Constitutive and basal secretion from the endocrine cell line, AtT-20

A variant of the ACTH-secreting pituitary cell line, AtT-20, has been isolated that does not make ACTH, sulfated proteins characteristic of the regulated secretory pathway, or dense-core secretory granules but retains constitutive secretion. Unlike wild type AtT-20 cells, the variant cannot store or release on stimulation, free glycosaminoglycan (GAG) chains. In addition, the variant cells cannot store trypsinogen or proinsulin, proteins that are targeted to dense core secretory granules in wild type cells. The regulated pathway could not be restored by transfecting with DNA encoding trypsinogen, a soluble regulated secretory protein targeted to secretory granules. A comparison of secretion from variant and wild type cells allows a distinction to be made between constitutive secretion and basal secretion, the spontaneous release of regulated proteins that occurs in the absence of stimulation.     

A 13-amino acid n-terminal domain of a basic proline-rich protein is necessary for storage in secretory granules and facilitates exit from the endoplasmic reticulum.

We have investigated the role of different domains of a salivary basic proline-rich protein in intracellular transport and sorting of proline-rich proteins to the secretory granules. We have cloned a full-length cDNA of a basic proline-rich protein from the rat parotid and expressed it in AtT-20 cells. It was correctly sorted into secretory granules as shown by EM immunolocalization and by its presence in 8-bromocyclic AMP-stimulated secretion. Deletion of the N-terminal thirteen amino acid domain upstream from the proline-rich domain eliminated storage whereas deletion of the C-terminal 20-amino acid domain downstream from the proline-rich domain had no effect. Intracellular transport of full-length and mutant proline-rich proteins was unusually slow due to slow exit from the endoplasmic reticulum. However, the rate of transport increased with increasing level of expression for the full-length protein and the C-terminal deletion mutant. In contrast, the rate of transport of the N-terminal deletion mutant was independent of the level of expression. These results imply that the N-terminal domain is necessary for both storage and efficient intracellular transport. Moreover, interactions (self-aggregation?) that mediate sorting may begin as early as the endoplasmic reticulum.     

Distinct Molecular Events during Secretory Granule Biogenesis Revealed by Sensitivities to Brefeldin A

The biogenesis of peptide hormone secretory granules involves a series of sorting, modification, and trafficking steps that initiate in the trans-Golgi and trans-Golgi network (TGN). To investigate their temporal order and interrelationships, we have developed a pulse–chase protocol that follows the synthesis and packaging of a sulfated hormone, pro-opiomelanocortin (POMC). In AtT-20 cells, sulfate is incorporated into POMC predominantly on N-linked endoglycosidase H-resistant oligosaccharides. Subcellular fractionation and pharmacological studies confirm that this sulfation occurs at the trans-Golgi/TGN. Subsequent to sulfation, POMC undergoes a number of molecular events before final storage in dense-core granules. The first step involves the transfer of POMC from the sulfation compartment to a processing compartment (immature secretory granules, ISGs): Inhibiting export of pulse-labeled POMC by brefeldin A (BFA) or a 20°C block prevents its proteolytic conversion to mature adrenocorticotropic hormone. Proteolytic cleavage products were found in vesicular fractions corresponding to ISGs, suggesting that the processing machinery is not appreciably activated until POMC exits the sulfation compartment. A large portion of the labeled hormone is secreted from ISGs as incompletely processed intermediates. This unregulated secretory process occurs only during a limited time window: Granules that have matured for 2 to 3 h exhibit very little unregulated release, as evidenced by the efficient storage of the 15-kDa N-terminal fragment that is generated by a relatively late cleavage event within the maturing granule. The second step of granule biogenesis thus involves two maturation events: proteolytic activation of POMC in ISGs and a transition of the organelle from a state of high unregulated release to one that favors intracellular storage. By using BFA, we show that the two processes occurring in ISGs may be uncoupled: although the unregulated secretion from ISGs is impaired by BFA, proteolytic processing of POMC within this organelle proceeds unaffected. The finding that BFA impairs constitutive secretion from both the TGN and ISGs also suggests that these secretory processes may be related in mechanism. Finally, our data indicate that the unusually high levels of unregulated secretion often associated with endocrine tumors may result, at least in part, from inefficient storage of secretory products at the level of ISGs.     

