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

Ectopic adrenocorticotropic hormone syndrome: a diagnostic challenge and review of the literature

We present a case of a 60-year-old woman who initially presented with pneumonia and abdominal pain and was diagnosed with ectopic adrenocorticotropic hormone (ACTH) syndrome secondary to small cell lung cancer. We review published literature and summarize the typical challenges in the diagnosis and treatment of ectopic ACTH syndrome. Recent research has shed new light on the mechanism of ectopic ACTH production and provided a potential new target for treatment.

The human POMC gene promoter: where do we stand?

Proopiomelanocortin (POMC) is crucial for several life-essential functions and its regulation has been studied extensively in the past decades. The first studies provided the framework for POMC promoter activity, namely the identification for the major response elements contained in the promoter, e.g., the glucocorticoid response element, the Nur response element, while subsequent studies showed the importance of cooperation and interplay between transcription factors to achieve optimal promoter activity. The involvement of constitutive repressors of POMC transcription, such as Bmp4, provided the latest clues to our understanding of POMC promoter activity. This increased knowledge benefits the clinician as it allows genetic testing and early recognition of patients with congenital ACTH deficiency due to mutations in TPIT and paves the way to new medical treatments in Cushing's disease. The present review will illustrate the current standing on regulation of the human POMC promoter, focusing on its activity in corticotropes.

Ontogenetic changes in the extra-pituitary expression of pro-opiomelanocortin in the developing ovine fetus

Previous studies performed in this laboratory have demonstrated that the fetal lung contains immunoreactive adrenocorticotropin (irACTH), and that the lung both clears and secretes irACTH under basal and stimulated conditions. Furthermore, we have demonstrated that the irACTH in fetal lung is accounted for by proopiomelanocortin (POMC), and that there is an evidence of post-translational processing that is distinct from the pattern of processing typical of the anterior pituitary. The present study was designed to test the hypothesis that POMC is synthesized in the fetal lung, and that there is decreased synthesis in the late-gestation ovine fetal lung. Lungs were collected from fetal sheep at 80, 100, 120, 130, and 145 days gestation (n=4/group; term=147 days). POMC mRNA was measured using reverse transcription and real-time polymerase chain reaction with probe and primers designed in this laboratory. The greatest abundance of POMC mRNA was in the 80-days fetal sheep, and the relative abundance decreased as a function of fetal gestational age. POMC protein was measured using immunoblot analysis in lungs from 80, 120, and 145-days fetal sheep. The pattern of POMC protein abundance was consistent with that of the mRNA (highest at 80 days, lowest at 145 days). The POMC immunoblot revealed specific staining of a peptide with molecular weight of 27 kDa and another peptide with a molecular weight slightly higher than that of native POMC (32 kDa). For comparison, we measured POMC mRNA in skeletal muscle and small intestine. We found POMC expression in both fetal tissues, but no statistically significant ontogenetic pattern of expression. We conclude that POMC is synthesized in the ovine fetal lung, and that the rate of synthesis decreases as the fetus matures in utero. We speculate that the decreasing abundance of POMC mRNA and protein reflects decreased release of POMC and POMC-related peptides into the fetal bloodstream.

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.

Corticotropin releasing hormone and proopiomelanocortin involvement in the cutaneous response to stress

The skin is a known target organ for the proopiomelanocortin (POMC)-derived neuropeptides alpha-melanocyte stimulating hormone (alpha-MSH), beta-endorphin, and ACTH and also a source of these peptides. Skin expression levels of the POMC gene and POMC/corticotropin releasing hormone (CRH) peptides are not static but are determined by such factors as the physiological changes associated with hair cycle (highest in anagen phase), ultraviolet radiation (UVR) exposure, immune cytokine release, or the presence of cutaneous pathology. Among the cytokines, the proinflammatory interleukin-1 produces important upregulation of cutaneous levels of POMC mRNA, POMC peptides, and MSH receptors; UVR also stimulates expression of all the components of the CRH/POMC system including expression of the corresponding receptors. Molecular characterization of the cutaneous POMC gene shows mRNA forms similar to those found in the pituitary, which are expressed together with shorter variants. The receptors for POMC peptides expressed in the skin are functional and include MC1, MC5 and mu-opiate, although most predominant are those of the MC1 class recognizing MSH and ACTH. Receptors for CRH are also present in the skin. Because expression of, for example, the MC1 receptor is stimulated in a similar dose-dependent manner by UVR, cytokines, MSH peptides or melanin precursors, actions of the ligand peptides represent a stochastic (predictable) nonspecific response to environmental/endogenous stresses. The powerful effects of POMC peptides and probably CRH on the skin pigmentary, immune, and adnexal systems are consistent with stress-neutralizing activity addressed at maintaining skin integrity to restrict disruptions of internal homeostasis. Hence, cutaneous expression of the CRH/POMC system is highly organized, encoding mediators and receptors similar to the hypothalamic-pituitary-adrenal (HPA) axis. This CRH/POMC skin system appears to generate a function analogous to the HPA axis, that in the skin is expressed as a highly localized response which neutralizes noxious stimuli and attendant immune reactions.

