Identification of the type 2 proinsulin processing endopeptidase as PC2, a member of the eukaryote subtilisin family

Interaction of a viral insulin-like peptide with the IGF-1 receptor produces a natural antagonist

Lymphocystis disease virus-1 (LCDV-1) and several other Iridoviridae encode viral insulin/IGF-1 like peptides (VILPs) with high homology to human insulin and IGFs. Here we show that while single-chain (sc) and double-chain (dc) LCDV1-VILPs have very low affinity for the insulin receptor, scLCDV1-VILP has high affinity for IGF1R where it can antagonize human IGF-1 signaling, without altering insulin signaling. Consequently, scLCDV1-VILP inhibits IGF-1 induced cell proliferation and growth hormone/IGF-1 induced growth of mice in vivo. Cryo-electron microscopy reveals that scLCDV1-VILP engages IGF1R in a unique manner, inducing changes in IGF1R conformation that led to separation, rather than juxtaposition, of the transmembrane segments and hence inactivation of the receptor. Thus, scLCDV1-VILP is a natural peptide with specific antagonist properties on IGF1R signaling and may provide a new tool to guide development of hormonal analogues to treat cancers or metabolic disorders sensitive to IGF-1 without affecting glucose metabolism.

Adaptation to chronic ER stress enforces pancreatic β-cell plasticity

Pancreatic β-cells are prone to endoplasmic reticulum (ER) stress due to their role in insulin secretion. They require sustainable and efficient adaptive stress responses to cope with this stress. Whether episodes of chronic stress directly compromise β-cell identity is unknown. We show here under reversible, chronic stress conditions β-cells undergo transcriptional and translational reprogramming associated with impaired expression of regulators of β-cell function and identity. Upon recovery from stress, β-cells regain their identity and function, indicating a high degree of adaptive plasticity. Remarkably, while β-cells show resilience to episodic ER stress, when episodes exceed a threshold, β-cell identity is gradually lost. Single cell RNA-sequencing analysis of islets from type 1 diabetes patients indicates severe deregulation of the chronic stress-adaptation program and reveals novel biomarkers of diabetes progression. Our results suggest β-cell adaptive exhaustion contributes to diabetes pathogenesis.

Altered islet prohormone processing: A cause or consequence of diabetes?

Peptide hormones are first produced as larger precursor prohormones that require endoproteolytic cleavage to liberate the mature hormones. A structurally conserved but functionally distinct family of nine prohormone convertase enzymes (PCs) are responsible for cleavage of protein precursors of which PC1/3 and PC2 are known to be exclusive to neuroendocrine cells and responsible for prohormone cleavage. Differential expression of PCs within tissues define prohormone processing; whereas glucagon is the major product liberated from proglucagon via PC2 in pancreatic α-cells, proglucagon is preferentially processed by PC1/3 in intestinal L cells to produce glucagon-like peptides 1 and 2 (GLP-1, GLP-2). Beyond our understanding of processing of islet prohormones in healthy islets, there is convincing evidence that proinsulin, proIAPP, and proglucagon processing is altered during prediabetes and diabetes. There is predictive value of elevated circulating proinsulin or proinsulin : C-peptide ratio for progression to type 2 diabetes and elevated proinsulin or proinsulin : C-peptide is predictive for development of type 1 diabetes in at risk groups. After onset of diabetes, patients have elevated circulating proinsulin and proIAPP and proinsulin may be an autoantigen in type 1 diabetes. Further, preclinical studies reveal that α-cells have altered proglucagon processing during diabetes leading to increased GLP-1 production. We conclude that despite strong associative data, current evidence is inconclusive on the potential causal role of impaired prohormone processing in diabetes, and suggest that future work should focus on resolving the question of whether altered prohormone processing is a causal driver or merely a consequence of diabetes pathology.

Isolation and Proteomics of the Insulin Secretory Granule

Insulin, a vital hormone for glucose homeostasis is produced by pancreatic beta-cells and when secreted, stimulates the uptake and storage of glucose from the blood. In the pancreas, insulin is stored in vesicles termed insulin secretory granules (ISGs). In Type 2 diabetes (T2D), defects in insulin action results in peripheral insulin resistance and beta-cell compensation, ultimately leading to dysfunctional ISG production and secretion. ISGs are functionally dynamic and many proteins present either on the membrane or in the lumen of the ISG may modulate and affect different stages of ISG trafficking and secretion. Previously, studies have identified few ISG proteins and more recently, proteomics analyses of purified ISGs have uncovered potential novel ISG proteins. This review summarizes the proteins identified in the current ISG proteomes from rat insulinoma INS-1 and INS-1E cell lines. Here, we also discuss techniques of ISG isolation and purification, its challenges and potential future directions.