Isoproterenol increases sorting of parotid gland cargo proteins to the basolateral pathway

Exocrine cells have an essential function of sorting secreted proteins into the correct secretory pathway. A clear understanding of sorting in salivary glands would contribute to the correct targeting of therapeutic transgenes. The present work investigated whether there is a change in the relative proportions of basic proline-rich protein (PRP) and acidic PRPs in secretory granules in response to chronic isoproterenol treatment, and whether this alters the sorting of endogenous cargo proteins. Immunoblot analysis of secretory granules from rat parotids found a large increase of basic PRP over acidic PRPs in response to chronic isoproterenol treatment. Pulse chase experiments demonstrated that isoproterenol also decreased regulated secretion of newly synthesized secretory proteins, including PRPs, amylase and parotid secretory protein. This decreased efficiency of the apical regulated pathway may be mediated by alkalization of the secretory granules since it was reversed by treatment with mild acid. We also investigated changes in secretion through the basolateral (endocrine) pathways. A significant increase in parotid secretory protein and salivary amylase was detected in sera of isoproterenol-treated animals, suggesting increased routing of the regulated secretory proteins to the basolateral pathway. These studies demonstrate that shifts of endogenous proteins can modulate regulated secretion and sorting of cargo proteins. amylase; parotid secretory protein; polarized secretion     

Sorting and secretion of adrenocorticotropin in a pituitary tumor cell line after perturbation of the level of a secretory granule-specific proteoglycan

A mouse anterior pituitary tumor cell line (AtT-20) that secretes adrenocorticotropin and beta endorphin sorts the proteins it transports to the surface into two exocytotic pathways. AtT-20 cells also synthesize a secretory granule-specific sulfated molecule and secrete it on stimulation (Moore, H.-P., B. Gumbiner, and R. B. Kelly, 1983, J. Cell Biol., 97:810-817). We show here that this molecule is sensitive to proteolysis and that the residual sulfated material co-migrates with a chondroitin sulfate standard on thin-layer electrophoresis. Furthermore, this sulfated molecule is completely sensitive to chondroitinase ABC digestion. Thus the secretory granule-specific sulfated molecule is a proteoglycan with chondroitin sulfate side chains. We examined the role of proteoglycans in the sorting and secretion of adrenocorticotropin in AtT-20 cells by severely decreasing the amount of this vesicle-specific proteoglycan in two ways. First, a xyloside was used to inhibit proteoglycan biosynthesis; second, a variant of the AtT-20 cell line was isolated that synthesized little of the sulfated proteoglycan. In neither case was the sorting or secretion of adrenocorticotropin detectably altered, suggesting that the proteoglycan is not required for these processes.     

Processing of proaugurin is required to suppress proliferation of tumor cell lines

Augurin is a secretory molecule produced in pituitary, thyroid, and esophagus and implicated in a wide array of physiological processes, from ACTH release to tumor suppression. However, the specific proaugurin-derived peptides present in various cell types are not yet known. In order to shed light on the posttranslational modifications required for biological activity, we here describe the posttranslational processing of proaugurin in AtT-20 and Lovo cells and identify proaugurin-derived products generated by convertases. In vitro cleavage of proaugurin with proprotein convertases produced multiple peptides, including a major product with a mass of 9.7 kDa by mass spectrometry. Metabolic labeling of C-terminally tagged proaugurin in AtT-20 and AtT-20/PC2 cells resulted in a major 15-kDa tagged form on SDS-PAGE, which likely corresponds to the 9.7-kDa in vitro fragment, with the added tag, its linker, and posttranslational modification(s). The secretion of neither proaugurin nor this cleavage product was stimulated by forskolin, indicating its lack of storage in regulated secretory granules and lack of cleavage by PC2. Incubation of cells with the furin inhibitor nona-d-arginine resulted in impaired cleavage of proaugurin, whereas metalloprotease inhibitors did not affect proaugurin proteolysis. These data support the idea that proaugurin is cleaved by furin and secreted via the constitutive secretory pathway. Interestingly, proaugurin was sulfated during trafficking; sulfation was completely inhibited by brefeldin A. Proliferation assays with three different tumor cell lines demonstrated that only furin-cleaved proaugurin could suppress cell proliferation, suggesting that proteolytic cleavage is a posttranslational requirement for proaugurin to suppress cell proliferation.     

