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MARCH 2012 CANCER DISCOVERY | 249The BATTLE Trial: Personalizing Therapy for Lung Cancer RESEARCH ARTICLE efficiently induces EMT. In a clinically relevant orthotopic breast cancer mouse model, Twist1 expression is essential for the formation of lung metastasis (5). In hepatocelluar carcinomas, overexpression of Twist1 correlates with EMTassociated changes and metastasis (6). Twist1 expression can also negatively regulate programmed cell death (7) and overcome oncogene-induced senescence (4).These studies indicate that Twist1 plays multiple roles in the regulation of antiapoptosis and metastasis during tumor progression. Active Akt directly phosphorylates Twist1 on serine 42 (S42) and protects cells from apoptosis induced by DNA damage (8). Twist1 promotes crucial prometastatic roles in tumor development and is frequently associated with aberrant hyperactivation of phosphoinositide 3-kinase (PI3K)/Akt. This prompted us to explore whether phosphorylation of Twist1 by Akt is part of a molecular mechanism contributing to Twist1 activity in cancer and particularly in metastasis.We analyzed Twist1 phosphorylation in human cancer cell lines (with different genetic backgrounds) and invasive human breast tumors and addressed its functional importance in a mouse model of spontaneous breast tumor metastasis. We show that Twist1 is ubiquitously phosphorylated in invasive human breast tumors and is required for breast cancer lung metastasis in vivo. Twist1 phosphorylation supports metastasis by transcriptionally upregulating TGF-β2, leading to enhanced activation of TGF-βR/Smad2 signaling, and these effects could be significantly suppressed by the knockdown of TGF-β2 expression. These data suggest that Twist1 phosphorylation not only mediates a feedback loop allowing cross-talk with TGF-β to maintain hyperactivation of PI3K/Akt but can also potentiate cancer metastasis, at least in part, through enhanced TGF-β signaling in cancer cells. Collectively, our data uncover a novel mechanism Metastatic breast tumor cells display an epithelial-mesenchymal transition (EMT) that increases cell motility, invasion, and dissemination. Although the transcription factor Twist1 has been shown to contribute to EMT and cancer metastasis, the signaling pathways regulating Twist1 activity are poorly understood. Here, we show that Twist1 is ubiquitously phosphorylated in 90% of 1,532 invasive human breast tumors. Akt/protein kinase B (PKB)-mediated Twist1 phosphorylation promotes EMT and breast cancer metastasis by modulating its transcriptional target TGF-β2, leading to enhanced TGF-β receptor signaling, which in turn maintains hyperactive phosphoinositide 3-kinase (PI3K)/Akt signaling. Preventing phosphorylation of Twist1, as well as depletion of TGF-β2, significantly impaired the metastatic potential of cancer cells in vivo, indicating a key role of phosphorylated Twist1 (phospho-Twist1) in mediating cross-talk between the PI3K/Akt and TGF-β/Smad signaling axes that supports metastatic tumor development. Our results describe a novel signalin...