Distinct patterns of chromogranin A-related species can be demonstrated in endocrine cells

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.

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.

Processing of chromogranin A in the parathyroid: generation of parastatin-related peptides

Chromogranin A (CgA) is a glycoprotein present in secretory granules of endocrine cells. In the parathyroid, it is costored and cosecreted with parathormone (PTH) in response to hypocalcemia. CgA is the precursor of several bioactive peptides including pancreastatin and betagranin. Parastatin (PARA, pCgA(347-419)) is a novel peptide that we generated in vitro by enzymatic digestion of pCgA. In vitro, it inhibits low Ca(2+)-stimulated parathyroid secretion. Full activity resides in its first 19 residues. In order to determine if PARA or PARA-derived peptides are natural products of the parathyroid, we generated an antiserum directed against pCgA(347-359) corresponding to the bioactive N-terminal sequence of pPARA (pPARA(1-13) antiserum), and developed a specific radioimmunoassay that we used in conjunction with various chromatographic separations. We identified small peptides carrying the pPARA(1-13) immunoactivity in extracts and secretion medium of porcine parathyroid glands. Continuous and pulse-chase radiolabeling studies, along with immunoprecipitation using PARA(1-13) antiserum demonstrate that a newly-synthesized PARA-related peptide fraction with a Mr of 11 kDa is secreted by the parathyroid cells and accumulates in the secretion medium. Edman degradation of the 11 kDa PARA-related peptide band by Edman degradation yielded three major N-terminal sequences: S-K-M-D-R-L-A-K-E-L-(residues 313-322), D-R-L-A-K-E-L-T-A-E-(residues 316-325), and A-K-E-L-T-A-E-K-R-L-(residues 319-329), in a molar ratio of approximately 1:2:1. The peptide bonds required to be cleaved to yield these peptides, Trp-Ser, Met-Asp and Leu-Ala, suggest that a chymotrypsin-like endopeptidase participated in their formation. The molecular size and the results of amino acid compositional analysis, indicate that the C-termini of these peptides extended variably to residues 384-401 of pCgA. These results demonstrate that processing of CgA by the parathyroid gland generates bioactive PARA-related peptides that could affect the gland's secretory activity.

Library of Sequence-specific Radioimmunoassays for Human Chromogranin A

Background: Human chromogranin A (CgA) is an acidic protein widely expressed in neuroendocrine tissue and tumors. The extensive tissue- and tumor-specific cleavages of CgA at basic cleavage sites produce multiple peptides.

Methods: We have developed a library of RIAs specific for different epitopes, including the NH2 and COOH termini and three sequences adjacent to dibasic sites in the remaining part of CgA.

Results: The antisera raised against CgA(210–222) and CgA(340–348) required a free NH2 terminus for binding. All antisera displayed high titers, high indexes of heterogeneity (∼1.0), and high binding affinities (Keff0 ∼ 0.1 × 1012 to 1.0 × 1012 L/mol), implying that the RIAs were monospecific and sensitive. The concentration of CgA in different tissues varied with the assay used. Hence, in a carcinoid tumor the concentration varied from 0.5 to 34.0 nmol/g tissue depending on the specificity of the CgA assay. The lowest concentration in all tumors was measured with the assay specific for the NH2 terminus of CgA. This is consistent with the relatively low concentrations measured in plasma from carcinoid tumor patients by the N-terminal assay, whereas the assays using antisera raised against CgA(210–222) and CgA(340–348) measured increased concentrations.

Conclusion: Only some CgA assays appear useful for diagnosis of neuroendocrine tumors, but the entire library is valuable for studies of the expression and processing of human CgA.

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.

Production and structure characterisation of recombinant chromogranin A N-terminal fragments (vasostatins) -- evidence of dimer-monomer equilibria

Vasostatins (VS) are vasoinhibitory peptides derived from the N-terminal domain of chromogranin A, a secretory protein present in the electron-dense granules of many neuroendocrine cells. In this work we describe a method for the production in Escherichia coli of large amounts of recombinant vasostatins, corresponding to chromogranin A residues 1-78 (VS-1), and 1-115 (VS-2), and the use of these materials for structure characterisation. The masses of both products were close to the expected values, by SDS/PAGE and mass spectrometry analysis. However, their hydrodynamic behaviours in size-exclusion chromatography corresponded to that of proteins with a larger size. SDS/PAGE analysis of VS-1 and VS-2 after cross-linking with disuccinimidyl suberate indicated that both polypeptides form dimers. VS-2 was almost entirely dimeric at > 4 microM, but rapidly converted to monomer after dilution to 70 nM. The rapid dimer-monomer transition of VS-2 after dilution could be part of a mechanism for regulating its activity and localising its action. Immunological studies of VS-1 have shown that residues 37-70 constitute a highly antigenic region characterised by an abundance of linear epitopes efficiently mimicked by synthetic peptides. The recombinant products and the immunological reagents developed in this work could be valuable tools for further investigating the structure and the function of chromogranin A and its fragments.

