Reduction of nocturnal rise in bone resorption by subcutaneous GLP-2

Effects of Exogenous GIP and GLP-2 on Bone Turnover in Individuals With Type 2 Diabetes

CONTEXT

Individuals with type 2 diabetes (T2D) have an increased risk of bone fractures despite normal or increased bone mineral density. The underlying causes are not well understood but may include disturbances in the gut-bone axis, in which both glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2) are regulators of bone turnover. Thus, in healthy fasting participants, both exogenous GIP and GLP-2 acutely reduce bone resorption.

OBJECTIVE

The objective of this study was to investigate the acute effects of subcutaneously administered GIP and GLP-2 on bone turnover in individuals with T2D.

METHODS

We included 10 men with T2D. Participants met fasting in the morning on 3 separate test days and were injected subcutaneously with GIP, GLP-2, or placebo in a randomized crossover design. Blood samples were drawn at baseline and regularly after injections. Bone turnover was estimated by circulating levels of collagen type 1 C-terminal telopeptide (CTX), procollagen type 1 N-terminal propeptide (P1NP), sclerostin, and PTH.

RESULTS

GIP and GLP-2 significantly reduced CTX to (mean ± SEM) 66 ± 7.8% and 74 ± 5.9% of baseline, respectively, compared with after placebo (P = .001). In addition, P1NP and sclerostin increased acutely after GIP whereas a decrease in P1NP was seen after GLP-2. PTH levels decreased to 67 ± 2.5% of baseline after GLP-2 and to only 86 ± 3.4% after GIP.

CONCLUSION

Subcutaneous GIP and GLP-2 affect CTX and P1NP in individuals with T2D to the same extent as previously demonstrated in healthy individuals.

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.

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.

Gut-Derived Peptide Hormone Analogues and Potential Treatment of Bone Disorders in Obesity and Diabetes Mellitus

Obesity and diabetes mellitus are prevalent metabolic disorders that have a detrimental impact on overall health. In this regard, there is now a clear link between these metabolic disorders and compromised bone health. Interestingly, both obesity and diabetes lead to elevated risk of bone fracture which is independent of effects on bone mineral density (BMD). In this regard, gastrointestinal (GIT)-derived peptide hormones and their related long-acting analogues, some of which are already clinically approved for diabetes and/or obesity, also seem to possess positive effects on bone remodelling and microarchitecture to reduce bone fracture risk. Specifically, the incretin peptides, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), as well as glucagon-like peptide-2 (GLP-2), exert key direct and/or indirect benefits on bone metabolism. This review aims to provide an initial appraisal of the relationship between obesity, diabetes and bone, with a focus on the positive impact of these GIT-derived peptide hormones for bone health in obesity/diabetes. Brief discussion of related peptides such as parathyroid hormone, leptin, calcitonin and growth hormone is also included. Taken together, drugs engineered to promote GIP, GLP-1 and GLP-2 receptor signalling may have potential to offer therapeutic promise for improving bone health in obesity and diabetes.

Acarbose diminishes postprandial suppression of bone resorption in patients with type 2 diabetes

AIMS

The alpha-glucosidase inhibitor acarbose is an antidiabetic drug delaying assimilation of carbohydrates and, thus, increasing the amount of carbohydrates in the distal parts of the intestines, which in turn increases circulating levels of the gut-derived incretin hormone glucagon-like peptide 1 (GLP-1). As GLP-1 may suppress bone resorption, acarbose has been proposed to potentiate meal-induced suppression of bone resorption. We investigated the effect of acarbose treatment on postprandial bone resorption in patients with type 2 diabetes and used the GLP-1 receptor antagonist exendin(9-39)NH₂ to disclose contributory effect of acarbose-induced GLP-1 secretion.

METHODS

In a randomised, placebo-controlled, double-blind, crossover study, 15 participants with metformin-treated type 2 diabetes (2 women/13 men, age 71 (57-85 years), BMI 29.7 (23.6-34.6 kg/m²), HbA1c 48 (40-74 mmol/mol)/6.5 (5.8-11.6 %) (median and range)) were subjected to two 14-day treatment periods with acarbose and placebo, respectively, separated by a six-week wash-out period. At the end of each period, circulating bone formation and resorption markers were assessed during two randomised 4-h liquid mixed meal tests (MMT) with infusions of exendin(9-39)NH₂ and saline, respectively. Glucagon-like peptide 2 (GLP-2) was also assessed.

RESULTS

Compared to placebo, acarbose impaired the MMT-induced suppression of CTX as assessed by baseline-subtracted area under curve (P = 0.0037) and nadir of CTX (P = 0.0128). During acarbose treatment, exendin(9-39)NH₂ infusion lowered nadir of CTX compared to saline (P = 0.0344). Neither parathyroid hormone or the bone formation marker procollagen 1 intact N-terminal propeptide were affected by acarbose or GLP-1 receptor antagonism. Acarbose treatment induced a greater postprandial GLP-2 response than placebo treatment (P = 0.0479) and exendin(9-39)NH₂ infusion exacerbated this (P = 0.0002).

CONCLUSIONS

In patients with type 2 diabetes, treatment with acarbose reduced postprandial suppression of bone resorption. Acarbose-induced GLP-1 secretion may contribute to this phenomenon as the impairment was partially reversed by GLP-1 receptor antagonism. Also, acarbose-induced reductions in other factors reducing bone resorption, e.g. glucose-dependent insulinotropic polypeptide, may contribute.

Long-acting agonists of human and rodent GLP-2 receptors for studies of the physiology and pharmacological potential of the GLP-2 system

BACKGROUND AND PURPOSE

Glucagon-like peptide-2 (GLP-2) is secreted postprandially from enteroendocrine Lcells and has anabolic action on gut and bone. Short-acting teduglutide is the only approved GLP-2 analog for the treatment of short-bowel syndrome (SBS). To improve the therapeutic effect, we created a series of lipidated GLP-2R agonists.

EXPERIMENTAL APPROACH

Six GLP-2 analogs were studied in vitro for cAMP accumulation, β-arrestin 1 and 2 recruitment, affinity, and internalization. The trophic actions on intestine and bone were examined in vivo in rodents.

KEY RESULTS

Lipidations at lysines introduced at position 12, 16, and 20 of hGLP-2(1-33) were well-tolerated with less than 2.2-fold impaired potency and full efficacy at the hGLP-2R in cAMP accumulation. In contrast, N- and C-terminal (His1 and Lys30) lipidations impaired potency by 4.2- and 45-fold and lowered efficacy to 77% and 85% of hGLP-2, respectively. All variants were similarly active on the rat and mouse GLP-2Rs and the three most active variants displayed increased selectivity for hGLP-2R over hGLP-1R activation, compared to native hGLP-2. Impact on arrestin recruitment and receptor internalization followed that of Gαs-coupling, except for lipidation in position 20, where internalization was more impaired, suggesting desensitization protection. A highly active variant (C16 at position 20) with low internalization and a half-life of 9.5 h in rats showed improved gut and bone tropism with increased weight of small intestine in mice and decreased CTX levels in rats.

