The N221D variant in PCSK1 is highly prevalent in childhood obesity and can influence the metabolic profile

OBJECTIVES

To study the prevalence and influence on metabolic profile of the prohormone-convertase-1 (PCSK1) N221D variant in childhood obesity, proven its role in the leptin-melanocortin signaling pathway as in proinsulin and other prohormone cleavage.

METHODS

Transversal study of 1066 children with obesity (mean age and BMI Z-score 10.38 ± 3.44 years and +4.38 ± 1.77, respectively), 51.4 % males, 54.4 % prepubertal, 71.5 % Caucasians and 20.8 % Latinos. Anthropometric and metabolic features were compared between patients carrying the N221D variant in PCSK1 and patients with no variants found after next generation sequencing analysis of 17 genes (CREBBP, CPE, HTR2C, KSR2, LEP, LEPR, MAGEL2, MC3R, MC4R, MRAP2, NCOA1, PCSK1, POMC, SH2B1, SIM1, TBX3 and TUB) involved in the leptin-melanocortin pathway.

RESULTS

No variants were found in 531 patients (49.8 %), while 68 patients carried the PCSK1 N221D variant (42 isolately, and 26 with at least one additional gene variant). Its prevalence was higher in Caucasians vs. Latinos (χ2 7.81; p<0.01). Patients carrying exclusively the PCSK1 N221D variant (n=42) showed lower insulinemia (p<0.05), HOMA index (p<0.05) and area under the curve for insulin in the oral glucose tolerance test (p<0.001) and higher WBISI (p<0.05) than patients with no variants, despite similar obesity severity, age, sex and ethnic distribution.

CONCLUSIONS

The N221D variant in PCSK1 is highly prevalent in childhood obesity, influenced by ethnicity. Indirect estimation of insulin resistance, based on insulinemia could be byassed in these patients and underestimate their type 2 diabetes mellitus risk.

Prader-Willi Syndrome and PCSK1 mutation: a novel presentation of combined syndromic and monogenic obesity

OBJECTIVE

Prader-Willi syndrome (PWS) is a genomic imprinting disorder predominantly caused by the absence of paternally expressed imprinted genes at chromosome 15q11.2-q13. The PCSK1 gene is vital for the processing of hypothalamic POMC to ACTH and α-MSH, leading to food intake suppression and increased energy expenditure. The aim of this study was to investigate whether our PWS patient had a defect in genes involved in the hypothalamic melanocortin-4 receptor (MC4R) pathway.

PATIENTS AND METHODS

A 27-year-old Greek man with PWS presented to the Adult Endocrine Clinic with morbid obesity and hyperphagia. He also had obstructive sleep apnea, growth hormone deficiency, gonadal failure and metabolic disturbances. At 6 years of age, chromosomal testing confirmed PWS with a deletion in the q11q13 region of the long arm of paternal chromosome 15.

RESULTS

At the age of 27 years, further genetic testing was conducted, and next generation sequencing revealed a PCSK1_pN221D_HET mutation which was confirmed by Sanger sequencing.

CONCLUSIONS

Our findings suggest that different genetic abnormalities may be present in an individual with PWS and that patients with PWS may need to be investigated for PCSK1 mutations, as the finding may potentially offer a novel treatment perspective for them.

Mof acetyltransferase inhibition ameliorates glucose intolerance and islet dysfunction of type 2 diabetes via targeting pancreatic α-cells

BACKGROUND

Previously, we reported that Mof was highly expressed in α-cells, and its knockdown led to ameliorated fasting blood glucose (FBG) and glucose tolerance in non-diabetic mice, attributed by reduced total α-cell but enhanced prohormone convertase (PC)1/3-positive α-cell mass. However, how Mof and histone 4 lysine 16 acetylation (H4K16ac) control α-cell and whether Mof inhibition improves glucose handling in type 2 diabetes (T2DM) mice remain unknown.

METHODS

Mof overexpression and chromatin immunoprecipitation sequence (ChIP-seq) based on H4K16ac were applied to determine the effect of Mof on α-cell transcriptional factors and underlying mechanism. Then we administrated mg149 to α-TC1-6 cell line, wild type, db/db and diet-induced obesity (DIO) mice to observe the impact of Mof inhibition in vitro and in vivo. In vitro, western blotting and TUNEL staining were used to examine α-cell apoptosis and function. In vivo, glucose tolerance, hormone levels, islet population, α-cell ratio and the co-staining of glucagon and PC1/3 or PC2 were examined.

RESULTS

Mof activated α-cell-specific transcriptional network. ChIP-seq results indicated that H4K16ac targeted essential genes regulating α-cell differentiation and function. Mof activity inhibition in vitro caused impaired α-cell function and enhanced apoptosis. In vivo, it contributed to ameliorated glucose intolerance and islet dysfunction, characterized by decreased fasting glucagon and elevated post-challenge insulin levels in T2DM mice.

CONCLUSION

Mof regulates α-cell differentiation and function via acetylating H4K16ac and H4K16ac binding to Pax6 and Foxa2 promoters. Mof inhibition may be a potential interventional target for T2DM, which led to decreased α-cell ratio but increased PC1/3-positive α-cells.

GLP-1 receptor signaling increases PCSK1 and β cell features in human α cells

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that potentiates glucose-stimulated insulin secretion. GLP-1 is classically produced by gut L cells; however, under certain circumstances α cells can express the prohormone convertase required for proglucagon processing to GLP-1, prohormone convertase 1/3 (PC1/3), and can produce GLP-1. However, the mechanisms through which this occurs are poorly defined. Understanding the mechanisms by which α cell PC1/3 expression can be activated may reveal new targets for diabetes treatment. Here, we demonstrate that the GLP-1 receptor (GLP-1R) agonist, liraglutide, increased α cell GLP-1 expression in a β cell GLP-1R-dependent manner. We demonstrate that this effect of liraglutide was translationally relevant in human islets through application of a new scRNA-seq technology, DART-Seq. We found that the effect of liraglutide to increase α cell PC1/3 mRNA expression occurred in a subcluster of α cells and was associated with increased expression of other β cell-like genes, which we confirmed by IHC. Finally, we found that the effect of liraglutide to increase bihormonal insulin+ glucagon+ cells was mediated by the β cell GLP-1R in mice. Together, our data validate a high-sensitivity method for scRNA-seq in human islets and identify a potentially novel GLP-1-mediated pathway regulating human α cell function.

Dapagliflozin promotes beta cell regeneration by inducing pancreatic endocrine cell phenotype conversion in type 2 diabetic mice

BACKGROUND

Clinical trials and animal studies have shown that sodium-glucose co-transporter type 2 (SGLT2) inhibitors improve pancreatic beta cell function. Our study aimed to investigate the effect of dapagliflozin on islet morphology and cell phenotype, and explore the origin and possible reason of the regenerated beta cells.

METHODS

Two diabetic mouse models, db/db mice and pancreatic alpha cell lineage-tracing (glucagon-β-gal) mice whose diabetes was induced by high fat diet combined with streptozotocin, were used. Mice were treated by daily intragastric administration of dapagliflozin (1 mg/kg) or vehicle for 6 weeks. The plasma insulin, glucagon and glucagon-like peptide-1 (GLP-1) were determined by using ELISA. The evaluation of islet morphology and cell phenotype was performed with immunofluorescence. Primary rodent islets and αTC1.9, a mouse alpha cell line, were incubated with dapagliflozin (0.25-25 μmol/L) or vehicle in the presence or absence of GLP-1 receptor antagonist for 24 h in regular or high glucose medium. The expression of specific markers and hormone levels were determined.

RESULTS

Treatment with dapagliflozin significantly decreased blood glucose in the two diabetic models and upregulated plasma insulin and GLP-1 levels in db/db mice. The dapagliflozin treatment increased islet and beta cell numbers in the two diabetic mice. The beta cell proliferation as indicated by C-peptide and BrdU double-positive cells was boosted by dapagliflozin. The alpha to beta cell conversion, as evaluated by glucagon and insulin double-positive cells and confirmed by using alpha cell lineage-tracing, was facilitated by dapagliflozin. After the dapagliflozin treatment, some insulin-positive cells were located in the duct compartment or even co-localized with duct cell markers, suggestive of duct-derived beta cell neogenesis. In cultured primary rodent islets and αTC1.9 cells, dapagliflozin upregulated the expression of pancreatic endocrine progenitor and beta cell specific markers (including Pdx1) under high glucose condition. Moreover, dapagliflozin upregulated the expression of Pcsk1 (which encodes prohormone convertase 1/3, an important enzyme for processing proglucagon to GLP-1), and increased GLP-1 content and secretion in αTC1.9 cells. Importantly, the dapagliflozin-induced upregulation of Pdx1 expression was attenuated by GLP-1 receptor antagonist.

CONCLUSIONS

Except for glucose-lowering effect, dapagliflozin has extra protective effects on beta cells in type 2 diabetes. Dapagliflozin enhances beta cell self-replication, induces alpha to beta cell conversion, and promotes duct-derived beta cell neogenesis. The promoting effects of dapagliflozin on beta cell regeneration may be partially mediated via GLP-1 secreted from alpha cells.

Mof regulates glucose level via altering different α-cell subset mass and intra-islet glucagon-like peptide-1, glucagon secretion

BACKGROUND

Males absent on the first (Mof) is implicated in gene control of diverse biological processes, such as cell growth, differentiation, apoptosis and autophagy. However, the relationship between glucose regulation and Mof-mediated transcription events remains unexplored. We aimed to unravel the role of Mof in glucose regulation by using global and pancreatic α-cell-specific Mof-deficient mice in vivo and α-TC1-6 cell line in vitro.

