Meena Asmar

The Glucagonostatic and Insulinotropic Effects of Glucagon-Like Peptide 1 Contribute Equally to Its Glucose-Lowering Action

OBJECTIVE Glucagon-like peptide 1 (GLP-1) exerts beneficial antidiabetic actions via effects on pancreatic β- and α-cells. Previous studies have focused on the improvements in β-cell function, while the inhibition of α-cell secretion has received less attention. The aim of this research was to quantify the glucagonostatic contribution to the glucose-lowering effect of GLP-1 infusions in patients with type 2 diabetes. RESEARCH DESIGN AND METHODS Ten male patients with well-regulated type 2 diabetes (A1C 6.9 ± 0.8%, age 56 ± 10 years, BMI 31 ± 3 kg/m2 [means ± SD]) were subjected to five 120-min glucose clamps at fasting plasma glucose (FPG) levels. On day 1, GLP-1 was infused to stimulate endogenous insulin release and suppress endogenous glucagon. On days 2–5, pancreatic endocrine clamps were performed using somatostatin infusions of somatostatin and/or selective replacement of insulin and glucagon; day 2, GLP-1 plus basal insulin and glucagon (no glucagon suppression or insulin stimulation); day 3, basal insulin only (glucagon deficiency); day 4, basal glucagon and stimulated insulin; and day 5, stimulated insulin. The basal plasma glucagon levels were chosen to simulate portal glucagon levels. RESULTS Peptide infusions produced the desired hormone levels. The amount of glucose required to clamp FPG was 24.5 ± 4.1 (day 1), 0.3 ± 0.2 (day 2), 10.6 ± 1.1 (day 3), 11.5 ± 2.7 (day 4), and 24.5 ± 2.6 g (day 5) (day 2 was lower than days 3 and 4, which were both similar and lower than days 1 and 5). CONCLUSIONS We concluded that insulin stimulation (day 4) and glucagon inhibition (day 3) contribute equally to the effect of GLP-1 on glucose turnover in patients with type 2 diabetes, and these changes explain the glucose-lowering effect of GLP-1 (day 5 vs. day 1).

An overview of once-weekly glucagon-like peptide-1 receptor agonists—available efficacy and safety data and perspectives for the future

Incretin‐based therapies, such as the injectable glucagon‐like peptide‐1 (GLP‐1) receptor agonists and orally administered dipeptidyl peptidase‐4 (DPP‐4) inhibitors, have recently been introduced into clinical practice. At present, the GLP‐1 receptor agonists need to be administered once or twice daily. Several once‐weekly GLP‐1 receptor agonists are in phase 3 development. This review examines the efficacy, safety and perspective for the future of the once‐weekly GLP‐1 receptor agonists: exenatide once weekly, taspoglutide, albiglutide, LY2189265 and CJC‐1134‐PC, and compared them to the currently available agonists, exenatide BID and liraglutide QD. A greater reduction in haemoglobin A1c (HbA1c) and fasting plasma glucose was found with the once‐weekly GLP‐1 receptor agonists compared with exenatide BID, while the effect on postprandial hyperglycaemia was modest with the once‐weekly GLP‐1 receptor agonist. The reduction in HbA1c was in most studies greater compared to oral antidiabetic drugs and insulin glargine. The reduction in weight did not differ between the short‐ and long‐acting agonists. The gastrointestinal side effects were less with the once‐weekly agonists compared with exenatide BID, except for taspoglutide. Antibodies seem to be most frequent with exenatide once weekly, while hypersensitivity has been described in few patients treated with taspoglutide. Injection site reactions differ among the long‐acting GLP‐1 receptor agonists and are observed more frequently than with exenatide BID and liraglutide. In humans, no signal has been found indicating an association between the once‐weekly agonists and C‐cell cancer. The cardiovascular safety, durability of glucose control and effect on weight will emerge from several ongoing major long‐term trials. The once‐weekly GLP‐1 receptor analogues are promising candidates for the treatment of type 2 diabetes, although their efficacy may not be superior to once‐daily analogue liraglutide.