Production of a novel heterodimeric two-chain insulin-Fc fusion protein

Insulin is a peptide hormone produced by the pancreas. The physiological role of insulin is the regulation of glucose metabolism. Under certain pathological conditions the insulin levels can be reduced leading to the metabolic disorder diabetes mellitus (DM). For type 1 DM and, dependent on the disease progression for type 2 DM, insulin substitution becomes indispensable. To relieve insulin substitution therapy for patients, novel insulin analogs with pharmacokinetic and pharmacodynamic profiles aiming for long-lasting or fast-acting insulins have been developed. The next step in the evolution of novel insulins should be insulin analogs with a time action profile beyond 1-2 days, preferable up to 1 week. Nowadays, insulin is produced in a recombinant manner. This approach facilitates the design and production of further insulin-analogs or insulin-fusion proteins. The usage of the Fc-domain from immunoglobulin as a fusion partner for therapeutic proteins and peptides is widely used to extend their plasma half-life. Insulin consists of two chains, the A- and B-chain, which are connected by two disulfide-bridges. To produce a novel kind of Fc-fusion protein we have fused the A-chain as well as the B-chain to Fc-fragments containing either 'knob' or 'hole' mutations. The 'knob-into-hole' technique is frequently used to force heterodimerization of the Fc-domain. Using this approach, we were able to produce different variants of two-chain-insulin-Fc-protein (tcI-Fc-protein) variants. The tcI-Fc-fusion variants retained activity as shown in in vitro assays. Finally, prolonged blood glucose lowering activity was demonstrated in normoglycemic rats. Overall, we describe here the production of novel insulin-Fc-fusion proteins with prolonged times of action.

Revisiting Proinsulin Processing: Evidence That Human β-Cells Process Proinsulin With Prohormone Convertase (PC) 1/3 but Not PC2

Insulin is first produced in pancreatic β-cells as the precursor prohormone proinsulin. Defective proinsulin processing has been implicated in the pathogenesis of both type 1 and type 2 diabetes. Though there is substantial evidence that mouse β-cells process proinsulin using prohormone convertase 1/3 (PC1/3) and then prohormone convertase 2 (PC2), this finding has not been verified in human β-cells. Immunofluorescence with validated antibodies revealed that there was no detectable PC2 immunoreactivity in human β-cells and little PCSK2 mRNA by in situ hybridization. Similarly, rat β-cells were not immunoreactive for PC2. In all histological experiments, PC2 immunoreactivity in neighboring α-cells acted as a positive control. In donors with type 2 diabetes, β-cells had elevated PC2 immunoreactivity, suggesting that aberrant PC2 expression may contribute to impaired proinsulin processing in β-cells of patients with diabetes. To support histological findings using a biochemical approach, human islets were used for pulse-chase experiments. Despite inhibition of PC2 function by temperature blockade, brefeldin A, chloroquine, and multiple inhibitors that blocked production of mature glucagon from proglucagon, β-cells retained the ability to produce mature insulin. Conversely, suppression of PC1/3 blocked processing of proinsulin but not proglucagon. By demonstrating that healthy human β-cells process proinsulin by PC1/3 but not PC2, we suggest that there is a need to revise the long-standing theory of proinsulin processing.

Biogenesis of the Insulin Secretory Granule in Health and Disease

The secretory granules of pancreatic beta cells are specialized organelles responsible for the packaging, storage and secretion of the vital hormone insulin. The insulin secretory granules also contain more than 100 other proteins including the proteases involved in proinsulin-to insulin conversion, other precursor proteins, minor co-secreted peptides, membrane proteins involved in cell trafficking and ion translocation proteins essential for regulation of the intragranular environment. The synthesis, transport and packaging of these proteins into nascent granules must be carried out in a co-ordinated manner to ensure correct functioning of the granule. The process is regulated by many circulating nutrients such as glucose and can change under different physiological states. This chapter discusses the various processes involved in insulin granule biogenesis with a focus on the granule composition in health and disease.

Characterization of Transplantable Insulinoma Cells

This chapter describes the propagation and characterization of transplantable insulinoma cells as model of insulin-producing pancreatic islet cells in the rat. Here, the cells are propagated by transplantation into rats followed by harvesting after growth for approximately 1 month. The cells are then purified by Percoll density gradient centrifugation and characterized by pulse-chase radiolabelling and immunoprecipitation of the insulin-related peptides. The results show that the transplantable insulinoma cells produce insulin in a manner similar to that found in normal pancreatic islets.

Multiplex Sequential Immunoprecipitation of Insulin Secretory Granule Proteins from Radiolabeled Pancreatic Islets

Pulse radiolabeling of cells with radioactive amino acids is a common method for tracking the biosynthesis of proteins. Specific proteins can then be immunoprecipitated and analyzed by electrophoresis and imaging techniques. This chapter presents a protocol for the biosynthetic labeling of pancreatic islets with ³⁵S-methionine, followed by multiplex sequential immunoprecipitation of insulin and three other secretory granule accessory proteins. This provided a means of distinguishing those pancreatic islet proteins with different biosynthetic rates in response to the media glucose concentrations.

Pulse-Chase Biosynthetic Radiolabeling of Pancreatic Islets to Measure Beta Cell Function

Pulse-chase radiolabeling of cells with radioactive amino acids is a common method for studying the biosynthesis of proteins. The labeled proteins can then be immunoprecipitated and analyzed by electrophoresis and gel imaging techniques. This chapter presents a protocol for the biosynthetic labeling and immunoprecipitation of pancreatic islet proteins which are known to be affected in disorders such as diabetes, obesity, and metabolic syndrome.

Time-Resolved Fluorescence Assays for Quantification of Insulin Precursors in Plasma and Serum

In metabolic diseases such as obesity and type 2 diabetes mellitus, the conversion of proinsulin to mature insulin can be impaired. This could mean that insulin molecules with lower activity toward the insulin receptor can be released under conditions of high metabolic demand, resulting in an inadequate glucoregulatory response. The chapter describes a fluorescent monoclonal antibody-based protocol for measurement of human proinsulin and the proinsulin conversion intermediates (split proinsulins). An example assay is presented using serum from non-diabetic, normal body mass index individuals.

