In Vivo Models for Incretin Research: From the Intestine to the Whole Body

Article information

Endocrinol Metab. 2016;31(1):45-51
Publication date (electronic) : 2016 March 16
doi : https://doi.org/10.3803/EnM.2016.31.1.45
Department of Internal Medicine, Seoul National University Bundang Hospital, Seoul National University College of Medicine, Seongnam, Korea.
Corresponding author: Tae Jung Oh. Department of Internal Medicine, Seoul National University Bundang Hospital, Seoul National University College of Medicine, 82 Gumi-ro 173beon-gil, Bundang-gu, Seongnam 13620, Korea. Tel: +82-31-787-7078, Fax: +82-31-787-4051, ohtjmd@gmail.com
Received 2016 January 15; Revised 2016 February 11; Accepted 2016 February 18.

Abstract

Incretin hormones are produced by enteroendocrine cells (EECs) in the intestine in response to ingested nutrient stimuli. The incretin effect is defined as the difference in the insulin secretory response between the oral glucose tolerance test and an isoglycemic intravenous glucose infusion study. The pathophysiology of the decreased incretin effect has been studied as decreased incretin sensitivity and/or β-cell dysfunction per se. Interestingly, robust increases in endogenous incretin secretion have been observed in many types of metabolic/bariatric surgery. Therefore, metabolic/bariatric surgery has been extensively studied for incretin physiology, not only the hormones themselves but also alterations in EECs distribution and genetic expression levels of gut hormones. These efforts have given us an enormous understanding of incretin biology from synthesis to in vivo behavior. Further innovative studies are needed to determine the mechanisms and targets of incretin hormones.

INTRODUCTION

In the early nineteenth century, physicians found that oral glucose administration was superior to intravenous glucose infusion in terms of reduced systemic glucose excursion [1]. Several decades ago, the concept of incretin was proposed [1]. Incretin hormones are a component of the enteroinsular axis, and are synthesized in the intestine [123]. Until now, two incretin hormones have been discovered: glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). The incretin hormones are synthesized by enteroendocrine cells (EECs) and stimulate insulin secretion from pancreatic β-cell [234]. Due to the incretins' insulinotropic and extrapancreatic effects, incretin-based therapy has been widely used for anti-diabetic treatment [5]. In addition, a GLP-1 analogue was proposed as an anti-obesity drug [6] because GLP-1 enhanced satiety and reduced appetite [3]. Although outstanding scientific knowledge is increasing, the actions and pathophysiological role of incretin are not totally understood. In this review, we overview the key experiments in incretin research, from the level of the intestine to the whole body.

INCRETIN FROM INTESTINE

GLP-1 and GIP are produced by L-cell in the distal small intestine and K-cell in the proximal small intestine, respectively. L-cell produces GLP-1, GLP-2, peptide-YY (PYY), and oxyntomodulin [7]. These L-cell hormones exert paracrine and endocrine effects such as intestinal proliferation [8] and energy homeostasis [910]. Interestingly, when the distal intestine was transposed to proximal jejunum, EECs which were supposed to be L-cell expressed both GLP-1 and GIP and were termed K/L-cell [11]. The co-expression of more than two peptides in a single EEC can be easily observed with immunohistochemistry [12]. For this process, the intestines were marked to identify its proximal-to-distal axis. For example, we harvested jejunum (10 cm distal from the ligament of Treitz) and ileum (5 cm proximal from the ileocecal valve) from rats, and each proximal site was tagged with a non-absorbable suture. To evaluate the density of EECs, we cut the intestines into longitudinal or cross-sectional sections and then counted the number of EECs per villus. However, it is hard to obtain the whole thickness of the intestine in human subjects. Therefore, mucosal biopsies were used in human experiments and the number of EECs per mucosal area was counted [13]. In this human study, regional differences of EECs were also observed and could be an important mechanism in metabolic/bariatric surgery.

