Vascular endothelial growth factor (VEGF) stimulates angiogenesis, or the growth of blood vessels, in both normal and tumour tissues. This process is essential for the growth of solid tumours, and VEGF inhibitors can be effective in cancer therapy.

It is becoming clear, however, that these important new drugs can have anti-cancer effects other than just blocking angiogenesis. A group led by Cédric Blanpain of Universite Libre de Bruxelles, Brussels, Belgium has investigated the effect of VEGF on controlling the properties of cancer stem cells using a mouse model of squamous cell carcinoma.

Squamous cell carcinoma is a common and easily treated form of skin cancer, affecting about 500,000 people worldwide every year. Mouse models of this cancer are particularly useful because these tumours in mice are almost identical to human tumours. Tumour epithelial cells that express the glycoprotein CD34 have been identified as stem cells in this tumour.

Blanpain and his co-workers first set out to explore whether these stem cells inhabit a vascular niche that can regulate their activity, as has been proposed for brain tumour stem cells, using a chemically-induced mouse tumour model.

They used immunostaining for stem cell markers including CD34 to show that stem cells were preferentially located in what may be such a vascular niche, close to endothelial cells. They then used an antibody to block signalling through the VEGF receptor VEGFR2, which is expressed in skin papillomas only in endothelial cells, and showed that this blockade caused tumour regression.

This antibody, DC101, also reduced the proportion of CD34+ stem cells in the tumours by reducing proliferation rather than by increasing apoptosis.

Blanpain and his co-workers then explored mechanisms through which VEGF signalling might directly affect the number and properties of cancer stem cells in this tumour. Conditional deletion of the gene Vegfa in tumour epithelial cells (TECs) once papillomas had been established in a transgenic mouse model was shown to cause tumour regression accompanied by a decrease in vascular density and endothelial cell proliferation.

The extent of tumour regression was too great to be explained by inhibition of angiogenesis alone. Interestingly, Vegfa deletion was also shown to alter the balance between self-renewing and differentiating cell divisions in these stem cells in favour of differentiation. In contrast, conditional over-expression of Vegfa in TECs in the same model resulted in increases in endothelial cell proliferation, vascular density and tumour growth, and an expansion in CD34+ stem cell numbers.

In order to investigate the molecular mechanism through which VEGF signalling might affect the stem-like properties of CD34+ cells, the researchers then compared the transcriptional profiles of VEGF-overexpressing and control stem cells. About 20% of all the genes that are known to be over-expressed in CD34+ cells compared to CD34- cells were further over-expressed in cells in which VEGF was up-regulated.

These include a number of genes already known to regulate proliferation and other properties consistent with a stem-like phenotype. Neuropilin (Nrp1) is highly expressed in CD34+ tumour stem cells in mice, and its human equivalent, NRP1, is expressed in several human tumours and cancer cell lines. Blanpain and his co-workers conditionally deleted this gene in the epidermis of their mouse model.

These mice developed normal skin but, crucially, did not develop papillomas when exposed to carcinogenic chemicals, and VEGF stimulation was suppressed by blocking Nrp1 in stem cells.

Taken together, these results suggest a dual role for VEGF in the stem cell-mediated initiation of squamous cell carcinoma in mice. Firstly, it creates, through angiogenesis, a vascular niche in which CD34+ cancer stem cells can proliferate, and secondly, it acts directly on these cells to foster their intrinsic stem-like properties through a still unknown mechanism that depends on Nrp1.

If, as is likely, these mechanisms have direct counterparts in human cancer, the results suggest novel targets for the prevention and treatment of epithelial cancers.

Reference

Beck, B,. Driessens, G., Goossens, S. and 13 others (2011). A vascular niche and a VEGF–Nrp1 loop regulate the initiation and stemness of skin tumours. Nature 478, 399-403. doi:10.1038/nature10525