Altered proopiomelanocortin gene expression in adrenocorticotropin-producing nonpituitary tumors. Comparative studies with corticotropic adenomas and normal pituitaries

Aggressive Cushing's Disease: Molecular Pathology and Its Therapeutic Approach

Cushing's disease is a syndromic pathological condition caused by adrenocorticotropic hormone (ACTH)-secreting pituitary adenomas (ACTHomas) mediated by hypercortisolemia. It may have a severe clinical course, including infection, psychiatric disorders, hypercoagulability, and metabolic abnormalities, despite the generally small, nonaggressive nature of the tumors. Up to 20% of ACTHomas show aggressive behavior, which is related to poor surgical outcomes, postsurgical recurrence, serious clinical course, and high mortality. Although several gene variants have been identified in both germline and somatic changes in Cushing's disease, the pathophysiology of aggressive ACTHomas is poorly understood. In this review, we focused on the aggressiveness of ACTHomas, its pathology, the current status of medical therapy, and future prospects. Crooke's cell adenoma (CCA), Nelson syndrome, and corticotroph pituitary carcinoma are representative refractory pituitary tumors that secrete superphysiological ACTH. Although clinically asymptomatic, silent corticotroph adenoma is an aggressive ACTH-producing pituitary adenoma. In this review, we summarize the current understanding of the pathophysiology of aggressive ACTHomas, including these tumors, from a molecular point of view based on genetic, pathological, and experimental evidence. The treatment of aggressive ACTHomas is clinically challenging and usually resistant to standard treatment, including surgery, radiotherapy, and established medical therapy (e.g., pasireotide and cabergoline). Temozolomide is the most prescribed pharmaceutical treatment for these tumors. Reports have shown that several treatments for patients with refractory ACTHomas include chemotherapy, such as cyclohexyl-chloroethyl-nitrosourea combined with 5-fluorouracil, or targeted therapies against several molecules including vascular endothelial growth factor receptor, cytotoxic T lymphocyte antigen 4, programmed cell death protein 1 (PD-1), and ligand for PD-1. Genetic and experimental evidence indicates that some possible therapeutic candidates are expected, such as epidermal growth factor receptor tyrosine kinase inhibitor, cyclin-dependent kinase inhibitor, and BRAF inhibitor. The development of novel treatment options for aggressive ACTHomas is an emerging task.

60 YEARS OF POMC: The proopiomelanocortin gene: discovery, deletion and disease

The cloning of the bovine proopiomelanocortin (POMC) cDNA in 1978 by Nakanishi and colleagues was the result of a remarkable series of exacting and ingenious experiments. With this work, they instantly confirmed the single precursor hypothesis for adrenocorticotrophic hormone-β-lipotropin, as it was then known, and in so doing revealed the existence of additional, largely unpredicted, N-terminal peptides that together formed the POMC precursor peptide. This work paved the way for a host of additional studies into the physiology of these peptides and their regulation. Furthermore, the cloning of the murine Pomc gene was essential for subsequent studies, in which Pomc was intentionally deleted in the mouse illuminating its substantial role in body weight regulation and adrenal function. Contemporaneously with this work, naturally occurring mutations in human POMC came to light underlining the vital role of this gene in appetite regulation. This article reviews each of these aspects of POMC with the benefit of several decades of hindsight and informed by more recent genomic and transcriptomic data.

Nelson's syndrome: a review of the clinical manifestations, pathophysiology, and treatment strategies

Nelson's syndrome is a rare clinical manifestation that occurs in 8%-47% of patients as a complication of bilateral adrenalectomy, a procedure that is used to control hypercortisolism in patients with Cushing's disease. First described in 1958 by Dr. Don Nelson, the disease has since become associated with a clinical triad of hyperpigmentation, excessive adrenocorticotropin secretion, and a corticotroph adenoma. Even so, for the past several years the diagnostic criteria and management of Nelson's syndrome have been inadequately studied. The primary treatment for Nelson's syndrome is transsphenoidal surgery. Other stand-alone therapies, which in many cases have been used as adjuvant treatments with surgery, include radiotherapy, radiosurgery, and pharmacotherapy. Prophylactic radiotherapy at the time of bilateral adrenalectomy can prevent Nelson's syndrome (protective effect). The most promising pharmacological agents are temozolomide, octreotide, and pasireotide, but these agents are often administered after transsphenoidal surgery. In murine models, rosiglitazone has shown some efficacy, but these results have not yet been found in human studies. In this article, the authors review the clinical manifestations, pathophysiology, diagnostic criteria, and efficacy of multimodal treatment strategies for Nelson's syndrome.

Clinical utility of plasma POMC and AgRP measurements in the differential diagnosis of ACTH-dependent Cushing's syndrome

CONTEXT

Distinguishing between pituitary [Cushing's disease (CD)] and ectopic causes [ectopic ACTH syndrome (EAS)] of ACTH-dependent Cushing's syndrome can be challenging. Inferior petrosal sinus sampling (IPSS) best discriminates between CD and occult EAS but is a specialized procedure that is not widely available. Identifying adjunctive diagnostic tests may prove useful. In EAS, abnormal processing of the ACTH precursor proopiomelanocortin (POMC) and the accumulation of POMC-derived peptides might be expected and abnormal levels of other neuropeptides may be detected.

OBJECTIVE

The objective of the study was to evaluate the diagnostic utility of POMC measurements for distinguishing between CD and occult EAS in patients referred for IPSS. Another objective of the study was to evaluate in parallel the diagnostic utility of another neuropeptide, agouti-related protein (AgRP), because we have observed a 10-fold elevation of AgRP in plasma in a patient with EAS from small-cell lung cancer.

DESIGN AND PARTICIPANTS

Plasma POMC and AgRP were measured in 38 Cushing's syndrome patients presenting for IPSS, with either no pituitary lesion or a microadenoma on magnetic resonance imaging, and in 38 healthy controls.

RESULTS

Twenty-seven of 38 patients had CD; 11 of 38 had EAS. The mean POMC was higher in EAS vs CD [54.5 ± 13.0 (SEM) vs 17.2 ± 1.5 fmol/mL; P < .05]. Mean AgRP was higher in EAS vs CD (280 ± 76 vs 120 ± 16 pg/mL; P = .01). Although there was an overlap in POMC and AgRP levels between the groups, the POMC levels greater than 36 fmol/mL (n = 7) and AgRP levels greater than 280 pg/mL (n = 3) were specific for EAS. When used together, POMC greater than 36 fmol/mL and/or AgRP greater than 280 pg/mL detected 9 of 11 cases of EAS, indicating that elevations in these peptides have a high positive predictive value for occult EAS.

CONCLUSIONS

Expanding upon previous observations of high POMC in EAS, this study specifically demonstrates elevated POMC levels can identify occult ectopic tumors. Elevations in AgRP also favor the diagnosis of EAS, suggesting AgRP should be further evaluated as a potential neuroendocrine tumor marker.

Prolonged remission after long-term treatment with steroidogenesis inhibitors in Cushing's syndrome caused by ectopic ACTH secretion

Spontaneous remission is rare in ectopic ACTH syndrome (EAS). We describe four patients with presumed EAS in whom long-term treatment with steroidogenesis inhibitors was followed by prolonged remission of hypercortisolemia. Biochemical testing was consistent with EAS, but imaging failed to identify a tumor. Patients were treated with ketoconazole alone or with mitotane and/or metyrapone to control hypercortisolemia. Dexamethasone was added when a block and replace strategy was used. Treatment with steroidogenesis inhibitors for 3-10 years in these patients was followed by a prolonged period of remission (15-60 months). During remission, the first patient had an elevated ACTH, low cortisol and 24-h urinary free cortisol (UFC), and adrenal atrophy on computerized tomography scan during remission, suggesting a direct toxic effect on the adrenal glands. Cases 2 and 3 had normal to low ACTH levels and low-normal UFC, consistent with an effect at the level of the ectopic tumor. They did not have a history of cyclicity and case 3 has been in remission for ~5 years, making cyclic Cushing's syndrome less likely. Case 4, with a history of cyclic hypercortisolism, had normal to slightly elevated ACTH levels and low-normal UFC during remission. The most likely etiology of remission is cyclic production of ACTH by the ectopic tumor. Spontaneous and sustained remission of hypercortisolemia is possible in EAS after long-term treatment with steroidogenesis inhibitors; a drug holiday may be warranted during chronic therapy to evaluate this. The pathophysiology remains unclear but may involve several different mechanisms.

