Glycoprotein III (clusterin, sulfated glycoprotein 2) in endocrine, nervous, and other tissues: immunochemical characterization, subcellular localization, and regulation of biosynthesis

Expanding the family of extracellular chaperones: Identification of human plasma proteins with chaperone activity

Proteostasis, the balance of protein synthesis, folding and degradation, is essential to maintain cellular function and viability, and the many known intracellular chaperones are recognized as playing key roles in sustaining life. In contrast, the identity of constitutively secreted extracellular chaperones (ECs) and their physiological roles in extracellular proteostasis is less completely understood. We designed and implemented a novel strategy, based on the well-known propensity of chaperones to bind to regions of hydrophobicity exposed on misfolding proteins, to discover new ECs present in human blood. We used a destabilized protein that misfolds at 37°C as "bait" to bind to potential ECs in human serum and captured the complexes formed on magnetic beads. Proteins eluted from the beads were identified by mass spectrometry and a group of seven abundant serum proteins was selected for in vitro analysis of chaperone activity. Five of these proteins were shown to specifically inhibit protein aggregation. Vitronectin and plasminogen activator-3 inhibited both the in vitro aggregation of the Alzheimer's β peptide (Aβ1-42 ) to form fibrillar amyloid, and the aggregation of citrate synthase (CS) to form unstructured (amorphous) aggregates. In contrast, prothrombin, C1r, and C1s inhibited the aggregation of Aβ1-42 but did not inhibit CS aggregation. This study thus identified five novel and abundant putative ECs which may play important roles in the maintenance of extracellular proteostasis, and which apparently have differing abilities to inhibit the amorphous and amyloid-forming protein aggregation pathways.

Clusterin in Neuroendocrine Epithelial Neoplasms: Absence of Expression in a Well-differentiated Tumor Suggests a Jejunoileal Origin

Clusterin, a widely expressed, tissue-specific glycoprotein, is a diagnostic marker of several tumor types, including anaplastic large cell lymphoma, follicular dendritic cell sarcoma, and tenosynovial giant cell tumor. A recent study has suggested it is highly expressed by well-differentiated neuroendocrine tumors (NET) arising at most anatomic sites, with the exception of jejunoileal tumors, and that it is similarly not expressed by poorly differentiated neuroendocrine carcinomas (NEC). We sought to validate this result in a large cohort of NETs and NECs. Clusterin immunohistochemistry was performed on tissue microarrays of 255 NETs [45 lung, 4 stomach, 8 duodenum, 75 pancreas (62 primary, 13 metastatic), 107 jejunoileum (69 primary, 38 metastatic), 16 appendix] and 88 NECs (43 visceral, 45 Merkel cell). Extent (%) and intensity (0, 1+, 2+, 3+) of staining were assessed and an H-score (extent x intensity) calculated. An average H-score >5 was considered positive. Clusterin expression was noted in 82.4% of 148 nonjejunoileal NETs (average H-score 183) and only 8.4% of 107 jejunoileal NETs (average H-score, 31), as well as 19.3% of NECs (average H-score, 36). Clusterin is frequently, strongly expressed by NETs of diverse anatomic sites, with the exception of jejunoileal tumors, in which it is only rarely, weakly expressed. It is occasionally, weakly expressed by NECs. Most metastatic NETs of occult origin arise in the pancreas or the jejunoileum. For cases in which an initial site of origin immunopanel (eg, islet 1, PAX6, CDX2) is ambiguous, addition of clusterin may be diagnostically useful, with absence of expression suggesting a jejunoileal origin.

