Defective prohormone processing and altered pancreatic islet morphology in mice lacking active SPC2

Significance of prohormone convertase 2, PC2, mediated initial cleavage at the proglucagon interdomain site, Lys70-Arg71, to generate glucagon

To define the biological significance of the initial cleavage at the proglucagon (PG) interdomain site, K70-R71 downward arrow, we created two interdomain mutants, K70Q-R71Q and R71A. Cotransfection studies in GH4C1 cells show significant amounts of glucagon production by PC2 along with some glicentin, glicentin-related polypeptide-glucagon (GRPP-glucagon) and oxyntomodulin from wild-type PG. In contrast, a larger peptide, PG 33-158, and low amounts of GRPP-glucagon are predominantly generated from interdomain mutants. HPLC analysis shows a 5-fold increase in glucagon production by PC2 from wild-type PG and a corresponding 4-fold lower accumulation and secretion of unprocessed precursor relative to interdomain mutants. PC2 generates significant levels of glucagon from a glicentin (PG 1-69) expression plasmid, whereas PC1/3 produces only modest amounts of oxyntomodulin. Employing a major PG fragment (PG 72-158) expression plasmid, we show that PC1/3 predominantly generates glucagon-like peptide (GLP)-1, whereas PC2 produces only N-terminally extended GLP-1. Surprisingly, production of GLP-1 and GLP-2 by PC1/3 from interdomain mutants, compared with wild-type PG, is not significantly impaired. In addition to PC2 and PC1/3, PC5/6A and furin are also able to cleave the sites, K70-R71 downward arrow and R107-X-R-R110 downward arrow in PG. We show a much greater ability of furin to cleave the monobasic site, R77 downward arrow, than at the dibasic site, R124-R125 downward arrow, which is also weakly processed by PC5/6A, indicating overlapping specificities of these two convertases mainly with PC1/3. We propose here a trimer-like model of the spatial organization of the hormonal sequences within the PG molecule in which the accessibility to prohormone convertase action of most cleavage sites is restricted with the exception of the interdomain site, K70-R71, which is maximally accessible.

Cathepsin L and Arg/Lys aminopeptidase: a distinct prohormone processing pathway for the biosynthesis of peptide neurotransmitters and hormones

Peptide neurotransmitters and hormones are synthesized as protein precursors that require proteolytic processing to generate smaller, biologically active peptides that are secreted to mediate neurotransmission and hormone actions. Neuropeptides within their precursors are typically flanked by pairs of basic residues, as well as by monobasic residues. In this review, evidence for secretory vesicle cathepsin L and Arg/Lys aminopeptidase as a distinct proteolytic pathway for processing the prohormone proenkephalin is presented. Cleavage of prohormone processing sites by secretory vesicle cathepsin L occurs at the NH2-terminal side of dibasic residues, as well as between the dibasic residues, resulting in peptide intermediates with Arg or Lys extensions at their NH2-termini. A subsequent Arg/Lys aminopeptidase step is then required to remove NH2-terminal basic residues to generate the final enkephalin neuropeptide. The cathepsin L and Arg/Lys aminopeptidase prohormone processing pathway is distinct from the proteolytic pathway mediated by the subtilisin-like prohormone convertases 1/3 and 2 (PC1/3 and PC2) with carboxypeptidase E/H. Differences in specific cleavage sites at paired basic residue sites distinguish these two pathways. These two proteolytic pathways demonstrate the increasing complexity of regulatory mechanisms for the production of peptide neurotransmitters and hormones.

Regulation of regional expression in rat brain PC2 by thyroid hormone/characterization of novel negative thyroid hormone response elements in the PC2 promoter

The prohormone convertases (PCs) PC1 and PC2 are involved in the tissue-specific endoproteolytic processing of neuropeptide precursors within the secretory pathway. We previously showed that changes in thyroid status altered pituitary PC2 mRNA and that this regulation was due to triiodothyronine-dependent interaction of the thyroid hormone receptor (TR) with negative thyroid hormone response elements (nTREs) contained in a large proximal region of the human PC2 promoter. In the current study, we examined the in vivo regulation of brain PC2 mRNA by thyroid status and found that 6-n-propyl-2-thiouracil-induced hypothyroidism stimulated, whereas thyroxine-induced hyperthyroidism suppressed, PC2 mRNA levels in the rat hypothalamus and cerebral cortex. To address the mechanism of T3 regulation of the PC2 gene, we used human PC2 (hPC2) promoter constructs transiently transfected into GH3 cells and found that triiodothyronine negatively and 9-cis-retinoic acid positively regulated hPC2 promoter activity. EMSAs, using purified TRalpha1 and retinoid X receptor-beta (RXRbeta) proteins demonstrated that TRalpha bound the distal putative nTRE-containing oligonucleotide in the PC2 promoter, and RXR bound to both nTRE-containing oligonucleotides. EMSAs with oligonucleotides containing deletion mutations of the nTREs demonstrated that the binding to TR and RXR separately is reduced, but specific binding to TR and RXR together persists even with deletion of each putative nTRE. We conclude that there are two novel TRE-like sequences in the hPC2 promoter and that these regions act in concert in a unique manner to facilitate the effects of thyroid hormone and 9-cis-retinoic acid on PC2.

Disruption of IFT results in both exocrine and endocrine abnormalities in the pancreas of Tg737(orpk) mutant mice

While relatively ignored for years as vestigial, cilia have recently become the focus of intense interest as organelles that result in severe pathologies when disrupted. Here, we further establish a connection between cilia dysfunction and disease by showing that loss of polaris (Tg737), an intraflagellar transport (IFT) protein required for ciliogenesis, causes abnormalities in the exocrine and endocrine pancreas of the Tg737(orpk) mouse. Pathology is evident late in gestation as dilatations of the pancreatic ducts that continue to expand postnatally. Shortly after birth, the acini become disorganized, undergo apoptosis, and are largely ablated in late stage pathology. In addition, serum amylase levels are elevated and carboxypeptidase is abnormally activated within the pancreas. Ultrastructural analysis reveals that the acini undergo extensive vacuolization and have numerous 'halo-granules' similar to that seen in induced models of pancreatitis resulting from duct obstruction. Intriguingly, although the acini are severely affected in Tg737(orpk) mutants, cilia and Tg737 expression are restricted to the ducts and islets and are not detected on acinar cells. Analysis of the endocrine pancreas in Tg737(orpk) mutants revealed normal differentiation and distribution of cell types in the islets. However, after fasting, mutant blood glucose levels are significantly lower than controls and when challenged in glucose tolerance tests, Tg737(orpk) mutants exhibited defects in glucose uptake. These findings are interesting in light of the recently proposed role for polaris, the protein encoded by the Tg737 gene, in the hedgehog pathway and hedgehog signaling in insulin production and glucose homeostasis.

