[The physiology of incretins]. / Az inkretinek elettana.

The discovery of incretins-glucagon-like peptide (GLP)-1 and glucose-dependent insulinotrop peptide (GIP)-, clarification of their physiological properties as well as therapeutic application of incretin-based blood glucose lowering drugs opened new perspectives in the medical management of type 2 diabetes. New results of basic research investigations led to revaluation of the role of GIP in metabolic processes and a more established use of GLP-1 action. The article overviews the most relevant data of production and effects of incretins, as well as future possibilities of their therapeutic use.

The physiology of incretin hormones and the basis for DPP-4 inhibitors.

With the rising prevalence of diabetes, new therapies that provide glucose control are needed. Although many medications are available, tight glucose control is still a challenge. In this article, the physiology of glucose homeostasis is explored with respect to type 2 diabetes. The incretin effect is explained in detail, and the incretin hormones, glucose-dependent insulinotrophic polypeptide and glucagon-like peptide 1, are investigated as well as their contribution to type 2 diabetes therapy. Studies involving dipeptidyl-peptidase 4 (DPP-4) inhibitors are summarized as to their effects on glucose homeostasis. Specifically, vildagliptin (Galvus; Novartis International AG, Basel, Switzerland) and sitagliptin (Januvia; Merck & Co, Inc, Whitehouse Station, NJ) are described. The use and efficacy of the currently available incretin mimetic, exenatide (Byetta; Amylin Pharmaceuticals, Inc and Eli Lilly and Company, San Diego, Calif, and Indianapolis, Ind), are briefly discussed. Throughout this article, the rationale for the use of DPP-4 inhibitors is presented.

Gut peptides and the regulation of appetite.

There is a growing worldwide epidemic of obesity. Obese people have a higher incidence of type 2 diabetes and cardiovascular disease, and hence present increasing social, financial and health burdens. Weight loss is always difficult to achieve through lifestyle changes alone, and currently licensed anti-obesity drug treatments, such as orlistat and sibutramine, if tolerated, only achieve modest weight loss. Therefore, there is a need to identify more potent pharmacological targets. In the last 10 years, discoveries of new hormones such as leptin and ghrelin, together with greater understanding of previously described hormones such as cholecystokinin (CCK), pancreatic polypeptide (PP), peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), have led to a rapid increase in our knowledge of the regulation of energy balance. Among the most important factors, controlling appetite and satiety are peptide hormones released from the gut. In this paper, we provide a full up-to-date overview of the current state of knowledge of this field, together with the potential of these peptides as drugs, or as other therapeutic targets, in the treatment of obesity. Finally, we propose an integrated model to describe the complex interplay of these hormones in the broader physiology of energy balance.

Gut hormones, and short bowel syndrome: the enigmatic role of glucagon-like peptide-2 in the regulation of intestinal adaptation.

Short bowel syndrome (SBS) refers to the malabsorption of nutrients, water, and essential vitamins as a result of disease or surgical removal of parts of the small intestine. The most common reasons for removing part of the small intestine are due to surgical intervention for the treatment of either Crohn's disease or necrotizing enterocolitis. Intestinal adaptation following resection may take weeks to months to be achieved, thus nutritional support requires a variety of therapeutic measures, which include parenteral nutrition. Improper nutrition management can leave the SBS patient malnourished and/or dehydrated, which can be life threatening. The development of therapeutic strategies that reduce both the complications and medical costs associated with SBS/long-term parenteral nutrition while enhancing the intestinal adaptive response would be valuable. Currently, therapeutic options available for the treatment of SBS are limited. There are many potential stimulators of intestinal adaptation including peptide hormones, growth factors, and neuronally-derived components. Glucagon-like peptide-2 (GLP-2) is one potential treatment for gastrointestinal disorders associated with insufficient mucosal function. A significant body of evidence demonstrates that GLP-2 is a trophic hormone that plays an important role in controlling intestinal adaptation. Recent data from clinical trials demonstrate that GLP-2 is safe, well-tolerated, and promotes intestinal growth in SBS patients. However, the mechanism of action and the localization of the glucagon-like peptide-2 receptor (GLP-2R) remains an enigma. This review summarizes the role of a number of mucosal-derived factors that might be involved with intestinal adaptation processes; however, this discussion primarily examines the physiology, mechanism of action, and utility of GLP-2 in the regulation of intestinal mucosal growth.

