Abstract-Interactions between integrins and growth factor receptors play a critical role in the development and healing of the vasculature. This study mapped two binding domains on fibronectin (FN) that modulate the activity of the angiogenic factor, vascular endothelial growth factor (VEGF T he growth, repair, and regeneration of blood vessels are complex processes that involve coordinated regulation of endothelial cell proliferation, migration, and differentiation. 1 One of the most important vascular morphogens is vascular endothelial growth factor (VEGF). VEGF has been shown to play a major role in vasculogenesis and angiogenesis by gene deletion studies. 2,3 Targeted disruption of the VEGF receptor Flk-1 (VEGFR-2) in mice resulted in failure of blood-island formation and endothelial differentiation. 4 Flk-1 is also the first endothelial receptor tyrosine kinase to be expressed in the hemangioblast. 5 We and others recently demonstrated that the hematopoietic progenitor cell CD34 ϩ can differentiate into endothelial cells, and that VEGF was one of the critical factors promoting this differentiation. 6,7 Interactions between cells and their extracellular matrix (ECM) play an integral role in blood vessel development. The earliest ECM protein expressed in the embryo during vasculogenesis is fibronectin (FN). 8 Gene deletion studies have demonstrated that both FN and its major integrin receptor, ␣ 5 ␤ 1 , are critical for vasculogenesis and angiogenesis in the developing embryo. 9 -11 Collectively, these observations suggest important roles for FN and its integrin receptor, ␣ 5 ␤ 1 , in vasculogenesis and angiogenesis.In this study, we show that novel VEGF binding domains of FN are required for promoting the specific association of the FN receptor integrin ␣ 5 ␤ 1 with the VEGF receptor, Flk-1. This association between VEGF and FN is required for the full effects of VEGF-induced endothelial cell migration and proliferation. This study demonstrates that FN can profoundly affect VEGF biological activity and consequently the behavior of endothelial cells through their coordinated effects on Flk-1 and ␣ 5 ␤ 1 . Materials and Methods Solid-Phase VEGF Binding AssayECM proteins and FN peptides were purchased from Sigma and Gibco and were purified further by gel filtration and ion exchange chromatography. Microtiter plates were coated with the appropriate ECM proteins (50 L; 10 g/mL) in 100 mmol/L bicarbonate buffer (pH 9) overnight at 4°C.

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