Thyrotropin releasing hormone stimulates transient phosphorylation of the tissue-specific transcription factor, Pit-1

GATA2 mediates thyrotropin-releasing hormone-induced transcriptional activation of the thyrotropin β gene

Thyrotropin-releasing hormone (TRH) activates not only the secretion of thyrotropin (TSH) but also the transcription of TSHβ and α-glycoprotein (αGSU) subunit genes. TSHβ expression is maintained by two transcription factors, Pit1 and GATA2, and is negatively regulated by thyroid hormone (T3). Our prior studies suggest that the main activator of the TSHβ gene is GATA2, not Pit1 or unliganded T3 receptor (TR). In previous studies on the mechanism of TRH-induced activation of the TSHβ gene, the involvements of Pit1 and TR have been investigated, but the role of GATA2 has not been clarified. Using kidney-derived CV1 cells and pituitary-derived GH3 and TαT1 cells, we demonstrate here that TRH signaling enhances GATA2-dependent activation of the TSHβ promoter and that TRH-induced activity is abolished by amino acid substitution in the GATA2-Zn finger domain or mutation of GATA-responsive element in the TSHβ gene. In CV1 cells transfected with TRH receptor expression plasmid, GATA2-dependent transactivation of αGSU and endothelin-1 promoters was enhanced by TRH. In the gel shift assay, TRH signal potentiated the DNA-binding capacity of GATA2. While inhibition by T3 is dominant over TRH-induced activation, unliganded TR or the putative negative T3-responsive element are not required for TRH-induced stimulation. Studies using GH3 cells showed that TRH-induced activity of the TSHβ promoter depends on protein kinase C but not the mitogen-activated protein kinase, suggesting that the signaling pathway is different from that in the prolactin gene. These results indicate that GATA2 is the principal mediator of the TRH signaling pathway in TSHβ expression.

The transcription factor prolactin regulatory element-binding protein mediates prolactin transcription induced by thyrotropin-releasing hormone in GH3 cells

The prolactin regulatory element-binding protein (PREB) is a transcription factor that regulates prolactin (PRL) promoter activity in the rat anterior pituitary. PRL gene expression and secretion are regulated by various hormones and growth factors, including dopamine, epidermal growth factor, and thyrotropin-releasing hormone (TRH). We examined the effect of TRH on PREB expression in pituitary cells. Western blots probed with a PREB-specific antiserum showed that the relative abundance of PREB in GH3 cells increased on treatment with TRH in a dose-dependent manner. The relative abundance of PREB mRNA also increased in a dose-dependent manner after treatment with TRH. TRH induced the expression of the luciferase reporter protein under the PREB promoter control. We used inhibitors of certain signal transduction pathways to show that TRH-induced PREB induction is sensitive to the protein kinase A (PKA) inhibitor. TRH stimulated the activity of the wild-type PRL promoter, whereas mutation of the PREB core-binding element on the PRL promoter reduced this ability. In summary, we have shown that TRH stimulated PREB expression in GH3 cells via the PKA pathway. PREB can function as a transcriptional regulator of PRL promoter activity and might be involved in TRH-induced PRL gene transcription.

Functional interactions with Pit-1 reorganize co-repressor complexes in the living cell nucleus

The co-repressor proteins SMRT and NCoR concentrate in specific subnuclear compartments and function with DNA-binding factors to inhibit transcription. To provide detailed mechanistic understanding of these activities, this study tested the hypothesis that functional interactions with transcription factors, such as the pituitary-gland-specific Pit-1 homeodomain protein, direct the subnuclear organization and activity of co-repressor complexes. Both SMRT and NCoR repressed Pit-1-dependent transcription, and NCoR was co-immunoprecipitated with Pit-1. Immunofluorescence experiments confirmed that endogenous NCoR is concentrated in small focal bodies and that incremental increases in fluorescent-protein-tagged NCoR expression lead to progressive increases in the size of these structures. In pituitary cells, the endogenous NCoR localized with endogenous Pit-1 and the co-expression of a fluorescent-protein-labeled Pit-1 redistributed both NCoR and SMRT into diffuse nucleoplasmic compartments that also contained histone deacetylase and chromatin. Automated image-analysis methods were applied to cell populations to characterize the reorganization of co-repressor proteins by Pit-1 and mutation analysis showed that Pit-1 DNA-binding activity was necessary for the reorganization of co-repressor proteins. These data support the hypothesis that spherical foci serve as co-repressor storage compartments, whereas Pit-1/co-repressor complexes interact with target genes in more widely dispersed subnuclear domains. The redistribution of co-repressor complexes by Pit-1 might represent an important mechanism by which transcription factors direct changes in cell-specific gene expression.

