IDENTIFICATION OF HEPATIC NUCLEAR FACTOR-1 BINDING-SITES IN THE 5'-FLANKING REGION OF THE HUMAN PHENYLALANINE-HYDROXYLASE GENE - IMPLICATION OF A DUAL FUNCTION OF PHENYLALANINE-HYDROXYLASE STIMULATOR IN THE PHENYLALANINE HYDROXYLATION SYSTEM

Phenylalanine hydroxylase stimulator (PHS) is a component of the pheny lalanine hydroxylation system that is involved in the regeneration of the cofactor tetrahydrobiopterin. It is also identical to the dimeriza tion cofactor of hepatocyte nuclear factor 1 (HNF1) (DCoH) that is abl e to enhance the transcriptional activity of HNF1. Moreover, it has th e structural potential for binding macromolecules such as proteins and nucleic acids, consistent with its involvement in gene expression. We investigated whether PHS/DCoH could enhance the expression of phenyla lanine hydroxylase (PAH). Cotransfection assays showed that DCoH itsel f could not transactivate the 9-kb human PAH 5' flanking fragment. How ever, this 9-kb fragment was transactivated by HNF1 in a dose-dependen t manner with a maximum of nearly 8-fold activation; DCoH potentiated this transactivation by another 1.6-fold. The HNF1 binding sites were located at -3.5 kb in a region that is 77.5% identical to the mouse li ver-specific hormone-inducible PAH gene enhancer. This study suggests a possible dual function of PBS in vivo in the human phenylalanine hyd roxylation system: it is involved in the regeneration of the cofactor tetrahydrobiopterin and can also enhance the expression of the human P AH gene.