Sequential Immunoprecipitation of Secretory Vesicle Proteins from Biosynthetically Labelled Cells

Pulse radiolabelling of cells with radioactive amino acids is a common method for studying the biosynthesis of proteins. The labelled proteins can then be immunoprecipitated and analysed by electrophoresis and imaging techniques. This chapter presents a protocol for the biosynthetic labelling and immunoprecipitation of pancreatic islet proteins which are known to be affected in psychiatric disorders such as schizophrenia.

2D Gel Electrophoresis of Insulin Secretory Granule Proteins from Biosynthetically Labelled Pancreatic Islets

Pulse radiolabelling of cells with radioactive amino acids such is a common method for investigating the biosynthetic rates of proteins. In this way, the abundance of newly synthesized proteins can be determined by several proteomic techniques including 2D gel electrophoresis (2DE). This chapter describes a protocol for labelling pancreatic islets with ³⁵S-methionine in the presence of low and high concentrations of glucose, followed by subcellular fractionation enrichment of secretory granule proteins and analysis of the granule protein contents by 2DE. This demonstrated that the biosynthetic rates of most of the granule proteins are co-ordinately regulated in the presence of stimulatory glucose concentrations.

Two Dimensional Gel Electrophoresis of Insulin Secretory Granule Proteins from Biosynthetically-Labeled Pancreatic Islets

Pulse-chase radiolabeling of cells with radioactive amino acids is a common method for tracking the biosynthesis of proteins. Radiolabeled newly synthesized proteins can be analyzed by a number of techniques such as two dimensional gel electrophoresis (2DE). This chapter presents a protocol for the biosynthetic labeling of pancreatic islets with ³⁵S-methionine in the presence of basal and stimulatory concentrations of glucose, followed by subcellular fractionation to produce a secretory granule fraction and analysis of the granule protein contents by 2DE. This provides a means of determining whether or not the biosynthetic rates of the entire granule constituents are coordinately regulated.

Proteasomal Degradation of Proinsulin Requires Derlin-2, HRD1 and p97

Patients with type 1 diabetes (T1D) suffer from beta-cell destruction by CD8⁺ T-cells that have preproinsulin as an important target autoantigen. It is of great importance to understand the molecular mechanism underlying the processing of preproinsulin into these CD8⁺ T-cell epitopes. We therefore studied a pathway that may contribute to the production of these antigenic peptides: degradation of proinsulin via ER associated protein degradation (ERAD). Analysis of the MHC class I peptide ligandome confirmed the presentation of the most relevant MHC class I-restricted diabetogenic epitopes in our cells: the signal peptide-derived sequence A15-A25 and the insulin B-chain epitopes H29-A38 and H34-V42. We demonstrate that specific silencing of Derlin-2, p97 and HRD1 by shRNAs increases steady state levels of proinsulin. This indicates that these ERAD constituents are critically involved in proinsulin degradation and may therefore also play a role in subsequent antigen generation. These ERAD proteins therefore represent interesting targets for novel therapies aiming at the reduction and possibly also prevention of beta-cell directed auto-immune reactions in T1D.

Group X secretory phospholipase A2 regulates insulin secretion through a cyclooxygenase-2-dependent mechanism

Group X secretory phospholipase A2 (GX sPLA2) potently hydrolyzes membrane phospholipids to release arachidonic acid (AA). While AA is an activator of glucose-stimulated insulin secretion (GSIS), its metabolite prostaglandin E2 (PGE2) is a known inhibitor. In this study, we determined that GX sPLA2 is expressed in insulin-producing cells of mouse pancreatic islets and investigated its role in beta cell function. GSIS was measured in vivo in wild-type (WT) and GX sPLA2-deficient (GX KO) mice and ex vivo using pancreatic islets isolated from WT and GX KO mice. GSIS was also assessed in vitro using mouse MIN6 pancreatic beta cells with or without GX sPLA2 overexpression or exogenous addition. GSIS was significantly higher in islets isolated from GX KO mice compared with islets from WT mice. Conversely, GSIS was lower in MIN6 cells overexpressing GX sPLA2 (MIN6-GX) compared with control (MIN6-C) cells. PGE2 production was significantly higher in MIN6-GX cells compared with MIN6-C cells and this was associated with significantly reduced cellular cAMP. The effect of GX sPLA2 on GSIS was abolished when cells were treated with NS398 (a COX-2 inhibitor) or L-798,106 (a PGE2-EP3 receptor antagonist). Consistent with enhanced beta cell function, GX KO mice showed significantly increased plasma insulin levels following glucose challenge and were protected from age-related reductions in GSIS and glucose tolerance compared with WT mice. We conclude that GX sPLA2 plays a previously unrecognized role in negatively regulating pancreatic insulin secretion by augmenting COX-2-dependent PGE2 production.

Insulin regulates carboxypeptidase E by modulating translation initiation scaffolding protein eIF4G1 in pancreatic β cells

Insulin resistance, hyperinsulinemia, and hyperproinsulinemia occur early in the pathogenesis of type 2 diabetes (T2D). Elevated levels of proinsulin and proinsulin intermediates are markers of β-cell dysfunction and are strongly associated with development of T2D in humans. However, the mechanism(s) underlying β-cell dysfunction leading to hyperproinsulinemia is poorly understood. Here, we show that disruption of insulin receptor (IR) expression in β cells has a direct impact on the expression of the convertase enzyme carboxypeptidase E (CPE) by inhibition of the eukaryotic translation initiation factor 4 gamma 1 translation initiation complex scaffolding protein that is mediated by the key transcription factors pancreatic and duodenal homeobox 1 and sterol regulatory element-binding protein 1, together leading to poor proinsulin processing. Reexpression of IR or restoring CPE expression each independently reverses the phenotype. Our results reveal the identity of key players that establish a previously unknown link between insulin signaling, translation initiation, and proinsulin processing, and provide previously unidentified mechanistic insight into the development of hyperproinsulinemia in insulin-resistant states.