Clinical endocrinology and metabolism. Glucagon-like peptide-1 and glucagon-like peptide-2

The glucagon-like peptides (glucagon-like peptide-1 (GLP-1) and glucagon-like peptide-2 (GLP-2)) are released from enteroendocrine cells in response to nutrient ingestion. GLP-1 enhances glucose-stimulated insulin secretion and inhibits glucagon secretion, gastric emptying and feeding. GLP-1 also has proliferative, neogenic and antiapoptotic effects on pancreatic beta-cells. More recent studies illustrate a potential protective role for GLP-1 in the cardiovascular and central nervous systems. GLP-2 is an intestinal trophic peptide that stimulates cell proliferation and inhibits apoptosis in the intestinal crypt compartment. GLP-2 also regulates intestinal glucose transport, food intake and gastric acid secretion and emptying, and improves intestinal barrier function. Thus, GLP-1 and GLP-2 exhibit a diverse array of metabolic, proliferative and cytoprotective actions with important clinical implications for the treatment of diabetes and gastrointestinal disease, respectively. This review will highlight our current understanding of the biology of GLP-1 and GLP-2, with an emphasis on both well-characterized and more novel therapeutic applications of these peptides.

Limited redundancy of the proprotein convertase furin in mouse liver

Furin is an endoprotease of the family of mammalian proprotein convertases and is involved in the activation of a large variety of regulatory proteins by cleavage at basic motifs. A large number of substrates have been attributed to furin on the basis of in vitro and ex vivo data. However, no physiological substrates have been confirmed directly in a mammalian model system, and early embryonic lethality of a furin knock-out mouse model has precluded in vivo verification of most candidate substrates. Here, we report the generation and characterization of an interferon inducible Mx-Cre/loxP furin knock-out mouse model. Induction resulted in near-complete ablation of the floxed fur exon in liver. In sharp contrast with the general furin knock-out mouse model, no obvious adverse effects were observed in the transgenic mice after induction. Histological analysis of the liver did not reveal any overt deviations from normal morphology. Analysis of candidate substrates in liver revealed complete redundancy for the processing of the insulin receptor. Variable degrees of redundancy were observed for the processing of albumin, alpha(5) integrin, lipoprotein receptor-related protein, vitronectin and alpha(1)-microglobulin/bikunin. None of the tested substrates displayed a complete block of processing. The absence of a severe phenotype raises the possibility of using furin as a local therapeutic target in the treatment of pathologies like cancer and viral infections, although the observed redundancy may require combination therapy or the development of a more broad spectrum convertase inhibitor.

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.

Altered biosynthesis of neuropeptide processing enzyme carboxypeptidase E after brain ischemia: molecular mechanism and implication

In this study, using both in vivo and in vitro ischemia models, the authors investigated the impact of brain ischemia on the biosynthesis of a key neuropeptide-processing enzyme, carboxypeptidase E (CPE). The response to brain ischemia of animals that lacked an active CPE was also examined. Combined in situ hybridization and immunocytochemical analyses for CPE showed reciprocal changes of CPE mRNA and protein, respectively, in the same cortical cells in rat brains after focal cerebral ischemia. Western blot analysis revealed an accumulation of the precursor protein of CPE in the ischemic cortex in vivo and in ischemic cortical neurons in vitro. Detailed metabolic labeling experiments on ischemic cortical neurons showed that ischemic stress caused a blockade in the proteolytic processing of CPE. When mice lacking an active CPE protease were subjected to a sublethal episode of focal cerebral ischemia, abundant TUNEL-positive cells were seen in the ischemic cortex whereas only a few were seen in the cortex of wild-type animals. These findings suggest that ischemia has an adverse impact on the neuropeptide-processing system in the brain and that the lack of an active neuropeptide-processing enzyme exacerbates ischemic brain injury.

Proopiomelanocortin and energy balance: insights from human and murine genetics

Proopiomelanocortin (POMC) undergoes extensive and tissue-specific posttranslational processing to yield a range of biologically active peptides. Historically, the most clearly defined roles of these peptides are in the control of adrenal steroidogenesis by corticotroph-derived ACTH and skin pigmentation by alphaMSH. However, a rapidly expanding body of work has established that POMC-derived peptides synthesized in neurons of the hypothalamus play a central role in the control of energy homeostasis. We review how inherited abnormalities in POMC synthesis and processing and defects in the action of POMC-derived peptides in both humans and mice have helped shape our current understanding of the importance of the melanocortin system in human energy balance.

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.

Manipulating the reversible aggregation of protein hormones in secretory granules: potential impact on biopharmaceutical development

Neuroendocrine cells and other secretory cell types are able to store secretory proteins in a concentrated form for extended periods until the release of large quantities of protein is triggered. The proteins are stored in dense core secretory granules. The dense cores of these granules are made up of large, insoluble aggregates that form by self-association. These aggregates solubilise rapidly into monomeric proteins in their native conformations when released from the cells by exocytosis of secretory granules. Formation of aggregates is an early event in secretory granule formation in at least some cell types. The function of secretory granules containing protein aggregates varies, depending upon the contents. This may occur because recognition of an aspect, such as a surface motif, of the aggregate facilitates correct assembly of the membrane proteins necessary for transport and exocytosis of the granules. Understanding the principles necessary for aggregation of protein hormones may help in the formulation of proteins for clinical use. Formation of aggregates of human prolactin has been investigated both in cells and in solution. In cells, the aggregation of human prolactin requires a mildly acidic pH, and is slowed in the presence of a membrane-permeable chelator of zinc. In solution, the aggregation of human prolactin at mildly acidic pH and physiological concentrations of Zn(2+) resembles that which occurs in cells if the reaction is performed with macromolecular crowding, which will mimic the conditions in cells. The factors causing protein aggregation and the extent to which aggregation plays a role in secretory granule formation are likely to vary with the protein and cell type. Further understanding of the principles involved in forming these aggregates that readily disassociate may enhance the ability to formulate protein preparations. Knowledge of the exact residues involved in the protein : protein interfaces in the aggregates of secretory granule proteins may lead to the ability to use small molecules to interfere with self-association and to regulate the storage of secretory granule proteins.

