Pro-protein convertases in intermediary metabolism: islet hormones, brain/gut hormones and integrated physiology

Low Fasting Concentrations of Glucagon in Patients with Very Long-Chain Acyl-CoA Dehydrogenase Deficiency

(1) Background: Deficiencies of mitochondrial fatty acid oxidation (FAO) define a subgroup of inborn errors of metabolism, with medium-chain acyl-CoA dehydrogenase deficiency (MCAD) and very long-chain acyl-CoA dehydrogenase deficiency (VLCAD) being two of the most common. Hypoketotic hypoglycemia is a feared clinical complication and the treatment focuses on avoiding hypoglycemia. In contrast, carnitine uptake deficiency (CUD) is treated as a mild disease without significant effects on FAO. Impaired FAO has experimentally been shown to impair glucagon secretion. Glucagon is an important glucose-mobilizing hormone. If and how glucagon is affected in patients with VLCAD or MCAD remains unknown. (2) Methods: A cross-sectional study was performed with plasma hormone concentrations quantified after four hours of fasting. Patients with VLCAD (n = 10), MCAD (n = 7) and CUD (n = 6) were included. (3) Results: The groups were similar in age, sex, weight, and height. The glucagon and insulin levels were significantly lower in the VLCAD group compared to the CUD group (p < 0.05, respectively). The patients with CUD had glucagon concentrations similar to the normative data. No significant differences were seen in GLP-1, glicentin, glucose, amino acids, or NEFAs. (4) Conclusions: Low fasting concentrations of glucagon are present in patients with VLCAD and cannot be explained by altered stimuli in plasma.

Variation in the Evolution and Sequences of Proglucagon and the Receptors for Proglucagon-Derived Peptides in Mammals

The mammalian proglucagon gene (Gcg) encodes three glucagon like sequences, glucagon, glucagon-like peptide-1 (GLP-1), and glucagon-like peptide-2 that are of similar length and share sequence similarity, with these hormones having cell surface receptors, glucagon receptor (Gcgr), GLP-1 receptor (Glp1r), and GLP-2 receptor (Glp2r), respectively. Gcgr, Glp1r, and Glp2r are all class B1 G protein-coupled receptors (GPCRs). Despite their sequence and structural similarity, analyses of sequences from rodents have found differences in patterns of sequence conservation and evolution. To determine whether these were rodent-specific traits or general features of these genes in mammals I analyzed coding and protein sequences for proglucagon and the receptors for proglucagon-derived peptides from the genomes of 168 mammalian species. Single copy genes for each gene were found in almost all genomes. In addition to glucagon sequences within Hystricognath rodents (e.g., guinea pig), glucagon sequences from a few other groups (e.g., pangolins and some bats) as well as changes in the proteolytic processing of GLP-1 in some bats are suggested to have functional effects. GLP-2 sequences display increased variability but accepted few substitutions that are predicted to have functional consequences. In parallel, Glp2r sequences display the most rapid protein sequence evolution, and show greater variability in amino acids at sites involved in ligand interaction, however most were not predicted to have a functional consequence. These observations suggest that a greater diversity in biological functions for proglucagon-derived peptides might exist in mammals.

The Effect of High-Fat Diet-Induced Obesity on the Expression of Nutrient Chemosensors in the Mouse Stomach and the Gastric Ghrelin Cell

