Human small cell lung cancer cell lines expressing the proopiomelanocortin gene have aberrant glucocorticoid receptor function

The Glucocorticoid Receptor: Isoforms, Functions, and Contribution to Glucocorticoid Sensitivity

Glucocorticoids exert pleiotropic effects on all tissues to regulate cellular and metabolic homeostasis. Synthetic forms are used therapeutically in a wide range of conditions for their anti-inflammatory benefits, at the cost of dose and duration-dependent side effects. Significant variability occurs between tissues, disease states, and individuals with regard to both the beneficial and deleterious effects. The glucocorticoid receptor (GR) is the site of action for these hormones and a vast body of work has been conducted understanding its function. Traditionally, it was thought that the anti-inflammatory benefits of glucocorticoids were mediated by transrepression of pro-inflammatory transcription factors, while the adverse metabolic effects resulted from direct transactivation. This canonical understanding of the GR function has been brought into question over the past 2 decades with advances in the resolution of scientific techniques, and the discovery of multiple isoforms of the receptor present in most tissues. Here we review the structure and function of the GR, the nature of the receptor isoforms, and the contribution of the receptor to glucocorticoid sensitivity, or resistance in health and disease.

Gene Mutations in Cushing's Syndrome

Background

Prolonged exposure to glucocorticoids can result in the development of Cushing's syndrome. Excess serum cortisol can occur due to several factors including exogenous steroids, pituitary and adrenal adenoma, and ectopic ACTH secretion.

Summary

The last 2 decades have seen significant progress in identifying new genetic and molecular mechanisms underlying hypercortisolemia. This has implicated mutations seen in a multitude of aberrant pathways that underpin the pathophysiology of Cushing's syndrome.

Key Messages

There is much overlap between the different, with mutations affecting well-understood molecular pathways such as the cAMP/PKA/MAPK and Wnt signalling systems. Further work should delineate the exact involvement of specific mutations in Cushing's syndrome and the effect of epigenetic/microenvironmental interactions. This could have implications for screening, as the identification of specific mutations may lead to earlier identification and subsequently improved prognosis.

β-Endorphin mediates radiation therapy fatigue

Fatigue is a common adverse effect of external beam radiation therapy in cancer patients. Mechanisms causing radiation fatigue remain unclear, although linkage to skin irradiation has been suggested. β-Endorphin, an endogenous opioid, is synthesized in skin following genotoxic ultraviolet irradiation and acts systemically, producing addiction. Exogenous opiates with the same receptor activity as β-endorphin can cause fatigue. Using rodent models of radiation therapy, exposing tails and sparing vital organs, we tested whether skin-derived β-endorphin contributes to radiation-induced fatigue. Over a 6-week radiation regimen, plasma β-endorphin increased in rats, paralleled by opiate phenotypes (elevated pain thresholds, Straub tail) and fatigue-like behavior, which was reversed in animals treated by the opiate antagonist naloxone. Mechanistically, all these phenotypes were blocked by opiate antagonist treatment and were undetected in either β-endorphin knockout mice or mice lacking keratinocyte p53 expression. These findings implicate skin-derived β-endorphin in systemic effects of radiation therapy. Opioid antagonism may warrant testing in humans as treatment or prevention of radiation-induced fatigue.

Molecular Derangements and the Diagnosis of ACTH-Dependent Cushing's Syndrome

Endogenous Cushing's syndrome (CS) is associated with morbidities (diabetes, hypertension, clotting disorders) and shortens life because of infections, pulmonary thromboembolism, and cardiovascular disease. Its clinical presentation is immensely variable, and diagnosis and treatment are often delayed. Thus, there are many opportunities for basic and clinical research leading to better tests, faster diagnosis, and optimized medical treatments. This review focuses on CS caused by excessive adrenocorticotropin (ACTH) production. It describes current concepts of the regulation of ACTH synthesis and secretion by normal corticotropes and mechanisms by which dysregulation occurs in corticotrope (termed "Cushing's disease") and noncorticotrope (so-called ectopic) ACTH-producing tumors. ACTH causes adrenal gland synthesis and pulsatile release of cortisol; the excess ACTH in these forms of CS leads to the hypercortisolism of endogenous CS. Again, the differences between healthy individuals and those with CS are highlighted. The clinical presentations and their use in the interpretation of CS screening tests are described. The tests used for screening and differential diagnosis of CS are presented, along with their relationship to cortisol dynamics, pathophysiology, and negative glucocorticoid feedback regulation in the two forms of ACTH-dependent CS. Finally, several gaps in current understanding are highlighted in the hope of stimulating additional research into this challenging disorder.