Gut hormones and the control of appetite.

Obesity is the main cause of premature death in the UK. Worldwide its prevalence is accelerating. It has been hypothesized that a gut nutriment sensor signals to appetite centres in the brain to reduce food intake after meals. Gut hormones have been identified as an important mechanism for this. Ghrelin stimulates, and glucagon like peptide-1, oxyntomodulin, peptide YY (PYY), cholecystokinin and pancreatic polypeptide inhibit, appetite. At physiological postprandial concentrations they can alter food intake markedly in humans and rodents. In addition, in obese humans fasting levels of PYY are suppressed and postprandial release is reduced. Administration of gut hormones might provide a novel and physiological approach in anti-obesity therapy. Here, we summarize some of the recent advances in this field.

Lipid malabsorption persists after weaning in rats whose dams were given GLP-2 and dexamethasone.

Glucagon-like peptide-2 (GLP-2) enhances intestinal growth and absorption in mature animals, and glucocorticosteroids (GC) increase the sugar and lipid uptake in adult animals. However, the role of GC and GLP-2 in the ontogeny of lipid absorption is unknown. We hypothesized that GLP-2 and the GC dexamethasone (DEX), when administrated to rat dams during pregnancy and lactation, would enhance lipid uptake in the offspring. Rat dams were treated in the last 10 d of pregnancy and during lactation with GLP-2 [0.1 microg/g/d subcutaneous (sc)], DEX (0.128 microg/g/d sc), GLP-2 + DEX, or a placebo. Sucklings were sacrificed at 19-21 d of age, and weanlings were sacrificed 4 wk later. Lipid uptake was assessed using an in vitro ring uptake method. Although DEX and GLP-2 + DEX increased the jejunal mass, the jejunal lipid uptake was unchanged. In contrast, GLP-2, DEX, and GLP-2 + DEX reduced the ileal lipid uptake in suckling and weanling rats. This reduction was not due to alterations in intestinal morphology or to changes in fatty acid-binding protein abundance, but it was partially explained by an increase in the effective resistance of the intestinal unstirred water layer. In sucklings, DEX dramatically reduced the jejunal lipid uptake to levels similar to those seen in weanlings, such that the normal ontogenic decline in lipid uptake was not observed. Giving dams GLP-2 or DEX during pregnancy and lactation reduced lipid uptake in the offspring, and this persisted for at least 1 mon. The impact this may have on the nutritional well-being of the animal in later life is unknown.

Peripheral oxyntomodulin reduces food intake and body weight gain in rats.

Oxyntomodulin (OXM) is a circulating gut hormone released post prandially from cells of the gastrointestinal mucosa. Given intracerebroventricularly to rats, it inhibits food intake and promotes weight loss. Here we report that peripheral (ip) administration of OXM dose-dependently inhibited both fast-induced and dark-phase food intake without delaying gastric emptying. Peripheral OXM administration also inhibited fasting plasma ghrelin. In addition, there was a significant increase in c-fos immunoreactivity, a marker of neuronal activation, in the arcuate nucleus (ARC). OXM injected directly into the ARC caused a potent and sustained reduction in refeeding after a fast. The anorectic actions of ip OXM were blocked by prior intra-ARC administration of the glucagon-like peptide-1 (GLP-1) receptor antagonist, exendin(9-39), suggesting that the ARC, lacking a complete blood-brain barrier, could be a potential site of action for circulating OXM. The actions of ip GLP-1, however, were not blocked by prior intra-ARC administration of exendin(9-39), indicating the potential existence of different OXM and GLP-1 pathways. Seven-day ip administration of OXM caused a reduction in the rate of body weight gain and adiposity. Circulating OXM may have a role in the regulation of food intake and body weight.