Evaluation of proopiomelanocortin mRNA in the peripheral blood from patients with Cushing's syndrome of different origin

ACTH-dependent Cushing's syndrome is due to ACTH overproduction originating from a pituitary corticotroph adenoma (Cushing's disease) or from ectopic tumors (ectopic ACTH syndrome). Due to difficulties in the differential diagnosis between these two forms of hypercortisolism it would be important to have molecular tools able to discriminate the two conditions. It is known that proopiomelanocortin (POMC) gene transcription can originate messengers of different length. ACTHomas show the normal 1072 nucleotides (nt) transcript, whereas ectopic tumors seem to be associated with a longer mRNA form (1450 nt). In order to analyse the presence of different POMC transcripts, we extracted total RNA from peripheral lymphocytes of 10 patients with Cushing's disease, 10 with ectopic Cushing syndrome, and 20 controls as well as from pituitary tissues (2 ACTH-omas and a normal pituitary polyA+ sample). Northern blot analysis correctly revealed a 1072 nt mRNA molecule in pituitary ACTH-oma and in the normal pituitary polyA+ RNA samples, whereas neither this molecule nor other alternative transcripts were detected in blood samples from patients and controls. These data were confirmed by the more sensitive RT-PCR technique. This study further underlines the need for alternative approaches in the diagnosis of ACTH-dependent Cushing's syndrome.

RT-PCR analysis of corticotroph-associated genes expression in carcinoid tumours in the ectopic-ACTH syndrome

OBJECTIVE

ACTH is frequently produced in non-pituitary tumours, leading to the ectopic-ACTH syndrome, but the molecular mechanisms of its expression remain obscure. This study was aimed at understanding the transcription mechanisms of the ACTH-precursor gene in carcinoid tumours of the lung or thymus.

DESIGN

Transcripts coding for a series of corticotroph-associated transcription factor genes were detected, together with markers of the corticotroph phenotype. We studied a series of 41 carcinoid tumours including 15 with proven ectopic-ACTH syndrome.

METHODS

Specific RT-PCR reactions were designed for each gene including alternatively spliced isoforms.

RESULTS

The markers of the corticotroph phenotype were detected in all ACTH-positive tumours. Expression of the Tpit and Pitx1 genes were not restricted to ACTH-positive tumours but were also detected in many ACTH-negative carcinoids. Only a subset of ACTH-negative tumours expressed NAK-1/Nur77, and NeuroD1 expression was detected in approximately 50% of the tumours regardless of their secretory status. The glucocorticoid receptor alpha was detected in every tumour in contrast to its beta isoform detectable in a few tumours only. Chicken ovalbumin upstream promoter-transcription factor 1 (COUP-TF1) and peroxisome proliferator-activated receptor (PPAR) gamma2 were expressed in 50% of the tumours of each group whereas PPARgamma1 was expressed in almost every tumour.

CONCLUSIONS

ACTH-positive carcinoids do not share a characteristic expression pattern of the corticotroph-associated transcription factor genes, suggesting that the transcriptional mechanisms of the ACTH-precursor gene differ from those in normal pituitary corticotrophs. Expression of Tpit and Pitx1 genes in most carcinoids suggests that some aspects of the pituitary corticotroph phenotype may belong to general carcinoid differentiation.

The Nelson's syndrome... revisited

Adrenalectomy is a radical therapeutic approach to control hypercortisolism in some patients with Cushing's disease. However it may be complicated by the Nelson's syndrome, defined by the association of a pituitary macroadenoma and high ACTH secretion after adrenalectomy. This definition has not changed since the end of the fifties. Today the Nelson's syndrome must be revisited with new to criteria using more sensitive diagnostic tools, especially the pituitary magnetic resonance imaging. In this paper we will review the pathophysiological aspects of corticotroph tumor growth, with reference to the impact of adrenalectomy. The main epidemiological data on the Nelson's syndrome will be presented. More importantly, we will propose a new pathophysiological and practical approach to this question which attempts to evaluate the Corticotroph Tumor Progression after adrenalectomy, rather than to diagnose the Nelson's syndrome. We will discuss the consequences for the management of Cushing's disease patients after adrenalectomy, and will also draw some perspectives.

Overexpression of the V3 vasopressin receptor in transgenic mice corticotropes leads to increased basal corticosterone

The vasopressin V3 receptor (V3) is specifically expressed in pituitary corticotropes and mediates the stimulatory effect of vasopressin on adrenocorticotropic hormone (ACTH) release. The V3 gene is overexpressed in corticotrope pituitary tumours compared to normal pituitaries. We hypothesized that V3 overexpression might induce changes in corticotrope function and alter the regulation of the hypothalamic-pituitary-adrenal axis. Thus, we generated transgenic mice (POMV3) expressing the human V3 receptor in the pituitary under the control of rat pro-opiomelanocortin (POMC) promoter sequences. The transgene was efficiently transcribed and vasopressin binding was increased in both corticotropes and melanotropes. In-vitro ACTH release and inositol phosphate formation were unchanged in POMV3 pituitaries, but the responses to vasopressin were significatively increased. In vivo, basal circulating concentrations of ACTH in POMV3 mice were similar to those of controls but corticosterone concentrations were moderately increased. In addition, the levels of POMC mRNA in the transgenic pituitaries were comparable to those of control mice. Finally, POMV3 mice responded with a similar maximal increase of ACTH and corticosterone to a 20-min acute restraint stress. Together, these results show that hypophyseal V3 overexpression led to increased basal concentrations of corticosterone and suggest that the negative glucocorticoid feedback may be altered at the pituitary level.

Loss of ACTH expression in cultured human corticotroph macroadenoma cells is consistent with loss of the POMC gene signal sequence

The proopiomelanocortin (POMC) gene is highly expressed in the pituitary gland where the resulting mRNA of 1200 base pairs (bp) gives rise to a full-length protein sequence. In peripheral tissues however both shorter and longer POMC variants have been described, these include for example placental tissue which contain 800 (truncated at the 5' end) and 1500 as well as the 1200 bp transcripts. The importance of the 800 bp transcript is unclear as the lack of a signal sequence renders the molecule to be non-functional. This transcript has not been previously demonstrated in the pituitary gland. In this report we show evidence of a 5' truncated POMC gene in human pituitary corticotroph macroadenoma cells (JE) maintained in primary culture for >1 year. The original tumour tissue and the derived cells during early passage (up to passage 4-5) immunostained for ACTH and in situ hybridisation confirmed the presence of the POMC gene in the cultured cells. These cells also secreted 15-40 pg/10(5) cells/24 h ACTH. In addition, as expected RT-PCR demonstrated the presence of all three POMC gene exons and is thus indicative of a full-length POMC gene. In late culture passages (passages 8-15) JE cells ceased to express ACTH and cell growth became very slow due presumably to cells reaching their Hayflick limit. ACTH immunostaining in these cells was undetectable and ACTH secretion was also at the detection limits of the assay and no greater than 10 pg/10(5) cells/24 h. ACTH precursor molecules were also undetectable. RT-PCR for the POMC gene in these late passage cells showed that only exon 3 was detectable, in contrast to early passage cells where all three exons were present. In summary we isolated in culture, human pituitary cells that possessed initially all three exons of the POMC gene and immunostained for ACTH. On further passaging these cells showed a loss of exons 1 and 2 in the POMC gene and a loss of ACTH immunostaining and secretion. We would like to suggest that the loss of ACTH peptide expression in these late passage cells is in part due to the loss of the POMC signal sequence. An alternative explanation for our findings is that there were originally two populations of corticotrophs in the cultures, one of which possessed the full-length POMC gene and the other only the 5' truncated POMC transcript and it is these latter cells which survived in culture. In either scenario this is the first report of the 5' truncated POMC gene occurring in pituitary cells.