ADAM12 induces actin cytoskeleton and extracellular matrix reorganization during early adipocyte differentiation by regulating β1 integrin function

Changes in cell shape are a morphological hallmark of differentiation. In this study we report that the expression of ADAM12, a disintegrin and metalloprotease, dramatically affects cell morphology in preadipocytes, changing them from a flattened, fibroblastic appearance to a more rounded shape. We showed that the highest levels of ADAM12 mRNA were detected in preadipocytes at the critical stage when preadipocytes become permissive for adipogenic differentiation. Furthermore, as assessed by immunostaining, ADAM12 was transiently expressed at the cell surface concomitant with the reduced activity of β1 integrin. Co-immunoprecipitation studies indicated the formation of ADAM12/β1 integrin complexes in these preadipocytes. Overexpression of ADAM12 at the cell surface of 3T3-L1 preadipocytes achieved by transient transfection or retroviral transduction led to the disappearance of the extensive network of actin stress fibers that are characteristic of these cells, and its reorganization into a cortical network located beneath the cell membrane. The cells became more rounded, exhibited fewer vinculin-positive focal adhesions, and adhered less efficiently to fibronectin in attachment assays. Moreover, ADAM12-expressing cells were more prone to apoptosis, which could be prevented by treating the cells with β1-activating antibodies. A reduced and re-organized fibronectin-rich extracellular matrix accompanied these changes. In addition, β1 integrin was more readily extracted with Triton X-100 from cells overexpressing ADAM12 than from control cells. Collectively, these results show that surface expression of ADAM12 impairs the function of β1 integrins and, consequently, alters the organization of the actin cytoskeleton and extracellular matrix. These events may be necessary for early adipocyte differentiation.

Preserved Inhibitory Potency of GLP-1 on Glucagon Secretion in Type 2 Diabetes Mellitus

OBJECTIVE Glucagon-like peptide-1 (GLP-1) is insulinotropic, but its effect on the alpha-cell is less clear. We investigated the dose-response relationship for GLP-1-induced glucagon suppression in patients with type 2 diabetes (T2DM) and healthy controls. DESIGN Ten patients with T2DM (duration of DM, 4 +/- 1 yr; glycosylated hemoglobin, 7.1 +/- 0.3%) were studied on 2 d, with stepwise increasing GLP-1 infusions (0.25, 0.5, 1.0, and 2.0 pmol x kg(-1) x min(-1)) (d 1) or saline (d 2) with plasma glucose (PG) clamped at fasting level. On d 3, patient PG was normalized overnight using a variable insulin infusion, followed by a 3-h GLP-1 infusion as on d 1. Ten healthy subjects were examined with the same protocol on d 1 and 2. RESULTS We observed similar dose-dependent stepwise suppression of glucagon secretion in both patients and controls. Significant suppression was observed at a GLP-1 infusion rate of 0.25 pmol x kg(-1) x min(-1) (resulting in physiological plasma concentrations) as early as time 15 min in healthy controls and time 30 min in patients (d 1 and d 3). AUC for glucagon was significantly reduced on d 1 and 3 (1096 +/- 109 and 1116 +/- 108 3h x pmol/liter; P = NS) as compared to d 2 (1733 +/- 193 3h x pmol/liter; P < 0.01) in patients with T2DM. A similar reduction in AUC for glucagon was observed in healthy controls [1122 +/- 186 (d 1) vs. 1733 +/- 312 3h x pmol/liter (d 2); P < 0.001]. CONCLUSIONS The diabetic alpha-cell appears to be highly sensitive to the inhibitory action of GLP-1 both during high and near-normalized PG levels, but responds with a short, nevertheless significant delay.

Glucose-Dependent Insulinotropic Polypeptide May Enhance Fatty Acid Re-esterification in Subcutaneous Abdominal Adipose Tissue in Lean Humans