A recent study showed that GLP-1 signaling is critical for intestinal growth [14]. One-month treatment with exendin-4, a GLP-1 receptor agonist, increased small bowel weight and length and crypt number, but these effects were diminished in GLP-1 receptor knockout mice [14]. Therefore, histological changes such as villus growth and cellular proliferation are important indicators of local incretin action. Similar to GLP-1, another L-cell hormone, PYY showed intestinotrophic effects [15], and GLP-2 improved intestinal integrity [16]. These paracrine effects can be assessed simply by measuring of villus length and muscle thickness [111718], and more precise stereological analyses can be adopted [19]. These changes at the level of the intestine might be a clue as to whether incretin or other gut hormones are working.

INCRETIN EFFECT

Insulin secretion is higher after glucose ingestion than intravenous administration of glucose, even though the blood glucose levels are identical, the difference thus being caused by the contribution of incretin hormones [20]. To quantitate this incretin effect, two separate glucose challenge studies are performed: an oral glucose tolerance test (OGTT) and an isoglycemic intravenous glucose infusion (IIGI) study. Plasma glucose levels are obtained at 5-minute intervals during standard OGTT procedures, and IGII studies were followed. In the IIGI studies, a gradual increase in dextrose infusion and frequent adjustment of the infusion rate are very important because plasma glucose levels should not be exceeded. In our experience [21], the amount of glucose infused during the initial 5 minutes is ~0.6 g both in healthy volunteers and type 2 diabetes patients. During the next 5 minutes, 1.0 and 1.4 g of glucose was needed to be infused in healthy volunteers and type 2 diabetes patients, respectively, to copy the glucose profiles of the 75-g OGTTs (Fig. 1). We developed mathematical models to calculate the glucose infusion rate [22], but further validation is needed. After both OGTTs and IIGI studies, the plasma levels of C-peptide and insulin are measured. The incretin effect is calculated using the area under the curve (AUC) value for C-peptide or insulin. The formula is 100×(AUCOGTT–AUCIIGI)/AUCIIGI [23].

Fig. 1

Intravenous glucose infusion rates during an isoglycemic intravenous glucose infusion (IIGI) study in subjects with (A) normal glucose tolerance (NGT) or (B) type 2 diabetes mellitus (T2DM). Plasma glucose profiles of oral glucose tolerance test (OGTTs) (filled symbol) and IIGI studies (open symbol) in subjects with (C) NGT or (D) T2DM. Adapted from Oh et al. [21], with permission from John Wiley and Sons.

The above-mentioned method to calculate the incretin effect has been widely adopted in human studies, can be applied with various dosages of glucose (25, 50, 75, 100, and 125 g) [2123242526] and can be repeated before and after therapeutic interventions (e.g., to test the effect of medications or procedures) [272829]. However, it is hard to apply the method to rodent models because of the complicated nature of the procedure, the frequent sampling and the precise adjustments of the infusion rate. Indirectly, the IIGI study can be substituted by an intraperitoneal glucose tolerance test (IPGTT) in rodent models. During the IPGTT, theoretically, endogenous incretin secretion and neural signaling of the "enteroinsular axis" are not stimulated like with intravenous glucose. Therefore, the difference in insulin secretion between the OGTT and IPGTT could denote the incretin effect. However, the glucose profiles of the OGTT and IPGTT are not identical, so there is a large limitation in the estimation of the incretin effect with these two procedures. In summary, OGTTs and IIGI studies have been used to calculate the incretin effect, but there are some drawbacks: a substantial trial-and-error stage is required, and the application to rodent models is very tricky.

