Bone: incretin hormones perceiver or receiver?

Chemical and Chemoenzymatic Synthesis of Peptide and Protein Therapeutics Conjugated with Human N-Glycans

Glycosylation is one of the most ubiquitous post-translational modifications. It affects the structure and function of peptides/proteins and consequently has a significant impact on various biological events. However, the structural complexity and heterogeneity of glycopeptides/proteins caused by the diversity of glycan structures and glycosylation sites complicates the detailed elucidation of glycan function and hampers their clinical applications. To address these challenges, chemical and/or enzyme-assisted synthesis methods have been developed to realize glycopeptides/proteins with well-defined glycan morphologies. In particular, N-glycans are expected to be useful for improving the solubility, in vivo half-life and aggregation of bioactive peptides/proteins that have had limited clinical applications so far due to their short duration of action in the blood and unsuitable physicochemical properties. Chemical glycosylation performed in a post-synthetic procedure can be used to facilitate the development of glycopeptide/protein analogues or mimetics that are superior to the original molecules in terms of physicochemical and pharmacokinetic properties. N-glycans are used to modify targets because they are highly biodegradable and biocompatible and have structures that already exist in the human body. On the practical side, from a quality control perspective, close attention should be paid to their structural homogeneity when they are to be applied to pharmaceuticals.

Bone resorption and incretin hormones following glucose ingestion in healthy emerging adults

Background

Studies in adults indicate that macronutrient ingestion yields an acute anti-resorptive effect on bone, reflected by decreases in C-terminal telopeptide (CTX), a biomarker of bone resorption, and that gut-derived incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), facilitate this response. There remain knowledge gaps relating to other biomarkers of bone turnover, and whether gut-bone cross-talk is operative during the years surrounding peak bone strength attainment. This study first, describes changes in bone resorption during oral glucose tolerance testing (OGTT), and second, tests relationships between changes in incretins and bone biomarkers during OGTT and bone micro-structure.

Methods

We conducted a cross-sectional study in 10 healthy emerging adults ages 18-25 years. During a multi-sample 2-hour 75 g OGTT, glucose, insulin, GIP, GLP-1, CTX, bone-specific alkaline phosphatase (BSAP), osteocalcin, osteoprotegerin (OPG), receptor activator of nuclear factor kappa-β ligand (RANKL), sclerostin, and parathyroid hormone (PTH) were assayed at mins 0, 30, 60, and 120. Incremental areas under the curve (iAUC) were computed from mins 0-30 and mins 0-120. Tibia bone micro-structure was assessed using second generation high resolution peripheral quantitative computed tomography.

Results

During OGTT, glucose, insulin, GIP, and GLP-1 increased significantly. CTX at min 30, 60, and 120 was significantly lower than min 0, with a maximum decrease of about 53 % by min 120. Glucose-iAUC_0-30 inversely correlated with CTX-iAUC_0-120 (rho = -0.91, P < 0.001), and GLP-1-iAUC_0-30 positively correlated with BSAP-iAUC_0-120 (rho = 0.83, P = 0.005), RANKL-iAUC_0-120 (rho = 0.86, P = 0.007), and cortical volumetric bone mineral density (rho = 0.93, P < 0.001).

Conclusions

Glucose ingestion yields an anti-resorptive effect on bone metabolism during the years surrounding peak bone strength. Cross-talk between the gut and bone during this pivotal life stage requires further attention.

Bone metabolism and incretin hormones following glucose ingestion in young adults with pancreatic insufficient cystic fibrosis

Background

Gut-derived incretin hormones, including glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 1 (GLP-1), regulate post-prandial glucose metabolism by promoting insulin production. GIP, GLP-1, and insulin contribute to the acute bone anti-resorptive effect of macronutrient ingestion by modifying bone turnover. Cystic fibrosis (CF) is associated with exocrine pancreatic insufficiency (PI), which perturbs the incretin response. Cross-talk between the gut and bone ("gut-bone axis") has not yet been studied in PI-CF. The objectives of this study were to assess changes in biomarkers of bone metabolism during oral glucose tolerance testing (OGTT) and to test associations between incretins and biomarkers of bone metabolism in individuals with PI-CF.

