Identification and processing of proglucagon in pancreatic islets

Glucagon and other proglucagon-derived peptides in the pathogenesis of obesity

Because of differential processing of the hormone precursor, proglucagon, numerous peptide products are released from the pancreatic alpha cells and the intestinal L-cells in which the (pro)glucagon gene is expressed. Of particular interest in relation to obesity are glucagon from the pancreas and oxyntomodulin and GLP-1 from the gut, all of which inhibit food intake, but the other products are also briefly discussed, because knowledge about these is required for selection and evaluation of the methods for measurement of the hormones. The distal intestinal L-cells also secrete the appetite-inhibiting hormone PYY. Characteristics of the secretion of the pancreatic and intestinal products are described, and causes of the hypersecretion of glucagon in obesity and type 2 diabetes are discussed. In contrast, the secretion of the products of the L-cells is generally impaired in obesity, raising questions about their role in the development of obesity. It is concluded that the impairment probably is secondary to obesity, but the lower plasma levels may contribute to the development.

Striking the Balance: GLP-1/Glucagon Co-Agonism as a Treatment Strategy for Obesity

Obesity and Type 2 diabetes represent global health challenges, and there is an unmet need for long-lasting and effective pharmacotherapies. Although long-acting glucagon-like peptide-1 (GLP-1) analogues are now in routine use for diabetes and are now being utilised for obesity per se, the need for ever better treatments has driven the development of co-agonists, with the theoretical advantages of improved efficacy by targeting multiple pathways and reduced adverse effects. In this review, we highlight the past and present progress in our understanding and development of treatments based on GLP-1/glucagon co-agonism. We also reflect on the divergent effects of varying the GLP-1:glucagon activity and ratio in the context of pre-clinical and human clinical trial findings. In particular, the multiple metabolic actions of glucagon highlight the importance of understanding the contributions of individual hormone action to inform the safe, effective and tailored use of GLP-1/glucagon co-agonists to target weight loss and metabolic disease in the future.

Insights into incretin-based therapies for treatment of diabetic dyslipidemia

Derangements in triglyceride and cholesterol metabolism (dyslipidemia) are major risk factors for the development of cardiovascular diseases in obese and type-2 diabetic (T2D) patients. An emerging class of glucagon-like peptide-1 (GLP-1) analogues and next generation peptide dual-agonists such as GLP-1/glucagon or GLP-1/GIP could provide effective therapeutic options for T2D patients. In addition to their role in glucose and energy homeostasis, GLP-1, GIP and glucagon serve as regulators of lipid metabolism. This review summarizes the current knowledge in GLP-1, glucagon and GIP effects on lipid and lipoprotein metabolism and frames the emerging therapeutic benefits of GLP-1 analogs and GLP-1-based multiagonists as add-on treatment options for diabetes associated dyslipidemia.

Glucagon-like peptide 1 (GLP-1)

BACKGROUND

The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases.

MAJOR CONCLUSIONS

Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders.

From the Incretin Concept and the Discovery of GLP-1 to Today's Diabetes Therapy

Researchers have been looking for insulin-stimulating factors for more than 100 years, and in the 1960ties it was definitively proven that the gastrointestinal tract releases important insulinotropic factors upon oral glucose intake, so-called incretin hormones. The first significant factor identified was the duodenal glucose-dependent insulinotropic polypeptide, GIP, which however, turned out not to stimulate insulin secretion in patients with type 2 diabetes. But resection experiments clearly indicated the presence of an additional incretin, and in 1986, an unexpected processing fragment of the recently identified glucagon precursor, proglucagon, namely truncated glucagon-like peptide 1 (GLP-1 7-36 amide), was isolated from the gut and found to both stimulate insulin secretion and inhibit glucagon secretion. The peptide also inhibited appetite and food intake. Unlike GIP, this peptide had preserved effects in patients with type 2 diabetes and it was soon documented to have powerful antidiabetic effects in clinical studies. Its utility was limited, however, because of an extremely short half-life in humans, but this problem had two solutions, both of which gave rise to important antidiabetic drugs: (1) orally active inhibitors of the enzyme dipeptidylpeptidase 4 (DPP-4 inhibitors), which was responsible for the rapid degradation; the inhibitors protect endogenous GLP-1 from degradation and thereby unfold its antidiabetic activity, and (2) long-acting injectable analogs of GLP-1 protected against DPP-4 degradation. Particularly, the latter, the GLP-1 receptor agonists, either alone or in various combinations, are so powerful that treatment allows more than 2/3 of type 2 diabetes patients to reach glycemic targets. In addition, these agents cause a weight loss which, with the most successful compounds, may exceed 10% of body weight. Most recently they have also been shown to be renoprotective and reduce cardiovascular risk and mortality.

Extrapancreatic glucagon: Present status

Pancreatic alpha cells are generally considered the only source of glucagon secretion in humans. In the 1970s several groups investigating totally pancreatectomised animals reported that glucagon-like immunoreactive material could be detected in the gastrointestinal tract and reopened the question of an extrapancreatic source of glucagon proposed in 1948 when a hyperglycaemic substance was found in the gastrointestinal tract of dogs and rabbits. Nevertheless, over the years, controversy about the existence of extrapancreatic glucagon has flourished as it proved difficult to accurately measure fully processed 29-amino acid glucagon. Recent advances in analytical methods have increased sensitivity and specificity of glucagon assays and, furthermore, technical advances in mass spectrometry-based proteomics have made the detection of low-abundant peptides, such as glucagon, in human plasma more accurate. Here we review new data on extrapancreatic glucagon secretion in the context of historical data and recent analytical breakthroughs. Furthermore, the source, regulation and potential physiological role of extrapancreatic glucagon are discussed and ongoing challenges and knowledge-gaps are outlined.