INCRETIN SENSITIVITY

A large body of evidence has indicated that alterations in incretin secretion is not the key pathophysiological mechanism of type 2 diabetes [3031]. Attenuation of the incretin effect, rather than decreases in incretin secretion, might be the important pathophysiological mechanism of type 2 diabetes, especially in Caucasians [23]. A diminished incretin effect without diminished secretion of incretin hormones could indicate decreased incretin sensitivity. Similar to insulin sensitivity, which is analyzed with a hyperinsulinemic euglycemic clamp, incretin sensitivity is evaluated using a hyperglycemic clamp with incretin infusion [32]. During hyperglycemic clamp, endogenous incretion secretion is blocked and insulin secretion is augmented by exogenously administered incretin hormones. For example, after infusion of a physiological dose of GIP, insulin secretion was lower in subjects with type 2 diabetes than in subjects with normal glucose tolerance [32]. This result indicated that GIP sensitivity is decreased in subjects with type 2 diabetes. Interestingly, GIP sensitivity was partially recovered after near-normalization of blood glucose levels with intensive insulin therapy, which was calculated by insulin secretion during hyperglycemic clamp with GIP infusion before and after the treatment [33]. Although the hyperglycemic state is not physiological, hyperglycemic clamp and incretin infusion study is a good model to evaluate incretin sensitivity, as both blood glucose levels and endogenous incretin secretion are controlled.

In rodents, we have a simple way to estimate incretin sensitivity: intraperitoneal injection of incretin followed by IPGTT. For example, exendin-4 was injected intraperitoneally or infused intravenously and then an IPGTT was performed [34]. This procedure did not augment endogenous incretin secretion but reflected the action of exogenously administered GLP-1. Higher insulin secretion represented higher incretin sensitivity during the exendin-4 and IPGTT study. However, severe β-cell dysfunction could mask the insulinotropic effect of GLP-1 [35]. Thus, we assessed incretin sensitivity under consideration of β-cell function, even though it is not conclusive whether β-cell dysfunction influences incretin sensitivity [36].

INCRETIN RESPONSE AFTER METABOLIC/BARIATRIC SURGERY

Changes of the intestinal configuration after metabolic/bariatric surgery produced alterations in gut hormone secretion [37]. In particular, L-cell hormones were robustly increased after Roux-en-Y gastric bypass (RYGB) [38]. Rapid stimulation of the small intestine (intestinal roux limb) with chyme from the directly anastomosed gastric pouch was found to be a key component of the enhanced secretion of L-cell hormones [39]. Ileal transposition (IT) and duodenal jejunal bypass (DJB) surgery shared this component of RYGB [40], so GLP-1 levels were consistently increased after RYGB [38], IT [1741], and DJB [1140]. Interestingly, sleeve gastrectomy, which does not change the configuration of the distal ileum, also enhanced GLP-1 secretion [42]; this effect was partially explained by acceleration of gastric emptying [37]. To summarize, metabolic/bariatric surgery is a good model with which to evaluate the metabolic effects of increased endogenous GLP-1.

After metabolic/bariatric surgery, the number of L-cell hormone-positive cells and the genetic expression of each hormone were dramatically changed in small intestine (Table 1) [13434445464748]. For example, the number of GLP-1-positive cells and the level of proglucagon mRNA were found to be increased in the alimentary and common limbs in obese patients [13] and rats [1843] after RYGB. This cellular change could be one mechanism for the increased plasma levels of GLP-1 [13], another one being the enhanced exposure of L-cell-rich intestinal section to nutrients. In animal models, the number of cells co-expressing GIP and GLP-1 was increased in the jejunum after DJB [11] and in the transposed ileum after IT [17]. However, the increased number of K/L cells did not increase the plasma levels of GIP [1134]. In addition, the level of proglucagon mRNA was not consistently increased in various metabolic surgery models [44454649]. Post-translational modification of incretin hormones, such as the regulation of prohormone convertase 2, might be important. Furthermore, glucose lowering effect was still observed in GLP-1 receptor knockout mice underwent RYGB [50] and sleeve gastrectomy [51], which result doubt the role of GLP-1 in metabolic/bariatric surgery. Further study is needed to shed light on the expression of incretin hormones after metabolic/bariatric surgery, from synthesis to modification, secretion, and biological action.

The Intestinal mRNA Levels of Proglucagon and Peptide-YY after Metabolic Surgery

NOVEL APPROACHES

Incretin hormones are produced by EECs, and EECs exhibit dynamic changes during differentiation. In addition, more than one peptide is frequently expressed in a single EEC [12]. This pluripotent nature has been studied using a green fluorescent protein tagging system [12]. Novel signals that induce EEC differentiation into GLP-1 secreting cells could be a potential treatment target for type 2 diabetes and obesity. In addition, mRNA expression changes in the intestine will give us new insights into the mechanisms causing diabetes remission after metabolic/bariatric surgery [52].

