Identification of cancer stem cells in human melanomas (2006-2011)

Stem cells play a critical role in normal tissue maintenance, and mutations in these stem cells may give rise to cancer. According to the cancer stem cell hypothesis, only a subpopulation of cells within a cancer has the capacity to sustain tumor growth. This subpopulation of cells is made up of cancer stem cells, which are defined as the population of cells within a tumor that can self-renew, differentiate, and generate a phenocopy of the cancer when injected in vivo. Cancer stem cells have been prospectively isolated from human cancers of the blood, breast, and brain. Several evidences strongly suggest that melanocytic neoplasias are derived from immature melanocytic cells, which are neural crest derived, and we thus hypothesize that melanoma develops from mutated neural crest stem cells. Accordingly, at least a fraction of melanoma cells should display features of neural crest stem cells. I have evidence that metastatic human melanomas, as well as established human melanoma cell lines, contain a cell subpopulation resembling neural crest stem cells. These cells are potent in tumor formation when a limited number of cells are injected into an orthotopic in vivo microenvironment. Very importantly, this cell population is able to self-renew and can generate melanoma after serial transplantation when re-isolated from secondary recipients. Moreover, these tumors contain the cell types observed in the original cancer. These results all together strongly support that melanomas are generated by cancer stem cells with characteristics similar to neural crest stem cells. The identification of cancer stem cells in melanoma has fundamental implications for the development of new therapeutic agents. Eradication of cancer may require the targeting and elimination of cancer stem cells.

 

Increased Wnt signaling triggers oncogenic conversion of human breast epithelial cells by a Notch-dependent mechanism (2004-2006)

Wnt and Notch signaling have long been established as strongly oncogenic in the mouse mammary gland. Aberrant expression of several Wnts and other components of this pathway in human breast carcinomas have been reported, but evidence for a causative role in the human disease has been missing. We found that increased Wnt signaling, as achieved by ectopic expression of Wnt‑1, results in tumorigenic conversion of primary human mammary epithelial cells. Wnt‑1-transformed cells have high telomerase activity, compromised p53 and Rb function, grow as spheres in suspension, and form tumors in mice, which closely resemble medullary carcinomas of the breast. Ectopic expression of Wnt‑1 induces activation of Notch signaling pathway. This is achieved is by over-expression of the Notch ligands Dll1, Dll3 and Dll4 and is required for expression of the tumorigenic phenotype. Increased Notch signaling in primary HMECs is sufficient to reproduce some aspects of Wnt‑induced transformation. The relevance of these findings for human breast cancer is supported by the fact that expression of Wnt‑1 and Wnt‑4 and of established Wnt target genes, such as Axin‑2 and Lef‑1, as well as the Notch ligands, such as Dll3 and Dll4, is upregulated in human breast carcinomas.

 

Wnt signaling via the epidermal growth factor receptor (1999-2003)

Recent data have suggested the epidermal-growth-factor receptor (EGFR) as a point of convergence for several different classes of receptor. I have demonstrated crosstalk between Wnt signaling and the EGFR, showing that in breast epithelial cells Wnts activate downstream targets of the EGFR, including cyclin D1. Given the role of members of these pathways in the aetiology of breast cancer and as markers of outcome and potential therapeutic targets in breast cancer, this observation has a number of potential implications important for both the basic biology of breast cancer and the clinical management of the disease. The finding that Wnts activate erbB signaling in addition to stimulating the prototypic Wnt/β-catenin signaling pathway is provocative, revealing a new level of regulation by members of the Wnt family and providing further impetus for increased interest in these and other developmental pathways in breast cancer. One question raised by these studies is the relation between the expression patterns of Wnts, the erbB ligands potentially mediating their effects, and the metalloproteinases necessary for Wnt-mediated activation of EGFR signaling in breast cancer, and whether any relations identified are correlated with outcome in breast cancer. These findings also emphasise the interconnected nature of many signaling pathways in breast cancer: crosstalk between Wnt and EGFR signaling is one example, but another example currently under intense investigation is the emerging link between steroid receptor and receptor tyrosine kinase signaling. In both cases, better understanding of these pathways is likely to have implications beyond the biology of breast cancer, by identifying potential new markers of disease outcome and therapeutic targets. As one example, the ability of sFRP-1, which is down-regulated in a proportion of breast cancers, to inhibit Wnt activation of EGFR signaling suggests that therapies aimed at increasing sFRP levels or activity may be effective.

Keywords

Cancer stem cells, melanoma, breast cancer, tissue recombination techniques, xenograft models, mouse orthotopic tumor models.

© 2014 Gianluca Civenni