Immunocytochemical localization of the neuropeptide-synthesizing enzyme PC1 in AtT-20 cells

Human Islets Contain a Beta Cell Type That Expresses Proinsulin But Not the Enzyme That Converts the Precursor to Insulin

In pancreatic beta cells, proinsulin (ProIN) undergoes folding in endoplasmic reticulum/Golgi system and is translocated to secretory vesicles for processing into insulin and C-peptide by the proprotein convertases (PC)1/3 and PC2, and carboxypeptidase E. Human beta cells show significant variation in the level of expression of PC1/3, the critical proconvertase involved in proinsulin processing. To ascertain whether this heterogeneity is correlated with the level of expression of the prohormone and mature hormone, the expression of proinsulin, insulin, and PC1/3 in human beta cells was examined. This analysis identified a human beta cell type that expressed proinsulin but lacked PC1/3 (ProIN⁺PC1/3^-). This beta cell type is absent in rodent islets and is abundant in human islets of adults but scarce in islets from postnatal donors. Human islets also contained a beta cell type that expressed both proinsulin and variable levels of PC1/3 (ProIN⁺PC1/3⁺) and a less abundant cell type that lacked proinsulin but expressed the convertase (ProIN^-PC1/3⁺). These cell phenotypes were altered by type 2 diabetes. These data suggest that these three cell types represent different stages of a dynamic process with proinsulin folding in ProIN⁺PC1/3^- cells, proinsulin conversion into insulin in ProIN⁺PC1/3⁺cells, and replenishment of the proinsulin content in ProIN^-PC1/3⁺ cells.

Increased expression and retention of the secretory chaperone proSAAS following cell stress

The secretory pathway of neurons and endocrine cells contains a variety of mechanisms designed to combat cellular stress. These include not only the unfolded protein response pathways but also diverse chaperone proteins that collectively work to ensure proteostatic control of secreted and membrane-bound molecules. One of the least studied of these chaperones is the neural- and endocrine-specific molecule known as proSAAS. This small chaperone protein acts as a potent anti-aggregant both in vitro and in cellulo and also represents a cerebrospinal fluid biomarker in Alzheimer's disease. In the present study, we have examined the idea that proSAAS, like other secretory chaperones, might represent a stress-responsive protein. We find that exposure of neural and endocrine cells to the cell stressors tunicamycin and thapsigargin increases cellular proSAAS mRNA and protein in Neuro2A cells. Paradoxically, proSAAS secretion is inhibited by these same drugs. Exposure of Neuro2A cells to low concentrations of the hypoxic stress inducer cobalt chloride, or to sodium arsenite, an oxidative stressor, also increases cellular proSAAS content and reduces its secretion. We conclude that the cellular levels of the small secretory chaperone proSAAS are positively modulated by cell stress.

60 YEARS OF POMC: From the prohormone theory to pro-opiomelanocortin and to proprotein convertases (PCSK1 to PCSK9)

Pro-opiomelanocortin (POMC), is a polyprotein expressed in the pituitary and the brain where it is proteolytically processed into peptide hormones and neuropeptides with distinct biological activities. It is the prototype of multipotent prohormones. The prohormone theory was first suggested in 1967 when Chrétien and Li discovered γ-lipotropin and observed that (i) it was part of β-lipotropin (β-LPH), a larger polypeptide characterized 2 years earlier and (ii) its C-terminus was β-melanocyte-stimulating hormone (β-MSH). This discovery led them to propose that the lipotropins might be related biosynthetically to the biologically active β-MSH in a precursor to end product relationship. The theory was widely confirmed in subsequent years. As we celebrate the 50th anniversary of the sequencing of β-LPH, we reflect over the lessons learned from the sequencing of those proteins; we explain their extension to the larger POMC precursor; we examine how the theory of precursor endoproteolysis they inspired became relevant for vast fields in biology; and how it led, after a long and arduous search, to the novel proteolytic enzymes called proprotein convertases. This family of nine enzymes plays multifaceted functions in growth, development, metabolism, endocrine, and brain functions. Their genetics has provided many insights into health and disease. Their therapeutic targeting is foreseeable in the near future. Thus, what started five decades ago as a theory based on POMC fragments, has opened up novel and productive avenues of biological and medical research, including, for our own current interest, a highly intriguing hypocholesterolemic Gln152His PCSK9 mutation in French-Canadian families.

