Identification of a cDNA encoding a second putative prohormone convertase related to PC2 in AtT20 cells and islets of Langerhans

Discovery of the Proprotein Convertases and their Inhibitors

The members of the convertase family play a central role in the processing of various protein precursors ranging from hormones and growth factors to viral envelope proteins and bacterial toxins. The proteolysis of these precursors that occurs at basic residues is mediated by the proprotein convertases (PCs), namely: PC1, PC2, Furin, PACE4, PC4, PC5 and PC7. The proteolysis at non-basic residues is performed by subtilisin/kexin-like isozyme-1 (S1P/SKI-1) and the newly identified neural apoptosis-regulated convertase-1 (NARC-1/PCSK9). These proteases have key roles in many physiological processes and various pathologies including cancer, obesity, diabetes, neurodegenerative diseases and autosomal dominant hypercholesterolermia. Here we summarize the discovery of the proprotein convertases and their inhibitors, discuss their properties, roles, resemblance and differences

Proprotein convertases: "master switches" in the regulation of tumor growth and progression

Proprotein convertases (PCs) are a group of Ca2+-dependent serine proteases that have homology to the endoproteases subtilisin (bacteria) and kexin (yeast). This group is comprised of less than a dozen members, known as furin/PACE, PC1/PC3, PC2, PC4, PACE4, PC5/PC6, PC7/PC8/LPC, SKI/S1P, and NARC-1/PCSK9. Four PCs (Furin, PACE4, PC5, and PC7) have been localized to several different tissues and epithelial or nervous system tumors. PCs activate their cognate substrates by limited proteolysis at the consensus sequence RXR/KR downward arrow. Many PC substrates are well known cancer-associated proteins such as growth factors, growth factor receptors, integrins, and matrix metalloproteases (MMPs). For example, IGF-1 and its receptor, TGF-beta, VEGF-C, and MT-MMPs have direct roles in tumor progression and metastasis. Furin, a well-studied member of the PC family, has been associated with enhanced invasion and proliferation in head and neck, breast, and lung cancer. Conversely, inhibition of PC activity by PDX or several PC pro-segments, resulted in reduced processing of these key cancer-related substrates in human squamous cell carcinomas (SCC), colon adenocarcinoma, and astrocytoma cell lines. In parallel to these changes in cell proliferation and invasiveness as well as metastatic ability were markedly impaired. By controlling the maturation/activation of key cancer-associated proteins, PCs act as "master switches" at different levels during tumor development and progression. The manifold effects of PCs, influencing tumor cell proliferation, motility, adhesiveness, and invasiveness, should be exploited by further developing competitive/inhibitory therapeutic strategies that would be able to neutralize simultaneously the most salient cancer cell properties.

Neuropeptide-processing enzymes: applications for drug discovery

Neuropeptides serve many important roles in communication between cells and are an attractive target for drug discovery. Neuropeptides are produced from precursor proteins by selective cleavages at specific sites, and are then broken down by further cleavages. In general, the biosynthetic cleavages occur within the cell and the degradative cleavages occur postsecretion, although there are exceptions where intracellular processing leads to inactivation, or extracellular processing leads to activation of a particular neuropeptide. A relatively small number of peptidases are responsible for processing the majority of neuropeptides, both inside and outside of the cell. Thus, inhibition of any one enzyme will lead to a broad effect on several different neuropeptides and this makes it unlikely that such inhibitors would be useful therapeutics. However, studies with mutant animals that lack functional peptide-processing enzymes have facilitated the discovery of novel neuropeptides, many of which may be appropriate targets for therapeutics.

Expression of proprotein convertase 2 mRNA in the ovarian follicles of the medaka, Oryzias latipes

Proprotein convertases (PCs) are enzymes responsible for processing the precursors of many bioactive peptides in vertebrates and invertebrates. In the present study, a cDNA for proprotein convertase 2 (PC2) was cloned for the first time from a fish. The clone, which was isolated from the ovary of the medaka, Oryzias latipes, by a combination of RT-PCR cloning and 5'- and 3'-rapid amplification of cDNA ends, codes for a protein of 641 amino acid residues highly homologous to other vertebrate PC2. The medaka preproPC2 consists of a signal sequence, a propeptide with sites for autocatalytic activation, a Kex2-like catalytic domain, and a P-domain. The catalytic triad residues (Asp-169, His-210, and Ser-386) were all conserved. Northern blot analysis revealed that PC2 was expressed in the brain, ovary, and kidney of the fish. The size of PC2 mRNA expressed in the ovary was 2.3 kb, whereas those of the brain and kidney were 2.8 kb. This size difference was attributed to the lack of an approximately 300-bp nucleotide sequence just before the poly(A)+ tail of the ovarian PC2 mRNA. Ovarian expression of the PC2 gene was found in the medaka but not in the mouse, and therefore further analysis was conducted for the fish ovary. The greatest expression of PC2 mRNA in the oocytes of small growing follicles in the mature medaka was demonstrated by Northern blotting, RT-PCR and in situ hybridization analysis. These results suggest that PC2 may play a role in the processing of proproteins and/or pro-hormones expressed in the growing oocytes.

Engineering a new β-cell: a critical venture requiring special attention to constantly changing physiological needs

As both type 1 and type 2 diabetes are characterized by reduced islet beta-cell mass and impaired insulin secretion, engineering new beta-cells for replacement therapy is appealing. For this to be successful, the intricate peptide processing and secretory machinery of the beta-cell must be duplicated. Further, the engineered beta-cell must be capable of modulating its function in response to physiological changes such as puberty, pregnancy and aging, and to more short-term challenges such as infection, exercise and weight fluctuation. The new cell should have a low risk for recurrence of the primary disease and ensuring its survival should not be worse than diabetes itself.

