Glucagon-like peptide-1 binding to rat hepatic membranes

Gut hormone analogues and skeletal health in diabetes and obesity: Evidence from preclinical models

Diabetes mellitus and obesity are rapidly growing worldwide. Aside from metabolic disturbances, these two disorders also affect bone with a higher prevalence of bone fractures. In the last decade, a growing body of evidence suggested that several gut hormones, including ghrelin, gastrin, glucose-dependent insulinotropic polypeptide (GIP), glucagon, and glucagon-like peptide-1 and 2 (GLP-1 and GLP-2, respectively) may affect bone physiology. Several gut hormone analogues have been developed for the treatment of type 2 diabetes and obesity, and could represent a new alternative in the therapeutic arsenal against bone fragility. In the present review, a summary of the physiological roles of these gut hormones and their analogues is presented at the cellular level but also in several preclinical models of bone fragility disorders including type 2 diabetes mellitus, especially on bone mineral density, microarchitecture and bone material properties. The present review also summarizes the impact of GLP-1 receptor agonists approved for the treatment of type 2 diabetes mellitus and the more recent dual or triple analogue on bone physiology and strength.

Semaglutide reduces tumor burden in the GAN diet-induced obese and biopsy-confirmed mouse model of NASH-HCC with advanced fibrosis

Non-alcoholic steatohepatitis (NASH) is emerging as a major cause of hepatocellular carcinoma (HCC), however, it is not resolved if compounds in late-stage clinical development for NASH may have additional therapeutic benefits in NASH-driven HCC (NASH-HCC). Here, we profiled monotherapy with semaglutide (glucagon-like-receptor-1 receptor agonist) and lanifibranor (pan-peroxisome proliferator-activated receptor agonist) in a diet-induced obese (DIO) mouse model of NASH-HCC. Disease progression was characterized in male C57BL/6 J mice fed the GAN (Gubra Amylin NASH) diet high in fat, fructose and cholesterol for 12-72 weeks (n = 15 per group). Other GAN DIO-NASH-HCC mice fed the GAN diet for 54 weeks and with biopsy-confirmed NASH (NAFLD Activity Score ≥ 5) and advanced fibrosis (stage F3) received vehicle (n = 16), semaglutide (30 nmol/kg, s.c., n = 15), or lanifibranor (30 mg/kg, p.o., n = 15) once daily for 14 weeks. GAN DIO-NASH-HCC mice demonstrated progressive NASH, fibrosis and HCC burden. Tumors presented with histological and molecular signatures of poor prognostic HCC. Consistent with clinical trial outcomes in NASH patients, both lanifibranor and semaglutide improved NASH while only lanifibranor reduced fibrosis in GAN DIO-NASH-HCC mice. Notably, only semaglutide reduced tumor burden in GAN DIO-NASH-HCC mice. In conclusion, the GAN DIO-NASH-HCC mouse is a clinical translational model of NASH-HCC. Semaglutide improves both NASH and tumor burden in GAN DIO-NASH-HCC mice, highlighting the suitability of this preclinical model for profiling novel drug therapies targeting NASH-HCC.

Incretin-based therapy for the treatment of bone fragility in diabetes mellitus

Bone fractures are common comorbidities of type 2 diabetes mellitus (T2DM). Bone fracture incidence seems to develop due to increased risk of falls, poor bone quality and/or anti-diabetic medications. Previously, a relation between gut hormones and bone has been suspected. Most recent evidences suggest indeed that two gut hormones, namely glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), may control bone remodeling and quality. The GIP receptor is expressed in bone cells and knockout of either GIP or its receptor induces severe bone quality alterations. Similar alterations are also encountered in GLP-1 receptor knock-out animals associated with abnormal osteoclast resorption. Some GLP-1 receptor agonist (GLP-1RA) have been approved for the treatment of type 2 diabetes mellitus and although clinical trials may not have been designed to investigate bone fracture, first results suggest that GLP-1RA may not exacerbate abnormal bone quality observed in T2DM. The recent design of double and triple gut hormone agonists may also represent a suitable alternative for restoring compromised bone quality observed in T2DM. However, although most of these new molecules demonstrated weight loss action, little is known on their bone safety. The present review summarizes the most recent findings on peptide-based incretin therapy and bone physiology.

Exendin-4 does not modify growth or apoptosis of human colon cancer cells

AIM

Glucagon-like peptide-1 (GLP-1) receptor agonists are a kind of very popular antidiabetes drugs. They promote cell proliferation and survival through activation of signaling pathways in human islet cells involving phosphate idylinositol 3 kinase (PI3K) and extracellular regulated kinases 1 and 2 (ERK1/2), which are frequently activated in human colon cancer cells. Then, it is possible that taking GLP-1 receptor (GLP-1R) agonists persistently would induce proliferation of β cells as well as colon cancer cells. So, clarifying the effects and mechanisms of GLP-1R agonists on colon cancer cells has important clinical implications.

MATERIALS AND METHODS

We investigated GLP-1R expression in human colon cancer tissue samples with immunohistochemisty analysis and explored the effects of exendin-4, a GLP-1 receptor agonist, on colon cancer cells in vitro and in vivo.

RESULTS

The results showed lack of GLP-1R expression in both human colon cancer tissues and colon cancer cell lines. Exendin-4 did not enhance the proliferation and migration of colon cancer cell lines in vitro, and nor did it inhibit apoptosis induced by cytotoxic agents such as 5-fluorouracil (5-FU) or irinotecan. In addition, exendin-4 did not promote the propagation of colon cancer cells in vivo.

CONCLUSION

Our study suggests that GLP-1R agonists do not modify the growth or survival of human colon cancer cells and may be safe for diabetic patients with colon cancer.

Interplay between bone and incretin hormones: A review

Bone is a tissue with multiple functions that is built from the molecular to anatomical levels to resist and adapt to mechanical strains. Among all the factors that might control the bone organization, a role for several gut hormones called "incretins" has been suspected. The present review summarizes the current evidences on the effects of glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) in bone physiology.

The Impact of Glucagon-Like Peptide-1 on Bone Metabolism and Its Possible Mechanisms

The impact of antidiabetic drugs on bone metabolism is drawing increasing attention due to the discovery of a correlation between type 2 diabetes mellitus (T2DM) and osteoporosis. Glucagon-like peptide-1 (GLP-1) receptor agonists are a novel and promising class of drugs for T2DM, which may also have clinical applications in bone tissue disorders. This review examines the impact of GLP-1 on bone metabolism, including enhancement of bone mineral density and improvement of bone quality. However, the precise effect of GLP-1 on fracture risk has not been unambiguously defined. This review also summarizes our current understanding of the mechanisms by which GLP-1 affects bone metabolism. GLP-1 may act on bone by promoting bone formation, inhibiting bone resorption, and affecting the coordination of the two processes. We describe molecular pathways and proteins, such as Wnt and calcitonin, that are associated with GLP-1 and bone tissue. The specific processes and related molecular mechanisms of the effects of GLP-1 on bone metabolism need to be further explored and clarified.

Control of liver glucokinase activity: A potential new target for incretin hormones?

