Characterisation of tissue-specific trans-acting factor binding to a proximal element in the rat growth hormone gene promoter

Transcriptional control during mammalian anterior pituitary development

The mammalian anterior pituitary gland is a compound endocrine organ that regulates reproductive development and fitness, growth, metabolic homeostasis, the response to stress, and lactation, by actions on target organs such as the gonads, the liver, the thyroid, the adrenals, and the mammary gland. The protein and peptide hormones that control these physiological parameters are secreted by specialized pituitary cell types that derive from a common origin in the early ectoderm. Collectively, the broad physiological importance of the pituitary gland, its intriguing organogenesis, and the clinical and agricultural significance of its actions, have established pituitary development as an excellent model system for the study of the gene-regulatory cascades that guide vertebrate cell determination and differentiation. We review the transcriptional pathways that regulate the commitment of the individual pituitary cell lineages and that subsequently modulate trophic hormone gene activity in the differentiated cells of the mature gland.

An alternatively spliced Pit-1 isoform altered in its ability to trans-activate

Although alternative splicing has been shown to give rise to isoforms of a number of transcription factors, such isoforms have not previously been detected for the POU homeodomain protein Pit-1. Screening of a rat pituitary GH3 cell cDNA expression library yielded a clone, termed pCMVPit-1a, encoding a 35.8 kD protein (Pit-1a) containing a 26 amino acid insert in the Pit-1 trans-activation domain. The position of the insert, plus Southern blot analysis, implied that Pit-1a mRNA arises by alternative splicing of the Pit-1 gene transcript. Pit-1a mRNA was detected in GH3 rat pituitary tumor cells at levels about 1/7 that of Pit-1 mRNA. Pit-1a mRNA-specific sequences were also detected in rat and mouse pituitary, and in mouse thyrotropic tumor TtT cells. DNA mobility shift assays showed that Pit-1a binds specifically to Pit-1 binding sites in the proximal prolactin promoter, but produces DNA-protein complexes of markedly different mobilities than Pit-1. In stably transfected CHO cells which accumulated approximately equal levels of either of the two proteins, Pit-1 trans-activated a prolactin promoter-driven CAT construct, while Pit-1a yielded no detectable transactivation, implying a trans-activation ratio for Pit-1a/Pit-1 of less than 0.05. Thus, the insertion of 26 amino acids of similar composition into the activation domain of Pit-1 has at once affected both the mode of binding of this protein and its ability to function as a trans-activator.

POU-domain proteins Pit-1 and Oct-1 interact to form a heteromeric complex and can cooperate to induce expression of the prolactin promoter

Two members of the POU-domain family of transcriptional activators, Pit-1 and Oct-1, are coexpressed in cells of the anterior pituitary gland. We demonstrate that the pituitary-specific developmental regulator Pit-1 can bind as a heterodimer with the widely expressed 100-kD Oct-1 protein to critical tissue-specific cis-active elements in the rat prolactin gene. Pit-1 and Oct-1 can also associate, in the absence of DNA, via their POU domains. Coexpression of Pit-1 and Oct-1 in a heterologous cell type results in reproducible 2- to 2.5-fold synergistic transcriptional effects on genes under control of the native prolactin promoter or of a single Pit-1 response element. These results suggest that a combinatorial pattern of heterodimeric and homodimeric interactions between different members of the POU-domain gene family can result when members of this large family are coexpressed in cells of developing and established organ systems, potentially regulating differential developmental gene activation.

The POU-specific domain of Pit-1 is essential for sequence-specific, high affinity DNA binding and DNA-dependent Pit-1-Pit-1 interactions

Pit-1 is a member of a family of transcription factors sharing two regions of homology: a highly conserved POU-specific (POUS) domain and a more divergent homeodomain (POUHD). Analysis of mutant Pit-1 proteins suggests that, while the POUHD is required and sufficient for low affinity DNA binding, the POUS domain is necessary for high affinity binding and accurate recognition of natural Pit-1 response elements. Pit-1 is monomeric in solution but associates as a dimer on its DNA response element, exhibiting DNA-dependent protein-protein interactions requiring the POUS domain. Analysis of alpha-helical domains and conserved structures in Pit-1 suggests that POU domain proteins interact with their DNA recognition sites differently than classic homeodomain proteins, with both the POUHD and the POUS domain contacting DNA. Transcriptional activity of Pit-1 on enhancer elements is conferred primarily by a Ser- and Thr-rich N-terminal region unrelated to other known transcription-activating motifs.

Pituitary cell phenotypes involve cell-specific Pit-1 mRNA translation and synergistic interactions with other classes of transcription factors

Development of the anterior pituitary gland involves proliferation and differentiation of ectodermal cells in Rathke's pouch to generate five distinct cell types that are defined by the trophic hormones they produce. A detailed ontogenetic analysis of specific gene expression has revealed novel aspects of organogenesis in this model system. The expression of transcripts encoding the alpha-subunit common to three pituitary glycoprotein hormones in the single layer of somatic ectoderm on embryonic day 11 established that primordial pituitary cell commitment occurs prior to formation of a definitive Rathke's pouch. Activation of Pit-1 gene expression occurs as an organ-specific event, with Pit-1 transcripts initially detected in anterior pituitary cells on embryonic day 15. Levels of Pit-1 protein closely parallel those of Pit-1 transcripts without a significant lag. Unexpectedly, Pit-1 transcripts remain highly expressed in all five cell types of the mature pituitary gland, but the Pit-1 protein is detected in only three cell types--lactotrophs, somatotrophs, and thyrotrophs and not in gonadotrophs or corticotrophs. The presence of Pit-1 protein in thyrotrophs suggests that combinatorial actions of specific activating and restricting factors act to confine prolactin and growth hormone gene expression to lactotrophs and somatotrophs, respectively. A linkage between the initial appearance of Pit-1 protein and the surprising coactivation of prolactin and growth hormone gene expression is consistent with the model that Pit-1 is responsible for the initial transcriptional activation of both genes. The estrogen receptor, which has been reported to be activated in a stereotypic fashion subsequent to the appearance of Pit-1, appears to be capable, in part, of mediating the progressive increase in prolactin gene expression characteristic of the mature lactotroph phenotype. This is a consequence of synergistic transcriptional effects with Pit-1, on the basis of binding of the estrogen receptor to a response element in the prolactin gene distal enhancer. These data imply that both transcriptional and post-transcriptional regulation of Pit-1 gene expression and combinatorial actions with other classes of transcription factors activated in distinct temporal patterns, are required for the mature physiological patterns of gene expression that define distinct cell types within the anterior pituitary gland.