Negative growth regulation of SK-N-MC cells by bFGF defines a growth factor-sensitive point in G(2)

Basic fibroblast growth factor (bFGF) has been shown to induce growth inhib ition of the neuroepithelioma cell line SK-N-MC, Here we show that this gro wth inhibition occurs in G(2). We show that bFGF is active on these cells d uring S and early G(2) phase. Therefore, this constitutes a rather unusual mechanism of growth inhibition, because it is generally believed that cells become refractory to extracellular signals after passage through the restr iction point. We show that bFGF treatment inhibits Tyr-15 dephosphorylation of cdc2 and prevents activation of Cdc25C, similar to what is seen upon ac tivation of the G(2) DNA damage checkpoint. Interestingly, both DNA damage- and bFGF-induced effects on cdc2 phosphorylation are reverted by caffeine. To confirm the involvement of similar pathways induced by bFGF and DNA dam age, we generated tetracycline-regulatable SK-N-MC clones expressing Cdc25C -S216A. Expression of this Cdc25C mutant can revert the bFGF-induced effect s on cdc2 phosphorylation and can rescue cells from the block in G(2) impos ed by bFGF. Taken together, these data define a growth factor-sensitive poi nt in G(2) that most likely involves regulation of Cdc25C phosphorylation.