HNF4-BETA, A NEW GENE OF THE HNF4 FAMILY WITH DISTINCT ACTIVATION ANDEXPRESSION PROFILES IN OOGENESIS AND EMBRYOGENESIS OF XENOPUS-LAEVIS

The transcription factor hepatocyte nuclear factor 4 (HNF4) is an orph an member of the nuclear receptor superfamily expressed in mammals in liver, kidney, and the digestive tract, Recently, we isolated the Xeno pus homolog of mammalian HNF4 and revealed that it is not only a tissu e-specific transcription factor but also a maternal component of the X enopus egg and distributed within an animal-to-vegetal gradient, We sp eculate that this gradient cooperates with the vegetally localized emb ryonic induction factor activin A to activate expression of HNF1 alpha , a tissue-specific transcription factor with an expression pattern ov erlapping that of HNF4. We have now identified a second Xenopus HNF4 g ene, which is more distantly related to mammalian HNF4 than the previo usly isolated gene, This new gene was named HNF4 beta to distinguish i t from the known HNF4 gene, which is now called HNF4 alpha. By reverse transcription-PCR, we detected within the 5' untranslated region of H NF4 beta two splice variants (HNF4 beta 2 and HNF4 beta 3) with additi onal exons, which seem to affect RNA stability, HNF4 beta is a functio nal transcription factor acting sequence specifically on HNF4 binding sites known for HNF4 alpha, but it seems to have a lower DNA binding a ctivity and is a weaker transactivator than the alpha isoform, Further more, the two factors differ with respect to tissue distribution in ad ult frogs: whereas HNF4 alpha is expressed in liver and kidney, HNF4 b eta is expressed in addition in stomach, intestine, lung, ovary, and t estis. Both factors are maternal proteins and present at constant leve ls throughout embryogenesis, However, using reverse transcription-PCR, we found the RNA levels to change substantially: whereas HNF4 alpha i s expressed early during oogenesis and is absent in the egg, HNF4 beta is first detected in the latest stage of oogenesis, and transcripts a re present in the egg and early cleavage stages, Furthermore, zygotic HNF4 alpha transcripts appear in early gastrula and accumulate during further embryogenesis, whereas HNF4 beta mRNA transiently appears duri ng gastrulation before it accumulates again at the tail bud stage, All of these distinct characteristics of the newly identified HNF4 protei n imply that the alpha and beta isoform have different functions in de velopment and in adult tissues.