Sustained activation of SMAD3/SMAD4 by FOXM1 promotes TGF-β–dependent cancer metastasis
Jianfei Xue, … , Mien-Chie Hung, Suyun Huang
Jianfei Xue, … , Mien-Chie Hung, Suyun Huang
Published January 2, 2014
Citation Information: J Clin Invest. 2014;124(2):564-579. https://doi.org/10.1172/JCI71104.
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Research Article Oncology

Sustained activation of SMAD3/SMAD4 by FOXM1 promotes TGF-β–dependent cancer metastasis

Abstract

A key feature of TGF-β signaling activation in cancer cells is the sustained activation of SMAD complexes in the nucleus; however, the drivers of SMAD activation are poorly defined. Here, using human and mouse breast cancer cell lines, we found that oncogene forkhead box M1 (FOXM1) interacts with SMAD3 to sustain activation of the SMAD3/SMAD4 complex in the nucleus. FOXM1 prevented the E3 ubiquitin-protein ligase transcriptional intermediary factor 1 γ (TIF1γ) from binding SMAD3 and monoubiquitinating SMAD4, which stabilized the SMAD3/SMAD4 complex. Loss of FOXM1 abolished TGF-β–induced SMAD3/SMAD4 formation. Moreover, the interaction of FOXM1 and SMAD3 promoted TGF-β/SMAD3–mediated transcriptional activity and target gene expression. We found that FOXM1/SMAD3 interaction was required for TGF-β–induced breast cancer invasion, which was the result of SMAD3/SMAD4-dependent upregulation of the transcription factor SLUG. Importantly, the function of FOXM1 in TGF-β–induced invasion was not dependent on FOXM1’s transcriptional activity. Knockdown of SMAD3 diminished FOXM1-induced metastasis. Furthermore, FOXM1 levels correlated with activated TGF-β signaling and metastasis in human breast cancer specimens. Together, our data indicate that FOXM1 promotes breast cancer metastasis by increasing nuclear retention of SMAD3 and identify crosstalk between FOXM1 and TGF-β/SMAD3 pathways. This study highlights the critical interaction of FOXM1 and SMAD3 for controlling TGF-β signaling during metastasis.

Authors

Jianfei Xue, Xia Lin, Wen-Tai Chiu, Yao-Hui Chen, Guanzhen Yu, Mingguang Liu, Xin-Hua Feng, Raymond Sawaya, René H. Medema, Mien-Chie Hung, Suyun Huang

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Figure 9

FOXM1 expression significantly correlates with activated TGF-β signaling in human breast cancer.

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FOXM1 expression significantly correlates with activated TGF-β signaling...
(A) The expression of FOXM1, p-SMAD3, TIF1γ, and SLUG in representative cases of primary breast tumors (P) and matched lymph node metastasis tissue specimens (M). Original magnification, ×400. Scale bars: 200 μm. (B) Boxes indicate interquartile range. Bars from each box extend to the largest and smallest observations. (C) In the working model of interaction of FOXM1 with SMAD3 and SMAD4, FOXM1 promotes metastasis via the TGF-β1 pathway. Left: in the absence of FOXM1, TIF1γ sets up a negative-feedback mechanism to the TGF-β1 pathway by inducing monoubiquitination of SMAD4 and thus disrupting formation of the SMAD3/SMAD4 complex. Thus, SMAD3 is not retained in the nucleus, which decreases SLUG expression and prevents metastasis. Right: in the presence of FOXM1, FOXM1 interacts with SMAD3/SMAD4, which results in inhibition of binding of TIF1γ with SMAD4 and thus prevention of monoubiquitination of SMAD4 by TIF1γ. Thus, p-SMAD3 is retained in the nucleus via sustenance of the SMAD3/SMAD4 complex, which ultimately promotes activation of SLUG transcription and metastasis.