CONCLUSIONS

Incretin hormone-producing EECs are scattered through the entire intestine, and the gut has been thought to be "the largest endocrine organ in the body" [53]. To understand incretin physiology, we need to investigate from intestines to whole body. At the level of the intestine, identification of incretin-producing cells and assessment of intestinal expression of related genes would be fundamental. At the level of the whole body, calculation of the incretin effect, measurement of the blood levels of incretin hormones and analysis of incretin sensitivity were widely studied. In the future, an in-depth understanding of incretin physiology coupled with creative methodology will give us a new treatment target for diabetes and obesity.

Notes

CONFLICTS OF INTEREST: No potential conflict of interest relevant to this article was reported.

References

1. Creutzfeldt W. The incretin concept today. Diabetologia 1979;16:75–85. 32119.
2. Cho YM, Kieffer TJ. K-cells and glucose-dependent insulinotropic polypeptide in health and disease. Vitam Horm 2010;84:111–150. 21094898.
3. Cho YM, Fujita Y, Kieffer TJ. Glucagon-like peptide-1: glucose homeostasis and beyond. Annu Rev Physiol 2014;76:535–559. 24245943.
4. Fehmann HC, Goke R, Goke B. Cell and molecular biology of the incretin hormones glucagon-like peptide-I and glucose-dependent insulin releasing polypeptide. Endocr Rev 1995;16:390–410. 7671853.
5. Nauck MA. Incretin-based therapies for type 2 diabetes mellitus: properties, functions, and clinical implications. Am J Med 2011;124(1 Suppl):S3–S18. 21194578.
6. Pi-Sunyer X, Astrup A, Fujioka K, Greenway F, Halpern A, Krempf M, et al. A randomized, controlled trial of 3.0 mg of liraglutide in weight management. N Engl J Med 2015;373:11–22. 26132939.
7. Cho YM, Merchant CE, Kieffer TJ. Targeting the glucagon receptor family for diabetes and obesity therapy. Pharmacol Ther 2012;135:247–278. 22659620.
8. Drucker DJ, Yusta B. Physiology and pharmacology of the enteroendocrine hormone glucagon-like peptide-2. Annu Rev Physiol 2014;76:561–583. 24161075.
9. Manning S, Batterham RL. The role of gut hormone peptide YY in energy and glucose homeostasis: twelve years on. Annu Rev Physiol 2014;76:585–608. 24188711.
10. Pocai A. Action and therapeutic potential of oxyntomodulin. Mol Metab 2014;3:241–251. 24749050.
11. Speck M, Cho YM, Asadi A, Rubino F, Kieffer TJ. Duodenal-jejunal bypass protects GK rats from {beta}-cell loss and aggravation of hyperglycemia and increases enteroendocrine cells coexpressing GIP and GLP-1. Am J Physiol Endocrinol Metab 2011;300:E923–E932. 21304061.
12. Egerod KL, Engelstoft MS, Grunddal KV, Nohr MK, Secher A, Sakata I, et al. A major lineage of enteroendocrine cells coexpress CCK, secretin, GIP, GLP-1, PYY, and neurotensin but not somatostatin. Endocrinology 2012;153:5782–5795. 23064014.
13. Rhee NA, Wahlgren CD, Pedersen J, Mortensen B, Langholz E, Wandall EP, et al. Effect of Roux-en-Y gastric bypass on the distribution and hormone expression of small-intestinal enteroendocrine cells in obese patients with type 2 diabetes. Diabetologia 2015;58:2254–2258. 26186884.
14. Koehler JA, Baggio LL, Yusta B, Longuet C, Rowland KJ, Cao X, et al. GLP-1R agonists promote normal and neoplastic intestinal growth through mechanisms requiring Fgf7. Cell Metab 2015;21:379–391. 25738454.
15. Zhu W, Zhang W, Gong J, Huang Q, Shi Y, Li Q, et al. Peptide YY induces intestinal proliferation in peptide YY knockout mice with total enteral nutrition after massive small bowel resection. J Pediatr Gastroenterol Nutr 2009;48:517–525. 19367178.
16. Cani PD, Possemiers S, Van de Wiele T, Guiot Y, Everard A, Rottier O, et al. Changes in gut microbiota control inflammation in obese mice through a mechanism involving GLP-2-driven improvement of gut permeability. Gut 2009;58:1091–1103. 19240062.
17. Oh TJ, Lee HJ, Cho YM. Ileal transposition decreases plasma lipopolysaccharide levels in association with increased L cell secretion in non-obese non-diabetic rats. Obes Surg 2015;9. 03. [Epub]. 10.1007/s11695-015-1879-0.