Clusterin: full-length protein and one of its chains show opposing effects on cellular lipid accumulation

Proteins, made up of either single or multiple chains, are designed to carry out specific biological functions. We found an interesting example of a two-chain protein where administration of one of its chains leads to a diametrically opposite outcome than that reported for the full-length protein. Clusterin is a highly glycosylated protein consisting of two chains, α- and β-clusterin. We have investigated the conformational features, cellular localization, lipid accumulation, in vivo effects and histological changes upon administration of recombinant individual chains of clusterin. We demonstrate that recombinant α- and β-chains exhibit structural and functional differences and differ in their sub-cellular localization. Full-length clusterin is known to lower lipid levels. In contrast, we find that β-chain-treated cells accumulate 2-fold more lipid than controls. Interestingly, α-chain-treated cells do not show such increase. Rabbits injected with β-chain, but not α-chain, show ~40% increase in weight, with adipocyte hypertrophy, liver and kidney steatosis. Many, sometimes contrasting, roles are ascribed to clusterin in obesity, metabolic syndrome and related conditions. Our findings of differential localization and activities of individual chains of clusterin should help in understanding better the roles of clusterin in metabolism.

Clusterin glycopeptide variant characterization reveals significant site-specific glycan changes in the plasma of clear cell renal cell carcinoma

Cancer-related alterations in protein glycosylation may serve as diagnostic or prognostic biomarkers or may be used for monitoring disease progression. Clusterin is a medium abundance, yet heavily glycosylated, glycoprotein that is upregulated in clear cell renal cell carcinoma (ccRCC) tumors. We recently reported that the N-glycan profile of clusterin is altered in the plasma of ccRCC patients. Here, we characterized the occupancy and the degree of heterogeneity of individual N-glycosylation sites of clusterin in the plasma of patients diagnosed with localized ccRCC, before and after curative nephrectomy (n = 40). To this end, we used tandem mass spectrometry of immunoaffinity-enriched plasma samples to analyze the individual glycosylation sites in clusterin. We determined the levels of targeted clusterin glycoforms containing either a biantennary digalactosylated disialylated (A2G2S2) glycan or a core fucosylated biantennary digalactosylated disialylated (FA2G2S2) glycan at N-glycosite N374. We showed that the presence of these two clusterin glycoforms differed significantly in the plasma of patients prior to and after curative nephrectomy for localized ccRCC. Removal of ccRCC led to a significant increase in the levels of both FA2G2S2 and A2G2S2 glycans in plasma clusterin. These changes were further confirmed by lectin blotting of plasma clusterin. It is envisioned that these identified glycan alterations may provide an additional level of therapeutic or biomarker sensitivity than levels currently achievable by monitoring expression differences alone.

Multidimensional liquid chromatography platform for profiling alterations of clusterin N-glycosylation in the plasma of patients with renal cell carcinoma

Identification of potential changes in the glycosylation of existing cancer biomarkers can result in a higher level of diagnostic sensitivity and specificity. Clusterin (Apolipoprotein J) has been implicated in renal cell carcinoma (RCC) and other types of malignancy as potential biomarker. In the present work, an automated multi-dimensional HPLC platform enabling high throughput affinity enrichment of clusterin from plasma samples was developed. Integrated with two dimensional gel electrophoresis, high purity clusterin in microgram quantities suitable for glycan characterization was isolated. The analytical platform was applied to study clusterin glycosylation in a small group of RCC patients before and after nephrectopy as a pilot study to evaluate the performance of the platform. A statistically significant decrease was observed in the levels of a bi-antennary digalactosyl disialylated (A2G2S(3)2) glycans while the levels of a core fucosylated bi-antennary digalactosyl disialylated glycan (FA2G2S(6)2) and a tri-antennary trigalactosyl disialylated glycan (A3G3S(6)2) were increased in the post-surgery plasma samples.

