Glucose-dependent insulinotropic peptide is an integrative hormone with osteotropic effects

Antidiabetic Agents and Bone Quality: A Focus on Glycation End Products and Incretin Pathway Modulations

Diabetes mellitus is associated with inadequate bone health and quality and heightened susceptibility to fractures, even in patients with normal or elevated bone mineral density. Elevated advanced glycation end-products (AGEs) and a suppressed incretin pathway are among the mechanisms through which diabetes affects the bone. Accordingly, the present review aimed to investigate the effects of antidiabetic medications on bone quality, primarily through AGEs and the incretin pathway. Google Scholar, Cochrane Library, and PubMed were used to examine related studies until February 2024. Antidiabetic medications influence AGEs and the incretin pathway directly or indirectly. Certain antidiabetic drugs including metformin, glucagon-like peptide-1 receptor agonists (GLP-1RA), dipeptidyl-peptidase-4 (DDP-4) inhibitors, α-glucosidase inhibitors (AGIs), sodium-glucose co-transporter-2 inhibitors, and thiazolidinediones (TZDs), directly affect AGEs through multiple mechanisms. These mechanisms include decreasing the formation of AGEs and the expression of AGEs receptor (RAGE) in tissue and increasing serum soluble RAGE levels, resulting in the reduced action of AGEs. Similarly, metformin, GLP-1RA, DDP-4 inhibitors, AGIs, and TZDs may enhance incretin hormones directly by increasing their production or suppressing their metabolism. Additionally, these medications could influence AGEs and the incretin pathway indirectly by enhancing glycemic control. In contrast, sulfonylureas have not demonstrated any obvious effects on AGEs or the incretin pathway. Considering their favorable effects on AGEs and the incretin pathway, a suitable selection of antidiabetic drugs may facilitate more protective effects on the bone in diabetic patients.

The Regulation of Metabolic Homeostasis by Incretins and the Metabolic Hormones Produced by Pancreatic Islets

In healthy humans, the complex biochemical interplay between organs maintains metabolic homeostasis and pathological alterations in this process result in impaired metabolic homeostasis, causing metabolic diseases such as diabetes and obesity, which are major global healthcare burdens. The great advancements made during the last century in understanding both metabolic disease phenotypes and the regulation of metabolic homeostasis in healthy individuals have yielded new therapeutic options for diseases like type 2 diabetes (T2D). However, it is unlikely that highly desirable more efficacious treatments will be developed for metabolic disorders until the complex systemic regulation of metabolic homeostasis becomes more intricately understood. Hormones produced by pancreatic islet beta-cells (insulin) and alpha-cells (glucagon) are pivotal for maintaining metabolic homeostasis; the activity of insulin and glucagon are reciprocally correlated to achieve strict control of glucose levels (normoglycaemia). Metabolic hormones produced by other pancreatic islet cells and incretins produced by the gut are also crucial for maintaining metabolic homeostasis. Recent studies highlighted the incomplete understanding of metabolic hormonal synergism and, therefore, further elucidation of this will likely lead to more efficacious treatments for diseases such as T2D. The objective of this review is to summarise the systemic actions of the incretins and the metabolic hormones produced by the pancreatic islets and their interactions with their respective receptors.

Glucagon-Like Peptide Receptor-1 Agonists Used for Medically-Supervised Weight Loss in Patients With Hip and Knee Osteoarthritis: Critical Considerations for the Arthroplasty Surgeon

Patients with morbid obesity and concomitant hip or knee osteoarthritis represent a challenging patient demographic to treat as these patients often present earlier in life, have more severe symptoms, and have worse surgical outcomes following total hip and total knee arthroplasty. Previously, bariatric and metabolic surgeries represented one of the few weight loss interventions that morbidly obese patients could undergo prior to total joint arthroplasty. However, data regarding the reduction in complications with preoperative bariatric surgery remain mixed. Glucagon-like peptide receptor-1 (GLP-1) agonists have emerged as an effective treatment option for obesity in patients with and without diabetes mellitus. Furthermore, recent data suggest these medications may serve as potential anti-inflammatory and disease-modifying agents for numerous chronic conditions, including osteoarthritis. This review will discuss the GLP-1 agonists and GLP-1/glucose-dependent insulinotropic polypeptide dual agonists currently available, along with GLP-1/glucose-dependent insulinotropic polypeptide/glucagon triple agonists presently being developed to address the obesity epidemic. Furthermore, this review will address the potential problem of GLP-1-related delayed gastric emptying and its impact on the timing of elective total joint arthroplasty. The review aims to provide arthroplasty surgeons with a primer for implementing this class of medication in their current and future practice, including perioperative instructions and perioperative safety considerations when treating patients taking these medications.

Effect of gut hormones on bone metabolism and their possible mechanisms in the treatment of osteoporosis

Bone is a highly dynamic organ that changes with the daily circadian rhythm. During the day, bone resorption is suppressed due to eating, while it increases at night. This circadian rhythm of the skeleton is regulated by gut hormones. Until now, gut hormones that have been found to affect skeletal homeostasis include glucagon-like peptide-1 (GLP-1), glucagon-like peptide-2 (GLP-2), glucose-dependent insulinotropic polypeptide (GIP), and peptide YY (PYY), which exerts its effects by binding to its cognate receptors (GLP-1R, GLP-2R, GIPR, and Y1R). Several studies have shown that GLP-1, GLP-2, and GIP all inhibit bone resorption, while GIP also promotes bone formation. Notably, PYY has a strong bone resorption-promoting effect. In addition, gut microbiota (GM) plays an important role in maintaining bone homeostasis. This review outlines the roles of GLP-1, GLP-2, GIP, and PYY in bone metabolism and discusses the roles of gut hormones and the GM in regulating bone homeostasis and their potential mechanisms.

(D-Ala2)GIP Inhibits Inflammatory Bone Resorption by Suppressing TNF-α and RANKL Expression and Directly Impeding Osteoclast Formation

Glucose-insulinotropic polypeptide (GIP) is an incretin hormone that induces insulin secretion and decreases blood glucose levels. In addition, it has been reported to suppress osteoclast formation. Native GIP is rapidly degraded by dipeptidyl peptidase-4 (DPP-4). (D-Ala2)GIP is a newly developed GIP analog that demonstrates enhanced resistance to DPP-4. This study aimed to evaluate the influence of (D-Ala2)GIP on osteoclast formation and bone resorption during lipopolysaccharide (LPS)-induced inflammation in vivo and in vitro. In vivo, mice received supracalvarial injections of LPS with or without (D-Ala2)GIP for 5 days. Osteoclast formation and bone resorption were evaluated, and TNF-α and RANKL expression were measured. In vitro, the influence of (D-Ala2)GIP on RANKL- and TNF-α-induced osteoclastogenesis, LPS-triggered TNF-α expression in macrophages, and RANKL expression in osteoblasts were examined. Compared to the LPS-only group, calvariae co-administered LPS and (D-Ala2)GIP led to less osteoclast formation, lower bone resorption, and decreased TNF-α and RANKL expression. (D-Ala2)GIP inhibited osteoclastogenesis induced by RANKL and TNF-α and downregulated TNF-α expression in macrophages and RANKL expression in osteoblasts in vitro. Furthermore, (D-Ala2)GIP suppressed the MAPK signaling pathway. The results suggest that (D-Ala2)GIP dampened LPS-triggered osteoclast formation and bone resorption in vivo by reducing TNF-α and RANKL expression and directly inhibiting osteoclastogenesis.

The Gut-Bone Axis in Diabetes

PURPOSE OF REVIEW

To describe recent advances in the understanding of how gut-derived hormones regulate bone homeostasis in humans with emphasis on pathophysiological and therapeutic perspectives in diabetes.

