The anterior pituitary gland of vertebrates is a key link between the hypothalamus of the central nervous system and the endocrine system. The anterior pituitary produces at least 6 different major hormones that regulate a number of important physiological functions. We have studied mechanisms regulating gene expression for three of the hormones, luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL). LH and FSH play key roles in reproduction and PRL is required for lactation to provide nutrition for a newborn. Thus, all three hormones play a role in reproductive success and survival of the species. The ability to secrete the necessary amounts of these hormones depends on appropriately regulated gene expression. Many of the studies have involved analysis of the DNA elements and transcription factors which are required for the tissue-specific, regulated expression of these hormone genes. Intensive studies by many laboratories have identified a number of transcriptional regulatory proteins that control the expression of the pituitary hormone genes. Current studies seek to further explore the mechanisms mediating the ability of these transcription factors and co-activators to regulate pituitary gene expression. The studies have implications for the general areas of hormone action, reproductive biology, fertility, sterility and oncology.
Identification of synaptotagmin I (SytI) as a gene activated by the pituitary-specific transcription factor, POU1F1 in PRL-producing cells. Studies from a number of labs have shown that POU1F1 is a key activator of PRL transcription. In addition, POU1F1 has been shown to play a crucial in the development of the pituitary and disruption of the POU1F1 gene leads to the loss of PRL-secreting cells from the pituitary. Presumably POU1F1 is involved in activation of a number of genes that contribute to the differentiated phenotype of PRL-secreting cells. We performed a genome-wide screen of POU1F1 chromatin binding sites in PRL-producing cells to look for target genes activated by POU1F1. Syt1 was identified in this screen as one of the genes that bind POU1F1. SYT1 has been extensively studied as an important transducer of Ca2+ signaling in regulated secretion. It seems likely that POU1F1-mediated activation of Syt1 gene expression is part of a differentiation mechanism supporting regulated secretion of hormones. Continuing studies seek to explore gene targets regulated by several transcription factors that are known to have crucial functions in regulating pituitary hormone gene expression.
Regulation of the expression of the LIM-domain binding 1 transcriptional co-activator by ubiquitination. LIM homeodomain transcription factors (Lhx) are a subset of homeodomain transcription factors which contain a zinc-finger protein-protein interaction domain, the LIM domain. A particular member of the Lhx family, Lhx3, has been shown to play a crucial role in pituitary development and Lhx3 also contributes to expression of pituitary hormone genes in the fully differentiated pituitary cells of the adult. Studies in several organisms have shown that the function of Lhx factors depends on interaction with the co-activator, LIM domain binding 1 (LDB1). The functional transcription complex contains an Lhx dimer binding to an LDB1 dimer to form a tetramer. Alterations in the concentration of either partner can disrupt formation of the tetramer complex. Maintenance of the proper stoichiometry of Lhx3 and LDB1 is therefore required for appropriate formation of the functional complex and expression of pituitary genes. Indeed, we and others have shown that overexpression of either Lhx3 or LDB1 can inhibit transcription of Lhx3 target genes. Thus anything that changes Lhx3 or LDB1 protein levels would be predicted to alter production of pituitary hormone genes. Recently we have studied the post-transcriptional regulation of LDB1 by ubiquitination. We have found that Lys-134 of LDB1 can either be mono-ubiquitinated, leading to stabilization of LDB1 or poly-ubiquitinated leading to degradation by the proteasome pathway. Differential regulation of LDB1 ubiquitination has the potential to substantially alter LDB1 expression, in turn altering pituitary hormone gene expression. Further studies will seek to examine the mechanisms mediating mono-ubiquitination versus poly-ubiquitination and explore physiological regulation of this process.