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

Pituitary protein 7B2 plasma levels in patients with liver disease: Comparisons with other hormones and neuropeptides

7B2, a protein initially isolated from the porcine pituitary gland, has been identified in numerous animal and human tissues, with the highest concentrations in the pituitary and hypothalamus. The 7B2 molecule is highly evolutionarily conserved and is considered to be indispensable in the function and regulation of proprotein convertase 2 (PC2). In the present study, the plasma 7B2 immunoreactivity (7B2-IR) of 18 patients with liver disease was studied. Of these patients, seven (three male and four female), aged 37–67 [54.6±13.5 (SD)] years, suffered from liver cirrhosis of cryptogenic (n=2) or alcoholic (n=5) aetiology. The remaining 11 patients (four male and seven female), aged 22–76 [56.1±17.6 (SD)] years, suffered from miscellaneous liver abnormalities. The clinical diagnosis was confirmed in the majority of patients by the histological examination of a percutaneous liver biopsy or by appropriate radiological investigations. Plasma bilirubin, alkaline phosphatase, aspartate aminotransferase, albumin, prothrombin time, electrolytes, urea and creatinine were measured. The plasma 7B2-IR levels were estimated using a sensitive radioimmunoassay (RIA), and the elution position of 7B2-IR was verified by gel chromatography. The mean plasma 7B2-IR concentration in patients with liver disease was 99.44±15.9 pmol/l. In the patients with hepatocellular damage due to metastatic tumours [Ca bronchus, carcinoid (n=6)], the 7B2-IR concentrations were significantly higher [185±36.9 pmol/l, (P<0.05)] compared with the overall subjects with liver damage. The results of the present study demonstrate that 7B2-IR is increased in liver disease, with the highest levels detected in patients with tumourous liver conditions.

Clinical features and outcomes of primary hepatic neuroendocrine carcinomas

BACKGROUND AND AIM

Primary hepatic neuroendocrine carcinomas (PHNECs) are extremely rare, with only about 150 cases having been reported in the English-language literature. Because of the rarity of PHNECs, its clinical features and treatment outcomes are not well understood. Here, we report our experiences with PHNECs.

METHODS

We identified patients diagnosed with PHNEC and analyzed their demographics, baseline laboratory data, tumor characteristics, treatment modalities and outcomes.

RESULTS

A total of 218 consecutive patients were identified with pathologically confirmed neuroendocrine carcinoma. Of these, 12 patients were diagnosed with PHNECs; the median age was 66.5 years (range, 37 to 80 years), and seven patients (58.3%) were male. Two patients who each had a single hepatic mass underwent curative surgical resection. One patient who was of inoperable status at the initial diagnosis because of multiple intrahepatic metastases showed a partial response after the ninth round of systemic chemotherapy and then underwent surgical resection. The median overall survival in the 12 patients was 16.5 months (range, 0.7 to 41.7 months). Three patients who underwent surgical treatment are alive without recurrence for 15.2 months, 18.0 months, and 36.9 months, respectively.

CONCLUSIONS

Primary hepatic neuroendocrine carcinoma should be considered as a possible differential diagnosis in the management of hepatic tumors. The liver can be the primary origin of neuroendocrine tumors, and if the tumors are diagnosed as primary hepatic neuroendocrine tumors, surgical resection must be considered for curative treatment.

The extended granin family: structure, function, and biomedical implications

The chromogranins (chromogranin A and chromogranin B), secretogranins (secretogranin II and secretogranin III), and additional related proteins (7B2, NESP55, proSAAS, and VGF) that together comprise the granin family subserve essential roles in the regulated secretory pathway that is responsible for controlled delivery of peptides, hormones, neurotransmitters, and growth factors. Here we review the structure and function of granins and granin-derived peptides and expansive new genetic evidence, including recent single-nucleotide polymorphism mapping, genomic sequence comparisons, and analysis of transgenic and knockout mice, which together support an important and evolutionarily conserved role for these proteins in large dense-core vesicle biogenesis and regulated secretion. Recent data further indicate that their processed peptides function prominently in metabolic and glucose homeostasis, emotional behavior, pain pathways, and blood pressure modulation, suggesting future utility of granins and granin-derived peptides as novel disease biomarkers.

