Principles of stem cell biology and cancer: future applications and therapeutics. Edited by T. Regad, T. J. Sayers and R. C. Rees. John Wiley & Sons (2015)
Part II. Cancer stem cells
The epithelial – mesenchymal transition (EMT) is a highly conserved cellular programme that is central to the normal development of multicellular organisms and that, when engaged, allows stationary epithelial cells to acquire mesenchymal characteristics and gain the ability to migrate and invade (Kalluri and Weinberg, 2009; Nieto, 2013). The EMT programme comprises several key events that are considered hallmarks of the process. These include the loss of epithelial cell – cell junctions, the loss of apical – basal polarity concomitant with the acquisition of front – rear polarity, the coordinated downregulation of an epithelial gene signature – particularly repression of E-cadherin – and the upregulation/activation of genes that define the mesenchymal phenotype, such as vimentin, N-cadherin and smooth-muscle actin (SMA) (Nieto, 2013; Tam and Weinberg, 2013; Lamouille et al., 2014). These phenotypic changes are associated with substantive alterations in cell behaviour, notably the ability to degrade surrounding extracellular matrix (ECM) and basement membrane components, increase motility and invasive properties and resist apoptosis (Nieto, 2013; Lamouille et al., 2014), traits that allow mesenchymal cells to move from one location to another. Importantly, upon reaching their destination, mesenchymal cells revert to an epithelial phenotype, through a process called mesenchymal – epithelial transition (MET), allowing them to establish, proliferate and differentiate, resulting in the formation of complex structures and organs and demonstrating the high degree of cellular plasticity (i.e. the ability of cells to change phenotype in a reversible manner) inherent within the EMT programme (Thiery, 2002; Nieto, 2013; Tam and Weinberg, 2013). In addition to the well-established role of EMT in normal development, it is now known that this core cellular programme, which is silenced in adult tissues, can be activated in a variety of pathologies. This has resulted in the recognition of three distinct types of EMT: developmental EMTs (Type 1), EMTs related to wound-healing fibrosis and tissue regeneration (Type 2) and EMTs associated with cancer progression and metastasis (Type 3) (Kalluri and Weinberg, 2009; Nieto, 2013). Indeed, research over the past decade has demonstrated that epithelial tumour cells can co-opt the EMT programme in order to promote cancer progression and, in particular, metastasis.
Cancer metastasis is an inherently inefficient process and is generally viewed as a cascade of several rate-limiting stages or steps (invasion, intravasation, systemic transport, extravasation and colonization) (Nguyen et al., 2009; Chaffer and Weinberg, 2011). Initial invasion at the primary tumour site requires that epithelial tumour cells degrade the underlying basement membrane and ECM, acquire migratory properties and invade the adjacent tissue stroma. Invading tumour cells must then penetrate the endothelial lining of adjacent vasculature, in a process called intravasation. Successful intravasation results in the entrance of cells into the systemic circulation, allowing tumour cells that have adapted for survival in this environment to travel to distant locations. Ultimately, select circulating tumour cells (CTCs) may arrest in the microvessels and capillaries of distant organs, penetrate the endothelial barrier once more (in a process called extravasation) and enter the stromal environment of the organs. In the final step of metastasis, called colonization, a small subset of these cells establish in the new environment and eventually proliferate to form clinically detectable macrometastases. EMT represents a vital programme required by epithelial cancer cells for successful completion of these steps (Chaffer and Weinberg, 2011). Indeed, EMT in cancer cells shares many of the hallmarks of EMT in development, and ultimately leads to cell dissociation and motility, two key steps in metastasis (Nieto, 2013; Tam and Weinberg, 2013).
An appreciation of the role of EMT in cancer progression and metastasis is evolving rapidly. Many studies using in vitro and in vivo models of cancer have clearly shown the importance of EMT in cancer progression (Yang and Weinberg, 2008; Thiery et al., 2009; Scheel and Weinberg, 2012; Nieto, 2013). However, the clinical relevance of EMT in the progression and metastasis of human cancer has been debated. While epithelial cancer cells in primary human tumours and CTCs exhibit hallmarks of EMT (Wan et al., 2013; Krebs et al., 2014), cells comprising distant macrometastases generally show an epithelial phenotype (Nieto, 2013; Tam and Weinberg, 2013), suggesting that mesenchymal disseminating tumour cells must revert to an epithelial phenotype (the MET) in order to successfully establish macrometastases. In addition, cancer cells that undergo EMT share many morphological and functional attributes with stromal cells, such as cancer-associated fibroblasts (CAFs), making the identification of tumour cells that have undergone EMT in complex tissues a particular challenge. The relevance of EMT to human cancer metastasis is highlighted by studies demonstrating that the expression of master regulators of EMT, including Twist1, Zeb1, Snail1 and Snail2, in primary tumours is linked to an increased risk of metastasis (De Craene and Berx, 2013). Also, breast and prostate cancer patients with a poor prognosis have been shown to have CTCs that have undergone EMT. Thus, the EMT programme must be very dynamic, and a model of EMT incorporating the concept of reversibility is key to understanding its role in cancer metastasis.
In this chapter, we will provide an overview of the basic components of EMT activation as they relate to metastasis, including the core inducers, transcriptional regulators and effector molecules. We will examine the role of EMT in the various steps of the metastatic cascade and discuss the important concept of epithelial – mesenchymal plasticity as it relates to the EMT programme in metastasis. We will then turn our attention to cancer stem cells (CSCs) and the role of EMT in their biology. We will also discuss the contribution of hypoxia to EMT and CSCs. Finally, we will highlight the role of EMT in the resistance of tumour cells to cancer treatment and discuss ways of interfering with EMT and CSC stemness to specifically target these treatment-resistant cells.