OBJECTIVE Glucose-dependent insulinotropic polypeptide (GIP) has been implicated in lipid metabolism in animals. In humans, however, there is no clear evidence of GIP effecting lipid metabolism. The present experiments were performed in order to elucidate the effects of GIP on regional adipose tissue metabolism. RESEARCH DESIGN AND METHODS Eight healthy subjects were studied on four different occasions. Abdominal subcutaneous adipose tissue metabolism was assessed by measuring arterio-venous concentration differences and regional adipose tissue blood flow during GIP (1.5 pmol/kg/min) or saline infused intravenously alone or in combination with a hyperinsulinemic-hyperglycemic (HI-HG) clamp. RESULTS During GIP and HI-HG clamp, abdominal subcutaneous adipose tissue blood flow, hydrolysis of circulating triacylglycerol (TAG) (P = 0.009), and glucose uptake (P = 0.03) increased significantly while free fatty acid (FFA) output (P = 0.04) and FFA/glycerol release ratio (P = 0.02) decreased compared with saline and HI-HG clamp. CONCLUSIONS In conclusion, GIP in combination with hyperinsulinemia and slight hyperglycemia increased adipose tissue blood flow, glucose uptake, and FFA re-esterification, thus resulting in increased TAG deposition in abdominal subcutaneous adipose tissue.

Glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide: new advances.

Purpose of reviewThis article highlights recent advances in our understanding of glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) physiology and their various sites of action beyond the incretin effect. Recent findingsBoth GLP-1 and GIP stimulate insulin secretion in a glucose-dependent manner and are thus classified as incretins. Beyond glucose-dependent insulin secretion, the peptides have common actions on islet β cells, leading β-cell proliferation and resistance to apoptosis. However, the action of GLP-1 and GIP is not limited to the islet cells; they have regulatory functions in many organs. Recent evidence has suggested that GLP-1 has important beneficial effects in the cardiovascular system and central nervous system. GIP may play a role in promoting energy storage in humans, enhances bone formation via stimulation of osteoblast proliferation and inhibition of apoptosis and may play a role in central nervous system function. SummaryThese new findings suggest further application of these hormones for the treatment of conditions such as cardiovascular disease and obesity.

On the role of glucose-dependent insulintropic polypeptide in postprandial metabolism in humans

We investigated the role of glucose-dependent insulintropic polypeptide (GIP) in the regulation of gastric emptying (GE), appetite, energy intake (EI), energy expenditure (EE), plasma levels of triglycerides (TAG), and free fatty acids (FFA) in humans. First, 20 healthy males received intravenous infusion of GIP (0.8 pmol.kg(-1).min(-1)) or saline for 300 min during and after a fixed meal (protocol 1). GE was measured using paracetamol, appetite sensations using visual analog scales, EE using indirect calorimetry, and EI during a subsequent ad libitum meal (at 300 min). Next, 10 healthy males received intravenous infusions of Intralipid, glucose, or Intralipid plus glucose, with and without GIP (1.5 pmol.kg(-1).min(-1)) for 300 min (protocol 2). In protocol 1, GIP did not have any effect on GE, EI, EE, removal of TAG, or FFA and did not influence the subjective feeling of hunger, satiety, fullness or prospective food consumption compared with saline. In protocol 2, no difference was seen in the plasma TAG on Intralipid + GIP/saline and Intralipid + glucose + GIP/saline days. FFA concentrations were lower on Intralipid + glucose + GIP/saline days (P < 0.05) compared with Intralipid + GIP/saline days and on Intralipid + GIP day (P < 0.004) compared with Intralipid + saline day. Insulin increased on all GIP days compared with saline days (P < 0.05). In conclusion, while confirming its insulinotropic effects, these data suggest that GIP does not affect GE, appetite, energy intake, EE, or the clearance rate of the applied TAG formulation in humans. However, both insulin and GIP lower post-Intralipid FFA concentration, GIP probably via stimulation of insulin secretion, increasing FFA reesterification.

Renal extraction and acute effects of glucagon-like peptide-1 on central and renal hemodynamics in healthy men

The present experiments were performed to elucidate the acute effects of intravenous infusion of glucagon-like peptide (GLP)-1 on central and renal hemodynamics in healthy men. Seven healthy middle-aged men were examined on two different occasions in random order. During a 3-h infusion of either GLP-1 (1.5 pmol·kg⁻¹·min⁻¹) or saline, cardiac output was estimated noninvasively, and intraarterial blood pressure and heart rate were measured continuously. Renal plasma flow, glomerular filtration rate, and uptake/release of hormones and ions were measured by Ficks Principle after catheterization of a renal vein. Subjects remained supine during the experiments. During GLP-1 infusion, both systolic blood pressure and arterial pulse pressure increased by 5±1 mmHg (P=0.015 and P=0.002, respectively). Heart rate increased by 5±1 beats/min (P=0.005), and cardiac output increased by 18% (P=0.016). Renal plasma flow and glomerular filtration rate as well as the clearance of Na⁺ and Li⁺ were not affected by GLP-1. However, plasma renin activity decreased (P=0.037), whereas plasma levels of atrial natriuretic peptide were unaffected. Renal extraction of intact GLP-1 was 43% (P<0.001), whereas 60% of the primary metabolite GLP-1 9-36amide was extracted (P=0.017). In humans, an acute intravenous administration of GLP-1 leads to increased cardiac output due to a simultaneous increase in stroke volume and heart rate, whereas no effect on renal hemodynamics could be demonstrated despite significant extraction of both the intact hormone and its primary metabolite.