METHODS

We used tamoxifen-induced temporal Mof-deficient mice first to show Mof regulate glucose homeostasis, islet cell proportions and hormone secretion. Then we used α-cell-specific Mof-deficient mice to clarify how α-cell subsets and β-cell mass were regulated and corresponding hormone level alterations. Ultimately, we used small interfering RNA (siRNA) to knockdown Mof in α-TC1-6 and unravel the mechanism regulating α-cell mass and glucagon secretion.

RESULTS

Mof was mainly expressed in α-cells. Global Mof deficiency led to lower glucose levels, attributed by decreased α/β-cell ratio and glucagon secretion. α-cell-specific Mof-deficient mice exhibited similar alterations, with more reduced prohormone convertase 2 (PC2)-positive α-cell mass, responsible for less glucagon, and enhanced prohormone convertase 1 (PC1/3)-positive α-cell mass, leading to more glucagon-like peptide-1 (GLP-1) secretion, thus increased β-cell mass and insulin secretion. In vitro, increased DNA damage, dysregulated autophagy, enhanced apoptosis and altered cell fate factors expressions upon Mof knockdown were observed. Genes and pathways linked to impaired glucagon secretion were uncovered through transcriptome sequencing.

CONCLUSION

Mof is a potential interventional target for glucose regulation, from the aspects of both α-cell subset mass and glucagon, intra-islet GLP-1 secretion. Upon Mof deficiency, Up-regulated PC1/3 but down-regulated PC2-positive α-cell mass, leads to more GLP-1 and insulin but less glucagon secretion, and contributed to lower glucose level.

Transcription factor Creb3l1 regulates the synthesis of prohormone convertase enzyme PC1/3 in endocrine cells

Transcription factor cAMP responsive element-binding protein 3 like 1 (Creb3l1) is a non-classical endoplasmic reticulum stress molecule that is emerging as an important component for cellular homeostasis, particularly within cell types with high peptide secretory capabilities. We have previously shown that Creb3l1 serves an important role in body fluid homeostasis through its transcriptional control of the gene coding for antidiuretic hormone arginine vasopressin in the neuropeptide-rich magnocellular neurones of the supraoptic nucleus. In response to osmotic stimuli such as dehydration, vasopressin magnocellular neurones undergo remarkable transcriptome changes, including increased Creb3l1 expression, to ensure that the supply of vasopressin meets demand. To determine where else Creb3l1 fits into the secretory cell supply chain, we performed RNA-sequencing of Creb3l1 knockdown anterior pituitary mouse corticotroph cell line AtT20. The target chosen for further investigation was Pcsk1, which encodes proprotein convertase enzyme 1 (PC1/3). PC1/3 is crucial for processing of neuropeptides and peptide hormones such as pro-opiomelanocortin (POMC), proinsulin, proglucagon, vasopressin and oxytocin. Viral manipulations in supraoptic nuclei by over-expression of Creb3l1 increased Pcsk1, whereas Creb3l1 knockdown decreased Pcsk1 expression. In vitro promoter activity and binding studies showed that Creb3l1 was a transcription factor of the Pcsk1 gene binding directly to a G-box motif in the promoter. In the dehydrated rat anterior pituitary, Creb3l1 and Pcsk1 expression decreased in parallel compared to control, supporting our findings from manipulations in AtT20 cells and the supraoptic nucleus. No relationship was observed between Creb3l1 and Pcsk1 expression in the neurointermediate lobe of the pituitary, indicating a different mechanism of PC1/3 synthesis by these POMC-synthesising cells. Therefore, Creb3l1, by regulating the expression of Pcsk1, does not control the processing of POMC peptides in the intermediate lobe.

Reduced Stability and pH-Dependent Activity of a Common Obesity-Linked PCSK1 Polymorphism, N221D

Common mutations in the human prohormone convertase (PC)1/3 gene (PCKSI) are linked to increased risk of obesity. Previous work has shown that the rs6232 single-nucleotide polymorphism (N221D) results in slightly decreased activity, although whether this decrease underlies obesity risk is not clear. We observed significantly decreased activity of the N221D PC1/3 enzyme at the pH of the trans-Golgi network; at this pH, the mutant enzyme was less stable than wild-type enzyme. Recombinant N221D PC1/3 also showed enhanced susceptibility to heat stress. Enhanced susceptibility to tunicamycin-induced endoplasmic reticulum stress was observed in AtT-20/PC2 cell clones in which murine PC1/3 was replaced by human N221D PC1/3, as compared with wild-type human PC1/3. However, N221D PC1/3-expressing AtT-20/PC2 clones processed proopiomelanocortin to α-MSH similarly to wild-type PC1/3. We also generated a CRISPR-edited mouse line expressing the N221D mutation in the PCKSI gene. When homozygous N221D mice were fed either a standard or a high-fat diet, we found no increase in body weight compared with their wild-type sibling controls. Sexual dimorphism was observed in pituitary ACTH for both genotypes, with females exhibiting lower levels of pituitary ACTH. In contrast, hypothalamic α-MSH content for both genotypes was higher in females compared with males. Hypothalamic corticotropin-like intermediate peptide content was higher in wild-type females compared with wild-type, but not N221D, males. Taken together, these data suggest that the increased obesity risk linked to the N221D allele in humans may be due in part to PC1/3-induced loss of resilience to stressors rather than strictly to decreased enzymatic activity on peptide precursors.

C-Terminal Binding Protein 1 Modulates Cellular Redox via Feedback Regulation of MPC1 and MPC2 in Melanoma Cells

BACKGROUND Recent studies have illustrated that the transcription co-repressor, C-terminal binding protein 1 (CtBP1), links the metabolic alterations to transcription controls in proliferation, EMT, genome stability, metabolism, and lifespan, but whether CtBP1 affects the cellular redox homeostasis is unexplored. This study was designed to investigate the mechanism of CtBP1-mediated transcription repression that contributes to the metabolic reprogramming. MATERIAL AND METHODS Knockdown of CtBP1 in both mouse MEF cells and human melanoma cells changed cell redox homeostasis. Further, chromatin immunoprecipitation (ChIP) and luciferase reporter assay were performed for identification of CtBP1 downstream targets, pyruvate carrier 1 and 2 genes (MPC1 and MPC2), which contribute to redox homeostasis and are transcriptionally regulated by CtBP1. Moreover, blockage of the cellular NADH level with the glycolysis inhibitor 2-Deoxy-D-Glucose (2-DG) rescued MPC1 and MPC2 expression. MTT assay and scratch assay were performed to investigate the effect of MPC1 and MPC2 expression on malignant properties of melanoma cells. RESULTS The data demonstrated that CtBP1 directly bound to the promoters of MPC1 and MPC2 and transcriptionally repressed them, leading to increased levels of free NADH in the cytosol and nucleus, thus positively feeding back CtBP1's functions. Consequently, restoring MPC1 and MPC2 in human tumor cells decreases free NADH and inhibits melanoma cell proliferation and migration. CONCLUSIONS Our data indicate that MPC1 and MPC2 are principal mediators that link CtBP1-mediated transcription regulation to NADH production. The discovery of CtBP1 as an NADH regulator in addition to being an NADH sensor shows that CtBP1 is at the center of tumor metabolism and transcription control.

Pancreatic cancer-derived exosomes suppress the production of GIP and GLP-1 from STC-1 cells in vitro by down-regulating the PCSK1/3

One hallmark of pancreatic cancer (PC) is the high prevalence of pancreatic cancer-associated diabetes mellitus (PC-DM), but the mechanisms remain to be elucidated. Patients with PC who are diagnosed with new-onset diabetes/prediabetes have recently been shown to display significantly lower levels of glucose-dependent insulinotropic peptide (GIP) secreted mainly by enteroendocrine cells. We hypothesized that PC-derived exosomes are responsible for the decreased levels of incretins in patients with PC-DM. In this study, exosomes were successfully isolated from PANC-1, MIA PaCa-2 and SW620 cells and characterized. Only the exosomes from MIA PaCa-2 cells (Exo-Mia) reduce the production of GIP and glucagon-like peptide-1 (GLP-1) from STC-1 cells in vitro in a concentration- and time-dependent manner. Moreover, Exo-Mia increased the levels of the Gip and proglucagon mRNAs and decreased the expression of proprotein convertase subtilisin/kexin type 1/3 (PCSK1/3), which is responsible for the post-translational processing of Gip and proglucagon. Furthermore, differentially expressed exosomal miRNAs (miR-6796-3p, miR-6763-5p, miR-4750-3p and miR-197-3p) were identified and considered to be responsible for the inhibitory effects on GIP and GLP-1 production. To further determine the approach of cancer-derived exosomes reaching enteroendocrine cells, we analyzed the uptake and distribution of exosomes in animal model. It was observed that exosomes infused into the intestinal cavity were more easily internalized by the intestinal epithelium than exosomes injected into blood. In conclusion, pancreatic cancer-derived exosomes (Exo-Mia) suppress the synthesis of GIP and GLP-1 from STC-1 cells in vitro by down-regulating the PCSK1/3. Moreover, it may be the pancreatic juice that transport cancer-derived exosomes to target cells (K and L cells) in the gut.