The stomach is the primary source of the orexigenic and adiposity-promoting hormone, ghrelin. There is emerging evidence on the nutrient-mediated modulation of gastric ghrelin secretion. However, limited information is available on gastric nutrient-sensing mechanisms in high-fat diet (HFD)-induced obesity. This study investigated the impact of HFD-induced obesity on the expression of nutrient chemosensors in mouse stomach, particularly ghrelin cells. Male C57BL/6 mice were fed either a standard laboratory diet (SLD) or HFD for 12 weeks. The expression of ghrelin, enzymes involved in ghrelin production (PC1/3, GOAT) and nutrient chemosensors (CD36, FFAR2&4, GPR93, CaSR, mGluR4 and T1R3) was determined by quantitative RT-PCR in the mouse corpus and antrum. Immunohistochemistry assessed the protein expression of CaSR and ghrelin in the corpus and antrum. Antral mRNA levels of CaSR and PC1/3 were increased in HFD compared to SLD mice, while mRNA levels of all other nutrient chemosensors examined remained unchanged. CaSR immunolabelling was observed in the gastric antrum only. Nearly 80% of antral ghrelin cells expressed CaSR, with a similar cell density and co-expression in SLD and HFD mice. In conclusion, HFD-induced obesity increased CaSR mRNA expression in mouse antrum. However, the high antral co-expression of CaSR and ghrelin was unaltered in HFD compared to SLD mice.

Localization of amylin-like immunoreactivity in the striped velvet gecko pancreas

Immunohistochemical techniques were employed to investigate the distribution of amylin-like immunoreactive cells in the pancreas of gecko Homopholis fasciata. Four types of endocrine cells were distinguished: insulin immunoreactive (B cells), pancreatic polypeptide immunoreactive (PP cells), glucagon and pancreatic polypeptide immunoreactive (A/PP cells) and somatostatin immunoreactive cells (D cells). Pancreatic islets contained B, A/PP and D cells, whereas extrainsular regions contained B, D and PP cells. In the pancreatic islets, amylin-like immunoreactive cells corresponded to B cells, but not to A/PP or D cells. In the extrainsular regions, amylin-like immunoreactive cells corresponded to either B or PP cells. Amylin secreted from intrainsular B cells may regulate pancreatic hormone secretion in an autocrine and/or a paracrine fashion. On the other hand, amylin secreted from extrainsular PP and B cells, and/or intrainsular B cells may participate in the modulation of calcium homoeostasis in an endocrine fashion.

Insights on glicentin, a promising peptide of the proglucagon family

Glicentin is a proglucagon-derived peptide mainly produced in the L-intestinal cells. While the roles of other members of the proglucagon family including glucagon-like peptide 1, glucagon-like peptide 2 and oxyntomodulin has been well studied, the functions and variation of glicentin in human are not fully understood. Experimental and clinical studies have highlighted its role in both intestinal physiology and glucose metabolism, pointing to its potential interest in a wide range of pathological states including gastrointestinal and metabolic disorders. Due to its structure presenting many similarities with the other proglucagon-derived peptides, its measurement is technically challenging. The recent commercialization of specific detection methods has offered new opportunities to go further in the understanding of glicentin physiology. Here we summarize the current knowledge on glicentin biogenesis and physiological roles. In the limelight of clinical studies investigating glicentin variation in human, we discuss future directions for potential applications in clinical practice.

Modified Western blotting for insulin and other diabetes-associated peptide hormones

Now, the quantification of proinsulin/insulin contents within organisms tends to be evaluated only by enzyme-linked immunosorbent assay (ELISA), although assessing the adequacy of results by some quantification method is important. Remarkably, few scientific papers use detection by Western blotting (WB), another immunological assay, of proinsulin/insulin. We found two problems with quantification of insulin and proinsulin by general WB: the shape of an insulin band in gel electrophoresis is distorted, and the retention potency to a blotting membrane of the peptide hormones (mainly insulin) is low. We solved the first problem by optimizing the sodium dodecyl sulfate concentration in the sample buffer and the second problem by glutaraldehyde fixation following treatment with a blocking solution for a short time. The improvements were confirmed by quantification of proinsulin/insulin in standards, MIN6c4 cell lysates, and MIN6c4 culture supernatants. Furthermore, we showed that the modified WB is applicable to other diabetes-associated peptide hormones: insulin analogs, glucagon, GLP-1s, somatostatins, ghrelins, and pancreatic polypeptide. Our data showed that the modified WB can contribute to qualitative or quantitative analyses of diabetes-associated peptides by providing analytical information based on electrophoresis, although ELISA, which is an almost exclusive method in the quantification of peptide hormones, supplies only numerical data.