Altered proglucagon processing in an alpha-cell line derived from prohormone convertase 2 null mouse islets

The endoproteolytic processing of proproteins in the secretory pathway depends on the expression of selected members of a family of subtilisin-like endoproteases known as the prohormone convertases (PCs). The main PC family members expressed in mammalian neuroendocrine cells are PC2 and PC1/3. The differential processing of proglucagon in pancreatic alpha-cells and intestinal L cells leads to production of distinct hormonal products with opposing physiological effects from the same precursor. Here we describe the establishment and characterization of a novel alpha-cell line (alphaTC-DeltaPC2) derived from PC2 homozygous null animals. The alphaTC-DeltaPC2 cells are shown to be similar to the well characterized alphaTC1-6 cell line in both morphology and overall gene expression. However, the absence of PC2 activity in alphaTC-DeltaPC2 leads to a complete block in the production of mature glucagon. Surprisingly, alphaTC-DeltaPC2 cells are able to efficiently cleave the interdomain site in proglucagon (KR 70-71). Further analysis reveals that alphaTC-DeltaPC2 cells, unlike alphaTC1-6 cells, express low levels of PC1/3 that lead to the generation of glicentin as well as low amounts of oxyntomodulin, GLP-1, truncated GLP-1, and N-terminally extended GLP-2. We conclude that alphaTC-DeltaPC2 cells provide additional evidence for PC2 as the major convertase in alpha-cells leading to mature glucagon production and provide a robust model for further analysis of the mechanisms of proprotein processing by the prohormone convertases.

Furin and prohormone convertase 1/3 are major convertases in the processing of mouse pro-growth hormone-releasing hormone

We investigated the proteolytic processing of mouse pro-GHRH [84 amino acids (aa)] by furin, PC1/3, PC2, and PC5/6A. We created six point mutations in the N- and C-terminal cleavage sites, RXXR decreased and RXRXXR decreased, respectively. The following results were obtained after transient transfection/cotransfection and metabolic pulse-chase labeling studies in several neuroendocrine cells. 1) Furin was the most efficient convertase in cleaving the N-terminal RXXR/RXRR site to generate intermediate I, 12-84aa, whereas PC1/3 was the most potent in processing the C-terminal RXRXXR site to yield mature GHRH, 12-53aa. 2) Both PC1/3 and PC5/6A also processed the N-terminal site but less efficiently than furin. 3) PC2 was much weaker in cleaving the C-terminal site relative to PC1/3 to generate mature GHRH. 4) The Q10R mutant was significantly more susceptible to furin cleavage at the N-terminal site than the wild-type pro-GHRH. And 5) the N- and C-terminal P1 Arg residues, R11 and R54, respectively, were essential for mature GHRH production. We also showed localization of the GHRH immunoreactive peptides in Golgi and secretory granules in neuroendocrine cells by an immunofluorescence assay. We conclude that the efficient production of mature GHRH from pro-GHRH is a stepwise process mediated predominantly by furin at the N-terminal cleavage site followed by PC1/3 at the C terminus.

Production of recombinant human relaxin 3 in AtT20 cells

Relaxin 3 has been reported recently as a member of the insulin/IGF/relaxin family. To clarify the function of relaxin 3, we prepared recombinant human relaxin 3 using a mouse adrenocorticotrophic hormone (ACTH)-secreting cell line, AtT20. To detect a mature form of recombinant human relaxin 3, a competitive enzyme immunoassay (EIA) was developed using a monoclonal antibody (mAb; HK4-144-10), which was raised for the N-terminal peptide of human relaxin 3 A-chain. We detected immunoreactive (ir-) relaxin 3 in the culture supernatant of AtT20 cells stably transfected with human relaxin 3 cDNA. After treatment with 5 microM forskolin for 3 days, the concentration of the ir-relaxin 3 in the culture supernatant reached 12 nM. Ir-relaxin 3 was purified from the culture supernatant by a combination of various chromatographies. By analyses of N-terminal amino acid sequence and electrospray ionization mass spectrometry (ESI-MS), we confirmed that the purified material was a mature form of human relaxin 3. The recombinant human relaxin 3 thereby obtained increased intracellular cAMP production in THP-1 cells. Our results demonstrate that the expression of relaxin 3 cDNA in AtT20 cells is a useful tool to produce a bioactive and mature form of relaxin 3.

Mutational analysis of predicted interactions between the catalytic and P domains of prohormone convertase 3 (PC3/PC1)

The subtilisin-like prohormone convertases (PCs) contain an essential downstream domain (P domain), which has been predicted to have a beta-barrel structure that interacts with and stabilizes the catalytic domain (CAT). To assess possible sites of hydrophobic interaction, a series of mutant PC3-enhanced GFP constructs were prepared in which selected nonpolar residues on the surface of CAT were substituted by the corresponding polar residues in subtilisin Carlsberg. To investigate the folding potential of the isolated P domain, signal peptide-P domain-enhanced GFP constructs with mutated andor truncated P domains were also made. All mutants were expressed in betaTC3 cells, and their subcellular localization and secretion were determined. The mutants fell into three main groups: (i) Golgisecreted, (ii) ERnonsecreted, and (iii) apoptosis inducing. The destabilizing CAT mutations indicate that the side chains of V292, T328, L351, Q408, H409, V412, and F441 and nonpolar fragments of the side chains of R405 and W413 form a hydrophobic patch on CAT that interacts with the P domain. We also have found that the P domain can fold independently, as indicated by its secretion. Interestingly, T594, which is near the P domain C terminus, was not essential for P domain secretion but is crucial for the stability of intact PC3. T594V produced a stable enzyme, but T594D did not, which suggests that T594 participates in important hydrophobic interactions within PC3. These findings support our conclusion that the catalytic and P domains contribute to the folding and thermodynamic stability of the convertases through reciprocal hydrophobic interactions.

Gene therapy of streptozotocin-induced diabetes by intramuscular delivery of modified preproinsulin genes

BACKGROUND

Despite improvements in insulin preparation and delivery, physiological normoglycemia is not easily achieved in diabetics. Therefore, there has been considerable interest in developing gene therapy approaches to supply insulin. We studied a nonviral muscle-based method of gene therapy and demonstrated that it could prevent hyperglycemia in murine streptozotocin (STZ)-induced diabetes.

METHODS

A plasmid encoding mouse furin-cleavable preproinsulin II cDNA (FI), or its B10-analogue (B10FI), and a plasmid encoding furin were coinjected into muscle of CD-1 mice, who were treated a day later with STZ to induce diabetes. Electroporation was applied to increase gene transfer. Blood glucose was measured in fed and fasting mice, and fasting plasma insulin was measured by radioimmunoassay. The form of insulin produced and the presence of C-peptide were analyzed by gel filtration chromatography.

