[MUSIC] Hello, my name is Sarah Armend and I'm a Cancer Researcher here at Johns Hopkins. Today, I'm going to talk to you about the next steps after the initial primary tumor formation, epithlial-mesenchymal transition of cancer cells Local tumor invasion and intravasation into blood or lymph vessels. By the end of this session, you will be able to define epithelial to mesenchymal transition and describe the characteristics of epithelial cells and mesenchymal cells. You'll be able to describe the positive and negative role of the extracellular matrix in local invasion, and define intravasation as a specialized invasion event. Cancer is often described as invasive, but what does that really mean? We'll be talking about four aspects of tumor invasion today. Epithelial to mesenchymal transition or EMT. The gain of motility characteristics. Invasion into the local area of the tumor and specialized invasion. Intravasation to travel to distance sites. Our first sections, we'll explore the go versus grow hypothesis and EMT. In other introductory lectures, you've been introduce to the idea of cancer and to the idea of metastasis. As previously defined, cancer is the uncontrolled cell division of a single cell to form a tumor. Metastasis is the spreading of the primary tumor to a distant site. That means to metastasize, primary tumor cells must move. Initially, these two ideas seem an odds. Our metastatic cancer cells proliferative or they model. Scientists have addressed this question by the application of the Go versus Grow hypothesis. A normal cell can do one of three things at any given time. It can proliferate or grow. It can move or go, or it can differentiate into a terminal cell. Within a single cell, this going and growing are separated in space and in time. The separation of this functional characteristics and the switching among them is important for embryonic development, wound healing and organ maintenance. Let's go back to the idea then of the metastasizing cancer cell. How does the go versus grow hypothesis apply here? Primary cells on the left have a high proliferation rate. Metastasizing cells on the right have high motility. The normal model of Go versus Grow hypothesis doesn't quite fit this malignant cell phenotype. Rather than an all or none phenomenon in cancer cells, cell proliferation and motility are on a spectrum. That means that the same ideas apply, but to lesser degrees. Proliferating cells have low motility, and migratory cells have low proliferation. Importantly, however, many cancer cells likely show a mixed phenotype of moderate levels of proliferation and movement simultaneously. We're still left with a fundamental question of the primary cancer cells, though. How do the proliferating epithelial cells of a growing carcinoma grain movement ability? This brings us to the idea of the epithelial to mesenchymal transition or EMT. EMT is the reversible transition process in which epithelial cells lose characteristic epithelial characters while simultaneously gaining mesenchymal stem cell characteristics, as illustrated here with epithelial cells on the far left and mesenchymal cells on the far right. EMT, important in physiologic processes, including organ development, wound healing and fibrosis was first described by Elizabeth Hay in 1968. It remains an active area of research today. Before we can understand the process of EMT, we need to know more about the phenotypic characteristics of the two extreme cell types, epithelial cells and mesenchymal cells. Epithelial cells make up the epithelial tissues that line all of the outer and inner cavities of the body, including the skin and the lining of the intestine as well as many other organs where they offer strong structural integrity. Epithelial cells have apicobasal polarity, which means that they have different characteristics on their top and bottom sides and have cuboidal morphology. They strongly adhere to the extracellular matrix on the basil side and form tight cell-cell interactions. They are highly proliferative and they do not move. In the face contrast image of in vitro cultured epithelial cells, you can see that the cells grow tightly together in colonies and have a cuboidal shape. Epithelial cells, as I mentioned before have those tight cell-cell interactions. This occurs through several different types of interactions. Near the apical surface of the cell, tight junctions create a seal between cells to prevent any infiltration of compounds from the surrounding area. They're made primarily of colodons and can be seen in the illustration on the right as the two green blobs. Adherence junctions link neighboring cells actin cytoskeleton to provide mechanical attachment to each other and are primarily made of cadherins. You can see that with the red linkers attaching the blue dots that indicate both cadherins, as well as the cytoskeleton. Gap junctions, which is illustrated here in light blue are a specialized cell-cell channel. Allowing direct communication between the cytoplasm of neighboring cells. Finally, desmosomes are another mechanical interaction that link cells intermediate filaments to provide tight cell-cell adhesion and are primarily made of desmoplakin. Now, let's turn to the mesenchymal cells. Mesenchymal cells are essential in embryonic development and in wound healing. They have many properties including the ability to differentiate into many cell types, to regulate immune and inflammatory response and to modulate surrounding cells. They are different from epithelial cells in a few striking ways. First, they do not directly bind with other cells. They look different as well with front to back polarity and specialized cytoskeleton components that allow for cell movement. What you can see well on face contrast image of the in vitro cultured mesenchymal cells is their spindle like morphology. Even though they don't form tight intracellular interactions, they do bind to the extracellular matrix using similar mechanisms as epithelial cells. Both cells attach to the ECM via focal adhesions which links the intracellular actin to the ECM through integrin clusters. So now you know about epithelial cells and you know about mesenchymal cells, but what do we use in the lab or in the clinic to distinguish between the two? First off, it's important to remember that in cancer cells EMT just like go or grow is a continuum. Some cells will retain, some epithelial characteristics when they transition or cells may show different mesenchymal characteristics. It's all context and cell dependent. In general, these are the characteristics that we look for. We first look at morphology, so epithelial cells are more cuboidal while mesenchymal cells are more spindle like. Their adhesion proteins are different with epithelial cells expressing e-cadherin and mesenchymal cells expressing n--cadherin and fibronectin. Epithelial cells, of course, have high cell-cell protein expression and they have a specific intermediate filament characteristics while mesenchymal cells have specific cytoskeletel proteins. Finally, we can look at their function. Remember, epithelial cells are highly proliferative while mesenchymal cells are highly migratory. Communication between tumor cells and cells in microenvironment drive progression. You heard Dr. Malcomberg talk about that in another lecture where he talked about the many factors and cells that drive that tumor progression, and you'll hear later about many microenvironment cells that can drive metastasis. Of course, all of these cells will also influence EMT. Some of the most well-studied factors of the tumor microenvironment are TGFbeta secreted by fibroblasts, M2 macrophages that secrete IL-6 and IL-4 and the reactive oxygen species that results from hypoxic conditions. This of course is not a comprehensive list and what induces EMT is an active area of research. An important thing to remember about EMT is that it is a transition. That means it can be a reversible process. Once a metastatic cell reaches a secondary site, it must regain the proliferative characteristics of epithelial cells in order to grow and form a secondary tumor. This is called the mesenchymal to epithelial transition or MET. This EMT to MET transitioning is regulated by gene expression through epigenetic regulation not gene mutation. To learn more about epigenetics, have a look at our Introduction to cancer course also available on Coursera. This concludes the section on Go versus Grow hypothesis and EMT. Next up, we'll be talking about the process of local tumor invasion.