Glucocorticoid Receptor Antagonism Upregulates Somatostatin Receptor Subtype 2 Expression in ACTH-Producing Neuroendocrine Tumors: New Insight Based on the Selective Glucocorticoid Receptor Modulator Relacorilant

Somatostatin exhibits an inhibitory effect on pituitary hormone secretion, including inhibition of growth hormone and adrenocorticotropic hormone (ACTH), and it can have antisecretory and antitumor effects on neuroendocrine tumors (NETs) that express somatostatin receptors. Although the precise mechanism remains unclear, the finding that glucocorticoids downregulate somatostatin receptor subtype 2 (SSTR2) expression has been used to explain the lack of efficacy of traditional SSTR2-targeting analogs in patients with ACTH-secreting NETs. Glucocorticoid receptor (GR) antagonism with mifepristone has been shown to reverse the glucocorticoid-induced downregulation of SSTR2; however, the effects of GR modulation on SSTR2 expression in ACTH-secreting NETs, particularly corticotroph pituitary tumors, are not well known. The current study presents new insight from in vitro data using the highly selective GR modulator relacorilant, showing that GR modulation can overcome dexamethasone-induced suppression of SSTR2 in the murine At-T20 cell line. Additional data presented from clinical case observations in patients with ACTH-secreting NETs suggest that upregulation of SSTR2 via GR modulation may re-sensitize tumors to endogenous somatostatin and/or somatostatin analogs. Clinical, laboratory, and imaging findings from 4 patients [2 ACTH-secreting bronchial tumors and 2 ACTH-secreting pituitary tumors (Cushing disease)] who were treated with relacorilant as part of two clinical studies (NCT02804750 and NCT02762981) are described. In the patients with ectopic ACTH secretion, SSTR2-based imaging (Octreoscan and ⁶⁸Ga-DOTATATE positron emission tomography) performed before and after treatment with relacorilant showed increased radiotracer uptake by the tumor following treatment with relacorilant without change in tumor size at computed tomography. In the patients with Cushing disease who received relacorilant prior to scheduled pituitary surgery, magnetic resonance imaging after a 3-month course of relacorilant showed a reduction in tumor size. Based on these findings, we propose that GR modulation in patients with ACTH-secreting NETs upregulates previously suppressed SSTR2s, resulting in tumor-specific antisecretory and anti-proliferative effects. The effect of relacorilant on pituitary corticotroph tumors is being investigated in an ongoing phase 3 study (NCT03697109; EudraCT 2018-003096-35).

The Mechanisms Underlying Autonomous Adrenocorticotropic Hormone Secretion in Cushing's Disease

Cushing’s disease caused due to adrenocorticotropic hormone (ACTH)-secreting pituitary adenomas (ACTHomas) leads to hypercortisolemia, resulting in increased morbidity and mortality. Autonomous ACTH secretion is attributed to the impaired glucocorticoid negative feedback (glucocorticoid resistance) response. Interestingly, other conditions, such as ectopic ACTH syndrome (EAS) and non-neoplastic hypercortisolemia (NNH, also known as pseudo-Cushing’s syndrome) also exhibit glucocorticoid resistance. Therefore, to differentiate between these conditions, several dynamic tests, including those with desmopressin (DDAVP), corticotrophin-releasing hormone (CRH), and Dex/CRH have been developed. In normal pituitary corticotrophs, ACTH synthesis and secretion are regulated mainly by CRH and glucocorticoids, which are the ACTH secretion-stimulating and -suppressing factors, respectively. These factors regulate ACTH synthesis and secretion through genomic and non-genomic mechanisms. Conversely, glucocorticoid negative feedback is impaired in ACTHomas, which could be due to the overexpression of 11β-HSD2, HSP90, or TR4, or loss of expression of CABLES1 or nuclear BRG1 proteins. Genetic analysis has indicated the involvement of several genes in the etiology of ACTHomas, including USP8, USP48, BRAF, and TP53. However, the association between glucocorticoid resistance and these genes remains unclear. Here, we review the clinical aspects and molecular mechanisms of ACTHomas and compare them to those of other related conditions.

The tobacco carcinogen NNK drives accumulation of DNMT1 at the GR promoter thereby reducing GR expression in untransformed lung fibroblasts

Small cell lung cancer (SCLC) is a highly aggressive, predominantly cigarette smoke-induced tumour with poor prognosis. The glucocorticoid receptor (GR), a SCLC tumour suppressor gene, is typically reduced in SCLC. We now show that SCLC cells express high levels of DNA methyltransferase 1 (DNMT1) which accumulates at the GR promoter. DNMT1 expression is further increased by exposure to the tobacco carcinogen NNK. In the untransformed human lung fibroblast cell line, MRC-5, short term NNK treatment decreases GRα mRNA and protein expression due to accumulation of DNMT1 at the GR promoter. Long term NNK treatment results in persistently augmented DNMT1 levels with lowered GR levels. Long term exposure to NNK slows cell proliferation and induces DNA damage, while the GR antagonist RU486 stimulates proliferation and protects against DNA damage. Although both NNK and RU486 treatment increases methylation at the GR promoter, neither are sufficient to prevent senescence in this context. NNK exposure results in accumulation of DNMT1 at the GR promoter in untransformed lung cells mimicking SCLC cells, directly linking tobacco smoke exposure to silencing of the GR, an important step in SCLC carcinogenesis.

