Prostate cancer presents a significant paradox: it is very common, yet rarely fatal. To wit, the prostate is the most common non-skin tissue for cancer diagnosis in men in the United States. Despite its high incidence, fatal malignancy occurs in only a small fraction of diagnosed men. The fatal cases are characteristically defined by distant spread in the body, also known as metastasis. In order to metastasize a cancer cell must complete several sequential steps. These include degradation of and invasion through the epithelial basement membrane, typically through the loss of static intracellular adhesions with fellow epithelial cells; entrance into the blood stream (intravasation); survival within circulation; exit from the blood stream upon arrival at a new tissue (extravasation); and survival and colonization at the secondary site.
At the time of diagnosis, it is not currently possible to accurately predict future metastasis and thereby clinicians cannot delineate those men at high risk for fatal disease from the vast majority of men who are likely to experience an indolent disease course. Consequently, we examined the behavior of cancer cells in several steps of the metastatic cascade. In doing so, we uncovered both molecular and biophysical characteristics of cancer cells that may facilitate successful metastatic dissemination and tumor outgrowth.
Epithelial-to-mesenchymal transition (EMT) is physiological process of transdifferentiation that is normally initiated during vertebrate development, but has recently been implicated in tumor development, progression, and metastases. The EMT program results in dramatic changes, including the exchange of epithelial for mesenchymal markers, altered cellular morphology, and gain of motility. EMT-like cellular alterations have been implicated most strongly in the metastasis steps of invasion and survival of cells at primary tumors sites. How EMT-like changes may facilitate survival and growth in the microenvironment of a micrometastatic niche has been less clearly elucidated. Consequently, we evaluated how EMT-like changes may affect the survival and subsequent outgrowth of prostate cancer cell lines following restrictive growth conditions. We observed that EMT-like cells as compared to their more epithelial counterparts displayed enhanced maintenance of their proliferative potential following extended culture in nutrient restriction. This phenotype depended on an EMT-associated increase in autophagy. Notably, the post-stress outgrowth phenotype could be conferred through a paracrine signaling mechanism that may involve autophagy-derived exosome-like extracellular vesicles. These studies demonstrated that EMT-like cells have a resistance to nutrient restriction through enhanced autophagy and may have uncovered a novel autophagy-dependent exosomal secretion pathway.
Metastatic efficiency is thought to be strongly limited by the destruction of circulating tumor cells by the hemodynamic shear forces within the vasculature. However, such a persistent belief has little appropriate published experimental evidence. We developed an in vitro assay to expose cells to fluid shear stress (FSS). By monitoring the viability of the cells, we determined that transformed cells had a highly conserved ability to resist even very high FSS. The mechanism depended on the capacity to patch membrane defects, extracellular calcium, and a dynamic cytoskeleton. We also observed a stiffening of cancer cell membranes after exposure to FSS. Taken together, these studies expand the understanding of how cancer cells survive in circulation and indicate that metastatic efficiency is less limited by hemodynamic forces than previously thought.
The steps of hematogenous metastasis between intravasation and extravasation necessitate the existence of circulating tumor cells (CTCs). Collection, enumeration, and study of CTCs have the potential to serve as a "liquid biopsy" of the metastatic cascade. In prostate cancer, the enumeration of CTCs by detection of the expression of epithelial markers has displayed limited clinical utility. We hypothesized that the prognostic value of CTC number may be enhanced by detection of cells which have undergone the pro-metastatic EMT-like program. We developed a flow cytometry-based experimental assay for enumeration of CTCs using epithelial (EpCAM) and mesenchymal-like (N-cadherin) surface proteins. We detected from prostatectomy patients before and after surgery events expressing EpCAM, N-cadherin, and both. However, the detection of background events from healthy control subjects was unacceptably high. These studies support the idea of mesenchymal-like tumor cells in circulation, but will require further assay development for reliable conclusions to be drawn.
In sum, the work described above has provided descriptive and mechanistic insight to molecular and biophysical properties that may facilitate prostate cancer metastasis. It is our hope that these data will result in the development of relevant preventative, diagnostic, and therapeutic clinical strategies for prostate cancer.