Characterization of MCH-gene-overprinted-polypeptide-immunoreactive material in hypothalamus reveals an inhibitory role of pro-somatostatin1-64 on somatostatin secretion

The melanin-concentrating hormone (MCH) gene encodes two proteins, pro-MCH and MCH-gene-overprinted polypeptide (MGOP), produced through alternative splicing of the primary transcript. Our initial purpose was to characterize the MGOP-immunoreactive material. First, MGOP mRNA was clearly found in rat and mouse hypothalami but Western blot analysis failed to unambiguously identify MGOP in protein extracts. Immunohistochemical experiments with wild-type and MCH gene-null mice demonstrated genuine expression of MGOP confined to the MCH-containing neurons in the lateral hypothalamus area and the presence of an 'MGOP-like' antigen in periventricular nucleus and arcuate nucleus neurons and their area of projection. This suggested a colocalization in somatostatin (SRIF) hypophysiotropic neurons. Further characterization, using SRIF gene-null mice and Western blot analysis with recombinant proteins, revealed that the MGOP-like product was pro-SRIF1-64. The role of pro-SRIF1-64 on fetal hypothalamic neurons was evaluated and a strong tonic inhibitory effect on SRIF secretion was found. These results (i) indicate that MGOP expression is restricted to the MCH neurons in the lateral hypothalamus and that MGOP-like immunoreactivity outside this system corresponds to pro-SRIF1-64, and (ii) provide the first evidence for a negative feedback regulation by pro-SRIF1-64 on SRIF secretion, suggesting new mechanisms by which the pro-region of a neuropeptide precursor may control the regulated secretion of a neuropeptide derived from the same precursor.

Deficits in reproduction and pro-gonadotropin-releasing hormone processing in male Cpefat mice

Cpe(fat/fat) mice are obese, diabetic, and infertile. These animals have a point mutation in carboxypeptidase E (CPE), an exopeptidase that removes C-terminal basic amino acids from peptide intermediates. The mutation renders the enzyme unstable, and it is rapidly degraded. Although the infertility of Cpe(fat/fat) mice has not been systematically investigated, it is thought to be due to a deficit in GnRH processing. We have evaluated this hypothesis and found hypothalamic GnRH levels to be reduced by 65-78% and concentrations of pro-GnRH and C-terminal-extended intermediates to be high. Basal serum gonadotropin contents are similar among wild-type, heterozygous, and homozygous mice. Testis morphology and function are abnormal in older obese Cpe(fat/fat) mice. Matings between homozygous mutants yield a 5% pregnancy rate. By comparison, when 50-d-old Cpe(fat/fat) males are paired with heterozygous females, rates increase to 43%, and they rapidly decrease to negligible levels by 120 d. As fertility declines without accompanying changes in the hypothalamic-pituitary-gonadal axis and before obesity is evident, reproduction is more complex than originally thought. This suspicion is confirmed in 90-d-old Cpe(fat/fat) males, who readily interact with females, but rarely mount and fail to show intromission or ejaculation behaviors. Together, these findings show that CPE is a key enzyme for pro-GnRH processing in vivo; however, the reproductive deficits in Cpe(fat/fat) males appear to be due primarily to abnormal sexual behavior.

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.

Genetic inactivation of prohormone convertase (PC1) causes a reduction in cholecystokinin (CCK) levels in the hippocampus, amygdala, pons and medulla in mouse brain that correlates with the degree of colocalization of PC1 and CCK mRNA in these structures

Prohormone convertase (PC1) is found in endocrine cell lines that express cholecystokinin (CCK) mRNA and process pro CCK to biologically active products. Other studies have demonstrated that PC1 may be a one of the enzymes responsible for the endoproteolytic cleavages that occur in pro CCK during its biosynthesis and processing. Prohormone convertase 1 (PC1) has a distribution that is similar to cholecystokinin (CCK) in rat brain. A moderate to high percentage of CCK mRNA-positive neurons express PC1 mRNA. CCK levels were measured in PC1 knockout and control mice to assess the degree to which loss of PC1 changed CCK content. CCK levels were decreased 62% in hippocampus, 53% in amygdala and 57% in pons-medulla in PC1 knockout mice as compared to controls. These results are highly correlated with the colocalization of CCK and PC1. The majority of CCK mRNA-positive neurons in the pyramidal cell layer of the hippocampus express PC1 mRNA and greater than 50% of CCK mRNA-positive neurons in several nuclei of the amygdala also express PC1. These results demonstrate that PC1 is important for CCK processing. PC2 and PC5 are also widely colocalized with CCK. It may be that PC2, PC5 or another non-PC enzyme are able to substitute for PC1 and sustain production of some amidated CCK. Together these enzymes may represent a redundant system to insure the production of CCK.

Gamma-MSH, sodium metabolism, and salt-sensitive hypertension

Alpha-, beta-, and gamma-melanocyte stimulating hormones (MSHs) are melanotropin peptides that are derived from the ACTH/beta-endorphin prohormone proopiomelanocortin (POMC). They have been highly conserved through evolutionary development, although their functions in mammals have remained obscure. The identification in the last decade of a family of five membrane-spanning melanocortin receptors (MC-Rs), for which the melanotropins are the natural ligands, has permitted the characterization of a number of important actions of these peptides, although the physiological function(s) of gamma-MSH have remained elusive. Much evidence indicates that gamma-MSH stimulates sympathetic outflow and raises blood pressure through a central mechanism. However, this review focuses on newer cardiovascular and renal actions of the peptide, acting in most cases through the MC3-R. In rodents, a high-sodium diet (HSD) increases the pituitary abundance of POMC mRNA and of gamma-MSH content and results in a doubling of plasma gamma-MSH concentration. The peptide is natriuretic and acts through renal MC3-Rs, which are also upregulated by the HSD. Thus the system appears designed to participate in the integrated response to dietary sodium excess. Genetic or pharmacologic induction of gamma-MSH deficiency results in marked salt-sensitive hypertension that is corrected by the administration of the peptide, probably through a central site of action. Deletion of the MC3-R also produces salt-sensitive hypertension, which, however, is not corrected by infusion of the hormone. These observations in aggregate suggest the operation of a hormonal system important in blood pressure control and in the regulation of sodium excretion. The relationship of these two actions to each other and the significance of this system in humans are important questions for future research.

The proprotein convertase PC2 is involved in the maturation of prosomatostatin to somatostatin-14 but not in the somatostatin deficit in Alzheimer's disease

A somatostatin deficit occurs in the cerebral cortex of Alzheimer's disease patients without a major loss in somatostatin-containing neurons. This deficit could be related to a reduction in the rate of proteolytic processing of peptide precursors. Since the two proprotein convertases (PC)1 and PC2 are responsible for the processing of neuropeptide precursors directed to the regulated secretory pathway, we examined whether they are involved first in the proteolytic processing of prosomatostatin in mouse and human brain and secondly in somatostatin defect associated with Alzheimer's disease. By size exclusion chromatography, the cleavage of prosomatostatin to somatostatin-14 is almost totally abolished in the cortex of PC2 null mice, while the proportions of prosomatostatin and somatostatin-28 are increased. By immunohistochemistry, PC1 and PC2 were localized in many neuronal elements in human frontal and temporal cortex. The convertases levels were quantified by Western blot, as well as the protein 7B2 which is required for the production of active PC2. No significant change in PC1 levels was observed in Alzheimer's disease. In contrast, a marked decrease in the ratio of the PC2 precursor to the total enzymatic pool was observed in the frontal cortex of Alzheimer patients. This decrease coincides with an increase in the binding protein 7B2. However, the content and enzymatic activity of the PC2 mature form were similar in Alzheimer patients and controls. Therefore, the cortical somatostatin defect is not due to convertase alteration occuring during Alzheimer's disease. Further studies will be needed to assess the mechanisms involved in somatostatin deficiency in Alzheimer's disease.

