Cytotoxicity of VEGF121 /rGel on vascular endothelial cells resulting in inhibition of angiogenesis is mediated via VEGFR-2

Abstract

Abstract

Background: The fusion protein VEGF₁₂₁ /rGel composed of the growth factor VEGF₁₂₁ and the plant toxin gelonin targets the tumor neovasculature and exerts impressive anti-vascular effects. We have previously shown that VEGF₁₂₁ /rGel is cytotoxic to endothelial cells overexpressing VEGFR-2 but not to endothelial cells overexpressing VEGFR-1. In this study, we examined the basis for the specific toxicity of this construct and assessed its intracellular effects in vitro and in vivo .

Methods: We investigated the binding, cytotoxicity and internalization profile of VEGF₁₂₁ /rGel on endothelial cells expressing VEGFR-1 or VEGFR-2, identified its effects on angiogenesis models in vitro and ex vivo , and explored its intracellular effects on a number of molecular pathways using microarray analysis.

Results: Incubation of PAE/VEGFR-2 and PAE/VEGFR-1 cells with ¹²⁵ I-VEGF₁₂₁ /rGel demonstrated binding specificity that was competed with unlabeled VEGF₁₂₁ /rGel but not with unlabeled gelonin. Assessment of the effect of VEGF₁₂₁ /rGel on blocking tube formation in vitro revealed a 100-fold difference in IC₅₀ levels between PAE/VEGFR-2 (1 nM) and PAE/VEGFR-1 (100 nM) cells. VEGF₁₂₁ /rGel entered PAE/VEGFR-2 cells within one hour of treatment but was not detected in PAE/VEGFR-1 cells up to 24 hours after treatment. In vascularization studies using chicken chorioallantoic membranes, 1 nM VEGF₁₂₁ /rGel completely inhibited bFGF-stimulated neovascular growth. The cytotoxic effects of VEGF₁₂₁ /rGel were not apoptotic since treated cells were TUNEL-negative with no evidence of PARP cleavage or alteration in the protein levels of select apoptotic markers. Microarray analysis of VEGF₁₂₁ /rGel-treated HUVECs revealed the upregulation of a unique "fingerprint" profile of 22 genes that control cell adhesion, apoptosis, transcription regulation, chemotaxis, and inflammatory response.

Conclusions: Taken together, these data confirm the selectivity of VEGF₁₂₁ /rGel for VEGFR-2-overexpressing endothelial cells and represent the first analysis of genes governing intoxication of mammalian endothelial cells by a gelonin-based targeted therapeutic agent.