18. Mumphrey MB, Patterson LM, Zheng H, Berthoud HR. Roux-en-Y gastric bypass surgery increases number but not density of CCK-, GLP-1-, 5-HT-, and neurotensin-expressing enteroendocrine cells in rats. Neurogastroenterol Motil 2013;25:e70–e79. 23095091.
19. Hansen CF, Vassiliadis E, Vrang N, Sangild PT, Cummings BP, Havel P, et al. The effect of ileal interposition surgery on enteroendocrine cell numbers in the UC Davis type 2 diabetes mellitus rat. Regul Pept 2014;189:31–39. 24512816.
20. Nauck MA, Homberger E, Siegel EG, Allen RC, Eaton RP, Ebert R, et al. Incretin effects of increasing glucose loads in man calculated from venous insulin and C-peptide responses. J Clin Endocrinol Metab 1986;63:492–498. 3522621.
21. Oh TJ, Kim MY, Shin JY, Lee JC, Kim S, Park KS, et al. The incretin effect in Korean subjects with normal glucose tolerance or type 2 diabetes. Clin Endocrinol (Oxf) 2014;80:221–227. 23405851.
22. Choi K, Lee JC, Oh TJ, Kim M, Kim HC, Cho YM, et al. A computational method to determine glucose infusion rates for isoglycemic intravenous glucose infusion study. IEEE J Biomed Health Inform 2016;20:4–10. 26259207.
23. Nauck M, Stockmann F, Ebert R, Creutzfeldt W. Reduced incretin effect in type 2 (non-insulin-dependent) diabetes. Diabetologia 1986;29:46–52. 3514343.
24. Bagger JI, Knop FK, Lund A, Vestergaard H, Holst JJ, Vilsboll T. Impaired regulation of the incretin effect in patients with type 2 diabetes. J Clin Endocrinol Metab 2011;96:737–745. 21252240.
25. Knop FK, Aaboe K, Vilsboll T, Volund A, Holst JJ, Krarup T, et al. Impaired incretin effect and fasting hyperglucagonaemia characterizing type 2 diabetic subjects are early signs of dysmetabolism in obesity. Diabetes Obes Metab 2012;14:500–510. 22171657.
26. Bagger JI, Knop FK, Lund A, Holst JJ, Vilsboll T. Glucagon responses to increasing oral loads of glucose and corresponding isoglycaemic intravenous glucose infusions in patients with type 2 diabetes and healthy individuals. Diabetologia 2014;57:1720–1725. 24879388.
27. Vardarli I, Nauck MA, Kothe LD, Deacon CF, Holst JJ, Schweizer A, et al. Inhibition of DPP-4 with vildagliptin improved insulin secretion in response to oral as well as "isoglycemic" intravenous glucose without numerically changing the incretin effect in patients with type 2 diabetes. J Clin Endocrinol Metab 2011;96:945–954. 21239518.
28. Nauck MA, Busing M, Orskov C, Siegel EG, Talartschik J, Baartz A, et al. Preserved incretin effect in type 1 diabetic patients with end-stage nephropathy treated by combined heterotopic pancreas and kidney transplantation. Acta Diabetol 1993;30:39–45. 8329730.
29. Kosinski M, Knop FK, Vedtofte L, Grycewiczv J, Swierzewska P, Cypryk K, et al. Postpartum reversibility of impaired incretin effect in gestational diabetes mellitus. Regul Pept 2013;186:104–107. 23958841.
30. Calanna S, Christensen M, Holst JJ, Laferrere B, Gluud LL, Vilsboll T, et al. Secretion of glucagon-like peptide-1 in patients with type 2 diabetes mellitus: systematic review and meta-analyses of clinical studies. Diabetologia 2013;56:965–972. 23377698.
31. Calanna S, Christensen M, Holst JJ, Laferrere B, Gluud LL, Vilsboll T, et al. Secretion of glucose-dependent insulinotropic polypeptide in patients with type 2 diabetes: systematic review and meta-analysis of clinical studies. Diabetes Care 2013;36:3346–3352. 24065842.
32. Nauck MA, Heimesaat MM, Orskov C, Holst JJ, Ebert R, Creutzfeldt W. Preserved incretin activity of glucagon-like peptide 1 [7-36 amide] but not of synthetic human gastric inhibitory polypeptide in patients with type-2 diabetes mellitus. J Clin Invest 1993;91:301–307. 8423228.
33. Hojberg PV, Vilsboll T, Rabol R, Knop FK, Bache M, Krarup T, et al. Four weeks of near-normalisation of blood glucose improves the insulin response to glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide in patients with type 2 diabetes. Diabetologia 2009;52:199–207. 19037628.
34. Oh TJ, Shin JY, Kang GH, Park KS, Cho YM. Effect of the combination of metformin and fenofibrate on glucose homeostasis in diabetic Goto-Kakizaki rats. Exp Mol Med 2013;45:e30. 23827952.
