Tube formation of endothelial cells is an important step of angiogenesis. However, little is known about the molecular mechanisms underlying growth factor-mediated tube formation by endothelial cells. FGF-2 stimulates tube formation by a murine brain capillary endothelial cell line, IBE cells, when cultured on collagen gels (differentiation-associated culture condition), whereas cells proliferate and migrate without forming tube on fibronectin-coated surface (proliferation/migration-associated condition). To elucidate FGF-2-mediated signal transduction pathways leading to tube formation by endothelial cells, we focused on the contribution of Src family kinases. Src family kinase inhibitor PP2 attenuated FGF-2-induced tube formation. Stable expression of kinase-inactive c-Src in IBE cells demonstrated no dominant negative effect on FGF-2-induced tube formation. In vitro kinase assay revealed that c-Fyn was activated by FGF-2 only in cells cultured on collagen gels. Three independent cell lines, expressing kinase-inactive c-Fyn, all exhibited attenuation of FGF-2-mediated tube formation. However, FGF-2-mediated proliferation or migration was not clearly perturbed in these cells. These results show the first time that c-Fyn plays a pivotal role in tube formation by endothelial cells. (C) 2002 Elsevier Science (USA).