The multifaceted proprotein convertases: their unique, redundant, complementary, and opposite functions

The secretory proprotein convertase (PC) family comprises nine members: PC1/3, PC2, furin, PC4, PC5/6, PACE4, PC7, SKI-1/S1P, and PCSK9. The first seven PCs cleave their substrates at single or paired basic residues, and SKI-1/S1P cleaves its substrates at non-basic residues in the Golgi. PCSK9 cleaves itself once, and the secreted inactive protease escorts specific receptors for lysosomal degradation. It regulates the levels of circulating LDL cholesterol and is considered a major therapeutic target in phase III clinical trials. In vivo, PCs exhibit unique and often essential functions during development and/or in adulthood, but certain convertases also exhibit complementary, redundant, or opposite functions.

Post-endoplasmic reticulum rescue of unstable MHC class I requires proprotein convertase PC7

The function of the peptide-loading complex (PLC) is to facilitate loading of MHC class I (MHC I) molecules with antigenic peptides in the endoplasmic reticulum and to drive the selection of these ligands toward a set of high-affinity binders. When the PLC fails to perform properly, as frequently observed in virus-infected or tumor cells, structurally unstable MHC I peptide complexes are generated, which are prone to disintegrate instead of presenting Ags to cytotoxic T cells. In this study we show that a second quality control checkpoint dependent on the serine protease proprotein convertase 7 (PC7) can rescue unstable MHC I, whereas the related convertase furin is completely dispensable. Cells with a malfunctioning PLC and silenced for PC7 have substantially reduced MHC I surface levels caused by high instability and significantly delayed surface accumulation of these molecules. Instead of acquiring stability along the secretory route, MHC I appears to get largely routed to lysosomes for degradation in these cells. Moreover, mass spectrometry analysis provides evidence that lack of PLC quality control and/or loss of PC7 expression alters the MHC I-presented peptide profile. Finally, using exogenously applied peptide precursors, we show that liberation of MHC I epitopes may directly require PC7. We demonstrate for the first time an important function for PC7 in MHC I-mediated Ag presentation.

Human pituitary contains dual cathepsin L and prohormone convertase processing pathway components involved in converting POMC into the peptide hormones ACTH, alpha-MSH, and beta-endorphin

The production of the peptide hormones ACTH, alpha-MSH, and beta-endorphin requires proteolytic processing of POMC which is hypothesized to utilize dual cysteine- and subtilisin-like protease pathways, consisting of the secretory vesicle cathepsin L pathway and the well-known subtilisin-like prohormone convertase (PC) pathway. To gain knowledge of these protease components in human pituitary where POMC-derived peptide hormones are produced, this study investigated the presence of these protease pathway components in human pituitary. With respect to the cathepsin L pathway, human pituitary contained cathepsin L of 27-29 kDa and aminopeptidase B of approximately 64 kDa, similar to those in secretory vesicles of related neuroendocrine tissues. The serpin inhibitor endopin 2, a selective inhibitor of cathepsin L, was also present. With respect to the PC pathway, human pituitary expresses PC1/3 and PC2 of approximately 60-65 kDa, which represent active PC1/3 and PC2; peptide hormone production then utilizes carboxypeptidase E (CPE) which is present as a protein of approximately 55 kDa. Analyses of POMC products in human pituitary showed that they resemble those in mouse pituitary which utilizes cathepsin L and PC2 for POMC processing. These findings suggest that human pituitary may utilize the cathepsin L and prohormone convertase pathways for producing POMC-derived peptide hormones.

(Pro)Insulin processing: a historical perspective

Insulin, the major secreted product of the beta-cells of the islets of Langerhans, is initially synthesized as a precursor (preproinsulin), from which the mature hormone is excised by a series of proteolytic cleavages. This review provides a personal narrative of some of the key research projects leading to the identification of the central processing enzymes as proprotein convertase 1, proprotein convertase 2, and carboxypeptidase E. It also discusses the central roles of the intragranular environment and chaperone-like proteins in modulating processing activity.

The A-chain of insulin is a hot-spot for CD4+ T cell epitopes in human type 1 diabetes

Type 1 diabetes (T1D) is caused by T cell-mediated destruction of the pancreatic insulin-producing beta cells. While the role of CD4(+) T cells in the pathogenesis of T1D is accepted widely, the epitopes recognized by pathogenic human CD4(+) T cells remain poorly defined. None the less, responses to the N-terminal region of the insulin A-chain have been described. Human CD4(+) T cells from the pancreatic lymph nodes of subjects with T1D respond to the first 15 amino acids of the insulin A-chain. We identified a human leucocyte antigen-DR4-restricted epitope comprising the first 13 amino acids of the insulin A-chain (A1-13), dependent upon generation of a vicinal disulphide bond between adjacent cysteines (A6-A7). Here we describe the analysis of a CD4(+) T cell clone, isolated from a subject with T1D, which recognizes a new HLR-DR4-restricted epitope (KRGIVEQCCTSICS) that overlaps the insulin A1-13 epitope. This is a novel epitope, because the clone responds to proinsulin but not to insulin, T cell recognition requires the last two residues of the C-peptide (Lys, Arg) and recognition does not depend upon a vicinal disulphide bond between the A6 and A7 cysteines. The finding of a further CD4(+) T cell epitope in the N-terminal A-chain region of human insulin underscores the importance of this region as a target of CD4(+) T cell responses in human T1D.

