Pituitary Cell Line AtT-20 Research Articles

THU083 Disulfiram Mediated Anti-tumor Effect In Pituitary Neuroendocrine Tumors By Inducing Cuproptosis

Abstract Disclosure: N. Huang: None. Y. Zhang: None. Y. Liu: None. Y. Feng: None. L. Xue: None. Z. Wu: None. Pituitary neuroendocrine tumors (PitNETs) are common intracranial tumors. Dopamine agonists and somatostatin analogues are the first-line drugs for PRL and GH-secreting PitNETs clinically. However, there is no effective drug for other subtypes and drug-resistant PitNETs. Thus, it’s impending to discover new therapeutic targets and medicine. Cuproptosis is a copper-dependent form of cell death driven by lipoylated protein aggregation, loss of Fe-S cluster-containing proteins, and induction of HSP70. However, the role of cuproptosis in PitNETs is not clear. In this study, a total of 17 primary samples of PitNETs were collected to perform high-throughput drug screening, an anti-abuse drug disulfiram (DSF) was identified that can induce cell death effectively. We found that disulfiram also has a strong inhibitory effect on pituitary tumor cell lines MMQ, Att20 and GH3 (-87.1%, -62.5%, -92.7%, p<0.001), but the inhibitory effect was eliminated by copper chelator TTM/BCS. This indicated that DSF played the anti-tumor effect requiring the participation of copper ions. To explore the underlying mechanism that DSF induces the cell death of PitNETs, we detected the hallmark of cuproptosis and found that DSF leads to a decrease in the acylation level of proteins related to the tricarboxylic acid cycle, limiting the function of the protein. On the one hand, DSF leads to the downregulation of iron-sulfur cluster-associated proteins, and the oligomerization of dihydrolipoamide S-acetyltransferase (DLAT), which is eventually accompanied by an increase in the protein level of heat shock protein 70 (HSP70) at protein and mRNA level, causing proteotoxic stress leading to cell death. Finally, DSF treatment inhibited the tumor growth in MMQ cell transplanted mouse model and destructed the morphology of the pituitary tumor 3D sphere similarly demonstrating the inhibition effect. In conclusion, we find that the anti-abuse drug disulfiram can induce coproptosis in pituitary tumor cells in a copper-dependent manner and is a potential drug for the treatment of PitNETs. Presentation: Thursday, June 15, 2023

Transcription factor Creb3l1 regulates the synthesis of prohormone convertase enzyme PC1/3 in endocrine cells.

Transcription factor cAMP responsive element‐binding protein 3 like 1 (Creb3l1) is a non‐classical endoplasmic reticulum stress molecule that is emerging as an important component for cellular homeostasis, particularly within cell types with high peptide secretory capabilities. We have previously shown that Creb3l1 serves an important role in body fluid homeostasis through its transcriptional control of the gene coding for antidiuretic hormone arginine vasopressin in the neuropeptide‐rich magnocellular neurones of the supraoptic nucleus. In response to osmotic stimuli such as dehydration, vasopressin magnocellular neurones undergo remarkable transcriptome changes, including increased Creb3l1 expression, to ensure that the supply of vasopressin meets demand. To determine where else Creb3l1 fits into the secretory cell supply chain, we performed RNA‐sequencing of Creb3l1 knockdown anterior pituitary mouse corticotroph cell line AtT20. The target chosen for further investigation was Pcsk1, which encodes proprotein convertase enzyme 1 (PC1/3). PC1/3 is crucial for processing of neuropeptides and peptide hormones such as pro‐opiomelanocortin (POMC), proinsulin, proglucagon, vasopressin and oxytocin. Viral manipulations in supraoptic nuclei by over‐expression of Creb3l1 increased Pcsk1, whereas Creb3l1 knockdown decreased Pcsk1 expression. In vitro promoter activity and binding studies showed that Creb3l1 was a transcription factor of the Pcsk1 gene binding directly to a G‐box motif in the promoter. In the dehydrated rat anterior pituitary, Creb3l1 and Pcsk1 expression decreased in parallel compared to control, supporting our findings from manipulations in AtT20 cells and the supraoptic nucleus. No relationship was observed between Creb3l1 and Pcsk1 expression in the neurointermediate lobe of the pituitary, indicating a different mechanism of PC1/3 synthesis by these POMC‐synthesising cells. Therefore, Creb3l1, by regulating the expression of Pcsk1, does not control the processing of POMC peptides in the intermediate lobe.

