Ricinus communis Agglutinin I Leads to Rapid Down-Regulation of VEGFR-2 and Endothelial Cell Apoptosis in Tumor Blood Vessels

Abstract

Ricinus communis agglutinin I (RCA I), a galactosebinding lectin from castor beans, binds to endothelial cells at sites of plasma leakage, but little is known about the amount and functional consequences of binding to tumor endothelial cells. We addressed this issue by examining the effects of RCA I on blood vessels of spontaneous pancreatic islet-cell tumors in RIP-Tag2 transgenic mice. After intravenous injection, RCA I bound strongly to tumor vessels but not to normal blood vessels. At 6 minutes, RCA I fluorescence of tumor vessels was largely diffuse, but over the next hour, brightly fluorescent dots appeared as the lectin was internalized by endothelial cells. RCA I injection led to a dose-and time-dependent decrease in vascular endothelial growth factor receptor-2 (VEGFR-2) immunoreactivity in tumor endothelial cells, with 95% loss over 6 hours. By comparison, VEGFR-3 , CD31 , and CD105 had decreases in the range of 21% to 33%. Loss of VEGFR-2 was followed by increased activated caspase-3 in tumor vessels. Prior inhibition of VEGF signaling by AG-028262 decreased RCA I binding and internalization into tumor vessels. These findings indicate RCA I preferentially binds to and is internalized by tumor endothelial cells , which leads to VEGFR-2 downregulation , endothelial cell apoptosis , and tumor vessel regression. Together , the results illustrate the selective impact of RCA I on VEGF signaling in tumor blood vessels.

Reduced VEGF Production, Angiogenesis, and Vascular Regrowth Contribute to the Antitumor Properties of Dual mTORC1/mTORC2 Inhibitors

“…This clustered pattern of VEGF immunofluorescence likely represents internalization of VEGF and VEGFR complexes into endothelial cells, as previously described (36–38). Intracellular VEGF complexes are dependent on Akt activation (37).…”

COG Complex Complexities: Detailed Characterization of a Complete Set of HEK293T Cells Lacking Individual COG Subunits

“…R. communis , or more commonly known as the castor bean plant, produces two kinds of lectins: R. communis toxin (RCA-II) (which is well known for its toxicity even at low doses), and RCA-I which is less toxic, but binds galactose residues with high affinity. RCA-1 has been shown to bind tumor cells (which also have altered secretion) with higher affinity than normal cells (You et al, 2010 ). Interestingly, knocking out COG subunits in either lobe caused a modest increase in RCA-I binding, assessed by fluorescence (Figure 6B ).…”

Blebbishields, the emergency program for cancer stem cells: sphere formation and tumorigenesis after apoptosis

“…VEGFR2 is known to get downregulated during apoptosis. 34 Alternatively, VEGF stimulation can downregulate VEGFR2. 35 230-kDa VEGFR2 downregulation in blebbishields is already in the context of apoptosis and hence we examined the conditioned BE medium and detected the presence of VEGF-A autocrine loop during blebbishield generation (Figure 8c).…”