Intra-islet glucagon-like peptide 1

PURPOSE

Glucagon-like peptide-1 (GLP-1) is originally identified in the gut as an incretin hormone, and it is potent in stimulating insulin secretion in the pancreas. However, increasing evidence suggests that GLP-1 is also produced locally within pancreatic islets. This review focuses on the past and current discoveries regarding intra-islet GLP-1 production and its functions.

MAIN FINDINGS

There has been a long-standing debate with regard to whether GLP-1 is produced in the pancreatic α cells. Early controversies lead to the widely accepted conclusion that the vast majority of proglucagon is processed to form glucagon in the pancreas, whereas an insignificant amount is cleaved to produce GLP-1. With technological advancements, recent studies have shown that bioactive GLP-1 is produced locally in the pancreas, and the expression and secretion of GLP-1 within islets are regulated by various factors such as cytokines, hyperglycemia, and β cell injury.

CONCLUSIONS

GLP-1 is produced by the pancreatic α cells, and it is fully functional as an incretin. Therefore, intra-islet GLP-1 may exert insulinotropic and glucagonostatic effects locally via paracrine and/or autocrine actions, under both normal and diabetic conditions.

Altered Plasma Levels of Glucagon, GLP-1 and Glicentin During OGTT in Adolescents With Obesity and Type 2 Diabetes

CONTEXT

Proglucagon-derived hormones are important for glucose metabolism, but little is known about them in pediatric obesity and type 2 diabetes mellitus (T2DM).

OBJECTIVE

Fasting and postprandial levels of proglucagon-derived peptides glucagon, GLP-1, and glicentin in adolescents with obesity across the glucose tolerance spectrum were investigated.

DESIGN

This was a cross-sectional study with plasma hormone levels quantified at fasting and during an oral glucose tolerance test (OGTT).

SETTING

This study took place in a pediatric obesity clinic at Uppsala University Hospital, Sweden.

PATIENTS AND PARTICIPANTS

Adolescents with obesity, age 10-18 years, with normal glucose tolerance (NGT, n = 23), impaired glucose tolerance (IGT, n = 19), or T2DM (n = 4) and age-matched lean adolescents (n = 19) were included.

MAIN OUTCOME MEASURES

Outcome measures were fasting and OGTT plasma levels of insulin, glucagon, active GLP-1, and glicentin.

RESULTS

Adolescents with obesity and IGT had lower fasting GLP-1 and glicentin levels than those with NGT (0.25 vs 0.53 pM, P < .05; 18.2 vs 23.6 pM, P < .01) and adolescents with obesity and T2DM had higher fasting glucagon levels (18.1 vs 10.1 pM, P < .01) than those with NGT. During OGTT, glicentin/glucagon ratios were lower in adolescents with obesity and NGT than in lean adolescents (P < .01) and even lower in IGT (P < .05) and T2DM (P < .001).

CONCLUSIONS

Obese adolescents with IGT have lowered fasting GLP-1 and glicentin levels. In T2DM, fasting glucagon levels are elevated, whereas GLP-1 and glicentin levels are maintained low. During OGTT, adolescents with obesity have more products of pancreatically than intestinally cleaved proglucagon (ie, more glucagon and less GLP-1) in the plasma. This shift becomes more pronounced when glucose tolerance deteriorates.

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.

Molecular mechanisms underlying physiological and receptor pleiotropic effects mediated by GLP-1R activation

The incidence of type 2 diabetes in developed countries is increasing yearly with a significant negative impact on patient quality of life and an enormous burden on the healthcare system. Current biguanide and thiazolidinedione treatments for type 2 diabetes have a number of clinical limitations, the most serious long-term limitation being the eventual need for insulin replacement therapy (Table 1). Since 2007, drugs targeting the glucagon-like peptide-1 (GLP-1) receptor have been marketed for the treatment of type 2 diabetes. These drugs have enjoyed a great deal of success even though our underlying understanding of the mechanisms for their pleiotropic effects remain poorly characterized even while major pharmaceutical companies actively pursue small molecule alternatives. Coupling of the GLP-1 receptor to more than one signalling pathway (pleiotropic signalling) can result in ligand-dependent signalling bias and for a peptide receptor such as the GLP-1 receptor this can be exaggerated with the use of small molecule agonists. Better consideration of receptor signalling pleiotropy will be necessary for future drug development. This is particularly important given the recent failure of taspoglutide, the report of increased risk of pancreatitis associated with GLP-1 mimetics and the observed clinical differences between liraglutide, exenatide and the newly developed long-acting exenatide long acting release, albiglutide and dulaglutide.

Nutrient regulation of glucagon secretion: involvement in metabolism and diabetes

Glucose homeostasis is precisely regulated by glucagon and insulin, which are released by pancreatic α- and β-cells, respectively. While β-cells have been the focus of intense research, less is known about α-cell function and the actions of glucagon. In recent years, the study of this endocrine cell type has experienced a renewed drive. The present review contains a summary of established concepts as well as new information about the regulation of α-cells by glucose, amino acids, fatty acids and other nutrients, focusing especially on glucagon release, glucagon synthesis and α-cell survival. We have also discussed the role of glucagon in glucose homeostasis and in energy and lipid metabolism as well as its potential as a modulator of food intake and body weight. In addition to the well-established action on the liver, we discuss the effects of glucagon in other organs, where the glucagon receptor is expressed. These tissues include the heart, kidneys, adipose tissue, brain, small intestine and the gustatory epithelium. Alterations in α-cell function and abnormal glucagon concentrations are present in diabetes and are thought to aggravate the hyperglycaemic state of diabetic patients. In this respect, several experimental approaches in diabetic models have shown important beneficial results in improving hyperglycaemia after the modulation of glucagon secretion or action. Moreover, glucagon receptor agonism has also been used as a therapeutic strategy to treat obesity.