Methods

We performed a secondary analysis of previously acquired blood specimens from multi-sample OGTT from individuals with PI-CF ages 14-30 years (n = 23). Changes in insulin, incretins, and biomarkers of bone resorption (C-terminal telopeptide of type 1 collagen [CTX]) and formation (procollagen type I N-terminal propeptide [P1NP]) during OGTT were computed.

Results

CTX decreased by 32% by min 120 of OGTT (P < 0.001), but P1NP was unchanged. Increases in GIP from 0 to 30 mins (rho = -0.48, P = 0.03) and decreases in GIP from 30 to 120 mins (rho = 0.62, P = 0.002) correlated with decreases in CTX from mins 0-120. Changes in GLP-1 and insulin were not correlated with changes in CTX, and changes in incretins and insulin were not correlated with changes in P1NP.

Conclusions

Intact GIP response was correlated with the bone anti-resorptive effect of glucose ingestion, represented by a decrease in CTX. Since incretin hormones might contribute to development of diabetes and bone disease in CF, the "gut-bone axis" warrants further attention in CF during the years surrounding peak bone mass attainment.

GLP-1 promotes osteogenic differentiation of human ADSCs via the Wnt/GSK-3β/β-catenin pathway

Glucagon-like peptide-1 (GLP-1) analogues are promising anti-diabetic drugs which had been shown to have beneficial effects on bone metabolism in clinical practice, but the molecular mechanism remains unclear. In this study, we evaluated whether GLP-1 can affect the "intestine-fat-bone axis" via the Wnt/GSK-3β/β-catenin pathway. We established a diabetic mouse model and then treated mice with GLP-1 analogue liraglutide. The results showed that after liraglutide treatment, glucose tolerance and insulin tolerance were significantly improved in diabetic mice as expected. Moreover, osteogenic markers such as collagenⅠ, Runx2 and OCN were upregulated; and the adipogenic differentiation markers C/EBP-α and PPAR-γ were downregulated, these results indicated that liraglutide could ameliorate the osteogenic metabolism in diabetic mice. In the cell model, human ADSCs (hADSCs) were cultured and induced to undergo osteogenic and adipogenic differentiation under high glucose conditions in vitro and then treated with GLP-1. The results showed that GLP-1 repressed the induction of adipocyte differentiation biomarkers and the secretion of GSK-3β in a dose-dependent manner. In addition, GLP-1 enhanced the expression of osteoblastogenic biomarkers, such as OCN, Runx2 and collagenⅠ, and promoted osteoblastic mineralization. These effects were substantially suppressed by the Wnt signal recombinant human DKK-1 or activated by Wnt pathway agonist LiCl. Silencing of GSK-3β showed that the levels of β-catenin, GSK-3β and Runx2 were significantly increased by 2.46-, 2.05-, 4.44-fold after GLP-1 treatment compared to that observed in the GSK-3β lentiviral group, respectively. We conclude that GLP-1 promotes the osteogenic differentiation of hADSCs via the Wnt/GSK-3β/β-catenin pathway.

Glucagon-like peptide-1 receptor agonists and risk of bone fracture in patients with type 2 diabetes: A meta-analysis of randomized controlled trials

AIMS

To evaluate the association between glucagon-like peptide-1 receptor agonists (GLP-1 RAs) and the risk of bone fracture in patients with type 2 diabetes mellitus (T2DM).

MATERIALS AND METHODS

We conducted a systematic literature search in PubMed, Embase, the Cochrane Library, and Web of Science from inception to 28 February 2018 and identified eligible randomized controlled trials. The following data were extracted from each study: first author, year of publication, sample size, patient characteristics, study design, intervention drug, control drug, follow-up time, and incident bone fracture events. A meta-analysis was conducted using Review Manager 5.3 software to calculate the odds ratio (OR) and 95% confidence intervals (CI) for dichotomous variables.

