Regulation of the biosynthesis of large dense-core vesicles in chromaffin cells and neurons

1. The proteins of large dense-core vesicles (LDV) in neuroendocrine tissues are well characterized. Secretory components comprise chromogranins and neuropeptides. Intrinsic membrane proteins include cytochrome b-561, transporters, SV2, synaptotagmin, and synaptobrevin. 2. The effects of stimulation and of second messengers on the biosynthesis of LDV have been studied in detail. 3. Regulation of biosynthesis is complex. The cell can adapt to prolonged stimulation either by producing vesicles of normal size filled with a higher quantum of secretory peptides or by forming larger vesicles. In addition, some components, e.g., enzymes, can be upregulated specifically.

G protein-coupled cholecystokinin-B/gastrin receptors are responsible for physiological cell growth of the stomach mucosa in vivo

Many peptide hormone and neurotransmitter receptors belonging to the seven membrane-spanning G protein-coupled receptor family have been shown to transmit ligand-dependent mitogenic signals in vitro. However, the physiological roles of the mitogenic activity through G protein-coupled receptors in vivo remain to be elucidated. Here we have generated G protein-coupled cholecystokinin (CCK)-B/gastrin receptor deficient-mice by gene targeting. The homozygous mice showed a remarkable atrophy of the gastric mucosa macroscopically, even in the presence of severe hypergastrinemia. The atrophy was due to a decrease in parietal cells and chromogranin A-positive enterochromaffin-like cells expressing the H+,K(+)-ATPase and histidine decarboxylase genes, respectively. Oral administration of a proton pump inhibitor, omeprazole, which induced hypertrophy of the gastric mucosa with hypergastrinemia in wild-type littermates, did not eliminate the gastric atrophy of the homozygotes. These results clearly demonstrated that the G protein-coupled CCK-B/gastrin receptor is essential for the physiological as well as pathological proliferation of gastric mucosal cells in vivo.

Chromogranin B (secretogranin I) promotes sorting to the regulated secretory pathway of processing intermediates derived from a peptide hormone precursor

Chromogranin B (CgB, secretogranin I) is a widespread constituent of neuroendocrine secretory granules whose function is unknown. To determine whether CgB affects the sorting of peptide hormone and neuropeptide precursors to secretory granules, we overexpressed CgB in AtT-20 cells, which exhibit an only moderate capacity to sort proopiomelanocortin and proteolytic fragments derived therefrom. In mock-transfected AtT-20 cells, a substantial proportion of newly synthesized proopiomelanocortin and its two primary proteolytic products generated in the trans-Golgi network, the N-terminal 23-kDa fragment containing adrenocorticotropin and the C-terminal beta-lipotropin fragment, was secreted via the constitutive pathway. Two- to three-fold overexpression of CgB markedly reduced the constitutive secretion of the 23-kDa fragment, but not beta-lipotropin and tripled the amount of adrenocorticotropin generated and stored in secretory granules. Our results indicate the existence of neuroendocrine-specific helper proteins which promote the sorting from the trans-Golgi network to secretory granules of certain processing intermediates derived from peptide hormone and neuropeptide precursors and demonstrate that CgB functions as such.

The biological significance of storage granules in rat parathyroid cells

Both prosecretory and storage granules are concomitantly formed at the trans Golgi network including the innermost Golgi cisterna. Prosecretory granules develop into small secretory granules that release their contents by exocytosis finely regulated by a complex mechanism for maintaining calcium homeostasis. In the rat parathyroid cells, storage granules are large secretory granules storing parathyroid hormone for an emergency supply. The hormone is rapidly discharged by exocytosis when serum calcium concentration is decreased. The granules are constantly produced even under conditions of low serum calcium concentration in the regions of 8 mg/dl. The granule content is constantly hydrolyzed when not discharged, leading to a decreased core and finally to the formation of vacuolar bodies. The fate of the vacuolar bodies is unknown. Hypercalcemic conditions accelerate hydrolysis. The threshold value of calcium concentration required for the release of storage granule contents is between 8.0 and 7.5 mg/dl and that of calcium concentration for accelerating degradation of storage granules is about 11.5 mg/dl. Sympathetic stimulation causes storage granules to be discharged regardless of hypercalcemia or hypocalcemia. Parasympathetic stimulation accelerates hydrolysis. The degradation of storage granules seems to be closely associated with an intracellular regulatory mechanism for parathyroid hormone secretion.