Peak stimulated insulin secretion is associated with specific changes in gene expression profiles in sporadic insulinomas

BACKGROUND

The molecular pathways that are responsible for pathologic insulin secretion by insulinomas have not been characterized. We studied gene expression profiles from insulinomas and determined associations between these changes and preoperative peak serum insulin levels.

METHODS

Ten patients with insulinomas underwent calcium-stimulated arteriography and surgical resection. Tumor RNA was isolated; corresponding complementary DNA was hybridized to 10K human complementary DNA arrays. Pooled human islet cell complementary DNA served as the control. Cluster analysis of gene expression and analysis of expression ratios was performed.

RESULTS

Nineteen genes were up-regulated at least 3-fold in insulinomas compared with controls, which included the genes for islet amyloid polypeptide and proprotein convertase type 2. Cluster analysis revealed 2 groups of patients with insulinoma and with distinct patterns of gene expression. Mean peak serum insulin values between groups were 196 and 1100 (U/mL (P<.05), which demonstrates a significant difference in insulin response to calcium stimulation between these 2 groups.

CONCLUSION

We show that genes that are relevant to the pathogenesis of hyperinsulinemia are expressed preferentially in insulinomas. In addition, patients with a distinct and common pattern of gene expression had significantly higher stimulated insulin secretion levels. The study of these genes may help to identify the biochemical pathways that are responsible for pathologic insulin secretion.

Distribution and colocalization of cholecystokinin with the prohormone convertase enzymes PC1, PC2, and PC5 in rat brain

During posttranslational processing to generate CCK 8, pro-cholecystokinin (CCK) undergoes endoproteolytic cleavage at three sites. Several studies using endocrine and neuronal tumor cells in culture and recombinant enzymes and synthetic substrates in vitro have pointed to the subtilisin/kexin-like enzymes prohormone convertase (PC) 1, PC2, and PC5 as potential candidates for these endoproteolytic cleavages. In these experimental models, they all appear to be able to cleave pro-CCK to make the correct products. One rodent model has provided information about the role of PC2. PC2 knockout mouse brains had less CCK 8 than wild-type, although a substantial amount of CCK was still present. The degree to which CCK levels were reduced in these mice was regionally specific. These data indicated that PC2 is important for normal production of CCK but that it is not the only endoprotease that is involved in CCK processing. To evaluate whether PC1 and PC5 are possible candidates for the other enzymes involved in CCK processing, the distribution of PC1, PC2, and PC5 mRNA was studied in rat brain. Their colocalization with CCK mRNA was examined using double-label in situ hybridization. PC2 was the most abundant of these enzymes in terms of the intensity and number of cells labeled. It was widely colocalized with CCK. PC1 and PC5 mRNA-positive cells were less abundant, but they were also widely distributed and strongly colocalized with CCK in the cerebral cortex, hippocampus, amygdala, ventral tegmental area, and substantia nigra zona compacta. The degree of colocalization of the enzymes with CCK was regionally specific. It is clear that PC1 and PC5 are extensively colocalized with CCK and could be participating in CCK processing in the rat brain and may be able to substitute for PC2 in its absence. These three enzymes may represent a redundant system to ensure production of biologically active CCK.

Role of beta-cell prohormone convertase (PC)1/3 in processing of pro-islet amyloid polypeptide

Islet amyloid polypeptide (IAPP) (amylin), the major component of islet amyloid, is produced by cleavage at the COOH- and NH(2)-termini of its precursor, proIAPP, likely by the beta-cell prohormone convertases (PC) 1/3 and PC2. Mice lacking PC2 can process proIAPP at its COOH- but not its NH(2)-terminal cleavage site, suggesting that PC1/3 is capable of initiating proIAPP cleavage at its COOH-terminus. To determine the precise role of PC1/3 in proIAPP processing, Western blot analysis was performed on islets isolated from mice lacking PC1/3 (PC1/3(-/-)). These islets contained not only fully processed IAPP as in PC1/3(+/+) islets, but also elevated levels of a COOH-terminally unprocessed intermediate form, suggesting impaired processing at the COOH-terminus. Next, GH3 cells that do not normally express proIAPP or detectable levels of PC1/3 or PC2 were cotransduced with adenoviruses expressing rat proIAPP and either PC2 or PC1/3. As expected, in GH3 cells transduced to express only proIAPP, no processing was observed. Coexpression of proIAPP and PC2 resulted in production of mature IAPP, whereas in cells that coexpressed proIAPP and PC1/3 only a 6-kDa intermediate was produced. We conclude that PC1/3 is important for processing of proIAPP at the COOH-terminus, but in its absence, PC2 can initiate complete processing of proIAPP to IAPP by cleaving the precursor at either its NH(2)- or COOH-terminal cleavage sites.

Small-intestinal dysfunction accompanies the complex endocrinopathy of human proprotein convertase 1 deficiency

We have previously described the only reported case of human proprotein convertase 1 (PC1) deficiency, in a female (Subject A) with obesity, hypogonadism, hypoadrenalism, and reactive hypoglycemia. We now report the second case of human PC1 deficiency (Subject B), also due to compound heterozygosity for novel missense and nonsense mutations. While both subjects shared the phenotypes of obesity, hypoadrenalism, reactive hypoglycemia, and elevated circulating levels of certain prohormones, the clinical presentation of Subject B was dominated by severe refractory neonatal diarrhea, malabsorptive in type. Subsequent investigation of Subject A revealed marked small-intestinal absorptive dysfunction, which was not previously clinically suspected. We postulate that PC1, presumably in the enteroendocrine cells, is essential for the normal absorptive function of the human small intestine. The differences in the nature and severity of presentation between the two cases cannot readily be explained on the basis of allelic heterogeneity, as the nonsense and missense mutations from both subjects had comparably severe effects on the catalytic activity of PC1. Despite Subject A's negligible PC1 activity, some mature ACTH and glucagon-like peptide 17-36(amide) were detectable in her plasma, suggesting that the production of these hormones, at least in humans, does not have an absolute dependence on PC1. The presence of severe obesity and the absence of growth retardation in both subjects contrast markedly with the phenotype of mice lacking PC1 and suggest that the precise physiological repertoire of this enzyme may vary between mammalian species.