The glucagon-like peptide-1 receptor agonist oxyntomodulin enhances beta-cell function but does not inhibit gastric emptying in mice

The proglucagon gene gives rise to multiple peptides that play diverse roles in the control of energy intake, gut motility, and nutrient disposal. Glucagon-like peptide-1 (GLP-1), a 30-amino-acid peptide regulates glucose homeostasis via control of insulin and glucagon secretion and by inhibition of gastric emptying and food intake. Oxyntomodulin (OXM) a 37-amino-acid peptide also derived from the proglucagon gene, binds to both the glucagon and GLP-1 receptor (GLP-1R); however, a separate OXM receptor has not yet been identified. Here we show that OXM, like other GLP-1R agonists, stimulates cAMP formation and lowers blood glucose after both oral and ip glucose administration, actions that require a functional GLP-1R. OXM also directly stimulates insulin secretion from murine islets and INS-1 cells in a glucose- and GLP-1R-dependent manner. Moreover, OXM ameliorates hyperglycemia and significantly reduces apoptosis in murine beta-cells after streptozotocin administration and directly reduces apoptosis in thapsigargin-treated INS-1 cells. Unexpectedly, OXM, but not the GLP-1R agonist exendin-4, increased plasma levels of insulin after oral glucose administration. Moreover, OXM administered at doses that potently lower blood glucose had no effect on inhibition of gastric emptying but reduced food intake in WT mice. Taken together, these findings illustrate that although structurally distinct proglucagon-derived peptides such as GLP-1 and OXM engage the GLP-1R, OXM mimics some but not all of the actions of GLP-1R agonists in vivo. These findings may have implications for therapeutic efforts using OXM as a long-acting GLP-1R agonist for the treatment of metabolic disorders.

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

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

Miniglucagon (glucagon 19-29): a novel regulator of the pancreatic islet physiology

Miniglucagon, the COOH-terminal (19-29) fragment processed from glucagon, is a potent and efficient inhibitor of insulin secretion from the MIN 6 beta-cell line. Using the rat isolated-perfused pancreas, we investigated the inhibitory effect of miniglucagon on insulin secretion and evaluated the existence of an inhibitory tone exerted by this peptide inside the islet. Miniglucagon dose-dependently inhibited insulin secretion stimulated by 8.3 mol/l glucose, with no change in the perfusion flow rate. A concentration of 1 nmol/l miniglucagon had a significant inhibitory effect on a 1 nmol/l glucagon-like peptide 1 (7-36) amide-potentiated insulin secretion. A decrease in extracellular glucose concentration simultaneously stimulated glucagon and miniglucagon secretion from pancreatic alpha-cells. Using confocal and electron microscopy analysis, we observed that miniglucagon is colocalized with glucagon in mature secretory granules of alpha-cells. Perfusion of an anti-miniglucagon antiserum directed against the biologically active moiety of the peptide resulted in a more pronounced effect of a glucose challenge on insulin secretion, indicating that miniglucagon exerts a local inhibitory tone on beta-cells. We concluded that miniglucagon is a novel local regulator of the pancreatic islet physiology and that any abnormal inhibitory tone exerted by this peptide on the beta-cell would result in an impaired insulin secretion, as observed in type 2 diabetes.

Severe defect in proglucagon processing in islet A-cells of prohormone convertase 2 null mice

Mice homozygous for a deletion in the gene encoding prohormone convertase 2 (PC2) are generally healthy but have mild hypoglycemia and flat glucose-tolerance curves. Their islets show marked alpha (A)-cell hyperplasia, suggesting a possible defect in glucagon processing (Furuta, M., Yano, H., Zhou, A., Rouille, Y., Holst, J., Carroll, R., Ravazzola, M., Orci, L., Furuta, H., and Steiner, D. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 6646-6651). In this report we have examined the biosynthesis and processing of proglucagon in isolated islets from these mice via pulse-chase labeling and find that proglucagon undergoes essentially no processing in chase periods up to 8 h in duration. Only a small percent of cleavage at the sensitive interdomain site (residues 71 and 72) appears to occur. These observations thus conclusively demonstrate the essentiality of PC2 for the production of glucagon in the islet A-cells. Ultrastructural and immunocytochemical studies indicate the presence of large amounts of proglucagon in atypical appearing secretory granules in the hyperplastic and hypertrophic A-cells, along with morphological evidence of high rates of proglucagon secretion in PC2 null islets. These findings provide strong evidence that active glucagon is required to maintain normal blood glucose levels, counterbalancing the action of insulin at all times.

Role of the prohormone convertase PC3 in the processing of proglucagon to glucagon-like peptide 1

Proglucagon is processed differentially in pancreatic alpha-cells and intestinal endocrine L cells to release either glucagon or glucagon-like peptide-1-(7-36amide) (tGLP-1), two peptide hormones with opposing biological actions. Previous studies have demonstrated that the prohormone convertase PC2 is responsible for the processing of proglucagon to glucagon, and have suggested that the related endoprotease PC3 is involved in the formation of tGLP-1. To understand better the biosynthetic pathway of tGLP-1, proglucagon processing was studied in the mouse pituitary cell line AtT-20, a cell line that mimics the intestinal pathway of proglucagon processing and in the rat insulinoma cell line INS-1. In both of these cell lines, proglucagon was initially cleaved to glicentin and the major proglucagon fragment (MPGF) at the interdomain site Lys70-Arg71. In both cell lines, MPGF was cleaved successively at the monobasic site Arg77 and then at the dibasic site Arg109-Arg110, thus releasing tGLP-1, the cleavages being less extensive in INS-1 cells. Glicentin was completely processed to glucagon in INS-1 cells, but was partially converted to oxyntomodulin and very low levels of glucagon in AtT-20 cells in the face of generation of tGLP-1. Adenovirus-mediated co-expression of PC3 and proglucagon in GH4C1 cells (normally expressing no PC2 or PC3) resulted in the formation of tGLP-1, glicentin, and oxyntomodulin, but no glucagon. When expressed in alphaTC1-6 (transformed pancreatic alpha-cells) or in rat primary pancreatic alpha-cells in culture, PC3 converted MPGF to tGLP-1. Finally, GLP-1-(1-37) was cleaved to tGLP-1 in vitro by purified recombinant PC3. Taken together, these results indicate that PC3 has the same specificity as the convertase that is responsible for the processing of proglucagon to tGLP-1, glicentin and oxyntomodulin in the intestinal L cell, and it is concluded that this enzyme is thus able to act alone in this processing pathway.