Identification of a potential functional single nucleotide polymorphism for fatness and growth traits in the 3'-untranslated region of the PCSK1 gene in chickens

Prohormone convertase 1/3 is a serine endoprotease belonging to the subtilisin-like proprotein convertase family that is encoded by the () gene, and its major function is the processing and bioactivation of the proproteins of many kinds of neuroendocrine hormones, including insulin, cholecystokinin, and adrenocorticotropic hormone. The results of our previous genomewide association study indicated that the gene might be an important candidate gene for fatness traits in chickens. The objectives of this study were to investigate the tissue expression profiles of gene and to identify functional variants associated with fatness and growth traits in the chicken. The results indicated that mRNA was widely expressed in various tissues, especially neuroendocrine and intestinal tissues. Of these 2 tissue types, mRNA expression in lean males was significantly higher than in fat males. A SNP in the 3' untranslated region of (c.*900G > A) was identified. Association analysis in the Arbor Acres commercial broiler population and Northeast Agricultural University broiler lines divergently selected for abdominal fat content (NEAUHLF) population showed that the SNP c.*900G > A was associated with abdominal fat weight, abdominal fat percentage, BW, metatarsus length, and metatarsal circumference. In the 5th to 19th generation (G to G) of NEAUHLF, the allele frequency of c.*900G > A changed along with selection for abdominal fat content. At G, allele G of c.*900G > A was predominate in the lean line, whereas allele A was predominate in the fat line. Functional analysis demonstrated that allele A of c.*900G > A reduced mRNA stability and consequently downregulated gene expression. These results suggested that c.*900G > A was a functional SNP for fatness and growth traits in the chicken. The results of this study provide basic molecular information for the role of gene in avian growth and development, especially obesity.

PICK1 is essential for insulin production and the maintenance of glucose homeostasis

Protein interacting with C-kinase 1 (PICK1) is a peripheral membrane protein that controls insulin granule formation, trafficking, and maturation in INS-1E cells. However, global Pick1-knockout mice showed only a subtle diabetes-like phenotype. This raises the possibility that compensatory effects from tissues other than pancreatic beta cells may obscure the effects of insulin deficiency. To explore the role of PICK1 in pancreatic islets, we generated mice harboring a conditional Pick1 allele in a C57BL/6J background. The conditional Pick1-knockout mice exhibited impaired glucose tolerance, profound insulin deficiency, and hyperglycemia. In vitro experiments showed that the ablation of Pick1 in pancreatic beta cells selectively decreased the initial rapid release of insulin and the total insulin levels in the islets. Importantly, the specific ablation of Pick1 induced elevated proinsulin levels in the circulation and in the islets, accompanied by a reduction in the proinsulin processing enzymes prohormone convertase 1/3 (PC1/3). The deletion of Pick1 triggered the specific elimination of chromogranin B in pancreatic beta cells, which is believed to control granule formation and release. Collectively, these data demonstrate the critical role of PICK1 in secretory granule biogenesis, proinsulin processing, and beta cell function. We conclude that the beta cell-specific deletion of Pick1 in mice led to hyperglycemia and eventually to diabetes.

Enteroendocrine K and L cells in healthy and type 2 diabetic individuals

AIMS/HYPOTHESIS

Enteroendocrine K and L cells are pivotal in regulating appetite and glucose homeostasis. Knowledge of their distribution in humans is sparse and it is unknown whether alterations occur in type 2 diabetes. We aimed to evaluate the distribution of enteroendocrine K and L cells and relevant prohormone-processing enzymes (using immunohistochemical staining), and to evaluate the mRNA expression of the corresponding genes along the entire intestinal tract in individuals with type 2 diabetes and healthy participants.

METHODS

In this cross-sectional study, 12 individuals with type 2 diabetes and 12 age- and BMI-matched healthy individuals underwent upper and lower double-balloon enteroscopy with mucosal biopsy retrieval from approximately every 30 cm of the small intestine and from seven specific anatomical locations in the large intestine.

RESULTS

Significantly different densities for cells positive for chromogranin A (CgA), glucagon-like peptide-1, glucose-dependent insulinotropic polypeptide, peptide YY, prohormone convertase (PC) 1/3 and PC2 were observed along the intestinal tract. The expression of CHGA did not vary along the intestinal tract, but the mRNA expression of GCG, GIP, PYY, PCSK1 and PCSK2 differed along the intestinal tract. Lower counts of CgA-positive and PC1/3-positive cells, respectively, were observed in the small intestine of individuals with type 2 diabetes compared with healthy participants. In individuals with type 2 diabetes compared with healthy participants, the expression of GCG and PYY was greater in the colon, while the expression of GIP and PCSK1 was greater in the small intestine and colon, and the expression of PCSK2 was greater in the small intestine.

CONCLUSIONS/INTERPRETATION

Our findings provide a detailed description of the distribution of enteroendocrine K and L cells and the expression of their products in the human intestinal tract and demonstrate significant differences between individuals with type 2 diabetes and healthy participants.

TRIAL REGISTRATION

NCT03044860.

IL-6-Type Cytokine Signaling in Adipocytes Induces Intestinal GLP-1 Secretion

We recently showed that interleukin (IL)-6-type cytokine signaling in adipocytes induces free fatty acid release from visceral adipocytes, thereby promoting obesity-induced hepatic insulin resistance and steatosis. In addition, IL-6-type cytokines may increase the release of leptin from adipocytes and by those means induce glucagon-like peptide 1 (GLP-1) secretion. We thus hypothesized that IL-6-type cytokine signaling in adipocytes may regulate insulin secretion. To this end, mice with adipocyte-specific knockout of gp130, the signal transducer protein of IL-6, were fed a high-fat diet for 12 weeks. Compared with control littermates, knockout mice showed impaired glucose tolerance and circulating leptin, GLP-1, and insulin levels were reduced. In line, leptin release from isolated adipocytes was reduced, and intestinal proprotein convertase subtilisin/kexin type 1 (Pcsk1) expression, the gene encoding PC1/3, which controls GLP-1 production, was decreased in knockout mice. Importantly, treatment with the GLP-1 receptor antagonist exendin 9-39 abolished the observed difference in glucose tolerance between control and knockout mice. Ex vivo, supernatant collected from isolated adipocytes of gp130 knockout mice blunted Pcsk1 expression and GLP-1 release from GLUTag cells. In contrast, glucose- and GLP-1-stimulated insulin secretion was not affected in islets of knockout mice. In conclusion, adipocyte-specific IL-6 signaling induces intestinal GLP-1 release to enhance insulin secretion, thereby counteracting insulin resistance in obesity.

Proopiomelanocortin processing and prohormone convertase 1 level in dogs with pituitary corticotroph tumors

Preliminary data suggest that prohormone convertase 1 (PC1/3) protein expression and proopiomelanocortin (POMC) processing are altered in large corticotroph tumors. The aim of this study was to characterize the levels of ACTH precursors and PC1/3 protein in small and large corticotroph tumors of dogs with Cushing's disease. Pituitary tumors of dogs with Cushing's disease were collected postmortem 30 min to 12h after natural death or euthanasia, and classified as small or large. POMC, pro-ACTH, and PC1/3 were detected by Western blotting. Five small and 6 large corticotroph tumors were collected. POMC and pro-ACTH signals were visualized in 5/6 large tumors and in 4/5 small tumors. The strongest signal intensity was observed in 2 large tumors. The PC1/3 signal was weak to undetectable in 6/6 large tumors but strong in 5/5 small tumors. These results suggest differences in PC1/3 protein levels and patterns of POMC processing between large and small corticotroph tumors. If confirmed in larger groups of tumors, further studies will be required to characterize the mechanism involved in these differences.

Proprotein convertase 1 mediated proneuropeptide proteolytic processing in ischemic neuron injury

BACKGROUND

Pro-protein processing mechanism plays an important role in neuron injury.

OBJECTIVE

To study the protein convertase 1 (PC1) mediated processing mechanism, the ischemic cellular or tissue proPC1/PC1 or proCgA/CgA (pro-chromogranin A) was analyzed.

METHODS

NS20Y differentiated cells were stressed by 0-6 h of oxygen and glucose deprivation (OGD) in glucose-free DMEM and an anaerobic jar environment. Ischemic C57BL/J mouse model was established by performing 60-min of middle cerebral artery occlusion (MCAO) operation and subsequent 4 or 24-h reperfusion. The TUNEL, immunochemistry, and Western blot methods were used to detect protein expression in ischemic cells or tissues.

RESULTS

The OGD or MCAO stress caused substantial cell death in a dose-dependent manner (p < 0.05 or 0.01). With the increasing OGD dose, proPC1 and PC1 proteins gradually increased (p < 0.05 or 0.01) whereas proCgA and CgA proteins decreased (p < 0.05). In vivo the proPC1 and PC1 expressions presented with a peak at 4-h and then decreased at 24-h reperfusion (p < 0.05 or 0.01). The tissue proCgA and CgA proteins decreased with the increasing reperfusion time (p < 0.05).

CONCLUSIONS

The results suggest that the increasing PC1 expression promoted the transformation of proCgA into CgA or smaller peptides, i.e. Pancreastatin or Secretoneurin, and the PC1 mediated processing plays a critical role (Fig. 4, Ref. 15).

Metabolic regulation of GLP-1 and PC1/3 in pancreatic α-cell line

Background and aims

An intra-islet incretin system has been recently suggested to operate through modulation of the expression and activity of proconvertase 1/3 and 2 (PC1/3, PC2) in pancreatic alpha-cell accounting for local release of GLP-1. Little is known, whether this alpha-cell activity can be affected by the metabolic alterations occurring in type 2 diabetes, such as hyperglycemia, hyperlipidemia or hyperglucagonemia.

Materials and methods

AlphaTC1/6 cells from a mice pancreatic cell line were incubated in the presence of two glucose (G) concentration (5.5 and 16.7 mM) for 16 h with or without free fatty acid, IL6 or glucagon. GLP-1 secretion was measured by ELISA and expression of PC1/3 and PC2 by RT-PCR and western blot; cell viability was determined by MTT method, Reactive Oxygen Species generation (ROS) by H₂DCFDA fluorescence and apoptosis by Annexin staining and Propidium Iodine (PI) fluorescence.