Decreased serum glicentin concentration in patients with severe and morbid obesity

Background

Proglucagon-derived hormones represent a family of peptides mainly produced in the pancreas and the intestine. While several proglucagon-derived peptides play key roles in metabolic diseases, little is known about glicentin. The aim of the present study was to investigate serum glicentin concentrations in individuals with adult obesity and to study its potential link with various metabolic parameters.

Methods

Fifty-two individuals with normal body mass index (BMI < 25 kg/m2) and 39 patients with severe or morbid obesity (BMI > 35 kg/m2) were prospectively included at the University Hospital of Nice between January 2014 and April 2016. Clinical data were recorded, and a fasting blood sample was collected to measure glicentin, glucose, insulin, C-peptide, total cholesterol, triglyceride, LDL and HDL-cholesterol. In addition, a homeostasis model assessment for insulin resistance (HOMA2-IR) was also calculated.

Results

Patients with severe and morbid obesity had significantly higher plasma glucose, together with higher serum concentrations of insulin, C-peptide, HOMA2-IR, triglyceride, LDL-cholesterol and lower serum concentrations of HDL-cholesterol compared with individuals with a normal body mass index. The obese patients displayed significantly lower fasting serum concentrations of glicentin compared with subjects with a normal body mass index (12 pmol/L vs. 24 pmol/L, P < 0.0001). In the total population, fasting glicentin concentrations did not correlate with BMI, glycaemic parameters (glucose, insulin, C-peptide, HOMA-IR) or lipid parameters (total cholesterol, triglyceride, LDL and HDL-cholesterol).

Conclusion

To the best of our knowledge, this is the first study reporting serum glicentin concentrations in healthy lean and obese adult subjects. We found that fasting serum glicentin concentrations are decreased in patients with severe or morbid obesity suggesting the potential interest of this peptide in obesity and metabolic-related disorders.

Evolving function and potential of pancreatic alpha cells

The alpha cells that co-occupy the islets in association with beta cells have been long recognized as the source of glucagon, a hyperglycemia-producing and diabetogenic hormone. Although the mechanisms that control the functions of alpha cells, glucagon secretion, and the role of glucagon in diabetes have remained somewhat enigmatic over the fifty years since their discovery, seminal findings during the past few years have moved alpha cells into the spotlight of scientific discovery. These findings obtained largely from studies in mice are: Alpha cells have the capacity to trans-differentiate into insulin-producing beta cells. Alpha cells contain a GLP-1 generating system that produces GLP-1 locally for paracrine actions within the islets that likely promotes beta cell growth and survival and maintains beta cell mass. Impairment of glucagon signaling both prevents the occurrence of diabetes in conditions of the near absence of insulin and expands alpha cell mass. Alpha cells appear to serve as helper cells or guardians of beta cells to ensure their health and well-being. Of potential relevance to the possibility of promoting the transformation of alpha to beta cells is the observation that impairment of glucagon signaling leads to a marked increase in alpha cell mass in the islets. Such alpha cell hyperplasia provides an increased supply of alpha cells for their transdifferentiation into new beta cells. In this review we discuss these recent discoveries from the perspective of their potential relevance to the treatment of diabetes.

Postabsorptive hyperglucagonemia in patients with type 2 diabetes mellitus analyzed with a novel enzyme-linked immunosorbent assay

Aims/introduction

The aims of the present study were to investigate the performance of a novel sandwich enzyme‐linked immunosorbent assay (ELISA) for measuring glucagon (1–29) with monoclonal antibodies against both the C‐ and N‐terminal regions of glucagon (1–29), and to analyze the differences in plasma levels and responses of glucagon (1–29) to oral glucose loading in normal glucose tolerance (NGT) subjects and patients with type 2 diabetes mellitus.