Role of the prohormone convertase PC2 in the processing of proglucagon to glucagon

Proglucagon is alternatively processed to glucagon in pancreatic alpha-cells, or to glucagon-like peptide-1 in intestinal L cells. Here, the specificity of PC2, the major prohormone convertase of alpha-cells, was examined both in vivo and in vitro. Adenovirus-mediated co-expression of proglucagon and PC2 in GH4C1 cells resulted in a pattern of processing products very similar to that observed in alpha-cells. Oxyntomodulin, an intermediate in the processing of proglucagon, was quantitatively converted to glucagon in vitro by purified recombinant PC2, in combination with carboxypeptidase E. It is concluded that PC2 is able to act alone in the pancreatic pathway of proglucagon processing.

Identification of amidated forms of GLP-1 in rat tissues using a highly sensitive radioimmunoassay

The development of a sensitive radioimmunoassay (RIA) for C-terminally amidated forms of glucagon-like peptide-1 (GLP-1) is described. Rabbits immunized with GLP-1(7-36)amide conjugated to bovine serum albumin with glutaraldehyde produced antisera containing high-affinity antibodies directed against an epitope that included the free amidated C-terminus of the peptide. These antisera could be used in a sensitive RIA (detection limit 0.1 fmol/tube) that measured GLP-1(7-36)amide and GLP-1(1-36)amide equally. Total concentrations of amidated GLP-1 immunoreactivity in extracts of rat hypothalamus, pancreas and intestine were determined by RIA, and resolved into GLP-1(7-36)amide, GLP-1(1-36)amide and unidentified cross-reacting substances by HPLC. Whereas only GLP-1(7-36)amide could be identified in the hypothalamus, in amounts that represented 55-94% of total glucagon-like immunoreactivity (GLI), the pancreas produced chiefly GLP-1(1-36)amide, representing 0.8-3.4% of total GLI, and only trace or undetectable amounts of GLP-1(7-36)amide (0-0.36% of total GLI). This argues against any role of intrapancreatic GLP-1(7-36)amide in the secretion of insulin. In the terminal ileum total amidated GLP-1 immunoreactivity represented 27-73% of total GLI, and in five of six specimens only GLP-1(7-36)amide could be identified on HPLC, in amounts representing 13-17% of total GLI. Only one specimen of terminal ileum contained HPLC-identified GLP-1(1-36)amide (13% of total GLI) in addition to GLP-1(7-36)amide (31% of total GLI). Acid-ethanol extraction of peptide-free rat plasma with added GLP-1(7-36)amide gave recoveries of 91+/-SEM 2% in the range 20-200 pmol/l. Basal plasma amidated GLP-1 in six unanaesthetized rats was 4.1+/-1.1 pmol/l and rose to a maximum of 15.4+/-3.0 pmol/l 10 min after intragastric glucose 1 g/kg, illustrating the modest level of plasma responses of amidated forms of GLP-1.

Enteroglucagon

The gene encoding proglucagon, the biosynthetic precursor of glucagon, is expressed not only in the pancreatic islets but also in endocrine cells of the gastrointestinal mucosa. The proglucagon (PG)-derived peptides from the gut include glicentin (corresponding to PG 1-69); smaller amounts of oxyntomodulin (PG 33-69) and glicentin-related pancreatic polypeptide (GRPP, PG 1-30); glucagon-like peptide-1 (GLP-1, PG 78-107 amide); intervening peptide-2 (IP-2, PG 111-122 amide); and glucagon-like peptide-2 (GLP-2, PG 126-158). All are secreted into the blood in response to ingestion of carbohydrates and lipids. Only oxyntomodulin and GLP-1 have proven biological activity; oxyntomodulin possibly because it interacts (but with lower potency) with GLP-1 and glucagon receptors. GLP-1 is the most potent insulinotropic hormone known and functions as an incretin hormone. It also inhibits glucagon secretion and, therefore, lowers blood glucose. This effect is preserved in patients with non-insulin-dependent diabetes mellitus, in whom infusions of GLP-1 may completely normalize blood glucose. However, GLP-1 also potently inhibits gastrointestinal secretion and motility, and its physiological functions include mediation of the "ileal-brake" effect, i.e. the inhibition of upper gastrointestinal functions elicited by the presence of unabsorbed nutrients in the ileum. As such it may serve to regulate food intake.

Differential processing of proglucagon by the subtilisin-like prohormone convertases PC2 and PC3 to generate either glucagon or glucagon-like peptide

Proglucagon is processed differently in the islet alpha cells and the intestinal endocrine L cells to release either glucagon or glucagon-like peptide 1-(7-37) (GLP1-(7-37)), peptide hormones with opposing actions in vivo. In previous studies with a transformed alpha cell line (alpha TC1-6) we demonstrated that the kexin/subtilisin-like prohormone convertase, PC2 (SPC2), is responsible for generating the typical alpha cell pattern of proglucagon processing, giving rise to glucagon and leaving unprocessed the entire C-terminal half-molecule known as major proglucagon fragment or MPGF (Rouillé, Y., Westermark, G., Martin, S. K., Steiner. D. F. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 3242-3246). Here we present evidence, using mouse pituitary AtT-20 cells infected with a vaccinia viral vector encoding proglucagon, that PC3 (SPC3), the major neuroendocrine prohormone convertase in these cells, reproduces the intestinal L cell processing phenotype, in which MPGF is processed to release two glucagon-related peptides, GLP1 and GLP2, while the glucagon-containing N-terminal half-molecule (glicentin) is only partially processed to oxyntomodulin and small amounts of glucagon. Moreover, in AtT-20 cells stably transfected with PC2 (AtT-20/PC2 cells), glicentin was efficiently processed to glucagon, providing further support for the conclusion that PC2 is the enzyme responsible for the alpha cell processing phenotype. In other cell lines expressing both PC2 and PC3 (STC-1 and beta TC-3), proglucagon was also processed extensively to both glucagon and GLP1-(7-37), although STC-1 cells express lower levels of PC2 and processed the N-terminal domain to glucagon less efficiently. In contrast, GH4C1 and COS 7 cells, which express very little or no PC2 or PC3, failed to process proglucagon, aside from a low level of interdomain cleavage which occurred only in the GH4C1 cells. In vitro PC3 did not cleave at the single Arg residue in GLP1 to generate GLP1-(7-37), its truncated biologically active form, indicating the likelihood that another convertase is required for this cleavage.