Chromogranin A: a novel regulator of parathyroid gland secretion

Chromogranin A (CgA) is a major protein of the parathyroid gland that is costored and cosecreted with parathyroid hormone (PTH). CgA, widely distributed in endocrine and neuroendocrine cells in addition to parathyroid cells, appears to be a precursor of biologically active autocrine or paracrine peptides that include pancreastatin (CgA240-288) and parastatatin (CgA347-419), compounds that strongly inhibit stimulated secretion of PTH and costored CgA. The biosynthesis of CgA and PTH via gene expression and mRNA translation are noncoordinately altered by agents that affect parathyroid cell secretion including 1,25 dihydroxycholecalciferol, dexamethasone and Ca2+. These data have been interpreted to support an important physiological role for CgA-derived peptides as autocrine, paracrine and endocrine agents in the regulation of parathyroid cell function and secretion.

[Factors regulating parathyroid hormone and the mechanism of signal transduction]

The secretion of parathyroid hormone (PTH) is influenced by various factors, but extracellular calcium concentration ([Ca2+]e) is the sole factor established to be the physiological regulator. [Ca2+]e acts on Ca2+ sensing receptor recently cloned, and elevates intracellular calcium ([Ca2+]i) and inositol trisphosphate. Although the changes in cAMP level and C-kinase activity also have been demonstrated, the mediator directly operating secretory mechanism of PTH has not been defined. Recently, the activity of chromogranin A-related peptides to modulate PTH secretion was found and their role as the autocrine-paracrine regulator of PTH secretion has been proposed.

Regulated secretion. Helper proteins for neuroendocrine secretion

The granins, known to be general constituents of neuroendocrine secretory granules, are regulators of the proteolytic processing of peptide precursors and promote their aggregation-mediated sorting into secretory granules.

Chromogranin A

Chromogranin A (CgA) is the major member of the granin family of acidic secretory glycoproteins that are expressed in all endocrine and neuroendocrine cells. Granins have been proposed to play multiple roles in the secretory process. Intracellularly, granins play a role in targeting peptide hormones and neurotransmitters to granules of the regulated pathway by virtue of their ability to aggregate in the low-pH, high-calcium environment of the trans-Golgi network. Extra-cellularly, peptides formed as a result of proteolytic processing of granins regulate hormone secretion. Some conserved features of the mature CgA protein are polyglutamic acids, calcium-binding sites, and several pairs of basic amino acids. The first 2 features are important for its intracellular functions, and the latter characteristic suggested that peptides could be released from the molecule by precursor processing enzymes. Several biologically active peptides encoded within the CgA molecule, such as vasostatin, beta-granin, chromostatin, pancreastatin, and parastatin act predominantly to inhibit hormone and neurotransmitter release in an autocrine or paracrine fashion. The biosynthesis of CgA is regulated by many different factors, including steroid hormones and agents that act through a variety of signalling pathways. CgA biosynthesis and that of the resident hormone or neurotransmitter can be regulated differentially. The widespread distribution of CgA has made the measurement of circulating immunoreactive CgA a valuable tool in the diagnosis of neuroendocrine neoplasia, and CgA immunohistochemistry can help to identify the neuroendocrine nature of tumours. Recent molecular biology studies are identifying those elements in the CgA gene promoter responsible for its specific neuroendocrine cell expression.

