In this next section we'll be defining the different types of cancer dormancy at the cellular level. The two main types of cancer dormancy are tumor mass dormancy and cellular dormancy. Two subtypes of tumor mass dormancy are angiogenic dormancy and immunologic dormancy. Regardless of the type, they're all forms of micrometastasis because they are too small to detect. Tumor mass dormancy as a concept refers to the balance of proliferation and apoptosis occurring within a mass of cancer cells. Some cells will be proliferating, but some cells will be dying. So there's no net increase in tumor mass, and it remains undetected. There are multiple reasons for some cells to be dying and some to be proliferating. And that is what the two subtypes of tumor mass dormancy describe. The first subtype is called angiogenic dormancy. This is when the growth of the micrometastasis is restricted due to the lack of sufficient vascularization. Recall that in order for a tumor mass to grow beyond 1 millimeter cubed, it needs to be able to promote angiogenesis which is the formation of new blood vessels. These vessels are required because they allow for the delivery of growth factors and oxygen. To control angiogenesis, the cancer cells can secrete pro or anti-angiogenic factors. So in angiogenic dormancy the regulation of apoptosis versus proliferation is controlled by the ability of the mass to promote angiogenesis which is ultimately controlled by the balance of pro and anti-angiogenetic factors. Cells that are unable to reach oxygen and growth factors because they are too far from a blood vessel end up dying. And the tumor mass is restricted in growth because of its limited ability to promote angiogenesis. The second subtype of tumor mass dormancy is immunologic dormancy. This is when the growth of the micrometastasis is restricted due to the immune system. The role of the immune system is to recognize and target foreign invaders. And it has the ability to do this with cancer cells. However, there are some methods by which cancer cells can overcome this recognition in targeting. So the balance of these immune evading cells and immune susceptible cells are what regulates the overall growth of the mass. The other major type of dormancy is called cellular dormancy. This refers to individual cells that are growth-arrested and are not proliferating, but are also not dead. You may have heard of the terms quiescence and senescence as also referring to growth-arrested cells, and now wonder how they are related to cellular dormancy. Well, the cellular dormancy that we are referring to here follows the same principles as quiescence does in normal physiology, as it must be a reversible state in order for the cell to eventually develop into a metastasis. Cellular dormancy therefore cannot represent senescent cells or the differentiated post-mitotic cells, which are most of the cells in your body, because those cells are all irreversibly growth-arrested. Cellular dormancy is the most accepted explanation for the long period of clinical dormancy compared to tumor mass dormancy. So when we talk about dormancy for the remainder of the lecture, we will be referring to cellular dormancy. Because the terms dormancy, quiescence, and senescence are often incorrectly used interchangeably, it is important to understand their differences. Quiescence in normal physiology is defined as a reversible cell cycle arrest and specifically refers to cells that exist in the cell cycle in a state called G0. They can sit in this state for years where they are always poised to reenter the cell cycle. Cells may undergo quiescence in response to deprivation of extracellular growth factors, anti-proliferative cytokines, and by making contact with other cells called contact-inhibition. The one difference between healthy cells and cancer cells is that cancer cells do not exhibit contact inhibition and keep growing instead. So this is not a factor that will induce cancer cells to undergo quiescence. Some examples of healthy quiescent cells in the body include adult stem cells, progenitor cells, fibroblasts, and hepatocytes. Quiescence is extremely important because it was required for tissue homeostasis. For example, quiescence of hepatocytes allows for the ability of the liver to renew and regenerate because they are able to reenter the cell cycle and proliferate when they are needed. Quiescence also protects long lived cells against stress and toxicity. And this is especially important for cells like hemataopetic stem cells which are responsible for generating immune cells. With each cell division there's a potential for mutations to occur, and cells are more susceptible to stress and toxicity. So it becomes crucial that such important and long lived cells are able to go into a quiescent state to avoid this susceptibility. In contrast to quiescence and dormancy, senescence is an irreversible growth arrest. And these cells have been reported to arrest in G1 or G2, as opposed to G0. Senescence can be induced by many factors, but the main one is due to telomere shortening called replicative senescence. This is shown in the figure to the right. With each cell division and DNA replication cycle, DNA on the ends of the chromosomes get shorter and shorter, and eventually at a certain threshold signals a DNA damage response to the cell to undergo senescence. Any cell can undergo senescence, and this is a major factor that contributes to again because these cells no longer have the functional capacity that healthy cells do. Because senescence can be triggered by a DNA damage response in general and not only from telomere shortening, it prevents the outgrowth of damaged cells that may lead to cancer. However, when cancer forms, it is because it has gained the ability to evade the senescent response. And thus, this evasion is actually a property of cancer cells. Quiescence and senescence can also be differentiated by their molecular markers, which is important for research. Positive markers are those that mark the cell in question, whereas negative markers mark any other cell. Quiescence and senescence are negatively marked in the same way since they are both growth-arrested, and those markers positively mark proliferating cells. For example, EdU is a molecule that can intercalate into DNA, but only when the DNA is actively replicating which happens in proliferating cells. Therefore, EdU would only mark proliferating cells, but neither quiescent nor senescent cells. Quiescence is typically positively marked by cell cycle regulators, whereas senescence can be marked by proteins involved in the damage response. The most robust and commonly used positive marker for senescence though is the enzyme beta galactosidase. There are many positive markers for proliferation, and negative markers are essentially the positive markers for quiescence. Now that we've defined cellular dormancy, we must understand how it is controlled. It may be that there's a lack of extracellular growth factors in the host site so the disseminated cells are unable to receive pro-proliferation signals. Thus the extrinsic microenvironment is not conducive for proliferation of that type of cell. Using the seed and soil analogy, the soil is not fertile. On the other hand, the host organ may contain anti-proliferative cytokines that actively tell the cell to stop proliferating despite the presence of any proliferation signals. This is how quiescent stem cells can be regulated. And it can be useful to try to think about cancer dormancy in terms of stem cell quiescence, as hijacking of normal processes by tumor cells tends to be a common theme in cancer. A third way that dormancy may be controlled is by a change in the cell's metabolism. Here we are specifically referring to autophagy or autophagocytosis which is a controlled process that degrades and recycles any cellular components that are not absolutely necessary at that time. The figure to the right shows the presence of autophagosomes and autolysosomes, which are where the cellular components are phagocytosed and broken down respectively. This can happen in response to cellular stress as a way for the cell to continue to survive on low resources by limiting its energy expenditure. The use of autophagy to maintain survival in a stressful environment has been shown to induce quiescence. To conclude this section we have defined different types of dormancy that may exist and how cellular dormancy in particular may be controlled. I want to emphasize the point that much of this area is under current investigation, and there's evidence for all these types of dormancy and regulation. But it is still not fully understood why or when each type is used and what the predominant regulatory mechanisms are.
