Inhibition of parathyroid hormone secretion by amino-terminal chromogranin peptides

Intricacies of the Molecular Machinery of Catecholamine Biosynthesis and Secretion by Chromaffin Cells of the Normal Adrenal Medulla and in Pheochromocytoma and Paraganglioma

The adrenal medulla is composed predominantly of chromaffin cells producing and secreting the catecholamines dopamine, norepinephrine, and epinephrine. Catecholamine biosynthesis and secretion is a complex and tightly controlled physiologic process. The pathways involved have been extensively studied, and various elements of the underlying molecular machinery have been identified. In this review, we provide a detailed description of the route from stimulus to secretion of catecholamines by the normal adrenal chromaffin cell compared to chromaffin tumor cells in pheochromocytomas. Pheochromocytomas are adrenomedullary tumors that are characterized by uncontrolled synthesis and secretion of catecholamines. This uncontrolled secretion can be partly explained by perturbations of the molecular catecholamine secretory machinery in pheochromocytoma cells. Chromaffin cell tumors also include sympathetic paragangliomas originating in sympathetic ganglia. Pheochromocytomas and paragangliomas are usually locally confined tumors, but about 15% do metastasize to distant locations. Histopathological examination currently poorly predicts future biologic behavior, thus long term postoperative follow-up is required. Therefore, there is an unmet need for prognostic biomarkers. Clearer understanding of the cellular mechanisms involved in the secretory characteristics of pheochromocytomas and sympathetic paragangliomas may offer one approach for the discovery of novel prognostic biomarkers for improved therapeutic targeting and monitoring of treatment or disease progression.

Proteomic Analysis of the Human Anterior Pituitary Gland

The pituitary function is regulated by a complex system involving the hypothalamus and biological networks within the pituitary. Although the hormones secreted from the pituitary have been well studied, comprehensive analyses of the pituitary proteome are limited. Pituitary proteomics is a field of postgenomic research that is crucial to understand human health and pituitary diseases. In this context, we report here a systematic proteomic profiling of human anterior pituitary gland (adenohypophysis) using high-resolution Fourier transform mass spectrometry. A total of 2164 proteins were identified in this study, of which 105 proteins were identified for the first time compared with high-throughput proteomic-based studies from human pituitary glands. In addition, we identified 480 proteins with secretory potential and 187 N-terminally acetylated proteins. These are the first region-specific data that could serve as a vital resource for further investigations on the physiological role of the human anterior pituitary glands and the proteins secreted by them. We anticipate that the identification of previously unknown proteins in the present study will accelerate biomedical research to decipher their role in functioning of the human anterior pituitary gland and associated human diseases.

Granin-derived peptides

The granin family comprises altogether 7 different proteins originating from the diffuse neuroendocrine system and elements of the central and peripheral nervous systems. The family is dominated by three uniquely acidic members, namely chromogranin A (CgA), chromogranin B (CgB) and secretogranin II (SgII). Since the late 1980s it has become evident that these proteins are proteolytically processed, intragranularly and/or extracellularly into a range of biologically active peptides; a number of them with regulatory properties of physiological and/or pathophysiological significance. The aim of this comprehensive overview is to provide an up-to-date insight into the distribution and properties of the well established granin-derived peptides and their putative roles in homeostatic regulations. Hence, focus is directed to peptides derived from the three main granins, e.g. to the chromogranin A derived vasostatins, betagranins, pancreastatin and catestatins, the chromogranin B-derived secretolytin and the secretogranin II-derived secretoneurin (SN). In addition, the distribution and properties of the chromogranin A-derived peptides prochromacin, chromofungin, WE14, parastatin, GE-25 and serpinins, the CgB-peptide PE-11 and the SgII-peptides EM66 and manserin will also be commented on. Finally, the opposing effects of the CgA-derived vasostatin-I and catestatin and the SgII-derived peptide SN on the integrity of the vasculature, myocardial contractility, angiogenesis in wound healing, inflammatory conditions and tumors will be discussed.

Chromogranin A: a paradoxical player in angiogenesis and vascular biology

Half a century after the discovery of chromogranin A as a secreted product of the catecholamine storage granules in the bovine adrenal medulla, the physiological role for the circulating pool of this protein has been recently coined, namely as an important player in vascular homeostasis. While the circulating chromogranin A since 1984 has proved to be a significant and useful marker of a wide range of pathophysiological and pathological conditions involving the diffuse neuroendocrine system, this protein has now been assigned a physiological "raison d'etre" as a regulator in vascular homeostasis. Moreover, chromogranin A processing in response to tissue damage and blood coagulation provides the first indication of a difference in time frame of the regulation of angiogenesis evoked by the intact chromogranin A and its two major peptide products, vasostatin-1 and catestatin. The impact of these discoveries on vascular homeostasis, angiogenesis, cancer, tissue repair and cardio-regulation will be discussed.

Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones

Mineral and bone metabolism are regulated differently in utero compared with the adult. The fetal kidneys, intestines, and skeleton are not dominant sources of mineral supply for the fetus. Instead, the placenta meets the fetal need for mineral by actively transporting calcium, phosphorus, and magnesium from the maternal circulation. These minerals are maintained in the fetal circulation at higher concentrations than in the mother and normal adult, and such high levels appear necessary for the developing skeleton to accrete a normal amount of mineral by term. Parathyroid hormone (PTH) and calcitriol circulate at low concentrations in the fetal circulation. Fetal bone development and the regulation of serum minerals are critically dependent on PTH and PTH-related protein, but not vitamin D/calcitriol, fibroblast growth factor-23, calcitonin, or the sex steroids. After birth, the serum calcium falls and phosphorus rises before gradually reaching adult values over the subsequent 24-48 h. The intestines are the main source of mineral for the neonate, while the kidneys reabsorb mineral, and bone turnover contributes mineral to the circulation. This switch in the regulation of mineral homeostasis is triggered by loss of the placenta and a postnatal fall in serum calcium, and is followed in sequence by a rise in PTH and then an increase in calcitriol. Intestinal calcium absorption is initially a passive process facilitated by lactose, but later becomes active and calcitriol-dependent. However, calcitriol's role can be bypassed by increasing the calcium content of the diet, or by parenteral administration of calcium.

Chromogranin A and the tumor microenvironment

Chromogranin A (CgA) is an acidic glycoprotein belonging to a family of regulated secretory proteins stored in the dense core granules of the adrenal medulla and of many other neuroendocrine cells and neurons. This protein is frequently used as a diagnostic and prognostic serum marker for a range of neuroendocrine tumors. Circulating CgA is also increased in patients with other diseases, including subpopulations of patients with non-neuroendocrine tumors, with important prognostic implications. A growing body of evidence suggests that CgA is more than a diagnostic/prognostic marker for cancer patients. Indeed, results of in vitro experiments and in vivo studies in animal models suggest that this protein and its fragments can affect several elements of the tumor microenvironment, including fibroblasts and endothelial cells. In this article, recent findings implicating CgA as a modulator of the tumor microenvironment and suggesting that abnormal secretion of CgA could play important roles in tumor progression and response to therapy in cancer patients are reviewed and discussed.

Catestatin in rat RVLM is sympathoexcitatory, increases barosensitivity, and attenuates chemosensitivity and the somatosympathetic reflex

The fundamental role and corollary effects of neuropeptides that govern cardiorespiratory control in the brain stem are poorly understood. One such regulatory peptide, catestatin [Cts, human chromogranin A-(352-372)], noncompetitively inhibits nicotinic-cholinergic-stimulated catecholamine release. Previously, we demonstrated the presence of chromogranin A mRNA in brain stem neurons that are important for the maintenance of arterial pressure. In the present study, using immunofluorescence histochemistry, we show that Cts immunoreactivity is colocalized with tyrosine hydroxylase in C1 neurons of the rostral ventrolateral medulla (RVLM, n = 3). Furthermore, we investigated the effects of Cts on resting blood pressure, splanchnic sympathetic nerve activity, phrenic nerve activity, heart rate, and adaptive reflexes. Cts (1 mM in 50 nl or 100 μM in 50-100 nl) was microinjected into the RVLM in urethane-anesthetized, vagotomized, ventilated Sprague-Dawley rats (n = 19). Cardiovascular responses to stimulation of carotid baroreceptors, peripheral chemoreceptors, and the sciatic nerve (somatosympathetic reflex) were analyzed. Cts (1 mM in 50 nl) increased resting arterial pressure (28 ± 3 mmHg at 2 min postinjection), sympathetic nerve activity (15 ± 3% at 2 min postinjection), and phrenic discharge amplitude (31 ± 4% at 10 min postinjection). Cts increased sympathetic barosensitivity 40% (slope increased from -0.05 ± 0.01 before Cts to -0.07 ± 0.01 after Cts) and attenuated the somatosympathetic reflex [1st peak: 36% (from 132 ± 32.1 to 84.0 ± 17.0 μV); 2nd peak: 44% (from 65.1 ± 21.4 to 36.6 ± 14.1 μV)] and chemoreflex (blood pressure response to anoxia decreased 55%, sympathetic response decreased 46%). The results suggest that Cts activates sympathoexcitatory bulbospinal neurons in the RVLM and plays an important regulatory role in adaptive reflexes.