Contrasting patterns of expression of transcription factors in pancreatic alpha and beta cells

Pancreatic alpha and beta cells are derived from the same progenitors but play opposing roles in the control of glucose homeostasis. Disturbances in their function are associated with diabetes mellitus. To identify many of the proteins that define their unique pathways of differentiation and functional features, we have analyzed patterns of gene expression in alphaTC1.6 vs. MIN6 cell lines by using oligonucleotide microarrays. Approximately 9-10% of >11,000 transcripts examined showed significant differences between the two cell types. Of >700 known transcripts enriched in either cell type, transcription factors and their regulators (TFR) was one of the most significantly different categories. Ninety-six members of the basic zipper, basic helix-loop-helix, homeodomain, zinc finger, high mobility group, and other transcription factor families were enriched in alpha cells; in contrast, homeodomain proteins accounted for 51% of a total of 45 TFRs enriched in beta cells. Our analysis thus highlights fundamental differences in expression of TFR subtypes within these functionally distinct islet cell types. Interestingly, the alpha cells appear to express a large proportion of factors associated with progenitor or stem-type cells, perhaps reflecting their earlier appearance during pancreatic development. The implications of these findings for a better understanding of alpha and beta cell dysfunction in diabetes mellitus are also considered.

Abrogation of protein convertase 2 activity results in delayed islet cell differentiation and maturation, increased alpha-cell proliferation, and islet neogenesis

To date, the role of pancreatic hormones in pancreatic islet growth and differentiation is poorly understood. To address this issue, we examined mice with a disruption in the gene encoding prohormone convertase 2 (PC2). These mice are unable to process proglucagon, prosomatostatin, and other neuroendocrine precursors into mature hormones. Initiation of insulin (IN) expression during development was delayed in PC2 mutant mice. Cells containing IN were first detected in knockout embryos on d 15 of development, 5 d later than in wild-type littermates. However, the IN(+) cells of d 15 PC2 mutant mice coexpressed glucagon, as did the first appearing beta-cells of controls. In addition, lack of PC2 perturbed the pattern of expression of transcription factors presumed to be involved in the determination of the mature alpha-cell phenotype. Thus, in contrast to controls, alpha-cells of mutant mice had protracted expression of Nkx 6.1 and Pdx-1, but did not express Brn-4. Islets of adult mutant mice also contained cells coexpressing insulin and somatostatin, an immature cell type found only in islets of the wild-type strain during development. In addition to the effects on islet cell differentiation, the absence of PC2 activity resulted in a 3-fold increase in the rate of proliferation of proglucagon cells during the perinatal period. This increase contributed to the development of alpha-cell hyperplasia during postnatal life. Furthermore, the total beta-cell volume was increased 2-fold in adult mutants compared with controls. This increase was due to islet neogenesis, as the number of islets per section was significantly higher in knockout mice compared with wild-type mice, whereas both strains had similar rates of IN cell proliferation. These results indicate that hormones processed by PC2 affected processes that regulate islet cell differentiation and maturation in embryos and adults.

Regulation of proglucagon transcription by activated transcription factor (ATF) 3 and a novel isoform, ATF3b, through the cAMP-response element/ATF site of the proglucagon gene promoter

Glucagon, the second major glucose-regulated hormone in the control of glucose homeostasis, functions as a counter-regulator to insulin and is specifically produced by the pancreatic alpha cells. Its excessive biosynthesis and secretion is associated with diabetes mellitus. The expression of the proglucagon gene has been demonstrated to be regulated by a cAMP-dependent pathway through cAMP-response element-binding protein (CREB) and possibly other transcription factors bound to its cAMP-response element (CRE)/activated transcription factor (ATF) site. Elsewhere we have shown that ATF3, a member of the ATF/CREB subfamily of the basic leucine zipper domain proteins, is expressed predominantly in the alpha cells of the pancreatic islets. In our attempts to further dissect the role of ATF3 proteins in alpha cells, we have identified and characterized a novel alternatively spliced form, ATF3b, and have compared the specific binding ability of ATF3 and ATF3b on the CRE/ATF motif of the proglucagon promoter. Our findings indicate the existence of a novel mechanism by which the transcription of the proglucagon gene is regulated in response to cAMP signals, in addition to CREB and in relation to glucose fluctuations in pancreatic alpha cells.

Obliteration of alpha-melanocyte-stimulating hormone derived from POMC in pituitary and brains of PC2-deficient mice

Alpha-melanocyte-stimulating hormone (alpha-MSH) is a neuropeptide expressed in pituitary and brain that is known to regulate energy balance, appetite control, and neuroimmune functions. The biosynthesis of alpha-MSH requires proteolytic processing of the proopiomelanocortin (POMC) precursor. Therefore, this study investigated the in vivo role of the prohormone convertase 2 (PC2) processing enzyme for production of alpha-MSH in PC2-deficient mice. Specific detection of alpha-MSH utilized radioimmunoassay (RIA) that does not crossreact with the POMC precursor, and which does not crossreact with other adrenocorticotropin hormone (ACTH) and beta-endorphin peptide products derived from POMC. alpha-MSH in PC2-deficient mice was essentially obliterated in pituitary, hypothalamus, cortex, and other brain regions (collectively), compared to wild-type controls. These results demonstrate the critical requirement of PC2 for the production of alpha-MSH. The absence of alpha-MSH was accompanied by accumulation of ACTH, ACTH-containing imtermediates, and POMC precursor. ACTH was increased in pituitary and hypothalamus of PC2-deficient mice, evaluated by RIA and reversed-phase high pressure liquid chromatography (RP-HPLC). Accumulation of ACTH demonstrates its role as a PC2 substrate that can be converted for alpha-MSH production. Further analyses of POMC-derived intermediates in pituitary, conducted by denaturing western blot conditions, showed accumulation of ACTH-containing intermediates in pituitaries of PC2-deficient mice, which implicate participation of such intermediates as PC2 substrates. Moreover, accumulation of POMC was observed in PC2-deficient mice by western blots with anti-ACTH and anti-beta-endorphin. In addition, increased beta-endorphin1-31 was observed in pituitary and hypothalamus of PC2-deficient mice, suggesting beta-endorphin1-31 as a substrate for PC2 in these tissues. Overall, these studies demonstrated that the PC2 processing enzyme is critical for the in vivo production of alpha-MSH in pituitary and brain.

Curbing activation: proprotein convertases in homeostasis and pathology

The proprotein convertases (PCs) are a seven-member family of endoproteases that activate proproteins by cleavage at basic motifs. Expression patterns for individual PCs vary widely, and all cells express several members. The list of substrates activated by PCs has grown to include neuropeptides, peptide hormones, growth and differentiation factors, receptors, enzymes, adhesion molecules, blood coagulation factors, plasma proteins, viral coat proteins, and bacterial toxins. It has become clear that the PC family plays a crucial role in a variety of physiological processes and is involved in the pathology of diseases such as cancer, viral infection, and Alzheimer's disease. Recent studies using PC inhibitors have demonstrated their potential as therapeutic targets. Despite the avalanche of in vitro data, the physiological role of individual PCs has remained largely elusive. Recently, however, knockout mouse models have been developed for furin, PC1, PC2, PC4, PC6B, LPC, and PACE4, and human patients with PC1 deficiency have been identified. The phenotypes range from undetectable to early embryonic lethality. The major lesson learned from these studies is that specific PC-substrate pairs do exist, but that there is substantial redundancy for the majority of substrates. To some extent, redundancy may be cell type and even species dependent.