A subclass of proteins and sulfated macromolecules secreted by AtT-20 (mouse pituitary tumor) cells is sorted with adrenocorticotropin into dense secretory granules

The AtT-20 cell, a mouse pituitary tumor line that secretes adrenocorticotropin and beta-endorphin, sorts the proteins it externalizes into two exocytotic pathways. Cells that are labeled with [35S]methionine or [35S]sulfate can be shown to transport three acidic polypeptides (65,000, 60,000, and 37,000 mol wt) and at least two sulfated macromolecules into storage secretory granules. When the cells are stimulated by the secretagogue 8-bromo-cAMP, these polypeptides are coordinately secreted with mature adrenocorticotropin into the culture medium. In contrast, a completely different set of secreted polypeptides and sulfated macromolecules does not enter a storage form and is transported to the cell surface more rapidly. Their secretion from the cells is constitutive and does not require the presence of secretagogues. These molecules, like a viral membrane glycoprotein described previously (Gumbiner, B., and R. B. Kelly, 1982, Cell, 28:51-59) are not found in isolated secretory granules and therefore must reach the cell surface in a different exocytotic vesicle. The segregation of a subclass of secretory macromolecules into the secretory granules, despite the existence of another potential secretory pathway, suggests that these molecules have specific functions related to regulated hormone secretion or storage. Presumably all of the proteins secreted by the regulated secretory granule pathway share some common property that targets them to the secretory granule.     

In vitro mutagenesis of trypsinogen: role of the amino terminus in intracellular protein targeting to secretory granules

The mouse anterior pituitary tumor cell line, AtT-20, targets secretory proteins into two distinct intracellular pathways. When the DNA that encodes trypsinogen is introduced into AtT-20 cells, the protein is sorted into the regulated secretory pathway as efficiently as the endogenous peptide hormone ACTH. In this study we have used double-label immunoelectron microscopy to demonstrate that trypsinogen colocalizes in the same secretory granules as ACTH. In vitro mutagenesis was used to test whether the information for targeting trypsinogen to the secretory granules resides at the amino (NH2) terminus of the protein. Mutations were made in the DNA that encodes trypsinogen, and the mutant proteins were expressed in AtT-20 cells to determine whether intracellular targeting could be altered. Replacing the trypsinogen signal peptide with that of the kappa-immunoglobulin light chain, a constitutively secreted protein, does not alter targeting to the regulated secretory pathway. In addition, deletion of the NH2-terminal "pro" sequence of trypsinogen has virtually no effect on protein targeting. However, this deletion does affect the signal peptidase cleavage site, and as a result the enzymatic activity of the truncated trypsin protein is abolished. We conclude that neither the signal peptide nor the 12 NH2-terminal amino acids of trypsinogen are essential for sorting to the regulated secretory pathway of AtT-20 cells.     

A targeting sequence for dense secretory granules resides in the active renin protein moiety of human preprorenin

