PTX1, A BICOID-RELATED HOMEO BOX TRANSCRIPTION FACTOR INVOLVED IN TRANSCRIPTION OF THE PRO-OPIOMELANOCORTIN GENE

The pituitary gland contains six distinct hormone-producing cell types that arise sequentially during organogenesis. The first cells to differentiate are those that express the pro-opiomelanocortin (POMC) gene in the anterior pituitary lobe. The other lineages, which appear later, include cells that are dependent on the POU factor Pit-1 and another POMC-expressing lineage in the intermediate pituitary lobe. Using AtT-20 cells as a model for early expression of POMC in the anterior pituitary, we have defined a regulatory element conferring cell specificity of transcription and cloned a cognate transcription factor. This factor, Ptxl (pi!uitary homeo box I), contains a homeo box related to those of the anterior-specific genes bicoid and orthodenticle in Drosophila, and Otx-1 and Otx-2 in mammals. Ptxl activates transcription upon binding a sequence related to the Drosophila bicoid target sites. Ptxl is the only nuclear factor of this DNA-binding specificity that is detected in AtT-20 cells, and it is expressed at high levels in a subset of adult anterior pituitary cells that express POMC. However, Ptxl is expressed in most cells of Rathke's pouch at an early time during pituitary development and before final differentiation of hormone-producing cells. Thus, Ptxl may have a role in differentiation of pituitary cells, and its early expression pattern suggests that i t may have a role in pituitary formation. In the adult pituitary gland, Ptxl appears to be recruited for cell-specific transcription of the POMC gene.