Do the Actions of Glucagon-Like Peptide-1 on Gastric Emptying, Appetite, and Food Intake Involve Release of Amylin in Humans?

OBJECTIVE Amylin, cosecreted with insulin, has like glucagon-like peptide-1 (GLP-1) been reported to inhibit glucagon secretion, delay gastric emptying, and reduce appetite and food intake. We investigated whether the effects of GLP-1 on gastric emptying, appetite, and food intake are mediated directly or indirectly via release of amylin. DESIGN Eleven C-peptide and amylin-negative patients with type 1 diabetes mellitus (T1DM) and 12 matched healthy controls participated in a placebo-controlled, randomized, single-blinded, crossover study. With glucose clamped between 6 and 9 mm, near-physiological infusions of GLP-1, human amylin, pramlintide, or saline were given for 270 min during and after a fixed meal. Gastric emptying was measured using paracetamol, appetite using visual analog scales, and food intake during a subsequent ad libitum meal (at 240 min). RESULTS In T1DM, gastric emptying, food intake, and appetite were reduced equally during low GLP-1 and amylin infusion compared with the saline infusion (P < 0.05). The controls showed stronger suppression of gastric emptying (P < 0.0001) and food intake (P < 0.01) with GLP-1 compared to amylin. Postprandial glucagon responses were reduced in controls and T1DM during GLP-1 and amylin infusions (P < 0.05). Amylin and pramlintide infusion had similar effects. CONCLUSIONS GLP-1 exerts its effect on gastric emptying, appetite, food intake, and glucagon secretion directly, although secretion of amylin may contribute to some of these effects in healthy control subjects.

Glucose-dependent insulinotropic polypeptide has impaired effect on abdominal, subcutaneous adipose tissue metabolism in obese subjects

Objective:Glucose-dependent insulinotropic polypeptide (GIP) appears to have a role in lipid metabolism. Recently, we showed that GIP in combination with hyperinsulinemia and hyperglycemia increases triglyceride uptake in abdominal, subcutaneous adipose tissue in lean humans. It has been suggested that increased GIP secretion in obesity will promote lipid deposition in adipose tissue. In light of the current attempts to employ GIP antagonists in the treatment and prevention of human obesity, the present experiments were performed in order to elucidate whether the adipose tissue lipid metabolism would be enhanced or blunted during a GIP, hyperinsulinemic and hyperglycemic (HI–HG) clamp in obese subjects with either normal glucose tolerance (NGT) or impaired glucose tolerance (IGT).Design:Sixteen obese (BMI>30 kg m−2) subjects were divided into two groups, based on their plasma glucose response to an oral glucose challenge: (i) NGT and (ii) IGT. Abdominal, subcutaneous adipose tissue lipid metabolism was studied by conducting measurements of arteriovenous concentrations of metabolites and regional adipose tissue blood flow (ATBF) during GIP (1.5 pmol kg−1 min−1) in combination with a HI–HG clamp.Results:In both groups, ATBF responses were significantly lower than what we have found previously in healthy, lean subjects (P<0.0001). The flow response was significantly lower in the IGT group than in the NGT group (P=0.03). It was not possible to show any increase in the lipid deposition in adipose tissue under the applied experimental conditions and likewise the circulating triglyceride (TAG) concentrations remained constant.Conclusion:The applied GIP, HI–HG clamp did not induce any changes in TAG uptake in adipose tissue in obese subjects. This may be due to a blunted increase in ATBF. These experiments therefore suggest that GIP does not have a major role in postprandial lipid metabolism in obese subjects.