Human renin plays an important role in blood pressure homeostasis and is secreted in a regulated manner from the juxtaglomerular apparatus of the kidney in response to various physiological stimuli. Many aspects of the regulated release of renin (including accurate processing of prorenin to renin, subcellular targeting of renin to dense secretory granules, and regulated release of active renin) can be reproduced in mouse pituitary AtT-20 cells transfected with a human preprorenin expression vector. Using protein engineering, we have attempted to define the roles of various structures in prorenin that affect its production and trafficking to dense core secretory granules, resulting in its activation and regulated secretion. Replacement of the native signal peptide of human preprorenin with that of a constitutively secreted protein (immunoglobulin M) had no apparent effect on either the constitutive secretion of prorenin or the regulated secretion of active renin in transfected AtT-20 cells. Removal of the pro segment resulted in a marked reduction in total renin secretion, but did not prevent renin from entering the regulated secretory pathway. Single or combined mutations in the two glycosylation sites of human renin did not prevent its regulated secretion; however, the complete elimination of glycosylation resulted in a significant increase in the ratio of renin/prorenin secreted by the transfected cells. Thus, these results suggest that 1) at least one of the sequences that target human renin to dense secretory granules lies within the protein moiety of active renin; 2) the presence of the pro segment is important for efficient prorenin and renin production; and 3) glycosylation can quantitatively affect the proportion of active renin secreted.     

Targeting of secretory vesicles to cytoplasmic domains in AtT-20 and PC- 12 cells

Organelles are not uniformly distributed throughout the cytoplasm but have preferred locations that vary between tissues and during development. To investigate organelle targeting to cytoplasmic domains we have taken advantage of the mouse pituitary cell line, AtT-20, which, when induced to extend long processes, accumulates dense core secretory granules at the tips of the processes. During mitosis, these secretory granules accumulate along the plane of division. Protein synthesis is not mandatory for such redistribution of secretory granules. To explore the specificity of the redistribution we have used transfected AtT-20 cells that express the immunoglobulin kappa light chain. While the endogenous hormone ACTH is found in secretory granules, the kappa chain is a marker for organelles involved in constitutive secretion. By immunofluorescence, kappa also accumulates at the tips of growing processes, and along the midline of dividing cells, suggesting that the redistribution of vesicles is not specific for dense-core secretory granules. Since there is evidence for selective organelle transport along processes in neuronal cells, the rat pheochromocytoma cell PC-12 was transfected with DNA encoding markers for regulated and constitutive secretory vesicles. Again regulated and constitutive vesicles co-distribute, even in cells grown in the presence of nerve growth factor. We suggest that at least in the cells studied here, cytoskeletal elements normally carry exocytotic organelles to the surface; when the cytoskeletal elements coalesce in an extending process, exocytotic organelles of both the constitutive and regulated pathway are transported nonselectively to the tips of the cytoskeletal elements where they accumulate.     

Cell type-specific storage of dopamine beta-monooxygenase

Expression of dopamine beta-monooxygenase (DBM), the enzyme that converts dopamine into norepinephrine, is limited to adrenal chromaffin cells and a small population of neurons. We studied DBM trafficking to regulated granules by stably expressing rat DBM in AtT-20 corticotrope tumor cells, which contain regulated granules, and in Chinese hamster ovary (CHO) cells, which lack regulated granules. The behavior of exogenous DBM in both cell lines was compared with endogenous DBM in adrenal chromaffin cells. CHO cells secreted active DBM, indicating that production of active enzyme does not require features unique to neuroendocrine cells. Pulse-chase experiments indicated that early steps in DBM maturation followed a similar time course in AtT-20, CHO, and adrenal chromaffin cells. Use of a conformation-sensitive DBM antiserum indicated that acquisition of a folded structure occurred with a similar time course in all three cell types. Cell type-specific differences in DBM trafficking became apparent only when storage in granules was examined. As expected, DBM was stored in secretory granules in chromaffin cells; CHO cells failed to store DBM. Despite the fact that AtT-20 cells have regulated granules, exogenous DBM was not stored in these granules. Thus storage of DBM in secretory granules requires cell type specific factors.     

