CHICKEN OVALBUMIN UPSTREAM PROMOTER-TRANSCRIPTION FACTOR, HEPATOCYTE NUCLEAR FACTOR-3, AND CCAAT ENHANCER BINDING-PROTEIN CONTROL THE FAR-UPSTREAM ENHANCER OF THE RAT ALPHA-FETOPROTEIN GENE/

We have further characterized the most distal of the three alpha-fetop rotein (AFP) enhancers required for expression of the AFP gene in feta l hepatocytes and yolk sac endodermal cells, Almost total rat AFP enha ncer 3 (E3) activity is driven by a 160-bp fragment at -6 kb containin g three target regions for nuclear proteins that cooperate to stimulat e transcription from the AFP and the thymidine kinase promoters in Hep G2 hepatoma cells, Region 1, recently shown to be crucial for correct function of the enhancer in liver of transgenic mice, is recognized by two sets of transcription factors that bind to partly overlapping sit es, la and Ib, in a noncooperative and nonexclusive manner, Site la co ntains a moth, AGGTCA, which is recognized by chicken ovalbumin upstre am promoter transcription factors (COUP-TFs), but not by hepatocyte nu clear factor 4. Hepatocyte nuclear factor 3 (HNF3) and CCAAT/enhancer binding protein (C/EBP), which bind to regions 2 and 3, respectively, are likely responsible for the liver-specific E3 action, They play a k ey role by acting in synergy, The participation of nuclear receptors s uch as COUP-TFs, with C/EBP and HNF3, in the tight control of the dist al AFP enhancer is a new, and perhaps key, step toward understanding t he regulation and function of this enhancer, which may remain active t hroughout development.