Abstract
Metastatic growth is the dominant cause of cancer related deaths, and therapeutic targeting of metastases remains challenging. Metastasis refers to a multistep process in which cancer cells migrate away from the primary tumor, disseminate throughout the body and colonize a distant tissue. We have studied metastasis by visualizing tumor cells
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at high resolution in living mice using intravital microscopy (IVM). IVM is an excellent technique to study dynamic processes; however, it can only be used to study metastatic prone organs such as the liver for ~ 36 hours. Because metastatic colonization takes place over multiple days we developed a new IVM instrument called the Abdominal Imaging Window that allowed us to intravitally image abdominal organs over weeks. Using this window we found a new stage during metastatic colonization, which is referred to as a pre-micrometastasis stage. During this stage the tumor cells are highly migratory and proliferative. Blocking migration during this pre-micrometastasis stage reduced metastatic outgrowth. As such, it may be used as a new target for pharmacological inhibition of metastatic growth. Next to studying migration during colonization we have also studied tumor cell migration during other steps of metastasis. We found that the death receptor CD95, which is well known for its apoptosis inducing effects, can stimulate migration and invasion in colorectal cancer cells. It does so by promoting Tyr 783 phosphorylation of phospholipase C-g1, resulting in phosphatidylinositol (4,5)-biphosphate (PIP2) hydrolysis. This activates cofilin, an actin-severing protein which is required for migration. To study the role of the microenvironment in inducing tumor cell migration we have developed a tool called Cryosection Labeling and Intravital Microscopy (CLIM). It allowed us to re-identify intravitally imaged regions in cryosections. By performing immune cell staining on the imaged areas we found a positive correlation between the in vivo migratory behavior of tumor cells and the presence of CD3+ T cells. We could confirm that CD3+ T cells stimulate tumor cell migration by depleting T cells in vivo. Taken together, this shows the use of IVM as an important tool to study complex dynamic processes in vivo such as metastasis. Furthermore, we found that migration is an important parameter during the entire metastatic process and we have identified the microenvironment and a new pathway as important players during tumor cell migration
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