Oestrogen requires the insulin-like growth factor-I receptor for stimulation of prolactin synthesis via mitogen-activated protein kinase

Sex steroids and growth factors interact at the intracellular level in a variety of tissues to control numerous physiological functions. Oestrogen is known to stimulate prolactin synthesis and secretion, but the effect of insulin-like growth factor (IGF)-I is less clear. We used GH3 cells, a somatolactotroph cell line, to study the interaction of 17beta-oestradiol (E(2)) and IGF-I on prolactin protein levels and the intracellular mechanisms involved. Cell cultures were treated with E(2) (10 nM) and/or IGF-I (10 ng/ml) for 8 h. The real-time reverse transcriptase-polymerase chain reaction, Western blot and enzyme-immunoassay were used to determine changes in prolactin mRNA and protein levels. At this time-point, there were no significant changes in cell number, prolactin mRNA expression, or the amount of secreted prolactin. However, E(2) increased intracellular prolactin concentrations. IGF-I alone had no effect, but blocked the stimulatory effect of E(2). MAPK (ERK1/2) activation, as determined by Western blot analysis, increased with both E(2) and IGF-I, but not with the combination of these factors. The MAPK inhibitor PD98059 blocked the ability of E(2) to increase intracellular prolactin concentrations. Similarly, the IGF-I receptor antagonist, JB1, blocked the effect of E(2) on prolactin synthesis and MAPK activation, as did the oestrogen receptor antagonist ICI182 780. These results suggest that, to stimulate prolactin synthesis, E(2) activates the MAPK cascade and that this requires the presence of both oestrogen and IGF-I receptors.

The de novo Q167K mutation in the POU1F1 gene leads to combined pituitary hormone deficiency in an Italian patient

The POU1F1 gene encodes a transcription factor that is important for the development and differentiation of the cells producing GH, prolactin, and TSH in the anterior pituitary gland. Patients with POU1F1 mutations show a combined pituitary hormone deficiency with low or absent levels of GH, prolactin, and TSH. Fourteen mutations have been reported in the POU1F1 gene up to now. These genetic lesions can be inherited either in an autosomal dominant or an autosomal recessive mode. We report on the first Italian patient, a girl, affected by combined pituitary hormone deficiency. The patient was found to be positive for congenital hypothyroidism (with low TSH levels) at neonatal screening. Substitutive therapy was started, but subsequent growth was very poor, although psychomotor development was substantially normal. Hospitalized at 10 mo she showed hypotonic crises, growth retardation, delayed bone age, and facial dysmorphism. In addition to congenital hypothyroidism, GH and prolactin deficiencies were found. Mutation DNA analysis of the patient's POU1F1 gene identified the novel Q167K amino acid change at the heterozygous level. The highly conserved Q167 residue is located in the POU-specific domain. No mutation was detected in the other allele. DNA analysis in the proband's parents did not identify this amino acid substitution, suggesting a de novo genetic lesion. From these data it can be hypothesized that the Q167K mutation has a dominant negative effect.

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.

Identification of a conserved protein that interacts with specific LIM homeodomain transcription factors

Lhx3, a member of the LIM homeodomain family of transcription factors, is required for development of the pituitary and is implicated in the transcription of pituitary-specific hormone genes. In this report we describe a novel gene product, SLB, that selectively interacts with Lhx3 and the closely related LIM factor, Lhx4. The SLB cDNA encodes a 1749-residue protein that contains seven WD40 repeats near the amino terminus and a putative nuclear localization signal and does not contain other recognizable motifs. SLB is expressed in a tissue-specific manner with the highest concentrations of SLB mRNA in the testis and pituitary cells. We demonstrate that SLB specifically binds to Lhx3 and Lhx4 with high affinity both in vitro and in vivo. SLB has much lower affinity or no detectable affinity for other LIM domains. An expression vector for a fragment of SLB containing the LIM-interaction domain was shown to reduce expression of Lhx3-responsive reporter genes. The ability of the LIM-interacting domain of SLB to alter reporter gene activity as well as the tissue-specific expression and the specificity of SLB binding to LIM factors suggest a possible role in modulating the transcriptional activity of specific LIM factors.

