Normoxic and hypoxic regulation of vascular endothelial growth factor (VEGF) by astrocytoma cells is mediated by Ras

Abstract

Vascular endothelial growth Factor (VEGF) has been identified as a key angiogenic factor involved in the growth and malignant progression of tumours. Glioblastoma multiforme (GBM) are the most common primary human brain tumours, histo-pathologically characterized by intense tumour angiogenesis. GBMs do not harbour oncogenic Ras mutations, but there is a functional up-regulation of Ras signaling through activation of receptor tyrosine kinases overexpressed by these tumours. We demonstrate that Ras pathway activation regulates VEGF secretion in astrocytoma cell lines. Ras pathway inhibition was carried out using genetic and pharmacologic techniques. Astrocytoma cells that were transfected to express the dominant inhibitory mutant H-Ras(N17) demonstrated a reduction in VEGF secretion under both normoxic and hypoxic conditions. Cells treated with the farnesyl transferase inhibitor L-744,832 demonstrated similar reductions in VEGF secretion. Furthermore, astrocytoma cells expressing a constitutively phosphorylated and truncated EGF-R common in GBMs (EGFRvIII or p140(EGF-R)) demonstrate further elevations in Ras activation, resulting in a further increase in VEGF secretion. We have previously demonstrated that activation of Ras plays a vital role in transducing mitogenic signals in human malignant astrocytoma cells. Our present results further extend the role of Ras activation in modulating tumour angiogenesis in these tumours. We propose that Ras may contribute to the angiogenic switch in astrocytomas.