RESULTS

A B10FI plasmid codelivered with a furin plasmid reduced fed and fasting blood glucose levels in STZ-treated diabetic mice. The (pro)insulin levels in plasma were increased by up to 70-fold versus blank plasmid-treated diabetic mice. The administration of FI with furin was less effective. (Pro)insulin levels were greatly increased by using two plasmids carrying different promoter elements (CMV and SV40). Insulin was identified in muscle cells by immunohistochemistry. In plasma, 40-70% of the (pro)insulin was processed to the mature form and free C-peptide was identified. Insulin gene-treated mice had improved growth rates and appeared healthier. A single injection of B10FI with SV40Furin DNA increased plasma (pro)insulin for at least 8 weeks and reduced fed blood glucose levels for 5 weeks and fasting levels for 8 weeks.

CONCLUSIONS

This is the first report that electroporation-enhanced intramuscular gene therapy with B10FI can prevent hyperglycemia in murine STZ-induced diabetes. Gene therapy using various routes and methods of furin-cleavable insulin gene delivery has been previously explored but, in muscle, results comparable to ours have not been reported.

Biological processing of the cocaine and amphetamine-regulated transcript precursors by prohormone convertases, PC2 and PC1/3

Cocaine and amphetamine-regulated transcript (CART), a neuroendocrine peptide influencing reward, feeding/appetite, and stress responses is derived from two peptide precursors of 129 and 116 amino acid (aa) residues that arise via alternative splicing from a single Cart gene in rats and mice. The signal peptide constitutes the first 27 aa resulting in pro-CART molecules of either 102 or 89 aa. In the present study, we have shown that pro-CART is a substrate for the neuroendocrine subtilisin/kexin-like prohormone convertases, PC2 (SPC2) and PC1/3 (SPC3). By using different neuroendocrine cell lines, with or without endogenous expression of either PC2 or PC1/3 or both enzymes, we have demonstrated through transient transfection studies that long pro-CART gives rise to an intermediate peptide, residues 33-102, and the two major bioactive CART forms, residues 55-102 (I) and 62-102 (II), respectively. Likewise, short pro-CART also generates three peptides, an intermediate, residues 10-89, and the two identical bioactive CART forms. We have confirmed the identities of the bioactive and intermediate CART molecules by microsequencing and/or high performance liquid chromatography and mass spectrometry. We have shown that PC2 is more efficient in generating bioactive CART I compared with PC1/3, whereas the production of the smaller bioactive CART II is exclusively carried out by PC2. PC1/3 is predominantly responsible for generating the intermediate CART fragments, 33-102 and 10-89, from long and short pro-CART, respectively. To compare in vitro and in vivo processing of pro-CART, we have examined its processing in PC2, 7B2, and PC1/3 knock-out mouse hypothalamic extracts and demonstrated that, as in vitro, PC2 is more potent than PC1/3 in generating bioactive CART I whereas bioactive CART II is solely generated by PC2. Also, in vivo, we have shown that PC1/3 is predominantly active in liberating the two intermediate CART fragments, 33-102 and 10-89. These findings confirm the key roles of PC2 and PC1/3 acting together or separately to carry out CART processing in selected sites in vivo.

Prohormone convertase 2 enzymatic activity and its regulation in neuro-endocrine cells and tissues

We used the fluorometric substrate, pGlu-Arg-Thr-Lys-Arg-MCA and the C-terminal peptide of human 7B2(155-185), a specific inhibitor of prohormone convertase 2 (PC2), to specifically measure the enzymatic activity of the prohormone convertases, PC2. Using lysates from the pancreatic alpha cell line, alphaTC1-6 cells, which contain moderate levels of PC2 enzymatic activity, we determined that the PC2 assay was linear with respect to time of incubation and protein added and had a pH optimum of 5.5 and a calcium optimum of 2.5 mM. Rat pituitary contained high levels of PC2 enzymatic activity, while the hypothalamus and other brain regions contained moderate levels. This enzyme assay was used to document that both mice null for PC2 as well as mice null for the PC2 cofactor, 7B2, had only trace levels of PC2 activity in various brain regions, while mice heterozygous for these alleles had approximately half of the PC2 activity in most brain regions. PC2 enzymatic activity and PC2 mRNA levels were somewhat discordant suggesting that PC2 mRNA levels do not always reflect PC2 enzymatic activity.

Inhibitory effect of the alpha1-antitrypsin Pittsburgh type-mutant (alpha1-PIM/R) on proinsulin processing in the regulated secretory pathway of the pancreatic beta-cell line MIN6

To elucidate its effect on proinsulin processing, we introduced the expression of a Pittsburgh type-mutant, alpha1-protease inhibitor M/R (alpha1-PIM/R) and its chimera protein with growth hormone (GH) (GHalpha1-PIM/R) into MIN6 cells. In metabolic labeling and chasing experiments with [3H]-Leu and [35S]-Met, proinsulin appeared in the medium during stimulatory secretion only from MIN6 clones expressing GHalpha1-PIM/R and, surprisingly, alpha1-PIM/R, but not from the clones of either the control or alpha1-PI. The major part of alpha1-PIM/R was secreted through the constitutive pathway and about 10% of total secreted alpha1-PIM/R in the chase periods entered the regulated pathway. On the other hand, GHalpha1-PIM/R was mainly transported to the secretory granules and about 80% of the total secreted GHalpha1-PIM/R in the chase periods was secreted during stimulatory secretion. In the first 3 h chase periods without stimulation, only alpha1-PIM/R and no GHalpha1-PIM/R appeared in the medium, thus suggesting that alpha1-PIM/R might be transported through a constitutive-like pathway for those periods. The alpha1-PI, which had no inhibitory effect on proinsulin processing, showed similar secretion pathways to those of alpha1-PIM/R. This implies that some part of alpha1-PIM/R and alpha1-PI entered the regulated pathway, not due to any specific interaction between the processing endoproteases and serine protease inhibitors, but due to some type of passive transport in a nonselective manner. The inhibitory effect of alpha1-PIM/R in the regulated secretory pathway was slightly but clearly evident when it was expressed in MIN6 beta-cells.

