Blockade of Vascular Endothelial Growth Factor Receptor Signal Pathway and Antitumor Activity of ON-III (2,4- Dihydroxy-6-methoxy-3,5-dimethylchalcone), a Component from Chinese Herbal Medicine

Antiangiogenesis is a promising strategy of cancer treatment. Vascular endothelial growth factor receptor [fetal liver kinase/ kinase-inserting domain-containing receptor (KDR)] is a ty-rosine kinase receptor and has been strongly implicated in tumor angiogenesis. In this study, we report that 2,4-dihy-droxy-6-methoxy-3,5-dimethylchalcone (ON-III), extracted from the dried flower Cleistocalyx operculatus, used in tradi-tional Chinese medicine, reversibly inhibited KDR tyrosine ki-nase phosphorylation, but epidermal growth factor receptor tyrosine kinase phosphorylation was unaffected under the same concentrations of ON-III. ON-III also inhibited mitogen-activated protein kinase (MAPK) and AKT activation of KDR signal transduction in downstream molecules without reduced total MAPK and AKT. The results in vitro showed that ON-III inhibited growth of human vascular endothelial HDMEC cells in the presence of VEGF preferentially, compared with epidermal growth factor. Systemic administration of ON-III at nontoxic doses in nude mice resulted in inhibition of subcutaneous tu-mor growth of human hepatocarcinoma Bel7402 and lung can-cer GLC-82 xenografts. The tumor vessel density decreased, as determined by immunohistochemical staining, for CD31 af-ter ON-III treatment. These results indicated that ON-III inhib-ited KDR tyrosine kinase, shut down KDR-mediated signal transduction, and inhibited tumor growth of human xenografts in vivo. Angiogenesis, the formation of new blood vessels by sprout-ing from pre-existing endothelium, is a significant compo-nent of a wide variety of biological processes, including em-bryonic vascular development and differentiation, wound healing, organ regeneration, and pathological processes in