Reconstitution of the protein kinase A response of the rat prolactin promoter: differential effects of distinct Pit-1 isoforms and functional interaction with Oct-1

The mechanism of selfheal extract in treating hyperprolactinemia

OBJECTIVE

Selfheal has been used for many years in hyperprolactinemia induced galactorrhea, menstrual disorders, and dysgenesis with satisfactory curative effect. However, its mechanism is still unclear. This study intended to investigate the effect of selfheal extract on hyperprolactinemia in vivo and in vitro, in order to elucidate its mechanism of anti-hyperprolactinemia.

PATIENTS AND METHODS

Hyperprolactinemia rat model was established. High dose (28.8 g/(kg⋅d)), middle dose (14.4 g/ (kg⋅d)), and low dose (7.2 g/(kg⋅d)) of selfheal extract were used to treat the model to observe impact on serum estradiol (E2), progesterone (P), prolactin (PRL), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) levels. Three cell lines MMQ, GH3, and PC12 were applied to investigate selfheal extract effect on PRL secretion, dopamine D2 receptor, and dopamine transporter (DAT).

RESULTS

High and middle dose of selfheal extract significantly reduced PRL level in hyperprolactinemia rat compared with model group (P< 0.01). Compared with normal control, 5 mg/ml and 10 mg/ml selfheal extract obviously inhibited PRL secretion in MMQ cells that high expressed D2 receptor after 24 hours (P< 0.01), but did not affect PRL secretion in GH3 cells lack of D2 receptor. 8 mg/ml selfheal extract markedly suppressed D2 receptor and DAT expression in PC12 cells that strongly expressed D2 receptor and DAT (P< 0.01).

CONCLUSIONS

Selfheal extract treated hyperprolactinemia through dopamine D2 receptor with significant effect.

Discovery of novel determinants of endothelial lineage using chimeric heterokaryons

We wish to identify determinants of endothelial lineage. Murine embryonic stem cells (mESC) were fused with human endothelial cells in stable, non-dividing, heterokaryons. Using RNA-seq, it is possible to discriminate between human and mouse transcripts in these chimeric heterokaryons. We observed a temporal pattern of gene expression in the ESCs of the heterokaryons that recapitulated ontogeny, with early mesodermal factors being expressed before mature endothelial genes. A set of transcriptional factors not known to be involved in endothelial development was upregulated, one of which was POU class 3 homeobox 2 (Pou3f2). We confirmed its importance in differentiation to endothelial lineage via loss- and gain-of-function (LOF and GOF). Its role in vascular development was validated in zebrafish embryos using morpholino oligonucleotides. These studies provide a systematic and mechanistic approach for identifying key regulators in directed differentiation of pluripotent stem cells to somatic cell lineages.

DOI:http://dx.doi.org/10.7554/eLife.23588.001

Endothelial cells form the inner surface of blood vessels, acting like a non-stick coating. In addition to making substances that keep blood from sticking to the vessel wall, endothelial cells generate compounds that relax the vessel, and prevent it from thickening. Endothelial cells also form capillaries, the smallest vessels that provide oxygen and nutrients for all tissues.

A regenerating organ, or a bioengineered tissue, requires a system of capillaries and other microvessels. Thus, regenerative medicine could benefit from a knowledge of how to generate endothelial cells from pluripotent stem cells – cells that can “differentiate” to form almost any type of cell in the body.

Wong, Matrone et al. have now used a cell fusion model (named heterokaryon) to track the changes in gene expression that occur as a pluripotent stem cell differentiates to ultimately become an endothelial cell. In this model, mouse embryonic stem cells (ESCs) are fused to human endothelial cells. Over time the human endothelial cells drive gene expression in the ESCs toward that of endothelial cells.

Wong, Matrone et al. discovered changes in gene expression in many genes that have not previously been described as involved in the differentiation of endothelial cells. When one of these genes – named Pou3f2 – was inactivated in ESCs, they could not be differentiated into endothelial cells. The absence of Pou3f2 also drastically impaired how blood vessels developed in zebrafish embryos.

