Editors' ChoiceCancer

Wnt Signaling Heterogeneity

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Science Signaling  11 May 2010:
Vol. 3, Issue 121, pp. ec140
DOI: 10.1126/scisignal.3121ec140

Some tumors are composed of heterogeneous cells, and an emerging concept is that a subset of these cells are “cancer stem cells,” which can undergo proliferation and self-renewal or can differentiate and contribute to the mass of the tumor (see Korkaya and Wicha). Noting that colon carcinomas, which have mutations that should increase Wnt signaling (APC mutations), exhibit heterogeneity in the abundance of nuclear β-catenin (the effector of Wnt signaling), Vermeulen et al. expressed a Wnt reporter gene (TOP-GFP) in cultured patient colon cancer cells that were positive for markers of cancer stem cells to explore how Wnt signaling heterogeneity affected tumorigenic potential and the maintenance of “stemness.” Clones from single cells exhibited a large variation in β-catenin nuclear localization and in green fluorescent protein (GFP) intensity, which allowed the authors to divide the cells into high (TOP-GFPhigh), intermediate (TOP-GFPintermediate), and low (TOP-GFPlow) populations. Microarray analysis of TOP-GFPhigh cells showed that these cells had higher expression of genes associated with stem cells, whereas TOP-GFPlow cells had higher expression of genes associated with epithelial differentiation. The clonogenic potential and the ability to form tumors in immunocompromised mice also correlated with the strength of Wnt signaling, with TOP-GFPhigh exhibiting the highest frequency of clone and tumor formation and TOP-GFPlow exhibiting the lowest. The tumors from the xenografted TOP-GFP cells exhibited morphological characteristics of the tumor from which the TOP-GFP cells were created and, like the tumor from which they were created, the TOP-GFP tumors showed variation in nuclear β-catenin and GFP intensity, suggesting that the tumor microenvironment controls the intensity of the Wnt signal. Myofibroblasts are stromal cells associated with colorectal cancer. When colon cancer cells positive for markers of cancer stem cells were cultured in the presence of serum, markers of differentiation were expressed and morphological changes associated with differentiation occurred. Coculture of these colon cancer cells with myofibroblasts or with myofibroblast-conditioned medium (MFCM) suppressed this in vitro differentiation. Cells cultured with MFCM exhibited increased nuclear β-catenin, GFP intensity, and clonogenic potential compared to cells grown in serum-containing medium. Hepatocyte growth factor (HGF), which is secreted by myofibroblasts, recapitulated the effects of MFCM, and cells exposed to HGF exhibited decreased activity of glycogen synthase kinase 3β (a negative regulator of Wnt signaling), increased activity of the kinase Akt, and increased stability of β-catenin. Exposure of TOP-GFPlow cells to MFCM induced transcripts associated with the cancer stem cell phenotype and increased clonogenic potential to nearly that of the TOP-GFPhigh cells, suggesting that stromal-derived signals can induce the cancer stem cell fate. Indeed, analysis of either xenografted tumors or patient tumor samples revealed that the cells with the greatest nuclear β-catenin (patient samples) or highest GFP intensity (TOP-GFP xenografts) were located adjacent to myofibroblasts. Finally, coinjection of TOP-GFPlow cells with myofibroblasts or with MFCM and artificial extracellular matrix resulted in increased tumorigenesis. Thus, by increasing the intensity of Wnt signaling, stromal signals drive a subpopulation of the tumor cells to adopt a cancer stem cell phenotype.

L. Vermeulen, F. De Sousa E Melo, M. van der Heijden, K. Cameron, J. H. de Jong, T. Borovski, J. B. Tuynman, M. Todaro, C. Merz, H. Rodermond, M. R. Sprick, K. Kemper, D. J. Richel, G. Stassi, J. P. Medema, Wnt activity defines colon cancer stem cells and is regulated by the microenvironment. Nat. Cell Biol. 12, 468–476 (2010). [PubMed]

H. Korkaya, M. S. Wicha, Cancer stem cells: Nature versus nurture. Nat. Cell Biol. 12, 419–421 (2010). [PubMed]

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