RECENT FINDINGS

The gut-derived incretin hormone glucose-dependent insulinotropic polypeptide (GIP) is important for postprandial suppression of bone resorption. The other incretin hormone, glucagon-like peptide 1 (GLP-1), as well as the intestinotrophic glucagon-like peptide 2 (GLP-2) has been shown to suppress bone resorption in pharmacological concentrations, but the role of the endogenous hormones in bone homeostasis is uncertain. For ambiguous reasons, both patients with type 1 and type 2 diabetes have increased fracture risk. In diabetes, the suppressive effect of endogenous GIP on bone resorption seems preserved, while the effect of GLP-2 remains unexplored both pharmacologically and physiologically. GLP-1 receptor agonists, used for the treatment of type 2 diabetes and obesity, may reduce bone loss, but results are inconsistent. GIP is an important physiological suppressor of postprandial bone resorption, while GLP-1 and GLP-2 may also exert bone-preserving effects when used pharmacologically. A better understanding of the actions of these gut hormones on bone homeostasis in patients with diabetes may lead to new strategies for the prevention and treatment of skeletal frailty related to diabetes.

The associations of gut microbiota, endocrine system and bone metabolism

Gut microbiota is of great importance in human health, and its roles in the maintenance of skeletal homeostasis have long been recognized as the "gut-bone axis." Recent evidence has indicated intercorrelations between gut microbiota, endocrine system and bone metabolism. This review article discussed the complex interactions between gut microbiota and bone metabolism-related hormones, including sex steroids, insulin-like growth factors, 5-hydroxytryptamine, parathyroid hormone, glucagon-like peptides, peptide YY, etc. Although the underlying mechanisms still need further investigation, the regulatory effect of gut microbiota on bone health via interplaying with endocrine system may provide a new paradigm for the better management of musculoskeletal disorders.

GIPR rs10423928 and bone mineral density in postmenopausal women in Shanghai

We demonstrated previously that there is a correlation between glucagon-like peptide-1 (GLP-1) single-nucleotide polymorphism (SNP) and bone mineral density in postmenopausal women. Both GLP-1 and glucose-dependent insulinotropic peptide are incretins. The glucose-dependent insulinotropic peptide receptor (GIPR) SNP rs10423928 has been extensively studied. However, it is not clear whether GIPR gene mutations affect bone metabolism. The aim of this study was to investigate the association between rs10423928 and bone mineral density in postmenopausal women in Shanghai. rs10423928 was detected in 884 postmenopausal women in Shanghai, and the correlation between the GIPR SNP and bone mineral density was assessed. The dominant T/T genotype of rs10423928 was found to be related to the bone mineral density of the femoral neck (P = 0.035). Overall, our findings indicate that the dominant T/T genotype of rs10423928 in postmenopausal women is significantly associated with a higher bone mineral density and that the T/T genotype exerts a bone-protective effect.

GIP and GLP-2 together improve bone turnover in humans supporting GIPR-GLP-2R co-agonists as future osteoporosis treatment

The intestinal hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2) are key regulators of postprandial bone turnover in humans. We hypothesized that GIP and GLP-2 co-administration would provide stronger effect on bone turnover than administration of the hormones separately, and tested this using subcutaneous injections of GIP and GLP-2 alone or in combination in humans. Guided by these findings, we designed series of GIPR-GLP-2R co-agonists as template for new osteoporosis treatment. The clinical experiment was a randomized cross-over design including 10 healthy men administered subcutaneous injections of GIP and GLP-2 alone or in combination. The GIPR-GLP-2R co-agonists were characterized in terms of binding and activation profiles on human and rodent GIP and GLP-2 receptors, and their pharmacokinetic (PK) profiles were improved by dipeptidyl peptidase-4 protection and site-directed lipidation. Co-administration of GIP and GLP-2 in humans resulted in an additive reduction in bone resorption superior to each hormone individually. The GIPR-GLP-2R co-agonists, designed by combining regions of importance for cognate receptor activation, obtained similar efficacies as the two native hormones and nanomolar potencies on both human receptors. The PK-improved co-agonists maintained receptor activity along with their prolonged half-lives. Finally, we found that the GIPR-GLP-2R co-agonists optimized toward the human receptors for bone remodeling are not feasible for use in rodent models. The successful development of potent and efficacious GIPR-GLP-2R co-agonists, combined with the improved effect on bone metabolism in humans by co-administration, support these co-agonists as a future osteoporosis treatment.

Subcutaneous GIP and GLP-2 inhibit nightly bone resorption in postmenopausal women: A preliminary study

BACKGROUND

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2) are gut hormones secreted in response to food ingestion, and they have been suggested to regulate bone turnover. In humans, exogenous GIP and GLP-2 acutely inhibit bone resorption as measured by circulating levels of carboxy-terminal type 1 collagen crosslinks (CTX).

OBJECTIVE

The objective was to study the individual and combined acute effects of GIP and GLP-2 on bone turnover in postmenopausal women during nighttime - a period of increased bone resorption.

METHODS

Using a randomized, placebo-controlled, double-blinded, crossover design, each participant (n = 9) received on four separate study days: GIP, GLP-2, GIP + GLP-2, and placebo (saline) as subcutaneous injections at bedtime. Main outcomes were levels of CTX and procollagen type 1 N-terminal propeptide (P1NP).

RESULTS

Compared with placebo, GIP and GLP-2 alone significantly inhibited bone resorption (measured by CTX). GIP rapidly reduced CTX levels in the period from 45 to 120 min after injection, while GLP-2 had a more delayed effect with reduced CTX levels in the period from 120 to 240 min after injection. Combining GIP and GLP-2 showed complementary effects resulting in a sustained inhibition of CTX with reduced levels from 45 to 240 min after injection. Furthermore, GIP acutely increased bone formation (measured by P1NP).

CONCLUSION

Both GIP and GLP-2 reduced CTX during the night and had complementary effects when combined.

Alliances of the gut and bone axis

Gut hormones secreted from enteroendocrine cells following nutrient ingestion modulate metabolic processes including glucose homeostasis and food intake, and several of these gut hormones are involved in the regulation of the energy demanding process of bone remodelling. Here, we review the gut hormones considered or known to be involved in the gut-bone crosstalk and their role in orchestrating adaptions of bone formation and resorption as demonstrated in cellular and physiological experiments and clinical trials. Understanding the physiology and pathophysiology of the gut-bone axis may identify adverse effects of investigational drugs aimed to treat metabolic diseases such as type 2 diabetes and obesity and new therapeutic candidates for the treatment of bone diseases.

Effects of Incretin-Related Diabetes Drugs on Bone Formation and Bone Resorption

Patients with type 2 diabetes have an increased risk of fracture compared to the general population. Glucose absorption is accelerated by incretin hormones, which induce insulin secretion from the pancreas. The level of the incretin hormone, glucagon-like peptide-1 (GLP-1), shows an immediate postprandial increase, and the circulating level of intact GLP-1 is reduced rapidly by dipeptidyl peptidase-4 (DPP-4)-mediated inactivation. Therefore, GLP-1 receptor agonists and DPP-4 inhibitors are effective in the treatment of type 2 diabetes. However, these incretin-related diabetic agents have been reported to affect bone metabolism, including bone formation and resorption. These agents enhance the expression of bone markers, and have been applied to improve bone quality and bone density. In addition, they have been reported to suppress chronic inflammation and reduce the levels of inflammatory cytokine expression. Previously, we reported that these incretin-related agents inhibited both the expression of inflammatory cytokines and inflammation-induced bone resorption. This review presents an overview of current knowledge regarding the effects of incretin-related diabetes drugs on osteoblast differentiation and bone formation as well as osteoclast differentiation and bone resorption. The mechanisms by which incretin-related diabetes drugs regulate bone formation and bone resorption are also discussed.

Glucose-Dependent Insulinotropic Polypeptide (GIP) Reduces Bone Resorption in Patients With Type 2 Diabetes

CONTEXT

In healthy individuals, glucose-dependent insulinotropic polypeptide (GIP) enhances insulin secretion and reduces bone resorption by up to 25% estimated by absolute placebo-corrected changes in carboxy-terminal type 1 collagen crosslinks (CTX) during GIP and glucose administration. In patients with type 2 diabetes (T2D), GIP's insulinotropic effect is impaired and effects on bone may be reduced.

