Modulation of tumor angiogenesis by conditional expression of fibroblast growth factor-2 affects early but not established tumors

Fibroblast growth factor-2 (FGF2) is a pleiotropic heparin-binding growth factor endowed with a potent angiogenic activity in vitro and in vivo. To investigate the impact of the modulation of FGF2 expression on the neovascularization at different stages of tumor growth, we generated stable transfectants (Tet-FGF2) from the human endometrial adenocarcinoma HEC-1-B cell line in which FGF2 expression is under the control of the tetracycline-responsive promoter (Tet-off system). After transfection, independent clones were obtained in which FGF2 mRNA and protein were up-regulated compared with parental cells. Also, the conditioned medium of Tet-FGF2 transfectants caused proliferation, urokinase-type plasminogen activator up-regulation, migration, and sprouting of cultured endothelial cells. A 3-day treatment of Tet-FGF2 cell cultures with tetracycline abolished FGF2 overexpression and the biological activity of the conditioned medium without affecting their proliferative capacity. Tet-FGF2 cells formed tumors when nude mice received s.c. injections. The administration of 2.0 mg/ml tetracycline in the drinking water before cell transplantation, continued throughout the whole experiment, inhibited FGF2 expression in Tet-FGF2 tumor lesions. This was paralleled by a significant decrease in the rate of tumor growth and vascularization to values similar to those observed in lesions generated by parental HEC-1-B cells. Tetracycline administration 20 days after tumor cell implant, although equally effective in reducing FGF2 expression and inhibiting tumor vascularity, only minimally impaired the growth of established Tet-FGF2 tumors. The results indicate that FGF2 expression deeply affects the initial tumor growth and neovascularization of HEC-1-B human endometrial adenocarcinoma in nude mice. On the contrary, the growth of established tumors appears to be independent of the inhibition of FGF2 expression and decreased vascular density. The possibility that a significant reduction of angiogenesis may not affect the progression of large tumors points to the use of antiangiogenic therapy in early tumor stage.