CONCLUSION AND IMPLICATION

We present novel hGLP-2 agonists suitable for in vivo studies of the GLP-2 system to uncover its pharmacological potential.

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.

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.

Bone Response to Weight Loss Following Bariatric Surgery

Obesity is a global health challenge that warrants effective treatments to avoid its multiple comorbidities. Bariatric surgery, a cornerstone treatment to control bodyweight excess and relieve the health-related burdens of obesity, can promote accelerated bone loss and affect skeletal strength, particularly after malabsorptive and mixed surgical procedures, and probably after restrictive surgeries. The increase in bone resorption markers occurs early and persist for up to 12 months or longer after bariatric surgery, while bone formation markers increase but to a lesser extent, suggesting a potential uncoupling process between resorption and formation. The skeletal response to bariatric surgery, as investigated by dual-energy X-ray absorptiometry (DXA), has shown significant loss in bone mineral density (BMD) at the hip with less consistent results for the lumbar spine. Supporting DXA studies, analyses by high-resolution peripheral quantitative computed tomography (HR-pQCT) showed lower cortical density and thickness, higher cortical porosity, and lower trabecular density and number for up to 5 years after bariatric surgery. These alterations translate into an increased risk of fall injury, which contributes to increase the fracture risk in patients who have been subjected to bariatric surgery procedures. As bone deterioration continues for years following bariatric surgery, the fracture risk does not seem to be dependent on acute weight loss but, rather, is a chronic condition with an increasing impact over time. Among the post-bariatric surgery mechanisms that have been claimed to act globally on bone health, there is evidence that micro- and macro-nutrient malabsorptive factors, mechanical unloading and changes in molecules partaking in the crosstalk between adipose tissue, bone and muscle may play a determining role. Given these circumstances, it is conceivable that bone health should be adequately investigated in candidates to bariatric surgery through bone-specific work-up and dedicated postsurgical follow-up. Specific protocols of nutrients supplementation, motor activity, structured rehabilitative programs and, when needed, targeted therapeutic strategies should be deemed as an integral part of post-bariatric surgery clinical support.

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.

Anabolic Bone Stimulus Requires a Pre-Exercise Meal and 45-Minute Walking Impulse of Suprathreshold Speed-Enhanced Momentum to Prevent or Mitigate Postmenopausal Osteoporosis within Circadian Constraints

Osteoporosis currently afflicts 8 million postmenopausal women in the US, increasing the risk of bone fractures and morbidity, and reducing overall quality of life. We sought to define moderate exercise protocols that can prevent postmenopausal osteoporosis. Our previous findings singled out higher walking speed and pre-exercise meals as necessary for suppression of bone resorption and increasing of markers of bone formation. Since both studies were amenable to alternate biomechanical, nutritional, and circadian interpretations, we sought to determine the relative importance of higher speed, momentum, speed-enhanced load, duration of impulse, and meal timing on osteogenic response. We hypothesized that: (1) 20 min of exercise one hour after eating is sufficient to suppress bone resorption as much as a 40-min impulse and that two 20 min exercise bouts separated by 7 h would double the anabolic effect; (2) early morning exercise performed after eating will be as effective as mid-day exercise for anabolic outcome; and (3) the 08:00 h 40-min. exercise uphill would be as osteogenic as the 40-min exercise downhill. Healthy postmenopausal women, 8 each, were assigned to a no-exercise condition (SED) or to 40- or 20-min exercise bouts, spaced 7 h apart, for walking uphill (40 Up and 20 Up) or downhill (40 Down and 20 Down) to produce differences in biomechanical variables. Exercise was initiated at 08:00 h one hour after eating in 40-min groups, and also 7 h later, two hours after the midday meal, in 20-min groups. Measurements were made of CICP (c-terminal peptide of type I collagen), osteocalcin (OC), and bone-specific alkaline phosphatase (BALP), markers of bone formation, and of the bone resorptive marker CTX (c-terminal telopeptide of type 1 collagen). The osteogenic ratios CICP/CTX, OC/CTX, and BALP/CTX were calculated. Only the 40-min downhill exercise of suprathreshold speed-enhanced momentum, increased the three osteogenic ratios, demonstrating the necessity of a 40-min, and inadequacy of a 20-min, exercise impulse. The failure of anabolic outcome in 40-min uphill exercise was attributed to a sustained elevation of PTH concentration, as its high morning elevation enhances the CTX circadian rhythm. We conclude that postmenopausal osteoporosis can be prevented or mitigated in sedentary women by 45 min of morning exercise of suprathreshold speed-enhanced increased momentum performed shortly after a meal while walking on level ground, or by 40-min downhill, but not 40-min uphill, exercise to avoid circadian PTH oversecretion. The principal stimulus for the anabolic effect is exercise, but the prerequisite for a pre-exercise meal demonstrates the requirement for nutrient facilitation.

The Antiresorptive Effect of GIP, But Not GLP-2, Is Preserved in Patients With Hypoparathyroidism-A Randomized Crossover Study

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2) are gut hormones secreted postprandially. In healthy humans, both hormones decrease bone resorption accompanied by a rapid reduction in parathyroid hormone (PTH). The aim of this study was to investigate whether the changes in bone turnover after meal intake and after GIP- and GLP-2 injections, respectively, are mediated via a reduction in PTH secretion. This was tested in female patients with hypoparathyroidism given a standardized liquid mixed-meal test (n = 7) followed by a peptide injection test (n = 4) using a randomized crossover design. We observed that the meal- and GIP- but not the GLP-2-induced changes in bone turnover markers were preserved in the patients with hypoparathyroidism. To understand the underlying mechanisms, we examined the expression of the GIP receptor (GIPR) and the GLP-2 receptor (GLP-2R) in human osteoblasts and osteoclasts as well as in parathyroid tissue. The GIPR was expressed in both human osteoclasts and osteoblasts, whereas the GLP-2R was absent or only weakly expressed in osteoclasts. Furthermore, both GIPR and GLP-2R were expressed in parathyroid tissue. Our findings suggest that the GIP-induced effect on bone turnover may be mediated directly via GIPR expressed in osteoblasts and osteoclasts and that this may occur independent of PTH. In contrast, the effect of GLP-2 on bone turnover seems to depend on changes in PTH and may be mediated through GLP-2R in the parathyroid gland. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

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.

Diabetes and Bone Involvement in Primary Hyperparathyroidism: Literature Review and Our Personal Experience

BACKGROUND

Primary hyperparathyroidism (PHPT) and type 2 diabetes mellitus (T2DM) are common endocrine disorders impacting on skeletal health, whose concomitant occurrence is becoming more frequent.