OBJECTIVE

To investigate GIP's effect on bone biomarkers in patients with T2D.

DESIGN

Randomized, double-blinded, crossover study investigating 6 interventions.

PATIENTS

Twelve male patients with T2D.

INTERVENTIONS

A primed continuous 90-minute GIP infusion (2 pmol/kg/min) or matching placebo (saline) administered at 3 plasma glucose (PG) levels (i.e., paired days with "insulin-induced hypoglycemia" (PG lowered to 3 mmol/L), "fasting hyperglycemia" (mean PG ~8 mmol/L), or "aggravated hyperglycemia" (mean PG ~12 mmol/L).

MAIN OUTCOME MEASURES

Bone biomarkers: CTX, procollagen type 1 N-terminal propeptide (P1NP) and PTH.

RESULTS

On days with insulin-induced hypoglycemia, CTX was suppressed by up to 40 ± 15% during GIP administration compared with 12 ± 11% during placebo infusion (P < 0.0001). On days with fasting hyperglycemia, CTX was suppressed by up to 36 ± 15% during GIP administration, compared with 0 ± 9% during placebo infusion (P < 0.0001). On days with aggravated hyperglycemia, CTX was suppressed by up to 47 ± 23% during GIP administration compared with 10 ± 9% during placebo infusion (P = 0.0005). At all glycemic levels, P1NP and PTH concentrations were similar between paired days after 90 minutes.

CONCLUSIONS

Short-term GIP infusions reduce bone resorption by more than one-third (estimated by absolute placebo-corrected CTX reductions) in patients with T2DM, suggesting preserved bone effects of GIP in these patients.

PRÉCIS

Short-term GIP infusions reduce the bone resorption marker CTX by one-third in patients with type 2 diabetes independent of glycemic levels.

GIP's involvement in the pathophysiology of type 2 diabetes

During the past four decades derangements in glucose-dependent insulinotropic polypeptide (GIP) biology has been viewed upon as contributing factors to various parts of the pathophysiology type 2 diabetes. This overview outlines and discusses the impaired insulin responses to GIP as well as the effect of GIP on glucagon secretion and the potential involvement of GIP in the obesity and bone disease associated with type 2 diabetes. As outlined in this review, it is unlikely that the impaired insulinotropic effect of GIP occurs as a primary event in the development of type 2 diabetes, but rather develops once the diabetic state is present and beta cells are unable to maintain normoglycemia. In various models, GIP has effects on glucagon secretion, bone and lipid homeostasis, but whether these effects contribute substantially to the pathophysiology of type 2 diabetes is at present controversial. The review also discusses the substantial uncertainty surrounding the translation of preclinical data relating to the GIP system and outline future research directions.

GIP and the gut-bone axis - Physiological, pathophysiological and potential therapeutic implications

The influence by gut-derived hormones on bone remodelling appears increasingly important as research on the enteroendocrine-osseous axis accelerates. Glucose-dependent insulinotropic polypeptide (GIP) is secreted from the gut and potentiates insulin secretion in a glucose-dependent manner. GIP has, like the two other gut-derived hormones, glucagon-like peptide 1 and glucagon-like peptide 2, been shown to affect bone remodelling as part of the enteroendocrine-osseous axis. Observational studies have shown that a mutation in the GIP receptor causing reduced receptor signalling leads to lower bone mineral density and increased fracture risk. Rodent as well as human studies have shown that GIP causes serum levels of the bone resorption marker carboxy-terminal type 1 collagen crosslinks to decline. GIP may also increase bone formation indicating a potential uncoupling of bone resorption and formation. Here, we review past and recent discoveries elucidating the enteroendocrine-osseous axis with a special focus on GIP.

GIP as a Potential Therapeutic Target for Atherosclerotic Cardiovascular Disease-A Systematic Review

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are gut hormones that are secreted from enteroendocrine L cells and K cells in response to digested nutrients, respectively. They are also referred to incretin for their ability to stimulate insulin secretion from pancreatic beta cells in a glucose-dependent manner. Furthermore, GLP-1 exerts anorexic effects via its actions in the central nervous system. Since native incretin is rapidly inactivated by dipeptidyl peptidase-4 (DPP-4), DPP-resistant GLP-1 receptor agonists (GLP-1RAs), and DPP-4 inhibitors are currently used for the treatment of type 2 diabetes as incretin-based therapy. These new-class agents have superiority to classical oral hypoglycemic agents such as sulfonylureas because of their low risks for hypoglycemia and body weight gain. In addition, a number of preclinical studies have shown the cardioprotective properties of incretin-based therapy, whose findings are further supported by several randomized clinical trials. Indeed, GLP-1RA has been significantly shown to reduce the risk of cardiovascular and renal events in patients with type 2 diabetes. However, the role of GIP in cardiovascular disease remains to be elucidated. Recently, pharmacological doses of GIP receptor agonists (GIPRAs) have been found to exert anti-obesity effects in animal models. These observations suggest that combination therapy of GLP-1R and GIPR may induce superior metabolic and anti-diabetic effects compared with each agonist individually. Clinical trials with GLP-1R/GIPR dual agonists are ongoing in diabetic patients. Therefore, in this review, we summarize the cardiovascular effects of GIP and GIPRAs in cell culture systems, animal models, and humans.

Incretin accelerates platelet-derived growth factor-BB-induced osteoblast migration via protein kinase A: The upregulation of p38 MAP kinase

Incretins, including glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), secreted from enteroendocrine cells after food ingestion, are currently recognized to regulate glucose metabolism through insulin secretion. We previously demonstrated that platelet-derived growth factor-BB (PDGF-BB) induces the migration of osteoblast-like MC3T3-E1 cells through mitogen-activated protein (MAP) kinases, including p38 MAP kinase. In the present study, we investigated whether or not incretins affect the osteoblast migration. The PDGF-BB-induced cell migration was significantly reinforced by GLP-1, GIP or cAMP analogues in MC3T3-E1 cells and normal human osteoblasts. The upregulated migration by GLP-1 or cAMP analogues was suppressed by H89, an inhibitor of protein kinase A. The amplification by GLP-1 of migration induced by PDGF-BB was almost completely reduced by SB203580, a p38 MAP kinase inhibitor in MC3T3-E1 cells and normal human osteoblasts. In addition, GIP markedly strengthened the PDGF-BB-induced phosphorylation of p38 MAP kinase. Exendin-4, a GLP-1 analogue, induced Rho A expression and its translocation from cytoplasm to plasma membranes in osteoblasts at the epiphyseal lines of developing mouse femurs in vivo. These results strongly suggest that incretins accelerates the PDGF-BB-induced migration of osteoblasts via protein kinase A, and the up-regulation of p38 MAP kinase is involved in this acceleration. Our findings may highlight the novel potential of incretins to bone physiology and therapeutic strategy against bone repair.

GIP's effect on bone metabolism is reduced by the selective GIP receptor antagonist GIP(3-30)NH2

Infusion of the incretin hormone glucose-dependent insulinotropic polypeptide (GIP) suppresses the bone resorption marker carboxy-terminal type 1 collagen crosslinks (CTX). Using separate and combined infusions of the selective GIP receptor (GIPR) antagonist, GIP(3-30)NH₂, and GIP, we investigated how GIPR inhibition affects bone turnover markers. Ten healthy men (median age 22.5 years (range 21-25), BMI 21.3kg/m² (19.9-24.7)) participated in a randomized, doubled blinded, placebo-controlled, crossover study with four 1h 12mmol/l-hyperglycemic clamps on four separate study days with concomitant infusions of GIP, GIP+GIP(3-30)NH₂, GIP(3-30)NH₂, and placebo, respectively, separated by a period of at least one week. GIP was infused at 1.5pmol/kg/min and GIP(3-30)NH₂ at 800pmol/kg/min. Plasma glucose was clamped at 12.0±1.2mmol/l and plasma levels of GIP and GIP(3-30)NH₂ amounted to ∼80pmol/l and ∼50nmol/l, respectively. GIP suppressed CTX more than placebo (baseline-subtracted AUC -6,811±1,260 vs. -3,012±3,018ng/l×min, P= 0.002) and resulted in CTX values of 53 ± 6.9% (GIP) versus 81 ± 10% of baseline (placebo), respectively (P = 0.0006), at the end of the hyperglycemic clamp. Co-infusion of GIP and GIP(3-30)NH₂ attenuated the GIP-induced CTX suppression by 51±33% (P = 0.01). The peak value of the bone formation marker N-terminal propeptide of type 1 procollagen (P1NP) peaked at higher levels during GIP (109±6.7% of baseline) than during GIP(3-30)NH₂ infusion (101±8.9%) (P = 0.049) and GIP suppressed PTH levels compared to GIP(3-30)NH₂ alone (P = 0.0158). In conclusion, blockade of the GIPR with GIP(3-30)NH₂ diminished GIP-induced CTX and P1NP responses, showing that these effects are GIPR-mediated and that GIPR antagonism might interfere with bone resorption.