Molecular characterization of the cDNA and localization of the mRNA encoding the prohormone convertase PC5-A in the European green frog

The structure and distribution of PC5-A, a prohormone convertase that is thought to be involved in post-translational processing of peptide hormone and neuropeptide precursors, have not been investigated in submammalian vertebrates. In the present study, we characterized the cDNA encoding PC5-A in the European green frog Rana esculenta. The frog PC5-A cDNA encodes a 913-amino acid protein that encompasses a 28-amino acid signal peptide, the Asp/His/Ser catalytic triad found in all serine proteinases of the subtilisin family, and two potential N-linked glycosylation sites located in a C-terminal cysteine-rich domain. Reverse transcriptase polymerase chain reaction amplification showed that PC5-A mRNA is expressed in various organs including the brain, spinal cord, pituitary, lung, liver, intestine, and testis, but not in the stomach and pancreas. The distribution of PC5-A mRNA in the frog brain was studied by in situ hybridization histochemistry. Intense expression was observed in the mitral cellular layer of the olfactory bulb, the nucleus of the diagonal band of Broca, the anterior preoptic area, and the suprachiasmatic and ventral hypothalamic nuclei. The expression pattern of PC5-A mRNA in the central nervous system of anuran amphibians was consistent with the implication of this prohormone convertase in the processing of various neuropeptide precursors.

amontillado, the Drosophila homolog of the prohormone processing protease PC2, is required during embryogenesis and early larval development

Biosynthesis of most peptide hormones and neuropeptides requires proteolytic excision of the active peptide from inactive proprotein precursors, an activity carried out by subtilisin-like proprotein convertases (SPCs) in constitutive or regulated secretory pathways. The Drosophila amontillado (amon) gene encodes a homolog of the mammalian PC2 protein, an SPC that functions in the regulated secretory pathway in neuroendocrine tissues. We have identified amon mutants by isolating ethylmethanesulfonate (EMS)-induced lethal and visible mutations that define two complementation groups in the amon interval at 97D1 of the third chromosome. DNA sequencing identified the amon complementation group and the DNA sequence change for each of the nine amon alleles isolated. amon mutants display partial embryonic lethality, are defective in larval growth, and arrest during the first to second instar larval molt. Mutant larvae can be rescued by heat-shock-induced expression of the amon protein. Rescued larvae arrest at the subsequent larval molt, suggesting that amon is also required for the second to third instar larval molt. Our data indicate that the amon proprotein convertase is required during embryogenesis and larval development in Drosophila and support the hypothesis that AMON acts to proteolytically process peptide hormones that regulate hatching, larval growth, and larval ecdysis.

Solution structure of the pro-hormone convertase 1 pro-domain from Mus musculus

The solution structure of the mouse pro-hormone convertase (PC) 1 pro-domain was determined using heteronuclear NMR spectroscopy and is the first structure to be obtained for any of the domains in the convertase family. The ensemble of NMR-derived structures shows a well-ordered core consisting of a four-stranded antiparallel beta-sheet with two alpha-helices packed against one side of this sheet. Sequence homology suggests that the other eukaryotic PC pro-domains will have the same overall fold and most of the residues forming the hydrophobic core of PC1 are highly conserved within the PC family. However, some of the core residues are predicted by homology to be replaced by polar amino acid residues in other PC pro-domains and this may help to explain their marginal stability. Interestingly, the folding topology observed here is also seen for the pro-domain of bacterial subtilisin despite little or no sequence homology. Both the prokaryotic and eukaryotic structures have hydrophobic residues clustered on the solvent-accessible surface of their beta-sheets although the individual residue types differ. In the bacterial case this region is buried at the binding interface with the catalytic domain and, in the eukaryotic PC family, these surface residues are conserved. We therefore propose that the hydrophobic patch in the PC1 pro-domain is involved in the binding interface with its cognate catalytic domain in a similar manner to that seen for the bacterial system. The PC1 pro-domain structure also reveals potential mechanisms for the acid-induced dissociation of the complex between pro- and catalytic domains.

Gastrin biosynthesis in canine G cells

Gastrin requires extensive posttranslational processing for full biological activity. It is presumed that progastrin is cleaved at pairs of basic amino acids by a prohormone convertase to form a glycine-extended intermediate (G-Gly) that serves as a substrate for peptidyl-glycine alpha-amidating monooxygenase (PAM), resulting in COOH-terminally amidated gastrin. To confirm the nature of progastrin processing in a primary cell line, we performed [(35)S]methionine-labeled pulse-chase biosynthetic experiments in canine antral G cells. Radiolabeled progastrin reached a peak earlier than observed for G-Gly or amidated gastrin. G-Gly radioactivity accumulated in G cells and preceded the appearance of radioactivity in amidated gastrin. The conversion of G-Gly to amidated gastrin was enhanced by the PAM cofactor ascorbic acid. To determine whether one member of the prohormone convertase family (PC2) was responsible for progastrin cleavage, G cells were incubated with PC2 antisense oligonucleotide probes. Cells treated with antisense probes had reduced PC2 expression, an accumulation of radiolabeled progastrin, and a delay in the formation of amidated gastrin. Progastrin in antral G cells is cleaved via PC2 to form G-Gly that is converted to amidated gastrin via the actions of PAM.

Development of selectivity of alpha1-antitrypsin variant by mutagenesis in its reactive site loop against proprotein convertase. A crucial role of the P4 arginine in PACE4 inhibition

PACE4, furin and PC6 are Ca2+-dependent serine endoproteases that belong to the subtilisin-like proprotein convertase (SPC) family. Recent reports have supported the involvement of these enzymes in processing of growth/differentiation factors, viral replication, activation of bacterial toxins and tumorigenesis, indicating that these enzymes are a fascinating target for therapeutic agents. In this work, we evaluated the sensitivity and selectivity of three rat alpha1-antitrypsin variants which contained RVPR352, AVRR352 and RVRR352, respectively, within their reactive site loop using both inhibition of enzyme activity toward a fluorogenic substrate in vitro and formation of a SDS-stable protease/inhibitor complex ex vivo. The RVPR variant showed relatively broad selectivity, whereas the AVRR and RVRR variants were more selective than the RVPR variant. The AVRR variant inhibited furin and PC6 but not PACE4. This selectivity was further confirmed by complex formation and inhibition of pro-complement C3 processing. On the other hand, although the RVRR variant inhibited both PACE4 and furin effectively, it needed a 600-fold higher concentration than the RVPR variant to inhibit PC6 in vitro. These inhibitors will be useful tools in helping us to understand the roles of PACE4, furin and PC6.