Essential role of clusterin in pancreas regeneration

Based on our previous observations that clusterin induction accompanies pancreas regeneration in the rat, we sought to determine if regeneration might be impaired in mice that lacked clusterin. We studied the impact of absent clusterin on morphogenic and functional features of regenerating pancreas. Clusterin induction was accompanied in the regenerating pancreas by a robust development of new lobules with ductules, acini, and endocrine islets in wild type after partial pancreatectomy. In clusterin knock-out mice, however, pancreatectomy resulted in a poor formation of regenerating lobule. In particular, regeneration of beta-cells was also significantly reduced and was associated with persistent hyperglycemia. Duct cells obtained from pancreatectomized clusterin knock-out mice exhibited impaired beta-cell formation in vitro; this was restored by administration of exogenous clusterin. We suggest that clusterin plays a critical role to promote both exocrine and endocrine regeneration following pancreas injury, as well as for in vitro beta-cell regeneration.

Cellular expression and subcellular localization of secretogranin II in the mouse hippocampus and cerebellum

Secretogranin II (SgII), or chromogranin C, is thought to participate in the sorting and packaging of peptide hormones and neuropeptides into secretory granules and large dense-core vesicle (LDCVs), and also functions as a precursor of neuropeptide secretoneurin. Although SgII is widely distributed in the brain and is predominantly localized at terminals of mossy fibers in the hippocampus and cerebellum and climbing fibers in the cerebellum, its cellular expression and ultrastructural localization remain largely unknown. In the present study, we addressed this issue in the adult mouse brain by multiple-labeling fluorescence in situ hybridization and immunofluorescence and by preembedding and postembedding immunoelectron microscopies. SgII was expressed in various neurons, distributed as either tiny puncta or coarse aggregates in the neuropil, and intensely accumulated in perikarya of particular neurons, such as parvalbumin-positive interneurons and mossy cells in the hippocampus and Purkinje cells in the cerebellum. Coarse aggregates were typical of terminals of mossy fibers and climbing fibers. In these terminals, numerous immunogold particles were clustered on individual LDCVs, and one or two particles also fell within small synaptic vesicle-accumulating portions. SgII was further detected as tiny puncta in neural elements lacking LDCVs, such as parallel fibers of cerebellar granule cells, somatodendritic elements of various neurons and Bergmann glia. Thus, SgII is present in LDCV and non-LDCV compartments of various neural cells. The wide subcellular localization of SgII may reflect diverse release sites of neuropeptides and secretorneurin, or suggests its role in the sorting and packaging of molecules other than neuropeptides in non-LDCV compartments.

Phagocytotic removal of apoptotic endocrine cells by folliculostellate cells and its functional implications in clusterin accumulation in pituitary colloids in helmeted guinea fowl (Numida meleagris)

The different cell types in the anterior pituitary behave as dynamic populations. The gland maintains a continuous renewal of cells to ensure a dynamic balance between cell division, differentiation, growth arrest and apoptosis. Apoptosis is a frequent event in the anterior pituitary in which unwanted cells are eliminated without affecting neighboring cells. We examined the link between apoptosis and the occurrence of colloids in the guinea fowl (Numida meleagris) pituitary gland and the relationship of clusterin accumulation in the colloids. S-100 positive folliculostellate (FS) cells were found surrounding colloids. Apoptotic cells detected by single stranded DNA (ssDNA) immunohistochemistry were observed in the whole anterior pituitary and preferentially near colloid masses. Clusterin protein was detected in endocrine cells, FS cells and in the colloids. In situ hybridization showed clusterin mRNA in endocrine cells and FS cells. Simultaneous localization was performed to determine whether clusterin mRNA and ssDNA within anterior pituitary was present within the same cell. Clusterin mRNA was not detected in apoptotic cells but was present in neighboring surviving cells. At the ultrastructural level, numerous endocrine cells at different stages of apoptosis were found phagocytosed by FS cells. Our results suggest that clusterin is produced by endocrine cells for cytoprotection before death. Apoptotic endocrine cells are phagocytosed by FS cells and digested by their lysosomal enzymes. In FS cells, clusterin interacts and aggregates with by-products of digestion that subsequently become stored in colloid as a residual body.