We tested the exendin-4 and des-fluoro-sitagliptin effects on fructose-induced increase in liver glucokinase activity in rats with impaired glucose tolerance and the exendin-4 effect on glucokinase activity in HepG2 cells incubated with fructose in the presence/absence of exendin-9-39. After 3 weeks of in vivo fructose administration we measured: (1) serum glucose, insulin and triglyceride levels; (2) liver and HepG2 cells glucokinase activity and (3) liver glucokinase and 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase mRNA and protein levels. Fructose fed rats had: hypertriglyceridemia, hyperinsulinemia and high liver glucokinase activity (mainly located in the cytosolic fraction) together with higher glucokinase and 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase mRNA and protein concentrations compared to control rats. Co-administration of either exendin-4 or des-fluoro-sitagliptin prevented serum and liver changes except glucokinase protein expression. Exendin-4 also prevented fructose-induced increase in glucokinase activity in cultured HepG2 cells, effect blunted by co-incubation with exendin-9-36. In conclusion exendin-4/des-fluro-sitagliptin prevented fructose-induced effect on glucokinase activity, mainly affecting enzyme activity modulators. Exendin 9-39 blunted in vitro protective exendin-4 effect on glucokinase activity, thus suggesting a direct effect of the later on hepatocytes through GLP-1 receptor. Alterations of glucokinase activity modulators could play a role in the pathogenesis of liver dysfunction, becoming a potential new treatment target for GLP-1 receptor agonists.

Glucagon-like peptide 1 interacts with ghrelin and leptin to regulate glucose metabolism and food intake through vagal afferent neuron signaling

Emerging evidence has suggested a possible physiologic role for peripheral glucagon-like peptide 1 (GLP-1) in regulating glucose metabolism and food intake. The likely site of action of GLP-1 is on vagal afferent neurons (VANs). The vagal afferent pathway is the major neural pathway by which information about ingested nutrients reaches the central nervous system and influences feeding behavior. Peripheral GLP-1 acts on VANs to inhibit food intake. The mechanism of the GLP-1 receptor (GLP-1R) is unlike other gut-derived receptors; GLP-1Rs change their cellular localization according to feeding status rather than their protein concentrations. It is possible that several gut peptides are involved in mediating GLP-1R translocation. The mechanism of peripheral GLP-1R translocation still needs to be elucidated. We review data supporting the role of peripheral GLP-1 acting on VANs in influencing glucose homeostasis and feeding behavior. We highlight evidence demonstrating that GLP-1 interacts with ghrelin and leptin to induce satiation. Our aim was to understand the mechanism of peripheral GLP-1 in the development of noninvasive antiobesity treatments.

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.

Potential of incretin-based therapies for non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is becoming an epidemic, paralleling the increased prevalence of obesity and diabetes, which are risk factors. In this review, we present the current pre-clinical evidence showing that GLP-1 analogues and DPP4 inhibitors can improve hepatic steatosis. Although some of the effects could be due to overall improvement in metabolic parameters, there are data to support improvements independent of weight loss, as well as direct effects on the hepatocyte in vitro. Multiple hepatocyte signal transduction pathways appear to be activated by GLP-1 and its analogues, with both AMP-activated protein kinase and Akt proposed to be key players in improving hepatic steatosis. However, it is controversial as to whether the pancreatic-type GLP-1 receptor is present or responsible for conferring the GLP-1 signal in the hepatocyte. In total, the data support the need for more rigorous prospective clinical trials to further investigate the potential of incretin therapies for treatment of NAFLD.

Effects of intraportal exenatide on hepatic glucose metabolism in the conscious dog

Incretins improve glucose metabolism through multiple mechanisms. It remains unclear whether direct hepatic effects are an important part of exenatide's (Ex-4) acute action. Therefore, the objective of this study was to determine the effect of intraportal delivery of Ex-4 on hepatic glucose production and uptake. Fasted conscious dogs were studied during a hyperglycemic clamp in which glucose was infused into the hepatic portal vein. At the same time, portal saline (control; n = 8) or exenatide was infused at low (0.3 pmol·kg⁻¹·min⁻¹, Ex-4-low; n = 5) or high (0.9 pmol·kg⁻¹·min⁻¹, Ex-4-high; n = 8) rates. Arterial plasma glucose levels were maintained at 160 mg/dl during the experimental period. This required a greater rate of glucose infusion in the Ex-4-high group (1.5 ± 0.4, 2.0 ± 0.7, and 3.7 ± 0.7 mg·kg⁻¹·min⁻¹ between 30 and 240 min in the control, Ex-4-low, and Ex-4-high groups, respectively). Plasma insulin levels were elevated by Ex-4 (arterial: 4,745 ± 428, 5,710 ± 355, and 7,262 ± 1,053 μU/ml; hepatic sinusoidal: 14,679 ± 1,700, 15,341 ± 2,208, and 20,445 ± 4,020 μU/ml, 240 min, area under the curve), whereas the suppression of glucagon was nearly maximal in all groups. Although glucose utilization was greater during Ex-4 infusion (5.92 ± 0.53, 6.41 ± 0.57, and 8.12 ± 0.54 mg·kg⁻¹·min⁻¹), when indices of hepatic, muscle, and whole body glucose uptake were expressed relative to circulating insulin concentrations, there was no indication of insulin-independent effects of Ex-4. Thus, this study does not support the notion that Ex-4 generates acute changes in hepatic glucose metabolism through direct effects on the liver.

Possible role of GLP-1 and its agonists in the treatment of type 1 diabetes mellitus

Unfortunately, the only approved medical treatment for type 1 diabetes mellitus (DM) is insulin, despite the fact that tight control cannot be reached without some serious side effects such as hypoglycemia and weight gain. More and more importance is now shifted towards developing new drugs that can reach a better glycemic control with lesser side effects. Some of these promising drugs are the glucagon-like peptides 1 (GLP-1) and their agonists, which have been FDA approved for the treatment of type 2 DM. The purpose of this article is to review all of the relevant literature on the potential role of GLP-1 in the treatment of type 1 DM. The major source of data acquisition included Medline search strategies, using the words "type 1 diabetes mellitus" and "GLP-1." Articles published in the last 20 years were screened. GLP-1 increases insulin secretion in humans with existing beta cells; it also decreases glucagon secretion, and blunts appetite. Of note, new animal studies demonstrate a role in beta cell-proliferation and decreased apoptosis. Because of all the effects mentioned above, GLP-1 seems to be a promising drug for type 1 DM treatment, but more studies are still needed before solid conclusions can be drawn.

Bone: incretin hormones perceiver or receiver?

Novel incretin-based drugs, such as glucagon-like peptide-1 receptor agonists (GLP-1 RA) and dipeptidyl peptidase-4 inhibitors (DPP-4i), have been last introduced in the pharmacological treatment of type 2 diabetes. In the last few years, the interest on the relationship of gut hormones with bone metabolism in diabetes has been increasing. The aim of present paper is to examine in vitro and in vivo evidence on the connections between incretin hormones and bone metabolism. We also discuss results of clinical trials and metaanalysis, explore the effects of incretin drugs in vitro on osteogenic cells and osteoclasts, and speculate on the possibility of different effects of GLP-1 RA and DPP-4i on the risk of bone fractures risk in humans. Although existing preliminary evidence suggests a protective effect on the bone, at least for DPP-4i, further controlled, long-term studies with measurement of bone markers, bone density, and clinical fractures rates are needed to substantiate and confirm those findings.

