FIBROBLAST GROWTH-FACTOR, BUT NOT ACTIVIN, IS A POTENT ACTIVATOR OF MITOGEN-ACTIVATED PROTEIN-KINASE IN XENOPUS EXPLANTS

Isolated explants from the animal hemisphere of Xenopus embryos were i ncubated with Xenopus basic fibroblast growth factor (XbFGF) or human activin A. XbFGF incubation resulted in the rapid activation of mitoge n-activated protein kinase (MAPK) and ribosomal S6 protein kinase (pp9 0(rsk)) in a dose-dependent manner with the highest levels of activati on occurring at 50 ng/ml. Maximal activation occurred within 6-10 min after the addition of growth factor, and the activity of both kinases declined to unstimulated levels after 30 min. Activin was unable to ac tivate either MAPK or pp90(rsk) in the Xenopus explants to a substanti al level, although it induced dorsal mesoderm better than XbFGF under the same experimental conditions. The regulatory protein Xwnt-8 did no t activate MAPK, nor did it enhance the activation of MAPK by XbFGF. X bFGF was able to activate MAPK through at least the midgastrula stage, suggesting that this family of growth factors may have a role in gast rula-stage events.