Cloning and expression of the mouse Pse gene encoding a novel Ets family member

Human prostate-specific Ets (hPSE) is a novel Ets transcription factor and is exclusively expressed in human prostate glandular epithelium. To explore the role of PSE, we cloned the mouse Pse (mPse) and examined its pattern of expression. A sequence analysis indicated that mPse contains a conserved carboxy-terminal ETS DNA-binding domain and central Pointed domain, and the overall amino acid sequence shares 86% identity with that of hPSE. The ETS DNA-binding domain is highly conserved between human and mouse (98.8% sequence identity) and is similar to Drosophila dets4 (76.7% identity), but not similar to other Ets factors. A Northern blotting analysis revealed that mPse shows organ-specific expression. An in situ hybridization analysis of the prostate and intestine showed that mPse transcripts were present in their epithelial cells. mPse transactivates the promoter of the MASPIN gene in transient transfection assay. These results suggest that mPse encodes a novel Ets family member and is expressed in epithelial cells of restricted organs.

A role for the mitogen-activated protein kinase in mediating the ability of thyrotropin-releasing hormone to stimulate the prolactin promoter

The hypothalamic hormone, TRH, stimulates PRL secretion and gene transcription. We have examined the possibility that the mitogen-activated protein kinase (MAPK) may play a role in mediating TRH effects on the PRL gene. TRH was found to stimulate sustained activation of MAPK in PRL-producing, GH3 cells, consistent with a possible role in transcriptional regulation. A kinase-defective, interfering MAPK kinase (MAPKK) mutant reduced TRH induction of the PRL promoter. Treatment with the MAPKK inhibitor, PD98059, blocked TRH-induced activation of MAPK and also reduced TRH induction of a PRL-luciferase reporter gene, confirming that MAPK activation is necessary for TRH effects on PRL gene expression. Previous studies have demonstrated that the PRL promoter contains binding sites for members of the Ets family of transcription factors, which are important for mediating MAPK responsiveness of the PRL promoter. Mutation of specific Ets sites within the PRL promoter reduced responsiveness to both TRH and MAPK. The finding that DNA elements required for MAPK responsiveness of the PRL gene colocalize with DNA elements required for TRH responsiveness further supports a role for MAPK in mediating TRH effects on the PRL gene. We also explored the signaling mechanisms that link the TRH receptor to MAPK induction. Occupancy of the TRH receptor results in activation of protein kinase C (PKC) as well as increases in the concentration of Ca2+ due to release from intracellular stores and entry of Ca2+ through Ca2+ channels. A PKC inhibitor, GF109203X, and an L-type Ca2+ channel blocker, nimodipine, both partially reduced TRH-induced MAPK activation and PRL promoter activity. The effects of the two inhibitors were additive. These studies are consistent with a signaling pathway involving PKC- and Ca2+-dependent activation of MAPK, which leads to phosphorylation of an Ets transcription factor and activation of the PRL promoter.

Expression cloning and characterization of PREB (prolactin regulatory element binding), a novel WD motif DNA-binding protein with a capacity to regulate prolactin promoter activity

Previous studies have implied that a transcription factor(s) other than Pit-1 is involved in homeostatic regulation of PRL promoter activity via Pit-1-binding elements. One such element, 1P, was employed to clone from a rat pituitary cDNA expression library a novel 417-amino acid WD protein, designated PREB (PRL regulatory element binding) protein. PREB contains two PQ-rich potential transactivation domains, but no apparent DNA-binding motif, and exhibits sequence-specific binding to site 1P, to a site nonidentical to that for Pit-1. The PREB gene (or a related gene) is conserved, as an apparently single copy, in rat, human, fly, and yeast. A single approximately 1.9-kb PREB transcript accumulates in GH3 rat pituitary cells, to levels similar to Pit-1 mRNA. PREB transcripts were detected in all human tissues examined, but the observation of tissue-specific multiple transcript patterns suggests the possibility of tissue-specific alternative splicing. RT-PCR analysis of human brain tumor RNA samples suggested region-specific expression of PREB transcripts in brain. Western and immunocytochemical analysis implied that PREB accumulates specifically in GH3 cell nuclei. Transient transfection employing PREB-negative C6 rat glial cells showed that PREB is as active as, and additive with, Pit-1 in transactivation of a PRL promoter construct, and that PREB, but not Pit-1, can mediate transcriptional activation by protein kinase A (PKA). Expression in GH3 cells of a GAL4-PREB fusion protein both strongly transactivated a 5XGAL indicator construct and yielded a further stimulation of expression of this construct by coexpressed PKA, implying that PREB can mediate both basal and PKA-stimulated transcriptional responses in pituitary cells. These observations imply that PREB will prove to play a significant transcriptional regulatory role, both in the pituitary and in other organs in which transcripts of its gene are expressed.

