Cell lines of the pituitary gonadotrope lineage derived by targeted oncogenesis in transgenic mice

Study of the molecular and cellular biology of the gonadotropin hormones would be greatly facilitated by the availability of immortalized anterior pituitary gonadotrope cell lines. We directed expression of the simian virus-40 (SV40) T-antigen (Tag) oncogene to specific cells in the anterior pituitary of transgenic mice using the promoter/enhancer region from the human glycoprotein hormone alpha-subunit gene. Transgenic mice carrying this fusion gene developed anterior pituitary tumors. Clonal cell lines established from these tumors express the endogenous mouse alpha-subunit gene and synthesize and secrete alpha-subunit protein. However, they do not express beta-subunit genes. Alpha-subunit mRNA is induced by GnRH in a dose- and time-dependent manner, but is not regulated by TRH. Thus, we have targeted tumorigenesis in transgenic mice to anterior pituitary cells of the gonadotrope lineage to immortalize this specific endocrine cell while maintaining several highly differentiated functions unique to gonadotropes.

Pathogenesis of pituitary tumors

Pituitary adenomas may hypersecrete hormones (including prolactin, growth hormone and adrenocorticotropic hormone, and rarely follicle-stimulating hormone, luteinizing hormone or TSH) or may be nonfunctional. Despite their high prevalence in the general population, these tumors are invariably benign and exhibit features of differentiated pituitary cell function as well as premature proliferative arrest. Pathogenesis of dysregulated pituitary cell proliferation and unrestrained hormone hypersecretion may be mediated by hypothalamic, intrapituitary and/or peripheral factors. Altered expression of pituitary cell cycle genes, activation of pituitary selective oncoproteins or loss of pituitary suppressor factors may be associated with aberrant growth factor signaling. Considerable information on the etiology of these tumors has been derived from transgenic animal models, which may not accurately and universally reflect human tumor pathophysiology. Understanding subcellular mechanisms that underlie pituitary tumorigenesis will enable development of tumor aggression markers as well as novel targeted therapies.

CDK inhibitors p18(INK4c) and p27(Kip1) mediate two separate pathways to collaboratively suppress pituitary tumorigenesis

INK4 and CIP/KIP are two distinct families of cyclin-dependent kinase (CDK) inhibitors implicated in mediating a wide range of cell growth control signals. We have created p18(INK4c)-deficient mice. These mice develop gigantism and widespread organomegaly. The pituitary gland, spleen, and thymus are disproportionately enlarged and hyperplastic. T and B lymphocytes develop normally in p18-deficient mice, but both exhibit increased cellularity and a higher proliferative rate upon mitogenic stimulation. Loss of p18, like that of p27, but not other CDK inhibitor genes, leads to a gradual progression from intermediate lobe pituitary hyperplasia in young mice to an adenoma by 10 months of age with a nearly complete penetrance. Mice lacking both p18 and p27, like mice chimeric for Rb deficiency, invariably died from pituitary adenomas by 3 months. Hence, p18 and p27 mediate two separate pathways to collaboratively suppress pituitary tumorigenesis, likely by controlling the function of Rb.

Mechanisms for pituitary tumorigenesis: the plastic pituitary

The anterior pituitary gland integrates the repertoire of hormonal signals controlling thyroid, adrenal, reproductive, and growth functions. The gland responds to complex central and peripheral signals by trophic hormone secretion and by undergoing reversible plastic changes in cell growth leading to hyperplasia, involution, or benign adenomas arising from functional pituitary cells. Discussed herein are the mechanisms underlying hereditary pituitary hypoplasia, reversible pituitary hyperplasia, excess hormone production, and tumor initiation and promotion associated with normal and abnormal pituitary differentiation in health and disease.

Pituitary apoplexy: a review

The authors present a critical review of the literature on the hemorrhagic complications of pituitary adenomas, especially those leading to apoplexy. They emphasize the distinction between pituitary apoplexy, hemorrhages leading to sudden endocrine alterations, and asymptomatic hemorrhages. Moreover, they speculate upon the possible pathophysiology of pituitary apoplexy and its predisposing factors. The clinical presentation, natural history, radiological findings, and differential diagnosis are also discussed. Finally, the historical evolution of the treatment of pituitary apoplexy is reviewed, with emphasis on the surgical treatment.