Ectopic pro-opiomelanocortin syndrome

Ectopic POMC syndrome remains one of the most challenging differential diagnoses in endocrinology. Recent progress in the understanding of the tissue specific regulation of POMC gene expression and new insights into the processing of the POMC peptide in nonpituitary tissues has helped elucidate some of the molecular events leading to ectopic expression and secretion of POMC peptides. Corticotropin and other POMC-derived peptides have diverse effects on adrenal steroidogenesis, growth, and extra-adrenal tissues. Differences in POMC gene regulation in the corticotrope versus ectopic POMC-producing tumors provides a scientific framework for the clinical distinction between eutopic and ectopic Cushing's syndrome. In an attempt to revisit recent basic and clinical advances in the diagnosis of ectopic POMC syndrome the authors undertook an extensive literature review of 530 cases in 197 published papers and provided a molecular biologic, demographic and diagnostic update. According to this review, the four most common causes of ectopic POMC syndrome are the small cell carcinoma of the lung (27%), bronchial carcinoids (21%), islet cell tumor of the pancreas (16%), and thymic carcinoids (10%). Although the clinical features of patients with ectopic POMC syndrome are similar to those with Cushing's disease, subgroup analysis reveals a broad spectrum of severity and progression of signs and symptoms of hypercortisolism. The endocrine workup of a patient with suspected ectopic POMC syndrome includes the establishment of pathologic hypercortisolism, diagnosis of corticotropin dependency, and the differential diagnosis of corticotropin-dependent Cushing's syndrome. The use of a variety of baseline endocrine values, dynamic endocrine testing, and invasive procedures leads to the correct diagnosis in the majority of patients with ectopic POMC syndrome. Diagnostic imaging, including conventional radiological techniques and somatostatin receptor scintigraphy, aids in the correct localization and eventual treatment of ectopic POMC production.

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.

Analysis of the human proopiomelanocortin gene promoter in a small cell lung carcinoma cell line reveals an unusual role for E2F transcription factors

The small cell lung carcinoma (SCLC) cell line DMS-79 has been used as a model for studying the molecular mechanism underlying the ectopic ACTH syndrome. We previously showed that two domains of the human Proopiomelanocortin (POMC) gene promoter were specifically active in DMS-79 cells. The present study focuses on the more distal one, Domain IV (-376/-417). DNaseI footprinting experiments identified a single binding site for DMS-79 cell proteins in this domain. Gel-shift and sequence analysis indicated that E2F proteins might bind this site. Indeed, proteins from DMS-79 cells which bind this site (i) have in vitro DNA binding properties indistinguishable from those of E2F proteins (ii) form, like E2F proteins, multiprotein complexes which can be dissociated by sodium deoxycholate and (iii) are recognized by antibodies directed against E2F proteins. Further, we show that the rat POMC distal promoter domain contains a homologous sequence which constitutes a natural mutant of the human POMC E2F binding site, since it does not bind E2F. We show by transient transfection that this natural mutant, in the context of the rat POMC promoter, is not active in DMS-79 cells by contrast to the human POMC E2F binding site. We conclude that E2F binding is required for the activity of Domain IV in DMS-79 cells and contributes to the expression of the POMC gene in SCLC. Further studies are required to elucidate the role of E2F factors in POMC gene transcription in SCLC cells, but our results have identified mechanisms different from those in pituitary corticotroph cells that are used by these SCLC tumor cells.

Overexpression of vasopressin (V3) and corticotrophin-releasing hormone receptor genes in corticotroph tumours

OBJECTIVE

The molecular mechanisms underlying ACTH-secreting tumour formation remain unknown. Transmembrane signalling pathways play an important role in several endocrine disorders including pituitary tumours. To investigate the role of the pituitary vasopressin (V3) receptor (R) in ACTH-secreting tumours we have qualitatively and quantitatively analysed its mRNA.

DESIGN

RT-PCR, denaturing gradient gel electrophoresis and S1 nuclease protection experiments were used to analyse V3 mRNA structure in ACTH-secreting tumours. We also developed a competitive RT-PCR system to compare the levels of expression of POMC, V3 and CRH-R genes. This system used as competitor a single mutant template (termed multi-mutant) containing primers for the three genes flanking an unrelated core sequence allowing multiple quantifications from the same cDNA preparations. We analysed 12 normal pituitaries, 15 corticotroph pituitary adenomas and 6 ACTH-secreting bronchial carcinoids.

RESULTS

The V3 mRNA structure and sequence were found to be identical in normal and tumoural pituitary indicating that the tumoural Vs mRNA codes for a normal receptor. POMC RT-PCR signals in the pituitary tumour group were approximately 7-fold higher than in the normal pituitary group. Similarly, V3 and CRH-R signal were increased in pituitary tumors (mean +/- SEM: 5.87 x 10(-6) +/- 1.73 x 10(-6), and 2.33 x 10(-4) +/- 1.4 x 10(-4), respectively), when compared to normal pituitaries (1.19 x 10(-7) +/- 2.39 x 10(-8), and 1.7 x 10(-6) +/- 4.65 x 10(-7), respectively) suggesting that these two genes are expressed at very high levels in corticotroph tumours. When expressed relative to the corresponding POMC signals, increases in V3 and CRH-R signals reached 49-fold and 137-fold, respectively, in pituitary tumours. In ACTH-secreting bronchial carcinoids V3 gene expression level was also higher than in normal pituitary, whereas CRH-R signals were detected in only 4 of the 6 tumours with wide variations.

CONCLUSION

Our results show that both vasopressin and CRH receptor genes are overexpressed in ACTH-secreting pituitary tumours. They suggest that overexpression of G protein-coupled receptors may be an additional mechanism through which membrane receptors may play a role in human tumours.

Identification and sequencing of a putative variant of proopiomelanocortin in human epidermis and epidermal cells in culture

Proopiomelanocortin (POMC) is a precursor polypeptide for various bioactive peptides, including adrenocorticotropic hormone, alpha-, beta-, and gamma-melanotropin, beta-endorphin, and beta-lipotropin. Although the classical source of POMC is the pituitary, various studies indicate the expression of POMC in several nonpituitary tissues. In this study, in situ hybridization with anti-sense cRNA riboprobe was used to show expression of POMC mRNA in human epidermis and cultured human epidermal cells (melanocytes and keratinocytes). POMC mRNA was amplified by reverse transcriptase-polymerase chain reaction using anti-sense and sense primers designed from Exons 2 and 3 of POMC gene. A approximately 300 bp product was present in normal human skin, grafted human skin, and cultured normal human melanocytes and keratinocytes. By Southern analysis this product was hybridized specifically to the POMC cDNA. Sequence analysis of the reverse transcriptase polymerase chain reaction product from tissues or cells showed 85% homology to POMC cDNA from human, bovine, pig, and monkey sources. This suggests the existence of a putative isoform or variant of POMC mRNA in human epidermis.

The pituitary V3 vasopressin receptor and the corticotroph phenotype in ectopic ACTH syndrome

Ectopic ACTH secretion occurs in highly differentiated and rather indolent tumors like bronchial carcinoids or, in contrast, in various types of aggressive and poorly differentiated neuroendocrine tumors. We explored this phenomenon using the recently cloned human pituitary V3 vasopressin receptor as an alternate molecular marker of the corticotroph phenotype. Expression of V3 receptor, corticotrophin releasing hormone (CRH) receptor, and proopiomelanocortin (POMC) genes was examined in tumors of pituitary and nonpituitary origin. A comparative RT-PCR approach revealed signals for both V3 receptor and CHR receptor mRNAs in 17 of 18 ACTH-secreting pituitary adenomas, and 6 of 6 normal pituitaries; in six growth hormone- or prolactin-secreting adenomas, a very faint V3 receptor signal was observed in three cases, and CRH receptor signal was undetected in all. Six of eight bronchial carcinoids responsible for the ectopic ACTH syndrome had both POMC and V3 receptor signals as high as those in ACTH-secreting pituitary adenomas; in contrast, no POMC signal and only a very faint V3 receptor signal were detected in six of eight nonsecreting bronchial carcinoids. Northern blot analysis showed V3 receptor mRNA of identical size in ACTH-secreting bronchial carcinoids and pituitary tumors. Other types of nonpituitary tumors responsible for ectopic ACTH syndrome presented much lower levels of both POMC and V3 receptor gene expression than those found in ACTH-secreting bronchial carcinoids. In contrast with the V3 receptor, CRH receptor mRNA was detected in the majority of neuroendocrine tumors irrespective of their POMC status. These results show that expression of the V3 receptor gene participates in the corticotroph phenotype. Its striking association with ACTH-secreting bronchial carcinoids defines a subset of nonpituitary tumors in which ectopic POMC gene expression is but one aspect of a wider process of corticotroph cell differentiation, and opens new possibilities of pharmacological investigations and even manipulations of this peculiar ACTH hypersecretory syndrome.