RESULTS

A total of 38 studies with 39 795 patients with T2DM were included. There were 241 incident bone fracture cases (107 in the GLP-1 RAs group and 134 in the control group). Compared with patients who received placebo and other anti-diabetic drugs, those who received GLP-1 RAs treatment showed a pooled OR of 0.71 (95% CI, 0.56-0.91) for bone fracture. Subgroup analysis showed that treatments with liraglutide and lixisenatide were associated with significantly reduced risk of bone fractures (ORs, 0.56; 95% CI, 0.38-0.81 and 0.55; 95% CI, 0.31-0.97, respectively). However, other GLP-1 RAs did not show superiority to placebo or other anti-diabetic drugs. Moreover, these beneficial effects were dependent on the duration of GLP-1 RAs treatment, only a GLP-1 RAs treatment period of more than 52 weeks could significantly lower the risk of bone fracture in patients with T2DM (OR, 0.71; 95% CI, 0.56-0.91).

CONCLUSIONS

Compared with placebo and other anti-diabetic drugs, liraglutide and lixisenatide were associated with a significant reduction in the risk of bone fractures, and the beneficial effects were dependent on the duration of treatment.

Involvement of Bone in Systemic Endocrine Regulation

The skeleton shows an unconventional role in the physiology and pathophysiology of the human organism, not only as the target tissue for a number of systemic hormones, but also as endocrine tissue modulating some skeletal and extraskeletal systems. From this point of view, the principal cells in the skeleton are osteocytes. These cells primarily work as mechano-sensors and modulate bone remodeling. Mechanically unloaded osteocytes synthetize sclerostin, the strong inhibitor of bone formation and RANKL, the strong activator of bone resorption. Osteocytes also express hormonally active vitamin D (1,25(OH)2D) and phosphatonins, such as FGF23. Both 1,25(OH)2D and FGF23 have been identified as powerful regulators of the phosphate metabolism, including in chronic kidney disease. Further endocrine cells of the skeleton involved in bone remodeling are osteoblasts. While FGF23 targets the kidney and parathyroid glands to control metabolism of vitamin D and phosphates, osteoblasts express osteocalcin, which through GPRC6A receptors modulates beta cells of the pancreatic islets, muscle, adipose tissue, brain and testes. This article reviews some knowledge concerning the interaction between the bone hormonal network and phosphate or energy homeostasis and/or male reproduction.

Epithelial Barrier Function in Gut-Bone Signaling

The intestinal epithelial barrier plays an essential role in maintaining host homeostasis. The barrier regulates nutrient absorption as well as prevents the invasion of pathogenic bacteria in the host. It is composed of epithelial cells, tight junctions, and a mucus layer. Several factors, such as cytokines, diet, and diseases, can affect this barrier. These factors have been shown to increase intestinal permeability, inflammation, and translocation of pathogenic bacteria. In addition, dysregulation of the epithelial barrier can result in inflammatory diseases such as inflammatory bowel disease. Our lab and others have also shown that barrier disruption can have systemic effects including bone loss. In this chapter, we will discuss the current literature to understand the link between intestinal barrier and bone. We will discuss how inflammation, aging, dysbiosis, and metabolic diseases can affect intestinal barrier-bone link. In addition, we will highlight the current suggested mechanism between intestinal barrier and bone.

Weak bones in diabetes mellitus – an update on pharmaceutical treatment options

Objectives

Diabetes mellitus is often associated with a number of complications such as nephropathy, neuropathy, retinopathy and foot ulcers. However, weak bone is a diabetic complication that is often overlooked. Although the exact mechanism for weak bones within diabetes mellitus is unclear, studies have shown that the mechanism does differ in both type I (T1DM) and type II diabetes (T2DM). This review, however, investigates the application of mesenchymal stem cells, recombinant human bone morphogenetic protein-2, teriparatide, insulin administration and the effectiveness of a peroxisome proliferator-activated receptor-ϒ modulator, netoglitazone in the context of diabetic weak bones.