Thus the heterokaryon model can generate important information regarding the dynamic changes in gene expression that occur as a pluripotent cell differentiates to become an endothelial cell. This model may also be useful for discovering other genes that control the differentiation of other cell types.

DOI:http://dx.doi.org/10.7554/eLife.23588.002

The role of cyclic nucleotides in pituitary lactotroph functions

Lactotrophs are one of the five secretory anterior pituitary cell types specialized to synthesize and release prolactin. In vitro, these cells fire action potentials (APs) spontaneously and the accompanied Ca(2+) transients are of sufficient amplitude to keep the exocytotic pathway, the transcription of prolactin gene, and de novo hormone synthesis continuously active. Basal cyclic nucleotide production is also substantial in cultured cells but not critical for the APs secretion/transcription coupling in lactotrophs. However, elevated intracellular cAMP levels enhance the excitability of lactotrophs by stimulating the depolarizing non-selective cationic hyperpolarization-activated cyclic nucleotide-regulated and background channels, whereas cGMP inhibits it by activating Ca(2+)-controlled K(+) channels. Elevated cAMP also modulates prolactin release downstream of Ca(2+) influx by changing the kinetic of secretory pores: stimulate at low and inhibit at high concentrations. Induction of prolactin gene and lactotroph proliferation is also stimulated by elevated cAMP through protein kinase A. Together, these observations suggest that in lactotrophs cAMP exhibits complex regulatory effects on voltage-gated Ca(2+) influx and Ca(2+)-dependent cellular processes.

Differential abilities of chicken Pit1 isoforms to regulate the GH promoter: evidence for synergistic activation

Pit1, pituitary-specific transcription factor 1, regulates differentiation of cells of the Pit1 lineage in the anterior pituitary and the synthesis of peptide hormones by these cell types, including GH. Pit1 is characterized by an N-terminal transactivation domain and a C-terminal POU domain. Alternative forms of Pit1, differing from each other in the N-terminal domain, have been reported in several species, but the functional implication of having multiple isoforms is not known. Several PIT1 mRNA transcripts exist in chickens that have not been characterized. This study was conducted to determine which, if any, of the chicken Pit1 isoforms regulate the chicken GH (cGH) promoter. During the course of this work, Pit1β2, a novel isoform of chicken Pit1, was discovered. Effects of known and novel isoforms (Pit1α, Pit1β1, Pit1β2, and Pit1γ) on cGH promoter activity were characterized in chicken Leghorn male hepatoma cells. Three of the isoforms, Pit1α, Pit1β1, and Pit1β2, activated the cGH promoter, whereas Pit1γ did not. Results from gel-shift assays indicated that Pit1γ does not bind to the proximal Pit1-bindng site of the cGH promoter, suggesting a possible mechanism underlying its inactivity. We found a functional advantage for having multiple isoforms expressed. When Pit1β1 was coexpressed with Pit1α or Pit1β2, significantly greater activation of the cGH promoter occurred than with any one isoform alone, with synergistic activation occurring when Pit1α and Pit1β1 were coexpressed. Whether this increased activation required, or was facilitated by, heterodimerization of two isoforms is not known. Identification of isoforms with specific functions will facilitate identification of their respective interacting partners that are essential for GH gene expression.

Phosphorylation of BRN2 modulates its interaction with the Pax3 promoter to control melanocyte migration and proliferation

MITF-M and PAX3 are proteins central to the establishment and transformation of the melanocyte lineage. They control various cellular mechanisms, including migration and proliferation. BRN2 is a POU domain transcription factor expressed in melanoma cell lines and is involved in proliferation and invasion, at least in part by regulating the expression of MITF-M and PAX3. The T361 and S362 residues of BRN2, both in the POU domain, are conserved throughout the POU protein family and are targets for phosphorylation, but their roles in vivo remain unknown. To examine the role of this phosphorylation, we generated mutant BRN2 in which these two residues were replaced with alanines (BRN2TS→BRN2AA). When expressed in melanocytes in vitro or in the melanocyte lineage in transgenic mice, BRN2TS induced proliferation and repressed migration, whereas BRN2AA repressed both proliferation and migration. BRN2TS and BRN2AA bound and repressed the MITF-M promoter, whereas PAX3 transcription was induced by BRN2TS but repressed by BRN2AA. Expression of the BRN2AA transgene in a Mitf heterozygous background and in a Pax3 mutant background enhanced the coat color phenotype. Our findings show that melanocyte migration and proliferation are controlled both through the regulation of PAX3 by nonphosphorylated BRN2 and through the regulation of MITF-M by the overall BRN2 level.

