The embryonic expression of the tissue-specific transcription factor HNF1α in Xenopus: Rapid activation by HNF4 and delayed induction by mesoderm inducers

Abstract

The tissue-specific transcription factor HNF1α is expressed in kidney, liver, intestine and stomach of Xenopus. We show that the HNF1α gene is transcriptionally activated at the onset of zygotic gene transcription and that this transcription is maintained throughout development. Ectodermal explants of blastulae (animal caps) express HNF1α mRNA upon stimulation with the mesoderm inducers activin A and BMP4 as well as on overexpression of Smad2 and Smad1, the corresponding members of the intracellular TGF-β signal transducers, respectively. Beside these factors that mediate their response through serine/threonine kinase receptors, bFGF, which acts via tyrosine kinase receptors, leads to HNF1α expression, too. These embryonic inducers result in a delayed appearance of HNF1α mRNA, excluding a direct activation of HNF1α. In contrast, the maternally expressed nuclear receptors HNF4α and HNF4β activate the initial HNF1α transcription, since overexpression of HNF4 leads to a rapid expression of HNF1α mRNA in animal caps. Similarly, in entire neurulae HNF4 overexpression results in increased HNF1α transcription. Therefore, we assume that the initial activation is dependent on maternal HNF4α and HNF4β transcription factors whereas HNF1α induction by growth factors reflects the property of these factors to induce the differentiation of mesodermal and entodermal cell types expressing HNF1α.