Adult pituitary stem cells: from pituitary plasticity to adenoma development

Phenotype Transformation of PitNETs

Phenotype transformation in pituitary neuroendocrine tumors is a little-known and unpredictable clinical phenomenon. Previous studies have not clearly defined and systematically concluded on the causes of this rare phenomenon. Additionally, the mechanisms of phenotype transformation are not well known. We reviewed cases reported in the literature with the aim of defining phenotype transformation in pituitary neuroendocrine tumors. We present an overview of the wide spectrum of phenotype transformation and its clinical features. We also discuss findings on the potential mechanism of this rare transformation, which may be related to PC1/3, the bioactivity of secretory hormones, gene mutations and the plasticity of pituitary neuroendocrine tumors. Clinicians should be aware of this rare phenomenon and more studies on the underlying mechanisms are required.

A Case of a Metastatic Pancreatic Neuroendocrine Tumor: A Surgical Conundrum Wrapped in Functionality's Embrace

This case study reports a rare case of a non-functioning metastatic pancreatic neuroendocrine tumor (pNET) transforming into a functioning pNET. A 59-year-old male, previously treated with distal pancreatectomy, splenectomy, lymph node dissection, liver metastasectomy, and pharmacotherapy, presented with weakness, hypoglycemia, and daily episodes of watery diarrhea. A functioning neuroendocrine liver metastasis expressing insulin and gastrin was identified. Surgical intervention, including left lateral hepatectomy and microwave ablation of multiple intrahepatic lesions, resulted in symptom resolution and uneventful recovery. However, metastatic liver disease re-emerged seven months post-surgery, necessitating chemotherapy. This case highlights the importance of vigilance for symptom development in non-functioning pNETs, signaling potential disease relapse and phenotype transformation, and suggests surgical treatment as a viable option in select cases.

Case Report: Giant insulinoma, a very rare tumor causing hypoglycemia

Insulinomas, with an incidence of 4 cases per million individuals per year, remain amongst the most frequent functional neuroendocrine tumors. The usual diameter of insulinomas usually remains under 3 cm of major axis. However, 44 exceptional cases of "giant insulinomas", have been reported worldwide, generally exceeding 9 cm in major axis. In this article, we report the case of a 38-year-old woman whom suffered from chronic hypoglycemia despite treatment with diazoxide. Abdominal CT-scan revealed a 88 x 73 mm mass located at the tail of the pancreas. Following surgical excision, histopathological analysis confirmed G1 neuroendocrine tumor, with focal cytoplasmic expression of insulin in tumor cells. After a 16-month follow-up period, the patient didn't address any specific complaint, and no disease recurrence and/or metastasis were observed. A ⁶⁸Ga-DOTATATE-PET scan was performed 6 months after surgery, which came back normal. Genetic evaluation has not been performed in our patient. The physiopathology of giant insulinomas remain unexplained, however with possible relationship with type 1 multiple endocrine neoplasia, sporadic somatic YY1 mutations and possible transformation of bulky non-functional pancreatic neuroendocrine tumors to a functional phenotype, with slow insulin secretion. While giant insulinomas remain rare in the literature, multicentric genetic analysis of tumor samples might reveal unique features of this rare subtype of neuroendocrine pancreatic tumors. Insulinomas of large size tend to have greater malignancy and higher rates of invasiveness. Careful follow-up, especially for liver and lymph node metastases, must be performed using functional imaging techniques to avoid disease relapse.

Malignant Insulinoma Arising From Nonfunctioning Pancreatic Neuroendocrine Tumor

Pancreatic neuroendocrine tumors are rare neoplasms characterized into nonfunctioning (NF-PNET) and functioning (F-PNET) subtypes. F-PNETs typically involve overt symptoms related to excessive hormone secretion but may rarely present first as NF-PNETs with delayed transformation. We present a patient with known NF-PNET with liver metastases who developed hypoglycemia 2 years after initial diagnosis due to malignant insulinoma. Hypoglycemia was refractory to continuous dextrose but improved temporarily after diazoxide and hepatic artery embolization. Malignant insulinomas are usually metastatic at presentation and portend poor prognosis. Hypoglycemia may be medically managed with steroids, somatostatin analogues, and diazoxide, along with therapies to reduce tumor burden.

Pituitary Remodeling Throughout Life: Are Resident Stem Cells Involved?

The pituitary gland has the primordial ability to dynamically adapt its cell composition to changing hormonal needs of the organism throughout life. During the first weeks after birth, an impressive growth and maturation phase is occurring in the gland during which the distinct hormonal cell populations expand. During pubertal growth and development, growth hormone (GH) levels need to peak which requires an adaptive enterprise in the GH-producing somatotrope population. At aging, pituitary function wanes which is associated with organismal decay including the somatopause in which GH levels drop. In addition to these key time points of life, the pituitary's endocrine cell landscape plastically adapts during specific (patho-)physiological conditions such as lactation (need for PRL) and stress (engagement of ACTH). Particular resilience is witnessed after physical injury in the (murine) gland, culminating in regeneration of destroyed cell populations. In many other tissues, adaptive and regenerative processes involve the local stem cells. Over the last 15 years, evidence has accumulated that the pituitary gland houses a resident stem cell compartment. Recent studies propose their involvement in at least some of the cell remodeling processes that occur in the postnatal pituitary but support is still fragmentary and not unequivocal. Many questions remain unsolved such as whether the stem cells are key players in the vivid neonatal growth phase and whether the decline in pituitary function at old age is associated with decreased stem cell fitness. Furthermore, the underlying molecular mechanisms of pituitary plasticity, in particular the stem cell-linked ones, are still largely unknown. Pituitary research heavily relies on transgenic in vivo mouse models. While having proven their value, answers to pituitary stem cell-focused questions may more diligently come from a novel powerful in vitro research model, termed organoids, which grow from pituitary stem cells and recapitulate stem cell phenotype and activation status. In this review, we describe pituitary plasticity conditions and summarize what is known on the involvement and phenotype of pituitary stem cells during these pituitary remodeling events.

