The embryonic expression of the tissue-specific transcription factor HNF1 alpha in Xenopus: rapid activation by HNF4 and delayed induction by mesoderm inducers

The tissue-specific transcription factor HNF1 alpha is expressed in kidney, liver, intestine and stomach of Xenopus, We show that the HNF1 alpha gene is transcriptionally activated at the onset of zygotic gene transcription a nd that this transcription is maintained throughout development. Ectodermal explants of blastulae (animal caps) express HNF1 alpha mRNA upon stimulati on with the mesoderm inducers activin A and BMP4 as well as on overexpressi on of Smad2 and Smad1, the corresponding members of the intracellular TGF-b eta signal transducers, respectively, Beside these factors that mediate the ir response through serine/threonine kinase receptors, bFGF, which acts via tyrosine kinase receptors, leads to HNF1 alpha expression, too. These embr yonic inducers result in a delayed appearance of HNF1 alpha mRNA, excluding a direct activation of HNF1 alpha. In contrast, the maternally expressed n uclear receptors HNF4 alpha and HNF4 beta activate the initial HNF1 alpha t ranscription, since overexpression of HNF4 leads to a rapid expression of H NF1 alpha mRNA in animal caps, Similarly, in entire neurulae HNF4 overexpre ssion results in increased HNF1 a transcription, Therefore, we assume that the initial activation is dependent on maternal HNF4 alpha and HNF4 beta tr anscription factors whereas HNF1 alpha induction by growth factors reflects the property of these factors to induce the differentiation of mesodermal and entodermal cell types expressing HNF1 alpha.