Mutant Ras-Expressing Fibroblasts Induce Expansion of Circulating Angio- and Lymphangiogenic Precursor Cells: Potential Role in Tumour Vascularisation.

Fibroblasts have a prominent role in the initiation, enlargement and metastasis of cancers. Their production of growth factors, chemokines and extracellular matrix facilitate the angiogenic recruitment of endothelial cells and pericytes. The oncogene Ras, known to be frequently mutated in many cancers, has also been shown to promote the expression of pro-angiogenic growth factors, chemokines and extracellular matrix components similar to those of fibroblasts. Here we show that mutant N-ras (N-ras^m)/GFP - transduced NIH3T3 fibroblasts act to promote the ex vivo expansion of umbilical cord blood (UCB)-derived primitive endothelial progenitor cells through paracrine mechanisms. Biphasic expansion of both angiogenic and lymphangiogenic precursors was observed although they exhibited different dynamics in their expansion pattern. The first peak of expansion for both types of precursor cells was seen on day 10 with a 69-fold expansion of the CD34+VEGFR2+(endothelial) and a 23-fold expansion of CD34+VEGFR3+(lymphatic) cells co-cultured with media conditioned by N-ras^m-transduced 3T3 cells. This compared to a 20-fold and 14-fold expansion respectively, in cells co-cultured with media conditioned by GFP (only)-transduced fibroblasts. Endothelial cell differentiation accounts for the dramatic reduction in the numbers of CD34+VEGFR2+ and CD34+VEGFR3+ precursor cells on day 14 with concomitant expansion of CD34−VEGFR2+ and CD34−VEGFR3+cells. The second peak for the expansion of endothelial precursor cells was seen on day 19 with a 214-fold expansion compared to 28-fold in the control cells. In addition, the expansion of CD14+ VEGFR2+ and CD14+VEGFR3+ cells was observed on days 14 and 19, the later correlated with the expansion of monocytic CD34−CD14+ cells promoted by N-ras^m-transduced fibroblasts. The expanded monocytes appear to contribute to the expansion of endothelial and lymphatic precursor cells induced by N-ras^m-transduced fibroblasts. We propose that the angiogenic factors secreted by mutant Ras-expressing fibroblasts in a tumour microenvironment promote tumour angiogenesis through the expansion of the circulated endothelial and lymphatic precursor cells. The recruitment of the expanded endothelial and lymphatic cells into the tumour’s vascular system is currently being investigated. We propose that targeting aberrant Ras signaling in tumour fibroblasts may represent an important target for cancer therapies.