Transcriptome and methylome analysis reveals three cellular origins of pituitary tumors

Pituitary adenomas (PA) are the second most common intracranial tumors. These neoplasms are classified according to the hormone they produce. The majority of PA occur sporadically, and their molecular pathogenesis is incompletely understood. The present transcriptomic and methylomic analysis of PA revealed that they segregate into three molecular clusters according to the transcription factor driving their terminal differentiation. First cluster, driven by NR5A1, consists of clinically non-functioning PA (CNFPA), comprising gonadotrophinomas and null cell; the second cluster consists of clinically evident ACTH adenomas and silent corticotroph adenomas, driven by TBX19; and the third, POU1F1-driven TSH-, PRL- and GH-adenomas, segregated together. Genes such as CACNA2D4, EPHA4 and SLIT1, were upregulated in each of these three clusters, respectively. Pathway enrichment analysis revealed specific alterations of these clusters: calcium signaling pathway in CNFPA; renin-angiotensin system for ACTH-adenomas and fatty acid metabolism for the TSH-, PRL-, GH-cluster. Non-tumoral pituitary scRNAseq data confirmed that this clustering also occurs in normal cytodifferentiation. Deconvolution analysis identify potential mononuclear cell infiltrate in PA consists of dendritic, NK and mast cells. Our results are consistent with a divergent origin of PA, which segregate into three clusters that depend on the specific transcription factors driving late pituitary cytodifferentiation.

Case Reports: Transformation of End-Stage Neuroendocrine Tumors With Uncontrollable Liver Metastasis Into a Novel or Additional Functional Phenotype

Background: Neuroendocrine tumors (NETs) are rare, but their worldwide incidence is gradually increasing. NETs are generally heterogeneous; however, in rare cases, they have been shown to change their phenotype (i.e., nonfunctional to functional or one functional phenotype to the addition of another functional phenotype). Here, we present two cases of liver metastatic NETs with phenotype transformation at the advanced stage that led to life-threatening events.

Case presentation: A 73-year-old woman had a small intestinal nonfunctional NET with liver metastasis. After uncontrollable liver metastasis at the advanced stage, she developed duodenal perforation with hypergastremia. The patient was treated with octreotide and proton pump inhibitors and underwent endoscopic closure for duodenal perforation, but her general condition gradually deteriorated, and she died 2 weeks after duodenal perforation. Another patient, a 50-year-old man, had a functional NET (gastrinoma) with liver metastasis and duodenal ulcer. After uncontrollable liver metastasis at the advanced stage, he developed hypoglycemia. Although octoreotide and diazoxide were administrated for hyperalimentation, his hypoglycemia was uncontrollable, and he died after 4 months owing to general deterioration.

Conclusion: The present cases show that advanced NETs with treatment-uncontrollable liver metastasis can transform their phenotype, specifically from a nonfunctional NET into a functional NET, and from one functional NET into the addition of another functional NET. These experiences suggest that the presence of treatment-resistant liver metastasis might be a hallmark of the potential to gain novel functions.

Recent Progress in Stem Cell Research of the Pituitary Gland and Pituitary Adenoma

Regenerative medicine and anti-tumoral therapy have been developed through understanding tissue stem cells and cancer stem cells (CSCs). The concept of tissue stem cells has been applied to the pituitary gland (PG). Recently, PG stem cells (PGSCs) were successfully differentiated from human embryonic stem cells and induced pluripotent stem cells, showing an in vivo therapeutic effect in a hypopituitary model. Pituitary adenomas (PAs) are common intracranial neoplasms that are generally benign, but treatment resistance remains a major concern. The concept of CSCs applies to PA stem cells (PASCs). Genetic alterations in human PGSCs result in PASC development, leading to treatment-resistant PAs. To determine an efficient treatment against refractory PAs, it is of paramount importance to understand the relationship between PGSCs, PASCs and PAs. The goal of this review is to discuss several new findings about PGSCs and the roles of PASCs in PA tumorigenesis.

Plasticity in medaka gonadotropes via cell proliferation and phenotypic conversion

Follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) produced by the gonadotropes play a major role in control of reproduction. Contrary to mammals and birds, Lh and Fsh are mostly produced by two separate cell types in teleost. Here, we investigated gonadotrope plasticity, using transgenic lines of medaka (Oryzias latipes) where DsRed2 and hrGfpII are under the control of the fshb and lhb promotors respectively. We found that Fsh cells appear in the pituitary at 8 dpf, while Lh cells were previously shown to appear at 14 dpf. Similar to Lh cells, Fsh cells show hyperplasia from juvenile to adult stages. Hyperplasia is stimulated by estradiol. Both Fsh and Lh cells show hypertrophy during puberty with similar morphology. They also share similar behavior, using their cellular extensions to make networks. We observed bi-hormonal gonadotropes in juveniles and adults but not in larvae where only mono-hormonal cells are observed, suggesting the existence of phenotypic conversion between Fsh and Lh in later stages. This is demonstrated in cell culture, where some Fsh cells start to produce Lhβ, a phenomenon enhanced by gonadotropin-releasing hormone (Gnrh) stimulation. We have previously shown that medaka Fsh cells lack Gnrh receptors, but here we show that with time in culture, some Fsh cells start responding to Gnrh, while fshb mRNA levels are significantly reduced, both suggestive of phenotypic change. All together, these results reveal high plasticity of gonadotropes due to both estradiol-sensitive proliferation and Gnrh promoted phenotypic conversion, and moreover, show that gonadotropes lose part of their identity when kept in cell culture.

