The embryonic expression of the tissue-specific transcription factor HNF1α 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 and that this transcription is maintained throughout development. Ectodermal explants of blastulae (animal caps) express HNF1 alpha 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-beta signal transducers, respectively, Beside these factors that mediate their response through serine/threonine kinase receptors, bFGF, which acts via tyrosine kinase receptors, leads to HNF1 alpha expression, too. These embryonic inducers result in a delayed appearance of HNF1 alpha mRNA, excluding a direct activation of HNF1 alpha. In contrast, the maternally expressed nuclear receptors HNF4 alpha and HNF4 beta activate the initial HNF1 alpha transcription, since overexpression of HNF4 leads to a rapid expression of HNF1 alpha mRNA in animal caps, Similarly, in entire neurulae HNF4 overexpression results in increased HNF1 a transcription, Therefore, we assume that the initial activation is dependent on maternal HNF4 alpha and HNF4 beta transcription 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.