Ectopic ACTH syndrome

Ectopic ACTH syndrome represents a cancer-induced amplification of a property [proopiomelanocortin (POMC) peptides production] normally present in the cells from which the cancer originated but with aberrant posttranslational processing of POMC resulting in a greatly elevated secretion of ACTH precursors. The classic ectopic ACTH-producing tumors described in the 1960s were highly malignant but more recently slowly growing tumors such as carcinoids are reported with increasing frequency. Clinical features of patients with ectopic ACTH were analyzed, including biochemical abnormalities, plasma ACTH, cortisol and urinary steroids. Dynamic tests such as high-dose dexamethasone suppression, metyrapone and ovine-CRH (oCRH) stimulation were explored, as well as inferior petrosal sinus ACTH sampling before and after oCRH. Among the tumor markers examined, elevation of ACTH precursors was uniformly present followed by increased output of calcitonin, gut hormones, oncofetal and placental hormones in decreasing order. Since more than 90% of ectopic ACTH tumors are neuroendocrine in nature exhibiting APUD characteristics, their 2 markers, neuron-specific enolase and chromogranins are very useful. The imaging procedures for localization of the tumor ranged from chest X-rays to computed tomography and magnetic resonance of the chest and abdomen. Abdominal ultrasonography was also useful. Finally somatostatin receptor scintigraphy permitted demonstration of unrecognized tumors and/or metastases, even when the tumors were occult. The ACTH content, immunostaining for APUD markers and altered POMC processing were evaluated in ectopic tumors and/or metastases. Occult ectopic ACTH syndrome of more than 4-6 months of symptoms without the emergence of an obvious source was reviewed. Since the tumors are often clinically and biochemically undistinguishable from pituitary-dependent Cushing's disease, inferior petrosal sinus sampling for ACTH after oCRH stimulation established the diagnosis in over 90% of the cases. 60% of the occult tumors were thoracic carcinoids (3/4 bronchial carcinoids), followed by small cell lung cancer and pancreatic neuroendocrine tumors. In 12% the primary etiology was not detected. The rare syndrome of ectopic CRH syndrome (6 published cases) leading to excessive stimulation of the pituitary which became hyperplastic and secreted excessive amounts of ACTH is discussed. Finally, the 12 published cases and 1 unreported patient with ectopic CRH-ACTH tumors were reviewed, the majority being metastatic small cell lung carcinomas, bronchial and thymic carcinoids.

Cushing's syndrome

Cushing's syndrome is usually caused by the secretion of corticotropin or cortisol by a pituitary or adrenal tumor, respectively, or by ectopic secretion of corticotropin. It is possible to determine the specific abnormality in most patients, but it can sometimes be difficult to decide whether the patient has hypercortisolism and whether it is primary or due to major depressive disorder or to the stress of other diseases. Determining the cause of the hypercortisolism involves performing multiple tests in a logical sequence; the results should all be consistent with the same diagnosis. Treatment should aim to cure the hypercortisolism and to eliminate any tumor that threatens the patient's health, while minimizing the chance of an endocrine deficiency or long-term dependence on medications.

Production of immunoreactive atrial natriuretic polypeptide in neuroendocrine tumors

BACKGROUND

Peptide hormone synthesis in neuroendocrine tumors is a well-recognized phenomenon. However, production in neuroendocrine tumors of atrial natriuretic polypeptide (ANP), a newly discovered peptide hormone from the heart, has not been studied extensively.

METHODS

The presence of immunoreactive human ANP (IR-hANP) in neuroendocrine tumors was determined using a specific human ANP radioimmunoassay. Neuroendocrine tumors examined included 9 small cell carcinomas of the uterus, 28 small cell carcinomas of the lung, 20 carcinoid tumors, 54 pancreatic endocrine tumors, 17 neuroblastic tumors, 14 pheochromocytomas, and 14 medullary carcinomas of the thyroid. Twenty atrial tissues also were examined as the control. Molecular size of IR-hANP in the extracts of atrial and tumor tissues was determined by gel chromatography.

RESULTS

IR-hANP was detected in the extracts of small cell carcinoma of the uterus, small cell carcinoma of the lung, and carcinoid tumor, with concentrations ranging from 3.1 to 210 ng/g wet weight tissue. No IR-hANP was detected in the extracts of pancreatic endocrine tumor, neuroblastic tumor, pheochromocytoma, and medullary carcinoma of the thyroid. The frequency of production of IR-hANP in neuroendocrine tumors was highest in small cell carcinoma of the uterus (44%), followed by small cell carcinoma of the lung (18%) and carcinoid tumor (15%). IR-hANP present in the extracts of small cell carcinomas of the uterus had molecular size heterogeneity, with three fragments in addition to alpha-, beta- and gamma-human ANP.

CONCLUSIONS

These results indicate that IR-hANP is produced by neuroendocrine tumors and that the molecular size of IR-hANP in tumor tissues is different from that in atrial tissues.

Corticotropin-releasing hormone, proopiomelanocortin, and glucocorticoid receptor gene expression in adrenocorticotropin-producing tumors in vitro

To differentiate between ectopic ACTH syndrome and Cushing's disease, gene expression of corticotropin-releasing hormone (CRH), proopiomelanocortin (POMC), and glucocorticoid receptor was examined in 10 pituitary adenomas (Cushing's disease) and in 10 ectopic ACTH-producing tumors. CRH increased plasma ACTH levels in all patients with Cushing's disease and in five patients with ectopic ACTH syndrome whose tumors contained CRH and CRH mRNA. In five CRH nonresponders, CRH was not detected in tumors that contained no CRH mRNA or that contained only long-size CRH mRNA. Dexamethasone (Dex) decreased plasma ACTH levels in all patients with Cushing's disease and in three patients with ectopic ACTH-producing bronchial carcinoid. These tumors contained glucocorticoid receptor mRNA. CRH increased and Dex decreased ACTH release and POMC mRNA levels in pituitary adenoma and bronchial carcinoid cells. PMA increased POMC mRNA levels only in carcinoid cells. These results reveal characteristics of ectopic ACTH-producing tumors: long-size CRH mRNA and PMA-induced POMC gene expression. In addition, there are two ectopic ACTH syndrome subtypes: tumors containing ACTH with CRH (CRH responder) and tumors without CRH. Dex decreases ACTH release and POMC mRNA levels in some bronchial carcinoids. Therefore, CRH and Dex tests have limited usefulness in differentiating between Cushing's disease and ectopic ACTH syndrome.

The cellular and molecular basis of the ectopic ACTH syndrome

In recent years the techniques of molecular and cellular biology have made it possible to begin to dissect the origins and behaviour of the ACTH-secreting tumour cell. It is becoming apparent that these tumours represent undifferentiated neuroendocrine cells, and it may be that their peptide-secreting properties may have no more sinister oncological significance. However, an autocrine role for beta-endorphin may confer a selective growth advantage on the POMC-expressing cell. It is still not clear why glucocorticoids fail to inhibit the POMC gene in these extra-pituitary tumours despite the presence of glucocorticoid receptors. This may not be resolved until the mechanism for inhibition of POMC by glucocorticoids in the normal pituitary is understood, although it is tempting to speculate that a mutation in the glucocorticoid receptor or a tissue specific interaction is responsible for the resistance of POMC observed in the ectopic ACTH syndrome. In studying the peptides secreted by the extra-pituitary tumours responsible for the ectopic ACTH syndrome it would appear that direct measurement of ACTH precursors and comparison with the circulating concentrations of ACTH can give valuable information on the percentage of tumours which do not effectively process the ACTH precursors. However, far more data have to be collected on patients with occult tumours in order to identify whether this type of processing is tissue specific. Nevertheless, these studies provide useful insights into the mechanisms of intracellular signalling and regulation in such tumours which may identify unique pharmacological tools to inhibit ACTH secretion or more importantly tumour growth.

