Cancer theory faces doubts

To differentiate or not--routes towards metastasis

Why are many metastases differentiated? Invading and disseminating carcinoma cells can undergo an epithelial-mesenchymal transition (EMT), which is associated with a gain of stem cell-like behaviour. Therefore, EMT has been linked to the cancer stem cell concept. However, it is a matter of debate how subsequent mesenchymal-epithelial transition (MET) fits into the metastatic process and whether a MET is essential. In this Opinion article, I propose two principle types of metastatic progression: phenotypic plasticity involving transient EMT-MET processes and intrinsic genetic alterations keeping cells in an EMT and stemness state. This simplified classification integrates clinically relevant aspects of dormancy, metastatic tropism and therapy resistance, and implies perspectives on treatment strategies against metastasis.

EMT and dissemination precede pancreatic tumor formation

Metastasis is the leading cause of cancer-associated death but has been difficult to study because it involves a series of rare, stochastic events. To capture these events, we developed a sensitive method to tag and track pancreatic epithelial cells in a mouse model of pancreatic cancer. Tagged cells invaded and entered the bloodstream unexpectedly early, before frank malignancy could be detected by rigorous histologic analysis; this behavior was widely associated with epithelial-to-mesenchymal transition (EMT). Circulating pancreatic cells maintained a mesenchymal phenotype, exhibited stem cell properties, and seeded the liver. EMT and invasiveness were most abundant at inflammatory foci, and induction of pancreatitis increased the number of circulating pancreatic cells. Conversely, treatment with the immunosuppressive agent dexamethasone abolished dissemination. These results provide insight into the earliest events of cellular invasion in situ and suggest that inflammation enhances cancer progression in part by facilitating EMT and entry into the circulation.

EMT and dissemination precede pancreatic tumor formation

Metastasis is the leading cause of cancer-associated death but has been difficult to study because it involves a series of rare, stochastic events. To capture these events, we developed a sensitive method to tag and track pancreatic epithelial cells in a mouse model of pancreatic cancer. Tagged cells invaded and entered the bloodstream unexpectedly early, before frank malignancy could be detected by rigorous histologic analysis; this behavior was widely associated with epithelial-to-mesenchymal transition (EMT). Circulating pancreatic cells maintained a mesenchymal phenotype, exhibited stem cell properties, and seeded the liver. EMT and invasiveness were most abundant at inflammatory foci, and induction of pancreatitis increased the number of circulating pancreatic cells. Conversely, treatment with the immunosuppressive agent dexamethasone abolished dissemination. These results provide insight into the earliest events of cellular invasion in situ and suggest that inflammation enhances cancer progression in part by facilitating EMT and entry into the circulation.

Heat shock proteins as danger signals for cancer detection

First discovered in 1962, heat shock proteins (HSPs) are highly studied with about 35,500 publications on the subject to date. HSPs are highly conserved, function as molecular chaperones for a large panel of “client” proteins and have strong cytoprotective properties. Induced by many different stress signals, they promote cell survival in adverse conditions. Therefore, their roles have been investigated in several conditions and pathologies where HSPs accumulate, such as in cancer. Among the diverse mammalian HSPs, some members share several features that may qualify them as cancer biomarkers. This review focuses mainly on three inducible HSPs: HSP27, HPS70, and HSP90. Our survey of recent literature highlights some recurring weaknesses in studies of the HSPs, but also identifies findings that indicate that some HSPs have potential as cancer biomarkers for successful clinical applications.

A novel platform for detection of CK+ and CK- CTCs

Metastasis is a complex, multistep process that begins with the epithelial-mesenchymal transition (EMT). Circulating tumor cells (CTC) are believed to have undergone EMT and thus lack or express low levels of epithelial markers commonly used for enrichment and/or detection of such cells. However, most current CTC detection methods target only EpCAM and/or cytokeratin (CK) to enrich epithelial CTCs, resulting in failure to recognize other, perhaps more important, CTC phenotypes that lack expression of these markers. Here, we describe a population of complex aneuploid CTCs that do not express CK or CD45 antigen in patients with breast, ovarian, or colorectal cancer. These cells were not observed in healthy subjects. We show that the primary epithelial tumors were characterized by similar complex aneuploidy, indicating conversion to an EMT phenotype in the captured cells. Collectively, our study provides a new method for highly efficient capture of previously unrecognized populations of CTCs.

