The role of the hGH locus control region in somatotrope restriction of hGH-N gene expression

Dexamethasone Rescues an Acute High-Fat Diet-Induced Decrease in Human Growth Hormone Gene Expression in Male Partially Humanized CD-1 Mice

Obesity in puberty, already a time of insulin resistance, increases the risk of developing type 2 diabetes. Human (h) growth hormone (GH) levels also peak during puberty, where it contributes to growth and energy homeostasis through positive effects on maintaining pancreatic β cell mass. Thus, it is important to understand the effects of overeating and obesity on hGH production in puberty. Three days of overeating in young male adults or high-fat diet (HFD) in pubescent male transgenic (171hGH/CS) CD-1 mice containing the hGH gene (hGH-N) results in excess insulin and a decrease in hGH production. This reduction in these mice occurred during the light phase of the daily cycle, and was associated with decreased availability of the clock-related transcription factor Brain and Muscle ARNT-Like 1 (Bmal1). However, the HFD-induced decrease in hGH-N expression was blocked by forced daily swim activity, which is expected to increase glucocorticoid (GC) levels. The aim of the study was to assess whether GCs, specifically daily injections with a pharmacological dose of dexamethasone (DEX) in the light or dark phase of the daily cycle, can limit the negative effect of HFD for 3 days on hGH-N expression in male 171hGH/CS mice. DEX treatment increased or rescued hGH-N RNA levels, and was associated with elevated Bmal1 transcripts when assessed 12 h after final treatment, and at a time when serum corticosterone levels were suppressed >90%. In addition, a diet-dependent effect on hGH-N RNA levels was observed at 36 h after final treatment, but only in the light stage, presumably due to residual effects of DEX treatment and/or recovery of endogenous corticosterone levels. This is the first evidence for a direct effect of GCs on hGH-N expression in vivo and the ability to potentially limit the negative effect of overeating/obesity on hGH production in puberty.

Obesity and regulation of human placental lactogen production in pregnancy

The four genes coding for placental members of the human (h) growth hormone (GH) family include two that code independently for placental lactogen (PL), also known as chorionic somatomammotrophin hormone, one that codes for placental growth hormone (PGH) and a pseudogene for which RNA but no protein product is reported. These genes are expressed preferentially in the villus syncytiotrophoblast of the placenta in pregnancy. In higher primates, the placental members, including hPL and PGH, are the result of multiple duplication events of the GH gene. This contrasts with rodents and ruminants, where PLs result from duplication of the prolactin (PRL) gene. Thus, unlike their mouse counterparts, the hPL and PGH hormones bind both lactogenic and somatogenic receptors with varying affinity. Roles influenced by nutrient availability in both metabolic control in pregnancy and maternal behaviour are supported. However, the effect maternal obesity has on the activation of placental members of the hGH gene family, particularly the expression and function of those genes, is poorly understood. Evidence from partially humanised hGH/PL transgenic mice indicates that both the remote upstream hPL locus control region (LCR) and more gene-related regulatory regions are required for placental expression in vivo. Furthermore, a specific pattern of interactions between the LCR and hPL gene promoter regions is detected in term placenta chromatin from women with a normal body mass index (BMI) in the range 18.5-25 kg m^-2 by chromosome conformation capture assay. This pattern is disrupted with maternal obesity (class II BMI > 35 kg m^-2 ) and associated with a > 40% decrease in term hPL RNA levels, as well as serum hPL but not PRL levels, during pregnancy. The relative importance of the chromosomal architecture and predicted properties for transcription factor participation in terms of hPL production and response to obesity are considered, based on comparison with components required for efficient human pituitary GH gene expression.

Single-cell transcriptomic analysis of adult mouse pituitary reveals sexual dimorphism and physiologic demand-induced cellular plasticity

The anterior pituitary gland drives highly conserved physiologic processes in mammalian species. These hormonally controlled processes are central to somatic growth, pubertal transformation, fertility, lactation, and metabolism. Current cellular models of mammalian anteiror pituitary, largely built on candidate gene based immuno-histochemical and mRNA analyses, suggest that each of the seven hormones synthesized by the pituitary is produced by a specific and exclusive cell lineage. However, emerging evidence suggests more complex relationship between hormone specificity and cell plasticity. Here we have applied massively parallel single-cell RNA sequencing (scRNA-seq), in conjunction with complementary imaging-based single-cell analyses of mRNAs and proteins, to systematically map both cell-type diversity and functional state heterogeneity in adult male and female mouse pituitaries at single-cell resolution and in the context of major physiologic demands. These quantitative single-cell analyses reveal sex-specific cell-type composition under normal pituitary homeostasis, identify an array of cells associated with complex complements of hormone-enrichment, and undercover non-hormone producing interstitial and supporting cell-types. Interestingly, we also identified a Pou1f1-expressing cell population that is characterized by a unique multi-hormone gene expression profile. In response to two well-defined physiologic stresses, dynamic shifts in cellular diversity and transcriptome profiles were observed for major hormone producing and the putative multi-hormone cells. These studies reveal unanticipated cellular complexity and plasticity in adult pituitary, and provide a rich resource for further validating and expanding our molecular understanding of pituitary gene expression programs and hormone production.