Cushing's syndrome associated with ectopic production of corticotrophin-releasing hormone, corticotrophin and vasopressin by a phaeochromocytoma

We describe a case of Cushing's syndrome caused by a phaeochromocytoma secreting corticotrophin-releasing hormone (CRH) and corticotrophin (ACTH). A 49-year-old white woman presented with a 1-month history of lower limb oedema, polydipsia and polyuria. Physical examination revealed a patient with plethoric facies, lanugo-type facial hair, central obesity, red abdominal striae, lower limb oedema, and blood pressure of 210/115 mmHg. Laboratory studies showed high plasma ACTH and markedly elevated urinary cortisol excretion that suppressed more than 50% with high-dose dexamethasone administration. Computed tomographic scan of the abdomen showed a 4-cm left adrenal tumour. Catecholamines and metabolites were markedly increased in a 24-hour urine collection. Results of venous catheterization studies showed that CRH and ACTH were secreted by the tumour. In addition, with ovine CRH administration, inferior petrosal sinus sampling showed pituitary secretion of ACTH. Left adrenalectomy resulted in complete remission of Cushing's syndrome. Light microscopic and immunohistochemical studies revealed a phaeochromocytoma that produced CRH, ACTH and vasopressin. RNA studies showed that this tumour, in contrast to normal adrenal and other reported phaeochromocytomas, transcribed a lone pituitary-sized (1200 nucleotide) pro-opiomelanocortin mRNA. This is the second reported case of a CRH-secreting phaeochromocytoma.

Unrestrained production of proopiomelanocortin (POMC) and its peptide fragments by pituitary corticotroph adenomas in Cushing's disease

The hallmark of ACTH oversecretion in Cushing's disease is its partial resistance to the normal suppressive effect of glucocorticoids. Because ACTH secretion by the pituitary tumor is not normally restrained ACTH is overproduced with subsequent chronic hypercortisolism. Since peripheral tissues have retained their normal sensitivity to the action of cortisol they appropriately develop the features of Cushing's disease. The question of whether a collection of corticotroph cells, eventually arranged in an adenomatous-like fashion, is a primary pituitary event or is corticotropin-releasing factor driven has had no response so far. Clonal composition of such lesions has been determined by X chromosome inactivation using DNA probes which detect multiallelic polymorphism in females. A monoclonal pattern is found in all macroadenomas. ACTH is co-secreted with other peptide fragments derived from their common polypeptide precursor, proopiomelanocortin (POMC). As a rule POMC processing in pituitary tumors is qualitatively unaltered: plasma values of the N-terminal fragment, the joining peptide, the beta- and gamma-lipotropins, and beta-endorphin all are valid alternate markers of the tumor activity. Tumor POMC peptides including ACTH and its phosphorylated form usually show no peculiar or unexpected molecular forms in contrast with what is often found when POMC expression occurs in a non-pituitary tumor.

Expression of the neuroendocrine cell marker 7B2 in human ACTH secreting tumours

OBJECTIVE AND DESIGN

Pro-opiomelanocortin gene expression is a ubiquitous phenomenon which takes place not only in the pituitary but also in many normal and tumoral non-pituitary tissues. However, the clinical features of the ectopic ACTH syndrome are rarely encountered. To further investigate this problem we examined series of normal human pituitaries and endocrine tumours evaluating the tissue content of pro-opiomelanocortin peptides, and the state of neuroendocrine differentiation as indicated by the biochemical marker 7B2.

PATIENTS AND MEASUREMENTS

Tissue concentration of 7B2, pro-opiomelanocortin products (joining peptide and beta-endorphin) were measured in 13 pituitary corticotrophic adenomas and 13 non-pituitary tumours associated with the ectopic ACTH syndrome (five out of 20 bronchial carcinoid tumours, two out of 19 phaeochromocytomas, one out of 11 medullary thyroid carcinomas, three pancreatic and two thymic carcinoid tumours). Molecular weight forms of immunoreactive 7B2 and 7B2 RNA messenger were determined using Western and Northern blot analysis respectively.

RESULTS

In all tissues examined, concentrations of immunoreactive beta-endorphin (fmol/mg tissue wet weight) showed widely distributed values from less than 0.7 to 1,340,000, which were correlated (r = 0.975, P less than 0.01) with that of immunoreactive joining peptide, another pro-opiomelanocortin fragment. In the 13 non-pituitary tumours associated with the ectopic ACTH syndrome, immunoreactive beta-endorphin concentrations ranged from 8.6 to 548,000, whereas in normal and tumoral pituitaries they varied from 16,600 to 364,800, and 5000 to 1,340,000, respectively. Immunoreactive 7B2 was detected in 67 of 68 neuroendocrine tumours. Tissue concentrations (fmol/mg tissue wet weight) of immunoreactive 7B2 varied from 135 to 1787 in pituitary tumours; from less than 0.5 to 555 in bronchial carcinoids; from 21.7 to 793 in phaeochromocytomas; from less than 1.6 to 948 in medullary thyroid carcinomas. Western blot analysis showed a predominant molecular weight form of immunoreactive 7B2 at 22 kDa. Northern blot analysis of RNA extracted from ACTH secreting pituitary and non-pituitary tumours showed a predominant signal hybridizing at 1.5 kb with a 7B2 probe.

CONCLUSION

These results show that all ACTH secreting tumours have biochemical markers for neuroendocrine differentiation. Tissue concentrations of pro-opiomelanocortin peptides are variable, being extremely high in the most benign tumours and low in those with an aggressive growing pattern, and are not correlated with the biochemical neuroendocrine markers.

Characteristics and kinetics of proopiomelanocortin mRNA expression by human leucocytes

Northern blot analysis of total and poly(A)+ RNA demonstrated that human leucocytes contain several proopiomelanocortin (POMC) mRNA species, including 0.8-, 1.2-, 1.5-, and 9.5-kb transcripts. Peripheral blood lymphocytes (PBL) were found to express all four species. The other cell types had either the 0.8-kb species alone or both 0.8- and 9.5-kb species. Neutrophils were the only cells to express the 9.5-kb transcript alone. In T cell clones, both interleukin (IL)-2 alone or the antigen for which the clone was specific induced POMC accumulation within 18-24 h. Cytoplasmic dot blot analysis of PBL RNA demonstrated that POMC expression could be induced by corticotrophin releasing factor, rIL-1, and phorbol ester, but not by calcium ionophore (A23187). In PBL activated in a mixed lymphocyte culture there was increased expression, particularly of the smaller species (0.8, 1.2, and 1.5 kb), within 1-3 days of activation. The intensity of the bands began to decline thereafter. The 9.5-kb POMC transcript essentially disappeared by Day 3.

Pituitary-like proopiomelanocortin transcripts in human Leydig cell tumors

Proopiomelanocortin is a polypeptide precursor molecule, the processing of which generates ACTH, beta-endorphin, the beta- and gamma-lipotropins, the joining peptide, and the NH2-terminal fragment. Anterior pituitary corticotrophs are the major site of proopiomelanocortin gene expression in man and the predominant, if not sole source of circulating ACTH. Recent data have established that proopiomelanocortin gene expression also occurs in various normal nonpituitary tissues, one of the best studied being the testis. In this latter organ the dominant gene products are short transcripts of approximately 800 nucleotides, which lack the first two exons of the gene and cannot encode a complete proopiomelanocortin molecule. In this report we show that the mode of proopiomelanocortin gene expression is occasionally modified in human Leydig cell tumors: a 1,200-nucleotide mRNA species identical to that in the pituitary is produced. It results from the usual (pituitary) start site of transcription and thus can encode the complete proopiomelanocortin molecule. In two out of six tumors, large amounts of the 1,200-nucleotide transcript led to a dramatic increase of approximately 1,000-fold in proopiomelanocortin peptide concentrations as compared with the normal and peritumoral testis. Proopiomelanocortin processing in these tumors generates various peptide fragments including ACTH. These results may help to understand the mechanism of proopiomelanocortin expression in nonpituitary tumors and have implications for the more general phenomenon of ectopic hormone secretion.