Proton Sensitivity of Corticotropin-Releasing Hormone Receptor 1 Signaling to Proopiomelanocortin in Male Mice.

Because an acidic cellular microenvironment is engendered by inflammation and may determine cell differentiation, we elucidated the impact of acidic conditions on induction of proopiomelanocortin (POMC) expression. Here, we demonstrate mechanisms for proton sensitivity of CRH receptor 1 (CRHR1) signaling to POMC and ACTH production. Low pH (6.8) resulted in doubling of POMC expression and ACTH production in pituitary cell line AtT-20 and in primary mouse pituitary cells. Using CRISPR knockout, we show that CRHR1 is necessary for acid-induced POMC expression, and this induction is mediated by CRHR1 histidine residues and calmodulin-dependent protein kinase II in both pituitary corticotroph cells and in nonpituitary cell lines expressing ectopic ACTH. In contrast, CRH ligand binding affinity to CRHR1 was decreased with acidic pH, implying that proton-induced POMC expression prevails in acidic conditions independently of CRH ligand binding. The results indicate that proton-induced CRHR1 signaling regulates ACTH production in response to an acidic microenvironment.

Effects of anti-estrogens on cell invasion and survival in pituitary adenoma cells: A systematic study

Although the molecular mechanisms underlying the formation of pituitary adenomas are largely unknown, it is clear that estrogen plays a key role in the pathogenesis of pituitary adenomas. Though this is exemplified by an investigation of fulvestrant in the pituitary adenoma cell line GH3, no systematic studies on the effects of selective estrogen receptor modulators (SERMs) on functional properties of pituitary adenoma cell lines to modulate cell migration, cell invasion, and cell survival are available. Here we analyzed the effects of fulvestrant and three SERMs, bazedoxifene, clomifene, and raloxifene, on pituitary adenomas cell lines AtT20, TtT/GF, and GH3. In cell survival assays, clomifene was shown to be the most potent compound in all three cell lines with IC50 values ranging between 2, 6, and 10 μM, respectively, depending on the cell type. Raloxifene and bazedoxifene were also effective but to a lower extent. Also, all SERMs affected migratory and invasive behavior of pituitary adenoma cells. Mechanistically, treatment of cells with SERMs caused cell apoptosis, as demonstrated by Caspase 3/7 activity and western blot assays. In addition, western blots demonstrate activation of p53 in TtT/GF cells and loss of ERK1/2 activation in AtT20 cells. In contrast, fulvestrant was only effective in GH3 cells. Thus, the general applicability of SERMs for pituitary adenoma cells might be promising in clinical applications for the treatment of pituitary adenomas.

Rasd1, a small G protein with a big role in the hypothalamic response to neuronal activation.

BackgroundRasd1 is a member of the Ras family of monomeric G proteins that was first identified as a dexamethasone inducible gene in the pituitary corticotroph cell line AtT20. Using microarrays we previously identified increased Rasd1 mRNA expression in the rat supraoptic nucleus (SON) and paraventricular nucleus (PVN) of the hypothalamus in response to increased plasma osmolality provoked by fluid deprivation and salt loading. RASD1 has been shown to inhibit adenylyl cyclase activity in vitro resulting in the inhibition of the cAMP-PKA-CREB signaling pathway. Therefore, we tested the hypothesis that RASD1 may inhibit cAMP stimulated gene expression in the brain.ResultsWe show that Rasd1 is expressed in vasopressin neurons of the PVN and SON, within which mRNA levels are induced by hyperosmotic cues. Dexamethasone treatment of AtT20 cells decreased forskolin stimulation of c-Fos, Nr4a1 and phosphorylated CREB expression, effects that were mimicked by overexpression of Rasd1, and inhibited by knockdown of Rasd1. These effects were dependent upon isoprenylation, as both farnesyltransferase inhibitor FTI-277 and CAAX box deletion prevented Rasd1 inhibition of cAMP-induced gene expression. Injection of lentiviral vector into rat SON expressing Rasd1 diminished, whereas CAAX mutant increased, cAMP inducible genes in response to osmotic stress.ConclusionsWe have identified two mechanisms of Rasd1 induction in the hypothalamus, one by elevated glucocorticoids in response to stress, and one in response to increased plasma osmolality resulting from osmotic stress. We propose that the abundance of RASD1 in vasopressin expressing neurons, based on its inhibitory actions on CREB phosphorylation, is an important mechanism for controlling the transcriptional responses to stressors in both the PVN and SON. These effects likely occur through modulation of cAMP-PKA-CREB signaling pathway in the brain.