Glucose-Dependent Insulinotropic Polypeptide Receptor Therapies for the Treatment of Obesity, Do Agonists = Antagonists?

Glucose-dependent insulinotropic polypeptide receptor (GIPR) is associated with obesity in human genome-wide association studies. Similarly, mouse genetic studies indicate that loss of function alleles and glucose-dependent insulinotropic polypeptide overexpression both protect from high-fat diet-induced weight gain. Together, these data provide compelling evidence to develop therapies targeting GIPR for the treatment of obesity. Further, both antagonists and agonists alone prevent weight gain, but result in remarkable weight loss when codosed or molecularly combined with glucagon-like peptide-1 analogs preclinically. Here, we review the current literature on GIPR, including biology, human and mouse genetics, and pharmacology of both agonists and antagonists, discussing the similarities and differences between the 2 approaches. Despite opposite approaches being investigated preclinically and clinically, there may be viability of both agonists and antagonists for the treatment of obesity, and we expect this area to continue to evolve with new clinical data and molecular and pharmacological analyses of GIPR function.

Mechanisms Underlying Bone Loss Associated with Gut Inflammation

Patients with gastrointestinal diseases frequently suffer from skeletal abnormality, characterized by reduced bone mineral density, increased fracture risk, and/or joint inflammation. This pathological process is characterized by altered immune cell activity and elevated inflammatory cytokines in the bone marrow microenvironment due to disrupted gut immune response. Gastrointestinal disease is recognized as an immune malfunction driven by multiple factors, including cytokines and signaling molecules. However, the mechanism by which intestinal inflammation magnified by gut-residing actors stimulates bone loss remains to be elucidated. In this article, we discuss the main risk factors potentially contributing to intestinal disease-associated bone loss, and summarize current animal models, illustrating gut-bone axis to bridge the gap between intestinal inflammation and skeletal disease.

Glucose-dependent insulinotropic polypeptide deficiency reduced fat accumulation and insulin resistance, but deteriorated bone loss in ovariectomized mice

Given the established roles of glucose-dependent insulinotropic polypeptide (GIP) in promoting fat storage and bone formation, we assessed the contribution of GIP to obesity and osteopenia in ovariectomized mice with a gene encoding green fluorescent protein (GFP) inserted into the GIP locus, in which GIP was either reduced (GIPgfp/+ ) or absent (GIPgfp/gfp ). In GIPgfp/gfp mice, weight gain, subcutaneous and visceral fat mass were reduced, and glucose intolerance was improved compared with wild-type mice with the same magnitude of insulin responses. Cancellous bone mineral density and bone cortical thickness were reduced in GIPgfp/gfp mice compared with wild-type mice. In GIPgfp/+ mice, weight gain, glucose intolerance and cancellous bone mineral density were not different from that of wild-type mice. These results indicate that the total elimination of GIP ameliorates weight gain and adiposity in ovariectomized mice, but it enhances osteopenia, particularly in cancellous bone by partly suppressing bone formation.

Temporal Change in Biomarkers of Bone Turnover Following Late Evening Ingestion of a Calcium-Fortified, Milk-Based Protein Matrix in Postmenopausal Women with Osteopenia

The diurnal rhythm of bone remodeling suggests nocturnal dietary intervention to be most effective. This study investigated the effect of bedtime ingestion of a calcium-fortified, milk-derived protein matrix (MBPM) or maltodextrin (CON) on acute (0–4 h) blood and 24-h urinary change in biomarkers of bone remodeling in postmenopausal women with osteopenia. In CON, participants received 804 ± 52 mg calcium, 8.2 ± 3.2 µg vitamin D and 1.3 ± 0.2 g/kg BM protein per day. MBPM increased calcium intake to 1679 ± 196 mg, vitamin D to 9.2 ± 3.1 µg and protein to 1.6 ± 0.2 g/kg BM. Serum C-terminal cross-linked telopeptide of type I collagen (CTX) and procollagen type 1 amino-terminal propeptide (P1NP), and urinary N-telopeptide cross-links of type I collagen (NTX), pyridinoline (PYD) and deoxypyridinoline (DPD) was measured. Analyzed by AUC and compared to CON, a −32% lower CTX (p = 0.011, d = 0.83) and 24% (p = 0.52, d = 0.2) increase in P1NP was observed for MBPM. Mean total 24 h NTX excreted in MBPM was −10% (p = 0.035) lower than CON. Urinary PYD and DPD were unaffected by treatment. This study demonstrates the acute effects of bedtime ingestion of a calcium-fortified, milk-based protein matrix on bone remodeling.

Fasting blood glucose levels in patients with different types of diseases

Normal human physiology is dependent on a tight control of the fasting blood glucose (FBG) levels. The islets of pancreas maintains FBG levels within a narrow range of 4-6mmol/L by secreting various hormones, especially insulin and glucagon. However, the hormone secretions by the islets of pancreas are governed by a collective effort among pancreas-islet axis, brain-islet axis, liver-islet axis, gut-islet axis, and adipocyte/myocyte-islet axis. Furthermore, the damage of pancreas, vascular system, brain, liver, intestine, adipose, muscle, and other organs and tissues might affect FBG levels through insulin resistance or impaired insulin signaling, which is the hallmark of type 2 diabetes. In this study, 320,572 clinical lab test results of FBG levels from healthy individuals and patients with 64 different types of diseases during the past 5 years in our hospital were retrieved and analyzed. Based on the mean (SD), median, and p (-Log₁₀p) values, we found 57/64 diseases including type 2 diabetes, pancreatitis, diabetic nephropathy, and pancreatic cancer had significantly (p<0.05, -Log₁₀p>1.30) increased whereas 6/64 diseases including preeclampsia, Wilms' tumor, and lupus erythematous had significantly decreased FBG levels compared to that of healthy controls. These data indicated that the increased FBG levels might be a general pathophysiological property of diseased tissues or organs and the increased FBG levels might be a consequence but not the cause for either prediabetes or type 2 diabetes.

Gut Hormones and Their Effect on Bone Metabolism. Potential Drug Therapies in Future Osteoporosis Treatment

Bone homeostasis displays a circadian rhythm with increased resorption during the night time as compared to day time, a difference that seems-at least partly-to be caused by food intake during the day. Thus, ingestion of a meal results in a decrease in bone resorption, but people suffering from short bowel syndrome lack this response. Gut hormones, released in response to a meal, contribute to this link between the gut and bone metabolism. The responsible hormones appear to include glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), known as incretin hormones due to their role in regulating glucose homeostasis by enhancing insulin release in response to food intake. They interact with their cognate receptors (GIPR and GLP-1R), which are both members of the class B G protein-coupled receptors (GPCRs), and already recognized as targets for treatment of metabolic diseases, such as type 2 diabetes mellitus (T2DM) and obesity. Glucagon-like peptide-2 (GLP-2), secreted concomitantly with GLP-1, acting via another class B receptor (GLP-2R), is also part of this gut-bone axis. Several studies, including human studies, have indicated that these three hormones inhibit bone resorption and, moreover, that GIP increases bone formation. Another hormone, peptide YY (PYY), is also secreted from the enteroendocrine L-cells (together with GLP-1 and GLP-2), and acts mainly via interaction with the class A GPCR NPY-R2. PYY is best known for its effect on appetite regulation, but recent studies have also shown an effect of PYY on bone metabolism. The aim of this review is to summarize the current knowledge of the actions of GIP, GLP-1, GLP-2, and PYY on bone metabolism, and to discuss future therapies targeting these receptors for the treatment of osteoporosis.

