gms | German Medical Science

Objective: Neoangiogenesis is a critical feature in malignant solid tumors. Thus, anti-angiogenic therapies targeting the VEGF-A/VEGFR-2 system are promising adjuvant treatment modalities. First clinical and pre-clinical studies demonstrate that therapies using anti-VEGF-A antibodies (e.g. Bevacizumab, Avastin^®) are capable to improve progression free survival in some solid tumors. However, increasing data show that during long-term anti-VEGF-A therapy, resistance phenomena resulting in secondary re-angiogenesis occur after a transient reduction and normalisation of tumor vascularisation. Thus, alternative signalling molecules are discussed to be involved in escaping anti-VEGF therapies.

Methods: Human brain derived endothelial cell lines and CD31 positive endothelial cells freshly isolated from glioblastoma as well as glioma cell lines U87, U373 and U251, were treated with bevacizumab (Avastin^®) for different time periods. Cell growth, expression of vascular endothelial growth factors VEGF-A, VEGF-C, VEGF-D and VEGFR-3 were assessed. Additionally, intracellular and functional response of the cells to these factors were investigated.

Results: Bevacizumab decreased proliferation by 10% after 12 days of treatment in all cell types. No growth reduction was observed during short time treatment (four days). After long-term treatment all cell lines showed a significant up-regulation of VEGF-D protein expression. Even more, cells developed reactivity to VEGF-C and -D by means of increased proliferation while being unreactive to these substances before Bevacizumab treatment. VEGFR3 protein expression was not increased significantly, however, phosphorylation of VEGFR3 by VEGF-A, VEGF-C and -D was enhanced in cells treated with Bevacizumab. Intracellular response to VEGF, VEGF-C and -D changed in a cell type specific manner with a shift from Erk1/2 to p38 and SAP/JNK phosphorylation.

Conclusions: Treatment with anti-VEGF antibodies in glioma leads to a phenotypic change with upregulation of VEGF-D and increased reactivity to VEGF-C and -D accompanied by intracellular changes in signal transduction. This may represent an escape mechanism of the tumor to therapies targeting the VEGF-A/VEGFR-2 system with a secondary activation of the VEGF-C/D-VEGFR3 system.