35. Meier JJ, Nauck MA. Is the diminished incretin effect in type 2 diabetes just an epi-phenomenon of impaired beta-cell function? Diabetes 2010;59:1117–1125. 20427697.
36. Oh TJ, Park KS, Cho YM. Correlation of the incretin effect with first- and second-phase insulin secretions in Koreans with various glucose tolerance statuses. Clin Endocrinol (Oxf) 2015;83:59–66. 25267549.
37. Cho YM. A gut feeling to cure diabetes: potential mechanisms of diabetes remission after bariatric surgery. Diabetes Metab J 2014;38:406–415. 25541603.
38. Vincent RP, le Roux CW. Changes in gut hormones after bariatric surgery. Clin Endocrinol (Oxf) 2008;69:173–179. 18167136.
39. Dirksen C, Jorgensen NB, Bojsen-Moller KN, Jacobsen SH, Hansen DL, Worm D, et al. Mechanisms of improved glycaemic control after Roux-en-Y gastric bypass. Diabetologia 2012;55:1890–1901. 22538359.
40. Liu S, Zhang G, Wang L, Sun D, Chen W, Yan Z, et al. The entire small intestine mediates the changes in glucose homeostasis after intestinal surgery in Goto-Kakizaki rats. Ann Surg 2012;256:1049–1058. 23001083.
41. Strader AD, Clausen TR, Goodin SZ, Wendt D. Ileal interposition improves glucose tolerance in low dose streptozotocin-treated diabetic and euglycemic rats. Obes Surg 2009;19:96–104. 18989728.
42. Salehi M, D'Alessio DA. Effects of glucagon like peptide-1 to mediate glycemic effects of weight loss surgery. Rev Endocr Metab Disord 2014;15:171–179. 24951252.
43. Hansen CF, Bueter M, Theis N, Lutz T, Paulsen S, Dalboge LS, et al. Hypertrophy dependent doubling of L-cells in Roux-en-Y gastric bypass operated rats. PLoS One 2013;8:e65696. 23776529.
44. Strader AD, Vahl TP, Jandacek RJ, Woods SC, D'Alessio DA, Seeley RJ. Weight loss through ileal transposition is accompanied by increased ileal hormone secretion and synthesis in rats. Am J Physiol Endocrinol Metab 2005;288:E447–E453. 15454396.
45. Patriti A, Aisa MC, Annetti C, Sidoni A, Galli F, Ferri I, et al. How the hindgut can cure type 2 diabetes. Ileal transposition improves glucose metabolism and beta-cell function in Goto-kakizaki rats through an enhanced Proglucagon gene expression and L-cell number. Surgery 2007;142:74–85. 17630003.
46. Nausheen S, Shah IH, Pezeshki A, Sigalet DL, Chelikani PK. Effects of sleeve gastrectomy and ileal transposition, alone and in combination, on food intake, body weight, gut hormones, and glucose metabolism in rats. Am J Physiol Endocrinol Metab 2013;305:E507–E518. 23800881.
47. Mencarelli A, Renga B, D'Amore C, Santorelli C, Graziosi L, Bruno A, et al. Dissociation of intestinal and hepatic activities of FXR and LXRalpha supports metabolic effects of terminal ileum interposition in rodents. Diabetes 2013;62:3384–3393. 23835330.
48. Ramzy AR, Nausheen S, Chelikani PK. Ileal transposition surgery produces ileal length-dependent changes in food intake, body weight, gut hormones and glucose metabolism in rats. Int J Obes (Lond) 2014;38:379–387. 24166069.
49. Cummings BP, Strader AD, Stanhope KL, Graham JL, Lee J, Raybould HE, et al. Ileal interposition surgery improves glucose and lipid metabolism and delays diabetes onset in the UCD-T2DM rat. Gastroenterology 2010;138:2437–2446. 20226188.
50. Mokadem M, Zechner JF, Margolskee RF, Drucker DJ, Aguirre V. Effects of Roux-en-Y gastric bypass on energy and glucose homeostasis are preserved in two mouse models of functional glucagon-like peptide-1 deficiency. Mol Metab 2014;3:191–201. 24634822.
51. Wilson-Perez HE, Chambers AP, Ryan KK, Li B, Sandoval DA, Stoffers D, et al. Vertical sleeve gastrectomy is effective in two genetic mouse models of glucagon-like peptide 1 receptor deficiency. Diabetes 2013;62:2380–2385. 23434938.
52. Ryan KK, Tremaroli V, Clemmensen C, Kovatcheva-Datchary P, Myronovych A, Karns R, et al. FXR is a molecular target for the effects of vertical sleeve gastrectomy. Nature 2014;509:183–188. 24670636.
53. Ahlman H, Nilsson . The gut as the largest endocrine organ in the body. Ann Oncol 2001;12(Suppl 2):S63–S68. 11762354.