Experimental Evidence and Clinical Implications of Pituitary Adenoma Stem Cells

Pituitary adenomas, accounting for 15% of diagnosed intracranial neoplasms, are usually benign and pharmacologically and surgically treatable; however, the critical location, mass effects and hormone hypersecretion sustain their significant morbidity. Approximately 35% of pituitary tumors show a less benign course since they are highly proliferative and invasive, poorly resectable, and likely recurring. The latest WHO classification of pituitary tumors includes pituitary transcription factor assessment to determine adenohypophysis cell lineages and accurate designation of adenomas, nevertheless little is known about molecular and cellular pathways which contribute to pituitary tumorigenesis. In malignant tumors the identification of cancer stem cells radically changed the concepts of both tumorigenesis and pharmacological approaches. Cancer stem cells are defined as a subset of undifferentiated transformed cells from which the bulk of cancer cells populating a tumor mass is generated. These cells are able to self-renew, promoting tumor progression and recurrence of malignant tumors, also conferring cytotoxic drug resistance. On the other hand, the existence of stem cells within benign tumors is still debated. The presence of adult stem cells in human and murine pituitaries where they sustain the high plasticity of hormone-producing cells, allowed the hypothesis that putative tumor stem cells might exist in pituitary adenomas, reinforcing the concept that the cancer stem cell model could also be applied to pituitary tumorigenesis. In the last few years, the isolation and phenotypic characterization of putative pituitary adenoma stem-like cells was performed using a wide and heterogeneous variety of experimental models and techniques, although the role of these cells in adenoma initiation and progression is still not completely definite. The assessment of possible pituitary adenoma-initiating cell population would be of extreme relevance to better understand pituitary tumor biology and to identify novel potential diagnostic markers and pharmacological targets. In this review, we summarize the most updated studies focused on the definition of pituitary adenoma stem cell phenotype and functional features, highlighting the biological processes and intracellular pathways potentially involved in driving tumor growth, relapse, and therapy resistance.

Gonadotrope plasticity at cellular, population and structural levels: A comparison between fishes and mammals

Often referred to as "the master gland", the pituitary is a key organ controlling growth, maturation, and homeostasis in vertebrates. The anterior pituitary, which contains several hormone-producing cell types, is highly plastic and thereby able to adjust the production of the hormones governing these key physiological processes according to the changing needs over the life of the animal. Hypothalamic neuroendocrine control and feedback from peripheral tissues modulate pituitary cell activity, adjusting levels of hormone production and release according to different functional or environmental requirements. However, in some physiological processes (e.g. growth, puberty, or metamorphosis), changes in cell activity may be not sufficient to meet the needs and a general reorganization of cell composition and pituitary structure may occur. Focusing on gonadotropes, this review examines plasticity at the cellular level, which allows precise and rapid control of hormone production and secretion, as well as plasticity at the population and structural levels, which allows more substantial changes in hormone production. Further, we compare current knowledge of the anterior pituitary plasticity in fishes and mammals in order to assess what has been conserved or not throughout evolution, and highlight important remaining questions.

Expression of P2X receptors in the rat anterior pituitary

In this study, the distribution patterns of P2X1 to P2X7 receptors in the anterior pituitary cells of rat were studied with single-, double-, and triple-labeling immunofluorescence, combined method of immunofluorescence and in situ hybridization, and Western blot. The results showed that the expression level of the P2X4 receptor protein was highest, followed by P2X5, P2X3, P2X2, P2X6, and P2X7 receptor proteins, but no P2X1 receptor protein was detected. Strong P2X4 receptor-immunoreactivity was detected in almost all the anterior pituitary cells. Different combinations of P2X receptors were detected in each individual cell type of the rat anterior pituitary. Gonadotrophs express P2X4, P2X5, and P2X6 receptors. Corticotrophs express P2X3 and P2X4 receptors. Folliculo-stellate cells express P2X2 and P2X4 receptors, and somatotrophs, lactotrophs, and thyrotrophs express only P2X4 receptors. The macrophages with Iba-1-ir expressed P2X7 receptors. The possible functions of these P2X receptors in each individual cell type of the rat anterior pituitary are discussed.

SOX2 mediates crosstalk between Sonic Hedgehog and the Wnt/β-catenin signaling pathway to promote proliferation of pituitary adenoma cells

The Sonic hedgehog (Shh) and Wnt/β-catenin signaling pathways are important regulators of early tissue patterning and stem cell propagation, and aberrant regulation of these two signaling pathways has been associated with a number of types of cancer. The identification of adult stem cells in the pituitary raised the question of whether these two signaling pathways are involved in pituitary adenoma formation. In the present study, it was identified that treating a human pituitary adenoma cell line with Shh was not able to promote the proliferation of cancer cells, but Shh was able to upregulate the expression of sex-determining region Y box 2 (SOX2) which is characteristic of stem cells. The addition of Shh into β-catenin-expressing cells also promoted cell proliferation. On the other hand, addition of Wnt3a into or overexpression of β-catenin in SOX2-expressing cancer cells was able to promote cell proliferation, Further investigation revealed that SOX2 is required to mediate crosstalk between the Shh and Wnt/β-catenin signaling pathways to promote the proliferation of pituitary adenoma cells.