Vasostatin 1 activates eNOS in endothelial cells through a proteoglycan-dependent mechanism

Accumulating evidences point to a significant role for the chromogranin A (CgA)-derived peptide vasostatin 1 (VS-1) in the protective modulation of the cardiovascular activity, because of its ability to counteract the adrenergic signal. We have recently shown that VS-1 induces a PI3K-dependent-nitric oxide (NO) release by endothelial cells, contributing to explain the mechanism of its cardio-suppressive and vasodilator properties. However, the cellular processes upstream the eNOS activation exerted by this peptide are still unknown, as typical high-affinity receptors have not been identified. Here we hypothesize that in endothelial cells VS-1 acts, on the basis of its cationic and amphipathic properties, as a cell penetrating peptide, binding to heparan sulfate proteoglycans (HSPGs) and activating eNOS phosphorylation (Ser1179) through a PI3K-dependent, endocytosis-coupled mechanism. In bovine aortic endothelial cells (BAE-1 cells) endocytotic vesicles trafficking was quantified by confocal microscopy with a water-soluble membrane dye; caveolin 1 (Cav1) shift from plasma membrane was studied by immunofluorescence staining; VS-1-dependent eNOS phosphorylation was assessed by immunofluorescence and immunoblot analysis. Our experiments demonstrate that VS-1 induces a marked increase in the caveolae-dependent endocytosis, (115 +/- 23% endocytotic spots/cell/field in VS-1-treated cells with respect to control cells), that is significantly reduced by both heparinase III (HEP, 17 +/- 15% above control) and Wortmannin (Wm, 7 +/- 22% above control). Heparinase, Wortmannin, and methyl-beta-cyclodextrin (MbetaCD) abolish the VS-1-dependent eNOS phosphorylation (P(Ser1179)eNOS). These results suggest a novel signal transduction pathway for endogenous cationic and amphipathic peptides in endothelial cells: HSPGs interaction and caveolae endocytosis, coupled with a PI3K-dependent eNOS phosphorylation.

Chromogranin A: a novel susceptibility gene for essential hypertension

Chromogranin A (CHGA) is ubiquitously expressed in secretory cells of the endocrine, neuroendocrine, and neuronal tissues. Although this protein has long been known as a marker for neuroendocrine tumors, its role in cardiovascular disease states including essential hypertension (EH) has only recently been recognized. It acts as a prohormone giving rise to bioactive peptides such as vasostatin-I (human CHGA(1-76)) and catestatin (human CHGA(352-372)) that exhibit several cardiovascular regulatory functions. CHGA is over-expressed but catestatin is diminished in EH. Moreover, genetic variants in the promoter, catestatin, and 3'-untranslated regions of the human CHGA gene alter autonomic activity and blood pressure. Consistent with these findings, targeted ablation of this gene causes severe arterial hypertension and ventricular hypertrophy in mice. Transgenic expression of the human CHGA gene or exogenous administration of catestatin restores blood pressure in these mice. Thus, the accumulated evidence establishes CHGA as a novel susceptibility gene for EH.

Chromogranins A and B and secretogranin II as prohormones for regulatory peptides from the diffuse neuroendocrine system

Chromogranin A (CgA), chromogranin B (CgB), and secretogranin II (SgII) belong to a family of uniquely acidic secretory proteins in elements of the diffuse neuroendocrine system. These "granins" are characterized by numerous pairs of basic amino acids as potential sites for intra- and extragranular processing. In response to adequate stimuli, the granins are coreleased with neurotransmitters and hormones and appear in the circulation as potential modulators of homeostatic processes. This review is directed towards functional aspects of the secreted CgA, CgB, and SgII and their biologically active sequences. Widely different effects and targets have been reported for granin-derived peptides. So far, the CgA peptides vasostatin-I, pancreastatin, and catestatin, the CgB peptides CgB(1-41) and secretolytin, and the SgII peptide secretoneurin are the most likely candidates for granin-derived regulatory peptides. Most of their effects fit into patterns of direct or indirect modulations of major functions, in particular associated with inflammatory conditions.

Two chromogranin a-derived peptides induce calcium entry in human neutrophils by calmodulin-regulated calcium independent phospholipase A2

Background

Antimicrobial peptides derived from the natural processing of chromogranin A (CgA) are co-secreted with catecholamines upon stimulation of chromaffin cells. Since PMNs play a central role in innate immunity, we examine responses by PMNs following stimulation by two antimicrobial CgA-derived peptides.

Methodology/Principal Findings

PMNs were treated with different concentrations of CgA-derived peptides in presence of several drugs. Calcium mobilization was observed by using flow cytometry and calcium imaging experiments. Immunocytochemistry and confocal microscopy have shown the intracellular localization of the peptides. The calmodulin-binding and iPLA2 activating properties of the peptides were shown by Surface Plasmon Resonance and iPLA2 activity assays. Finally, a proteomic analysis of the material released after PMNs treatment with CgA-derived peptides was performed by using HPLC and Nano-LC MS-MS. By using flow cytometry we first observed that after 15 s, in presence of extracellular calcium, Chromofungin (CHR) or Catestatin (CAT) induce a concentration-dependent transient increase of intracellular calcium. In contrast, in absence of extra cellular calcium the peptides are unable to induce calcium depletion from the stores after 10 minutes exposure. Treatment with 2-APB (2-aminoethoxydiphenyl borate), a store operated channels (SOCs) blocker, inhibits completely the calcium entry, as shown by calcium imaging. We also showed that they activate iPLA2 as the two CaM-binding factors (W7 and CMZ) and that the two sequences can be aligned with the two CaM-binding domains reported for iPLA2. We finally analyzed by HPLC and Nano-LC MS-MS the material released by PMNs following stimulation by CHR and CAT. We characterized several factors important for inflammation and innate immunity.