The pregnancy hormone relaxin is a player in human heart failure

Human congestive heart failure is characterized by complex neurohumoral activation associated with the up-regulation of vasoconstricting and salt-retaining mediators and the compensatory rise of counter-regulatory hormones. In the present study, we provide the first evidence that relaxin (RLX), known as a pregnancy hormone, represents a potential compensatory mediator in human heart failure: plasma concentrations of RLX and myocardial expression of the two RLX genes (H1 and H2) correlate with the severity of disease and RLX responds to therapy. The failing human heart is a relevant source of circulating RLX peptides, and myocytes as well as interstitial cells produce RLX. Elevation of ventricular filling pressure up-regulates RLX expression and the hormone acts as a potent inhibitor of endothelin 1, the most powerful vasoconstrictor in heart failure. Furthermore, RLX modulates effects of angiotensin II, another crucial mediator. Our data identify RLX as a new player in human heart failure with potential diagnostic and therapeutic relevance.

Neuronal cell lines expressing PC5, but not PC1 or PC2, process Pro-CCK into glycine-extended CCK 12 and 22

Endocrine tumor cells in culture and in vitro cleavage assays have shown that PC1 and PC2 are capable of processing pro-CCK into smaller, intermediate and final, bioactive forms. Similar studies have shown that PC5 has the ability to process a number of propeptides. Here, we use GT1-7 (mouse hypothalamic) and SK-N-MC and SK-N-SH (human neuroblastoma) tumor cell lines to study the ability of PC5 to process pro-CCK. RT-PCR and Western blot analysis showed that the cells express PC5 mRNA and protein, but not PC1 or PC2. They were engineered to stably overexpress CCK and cell media was analyzed for pro-CCK expression and cleavage of the prohormone. Radioimmunoassays showed that pro-CCK was expressed, but no amidated CCK was detected. Lack of production of amidated CCK may be due to the lack of the appropriate carboxypeptidase and amidating enzymes. Production of glycine-extended CCK processing products was evaluated by treatment of media with carboxypeptidase B followed by analysis with a CCK Gly RIA. Glycine-extended forms of the peptide were found in the media. The predominant forms co-eluted with CCK 12 Gly and CCK 22 Gly on gel filtration chromatography. The results demonstrate that these cell lines which express PC5 and not PC1 or PC2 have the ability to process pro-CCK into intermediate, glycine-extended forms more closely resembling pro-CCK products in intestine than in brain.

Is the defect in pro-hormone processing in Type 2 diabetes mellitus restricted to the beta cell?

AIMS

To investigate whether the defect in pro-hormone processing in people with Type 2 diabetes mellitus is restricted to the pancreatic beta cell or whether there is evidence of a more generalized abnormality.

METHODS

Ten Indian subcontinent Asian women with diet-controlled Type 2 diabetes were compared with a control group of nine non-diabetic Asian women who were matched for body mass index. All subjects underwent a standard 75 g oral glucose tolerance test during which plasma glucose, insulin, proinsulin and 32,33 split proinsulin were measured. Subjects in both groups then underwent an intravenous corticotrophin-releasing hormone (CRH) test with 100 microg human CRH. Plasma cortisol, adrenocorticotrophic hormone (ACTH) and ACTH precursors were measured.

RESULTS

Basal levels of insulin were lower in diabetic subjects (32.5 (12.5-52) pmol/l) than in the control group (42.2 (21.4-63.0) pmol/l); normalized geometric mean (95% confidence interval), P<0.05; as were the levels at 30 min (29.5 (3.7-55.3) pmol/l) and (34.4 (10.7-58.2) pmol/l), P<0.05. The levels at 60 min were (26.7 (6.4-47.0) pmol/l) in subjects with diabetes and (13.6 (-11.3-38.5) pmol/l) in controls, P<0.05 and at 90 min these were (43.4 (19.4-67.4) pmol/l) and (19.3 (-4.6-43.3) pmol/l), P<0.05, respectively, after the 75-g oral glucose load. The acute insulin response was markedly reduced in the subjects with diabetes (1.8 (0.60-3.1) pmol insulin/ mmol glucose), compared with the control group (31.4 (8.0-54) pmol/l insulin/mmol glucose), P<0.005. Intact proinsulin was much higher in the diabetic subjects (23.9 (10.1-37) pmol/1) than in the control group (7.2 (3.9-10) pmol/1), P<0.002, as was the percentage of proinsulin-like molecules (28 (14-42) and 12 (8-17)%), respectively, P<0.01. There were no differences between basal or stimulated levels of ACTH precursors, ACTH or cortisol between the diabetic subjects and the controls.

CONCLUSIONS

The defect in insulin secretion in Indian subcontinent Asian women with Type 2 diabetes is similar to that described in other populations with an increased prevalence of Type 2 diabetes. The absence of any defect in the processing of cortisol precursors in the women with Type 2 diabetes suggests that they do not have a generalized defect of hormone processing.

A predicted alpha -helix mediates targeting of the proprotein convertase PC1 to the regulated secretory pathway

The proprotein convertase PC1 is a protease whose activity is largely confined to the dense core secretory granules of neuroendocrine cells. Efficient processing of PC1 substrates in granules requires a mechanism that will both limit the activity of the enzyme to these organelles and promote its targeting to the nascent secretory granules. In the current study, we provide evidence that targeting of PC1 to secretory granules is mediated by alpha-helical structures in its C-terminal tail and, at least in part, is dependent on interactions with specific components of the secretory granule membrane.

Expression and localization of prohormone convertase 1/3 (SPC3) in porcine ovary

Tissue distribution and cellular localization of PC1/3 mRNA in porcine tissues were examined by ribonuclease protection assay and in situ hybridization. PC1/3 mRNA was detected mainly in the corpus luteum of pregnant sow and brain. Within the ovary, PC1/3 and relaxin transcripts colocalized within large luteal cells. Levels of PC1/3 transcripts in corpora lutea increased as gestation advanced, parallel with an observed increase in relaxin transcripts. A role for PC1/3 in proprotein processing in the ovary is discussed.