EZH2 is highly expressed in pituitary adenomas and associated with proliferation.

Enhancer of zeste homolog 2 (EZH2) is a core epigenetic regulator, playing a crucial role in cell cycle regulation. The protein is known to be associated with proliferation and worse outcome in several tumor entities. In this study, we immunohistochemically investigated the expression pattern of EZH2 in a large cohort of pituitary tumors. These results were correlated with clinical features and double immunofluorescence stainings (DIS) were conducted to evaluate co-expression of EZH2 and proliferation marker Ki-67. Furthermore, we analyzed the effect of EZH2 inhibition on cell proliferation in vitro using the pituitary cell line AtT-20. While in the normal anterior pituitary EZH2 was almost absent, the cohort of tumors showed enhanced expression levels (p ≤ 0.0005). This was positively associated with Ki-67 indices (r = 0.834, p ≤ 0.0005) and DIF confirmed a predominant co-expression of both markers. In vitro experiments revealed a significant (p ≤ 0.05) decrease of tumor cell proliferation using the EZH2 inhibitor GSK126. Our results further support that epigenetic events are involved in the pathogenesis and biology of pituitary adenomas (PA). Therefore, EZH2 may function as a new potential target for therapeutic interventions in PA.

Ketoconazole Treatment Decreases the Viability of Immortalized Pituitary Cell Lines Associated with an Increased Expression of Apoptosis-Related Genes and Cell Cycle Inhibitors.

Ketoconazole, which was initially developed as an antifungal agent, is a potent inhibitor of adrenal steroidogenesis and has therefore been used in the management of Cushing's disease. Surprisingly, the reduction of cortisol levels during ketoconazole treatment is not accompanied by the expected elevation in plasma adrenocorticotrophic hormone (ACTH) at the loss of negative cortisol feedback from corticotrophic cells, suggesting a direct effect of ketoconazole on these cells. To characterize the direct effects of ketoconazole, we evaluated its invitro effect on cell viability using the pituitary tumoural cell lines AtT-20 (which secretes ACTH), GH3 (which secretes growth hormone and prolactin) and αT3.1 (which secretes α-subunit) and we also determined the expression levels of genes involved in apoptosis and DNA replication by the quantitative reverse transcription polymerase chain reaction (qRT-PCR). We also evaluated ACTH levels in AtT-20 cells during ketoconazole treatment. We observed a ketoconazole concentration-dependent decrease in pituitary cell viability and reduced ACTH levels in AtT-20 cells after removal of the drug. We also observed increased expression of cell death receptors (e.g. Fas, tumour necrosis factor receptor) and caspases (e.g., caspase-6, caspase-7, caspase-9), suggesting activation of the apoptosis pathway. In addition, we observed increased gene expression of the cell cycle inhibitors p21 and p27 in GH3 cells and increased expression of p21 in αT3.1 cells. In conclusion, our findings suggest that ketoconazole significantly reduces cell viability in a concentration-dependent manner in pituitary tumour cell lines and is associated with an increase in apoptosis- and cell cycle regulation-related gene expression.

Pituitary tumors contain a side population with tumor stem cell-associated characteristics.

Pituitary adenomas cause significant endocrine and mass-related morbidity. Little is known about the mechanisms that underlie pituitary tumor pathogenesis. In the present study, we searched for a side population (SP) in pituitary tumors representing cells with high efflux capacity and potentially enriched for tumor stem cells (TSCs). Human pituitary adenomas contain a SP irrespective of hormonal phenotype. This adenoma SP, as well as the purified SP (pSP) that is depleted from endothelial and immune cells, is enriched for cells that express 'tumor stemness' markers and signaling pathways, including epithelial-mesenchymal transition (EMT)-linked factors. Pituitary adenomas were found to contain self-renewing sphere-forming cells, considered to be a property of TSCs. These sphere-initiating cells were recovered in the pSP. Because benign pituitary adenomas do not grow in vitro and have failed to expand in immunodeficient mice, the pituitary tumor cell line AtT20 was further used. We identified a SP in this cell line and found it to be more tumorigenic than the non-SP 'main population'. Of the two EMT regulatory pathways tested, the inhibition of chemokine (C-X-C motif) receptor 4 (CXCR4) signaling reduced EMT-associated cell motility in vitro as well as xenograft tumor growth, whereas the activation of TGFβ had no effect. The human adenoma pSP also showed upregulated expression of the pituitary stem cell marker SOX2. Pituitaries from dopamine receptor D2 knockout (Drd2(-/-)) mice that bear prolactinomas contain more pSP, Sox2(+), and colony-forming cells than WT glands. In conclusion, we detected a SP in pituitary tumors and identified TSC-associated characteristics. The present study adds new elements to the unraveling of pituitary tumor pathogenesis and may lead to the identification of new therapeutic targets.