Localization of aldolase C mRNA in brain cells

The expression of aldolase C and aldolase A mRNA was assessed by Northern blot hybridization using RNAs purified from cultured rat and mouse brain neurons and astroglial cells. Neurons were found to contain about 4-fold more aldolase C mRNA and about twice as much aldolase A mRNA than astroglia. Analysis of the cellular localization of aldolase C mRNA by in situ hybridization to brain slices showed a predominantly neuronal labeling with an irregular distribution. A strong signal was observed in Purkinje cell somata and a weaker signal in subpopulations of neurons in cerebral cortex, striatum, hippocampus, hypothalamic nuclei and primary olfactory cortex.

Neuroendocrine alterations in nude mice with a human lung carcinoma producing pro-opiomelanocortin, corticotrophin-releasing hormone and arginine vasopressin

A mucoepidermoid carcinoma of the lung (ICD classification 8430/3) resected from a patient with no clinical signs of pituitary-adrenal alterations was transplanted into 2-month-old athymic nu/nu nude mice, with the purpose of studying the effects exerted by the human tumour on the host hypothalamo-pituitary-adrenal axis. The tumour produces peptides derived from different regions of pro-opiomelanocortin (POMC: ACTH, 7.6 +/- 0.7; N-terminal POMC, 6.6 +/- 0.6; beta-LPH/endorphin, 7.3 +/- 0.7; and alpha-MSH;3.8 +/- 0.5 pmol/g wet tissue) and the neuropeptides corticotrophin-releasing hormone and arginine vasopressin (CRH: 3.6 +/- 0.4 and AVP: 1.1 +/- 0.2 pmol/g wet tissue). Immunohistochemical staining of consecutive sections of the tumour indicated that staining of tumour cells for the different peptides was not uniform and although some cells co-stained with CRH and AVP, POMC-positive cells appeared to be distinct from CRH and AVP cells. Tumour extracts were chromatographed on Sephadex G-75 and fractions monitored for POMC-derived peptides. A single peak with characteristics of alpha-MSH was detected. The ACTH, N-POMC and beta-LPH/endorphin radioimmunoassays (RIA) detected a peak at large molecular weight, eluting at the position expected for POMC. These RIA systems also revealed an ACTH(1-39) peak and another peak which probably correspond to 13 kDa ACTH, a peak eluting at the position of hN-POMC(1-48), a beta-LPH-like peak, and a smaller sized peak which may represent alpha- or gamma-endorphin. The ACTH, N-POMC and beta-LPH/endorphin contents of anterior lobe (AL) extracts, but not neutrointermediate lobe (NIL) extracts, showed a striking decrease in tumour-bearing (TB) nude mice. However, while no difference was seen in the alpha-MSH content of AL extract between TB and control (C) nude mice, it decreased in NIL extracts of TB animals. The contents of CRH and AVP in stalk-median eminence extracts of TB nude mice was significantly lower than that of C nude mice. Basal plasma corticosteroids were raised in TB nude mice at levels comparable to those in stressed C nude mice, and although adrenal weights did not vary between TB and C nude mice, morphological changes indicating hypertrophy were found in the adrenal glands of the host animals. It was concluded that the tumour dramatically alters the hypothalamo-pituitary-adrenal axis of the host, and that it may be a useful model for studying tumour-host interactions in ectopic hormone-producing tumours.

The synthesis of ACTH and related peptides by tumours

Despite all we have learned, the reason why certain tumours and particularly non-pituitary tumours synthesize ACTH remains an enigma. There is no clear theory which links the neoplastic process with the expression of peptide hormones but it is interesting to speculate that the amplification of certain oncogenes may be linked to de-repression of hormone genes. Once the gene has been switched on, there should be some mechanism for preventing continuous expression and in the pituitary the POMC gene is normally inhibited by glucocorticoids. Therefore it is crucial to investigate the role of glucocorticoids in non-pituitary tumours and this requires an understanding of the molecular mechanisms involved in glucocorticoid inhibition of the normal POMC gene in the pituitary. The evidence presented in this chapter describing the glucocorticoid receptor binding site in the promoter region of the POMC gene in rat pituitary gives an exciting insight into the regulatory mechanisms and their potential for aberrant control. Taken with the presence of pituitary-specific regions regulating the POMC gene promoter there appear to be multiple approaches to dissecting out the differences in non-pituitary tumours. Thus in a relatively short period of time there has been a marked increase in our understanding of the molecular mechanisms underlying POMC gene expression. At the level of the peptides, progress has been slower. We are now aware that secretion of ACTH implies that a number of other peptides will be found in the circulation, even though there is limited evidence for a specific role for any of the co-secreted peptides. However, it is hard to understand the conflicting reports that N-POC is synthesized by non-small cell and small cell carcinoma of the lung when ACTH, which we assume to be co-secreted, is thought to be synthesized only by small cell carcinoma. The most likely explanation for this is the difficult nature of the radio-immunoassays for these hormones and the problems associated with studying large groups of clearly defined patients. Development of very simple methods for measuring the ACTH precursors has demonstrated that they are released into the circulation in normal subjects and that the levels are markedly elevated in non-pituitary tumours, suggesting that they are the major circulating forms in the ectopic ACTH syndrome. This implies that these tumours cannot process the precursor molecules suggesting that the processing enzymes are lacking.(ABSTRACT TRUNCATED AT 400 WORDS)

Pro-opiomelanocortin gene expression in silent corticotroph-cell adenoma and Cushing's disease

The silent corticotroph-cell adenoma (SCCA) is characterized by the presence of immunoreactive adrenocorticotropic hormone (ACTH) in the tumor tissue in patients without symptoms of Cushing's disease. To elucidate the pathophysiology of SCCA, the expression of pro-opiomelanocortin (a ACTH precursor) genes was studied in a patient with SCCA and in three patients with Cushing's disease. Pro-opiomelanocortin messenger ribonucleic acid (mRNA) was found in the SCCA tissue to a greater degree than in the adenomas of the patients with Cushing's disease. Northern blot analysis revealed that the size of pro-opiomelanocortin mRNA present in the SCCA tissue was indistinguishable from that in the adenomas associated with Cushing's disease. A ribonuclease mapping study indicated that there were no point mutations in the coding sequence of pro-opiomelanocortin mRNA present in the SCCA tissue. Because of the presence of pro-opiomelanocortin mRNA and immunoreactive ACTH in the adenoma tissue, it is proposed that translation of the mRNA and subsequent accumulation of ACTH precursor occurred in the SCCA. Thus, the absence of Cushing's disease symptoms in this SCCA could not be caused by abnormality in the coding sequence of the pro-opiomelanocortin gene or in ribonucleic acid processing. The occurrence of abnormality at or after the translational steps was strongly suggested.

Amidated joining peptide in the human pituitary, gut, adrenal gland and bronchial carcinoids. Immunocytochemical and immunochemical evidence

The distribution of the proopiomelanocortin-derivated amidated joining peptide (JP-N) was examined in the human pituitary gland, adrenal gland, gut and in three bronchial carcinoids. Double immunostaining showed coexistence of immunoreactive JP-N and other proopiomelanocortin derivatives, e.g., ACTH, beta-endorphin, Pro-tau-MSH, in the pituitary gland and adrenal medulla. The JP-N immunoreactive cells in the adrenal medulla were identified as a subpopulation of adrenaline-producing cells by means of an antiserum against phenylethanolamine N-methyltransferase. In the gut immunoreactive JP-N was costored with somatostatin in endocrine cells. Using radioimmunoassay, JP-N was found in higher concentrations than ACTH and alpha-MSH in the gut but not in the adrenal gland. Gel chromatography of gastric antrum and adrenal gland extracts showed three and two dominating components of immunoreactive JP-N, respectively, but under reduced conditions most of the immunoreactive material appeared as of low molecular weight in both extracts. In conclusion, immunoreactive JP-N is a major product from the processing of proopiomelanocortin in human extrapituitary tissues. The molecular forms of immunoreactive JP-N correspond to previous findings in the human pituitary gland.