Deletions of the Synenkephalin Domain Which Do Not Alter Cell-Specific Proteolytic Processing or Secretory Targeting of Human Proenkephalin

Abstract: To identify signals that direct the proteolytic processing and regulated secretion of human proenkephalin (hPE), we have transfected the hPE gene or minigene constructs into pituitary tumor cells, either rat GH4C1 cells or mouse AtT-20 cells. Cells transfected with either the hPE gene or minigene contained similar levels of methionine-enkephalin (ME)-containing peptides and hPE mRNA. In the GH4C1 clones, ME was present predominantly in high-molecular-mass forms (5–25 kDa). In contrast, the AtT-20 clones contained almost exclusively free ME and low-molecular-mass forms (<5 kDa), with very little high-molecular-mass species present. Thus, among pituitary cells, corticotroph-derived cells appear better equipped to process hPE than lactotroph-derived cells. Despite limited proteolytic processing, GH4C1 clones secreted large amounts of unprocessed (>20 kDa) hPE into the medium, making up to 10% of endogenous rat prolactin secretion. Both precursor and processed forms of ME were cosecreted acutely (<1 h) with rat prolactin, and release of both polypeptides was stimulated up to 12-fold by secretagogues. Thus, complete proteolytic processing was not required for accurate targeting of hPE to the regulated secretory pathway. When transfected with constructs bearing deletions of amino-terminal amino acids 2–43 or 2–67, i.e., part or nearly all of the synenkephalin moiety, GH4C1 cells handled the modified protein much like cells expressing the complete protein. They did not process the modified hPE extensively, but the protein was correctly targeted to the regulated secretory pathway. AtT-20 cells transfected with truncated hPE cDNA constructs expressed and processed the protein as efficiently as cells expressing unmodified hPE and expressed predominantly low-molecular-mass forms of ME. Therefore, the structural features required for correct targeting and processing are not present in the cysteine-rich amino-terminal third of the prohormone. It is interesting that the deletions did not include the SHLL peptide motif in synenkephalin, a motif that has been proposed as a sorting signal.     

Secretory protein targeting in a pituitary cell line: differential transport of foreign secretory proteins to distinct secretory pathways

The mouse pituitary cell line, AtT-20, packages the adrenocorticotropic hormone (ACTH) in secretory vesicles and releases it when the cell is stimulated with secretagogues. These cells have the capacity, after transfection with the appropriate DNA, to package heterologous peptide hormones into the regulated secretory vesicles (Moore, H. P. H., M. D. Walker, F. Lee, and R. B. Kelly, 1983, Cell, 35:531-538). To test if other secreted proteins prefer a different route to the surface, we have transfected AtT-20 cells with DNAs coding for a fragment of a membrane protein, the vesicular stomatitis virus G protein from which the membrane spanning domain has been deleted (Rose, J. K., and J. E. Bergmann, 1982, Cell, 17:813-819). We found that the secreted vesicular stomatitis virus G proteins were not transported to the regulated secretory vesicles. Instead they preferentially exited the cell by the constitutive pathway previously found in these cells (Gumbiner, B., and R. B. Kelly, 1982, Cell, 28:51-59). In contrast, human growth hormone transfected into the cells by the same procedure was transported to the regulated pathway with a similar efficiency as the endogenous hormone ACTH. Transport of the secreted G protein to the regulated pathway, if it occurs at all, is at least 30-fold less efficient than peptide hormones. We conclude that the transport machinery in AtT-20 cells must selectively recognize different secreted proteins and sort them into distinct secretory pathways.     

Oxidative modifications, mitochondrial dysfunction, and impaired protein degradation in Parkinson's disease: how neurons are lost in the Bermuda triangle