The role of glucocorticoid receptors in metabolic syndrome and psychiatric illness

Glucocorticoids (GCs) are involved in a large number of the physiological changes associated with metabolic syndrome and certain psychiatric illness. Although significance is often given to the concentration of GC, its biological action is determined by the activation of intracellular GC receptors (GR). Genetic polymorphisms of the GR and the large array of GR related cofactors can directly or indirectly affect the pathophysiology and evolution of these conditions. This review will discuss the effects of GR mutations on metabolic syndrome and psychotic depression.

Glucocorticoid receptors in lung cancer: new perspectives

Proper expression of the glucocorticoid receptor (GR) plays an essential role in the development of the lung. GR expression and signalling in the lung is manipulated by administration of synthetic glucocorticoids (Gcs) for the treatment of neonatal, childhood and adult lung diseases. In lung cancers, Gcs are also commonly used as co-treatment during chemotherapy. This review summarises the effect of Gc monotherapy and co-therapy on lung cancers in vitro, in mouse models of lung cancer, in xenograft, ex vivo and in vivo The disparity between the effects of pre-clinical and in vivo Gc therapy is commented on in light of the recent discovery of GR as a novel tumour suppressor gene.

Mifepristone Improves Octreotide Efficacy in Resistant Ectopic Cushing's Syndrome

A 30-year-old Caucasian man presented with severe Cushing's syndrome (CS) resulting from ectopic adrenocorticotropin syndrome (EAS) from a metastatic pancreatic neuroendocrine tumor. The patient remained hypercortisolemic despite treatment with steroidogenesis inhibitors, chemotherapy, and octreotide long-acting release (LAR) and was enrolled in a 24-week, phase 3 clinical trial of mifepristone for inoperable hypercortisolemia. After mifepristone was added to ongoing octreotide LAR treatment, EAS symptoms essentially resolved. Cortisol decreased dramatically, despite mifepristone's competitive glucocorticoid receptor antagonist effects. The clinical and biochemical effects reversed upon mifepristone discontinuation despite the continued use of octreotide LAR therapy. Substantial improvement in octreotide LAR efficacy with mifepristone use was noted in this patient with ectopic CS, consistent with upregulation of somatostatin receptors previously downregulated by hypercortisolemia.

Glucocorticoid receptor-mediated apoptosis in small-cell lung cancer requires interaction with BCL2

Small-cell lung cancer (SCLC) tumours are highly aggressive. At the time of diagnosis, patients have often developed metastases, and overall prognosis is particularly poor, making effective treatment difficult. Novel mechanisms need to be identified as treatment targets. We have previously found low levels of the glucocorticoid receptor (GR) in SCLC cell lines and demonstrated that over-expression of GR increases tumour cell death both in vitro and in vivo. We hypothesise that low levels of GR impair its inhibitory effect on BCL2 and thus provide a survival advantage to SCLC cell lines. The mechanism behind GR-induced apoptosis is currently unknown; therefore, pro- and anti-apoptotic genes were investigated for their role in GR-mediated apoptosis signalling. We found that over-expression of wtGR via retroviral transduction causes the DMS 79 SCLC cell line to undergo caspase-mediated apoptosis within 72  h. Neither BAD nor BCL2L11 (BIM) mRNA and protein levels were affected by GR restoration implying that GR does not trigger apoptosis in the SCLC cell lines by up-regulating these pro-apoptotic genes. The anti-apoptotic BCL2 gene was significantly overexpressed in six SCLC cell lines and the BCL2 inhibitor ABT-737 increased apoptosis in all three cell lines tested. GR interacted with BCL2 in DMS 153, DMS 79 and COR-L42 cell lines, suggesting that a protein interaction between GR and BCL2 could play a role in GR-induced apoptosis. A deeper understanding of the molecular mechanism for increasing GR expression in SCLC could provide novel treatment strategies in the future.

Mifepristone effects on tumor somatostatin receptor expression in two patients with Cushing's syndrome due to ectopic adrenocorticotropin secretion

CONTEXT

Two patients presented with Cushing's syndrome due to ectopic ACTH secretion. Initial localization studies included computed tomography, magnetic resonance imaging, and octreoscans ((111)In-pentreotide scintigraphy), which were negative in both patients. They were treated with the glucocorticoid receptor antagonist mifepristone, with improvement in their clinical symptoms. Follow-up octreoscans after, respectively, 6 and 12 months showed the unequivocal presence of a bronchial carcinoid in both patients.

OBJECTIVE

The objective of the study was to correlate in vivo and in vitro findings in patients with ectopic ACTH-producing syndrome.

METHODS

We determined the expression of somatostatin and dopamine receptors by immunohistochemistry (patients 1 and 2), quantitative PCR, and in vitro culturing of tumor cells (patient 1 only). IN VITRO RESULTS: Both tumors were strongly positive for somatostatin receptor type 2 (sst(2)) on immunohistochemistry, whereas one of the tumors (patient 1) was also dopamine receptor subtype 2 (D(2)) positive on both immunohistochemistry and quantitative PCR. Octreotide (a sst(2) preferring analog) and cabergoline (D(2) agonist) both decreased the ACTH levels in the cultured tumor cells of patient 1.