Increased Body Weight and Fat Mass After Subchronic GIP Receptor Antagonist, but Not GLP-2 Receptor Antagonist, Administration in Rats

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2) are hormones secreted from the enteroendocrine cells after a meal. They exert their actions through activation of G protein-coupled receptors (R), the GIPR and GLP-2R, respectively. Both have been reported to influence metabolism. The purpose of the study was to investigate the role of the hormones in the regulation of lipid and bone homeostasis by subchronic treatment with novel GIPR and GLP-2R antagonists. Rats were injected once daily with vehicle, GIPR, or GLP-2R antagonists for 3 weeks. Body weight, food intake, body composition, plasma lipoprotein lipase (LPL), adipokines, triglycerides and the marker of bone resorption carboxy-terminal collagen crosslinks (CTX), were examined. In rats, subchronic treatment with GIPR antagonist, rat GIP (3-30)NH₂, did not modify food intake and bone resorption, but significantly increased body weight, body fat mass, triglycerides, LPL, and leptin levels compared with vehicle treated rats. Subchronic (Pro3)GIP (a partial GIPR agonist), GLP-2(11-33), and GLP-2(3-33) (GLP-2R antagonists) treatment did not affect any parameter. The present results would be consistent with a role for GIP, but not GLP-2, in the maintenance of lipid homeostasis in rats, while neither GIPR nor GLP-2R antagonism appeared to influence bone resorption in rats.

Gut hormone polyagonists for the treatment of type 2 diabetes

Chemical derivatives of the gut-derived peptide hormone glucagon-like peptide 1 (GLP-1) are among the best-in-class pharmacotherapies to treat obesity and type 2 diabetes. However, GLP-1 analogs have modest weight lowering capacity, in the range of 5-10%, and the therapeutic window is hampered by dose-dependent side effects. Over the last few years, a new concept has emerged: combining the beneficial effects of several key metabolic hormones into a single molecular entity. Several unimolecular GLP-1-based polyagonists have shown superior metabolic action compared to GLP-1 monotherapies. In this review article, we highlight the history of polyagonists targeting the receptors for GLP-1, GIP and glucagon, and discuss recent progress in expanding of this concept to now allow targeted delivery of nuclear hormones via GLP-1 and other gut hormones, as a novel approach towards more personalized pharmacotherapies.

Incretin-based therapy for type 2 diabetes mellitus is promising for treating neurodegenerative diseases

Incretin hormones include glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Due to their promising action on insulinotropic secretion and improving insulin resistance (IR), incretin-based therapies have become a new class of antidiabetic agents for the treatment of type 2 diabetes mellitus (T2DM). Recently, the links between neurodegenerative diseases and T2DM have been identified in a number of studies, which suggested that shared mechanisms, such as insulin dysregulation or IR, may underlie these conditions. Therefore, the effects of incretins in neurodegenerative diseases have been extensively investigated. Protease-resistant long-lasting GLP-1 mimetics such as lixisenatide, liraglutide, and exenatide not only have demonstrated promising effects for treating neurodegenerative diseases in preclinical studies but also have shown first positive results in Alzheimer's disease (AD) and Parkinson's disease (PD) patients in clinical trials. Furthermore, the effects of other related incretin-based therapies such as GIP agonists, dipeptidyl peptidase-IV (DPP-IV) inhibitors, oxyntomodulin (OXM), dual GLP-1/GIP, and triple GLP-1/GIP/glucagon receptor agonists on neurodegenerative diseases have been tested in preclinical studies. Incretin-based therapies are a promising approach for treating neurodegenerative diseases.

Glucose-Dependent Insulinotropic Polypeptide (GIP) Inhibits Bone Resorption Independently of Insulin and Glycemia

CONTEXT

The gut hormone glucose-dependent insulinotropic polypeptide (GIP) causes postprandial insulin release and inhibits bone resorption assessed by carboxy-terminal collagen crosslinks (CTX).

OBJECTIVE

To study if GIP affects bone homeostasis biomarkers independently of insulin release and glycemic level.

DESIGN

Randomized, double-blinded, crossover study with 5 study days.

PATIENTS

Ten male C-peptide-negative patients with type 1 diabetes.

INTERVENTIONS

On 3 matched days with "low glycemia" (plasma glucose in the interval 3 to 7 mmol/L for 120 minutes), we administered intravenous (IV) GIP (4 pmol × kg-1 × min-1), glucagon-like peptide 1 (1 pmol × kg-1 × min-1), or placebo (saline), and on 2 matched days with "high glycemia" (plasma glucose 12 mmol/L for 90 minutes), we administered either GIP or saline.

MAIN OUTCOME MEASURES

CTX, procollagen type 1 N-terminal propeptide (P1NP), and parathyroid hormone (PTH).

RESULTS

During low glycemia: GIP progressively suppressed CTX from baseline by up to 59 ± 18% compared with 24 ± 10% during saline infusion (P < 0.0001). Absolute values of P1NP and PTH did not differ between days. During high glycemia: GIP suppressed CTX from baseline by up to 59 ± 19% compared with 7 ± 9% during saline infusion (P < 0.0001). P1NP did not differ between days. GIP suppressed PTH after 60 minutes compared with saline (P < 0.01), but this difference disappeared after 90 minutes.

CONCLUSIONS

Short-term GIP infusions robustly reduce bone resorption independently of endogenous insulin secretion and during both elevated and low plasma glucose, but have no effect on P1NP or PTH after 90 minutes.

Effects of gastric inhibitory polypeptide, glucagon-like peptide-1 and glucagon-like peptide-1 receptor agonists on Bone Cell Metabolism

The relationship between gut and skeleton is increasingly recognized as part of the integrated physiology of the whole organism. The incretin hormones gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are secreted from the intestine in response to nutrient intake and exhibit several physiological functions including regulation of islet hormone secretion and glucose levels. A number of GLP-1 receptor agonists (GLP-1RAs) are currently used in treatment of type 2 diabetes and obesity. However, GIP and GLP-1 cognate receptors are widely expressed suggesting that incretin hormones mediate effects beyond control of glucose homeostasis, and reports on associations between incretin hormones and bone metabolism have emerged. The aim of this MiniReview was to provide an overview of current knowledge regarding the in vivo and in vitro effects of GIP and GLP-1 on bone metabolism. We identified a total of 30 pre-clinical and clinical investigations of the effects of GIP, GLP-1 and GLP-1RAs on bone turnover markers, bone mineral density (BMD), bone microarchitecture and fracture risk. Studies conducted in cell cultures and rodents demonstrated that GIP and GLP-1 play a role in regulating skeletal homeostasis, with pre-clinical data suggesting that GIP inhibits bone resorption whereas GLP-1 may promote bone formation and enhance bone material properties. These effects are not corroborated by clinical studies. While there is evidence of effects of GIP and GLP-1 on bone metabolism in pre-clinical investigations, clinical trials are needed to clarify whether similar effects are present and clinically relevant in humans.