Normalizing action of exendin-4 and GLP-1 in the glucose metabolism of extrapancreatic tissues in insulin-resistant and type 2 diabetic states

Exendin-4 (Ex-4) mimics glucagon-like peptide-1 (GLP-1 or GCG as listed in the HUGO database), being anti-diabetic and anorectic, in stimulating glucose and lipid metabolism in extrapancreatic tissues. We studied the characteristics of Ex-4 and GLP-1 action, during prolonged treatment, on GLUTs expression (mRNA and protein), glycogen content (GC), glucose transport (GT), glycogen synthase a (GSa), and kinase (PI3K and MAPKs) activity, in liver, muscle, and fat of insulin-resistant (IR, by fructose) and type 2 diabetic (T2D, streptozotocin at birth) rats compared with normal rats. In both IR and T2D, the three tissues studied presented alterations in all measured parameters. In liver, GLP-1 and also Ex-4 normalized the lower than normal Glut2 (Slc2a2) expression and showed a trend to normalize the reduced GC in IR, and GLP-1, like Ex-4, also in T2D, effects mediated by PI3K and MAPKs. In skeletal muscle, neither GLP-1 nor Ex-4 modified Glut4 (Slc2a4) expression in either experimental model but showed normalization of reduced GT and GSa, in parallel with the normalization of reduced PI3K activity in T2D and MAPKs in both models. In adipose tissue, the altered GLUT4 expression in IR and T2D, along with reduced GT in IR and increased GT in T2D, and with hyperactivated PI3K in both, became normal after GLP-1 and Ex-4 treatment; yet, MAPKs, that were also higher, became normal only after Ex-4 treatment. The data shows that Ex-4, as well as GLP-1, exerts a normalizing effect on IR and T2D states through a distinct post-receptor mechanism, the liver being the main target for Ex-4 and GLP-1 to control glucose homeostasis.

GLP-1 signals via ERK in peripheral nerve and prevents nerve dysfunction in diabetic mice

AIM

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that induces glucose-dependent insulin secretion and may have neurotrophic properties. Our aim was to identify the presence and activity of GLP-1 receptors (GLP-1Rs) in peripheral nerve and to assess the impact of GLP-1R agonists on diabetes-induced nerve disorders.

METHODS

Tissues were collected from streptozotocin-diabetic rats. GLP-1R function was assessed by incubating tissues from normal and diabetic rats with GLP-1R agonists and antagonists and measuring induction of ERK1/2 phosphorylation by Western blot. Streptozotocin-diabetic mice were also treated with the GLP-1R agonist exenatide for 8 weeks to assess the impact of GLP-1R signalling on peripheral nerve function and structure.

RESULTS

GLP-1R protein was detected in rat dorsal root ganglia and the neurons and Schwann cells of the sciatic nerve. Protein levels were not affected by streptozotocin-induced diabetes. GLP-1R agonists did not signal via ERK1/2 in sciatic nerve of normal rats. However, GLP-1R agonists significantly increased pERK1/2 levels in sciatic nerves from diabetic rats, indicating that GLP-1Rs are functional in this tissue. Exenatide treatment did not affect blood sugar, insulin levels or paw thermal response latencies in either control or diabetic mice. However, the reductions of motor nerve conduction velocity and paw intraepidermal fibre density seen in diabetic mice were attenuated by exenatide treatment.

CONCLUSIONS

These data show that the peripheral nerve of diabetic rodents exhibits functional GLP-1R and suggest that GLP-1R-mediated ERK-signalling in sciatic nerve of diabetic rodents may protect large motor fibre function and small C fibre structure by a mechanism independent of glycaemic control.

The peptide-hormone glucagon-like peptide-1 activates cAMP and inhibits growth of breast cancer cells

The incretin hormone glucagon-like peptide (GLP)-1 is secreted from intestinal L cells in response to food intake, and promotes insulin secretion and pancreatic β-cell proliferation. Reduced GLP-1 levels are observed in obesity and type 2 diabetes mellitus (T2DM) and are associated with reduced insulin secretion and increased insulin resistance. GLP-1 mediates its activities through activation of a G-protein coupled receptor, which is expressed in the pancreas, as well as other tissues. Long-acting GLP-1 receptor (GLP-1R) agonists, such as exendin-4, are currently approved for the treatment of T2DM. As obesity and T2DM are associated with increased risk of breast cancer, we aimed to explore the effects of GLP-1 and exendin-4, on breast cancer cells. Treatment with GLP-1 or exendin-4 reduced viability and enhanced apoptosis of breast cancer cells but did not affect viability of nontumorigenic cells. Moreover, exendin-4 attenuated tumor formation by breast cancer cells in athymic mice. Treatment with either GLP-1 or exendin-4 elevated cAMP levels, activated the down-stream target CREB, and enhanced CRE promoter transcription, in breast cancer cells. Moreover, inhibition of exendin-4-induced adenylate cyclase activation restored cell viability, thus suggesting cAMP as a principle mediator of exendin-4 anti-tumorigenic activity. While the pancreatic form of the GLP-1R could not be detected in breast cancer cells, several lines of evidence indicated the existence of an alternative GLP-1R in mammary cells. Thus, internalization of GLP-1 into MCF-7 cells was evidenced, infection of MCF-7 cells with the pancreatic receptor enhanced proliferation, and treatment with exendin-(9-39), a GLP-1R antagonist, further increased cAMP levels. Our studies indicate the incretin hormone GLP-1 as a potent inducer of cAMP and an inhibitor of breast cancer cell proliferation. Reduced GLP-1 levels may, therefore, serve as a novel link between obesity, diabetes mellitus, and breast cancer.

Characteristic of GLP-1 effects on glucose metabolism in human skeletal muscle from obese patients

Direct effects of GLP-1, kinase-mediated, on glucose and lipid metabolism in rat and human extrapancreatic tissues, are amply documented and also changes in type-2 diabetic (T2D) patients. Here, we explored the characteristics of the GLP-1 action and those of its analogs Ex-4 and Ex-9, on muscle glucose transport (GT) and metabolism in human morbid obesity (OB), as compared with normal and T2D subjects. In primary cultured myocytes from OB, GT and glycogen synthase a (GSa) activity values were lower than normal, and comparable to those reported in T2D patients; GT was increased by either GLP-1 or Ex-9 in a more efficient manner than in normal or T2D, up to normal levels; the Ex-4 increasing effect on GSa activity was two times that in normal cells, while Ex-9 failed to modify the enzyme activity. In OB, the control value of all kinases analyzed - PI3K, PKB, MAPKs, and p70s6K - although lower than that in normal or T2D subjects, the cells maintained their response capability to GLP-1, Ex-4, Ex-9 and insulin, with some exceptions. GLP-1 and exendins showed a direct normalizing action in the altered glucose uptake and metabolism in the muscle of obese subjects, which in the case of GLP-1 could account, at least in part, for the reported restoration of the metabolic conditions of these patients after restrictive surgery.