CONCLUSION

We describe two patients with ACTH-producing bronchial carcinoids, in whom a direct down-regulatory effect of glucocorticoid levels on tumoral sst(2) receptor expression is suggested by a remarkable change in octreoscan status after successful mifepristone therapy. Further studies will have to demonstrate whether glucocorticoid lowering or antagonizing therapy may be used to improve the diagnostic accuracy of somatostatin receptor scintigraphy in patients with ectopic ACTH production of unknown primary origin.

Loss of glucocorticoid receptor expression by DNA methylation prevents glucocorticoid induced apoptosis in human small cell lung cancer cells

Human small cell lung cancer (SCLC) is highly aggressive, and quickly develops resistance to therapy. SCLC cells are typically insensitive to glucocorticoids due to impaired glucocorticoid receptor (GR) expression. This is important as we have previously shown that expression of a GR transgene induces cell death in-vitro, and inhibits tumor growth in-vivo. However, the underlying mechanism for loss of GR expression is unknown. The SCLC cell line, DMS79, has low GR expression, compared to non-SCLC cell lines and normal bronchial epithelial cells. Retroviral GR expression in DMS79 cells caused activation of the apoptotic pathway as evidenced by marked induction of caspase-3 activity. Methylation analysis of the GR promoter revealed some methylation in the 1D, and 1E promoters of the GR gene, however the ubiquitous constitutively active 1C promoter was heavily methylated. In the 1C promoter there was a highly significant increase in DNA methylation in a panel of 14 human SCLC cell lines compared to a mixed panel of GR expressing, and non-expressing cell lines, and to peripheral blood mononuclear cells. Furthermore, within the panel of SCLC cell lines there was a significant negative correlation seen between methylation of the 1C promoter, and GR protein expression. Reversal of GR gene methylation with DNA methyltransferase inhibition caused increased GR mRNA and protein expression in SCLC but not non-SCLC cells. This resulted in increased Gc sensitivity, decreased Bcl-2 expression and increased caspase-3 activity in SCLC cells. These data suggest that DNA methylation decreases GR gene expression in human SCLC cells, in a similar manner to that for conventional tumor suppressor genes.

Glucocorticoid receptor-mediated apoptosis: mechanisms of resistance in cancer cells

Glucocorticoids (Gcs) are commonly used to treat patients suffering from a wide range of cancers. Their main therapeutic role is based on Gc receptor (GR)-mediated mechanisms that trigger cell death but this varies depending on the cancer type. This review aims to provide an overview of the mechanisms of Gc-induced cell death and more importantly the changes in GR that lead to resistance to Gc treatment in cancer. The three main cancer types, which are susceptible to Gc resistance and therefore loss of Gc-induced apoptotic effects, are acute lymphoblastic leukaemia, osteosarcoma and small-cell lung carcinoma. A common theme is the loss of GR function and/or a downregulation of GR expression which leads to failure of the cell death-inducing effects of Gcs. Loss of GR function is attributed to mutations in the GR gene, and in some cases a dominant-negative effect on any functional GR still present. The downregulation of GR expression can be due to decreased GR promoter activation, increased GR promoter methylation or increased expression of alternative splice isoforms of GR that have decreased transcriptional activity. Understanding the mechanisms behind Gc-triggered apoptosis and the resistance to it in these cancer types will help in further refining treatment regimens for patients and will decrease the chance of relapse caused by Gc-resistant cancer phenotypes.

Somatostatin and dopamine receptors as targets for medical treatment of Cushing's Syndrome

Somatostatin (SS) and dopamine (DA) receptors are widely expressed in neuroendocrine tumours that cause Cushing's Syndrome (CS). Increasing knowledge of specific subtype expression within these tumours and the ability to target these receptor subtypes with high-affinity compounds, has driven the search for new SS- or DA-based medical therapies for the various forms of CS. In Cushing's disease, corticotroph adenomas mainly express dopamine receptor subtype 2 (D(2)) and somatostatin receptor subtype 5 (sst(5)), whereas sst(2) is expressed at lower levels. Activation of these receptors can inhibit ACTH-release in primary cultured corticotroph adenomas and compounds that target either sst(5) (pasireotide, or SOM230) or D(2) (cabergoline) have shown significant efficacy in subsets of patients in recent clinical studies. Combination therapy, either by administration of both types of compounds separately or by treatment with novel somatostatin-dopamine chimeric molecules (e.g. BIM-23A760), appears to be a promising approach in this respect. In selected cases of Ectopic ACTH-producing Syndrome (EAS), the sst(2)-preferring compound octreotide is able to reduce cortisol levels effectively. A recent study showed that D(2) receptors are also significantly expressed in the majority of EAS and that cabergoline may decrease cortisol levels in subsets of these patients. In both normal adrenal tissue as well as in adrenal adenomas and carcinomas that cause CS, sst and DA receptor expression has been demonstrated. Although selected cases of adrenal CS may benefit from sst or DA-targeted treatment, its total contribution to the treatment of these patients is likely to be low as surgery is effective in most cases.