SOX 2 Expression in Human Pituitary Adenomas-Correlations With Pituitary Function

BACKGROUND/AIM

The aim of this study was to evaluate SOX2 expression in pituitary adenomas and its correlation to their secretory state and clinicopathological parameters.

PATIENTS AND METHODS

Thirty-four patients were clinically evaluated and surgery was recommended for tumor removal. Histopathological diagnosis by hematoxylin eosin staining was followed by immunohistochemistry for pituitary hormones and SOX2 co-expression.

RESULTS

Fourteen of the 34 cases were GH-secreting adenomas, 10 were prolactinomas and 10 non-functioning pituitary adenomas. SOX2-positive expression was detected in 47.05% of total cases: 8 GH-secreting adenomas (57.14%), 6 prolactinomas (60%) and 2 non-functioning adenomas (20%). SOX2 positivity was significantly higher amongst secreting adenomas (p=0.041). SOX2-negative tumors were significantly associated with corticotrophin deficiency (p=0.047) and gonadotrophin deficiency (p=0.041). No correlation with tumor size or extrasellar extension was detected.

CONCLUSION

SOX2 is differentially expressed in pituitary adenomas and influences the secretory state or clinical behavior of pituitary adenomas.

Primary Cultures from Human GH-secreting or Clinically Non-functioning Pituitary Adenomas

Pituitary adenomas are among the more frequent intracranial tumors usually treated with both surgical and pharmacological-based on somatostatin and dopamine agonists-approaches. Although mostly benign tumors, the occurrence of invasive behaviors is often detected resulting in poorer prognosis. The use of primary cultures from human pituitary adenomas represented a significant advancement in the knowledge of the mechanisms of their development and in the definition of the determinants of their pharmacological sensitivity. Moreover, recent studies identified also in pituitary adenomas putative tumor stem cells representing, according to the current hypothesis, the real cellular targets to eradicate most malignancies. In this protocol, we describe the procedure to establish primary cultures from human pituitary adenomas, and how to select, in vitro expand, and phenotypically characterize putative pituitary adenoma stem cells.

Anterior Pituitary Transcriptome Suggests Differences in ACTH Release in Tame and Aggressive Foxes

Domesticated species exhibit a suite of behavioral, endocrinological, and morphological changes referred to as "domestication syndrome." These changes may include a reduction in reactivity of the hypothalamic-pituitary-adrenal (HPA) axis and specifically reduced adrenocorticotropic hormone release from the anterior pituitary. To investigate the biological mechanisms targeted during domestication, we investigated gene expression in the pituitaries of experimentally domesticated foxes (Vulpes vulpes). RNA was sequenced from the anterior pituitary of six foxes selectively bred for tameness ("tame foxes") and six foxes selectively bred for aggression ("aggressive foxes"). Expression, splicing, and network differences identified between the two lines indicated the importance of genes related to regulation of exocytosis, specifically mediated by cAMP, organization of pseudopodia, and cell motility. These findings provide new insights into biological mechanisms that may have been targeted when these lines of foxes were selected for behavior and suggest new directions for research into HPA axis regulation and the biological underpinnings of domestication.

Stem cell therapy and its potential role in pituitary disorders

PURPOSE OF REVIEW

The pituitary gland is one of the key components of the endocrine system. Congenital or acquired alterations can mediate destruction of cells in the gland leading to hormonal dysfunction. Even though pharmacological treatment for pituitary disorders is available, exogenous hormone replacement is neither curative nor sustainable. Thus, alternative therapies to optimize management and improve quality of life are desired.

RECENT FINDINGS

An alternative modality to re-establish pituitary function is to promote endocrine cell regeneration through stem cells that can be obtained from the pituitary parenchyma or pluripotent cells. Stem cell therapy has been successfully applied to a plethora of other disorders, and is a promising alternative to hormonal supplementation for resumption of normal hormone homeostasis.

SUMMARY

In this review, we describe the common causes for pituitary deficiencies and the advances in cellular therapy to restore the physiological pituitary function.

Programmed cell senescence: role of IL-6 in the pituitary

IL-6 is a pleiotropic cytokine with multiple pathophysiological functions. As a key factor of the senescence secretome, it can not only promote tumorigenesis and cell proliferation but also exert tumor suppressive functions, depending on the cellular context. IL-6, as do other cytokines, plays important roles in the function, growth and neuroendocrine responses of the anterior pituitary gland. The multiple actions of IL-6 on normal and adenomatous pituitary function, cell proliferation, angiogenesis and extracellular matrix remodeling indicate its importance in the regulation of the anterior pituitary. Pituitary tumors are mostly benign adenomas with low mitotic index and rarely became malignant. Premature senescence occurs in slow-growing benign tumors, like pituitary adenomas. The dual role of IL-6 in senescence and tumorigenesis is well represented in pituitary tumor development, as it has been demonstrated that effects of paracrine IL-6 may allow initial pituitary cell growth, whereas autocrine IL-6 in the same tumor triggers senescence and restrains aggressive growth and malignant transformation. IL-6 is instrumental in promotion and maintenance of the senescence program in pituitary adenomas.

The Stem Cell Connection of Pituitary Tumors

Tumors in the pituitary gland are typically benign but cause serious morbidity due to compression of neighboring structures and hormonal disruptions. Overall, therapy efficiency remains suboptimal with negative impact on health and comfort of life, including considerable risk of therapy resistance and tumor recurrence. To date, little is known on the pathogenesis of pituitary tumors. Stem cells may represent important forces in this process. The pituitary tumors may contain a driving tumor stem cell population while the resident tissue stem cells may be directly or indirectly linked to tumor development and growth. Here, we will briefly summarize recent studies that afforded a glance behind the scenes of this stem cell connection. A better knowledge of the mechanisms underlying pituitary tumorigenesis is essential to identify more efficacious treatment modalities and improve clinical management.

