Two independent mechanisms essential for tumor angiogenesis: Inhibition ofhuman melanoma xenograft growth by interfering with either the vascular endothelial growth factor receptor pathway or the Tie-2 pathway

Protein ligands and receptor tyrosine kinases that specifically regulate en dothelial cell function are mainly involved in physiological as well as in disease-related angiogenesis. These ligand/receptor systems include the vas cular endothelial growth factor (VEGF) and the angiopoietin (Ang) families, and their receptors, the VEGF receptor family and the tyrosine kinase with immunoglobulin-like and epidermal growth factor homology domains (Tie) fam ily. In the present study, the contribution of these endothelium-specific l igand/receptor systems to tumor angiogenesis was evaluated. A375v human mel anoma cells, which express at least the angiogenic growth factors VEGF, VEG F-C, and Ang-1, were stably transfected to overexpress the extracellular li gand-binding domains of the endothelium-specific receptor tyrosine kinases fats-like tyrosine kinase-1 (Flt-1), Flt-4, Tie-1, and Tie-2, respectively. In vitro proliferation and colony formation assays confirmed that expressi on of the extracellular receptor domains inhibited neither tumor cell mitog enesis nor the ability to produce anchorage-independent growth. Nude mouse xenografts revealed that interference with either the VEGF receptor pathway or the Tie-2 pathway resulted in a significant inhibition of tumor growth and tumor angiogenesis. In contrast, interference with the Flt-4 pathway or the Tie-1 pathway was without significant effect. Our results show that bo th the VEGF receptor pathway and the Tie-2 pathway are essential for A375v melanoma xenograft growth. The inhibition of the VEGF receptor pathway cann ot be compensated by the Tie-2 pathway, nor vice versa, These findings sugg est that the VEGF receptor pathway and the Tie-2 pathway have to be conside red as two independent mediators essential for the process of in vivo angio genesis.