Development and characterization of pituitary GH3 cell clones stably transfected with a human proinsulin cDNA

Successful beta-cell replacement therapy in insulin-dependent (type I) diabetes is hindered by the scarcity of human donor tissue and by the recurrence of autoimmune destruction of transplanted beta cells. Availability of non-beta cells, capable of releasing insulin and escaping autoimmune recognition, would therefore be important for diabetes cell therapy. We developed rat pituitary GH3 cells stably transfected with a furin-cleavable human proinsulin cDNA linked to the rat PRL promoter. Two clones (InsGH3/clone 1 and 7) were characterized in vitro with regard to basal and stimulated insulin release and proinsulin transgene expression. Mature insulin secretion was obtained in both clones, accounting for about 40% of total released (pro)insulin-like products. Immunocytochemistry of InsGH3 cells showed a cytoplasmic granular insulin staining that colocalized with secretogranin II (SGII) immunoreactivity. InsGH3 cells/clone 7 contained and released in vitro significantly more insulin than clone 1. Secretagogue-stimulated insulin secretion was observed in both InsGH3 clones either under static or dynamic conditions, indicating that insulin was targeted also to the regulated secretory pathway. Proinsulin mRNA levels were elevated in InsGH3 cells, being significantly higher than in betaTC3 cells. Moreover, proinsulin gene expression increased in response to various stimuli, thereby showing the regulation of the transfected gene at the transcriptional level. In conclusion, these data point to InsGH3 cells as a potential beta-cell surrogate even though additional engineering is required to instruct them to release insulin in response to physiologic stimulations.

Interactions between the prohormone convertase 2 promoter and the thyroid hormone receptor

The majority of prohormones are cleaved at paired basic residues to generate bioactive hormones by prohormone convertases (PCs). As PC1 and PC2, two neuroendocrine-specific PCs, appear to be the key enzymes capable of processing a variety of prohormones, alterations of PC2 and/or PC1 levels will probably have a profound effect on hormonal homeostasis. We investigated the regulation of PC2 messenger RNA (mRNA) by thyroid hormone using GH3 cells to demonstrate that T3 negatively regulated PC2 mRNA levels in a dose- and time-dependent fashion. Functional analysis of progressive 5'-deletions of the human (h) PC2 promoter luciferase constructs in GH3 cells demonstrated that the regulation probably occurs at the transcriptional level, and that putative negative thyroid hormone response elements were located within the region from -44 to + 137 bp relative to the transcriptional start site. Transient transfections in JEG-3 cells and COS-1 cells showed that the suppressive effect of T3 was equally mediated by the thyroid hormone receptor (TR) isoforms TRalpha1 and TRbeta1. Electrophoretic mobility shift assays using purified TRal and retinoid X receptor-beta protein as well as GH3 nuclear extracts showed that regions from +51 to +71 bp and from +118 to +137 bp of the hPC2 promoter bind to TRalpha1 as both a monomer and a homodimer and with TRalpha1/retinoid X receptor-beta as a heterodimer. Finally, the in vivo regulation of pituitary PC2 mRNA by thyroid status was demonstrated in rats. These results demonstrate that T3 negatively regulates PC2 expression at the transcriptional level and that functional negative thyroid hormone response elements exist in the hPC2 promoter. We postulate that the alterations of PC2 activity may mediate some of the pathophysiological consequences of hypo- or hyperthyroidism.

Immunohistochemical evidence for the involvement of protein convertases 5A and 2 in the processing of pro-neurotensin in rat brain

The neuropeptides/neurotransmitters neurotensin (NT) and neuromedin (NN) are synthesized by endoproteolytic cleavage of a common inactive precursor, pro-NT/NN. In vitro studies have suggested that the prohormone convertases PC5A and PC2 might both be involved in this process. In the present study, we used dual immunohistochemical techniques to determine whether either one or both of these two convertases were co-localized with pro-NT/NN maturation products and could therefore be involved in the physiological processing of this propeptide in rat brain. PC2-immunoreactive neurons were present in all regions immunopositive for NT. All but three regions expressing NT were also immunopositive for PC5A. Dual localization of NT with either convertase revealed that NT was extensively co-localized with both PC5A and PC2, albeit with regional differences. These results strongly suggest that PC5A and PC2 may play a key role in the maturation of pro-NT/NN in mammalian brain. The regional variability in NT/PC co-localization patterns may account for the region-specific maturation profiles previously reported for pro-NT/NN. The high degree of overlap between PC5A and PC2 in most NT-rich areas further suggests that these two convertases may act jointly to process pro-NT/NN. At the subcellular level, PC5A was largely co-localized with the mid-cisternae Golgi marker MG-160. By contrast, PC2 was almost completely excluded from MG-160-immunoreactive compartments. These results suggest that PC5A, which is particularly efficient at cleaving the two C-terminal-most dibasics of pro-NT/NN, may be acting as early as in the Golgi apparatus to release NT, whereas PC2, which is considerably more active than PC5A in cleaving the third C-terminal doublet, may be predominantly involved further distally along the secretory pathway to release NN.

Spatiotemporal expression, distribution, and processing of POMC and POMC-derived peptides in murine skin

In murine skin, after depilation-induced anagen, there was a differential spatial and temporal expression of pro-opiomelanocortin (POMC) mRNA, of the POMC-derived peptides beta-endorphin, ACTH, beta-MSH, and alpha-MSH, and of the prohormone convertases PC1 and PC2 in epidermal and hair follicle keratinocytes and in the cells of sebaceous units. Using a combination of in situ hybridization histochemistry and immunohistochemistry, we found cell-specific variations in the expression of POMC mRNA that were consistent with immunoreactivities for POMC-derived peptides. Cells that contained POMC peptide immunoreactivity (IR) also expressed POMC mRNA, and where the IR increased there was a parallel increase in mRNA. The levels of PC1-IR and PC2-IR also showed cell-specific variations and were present in the same cells that contained the POMC peptides. Based on the cleavage specificities of these convertases and on the spatial and temporal expression of the convertases and of ACTH, beta-endorphin, beta-MSH, and alpha-MSH, we can infer that the activities of PC1 and PC2 are responsible for the cell-specific differential processing of POMC in murine skin.