In AtT-20 cells ACTH secretion is regulated by both Ca²⁺ and G proteins. We previously demonstrated that calnuc, an EF-hand Ca²⁺ binding protein which regulates Alzheimer's β-amyloid precursor protein (APP) biogenesis, binds both Ca²⁺ as well as Gα subunits. Here we investigate calnuc's role in G protein-mediated regulation of ACTH secretion in AtT-20 neuroendocrine secretory cells stably overexpressing calnuc-GFP. Similar to endogenous calnuc, calnuc-GFP is mainly found in the Golgi, on the plasma membrane (PM), and associated with regulated secretion granules (RSG). By deconvolution immunofluorescence, calnuc-GFP partially colocalizes with Gαi1/2 and Gαi3 at the PM and on RSG. Cytosolic calnuc(ΔSS)-CFP with the signal sequence deleted also partially colocalizes with RSG and partially cosediments with Gαi1/2 in fractions enriched in RSG. Overexpression of calnuc-GFP specifically increases the distribution of Gαi1/2 on the PM whereas the distribution of Gβ subunits and synaptobrevin 2 (Vamp 2) is unchanged. Overexpression of calnuc-GFP or cytosolic calnuc(ΔSS)-CFP enhances ACTH secretion two-fold triggered by mastoparan or GTPγS but does not significantly affect glycosaminoglycan (GAG) chain secretion along the constitutive pathway or basal secretion of ACTH. Calnuc's facilitating effects on ACTH secretion are decreased after introducing anti-Gαi1/2, Gαi3, Gβ or calnuc IgG into permeabilized cells but not when Gα12 or preimmune IgG is introduced. The results suggest that calnuc binds to Gα subunits on the Golgi and on RSG and that overexpression of calnuc causes redistribution of Gαi subunits to the PM and RSG, indicating that calnuc plays a role in dynamic distribution of only Gα but not Gβ subunits. Thus calnuc may connect G protein signaling and calcium signaling during regulated secretion.     

Occurrence of tyrosine sulfate in proteins--a balance sheet. 1. Secretory and lysosomal proteins

1. The abundance of tyrosine sulfate in secretory proteins and in various classes of cellular proteins has been quantified and compared to protein-bound carbohydrate sulfate. 2. HepG2 cells and fibroblasts, two cell types showing only the constitutive pathway of secretion, and PC12 cells, which show both the constitutive and the regulated pathway of secretion, were subjected to pulse-chase and/or long-term labelling with [35S]sulfate and [3H]tyrosine, followed by analysis of proteins in the cells and medium. Under both conditions of labelling, 65-92% of the protein-bound tyrosine sulfate and 44-84% of the protein-bound carbohydrate sulfate were found to be secretory. In HepG2 cells, the frequency of sulfation of tyrosine residues, which can be determined independently from protein abundance and the rate of protein synthesis, was 8-22 times higher in proteins secreted into the medium than in cellular proteins. 3. All cell lines studied contained significant amounts, not only of carbohydrate sulfate, but also of tyrosine sulfate in specific cellular proteins. As shown for fibroblasts, these tyrosine-sulfated proteins were retained within the cells for at least 100 min of chase following a pulse with [35S]sulfate and were almost completely recovered in a light membrane fraction after subcellular fractionation. 4. Lysosomes were found to contain small, but significant, amounts of protein-bound tyrosine sulfate in addition to protein-bound carbohydrate sulfate. Protein-bound tyrosine sulfate in lysosomes reached a peak at 20 min of chase and rapidly disappeared thereafter, whereas protein-bound carbohydrate sulfate accumulated after 20 min of chase. Examination of the known sequences of eleven lysosomal enzymes revealed the presence of potential tyrosine sulfation sites in five of them. 5. Our results show that secretory proteins are the most abundant, but not exclusive, in vivo substrates for tyrosine sulfation and suggest the presence of soluble tyrosine-sulfated proteins in lysosomes and other, as yet unidentified, organelles of the secretory pathway. In the following paper in this journal we describe the abundance of tyrosine sulfate in integral membrane proteins.     