AP-1A controls secretory granule biogenesis and trafficking of membrane secretory granule proteins

The adaptor protein 1A complex (AP-1A) transports cargo between the trans-Golgi network (TGN) and endosomes. In professional secretory cells, AP-1A also retrieves material from immature secretory granules (SGs). The role of AP-1A in SG biogenesis was explored using AtT-20 corticotrope tumor cells expressing reduced levels of the AP-1A μ1A subunit. A twofold reduction in μ1A resulted in a decrease in TGN cisternae and immature SGs and the appearance of regulated secretory pathway components in non-condensing SGs. Although basal secretion of endogenous SG proteins was unaffected, secretagogue-stimulated release was halved. The reduced μ1A levels interfered with the normal trafficking of carboxypeptidase D (CPD) and peptidylglycine α-amidating monooxygenase-1 (PAM-1), integral membrane enzymes that enter immature SGs. The non-condensing SGs contained POMC products and PAM-1, but not CPD. Based on metabolic labeling and secretion experiments, the cleavage of newly synthesized PAM-1 into PHM was unaltered, but PHM basal secretion was increased in sh-μ1A PAM-1 cells. Despite lacking a canonical AP-1A binding motif, yeast two-hybrid studies demonstrated an interaction between the PAM-1 cytosolic domain and AP-1A. Coimmunoprecipitation experiments with PAM-1 mutants revealed an influence of the luminal domains of PAM-1 on this interaction. Thus, AP-1A is crucial for normal SG biogenesis, function and composition.

Defective transport of the obesity mutant PC1/3 N222D contributes to loss of function

Mutations in the PCSK1 gene encoding prohormone convertase 1/3 (PC1/3) are strongly associated with obesity in humans. The PC1/3(N222D) mutant mouse thus far represents the only mouse model that mimics the PC1/3 obesity phenotype in humans. The present investigation addresses the cell biology of the N222D mutation. Metabolic labeling experiments reveal a clear defect in the kinetics of insulin biosynthesis in islets from PC1/3(N222D) mutant mice, resulting in an increase in both proinsulin and its processing intermediates, predominantly lacking cleavage at the Arg-Arg site. Although the mutant PC1/3 zymogen is correctly processed to the 87-kDa form, pulse-chase immunoprecipitation experiments, labeling, and immunohistochemical experiments using uncleavable variants all demonstrate that the PC1/3-N222D protein is largely mislocalized compared with similar wild-type (WT) constructs, being predominantly retained in the endoplasmic reticulum. The PC1/3-N222D mutant also undergoes more efficient degradation via the ubiquitin-proteasome system than the WT enzyme. Lastly, the mutant PC1/3-N222D protein coimmunoprecipitates with WT PC1/3 and exerts a modest effect on intracellular retention of the WT enzyme. These profound alterations in the cell biology of PC1/3-N222D are likely to contribute to the defective insulin biosynthetic events observed in the mutant mice and may be relevant to the dramatic contributions of polymorphisms in this gene to human obesity.

Proprotein convertase 1/3 (PC1/3) in the rat alveolar macrophage cell line NR8383: localization, trafficking and effects on cytokine secretion

The proprotein convertase 1/3 (PC1/3) is an important post-translational processing enzyme for the activation of precursor proteins within the regulated secretory pathway. Well characterized for its role in the neural and endocrine systems, we recently reported an unconventional role of PC1/3 as a modulator of the Toll-like receptor innate immune response. There are only a few reports that have studied PC1/3 expression in macrophages, and more investigation is needed to better characterize its function. These studies would greatly benefit from model cell lines. Our study aims to identify and characterize PC1/3 in a relevant model macrophage cell line and to determine the links between PC1/3 and innate immune cellular responses. We describe the rat alveolar cell line, NR8383, as expressing PC1/3 and the most common Toll-like receptors. In NR8383 cells, PC1/3 is localized at the Trans-Golgi network and traffics to lysosome related vesicles upon lipopolysaccharide stimulation. Moreover, we report the co-localization of PC1/3 and Toll-like receptor 4 upon lipopolysaccharide stimulation. Down regulation of PC1/3 by shRNA produce a similar phenotype in NR8383 to what we previously reported in isolated peritoneal macrophages. PC1/3 shRNA induced changes in the cellular organization and expression of the specific trafficking regulator RAB GTPase. As a consequence, NR8383 down-regulated for PC1/3, present an abnormal cytokine secretion profile. We conclude that the NR8383 cell line represents a good model to study PC1/3 in macrophages and we present PC1/3 as an important regulator of vesicle trafficking and secretion in macrophages.