Farnesol, a fungal quorum-sensing molecule triggers apoptosis in human oral squamous carcinoma cells

Farnesol is a catabolite within the isoprenoid/cholesterol pathway that has exhibited significant antitumor activity. Farnesol was recently identified as a quorum-sensing molecule produced by the fungal pathogen Candida albicans. In this study, we hypothesize that synthetic and Candida-produced farnesol can induce apoptosis in vitro in oral squamous cell carcinoma (OSCC) lines. Cell proliferation, apoptosis, mitochondrial degradation, and survivin and caspase expressions were examined. In addition, global protein expression profiles were analyzed using proteomic analysis. Results demonstrated significant decrease in proliferation and increase in apoptosis in cells exposed to farnesol and C. albicans culture media. Concurrently, protein expression analysis demonstrated a significant decrease in survivin and an increase in cleaved-caspase expression, whereas fluorescent microscopy revealed the presence of active caspases with mitochondrial degradation in exposed cells. A total of 36 differentially expressed proteins were identified by proteomic analysis. Among the 26 up-regulated proteins were those involved in the inhibition of carcinogenesis, proliferation suppression, and aging. Most notable among the 10 down-regulated proteins were those involved in the inhibition of apoptosis and proteins overexpressed in epithelial carcinomas. This study demonstrates that farnesol significantly inhibits the proliferation of OSCCs and promotes apoptosis in vitro through both the intrinsic and extrinsic apoptotic signaling pathways. In addition, we report for the first time the ability of Candida-produced farnesol to induce a similar apoptotic response through the same pathways. The capability of farnesol to trigger apoptosis in cancer cells makes it a potential tool for studying tumor progression and an attractive candidate as a therapeutic agent.

Identification and functional analysis of SKA2 interaction with the glucocorticoid receptor

Glucocorticoid (GC) receptors (GRs) have profound anti-survival effects on human small cell lung cancer (SCLC). To explore the basis of these effects, protein partners for GRs were sought using a yeast two-hybrid screen. We discovered a novel gene, FAM33A, subsequently identified as a SKA1 partner and involved in mitosis, and so renamed Ska2. We produced an anti-peptide antibody that specifically recognized full-length human SKA2 to measure expression in human cell lines and tissues. There was a wide variation in expression across multiple cell lines, but none was detected in the liver cell line HepG2. A xenograft model of human SCLC had intense staining and archival tissue revealed SKA2 in several human lung and breast tumours. SKA2 was found in the cytoplasm, where it co-localized with GR, but nuclear expression of SKA2 was seen in breast tumours. SKA2 overexpression increased GC transactivation in HepG2 cells while SKA2 knockdown in A549 human lung epithelial cells decreased transactivation and prevented dexamethasone inhibition of proliferation. GC treatment decreased SKA2 protein levels in A549 cells, as did Staurosporine, phorbol ester and trichostatin A; all agents that inhibit cell proliferation. Overexpression of SKA2 potentiated the proliferative response to IGF-I exposure, and knockdown with shRNA caused cells to arrest in mitosis. SKA2 has recently been identified in HeLa S3 cells as part of a complex, which is critical for spindle checkpoint silencing and exit from mitosis. Our new data show involvement in cell proliferation and GC signalling, with implications for understanding how GCs impact on cell fate.

Glucocorticoid receptor overexpression exerts an antisurvival effect on human small cell lung cancer cells

Small cell lung cancer (SCLC) is an aggressive tumour with an abysmal prognosis. These cancers are characteristically resistant to glucocorticoid (Gc) action, owing to impaired expression of the glucocorticoid receptor (GR). We identified reduced GR expression in human SCLC cell lines, compared to a non-SCLC cell line. The SCLC cells also showed no Gc inhibition of proliferation, in contrast to non-SCLC cells. Retroviral overexpression of GR resulted in significantly increased cell death, which was partially blocked by the GR antagonist, RU486. Indeed, in cells sorted for GR expression, there was rapid, near complete loss of live cells by 72 h, in contrast to control cells that proliferated as expected. Flow cytometry using Annexin V revealed that cell death was by apoptosis. In addition, confocal analysis of fixed cells showed that cells overexpressing GR displayed a significant increase in fragmenting apoptotic nuclei. Microarray studies showed that transgenic GR expression upregulated the proapoptotic genes, BAD and BAX. We have, therefore, identified a profound apoptotic effect of GR in SCLC cells, which may explain the low levels of endogenous GR in SCLC cells. Understanding how GR overexpression leads to apoptotic cell death in SCLC cells may uncover new therapeutic strategies.

A Brief Update of Glucocorticoid Receptor Variants and Obesity Risk

Excess body fat, obesity, is one of the most common disorders in clinical practice. Obese individuals are at increased risk for physical ailments, such as type 2 diabetes, coronary heart disease, hypertension, and several types of cancer. The location of the body fat is a major determinant of the degree of excess morbidity and mortality due to obesity. More specifically, the amount of subcutaneous truncal or abdominal fat, and the amount of visceral fat located in the abdominal cavity independently predicts obesity-related adverse health outcomes. The obesity gene map shows putative loci on all chromosomes except Y. More than 300 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. These genes can be divided into two broad categories: (a) rare gene variants that have a strong influence, and (b) common gene variants that have a weaker influence on obesity phenotypes. Studies in humans have suggested a positive association between obesity, hypertension, and insulin resistance, with alleles at the glucocorticoid receptor gene. In this article, we will estimate the risk by which such gene polymorphism mediates a role in obesity.