Activation of the dopamine receptor type-2 (DRD2) promoter by 9-cis retinoic acid in a cellular model of Cushing's disease mediates the inhibition of cell proliferation and ACTH secretion without a complete corticotroph-to-melanotroph transdifferentiation.

Cushing's disease (CD) is a rare condition in which hypercortisolemia is secondary to excessive ACTH release from a pituitary corticotroph adenoma. CD is associated with significant morbidity and mortality, and a safe therapy that effectively targets the pituitary tumor is still lacking. Retinoic acid (RA) and dopamine agonists (DAs) have recently been considered as monotherapy in CD patients, and satisfactory results have been reported, albeit in a limited number of patients. Given the permissive role of RA on the dopamine receptor type-2 (DRD2), the aim of the present study was to see whether a combination of 9-cis RA and the DA bromocriptine (Br) might represent a possible treatment for CD. Here we show that 9-cis RA induces a functional DRD2 in the pituitary corticotroph cell line AtT20, and increases cell sensitivity to Br via a mechanism only partially related to corticotroph-to-melanotroph transdifferentiation. In addition, 9-cis RA and Br act synergistically to modulate cell viability, with favorable implications for clinical use. In nearly 45% of corticotropinoma-derived primary cultures, the combined administration of 9-cis RA and Br lowered the steady-state level of the ACTH precursor proopiomelanocortin (POMC) more efficiently than either of the drugs alone. In conclusion, the effects of a combination of 9-cis RA and Br on ACTH synthesis/secretion and cell viability in AtT20, and on POMC transcriptional activity in human corticotropinomas might represent a suitable starting point for assessing the potential of this treatment regimen for ACTH-secreting pituitary adenomas. This study thus has potentially important implications for novel therapeutic approaches to CD.

Agonist-biased Trafficking of Somatostatin Receptor 2A in Enteric Neurons

Somatostatin (SST) 14 and SST 28 activate somatostatin 2A receptors (SSTR2A) on enteric neurons to control gut functions. SST analogs are treatments of neuroendocrine and bleeding disorders, cancer, and diarrhea, with gastrointestinal side effects of constipation, abdominal pain, and nausea. How endogenous agonists and drugs differentially regulate neuronal SSTR2A is unexplored. We evaluated SSTR2A trafficking in murine myenteric neurons and neuroendocrine AtT-20 cells by microscopy and determined whether agonist degradation by endosomal endothelin-converting enzyme 1 (ECE-1) controls SSTR2A trafficking and association with β-arrestins, key regulators of receptors. SST-14, SST-28, and peptide analogs (octreotide, lanreotide, and vapreotide) stimulated clathrin- and dynamin-mediated internalization of SSTR2A, which colocalized with ECE-1 in endosomes and the Golgi. After incubation with SST-14, SSTR2A recycled to the plasma membrane, which required active ECE-1 and an intact Golgi. SSTR2A activated by SST-28, octreotide, lanreotide, or vapreotide was retained within the Golgi and did not recycle. Although ECE-1 rapidly degraded SST-14, SST-28 was resistant to degradation, and ECE-1 did not degrade SST analogs. SST-14 and SST-28 induced transient interactions between SSTR2A and β-arrestins that were stabilized by an ECE-1 inhibitor. Octreotide induced sustained SSTR2A/β-arrestin interactions that were not regulated by ECE-1. Thus, when activated by SST-14, SSTR2A internalizes and recycles via the Golgi, which requires ECE-1 degradation of SST-14 and receptor dissociation from β-arrestins. After activation by ECE-1-resistant SST-28 and analogs, SSTR2A remains in endosomes because of sustained β-arrestin interactions. Therapeutic SST analogs are ECE-1-resistant and retain SSTR2A in endosomes, which may explain their long-lasting actions.