Key findings

In T1DM, weak bones may be the result of defective osteoblast activity, the absence of insulin's anabolic effects on bone, the deregulation of the bone–pancreas negative feedback loop and advanced glycation end product (AGE) aggregation within the bone matrix as a result of hyperglycaemia. Interestingly, T2DM patients placed on insulin administration, thiazolidinediones, SGLT2 inhibitors and sulfonylureas have an associated increased fracture risk. T2DM patients are also observed to have high sclerostin levels that impair osteoblast gene transcription, AGE aggregation within bone, which compromises bone strength and a decrease in esRAGE concentration resulting in a negative association with vertebral fractures.

Summary

Effective treatment options for weak bones in the context of diabetes are currently lacking. There is certainly scope for discovery and development of novel agents that could alleviate this complication in diabetes patients.

Vildagliptin has the same safety profile as a sulfonylurea on bone metabolism and bone mineral density in post-menopausal women with type 2 diabetes: a randomized controlled trial

BACKGROUND

Several antidiabetic therapies affect bone metabolism. Sulfonylureas have the lowest impact on bone among oral antidiabetics. The objective of this study is to compare the effects of vildagliptin and gliclazide modified release (MR) on bone turnover markers (BTMs) and bone mineral density (BMD) in postmenopausal women with uncontrolled type 2 diabetes (T2D).

METHODS

Forty-two postmenopausal women with uncontrolled T2D were randomly allocated into vildagliptin or gliclazide MR (control) groups. The primary endpoint was the change in the BTMs in months 6 and 12 compared with the baseline. The secondary endpoint was the variation in the BMD, which was assessed via dual-energy X-ray absorptiometry at the lumbar spine, femoral neck and total hip at baseline and month 12.

RESULTS

After a 12-month treatment, the BTM serum carboxy-terminal telopeptide of type 1 collagen increased 0.001 ± 0.153 ng/mL in the vildagliptin group versus 0.008 ± 0.060 ng/mL in the gliclazide MR group (p = 0.858). The serum osteocalcin, serum amino-terminal propeptide of procollagen type I and urinary amino-terminal telopeptide of type 1 collagen remained stable in both groups, and there was no statistically significant difference between the effect of vildagliptin and gliclazide MR on these variables. The lumbar spine BMD did not change in the vildagliptin or gliclazide MR groups after a 12-month treatment (0.000 ± 0.025 g/cm² versus -0.008 ± 0.036, respectively, p = 0.434). Furthermore, there was a similar lack of change in the femoral neck and total hip BMD values in both treatments.

CONCLUSIONS

Bone turnover markers and BMD remained unchanged after a 12-month treatment in both groups, which suggests that vildagliptin has the same safety profile as gliclazide MR on bone metabolism. Trial Registration ClinicalTrials.gov number NCT01679899.

Glucose-Dependent Insulinotropic Peptide Prevents Serum Deprivation-Induced Apoptosis in Human Bone Marrow-Derived Mesenchymal Stem Cells and Osteoblastic Cells

Human bone marrow-derived mesenchymal stem cells (hBMSC) are able to differentiate into cells of connective tissue lineages, including bone and cartilage. They are therefore considered as a promising tool for the treatment of bone degenerative diseases. One of the major issues in regenerative cell therapy is the biosafety of fetal bovine serum used for cell culture. Therefore, the development of a culture medium devoid of serum but preserving hBMSC viability will be of clinical value. The glucose-dependent insulinotropic peptide (GIP) has an anti-apoptotic action in insulin-producing cells. Interestingly, GIP also exerts beneficial effects on bone turnover by acting on osteoblasts and osteoclasts. We therefore evaluated the ability of GIP to prevent cell death in osteoblastic cells cultured in serum-free conditions. In hBMSC and SaOS-2 cells, activation of the GIP receptor increased intracellular cAMP levels. Serum deprivation induced apoptosis in SaOS-2 and hBMSC that was reduced by 30 and 50 %, respectively, in the presence of GIP. The protective effect of GIP involves activation of the adenylate cyclase pathway and inhibition of caspases 3/7 activation. These findings demonstrate that GIP exerts a protective action against apoptosis in hBMSC and suggest a novel approach to preserve viability of hBMSC cultured in the absence of serum.