Clusterin is a secreted marker for a hypoxia-inducible factor-independent function of the von Hippel-Lindau tumor suppressor protein

Germline mutations in the von Hippel-Lindau (VHL) tumor suppressor gene predispose people to renal cancer, hemangioblastomas, and pheochromocytomas in an allele-specific manner. The best documented function of the VHL gene product (pVHL) relates to its ability to polyubiquitinate, and hence target for destruction, the alpha subunits of the heterodimeric transcription factor hypoxia-inducible factor (HIF). pVHL mutants linked to familial pheochromocyctoma (type 2C VHL disease), in contrast to classical VHL disease, appear to be normal with respect to HIF regulation. Using a simple method for identifying proteins that are differentially secreted by isogenic cell line pairs, we confirmed that the HIF targets IGBP3 and PAI-1 are overproduced by pVHL-defective renal carcinoma cells. In addition, cells lacking wild-type pVHL, including cells producing type 2C pVHL mutants, were defective with respect to expression and secretion of clusterin, which does not behave like a HIF target. Decreased clusterin secretion by pVHL-defective tumors was confirmed in vivo by immunohistochemistry. Therefore, clusterin is a secreted marker for a HIF-independent pVHL function that might be especially important in pheochromocytoma development.

Involvement of the membrane lipid bilayer in sorting prohormone convertase 2 into the regulated secretory pathway

Prohormone convertase 2 (PC2) is a neuroendocrine-specific protease involved in the intracellular maturation of prohormones and proneuropeptides. PC2 is synthesised as a proprotein (proPC2) that undergoes proteolysis, aggregation and membrane association during its transit through the regulated secretory pathway. We have previously shown that the pro region of proPC2 plays a key role in its aggregation and membrane association. To investigate this further, we determined the binding properties of a peptide containing amino acids 45-84 of proPC2 (proPC2(45-84)) to trans-Golgi network/granule-enriched membranes from the AtT20 cell line. Removal of peripheral membrane proteins or hydrolysis of integral membrane proteins did not affect the binding properties of proPC2(45-84). Rather, proPC2(45-84) was shown to bind to protein-free liposomes in a pH- and Ca(2+)-dependent manner. To identify the component of the lipid bilayer involved in this membrane association, we used chromaffin-granule membranes and studied the binding properties of the endogenous PC2. Treatment of the membranes with saponin, a cholesterol-depleting detergent, failed to extract PC2 from the membranes, whereas chromogranin A (CgA) was removed. Treatment of the membranes with Triton X-100 yielded a low-density detergent-insoluble fraction enriched in PC2, but not CgA. The detergent-insoluble fraction also contained glycoprotein III, known to be part of the lipid rafts (membrane microdomains rich in sphingolipids). Finally, sphingolipid depletion of AtT20 cells resulted in the mis-sorting of PC2, suggestive of a link between the association of PC2 with lipid rafts in the membrane and its sorting into the regulated secretory pathway.

Transforming growth factor beta-1 up-regulates clusterin synthesis in thyroid epithelial cells

Porcine epithelial cells in primary culture seeded on plastic substratum form a monolayer containing pseudo-vesicles. When cultured in the presence of thyreotropin (TSH) thyrocytes adopted a follicular-like structure and formed clusters. Transforming growth factor beta-1 (TGFbeta1) induced a rapid spreading of the TSH-treated cells only. At the same time, TGFbeta1 enhanced clusterin protein and mRNA expression. The increase of clusterin synthesis was proportional to the TGFbeta1 concentration in the culture medium. Tunicamycin abolished the up-regulation of whole clusterin synthesis and morphological changes. The activator protein-1 binding site partly directed the TGFbeta1-stimulated clusterin expression. Phorbol ester caused rapid spreading of the cells with disappearance of vesicular and follicular structures. It decreased clusterin mRNA and protein expression, but increased Mr 45, 000 protein secretion in both TSH-treated and nontreated cells. Up-regulation of clusterin expression may be a marker of TGFbeta-mediated thyrocyte dedifferentiation.