PATIENTS AND METHODS

We searched the PubMed database from the National Library of Medicine about the relationship between T2DM and its treatment and bone manifestations of PHPT. Thereafter, we retrospectively evaluated a consecutive series of 472 PHPT patients. Among them 55 were also affected by T2DM. At the diagnosis of PHPT we compared bone turnover markers and bone densitometry between 55 patients with and 417 without T2DM and in the former group according to antidiabetic treatment.

RESULTS

Few data are available about T2DM and PHPT bone involvement, studies about T2DM treatments and PHPT bone manifestations are lacking. Among patients with PHPT of our series, those with T2DM were older, had a lower prevalence of osteitis fibrosa cystica, higher lumbar and femoral T-scores than the remaining patients. No difference was disclosed among the diabetic patients according to ongoing antidiabetic treatment, even though modern treatments were under-represented.

CONCLUSIONS

No clinical study specifically evaluated the impact of T2DM on bone involvement in PHPT. In our experience, diabetic patients resulted more frequently "mild asymptomatic" than non-diabetic patients and showed a lower prevalence of radiological PHPT bone manifestations. The treatment of T2DM does not seem to affect the biochemical or clinical features of PHPT in our series. Further studies are needed to fully disclose the influence of T2DM and antidiabetic treatment on bone health in patients with PHPT.

Update on the Acute Effects of Glucose, Insulin, and Incretins on Bone Turnover In Vivo

PURPOSE OF REVIEW

To provide an update on the acute effects of glucose, insulin, and incretins on markers of bone turnover in those with and without diabetes.

RECENT FINDINGS

Bone resorption is suppressed acutely in response to glucose and insulin challenges in both healthy subjects and patients with diabetes. The suppression is stronger with oral glucose compared with intravenous delivery. Stronger responses with oral glucose may be related to incretin effects on insulin secretion or from a direct effect on bone turnover. Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2) infusion acutely suppresses bone resorption without much effect on bone formation. The bone turnover response to a metabolic challenge may be attenuated in type 2 diabetes, but this is an understudied area. A knowledge gap exists regarding bone turnover responses to a metabolic challenge in type 1 diabetes. The gut-pancreas-bone link is potentially an endocrine axis. This linkage is disrupted in diabetes, but the mechanism and progression of this disruption are not understood.

Changes in Bone Mineral Density in Patients with Type 2 Diabetes After Different Bariatric Surgery Procedures and the Role of Gastrointestinal Hormones

BACKGROUND

To compare changes in bone mineral density (BMD) in patients with morbid obesity and type 2 diabetes (T2D) a year after being randomized to metabolic gastric bypass (mRYGB), sleeve gastrectomy (SG), and greater curvature plication (GCP). We also analyzed the association of gastrointestinal hormones with skeletal metabolism.

METHODS

Forty-five patients with T2D (mean BMI 39.4 ± 1.9 kg/m²) were randomly assigned to mRYGB, SG, or GCP. Before and 12 months after surgery, anthropometric, body composition, biochemical parameters, fasting plasma glucagon, ghrelin, and PYY as well as GLP-1, GLP-2, and insulin after a standard meal were determined.

RESULTS

After surgery, the decrease at femoral neck (FN) was similar but at lumbar spine (LS), it was greater in the mRYGB group compared with SG and GCP - 7.29 (4.6) vs. - 0.48 (3.9) vs. - 1.2 (2.7)%, p < 0.001. Osteocalcin and alkaline phosphatase increased more after mRYGB. Bone mineral content (BMC) at the LS after surgery correlated with fasting ghrelin (r = - 0.412, p = 0.01) and AUC for GLP-1 (r = - 0.402, p = 0.017). FN BMD at 12 months correlated with post-surgical fasting glucagon (r = 0.498, p = 0.04) and insulin AUC (r = 0.384, p = 0.030) and at LS with the AUC for GLP-1 in the same time period (r = - 0.335, p = 0.049). However, in the multiple regression analysis after adjusting for age, sex, and BMI, the type of surgery (mRYGB) remained the only factor associated with BMD reduction at LS and FN.

CONCLUSIONS

mRYGB induces greater deleterious effects on the bone at LS compared with SG and GCP, and gastrointestinal hormones do not play a major role in bone changes.

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.

GLP-2 administration in ovariectomized mice enhances collagen maturity but did not improve bone strength

Osteoporosis and bone fragility are progressing worldwide. Previous published literature reported a possible beneficial role of gut hormones, and especially glucagon-like peptide-2 (GLP-2), in modulating bone remodeling. As now (Gly²)GLP-2 is approved in the treatment of short bowel syndrome, we thought to investigate whether such molecule could be beneficial in bone fragility.

MC3T3 and Raw 264.7 were cultured in presence of ascending concentrations of (Gly²)GLP-2. Collagen crosslinks, maturity, lysyl oxidase activity and osteoclastogenesis were then analyzed. Furthermore, (Gly²)GLP-2, at the clinical approved dose of 50 μg/kg/day, was also administered to ovariectomized Balb/c mice for 8 weeks. Hundred μg/kg zoledronic acid (once iv) was also used as a positive comparator. Bone strength, microarchitectures and bone tissue composition were analyzed by 3-point bending, compression test, microCT and Fourier transform infrared imaging, respectively.

In vitro, (Gly²)GLP-2 was potent in enhancing bone matrix gene expression but also to dose-dependently enhanced collagen maturation and post-processing. (Gly²)GLP-2 was also capable of reducing dose-dependently the number of newly generated osteoclasts. However, in vivo, (Gly²)GLP-2 was not capable of improving neither bone strength, at the femur diaphysis or lumbar vertebrae, nor bone microarchitecture. On the other hand, at the tissue material level, (Gly²)GLP-2 significantly enhances collagen maturity and reduce phosphate/amide ratio.

Overall, this study highlights that despite modification of bone tissue composition, (Gly²)GLP-2, at the clinical approved dose of 50 μg/kg/day, did not provide real beneficial effects in improving bone strength in a mouse model of bone fragility. Further studies are recommended to validate the best dose and regimen of administration to significantly enhance bone strength.

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In vitro, (Gly²)GLP-2 enhances dose-dependently bone matrix deposition and quality.

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In vitro, (Gly²)GLP-2 reduces dose-dependently osteoclast formation.

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In vivo, (Gly²)GLP-2 failed to improve bone strength in ovariectomy-induced bone loss.

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In vivo, (Gly²)GLP-2 failed to improve bone microarchitecture.

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In vivo, (Gly²)GLP-2 increased collagen maturity and phosphate/amide ratios.