Article information Continued

Fig. 1

Intravenous glucose infusion rates during an isoglycemic intravenous glucose infusion (IIGI) study in subjects with (A) normal glucose tolerance (NGT) or (B) type 2 diabetes mellitus (T2DM). Plasma glucose profiles of oral glucose tolerance test (OGTTs) (filled symbol) and IIGI studies (open symbol) in subjects with (C) NGT or (D) T2DM. Adapted from Oh et al. [21], with permission from John Wiley and Sons.

Table 1

The Intestinal mRNA Levels of Proglucagon and Peptide-YY after Metabolic Surgery

Study Subject/Animal Surgery Proglucagon Peptide-YY
Strader et al. (2005) [44] Long-Evans rat on a high-fat diet Ileal transposition
Patriti et al. (2007) [45] GK rat Ileal transposition NA
Nausheen et al. (2013) [46] SD rat Sleeve gastrectomy
Nausheen et al. (2013) [46] SD rat Ileal transposition
Mencarelli et al. (2013) [47] Wistar rat Ileal transposition NA
Hansen et al. (2013) [43] Wistar rat RYGB
Ramzy et al. (2014) [48] SD rat Ileal transposition
Rhee et al. (2015) [13] Type 2 diabetes patients RYGB

GK, Goto-Kakizaki; NA, not available; SD, Sprague-Dawley; RYGB, Roux-en-Y gastric bypass.