Membrane composition of adrenergic large and small dense cored vesicles and of synaptic vesicles: consequences for their biogenesis

The membrane proteins of adrenergic large dense cored vesicles, in particular those of chromaffin granules, have been characterized in detail. With the exception of the nucleotide carrier all major peptides have been cloned. There has been a controversy whether these vesicles contain antigens like synaptophysin, synaptotagmin and VAMP or synaptobrevin found in high concentration in synaptic vesicles. One can now conclude that large dense core vesicles also contain these peptides although in lower concentrations. The biosynthesis of large dense core vesicles is analogous to that of other peptide secreting vesicles of the regulated pathway. One cannot yet definitely define the biosynthesis of small dense core vesicles which apparently have a very similar membrane composition to that of large dense core vesicles. They may form directly from large dense core vesicles when their membranes have been retrieved after exocytosis. These membranes may become sorted in an endosomal compartment where peptides may be deleted or added. Such an addition could be derived from synaptophysin-rich vesicles present in adrenergic axons. However small dense core vesicle peptides may also be transported axonally independent of large dense core vesicles. For proving one of these possibilities some crucial experiments have been suggested.

Chromaffin granule membrane glycoprotein IV is identical with Ac45, a membrane-integral subunit of the granule's H(+)-ATPase

Glycoprotein IV of bovine adrenal chromaffin granule membranes was purified by membrane fractionation with Triton X-114 and lectin affinity chromatography. An antiserum raised against this protein recognized the same component as one directed against subunit Ac45 of the proton-translocating adenosine triphosphatase in the granule membrane. Amino acid sequencing confirmed that glycoprotein IV and Ac45 are identical proteins, and also showed that they are derived from a larger precursor by removal of a 246-amino acid N-terminal sequence. Enzymatic deglycosylation indicated an apparent polypeptide molecular mass of 29 kDa for the mature Ac45/glycoprotein IV. Blue Native electrophoresis confirmed that this protein is a component of the membrane sector of the V-ATPase.

Role of clusterin in cell adhesion during early phases of programmed cell death in P19 embryonic carcinoma cells

This study explored the role of clusterin in mechanisms of cell adhesion and apoptosis in P19 embryonic carcinoma cells. We found that serum deprivation induced transient but dramatic elevation in cell adhesion strength to the culture substrate and eventually led to apoptotic cell death. The time course of cell-adhesion increase overlapped temporally with the elevation of clusterin mRNA (peak 8 h after serum deprivation). The coincidental elevation of clusterin expression and cell adhesion strength preceded the schedule of apoptotic cell death. Clusterin antiserum partially antagonized cell adhesion, but did not modify the course of apoptosis. These data suggest that clusterin expression may partially control cell adhesion with no influence on apoptosis in P19 cells, under defined conditions.

Clusterin: a role in cell survival in the face of apoptosis?

Clusterin is a multifunctional glycoprotein complex found in virtually all body fluids and on the surface of cells lining body cavities. Demonstrated and proposed functions include the transport of lipoproteins, the inhibition of complement-mediated cell lysis and the modulation of cell-cell interactions. On the basis of its elevated expression in apoptotic tissues, it was originally proposed that the protein might be casually involved in apoptosis. Here, we discuss the recent data that, in contrast to the earlier notion, suggest that clusterin expression is not enhanced, but rather is down-regulated in the cells undergoing apoptosis and that its expression in the apoptotic tissue is restricted to the vital neighboring cells. These results led to the proposal that rather than being a cell death gene, clusterin is a cell survival gene, exerting a protective function on the surviving bystander cells.