Glucagon-like peptide 1 and Glucagon-like peptide 2 in relation to osteoporosis in non-diabetic postmenopausal women

Osteoporosis results from an imbalance in bone remodeling, which is known to follow a circadian rhythm determined by a functional relationship between intestine and bone tissue. Specific intestinal peptides have been identified as mediators. Glucagon-like peptide 1 and glucagon-like peptide 2, have been associated with bone health. Our main objective was to determine whether postprandial plasma levels of glucagon-like peptide 1, glucagon-like peptide 2 and dipeptidyl-peptidase 4 activity, are associated with osteoporosis in non-diabetic postmenopausal women. We studied non-diabetic postmenopausal women with osteoporosis diagnosed by dual-energy X-ray absorptiometry (cases, n = 43) and age-matched (±1 yr) controls without osteoporosis or a history of osteoporotic fracture (n = 43). We measured postprandial plasma levels of glucagon-like peptide 1, glucagon-like peptide 2, and dipeptidyl-peptidase 4 activity, bone mineral density, and baseline levels of bone remodeling markers and analyzed the food intake using a food-frequency questionnaire. Postprandial glucagon-like peptide 1 values were lower (p < 0.001) in cases, μ (SEM) = 116.25 (2.68), than in controls, μ (SEM) = 126.79 (2.68). Glucagon-like peptide 1 was associated with reduced osteoporosis risk in the crude logistic regression analysis [OR (95% CI) = 0.724 (0.53–0.97), p = 0.031] and adjusted analysis [OR = 0.603 (0.38–0.94), p = 0.027]. We found no association of glucagon-like peptide 2, or dipeptidyl-peptidase 4 activity with osteoporosis. Postprandial glucagon-like peptide 1 levels are related to osteoporosis and osteoporosis risk in non-diabetic postmenopausal women. Further studies are required to verify these findings.

GLP-2 and GIP exert separate effects on bone turnover: A randomized, placebo-controlled, crossover study in healthy young men

BACKGROUND

Glucagon-like peptide-2 (GLP-2) and glucose-dependent insulinotropic polypeptide (GIP) both inhibit bone resorption in humans but the underlying mechanisms are poorly understood. In vitro, GLP-2 activates the GIP-receptor (GIPR).

OBJECTIVE

Based on in vitro studies, we hypothesized that the antiresorptive effect of GLP-2 was mediated through the GIPR. This was tested using the selective GIPR-antagonist GIP(3-30)NH₂.

METHODS

The study was a randomized, single-blinded, placebo-controlled, crossover study conducted at Hvidovre University Hospital, Denmark. Eight healthy young men were included and studied on four study days: GIP (200 μg), GLP-2 (800 μg), GIP(3-30)NH₂ (800 pmol/kg/min) + GLP-2 (800 μg), and placebo. The main outcomes were bone resorption measured as collagen type 1 C-terminal telopeptide (CTX) and bone formation measured as procollagen type 1 N-terminal propeptide (P1NP).

RESULTS

CTX (mean ± SEM) significantly decreased after both GIP (to 55.3 ± 6.3% of baseline at t = 90 min) and GLP-2 (to 60.5 ± 5.0% of baseline at t = 180 min). The maximal reduction in CTX after GIP(3-30)NH₂ + GLP-2 (to 63.2 ± 3.1% of baseline) did not differ from GLP-2 alone (p = 0.95) nor did net AUC_0-240 (-6801 ± 879%*min vs -6027 ± 648%*min, p = 0.56). At t = 30 min, GIP significantly (p < 0.0001) increased P1NP to 115.1 ± 2.2% of baseline compared with 103.1 ± 1.5% after placebo. Both GLP-2 and GIP(3-30)NH₂ + GLP-2 significantly (p < 0.0001) decreased P1NP to 91.3 ± 1.1% and 88.1 ± 3.0% of baseline, respectively (at t = 45 min) compared with placebo.

CONCLUSIONS

GIPR antagonism did not inhibit the GLP-2-induced reduction in bone resorption (CTX) in healthy young men. In contrast to GLP-2, GIP increased P1NP despite decreasing CTX indicating an uncoupling of bone resorption from formation. Thus, GLP-2 and GIP seem to exert separate effects on bone turnover in humans.

CLINICAL TRIALS INFORMATION

ClinicalTrials.gov (NCT03159741).

Glucagon like peptide 2 has a positive impact on osteoporosis in ovariectomized rats

AIMS

In this study, we evaluate the effects of glucagon-like peptide 2 (GLP-2) on bone microarchitecture, bone turnover markers (BTMs) and inflammation markers in ovariectomized (OVX) rats.

MATERIAL AND METHODS

In total, 31 Sprague-Dawley rats were divided into the following three groups: sham (control sham-operated with vehicle, n = 7), OV (OVX with vehicle, n = 12), and GLP-2 (OVX with GLP-2, n = 12). Intervention began at the 12th week after surgery and lasted for 4 weeks. The dosage of the GLP-2 was 160 μg/kg/d through subcutaneous injections, and normal saline was used as the vehicle agent. After 4 weeks of treatment, serum BTM and inflammation marker levels were measured by ELISA, and femora samples were analyzed by qRT-PCR, micro-CT, histology and histomorphometry.

KEY FINDINGS

After 4 weeks of treatment, serum TRAcP-5b and RANKL levels as well as the CTX-1/P1NP ratio in the GLP-2 group decreased, and ALP activity, P1NP level, and OPG/RANKL ratio increased significantly; qRT-PCR analysis showed that mRNA levels of RANKL decreased, and Runx2, ALP, and Col-1 levels as well as the OPG/RANKL ratio increased significantly in the GLP-2 group compared with the OV group. In bone histology analysis, GLP-2 significantly decreased the AV/MV, Oc.N and Oc.S but increased the Ob.N, BFR and MAR. Analysis with μ-CT showed that the BMD, BV/TV, Tb.N and Conn.D increased significantly in the GLP-2 group compared with the OV group. The levels of serum inflammation markers TNF-α, IL-1β and IL-6 decreased, and TGF-β levels increased in the GLP-2 group compared with the OV group.

SIGNIFICANCE

GLP-2 may have a positive impact on osteoporosis by promoting bone formation, inhibiting bone resorption and decreasing circulatory inflammation in ovariectomized rats.

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.

GLP-2 Dysregulates Hepatic Lipoprotein Metabolism, Inducing Fatty Liver and VLDL Overproduction in Male Hamsters and Mice

Fundamental complications of insulin resistance and type 2 diabetes include the development of nonalcoholic fatty liver disease and an atherogenic fasting dyslipidemic profile, primarily due to increases in hepatic very-low-density lipoprotein (VLDL) production. Recently, central glucagon-like peptide-2 receptor (GLP2R) signaling has been implicated in regulating hepatic insulin sensitivity; however, its role in hepatic lipid and lipoprotein metabolism is unknown. We investigated the role of glucagon-like peptide-2 (GLP-2) in regulating hepatic lipid and lipoprotein metabolism in Syrian golden hamsters, C57BL/6J mice, and Glp2r-/- mice consuming either a normal chow or high-fat diet (HFD). In the chow-fed hamsters, IP GLP-2 administration significantly increased fasting dyslipidemia, hepatic VLDL production, and the expression of key genes involved in hepatic de novo lipogenesis. In HFD-fed hamsters and chow-fed mice, GLP-2 administration exacerbated or induced hepatic lipid accumulation. HFD-fed Glp2r-/- mice displayed reduced glucose tolerance, VLDL secretion, and microsomal transfer protein lipid transfer activity, as well as exacerbated fatty liver. Thus, we conclude that GLP-2 plays a lipogenic role in the liver by increasing lipogenic gene expression and inducing hepatic steatosis, fasting dyslipidemia, and VLDL overproduction. In contrast, the lack of Glp2r appears to interfere with VLDL secretion, resulting in enhanced hepatic lipid accumulation. These studies have uncovered a role for GLP-2 in maintaining hepatic lipid and lipoprotein homeostasis.

