Date of Degree
PhD (Doctor of Philosophy)
Molecular Physiology and Biophysics
Michael D. Henry
Metastasis is the most common cause of lethality in patients with solid tumors. This complex cascade of events begins with invasion of local tissue by cancer cells of the primary tumor and eventually leads to dissemination of cancer cells through the bloodstream. In order to colonize a distant tissue, circulating cancer cells must first survive the physical stresses within the vasculature, and then traverse the endothelium, by a process called extravasation. After extravasation, colonized cancer cells face several additional challenges including proliferation in a nutrient-deprived microenvironment. Epithelial-mesenchymal transition (EMT) is a process by which cells lose their epithelial characteristics and gain a mesenchymal, often migratory phenotype. There is much evidence that EMT augments cancer cell invasion, however little is known about how EMT-like cells interact with their microenvironment during metastasis. We investigated the migratory behavior of EMT-like cancer cells on different basement membrane constituents as well as in the presence of other cell types. We showed that ZEB1, a driver of EMT, regulates pro-migratory genes, resulting in cells which must co-opt with their matrix and cellular surroundings to elicit invasive migration. Additionally, we show that RNAi-mediated knockdown of ZEB1 results in significantly reduced anchorage-independent growth as well as metastatic colonization in mice. Thus, ZEB1 and EMT-states may facilitate both extravasation and survival of cancer cells in vivo.
In experimental and clinical settings, metastasis is viewed as an inefficient process; of the many cancer cells which enter the bloodstream, very few go on to form secondary tumors. The events which contribute to this inefficiency are debated. A popular theory is that most cancer cells die in circulation, under hemodynamic shear forces. There is evidence, however, which challenges this paradigm. Direct analyses of the response of cancer cells to shear forces are lacking. Therefore, we designed an in vitro model of fluid shear stress, which allows high throughput analysis of various cell types. In a broad panel of cancer cell lines, derived from various tissues, we found a remarkably conserved inducible shear stress resistance response. This response was absent in normal epithelial cells or non-transformed cell lines. Mechanistically, this response requires extracellular calcium and actin polymerization. These studies revealed a novel mechanism which may be necessary for progressive metastasis, and has practical implications in the study of circulating tumor cells. To gain insight into the metastatic phenotype, we analyzed of a panel of cancer cell lines derived from metastatic passage in mice. We noticed that derivative cells were physically smaller than their respective parental cell lines. Reduced cell size was correlated with attenuated activation of the mTOR pathway, and an increase in autophagic flux. Autophagy allows cells to digest their own proteins and organelles, and thus benefits cells residing in a nutrient-depleted environment. Our data suggest that autophagic cells are selected for in the metastatic microenvironment Future directions aim to determine the role of autophagy in metastasis. Finally, we show that an aggressive subpopulation of prostate cancer cells exhibit stem cell-like features, which may be regulated by ZEB1. In sum, these studies provide mechanistic details underlying the interactions of cancer cells with matrix and fluid microenvironments, which in turn affect migration, survival, and metabolism during metastasis.
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Copyright 2011 James Matthew Barnes