Materials and Methods

The cross‐reactivity against proglucagon fragments using the ELISA kit and two types of conventional radioimmunoassay (RIA) kits was evaluated. A 75‐g oral glucose tolerance test was carried out with NGT subjects and patients with type 2 diabetes mellitus, and the glucagon (1–29) concentration was measured using three types of kit.

Results

The ELISA kit clearly had the lowest cross‐reactivity against miniglucagon (19–29) and glicentin (1–61). The oral glucose tolerance test was carried out with 30 NGT and 17 patients with type 2 diabetes mellitus. The glucagon (1–29) levels measured by the ELISA kit after glucose loading were significantly higher at all time‐points in the type 2 diabetes mellitus group than in the NGT group. However, the glucagon (1–29) levels measured by one RIA kit were significantly higher in the NGT group, and those measured with the other RIA kit were approximately the same among the groups.

Conclusions

The novel sandwich ELISA accurately determines plasma glucagon (1–29) concentrations with much less cross‐reactivity against other proglucagon fragments than conventional RIA kits.

Glucagon-reactive islet-infiltrating CD8 T cells in NOD mice

Type 1 diabetes is characterized by T-cell-mediated destruction of the insulin-producing β cells in pancreatic islets. A number of islet antigens recognized by CD8 T cells that contribute to disease pathogenesis in non-obese diabetic (NOD) mice have been identified; however, the antigenic specificities of the majority of the islet-infiltrating cells have yet to be determined. The primary goal of the current study was to identify candidate antigens based on the level and specificity of expression of their genes in mouse islets and in the mouse β cell line MIN6. Peptides derived from the candidates were selected based on their predicted ability to bind H-2K(d) and were examined for recognition by islet-infiltrating T cells from NOD mice. Several proteins, including those encoded by Abcc8, Atp2a2, Pcsk2, Peg3 and Scg2, were validated as antigens in this way. Interestingly, islet-infiltrating T cells were also found to recognize peptides derived from proglucagon, whose expression in pancreatic islets is associated with α cells, which are not usually implicated in type 1 diabetes pathogenesis. However, type 1 diabetes patients have been reported to have serum autoantibodies to glucagon, and NOD mouse studies have shown a decrease in α cell mass during disease pathogenesis. Our finding of islet-infiltrating glucagon-specific T cells is consistent with these reports and suggests the possibility of α cell involvement in development and progression of disease.

Expression of Secretogranin III in Chicken Endocrine Cells: Its Relevance to the Secretory Granule Properties of Peptide Prohormone Processing and Bioactive Amine Content

The expression of secretogranin III (SgIII) in chicken endocrine cells has not been investigated. There is limited data available for the immunohistochemical localization of SgIII in the brain, pituitary, and pancreatic islets of humans and rodents. In the present study, we used immunoblotting to reveal the similarities between the expression patterns of SgIII in the common endocrine glands of chickens and rats. The protein-protein interactions between SgIII and chromogranin A (CgA) mediate the sorting of CgA/prohormone core aggregates to the secretory granule membrane. We examined these interactions using co-immunoprecipitation in chicken endocrine tissues. Using immunohistochemistry, we also examined the expression of SgIII in a wide range of chicken endocrine glands and gastrointestinal endocrine cells (GECs). SgIII was expressed in the pituitary, pineal, adrenal (medullary parts), parathyroid, and ultimobranchial glands, but not in the thyroid gland. It was also expressed in GECs of the stomach (proventriculus and gizzard), small and large intestines, and pancreatic islet cells. These SgIII-expressing cells co-expressed serotonin, somatostatin, gastric inhibitory polypeptide, glucagon-like peptide-1, glucagon, or insulin. These results suggest that SgIII is expressed in the endocrine cells that secrete peptide hormones, which mature via the intragranular enzymatic processing of prohormones and physiologically active amines in chickens.