Conclusions/Significance

For the first time, we demonstrate that CHR and CAT, penetrate into PMNs, inducing extracellular calcium entry by a CaM-regulated iPLA2 pathway. Our study highlights the role of two CgA-derived peptides in the active communication between neuroendocrine and immune systems.

The chromaffin vesicle: advances in understanding the composition of a versatile, multifunctional secretory organelle

Chromaffin vesicles (CV) are highly sophisticated secretory organelles synthesized in adrenal medullary chromaffin cells. They contain a complex mixture of structural proteins, catecholamine neurotransmitters, peptide hormones, and the relative processing enzymes, as well as protease inhibitors. In addition, CV store ATP, ascorbic acid, and calcium. During the last decades, extensive studies have contributed to increase our understanding of the molecular composition of CV. Yet, the recent development of biochemical and imaging procedures has greatly increased the list of CV-soluble constituents and opened new horizons as to the complexity of CV involvement in acute stress responses. Thus, a coherent picture of CV molecular composition is still to be drawn. This review article will provide a detailed account of the content of CV soluble molecules as it emerges from the most recent analytical studies. Moreover, this review article will attempt at focussing on the physiological and pathophysiological implications of the products released by CV.

Chromogranins A and B and secretogranin II: evolutionary and functional aspects

Chromogranins/secretogranins or granins are a class of acidic, secretory proteins that occur in endocrine, neuroendocrine, and neuronal cells. Granins are the precursors of several bioactive peptides and may be involved in secretory granule formation and neurotransmitter/hormone release. Characterization and analysis of chromogranin A (CgA), chromogranin B (CgB), and secretogranin II (SgII) in distant vertebrate species confirmed that CgA and CgB belong to related monophyletic groups, probably evolving from a common ancestral precursor, while SgII sequences constitute a distinct monophyletic group. In particular, selective sequences within these proteins, bounded by potential processing sites, have been remarkably conserved during evolution. Peptides named vasostatin, secretolytin and secretoneurin, which occur in these regions, have been shown to exert various biological activities. These conserved domains may also be involved in the formation of secretory granules in different vertebrates. Other peptides such as catestatin and pancreastatin may have appeared late during evolution. The function of granins as propeptide precursors and granulogenic factors is discussed in the light of recent data obtained in various model species and using knockout mice strains.

Functional single-nucleotide polymorphisms in the secretogranin III (SCG3) gene that form secretory granules with appetite-related neuropeptides are associated with obesity

CONTEXT

Genetic factors are important for the development of obesity. However, the genetic background of obesity still remains unclear.

OBJECTIVE

Our objective was to search for obesity-related genes using a large number of gene-based single-nucleotide polymorphisms (SNPs).

DESIGN AND SETTING

We conducted case-control association analyses using 94 obese patients and 658 controls with 62,663 SNPs selected from the SNP database. SNPs that possessed P < or = 0.02 were further analyzed using 796 obese and 711 control subjects. One SNP (rs3764220) in the secretogranin III (SCG3) gene showed the lowest P value (P = 0.0000019). We sequenced an approximately 300-kb genomic region around rs3764220 and discovered SNPs for haplotype analyses. SCG3 was the only gene within a haplotype block that contained rs3764220. The functions of SCG3 were studied.

PATIENTS

Obese subjects (body mass index > or = 30 kg/m(2), n = 890) and control subjects (general population; n = 658, body mass index < or = 25 kg/m(2); n = 711) were recruited for this study.

RESULTS

Twelve SNPs in the SCG3 gene including rs3764220 were in almost complete linkage disequilibrium and significantly associated with an obesity phenotype. Two SNPs (rs16964465, rs16964476) affected the transcriptional activity of SCG3, and subjects with the minor allele seemed to be resistant to obesity (odds ratio, 9.23; 95% confidence interval, 2.77-30.80; chi(2) = 19.2; P = 0.0000067). SCG3 mRNA and immunoreactivity were detected in the paraventricular nucleus, lateral hypothalamic area, and arcuate nucleus, and the protein coexisted with orexin, melanin-concentrating hormone, neuropeptide Y, and proopiomelanocortin. SCG3 formed a granule-like structure together with these neuropeptides.

CONCLUSIONS

Genetic variations in the SCG3 gene may influence the risk of obesity through possible regulation of hypothalamic neuropeptide secretion.