Single-cell analysis reveals a nestin+ tendon stem/progenitor cell population with strong tenogenic potentiality

The repair of injured tendons remains a formidable clinical challenge because of our limited understanding of tendon stem cells and the regulation of tenogenesis. With single-cell analysis to characterize the gene expression profiles of individual cells isolated from tendon tissue, a subpopulation of nestin⁺ tendon stem/progenitor cells (TSPCs) was identified within the tendon cell population. Using Gene Expression Omnibus datasets and immunofluorescence assays, we found that nestin expression was activated at specific stages of tendon development. Moreover, isolated nestin⁺ TSPCs exhibited superior tenogenic capacity compared to nestin^- TSPCs. Knockdown of nestin expression in TSPCs suppressed their clonogenic capacity and reduced their tenogenic potential significantly both in vitro and in vivo. Hence, these findings provide new insights into the identification of subpopulations of TSPCs and illustrate the crucial roles of nestin in TSPC fate decisions and phenotype maintenance, which may assist in future therapeutic strategies to treat tendon disease.

Notch-Dependent Pituitary SOX2(+) Stem Cells Exhibit a Timed Functional Extinction in Regulation of the Postnatal Gland

Although SOX2⁺ stem cells are present in the postnatal pituitary gland, how they are regulated molecularly and whether they are required for pituitary functions remain unresolved questions. Using a conditional knockout animal model, here we demonstrate that ablation of the canonical Notch signaling in the embryonic pituitary gland leads to progressive depletion of the SOX2⁺ stem cells and hypoplastic gland. Furthermore, we show that the SOX2⁺ stem cells initially play a significant role in contributing to postnatal pituitary gland expansion by self-renewal and differentiating into distinct lineages in the immediate postnatal period. However, we found that within several weeks postpartum, the SOX2⁺ stem cells switch to an essentially dormant state and are no longer required for homeostasis/tissue adaptation. Our results present a dynamic tissue homeostatic model in which stem cells provide an initial contribution to the growth of the neonatal pituitary gland, whereas the mature gland can be maintained in a stem cell-independent fashion.

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Notch signaling is necessary to maintain Sox2⁺ stem cells in the pituitary gland

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Sox2⁺ cells and differentiated cells contribute to postnatal pituitary expansion

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Sox2⁺ stem cells prove to be dispensable for adult pituitary gland homeostasis

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Differentiated cells retain mitotic capacity and respond to physiological demands

Medical management of secretory syndromes related to gastroenteropancreatic neuroendocrine tumours

Although recent epidemiological evidence indicates that the prevalence of non-functioning gastroenteropancreatic (GEP) neuroendocrine tumours (NETs) is rising, a significant number of GEP-NETs still present with symptoms related to the secretion of biologically active substances leading to the development of distinct clinical syndromes. In the past, these syndromes were associated with substantial morbidity and mortality due to the lack of specific therapies; however, since the introduction of long-acting somatostatin analogues and medications such as proton pump inhibitors, their control has been greatly improved. As a result, nowadays, the main cause of morbidity and mortality in GEP-NETs is mostly directly related to tumour growth and the extent of metastatic disease. However, in some patients with functioning tumours and extensive disease, control of the secretory syndrome still remains problematic, necessitating the employment of several cytoreductive techniques, which may not always be sufficient. Recently, new agents directed against tumour growth, or exerting increased binding activity to receptors expressed in these tumours, or interfering with the synthetic pathway of some of the compounds secreted by these tumours, have been developed. Since there are no specific guidelines addressing the totality of the management of the secretory syndromes related to GEP-NETs, this review aims at critically analysing the medical management of previously recognised secretory syndromes; it also addresses areas of uncertainty, assesses the newer therapeutic developments and also addresses recently described but poorly characterised secretory syndromes related to GEP-NETs.

Phenotypical and Pharmacological Characterization of Stem-Like Cells in Human Pituitary Adenomas

The presence and functional role of tumor stem cells in benign tumors, and in human pituitary adenomas in particular, is a debated issue that still lacks a definitive formal demonstration. Fifty-six surgical specimens of human pituitary adenomas were processed to establish tumor stem-like cultures by selection and expansion in stem cell-permissive medium or isolating CD133-expressing cells. Phenotypic and functional characterization of these cells was performed (1) ex vivo, by immunohistochemistry analysis on paraffin-embedded tissues; (2) in vitro, attesting marker expression, proliferation, self-renewal, differentiation, and drug sensitivity; and (3) in vivo, using a zebrafish model. Within pituitary adenomas, we identified rare cell populations expressing stem cell markers but not pituitary hormones; we isolated and expanded in vitro these cells, obtaining fibroblast-free, stem-like cultures from 38 pituitary adenoma samples. These cells grow as spheroids, express stem cell markers (Oct4, Sox2, CD133, and nestin), show sustained in vitro proliferation as compared to primary cultures of differentiated pituitary adenoma cells, and are able to differentiate in hormone-expressing pituitary cells. Besides, pituisphere cells, apparently not tumorigenic in mice, engrafted in zebrafish embryos, inducing pro-angiogenic and invasive responses. Finally, pituitary adenoma stem-like cells express regulatory pituitary receptors (D2R, SSTR2, and SSTR5), whose activation by a dopamine/somatostatin chimeric agonist exerts antiproliferative effects. In conclusion, we provide evidence that human pituitary adenomas contain a subpopulation fulfilling biological and phenotypical signatures of tumor stem cells that may represent novel therapeutic targets for therapy-resistant tumors.

