Differential inhibition of tumor angiogenesis by TIE2 and vascular endothelial growth factor receptor-2 dominant-negative receptor mutants

Tumor growth is angiogenesis-dependent. Current evidence suggests that vasc ular endothelial growth factor (VEGF), a major regulator of embryonic and h ypoxia-mediated angiogenesis, is necessary for tumor angiogenesis. VEGF is expressed in tumor cells in vivo, and its tyrosine kinase receptors VEGFR-1 and VEGFR-2 are up-regulated in the tumor endothelium. A second endothelia l cell-specific ligand/receptor tyrosine kinase system, consisting of the t ie2 receptor, its activating ligand angiopoietin-1 and the inhibitory ligan d angiopoietin-2, has been characterized. We have examined 6 human primary breast-cancer samples and 4 murine breast-cancer cell lines (M6363, M6378, M6444, M6468), transplanted into nude mice, by in situ hybridization and/or Northern analysis. Expression of angiopoietin-1, angiopoietin-2 and tie2 w as compared to VEGF and VEGFR-2 expression. Human tumors expressed VEGFR-2 and tie2 but varied considerably in VEGF and angiopoietin-1/-2 expression. In the murine tumor models, we observed high heterogeneity of receptor and ligand expression. M6363 and M6378 tumors were analyzed in detail because t hey showed different expression of components of the tie2/angiopoietin sign aling system. M6363 tumors expressed VEGF, VEGFR-2 and angiopoietin-1 but n ot tie2 or angiopoietin-1, suggesting activation of VEGFR-2 and inhibition of tie2 signaling pathways, whereas M6378 tumors expressed VEGF, VEGFR-2, t ie2 and angiopoietin-1 but little angiopoietin-2, suggesting activation of both VEGFR-2 and tie2 signaling pathways. In viva studies using truncated d ominant-negative tie2 and VEGFR-2 mutants revealed inhibition of M6363 tumo r growth by 15% (truncated tie2) and 36% (truncated VEGFR-2), respectively. In contrast, M6378 tumor growth was inhibited by 57% (truncated tie2) and 47% (truncated VEGFR-2), respectively. These findings support the hypothesi s that tumor angiogenesis is dependent on VEGFR-2 but suggest that, in addi tion, tie2-dependent pathways of tumor angiogenesis may exist. For adequate application of angiogenesis inhibitors in tumor patients, analysis of prev ailing angiogenesis pathways may be a prerequisite. (C) 2001 Wiley-Liss, In c.