The 26-amino acid beta-motif of the Pit-1beta transcription factor is a dominant and independent repressor domain

The POU-homeodomain transcription factor Pit-1 governs the pituitary cell-specific expression of Pit-1, GH, prolactin (PRL), and TSHbeta genes. Alternative splicing generates Pit-1beta, which contains a 26-amino acid beta-domain inserted at amino acid 48, in the middle of the Pit-1 transcription activation domain (TAD). Pit-1beta represses GH, PRL, and TSHbeta promoters in a pituitary-specific manner, because Pit-1beta activates these same promoters in HeLa nonpituitary cells. Here we comprehensively analyze the role of beta-domain sequence, position, and context, to elucidate the mechanism of beta-dependent repression. Repositioning the beta-motif to the Pit-1 amino terminus, hinge, linker, and carboxyl terminus did not affect its ability to repress basal rat (r) PRL promoter activity in GH4 pituitary cells, but all lost the ability to repress Ras-induced rPRL promoter activity. To determine whether beta-domain repression is independent of Pit-1 protein and DNA binding sites, we generated Gal4-Pit-1TAD, Gal4-Pit-1betaTAD, and Gal4-beta-domain fusions and demonstrated that the beta-motif is sufficient to actively repress VP16-mediated transcription of a heterologous promoter. Moreover, beta-domain point mutants had the same effect whether fused to Gal4 or within the context of intact Pit-1beta. Surprisingly, Gal4-beta repression lost histone deacetylase sensitivity and pituitary specificity. Taken together, these results reveal that the beta-motif is a context-independent, modular, transferable, and dominant repressor domain, yet the beta-domain repressor activity within Pit-1beta contains cell type, promoter, and Pit-1 protein context dependence.

Mechanisms of glucose-induced expression of pancreatic-derived factor in pancreatic beta-cells

Pancreatic-derived factor (PANDER) is a cytokine-like peptide highly expressed in pancreatic beta-cells. PANDER was reported to promote apoptosis of pancreatic beta-cells and secrete in response to glucose. Here we explored the effects of glucose on PANDER expression, and the underlying mechanisms in murine pancreatic beta-cell line MIN6 and primary islets. Our results showed that glucose up-regulated PANDER mRNA and protein levels in a time- and dose-dependent manner in MIN6 cells and pancreatic islets. In cells expressing cAMP response element-binding protein (CREB) dominant-negative construct, glucose failed to induce PANDER gene expression and promoter activation. Treatment of the cells with calcium chelator [EGTA, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl)ester (BAPTA/AM)], the voltage-dependent Ca(2+) channel inhibitor (nifedipine), the protein kinase A (PKA) inhibitor (H89), the protein kinase C (PKC) inhibitor (Go6976), or the MAPK kinase 1/2 inhibitor (PD98059), all significantly inhibited glucose-induced PANDER gene expression and promoter activation. Further studies showed that glucose induced CREB phosphorylation through Ca(2+)-PKA-ERK1/2 and Ca(2+)-PKC pathways. Thus, the Ca(2+)-PKA-ERK1/2-CREB and Ca(2+)-PKC-CREB signaling pathways are involved in glucose-induced PANDER gene expression. Wortmannin (phosphatidylinositol 3-kinase inhibitor), ammonium pyrrolidinedithiocarbamate (nuclear factor-kappaB inhibitor and nonspecific antioxidant), and N-acetylcysteine (antioxidant) were also found to inhibit glucose-induced PANDER promoter activation and gene expression. Because there is no nuclear factor-kappaB binding site in the promoter region of PANDER gene, these results suggest that phosphatidylinositol 3-kinase and reactive oxygen species be involved in glucose-induced PANDER gene expression. In conclusion, glucose induces PANDER gene expression in pancreatic beta-cells through multiple signaling pathways. Because PANDER is expressed by pancreatic beta-cells and in response to glucose in a similar way to those of insulin, PANDER may be involved in glucose homeostasis.