Initiation of pro-opiomelanocortin mRNA from a normally quiescent promoter in a human small cell lung cancer cell line

We describe the characteristics of pro-opiomelanocortin (POMC) mRNA synthesized by a human small cell lung cancer (SCLC) cell line that secretes a peptide immunoreactive with antibodies to the POMC-derived component, adrenocorticotropin. While no alteration in restriction endonuclease pattern or structure was found for the SCLC-derived pomc gene vs. the previously described human pomc gene cloned from a fetal liver library, Northern-blot analysis of SCLC RNA using pomc-derived probes showed a hybridizing transcript more than 300 nucleotides longer than POMC mRNA isolated from human pituitaries, as well as a pomc-gene-hybridizing mRNA the same length as pituitary-derived transcripts. 5' end mapping and primer extension analyses showed that the novel mRNA species is initiated at a site 371 bp upstream from the 5' end identified for pituitary-derived POMC mRNA. We conclude that synthesis of POMC transcripts occurs from an ordinarily quiescent promoter in the SCLC cell line we have studied, as well as from the pomc promoter normally used in pituitary cells.

Corticotrophin-like intermediary lobe peptide as a marker of alternate pro-opiomelanocortin processing in ACTH-producing non-pituitary tumours

In order to evaluate which of human (h) corticotrophin-like intermediary lobe peptide (CLIP) or h beta-melanocyte stimulating hormone5-22 (h beta MSH5-22) was the better marker of alternate pro-opiomelanocortin (POMC) processing, both peptides were simultaneously sought in the same tissue extracts from a normal human pituitary, six corticotrophic adenomas, and four non-pituitary tumours responsible for an ectopic ACTH syndrome. Human CLIP was detected using a combination of gel exclusion chromatography and two different radioimmunoassays (RIAs): a mid-ACTH RIA which recognized ACTH but not CLIP, and a COOH-ACTH RIA which recognized both molecules. Human beta MSH5-22 had been measured previously. Neither hCLIP nor h beta MSH5-22 were detected in the normal or tumoural pituitaries. The four non-pituitary tumours, in contrast, contained both peptides; the hCLIP and h beta MSH5-22 ratios (CLIP/CLIP + ACTH and h beta MSH5-22/h beta MSH5-22 + h gamma LPH) ranged from 40 to 94% and from 24 to 46%, respectively. In a given tissue the hCLIP ratio was always higher than the h beta MSH5-22 ratio. hCLIP is therefore the better marker of alternate POMC processing.

Microsequencing evidence for the maturation of human proopiomelanocortin into an 18 amino acid beta-melanocyte stimulating hormone [h beta MSH(5-22)] in nonpituitary tissue

Sixty pmoles of a material with molecular size, immunological, and RP-HPLC characteristics identical to that of h beta MSH(5-22) were purified from a bronchial carcinoid tumor responsible for the ectopic ACTH syndrome. The first 16 cycles of microsequencing revealed the following sequence: Asp-Glu-Gly-Pro-Tyr-Arg-Met-Glu-X-Phe-Arg-Trp-Gly-X-Pro- Pro-, identical to the first 16 amino acids of h beta MSH(5-22). Since this material was recognized by an antibody which requires the free COOH-terminal Asp22 residue, it can be assumed that it is indeed h beta MSH(5-22). We also show that neither the 5 N acetic acid nor the 1 N HCl extraction procedure artefactually generated h beta MSH-like material in normal or tumoral human pituitaries and in nonpituitary tumors. We conclude that h beta MSH(5-22) is a normal maturation product of proopiomelanocortin in the human nonpituitary tissues which express its gene, including the hypothalamus and ACTH-secreting tumors.

Characterization of human corticotrophin-releasing hormone and pro-opiomelanocortin-related peptides in a thymic carcinoid tumour responsible for Cushing's syndrome

Severe Cushing's syndrome developed in a man of 35 years. Plasma ACTH and lipotrophin hormone levels were supranormal, and dexamethasone failed to stop their production. An ACTH-producing thymic carcinoid tumour was found to be responsible for the Cushing's syndrome. The tumour tissue contained pro-opiomelanocortin (POMC)-mRNA and POMC-related peptides. In addition, human corticotrophin-releasing hormone (h-CRH) (0.25 ng/mg wet tissue) was identified in the tumour extract. Among a series of extracts from two normal and three tumoral (Nelson's syndrome) pituitary glands, six non-pituitary POMC-producing tumours and five normal thymuses examined, only the extract from the thymic tumour of our patient contained h-CRH. The molecule isolated had the same properties as synthetic h-CRH (dilution, Sephadex G 50 chromatography). Circulating h-CRH levels, however, were normal. The possible involvement of such ectopic CRH production in the aetiology of Cushing's syndrome remains uncertain.

Ectopic hormone production

Current understanding of the phenomenon of ectopic hormone production is largely based on a histopathological and immunocytochemical analysis of peptide hormone secreting tumours arising in non-endocrine tissues. Recent advances in the study of gene regulation show that the tissue-specific expression of genes is a highly sophisticated process and is unlikely to be disturbed by a spontaneous event such as point mutation in DNA. Study of several genes for frequently found ectopic hormones, i.e. prop-opiomelanocortin, vasopressin/neurophysin II, gastrin-releasing peptide, parathyroid hormone-related peptide, calcitonin gene-related peptide and beta-chorionic gonadotropin, suggests they are transcribed as they would be in their natural cell of origin. It is argued therefore that these data are compatible with the concept that the tumour cell of origin was capable of expressing these peptides, if only in a minor or transient manner. In one example, the ectopic ACTH syndrome, it is also necessary to explain the non-suppression of this gene's expression by elevated levels of glucocorticoids. Recent work suggests that this may result from physically present, but biologically inactive glucocorticoid receptors, a phenomenon that has occasionally been noted in hormonally inactive tumour tissue and cell lines.

Proopiomelanocortin gene expression in a pheochromocytoma using upstream transcription initiation sites

The proopiomelanocortin (POMC) gene is expressed in many normal nonpituitary tissues, in addition to the pituitary. POMC-derived peptides have also been detected in many pheochromocytomas. We examined poly(A)+ RNA from 5 pheochromocytomas with a POMC exon 3 riboprobe and show that 3 tumors contained POMC-like mRNAs that were 50 to 350 bases longer than pituitary POMC mRNA. S1 nuclease analyses of tumor poly(A)+ RNA demonstrate that some of the POMC-like mRNAs contain additional segments of 238, 318 or 374 nucleotides derived from the region immediately upstream from the normal transcription initiation site. We conclude that pheochromocytomas express the POMC gene and that the long POMC-like mRNAs in these tumors and, probably, those in normal adrenal and testis arise from transcription initiating at sites upstream from the normal site used in pituitary.

Effects of corticotropin-releasing hormone and dexamethasone on proopiomelanocortin messenger RNA level in human corticotroph adenoma cells in vitro

The effects of corticotropin-releasing hormone (CRH) and dexamethasone on proopiomelanocortin (POMC) mRNA levels in cultured pituitary adenoma cells were studied in 10 patients with Cushing's disease. As a control, POMC mRNA levels in cells from nonadenomatous tissues were examined in four patients. Human POMC mRNA in the cells was analyzed by Northern blot hybridization. Human POMC DNA probe hybridized with only a single size class of RNA (approximately 1,200 nucleotides) from the adenoma and nonadenoma cells of each patient. The size of POMC mRNA did not change through the culture or after incubation with CRH or dexamethasone. CRH increased POMC mRNA levels in these cells in a dose- and time-dependent manner. The minimum concentration of CRH required to elevate POMC mRNA levels in these cells exposed for 15 h was 0.1 nM. The minimum duration of 1 nM CRH treatment required to increase these levels was 3 h under our conditions. Inhibitory effects of 1 and 10 micrograms/dl dexamethasone on ACTH release and POMC mRNA levels in nonadenoma cells were greater than those in adenoma cells. These results suggest the following: (a) that the mRNA in cultured pituitary adenoma cells is qualitatively the same as that in vivo; (b) that responses of mRNA levels to CRH are time- and dose-dependent; and (c) that adenoma cells resist the inhibitory effect of dexamethasone on POMC mRNA levels and ACTH release.