Results

Upon 16.7G incubation, GLP-1 secretion (total and active) was significantly increased in parallel with a significant increment in PC1/3 expression, a slight increase in cell viability and ROS generation and by a decrement in PC2 expression with no change in cell apoptosis. When cells were incubated at 5.5mM glucose with FFA, also an increment in GLP-1 secretion and PC1/3 expression was observed together an increment in ROS generation, a decrement in cell viability, and a modest increment in apoptosis. When incubated with 16.7mM glucose with FFA, the increment in GLP-1 secretion was reduced to basal, accompanied by an increment in apoptosis and ROS generation. This was also observed with IL-6, but in this case, no modification in ROS generation or apoptosis was observed when compared to 16.7mM glucose. The presence of glucagon did not modify any of the parameters studied.

Conclusion

These data suggest that under hyperglycemic, hyperlipidemia or inflammatory conditions, alpha cells can increase expression PC1/3 and activate GLP-1 secretion, which may contribute protecting both alpha and beta-cells from glucose and lipotoxicity, while this effect seems to be lost in the presence of both pathophysiological conditions.

Targeted high-resolution quadrupole-Orbitrap mass spectrometry analyses reveal a significant reduction of tachykinin and opioid neuropeptides level in PC1 and PC2 mutant mouse spinal cords

Tachykinin and opioid neuropeptides play a fundamental role in pain transmission, modulation and inhibition. The proteolysis control of endogenous tachykinin and opioid neuropeptides has a significant impact on pain perception. The role of proprotein convertases (PCs) in the proteolysis of proneuropeptides was previously established but very few studies have shown the direct impact of PCs on the regulation of specific tachykinin and opioid peptides in the central nervous system. There is an increasing interest in the therapeutic targeting of PCs for the treatment of pain but it is imperative to assess the impact of PCs on the pronociceptive and the endogenous opioid systems. The objective of this study was to determine the relative concentration of targeted neuropeptides in the spinal cord of WT, PC1^-/+ and PC2^-/+ animals to establish the impact of a restricted PCs activity on the regulation of specific neuropeptides. The analysis of tachykinin and opioid neuropeptides were performed on a HPLC-MS/MS (High-Resolution Quadrupole-Orbitrap Mass Spectrometer). The results revealed a significant decrease of Dyn A (p<0.01), Leu-Enk (p<0.001), Met-Enk (p<0.001), Tach_58-71 (p<0.05), SP (p<0.01) and NKA (p<0.001) concentrations in both, PC1^-/+ and PC2^-/+ animals. Therefore, the modulation of PCs activity has an important impact on specific pronociceptive peptides (SP and NKA), but the results also shown that endogenous opioid system is hindered and consequently it will affect significantly the pain modulatory pathways. These observations may have insightful impact on future analgesic drug developments and therapeutic strategies.

Survival of an Islet Allograft Deficient in iNOS after Implantation into Diabetic NOD Mice

Proinflammatory cytokines play a major role in rejection of pancreatic islet allografts and in type 1 diabetes (T1D). In rodent islets, exposure to IL-1β alone or combined with IFN-γ induces expression of inducible nitric oxide synthase (iNOS). Inhibition of iNOS or a deletion of the iNOS gene has been shown to be protective in animal models of T1D. In the present study we tested the hypothesis that transplantation of pancreatic islets deficient in iNOS (iNOS–/–) would permit increased graft survival. Pancreatic islets isolated from wild-type (wt) mice and iNOS–/– mice were allogeneically transplanted beneath the kidney capsule of spontaneously diabetic NOD mice. When blood glucose increased above 12.0 mM after preceding normalization of hyperglycemia, animals were sacrificed. Histological examinations of grafts were performed and graft gene expression was analyzed by real-time PCR. Transplantations of the two types of islets could reverse hyperglycemia and the grafts functioned for on average 1 week posttransplantation. Morphological examination of both types of islet grafts showed immune cell infiltration around and within the grafts. Remaining endocrine cells could be observed in wt and iNOS–/– islet grafts. In the removed grafts iNOS-/islet tissue contained higher mRNA levels of insulin, proinsulin convertases (PC-1 and PC-2), and IL-1β compared to transplanted wt islets. The assessments of insulin, PC-1 and PC-2 mRNAs of the grafts suggest that the iNOS–/– islets may be more resistant to destruction in the transplantation model used; however, this was not sufficient to prolong the period of normoglycemia posttransplantation.

The changing faces of corticotroph cell adenomas: the role of prohormone convertase 1/3

The spectrum of corticotroph cell adenomas is very wide. Though rarely, silent corticotroph cell adenomas (SCA) may transform into corticotroph cell adenomas associated with Cushing's disease (CD). The aim of the study was to investigate the role of prohormone convertase 1/3 (PC1/3) in the transformation of SCA into CD. We reviewed the records of 1259 consecutive endoscopic endonasal procedures for pituitary adenomas from 1998 to 2013. Of these, 132 were CD and 44 were SCA. During the follow-up, three patients with SCA showed a clear transformation from SCA into CD and underwent surgery once again to remove the recurrent tumour. The PC1/3 expression was analysed by both immunohistochemistry and quantitative real time-polymerase chain reaction (qRT-PCR) in primary and recurrent tumours. The immunohistochemical PC1/3 expression was negative or weak in the three patients in the initial phase of SCA, while a strong expression was observed in the majority of neoplastic cells in tissue specimens obtained from the same three patients at the time of recurrence as CD. The immunohistochemical PC1/3 expression showed a strict correlation with the PC1/3 levels obtained by qRT-PCR. In 14 cases of SCA with no change of phenotype during the follow-up, the immunohistochemical PC1/3 expression was low and strictly associated with the level of PC1/3 obtained by qRT-PCR both in primary (14/14 cases) and in recurrent tumours (4/4 cases). Our study provides insight into the crucial role of the PC1/3 protein in the transformation of phenotype from SCA to CD.

Loss of prohormone convertase 2 promotes beta cell dysfunction in a rodent transplant model expressing human pro-islet amyloid polypeptide

AIMS/HYPOTHESIS

A contributor to beta cell failure in type 2 diabetes and islet transplants is amyloid formation by aggregation of the beta cell peptide, islet amyloid polypeptide (IAPP). Similar to the proinsulin processing pathway that generates insulin, IAPP is derived from a prohormone precursor, proIAPP, which requires cleavage by prohormone convertase (PC) 1/3 and PC2 in rodent pancreatic beta cells. We hypothesised that loss of PC2 would promote beta cell death and dysfunction in a rodent model of human beta cell proIAPP overexpression.

METHODS

We generated an islet transplant model wherein immune-deficient mouse models of diabetes received islets expressing amyloidogenic human proIAPP and lacking PC2, leading to restoration of normoglycaemia accompanied by increased secretion of human proIAPP. Blood glucose levels were analysed for up to 16 weeks in transplant recipients and grafts were assessed for islet amyloid and beta cell number and death.

RESULTS

Hyperglycaemia (blood glucose >16.9 mmol/l) returned in 94% of recipients of islets expressing human proIAPP and lacking PC2, whereas recipients of islets that express human proIAPP and normal PC2 levels remained normoglycaemic for at least 16 weeks. Islet graft failure was accompanied by a ∼20% reduction in insulin-positive cells, yet the degree of amyloid deposition and beta cell apoptosis was similar to those of controls expressing human proIAPP with functional PC2 levels.

CONCLUSIONS/INTERPRETATION

PC2 deficiency in transplanted mouse islets expressing human proIAPP promotes beta cell loss and graft failure. Our data suggest that impaired NH₂-terminal processing and increased secretion of human proIAPP promote beta cell failure.

Interferon alpha impairs insulin production in human beta cells via endoplasmic reticulum stress

Despite substantial advances in the research exploring the pathogenesis of Type 1 Diabetes (T1D), the pathophysiological mechanisms involved remain unknown. We hypothesized in this study that interferon alpha (IFNα) participates in the early stages of T1D development by triggering endoplasmic reticulum (ER) stress. To verify our hypothesis, human islets and human EndoC-βH1 cells were exposed to IFNα and tested for ER stress markers, glucose stimulated insulin secretion (GSIS) and insulin content. IFNα treatment induced upregulation of ER stress markers including Binding immunoglobulin Protein, phospho-eukaryotic translation initiation factor 2α, spliced- X-box binding protein-1, C/EBP homologous protein and activating transcription factor 4. Intriguingly, IFNα treatment did not impair GSIS but significantly decreased insulin production in both human islets and EndoC-βH1 cells. Furthermore, IFNα decreased the expression of both proinsulin convertase 1 and proinsulin convertase 2, suggesting an altered functional state of the beta cells characterized by a slower proinsulin-insulin conversion. Pretreatment of both human islets and EndoC-βH1 cells with chemical chaperones 4-phenylbutyric acid and tauroursodeoxycholic acid completely prevented IFNα effects, indicating an ER stress-mediated impairment of insulin production. We demonstrated for the first time that IFNα elicits ER stress in human beta cells providing a novel mechanistic role for this virus-induced cytokine in the development of T1D. Compounds targeting molecular processes altered in ER-stressed beta cells could represent a potential therapeutic strategy to prevent IFNα-induced beta cell dysfunction in the early onset of T1D.

Exenatide substantially improves proinsulin conversion and cell survival that augment Ins2+/Akita beta cell function

Proinsulin folding imperfections cause extensive beta-cell defects known in diabetes. Here, we investigated whether exenatide can alleviate such defects in proinsulin conversion, beta-cell survival, and insulin secretion, in the Ins2^+/Akita beta-cells that have a spontaneous mutation (Cys 96 Tyr) in the insulin 2 gene caused dominant negative misfolding problem. 15 or 120 min exenatide administration substantially improves glucose-stimulated insulin secretion, marked in the secreted insulin levels and proinsulin/insulin ratio. This improvement is mainly due to enhanced conversion of proinsulin to insulin, having nothing to do with the prohormone convertase PC1/3 and PC2 levels. The 15 min improvement is calcium-independent. The 120 min improvement is linked to calcium and/or cAMP dependent mechanisms. This efficacy is validated during longer treatment and in Akita islets. Exenatide improves Ins2^+/Akita beta-cell survival and Akita mouse's glucose tolerance. The results suggest a potential of incretin mimetics in alleviating defective proinsulin conversion and other proinsulin misfolding consequences.