Inhibition of insulin secretion by betagranin, an N-terminal chromogranin A fragment

Betagranin, an N-terminal fragment of chromogranin A, results from a proteolytic processing, and is co-secreted with insulin. While other chromogranin A-derived peptides negatively modulate hormone secretion, the role of betagranin in pancreatic beta-cells is so far unknown. We have recently shown that pancreatic islet betagranin levels are down-regulated in obese, leptin-deficient mice. In the present study, we have investigated the distribution of betagranin in primary mouse islets and cells of the MIN6 line and have evaluated its effects on insulin secretion. We showed that betagranin co-localizes with insulin within secretory granules and strongly inhibited insulin secretion in response to both glucose and potassium, by blocking the influx of calcium. The data demonstrated a hitherto unknown inhibitory effect of betagranin on insulin secretion.

Normocalcemic primary hyperparathyroidism in patients with recurrent kidney stones: pathological analysis of parathyroid glands

The lack of overt elevation of serum calcium concentration in some patients suffering from primary hyperparathyroidism is an intriguing clinical phenomenon. Previous studies have substantiated abnormal parathyroid tissue in these patients, but the extent and mode of derangements remained largely undefined. The parathyroid tissues from patients of normocalcemic primary hyperparathyroidism (NCPHPT) and those having normal parathyroid glands, hypercalcemic primary hyperplasia, secondary hyperplasia, and adenoma were compared by undertaking quantitative immunohistochemistry analysis on tissue microarray. The statistic results suggested that the parathyroid tissue of NCPHPT approximates more to normal gland than to its counterpart in other groups of parathyroid proliferative diseases in terms of the lack of significant alterations of calcium-sensing receptor (CaSR), chromogranin A (CGA), parathyroid hormone (PTH), and proliferation index (Ki67). On the other hand, the depressed vitamin D receptor (VitDR) and elevated cyclin D1 (CyD1) of NCPHPT indicated the inherent functional abnormalities in parathyroid cells. Our results imply that inherent functional disengagement may exist between CaSR and CyD1 or between CaSR and VitDR or both in parathyroid cells of symptomatic NCPHPT. Lack of enhanced release of CGA and PTH and discordance between proliferative activity and CyD1 expression in parathyroid cells may further hinder the development of hypercalcemia.

Antimicrobial chromogranins and proenkephalin-A-derived peptides: Antibacterial and antifungal activities of chromogranins and proenkephalin-A-derived peptides

The secretory granules from adrenal medullary chromaffin cells contain a complex mixture of low-molecular mass constituents such as catecholamines, ascorbate, nucleotides, calcium, peptides, and several high-molecular mass water-soluble proteins including chromogranins and proenkephalin-A. These proteins are sequestered into secretory granules in which processing yields a large variety of peptides. These fragments are released into the extracellular space upon cell stimulation and are recovered in blood, lymph, cerebrospinal fluid, and synovial fluid. Some of them have biological activity on cells in an autocrine, paracrine, or endocrine fashion. In addition, we have shown that peptides with antimicrobial activity are present with the secretory chromaffin granules and demonstrated that they are released from stimulated chromaffin cells. We have shown that posttranslational modifications modulate the antimicrobial activities. For some peptides, using confocal laser microscopy, we have examined the interaction of the rhodaminated peptides with biological membranes. In addition, we have shown that chromofungin, the antifungal peptide corresponding to chromogranin A(47-66), can bind calmodulin in the presence of calcium and induce inhibition of calcineurin, a calmodulin-dependent enzyme. Because these antibacterial peptides are colocalized with catecholamines, they may be activated during stress, playing a role as a first protective barrier against bacterial infection, and thus act as factors of the innate immunity shortly after infection and before the induction and mobilization of an adaptative immune system.

The chromogranins: their roles in secretion from neuroendocrine cells and as markers for neuroendocrine neoplasia

Chromogranins are the major components of the secretory granules of most neuroendocrine cells. Within the secretory pathway, chromogranins are involved in granulogenesis, and in sorting and processing of secretory protein cargo prior to secretion. Once secreted, they have hormonal, autocrine, and paracrine activities. The chromogranin family includes chromogranins A (CgA) and B (CgB) and secretogranin II (SgII, once called chromogranin C). The related "granins" NESP55, 7B2, secretogranin III/1B 1075 (SgIII), and secretogranin IV/HISL-19 antigen (SgIV), are also sometimes included when considering the chromogranins. While it is useful to consider the granin proteins as a family with many common features, it is also necessary to examine the distinct features and properties of individual members of the granin family to understand fully their functions, employ them efficiently as tissue, serum, and urinary markers for neuroendocrine neoplasia, and develop an evolutionary-biologic perspective on their contribution to mammalian physiology. Recent advances in chromogranin research include establishing the role of CgA in granulogenesis and the role of CgB in nuclear transcription; new biologic activities for CgA-, CgB-, and SgII-derived peptides; and new marker functions for granins and their proteolytically processed products in endocrine neoplasias.

