Identification of hepatic nucleal flanking region of the human ph Implication of a dual function ol stimulator in the phenylalanine I

Phenylalanine hydroxylase stimulator (PHS) is a component of the phenylalanine hydroxylation system that is involved in the regeneration of the cofactor tetrahydrobiopterin. It is also identical to the dimerization cofactor of hepatocyte nuclear factor 1 (HNF1) (DCoH) that is able to enhance the transcriptional activity of HNF1. Moreover, it has the structural potential for binding macro- molecules such as proteins and nucleic acids, consistent with its involvement in gene expression. We investigated whether PHS/DCoH could enhance the expression of phenylalanine hydroxylase (PAH). Cotransfection assays showed that DCoH itself could not transactivate the 9-kb human PAH 5' flanking fragment. However, this 9-kb fragment was transactivated by HNF1 in a dose-dependent manner with a maximum of nearly 8-fold activation; DCoH potentiated this transactivation by another 1.6-fold. The HNFI binding sites were located at -3.5 kb in a region that is 77.5% identical to the mouse liver- specific hormone-inducible PAH gene enhancer. This study suggests a possible dual function of PHS in vivo in the human phenylalanine hydroxylation system: it is involved in the regeneration of the cofactor tetrahydrobiopterin and can also enhance the expression of the human PAH gene.