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The POU-homeodomain transcription factor Pit-1 is required for the differentiation of the anterior pituitary cells and the expression of their hormone products. Pit-1β, an alternate splicing isoform, has diametrically different outcomes when it is expressed in different cell types. Pit-1β acts as a transcriptional repressor of prolactin (PRL) and growth hormone genes in pituitary cells, and as a transcriptional activator in non-pituitary cells. In order to explore these differences, we: (1) identified the transcriptional cofactors necessary for reconstitution of repression in non-pituitary cells; (2) tested the effect of the β-domain on heterodimerization with Pit-1 and physical interaction with the co-activator CREB binding protein (CBP); and (3) determined the β-domain sidechain chemistry requirements for repression. Co-expression of both Pit-1 isoforms reconstituted the repression of the PRL promoter in non-pituitary cells. The β-domain allowed heterodimerization with Pit-1 but blocked physical interaction with CBP, and specific chemical properties of the β-domain beyond hydrophobicity were dispensable. These data strongly suggest that Pit-1β represses hormone gene expression by heterodimerizing with Pit-1 and interfering with the assembly of the Pit-1–CBP complex required for PRL promoter activity in pituitary cells.

cAMP-response element (CRE) binding (CREB) and modulator (CREM) proteins, activated by protein kinase A-mediated phosphorylation, bind as homo- and heterodimers to promoters containing CRE and activator protein 1 (AP-1) sites to alter target-gene expression. We have previously reported differential expression of CREB and CREM splice variants CREMα and CREMτ₂α in human myometrium during pregnancy and labour. Via microarray studies with cultured myometrial cells stably transfected with CREB, CREMα and CREMτ₂α cDNAs, CREB affected the expression of 958 genes; 522 being up-regulated and 436 down-regulated. CREMα altered the expression of 118 genes; 71 were increased and 47 decreased. CREMτ₂α affected 220 genes; 148 were activated and 72 repressed. Notably, genes affected by CREB, CREMα and CREMτ₂α belong to largely discrete groups: less than 9% were affected by more than one factor. Genes involved in regulation of cell death and apoptosis, growth and maintenance, signal transduction, physiological and developmental processes, protein kinase cascades, extracellular matrix, cytoskeleton, cell-cycle regulation, transport, and a variety of enzymes, intracellular components and nucleic acid-binding proteins have been described, many of which are involved in the modulation of myometrial activity during pregnancy and parturition.