Functional Characteristics of Multipotent Mesenchymal Stromal Cells from Pituitary Adenomas

Pituitary adenomas are one of the most common endocrine and intracranial neoplasms. Although they are theoretically monoclonal in origin, several studies have shown that they contain different multipotent cell types that are thought to play an important role in tumor initiation, maintenance, and recurrence after therapy. In the present study, we isolated and characterized cell populations from seven pituitary somatotroph, nonhormonal, and lactotroph adenomas. The obtained cells showed characteristics of multipotent mesenchymal stromal cells as observed by cell morphology, cell surface marker CD90, CD105, CD44, and vimentin expression, as well as differentiation to osteogenic and adipogenic lineages. They are capable of growth and passaging under standard laboratory cell culture conditions and do not manifest any hormonal cell characteristics. Multipotent mesenchymal stromal cells are present in pituitary adenomas regardless of their clinical manifestation and show no considerable expression of somatostatin 1–5 and dopamine 2 receptors. Most likely obtained cells are a part of tissue-supportive cells in pituitary adenoma microenvironment.

Concise Review: Paracrine Role of Stem Cells in Pituitary Tumors: A Focus on Adamantinomatous Craniopharyngioma

The existence of tissue‐specific progenitor/stem cells in the adult pituitary gland of the mouse has been demonstrated recently using genetic tracing experiments. These cells have the capacity to differentiate into all of the different cell lineages of the anterior pituitary and self‐propagate in vitro and can therefore contribute to normal homeostasis of the gland. In addition, they play a critical role in tumor formation, specifically in the etiology of human adamantinomatous craniopharyngioma, a clinically relevant tumor that is associated with mutations in CTNNB1 (gene encoding β‐catenin). Mouse studies have shown that only pituitary embryonic precursors or adult stem cells are able to generate tumors when targeted with oncogenic β‐catenin, suggesting that the cell context is critical for mutant β‐catenin to exert its oncogenic effect. Surprisingly, the bulk of the tumor cells are not derived from the mutant progenitor/stem cells, suggesting that tumors are induced in a paracrine manner. Therefore, the cell sustaining the mutation in β‐catenin and the cell‐of‐origin of the tumors are different. In this review, we will discuss the in vitro and in vivo evidence demonstrating the presence of stem cells in the adult pituitary and analyze the evidence showing a potential role of these stem cells in pituitary tumors. Stem Cells2016;34:268–276

Pancreatic neuroendocrine tumors with transformation to insulinoma: an unusual presentation of a rare disease

Approximately 35% of the pancreatic neuroendocrine tumors (pNETs) are functional, the most common of which is an insulinoma. Rarely can initially nonfunctioning tumor undergo biological transformation to a hormone-secreting tumor with subsequent changes in the clinical picture. We present here three unique patients with long-standing pNETs who developed life-threatening hyperinsulinemic hypoglycemia along with tumor progression. In two of the patients, everolimus (Afinitor) was administered in an attempt to control both tumor growth and hypoglycemia. In two cases everolimus therapy resulted in the abolishment of hypoglycemia and induced significant tumor regression; however these beneficial responses were transient. These cases highlight the exceptional ability of pNETs to change biological behavior in parallel with disease progression. Our experience concurs with recently published studies demonstrating the utility of everolimus for the control of both hypoglycemia and tumor progression.

Disulfiram sensitizes pituitary adenoma cells to temozolomide by regulating O6-methylguanine-DNA methyltransferase expression

O6-methylguanine-DNA methyltransferase (MGMT) activity is responsible for temozolomide (TMZ) resistance in patients harboring aggressive pituitary adenomas. Recently, disulfiram (DSF) has been shown to induce the loss of MGMT protein and increase TMZ efficacy in glioblastoma cells, while CD133+ nestin+ cells isolated from the cell population have been implicated as pituitary adenoma stem-like cells. However, whether DSF is able to potentiate the cytotoxic effects of TMZ on human pituitary adenoma cells has not been investigated to date. In the present study, CD133+ nestin+ phenotype cells were isolated from primary cultured human pituitary adenoma cells using microbeads. It was found that DSF reduced MGMT protein expression and sensitized human pituitary adenoma cells and stem-like cells to TMZ in vitro, while the proteasome inhibitor PS-341 abrogated the inhibitory effect of DSF on MGMT in vitro. The sensitizing effect of DSF was also verified in primary cultured human pituitary adenoma cells in vivo. The results of the present study suggested that DSF can increase the efficacy of the anti-tumor effect of TMZ on human pituitary adenoma cells and CD133+ nestin+ stem like cells via the ubiquitin-proteasomal MGMT protein elimination route. DSF combined with TMZ may be an effective therapeutic strategy against aggressive pituitary adenomas.

Expression and clinical relevance of paired box protein 7 and sex determining region Y-box 2 in canine corticotroph pituitary adenomas

Pituitary-dependent hypercortisolism is a common endocrinopathy in dogs, caused by an adrenocorticotrophic hormone secreting pituitary tumour of the anterior or intermediate lobe. The prognosis of intermediate lobe adenomas is worse than that of anterior lobe adenomas, indicating the possible usefulness of melanotropic markers as prognosticators. Another possible origin of pituitary adenomas is found in cancer stem cells. The aim of the present study was to investigate the expression of melanotroph specific transcription factor paired box protein 7 (Pax7) and stem cell marker and reprogramming factor sex determining region Y-box 2 (Sox2) and to relate their expression to clinical parameters. The mean ± SD of labelling index (LI) for Pax7 was 8.6% ± 21.7% in the adenomas; 1/6 controls had positive staining (LI, 15.2%). For Sox2, the LI in the adenomas was 16.9% ± 15.2% and 19.5% ± 11.6% in the controls. Pax7 expression was significantly higher in enlarged pituitaries, compared to non-enlarged pituitaries (P = 0.05), but Pax7 or Sox2 immunopositivity did not correlate to other clinical parameters such as histological diagnosis, survival time or disease-free interval. Gene expression of Pax7 target genes, such as proconvertase 2 (PC2), pro-opiomelanocortin (POMC), and dopamine D2 receptor (DRD2), was significantly lower in the adenoma samples compared to normal tissue, indicating that Pax7 signalling was not activated in adenomas. It was suggested that Pax7 and Sox2 remain interesting targets for molecular investigations into their role in pituitary tumorigenesis, but were unsuitable as clinical prognosticators in dogs.