Granins and their derived peptides in normal and tumoral chromaffin tissue: Implications for the diagnosis and prognosis of pheochromocytoma

Pheochromocytomas are rare catecholamine-secreting tumors that arise from chromaffin tissue within the adrenal medulla and extra-adrenal sites. Typical clinical manifestations are sustained or paroxysmal hypertension, severe headaches, palpitations and sweating resulting from hormone excess. However, their presentation is highly variable and can mimic many other diseases. The diagnosis of pheochromocytomas depends mainly upon the demonstration of catecholamine excess by 24-h urinary catecholamines and metanephrines or plasma metanephrines. Occurrence of malignant pheochromocytomas can only be asserted by imaging of metastatic lesions, which are associated with a poor survival rate. The characterization of tissue, circulating or genetic markers is therefore crucial for the management of these tumors. Proteins of the granin family and their derived peptides are present in dense-core secretory vesicles and secreted into the bloodstream, making them useful markers for the identification of neuroendocrine cells and neoplasms. In this context, we will focus here on reviewing the distribution and characterization of granins and their processing products in normal and tumoral chromaffin cells, and their clinical usefulness for the diagnosis and prognosis of pheochromocytomas. It appears that, except SgIII, all members of the granin family i.e. CgA, CgB, SgII, SgIV-SgVII and proSAAS, and most of their derived peptides are present in adrenomedullary chromaffin cells and in pheochromocytes. Moreover, besides the routinely used CgA test assays, other assays have been developed to measure concentrations of tissue and/or circulating granins or their derived peptides in order to detect the occurrence of pheochromocytomas. In most cases, elevated levels of these entities were found, in correlation with tumor occurrence, while rarely discriminating between benign and malignant neoplasms. Nevertheless, measurement of the levels of granins and derived peptides improves the diagnostic sensitivity and may therefore provide a complementary tool for the management of pheochromocytomas. However, the existing data need to be substantiated in larger groups of patients, particularly in the case of malignant disease.

Dysregulation of dynorphins in Alzheimer disease

The opioid peptides dynorphins may be involved in pathogenesis of Alzheimer disease (AD) by inducing neurodegeneration or cognitive impairment. To test this hypothesis, the dynorphin system was analyzed in postmortem samples from AD and control subjects, and subjects with Parkinson or cerebro-vascular diseases for comparison. Dynorphin A, dynorphin B and related neuropeptide nociceptin were determined in the Brodmann area 7 by radioimmunoassay. The precursor protein prodynorphin, processing convertase PC2 and the neuroendocrine pro7B2 and 7B2 proteins required for PC2 maturation were analyzed by Western blot. AD subjects displayed robustly elevated levels of dynorphin A and no differences in dynorphin B and nociceptin compared to controls. Subjects with Parkinson or cerebro-vascular diseases did not differ from controls with respect to any of the three peptides. PC2 levels were also increased, whereas, those of prodynorphin and pro7B2/7B2 were not changed in AD. Dynorphin A levels correlated with the neuritic plaque density. These results along with the known non-opioid ability of dynorphin A to induce neurodegeneration suggest a role for this neuropeptide in AD neuropathology.

The proprotein convertase PC2 is involved in the maturation of prosomatostatin to somatostatin-14 but not in the somatostatin deficit in Alzheimer's disease

A somatostatin deficit occurs in the cerebral cortex of Alzheimer's disease patients without a major loss in somatostatin-containing neurons. This deficit could be related to a reduction in the rate of proteolytic processing of peptide precursors. Since the two proprotein convertases (PC)1 and PC2 are responsible for the processing of neuropeptide precursors directed to the regulated secretory pathway, we examined whether they are involved first in the proteolytic processing of prosomatostatin in mouse and human brain and secondly in somatostatin defect associated with Alzheimer's disease. By size exclusion chromatography, the cleavage of prosomatostatin to somatostatin-14 is almost totally abolished in the cortex of PC2 null mice, while the proportions of prosomatostatin and somatostatin-28 are increased. By immunohistochemistry, PC1 and PC2 were localized in many neuronal elements in human frontal and temporal cortex. The convertases levels were quantified by Western blot, as well as the protein 7B2 which is required for the production of active PC2. No significant change in PC1 levels was observed in Alzheimer's disease. In contrast, a marked decrease in the ratio of the PC2 precursor to the total enzymatic pool was observed in the frontal cortex of Alzheimer patients. This decrease coincides with an increase in the binding protein 7B2. However, the content and enzymatic activity of the PC2 mature form were similar in Alzheimer patients and controls. Therefore, the cortical somatostatin defect is not due to convertase alteration occuring during Alzheimer's disease. Further studies will be needed to assess the mechanisms involved in somatostatin deficiency in Alzheimer's disease.