Clusterin: physiologic and pathophysiologic considerations

Clusterin is a heterodimeric glycoprotein produced by a wide array of tissues and found in most biologic fluids. A number of physiologic functions have been proposed for clusterin based on its distribution and in vitro properties. These include complement regulation, lipid transport, sperm maturation, initiation of apoptosis, endocrine secretion, membrane protection, and promotion of cell interactions. A prominent and defining feature of clusterin is its induction in such disease states as glomerulonephritis, polycystic kidney disease, renal tubular injury, neurodegenerative conditions including Alzheimer's disease, atherosclerosis, and myocardial infarction. The expression of clusterin in these states is puzzling, from the specific molecular species and cellular pathways eliciting such expression, to the roles subserved by clusterin once induced. This review will discuss these physiologic and pathophysiologic aspects of clusterin and speculate on its role in disease.

Biosynthesis of large dense-core vesicles in PC12 cells: effects of depolarization and second messengers on the mRNA levels of their constituents

mRNA levels of various constituents of large dense-core vesicles were determined in PC12 cells during depolarization and/or in the presence of BayK 8644, forskolin or phorbolester. For the soluble (secretory) proteins of the vesicles the mRNAs of chromogranin A and B, secretogranin II, neuropeptide Y and VGF were analyzed. Depolarization in the presence of BayK induced a strong up-regulation of the messages for chromogranin B, neuropeptide Y and VGF. Addition of forskolin enhanced this response for neuropeptide Y and VGF, phorbolester did the same only for VGF. Partly membrane-bound and membrane-spanning components analyzed were carboxypeptidase H, dopamine beta-hydroxylase and glycoprotein III (clusterin), peptidylglycine alpha-amidating mono-oxygenase and cytochrome b-561, respectively. Changes of mRNAs for these components were in general smaller and delayed. Six days of depolarization caused an up-regulation of glycoprotein III, peptidylglycine alpha-amidating mono-oxygenase and carboxypeptidase H mRNA levels which were not further increased by cyclic AMP and phorbolester. The dopamine beta-hydroxylase message increased after 6 days of depolarization, however, addition of phorbolester reduced this effect. For cytochrome b-561 there was no change after any of the conditions employed. These in vitro results are compared with those obtained for the biosynthesis regulation of large dense-core vesicles under in vivo conditions. It is suggested that in vivo acetylcholine and vasoactive intestinal polypeptide released from splanchnic nerve induce a differential change in the biosynthesis of large dense-core vesicles by acting via calcium and protein kinase A and C.

Identification of glycoprotein 330 as an endocytic receptor for apolipoprotein J/clusterin

Glycoprotein 330 (gp330) is a member of a family of endocytic receptors related to the low density lipoprotein receptor. gp330 has previously been shown to bind a number of ligands in common with its family member, the low density lipoprotein receptor-related protein (LRP). To identify ligands specific for gp330 and relevant to its localization on epithelia such as in the mammary gland, gp330-Sepharose affinity chromatography was performed. As a result, a 70-kDa protein was selected from human milk and identified by protein sequencing to be apolipoprotein J/clusterin (apoJ). Solid-phase binding assays confirmed that gp330 bound to apoJ with high affinity (Kd = 14.2 nM). Similarly, gp330 bound to apoJ transferred to nitrocellulose after SDS-polyacrylamide gel electrophoresis. LRP, however, showed no binding to apoJ in either type of assay. The binding of gp330 to apoJ could be competitively inhibited with excess apoJ as well as with the gp330 ligands apolipoprotein E, lipoprotein lipase, and the receptor-associated protein, a 39-kDa protein that acts to antagonize binding of all known ligands for gp330 and LRP. Several cultured cell lines that express gp330 and ones that do not express the receptor were examined for their ability to bind and internalize 125I-apoJ. Only cells that expressed gp330 endocytosed and degraded radiolabeled apoJ. Furthermore, F9 cells treated with retinoic acid and dibutyryl cyclic AMP to increase expression levels of gp330 displayed an increased capacity to internalize and degrade apoJ. Cellular internalization and degradation of radiolabeled apoJ could be inhibited with unlabeled apoJ, receptor-associated protein, and gp330 antibodies. The results indicate that gp330 but not LRP can bind to apoJ in vitro and that gp330 expressed by cells can mediate apoJ endocytosis leading to lysosomal degradation.