Proteoglycans in haemopoietic cells

Proteoglycans are produced by all types of haemopoietic cells including mature cells and the undifferentiated stem cells. The proteinase-resistant secretory granule proteoglycan (serglycin; Ref. 14), is the most prevalent and best characterised of these proteoglycans. Although its complete pattern of distribution in the haemopoietic system is unknown, serglycin has been identified in the mast cells, basophils and NK cells, in which secretion is regulated, and in HL-60 cells and a monocytoid cell line (Kolset, S.O., unpublished data) in which secretion is constitutive. Proteinase-resistant proteoglycans have been detected in human T-lymphocytes and murine stem cells (FDCP-mix) and the core proteins may be closely related to serglycin. A variety of glycosaminoglycan chains are assembled on the serglycin protein and it is likely that this class of proteoglycan can carry out a wide variety of functions in haemopoietic cells including the regulation of immune responses, inflammatory reactions and blood coagulation. There is strong evidence that in mast cells, NK cells and platelets, the proteoglycans are complexed to basic proteins (including enzymes and cytolytic agents) and amines in secretory granules and such complexes may dissociate following secretion from the cell. The stability of the complexes may be regulated by the ambient pH which may be acidic in the granules and neutral or above in the external medium. However, proteinase-proteoglycan complexes in mast cell granules seem to remain stable after secretion and it has been proposed that the proteoglycan regulates activity of proteinases released into the pericellular domain. The functions of proteoglycans which are constitutively secreted from cells are less clear. If cells have no requirement for storage of basic proteins why do they utilise the same design of proteoglycan as cells which accumulate secretory material prior to regulated release? We should stress that the so-called constitutive secretory pathway has been identified in haemopoietic cells in culture, which are usually maintained and grown in the presence of mitogenic factors (e.g., IL-2, IL-3). the cells are therefore activated and it has not been established that continuous proteoglycan secretion occurs in quiescent cells circulating in the peripheral blood. It is possible that lymphocytes, monocytes and macrophages, in which the constitutive secretion pathway operates in vitro, may store proteoglycan in vivo unless stimulated by mitogens or other activating agents.(ABSTRACT TRUNCATED AT 400 WORDS)     

Expression and sorting of rat plasma kallikrein in POMC-producing AtT-20 cells.

A vaccinia virus (VV) vector was used to express rat plasma kallikrein (rPK) in the constitutively secreting cells, BSC-40, and in the endocrine regulated cells, AtT-20. Using a specific rPK antibody and a fluorogenic substrate, Phe-Phe-Arg-AMC, we demonstrated that in both cell lines VV infections resulted in the synthesis of an immunoreactive enzyme predominantly present as a zymogen which can be activated with trypsin. Stimulation of VV:rPK-infected AtT-20 cells with either 5mM 8-bromo-cAMP or 56 mM KCl resulted in a different pattern of rPK and ACTH secretion, strongly suggesting that rPK follows the constitutive secretory pathway. Finally, the 10% rPK activity found within AtT-20 cell extracts had no effect on pro-opiomelanocortin (POMC) processing either intracellularly or extracellularly. The above data show that the biosynthetic machinery of both cell lines analyzed does not allow the efficient activation of plasma prekallikrein. Finally, despite the PK's demonstrated ability to cleave various hormone precursors in vitro at pairs of basic residues, in vivo, we did not obtain evidence that this hepatic enzyme can also act as an intracellular pro-protein processing enzyme.     

Secretion and molecular forms of NESP55, a novel genomically imprinted neuroendocrine-specific protein from AtT-20 cells

NESP55 (neuroendocrine secretory protein of M(r) 55,000) is a paternally imprinted proteoglycan, expressed specifically in endocrine cells and the nervous system. We investigated the subcellular localization and secretion of NESP55 in AtT-20 cells. NESP55 accumulated in the medium linearly over 24 h exceeding its intracellular content 3.7-fold by that time. Incubation of cells at 16 degrees C, to block protein export, inhibited basal secretion by 79%. Stimulation of AtT-20 cells with 8-Br-cAMP increased secretion of NESP55 by only 45%. The NESP55 secretory vesicles sedimented at a density of 1.2-1.4 M, which is slightly lighter than that of the large dense core vesicles. Immunofluorescence studies revealed immunoreactivity in the Golgi apparatus and a punctuate staining of processes or neurites. Our data demonstrate that NESP55 is mainly sorted to and released from a population of constitutive secretory vesicles, which are transported out of the perikarya into processes or axons. In addition, some NESP55 is also routed to the regulated pathway. The signal peptide of NESP55, as determined with peptide antisera, is 46 amino acids long and represents the best conserved region of this molecule suggesting that the signal peptide may have a function of its own. The subcellular localization and export of NESP55 from cells are reminiscent of neuronal proteoglycans forming the extracellular matrix, which are implicated in the development and maintenance of neuronal circuits and mechanisms of axonal guidance.     