Activin inhibits the human Pit-1 gene promoter through the p38 kinase pathway in a Smad-independent manner

The pituitary transcription factor Pit-1 regulates hormonal production from the anterior pituitary gland. However, the mechanisms by which Pit-1 gene expression is regulated in humans are poorly understood. Activin, a member of the TGFbeta superfamily, acts as a negative regulator of cell growth and prolactin gene expression in lactotrope cells. In this study, we show that activin negatively regulates the human Pit-1 gene promoter. We defined a 117-bp element within the Pit-1 promoter that is sufficient to relay these inhibitory effects. We further investigated the signaling pathways that mediate activin-induced inhibition of Pit-1 gene promoter in pituitary lactotrope cells. We found that the activin effects on Pit-1 gene regulation are Smad independent and require the p38 MAPK pathway. Specifically, blocking p38 kinase activity reverses activin-mediated inhibition of the Pit-1 gene promoter. Together, our results highlight the p38 MAPK pathway as a key regulator of activin function in pituitary lactotrope cells and further emphasizes the critical role played by activin in regulating hormonal production in the pituitary gland.

Dynamic interactions between Pit-1 and C/EBPalpha in the pituitary cell nucleus

The homeodomain (HD) transcription factors are a structurally conserved family of proteins that, through networks of interactions with other nuclear proteins, control patterns of gene expression during development. For example, the network interactions of the pituitary-specific HD protein Pit-1 control the development of anterior pituitary cells and regulate the expression of the hormone products in the adult cells. Inactivating mutations in Pit-1 disrupt these processes, giving rise to the syndrome of combined pituitary hormone deficiency. Pit-1 interacts with CCAAT/enhancer-binding protein alpha (C/EBPalpha) to regulate prolactin transcription. Here, we used the combination of biochemical analysis and live-cell microscopy to show that two different point mutations in Pit-1, which disrupted distinct activities, affected the dynamic interactions between Pit-1 and C/EBPalpha in different ways. The results showed that the first alpha-helix of the POU-S domain is critical for the assembly of Pit-1 with C/EBPalpha, and they showed that DNA-binding activity conferred by the HD is critical for the final intranuclear positioning of the metastable complex. This likely reflects more general mechanisms that govern cell-type-specific transcriptional control, and the results from the analysis of the point mutations could indicate an important link between the mislocalization of transcriptional complexes and disease processes.

Differential involvement of the Ras and Rap1 small GTPases in vasoactive intestinal and pituitary adenylyl cyclase activating polypeptides control of the prolactin gene

In pituitary cells, transcriptional regulation of the prolactin (PRL) gene and prolactin secretion are controlled by multiple transduction pathways through the activation of G protein coupled receptors and receptor tyrosine kinases. In the somatolactotrope GH4C1 cell line, we have previously identified crosstalk between the MAPKinase cascade ERK1/2 and the cAMP/protein kinase A pathway after the activation of the VPAC2 receptor by vasoactive intestinal polypeptide (VIP) or pituitary adenylyl cyclase-activating polypeptide (PACAP38). In the present study, we focus on the involvement of the GTPases Ras and Rap1 as downstream components of signal transmission initiated by activation of the VPAC2 receptor. By using pull-down experiments, we show that VIP and PACAP38 preferentially activate Rap1, whereas thyrotropin releasing hormone (TRH) and epidermal growth factor (EGF) mainly activate Ras GTPase. Experiments involving the expression of the dominant-negative mutants of Ras and Rap1 signaling (RasN17 or Rap1N17) indicate that both GTPases Ras and Rap1 are recruited for the ERK activation by VIP and PACAP38, whereas Rap1 is poorly involved in TRH or EGF-induced ERK activation. The use of U0126, a selective inhibitor of MAPKinase kinase, provides evidence that MAPKinase contributes to the regulation of the PRL gene. Moreover, cotransfection of RasN17 or Rap1N17 with the PRL proximal promoter luciferase reporter construct indicates that Rap1 may be responsible for VIP/PACAP-induced activation of the PRL promoter. Interestingly, Ras would be involved as a negative regulator of VIP/PACAP-induced PRL gene activation, in contrast to its stimulatory role in the regulation of the PRL promoter by TRH and EGF.