Prohormone convertases 1/3 and 2 together orchestrate the site-specific cleavages of progastrin to release gastrin-34 and gastrin-17

Cellular synthesis of peptide hormones requires PCs (prohormone convertases) for the endoproteolysis of prohormones. Antral G-cells synthesize the most gastrin and express PC1/3, 2 and 5/6 in the rat and human. But the cleavage sites in progastrin for each PC have not been determined. Therefore, in the present study, we measured the concentrations of progastrin, processing intermediates and alpha-amidated gastrins in antral extracts from PC1/3-null mice and compared the results with those in mice lacking PC2 and wild-type controls. The expression of PCs was examined by immunocytochemistry and in situ hybridization of mouse G-cells. Finally, the in vitro effect of recombinant PC5/6 on progastrin and progastrin fragments containing the relevant dibasic cleavage sites was also examined. The results showed that mouse G-cells express PC1/3, 2 and 5/6. The concentration of progastrin in PC1/3-null mice was elevated 3-fold. Chromatography showed that cleavage of the Arg(36)Arg(37) and Arg(73)Arg(74) sites were grossly decreased. Accordingly, the concentrations of progastrin products were markedly reduced, alpha-amidated gastrins (-34 and -17) being 25% of normal. Lack of PC1/3 was without effect on the third dibasic site (Lys(53)Lys(54)), which is the only processing site for PC2. Recombinant PC5/6 did not cleave any of the dibasic processing sites in progastrin and fragments containing the relevant dibasic processing sites. The complementary cleavages of PC1/3 and 2, however, suffice to explain most of the normal endoproteolysis of progastrin. Moreover, the results show that PCs react differently to the same dibasic sequences, suggesting that additional structural factors modulate the substrate specificity.

Proopiomelanocortin processing in the anterior pituitary of the ovine fetus after lesion of the hypothalamic paraventricular nucleus

The hypothalamic-pituitary-adrenocortical axis plays an essential role in the maturation of fetal organs and, in sheep, birth. Lesioning the paraventricular nucleus (PVN) in fetal sheep prevents adrenocortical maturation and parturition without altering plasma immunoreactive ACTH concentrations. The purpose of this study was to determine the effect of PVN lesion on anterior pituitary processing of proopiomelanocortin (POMC) to ACTH, plasma concentrations of ACTH and ACTH precursors (POMC; 22-kDa proACTH), and expression of subtilisin-like prohormone convertase 3 (SPC3) in corticotropes in fetal sheep. PVN lesion did not affect anterior pituitary POMC and 22-kDa proACTH levels, whereas ACTH was significantly affected. The ACTH precursor (POMC plus 22-kDa proACTH) to ACTH ratio in the anterior pituitary was significantly increased after PVN lesion. Post-PVN lesion, fetal plasma ACTH(1-39), was below the limit of detection, whereas ACTH precursors (POMC plus 22-kDa proACTH) were not affected. In the inferior region of the anterior pituitary, 40-50% of corticotropes had detectable SPC3 hybridization signal, and PVN lesion did not change the extent of colocalization of POMC and SPC3, or SPC3 mRNA levels within corticotropes. Neither the percent of corticotropes in the superior region containing SPC3 hybridization (7-12%) or hybridization signal strength was altered in response to PVN lesion. In conclusion, the fetal PVN is necessary for sustaining adequate anterior pituitary processing of POMC to ACTH and ACTH release needed for maturing the adrenal cortex in the sheep fetus.