Increased Wingless (Wnt) signaling in pituitary progenitor/stem cells gives rise to pituitary tumors in mice and humans

Wingless (Wnt)/β-catenin signaling plays an essential role during normal development, is a critical regulator of stem cells, and has been associated with cancer in many tissues. Here we demonstrate that genetic expression of a degradation-resistant mutant form of β-catenin in early Rathke's pouch (RP) progenitors leads to pituitary hyperplasia and severe disruption of the pituitary-specific transcription factor 1-lineage differentiation resulting in extreme growth retardation and hypopituitarism. Mutant mice mostly die perinatally, but those that survive weaning develop lethal pituitary tumors, which closely resemble human adamantinomatous craniopharyngioma, an epithelial tumor associated with mutations in the human β-catenin gene. The tumorigenic effect of mutant β-catenin is observed only when expressed in undifferentiated RP progenitors, but tumors do not form when committed or differentiated cells are targeted to express this protein. Analysis of affected pituitaries indicates that expression of mutant β-catenin leads to a significant increase in the total numbers of pituitary progenitor/stem cells as well as in their proliferation potential. Our findings provide insights into the role of the Wnt pathway in normal pituitary development and demonstrate a causative role for mutated β-catenin in an undifferentiated RP progenitor in the genesis of murine and human craniopharyngioma.

Clinical review: Diagnosis and management of pituitary carcinomas

Pituitary carcinomas are rare, making up some 0.2% of all pituitary tumors, but represent a particular challenge to clinical practice. The diagnosis of a pituitary carcinoma requires evidence of metastatic disease, either outside the central nervous system (CNS) or as separate noncontiguous foci within the CNS. They may present as typical pituitary adenomas, which reveal their malignant character only as time progresses, or as peculiarly aggressive tumors ab initio. Recent changes in histopathological classification have clarified many of the features of such tumors, including immunohistochemical staining for Ki-67 and p53, but to date none has been found to be pathognomonic. The majority of carcinomas are secretory, usually arising from corticotroph tumors or prolactinomas, but all histological types and secretory patterns are represented. Treatment is by surgery, transsphenoidal wherever possible, and conventional and stereotactic radiotherapy, but ultimately, a plethora of therapies may be required, including various attempts at medical therapy. Chemotherapy in some instances probably prolongs survival, but, in general, their progress from the diagnosis of carcinomatous changes is progressive and inexorable. However, we do not believe there will be any real prospect of long-term survival until the development and use of therapies targeted at specific molecular abnormalities.

Tumors of the nervous system and pituitary gland associated with atomic bomb radiation exposure

BACKGROUND

The risk of developing nervous system tumors following exposure to ionizing radiation is not well quantified. We characterized the incidence of nervous system tumors among atomic bomb survivors as a function of radiation dose.

METHODS

Tumors of the nervous system and pituitary gland diagnosed between 1958 and 1995 among 80 160 atomic bomb survivors were ascertained using the Hiroshima and Nagasaki tumor registries, medical records, and death certificates. Pathologists reviewed slides and medical records to provide histologic diagnoses. Poisson regression analyses were used to characterize radiation effects on tumor incidence, which are expressed as excess relative risk per sievert (ERR(Sv)). All statistical tests were two-sided.

RESULTS

A statistically significant dose-related excess of nervous system tumors was observed in the cohort (ERR(Sv) = 1.2, 95% confidence interval [CI] = 0.6 to 2.1). The highest ERR(Sv) was seen for schwannoma (4.5, 95% CI = 1.9 to 9.2). The risk for all other nervous system tumors as a group is also statistically significantly elevated (ERR(Sv) = 0.6, 95% CI = 0.1 to 1.3). Risk increases, although not statistically significant, were seen for meningiomas (ERR(Sv) = 0.6, 95% CI = -0.01 to 1.8), gliomas (ERR(Sv) = 0.6, 95% CI = -0.2 to 2.0), other nervous system tumors (ERR(Sv) = 0.5, 95% CI = <-0.2 to 2.2), and pituitary tumors (ERR(Sv) = 1.0, 95% CI = <-0.2 to 3.5). The dose-response relationships were linear. For nervous system tumors other than schwannoma, excess risks were higher for men than for women and for those exposed during childhood than for those exposed during adulthood.

CONCLUSIONS

A statistically significant dose response was observed for all nervous system tumors combined and for schwannoma considered separately, indicating that exposure to even moderate doses (i.e., <1 Sv) of radiation is associated with an elevated incidence of nervous system tumors.

Damage to hypothalamic dopaminergic neurons is associated with development of prolactin-secreting pituitary tumors

Old female rats with spontaneous prolactin-secreting pituitary tumors (prolactinomas) and young females with prolactinomas produced by prolonged estrogen treatment had damaged tuberoinfundibular dopaminergic neurons. Since these neurons inhibit the function of pituitary prolactin-secreting cells, their destruction may lead to development of prolactinomas.