Trophic factors from chromaffin granules promote survival of peripheral and central nervous system neurons

Chromaffin cells of the adrenal medulla were used to study the release of neurotrophic factors operationally defined by their capacity to promote the in vitro survival of embryonic neurons from the peripheral and central nervous system. Chromaffin cells are closely related to sympathetic neurons in terms of their transmitters and specific proteins and, like sympathetic neurons, receive preganglionic cholinergic, aminergic and peptidergic neuronal inputs. The issue of whether chromaffin cells store and secrete neurotrophic factors is therefore pertinent to the question whether trophic mechanisms may be involved in neuronal interactions and what modes of secretion are employed to liberate neurotrophic factors from neurons. Cell culture media conditioned by purified bovine chromaffin cells supported several neuron populations in vitro. Stimulation of the chromaffin cells with the cholinergic agonist carbachol (10(-4) M) increased in parallel the output of neurotrophic factor activity (assayed on chick ciliary ganglionic neurons) as well as two components specifically located in chromaffin granules, chromogranin A and catecholamines. The release of all three components was partially blocked by the Ca2+ channel blocker verapamil (10(-5) M), suggesting co-storage and -release of neurotrophic factors, chromogranin A and catecholamines in/from chromaffin granules. Neurotrophic factor activity for ciliary ganglionic neurons accumulating in the medium of unstimulated chromaffin cells decreased with time, and so did catecholamines. In contrast, amounts of neurotrophic factors and catecholamines released by challenging cells with carbachol did not significantly decline up to 62 h. The neurotrophic factor activity tested on chick ciliary, sensory and spinal cord neurons as well as on rat hippocampal neurons was heat- and trypsin-labile and could not be blocked by polyclonal antibodies against bovine nerve growth factor and the chromogranin A, B, and C. Defined fragments of chromogranin A and pancreastatin were devoid of neurotrophic activity. Our results suggest the presence of one or several neurotrophic factors in chromaffin granules, which can be released by exocytosis and may be potentially relevant for the maintenance of neurons innervating the adrenal medulla.

Production and secretion of chromogranin A and pancreastatin by the human pancreatic carcinoma cell line QGP-1N on stimulation with carbachol

Chromogranin A (CGA) is thought to be a precursor of pancreastatin (PST). Carbachol (Cch) stimulated the secretion of CGA and PST from QGP-1N cells derived from a human pancreatic islet cell tumor. Atropine inhibited the secretion of both. Sodium fluoride, phorbol ester, and calcium ionophore also stimulated the secretion of both. Cch (10(-5) M) stimulated inositol 1,4,5-trisphosphate production in QGP-1N cells. Stimulation with Cch increased the total amount of PST in the cells and the medium 1.7-fold and decreased the amount of CGA in the cells and medium. QGP-1N cells were labelled with [35S]methionine, and then CGA and PST in the cells and medium were immunoprecipitated with specific antisera, and separated by electrophoresis in polyacrylamide gel. Stimulation with Cch resulted in an increase in the intensity of PST-immunoreactive bands and a decrease in those of CGA-immunoreactive bands. Cch did not increase the cellular level of CGA messenger RNA. These results suggested that (1) the secretion of CGA and PST from QGP-1N cells is regulated mainly through muscarinic receptors coupled with activation of polyphosphoinositide breakdown by a G protein, with intracellular calcium ion and protein kinase C playing a role in the stimulus-secretion coupling and that (2) Cch may induce the secretion of PST and CGA and processing from CGA to PST.

Inhibition of parathyroid hormone secretion by amino-terminal chromogranin peptides

The parathyroid gland responds to decreases in levels of extracellular calcium by increasing the secretion of both PTH and chromogranin-A (CGA) in approximately equal molar ratios. Because CGA has been suggested to be a precursor for biologically active peptides, we used primary cultures of bovine parathyroid cells to examine the effects of various peptides from CGA as well as analogous peptides from chromogranin-B (CGB) on PTH secretion. In concentrations from 10(-8)-10(-7) M, amino-terminal peptide CGA-(1-76) effectively inhibited the release of PTH in response to low extracellular calcium. Truncated analogs of this peptide, CGA-(1-40), CGB-(1-41), and CGA-(17-38) were also found to be active in the following order: CGA-(1-76) = CGA-(1-40) = CGB-(1-41) > CGA-(17-38). The biological activity of CGA-(1-40) was markedly reduced after reduction and alkylation, which resulted in disruption of the single disulfide bond between Cys17 and Cys38. Moreover, peptides derived from other regions of CGA and CGB, which included CGA-(403-428), CGB-(1-16), CGB-(316-326), and CGB-(635-657) were inactive. Pulse-chase experiments, using primary cultures of bovine parathyroid cells, revealed the presence of a CGA peptide in the culture medium that had the same amino-terminal sequence and mobility on sodium dodecyl sulfate-polyacrylamide gels as synthetic CGA-(1-76). Furthermore, in binding and cross-linking studies using intact parathyroid cells, CGA-(1-40) formed a single, covalently linked protein complex with a mol wt of 78,000. Formation of the protein complex could be completely inhibited in the presence of an excess of either CGB-(1-41) or CGA-(17-38). These results show that a naturally occurring amino-terminal peptide from CGA as well as shorter analogs can act as potent inhibitors of PTH secretion, and that their biological activity may be mediated through binding to a specific cell surface protein.