Calcium homeostasis in a clonal pituitary cell line of mouse corticotropes

Calcium homeostasis was studied following a depolarization-induced transient increase in [Ca2+]i in single cells of the clonal pituitary cell line of corticotropes, AtT-20 cells. The KCl-induced increase in [Ca2+]i was blocked in (i) extracellular calcium-deficient solutions, (ii) external cobalt (2.0 mM), (iii) cadmium (200 µM), and (iv) nifedipine (2.0 µM). The mean increase in [Ca2+]i in single cells in the presence of an uncoupler of mitochondrial function [carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, FCCP, 1 µM] was 54 ± 13 nM (n = 9). The increase in [Ca2+]i produced by FCCP was greater either during or following a KCl-induced [Ca2+]i load. However, FCCP did not significantly alter the clearance of calcium during a KCl-induced rise in [Ca2+]i. Fifty percent of the cells responded to caffeine (10 mM) with an increase in [Ca2+]i (191 ± 24 nM; n = 21) above resting levels; this effect was blocked by ryanodine (10 µM). Thapsigargin (2 µM) and 2,5 di(-t-butyl)-1,4 hydroquinone (BuBHQ, 10 µM) produced increases in [Ca2+]i (47 ± 11 nM, n = 6 and 22 ± 4 nM, n = 8, respectively) that increased cell excitability. These results support a role for mitochondria and sarco-endoplasmic reticulum calcium stores in cytosolic [Ca2+]i regulation; however, none of these organelles are primarily responsible for the return of [Ca2+]i to resting levels following this KCl-induced [Ca2+]i load.Key words: calcium homeostasis, intracellular calcium stores, anterior pituitary cells, mitochondria.

Carboxypeptidase Z is present in the regulated secretory pathway and extracellular matrix in cultured cells and in human tissues

Carboxypeptidase Z (CPZ) is a newly reported member of the metallocarboxypeptidase gene family, but unlike other members of this family, CPZ contains an N-terminal domain that has amino acid sequence similarity to Wnt-binding proteins. In order to gain insights as to the potential function of CPZ, the intracellular localization of this protein was determined in cell culture and in human tissues. When expressed in the AtT-20 mouse pituitary cell line, CPZ protein is routed to the regulated secretory pathway and secreted upon stimulation. A fraction of the secreted CPZ remains associated with the extracellular matrix. Endogenous CPZ in the PC12 rat pheochromocytoma cell line is also associated with the extracellular matrix. In human placenta, CPZ is present within invasive trophoblasts and in the surrounding extracellular space, indicating an association with extracellular matrix. CPZ is also present in amnion cells, but is not readily apparent in the extracellular matrix of this cell type. A human adenocarcinoma of the colon shows expression of CPZ in the extracellular matrix adjacent to malignant cells. Taken together, CPZ appears to be a component of the extracellular matrix in some cell types, where it may function in the binding of Wnt.

Characterization of the enzymatic properties of the first and second domains of metallocarboxypeptidase D

Carboxypeptidase D (CPD) contains three domains with homology to other metallocarboxypeptidases. To further characterize the various domains, we constructed a series of point mutants with a critical active site Glu of duck CPD converted to Gln. The proteins were expressed in the baculovirus system, purified to homogeneity, and characterized. Point mutations within both the first and second domains eliminated enzyme activity, indicating that the third domain is inactive toward dansyl-Phe-Ala-Arg. CPD removed only the C-terminal Lys or Arg from peptides, with the first domain more efficient toward Arg and the second domain more efficient toward Lys. Peptides containing Pro in the penultimate position were poorly cleaved by either domain. Cleavage of a peptide with Ala in the penultimate position was most efficient, with the relative order Ala >/= Met > Ser, Phe > Tyr > Trp > Thr >/= Gln, Asp, Leu, Gly >> Pro for CPD with both domains active. There were only minor differences between the first and the second domains regarding the influence of the penultimate amino acid. The first domain was optimally active at pH 6.3-7.5, whereas the second domain was optimally active at pH 5. 0-6.5. Thus, the first and second carboxypeptidase domains have complementary enzyme activities. Furthermore, the finding that CPD with both domains active shows a broad activity to a wide range of substrates is consistent with a role for this enzyme in the processing of many proteins that transit the secretory pathway.

