UP-REGULATION OF VASCULAR ENDOTHELIAL GROWTH-FACTOR AND ITS COGNATE RECEPTORS IN A RAT GLIOMA MODEL OF TUMOR ANGIOGENESIS

We have recently shown that vascular endothelial growth factor (VEGF) is produced by human malignant glioma cells and acts on tumor endothel ial cells, which express VEGF receptors, suggesting that VEGF is a reg ulator of tumor angiogenesis. To investigate the feasibility of antian giogenic brain tumor therapy, we developed an intracerebral (i.c.) rat glioma model. We used two transplantable rat glioma cells lines, C6 a nd GS-9L, to analyze VEGF regulation in vitro and expression of VEGF a nd its high affinity tyrosine kinase receptors, flt-1 and flk-1, in vi vo. Glioma cells were transplanted i.c. or s.c. into syngeneic rats. C 6 gliomas exhibit morphological characteristics of human glioblastoma multiforme such as necroses with palisading cells. Immunocytochemistry with von Willebrand factor showed that C6 gliomas are highly vascular ized and therefore show another prominent feature of human glioblastom a. GS-9L gliosarcomas were less vascularized. In situ hybridization sh owed that VEGF is expressed in vivo in rat glioma cells which reside a long necrotic areas and therefore closely mimicks the expression patte rn of VEGF observed in human glioblastoma. flt-1 and flk-1 are specifi cally expressed in endothelial cells in the tumor and at the border be tween tumor and normal brain but are absent from endothelial cells in the normal brain proper. The action of VEGF may therefore be restricte d to tumor endothelium. Up-regulation of VEGF, but not acid fibroblast growth factor, basic fibroblast growth factor, and platelet-derived g rowth factor B messenger RNA was observed in hypoxic C6 and GS-9L cell s in vitro. These observations are consistent with a role for VEGF in tumor- and hypoxia-induced angiogenesis. Since the expression pattern of VEGF and its receptors in rat glioma appears to be indistinguishabl e from human glioblastoma multiforme, this model provides an excellent tool to study anti-angiogenic therapy.