Cleavage of Chromogranin A N-terminal Domain by Plasmin Provides a New Mechanism for Regulating Cell Adhesion

It has been proposed that chromogranin A (CgA), a protein secreted by many normal and neoplastic neuroendocrine cells, can play a role as a positive or a negative modulator of cell adhesion. The mechanisms that regulate these extracellular functions of CgA are unknown. We show here that plasmin can regulate the anti/pro-adhesive activity of CgA by proteolytic cleavage of the N-terminal domain. Limited proteolytic processing decreased its anti-adhesive activity and induced pro-adhesive effects in fibronectin or serum-dependent fibroblast adhesion assays. Cleavage of Lys(77)-Lys(78) dibasic site in CgA(1-115) was relatively rapid and associated with an increase of pro-adhesive effect. In contrast, antibodies against the region 53-90 enhanced the anti-adhesive activity of CgA and CgA(1-115). Structure-activity relationship studies showed that the conserved region 47-64 (RILSILRHQNLLKELQDL) is critical for both pro- and anti-adhesive activity. These findings suggest that CgA might work on one hand as a negative modulator of cell adhesion and on the other hand as a precursor of positive modulators, the latter requiring proteolytic processing for activation. Given the importance of plasminogen activation in tissue invasion and remodeling, the interplay between CgA and plasmin could provide a novel mechanism for regulating fibroblast adhesion and function in neuroendocrine tumors.

Structural and biological characterization of chromofungin, the antifungal chromogranin A-(47-66)-derived peptide

Vasostatin-I, the natural fragment of chromogranin A-(1-76), is a neuropeptide able to kill a large variety of fungi and yeast cells in the micromolar range. We have examined the antifungal properties of synthetic vasostatin-I-related peptides. The most active shortest peptide, named chromofungin, corresponds to the sequence Arg(47)-Leu(66). Extensive (1)H NMR analysis revealed that it adopts a helical structure. The biophysical mechanism implicated in the interaction of chromofungin with fungi and yeast cells was studied, showing the penetration of this peptide with different lipid monolayers. In order to examine thoroughly the antifungal activity of chromofungin, confocal laser microscopy was used to demonstrate the ability of the rhodamine-labeled peptide to interact with the fungal cell wall, to cross the plasma membrane, and to accumulate in Aspergillus fumigatus, Alternaria brassicola, and Candida albicans. Our present data reveal that chromofungin inhibits calcineurin activity, extending a previous observation that the N-terminal region of chromogranin A interacts with calmodulin in the presence of calcium. Therefore, the destabilization of fungal wall and plasma membrane, together with the possible intracellular inhibition of calmodulin-dependent enzymes, is likely to represent the mechanism by which vasostatin-I and chromofungin exert antifungal activity.

Analysis of the post-translational processing of chromogranin A in rat neuroendocrine tissue employing an N-terminal site-specific antiserum

Chromogranin A (CgA) is a complex prohormone expressed as a constituent of the regulated secretory pathway of numerous neuroendocrine cells. Recent investigations have demonstrated that CgA is selectively cleaved to generate distinct peptides in different neuroendocrine tissues. This investigation employed a site-specific antiserum that detects residues 98-106 rat CgA to examine the amino-terminal processing of CgA to generate beta-granin and related peptides in rat neuroendocrine tissues. Immunohistochemistry revealed moderate to intense beta-granin-like immunostaining in cells scattered throughout the anterior pituitary, thyroid, in the islets of Langerhans and in the mucosa of the gastrointestinal tract. Variable intensities of immunostaining were observed in distinct clusters of chromaffin cells. Quantitatively, the highest concentration of beta-granin-like immunoreactivity was detected in pituitary extracts. Significantly lower concentrations were detected in adrenal and thyroid glands, brain, ventral and dorsal pancreatic lobes and gastrointestinal tissue extracts. Chromatography resolved three distinct beta-granin-like immunoreactants; a large CgA-like form, an intermediate molecular form presumably corresponding to beta-granin (rat CgA1-128) and small immunoreactants that coeluted with the synthetic peptide. Two beta-granin-like immunoreactants, 21 and 22 kDa, were detected following immunoblot analysis of pituitary extracts. This study has demonstrated that chromogranin A is subject to distinct amino-terminal patterns of tissue-and cell-specific processing to generate a beta-granin-like immunoreactant which is additionally cleaved in pancreatic, fundic and colonic tissue to generate previously unidentified peptides.