Presence of a functional receptor for GLP-1 in osteoblastic cells, independent of the cAMP-linked GLP-1 receptor

Glucagon-like peptide 1 (GLP-1) controls glucose metabolism in extrapancreatic tissues through receptors other than the pancreatic cAMP-linked GLP-1 receptor; also, GLP-1 induces an insulin- and PTH-independent bone anabolic action in insulin-resistant and type-2 diabetic rats. Here we searched for the presence and characteristics of GLP-1 receptors in osteoblastic MC3T3-E1 cells. [(125)I]-GLP-1 specific binding to MC3T3-E1 cells was time- and temperature-dependent, reaching maximal value at 30 min at 25 degrees C; in these conditions, [(125)I]-GLP-1 binding was dissociable, and displaced by GLP-1, partially by GLP-2, but not by exendin-4 (Ex-4), exendin-9 (Ex-9), glucagon or insulin; Scatchard analysis of the unlabeled GLP-1 data showed high and low affinity binding sites; cross-linking of GLP-1 binding revealed an estimated 70 kDa band, almost undetectable in the presence of 10(-6) M GLP-1. GLP-1, Ex-9, insulin or glucagon failed to modify cellular cAMP content, while GLP-2 and Ex-4 increased it. However, GLP-1 induced an immediate hydrolysis of glycosylphosphatidylinositols (GPIs) generating short-lived inositolphosphoglycans (IPGs), and an increase in phosphatidylinositol-3 kinase (PI3K) and mitogen activated protein kinase (MAPK) activities; Ex-4 also affected GPIs, but its action was delayed with respect to that of GLP-1. This incretin was found to decrease Runx2 but increased osteocalcin gene expression, without affecting that of osteoprotegerin or the canonical Wnt pathway activity in MC3T3-E1 cells which do not express the pancreatic GLP-1 receptor. Our data demonstrate for the first time that GLP-1 can directly and functionally interact with osteoblastic cells, possibly through a GPI/IPG-coupled receptor.

Exendin-4 exerts osteogenic actions in insulin-resistant and type 2 diabetic states

Poor control of glucose homeostasis accounts for diabetes-related bone loss. Incretins - GLP-1 and GIP - have been proposed to affect bone turnover. GLP-1, apart from its anti-diabetic and other actions, has shown to exert a bone anabolic effect in streptozotocin-induced type 2 diabetic (T2D) and fructose-induced insulin-resistant (IR) rats. Exendin-4 (Ex-4), a peptide of non-mammalian nature, is sharing with GLP-1 part of its structural sequence, and also several glucoregulatory effects in mammals in an even more efficient manner. We have explored the effect of continuous administration (3 days by osmotic pump) of Ex-4 or saline (control) on bone turnover factors and bone structure in T2D and IR rats, compared to N, and the possible interaction of Ex-4 with the Wnt signalling pathway. Blood was taken before and after treatment for plasma measurements; tibiae and femurs were collected for gene expression of bone markers (RT-PCR) and structure (microCT) analysis; we also measured the mRNA levels of LRP5 - an activator of the Wnt pathway - and those of DKK1 and sclerostin (SOST) - both blockers of LRP5 activity. Compared to N-control, plasma glucose and insulin were respectively higher and lower in T2D; osteocalcin (OC) and tartrate-resistant alkaline phosphatase 5b (TRAP5b) were lower; after Ex-4, these turnover markers were further reduced in T2D and IR, while TRAP5b increased in N. Bone OC, osteoprogeterin (OPG) and receptor activator of NF-kB ligand (RANKL) mRNA were lower in T2D and IR; Ex-4 increased OC in all groups and OPG in N and IR, reduced RANKL in N and T2D but increased it in IR; the LRP5/DKK1 and LRP5/SOST mRNA ratios were similarly decreased in T2D, but in IR, the latter ratio was reduced while the former was increased; after Ex-4, both ratios augmented in N, and that of LRP5/DKK1 tended to normalize in T2D and IR. In conclusion, Ex-4 exerts osteogenic effects in T2D and IR models, and interacts with the Wnt pathway to promote bone formation.

Insulin-like actions of glucagon-like peptide-1: a dual receptor hypothesis

GLP-1 (9-36)amide is the cleavage product of GLP-1(7-36) amide, formed by the action of diaminopeptidyl peptidase-4 (Dpp4), and is the major circulating form in plasma. Whereas GLP-1(7-36)amide stimulates glucose-dependent insulin secretion, GLP-1(9-36)amide has only weak partial insulinotropic agonist activities on the GLP-1 receptor, but suppresses hepatic glucose production, exerts antioxidant cardioprotective actions and reduces oxidative stress in vasculature tissues. These insulin-like activities suggest a role for GLP-1 (9-36)amide in the modulation of mitochondrial functions by mechanisms independent of the GLP-1 receptor. In this paper, we discuss the current literature suggesting that GLP-1(9-36)amide is an active peptide with important insulin-like actions. These findings have implications in nutrient assimilation, energy homeostasis, obesity, and the use of Dpp4 inhibitors for the treatment of diabetes.

Exendin-4 promotes liver cell proliferation and enhances the PDX-1-induced liver to pancreas transdifferentiation process

Over the last few years, evidence has accumulated revealing the unexpected potential of committed mammalian cells to convert to a different phenotype via a process called transdifferentiation or adult cell reprogramming. These findings may have major practical implications because this process may facilitate the generation of functional autologous tissues that can be used for replacing malfunctioning organs. An instructive role for transcription factors in diverting the developmental fate of cells in adult tissues has been demonstrated when adult human liver cells were induced to transdifferentiate to the pancreatic endocrine lineage upon ectopic expression of the pancreatic master regulator PDX-1 (pancreatic and duodenal homeobox gene 1). Since organogenesis and lineage commitment are affected also by developmental signals generated in response to environmental triggers, we have now analyzed whether the hormone GLP-1 (glucogen-like peptide-1) documented to play a role in pancreatic beta cell differentiation, maturation, and survival, can also increase the efficiency of liver to pancreas transdifferentiation. We demonstrate that the GLP-1R agonist, exendin-4, significantly improves the efficiency of PDX-1-mediated transdifferentiation. Exendin-4 affects the transdifferentiation process at two distinct steps; it increases the proliferation of liver cells predisposed to transdifferentiated in response to PDX-1 and promotes the maturation of transdifferentiated cells along the pancreatic lineage. Liver cell reprogramming toward the pancreatic beta cell lineage has been suggested as a strategy for functional replacement of the ablated insulin-producing cells in diabetics. Understanding the cellular and molecular basis of the transdifferentiation process will allow us to increase the efficiency of the reprogramming process and optimize its therapeutic merit.

Effect of GLP-1 treatment on bone turnover in normal, type 2 diabetic, and insulin-resistant states

It has been suggested that hormones released after nutrient absorption, such as glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 2 (GLP-2), could be responsible for changes in bone resorption. However, information about the role of GLP-1 in this regard is scanty. Diabetes-related bone loss occurs as a consequence of poor control of glucose homeostasis, but the relationship between osteoporosis and type 2 diabetes remains unclear. Since GLP-1 is decreased in the latter condition, we evaluated some bone characteristics in streptozotocin-induced type 2 diabetic (T2D) and fructose-induced insulin-resistant (IR) rat models compared to normal (N) and the effect of GLP-1 or saline (control) treatment (3 days by osmotic pump). Blood was taken before and after treatment for plasma measurements; tibiae and femora were collected for gene expression of bone markers (RT-PCR) and structure (microCT) analysis. Compared to N, plasma glucose and insulin were, respectively, higher and lower in T2D; osteocalcin (OC) and tartrate-resistant alkaline phosphatase 5b were lower; phosphate in IR showed a tendency to be higher; PTH was not different in T2D and IR; all parameters were unchanged after GLP-1 infusion. Bone OC, osteoprotegerin (OPG) and RANKL mRNA were lower in T2D and IR; GLP-1 increased OC and OPG in all groups and RANKL in T2D. Compared to N, trabecular bone parameters showed an increased degree of anisotropy in T2D and IR, which was reduced after GLP-1. These findings show an insulin-independent anabolic effect of GLP-1 and suggest that GLP-1 could be a useful therapeutic agent for improving the deficient bone formation and bone structure associated with glucose intolerance.