Selective roles for the PC2 processing enzyme in the regulation of peptide neurotransmitter levels in brain and peripheral neuroendocrine tissues of PC2 deficient mice

The prohormone convertase 2 (PC2) is hypothesized to convert multiple pro-neuropeptides into active peptides that function as neurotransmitters. To examine the in vivo role of PC2 in neuropeptide production, the tissue contents of six different neuropeptides in brain and peripheral nervous tissues were examined in PC2 deficient mice. Specific neuropeptide radioimmunoassays and RP-HPLC (reverse-phase HPLC) provided evaluation of processed, active neuropeptides in brain and neuroendocrine tissues of PC2 deficient mice. Results demonstrated three features with regard to the selective roles of PC2 in determining the production of NPY, somatostatin-28, enkephalin, VIP, galanin, and CRF in neuroendocrine tissues. Firstly, PC2 deficient mice showed changes in several neuropeptides, but not all neuropeptides examined. The absence of active PC2 resulted in altered cellular levels of NPY, somatostatin-28, and (Met)enkephalin; few changes in VIP or galanin occurred in the tissues examined. CRF content was not altered in brains of PC2 deficient mice. Secondly, comparison of a single neuropeptide among different tissues of PC2 deficient mice demonstrated tissue-selective roles for PC2 in production of the neuropeptide. For example, NPY levels were decreased in ileum of PC2 deficient mice, but NPY content was not altered in hypothalamus that is abundant in NPY. In addition, (Met)enkephalin levels in hypothalamus and cortex were decreased in PC2 deficient mice, but no changes were observed in adrenal or intestine. Thirdly, a single tissue region often showed selective alterations among different neuropeptides. For example, the neuropeptide-rich hypothalamus region showed decreased (Met)enkephalin in PC2 deficient mice, but NPY, VIP, galanin, and CRF were not altered. These results demonstrate the selective role of PC2 in neuropeptide production that provides active peptide neurotransmitter or hormones for biological functions in brain and neuroendocrine systems.

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.

The EGL-21 carboxypeptidase E facilitates acetylcholine release at Caenorhabditis elegans neuromuscular junctions

Proneuropeptides are packaged into dense-core vesicles in which they are processed into active peptides by copackaged enzymes. Proprotein convertases (PCs) cleave precursors after dibasic residues, and carboxypeptidases remove basic residues from the C terminals. We show here that the Caenorhabditis elegans egl-21 gene encodes a protein that is very similar to carboxypeptidase E (CPE) and is broadly expressed in the nervous system. Mutants lacking either egl-21 CPE or egl-3, which encodes the C. elegans ortholog of PC type 2 (PC2), were defective for processing endogenously expressed FMRFamide (Phe-Met-Arg-Phe-NH2)-related peptides (FaRPs). Mutants lacking the unc-104 kinesin motor protein were defective for anterograde movement of dense-core vesicle components, including egl-3 PC2, egl-21 CPE, and FaRPs. We provide evidence that egl-3 PC2 and egl-21 CPE mutants have diminished acetylcholine release at neuromuscular junctions (NMJs). Taken together, these results suggest that egl-21 CPE and egl-3 PC2 process endogenous neuropeptides that facilitate acetylcholine release at C. elegans NMJs.

Glucagon and regulation of glucose metabolism

As a counterregulatory hormone for insulin, glucagon plays a critical role in maintaining glucose homeostasis in vivo in both animals and humans. To increase blood glucose, glucagon promotes hepatic glucose output by increasing glycogenolysis and gluconeogenesis and by decreasing glycogenesis and glycolysis in a concerted fashion via multiple mechanisms. Compared with healthy subjects, diabetic patients and animals have abnormal secretion of not only insulin but also glucagon. Hyperglucagonemia and altered insulin-to-glucagon ratios play important roles in initiating and maintaining pathological hyperglycemic states. Not surprisingly, glucagon and glucagon receptor have been pursued extensively in recent years as potential targets for the therapeutic treatment of diabetes.

Genetic disruption of gamma-melanocyte-stimulating hormone signaling leads to salt-sensitive hypertension in the mouse

The gamma-melanocyte-stimulating hormone (gamma-MSH) is a natriuretic peptide derived from the N-terminal region of proopiomelanocortin (POMC). Evidence suggests that it may be part of the coordinated response to a low-sodium diet (LSD). We tested the effect of the HSD (8% NaCl) compared with LSD (0.07%) on mean arterial pressure (MAP) in mice with targeted disruption of the PC2 gene (PC2(-/-)), necessary for processing of POMC into gamma-MSH, or the melanocortin receptor 3 gene (Mc3r(-/-); the receptor for MSH). In wild-type mice, HSD for 1 week did not alter MAP versus LSD mice, but plasma gamma-MSH immunoreactivity was more than double the LSD value. In contrast, in PC2(-/-) mice, MAP on the LSD was not greater than in wild-type mice, but plasma gamma-MSH was reduced to one-seventh the wild-type value. On the HSD, MAP rose to a markedly hypertensive level while plasma gamma-MSH concentration remained severely depressed. Intravenous infusion of gamma-MSH (0.2 pmol/min) for 30 min to PC2(-/-) mice after 1 week of HSD lowered MAP from hypertensive levels to normal; infusion of alpha-MSH at the same rate had no effect. Injection of 60 fmol of gamma-MSH into the lateral cerebral ventricle of hypertensive mice also lowered MAP to normal. Administration of a stable analogue of gamma-MSH intra-abdominally by microosmotic pump to PC2(-/-) mice prevented the development of hypertension when ingesting the HSD. In mice with targeted disruption of the Mc3r gene, the HSD also led to marked hypertension accompanied by elevated plasma levels of gamma-MSH; infusion of exogenous gamma-MSH to these mice had no effect on MAP. These results strongly suggest that PC2-dependent processing of POMC into gamma-MSH is necessary for the normal response to the HSD. gamma-MSH deficiency results in marked salt-sensitive hypertension that is rapidly improved with exogenous gamma-MSH through a central site of action. alpha-MSH infused at the same rate had no effect on MAP, indicating that the hypertension is a specific consequence of impaired POMC processing into gamma-MSH. Absence of Mc3r produces gamma-MSH resistance and hypertension on the HSD. These findings demonstrate a novel pathway mediating salt-sensitivity of blood pressure.