A new role for enteric glucagon-37: acute stimulation of glucose absorption in rat small intestine.

Glucagon-37 is secreted by intestinal L-cells following carbohydrate uptake. It is known to inhibit gastric acid secretion (hence also named oxyntomodulin) and appears to increase intracellular cyclic AMP concentrations. Since cyclic AMP could enhance intestinal glucose absorption, a possible stimulatory effect of glucagon-37 on glucose transport was examined. Glucagon-37 acutely increased glucose absorption in the isolated, vascularly perfused small intestine and in isolated enterocytes of the rat. In these cells the stimulation by glucagon-37 could be completely blocked by the cAMP antagonist Rp-cAMPS and was therefore mediated by cAMP. The stimulation of intestinal glucose absorption by glucagon-37 appears to be a major new physiological function.

Glicentin, an active enteroglucagon, has a significant trophic role on the small intestine but not on the colon in the rat.

BACKGROUND: Many experiments have indicated that the gut glucagons (enteroglucagons) are associated with cell proliferation in the small intestine. However, recent studies have failed to show trophic effects of glicentin (enteroglucagon) on the intestine. AIMS: To examine the effects of glicentin on intestinal proliferation in vivo in the rat. METHODS: Rats were established on total parenteral nutrition for 6 days. Four experimental groups were given daily doses of 1, 4, 20 and 80 microg/rat of glicentin via the jugular vein. Rats fed by total parenteral nutrition and rats fed chow ad libitum were used as controls. Tissues taken from the duodenum, jejunum, ileum and colon were fixed in Carnoy's fluid and microdissected to determine the metaphase arrest scores and crypt fission ratios. RESULTS: The mean metaphase arrest scores per crypt of the small intestine were significantly increased in the rats given 4, 20 and 80 microg of glicentin. These responses were dose-dependent, and were most prominent in the ileum. Crypt fission of the ileum was significantly decreased in the 20 and 80 microg glicentin groups. Glicentin had no effects on proliferation or fission in the colon. CONCLUSIONS: Glicentin is trophic to the rat small intestine, but not the colon.

The biological significance of "enteroglucagon." Present status.

"Enteroglucagon" refers to glucagon-like peptides present in intestine that cross react with N-terminally directed antiglucagon antisera but not with C-terminally directed antisera. Two peptides having these features have been isolated from the lower small intestine: glicentin (69 amino acids) and oxyntomodulin (37 amino acids). The sequence of the pancreatic preproglucagon gene suggests that glucagon, glicentin and oxyntomodulin derive from the same translational pathway, each individual peptide being produced by different posttranslational processing. Both glicentin and oxyntomodulin contain the glucagon sequence that bears the N-terminal epitope and are C-terminally extended by the same octapeptide masking the C-terminal epitope. The N-terminal 32 amino acid extension of glicentin renders the molecule unable to bind to hepatic glucagon receptors, unlike glucagon and oxyntomodulin. An original tissue specificity of oxyntomodulin, mediated by a novel type of receptor, has been observed in acid secreting gastric oxyntic glands. Oxyntomodulin and glicentin containing the C-terminal octapeptide, as well as the octapeptide itself, are able to inhibit gastric acid secretion. This biological activity is likely to represent the main physiological regulatory pattern in which "Enteroglucagon" is involved.

Decreased food intakes and body weights in rats immunized against pancreatic glucagon.