Mechanisms of disease: The pathogenesis of pituitary tumors

Pituitary tumors exhibit a spectrum of biology, with variable growth and hormonal behaviors. They therefore provide an opportunity to examine pathogenetic mechanisms that underlie the neoplastic process. These include alterations in hormone regulation, growth-factor stimulation, cell-cycle control and cell-stromal interactions that result from genetic mutations or epigenetic disruption of gene expression. Mouse models have validated the roles of these alterations, which can be targets for the development of therapies that can manage these lesions. These therapies are increasingly recognized as critical for quality of life.

The role of hormones, growth factors and their receptors in pituitary tumorigenesis

Numerous factors have been shown to govern adenohypophysial cell proliferation. Human and animal models have documented that the hypothalamic trophic hormone growth hormone-releasing hormone stimulates cell proliferation, and prolonged stimulation leads to tumor formation. Similarly, lack of dopaminergic inhibition of lactotrophs and lack of feedback suppression by adrenal, gonadal or thyroid hormones are implicated, perhaps through hypothalamic stimulatory mechanisms, in pituitary adenoma formation superimposed on hyperplasia. However, most pituitary tumors are not associated with underlying hyperplasia. Overexpression of growth factors and their receptors, such as EGF, TGFalpha, EGF-R and VEGF has been identified in pituitary adenomas, and reduction of follistatin expression has been implicated in gonadotroph adenomas. Aberrant expression of members of the FGF family, an FGF antisense gene and FGF receptors have all been described in pituitary adenomas. The clonal composition of pituitary adenomas attests to the molecular basis of pituitary tumorigenesis, however, the evidence suggests that these various hypophysiotropic hormones and growth factors likely play a role as promoters of tumor cell growth in genetically transformed cells.

Arsenic alters the function of the glucocorticoid receptor as a transcription factor

Chronic human exposure to nonovertly toxic doses of arsenic is associated with an increased risk of cancer. Although its carcinogenic mechanism is still unknown, arsenic does not directly cause DNA damage or mutations and is therefore thought to act principally as a co-mutagen, co-carcinogen, and/or tumor promoter. Previous studies in our laboratory demonstrated that effects of low-dose arsenic (III) (arsenite) on expression of the hormone-regulated phosphoenolpyruvate carboxykinase (PEPCK) gene were strongly associated with the glucocorticoid receptor (GR)-mediated regulatory pathway. We therefore examined specifically the effects of arsenite on the biochemical function of GR in hormone-responsive H4IIE rat hepatoma cells. Completely noncytotoxic arsenite treatments (0.3-3.3 microM) significantly decreased dexamethasone-induced expression of transiently transfected luciferase constructs containing either an intact hormone-responsive promoter from the mammalian PEPCK gene or two tandem glucocorticoid response elements (GRE). Western blotting and confocal microscopy of a green fluorescent protein-tagged-GR fusion protein demonstrated that arsenite pretreatment did not block the normal dexamethasone-induced nuclear translocation of GR. These data indicate that nontoxic doses of arsenite can interact directly with GR complexes and selectively inhibit GR-mediated transcription, which is associated with altered nuclear function rather than a decrease in hormone-induced GR activation or nuclear translocation.

Expression of nerve growth factor-induced clone B subfamily and pro-opiomelanocortin gene in lung cancer cell lines

Nerve growth factor-induced clone B (NGFI-B), Nur-related factor 1, and neuron-derived orphan receptor-1 have structural features of ligand-activated transcriptional regulators and constitute the NGFI-B subfamily within the nuclear receptor superfamily. The NGFI-B subfamily is highly expressed in neuroendocrine organs and regulates the pro-opiomelanocortin (POMC) gene. Small-cell lung cancer (SCLC) is considered to be a neuroendocrine tumor that produces large numbers of polypeptide hormones. In this study we measured the NGFI-B subfamily and POMC messenger RNA (mRNA) levels in various lung-cancer cell lines by means of the quantitative reverse transcription-polymerase chain reaction, and evaluated the correlations between expression of these genes and polypeptide hormone productions. We also examined the effect of antisense oligonucleotide to NGFI-B mRNA on the expression of POMC mRNA. The NGFI-B subfamily and POMC mRNAs were highly expressed in SCLC cell lines. In addition, there were strong correlations between the NGFI-B, POMC genes, and the adrenocorticotropin hormone (ACTH) level. Further, the antisense oligonucleotide significantly suppressed POMC gene expression. We conclude that the NGFI-B subfamily was a significant molecule in SCLC and that the NGFI-B was a positive transcriptional factor for ACTH production.