Proteolytic processing of chromogranin A by the prohormone convertase PC2

The neuroendocrine secretory protein chromogranin A (CgA) is a precursor for various biologically active peptides. Several single and paired basic residues are present within its primary amino acid sequence comprising cleavage sites for prohormone convertases. In this study, SH-SY5Y human neuroblastoma cells were stably transfected with the prohormone convertase PC2 to analyse the proteolytic processing of endogenous chromogranin A and, in particular, the formation of the chromogranin-A-derived peptide GE-25. Our analyses revealed a significant change in the pattern of proteolytic conversion of chromogranin A in cells expressing PC2. Mock-transfected control cells contained mainly the intact chromogranin A molecule and hardly any shorter products were found. On the other hand, PC2-transfected cells showed extensive processing of chromogranin A, resulting in significantly lower amounts of the intact precursor and especially high levels of the free peptide GE-25.

The EGL-21 carboxypeptidase E facilitates acetylcholine release at Caenorhabditis elegans neuromuscular junctions

Proneuropeptides are packaged into dense-core vesicles in which they are processed into active peptides by copackaged enzymes. Proprotein convertases (PCs) cleave precursors after dibasic residues, and carboxypeptidases remove basic residues from the C terminals. We show here that the Caenorhabditis elegans egl-21 gene encodes a protein that is very similar to carboxypeptidase E (CPE) and is broadly expressed in the nervous system. Mutants lacking either egl-21 CPE or egl-3, which encodes the C. elegans ortholog of PC type 2 (PC2), were defective for processing endogenously expressed FMRFamide (Phe-Met-Arg-Phe-NH2)-related peptides (FaRPs). Mutants lacking the unc-104 kinesin motor protein were defective for anterograde movement of dense-core vesicle components, including egl-3 PC2, egl-21 CPE, and FaRPs. We provide evidence that egl-3 PC2 and egl-21 CPE mutants have diminished acetylcholine release at neuromuscular junctions (NMJs). Taken together, these results suggest that egl-21 CPE and egl-3 PC2 process endogenous neuropeptides that facilitate acetylcholine release at C. elegans NMJs.

Proopiomelanocortin-derived peptides in rat cerebrospinal fluid and hypothalamic extracts: evidence that secretion is regulated with respect to energy balance

Regulation of proopiomelanocortin (POMC) is an important means of controlling the central melanocortin system. It has never been established whether the spectrum of POMC-derived peptides synthesized and secreted from the hypothalamus is altered in response to changes in energy homeostasis in vivo. To monitor secretion, we analyzed peptide content of rat cerebrospinal fluid. Strikingly, both the POMC precursor and ACTH were readily detected. Moreover, levels of both were lower in samples from obese Zucker rats (fa/fa) vs. lean Zucker rats (+/+, fa/+) and from fasted vs. fed rats, whereas alpha MSH could not be detected. POMC levels were also decreased in hypothalamic extracts from obese and fasted animals. In contrast, despite being the most predominant peptide in extracts, alpha MSH levels were not significantly changed in any of the rat models. The ratio of precursor to derived peptides in cerebrospinal fluid was significantly higher in obese vs. lean and fed vs. fasted rats, indicating that secretion of POMC-derived peptides is differentially down-regulated during negative energy balance. In contrast to peptide analysis, we found that POMC gene expression was not significantly decreased in fasted rat hypothalami. We conclude that regulation of peptide secretion is an important mechanism by which the POMC system is controlled.

Expression, purification, and PC1-mediated processing of human proglucagon, glicentin, and major proglucagon fragment

To examine the cleavage specificity of different members of the furin/propeptide convertase (PC) family of enzymes, we have selected proglucagon (PG) as a model substrate. PG was selected because it is subject to differential processing in vivo. PG is thought to be cleaved initially at an interdomain site to produce glicentin and the major proglucagon fragment (MPGF). These intermediates are subsequently cleaved, most likely by the convertases PC2 and PC1, respectively. To determine the exact sites within PG that are cleaved by PC1 and PC2, we attempted to produce milligram quantities of human PG, glicentin, and MPGF for use in an in vitro conversion assay. A methionine residue was added to the N-terminus of each protein to initiate translation. Purification was achieved using cation exchange and reversed-phase chromatography, and the integrity of the methionylated proteins was confirmed by both electrospray ionization-mass spectrometry and amino acid analysis. The combined expression and purification scheme is fast, efficient, and results in milligram quantities of > or =95% pure proglucagon, > or =95% pure MPGF, and > or =93% pure glicentin. These prohormones are cleaved by PC1 to produce product peptides consistent with the processing of PG observed in vivo, and should therefore be suitable for further analysis of the post-translational processing of PG.

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.

Lower blood glucose, hyperglucagonemia, and pancreatic alpha cell hyperplasia in glucagon receptor knockout mice

Glucagon, the counter-regulatory hormone to insulin, is secreted from pancreatic alpha cells in response to low blood glucose. To examine the role of glucagon in glucose homeostasis, mice were generated with a null mutation of the glucagon receptor (Gcgr(-/-)). These mice display lower blood glucose levels throughout the day and improved glucose tolerance but similar insulin levels compared with control animals. Gcgr(-/-) mice displayed supraphysiological glucagon levels associated with postnatal enlargement of the pancreas and hyperplasia of islets due predominantly to alpha cell, and to a lesser extent, delta cell proliferation. In addition, increased proglucagon expression and processing resulted in increased pancreatic glucogen-like peptide 1 (GLP-1) (1-37) and GLP-1 amide (1-36 amide) content and a 3- to 10-fold increase in circulating GLP-1 amide. Gcgr(-/-) mice also displayed reduced adiposity and leptin levels but normal body weight, food intake, and energy expenditure. These data indicate that glucagon is essential for maintenance of normal glycemia and postnatal regulation of islet and alpha and delta cell numbers. Furthermore, the lean phenotype of Gcgr(-/-) mice suggests glucagon action may be involved in the regulation of whole body composition.

Biosynthesis and processing of pro CCK: recent progress and future challenges

Pro Cholecystokinin (CCK) like other prohormones that pass through the regulated secretory pathway, undergoes a number of post-translational modifications during its biosynthesis including tyrosine sulfation, endoproteolytic cleavages, trimming by carboxypeptidase and c-terminal amidation. This minireview summarizes what is known about this process, what specific enzymes are involved in endocrine and neuronal tumor cells and in mutant and knockout mouse strains. It also points out the major challenges that remain for future research.

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.