Postprandial GLP-2 Levels Are Increased After Biliopancreatic Diversion in Diabetic Individuals with Class I Obesity: a Prospective Study

BACKGROUND

Biliopancreatic diversion (BPD) is a predominantly malabsorptive procedure. Glucagon-like peptide 2 (GLP-2) plays predominantly trophic effects on the gut. A significant increase in GLP-2 after BPD in rats was previously observed, but there are no studies investigating the effect of BPD in GLP-2 levels in humans.

OBJECTIVE

The aim of this study is to evaluate the influence of BPD on the release of GLP-2.

METHODS

This is a prospective cohort study that evaluated diabetic individuals with class I obesity which underwent BPD (Scopinaro operation) and were followed up for 12 months. Of 12 individuals, four did not comply with the proposed follow-up and were excluded from the analysis. GLP-2 levels were determined by means of an enzyme-linked immunosorbent assay (ELISA), and we collected serial lab samples through a standard meal tolerance test (MTT) in the immediate preoperative period and 12 months after surgery.

RESULTS

During standard MTT, we observed significant increases of GLP-2 levels from 15 to 60 min (respectively, at 15 min, 5.7 ± 3.4 versus 12.4 ± 4.3, p = 0.029; 30 min, 6 ± 3.5 versus 14.6 ± 3.9; p = 0.004; 45 min, 5.6 ± 4.1 versus 12.6 ± 5.2, p = 0.013; 60 min, 5.8 ± 2.9 versus 10.6 ± 5.6, p = 0.022); then it began to gradually decrease to levels close to the basal.

DISCUSSION

Our findings have confirmed that there is a significant increase in GLP-2 levels after BPD in humans. GLP-2 plays a number of roles which may be adaptive, compensatory, and beneficial in the context of BPD. The clinical implications of this finding remain to be completely understood.

Review article: effects of type 2 diabetes therapies on bone metabolism

Diabetes complications and osteoporotic fractures are two of the most important causes of morbidity and mortality in older patients, and they share many features, including genetic susceptibility, molecular mechanisms, and environmental factors. Type 2 diabetes mellitus (T2DM) compromises bone microarchitecture by inducing abnormal bone cell function and matrix structure with increased osteoblast apoptosis, diminished osteoblast differentiation, and enhanced osteoclast-mediated bone resorption. The linkage between these two chronic diseases creates a possibility that certain antidiabetic therapies may affect bone function. The treatment of T2DM has been improved in the past two decades with the development of new therapeutic drugs. Each class has a pathophysiologic target related to the regulation of the energy metabolism and insulin secretion. However, both glycemic homeostasis and bone homeostasis are under the control of common regulatory factors. This background allows the individual pharmacological targets of antidiabetic therapies to affect bone quality due to their indirect effects on bone cell differentiation and the bone remodeling process. With a greater number of diabetic patients and antidiabetic agents being launched, it is critical to highlight the consequences of this disease and its pharmacological agents on bone health and fracture risk. Currently, there is little scientific knowledge approaching the impact of most anti-diabetic treatments on bone quality and fracture risk. Thus, this review aims to explore the pros and cons of the available pharmacologic treatments for T2DM on bone mineral density and risk fractures in humans.

Association of DPP-4 activity with BMD, body composition, and incident hip fracture: the Cardiovascular Health Study

There was no association of plasma DPP-4 activity levels with bone mineral density (BMD), body composition, or incident hip fractures in a cohort of elderly community-dwelling adults.

INTRODUCTION

Dipeptidyl peptidase IV (DPP-4) inactivates several key hormones including those that stimulate postprandial insulin secretion, and DPP-4 inhibitors (gliptins) are approved to treat diabetes. While DPP-4 is known to modulate osteogenesis, the relationship between DPP-4 activity and skeletal health is uncertain. The purpose of the present study was to examine possible associations between DPP-4 activity in elderly subjects enrolled in the Cardiovascular Health Study (CHS) and BMD, body composition measurements, and incident hip fractures.

METHODS

All 1536 male and female CHS participants who had evaluable DXA scans and plasma for DPP-4 activity were included in the analyses. The association between (1) BMD of the total hip, femoral neck, lumbar spine, and total body; (2) body composition measurements (% lean, % fat, and total body mass); and (3) incident hip fractures and plasma levels of DPP-4 activity were determined.

RESULTS

Mean plasma levels of DPP-4 activity were significantly higher in blacks (227 ± 78) compared with whites (216 ± 89) (p = 0.04). However, there was no significant association of DPP-4 activity with age or gender (p ≥ 0.14 for both). In multivariable adjusted models, there was no association of plasma DPP-4 activity with BMD overall (p ≥ 0.55 for all) or in gender stratified analyses (p ≥ 0.23). There was also no association of DPP-4 levels and incident hip fractures overall (p ≥ 0.24) or in gender stratified analyses (p ≥ 0.39).

CONCLUSION

Plasma DPP-4 activity, within the endogenous physiological range, was significantly associated with race, but not with BMD, body composition, or incident hip fractures in elderly community-dwelling subjects.

Diabetes Drug Effects on the Skeleton

Diabetes be it type 1 or type 2 is associated with an increased risk of fragility fractures. The mechanisms underlying this increased risk are just being elucidated. Anti-diabetes medications are crucial for maintaining glucose control and for preventing micro- and macrovascular complications in diabetes. However, they may modulate fracture risk in diabetes in different ways. Thiazolidinediones have demonstrated an unfavorable effect on the skeleton, while metformin and sulfonylureas may have a neutral if not beneficial effect on bone. The use of insulin has been associated with an increased risk of fragility fractures though it is not clear whether it is due to direct influence of insulin or whether it is mediated through hypoglycemia and increased falls risk. The overall effect of incretin mimetics appears to be beneficial; however, this has to be elucidated further. The bone effects of pramlintide, a synthetic analog of amylin, have not been explored fully. Finally, issues regarding bone safety of SGLT2 (sodium-dependent glucose transporter 2) inhibitors, the newest anti-diabetic medications on the market are of concern. The purpose of this review is to provide a comprehensive overview of the effect of these medications on bone metabolism and the studies exploring the risk or lack thereof of these medications on bone loss and fragility fractures.