Glucose-Dependent Insulinotropic Polypeptide Receptor Deficiency Leads to Impaired Bone Marrow Hematopoiesis

The bone marrow (BM) contains controlled specialized microenvironments, or niches, that regulate the quiescence, proliferation, and differentiation of hematopoietic stem and progenitor cells (HSPC). The glucose-dependent insulinotropic polypeptide (GIP) is a gut-derived incretin hormone that mediates postprandial insulin secretion and has anabolic effects on adipose tissue. Previous studies demonstrated altered bone microarchitecture in mice deficient for GIP receptor (Gipr^-/- ), as well as the expression of high-affinity GIP receptor by distinct cells constructing the BM HSPC niche. Nevertheless, the involvement of GIP in the process of BM hematopoiesis remains elusive. In this article, we show significantly reduced representation and proliferation of HSPC and myeloid progenitors in the BM of Gipr^-/- mice. This was further manifested by reduced levels of BM and circulating differentiated immune cells in young and old adult mice. Moreover, GIP signaling was required for the establishment of supportive BM HSPC niches during HSPC repopulation in radioablated BM chimera mice. Finally, molecular profiling of various factors involved in retention, survival, and expansion of HSPC revealed significantly lower expression of the Notch-receptor ligands Jagged 1 and Jagged 2 in osteoblast-enriched bone extracts from Gipr^-/- mice, which are important for HSPC expansion. In addition, there was increased expression of CXCL12, a factor important for HSPC retention and quiescence, in whole-BM extracts from Gipr^-/- mice. Collectively, our data suggest that the metabolic hormone GIP plays an important role in BM hematopoiesis.

Incretins amplify TNF-α-stimulated IL-6 synthesis in osteoblasts: Suppression of the IκB/NF-κB pathway

Incretins including glucagon-like peptide-1 (GLP-1) and glucose‑dependent insulinotropic polypeptide (GIP) secreted from the small intestine after oral food ingestion are currently recognized to stimulate insulin secretion from pancreatic β cells. We previously reported that p70 S6 kinase limits the tumor necrosis factor‑α (TNF‑α)‑stimulated interleukin-6 (IL‑6) synthesis in osteoblast‑like MC3T3‑E1 cells. In the present study, we investigated the effects of incretins on the TNF‑α‑induced IL‑6 synthesis and the underlying mechanism in MC3T3‑E1 cells. GLP‑1 and GIP significantly upregulated both TNF‑α‑stimulated IL‑6 release and mRNA levels. Wedelolactone, an inhibitor of IκB kinase, amplified the TNF-α-induced IL‑6 release. GLP‑1 significantly attenuated the TNF‑α‑induced phosphorylation of IκB without affecting the phosphorylation of p70 S6 kinase. On the other hand, GLP‑1 markedly induced the phosphorylation of cAMP response element-binding protein (CREB). H‑89, an inhibitor of protein kinase A, significantly suppressed the enhancement by GLP-1 of TNF-α-stimulated IL‑6 release. Dibutyryl cAMP, a permeable analogue of cAMP, which suppressed the TNF-α-induced IκB phosphorylation, amplified the IL‑6 release. These results strongly suggest that incretins upregulate the TNF-α-stimulated IL‑6 synthesis in osteoblasts, and that the amplifying effect of incretin is exerted via reducing the IκB/NF‑κB pathway through the adenylyl cyclase-cAMP system.

Intestinal Incretins and the Regulation of Bone Physiology

Although originally identified as modulators of nutrient absorption, the gut hormones gastric inhibitory polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucagon-like peptide-2 (GLP-2) have also been found to play an important role in the regulation of bone turnover. These "incretin" hormones promote bone anabolism by stimulating osteoblast differentiation as well as increasing osteoblast longevity. In addition, GIP and perhaps GLP-2 attenuate the activity of osteoclastic cells, leading to a net increase in bone deposition and ultimately increasing bone mass. Studies have demonstrated that these hormones are important for bone mineralization and overall bone quality and function evolutionarily as important nutritional links signaling nutrient availability for skeletal anabolic functions. Accordingly, these entero-osseous hormones (EOH) have therapeutic potential for the management of osteoporosis. Although this chapter primarily focuses on skeletal effects of these incretin hormones, the GIP, GLP-1, and GLP-2 receptors are actually widely expressed throughout the body. Therefore, we will also briefly discuss these extraosseous receptors/effects and how they may indirectly impact the skeleton.

Boning up on DPP4, DPP4 substrates, and DPP4-adipokine interactions: Logical reasoning and known facts about bone related effects of DPP4 inhibitors

Dipeptidyl peptidase 4 (DPP4) is a conserved exopeptidase with an important function in protein regulation. The activity of DPP4, an enzyme which can either be anchored to the plasma membrane or circulate free in the extracellular compartment, affects the glucose metabolism, cellular signaling, migration and differentiation, oxidative stress and the immune system. DPP4 is also expressed on the surface of osteoblasts, osteoclasts and osteocytes, and was found to play a role in collagen metabolism. Many substrates of DPP4 have an established role in bone metabolism, among which are incretins, gastrointestinal peptides and neuropeptides. In general, their effects favor bone formation, but some effects are complex and have not been completely elucidated. DPP4 and some of its substrates are known to interact with adipokines, playing an essential role in the energy metabolism. The prolongation of the half-life of incretins through DPP4 inhibition led to the development of these inhibitors to improve glucose tolerance in diabetes. Current literature indicates that the inhibition of DPP4 activity might also result in a beneficial effect on the bone metabolism, but the long-term effect of DPP4 inhibition on fracture outcome has not been entirely established. Diabetic as well as postmenopausal osteoporosis is associated with an increased activity of DPP4, as well as a shift in the expression levels of DPP4 substrates, their receptors, and adipokines. The interactions between these factors and their relationship in bone metabolism are therefore an interesting field of study.

Attenuation by incretins of thyroid hormone-stimulated osteocalcin synthesis in osteoblasts

Incretins, the polypeptide hormone glucose- dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) secreted from the small intestine after nutrient ingestion, are generally known to stimulate insulin secretion from pancreatic β-cells. We previously demonstrated that triiodothyronine (T₃) stimulates osteocalcin synthesis at least in part through p38 mitogen-activated protein kinase in osteoblast-like MC3T3-E1 cells. In the present study, we investigated the effects of GIP and GLP-1 on T₃-stimulated osteocalcin synthesis and the mechanism of action involved in MC3T3-E1 cells. GIP and GLP-1 markedly suppressed the T₃-stimulated osteocalcin release. GIP and GLP-1 significantly attenuated the expression levels of osteocalcin mRNA induced by T₃. The T₃-stimulated transactivation activity of the thyroid hormone-responsive element was reduced by GIP and GLP-1. These results suggest that incretins repressed the T₃-stimulated osteocalcin synthesis in osteoblast-like MC3T3-E1 cells, and the suppressive effect of incretins was mediated through transcriptional levels.

c-Fos induction by gut hormones and extracellular ATP in osteoblastic-like cell lines

It is widely accepted that the c-Fos gene has a role in proliferation and differentiation of bone cells. ATP-induced c-Fos activation is relevant to bone homeostasis, because nucleotides that are present in the environment of bone cells can contribute to autocrine/paracrine signalling. Gut hormones have previously been shown to have an effect on bone metabolism. In this study, we used the osteoblastic Saos-2 cell line transfected with a c-Fos-driven reporter stimulated with five gut hormones: glucose inhibitory peptide (GIP), glucagon-like peptide-1 (GLP-1), glucagon-like peptide-2 (GLP-2), ghrelin and obestatin, in the presence or absence of ATP. In addition, TE-85 cells were used to determine the time course of c-Fos transcript induction following stimulation with GLP-1, and GLP-2 with or without ATP, using reverse transcription qPCR. The significant results from the experiments are as follows: higher level of c-Fos induction in presence of GIP, obestatin (p = 0.019 and p = 0.011 respectively), and GIP combined with ATP (p < 0.001) using the luciferase assay; GLP-1 and GLP-2 combined with ATP (p = 0.034 and p = 0.002, respectively) and GLP-2 alone (p < 0.001) using qPCR. In conclusion, three of the gut peptides induced c-Fos, providing a potential mechanism underlying the actions of these hormones in bone which can be directed or enhanced by the presence of ATP.