The Transcription Factor NR4A2 Plays an Essential Role in Driving Prolactin Expression in Female Pituitary Lactotropes

Differentiation of the hormone-producing cells of the pituitary represents an informative model of cell fate determination. The generation and maintenance of 2 pituitary lineages, the growth hormone (GH)- producing somatotropes and the prolactin (PRL)- producing lactotropes, are dependent on the pituitary-specific transcription factor, POU1F1. While POU1F1 is expressed in both cell types, and plays a role in activation of both the Gh and Prl genes, expression of Gh and Prl is restricted to somatotropes and lactotropes, respectively. These observations imply the existence of additional factors that contribute to the somatotrope and lactotrope identities and their hormone expressions. Prior transcriptome analysis of primary somatotropes and lactotropes isolated from the mouse pituitary identified enrichment of a transcription factor, Nr4a2, in the lactotropes. Nr4a2 was shown in a cell culture model to bind the Prl promoter at a position adjacent to Pou1f1 and to synergize with Pou1f1 in driving Prl transcription. Here we demonstrate in vivo the role of Nr4a2 as an enhancer of Prl expression by conditional gene inactivation of the Nr4a2 gene in mouse lactotropes. We demonstrate that nuclear orphan receptor transcription factor (NR4A2) binding at the Prl promoter is dependent on actions of POU1F1; while POU1F1 is essential to loading polymerase (Pol) II on the Prl promoter, Nr4a2 plays a role in enhancing Pol II release into the Prl gene body. These studies establish an in vivo role of Nr4a2 in enhancing Prl expression in mouse lactotropes, explore its mechanism of action, and establish a system for further study of the lactotrope lineage in the pituitary.

Transcriptome Analyses of Female Somatotropes and Lactotropes Reveal Novel Regulators of Cell Identity in the Pituitary

The differentiation of the hormone-producing cell lineages of the anterior pituitary represents an informative model of mammalian cell fate determination. The generation and maintenance of two of these lineages, the GH-producing somatotropes and prolactin (PRL)-producing lactotropes, are dependent on the pituitary-specific transcription factor POU1F1. Whereas POU1F1 is expressed in both cell types, and plays a direct role in the activation of both the Gh and Prl genes, GH expression is restricted to somatotropes and PRL expression is restricted to lactotropes. These observations imply the existence of additional, cell type-enriched factors that contribute to the somatotrope and lactotrope cell identities. In this study, we use transgenic mouse models to facilitate sorting of somatotrope and lactotrope populations based on the expression of fluorescent markers expressed under Gh and Prl gene transcriptional controls. The transcriptomic analyses reveal a concordance of gene expression profiles in the two populations. The limited number of divergent mRNAs between the two populations includes a set of transcription factors that may have roles in pituitary lineage divergence and/or in regulating expression of cell type-specific genes after differentiation. Four of these factors were validated for lineage enrichment at the level of protein expression, two somatotrope enriched and two lactotrope enriched. Three of these four factors were shown to have corresponding activities in appropriate enhancement or repression of landmark genes in a cell culture model system. These studies identify novel regulators of the somatotropes and lactotropes, and they establish a useful database for further study of these lineages in the anterior pituitary.

Asymmetry between Activation and Deactivation during a Transcriptional Pulse

Transcription in eukaryotic cells occurs in gene-specific bursts or pulses of activity. Recent studies identified a spectrum of transcriptionally active “on-states,” interspersed with periods of inactivity, but these “off-states” and the process of transcriptional deactivation are poorly understood. To examine what occurs during deactivation, we investigate the dynamics of switching between variable rates. We measured live single-cell expression of luciferase reporters from human growth hormone or human prolactin promoters in a pituitary cell line. Subsequently, we applied a statistical variable-rate model of transcription, validated by single-molecule FISH, to estimate switching between transcriptional rates. Under the assumption that transcription can switch to any rate at any time, we found that transcriptional activation occurs predominantly as a single switch, whereas deactivation occurs with graded, stepwise decreases in transcription rate. Experimentally altering cAMP signalling with forskolin or chromatin remodelling with histone deacetylase inhibitor modifies the duration of defined transcriptional states. Our findings reveal transcriptional activation and deactivation as mechanistically independent, asymmetrical processes.