Interaction of Drosophila melanogaster prohormone convertase 2 and 7B2. Insect cell-specific processing and secretion

The prohormone convertases (PCs) are an evolutionarily ancient group of proteases required for the maturation of neuropeptide and peptide hormone precursors. In Drosophila melanogaster, the homolog of prohormone convertase 2, dPC2 (amontillado), is required for normal hatching behavior, and immunoblotting data indicate that flies express 80- and 75-kDa forms of this protein. Because mouse PC2 (mPC2) requires 7B2, a helper protein for productive maturation, we searched the fly data base for the 7B2 signature motif PPNPCP and identified an expressed sequence tag clone encoding the entire open reading frame for this protein. dPC2 and d7B2 cDNAs were subcloned into expression vectors for transfection into HEK-293 cells; mPC2 and rat 7B2 were used as controls. Although active mPC2 was detected in medium in the presence of either d7B2 or r7B2, dPC2 showed no proteolytic activity upon coexpression of either d7B2 or r7B2. Labeling experiments showed that dPC2 was synthesized but not secreted from HEK-293 cells. However, when dPC2 and either d7B2 or r7B2 were coexpressed in Drosophila S2 cells, abundant immunoreactive dPC2 was secreted into the medium, coincident with the appearance of PC2 activity. Expression and secretion of dPC2 enzyme activity thus appears to require insect cell-specific posttranslational processing events. The significant differences in the cell biology of the insect and mammalian enzymes, with 7B2 absolutely required for secretion of dPC2 and zymogen conversion occurring intracellularly in the case of dPC2 but not mPC2, support the idea that the Drosophila enzyme has specific requirements for maturation and secretion that can be met only in insect cells.

Proinsulin endoproteolysis confers enhanced targeting of processed insulin to the regulated secretory pathway

Recently, two different prohormone-processing enzymes, prohormone convertase 1 (PC1) and carboxypeptidase E, have been implicated in enhancing the storage of peptide hormones in endocrine secretory granules. It is important to know the extent to which such molecules may act as "sorting receptors" to allow the selective trafficking of cargo proteins from the trans-Golgi network into forming granules, versus acting as enzymes that may indirectly facilitate intraluminal storage of processed hormones within maturing granules. GH4C1 cells primarily store prolactin in granules; they lack PC1 and are defective for intragranular storage of transfected proinsulin. However, proinsulin readily enters the immature granules of these cells. Interestingly, GH4C1 clones that stably express modest levels of PC1 store more proinsulin-derived protein in granules. Even in the presence of PC1, a sizable portion of the proinsulin that enters granules goes unprocessed, and this portion largely escapes granule storage. Indeed, all of the increased granule storage can be accounted for by the modest portion converted to insulin. These results are not unique to GH4C1 cells; similar results are obtained upon PC1 expression in PC12 cells as well as in AtT20 cells (in which PC1 is expressed endogenously at higher levels). An in vitro assay of protein solubility indicates a difference in the biophysical behavior of proinsulin and insulin in the PC1 transfectants. We conclude that processing to insulin, facilitated by the catalytic activities of granule proteolytic enzymes, assists in the targeting (storage) of the hormone.

Diminished prohormone convertase 3 expression (PC1/PC3) inhibits progastrin post-translational processing

Gastrin is initially synthesized as a large precursor that requires endoproteolytic cleavage by a prohormone convertase (PC) for bioactivation. Gastric antral G-cells process progastrin at Arg(94)Arg(95) and Lys(74)Lys(75) residues generating gastrin heptadecapeptide (G17-NH(2)). Conversely, duodenal G-cells process progastrin to gastrin tetratriacontapeptide (G34-NH(2)) with little processing at Lys(74)Lys(75). Both tissues express PC1/PC3 and PC2. Previously, we demonstrated that heterologous expression of progastrin in an endocrine cell line that expresses PC1/PC3 and little PC2 (AtT-20) resulted in the formation of G34-NH(2). To confirm that PC1/PC3 was responsible for progastrin processing in AtT-20 cells and capable of processing progastrin in vivo we coexpressed either human wild-type (Lys(74)Lys(75)) or mutant (Arg(74)Arg(75), Lys(74)Arg(75), and Arg(74)Lys(75)) progastrins in AtT-20 cells with two different antisense PC1/PC3 constructs. Coexpression of either antisense construct resulted in a consistent decrease in G34-NH(2) formation. Gastrin mRNA expression and progastrin synthesis were equivalent in each cell line. Although mutation of the Lys(74)Lys(75) site within G34-NH(2) to Lys(74)Arg(75) resulted in the production of primarily G17-NH(2) rather than G34-NH(2), inhibition of PC1/PC3 did not significantly inhibit processing at the Lys(74)Arg(75) site. We conclude that PC1/PC3 is a progastrin processing enzyme, suggesting a role for PC1/PC3 progastrin processing in G-cells.

Favourable side-chain orientation of cleavage site dibasic residues of prohormone in proteolytic processing by prohormone convertase 1/3

Previous studies using selectively modified pro-ocytocin/neurophysin substrate analogues and the purified metalloprotease, pro-ocytocin/neurophysin convertase (magnolysin; EC 3.4 24.62), have shown that dibasic cleavage site processing is associated with a prohormone sequence organized in a beta-turn structure. We have used various peptide analogues of the pro-ocytocin-neurophysin processing domain, and recombinant prohormone convertase 1/3, to test the validity of this property towards this member of the family of prohormone convertases (PCs). The enzymatic cleavage analysis and kinetics showed that: (a) with methyl amide (N-Met) modification, a secondary structure beta-turn breaker, the enzyme substrate interaction was abolished; (b) cleavage was favoured when the dibasic substrate side-chains were oriented in opposite directions; (c) the amino acid present at the P'1 position is important in the enzyme-substrate interaction; (d) the flexibility of the peptide substrate is necessary for the interaction; (e) Addition of dimethylsulfoxide to the cleavage assay favoured the cleavage of the pro-ocytocin/neurophysin large substrate over that of the smaller one pGlu-Arg-Thr-Lys-Arg-methyl coumarin amide. These data allowed us to conclude that proteolytic processing of pro-ocytocin-related peptide substrates by PC1/3 as well as by the metalloenzyme, magnolysin, involves selective recognition of precise cleavage site local secondary structure by the processing enzyme. It is hypothesized that this may represent a general property of peptide precursor proteolytic processing systems.

