Glucose-dependent insulinotropic polypeptide (GIP) and GIP receptor (GIPR) genes: An association analysis of polymorphisms and bone in young and elderly women

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.

Gut hormones and bone homeostasis: potential therapeutic implications

Bone resorption follows a circadian rhythm, with a marked reduction in circulating markers of resorption (such as carboxy-terminal telopeptide region of collagen type I in serum) in the postprandial period. Several gut hormones, including glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide 1 (GLP1) and GLP2, have been linked to this effect in humans and rodent models. These hormones are secreted from enteroendocrine cells in the gastrointestinal tract in response to a variety of stimuli and effect a wide range of physiological processes within and outside the gut. Single GLP1, dual GLP1-GIP or GLP1-glucagon and triple GLP1-GIP-glucagon receptor agonists have been developed for the treatment of type 2 diabetes mellitus and obesity. In addition, single GIP, GLP1 and GLP2 analogues have been investigated in preclinical studies as novel therapeutics to improve bone strength in bone fragility disorders. Dual GIP-GLP2 analogues have been developed that show therapeutic promise for bone fragility in preclinical studies and seem to exert considerable activity at the bone material level. This Review summarizes the evidence of the action of gut hormones on bone homeostasis and physiology.

Molecular Characterization of Grass Carp GIPR and Effect of Nutrition States, Insulin, and Glucagon on Its Expression

GIP plays an important regulatory role in glucose and lipid metabolism. As the specific receptor, GIPR is involved in this physiological process. To assess the roles of GIPR in teleost, the GIPR gene was cloned from grass carp. The ORF of cloned GIPR gene was 1560 bp, encoding 519 amino acids. The grass carp GIPR was the G-protein-coupled receptor which contains seven predicted transmembrane domains. In addition, two predicted glycosylation sites were contained in the grass carp GIPR. The grass carp GIPR expression is in multiple tissues and is highly expressed in the kidney, brain regions, and visceral fat tissue. In the OGTT experiment, the GIPR expression is markedly decreased in the kidney, visceral fat, and brain by treatment with glucose for 1 and 3 h. In the fast and refeeding experiment, the GIPR expression in the kidney and visceral fat tissue was significantly induced in the fast groups. In addition, the GIPR expression levels were markedly decreased in the refeeding groups. In the present study, the visceral fat accumulation of grass carp was induced by overfed. The GIPR expression was significantly decreased in the brain, kidney, and visceral fat tissue of overfed grass carp. In primary hepatocytes, the GIPR expression was promoted by treatment with oleic acid and insulin. The GIPR mRNA levels were significantly reduced by treatment with glucose and glucagon in the grass carp primary hepatocytes. To our knowledge, this is the first time the biological role of GIPR is unveiled in teleost.

The role of GIP in α-cells and glucagon secretion

Glucose-dependent insulinotropic polypeptide (GIP) is an intestinally derived peptide that is secreted in response to feeding. The GIP receptor (GIPR) is expressed in many cell types involved in the regulation of metabolism, including α- and β-cells. Glucagon and insulin exert tremendous control over glucose metabolism. Thus, GIP action in islets strongly dictates metabolic control in the postprandial state. Loss of GIPR activity in β-cells is a characteristic of type 2 diabetes (T2D) which associates with reduced postprandial insulin secretion and hyperglycemia. Less is known about GIPR activity in α-cells or the control of glucagon secretion. GIP stimulates glucagon secretion in a glucose-dependent manner in healthy people, with enhanced activity at lower glycemia. However, GIP stimulates glucagon secretion even at hyperglycemia in people with T2D, suggesting that inappropriate GIPR activity in α-cells contributes to the pathogenesis of T2D. Here, we review the literature describing GIP action and GIPR activity in the α-cell, detailing the basic science that has shaped the view of how GIP regulates glucagon secretion. We also contrast the effects of GIP on glucagon secretion in healthy and T2D people. Finally, we contextualize these observations in light of recent work that redefines the role of glucagon in glucose homeostasis, suggesting that hyperglucagonemia per se does not drive hyperglycemia. As new medications for T2D that incorporate GIPR activity are being developed, it is clear that a better understanding of GIPR activity beyond the β-cell is necessary. This work highlights the importance of focusing on the GIPR in α-cells.

