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 f actor endowed with a potent angiogenic activity in vitro and in vivo. To in vestigate the impact of the modulation of FGF2 expression on the neovascula rization at different stages of tumor growth, we generated stable transfect ants (Tet-FGF2) from the human endometrial adenocarcinoma HEC-1-B cell line in which FGF2 expression is under the control of the tetracycline-responsi ve 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 conditio ned medium of Tet-FGF2 transfectants caused proliferation, urokinase-type p lasminogen activator up-regulation, migration, and sprouting of cultured en dothelial cells. A 3-day treatment of Tet-FGF2 cell cultures with tetracycl ine abolished FGF2 overexpression and the biological activity of the condit ioned medium without affecting their proliferative capacity. Tet-FGF2 cells formed tumors when nude mice received s.c. injections. The a dministration of 2.0 mg/ml tetracycline in the drinking water before cell t ransplantation, continued throughout the whole experiment, inhibited FGF2 e xpression in Tet-FGF2 tumor lesions. This was paralleled by a significant d ecrease in the rate of tumor growth and vascularization to values similar t o those observed in lesions generated by parental HEC-1-B cells. Tetracycli ne administration 20 days after tumor cell, implant, although equally effec tive in reducing FGF2 expression and inhibiting tumor vascularity, only min imally 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 i n nude mice. On the contrary, the growth of established tumors appears to b e independent of the inhibition of FGF2 expression and decreased vascular d ensity. The possibility that a significant reduction of angiogenesis may no t affect the progression of large tumors points to the use of antiangiogeni c therapy in early tumor stage.