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Gene transcription switches between variable rates

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Single-cell microscopy and mathematical modeling quantifies switch dynamics

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We observe an asymmetry in the activation/deactivation of transcriptional bursts

Autonomous actions of the human growth hormone long-range enhancer

The human growth hormone (hGH) gene is controlled by a long-range enhancer, HSI, located 14.5 kb 5′ to the hGH promoter. HSI establishes a domain of noncoding transcription that is ‘looped’ to the hGH promoter as an essential step in initiating hGH gene expression. Thus, defining how HSI generates its domain of noncoding transcription is central to understanding its long-range function. Here, we demonstrate that activation of noncoding transcription reflects an HSI-autonomous activity fully independent of interactions with linked gene promoters and occurring in spatial and temporal synchrony with initiation of GH expression in the embryonic pituitary. HSI establishes its noncoding transcription start sites (TSS) over a defined distance from its core determinants and in a manner independent of local primary sequences. The interval between HSI and it TSS co-maps with a domain of disordered and/or highly mobile nucleosomes specific to the pituitary locus. Thus, a localized chromatin reconfiguration by HSI and consequent establishment of an adjacent domain of noncoding transcription constitute initiating events in long-range enhancer function within the hGH locus.

Growth hormone is a cellular senescence target in pituitary and nonpituitary cells

Premature proliferative arrest in benign or early-stage tumors induced by oncoproteins, chromosomal instability, or DNA damage is associated with p53/p21 activation, culminating in either senescence or apoptosis, depending on cell context. Growth hormone (GH) elicits direct peripheral metabolic actions as well as growth effects mediated by insulin-like growth factor 1 (IGF1). Locally produced peripheral tissue GH, in contrast to circulating pituitary-derived endocrine GH, has been proposed to be both proapoptotic and prooncogenic. Pituitary adenomas expressing and secreting GH are invariably benign and exhibit DNA damage and a senescent phenotype. We therefore tested effects of nutlin-induced p53-mediated senescence in rat and human pituitary cells. We show that DNA damage senescence induced by nutlin triggers the p53/p21 senescent pathway, with subsequent marked induction of intracellular pituitary GH in vitro. In contrast, GH is not induced in cells devoid of p53. Furthermore we show that p53 binds specific GH promoter motifs and enhances GH transcription and secretion in senescent pituitary adenoma cells and also in nonpituitary (human breast and colon) cells. In vivo, treatment with nutlin results in up-regulation of both p53 and GH in the pituitary gland, as well as increased GH expression in nonpituitary tissues (lung and liver). Intracrine GH acts in pituitary cells as an apoptosis switch for p53-mediated senescence, likely protecting the pituitary adenoma from progression to malignancy. Unlike in the pituitary, in nonpituitary cells GH exerts antiapoptotic properties. Thus, the results show that GH is a direct p53 transcriptional target and fulfills criteria as a p53 target gene. Induced GH is a readily measurable cell marker for p53-mediated cellular senescence.

Constitutive somatostatin receptor subtype 2 activity attenuates GH synthesis

Somatostatin signals predominantly through somatostatin receptor (SSTR) subtype 2 to attenuate GH release. However, the independent role of the receptor in regulating GH synthesis is unclear. Because we had previously demonstrated constitutive SSTR2 activity in mouse corticotrophs, we now analyzed GH regulation in rat pituitary somatotroph (GC) tumor cells, which express SSTR2 exclusively and are devoid of endogenous somatostatin ligand. We demonstrate that moderately stable SSTR2 overexpression (GpSSTR2(WT) cells) was associated with decreased GH promoter activity, GH mRNA, and hormone levels compared with those of control transfectants (GpCon cells). In contrast, levels of GH mRNA and peptide and GH promoter activity were unchanged in GpSSTR2(DRY) stable transfectants moderately expressing DRY motif mutated SSTR2 (R140A). GpSSTR(2DRY) did not exhibit an enhanced octreotide response as did GpSSTR2(WT) cells; however, both SSTR2(WT)-enhanced yellow fluorescent protein (eYFP) and SSTR2(DRY)-eYFP internalized on octreotide treatment. Suberoylanilide hydroxamic acid (SAHA), a histone deacetylase inhibitor, increased GH synthesis in wild-type GC cells and primary pituitary cultures. GpSSTR2(WT) cells induced GH synthesis more strongly on SAHA treatment, evident by both higher GH peptide and mRNA levels compared with the moderate but similar GH increase observed in GpCon and GpSSTR2(DRY) cells. In vivo SAHA also increased GH release from GpSSTR2(WT) but not from control xenografts. Endogenous rat GH promoter chromatin immunoprecipitation showed decreased baseline acetylation of the GH promoter with exacerbated acetylation after SAHA treatment in GpSSTR2(WT) compared with that of either GpSSTR(2DRY) or control cells, the latter 2 transfectants exhibiting similar GH promoter acetylation levels. In conclusion, modestly increased SSTR2 expression constitutively decreases GH synthesis, an effect partially mediated by GH promoter histone deacetylation.