Glucagon-like peptide-1 receptor in the brain: role in neuroendocrine control of energy metabolism and treatment target for obesity

The central glucagon-like peptide (GLP)-1 receptor mediates a number of metabolic processes, including feeding, body weight and glucose homeostasis. More recently, roles in energy expenditure and reward pathway modulation have been described. GLP-1 receptor agonism promotes insulin release and is currently used to treat type 2 diabetes humans, with a common side effect being weight loss. It is likely that many of these metabolic effects are mediated by GLP-1Rs located in the central nervous system, throughout areas known to be important in control of energy homeostasis. The physiological role of the GLP-1 receptor signalling in each of these brain nuclei is becoming clearer and the GLP-1 system appears to act as an integrator of peripheral energy availability and effector of metabolic regulation. This makes it an attractive target for obesity therapies, with excellent pre-clinical efficacy seen in molecules combining GLP-1 receptor agonism with glucagon receptor agonism. This review discusses what is known about the functions of the GLP-1 receptor in the central nervous system, and highlights the ways that this system may be targeted for development of new obesity therapeutics.

The sorting of proglucagon to secretory granules is mediated by carboxypeptidase E and intrinsic sorting signals

Proglucagon is expressed in pancreatic alpha cells, intestinal L cells and brainstem neurons. Tissue-specific processing of proglucagon yields the peptide hormones glucagon in the alpha cell and glucagon-like peptide (GLP)-1 and GLP-2 in L cells. Both glucagon and GLP-1 are secreted in response to nutritional status and are critical for regulating glycaemia. The sorting of proglucagon to the dense-core secretory granules of the regulated secretory pathway is essential for the appropriate secretion of glucagon and GLP-1. We examined the roles of carboxypeptidase E (CPE), a prohormone sorting receptor, the processing enzymes PC1/3 and PC2 and putative intrinsic sorting signals in proglucagon sorting. In Neuro 2a cells that lacked CPE, PC1/3 and PC2, proglucagon co-localised with the Golgi marker p115 as determined by quantitative immunofluorescence microscopy. Expression of CPE, but not of PC1/3 or PC2, enhanced proglucagon sorting to granules. siRNA-mediated knockdown of CPE disrupted regulated secretion of glucagon from pancreatic-derived alphaTC1-6 cells, but not of GLP-1 from intestinal cell-derived GLUTag cells. Mutation of the PC cleavage site K70R71, the dibasic R17R18 site within glucagon or the alpha-helix of glucagon, all significantly affected the sub-cellular localisation of proglucagon. Protein modelling revealed that alpha helices corresponding to glucagon, GLP-1 and GLP-2, are arranged within a disordered structure, suggesting some flexibility in the sorting mechanism. We conclude that there are multiple mechanisms for sorting proglucagon to the regulated secretory pathway, including a role for CPE in pancreatic alpha cells, initial cleavage at K70R71 and multiple sorting signals.

Origin and convergent evolution of exendin genes

Exendins are secretin hormone-like peptides that are components of the toxins from two venomous lizards, Heloderma suspectum (Gila monster) and Heloderma horridium (Mexican bearded lizard). Exendins-1 and -2 are vasoactive intestinal peptide (VIP)-like, both in sequence and function, while exendins-3 and -4 are glucagon-like peptide-1 (GLP-1)-like. The evolutionary origin of these peptides, and the genes that encode them, has been unclear. Recently, genes orthologous to exendin have been identified in reptiles, birds and amphibians. Analysis of the orthologous sequences demonstrates that the Heloderma exendins diversified by gene duplication from a common exendin ancestor on the Heloderma lineage after divergence from other reptiles, including the anole lizard and Burmese python. In addition, the exendin toxin peptide sequences, but not their pro or signal peptides, have evolved very rapidly on the Heloderma lineage, likely as they adapted to their new function as toxins. Exendins-1 and -2 not only evolved rapidly but their sequences have evolved convergently upon that of VIP, resulting in a doubling of its identity with VIP, while exendins-3 and -4 have retained an ancestral property of being more GLP-1-like sequences. These results suggest that the ancestral role of exendin, which is potentially still retained in some species, had greater similarity with proglucagon-derived peptides or GIP.