The cell biology of the prohormone convertases PC1 and PC2

Mature peptide hormones and neuropeptides are typically synthesized from much larger precursors and require several posttranslational processing steps--including proteolytic cleavage--for the formation of the bioactive species. The subtilisin-related proteolytic enzymes that accomplish neuroendocrine-specific cleavages are known as prohormone convertases 1 and 2 (PC1 and PC2). The cell biology of these proteases within the regulated secretory pathway of neuroendocrine cells is complex, and they are themselves initially synthesized as inactive precursor molecules. ProPC1 propeptide cleavage occurs rapidly in the endoplasmic reticulum, yet its major site of action on prohormones takes place later in the secretory pathway. PC1 undergoes an interesting carboxyl terminal processing event whose function appears to be to activate the enzyme. ProPC2, on the other hand, exhibits comparatively long initial folding times and exits the endoplasmic reticulum without propeptide cleavage, in association with the neuroendocrine-specific protein 7B2. Once the proPC2/7B2 complex arrives at the trans-Golgi network, 7B2 is internally cleaved into two domains, the 21-kDa fragment and a carboxy-terminal 31 residue peptide. PC2 propeptide removal occurs in the maturing secretory granule, most likely through autocatalysis, and 7B2 association does not appear to be directly required for this cleavage event. However, if proPC2 has not encountered 7B2 intracellularly, it cannot generate a catalytically active mature species. The molecular mechanism behind the intriguing intracellular association of 7B2 and proPC2 is still unknown, but may involve conformational rearrangement or stabilization of a proPC2 conformer mediated by a 36-residue internal segment of 21-kDa 7B2.

Activation and routing of membrane-tethered prohormone convertases 1 and 2

Many peptide hormones and neuropeptides are processed by members of the subtilisin-like family of prohormone convertases (PCs), which are either soluble or integral membrane proteins. PC1 and PC2 are soluble PCs that are primarily localized to large dense core vesicles in neurons and endocrine cells. We examined whether PC1 and PC2 were active when expressed as membrane-tethered proteins, and how tethering to membranes alters the biosynthesis, enzymatic activity, and intracellular routing of these PCs. PC1 and PC2 chimeras were constructed using the transmembrane domain and cytoplasmic domain of the amidating enzyme, peptidylglycine alpha-amidating monooxygenase (PAM). The membrane-tethered PCs were rerouted from large dense core vesicles to the Golgi region. In addition, the chimeras were transiently expressed at the cell surface and rapidly internalized to the Golgi region in a fashion similar to PAM. Membrane-tethered PC1 and PC2 exhibited changes in pro-domain maturation rates, N-glycosylation, and in the pH and calcium optima required for maximal enzymatic activity against a fluorogenic substrate. In addition, the PC chimeras efficiently cleaved endogenous pro-opiomelanocortin to the correct bioactive peptides. The PAM transmembrane domain/cytoplasmic domain also prevented stimulated secretion of pro-opiomelanocortin products in AtT-20 cells.

Localization of metallocarboxypeptidase D in AtT-20 cells. Potential role in prohormone processing

Carboxypeptidase D (CPD) is a recently discovered metallocarboxypeptidase that is predominantly located in the trans-Golgi network (TGN), and also cycles between the cell surface and the TGN. In the present study, the intracellular distribution of CPD was examined in AtT-20 cells, a mouse anterior pituitary-derived corticotroph. CPD-containing compartments were isolated using antibodies to the CPD cytosolic tail. The immunopurified vesicles contained TGN proteins (TGN38, furin, syntaxin 6) but not lysosomal or plasma membrane proteins. The CPD-containing vesicles also contained neuropeptide-processing enzymes and adrenocorticotropic hormone, a product of proopiomelanocortin proteolysis. Electron microscopic analysis revealed that CPD is present within the TGN and immature secretory granules but is virtually absent from mature granules, suggesting that CPD is actively removed from the regulated pathway during the process of granule maturation. A second major finding of the present study is that a soluble truncated form of CPD is secreted mainly via the constitutive pathway in AtT-20 cells, indicating that the lumenal domain does not contain signals for the sorting of CPD to mature secretory granules. Taken together, these data are consistent with the proposal that CPD participates in the processing of proteins within the TGN and immature secretory vesicles.

The pro-protein convertase PC1 is induced in the transected sciatic nerve and is present in cultured Schwann cells: comparison with PC5, furin and PC7, implication in pro-BDNF processing

Injury of peripheral nerves induces expression of several pro-protein convertases (PCs) involved in processing of precursor proteins into their diverse active end-products. In this study, the focus was on convertase PC1 which, although undetectable in control nerves, is strongly induced in injured nerves. High concentrations of PC1 mRNA of 9.0, 5.5, 3.0, 2.5 and 1.6 kb were observed on day 4 post-lesion in proximal and distal segments. By in situ hybridization PC1 mRNA was detected in most of endoneurial cells, which were further identified by immunocytochemistry as myelin 2', 3'-cyclic nucleotide 3'-phosphodiesterase containing Schwann cells. PC1 mRNA and protein were also present in cultured Schwann cells also containing convertases PC5, furin and PC7 as well as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). Mostly unprocessed pro-NGF of 35 kDa and pro-BDNF of 35 kDa were found on Western blotting of Schwann cells. Expression of exogenous neurotrophins by infection with vaccinia virus vector showed that mouse pro-NGF and rat pro-BDNF are cleaved intracellularly on smaller forms of 13.5 kDa NGF and 14 kDa BDNF. Infection experiments demonstrated that Schwann cells contain active processing enzymes. In conclusion, this work provides in vivo evidence of the presence of several PCs in the injured rat sciatic nerve and ex vivo in cultured Schwann cells.