Targeting the CNS to treat type 2 diabetes

Research on the role of peripheral organs in the regulation of glucose homeostasis has led to the development of various monotherapies that aim to improve glucose uptake and insulin action in these organs for the treatment of type 2 diabetes. It is now clear that the central nervous system (CNS) also plays an important part in orchestrating appropriate glucose metabolism, with accumulating evidence linking dysregulated CNS circuits to the failure of normal glucoregulatory mechanisms. There is evidence that there is substantial overlap between the CNS circuits that regulate energy balance and those that regulate glucose levels, suggesting that their dysregulation could link obesity and diabetes. These findings present new targets for therapies that may be capable of both inducing weight loss and improving glucose regulation.

Inhibition of dipeptidyl peptidase-4 by vildagliptin during glucagon-like Peptide 1 infusion increases liver glucose uptake in the conscious dog

OBJECTIVE

This study investigated the acute effects of treatment with vildagliptin on dipeptidyl peptidase-4 (DPP-4) activity, glucagon-like peptide 1 (GLP-1) concentration, pancreatic hormone levels, and glucose metabolism. The primary aims were to determine the effects of DPP-4 inhibition on GLP-1 clearance and on hepatic glucose uptake.

RESEARCH DESIGN AND METHODS

Fasted conscious dogs were studied in the presence (n = 6) or absence (control, n = 6) of oral vildagliptin (1 mg/kg). In both groups, GLP-1 was infused into the portal vein (1 pmol . kg(-1) . min(-1)) for 240 min. During the same time, glucose was delivered into the portal vein at 4 mg . kg(-1) . min(-1) and into a peripheral vein at a variable rate to maintain the arterial plasma glucose level at 160 mg/dl.

RESULTS

Vildagliptin fully inhibited DPP-4 over the 4-h experimental period. GLP-1 concentrations were increased in the vildagliptin-treated group (50 +/- 3 vs. 85 +/- 7 pmol/l in the portal vein in control and vildagliptin-treated dogs, respectively; P < 0.05) as a result of a 40% decrease in GLP-1 clearance (38 +/- 5 and 22 +/- 2 ml . kg(-1) . min(-1), respectively; P < 0.05). Although hepatic insulin and glucagon levels were not significantly altered, there was a tendency for plasma insulin to be greater (hepatic levels were 73 +/- 10 vs. 88 +/- 15 microU/ml, respectively). During vildagliptin treatment, net hepatic glucose uptake was threefold greater than in the control group. This effect was greater than that predicted by the change in insulin.

CONCLUSIONS

Vildagliptin fully inhibited DPP-4 activity, reduced GLP-1 clearance by 40%, and increased hepatic glucose disposal by means beyond the effects of GLP-1 on insulin and glucagon secretion.

CNS GLP-1 regulation of peripheral glucose homeostasis

Current models hold that peripheral and CNS GLP-1 signaling operate as distinct systems whereby CNS GLP-1 regulates food intake and circulating GLP-1 regulates glucose homeostasis. There is accumulating evidence that the arcuate nucleus, an area of the CNS that regulates energy homeostasis, responds to hormones and nutrients to regulate glucose homeostasis as well. Recent data suggest that GLP-1 may be another signal acting on the arcuate to regulate glucose homeostasis challenging the conventional model of GLP-1 physiology. This review discusses the peripheral and central GLP-1 systems and presents a model whereby these systems are integrated in regulation of glucose homeostasis.

The integrative role of CNS fuel-sensing mechanisms in energy balance and glucose regulation

The incidences of both obesity and type 2 diabetes mellitus are rising at epidemic proportions. Despite this, the balance between caloric intake and expenditure is tremendously accurate under most circumstances. Growing evidence suggests that nutrient and hormonal signals converge and directly act on brain centers, leading to changes in fuel metabolism and, thus, stable body weight over time. Growing evidence also suggests that these same signals act on the central nervous system (CNS) to regulate glucose metabolism independently. We propose that this is not coincidental and that the CNS responds to peripheral signals to orchestrate changes in both energy and glucose homeostasis. In this way the CNS ensures that the nutrient demands of peripheral tissues (and likely of the brain itself) are being met. Consequently, dysfunction of the ability of the CNS to integrate fuel-sensing signals may underlie the etiology of metabolic diseases such as obesity and diabetes.

The physiology of glucagon-like peptide 1

Glucagon-like peptide 1 (GLP-1) is a 30-amino acid peptide hormone produced in the intestinal epithelial endocrine L-cells by differential processing of proglucagon, the gene which is expressed in these cells. The current knowledge regarding regulation of proglucagon gene expression in the gut and in the brain and mechanisms responsible for the posttranslational processing are reviewed. GLP-1 is released in response to meal intake, and the stimuli and molecular mechanisms involved are discussed. GLP-1 is extremely rapidly metabolized and inactivated by the enzyme dipeptidyl peptidase IV even before the hormone has left the gut, raising the possibility that the actions of GLP-1 are transmitted via sensory neurons in the intestine and the liver expressing the GLP-1 receptor. Because of this, it is important to distinguish between measurements of the intact hormone (responsible for endocrine actions) or the sum of the intact hormone and its metabolites, reflecting the total L-cell secretion and therefore also the possible neural actions. The main actions of GLP-1 are to stimulate insulin secretion (i.e., to act as an incretin hormone) and to inhibit glucagon secretion, thereby contributing to limit postprandial glucose excursions. It also inhibits gastrointestinal motility and secretion and thus acts as an enterogastrone and part of the "ileal brake" mechanism. GLP-1 also appears to be a physiological regulator of appetite and food intake. Because of these actions, GLP-1 or GLP-1 receptor agonists are currently being evaluated for the therapy of type 2 diabetes. Decreased secretion of GLP-1 may contribute to the development of obesity, and exaggerated secretion may be responsible for postprandial reactive hypoglycemia.

Newer therapeutic options for children with diabetes mellitus: theoretical and practical considerations

Recent studies in adult patients with type 1 diabetes mellitus (T1DM) and T2DM have examined the potential utility, benefits, and side effects of agents that augment insulin secretion after oral ingestion of nutrients in comparison with intravenous nutrient delivery, the so-called incretins. Two families of incretin-like substances are now approved for use in adults. Glucagon-like peptide-1 (GLP-1) or agents that bind to its receptor (exenatide, Byetta) or agents that inhibit its destruction [dipeptidyl peptidase-IV (DPP-IV) inhibitors, Vildagliptin] improve insulin secretion, delay gastric emptying, and suppress glucagon secretion while decreasing food intake without increasing hypoglycemia. Pramlintide, a synthetic amylin analog, also decreases glucagon secretion and delays gastric emptying, improves hemoglobin A1c (HbA1C), and facilitates weight reduction without causing hypoglycemia. We review the historical discovery of these agents, their physiology [corrected] and their current applications. Remarkably, only one or two studies have been reported in children. Pediatricians caring for children with T1DM and T2DM should become familiar with these agents and investigate their applicability, as they seem likely to enhance our therapeutic armamentarium to treat children with diabetes mellitus.

