Abstract:
SMAD4 has been suggested to inhibit the activity of the WNT/beta-catenin signaling pathway in cancer. However, the mechanism by which SMAD4 antagonizes WNT/b-catenin signaling in cancer remains largely unknown. Aurora A kinase (AURKA), which is frequently overexpressed in cancer, increases the transcriptional activity of beta-catenin/T-cell factor (TCF) complex by stabilizing beta-catenin through the inhibition of GSK-3 beta. Here, SMAD4 modulated AURKA in a TGF beta-independent manner. Overexpression of SMAD4 significantly suppressed AURKA function, including colony formation, migration, and invasion of cell lines. In addition, SMAD4 bound to AURKA induced degradation of AURKA by the proteasome. A luciferase activity assay revealed that the transcriptional activity of the beta-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 beta-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 beta-catenin was independent of TGF beta signaling because blockage of TGF beta R1 or restoration of TGF beta signaling did not prevent suppression of AURKA and beta-catenin signaling by SMAD4. These results indicate that the tumor-suppressive function of SMAD4 is mediated by down regulation of b-catenin transcriptional activity via AURKA degradation in a TGF beta-independent manner.