Nelson's syndrome: frequency, prognosis, and effect of prior pituitary irradiation

Previous reports differ regarding the frequency and course of pituitary tumors occurring after adrenalectomy for bilateral adrenal hyperplasia (Nelson's syndrome). In this report, 120 patients who were adrenalectomized for bilateral adrenal hyperplasia were followed for 2 to 20 years. Nine of the 120 developed Nelson's syndrome (8%), the tumors appearing 6 months to 16 years after adrenalectomy. In the majority of cases, the course was benign; seven patients are living an average of 9.7 years after discovery of their tumors. Finally, contrary to previous reports, pituitary irradiation before adrenalectomy did not prevent Nelson's syndrome. Twenty of 120 patients had pituitary irradiation as the initial treatment for bilateral adrenal hyperplasia and two subsequently developed pituitary tumors. Thus, after adrenalectomy for bilateral adrenal hyperplasia, all patients, regardless of previous pituitary irradiation, should be followed indefinitely with periodic X rays of the sella turcica for the possible occurrence of Nelson's syndrome.

Pathophysiology of acromegaly

This review discusses the pathophysiology of acromegaly. Acromegaly has been classified in this paper into distinct entities based on etiology, ultrastructural features of the pituitary, and cytogenesis. This classification has been proposed based on clinical signs, immunoperoxidase techniques, transmission electromicroscopy and immunoelectron microscopy. Pituitary causes of acromegaly include densely granulated adenomas, sparsely granulated adenomas, mixed growth hormone and prolactin cell adenomas, acidophil stem cell adenomas, mammosomatotroph cell adenomas, and pleurihormonal adenomas. GH cell hyperplasia and GH cell carcinoma are also discussed. Extrapituitary causes of acromegaly include eutopic GH cell adenoma in the sphenoid sinus or parapharyngeal region and excess GHRF secretion which may be eutopic or ectopic. The pathological, clinical, and biochemical evidence in favor of a pituitary or hypothalamic etiology of acromegaly has been reviewed. Finally, a multistage theory of GH cell tumorigenesis has been proposed as a model in an attempt to unify the genetic, environmental and biochemical factors implicated in the pathogenesis of acromegaly.

Pituitary tumours: TSH-secreting adenomas

Thyrotropin-secreting pituitary adenomas (TSHomas) are a rare cause of hyperthyroidism and account for less than 2% of all pituitary adenomas. In the last years, the diagnosis has been facilitated by the routine use of ultra-sensitive TSH immunometric assays. Failure to recognise the presence of a TSHoma may result in dramatic consequences, such as improper thyroid ablation that may cause the pituitary tumour volume to further expand. The diagnosis mainly rests on dynamic testing, such as T3 suppression tests and TRH, which are useful in differentiating TSHomas from the syndromes of thyroid hormone resistance. The first therapeutical approach to TSHomas is the pituitary neurosurgery. The medical treatment of TSHomas mainly rests on the administration of somatostatin analogues, such as octreotide and lanreotide, which are effective in reducing TSH secretion in more than 90% of patients with consequent normalisation of FT4 and FT3 levels and restoration of the euthyroid state.

Targeted expression of a human pituitary tumor-derived isoform of FGF receptor-4 recapitulates pituitary tumorigenesis

It is estimated that up to one in five individuals develop pituitary gland tumors. Despite the common occurrence of these tumors, the pathogenetic mechanisms underlying their development remain largely unknown. We report the identification of a novel pituitary tumor-derived, N-terminally truncated isoform of FGF receptor-4 (ptd-FGFR4). The corresponding mRNA results from alternative transcription initiation and encodes a polypeptide that lacks a signal peptide and the first two extracellular Ig-like domains. ptd-FGFR4 has a distinctive cytoplasmic residence, is constitutively phosphorylated, and is transforming in vitro and in vivo. Here we show that targeted expression of ptd-FGFR4, but not FGFR4, results in pituitary tumors that morphologically recapitulate the human disease.