Expression of the pituitary stem/progenitor marker GFRα2 in human pituitary adenomas and normal pituitary

PURPOSE

Recent studies suggest that adult pituitary stem cells may play a role in pituitary tumorigenesis. We sought to explore whether the Glial cell-line derived neurotrophic factor receptor alpha 2 (GFRα2), a recently described pituitary stem/progenitor marker, might be differentially expressed in pituitary adenomas versus normal pituitary.

METHODS

The expression of GFRα2 and other members of the GFR receptor family (GFRα1, α3, α4) were analyzed using RT-PCR, western blot, and immunohistochemistry in 39 pituitary adenomas, 14 normal pituitary glands obtained at autopsy, and cDNA from 3 normal pituitaries obtained commercially.

RESULTS

GFRα2 mRNA was ~2.6 fold under-expressed in functioning adenomas (p < 0.01) and ~3.5 fold over-expressed in non-functioning adenomas (NFAs) (p < 0.05) compared to normal pituitary. Among NFAs, GFRα2 was significantly over-expressed (~5-fold) in the gonadotropinoma subtype only (p < 0.05). GFRα2 protein expression appeared to be higher in most NFAs, although there was heterogeneity in protein expression in this group. GFRα2 protein expression appeared consistently lower in functioning adenomas by IHC and western blot. In normal pituitary, GFRα2 was localized in Rathke's remnant, the putative pituitary stem cell niche, and in corticotropes.

CONCLUSION

Our results suggest that the pituitary stem cell marker GFRα2 is under-expressed in functioning adenomas and over-expressed in NFAs, specifically gonadotropinomas. Further studies are required to elucidate whether over-expression of GFRα2 in gonadotropinomas might play a role in pituitary tumorigenesis.

Pituitary stem cells: where do we stand?

Some 5 years ago, the stem cells of the adult pituitary gland were discovered. Subsequent in-depth characterization revealed expression of several stemness markers and embryo-typical factors. Now, the quest is open to decipher their role in the gland. When and how pituitary stem cells differentiate to contribute to the mature hormone-producing cell populations is not known. New research models support their involvement in cell regeneration after injury in the gland, and suggest a possible role in pituitary tumor formation. From their expression phenotype, pituitary stem cells seem to re-use embryonic developmental programs during the creation of new hormonal cells. Here, we will review the latest progression in the domain of pituitary stem cells, including the uncovering of some new molecular flavors and of the first potential functions. Eventually, we will speculate on their differentiation programs towards hormonal cells, with a particular focus on gonadotropes.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Mixed pituitary adenoma/craniopharyngioma: clinical, morphological, immunohistochemical and ultrastructural study of a case, review of the literature, and pathogenetic and nosological considerations

Mixed pituitary adenoma/craniopharyngiomas are very rare tumors. Their pathogenesis is still unclear and it is not known whether they are collision tumors derived from independent stem cells or whether they originate from a single stem cell undergoing divergent differentiation. The latter hypothesis is supported by the close commixture between the two tumor components with transition areas that has been previously described. However, "hybrid" cells with both pituitary adenoma and craniopharyngioma features have never been described. In this paper we report a case of mixed pituitary adenoma/craniopharyngioma observed in a 75-year-old woman presenting with diplopia and slight increase of serum prolactin, who underwent endoscopic endonasal trans-sphenoidal tumor resection. Histologically, the tumor was composed of a typical pituitary silent subtype 2 ACTH cell adenoma admixed with islands of adamantinomatous craniopharyngioma. Electron microscopy showed that, in addition to distinct silent subtype 2 ACTH and craniopharyngioma cells, there were "hybrid" cells, showing characteristics of both pituitary adenoma and craniopharyngioma, consisting of small dense secretory granules, bundles of cytoplasmic filaments, and desmosomes. This ultrastructural finding was also confirmed by the presence of cells showing nuclear p40 expression and chromogranin A immunoreactivity. The close commixture between the two components and the ultrastructural and immunohistochemical findings demonstrate a common histogenesis of the two components and support the classification of the neoplasm as a mixed tumor. The patient completely recovered and, 10 months after surgery, head MR confirmed the complete resection of the lesion.

Stem cells in the canine pituitary gland and in pituitary adenomas

Cushing's disease (CD) or pituitary-dependent hypercortisolism is a common endocrinopathy in dogs, with an estimated prevalence of 1 or 2 in 1000 dogs per year. It is caused by an adrenocorticotropic hormone secreting adenoma in the pars distalis or pars intermedia of the pituitary gland. The pituitary gland is a small endocrine gland located in the pituitary fossa. In the postnatal individual, the hypothalamus-pituitary axis plays a central role in maintaining homeostatic functions, like control of metabolism, reproduction, and growth. Stem cells are suggested to play a role in the homeostatic adaptations of the adult pituitary gland, such as the rapid specific cell-type expansion in response to pregnancy or lactation. Several cell populations have been suggested as pituitary stem cells, such as Side Population cells and cells expressing Sox2 or Nestin. These cell populations are discussed in this review. Also, stem and progenitor cells are thought to play a role in pituitary tumorigenesis, such as the development of pituitary adenomas in dogs. There are limited reports on the role of stem cells in pituitary adenomas, especially in dogs. Further studies are needed to identify and characterize this cell population and to develop specific cell targeting therapeutic strategies as a new way of treating canine CD.