Proopiomelanocortin production by epidermal cells: evidence for an immune neuroendocrine network in the epidermis

Proopiomelanocortin (POMC) is known to be synthesized in the pituitary gland and is subsequently cleaved by specific prohormone convertases into biologically active peptide hormones such as melanocyte stimulating hormones (MSH), adrenocorticotropin (ACTH) and endorphins (EP). Guanine nucleotide-binding protein (G-protein)-coupled receptors, which have only recently been discovered, are involved in the transmission of their message. There is also evidence indicating that POMC is not only produced by pituitary cells but is an ubiquitous molecule, that is cleaved cell- and tissue-specific. It has also been shown that the epidermis keratinocytes as well as melanocytes express POMC upon stimulation and release alpha MSH and ACTH. In addition to their function as hormones, POMC peptides have been shown to exert a variety of immunoregulatory effects by modulating the function of immunocompetent cells as well as cytokines. These findings provide further evidence for the immunoneuroendocrine network playing a crucial role during the pathogenesis of immune and inflammatory skin disease.

A novel hypothesis: specific oncogenes and tumor suppression genes are involved in the expression of the proopiomelanocortin gene by small cell lung cancer

The endogenous opioid beta-endorphin, a derivative of proopiomelanocortin, stimulates the growth of cloned human small cell lung carcinoma. The present hypothesis states that mutations of the retinoblastoma gene (a tumor suppressor gene) associated to the malignant transformation of bronchial cells would trigger a cascade of biomolecular events leading to 'de novo' proopiomelanocortin expression in small cell lung carcinoma.

On the potential role of proopiomelanocortin in skin physiology and pathology

Pro-opiomelanocortin (POMC) is the precursor of ACTH, alpha-MSH and beta-endorphin, neuropeptides with multiple regulatory functions. Both the pituitary gland and peripheral tissues such as mammalian skin are capable of generating POMC-derived neuropeptides. Mammalian skin is also a target for POMC products; their possible roles in skin physiology and pathology are discussed in this communication.

Expression of the pro-opiomelanocortin gene in lung neuroendocrine tumours: in situ hybridization and immunohistochemical studies

Neuroendocrine tumours of the lung may be associated with the ectopic adrenocorticotrophin (ACTH) syndrome and may synthesize and secrete ACTH-related peptides in the absence of the syndrome. However, immunocytochemical analysis may not confirm these biochemical findings, particularly in small cell carcinoma, which is poorly granulated. To investigate further the morphological evidence for expression of the pro-opiomelanocortin (POMC) gene in neuroendocrine lung tumours, we have examined a series of 46 small cell carcinomas and 13 carcinoid tumours of the lung by in situ hybridization for POMC mRNA using a digoxigenin-labelled oligoprobe. We have compared the findings with the immunocytochemical detection of ACTH and beta-endorphin. In situ hybridization was positive in 15 of 46 small cell carcinomas (33 per cent) and in 8 of 13 carcinoid tumours (62 per cent). Immunocytochemical staining was positive in only one carcinoid tumour. These in situ hybridization studies have corroborated biochemical data indicating POMC gene expression in a high proportion of lung neuroendocrine tumours. This suggests that the low levels of expression detected by immunocytochemistry may be due to low levels of hormone storage. Multivariate analysis showed a weak negative association between POMC expression and survival in small cell carcinomas, although this did not reach statistical significance.

POMC gene expression in mouse and hamster melanoma cells

Expression of mRNA hybridizable to murine POMC cDNA, and immunoreactivity specific for POMC derived peptide, beta-endorphin, are reported for the first time in one murine and two hamster melanoma lines. The apparent sizes of POMC mRNA transcript were 3.5, 1.5 and 1 kb. It is suggested that POMC gene expression may be involved in the autoregulation of the melanoma phenotype at the cellular and tissue levels.