Inhibitor of the tissue-specific transcription factor HNF4, a potential regulator in early Xenopus development

Hepatocyte nuclear factor 4 alpha (HNF4 alpha) is an orphan receptor of the nuclear receptor superfamily and expressed in vertebrates as a tissue-spec ific transcription factor in liver, kidney, intestine, stomach, and pancrea s. It also plays a crucial role in early embryonic development and has been identified as a maternal component in the Xenopus egg. We now report on an activity present in Xenopus embryos that inhibits the DNA binding of HNF4. This HNF4 inhibitor copurifies with a 25-kDa protein under nondenaturing c onditions but can be separated from this protein by sodium dodecyl sulfate treatment. Protease treatment of the inhibitor results in a core fragment o f about 5 kDa that retains full inhibitory activity. The activity of Be HNF 4 inhibitor fan also be monitored in the absence of DNA, as it alters the m obility of the HNF4 protein in native polyacrylamide gels and the accessibi lity of antibodies. Comparing the activity of the HNF4 inhibitor with acyl coenzyme A's, recently proposed to be ligands of HNF4, we observe a more st ringent specificity for the HNF4 inhibitor activity. Using deletion constru cts of the HNF4 protein, we could show that the potential ligand-binding do main of HNF4 is not required, and thus the HNF4 inhibitor does not represen t a classical ligand as defined for the nuclear receptor superfamily. Based on our previous finding that maternal HNF4 is abundantly present in Xenopu s embryos but the target gene HNF1 alpha is only marginally expressed, we p ropose that the HNF4 inhibitor functions in the embryo to restrict the acti vity of the maternal HNF4 proteins.