Profiling synaptic proteins identifies regulators of insulin secretion and lifespan

Cells are organized into distinct compartments to perform specific tasks with spatial precision. In neurons, presynaptic specializations are biochemically complex subcellular structures dedicated to neurotransmitter secretion. Activity-dependent changes in the abundance of presynaptic proteins are thought to endow synapses with different functional states; however, relatively little is known about the rules that govern changes in the composition of presynaptic terminals. We describe a genetic strategy to systematically analyze protein localization at Caenorhabditis elegans presynaptic specializations. Nine presynaptic proteins were GFP-tagged, allowing visualization of multiple presynaptic structures. Changes in the distribution and abundance of these proteins were quantified in 25 mutants that alter different aspects of neurotransmission. Global analysis of these data identified novel relationships between particular presynaptic components and provides a new method to compare gene functions by identifying shared protein localization phenotypes. Using this strategy, we identified several genes that regulate secretion of insulin-like growth factors (IGFs) and influence lifespan in a manner dependent on insulin/IGF signaling.

Cells are divided into multiple subcellular compartments that perform diverse functions. In neurons, synapses mediate transmission of information between cells and they comprise hundreds of proteins dedicated for this purpose. Changes in the protein composition of synapses are thought to produce changes in synaptic transmission, such as those that occur during development, learning, and memory. Here, we describe a systematic genetic strategy for analyzing the protein composition of synapses. Using this strategy, we identified sets of genes that alter synapses in similar ways, and identified novel regulatory relationships between particular synaptic proteins. One set of genes regulated secretion of insulin-like hormones from neurons and had corresponding effects on lifespan, which is controlled by insulin signaling. These results illustrate how changes in synaptic composition can be utilized as a probe to explain changes in physiology. Our approach can be expanded to include a larger set of synaptic proteins or to analyze other subcellular compartments.

Hormonal regulation of proprotein convertase subtilisin/kexin type 5 expression during ovarian follicle development in the rat

The proprotein convertase subtilisin/kexin (PCSKs), a family of subtilisin-like proteases, is the processing enzymes for the activation of many hormone precursors. The present study was designed to identify the PCSK isoform expressed in the ovary and to examine its expression in gonadotropin-stimulated rat ovary. Northern blot analysis of ovaries obtained from prepubertal rats revealed an increased expression of Pcsk5 messenger RNA (mRNA) during development with the highest levels at 21 days of age. Treatment of immature rats with PMSG further increased ovarian Pcsk5 expression, and in situ hybridization analysis revealed the localization of Pcsk5 mRNA in theca-interstitial cells of follicles in different sizes. Interestingly, treatment of PMSG-primed rats with hCG resulted in a transient stimulation of ovarian Pcsk5 mRNA levels within 3-6 h. In addition to theca-interstitial cells, hCG treatment induced the expression of Pcsk5 in granulosa cells of preovulatory follicles. Pcsk1, 2 and 4 mRNAs were not detected whereas Pcsk7 mRNA was slightly expressed. Injection of a progestin antagonist RU486 or an inhibitor of 3beta-hydroxysteroid dehydrogenase epostane at 1h before hCG treatment inhibited hCG-induced Pcsk5 mRNA levels. Treatment with LH stimulated both Pcsk5 mRNA and protein levels in preovulatory follicles cultured in vitro. In addition, forskolin but not TPA stimulated Pcsk5 mRNA levels. RNase protection assay revealed that the soluble Pcsk5A variant was the predominant form stimulated by gonadotropins in the ovary. Finally, the predicted proprotein substrates cleaved by PCSK5 were analyzed in preovulatory follicles using regular expressions. The present study demonstrates PCSK5A as the gonadotropin-regulated PCSK isoform in the ovary, and its possible contribution to ovulation by processing pro-TGFbeta and matrix metalloproteinase family.

Direct visualisation of peptide hormones in cultured pancreatic islet alpha- and beta-cells by intact-cell mass spectrometry

The application of intact-cell mass spectrometry (ICM) by matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) mass spectrometry to achieve direct protein-profiling of bacterial species is now well established. However, this methodology has not to our knowledge been applied to the analysis of mammalian cells in routine culture. Here, we describe a novel application of ICM by which we have identified proteins in intact cells from two lines representative of pancreatic islet alpha- and beta-cells. Adherent alphaTC1 clone 9 and betaTC6 F7 cells were harvested into phosphate-buffered saline (PBS) using enzyme-free dissociation buffer before 1 microL of cell suspension was spotted onto MALDI plates. Cells were overlaid with sinapinic acid then washed with pure water before application of a final coat of sinapinic acid. Data in the 2000-20,000 m/z range were acquired in linear mode on a Voyager DE-Pro mass spectrometer. The proteins which ionised were composed in large part of peptide hormones (e.g. insulin and glucagon) known to be packaged into the secretory granules of the beta- and alpha-cells respectively. However, in addition to visualising the peptides expected to be associated with these cells, a mass consistent with oxyntomodulin was identified in the cultured alpha-cells, a finding not previously reported to our knowledge. In summary, this paper describes, for the first time, a rapid and direct method useful for identifying secretory products in intact endocrine cells.

