SYNERGISTIC INTERACTIONS BETWEEN PIT-1 AND OTHER ELEMENTS ARE REQUIRED FOR EFFECTIVE SOMATOTROPH RAT GROWTH-HORMONE GENE-EXPRESSION IN TRANSGENIC MICE

The role of the pituitary-specific POU-domain protein, Pit-1, in GH ge ne activation has been established by in vitro analyses and by the obs ervation that mutations affecting the Pit-1 genomic locus result in ge netically transmitted dwarfism. To define the quantitative contributio n of the two Pit-1 response elements and the potential role of other f actors in GH gene activation, we systematically assessed the ability o f a series of GH promoter regions to activate transgenes in the mouse anterior pituitary gland. These studies revealed that the two GH Pit-1 binding sites are necessary, but not sufficient, for efficient transc riptional activation. Transgenes containing information including only these cis-active regions are expressed at extremely low levels in the pituitary glands of transgenic mice. The addition of 35 base pairs of 5'-flanking information, contributing other elements including a thyr oid hormone/retinoic acid response element, results in much higher lev els of transgene expression. Sequences located upstream of this segmen t contribute a further 5- to 10-fold activation. Thus, while Pit-1 is required for GH gene activation, it alone can only direct minimal expr ession in transgenic animals. Rather, synergistic interactions between other promoter elements and Pit-1 appear to be required for expressio n of the transgenes at approximately the 100-fold higher levels that a re characteristic of somatotrophs, and are therefore likely to be crit ical components of somatotroph-specific expression of the GH gene.