Immunocytochemical localization of the prohormone convertases PC1 and PC2 in rat prolactin cells

The prohormone convertases PC1 and PC2 are subtilisin-related endopeptidases that process prohormone and neuropeptide precursors. Using different ultrastructural immunocytochemical approaches, we have investigated their intracellular distribution in a neuroendocrine cell type that has not been examined thus far, the rat anterior pituitary lactotrope. These cells secrete mainly prolactin and also express the neuroendocrine-specific protein secretogranin II, which is considered a peptide precursor. Our study provides evidence for the expression of PC1 and PC2 in rat lactotropes and provides new information on their subcellular localization. Apart from their presence in the secretory granules, PC1 and PC2 displayed different major localization along the secretory pathway. PC1 immunoreactivity was concentrated in the Golgi apparatus, whereas PC2 immunoreactivity was prominent in the rough endoplasmic reticulum (RER). These observations provide morphological support for previous biochemical analysis of proPC1 and proPC2 post-translational processing, which has demonstrated that PC1 exits very rapidly from the RER, whereas PC2 is retained much longer in this compartment. (J Histochem Cytochem 46:101-108, 1998)

Proteolytic processing of pro-opiomelanocortin occurs in acidifying secretory granules of AtT-20 cells

Using antibodies specific for pro-opiomelanocortin (POMC), amidated joining peptide (JP), and the prohormone convertase PC1, we showed immunocytochemically that PC1 in a corticotrophic tumor cell line, AtT-20, was co-localized either with POMC or with amidated JP in secretory granules, and also confirmed that POMC was cleaved mainly in secretory granules. Analysis using DAMP (3- [2,4-dinitroanilino]-3'-amino-N-methyldipropylamine) as the pH probe suggested a correlation between POMC processing and acidic pH in the secretory granules. Bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPase, completely inhibited POMC processing and caused constitutive secretion of the unprocessed precursor. By contrast, chloroquine, a weak base that is known to neutralize acidic organelles, was unable to inhibit POMC processing. Electron microscopic analysis revealed that, in AtT-20 cells treated with bafilomycin A1, the trans-Golgi cisternae were dilated and few secretory granules were present in the cytoplasm. These observations suggest that acidic pH provides a favorable environment for proteolytic processing of POMC by PC1 but is not required, and that integrity of the trans-Golgi network and sorting of POMC into secretory granules are important for POMC processing.

Induced and spontaneous mutations at Ser202 of carboxypeptidase E. Effect on enzyme expression, activity, and intracellular routing

Carboxypeptidase E (CPE) is involved in peptide processing in the brain and various neuroendocrine tissues. In mice homozygous for the Cpefat mutation, the virtual absence of CPE activity in islets of Langerhans and pituitary was associated with a missense mutation effecting a Ser202 to Pro shift (Naggert, J. K., Fricker, L. D., Varlamov, O., Nishina, P. M., Rouille, Y., Steiner, D. F., Carroll, R. J., Paigen, B. J., and Leiter, E. H. (1995) Nat. Genet. 10, 135-142). To examine the importance of Ser202 in CPE function, several mutations in this position were generated (Pro202, Ala202, Gly202, and Phe202). When the mutant proteins were expressed in a Baculovirus system, both Phe202 and Pro202CPE were enzymatically inactive, were unable to bind to a substrate affinity column, and were not secreted from Sf9 cells. In contrast, Ala202CPE or Gly202CPE exhibited enzymatic properties similar to those of wild-type CPE and were secreted from Sf9 cells. When expressed in AtT-20 cells, a mouse pituitary-derived cell line, CPE with Pro202 and Phe202 were not secreted. Pulse-chase analysis with [35S]Met indicated that Pro202CPE was degraded in AtT-20 cells within several hours. This degradative process was blocked by incubation at 15 degrees C but not by brefeldin A or by lysosomotrophic drugs. Pulse-chase analysis using dispersed pituitary cells from C57BLKS/Lt-Cpefat/Cpefat mutant mice shows similar results; Pro202-CPE produced in these cells was not secreted but rather was degraded within 5 h. Immunofluorescence analysis of epitope-tagged CPE revealed Ser202CPE to be present primarily in secretory vesicles, whereas Pro202CPE was localized to the endoplasmic reticulum and not the secretory vesicle-like structures. These results support the previous finding that Cpefat/Cpefat mice are defective in CPE activity because of the point mutation producing the Ser202 to Pro substitution. Furthermore, these results are consistent with a model that Ser202 is important for the intracellular folding of CPE.

