J Cell Physiol. 2015 230(12):3029-3036
GLP-2 as Beneficial Factor in the Glucose Homeostasis in Mice Fed a High Fat Diet
Sara Baldassano1, Francesca Rappa2, 3, Antonella Amato1, Francesco Cappello2, 3, Flavia Mulè1
1Dipartimento di Scienze e Tecnologie Biologiche Chimiche e Farmaceutiche (STEBICEF) Università di Palermo, 90128 Italy,
2Dipartimento di Biomedicina Sperimentale e Neuroscienze Cliniche – Università di Palermo, 90128 Italy,
3Istituto Euro-Mediterraneo di Scienza e Tecnologia, Palermo, Italy
Glucagon like peptide-2 (GLP-2) is a gastrointestinal hormone released in response to dietary nutrients, which acts through a specific receptor, the GLP-2 receptor (GLP-2R). The physiological effects of GLP-2 are multiple, involving also the intestinal adaptation to high fat diet (HFD). In consideration of the well-known relationship between chronic HFD and impaired glucose metabolism, in the present study we examined if the blocking of the GLP-2 signaling by chronic treatment with the GLP-2R antagonist, GLP-2 (3–33), leads to functional consequences in the regulation of glucose metabolism in HFD-fed mice. Compared with animals fed standard diet (STD), mice at the 10th week of HFD showed hyperglycemia, glucose intolerance, high plasma insulin level after glucose load, increased pancreas weight and β cell expansion, but not insulin resistance. In HFD fed mice, GLP-2 (3–33) treatment for 4 weeks (from the 6th to the 10th week of diet) did not affect fasting glycaemia, but it significantly increased the glucose intolerance, both fasting and glucose-induced insulin levels, and reduced the sensitivity to insulin leading to insulin-resistance. In GLP-2 (3–33)-treated HFD mice pancreas was significantly heavier and displayed a significant increase in β-cell mass in comparison with vehicle-treated HFD mice. In STD mice, the GLP-2 (3–33) treatment did not affect fasted or glucose-stimulated glycaemia, insulin, insulin sensitivity, pancreas weight and beta cell mass. The present study suggests that endogenous GLP-2 may act as a protective factor against the dysregulation of the glucose metabolism that occurs in HFD mice, because GLP-2 (3-33) worsens glucose metabolism disorders.
Glucagon-like peptide 2 (GLP-2) is a 33-amino-acid proglucagon-derived peptide, produced by intestinal enteroendocrine L cells and by a discrete population of neurons in the brainstem, which projects mainly to the hypothalamus. Prohormone convertase 1/3 (PC1/3) processes proglucagon in the gastrointestinal tract and in the brain, resulting in glucagon-like peptide-1 (GLP-1), GLP-2 and other peptides. The studies on proglucagon-derived peptides have supplied two classes of glucose lowering agents, the dipeptidyl peptidase IV (DPP-IV) inhibitors and GLP-1 receptor agonists, useful tools for treatment of type 2 diabetes (T2D). However, the overwhelming interest attracted by GLP-1 analogues as potent incretins has somewhat clouded the efforts to understand the importance of other proglucagon-derived peptides in the control of glucose homeostasis.
The main biological actions of GLP-2 are related to the regulation of energy absorption and maintenance of mucosal morphology, function, and integrity of the intestine and are mediated by the activation of a specific receptor, the GLP-2R (1). Our laboratory has analyzed different actions of GLP-2 (2-9) and previously showed that GLP-2 is involved in the regulation of the small intestine morphological changes following chronic high fat diet (HFD) (10). Indeed, the chronic treatment with the GLP-2R antagonist, GLP-2 (3–33), reduces the increase in crypt–villus height and in the cell number per villus in HFD mice. In the present study, we hypothesized that changes in mucosal morphology induced by chronic treatment with the GLP-2R antagonist, GLP-2 (3-33), are associated with changes in glucose homeostasis.
Fig. 1: Blood glucose concentrations during intraperitoneal glucose tolerance test in high-fat fed mice injected for 4 weeks with PBS (vehicle), GLP-2 (3-33) (60 ng), or Gly2-GLP-2 (5 µg) once a day. Mice were intraperitoneally injected with 2 mg/kg glucose solution and blood glucose was measured at 0 min (before glucose injection) and at 15, 30, 60, 90 and 120 min (after glucose injection) by tail vein using a commercial glucometer. *P < 0.05 compared with PBS.