Revisiting the regulated secretory pathway: from frogs to human

The regulated secretory pathway is a hallmark of endocrine and neuroendocrine cells. This process comprises different sequential steps, including ER-associated protein synthesis, ER-to-Golgi protein transport, Golgi-associated posttranslational modification, sorting and packing of secretory proteins into carrier granules, cytoskeleton-based granule transport towards the plasma membrane and tethering, docking and fusion of granules with specialized releasing zones in the plasma membrane. Each one of these steps is tightly regulated by a large number of factors that function in a spatially and temporarily coordinated fashion. During the past three decades, much effort has been devoted to characterize the precise role of the yet-known proteins participating in the different steps of this process and to identify new regulatory factors in order to obtain a unifying picture of the secretory pathway. In spite of this and given the enormous complexity of the process, certain steps are not fully understood yet and many players remain to be identified. In this review, we offer a summary of the current knowledge on the main molecular mechanisms that govern and ensure the correct release of secretory proteins. In addition, we have integrated the advance on the field made possible by studies carried out in non-mammalian vertebrates, which, although not very numerous, have substantially contributed to acquire a mechanistic understanding of the regulated secretory pathway.

Evolution of genes for incretin hormones and their receptors

The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are essential components in the regulation of blood glucose levels in mammals. These two incretins are produced by evolutionarily related genes and these hormones show similarity in sequence as both are glucagon-like sequences. Genes for these hormones have been identified in a number of diverse vertebrate species indicating that they originated prior to the earliest divergences of vertebrate species. However, analysis of functional and sequence data suggest that each of these hormones acquired incretin activity independently, and only since the divergence of tetrapods from fish. Not only are the hormones related, but so are their receptors. Like the hormones, the incretin action of the receptors is not a product of a shared common ancestral history, as the receptors for GLP-1 and GIP are not most closely related. Further study of the physiological functions of GLP-1 and GIP in additional vertebrates is required to better understand the origin of incretin action.

The avian proglucagon system

Understanding how the proglucagon system functions in maintaining glycemic control and energy balance in birds, as well as defining its specific roles in regulating metabolism, gastrointestinal tract function and food intake requires detailed knowledge of the components that comprise this system. These include proglucagon, a precursor protein from which glucagon and two glucagon-like peptide hormones (GLP-1 and GLP-2) are derived, and the membrane bound G-protein-coupled receptors that specifically bind glucagon, GLP-1 and GLP-2 to mediate their individual physiological actions. Another key feature of the proglucagon system that is important for regulating its activity in different tissues involves post-translational processing of the proglucagon precursor protein and the individual peptide hormones derived from it. Currently, there is limited information about the proglucagon system in birds with the majority of that coming from studies involving chickens. By summarizing what is currently known about the proglucagon system in birds, this review aims to provide useful background information for future investigations that will explore the nature and actions of this important hormonal system in different avian species.

Processing of peptide and hormone precursors at the dibasic cleavage sites

Many functionally important cellular peptides and proteins, including hormones, neuropeptides, and growth factors, are synthesized as inactive precursor polypeptides, which require post-translational proteolytic processing to become biologically active polypeptides. This is achieved by the action of a relatively small number of proteases that belong to a family of seven subtilisin-like proprotein convertases (PCs) including furin. In view of this, this review focuses on the importance of privileged secondary structures and of given amino acid residues around basic cleavage sites in substrate recognition by these endoproteases. In addition to their participation in normal cell functions, PCs are crucial for the initiation and progress of many important diseases. Hence, these proteases constitute potential drug targets in medicine. Accordingly, this review also discusses the approaches used to shed light on the cleavage preference and the substrate specificity of the PCs, a prerequisite to select which PCs are promising drug targets in each disease.