New Target Sites for Treatment of Osteoporosis

In the last few years, much progress has been achieved in the discovery of new drug target sites for treatment of osteoporotic disorders, one of the main challenging diseases with a large burden for the public health systems. Among these new agents promoting bone formation, shifting the impaired equilibrium between bone anabolism and bone catabolism in the direction of bone synthesis are inorganic polymers, in particular inorganic polyphosphates that show strong stimulatory effects on the expression of bone anabolic marker proteins and hydroxyapatite formation. The bone-forming activity of these polymers can even be enhanced by combination with certain small molecules like quercetin, or if given as functionally active particles with certain divalent cations like strontium ions even showing by itself biological activity. This chapter summarizes recent developments in the search and development of novel anti-osteoporotic agents, with a particular focus on therapeutic approaches based on the potential application of inorganic polymers and combinations.

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.

Effects of Incretin-Based Therapies and SGLT2 Inhibitors on Skeletal Health

Anti-diabetic drugs are widely used and are essential for adequate glycemic control in patients with type 2 diabetes. Recently, marketed anti-diabetic drugs include incretin-based therapies (GLP-1 receptor agonists and DPP-4 inhibitors) and sodium-glucose co-transporter 2 (SGLT2) inhibitors. In contrast to well-known detrimental effects of thiazolidinediones on bone metabolism and fracture risk, clinical data on the safety of incretin-based therapies is limited. Based on meta-analyses of trials investigating the glycemic-lowering effect of GLP-1 receptor agonists and DPP4 inhibitors, it seems that incretin-based therapies are not associated with an increase in fracture risk. Sodium-glucose co-transporter 2 inhibitors may alter calcium and phosphate homeostasis as a result of secondary hyperparathyroidism induced by increased phosphate reabsorption. Although these changes may suggest detrimental effects of SGLT-2 inhibitors on skeletal integrity, treatment-related direct effects on bone metabolism seem unlikely. Observed changes in BMD, however, seem to result from increased bone turnover in the early phase of drug-induced weight loss. Fracture risk, which is observed in older patients with impaired renal function and elevated cardiovascular disease risk treated with SGLT2 inhibitors, seems to be independent of direct effects on bone but more likely to be associated with falls and changes in hydration status secondary to osmotic diuresis.

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.

Glucose-dependent insulinotropic polypeptide (GIP) dose-dependently reduces osteoclast differentiation and resorption

A role for glucose-dependent insulinotropic polypeptide (GIP) in controlling bone resorption has been suspected. However uncertainty remains to identify whether GIP act directly on osteoclasts. The aim of the present study were (i) to identify in different osteoclast differentiation models (human peripheral blood mononuclear cells-PBMC, murine bone marrow macrophage-BMM and murine Raw 264.7 cells) whether GIP was capable of reducing osteoclast formation and resorption; (ii) ascertain whether the highly potent GIP analogue N-AcGIP was capable of inducing a response at lower concentrations and (iii) to decipher the molecular mechanisms responsible for such effects. [d-Ala(2)]-GIP dose-dependently reduced osteoclast formation at concentration as low as 1nM in human PBMC and 10nM in murine BMM cultures. Furthermore, [d-Ala(2)]-GIP also reduced the extent of osteoclast resorption at concentration as low as 1nM in human PBMC and murine BMM cultures. The mechanism of action of [d-Ala(2)]-GIP appeared to be mediated by reduction in intracellular calcium concentration and oscillation that subsequently inhibited calcineurin activity and NFATc1 nuclear translocation. The potency of the highly potent N-AcGIP was determined and highlighted an effect on osteoclast formation and resorption at concentration ten times lower than observed with [d-Ala(2)]-GIP in vitro. Furthermore, N-AcGIP was also capable of reducing the number of osteoclast in ovariectomized mice as well as the circulating level of type I collagen C-telopeptide. Pharmacological concentrations required for reducing osteoclast formation and resorption provide the impetus to design and exploit enzymatically stable GIP analogues for the treatment of bone resorption disorders in humans.

Effect of Intravenous Glucose Tolerance Test on Bone Turnover Markers in Adults with Normal Glucose Tolerance

Background

It is well known that enteral nutrients result in acute suppression of bone turnover markers (BTMs), and incretin hormones are believed to play a significant role in this physiological skeletal response. However, there is limited research exploring the impact of parenteral nutrients on BTMs. Our aim was to assess the influence of intravenous glucose on BTMs in adults with normal glucose tolerance (NGT).

Material/Methods

We conducted 1-h intravenous glucose tolerance test (IVGTT) in 24 subjects with NGT. Blood samples were collected before and 5, 10, 15, 20, 30, 60 min after administration of glucose, then serum levels of bone formation marker procollagen type I N-terminal propeptide (P1NP) and resorption marker C-terminal cross-linking telopeptides of collagen type I (CTX) were measured.

Results

During IVGTT, the fasting CTX level fell gradually and reached a nadir of 80.4% of the basal value at 60 min. Conversely, the fasting P1NP level decreased mildly and reached a nadir of 90.6% of the basal value at 15 min, then gradually increased and reached 96.6% at 60 min. The CTX-to-P1NP ratio increased slightly and reached a peak of 104.3% of the basal value at 10 min, then fell gradually and reached a nadir of 83% at 60 min.

Conclusions

Our study indicates that intravenous glucose results in an acute suppression of BTMs in the absence of incretin hormones. The mechanism responsible for this needs further investigation.

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.

Searching for the physiological role of glucose-dependent insulinotropic polypeptide

Glucose‐dependent insulinotropic polypeptide (GIP) was established as a gut hormone more than 40 years ago, and there is good experimental support for its role as an incretin hormone although deletion of the GIP receptor or the GIP cells or GIP receptor mutations have only minor effects on glucose metabolism. Unlike the related hormone, GLP‐1, GIP stimulates the secretion of glucagon, which in healthy individuals may help to stabilize glucose levels, but in people with type 2 diabetes may contribute to glucose intolerance. A role in lipid metabolism is supported by numerous indirect observations and by resistance to diet‐induced obesity after deletion of the GIP receptor. However, a clear effect on lipid clearance could not be identified in humans, raising doubt about its importance. The GIP receptor is widely expressed in the body and also appears to be expressed on bone cells, and experimental studies in rodent point to effects on bone metabolism. Recent studies revealed pronounced inhibitory effects of GIP on bone resorption markers in humans and suggest that GIP may be (one of the) gastrointestinal regulators of bone turn‐over. In support of this, a loss‐of‐function GIP receptor mutation in humans is associated with a marked increase in fracture risk. The lack of a reliable GIP receptor antagonist contributes to the uncertainty regarding the physiological role of GIP.