Impaired prohormone processing: a grand unified theory for features of Prader-Willi syndrome?

Prader-Willi syndrome (PWS) is a complex disorder that manifests with an array of phenotypes, such as hypotonia and difficulties in feeding during infancy and reduced energy expenditure, hyperphagia, and developmental delays later in life. While the genetic cause has long been known, it is still not clear how mutations at this locus produce this array of phenotypes. In this issue of the JCI, Burnett and colleagues used a comprehensive approach to gain insight into how PWS-associated mutations drive disease. Using neurons derived from PWS patient induced pluripotent stem cells (iPSCs) and mouse models, the authors provide evidence that neuroendocrine PWS-associated phenotypes may be linked to reduced expression of prohormone convertase 1 (PC1). While these compelling results support a critical role for PC1 deficiency in PWS, more work needs to be done to fully understand how and to what extent loss of this prohormone processing enzyme underlies disease manifestations in PWS patients.

Characterization of endoproteolytic processing of dynorphins by proprotein convertases using mouse spinal cord S9 fractions and mass spectrometry

Dynorphins are important neuropeptides with a central role in nociception and pain alleviation. Many mechanisms regulate endogenous dynorphin concentrations, including proteolysis. Proprotein convertases (PCs) are widely expressed in the central nervous system and specifically cleave at C-terminal of either a pair of basic amino acids, or a single basic residue. The proteolysis control of endogenous big dynorphin (BDyn) and dynorphin A (Dyn A) levels has a profound impact on pain perception and the role of PCs remain unclear. The objective of this study was to decipher the role of PC1 and PC2 in the proteolysis control of BDyn and Dyn A levels using cellular fractions of spinal cords from wild-type (WT), PC1(-/+) and PC2(-/+) animals and mass spectrometry. Our results clearly demonstrate that both PC1 and PC2 are involved in the proteolysis regulation of BDyn and Dyn A with a more important role for PC1. C-terminal processing of BDyn generates specific peptide fragments dynorphin 1-19, dynorphin 1-13, dynorphin 1-11 and dynorphin 1-7, and C-terminal processing of Dyn A generates dynorphin 1-13, dynorphin 1-11 and dynorphin 1-7, all these peptide fragments are associated with PC1 or PC2 processing. Moreover, the proteolysis of BDyn leads to the formation of Dyn A and Leu-Enk, two important opioid peptides. The rate of formation of both is significantly reduced in cellular fractions of spinal cord mutant mice. As a consequence, even the partial inhibition of PC1 or PC2 may impair the endogenous opioid system.

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

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

Human 'brite/beige' adipocytes develop from capillary networks, and their implantation improves metabolic homeostasis in mice

Uncoupling protein 1 (UCP1) is highly expressed in brown adipose tissue, where it generates heat by uncoupling electron transport from ATP production. UCP1 is also found outside classical brown adipose tissue depots, in adipocytes that are termed 'brite' (brown-in-white) or 'beige'. In humans, the presence of brite or beige (brite/beige) adipocytes is correlated with a lean, metabolically healthy phenotype, but whether a causal relationship exists is not clear. Here we report that human brite/beige adipocyte progenitors proliferate in response to pro-angiogenic factors, in association with expanding capillary networks. Adipocytes formed from these progenitors transform in response to adenylate cyclase activation from being UCP1 negative to being UCP1 positive, which is a defining feature of the beige/brite phenotype, while displaying uncoupled respiration. When implanted into normal chow-fed, or into high-fat diet (HFD)-fed, glucose-intolerant NOD-scid IL2rg(null) (NSG) mice, brite/beige adipocytes activated in vitro enhance systemic glucose tolerance. These adipocytes express neuroendocrine and secreted factors, including the pro-protein convertase PCSK1, which is strongly associated with human obesity. Pro-angiogenic conditions therefore drive the proliferation of human beige/brite adipocyte progenitors, and activated beige/brite adipocytes can affect systemic glucose homeostasis, potentially through a neuroendocrine mechanism.

Revisiting PC1/3 Mutants: Dominant-Negative Effect of Endoplasmic Reticulum-Retained Mutants

Prohormone convertase 1/3 (PC1/3), encoded by the gene PCSK1, is critical for peptide hormone synthesis. An increasing number of studies have shown that inactivating mutations in PCSK1 are correlated with endocrine pathologies ranging from intestinal dysfunction to morbid obesity, whereas the common nonsynonymous polymorphisms rs6232 (N221D) and rs6234-rs6235 (Q665E-S690T) are highly associated with obesity risk. In this report, we revisited the biochemical and cellular properties of PC1/3 variants in the context of a wild-type PC1/3 background instead of the S357G hypermorph background used for all previous studies. In the wild-type background the PC1/3 N221D variant exhibited 30% lower enzymatic activity in a fluorogenic assay than wild-type PC1/3; this inhibition was greater than that detected in an equivalent experiment using the PC1/3 S357G background. A PC1/3 variant with the linked carboxyl-terminal polymorphisms Q665E-S690T did not show this difference. We also analyzed the biochemical properties of 2 PC1/3 mutants, G209R and G593R, which are retained in the endoplasmic reticulum (ER), and studied their effects on wild-type PC1/3. The expression of ER-retained mutants induced ER stress markers and also resulted in dominant-negative blockade of wild-type PC1/3 prodomain cleavage and decreased expression of wild-type PC1/3, suggesting facilitation of the entry of wild-type protein to a degradative proteasomal pathway. Dominant-negative effects of PC1/3 mutations on the expression and maturation of wild-type protein, with consequential effects on PC1/3 availability, add a new element which must be considered in population and clinical studies of this gene.

Progressive change of intra-islet GLP-1 production during diabetes development

BACKGROUND

Glucagon-like peptide 1 (GLP-1) and glucagon share the same precursor molecule proglucagon, but each arises from a distinct posttranslational process in a tissue-specific manner. Recently, it has been shown that GLP-1 is co-expressed with glucagon in pancreatic islet cells. This study was aimed to investigate the progressive changes of GLP-1 versus glucagon production in pancreatic islets during the course of diabetes development.

METHODS

Both type 1 (non-obese diabetes mice) and type 2 (db/db mice) diabetes models were employed in this study. The mice were monitored closely for their diabetes progression and were sacrificed at different stages according to their blood glucose levels. GLP-1 and glucagon expression in the pancreatic islets was examined using immunohistochemistry assays. Quantitative analysis was performed to evaluate the significance of the changes.

RESULTS

The ratio of GLP-1-expressing cells to glucagon-expressing cells in the islets showed significant, progressive increase with the development of diabetes in db/db mice. The increase of GLP-1 expression was in agreement with the upregulation of PC1/3 expression in these cells. Interestingly, intra-islet GLP-1 expression was not significantly changed during the development of type 1 diabetes in non-obese diabetes mice.

CONCLUSIONS

The study demonstrated that GLP-1 was progressively upregulated in pancreatic islets during type 2 diabetes development. In addition, the data suggest clear differences in intra-islet GLP-1 production between type 1 and type 2 diabetes developments. These differences may have an effect on the clinical and pathophysiological processes of these diseases and may be a target for therapeutic approaches.

A novel familial mutation in the PCSK1 gene that alters the oxyanion hole residue of proprotein convertase 1/3 and impairs its enzymatic activity

Four siblings presented with congenital diarrhea and various endocrinopathies. Exome sequencing and homozygosity mapping identified five regions, comprising 337 protein-coding genes that were shared by three affected siblings. Exome sequencing identified a novel homozygous N309K mutation in the proprotein convertase subtilisin/kexin type 1 (PCSK1) gene, encoding the neuroendocrine convertase 1 precursor (PC1/3) which was recently reported as a cause of Congenital Diarrhea Disorder (CDD). The PCSK1 mutation affected the oxyanion hole transition state-stabilizing amino acid within the active site, which is critical for appropriate proprotein maturation and enzyme activity. Unexpectedly, the N309K mutant protein exhibited normal, though slowed, prodomain removal and was secreted from both HEK293 and Neuro2A cells. However, the secreted enzyme showed no catalytic activity, and was not processed into the 66 kDa form. We conclude that the N309K enzyme is able to cleave its own propeptide but is catalytically inert against in trans substrates, and that this variant accounts for the enteric and systemic endocrinopathies seen in this large consanguineous kindred.

Biochemical and cell biological properties of the human prohormone convertase 1/3 Ser357Gly mutation: a PC1/3 hypermorph

Satiety and appetite signaling are accomplished by circulating peptide hormones. These peptide hormones require processing from larger precursors to become bioactive, often by the proprotein convertase 1/3 (PC1/3). Several subcellular maturation steps are necessary for PC1/3 to achieve its optimal enzymatic activity. Certain PC1/3 variants found in the general population slightly attenuate its enzymatic activity and are associated with obesity and diabetes. However, mutations that increase PC1/3 activity and/or affect its specificity could also have physiological consequences. We here present data showing that the known human Ser357Gly PC1/3 mutant (PC1/3(S357G)) represents a PC1/3 hypermorph. Conditioned media from human embryonic kidney-293 cells transfected with PC1/3(WT) and PC1/3(S357G) were collected and enzymatic activity characterized. PC1/3(S357G) exhibited a lower calcium dependence; a higher pH optimum (neutral); and a higher resistance to peptide inhibitors than the wild-type enzyme. PC1/3(S357G) exhibited increased cleavage to the C-terminally truncated form, and kinetic parameters of the full-length and truncated mutant enzymes were also altered. Lastly, the S357G mutation broadened the specificity of the enzyme; we detected PC2-like specificity on the substrate proCART, the precursor of the cocaine- and amphetamine regulated transcript neuropeptide known to be associated with obesity. The production of another anorexigenic peptide normally synthesized only by PC2, αMSH, was increased when proopiomelanocortin was coexpressed with PC1/3(S357G). Considering the aberrant enzymatic profile of PC1/3(S357G), we hypothesize that this enzyme possesses unusual processing activity that may significantly change the profile of circulating peptide hormones.