Glucocorticoid-induced loss of DNA methylation in non-neuronal cells and potential involvement of DNMT1 in epigenetic regulation of Fkbp5

Glucocorticoids may play a significant role in the etiology of neuropsychiatric illnesses. Abnormalities in plasma cortisol levels, glucocorticoid sensitivity, and HPA-axis function often accompany clinical symptoms of stress-related illnesses such as PTSD and depression. Of particular interest are genetic association studies that link single nucleotide polymorphisms of HPA-axis genes with illnesses only in the context of an early-life trauma exposure such as child abuse. These studies suggest that dysregulation of HPA-axis function can have lasting repercussions in shaping mood and anxiety, long after termination of the traumatic experience. As persistent glucocorticoid-induced loss of DNA methylation in FK506 binding protein 5 (Fkbp5) was previously observed in the hippocampus and blood and in the neuronal cell line HT-22, we asked whether these epigenetic alterations occur in non-neuronal, HPA-axis relevant cells. We used the pituitary adenoma cell line AtT-20 to demonstrate that the intronic enhancer region of Fkbp5 undergoes loss of DNA methylation in response to dexamethasone treatment in a dose-dependent manner. We also focused on the mouse hippocampal dentate gyrus to test whether these changes would be enriched in a region implicated in the HPA-axis stress response, neurogenesis, and synaptic plasticity. We observed an increase in enrichment of DNA methylation loss in the dentate gyrus, as compared to whole hippocampal tissues that were similarly treated with glucocorticoids. We then asked whether DNA methyltransferase 1 (Dnmt1), a methyltransferase enzyme involved in maintaining DNA methylation following cell division, is involved in the observed epigenetic alterations. We found a dose-dependent decrease of Dnmt1 expression in the AtT-20 cells following dexamethasone treatment, and a similar decrease in corticosterone-treated mouse hippocampus. Taken together, we provide evidence that these glucocorticoid-induced epigenetic alterations have a broader validity in non-neuronal cells and that they may involve the DNA methylation machinery.

Tissue specific epigenetic differences in CRH gene expression

Corticotropin Releasing Hormone (CRH), a 41-amino acid peptide, is a major regulator of hypothalamic-pituitary-adrenal axis function. CRH also has important roles in several processes pertaining to pregnancy and parturition, including being a possible regulator of gestational length and predictor of pre-term birth. Regulation of the CRH promoter exhibits some tissue-specificities, the most well characterized example being glucocorticoids, which can stimulate placental CRH production but suppress hypothalamic CRH. In the last decade there has been growing interest in the role of epigenetic regulation of gene expression. Modification of the structure of chromatin is an example of epigenetic change affecting gene expression. We have found that inhibition of histone deacetylases results in an increase in CRH expression in the AtT20 pituitary cell line, but a decrease in CRH expression in the placenta. In this paper we review tissue specific differences in CRH gene expression, and discuss how epigenetic chromatin modification mechanisms can relate to tissue specific differences in expression of CRH.

Deducing corticotropin-releasing hormone receptor type 1 signaling networks from gene expression data by usage of genetic algorithms and graphical Gaussian models.

BackgroundDysregulation of the hypothalamic-pituitary-adrenal (HPA) axis is a hallmark of complex and multifactorial psychiatric diseases such as anxiety and mood disorders. About 50-60% of patients with major depression show HPA axis dysfunction, i.e. hyperactivity and impaired negative feedback regulation. The neuropeptide corticotropin-releasing hormone (CRH) and its receptor type 1 (CRHR1) are key regulators of this neuroendocrine stress axis. Therefore, we analyzed CRH/CRHR1-dependent gene expression data obtained from the pituitary corticotrope cell line AtT-20, a well-established in vitro model for CRHR1-mediated signal transduction. To extract significantly regulated genes from a genome-wide microarray data set and to deduce underlying CRHR1-dependent signaling networks, we combined supervised and unsupervised algorithms.ResultsWe present an efficient variable selection strategy by consecutively applying univariate as well as multivariate methods followed by graphical models. First, feature preselection was used to exclude genes not differentially regulated over time from the dataset. For multivariate variable selection a maximum likelihood (MLHD) discriminant function within GALGO, an R package based on a genetic algorithm (GA), was chosen. The topmost genes representing major nodes in the expression network were ranked to find highly separating candidate genes. By using groups of five genes (chromosome size) in the discriminant function and repeating the genetic algorithm separately four times we found eleven genes occurring at least in three of the top ranked result lists of the four repetitions. In addition, we compared the results of GA/MLHD with the alternative optimization algorithms greedy selection and simulated annealing as well as with the state-of-the-art method random forest. In every case we obtained a clear overlap of the selected genes independently confirming the results of MLHD in combination with a genetic algorithm.With two unsupervised algorithms, principal component analysis and graphical Gaussian models, putative interactions of the candidate genes were determined and reconstructed by literature mining. Differential regulation of six candidate genes was validated by qRT-PCR.ConclusionsThe combination of supervised and unsupervised algorithms in this study allowed extracting a small subset of meaningful candidate genes from the genome-wide expression data set. Thereby, variable selection using different optimization algorithms based on linear classifiers as well as the nonlinear random forest method resulted in congruent candidate genes. The calculated interacting network connecting these new target genes was bioinformatically mapped to known CRHR1-dependent signaling pathways. Additionally, the differential expression of the identified target genes was confirmed experimentally.