Use of molecular probes in the study of endocrine diseases

DNA hybridization techniques are now commonly used in studies of endocrine disease. Although solution and filter hybridization have been more widely used in endocrine studies, ISH offers many advantages to the pathologist. Individual cells can be visualized directly with the microscope to study the expression of specific gene products. ISH is somewhat similar to immunohistochemistry and can be set up rapidly in most histology laboratories, especially when nonradioactive biotinylated probes are used. The ability to combine ISH with immunochemical techniques offers to the pathologist the use of two powerful diagnostic tools simultaneously. Although DNA hybridization has only begun to have an impact in diagnostic pathology, many studies using this technique in the diagnosis of endocrine diseases are emerging. The availability of more probes, especially with nonradioactive detection systems, should accelerate the application of these techniques in diagnostic pathology. As the number of molecular probes available for hybridization studies increases, pathologists can have a major impact in this field by carefully evaluating probes that have potential diagnostic value for their specificity and sensitivity in a wide variety of normal and abnormal human tissues. Such careful and critical analyses will contribute greatly to the confidence with which these molecular probes can be used as another tool in the analysis of difficult diagnostic lesions. Molecular hybridization techniques will also contribute greatly to our knowledge of basic biology and pathophysiology in the study of abnormal gene expression in endocrine and other pathologic conditions.

Characterization of proopiomelanocortin transcripts in human nonpituitary tissues

Proopiomelanocortin (POMC), the precursor to adrenocorticotropic hormone and other related peptides, was originally identified in the corticotropic cell. Recent evidence shows that POMC products are also normally present in a variety of nonpituitary tissues. To investigate this phenomenon in humans we looked for the presence and characteristics of POMC transcripts in various adult tissues. Blot hybridization analysis of normal adrenal, thymus, and testis RNAs revealed a small RNA species approximately 400 nucleotides shorter than the 1200-nucleotide pituitary species. Primer extension and S1 nuclease mapping studies showed that this small RNA lacked exon 1 and exon 2 of the gene, and it corresponded to a set of at least six molecules starting 41 to 162 nucleotides downstream from the 5' end of exon 3. These RNAs appear to result from heterogeneous transcription initiation sites presumably under the control of "GC box" promoter sequences located in the 3' end of intron 2. They cannot encode a complete POMC molecule, and the only truncated POMC molecules that could be translated would lack a signal peptide necessary for membrane translocation and precursor processing. The use of highly sensitive S1 nuclease mapping techniques with uniformly labeled single-stranded DNA probes allowed the detection of a small but definite amount of the "normal," 1200-nucleotide, mRNA species. It is suggested that it is this POMC mRNA that is responsible for the local production of all the POMC peptides.

Cushing's syndrome due to ectopic proopiomelanocortin gene expression by islet cell carcinoma of the pancreas

Expression of proopiomelanocortin (POMC) was studied in a male patient with Cushing's syndrome and ectopic production of ACTH by a pancreatic carcinoma. Plasma ACTH levels (greater than 200 pg/ml) were elevated, and elevated serum cortisol and urinary free cortisol were partially suppressed to 25% of basal levels by high-dose dexamethasone. Petrosal and jugular vein sampling did not yield a gradient of ACTH. Immunohistochemical staining of tumor tissue removed at pancreatectomy was positive for ACTH and beta endorphin, and negative for corticotropin-releasing factor (CRF). Tumor cells cultured in vitro secreted ACTH and beta-endorphin, which comigrated with their respective radiolabeled standards on gel chromatography. Hydrocortisone suppressed in vitro ACTH secretion and CRF (100 nM) stimulated ACTH by 50% during 72 hours of incubation. Agarose gel electrophoresis of poly-(A) mRNA extracts of tumor tissue followed by hybridization with 32P-cDNA for POMC revealed 2 distinct RNA species. The major RNA species (about 1.0 kb) was smaller than authentic pituitary POMC mRNA (about 1.1 kb); a larger precursor band also was visualized, suggesting either processing or degradation of tumor-POMC mRNA. Cytoplasmic dot blot hybridization of tumor mRNA with POMC cDNA yielded a positive signal with increasing amounts of RNA blotted. Immunohistochemistry and radioimmunoassay (RIA) of ACTH, in vitro regulation of ACTH secretion, and expression of POMC mRNA species by this tumor document expression of the human POMC gene by an islet carcinoma associated with Cushing's syndrome.

Human beta-melanocyte-stimulating hormone revisited

It is generally accepted that human beta-melanocyte-stimulating hormone (h beta MSH) does not normally exist in humans but was merely an artifactually generated 22-amino acid peptide corresponding to a lipotropin (LPH) fragment (residues 35-56). We examined whether the shorter 18-amino acid peptide h beta MSH-(5-22) could be detected in some human tissues. Normal human pituitaries and hypothalami as well as corticotropin-secreting pituitary and nonpituitary tumors were extracted and chromatographed on Sephadex G-50, and the fractions were measured with two radioimmunoassays using either a COOH-terminal human gamma LPH (h gamma LPH) antiserum that recognized equally h gamma LPH, h beta MSH, and h beta MSH-(5-22) or a mid-portion h gamma LPH antiserum that recognized h gamma LPH and h beta MSH but not h beta MSH-(5-22). Normal pituitaries and pituitary tumors contained a single immunoreactive material coeluting with h gamma LPH. The hypothalami and the nonpituitary tumors all contained h gamma LPH and a smaller molecular weight material that was only detected in the COOH-terminal h gamma LPH radioimmunoassay; its elution volume (Ve/V, 0.75) was identical to that of h beta MSH-(5-22) but different from that of h beta MSH (Ve/V, 0.60); on reversed-phase HPLC, it coeluted with synthetic h beta MSH-(5-22) with a retention time different from that of h beta MSH. It is concluded that h beta MSH-(5-22) that corresponds to the 18-amino acid peptide h beta LPH-(39-56), flanked by two pairs of basic amino acids within the h beta LPH molecule, is a normal maturation product of proopiomelanocortin in human nonpituitary tissues.

Cloning and expression of cDNA for anti-müllerian hormone

Messenger RNA, prepared from fetal bovine testicular tissue, was used to construct a cDNA library in lambda gt11 phage. The library was screened with an antibody probe directed against bovine anti-Müllerian hormone and three positive clones were isolated. Cross-hybridizing cDNA inserts carried by clones 4 and 5 (1.2 and 0.08 kilobases long, respectively) code for a fragment of authentic anti-Müllerian hormone, as shown by the ability of the anti-epitope antibodies eluted from fusion protein 4 to bind strongly to anti-Müllerian hormone on immunoblots and by the capacity of anti-epitope antibodies 4 and 5 to precipitate radioiodinated bovine anti-Müllerian hormone. A probe prepared from insert 4 hybridizes with an mRNA present only in tissues that are known producers of anti-Müllerian hormone, such as the fetal testis and adult ovarian follicles. The amount of specific mRNA in tissues of males and females is related to the rate of their anti-Müllerian hormone production. The 2.1-kilobase size of this mRNA species is large enough to code for the Mr 62,000 anti-Müllerian hormone polypeptide chain. Insert 4 also hybridizes with an mRNA of similar size in human and rat fetal testicular tissue. The third isolated clone, clone 8, which does not cross-hybridize with the others, carries a cDNA insert coding for a ubiquitous protein, smaller than anti-Müllerian hormone, with which it apparently shares an epitope.