Cloning of a protein binding to the most proximal Pit-1 binding element of prolactin gene from human pituitary cDNA library

A human pituitary cDNA library was screened using a yeast one-hybrid system to find a factor binding Pit-1 binding elements in the PRL gene other than Pit-1. Beside colonies containing Pit-1 or Oct-1 cDNA, three colonies contained mPOU cDNA, a member of the POU protein family. Immunohistochemical analysis showed mPOU-like immunoreactivity was present in human PRL-producing pituitary tumors but not in non-functioning pituitary tumors. Mobility shift analysis revealed that mPOU bound to Pit-1 binding elements of the PRL gene, 1P and 3P. mPOU activated the expression of 0.6 k PRL and 7x1P reporter genes in the presence of Pit-1 and cAMP, although it did not enhance Pit-1-induced expression of 7x3P reporter gene. These findings suggest that mPOU is involved in the activation of the PRL gene by cAMP through 1P in the presence of Pit-1.

A PIT-1 homeodomain mutant blocks the intranuclear recruitment of the CCAAT/enhancer binding protein alpha required for prolactin gene transcription

The pituitary-specific homeodomain protein Pit-1 cooperates with other transcription factors, including CCAAT/enhancer binding protein alpha (C/EBPalpha), in the regulation of pituitary lactotrope gene transcription. Here, we correlate cooperative activation of prolactin (PRL) gene transcription by Pit-1 and C/EBPalpha with changes in the subnuclear localization of these factors in living pituitary cells. Transiently expressed C/EBPalpha induced PRL gene transcription in pituitary GHFT1-5 cells, whereas the coexpression of Pit-1 and C/EBPalpha in HeLa cells demonstrated their cooperativity at the PRL promoter. Individually expressed Pit-1 or C/EBPalpha, fused to color variants of fluorescent proteins, occupied different subnuclear compartments in living pituitary cells. When coexpressed, Pit-1 recruited C/EBPalpha from regions of transcriptionally quiescent centromeric heterochromatin to the nuclear regions occupied by Pit-1. The homeodomain region of Pit-1 was necessary for the recruitment of C/EBPalpha. A point mutation in the Pit-1 homeodomain associated with the syndrome of combined pituitary hormone deficiency in humans also failed to recruit C/EBPalpha. This Pit-1 mutant functioned as a dominant inhibitor of PRL gene transcription and, instead of recruiting C/EBPalpha, was itself recruited by C/EBPalpha to centromeric heterochromatin. Together our results suggest that the intranuclear positioning of these factors determines whether they activate or silence PRL promoter activity.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

POU domain factors in the neuroendocrine system: lessons from developmental biology provide insights into human disease

POU domain factors are transcriptional regulators characterized by a highly conserved DNA-binding domain referred to as the POU domain. The structure of the POU domain has been solved, facilitating the understanding of how these proteins bind to DNA and regulate transcription via complex protein-protein interactions. Several members of the POU domain family have been implicated in the control of development and function of the neuroendocrine system. Such roles have been most clearly established for Pit-1, which is required for formation of somatotropes, lactotropes, and thyrotropes in the anterior pituitary gland, and for Brn-2, which is critical for formation of magnocellular and parvocellular neurons in the paraventricular and supraoptic nuclei of the hypothalamus. While genetic evidence is lacking, molecular biology experiments have implicated several other POU factors in the regulation of gene expression in the hypothalamus and pituitary gland. Pit-1 mutations in humans cause combined pituitary hormone deficiency similar to that found in mice deleted for the Pit-1 gene, providing a striking example of how basic developmental biology studies have provided important insights into human disease.