Anterograde axonal transport of Boc-Arg-Val-Arg-Arg-MCA hydrolyzing enzyme in rat sciatic nerves: cleavage occurs between basic residues

Axonal transport of Boc-Arg-Val-Arg-Arg-MCA hydrolyzing enzyme activity was studied in rat sciatic nerves from 12 to 120 h after double ligations. The anterograde axonal transport increased and peaked 72 h after ligation. The optimum pH for Boc-Arg-Val-Arg-Arg-MCA hydrolyzing enzyme activity was 6.5 to 6.9 and did not require Ca(2+) for the activity. Two molecular forms with enzyme activity were identified by size-exclusion chromatography and the molecular masses of the two enzymes were estimated to be 98 and 52 kDa. Two enzyme activities were strongly inhibited by Hg(2+), Cu(2+) and trypsin inhibitors such as TLCK, antipain and leupeptin. It cleaved the substrate, Boc-Arg-Val-Arg-Arg-MCA, between the dibasic sequence Arg-Arg, and needed a support of aminopeptidase B-like enzyme activity for the liberation of 7-amino-4-methylcoumarin. These results suggest that the enzyme is transported in rat sciatic nerves and involved in the post-translational processing of precursor proteins under the anterograde axonal transport. But there is absolutely no evidence for a role in precursor processing and such a putative role is purely speculative.

Immunocytochemical Localization of Prohormone Convertase 1/3 and 2 in Thyroid C-Cells and Medullary Thyroid Carcinomas

Prohormone convertases (PCs) are the key enzymes in the regulated pathways for the post-translational processing of peptide hormones and are involved in converting larger prohormones to smaller, biologically active hormones. Immunolocalization of PC1/3 and PC2 was performed with thyroid C-cells from normal thyroid glands and medullary thyroid carcinomas (MTCs); thyroid C-cells were not consistently positive for PCs, whereas MTCs were consistently positive for PCs. Positive staining for PCs included tumor cell nests adjacent to the main MTCs in cases of multiple endocrine neoplasia type 2. Procalcitonin was originally detected in MTCs and a large amount of procalcitonin was present together with abundant PCs in MTCs. Thus, immunocytochemical staining for PCs may be another characteristic of MTCs.

Regulation of prohormone convertase 1 (PC1) by gp130-related cytokines

The processing of pro-opiomelanocortin (POMC) to generate bioactive ACTH in the anterior pituitary is mediated by prohormone convertase 1 (PC1). Leukemia inhibitory factor (LIF) and interleukin 6 (IL-6), two cytokines sharing the common gp130 receptor subunit and functioning through activation of the intracellular JAK/STAT pathway, induce POMC synthesis and ACTH release. We investigated the effects of LIF and IL-6 on PC1 expression and its subsequent processing of POMC. A significant time-dependent up-regulation of both PC1 protein and mRNA by LIF and IL-6 was seen in mouse corticotroph AtT-20 cells. IL-6 or LIF increased the synthesis of ACTH-related products with a concomitant increase in bioactive 5 and 13 kDa ACTH indicating coordinated regulation of substrate and processing enzyme. AtT-20 cells transiently transfected with a human PC1-promoter-luciferase reporter construct and treated with LIF or IL-6 showed significantly increased luciferase activity. Additionally, lipopolysaccharide (LPS) administration to rats resulted in an increase in both pituitary PC1 and POMC mRNA. These findings suggest that the ACTH increase induced by LIF and IL-6 is due to both increased POMC synthesis as well as increased POMC processing by up-regulation of PC1. These two coordinately regulated processing events probably exert central roles in the pathophysiological response to some stresses, such as inflammatory stress.

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.

A role for amontillado, the Drosophila homolog of the neuropeptide precursor processing protease PC2, in triggering hatching behavior

Accurate proteolytic processing of neuropeptide and peptide hormone precursors by members of the kexin/furin family of proteases is key to determining both the identities and activities of signaling peptides. Here we identify amontillado (amon), the Drosophila melanogaster homolog of the mammalian neuropeptide processing protease PC2, and show that in contrast to vertebrate PC2, amontillado expression undergoes extensive regulation in the nervous system during development. In situ hybridization reveals that expression of amontillado is restricted to the final stages of embryogenesis when it is found in anterior sensory structures and in only 168 cells in the brain and ventral nerve cord. After larvae hatch from their egg shells, the sensory structures and most cells in the CNS turn off or substantially reduce amontillado expression, suggesting that amontillado plays a specific role late in embryogenesis. Larvae lacking the chromosomal region containing amontillado show no gross anatomical defects and respond to touch. However, such larvae show a greatly reduced frequency of a hatching behavior of wild-type Drosophila in which larvae swing their heads, scraping through the eggshell with their mouth hooks. Ubiquitous expression of amontillado can restore near wild-type levels of this behavior, whereas expression of amontillado with an alanine substitution for the catalytic histidine cannot. These results suggest that amontillado expression is regulated as part of a programmed modulation of neural signaling that controls hatching behavior by producing specific neuropeptides in particular neurons at an appropriate developmental time.

The role of the 7B2 CT peptide in the inhibition of prohormone convertase 2 in endocrine cell lines

Prohormone convertase (PC) 2 plays an important role in the processing of neuropeptide precursors via the regulated secretory pathway in neuronal and endocrine tissues. PC2 interacts with 7B2, a neuroendocrine protein that is cleaved to a 21-kDa domain involved in proPC2 maturation and a carboxyl-terminal peptide (CT peptide) that represents a potent inhibitor of PC2 in vitro. A role for the CT peptide as an inhibitor in vivo has not yet been established. To study the involvement of the CT peptide in PC2-mediated cleavages in neuroendocrine cells, we constructed a mutant proenkephalin (PE) expression vector containing PE with its carboxyl-terminal peptide (peptide B) replaced with the 7B2 inhibitory CT peptide. This PECT chimera was stably transfected into two PC2-expressing cell lines, AtT-20/PC2 and Rin cells. Although recombinant PECT proved to be a potent (nM) inhibitor of PC2 in vitro, cellular PC2-mediated cleavages of PE were not inhibited by the PECT chimera, nor was proopiomelanocortin cleavage (as assessed by adrenocorticotropin cleavage to alpha-melanocyte-stimulating hormone) inhibited further than in control cells expressing only the competitive substrate PE. Tests of stimulated secretion showed that both the CT peptide and the PE portion of the chimera were stored in regulated secretory granules of transfected clones. In both AtT-20/PC2 and Rin cells expressing the chimera, the CT peptide was substantially internally hydrolyzed, potentially accounting for the observed lack of inhibition. Taken together, our data suggest that overexpressed CT peptide derived from PECT is unable to inhibit PC2 in mature secretory granules, most likely due to its inactivation by PC2 or by other enzyme(s).