Radioautographic demonstration and localization of glucagon receptors in duck brain

The discovery of glucagon biosynthesis and receptors within mammalian brain has led one to suspect a neurotransmitter role for glucagon. In order to address this hypothesis in birds, we investigated the existence of glucagon receptors in duck brain by radioligand binding on fresh tissue sections and radioautography. Specific high-affinity [125I]glucagon binding sites similar to those in the liver were demonstrated in the avian brain. Mapping of these putative glucagon receptors revealed a discrete distributional pattern. Most of the [125I]glucagon binding capacity in duck brain is concentrated within the telencephalon, mainly in components of motor and limbic systems. Specific labeling densities were associated with avian equivalents of the mammalian pyramidal system (hyperstriatum accessorium; archistriatum intermedium and tractus occipitomesencephalicus) and extrapyramidal system (paleostriatum augmentatum, paleostriatum primitivum and lobus parolfactorius), as well as several limbic structures (hippocampal formation, nucleus taeniae and the caudal part of the archistriatum). Few glucagon-reactive foci were detected in the diencephalon (the nucleus dorsomedialis of hypothalamus, the two circumventricular organs, organum vasculosum of the lamina terminalis and median eminence and the nucleus habenularis medialis). These findings suggest that glucagon might be involved in the central control of somatic motricity and basic behaviors and point therefore to glucagon as a new neuroactive messenger in avian brain. The extensive difference between the distribution of glucagon binding sites observed in duck brain and that previously reported in rat brain suggests that glucagon does not subserve the same physiological role(s) in avian and mammalian brains.

Proglucagon is processed to glucagon by prohormone convertase PC2 in alpha TC1-6 cells

Proglucagon is processed differentially in the pancreatic alpha cells and the intestinal L cells to yield either glucagon or glucagon-like peptide 1, respectively, structurally related hormones with opposing metabolic actions. Here, we have studied the processing of proglucagon in alpha TC1-6 cells, an islet-cell line transformed by simian virus 40 large tumor (T) antigen, a model of the pancreatic alpha cell. We found that these cells process proglucagon at certain dibasic cleavage sites to release glucagon and only small amounts of glucagon-like peptide 1, as demonstrated by both continuous and pulse-chase labeling experiments. Both normal islet alpha cells and alpha TC1-6 cells were shown to express the prohormone convertase PC2 at high levels, but not the related protease PC3. Expression of PC2 antisense RNA in alpha TC1-6 cells inhibited both PC2 production and proglucagon processing concomitantly. We conclude that PC2 is the key endoprotease responsible for proglucagon processing in cells with the alpha-cell phenotype.

Analysis of proinsulin and its conversion products by reversed-phase high-performance liquid chromatography

Proinsulin is synthesized in the beta-cells of the endocrine pancreas, one of the four cell types found in the islets of Langerhans. Specific enzymatic cleavage of proinsulin results in the formation of equimolar amounts of insulin and C-peptide, via several intermediate split-proinsulin forms. Most mammals produce a single insulin, but in rodents two non-allelic insulin genes are expressed. There is an inverse ratio between the two insulins in rats and mice, the reason for this being unknown. It has been suggested that differences in transcription, translation (biosynthesis) and/or posttranslational processes (enzymatic conversion, intracellular degradation) could be possible explanations. Elevated amounts of proinsulin-immunoreactive material (PIM) have been described to occur in various conditions/diseases, suggesting alterations in beta-cell function, but the composition of the secreted PIM (intact proinsulin or its intermediates) has been incompletely determined. Studies of the biosynthesis of proinsulins and their conversion with the purpose of revealing some of these points depend on accessible reversed-phase high-performance liquid chromatographic (RP-HPLC) analyses capable of separating all the relevant, closely related polypeptides involved. This review will deal with the optimization of the RP-HPLC separations as well as sample preparation and recovery. Applications of the selected methods in the study of proinsulin biosynthesis and its conversion will also be presented.

Glucagon-like peptide-1, a new hormone of the entero-insular axis

The post-translational processing of proglucagon in the small intestine gives rise to glucagon-like peptide-1 (PG 78-107 amide) which has profound effects on the endocrine pancreas, and in many species also on the stomach. Glucagon-like peptide-1 (PG 78-107 amide) is secreted in man in response to physiological stimuli e.g. a mixed meal. Glucagon-like peptide-1, in concentrations corresponding to those observed in response to meals, strongly stimulates insulin secretion, in all mammals studied, even more potently than the gastric inhibitory peptide. Thus, glucagon-like peptide-1 fulfills the classic criteria for being a hormone and is likely to be a new incretin. The glucagon inhibitory effect of glucagon-like peptide-1 (PG 78-107 amide) probably further potentiates the effect of glucagon-like peptide-1 on glucose metabolism and distinguished this peptide from other intestinal peptides which have been proposed as incretins. Glucagon-like peptide-1 also inhibits gastric acid secretion and gastric emptying in man. The latter delays nutrient entry to the intestine and thereby diminishes meal-induced glucose excursions. Elevated plasma concentrations of immunoreactive glucagon-like peptide-1 have been reported in Type 2 (noninsulin-dependent) diabetic patients, however, the consequences of the elevation are not yet known. However, elevated levels of glucagon-like peptide-1 in patients with increased gastric emptying rate (post-gastrectomy syndromes) may be responsible for the exaggerated insulin secretion seen in these patients.