Imbalanced expression of the glucocorticoid receptor isoforms in cultured lymphocytes from a patient with systemic glucocorticoid resistance and chronic lymphocytic leukemia

The human glucocorticoid receptor (GR) is expressed as two alternatively spliced isoforms, GRalpha and GRbeta. Whereas GRalpha is a hormone-activated transcription factor, GRbeta does not bind glucocorticoids (GCs), is transcriptionally inactive, and is a potential inhibitor of activated GRalpha. Differential expression of GR isoforms may play a role in generalized or tissue-specific GC resistance. GCs induce apoptosis in neoplastic lymphoid cells; and, defective apoptosis is implicated in the genesis of chronic lymphocytic leukemia (CLL). We studied a patient with generalized GC resistance and CLL. GR number in the patient's transformed lymphocytes was approximately one half that of control cells with a approximately 10-fold reduction in binding affinity for dexamethasone. In vitro apoptosis induction in CLL cells was delayed in response to GCs, but not to other apoptosis inducers. Sequencing of the GR cDNA and gene including the 2.3-kb coding region, the intron/exon junctions, the known 5'-regulatory region, and approximately 300 bp of the 3'-region revealed no alterations. Western blot with an N-terminal antibody showed normal levels of immunoreactive GR, but quantitative analysis with isoform-specific C-terminal antibodies revealed a markedly reduced GRalpha expression, and high GRbeta expression. These findings indicate that imbalanced expression of the GR isoforms may be a mechanism of GC resistance, and may have implications for tumorigenesis by enhancing cell survival.

Analysis of proopiomelanocortin gene transcription mechanisms in bronchial tumour cells

The ectopic ACTH syndrome results from the transcription of the proopiomelanocortin (POMC) gene in non pituitary tumors. To determine its mechanisms, we examined in the human bronchial carcinoma cell line DMS-79 transacting factors binding to the human POMC gene promoter. Three binding sites were identified in the proximal promoter and proteins were studied by gel-shift assays. One of them is a binding site for Nur77/Nurr1 proteins in corticotroph cells but is bound in DMS-79 cells by factor(s) distinct from these proteins. The remaining two binding sites bound yet unidentified proteins and were both functionally active in DMS-79 cells. We also showed that DMS-79 cells lacked a factor required for tissue-restricted POMC gene expression in corticotroph cells. Altogether, our results indicate that POMC gene expression in DMS-79 cells is achieved without several of the corticotroph factors and provide a preliminary characterization of some factors involved in this process. They also reveal that DMS-79 cells are deficient in proteins involved in the regulation by cAMP and glucocorticoids.

Expression of 11beta-hydroxysteroid dehydrogenase type 2 in an ACTH-producing small cell lung cancer

Non-pituitary tumors that produce adrenocorticotropic hormone (ACTH) exhibit resistance to the normal feedback effects of glucocorticoids on proopiomelanocortin (POMC) gene expression. This glucocorticoid resistance is typically complete, although some tumors show only relative glucocorticoid resistance in the clinical setting. The molecular mechanisms responsible for these clinical pathophysiologic observations are unknown, but might include glucocorticoid receptor defects or aberrant expression of enzymes or transporters that exclude glucocorticoids from access to their intracellular receptors. We examined whether ACTH-producing non-pituitary tumor cells might express 11beta-hydroxysteroid dehydrogenase (11beta-HSD), the principal 'gatekeeper' enzyme known to metabolize glucocorticoids. 11Beta-HSD mRNA and enzyme activity were assessed in DMS-79 cells, a line derived from an ACTH-producing small cell lung cancer. RT-PCR studies showed expression of mRNA encoding 11beta-HSD2 but not 11beta-HSD1 in DMS-79 cells. Control human fibroblasts expressed predominantly 11beta-HSD1 but also had detectable 11beta-HSD2 mRNA, while HepG2 hepatoma cells also expressed only 11beta-HSD2 mRNA. Whole cell assays in DMS-79 cells revealed 11beta-HSD activity with a Km for cortisol of 26.1 +/- 9.0 nM and Vmax of 57.0 +/- 5.9 pmol/h/mg protein. HepG2 cells expressed a similar high affinity enzyme activity, while control fibroblasts expressed 11beta-HSD activity with a Km for cortisol of 652 nM. Conversion of cortisol to cortisone in DMS-79 cells was inhibited to 7% of baseline by addition of 10 microM glycyrrhetinic acid. Dexamethasone (20 nM) was converted to a single product in DMS-79 cells at a rate of 17.2 pmol/h/mg protein; this activity was also inhibited by glycyrrhetinic acid. We conclude that DMS-79 cells express 11beta-HSD2. While DMS-79 cells harbor additional defects in glucocorticoid signaling, these data suggest that expression of 11beta-HSD2 might contribute to the development of the glucocorticoid-resistant phenotype of some ACTH-producing tumors.

