Cooperation between integrin α₅ and tetraspan TM4SF5 regulates VEGF-mediated angiogenic activity

Tetraspan TM4SF5 is highly expressed in a diverse number of tumor types. Here we explore the mechanistic roles of TM4SF5 in angiogenesis. We found that TM4SF5 overexpression correlates with vascular endothelial growth factor (VEGF) expression in SNU449 hepatocytes and with vessel formation in clinical hepatocarci-noma samples. Conditioned media from TM4SF5-expressing cells enhanced viability and tube formation of primary human umbilical vein endothelial cells, and out-growth of endothelial cells from aorta ring segments, which was abolished by treatment with an anti-VEGF antibody. TM4SF5 retained integrin α₅ on the cell surface for VEGF induction, and preincubation with anti-integrin α₅ antibody abolished TM4SF5-mediated VEGF expression and secretion. TM4SF5-mediated effects required integrin α₅, c-Src, and signal transducer and activator of transcription 3 (STAT3). In addition, tumors from nude mice injected with TM4SF5-expressing cells and from clinical human hepatocarcinoma tissues showed enhanced integrin α₅ expression, vessel formation, and signaling activity, which were inhibited by administration of anti-integrin α₅ or -VEGF antibody. This study suggests that TM4SF5 facilitates angio-genesis of neighboring endothelial cells through VEGF induction, mediated by cooperation between TM4SF5 and integrin α₅ of epithelial cells.