Glucagon, a putative satiety peptide, has decreased food intake and antibodies to glucagon have increased food intake when administered acutely. It may be hypothesized if rats were immunized against glucagon, antibodies would chronically sequester glucagon released during meals and food intake and weight gain would increase. Female Zucker obese (n = 16, BW = 160 +/- 5 g) and lean (n = 16, BW = 123 +/- 3 g) rats were immunized against pancreatic glucagon conjugated to BSA (GG-AB) or BSA alone (BSA-AB). Only GG-AB rats developed glucagon antibody titers (p less than 0.01). During a 16 week period average daily food intakes and body weight gains were decreased 5.0 (p less than 0.001) and 9.4% (p less than 0.001) respectively in GG-AB compared with BSA-AB rats. Free glucagon, measured by RIA using a pancreatic glucagon specific antibody, was decreased 60% at 8 weeks (130 vs. 322 pg/ml, p less than 0.001) and 33% at 16 weeks (206 vs. 307 pg/ml, p less than 0.02). However, total pancreatic glucagon (free and antibody-bound) was increased 226% (428 vs. 129 pg/ml, p less than 0.02) at 16 weeks. Thus, although sufficient antibody titers were developed in rats to sequester 76% of the free circulating glucagon from both pancreatic and gut sources, food intakes and body weight gains were decreased, likely as a consequence of an over-compensatory increase in total glucagon concentrations.

Large bowel growth.

Developmental events in the large bowel have been studied pre- and post-natally and indicate that by birth, crypt organisation and kinetic activity are organised along adult lines. The period from birth to maturity is marked by an increase in crypt size and a massive increase in their number, with new crypts developing by a process of longitudinal fission from the base of existing ones. We know little of the fate of crypt size and number thereafter. Adaptive responses to resection or bypass of intestine are much less marked in large bowel when compared to small bowel, but in general postoperative responses have not been as extensively examined. Of the factors maintaining mass and cell turnover in large bowel mucosa simple luminal bulk seems to be most important, although a role exists for endocrine and neurovascular influences. Knowledge of growth, kinetic activity and adaptive responses in human large bowel is scanty and represents a large area for further study.

The hormonal pattern of intestinal adaptation. A major role for enteroglucagon.

A number of human diseases with intestinal adaptation have been investigated, including acute infective diarrhoea, intestinal resection, jejuno-ileal bypass, coeliac disease, tropical sprue, chronic pancreatitis and cystic fibrosis. In all, the newly isolated hormone enteroglucagon appeared to be elevated in proportion to the degree of adaptation. In rats after gut resection and cold adaptation, enteroglucagon was also elevated and the degree of elevation correlated closely with the crypt cell production rate (CCPR). Chronic administration of somatostatin suppressed both enteroglucagon and CCPR, while bombesin stimulated both. A crude preparation of enteroglucagon was found to directly stimulate DNA synthesis in enterocyte cultures. It is thus concluded that, at present, the most likely candidate for the humoral component of intestinal adaptation is the hormonal peptide enteroglucagon.

Hormones and polyamines in intestinal and pancreatic adaptation.

The evidence for and against an enteropancreatic trophic axis is reviewed. Luminal nutrition is essential for the maintenance of normal intestinal mucosal, and exocrine pancreatic, structure and function. Exclusion of luminal nutrition leads to mucosal hypoplasia and hypofunction with similar changes in the pancreas. The trophic effect of luminal nutrition may be mediated through the release of regulatory peptides with endocrine or paracrine effects. Enteroglucagon is the strongest candidate for the role of 'enterotrophin' while cholecystokinin (CCK) markedly influences pancreatic growth. Thus, CCK not only stimulates exocrine pancreatic secretion but makes acinar cells divide and the pancreas grow. The cellular mechanisms whereby trophic peptides influence normal and adaptive growth are also discussed with emphasis on polyamines (putrescine, spermidine and spermine) and the key enzymes controlling their synthesis (ornithine decarboxylase; ODC) and degradation (diamine oxidase; DAO). When polyamine synthesis is blocked with the ODC inhibitor, difluoromethyl ornithine (DFMO), the adaptive intestinal hyperplasia of pancreatico-biliary diversion is either inhibited or completely prevented. A proposed sequence of events might be as follows: luminal nutrients, particularly long-chain fats, reach the ileum and colon and stimulate increased enteroglucagon release. Enteroglucagon binds to cell receptors and triggers an intracellular cascade involving ODC and the polyamines, which, in turn, stimulate RNA polymerase, DNA, RNA and protein synthesis, cell division, and adaptive tissue growth.