Distinct chromatin configurations regulate the initiation and the maintenance of hGH gene expression

For many mammalian genes, initiation of transcription during embryonic development must be subsequently sustained over extensive periods of adult life. It remains unclear whether maintenance of gene expression reflects the same set of pathways as are involved in initial gene activation. The human pituitary growth hormone (hGH-N) locus is activated in the differentiating somatotrope midway through embryogenesis by a multicomponent locus control region (LCR). DNase I-hypersensitive site I (HSI) of the LCR is essential to full developmental activation of the hGH-N locus. Here we demonstrate that conditional deletion of HSI from the active hGH locus in the adult pituitary effectively silences hGH-N expression. Analyses of chromatin structure and locus positioning demonstrate that a specific subset of the HSI functions active in the embryo retain their HSI dependence in the adult pituitary. These functions sustain engagement of the hGH locus with polymerase II (Pol II) factories, histone acetylation at the hGH-N promoter, and looping of the LCR to its target promoter. These data reveal that HSI is essential to both the maintenance and the initiation phases of gene expression. These observations contribute to our mechanistic understanding of how stable patterns of mammalian gene expression are established in a terminally differentiated cell.

The prolactin gene: a paradigm of tissue-specific gene regulation with complex temporal transcription dynamics

Transcription of numerous mammalian genes is highly pulsatile, with bursts of expression occurring with variable duration and frequency. The presence of this stochastic or 'noisy' expression pattern has been relatively unexplored in tissue systems. The prolactin gene provides a model of tissue-specific gene regulation resulting in pulsatile transcription dynamics in both cell lines and endocrine tissues. In most cell culture models, prolactin transcription appears to be highly variable between cells, with differences in transcription pulse duration and frequency. This apparently stochastic transcription is constrained by a transcriptional refractory period, which may be related to cycles of chromatin remodelling. We propose that prolactin transcription dynamics result from the summation of oscillatory cellular inputs and by regulation through chromatin remodelling cycles. Observations of transcription dynamics in cells within pituitary tissue show reduced transcriptional heterogeneity and can be grouped into a small number of distinct patterns. Thus, it appears that the tissue environment is able to reduce transcriptional noise to enable coordinated tissue responses to environmental change. We review the current knowledge on the complex tissue-specific regulation of the prolactin gene in pituitary and extra-pituitary sites, highlighting differences between humans and rodent experimental animal models. Within this context, we describe the transcription dynamics of prolactin gene expression and how this may relate to specific processes occurring within the cell.

DNase I hypersensitive site II of the human growth hormone locus control region mediates an essential and distinct long-range enhancer function

Locus control regions (LCRs) comprise sets of DNA elements capable of establishing autonomous chromatin domains that support robust and physiologically appropriate expression of target genes, often working over extensive distances. Human growth hormone (hGH-N) expression in the pituitary is under the regulation of a well characterized LCR containing four DNase I hypersensitive sites (HSs). The two pituitary-specific HS, HSI and HSII, are located 14.5 and 15.5 kb 5' to the hGH-N promoter. HSI is essential for activation of hGH-N during pituitary development and for sustaining robust activity in the adult. To determine whether the closely linked HSII has a role in hGH-N expression, it was deleted from a previously validated hGH/P1 transgene. Analysis of three independent hGH/P1(ΔHSII) transgenic mouse lines revealed that this deletion had no adverse effect on the formation of HSI, yet resulted in a substantial loss (70%) in hGH-N mRNA expression. This loss of expression was accompanied by a corresponding reduction in recruitment of the pituitary-specific transcription factor Pit-1 to the hGH-N promoter and a selective decrease in promoter occupancy of the elongation-linked isoform of RNA polymerase II. Sufficiency of HSI and HSII in LCR activity was explored by establishing two additional sets of mouse transgenic lines in which DNA segments containing these HS were positioned within the λ phage genome. In this "neutral" DNA context, HSII was required for the recruitment of HAT activity. These data establish HSII as a nonredundant component of the hGH LCR essential for establishment of robust levels of hGH-N gene expression.