Combined inhibition of vascular endothelial growth factor receptor signaling with temozolomide enhances cytotoxicity against human glioblastoma cells via downregulation of Neuropilin-1

Glioblastoma multiforme (GBM) is the most common and aggressive type of primary brain tumor with grave prognosis. Despite the growing understanding of the complex signaling networks responsible for the initiation and progression of GBM, many experimental therapies have fallen short of their treatment goals. In the present study, we investigated the novel molecular mechanisms responsible for synergistic action of temozolomide (TMZ) and anti-VEGF therapy in GBM cells. We tested the combined effects of TMZ and VEGF blockade in four human GBM cell lines: TMZ-sensitive U251-MG and U373-MG cells, and TMZ-resistant CRT-MG and LN215-MG cells, which correlated with MGMT promoter methylation status. Treatment of TMZ along with a sublethal dosage range of SU1498, a chemical inhibitor of the VEGF receptor signaling, induced significant cell death in both TMZ-sensitive and TMZ-resistant GBM cells without changing the status of the MGMT promoter methylation. Treatment with TMZ specifically reduced the expression of NRP-1, a coreceptor of VEGF but not those of VEGF-R1 and VEGF-R2. We further confirmed the key role of NRP-1 by showing that the reduction of NRP-1 by siRNA also increased the SU1498-induced cytotoxicity of LN215-MG. These results collectively indicate that combined treatment of TMZ can sensitize GBM cells to blockade of autocrine VEGF signaling through specific down-regulation of NRP-1, which provide a rationale for further evaluation and a potential clinical trial of combinatorial therapy of TMZ and SU1498 or other VEGF inhibitors for intractable brain tumors.