High-fat diet induces early-onset diabetes in heterozygous Pax6 mutant mice

BACKGROUND

Type 2 diabetes is caused by interactions between genetic and environmental factors. Our previous studies reported that paired box 6 mutation heterozygosity (Pax6(m/+)) led to defective proinsulin processing and subsequent abnormal glucose metabolism in mice at 6  months of age. However, high-fat diet exposure could be an important incentive for diabetes development. In this study, we aimed to develop a novel diabetic model imitating human type 2 diabetes by exposing Pax6(m/+) mice to high-fat diet and to explore the underlying mechanism of diabetes in this model.

METHODS

Over 300 Pax6(m/+) and wild-type male weanling mice were randomly divided into two groups and were fed an high-fat diet or chow diet for 6-10  weeks. Blood glucose and glucose tolerance levels were monitored during this period. Body weights, visceral adipose weights, blood lipid profiles and insulin sensitivity (determined with an insulin tolerance test) were used to evaluate obesity and insulin resistance. Proinsulin processing and insulin secretion levels were used to evaluate pancreatic β cell function.

RESULTS

After 6  weeks of high-fat diet exposure, only the Pax6(m/+) mice showed dramatic postloading hyperglycaemia. These mice exhibited significant high-fat diet-induced visceral obesity and insulin resistance and displayed defective prohormone convertase 1/3 production, an increased proinsulin:total insulin ratio and impaired early-phase insulin secretion, because of the Pax6 mutation. Hyperglycaemia worsened progressively over time with the high-fat diet, and most Pax6(m/+) mice on high-fat diet developed diabetes or impaired glucose tolerance after 10  weeks. Furthermore, high-fat diet withdrawal partly improved blood glucose levels in the diabetic mice.

CONCLUSIONS

By combining the Pax6(m/+) genetic background with an high-fat diet environment, we developed a novel diabetic model to mimic human type 2 diabetes. This model is characterized by impaired insulin secretion, caused by the Pax6 mutation, and high-fat diet-induced insulin resistance and therefore provides an ideal tool for research on type 2 diabetes pathogenesis and therapies.

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

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

Chronic exposure to GLP-1 increases GLP-1 synthesis and release in a pancreatic alpha cell line (α-TC1): evidence of a direct effect of GLP-1 on pancreatic alpha cells

Aims/Hypothesis

Incretin therapies, which are used to treat diabetic patients, cause a chronic supra-physiological increase in GLP-1 circulating levels. It is still unclear how the resulting high hormone concentrations may affect pancreatic alpha cells. The present study was designed to investigate the effects of chronic exposure to high GLP-1 levels on a cultured pancreatic alpha cell line.

Methods

α-TC1-6 cell line was cultured in the presence or absence of GLP-1 (100 nmol/l) for up to 72 h. In our model GLP-1 receptor (GLP-1R) was measured. After the cells were exposed to GLP-1 the levels of glucagon secretion were measured. Because GLP-1 acts on intracellular cAMP production, the function of GLP-1R was studied. We also investigated the effects of chronic GLP-1 exposure on the cAMP/MAPK pathway, Pax6 levels, the expression of prohormone convertases (PCs), glucagon gene (Gcg) and protein expression, glucagon and GLP-1 production.

Results

In our model, we were able to detect GLP-1R. After GLP-1 exposure we found a reduction in glucagon secretion. During further investigation of the function of GLP-1R, we found an activation of the cAMP/MAPK/Pax6 pathway and an increase of Gcg gene and protein expression. Furthermore we observed a significant increase in PC1/3 protein expression, GLP-1 intracellular content and GLP-1 secretion.

Conclusions/Interpretation

Our data indicate that the chronic exposure of pancreatic alpha cells to GLP-1 increases the ability of these cells to produce and release GLP-1. This phenomenon occurs through the stimulation of the transcription factor Pax6 and the increased expression of the protein convertase PC1/3.

Estimated proinsulin processing activity of prohormone convertase (PC) 1/3 rather than PC2 is decreased in pancreatic β-cells of type 2 diabetic patients

Type 2 diabetic (T2D) patients exhibit fasting relative hyperproinsulinemia owing to pancreatic β-cell dysfunction. To clarify the mechanism underlying this hyperproinsulinemic state, we evaluated the activities of the endopeptidases prohormone convertase (PC) 1/3 and PC2 in T2D patients. Fasting blood levels of intact proinsulin (IPI), total proinsulin (t-PI) and C-peptide were measured simultaneously, and intravenous glucagon loading was performed to investigate the dynamics of circulating proinsulin-related molecules released from pancreatic β-cells in 12 healthy volunteers and 18 T2D patients. Taking advantage of the 95% cross-reactivity between proinsulin and des-31,32-proinsulin (des-31,32-PI) with the human proinsulin radioimmunoassay kit used in this study, we estimated PC1/3 and PC2 activities using the following formulas: des-31,32-PI = (t-PI-IPI)/0.95; PC1/3 activity = des-31,32-PI/IPI; and PC2 activity = C-peptide/des-31,32-PI. C-peptide responses to glucagon were slightly lower among T2D patients. IPI and the IPI/C-peptide ratio were significantly higher in T2D patients (p<0.05 and p<0.01, respectively). There was no difference in des-31,32-PI levels or PC2 activity between the two groups. However, PC1/3 activity was significantly lower in T2D patients than in the control group (p<0.01). We propose that decreased activity of PC1/3 rather than PC2 in pancreatic β-cells is involved in the impaired proinsulin processing, resulting in elevated IPI levels in T2D patients.

Differential processing of neuropeptide proprotein in human breast adenocarcinoma

BACKGROUND

The processing of proprotein convertase (PC)-mediated neuropeptide plays a very important role in carcinogenesis and tumor proliferation.

AIM

To investigate proneuropeptide processing mechanism in tumorigenesis and tumor proliferation.

MATERIALS AND METHODS

The expression and processing profiles of PC1, carboxypeptidase E (CPE), PC2, GHRH, or neuropeptide Y (NPY) gene and protein level were investigated between 42 human breast tumor tissues and 21 tumor-adjacent normal tissues.

RESULTS

Gene analyses indicated that the proPC1, CPE, or preproNPY gene had higher expression in the breast tumor tissues, whereas the proPC2 or preproGHRH gene showed lower expression in the tissues. Protein analyses showed that the proPC1, PC1, CPE, GHRH, and preproNPY proteins were upregulated in the tumor tissues, whereas the proPC2, PC2, preproGHRH, and NPY proteins were down-regulated in them. The tissue results were highly corroborated with the serum data from the tumor patients and healthy women.

CONCLUSIONS

The higher PC1 and CPE expressions as well as the transformation of more proGHRH into active GHRH peptide suggest stronger PC1/CPE-mediated neuropeptide processing in the tumor, whereas the lower PC2 expression as well as the transformation of less proNPY into active NPY peptide suggests a weak PC2-mediated processing in it. The alterations of the convertase expressions and processing show that there is a differential proprotein processing system in the tumor, which leads to the abnormal distributions of species, ratio, and concentration of (pro)peptide(s) in the microenvironment of cells. The latter may contribute to cancer progression.

Morphine treatment selectively regulates expression of rat pituitary POMC and the prohormone convertases PC1/3 and PC2

The prohormone convertases, PC1/3 and PC2 are thought to be responsible for the activation of many prohormones through processing including the endogenous opioid peptides. We propose that maintenance of hormonal homeostasis can be achieved, in part, via alterations in levels of these enzymes that control the ratio of active hormone to prohormone. In order to test the hypothesis that exogenous opioids regulate the endogenous opioid system and the enzymes responsible for their biosynthesis, we studied the effect of short-term morphine or naltrexone treatment on pituitary PC1/3 and PC2 as well as on the level of pro-opiomelanocortin (POMC), the precursor gene for the biosynthesis of the endogenous opioid peptide, β-endorphin. Using ribonuclease protection assays, we observed that morphine down-regulated and naltrexone up-regulated rat pituitary PC1/3 and PC2 mRNA. Immunofluorescence and Western blot analysis confirmed that the protein levels changed in parallel with the changes in mRNA levels and were accompanied by changes in the levels of phosphorylated cyclic-AMP response element binding protein. We propose that the alterations of the prohormone processing system may be a compensatory mechanism in response to an exogenous opioid ligand whereby the organism tries to restore its homeostatic hormonal milieu following exposure to the opioid, possibly by regulating the levels of multiple endogenous opioid peptides and other neuropeptides in concert.

Ghrelin O-acyltransferase (GOAT) is expressed in prostate cancer tissues and cell lines and expression is differentially regulated in vitro by ghrelin

BACKGROUND

Ghrelin is a 28 amino acid peptide hormone that is expressed in the stomach and a range of peripheral tissues, where it frequently acts as an autocrine/paracrine growth factor. Ghrelin is modified by a unique acylation required for it to activate its cognate receptor, the growth hormone secretagogue receptor (GHSR), which mediates many of the actions of ghrelin. Recently, the enzyme responsible for adding the fatty acid residue (octanoyl/acyl group) to the third amino acid of ghrelin, GOAT (ghrelin O-acyltransferase), was identified.

METHODS

We used cell culture, quantitative real-time reverse transcription (RT)-PCR and immunohistochemistry to demonstrate the expression of GOAT in prostate cancer cell lines and tissues from patients. Real-time RT-PCR was used to demonstrate the expression of prohormone convertase (PC)1/3, PC2 and furin in prostate cancer cell lines. Prostate-derived cell lines were treated with ghrelin and desacyl ghrelin and the effect on GOAT expression was measured using quantitative RT-PCR.