The pathogenic role of the GIP/GIPR axis in human endocrine tumors: emerging clinical mechanisms beyond diabetes

The glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone produced in the gastrointestinal tract in response to nutrients. GIP has a variety of effects on different systems, including the potentiation of insulin secretion from pancreatic β-cells after food intake (i.e. incretin effect), which is probably the most important. GIP effects are mediated by the GIP receptor (GIPR), a G protein-coupled receptor expressed in several tissues, including islet β-cells, adipocytes, bone cells, and brain. As well as its involvement in metabolic disorders (e.g. it contributes to the impaired postprandial insulin secretion in type 2 diabetes (T2DM), and to the pathogenesis of obesity and associated insulin resistance), an inappropriate GIP/GIPR axis activation of potential diagnostic and prognostic value has been reported in several endocrine tumors in recent years. The ectopic GIPR expression seen in patients with overt Cushing syndrome and primary bilateral macronodular adrenal hyperplasia or unilateral cortisol-producing adenoma has been associated with an inverse rhythm of cortisol secretion, with low fasting morning plasma levels that increase after eating. On the other hand, most acromegalic patients with an unusual GH response to oral glucose suppression have GIPR-positive somatotropinomas, and a milder phenotype, and are more responsive to medical treatment. Neuroendocrine tumors are characterized by a strong GIPR expression that may correlate positively or inversely with the proliferative index MIB-1, and that seems an attractive target for developing novel radioligands. The main purpose of this review is to summarize the role of the GIP/GIPR axis in endocrine neoplasia, in the experimental and the clinical settings.

Risk of Fractures Associated with Dipeptidyl Peptidase-4 Inhibitor Treatment: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

INTRODUCTION

More and more studies suggest that type 2 diabetes mellitus (T2DM) can lead to an increased fracture risk. Some previous clinical studies and experimental data have shown that some antidiabetic drugs can increase or decrease the incidence of fractures.

METHODS

We searched Medline, Embase, Cochrane Library, and the ClinicalTrials.gov website ( https://www.clinicaltrials.gov ) for published or unpublished randomized controlled trials (RCTs) from inception through 2 December 2018 to compare the effects of dipeptidyl peptidase-4 (DDP-4) inhibitors with active control drugs or placebo in T2DM patients. All RCTs had a duration of at least 12 weeks, and the ultimate measure was whether a fracture occurs or not. We calculated odds ratios and their 95% confidence intervals by the fixed effect Mantel-Haenszel model. Publication bias was investigated firstly through visual observation of funnel plot asymmetry and then through Begg's test or Egger's test. The Cochrane bias risk tools were used to assess the quality of included studies.

RESULTS

Eighty-seven eligible RCTs were included in this study. Of 93,772 participants, 49,270 patients received therapy and 44,502 were control patients. Five kinds of DDP-4 inhibitors were included: sitagliptin, saxagliptin, alogliptin, linagliptin and vildagliptin. There were 676 fractures in the DDP-4 inhibitor treatment group and 646 in the control group. The median average glycosylated hemoglobin level was 8.2%. DDP-4 inhibitor treatment did not seem to influence the fracture risk, no matter whether compared with placebo or active comparators in T2DM patients (Mantel-Haenszel odds ratio (MH-OR) = 1.01, 95% CI 0.90-1.12, P = 0.92). After three subgroup analyses which were defined by drug type, control regimen and duration, the results were still stable.

CONCLUSION

This systematic review and meta-analysis shows that DDP-4 inhibitors do not affect the fracture risk when compared with antidiabetic drugs or placebo in T2DM patients.

Incretin hormones: Their role in health and disease

Incretin hormones are gut peptides that are secreted after nutrient intake and stimulate insulin secretion together with hyperglycaemia. GIP (glucose-dependent insulinotropic polypeptide) und GLP-1 (glucagon-like peptide-1) are the known incretin hormones from the upper (GIP, K cells) and lower (GLP-1, L cells) gut. Together, they are responsible for the incretin effect: a two- to three-fold higher insulin secretory response to oral as compared to intravenous glucose administration. In subjects with type 2 diabetes, this incretin effect is diminished or no longer present. This is the consequence of a substantially reduced effectiveness of GIP on the diabetic endocrine pancreas, and of the negligible physiological role of GLP-1 in mediating the incretin effect even in healthy subjects. However, the insulinotropic and glucagonostatic effects of GLP-1 are preserved in subjects with type 2 diabetes to the degree that pharmacological stimulation of GLP-1 receptors significantly reduces plasma glucose and improves glycaemic control. Thus, it has become a parent compound of incretin-based glucose-lowering medications (GLP-1 receptor agonists and inhibitors of dipeptidyl peptidase-4 or DPP-4). GLP-1, in addition, has multiple effects on various organ systems. Most relevant are a reduction in appetite and food intake, leading to weight loss in the long term. Since GLP-1 secretion from the gut seems to be impaired in obese subjects, this may even indicate a role in the pathophysiology of obesity. Along these lines, an increased secretion of GLP-1 induced by delivering nutrients to lower parts of the small intestines (rich in L cells) may be one factor (among others like peptide YY) explaining weight loss and improvements in glycaemic control after bariatric surgery (e.g., Roux-en-Y gastric bypass). GIP and GLP-1, originally characterized as incretin hormones, have additional effects in adipose cells, bone, and the cardiovascular system. Especially, the latter have received attention based on recent findings that GLP-1 receptor agonists such as liraglutide reduce cardiovascular events and prolong life in high-risk patients with type 2 diabetes. Thus, incretin hormones have an important role physiologically, namely they are involved in the pathophysiology of obesity and type 2 diabetes, and they have therapeutic potential that can be traced to well-characterized physiological effects.