Newer therapeutic options for children with diabetes mellitus: theoretical and practical considerations

Recent studies in adult patients with type 1 diabetes mellitus (T1DM) and T2DM have examined the potential utility, benefits, and side effects of agents that augment insulin secretion after oral ingestion of nutrients in comparison with intravenous nutrient delivery, the so-called incretins. Two families of incretin-like substances are now approved for use in adults. Glucagon-like peptide-1 (GLP-1) or agents that bind to its receptor (exenatide, Byetta) or agents that inhibit its destruction [dipeptidyl peptidase-IV (DPP-IV) inhibitors, Vildagliptin] improve insulin secretion, delay gastric emptying, and suppress glucagon secretion while decreasing food intake without increasing hypoglycemia. Pramlintide, a synthetic amylin analog, also decreases glucagon secretion and delays gastric emptying, improves hemoglobin A1c (HbA1C), and facilitates weight reduction without causing hypoglycemia. We review the historical discovery of these agents, their physiology [corrected] and their current applications. Remarkably, only one or two studies have been reported in children. Pediatricians caring for children with T1DM and T2DM should become familiar with these agents and investigate their applicability, as they seem likely to enhance our therapeutic armamentarium to treat children with diabetes mellitus.

Regulation of hypothalamic prohormone convertases 1 and 2 and effects on processing of prothyrotropin-releasing hormone

Regulation of energy balance by leptin involves regulation of several neuropeptides, including thyrotropin-releasing hormone (TRH). Synthesized from a larger inactive precursor, its maturation requires proteolytic cleavage by prohormone convertases 1 and 2 (PC1 and PC2). Since this maturation in response to leptin requires prohormone processing, we hypothesized that leptin might regulate hypothalamic PC1 and PC2 expression, ultimately leading to coordinated processing of prohormones into mature peptides. Using hypothalamic neurons, we found that leptin stimulated PC1 and PC2 mRNA and protein expression and also increased PC1 and PC2 promoter activities in transfected 293T cells. Starvation of rats, leading to low serum leptin levels, decreased PC1 and PC2 gene and protein expression in the paraventricular nucleus (PVN) of the hypothalamus. Exogenous administration of leptin to fasted animals restored PC1 levels in the median eminence (ME) and the PVN to approximately the level found in fed control animals. Consistent with this regulation of PCs in the PVN, concentrations of TRH in the PVN and ME were substantially reduced in the fasted animals relative to the fed animals, and leptin reversed this decrease. Further analysis showed that proteolytic cleavage of pro-thyrotropin-releasing hormone (proTRH) at known PC cleavage sites was reduced by fasting and increased in animals given leptin. Combined, these findings suggest that leptin-dependent stimulation of hypothalamic TRH expression involves both activation of trh transcription and stimulation of PC1 and PC2 expression, which lead to enhanced processing of proTRH into mature TRH.

Expression, purification, and PC1-mediated processing of (H10D, P28K, and K29P)-human proinsulin

Our previous methods for the generation of recombinant human proinsulin were inadequate in terms of reproducibility and yield. In addition, it was difficult to perform structure/function studies on proinsulin because of its tendency to form hexamers. We have developed an improved procedure, which overcomes many of the technical purification problems, and results in a potentially monomeric version of modified proinsulin. Inclusion bodies were prepared using a commercial bacterial lysis solution. The inclusion bodies were solubilized and the fusion protein's affinity tag was removed by chemical cleavage. The polypeptide was then reduced and transferred into a refolding buffer. Following an overnight incubation, only a single form of proinsulin was detected using analytical reversed-phase high-performance liquid chromatography. The refolded (H10D, P28K, and K29P)-human proinsulin (DKP-hPI) was subjected to a final purification step using reversed-phase chromatography. The method is reproducible and produces milligram quantities of purified DKP-hPI from a single liter of bacterial culture. The final product is greater than 95% pure and is suitable for use as a substrate for the propeptide convertase PC1.

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.

Functional characterization of ProSAAS: similarities and differences with 7B2

Prohormone convertases (PC) 1 and 2, enzymes found primarily in neuroendocrine tissues, are thought to mediate the proteolytic cleavage of many peptide precursors. To date, endogenous binding proteins for both PC2 (7B2) and PC1 (proSAAS) have been identified. Although 7B2 represents a potent inhibitor of PC2, the most important function of 7B2 as regards this enzyme appears to be the absolute requirement of PC2 for 7B2 in the generation of active enzyme, recently corroborated through production of a null animal that lacks PC2 activity. The purpose of the present study was to determine whether proSAAS exerts effects on PC1 other than inhibition, and to establish functional similarities and differences between 7B2 and proSAAS. We first asked whether the N-terminal domain of proSAAS (proSAAS-(1-180)) could stabilize PC1 activity, similar to the effect of the N-terminal domain of 7B2 on PC2. Recombinant His-tagged proSAAS-(1-180) had no effect on PC1 activity in vitro and was unable to protect PC1 from thermal denaturation. Transient cotransfection of proSAAS-(1-225) cDNA with PC1 cDNA into HEK 293 cells reduced the amount of PC1 activity detected in the medium. Surprisingly, cotransfection of proSAAS-(1-180) cDNA, encoding a protein that lacks the inhibitory C-terminal domain peptide, also reduced the activity of PC1 detected in the medium, but the mass of PC1 secreted into the medium was increased, suggesting a proSAAS-mediated inactivation reaction. Similar results were observed in CHO/PC1 cells stably transfected with pro-SAAS-(1-180). Stable transfection of SAAS cDNAs into AtT-20 cells was used to examine the role of proSAAS in a neuroendocrine setting. Unlike 7B2, proSAAS-(1-225) was able to slow convertase-mediated processing of proopiomelanocortin and proenkephalin; however, similarly to 7B2, proSAAS expression did not result in any accumulated differences in the content of cellular processed peptide. In summary, although both proSAAS and 7B2 potently inhibit PC enzymes via a C-terminal peptide, their intracellular interactions with PCs appear to differ significantly, with each binding protein exhibiting unique properties.

