KDR activation in astrocytic neoplasms.

The development of new capillary networks appears to be necessary for the growth of solid tumors. Tumor angiogenesis is believed to be mediated by soluble factors released from tumor cells that then act on endothelial cells in a paracrine manner. Vascular endothelial growth factor (VEGF) is a prime regulator of normal and tumor angiogenesis as well as vasculogenesis. VEGF is expressed in glioma cells and its receptors (Flt-1 and KDR) are expressed in the same gliomas. The two receptors are tyrosine kinases and have an extracellular domain containing seven immunoglobulin-like loops and a split tyrosine-kinase domain. KDR is a receptor for the various VEGF isoforms and for VEGF-C; Flt-1 is a receptor for the various isoforms. Studies suggest that the VEGF receptors are induced in endothelial cells during tumor angiogenesis. Stimulation of aortic endothelial cells results in receptor tyrosine phosphorylation (receptor activation). In this study the activation state of the KDR receptors was determined in low grade, anaplastic, and high grade gliomas. A synthetic tyrosine phosphopeptide was used to raise an antibody that recognizes the phosphorylation state of tyrosine 1054/1059 in the KDR receptor. Western blot analysis was performed on 37 astrocytic neoplasms (7 low grade astrocytomas, 13 anaplastic astrocytomas, and 17 cases of glioblastoma multiforme). Immunoblotting with this antibody found that tyrosines 1054/1059 were phosphorylated constitutively within multiple fresh surgical specimens of glioblastomas (71%) and anaplastic gliomas (15%), but not in low grade gliomas. The findings of the current study strongly support the hypothesis that the onset of angiogenesis is an important event during the disease progression of gliomas.