Stav Cullum

Introduction. Epithelial-mesenchymal transition (EMT) is a process by which cells lose epithelial and gain mesenchymal characteristics, allowing migration for physiological (development) or pathological (metastatic) purposes.¹ Once a tumor cell loses expression of cellular adhesion protein E-cadherin, it is capable of invading the extracellular matrix and vascular dissemination.^2,7 It has been hypothesized that metastasis occurs through highly migratory, mesenchymal-like single cells in a primary tumor site.² However, new data contrasts this hypothesis and suggests EMT occurs in circulation through an interaction between circulating tumor cells (CTCs), the release of transforming growth-factor β (TGF-β) by platelets, and subsequent induction of EMT.² Methods. To determine the role(s) of TGF-β in EMT, an in-silico model was constructed.³ Findings were confirmed in murine and human liver cancer models. Another study measured the abundance of proteins, mRNAs and microRNAs representing core regulators of EMT in a human mammary epithelial cell line.⁵ Additionally, CTCs were obtained using microfluidic capture with epithelial and tumor-specific antibodies.² The EMT state of these CTCs was then analyzed using RNA-in situ hybridization (ISH). Results. The in-silico model showed TGF-β is a major signal for EMT induction.³ Additionally, activation of EMT showed repressed transcription of E-cadherin in murine and human epithelial cell lines. Increasing concentrations of TGF-β in the human cell line resulted in transition to a mesenchymal state.⁵ RNA expression analysis supported a two-step EMT, with each step associated with abrupt increase in a transcription factor (TF) and reduction in the corresponding inhibitory miRNA.⁵ The data suggest TF SNAIL1 is involved in the first step, and TF ZEB1 in the second. Interestingly, two markers of mesenchymal cells, N-cadherin and vimentin, are promoted by SNAIL1 and ZEB1. RNA ISH detected hybrid biphenotypic epithelial/mesenchymal cells and clusters of CTCs strongly expressed mesenchymal markers in comparison to single migratory cells.² Finally, when stained with CD61 marker for platelets, CTCs were abundantly positive and RNA transcripts in these cells showed a strong TGF-β signature.² Conclusions. New data suggests metastasis may occur through the proliferation of a single cell that has undergone EMT in a CTC.² Additionally, given the critical role of TGF-β in EMT, inhibiting components of its signaling pathway has potential as anti-metastatic therapy.^1-6 Further research embodies significant clinical relevance, as a high level of TGF-β has shown to be an important prognostic indicator of shorter overall survival in cancer.⁶

1. Steinway SN, Zanudo JGT, Michel PJ, Feith DJ, Loughran TP, Albert R. Combinatorial interventions inhibit TGFbeta-driven epithelial-to-mesenchymal transition and support hybrid cellular phenotypes. NPJ systems biology and applications. 2015;1:15014.
2. Yu M, Bardia A, Wittner BS, et al. Circulating breast tumor cells exhibit dynamic changes in epithelial and mesenchymal composition. Science (New York, NY). 2013;339(6119):580-584.
3. Steinway SN, Zanudo JG, Ding W, et al. Network modeling of TGFbeta signaling in hepatocellular carcinoma epithelial-to-mesenchymal transition reveals joint sonic hedgehog and Wnt pathway activation. Cancer research. 2014;74(21):5963-5977.
4. Steinway SN, Zanudo JGT, Michel PJ, Feith DJ, Loughran TP, Albert R. Combinatorial interventions inhibit TGFbeta-driven epithelial-to-mesenchymal transition and support hybrid cellular phenotypes. NPJ systems biology and applications. 2015;1:15014.
5. Zhang J, Tian XJ, Zhang H, et al. TGF-beta-induced epithelial-to-mesenchymal transition proceeds through stepwise activation of multiple feedback loops. Science signaling. 2014;7(345):ra91.
6. Peng L, Yuan XQ, Zhang CY, et al. High TGF-beta1 expression predicts poor disease prognosis in hepatocellular carcinoma patients. Oncotarget. 2017;8(21):34387-34397.
7. Strilic B, Offermanns S. Intravascular Survival and Extravasation of Tumor Cells. Cancer cell. 2017;32(3):282-293.