Selective inhibition of prolactin gene transcription by the ETS-2 repressor factor

Regulation of prolactin gene transcription requires cooperative interactions between the pituitary-specific POU domain protein Pit-1 and members of the ETS transcription factor family. We demonstrate here that the ETS-2 repressor factor (ERF) is expressed in pituitary tumor cells and that overexpression of recombinant ERF inhibits prolactin promoter activity, but not the closely related growth hormone promoter. In non-pituitary cell lines, coexpression of ERF disrupts the cooperative interactions between Pit-1 and ETS-1 and blocks the induction of Pit-1-dependent prolactin promoter activity by cAMP. The potential role of ERF in the inhibitory response of the prolactin promoter to dopamine was examined using pituitary tumor cells stably expressing dopamine D2 receptors. The inhibitory responses of the prolactin promoter to ERF and dopamine are additive, suggesting that ERF has a complementary role in this hormonal response. A single Pit-1 DNA-binding element from the prolactin promoter is sufficient to reconstitute the inhibitory response to ERF. DNA binding analysis using either a composite Pit-1/ETS protein-binding site or a Pit-1 element with no known affinity for ETS proteins revealed that ERF interferes with Pit-1 binding. Together, these results demonstrate that ERF is a specific inhibitor of basal and hormone-regulated transcription of the prolactin gene and suggest a new level of complexity for the interaction of ETS factors with Pit-1 target genes.

Characterization of DNA regions mediating the ability of Ca2+/calmodulin dependent protein kinase II to stimulate prolactin promoter activity

The ability of Ca2+/calmodulin-dependent protein kinases (CaMKs) to regulate transcription of the rat prolactin (PRL) gene has been examined. We found that KN-62, a potent inhibitor of CaM kinases, blunted the ability of TRH to activate the prolactin promoter. Transfection experiments using expression plasmids for constitutively active forms of CaMKI, CaMKII, or CaMKIV show that CaMKII is the most effective activator of prolactin promoter expression. Deletion studies demonstrated that the upstream boundary of sequences necessary to respond to CaMKII is located within the distal enhancer of the prolactin gene. Neither the distal enhancer alone nor the proximal region of the prolactin gene are sufficient to mediate a response to CaMKII. Mutational analysis suggests that several Pit-1 binding sites contribute to CaMKII responsiveness. These findings suggest that CaMKII responsiveness of the prolactin promoter requires multiple factor binding sites in both the distal and proximal regions of the gene.

Regulation of activating transcription factor-1 and the cAMP response element-binding protein by Ca2+/calmodulin-dependent protein kinases type I, II, and IV

The ability of activating transcription factor-1 (ATF1) or the cAMP response element-binding protein (CREB) to enhance transcription can be stimulated by increases in intracellular Ca2+ concentrations. To identify protein kinases which may mediate the ability of Ca2+ to activate these transcription factors, we compared the ability of constitutively active forms of several Ca2+/calmodulin-dependent protein kinases (CaM kinases) to activate ATF1 or CREB. We find that constitutively active CaM kinase I and IV can activate both ATF1 and CREB. In addition, expression vectors for full-length CaM kinase I and IV were able to augment the ability of Ca2+ influx to activate ATF1 or CREB consistent with a role for these kinases in mediating transcriptional responses to Ca2+ signaling. In contrast, CaM kinase II was unable to activate either ATF1 or CREB. These findings provide a potential mechanism that may permit variation in the ability of ATF1 and CREB to respond to changes in intracellular Ca2+ concentrations depending on differences in the relative concentrations of specific CaM kinases.

Anterior pituitary cells defective in the cell-autonomous factor, df, undergo cell lineage specification but not expansion

The Ames dwarf mouse transmits a recessive mutation (df) resulting in a profound anterior pituitary hypocellularity due to a general lack of thyrotropes, somatotropes and lactotropes. These cell types are also dependent on the pituitary-specific transcription factor, Pit-1. We present evidence that expression of Pit-1 and limited commitment to these cells lineages occurs in df/df pituitaries. Thus, the crucial role of df may be in lineage-specific proliferation, rather than cytodifferentiation. The presence of all three Pit-1-dependent cell types in clonally derived clusters provides compelling evidence that these three lineages share a common, pluripotent precursor cell. Clusters containing different combinations of Pit-1-dependent cell types suggests that the Pit-1+ precursor cells choose from multiple developmental options during ontogeny. Characterization of df/df<-->+/+ chimeric mice demonstrated that df functions by a cell-autonomous mechanism. Therefore, df and Pit-1 are both cell-autonomous factors required for thyrotrope, somatotrope and lactotrope ontogeny, but their relative roles are different.

