THE ENHANCERS OF THE HUMAN PLACENTAL-LACTOGEN B-GENE, A-GENE, AND L-GENE - PROGRESSIVE ACTIVATION DURING IN-VITRO TROPHOBLAST DIFFERENTIATION AND IMPORTANCE OF THE DF-3 ELEMENT IN DETERMINING THEIR RESPECTIVE ACTIVITIES

The hCS-A and hCS-B genes encoding human chorionic somatomammotropin a nd the related hCS-L gene are very similar in their coding and flankin g sequences, For each of these genes, downstream enhancers, varying in strength, have been identified with the help of cytotrophoblast-deriv ed JEG-3 cells, which do not express the hCS genes, Here we study the activity of the hCS enhancers in human syncytiotrophoblast in primary culture, which naturally expresses the hCS genes, We show that the act ivity of the hCS-B gene enhancer is mediated by two elements, DF-3 and DF-4, whereas the hCS-L and hCS-A gene enhancers display weaker activ ity due to mutations in their respective DF-3 sites, Replacement of th e hCS-B DF-3 site with the homologous hCS-A sequence causes hCS-B enha ncer activity to decrease, Primary cytotrophoblasts differentiate in c ulture to form the syncytiotrophoblast. We show that during this proce ss the production of hCS progressively increases and that concomitantl y all three hCS enhancers are progressively activated, A targeted muta tion in the 3' part of the DF-4 element abolishes the binding of a pro tein present only in syncytiotrophoblast extracts and inactivates the DF-4 element, Thus, a direct correlation exists between the appearance of this syncytiotrophoblast-specific protein and hCS enhancer activit y, This primary culture model proves useful in studying the regulation of the hCS genes.