An ACTH-producing small cell lung cancer expresses aberrant glucocorticoid receptor transcripts from a normal gene

ACTH production by non-pituitary tumors is generally not suppressible by exogenous glucocorticoid administration. We had postulated that defects in the glucocorticoid receptor (GR) signaling system might be responsible for this apparent glucocorticoid resistance and had previously demonstrated that DMS-79 cells, derived from an ectopic ACTH-producing tumor, express an abnormal GR mRNA. In this DMS-79 cell GR the sequence normally derived from exons 8 and 9 is replaced by sequence unmatched in the DNA databases. The protein encoded by this mRNA lacks the steroid-binding domain and does not function as a ligand-activated transcription factor. In the present work, we sought to identify the origin of the novel GR mRNA sequence. Southern blot analysis of DMS-79 genomic DNA showed no major structural alteration of the GR gene. Southern blotting of cosmid clones of the normal GR gene revealed that the novel DMS-79 GR mRNA sequence is derived from intron G, between exons 7 and 8. No splice site mutations were found in PCR-amplified DMS-79 DNA fragments surrounding the downstream splice junctions. Further sequencing indicated that the aberrant GR transcript appears to be generated by use of a consensus cleavage/polyadenylation signal found 3650 base pairs into the normal intron G. We conclude that abnormal GR pre-mRNA processing rather than a GR gene mutation confers glucocorticoid resistance on DMS-79 cells.

Small cell carcinoma of the vagina causing Cushing's syndrome by ectopic production and secretion of ACTH: a case report

BACKGROUND

Small cell carcinomas of pulmonary or extrapulmonary origin are neuroendocrine tumors classically associated with ectopic hormone production, particularly ACTH secretion resulting in Cushing's syndrome. However, ectopic Cushing's syndrome has not previously been reported in the setting of small cell carcinoma of the vagina.

METHODS

A primary vaginal tumor with hepatic metastases was evaluated with light microscopy. Serum cortisol and plasma ACTH levels were evaluated by radioimmunoassay and immunoradiometric assay, respectively, during a standard high-dose (8 mg) overnight dexamethasone suppression test.

RESULTS

Vaginal small cell carcinoma with hepatic metastases was demonstrated. Electrolyte abnormalities, elevated cortisol and ACTH levels, and failure to suppress ACTH secretion during high-dose dexamethasone administration confirmed the diagnosis of ectopic ACTH syndrome.

CONCLUSIONS

This case report establishes a clinical association between vaginal small cell carcinoma and ectopic Cushing's syndrome, confirming the neuroendocrine potential of this malignancy and features common to small cell neoplasms originating in other sites.

Clinical aspects of glucocorticoid sensitivity

Recent studies demonstrate that primary (hereditary) abnormalities in the glucocorticoid receptor gene make 6.6% of the normal population relatively "hypersensitive" to glucocorticoids, while 2.3% are relatively "resistant." These abnormalities might explain why some individuals develop severe adverse effects during low dose glucocorticoid therapy, while others do not develop side effects even during long-term therapy with a much higher dose. Awareness of this heterogeneity in glucocorticoid sensitivity in the normal population might eventually allow the prediction of a "safe" dose of glucocorticoid in individual patients. "Resistance" to the beneficial clinical effects of glucocorticoid therapy in part of the patients with severe rheumatoid arthritis and asthma is probably rarely related to generalized primary (hereditary) glucocorticoid resistance. In the majority of patients this "resistance" seems to be acquired and localized to the sites of inflammation, where it reflects high local cytokine production, which interferes with glucocorticoid action. Recognition of localized, acquired glucocorticoid resistance is of great importance indicating as alternative drug therapy with other immune-modulating drugs like cyclosporin and methotrexate. Chronic high dose glucocorticoid treatment in such patients is ineffective in alleviating symptomatology, while generalized side effects occur, reflecting the patient's normal systemic sensitivity to these drugs.

The glucocorticoid receptor and RU 486 in man

RU 486 is a prototype glucocorticoid antagonist with strong antiglucocorticoid activity in vitro and in vivo. Studies of its molecular mechanism of action have provided invaluable insights in the complex activation cascade of the GR itself. RU 486 effectively antagonizes glucocorticoids in most glucocorticoid-sensitive systems. Agonist effects, however, have been observed in some in vitro and in vivo systems, but remain the exception. In humans, administration of RU 486 causes generalized glucocorticoid resistance with high levels of cortisol compensating for the peripheral receptor blockade. This state is similar to that of patients with generalized familial glucocorticoid resistance. Because of its HPA axis stimulatory effects, RU 486 has limited chronic utility as an antiglucocorticoid. To date, the application of RU 486 can only be recommended in inoperable patients with ectopic ACTH secretion or adrenal carcinoma who have failed to respond to other treatments. The unusually long half-life of this drug also poses problems with titrating its dose within a therapeutic range. The development of a short-acting, selective glucocorticoid antagonist is therefore, a desirable goal of future research.

Glucocorticoid resistance in humans

Generalized inherited glucocorticoid resistance is a rare disorder caused by glucocorticoid receptor mutations and characterized clinically by hypercortisolism. Pituitary and peripheral resistance are balanced so that neither adrenal insufficiency nor Cushingoid features develop. Clinical characteristics reflect overproduction of adrenal androgens and mineralocorticoids, which are caused by excess stimulation of the adrenal gland by ACTH. These clinical characteristics respond to dexamethasone therapy. Localized and/or acquired glucocorticoid resistance may occur in some circumstances.