Pharmacological Actions of Glucagon-Like Peptide-1, Gastric Inhibitory Polypeptide, and Glucagon

Glucagon family of peptide hormones is a group of structurally related brain-gut peptides that exert their pleiotropic actions through interactions with unique members of class B1 G protein-coupled receptors (GPCRs). They are key regulators of hormonal homeostasis and are important drug targets for metabolic disorders such as type-2 diabetes mellitus (T2DM), obesity, and dysregulations of the nervous systems such as migraine, anxiety, depression, neurodegeneration, psychiatric disorders, and cardiovascular diseases. The current review aims to provide a detailed overview of the current understanding of the pharmacological actions and therapeutic advances of three members within this family including glucagon-like peptide-1 (GLP-1), gastric inhibitory polypeptide (GIP), and glucagon.

Pancreatic regulation of glucose homeostasis

In order to ensure normal body function, the human body is dependent on a tight control of its blood glucose levels. This is accomplished by a highly sophisticated network of various hormones and neuropeptides released mainly from the brain, pancreas, liver, intestine as well as adipose and muscle tissue. Within this network, the pancreas represents a key player by secreting the blood sugar-lowering hormone insulin and its opponent glucagon. However, disturbances in the interplay of the hormones and peptides involved may lead to metabolic disorders such as type 2 diabetes mellitus (T2DM) whose prevalence, comorbidities and medical costs take on a dramatic scale. Therefore, it is of utmost importance to uncover and understand the mechanisms underlying the various interactions to improve existing anti-diabetic therapies and drugs on the one hand and to develop new therapeutic approaches on the other. This review summarizes the interplay of the pancreas with various other organs and tissues that maintain glucose homeostasis. Furthermore, anti-diabetic drugs and their impact on signaling pathways underlying the network will be discussed.

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

INTRODUCTION

The gastro-intestinal hormone glucose-dependent insulinotropic polypeptide (GIP) potentiates glucose-induced insulin secretion, with bone anabolic effects through GIP receptor (GIPR) in animal models. We explore its potential in humans by analyzing association between polymorphisms (SNPs) in the GIP and GIPR genes with bone phenotypes in young and elderly women.

METHODS

Association between GIP (rs2291725) and GIPR (rs10423928) and BMD, bone mineral content (BMC), bone microarchitecture, fracture and body composition was analyzed in the OPRA (75y, n = 1044) and PEAK-25 (25y; n = 1061) cohorts and serum-GIP in OPRA.

RESULTS

The GIP receptor AA-genotype was associated with lower ultrasound values in young women (BUA p = 0.011; SI p = 0.030), with no association to bone phenotypes in the elderly. In the elderly, the GIP was associated with lower ultrasound (GG vs. AA; SOS p_adj = 0.021) and lower femoral neck BMD and BMC after adjusting for fat mass (p_adj = 0.016 and p_adj = 0.03). In young women, neither GIPR nor GIP associated with other bone phenotypes including spine trabecular bone score. In the elderly, neither SNP associated with fracture. GIP was associated with body composition only in Peak-25; GIPR was not associated with body composition in either cohort. Serum-GIP levels (in elderly) were not associated with bone phenotypes, however lower levels were associated with the GIPR A-allele (β = - 6.93; p_adj = 0.03).

CONCLUSIONS

This first exploratory association study between polymorphisms in GIP and GIPR in relation to bone phenotypes and serum-GIP in women at different ages indicates a possible, albeit complex link between glucose metabolism genes and bone, while recognizing that further studies are warranted.

Incidence of Fractures in Patients With Type 2 Diabetes in the SAVOR-TIMI 53 Trial

OBJECTIVE

Patients with type 2 diabetes have an increased risk of bone fractures, the predisposing factors for which are unknown. Treatment with thiazolidinediones (TZDs) further increases the incidence of osteoporotic fractures. In the Saxagliptin Assessment of Vascular Outcomes Recorded in Patients with Diabetes Mellitus-Thrombolysis in Myocardial Infarction 53 (SAVOR-TIMI 53) trial, fractures were considered an adverse event of special interest, and information regarding fractures was collected.

RESEARCH DESIGN AND METHODS

We compared the incidence of fractures among the 8,280 patients who were assigned to treatment with saxagliptin with that in the 8,212 patients who were assigned to placebo. We further analyzed the participants' baseline characteristics and fracture risk.

RESULTS

During a median follow-up of 2.1 years, 241 patients (2.9%) in the saxagliptin group and 240 (2.9%) in the placebo group experienced a fracture (hazard ratio [HR] 1.00 [95% CI 0.83-1.19]). Event rates for fractures were the same in both treatment arms: 14.7 per 1,000 patient-years in the entire population and 14.0 in the on-treatment population (first event only). Fracture risk was similar in patients treated with saxagliptin or placebo across different subgroups defined by race, cardiovascular risk, and renal function. A multivariable Cox regression analysis showed that risk of fracture was associated with female sex (P < 0.0001), longer diabetes duration (P < 0.0001), older age (P = 0.002), major hypoglycemic events (P = 0.01), noncompliance with study drug (P = 0.01), and treatment with TZDs (P = 0.03).

CONCLUSIONS

In a large population of older patients with type 2 diabetes, treatment with saxagliptin was not associated with an increased risk of fractures. The association between longer diabetes duration and increased risk of bone fracture is an intriguing finding.

The Place of Dipeptidyl Peptidase-4 Inhibitors in Type 2 Diabetes Therapeutics: A "Me Too" or "the Special One" Antidiabetic Class?

Incretin-based therapies, the most recent therapeutic options for type 2 diabetes mellitus (T2DM) management, can modify various elements of the disease, including hypersecretion of glucagon, abnormal gastric emptying, postprandial hyperglycaemia, and, possibly, pancreatic β cell dysfunction. Dipeptidyl peptidase-4 (DPP-4) inhibitors (gliptins) increase glucagon-like peptide-1 (GLP-1) availability and correct the "incretin defect" seen in T2DM patients. Clinical studies have shown good glycaemic control with minimal risk of hypoglycaemia or any other adverse effects, despite the reports of pancreatitis, whose association remains to be proved. Recent studies have been focusing on the putative ability of DPP-4 inhibitors to preserve pancreas function, in particular due to the inhibition of apoptotic pathways and stimulation of β cell proliferation. In addition, other cytoprotective effects on other organs/tissues that are involved in serious T2DM complications, including the heart, kidney, and retina, have been increasingly reported. This review outlines the therapeutic potential of DPP-4 inhibitors for the treatment of T2DM, focusing on their main features, clinical applications, and risks, and discusses the major challenges for the future, in particular the possibility of becoming the preferred therapy for T2DM due to their ability to modify the natural history of the disease and ameliorate nephropathy, retinopathy, and cardiovascular complications.

Pros and cons of fatty acids in bone biology

Despite the growing interest in deciphering the causes and consequences of obesity-related disorders, the mechanisms linking fat intake to bone behaviour remain unclear. Since bone fractures are widely associated with increased morbidity and mortality, most notably in elderly and obese people, bone health has become a major social and economic issue. Consistently, public health system guidelines have encouraged low-fat diets in order to reduce associated complications. However, from a bone point of view, mechanisms linking fat intake to bone alteration remain quite controversial. Thus, after more than a decade of dedicated studies, this timely review offers a comprehensive overview of the relationships between bone and fatty acids. Using clinical evidences as a starting-point to more complex molecular elucidation, this work highlights the complexity of the system and reveals that bone alteration that cannot be solved simply by taking ω-3 pills. Fatty acid effects on bone metabolism can be both direct and indirect and require integrated investigations. Furthermore, even at the level of a single cell, one fatty acid is able to trigger several different independent pathways (receptors, metabolites…) which may all have a say in the final cellular metabolic response.

The aromatic amino acid tryptophan stimulates skeletal muscle IGF1/p70s6k/mTor signaling in vivo and the expression of myogenic genes in vitro

OBJECTIVES

Nutrition plays a key role in the maintenance of muscle and bone mass, and dietary protein deficiency has in particular been associated with catabolism of both muscle and bone tissue. One mechanism thought to link protein deficiency with loss of muscle mass is deficiency in specific amino acids that play a role in muscle metabolism. The aim of this study was to test the hypothesis that the essential amino acid tryptophan, and its metabolite kynurenine, might directly affect muscle metabolism in the setting of protein deficiency.