RESULTS

We have demonstrated that GOAT mRNA and protein are expressed in the normal prostate and human prostate cancer tissue samples. The RWPE-1 and RWPE-2 normal prostate-derived cell lines and the LNCaP, DU145, and PC3 prostate cancer cell lines express GOAT and at least one other enzyme that is necessary to produce mature, acylated ghrelin from proghrelin (PC1/3, PC2 or furin). Finally, ghrelin, but not desacyl ghrelin (unacylated ghrelin), can directly regulate the expression of GOAT in the RWPE-1 normal prostate derived cell line and the PC3 prostate cancer cell line. Ghrelin treatment (100nM) for 6 hours significantly decreased GOAT mRNA expression two-fold (P < 0.05) in the PC3 prostate cancer cell line, however, ghrelin did not regulate GOAT expression in the DU145 and LNCaP prostate cancer cell lines.

CONCLUSIONS

This study demonstrates that GOAT is expressed in prostate cancer specimens and cell lines. Ghrelin regulates GOAT expression, however, this is likely to be cell-type specific. The expression of GOAT in prostate cancer supports the hypothesis that the ghrelin axis has autocrine/paracrine roles. We propose that the RWPE-1 prostate cell line and the PC3 prostate cancer cell line may be useful for investigating GOAT regulation and function.

Decreased immunoreactivity of the polypeptide precursor pro-opiomelanocortin (POMC) and the prohormone convertase pc1/3 after chronic ethanol exposure in Sprague-Dawley rats

BACKGROUND

The melanocortin (MC) peptides and opioid peptide β-endorphin are cleaved from the polypeptide precursor pro-opiomelanocortin (POMC). POMC-derived peptides are generated by extensive posttranslational processing that involves several enzymes including prohormone convertase 1/3 and 2 (PC1/3 and PC2). Because ethanol (EtOH) decreases POMC mRNA levels, we determined whether the exposure to an EtOH-containing diet (ED) would significantly reduce central immunoreactivity (IR) of POMC, PC1/3, PC2, and β-endorphin.

METHODS

Male Sprague-Dawley rats were given 18 days of access to a normal rodent chow or a control diet (CD), or short-term (4 days) or long-term (18 days) access to an ED. At the end of the study, rats were perfused with 4% paraformaldehyde, and their brains were sectioned into sets for processing with POMC, PC1/3, PC2, and β-endorphin IR.

RESULTS

Rats exposed to an ED for 18 days (ED18) exhibited significant reductions of POMC and PC1/3 IR in the arcuate nucleus of the hypothalamus (Arc) relative to rats pair-fed a CD. On the other hand, rats exposed to an ED did not show any changes of central β-endorphin or PC2 IR relative to rats pair-fed a CD, regardless of length of exposure. Because there were no differences in body weights or caloric intake between the CD and ED groups, reductions of POMC and PC1/3 IR in ED-treated rats are best explained by EtOH exposure rather than altered energy balance.

CONCLUSIONS

This study shows that EtOH site-specifically reduces POMC and PC1/3 IR in rat brain. These observations are consistent with EtOH-induced reductions of α-melanocyte-stimulating hormone (α-MSH) and POMC IR that were previously reported. As MC agonists have been shown to blunt EtOH intake in rodents, exogenous MC receptor agonists, as well as targets that may increase the synthesis of endogenous α-MSH (e.g., PC1/3), may have therapeutic value for treating alcohol abuse disorders and alcoholism.

Pax6 directly down-regulates Pcsk1n expression thereby regulating PC1/3 dependent proinsulin processing

Background

Heterozygous paired box6 (Pax6) mutations lead to abnormal glucose metabolism in mice older than 6 months as well as in human beings. Our previous study found that Pax6 deficiency caused down-expression of prohormone convertase 1/3 (Pcsk1), resulting in defective proinsulin processing. As a protein cleaving enzyme, in addition to its expression, the activity of PC1/3 is closely related to its function. We therefore hypothesize that Pax6 mutation alters the activity of PC1/3, which affects proinsulin processing.

Methodology/Principal Findings

Using quantitative RT-PCR, western blot and enzyme assay, we found that PC1/3 C-terminal cleavage and its activity were compromised in Pax6 R266Stop mutant mice, and the expression of Pcsk1n, a potent inhibitor of PC1/3, was elevated by Pax6 deficiency in the mutant mice and MIN6 cells. We confirmed the effect of proSAAS, the protein encoded by Pcsk1n, on PC1/3 C-terminal cleavage and its activity by Pcsk1n RNAi in MIN6 cells. Furthermore, by luciferase-reporter analysis, chromatin immunoprecipitation, and electrophoretic mobility shift assay, we revealed that Pax6 bound to Pcsk1n promoter and directly down-regulated its expression. Finally, by co-transfecting Pax6 siRNA with Pcsk1n siRNA, we showed that Pax6 knock-down inhibited proinsulin processing and that this effect could be rescued by proSAAS down-regulation. These findings confirm that Pax6 regulates proinsulin processing partially through proSAAS-mediated PC1/3 processing and activity.

Conclusions/Significance

Collectively, the above experiments demonstrate that Pax6 can directly down-regulate Pcsk1n expression, which negatively affects PC1/3 C-terminal cleavage and activity and subsequently participates in proinsulin processing. We identified proSAAS as a novel down-regulated target of Pax6 in the regulation of glucose metabolism. This study also provides a complete molecular mechanism for the Pax6 deficiency-caused diabetes.

Functional melanocortin-2 receptors are expressed by mouse aorta-derived mesenchymal progenitor cells

A local melanocortin system is active during tissue injury and inflammation. Thus far this system has been described as autocrine in nature where local production of pro-opiomelanocortin (POMC) peptides by leukocytes feeds back on melanocortin receptor (MC-R) expressing immune cells to quell inflammatory cytokine production. Here we present evidence that POMC peptides may generate extracellular matrix (ECM) changes by inducing matrix production by cells of the mesenchymal lineage through activation of the MC2-R. Using immunoblot, we determined that mouse aorta-derived mesenchymal progenitor cells express both MC2-R and MC3-R. These progenitors respond to treatment with ACTH by increasing collagen matrix synthesis as assessed by picrosirius red stain and (3)H-proline incorporation. ACTH also induces transient increases in intracellular calcium ([Ca(2+)](i)) as assessed using the fluorescent Ca(2+) indicator, fura-2. The ACTH-induced changes in [Ca(2+)](i) are consistent with MC2-R signaling and consist of both an intracellular release and an extracellular influx of Ca(2+). Both mouse aortic mesenchymal progenitors and mouse macrophage cells express POMC and the prohormone convertase 1/3 (PC1/3) indicating they have the potential to contribute to the local production of POMC peptides. These data demonstrate functional MC2-R expression in mouse aorta-derived mesenchymal progenitors and implicate both macrophage and mesenchymal cells as relevant sources of local POMC peptides.

Differential expression and processing of secretogranin II in relation to the status of pheochromocytoma: implications for the production of the tumoral marker EM66

We have previously demonstrated that measurement of tissue concentrations of the secretogranin II (SgII or SCG2 as listed in the HUGO database)-derived peptide EM66 may help to discriminate between benign and malignant pheochromocytomas and that EM66 represents a sensitive plasma marker of pheochromocytomas. Here, we investigated the gene expression and protein production of SgII in 13 normal adrenal glands, and 35 benign and 16 malignant pheochromocytomas with the goal to examine the molecular mechanisms leading to the marked variations in the expression of EM66 in tumoral chromaffin tissue. EM66 peptide levels were 16-fold higher in benign than in malignant pheochromocytomas and had an area under the receiver-operating characteristic curve of 0.95 for the distinction of benign and malignant tumors. Q-PCR experiments indicated that the SgII gene was significantly underexpressed in malignant tumors compared with benign tumors. Western blot analysis using antisera directed against SgII and SgII-derived fragments revealed lower SgII protein and SgII-processing products in malignant tumors. Western blot also showed that low p-cAMP-responsive element-binding (CREB) concentrations seemed to be associated with the malignant status. In addition, the prohormone convertase PC1 and PC2 genes and proteins were overexpressed in benign pheochromocytomas compared with malignant pheochromocytomas. Low concentrations of EM66 found in malignant tumors are associated with reduced expression and production of SgII and SgII-derived peptides that could be ascribed to a decrease in SgII gene transcription, probably linked to p-CREB down-regulation, and to lower PC levels. These findings highlight the mechanisms leading to lower concentrations of EM66 in malignant pheochromocytoma and strengthen the notion that this peptide is a suitable marker of this neuroendocrine tumor.

A common variant upstream of the PAX6 gene influences islet function in man

AIMS/HYPOTHESIS

Impaired glucose tolerance and impaired insulin secretion have been reported in families with PAX6 mutations and it is suggested that they result from defective proinsulin processing due to lack of prohormone convertase 1/3, encoded by PCSK1. We investigated whether a common PAX6 variant would mimic these findings and explored in detail its effect on islet function in man.

METHODS

A PAX6 candidate single nucleotide polymorphism (rs685428) was associated with fasting insulin levels in the Diabetes Genetics Initiative genome-wide association study. We explored its potential association with glucose tolerance and insulin processing and secretion in three Scandinavian cohorts (N = 8,897 individuals). In addition, insulin secretion and the expression of PAX6 and transcriptional target genes were studied in human pancreatic islets.