The chromaffin cell: paradigm in cell, developmental and growth factor biology

This article reviews the chromaffin cell in relation to studies that have elucidated fundamental phenomena in cell biology (the molecular anatomy of exocytosis) and developmental neuroscience (the principle of neuropoiesis in the development of the sympathoadrenal cell lineage). A final section addresses growth factor synthesis and storage in chromaffin cells and their implications for the treatment of neurological disorders, such as Parkinson's disease.

Human chromostatin inhibits endothelin-1-induced contractures in human blood vessels

Isolated endothelium-denuded segments of the human internal thoracic artery (ITA) and saphenous vein (SV) have been used for characterization of vasoinhibitory effects of the chromostatin (hChs) sequence of human chromogranin A (CGA124-143). In SV preincubation with hChs inhibited the response to depolarizing high K+ in Ca(2+)-free medium in a concentration dependent manner (EC50 approximately 2 nM). At 200 nM hChs the tension response to high K+ (80 mM) was inhibited by 44% (n = 8) and the tension response to noradrenaline (2.6 microM) was inhibited by 20% (n = 6), but the tension response to endothelin-1 (65 nM) (ET-1) was not affected. In ITA no effect of hChs was observed on tension response to K+ or ET-1 in Ca(2+)-free medium. On the other hand, in Ca(2+)-containing medium the tension evoked by 65 nM ET-1 was no longer sustained in segments preincubated with 200 nM hChs and declined spontaneously to 76 +/- 12% (n = 6) of maximal tension after 6 min. A vascular function for the Chs sequence of the human CGA is thus indicated, inhibiting different components of vasoconstrictor responses in the human SV and ITA segments.

Current status on chromogranin A and pancreastatin

The authors review the biochemical and biological properties of chromogranins and pancreastatin. Chromogranins A, B and C are acidic proteins of a molecular mass of 48,000, 76,000 and 67,000, respectively, located in the secretory granules of the neuroendocrine cells. Since large amounts of chromogranin A were found in most neuroendocrine tumours, chromogranin A plasma determination is a diagnostic tool even in silent tumours. Pancreastatin is a peptide derived from chromogranin A, which inhibits insulin secretion, exocrine pancreatic secretion and gastric acid secretion, and which stimulates glucagon secretion. Pancreastatin has different molecular forms, the major form being a high molecular form of 92 amino acids, found by the authors in human stomach- and colon extracts and in a liver metastasis of a gastrinoma. The controlled proteolysis of chromogranin A in gut neuroendocrine cells generates predominantly the high molecular weight form.

Chromogranin A does not mediate glucocorticoid inhibition of adrenocorticotropin secretion

