Xenopus ectodermal cells have previously been shown to respond to acidic and basic FGF by differentiating into mesodermal tissue. In the present study, ectodermal explants from Xenopus blastulae were shown to have high affinity binding sites for 125I-aFGF (Kd = 1.4 X 10(−10) M). The total number of sites, determined by Scatchard analysis, was 3 X 10(8) per explant (surface area of approximately 1 mm2). Two putative receptors of relative molecular mass 130,000 and 140,000 were identified by chemical crosslinking to 125I-aFGF. Both acidic and basic FGF, but not TGF beta 2, could compete for affinity labelling of these bands. The receptor density at the cell surface parallels the developmental competence of Xenopus animal pole cells to respond to FGF. Receptors are present at highest density in the marginal zone but are not restricted to cells in this region.
Many theories of neoplasia suggest that oncogenic transformations result from aberrations in the control mechanisms which normally regulate growth and differentiation during embryonic development. It has recently become clear that many proto-oncogenes are differentially expressed during embryonic development and may thus be important embryonic regulatory molecules. We report here that the products of two transforming oncogenes int-2 and hst/ks (now called kfgf) can, with different potencies, induce mesoderm formation in isolated Xenopus laevis animal pole explants and stimulate DNA synthesis in mammalian fibroblasts. The results suggest that these proteins may function as mesoderm inducers in mammalian embryogenesis and that similar receptor/signalling pathways may be utilized for developmental and oncogenic processes. Finally, we have shown that the Xenopus assay system used in this study provides a powerful screen for protein factors that are active in development.