Activin increases phosphorylation and decreases stability of the transcription factor Pit-1 in MtTW15 somatotrope cells

Activin is a polypeptide growth factor which exerts endocrine, paracrine, and autocrine effects in a variety of tissues. In the pituitary somatotrope, activin represses proliferation and growth hormone (GH) biosynthesis and secretion. We previously demonstrated that decreases in GH biosynthesis in MtTW15 somatotrope cells are due at least in part to decreased binding of the tissue-specific transcription factor, Pit-1, to the GH promoter, resulting in decreased transcription of the GH gene. The objective of the current study was to determine the extent to which activin-mediated decreases in GH transcription were the result of decreased Pit-1 activity and/or decreased Pit-1 protein content in MtTW15 cells. Activin caused rapid increases in Pit-1 phosphorylation, which were temporally correlated with decreases in GH DNA binding. Pit-1 phosphorylation preceded marked decreases in steady-state levels of Pit-1 protein. The rate of Pit-1 synthesis was only moderately decreased by activin, with a time-course similar to that observed for decreases in GH biosynthesis. However, Pit-1 stability was markedly decreased after more than 4 h of activin treatment. These data demonstrate that activin decreases GH expression in MtTW15 cells through multilevel regulation of Pit-1, which may represent a more general mechanism whereby activin and other transforming growth factor beta family members modulate gene expression through regulation of transcription factor activity as well as content.

In vivo mutational analysis of the DNA binding domain of the tissue-specific transcription factor, Pit-1

Pit-1 is a member of the POU family of transcription factors, which contain a bipartite DNA binding domain. The DNA binding domain consists of a POU-specific domain and a POU homeodomain. Each of the subdomains can interact with DNA independently, but both subdomains are required for high affinity, sequence-specific DNA binding. To examine the contributions of individual amino acids to the function of the DNA binding domain of Pit-1, we developed an approach involving random, in vitro mutagenesis followed by functional screening in Saccharomyces cerevisiae. Using this strategy, we identified a number of point mutations that altered the function of the Pit-1 DNA binding domain. Mutations that altered Pit-1 function were found in both the POU-specific and the POU homeodomain. Most of the mutations involve amino acid residues that are conserved in POU factors. One of the more frequent kinds of mutation affected residues located in the hydrophobic core of the protein. Another common mutation involved amino acids that are thought to make specific contacts with DNA. These mutations define a number of amino acid residues that are important for the function of the DNA binding domain of Pit-1.

A composite Ets/Pit-1 binding site in the prolactin gene can mediate transcriptional responses to multiple signal transduction pathways

Binding sites for the tissue-specific transcription factor, Pit-1, are required for basal and hormonally induced prolactin gene transcription. Although Pit-1 is phosphorylated in response to several signaling pathways, the mechanism by which Pit-1 contributes to hormonal induction of gene transcription has not been defined. Recent reports suggest that phosphorylation of Pit-1 may not be required for hormonal regulation of the prolactin promoter. Analysis of the contribution of individual Pit-1 binding sites has been complicated due to the fact that some of the elements appear to be redundant. To better understand the role of Pit-1 sites in mediating hormonal regulation of the prolactin gene, we have performed enhancer tests using the three most proximal Pit-1 binding sites of the rat prolactin gene which are designated the 1P, 2P, and 3P sites. The results demonstrate that multimers of the 3P Pit-1 binding site are much more responsive to several hormonal and intracellular signaling pathways than multimers of the 1P or 2P sites. The 3P DNA element was found to contain a consensus binding site for the Ets family of proteins. Mutation of the Ets binding site greatly decreased the ability of epidermal growth factor, phorbol esters, Ras, or the Raf kinase to induce reporter gene activity. Mutation of the Ets site had little effect on basal enhancer activity. In contrast, mutation of the consensus Pit-1 binding site in the 3P element essentially abolished all basal enhancer activity. Overexpression of Ets-1 in GH3 pituitary cells enhanced both basal and Ras induced activity from the 3P enhancer. These data describe a composite element in the prolactin gene containing binding sites for two different factors and the studies suggest a mechanism by which Ets proteins and Pit-1 functionally cooperate to permit transcriptional regulation by different signaling pathways.