SIGNIFICANCE

Current assays for CTC capture likely miss populations of cells that have undergone EMT. Capture and study of CTCs that have undergone EMT would allow a better understanding of the mechanisms driving metastasis.

Circulating tumor cells and circulating tumor DNA

Solid tumors derived from epithelial tissues (carcinomas) are responsible for 90% of all new cancers in Europe, and the main four tumor entities are breast, prostate, lung, and colon cancer. Present tumor staging is mainly based on local tumor extension, metastatic lymph node involvement, and evidence of overt distant metastasis obtained by imaging technologies. However, these staging procedures are not sensitive enough to detect early tumor cell dissemination as a key event in tumor progression. Many teams have therefore focused on the development of sensitive assays that allow the specific detection of single tumor cells or small amounts of cell-free tumor DNA in the peripheral blood of cancer patients. These methods allow the detection and characterization of early metastatic spread and will provide unique insights into the biology of metastatic progression of human tumors, including the effects of therapeutic interventions.

Components in melanoma cytoplasm might induce murine BMSCs transformation and expression of Melan-A

This study explored the possibility that the components in melanoma cytoplasm induce murine BMSCs transformation and expression of Melan-A by morphologically observing the changes of BMSCs and immunocytochemically detecting Melan-A in the cells after culturing BMSCs in medium containing melanoma cytoplasm components (MCC). MCC of B16 melanoma cells was prepared and BMSCs were cultured and induced by adding the MCC into culture medium. The cells were morphologically observed and Melan-A was immunohistochemically detected to confirm BMSCs transformation. MCC-induced BMSCs underwent morphological changes. A number of melanin granules appeared in the cytoplasm of the cells and some were released into surrounding areas. Several cells that might come from one cell formed a cluster, and their granules, together with those secreted by other induced BMSCs, formed a so-called "sphere-formed structure". The induced BMSCs expressed Melan-A. We are led to conclude that there might be some factors in the cytoplasm of melanoma cells that might induce BMSCs transformation toward melanogenic cell, or even melanoma.

Mesenchymal transition and dissemination of cancer cells is driven by myeloid-derived suppressor cells infiltrating the primary tumor

In order to metastasize, cancer cells need to acquire a motile phenotype. Previously, development of this phenotype was thought to rely on the acquisition of selected, random mutations and thus would occur late in cancer progression. However, recent studies show that cancer cells disseminate early, implying the existence of a different, faster route to the metastatic motile phenotype. Using a spontaneous murine model of melanoma, we show that a subset of bone marrow-derived immune cells (myeloid-derived suppressor cells or MDSC) preferentially infiltrates the primary tumor and actively promotes cancer cell dissemination by inducing epithelial-mesenchymal transition (EMT). CXCL5 is the main chemokine attracting MDSC to the primary tumor. In vitro assay using purified MDSC showed that TGF-β, EGF, and HGF signaling pathways are all used by MDSC to induce EMT in cancer cells. These findings explain how cancer cells acquire a motile phenotype so early and provide a mechanistic explanation for the long recognized link between inflammation and cancer progression.

Cancer progression has been depicted as a linear process, during which the incipient cancer cell sequentially accumulates mutations that confer the ability to metastasize. However, recent studies show that cancer cells disseminate early, before such mutations can accumulate, implying the existence of a different, faster route to the metastatic phenotype. Using a mouse model of melanoma, we show that the primary tumor attracts a subset of immune cells that actively promote cancer cell motility, dissemination, and metastasis. These tumor-infiltrating immune cells do so by reactivating a cellular program (mesenchymal transition) used by melanocytes during their development to colonize the skin, and also believed to be an essential step in cancer cell dissemination and metastasis. Once the melanoma cells migrate out of the primary tumor, they can lapse back to their original phenotype and lose their migratory potential. This transient phenotypic switch may accelerate carcinogenesis and participate in the plasticity of cancer. It explains how cancer cells might spread to other organs even before the original tumor is detected. In addition to the evidence gleaned from our mouse melanoma model, we show that these immune cells induce typical features of epithelial-mesechymal transition in both melanoma and bladder human cell lines when examined in culture dishes. These findings provide an underlying mechanism for the long-recognized link between inflammation and cancer progression.