Nuclear-encoded chloroplast ribosomal protein L27 of Nicotiana tabacum: cDNA sequence and analysis of mRNA and genes

A tobacco (Nicotiana tabacum cv. Petite Havana) leaf cDNA library was constructed in the expression vector lambda gt11. Immunological and nucleic acid hybridization screening yielded several cDNAs encoding an M(r) 19,641 precursor to an M(r) 14,420 mature protein which is homologous to Escherichia coli ribosomal protein L27. One cDNA (L27-1; 882 nucleotides long) contains 104 bp of 5'-noncoding sequence, 51 codons for a transit peptide, 128 codons for the predicted mature L27 polypeptide, and 241 bp of 3'-noncoding sequence, including the poly(A)29 tail. A beta-galactosidase-L27 fusion protein was bound to nitrocellulose filters, expressed, and used as an affinity matrix to purify monospecific antibody to L27 protein from an antiserum of rabbits immunized with 50S chloroplast ribosomal proteins. Using this monospecific antibody, protein L27 was identified among HPLC-purified tobacco chloroplast ribosome 50S subunit proteins. The predicted amino terminus of the mature L27 protein was confirmed by partial sequencing of the HPLC-purified L27 protein. The mature L27 protein has 66%, 61%, 56%, and 48% amino acid sequence identity with the L27-type ribosomal proteins of Bacillus subtilis, E. coli, Bacillus stearo-thermophilus, and yeast mitochondria (MRP7), respectively, in the homologous overlapping regions. The transit peptide of tobacco chloroplast ribosomal protein L27 has 41% amino acid sequence similarity with the MRP7 mitochondrial targeting sequence. Tobacco chloroplast L27 protein also has a 40 amino acid long carboxyl-terminal extension (compared to its bacterial counterparts) which is similar to the corresponding portion of yeast MRP7.(ABSTRACT TRUNCATED AT 250 WORDS)

Proglucagon expression, posttranslational processing and secretion in SV40-transformed islet cells

HIT T15 is a B cell line derived from SV40 transformation of hamster islets. We describe here a HIT T15 variant, designated HIT T15-G, which appears to have evolved spontaneously and which expresses glucagon. Regulation of glucagon gene expression, posttranslational processing of proglucagon, and secretion of glucagon were studied in this cell line. Glucagon mRNA concentrations were increased approx. 2-fold following incubation of cells for 18 h in 10 microM forskolin but were unaffected by treatment with a phorbol ester (12-O-tetradecanoylphorbol 13-acetate; TPA) or with ionomycin. Proglucagon was processed to glucagon, and several large molecular weight forms of GLP-I and GLP-II which may include the major proglucagon fragment (MPF). The secretion of glucagon was stimulated by forskolin (5-fold), adrenalin (2-fold), arginine (3-fold) and KCl (2-fold) but was unaffected by glucose. These results suggest that the HIT T15-G cells may represent a less differentiated form of the parental HIT T15 cell line in which A cell phenotype is dominant but not complete.

Rapid chromatographic isolation and immunoblot characterization of immunoreactive fibroblast growth factor-related polypeptides from various tissues

Procedures to rapidly isolate fibroblast growth factor (FGF)-like activity from a number of tissue sources (lung, plasma, brain, ovary, corpus luteum, pituitary, chondrosarcoma) of bovine, porcine or rat origin are described. In addition, immunoblotting experiments using well characterized and specific rabbit polyclonal anti-fibroblast growth factor beta (anti-FGF-beta) sera have been performed. Besides documenting the first report of the isolation of FGF-beta from bovine lung and plasma, these studies provide evidence for the existence of higher-molecular-mass proteins with FGF-beta-like immunoreactivity. For example, in addition to new truncated forms of the acidic and basic FGF (FGF-alpha and FGF-beta), respectively, other higher-molecular-mass immunoreactive proteins were detected in bovine, pig and rat brain, and in rat chondrosarcoma. The tissue distribution of these immunoreactive proteins and their competitive inhibition characteristics mitigate against the possibility that the polyclonal antisera are cross-reacting non-specifically with common cellular proteins. Rather, the data suggest that the immunoblotting technique is either detecting other proteins structurally related to FGF-beta or alternatively FGF-beta is strongly bound to specific carrier proteins (e.g. heparan sulphate proteoglycan fragments) associated with their transport and recognition at the cellular level.

Alterations of the glutamine residues of human apolipoprotein AI propeptide by in vitro mutagenesis. Characterization of the normal and mutant protein forms

We have used site-directed mutagenesis to independently alter the Gln residues at positions -1 and -2 of the human apoAI propeptide to Arg residues. The normal and mutated genes were placed under the control of the mouse metallothionein 1 promoter in a bovine papilloma virus (BPV) vector which also carries a copy of the human metallothionein 1A gene. Following transfection of mouse C127 cells [corrected] with the vectors, cell clones resistant to CdCl2 were selected and analyzed for production of apoAI mRNAs and protein. The RNA blotting analysis showed that the steady-state apoAI mRNA levels of cell clones expressing either the normal or the mutant apoAI gene are 3-5-fold higher than that of the liver or HepG2 cells. Two-dimensional gel electrophoresis of radiolabeled apoAI showed that the apoAI-expressing clones secreted mainly the proapoAI form. Furthermore, both mutant proapoAI's differed by one positive charge from the normal apoAI. Secretion of apoAI into the culture medium follows apparent first-order kinetics and gives similar rate constants for the normal and mutant apoAI forms. Separation of secreted apoAI by density gradient ultracentrifugation in the presence of human plasma or HDL shows identical distribution of plasma and nascent (normal and mutant) apoAI. The findings indicate that in the cell system used the modification of either of the two glutamines of the apoAI prosegment does not affect the intracellular transport and secretion of apoAI, and its ability to associate with HDL.