Induction of Hypothalamic Sirt1 Leads to Cessation of Feeding via Agouti-Related Peptide

Silent information regulator (SIR)2 is an nicotinamide adenine dinucleotide dependent deacetylase implicated in the regulation of life span in species as diverse as yeast, worms, and flies. Mammalian Sirt1 is the most closely related homolog of the SIR2 gene. Pharmacological activators of Sirt1 have been reported to increase the life span and improve the health of mice fed a high-fat diet and to reverse diabetes in rodents. Sirt1 links the energy availability status with cellular metabolism in peripheral organs including liver, pancreas, muscle, and white adipose tissue. Insulin and leptin signaling regulate food intake by controlling the expression of orexigenic and anorexigenic neuropeptides in the arcuate nucleus of the hypothalamus via Forkhead box O (Foxo)-1 and signal transducer and activator of transcription-3. Sirt1 has been reported to improve insulin sensitivity in vitro, but the role of hypothalamic Sirt1 in regulating feeding has not been addressed. We found that hypothalamic Sirt1 protein levels increase on feeding, and this induction is abrogated in diet-induced obese mice and db/db mice. We also demonstrate for the first time that Sirt1 protein turnover is regulated by the proteasome and ubiquitination in a hypothalamic cell line and in vivo by feeding, and this regulation is not seen in a pituitary cell line AtT20. Forced expression of wild-type Sirt1 in the mediobasal hypothalamus by adenovirus microinjection suppressed Foxo1-induced hyperphagia, a model for central insulin resistance. Moreover, Sirt1 suppressed Foxo1-dependent expression of the orexigenic neuropeptide Agouti-related peptide in vitro. We propose that on feeding, Sirt1 protein is stabilized in the hypothalamus, leading to decreased Foxo1-dependent expression of orexigenic neuropeptide Agouti-related peptide and cessation of feeding.

Genome-Wide Analysis in a Murine Dnmt1 Knockdown Model Identifies Epigenetically Silenced Genes in Primary Human Pituitary Tumors

DNA methylation at promoter CpG islands (CGI) is an epigenetic modification associated with inappropriate gene silencing in multiple tumor types. In the absence of a human pituitary tumor cell line, small interfering RNA-mediated knockdown of the maintenance methyltransferase DNA methyltransferase (cytosine 5)-1 (Dnmt1) was used in the murine pituitary adenoma cell line AtT-20. Sustained knockdown induced reexpression of the fully methylated and normally imprinted gene neuronatin (Nnat) in a time-dependent manner. Combined bisulfite restriction analysis (COBRA) revealed that reexpression of Nnat was associated with partial CGI demethylation, which was also observed at the H19 differentially methylated region. Subsequent genome-wide microarray analysis identified 91 genes that were significantly differentially expressed in Dnmt1 knockdown cells (10% false discovery rate). The analysis showed that genes associated with the induction of apoptosis, signal transduction, and developmental processes were significantly overrepresented in this list (P < 0.05). Following validation by reverse transcription-PCR and detection of inappropriate CGI methylation by COBRA, four genes (ICAM1, NNAT, RUNX1, and S100A10) were analyzed in primary human pituitary tumors, each displaying significantly reduced mRNA levels relative to normal pituitary (P < 0.05). For two of these genes, NNAT and S100A10, decreased expression was associated with increased promoter CGI methylation. Induced expression of Nnat in stable transfected AtT-20 cells inhibited cell proliferation. To our knowledge, this is the first report of array-based "epigenetic unmasking" in combination with Dnmt1 knockdown and reveals the potential of this strategy toward identifying genes silenced by epigenetic mechanisms across species boundaries.