It has recently been proposed that chromogranin A (CgA), a 50-kilodalton acidic glycoprotein that is costored and cosecreted with hormones and neurotransmitters in a variety of tissues, mediates glucocorticoid inhibition of ACTH secretion from AtT20/D16v mouse anterior pituitary corticotroph tumor cells by an undefined autocrine mechanism. We used AtT20/D16v cells, RIAs for murine CgA and ACTH, complementary DNA probes for CgA and POMC, the precursor of ACTH, antiserum that reacts with murine CgA, highly purified bovine CgA, and synthetic rat and porcine pancreastatin, a bioactive cleavage product of CgA in some systems, to study the kinetics of the effect of glucocorticoids on CgA and ACTH synthesis and secretion and of CgA's subsequent effects on ACTH secretion. Exposure to 100 nM dexamethasone (DEX) did not alter the size of CgA or POMC messenger RNA (mRNA) transcripts but increased cell CgA mRNA content 42% by 3 h and 192% by 48 h. DEX decreased cell POMC mRNA content 22% by 6 h and 57% by 48 h. These divergent effects of DEX on steady state mRNA levels were accompanied by similar divergent effects on the production of CgA and ACTH protein. Thirty-minute exposure to 10 nM ovine CRF increased CgA and ACTH release to 300% and 360% of basal levels, respectively. One-hour DEX pretreatment inhibited CRF-stimulated CgA and ACTH release 58% and 49% at 30 min and 67% and 66% at 60 min, respectively. There was a positive correlation between CgA and ACTH release under all conditions at both times (r = 0.976 and 0.964, respectively, P < 0.001), consistent with costorage and cosecretion of the two proteins. The ratio of secreted ACTH to CgA decreased progressively with DEX treatment. Purified bovine CgA (100 nM) had little or no effect on basal or CRF-stimulated ACTH secretion from AtT20/D16v cells, 100 nM synthetic pancreastatin had no significant effect on basal or CRF-stimulated ACTH release from AtT20/D16v cells or dispersed normal male rat anterior pituitary cells, and anti-CgA sera had no significant effect on basal or CRF-stimulated ACTH release from AtT20/D16v cells. These results indicate: 1) that DEX stimulates CgA synthesis, whereas it inhibits POMC synthesis; 2) that CgA and ACTH are cosecreted; 3) that DEX increases CgA secretion relative to ACTH secretion, but decreases the absolute secretion of both proteins; and 4) that neither CgA nor its proteolytic product, pancreastatin, inhibits ACTH secretion. Thus, CgA does not mediate the inhibitory effect of DEX on ACTH secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

SIIp: a unique secretogranin/chromogranin of the pituitary released in response to gonadotropin-releasing hormone

A monoclonal antibody prepared by immunization of mice with a rat pituitary granule fraction stained a single band on a Western blot of pituitary homogenate (bovine, ovine, porcine, or rat) with an apparent mol wt of 78,000 (7.5% acrylamide gel in sodium dodecyl sulfate) and pI 5.0-5.1 (isoelectric focusing). Subcellular fractionation studies of rat pituitaries indicated that the determinant of the monoclonal antibody was markedly enriched in the secretory granule fraction, an observation that was independently confirmed by immunohistochemistry of intact cells. Immunohistochemistry also indicated that this determinant was selectively located in gonadotropes and thyrotropes. On Western blots, this band comigrated with adrenal secretogranin-II (SII; chromogranin-C), had the same N-terminal sequence (six amino acids), and was heat stable (95 C; 10 min). The pituitary protein containing the determinant for the monoclonal antibody could be precipitated by a polyclonal antibody prepared by immunization of rabbits with the C-terminal sequence of adrenal SII (triodecapeptide). Conversely, the monoclonal antibody precipitated the protein containing the determinant for the polyclonal antibody. While both the monoclonal and polyclonal antisera recognized the pituitary molecule, only the polyclonal antibody recognized SII from the adrenal. A RIA was established and used to assess the release pattern of this molecule from pituitary cell cultures. Release was stimulated by GnRH and blocked by a GnRH antagonist. Release was Ca2+ dependent and stimulated by either phorbol myristyl acetate (a protein kinase-C activator) or NaF (a G-protein activator). GHRH and TRH were not as effective secretogogues as GnRH. The observations that a unique form of SII is present in the pituitary gonadotrope and secreted in response to a specific endocrine stimulus present the possibility that this substance has an endocrine function. Further, the tissue specificity of the determinant suggest that it may be useful for the specific diagnosis and monitoring of pituitary tumors.