Differential inhibitory action of cationic amino acids on protein synthesis in pancreatic rat islets

The effect of cationic amino acids, i.e. L-arginine and L-lysine, on protein synthesis in isolated rat islets of Langerhans has been investigated. Except for prosomatostatin, the formation of islet proteins is strongly depressed by these amino acids. This effect can be demonstrated within a few minutes and is rapidly reversible. For proglucagon, efficient concentrations of arginine are in the range of 1 to 10 mmol/l. The sensitivity of proinsulin formation to arginine is glucose-dependent: at 2.5 mmol/l, inhibitory concentrations of arginine are 10-fold lower than in the case of proglucagon. High glucose (20 mmol/l) almost completely protects proinsulin synthesis from this inhibition. The proteolytic conversion steps in hormonal precursor processing are not influenced by cationic amino acids as studied in intact islets and in a cell-free translational system. It is concluded that arginine and lysine inhibit protein synthesis in islet cells at the translational level. The release of these amino acids by prohormone conversion may exert a feed-back control on proinsulin formation that is modulated by glucose.

Ontogeny of glucagon-like immunoreactive peptides in rat intestine

The ontogeny of the intestinal glucagon-like peptides was investigated in rats between 16 days of gestation and 4 postnatal days. The intestinal content of glucagon-like immunoreactive (GLI) peptides increased from 0.09 +/- 0.02 pmol/nmol protein at 16-17 days to plateau at 2.8 +/- 0.4 pmol/nmol protein by 20 days of gestation (P less than 0.001). The apparent immunoreactive glucagon (IRGa) content of the gut ranged from 0.03 +/- 0.01 to 0.08 +/- 0.01 pmol/nmol protein. No developmental trends in IRGa peptide content were observed. Following gel filtration of intestines extracted from rats of 18 days of gestation or greater, two main GLI peptides were detected with apparent mol. wts. of 11-12 and 5-6 kDa. Significant peaks of GLI peptides were not detected following gel filtration of intestines extracted from 16- or 17-day fetuses, nor were peaks of IRGa found at any age. In conclusion, the fetal rat intestine undergoes maturational development between 17 and 19 days of gestation to produce the GLI peptides.

Mature bovine adrenodoxin contains a 14-amino-acid COOH-terminal extension originally detected by cDNA sequencing

Since the nucleotide sequence of bovine adrenodoxin cDNA is at variance with protein sequencing data in that it encodes an additional 14 amino acids at the COOH terminus, we used a specific antibody raised against this 14-amino-acid segment to examine its presence in: nascent precursor polypeptide chains, the processed mature adrenodoxin and mitochondria of both steroidogenic and nonsteroidogenic tissues. These studies reveal the presence of the extra peptide in the precursor form derived from in vitro translation and in the newly synthesized mature form as shown by [35S]methionine labeling of proteins in adrenocortical cells. Both the purified COOH-terminal synthetic peptide and purified mature adrenodoxin competed with radiolabeled adrenodoxin for immunoprecipitation by the anti-peptide antibody. Immunoblots revealed the presence of the extra peptide in purified adrenodoxin and in bovine adrenocortical, corpus luteal, kidney and liver mitochondria while it was not detectable in heart mitochondria. Thus, we conclude that mature adrenodoxin and its homologs in non-steroidogenic tissues contain the C-terminal extension following uptake into mitochondria. These results indicate structural homology between adrenodoxin and the iron-sulfur proteins of the kidney and liver and also suggest the presence of a second iron-sulfur protein in kidney and liver.

Immunohistochemical localization of glucagon-like peptide 1. Use of poly- and monoclonal antibodies

We report the use of poly- and monoclonal antibodies to study the immunohistochemical distribution of glucagon-like peptide-1 immunoreactivity (GLP-1-IR) in various tissues. The polyclonal antibodies against GLP-1 reacted with pancreatic A cells, enteroglucagon (L) cells in the gut, and some neurons in the central nervous system of all species tested. In pancreas and gut the monoclonal antibodies against GLP-1 exhibited a similar, but species specific distribution, relative to the polyclonal antibodies. The colocalization of GLP-1 and glucagon immunoreactivity in pancreatic, intestinal, and nervous tissues is in agreement with previously reported findings that both peptides are part of a single precursor molecule (preproglucagon).

Glucagon-like immunoreactive peptides in a rat ileal epithelial cell line (IEC-18)

The presence of cells containing glucagon-like immunoreactive (GLI) peptides was demonstrated in a rat ileal epithelial cell line (IEC-18) by both immunofluorescence and radioimmunoassay. When cell extracts were subjected to gel filtration chromatography, the cells were found to contain 3.5 Kd glucagon in addition to significant quantities of large molecular weight GLI peptides (apparent molecular weights of 4, 6, 8 and 10 Kd) and a 9 Kd peptide with apparent glucagon immunoreactivity. This was in contrast to extracts of adult rat ileum, which contained only large molecular weight GLI peptides (apparent molecular weights of 6 and 12 Kd). Production of GLI peptides by the IEC-18 cells was stimulated by glucose (p less than 0.02) and inhibited by insulin (p less than 0.01). In conclusion, these results demonstrate that the IEC-18 cells produce both GLI peptides and glucagon, and thus support the notion that proglucagon processing is cell-specific. IEC-18 cells may therefore provide a tool for investigations of some aspects of GLI peptide and glucagon synthesis.