Secretogranin II: molecular properties, regulation of biosynthesis and processing to the neuropeptide secretoneurin

Secretogranin II is an acidic secretory protein in large dense core vesicles of endocrine, neuroendocrine and neuronal tissues. It comprises, together with chromogranins A and B, the class of proteins collectively called chromogranins. In this review the physico-chemical properties, genomic organization, tissue distribution, synthesis regulation, ontogeny and physiological function of this protein are discussed. Secretogranin II gained interest recently for mainly three reasons: (1) secretogranin II is an excellent marker for the regulated secretory pathway due to its simple and specific metabolic labeling by inorganic sulfate; (2) secretogranin II occurs in a variety of neoplasms arising from endocrine and neuroendocrine cells and was shown to be a useful histological tumor marker for these cells; (3) secretogranin II is the precursor of the recently discovered neuropeptide secretoneurin which induces dopamine release in the striatum of the rat brain.

Large dense-core vesicles in rat adrenal after reserpine: levels of mRNAs of soluble and membrane-bound constituents in chromaffin and ganglion cells indicate a biosynthesis of vesicles with higher secretory quanta

Rats were injected with a large dose of reserpine known to stimulate the adrenal medulla. Various times after drug treatment the mRNA levels of several constituents of large dense-core vesicles were determined by northern blot analysis and in situ hybridization. The latter method allowed detection of changes in mRNA levels not only in chromaffin cells, but also in the ganglion cells found in adrenal medulla. Levels of the mRNAs of secretory components of large dense-core vesicles (chromogranins A and B, secretogranin II, VGF, and neuropeptide Y) increased in chromaffin cells by 215-857% after 1-3 days of drug treatment. For partly membrane-bound components (dopamine beta-hydroxylase, prohormone convertase 2, carboxypeptidase H, and peptidylglycine alpha-amidating monooxygenase) the changes ranged from 182 to 315%, whereas for glycoprotein III and for intrinsic membrane proteins (cytochrome b561 and vesicle monoamine transporter 2) no change occurred. In ganglion cells the mRNAs that could be detected for VGF, neuropeptide Y, secretogranin II, carboxypeptidase H, and vesicle monoamine transporter 1 showed an analogous pattern of change, with significant increases for the secretory proteins and no change for the membrane components. From these and previous results we suggest the following concept: Long-lasting stimulation of chromaffin cells or neurons does not induce the biosynthesis of a larger number of vesicles but rather leads to the formation of vesicles containing higher secretory quanta of chromogranins and neuropeptides.

Biosynthesis of proteins of large dense-core vesicles in rat PC12 cells: regulation by forskolin and phorbol ester

We have investigated the influence of various second messengers on the biosynthesis of large dense-core vesicle constituents in rat PC12 cells. After treatment with forskolin, phorbol ester or a combination of both substances for up to six days, the messenger RNA levels of several vesicle components were determined by northern blotting. Forskolin increased the expression of messenger RNA encoding the soluble proteins chromogranin B, neuropeptide Y and VGF. Addition of phorbol ester markedly enhanced the effects of forskolin. On the other hand, the expression of two further soluble proteins, chromogranin A and secretogranin II, remained fairly unchanged with all treatments tested. Amongst partly membrane-bound vesicle components, the biosynthesis of glycoprotein III and peptidylglycine alpha-amidating mono-oxygenase was significantly up-regulated by combined treatment with forskolin plus phorbol ester. The carboxypeptidase H messenger RNA increased due to phorbol ester and after long-term application of both drugs. In contrast, phorbol ester alone or plus forskolin down-regulated the expression of dopamine beta-hydroxylase. Essentially the same applies to the intrinsic membrane protein cytochrome b-561, whose messenger RNA level declined in all treatment groups. In conclusion, our results show that forskolin and phorbol ester can regulate the composition of large dense-core vesicles in quite distinct patterns.