Insulin secretion-independent effects of GLP-1 on canine liver glucose metabolism do not involve portal vein GLP-1 receptors

Whether glucagon-like peptide (GLP)-1 requires the hepatic portal vein to elicit its insulin secretion-independent effects on glucose disposal in vivo was assessed in conscious dogs using tracer and arteriovenous difference techniques. In study 1, six conscious overnight-fasted dogs underwent oral glucose tolerance testing (OGTT) to determine target GLP-1 concentrations during clamp studies. Peak arterial and portal values during OGTT ranged from 23 to 65 pM and from 46 to 113 pM, respectively. In study 2, we conducted hyperinsulinemic-hyperglycemic clamp experiments consisting of three periods (P1, P2, and P3) during which somatostatin, glucagon, insulin and glucose were infused. The control group received saline, the PePe group received GLP-1 (1 pmol.kg(-1).min(-1)) peripherally, the PePo group received GLP-1 (1 pmol.kg(-1).min(-1)) peripherally (P2) and then intraportally (P3), and the PeHa group received GLP-1 (1 pmol.kg(-1).min(-1)) peripherally (P2) and then through the hepatic artery (P3) to increase the hepatic GLP-1 load to the same extent as in P3 in the PePo group (n = 8 dogs/group). Arterial GLP-1 levels increased similarly in all groups during P2 ( approximately 50 pM), whereas portal GLP-1 levels were significantly increased (2-fold) in the PePo vs. PePe and PeHa groups during P3. During P2, net hepatic glucose uptake (NHGU) increased slightly but not significantly (vs. P1) in all groups. During P3, GLP-1 increased NHGU in the PePo and PeHa groups more than in the control and PePe groups (change of 10.8 +/- 1.3 and 10.6 +/- 1.0 vs. 5.7 +/- 1.0 and 5.4 +/- 0.8 micromol.kg(-1).min(-1), respectively, P < 0.05). In conclusion, physiological GLP-1 levels increase glucose disposal in the liver, and this effect does not involve GLP-1 receptors located in the portal vein.

Effect of GLP-1 on glucose transport and its cell signalling in human myocytes

Glucagon-like peptide-1 (GLP-1) controls glucose metabolism in extrapancreatic tissues participating in glucose homeostasis, through receptors not associated to cAMP. In rat hepatocytes, activation of PI3K/PKB, PKC and PP-1 mediates the GLP-1-induced stimulation of glycogen synthase. We have investigated the effect of GLP-1 in normal human myocytes, and that of its structurally related peptides exendin-4 (Ex-4) and its truncated form 9-39 (Ex-9) upon glucose uptake, and the participation of cellular enzymes proposed to mediate insulin actions. GLP-1 and both exendins activated, like insulin, PI3K/PKB and p42/44 MAPK enzymes, but p70s6k was activated only by GLP-1 and insulin. GLP-1, Ex-4 and Ex-9, like insulin, stimulated glucose uptake; wortmannin blocked the action of GLP-1, insulin and Ex-9, and reduced that of Ex-4; PD98059 abolished the effect of all peptides/hormones, while rapamycin blocked that of insulin and partially prevented that of GLP-1. H-7 abolished the action of GLP-1, insulin and Ex-4, while Ro 31-8220 prevented only the Ex-4 and Ex-9 effect. In conclusion, GLP-1, like insulin, stimulates glucose uptake, and this involves activation of PI3K/PKB, p44/42 MAPKs, partially p70s6k, and possibly PKC; Ex-4 and Ex-9 both have GLP-1-like effect upon glucose transport, in which both share with GLP-1 an activation of PI3K/PKB--partially in the case of Ex-4--and p44/42 MAPKs but not p70s6k.

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

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

Insulin-independent effects of GLP-1 on canine liver glucose metabolism: duration of infusion and involvement of hepatoportal region

Whether glucagon-like peptide-1 (GLP-1) has insulin-independent effects on glucose disposal in vivo was assessed in conscious dogs by use of tracer and arteriovenous difference techniques. After a basal period, each experiment consisted of three periods (P1, P2, P3) during which somatostatin, glucagon, insulin, and glucose were infused. The control group (C) received saline in P1, P2, and P3, the PePe group received saline in P1 and GLP-1 (7.5 pmol.kg(-1).min(-1)) peripherally (Pe; iv) in P2 and P3, and the PePo group received saline in P1 and GLP-1 peripherally (iv) (P2) and then into the portal vein (Po; P3). Glucose and insulin concentrations increased to two- and fourfold basal, respectively, and glucagon remained basal. GLP-1 levels increased similarly in the PePe and PePo groups during P2 ( approximately 200 pM), whereas portal GLP-1 levels were significantly increased (3-fold) in PePo vs. PePe during P3. In all groups, net hepatic glucose uptake (NHGU) occurred during P1. During P2, NHGU increased slightly but not significantly in all groups. During P3, NHGU increased in PePe and PePo groups to a greater extent than in C, but no significant effect of the route of infusion of GLP-1 was demonstrated (16.61 +/- 2.91 and 14.67 +/- 2.09 vs. 4.22 +/- 1.57 micromol.kg(-1).min(-1), respectively).

IN CONCLUSION

GLP-1 increased glucose disposal in the liver independently of insulin secretion; its full action required long-term infusion. The route of infusion did not modify the hepatic response.

The gastrointestinal tract and glucose tolerance

PURPOSE OF REVIEW

The development of incretin hormones and incretin analogues for the therapy of diabetes highlights the importance of the gastrointestinal tract in the maintenance of glucose tolerance.

RECENT FINDINGS

The review focuses on recent information on the role of incretins and their breakdown products on insulin secretion, gastric emptying, and satiety. The importance of gastric emptying and its absorptive potential as well as of dietary composition on gastric emptying and glucose tolerance is highlighted. The concept of a portal glucose sensor in humans has been the subject of some controversy but has been recently revisited.

SUMMARY

The gastrointestinal tract plays an important part in glucose tolerance. In this review we have examined how factors altering gastric emptying, insulin secretion in response to meal ingestion, and gastric emptying contribute to the maintenance and deterioration of glucose tolerance.

Glucagon-like peptide-1 inhibits glucagon-induced glycogenolysis in perivenous hepatocytes specifically

Hepatocytes form the hepatic acinus as a unit of microcirculation. Following the bloodstream, at least two different zones can be discerned: the periportal (PPH) and the perivenous (PVH) zones. Recently, we found that insulin inhibits glucagon-induced glycogenolysis in PVH specifically. We therefore investigated the region-specific functional effects of glucagon-like peptide-1 (GLP-1), which is known to have an insulin-like activity, on glucagon-induced glycogenolysis in isolated PPH and PVH prepared by the digitonin-collagenase method. GLP-1 inhibited 0.1 nM glucagon-induced increase in glucose release from the PVH of fed rats specifically (p < 0.01) and had an additive effect with insulin. Insulin binding did not differ between PPH and PVH of fed rats. GLP-1 did not displace [125I]-glucagon binding to the purified hepatic cell membrane. Thus, it is directly confirmed that GLP-1 has an insulin-like activity in the liver.