Mutations in the guinea pig preproglucagon gene are restricted to a specific portion of the prohormone sequence

A cDNA clone encoding guinea pig preproglucagon has been isolated from a pancreatic cDNA library. The predicted amino acid sequence of proglucagon is highly conserved in all regions, in comparison to other mammals, except for the C-terminal portion of the 29-residue glucagon region, in which 5 amino acid substitutions have occurred. These changes may serve to offset the reduced receptor-binding potency of the highly mutated insulin in this New World species.

Structure of the human glucagon gene

A clone containing the complete human glucagon gene was isolated and sequenced. The gene is approximately 9.4 kilobases in length and comprises six exons and five introns. The putative preproglucagon encoded by this gene, 180 amino acids in length and containing glucagon and two glucagon-like peptides, is very similar to that of other mammalian species (greater than 90% amino acid sequence homology). There is 88% nucleotide sequence homology between the proximal 130 base pairs of the 5' flanking regions of the human and rat glucagon genes. These sequences, highly conserved throughout evolution, are likely involved in the regulation of glucagon gene transcription.

Production and characterization of N-terminally and C-terminally directed monoclonal antibodies against pancreatic glucagon

Hybridoma technology has been successfully applied to the production of monoclonal antibodies against a variety of small soluble peptides. We report herein for the first time on the development of monoclonal antibodies to pancreatic glucagon. Twenty-three stable positive hybridomas were detected by radioimmunoassay from five separate fusions and cloned by the limiting dilution method. Four selected monoclonal antibodies were all of the IgG 2a subclass type kappa and bound to protein A. One monoclonal antibody (23.8B6) was shown to be directed toward the C-terminal region and another (23.6B4) toward the N-terminal to central region of the glucagon molecule. These antibodies did not cross-react with any of the other peptides tested. Two further monoclonal antibodies (23.4A1, 22.3A6) reacted with the C-terminal third of the glucagon molecule and showed a cross-reaction with the structurally related gastric inhibitory polypeptide of 0.7% and 9.1%, respectively. All but the C-terminal monoclonal antibody 23.8B6 showed a marked cross-reaction with ileal extracts. The N-terminally directed monoclonal antibody 23.6B4 was of sufficient avidity for use in the radioimmunoassay of pancreatic glucagon and gut glucagon-like immunoreactivity in tissue extracts, being sensitive to changes of pancreatic glucagon of 2.0 fmol/tube at a final titer of culture supernatant of 1:1.4 X 10(5). In gel permeation chromatography of intestinal extracts, two major peaks were detectable (Kav 0.27 and 0.54). The present findings show that monoclonal antibodies provide sensitive tools for detecting pancreatic glucagon and gut glucagon-like immunoreactivity. They will be valuable immunoreactants for the development of immunoradiometric assays as well as for large-scale immunoaffinity purification of gut glucagon-like immunoreactivity.

Molecular cloning and amino acid sequence of the precursor form of bovine adrenodoxin: evidence for a previously unidentified COOH-terminal peptide

Several recombinant cDNA clones specific for the mitochondrial iron-sulfur protein adrenodoxin have been identified in a bovine adrenocortical cDNA library. One clone (pBAdx4) contains a 900-base-pair insert that includes the entire amino acid coding region of the adrenodoxin precursor protein. The amino acid sequence of mature adrenodoxin deduced from the nucleotide sequence of pBAdx4 is identical with that determined by protein sequencing except for three amide changes. The previously undetermined sequence of the adrenodoxin NH2-terminal precursor segment (58 amino acids) contains several basic residues, a characteristic feature of the precursor segment of proteins destined for mitochondria. In addition, a 14 amino acid extension is present at the COOH terminus of the mature adrenodoxin sequence. Whether this represents a COOH-terminal precursor segment is not clear. Three different adrenodoxin mRNAs are present [1.75, 1.4, and 0.95 kilobase(s) long] in bovine adrenocortical RNA. RNA from bovine corpus luteum, liver, and kidney contains transcripts that hybridize to adrenodoxin cDNA. Accumulation of adrenodoxin mRNA occurs in cultured bovine adrenocortical cells after treatment with ACTH or dibutyryl-cAMP, similar to that observed for the mitochondrial steroid hydroxylases that it services--namely, the cholesterol side-chain-cleavage cytochrome P-450 and the steroid 11 beta-hydroxylase cytochrome P-450.

Large glucagon-like immunoreactivity in a primary ovarian carcinoid

A primary ovarian carcinoid composed of both trabecular and strumal types was studied by histochemical, immunocytochemical, and biochemical techniques. High contents of glucagon, secretin, and calcitonin were demonstrated in the tumor homogenate. All of the tumor cells, irrespective of histologic type, showed properties of argyrophilia and neurosecretory granules on electron microscopy. Glucagon-producing cells were positive in trabecular carcinoid by immunoperoxidase techniques. Bio-Gel P10 gel filtration showed that the molecular weight of major immunoreactive glucagon in tumor was 20,000. It migrated faster than true glucagon after polyacrylamide gel electrophoresis. No clinical symptoms of glucagonoma developed.

Glucagon and small-bowel mucosa

Numerous functional and structural effects of pharmacological dosages of glucagon on the small-intestinal mucosa have been demonstrated. In addition, clinical conditions associated with elevated concentrations of plasma glucagon may go along with alterations of the intestinal mucosa. The physiological and pathophysiological relevance of these findings, however, is questionable in view of the heterogeneity of the findings, of the complexity of the experimental systems used and of the methodological problems involved. With respect to possible trophic effects on the small-bowel mucosa enteroglucagon is of special importance. Numerous diseases in which increased intestinal mucosal growth has been shown are associated with elevated plasma concentrations of enteroglucagon. Our results concerning radiation damage, the time course of plasma enteroglucagon levels during antimitotic treatment, the small intestinal resection and the experimental blind loop syndrome are discussed. An outlook will be given as to the use of monoclonal antibodies in the development of glucagon as well as enteroglucagon deficiency states for the study of the physiological relevance of these two regulatory peptides.