Endothelial monocyte activating polypeptide II (EMAP II) is a proinflammatory cytokine with antiangiogenic properties. EMAP II functions as a potent inhibitor of primary and metastatic tumor growth, has strong inhibitory effects on endothelial cells (ECs), and can reduce intratumoral expression of the angiogenesis inducer vascular endothelial growth factor (VEGF). VEGF influences EC functions such as proliferation, migration, survival and tube formation. Therapeutic strategies that target VEGF have been demonstrated to reduce the tumor growth. We investigated the effects of EMAP II on VEGF-induced angiogenesis signaling. Primary human fetal lung ECs (HFLECs) and human umbilical vein ECs (HUVECs) were grown in E-Stim medium. Protein binding was analyzed using enzyme-linked immunosorbent assay (ELISA). Protein expression was determined by western blot analysis. EC proliferation and migration was determined using WST-1 reagent and transwell membrane, respectively. EMAP II efficiently and dose dependently binds to VEGF receptor 1 (VEGFR1) and VEGF receptor 2 (VEGFR2) as observed by ELISA. Bmax values for VEGFR1 and VEGFR2 were 0.45 and 0.17, respectively. In addition, EMAP II inhibited binding of VEGF to VEGFR1 and VEGFR2. EMAP II significantly reduced VEGF-induced expression of phosphorylated VEGFR1 (in HFLEC and HUVEC) by >50%, and of phosphorylated VEGFR2 (in HUVEC) by 66%. EMAP II also inhibited downstream VEGF signaling. Although VEGF-induced phosphorylation of Akt, Erk1/2, p38 and Raf 2.8-, 1.5-, 2.2- and 3.6-fold, respectively, EMAP II preincubation blocked this induction in phosphorylation to control levels. VEGF-induced EC proliferation 2.5-fold, and EMAP II pretreatment abrogated this effect. Similarly, VEGF-induced EC migration (2.5-fold) was significantly inhibited by EMAP II. These finding suggest that inhibition of VEGF signaling is one possible antiangiogenic mechanism of EMAP II, which may explain its in vivo antitumor activity and delineate therapeutic strategies to enhance anti-VEGF therapy to inhibit tumor growth.