Molecular approaches for the analysis of chromogranins and secretogranins

Recent molecular analyses have contributed to our knowledge about the chromogranin/secretogranin (Cg/Sg) family and their utility in diagnostic pathology. The genes for five of these proteins have been cloned, and the deduced amino acid sequences have provided insights into the structure and possible functions of the Cgs/Sgs, including their role as prohormones. Northern hybridization and in situ hybridization histochemistry have provided a great deal of information about the tissue distribution of the Cg/Sg gene products. Some neoplasms such as small cell lung carcinomas, which have little stored Cg/Sg protein, have abundant cytoplasmic mRNAs that can be readily detected by hybridization studies. Some other neoplasms such as neuroblastomas have decreased CgA and increased SgII mRNAs during maturation to ganglioneuromas. There is also a differential expression of Cgs/Sgs in some endocrine neoplasms such as parathyroid adenomas, which express abundant CgA mRNA and little CgB mRNA, and in pituitary prolactinomas, which express CgB mRNA but not CgA mRNA. The mRNA for CgA has been found unexpectedly in some neoplasms such as 15% of colonic adenocarcinomas. Thus, molecular approaches in the analysis of Cgs/Sgs should contribute to the diagnosis of endocrine neoplasms and may provide support for a molecular classification of neoplasms in diagnostic pathology.

Chromogranin A: its role in endocrine function and as an endocrine and neuroendocrine tumor marker

CgA is a 49 kilodalton protein that is present in the secretory granules of most endocrine and many neuroendocrine cells. Detection of CgA in cells by immunocytochemistry and measurement of CgA in serum by immunoassay can serve as tissue and serum markers for CgA-producing tumors. CgA is of diagnostic value in classical endocrine tumors, in hormone-negative tumors, and in endocrine tumors in which other diagnostic procedures have their limitations. Although the biological function of CgA is yet unknown, it may serve as a precursor molecule, like POMC, for a family of biologically active peptides. CgA is an important new tool for the endocrinologist in the diagnosis and management of patients with endocrine and neuroendocrine tumors.

A proposed role for chromogranin A as a glucocorticoid-responsive autocrine inhibitor of proopiomelanocortin secretion

Chromogranin A (CgA) is a 50 kilodalton (kDa) acidic glycoprotein that is costored and cosecreted from secretory granules with endogenous hormone from diverse endocrine cell types. The physiological role(s) of CgA is yet to be defined. In this study we used the AtT-20 mouse corticotropic cell line, which produces both CgA and POMC-derived peptides, to study 1) the regulation of CgA and POMC synthesis and secretion, and 2) the influence of CgA on POMC secretion. To study regulation of CgA and POMC biosynthesis and secretion, cells were treated with dexamethasone (DEX) or CRF for 48 h and CgA and POMC messenger RNAs and proteins were analyzed. Exposure to DEX for 48 h (10 nM) inhibited secretion of the 16 K fragment of POMC by 60% while stimulating CgA secretion 500% of control value. Consonant with these changes in protein, POMC mRNA levels fell to 40% of control levels while CgA mRNA levels increased to 250% of control values with DEX treatment. DEX treatment had no effect on the sizes of the CgA [2.1 kilobase (kb)] and POMC (1.0 kb) mRNAs. CRF (100 nM) stimulated secretion of both CgA (4-fold) and ACTH (2.5-fold) above basal values. By contrast, CRF increased POMC mRNA levels but had no effect on levels of CgA mRNA. Changes in total peptide production paralleled the changes in mRNA levels. Because DEX differentially regulated CgA and POMC synthesis and secretion, we questioned whether CgA could function as an autocrine inhibitor of hormone secretion. CgA inhibited CRF-stimulated secretion of 16 K fragment in a concentration-dependent manner (100% at 100 nM) without affecting basal 16 K fragment secretion. Moreover, anti-CgA antiserum, but not nonimmune serum, increased basal 16 K fragment secretion 2-fold and CRF-stimulated 16 K fragment secretion 1.5-fold. These results suggest that CgA plays an autocrine role as a glucocorticoid responsive inhibitor of POMC-derived peptide secretion.