Mutant Ras-induced proliferation of human thyroid epithelial cells requires three effector pathways

Ras mutations occur as an early event in many human tumours of epithelial origin, including thyroid. Using primary human thyroid epithelial cells to model tumour initiation by Ras, we have shown previously that activation of both the MAP kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) effector pathways are necessary, but even when activated together are not sufficient, for Ras-induced proliferation. Here, we show that a third effector, RalGEF, is also activated by Ras in these cells, that this activation is necessary for Ras-induced proliferation, and furthermore that in combination with the MAPK and PI3K effectors, it is able to reproduce the proliferative effect of activated Ras. The requirement for three effector pathways indicates a more robust control of cell proliferation in this normal human epithelial cell type than has been displayed in previous similar studies using rodent and human cell lines. Our findings highlight the importance of the appropriate cellular context in models of Ras-induced tumour development.