The specific expression of growth hormone (GH) in the somatotrophic cells of the anterior pituitary is largely attributable to a short promoter in the 5' flanking region of the GH gene. This promoter contains two binding sites for the transcription factor GHF-1, the expression of which is also specific to cells of the somatotrophic lineage and correlates with activation of the GH gene in the developing mouse pituitary. Various studies indicate that GHF-1 is the main determinant of cell type-specific expression of the GH gene. GHF-1 is a member of the POU-domain class of proteins that each contain two highly conserved sequence motifs, the homoeodomain and the POU-specific domain. Here we report that the GHF-1 homoeodomain is sufficient for sequence-specific DNA binding, although its activity is stimulated by the POU-specific domain, which does not interact directly with the DNA. Transcriptional activation is mediated by a separate domain rich in hydroxylated amino-acid residues. Similar sequences are present in other cell type-specific transcription factors.

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Mutations that cause pituitary dwarfism in the mouse reside in the gene encoding the transcription factor growth hormone factor 1 (GHF1 or pit1). These dwarf mice (dw and dwJ) are deficient in growth hormone (GH) and prolactin (PRL) synthesis and exhibit pituitary hypoplasia, suggesting a stem cell defect. With antisense oligonucleotide technology, a cell culture model of this genetic defect was developed. Specific inhibition of GHF1 synthesis by complementary oligonucleotides led to a marked decrease in GH and PRL expression and to a marked decrease in proliferation of somatotrophic cell lines. These results provide direct evidence that the homeodomain protein GHF1 is required not only for the establishment and maintenance of the differentiated phenotype but for cell proliferation as well.

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The POU domain protein GHF‐1 has a critical role in generation, proliferation and phenotypic expression of three pituitary cell types. GHF‐1 functions in part by binding to and transactivating the promoters of both the growth hormone (GH) and prolactin (PRL) genes and that of the GHF1 gene itself. We describe a naturally occurring isoform of GHF‐1, GHF‐2, in which an additional 26 amino acids are inserted into the activation domain of the protein as a result of alternative splicing. GHF‐2 retains the DNA binding activity of GHF‐1 and can activate the GH promoter but has lost the ability to activate the PRL and GHF1 promoters. These results suggest that GHF‐2 may function in differential target gene activation during differentiation of the somatotrophic lineage. Both GHF‐1 and GHF‐2 transcripts are specifically expressed in the anterior pituitary. Analysis of the genomic GHF1 gene shows that most of the distinct functional domains of GHF‐1 (and GHF‐2) are encoded by separate exons. Gene segment duplication and exon shuffling may have contributed to the evolution of this cell type‐specific transcriptional regulatory gene.

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The pituitary-specific transcription factor Pit-1/ GHF-1 is a member of the POU domain family of regulatory proteins. It is involved in the commitment and expansion of the somatotropic cell lineage and activates the transcription of a set of anterior pituitary genes. We have cloned the human PIT1/GHF1 gene and characterized the regulatory mechanisms controlling its promoter activation and regulation. A minimal promoter region (؊102 to ؉15) contains the cis-acting elements that confer to the human PIT1/GHF1 gene a high basal transcriptional activity, the tissue-specific expression, and the autoregulation by Pit-1/GHF-1 protein. The upstream promoter region contains a multiplicity of Pit-1/ GHF-1 binding sites that do not show any synergistic interaction with the minimal promoter. The transcriptional activity is negatively regulated by Oct-1 and mediated by an octamer-binding site (OTF). In addition, we have also identified a 12-O-tetradecanoylphorbol-13-acetate-responsive element, which overlaps with a Pit-1/ GHF-1 binding site. A mutually exclusive binding of the activator protein-1 (AP-1) and Pit-1/GHF-1 has been observed on this composite site, and AP-1 was shown to down-regulate PIT1/GHF1 transcription.

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