Expression of the Ets-1 transcription factor in human astrocytomas is associated with Fms-like tyrosine kinase-1 (Flt-1)/vascular endothelial growth factor receptor-1 synthesis and neoangiogenesis

Abstract

Marked neovascularization and vascular endothelial proliferation are characteristic features of malignant gliomas. Vascular endothelial growth factor (VEGF), an angiogenic protein secreted by glioma cells, appears to play a crucial role for induction of neoangiogenesis. The VEGF receptors fms-like tyrosine kinase-1 (Flt-1)/VEGFR-1 and kinase insert domain-containing receptor (KDR)/ VEGFR-2 are up-regulated on the surface of endothelial cells (ECs) in gliomas. Both receptor genes contain an Ets-responsible element in their promoters. The proto-oncogene ets-1 encodes a transcription factor that has been associated with blood vessel formation in vivo under physiological and pathophysiological conditions including tumor neovascularization. Ets-1 is induced by VEGF in cultured ECs. In vitro data also point to a role of Ets-1 as a transcriptional activator of Flt-1. These properties prompted us to investigate Ets-1 expression in 32 human astroglial tumors of WHO grades I-IV and to correlate the data with the expression pattern of VEGF, Flt-1, and KDR. By in situ hybridization, high ets-1 mRNA levels were found in the glioma microvasculature with particularly prominent signals in glomeruloid vascular endothelial proliferations of glioblastomas (WHO grade IV). Semiquantitative reverse transcription-PCR identified the full-length ets-1 transcript but none of three known splice variants encoding isoforms with different functional domains. Immunohistochemical staining demonstrated Ets-1 protein preferentially in the nucleus of those ECs with an epithelioid morphology consistent with an activated state, whereas quiescent flat-shaped ECs predominantly displayed cytosolic immunoreactivity. This observation proposes nuclear translocation of Ets-1 during neoangiogenesis. VEGF synthesis by glioma cells was accompanied by Ets-1 expression in adjacent microvascular ECs. Furthermore, a highly significant correlation was observed between Ets-1 and Flt-1 (but not KDR) expression in ECs of the glioma microvasculature. Our data suggest that VEGF secreted by glioma cells induces Ets-1 in adjacent microvascular ECs, which subsequently transactivates the VEGF receptor Flt-1. This cascade may crucially promote neoangiogenesis in human gliomas.