Indeed, mice after 10 weeks on high fat diet, have greater mass gain compared to standard diet fed mice, display hyperglycemia, an impaired glycemic response following intraperitoneal glucose load, high plasma insulin level after glucose load, increased pancreas weight and β cell expansion, but not insulin resistance. In HFD fed mice, GLP-2 (3-33) treatment for four weeks (from the sixth to the tenth week of diet) does not affect fasting glycaemia, but it significantly increases the glucose intolerance (Fig. 1), enhances both fasting and glucose-induced insulin concentrations, and reduces the sensitivity to exogenous insulin (Fig. 2). Therefore, the impairment of the GLP-2R signaling accelerates the process leading to insulin resistance in HFD mice and suggests that GLP-2 may act as a protective factor against the deregulation of the glucose metabolism.
Interestingly, in a subsequent study we found that long-term exposure to GLP-2 stable analogue improves the obesity-related glucose dysmetabolism in a concentration-dependent manner. In fact, HFD mice treated with pharmacological doses of Gly2-GLP-2 for four weeks show significant increase in glucose tolerance and exogenous insulin sensitivity (Figs. 1 and 2), reduction in glucose-stimulated plasma insulin levels in comparison with pair-aged HFD untreated animals, suggesting that the peptide is able to delay the onset of insulin resistance (11).
In conclusion, our findings suggest that endogenous GLP-2 is functionally important for the maintenance of glucose homeostasis in HFD mice because loss of the GLP-2R signaling worsens glucose control leading to insulin resistance. Although the mechanisms underlying the beneficial impact of GLP-2 on glucose metabolism remain to be established and results from human studies so far remain inconsistent, greater attention should be given to investigating on this hormone and its receptor as targets for treatment of obesity-associated diabetes also in consideration of the chemical strategies for the design of multifunctional peptides.
Fig. 2: Blood glucose concentrations during insulin tolerance test in high-fat fed mice injected for 4 weeks with PBS (vehicle), GLP-2 (3-33) (60 ng), or Gly2-GLP-2 (5 µg) once a day. Mice were intraperitoneally injected with insulin (0.5 U/kg) and blood glucose was measured up to 120 min (0,15, 30, 60, 90, 120 min) by tail vein using a commercial glucometer. *P < 0.05 compared with PBS.
- Janssen P, Rotondo A, Mulè F & Tack J 2013 Review article: a comparison of glucagon-like peptides 1 and 2. Alimentary Pharmacology & Therapeutics 37 18-36.
- Amato A, Baldassano S, Serio R & Mulè F 2009 Glucagon-like peptide-2 relaxes mouse stomach through vasoactive intestinal peptide release. American Journal of Physiology. Gastrointestinal and Liver Physiology 296 G678-G684.
- Amato A, Rotondo A, Cinci L, Baldassano S, Vannucchi MG & Mulè F 2010 Role of cholinergic neurons in the motor effects of glucagon-like peptide-2 in mouse colon. American Journal of Physiology. Gastrointestinal and Liver Physiology 299 G1038-G1044.
- Angelone T, Filice E, Quintieri AM, Imbrogno S, Amodio N, Pasqua T, Pellegrino D, Mulè F & Cerra MC 2012 Receptor identification and physiological characterization of glucagon-like peptide-2 in the rat heart. Nutrition, Metabolism, and Cardiovascular Diseases 22 486-494.
- Baldassano S, Liu S, Qu MH, Mulè F & Wood JD 2009 Glucagon-like peptide-2 modulates neurally evoked mucosal chloride secretion in guinea pig small intestine in vitro. American Journal of Physiology. Gastrointestinal and Liver Physiology 297 G800-G805.
- Baldassano S, Bellanca AL, Serio R & Mulè F 2012 Food intake in lean and obese mice after peripheral administration of glucagon-like peptide 2. Journal of Endocrinology 213 277-84.
- Baldassano S, Amato A, Rappa F, Cappello F & Mulè F 2016 Influence of endogenous glucagon like peptide-2 on lipid disorders in mice fed a high fat diet. Endocrine research DOI: 10.3109/07435800.2016.1141950.
- Cinci L, Faussone-Pellegrini MS, Rotondo A, Mulè F & Vannucchi MG 2011 GLP-2 receptor expression in excitatory and inhibitory enteric neurons and its role in mouse duodenum contractility. Neurogastroenterology and Motility 23 e383-392.
- Rotondo A, Amato A, Baldassano S, Lentini L & Mulè F 2011b Gastric relaxation induced by glucagon-like peptide-2 in mice fed a high-fat diet or fasted. Peptides 32 1587-1592.
- Baldassano S, Amato A, Cappello F, Rappa F & Mulè F 2013 Glucagon-like peptide-2 and mouse intestinal adaptation to a high-fat diet. Journal of Endocrinology 217 11-20.
- Baldassano S, Amato A, Caldara GF & Mulè F 2016 Glucagon-like peptide-2 treatment improves glucose dysmetabolism in mice fed a high fat diet. Endocrine in press DOI: 10.1007/ s12020-016-0871-3