Chromogranin A fragments modulate cell adhesion. Identification and characterization of a pro-adhesive domain

Although several functions have been suggested for chromogranin A, a glycoprotein secreted by many neuroendocrine cells, the physiological role of this protein and of its proteolytic fragments has not been established. We have found that mixtures of chromogranin A fragments can inhibit fibroblast adhesion. The anti-adhesive activity was converted into pro-adhesive activity by limited trypsin treatment. Pro-adhesive effects were observed also with recombinant N-terminal fragments corresponding to residues 1-78 and 1-115 and with a synthetic peptide encompassing the residues 7-57. These fragments induced adhesion and spreading of fibroblasts on plates coated with collagen I or IV, laminin, fetal calf serum (FCS) but not on bovine serum albumin. The long incubation time required for adhesion assays (4 h) and the FCS requirements for optimal adhesion suggest that the adhesive activity is likely indirect and requires other proteins present in the FCS or made by the cells. These findings suggest that chromogranin A and its fragments could play a role in the regulation of cell adhesion. Since chromogranin A is concentrated and stored within granules and rapidly released by neuroendocrine cells and neurons after an appropriate stimulus, this protein could be important for the local control of cell adhesion by stimulated cells.

Characterisation of circulating chromogranin A in human cancer patients

The structure of circulating chromogranin A (CgA) of phaeochromocytoma patients was characterised and compared with that of CgA extracted from tumours. Size exclusion chromatography experiments provided evidence that CgA is present in the blood of different patients, as well as in tumour extracts, as multiple forms having different hydrodynamic sizes of 600 kDa (CgA-I), 100 kDa (CgA-II) and 55 kDA (CgA-III). The amount of each CgA form as a proportion of the total antigenic material was different in different patients. Western blot analysis of chromatographic fractions indicated that these forms are made up by polypeptides of similar molecular weight (about 60-70 kDa). All CgA forms express the epitopes recognised by two monoclonal antibodies (A11 and B4E11), directed against residues 68-70 and 81-90 of human CgA. However, their relative immunoreactivity was markedly different. No evidence for the presence of multimeric complexes in the CgA-I fraction was obtained by various immunological and biochemical methods. These results suggest that circulating CgA in phaeochromocytoma patients consists of at least three forms that appear to be made up by polypeptides with similar molecular weight and different hydrodynamic properties and immunoreactivity. We hypothesise that different conformations and shapes contribute to the heterogeneity of circulating CgA.

Antigenic regions of human chromogranin A and their topographic relationships with structural/functional domains

Chromogranin A is a protein contained in the secretory granules of many neuroendocrine cells. The linear antigenic sites of human chromogranin A were studied by examining the cross-reaction of polyclonal and monoclonal anti-chromogranin A antibodies with native chromogranin A and with synthetic peptides encompassing most of the chromogranin A sequence. Chromogranin A residues 1-20, 47-67, 107-158, 254-297, 331-375, and 395-419 were found to be poorly or not antigenic, while residues 25-46, 163-210, 231-253, 298-314 and 68-106, 222-230, 315-330, 376-394 were found to contain weak and strong antigenic sites, respectively. Residues 68-70 (GAK) and 81-90 (GFEDELSEVL) were strongly recognized by two mouse mAbs (B4E11 and A11, respectively). Since mAb A11 has been previously used for immunohistochemical analysis of chromogranin-A-producing tissues from different species and for in vivo imaging of chromogranin-A-positive endocrine tumors, these results imply that at least part of the 81-90 region is surface-exposed in cryostat tissue sections as well as in vivo. The results may help in selecting new antibodies with improved affinity and immunogenicity for in vivo targeting of chromogranin-A-producing tumors.

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.

Processing of chromaffin granule proteins: a profusion of proteases?