Structure-activity relationships of chromogranin A in cell adhesion. Identification of an adhesion site for fibroblasts and smooth muscle cells

Previous studies showed that chromogranin A (CgA), a glycoprotein stored and co-released with various hormones by neuroendocrine cells and neurons, can modulate cell adhesion. We have investigated the structure-activity relationships of CgA using fibroblasts and coronary artery smooth muscle cells in adhesion assays. A recombinant CgA fragment 1-78 and a peptide 7-57 containing reduced and alkylated cysteines (Cys(17) and Cys(38)) induced cell adhesion after adsorption onto solid phases at 50-100 nm. Peptides lacking the disulfide loop region, including residues 47-68, 39-59, and 39-68, induced cell adhesion, either bound to solid phases at 200-400 nm or added to the liquid phase at 5-10 microm, whereas peptide 60-68 was inactive, suggesting that residues 47-57 are important for activity. The effect of CgA-(1-78) was blocked by anti-CgA antibodies against epitopes including residues Arg(53), His(54), and Leu(57). Substitutions of residues His(54), Gln(55), and Asn(56) with alanine decreased the cell adhesion activity of peptide 47-68. These results suggest that the region 47-57 (RILSILRHQNL) contains a cell adhesion site and that the disulfide bridge is not necessary for the proadhesive activity. The ability of soluble peptides to elicit proadhesive effects suggests an indirect mechanism. The high sequence conservation and accessibility to antibodies suggest that this region is important for the physiological role of CgA.

Antibacterial and antifungal activities of vasostatin-1, the N-terminal fragment of chromogranin A

Vasostatin-1, the natural N-terminal 1-76 chromogranin A (CGA)-derived fragment in bovine sequence, has been purified from chromaffin secretory granules and identified by sequencing and matrix-assisted laser desorption time-of-flight mass spectrometry. This peptide, which displays antibacterial activity against Gram-positive bacteria at micromolar concentrations, is also able to kill a large variety of filamentous fungi and yeast cells in the 1-10 microM range. We have found that the C-terminal moiety of vasostatin-1 is essential for the antifungal activity, and shorter active peptides have been synthesized. In addition, from the comparison with the activity displayed by related peptides (human recombinant and rat synthetic fragments), we could determine that antibacterial and antifungal activities have different structural requirements. To assess for such activities in vivo, CGA and CGA-derived fragments were identified in secretory material released from human polymorphonuclear neutrophils upon stimulation. Vasostatin-1, which is stored in a large variety of cells (endocrine, neuroendocrine, and neurons) and which is liberated from stimulated chromaffin and immune cells upon stress, may represent a new component active in innate immunity.

Frog chromogranin A messenger ribonucleic acid encodes three highly conserved peptides. Coordinate regulation of proopiomelanocortin and chromogranin A gene expression in the pars intermedia of the pituitary during background color adaptation

Chromogranin A (CgA) is a neuroendocrine secretory protein that is widely used as a marker for endocrine neoplasms but whose function is not completely understood. In mammals, it is thought that CgA is a precursor for biologically active peptides. Here, we describe the cloning of a complementary DNA encoding CgA from a nonmammalian vertebrate, the frog Rana ridibunda. Sequence analysis revealed that frog CgA exhibits only 40-44% amino acid sequence similarity with its mammalian homologues. The amino acid identity is confined to three regions (70-80% identity) of the protein that are flanked by conserved pairs of basic amino acid residues, suggesting that proteolytic processing at these cleavage sites may give rise to three biologically active peptides whose sequences have been highly preserved during evolution. Tissue distribution analysis by Northern blot and in situ hybridization revealed the widespread expression of frog CgA messenger RNA in the brain and in endocrine tissues, the highest concentration occurring in the distal lobe of the pituitary. Adaptation of frog skin color to a dark background caused a concomitant increase in CgA and POMC messenger RNA levels in the intermediate lobe of the pituitary. Taken together, these data indicate that CgA may function as a precursor to three highly conserved peptides that may exert regulatory functions in the neuroendocrine system.

Ca2+-induced increases in steady-state concentrations of intracellular calcium are not required for inhibition of parathyroid hormone secretion

It has been well established that increases in extracellular calcium concentration ([Ca2+]) inhibit parathyroid hormone (PTH) secretion. The effects of [Ca2+] are mediated through a G-protein-coupled receptor that has been cloned and characterized. Additionally, it has been demonstrated in parathyroid cells that an increase in [Ca2+] results in an increase in steady-state levels of intracellular calcium ([Ca2+]i). At present, it has not been fully resolved whether changes in [Ca2+]i are related to changes in PTH secretion. In the current study, the effect of increased [Ca2+] on PTH secretion and the connection regarding changes in concentrations of intracellular calcium [Ca2+]i have been examined in primary cultures of bovine parathyroid cells. PTH secretion was measured by radioimmunoassay and intracellular calcium was determined by single cell calcium imaging. Bovine parathyroid cells pre-incubated with either 0.5 or 1 mM calcium responded to rapid increases in [Ca2+] (> or = 0.5 mM) with an immediate and sustained increase in steady-state levels of [Ca2+]i that persisted for time intervals greater than 15 minutes. Although the magnitude of the sustained increase in [Ca2+]i varied among individual cells (approximately 40% to > 300%), the overall pattern and course of time were similar in all cells examined (n = 142). In all trials, [Ca2+]i immediately returned to baseline levels following the addition of the calcium chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). Additional control studies, however, suggest that sustained increases in [Ca2+]i do not correlate with regulation of parathyroid hormone secretion. Sustained elevations of [Ca2+]i were not observed when [Ca2+] was gradually increased by the addition of 0.1 mM increments at 1 minute intervals. Furthermore, the effect on inhibition of PTH secretion was the same regardless of whether [Ca2+] was increased by gradual or rapid addition.

