Mesoderm and endoderm differentiation in animal cap explants: identification of the HNF4-binding site as an activin A responsive element in the Xenopus HNF1alpha promoter

The gene encoding the tissue-specific transcription factor HNF1alpha (LFB1) is transcriptionally activated shortly after mid-blastula transition in Xenopus embryos. We have now shown that the HNF1alpha protein is localized in the nuclei of the liver, gall bladder, gut and pronephros of the developing larvae. In animal cap explants treated with activin A together with retinoic acid, we induced HNF1alpha in pronephric tubules and epithelial gut cells, i.e. in mesodermal as well as in endodermal tissues. HNF1alpha can also be induced by activin A, but not by retinoic acid alone. To define the promoter element responding to the activin A signal, we injected various HNF1alpha promoter luciferase constructs into fertilized eggs and cultured the isolated animal caps in the presence of activin A. From the activity profiles of the promoter mutants used, we identified the HNF4-binding site as an activin-A-responsive element. As HNF4 is a maternal protein in Xenopus and localized in an animal-to-vegetal gradient in the cleaving embryo, we speculate that the activin A signal emanating from the vegetal pole cooperates with the maternal transcription factor HNF4 to define the embryonic regions expressing HNF1alpha.