Glucagon and GLP-1 stimulate IGFBP-1 secretion in Hep G2 cells without effect on IGFBP-1 mRNA

Glucagon has previously been reported to increase serum levels of insulin-like growth factor binding protein-1 (IGFBP-1) in humans. The in vitro effect of glucagon and glucagon-like peptide-1 (7-36) amide (GLP-1) was investigated in Hep G2 human hepatoma cells. The expression of IGFBP-1 mRNA was determined by solution hybridization assay and IGFBP-1 secretion was measured by radioimmunoassay. In contrast to forskolin the peptides glucagon and GLP-1 had no effect on IGFBP-1 mRNA at 3, 6 and 24 h incubation or any detectable effect on the apparent half-life of IGFBP-1 mRNA. However, the exposure to glucagon (10 microg/mL, 2.87 microM) and GLP-1 (1 microM) caused a two-fold stimulation in protein levels of IGFBP-1 after 6 h incubation, declining to control levels after 24 h. This transient effect was dose dependent, remained when transcription was inhibited and required protein synthesis. The regulation of IGFBP-1 secretion by glucagon and GLP-1 appeared to be cAMP independent. In conclusion, glucagon and GPL-1 were shown to have a post-transcriptional stimulatory effect on IGFBP-1 release.

Identification of a proglucagon cDNA from Rana tigrina rugulosa that encodes two GLP-1s and that is alternatively spliced in a tissue-specific manner

Glucagon plays a pivotal role in the regulation of metabolism. A glucagon receptor has been previously characterized in the frog, Rana tigrina rugulosa, and the frog and human glucagon receptors have been shown to possess similar binding affinities toward human glucagon. To study the structural evolution of glucagon peptide and its receptor in vertebrates, in the current study, a proglucagon cDNA from the same frog species was cloned. Interestingly, in contrast to the mammalian proglucagons that contain only one GLP-1 peptide, the frog proglucagon cDNA encodes two GLP-1 peptides (GLP-1A and GLP-1B) in addition to a glucagon peptide and a glucagon-like peptide 2 (GLP-2). By reverse transcriptase-PCR (RT-PCR) analysis, the proglucagon gene expression was widely detected in the brain, colon, small intestine, liver, lung, and pancreas, suggesting that the proglucagon-derived peptides have diverse functions in frogs. Moreover, tissue-specific alternative mRNA splicing was observed in the brain, colon, and pancreas. In these tissues, proglucagon transcripts with a 135 bp in frame deletion encoding GLP-1A were found. This splicing event in R. tigrina rugulosa is novel because it deletes a GLP-1 encoding sequence instead of the GLP-2 observed in other vertebrates. These findings should enhance understanding of the proglucagon evolution, structure, and expression in vertebrates.

Effect of glucagon-like peptide-1(7-36)-amide on initial splanchnic glucose uptake and insulin action in humans with type 1 diabetes

In vitro studies indicate that glucagon-like peptide-1(7-36)-amide (GLP-1) can enhance hepatic glucose uptake. To determine whether GLP-1 increases splanchnic glucose uptake in humans, we studied seven subjects with type 1 diabetes on two occasions. On both occasions, glucose was maintained at approximately 5.5 mmo/l during the night using a variable insulin infusion. On the morning of the study, a somatostatin, glucagon, and growth hormone infusion was started to maintain basal hormone levels. Glucose (containing [3H]glucose) was infused via an intraduodenal tube at a rate of 20 micromol.kg(-1).min(-1). Insulin concentrations were increased to approximately 500 pmol/l while glucose was clamped at approximately 8.8 mmol/l for the next 4 h by means of a variable intravenous glucose infusion labeled with [6,6-2H2]glucose. Surprisingly, the systemic appearance of intraduodenally infused glucose was higher (P = 0.01) during GLP-1 infusion than saline infusion, indicating a lower (P < 0.05) rate of initial splanchnic glucose uptake (1.4 +/- 1.5 vs. 4.8 +/- 0.8 micromol.kg(-1).min(-1)). On the other hand, flux through the hepatic uridine-diphosphate- glucose pool did not differ between study days (14.2 +/- 5.5 vs. 13.0 +/- 4.2 micromol.kg(-1).min(-1)), implying equivalent rates of glycogen synthesis. GLP-1 also impaired (P < 0.05) insulin-induced suppression of endogenous glucose production (6.9 +/- 2.9 vs. 1.3 +/- 1.4 micromol.kg(-1).min(-1)), but caused a time-dependent increase (P < 0.01) in glucose disappearance (93.7 +/- 10.0 vs. 69.3 +/- 6.3 micromol.kg(-1).min(-1); P < 0.01) that was evident only during the final hour of study. We conclude that in the presence of hyperglycemia, hyperinsulinemia, and enterally delivered glucose, GLP-1 increases total body but not splanchnic glucose uptake in humans with type 1 diabetes.

Combined intraportal infusion of acetylcholine and adrenergic blockers augments net hepatic glucose uptake

Portal glucose delivery in the conscious dog augments net hepatic glucose uptake (NHGU). To investigate the possible role of altered autonomic nervous activity in the effect of portal glucose delivery, the effects of adrenergic blockade and acetylcholine (ACh) on hepatic glucose metabolism were examined in 42-h-fasted conscious dogs. Each study consisted of an equilibration (-120 to -20 min), a control (-20 to 0 min), and a hyperglycemic-hyperinsulinemic period (0 to 300 min). During the last period, somatostatin (0.8 microg. kg(-1). min(-1)) was infused along with intraportal insulin (1.2 mU. kg(-1). min(-1)) and glucagon (0.5 ng. kg(-1). min(-1)). Hepatic sinusoidal insulin was four times basal (73 +/- 7 microU/ml) and glucagon was basal (55 +/- 7 pg/ml). Glucose was infused peripherally (0-300 min) to create hyperglycemia (220 mg/dl). In test protocol, phentolamine and propranolol were infused intraportally at 0.2 microg and 0.1 microg. kg(-1). min(-1) from 120 min on. ACh was infused intraportally at 3 microg. kg(-1). min(-1) from 210 min on. In control protocol, saline was given in place of the blockers and ACh. Hyperglycemia-hyperinsulinemia switched the net hepatic glucose balance (mg. kg(-1). min(-1)) from output (2.1 +/- 0.3 and 1.1 +/- 0.2) to uptake (2.8 +/- 0.9 and 2.6 +/- 0.6) and lactate balance (micromol. kg(-1). min(-1)) from uptake (7.5 +/- 2.2 and 6.7 +/- 1.6) to output (3.7 +/- 2.6 and 3.9 +/- 1.6) by 120 min in the control and test protocols, respectively. Thereafter, in the control protocol, NHGU tended to increase slightly (3.0 +/- 0.6 mg. kg(-1). min(-1) by 300 min). In the test protocol, adrenergic blockade did not alter NHGU, but ACh infusion increased it to 4.4 +/- 0.6 and 4.6 +/- 0.6 mg. kg(-1). min(-1) by 220 and 300 min, respectively. These data are consistent with the hypothesis that alterations in nerve activity contribute to the increase in NHGU seen after portal glucose delivery.

