The cancer stem cell hypothesis maintains that the growth of many, if not all, tumours is driven by a small subset of cells known as cancer stem cells (CSCs).

These cells share characteristics with normal stem cells in that they are able both to self-renew and to differentiate into non-self-renewing cells that form the bulk of the tumour.

However, although cancer stem cells have been identified and characterised in some tumour types, little is known yet about their origin and properties.

Analysis of these cells has been hampered particularly by difficulties in isolating pure samples from tumour tissue. Paola Scaffidi and Tom Misteli of the National Cancer Institute, Bethesda, Maryland, USA have now shown that it is possible to generate human cells with all the characteristics of cancer stem cells in vitro by the oncogenic re-programming of somatic cells.

Scaffidi and Misteli first transformed primary human skin fibroblasts into cells capable of nucleating tumours in immunocompromised mice through the expression of telomerase and viral antigens, and injected them into mice. The resulting tumours were undifferentiated and contained cells with varying morphology.

Some of these were, like normal mesenchymal stem cells, able to differentiate into adipocytes and osteoblasts, indicating that they had acquired cancer stem cell-like properties during transformation. Scaffidi and Misteli therefore examined the tumourigenic cells for known cancer stem cells markers.

The stage-specific embryonic antigen SSEA-1, which is known as a CSC marker in human and mouse brain tumours, was detected in about 1% of the transformed cells, but not at all in a similar cell line that had been immortalised but not transformed.

This antigen was also observed in a subpopulation of cells in transformed mammary epithelial cells, indicating that it was not simply a characteristic of fibroblasts, and in some clinical tumour samples.

Further analysis suggested that the SSEA-1+ putative cancer stem cells acquired these characteristics along with tumourigenicity during transformation.

When the SSEA-1+ cell subpopulation was cultured separately, the daughter cells were found to be heterogeneous, with the proportion of cells carrying the marker decreasing to that found in the original tumours within six days.

About one-fifth of cell divisions were asymmetrical, resulting in one SSEA-1+ and one SSEA-1- daughter cell.

The expression properties of both SSEA-1+ and SSEA-1- fibroblasts were then examined using microarrays. About 300 genes were differentially expressed between the cell types, with genes involved in tumourigenesis up-regulated and tumour suppressor genes down-regulated in the SSEA-1+ cells.

The SSEA-1+ cells were further shown to initiate the bulk of the tumours, and to do so by differentiating into heterogeneous, non-tumourigenic, SSEA-1- cancer cells.

Only a small proportion of differentiated cells in the tumours had the characteristics of fibroblasts. The tumours were found to contain about 1% of fibroblastic, CSC-like SSEA-1+ cells, which is a very similar proportion to that in the original tumours.

A comparison of the expression profiles of these SSEA-1+ cells and their non-stem-like progeny in three tumours revealed that the tumour-maintaining stem-like cells have a distinct profile with up-regulated genes characteristic both of fibroblasts and of tumour growth.

Genes up-regulated in the SSEA-1- progeny included some involved in differentiation and in inhibition of cell proliferation.

Scaffidi and Misteli have shown that it is possible to generate a cell sub-population with all the properties of cancer stem cells in vitro by tumourigenic transformation of a human cell line.

These cells form the source of heterogeneous, hierarchically organised tumours. They will also be valuable as a tool for the further analysis of the properties of cancer stem cells and the mechanisms of stem cell-directed carcinogenesis.

Reference: Scaffidi, P. and Misteli, T. (2011). In vitro generation of human cells with cancer stem cell properties. Nature Cell Biology, published online ahead of print 21 August 2011. DOI: 10.1038/ncb2308