Pathology and pathogenesis of craniopharyngiomas

Craniopharyngiomas are benign but locally invasive tumours of the sellar region that occur as two subtypes. The adamantinomatous type (aCP) occurs mainly during childhood while the papillary type (pCP) is found almost exclusively in adults. It is thought that aCPs arise from ectopic embryonic remnants of Rathke's pouch and these tumours share features with odontogenic tumours suggesting a common origin. The pathogenesis of pCPs is less understood but these tumours may arise from metaplastic transformation of anterior pituitary epithelial cells. Mutations in CTNNB1 that encodes β-catenin are found in around 70 % of aCPs. These mutations stabilise β-catenin, which evades destruction and accumulates in the nucleus and cytosol leading to constitutive activation of the Wnt signaling pathway. Expression of mutant β-catenin early in mouse pituitary development promotes the formation of tumours similar to aCPs. However, accumulation of β-catenin occurs only in small clusters of tumour cells even though the mutation is ubiquitous. These cell clusters are slow-growing and share some characteristics with pituitary stem cells. They are often present at the invading edge and express growth factors that may participate in paracrine signaling to surrounding cells. β-Catenin nuclear translocation may also occur in the absence of CTNNB1 mutations, suggesting that other genetic or epigenetic events can activate Wnt signaling in aCP. These mechanisms, as well as those underlying the molecular pathogenesis of pCPs remain to be identified.

Craniopharyngioma: modern concepts in pathogenesis and treatment

PURPOSE OF REVIEW

Craniopharyngioma is a benign tumour. Its tendency to recur after excision and the high surgical risk due to involvement of the most vital structures of the brain mean that alternatives to radical surgery should be considered, namely limited surgical procedures followed by radiotherapy. Since both options present inherent risks, optimal craniopharyngioma treatment remains controversial. This paper aims to critically review the recent literature on craniopharyngioma.

RECENT FINDINGS

The management of children with craniopharyngioma has benefited from concerted efforts by national and international groups to improve outcome and reduce morbidity. From the current literature it is evident that there is a trend to better integrate all treatment modalities available, tailoring therapies to specific risk factors. Modern imaging and new surgical and radiotherapy techniques are increasing the possibility of cure. Biological markers are under investigation and this will increase our knowledge on craniopharyngioma.

SUMMARY

Studies on treatment, biology and pathogenesis of craniopharyngioma, available in the current literature, grew considerably in the last year. Although a consensus has not been reached on all aspects of this complex disease, there is a trend in the field to move quickly towards a better understanding of the disease to improve treatment strategies and to produce clinical cooperative trials.

Long-term exposure of E-mu-Pim1 transgenic mice to 898.4 MHz microwaves does not increase lymphoma incidence

A total of 120 E mu-Pim1 heterozygous mice and 120 wild-type mice were exposed for 1 h/day 5 days/week at each of the four exposure levels in "Ferris-wheel" exposure systems for up to 104 weeks to GSM-modulated 898.4 MHz radiation at SARs of 0.25, 1.0, 2.0 and 4.0 W/kg. In addition, 120 heterozygous and 120 wild-type mice were sham-exposed; there was also an unrestrained negative control group. Four exposure levels were used to investigate whether a dose-response effect could be detected. Independent verification confirmed that the exposures in the current study were nonthermal. There was no significant difference in the incidence of lymphomas between exposed and sham-exposed groups at any of the exposure levels. A dose-response effect was not detected. The findings showed that long-term exposures of lymphoma-prone mice to 898.4 MHz GSM radiofrequency (RF) radiation at SARs of 0.25, 1.0, 2.0 and 4.0 W/kg had no significant effects when compared to sham-irradiated animals. A previous study (Repacholi et al., Radiat. Res. 147, 631-640, 1997) reported that long-term exposure of lymphoma-prone mice to one exposure level of 900 MHz RF radiation significantly increased the incidence of non-lymphoblastic lymphomas when compared to sham-irradiated animals.

Pituitary lactotroph adenomas develop after prolonged lactotroph hyperplasia in dopamine D2 receptor-deficient mice

Tuberoinfundibular dopamine tonically inhibits PRL expression and secretion from the pituitary gland by the activation of dopamine D2 receptors (D2R) localized on lactotrophs. Mutant female mice that lack D2Rs have persistent hyperprolactinemia but also develop extensive hyperplasia of pituitary lactotrophs and peliosis of the adenohypophysis at 9 to 12 months of age, while age-matched male D2R-deficient mice have no morphologic adenohypophysial lesion. We now report that both female and male D2R-deficient mice 17 to 20 months of age develop pituitary lactotroph adenomas. Of 12 aged female mice examined, all developed monohormonal PRL-immunoreactive neoplasms that had a characteristic juxtanuclear Golgi pattern of PRL staining and loss of the reticulin fiber network. Several of these adenomas were 50-fold larger than normal glands with marked suprasellar extension and invasion of brain but no gross evidence of distant metastases. They also had striking peliosis that was more marked than the lesion seen in the hyperplastic pituitaries of the younger females. These findings demonstrate that a chronic loss of neurohormonal dopamine inhibition promotes the hyperplasia-neoplasia sequence in adenohypophysial lactotrophs. Our results are analogous to previous data indicating that protracted stimulation of adenohypophysial cells by hormones or growth factors results in proliferation with initial hyperplasia followed by the development of neoplasia. Six aged male D2R-deficient mice had slightly enlarged anterior pituitaries similar in size to normal female glands. However, each case exhibited multifocal, microscopic lactotroph adenomas with strong nuclear immunoreactivity for estrogen receptors and Pit-1 transcription factor. The unexpected development of adenomas in males without preexisting or concomitant hyperplasia suggests that prolonged loss of dopamine inhibition may also cause neoplasia by distinct cellular mechanisms in male and female animals.

