or restoration ofTGFb signaling did not prevent suppression ofAURKAand β-catenin signaling by SMAD4. These results indicate that the tumor-suppressive function of SMAD4 is mediated by downregulation of β-catenin transcriptional activity via AURKA degradation in a TGFβ-independent manner.