Increased synthesis but decreased processing of neuronal proCCK in prohormone convertase 2 and 7B2 knockout animals

In addition to its role as a gut hormone, cholecystokinin (CCK) is a widespread and potent neurotransmitter. Its biosynthesis requires endoproteolytic cleavage of proCCK at several mono- and dibasic sites by subtilisin-like prohormone convertases (PCs). Of these, PC1 and PC2 are specific for neuroendocrine cells. We have now examined the role of PC2 and its binding protein, 7B2, in the neuronal processing of proCCK by measurement of precursor, processing-intermediates and bioactive end-products in brain extracts from PC2- and 7B2-null mice and from corresponding controls. PC2-null mice displayed a nine-fold increase of cerebral proCCK concentrations, and a two-fold increase in the concentrations of the processing-intermediate, glycine-extended CCK, whereas the concentrations of transmitter-active (i.e. alpha-amidated and O-sulfated) CCK peptides were reduced (61%). Chromatography showed that O-sulfated CCK-8 still is the predominant transmitter-active CCK in PC2-null brains, but that the fraction of intermediate-sized CCK-peptides (CCK-58, -33 and -22) was eight-fold increased. 7B2-null brains displayed a similar pattern but with less pronounced precursor accumulation. In contrast with the cerebral changes, PC2 deficiency was without effect on proCCK synthesis and processing in intestinal endocrine cells, whereas 7B2 deficiency halved the concentration of bioactive CCK in the intestine. The results show that PC2 plays a major neuron-specific role in the processing of proCCK.

Hunting for a hypoglycemia gene: severe neonatal hypoglycemia in a consanguineous family

Hypoglycemia is a dreaded complication in diabetes mellitus patients treated with insulin, but is also a symptom that is observed in many disorders. In some metabolic diseases of early infancy, low blood glucose is the major presentation and the condition can become life-threatening. Such cases are often attributed to inherited hyperinsulinism. Vidnes and Øyasaeter [1977: Pediatr Res 11:943-949] described a son of consanguineous Pakistani parents with severe neonatal hypoglycemia and concluded that the patient probably suffered from an isolated glucagon deficiency. We have continued the investigation of this family, which now includes a hypoglycemic daughter and two healthy children. The original diagnosis is questioned because the second case of hypoglycemia can be explained by hyperinsulinism. We proceeded with microsatellite marker analysis for selected candidate genes under the assumption that the condition is autosomal recessive and that affected children are homozygous for a mutated allele. The four known genetic causes for inborn hyperinsulinism (mutations in the genes ABCC8, KCNJ11, GLUD1, and GCK) were excluded. Furthermore, we eliminated 13 candidate genes coding for transcription factors involved in pancreas development and differentiation. The analysis was also negative for the genes encoding insulin and glucagon, their receptors, and processing enzymes. The identification of a novel gene for persistent neonatal hypoglycemia can be expected to yield fundamental information about glucose homeostasis, and will therefore have implications for the understanding of diabetes as well.

Furin at the cutting edge: from protein traffic to embryogenesis and disease

Furin catalyses a simple biochemical reaction--the proteolytic maturation of proprotein substrates in the secretory pathway. But the simplicity of this reaction belies furin's broad and important roles in homeostasis, as well as in diseases ranging from Alzheimer's disease and cancer to anthrax and Ebola fever. This review summarizes various features of furin--its structural and enzymatic properties, intracellular localization, trafficking, substrates, and roles in vivo.

Severe block in processing of proinsulin to insulin accompanied by elevation of des-64,65 proinsulin intermediates in islets of mice lacking prohormone convertase 1/3

The neuroendocrine processing endoproteases PC2 and PC1/3 are expressed in the beta cells of the islets of Langerhans and participate in the processing of proinsulin to insulin and C-peptide. We have previously shown that disruption of PC2 (SPC2) expression significantly impairs proinsulin processing. Here we report that disruption of the expression of PC1/3 (SPC3) produces a much more severe block in proinsulin conversion. In nulls, pancreatic and circulating proinsulin-like components comprise 87% and 91%, respectively, of total insulin-related immunoreactivity. Heterozygotes also show a more than 2-fold elevation in proinsulin levels to approximately 12%. Immunocytochemical and ultrastructural studies of the beta cells reveal the nearly complete absence of mature insulin immunoreactivity and its replacement by that of proinsulin in abundant immature-appearing secretory granules. In contrast, alpha cell morphology and glucagon processing are normal, and there is also no defect in somatostatin-14 generation. Pulse-chase labeling studies confirm the existence of a major block in proinsulin processing in PC1/3 nulls with prolongation of half-times of conversion by 7- and 10-fold for proinsulins I and II, respectively. Lack of PC1/3 also results in increased levels of des-64,65 proinsulin intermediates generated by PC2, in contrast to PC2 nulls, in which des- 31,32 proinsulin intermediates predominate. These results confirm that PC1/3 plays a major role in processing proinsulin, but that its coordinated action with PC2 is necessary for the most efficient and complete processing of this prohormone.

Disruption of PC1/3 expression in mice causes dwarfism and multiple neuroendocrine peptide processing defects

The subtilisin-like proprotein convertases PC1/3 (SPC3) and PC2 (SPC2) are believed to be the major endoproteolytic processing enzymes of the regulated secretory pathway. They are expressed together or separately in neuroendocrine cells throughout the brain and dispersed endocrine system in both vertebrates and invertebrates. Disruption of the gene-encoding mouse PC1/3 has now been accomplished and results in a syndrome of severe postnatal growth impairment and multiple defects in processing many hormone precursors, including hypothalamic growth hormone-releasing hormone (GHRH), pituitary proopiomelanocortin to adrenocorticotropic hormone, islet proinsulin to insulin and intestinal proglucagon to glucagon-like peptide-1 and -2. Mice lacking PC1/3 are normal at birth, but fail to grow normally and are about 60% of normal size at 10 weeks. They lack mature GHRH, have low pituitary growth hormone (GH) and hepatic insulin-like growth factor-1 mRNA levels and resemble phenotypically the "little" mouse (Gaylinn, B. D., Dealmeida, V. I., Lyons, C. E., Jr., Wu, K. C., Mayo, K. E. & Thorner, M. O. (1999) Endocrinology 140, 5066-5074) that has a mutant GHRH receptor. Despite a severe defect in pituitary proopiomelanocortin processing to mature adrenocorticotropic hormone, blood corticosterone levels are essentially normal. There is marked hyperproinsulinemia but without impairment of glucose tolerance. In contrast, PC2-null mice lack mature glucagon and are chronically hypoglycemic (Furuta, M., Yano, H., Zhou, A., Rouille, Y., Holst, J., Carroll, R., Ravazzola, M., Orci, L., Furuta, H. & Steiner, D. (1997) Proc. Natl. Acad. Sci. USA 94, 6646-6651). The PC1/3-null mice differ from a human subject reported with compound heterozygosity for defects in this gene, who was of normal stature but markedly obese from early life. The PC1/3-null mice are not obese. The basis for these phenotypic differences is an interesting topic for further study. These findings prove the importance of PC1/3 as a key neuroendocrine convertase.