Dysregulation of the Wnt/ß-catenin pathway in ACTH-secreting pituitary adenomas and its inhibition through SRIF-14 in AtT-20 cells

Introduction: Altered Wnt/ß-catenin signalling has recently been recognized to contribute to the development of various human cancers. Wnt stabilizes ß-catenin (free ß-catenin) by preventing its phosphorylation which targets ß-catenin for degradation. Free ß-catenin is translocated to the nucleus where it stimulates a number of genes that modulate proliferation and differentiation upon binding to TCF/LEF-1. Somatostatin (SRIH) has been proposed as a therapeutic agent in the medical treatment of adrenocorticotropin (ACTH)-secreting pituitary adenomas. The mechanisms, however, and the intracellular signalling pathways involved herein are only partly understood. The aim of the present study was to investigate a potential role of the Wnt/ß-catenin pathway in ACTH-secreting pituitary adenomas and its modulation through SRIH. Methods: The ACTH secreting mouse pituitary cell line AtT-20 and samples from 10 ACTH-secreting pituitary adenomas were studied by Western blot analyses and RT-PCR for the expression of ß-catenin, TCF-4 and cyclin D1. The effects of SRIF-14 and of corticotropin releasing hormone (CRH) on the expression of ß-catenin, TCF-4 and cyclin D1 were studied in AtT-20 cells. Results: In ACTH-secreting adenomas the expression of ß-catenin, TCF-4 and cyclin D1 was higher than in normal control tissue both at the RNA and protein level. In the ACTH-secreting cell line AtT20 treatment with SRIF-14 rapidly decreased beta-catenin, TCF-4 and cyclin D1 mRNA expression at 4–6h and caused a potent and long-lasting decrease at the protein level. In contrast, stimulation of AtT-20 cells with CRH significantly increased both the transcriptional and the translational responses of ß-catenin, TCF-4 and cyclin D1. Conclusions: We demonstrate that key elements of the Wnt/ß-catenin signalling pathway are expressed in ACTH-secreting pituitary cells. The expression of ß-catenin, TCF-4 and cyclin D1 is inhibited by SRIF-14 and stimulated by CRH, which suggests a functional role in corticotrope cell function. In human ACTH-secreting pituitary adenomas ß-catenin, TCF-4 and cyclin D1 expression is upregulated, which may contribute to their pathologic behaviour. Down-regulation of these genes by somatostatin or its analogs might exert beneficial effects corticotrope dysfunction Cushing's disease.

Coexpression of Somatostatin Receptor Subtype 5 Affects Internalization and Trafficking of Somatostatin Receptor Subtype 2

The somatostatin [somatotropin release-inhibiting factor (SRIF)] receptor subtypes sst(2A) and sst(5) are frequently coexpressed in SRIF-responsive cells, including endocrine pituitary cells. We previously demonstrated that sst(2A) and sst(5) exhibit different subcellular localizations and regulation of cell surface expression, although they have similar signaling properties. We investigated here whether sst(2A) and sst(5) functionally interact in cells coexpressing the two receptor subtypes. We stimulated both transfected cells stably expressing sst(2A) alone (CHO-sst(2A)) or together with sst(5) (CHO-sst(2A+5)) and the pituitary cell line AtT20, which endogenously expresses the two receptor subtypes, with either the nonselective agonist [D-Trp(8)]-SRIF-14 or the sst(2)-selective agonist L-779,976. In CHO-sst(2A) cells, stimulation with either ligand resulted in the loss of approximately 75% of cell surface SRIF binding sites and massive internalization of sst(2A) receptors. The cells were desensitized to subsequent stimulation with [D-Trp(8)]-SRIF-14, which failed to inhibit forskolin-evoked cAMP accumulation. Similarly, in CHO-sst(2A+5) and AtT20 cells, [D-Trp(8)]-SRIF-14 induced the loss of 60-70% of SRIF binding sites as well as massive sst(2A) endocytosis. By contrast, in cells expressing both sst(2A) and sst(5), selective stimulation of sst(2A) with L-779,976 resulted in only 20-40% loss of cell surface binding and markedly reduced sst(2A) internalization. Consequently, whereas CHO-sst(2A+5) and AtT20 cells stimulated with [D-Trp(8)]-SRIF-14 were desensitized to a second stimulation with the same agonist, cells prestimulated with L-779,976 were not desensitized to subsequent [D-Trp(8)]-SRIF-14 stimulation. These findings indicate that the presence of sst(5) in the same cells modulates trafficking and cell surface regulation of sst(2A) and cellular desensitization to the effects of SRIF.