Neuroendocrine differentiation in prostatic carcinoma

BACKGROUND

Information is presented on prostatic neuroendocrine cells and neuroendocrine differentiation in prostatic carcinoma. The prognostic and therapeutic implications of neuroendocrine differentiation in prostatic carcinoma are reviewed.

METHODS

Data are presented that support the intriguing link between neuroendocrine differentiation, tumor progression, and androgen-independent prostate cancer. The hormones, and the receptors, expressed by prostatic neuroendocrine cells are investigated in order to elucidate their significance for prognosis and therapy.

RESULTS

The prognostic significance of neuroendocrine differentiation in prostatic malignancy has been controversial, but recent studies employing markers such as chromogranin A and neuron-specific enolase suggest that neuroendocrine differentiation, as reflected by increased tissue expression and/or blood levels of these neuroendocrine secretory products, correlates with poor prognosis, tumor progression, and androgen-independence. Since all malignant neuroendocrine cells are devoid of androgen receptors and since neuroendocrine phenotypic expression is not suppressed by androgen ablation, clonal propagation of androgen receptor-negative neuroendocrine cells may play an important role in the pathway towards the androgen-independent state of prostatic carcinoma. This would have significant implications for the treatment of prostate cancer, as several of the hormones known to be expressed by neuroendocrine-differentiated, malignant prostatic cells are potential candidates for drug therapy. A limited number of hormones have been tested in this context, in particular somatostatin, bombesin, and serotonin.

CONCLUSIONS

Neuroendocrine differentiation in carcinoma of the prostate appears to be associated with poor prognosis, tumor progression, and the androgen-independent state, for which there is currently no successful therapy. Therefore, new therapeutic protocols and trials need to be developed to test drugs based on neuroendocrine hormones and/or their antagonists. An evaluation of this new therapeutic approach against prostatic carcinoma with neuroendocrine differentiation, including hormone-refractory cancer, is easily justified, since these tumors are unresponsive to current modes of therapy.

The chemokine interleukin-8 regulates parathyroid secretion

Interleukin-8 (IL-8) is a chemokine important in inflammatory processes. Homology cloning experiments performed using bovine parathyroid cDNA and degenerate primers encoding transmembrane regions III and VI of peptide and protein hormone G-protein coupled receptors identified a set of known receptors not previously identified in the parathyroid. Among these was the IL-8 type B receptor. Incubation of freshly isolated bovine parathyroid cells with recombinant IL-8 for 6-48 h produced an increase in the levels of mRNA for parathyroid hormone (PTH). The levels of PTH secreted in response to nanomolar amounts of IL-8 were also elevated in cells incubated for 1 h with IL-8. Differential display analysis of mRNA from parathyroid cells, incubated in the presence and absence of IL-8, permitted the identification of cDNA clones for RNA species whose expression was either elevated or suppressed. These experiments suggest that IL-8 and inflammatory events play a role in bone homeostasis through actions on the parathyroid gland.

Chromogranin A: its clinical value as marker of neuroendocrine tumours

Chromogranin A (CgA) belongs to a family of secretory proteins that are present in densecore vesicles of neuroendocrine cells. Owing to its widespread distribution in neuroendocrine tissues, it can be used as an excellent immunohistochemical marker of neoplasms of neuroendocrine origin. It can also serve as serum marker of neuroendocrine activity because it is co-released with the peptide hormone content of the secretory granules. The serum concentration of CgA is elevated in patients with various neuroendocrine tumours. Elevated levels are strongly correlated with tumour volume. Although its sensitivity and specificity cannot compete with that of the specific hormonal secretion products of most of these tumours, it can nevertheless have useful clinical applications. Neuroendocrine tumours for which no peptide marker is available usually retain the capacity to secrete CgA. CgA can thus be used as serum marker for these so-called 'non-functioning' endocrine tumours. Moreover, in patients with carcinoids and phaeochromocytomas, CgA is a more stable and thus more easily manageable marker than plasma levels of respectively serotonin and catecholamines and their urinary metabolites. Its role as an important general neuroendocrine marker may be extended in the future by the development of immunoscintigraphy of membrane-bound CgA, allowing in vivo visualization of neuroendocrine neoplasms.