Perspectives in GLP-1 Research: New Targets, New Receptors

The incretin hormone glucagon-like peptide-1 (GLP-1) binds to and activates its G-protein-coupled-receptor GLP-1R to reduce glycaemia through the stimulation of insulin and suppression of pancreatic glucagon secretion. Recently, GLP-1 effects unrelated to glucose homeostasis have been discovered in myocardium, bone, adipose tissue, and other target organs, which appear to be mainly mediated by GLP-1R-independent pathways. Here, we summarize knowledge on GLP-1R agonists (GLP-1RAs) as they relate to the improvement of glucose control, and focus on the most recently described effects, discussing the preclinical evidence of the involvement of alternative receptors and signalling mechanisms. It is now evident that the universe of GLP-1RAs is expanding further from the initial incretin effect, opening new unforeseen avenues for research and clinical applications.

Activation of GLP-1 Receptor Promotes Bone Marrow Stromal Cell Osteogenic Differentiation through β-Catenin

Glucagon-like peptide 1 (GLP-1) plays an important role in regulating bone remodeling, and GLP-1 receptor agonist shows a positive relationship with osteoblast activity. However, GLP-1 receptor is not found in osteoblast, and the mechanism of GLP-1 receptor agonist on regulating bone remodeling is unclear. Here, we show that the GLP-1 receptor agonist exendin-4 (Ex-4) promoted bone formation and increased bone mass and quality in a rat unloading-induced bone loss model. These functions were accompanied by an increase in osteoblast number and serum bone formation markers, while the adipocyte number was decreased. Furthermore, GLP-1 receptor was detected in bone marrow stromal cells (BMSCs), but not in osteoblast. Activation of GLP-1 receptor by Ex-4 promoted the osteogenic differentiation and inhibited BMSC adipogenic differentiation through regulating PKA/β-catenin and PKA/PI3K/AKT/GSK3β signaling. These findings reveal that GLP-1 receptor regulates BMSC osteogenic differentiation and provide a molecular basis for therapeutic potential of GLP-1 against osteoporosis.

High Levels of Serum DPP-4 Activity Are Associated with Low Bone Mineral Density in Obese Postmenopausal Women

BACKGROUND

Dipeptidyl peptidase 4/CD26 (DPP-4) is a widely expressed cell surface serine protease. DPP-4 inhibitors, one of common anti-diabetic agents play a protective role in bone metabolism in recent studies. A soluble form of DPP-4 is found in serum, and exhibits DPP-4 enzymatic activity. However, the physiological role of serum or soluble DPP-4 and its relationship with DPP-4 enzymatic function remain poorly understood. The aims of current study were to determine the association between serum DPP-4 activity and bone mineral density (BMD) in postmenopausal women.

METHODS

We recruited data and serum samples from 124 consecutive healthy postmenopausal women aged >50 years. We divided study subjects into obese (body mass index [BMI] ≥25 kg/m²) and non-obese (BMI <25 kg/m²) postmenopausal women and examined the correlation between serum DPP-4 activity and clinical variables in each groups.

RESULTS

A total of 124 postmenopausal women was enrolled, with a mean age of 59.9±7.1 years. The mean BMI of the study patients was 24.4±2.8 kg/m². Regarding bone turnover markers, serum DPP-4 activity was positively correlated with serum calcium concentrations, intact parathyroid hormone, and serum C-telopeptide levels in all of the study subjects. However, there was no association between serum DPP-4 activity and BMD in the spine or femoral neck in all of the study subjects. Serum DPP-4 activity was negatively correlated (R=-0.288, P=0.038) with BMD of the spine in obese postmenopausal women.

CONCLUSION

This study demonstrated for the first time that serum soluble DPP-4 activity was negatively correlated with BMD in obese postmenopausal women.