Structural elements that direct specific processing of different mammalian subtilisin-like prohormone convertases

PC1 and PC2 are two important subtilisin-like prohormone convertases (PC) that undergo differential endoproteolytic processing steps and sequentially mediate proopiomelanocortin (POMC) processing. To investigate the structural elements directing the processing of different PCs, we constructed a series of mutant and chimeric PC proteins and expressed them in cell lines with different patterns of expression of endogenous PCs: AtT-20, hEK293, and hLoVo cells. The COOH-terminally truncated PC1 underwent efficient proregion cleavage and rapid secretion in all three cell lines, while proregion cleavage and secretion were completely blocked in an active-site mutant of PC1. The truncated PC1 produced dramatic changes in POMC processing in AtT-20 cells. PC2 with the potential oxyanion hole Asp residue changed to Asn was processed and altered several aspects of POMC processing in a manner similar to that of wild-type PC2. PC1 protein with its proregion substituted with that of furin was cleaved after its proregion, producing active PC1 enzyme. A similar furin/PC2 fusion protein underwent proregion cleavage at low efficiency. By contrast, when the proregions of PC1 and PC2 were substituted with one another, both fusion proteins failed to cleave the foreign prosequences, were unable to undergo oligosaccharide maturation, and remained in the ER. Although inactive PC mutants could theoretically function as dominant negatives, none interfered with the processing of endogenous active PCs or with POMC processing. We conclude that the COOH-terminal of PC1 plays an important role in the routing or storage of PC1, the proregions of these PC proteins are replaceable in a molecule-specific manner, removal of proregion is essential for routing and for endoproteolytic activity, and the role of the potential oxyanion hole in PC2 is still unclear.

Prohormone convertases PC2 and PC3 in rat neutrophils and macrophages. Parallel changes with proenkephalin-derived peptides induced by LPS in vivo

Prohormone- or proneuropeptide-converting enzymes PC2 and PC3 have been observed exclusively in nervous and endocrine tissues. In this work the presence of these enzymes in cells of the immune system was demonstrated. PC2 was detected in peripheral and liver-infiltrating polymorphonuclear leukocytes (PMN) but not in alveolar macrophages (AM) or spleen mononuclear cells (SMC). PC2 proteins corresponded to 75, 71 and 56 kDa forms. PC3 appeared in AM and SMC but not in PMN, and a 66 kDa protein was the only PC3 form detected. Proenkephalin-derived peptides (PENKp) were observed in PMN and AM, showing peptides of 35, 28, 21, 18 and 14 kDa in the former cells and a doublet of 35 and 32 kDa in the latter. PC2 proteins and PENKp decreased in liver PMN and peripheral PMN 90 min after intravenous (i.v.) infusion of LPS, suggesting an increased release. However, in vitro assays showed that the chemotactic peptide FMLP but not LPS increased the basal secretion of PC2 proteins and PENKp in PMN. These results indicate that PC2 proteins are released from PMN, together with PENKp, and suggest that LPS in vivo may act through an indirect mechanism. Low levels of PC3 and PENK were detected in the AM of rats treated for 90 min with SAL or LPS. However, a significant increase of PC3 and PENKp appeared 30 h after LPS infusion. These results show for the first time that PC2 and PC3 are differentially expressed in PMN and AM, respectively, which were paralleled by the presence of different post-translational products of PENK. In addition, the in vivo effect of LPS on PC2, PC3 and PENKp levels in PMN and AM resembles the effect of LPS on prohormone levels in endocrine tissues, suggesting that similar mechanisms may control the turnover of PENK in endocrine and in these immune cells.