The infection of normal mouse mammary EF43 cells by a retroviral vector carrying either Fgf-3 (EF43.Fgf-3) or Fgf-4 (EF43.Fgf-4) cDNA resulted in the transformation of cells displaying different tumorigenic potentials in nude mice (A. Hajitou and C-M. Calberg-Bacq, Int. J. Cancer, 63: 702-709, 1995). EF43.Fgf-4 produced rapidly developing tumors at all sites of inoculation, whereas EF43.Fgf-3 produced slowly growing tumors only in the mammary fat pad. Cells infected with the vector carrying the selection gene alone (EF43.C) were not tumorigenic. The angiogenic properties of these cells were tested in an in vitro angiogenesis model using human umbilical vein endothelial cells (HUVECs) cultured at the surface of a type I collagen gel and their capacity to form tube-like structures on invasion of the gel. Only the conditioned medium (CM) of EF43.Fgf-4 induced an angiogenic morphotype in HUVECs. In parallel, the mRNA expression of matrix metalloproteinase 1 and c-ETS-1 was increased in the HUVECs displaying a differentiated phenotype, whereas the tissue inhibitor of matrix metalloproteinase 1 mRNA level was decreased. Recombinant human fibroblast growth factor 4 (FGF-4) did not induce an angiogenic phenotype in HUVECs by itself. By Western blot analysis, a high expression of vascular endothelial growth factor (VEGF) was detected in the EF43.Fgf-4 CM. This result was confirmed by Northern blot analysis of total RNA extracted from the three cell types; the steady-state level of VEGF mRNA was low and equivalent in EF43.C and EF43.Fgf-3, whereas it was strongly increased in EF43.Fgf-4. Culturing EF43 cells carrying only the selection gene with increasing concentrations of recombinant human FGF-4 resulted in a dose-dependent stimulation of VEGF. The induction of the angiogenic morphotype and the parallel modulations of the biosynthetic phenotype in HUVECs were completely suppressed by adding a neutralizing antibody directed against VEGF to EF43.Fgf-4 CM. Furthermore, inhibition of protein kinase C by bisindoylmaleimide suppressed the angiogenic phenotype induced by the CM of EF43.Fgf-4. Our results point to an indirect angiogenic activity of FGF-4 through the autocrine induction of VEGF secretion by EF43.Fgf-4 cells, an original signaling pathway that might be significant in tumor progression and metastasis.