Abstract 4130: Dual targeting of VEGF and endoglin inhibits tumor angiogenesis and metastatic spread
Authors: Paauwe Madelon , Heijkants Renier C. , Oudt Lotte H. , van Pelt Gabi W. , Sier Cornelis F.M. , Theuer Charles P. , Hawinkels Lukas J.A.C. 
Published: August 2015

Abstract
Endoglin, a co-receptor for TGF-β and BMP-9, is highly expressed on angiogenic endothelial cells and has been shown to be a predictive marker for patient survival in various solid tumors. TRC105 is a neutralizing antibody against endoglin which is currently being explored in clinical trials as anti-angiogenic therapy. Previously we have observed upregulation of the endoglin signaling pathway in endothelial cells upon anti-VEGF therapy in rectal cancer patients. In this project we have further explored the redundancy between VEGF and endoglin signaling in endothelial cells and angiogenesis. In addition, we assessed the effect of simultaneously targeting both pathways on breast cancer progression and metastasis.
In vitro, inhibition of endoglin signaling by TRC105 caused upregulation of VEGF signaling in human umbilical vein endothelial cells (HUVECs). Using in vitro angiogenesis assays we showed that combined targeting with TRC105 and the VEGF receptor kinase inhibitor SU5416 inhibited angiogenic capacity even further than targeting both pathways separately. To determine the effect of combined targeting in vivo, zebrafish embryos were treated with either TRC105, SU5416 or a combination. Both single treatments resulted in abnormal subintestinal vessel development, while treatment with TRC105 and SU5416 simultaneously increased this effect. Next, we determined whether combined targeting of the endoglin and VEGF pathway could affect tumor growth and metastasis. In a syngeneic model for invasive lobular carcinomas, KEP1-11 breast cancer cells were orthotopically transplanted in mice and tumor growth and metastasis formation were followed over time. Although TRC105 and SU5416 decreased tumor vascular density, tumor volume was not affected. Strikingly, we observed a strong inhibition of metastases in mice treated with TRC105 or a combination of TRC105 and SU5416. In a follow-up experiment, we observed that after resection of the primary tumor, which was accompanied by increased metastatic spread, TRC105 alone already showed a marked decrease in the number of metastases. To further establish the role of endoglin we assessed the effect of endoglin-Fc expression by the tumor cells on tumor growth and metastasis. The extracellular part of endoglin, coupled to the Fc tail of human IgG functions as a ligand trap for endoglin ligands in the circulation. Similar to treatment with TRC105, stable expression of endoglin-Fc significantly decreased vessel density, but did not result in decreased primary tumor formation. Resection of endoglin-Fc expressing tumors lead to drastically less tumor relapse compared to the Fc control group and a complete abrogation of distant metastases was observed.
These results imply that targeting endoglin signaling, either with a neutralizing antibody or ligand trap, reduces tumor vessel density or, importantly, inhibits the metastatic spread of breast cancer cells in vivo.
Citation Format: Madelon Paauwe, Renier C. Heijkants, Lotte H. Oudt, Gabi W. van Pelt, Cornelis F.M. Sier, Charles P. Theuer, Lukas J.A.C. Hawinkels. Dual targeting of VEGF and endoglin inhibits tumor angiogenesis and metastatic spread. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4130. doi:10.1158/1538-7445.AM2015-4130