The Pit-1beta domain dictates active repression and alteration of histone acetylation of the proximal prolactin promoter

A critical problem in current molecular biology is to gain a detailed understanding of the molecular mechanisms by which related transcription factor isoforms with identical DNA sequence specificity mediate distinct transcription responses. Pit-1 and Pit-1beta constitute such a pair of transcription factor isoforms. Pit-1 enhances the Ras signaling pathway to the prolactin promoter, and Pit-1beta represses basal prolactin promoter activity as well as Ras signaling to the prolactin promoter in pituitary cells. We have previously demonstrated that the beta-domain amino acid sequence dictates the transcriptional properties of Pit-1beta. Here, we show that five hydrophobic beta-domain residues are required for Pit-1 isoform-specific repression of Ras signaling, and we demonstrate that sodium butyrate and trichostatin A, pharmacological inhibitors of histone deacetylation, as well as viral Ski protein, a dominant-negative inhibitor of recruitment of N-CoR/mSin3 histone deacetylase complexes, specifically reverse beta isoform-specific repression of Ras signaling. Moreover, we directly demonstrate, with a chromatin immunoprecipitation assay, that the Pit-1beta isoform alters the histone acetylation state of the proximal prolactin promoter. This differential analysis of Pit-1/Pit-1beta isoform function provides significant insights into the structural determinants that govern how different transcription factors with identical DNA sequence specificity can display opposite effects on target gene activity.

The Pit-1 homeodomain and beta-domain interact with Ets-1 and modulate synergistic activation of the rat prolactin promoter

Pit-1/GHF-1 is a pituitary-specific, POU homeodomain transcription factor required for development of somatotroph, lactotroph, and thyrotroph cell lineages and regulation of the temporal and spatial expression of the growth hormone, prolactin (PRL), and thyrotropin-beta genes. Synergistic interaction of Pit-1 with a member of the Ets family of transcription factors, Ets-1, has been shown to be an important mechanism regulating basal and Ras-induced lactotroph-specific rat (r) PRL promoter activity. Pit-1beta/GHF-2, an alternatively spliced isoform containing a 26-amino acid insert (beta-domain) within its transcription-activation domain, physically interacts with Ets-1 but fails to synergize. By using a series of Pit-1 internal-deletion constructs in a transient transfection protocol to reconstitute rPRL promoter activity in HeLa cells, we have determined that the functional and physical interaction of Pit-1 and Ets-1 is mediated via the POU homeodomain, which is common to both Pit-1 and Pit-1beta. Although the Pit-1 homeodomain is both necessary and sufficient for direct binding to Ets-1 in a DNA-independent manner, an additional interaction surface was mapped to the beta-domain, specific to the Pit-1beta isoform. Thus, the unique transcriptional properties of Pit-1 and Pit-1beta on the rPRL promoter may be due to the formation of functionally distinct complexes of these two Pit-1 isoforms with Ets-1.

Differential activation of pituitary hormone genes by human Lhx3 isoforms with distinct DNA binding properties

Lhx3 is a LIM homeodomain transcription factor essential for pituitary development and motor neuron specification in mice. We identified two isoforms of human Lhx3, hLhx3a and hLhx3b, which differ in their ability to trans-activate pituitary gene targets. These factors are identical within the LIM domains and the homeodomain, but differ in their amino-terminal sequences preceding the LIM motifs. Both isoforms are localized to the nucleus and are expressed in the adult human pituitary, but gene activation studies demonstrate characteristic functional differences. Human Lhx3a trans-activated the alpha-glycoprotein subunit promoter and a reporter construct containing a high-affinity Lhx3 binding site more effectively than the hLhx3b isoform. In addition, hLhx3a synergized with the pituitary POU domain factor, Pit-1, to strongly induce transcription of the TSHbeta-subunit gene, while hLhx3b did not. We demonstrate that the differences in gene activation properties between hLhx3a and hLhx3b correlate with their DNA binding to sites within these genes. The short hLhx3b-specific amino-terminal domain inhibits DNA binding and gene activation functions of the molecule. These data suggest that isoforms of Lhx3 may play distinct roles during development of the mammalian pituitary gland and other neuroendocrine systems.