Transcriptional and Epigenetic Regulation of Epithelial-Mesenchymal Transition
Abstract: The transforming growth factor beta (TGF?) is a cytokine that regulates a plethora of cellular processes such as cell proliferation, differentiation, migration and apoptosis. TGF? signals via serine/threonine kinase receptors and activates the Smads to regulate gene expression. Enigmatically, TGF? has a dichotomous role as a tumor suppressor and a tumor promoter in cancer. At early stages of tumorigenesis, TGF? acts as a tumor suppressor by exerting growth inhibitory effects and inducing apoptosis. However, at advanced stages, TGF? contributes to tumor malignancy by promoting invasion and metastasis.The pro-tumorigenic TGF? potently triggers an embryonic program known as epithelial-mesenchymal transition (EMT). EMT is a dynamic process whereby polarized epithelial cells adapt a mesenchymal morphology, thereby facilitating migration and invasion. Downregulation of cell-cell adhesion molecules, such as E-cadherin and ZO-1, is an eminent feature of EMT. TGF? induces EMT by upregulating a non-histone chromatin factor, high mobility group A2 (HMGA2). This thesis focuses on elucidating the molecular mechanisms by which HMGA2 elicits EMT.We found that HMGA2 regulates a network of EMT transcription factors (EMT-TFs), such as members of the Snail, ZEB and Twist families, during TGF?-induced EMT. HMGA2 can interact with Smad complexes to synergistically induce Snail expression. HMGA2 also directly binds and activates the Twist promoter. We used mouse mammary epithelial cells overexpressing HMGA2, which are mesenchymal in morphology and highly invasive, as a constitutive EMT model. Snail and Twist have complementary roles in HMGA2-mesenchymal cells during EMT, and tight junctions were restored upon silencing of both Snail and Twist in these cells. Finally, we also demonstrate that HMGA2 can epigenetically silence the E-cadherin gene. In summary, HMGA2 modulates multiple reprogramming events to promote EMT and invasion.
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