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.

Translational regulation of proinsulin biosynthesis and proinsulin conversion in the pancreatic beta-cell

Insulin secretion from the pancreatic beta -cell can be initiated in minutes, vary as much as 50-100-fold, and be sustained for several hours without need for changes in insulin gene transcription. Remarkably, the cellular content of the hormone and its molecular composition do not vary appreciably in the face of changes of insulin granule exocytosis. Minimal morphological changes are apparent, further indicating that the movement of lipids and membrane proteins between the granule storage pool, the plasma membrane, and Golgi are likewise tightly controlled. Such homeostasis is achieved by an interplay of signaling pathways originating from the metabolism of glucose with downstream targets at the level of translation of dense-core granule proteins, granule biogenesis, and membrane trafficking. Our scant knowledge in this area is confined mostly to a descriptive account of the fate of the major secreted components, principally insulin and the enzymes PC1, PC2, and CPH involved in the proteolytic conversion of proinsulin to insulin. A common theme seems to be the role of intracellular energy homeostasis in integrating the stimulus-secretion and stimulus-biosynthetic responses of this cell.

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.

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.

Regulation of pancreatic PC1 and PC2 associated with increased glucagon-like peptide 1 in diabetic rats

The pancreatic processing enzymes, PC1 and PC2, convert proinsulin to insulin and convert proglucagon to glucagon and glucagon-like peptide 1 (GLP-1). We examined the effect of streptozotocin (STZ) treatment on the regulation of these enzymes and the production of insulin, glucagon, and GLP-1 in the rat. Pancreatic PC1 and PC2 mRNA increased >2-fold and >4-fold, respectively, in rats receiving intraperitoneal STZ (50 mg/kg) daily for 5 days. Immunocytochemistry revealed that, although pancreatic islet cells in the STZ-treated rats were sparse and atrophic PC1, PC2, glucagon, and GLP-1 immunoreactivity increased dramatically in the remaining islet cells. Heightened PC1 and PC2 expression was seen in cells expressing glucagon but not in insulin-expressing cells. Furthermore, in STZ-treated rats, bioactive GLP-1(7-36 amide) accumulated in pancreatic extracts and serum 3- and 2.5-fold, respectively, over control animals. This treatment also caused a 2-fold increase in the ratio of amidated forms of GLP-1 immunoreactivity to total glucagon immunoreactivity in the pancreas but did not affect the ratio of proinsulin to insulin. We conclude that hyperglycemic rats have an increased expression of prohormone converting enzymes in islet alpha cells, leading to an increase in amidated GLP-1, which can then exert an insulinotropic effect on the remaining beta cells.

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.

Streamlined procedure for the production of normal and altered versions of recombinant human proinsulin

A method for the simplified, reproducible production of both normal and altered versions of human proinsulin has been developed. A polyhistidine/proinsulin fusion protein was expressed using a prokaryotic expression system and partially purified by affinity chromatography. Disulfide bonds within the polypeptide were formed prior to removal of the affinity tag. The proinsulin cleaved from the fusion protein was then subjected to a final purification step of semipreparative reversed-phase high-performance liquid chromatography. Integrity of both the normal and mutant proinsulins was confirmed by peptide mapping and mass spectrometry. The different versions of proinsulin will be used to map those residues of the substrate used in cleavage site recognition by members of the furin/PC family of converting enzymes.

The proteolytic maturation of prohormone convertase 2 (PC2) is a pH-driven process

Recombinant proPC2 purified from the medium of CHO cells overexpressing both the prohormone convertase (PC) precursor proPC2 and the 21-kDa amino terminal portion of the neuroendocrine protein 7B2 can spontaneously convert to an active species. In the present report, we have characterized the proPC2 zymogen conversion process. Sequencing of the mature 66 kDa enzyme revealed a single site of cleavage at the paired basic site amino terminal to the GYRDI sequence. In contrast to mature PC2 activity, proPC2 conversion was inhibited neither by the eukaryotic subtilisin inhibitor pCMS nor by the specific PC2 inhibitor, 7B2 CT peptide, suggesting significant differences between the proPC2 conversion reaction and the hydrolysis of synthetic substrates by mature PC2. In support of this idea, proPC2 conversion was not calcium dependent and was unaffected by 5 mM EDTA. The rate of conversion of proPC2 remained similar with a 10-fold difference in zymogen concentration, implicating an intramolecular rather than intermolecular mechanism of activation. Interestingly, the rate of proPC2 conversion was extremely pH dependent, occurring most extensively between pHs 4.0 and 4.9. Taken together, our results suggest that cellular proPC2 maturation occurs via an autocatalytic, intramolecular process controlled not by 7B2 inhibition nor by calcium levels, but by the decreasing pH gradient along the secretory pathway.