METHODS

Adult mice (12 mo) were fed a normal diet (18% protein), as well as diets with low protein (8%) supplemented with increasing concentrations (50, 100, and 200 uM) of kynurenine (Kyn) or with tryptophan (Trp; 1.5 mM) for 8 weeks. Myoprogenitor cells were also treated with Trp and Kyn in vitro to determine their effects on cell proliferation and expression of myogenic differentiation markers.

RESULTS

All mice on the low-protein diets weighed less than the group fed normal protein (18%). Lean mass measured by dual-energy X-ray absorptiometry was lowest in mice on the high Kyn diet, whereas percent lean mass was highest in mice receiving Trp supplementation and percent body fat was lowest in mice receiving Trp. Enzyme-linked immunosorbent assays showed significant increases in skeletal muscle insulin-like growth factor-1, leptin, and the myostatin antagonist follistatin with Trp supplementation. mRNA microarray and gene pathway analysis performed on muscle samples demonstrate that mTor/eif4/p70s6k pathway molecules are significantly up-regulated in muscles from mice on Kyn and Trp supplementation. In vitro, neither amino acid affected proliferation of myoprogenitors, but Trp increased the expression of the myogenic markers MyoD, myogenin, and myosin heavy chain.

CONCLUSION

These findings suggest that dietary amino acids can directly affect molecular signaling in skeletal muscle, further indicating that dietary manipulation with specific amino acids could potentially attenuate muscle loss with dietary protein deficiency.

GIP and GLP-1, the two incretin hormones: Similarities and differences

Gastric inhibitory polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) are the two primary incretin hormones secreted from the intestine on ingestion of glucose or nutrients to stimulate insulin secretion from pancreatic β cells. GIP and GLP‐1 exert their effects by binding to their specific receptors, the GIP receptor (GIPR) and the GLP‐1 receptor (GLP‐1R), which belong to the G‐protein coupled receptor family. Receptor binding activates and increases the level of intracellular cyclic adenosine monophosphate in pancreatic β cells, thereby stimulating insulin secretion glucose‐dependently. In addition to their insulinotropic effects, GIP and GLP‐1 play critical roles in various biological processes in different tissues and organs that express GIPR and GLP‐1R, including the pancreas, fat, bone and the brain. Within the pancreas, GIP and GLP‐1 together promote β cell proliferation and inhibit apoptosis, thereby expanding pancreatic β cell mass, while GIP enhances postprandial glucagon response and GLP‐1 suppresses it. In adipose tissues, GIP but not GLP‐1 facilitates fat deposition. In bone, GIP promotes bone formation while GLP‐1 inhibits bone absorption. In the brain, both GIP and GLP‐1 are thought to be involved in memory formation as well as the control of appetite. In addition to these differences, secretion of GIP and GLP‐1 and their insulinotropic effects on β cells have been shown to differ in patients with type 2 diabetes compared to healthy subjects. We summarize here the similarities and differences of these two incretin hormones in secretion and metabolism, their insulinotropic action on pancreatic β cells, and their non‐insulinotropic effects, and discuss their potential in treatment of type 2 diabetes. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00022.x, 2010)

Glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1: Incretin actions beyond the pancreas

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are the two primary incretin hormones secreted from the intestine on ingestion of various nutrients to stimulate insulin secretion from pancreatic β-cells glucose-dependently. GIP and GLP-1 undergo degradation by dipeptidyl peptidase-4 (DPP-4), and rapidly lose their biological activities. The actions of GIP and GLP-1 are mediated by their specific receptors, the GIP receptor (GIPR) and the GLP-1 receptor (GLP-1R), which are expressed in pancreatic β-cells, as well as in various tissues and organs. A series of investigations using mice lacking GIPR and/or GLP-1R, as well as mice lacking DPP-4, showed involvement of GIP and GLP-1 in divergent biological activities, some of which could have implications for preventing diabetes-related microvascular complications (e.g., retinopathy, nephropathy and neuropathy) and macrovascular complications (e.g., coronary artery disease, peripheral artery disease and cerebrovascular disease), as well as diabetes-related comorbidity (e.g., obesity, non-alcoholic fatty liver disease, bone fracture and cognitive dysfunction). Furthermore, recent studies using incretin-based drugs, such as GLP-1 receptor agonists, which stably activate GLP-1R signaling, and DPP-4 inhibitors, which enhance both GLP-1R and GIPR signaling, showed that GLP-1 and GIP exert effects possibly linked to prevention or treatment of diabetes-related complications and comorbidities independently of hyperglycemia. We review recent findings on the extrapancreatic effects of GIP and GLP-1 on the heart, brain, kidney, eye and nerves, as well as in the liver, fat and several organs from the perspective of diabetes-related complications and comorbidities.

Expression of glucose-dependent insulinotropic polypeptide and its receptor in the rat major salivary glands

Glucose-dependent insulinotropic polypeptide receptors (GIPR) are expressed throughout the body. The expression of its ligand, glucose-dependent insulinotropic polypeptide (GIP) however, has only been reported in a limited numbers of organs. Although the rat submandibular salivary gland (SMG) has been found to express GIP, its biological role is still not understood. Moreover, nothing is known about the expression of GIP in other types of salivary glands, i.e. the parotid (PG) and sublingual (SLG) glands. We detected the expression of GIP mRNA in the rat PG, SMG and SLG. Immunohistochemical analyses revealed that GIP and GIPR were expressed only in the ductal area of all types of major salivary glands, and no immunostaining was found in the acini area. We also found GIP expression in the rat SMG to be age dependent, with 8-week-old rats showing 2-3-fold higher than those of 9- and 11-week-old rats, respectively. This is the first study to indicate both GIP and GIPR expression in the rat major salivary glands, as well as its variation in the rat SMG during the growth period. These findings are crucial for a better understanding of the physiological function of GIP in rat major salivary gland.

Bone and mineral metabolism in patients undergoing Roux-en-Y gastric bypass

Despite effective weight reduction, the impact of bariatric surgery on bone is a major concern. Mechanisms include decreased mechanical loading, calcium and vitamin D malabsorption, deficiency in other nutrients, and alterations in fat- and gut-derived hormones. The evidence to support clinical care pathways to prevent bone loss and fractures is at this point weak.

INTRODUCTION

There is a growing concern regarding the potential deleterious impact of bariatric surgery on bone metabolism. This comprehensive review addresses this controversial topic.

METHODS

We reviewed and analyzed articles evaluating bone metabolism and mechanisms for the ensuing putative bone loss in adult patients exclusively undergoing Roux-en-Y gastric bypass (RYGB) surgery, for the period spanning 1942 till September 2012.

RESULTS

Mechanisms identified to contribute to alterations in bone metabolism after bypass surgery include: decreased mechanical loading, calcium and vitamin D malabsorption with secondary hyperparathyroidism, deficiency in other nutrients, in addition to alterations in adipokines, gonadal steroids, and gut-derived hormones favoring bone loss, with the exception of serotonin and glucagon-like peptide-1. The relative contribution of each of these hormones to changes in bone homeostasis after bypass surgery remains undefined. Bone loss reflected by a decline in bone mineral density (BMD) and an increase in bone turnover markers have been reported in many studies, limited for the most part by the exclusive use of dual energy X-ray absorptiometry. Well-designed long-term prospective trials with fractures as an outcome, and studies investigating the magnitude, reversibility, and impact of the observed metabolic changes on fracture outcomes are lacking.

CONCLUSION

Robust conclusions regarding bone loss and fracture outcome after RYGB surgery cannot be drawn at this time. Although not evidence based, baseline evaluation and sequential monitoring with measurement of BMD and calciotropic hormones seem appropriate, with adequate calcium and vitamin D replacement. Beneficial interventions remain unclear.