Abstract 687: The mechanisms of anti-VEGF therapy resistance in renal cell carcinoma
Authors: Zhou Lijun , Liu Xiande , Sun Mianen , Tannir Nizar M. , Laird Douglas , Jonasch Eric 
Published: May 2015

Abstract
Anti-VEGF therapy is a mainstay of treatment for metastatic renal cell carcinoma (RCC). Treatment is either via targeting of the ligand itself, or via inhibition of the receptor with small molecule VEGF receptor inhibitors like sunitinib, pazopanib or axitinib. Unfortunately, between 10 and 20 percent of patients have tumors that are refractory to anti-VEGF therapy, and the majority of remaining patients eventually demonstrate tumor growth after a median of 10-12 months of therapy. Thus, an important goal is to develop strategies to overcome anti-VEGF therapy resistance. We hypothesized that the receptor tyrosine kinases AXL and MET, which are implicated in oncogenesis and the development of resistance to targeted therapies in some cancers, are involved in anti-VEGF therapy resistance in RCC, and this resistance can be overcome with an inhibitor of AXL and MET.
In the current study, we investigated both tumor autonomous and endothelial-specific drivers of anti-VEGF/sunitinib therapy resistance, bearing in mind that the primary presumed target of sunitinib in RCC is the endothelial cell. We observed that sunitinib treatment stimulated both AXL expression and kinase activation in the 786-0 RCC cell line. Chronic sunitinib treatment (four weeks, 5µM) also elevated MET activity, and stimulated the activation of key effector kinases, AKT and ERK. In addition, the ability of sunitinib to inhibit cell proliferation was lost in786-O cells chronically exposed to sunitinib. Cell migration in vitro was inhibited when AXL and MET expression was suppressed by shRNA knockdown. We also observed that cabozantinib, a novel MET/VEGFR/AXL inhibitor, effectively suppressed the activation of MET, AXL, AKT and ERK both in parental and sunitinib-resistant 786-O cells, and inhibited cell migration in vitro.
Angiogenesis is critical for RCC tumor development and metastasis. Therefore we explored the interaction between endothelial cells and RCC cells. Our preliminary data showed that conditioned medium (CM) from HUVEC cells activated oncogenic signaling pathways in 786-O cells, and this activity was further increased with CM from sunitinib-pretreated HUVEC cells. These data suggest the development of sunitinib resistance could be promoted by a microenviromental feedback loop.
In summary, our investigations into the molecular mechanisms of sunitinib resistance demonstrated that AXL and MET were drivers of sunitinib resistance in our cell model system. We found cabozantinib inhibited both AXL and MET signaling, and suppressed cell migration. Endothelial cells also contributed to the upregulation of oncogenic signaling pathways within the tumor epithelial cell through a positive feedback loop. Ongoing experiments will further characterize the detailed mechanisms of sunitinib resistance and explore the potential utility of using MET/AXL inhibitors to treat RCC that is resistant to therapies targeting the VEGF signaling pathway.
Citation Format: Lijun Zhou, Xiande Liu, Mianen Sun, Nizar M. Tannir, Douglas Laird, Eric Jonasch. The mechanisms of anti-VEGF therapy resistance in renal cell carcinoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 687. doi:10.1158/1538-7445.AM2014-687