Intestinal adaptation: factors that influence morphology.

The lining of the intestinal tract is constantly renewed in a brisk but orderly fashion. Further acceleration of cell renewal is elicited by various stimuli, notably surgical shortening of the intestine and hyperphagia, which lead to prompt but persistent increases in mucosal mass. Progressive hypoplasia ensues when the small and large bowel are deprived of their normal contents, either by fasting (with or without parenteral nutrition) or by exclusion from intestinal continuity. All atrophic changes are reversed by refeeding or restoration of the normal anatomical disposition. Intestine responds to mucosal damage by regeneration from the crypts. Pancreatobiliary secretions mediate some of the tropic effects of chyme; systemic influences, both neurovascular and humoral, also play a part in the adaptive response of the gut.

Glucagon-like peptide (GCGL) is a novel potential TSH-releasing factor (TRF) in Chickens: I) Evidence for its potent and specific action on stimulating TSH mRNA expression and secretion in the pituitary.

Our recent study proposed that the novel glucagon-like peptide (GCGL), encoded by a glucagon-like gene identified in chickens and other lower vertebrates, is likely a hypophysiotropic factor in nonmammalian vertebrates. To test this hypothesis, in this study, we investigated the GCGL action on chicken pituitaries. The results showed that: 1) GCGL, but not TRH, potently and specifically stimulates TSH secretion in intact pituitaries incubated in vitro or in cultured pituitary cells monitored by Western blotting or a cell-based luciferase reporter assay; 2) GCGL (0.1nM-10nM) dose dependently induces the mRNA expression of TSHß but not 5 other hormone genes in cultured pituitary cells examined by quantitative real-time RT-PCR, an action likely mediated by intracellular adenylate cyclase/cAMP/protein kinase A and phospholipase C/inositol 1,4,5-trisphosphate/Ca(2+) signaling pathways coupled to GCGL receptor (GCGLR); 3) GCGLR mRNA is mainly localized in pituitary cephalic lobe demonstrated by in situ hybridization, where TSH-cells reside, further supporting a direct action of GCGL on thyrotrophs. The potent and specific action of GCGL on pituitary TSH expression and secretion, together with the partial accordance shown among the temporal expression profiles of GCGL in the hypothalamus and GCGLR and TSHß in the pituitary, provides the first collective evidence that hypothalamic GCGL is most likely to be a novel TSH-releasing factor functioning in chickens. The discovery of this novel potential TSH-releasing factor (GCGL) in a nonmammalian vertebrate species, ie, chickens, would facilitate our comprehensive understanding of the hypothalamic control of pituitary-thyroid axis across vertebrates.

Gastrointestinal peptides and bone health.

PURPOSE OF REVIEW: To outline recent developments in research surrounding gastrointestinal peptides and their role in skeletal regulation. RECENT FINDINGS: Bone remodeling is influenced by many regulatory systems, which interact to ensure that the complex demands upon mineralized tissue are met without undue compromise. These include local actions such as mechanical factors, but are dominated by systemic endocrine factors. Although the involvement of hypothalamo-pituitary actions on bone homeostasis is well defined, growing evidence suggests that peripheral tissues and the circulating factors they produce represent an important regulatory axis in bone. Given the critical role of diet in mineral homeostasis, the gastrointestinal tract is a rich source of circulating factors capable of regulating bone homeostasis. After a review of manuscripts on known mechanisms and effects of gastrointestinal peptide on bone, these were summarized. SUMMARY: Although clearly an exciting and emergent field of research, more studies are required to define the specific actions of gastrointestinal regulator in bone, in particular, the relative contribution of systemic and local effects, to aid interpretation of their potential impact on human health and disease. Nonetheless, this exciting research will further our understanding on bone physiology and provide novel approaches to therapy in a wide range of skeletal conditions.