INTERACTION OF ETS-1 AND THE POU-HOMEODOMAIN PROTEIN GHF-1 PIT-1 RECONSTITUTES PITUITARY-SPECIFIC GENE-EXPRESSION/

The pituitary-specific, POU-homeodomain factor GHF-1/Pit-1 is necessar y, but not sufficient, for cell-specific expression of prolactin (PRL) , growth hormone (GH), and thyrotropin. Combinatorial interactions of GHF-1 with other factors are likely to be required; however, such fact ors and their mechanisms of action remain to be elucidated. Here we id entify Ets-l as a factor that functionally and physically interacts wi th GHF-1 to fully reconstitute proximal PRL promoter activity. In cont rast, Ets-2 has no effect, and the alternatively spliced GHF-2/Pit-1 b eta variant fails to synergize with Ets-l, The Ets-1-GHF-1 synergy req uires a composite Ets-1-GHF-1 cis element and is dependent on an Ets-l -specific protein domain. Furthermore, the ancestrally related and GHF -1-dependent GH promoter, which lacks this composite element, does not exhibit this response, Finally, Ets-l, but not Ets-2, binds directly to GHF-1 and GHF-2. These data show that a functional interaction of G HF-1 and Ets-l, acting via a composite DNA element, is required to est ablish lactotroph-specific PRL gene expression, thus providing a molec ular mechanism by which GHF-1 can discriminate between the GH and PRL genes. These results underscore the importance of transcription factor s that are distinct from, but interact with, homeobox proteins to esta blish lineage-specific gene expression.