Basic fibroblast growth factor stimulates DNA synthesis in cells overexpressing the insulin-like growth factor-I receptor.

Insulin-like growth factor-I (IGF-I), by itself, cannot sustain the growth of BALB/c 3T3 cells, but requires the cooperation of other growth factors, such as platelet-derived growth factor or epidermal growth factor. In 3T3 cells constitutively overexpressing the human IGF-I receptor, called p6 cells, IGF-I by itself is fully mitogenic. We show here that p6 cells are also stimulated to enter DNA synthesis by the sole addition of basic fibroblast growth factor (bFGF), which, by itself, is incapable of stimulating parental 3T3 cells. Although bFGF does not bind directly to the IGF-I receptor, it induces its autophosphorylation. Stimulation of p6 cells by bFGF is not inhibited by an antibody to the IGF-I receptor that inhibits IGF-I-mediated DNA synthesis, and IGF-I is not detectable in the medium of bFGF-treated p6 cells. Stimulation cannot be explained by an increased number of FGF receptors, because p6 cells actually have slightly fewer FGF receptors than parental BALB/c 3T3 cells. Basic FGF also stimulates DNA synthesis in 3T3 cells overexpressing a mutant IGF-I receptor that does not autophosphorylate in response to IGF-I and has lost its mitogenic potential. Although we were unable to demonstrate directly that bFGF causes transphosphorylation of the IGF-I receptor, we conclude that in cells overexpressing the IGF-I receptor, bFGF can stimulate DNA synthesis either by an unknown mechanism or through transphosphorylation of the IGF-I receptor.