Spatial segregation of the regulated and constitutive secretory pathways

Recent experiments using DNA transfection have shown that secretory proteins in AtT-20 cells are sorted into two biochemically distinct secretory pathways. These two pathways differ in the temporal regulation of exocytosis. Proteins secreted by the regulated pathway are stored in dense-core granules until release is stimulated by secretagogues. In contrast, proteins secreted by the constitutive pathway are exported continuously, without storage. It is not known whether there are mechanisms to segregate regulated and constitutive secretory vesicles spatially. In this study, we examined the site of insertion of constitutive vesicles and compared it with that of regulated secretory granules. Regulated granules accumulate at tips of processes in these cells. To determine whether constitutively externalized membrane proteins are inserted into plasma membrane at the cell body or at process tips, AtT-20 cells were infected with ts-O45, a temperature-sensitive mutant of vesicular stomatitis virus in which transport of the surface glycoprotein G is conditionally blocked in the ER. After switching to the permissive temperature, insertion of G protein was detected at the cell body, not at process tips. Targeting of constitutive and regulated secretory vesicles to distinct areas of the plasma membrane appears to be mediated by microtubules. We found that while disruption of microtubules by colchicine had no effect on constitutive secretion, it completely blocked the accumulation of regulated granules at special release sites. Colchicine also affected the proper packaging of regulated secretory proteins. We conclude that regulated and constitutive secretory vesicles are targeted to different areas of the plasma membrane, most probably by differential interactions with microtubules. These results imply that regulated secretory granules may have unique membrane receptors for selective attachment to microtubules.     

Rat pro-atrial natriuretic factor expression and post-translational processing in mouse corticotropic pituitary tumor cells

Atrial natriuretic factor (ANF) is stored within atrial myocyte secretory granules as pro-ANF (ANF-(1-126] and is proteolytically processed co-secretionally C-terminal to a single basic amino acid to form ANF-(1-98) and the bioactive product ANF-(99-126). Pro-ANF is also expressed in certain non-cardiac neuroendocrine cell types (e.g. brain, adrenal). Although the relatively low levels of the peptide in these cell types have precluded detailed processing and secretion studies using cultured cells, some work with tissue extracts suggests that pro-ANF is pre-secretionally processed between or C-terminal to Arg101-Arg102 in such cells. In order to assess whether cultured non-cardiac endocrine cells process pro-ANF pre- or co-secretionally, and to establish whether both paired and single basic amino acids can serve as cleavage sites, transfection studies were carried out using the adrenocorticotropic hormone (ACTH)-producing pituitary tumor cell line AtT-20/D-16v. These cells normally cleave pro-ACTH/endorphin pre-secretionally at selected, but not all, pairs of basic amino acids to a variety of product peptides. A prepro-ANF expression plasmid was constructed and transfected into the AtT-20 cells. The resulting ANF/AtT-20 cell clone selected for this study expressed ACTH at levels similar to the untransfected wild type cells and secreted immunoreactive ANF-related material at a rate of approximately 1 fmol/min/10(5) cells, which was about 10% the rate of ACTH secretion. The rates of secretion of both ANF and ACTH could be increased 3-5-fold with a variety of known AtT-20 cell secretagogues including phorbol esters and the beta-adrenergic agonist, isoproterenol, thus indicating that both peptides were routed through regulated secretory pathways. Utilizing a combination of specific antisera directed against various regions of pro-ANF, size exclusion and reversed phase high performance liquid chromatography, and peptide mapping, it was shown that the ANF/AtT-20 cells contained and secreted the bioactive peptide ANF-(103-126) and -(1-97). These results indicate that the ANF/AtT-20 cells specifically cleave pro-ANF pre-secretionally at the same single basic site used by cardiac tissue; this single basic cleavage is apparently followed by removal of Arg98 by carboxypeptidase H. It is also apparent that the cells can cleave at the sole paired basic site in pro-ANF, which is the probable cleavage site used by neurons and some other endocrine cells that express low levels of the prohormone.(ABSTRACT TRUNCATED AT 400 WORDS)