Pituitary tumours: the prolactinoma

This review focus on the epidemiology, diagnosis and treatment of prolactinomas. In particular, attention was given to recent data showing a high prevalence of these tumours in the general population, 3-5 times higher than previously reported. The diagnosis of hyperprolactinaemia has been simplified in recent years, and only prolactin (PRL) assay and magnetic resonance imaging of the sella are required. Nonetheless, macroprolactinaemia should be assessed in patients with hyperprolactinaemia in the absence of clinical symptoms of elevated PRL levels. The recent evidence that medical therapy with dopamine agonists should be continued lifelong has been confirmed by several studied. The patients achieving disappearance of the tumours and suppression of PRL levels during treatment are those showing the highest likelihood to have persistent remission of hyperprolactinaemia after treatment withdrawal.

An unliganded thyroid hormone beta receptor activates the cyclin D1/cyclin-dependent kinase/retinoblastoma/E2F pathway and induces pituitary tumorigenesis

Thyroid-stimulating hormone (TSH)-secreting tumors (TSH-omas) are pituitary tumors that constitutively secrete TSH. The molecular genetics underlying this abnormality are not known. We discovered that a knock-in mouse harboring a mutated thyroid hormone receptor (TR) beta (PV; TRbeta(PV/PV) mouse) spontaneously developed TSH-omas. TRbeta(PV/PV) mice lost the negative feedback regulation with highly elevated TSH levels associated with increased thyroid hormone levels (3,3',5-triiodo-l-thyronine [T3]). Remarkably, we found that mice deficient in all TRs (TRalpha1(-/-) TRbeta(-/-)) had similarly increased T3 and TSH levels, but no discernible TSH-omas, indicating that the dysregulation of the pituitary-thyroid axis alone is not sufficient to induce TSH-omas. Comparison of gene expression profiles by cDNA microarrays identified overexpression of cyclin D1 mRNA in TRbeta(PV/PV) but not in TRalpha1(-/-) TRbeta(-/-) mice. Overexpression of cyclin D1 protein led to activation of the cyclin D1/cyclin-dependent kinase/retinoblastoma protein/E2F pathway only in TRbeta(PV/PV) mice. The liganded TRbeta repressed cyclin D1 expression via tethering to the cyclin D1 promoter through binding to the cyclic AMP response element-binding protein. That repression effect was lost in mutant PV, thereby resulting in constitutive activation of cyclin D1 in TRbeta(PV/PV) mice. The present study revealed a novel molecular mechanism by which an unliganded TRbeta mutant acts to contribute to pituitary tumorigenesis in vivo and provided mechanistic insights into the understanding of pathogenesis of TSH-omas in patients.

Persistent Prop1 expression delays gonadotrope differentiation and enhances pituitary tumor susceptibility

The 'paired'-like homeodomain transcription factor Prop1 is essential for the expansion of the pituitary primordia and for the differentiation and/or function of the hormone-producing cells of the anterior pituitary gland. Prop1 expression is normally extinguished before transcription of most differentiation markers is initiated. We report that constitutive expression of Prop1 interferes with anterior pituitary cell differentiation and increases the susceptibility for pituitary tumors. The terminal differentiation of pituitary gonadotropes is delayed, resulting in transient hypogonadism and a delay in the onset of puberty. Thyrotrope differentiation occurs normally, but thyrotrope function is impaired resulting in mild hypothyroidism. Aged mice exhibit defects consistent with misregulation of pituitary cell proliferation, including adenomatous hyperplasia with the formation of Rathke's cleft cysts and tumors. Thus, silencing Prop1 is important for normal pituitary development and function. These data suggest that gain-of-function mutations in PROP1 could contribute to the most common human pituitary endocrinopathies and tumors.