In vivo dynamic distribution of 131I-glucagon-like peptide-1 (7-36) amide in the rat studied by gamma camera

The in vivo distribution of glucagon-like peptide-1 (7-36) amide (GLP-1) was studied in a rat model using radiolabeled GLP-1 (131I-GLP-1) depicted by a gamma-camera. The dynamic scan showed a rapid clearance from the blood circulation after an intravenous (i.v.) injection of 131I-GLP-1. After 10 min, the major part of the radioactivity was accumulated in the kidneys, whereas about 9% (of the blood value) was found in the brain. The pharmacokinetic study using 125I-GLP-1 demonstrated a rapid elimination from plasma, with a half-life of 3.3 +/- 0.6 min, a clearance of 117 +/- 15 mL/min, and a distribution volume of 557 +/- 61 mL. The elimination half-lives for the intact 125I-GLP-1 in lungs and kidneys were determined to 3.7 and 3.9 min, respectively. The metabolite GLP-1 (9-36) amide was followed in blood, lung, and kidney. All other organs assumed to contain low molecular weight fragments of GLP-1. The present study suggest that GLP-1 and/or its labeled metabolites cross the blood-brain barrier. Also the kidney plays an essential role in GLP-1 elimination after an i.v. administration, which can be of clinical interest especially in patients with kidney insufficiency who are treated with GLP-1.

Insulin, insulin-like growth factor-I (IGF-I) and glucagon: the evolution of their receptors

Insulin and glucagon, two of the most studied pancreatic hormones bind to specific membrane receptors to exert their biological actions. Insulin-like growth factors IGF-I and IGF-II are structurally related to insulin, although they are expressed ubiquitously. The biological functions of the IGFs are mediated by different transmembrane receptors, which includes the insulin, IGF-I and IGF-II receptors. The interaction of insulin, insulin related peptides and glucagon with the corresponding receptors has been studied extensively in mammals and continues to be so. At the same time, research on ectothermic animals has made enormous progress in the recent years. This paper summarizes current knowledge on insulin, IGF-I and glucagon receptors, from a comparative point of view with special attention to non-mammalian vertebrates. The review covers adult and mostly typical target tissues, and with very few exceptions, developmental aspects are not considered. Binding characteristics, tissue distribution and structure of insulin and IGF-I receptors will be considered first, because both ligands and receptors are structurally related and have overlapping functions. These sections will be followed by similar distribution of information on glucagon receptors. Readers interested in either structure or functions of insulin, IGFs and glucagon in nonmammalian vertebrates are referred to other reviews (Mommsen TP, Plisetskaya EM. Insulin in fishes and agnathans: history, structure and metabolic regulation. Rev Aquat Sci 1991;4:225-259; Mommsen TP, Plisetskaya EM. Metabolic and endocrine functions of glucagon-like peptides: evolutionary and biochemical perspectives. Fish Physiol Biochem 1993;11:429-438; Duguay SJ, Mommsen TP. Molecular aspects of pancreatic peptides. In: Sherwood NM, Hew CL, editors, Fish Physiology. vol 13. 1994:225-271; Plisetskaya EM, Mommsen TP. Glucagon and glucagon-like peptides in fishes. Int Rev Citol 1996;168:187-257.).

Inositolphosphoglycans possibly mediate the effects of glucagon-like peptide-1(7-36)amide on rat liver and adipose tissue

Insulin-like effects of glucagon-like peptide-1(7-36)amide (GLP-1) in rat liver, skeletal muscle and fat, and also the presence of GLP-1 receptors in these extrapancreatic tissues, have been documented. In skeletal muscle and liver, the action of GLP-1 is not associated with an activation of adenylate cyclase, and in cultured murine myocytes and hepatoma cell lines, it was found that GLP-1 provokes the generation of inositolphosphoglycan molecules (IPGs), which are considered second messengers of insulin action. In the present work, we document in isolated normal rat adipocytes and hepatocytes that GLP-1 exerts a rapid decrease of the radiolabelled glycosylphosphatidylinositols (GPIs)--precursors of IPGs--in the same manner as insulin, indicating their hydrolysis and the immediate short-lived generation of IPGs. Thus, IPGs could be mediators in the GLP-1 actions in adipose tissue and liver, as well as in skeletal muscle, through GLP-1 receptors which are, at least functionally, different from that of the pancreatic B-cell.

Novel signal transduction and peptide specificity of glucagon-like peptide receptor in 3T3-L1 adipocytes

Glucagon-like peptide-1 (7-36) amide (GLP-1), in addition to its well known effect of enhancing glucose-mediated insulin release, has been shown to have insulinomimetic effects and to enhance insulin-mediated glucose uptake and lipid synthesis in 3T3-L1 adipocytes. To elucidate the mechanisms of GLP-1 action in these cells, we studied the signal transduction and peptide specificity of the GLP-1 response. In 3T3-L1 adipocytes, GLP-1 caused a decrease in intracellular cAMP levels which is the opposite to the response observed in pancreatic beta cells in response to the same peptide. In 3T3-L1 adipocytes, free intracellular calcium was not modified by GLP-1. Peptide specificity was examined to help determine if a different GLP receptor isoform was expressed in 3T3-L1 adipocytes vs. beta cells. Peptides with partial homology to GLP-1 such as GLP-2, GLP-1 (1-36), and glucagon all lowered cAMP levels in 3T3-L1 adipocytes. In addition, an antagonist of pancreatic GLP-1 receptor, exendin-4 (9-39), acted as an agonist to decrease cAMP levels in 3T3-L1 adipocytes as did exendin-4 (1-39), a known agonist for the pancreatic GLP-1 receptor. Binding studies using 125I-GLP-1 also suggest that pancreatic GLP-1 receptor isoform is not responsible for the effect of GLP-1 and related peptides in 3T3-L1 adipocytes. Based on these results, we propose that the major form of the GLP receptor in 3T3-L1 adipocytes is functionally different from the pancreatic GLP-1 receptor.

Exendin-4 agonist and exendin(9-39)amide antagonist of the GLP-1(7-36)amide effects in liver and muscle

The GLP-1 structurally related peptides exendin-4 and exendin(9-39)amide were found to act, in rat liver and skeletal muscle, as agonist and antagonist, respectively, of the GLP-1(7-36)amide effects on glucose metabolism. Thus, like GLP-1(7-36)amide, exendin-4 increased glycogen synthase a activity and glucose incorporation into glycogen in both tissues and also stimulated exogenous D-glucose utilization and oxidation in muscle. These effects of GLP-1(7-36)amide and exendin-4 were inhibited by exendin(9-39)amide. Our findings provide further support to the proposed use of GLP-1, or exendin-4, as a tool in the treatment of diabetes mellitus. Thus, in addition to the well-known insulinotropic action of the peptides, they act both in liver and in muscle in a manner most suitable for restoration of glucose homeostasis, with emphasis on their positive effects upon glycogen synthesis in the two tissues and on the stimulation of exogenous glucose catabolism in muscle.

Inositolphosphoglycans are possible mediators of the glucagon-like peptide 1 (7-36)amide action in the liver

A potent glycogenic effect for GLP-1(7-36)amide has been found in rat hepatocytes and skeletal muscle, and the specific receptors detected for GLP-1(7-36)amide in these tissue membranes do not seem to be associated to adenylate cyclase. On the other hand, inositolphosphoglycan molecules (IPGs) have been implicated as second messengers in the action of insulin. In a human hepatoma cell line (HEP G-2), we have observed the presence of [125I]GLP-1(7-36)amide specific binding, and a stimulatory effect of the peptide upon glycogen synthesis, confirming the findings in isolated rat hepatocytes. Also, GLP-1(7-36)amide modulates the cell content of radiolabelled glycosylphosphatidylinositols (GPIs), in the same manner as insulin, indicating hydrolysis of GPIs and an immediate and short-lived generation of IPGs. Thus, IPGs could be mediators in the GLP-1(7-36)amide glycogenic action in the liver.