Evidence suggests that proenkephalin and members of the chromogranin/secretogranin family of proteins are prohormone precursors, giving rise to a variety of peptides with biologic activity. However, the specific proteases responsible for cleaving these proteins in vivo have not been fully established. Several candidate proteases have been described, some of which have been shown to cleave these proteins in vitro. Proteolytic processing of the chromogranins may be particularly complex, occurring in specific tissue-dependent patterns. To account for this level of complexity several protease systems may be operative, either alone or in concert, both within the neurosecretory granule and in the extracellular space. Specific proteases which are available within neurosecretory cells or in the local extracellular environment, and which may cleave these prohormones include PC1 and PC2 (recently described members of the Kex2/furin family of endoproteases), as well as kallikrein, acetylcholinesterase, and, more recently, the plasminogen/plasmin protease system. The potential role of these specific proteases in the processing of proenkephalin and the chromogranins is discussed, in particular, in the context of possible processing clues available from recent analysis of cDNA and genomic intron/exon structure.

Posttranslational processing of proenkephalins and chromogranins/secretogranins

Posttranslational processing of peptide-precursors is nowadays believed to play an important role in the functioning of neurons and endocrine cells. Both proenkephalins and chromogranins/secretogranins are considered as precursor molecules in these tissues, resulting in posttranslationally formed degradation products with potential biological activities. Among the proteins and peptides of neuronal and endocrine secretory granules, the enkephalins and enkephalin-containing peptides have been most extensively studied. The characterization of the post-translationally formed degradation products of the proenkephalins have enabled the understanding of their processing pathway. Chromogranins/secretogranins represent a group of acidic glycoproteins, contained within hormone storage granules. The biochemistry, biogenesis and molecular properties of these proteins have already been studied for 25 years. The chromogranins/secretogranins have a widespread distribution throughout the neuroendocrine system, the adrenal medullary chromaffin granules being the major source of these storage components. Recent data provide evidence for a precursor role for all members of the chromogranins/secretogranins family although also several other functions have been proposed. In this review, some of the methods applied to study proteolytic processing are described. In addition, the posttranslational processing of chromogranins/secretogranins and proenkephalins, especially the biochemical aspects, will be discussed and compared. Recent exciting developments on the generation and identification of potential physiologically active fragments will be covered.

Pancreastatin and bovine parathyroid cell secretion

Chromogranin A (CgA) is an acidic glycoprotein found in secretory granules of multiple peptidergic tissues and cosecreted with the resident peptide hormones. Pancreastatin is an amidated, biologically active peptide whose sequence is contained within CgA. We investigated the effect of the C-terminal fragment of bovine pancreastatin (bP32-47) on bovine parathyroid cell secretion. bP32-47 amide inhibited low-calcium-stimulated PTH secretion by 44% and chromogranin A (CgA) secretion by 33%. We were able to identify a pancreastatin-like peptide as a very minor component of the endogenous breakdown peptides from CgA. However, using several approaches, we were unable to detect pancreastatin in secretory granule extracts or in incubation media. We conclude that although exogenous bovine pancreastatin has inhibitory effects on secretion, detectable pancreastatin is not secreted under normal incubation conditions. Based on our current data, we would question the physiologic importance of pancreastatin in bovine parathyroid glands.

Autocrine inhibition of parathyroid cell secretion requires proteolytic processing of chromogranin A

Chromogranin A (CgA, Secretory Protein-I) is a protein of about 450 amino acids representing a major soluble component of the secretory granules of parathyroid and other endocrine and neuroendocrine cells. In the parathyroid, CgA is costored and cosecreted with parathormone (PTH). We earlier found that CgA and the derived peptide, pancreastatin, inhibited secretion of PTH and CgA by parathyroid cells in culture and that CgA antiserum stimulated secretion above the maximum achieved at low (0.5 mM) Ca2+. In the present study, porcine parathyroid cells were incubated at different cell concentrations at low Ca2+. The amount of secreted CgA increased over the 6-h incubation period at 1 x 10(6) to 4 x 10(6) cells/ml, but plateaued after 3 h at 6 x 10(6) cells/ml. Secretion did not plateau when antisera were added at 3 h. Conditioned medium contained a factor or factors that blocked secretion by fresh parathyroid cells at 0.5 mM Ca2+. Pulse-chase studies revealed that 40% of the secreted CgA was processed after 6 h of chase. alpha-2-macroglobulin, an inhibitor of proteolytic processing, increased the amount of CgA in the medium by 30% at 1 h of chase and decreased the amount processed to 20% by 6 h. Other protease inhibitors similarly enhanced the amount of CgA in the medium. These data indicate that proteolytic processing of intact CgA is requisite for its autocrine inhibitory activity.