Pax6 regulates the proglucagon processing enzyme PC2 and its chaperone 7B2

Pax6 is important in the development of the pancreas and was previously shown to regulate pancreatic endocrine differentiation, as well as the insulin, glucagon, and somatostatin genes. Prohormone convertase 2 (PC2) is the main processing enzyme in pancreatic alpha cells, where it processes proglucagon to produce glucagon under the spatial and temporal control of 7B2, which functions as a molecular chaperone. To investigate the role of Pax6 in glucagon biosynthesis, we studied potential target genes in InR1G9 alpha cells transfected with Pax6 small interfering RNA and in InR1G9 clones expressing a dominant-negative form of Pax6. We now report that Pax6 controls the expression of the PC2 and 7B2 genes. By binding and transactivation studies, we found that Pax6 indirectly regulates PC2 gene transcription through cMaf and Beta2/NeuroD1 while it activates the 7B2 gene both directly and indirectly through the same transcription factors, cMaf and Beta2/NeuroD1. We conclude that Pax6 is critical for glucagon biosynthesis and processing by directly and indirectly activating the glucagon gene through cMaf and Beta2/NeuroD1, as well as the PC2 and 7B2 genes.

Glucagon and glucagon-like peptides 1 and 2

The glucagon gene is expressed not only in the alpha cells of the pancreatic islets but also in the endocrine cells of the intestinal epithelium (so-called L-cells), and in certain neurons of the brain stem. Whereas in the pancreas, glucagon, the hyperglycaemic hormone, is cleaved out of the 160 amino acid precursor, proglucagon, leaving behind proglucagon fragments (PG 1-30 and PG 72-158, the so-called major proglucagon fragment (MPGF)) that are probably inactive, the intestinal processing leads to the formation of glicentin (PG 1-69; action uncertain) and glucagon-like peptides 1 (PG 78-107amide, a potent incretin homone, regulating insulin secretion, glucagon secretion, gastrointestinal motility and appetite) and 2 (PG 126-158, a regulator of gut mucosal growth and integrity). The two prohormone convertases PC2 and PC1/3, respectively, are responsible for the differential processing. After their release, the hormones are eliminated mainly in the kidneys, but both GLP-2 and in particular GLP-1, but not glucagon, are metabolized both locally and in the circulation and liver by dipeptidyl peptidase 4 (DPP-4) which inactivates the peptides, suggesting that GLP-1 acts locally rather than in an endocrine manner. A number of transcription factors have been identified that can at least partly explain the differential cellular expression of the glucagon gene as well as the differential tissue-specific processing of the precursor.

Ontogeny of ghrelin, obestatin, preproghrelin, and prohormone convertases in rat pancreas and stomach

The processing of preproghrelin in the stomach by prohormone convertase (PC) 1/3 produces ghrelin and possibly obestatin. In the neonate, the pancreas is also a major source of ghrelin. We compared the ontogeny of preproghrelin, ghrelin, obestatin, and PCs in the stomach and pancreas from rat embryos (day 21) and neonates (days 1, 6, 13, 21, and 28) by immunohistochemistry. In stomach, preproghrelin positive cells were present from embryonic day 21 and were in excess of ghrelin cells. The number of ghrelin positive cells progressively increased with age. When preproghrelin cells were immunoreactive for ghrelin, they were also immunoreactive for obestatin and PC1/3. In pancreas, we only found 0 to 2 preproghrelin positive cells per islet and each of these cells was also positive for ghrelin and obestatin. None of the ghrelin positive cells stained for insulin, but we observed ghrelin positive/glucagon negative and ghrelin positive/glucagon positive cells. Ghrelin positive cells contained PC1/3 or PC2. In summary, in stomach, an excess of preproghrelin positive cells compared with ghrelin/PC1/3 positive cells suggests that PC1/3 determines preproghrelin processing to ghrelin. In pancreas, the colocalization of PC1/3 or PC2 in ghrelin positive cells points to a role for both PCs in preproghrelin processing.