Emerging Targets in Pituitary Adenomas: Role of the CXCL12/CXCR4-R7 System

Chemokines are chemotactic regulators of immune surveillance in physiological and pathological conditions such as inflammation, infection, and cancer. Several chemokines and cognate receptors are constitutively expressed in the central nervous system, not only in glial and endothelial cells but also in neurons, controlling neurogenesis, neurite outgrowth, and axonal guidance during development. In particular, the chemokine CXCL12 and its receptors, CXCR4 and CXCR7, form a functional network that controls plasticity in different brain areas, influencing neurotransmission, neuromodulation, and cell migration, and the dysregulation of this chemokinergic axis is involved in several neurodegenerative, neuroinflammatory, and malignant diseases. CXCR4 primarily mediates the transduction of proliferative signals, while CXCR7 seems to be mainly responsible for scavenging CXCL12. Importantly, the multiple intracellular signalling generated by CXCL12 interaction with its receptors influences hypothalamic modulation of neuroendocrine functions, although a direct modulation of pituitary functioning via autocrine/paracrine mechanisms was also reported. Both CXCL12 and CXCR4 are constitutively overexpressed in pituitary adenomas and their signalling induces cell survival and proliferation, as well as hormonal hypersecretion. In this review we focus on the physiological and pathological functions of immune-related cyto- and chemokines, mainly focusing on the CXCL12/CXCR4-7 axis, and their role in pituitary tumorigenesis. Accordingly, we discuss the potential targeting of CXCR4 as novel pharmacological approach for pituitary adenomas.

Sox2(+) stem/progenitor cells in the adult mouse pituitary support organ homeostasis and have tumor-inducing potential

Sox2(+) adult mouse pituitary cells can self-renew and terminally differentiate in vitro, but their physiological role in vivo and possible contribution to oncogenesis remain largely unknown. Using genetic lineage tracing, we show here that the Sox2(+) cell compartment of both the embryonic and adult pituitary contains stem/progenitor cells that are able to differentiate into all hormone-producing lineages and contribute to organ homeostasis during postnatal life. In addition, we show that targeted expression of oncogenic β-catenin in Sox2(+) cells gives rise to pituitary tumors, but, unexpectedly, the tumor mass is not derived from the Sox2(+) mutation-sustaining cells, suggesting a paracrine role of Sox2(+) cells in pituitary oncogenesis. Our data therefore provide in vivo evidence of a role for Sox2(+) stem/progenitor cells in long-term physiological maintenance of the adult pituitary, and highlight an unexpected non-cell-autonomous role for these cells in the induction of pituitary tumors.

Stem cells and cancer stem-like cells in endocrine tissues

Cancer stem-like cells are a subpopulation of self-renewing cells that are more resistant to chemotherapy and radiation therapy than the other surrounding cancer cells. The cancer stem cell model predicts that only a subset of cancer cells possess the ability to self-renew and produce progenitor cells that can reconstitute and sustain tumor growth. Evidence supporting the existence of cancer stem-like cells in the thyroid, pituitary, and in other endocrine tissues is rapidly accumulating. These cells have been studied using specific biomarkers including: CD133, CD44, Nestin, Nanog, and aldehyde dehydrogenase enzyme. Putative cancer stem-like cells can be studied in vitro using serum-free media supplemented with basic fibroblast growth factor and epidermal growth factor grown in low attachment plates or in extracellular matrix leading to sphere formation in vitro. Cancer stem-like cells can also be separated by fluorescent cell sorting and used for in vitro or in vivo studies. Injection of enriched populations of cancer stem-like cells (also referred to as tumor initiating cells) into immunodeficient mice results in growth of xenografts which express cancer stem-like biomarkers. Human cancer stem-like cells have been identified in thyroid cancer cell lines, in primary thyroid cancers, in normal pituitary, and in pituitary tumors. Other recent studies suggest the existence of stem cells and cancer stem-like cells in endocrine tumors of the gastrointestinal tract, pancreas, lungs, adrenal, parathyroid, and skin. New discoveries in this field may lead to more effective therapies for highly aggressive and lethal endocrine cancers.

Pituitary adenoma pathogenesis: an update

PURPOSE OF REVIEW

To describe the recent efforts to understand the molecular and genetic mechanisms involved in the tumorigenesis of pituitary adenomas.

RECENT FINDINGS

There is rapidly accumulating evidence for the roles of molecular abnormalities in pituitary adenoma tumorigenesis, including dysregulation of the cell cycle, signal transduction pathways, oncogenes and tumor suppressor genes. MicroRNAs have been identified as important participants in gene regulation and may have a role in therapy. Stem cells have also provided novel concepts for tumorigenesis and potentially treatment.

SUMMARY

Pituitary adenomas are relatively common neoplasms, whose pathogenesis is still poorly understood. Although considered by many as benign monoclonal proliferations, their clinical spectrum is diverse including hormone hypersecretion, and various degrees of invasiveness, suggesting multiple steps and mechanisms. This review summarizes recent development in the molecular tumorigenesis of pituitary adenomas emphasizing the dysregulation of the cell cycle components, tumor suppressor genes, oncogenes, stem cells and microRNAs.