Expression of angiopoietin-2 in human glioma cells and its role for angiogenesis

In highly vascular malignant glioma, glioma cells themselves may express an giogenie factors and induce angiogenesis. Recent studies have shown that no vel angiogenie factors, angiopoietin-1 (Ang1) and -2 (Ang2), play important roles in the modulation of vasculogenesis and angiogenesis. In this study, we determined Ang2 mRNA expression in cultured human malignant glioma cell s (U105, U251, and U373 MG) by reverse transcriptase-PCR. Western blot anal ysis and immunocytochemical analysis with antihuman Ang2 antibody revealed that Ang2 protein was expressed and secreted by these cells. Furthermore, h ypoxia increased the Ang2 protein level in cultured glioma cells. Serial se ctions of 32 human glioma tissues (14 glioblastomas, eight anaplastic astro cytomas, seven astrocytomas, and three pilocytic astrocytomas) were immunos tained against Ang2, vascular endothelial growth factor, Tie2, von Willebra nd factor, and alpha smooth muscle actin. The immunoreactivity of each angi ogenic factor was higher in malignant gliomas than in low-grade gliomas. An g2 protein was detected not only in endothelial cells but also in glioma ce lls, and its expression was prominent in both the area surrounding the necr osis and the periphery of glioblastomas. In the area surrounding necrosis, Ang2 was highly expressed and tumor vessels showed regression. In the tumor periphery, Ang2 was highly expressed and many small vessels stained positi vely for von Willebrand factor but not for alpha smooth muscle actin, sugge sting angiogenesis. Statistical analysis revealed that the Ang2 expression was negatively correlated with vessel maturation in malignant gliomas and t hat vascular endothelial growth factor expression was positively correlated with vessel maturation in low-grade gliomas (P < 0.05). These results sugg est that glioma cells themselves express Ang2 and that expression may be in duced by hypoxic stimulation and may play a crucial role in the vessel matu ration, angiogenesis, and vessel regression in malignant glioma.