Effects of diabetes drugs on the skeleton

Type 2 diabetes is associated with increased fracture risk and the mechanisms underlying the detrimental effects of diabetes on skeletal health are only partially understood. Antidiabetic drugs are indispensable for glycemic control in most type 2 diabetics, however, they may, at least in part, modulate fracture risk in exposed patients. Preclinical and clinical data clearly demonstrate an unfavorable effect of thiazolidinediones on the skeleton with impaired osteoblast function and activated osteoclastogenesis. The negative effect of thiazolidinediones on osteoblastogenesis includes decreased activity of osteoblast-specific transcription factors (e.g. Runx2, Dlx5, osterix) and decreased activity of osteoblast-specific signaling pathways (e.g. Wnt, TGF-β/BMP, IGF-1). In contrast, metformin has a positive effect on osteoblast differentiation due to increased activity of Runx2 via the AMPK/USF-1/SHP regulatory cascade resulting in a neutral or potentially protective effect on bone. Recently marketed antidiabetic drugs include incretin-based therapies (GLP-1 receptor agonists, DPP-4 inhibitors) and sodium-glucose co-transporter 2 (SGLT2)-inhibitors. Preclinical studies indicate that incretins (GIP, GLP-1, and GLP-2) play an important role in the regulation of bone turnover. Clinical safety data are limited, however, meta-analyses of trials investigating the glycemic-lowering effect of both, GLP-1 receptor agonists and DPP4-inhibitors, suggest a neutral effect of incretin-based therapies on fracture risk. For SGLT2-inhibitors recent data indicate that due to their mode of action they may alter calcium and phosphate homeostasis (secondary hyperparathyroidism induced by increased phosphate reabsorption) and thereby potentially affect bone mass and fracture risk. Clinical studies are needed to elucidate the effect of SGLT2-inhibitors on bone metabolism. Meanwhile SGLT2-inhibitors should be used with caution in patients with high fracture risk, which is specifically true for the use of thiazolidinediones.

The glucagon-like peptide 2 receptor is expressed in enteric neurons and not in the epithelium of the intestine

Glucagon-like peptide 2 (GLP-2) is a potent intestinotrophic growth factor with therapeutic potential in the treatment of intestinal deficiencies. It has recently been approved for the treatment of short bowel syndrome. The effects of GLP-2 are mediated by specific binding of the hormone to the GLP-2 receptor (GLP-2R) which was cloned in 1999. However, consensus about the exact receptor localization in the intestine has never been established. By physical, chemical and enzymatic tissue fragmentation, we were able to divide rat jejunum into different compartments consisting of: (1) epithelium alone, (2) mucosa with lamina propria and epithelium, (3) the external muscle coat including myenteric plexus, (4) a compartment enriched for the myenteric plexus and (5) intestine without epithelium. Expression of Glp2r; chromogranin A; tubulin, beta 3; actin, gamma 2, smooth muscle, enteric and glial fibrillary acidic protein in these isolated tissue fractions was quantified with qRT-PCR. Expression of the Glp2r was confined to compartments containing enteric neurons and receptor expression was absent in the epithelium. Our findings provide evidence for the expression of the GLP-2R in intestinal compartments rich in enteric neurons and, importantly they exclude significant expression in the epithelium of rat jejunal mucosa.

The impact of diabetes and diabetes medications on bone health

Patients with type 2 diabetes mellitus (T2DM) have an increased risk of fragility fractures despite increased body weight and normal or higher bone mineral density. The mechanisms by which T2DM increases skeletal fragility are unclear. It is likely that a combination of factors, including a greater risk of falling, regional osteopenia, and impaired bone quality, contributes to the increased fracture risk. Drugs for the treatment of T2DM may also impact on the risk for fractures. For example, thiazolidinediones accelerate bone loss and increase the risk of fractures, particularly in older women. In contrast, metformin and sulfonylureas do not appear to have a negative effect on bone health and may, in fact, protect against fragility fracture. Animal models indicate a potential role for incretin hormones in bone metabolism, but there are only limited data on the impact of dipeptidyl peptidase-4 inhibitors and glucagon-like peptide-1 agonists on bone health in humans. Animal models also have demonstrated a role for amylin in bone metabolism, but clinical trials in patients with type 1 diabetes with an amylin analog (pramlintide) have not shown a significant impact on bone metabolism. The effects of insulin treatment on fracture risk are inconsistent with some studies showing an increased risk and others showing no effect. Finally, although there is limited information on the latest class of medications for the treatment of T2DM, the sodium-glucose co-transporter-2 inhibitors, these drugs do not seem to increase fracture risk. Because diabetes is an increasingly common chronic condition that can affect patients for many decades, further research into the effects of agents for the treatment of T2DM on bone metabolism is warranted. In this review, the physiological mechanisms and clinical impact of diabetes treatments on bone health and fracture risk in patients with T2DM are described.

Glugacon-like peptide-2: broad receptor expression, limited therapeutic effect on intestinal inflammation and novel role in liver regeneration

The glucagon-like peptide 2 (GLP-2) is an intestinotrophic hormone with growth promoting and anti-inflammatory actions. However, the full biological functions of GLP-2 and the localization of its receptor (GLP-2R) remain controversial. Among cell lines tested, the expression of GLP-2R transcript was detected in human colonic myofibroblasts (CCD-18Co) and in primary culture of rat enteric nervous system but not in intestinal epithelial cell lines, lymphocytes, monocytes, or endothelial cells. Surprisingly, GLP-2R was expressed in murine (GLUTag), but not human (NCI-H716) enteroendocrine cells. The screening of GLP-2R mRNA in mice organs revealed an increasing gradient of GLP-2R toward the distal gut. An unexpected expression was detected in the mesenteric fat, mesenteric lymph nodes, bladder, spleen, and liver, particularly in hepatocytes. In two mice models of trinitrobenzene sulfonic acid (TNBS)- and dextran sulfate sodium (DSS)-induced colitis, the colonic expression of GLP-2R mRNA was decreased by 60% compared with control mice. Also, GLP-2R mRNA was significantly downregulated in intestinal tissues of inflammatory bowel disease patients. Therapeutically, GLP-2 showed a weak restorative effect on intestinal inflammation during TNBS-induced colitis as assessed by macroscopic score and inflammatory markers. Finally, GLP-2 treatment accelerated mouse liver regeneration following partial hepatectomy as assessed by histological and molecular analyses. In conclusion, the limited therapeutic effect of GLP-2 on colonic inflammation dampens its utility in the management of severe inflammatory intestinal disorders. However, the role of GLP-2 in liver regeneration is a novelty that might introduce GLP-2 into the management of liver diseases and emphasizes on the importance of elucidating other extraintestinal functions of GLP-2.

Suppressed bone turnover in obesity: a link to energy metabolism? A case-control study

CONTEXT

Observations in rodents suggest that osteocalcin (OC) participates in glucose metabolism. Based on human studies, it remains unclear whether circulating OC is simply a bone turnover marker (BTM) or also a mediator in interactions between the skeleton and glucose homeostasis.

OBJECTIVE

The objective of the study was to determine the responses of BTMs, including OC, to oral glucose tolerance test (OGTT) in a case-control setting.

DESIGN AND PATIENTS

Thirty-four normoglycemic young adults [mean age 19 y (SD 2.3)] with severe childhood-onset obesity and their gender- and age-matched nonobese controls underwent a standard 2-hour OGTT.