Neuroendocrine secretory protein 7B2: structure, expression and functions

7B2 is an acidic protein residing in the secretory granules of neuroendocrine cells. Its sequence has been elucidated in many phyla and species. It shows high similarity among mammals. A Pro-Pro-Asn-Pro-Cys-Pro polyproline motif is its most conserved feature, being carried by both vertebrate and invertebrate sequences. It is biosynthesized as a precursor protein that is cleaved into an N-terminal fragment and a C-terminal peptide. In neuroendocrine cells, 7B2 functions as a specific chaperone for the proprotein convertase (PC) 2. Through the sequence around its Pro-Pro-Asn-Pro-Cys-Pro motif, it binds to an inactive proPC2 and facilitates its transport from the endoplasmic reticulum to later compartments of the secretory pathway where the zymogen is proteolytically matured and activated. Its C-terminal peptide can inhibit PC2 in vitro and may contribute to keep the enzyme transiently inactive in vivo. The PC2-7B2 model defines a new neuroendocrine paradigm whereby proteolytic activation of prohormones and proneuropeptides in the secretory pathway is spatially and temporally regulated by the dynamics of interactions between converting enzymes and their binding proteins. Interestingly, unlike PC2-null mice, which are viable, 7B2-null mutants die early in life from Cushing's disease due to corticotropin ('ACTH') hypersecretion by the neurointermediate lobe, suggesting a possible involvement of 7B2 in secretory granule formation and in secretion regulation. The mechanism of this regulation is yet to be elucidated. 7B2 has been shown to be a good marker of several neuroendocrine cell dysfunctions in humans. The possibility that anomalies in its structure and expression could be aetiological causes of some of these dysfunctions warrants investigation.

The neuroendocrine protein 7B2 is required for peptide hormone processing in vivo and provides a novel mechanism for pituitary Cushing's disease

The neuroendocrine protein 7B2 has been implicated in activation of prohormone convertase 2 (PC2), an important neuroendocrine precursor processing endoprotease. To test this hypothesis, we created a null mutation in 7B2 employing a novel transposon-facilitated technique and compared the phenotypes of 7B2 and PC2 nulls. 7B2 null mice have no demonstrable PC2 activity, are deficient in processing islet hormones, and display hypoglycemia, hyperproinsulinemia, and hypoglucagonemia. In contrast to the PC2 null phenotype, these mice show markedly elevated circulating ACTH and corticosterone levels, with adrenocortical expansion. They die before 9 weeks of severe Cushing's syndrome arising from pituitary intermediate lobe ACTH hypersecretion. We conclude that 7B2 is indeed required for activation of PC2 in vivo but has additional important functions in regulating pituitary hormone secretion.

Rho proteins are localized with different membrane compartments involved in vesicular trafficking in anterior pituitary cells

In order to explore the role of certain GTP binding proteins in the rat anterior pituitary, we have analyzed the subcellular distribution of the proteins rho and rab. They were found in both membrane and cytosolic fractions. Rab1 and rab2 were localized in both Golgi and endoplasmic reticulum (ER) membranes, while rab4 and rab6 were found in fractions enriched with Golgi and plasma membranes, implicating these proteins in the control of vesicular intracellular trafficking as described in other systems. Rab3 was localized like a fraction of synaptophysin, suggesting a role for rab3 in the targeting of "synaptic-like' microvesicles. We have identified three substrates of C. botulinum exoenzyme C3. A 26-kDa substrate with an isoelectric point (pI) of 5.2, probably rhoB, was localized in the lightest fractions such as rab3 and synaptophysin proteins. Two other 23-24 kDa substrates with pI of 5.5-5.8, probably rhoA and/or rhoC, were found in both fractions enriched with ER and secretory granules. Rho proteins have been implicated in the control of actin polymerization. Their localization in anterior pituitary suggests that rhoB could control the association of synaptic-like microvesicles and plasma membrane, and that rhoA/rhoC could play a role in secretory granule exocytosis; these two pathways being involved in cytoskeleton protein reorganisation in response to extracellular signals.

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.

A C-terminal domain, which prevents secretion of the neuroendocrine protein 7B2 in Saccharomyces cerevisiae, inhibits Kex2 yet is processed by the Yap3 protease

Recent reports reveal that the C-terminal half of the neuroendocrine polypeptide 7B2 selectively inhibits and binds PC2, a mammalian prohormone converting enzyme that is homologous to the yeast pro-alpha-factor processing protease Kex2. During attempted secretion of the 185 amino-acid human 7B2 in Saccharomyces cerevisiae, we observe that the protein is mostly retained inside the cell. However a mutant polypeptide (7B2 delta 1), where the C-terminal 48 amino acids of 7B2 are deleted, is efficiently secreted. Two shorter C-terminal truncations either permit poor secretion or no secretion at all. Surprisingly, full-length 7B2 but not 7B2 delta 1 abolishes the catalytic activity of Kex2, indicating that C-terminal residues of 7B2 might also be important for inhibition of the yeast protease. When the KEX2 gene is disrupted, yeast cells unexpectedly secrete a 7B2 variant similar in size to 7B2 delta 1, suggesting involvement of the alternate yeast prohormone convertase Yap3 in processing. Secretion is enhanced by overexpression of Yap3 and by the presence of a Lys-Arg residue at the processing site of precursor 7B2. These results purport that, in neuroendocrine cells too, secretion of 7B2 could be mediated by a homologue of Yap3.