The bone-preserving effects of exendin-4 in ovariectomized rats

Exendin-4 was found to be beneficial to the skeleton in diabetic rodents. In this study, we assessed the changes of bone mineral densities (BMDs) and quality in non-diabetic ovariectomized (OVX) rats after treatment with exendin-4. The regulatory role of exendin-4 on osteoblastogenesis and adipogenesis in rat bone marrow stromal cells (BMSCs) was also explored. Three months after sham surgery or OVX, 18 5-month-old female Wistar rats were divided into three groups and received the following treatment for 8 weeks: (1) Sham + vehicle; (2) OVX + vehicle; and (3) OVX + exendin-4 20 µg/kg/day. Micro-CT and three-point bending test were used to evaluate the BMDs, bone morphometric parameters, and biomechanical properties. Real-time PCR and Western blot were performed to measure gene and protein expression after exendin-4 treatment in adipogenesis and osteoblastogenesis of rat BMSCs. Exendin-4 could improve trabecular volume, thickness, and number, increase BMD, and reduce trabecular spacing in the lumbar spine and femur of OVX rats. Exendin-4 had little impact on the mechanical resistance of femurs to fracture. When rat BMSCs were treated with exendin-4, the mRNA expression levels of runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP), and collagen α1 (Coll-1) were increased, while those of peroxisome proliferators activated receptor γ (PPARγ) and CCAAT/enhancer-binding protein (C/EBPα) decreased. Exendin-4 treatment also resulted in increased expression levels of p38, p42/44, and β-catenin proteins. Exendin-4 was anabolic to bone in OVX rats possibly by facilitating osteoblastogenesis while repressing adipogenesis during BMSC lineage differentiation.

Differences in biochemical bone markers by diabetes type and the impact of glucose

BACKGROUND

Diabetes mellitus is associated with an increased fracture risk, however the fracture risk is 7 fold increased in patients with type 1 diabetes (T1D) and 1.4 fold increased in patients with type 2 diabetes (T2D) with decreased and increased bone mineral density, respectively. Oral ingestion of glucose causes an acute decrease in bone turnover markers, and thus glucose levels may affect bone turnover in diabetes.

OBJECTIVE

The aim was to examine disparities in bone turnover markers between patients with T1D and T2D and evaluate the effect of glucose on bone turnover.

METHODS

A cross-sectional study was conducted. Patients diagnosed with T1D (n=98) or T2D (n=96) were included from the outpatient clinics at two University Hospitals. All individuals had normal renal function. Glucose and bone turnover markers were measured in non-fasting blood samples.

RESULTS

P-procollagen type 1 amino terminal propeptide (P1NP), p-osteocalcin (OC), and s-Receptor Activator of Nuclear factor Kappa beta Ligand (RANKL) were lower in patients with T2D compared to T1D, and s-osteoprotegerin (OPG) was higher in T2D. P-C-terminal cross-linked telopeptide of type-I collagen (CTX), p-fibroblast growth factor-23 (FGF-23), p-sclerostin, and p-undercarboxylated osteocalcin (ucOC) were similar in between the two groups of patients. Increasing non-fasting glucose levels were inversely related to p-CTX, p-P1NP, p-OC, and p-ucOC and directly related to s-OPG in simple linear and multiple linear regressions adjusted for factors influencing bone turnover markers including HbA1c.

CONCLUSION

Bone turnover markers were lower in patients with T2D compared to T1D. Acute blood glucose alterations may change bone turnover mediated by OPG and have detrimental effects on bone health in diabetes.

TRIAL REGISTRATION NUMBER

ClinicalTrials.govNCT01870557.

