Activin A stimulates mitogenesis in Swiss 3T3 fibroblasts without activation of mitogen-activated protein kinases

Activin A stimulated DNA synthesis and transient c-fos expression in quiescent Swiss 3T3 fibroblasts. The activin A-induced DNA synthesis was dose-dependent with a half-maximal effect obtained at 0.3 nM. The maximal response obtained at 10 nM was comparable with that induced by 5 ng/ml basic fibroblast growth factor, Swiss 3T3 fibroblasts expressed abundant high affinity binding sites for ¹²⁵I-labeled activin A with a K(d) value of 0.63 nM and the number of binding sites at 24,000/cell. Northern blot analysis revealed that Swiss 3T3 fibroblasts express a high level of type II activin receptor mRNA. In an attempt to elucidate the mechanism of mitogenic action of activin A₂ we examined the effect of activin A on mitogen- activated protein kinase activation. Unexpectedly, however, activin A did not induce kinase activation under conditions in which basic fibroblast growth factor and endothelin-I at similar or even less potent mitogenic concentrations did. Furthermore, activin A did not induce phosphorylation of the Erk2 species of mitogen-activated protein kinase. These observations strongly suggest that the activation of mitogen-activated protein kinase is not a necessary step for activin A-induced DNA synthesis in Swiss 3T3 fibroblasts.