Cell death mechanisms in AtT20 pituitary cells exposed to polychlorinated biphenyls (PCB 126 and PCB 153) and methylmercury

Polychlorinated biphenyls (PCBs) are persistent food contaminants that can have adverse effects on the endocrine and nervous systems, including the pituitary. In the present study, we have investigated cell death in the AtT20 pituitary cell line after exposure to coplanar PCB 126 and non-coplanar PCB 153. In addition, co-exposure to the PCBs and another neurotoxic food contaminant, methylmercury (MeHg), was studied to test possible interactive effects. Our results show that mainly necrosis is induced after exposure to the selected toxicants. Simultaneous exposure to moderately toxic doses of PCBs and MeHg resulted in additive or slightly synergistic effects on the induction of cell death. Furthermore, our data suggest that both PCB congeners trigger cell death in AtT20 cells via activation of calcium regulated calpains and lysosomal cathepsins, possibly through disruption of mitochondrial function and intracellular calcium signaling. However, caspase-activity appears not to be critical for PCB induced cell death in these cells. Presence of reactive oxygen species (ROS) and protective effects of pre-treatment with antioxidants were only found after MeHg exposure, suggesting that oxidative stress plays a major role in MeHg but not PCB toxicity in this experimental model.

G-protein-coupled glucocorticoid receptors on the pituitary cell membrane

Rapid, nongenomic actions of glucocorticoids (GCs) have been well documented, but information about putative membrane receptors that mediate them is scarce. We used fluorescence correlation spectroscopy to search for membrane GC-binding on the mouse pituitary cell line AtT-20. A slowly diffusing fraction (tau3; diffusion constant 3x10(-10) cm2 s-1) of fluorescein-labeled dexamethasone on the cell membrane corresponds to fluorescein-dexamethasone binding. Preincubation experiments were performed to test binding specificity: a 500-fold excess of unlabeled dexamethasone abolished subsequent fluorescein-dexamethasone membrane binding from 58+/-2 (control) to 8+/-1 (% of tau3, mean+/-s.e.m.), the natural ligand corticosterone prevented it partially (29+/-2), while the sex steroids estradiol (56+/-4) and progesterone (50+/-4) and the GC-receptor antagonist RU486 (56+/-2) had no effect. Preincubation with pertussis toxin resulted in disappearance of the slowest diffusion component (11+/-4) suggesting association of the receptor with a G-protein. Varying the concentration of fluorescein-dexamethasone showed that membrane binding is highly cooperative with an apparent Kd of 180 nM and Bmax of 230 nM. Taken together, these results demonstrate high-affinity GC-binding on the cell membrane of AtT-20 cells with characteristics distinct from intracellular binding.

An Extended Tyrosine‐Targeting Motif for Endocytosis and Recycling of the Dense‐Core Vesicle Membrane Protein Phogrin

Integral membrane proteins of neuroendocine dense-core vesicles (DCV) appear to undergo multiple rounds of exocytosis; however, their trafficking and site of incorporation into nascent DCVs is unclear. Previous studies with phogrin (IA-2beta) identified sorting signals in the luminal domain that is cleaved post-translationally; we now describe an independent DCV targeting motif in the cytosolic domain that may function at the level of endocytosis and recycling. Pulse-chase radiolabeling and cell surface biotinylation experiments in the pituitary corticotroph cell line AtT20 showed that the mature 60/65 kDa form that resides in the DCV is generated by limited proteolysis in a post-trans Golgi network compartment with similar kinetics to the formation of the principal cargo, ACTH. Phogrin is exposed on the cell surface in response to stimuli and progressively internalized to a perinuclear compartment that overlaps with recycling endosomes marked by transferrin. Chimeric molecules of phogrin transmembrane and cytosolic sequences with the interleukin-2 receptor alpha chain (Tac) were sorted to DCVs through the action of an extended tyrosine-based motif Y(654)QELCRQRMA located in a 27aa sequence adjacent to the membrane-spanning domain. A 36aa domain terminating in this sequence conferred DCV localization to Tac in the absence of any other cytosolic or luminal phogrin components. The endocytosis and DCV targeting of phogrin Y(654) > A mutants correlated with the impaired binding of the phogrin cytosolic tail to the micro-subunit of the AP2 adaptor complex in vitro.