| Abstract: | The ability of platelet-derived growth factor and fibroblast growth factor to stimulate the initiation of DNA synthesis in quiescent BALB/c-3T3 cells was enhanced by cholera toxin. However, the addition of cholera toxin to unsupplemented medium was not mitogenic, nor did cholera toxin increase the mitogenic potential of mediuum supplemented with platelet-poor plasma. The enhancement of serum-induced DNA synthesis by cholera toxin was due to a specific effect on competence formation and not plasma-controlled progression. Cholera toxin increased the rate of competence formation during a transient exposure of quiescent cells to platelet-derived growth factor; this rate was further increased by the addition of isobutylmethylxanthine, a cyclic nucleotide phosphodiesterase inhibitor. Intracellular cyclic AMP concentrations in quiescent BALB/c-3T3 cells were increased 2- to 3-fold after the addition of cholera toxin. The addition of cholera toxin plus 30 m?M isobutylmethylxanthine caused an even greater (7- to 8-fold) increase in the cellular levels of cyclic AMP. That these increases in cyclic AMP concentrations mediated at least part of the increased sensitivity of quiescent cells to competence factors was substantiated by the observation that 0.01 to 1 mM monobutrylcyclic AMP or 8-bromocyclic AMP also caused a concentration-dependent potentiation of competence formation in quiescent cells during a transient exposure to platelet-derived growth factor. |