MAIN OUTCOME MEASURES

Glucose, insulin, and six BTMs including total and carboxylated OC (cOC) were determined at baseline and at 30, 60, 90, and 120 minutes during OGTT.

RESULTS

The obese and control subjects were similar in height; the mean body mass indices were 40.4 and 21.9 kg/m(2), respectively. The homeostasis model assessment index was 2.7 times greater in the obese subjects. All BTMs, except bone-specific alkaline phophatase, were lower in the obese subjects compared with the controls: the differences at baseline were 40%, 35%, 17%, 31%, and 32% for N-terminal propeptides of type I collagen, cross-linked telopeptides of type I collagen, tartrate-resistant acid phosphatase, total OC, and carboxylated OC (P < .05 for all) after adjusting for whole-body bone area. All BTMs decreased during OGTT. The relative values for the OGTT responses for total, but not for cOC (measured as area under the curve) differed between the two groups (P = .029 and P = .139, respectively): the decrease in total OC during the OGTT was less pronounced in the obese subjects. Responses in other BTMs were similar between the groups. No associations were observed between glucose metabolism and OCs during OGTT with linear regression.

CONCLUSIONS

Bone turnover markers were substantially lower in obese subjects compared with controls. Total OC and cOC showed less pronounced decrease during the OGTT in obese subjects compared with controls, whereas other BTMs responded similarly in the two groups. The role of OC, if anything, in glucose homeostasis is indirect and may be mediated via other factors than glucose or insulin.

Use of glucagon-like peptide-1 receptor agonists and bone fractures: a meta-analysis of randomized clinical trials

BACKGROUND

Patients with type 2 diabetes mellitus (T2DM) are at a higher risk of bone fractures independent of the use of antidiabetic medications. Furthermore, antidiabetic medications could directly affect bone metabolism. Recently, the use of dipeptidyl peptidase-4 inhibitors has been associated with a lower rate of bone fracture. The aim of the present meta-analysis was to assess whether patients with T2DM treated with glucagon-like peptide-1 receptor agonists (GLP-1Ra) present a lower incidence of bone fracture compared with patients using other antidiabetic drugs.

METHODS

A search on Medline, Embase, and http://www.clinicaltrials.gov, as well as a manual search for randomized clinical trials of T2DM treated with either a GLP-1Ra or another antidiabetic drug for a duration of ≥24 weeks was conducted by two authors (GM, AM) independently.

RESULTS

Although 28 eligible studies were identified, only seven trials reported the occurrence of at least a bone fracture in one arm of the trial. The total number of fractures was 19 (13 and six with GLP-1Ra and comparator, respectively). The pooled Mantel-Haenszel odds ratio for GLP-1Ra was 0.75 (95% confidence interval 0.28-2.02, P = 0.569) in trials versus other antidiabetic agents.

CONCLUSIONS

Although preliminary, our study highlighted that the use of GLP-1Ra does not modify the risk of bone fracture in T2DM compared with the use of other antidiabetic medications.

Physiology and pharmacology of the enteroendocrine hormone glucagon-like peptide-2

Glucagon-like peptide-2 (GLP-2) is a 33-amino-acid proglucagon-derived peptide secreted from enteroendocrine L cells. GLP-2 circulates at low basal levels in the fasting period, and plasma levels rise rapidly after food ingestion. Renal clearance and enzymatic inactivation control the elimination of bioactive GLP-2. GLP-2 increases mesenteric blood flow and activates proabsorptive pathways in the gut, facilitating nutrient absorption. GLP-2 also enhances gut barrier function and induces proliferative and cytoprotective pathways in the small bowel. The actions of GLP-2 are transduced via a single G protein-coupled receptor (GLP-2R), expressed predominantly within the gastrointestinal tract. Disruption of GLP-2R signaling increases susceptibility to gut injury and impairs the adaptive mucosal response to refeeding. Sustained augmentation of GLP-2R signaling reduces the requirement for parenteral nutrition in human subjects with short-bowel syndrome. Hence GLP-2 integrates nutrient-derived signals to optimize mucosal integrity and energy absorption.

Influence of hormonal appetite and energy regulators on bone

Nutritional status is an essential component in determining whole body energy homeostasis. The balance between energy/food intake and metabolism is governed by a range of hormones secreted from various parts of the body. Their subsequent dissemination via the blood results in a wide range of biological responses including satiety, hunger, and glucose uptake. The roles of these systemic hormones also extend to bone regulation with animal and clinical studies establishing a relationship between these regulatory pathways. This review covers the gastrointestinal hormones, ghrelin, PYY, GIP, GLP-1, and GLP-2, and the adipokines, leptin, and adiponectin and their roles in regulating bone homeostasis. Their known actions are reviewed, with an emphasis upon recent advances in understanding. Taken together, this review outlines an expanding appreciation of the interactions between bone mass and the nutritional control of whole body energy balance by gut and adipose tissue.

Exendin-4, a glucagon-like peptide-1 receptor agonist, prevents osteopenia by promoting bone formation and suppressing bone resorption in aged ovariectomized rats

Osteoporosis mainly affects postmenopausal women and older men. Gastrointestinal hormones released after meal ingestion, such as glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide (GLP)-2, have been shown to regulate bone turnover. However, whether GLP-1, another important gastrointestinal hormone, and its analogues also have antiosteoporotic effects, especially in aged postmenopausal situation, has not been confirmed. In the present study, we evaluated the effects of the GLP-1 receptor agonist exendin-4 on ovariectomy (OVX)-induced osteoporosis in old rats. Twelve-month-old female Sprague-Dawley rats were subjected to OVX, and exendin-4 was administrated 4 weeks after the surgery and lasted for 16 weeks. Bone characters and related serum and gene biomarkers were analyzed. Sixteen weeks of treatment with exendin-4 slowed down body weight gain by decreasing fat mass and prevented the loss of bone mass in old OVX rats. Exendin-4 also enhanced bone strength and prevented the deterioration of trabecular microarchitecture. Moreover, exendin-4 decreased the urinary deoxypyridinoline (DPD)/creatinine ratio and serum C-terminal cross-linked telopeptides of type I collagen (CTX-I) and increased serum alkaline phosphatase (ALP), osteocalcin (OC), and N-terminal propeptide of type 1 procollagen (P1NP) levels, key biochemical markers of bone turnover. Interestingly, gene expression results further showed that exendin-4 not only inhibited bone resorption by increasing the osteoprotegerin (OPG)/receptor activator of NF-κB ligand (RANKL) ratio, but also promoted bone formation by increasing the expression of OC, Col1, Runx2, and ALP, which exhibited dual regulatory effects on bone turnover as compared with previous antiosteoporotic agents. In conclusion, these findings demonstrated for the first time the antiosteoporotic effects of exendin-4 in old OVX rats and that it might be a potential candidate for treatment of aged postmenopausal osteoporosis.