RESULTS

rs685428 G allele carriers had lower islet mRNA expression of PAX6 (p = 0.01) and PCSK1 (p = 0.001) than AA homozygotes. The G allele was associated with increased fasting insulin (p (replication) = 0.02, p (all) = 0.0008) and HOMA-insulin resistance (p (replication) = 0.02, p (all) = 0.001) as well as a lower fasting proinsulin/insulin ratio (p (all) = 0.008) and lower fasting glucagon (p = 0.04) and gastric inhibitory peptide (GIP) (p = 0.05) concentrations. Arginine-stimulated (p = 0.02) insulin secretion was reduced in vivo, which was further reflected by a reduction of glucose- and potassium-stimulated insulin secretion (p = 0.002 and p = 0.04, respectively) in human islets in vitro.

CONCLUSIONS/INTERPRETATION

A common variant in PAX6 is associated with reduced PAX6 and PCSK1 expression in human islets and reduced insulin response, as well as decreased glucagon and GIP concentrations and decreased insulin sensitivity. These findings emphasise the central role of PAX6 in the regulation of islet function and glucose metabolism in man.

Processing of proglucagon to GLP-1 in pancreatic α-cells: is this a paracrine mechanism enabling GLP-1 to act on β-cells?

Proglucagon is cleaved to glucagon by prohormone convertase 2 (PC2) in pancreatic α-cells, but is cleaved to glucagon-like peptide-1 (GLP-1) by PC1 in intestinal L-cells. The aim of this study was to identify mechanisms which switch processing of proglucagon to generate GLP-1 in the pancreas, given that GLP-1 can increase insulin secretion and β-cell mass. The α-cell line, αTC1-6, expressed PC1 at low levels and GLP-1 was detected in cells and in culture media. GLP-1 was also found in isolated human islets and in rat islets cultured for 7 days. High glucose concentrations increased Pc1 gene expression and PC1 protein in rat islets. High glucose (25  mM) also increased GLP-1 but decreased glucagon secretion from αTC1-6 cells suggesting a switch in processing to favour GLP-1. Three G protein-coupled receptors, GPR120, TGR5 and GPR119, implicated in the release of GLP-1 from L-cells are expressed in αTC1-6 cells. Incubation of these cells with an agonist of TGR5 increased PC1 promoter activity and GLP-1 secretion suggesting that this is a mechanism for switching processing to GLP-1 in the pancreas. Treatment of isolated rat islets with streptozotocin caused β-cell toxicity as evidenced by decreased glucose-stimulated insulin secretion. This increased GLP-1 but not glucagon in the islets. In summary, proglucagon can be processed to GLP-1 in pancreatic cells. This process is upregulated by elevated glucose, activation of TGR5 and β-cell destruction. Understanding this phenomenon may lead to advances in therapies to protect β-cell mass, and thereby slow progression from insulin resistance to type 2 diabetes.

Modulation of PC1/3 activity by self-interaction and substrate binding

Prohormone convertase (PC)1/3 is a eukaryotic serine protease in the subtilase family that participates in the proteolytic maturation of prohormone and neuropeptide precursors such as proinsulin and proopiomelanocortin. Despite the important role of this enzyme in peptide synthesis, how PC1/3 activity is regulated is still poorly understood. Using ion exchange chromatography and two-dimensional gel electrophoresis we found that natural PC1/3 present in AtT-20 cells and bovine chromaffin granules, as well as recombinant PC1/3 secreted from overexpressing Chinese hamster ovary cells, exists as multiple ionic forms. Gel filtration and cross-linking studies revealed that protein oligomerization and aggregation contribute greatly to variability in surface charge. The most acidic forms of PC1/3 contained both inactive aggregates as well as oligomerized 87-kDa PC1/3 that exhibited stable activity which was partially latent and could be revealed by dilution. No such latency was observed for the more basic, 66/74-kDa forms of PC1/3. Fractions containing these species were stabilized by preincubation with micromolar concentrations of either fluorogenic substrate or peptides containing pairs of basic residues. In addition, the most active form of 87-kDa PC1/3, a probable homodimer, was activated by preincubation with these same peptides. Cleavage by PC1/3 is often the initiating step in the biosynthetic pathway for peptide hormones, implying that this is a natural step for regulation. Our data suggest that enzyme oligomerization and peptide stabilization represent important contributing factors for the control of PC1/3 activity within secretory granules.

On the discovery of precursor processing

Studies of the biosynthesis of insulin in a human insulinoma beginning in 1965 provided the first evidence for a precursor of insulin, the first such prohormone to be identified. Further studies with isolated rat islets then confirmed that the precursor became labeled more rapidly than insulin and later was converted to insulin by a proteolytic processing system located mainly within the secretory granules of the beta cell and was then stored or secreted. The precursor was designated "proinsulin" in 1967 and was isolated and sequenced from beef and pork sources. These structural studies confirmed that the precursor was a single polypeptide chain which began with the B chain of insulin, continued through a connecting segment of 30-35 amino acids and terminated with the A chain. Paired basic residues were identified at the sites of excision of the C-peptide. Human proinsulin and C-peptide were then similarly obtained and sequenced. The human C-peptide assay was developed and provided a useful tool for measuring insulin levels indirectly in diabetics treated with insulin. The discovery of other precursor proteins for a variety of peptide hormones, neuropeptides, or plasma proteins then followed, with all having mainly dibasic cleavage sites for processing. The subsequent discovery of a similar biosynthetic pathway in yeast led to the identification of eukaryotic families of specialized processing subtilisin-like endopeptidases coupled with carboxypeptidase B-like exopeptidases. Most neuroendocrine peptides are processed by two specialized members of this family - PC2 and/or PC1/3 - followed by carboxypeptidase E (CPE). This brief report concentrates mainly on the role of insulin biosynthesis in providing a useful early paradigm of precursor processing in the secretory pathway.

Ghrelin is produced in taste cells and ghrelin receptor null mice show reduced taste responsivity to salty (NaCl) and sour (citric acid) tastants

Background

The gustatory system plays a critical role in determining food preferences, food intake and energy balance. The exact mechanisms that fine tune taste sensitivity are currently poorly defined, but it is clear that numerous factors such as efferent input and specific signal transduction cascades are involved.

Methodology/Principal Findings

Using immunohistochemical analyses, we show that ghrelin, a hormone classically considered to be an appetite-regulating hormone, is present within the taste buds of the tongue. Prepro-ghrelin, prohormone convertase 1/3 (PC 1/3), ghrelin, its cognate receptor (GHSR), and ghrelin-O-acyltransferase (GOAT , the enzyme that activates ghrelin) are expressed in Type I, II, III and IV taste cells of mouse taste buds. In addition, ghrelin and GHSR co-localize in the same taste cells, suggesting that ghrelin works in an autocrine manner in taste cells. To determine a role for ghrelin in modifying taste perception, we performed taste behavioral tests using GHSR null mice. GHSR null mice exhibited significantly reduced taste responsivity to sour (citric acid) and salty (sodium chloride) tastants.

Conclusions/Significance

These findings suggest that ghrelin plays a local modulatory role in determining taste bud signaling and function and could be a novel mechanism for the modulation of salty and sour taste responsivity.

Modulation of prohormone convertase 1/3 properties using site-directed mutagenesis

Prohormone convertase (PC)1/3 and PC2 cleave active peptide hormones and neuropeptides from precursor proteins. Compared with PC2, recombinant PC1/3 exhibits a very low specific activity against both small fluorogenic peptides and recombinant precursors, even though the catalytic domains in mouse PC1/3 and PC2 share 56% amino acid sequence identity. In this report, we have designed PC2-specific mutations into the catalytic domain of PC1/3 in order to investigate the molecular contributions of these sequences to PC1/3-specific properties. The exchange of residues RQG(314) with the SY sequence present in the same location within PC2 paradoxically shifted the pH optimum of PC1/3 upward into the neutral range; other mutations in the catalytic domain had no effect. Although none of the full-length PC1/3 mutants examined exhibited increased specific activity, the 66-kDa form of the RQG(314)SY mutant was two to four times more active than the 66-kDa form of wild-type PC1/3. However, stable transfection of RQG(314)SY into PC12 cells did not result in greater activity against the endogenous substrate proneurotensin, implying unknown cellular controls of PC1/3 activity. Mutation of GIVTDA(243-248) to QPFMTDI, a molecular determinant of 7B2 binding, resulted in increased zymogen expression but no propeptide cleavage or secretion, suggesting that this mutant is trapped in the endoplasmic reticulum due to an inability to cleave its own propeptide. We conclude that many convertase-specific properties are attributable less to convertase-specific catalytic cleft residues than to convertase-specific domain interactions.

Intraislet production of GLP-1 by activation of prohormone convertase 1/3 in pancreatic α-cells in mouse models of ß-cell regeneration

The islet of Langerhans is a highly vascularized micro-organ consisting of not only ß-cells but multiple cell types such as α-, delta-, pancreatic polypeptide- and epsilon-cells that work together to regulate glucose homeostatis. We have recently proposed a new model of the neonatal islet formation in mice by a process of fission following contiguous endocrine cell proliferation in the form of branched cord-like structures in embryos and newborns. There exist large stretches of interconnected islet structures along large blood vessels in the neonatal pancreas, which, upon further development, segregate into smaller fragments (i.e., islets) that eventually become more spherical by internal proliferation as seen in the adult pancreas. α-cells span these elongated islet-like structures in the developing pancreas, which we hypothesize represent sites of fission and facilitate the eventual formation of discrete islets. The α-cells express both prohormone convertase 2 and 1/3 (PC 2 and PC 1/3, respectively), which resulted in the processing of the proglucagon precursor into glucagon-like peptide 1, thereby leading to local production of this important ß-cell growth factor. Furthermore, while α-cells in the adult basically only express PC 2, significant activation of PC 1/3 is also observed in mouse models of insulin resistance such as pregnant, ob/ ob, db/db and prediabetic NOD mice, which may be a common mechanism in proliferating ß-cells. Our study suggests an important role of α-cells for ß-cell proliferation and further for the endocrine cell network within an islet.