Glucagon-like peptide-1 receptor agonist Liraglutide has anabolic bone effects in ovariectomized rats without diabetes

Recently, a number of studies have demonstrated the potential beneficial role for novel anti-diabetic GLP-1 receptor agonists (GLP-1RAs) in the skeleton metabolism in diabetic rodents and patients. In this study, we evaluated the impacts of the synthetic GLP-1RA Liraglutide on bone mass and quality in osteoporotic rats induced by ovariectomy (OVX) but without diabetes, as well as its effect on the adipogenic and osteoblastogenic differentiation of bone marrow stromal cells (BMSCs). Three months after sham surgery or bilateral OVX, eighteen 5-month old female Wistar rats were randomly divided into three groups to receive the following treatments for 2 months: (1) Sham + normal saline; (2) OVX + normal saline; and (3) OVX + Liraglutide (0.6 mg/day). As revealed by micro-CT analysis, Liraglutide improved trabecular volume, thickness and number, increased BMD, and reduced trabecular spacing in the femurs in OVX rats; similar results were observed in the lumbar vertebrae of OVX rats treated with Liraglutide. Following in vitro treatment of rat and human BMSCs with 10 nM Liraglutide, there was a significant increase in the mRNA expression of osteoblast-specific transcriptional factor Runx2 and the osteoblast markers alkaline phosphatase (ALP) and collagen α1 (Col-1), but a significant decrease in peroxisome proliferator-activated receptor γ (PPARγ). In conclusion, our results indicate that the anti-diabetic drug Liraglutide can exert a bone protective effect even in non-diabetic osteoporotic OVX rats. This protective effect is likely attributable to the impact of Liraglutide on the lineage fate determination of BMSCs.

Bone tissue as a systemic endocrine regulator

Bone is a target tissue for hormones, such as the sex steroids, parathormon, vitamin D, calcitonin, glucocorticoids, and thyroid hormones. In the last decade, other "non-classic" hormones that modulate the bone tissue have been identified. While incretins (GIP and GLP-1) inhibit bone remodeling, angiotensin acts to promote remodeling. Bone morphogenetic protein (BMP) has also been found to have anabolic effects on the skeleton by activating bone formation during embryonic development, as well as in the postnatal period of life. Bone has also been identified as an endocrine tissue that produces a number of hormones, that bind to and modulate extra-skeletal receptors. Osteocalcin occupies a central position in this context. It can increase insulin secretion, insulin sensitivity and regulate metabolism of fatty acids. Moreover, osteocalcin also influences phosphate metabolism via osteocyte-derived FGF23 (which targets the kidneys and parathyroid glands to control phosphate reabsorption and metabolism of vitamin D). Finally, osteocalcin stimulates testosterone synthesis in Leydig cells and thus may play some role in male fertility. Further studies are necessary to confirm clinically important roles for skeletal tissue in systemic regulations.

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.

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.

The efficacy of incretin therapy in patients with type 2 diabetes undergoing hemodialysis

BACKGROUND

Although incretin therapy is clinically available in patients with type 2 diabetes undergoing hemodialysis, no study has yet examined whether incretin therapy is capable of maintaining glycemic control in this group of patients when switched from insulin therapy. In this study, we examined the efficacy of incretin therapy in patients with insulin-treated type 2 diabetes undergoing hemodialysis.

METHODS

Ten type 2 diabetic patients undergoing hemodialysis received daily 0.3 mg liraglutide, 50 mg vildagliptin, and 6.25 mg alogliptin switched from insulin therapy on both the day of hemodialysis and the non-hemodialysis day. Blood glucose level was monitored by continuous glucose monitoring. After blood glucose control by insulin, patients were treated with three types of incretin therapy in a randomized crossover manner, with continuous glucose monitoring performed for each treatment.

RESULTS

During treatment with incretin therapies, severe hyperglycemia and ketosis were not observed in any patients. Maximum blood glucose and mean blood glucose on the day of hemodialysis were significantly lower after treatment with liraglutide compared with treatment with alogliptin (p < 0.05), but not with vildagliptin. The standard deviation value, a marker of glucose fluctuation, on the non-hemodialysis day was significantly lower after treatment with liraglutide compared with treatment with insulin and alogliptin (p < 0.05), but not with vildagliptin. Furthermore, the duration of hyperglycemia was significantly shorter after treatment with liraglutide on both the hemodialysis and non-hemodialysis days compared with treatment with alogliptin (p < 0.05), but not with vildagliptin.

CONCLUSIONS

The data presented here suggest that patients with type 2 diabetes undergoing hemodialysis and insulin therapy could be treated with incretin therapy in some cases.