Homotypic fusion of immature secretory granules during maturation in a cell-free assay

The biogenesis of secretory granules embodies several morphological and biochemical changes. In particular, in neuroendocrine cells maturation of secretory granules is characterized by an increase in size which has been proposed to reflect homotypic fusion of immature secretory granules (ISGs). Here we describe an assay that provides the first biochemical evidence for such a fusion event and allows us to analyze its regulation. The assay reconstitutes homotypic fusion between one population of ISGs containing a [35S]sulfate-labeled substrate, secretogranin II (SgII), and a second population containing the prohormone convertase PC2. Both substrate and enzyme are targeted exclusively to ISGs. Fusion is measured by quantification of a cleavage product of SgII produced by PC2. With this assay we show that fusion only occurs between ISGs and not between ISGs and MSGs, is temperature dependent, and requires ATP and GTP and cytosolic proteins. NSF (N-ethylmaleimide-sensitive fusion protein) is amongst the cytosolic proteins required, whereas we could not detect a requirement for p97. The ability to reconstitute ISG fusion in a cell-free assay is an important advance towards the identification of molecules involved in the maturation of secretory granules and will increase our understanding of this process.

Prohormones: their clinical relevance

The majority of peptide hormones and neuropeptides are synthesized as precursors, which are cleaved in a sequence-specific and tissue-specific manner to yield the biologically active peptides. There has been considerable progress in the past ten years in understanding the nature and mechanism of action of the prohormone convertases that cleave these prohormones. Evidence from knockout technology and clinical examples of gene mutations has provided functional information on disruption of prohormone cleavage and the bioactivity of prohormones. There are specific examples of the clinical relevance of circulating prohormones, such as adrenocorticotrophin (ACTH) precursors and proinsulin. The central issues that still remain are: (1) What is the relative importance of each of the different processing pathways and processing enzymes in regulating hormone action? (2) How do the serum concentrations of prohormones compare with the mature hormone levels? (3) What are the biological consequences of prohormones in the circulation?

PC5-A-mediated processing of pro-neurotensin in early compartments of the regulated secretory pathway of PC5-transfected PC12 cells

Among the members of the proprotein convertase (PC) family, PC1 and PC2 have well established roles as prohormone convertases. Another good candidate for this role is PC5-A that has been shown to be present in the regulated secretory pathway of certain neuroendocrine tissues, but evidence that it can process prohormones is lacking. To determine whether PC5-A could function as a prohormone convertase and to compare its cleavage specificity with that of PC1 and PC2, we stably transfected the rat pheochromocytoma PC12 cell line with PC5-A and analyzed the biosynthesis and subcellular localization of the enzyme, as well as its ability to process pro-neurotensin/neuromedin N (pro-NT/NN) into active peptides. Our data showed that in transfected PC12 cells, PC5-A was converted from its 126-kDa precursor form into a 117-kDa mature form and, to a lesser extent, into a C-terminally truncated 65-kDa form of the 117-kDa product. Metabolic and immunochemical studies showed that PC5-A was sorted to early compartments of the regulated secretory pathway where it colocalized with immunoreactive NT. Furthermore, pro-NT/NN was processed in these compartments according to a pattern that differed from that previously described in PC1- and PC2-transfected PC12 cells. This pattern resembled that previously reported for pro-NT/NN processing in the adrenal medulla, a tissue known to express high levels of PC5-A. Altogether, these data demonstrate for the first time the ability of PC5-A to function as a prohormone convertase in the regulated secretory pathway and suggest a role for this enzyme in the physiological processing of pro-NT/NN.

Effect of high glucose concentration on proinsulin biosynthesis and conversion by human islets

In the present study we investigate whether glucose concentration could have an effect on proinsulin biosynthesis and processing. We cultured control human islets under chronic high and low glucose concentrations. After the culture period, islets were pulse-labeled and chased for different periods of time. Proteins from islets were collected, insulin immunoprecipitated, and analyzed by alkaline-urea gel electrophoresis. We have found an accelerated rate of proinsulin conversion by those islets exposed to high glucose concentration (at 24.4 mM of glucose), but not by those islets cultured at low glucose concentration (at 5.5 mM of glucose). However, we do not observe any decrease or increase on newly proinsulin synthesis in any of these conditions.

Effect of glucose on production and release of proinsulin conversion products by cultured human islets

Isolated human islets were examined for the rates of conversion and release of newly formed (pro)insulin-like peptides. The rate of proinsulin (PI) conversion was 2-fold slower in human beta-cells (t(1/2) = 50 min) than in rat beta-cells (t(1/2) = 25 min). During the first hour following labeling of newly synthesized proteins, PI represented the main newly formed hormonal peptide in the medium; its release was stimulated 2-fold over the basal level by 20 mmol/L glucose. During the second hour, newly synthesized hormone was mainly released as insulin, with 10- to 20-fold higher rates at 20 mmol/L glucose. Prolonged preculture of the islets at 20 mmol/L glucose did not delay PI conversion, but markedly increased the release of newly formed PI, des(31,32)-PI, and insulin at both low and high glucose levels. Our data demonstrate that 1) the release of PI provides an extracellular index for the hormone biosynthetic activity of human beta-cells; 2) an acute rise in glucose exerts a stronger amplification of the release of converted hormone than in that of nonconverted hormone; and 3) prolonged exposure to high glucose levels results in an elevated basal release of converted and nonconverted PI; this elevation is not associated with a delay in PI conversion, but is attributed to the hyperactivated state of the human beta-cell population, which was recently found to be responsible for an elevation in basal rates of hormone synthesis. These in vitro observations on human beta-cells provide a possible explanation for the altered circulating (pro)insulin levels measured in nondiabetic and noninsulin-dependent diabetic subjects.